G. W. Smith

Real-time laser-light scattering studies of surface topography development during GaAs MBE growth

Abstract In-situ laser light scattering is shown to yield valuable real-time information on the development of surface micro-topography during GaAs MBE growth. In-vacuo oxide desorption results in a large increase in scattered light intensity. Ex-situ atomic force microscopy has indicated the formation of a high density (>10 9 cm -2 ) of surface pits, attributed to localized GaAs consumption. Subsequent GaAs homoepitaxial growth (at 600°C) results in the development of an anisotropic ridged topography (ridge axes parallel to [110] with ridge dimensions increasing with epilayer thickness (peak-valley ≤ 12 nm for a 2.0 μm thick epilayer). Post growth in-vacuo annealing generates relatively flat surfaces, dominated by misorientation steps. For a 1 μm thick epilayer, annealing has a long time constant ( > 30 min at 600°C).

Model-based comparisons of phylogeographic scenarios resolve the intraspecific divergence of cactophilic Drosophila mojavensis.

The cactophilic fly Drosophila mojavensis exhibits considerable intraspecific genetic structure across allopatric geographic regions and shows associations with different host cactus species across its range. The divergence between these populations has been studied for more than 60 years, yet their exact historical relationships have not been resolved. We analysed sequence data from 15 intronic X‐linked loci across populations from Baja California, mainland Sonora‐Arizona and Mojave Desert regions under an isolation‐with‐migration model to assess multiple scenarios of divergence. We also compared the results with a pre‐existing sequence data set of eight autosomal loci. We derived a population tree with Baja California placed at its base and link their isolation to Pleistocene climatic oscillations. Our estimates suggest the Baja California population diverged from an ancestral Mojave Desert/mainland Sonora‐Arizona group around 230 000–270 000 years ago, while the split between the Mojave Desert and mainland Sonora‐Arizona populations occurred one glacial cycle later, 117 000–135 000 years ago. Although we found these three populations to be effectively allopatric, model ranking could not rule out the possibility of a low level of gene flow between two of them. Finally, the Mojave Desert population showed a small effective population size, consistent with a historical population bottleneck. We show that model‐based inference from multiple loci can provide accurate information on the historical relationships of closely related groups allowing us to set into historical context a classic system of incipient ecological speciation.

TRANSCRIPTOME-WIDE EXPRESSION VARIATION ASSOCIATED WITH ENVIRONMENTAL PLASTICITY AND MATING SUCCESS IN CACTOPHILIC DROSOPHILA MOJAVENSIS

Ecological speciation occurs with the adaptation of populations to different environments and concurrent evolution of reproductive isolation. Phenotypic plasticity might influence both ecological adaptation and reproductive traits. We examined environment‐specific gene expression and male mating success in cactophilic Drosophila mojavensis using transcriptome sequencing. This species exhibits cactus‐dependent mating success across different species of host plants, with genotype‐by‐environment interactions for numerous traits. We cultured flies from egg to eclosion on two natural cactus hosts and surveyed gene expression in adult males that were either successful or unsuccessful in achieving copulation in courtship trials. We identified gene expression differences that included functions involved with metabolism, most likely related to chemical differences between host cactus species. Several epigenetic‐related functions were identified that might play a role in modulating gene expression in adults due to host cactus effects on larvae, and mating success. Cactus‐dependent mating success involved expression differences of genes implicated in translation, transcription, and nervous system development. This suggests a role of neurological function genes in the mating success of D. mojavensis males. Together, these results suggest that the influence of environmental variation on mating success via regulation of gene expression might be an important aspect of ecological speciation.

Photoluminescence excitation spectroscopy on intermixed GaAs/AlGaAs quantum wires

Quantum wires with widths down to 45 nm have been realized by implantation induced intermixing of a surface near GaAs/AlGaAs quantum well. A very steep lateral potential has been achieved together with extreme low damage in the wire regions. As a result the optical transitions in photoluminescence excitation spectroscopy could be observed for all wire widths. With decreasing wire width an increasing Stokes shift has been determined due to the increasing importance of fluctuations in wire dimensions. A weak wire width dependence of transitions near the former two‐dimensional light‐hole level was observed, which is attributed to the predicted reduced energy shift of states near this level.

GaAs/AlGaAs quantum dots by implantation induced intermixing

By implantation induced intermixing we have realized GaAs/AlGaAs quantum dots with diameters down to 70 nm. In low‐temperature photoluminescence an increasing shift to higher energies, up to 7 meV, with decreasing dot diameter is observed. From a simple model we conclude that a very steep lateral potential has been achieved and that the shift is partly due to the radial confinement. All dot structures show a high luminescence intensity and in the time integrated measurements no indication for a strong reduction of the energy relaxation is observed. This could be attributed to the measured carrier capture from the lateral barrier into the dots and by the shape of the radial potential which determines the energy levels in the dots.

Selective area two-dimensional electron gas structures and in situ ohmic contacts patterned by focused ion beam doping during molecular beam epitaxial growth

Abstract For the first time, the technique of low energy focused ion beam (FIB) doping during molecular beam epitaxial (MBE) growth has been used to grow GaAs/AlGaAs based structures with three-dimensionally patterned doping profiles. Two applications are described. Selective area Sn ion doped two-dimensional electron gas (2DEG) structures were grown with a 1.2 K mobility of 200,000 cm 2 V -1 s -1 at a carrier concentration of 1×10 11 cm -2 . Further, selective area highly doped regions were patterned in situ to form low temperature, low resistance ohmic contacts to the 2DEG of conventional HEMT structures doped with a thermal Si source. The effects of Sn diffusion and ion beam damage were investigated in these structures.

Quantum dots and quantum wires with high optical quality by implantation-induced intermixing

The use of a near-surface quantum well as a starting material in conjunction with a low implantation energy is demonstrated to be a key parameter for the realization of homogeneous nanostructures for optical applications by implantation-induced intermixing. Using this technique we have realized GaAs/AlGaAs quantum wires with wire widths down to 45 nm and quantum dots with diameters ranging from 400 to 70 nm. The optical properties of the structures have been studied by low-temperature photoluminescence and photoluminescence excitation spectroscopy. In photoluminescence the structures show a high intensity, and from the observed increasing blue shift with decreasing size, steep confining potentials are concluded. The excitation spectra clearly demonstrate the advantage of the near-surface quantum wells by the reduction of level broadening compared to previous results on quantum wires.

Dopant patterning of MBE GaAs during growth by very low energy focussed tin ion beam deposition

Abstract For the first time a focussed ion beam has been used to successfully dope MBE GaAs during growth. GaAs with a mobility of 73 000 cm2 V−1 s−1 at a carrier concentration of 1 × 1014 cm−3 has been attained which is the highest mobility MBE GaAs achieved by the technique of ionised beam doping. The use of a tin focussed ion beam as an MBE dopant source has also allowed the three-dimensional patterning of the dopant concentration in GaAs, for the first time. The focussed ion beam apparatus did not interfere with or affect the conventional MBE growth of GaAs and AlGaAs in the chamber.

Intermixed GaAs/AlGaAs quantum wires and the influence of implantation species on the steepness of the lateral potential

We have studied the influence of the implantation species on the steepness of the lateral potential of GaAs/AlGaAs quantum wires fabricated by the implantation induced intermixing technique. Using Ga as the ion species wires with a very steep lateral potential have been fabricated. The steepness was limited by the lateral straggling of the ions during implantation. For the first time wires have been realized after In and Xe implantation and these results are compared to the steepness achieved by Ga implantation. Although ion straggling is reduced for heavier ions, Xe and In implantations result in more shallow lateral potentials than Ga implantation. This effect is attributed to a more effective intermixing in the low dose region for Xe and In compared to Ga as observed from homogeneously implanted samples. From the point of view of a patterning technique the intermixing dependence on dose can be compared to the resist contrast in lithography.

Effect of ion energy on Sn donor activation and defect production in molecular beam epitaxy GaAs doped with Sn ions during growth

The production of electrically active defect centers in molecular beam epitaxy (MBE) GaAs irradiated with low energy (50–500 eV) Sn ions during growth has been investigated as a function of ion energy. GaAs was doped n type during growth with a specially designed Sn focused ion beam column mounted on a MBE growth chamber. The 77 and 300 K Hall mobility and carrier concentration of the GaAs depended strongly on the ion energy, thus providing a sensitive measure of the concentration of ion‐induced acceptorlike defect centers. The material was found to be nonconducting for ion energies greater than 200 eV, while a systematic decrease in the acceptor concentration, and consequent increase in the mobility, was observed as the ion energy was decreased below this value. A peak mobility of 90 000 cm2 V−1 s−1 at a carrier concentration of 1×1014 cm−3 was achieved (at 60 K) which is in excess of that obtained in other reports of ion‐doped GaAs. A similar dependence on ion energy was found in the 4.2 K photoluminesc...