Self-organization of three-dimensional lead telluride nanoislands under conditions close to thermodynamic equilibrium

Three-dimensional lead telluride (PbTe) nanoislands were grown on (111)BaF₂ substrates by hotwall epitaxy (HWE) from vapor phase under conditions close to thermodynamic equilibrium and their surface morphology was studied by atomic force microscopy in various growth stages, including the initial stage of nucleation and the subsequent evolution of the size and shape of nanoislands. The distributions of island dimensions in the samples grown under various thermodynamic conditions were statistically analyzed. It is shown that the proposed HWE method ensures the formation of dense (∼8 × 10¹⁰ cm⁻²) self-organized arrays of PbTe quantum dots with parameters comparable with those of the quantum dots of the same material grown by molecular beam epitaxy according to the Volmer-Weber mechanism.     

Growth Kinetics and Microstructure of PbTe Films Produced on Si and BaF<Subscript>2</Subscript> Substrates by a Modified Hot-Wall Method

Lead telluride films have been grown on Si (100) and BaF₂ (100) substrates by a modified hot-wall method using a graphite reaction chamber. According to X-ray diffraction, X-ray microanalysis, and scanning electron microscopy characterization results, the average growth rate of PbTe films having compositions within the homogeneity range of lead telluride increases with increasing lead vapor partial pressure and decreases with increasing tellurium vapor partial pressure, independent of the nature of the substrate. The rate of PbTe film growth has been shown to be maximal in the initial stage of the process and decrease monotonically over time, independent of the nature of the substrate. Independent of the growth time, the average growth rate of the PbTe films on the Si (100) substrates is considerably higher than that on the BaF₂ (100) substrates. Reflection high-energy electron diffraction data indicate that the texture of the PbTe films on Si (100) corresponds to the substrate orientation and that the misorientation angle of the mosaic blocks does not exceed 20°. On the BaF₂ (100) substrates, we observe epitaxial PbTe film growth with the orientation relationship (100), [011] PbTe ║ (100), [011] BaF₂.     

Photoabsorption properties of β-FeSi2 nanoislands grown on Si(111) and Si(001): Dependence on substrate orientation studied by nano-spectroscopic measurements

Photoabsorption properties of β-FeSi₂ nanoislands epitaxially grown on Si(111) and Si(001) have been discussed using photoabsorption nano-spectroscopy based on scanning tunneling microscope. The obtained spectra exhibit clear features around 0.86-0.91 eV and around 0.71-0.74 eV, which are explained as a direct and an indirect photoabsorption edge of β-FeSi₂, respectively. We also observed a blue shift of spectrum obtained from β-FeSi₂ nanoislands on Si(111) substrates, compared to those on Si(001) substrates. We attributed the dependence on Si-substrate orientation not to a quantum confinement effect but to an effect of elastic strain in the? β-FeSi₂ nanoislands epitaxially grown on the substrate.     

Epitaxial lead chalcogenide films: Orientation on substrates with the fluoride structure

Epitaxial films of the lead chalcogenides on (111)-oriented CaF₂, SrF₂ and BaF₂ substrates are azimuthally rotated by $\text{(2n ? 1)π}\text{3}$ relative to the substrate, e.g. (111)∥(111) but [10$\text{1}$]∥[$\text{1}$01]. It is shown that a model based on nearest and next-nearest neighbor interactions between the film and its substrate (over a sequence of five atomic (111) layers) leads to an interface structure that accounts for the experimentally observed orientation.     

Single-crystal heteroepitaxial growth of PbxSn1−xTe films on germanium substrates by r.f. sputtering

Single-crystal heteroepitaxial growth of Pb_xSn_1?xTe films was achieved by r.f. sputtering deposition onto single crystals of germanium (both 〈100〉 and 〈111〉 orientations), although there is a large mismatch (～12%) in the lattice parameters between the grown film and the substrate.The structures of the r.f. sputtered films were investigated by high energy electron diffraction microscopy, electron surface replicas and scanning electron microscopy.Differences in the substrate temperature needed for the epitaxial growth on 〈100〉 or 〈111〉 orientations are attributed to the tendency of Pb_xSn_1?xTe films to grow parallel to the (100) plane, which is the cleavage plane of the NaCl structure. This causes a mixed (100)-oriented phase in addition to the (111) orientation expected from the symmetry of the substrate surface in the case of (111) germanium substrates.Examples of the effects on the heterogeneous nucleation of the substrate temperature and deposition rate are shown for both orientations.The first stages of growth (initial reactions with the substrates) and the thickness and concentration profiles were investigated by ion beam backscattering analysis.Channelling techniques confirmed the high degree of order in the single- crystal structure of films deposited on large areas (～1 cm²).     

Substrate dependence of structural and transport properties of R.F. sputtered pbte films

The nature of the substrates has a dramatic influence on the properties of PbTe films grown by R.F. sputtering technique. We have studied the structural and transport properties of PbTe films for the substrates NaCl, CaF₂, BaF₂ and Al₂O₃. A correlation between crystallographic properties and mobility has been found. In particular the crystallographic order in the plane of growth of the film has been shown to have a strong influence on mobility.     

PbTe films grown by hot wall epitaxy on sapphire substrates

A ‘hot wall epitaxy’ is applied to grow PbTe thin films on sapphire substrates with BaF₂ buffer layer deposited by molecular beam epitaxy (MBE). The microstructural and strain state characteristics of PbTe layer were examined with high resolution X-ray diffraction techniques. The epilayer is composed of two (111)PbTe‖(0001)Al₂O₃ epitaxially oriented domain variants. The domains are azimuthally rotated and their interfacial directions relative to the substrate are [011?]PbTe‖Al₂O₃ and [1?54?]PbTe‖Al₂O₃, respectively. Another possible alignment of the domain variant corresponds to PbTe‖Al₂O₃ orientation. The strain state analysis of PbTe layer points to its relaxation via domain formation and high dislocation density generation in the lattice. Despite the domains formation the measured mobility of electron carriers is approximately 1600 cm²/V s and 30 000 cm²/V s at 300 K and 77 K, respectively. The theoretical analysis of the measured electrical properties indicates that the scattering by acoustic and optical phonons is the factor affecting the conduction process.     

Epitaxial growth of CdS on ionic substrates

Transmission electron microscopy has been used to study the structure of films of CdS evaporatedin vacua in the 10^–5 torr range on to (100) cleavage faces and cut and polished (110) and (111) faces of NaCl, and also on to (111) cleavage faces of BaF₂, (111) substrate faces were found to produce wurtzite structure (hexagonal) films with great structural perfection over wider ranges of epitaxial growth temperature than (100) substrate faces. The (100) and (110) substrates produced sphalerite structure (cubic) films. Electron beam evaporation and generally clean growth conditions were found to produce good quality films at low substrate temperatures. The films were in general free of any included grains and the diffraction patterns free of satellite spots. 111 or 10¯10 streaking was present in the diffraction patterns, however, except in the case of films grown on BaF₂ above 170° C, and on NaCl (111) above 250° C. These films were also free of planar defects and only contained of the order of 10¹⁰ dislocations per cm².     

Selective growth of β-SiC whisker on a patterned Si (111) substrate for a field emission device

β-SiC whiskers were grown via a vapor-solid (VS) reaction on two types of substrates; bare Si (111) and SiO₂-coated Si (111). Different growth behaviors of β-SiC whiskers were observed for each substrate. β-SiC whiskers grew only on the bare Si substrate, not on the SiO₂-coated Si surface. Therefore, β-SiC whiskers could be selectively grown along the patterns on the SiO₂-coated Si substrate. The turn-on field and the maximum current density of the specimens using selectively grown whiskers along the patterns on the SiO₂-coated Si substrate were 2.0 V/μm and 1.01 mA/cm², respectively. The converted curve of I-V characteristics of β-SiC whiskers by the Fowler-Nordheim equation maintained a linear slope.     

Characterization of size-quantized PbTe thin films synthesized by an electrochemical co-deposition method

Size-quantized thin films of PbTe were electrodeposited on Au (1 1 1) substrates using a practical electrochemical method, based on the simultaneous underpotential deposition of Pb and Te from the same solution containing ethylenediamine tetraacetic acid, Pb²⁺, and TeO₃²⁻ at a constant potential. These thin films were characterized by X-ray diffraction (XRD), scanning tunneling microscopy (STM), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS), and reflection absorption-FTIR (RA-FTIR). AFM, STM, and XRD results indicate that the growth of PbTe thin films follows the nucleation and two dimensional growth mechanism, resulting in high crystalline films of PbTe (2 0 0) in cubic structure, which was grown at a kinetically preferred orientation on Au (1 1 1). The EDS analyses of the films reveal that Pb and Te are present in an atomic ratio of approximately 1:1. The quantum-confined effect of the PbTe thin films are confirmed by the RA-FTIR measurements.     

Cu films formed simultaneously on (111) and (100) NaCl

Copper films were deposited simultaneously in high vacuum on three different monocrystalline NaCl substrates: evaporated (111) NaCl on mica, evaporated (100) NaCl and air-cleaved (100) NaCl. The occurence and microstructure of monocrystalline or polycrystalline copper films were determined by transmission electron microscopy and diffraction as a function of deposition rate R and substrate temperature T. When log R was plotted against 1/T, straight lines could be drawn separating the monocrystalline and the polycrystalline regions. Activation energies for the polycrystalline to monocrystalline transition of Cu films were calculated to be 1.48, 1.22 and 1.27 eV for the (111), evaporated (100) and air-cleaved (100) NaCl substrates respectively. It is shown that these results can be related to the atomistic theory of nucleation by Walton. Moreover, the results indicate that both the binding energy U between a single adatom and a growing oriented cluster and the atomic adsorption energy Q_ad on the substrate surface are proportional to the planar atom densities in the growing cluster and in the substrate surface respectively. It is further shown that while the activation energies for Cu films formed on the two (100) substrate surfaces are about the same, the actual epitaxial temperatures for the same R are significantly different.     

Nucleation and growth of sputtered and evaporated trigonal selenium films

Critical nucleation temperature experiments, scanning electron microscopy and light microscopy were used to study the nucleation and growth of trigonal Se films deposited by evaporation and sputtering on both gold and anodized aluminum substrates. The major nucleation and growth processes were shown to be the condensation of Se₂ molecules followed by their polymerization into Se_n chains, where n can be very large. In sputtering, these chains have a higher surface mobility, due in part to their higher residual kinetic energy, and thus they can form large trigonal Se crystallites. The high incident kinetic energies also allow the Se to break through the surface contamination of the substrate. This is important since the Se chain mobility should also be dependent on the surface free energy and on the nucleation site density of the substrate. Very adherent large-grained trigonal Se films were grown by sputtering on Au substrates. It was shown that nucleation sites for the large trigonal Se crystallites were not created by the incident flux but are inherent on the substrate. The high energy incident flux did, however, increase nucleation probability for the initial Se₂ condensation. For sputtering on anodized aluminum substrates the combination of low surface energy and high nucleation site density decreased chain mobility and thus prevented formation of large crystallites.     

Defect induced shortening of excess carrier lifetime in single period nipi structures

Abstract

High sensitivity for detecting optically generated excess carriers in the mid infrared spectrum is offered by nipi structures of PbTe. The built in nipi potential separates the electron–hole pairs in real space which leads to a high excess carrier lifetime τ. The temperature dependence of τ in PbTe single period nipi structures grown on BaF₂ substrates under repeated thermal cycling ranging from 300 to 12 K was studied in the present paper. The typical behaviour of the lifetime τ deviates from theory. This was attributed to the fact that the lattice mismatch and the different thermal expansion of BaF₂ and PbTe generate a temperature dependent strain which causes strain induced lattice defects. Besides misfit dislocations at the interface, threading dislocations are generated, which form channels for enhanced recombination of the excess carriers. A special temperature cycling technique is presented which can change the number and the arrangement of the strain induced defects and leads to higher excess carrier lifetimes τ at low temperatures.     

Effect of oxidation on thickness dependencies of thermoelectric properties in PbTe/mica thin films

The dependencies of the Hall coefficient R_H and Seebeck coefficient S at room temperature on the thickness (d=10-550 nm) of thin PbTe films prepared by the thermal evaporation in vacuum of n-type PbTe crystals with various charge carrier concentrations (10¹⁷-10¹⁹ cm⁻³) and their deposition on mica substrates were obtained. It was established that, with decreasing thickness of PbTe films, a transition from an electron to a hole conductivity occurs, and the inversion point shifts to smaller d values as the electron concentration in the target material increases. The experimental R_H(d) and S(d) dependencies are interpreted in terms of the acceptor states created by oxygen on the film surface. These dependencies were also calculated theoretically, taking into account the existence of two types of charge carriers (electrons and holes). The theoretical curves are found to be in good agreement with the experimental data.     

InSb/InAs quantum dots grown by liquid phase epitaxy

The first original results on the growth of quantum dots (QDs) in the InSb/InAs system by liquid phase epitaxy (LPE) are reported. The density and dimensions of QDs were studied by methods of scanning probe microscopy and atomic force microscopy. The surface density, shapes, and dimensions of LPE-grown nanoislands depend on the growth conditions (temperature, cooling rate, and solution melt-substrate contact time). In the interval of temperatures T = 420–445°C, homogeneous arrays of InSb quantum dots on InAs(100) substrates were obtained with an average height of H = 3.4 ± 1nm, a radius of R = 27.2 ± 7.5 nm, and a density of up to 1.9 × 10¹⁰ cm^?2.     

The growth behavior of GaN film on Si(111) substrate by an ion-beam assisted evaporation process

Highly oriented GaN thin films were grown on Si(111) substrate using an ion beam assisted evaporation method. Nitrogen ions, with a kinetic energy of about 40 eV, was supplied by a Kaufman ion source; and Ga vapor was supplied by thermal evaporation. The surface morphology of the nucleation layer, and the crystalline properties of 200–300 nm thick GaN epi-layer were investigated by atomic force microscopy, transmission electron microscopy, and X-ray diffraction. Film grown under a Ga-rich flux condition produced film growth behavior of large islands of hexagonal configuration. Crystallinity on such film, however, was of poorer quality than other films with smaller islands, grown under high nitrogen ion flux conditions. The full width at half-maximum of (0002) diffraction peak was measured at 52 arcminutes for the GaN epilayer single-stepwise grown at 660°C. Ion-enhanced decomposition occurred, causing no film formation at substrate temperatures above 710°C. Additionally, the effect of predeposition of a buffer layer on GaN crystallinity was investigated for surface roughness. AFM measurement revealed that the GaN buffer layer grown on Si(111) showed smooth surface under the relatively N₂⁺-sufficient condition. The introduction of thin GaN buffer layer, grown at 600°C under N₂⁺-sufficient condition, worked on reducing the lattice-mismatch stress and in-plane misorientation of grains, and thus enhancing the crystallinity of the two-stepwise grown GaN epi-layer. Characteristic behavior of GaN epi-layers, single or two stepwise grown on Si(111), show a type of granular (columnar) epitaxy.     

Misfit and thermally induced strain in epitaxial IV–VI semiconductor films

Epitaxially grown single-crystal PbTe films on BaF₂ are subjected to a considerable strain due to the mismatch of the lattice constants and the thermal expansion coefficients. By the magneto-optical and X-ray experiments reported in this paper it was possible to investigate separately the contributions of the compressive misfit strain and the thermally induced tensile strain. As a consequence of the opposite directions of the two effects, the system PbTe/BaF₂ is suited to grow single-crystal PbTe films exhibiting low strain below room temperature.     

The electrical properties of CdTe films grown by hot wall epitaxy

The electrical properties of CdTe thin films deposited on BaF₂ substrates were investigated. Different treatments of the substrates prior to deposition were tried. For films grown on thermally precleaned substrates the mobility increased exponentially with temperature in the temperature range 200–300 K and decreased with increasing carrier concentration. These effects can be explained by means of a grain boundary model. However, the mobility of films deposited on substrates which have a very thin PbTe buffer layer showed a bulk-like temperature dependence.     

The growth and transformation of Pd2Si on (111), (110) and (100) Si

Thin Pd films on (111), (110), (100) and amorphous Si substrates form [001] fiber textured Pd₂Si in the temperature range 100°–700°C. The degree of texture is a function of substrate orientation, increasing in the order amorphous Si, (100) Si, (110) Si and (111) Si. Only on the (111) Si substrate is the Pd₂Si film epitaxially oriented. Temperature-dependent growth on this orientation can be characterized by [001] textured growth, epitaxial azimuth orientation at the Si interface and progressive layer by layer formation of the mosaic crystal to the thin film surface.During Pd deposition, rapid non-diffusion-controlled growth of epitaxial Pd₂Si on (111) Si occurs at substrate temperatures of 100° and 200°C. An unidentified palladium silicide of low crystallographic symmetry forms during Pd deposition onto a 50°C substrate. The diffusion-controlled growth of Pd₂Si on (111) Si follows a t^0.5 dependence. The velocity constant is

$\text{k = 7 × 10}{}^{\mathrm{?2}}\text{exp}\text{?}\text{29200±800}\text{RT}\text{cm}{}^2\text{/}\text{sec}$

Palladium deposited on 100°C (111) Ge substrates reacts during deposition to form epitaxially oriented Pd₂Ge. However, growth of this phase at higher temperatures results in a randomly oriented film. The transformation of Pd₂Ge to PdGe is kinetically controlled. After a 15 min anneal at 560°±10°C in N₂ only PdGe is detectable on (111) Ge.The high temperature stability of thin film Pd₂Si is controlled by time- temperature kinetics. For a given annealing cycle, the nucleation and growth rates of the PdSi phase are inversely related to the crystalline perfection of Pd₂Si. Decreasing transformation rates follow the order (100), (110), (111) Si. formation of thin film Pd₂Si occurs by the formation of PdSi and subsequent growth of Si within the PdSi phase. After a 30 min N₂ anneal, initial transformation occurs at 735°C on (100) Si, 760°C on (110) Si and 840°C on (111) Si. Extended high temperature annealing produces a two-phase structure of highly twinned and misoriented Si and small PdSi grains that penetrate as much as 3 μm into the Si.     

Wetting and infiltration of volatile fluorides by In-Ti melt

In this study, the effect of the volatile nature of thermodynamically stable fluorides (MgF₂, CaF₂, and BaF₂) on wetting and infiltration phenomenon is discussed. Specially designed sets of experiments under different conditions of fluoride vapor evacuation were conducted. Experiments with In-Ti (~1 at.%) drops covered by a small cap and with artificially closed and open capillaries suggested that under restricted conditions of vapor evacuation, a thin solid layer is formed on the surface of the liquid metal, mechanically preventing both the spreading of In-Ti melt at the surface of the dense substrates and liquid penetration into the porous preforms.