Growth of CdTe bulk crystals by the multi tube physical vapour transport process

Abstract

This thesis is primarily concerned with the growth of bulk cadmium telluride (CdTe) crystals. The work relates to a modified physical vapour transport system which also enables the growth of ternary ll-VI compounds, in particular cadmium zinc telluride (CdZnTe). A computer simulation to model the CdTe growth process is developed, along with a system to measure the partial pressures in situ during growth. The modified Multi Tube Physical Vapour Transport (MTPVT) system, which is essentially a combination of the Markov annulus and Rosenberger flow restrictor designs, consists of two source tubes and a growth tube connected by a crossmember. This crossmember, which is optically heated, contains two capillaries which act as flow restrictors, and allows physical displacement, and therefore better thermal decoupling, of the source and growth areas. The growth of CdTe in the MTPVT system is investigated, in particular, factors affecting the optimal design for the growth tube. The design of the pedestal, on which the seed crystal is located, is of prime importance in preventing polycrystalline growth, and the size and shape of the seed crystal is also critical. Increasing the inner diameter of the growth tube from 32 mm to 52 mm reduces the effect of the annulus gap around the seed, and increases the controllability of the growth process. A computer simulation is used to model the vapour flow from the subliming source, through the capillary, and to the growing crystal and down the annulus. The trends predicted closely match those obtained experimentally, and imply, as expected, the growth process is controlled by the mass transport rate through the capillary. Further comparison with experiment gives an upper limit of 0.45 eV for the activation energy of the incorporation of atoms into the CdTe seed, although no lower limit can be set. The partial pressure - optical density relationships are derived for Cd(_g), Zn(_g) and Te2(_g), with the 214 nm absorption line of Zn(_g) observed to deviate significantly from Beer’s Law. Optical monitoring during CdTe growth by a computer controlled optical absorption measurement system allows in situ monitoring of the partial pressures. The measurements of the source side partial pressures match those predicted by the computer simulation, with the partial pressure ratio of around 1.7 also consistent with the model.