Assessment of Organotellurium Compounds for Use as Movpe Precursors
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ASSESSMENT OF ORGANOTELLURIUM COMPOUNDS FOR USE AS MOVPE PRECURSORS J.E. HAILS, S.J.C. IRVINE, D. COLE-HAMILTON*
J.B.
MULLIN,
D.V.
SHENAI-KHATKHATE*
AND
Royal Signals & Radar Establishment, St. Andrews Road, Malvern, Worcs. WR14 3PS. UK *
University of St. Andrews, St. Andrews, Fife, UK
ABSTRACT In order to reduce the growth temperature of (Hg,Cd)Te by MOVPE below 350-400"C, alternative organometallic precursors will be required which either decompose at a lower temperature than existing precursors or which absorb strongly at a suitable wavelength in the ultraviolet. The features required for a programme of assessment of organometallics are discussed. UV absorption spectra for dimethyltelluride, dimethylditelluride, diethyltelluride, di-iso-propyltelluride and diallyltelluride are presented and their usefulness as photolytic MOVPE precursors discussed. INTRODUCTION Efforts are currently in progress to reduce the temperature at which the infra-red detector material (Hg,Cd)Te can be grown. This reduction in temperature is necessary in order to reduce and control the equilibrium mercury vacancy concentration which in turn would allow greater control of the electrical properties. In addition, the lowering of the growth temperature would also reduce the interdiffusion between substrates and epitaxial layers thus enabling more complex structures to be grown. The lowest temperature at which pyrolytic growth can occur is dependent upon the decomposition temperature of the more stable of the organometallic precursors. Among the most commonly used precursors t for the MOVPE growth of (Hg,Cd)Te are Me 2 Cd, Et 2Te and Pr 2 Te and it is the stability of the tellurium alkyl that limits the growth temperature to around 410"C for Et 2 Te and 350"C for Pri 2 Te. In order to reduce the growth temperature a less stable organotellurium compound is required. An alternative method of growth is by photolysis which also offers the potential for photo-patterning and an organotellurium compound which strongly absorbs in the UV at a wavelength corresponding to a laser or arc lamp emission, for example, 193, 248, 254 or 257nm would be invaluable in photolytic growth. For any organometallic compound to be suitable as an alternative to the existing ones it must possess several other properties. It must be stable at room temprature, under an inert atmosphere if necessary, for a period of at least several months and preferably indefinately. It must also be sufficiently volatile to be transported around the MOVPE system, ideally with a vapour pressure of >1 Torr at room temperature. Equally importantly an alternative organometallic should not undergo any undesirable reactions such as polymerisation or premature vapour phase reaction with the other organometalics in the system. It must also be obtainable chemically pure. Several techniques are likely to be of use in the assessment of organotellurium compounds for use as MOVPE precursors and these include UV spectroscopy, vapour pressure measurements, study of decomposition products and the
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