A. G. Cullis

An experimental and theoretical study of the formation and microstructure of porous silicon

Abstract We report a systematic study of the formation and properties of porous silicon formed by anodising silicon under a wide range of conditions. The current-voltage characteristics of the silicon-hydrofluoric acid system are presented. The detailed microstructure of the two types of porous silicon that form depending on the dopant concentration in the silicon, were determined by cross-sectional transmission electron microscopy. A theory for the formation of porous silicon is proposed which accounts for the dependence of the microstructure on the anodising conditions. The formation of porous gallium arsenide, analogous to porous silicon, is reported for the first time.

Microstructure and formation mechanism of porous silicon

A systematic study is presented of the effects of silicon dopant type, resistivity, current density, and hydrofluoric acid concentration on the formation and properties of porous silicon. Cross‐section transmission electron microscopy revealed the presence of two distinct microstructures. The structure formed is determined by the doping level with the transition occurring near degeneracy. A model of the anodisation process is presented which is based on the semiconducting properties of the material and which explains the formation of the two different types of porous structure observed.

The characteristics of strain-modulated surface undulations formed upon epitaxial Si1−xGex alloy layers on Si

Abstract When strained, continuous Si 1- x Ge x alloy layers are prepared by epitaxial growth on Si substrates, the growth surface can become nonplanar. In the present work, combined transmission electron microscope and atomic force microscope studies are employed to reveal the detailed nature of the surface ripples and undulations which form. Under specific ranges of growth conditions, crystallographically aligned, interlocking ripple arrays are produced. TEM contrast studies demonstrate that well-defined, oscillatory strain variations accompany these ripple structures, the presence of which is shown to be associated with partial elastic strain-relief and lowering of the energy of the strained-layer system. It is also demonstrated that the upper surface of a Si cap deposited on such a layer rapidly becomes planar with increasing thickness, this being likely to result from the reduction in surface energy so achieved.

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.

The preparation of transmission electron microscope specimens from compound semiconductors by ion milling

Abstract It is shown that transmission electron microscope specimens prepared from compound semiconductors by conventional argon ion milling may contain serious artefactual strucutures. The nature of these artefacts is characterised and correlated in detail with specific features of the ion milling process. Results obtained using argon ion milling, for a number of semiconducting materials over a range of milling conditions, are compared with those obtained using alternative ion species. These comparative results demonstrate the great improvement in specimen structural quality which may be achieved by combining a logical selection of ion species with a careful choice of ion milling conditions. Full information is given on specific processes for producing thin specimens of high structural quality from a wide range of compound semiconductors.

A model for heterogeneous growth of Si1−xGex films from hydrides

Growth rates for Si1−xGex films (0≤x≤0.19) have been measured between 610–750 °C using low pressure H2/SiH4/GeH4 mixtures and at different temperatures these rates show different dependencies on composition x. A model attributes this complex behavior to competition between an increasing rate for desorption of surface hydrogen and a decreasing sticking probability for the reactive hydrides as x increases. The latter effect is explicitly reported for the first time. GeH4 is found to be ∼4.7 times more reactive than SiH4, and relative surface hydrogen coverages on Si and Si0.87Ge0.13 films measured by secondary ion mass spectroscopy are compared with the model.

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.

Dependence of trapping and segregation of indium in silicon on the velocity of the liquid‐solid interface

The segregation phenomena of In‐implanted Si have been observed following the melting and epitaxial regrowth of surface layers by pulsed ruby laser irradiation. The velocity of the liquid‐solid interface on recrystallization has been varied from 1.8 to 5.2 m/s in two independent ways. Indium is observed to be trapped on substitutional sites, in excess of solid solubilities, or segregated in a thin surface layer. Trapping increases and segregation decreases as the interfacial velocity is raised. The complete depth profiles can be fitted with unique interfacial segregation coefficients which are velocity dependent. The material that has been segregated to the surface shows cell structure of approximately 100 A diameter arising from lateral segregation due to constitutional supercooling. The cells are not present at velocities of 5 m/s. The critical dependence of In trapping and segregation on velocity in the range 2–5 m/s is interpreted in terms of interfacial residence times.

Growth morphology evolution and dislocation introduction in the InGaAsGaAs heteroepitaxial system

Abstract The present work examines in detail heteroepitaxial In x Ga 1 − x As alloy layers on GaAs by use of complementary transmission electron microscopy and atomic force microscopy. The characteristics of the low In x -value, smooth growth regime are established in terms of surface step configurations. Progressively increasing irregularities in step fronts and monolayer island formation with increasing In concentration in the alloy are linked with the transition to undulating growth as an x -value of 0.25 is approached. The evolution of high x -value roughened layers is studied and the structure changes with increasing film thickness and In content are determined. The manner in which final ripple arrays evolve from isolated islands is described and the stress interaction between islands is highlighted. The magnitude of the periodic elastic stress field which accompanies the formation of the ripple structures is microscopically measured and is shown to yield essentially complete misfit relief within the ripple crests. The increased stress present at ripple troughs is shown to lead to misfit defect source behaviour, which is expected to be of wide-ranging importance for defect generation in strained, undulating epitaxial films in general.