Charge Transport Phenomena Unique to Diamond
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Charge Transport Phenomena Unique to Diamond Kiran K. Kovi1, Nattakarn Suntornwipat1, Saman Majdi1, Markus Gabrysch1, Johan Hammersberg1 and Jan Isberg1 1
Division of Electricity, Department of Engineering Sciences, Box 534, Ångström laboratory, Uppsala University, SE-75121,Uppsala, Sweden.
ABSTRACT Diamond is a unique material in many respects. One of the most well-known extreme properties of diamond is its ultrahardness. This property of diamond actually turns out to have interesting consequences for charge transport, in particular at low temperatures. In fact, the strong covalent bonds that give rise to the ultrahardness results in a lack of short wavelength lattice vibrations which has a strong impact on both electron and hole scattering. In some sense diamond behaves more like a vacuum than other semiconductor materials. In this paper we describe some interesting charge transport properties of diamond and discuss possible novel electronic applications. INTRODUCTION There are a range of techniques to carry out charge transport measurements. Unfortunately, most of these techniques, e.g. Hall measurements, are not applicable to materials with high resistivities, but there are some useful techniques that are more appropriate. The Time-of-Flight (ToF) technique is ideally suited for drift velocity measurements at very low carrier concentrations and for highly resistive materials. The ToF technique is a powerful method that has been used to investigate the electrical properties of semiconductors. It is based on a direct time-resolved measurement of the current, arising from the drift of free charge carriers in an applied electric field. If the electric field distribution across the sample is known, the transit time of the charge carriers is directly related to the drift mobility. This method has been used to measure, e.g., drift velocity in amorphous silicon [1], silicon [2,3], cadmium telluride [4] and also in natural and single-crystalline CVD diamond [5–16]. The ToF technique can also be used to measure carrier lifetimes [6,17] and to extract the electric field distribution in particle detectors and device structures [18]. Our group has used the ToF technique with low-intensity pulsed UV excitation to study charge transport in single-crystalline CVD diamond over a wide range of temperatures (5-500 K) [9,13,19]. At temperatures above 150 K, the drift velocities of holes and electrons exhibit a similar behaviour as functions of the applied electric field [9]. For both carrier types the drift velocities show a linear behaviour at low fields; at higher fields the drift velocities saturate. Below 150 K, however, holes and electrons behave very differently. This can be traced to the completely different structure of the valence and conduction bands in diamond [20,21] and to the high energy of optical phonons in diamond [22]. The very high energy of optical phonons leads to an absence of such phonons at low temperatures and therefore diamond behaves, due to the lack of optical phonon scattering, more like a vacuum than
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