Ge Melt Research Articles

POSSIBILITY OF GROWING BULK SI—GE CRYSTALS USING AXIAL HEAT FLOW METHOD NEAR THE CRYSTALLIZATION FRONT

A technique for crucibleless growth of single−crystal silicon and its alloys with germanium is developed. For this purpose, the setup of floating zone method was used, which was equipped with additional so−called AHP heater. The heater forms around itself a melt zone that is suspended between the growing crystal, the feeding rod and correspondingly the bottom and the top surfaces of the AHP heater by forces of surface tension. To protect the graphite casing of the heater against the aggressive action of molten silicon, the casing surface was coated with SiC having a special nano−crystalline structure. The system of automation control of the AHP crystallization mode is described. It allows controlling the thermal field near the growing crystal with an accuracy of about 0.05−0.1 K. Numerical computations of heat and mass transfer during the solidification of SixGe1−x alloy with a 2% Si content, as well as shaping of the free Si−Ge melt surface during the crystal pulling were performed. Uniform bulk crystals were obtained. The range of the highest melt layer at which the shaping process remains stable was found to be 10−20 mm. The grown As−doped Si single crystals showed to have strong twining directly caused by presence of the SiC inclusions revealed in the crystal bulk. The possibility to achieve a convex and nearly flat shape of the interface by means of the AHP heater was proved. The layered mechanism of Si crystallization was found to be present during crystal growth on a seed in the [111] direction, with the faceted area under certain conditions occupying almost the entire crystal cross section.

Optical diagnostics of the laser-induced phase transformations in thin germanium films on silicon, sapphire, and fused silica

The in-situ procedure is used to study the modification of thin (200–600 nm) germanium films induced by nanosecond pulses of a ruby laser. The films are produced using the ion-beam or magnetron sputtering on single-crystalline silicon (Si), sapphire (Al2O3), and fused silica (α-SiO2) substrates. The results on the dynamics of the laser-induced processes are obtained using the optical probing of the irradiated region at wavelengths of λ = 0.53 and 1.06 μm. The results of probing make it possible to determine the threshold laser energy densities that correspond to the Ge and Si melting and the generation of the Ge ablation plasma versus the amount of deposited Ge and thermophysical parameters of the substrate. The reflection oscillograms are used to obtain the dependences of the melt lifetime on the laser-pulse energy density.

Strain-induced Ge segregation on Si at high temperatures

Surface morphology transformation under high-temperature annealing of the Ge layers grown on Si(111) at about 450°C is studied with scanning tunneling and scanning reflection microscopies. The surface of the grown Ge layers, covered with relatively low-sloped facets, becomes composed of islands and continuous ridges with steep facets on their sidewalls under annealing at 700–850°C. Such Ge segregation on Si, taking place at temperatures significantly lower than the Ge melting temperature, is initiated by the Ge-Si lattice strain. The strain relaxation occurs through Ge amount reduction at the Ge/Si interface. This leads to an increase in the ridges height-to-width aspect ratio up to about 0.41 and, hence, to the increase in surface energy. The role of surface energy minimization consists in the formation of energetically preferable facets on sidewalls of ridges and in the determination of their configuration. The observed surface morphology is suggested to be similar to that of Ge thin-layer solidification on Si.

Temperature Dependent Internal Friction Behavior of Single Crystal Germanium Studied by the Impulse Excitation Technique

The anelastic relaxation process in crystalline solids is influenced by various mechanisms including diffusion of atoms and point defects, grain boundary viscosity, dislocation, phase transformation, etc. To analyze the type and concentration of relaxing units participating in a particular activated process, measuring the internal friction is a well-known approach. A fundamental measure of internal friction can be defined as the ratio between the mechanical energy dissipated during one cycle in a unit volume and the maximum stored elastic energy per unit volume during that cycle [1]. The impulse excitation technique (IET) is a well-established mechanical spectroscopic technique analyzing the internal friction and Young’s modulus behaviour of materials as function of temperature [2]. IET measures the resonant frequency of a specimen by exciting it mechanically with an impulse tool. The resonance frequencies are characteristic for the sample material and are related to its stiffness, mass and geometry. The amplitude decay of a free vibration can be related to the internal friction, i.e. the dissipation of the vibration energy or damping. Measurement of the resonance frequency allows the calculation of Young's modulus E. This technique has been previously applied to determine the temperature dependent Young's modulus of single crystal silicon and germanium in the <100>, <110> and <111> crystallographic directions [3,4,5]. More and more, single crystal germanium and silicon layers and substrates are used together to process advanced nano-eletronic devices. The thermal stresses during crystal growth and device processing have an important impact on generation and diffusion of point defects and in the worst case also can lead to dislocation generation. Although the temperature dependent mechanical properties of silicon are well known, this is much less the case for germanium.In the present study, the impulse excitation technique was used to analyze the temperature dependence of the internal friction in single crystal Ge in the <111>, <100> and <110> directions between room and melting temperatures. A similar set-up was used as in [6] but equipped with a specimen chamber and specimen suspension system allowing heating the samples in an inert atmosphere up to the Ge melt temperature. Between room temperature and melt temperature, three internal friction peaks were observed for each crystallographic orientation after internal friction spectrum analysis, a typical spectrum is shown in the figure. The mechanism leading to these peaks is thermally activated. To interpret the observed internal friction peaks, various aspects such as activation energy of peaks, peak positions and relaxation time will be discussed. The origin of the relaxation peaks is associated with intrinsic point defect processes and/or extended defect formation.

Strain-induced direct band gap shrinkage in local Ge-on-insulator structures fabricated by lateral liquid-phase epitaxy

Direct band gap shrinkage in Ge-on-insulator (GOI) structures fabricated by lateral liquid-phase epitaxy (LLPE) was investigated by means of micro(μ)-photoluminescence and μ-Raman spectroscopy. The LLPE method, based on the rapid thermal process, was found to be an effective and feasible way to produce highly strained local GOI structures. We observed a significant redshift of direct gap emission amounting to 45 meV from the tensile-strained GOI layer. Strain analysis and temperature dependent PL spectra indicated that a direct band gap shrinkage was mainly due to a tensile strain of about 0.4% induced by rapid crystallization from the Ge melting point during the LLPE process. The local optical properties along the LLPE-grown GOI wire were examined and discussed on the basis of μ-PL mapping images.

Analysis of dissolution and growth process of SiGe alloy semiconductor based on penetrated X-ray intensities

Si dissolution into Ge melt, solute transport in the Si–Ge solution and crystal growth of SiGe alloys were in situ observed by X-ray penetration method. The rectangular shaped sandwich sample of Si (seed)/Ge/Si (feed) was used for the experiment. X-ray intensities penetrated through the sample, which was heated up to the growth temperature of 1200°C, were recorded by rectangular shaped CdTe line sensor as a function of time and temperature. The experimental results demonstrated that the dissolution of Si seed was larger compared to Si feed crystal although Si feed temperature was relatively higher than that of seed. Crystal growth of SiGe was observed at the feed interface as the growth interface was observed clearly by an abrupt change of penetrated X-ray intensity near the growth interface. Since the crystal grew with Si rich composition (at high temperature 1200°C), solution becomes Ge richer which causes penetrated X-ray intensity variation at the growth interface. The growth mechanism for the observed SiGe growth process was discussed based on the penetrated X-ray intensity profile and a growth model. The composition of the grown sample was measured by FE-EPMA analysis.

Reconstructions and phase transition of clean Ge(110)

The structure of the clean Ge(110) surface is characterized between room temperature and the Ge melting temperature using scanning tunneling microscopy (STM) and low energy electron diffraction (LEED) and microscopy (LEEM). Rapid cooling from high temperature (~800°C) to room temperature yielded a surface composed of the c(8×10) reconstruction, {17 15 1} facets, and a previously unreported (8×2) reconstruction. Heating from room temperature to above 430°C extinguishes some, but not all, high-order LEED spots, indicating the presence of ordering up to at least 650°C. LEED observations of the phase transition between the c(8×10) and a disordered phase differ from earlier work but are consistent with previously published STM data.

Improved Czochralski Growth of Germanium Single Crystals from a Melt Covered by Boron Oxide

In germanium (Ge) crystal growth, a serious problem in growing high quality Ge crystals is the formation of germanium-oxide-particles on the melt surface. In addition, B doping in Ge crystals is very di cult because of the segregation coe cient, which is greater than unity, and the instability of free B atoms in the Ge melt. We have found a unique solution, which is CZ-Ge crystal growth from a melt covered by boron oxide (B2O3). Ge crystals which are dislocation-free and/or oxygen-enriched can be grown by these improved CZ techniques. Current and planned research using such Ge crystals is introduced.

Potential for growth of Si–Ge bulk crystals by modified FZ technique

The technique for crucibleless crystal growth of silicon and its alloy with germanium is developed on the basis of the floating zone (FZ) setup equipped with an additional so-called axial heating process (AHP) heater. The heater forms around itself the melt zone being suspended by forces of surface tension between its top surfaces and the growing crystal, as well as between the top surfaces of the AHP heater and the feeding rod. To investigate into the problem of the capillary stability of the pulling process, numerical computations of shaping of the free Si and Ge melt surface during the crystal pulling were performed. The dependences of the crystal radius and the thickness of the melt film on the parameters of the process are analyzed. It is demonstrated that, in the modified FZ method, the thickness of the melt layer between the crystal and the heater can be considerably larger than the capillary constant. A number of Si and Si–Ge crystals with a diameter up to 15mm are grown. The range of a Si–Ge melt layer, which one could establish as high as possible, was found to be equal to 11–18mm.

From sponge to dot arrays on (100) Ge by increasing the energy of ion impacts

Ge surfaces of up to 780 K temperature have been irradiated at normal incidence with up to 1017 Bi+ ions cm−2 having kinetic energies from 10 to 30 keV. The resulting surface morphologies have been studied by scanning electron microscopy. While at room temperature the impacts of high-energy Bi+ ions result in porous networks, at elevated irradiation temperatures hexagonally ordered dot arrays are formed, whereas after a further temperature increase the surface becomes smooth. The comprehensive experimental studies have been summarized in a phase diagram of surface morphologies in the ion energy versus substrate temperature plane. In this phase diagram, the onset of dot formation with increasing substrate temperature has been consistently modeled by nanomelting of the collision cascade volume of ion impacts, thereby taking into account the thermodynamic parameters of amorphous Ge (melt temperature, heat of fusion, and heat capacity) as well as the energy density deposited in the cascade volume as predicted by established simulation programs.

Liquidus projection of the Ag–Ba–Ge system and melting points of clathrate type-I compounds

The liquidus and solidus projection has been constructed for the Ag–Ba–Ge system up to 33.3at% Ba, using electron micro probe analysis (EPMA), X-ray powder diffraction (XRD) and differential thermal analysis (DSC/DTA). Eight different primary crystallization regions were found: (Ge), Ba8AgxGe46−x−y□y (κI) (□ is a vacancy), Ba6AgxGe25−x (κIx), BaGe2, Ba(Ag1−xGex)2 (τ1), BaAg2−xGe2+x (τ2) BaAg5 and (Ag). The ternary invariant reactions have been determined for the region investigated and are the basis for a Schulz–Scheil diagram.The second part of this work provides a comprehensive compilation of melting points of ternary A8TxM46−x and quaternary (A=Sr, Ba, Eu; T=Ni, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga; M=Si, Ge, Sn) clathrate type-I compounds and decomposition temperatures of inverse clathrate type-I Ge38{P,As,Sb}8{Cl,Br,I}8, Si46−xPxTey and tin based compounds.

Czochralski growth techniques of germanium crystals grown from a melt covered partially or fully by liquid B2O3

We propose two unique Czochralski (CZ) techniques for growing germanium (Ge) crystals with an extremely low dislocation density and high interstitial oxygen concentration ([Oi]) using boron oxide (B2O3) and a silica crucible. When a Ge melt is partially covered with liquid B2O3, but only on the outer region of the melt surface, germanium-oxide (GeO2)-related particles forming naturally in the melt are effectively dissolved by the liquid B2O3. The clean central portion of the melt produces dislocation-free undoped or Ga-doped Ge crystals. In addition, Ge crystals with [Oi] up to 6×1017cm−3 can be grown from a melt fully covered by liquid B2O3 with added GeO2 powder. The reaction and transportation of oxygen atoms during the growth process using B2O3 was investigated, revealing that liquid B2O3 acts like a catalyst without heavy contamination of the growing Ge crystal by B and Si atoms.

Nucleation of Si and Ge by rapid cooling using molecular-dynamics simulation

The recrystallization processes of supercooled Si and Ge melts were investigated using molecular-dynamics (MD) simulations. The incubation time Ti defined as the cooling time period to obtain 10% recrystallization of supercooled melts is found to be minimum when temperature is around 0.7Tm (Tm: melting temperature for both of crystal Si and Ge). The MD results suggest that 0.7Tm is an optimum annealing temperature to enhance the nucleation rate in the growth processes of Si and Ge films.

Macrosegregation in vertical Bridgman grown Ge 1− xSi x alloy with large melt volume: Limit of complete mixing

Vertical Bridgman growth of GeSi alloy with an average Si composition of 15 atom% was performed on a large 25 mm diameter by 90 mm long melt volume to significantly increase the buoyancy-driven convection within the melt. For one sample, the longitudinal Si composition along the sample length is described by a simple one-dimensional model including complete mixing of the Ge and Si melt constituents and the equilibrium Si segregation coefficient given by the GeSi phase diagram. The excellent agreement between theory and experiment shows that the GeSi phase diagram in the Ge-rich region is determined accurately.

3C–SiC Heteroepitaxial Growth by Vapor–Liquid–Solid Mechanism on Patterned 4H–SiC Substrate Using Si–Ge Melt

In this work, we report on the use of patterned 4H-SiC(0001) substrates for the heteroepitaxial growth of 3C-SiC by vapor-liquid-solid (VLS) mechanism using Ge(50)Si(50) melt. Mesas structures of various size and shape were obtained by standard photolithography and dry etching processes. On the temperature range investigated 1300-1450 degrees C, 3C-SiC deposit was obtained on top and outside the mesas. Some lateral enlargement of these mesas was observed, but it was systematically homoepitaxial. The lateral growth rate was found rather low compared to other techniques like chemical vapor deposition, with a maximum value of similar to 12 mu m/h. In addition, elimination of twin boundaries (TBs) inside the 3C-SiC deposit on top of the mesas was observed in the temperature range of 1400-1450 degrees C and for specific mesa shape or orientation of the sidewalls. The best case for eliminating these TBs was found to be with initially circular mesas, which spontaneously form well orientated hexagonal facets and then lead to TB-free deposit on top after VLS growth.

Integration of Ge nanowire arrays in glass micro-fibers

We report on a technological route for the integration of large arrays of Ge nanowires (NWs) in a human-hair-like glass micro-fiber, the length of the micro-fiber reaching one meter. The route comprises (a) the formation of semiconductor microwire in glass insulation by capillary drawing from the bottom of a glass tube softened by a conducting Ge melt drop levitating in the high-frequency electromagnetic induction field; (b) mechanical assembly of a bundle from equal-length cut microwires which are distributed in a two-dimensional quasi-hexagonal densely packed lattice encircled by a joint glass envelope; (c) stretching of the obtained preform under proper heating conditions to reduce the diameters of the stacked together microwires; (d) repeating the cut-assembly-stretching processes for the purpose of further decreasing in transverse dimensions of constituents down to 150 nm. The fascinating incorporation of huge amounts of Ge nanowires in glass micro-fibers opens new possibilities for the development of highly integrated photonic and quantumelectronic systems.

Investigation of the Growth of 3C-SiC on Si by Vapor-Liquid-Solid ( VLS ) Transport

In this work, the growth by Vapour-Liquid-Solid (VLS) mechanism of 3C-SiC on silicon substrate is reported. Firstly, a germanium layer is deposited on the substrate. Then the temperature of the sample is increased above Ge melting point in order to form a SiGe liquid phase by reaction with the substrate. Upon reaching the target temperature (1100-1300°C) the VLS growth starts with the injection of propane in the reactor. Both Raman spectrometry and X-Ray diffraction analyses evidenced the formation of 3C-SiC on every sample. However, this SiC deposit, a few micrometers thick, is always found to be polycrystalline though textured. In parallel, the presence of an epitaxial Si-Ge alloy, whose composition depends on the growth temperature, was systematically detected between Si and SiC.

Ge instability and the growth of Ge epitaxial layers in nanochannels on patterned Si (001) substrates

We report the Ge thermal instability against surface diffusion in extremely narrow channels isolated by SiO2 and demonstrate the selective growth of strain-relaxed Ge epitaxial layers in channels as narrow as 10 nm on patterned Si (001) substrates by a combination of low temperature growth and selective recrystallization using Ge melt and regrowth during a millisecond laser anneal. Filling narrow channels at high growth temperatures is prohibited by Ge outdiffusion due to the high Ge chemical potential in such narrow channels. The Ge thermal instability depends on the channel width and a critical channel width of 50 nm is extracted from our calculation and experimental results. The Ge surface diffusion rate is dominated by the surface diffusion activation energy barrier that is significantly enhanced by the adsorbed surface hydrides. At a low temperature below 450 °C, a hydride-terminated surface is maintained which increases the Ge surface diffusion barrier and retards the outdiffusion of the Ge adatoms, providing excellent channel filling. The resulting low crystalline quality can be restored with a millisecond laser melt and regrowth.

Czochralski-growth of germanium crystals containing high concentrations of oxygen impurities

Oxygen-containing germanium (Ge) single crystals with low density of grown-in dislocations were grown by the Czochralski (CZ) technique from a Ge melt, both with and without a covering by boron oxide (B 2O 3) liquid. Interstitially dissolved oxygen concentrations in the crystals were determined by the absorption peak at 855 cm −1 in the infrared absorption spectra at room temperature. It was found that oxygen concentration in a Ge crystal grown from melt partially or fully covered with B 2O 3 liquid was about 10 16 cm −3 and was almost the same as that in a Ge crystal grown without B 2O 3. Oxygen concentration in a Ge crystal was enhanced to be greater than 10 17 cm −3 by growing a crystal from a melt fully covered with B 2O 3; with the addition of germanium oxide powder, the maximum oxygen concentration achieved was 5.5×10 17 cm −3. The effective segregation coefficients of oxygen in the present Ge crystal growth were roughly estimated to be between 1.0 and 1.4.

Undercooling of Pure Cu and Ge Melts in a Static Magnetic Field

The glass fluxing technique was used to undercool melts of pure Cu and Ge in the presence of a static magnetic field generated by a superconducting magnet. It was found that the mean undercooling of liquid Cu increased with increasing magnetic field, whereas the mean undercooling of liquid Ge did not show any significant changes with increasing magnetic field. Such a difference in the undercooling behavior can be related to the Lorentz force imposed by the magnetic field, which was larger for liquid Cu because of a larger electrical conductivity than that of liquid Ge.