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L. BAUFAY, A. PIGEOLET, R. ANDREW and L.D. LAUDE Universitg de l'Etat, Avenue Maistriau, 23 7000 MONS

Belgium.

ABSTRACT Optical and electrical characterization of CdTe synthetized by laser irradiationofamultilayer film of alternately Cd and Te is achieved. Optical absorption measurements evidence the good quality of these films and show that they have behaviour comparable to the single crystal. The influence of the irradiation conditions on the electrical properties of such CdTe films is discussed; they are compared to single crystal from the point of view of resistivity. It is shown that it is possible to prepare by this means samples devoid of impurity states in the middle of the forbidden gap. Finally, the ohmicity of Au, Al, Cr, ITO and non irradiated Cd/Te sandwich contacts is tested.

INTRODUCTION Aside from the applications of laser annealing to essentially monoelement systems, as for example the recrystallisation of amorphous or implanted layers in Si or Ge, there are also a number of reports of what can be termed laser-induced synthesis of compounds involving two or more elements in stoechiometric ratio. We may include here the formation of metal silicides [1,2] by irradiation of metal overlayers on silicon wafers, and the formation of semiconducting oxides [37] by irradiation of the corresponding metallic films in oxygen rich ambiances. We have previously reported the formation of the compounds AlSb, AlAs, CdSe, CdTe and CdTexSel-x [8-101 by irradiation of metal sandwich structures of these materials on thermally insulating substrates and have proposed a model [ 11141 for the observed reaction which takes account of the heat of formation involved. Note that whereas laser annealing of compound semiconductors does not in general lead to very satisfactory results, as compared to the success with silicon, our synthesis technique differs in that the starting point is not the compound but the separate constituents, and we have reason to believe that the liberation of heat of formation during the reaction, leading as it does to high local temperature gradients and a kind of zone refining action, allows us to produce relatively high quality films even when the components before irradiation may be as much as 1-2% away from the correct stoichiometric ratio. The purpose of the present article is to examine more closely the electrical and optical properties of CdTe films produced in this way. FILMS PREPARATION Metallic films preparation Cd and Te layers are successively and alternately condensed onto sputtercleaned glass substrates using a multiple crucible e-gun source and a quartz thickness monitor at a base pressure of 10-6-10-7 torr. Thickness and number of elemental layers are adjusted to give as nearly as possible (uncertainty 2%) equal numbers of Cd and Te atoms in the overall film. In order to standardize production and have the more volatile component (Cd) buried in the film away from the Mat.

Res.

Soc.

Symp.

Proc. Vol

11 (1983)

EElsevier Science Publishing Co.,

Inc.

666

outer surfaces, the succe