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Single-phase monoclinic aluminum–gallium oxide powders, β−(AlxGa1−x)2O3, have been produced by solution combustion synthesis for Al fraction 0 ≤ x

We have investigated the growth of GaSe, a layered semiconductor, on single crystal Al2O3(0001)(sapphire), an ionic crystal. We have used reflection high energy electron diffraction, x-ray photoelectron spectroscopy, and transmission electron microscopy and diffraction to study the growth, interface reaction, and resultant film structure. When a clean, heated (T=620 °C) sapphire substrate is exposed to Ga and Se from a compound GaSe source (Ga2Se and Se2) and a separate uncracked Se source (Sex, x∼6), a polycrystalline film is formed with stoichiometry close to that of cubic Ga2Se3. After annealing to 1100 °C, the film evaporates, leaving a reacted interface layer containing both Ga and Se. Subsequent deposition on this reacted surface under the same conditions leads to growth of 500–1000 A grains of layered GaSe, which have their c axis normal to the substrate surface but random orientation parallel to the substrate. A mechanism is proposed that describes the formation of the interface layer and its effect on the subsequent growth of GaSe.

The growth and phase segregation properties of the potential dilute magnetic semiconductor alloy (MnSe)${}_{x}$(Ga${}_{2/3}$Se)${}_{1\ensuremath{-}x}$ are studied as a function of thickness, Mn concentration, postgrowth annealing, and the presence or absence of undoped Ga${}_{2}$Se${}_{3}$ buffer and capping layers. This system is an unusual case in heteroepitaxy where two-phase $\mathrm{MnSe}+{\mathrm{Ga}}_{2}{\mathrm{Se}}_{3}$ has better lattice matching than the (MnSe)${}_{x}$(Ga${}_{2/3}$Se)${}_{1\ensuremath{-}x}$ alloy. Despite this peculiarity, this system shows a modified form of Stranski-Krastonow growth: laminar films are observed up to a certain $x$-dependent critical thickness, above which islands are observed by scanning tunneling microscopy. The island morphology depends on the presence or absence of an undoped Ga${}_{2}$Se${}_{3}$ buffer layer and postgrowth annealing. A kinetically stabilized platelet morphology is observed at the crossover point between laminar and islanded films. Based on Mn and Se $K$-edge extended x-ray absorption fine structure and x-ray absorption near-edge structure spectroscopy, there are two types of Mn in islanded films: Mn that remains doped in the Ga${}_{2}$Se${}_{3}$ but oxidizes upon exposure to air, and Mn that participates in the islands, which are precipitates of the MnSe phase. Consistent with MnO or MnSe, $L$-edge x-ray absorption on air-exposed films suggests the Mn is in the formal $+$2 oxidation state. No $L$-edge x-ray magnetic circular dichroism signal is observed at 20 K, which may be due to surface effects or to a lack of magnetic order.

Chromium-doped gallium sesquiselenide, Cr:Ga${}_{2}$Se${}_{3}$, is a member of a new class of dilute magnetic semiconductors exploiting intrinsic vacancies in the host material. The correlation among room-temperature ferromagnetism, surface morphology, electronic structure, chromium concentration, and local chemical and structural environments in Cr:Ga${}_{2}$Se${}_{3}$ films grown epitaxially on silicon is investigated with magnetometry, scanning tunneling microscopy, photoemission spectroscopy, and x-ray absorption spectroscopy. Inclusion of a few percent chromium in Ga${}_{2}$Se${}_{3}$ results in laminar, semiconducting films that are ferromagnetic at room temperature with a magnetic moment $\ensuremath{\geqslant}4{\ensuremath{\mu}}_{B}/\text{Cr}$. The intrinsic-vacancy structure of defected-zinc-blende $\ensuremath{\beta}\ensuremath{-}$Ga${}_{2}$Se${}_{3}$ enables Cr incorporation in a locally octahedral site without disrupting long-range order, determined by x-ray absorption spectroscopy, as well as strong overlap between Cr $3d$ states and the Se $4p$ states lining the intrinsic-vacancy rows, observed with photoemission. The highest magnetic moment per Cr is observed near the solubility limit of roughly one Cr per three vacancies. At higher Cr concentrations, islanded, metallic films result, with a magnetic moment that depends strongly on surface morphology. The effective valence is Cr${}^{3+}$ in laminar films, with introduction of Cr${}^{0}$ upon islanding. A mechanism is proposed for laminar films whereby ordered intrinsic vacancies mediate ferromagnetism.

Electron transport mechanisms in reduced Sr0.5Ba0.5Nb2O6 (SBN50) are investigated from ∼100 to 955 K through an analysis of the electrical conductivity (σ) and the Seebeck coefficient (S) with respect to temperature (T). Notably, experimental evidence is presented that supports a scenario of Anderson localization below 600 K and carrier excitation across a mobility edge at higher temperature. As a relaxor ferroelectric, stoichiometric SBN has intrinsic disorder associated with both the distribution of Sr/Ba vacancies and the formation of polarized nanoregions. The removal of oxygen through reduction generates conduction electrons in SBN. At the lowest temperatures measured (100–155 K), the electrical conductivity exhibits a temperature dependence characteristic of variable range hopping, followed by a transition to small polaron hopping at intermediate temperatures (250–545 K). In both the variable range and small polaron hopping regimes, a semiconductor-like temperature dependence of the electrical condu...

The electronic structure of the Si(1 1 1):GaSe van der Waals-like surface termination has been determined by angle-resolved photoelectron spectro- scopy using photons in the energy range hν = 12–170 eV supplied by the BESSY and ALS synchrotron light sources. The Si(1 1 1):GaSe surface is isoelectronic to the passivated Si(1 1 1):H and Si(1 1 1):As surfaces, and also reflects the principal building block of layered chalcogenide GaSe single crystals. The electronic structure is discussed in relation to these systems. The chemical bond between the Si and Ga surface atoms is non-polar and therefore similar to the Ga–Ga bond in GaSe single crystals and also to the Si–Si bond in bulk silicon. This explains both the absence of a surface core-level shift in Si 2p photoelectron spectra of the terminated surface and the striking similarity between its observed band structure and that of bulk GaSe.

We have developed a new technique, photothermal displacement spectroscopy, for studying the optical and thermal properties of surfaces. Photothermal displacement spectroscopy is based on optical detection of the thermal expansion of a sample as it is heated by absorption of electromagnetic radiation. Details of the experimental configuration are shown. An optical absorption spectrum is generated as an intensity modulated, tunable light beam (pump beam) is focussed onto the sample, and, depending on the optical cross section, some fraction of the radiation is absorbed. As the excited electrons decay nonradiatively, the sample is heated. The illuminated surface is then displaced as the sample expands. A probe beam reflected from the surface is deflected by the slope of the surface displacement. This deflection is measured by a position sensitive photodiode whose output is then amplified by a phase sensitive lock-in amplifier referenced to the pump beam modulation. Thermal information can be obtained by measuring the shape and phase of the photothermal displacement relative to the illumination as a function of the modulation frequency.

A new technique, photothermal displacement spectroscopy, for probing the optical properties of surfaces is presented. Optical spectra are obtained directly, and surface and bulk contributions are easily and unambiguously differentiated. The technique is well suited for in situ ultra-high vacuum experiments.

The room temperature (111) cleavage faces of silicon and germanium are known to reconstruct to a metastable 2x1 unit cell, with period doubling along one of the three equivalent directions on the surface. The mechanism behind this doubling of the unit cell has been the subject of much inquiry, with numerous models proposed to explain the various experimental results. Many of the results which have been used to distinguish among competing reconstruction models require extensive calculations for this process. Photoemission, ion backscattering, and LEED all require the assumption of a specific atomic configuration to generate theoretical results with which to compare the experimental data. The polarization dependence of the surface optical absorption, however, is a consequence of the overall symmetry of the dangling-bond orbitals, and does not require exact knowledge of the atomic positions to allow comparison with a class of models. As we will discuss below, the angular dependence of the dipole matrix element is different for the various models, making the polarization-dependent dangling-bond absorption a crucial test of these models.

We have developed a new technique, photothermal displacement spectroscopy, for studying the optical and thermal properties of surfaces. The ability to distinguish surface and bulk optical absorptions is demonstrated. The signal is directly proportional to the surface absorption coefficient and is insensitive to variations in the real part of the dielectric constant.