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Studies of Minority Carrier Recombination Mechanisms in Beryllium Doped GaAs for Optimal High Speed LED Performance An Phuoc Doan Department of Electrical Engineering Senior Project Presentation April 30, 2002 Introduction • We are interested in LEDs as high speed emitters for optical communications. • Desirable to have bright, fast, and inexpensive devices. • LED’s high speed performance relies on recombination mechanisms. Minority Carrier Lifetime and Recombination • Minority carrier lifetime is the average time an excess minority carrier will exist before recombining with majority carriers. • is dependent on doping concentration. – Higher doping concentration, lower . • Lower means faster LEDs. • Higher doping results in faster LEDs. Motivation • Previous studies show that as the doping concentration increases, the internal quantum efficiency also decreases as the minority carrier lifetime decreases. • Degradation of performance due to nonradiative recombination mechanisms (i.e. Auger, impurity trappings, surface recombination, etc.), selfabsorption effects. • Optical characterization techniques designed to probe possible mechanisms of intensity degradation. GaAs Sample • Sample doped p-type because electron injection is generally more efficient than hole injection. Ga0.6Al0.4As: 0.2 mm; p=5*1018 Grading: 500Å GaAs: 1 mm S1: p=2.0x1018 to p=6.0x1019 Grading: 500 Å Ga0.6Al0.4As: 0.2 mm; p=5*1018 GaAs: Substrate Ec Ev • Purpose of structure: To confine carriers to region of interest and reduce surface effects and maximizes pumping efficiency. Doping Profile with Electrochemical Capacitance Voltage (ECV) Profiler -3 NA (cm ) 1E+20 082500a 083100a 071200a - 1E+19 1E+18 0.000 0.200 0.400 0.600 Depth (mm) 0.800 1.000 1.200 Minority Carrier Lifetimes 1.00E-06 1.00E-07 (sec) 1.00E-08 1.00E-09 1.00E-10 Nelson and Sobers Ge-doped LPE 1.00E-11 Dumpke Theoretical Auger Limit Takashima theoretical B and C Ahrenkiel et. al. C-doped MBE 1.00E-12 Ito Be-doped MBE Yale/NREL Be-doped MBE 1.00E-13 1.00E+15 1.00E+16 1.00E+17 1.00E+18 NA- (cm-3) 1.00E+19 1.00E+20 1.00E+21 Photoluminescence 4000 3500 NA=3E18 NA=4E19 NA=6E19 Intensity (A.U.) 3000 2500 2000 1500 1000 500 0 750 800 850 900 Wavelength (nm) 950 1000 Experimental Details • Measure Photoluminescence Intensity as Function of Pump Intensity: To quantify the effect of minority carrier concentration on the recombination mechanisms. • Self-Absorption: Varying the pumping depth within the sample by changing the pump energy. Pump Intensity v. PL Intensity Pump Intensity v. PL Power Intensity Self – Absorption Analysis 4000 3500 Intensity (A.U.) 3000 2500 2000 660 nm pump 13.3 mW 405 nm pump 2.1 mW 1500 1000 500 0 750 800 850 900 Wavelength (nm) 950 1000 Conclusion • We do not know why our samples have longer lifetimes, yet not very bright. • The two experiments presented here eliminated two possible failure mechanisms. • Much work needs to be done in order to have fast and bright light emitters. Double Heterostructure LEDs p – type active region • DH-LED as application of radiative recombination in direct bandgap semiconductors. p – type barrier n – type emitter – Recombination of electron from conduction band and hole from valence band result in photons. Auger Recombination Time Resolved Photoluminescence (TRPL) 14000 082500a 083100a 71300 Intesity (A.U.) 12000 10000 8000 6000 4000 2000 0 0.E+00 2.E-09 4.E-09 6.E-09 8.E-09 1.E-08 1.E-08 1.E-08 Time (sec) TRPL measures photoluminescence decay by photon counting over many excitation cycles.