Capacitance-Voltage Characteristics of Metallic Gate/Oxide/a-Si:H MOS Structures
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CAPACITANCE-VOLTAGE CHARACTERISTICS OF METALLIC GATE/OXIDE/a-Si:H STRUCTURES
MOS
RUUD E.I. SCHROPP, JAN SNIJDER, JAN F. VERWEY Department of Applied Physics, Groningen State University, Nijenborgh 18, 9747 AG Groningen, The Netherlands. ABSTRACT The density of states (DOS) has for the first time been calculated throughout the entire bandgap region of undoped amorphous silicon from quasi-static capacitance-voltage (QSCV) measurements using MOS structures. The QSCV DOS is compared with the DOS obtained by the field-effect method. It is shown, that the coexistence of states of a different nature at the same bandgap level can be revealed by the temperature dependence of low frequency MOS CV measurements. INTRODUCTION The study of metal/oxide/semicondictor (MOS) structures, where the semiconductor film is hydrogenated amorphous silicon (a-Si:H) gives valuable information with respect to application of these structures, especially in thin-film devices. An important physical quantity is the density of states (DOS) distribution N(E) in the bandgap. It has a great influence on the performance of thin-film devices, such as solar cells, and it can be determined by the field-effect (FE) technique [1,2] or by capacitance-voltage (CV) measurements on these structures [3]. The MOS CV method has in some respects advantages over the FE technique, because (i) the MOS CV method is independent of specific transport properties of amorphous semiconductors, such as the anomalous conduction behaviour according to the Meyer-Neldel (MN)relation for thermally activated conduction [4,5]. Therefore, the analysis can be considerably simplified and misinterpretation is avoided. (ii) The method does not solely depend on states communicating with majority carriers. Therefore the DOS information can be probed above as well as below the Fermi-level in the same a-Si:H sample. Moreover, the method can be applied to intrinsic semiconductors. This implies an advantage over CV measurements in metal/amorphous semiconductor Schottky barriers, because it is difficult to obtain good quality Schottky barriers on intrinsic [6] or p-type a-Si:H [7]. And,(iii) a possible error in the flat-band condition in CV analysis primarily has the effect of rigidly shifting the DOS profile along the energy axis [8], so that a time consuming flat-band voltage determination is not a necessary requirement if only a fast, comparative determination of the DOS is desired. The CV method has in common with the FE method that it is sensitive to the presence of interface states. This may be a problem if one has the aim of determining real bulk properties of a-Si:H, but on the contrary, it may be extremely useful in the optimalization of thin-film devices, because interface states have a large impact on device performance. Therefore, the CV method can be used to investigate the deposition technology for obtaining the best device-quality interfaces. In this paper, the DOS distribution is calculated for the first time throughout the entire energy region of the bandgap from a quasi-sta
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