Sub-bandgap photoluminescence in InGaAs/InAsP heterostructures lattice-mismatched to InP substrates J. Peter Campbell and Tim Gfroerer

Davidson College, Davidson, NC

Mark Wanlass

National Renewable Energy Laboratory

Project supported by Research Corporation and the American Chemical Society – Petroleum Research Fund

Previous Work • Previous work provides indirect evidence that increasing lattice mismatch changes the distribution of defect levels.

Increasing Mismatch

Motivation • Can we find direct evidence for these defect levels?

• Sub-bandgap PL spectra may reveal the states.

Increasing Mismatch

Phonons (Heat) Photons (Light)

Fourier Transform IR Spectroscopy 1.00E-03 0.00E+00 -1.00E-03 -2.00E-03 -3.00E-03 -4.00E-03 -5.00E-03 -6.00E-03 -200.00

-150.00

-100.00

-50.00

0.00

50.00

100.00

Interferogram 150.00

200.00

1.00E-02 1.00E-03 Fourier Transform 1.00E-04 1.00E-05 1.00E-06 1.00E-07 0.40

0.50

0.60

0.70

0.80

0.90

1.00

1.10

Spectrum 1.20

1.30

1.40

Sample Structure • • Bandgap Series: Increased [In] in active layer increases lattice mismatch relative to substrate.

Buffer Series: Varying [As] in InAsP buffer layer optimizes active/buffer layer interface.

InGaAs Substrate (InP) DEFECT

Bandgap Series Spectra 1.E+16 1.E+14 1.E+12 1.E+10 1.E+08 1.E+06 1.E+04 1.E+02 1.E+00 1.E-02 1.E-04 1.E-06 0.30

T = 77K 0.40

0.50

0.60

Energy (eV)

0.70

0.80

0.90

[In] = 0.53

0% MM [In] = 0.60

0.46% MM [In] = 0.66

0.87% MM [In] = 0.72

1.28% MM [In] = 0.78

1.69% MM

Temperature Dependence

Arrhenius Plot for Bandgap-Series 0.53 eV peak

-15.00

-15.50

-16.00

-16.50

-17.00

-17.50

-18.00

-18.50

-19.00

50 PL Intensity = e

Ea / kT

70 Thermal Activation Energies [In] = 0.53, Ea = 24 +/- 6 meV [In] = 0.60, Ea = 24 +/- 3 meV 90 110

1 / kT (eV)

130 150 170

[In]=0.53

[In]=0.60

Conclusions and Future Work • Nonradiative transition from mid-gap states appear to be phonon-assisted (phonon energy ~ 30 meV) • Shallow states can result when the buffer/active interface is mismatched.

• Transient Capacitance Spectroscopy will be used to further characterize sub-bandgap energy levels. (Summer 2003)

Buffer Series Spectra 1.00E+07 T = 77K 1.00E+04 1.00E+01 1.00E-02 1.00E-05 1.00E-08 0.20

0.30

0.40

Energy (eV)

0.50

0.60

0.70

[As] = .43

[As] = .52

[As] = .59

[As] = .62

[As] = .70

Buffer Series Results

[As] in InAsP Lum Intensity Lum Intensity .35 eV Peak .45 eV Peak (arb. units) (arb. units) 0.43

0.52

0.59

0.62

0.7

2 3 7 8 6 0 6 26 686 0

• • • All samples have an unidentified peak near 0.35 eV.

Peak B is strongly [As] dependent.

Other studies show that [As] = 0.52, 0.59 have the highest radiative efficiency.

-15.00

-15.50

-16.00

-16.50

-17.00

-17.50

-18.00

-18.50

-19.00

50 Temperature Dependence

Arrhenius Plot for Bandgap-Series .53 eV peak y = 2.43E-02x - 1.91E+01 R 2 = 8.99E-01

70 90

y = 2.40E-02x - 1.92E+01 R 2 = 9.75E-01

Lum = e^

Ea/kT

LN (Lum) = Ea (1/kT) The slope of this plot is a measure of the thermal activation energy of the defect state causing this luminescence.

[In] = .53 Ea = 24 +/- 6 meV [In] = .60 Ea = 24 +/- 3 meV 110

1/kT (eV)

130 150 170

[In] = .53

[In] = .60

Bandgap Series Spectra 1.E+16 1.E+14 1.E+12 1.E+10 1.E+08 1.E+06 1.E+04 1.E+02 1.E+00 1.E-02 1.E-04 1.E-06 0.30

0.40

0.50

0.60

Energy (eV)

0.70

0.80

0.90

[In] = .53

0% MM [In] = .60

.46% MM [In] = .66

.87% MM [In] = .72

1.28% MM [In] = .78

1.69% MM