Light-Excitation-Based Spectroscopy of Electronic Defects in Novel Materials
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A. CASTALDINI, A. CAVALLINI, L. POLENTA INFM and Department of Physics, University of Bologna, Italy, [email protected] ABSTRACT In recent years several spectroscopic methods based on deep level photoionization have been developed because of the demand to investigate materials spreading over a wide range of resistivities. This paper deals with some capacitance- and current-based analyses, that are mature techniques for the characterization of semiconducting as well as semi-insulating materials with respect of deep level identification as well as spatial distribution. Results relevant to novel materials are also reported. INTRODUCTION A number of deep level transient spectroscopy (DLTS) methods has been developed over two decades from the pioneering paper of Miller et al. [1], due to the growing importance of characterizing electronic defects in semiconductors. However, conventional DLTS is of limited use or cannot be used at all when investigating: i) materials where the free carrier concentration is comparable or lower than that of defect associated levels, and ii) wide-band-gap materials, where the range of level energies in the gap actually accessible to DLTS is restricted to about 1 eV of either band edge. Indeed, in recent years a large body of work has been successfully performed to study the electronic levels associated with defects in wide-band-gap materials, such as GaN [2] and SiC [3]. However, it is well known that conventional DLTS analyses are of limited use in the investigation
of these novel materials. Krtschil et al [4] evidenced that in molecular beam epitaxy (MBE) grown GaN layers DLTS measurements fail, due to the contact bad quality, and thermal admittance spectroscopy (TAS) could be alternatively used [5]. Besides this item, which depends on material peculiarities, the DLTS exploitation in studying wide-band-gap materials, where mid-gap levels should be of major importance, is of limited use because it relies on thermal ionization of deep levels. Consequently, energy levels with ErT> 1eV can be observed at temperatures higher than 450K, a value beyond the normal temperature threshold of most of the commercial apparatuses. Besides, conventional DLTS fails in studying deep levels in materials made semi-insulating by the presence of deep levels Nd as compensating centres, since a large series resistance can interfere with the measurement of the capacitance transient [6]. For these reasons, a few alternative ways have been recently explored to locate and identify energy levels. A number of these methods makes use of light excitation to alter the charge carrier population, in most of them by defect-to-band transitions, and to get information on the energy levels involved in the carrier density changes. It is worth noting that junction space charge techniques use either current or capacitance signals. The major difference consists in that the current originates only from the deep centres, while the capacitance signal related to the emission from the deep centres is added to the total diode capaci
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