Ohmic contacts to p-CuInSe2 crystals

Regular Issue Paper

Journal of Electronic Materials, Vol. 22, No. 3, 1993

Ohmic Contacts to p-CulnSe 2 Crystals ELLEN MOONS, TINA ENGELHARD, and DAVID CAHEN The Weizmann Institute of Science, Department of Materials and Interfaces, Rehovot, ISRAEL 76100 We report here on the optimization of ohmic contacts to p-CuInSe 2(CISe) single crystals. A low resistance ohmic contact is required to minimize current losses due to series resistance; e.g. in Schottky diodes. Both In-Ga (eutectic)/CISe and gold (evaporated)/CISe contacts have been fabricated on crystals with different orientations and bulk properties. Gold contacts were found to have a lower resistance and to be more stable than In-Ga ones, from the slope of the linear current-voltage plot of the junctions. The resistance of the Au/CISe ohmic contact was decreased by etching the CISe crystal surface chemically in a 0.5% solution ofBr 2in methanol for 30 sec at room temperature, prior to gold deposition, while that of the In-Ga contact increased by this etch. Wetting experiments and contact angle measurements showed evidence for changes in the polarity of the surface due to chemical etches. K e y words:

Chemical etch, contact angle, CuInSe2, ohmic contact, metalsemiconductor, I-V characteristics

INTRODUCTION The fabrication of metal/semiconductor contacts and more particularly of stable and low resistance ohmic contacts to semiconductors is an important and still existing problem in semiconductor engineering. Generally, each semiconductor requires its own specific contacting material for forming an electronically and atomically stable interface. 1 Minimizing contact resistance is of direct importance for optimizing the performance of solar cells .2.3 CuInS% (CISe) is a promising semiconductor absorber material for efficient polycrystalline thin film solar cells.4 Native defects play a crucial role in the solid state and surface chemistry of this material and therefore in the determination of its electronic properties. In particular, the photovoltaic performance of polycrystalline devices appears to be limited by grain boundary recombination.4~ Single crystals of CISe are studied to understand the basic chemistry and physics of this material. As with many other semiconductors, forming ohmic contacts appears to be less of a problem with n-type (CISe) than with p-type (CISe), which is of greater present interest for solar cells. In most polycrystal(Received September 23, 1992)

line CISe-based devices, molybdenum is used as a low-cost back contact metal, although formation of a small Schottky barrier with molybdenum has been reported. 7-11Furthermore, Russell et al. v found that photovoltaic cells with a gold back contact have a higher open circuit voltage (Voo)than identical cells with a molybdenum back contact. Matson et al.12have investigated alternative metal back contacts to pCISe thin films, and concluded that only gold and magnetron sputter-deposited nickel give reproducible ohmic contacts. In early work on CISe, local in-diffusion of indium was used for making ohmic contacts to p-CISe single crystals. 13,14 However, this changes the material's stoichiometry and the native defect chemical equilibria, and thus its electrical and optical properties. 15,16 In studies on homojunction formation in p-CISe, conducting silver paint has been used as an ohmic back contact, 17 in some cases after sandblasting the back side of the crystal. 18This material has also been used for back-contactingA1/CISe Schottky barriers. 19Sputtered 2~and electroplated21gold were reported to form a low resistance ohmic contact to p-CISe; evaporated gold22 has been used as well. However, it was found that sputtered gold contacts degrade with time, and a more reproducible method, with better stability over time, was developed by rubbing In-Ga alloy onto a c~rT~

276 polished surface. 23 Abou-Elfotouh et al. have compared the current-voltage (I-V) characteristics of thermally evaporated gold and gold-beryllium, electron beam evaporated platinum and radio frequency (rf)sputtered molybdenum contacts to p-CISe single crystals with different compositions. 24,2~Of all these, platinum contacts showed the most linear behavior. Linear current-voltage characteristics with gold were obtained only for crystals with a copper-rich composition (Cu/In>l). Annealing in forming gas at 200~ improved the gold contacts. Annealing at temperatures up to 480~ either in air or in forming gas (reducing agent) was necessary to improve the platinum, the molybdenum, and the gold-beryllium contacts. Several metal contacts (evaporated gold and direct current [dc]- sputtered molybdenum, nickel, and aluminum) to single crystals ofp-CISe have been studied in systematic work by Toro. 2GHe used a threeprobe ring structure as contact configuration to measure the current-voltage (I-V) characteristics with a negligible voltage drop across the contact. The contact resistivity was calculated directly from the slope of the I-V plot, in the forward range. In addition, he performed electron beam-induced current measurements, electron probe microanalysis, and Auger electron spectroscopy. His conclusions are as follows: 1. all the metals that were investigated form homojunctions buried about 1 ~ into the CISe. 2. Molybdenum, nickel, and gold show linear I-V characteristics at room temperature. As deposited nickel and gold yield a lower contact resistivity, 0.6 ~2 cm 2, than molybdenum, 5 ft cm 2. The lowest contact resistivity was obtained with nickel, annealed at 450~ in H 2 (