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.), self-absorption effects.
• Optical characterization techniques designed to probe possible mechanisms of intensity degradation.
GaAs Sample
• Purpose of structure: To confine carriers to region of interest and reduce surface effects and maximizes pumping efficiency.
• Sample doped p-type because electron injection is generally more efficient than hole injection.
Ga0.6Al0.4As: 0.2m; p=5*1018
GaAs: 1 mS1: p=2.0x1018 to p=6.0x1019
Ga0.6Al0.4As: 0.2 m; p=5*1018
Grading: 500Å
Grading: 500 Å
GaAs: Substrate
Ga0.6Al0.4As: 0.2m; p=5*1018
GaAs: 1 mS1: p=2.0x1018 to p=6.0x1019
Ga0.6Al0.4As: 0.2 m; p=5*1018
Grading: 500Å
Grading: 500 Å
GaAs: Substrate
Ec
Ev
Doping Profile with Electrochemical Capacitance Voltage (ECV) Profiler
1E+18
1E+19
1E+20
0.000 0.200 0.400 0.600 0.800 1.000 1.200
Depth (m)
NA
- (cm
-3)
082500a
083100a
071200a
Minority Carrier Lifetimes
1.00E-13
1.00E-12
1.00E-11
1.00E-10
1.00E-09
1.00E-08
1.00E-07
1.00E-06
1.00E+15 1.00E+16 1.00E+17 1.00E+18 1.00E+19 1.00E+20 1.00E+21
NA- (cm-3)
(s
ec)
Nelson and Sobers Ge-doped LPE
Dumpke Theoretical Auger Limit
Takashima theoretical B and C
Ahrenkiel et. al. C-doped MBE
Ito Be-doped MBE
Yale/NREL Be-doped MBE
Photoluminescence
0
500
1000
1500
2000
2500
3000
3500
4000
750 800 850 900 950 1000
Wavelength (nm)
Inte
nsi
ty (
A.U
.)
NA=3E18NA=4E19NA=6E19
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
0
500
1000
1500
2000
2500
3000
3500
4000
750 800 850 900 950 1000
Wavelength (nm)
Inte
nsit
y (A
.U.)
660 nm pump 13.3 mW 405 nm pump 2.1 mW
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
• DH-LED as application of radiative recombination in direct bandgap semiconductors. – Recombination of electron
from conduction band and hole from valence band result in photons.
n – type emitter
p – type barrier
p – type active region
Auger Recombination
Time Resolved Photoluminescence (TRPL)
0
2000
4000
6000
8000
10000
12000
14000
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)
Inte
sity
(A
.U.)
082500a083100a71300
TRPL measures photoluminescence decay by photon counting over many excitation cycles.
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