Manfred Mühlberg

Basic problems of vertical Bridgman growth of CdTe

Many efforts have been made to grow CdTe bulk crystals with a low defect content but improvements are limited. The best ingots with large extended single-crystal regions can be grown by the vertical Bridgman method. However, fundamental studies about the CdTe growth peculiarities are absent. Our investigations are concentrated on the following problems: (i) the influence of the melt structure, from which an associated state is assumed, on the crystalline quality, (ii) the composition instability in conventional ampoules, (iii) the segregation behaviour of the excess component (normally tellurium), (iv) the axial distribution of inclusions and precipitations, (v) the mass transport in modified ampoules with an additional cadmium source, (vi) the correlation between the vacancy and impurity segregation and (vii) the substrate purity as a function of the axial crystal position.

Crystalline and chemical quality of CdTe and Cd1-xZnxTe grown by the Bridgman method in low temperature gradients

Abstract CdTe and (Cd, Zn)Te grown by the vertical Bridgman method in low temperature gradients have been investigated with respect to the structural perfection using X-ray double crystal topography and an etching technique. Furthermore, information on the axial distribution has been obtained by means of lattice constant and infrared transmittance measurements, respectively.

Interface shape observation and calculation in crystal growth of CdTe by the vertical Bridgman method

Abstract The vertical Bridgman method is the most favored growth process for CdTe. However, the crystals include a large number of defects. Observations and calculations show a changing interface shape at several stages of growth. An optimization of the aspect ratio of the ingot can lead to more monocrystalline material because of the diminishing influence of the end effects.

The correlation between superheating and supercooling in CdTe melts during unseeded bridgman growth

Recently, the crystal growth of semiconducting compounds using a low temperature gradient profile ( ≤ 10 K/cm) has gained in interest. In the case of unseeded growth considerable supercooling in the tip region can be observed. These supercooling effects depend on the degree of superheating in the molten state. A decrease in the associated structure of molten CdTe is reflected by a step-like increase in the degree of supercooling for melts superheated by more than about 10 K. A large-extended polycrystalline region can result on crystallization if superheating > 9–10 K is used. This polycrystalline first-to-freeze region is followed by a section of monocrystalline crystal or by a region with only one or two grain boundaries. When a small degree of superheating is used (

Origin and evolution of background impurity content of materials used in the preparation of (Hg, Cd) Te LPE layers on CdTe substrates

Abstract The presence of unintentional background impurities found in LPE-grown Hg 1- x Cd x Te layers has been traced back to the starting materials and different technological steps in the course of preparation of the layers. The purified elements Cd and Te, the binary compounds HgTe and CdTe synthesized from them, VB-grown CdTe monocrystals, LPE source solutions and the final LPE (Hg 0.78 Cd 0.22 Te/CdTe) layer/substrate structures have been analysed with regard to their impurity content. Spark source mass spectrometry, atomic absorption spectrophotometry and secondary ion mass spectrometry were the analytical techniques employed. Generally, any high-temperature and handling procedures cause an increase in the concentration of most of the impurities. For CdTe Bridgman ingots, a non-uniform distribution with enrichment in the last-to-freeze part of the as-grown crystal is observed. Furthermore, it was found that the carrier concentration and conductivity type of annealed LPE layers are influenced by the varying impurity levels of substrates from different axial positions within the CdTe ingot. The impurity depth profiles of LPE layers show a gettering effect of the layer surface and the layer/substrate interface resulting in a reduced impurity level in the central part of the layers.

Crystal growth and elastic properties of orthorhombic Bi2Ga4O9

The combination of favourable oxygen conductivity at high temperatures with mechanical strength make Bi-containing compounds with mullite-type crystal structures strong candidates for use as electrolytes of solid fuel cells. Large single crystals of orthorhombic Bi2Ga4O9 with dimensions up to 20 × 20 × 10 mm3 were grown by the top-seeded solution growth technique. Their elastic constants at room temperature were determined for the first time using resonant ultrasound spectroscopy. The values given in GPa are c11 = 143.4(2), c22 = 161.7(2), c33 = 224.2(3), c44 = 69.6(1), c55 = 49.2(1), c66 = 76.5(2), c12 = 73.7(2), c13 = 62.2(3) and c23 = 70.3(3). Further, the crystal structure and the elastic properties of Bi2Ga4O9 were studied at 0 K by parameter-free ab initio calculations based on density-functional theory. On average the computed elastic constants differ from the experimental values by about 10%, indicating the reliability of the theoretical approach. Like in other mullite-type compounds the anisotropy of the longitudinal elastic stiffness is clearly controlled by the structurally dominant octahedral chains running parallel to [001]. The deviations from Cauchy relations show a significant anisotropy of the type g22>g11≈g33 which is related to the covalent character of the bonding interactions within the infinite –Bi–O–Bi–O– bond chains parallel to [010]. The mean elastic stiffness of Bi2Ga4O9 is about 40% smaller than for 2/1-mullite and sillimanite. This discrepancy can be attributed to the mechanically very soft behaviour of the Bi 6s2 lone electron pair. Its stereochemical activity is clearly evident from both the asymmetry of the bismuth coordination polyhedron and the calculated electron density maps.

Improved Single Crystal Growth of the Boron Sillenite “Bi24B2O39” and Investigation of the Crystal Structure

The boron sillenite, up to now known as the 12:1 compound Bi 24 B 2 O 39 in the system Bi 2 O 3 - B 2 O 3 and crystallizing in the space group 123, melts incongruently at 655 °C only about 25 K above the eutectic tie line and corresponding to a steep liquidus line. Single crystals with dimensions larger then 1 cm 3 have been successfully grown in [100], [110], and [111] direction by an improved Top Seeded Solution Growth (TSSG) technique equipped with crucible weighing, accelerated crystal rotation technique and air-cooled pulling rod. The structure of the boron sillenite was analyzed by X-ray diffraction method, which was possible due to the high crystalline quality achieved. A defect-free sublattice corresponding to a Bi-O framework is isostructural with all sillenites, but a 2 A environment around the origin is occupied by different cations with different population coefficients. The best calculation results in the formula Bi 24.5 BO 38.25 which is more Bi-rich than the 12:1 assumption.

Persistence of the stereochemical activity of the Bi3+ lone electron pair in Bi2Ga4O9 up to 50 GPa and crystal structure of the high-pressure phase

The crystal structure of the high-pressure phase of bismuth gallium oxide, Bi(2)Ga(4)O(9), was determined up to 30.5 (5) GPa from in situ single-crystal in-house and synchrotron X-ray diffraction. Structures were refined at ambient conditions and at pressures of 3.3 (2), 6.2 (3), 8.9 (1) and 14.9 (3) GPa for the low-pressure phase, and at 21.4 (5) and 30.5 (5) GPa for the high-pressure phase. The mode-Grüneisen parameters for the Raman modes of the low-pressure structure and the changes of the modes induced by the phase transition were obtained from Raman spectroscopic measurements. Complementary quantum-mechanical calculations based on density-functional theory were performed between 0 and 50 GPa. The phase transition is driven by a large spontaneous displacement of one O atom from a fully constrained position. The density-functional theory (DFT) model confirmed the persistence of the stereochemical activity of the lone electron pair up to at least 50 GPa in accordance with the crystal structure of the high-pressure phase. While the stereochemical activity of the lone electron pair of Bi(3+) is reduced at increasing pressure, a symmetrization of the bismuth coordination was not observed in this pressure range. This shows an unexpected stability of the localization of the lone electron pair and of its stereochemical activity at high pressure.

High-pressure phase transition of Bi2Fe4O9.

The high-pressure behaviour of Bi2Fe4O9 was analysed by in situ powder and single-crystal x-ray diffraction and Raman spectroscopy. Pressures up to 34.3(8) GPa were generated using the diamond anvil cell technique. A reversible phase transition is observed at approximately 6.89(6) GPa and the high-pressure structure is stable up to 26.3(1) GPa. At higher pressures the onset of amorphization is observed. The crystal structures were refined from single-crystal data at ambient pressure and pressures of 4.49(2), 6.46(2), 7.26(2) and 9.4(1) GPa. The high-pressure structure is isotypic to the high-pressure structure of Bi2Ga4O9. The lower phase transition pressure of Bi2Fe4O9 with respect to that of Bi2Ga4O9 (16 GPa) confirms the previously proposed strong influence of cation substitution on the high-pressure stability and the misfit of Ga3+ and Fe3+ in tetrahedral coordination at high pressure. A fit of a second-order Birch–Murnaghan equation of state to the p–V data results in K0 = 74(3) GPa for the low-pressure phase and K0 = 79(2) GPa for the high-pressure phase. The mode Grüneisen parameters were obtained from Raman-spectroscopic measurements.

Thermal expansion and elastic properties of mullite-type Bi2Ga4O9 and Bi2Fe4O9 single crystals

Abstract Resonant ultrasound spectroscopy was used to characterize the elastic properties of single crystal orthorhombic Bi2Ga4O9 and Bi2Fe4O9 between room temperature and about 1200 K. Additionally, the coefficients of thermal expansion were studied in the range 100 K to 1280 K using high-resolution dilatometry and X-ray powder diffraction. The elastic constants at 295 K are in GPa c11 = 143.4(1), c22 = 161.9(1), c33 = 224.5(1), c44 = 68.4(1), c55 = 49.3(1), c66 = 76.6(1), c12 = 74.2(1), c13 = 62.2(1), c23 = 70.5(1) for Bi2Ga4O9, and c11 = 106.7(1), c22 = 141.2(1), c33 = 183.7(2), c44 = 53.7(1), c55 = 41.9(1), c66 = 63.8(1), c12 = 63.5(1), c13 = 59.8(1), c23 = 63.4(2) for Bi2Fe4O9. In both mullite-type compounds the strong bond chains built up by edge-sharing coordination octahedra extending parallel to [001] dominate the anisotropy of their elastic and thermoelastic properties. Smaller variations of elastic anisotropy within the (001) plane can be attributed to the specific type of cross-linking of the octahedral chains. The temperature evolution of the cij shows no hint on any structural instability or glass-like transition that might be related to the suspected ion conductivity at high temperatures. However, in both crystal species characteristic anelastic relaxation phenomena occur in the ultrasonic frequency regime close to room temperature. The smallest thermal expansion is observed in the plane perpendicular to the stiffest octahedral chains. A model is discussed to explain the apparent discrepancy in terms of cross-correlations within the three-dimensional framework of edge- and corner-linked coordination polyhedra.