Germanium and Silicon Liquidus Curves

Estimation of interaction parameters in the Al-Ga-As-Sn-Bi system

The development of GaAs based high power side-input photovoltaic converters requires thick (50-100 μm) transparent gradient refraction layers that can be grown by liquid phase epitaxy. Such thick layers can also be used in LED structures. To solve the problem of Al_xGa_1-xAs conductivity reduction at the x∼40% point a five-component, Al-Ga-As-Sn-Bi system is proposed. The interaction parameters in the liquid phase (αij) in the Al-Ga-As-Sn-Bi system are determined within the framework of a quasi-regular solutions model. For an Al_xGa_1-xAs solid solution growing from a Ga-melt containing 10 at.% of Bi (as a neutral solvent) and 15 at.% of Sn (as an n-type dopant), liquidus and solidus isotherms for 900 °C are modeled based on the calculated αij. Satisfactory agreement between calculated and experimental data has been obtained. Hall data show that AlGaAs layers grown from Bi-containing melts have n-type conductivity. Doping by tin during growth from mixed Ga-Bi melts makes it possible to increase the electron concentration in the AlGaAs layer.

Synergetic effect in rolling GaIn alloy droplets enables ultralow temperature growth of silicon nanowires at 70 °C on plastics

Ultralow temperature growth of silicon nanowires (SiNWs) directly upon cheap plastics is highly desirable for building high performance soft logics and sensors based on mature Si technology. In this work, a low temperature growth of SiNWs at only 70 °C has been demonstrated for the first time, upon polyethylene terephthalate plastics, by using gallium-indium (GaIn) alloy droplets that consume an amorphous Si (a-Si) layer as the precursor. The GaIn alloy droplets enable a beneficial synergetic effect that helps not only to reduce the melting temperature, but also to install a protective Gibbs adsorption layer of In atoms, which are critical to stabilize the rolling catalyst droplet, against otherwise rapid diffusion loss of Ga into the a-Si matrix. Ultra-long SiNWs can be batch-produced with a precise location and preferred elastic geometry, which paves the way for large scale integration. At <70 °C, a transition from rolling to sprawling dynamics is observed by in situ scanning electron microscopy, caused by reduced diffusion transport and rapid formation of discrete nuclei in the alloy droplet, which provides the basis for continuous growth of SiNWs. This unique capability and critical new understanding open the way for integrating high quality c-Si electronics directly over flexible, lightweight and extremely low cost plastics.

Pressure-sensitive liquid phase epitaxy of highly-doped n-type SiGe crystals for thermoelectric applications

Based on recent works, the most desirable high-temperature thermoelectric material would be highly-doped Si_1−xGe_x crystals or films with sufficiently high Ge concentrations so that simultaneous enhancing the power factor and wave-engineering of phonons could be possible on the ballistic thermal conductor. However, available thin film deposition methods such as metal organic chemical vapor deposition, electron-beam evaporation, or sputtering are unable to produce highly-doped SiGe single crystals or thick films of high quality. To fabricate the desired material, we here employ liquid phase epitaxy to make highly-doped (up to 2% GaP doping) SiGe crystals with minimized concentration variations on Si (111) and (100) substrates. We find that growing Si_1−xGe_x (x = 0.05~0.25) crystals from Ga solvents at relatively high vacuum pressure (0.1 torr) displays significant deviations from previous calculated phase diagram. Moreover, doping GaP into SiGe is found to affect the solubility of the system but not the resulting Ge concentration. We thus plot a new pressure-dependent phase diagram. We further demonstrate that the new pressure-induced liquid phase epitaxy technique can yield Si_1−xGe_x crystals of much higher Ge concentrations (x > 0.8) than those grown by the conventional method.

Binary Phase Diagrams and Thermodynamic Properties of Silicon and Essential Doping Elements (Al, As, B, Bi, Ga, In, N, P, Sb and Tl)

Fabrication of solar and electronic silicon wafers involves direct contact between solid, liquid and gas phases at near equilibrium conditions. Understanding of the phase diagrams and thermochemical properties of the Si-dopant binary systems is essential for providing processing conditions and for understanding the phase formation and transformation. In this work, ten Si-based binary phase diagrams, including Si with group IIIA elements (Al, B, Ga, In and Tl) and with group VA elements (As, Bi, N, P and Sb), have been reviewed. Each of these systems has been critically discussed on both aspects of phase diagram and thermodynamic properties. The available experimental data and thermodynamic parameters in the literature have been summarized and assessed thoroughly to provide consistent understanding of each system. Some systems were re-calculated to obtain a combination of the best evaluated phase diagram and a set of optimized thermodynamic parameters. As doping levels of solar and electronic silicon are of high technological importance, diffusion data has been presented to serve as a useful reference on the properties, behavior and quantities of metal impurities in silicon. This paper is meant to bridge the theoretical understanding of phase diagrams with the research and development of solar-grade silicon production, relying on the available information in the literature and our own analysis.

Ising Model Simulations Of Impurity Trapping in Silicon

Laser annealing experiments on silicon have shown that rapid solidification can trap large amounts of certain impurities in the crystal lattice. Concentrations that exceed the equilibrium solubility limits by several orders of magnitude have been obtained. In this paper we discuss the impurity trapping process using Monte Carlo simulation data from the kinetic Ising model. The dependence of the impurity concentration in the crystalon the solidification rate is calculated. The simulation data are compared with recent laser annealing results for bismuth and indium. Excellent agreement between the model and the bismuth experiments is obtained. The larger trapping rate on the (111) relative to the (100) orientation is found to be caused by the slower crystallization kinetics on the (111) face. Similar results are obtained for indium, although the difference in trapping on the (111) and (100) faces is somewhat smaller in the model than in the experiment.

Gallium-assisted growth of silicon nanowires by electron cyclotron resonance plasmas

The use of gallium droplets for growing Si nanowires (SiNWs) by electron cyclotron resonance plasmas is investigated. First, the relationship between evaporation time and resultant size of the gallium droplets is studied. Through the use of spectroscopic ellipsometry, the dependence of the surface plasmon resonance (SPR) energy on the droplet size is determined. From these gallium droplets, SiNWs were grown at 300 and 550 °C in electron cyclotron resonance plasmas containing SiH(4), Ar, and H(2). Scanning electron microscopy results show that tapered NWs are obtained for a wide range of growth conditions. Besides, it is found that H(2) plays an important role in the parasitic axial growth of the SiNWs. Namely, H(2) inhibits the radial growth and contributes dramatically to increasing the SiNW defects.

Rationalization of Nanowire Synthesis Using Low-Melting Point Metals

In this paper, we provide a theoretical basis using thermodynamic stability analysis for explaining the spontaneous nucleation and growth of a high density of 1-D structures of a variety of materials from low-melting metals such as Ga, In, or Sn. The thermodynamic stability analysis provides a theoretical estimate of the extent of supersaturation of solute species in molten metal solvent. Using the extent of maximum supersaturation, the size and density of critical nucleus were estimated and compared with experimental results using nucleation and growth of Ge nanowires using Ga droplets. The consistency of the proposed model is validated with the size and density of the resulting nanowires as a function of the synthesis temperature and droplet size. Both the experimental evidence and the theoretical model predictions point that the diameters of the resulting nanowires decrease with the lowering of synthesis temperatures and that the nucleation density decreases with the size of metal droplet diameter and increasing synthesis temperature.