Non Destructive Electrical Defect Characterisation and Topography of Silicon Wafers and Epitaxial Layers
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Non destructive electrical defect characterisation and topography of silicon wafers and epitaxial layers K. Dornich, T. Hahn, and J.R. Niklas Technische Universität Bergakademie Freiberg, Silbermannstr.1, D-09596 Freiberg, Germany corresponding author: Electronic address: [email protected] Abstract Recent progress in experimental technique made it possible to improve the sensitivity of microwave detected photoconductivity by several orders of magnitude. This opens completely new possibilities for a contact less non-destructive electrical defect characterization of silicon wafers and even of epitaxial layers on substrates with extremely low resistivity. Electrical properties such as lifetime, mobility and diffusion length can be measured without contacts also at very low injection levels with a resolution only limited by the diffusion length of the charge carriers. The doping level of the material plays no major role. Owing to the high sensitivity, thermal excitation of charge carriers out of defect levels filled during the photo pulse can also be observed. This leads to defect specific photoconductivity transients which deliver pieces of information like DLTS, however, again without contacts, non critical doping, and with high spatial resolution. Experimental procedure Absorption of light inducing band-to-band transitions generates excess charge carriers giving rise to a photoconductivity signal. However, rather than to measure the photoconductivity by contacts the excess carriers are detected by microwave absorption. Using a laser generating a very small light spot on the wafer, the experiments can be carried out with high spatial resolution. A typical response to a rectangular light pulse is shown in Fig. 1. The rapid decay after switching off the light is due to recombination of free carriers (lifetime τ). With the light on, carriers are also trapped by different defect levels. These carriers are thermally re-emitted with the light pulse off giving rise to a sometimes extremely small slow transient referred to as “defect part” in Fig. 1. This defect part furnishes valuable information about electrical properties of the defect as will be shown later. The light pulse must be long enough in order to fill all defect levels of interest. Once a defect level is filled, the re-emission signal (defect part) does definitely not depend on the light intensity. It is proportional to the defect concentration. In order to detect the defect part of the transient signal also at low defect concentrations it is therefore necessary to keep the light intensity sufficiently low not to overload the detection system by a too high photo pulse signal. Presently commercially available systems fail in the detection of the defect part due to a sensitivity some orders of magnitude too low. The effect of light intensity (injection level) on the amount of the defect signal is illustrated in Fig. 2.
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