L7 10112016 ECE 5212 Fall 2016 UConn F. Jain nd LED structures and material selection Cont. L8 10192016 Blue LED with green and red phosphor layer to produce white light using flip chip p-GaN fully metalized design. 2009.7 (August 2009) Philips Lumileds Rebel warm white light lamp [4].

Truncated inverted pyramid LED P-GaP Cap layer (Window) Eg3 =2.24 P-AlGaInP (Eg2) P-AlGaInP (Eg1 different composition) n AlGaInP (Eg2) Eg3 > Eg2 > Eg1 Lift-off layer/buffer (Sacrificial) n-GaAs Substrate 1.42ev Fig 6. Epi layer transfer from GaAs to GaP substrate Double Heterostructure (DH) LED Epoxy Dome Ohmic contact to Substrate n-GaP Substrate DH LED AlGaInP W=1.5 2Ln=7.1*2=14 p-GaP Cap Contact Ohmic Fig 7. Truncated inverted pyramid (TIP) LED with 50% Extraction efficiency hextraction Truncated inverted pyramid LED 2.1ev Double Heterojunction

Figure 8(b) shows a chronological map presenting LED evolution and comparing them to other technologies. Note that Organic LEDs (OLEDs) is an emerging competitor.

Extraction efficiency External efficiency Internal efficiency Extraction efficiency External efficiency (1) 50% Loss due to travel towards the back contact Figure 14 shows that about 1/2 of the generated light is lost as it travels towards the back electrode. 2. Surface reflection 3. Critical angle related loss Three Losses

Overall conversion efficiency Overall efficiency is defined as the Light output (integrated in all directions) in watts/Electrical power input (VI) supplied to the device. Sometimes the power conversion efficiency is defined as Here, P(λ) = light output at wavelength

Spectral width of emitted radiation Density of states in bulk is N(E)dE = The electron concentration ‘n’ in the entire conduction band is given by (EC is the band edge) This equation assumes that the bottom of the conduction band is =0. Electron and hole concentration as a function of energy

Graphical method to find carrier concentration in quantum well Spectral width of emitted radiation Graphical method to find carrier concentration in quantum well

4.8. Methods to Reduce Losses in LEDs Various mechanisms are identified which lead to losses of generated photons in a p-n diode. Design methods are outlined to reduce these losses. We have seen that if all the loss mechanisms are considered, the light extraction efficiency comes out to be very low (1-2%). The following steps are taken to reduce photon losses ( or improve photon extraction): To recover part of the photons traveling towards the bottom contact. There are various ways to achieve this. One may be have a reflecting contact. This invariably results in poor electrical characteristics; due to increased Ohmic voltage drop (contact resistance * current). Another way is to incorporate a layer, shown dotted in Figure 16, having a lower index of refraction than the layer in which the photons are generated.

LED Design Design parameters: Operating wavelength, single (green, red, blue) or multi-color white light LEDs Optical power output, expected external and wall conversion efficiency, Cost.: Preferred packaging, (epoxy dome), Operating wavelength: Material selection

LED Design Index of refraction for ternary semiconductors 4.7. Refractive Index as a funciton of semiconductor composition This section provides information on the index of refraction for semiconductors we use for visible or infrared sources. The compositional dependence of refractive index in the case of AlxGa1-xAs is given by[1] (22) and for InxGa1-xAsyP1-y[2] [1]H.C. Casey Jr. & M.B. Panish, Heterostructure Lasers Part A: Fundamental Principles, Academic Press, 1978, Chapter 2 [2]G.H.Olsen et al, Journal of Electronic Materials, vol 9, pp. 977-987, 1980.

Material Selection Point A: Project GaAs-A-InAs curve on the horizontal O-C-2 line. Find the fraction OC/O2 = 0.528 C2/O2 = 0.472. Therefore, at point A the composition is Ga0.472In0.528As.

Material Selection

Material Selection

LED Design Index of refraction for ternary semiconductors 4.7. Refractive Index as a funciton of semiconductor composition This section provides information on the index of refraction for semiconductors we use for visible or infrared sources. The compositional dependence of refractive index in the case of AlxGa1-xAs is given by[1] (22) and for InxGa1-xAsyP1-y[2] [1]H.C. Casey Jr. & M.B. Panish, Heterostructure Lasers Part A: Fundamental Principles, Academic Press, 1978, Chapter 2 [2]G.H.Olsen et al, Journal of Electronic Materials, vol 9, pp. 977-987, 1980.

TIP

Fig. 7 Blue LED with green and red phosphor layer to produce white light using flip chip p-GaN fully metalized design. 2009.7 (August 2009) Philips Lumileds Rebel warm white light lamp [4].

Operating voltage V and current I to design a LED Performance parameters:[4] Operating conditions— Current I = 700 mA, area 1mm2, V=3.2V DC. Cumulative white light efficiency hL = 14%. It is composed of three terms. 1. Blue LED efficiency-- hB = 0.73W/2.24W = 33%. 2. Phosphor package efficiency hP = 0.4 W/0.73W = 54%. 3. Spectral efficacy hS = 128 lm/164 lm =78%. (it measures response of eye). Power Budget: DC power 2.2W (VI=0.7A *3.2V) is supplied by the power supply. 1. It produces 0.73W of light having following components 0.511 W blue photons input used by red phosphor which produces 0.263 W and out put is at 29% after various losses. 0.177 W blue power input is used by green phosphor which produces 0.104 W and after accounting for various losses, we are left with 18%, 0.046 W remaining blue input produces 0.036W). The losses in phosphor are due to Stokes loss (input at high energy and out put at low), scattering and absorption losses. 2. 0.39W is lost due to voltage droop due to reduction of internal quantum efficiency as compared with obtained under low injection condition (1- hIQE/ hIQE, lo inj), 30%. 3. Internal non-radiative transitions 0.56W (30%); (1- hIQE, lo inj). 4. Joule loss 0.3W (15%) 5. Un-extracted light (1- hEQE), here hEQE is external quantum efficiency. Operating voltage V and current I to design a LED DC power P (V*I) = Optical power emitted/ Po supplied by battery= DC power P (V*I) + I2R losses

Excitonic Transitions in Quantum Wires Excitonic Transitions: This gets modified when the exciton binding energy in a system is rather large as compared to phonon energies (~kT). In the case of excitonic transitions, the gain coefficient is: