Abstract
Accurate measurements of the bulk minority carrier lifetime in high‐quality silicon materials is challenging due to the influence of surface recombination. Conventional surface passivation processes such as thermal oxidation or dielectric deposition often modify the bulk lifetime significantly before measurement. Temporary surface passivation processes at room or very low temperatures enable a more accurate measurement of the true bulk lifetime, as they limit thermal reconfiguration of bulk defects and minimize bulk hydrogenation. In this article we review the state‐of‐the‐art for temporary passivation schemes, including liquid immersion passivation based upon acids, halogen‐alcohols and benzyl‐alcohols, and thin film passivation usually based on organic substances. We highlight how exceptional surface passivation (surface recombination velocity below 1 cm s−1) can be achieved by some types of temporary passivation. From an extensive review of available data in the literature, we find p‐type silicon can be best passivated by hydrofluoric acid containing solutions, with superacid‐based thin films showing a slight superiority in the n‐type case. We review the practical considerations associated with temporary passivation, including sample cleaning, passivation activation, and stability. We highlight examples of how temporary passivation can assist in the development of improved silicon materials for photovoltaic applications, and provide an outlook for the future of the field.
Highlights
Introduction surface passivation of siliconThe primary focus is on junction-less substrates forThe minority carrier lifetime is a key figure of merit in the development of silicon applicable to IC wafers.wafers for use in photovoltaics or for integrated circuits (ICs).The effectiveness of passivation is usually measured by aThe highest efficiency silicon solar cells, for example, surface recombination velocity (S) which depends on the doping require silicon substrates with lifetimes well into the millisecond type, doping level, and excess minority carrier density
Summary to Achieve the Best Results a high level of surface passivation has been achieved by treating the silicon wafers with dilute HF prior to thin film deposition, Grant et al have demonstrated that a superior level of passivation results if the silicon surface is etched, cleaned, and HF dipped
The best pre-treatment processes may have some dependence on the film being deposited and the chemical reactions that occur at the interface during passivation, and variations of the etch and clean procedures compared to Grant et al may be required for the best results
Summary
Very good surface passivation can be achieved by thermal oxidation, a process which usually takes place !800 C. The redox potential can be described by a solution’s (or a species’) tendency to either accept or donate electrons, and can be varied depending on the composition of the solution and its pH.[23] Figure 2 depicts the energetics when an n-type silicon wafer is brought into contact with an electrolyte In this case, the redox potential of the solution is below the silicon Fermi level EFS and electrons are transferred from the silicon to the solution, resulting in upward bending of the bands at the silicon surface and the formation of a space charge region. Bandbending at the silicon–thin film interface can be measured using surface voltage and Kelvin probe techniques.[27]
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