G. M. Williams

Molecular beam epitaxial growth of high structural perfection, heteroepitaxial CdTe films on InSb (001)

Heteroepitaxial films on CdTe have been grown by molecular beam epitaxy (MBE) on InSb (001) orientation substrates at growth temperatures ≲220 °C. The structural perfection of these films has been explored by double crystal x‐ray diffraction techniques, which reveal that films of ∼1 μm thick grown at 150 and 220 °C are free of low‐angle grain boundaries and are exactly lattice matched to the InSb substrate. The lattice parameter mismatch is accommodated by misfit elastic strain.

The growth of metastable, heteroepitaxial films of α-Sn by metal beam epitaxy

Abstract Heteroepitaxial films of α-Sn have been prepared for the first time. The films were grown in an MBE system by direct condensation of a beam of Sn atoms onto clean, ordered (001) surfaces of InSb and CdTe held at T ≈25°C. In-situ RHEED studies indicate that the films grow by a two-dimensional layer mechanism with a (2×2) surface reconstruction throughout growth. Above a film thickness of ∼0.5 μm nucleation and growth of β-Sn occurred. β-Sn films of ≲0.5 μm in thickness are a substrate-stabilized metastable phase which undergoes a reversible α→β phase transformation at ∼70°C. The presence of uniaxial strain in the films has been confirmed by double-crystal X-ray diffraction measurements which reveal that the films have tetragonal symmetry as a result of in-plane compression imposed by the constraint of epitaxy. Ge-doping of the α-Sn films permits growth of films thicker than 0.5 μm and reduces the degree of uniaxial strain.

CORRELATION OF THE STRUCTURAL AND OPTICAL PROPERTIES OF LUMINESCENT, HIGHLY OXIDIZED POROUS SILICON

The light‐emitting properties of rapid thermally oxidized porous Si are studied by both photoluminescence and cathodoluminescence methods. The structure of the material is examined by transmission electron microscopy, while its oxygen content is determined by x‐ray microanalysis. These investigations show that crystalline Si nanostructures remain in the heavily oxidized porous material and account for its ∼750 nm red photo‐ and cathodoluminescence. The work demonstrates that the previously speculated possible importance of either Si‐based amorphous phases or the interesting material, siloxene, in this regard is unrealistic. Furthermore, it is shown that the luminescence properties of silicon oxides are of paramount importance in interpreting the many additional (shorter wavelength) emission bands observed.

Heteroepitaxial growth of InSb on (100)GaAs using molecular beam epitaxy

Molecular beam epitaxy has been used to grow thin (0.5 μm<t<10 μm) InSb epilayers on (100) GaAs substrates. Reflection high‐energy electron diffraction studies indicate that the early stages of layer growth involve three‐dimensional nucleation and the formation of a nonpseudomorphic structure. High‐resolution electron microscopy studies of the interface are reported for the first time and directly confirm that the large lattice mismatch (14.6% at room temperature) is accommodated by the generation of misfit dislocations. Nevertheless, the structural quality of the InSb is observed to improve dramatically with increasing thickness. Detailed secondary‐ion mass spectrometry measurements also demonstrate that there is no large‐scale interdiffusion of constituent elements at the interface. Finally, electrical measurements show the InSb to be p type and comparable with homoepitaxial material.

Chemical and electronic structure of InSb‐CdTe interfaces

The microscopic interactions at heterojunctions formed between cleaned surfaces of InSb and CdTe have been investigated by low‐energy electron diffraction and soft x‐ray photoelectron spectroscopy. Layers of CdTe have been deposited on 1×1 (110) cleaved InSb and on c(2×8) (100) sputter cleaned and annealed surfaces, for various substrate temperatures. The valence‐band offset has been measured and compared with theoretical predictions for layers deposited on room‐temperature substrates. For layers deposited onto substrates at elevated temperatures typical of those employed in molecular beam epitaxial growth, the interface is complex and consists of a region rich in indium and tellurium, presumed to be indium telluride. The thickness of this layer is temperature dependent and may be several tens of angstroms.

Molecular‐beam epitaxy of (100) InSb for CdTe/InSb device applications

Interest in InSb and InSb‐based heterostructures has recently been renewed in view of their important potential applications in infrared, logic, and novel quantum‐well devices. Our work to date has concentrated on the growth of CdTe/InSb multilayer structures in which the properties of the InSb constituent layers have a very significant influence on subsequent device performance. The present paper describes data obtained during a systematic investigation of the growth, using molecular‐beam epitaxy (MBE), of (100) InSb homoepitaxial layers, specifically for CdTe/InSb device applications. Modulated‐beam mass spectrometry experiments have shown that polycrystalline InSb can be used as an MBE source of antimony, and the properties of InSb epilayers grown using either elemental antimony or polycrystalline InSb as the group‐V source are compared. Cross‐sectional transmission electron microscope analysis indicates that very high structural quality layers can be produced and has also identified the mechanisms whi...

Identification of tellurium precipitates in cadmium telluride layers grown by molecular beam epitaxy

The low‐temperature growth of crystalline layers of CdTe on InSb substrates by molecular beam epitaxy has been investigated using transmission electron microscopy. It has been shown that while layers grown at substrate temperatures of ∼180–200 °C are generally single crystal and of good structural quality, layers grown at temperatures of ∼150 °C, although initially single crystal, rapidly become polycrystalline with increasing layer thickness. The mechanism responsible for this growth breakdown at low temperatures is shown to be the formation of second phase precipitates. These precipitates have been identified using diffraction analysis as the low‐pressure, hexagonal phase of elemental Te.

Structure of hexagonal and cubic CdS heteroepitaxial layers on GaAs studied by transmission electron microscopy

The initial heteroepitaxial growth and structure of thin CdS layers on GaAs has been studied by conventional and high‐resolution transmission electron microscopy. The work highlights the dependence of CdS crystal type on GaAs substrate orientation. Wurtzite‐structure CdS is formed on (111)A GaAs and it is found to relieve misfit stresses by the introduction of interfacial defects, often associated with steps at the interface. Sphalerite‐structure CdS is produced by initial growth on (001)GaAs and, in this case, misfit stresses are more slowly relieved, first with the formation of an asymmetrical array of interfacial dislocations and inclined stacking faults.

Interfacial studies and electrical characterisation of heteroepitaxial InSb on GaAs (100) grown by MBE

The growth by molecular beam epitaxy of InSb layers (0.5 < t < 10 μm) on undoped GaAs (100) substrates has been undertaken. In situ reflection high energy electron diffraction and Auger electron spectroscopy have been used to investigate the initial stages of InSb layer nucleation. The GaAs/InSb interface has also been investigated ex vacuo using transmission electron microscopy and secondary ion mass spectrometry. The data reveal a maximum interface width of 25 nm and indicate that the large mismatch (14.6% at room temperature) is accommodated by a network of misfit dislocations at the epilayer substrate interface. Nevertheless, transmission electron microscopy studies and data obtained from double crystal X-ray rocking curves show that the structural quality of the heteroepitaxial InSb improves considerably with increasing layer thickness. The electrical and morphological properties of the InSb heteroepitaxial layers have also been assessed using 77 K Hall, 2 K photoluminescence and Nomarski optical interference microscopy. All layers grown have exhibited p-type carrier concentrations (p ∽ 2 x 1015 cm-3), a value somewhat higher than observed for the corresponding homoepitaxial material.

Growth of CdTe‐InSb multilayer structures on (100) InSb substrates using molecular beam epitaxy

Molecular beam epitaxy has been used to grow multilayer structures of CdTe/InSb on (100)InSb substrates. To minimize interdiffusion effects, a particularly low growth temperature was used. This study presents the first transmission electron microscope and secondary‐ion mass spectrometer investigations of this multilayer system. The work clearly demonstrates that the multilayers have high structural quality and that there is no large scale interdiffusion of the matrix elements. Roughening is observed at the ‘‘InSb grown on CdTe’’ interface, although only relatively minor undulations are present at the inverted interface. A possible explanation for this effect is described.