Steady‐State Photocarrier Grating Method

Abstract

The excess-carrier properties of a photoexcited semiconductor are important indicators of its quality with respect to applications in optoelectronic devices or for studying the recombination physics. In contrast to the majority-carrier properties, which can be determined rather straightforwardly by stationary photocurrent measurements, the minority-carrier properties can only be revealed by more sophisticated methods. In this respect the steady-state photocarrier grating (SSPG) method has had an enormous impact since it was suggested by Ritter, Zeldov, and Weiser, named RZW hereafter, in 1986 [1].The SSPG method is based on the carrier diffusion under the presence of a spatial sinusoidal modulation in the photogeneration rate G, which induces a so-called photocarrier grating. From photocurrent measurements at different grating periods L, the ambipolar diffusion lengthL can be determined by an analysis that assumes ambipolar transport and charge neutrality.The proposal by RZW, following papers on the analysis [2, 3] and the simple setup triggered a rapid widespread application [4][5][6][7][8]. In parallel, critical and more in-depth accounts on the underlying theory were given to put the technique on a firm ground or to describe its limits when applied to semiconductors with traps. Ritter et al. [9], Balberg et al. [10, 11], Li [12], and Shah et al. [13] analyzed the transport equations, also with respect to the lifetime or relaxation time regimes. In the lifetime regime, the carrier lifetimes are longer than the dielectric relaxation time. The previous publications were later criticized by Hattori et al. [14] who performed a second-order perturbation approach and pointed out deficiencies of other earlier analyses. Nevertheless, Hattori et al. showed that under the conditions in the lifetime regime the analysis and evaluation of the SSPG method are correct. These authors also suggested a correction method to avoid incorrect values of L.A novel aspect was introduced by Abel and Bauer [15] who numerically solved the transport and Poisson equations and compared the solutions with a generalized theory which enables to study the SSPG results by the variation of L and/or electric j177 field E. These authors derive their expressions in terms of the mobility-lifetime product ðmtÞ min of the minority carriers, which can be determined from the SSPG method and related to L.More recently, Schmidt and Longeaud [16] developed a generalized derivation of the solution of the SSPG equations at low E. They identified the shortcomings in the previous derivations which differ from the numerical solution. Their approach also allows the SSPG experiment to be used for the density-of-states (DOS) determination.An important aspect is the above-mentioned association of L with the ðmtÞ min product of the minority carriers. Together with the majority-carrier mobility-lifetime product ðmtÞ maj at the same G from the photoconductivity s ph via s ph ¼ eGðmtÞ maj , where e equals 1.6 Â 10 À19 C, the excess-carrier properties can be related to e...

Microcrystalline silicon thin film deposition from silicon tetrafluoride: Isolating role of ion energy using tailored voltage waveform plasmas

“…Notably, it fits quite well within the trend. Moreover, SSPG measurements allows for the determination of the ambipolar mobility-lifetime products (holes mobility-lifetime products, µτmin) through [63]:…”