Light-Induced Passivation of Si by Iodine Ethanol Solution
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Light-Induced Passivation of Si by Iodine Ethanol Solution Bhushan Sopori1, Przemyslaw Rupnowski1, Jesse Appel1, Debraj Guhabiswas1, LaTecia Anderson-Jackson2 1 National Renewable Energy Laboratory, Golden, CO 80401 2 North Carolina Agricultural and Technical State University, Greensboro, NC, 27411
ABSTRACT We report on our observations of light-activated passivation (LIP) of Si surfaces by iodine-ethanol (I-E) solution. Based on our experimental results, the mechanism of passivation appears to be related to dissociation of iodine by the photo-carriers injected from the Si wafer into the I-E solution. The ionized iodine (I-) then participates in the formation of a Si-ethoxylate bond that passivates the Si surface. Experiments with a large number of wafers of different material parameters indicate that under normal laboratory conditions, LIP can be observed only in some samples—samples that have moderate minority-carrier lifetime. We explain this observation and also show that wafer cleaning plays an extremely important role in passivation. INTRODUCTION It is well recognized that iodine-ethanol (I-E) or iodine-methanol (I-M) solutions can passivate Si surfaces [1–9]. This method of surface passivation can be very valuable when a temporary and removable passivation of a Si wafer surface(s) is needed. The most common application of a temporary passivation is for making lifetime measurements of Si wafers, where a very high-quality surface passivation is needed to measure the bulk lifetime, τb. This is a preferred method of passivation because, unlike other techniques of passivation such as deposition of nitride, oxide, or an n/p junction, I-E passivation is achieved at room temperature without any high-temperature wafer processing. Unfortunately, the mechanism of passivation is not well understood. As a result, it is a common experience that measurements made with I-E (IM) solution are not reproducible. Earlier studies to investigate sources of these variations have shown that measurement is influenced by wafer cleaning [3,7,10]. Furthermore, Refs. [3,7] have shown that multiple cleaning of wafers leads to an improved passivation. Recently, we developed a procedure for wafer cleaning that yields highly reproducible values of minoritycarrier lifetime as measured by photo-conductance decay (PCD) or quasi-steady-state photoconductance (QSSPC) techniques. This cleaning procedure involves cleaning the wafer in Piranha, followed by a low-temperature oxidation and a hydrofluoric (HF) dip to remove a thin layer from the surface [7]. Because our new procedure enabled reproducible measurement of lifetime, it led to the discovery of another source of variability in lifetime measurement—the influence of lightexposure on the passivation produced by I-E. We observed that surface passivation is greatly hastened if the Si wafer, immersed in I-E solution, is exposed to light [10]. Here, we present a mechanism of passivation by I-E solution, which also explains the results presented in this paper on the dependence of passiv
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