Vertically oriented gallium nitride (GaN) nanocolumns (NCs) approximately 90±10nm wide and 0.75μm tall were grown by plasma-assisted molecular beam epitaxy on Al2O3(0001) and Si(111). The dense packing of the NCs gives them the appearance of a continuous film in surface view, but cross-sectional analysis shows them to be isolated nanostructures. Low-temperature photoluminescence measurements of NCs show excitonic emission with a dominant, narrow peak centered at 3.472eV and FWHM of 1.26meV. This peak is identified as the ground state of the A free exciton as confirmed by reflection measurements. Cross-sectional transmission electron microscopy identifies the NC microstructure as wurtzite GaN and that the NCs are largely free of defects. The GaN NCs are subsequently utilized as a defect-free vehicle for optical studies of Si-doped GaN; and the donor state was identified through low-temperature photoluminescence experiments.

Hydrogen incorporation depths of >25 μm were obtained in bulk, single-crystal ZnO during exposure to H2 plasmas for 0.5 h at 300 °C, producing an estimated diffusivity of ∼8×10−10 cm2/V⋅s at this temperature. The activation energy for diffusion was 0.17±0.12 eV, indicating an interstitial mechanism. Subsequent annealing at 500–600 °C was sufficient to evolve all of the hydrogen out of the ZnO, at least to the sensitivity of secondary ion mass spectrometry (<5×1015 cm−3). The thermal stability of hydrogen retention is slightly greater when the hydrogen is incorporated by direct implantation relative to plasma exposure, due to trapping at residual damage in the former case.

Hydrogen is readily incorporated into bulk, single-crystal ZnO during exposure to plasmas at moderate (100–300°C) temperatures. Incorporation depths of >25 μm were obtained in 0.5 h at 300°C, producing a diffusivity of ∼8×10−10 cm2/Vs at this temperature. The activation energy for diffusion is 0.17±0.12 eV, indicating an interstitial mechanism. Subsequent annealing at 500–600 °C is sufficient to evolve all of the hydrogen out of the ZnO, at least to the sensitivity of Secondary Ion Mass Spectrometry (<5×1015 cm−3). The thermal stability of hydrogen retention is slightly greater when the hydrogen is incorporated by direct implantation relative to plasma exposure, due to trapping at residual damage.

Lattice-mismatched epitaxy produces a high concentration of dislocations (Ndis) in the interface region, and this region is often highly conductive, due to donor (ND) decoration of the dislocations. Here we show that a simple postulate, ND=α(Ndis/c), where c is the lattice constant and α a constant of order 1–2, predicts a nearly constant low-temperature mobility, independent of Ndis. This prediction is experimentally verified in GaN grown on Al2O3, and is also applied to other mismatched systems.

The accumulation of charge in InGaAs quantum dots has been measured at room temperature by the photoelectrochemical capacitance-voltage (CV) technique for the first time. A carrier per quantum dot ratio greater than four has been observed. The use of atomic force microscopy and low temperature and room temperature photoluminescence (PL) confirm the existence of quantum dots. Also, a possible excited state is indicated by room temperature PL in a sample with small quantum dots.

We quantify the rates and total amounts of the arsenic for antimony exchange on both the Sb-terminated and Ga (or In)-terminated GaSb (001) surfaces using in situ real time line-of-sight mass spectrometry (LOS-MS) during molecular beam epitaxy. On the Sb-terminated GaSb (001) surface, an As for Sb exchange is observed to occur at all values of incident As2 flux considered. At high substrate temperature, three-dimensional (3D) nanometer-sized clusters from as a consequence of As/Sb exchange and lattice mismatch strain between GaAs and GaSb. The 3D clusters are found to have lateral dimensions of ∼10–30 nm and heights of 1–3 nm by atomic force microscopy (AFM). By contrast, at lower substrate temperatures a two-dimensional surface morphology is maintained, and AFM reveals an array of atomically flat terraces. On the surface terminated by one monolayer (ML) of Ga or In, there exists a critical As2 flux below which the As/Sb exchange is greatly diminished. The net amounts of Sb leaving the surface during one period of InAs/GaSb type-II superlattice growth are measured in real time by LOS-MS and estimated to be in the range of 0–0.4 ML for the various conditions used. By supplying only an As2 beam to a GaSb surface covered by InAs, the Sb riding over the InAs layer is replaced by arsenic and the total amount of such Sb is measured. The amount of Sb riding on the InAs can be as large as 0.8 ML for the first 1 ML of InAs and it gradually decreases to zero as the number of InAs monolayer increases. X-ray diffraction data show that all the InAs/GaSb superlattices coherently match with the GaSb substrate in the growth plane. The average lattice constant along the growth direction reduces with decreasing Sb mole fraction shown by the increased Sb desorption signal. Using the information on As/Sb exchange and Sb riding on the InAs surface, we predict an average lattice constant along the growth direction to be consistent with the measured one to within 2×10−4.

Photoelectrochemical etching of GaN, using a KOH solution and a 325 nm wavelength UV laser, has been used to obtain carrier concentration depth profiles. The photoelectrochemical capacitance-voltage measurements are supported with conventional depletion mode capacitance-voltage, secondary ion mass spectroscopy, and Hall measurements. The data show that steps in carrier concentration profiles can be accurately reproduced.

Saturation spectroscopy of electronic states in ${\mathrm{I}\mathrm{n}\mathrm{A}\mathrm{s}/\mathrm{A}\mathrm{l}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{Sb}$ single-quantum-well structures has been carried out with the UCSB free-electron laser. An effective Landau-level lifetime is extracted from the cyclotron-resonance (CR) saturation results on a semiconducting sample $(x=0.5)$ with the help of an n-level rate equation model. The effective lifetime shows strong oscillations (greater than an order of magnitude) with laser frequency with minima shifted to higher frequencies than given by the simple parabolic magnetophonon resonance condition due to large nonparabolicity in the InAs conduction band. Similar saturation studies of two lines (the X line and CR) in a ``semimetallic'' sample $(x=0.1)$ show markedly different effective lifetimes, demonstrating that the two lines are of different origin.

Far-infrared magneto-transmission studies on two high mobility InAs/AlxGa1−xSb (x=0.1 and 0.2) type-II single quantum wells in magnetic fields up to 30T show that a magnetic-field-induced semimetal–semiconductor (SM–SC) transition occurs in the x=0.2 sample between 8 and 12T, and in the x=0.1 sample in the vicinity of 28T. The so-called X-lines vanish at high magnetic fields and temperatures, consistent with their assignment to internal transitions of stable, spatially separated excitons. A large splitting (∼30cm−1) between cyclotron-resonance-like transitions for 13T⩽B⩽23T for the x=0.1 sample is attributed to enhanced spin splitting due to conduction–valence-band Landau-level mixing across the interface.

We have carried out saturation spectroscopy of cyclotron resonance in a semiconducting InAs/Al0.5Ga0.5Sb single quantum well using the UCSB free electron laser and have extracted an effective Landau level lifetime using an n-level rate equation model. The effective lifetime shows strong oscillations (>an order of magnitude) with frequency. Minima are shifted to higher frequencies than those given by the simple parabolic magnetophonon resonance condition due to large nonparabolicity in the InAs conduction band. We have also used this technique to investigate the origins of two lines: the X-line and cyclotron resonance in a “semimetallic” InAs/Al0.1Ga0.9Sb single quantum-well structure. Results show that the two lines are of different origin.

Magnetron sputter deposition was used to fabricate body centered cubic (bcc) and A-15 crystal structure W thin films. Previous work demonstrated that the as-deposited crystal structure of the films was dependent on the deposition parameters and that the formation of a metastable A-15 structure was favored over the thermodynamically stable bcc phase when the films contained a few atomic percent oxygen. However, the A-15 phase was shown to irreversibly transform into the bec phase between 500°C and 650°C and that a significant decrease in the resistivity of the metallic films was measured after the transformation. The current investigation of 150 nm thick, sputter deposited A-15 and bcc tungsten thin films on silicon wafers consisted of a series of experiments in which the stress, resistivity and crystal structure of the films was measured as a function of temperature cycles in a Flexus 2900 thin film stress measurement system. The as-deposited film stress was found to be a function of the sputtering pressure and presputter time; under conditions in which the as-deposited stress of the film was ∼ 1.5 GPa compressive delamination of the W film from the substrate was observed. Data from the thermal studies indicated that bcc film stress was not affected by annealing but transformation of the A-15 structure resulted in a large tensile increase in the stress of the film, regardless of the as-deposited stress of the film. In several instances, complete transformation of the A-15 structure into the bec phase resulted in ≥ tensile increase in film stress.

We present results of a detailed far-infrared magneto-optical study on a series of high-mobility InAs/AlxGa1-xSb (x=1.0, 0.8, 0.5, 0.4, 0.2, and 0.1) type-II single quantum wells. A wide range of phenomena arising from the unusual properties of two-dimensional (2D) electrons and holes and their Coulomb interaction in high magnetic fields has been revealed. Semiconducting samples (x greater than or equal to 0.4), in which only 2D electrons exist in the InAs wells, exhibit cyclotron-resonance (CR) splittings due to large conduction-band nonparabolicity. Semimetallic samples (x=0.1 and 0.2), in which both 2D electrons (in InAs) and 2D holes (in AlxGa1-xSb) are present, show two additional lines (e and h-X lines) as well as electron and hole CR. The X-lines increase in intensity at the expense of CR with increasing electron-hole (e-h) pair density, decreasing temperature, or increasing magnetic field (at low field), suggesting that they are associated with e-h binding which is increased by the magnetic field. The electron CR shows strongly oscillatory linewidth, amplitude, and mass, part of which are interpreted in the light of the unusual ''antinonparabolic'' band structure resulting from band overlap and coupling between conduction-band states in InAs and valence-band states in AlxGa1-xSb; part of these results are qualitatively consistent with the predictions of Altarelli and co-workers. The X lines are attributed to internal transitions of correlated electron e-h pairs (excitons) in high magnetic fields mediated by the excess electron density.

The effect of parasitic series resistances on electrochemical capacitance-voltage (EC-V) profiling is simulated numerically for Al x Gaj 1-x As/InyGa 1-y As pseudomorphic high electron mobility transistor (p-HEMT) structures. The actual EC-V measurement is simulated numerically by reconstructing the charge distribution from an intrinsic distribution calculated from a self-consistent k p model. The calculated charge distribution then forms the basis for examining the possible profiles an EC-V measurement would produce when parasitics are taken into account. From a simple lumped-circuit model, it is shown that parasitic resistances distort the shape of the charge distribution by changing the relative heights of the 5-layer and channel charges. Hysteresis effects may also occur and may be accompanied by a change in material type from n to p even though the material is known to be n-type over the entire range of measurement. Additionally, an undesirable dependence of the charge profile on the probe signal frequency is found.

δ-doped PHEMT structures commercially grown by OMVPE and MBE on 3 inch wafers were examined using contactless resistance, magneto-Hall, PR, PL, DXRD, RBS, SIMS, and electrochemical capacitance-voltage measurements, and 0.1 μm gate length devices were fabricated from them and then characterized. The electron mobilities in the OMVPE 2DEG were a little smaller than those in the MBE 2DEG, but were still excellent, and the carrier concentrations, which were > 2.5 × 1012cm−2 at 77 K, were similar. The interface quality as measured by PR spectra was a little better in the MBE sample. Variation in the In areal density as determined by RBS was a little larger in the OMVPE wafer, but it was still less than 1 at%. The sheet resistance and the doping concentration as determined by SIMS and EC-V measurements showed the variation in the OMVPE wafers was slightly larger, but it was still considered to be small. PHEMTs fabricated from the most promising OMVPE wafer had gm's (753 mS/mm) that approached those for the MBE wafer (796 mS/mm) as did their fT's (115 versus 117 Ghz). The variations in these parameters were also found to be slightly larger in the OMVPE wafers.

Hall scattering factors for electrons and holes in molecular beam epitaxial GaAs layers have been determined by comparing carrier concentrations measured by the Hall effect with those measured by the electrochemical capacitance–voltage technique. The conclusion is that both the electron and hole scattering factors are near unity for n ranging from 2×1016 to 7×1017 cm−3, and p ranging from 5×1016 to 4×1019 cm−3. This conclusion is consistent with the present theory for electrons, but not with that for holes.

We present results of a detailed far-infrared magneto-optical study on a series of high-mobility InAs/AlxGa1-xSb (0.1 less than or equal to x less than or equal to 1.0) type-II single quantum wells. Semiconducting samples (x greater than or equal to 0.4), in which only 2D electrons exist in the InAs wells, exhibited cyclotron resonance (CR) splittings due to large nonparabolicity. Semimetallic samples (x = 0.2 and 0.1), in which both 2D electrons and 2D holes are present, showed a variety of absorption lines, including "X-lines" that we have ascribed to intra-exciton transitions and electron-CR with strongly oscillatory linewidth, amplitude, and mass, part of which are interpreted in light of peculiar "anti-nonparabolic" band structure resulting from band overlap and coupling between conduction-band states in InAs and valence-band states in AlGaSb.