Roger K. Crouch

Thermal convection during Bridgman crystal growth

Abstract Numerical experiments are used to study thermally driven flows which occur during vertical Bridgman crystal growth of a single component fluid. The solid-liquid interface was specified as parabolic and flow patterns were calculated for various insulation thicknesses, Grashof, Prandtl and Biot numbers. When the melt is on top and the gravity vector is axially downward it was shown that flow persists as long as a radial temperature gradient is present. If the interface is convex, as viewed from the liquid, a single cell is observed. A concave interface exhibits multiple counterrotating cells. The insulation thickness and Grashof, Prandtl and Biot numbers influence the flow in a quantitative manner.

Properties of GaN grown on sapphire substrates

Epitaxial growth of GaN on sapphire substrates using an open-tube growth furnace has been carried out to study the effects of substrate orientation and transfer gas upon the properties of the layers. It has been found that for the (0001) substrates, surface appearance was virtually independent of carrier gas and of doping levels. For the (1 ¯102) substrates surface faceting was greatly reduced when He was used as a transfer gas as opposed to H2. Faceting was also reduced when the GaN was doped with Zn and the best surfaces for the (1 ¯102) substrates were obtained in a Zn-doped run using He as the transfer gas. The best sample in terms of electrical properties for the (1¯102) substrate had a mobility greater than 400 cm2 V−1 sec−1 and a carrier concentration of about 2 × 1017 cm−3. This sample was undoped and used He as the transfer gas. The best (0001) sample was also grown undoped with He as the transfer gas and had a mobility of 300cm2V−1 sec−1 and a carrier concentration of 1 × 1018 cm−3.

Effects of supercooling in the initial solidification of PbTe-SnTe solid solutions

Abstract Deviations from compositions anticipated by the thermal equilibrium phase diagram have been observed in Bridgman-grown crystals of Pb 1− x Sn x Te, in the first to freeze region of the boule. A set of experiments were conducted to determine the extent of thermal supercooling of Pb 1− x Sn x Te in a Bridgman-like configuration. The results of the compositional profiles and the supercooling measurements are consistent with a diffusionless transformation occurring at the onset of solidification and the length of uncontrolled growth is inversely related to the temperature gradient of the furnace.

Vacuum tight quartz ampoule for Bridgman growth of crystals with interface demarcation

A growth ampoule for protecting the furnace lining and to prevent a change in the composition of the melt during growth of Pb-T-Te or Ge semiconductors doped with Ga is described. Mo foil is inserted into a quartz tube and sealed at the ends. A vacuum pumping system collapses the tube onto the foil. Spotwelded Mo wires lead in two directions from the collapsed section through quartz capillaries, one of which is also collapsed, on the wire, which provides a contact point for seeded growth of the semiconductor. The ampoule has been used for Bridgeman growth of crystals using a hot zone of 1150 C, with a temperature gradient of 240 C/m, and with current pulses up to 40 A/sq cm.

A study of beryllium-hydrogencomplexes in silicon

Abstract Single-crystal silicon, containing the impurity beryllium, has been doped with hydrogen, using two different techniques. Some samples were implanted with 3 MeV protons as a hydrogen source, and others were heated in a hydrogen atmosphere up to temperatures of 1000°C. Infrared absorption spectra of these samples at low temperatures have revealed two new series of absorptions identified as acceptor levels 91 meV and about 73 meV above the valence band. Quenching and annealing studies indicate that these absorptions are due to beryllium-hydrogen pairs, a direct analog to the beryllium-lithium pairs. Also a new series of absorptions has been found in samples which have a very high beryllium concentration. These lines at 298, 324 and 345 cm −1 disappear after short anneals at elevated temperatures and are believed to be an acceptor level associated with a more complex beryllium center.

Thermal ionization energy of lithium and lithium-oxygen complexes in single-crystal silicon

Abstract Recent optical studies[1, 2] have found that for float-zone crystals, having a relatively low oxygen content (less than 10 16 cm −3 ), the ionization energy of the lithium impurity was about 32 meV, whereas for pulled-crucible crystals, having a higher oxygen concentration (approximately 10 18 cm −3 ), the ionization energy was 39 meV. Previous measurements by means of Hall effect studies had indicated that the ionization energy in a pulled crucible sample was about 33 meV which seemed to agree with the optical float-zone value. In an attempt to clarify the situation, Hall effect measurements were made on pulled-crucible and float-zone silicon samples containing lithium as an impurity, and ionization energies were calculated. It was found that this new Hall effect data seemed to be in agreement with the optical data, the ionization energy being 37 meV for the pulled crucible sample and 29.5 meV for the float-zone.

Finite element analysis of the effect of a non-planar solid-liquid interface on the lateral solute segregation during unidirectional solidification

The effect of solid-liquid interface shape on lateral solute segregation during steady-state unidirectional solidification of a binary mixture is calculated under the assumption of no convection in the liquid. A finite element technique is employed to compute the concentration field in the liquid and the lateral segregation in the solid with a curved boundary between the liquid and solid phases. The computational model is constructed assuming knowledge of the solid-liquid interface shape; no attempt is made to relate this shape to the thermal field. The influence of interface curvature on the lateral compositional variation is investigated over a range of system parameters including diffusivity, growth speed, distribution coefficient, and geometric factors of the system. In the limiting case of a slightly non-planar interface, numerical results from the finite element technique are in good agreement with the analytical solutions of Coriell and Sekerka obtained by using linear theory. For the general case of highly non-planar interface shapes, the linear theory fails and the concentration field in the liquid as well as the lateral solute segregation in the solid can be calculated by using the finite element method.

Experimental investigation of the effects of gravity on thermosolutal convection and compositional homogeneity in bridgman grown, compound semiconductors☆

Lead-tin-telluride has been grown in a thermally stable mode (solutally unstable) and in a solutally stable (thermally unstable) mode in a Bridgman configuration. Significant differences in the crystal morphology and the compositional homogeneity have been found between the two configurations. In addition, for the solutally stable configuration, evidence has been found that the flow characteristics in the melt change drastically during the course of the run.

Thermophysical Properties of Germanium for Thermal Analysis of Growth from the Melt

The thermal diffusivity of Ge has been measured over a temperature range from 300 C to 1010 C which includes values for the melt. Specific heat has been measured from room temperature to 727 C. Thermal conductivity has been calculated over the same temperature range as the diffusivity measurements. These data are reported along with the best values from the literature for the other parameters which are required to calculate the temperature and convective fields for the growth of germanium by the Bridgman method. These parameters include the specific heat, the viscosity, the emissivity, and the density as a function of temperature.