Daniel A. Beaton

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.

Shubnikov-de Haas measurement of electron effective mass in GaAs1−xBix

Magnetic field and temperature dependent resistivity measurements on n-type GaAs1-xBix epitaxially grown films show clear Shubnikov de Haas oscillations in the range 0u2009≤u2009xu2009≤u20090.0088. An overall decrease in the electron effective mass is observed for this range of compositions. Accounting for the known giant bandgap bowing and giant spin orbit bowing, the measured changes in the effective mass are in qualitative agreement with perturbation theory applied to these energy band changes, confirming that bismuth mainly perturbs the valence band. The stronger compositional dependence of the measured mass is attributed to effects from the bismuth isolated state.

Insight into the epitaxial growth of high optical quality GaAs1–xBix

The ternary alloy GaAs1–xBix is a potentially important material for infrared light emitting devices, but its use has been limited by poor optical quality. We report on the synthesis of GaAs1–xBix epi-layers that exhibit narrow, band edge photoluminescence similar to other ternary GaAs based alloys, e.g., InyGa1–yAs. The measured spectral linewidths are as low as 14u2009meV and 37u2009meV at low temperature (6u2009K) and room temperature, respectively, and are less than half of previously reported values. The improved optical quality is attributed to the use of incident UV irradiation of the epitaxial surface and the presence of a partial surface coverage of bismuth in a surfactant layer during epitaxy. Comparisons of samples grown under illuminated and dark conditions provide insight into possible surface processes that may be altered by the incident UV light. The improved optical quality now opens up possibilities for the practical use of GaAs1–xBix in optoelectronic devices.

Fermi energy tuning with light to control doping profiles during epitaxy

The influence of light stimulation and photogenerated carriers on the process of dopant surface segregation during growth is studied in molecular beam epitaxially grown Si-doped GaAs structures. The magnitude of surface segregation decreases under illumination by above-bandgap photons, wherein splitting of the quasi Fermi levels reduces the band bending at the growth surface and raises the formation energy of compensating defects that can enhance atomic diffusion. We further show that light-stimulated epitaxy can be used as a practical approach to diminish dopant carry-forward in device structures and improve the performance of inverted modulation-doped quantum wells.

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 2.34u2009eV.

Precise Determination of the Direct–Indirect Band Gap Energy Crossover Composition in AlxGa1-xAs

MBE grown AlxGa1-xAs samples, with x in the range 0.282–0.424, were studied by using time-integrated and time-resolved photoluminescence spectroscopy, photo-modulation reflectance spectroscopy and photoluminescence excitation spectroscopy. The direct and indirect band gap transitions are observed simultaneously in two AlxGa1-xAs samples with x=0.387 and 0.396. An exact determination of the direct–indirect crossover composition at low temperatures is found at a direct band gap energy of 2.059 eV, which corresponds to a aluminum content of [Al]=38.4±0.3%.

Bismuth incorporation into gallium phosphide

Gallium phosphide bismide (GaP1-xBix) epilayers with bismuth fractions from 0.9% to 3.2%, as calculated from lattice parameter measurements, were studied with Rutherford backscattering spectrometry (RBS) to directly measure bismuth incorporation. The total bismuth fractions found by RBS were higher than expected from the lattice parameter calculations. Furthermore, in one analyzed sample grown by molecular beam epitaxy at 300 °C, 55% of incorporated bismuth was found to occupy interstitial sites. We discuss implications of this high interstitial incorporation fraction and its possible relationship to x-ray diffraction and photoluminescence measurements of GaP0.99Bi0.01.

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.