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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

Phase transformation of thin film (∼30 nm)In2Se3/Si(111) (amorphous→crystalline) was performed by resistive annealing and the reverse transformation (crystalline→amorphous) was performed by nanosecond laser annealing. As an intrinsic-vacancy, binary chalcogenide semiconductor, In2Se3 is of interest for non-volatile phase-change memory. Amorphous In x Se y was deposited at room temperature on Si(111) after pre-deposition of a crystalline In2Se3 buffer layer (0.64 nm). Upon resistive annealing to 380°C, the film was transformed into a γ-In2Se3 single crystal with its {0001} planes parallel to the Si(111) substrate and $(11\bar{2}0)$ parallel to Si $(1\bar{1}0)$ , as evidenced by scanning tunneling microscopy, low energy electron diffraction, and X-ray diffraction. Laser annealing with 20-ns pulses (0.1 millijoules/pulse, fluence≤50 mJ/cm2) re-amorphized the region exposed to the laser beam, as observed with photoemission electron microscopy (PEEM). The amorphous phase in PEEM appears dark, likely due to abundant defect levels inhibiting electron emission from the amorphous In x Se y film.

Deposition of Se and GaSe on $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ surfaces was studied with low-energy electron diffraction, x-ray photoelectron spectroscopy, and x-ray photoelectron diffraction to probe initial nucleation and interface structure for $\mathrm{GaSe}/\mathrm{Si}(111)$ heteroepitaxy. Room-temperature deposition of Se on $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ results in an amorphous film. Subsequent annealing leads to Se evaporation without ordering or interdiffusion. Se deposition at $450\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ saturates at submonolayer coverage with no diffusion of Se into the substrate. There is no clear evidence of ordered sites for the Se. Growth of GaSe on $\mathrm{Si}(111)7\ifmmode\times\else\texttimes\fi{}7$ above $500\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$ results in a pseudomorphic bilayer, with Si-Ga-Se bonding. Additional GaSe does not stick to the bilayer above $525\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ The resulting Se lone pair at the surface leads to an ideally passivated surface similar to $\mathrm{As}/\mathrm{Si}(111).$ This stable surface is similar to the layer termination in bulk GaSe. The single domain bilayer is oriented with the Ga-Se bond parallel to the substrate Si-Si bond.

We have studied diffusion of Ge into subsurface layers of Si(100). Auger electron diffraction measurements show Ge in the fourth layer after submonolayer growth at temperatures as low as 500 degrees C. Density functional theory predictions of equilibrium Ge subsurface distributions are consistent with the measurements. We identify a surprisingly low energy pathway resulting from low interstitial formation energy in the third and fourth layers. Doping significantly affects the formation energy, suggesting that n-type doping may lead to sharper Si/Ge interfaces.

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.

Bulk and interface photoemission satellite excitations, measured with x-ray photoelectron spectroscopy and x-ray photoelectron diffraction, are compared for thick, thin, and monolayer films of ${\mathrm{CaF}}_{2}$ and ${\mathrm{SrF}}_{2}$ on Si(111). Intrinsic satellites are observed for excitation of atoms in the first monolayer, both uncovered and at the buried interface, that differ from those associated with bulk atoms. For F 1s excitation, the bulk and interface satellites differ only in width and amplitude; for the cation core excitations (Ca 2p, Sr 3p, and Sr 3d), the observed excitation energies and intensities differ both from the equivalent bulk satellites and among the various core hole states. The results yield new information on the nature of the interface bonding, as well as on the origin of both bulk and interface satellites. Several models are considered, including differential screening, dielectric losses, crystal-field and multiplet effects, and interface excitation. The most likely explanation for the new cation satellites at the ${\mathrm{CaF}}_{2}$/Si(111) [${\mathrm{SrF}}_{2}$/Si(111)] interface is localized excitation of the interface bond, where the interface band gap is controlled by the collapse of the Ca 3d (Sr 4d) level in the presence of the Ca (Sr) core hole. \textcopyright{} 1996 The American Physical Society.

The thickness uniformity and the spatial distribution of lattice relaxation in thin ( $l$8 nm) Ca${\mathrm{F}}_{2}$/Si(111) films, observed with photoelectron spectroscopy and transmission electron microscopy, are seen to depend strongly on the initial nucleation kinetics. We develop a general model for heteroepitaxial growth that explains both these and literature results. Terrace or step nucleation leads to laminar films, although with different relaxation patterns; combined step and terrace nucleation leads to rough films due to different upper-layer nucleation rates on the differently sized islands.

Kinetic variations of the initial stages of CaF[sub 2] growth on Si(111) by molecular-beam epitaxy are studied with the [ital in] [ital situ] combination of x-ray photoelectron spectroscopy and diffraction. After the formation of a chemically reacted interface layer, the morphology of the subsequent bulk layers is found to be dependent on the substrate temperature and incident flux rate, as well as the initial interface structure. For substrate temperatures above [similar to]600 [degree]C, subsequent layers do not easily wet the interface layer, and a transition is observed from a three-dimensional island formation at low flux to a laminar growth following the coalescence of bilayer islands at higher flux. At lower substrate temperatures ([similar to]450 [degree]C), a different stoichiometry and structure of the interface layer leads to laminar growth at all fluxes, but with a different bulk nucleation behavior. Crystalline heteroepitaxy is not observed when growth initiates at room temperature, but homoepitaxy does proceed at room temperature if the first few layers are deposited at a high temperature. The different growth regimes are discussed in terms of a kinetic model separating step and terrace nucleation where, contrary to homoepitaxy, step nucleation leads to islanded growth.

Using x-ray-photoelectron spectroscopy and Auger-electron spectroscopy, we have resolved surface, bulk, and interface Ca and F core-level emission in thin films (3--8 triple layers) of CaF[sub 2] and SrF[sub 2] on Si(111). We confirmed these assignments using x-ray-photoelectron diffraction (XPD) and surface modification. XPD was also used to identify the growth modes of the films as being either laminar or layer plus islands; in the latter case we have resolved buried and uncovered interface F and Ca/Sr emission. We compare the observed energy differences between surface, bulk, and interface emission to theoretical estimates of the extra-atomic contributions to emission energies. We find excellent agreement considering only the Madelung (electrostatic) potentials for the initial-state contribution and polarization response for the final-state contribution, including the effect of tetragonal strain. Small discrepancies for emission from metal atoms bonded to the Si substrate are interpreted in terms of chemical shifts.

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.

The demands of chemisorption and epitaxy are quite different for electronically dissimilar systems. The transition between these two regimes in ${\mathrm{CaF}}_{2}$-Si(111) is studied with transmission-electron microscopy and photoemission. Changes in the electronic structure of the evolving growth surface are expressed in the composite growth mode, a Stranski-Krastanow pathway to layer-by-layer growth, which begins with ${\mathrm{CaF}}_{2}$ coherent island formation on a Si-CaF layer. After this transition, layer-by-layer ${\mathrm{CaF}}_{2}$ homoepitaxy is possible even at room temperature, and the critical thickness can be extended.

Gallium selenide Ga2Se3 is a III-VI chalcogenide material whose intrinsic cation vacancy ordering into one dimensional chains has been predicted to result in a one dimensional band at the valence-band maximum VBM. The electronic structure of Ga2Se3 thin films on Si001:As is studied with angle-resolved photoemission spectroscopy. The integrated density of states and the dispersion exhibit good agreement with that predicted by published density-functional theory results. Consistent with a one dimensional state, the electronic states at the VBM show no dispersion perpendicular to the vacancy chains. The Se-Ga bond length from extended x-ray absorption fine structure is 2.34 A. Low-energy electron-diffraction results indicate that the surface is characterized by nanometer-scale 111 facets, consistent with previous scanning tunneling microscopy results.

Addition of the transition-metal dopant Cr to ${\text{Ga}}_{2}{\text{Se}}_{3}$ during heteroepitaxial growth on Si(001), a system of interest as a prototype silicon-compatible, dilute magnetic semiconductor, has been studied with scanning-tunneling microscopy and scanning Auger microscopy as a function of Cr concentration and the presence or absence of an undoped buffer or capping layer. Chromium incorporates into laminar ${\text{Ga}}_{2}{\text{Se}}_{3}$ films up to a solubility limit of several atomic percent, after which Cr-rich islands nucleate. At low Cr concentrations, the vacancy-ordered nanoridge structure characteristic of pure ${\text{Ga}}_{2}{\text{Se}}_{3}$ remains but nanoridge aspect ratios decrease with Cr concentration; this is likely associated with Cr removing intrinsic vacancies. At higher Cr concentrations, faceted, Cr-rich islands nucleate, often surrounded by trenches, and the terrace morphology no longer resembles pure ${\text{Ga}}_{2}{\text{Se}}_{3}$. Growth of Cr-doped ${\text{Ga}}_{2}{\text{Se}}_{3}$ directly on Si(001):As is qualitatively similar to growth on a pure ${\text{Ga}}_{2}{\text{Se}}_{3}$ buffer layer; however, the island structure changes dramatically upon coverage of a highly doped layer with undoped ${\text{Ga}}_{2}{\text{Se}}_{3}$. Addition of Cr stabilizes cubic overlayer growth under Se-poor growth conditions beyond that of pure ${\text{Ga}}_{2}{\text{Se}}_{3}$; no growth of the hexagonal layered structure characteristic of bulk GaSe was observed.

Strain-induced lattice relaxation during overlayer growth is investigated by {ital in} {ital situ} monitoring of SrF{sub 2} and Ca{sub {ital x}}Sr{sub 1{minus}{ital x}}F{sub 2} growth on Si(111) with x-ray standing-wave fluorescence and low-energy electron diffracton. Submonolayer films show both intrinsic lateral disorder and variable interface stoichiometry with discommensuration parallel to the surface after completion of the first monolayer. In contrast, local commensuration is observed for Ca-rich alloy films. In the alloy, Sr and Ca occupy different sites in the first monolayer, but have similar average positions in subsequent layers.

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...

Andreas Klust, ∗ Taisuke Ohta, † Aaron A. Bostwick, † Eli Rotenberg, Qiuming Yu, Fumio S. Ohuchi, and Marjorie A. Olmstead University of Washington, Department of Physics, Box 351560, Seattle, WA 98195, USA University of Washington, Department of Materials Science and Engineering, Box 352120, Seattle, WA 98195, USA Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA. (Dated: October 19, 2005)

PACS numbers: 68.37.Ef, 68.55.–a The intrinsic structural vacancies of semiconducting chalcogenides lead to numerous interesting structural, electronic, and optical properties. In this Letter, we demonstrate a new mechanism for creating a highly anisotropic growth morphology based on these ordered, intrinsic vacancies. In particular, we observe a nanoscale wire

A highly anisotropic growth morphology is found for heteroepitaxial gallium sesquiselenide (Ga2Se3) on the lattice matched substrate, arsenic-terminated Si(001). Scanning tunneling microscopy of Ga2Se3 films reveals nanoscale, wirelike structures covering the surface in parallel lines, less than 1 nm wide and up to 30 nm long. Core-level photoemission spectroscopy and diffraction reveals the local structure of buried Ga and Se atoms to reflect the bulk, defected zinc-blende structure of beta-Ga2Se3, which contains ordered 110 arrays of Ga vacancies. These ordered vacancy lines are proposed to be responsible for the observed growth anisotropy in heteroepitaxial Ga2Se3.

The gallium-selenide (GaSe)/silicon (Si) heterointerface provides an excellent model system to investigate the role of defects in heterointerface formation between elemental and polar semiconductors. The first bilayer of GaSe on Si(111) exhibits negligible intermixing, high chemical stability, and no lateral surface reconstruction. In situ scanning tunneling microscopy (STM) was used to perform a planar investigation of the GaSe-Si heterointerface structure. In addition to large regions of a uniform, hexagonal $1\ifmmode\times\else\texttimes\fi{}1$ structure, STM revealed various point, line, and planar defects, including clustered point defects, orientational domains and their boundaries, and Ga terminated regions (Ga/Si) in the interface layer. A model atomic structure of the orientational domains and their boundaries is deduced from polarity-dependent atomically resolved STM images: Se atoms are located at ${T}_{4}$ sites for rotated domains (RD's) instead of ${H}_{3}$ sites for normal domains. Formation mechanisms of orientational domains and the domain boundaries together with Ga/Si is discussed in terms of growth conditions and surface dipole moments around defects. It is also shown that high growth temperature reduces formation of RD's, which might in turn enable the growth of single-domain epitaxial GaSe films. Plan-view information about interface defects is key to understanding nucleation, growth kinetics and the resulting electronic structures of the overlayer.

The CaF bilayer on Si(111) was imaged with atomic resolution scanning force microscopy (SFM). The CaF bilayer has almost exactly the same geometry as the uppermost two atomic layers of bulk CaF 2 ; the positions of the F atoms below the Ca layer, however, are vacant. We discuss the influence of this difference on atomic scale contrast formation in SFM. The SFM images obtained on the CaF bilayer are very similar to results from the literature obtained on bulk CaF 2 with positively terminated tips. SFM observations on the CaF bilayer are explained within a model based on the electrostatic and van der Waals forces between tip and sample. The model predicts that the atomic corrugation is independent of the charge located at the position of the F vacancies and thus explains the similarity of SFM images obtained on bulk CaF 2 and the CaF bilayer.

Chemical discrimination of bulk and interface Ca 2p x‐ray photoelectron diffraction modulations is used to identify three growth regimes during the initial stages of CaF2 epitaxy on Si(111). Low flux, high temperature conditions produce island growth atop a nonwetting, chemically reacted Ca‐F interface layer. Changing the growth kinetics by increasing the flux produces more laminar growth. Lowering the substrate temperature produces a more stoichiometric CaF2 interface layer that results in immediate wetting and laminar growth.

Site occupancy and cation binding states in the proposed thermoelectric n-type oxide SrxBa1−xNb2O6 (SBN100x) were investigated using X-ray photoelectron spectroscopy (XPS). Sr 3d XPS spectra from unreduced polycrystalline SBN100x with various compositions contained two distinct spin-orbit doublets corresponding to Sr occupying either A1 or A2 positions in the SBN lattice; the higher binding energy state was associated with Sr ions at A2 sites, presumably due to their increased coordination over Sr at A1 sites. To gain insight into optimizing the thermoelectric properties of reduced SBN, sintered SBN50 specimens were reduced in Ar/H2 or N2/H2 ambient. A decrease in the average Nb valence was observed in Nb 3d photoemission through the growth of low-binding energy components after reduction in either environment; evidence of surface NbN formation was apparent with longer reducing times in N2/H2. Both the single-component Ba 3d emission and the A2 component of the Sr 3d spectra show shifting to lower binding...

Single crystals of transition metal (TM) doped β–Ga2O3, a wide gap semiconductor system of interest for transparent conductive oxide and diluted magnetic semiconductor applications, have been studied in the dilute, non-interacting limit (≤0.06 cation %). Based on optical absorption, particle induced x-ray emission, and Rutherford backscattering measurements, Mn does not incorporate as well as Cr, and Mn degrades the crystal quality. Using superconducting quantum interference device (SQuID) magnetometry, a Brillouin type paramagnetic magnetization is observed for Mn or Cr doped crystals with an effective number of Bohr magnetons per TM ion of 5.88 ± 0.1 or 3.95 ± 0.1, respectively. A trace ferromagnetic signal is consistent with a very small concentration of secondary phases in the Mn-doped crystal. The position of the edge in x-ray absorption near edge structure (XANES) measurements suggests that the Cr takes the 3+ valence, while a mixture of Mn2+ and Mn3+ are present; based on the absence of a prominent...

Surface band bending and surface defects on the UV-transparent conducting oxide β-Ga2O3 (100) are studied with hard x-ray photoemission spectroscopy and scanning tunneling microscopy. Highly doped β-Ga2O3 shows flat bands near the surface, while the bands on nominally undoped (but still n-type), air-cleaved β-Ga2O3 are bent upwards by ≳0.5 eV. Negatively charged surface defects are observed on vacuum annealed β-Ga2O3, which also shows upward band bending. Density functional calculations show oxygen vacancies are not likely to be ionized in the bulk, but could be activated by surface band bending. The large band bending may also hinder formation of ohmic contacts.

Thin (0.5 to 8 triple layer) CaF2 on Si(111) films were grown using molecular beam epitaxy (MBE) and characterized using an in situ combination of x-ray photoelectron spectroscopy (XPS) and componentresolved x-ray photoelectron diffraction (XPD). We identified surface, bulk-like, and interface F and Ca core-level shifts and used the XPS shifts and XPD modulations to identify the growth modes as a function of the kinetic parameters of CaF2 flux and Si temperature. We identify 3 distinct regimes: (i) for high temperatures and flux we find a complete reacted F-Ca-Si layer, overlaid by 2 layer high islands which coalesce, followed by layer-by-layer growth, (ii) for high temperature and low flux, we find the reacted F-Ca-Si layer to be partially covered with thick islands, and (iii) for low temperatures we find an incompletely occupied F-Ca-Si layer followed by layer-by-layer growth. In all cases we find the buried interface to be structurally identical to the unburied F-Ca-Si layer

Nanoscale enrichments resulting from spinodal decomposition have been proposed to contribute to the interesting magnetic properties of diluted magnetic oxides such as cobalt-doped ZnO (Zn1-xCoxO), but little is known experimentally about the electronic structures or physical properties of such enrichments. Here, x-ray photoelectron spectroscopy (XPS) is used to examine wurtzite Zn1-xCoxO crystallites over the full composition range (0.0≤x≤1.0) that serve as models of the proposed spinodal decomposition nanostructures within Zn1-xCoxO bulk materials. With increasing x, the valence band edge shifts to smaller binding energies and the cobalt 2p peaks shift to greater binding energies, providing spectroscopic signatures that may allow identification of spinodal decomposition in bulk Zn1-xCoxO. Reduction of Co2+ to Co0 by argon ion (Ar+) sputtering was also found to become markedly more facile with increasing x, suggesting that locally-enriched Zn1-xCoxO is at greater risk of yielding false-positive Co0 XPS si...

We present a model for the extra-atomic contributions to core-level shifts in insulating thin films on polarizable substrates. The final-state shift is calculated from the screening-dependent local fields at a photoemitting atom and shown to be comparable to the initial-state Madelung potential shift in polar crystals. For Xe(111) films, our model completely accounts for experimental results. For NaCl(100) and ${\mathrm{CaF}}_{2}$(111) surfaces, we present predictions of surface core-level shifts for simple bulk terminations. We discuss corrections which can be incorporated into our model.

Heteroepitaxial thin films of Mn-doped Ga2Se3 are grown by molecular beam epitaxy on Si(001):As. Mn-doped films are laminar for the first 1–2 nm, after which oriented islands with flat tops are observed by scanning tunneling microscopy. In contrast with the bulk phase diagram, which predicts MnGa2Se4 precipitates, the precipitates are identified by bond length measurements from extended x-ray absorption fine structure as rocksalt MnSe. This difference is attributed to superior lattice matching of MnSe to the substrate, and an epitaxial relationship between the MnSe and Si substrate is inferred.

Experimental studies of the surface morphology and electronic structure of bulk single crystals of the transparent and wide gap semiconductor gallium oxide (β-Ga2O3) have been conducted using scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and angle-resolved photoemission spectroscopy (ARPES). Atomically resolved STM and LEED results for the β-Ga2O3(100) surface clarify that the predominant surface termination contains both gallium and oxygen, and this surface does not exhibit a reconstruction. The valence band structure was obtained with ARPES and shows good agreement with existing theoretical works at the zone center and along the a∗ and c∗ directions, except that the calculated bandwidth is ∼7% too small. There is poorer agreement along the b∗ direction, where the experimental bands disperse more strongly than the calculations.

The formation of ${\mathrm{SrF}}_{2}$/Si(111) and Ge/${\mathrm{CaF}}_{2}$/Si(111) interfaces is studied with photoemission and compared to previous results for the ${\mathrm{CaF}}_{2}$/Si(111) interface. The interface between ${\mathrm{SrF}}_{2}$ and Si(111) is found to be nonstoichiometric, similar to the interface between ${\mathrm{CaF}}_{2}$ and Si(111): the bonding is between Si and the cation, with a layer of fluorine missing at the interface. In the case of Ge growth on ${\mathrm{CaF}}_{2}$/Si(111), a variety of effects are noted: The ${\mathrm{CaF}}_{2}$/Si(111) valence-band offset is reduced by about 1 eV upon deposition of Ge at room temperature. The sticking coefficient of the Ge is significantly increased by preparing the ${\mathrm{CaF}}_{2}$ surface with electron bombardment to remove the top layer of fluorine. For both the irradiated and nonirradiated cases, annealing of thin room-temperature-deposited films resulted in Ge island formation.

We report a combined high-temperature scanning tunneling microscopy, ion scattering spectroscopy, and photoelectron spectroscopy study of bare lanthanum aluminate [LaAlO3(LAO)] and of the initial stages of anatase TiO2 growth on LAO(001). LAO(001) exhibits mixed La–O and Al–O2 surface terminations at 400°C. Heteroepitaxial TiO2, grown by evaporating Ti metal in O2, nucleates near step edges, growing out to cover both upper and lower terraces uniformly, regardless of termination, indicating that the substrate cations and perovskite surface polarity play little direct role in controlling the morphology of this single-metal oxide heteroepitaxy. TiO2 films of 1.5nm in thickness exhibit a surface reconstruction similar to the bulk anatase (1×4).

We report controlled laminar growth of a crystalline transition metal oxide on Si(001) without SiOx or silicide formation by utilizing the chalcogenide semiconductor gallium sesquiselenide (Ga2Se3) as a nonreactive buffer layer. Initial nucleation of both pure and Co-doped anatase (TiO2) is along Ga2Se3 nanowire structures, coalescing to a flat, multidomain film within two molecular layers. Arsenic-terminated Si(001) [Si(001):As] is stable against pure O2, but oxidizes when both Ti and O2 are present. The Si–TiO2 valence band offset using either buffer layer is about 2.8eV, producing a staggered band alignment.

The contrast in scanning probe microscopy images of ultrathin CaF2 films epitaxially grown on Si(111) is studied using scanning tunneling microscopy (STM) and scanning force microscopy (SFM). Material contrast between CaF2 and the underlying Si–CaF interface layer can be achieved by multiple scan modes. STM images of CaF2∕Si(111) exhibit a distinct contrast depending on the bias voltage. Using SFM, CaF2 and CaF can be distinguished by the contact potential difference (CPD) determined by measuring the bias voltage dependence of the cantilever resonance frequency. The CPD also causes a material contrast in the dissipation signal.

The chemical stability of Si(111), terminated with bilayer AlSe and GaSe, upon exposure to atmosphere, N2 and O2, was investigated with core-level and valence band photoelectron spectroscopy. Si(111):GaSe and Si(111):AlSe both form stable, unreconstructed surfaces with no states in the silicon energy gap; their atomic structures are nearly identical. However, similarities in surface electronic and atomic structure do not imply similar chemical passivity. While Si(111):GaSe is largely unaffected by the exposures, Si(111):AlSe reacts irreversibly with both pure O2 and atmosphere, removing over 13 of the Se and permanently destroying long-range order.

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 studied the epitaxial growth of GaSe, a layered van der Waals material, on GaAs, a zinc-blende-structure semiconductor. This heterostructure exhibits a 6% lattice mismatch, and is a prototypical example of van der Waals epitaxy, where the weak van der Waals interaction allows the misfit to be accommodated without the formation of electronically active defects. GaSe was supplied to the growing surface from a single GaSe Knudsen cell. Reflection high energy electron diffraction and x-ray photoemission spectroscopy studies of the nucleation of GaSe indicate Se reacts with the GaAs surface to remove the surface dangling bonds prior to GaSe formation. This is followed by the oriented growth of stoichiometric GaSe layers, that are rotationally aligned with the underlying GaAs substrate. The termination of the GaAs dangling bonds most likely occurs by Se substitution for As in the surface layer of GaAs(111) B and by direct bonding of Se to surface Ga on GaAs(111) A surfaces. In addition, photoemission me...

Using a combination of in situ x‐ray photoelectron spectroscopy and x‐ray photoelectron diffraction, we have resolved surface core‐level shifts for Ca and F atoms in thin (three–eight layers) CaF2‐on‐Si(111) films. These values are 0.7±0.1 eV towards higher binding energy for Ca 2p electrons and 0.6±0.1 eV towards lower kinetic energy for F KVV electrons. These shifts are in good agreement with a simple extra‐atomic electrostatic model in which the shifts are due to the change in both the Madelung potential and the polarization response (relaxation) at the surface. We find no evidence for altered ionicity at the CaF2(111) surface.

The bonding at the interface between calcium fluoride and the silicon (111) surface has been studied with surface-sensitive core-level photoemission spectroscopy. The interface is found to consist of predominately Si---Ca bonds, with a dissociation of the ${\mathrm{CaF}}_{2}$ molecule at the interface resulting in depletion of fluorine atoms. When the interface is grown at temperatures below the optimal epitaxial growth temperatures, there is both more fluorine and more disorder at the interface. This affects the valence-band offset between the two materials, which ranges between about 7.3 and 8.3 eV. When the interface is annealed at 700--800 \ifmmode^\circ\else\textdegree\fi{}C, the ${\mathrm{CaF}}_{2}$ film reevaporates until a 3\ifmmode\times\else\texttimes\fi{}1 reconstruction is obtained on which there is calcium but no fluorine. The bonding at this surface is similar to that at the ${\mathrm{CaF}}_{2}$-on-Si(111) interface.

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.

We present a sensitive technique for determining the optical and thermal properties of solids, surfaces and thin films. This technique, photothermal displacement Spectroscopy, is based on the detection of the thermal expansion of a sample upon absorption of electromagnetic radiation. The technique is well suited for in situ ultrahigh vacuum studies and for experiments where wide temperature ranges are required. We show that surface and bulk optical absorption can be distinguished and that surface absorptions of αL=10−6/W of incident power can be measured. The theoretical basis of the signal generation is given, and excellent experimental and theoretical agreement is demonstrated. The implications of our findings to imaging and microscopy are discussed.

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 polarization dependence of midgap optical absorption (0.38--0.51 eV) by Si(111)2 x 1 single-domain surfaces has been measured directly by photothermal displacement spectroscopy. The absorption is strongest when the light is polarized perpendicular to the period-doubling direction of the 2 x 1 reconstruction and is reduced by at least 95% when the polarization is rotated by 90/sup 0/. This result supports the ..pi..-bonded chain model of the Si(111)2 x 1 surface reconstruction.

The interfaces formed when films of Se and ZnSe are deposited on Si(100) substrates have been studied with photoemission core-level spectroscopy. For Se films deposited at room temperature and annealed, the presence of Si atoms with chemical shifts of $\ensuremath{\delta}$, $2\ensuremath{\delta}$, $3\ensuremath{\delta}$, and $4\ensuremath{\delta}$ where $\ensuremath{\delta}$ is 0.53\ifmmode\pm\else\textpm\fi{}0.03 eV were found, suggesting the formation of Si${\mathrm{Se}}_{2}$. At higher annealing temperatures, the results are consistent with the presence of a submonolayer of Se atoms in bridge sites on the Si(100) surface. ZnSe films were deposited at room temperature and at 300\ifmmode^\circ\else\textdegree\fi{}C on Si(100) and the primary bonding at the interface was found to be between Si and Se. Island formation occurred in the initial stages of growth at the higher temperature.

Recent results for the arsenic termination of the Ge(111), Si(111) and Si(100) surfaces are reviewed. Arsenic interaction with the Ge(111) and Si(111) surfaces has been found to result in the replacement of the outer Ge (Si) layer with an As layer. This arrangement has a 1 × 1 symmetry and the calculated positions of the As atoms are very close to the positions expected from bulk bond lengths. Ge(111): As and Si(111): As thus provide us with model ideal surfaces. Similar investigations of the Si(100): As 2 × 1 surface show that this system can be considered as a model for surface dimers. Experimental surface-state dispersions obtained from angle-resolved photoemission measurements and their comparison with results of total energy calculations for these surfaces are reviewed. Photoemission core level data for the As-terminated surfaces and for GaAs on Si(111) are also discussed in terms of the model surfaces.

The theoretical and experimental results on the cleaved surfaces of silicon and germanium (111) are reviewed. Emphasis is placed on the optical properties of these structures, and how the determination of the optical properties has led to structural information. These and other results show strong support for a structural model involving π-bonded chains of atoms along the surface.