Robert G. Long

Epitaxial films of semiconducting FeSi2 on (001) silicon

Epitaxial thin films of the semiconducting transition metal silicide, beta‐FeSi2, were grown on (001) silicon wafers. The observed matching face relationship is FeSi2(100)/Si(001), with the azimuthal orientation being FeSi2[010]‖‖Si〈110〉. This heteroepitaxial relationship has a common unit mesh of 59 A2 area, with a mismatch of 2.1%. There is a strong tendency toward island formation within this heteroepitaxial system.

A review of the geometrical fundamentals of reflection high‐energy electron diffraction with application to silicon surfaces

Reflection high‐energy electron diffraction (RHEED) is an experimentally simple technique, and yet a powerful one for examining the structure of a substrate surface and for monitoring the surface crystal structure and the crystallographic orientation of thin films during their growth. However, it can be difficult to learn to interpret the RHEED patterns of new materials, because a practical and adequately detailed introduction to the technique is not generally available. To address this need, we develop the geometrical principles of RHEED; using the kinematic approximation, we show how a particular point of the sample surface’s reciprocal net gives rise to a diffraction maximum at a particular location on the RHEED viewing screen. We explain the origins of ‘‘reciprocal lattice rods,’’ RHEED streaks, and Laue rings. We show how to calculate the streak spacing, and clarify the basic effect on the RHEED pattern of using a nonzero angle of incidence for the incident beam. Crystalline nets, reciprocal nets, an...

Surface electron-diffraction patterns of β-FeSi2 films epitaxially grown on silicon

Semiconducting β‐FeSi2 is drawing much current research interest because of hoped‐for silicon‐based optoelectronics applications. The study of heteroepitaxial film growth on silicon depends heavily upon several transmission and reflection electron‐diffraction techniques. Because of the complicated crystal structure of this material, the possibility of competing heteroepitaxial relationships, the propensity for formation of epitaxial variants by rotation twinning, and the uncertainty in the crystalline surface nets, the analysis of experimental diffraction patterns is complicated. A theoretical reference for a number of fundamental electron‐diffraction patterns is provided and they are illustrated with a broad range of experimentally obtained patterns from the surfaces of epitaxial films. In situ transmission reflection high‐energy electron diffraction (RHEED) (transmission electron diffraction with conventional RHEED instrumentation), from rough but epitaxial films, is of great utility and quite feasible ...

Reaction between SiC and W, Mo, and Ta at elevated temperatures

The stability of W, Mo, and Ta in contact with single‐crystal β‐SiC at elevated temperatures has been investigated using Auger sputter profiling. All three metals were found to form a thin‐mixed layer of metal carbide and silicide upon metal deposition at room temperature. This layer is thought to be the result of surface defects which weaken the Si—C bonds and allow a low‐temperature reaction to occur. Upon heating, the Ta readily reacts with the SiC substrate and forms a mixed layer of Ta carbide and silicide at annealing temperatures as low as 400 °C, however, the W/SiC and Mo/SiC systems are stable and change very little after annealing at 850 and 800 °C, respectively.

Epitaxial orientation and morphology of β‐FeSi2 on (001) silicon

Epitaxially aligned films of β‐FeSi2 were grown on (001) silicon by reactive deposition epitaxy (RDE), molecular‐beam epitaxy (MBE), and solid‐phase epitaxy (SPE). Although the matching crystallographic faces, FeSi2 (100)/Si(001), remained invariant throughout this study, two different azimuthal orientations predominated, depending on the deposition mode and growth temperature. Films with the FeSi2[010]∥Si〈110〉 orientation (grown by RDE at typically 500 °C) were of a genuine large‐area single‐crystal structure; however, the surface morphology was rough due to islanding which always preceeded the formation of a continuous film. Films of the alternative azimuthal orientation FeSi2[010]∥Si〈100〉 (which were grown by SPE at typically 250 °C or by MBE at temperatures as low as 200 °C on top of an SPE‐grown template) have a much smoother surface morphology. However, there was some loss of purity in the epitaxial alignment at these extremely low temperatures. Excellent RHEED (reflection high‐energy electron diffr...

Optical and electrical properties of semiconducting rhenium disilicide thin films

Abstract Thin polycrystalline films of rhenium disilicide, a narrow band gap semiconductor, were prepared by furnace reaction of thin films of rhenium with silicon wafers. The band structure of the material was probed by measuring the normally incident spectral transmittance and reflectance, together with the temperature dependence of the conductivity. The band gap appears to be indirect with a magnitude of slightly less than 0.12 eV. A direct transition at approximately 0.36 eV was also observed.

Epitaxial tendencies of ReSi2 on (001) silicon

ReSi2 thin films were grown on (001) silicon wafers by vacuum evaporation of rhenium onto hot substrates in ultrahigh vacuum. The preferred epitaxial relationship was found to be ReSi2 (100)/Si(001) with ReSi2 [010]∥Si〈110〉. The lattice matching consists of a common unit mesh of 120 A2 area, and a mismatch of 1.8%. Transmission electron microscopy revealed the existence of rotation twins corresponding to two distinct but equivalent azimuthal orientations of the common unit mesh. Although the lateral dimension of the twins is on the order of 100 A, MeV He+ backscattering spectrometry revealed a minimum channeling yield of 2% for a ∼1500‐A‐thick film grown at 650 °C. There is a very high degree of alignment between the ReSi2 (100) and the Si(001) planes.

Reflection high-energy electron diffraction patterns of carbide-contaminated silicon surfaces

Carbon contamination of silicon surfaces is a longstanding concern for growers of thin films who utilize silicon wafer substrates. This contamination often takes the form of epitaxial β‐SiC particles which grow after the decomposition of adsorbed carbon‐bearing molecules, and the subsequent reaction of the freed carbon with the silicon substrate. Positive identification of such SiC contamination is possible via reflection high‐energy electron diffraction (RHEED). To provide a complete demonstration and analysis of the relevant RHEED patterns, we prepared within a ‘‘silicon molecular beam epitaxy’’ system carbide‐contaminated silicon surfaces using procedures intended to foster such contamination. With conventional RHEED instrumentation, we obtained transmission electron diffraction patterns which resulted from the passage of the RHEED electron beam laterally through the SiC particles. Comparison with theoretically predicted patterns positively identifies the β‐SiC phase and shows that the particles are ep...

ReSi2 thin‐film infrared detectors

Two types of thin‐film infrared‐sensing devices have been investigated using the narrow band‐gap semiconductor, rhenium disilicide (Eg∼0.1 eV). These are the ReSi2/n‐Si heterojunction internal photoemission (HIP) detector and the ReSi2 thin‐film photoconductor. The HIP device was found to be rectifying and to obey a Fowler‐type law with a long‐wavelength cutoff of ∼2.1 μm (0.59 eV) at room temperature. In hopes of approaching the fundamental limit for a ReSi2‐based photonic detector, ∼12 μm (0.1 eV), the ReSi2 photoconductor was explored. Indeed, the spectral response (measured at 10 K) of the ohmic photoconductor was found to extend to 6 μm (the present limit of our measurement equipment), with no indication of a detection cutoff.

Two pseudobinary semiconducting silicides : RexMo1-xSi2 and CrxV1-xSi2

Two groups of thin‐film samples were grown on silicon wafer substrates of compositions spanning the entire range of the ternary disilicides: RexMo1−xSi2 and CrxV1−xSi2. In each case, the lattice parameters vary smoothly with composition. The optical and electrical properties of the films suggest that when molybdenum is added to semiconducting ReSi2 and when vanadium is added to semiconducting CrSi2, the forbidden energy gap in each case decreases smoothly to zero.