Studies of Minority Carrier Recombination Mechanisms in Beryllium Doped GaAs for Optimal High Speed...

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.