Optical vapour pressure monitoring and mass transport control during bulk CdTe crystal growth in a novel multi-tube PVT system
This work is concerned with the development of a new vapour growth system for the growth of high quality bulk CdTe crystals, and, in particular, the study of mass transport control and in situ vapour pressure monitoring during growth in this system. The Multi-tube Physical Vapour Transport (MPVT) apparatus designed in Durham incorporates several modifications and improvements from past PVT methods. The U-shaped system, which is based on the Markov design, consists of three main parts, namely the source, crossmember and growth tubes, and allows to some extent decoupling of the temperature fields between source and growth regions. The introduction of the crossmember is effectively the technological key that enables the use of a flow restrictor to control mass transport and the introduction of a non-intrusive optical absorption technique capable of monitoring the partial vapour pressures of the vapour elemental constituents, i.e. Cd atoms and Te(_2) molecules, as growth proceeds. The optical absorption in the near UV and Visible spectrum of Cd and Te(_2) vapours has been quantitatively studied. The non-overlapping of the tellurium and the cadmium absorption bands allowed the use of a simple and compact light and detection system for in situ monitoring of vapour partial Cd and Te(_2) pressures independently during growth in the MPVT apparatus. In particular, the optical absorption system is capable of monitoring, after suitable calibration, cadmium vapour pressures in the range 0.01 -20 mbar and tellurium vapour pressures in the range 0.1-18 mbar The use of porous silica discs for use as flow restrictors is quantitatively studied both using theory and experiment. The transport properties of these discs as a function of intrinsic parameters (porosity, dimensions and specific surface area) and extrinsic parameters (temperature, pressure and molecular weight of the gas) are evaluated experimentally in detail. The experimental results are found to agree with the predictions of standard theoretical expressions. In particular, for the case of a multicomponent vapour, i.e. Cd and Te(_2) vapour mixture, it is experimentally found that the porous silica flow restrictor can also set the stoichiometry of the vapour to a smaller value than that expected under congruent CdTe sublimation. It is shown that use of a porous silica disc in conjunction with dynamic pumping during growth allows the mass transport rate to be set to the desired value, and permits diffusionless transport. Modelling of the transport process under several possible flow regimes encountered during normal growth conditions is also presented. The model, which assumes that diffusion mechanisms are unimportant, allows expressions to be obtained for the mass transport rate through the flow restrictor (porous disc or capillary) in the crossmember, the growth rate and the stoichiometry of the vapour above the source and crystal. Optical monitoring during CdTe growth carried out in several growth runs confirms both the predictions of the model concerning the stoichiometry of the vapour and its usefulness as a possible tool to monitor the growth process and growth rate.