Vapour Pressure Measurements on Organotellurium Precursors for Movpe
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VAPOUR PRESSURE MEASUREMENTS ON ORGANOTELLURIUM PRECURSORS FOR MOVPE J.E. HAILS, S.J.C. IRVINE AND J.B. MULLIN Royal Signals & Radar Establishment, St Andrews Road, Malvern, Worcs, WR14 3PS, UK ABSTRACT Before an organometallic compound can be used as an MOVPE precursor, certain basic properties need to be known and one of the most important of these is vapour pressure. A technique for the measurement of vapour pressures using an MOVPE system designed for precursor assessment is described. Results on the assessment of di-is-propyl-telluride, di-n-propyl-telluride, di-allyl-telluride, methyl-allyl-telluride and di-t-butyl-telluride are presented. INTRODUCTION A well accepted objective for improved material quality in the MOVPE growth of the infra-red detector material (Hg,Cd)Te is deposition at lower temperatures. This will lead to reduction and control of the equilibrium mercury vacancy concentration, allowing greater control of the electrical properties. Lowering the growth temperature would also reduce the interdiffusion between substrates and epitaxial layers, thus enabling more complex structures to be grown. There are two ways in which the growth temperature can be lowered, both of which will require the use of tellurium precursors other than the commonly available Pri 2Te and Et 2Te. Firstly by the use of precursors which are thermally less stable than those currently limited to 410"C by Et 2Te or 350'C by Pri 2Te, this essentially means a less stable tellurium precursor. Secondly photolytic growth can also provide a way of growing (Hg,Cd)Te at lower temperatures[1]. Before a new organometallic compound can sensibly be used as an MOVPE precursor, certain basic chemical and physical properties need to be known about it and one of the most important of these is vapour pressure in the range of temperature typically used for bubblers, 0-30"C. Ideally the volatility should be above 1 Torr at 20'C and the precursor be thermally stable at these temperatures. Several tellurium precursors have been tried recently, but vapour pressures recorded in the literature are limited to single temperature measurements (Table 1, reference 2). Even the commonly used Pri 2Te is only recorded as 5.6 Torr at 30"C [3,4]. In order to remedy this situation, vapour pressure measurements over the range of temperatures 0-30'C on a number of possible tellurium precursors have been measured and are presented here. EXPERIMENTAL All the vapour pressure measurements which are discusssed here have been carried out using an MOVPE system specifically designed to investigate organotellurium compounds for suitability as MOVPE precursors[2]. The section of the system used in measuring vapour pressures, shown schematically in Fig. 1, is located between the gas handling section and the MOVPE reactor. The pipework shown in the diagram was held at 40'C throughout. Prior to collection of a sample for vapour pressure measurement, the reactor and the section in Fig. 1 up to valves A and B were evacuated to 9x10-7 Torr at the Penning gauge on the pumping sys
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