Structure and Properties of W and Mo-Subcontact Layers Influenced by Thermal Annealing
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ABSTRACT One of the most important problems to solve in order to improve the CdTe solar cell performance is the deposition of stable ohmic contacts. W and Mo with their high work function, high temperature stability and good mechanical properties are prospective materials for contacts to p-CdTe. This paper presents data for the electrophysical characteristics of W or Mo/CdTe and W or Mo/ CdS-CdTe heterostructure systems. W and Mo films are deposited by a W(CO) 6 based CVD process at atmospheric pressure and low temperatures (250-300)°C. We emphasize the improvment of the contact properties by developing a transitional layer heavily doped with acceptors. The defects arrangement in the layer should promote the diffusion of such impurities as Cu, P, As etc. A proper balance between the impurities and the defects could be achieved by suitable thermal treatment. Rapid thermal or traditional annealing in different gas atmospheres - N2, Ar or air were applied, followed by chemical or electrochemical treatment. Electrical characteristics measurements and structural studies were performed for CdTe layers as well as for the Sn0 2- CdS - CdTe structures INTRODUCTION W and Mo are suitable for back contacts of CdTe solar cells because of their high work
function, high mechanical and chemical stability and because of the possibility by low-cost technology to be deposited on the surface of semiconductor layers. We have experience in chemical vapor deposition (CVD) of these refractory metals using low-temperature processpyrolitical decomposition of W(CO) 6 at atmospheric pressure in argon atmosphere. Our previous studies of W and Mo films showed that if deposited on monocrystalline Si, W films grow textured [1]. The higher degree of structural arrangement and high density of the CVD films cause the low resistivities of W and Mo films [2]. When applied as back contacts to CdS/CdTe photocells, these films lead to open circuit voltage in the order of 700mv [3]. The IV characteristics of these structures show a decreased short circuit current assigned to a possible high resistive sublayer due to different surface ractions. In order to decrease the contact resistance, suitable surface treatments are usually applied. Mo and W can be used as sources for diffision of acceptor impurities into the CdTe layer. In this case one of the most important conditions for obtaining of stable ohmic contacts is the preparation of the CdTe surface before deposition of the metal. It is necessary for the treatment to be compatible with the metal contact deposition process, which requires a temperature of 290 °C. Usually p-CdTe is submitted to an etching procedure, leading to surface enriched with tellurium. The most practical etching solutions used are for instance Br in methanol or HNO 3 +
477 Mat. Res. Soc. Symp. Proc. Vol. 403 01996 Materials Research Society
H 3P0 4. They lead to forming on the CdTe surface of tellurium film with imbedded in it tellurium oxide. If combined with high temperature processing (CVD is one of those) , this may result in
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