Transport Properties and Conduction Band Offset of n-ZnO/n-6H-SiC Heterostructures
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Transport Properties and Conduction Band Offset of n-ZnO/n-6H-SiC Heterostructures Yahya Alivov1, Xiao Bo1, Fan Qian1, Daniel Johnstone2, Cole Litton3, and Hadis Morkoç1 1 Electrical Engineering, Virginia Commonwealth University, 601 W. Main Street, Richmond, VA, 23284 2 SEMETROL, Chesterfield, VA, 23838 3 Retired, Air Force Research Laboratory (AFRL/MLPS), Dayton, OH, 45433
ABSTRACT The conduction band offset of n-ZnO/n-6H-SiC heterostructures fabricated by rfsputtered ZnO on commercial n-type 6H-SiC substrates has been measured. Temperature dependent current-voltage characteristics, photocapacitance, and deep level transient spectroscopy measurements showed the conduction band offsets to be 1.25 eV, 1.1 eV, and 1.22 eV, respectively.
INTRODUCTION Zinc Oxide (ZnO) is being considered for optical devices owing to its large exciton binding energy (∼60 meV), high emission efficiency, relative ease of bulk material growth, and the possibility of growing highly conductive transparent layers. Because high quality, reproducible p-type ZnO layers are not yet available, p-n junctions are fabricated that take advantage of heteroepitaxy.1 Heteroepitaxial structures are also of interest on their own merits because of advantages provided by the valence and conduction band offsets.2 Various heteroepitaxial structures have been used as the p-type layer for device fabrication. For example, ZnO has been used as a channel for high quality field-effect transistors by several groups. 1,3,4 The component materials of the heterojunction can have a large impact on device performance. One of the vital criteria in heteroepitaxial based devices is to match the lattice parameters, and from this point of view GaN and its alloys with aluminum have been considered by many authors as one of the best candidates for such device fabrication.5,6,7,8,9 Another good candidate for ZnO based heterostructure devices is SiC, which matches the wurzite crystal structure, and has a relatively small lattice mismatch of ∼4% with ZnO. 10,11,12,13 Recently, we demonstrated high quality and highly reproducible n-ZnO/p-6H-SiC heterojunction diodes with a very good rectifying behavior with a leakage current less than 1x10−4 A/cm2 at −10 V, a forward current of 4 A/cm2 at 8 V, and a breakdown voltage greater than 20 V.10,11 In Ref. [11] it was shown from electroluminescence measurements that electron injection takes place primarily from ZnO into SiC without substantial hole injection into the ZnO. Although this is not beneficial for LED applications (because ZnO would not be the active photon emission layer in this case), it would be of interest for applications in which ZnO would serve as the emitter, such as photodiodes and n-p-n type heterostructure bipolar transistors (HBT).
To better understand heterostructure device performance, knowledge of the band offset between materials forming the heterostructure is necessary. In this report we report on the conduction band offset in n-ZnO/n-6H-SiC isotype heterodiodes with ohmic contacts by temperature
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