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The formation of GaAs islands is a major effect at the beginning of GaAs‐on‐Si epitaxy. The density of nucleation sites for the islands and the manner of their subsequent coalescence will influence the dislocation density of the final GaAs film. In this paper the effect on GaAs‐on‐Si epitaxy of a Ga‐prelayer treatment is studied with photoemission core level spectroscopy and high resolution transmission microscopy (HTEM). Experiments are carried out with GaAs film thicknesses in the range from one monolayer to around 50 nm. 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 HTEM using a wedge‐shaped GaAs‐on‐Si sample. This allows side‐by‐side comparisons of areas with and without the Ga prelayer as a function of GaAs thickness. At thicknesses of around 5 nm, it is shown that the Ga prelayer yields islands with a lower ...

Studies of silicon on gallium arsenide and gallium arsenide on silicon interface formation will be described and used as examples of how synchrotron radiation excited photoemission is a powerful probe of surface and interface phenomena. The samples and interfaces studied in this work were prepared in-situ using thermal Ga and As MBE sources and a resistive Si evaporation source. The MBE capability has allowed us to explore atomic and electronic structural issues of interface formation during hetero-epitaxy. We have combined these techniques to explore Si and GaAs surfaces, the adsorbate interactions of Ga and As on Si, and interface formation of Si on GaAs and the inverse system, GaAs on Si. Several of these surfaces and interfaces yield model systems which can provide general insights. As an example, studies of arsenic terminated silicon (100) and (111) surfaces will be described. A specific focus of the paper will be the interfacial development of Si on GaAs(100). Si deposited at various coverages below 10 ml onto a room temperature substrate yields a disordered surface. For coverages below two monolayers, subsequent annealing at 450–600°C leads to a single domain 1×2 surface from either the As rich c(2×8) or Ga rich 4×6 reconstructions. While the periodicity is the same for the different starting surfaces, evidence is presented that the atomic arrangement in the unit cell is different.

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 atomic structure occurring at the interface between Si(111) and thin GaAs epitaxial overlayers has a pronounced effect on the surface electronic structure, thus making it possible to infer structural information by comparing measured surface-state energies with those calculated for possible interface bonding arrangements. The atomically abrupt interface...SiAsGaAs, in addition to being energetically very unfavorable, is inconsistent with measured valence-band photoemission spectroscopy. Our results indicate that the interface is not atomically abrupt.

The interface structure of thin overlayers of GaAs grown using molecular-beam epitaxy techniques on on-axis Si(100), Si(111), and Ge(111) substrates has been studied using photoemission core-level spectroscopy. Results for As and Ga overlayers are also reported and are utilized to interpret the results for GaAs growth. The interface bonding is found to be the same for growth using predeposition of Ga as for predeposition of As. For GaAs on Si(111), the bonding at the interface is found to consist predominantly of Si-As bonds. We also find strong evidence of island formation before the completion of the first GaAs bilayer. The total area between islands can be reduced either by faster deposition rates or by using a Ga prelayer. For GaAs on Si(100), bonding takes place to both Ga and As but with fewer Si-Ga bonds than Si-As bonds. The tendency to island formation is less than for the Si(111) case. These results are compared with earlier data for As interaction with off-axis Si(100) surfaces to explain the absence of antiphase domains in GaAs grown on off-axis (100) substrates. Roughly equal numbers of Ge-As and Ge-Ga bonds were found for GaAs on Ge(111), and this result is interpreted as indicating thatmore » Ge interdiffuses more rapidly into GaAs than does Si.« less

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.

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.

The growth of SrF2 on Si(111) is investigated using photoemission spectroscopy and compared to the growth of CaF2 on Si(l11). Both CaF2 and SrF2 have a fluorite lattice structure closely related to silicons diamond lattice structure. However, while the room temperature lattice constant of Si is only 0.6% smaller than that of CaF2, it is 6.8% smaller than that of SrF2. The photoemission results for thin films of SrF2 on Si(111) are very similar to previous measurements of the CaF2-on-Si(111) interface, reflective of strong silicon-cation bonding and the removal of fluorine from the interface. These results indicate that the chemical and electronic driving forces to dissociate the CaF2 or SrF2 molecule at the fluoride-silicon interface dominate over the structural constraints induced by the lattice mismatch.

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.

Si heteroepitaxy on CaF2 was studied with x-ray photoelectron spectroscopy and diffraction and low-energy electron diffraction to determine the interface bonding and silicon overlayer growth mode. The CaF2 surface was prepared by irradiation with low-energy electrons and exposure to arsenic, which replaced surface fluorine atoms with arsenic. Thin Si films (1.3 nm) were subsequently deposited at 550 °C. The Si films completely cover the CaF2 substrate and have a type-B orientation. The resultant interface has Si–Ca bonds, with the As surfactant layer terminating the Si surface in a 1×1 structure.

The interfacial development of Si on GaAs(100) has been studied using core level photoemission. The studies included arsenic rich and gallium rich starting surfaces and a variety of annealing procedures. Si deposited at various coverages below 10 monolayers (ML) onto a room temperature substrate yields a disordered surface. For coverages below 2 ML, subsequent annealing leads to a predominantly single domain 1×2 surface. Deposition onto a heated substrate in the range of 450–600 °C yields the 1×2 reconstruction directly. Growth of a GaAs–Si alloy yielded a 1×2 surface with diffraction spot broadening indicating considerable strain.

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.

The initial stages of interface formation between CaF2 and Si(111) have been studied with both core‐level and angle‐resolved valence band photoemission spectroscopy. Both the Si 2p and Ca 3p core levels indicate that a Si–Ca bond is present at the CaF2–Si(111) interface. Annealing of thin CaF2 films at 700–800 °C results in preferential evaporation of F, and the surface undergoes a number of reconstructions until a stable Si(111):Ca 3×1 reconstruction is obtained on which there is calcium, but no fluorine. This surface exhibits a surface state with an upward dispersion of 0.4 eV towards the K point of the 1×1 surface Brillouin zone. Our results show that the CaF2 molecule can be dissociated on the Si(111) surface at typical epitaxial growth temperatures, with the Ca being more strongly adsorbed to the Si than is the F.