Determination of the minority carrier lifetime in crystalline silicon thin-film material

Abstract

The effective minority carrier lifetimes on epitaxial silicon thin-film material have been measured successfully using two independent microwave-detected photoconductivity decay setups. Both measurement setups are found to be equally suited to determine the minority carrier lifetime of crystalline silicon thin-film (cSiTF) material. The different measurement conditions to which the sample under investigation is exposed are critically analyzed by both simulations and measurements on a large number of lifetime samples. No systematic deviation between the lifetime results from different measurement setups could be observed, underlining the accuracy of the determined lifetime value. Subsequently, a method to separate the epitaxial bulk lifetime and the total recombination velocity, consisting of front surface and interface recombination between the epitaxial layer and the substrate, is presented. The method, based on different thicknesses of the epitaxial layer, is applied to all batches of this investigation. Each batch consists of samples with the same material quality but different epitaxial layer thicknesses whereas different batches differ in their material quality. In addition, the same method is also successfully applied on individual cSiTF samples. From the results, it can be concluded that the limiting factor of the effective minority carrier lifetime for the investigated solar-grade cSiTF material is the elevated recombination velocity at the interface between epitaxial layer and the substrate compared with microelectronic-grade material. In contrast, the samples cannot be classified into different material qualities by their epitaxial bulk lifetimes. Even on multicrystalline substrate, solar-grade material can exhibit high epitaxial bulk lifetimes comparable to microelectronic-grade material

Evaluation of the influence of an embedded porous silicon layer on the bulk lifetime of epitaxial layers and the interface recombination at the epitaxial layer/porous silicon interface

“…In other words, the BSF is very effective in shielding the epilayer-porous silicon interface from minority carriers. A comparison of values calculated on the bases of Equations (10), (11) and (12) is given in Table II, which shows that the approximations made towards the derivation of (11) and (12) are reasonable. In using Equation (10), the extreme case of a large t epi,PS = 100 ms was used.…”

“…Walter et al . have shown that microwave‐detected photoconductance decay may be used successfully to extract effective lifetimes of epilayers . Subsequently, the epitaxial layer bulk lifetime and the interface recombination velocity can be decoupled by considering a variation in the thickness of the epitaxial layer.…”

“…The techniques most suitable for measuring lifetime in thin-film layers attached to a substrate are those that are sensitive to the tens of microns closest to the sample surface. Walter et al have shown that microwavedetected photoconductance decay may be used successfully to extract effective lifetimes of epilayers [11,12]. Subsequently, the epitaxial layer bulk lifetime and the interface recombination velocity can be decoupled by considering a variation in the thickness of the epitaxial layer.…”

“…R PS=No PS % t s;PS t s; no PS (11) This says that the ratio of PL signals, R PS/no PS , is approximately equal to the ratio of the surface lifetimes.…”

“…Since the condition of S≪ Dnp 2 4d is also satisfied for the case when the porous silicon is shielded by a BSF, we can further simplify Equation (11) to obtain the following:…”