In-situ Ga doping of fully strained Ge1-xSnx heteroepitaxial layers grown on Ge(001) substrates

We have investigated the Ga and Sn content dependence of the crystallinity and electrical properties of Ga-doped Ge_1-xSn_x layers that are heteroepitaxially grown on Ge(001) substrates. The doping of Ga to levels as high as the solubility limit of Ga at the growth temperature leads to the introduction of dislocations, due to the increase in the strain of the Ge_1-xSn_x layers. We achieved the growth of a fully strained Ge_0.922Sn_0.078 layer on Ge with a Ga concentration of 5.5 × 10¹⁹ /cm³ without any dislocations and stacking faults. The resistivity of the Ga-doped Ge_1-xSn_x layer decreased as the Sn content was increased. This decrease was due to an increase in the carrier concentration, with an increase in the activation level of Ga atoms in the Ge_1-xSn_x epitaxial layers being induced by the introduction of Sn. As a result, we found that the resistivity for the Ge_0.950Sn_0.050 layer annealed at 600°C for 1 min is 3.6 times less than that of the Ga-doped Ge/Ge sample.     

Growth of Ge1 − xSnx heteroepitaxial layers with very high Sn contents on InP(001) substrates

We investigated the crystalline structures of Ge_1 − xSn_x heteroepitaxial layers with Sn contents greater than 20% grown on InP and Ge substrates. Considering the lattice mismatch between the Ge_1 − xSn_x layers and the substrates, we achieved epitaxial growth of Ge_1−xSn_x layers with very high Sn content by suppressing the Sn precipitation; in addition, we improved upon the crystalline quality of Ge_1 − xSn_x heteroepitaxial layers. As a result, we could successfully form a 130 nm-thick Ge_1 − xSn_x heteroepitaxial layer on an InP substrate with a Sn content as high as 27% without Sn precipitation. We also improved the crystalline quality of Ge_1 − xSn_x layers by annealing at a temperature as low as 290 °C.     

Laser assisted formation of binary and ternary Ge/Si/Sn alloys

Binary and ternary Si/Ge/Sn alloys were epitaxially grown on virtual Germanium buffer layers using pulsed laser induced epitaxy with a 193 nm Excimer laser source. The role of the processing parameters on the intermixing of the components (Sn, Ge and Si) has been studied. Characterization of the resulting Ge_1 − xSn_x and Si_{1 − y − x}Ge_ySn_x alloys yield up to 1% Sn concentration in substitutional sites of the Ge or SiGe matrix.     

Low-temperature growth of Ge1 − xSnx thin films with strain control by molecular beam epitaxy

High-quality Ge_1 − xSn_x thin films on InGaAs buffer layers have been demonstrated using low-temperature growth by molecular beam epitaxy. X-ray diffraction and secondary ion mass spectrometry are used to determine the strain and Sn concentration. Up to 10.5% Sn has been incorporated into the Ge_1 − xSn_x thin film without Sn precipitation, as verified by transmission electron microscopy. Roughened surfaces are found for tensile strained Ge_1 − xSn_x layers.     

Quest for an optoelectronic material: tetragonal SixGe1−x films

We report experiments in which we attempted to grow Ge and Si_0.2Ge_0.8 films with a tetragonal crystal structure on InP(001). The substrate was chosen because it has approximately the same lattice constant as the tetragonal Si_xGe_1−x a-axis lattice constant. The tetragonal structure has been observed previously in Ge, and has a 1.5-eV bandgap. Theory predicts that the gap is direct, which implies that tetragonal Si_xGe_1−x films are suitable for optoelectronic devices. Growth on the InP(001) (2×3) and (2×4) reconstructed surfaces was investigated. Some evidence for the tetragonal phase was obtained for the growth of Si_0.2Ge_0.8 at 100 °C on the two reconstructions, but no evidence was obtained for growth of either tetragonal Si_0.2Ge_0.8 or Ge on the (2×3) reconstruction for growth temperatures at or above 130 °C. We conclude that tetragonal Si_xGe_1−x films on InP(001) does not seem to be a technologically feasible material system.     

Non-substitutional Sn Defects in Ge1−x Sn x Alloys for Opto- and Nanoelectronics

Important technological applications are envisaged for Ge_1?x Sn _x alloys. They provide a route to obtain direct-gap group IV materials, tuneable by concentration. Therefore, these alloys are ideal candidates for optoelectronic devices, highly compatible with Si integrated circuits. Contrary to other binary alloys with group IV elements, homogeneous Ge_1?x Sn _x alloys, as required for device applications, have proven difficult to form above a certain temperature-dependent critical Sn concentration. Through a detailed ab-initio local defect study, and the proposal of a statistical model for the formation of these alloys, we predicted that a new type of Sn defect (β-Sn), consisting of a single Sn atom in the centre of a Ge divacancy, might be formed. The environment of this defect relaxes towards a cubic octahedral configuration, facilitating the nucleation of white tin and its segregation, as found in amorphous samples. We confirmed that Sn would enter substitutionally in the Ge lattice, but above a temperature-dependent critical concentration, non-substitutional β-Sn defects should be formed, consistent with experimental observations. In this paper we introduce a two-site substitutional equivalent for the non-substitutional β-Sn defect in Ge, as needed in order to be able to include β-Sn in electronic structure calculations with effective-field electronic models for disorder, like the Virtual Crystal Approximation (VCA). The equivalent substitutional model is derived in order to take into account the different symmetries in the immediate environment of the substitutional α and non-substitutional β defect sites and their effect on the electronic structure.     

Compositional dependence of the optical parameters for Bi5GexSe65−x (30 ≤ x ≤ 45) films

Different compositions of Bi₅Ge_xSe_95−x (x = 30, 35, 40 and 45 at %) thin films were deposited onto cleaned glass substrates by thermal evaporation method. The structural characterization revealed that, the as-prepared films of x = 30, 35 and 40 at. % are in amorphous state but there are few tiny crystalline peaks of relatively low intensity for the film with x = 45 at. %. The chemical composition of the as-prepared Bi₅Ge_xSe_65−x films has been checked using energy dispersive X-ray spectroscopy (EDX). The optical properties for the as-deposited Bi₅Ge_xSe_65−x thin films have been studied. The additions of Ge content were found to affect the optical constants (refractive index, n and the extinction coefficient, k). Tauc’s relation for the allowed indirect transition is successfully describing the mechanism of the optical absorption. It was found that, the optical energy gap (E_g) decreases with the increase in Ge content. These obtained results were discussed in terms of the chemical bond approach proposed by Bicermo and Ovshinsky. The composition dependence of the refractive index was discussed in terms of the single oscillator model.     

Composition and thickness dependant optical study of a-Pb–Se–Ge–Sn glassy thin films

Thickness and compositional dependence of optical properties of Pb₉Se₇₁Ge_20?xSn_x (8 ≤ x ≤ 12) glass has been studied. Various optical constants such as refractive index, extinction coefficient and optical band gap have been determined by analyzing optical transmittance data in the wavelength range of 200–3,500 nm. Density of localized states and disorder plays a crucial role in deciding the optical properties of amorphous semiconductors. Refractive index and extinction coefficient increase as Sn content increase in material. With the rise in thickness, there may be increase in order of short range order of the film and continuous random network simultaneously, result in reduced band gap. The isoelectronic substitutions of Ge by Sn in the glassy system also contribute to reduction in optical band gap of the material.     

Low temperature formation of Si1 − x − yGexSny-on-insulator structures by using solid-phase mixing of Ge1 − zSnz/Si-on-insulator substrates

We proposed the low temperature formation technique of strain-relaxed Si_{1 − x − y}Ge_xSn_y-on-insulator (SGTOI) structures. We found that the solid-phase reaction and the formation of single and uniform Si_{1 − x − y}Ge_xSn_y layer on an insulator after annealing SiO₂/Ge_1 − zSn_z/SOI structures even at a temperature as low as 400 °C. We characterized the crystalline structure of SGTOI, and investigated the effects of annealing, Sn incorporation, and a SiO₂ cap layer on the solid-phase reaction between Ge_1 − zSn_z and SOI layers. The solid-phase reaction is enhanced with a higher Sn content and a thicker SiO₂ cap layer, and then Si_{1 − x − y}Ge_xSn_y layers are more rapidly formed. The SGTOI layer exhibits very low mosaicity and have good crystallinity.     

Growth and applications of GeSn-related group-IV semiconductor materials

Abstract

We review the technology of Ge_1?xSn_x-related group-IV semiconductor materials for developing Si-based nanoelectronics. Ge_1?xSn_x-related materials provide novel engineering of the crystal growth, strain structure, and energy band alignment for realising various applications not only in electronics, but also in optoelectronics. We introduce our recent achievements in the crystal growth of Ge_1?xSn_x-related material thin films and the studies of the electronic properties of thin films, metals/Ge_1?xSn_x, and insulators/Ge_1?xSn_x interfaces. We also review recent studies related to the crystal growth, energy band engineering, and device applications of Ge_1?xSn_x-related materials, as well as the reported performances of electronic devices using Ge_1?xSn_x related materials.     

Decomposition of (Sn2xFe1−xSb1−x)O4 solid solutions with x≤0.50

Thermal stabilities of the rutile-type (Sn_2xFe_1−xSb_1−x)O₄ solid solutions with x≤0.5 were investigated by TG–DTA in flowing O₂ up to 1673 K. After thermal analysis the samples were characterised by means of powder X-ray diffraction analysis (XRD), optical and scanning electron microscopy (SEM) observation and electron dispersive spectrometry (EDS) analysis. The decomposition of the solid solution involves the formation of hematite and a volatile Sb oxide, probably Sb₄O₆. The decomposition temperature increases with the Sn content of the solid solution.     

Semiconductors of the type MeIIMeIVS3

Ternary compounds of the type Me^IIMe^IVS₃ with the elements Me^II = Sn, Pb and Me^IV = Ge, Sn form two isotypic groups, with orthorhombic (Me^IISn^IVS₃) and monoclinic (Me^IIGe^IVS₃) symmetry respectively. Single crystals of sufficiently large size have been prepared to examine some basic physical properties. Results on electrical conductivity, and optical absorption measurements at the fundamental electronic gap are reported. They will be discussed with respect to the crystal structures.     

Growth of homogeneous polycrystalline Si1-xGex and Mg2Si1-xGex for thermoelectric application

Homogeneous polycrystalline Si_1-xGe_x were grown using a Si(seed)/Ge/Si(feed) sandwich structure under the low temperature gradient less than 0.4 °C/mm. It was found that the composition of the Si_1-xGe_x was controlled by the growth temperature. The homogeneous Mg₂Si_1-xGe_x was synthesized by heat treatment of the homogeneous Si_1-xGe_x powders under Mg vapor. The Mg₂Si_1-xGe_x sample with the relative density of 95% was synthesized by spark plasma sintering technique. The resistivity and the Seebeck coefficient of the Si, Ge, Si_1-xGe_x and Mg₂Si_1-xGe_x samples were evaluated as a function of temperature. It indicated that Seebeck coefficients of the Si_1-xGe_x and Mg₂Si_1-xGe_x samples were higher than those of Si and Ge. Moreover, the Seebeck coefficient of Mg₂Si_0.7Ge_0.3 sample was higher than that of Mg₂Si_0.5Ge_0.5 and Si_0.5Ge_0.5 samples.     

A continuum model for the Liquid Phase Diffusion growth of bulk SiGe single crystals

In this article, we present a computational model for the growth of Si_xGe_1−x single crystals by Liquid Phase Diffusion (LPD) from the germanium-rich side of the binary Si_xGe_1−x phase diagram. The model accounts for some important physical features of the LPD growth process of Si_xGe_1−x such as a growth-zone design on the thermal field, the buoyancy induced convective flow and its effect on the growth process, the evolution of growth interface, and the variation of growth velocity with time. Two- and three-dimensional numerical simulations were carried out. Simulation results show that the natural convection in the solution is very strong during the initial stages of the growth process, and gets weaker as the growth progresses. Diffusion becomes the dominant mass transport mechanism during the rest of the growth process. Computed growth interface shapes and growth velocities agree with experimental observations.     

Inducing imperfections in germanium nanowires

Nanowires with inhomogeneous heterostructures such as polytypes and periodic twin boundaries are interesting due to their potential use as components for optical,electrical,and thermophysical applications.Additionally,the incorporation of metal impurities in semiconductor nanowires could substantially alter their electronic and optical properties.In this highlight article,we review our recent progress and understanding in the deliberate induction of imperfections,in terms of both twin boundaries and additional impurities in germanium nanowires for new/enhanced functionalities.The role of catalysts and catalyst-nanowire interfaces for the growth of engineered nanowires via a three-phase paradigm is explored.Three-phase bottom-up growth is a feasible way to incorporate and engineer imperfections such as crystal defects and impurities in semiconductor nanowires via catalyst and/or interfacial manipulation."Epitaxial defect transfer"process and catalyst-nanowire interfacial engineering are employed to induce twin defects parallel and perpendicular to the nanowire growth axis.By inducing and manipulating twin boundaries in the metal catalysts,twin formation and density are controlled in Ge nanowires.The formation of Ge polytypes is also observed in nanowires for the growth of highly dense lateral twin boundaries.Additionally,metal impurity in the form of Sn is injected and engineered via third-party metal catalysts resulting in above-equilibrium incorporation of Sn adatoms in Ge nanowires.Sn impurities are precipitated into Ge bi-layers during Ge nanowire growth,where the impurity Sn atoms become trapped with the deposition of successive layers,thus giving an extraordinary Sn content (＞6 at.％) in Ge nanowires.A larger amount of Sn impingement (＞9 at.％) is further encouraged by utilizing the eutectic solubility of Sn in Ge along with impurity trapping.     

Optical, thermal and phase transition studies in Sn1-x Ge x Te

The optical and thermal properties of the mixed semiconducting alloy, Sn_1-xGe_xTe, is studied by photo acoustics, for various Ge concentrations and phase transition for a particular concentration is also studied by the same method. The results are compared with the available literature values and discussed.     

Sims analysis of Gex Se1−x amorphous thin films

Ge_xSe_1−x amorphous films, 1 μm thick, prepared by vacuum thermal evaporation were analyzed by the SIMS method in order to test the distribution into the depth of the samples of the Ge and Se components. Several analyses were done; they allow us first to show that mass interferences could occur between different species of identical masses, how to avoid them and then to deduce a way to perform the evaporation process so as to get a better homogeneity. Both from depth profiles and ion optical images for two atomic concentrations (x = 0.08 and x = 0.12), we concluded within the sensitivity of the SIMS analysis that the studied samples exhibit a good homogeneity.     

Observation of phase separation in (Ge2)x(GaAs)1−x alloys grown by molecular beam epitaxy

(Ge₂)_x(GaAs)_1?x alloys with 0 < x < 1 have been grown by molecular beam epitaxy on various Ge and GaAs substrates. The structure of these alloys has been studied using transmission electron microscopy. We observe that in all cases studied, Ge tends to phase-separate from GaAs, with the Ge domain size ranging from 100 to 300 Å.     

Spectroellipsometric characterisation of thin epitaxial Si1–xGex layers

Abstract

Spectroscopic ellipsometry is assessed as a toolfor rapid and non-destructive determination of thicknesses and/or Ge concentrations of Si_1–xGe_x epitaxial layers. An algorithmfor the extraction of optical constants from ellipsometric measurements carried out on strained uncapped Si_1–xGe_x layers is developed. The smoothness of the extracted spectra of the Si_0·92Ge_0·08 optical constants indicates very high accuracy even in the region of the weak absorption. Spectroscopic ellipsometry measurements of Si capped Si_1–xGe_x layers are interpreted using the established reference data and correlated with results from other techniques. The accuracy of this interpretation for the spectroellipsometric determination of Ge concentration and Si_1–xGe_x layer thickness is numerically predicted. It is suggested that spectroscopic ellipsometry can also be applied to capped Si_1–xGe_x layers ifmore accurate reference data in the low absorption region become available.

MST/3295     

Properties of La0.75Sr0.25MnO3 films grown on Si substrate with Si1−xGex and Si1−yCy buffer layers

The structural and electrical properties of La_0.75Sr_0.25MnO₃ (LSMO) film on Bi₄Ti₃O₁₂ (BTO)/CeO₂/YSZ buffered Si_1−xGe_x/Si (0.05 ≤ x ≤ 0.2 for compressive strain), blank Si, and Si_1−yC_y/Si (y = 0.01 for tensile) were studied. X-ray high resolution reciprocal lattice mapping (HRRLM) and atomic force microscopy (AFM) show that structural properties of LSMO and buffer oxide layers are strongly related to the strain induced by amount of Ge and C contents. The RMS roughness of LSMO on Si_1−xGe_x/Si has a tendency to increase with increasing of Ge content. Electrical properties of LSMO film with Ge content up to 10% are slightly improved compared to blank Si whereas higher resistivity values were obtained for the samples with higher Ge content.