Theresa M. Christian

Growth, microstructure, and luminescent properties of direct-bandgap InAlP on relaxed InGaAs on GaAs substrates

Direct-bandgap InAlP alloy has the potential to be an active material in nitride-free yellow-green and amber optoelectronics with applications in solid-state lighting, display devices, and multi-junction solar cells. We report on the growth of high-quality direct-bandgap InAlP on relaxed InGaAs graded buffers with low threading dislocation densities. Structural characterization reveals phase-separated microstructures in these films which have an impact on the luminescence spectrum. While similar to InGaP in many ways, the greater tendency for phase separation in InAlP leads to the simultaneous occurrence of compositional inhomogeneity and CuPt-B ordering. Mechanisms connecting these two structural parameters are presented as well as results on the effect of silicon and zinc dopants on homogenizing the microstructure. Spontaneous formation of tilted planes of phase-separated material, with alternating degrees of ordering, is observed when InAlP is grown on vicinal substrates. The photoluminescence peak-wid...

Amber-green light-emitting diodes using order-disorder AlxIn1−xP heterostructures

We demonstrate amber-green emission from Al x In1– x P light-emitting diodes (LEDs) with luminescence peaked at 566 nm and 600 nm. The LEDs are metamorphically grown on GaAs substrates via a graded In y Ga1– y As buffer layer and feature electron confinement based on the control of Al x In1– x P CuPt atomic ordering. A control sample fabricated without order-disorder carrier confinement is used to illustrate device improvement up to a factor of 3 in light output due to confinement at drive currents of 40 A/cm2. The light output at room temperature from our Al x In1– x P LED structure emitting at 600 nm is 39% as bright as a Ga x In1– x P LED emitting at 650 nm.

Mysterious absence of pair luminescence in gallium phosphide bismide

Gallium phosphide bismide (GaP1−xBix) epilayers with x up to 1.0% were grown via molecular beam epitaxy and their photoluminescence spectra were investigated at low temperatures. Surprisingly, the emission spectrum of the GaP1−xBix epilayers was fully described by isolated bismuth-bound exciton recombination at the A and B lines (2.232 and 2.229 eV, respectively) together with their phonon replicas, without a need for any description of recombination from bismuth pair or cluster states. These observations contrast with the typical behavior of energy transfer to lower-lying nitrogen pair states in GaP1−yNy at similar impurity concentrations and offer insights into the electronic structure evolution of GaP1−xBix.

Determination of the direct to indirect bandgap transition composition in AlxIn1−xP

AlxIn1−xP semiconductor alloys grown by metalorganic chemical vapor deposition on InGaAs graded buffer layers with varied aluminum compositions that span the transition from a direct to indirect semiconductor alloy are explored. The direct and indirect band gap transitions are observed in a single AlxIn1−xP sample with 40.8% allowing for a precise determination of the direct-indirect cross-over composition, xc. The direct and indirect nature of observed luminescence peaks is verified using time-resolved photoluminescence. At low temperatures, xc is determined to be 40.5% at a corresponding direct band gap energy of 2.34 eV.

Spectrally resolved localized states in GaAs1−xBix

The role of localized states and their influence on the broader band structure remains a crucial question in understanding the band structure evolution in GaAs1− x Bi x . In this work, we present clear spectroscopic observations of recombination at several localized states in GaAs1− x Bi x . Sharp and recognizable photoluminescence features appear in multiple samples and redshift as a function of GaBi fraction between x = 0.16% and 0.4% at a linearized rate of 34 meV per % Bi, weaker than the redshift associated with band-to-band recombination. Interpreting these results in terms of radiative recombination between localized holes and free electrons sheds light on the relative movement of the conduction band minimum and the characteristics of localized bismuth-related trap states in GaAs1− x Bi x alloys.

Bismuth-induced Raman modes in GaP1–xBix

Dilute bismide semiconductor alloys are a promising material platform for optoelectronic devices due to drastic impacts of bismuth on the electronic structure of the alloy. At the same time, the details of bismuth incorporation in the lattice are not fully understood. In this work, we conduct Raman scattering spectroscopy on GaP1− x Bi x epilayers grown by molecular beam epitaxy (MBE) and identify several bismuth-related Raman features including gap vibration modes at 296, 303, and 314 cm−1. This study paves the way for more detailed analysis of the local symmetry at bismuth incorporation sites in the dilute bismide alloy regime.