12 Search Results

We investigate the influence of strain and dislocations on band alignment in GaSb/GaAs quantum dot systems. Composition profiles from cross-sectional scanning tunneling microscopy images are interpolated onto a finite element mesh in order to calculate the distribution of local elastic strain, which is converted to a spatially varying band alignment using deformation potential theory. Our calculations predict that dislocation-induced strain relaxation and charging lead to significant local variations in band alignment. Furthermore, misfit strain induces a transition from a nested (type I) to a staggered (type II) band alignment. Although dislocation-induced strain relaxation prevents the type I to type IImore » transition, electrostatic charging at dislocations induces the staggered band alignment once again.« less

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. Additionally, 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.

Applying a voltage to a semiconductor sample generates a current-induced electron spin polarization (CISP). Using an ultrafast mode-locked laser and lock-in detection scheme, we measure CISP on an indium gallium arsenide epilayer via Faraday rotation and extract the spin generation rate. While the measured spin polarization initially increases linearly with electric field as observed in previous work, larger applied voltages lead to a decreasing spin generation rate. Here we show that we can recover the linear dependence of spin generation rate with electric field even at larger applied voltages by modifying the applied voltage waveform to reduce heating and multiplyingmore » by an appropriate correction factor. Future CISP studies can utilize this technique to investigate CISP under larger applied electric fields.« less

We have examined the influence of spontaneous nano-patterning on the placement of InAs quantum dots (QDs) on (Al)GaAs surfaces using an experimental-computational approach. Both atomically flat and mounded surfaces, generated via a surface instability induced by the Ehrlich- Schwoebel barrier, are employed as templates for the subsequent deposition of InAs QDs. Using height profiles from atomic-force micrographs, we simulate QD deposition with a 2D phase field model, which describes the time evolution of the InAs layer driven by a chemical potential gradient. For flat surfaces, phase-field simulations result in QD densities comparable to experimental observations. For mounded surfaces, the simulationsmore » reveal QDs preferentially positioned in regions of positive curvature (substrate valleys), e.g., at the edge of surface mounds, consistent with the anisotropic QD placement observed experimentally. We discuss the role of curvaturedriven diffusion in the spontaneous ordering of QDs, demonstrating the applicability of this mechanism to AlGaAs mounds.« less

We have examined the influence of bismuth (Bi) and nitrogen (N) fluxes on N and Bi incorporation during molecular-beam epitaxy of GaAs_1-x-yN_xBi_y 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 andmore » the resulting enhancement in total N incorporation via the formation of additional (N-As)_As.« less

Significant composition-dependent incorporation of N into non-substitutional sites is often reported for dilute GaAsN alloys. To distinguish (N-N)_As, (N-As)_As, and (As_Ga-N_As) complexes, in this work we compare Rutherford backscattering spectrometry and nuclear reaction analysis (NRA) spectra with Monte Carlo-Molecular Dynamics simulations along the [100], [110], and [111] directions. For the Monte Carlo simulation, we assume that (N-N)_As is aligned along the [111] direction, while (N-As)_As is aligned along the [010] direction. The measured channeling NRA spectra exhibit the highest (lowest) yield in the [111] ([100]) directions. Similar trends are observed for simulations of (N-As)_As, suggesting that (N-As)_As is the dominantmore » interstitial complex in dilute GaAsN.« less

In this paper, we report the enhanced thermoelectric power factor in topologically insulating thin films of Bi_0.64Sb_1.36Te₃ with a thickness of 6–200 nm. Measurements of scanning tunneling spectroscopy and electronic transport show that the Fermi level lies close to the valence band edge, and that the topological surface state (TSS) is electron dominated. We find that the Seebeck coefficient of the 6 nm and 15 nm thick films is dominated by the valence band, while the TSS chiefly contributes to the electrical conductivity. In contrast, the electronic transport of the reference 200 nm thick film behaves similar to bulk thermoelectricmore » materials with low carrier concentration, implying the effect of the TSS on the electronic transport is merely prominent in the thin region. The conductivity of the 6 nm and 15 nm thick film is obviously higher than that in the 200 nm thick film owing to the highly mobile TSS conduction channel. As a consequence of the enhanced electrical conductivity and the suppressed bipolar effect in transport properties for the 6 nm thick film, an impressive power factor of about 2.0 mW m^–1 K^–2 is achieved at room temperature for this film. Further investigations of the electronic transport properties of TSS and interactions between TSS and the bulk band might result in a further improved thermoelectric power factor in topologically insulating Bi_0.64Sb_1.36Te₃ thin films.« less

Here, we have investigated the formation and coarsening of near-surface Ga nanoparticles (NPs) in SiN_x using Ga⁺ focused-ion-beam-irradiation of SiN_x, followed by rapid thermal annealing. For surfaces with minimal curvature, diffusive growth is apparent, leading to nearly close packed arrays with NP diameters as small as 3 nm and densities as high as ~4 × 10¹² cm^-2. The diffusive flux increases with annealing temperature, leading to NP coarsening by Ostwald ripening. For surfaces with increased curvature, diffusion towards the valleys also increases during annealing, leading to Ga NP coalescence and a bi-modal distribution of NP sizes.

We profile the local carrier concentration, n, across epitaxial InAs/GaAs quantum dots (QDs) consisting of 3D islands on top of a 2D alloy layer. Here, we use scanning thermoelectric microscopy to measure a profile of the temperature gradient-induced voltage, which is converted to a profile of the local Seebeck coefficient, S. The S profile is then converted to a conduction band-edge profile and compared with Poisson-Schrodinger band-edge simulations. Our combined computational-experimental approach suggests a reduced carrier concentration in the QD center in comparison to that of the 2D alloy layer. The relative roles of free carrier trapping and/or dopant expulsionmore » are discussed.« less

We have examined the influence of Bi on embedded nanocomposite formation and the resulting thermoelectric properties of GaAs. Bi implantation amorphizes the GaAs matrix, reducing both the free carrier concentration (n) and the electrical conductivity (σ). Following rapid thermal annealing, the matrix is transformed to single crystal GaAs with embedded Bi nanocrystals (NCs). In comparison to a GaAs reference, the Bi NC-containing films exhibit a sizeable reduction in thermal conductivity (κ), leading to a 30% increase in the thermoelectric figure-of-merit. We also present a universal trend for the influence of microstructure on the n-dependence of σ and κ.