Imaging method for laterally resolved measurement of minority carrier densities and lifetimes: Measurement principle and first applications

Abstract

Carrier density imaging, a further development of the infrared lifetime mapping technique [Bail et al., Proceedings 28th IEEE-PVSC (2000)], is presented as an extremely fast, spatially resolved lifetime measurement method. It is based on the principle of absorption of infrared radiation by free carriers. The physical principles and necessary calibration procedures are discussed, showing that the steady state carrier density and thus actual carrier lifetime at low-level injection is measured, whereas many standard lifetime measurement techniques measure differential lifetimes only. A charge coupled device camera operating in the infrared is applied for measuring the infrared transmissivity of the sample. This overcomes the necessity of scanning the sample in order to receive laterally resolved information. Thus measurement times can be cut from hours to minutes or even seconds. An additional advantage is the absence of any moving parts which may cause spatial errors or even failures in the measurement. As is shown in the article the use of lock in technique and an appropriate source of illumination is essential to receive good results on production type materials. The quantitative comparison to other lifetime measurement techniques and the application to silicon samples at different process stages is discussed. Finally methods for further reduction of measurement time are discussed, showing that this method may well be suitable for in-line process control.