I. S. T. Tsong

Scanning tunneling microscopy of an ion‐bombarded PbS(001) surface

Freshly cleaved (001) surfaces of single crystalline PbS were bombarded by 8 keV Kr+ ions at a dose of 3×1012 cm−2. Atomically resolved scanning tunneling microscopy (STM) images were taken showing damaged areas due to individual ion impacts. Analysis of a STM image shows a shallow impact crater, a stacking fault, displacement of Pb2+ and S2− ions from their regular surface sites, and migration of interstitials to the surface.

Low temperature chemical vapor deposition of Si-based compounds via SiH3SiH2SiH3 : Metastable SiSn/GeSn/Si(100) heteroepitaxial structures

Growth of Si1−xSnx alloys on Ge1−ySny-buffered Si(100) was achieved via reactions of SnD4 and SiH3SiH2SiH3 at 275°C. Kinetic studies indicate that unprecedented low growth temperatures are made possible by the highly reactive SiH2 groups. The authors obtain supersaturated metastable compositions (y∼25%) near the indirect to direct band gap crossover predicted by first principles simulations. Extensive characterizations of composition, structure, and morphology show that the SiSn∕GeSn films grow lattice matched via a “compositional pinning” mechanism. The initial Raman observations of Si–Sn bond vibrations in a condensed phase are discussed in the context of simulated bond distributions in the alloys.

Influence of 6H–SiC(0001) substrate surface morphology on the growth of AlN epitaxial layers

Epitaxial AlN films were grown on 6H–SiC(0001) substrates using an ammonia supersonic seeded beam. The films grown on substrates etched in hydrogen at high temperatures were shown by ion beam channeling to exhibit a higher degree of order relative to those grown on the as-received substrates. Cross-sectional electron microscopy revealed sharper SiC–AlN interfaces with extended flat terraces. In particular, very few stacking mismatch boundaries were observed to originate from the 1.5 nm steps which correspond to the 6H stacking sequence of the substrate.

Scanning tunneling microscopy of ion impacts on semiconductor surfaces

The effects of individual ion impacts on the surfaces of two semiconductors Si(100) and PbS(100) were studied by scanning tunneling microscopy (STM). As+ and Ge+ ions ranging in energies from 20 keV to 1 MeV were implanted into Si(100) through a SiO2 layer. STM was performed after removal of the SiO2 by HF etching. Kr+ ions of 8 keV bombarded freshly cleaved PbS(100) surfaces at 45° incidence. Atomically resolved STM images of the impact zone were obtained in this case. Low ion doses ∼1011 cm−2 were used throughout to avoid overlap of cascades. Surface features observed by STM include craters, structural disorder, and surface print defects. There is a 1:1 correlation between the number of ion impacts and the number of craters. The mean crater diameter correlates with the lateral extent of nuclear energy deposition at the surface.

FIELD-ION SCANNING TUNNELING MICROSCOPY STUDY OF THE ATOMIC STRUCTURE OF 6H-SIC(0001) SURFACES CLEANED BY IN SITU SI MOLECULAR BEAM ETCHING

Field ion‐scanning tunneling microscopy has been used to study 6H–SiC(0001) surfaces with Si adlayers on the Si‐terminated surface, formed by in situ Si molecular beam etching at 950u2009°C. The as‐cleaned surface showed a (∛×∛) reconstruction. The (2×3), (2∛×6√6), and (3×3) phases were formed by evaporating Si on this clean 6H–SiC(0001)‐(∛×∛) surface. A Si(111) film 6 monolayers thick was also epitaxially grown on the 6H–SiC(0001)‐(∛×∛) surface at 800u2009°C, and the surface exhibited the Si(111)–(7×7) reconstruction. A surface vacancy model for the (∛×∛) reconstruction is proposed, and a possible application of utilizing the various surface reconstructions to control the growth of different polytypes of SiC on the 6H–SiC substrate is discussed.

Sputtering of ices by keV ions

Abstract We have measured the sputtering yields of H 2 O, CO 2 and N 2 O ices at 78 K under Ar + , Ne + , N + , He + and e − bombardment in the keV energy range. The experimental technique involves the determination of the initial ice film thickness by optical interference and the fluence required to sputter through the film by SIMS. The measured yields were compared with predictions using Sigmunds linear cascade theory. The results indicate that processes in addition to the purely ballistic process contribute to the ejection.

Growth of AlN and GaN on 6H–SiC(0001) using a helium supersonic beam seeded with ammonia

We have grown AlN and GaN layers on 4° off-axis 6H–SiC (0001) substrates using He supersonic beams seeded with NH3. The AlN films were used as buffer layers for GaN growth at 800°C. We estimate 39% incorporation of the NH3 molecules impinging on the substrate surface during GaN film growth. High structural quality of the epitaxial GaN layers was confirmed by transmission electron microscopy and electron channeling patterns. The GaN films, which had a thickness of ∼105u2009nm, contained a defect density of ∼2×1010u2009cm−2.

Surface reconstructions induced by thin overlayers of indium on Si(111)

We have used the technique of impact collision ion scattering spectrometry (ICISS) to study the in‐plane geometry of both the 7/8 × 7/8 and 4×1 reconstructions of indium on Si(111). For the 7/8 × 7/8 reconstruction the In adatoms are generally thought to lie on either threefold hollow (H3) sites or on fourfold atop (T4) sites. Our ICISS polar‐angle scans agree with computer simulations of the T4 model. Separations of the In and Si layers have also been determined from our experimental data, in agreement with predictions from total energy calculations. The 4×1 reconstruction is known to consist of double rows of adatoms running in 〈110〉 directions, from scanning tunneling microscopy images. Assuming the top layer adatoms to be indium, we perform ICISS polar‐angle scans to determine whether the adatom positions lie in H3 or T4 sites. Our results show general agreement with the simulations for the H3 sites in the 4×1 geometry.

Low-temperature pathways to Ge-rich Si1−xGex alloys via single-source hydride chemistry

We report rapid low-temperature (300–470°C) growth of Si0.50Ge0.50, Si0.33Ge0.67, Si0.25Ge0.75, and Si0.20Ge0.80 alloys on Si(100) using heavy single-source hydride molecular compounds (H3Ge)nSiH4−n (n=1–4). Incorporation of the entire SiGe, SiGe2, SiGe3, and SiGe4 framework of these precursors into the film provides precise control of morphology, composition, and strain. Low-energy electron microscopy analysis indicates that the (H3Ge)xSiH4−x (x=2–4) species are highly reactive, with H2 desorption characteristics comparable to those of Ge2H6, despite the presence of strong Si–H bonds in their molecular structure.

Shadow cones formed by target atoms bombarded by 1 to 3 keV He+, Li+, Ne+ and Na+ ions

The shadow cones of 1 to 3 keV He + , Li + , Ne + and Na + ions scattered from a variety of target atoms ranging from C to Pb are calculated by classical scattering theory using the Thomas-Fermi-Moliere interatomic potential. These shadow cones are useful in the study of surface structures by the ICISS technique.