Low Temperature Metalorganic Chemical Vapor Deposition of Semiconductor Thin Films for Surface Passivation of Photovolta
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Low Temperature Metalorganic Chemical Vapor Deposition of Semiconductor Thin Films for Surface Passivation of Photovoltaic Devices Sneha Banerjee1, Rajendra Dahal1, and Ishwara Bhat1 1
ECSE Department, Rensselaer Polytechnic Institute, 110 8th Street, Troy, NY-12180, U.S.A.
ABSTRACT Three II-VI wide bandgap compound semiconductors have been investigated for surface passivation of various photovoltaic devices. First part of this work focuses on the surface passivation of HgCdTe IR detectors using CdTe. A new metalorganic chemical vapor deposition (MOCVD) process has been developed that involves depositing CdTe films at much lower temperature (< 175°C) than the conventional processes used till now. Deposition rate as high as 420nm/h was obtained using this novel experimental setup. Favorable conformal coverage on high aspect ratio HgCdTe devices along with a significant minority carrier lifetime improvement was obtained. Another II-VI semiconductor, namely, CdS was investigated as a surface passivant for HgCdTe IR detectors. It was deposited by MOCVD as well as atomic layer deposition (ALD) and was studied for optimal conformal coverage on high aspect ratio structures. Surface passivation of p-type Si wafer has also been demonstrated using p-ZnTe grown by MOCVD, for possible application in solar cells. Preliminary work showed a remarkable improvement in the minority carrier lifetime of Si light absorbing layer after passivation with a thin layer of ZnTe. INTRODUCTION Photovoltaic devices are an essential part of every phase of life today. A photovoltaic device is an application of a semiconductor p-n junction which converts light into an electrical signal. Photons are absorbed by the semiconductor lattice creating electron-hole pairs. The generated minority carriers diffuse though the lattice and are separated by the p-n junction electric field producing an external voltage. This principle of light conversion is mostly utilized in solar cells and infrared (IR) detectors. Starting from electricity generation using solar cell panels to medical imaging using IR detectors, these devices find a wide variety of applications. Over the years, structures of such devices and their fabrication processes have become increasingly intricate pushing the limits of their efficiencies. Such advancement in technology has been possible by developing advanced growth techniques like chemical vapor deposition (CVD) and molecular beam epitaxy (MBE) coupled with effective surface and interface engineering [1]. Deposition of a thin insulating film, called a surface passivation layer has become one of the major aspects of fabrication of photovoltaic devices. It is used to enhance the electrical performance of the device as well as its chemical stability. Semiconductor surfaces are generally quite different from the bulk due to contamination during the fabrication process or
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