Recombination in low-bandgap InGaAs

 Tim Gfroerer

Davidson College, Davidson, NC

with Mark WanlassNational Renewable Energy Lab, CO

~ Supported by Bechtel Bettis, Inc. and the American Chemical Society – Petroleum Research Fund ~

Experiments by . . .

Colleen Gillespie (Davidson ’06)

Patten Priestley (Davidson ’03)and Malu Fairley (Spelman ’03)

Pete Campbell(Davidson ’03)

5.6 5.7 5.8 5.9 6.0 6.10.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

I nAs

GaAs

SevereMismatch

InPSubstrate

Band

gap

(eV)

Lattice parameter (Angstroms)

Motivation: TPV converters

Increasing the Indium concentration in the InGaAs lowers the bandgap and increases the fraction of blackbody radiation that is absorbed in the cell.

0.0 0.5 1.0

1.5 2.0 2.5

0.0

0.2

0.4

0.6

0.8

1.0

Norm

aliz

ed

Inte

nsi

tyEnergy (eV)

T=1300 C

Variable-Bandgap Lattice-Mismatched Stuctures

Undoped InAsyP1-y, 30 nm

Undoped InxGa1-xAs, 1.5 μm

Undoped InAsyP1-y buffer, 1 μm

Undoped InAsyP1-y step-grade region:0.3 μm/step (~ -0.2% LMM/step), n

steps

Undoped InP substrate

Efficiency Measurements

: Laser Light

: Luminescence

CW YAG laser1 Watt @ 1064 nm

ND Filters

Variable Temp Cryostat

Sample

Photodiode

Lowpass Filter

heat

light inlight in light out

light in = heat + light out

radiative efficiency = light out / light in

0.0 0.1 0.2 0.3 0.4 0.5 0.6100

104

108

1012

1016

Den

sity

of S

tate

s (c

m-3eV

-1)

Energy (eV)

Defect-Related Density of States

Valence Band

Conduction Band

EN

ER

GY

The distribution of defect levels within the bandgap can be represented by a density of states (DOS) function as shown above.

0

20

40

60

80

100

EV

EC

Energy

Lo

g(D

OS

)

Eg = 0.80 eV

10231019 1021 1025

R

adia

tive

Effi

cien

cy (

%)

e-h Pair Gen. / Recombination (cm-3s-1)

Radiative Efficiency Measurements

heat

light

1018 1020 1022 1024

0

20

40

60

80

100

EC

EV

EC

EV

Eg = 0.68 eV

Energy

Log(

DO

S)

Energy

Log(

DO

S)

Rad

iativ

e E

ffici

ency

(%

)

e-h Pair Gen. / Recombination (cm-3s-1)

Radiative efficiency measurements at 77K. The theoretical fits correspond to the defect-related DOS functions indicated in the inset graphs.

Defect-Related Transition Probabilities

P ~ 10-

3

P ~ (0.5)10 ~ 10-3

P ~ 10-5

P ~ 10-1

P ~ (0.5)16 ~ 10-5

P ~ (0.5)4 ~ 10-1

The probability P of transitions involving phonon emission depends on the number of phonons required, which is determined by the position of the defect level in the gap.

-

+ + +

- -

Temperature Dependence

0.0

0.2

0.4

0.6

0.8

1.0

Eg ~ 0.77 eV

102310211019 1025

Rad

iativ

e E

ffici

ency

e-h Pair Gen. / Recombination (cm-3s-1)

77K 131K 185K 239K 296K

1018 1020 1022 1024 1026

0.0

0.2

0.4

0.6

0.8

1.0

Eg ~ 0.56 eV

Rad

iativ

e E

ffici

ency

e-h Pair Gen. / Recombination (cm-3s-1)

77K 131K 185K 239K 296K

Temperature dependence of radiative efficiency vs. excitation, showing how the SRH and Auger mechanisms depend on Indium concentration.

0.5 0.6 0.7 0.80.4

0.6

0.8

1

2

4

6

8

C (

10-2

8 cm

6 /s )

Bandgap Energy (eV)

185 K 239 K 296 K

Auger Recombination

40 60 80 1000.4

0.6

0.8

1

2

4

6

8

C

( 1

0-28 c

m6 /s

)

1 / kT (eV-1)

Eg ~ 0.77 eV

Eg ~ 0.71 eV

Eg ~ 0.65 eV

Eg ~ 0.56 eV

Temperature and bandgap dependence of the Auger coefficient C. The CHSH(band-to-band) mechanism dominates Auger recombination in low-bandgap InGaAs.

Sub-Bandgap Photoluminescence

0.4 0.5 0.6 0.7 0.8 0.9

10-4

10-3

10-2

10-1

100

T = 77K

x = 0.53 x = 0.60

N

orm

aliz

ed P

L In

tens

ity

Energy (eV)

0.3 0.4 0.5 0.6 0.7 0.8

10-4

10-3

10-2

10-1

100

T = 77K x = 0.72 x = 0.78

Nor

mal

ized

PL

Inte

nsity

Energy (eV)

FTIR spectra showing a deep transition in the lattice-matched material that abates and then disappears with increasing [In].

Four Conclusions• Deep defect levels → shallow near-bandedge states

with increasing [In].• The CHSH Auger mechanism is dominant in this alloy.• Sub-gap PL from deep (Ea > 0.2 eV) levels ↓ and then

disappears with increasing [In].• Structure-less sub-gap cathodoluminescence supports

assignment of this band to point defects.

Three References• T.H. Gfroerer, L.P. Priestley ('03), F.E. Weindruch

('01), and M.W. Wanlass, APL 80, 4570 (2002).• T.H. Gfroerer, L.P. Priestley ('03), Malu Fairley

(‘03), and M.W. Wanlass, JAP 94, 1738 (2003).• T.H. Gfroerer, C.E. Gillespie (‘05), J.P. Campbell

(‘03), and M.W. Wanlass, JAP 98, 093708 (2005).