Effects of annealing and surface preparation on the properties of polycrystalline CdZnTe films grown by molecular beam epitaxy

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Abstract
The effects of annealing and chemical etching on the chemical, compositional, and electrical properties of polycrystalline molecular-beam epitaxy (MBE)-grown Cd1−xZnxTe thin films were investigated in detail for the first time to identify and eliminate some of the undesirable process-induced side effects resulting from the CdZnTe/CdS solar cell fabrication procedure. Depth-resolved x-ray photoelectron and Auger electron spectroscopies on processed surfaces along with current-voltage and capacitance-voltage measurements on In/Cd1−xZnxTe Schottky diodes were used to characterize the treated Cd1−xZnxTe surface. Films were annealed in air at 200–400 °C and etched in Br:CH3OH and K2Cr2O7:H2SO4 solutions, similar to processes used to achieve high efficiency CdTe/CdS solar cells. Air annealing was found to enhance the uniformity of the p-type conductivity within the film, yielding a doping value of ∼3×1015 cm−3. However, the air anneal resulted in a highly Zn-rich surface compared to hydrogen and argon anneals. Furthermore, the air anneal resulted in the oxidation of ∼80% of the Zn at the surface which was found to extend ∼0.1 μm into the bulk. This is in contrast to Cd and Te oxides which were detected only at the surface. A model is proposed to explain the Zn pileup near the surface. According to this model, the oxygen diffuses into the film to oxidize Zn resulting in a concentration gradient and outdiffusion of Zn. Subsequent etching of the annealed films by the standard Br2:CH3OH etch used for CdTe processing removed the surface Te and Cd oxides but failed to remove the Zn-rich surface region and Zn–O and did not result in a p+ Te-rich surface which is required to facilitate ohmic contacts. In contrast, etching of the annealed Cd1−xZnxTe surfaces with a concentrated K2Cr2O7:H2SO4 solution removed all oxides and left a surface completely depleted in Cd and Zn. Subsequently fabricated In/Cd1−xZnxTe Schottky barrier diodes on etched surfaces confirmed a significant reduction in effective barrier height, from ∼1.2 eV for the Br:CH3OH surface to ∼0.65 eV for the K2Cr2O7:H2SO4 etched surface due to a Te-rich surface layer with a p-type doping concentration of ∼3×1017 cm−3. It is expected that these results will provide guidelines to increase present-day Cd1−xZnxTe cell performance by reducing the resistance of ‘‘ohmic’’ contacts.

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