We report on the influence of surface reconstruction on silicon dopant incorporation and transport properties during molecular-beam epitaxy of GaAs(Bi) alloys. GaAs(Bi) growth with an (n × 3) reconstruction leads to n-type conductivity, while growth with a (2 × 1) reconstruction leads to p-type conductivity. We hypothesize that the presence or absence of surface arsenic dimers prevents or enables dopant incorporation into arsenic lattice sites. We consider the influence of bismuth anions on arsenic-dimer mediated dopant incorporation and the resulting electronic transport properties, demonstrating the applicability of this mechanism to mixed anion semiconductor alloys.
The current-induced spin polarization and momentum-dependent spin-orbit field were measured in In x Ga 1−x As epilayers with varying indium concentrations and silicon doping densities. Samples with higher indium concentrations and carrier concentrations and lower mobilities were found to have larger electrical spin generation efficiencies. Furthermore, current-induced spin polarization was detected in GaAs epilayers despite the absence of measurable spin-orbit fields, indicating that the extrinsic contributions to the spin polarization mechanism must be considered. Theoretical calculations based on a model that includes extrinsic contributions to the spin dephasing and the spin Hall effect, in addition to the intrinsic Rashba and Dresselhaus spin-orbit coupling, are found to qualitatively agree with the experimental results.
We have examined the alloy composition dependence of the energy bandgap and electronic states in GaAsNBi alloys. Using direct measurements of N and Bi mole fractions, via ion beam analysis, in conjunction with direct measurements of the out-of-plane misfit via x-ray rocking curves, we determine the “magic ratio” for lattice-matching of GaAsNBi alloys with GaAs substrates. In addition, using a combination of photoreflectance and photoluminescence spectroscopy, we map the composition- and misfit-dependence of the energy bandgaps, along with revealing the energetic position of Bi-related states at approximately 0.18 eV above the valence band maximum.
We have examined the influence of bismuth (Bi) and nitrogen (N) fluxes on N and Bi incorporation during molecular-beam epitaxy of GaAs1-x-yNxBiy alloys. The incorporation of Bi is found to be independent of N flux, while the total N incorporation and the fraction of N atoms occupying non-substitutional lattice sites increase with increasing Bi flux. A comparison of channeling nuclear reaction analysis along the [100], [110], and [111] directions with Monte Carlo-Molecular Dynamics simulations indicates that the non-substitutional N primarily incorporate as (N-As)As interstitial complexes. We discuss the influence of Bi adatoms on the formation of arsenic-terminated [110]-oriented step-edges and the resulting enhancement in total N incorporation via the formation of additional (N-As)As.
We have examined the influence of an incorporating surfactant on chemical ordering in GaAsN:Bi alloys. Epitaxy with a (2 × 1) reconstruction leads to the formation of GaAsN alloys, while the introduction of a Bi flux induces long-range chemical ordering of the {111} planes of GaAsN:Bi. We propose a mechanism in which Bi enhances the alignment of dimer rows along the [110] direction, facilitating N incorporation beneath surface dimers and Bi incorporation between dimer rows to form alternating N-rich and Bi-rich {111} planes. These findings suggest a route to tailoring the local atomic environment of N and Bi atoms in a wide range of emerging dilute nitride-bismide alloys.
We have examined the influence of electron irradiation and rapid thermal annealing on photoluminescence emission from GaAsNBi alloys. Electron irradiation of a 1-eV compressively strained GaNAsBi-on-GaAs epilayer, grown by molecular beam epitaxy and subsequently rapidly thermally annealed, is found to induce much stronger photoluminescence than what is observed for an identical as-grown sample upon annealing. At the same time, annealing of the irradiated sample caused a negligible spectral blueshift and reduced alloy potential energy fluctuations. These irradiation-related phenomena occurred without a change in the alloy macroscopic composition as revealed by x-ray diffraction and are mainly related to the nitrogen incorporated into non-substitutional sites in the quaternary alloy.
Photoreflectance measurements were performed for GaAs1−x−yNxBiy layers in the temperature range of 20–300 K. For each sample a transition related to the band-gap was observed, which red-shifts with increasing nitrogen and bismuth content. The temperature dependencies of the band-gap were fitted by the Varshni and Bose–Einstein formulas and simulated within the band anticrossing model of the interaction between the extended band states of the GaAs and the localized states associated with nitrogen and bismuth atoms. The reduction of the band-gap was found to be ∼80–100 meV.
We examine the formation and properties of InGaN quantum dots (QDs) on free-standing GaN and GaN/sapphire templates, with and without buried InGaN/GaN QD superlattices (SLs). We use scanning tunneling microscopy (STM) and scanning tunneling spectroscopy to image the QDs and measure their electronic states. As the number of layers preceding the QDs increases (i.e., increasing substrate complexity), the total QD density increases. For free-standing GaN, STM reveals a mono-modal QD-size-distribution, consistent with a limited density of substrate threading dislocations serving as heterogeneous nucleation sites. For GaN/sapphire templates, STM reveals a bimodal QD-size-distribution, presumably due to the nucleation of additional ultra-small InN-rich QDs near threading dislocations. For multi-period QD SLs on GaN/sapphire templates, an ultra-high density of QDs, with a mono-modal size distribution is apparent, suggesting that QD nucleation is enhanced by preferential nucleation at strain energy minima directly above buried QDs. We discuss the relative influences of strain fields associated with threading dislocations and buried QD SLs on the formation of InGaN QDs and their effective bandgaps.
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