Photochemical Processes in Photo-Assisted Epitaxy of Cd x Hg 1-x Te
- PDF / 1,038,282 Bytes
- 9 Pages / 417.6 x 639 pts Page_size
- 5 Downloads / 167 Views
PHOTOCHEMICAL PROCESSES IN PHOTO-ASSISTED EPITAXY OF CdxHg 1 _xTe S J C IRVINE, J B MULLIN, G WBLACKMORE, 0 D DOSSER AND H HILL Royal Signals and Radar Establishment, St Andrews Road, Malvern, Worcestershire, WR14 3PS, UK. ABSTRACT Mechanisms for the low temperature photo-dissociation of alkyl precursors for the epitaxial growth of CdxHgI xTe (CMT) are discussed. The roles of vapour and surface nucleation are considered in the light of the free radical model which also can provide methods for controlling0 the vapour phase photochemistry. Higher quality CMT has been grown at 250 C by using dimethyl mercury as a source of free methyl radicals. Problems encountered in reducing growth temperature for multilayer epitaxy are considered for CdTe and HgTe and conditions established for epitaxial growth at 2000C. The roles of alkyl concentration, substrate temperature, UV intensity and free radical concentration are explored. Results on the first reported growth of CdTe deposition using a cw UV laser source are compared with the arc lamp grown layers. INTRODUCTION The application of photo-dissociation to low temperature epitaxy is now widely recognised but the processes are not well understood and only a small proportion of the potential benefits has been realised so far [1,2]. The objectives for Cd Hgi-xTe (CMT) low temperature photo-epitaxy are very specific, namely: (a) to achieve more abrupt heterostructure interfaces (b) control Hg vacancy concentration (c) achieve selective area deposition of detector structures (d) in-situ processing. Objectives (a) and (b) have been partially achieved with photo-epitaxial HgTe/Cfie interfaces of - 1OOA at 230°C and Hg vacancy concentrations below 1 x 10 cm- [1]. The photoexcitation of metal organics such as dimethylcadmium (Me2 Cd) and diethyltelluride (Et Te) can lead to either an enhancement of growth rate such as by a factor o' 3 at 350-C for CdTe [3] or a purely photon initiated reaction such as HgTe epitaxy at 250°C. The preferred photon sources have been arc lamps capable of illuminating large areas with high intensity UV radiation [2]. An excimer laser has been used for a purely vapour phase photo-dissociation by Morris [4] and in the present work some preliminary results on cw-laser irradiation will be reported. Although, all these approaches should be capable of fulfilling requirements (a) and (b), only the latter would be suitable for (c) and (d). By non-thermal photo-dissociation of the alkyl precursors, the thermal stability is no longer a limitation to growth temperature. Furthermore, growth temperatures below 200*C have been demonstrated for both HgTe and CdTe with 0 evidence of good crystallinity [4,5]. For growth temperatures above 200 C more detailed studies of epitaxy onto CdTe and InSb substrates have indicated a very high inherent epitaxial quality [1,3,6]. In this respect photo-excitation of the surface may actually aid the growth process by increased atom mobility or product desorption. Kisker and Feldman [3] showed that photo-enhanced CdTe growth at 350*C onto
Data Loading...
