Electroluminescence from 4H-SiC Schottky Diodes
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Electroluminescence from 4H-SiC Schottky Diodes F. H. C. Carlsson, Q. ul-Wahab, J. P. Bergman and E. Janzén Dept. of Phys. and Meas. Tech., Materials Science, Linköping University, SE-581 83 Linköping. Sweden ABSTRACT We have observed electroluminescence from 4H-SiC Ni-Schottky diodes on 1015cm-3 nitrogen doped n-type epilayers. A high barrier Schottky contact will form an inversion layer close to it. This creates minority carriers that can be injected into the epi and recombine to emit light. The spectral composition and its temperature dependence have been investigated from liquid He temperatures to room temperature. Band edge luminescence, Al related luminescence and DI bound exciton have been observed. To study the electroluminescence from Schottky diodes provides an easy and additional technique for defect characterization of epitaxial layers. INTRODUCTION SiC is a semiconductor of interest for mainly high power, high temperature and high frequency applications. To achieve this the material quality needs to be improved, regarding for example the presence and properties of residual defects. These defects are optically characterized by photoluminescence (PL) but also in some cases by electroluminescence [1-3] (EL). So far, all EL studies have been performed on pn-diodes. This is however not an ideal situation since the additional layer, i.e., p+ on n- or n+ on p-, introduces additional defects and recombination possibilities, which makes it more difficult to draw conclusions about defects in the low doped base layer. In this work we have studied the possibility to use EL from Schottky diodes where only one epitaxial layer is present. Schottky diodes can be fabricated without advanced processing. To obtain EL from any diode it is necessary to have both majority and minority carriers present. In a pn-junction the minority carriers are injected from the emitter side of the junction under forward bias. For a Schottky junction this is not possible. It is however possible to have an inversion layer close to the metal contact where minority carriers can be present, for example a hole in a n-type epilayer. In this region holes created by thermionic emission from the Schottky metal or ionization of compensating acceptors can be collected and by diffusion or drift enter the n-type region of the epilayer which is shown in figure 1. The main factor that Metal
n-type SiC
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Figure 1. Current transport mechanisms in a Schottky diode. The holes are created by thermionic emission from the metal and can recombine with electrons in the n-epi. H4.8.1
determines the presence of an inversion layer is that the barrier height Φb is larger than half the band gap. This will put the Fermi-level in the lower part of the band gap close to the contact and create an inversion layer. A second important factor is the doping, that decides how close to the conduction band the Fermi-level will be located and the carrier concentration. The carrier concentration will also to some extent determine the current contribution from the mino
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