P. Specht

Femtosecond response times and high optical nonlinearity in beryllium-doped low-temperature grown GaAs

We have investigated the effect of beryllium doping on the optical nonlinearity and on the carrier dynamics in low-temperature (LT) grown GaAs for various growth temperatures and doping levels. Pump–probe experiments with 20 fs pulses and quantitative measurements of the nonlinear absorption show that in undoped LT GaAs, ultrafast response times are only obtained at the expense of low absorption modulation. In contrast, in Be-doped LT GaAs, high absorption modulation is maintained for response times as short as 100 fs. These results are qualitatively explained accounting for the point-defect-related optical transitions in LT-GaAs.

Optical nonlinearity in low-temperature-grown GaAs: Microscopic limitations and optimization strategies

We have quantitatively measured the linear and the nonsaturable absorption as well as the absorption modulation and its recovery time in as-grown and annealed low-temperature (LT) GaAs. Correlation of the optical data with As antisite (AsGa) defect densities yields the absorption cross section and the saturation parameter of the dominant AsGa to the conduction-band defect transition. We show that this defect transition is mainly responsible for the large nonsaturable absorption in as-grown LT GaAs with fast recovery times. Reducing the AsGa density by annealing yields an optimized material with small nonsaturable absorption, high absorption modulation, and fast recovery times.

Phase separation in InxGa1-xN

Quantitative high-resolution transmission electron microscopy was used to study the distribution of indium atoms in In x Ga1− x N alloys by strain mapping. In GaN/In x Ga1− x N/GaN quantum wells with x < 0.1 we find that the sample thickness and the precision to which displacement fields can be extracted from a lattice image determine whether or not it is possible to discriminate between random alloy fluctuations and cluster formation. In miscible alloys such as SiGe or AlGaN a precision of better than 1 pm is required to reveal random alloy fluctuations, which presently exceeds experimental capabilities. In In x Ga1− x N with x > 0.1, a precision of about 3 pm suffices to distinguish random alloy fluctuations from indium clusters that are present. Thick In x Ga1− x N layers with x = 0.6 and x = 0.7 show phase separation with a wavelength between 2 and 4 nm and a fluctuation amplitude of Δx = 0.10 and 0.15, respectively. This produces striped composition fluctuations, which are modulated by dot-like structures. The similarity of the fluctuation magnitudes in quantum wells and thick layers suggests that spinodal decomposition occurs in both materials and our results place the centre of the miscibility gap around x = 0.5–0.6.

Observing gas-catalyst dynamics at atomic resolution and single-atom sensitivity

Transmission electron microscopy (TEM) has become an indispensable technique for studying heterogeneous catalysts. In particular, advancements of aberration-corrected electron optics and data acquisition schemes have made TEM capable of delivering images of catalysts with sub-Ångström resolution and single-atom sensitivity. Parallel developments of differentially pumped electron microscopes and of gas cells enable in situ observations of catalysts during the exposure to reactive gas environments at pressures of up to atmospheric levels and temperatures of up to several hundred centigrade. Here, we outline how to take advantage of the emerging state-of-the-art instrumentation and methodologies to study surface structures and dynamics to improve the understanding of structure-sensitive catalytic functionality. The concept of using low electron dose-rates in TEM in conjunction with in-line holography and aberration-correction at low voltage (80 kV) is introduced to allow maintaining atomic resolution and sensitivity during in situ observations of catalysts. Benefits are illustrated by exit wave reconstructions of TEM images of a nanocrystalline Co3O4 catalyst material acquired in situ during their exposure to either a reducing or oxidizing gas environment.

Improvement of molecular beam epitaxy-grown low-temperature GaAs through p doping with Be and C

Nonstoichiometric GaAs thin layers can be produced in molecular beam epitaxy if they are grown at temperatures below 400 °C [low-temperature (LT)-GaAs]. Due to the incorporation of excess As in the form of native point defects, namely As antisite defects (AsGa), these layers exhibit ultrashort time response and, after annealing at 600 °C, excellent semi-insulating behavior. The ultrashort time response, however, is governed by the concentration of ionized antisites ([AsGa+]), which are just a few percent of the total concentration of antisites ([AsGa]). Additionally, thermal annealing leads to As precipitate formation and out-diffusion of point defects into adjacent layers. Recent studies have shown that p-type doping with Be increases the thermal stability of point defects and shortens the time response due to an increase in ionized antisites, while maintaining the high electrical resistivity in as-grown material. We report on the studies of p doping of LT-GaAs with Be and, alternatively, with C in order...

Instrumental requirements for the detection of electron beam-induced object excitations at the single atom level in high-resolution transmission electron microscopy.

This contribution touches on essential requirements for instrument stability and resolution that allows operating advanced electron microscopes at the edge to technological capabilities. They enable the detection of single atoms and their dynamic behavior on a length scale of picometers in real time. It is understood that the observed atom dynamic is intimately linked to the relaxation and thermalization of electron beam-induced sample excitation. Resulting contrast fluctuations are beam current dependent and largely contribute to a contrast mismatch between experiments and theory if not considered. If explored, they open the possibility to study functional behavior of nanocrystals and single molecules at the atomic level in real time.

Atomic resolution phase contrast imaging and in-line holography using variable voltage and dose rate.

The TEAM 0.5 electron microscope is employed to demonstrate atomic resolution phase contrast imaging and focal series reconstruction with acceleration voltages between 20 and 300 kV and a variable dose rate. A monochromator with an energy spread of ≤0.1 eV is used for dose variation by a factor of 1,000 and to provide a beam-limiting aperture. The sub-Ångstrøm performance of the instrument remains uncompromised. Using samples obtained from silicon wafers by chemical etching, the [200] atom dumbbell distance of 1.36 Å can be resolved in single images and reconstructed exit wave functions at 300, 80, and 50 kV. At 20 kV, atomic resolution <2 Å is readily available but limited by residual lens aberrations at large scattering angles. Exit wave functions reconstructed from images recorded under low dose rate conditions show sharper atom peaks as compared to high dose rate. The observed dose rate dependence of the signal is explained by a reduction of beam-induced atom displacements. If a combined sample and instrument instability is considered, the experimental image contrast can be matched quantitatively to simulations. The described development allows for atomic resolution transmission electron microscopy of interfaces between soft and hard materials over a wide range of voltages and electron doses.

Analysis of twin defects in GaAs(111)B molecular beam epitaxy growth

The formation of twin is common during GaAs(111) and GaN(0001) molecular beam epitaxy (MBE) metalorganic chemical vapor deposition growth. A stacking fault in the zinc-blende (ZB)(111) direction can be described as an insertion of one monolayer of wurtzite structure, sandwiched between two ZB structures that have been rotated 60° along the growth direction. GaAs(111)A/B MBE growth within typical growth temperature regimes is complicated by the formation of pyramidal structures and 60° rotated twins, which are caused by faceting and stacking fault formation. Although previous studies have revealed much about the structure of these twins, a well-established simple nondestructive characterization method which allows the measurement of total aerial density of the twins does not exist at present. In this article, the twin density of AlGaAs layers grown on 1° miscut GaAs(111)B substrates has been measured using high resolution x-ray diffraction, and characterized with a combination of Nomarski microscopy, atomi...

High resolution transmission electron microscopy of InN

Hexagonal InN layers were grown by molecular beam epitaxy and studied by high resolution electron microscopy and by photoluminescence spectroscopy. Inclusions of a few nanometers in diameter were found, which are among the smallest reported. Image simulation, beam sensitivity, and photoluminescence of the samples corroborate that these inclusions are indeed metallic indium. This letter provides evidence that nanoscopic metallic indium inclusions can be present in InN crystals and have a strong influence on its optical properties.

Proton Radiation-Induced Void Formation in Ni/Au-Gated AlGaN/GaN HEMTs

AlGaN/GaN high-electron mobility transistors (HEMTs) were exposed to 2-MeV protons irradiation, at room temperature, up to a fluence of 6 × 1014 H+/cm2. Aside from degradation resulting from radiation-induced charge trapping, transmission electron microscopy and electrical measurements reveal a radiation-induced defect located at the edges of the Ni/Au Schottky gate in the proton-irradiated devices. At the edges of the Ni/Au gate, the Ni of the Ni/Au gate diffused up into the Au layer and migrated into the AlGaN barrier, leaving voids in the Ni layer at the gate edges after irradiation. These radiation-induced voids are caused by diffusion of Ni through vacancy exchange, known as the Kirkendall effect, resulting in reduced gate area and degrading the HEMT performance.