W.A. Sunder

Czochralski growth and characterization of GaSb

Abstract Single crystal GaSb is important as a substrate for long wavelength ( ⩾ 1.5 μm) detectors and lasers. Procedures for the reproducible synthesis of GaSb, and Czochralski growth of single crystals with varied stoichiometry are discussed. The distribution of dopants including Te, Se, Si and Ge was studied and the dependence of carrier concentration on dopant concentration (determined by atomic absorption analysis) is reported. In the Te concentration range studied, 1.5 × 10 17 -3 × 10 19 cm -3 , the carrier concentration is always substantially less than the Te concentration ranging from ≈ 0.25 at low Te to ≈ 0.14 at high Te. Room temperature electron mobilities > 3000 cm 2 V -1 s -1 are obtained at low carrier concentrations. The interface shape and its change with fraction frozen is shown to affect the impurity distribution. Normal freeze behavior is qualitatively observed for Te partition as a function of fraction of melt frozen. Equations for the effect on impurity distribution of the changing Ga/Sb ratio in the solid as freezing proceeds are developed and compared with experimental results. It is shown that the most important correction to the distribution constant is based on interface shape changes and on and off facet growth as freezing proceeds.

Solubility and P-V-T relations and the growth of potassium titanyl phosphate

Abstract KTP, potassium titanyl phosphate (KTiOPO4) single crystals are the material of choice for frequency doubling Nd: YAG laser light to visible green. Solubility and P-V-T measurements were made at the conditions used in a new low temperature hydrothermal process for growth. P-V-T data show that in 2M K2HPO4 and 2M K2HPO4 +0.125M KNO3 saturated with KTP the pressures are not substantially different although they are greatly reduced from those of water. The data are used to show the temperature at which autoclaves fill with a single fluid phase as a function of various initial degrees of fill. This is of aid in crystal growth since better quality and reproducibility occur in the one fluid phase region. Solubility data in 2M K2HPO4 and 2M K2HPO4+KPO3 show that KTP is probably incongruently saturating and not the stable phase except when excess KPO3 is present. These results explain the long equilibrium times and scatter in apparent solubilities in K2HPO4 and the formation of solid TiO2 inclusions during the initial stages of KTP growth. The phase equilibrium and solubility measurements suggested that growth in a slight excess of KPO3 would improve perfection by repressing TiO2 formation. This was experimentally verified, but with some diminishing of growth rate on (011).

Highly textured and single crystal Bi2CaSr2Cu2Ox prepared by laser heated float zone crystallization

Abstract A CO 2 laser heat source was used to float zone 0.36 cm diameter ceramic rods of Bi 2 CaSr 2 Cu 2 O x . Well behaved, stable molten zones were obtained in air and in O 2 pressures up to 2.6 atm. Highly textured elongated grains ∼ 1 cm long with the c -axis normal to the rod axis were obtained at slow (0.19 cm/h) growth rates. Crystals 3×1.2×0.3 mm were easily cleaved from such specimens. In the best cases (slow growth rates), X-ray powder diffraction indicated ∼ 100% Bi 2 CaSr 2 Cu 2 O x , the superconducting transition temperature was as high as 87 K, and diamagnetism measurements indicated 60–70% superconducting phase. Meissner measurements however, in common with most Meissner measurements on Bi-containing superconductors, indicated substantially less superconducting material. Critical currents were comparable to untextured ceramic and flux grown single crystals. Float zone crystallization shows great promise for preparing single crystals and textured ceramics of incongruently melting superconductors, even when detailed phase diagrams are not known.

Thermodynamic considerations in the synthesis and crystal growth of GaSb

Abstract We use a newly developed optimization algorithm, the “sticky trust region technique”, for Gibbs energy minimization to determine the gaseous species and liquid and solid phases present during the synthesis and crystal growth of GaSb. The growth system involves almost thirty species comprising a gaseous phase and nine condensed species. We model the system as a function of temperature, oxygen and hydrogen pressure in the presence of an SiO2 crucible. Ga2O3 is identified as the most stable contaminant compound and is seen as a phase that floats on the liquid melt during growth. This oxide often prevents the growth of high quality crystals. We show the effects on its thickness of increasing the ambient hydrogen concentration and determine that the SiO2 crucible is not an important source of the contaminant oxygen except at high temperatures. The Gibbs energy minimization method combined with the newly-developed optimization algorithm is shown to be an efficient tool for evaluating the problems of cystal growth, materials synthesis and purification.

Thermodynamic stability and reactivity of AlSb and their relationship to crystal growth

Abstract The vapor pressure, species concentration and reactivity of AlSb is modeled thermodynamically using the newly-described sticky trust region technique, STRT, for free energy minimization. The conditions chosen include oxygen and hydrogen concentrations appropriate to Czochralski crystal growth in SiO 2 , C, BN, Al 2 O 3 and BeO crucibles and growth in the absence of a crucible. Results are compared with crystal growth experiments where appropriate. At the melting point the principal vapor species is Sb 2 with a total vapor pressure of ∼10 −3 atm, which is about 10 2 larger than for GaSb. These results agree with the experimental data of Sirota and Golodushko. Reaction with the crucible is predicted for SiO 2 and BN in agreement with crystal growth experiments and also with C, where the slow kinetics of vitreous carbon reactions make it possible to use it for crucibles. Al 2 O 3 and BeO are predicted to be inert. Preliminary crystal growth results confirm these results and these materials are recommended for further investigation. The results indicate that thermodynamic “testing” of crucibles is an appropriate crystal growth strategy.

Raman characterization of single defect layers embedded in finite superlattices

Single defect layers embedded in finite, nominally periodic superlattices have been examined using Raman scattering. The spectrum of zone‐folded acoustic phonons exhibits defect‐associated peaks due to the broken symmetry of the lattice. The frequencies and intensities of these additional modes can be qualitatively estimated using a simple photoelastic coupling model. Shifts in the defect mode frequencies are observed when the spatial location of the defect layer is moved from the substrate to the air interface.

Molecular beam epitaxy growth and characterization of GaSb / AlSb strained-layer superlattices on nonvicinal (001) and 111 (B) GaSb substrates

Abstract A comparison of the MBE growth of GaSb/AlSb strained-layer structures has been carried out for (001) and (111)B GaSb substrates. Optimal in vacuo cleaning procedures were found to differ and are delineated. The growth rate for (111)B is 10% larger than (001) for the same beam fluxes and substrate temperature. Growth on the (001) orientation is specular, whereas growth on nonvicinal (111)B is faceted. Varying the substrate temperature and V/III flux ratio did not result in specular growth on the (111)B orientation. Strong excitonic luminescence is observed for single and multiple quantum well structure grown on both substrate orientations. Systematic excitonic peak shifts are observed from strained and unstrained GaSb wells by changing the buffer layer which separates the substrate and superlattice. The degree of strain retained in the GaSb wells has been examined from analysis of the optical mode frequency shifts measured by Raman scattering.

Czochralski crystal growth in the system PtMnxSb2-x

Abstract Czochralski growth of the system PtMn x Sb 2− x was investigated to explore its magnetic semiconducting potential. Using the LEC technique, PtSb 2 single crystals, p-type with ~ 10 18 carriers cm -3 , were obtained from BN crucibles; doping with Te exhibited a k (eff) of about one and produced n-type material. Melts of a composition PtMnSb crystallized as PtMnSb in single crystal form. Melt compositions of PtMn 0.25 Sb 1.75 resulted in the formation of PtSb 2 and with continuing growth in the appearance of several different phases, suggesting the likely absence of congruently melting compositions with x

Czochralski growth of single crystals of Ni3−xMnxSn

Abstract Single crystal growth in the Heusler alloy system Ni 3- x Mn x Sn was explored using the Czochralski technique. For x close to 1.0, single crystals (as indicated by X-ray Laue pictures and metallography) were grown under an Ar atmosphere, in vitreous C crucibles, at growth rages of ∼ 0.5 inch/h, using a stirring speed of 30 rpm. When x in the melt was not close to 1.0, constitutional supercooling problems and dendritic “globs” made it extremely difficult to increase boule diameter and control growth on seeds. Lattice parameters, pyrometer temperature readings and differential thermal analysis suggest that Ni 2 MnSb is close to a congruently melting composition, so that solids frozen very far from x =1.0 differ in composition from the liquid and as a consequence their growth would be expected to be difficult. However, with care boules with substantial sized single crystal grains with x as large as 1.5 and as small as 0.9 could be grown.

Czochralski growth of doped single crystals of Bi2Te3

Abstract Unencapsulated Czochralski growth normal to the c axis was used to produce good quality Bi2Te3 single crystals. Growth was successful both in H2 and in an inert atmosphere with a seed rotation speed of 30 rpm and a pull rate of 12 mm/h. Residual acceptor concentration for nominally stoichiometric crystals was about 1019 carriers cm-3. The distribution constant for Te is 10-2 so that with large Te excesses in the liquid and slow growth rates, n-type (1019) material could be prepared. Doping with Ti, Cr, Mn, Fe, Co, Ni, Ge and Sn was studied. Mn has a distribution constant of 10-1 so that samples with as much as 0.3% Mn could be prepared. The normal freeze equation was not obeyed probably because of facet effects and changes in melt stoichiometry as freezing proceeds. The relative ease of growth and doping suggests that single crystal growth and characterization of other nontetrahedral semiconductors could be worthwhile and that bismuth telluride could be an interesting host for the study of magnetic ions in semiconductor single crystals.