Determination of carrier bulk lifetime and surface recombination velocity in semiconductor from double-wavelength free carrier absorption

In microelectronic and photovoltaic industry, semiconductors are the basic materials in which impurities or defects have a serious influence on the properties of semiconductor-based devices. The determination of the electronic transport properties, i.e., the carrier bulk lifetime (τ) and the front surface recombination velocity (S1), is important for evaluating the semiconductor material. In this paper, a method of simultaneously measuring the bulk lifetime and the front surface recombination rate of semiconductor material by using double-wavelength free carrier absorption technique is presented. The effect of the carrier bulk lifetime and the front surface recombination rate on the modulated free carrier absorption signal (Ampratio and Phadiff) are qualitatively analyzed. The process of extracting the bulk lifetime and the front surface recombination rate by the proposed double-wavelength free carrier absorption method are also given. At the same time, the uncertainties of the parameters extracted by this method are calculated and compared with those obtained by the traditional frequency-scan free carrier absorption technique. The results show that the proposed method can significantly reduce the uncertainties of the measurement parameters, especially for the samples with higher surface recombination rate. For the sample with a lower front surface recombination rate (S1=102 m/s), the uncertainty of the carrier bulk lifetime and the front surface recombination velocity obtained by the proposed method are almost in agreement with those obtained by the conventional frequency-scan method. On the contrary, for the samples with higher front surface recombination rate (S1 ≥ 103 m/s), the uncertainties of the carrier transport parameters are much smaller than those from the conventional frequency-scan method. For example, the estimated uncertainty of the carrier bulk lifetime and the front surface recombination velocity for the sample with τ=10 μs and S1=103 m/s are approximately ±5.55% and ±2.83% by the proposed method, which are more improved than ±18.50% and ±31.46% by the conventional frequency-scan method with a wavelength of 405 nm. Finally, we explain the above phenomenon by analyzing the distribution of excess carrier concentration at different pump wavelengths. As the pump wavelength decreases, the more excess carriers are excited near the surface of the sample due to the greater absorption coefficient, and the influence of the surface recombination by the impurities and defects on the signal is more obvious. Therefore, the measurement accuracy of the front surface recombination rate can be improved effectively by using double wavelength pumping.