Morphological transformations of thin heteroepitaxial films

The features of two-and three-dimensional growth of thin heteroepitaxial films are analysed with allowance for the energy of misfit dislocation (MD) networks. Criteria for distinguishing between Frank-van der Merwe, Stranski-Krastanov and Volmer-Weber growth modes are suggested. For the Stanski-Krastanov mode the transition from layer to island growth is considered to be associated with an increase in the interface energy owing to MD formation. The critical thickness of the pseudomorphic film in the case of the formation of dislocated islands is much less than that in the case of the introduction of MDs into the continuous film and depends on an average migration length of the adatoms. Some theoretical results are compared with experimental results for germanium films obtained by molecular beam epitaxy on Si(001) and Si(111) substrates.     

Vapor growth of thin heteroepitaxial InP films on CdS

InP was heteroepitaxially deposited onto CdS single-crystal substrates by chemical vapor deposition using phosphine (PH₃), HCl and indium as reactants. Single-crystalline films were deposited at substrate temperatures as low as 450°C. Scanning Auger microscopy shows that the films are InP and that formation of solid solutions between the CdS and the InP is minimal. The as-grown films are n type with residual donor densities of 4 × 10¹⁶–4 × 10¹⁷ cm^-3.     

Electrical properties of electron-gun-evaporated InAs thin films

Measurements of the resistivity and the Hall coefficient versus the temperature (77–300 K) and the magnetic field (0–11 kG) were performed on InAs thin films (100–1500 Å thick) obtained by vacuum evaporation from an electron gun and condensation onto glass substrates at room temperature. Analysis of the electrical results reveals the typical behaviour of disordered structures. The conductivity versus temperature curves exhibit an anomalous sharp variation which appears to separate two temperature ranges characterized by different conductivity slopes. Observation of the samples in a transmission electron microscope reveals the polycrystalline structure of the films. Electron spectroscopy for chemical analysis shows the sample to be non-stoichiometric with an excess of indium in the inner layers and the presence of indium and arsenic oxides at the surface of the films. Numerical analysis, performed with a multiparametric best-fit procedure, shows that the conductivity conforms to the percolation conductivity model but with an exponential temperature dependence, in accordance in the high temperature region with the more recent hypotheses regarding disordered structures.     

Phase composition of thin oxide films on InSb

Two methods of phase analysis for solids with complex compositions have been developed in our previous investigations.The first method is thermodynamic analysis which enables a phase diagram to be plotted and by means of this the spatial distribution of phases as a result of chemical interactions between two different solids at an interface may be predicted.The second method is cyclic voltammetry using carbon paste electrodes; with this method it is possible to analyse the phase composition of small objects, e.g. thin (about 0.01 μm) films. The limit of detection of phases by this method is about 10¹⁵ cm^-3, i.e. about two orders of magnitude lower than that of modern expensive Auger electron spectroscopy, electron spectroscopy for chemical analysis and other surface analysis methods.Both methods were applied to the InSb/native oxide system. Using the first method three alternative ternary In-Sb-O diagrams were constructed and the experimental data obtained with the second method permitted us to select the most probable diagram.Our results showed that thin (about 0.1 μm) native oxide films on InSb have multilayer structures in which each layer has a different phase composition. When the oxidation occurs under quasi-equilibrium conditions the phase compositions from the InSb surface to the top of the film are as follows: I, In₂O₃+Sb; II, In₂O₃+Sb₂O₃+Sb; III, In₂O₃+Sb₂O₄; IV, InSbO₄. However, when the In-Sb-O system is far from equilibrium during oxidation, unstable phase complexes such as Sb₆O₁₃+Sb₂O₃ or In₂O₃+Sb₂O₅ may be observed; films obtained by anodic oxidation of InSb in 0.1 M KOH are an example of this situation.     

Strain control and spontaneous phase ordering in vertical nanocomposite heteroepitaxial thin films

Two-phase, vertical nanocomposite heteroepitaxial films hold great promise for (multi)functional device applications. In order to achieve practical devices, a number of hurdles need to be overcome, including the creation of ordered structures (and their formation on a large scale), achieving different combinations of materials and control of strain coupling between the phases. Here we demonstrate major advances on all these fronts: remarkable spontaneously ordered structures were produced in newly predicted compositions, vertical strain was proven to dominate the strain state in films above 20 nm thickness and strain manipulation was demonstrated by selection of phases with the appropriate elastic moduli. The work opens up a new avenue for strain control in relatively thick films and also promises new forms of ordered nanostructures for multifunctional applications.     

Ionized impurity scattering in heavily doped p-type InSb thin films

Hall coefficient and d.c. conductivity measurements were made on p-type polycrystalline InSb films which were heavily doped with copper and zinc. The ionized impurity scattering limited mobility μ_IIS was estimated, taking into account diffused and grain-boundary scattering. A large difference was observed between the μ_IIS values for the copper- and zinc-doped films with free carrier concentrations greater than 10¹⁸ cm^-3, which suggests a breakdown of screened coulombic interactions.     

Recrystallization of InSb thin films obtained by cathode sputtering

Thin polycrystalline n-type InSb films were prepared by cathode sputtering. The Hall mobility of the films was about $\text{300 cm}{}^2\text{V}\text{s}$. When the films were recrystallized by melting in an argon atmosphere at normal pressure, an influence of the sputtering conditions on the properties of the recrystallized films was found. Room temperature mobilities of $\text{4000 cm}{}^2\text{V}\text{s}$ were achieved after recrystallization.     

A Monte Carlo investigation of growth and characterization of heteroepitaxial thin films

We investigate the growth of mismatched thin films by a kinetic Monte Carlo computer simulation and including a local photoemission model with reflection high-energy electron diffraction (RHEED) intensity for comparison. The strain is introduced through an elastic energy term based on a valence force field approximation. We describe an atomistic mechanism for dislocation nucleation during first stage of GaSb/GaAs (001) growth and in situ variations of photoemission current (PE) and RHEED intensity are reported. We have shown the formation of grooves corresponding to (111) facets, a precursor to the formation of misfit defects. The surface roughening and facetting by creation of grooves explain the absence of photoemission and RHEED oscillations in accordance with experimental observations [J.J. Zinck and D.H. Chow, J. Cryst. Growth, 175/176 (1997) 323, J.J. Zinck and D.H Chow, Appl. Phys. Lett. 66 (1995) 3524].     

Effect of thin intermediate-layer of InAs quantum dots on the physical properties of InSb films grown on (001) GaAs

In this study the formation of a semiconducting InSb layer, preceded by the growth of an intermediate layer of InAs quantum dots, is attempted on (001) GaAs substrate. From the analysis of atomic-force-microscopy and transmission-electron-microscopy images together with Raman spectra of the InSb films, it is found that there exists a particular layer-thickness of ~ 0.5 μm above which the structural and transport qualities of the film are considerably enhanced. The resultant 2.60-μm-thick InSb layer, grown at the substrate temperature of 400 °C and under the Sb flux of 1.5 × 10^− 6 Torr, shows the electron mobility as high as 67,890 cm²/Vs.     

Modeling of InSb and InAs whiskers growth

For InSb whiskers grown by the chemical transport reaction (CTR) method in the InSb-J₂ system the relation between whisker diameter (d), crystallization zone temperature (T_cr) and thermoprocessing time (t) is found in a certain range of these values. This relation is of practical importance when whiskers are grown for using as active areas of magnetic sensors. The dependence of the InSb and InAs whisker growth rate on the diameter is also considered, main kinetic parameters of the whisker crystallization are determined. In particular, the crystallization energy for InSb and InAs whiskers is equal to 63.9 and 147 kJ mol⁻¹, respectively, that confirms the whisker formation by CTR-method in accordance with the vapour-liquid-crystal mechanism.     

Compositional profile of heteroepitaxial InAs on GaAs substrates

The nature of the interface region between semiconducting heteroepitaxial layers and their substrates has been experimentally studied using Auger electron spectroscopy and energy dispersive X-ray analysis. InAs grown epitaxially on single-crystal semi-insulating GaAs was investigated since it has exhibited electrical properties superior to those of bulk InAs in spite of the considerable crystal lattice mismatch between InAs and GaAs.Findings indicate the existence of a graded composition (In_xGa_1-xAs) region of approximately 1500 Å thickness between the substrate and epilayer which is thought to play a major role in strain relief and, consequently, in determining the electrical characteristics of the resulting InAs epilayer.     

Stress reduction and electric properties of InSb thin films grown by metalorganic vapor phase epitaxy on sapphire substrates with an InAs buffer layer

InSb thin films were grown by metalorganic vapor phase epitaxy using an InAs buffer layer on sapphire (0001) substrates. The stresses and strains in InSb were controlled by the thickness of the InAs buffer layer, and it was found that with decreasing compressive stress in InSb, the crystalline quality and the electrical properties improved. The thermoelectric properties of InSb were assessed and it was found that the power factor of InSb with a thickness of 5 μm reached as high as 5.8 × 10⁻³ W/mK² at 600 K.     

Structure-property relationships in graphitic cast irons

Some techniques used to study fatigue of cast irons are discussed and used to study the structure-property relationships in graphitic cast irons. A wide range of cast irons have been assessed to give a better understanding of the effects of metallurgical structure and graphite morphology on the fatigue behaviour of these materials. It is concluded that the ‘true’ fatigue strength reduction factor is one of the most significant parameters by which to consider improvements in fatigue properties. Increasing the amount of spheroidal graphite is suggested as a method of improving the fatigue properties of CG cast irons and yielding alloys with great potential importance.     

Structure-property relationships in polyimide molecular composites

Rigid rod-like polyimide (PI_R) / flexible-chain polyimide (PI_F) molecular composites, with 20, 40, 50, 60 and 80 wt. % of the rigid component, were prepared and their structure and molecular dynamics were investigated, in relation to those in pure PI_R and PI_F. SAXS, DSC, DMA, TSDC and laser-interferometric creep rate spectroscopy (CRS) techniques were used for characterization of nanostructure, glass transition and sub-T _g relaxations at temperatures from 100 to 600–650 K. All the experiments indicated pronounced deviations from additivity in both nanostructure and dynamics of molecular composites. Mixing of the PI_R and PI_F components led in particular to a smaller nanostructure, down to formation of the nanoscale-homogeneous composite. Changes of the dynamic characteristics in two opposite directions and arising of large dynamic heterogeneity around T _g were observed as a result of confinement/constraining effects. Received: 19 November 2001 / Reviewed/Accepted: 7 December 2001     

Microstructure-mechanical and chemical behavior relationships in passive thin films

The passive films play an important role in corrosion and stress corrosion cracking of austenitic stainless steels. The current research investigates the relationship between alloy chemistry, microstructure, and mechanical behavior of passive films formed on 316, 304, and 904L stainless steels (SS). X-ray photoelectron spectroscopy and transmission electron microscopy were used to investigate the effect of alloy chemistry and microstructure constituents on the thin film fracture properties determined by nanoindentation tests. The analyses showed that fracture loads are directly related to the crystallography of the thin films. It was found that decreasing the ratio of iron to other metallic elements in the film led to an increase in the load required to fracture the film. It was also found that films grown on 304, 316, and 904L stainless steels were the cubic polymorph of Cr₂O₃, rather than the lower energy rhombohedral form. In the case of 904L SS the film formed as an epitaxial layer. In the other two cases it consisted of small crystalline islands in an amorphous matrix. A dichromate treatment of 316 SS decreased the iron content in the oxide film and increased the hardness. It also resulted in an epitaxial film.     

Temperature-dependent refractive index measurements of wafer-shaped InAs and InSb

An experimental method is introduced to measure the refractive index and its temperature dependence for wafer-shaped infrared materials over a continuous temperature range. Using a combination of Michelson interferometry, Fabry-Perot interferometry, and a temperature-controlled cryostat in a laser micrometer, refractive index values and their temperature coefficients can be measured for any specific temperature within a desired temperature range. Measurements are reported for InAs and InSb for a laser wavelength of 10.59 microm.     

Interfacial coupling in heteroepitaxial vertically aligned nanocomposite thin films: From lateral to vertical control

Very recently, vertically aligned nanocomposite (VAN) thin films have served as an intriguing platform to obtain significant insights of the fundamental physics and achieve novel functionalities for potential technological applications. In this review article, we have investigated the lattice mismatch and vertical interfacial coupling in representative VAN systems for probing strain engineering in the vertical direction. Systematic studies of ferroelectricity, low field magnetoresistance and magnetoelectric coupling in VAN architectures have been reviewed and compared. The enhancement and tunability of the physical properties are attributed to the effective strain-, phase- and interface- couplings in VAN films. In the end, important and promising research directions in this field are proposed, including understanding the growth mechanisms of VAN structures, and creating more effective couplings for enhanced functionalities and ultimate device applications.