Carrier Lifetime: Free Carrier Absorption, Photoconductivity, and Photoluminescence

AbstractThe charge carrier lifetime is one of the most important parameters characterizing a semiconductor. This is mainly for two reasons: (1) the carrier lifetime determines the performance of devices while, at the same time, (2) it is a sensitive measure of material quality and cleanliness. Carrier lifetime may impact device operation directly, such as by affecting the turn‐off speed of a diode or a thyristor operating at high injections or the leakage current of a pn‐junction. Carrier lifetime may also affect operation indirectly. Thus, a high carrier lifetime is desirable in many cases, whereas a low lifetime favors specific applications. Furthermore, for optically active devices, one is interested in radiative recombination, and other recombination mechanisms must be suppressed to enhance quantum efficiency.This article reviews the theory and common concepts in carrier‐lifetime measurement methodology and focuses on a few important and general techniques used to characterize lifetime in semiconductor materials. Thus, device lifetime characterization methods have been left out. The our aim here is to present basic principles of the methods, methodology, and associated difficulties, such that the reader will be able to critically analyze data provided by the methods and to perform simple measurements using the techniques. Also, a selection guide is provided to direct users to the most appropriate method for their specific needs.The three methods described in this article are free carrier absorption (FCA), photoconductivity (PC), and photoluminescence (PL).Free carrier absorptionis not of widespread use but is a very general tool applicable to most semiconductor materials and also to devices. It provides an exceptional advantage, as different geometries and very different injection regimes can be realized, and therefore many different materials and recombination mechanisms may be characterized. It is used here to illustrate many common concepts in lifetime measurement methods.Photoconductivityis one of the few standard methods used both for wafer fabrication quality monitoring and for clean‐room process monitoring.Photoluminescenceis a particularly useful method for characterizing direct‐band‐gap semiconductors, but has also been applied to indirect band‐gap materials. The PL method has also become the workhorse when small volumes must be analyzed, such as in thin‐film studies, for quantum‐well characterization, or for the rapidly expanding field of semiconductor nanostructures.The article commences with a review of the carrier‐lifetime concept and various recombination mechanisms, also illustrating some major difficulties affecting lifetime characterization, such as trapping and surface recombination. Brief outlines of the different techniques and their fundamental principles of operation are given. The three different techniques are described in detail. A selection guide is provided for the different lifetime applications.