Krzysztof Grasza

A novel method of crystal growth by physical vapour transport and its application to CdTe

A novel method of in-situ “nucleation” and growth by physical vapour transport has been developed and applied to CdTe. The main feature of this method is the technique for formation of a perfect seed in the first stage of the crystal growth procedure. In this technique, due to imposing thermal conditions, the source material takes the form of a cone. The monocrystalline tip of this cone moves towards the surface of a crystal holder, adheres to it and then separates from the remaining part of the source. This single crystal serves subsequently as a seed for the bulk physical vapour transport growth process. The crystal grows inside a silica glass ampoule without wall contact. Large, high-quality single crystals of CdTe up to 8 cm3 of both p-type and n-type have been grown using this method. The etch pit density on the exposed {111} crystal faces was found to be as low as 2X103 cm-2. The half-maximum width of the rocking curve measured on the exposed {111} surfaces was between 24 and 100 arc sec. The resistivity and optical absorption of the crystals are also reported.

Bulk vapour growth of CdTe

Growth of CdTe crystals was performed reproducibly in 130 Torr hydrogen atmosphere by a method of self nucleation and growth with no contact between crystal and ampoule wall [K. Grasza et al., J. Crystal Growth 123 (1992) 519]. The growth temperature was 960 o C and the furnace translation rate 4 mm /day. Large CdTe single crystals up to 15 cm 3 have been grown using this method. The way to increase the size of crystals and results of growth of crystals up to 3.5 cm in length and 5.5 cm diameter are presented

Characterization of cadmium telluride crystals grown by different techniques from the vapour phase

Abstract Semi-insulting CdTe bulk crystals were grown from the vapour phase in both closed and semi-open arrangements. The results of the growth experiments are discussed in terms of various electrical and optical characterization methods. Van der Pauw measurements and time dependent charge measurements (TDCM) were used to determine the resistivity. Deep level defects were investigated by means of photoinduced current transient spectroscopy (PICTS). For one of the most important fields of application, detector spectra of the vapour phase material are measured and discussed.

Growth of cadmium-zinc telluride crystals by controlled seeding contactless physical vapor transport

Bulk crystals of cadmium-zinc telluride, 23 mm in diameter and up to 45 grams in weight were grown. Controlled seed formation procedure was used to limit the number of grains in the crystal. Most uniform distribution of ZnTe in the crystals was obtained using excess (Cd + Zn) pressure in the ampoule.

Surface morphology of vapour phase grown CdTe

Surfaces of large single crystals of CdTe grown from the vapour having no contact with the ampoule wall have been investigated by scanning electron microscopy. The morphology of the growth interface has been studied as a function of several growth parameters, such as temperature field, supersaturation and crystal orientation. The formation of flat facets, depressions and pipes parallel to the growth direction is discussed in relation to the growth conditions.

The optimal temperature profile in crystal growth from the vapour

Abstract An attempt at an analytical explanation of the role of the axial temperature profile in a tubular furnace in the growth of crystals from the vapour phase is reported. Three possible profile shapes are compared and discussed. The optimal profile is found to be characterized by an isothermal plateau at the location of the source material and a temperature hump between the source material and crystal. This profile was used for the growth of good quality ternary and quaternary ZnSe compounds with transition metals (ZnMnSe, ZnFeSe, ZnNiSe, ZnFeSSe, ZnSeS) by chemical vapour transport. The quality of the crystals was confirmed by double crystal X-ray diffraction, which showed the full width half maximum (FWHM) of the rocking curves to be as low as 12 arcsec on the natural faces of the crystals.

Estimation of the optimal conditions for directional crystal growth from the vapour phase with no contact between crystal and ampoule wall

In this paper an estimation of the optimal thermal conditions for directional crystallization of some semiconducting materials from the vapour phase with no contact between crystal and ampoule wall is presented. A graph of temperature versus ratio of emissivity and heat conductivity is shown which characterizes the temperature range suitable for the directional growth of crystals of a given material when our crystal growth system is used. An extension of this growth method for low thermal conductivity materials is proposed.

Stress birefringence in vapour-grown CdTe and its correlation to the growth techniques

The measurement of piezobirefringence in CdTe is a useful tool for the detection of long-range and local stress fields. By means of infrared polariscopy, we obtained both the orientation of principal stresses, and the size mapping of their differences. Wall contact of the crystal, self-seeding and formation of facets at the phase boundary are origins of highly strained regions. Local stress fields can be assigned to microscopic defects such as dislocations and inclusions. We will contribute to the classification of inclusions concerning their size and shape versus local stress generation.

Growth stability in high temperature vapour growth

Abstract Growth stability is studied in a system for high temperature vapour growth of PbSnTe. The transition from unstable to stable growth conditions is observed. The observation compared with theoretical analysis of the concentration and temperature field in the crystal and vapour at the growth interface confirms the recently published theory of Louchev [1] [J. Crystal Growth 140 (1994) 219] concerning diffusion, heat transfer, equilibrium molecular density and kinetics mechanism of morphological instability in physical vapour deposition and our conception of constitutional supersaturation criterion [2] [Grasza, in: Mass and Heat Transfer, Elementary Crystal Growth]. It is shown, that, additionally to the dependence of the morphological stability on the kinetic coefficient, the relation of the temperature gradient across the depositing layer to the gradient of concentration of the deposited substance in front of the growth interface, and the dependence of equilibrium molecular density (pressure) of the depositing substance on temperature, are the most important factors influencing the stability of the solid-vapour interface.

Characterization of cadmium-zinc telluride crystals grown by ‘contactless’ PVT using synchrotron white beam topography

Abstract Crystals of Cd 1 − x Zn x Te grown by PVT using self-seeding ‘contactless’ technique were characterized using synchrotron radiation (reflection, transmission, and Laue back-reflection X-ray topography). Crystals of low ( x = 0.04) and high (up to x ≈ 0.4) ZnTe content were investigated. Twins and defects such as dislocations, precipitates, and slip bands were identified. Extensive inhomogeneous strains present in some samples were found to be generated by interaction (sticking) with the pedestal and by composition gradients in the crystals. Large (up to about 5 mm) oval strain fields were observed around some Te precipitates. Low angle grain boundaries were found only in higher ZnTe content ( x ⩾ 0.2) samples.