The effect of narrowing of the band gap on surface recombination in silicon
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CONDUCTOR STRUCTURES, INTERFACES, AND SURFACES
The Effect of Narrowing of the Band Gap on Surface Recombination in Silicon A. V. Sachenko^ and I. O. Sokolovsky Lashkarev Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Kiev, 03028 Ukraine ^e-mail: [email protected] Submitted September 13, 2006; accepted for publication October 2, 2006
Abstract—The effect of narrowing of the band gap on the effective surface recombination rates in silicon is studied. It is shown that the effect results in an increase in the effective surface recombination rate if the concentration of charge carriers at the surface is higher that in the bulk; otherwise, the effect causes the rate to decrease. The changes induced by narrowing of the band gap in the open-circuit voltage of solar cells are calculated taking into account the contribution of surface recombination. PACS numbers: 71.20.Mq, 71.55.Cn, 72.20.Jv, 73.50.Gr, 73.50.Pz, 84.60.Jt DOI: 10.1134/S1063782607060115
1. INTRODUCTION It is known that, as a result of sufficiently heavy doping of a semiconductor, the energy band gap is narrowed [1–3]. The physical causes of the narrowing are different at dopant concentrations higher or lower than 1019 cm–3, and the change in the band gap at the dopant concentration ~1019 cm–3 in silicon is comparable to the temperature change in the band gap in the temperature range 0–300 K [3]. At the present time, the effect of narrowing of the band gap is commonly taken into account in modeling and developing semiconductor devices and solar cells (SCs) [4–6]. This effect is manifested firstly in an increase in the concentration of intrinsic charge carriers (ni) and in saturation currents at rather high levels of doping. At the same time, until recently, the effect of narrowing of the band gap was not treated in detail in analysis of the effective surface recombination rate (SRR). In particular, it was not taken into account that the effect may manifest itself not only because of a change in the level of doping but also because of high surface concentrations induced by the surface electric field. Due to the last-mentioned fact, one should expect a change in the mechanisms of a wide variety of SRR-sensitive photoelectric phenomena. In this study, we analyze the effect of narrowing of the band gap on the effective SRR and open-circuit voltage in silicon SCs at room temperature. Specifically, it is shown that the effect of narrowing of the band gap may result in a noticeable increase in the SRR in cases where there is rather pronounced enriched or inversed band bending at the surface of silicon. At the same time, for a heavily doped emitter, the effect of narrowing of the band gap is more complex. On the one hand, the effective SRR at the emitter boundary is lower
than the SRR without narrowing of the band gap, if there is a depletion layer at the emitter surface; on the other hand, the higher concentration of charge carriers in the emitter compared to the base is favorable for increasing the total rates of surface recombination a
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