P. Rudolph

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.

Distribution and genesis of inclusions in CdTe and (Cd,Zn)Te single crystals grown by the Bridgman method and by the travelling heater method

Abstract The density and distribution of inclusions of the excess component in CdTe and (Cd,Zn)Te crystals grown by the Bridgman method and by the travelling heater method (THM) have been investigated by transmission infrared (IR) microscopy. A correlation between composition of the metl (melt-solution) and inclusion density has been found. An inclusion-free CdTe Bridgman crystal was grown using a Cd overpressure, a source temperature of 850°C and a melt temperature of 1118°C. The highest inclusion density was detected in a (Cd,Zn)Te THM crystal grown from a Te-rich zone under microgravity conditions. For the first time the kinetics of the inclusion genesis are discussed in detail. The inclusion morphology is very sensitive to the temperature field. In contrast to as-grown crystals, cooling down solution-grown crystals in a very small temperature gradient causes a symmetrical inwards crystallization of the solution as entrapped droplets. In a large temperature gradient, temperature gradient zone melting (TGZM) of the included droplets occurs. Their migration results in {111} facet development at the dissolving interface, although the growing interface is rounded.

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.

Defects in CdTe bridgman monocrystals caused by nonstoichiometric growth conditions

The stoichiometry of the Cd/Te melt composition for the Bridgman growth of CdTe in an evacuated and sealed ampoule is critically evaluated. It is determined by the accuracy of the initial weighing and a remarkable Cd loss in the vapour phase. Experimental results on Te inclusion and precipitation will be given. Size, density and axial distribution of inclusions and precipitates are correlated to the nonstoichiometry of the melt.

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.

New method for the determination of VCd concentrations in p-CdTe

Abstract If p-CdTe crystals are dipped in an AgNO 3 solution followed by storage of the crystals at 300 K, one can observe a strong decrease in the hole concentration after some days. This is obviously due to fast diffusion of silver, which takes place most probably via interstitial sites (Ag i ), and is consistent with the increase in the degree of compensation detected by photoluminescence (PL) analysis. Deep-level transient spectroscopy investigations have shown the appearance of a new hole trap at 0.47 eV after the silver diffusion procedure, which is tentatively assigned to the Ag i deep donors. If the silver treatment is applied to p-CdTe crystals annealed in a tellurium atmosphere, the concentration of Ag Cd impurities grows owing to the defect reaction V Cd +Ag I →Ag Cd as can be seen from the enhancement of the corresponding (A 0 ,X) line in the PL spectra. Positron annihilation measurements with the same crystals have confirmed this reaction by the observation of a decrease in the average positron lifetime before the silver treatment (about 295 ps) to a value of about 285 ps which occurs after some hundred hours. By means of a quantitative PL analysis we are able to estimate the concentration of isolated V Cd in p-CdTe from the increased Ag Cd concentration. Other possible defect reactions are discussed on the basis of our PL investigations.

Systematic steps towards exactly stoichiometric and uncompensated CdTe Bridgman crystals

A modified vertical Bridgman arrangement with Cd extra source and variable CdTe melt surface temperature is used to find out conditions for the growth of near stoichiometric CdTe crystals. Growth experiments were carried out with different temperature courses according to predictions for the optimum temperature program obtained from thermodynamic calculations. The transition point from p- to n-type conductivity for inclusion-free crystals was observed at a CdTe melt surface temperature of 1118°C and a Cd source temperature of 850°C. The incorporation of shallow acceptors (Ag, Cu) as a function of the deviation from stoichiometry during the growth was analysed by photoluminescence, mass spectroscopy and atomic absorption spectrophotometry. The incorporation coefficients of atoms substituting Cd were deduced in dependence on their total concentration in the melt and the Te excess. The maintenance of nearly stoichiometric growth conditions drastically reduces the substitutional impurity fraction, acting as shallow acceptors, and therefore the carrier concentration.

Complex formation in In- and Ag/Cu-doped CdTe

Abstract Interactions between In donors and Ag, Cu and acceptor-like native defects have been studied in p- and n-type CdTe single crystals by perturbed angular correlation (PAC) spectroscopy on 111In probe atoms. Silver diffusion into the p-type samples at room temperature results in the formation of a distinct complex characterized by an interaction frequency eQVzz/h = 60.1 MHz and an asymmetry parameter η = 0.15. At room temperature the relative fraction of this complex increases within a few hours and decreases with a significantly longer time constant. After copper doping, a similar complex with 57.5 MHz and η = 0.16 was observed which, however, did not show any decrease. The observed behaviour is explained by fast diffusion of Ag (or Cu) via an interstitial mechanism and the interaction with cation vacancies. Finally, In-Ag (or In-Cu) complexes are formed.

The influence of thermal diffusion on the travelling heater method in a diffusion controlled growth system

Abstract The transport of matter in a diffusion controlled THM growth system is considered from a theoretical point of view. Particular attention is given to the influence of the thermal diffusion effect (Soret phenomenon). By means of a simple mathematical model it is shown, for the growth of PbTe from a Te-rich solution zone, that a separation of components of 4–5 at% may occur within the zone region of maximum temperature. This results in changes in the concentrations and temperatures of the solid-liquid interfaces, and it leads to modified maximum growth velocities up to those which ensure the macroscopic stability of the liquid zone. The model has been designed in such a manner that other material systems can be treated as well.