J. Peter Campbell and Tim Gfroerer Davidson College, Davidson, NC

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Presentation transcript:

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 ENERGY Increasing Mismatch Previous work provides indirect evidence that increasing lattice mismatch changes the distribution of defect levels. Fundamental semiconductor theory Efficiency curves

Motivation Can we find direct evidence for these defect levels? ENERGY Increasing Mismatch Phonons (Heat) Photons (Light) Can we find direct evidence for these defect levels? Sub-bandgap PL spectra may reveal the states. Mechanisms of recombination (Radiative vs. Nonradiative and PL)

Fourier Transform IR Spectroscopy Interference pattern is OPL-dependent, since there is more than one wavelength present. Centerburst is the result of constructive interference of all wavelengths. Sine wave = single wavelength. Fourier transform separates a waveform into a sum of different frequency sinusoids. Each point of the spectrum is essentially the amplitude of the sinusoid at that frequency. Vs. scanning monochromator Advantages: All energy hits sample (increased S:N), Better resolution (proportional to opl) Fourier Transform Interferogram Spectrum

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. Substrate (InP) InGaAs DEFECT

Bandgap Series Spectra T = 77K Samples [In] = .53, .60 contain an identical peak at .35 eV. Phonon-mediated transition? The other three samples contain no distinguishable peaks. Sample [In] = .66 may display hypothesized band-edge tail states. Spectrum of sample [In] = .53 may corroborate previously published energy levels. PL results suggest that [In] in InGaAs affects distribution of defect states.

Temperature Dependence 25 meV is on the order of the characteristic phonon energies for this material. Thus, the peak seems to reflect some sort of phonon-assisted transition. As T increases, more phonons of that energy are available, thus fewer photons are emitted. Resonance.

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 T = 77K

Buffer Series Results 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.

Temperature Dependence 25 meV is on the order of the characteristic phonon energies for this material. Thus, the peak seems to reflect some sort of phonon-assisted transition. As T increases, more phonons of that energy are available, thus fewer photons are emitted. Resonance.

Bandgap Series Spectra Samples [In] = .53, .60 contain an identical peak at .35 eV. Phonon-mediated transition? The other three samples contain no distinguishable peaks. Sample [In] = .66 may display hypothesized band-edge tail states. Spectrum of sample [In] = .53 may corroborate previously published energy levels. PL results suggest that [In] in InGaAs affects distribution of defect states.