The Dynamics of Cadmium Telluride Etching
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The Dynamics of Cadmium Telluride Etching K. D. Dobson, P. D. Paulson, B. E. McCandless and R. W. Birkmire Institute of Energy Conversion, University of Delaware, Newark, DE 19716 USA ABSTRACT CdTe etching was investigated using variable angle spectroscopic ellipsometry and glancing angle x-ray diffraction. Treatment with HNO3 :H3 PO4 (NP) based etches was shown to form amorphous-Te surface films which spontaneously crystallize following etching. Br2 /methanol (BM) etching forms very thin amorphous-Te films. NP-etched surfaces are stable in ambient air for ∼1 hr before beginning to oxidize, while BM etched films oxidize immediately following treatment. CdTe grain boundary etching by NP was minimized using more acidic etches. Device analysis suggests that a higher Te content produces more stable back contacts by attenuating Cu diffusion. Mechanistic details of NP etching are discussed. INTRODUCTION Wet chemical etching is used in a number of technologies and industries in order to achieve contaminant- free material surfaces. A critical aspect of back contact processing for CdTe/CdS cells is the preparation of a Te-rich CdTe surface to allow the formation of back contacts on reaction with metals. Most research groups utilize an oxidizing wet chemical etch to form Terich CdTe surfaces. Wet etches allow fast treatment times and are easy to use. Only small solution volumes are generally used and often no heating of the solution or substrate is required. However, penetration of etchants into the cell structure, the action of the atmosphere on the freshly etched surface and changes to film and grain boundary (GB) morphology during etching may affect device performance. Currently, two oxidizing etches are popularly employed for CdTe/CdS contacting at a research level; aqueous HNO3 /H3 PO4 (NP) and Br2 /methanol (BM). It is understood that these treatments do indeed produce Te-rich CdTe surfaces [1-3], however, despite their widespread use, the action of these etches on the CdTe surface is not completely understood. A knowledge of etchant behavior and the nature of the CdTe surface throughout the reaction, including its reactivity towards the atmosphere and back contact metals, will allow optimization of wet chemical treatments to form stable back contacts to CdTe/CdS cells. Generally the etch solutions are corrosive and toxic, therefore the development of a universal and safe wet etchant that can be easily applied to CdTe films, independent of thickness and history, would be advantageous for scale-up of the CdTe device processing. In this paper the action of various NP and BM etches on CdTe films has been investigated. Effects of concentration, acidity, and solution viscosity were studied, with the aim of understanding the mechanistic details of the etch process and to develop an optimized wet chemical etch for CdTe contact processing. Variable angle spectroscopic ellipsometry (VASE) and glancing angle x-ray diffraction (GIXRD), techniques that are sensitive to surface changes, were employed to monitor the effects o
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