AT PRESENT, several research institutions are active in

Transcription

1 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 59, NO. 5, MAY Deposition and Characterization of High-Efficiency Silicon Thin-Film Solar Cells by HF-PECVD and OES Technology Shui-Yang Lien, Member, IEEE, Yu-Cheng Chang, Yun-Shao Cho, Yin-Yu Chang, and Shuo-Jen Lee Abstract The optical emission spectrometer (OES) is an effective experimental tool for monitoring plasma states and the composition of gases during the growth of silicon thin films by plasma-enhanced chemical vapor deposition. In this paper, hydrogenated amorphous silicon (a-si) (a-si:h) and microcrystalline silicon (µc-si) thin films have been deposited in a parallel-plate radio frequency (RF) plasma reactor using silane and hydrogen gas mixtures. The plasma emission atmosphere was recorded using an OES system during the growth of the Si thin films. The plasma was simultaneously analyzed during the process using an OES method to study the correlation between growth rate and microstructure of the films. In the deposition, the emitted species (SiH, Si, and H ) were analyzed. The OES analysis supported a chemisorption-based deposition model of the growth mechanism. The effects of RF power, electron-to-substrate distance, and H 2 dilution of the emission intensities of excited SiH, Si, and H on the growth rate and microstructures of the film were studied. Finally, single-junction a-si:h and µc-si solar cells were obtained with initial aperture area efficiencies of 9.71% and 6.36%, respectively. A tandem a-si/µc-si cell was also realized with an efficiency of 12.3%. Index Terms Amorphous/microcrystalline silicon (a-si/µc-si) thin films, optical emission spectrometer (OES), plasma-enhanced chemical vapor deposition (PECVD). I. INTRODUCTION AT PRESENT, several research institutions are active in the research and development of various thin-film silicon solar cells. Process technology, yield rate, and the efficiency of cells are all gradually being improved. Currently, the most Manuscript received November 9, 2011; revised December 27, 2011; accepted January 24, Date of publication March 6, 2012; date of current version April 25, This work was supported by the National Science Council of the Republic of China under Contracts NSC E CC2, NSC E MY2, and NSC E CC2. The review of this paper was arranged by Editor A. G. Aberle. S.-Y. Lien and Y.-S. Cho are with the Department of Materials Science and Engineering, Mingdao University, Changhua 52345, Taiwan ( syl@mdu.edu.tw). Y.-Y. Chang is with the Department of Materials Science and Engineering, Mingdao University, Changhua 52345, Taiwan, and also with National Formosa University, Huwei 63201, Taiwan. Y.-C. Chang and S.-J. Lee are with the Department of Mechanical Engineering, Yuan Ze University, Jhongli 320, Taiwan ( mesjl@saturn. yzu.edu.tw). Color versions of one or more of the figures in this paper are available online at Digital Object Identifier /TED important apparatus in the production of various types of thinfilm silicon solar cells is the equipment required for plasmaenhanced chemical vapor deposition (PECVD). Their stability depends on understanding the behavior of plasma inside the chamber. Analyzing the same is a highly complex process and involves different techniques. The in situ measurement and control of the state of the plasma are critical to silicon thin-film solar cell. Numerous publications are concerned with the gases and surface processes that are involved in the nucleation and growth of films, and several related ideas have been speculated. Hydrogenated amorphous silicon (a-si) (a-si:h) and microcrystalline silicon (μc-si) thin films have been examined extensively in recent years for use as photovoltaic materials [1], [2]. The Institute of Photovoltaic has stated that initial solar energy conversion efficiency can be increased from 7.9% to 8.8% by applying process control that is monitored using an optical emission spectrometer (OES) [3]. Guláš et al. indicated that OES is the main tool used in experiments for comparing gas phases and the concentration of the active gas phase species for the growth of carbon nanotubes [4]. Parashar et al. examined the correlation among pressure, crystallinity, and conductivity using OES. They deposited the film at a pressure of 4 torr and observed a relatively low sheath/bulk field ratio [5]. Zhang et al. found the deposition rate as a function of film crystallinity using OES to record changes in the discharge [6]. Chen et al. elucidated the physical properties of TiO x films and utilized OES to monitor the intensity of the Ti emission line using a flow-supply feedback loop [7], [8]. OESs have been utilized to study the excited SiH, Si, and H [9] [12]. The equivalent SiH 3 and SiH 2 species cannot be detected using this OES method. In this paper, the excited species and their spatial distribution of optical emission between the grounded electrode and power electrode are observed. The effect of RF power on the microstructure and efficiency of a solar cell is determined using RF discharge at 27.1 MHz with the cells fed with pure SiH 4 precursor. The spatial distribution of the intensity of the emitted light is measured. The effects of RF power, electron-to-substrate distance (E/S distance), and H 2 dilution on the microstructure and efficiency of a solar cell are studied using RF discharge at 27.1 MHz. Finally, the singlejunction a-si:h and μc-si cells were obtained with initial aperture area efficiencies of 9.71% and 6.36%, respectively. The tandem a-si:h/μc-si cell was also realized with an efficiency of 12.3% /$ IEEE

2 1246 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 59, NO. 5, MAY 2012 TABLE I DEPOSITION PARAMETERS OF SILICON THIN FILM TABLE II DEPOSITION PARAMETERS OF INTRINSIC A-SI, µc-si, AND A-SI:H/µC-SI THIN FILMS Fig. 1. Schematic diagrams of the (a) deposition chamber and (b) OES photocollimator location of sampling points. (c) OES system integrated configuration. II. EXPERIMENTS Fig. 1(a) shows the experimental apparatus. Silicon thin-film solar cells are prepared in a stainless-steel plasma chamber, which has an internal diameter of 40 cm and a height of 30 cm. The diameter of the parallel-plate electrodes is 25 cm. They are separated by 7 11 cm, and the grounded electrode consists of a substrate holder that is suitable for measurement with an area of cm 2. RF power is applied to the upper electrode through an impedance-matching network. The spatial distribution of intensity of the emitted light is measured. The point on the photocollimator in Fig. 1(a) denotes the spot where OES measurements were made to determine the growth mechanism of the film. Fig. 1(b) shows the experimental setup that is used to record spatially resolved emission profiles. To measure the intensity of emitted light at various positions, slits are used, and the photocollimator can be moved over a distance of 1.0 cm (power electrode D = 1.0 cm and grounded electrode D = 0 cm). The discharge parameter nominal pressure, excitation frequency, SiH 4 gas flow, and the electrode gap were maintained constant at 20 Pa, MHz, 40 sccm, and 20 mm, respectively. Spatially resolved emission profiles of SiH and H radicals were recorded under these conditions, and the results were correlated with the performance of the cells that incorporated these layers. Solar cells have been fabricated under same conditions as the films investigated here. To confirm that the OES measurements provided effective monitoring during film growth, many experimental groups deposited films under various conditions.table I shows the deposition parameters of thin-film solar cells: The RF power, E/S distance, and H 2 dilution are W, 7 11 mm, and sccm, respectively. Eventually, by controlling these deposition parameters, thin films a-si:h, μc-si, and a- Si:H/μc-Si tandem cell were fabricated, and the cell characteristics were obtained with the OES system. Three types of thin-film solar cells have been developed. They are a-si:h, μc-si, and a-si:h/μc-si tandem cell. Table II shows the details of intrinsic a-si:h, μc-si, and a-si:h/μc-si thin films. All these silicon films were deposited by PECVD in an ultrahigh-vacuum single-chamber load-locked system at a constant temperature of 200 C. We used a conductive SnO 2 - coated glass as substrate of size 5 5cm 2. The a-si:h single cell was constructed by p-b-i-n deposition, with a p-layer (10 nm), buffer layer (10 nm), i-layer (300 nm), and n-layer (30 nm) on the Asahi glass substrate with an E/S distance of 20 mm, 20-sccm SiH 4, and 80-sccm H 2. A 0.3-μm-thick Aldoped zinc oxide (AZO) layer and a 0.5-μm-thick Ag layer as a back contact were deposited by sputtering on the cell. In the μc-si single cell, the hydrogen dilution was changed to 1800 sccm, and the distance was changed from 11 to 8 to ensure the quality of the film. The a-si/μc-si tandem cell was fabricated in a manner that was based on the method used to fabricate the single-junction cells. The effective aperture area of the cell is 1 cm 2 by laser scribing process. These solar cells were characterized by current voltage and quantum efficiency measurements at 100 mw/cm 2 using an AM 1.5G dual-beam light source sun simulator. For quantum efficiency measurement, the ac probe had a diameter bigger than the solar cell to ensure that the entire sample could be probed. Infrared and ultraviolet band light was applied to measure the external quantum efficiency (EQE) of the top and bottom cells separately.

3 LIEN et al.: DEPOSITION AND CHARACTERIZATION OF SILICON THIN-FILM SOLAR CELLS 1247 III. RESULTS AND DISCUSSION A. Deposition Mechanism The glow-discharge deposition processes and growth mechanism involve the deposition, bombardment, and selective etching of the silicon films [13], [14]. The deposition processes are very complicated because of the physical and chemical interactions that occurred in the plasma. The growth of films depends on the RF power and frequency, the substrate temperature, the gas pressure and composition, the magnitude and the pattern of the gas flow, the geometry of the electrodes, and other factors. The growth mechanism of silicon thin films in plasma has three effects on the surface of a growing film. In film deposition, the primary reactions in the gas phase are the electron-impact excitation, dissociation, and ionization of SiH 4 molecules. The plasma thus comprises neutral radicals and molecules, positive and negative ions, and electrons. The second effect is the ion bombardment effect, in which reactive neutral species move to the substrate by diffusion and positive ions bombard the growing film. The third effect consists of the surface reaction, such as hydrogen abstraction, radical diffusion, and chemical bonding. This step is the subsurface release of hydrogen molecules and relaxation of the silicon matrix. Further understanding of these processes is sought to improve film quality, as well as to understand the basic surface and gas chemistry. Neutral radical species are widely accepted to be responsible for most of the deposition of a-si from discharges. The electron-impact excitation process is well known to govern the emission of light through the subsequent radiative decay of electronic-excited species. Pioneering work on SiH 4 plasma has revealed that such processes directly produce excited SiH and Si species [15]. Gas molecules may be excited into a higher electronic state, from which recombination to the ground state results in the emission of photons and is the origin of the plasma glow. The gas molecules are also excited into higher vibrational or rotational states in which the vibrational temperature of silane has been measured to be approximately 850 K. However, neither of these excitations directly causes deposition of the film. More important is the dissociation of gas either neutral radicals or ions. Examples of silane dissociation reactions and their required energies are shown as follows: [16] SiH 4 + e SiH 3 + H + e (4.0 ev; τ>20 ms) (3.1) SiH 2 + H 2 + e (2.2 ev; τ<3ms) SiH + H 2 + H + e (5.7 ev; τ<3ms) Si + 2H 2 + e (4.2 ev; τ<3ms) SiH 4 + H SiH 3 + H 2 (3.2) SiH 3 SiH + H 2 (3.3) SiH 2 Si + H 2. (3.4) Their radiative decays can be expressed as SiH SiH + hν (3 ev) (3.5) Si Si + hν ( ev). (3.6) From (3.1), during deposition, most of the reactants dissociate into SiH 2 which has the lowest excitation energy. However, there is a shorter lifetime (τ) compared to SiH 3 radicals, and it can form a molecule group with SiH 4 by a secondary reaction resulting in contamination of dust. Introducing hydrogen into the plasma chamber is a common method for inhibiting this effect by enhancing the reaction that is described by (3.2). Hydrogen atoms can help to generate SiH 3 with silane rather than rich SiH 2, favoring deposition of the Si thin film. The radicals decay as described in (3.5) and (3.6), and the release of photons is detected by an OES. The excited energies facilitate the determination of the dissociation. For example, the reactions in plasma generally involve the dissociation of SiH 3, and the OES spectrum must detect a feature at around 400 nm. Restating, if the main reaction involves SiH 2, such radiative decay causes the release of photon (hν) that is detected by the OES system as a feature around 250 nm. Extensive dilution in H 2 results in films, in which the crystals either abut each other [17] or are embedded in an amorphous matrix [18] forming nanocrystals. The two mechanisms of deposition of plasma from SiH 4 gas are surface diffusion [19], [20] and selective etching [21] [23]. Both have been proposed to explain the deposition of nc-si:h. The surface diffusion was considered to improve the mobility of the deposition precursors by many hydrogen atoms to impinge on the surface. For selective etching, amorphous and crystalline phases are assumed to be deposited simultaneously; atomic hydrogen impinges on the film surface, and the amorphous material is selectively etched, leaving behind a crystal film. The chemical approach has been proposed to enable hydrogen atoms to promote crystallization by annihilating the strained Si Si bonds in the a-si:h film [24] [27]. The concentration of hydrogen in the film and with the silicon network depends on the chemical potential of the hydrogen in plasma state. The weaker Si Si bonds, below the hydrogen chemical potential, are broken, while the stronger bonds remain. The concentration of hydrogen should increase with the chemical potential, while a-si:h films actually fall as hydrogen is added to the plasma [28]. The reconstruction of the silicon network may move binding sites from below the chemical potential to above it, in effect replacing weak bonds with strong ones. The other effect of hydrogen in the silicon matrix is to break Si Si bonds. Following exposure of the surface to hydrogen atoms, defects are observed in the surface region [29], and these defects have no Burger vector and do not represent dislocations. Some unhealed structural defects remain at the interface between the amorphous and crystalline regions. The insertion of hydrogen into the Si Si bonds [30], [31] causes the formation of intermediate bond-centered Si H Si configurations, and so, the structure in the amorphous-to-crystalline transition is mediated by relaxation of strained Si Si bonds. After the hydrogen atoms move away from the bond-centered location, the strained Si Si bonds break or relax; subsequent local structural rearrangements yield bond lengths and angles that are closer to those of c-si. In summary, the chemical mechanism in this theory bonded the materials, in which the presence of hydrogen causes disorder-to-order transitions. [32]

4 1248 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 59, NO. 5, MAY 2012 Fig. 2. Optical emission spectra of plasma state with different RF powers. The OES photocollimator positioned at X = 3cmandD = 0.4 integrated configuration. Fig. 3. Results of the radical intensity and deposition rate as a function of different RF powers from 100 to 500 W. B. Effect Discussion Fig. 2 shows a series of optical emission spectra of pure SiH 4 RF glow discharges with various powers. The OES photocollimator sampling points are located at X = 3 cm and D = 0.4 cm due to the fact that the position has the best film deposition state in terms of uniformity and crystallinity quality. The intensity of each spectrum is related to the strength of the electrical signal of the photodiode array detector. The main features in the spectra correspond to the emission of SiH- (414 nm) and H- (656, 722, and 772 nm) excited radicals, which are formed directly by dissociative excitation in electron collision with pure SiH 4. SiH and H radicals affect the deposition rate and the rate of selective etching, respectively. The emission intensity of SiH depends more strongly on power than those of Si and H. A study of RF power is essential because its variation directly influences the total electron density. The SiH concentration appears to be determined only by the density of energetic electrons in the plasma bulk. Fig. 3 shows the effects of the intensity of excited radicals and the deposition rate on RF power. By increasing the injected plasma power, we observe an increase of SiH and H emission peaks and increased deposition rate from 0.2 to 0.8 nm/s as the power increased from 100 to 500 W. All of these samples are fabricated only on glass so that the film state and trend which fixed the thickness of 1.2 μm on different process conditions could be clearly observed. The high-power H 2 -rich plasma is shown in Fig. 4. The grain profile of the silicon film becomes sharper, and the size of the grain increased with the processing power. The distribution of grain sizes is strongly affected by the increase in the SiH and H emissions. Abundant excited radicals can be regarded as the deposition and dissociation in the chamber which act onto the substrate, resulting in the increase of the crystalline phase. In this experiment on various E/S distances, we gained the data with the position of the collimator at X = 3 cm and D = 0.2 cm at first at an E/S distance of 7 mm. Since the E/S distance increased, the collimator was moved upward to fix the gap from the substrate so that to make sure that the Fig. 4. SEM images with different RF powers. radical intensities have been detected accurately. Finally, we set the collimator with position of X = 3 cm and D = 0.8 cm, while the E/S distance was 11 mm. Fig. 5 shows the spatial distribution of the normalized emission profiles of the electrondissociative excitation of SiH and H. Fig. 6 plots the excited SiH and H radicals and deposition rate as functions of the E/S distance. The intensity of excited SiH radicals in the plasma decreases slightly from 500 to 420 a.u. as the E/S distance increases from 7 to 11 mm. The intensity of the signals from the excited H radicals initially decreased slightly, falling to 250 a.u. at an E/S distance of 11 mm. Since the E/S distance increased over 9 mm, the mean free path of the H radicals was too long to enable them to reach the surface of the thin film, reducing the effectiveness of selective etching. Therefore, the deposition rate was increased to 7.1 Å/s by weakening the selective etching effect. Fig. 7 shows the scanning electron microscope (SEM) images of the crystalline phase. The crystalline growth rate depended on the E/S distance. As the effectiveness of selective etching decreased, the grain size slightly decreased, and at an E/S distance of 11 mm, deposition of the film became easier.

5 LIEN et al.: DEPOSITION AND CHARACTERIZATION OF SILICON THIN-FILM SOLAR CELLS 1249 Fig. 5. Optical emission spectra of plasma state with different E/S distances. Fig. 8. Optical emission spectra of plasma state with different H 2 dilutions. Fig. 6. Results of the radical intensity and deposition rate as function of different E/S distances from 7 to 11 mm. Fig. 7. SEM images with different E/S distances. Fig. 8 shows the optical emission spectra of the H 2 dilution effect, which includes such features associated with the emission of excited SiH (414 nm) and H ( nm) radicals with five different values of the measured H 2 in standard cubic Fig. 9. Results of the radical intensity and deposition rate as function of different H 2 dilutions from 1200 to 2000 sccm. centimeters per minute. The number of excited SiH radicals observably decreased as H 2 dilution values increased, reducing the rate of deposition.fig. 9 plots the radical intensity and deposition rate as a function of H 2 dilution. The intensity of SiH observably decreased with increased H 2 dilution, whereas the intensity of H remained at around 450. The status of plasma dissociation is maintained by the constancy of the ratio of the number of SiH to the number of H radicals, explaining why the decrease in the intensity of the signal-associated SiH was accompanied by abundant H 2 dilution. Therefore, the shortage of dissociated SiH radicals reduced the deposition rate. Fig. 10 shows the SEM images of films that were deposited at various H 2 dilutions; the grain size evidently increased with H 2 dilution. The aforementioned results demonstrate that the OES efficiently monitors the conditions of film growth. In this investigation, various SiH signal intensities were obtained by changing the parameters at a high frequency of 27.1 MHz. In particular, OES measurements were made to obtain axial concentration profiles of SiH and H radicals with conditions. Fig. 11(a) (c) shows the domains of the plasma deposition space; the x-, y-, and z-axes represent the position of the sample, the distance from the grounded electrode, and the intensity of the signals associated with the radicals, respectively. With respect to the x-axis, X1 X5 represent five positions, and the intensities are shown in every horizontal line so that the uniformity or lack

6 1250 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 59, NO. 5, MAY 2012 Fig. 10. SEM images with different H 2 dilutions. thereof can be observed easily from the position of the sample. The y-axis is divided into D1 D4 expressed in each vertical line consisting of radical intensity flowed down to the sample position in plasma space.fig. 11(a) and (b) shows the distribution of the SiH and H radicals at various positions. Every node corresponds to their axis position which was identified as the intensity. By monitoring the state of plasma, we can obtain the radical emission status whether it is uniform or not while in the process of deposition, which is marked by a figure to easily control the film quality. Fig. 11(c) shows the thickness of the deposited film; the thickness is highly uniform with a standard deviation of approximately 2%. The OES is an efficient means of monitoring amorphous growth conditions. Torres et al. [33], [34] mentioned that the ratio of emission intensities SiH/H can be used to determine the microstructure of the deposited film. The deposition rate is a sensitive function of the SiH intensity. Therefore, the intensity of the SiH peak can be used to monitor the deposition rate of films. The real-time monitoring of SiH intensity at various positions between the RF electrode and the substrate can be applied to the in situ control of the quality of the film during the deposition of a-si:h or μc-si in a PECVD system. Additionally, an increase of the deposition rate is accompanied by an increase of the SiH intensity, causing the formation of more defects or dangling bonds in the film, affecting photovoltaic conversion efficiency [33]. The size of the grains in the thin films was also related to the OES ratio. For this investigation, we observed that, when the SiH/H ratio is above 1.7, the a-si:h film growth was obtained. The behavior that governed the surface morphology of the thin film is consistent with the results in other studies and references [35] [37]. C. Cell Performance In this paper, single-junction amorphous, microcrystalline, and tandem solar cells were prepared to validate process Fig. 11. Distribution of plasma state according to the sample position, distance from grounded electrode, and intensity of radicals as x-, y-, and z-axes, respectively. (a) SiH.(b)H. (c) Thickness distribution. control using OES technology. Fig. 12 plots the typical I V curves of a-si:h and μc-si single-junction solar cells with aperture area efficiencies of 1 1 cm 2. The best performances of the a-si:h single-junction cell were initial conversion efficiency = 9.71%, FF = 0.70, J sc = 16.9 ma/cm 2, and V oc = 0.81 V, and those of μc-si singlejunction cell were initial efficiency = 6.36%, FF = 0.68, J sc = 18.9 ma/cm 2, and V oc = 0.49 V. Fig. 13 shows the EQE s of two representative amorphous and microcrystalline thin-film solar cells that were fabricated on Asahi U-type glass. Theoretically, for solar cell, the effective cutoff wavelengths of a-si and μc-si cells are about 700 and 1100 nm due to materials bandgaps, respectively. In this paper, we validated these a-si:h, μc-si, and tandem cells with EQE measurements and showed spectrum response characteristics for our guaranty. These results reveal that the cells exhibited well spectrum response, with respect to the reference standard thin-film solar cell. The amorphous cell had a glass/sno 2 /

7 LIEN et al.: DEPOSITION AND CHARACTERIZATION OF SILICON THIN-FILM SOLAR CELLS 1251 Fig. 15. TEM images of µc-si single-junction cell on Asahi U-type glass for (a) cross-sectional image and (b) the diffraction pattern of silicon thin film. Fig. 12. J V curves of a-si:h and µc-si single-junction cells prepared on Asahi U-type glass, and the inset table shows the corresponding cell characteristics. Fig. 16. Presentation of the a-si:h/µc-si tandem cell performance and J V characteristics prepared on Asahi U- type glass. Fig. 13. EQE for a-si:h and µc-si single-junction cells with a standard AM1.5 spectrum; results show a normal spectral response for cell verification. Fig. 14. TEM images of a-si:h single-junction cell on Asahi U-type glass for (a) cross-sectional image and (b) the diffraction pattern of silicon thin film. p-b-i-n/azo/ag structure. Fig. 14(a) shows the transmission electron microscopy (TEM) images of the deposited a-si:h thin-film solar cell, and Fig. 14(b) shows the diffraction pattern of a-si:h intrinsic layer. The pattern of a-si:h is broad without obvious speckles or ring. Fig. 15(a) shows microcrystalline thin-film solar cells that have the same structure as a-si:h and are fabricated under different deposition conditions. The diffraction pattern shows a halo pattern and demonstrates the microcrystals formed [Fig. 15(b)]. Fig. 16 shows that the J V characteristic of a current tandem cell with an aperture area of 1 1cm 2 is the same as that of the single-junction cell that was presented earlier. The best Fig. 17. EQE of the top and bottom junctions of a tandem cell with a standard AM1.5 spectrum is shown with a normal spectral response. cell performance was efficiency = 12.3%, FF = 0.72, J sc = 13.1 ma/cm 2, and V oc = 1.3 V. Fig. 17 plots the EQE curves of the tandem cell in its initial state. The figure demonstrates that the tandem solar cell with a a-si:h top cell exhibits shortwavelength spectral response, and its μc-si bottom cell can be used for illumination by long wavelengths. Fig. 18 shows the TEM images of tandem solar cell to verify the quality of the film. The diffraction patterns in Fig. 18(a) and (c) demonstrate that the top cell had an amorphous structure and the bottom cell had a microcrystalline structure. Furthermore,

8 1252 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 59, NO. 5, MAY 2012 ratio of radicals was controlled below 1.7, and microcrystalline growth was obtained in the final stage. Finally, a solar cell with a back contact was fabricated by inline sputtering and characterized by current voltage and EQE measurements. This process is an example of the use of the OES method to fabricate a high-quality solar cell. Fig. 18. TEM images with diffraction pattern of a-si:h/µc-si tandem cell on Asahi U-type glass for (a) cross-sectional image of top cell, (b) the interface tunneling junction between top and bottom cells, and (c) cross-sectional image of the bottom cell. IV. CONCLUSION In this paper, the OES technique has been adopted to characterize the temporal behavior of pure silane plasma. The utility of the OES method for monitoring the state of plasma during deposition is thus proven. The ratio of amounts of SiH to H radicals is useful in controlling the microstructure of the deposited films and, so, can be adopted with an OES to control the quality of the film in real time. The main findings of this research concerning film deposition are as follows: 1) The grain profile became more distinct, and the size of the grains increased as the processing power increased; 2) the SiH signal intensity from the plasma decreased slightly as the E/S distance increased; 3) as the H 2 dilution increased, the intensity of the signal associated with SiH was decreased, because the rate of dissociation in the plasma was maintained by holding constant the ratio of SiH to H radicals. The rate of deposition of silicon films depended on the SiH emission intensity because the concentration of active species increased with RF power. Finally, single-junction amorphous and microcrystalline solar cells with efficiencies of 9.71% and 6.36%, respectively, were fabricated using high-frequency PECVD. A a-si:h/μc-si tandem cell with a favorable efficiency of 12.3% was also obtained. The OES method can be adopted to control the quality and crystalline nature of the silicon thin film. The findings herein provide guidance toward the fabrication of optimal solar cells. Fig. 19. Deposition phase diagram for in situ growth on silicon thin film. This diagram describes the relationship between radical intensity and deposition duration which is measured by the OES system. the tunneling junction between the top and bottom cells was of high quality, shown in Fig. 18(b). D. Deposition Process In this investigation, OES with a high-frequency 27.1-MHz PECVD was adopted. The intensity of signals associated with SiH and H was monitored to control the quality and crystallinity of silicon thin films. Fig. 19 plots the radical intensity measured in situ by OES during deposition of the tandem cell which was discussed earlier. First, the ratio of SiH to H in the top cell was maintained above 1.7 to ensure that the film was amorphous in the first 30 min. Second, as the bottom cell was deposited, the REFERENCES [1] Y. Tawada, H. Okamoto, and Y. Hamakawa, Properties and structure of a-sic:h for high-efficiency a-si solar cell, Appl. Phys., vol. 53, no. 7, pp , Jul [2] L. Feitknecht, O. Kluth, Y. Ziegler, X. Niquille, P. Torres, J. Meier, N. Wyrsch, and A. Shah, Microcrystalline n-i-p solar cells deposited at 10 Å/s by VHF-GD, Solar Energy Mater. Solar Cell, vol. 66, no. 1 4, pp , Feb [3] T. Kilper, M. N. van den Donker, R. Carius, B. Rech, G. Braüer, and T. Repmann, Process control of high rate microcrystalline silicon based solar cell deposition by optical emission spectroscopy, Thin Solid Films, vol. 516, no. 14, pp , May [4] M. Guláè, F. Le Normand, and P. Veis, Gas phase kinetic and optical emission spectroscopy studies in plasma-enhanced hot filament catalytic CVD production of carbon nanotubes, Appl. Surf. Sci., vol. 255, no. 10, pp , Mar [5] A. Parashar, S. Kumar, P. N. Dixit, J. Gope, C. M. S. Rauthan, and S. A. Hashmi, High-pressure condition of SiH 4 + Ar + H 2 plasma for deposition of hydrogenated nanocrystalline silicon film, Solar Energy Mater. Solar Cells, vol. 92, no. 10, pp , Oct [6] X. D. Zhang, F. R. Zhang, E. Amanatides, D. Mataras, and Y. Zhao, Effect of substrate bias on the plasma enhanced chemical vapor deposition of microcrystalline silicon thin films, Thin Solid Films, vol. 516, no. 20, pp , Aug [7] G. S. Chen, C. C. Lee, H. Niu, H. Welson, J. Robert, and T. Schütte, Sputter deposition of titanium monoxide and dioxide thin films with controlled properties using optical emission spectroscopy, Thin Solid Films, vol. 516, no. 23, pp , Oct [8] L. Feitknecht, J. Meier, P. Torres, J. Zürcher, and A. Shah, Plasma deposition of thin film silicon: Kinetics monitored by optical emission

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Phys, vol. 29, no. 10, pp , Oct Shui-Yang Lien (M 00) received the M.S. degree in electrical engineering from the National Yunlin University of Science and Technology, Yunlin, Taiwan, in 2003 and the Ph.D. degree in materials science and engineering from National Chung Hsing University, Taichung, Taiwan, in He is currently an Assistant Professor with the Department of Materials Science and Engineering, Mingdao University, Changhua, Taiwan. He has done work in the field of solar cell materials by plasma-enhanced chemical vapor deposition (CVD) and hot-wire CVD. He has authored or coauthored over 32 technical papers (SCI) in his fields of expertise. His research studies and main interests are silicon-based materials, thin-film solar cells, wafer-based high-efficiency solar cells, simulation technology of solar cell processes, CVD machine design, and thin-film growth. Yu-Cheng Chang received the B.S. degree from the Department of Mechanical Engineering, Chinese Culture University, Taipei, Taiwan, in 2008 and the M.S. degree in mechanical engineering from Yuan Ze University, Chungli, Taiwan, in He is currently with Yuan Ze University. His research is focused on the process of thin-film silicon solar cell. Yun-Shao Cho received the M.S. degree in materials science and engineering from Mingdao University, Changhua, Taiwan, in He is currently working toward the Ph.D. degree in the Department of Materials Science and Engineering, National Chung Hsing University, Taichung, Taiwan. His research is focused on silicon-based thin-film solar cells and the optical emission spectrometer technology.

10 1254 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 59, NO. 5, MAY 2012 Yin-Yu Chang received the Ph.D. degree in materials engineering from National Chung Hsing University, Taichung, Taiwan, in He was with Surftech Company as an R/D Manager for coating research and strategies for 13 years until After his industrial service, he has been with the Department of Materials Science and Engineering, Mingdao University, Changhua, Taiwan, since 2005 where he has been responsible for a research center (Surface Engineering Research Center) since He directed the research on thin-film applications including hard coatings, antibacterial surface engineering, and vacuum plasma surface modification. This led to the development of advanced coating systems with cathodic arc evaporation and computerized interfaces. He is currently an Assistant Professor with the Department of Mechanical and Computer-Aided Engineering, National Formosa University, Huwei, Taiwan. His main research studies are plasma analysis of vacuum deposition, thin-film technologies by PECVD, cathodic arc ion plating, magnetron sputtering, etc. Dr. Chang and his research team in Surftech Company were a recipient of the Innovation Reward by the Ministry of Economic Affairs, Taiwan, in 2000 and the R/D Innovation Reward by the Ministry of Economic Affairs and Taichung County Industry Association (2003). Shuo-Jen Lee received the Ph.D. degree from the University of Illinois, Urbana. He is a Professor from the Department of Mechanical Engineering, Yuan Ze University, Jhongli, Taiwan. His current research has been focused on electrochemical processes and MEMS for the applications in fuel cells, thin-film solar cells, and secondary Li-ion batteries.