CHAPTER 4 LED LIGHT EMITTING DIODE

Transcription

1 CHAPTER 4 LED LIGHT EMITTING DIODE 1

2 PART II LIGHT EMITTING DIODE

3 LED are semiconductor p-n junctions that under forward bias conditions can emit What is LED? radiation by electroluminescence in the UV, visible or infrared regions of the electromagnetic spectrum. The qaunta of light energy released is approximately proportional to the band gap of the semiconductor.

4 APPLICATION

5 LED BASIC STRUCTURE

6 MECHANISM OF PHOTON EMISSION injection Electroluminescence (EL) Electrical + optical phenomenon Apply strong electric field to the system Material will emit light as a result of radiative recombination of electron- hole pairs..

7 MECHANISM OF PHOTON EMISSION Luminescence part tells us that we are producing photons. Electro part tells us that the photons are being produced by an electric current. Injection tells us that photon production is by the injection of current carriers.

8 PRINCIPLES OF OPERATION Figure 1 : The energy band diagram for p-n + (heavily n- type dopes) junction without any bias (V = 0). Built in potential V o prevents electron from diffusing from n to p side. Figure 2 : The applied bias reduces V o and thereby allows electron to diffuse, be injected into p side. Recombination around the junction and within the diffusion length of the electron in the p-side leads to phonon emission

9 PRINCIPLES OF OPERATION LED is essentially a pn junction diode typically made from a direct bandgap semiconductor i.e. GaAs Electron-hole pair (EHP) recombination results in the emission of a photon energy equal to the bandgap energy, hv E g Figure 1 shows energy band diagram on an unbiased pn + junction device in which the n-side is more heavily doped (allow more electron diffusion) compared to the p-side This net electron diffusion however is prevented by the electron potential barrier, evo. Potential energy, PE (built in voltage) is given by : ev o = E c = E cp E cn +

10 PRINCIPLES OF OPERATION As soon as a forward bias V is applied, this voltage drops across the depletion region since this is the most resistive part of the device. Refer Figure 2. Consequently, the built-in potential is reduced to V 0 - V, which then allows the electron from n-side to diffuse into the p-side The hole injection component from p to n-side is much smaller than the electron injection component Recombination of electrons in the depletion region and the p-side results in spontaneous emission of photons Recombination primarily in depletion region and volume extending over the diffusion length (L e ) of the electrons on the p-side this recombination zone is called the active region Phenomenon of light emission from EHP (electron hole pair) recombination as a result of minority carrier injection as in this case is called injection electroluminescence

11 PRINCIPLES OF OPERATION Figure 3 : Spontaneous emission under forward bias ( current flow is permitted) Statistical nature of recombination leads to emission of photons in random directions results from spontaneous emission LED structure has to be such that the emitted photons can escape the device without being reabsorbed by the semiconductor material This means the p-side has to be sufficiently narrow or we have to use heterostructure devices as will be discussed later

12 DEVICE STRUCTURES The LED structure plays a crucial role in emitting light from the LED surface. The LEDs are structured to ensure most of the recombination's takes place on the surface by the following two ways. i. increase the doping concentration of the substrate, so that additional free minority charge carriers electrons move to the top, recombine and emit light at the surface. ii. increase the diffusion length L = Dτ, where D is the diffusion coefficient and τ is the carrier life time. But when increased beyond a critical lengththere is a chance of reabsorption of the photons into the device

13 DEVICE STRUCTURES The LED has to be structured so that the photons generated from the device are emitted without being reabsorbed. One solution is to make the p layer on the top thin, enough to create a depletion layer GaAs or GaP Light output Light output p n + Epitaxial layers p n + Insulator (oxide) Epitaxial layer n + Substrate n + Substrate (a) Metal electrode (b) A schematic illustration of typical planar surface emitting LED devices. (a) p-layer grown epitaxially on an n + substrate. (b) First n + is epitaxially grown and then p region is formed by dopant diffusion into the epitaxial layer S.O. Kasap, Optoelectronics (Prentice Hall)

14 DEVICE STRUCTURES LED are typically fabricated by epitaxially growing doped semiconductor layers on a suitable substrate (e.g. GaAs or GaP) as depicted in the next figure This type of planar pn junction is formed by the epitaxial growth of first the n-layer then the p-layer Substrate is essentially a mechanical support for the pn junction and can be of different material The p-side is on the surface from which light is emitted and is therefore made narrow (a few microns) to allow the photons to escape without being reabsorbed To ensure that most of the recombination is in the p-side, the n- side is heavily doped (n + ) Those photons that are emitted towards the n-side become either absorbed or reflected back at the substrate interface It is also possible to form p-side by using dopant diffusion

15 DEVICE STRUCTURES Not all light rays reaching the semiconductor-air interface can escape because of total internal reflection (TIR) Those with angle of incidence greater than the critical angle c become reflected as shown in the next figure GaAs-air interface, the C = 16 o which means that much of the light suffers TIR. To solve the problem we could: Need to shape the surface of the semiconductor into a dome or hemisphere so that light rays strike the surface at angles less than

16 C = 16 o which means that much of the light suffers TIR reduce TIR (a) (b) (c) Light output Plastic dome Light Domed semiconductor p n + n + pn Junction Substrate Electrodes Electrodes (a) Some light suffers total internal reflection and cannot escape. (b) Internal reflections can be reduced and hence more light can be collected by shaping the semiconductor into a dome so that the angles of incidence at the semiconductor-air surface are smaller than the critical angle. (b) An economic method of allowing more light to escape from the LED is to encapsulate it in a transparent plastic dome S.O. Kasap, Optoelectronics (Prentice Hall)

17 DEVICE STRUCTURES Drawback difficult to fabricate the dome and associated increase in expense Inexpensive and common procedure to reduce TIR is encapsulation with a transparent plastic medium that has higher refractive index than air and also has a domed surface on one side of the pn junction

18 DEVICE STRUCTURES UV AlGaN Blue GaN, InGaN Red, green GaP Red, yellow GaAsP IR- GaAs

19 LED - material, Colors & voltage drop Color Wavelength (nm) Voltage (V) Semiconductor Material Infrared λ > 760 ΔV < 1.9 Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs) Red 610 < λ < < ΔV < 2.03 Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP) Orange 590 < λ < < ΔV < 2.10 Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP)Gallium(III) phosphide (GaP) Yellow 570 < λ < < ΔV < 2.18 Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP) Green 500 < λ < < ΔV < 4.0 Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN) Gallium(III) phosphide (GaP)Aluminium gallium indium phosphide (AlGaInP) Aluminium gallium phosphide (AlGaP) Blue 450 < λ < < ΔV < 3.7 Zinc selenide (ZnSe), Indium gallium nitride (InGaN), Silicon carbide (SiC) as substrate, Silicon (Si) Violet 400 < λ < < ΔV < 4.0 Indium gallium nitride (InGaN) Purple multiple types 2.48 < ΔV < 3.7 Dual blue/red LEDs,blue with red phosphor,or white with purple plastic Ultraviolet λ < < ΔV < 4.4 diamond (235 nm), Boron nitride (215 nm), Aluminium nitride (AlN) (210 nm) Aluminium gallium nitride (AlGaN) (AlGaInN) (to 210 nm) White Broad spectrum ΔV = 3.5 Blue/UV diode with yellow phosphor

20

21 DEVICE MATERIAL Various direct bandgap semiconductor materials that can be readily doped to make a commercial pn junction LEDs that emits radiation in the visible and infrared range of wavelengths An important class of commercial semiconductor materials that cover the visible spectrum is the III-V ternary alloys based on alloying GaAs and GaP, which are denoted as GaAs 1-y P y In this compound, As and P atoms from group V are distributed randomly at normal As sites in the GaAs crystal structure

22 DEVICE MATERIAL Direct bandgap and Indirect bandgap When y < 0.45, the alloy GaAs 1-y P y is a direct bandgap semiconductor and hence EHP recombination process is direct The rate of recombination is directly proportional to the product of electron and hole concentrations Emitted wavelengths range from about 630nm for y = 0.45 (GaAs 1-y P y ) to 870nm for y=0 (GaAs) GaAs 1-y P y alloys with y>0.45 are indirect bandgap semiconductors An important class of commercial semiconductor materials that cover the visible spectrum is the III-V ternary alloys based on alloying GaAs and GaP, which are denoted as GaAs 1-y P y

23 DEVICE MATERIAL The EHP recombination process occur through recombination centers and involve lattice vibrations rather than photons emission However, if we add isoelectronic impurities such as nitrogen (in same group V as P) into the semiconductor crystal then some of these N atoms substitute for P atoms do not act as donor or acceptor Electronic cores of N and P are however different the positive nucleus of N is less shielded by electron compared to P This means that the conduction electron in the neighborhood of an N atom will be attracted and may become trapped at this site N atom therefore introduce localized energy level, or electron traps, E N, near the conduction band edge as depicted in figure When a conduction electron is captured at E N, it attract a hole in its vicinity by Coulombic attraction and eventually recombine with it directly and emit a photon energy slightly less than E g

24

25 DEVICE MATERIAL Isoelectronic Centre Isoelectronic means that the centre being introduced has the same number of valance electrons as the element it is replacing. For example, nitrogen can replace some of the phosphorus in GaP. It is isoelectronic with phosphorus, but behaves quite differently allowing reasonably efficient green emission.

26 Because the effective transition is occurring between the isoelectronic centre and VB edge, the photon that is emitted has a lower energy than the band-gap energy. Isoelectronic centres provide a stepping stone for electrons in E-k space so that transitions can occur that are radiatively efficient. The recombination event shown has no change in momentum, so it behaves like a direct transition. Isoelectronic centre Electron-hole recombination E Holes k = 0 CB edge electrons VB edge de

27 . Photon energy is less than the semiconductor band-gap energy it means that the photon is not absorbed by the semiconductor, and so the photon is easily emitted from the material. Isoelectronic centre Electron-hole recombination E CB edge electrons de Holes VB edge k = 0

28 Energy _ Addition of a nitrogen recombination center to indirect GaAsP. Both As and P are group V elements. (Hence the nomenclature of the materials as III-V compound semiconductors.) h + Momentum

29 As the recombination process depends on N doping, it is not as efficient as direct recombination less efficiency Nitrogen doped indirect bandgap GaAs 1-y P y alloys are widely used in inexpensive green, yellow,and orange LEDs Two types of blue LED materials GaN is a direct bandgap material (E g =3.4eV) blue GaN LEDs use GaN alloy; InGaN - Indium gallium nitride (E g =2.7eV) blue emission Less efficient type is Al doped silicon carbide (SiC) indirect bandgap acceptor type localized energy level captures a hole recombines and emit a photon not efficient limited brightness

30 White LED White Light When light from all parts of the visible spectrum overlap one another, the additive mixture of colors appears white. However, the eye does not require a mixture of all the colors of the spectrum to perceive white light. Primary colors from the upper, middle, and lower parts of the spectrum (red, green, and blue), when combined, appear white.

31 Red LEDs can be made in the GaAsP (gallium arsenide phosphide). GaAs 1-x P x for 0<x<0.45 has direct-gap for x>0.45 the gap goes indirect and for x=0.45 the band gap energy is 1.98 ev. Hence it is useful for red LEDs. p-gaasp region N-GaAsP P = 40 % N-GaAs substrate Ohmic Contacts Dielectric (oxide or nitride) Fig. GaAsP RED LED on a GaAs sub.

32 Orange-Yellow & Green LEDs Orange (620 nm) and yellow (590 nm) LEDs are commercially made using the GaAsP system. However, as we have just seen above, the required band-gap energy for emission at these wavelengths means the GaAsP system will have an indirect gap. The isoelectronic centre used in this instance is nitrogen, and the different wavelengths are achieved in these diodes by altering the phosphorus concentration.

33 Visible LED Definition: LED which could emit visible light, the band gap of the materials that we use must be in the region of visible wavelength = nm. This coincides with the energy value of 3.18eV- 1.61eV which corresponds to colours as stated below: Colour of an LED should emits Violet Blue Green Yellow Orange Red ~ 3.17eV ~ 2.73eV ~ 2.52eV ~ 2.15eV ~ 2.08eV ~ 1.62eV The band gap, E g that the semiconductor must posses to emit each light

34 GaN ZnSe GaP:N Relative response of the human eye to various colors 10 0 Relative eye response GaAs.14 p 86 GaAs.35 p 65 GaAs.6 p violet blue green yellow orange red Wavelength in nanometers The materials which are used for important light emitting diodes (LEDs) for each of the different spectral regions.

35 Calculate If GaAs has E g = 1.43ev What is the wavelength, g it emits? What colour corresponds to the wavelength? Given: E C -E V = E g g = hc/e g

36 Properties of InGaN A LED fabricated in a graded material where on either side of the junction region the material changes slowly from InN to GaN via InGaN alloys. Minority carriers need to get through the whole of this alloy region if efficient photon production at all visible wavelengths was to occur.

37 Concentration: GaN (direct) The highly gallium rich alloy InN (indirect) The highly indium rich alloy Band gap: 3.3eV 2 ev Wavelength of photons: 376 nm 620 nm Part of the electromagnetic spectrum: In the ultraviolet In the visible (orange)

38 GaN is a direct bandgap material (E g =3.4eV) blue GaN LEDs use GaN alloy; InGaN (E g =2.7eV) blue emission GaN InN ultraviolet 3.3 ev(376 nm)

39 Violet Blue Green Yellow Orange Red ~ 3.17eV ~ 2.73eV ~ 2.52eV ~ 2.15eV ~ 2.08eV ~ 1.62eV GaN InN violet 3 ev (414 nm)

40 Violet Blue Green Yellow Orange Red ~ 3.17eV ~ 2.73eV ~ 2.52eV ~ 2.15eV ~ 2.08eV ~ 1.62eV GaN InN 2.7 ev(460 nm)

41 Violet Blue Green Yellow Orange Red ~ 3.17eV ~ 2.73eV ~ 2.52eV ~ 2.15eV ~ 2.08eV ~ 1.62eV GaN InN 2.4 ev(517 nm)

42 Violet Blue Green Yellow Orange Red ~ 3.17eV ~ 2.73eV ~ 2.52eV ~ 2.15eV ~ 2.08eV ~ 1.62eV GaN InN 2.1 ev(591 nm)

43 Violet Blue Green Yellow Orange Red ~ 3.17eV ~ 2.73eV ~ 2.52eV ~ 2.15eV ~ 2.08eV ~ 1.62eV GaN 2 ev(620 nm) InN 2.00 ev

44 A number of the important LEDs are based on the GaAsP system. GaAs is a direct band-gap S/C with a band gap of 1.42 ev (in the infrared). GaP is an indirect band-gap material with a band gap of 2.26 ev (550nm, or green). GaAs GaP 1.42 ev

45 Violet Blue Green Yellow Orange Red ~ 3.17eV ~ 2.73eV ~ 2.52eV ~ 2.15eV ~ 2.08eV ~ 1.62eV GaAs GaP 1.52 ev

46 GaAs GaP 1.62 ev

47 Violet Blue Green Yellow Orange Red ~ 3.17eV ~ 2.73eV ~ 2.52eV ~ 2.15eV ~ 2.08eV ~ 1.62eV GaAs GaP 1.72 ev

48 GaAs GaP 1.80 ev

49 GaAs GaP 1.90 ev

50 GaAs GaP 2.00 ev

51 GaAs GaP 2.26 ev

52 Internal efficiency what fraction of EHP recombinations in the forward biased pn junction are radiative and therefore lead to photon emission internal Rate of radiative recombination Total rate of recombination (radiative and nonradiative) 1 τr 100% 1 1 τ τ r nr η int = photons emitted from active region per second electrons injected in to LED per second = P int / (hν) I / e

53 External efficiency efficiency of conversion of electrical energy into an emitted external optical energy incorporates efficiency of radiative recombination process and subsequent photon extraction from the device external P out Optical IV 100%

54 External Quantum Eficiency ext photons emitted from LED LED internally generated photons In order to calculate the external quantum efficiency, we need to consider the reflection effects at the surface of the LED. If we consider the LED structure as a simple 2D slab waveguide, only light falling within a cone defined by critical angle will be emitted from an LED.

55 d T c ) sin )(2 ( ext [4-11] ) ( 4 (0) Transmission Coefficient ) :Fresnel ( n n n n T T [4-12] ext 2 1) ( 1 1 If n n n [4-13] int int ext 1) ( powr, optical LED emitted n n P P P [4-14]

56 LEDs for optical communications Two types of LED devices Surface emitting LED (SLED) emitted radiation emerges from an area in the plane of recombination layer Edge emitting LED (ELED) emitted radiation emerges from an area on an edge of the crystal, i.e. perpendicular to the active layer

57 Burrus- Type LED Light emits from the surface Light emitting from the edge of a cleaved wafer, where the active region meets the cleaved surface. Double heterostructure (a) Surface emitting LED Light Light emits from the edge (b) Edge emitting LED 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

58 Burrus- Type LED Light emits from the surface Light greater intensity light relatively low refractive index Light guided to the edge by dielectric waveguide form by double heterostructure Double heterostructure (a) Surface emitting LED Light Light emits from the edge (b) Edge emitting LED 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

59 LED Characteristics Energy of emitted photon not simply equal to the bandgap energy because electrons and holes are distributed in the CB and VB next figure Electron concentration is given by g(e)f(e) g(e) is density of states and f(e) is the Fermi-Dirac function (probability of finding and electron in a state with energy E) Asymmetrical concentration with peak at (1/2)k B T above E c with energy spread ~2k B T Recall that the rate of recombination is proportional to both the electron and hole concentrations at the energies involved relate figure (b) and (c) By using =c/v we can get (d) from (c) The linewidth of the output spectrum is defined as the width between half-intensity points as in (c) and (d)

60 (a) E (b) (c) (d) Electrons in CB CB VB E c E g E v 2k B T Relative intensity Relative intensity 1 /2 k B T E g + k B T 1 (2.5-3)k B T 1 h 0 h Holes in VB h h h 0 Carrier concentration per unit energy E g (a) Energy band diagram with possible recombination paths. (b) Energy distribution of electrons in the CB and holes in the VB. The highest electron concentration is (1/2) k B T above E c. (c) The relative light intensity as a function of photon energy based on (b). (d) Relative intensity as a function of wavelength in the output spectrum based on (b) and (c) S.O. Kasap, Optoelectronics (Prentice Hall)

61 The wavelength and linewidth are obviously related to the energy distribution of electrons and holes density of states individual semiconductor properties Photon energy for the peak emission is roughly E g +k B T (peak-to-peak transitions) The linewidth is typically between 2.5k B T to 3k B T The output spectrum depends not only to the semiconductor material but also on the structure of the pn junction including the dopant concentration levels The spectrum in (d) represents and idealized spectrum without including the effects of heavy doping on the energy bands For a heavily doped n-type semiconductor there are so many donors that the electron wavefunctions at these donors overlap to generate a narrow impurity band centered at E d but extending into the CB Thus donor impurity band overlaps the CB and hence effectively lowers E c

62 The minimum emitted photon energy from a heavily doped semiconductors is therefore less than E g and depends on the amount of doping spectrum change from asymmetric to more symmetric Typical LED characteristics are shown in the next figure More symmetric spectrum with width of 24nm ~ 2.7k B T As LED current increases, so does the injected minority carrier concentration, thus the rate of recombination and hence the output light intensity However the increase is not linear, at high current levels, strong injection of minority carriers leads to the recombination time depending on the injected carrier concentration and hence the current itself Turn-on or the cut-in voltage is about 1.5V current increases sharply with voltage depends on the semiconductor and generally increases with energy bandgap Blue V, yellow 2V and infrared 1V

63 Relative intensity (a) 655nm (b) Relative light intensity V (c) nm I (ma) I (ma) (a) A typical output spectrum (relative intensity vs wavelength) from a red GaAsP LED. (b) Typical output light power vs. forward current. (c) Typical I-V characteristics of a red LED. The turn-on voltage is around 1.5V S.O. Kasap, Optoelectronics (Prentice Hall)

64 LEDs for optical communications Type of light source suitable for optical communications depends not only on the communication distance but also on the bandwidth requirement Short haul applications, e.g. LAN, LEDs are preferred as they are simper to drive, more economic, have a longer lifetime and provide the necessary output power even though have a wider spectrum compared to laser diode For long haul and wide bandwidth communications laser diodes are invariably used because of their narrow linewidth, high output power and higher signal bandwidth capability Two types of LED devices Surface emitting LED (SLED) emitted radiation emerges from an area in the plane of recombination layer Edge emitting LED (ELED) emitted radiation emerges from an area on an edge of the crystal, i.e. perpendicular to the active layer

65 Light Double heterostructure Light (a) Surface emitting LED (b) Edge emitting LED 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

66 The simplest method of coupling radiation from a SLED is to etch a well in the planar LED structure and lower the fiber as close as possible to the active region Burrus type device Epoxy resin is used to bond the fiber and provide index matching between glass fiber and LED material Another method is to use a truncated spherical lens ( a microlens) with a high refractive index (n=1.9 2) to focus the light into the fiber index matching cement is used to hold them together ELED provide greater intensity light and also more collimated light Light guided to the edge by dielectric waveguide form by double heterostructure Lens system is used to couple the emitted radiation from ELED to fiber Graded index (GRIN) rod lens is used to focus light into fiber especially single mode fiber

67 Epoxy resin Fiber (multimode) Electrode Fiber Microlens (Ti 2 O 3 :SiO 2 glass) Etched well Double heterostructure SiO 2 (insulator) Electrode (a) Light is coupled from a surface emitting LED into a multimode fiber using an index matching epoxy. The fiber is bonded to the LED structure. (b) A microlens focuses diverging light from a surface emitting LED into a multimode optical fiber S.O. Kasap, Optoelectronics (Prentice Hall)

68 60-70 m Stripe electrode Insulation p + -InP (E g = 1.35 ev, Cladding layer) p + -InGaAsP (E g 1 ev, Confining layer) n-ingaas (E g 0.83 ev, Active layer) n + -InGaAsP (E g 1 ev, Confining layer) 2 n InP (E g = 1.35 ev, Cladding/Substrate) Current Electrode paths Substrate L m Light beam Cleaved reflecting surface Active region (emission region) Schematic illustration of the the structure of a double heterojunction stripe contact edge emitting LED 1999 S.O. Kasap, Optoelectronics (Prentice Hall)

69 ELED Lens Multimode fiber ELED GRIN-rod lens Single mode fiber Active layer (a) Light from an edge emitting LED is coupled into a fiber typically by using a lens or a GRIN rod lens S.O. Kasap, Optoelectronics (Prentice Hall) (b)

70 Output spectra from SLED and ELED using the same material is not necessarily the same First reason is active layers have different doping levels Second, there is self-absorption of some of the photons guided along the active layers as in ELED Typically ELED has narrower linewidth than SLED InGaAs ELED at 1300nm has linewidth of 75nm where as corresponding SLED has linewidth of 125nm

71 eature=related DzXM&feature=related ature=related feature=related