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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.

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.

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.

The polarization dependence of surface-state absorption of Ge(111)2 x 1 single-domain surfaces has been directly measured using photothermal displacement spectroscopy. The absorption at 0.5 eV is maximum for incident light polarized perpendicular to the period-doubling direction of the 2 x 1 reconstruction. As the polarization direction is rotated, the absorption follows a cos/sup 2/theta dependence. This result supports the ..pi..-bonded chain model for the surface reconstruction, and is the same as that recently reported for the dangling-bond absorption of Si(111)2 x 1.

Results of photoemission core‐level spectroscopy measurements for coverages of around one monolayer of As, Ga, and GaAs on Si substrates are presented. The interfaces were formed on on‐axis Si(100) and Si(111) substrates using molecular beam epitaxy techniques. The bonding between As and the substrate surface leaves the As atoms fully coordinated and thus extremely unreactive. This causes the GaAs films to form islands at average coverages of less than one monolayer. The surface between the islands is found to be terminated by a single atomic layer of As. Use of a Ga predeposition technique shows evidence of decreasing the area between islands. Results for As interaction with stepped Si(100) surfaces and the implications for avoidance of antiphase domain boundaries are discussed.

Core level spectroscopy measurements have been made on Si substrates with coverages of around one monolayer of As, Ga and GaAs. The interfaces were formed on on-axis Si(100) and Si(111) substrates using molecular beam epitaxy. Results are also presented for an arsenic monolayer on a single-domain Si(100) surface prepared by cutting the crystal off-axis by 4 degrees. The strong bonding between As monolayers and the surface of the substrate causes the GaAs to begin to form islands at average coverages of less than one monolayer. The surface between the islands is found to be terminated by a single atomic layer of As. Use of a Ga predeposition technique shows evidence of reducing the tendancy towards island formation.

The temperature dependence of the dangling-bond optical absorption of cleaved silicon and germanium (111)2 x 1 surfaces has been studied using photothermal displacement spectroscopy. The polarization-dependent surface-state absorption was utilized as an order parameter to monitor the Ge(111)2 x 1 c-2 x 8 phase transition in real time. The temperature dependence of the Si(111)2 x 1 surface optical gap in the range 300--450 K was also measured. Unlike the corresponding bulk case, no shift of the surface gap was observed. Reasons for this temperature independence are discussed.

Several surface treatment techniques have been used to study the effect of the Si surface on subsequent GaAs-on-Si epitaxy. In particular, the effect on the degree of island formation is addressed. Experiments have been carried out with GaAs film thicknesses in the range from one monolayer to around 50nm. Core level spectroscopy results for the monolayer films give information about the bonding character at the interface and suggest methods of improving the degree of two-dimensional growth. A particular Ga-prelayer technique is examined with high resolution TEM and found to give significantly different results than the standard MBE growth technique.

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.

The interaction of arsenic with the hydrogen‐terminated Si(111) surface has been studied using photoemission valence band and core level spectroscopy. The results are compared with those obtained previously for the interaction of arsenic with Si(111) and hydrogen with the arsenic‐terminated Si(111) surface. We have found that neither As nor H fully passivates the Si(111) surface with respect to the other species. Hydrogen can be removed from both the Si(111):As + H and Si(111):H + As surfaces by annealing, leaving As bonded to the Si(111) surface. A comparison is also made between arsenic‐terminated surfaces made with (i) an As4 deposition onto a substrate whose temperature was varied from 800 °C down to 300 °C and (ii) a deposition at 300 °C followed by a 650 °C anneal. It is found that method (i) yields a surface with significantly better surface order.

Photoemission core level spectroscopy results obtained for the bonding of monolayers of As atoms to the Si(111) and Si(100) surfaces, are reviewed and compared to similar measurements for thin layers of Se atoms. For As‐terminated Si surfaces, a single, ordered, passivating monolayer is formed. For Se films deposited at room temperature on Si(100) surfaces, on the other hand, SiSi2 compound formation occurs after annealing to 300 °C. After annealing to higher temperatures, a submonolayer of Se atoms in bridge sites remains.