The Pennsylvania State University. The Graduate School. College of Engineering CHARACTERIZATION AND OPTIMIZATION OF LASER-DOPED

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1 The Pennsylvania State University The Graduate School College of Engineering CHARACTERIZATION AND OPTIMIZATION OF LASER-DOPED SELECTIVE EMITTERS FOR APPLICATIONS IN SILICON SOLAR CELLS A Thesis in Engineering Science by Jillian J. Woolridge 2011 Jillian J. Woolridge Submitted in Partial Fulfillment of the Requirements for the Degree of Master of Science December 2011

2 The thesis of Jillian J. Woolridge was reviewed and approved* by the following: Edward W. Reutzel Research Associate, Pennsylvania State University Applied Research Lab Thesis Co-Adviser S. Ashok Professor of Engineering Science and Mechanics Thesis Co-Adviser Suzanne Mohney Professor of Materials Science and Engineering Judith Todd Professor of Engineering Science and Mechanics Head of the Department of Engineering Science and Mechanics *Signatures are on file in the Graduate School

3 Abstract Commercial or standard silicon solar cells (crystalline silicon cells with screen-printed front contacts, a PECVD silicon nitride layer, and an aluminum back-surface field rear contact) have reached a relatively stable plateau in terms of conversion efficiency. New methods of increasing efficiencies involve cell design modifications, with use of selective emitters being among the most popular. Selective emitters are highly doped regions beneath front metal contacts which enhance collection, shield contacts from minority carriers, and lower device series resistance for an overall increase in device efficiency. Between the metal contacts, low doping is desirable to increase lifetime and blue-response. Laser processing with high-power, fast repetition rate lasers makes fast throughput and production adaptation feasible. By using lasers, selected areas or regions on cell surfaces can be heavily doped while maintaining light doping between these contact areas. In addition, lasers can produce these structures using a one-step process, in contrast to conventional techniques that require multiple steps such as deposition of masks, chemical etches, and hightemperature diffusion steps. Optical absorptivity in silicon is known to be a strong function of wavelength. UV is only absorbed within a few tens of nanometers, while near-ir wavelengths will travel a few hundred microns. Fluence and laser pulse duration define the peak irradiance, which is known to play a significant role in material interactions. This work investigates the influence of wavelength, fluence, and pulse duration on selective emitter performance to determine industrially feasible process conditions for producing selective emitter-based solar cells in a practical manner iii

4 Table of Contents List of Figures... vi List of Tables... viii Acknowledgements... ix Chapter 1/ Introduction... 1 Solar Cell History... 1 Silicon Solar Cells... 1 Selective Emitters... 2 Industrial Manufacturing Processes... 2 Goals and Objectives... 3 Chapter 2/Background... 4 History of Lasers... 4 Laser Fundamentals... 4 Laser Material Interactions... 9 Silicon Solar Cells Laser Doping and Laser-Doped Emitters Chapter 3/Parameter Development Experiments Goals and objectives Laser firing Set-up Wafer description Parameter arrays, cross-section arrays Electrical characterization probe set-up Diode Junction Characterization Current-Voltage Characteristics Cross-sectioning Comparison and Correlation of Parameters Discussion of Error Chapter 4/Spot-Size Affected Lifetime Decay Experiments Goals and objectives Experimental Design Procedures Wafer Description Parameter choices iv

5 Firing of wafer Lifetime measurements Lifetime Characterization Discussion of Error Chapter 5/Diagnostic Cell Experiments Goals and objectives Parameter choices Device design Firing of cell Metallization Solar Cell Characterization Discussion of Error Chapter 6/Summary and Conclusions Parameter Studies Spot-Size Affected Lifetime Decay Study Diagnostic Cell Study Chapter 7/Future Work Works Cited Appendix A: Lifetime Wafer Parameters, Layouts, and Lifetime Maps Appendix B: Diagnostic Cell Parameters... 91

6 List of Figures Figure 1: p-n junction Figure 2: Ideal solar cell curve Figure 3: Practical equivalent solar cell circuit Figure 4: Solar cell with selective and shallow emitters Figure 5: Solar cell with both selective and shallow emitters Figure 6: (a) Quantel beam optics, (b) IPG beam optics, (c) Coherent beam optics Figure 7: Screening experiment wafer stack-ups, single passivation stack (left) and dual passivation (right) Figure 8: Wafer layout with ID numbers Figure 9: Sample parameter layout Figure 10: Parameter layout for IPG Laser, 1070 nm, μs pulse duration Figure 11: Sample processed for cross-sectioning, including wafer layout, schematic of wafer sectioning, and final wafer halves glued face-to-face for polishing, etching, plating, and characterization Figure 12: Probe placement (top) single umetallized diode uncertainty and (bottom) metallized parameter set Figure 13: Comparison of gated CW pulse and Q-switched pulse Figure 14: SEM morphologies of 1070 nm CW fiber laser, dual passivation, unmetallized Figure 15: SEM morphologies of 1070 nm CW fiber laser, single passivation, unmetallized Figure 16: SEM morphologies of 355 nm, 30 ns pulsed laser, dual passivation, unmetallized. 40 Figure 17: SEM morphologies of 355 nm, 30 ns pulsed laser, single passivation, unmetallized Figure 18: SEM morphologies of 355 nm, 30 ns pulsed laser, dual passivation, single shot, unmetallized Figure 19: SEM morphologies of 1064 nm, 4 ns pulsed laser, single passivation (left) and dual passivation (right), unmetallized Figure 20: SEM morphologies of 532 nm, 4 ns pulsed laser, dual passivation, unmetallized Figure 21: Current-voltage characteristics of 1070 nm CW fiber laser, dual passivation (top) and single passivation (bottom), unmetallized, from -1.0to 1.0 V Figure 22: Current-Voltage characteristics of 355 nm, 30 ns pulsed laser, dual passivation (top) and single passivation (bottom), unmetallized Figure 23: Comparison of current-voltage characteristics for lower and higher fluencies at 355 nm, 30 ns Figure 24: Current-Voltage characteristics of 1064 nm, 4 ns pulsed laser, single passivation (left) and dual passivation (right), unmetallized Figure 25: Current-Voltage characteristics of 532 nm, 4 ns pulsed laser, dual passivation (right) and single passivation (left), unmetallized... 52

7 Figure 26: IPG cross sections, dual passivation layer Figure 27: IPG cross sections, single passivation layer Figure 28: Quantel cross sections, single passivation layer, 1064 nm Figure 29: Quantel cross section, dual passivation layer, Figure 30: Quantel cross sections, single passivation layer, Figure 31: AVIA cross section, single passivation layer Figure 32: IPG doping parameters, I-V, surface morphology, and cross section for dual passivation structure Figure 33: Quantel doping parameters at 1064 nm, I-V, surface morphology, and cross section for single passivation structure Figure 34: AVIA doping parameters, I-V, surface morphology, and cross section for single passivation structure Figure 35: Lifetime wafer stack-up, passivated Figure 36: Wafer lay-out of parameter sets Figure 37: Wafer Si_p_01, before (left) and after (right) anneal Figure 38: Diagnostic cell wafer flow Figure 39: Mask layout for diagnostic cell, 100 μm finger width and 1.5 mm finger spacing Figure 40: AVIA diagnostic cell illuminated curve (top), diode characteristics (bottom left), and diode surface morphology (bottom right) Figure 41: Quantel diagnostic cells illuminated curves (top left and right), diode characteristics (bottom left), and diode surface morphology (bottom right) Figure 42: Si_b_01 wafer layout for spot-size affected lifetime decay experiments Figure 43: Si_p_01 wafer layout for spot-size affected lifetime decay experiments... 87

8 List of Tables Table 1: Summary of diode parameters Table 2: Summary of laser parameters and characteristics Table 3: Beam diameters and fluencies tested Quantel (1064 nm, 4 ns pulse) Table 4: Beam diameters and fluencies tested IPG (1070 nm, μs pulse) Table 5: Beam diameters and fluencies tested - AVIA (355 nm, 30 ns) Table 6: Diameters and preserved ratios for a fired pad of 5 x 5 mm Table 7: Spot-size affected lifetime decay parameters Table 8: Diagnostic cell parameters Table 9: Diagnostic cell parameters and cell performance characteristics... 92

9 Acknowledgements Thank you first to my advisors, Dr. Reutzel and Dr. Ashok, who took someone who knew little to nothing about either lasers or semiconductors and did their best to teach me about both. You have been patient and supportive, and I greatly appreciate your guidance though this research and thesis. Thank you to Dr. Mohney and her past and present students, for their materials expertise and instruction, and to the rest of the BP Solar team at the Applied Research Lab, for all the hard work and collaboration on the project. To Drs. Lian Zou and Dave Carlson of BP Solar in Frederick, MD, we thank you for your funding, support, analysis, and research collaboration; I hope the joint efforts have enriched both of our institutions. Lastly, to dad, mom, Ainsley, Ginger, and all of my extra families and friends (you know who you are), who have supported me in my insanity, who have helped me through the roughest of times and celebrated in the best, and who, I hope, still love me no matter what.

10 Chapter 1/ Introduction Solar Cell History Edmund Becquerel first reported photovoltaic action in 1839, when an illuminated silvercoated platinum electrode, immersed in an electrolyte, produced an electric current. The photoconductivity of metal-semiconductor barriers fascinated researchers, but it was not until over one hundred years later, with the development of reasonable-quality silicon wafers in the 1950 s for the development of new solid-state electronics that the p-n junction solar cell was fabricated. The p-n junctions created showed much greater photovoltaic action, producing efficiencies six times higher than the Schottky barriers, but production costs were still in the hundreds of dollars per Watt. While this was too high to consider for everyday power, an obvious application was in extraterrestrial power for use in satellites, where the cost of fuel is inconsequential compared to the weight. Thus, solar cell technology took off with development of applications in space (1). Silicon Solar Cells Silicon was the material used for the first practical cells, and remains today the number one material for commercial solar cell production. Material costs, maturity of technology, and the fact that silicon can grow its own insulator in the form of silicon dioxide keep silicon cells at the forefront of the available technology. In particular, terrestrial solar cells, where the cost of the energy exceeds considerations such as weight and efficiency, are typically made from a form of silicon. Single-crystal silicon yields the highest efficiencies due to higher lifetimes of charge carriers with no grain boundaries, while multi-crystalline cells are cheaper to produce due to the lack of need for high-quality, single crystal growth. 1

11 Selective Emitters As silicon solar cells reach their maximum potential efficiency with state-of-the-art processing and materials, new structures or devices must be found to further increase the efficiency of standard cells. One of these emerging technologies is the selective emitter. Standard solar cells typically have a blanket shallow emitter, (2) doped to an optimum depth and dopant concentration to maximize both blue (or UV) response and minimize sheet resistance. Higher dopant concentrations increase carrier recombination by creating traps and recombination centers, and decrease blue response (via high concentration of dopants crating a dead layer at the surface) and diffusion length of majority carriers, but also improve resistivity and decreases contact resistance to front-side contact fingers. Typical cells with only a shallow lightly-doped emitter will optimize dopant concentration in order to compromise between these competing effects. A cell with selective emitters will have a fullsurface area shallow doped emitter to maximize blue response and electron lifetime, and create heavily-doped selective emitters beneath metal contacts to minimize contact resistance. Current efforts of research and development are focused on industrially feasible methods of selective emitter formation. Industrial Manufacturing Processes Industrial manufacturing involves large batch processes for cleaning, etching, diffusing, and screen-printing of front contacts. As previously mentioned, large batch processing steps for silicon are technologically far more advanced than for any other semiconductor material, and full-surface emitters are already made during batch processes in industrial diffusion furnaces. Selective emitters, the next step in further increasing cell efficiency, have yet to be incorporated industry-wide. Laboratory processes to produce selective emitters have proven their effectiveness in increasing cell performance, but conventional fabrication methods 2

12 involve time-, material-, and cost-prohibitive lithographic methods. Such processes would cost more than they would save in cost per Watt of a final cell, and therefore industriallyfeasible methods for incorporating selective emitter technology into current production methods are being sought. A laser-based processing solution is one of these technologies, either for selective doping or selective removal to create openings through surface dielectrics or anti-reflection coatings. Goals and Objectives The goal of this thesis is to investigate the properties of laser-doped selective emitters created using various types of lasers and processing conditions and characterize their performance in solar cells. Lasers to be investigated include an infrared (IR) continuous wave (CW) fiber laser operating at 1070 nm and gated to produce pulses in the 10 s to 100 s of microseconds, a Q-switched Nd:YAG laser with a 4 ns pulse operating at 1064 and 532 nm, and a Q-switched 3 rd harmonic Nd:YVO 4 laser with a 30 ns pulse operating at 355 nm. Characterization methods applied to the laser-doped spots include current-voltage characteristic analysis and comparison to an ideal diode, SEM micrograph investigation for the observation of surface morphologies, cross-sectional junction delineation via selective electroless plating techniques, and computational modeling. 3

13 Chapter 2/Background To design experiments in laser doping of selective emitters, one must have an understanding of both lasers and solar cells. This chapter will provide a brief introduction to lasers and laser operations, as well as laser material interactions. A description of silicon solar cell principals of operations and components is included, to understand the underlying physical properties and materials needed here, and then a summary of current work in the area of laser material surface modification and doping concludes this introduction. History of Lasers The term Laser, though now recognized as a noun, is actually an acronym for Light Amplification by Stimulated Emission of Radiation. Lasers actually began as masers, microwave amplifiers which were designed to use coherent beams of microwaves in communication applications (3). The first masers used wavelengths of 1.25 cm, which was related to the dimensions of the amplifier. In 1958, Charles Townes and his associate Arthur Schawlow proposed an optical amplifier based on the principals of the maser and using optical mirrors to amplify the beam. Two years later, Theodore Maiman of Hughes Research Laboratories developed the ruby laser, using flash lamps to pump a ruby crystal rod which had been coated at both ends with a reflective material. When the flash lamp was fired, a red beam was emitted from the end of the rod. Thus the laser was born, and half a century of research, development, and experimentation has led to a variety of lasers, each using different mediums and methods to amplify light. Laser Fundamentals Principals of Operation Albert Einstein proposed the idea of stimulated emission in a 1917 paper (4), which mathematically predicted the phenomenon of wave amplification. Stimulated emission relies 4

14 on the concept of quantization of energy and discrete energy states within molecules and atoms. Atoms can be at varying energy states, but according to Boltzmann s Principal (5), at thermal equilibrium, more atoms will be in lower energy states than in higher energy states. However, for stimulated emission to occur, it has been proven that there must be a population inversion; that is, there must be more atoms in an excited state. For energy states 1 and 2, with a given number of atoms N 1 and N 2 in each state, where net absorption by a number of atoms (N 1 ) and emission of a second number of atoms (N 2 ) with light of intensity I(t) (and proportionality constant K) is given by the equation. (1) We can infer from this equation that if the number of atoms in the lower state 1 is greater than then number of atoms in the upper state 2, light will be absorbed. Inversely, if N 2 is greater than N 1 then an incoming energy wave will be strengthened by electromagnetic emission from the atom as the N 2 atoms relax to the lower energy state N 1. However, since at equilibrium N 2 would not exceed N 1, a lasing medium must be externally pumped with enough energy to raise over half the atoms out of the low-energy ground state. Sometimes a second or even third energy state is required to allow for enough atoms to maintain the N 2 state; that is, atoms are pumped to the N 3 state, where they decay rapidly to a nearby N 2 state. Then, a population inversion between N 2 and N 1 allows for the stimulated emission phenomenon to occur. A photon of wavelength λ, corresponding to the energy difference between the energy states E 2 - E 1, interacts with an excited atom in the 2 nd state, and the atom drops to the ground state, emitting an additional photon of the same wavelength and phase. Thus, the energy has effectively been doubled, and the light has been amplified. The light thus released is contained within the laser cavity or active medium. To contain the light, both ends of the cavity are sealed with reflective mirrors. The light therefore is reflected between the two mirrors back and forth through the medium, increasing the 5

15 amplification with each pass. Different cavity designs and reflection methods are used depending on the lasing material. Generally, though, a totally reflecting curved mirror encloses one end, and the other end is either partially reflecting or has a window or aperture for releasing the beam. Pulsed versus Continuous Wave Operation Pulsed or continuous wave operation refers to the duration of lasing activity, and to the method of pumping of the active lasing medium (6). Pulsed lasers emit short-duration pulses of energy, while continuous wave lasers emit a continuous beam until the pumping energy is released. Pulsed lasers may use medium pressure xenon or krypton lamps, between Torr, to provide the external pumping energy. The intense flash from these lamps, in the millisecond duration regime, supplies the necessary energy to raise the active lasing medium into an excited state. Continuous wave lasers, on the other hand, have historically been pumped by high pressure (roughly Torr) krypton or tungsten-iodine lamps which are applied continuously, instead of flashed, thus providing an unbroken supply of energy to pump the lasing medium. Lasers can also be pumped using diodes or diode lasers (6), which are of higher electrical efficiency than flash lamps. Additionally, the output wavelength of the diode laser is matched to the absorption bandwidth of the actual lasing material, and thus less energy is wasted in pumping the lasing medium. Solid State and Fiber Lasers Solid state lasers are crystal rods mounted in a cavity with a pumping medium (flash lamp, etc.) (4). The medium lases like any other type of laser, and sometimes a Q-switch is employed for quick shuttering and pulse length control. Q-switches work by using a variable attenuator to limit or eliminate resonance in a gain medium. The gain medium is pumped to 6

16 cause population inversion without allowing amplification, until saturation is reached, and then the Q-switch is reversed, allowing feedback. This produces a short, intense pulse due to the high amount of stored energy in the medium. These types of lasers are typically made of neodymium-doped yttrium aluminum garnet (Nd:YAG) or yttrium orthovanadate (Nd:YVO 4 ), but other common materials are yttrium lithium fluoride (YLF), yttrium aluminum phosphate (YAP), and phosphate or silica glass, all doped with rare earth elements. Fiber lasers have a glass fiber doped with rare earth elements and pumped via diodes, thus making them extremely efficient (7). The entire fiber acts as both resonator (with dielectric coatings or Bragg gratings acting as reflectors for each end) and delivery system. The beam quality can be quite good, and fiber lasers can operate in both pulsed and CW modes. Other Common Lasing Mediums Ruby, CO 2 and Excimer Lasers The first successful laser, invented in 1960 (4), consisted of a ruby crystal coated on both ends with reflective material. Ruby crystals are comprised of active chromium held in an aluminum oxide matrix, and can be pumped to an excited energy state which then falls quickly to a more stable state at 1.77 ev. This corresponds to visible emission at 694 nm. Due to the heavy pumping needed for ruby lasers, they are limited to pulsed operation, as opposed to continuous wave operation. Ruby lasers can be pulsed using a megawatt flash lamp, and provide pulses in the nanosecond range (6). Carbon dioxide, as pure CO 2, is not in itself very powerful or effective at lasing. However, excited nitrogen molecules, N 2, oscillate at a frequency essentially close enough to that of CO 2 that an excited N 2 molecule can collide with a CO 2 molecule and excite it to the necessary level for lasing (4). This transfer of energy only happens with cold CO 2, however, so cooling can be a major issue for such lasers. Therefore, such lasers are built with the 7

17 cooling of the lasing medium as a high priority. The lasing mixture of gas is typically 78% helium for stability and thermal conduction, and then 13% nitrogen and 10% carbon dioxide. Different designs exist for efficient cooling of the gas mixture. Excimer lasers are made of excited dimers of inert gases, such as argon, krypton, or xenon, and halogens, such as fluorine or chlorine (5). They are excited electrically, and, due to short lifetimes of the unstable ions, can operate only in pulsed modes, and usually have wavelengths in the UV range. Beam Manipulation Once a beam exits the resonating cavity, it must be directed to the working surface. This is done via a series of optical elements used to manipulate, shape, guide, or otherwise direct the beam. (4) Simple optics can be used to expand, collimate, or focus a beam. Mirrors can be used to turn or even turn and focus a beam off-axis if space does not allow a beam to directly access a processing surface. Coatings or filters can be used to absorb or deflect a certain portion of the energy to reduce the beam energy without changing any laser parameters. More advanced optics can shape a certain energy distribution (e.g. Gaussian) to a desired output energy profile (e.g. top hat, square, line, etc.) or even split a beam into even identical beamlets. This is useful for employing a more powerful laser to process many regions at once. Lasers in Industry The laser was dubbed a solution looking for a problem (4) when it was first invented in the 1960 s, but now the laser is employed in a wide range of diverse industries. Laser energy and light can be used for measurement of distances and velocities, imaging, communications, medical or surgical procedures, and as a heat source. Lasers are used in welding, cutting, drilling, cladding, heat treating, scribing, marking, and many other high-energy and/or high- 8

18 precision areas. Power levels ranging from microwatts to kilowatts are used depending on the amount of energy needed for a given application. For this thesis, the use of laser energy to heat and process material is of interest. Laser Material Interactions Lasers are unique in the material processing field for the way that they can very locally and selectively apply a large amount of energy to a small area. There are many considerations when processing with lasers, both concerning the laser processing conditions to be employed and the material to be processed. For lasers, properties such as the operating wavelength, operating mode (pulsed versus continuous wave), power or energy, beam mode or energy distribution, polarization, and many other factors can affect the way a laser interacts with a substrate material. Material properties are also crucial to consider. Absorption or reflection at the laser s operating wavelength is one of the most important characteristics, as it affects the coupling of the beam to the work piece. Others include the physical size and thickness of the work piece, the thermal expansion, refractive index, thermal diffusivity and conductivity of the material, and more. The desired type of processing also influences the selection of processing parameters. Laser conditions for welding may not be the best for cutting, and the difference may be as small as different optics to change the focal spot size and thus the fluence, or as large as an entire change of laser. Absorptivity at the substrate can be affected by polarization of the laser beam, and therefore cutting in one direction could be different than cutting perpendicular to that direction. Welding of inch-thick steels requires different power levels than micro-welding of medical instruments. These are just a few examples of how the processing requirements can also dictate the parameters used in a given situation. 9

19 Melting versus Ablation Melting can be realized through the use of laser energy to heat and cause a phase transformation from the solid to the liquid state in the substrate. If enough energy is applied at the surface, the molten material may also vaporize, thus removing material from the substrate. Ablation is the removal of material from the surface by application of high enough energy to vaporize the surface material and expel it from the bulk. Generally, pulses between a few to tens of nanoseconds are used for industrial ablation removal processes (8). Some laser ablation models assume that the laser energy is directly converted to lattice vibrations, breaking bonds and expelling material without resulting in the large thermal transfers which cause melting seen with longer pulses (9). Solid-state versus liquid-state diffusion and mixing Diffusion rates in materials are strongly dependent on the states of the materials. Solid materials are held in a lattice, and thus diffusion is much slower between two solid materials. Liquid-state diffusion occurs much more rapidly due to the weaker bonds between the molecules. In addition, within the liquid phase, convective forces induce mixing of the materials (10); thus, both diffusion and convection are responsible for the mass transport between the two materials. Due to the time scales in certain cases of pulsed laser processing, the rapid melting and solidification of materials can result in doping higher than the solubility limit of one material in another as rapid cooling rates can cause material to be frozen in their non-equilibrium liquid concentrations. Silicon Solar Cells The development of practical solar cells was first possible in the 1950 s with the introduction of higher quality silicon wafers, and useful amounts of power were producible (1). The first cell with 6% efficiency was reported by Cahpin, Fuller, and Pearson 10

20 in The high cost per Watt meant that commercial cells were not considered at this point. However, with the growth of satellites and space exploration, cost was a small issue compared to payload weights, and solar cell research continued. Interest was again sparked with the energy crises in the 1970 s, as all forms of renewable energy became high priority. Intense research was undertaken to improve material cost and module production, as well as to develop advanced designs such as multi-junction cells. Solar for remote areas generated the most interest, where the electricity supply was most expensive. As prices fell, newer markets opened (11). Principles of Operation Solar cells are basically large-area semiconductor p-n homo- or heterojunctions. Silicon has a band-gap of roughly 1.1 ev, allowing it to absorb light of about 1100 nm and shorter, corresponding with the majority of the radiation of the sun. In the dark, no current is generated. However, when illuminated with irradiation of high enough energy, i.e. anything in the visible spectrum, photons of light are absorbed in the material. The energy of the photons is used to energize an electron from a ground state, in the valence band, to an excited state in the conduction band of the material. This creates an electron-hole pair, the charge carriers of current. Once these charge carriers are generated, they must be separated in order to do useful work (12). Silicon can be doped into either p- or n-type by incorporating dopants into the crystal. Boron is a typical p-type dopant for crystalline silicon, and phosphorous is a typical n-type dopant. To create a p-n junction for charge separation in a solar cell, silicon wafers doped to either p- or n-type are chosen, and the opposite type of dopant is diffused into the wafer at one surface. This is typically done in a thermal diffusion furnace, by introducing a gas containing the opposite dopant atoms at elevated temperatures. The dopant is diffused to 11

21 reverse the species of majority carrier of the surface region of the wafer; where the majority carrier changes from n- to p-type, there exists an electric field between the more negatively and positively doped regions. This electric field is the means of separation of the photogenerated electrons and holes. Electrons and holes generated in this space charge region (SCR) are pushed in opposite directions via an electrostatic drift force. Figure 1 shows a simple diagram of a solar cell with n-type emitter, p-type wafer, front and back contacts, and space charge region. Note that the wafer is not drawn to scale and would be many times thicker than shown. Figure 1: p-n junction Once they reach their respective edges, electrons towards the n-type and holes towards the p-type, they are collected by the contacts following majority carrier drift in these (quasi-) neutral regions. Electrons and holes generated outside of the electric field of the p-n junction must first diffuse towards this region in order to be separated. Hence it is necessary to use high-quality silicon, with high lifetime for the photogenerated carriers. This corresponds to long diffusion lengths, allowing charge carriers the maximum possibility of reaching contacts before recombining and releasing the absorbed energy. Characterization An ideal solar cell can be described by the Shockley solar cell equation 12

22 1 (2) which describes a current source (the sun) in parallel with a rectifying diode (the p-n junction), where q is the elementary electron charge, V is the voltage, and k b is the Boltzmann constant. I ph represents the photogenerated current under illumination, and I 0 is the diode saturation current in the dark, a property of the quality of the junction. (11) Ideally, the short circuit current I sc, or the current under illumination with zero load resistance, is equal to the photogenerated current and the open circuit voltage V oc, the voltage under illumination with no external load (R L ), is theoretically calculated by the equation ln 1 (3) A theoretical I-V curve of a solar cell under illumination is shown in figure 2. The maximum power point P max illustrates the point at which the device will produce the most power. Note that this does not occur at V oc or I sc, but rather at a voltage and current slightly below each. Figure 2: Ideal solar cell curve 13

23 The fill factor FF is a figure of merit concerning cell operation, and is a ratio of the max power to the power theoretically obtained if the cell were to operate at its short circuit current and open circuit voltage, that is: (4) However, in practice, many factors contribute to a variation in the characteristics from the ideal. A two diode model, shown in figure 3, is used with an ideality factor in the exponential to fit a curve showing two distinct regions, one relating to the SCR region and one to the quasi-neutral region (QNR), the region further from the junction experiencing no electric field. In addition, series and shunt resistances (R s and R sh, respectively) almost always exist and affect the end maximum power output. A shunt resistance is a conductive path in parallel with the diode and allows a leakage of current that does not contribute to the power, and a series resistance, in series with the output current, causes a drop in the total voltage available. Figure 3: Practical equivalent solar cell circuit The practical equation relating all variables is now 1 1 (5) 14

24 A qualitative analysis of the diode equation and circuit diagram can show that one would design for the highest possible shunt resistance (near infinite) and the lowest possible series resistance (near zero) to maximize I and V oc. A final figure of merit when evaluating the performance of a cell is the efficiency, which is a ratio of the measured power output to the input. In solar cell testing, the standard of Air Mass 1.5, or AM 1.5, equivalent to 1000 W/m 2 is used to simulate the intensity of the sun s radiation after traversing approximately 1.5 lengths of the earth s atmosphere to account for diurnal and other variations of the solar intensity on earth s surface. The power output per unit area is divided by the AM 1.5 standard to give a total conversion efficiency. Metallization In order to effectively connect a solar cell to an external circuit, ohmic contacts must be formed with either side of the p-n junction to extract the charge carriers. Metallization techniques are important to improve cell performance, and there are many factors to consider. Material, design, and fabrication methods all play different roles at different stages of research, development, optimization, and commercialization. Pattern design A large factor in the efficiency of front-contact solar cells is the shadowing factor, or percent of the front surface that is covered by metallization. Front side metallization covers a part of the cell, blocking the incoming sunlight and lowering the amount of incident energy that gets converted to electricity. For this reason, minimal metallization allows maximum conversion. However, less metallization also results in higher resistances through the fingers and bus bars which carry the current up and out of the cell (13). Therefore, optimization of finger width and spacing is crucial to overall cell efficiency and performance. 15

25 Lithography and Metal Deposition Lithographic methods of patterning front side contacts are primarily used in research, and can involve many labor- and material-intensive steps. This method is ideal for precise control of contact openings and other processes which require patterning. For example, a mask could use lift-off techniques to pattern a contact grid to the front surface, and a metallization technique such as sputtering, evaporation, etc. could be used to apply the desired metal to form the front contact. However, the slow throughput and large material and equipment requirement limit use of lithography in industrial production to necessary processes unrelated to metallization contact patterning. Screen Printing Silver screen printing is a mature, industrially-feasible method employed widely in commercial production of solar cells. Benefits of this method are the mature technology, high throughput, relatively low cost and repeatability of the process. However, silver screen printing is a contact process and can induce cracking of the wafer if too much mechanical stress is applied to the cell. Also, relatively large finger widths lead to larger than ideal shading losses. Selective Plating Recent research in metallization for solar cells has focused on selective deposition or plating of metals to doped areas of the surface of the cell, either using more highly-doped areas or a dielectric layer as a mask to pattern the contacts (14) (15) (16) (17) (18) (19). Researchers at the Fraunhofer Institute for Solar Energy Systems have developed a method of electroless nickel sulfate bath-plating using patterned openings in a silicon nitride layer as a mask to selectively plate front contact fingers. The nickel is deposited in an alkaline ph 16

26 solution, and then annealed to form a conductive nickel silicide. The initial nickel seed layer can then be thickened using light-induced plating of silver to increase conductivity. Another method investigated at Fraunhofer was formation of the seed layer via aerosol printing, which uses a sprayed metal aerosol from a nozzle tip, guided by a focused cone of gas, to direct the metal to plate in a specific area, followed by light-induced plating of silver. Silicon Solar Cell Components Solar cells can be divided into front and back contact cells. Front contact cells have metallization on both sides of the cell and require design of metal grid patterns to minimize shading and maximize the amount of useful absorber area on the surface. Interdigitated back contact cells incorporate emitters and base contacts on the same side; they eliminate shading efficiency losses but give rise to new manufacturing hurdles. Films of different materials are usually employed as passivation and anti-reflection coatings on silicon solar cells. Passivation is a technique used in solar cells to decrease surface recombination and lifetime degradation at the wafer surface by either completing dangling silicon bonds by hydrogen incorporation or by supplying a fixed charge to repel the minority carriers away from the dangling bonds (or sometimes, a combination of both). It is an important part in every commercial cell available today, as passivation is a simple way to increase cell efficiencies. For silicon, typical passivation materials are amorphous silicon (a-si:h) or silicon dioxide. Passivation can be used on both front and back sides of the cell, as long as the metal contacts can still penetrate to the active regions and collect the carriers. Anti-reflection coatings (ARCs) and texturing are used to increase the light absorptivity of the cell; ARC s are thin films which have a refractive index between that of air (or the outer environment) and the substrate (the solar cell) and whose thickness is equivalent to a quarterwave of the wavelength to be absorbed. For the sun, this is maximized at the peak of the solar 17

27 spectrum, around 550 nm. Silicon nitride is the most widely used ARC industrially. Texturing of surfaces causes scattering of the light, trapping it within the cell and increasing the chance of absorption by effectively increasing the length of travel of longer, less absorbed wavelengths. The simplest solar cells have an optimized full front surface emitter with an optimized doping concentration. The concentration of the dopants must be high enough to provide sufficient conductivity and charge separation, but must be low enough to not cause degradation of lifetime and blue-response of the cell. These shallow emitters however tend to have a higher resistance than ideal at the front contacts due to a lower doping concentration, and the newest trend to become commercially viable is the selective emitter. Figure 4 shows an example of a solar cell, connected to an external load, with both selective and shallow emitters. Figure 4: Solar cell with selective and shallow emitters A selective emitter is a highly doped region that exists only under the front metal contacts. The higher doping decreases contact resistance, and since the higher concentration is limited to below the contacts, it has little effect on the rest of the absorptive area of the cell, thus increasing efficiency by decreasing the contact resistance. Laser doping is a technology that may enable efficient formation of these selective emitters. 18

28 Laser Doping and Laser-Doped Emitters Due to the rapid miniaturization of electronics, smaller circuit elements with the same or better performance are required. In terms of transistors, for example, this can be achieved in part by using increasingly shallow p-n junctions, and the limit to which traditional furnace diffusion is a viable fabrication method is being reached. New methods of forming highlydoped shallow junctions are being sought, and laser doping is becoming as proficient a solution as any (9). Laser doping involves the rapid melting and solidification of a substrate material, incorporating dopant atoms during the solidification process. The dopant can be supplied in various ways, either via a gaseous, liquid or solid pre-placed source (2). The Gas-Immersion Laser Doping or GILD method incorporates dopants from a gaseous source by using laser energy to break the bonds of the gas in order to incorporate single dopant atoms into the melt pool produced by the beam. In contrast, a bath or jet of liquid with the dopant precursor can also be employed in a similar manner. Recent work in this area by Kray, et al. at the Fraunhofer Institute for Solar Energy Systems (20) (21) involves a liquid jet of dopant precursor that acts as both dopant source and laser guide, since the laser beam is contained within the jet via total internal reflection. This method is now in production at RENA GmbH in Germany for the production of selective emitters in solar cell modules. The Laser Induced Melting of Predeposited Impurity Doping or LIMPID method uses a thin film of dopant source on the substrate surface and uses the laser to melt the film and the substrate (2). The melt pool dynamics cause incorporation of the dopant into the substrate, where solidification and recrystallization freezes the dopant atoms in the substrate lattice. Laser Doping of Silicon Generally a shorter wavelength (e.g., ultraviolet laser) is used on silicon due to its higher absorptivity; the energy is absorbed near the surface within a few hundred nanometers and 19

29 the resulting junction is ultra-shallow (22) (23). Wavelengths around the green area of the visible spectrum, namely the frequency-doubled wavelength of an Nd:YAG laser (532 nm) with hundreds of nanosecond pulses can give a junction around one micrometer (24), and, for similar pulse durations, longer near IR and IR wavelengths show deeper penetration and heat-affected zones due to low absorptivity (25). It is believed that some undesirable impurities can be incorporated during the liquid diffusion and melting; these impurities include oxygen, carbon, and other atmospheric gases. It is believed that such impurities serve as recombination centers and decrease carrier lifetime (26). Laser-Doped Full-Area Emitters in Solar Cells Lasers of a variety of wavelengths and pulse durations, from ultraviolet to infrared and nanosecond to continuous wave, have been used to irradiate full-surface areas in attempts to dope shallow emitters for solar cells. Laser doping has the advantage of locally heating at the surface, as opposed to subjecting an entire wafer to thermal stresses seen in conventional furnace diffusion. Recent investigations are discussed below. Continuous Wave Laser Doping Hasegawa, et al., at the Graduate School of Materials Science of the Nara Institute of Science and Technology in Japan demonstrated a full-area doped emitter via CW irradiation at 532 nm using an Nd:YVO 4 laser, processing with a spot size of 6 μm, scanning speed of 6 cm/s along the x axis and y-shift after each scan of 1-6 μm, on a p-type multicrystalline silicon substrate with a preplaced spun-on phosphorous source liquid. Pitting was observed on the surface of the sample. Samples were not optimized via antireflection or passivation 20

30 layers. Open circuit voltages of 501 mv, short circuit currents of 13.6 ma/cm 2 and a fill factor of 58.6% were achieved (27). Sameshima, of the Tokyo University of Agriculture and Technology in Japan used a CW IR semiconductor laser with a mean spot size of diameter 180 µm, wavelength 940 nm and power of 20 W, travel speed of 3-20 cm/s, effective dwell time of 0.9 to 6.0 ms, peak power density of 70 kw/cm 2 to recrystallize amorphous silicon to electrically activate ion-implanted phosphorus dopants (28). It was shown that recrystallization could be achieved while maintaining dopant profiles, demonstrating that the annealing method was not pushing the dopant atoms further into the material. Recrystallization occurred just below the melting temperature of silicon. The same group, later in a paper by Ukawa, et al., used a semiconductor CW laser at 976 nm wavelength, travel speeds of mm/s, effective irradiance of 375 kw/cm 2, 40 W power and equivalent dwell times of µs to study the doping activation depths and recrystallization patterns of full-surface-irradiated silicon wafers (29). In this work, a line-focused beam of area 500 µm by 15 µm was scanned across the surface of p-type silicon with ion-implanted boron and phosphorus atoms/clusters. I-V characteristics of the phosphorus-doped samples show diode-qualities with ideality factors of 1.58 to 1.75, and reverse bias currents of 2.7 to 7.4 µa/cm 2. Depths from about 150 to 600 nm, determined by the implantation energy (from 100 KeV to 500 KeV), were achieved, and the laser was used to irradiate and recrystallize the silicon in the solid state, incorporating the atoms into active sites in the lattice. IR Pulsed Laser Surface Modification Tian, et al., compared excimer and first harmonic (1064 nm) Nd:YAG pulsed lasers for doping of aluminum and nitrogen into silicon carbide. Pulse energy densities lower than the decomposition and ablation thresholds were used to achieve both shallow and deep doping 21

31 profiles. These profiles ranged from 600 nm to 4 µm for 1064 nm, using pulse durations of 270 ns and 70 ns and pulse energy for both wavelengths of mj/cm 2 (25). Tian calculated temperatures at different depths for the two different lasers (UV and IR) and found a much higher surface temperature and deeper heat conduction with a 1064 nm wavelength laser compared to UV excimer laser. He also found higher-than-maximum-solubility concentrations of dopants in the surface of the wafers due to non-equilibrium processes of heating and cooling during laser doping. Researchers at the Universidad Autónoma Metroplitana-Iztapalapa in Mexico used a pulsed Nd:YAG laser at the fundamental and third harmonics (1064 nm and 355 nm) to irradiate n-type silicon in oxygen atmosphere and observe morphological changes at the surface (30). The laser was pulsed at energy densities of 6 J/cm 2 at a frequency of 10 Hz with a 10 ns width, and a random scanning pattern was used to irradiate the entire surface. The area for 1064 nm was 2.1 x 10-2 cm 2, and for 355 nm was 3.2 x 10-3 cm 2. For the IR wavelength, the first features seen were fractures at the surface of the silicon. After continued irradiation, these fractures develop and become shaped into cones. The UV wavelength produced ripples or waves at the surface but no fractures, and after prolonged exposure, steep cones were formed. The end result was similar to the IR irradiation; however, initial fluencies produced surface cracks using the IR wavelength. Though doping was not performed here, surface morphologies should be considered when doping. Molpeceres, et al., of the Universidad Politécnica de Madrid in Spain, investigated the ablation characteristics of UV and IR pulsed lasers (less than 12 and 70 ns each, respectively) on both amorphous silicon and the transparent conductive oxide indium-tin-oxide (ITO) (31). Results showed that at small pulse energies for all lasers of 0.75 mj, did not fully ablate material, while energies higher (from 1.25 to 2.5 mj) achieved ablation, and higher (>3.5 mj) affected the underlying substrate. The paper notes that absorption for a-si is two orders of 22

32 magnitude higher for UV wavelengths than for IR, and consequently differences in the ablation profiles for the two wavelengths were noted. Most importantly, thermal damage was seen on the edge of the kerf widths. For such high energies and ablation, this could be undesirable; however, for the purpose of doping, low thermal gradients in the material below damage thresholds could be useful for the incorporation and diffusion of dopants, though that was not the specific focus of this study. UV Laser Doping Due to the high absorptivity of ultraviolet (UV) wavelengths in silicon, most of the energy of these wavelengths is absorbed in a very shallow region near the surface of the substrate. For this reason, ultra-shallow junctions with high dopant concentrations are often created with frequency-tripled Nd:YAG or Nd:YVO 4 crystals or excimer lasers, all with a few hundreds of nanometer wavelengths. Wong and group from Hong Kong Polytechnic successfully created shallow doping profiles using a XeCl 308 nm excimer laser with a 10 ns pulse and fluence variation from about 0.6 to 1.3 J/cm 2 (32). A spin-on phosphorus-containing dopant was preplaced and baked, then fired with multiple pulses with different fluencies to determine spreading resistance and junction depth. Depths of about 200 nm were achieved with 130 pulses (unknown frequency) at 0.7 J/cm 2, and I-V characteristics of junctions showed typical rectifier behavior with high leakage currents, which the authors attributed to defects introduced in the doping process, though exact processes were not discussed. Green/Visible Laser Doping Wavelengths around the visible green range of the electromagnetic spectrum have an absorption depth of roughly 1 micrometer, which is about the depth desired for doping of 23

33 shallow emitters for solar cells. This corresponds to the absorption depth of the peak wavelength of the solar spectrum, and optimal solar cell design allows for maximum absorption at the junction, where charge carriers are separated more efficiently. Therefore, there has been a strong push to research wavelengths in this range for doping of solar cells. Nd:YAG and similar crystals can operate at their second harmonic, at 532 nm, and there exist numerous mature laser technologies available. The Institut für Physikalische Elektronik at the Universität Stuttgart in Germany has developed a process using both scanned line- and circular-focused 2 nd harmonic Nd:YVO 4 and Nd:YAG lasers at 532 nm with pulse durations of 10s to 100s of nanoseconds to irradiate a full-surface for shallow emitter formation (26) (33) (34). Pre-placed dopant films are applied to the surface, and the laser is scanned, allowing for a certain overlap of the lineshaped focus (roughly of width 10 µm). Doping parameters used are 65 ns pulse, 2-6 J/cm 2, frequencies of khz, has a linear focus with width <10 µm and length of several hundred µm, and scanning velocity of m/s. Dopant concentrations are measured to approximately 1 µm for various pulse energy densities, and have a limit near phosphorus atoms per cm 3 (35). Sheet resistances and dopant profiles between 20 and 400 Ω/ have been achieved by varying laser parameters such as repetition rate for circular beam focus; carrier lifetimes for these cells are shown to decrease with increasing pulse energy density, and open circuit voltage limits are estimated at 692 mv (36). Laser-Doped Selective Emitters in Solar Cells Selective emitters, or highly-doped regions under front metal contact fingers, help to lower contact resistance between the cell and the contacts. However, such high doping is detrimental when applied to the entire front surface of the cell. Therefore, patterning of these highly-doped regions must be done to ensure that they only exist under the metal contacts. 24

34 Historically, such patterning was only available via lithography. With the development and research in laser doping, however, selective doping can be achieved by employing a focused laser spot with widths or diameters of microns while leaving the bulk substrate and majority of the surface unaffected. Figure 5 shows a solar cell with front and back contacts and both selective (n+-type) and shallow full area (n-type) emitters for reference in this section. Figure 5: Solar cell with both selective and shallow emitters Researchers at the University of New South Wales created n-type silicon solar cells using a conventional furnace diffusion method to create a shallow p+ emitter on the front surface and n+ emitter on the back surface of 100 Ω/ each, then grew passivating anti-reflective oxide layers on both surfaces. Afterwards, a spin-on dopant film was applied to each of the wafer surfaces, and a Q-switched Nd:YAG laser with 200 ns pulse duration was used to locally remove the dielectric oxide and melt the underlying wafer, incorporating the dopants from the source film into selected regions. The remaining surface and anti-reflective passivation was unaffected, and after removal of the unused dopant film, was employed as a mask for electroless nickel and copper plating of the openings at the selectively-doped regions. Open circuit voltages were improved when compared to a cell with no laser doping, and amongst a range of concentrations of selective doping, heavier doping produced larger fill-factors of around 78% and efficiencies of 17.4%, with a V OC of 672 mv and I SC of 25

35 33.2 ma/cm 2 (37). Further process improvements with commercial-grade materials and processes resulted in a fill factor of 78%, efficiency of 18.7%, V OC of 632 mv and I SC of 38 ma/cm 2 (38). The Fraunhofer Institute for Solar Energy Systems has also done significant research into the area of laser doping for selective emitters (39). A 3 rd harmonic Nd:YVO 4 laser (355 nm) with 25 ns pulse duration was used to selectively dope patterns for subsequent screen-printed metallization. Selective emitters of 23 and 26 Ω/ (fabricated using unknown pulse energy densities) were compared to a blanket sheet resistance of 65 Ω/, and it was observed that the lower sheet resistances resulted in higher open circuit voltages compared to a homogenous cell. The University of Stuttgart investigated the use of a line-shaped focus beam of size 300 µm x 5 µm with a 532 nm Nd:YAG second harmonic laser with pulse duration of 65 ns, frequency of 30 khz, (40). They use a traditional phosphosilicate glass (PSG) layer, formed during POCl 3 furnace diffusion, as a source of phosphorus atoms during laser doping of selective emitter areas. The PSG layer is then removed and standard AR SiN x layers are deposited. Contacts are screen printed Ag-front and Al-back contacts. Comparisons of laserdoped selective emitter cells with conventional cells (e.g. cells with no additional laser doping step) show an increase of 0.5% efficiency, from 17.5% to 18%, and an increase in short circuit current and open circuit voltage. The fill factor decreased from 78.4% to 77.1%, but the decrease was attributed to a non-optimized grid pattern. It was noted that the maximum melt depth increased significantly from 200 nm to 1 µm with longer pulses within the 100s of nanoseconds range. 26

36 Laser-Induced Damage Processing with lasers inherently modifies a substrate, and there is a potential for a small or large amount of damage to occur. In some applications, damage is less critical than in solar cells; however, surface damage of any form induces defects in the crystalline silicon lattice, which act as recombination centers that can drastically reduce carrier lifetime, and thus cell efficiency. Therefore, it is of utmost importance to avoid laser damage during processing of selective emitters. University of Stuttgart researchers claim their narrow line-focus beam enables defect-free recrystallization due to the fact that the heat diffusion depth is on the same order of magnitude of the beam width (<10 µm) (41). Numerical modeling of surface temperatures and melt fronts imply that the thermal stresses in the material are lower for a narrow lineshaped beam with low penetration depth. Melt times were also significantly shorter for a line shaped beam, about 200 ns for a 25 ns pulse, versus over 500 ns for the same length circular Gaussian beam. TEM micrographs of experimentally processed samples with pulse duration of 25 ns, 3.3 J/cm 2, and dimensions of 6 µm x 200 µm with pulse energy density of 3.3 J/cm 2 showed no defects, whereas a 50 µm circular Gaussian beam with constant pulse energy density of 3 J/cm 2 produced defects. 27

37 Chapter 3/Parameter Development Experiments Goals and objectives The laser doping screening experiments are designed to determine processing parameters to produce optimal diodes for solar cell selective emitters. An optimal selective emitter would exhibit a very low series resistance and high currents in forward bias, ideally improving cell efficiency. Laser doping would increase manufacturability of selective emitters for solar cells, and processing parameters to create effective selective emitters are investigated in this study. Screening experiments are performed to vary laser conditions and parameters, and the impact of these parameters are evaluated though use of current-voltage characterization, surface morphologies, and cross-sectional dopant profiles. The parameters screened for reasonable characteristics are later implemented in diagnostic solar cell structures. Laser firing Set-up Three lasers are employed for doping in this thesis: a Coherent AVIA-355 frequencytripled (355 nm) Nd:YVO 4 Q-switched laser with a 30 ns pulse, a Quantel Brilliant Nd:YAG Q-switched laser, operating at fundamental (1064 nm) and doubled (532 nm) frequencies with a 4 ns pulse, and an IPG Photonics continuous wave fiber laser of wavelength 1070 nm, gated to create various pulse durations in the lower microsecond range. The Quantel and IPG lasers employ a train of optics to expand the beam, collimate, and focus it to a point along the beam axis. A vacuum stage is attached to a three-axis Aerotech stage and holds the sample vertical in space and perpendicular to the beam. The Coherent laser uses a GSI Lumonics (currently Cambridge Technology) 3-axis scanner and turns the beam off-axis to a plane parallel to the floor. Figure 6 shows simple diagrams of the three laser optics set-ups. Assumed spot sizes at focus for the IPG were about 20 microns diameter, for the Quantel about 40 microns diameter at 1064 nm and 20 microns diameter at 532 nm, and for the AVIA 28

38 about 30 microns diameter, and are used in later calculations for fluence. Spot size was determined via observation of burn marks and processed spot sizes. The Quantel laser was optimized after initial experiments by processing closer to the aperture, and the beam mode improved closer to a single-mode beam. This can be seen in the more circular spots obtained for the 532 parameter experiments and the lifetime experiments. (a) (b) (c) 29

39 Figure 6: (a) Quantel beam optics, (b) IPG beam optics, (c) Coherent beam optics Wafer description Four inch diameter p-type FZ Si wafers, 1-5 Ω-cm and 275 µm thick, were cleaned in 10:1 Buffered Oxide Etch (BOE) to remove native oxides. Two types of wafer stack-ups were used for this study: a single passivation layer of P-doped a-si:h, and a dual passivation of 100 nm of SiO x overlaying a 20 nm P-doped a-si:h layer. Films of 10 nm a-si:h, followed by either 20 nm (dual) or 100 nm (single) P-doped a-si:h and, in the case of the dual stack, 100 nm SiO x, were deposited by the Penn State University Nanofabrication Laboratory s Applied Materials P-5000 PECVD Cluster Tool on the emitter side. Figure 7 shows diagrams of the two wafer stack-ups. The phosphorus dopant source was incorporated into the n+ doped a-si:h films by introducing phosphine (PH 3 ) gas during deposition of the passivation stacks. To deposit the n+ a-si:h the respective flow rates of PH 3, SiH 4 and H 2 were 15 sccm, 30 sccm, and 285 sccm. Deposition temperature was 200 C, with a pressure of 2 Torr and a power of 50 W. The undoped a-si:h films were deposited by flowing silane (SiH 4 ) and hydrogen (H 2 ) at rates of 30 sccm and 300 sccm, respectively, at a temperature of 120 C, pressure of 2 Torr, and power of 25 W. The SiO x films were deposited using N 2 O, SiH 4, and an N 2 purge gas, with flow rates of 1800 sccm, 18 sccm, and 1800 sccm, respectively. Deposition temperature was 250 C at a pressure of 3.5 Torr and a power of 100 W. This material design was inspired by a patent from Dr. Carlson of BP Solar (42). 30

40 Figure 7: Screening experiment wafer stack-ups, single passivation stack (left) and dual passivation (right) After deposition, wafers were sectioned via laser into 13 mm x 13 mm squares for parameter testing, and identification numbers were scribed on each square, as shown in figure 8. The circle represents the wafer, while red lines represent cuts and green labels represent scribed ID numbers. 31

41 Figure 8: Wafer layout with ID numbers Parameter arrays, cross-section arrays Parameters were tested on 13 mm x 13 mm squares in arrays varying parameters along each axis. Figure 9 shows the layout of a sample. Figure 9: Sample parameter layout Parameters were varied along each axis, creating a 2D array of different laser parameters for characterization. Each area of the array was fired with a three by three array of identical parameters. Spot size ranged from approximately 20 to 60 microns, depending on the laser and optics, and spacing was set at 200 microns in each direction. Figure 10 gives an example for one of the lasers. 32

42 Figure 10: Parameter layout for IPG Laser, 1070 nm, μs pulse duration Current-voltage characteristics were measured via force-probe measurements, and the voltage was varied from -1.0 V to +1.0 V. Spots with promising current-voltage characteristics were selected for further characterization, including cross-sectional examination of junction depth using electroless gold plating to selectively plate the n-type laser-doped region. In order to make cross sections of these small spots possible, a single parameter was repeated in an array with spacing a few microns greater than the diameter of a single spot. For example, if a spot had a nominal diameter of 50 microns, a spot spacing of 60 microns in each direction was used, and an array ranging from 50 to 80 spots along each axis was created. A cut was made through the array, which was sectioned down the center, and the two halves were glued face-to face for ease of wafer handling and increase in probability of cross sectioning an individual spot. Figure 11 shows an example of the layout of a sample processed for cross-sectioning. 33

43 Figure 11: Sample processed for cross-sectioning, including wafer layout, schematic of wafer sectioning, and final wafer halves glued face-to-face for polishing, etching, plating, and characterization Electrical characterization probe set-up A Karl Suss four-probe station was used to measure diodes via a Keithley 4200 semiconductor characterization system. Measurements were made as-fired using a two-probe through-measurement set-up. Samples were placed on a conductive copper tape surface, and a probe was used to contact the copper. The second probe was used to contact the surface of the sample at a doped diode. One of the nine identical parameters at a given array point was measured from either -1, -0.8, or -0.5 Volt to 0.5, 0.8 or 1 Volt. Certain spots were later metalized using either sputtered nickel, or e-beam evaporated aluminum followed by titanium. It is believed that metallization before measurements could eliminate variation in the I-V characteristics caused by probe placement. However, due to shunting through the P- doped a-si:h, only the dual stack wafers could be metalized. Figure 12 gives an example of probe placement of an unmetallized diode versus a blanket metallization over the set of identical parameters. Size scales of the probe tips were on the order of the size of the fired spots, and the probe tips could have been in contact with the edges of the fired spot. Electrical characteristics were called into question, if the current paths were along the edges of the doped regions compared to the center of the laser-doped diode. 34

44 Figure 12: Probe placement (top) single umetallized diode uncertainty and (bottom) metallized parameter set Placement of the probe at various places on the diode surface gives different diode characteristics, while placement on the metallized pad gives significantly less variation due to probe placement. 35

45 Diode Junction Characterization SEM Micrographs SEM micrographs of parameter sets were used to compare surface morphologies. The belief in this case was that the gentler or less damaged surfaces would indicate a lower concentration of laser-induced defects or removal of dopant source through ablation. Damage and defects cause recombination centers, which increase resistance and reduce cell efficiency. Most notable was the difference between the gated CW pulses (hundreds of microseconds) and the Q-switched pulses (4 and 30 nanoseconds). Though thousands of times of more energy per unit area was deposited with the CW pulses, the peak power was only as high as the average power of the laser, 100 Watts or less. The peak power of the Q- switched lasers was in single- to double-digit kilowatt range. Such high instantaneous powers at the levels tested caused visible surface damage and ablation of material. Figure 13: Comparison of gated CW pulse and Q-switched pulse Figure 13 shows a gated CW pulse, 350 μs and 100 W, estimated 20 micron diameter, for a total of 11.1 kj/cm 2, and a 4 ns Q-switched pulse, energy 90.7 J/cm 2. It can be seen that while an amount of energy orders of magnitude larger was used for the gated CW pulse, the peak energy of the pulse has a significant effect on the surface morphology. The peak energy 36

46 of the gated CW pulse was 100 W, while the peak energy of the Q-switched pulse was 71.3 kw. Such high powers caused ablation and surface damage, while, even with higher total energy, a magnitudes-larger duration allowed for low surface damage. The following figures show arrays of parameters for the IPG laser at 1070 nm with a gated CW pulse. It is noted that while parameters with lower total energies were performed, they could not be identified on the wafer surface. There also exists a significant difference between the two wafer stack-ups for each identical set of parameters. The dual passivation layer with a SiO x capping dielectric layer typically shows less surface effects for every laser when compared to the same parameters on the single a-si:h passivation layer. This is believed to be a consequence of the robustness of the layer and the fact that silicon oxide is transparent to all wavelengths used in this study. The features seen here are most likely to a melting or ablation of the material beneath the oxide layer, which expands and forces the capping oxide to ablate, or, in the case of the CW gated pulse, the thermal energy of the material just below the oxide causes it to melt from below. 37

47 Figure 14: SEM morphologies of 1070 nm CW fiber laser, dual passivation, unmetallized 38

48 Figure 15: SEM morphologies of 1070 nm CW fiber laser, single passivation, unmetallized The following set of figures shows SEM morphologies of results after processing with the AVIA 355 nm, 30 ns pulsed laser. Significant ablation is observed for all parameters, and drilling was noticeable for multiple pulses. Again seen here, there is slightly less ablation with the dual passivation stack compared to the single passivation stack. 39

49 Figure 16: SEM morphologies of 355 nm, 30 ns pulsed laser, dual passivation, unmetallized 40

50 Figure 17: SEM morphologies of 355 nm, 30 ns pulsed laser, single passivation, unmetallized A second round of parameter studies was conducted using the AVIA laser. Lower pulse energies were tested. The AVIA parameters with lower fluencies showed noticeably less surface damage compared to earlier studies. Only single shots were examined. 41

51 Figure 18: SEM morphologies of 355 nm, 30 ns pulsed laser, dual passivation, single shot, unmetallized The final set of SEM micrographs shows the Quantel 1064 nm, 4 ns pulsed laser morphologies. As for the other pulsed laser, significant damage was seen, though it appears as though less material was ablated, since there is less splatter. This is most likely due to the smaller absorption coefficient of this wavelength, leading to less material effects at the surface. 42

52 Figure 19: SEM morphologies of 1064 nm, 4 ns pulsed laser, single passivation (left) and dual passivation (right), unmetallized The Quantel 532 nm showed similar trends to the 1064 nm wavelength, with significant ablation. Higher absorptivity near the surface of this wavelength would cause more material effects. The measured pulse energies were at the lower limit of the characterization 43

53 equipment. Compared to the 1064 nm wavelength of the same laser and pulse duration, these show significantly more surface ablation. The 1064 nm laser shows significantly more of the underlying silicon with less splatter, most likely due to the deeper penetration, while the 532 shows more splatter and surface damage due to the high absorptivity near the surface. The 532 nm wavelength shows less splatter and surface damage when compared to the 355 nm 30 ns AVIA laser (figure 16) for similar fluencies, again due to the higher absorptivity of the UV to visible wavelengths. The higher absorptivity of UV in silicon causes more energy to be absorbed at the surface, thus affecting the material more and causing more surface damage. 44

54 Figure 20: SEM morphologies of 532 nm, 4 ns pulsed laser, dual passivation, unmetallized 45

55 Current-Voltage Characteristics Diode-like characteristics and rectifying behavior is desirable in a solar cell emitter, and therefore the laser-doped selective emitters from this study were characterized electrically to determine their current-voltage behavior. Ideally, a solar cell selective emitter would have a very low saturation current, in the low pico-ampere range, and almost no series resistance in the range of operation (for current solar cells, between 600 and 750 mv). Leakage current is undesirable in a diode; however, as a solar cell never should be operating in reverse bias, it is of less consequence in this study. The most important characteristics examined in this study were the series resistance and the saturation current. Minimization of series resistances was of significant importance for a selective emitter, as the goal of the selective emitter is to minimize resistance to the contacts, and any additional resistance added is detrimental. Also examined qualitatively was the shunt resistance, which provides a path for current around the diode. Shunt resistances should be maximized to avoid current shunting around the diode. Significant electrical characterization of the laser-doped selective emitters fabricated in this study was performed by Brittany Hedrick and reported in her thesis (43), and the results are summarized here for comparison with morphology and cross-sections. The two different wafer passivation stack-ups were fired with identical parameter sets, and the current-voltage characteristics were fitted to approximate characteristics such as ideality factor and saturation current in the quasi-neutral region, and the series resistance of the diode. The following figures summarize the parameter I-V curves. Green boxes indicate promising parameters with low series resistance and low saturation currents. The first set show the results of the 1070 nm, CW gated pulse laser. 46

56 Figure 21: Current-voltage characteristics of 1070 nm CW fiber laser, dual passivation (top) and single passivation (bottom), unmetallized, from -1.0to 1.0 V Parameters were tested from 30 W to 100 W, with durations of 50 to 350 µs. However, only the parameters shown could be measured due to the physical limitations of finding and contacting the diodes on the surface of the wafers. The range of extrapolated currents from high forward bias region is 10-6 to 10-3 A. The next set of figures shows the I-V characteristics of the 355 nm, 30 ns pulsed laser. Here, the parameters varied were pulse energy and number of shots fired. It was quickly 47

57 noted that multiple shots on one spot, while possibly providing (in some cases) reasonable I- V curves, also drilled the wafer, as seen in the surface SEMs. 48

58 Figure 22: Current-Voltage characteristics of 355 nm, 30 ns pulsed laser, dual passivation (top) and single passivation (bottom), unmetallized 49

59 The second round of parameter studies was conducted using the AVIA and Quantel lasers. The AVIA parameters were metalized before diode characterization, and the average of 1/9 of the current (to represent one of the 9 identical diodes fired under the metallization). Results from the AVIA were much more promising than previous studies on the same laser with higher fluencies. Figure 23 compares lower fluencies with the higher fluencies from the first round of studies. It is noted that the lower fluencies show a higher shunt resistance (through smaller currents at the lower voltages) and lower series resistance (through the reduced bending-over of the curve near 0.8 V), both desirable characteristics for a selective emitter. Figure 23: Comparison of current-voltage characteristics for lower and higher fluencies at 355 nm, 30 ns 50

60 The final set of figures shows the I-V curves for the Quantel 1064/532 nm laser with a 4 ns pulse. The curves shown represent the 1064 nm mode; 532 nm pulses did not produce reasonable results for comparison. Figure 24: Current-Voltage characteristics of 1064 nm, 4 ns pulsed laser, single passivation (left) and dual passivation (right), unmetallized The Quantel, operating at 532 nm, showed unpromising results. When compared to the results in the literature from Fraunhofer Institute and Stuttgart University, it is hypothesized that the pulse duration was too short and the pulse energy and fluence too high to replicate or produce similar results. Dissimilar dopant sources were also employed, which could also lead to a lack of agreement between the results of this research and those reported in the literature. 51

61 Figure 25: Current-Voltage characteristics of 532 nm, 4 ns pulsed laser, dual passivation (right) and single passivation (left), unmetallized A two diode model was used to manually fit the data in Microsoft Excel to simulate and extract parameters such as saturation currents, ideality factors, and resistances. The best parameters are summarized below. Idealities were fit to the curve and represent roughly 1-2 decades of linearity in the QNR region, currents and series resistances were fit to the curve to give a best-fit. 52

62 Table 1: Summary of diode parameters Wavelength Pulse QNR Saturation Series Fluence (nm) Duration Ideality Current Resistance Metallization µs 2.8 kj/cm pa > 1000 Ω None ns 22 J/cm pa > 1000 Ω None ns 22 J/cm pa > 1000 Ω None ns 0.59 J/cm pa 11 Ω 100 nm Ni, sputtered ns 4.26 J/cm pa 35 Ω 100 nm Ni, sputtered From these results, it can be seen that all lasers were capable of producing diodes. More promising results were typically seen on the single passivation structure with a thick doped amorphous silicon film as dopant source; however, for subsequent metallization, a dielectric mask was necessary to eliminate possible shunting of current through this structure. Therefore, for diagnostic cell comparison, only parameters from the dual stack structures were chosen. Cross-sectioning Parameters that showed potentially acceptable current-voltage characteristics and reasonable surface morphologies were chosen for a select study involving cross section techniques to examine the dopant profile within the substrate. Samples were fired in arrays and then a cut was made through the array. The two halves were glued face-to-face and the edge of exposed fired spots was polished, then etched to remove the native oxide and immersed in a selective electroless gold-plating solution to plate the n+-type laser-doped region. Comparisons were made between the various wavelengths and pulse durations. It can be seen from the gold plating that the n+-doped regions extend much more deeply in the samples doped with the CW gated pulses. These pulses were thousands of times longer than the Q-switched lasers (hundred of microseconds versus nanoseconds) and imparted a 53

63 thousand times the energy per unit area. Depth penetration was close to 10 microns in each case, and widths were much wider than the declared beam width at focus. This demonstrates that the laser was able to create a melt pool that penetrated both laterally and vertically into the sample. In contrast, the Q-switched lasers (at wavelengths of 355 nm, 532 nm, and 1064 nm) penetrated a few microns or less, if a distinct spot could be observed. Pulse duration therefore played a significant role in the depth of dopant penetration. For the 1064 nm 4 ns pulse, no distinct spot was observed, while for the 532 nm 4 ns pulse, the higher absorptivity concentrated the energy absorption near the surface and a shallow, wide doped region roughly 40 microns across could be observed. For a 355 nm 30 ns pulse, a 40 micron wide and 2 micron deep dopant profile was observed, which could be attributed both the higher absorptivity of the shorter wavelengths in silicon and the longer duration of the pulse. Figure 26 shows dual passivation layer cross sections, and figure 27 shows single passivation structure cross sections. It can be seen that passivation structure had little effect on the overall doping profile, and did not significantly reduce the doping width or depth. For various parameters and energy densities on the same single passivation structure, it was expected and observed that a larger input fluence had a deeper doping depth. This depth was much deeper than the assumed ideal of 1 micron for a shallow emitter; however, it could be proposed that a deeper doping profile under a shaded area, such as under a front contact (as is the case of our selective emitter), is not collecting light from absorption at the surface directly above it, and the doping depth is of less consequence. 54

64 Figure 26: IPG cross sections, dual passivation layer Figure 27: IPG cross sections, single passivation layer 55

65 Figure 28: Quantel cross sections, single passivation layer, 1064 nm Figure 29: Quantel cross section, dual passivation layer, 532 Figure 30: Quantel cross sections, single passivation layer,

66 The Quantel laser, operating at 1064 nm with a 4 ns pulse, showed no distinct doping regions in the single passivation layer, figure 28. This was attributed to the low absorptivity of the near IR wavelength, resulting in a low energy deposition close to the surface. Likely, the energy penetrated much more deeply into the wafer without causing doping, only localized heating. Defect characterization on the cross section could potentially reveal subsurface damage to the crystal lattice, but was not performed in this study. Figures 29 and 30, however, showed some distinct doping regions for the same laser operating at the second harmonic, 532 nm, with a 4 ns pulse. The significantly higher absorption of the visible wavelength compared to the IR wavelength resulted in higher energy deposition and absorption at the surface, and therefore higher doping, creating visible doping and subsequent plating. The energy of the IR wavelength penetrated deeper and was not as absorbed near the surface, causing only shallow doping in the nearest surface regions. Finally, the 355 nm 30 ns AVIA laser also created visible doped regions at the surface. The high absorptivity of the UV wavelength allowed for high energy absorption in the surface regions, and high doping resulted. 57

67 Figure 31: AVIA cross section, single passivation layer Corroboration of the doping depths seen here could potentially be performed using SIMS or other depth profiling methods to confirm the characteristics observed in the cross sections, but was not performed in this study. Comparison and Correlation of Parameters From the three methods of characterization mentioned here, it was observed that different doping parameters produced a wide variety of surface morphologies and doping profiles, but also produced reasonable diode-like electrical characteristics. Figures summarize good parameters for each laser, showing various profiles, surface morphologies, and even stackups. 58

68 Table 2: Summary of laser parameters and characteristics Laser Pulse Beam Doping Doping Pulse Fluence Wavelength Duration Diameter Depth Width µs 3.64 kj/cm 2 20 µm 11 µm 52 µm ns ~ 50 J/cm 2 40 µm ~1-2 µm Unknown ns 9 J/cm 2 30 µm ~4 µm 30 µm Figure 32: IPG doping parameters, I-V, surface morphology, and cross section for dual passivation structure 59

69 Figure 33: Quantel doping parameters at 1064 nm, I-V, surface morphology, and cross section for single passivation structure Figure 34: AVIA doping parameters, I-V, surface morphology, and cross section for single passivation structure 60

70 Trends across the lasers can be observed. It is noted that for the ns pulsed lasers (Quantel at 4 ns, AVIA at 30 ns), relatively shallow wide doping regions, if any, are observed. Doping profiles are flat and do not extend deep into the substrate. This is attributed to the short timeframe of the pulse, which does not allow for significant melting or heat transfer deep into the substrate during the relatively short duration. In contrast, the longer microsecond pulses of the IPG laser allow for the melting of regions significantly larger than the assumed spot size at the surface of the substrate. The low peak power, in addition, limits the amount of surface damage seen, while the high (kw) peak powers of the nanosecond pulsed lasers cause a large amount of energy to reach the surface in a short time, causing material ablation. However, all parameters have produced curves showing low series resistance in the 0.6 to 0.8 V range, indicated by the curves lack of bending over or domination by the series resistance in this range. They also show curves similar to the ideal solar cell equations outlined in the background of this thesis. While the reverse leakage current is significantly higher than an ideal diode, it is of less consequence in this report, as a solar cell is not expected to operate in reverse bias. Discussion of Error It is believed that inconsistencies in the data stem from variations in the laser optics setups. While care was taken to maintain consistency between experiments, there was likely some variation from one set of experiments to another during this study. The Quantel in particular was found to have a poor quality multi-mode beam at the operating distance used in the initial parameter screening experiments; however, for later studies, processing was performed much closer to the aperture, where the beam exited in a near-gaussian energy profile. The original IPG laser used in this study was decommissioned after the initial parameter sets were analyzed, and a second laser, similar but with a larger, uncollimated 61

71 beam, was used to complete additional experiments. Other inconsistencies could be attributed to the measurement methods for the I-V characteristics, which while sufficient without metallization to give an idea of the diode qualities of a set of parameters, was difficult to repeat results even on the same set of samples. Samples were processed in atmosphere, and impurities incorporated during processing could have varied from day to day conditions (such as humidity) in the laboratory processing room. Inconsistencies resulting from a lack of metallization for most samples also could cause variation in measured I-V characteristics. 62

72 Chapter 4/Spot-Size Affected Lifetime Decay Experiments Goals and objectives The goal of the spot-size affected lifetime decay experiment is to compare the effect of spot size and energy distribution on lifetime by processing a range of fluencies over equal areas, including those with previously-determined acceptable I-V characteristics. Experiments will also determine the effects of pre- and post-laser processing passivation and annealing. Experimental Design Fluencies were held constant for various spot sizes, and equal areas were processed to deposit total equal energies for various fluence and spot-size parameters. Lifetime maps were taken at various points along the process to compare the effects of various processing steps. Procedures Passivate one of two wafers with 10 nm a-si:h/100 nm SiO x, shown in figure 35 Laser process wafers with identical parameters Passivate second wafer with 10 nm a-si:h/100 nm SiO x Map lifetime Anneal 325 C for 10 minutes Map lifetime Wafer Description Two FZ <100> Si wafers, one to be passivated by PECVD with 100 nm SiO 2, were processed with identical parameters, and a third was passivated and left unprocessed for comparison of lifetimes. 63

73 Figure 35: Lifetime wafer stack-up, passivated Figure 36 shows the square 5 x 5 mm areas that designate the processing areas for each set of parameters. A total of 66 squares offered five (5) sets of fluence/spot size parameters per wafer, allowing for 2 sets of six (6) or nine (9) parameters for redundancy. Each laser (1-5) was duplicated on the same wafer. Four (4) sets of six (6) parameters and one (1) set of nine (9) offered insight to the effect of pulse duration (gated, longer versus Q-switched ns), beam energy profile, and wavelength on lifetime after laser processing with various fluencies and spot sizes. 64

74 Figure 36: Wafer lay-out of parameter sets Parameter choices With 5 x 5 mm sample areas on a pad, there was space for up to 66 samples per wafer. For each available laser when possible, two (2) different spot sizes at three (3) fluencies were tested, which yielded six (6) parameters total per laser. Fluencies were selected to span the ranges which produced promising I-V characteristics for each laser. Fluence and spot size were varied via variation of the focal length of the optics, and the total area processed was held equal for all six (6) parameter sets. Select samples from the Quantel laser (1064 nm) were processed with Gaussian and top hat energy profiles. Table 3 shows proposed parameters for the Quantel (1064 nm, 4 ns), table 4 shows proposed parameters for the IPG (1070 nm, μs range), and table 5 shows proposed parameters for the AVIA (355 nm, 30 ns), including the pulse area and required pulse energy to achieve the desired fluence. 65

75 Table 3: Beam diameters and fluencies tested Quantel (1064 nm, 4 ns pulse) Beam Area Desired Fluence Required Pulse Diameter (µm) (cm^2) (J/cm 2 ) Energy (µj) x x x x x x Table 4: Beam diameters and fluencies tested IPG (1070 nm, μs pulse) Beam Area Desired Fluence Required Pulse Pulse Power Diameter (µm) (cm^2) (J/cm 2 ) Energy (mj) Duration (us) (W) x x x Table 5: Beam diameters and fluencies tested - AVIA (355 nm, 30 ns) Beam Area Desired Fluence Required Pulse Diameter (µm) (cm^2) Fluence Energy (µj) x x x x x x By varying the numbers of spots, the total area fired was held relatively constant for both diameters (20 and 60 um). The spot spacing and number of spots per 5 mm square area were required to preserve ratios for total fired areas, demonstrated in table 6. These spacing and spot area variations gave total covered area of about 50% of a 5 x 5 mm square. 66

76 Table 6: Diameters and preserved ratios for a fired pad of 5 x 5 mm No. of spots in Total Area of Diameter (um) 5 x 5 mm square All Spots (mm^2) Spacing (um) , Firing of wafer Whole wafers were mounted on a vacuum stage normal to the axis of the beam, as for the parameter and diagnostic cell procedures for smaller 13 mm x 13 mm samples. Parameters were fired with the indicated spacing from table 6 to cover a 5 mm x 5 mm area on the wafer. Identical parameters were fired twice on separate areas of the wafer for redundancy. Parameter sets were spaced approximately 5 mm apart for resolution purposes. However, equipment limitations included, in some instances, lack of alignment verification and parameter sets could not be entirely aligned square at a set distance. In addition, a wafer (Si_p_01) cracked while being placed in contact with the vacuum stage during processing. Appendix A details parameters and wafer layouts. Lifetime measurements Lifetime measurements were performed by BP Solar in Frederick, Maryland via microwave photoconductive decay (μ-pcd) measurements. Wafers which had been passivated prior to laser firing were measured, annealed, and measured a second time to observe the effects of laser doping and annealing on wafer lifetime. A second wafer was fired with identical parameters, then passivated post-firing. Lifetimes were measured, annealed, and then remeasured. A passivated wafer with no parameters fired was measured, annealed, and remeasured as a control for comparison of an unfired wafer. Comparisons of lifetimes for wafers fired though a silicon dioxide passivation layer and on bare silicon were made. 67

77 Lifetime Characterization Lifetime maps provided by BP Solar USA in Frederick, MD, are shown in Appendix B, and correspond to the parameter layouts listed in the same section. As expected, significant lifetime degradation was induced with all but the mildest of parameters. Figure 37 shows the results for the wafer passivated before firing. Keeping in mind the differences in scale, it can be seen that certain parameters had a more detrimental effect after firing, and only the lowest powers for the Q-switched laser, operating at 1064 with a 4 ns pulse, have a noticeably reduced detrimental effect on lifetime. It was shown in previous work on laser fired contacts (LFCs) in the thesis of Brennan DeCesar (44) that a 1070 nm CW laser operating at W with millisecond pulses showed little to no lifetime degradation. This was performed with a similar laser on different wafer stack ups to the laser operating here at 100 W and hundreds of microsecond pulses on the previously-described wafer stack-up, which showed poor lifetimes post-firing. The discrepancies here are interesting to note, and could be due to both the heightened power and the shorter pulses, which lead to greater temperature gradients and faster cooling rates within the wafer. 68

78 Figure 37: Wafer Si_p_01, before (left) and after (right) anneal Discussion of Error Sources of error for this study are as mentioned for the other studies done in this thesis. It is noted that for the control wafer, lifetimes were poor before and after annealing, and therefore overall lifetimes for this study could potentially be low due to wafer or passivation quality, not necessarily due to laser damage. 69

79 Chapter 5/Diagnostic Cell Experiments Goals and objectives The goal of the solar cell diagnostic cell experiment is to correlate diode characteristics, surface morphologies and cross-sections of the parameter development studies to diagnostic device characteristics and performance. Optimal doping parameters investigated in the parameter development experiments are screened and chosen for further study in complete diagnostic cell devices. Laser parameters are duplicated and implemented along finger regions of a wafer, and then metallization techniques plate the front surface collection fingers over the laser-doped selective emitters. Solar cell characterization techniques are employed to illuminate the cell and measure figures of merit to determine effectiveness of selective emitters with no shallow emitters. Parameter choices Parameters were chosen from the development study based on favorable diode characteristics, as well as reasonable surface morphologies that showed little damage to the surrounding passivation layer, and are outlined in appendix B. Favorable diode characteristics were evaluated using Microsoft Excel curve-fitting of a double-diode model to approximate the IV curve of the space charge region (SCR) and quasi-neutral region (QNR). Quasi-neutral region ideality factors should be 1, and saturation current densities are generally reported in the pico-amp range. Surface morphologies were used to determine parameters which had the least ablative results, which would lead to increased removal of dopant source material and a decrease in doping for a given diode. One would also expect highly-ablative processing to cause significant sub-surface wafer damage, introducing defects and recombination centers into the wafer lattice. 70

80 Device design Test wafers (13 x 13 mm) used the same dual passivation and dopant structure studied in the parameter development experiments (see chapter 3) on the emitter side. Emitter side passivation and dopants were deposited at the PSU Nanofabrication facility. A layer of aluminum 500 nm thick was electron-beam deposited on the back contact side and back contacts were laser-fired at BP Solar in Frederick, Maryland. Parameters for the laser fired contacts (LFCs) were wavelength of 1064 nm, repetition frequency of 10 khz, pulse energy of 1.7 mj, spot diameter of 100 um, and spot spacing of 1 mm. Finally, selective emitters were fired through the front passivation structure, and metallized for characterization. Figure 38 shows the process flow for the creation of diagnostic cells used in this study. 71

81 Figure 38: Diagnostic cell wafer flow Firing of cell Diagnostic cells were fired with 100 μm spacing in finger patterns with finger widths of 100 μm, a bus-bar width of 300 μm, and edge-to-edge finger spacing of both 0.5 and 1.0 mm. Individual spot sizes were on the order of microns. Figure 39 shows an example grid pattern for a cell with 100 μm finger width and 1 mm finger spacing. 72

82 Figure 39: Mask layout for diagnostic cell, 100 μm finger width and 1.5 mm finger spacing Laser parameters were chosen from parameter development and screening studies. Parameters were chosen from the Quantel and AVIA lasers (1064 nm/4 ns and 355 nm/30 ns, respectively). Wafers were mounted in a manner similar to the parameter development studies for each laser; for the Quantel laser, samples were mounted on a vacuum stage normal to the axis of the beam and the laser was fired every 100 microns along 10 mm for each finger for a total of 100 spots per finger. 73

83 Metallization Metallization procedures were similar to those used for the parameter development study. Lithographic processes were used to transfer the mask pattern to the diagnostic cell surfaces, and 100 nm of nickel was sputtered to form the contact fingers, bus bar and contact pad. PG Remover was used to remove the sputtered nickel from the areas masked by photoresist. Diagnostic cells were then hot-plate annealed at 350 C for 7 minutes. Annealing in air formed a layer of nickel oxide on the surface of the metallization, and crated a barrier to electrical probe contacts for characterization. This oxide layer was removed by etching at toom temperature in 10:1 HCl for 2 minutes. Transene PC Electroless Copper Plating solution was then used to create a thicker layer for electrical contact via plating for 10 minutes (estimated 30 microns of copper deposited). Solar Cell Characterization Solar cells were tested using a class A Oriel solar characterization device under 1-Sun conditions at ambient temperature (about 22 C). Wafers were placed on a gold slide to make contact with the aluminum back-side, and a stainless steel probe was contacted to each the gold plate and the metal contact pad on the front. The probes were placed at the shallowest angle to maximize contact area. Characteristic parameters such as V oc, I sc, FF, and efficiency were measured. Results were compared with previous I-V curves of diodes to relate firing conditions to cell performance. A cell area of 125 mm 2 was used for calculations. Cells with 1 mm finger spacing had a total shading of 12.5%, and cells with 0.5 mm spacing had a total shading of 18%. Initial testing of the cells prior to electroless copper plating was unpromising. Likely causes of high resistances were oxides formed on the nickel and aluminum front and back contacts, respectively, during annealing after the sputtering of the nickel contacts and also due to the 74

84 small dimensions of the nickel contacts. Results after oxide removal and electroless copper plating showed much more promise. Parameters tested from the Quantel (1064 nm) and AVIA (355 nm) lasers are listed in Appendix B. Figures 40 and 41 show the best three illuminated current characteristics from the cells fabricated, along with corresponding surface morphologies and I-V curves of the diodes from the parameter studies. Figure 40: AVIA diagnostic cell illuminated curve (top), diode characteristics (bottom left), and diode surface morphology (bottom right) 75

85 Figure 41: Quantel diagnostic cells illuminated curves (top left and right), diode characteristics (bottom left), and diode surface morphology (bottom right) It can be seen here that the shorter wavelength of the AVIA created a much higher current collected in the cell (all illuminated diagnostic cell curves are set on the same scale), between 7 and 8 ma. This is an extremely small amount of current, and together with the open circuit voltages between 200 and 300 mv, the efficiencies (represented as η) of less than 1 and fill factors around 30%, one would consider solar cells fabricated with only selective emitters a poor choice for any practical application. However, the efficiencies, open circuit voltages and short circuit currents can be considered low simply because there was very little collective area. Only carriers created in the proximity of the fingers would be able to reach the emitters and exit the cell, due to the lack of full-area p-n junction shallow emitter. The poor fill factors, however, are due to resistances in the cell and contacts. Steps for better metallization would significantly reduce the contact resistance and enable full 76

86 characterization of the resistances of the emitters themselves; however, it can be qualitatively observed, from the diode curves as well as the illuminated cell curves, that the laser doped selective emitters have a characteristic resistance of their own which would detract from cell performance, in many cases significantly. The parameters chosen for this diagnostic study were chosen in part because of their relatively low series resistances at the operating voltages of a typical silicon solar cell, between 600 and 700 mv at V oc. Further improvements to the cells, including the addition of a full-area emitter and better contact metallization, would improve cell performance. Discussion of Error Sources of error in this study are similar to those mentioned above, for the parameter study. Laser optics set-ups were changed between the two studies, and while all attempts were made to minimize variation between the studies, some differences were inevitable. Another source of error was the passivation and dopant layer deposition. Wafers used for both studies were created using the same structures and recipes on the same machine, but the same machine suffered from a series of maintenance issues between the depositions, and the consistency and quality of the passivation structures is questioned, due to possible changes in calibration between the two different depositions. A change in color between the two wafers was the most noticeable variation; the wafers for the parameter study had a grayish tint when viewed normal to the surface, while the wafers for the diagnostic cell and spot size affected lifetime decay studies were blue/violet in color. A change in color indicates a variation in the thickness of the oxide layer, which could alter the parameters for ideal doping. Metallization was a final source of error for this study. Similar metallization techniques to those in the later parameter development studies were used, and with such narrow finger widths, nickel-silicon adhesion was an issue. Some cells had less-than-ideal coverage and 77

87 metal peeled off of some of the fingers. To promote adhesion, samples were hot-plate annealed in atmosphere, which caused a nickel oxide to form on the front, and an aluminum oxide to form on the back contacts, giving rise to a higher series resistance and poor contact during testing. This oxide was removed and the remaining metal was electrolessly plated with copper to enhance conductivity of the fingers. 78

88 Chapter 6/Summary and Conclusions Parameter Studies Parameter studies showed that reasonable diode-like characteristics could be obtained from a variety of processing parameters. Balancing fluence, wavelength, and pulse duration for any number of processing conditions yielded reasonable diode-like rectifying characteristics for a given wafer stack-up and dopant structure. However, when analyzed considering assumed currents, the values of each are very large when compared to the literature. This is likely due to the high fluences investigated here. Surface morphologies showed ablation, which causes damage and dopant source material removal, leading to the assumption that damage gave rise to recombination centers and possible sources of shunt and series resistance paths. Characterization of dopant profiles via cross-sectioning methods showed that processing pulse length had a large effect on dopant penetration. High peak powers showed much more surface damage, while lower peak powers for long durations showed deep doping profiles, reaching to 10 μm. However, all lasers were able to produce diodes when the correct fluences were chosen with the wafer stack-ups used in this study. Spot-Size Affected Lifetime Decay Study Parallel studies concerning the effects of spot size on lifetime decay of a wafer were performed. The study intended to test if, for similar total energy deposited for a large spot size versus a very small (~10 micron diameter) spot size, a smaller spot size would show significantly less material damage due to a reduced thermal gradient. This was the idea behind the line-shaped beams performed by the Stuttgart University, using a very narrow line beam that they claimed was the reason for the lack of defects found in their samples. 79

89 However, the only promising results for low lifetime decay was in the very low fluences of the 4 ns, 1064 nm laser, which showed no visible surface effects. Previous studies using the IPG laser (CW gated pulses) showed no lifetime degradation for millisecond pulses at half the power used in this study. However, the laser used in this study was, as mentioned above, similar but not exact to that used for the parameter development study. Promising parameters developed there were to be replicated for this study; however, the exact replication of earlier results was doubtable due to the variances in beam quality and diameter out of the aperture. It was known, however, that the pulse from this laser was in the hundreds of microsecond range with a smaller focused spot size and higher powers than previous work (44). The difference in lifetime for a shorter pulse at higher power is notable, and could be attributed to the length of the pulse, causing faster cooling rates, or the higher temperature, causing higher temperature gradients. Diagnostic Cell Study Diagnostic cells were made with parameters selected from the development study. The most reasonable parameters were chosen to compare diode qualities to actual cell performance under standard testing conditions. Cells showed universally poor efficiencies and high resistances, but did exhibit nonlinear IV characteristics, indicating existence of a p-n junction and rectifying behavior. Correlation of high resistances at long wavelength, short pulse duration between both screening experiments and diagnostic cells is seen, and improvement is seen in the parasitic resistances for shorter wavelengths at short pulse durations. There are many contributing factors beyond diode performance, however, and strong correlations between processing parameters, screening parameter characteristics, and diagnostic cell characteristics are questionable. 80

90 Chapter 7/Future Work Future work in this area could be done in many areas. Of interest would be to use a green laser with longer pulse duration to replicate studies reported. Particularly, a CW green laser could give promising results, with low peak power pulses and an ideal absorption depth. Lower fluencies at all wavelengths should be investigated to determine if approaching the ablation threshold would minimize damage while still incorporating dopants into the wafer and forming a p-n junction for both micro- and nanosecond pulses. Modeling of the pulses could show theoretical material effects on a time-scale. Other work to be done would be to simulate a full solar cell, with both shallow full area emitter and laser doped selective emitters under the front side metallization. Optimal spacing of laser doped spots under the contact fingers should be investigated, as well as optimal contact finger spacing and complete cell front-side contact design. Further studies for metallization techniques could be performed to minimize errors and inefficiencies stemming from poor metallization. As to material parameters, different doping stack-ups, varying both concentration and thickness of the doping layers, could be performed to determine a method to incorporate heavier or lighter doping, if necessary. Capping dielectric layers and ARCs could be investigated to ensure maximum cell performance. Finally, a full study of lasers and other equipment available for industrial production should be made to determine if the end production methods are economically feasible; that is, if the increase in solar cell efficiency via laser doped selective emitters justifies the processing time and expense of the industrial process implementation. 81

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94 Appendix A: Lifetime Wafer Parameters, Layouts, and Lifetime Maps Wafers Si_b_01 and Si_p_01 were used to perform the spot-size affected lifetime decay experiments. Lists of parameters, as well as detailed diagrams indicating placement of each set of parameters on the arrays are listed here. Table 7: Spot-size affected lifetime decay parameters Number Wavelength Spot Size Pulse Energy Array size Spot Spacing Beam Profile μm 20 μj 50 x 50 spots 100 x 100 μm Gaussian μm 43 μj 50 x 50 spots 100 x 100 μm Gaussian μm 60 μj 50 x 50 spots 100 x 100 μm Gaussian μm 20 μj 50 x 50 spots 100 x 100 μm Top Hat μm 43 μj 50 x 50 spots 100 x 100 μm Top Hat μm 60 μj 50 x 50 spots 100 x 100 μm Top Hat μm 20 μj 15 x 18 spots 330 x 275 μm Gaussian μm 43 μj 15 x 18 spots 330 x 275 μm Gaussian μm 60 μj 15 x 18 spots 330 x 275 μm Gaussian μm 20 μj 15 x 18 spots 330 x 275 μm Top Hat μm 43 μj 15 x 18 spots 330 x 275 μm Top Hat μm 60 μj 15 x 18 spots 330 x 275 μm Top Hat μm 85 W x 85 μs 15 x 18 spots 330 x 275 μm Gaussian μm 85 W x 170 μs 15 x 18 spots 330 x 275 μm Gaussian μm 85 W x 255 μs 15 x 18 spots 330 x 275 μm Gaussian μm 6 μj 50 x 50 spots 100 x 100 μm Gaussian μm 13 μj 50 x 50 spots 100 x 100 μm Gaussian μm 18 μj 50 x 50 spots 100 x 100 μm Gaussian μm 52 μj 15 x 18 spots 330 x 275 μm Gaussian μm 99 μj 15 x 18 spots 330 x 275 μm Gaussian μm 163 μj 15 x 18 spots 330 x 275 μm Gaussian 85

95 Figure 42: Si_b_01 wafer layout for spot-size affected lifetime decay experiments 86

96 Figure 43: Si_p_01 wafer layout for spot-size affected lifetime decay experiments 87

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