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.