Metallic Impurities in n- and p- Type Silicon: DLTS Studies
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METALLIC IMPURITIES IN n- AND p- TYPE SILICON: DLTS STUDIES Aditya Agarwal, Z. J. Radzimski*, A. Buczkowski, F. Shimura and G. A. Rozgonyi North Carolina State University, Department of Materials Science and Engineering, Raleigh NC 27695-7916 *Analytical Instrumentation Facility, North Carolina State University, Raleigh NC 27695 ABSTRACT Deep level majority and minority carrier traps in p+/n and n+/p junction diodes have been investigated. The junctions were fabricated on n- and p- type silicon which was intentionally and uniformly doped with heavy metals Cr, Fe, Ni, and Au during Czochralski crystal growth. The activation energies of the traps in these devices has been determined using a computer based Deep Level Transient Spectroscopy system which stores and analyzes entire capacitancetime transients. The capacitance-time data sets have been analyzed using the standard ratewindow method as well as by a new algorithm which is able to test for the existence of a single exponential. The new algorithm has shown that only one of the ten traps measured contained a single exponential. Rate-window analysis of all the data sets, however, yielded energy levels based on the expectation of a single-exponential, despite the inherent non-exponentiality of the transients. Implications on the reliability of results obtained by the rate window method have been discussed. Possible reasons for the observed non-exponentiality in the data have been suggested based on a study of simulated data. INTRODUCTION Deep Level Transient Spectroscopy (DLTS) has been used extensively in recent years for the characterization of deep levels in semiconductors. Although many have studied deep levels caused by metallic impurities, in most cases the impurities were introduced via diffusion [1-3]. Also, most studies have been performed either on n-type or on p-type Si but not on both types at the same time while comparing the two [2-41. In this study both n- and p- type Si with the same metallic impurities introduced during melt, under the same conditions, have been investigated. Just as diffusion of metals under different conditions has led to the formation of different deep level centers, each with a different activation energy (for example, diffusion of Au, as shown by Morooka et al. [4]), it is expected that introduction of the metals during melt may cause deep levels centers to be formed which are different than those caused by diffusion of the impurities. The deep levels have been investigated with a computer based DLTS system developed by the authors. The system allows viewing of entire capacitance-time (C-t) transients which it then digitizes and stores for later analysis. The stored C-t data has been analyzed by a ratewindow method (RW) and also by a more novel approach of plotting the capacitance values against themselves (C-C). The rate-window method, first suggested by Lang [51 has been the basis of the most popular method (the box-car) for the identification of the deep level activation energies [1-6]. The rate-window method is based on th
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