J.M. Corbett

Determination of elastic strains in epitaxial layers by HREM

Abstract A new technique is presented to directly measure strains in epitaxial systems from high-resolution electron microscope (HREM) images. This method involves the calculation of the cumulative sum (CUSUM) of deviations from an average lattice fringe spacing value. Even when the variation in individual lattice fringe spacings is large compared to the difference in fringe spacing due to strain, the CUSUM method is capable of providing reliable strain determinations. The CUSUM approach was applied to three samples of semiconductor In 1−x Al x Sb/InSb (0≤x≤0.5) bilayer and superlattice systems in [11¯0] zone-axis projection. It was found that the epitaxial strains obtained from the HREM CUSUM method agreed with the bulk X-ray diffraction values when the sample thicknesses were on the order of 80–100 nm. Thinner specimens, on the order of 10–20 nm, displayed significant surface relaxation effects.

ELASTIC STRAIN DETERMINATION IN SEMICONDUCTOR EPITAXIAL LAYERS BY HREM

Abstract A new technique that is independent of image contrast and robust to the presence of experimental noise is presented to analyze strains from high resolution electron microscopy (HREM) lattice images. This approach involves the analysis of the cumulative sum of deviations (CUSUM) in lattice-fringe spacings from a target value. The effects of surface roughness at an interface, and surface relaxation due to transmission electron microscope (TEM) sample preparation are discussed. The CUSUM method was applied to two simulated and two experimental HREM images of semiconductor strained layer structures in [110] zone axis projection. The CUSUM technique was able to accurately reproduce the strain profiles from the simulated and experimental images in all cases studied except for the component of the strain in the slip direction (exx) of an edge dislocation in a simulated image. In this case, the strain field near the core appeared hemispherical rather than lobed as expected.

Transmission electron microscopy characterization of InAlSb/InSb bilayers and superlattices

Abstract High quality InAlSb/InSb bilayers and superlattices have been grown by magnetron sputter epitaxy and the physical structure has been characterized by transmission electron microscopy. It was found that single layers of InAlSb, whose thicknesses greatly exceeded the equilibrium critical thickness, could be grown coherently on (001) InSb for Al concentrations approaching 13–15%. Also, it was observed that planar defects increased in density for both the bilayer and superlattice structures as the Al concentration increased.

Determination of the symmetry of charge-density-wave modulations in TaTe4 by HREM

Abstract TaTe4 is a quasi-one-dimensional crystal which at room temperature displays a periodic lattice distortion associated with charge-density waves [Boswell et al., J. Phys. C16 (1983) 1067]. The structure of TaTe4 has been studied in detail over the past few years, both by X-ray and electron diffraction and by electron microscopy. The literature, however, contains conflicting reports concerning the symmetry of the superlattice modulations. Eaglesham et al. [J. Phys. C18 (1985) 1] have investigated TaTe4 by CBED and determined the superstructure space group to be P4/mcc. Sahu and Walker [Phys. Rev. B32 (1985) 1643], on the other hand, predicted that the space group should be P4/ncc, on the basis of Landau theory calculations. Recently, Bronsema et al. [Acta Cryst. B43 (1987) 305] investigated the structure by X-ray diffraction and concluded that the symmetry is P4/ncc [4]. In the present study the modulations were investigated by high resolution electron microscopy (HREM). Images recorded in the [210] orientation confirm the absence of a simple mirror plane normal to the c- axis in the tetragonal structure. A comparison of experimental and computed images supports the model for the modulated structure deduced by X-ray diffraction.

Investigation of order in AgxNbS2 by electron diffraction

Abstract Electron diffraction investigation of single crystals of Ag x NbS 2 (x in the range 0.2 to 0.4) has shown that a variety of diffuse scattering and superlattice patterns is observed. The Ag is mobile in the crystals and some regions were observed where the Ag content had decreased to almost zero. In such regions, strong diffuse scattering was observed above a critical temperature, and below this temperature diffraction patterns corresponding to long-range orthorhombic superlattices were observed. A possible explanation of these observations is presented and discussed.

The epitaxy of gold on sodium chloride with a continuous carbon intermediate film

Abstract There are conflicting reports in the literature as to whether epitaxial films can be grown over intermediate thin continuous carbon layers. A series of experiments was carried out on the NaClAu system in which the effect of intermediate carbon films on the epitaxy of the gold was studied. For a very limited thickness range of carbon intermediate layers, good epitaxial gold films were formed. The corresponding carbon films were stripped, and examination in the electron microscope showed that the films were continuous and free from holes greater than 10 A in diameter. A model based on “pseudomorphic islands” in the carbon film is proposed to account for the observations.

Elastic strains in epitaxial vacuum-deposited germanium on silicon

Abstract The lattice parameters of epitaxial Ge films vacuum-deposited on chemically thinned (111) Si crystals held at 700°–900°C have been determined by selected are electron diffraction. The films were deposited at 12 A/s and their average thicknesses ranged from 25–600 A. In each deposit the Ge parameter was found to be contracted significantly from the bulk value. Contractions ranged from 0.2 to 2% for films prepared at substrate temperatures ranging from 700°–900°C, but were relatively insensitive to the amount of material deposited. In the case of the 25 A deposit prepared at 900°C, the lattice parameters of individual islands in the deposit were found to be contracted from 1.4% for an island on a relatively thick portion of the substrate to 4.2% (complete accommodation of the atomic misfit) for islands near the edge of the polished hole in the substrate. In the latter area, the GeSi bicrystal regions were found to be symmetrically bent, with radii of curvature of about 1 μ. The results are discussed in terms of the theory of crystal interfaces.

A low energy ion beam assisted deposition technique for realizing iso-type SiGe/Si hetero-interface diodes

Abstract Amorphous SiGe/Si iso-type diodes have been successfully fabricated at a low temperature of about 250°C, using a low energy ion-beam assisted vacuum co-evaporation technique. The physical characteristics of the prepared structure is studied using RBS, SIMS and high resolution TEM. The electrical characteristics of the iso-type diodes are studied at different ambient temperatures, which show an ideal behavior over a significant range of temperatures (90–200 K). An energy barrier of 0.38 V is extracted from the current-voltage study.

Study of in-situ surface cleaning for Si and SiGe epitaxy on Si with a novel ion beam vapor/assisted deposition technique

Abstract Ion beam vapor deposition is a new technique to grow Si and SiGe layers on Si substrates at low temperatures. The in situ surface cleaning prior to the deposition is a crucial step in the epitaxial growth of Si and SiGe films and is achieved by Ar ion bombardment with substrates kept at ambient temperature. A high temperature annealing (800 °C) is needed to repair the damage caused by this bombardment. We studied the effects of ion beam energy and the substrate temperature during the in situ cleaning on the quality of the grown films. An ion beam energy of 150–200eV is found to be sufficient to clean the surface for epitaxial growth. While the films deposited on properly cleaned surfaces are epitaxially grown, the inadequately cleaned surface leads to the formation of polycrystalline layers especially at low substrate temperatures.

ION BEAM VAPOR DEPOSITION FOR SI EPITAXY AT LOW SUBSTRATE TEMPERATURES

We report the results of an investigation to grow thin Si films on Si substrates at low substrate temperatures using ionized SiH4 gas generated with a Kaufman type ion gun. This investigation shows island‐growth at higher substrate temperatures (500–700 °C) in the form of square‐based pyramids. By lowering the substrate temperature to 300 °C, we were able to achieve a planar growth with a growth rate of 8 A per minute. Transmission electron microscopy (TEM) is used to study the crystalline quality of the samples prepared at different temperatures. The study by high resolution TEM of the sample grown at 300 °C indicates the presence of microtwins in the Si epi‐layer.