Minority Carrier Lifetime Estimation by Photoconductance Decay for Silicon Wafers Immersed in HF

The measurement of minority carrier bulk lifetimes of Si wafers immersed in HF is known to have an immersion time dependence, which is specially strong for high lifetime wafers and can vary significantly for different surface treatments performed previously. These complications can make the lifetime measurement become difficult to interpret and inaccurate. Therefore, a method for extracting the bulk lifetime from the measured curves of lifetime versus immersion time is suggested. A model of the reaction between Si and HF is considered, consisting in a first-order reaction between Si and HF for polished wafers and two first-order reactions for textured wafers. Experiments on polished and textured p-type Si wafers were conducted with the photoconductance decay technique to test the model proposed and to show that it is more adequate to estimate the bulk lifetime by using the reactions models than to report it as the lifetime measured at a particular immersion time, which is the usual procedure.

I. Introduction

It is well established that the minority carrier lifetime () and the surface recombination velocity () are important parameters for characterizing photovoltaic cells [1]–[3]. Because of this importance, the dependence of on the various microscopic parameters of Si wafers has been extensively studied in the past few decades, and effective and fast methods of measuring it have been proposed. The usual method is measuring the photoconductance decay (PCD) while the Si wafer surfaces are covered with a passivating thin film [3], [4], such as a-Si:H, Al O, SiO , or SiN, or a passivating liquid [5], such as HF [6] or I/ethanol [7]. Although thin film passivation may be a more practical route in most cases, this passivation scheme usually involves high-temperature steps, which may degrade the wafer bulk and change . In these cases, liquid passivation may be more appropriate.