Band offset measurements of ZnO∕6H-SiC heterostructure system

The conduction band offset of n-ZnO∕n-6H-SiCheterostructures fabricated by rf-sputtered ZnO on commercial n-type 6H-SiC substrates has been measured by a variety of methods. Temperature dependent current-voltage characteristic, photocapacitance, and deep level transient spectroscopy measurements showed the conduction band offsets to be 1.25, 1.1, and 1.22eV, respectively

Photoresponse of n-ZnO∕p-SiC heterojunction diodes grown by plasma-assisted molecular-beam epitaxy

High quality n-ZnOfilms on commercial p-type 6H–SiC substrates have been grown by plasma-assisted molecular-beam epitaxy, and n-ZnO∕p-SiCheterojunction mesa structures have been fabricated. Current-voltage characteristics of the structures had a very good rectifying diode-like behavior with a leakage current less than 2×10−4A/cm2 at −10V, a breakdown voltage greater than 20V, a forward turn on voltage of ∼5V, and a forward current of ∼2A/cm2 at 8V. Photosensitivity of the diodes was studied at room temperature and a photoresponsivity of as high as 0.045A∕W at −7.5V reverse bias was observed for photonenergies higher than 3.0eV

A comprehensive review of ZnO materials and devices

The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev.142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys.6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. Lett.16, 439 (1970)]. In terms of devices, Au Schottky barriers in 1965 by Mead [Phys. Lett.18, 218 (1965)], demonstration of light-emitting diodes (1967) by Drapak [Semiconductors 2, 624 (1968)], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by Minami et al. [Jpn. J. Appl. Phys.13, 1475 (1974)], ZnO∕ZnSe n-p junctions (1975) by Tsurkan et al. [Semiconductors 6, 1183 (1975)], and Al∕Au Ohmic contacts by Brillson [J. Vac. Sci. Technol.15, 1378 (1978)] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by Look and Claflin [Phys. Status Solidi B241, 624 (2004)]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures

Deposition of earth-abundant p-type CuBr films with high hole conductivity and realization of p-CuBr/n-Si heterojunction solar cell

We present details of the deposition of transparent and earth-abundant p-type CuBr films with high hole conductivity and the fabrication and characterization of a prototype solar cell based on p-CuBr/n-Si heterojunctions. p-type CuBr films with typical resistivities and hole concentrations of 7×10-1 Ωcm and 7.5×1019 cm-3, respectively, are deposited by thermal evaporation followed by oxygen plasma treatment. The transparent p-type films show strong room temperature photoluminescence at ~2.97 eV. The current voltage (I-V) characteristics of the heterojunctions show good diode behaviour. Power conversion efficiency of ~ 2 % was achieved for the heterojunction device without any optimization of the cell structure under AM 1.5 illumination condition with a short circuit current (Jsc) and open circuit voltage (Voc) of 13.2 mA/cm2 and 0.44 V, respectively

Fabrication and current-voltage characterization of a ferroelectric lead zirconate titanate/AlGaN∕GaN field effect transistor

We demonstrated ferroelectricfield effect transistors (FFETs) with hysteretic I-V characteristics in a modulation-doped field effect transistors(MODFET)AlGaN∕GaN platform with ferroelectricPb(Zr,Ti)O3 between a GaN channel and a gate metal. The pinch-off voltage was about 6–7V comparable to that of conventional Schottky gate MODFET. Counterclockwise hysteresis appeared in the transfer characteristics with a drain current shift of ∼5mA for zero gate-to-source voltage. This direction is opposite and much more pronounced than the defect induced clockwise hysteresis in conventional devices, which suggests that the key factor contributing to the counterclockwise hysteresis of the FFET is the ferroelectric switching effect of the lead zirconate titanate gate

SiC polytypes and doping nature effects on electrical properties of ZnO-SiC Schottky diodes

Electrical properties of ZnO/SiC Schottky diodes with two SiC polytypes and N and P doping are investigated. Characterization was performed through I–V and C–V–f measurements. Schottky barrier height (Φb), ideality factor (n), and series resistance (Rs) were extracted from forward I–V characteristics. (Φb), carrier’s concentrations (Nd-Na) and (Rs) frequency dependence were extracted from C–V–f characteristics. The extracted n values suggest that current transport is dominated by interface generation-recombination and/or barrier tunneling mechanisms. When changing SiC polytypes, the rectifying ratio of ZnO/n-4HSiC is found to be twice that of ZnO/n-6HSiC. A change in doping nature gave a leakage current ratio of 40 between ZnO/p-4HSiC and ZnO/n- 4HSiC. These results indicate that ZnO/p-4HSiC diodes have a complex current transport compared to diodes on n-type SiC. From I-V measurements, barrier height values are 0.63eV, 0.65eV and 0.71 eV for heterojunction grown on n-6HSiC, n-4HSiC and p-4HSiC, respectively. C-V measurements gave higher values indicating the importance of interface density of states. Nss values at 1MHz frequency are 4.54×1011 eV-1 cm-2, 3×1012 eV-1 cm-2 and 8.13×1010 eV-1 cm-2 for ZnO/n-6HSiC, ZnO/n-4HSiC and ZnO/p-4HSiC, respectively. Results indicate the importance of SiC polytypes and its doping natur

White Electroluminescence Using ZnO Nanotubes/GaN Heterostructure Light-Emitting Diode

We report the fabrication of heterostructure white light–emitting diode (LED) comprised of n-ZnO nanotubes (NTs) aqueous chemically synthesized on p-GaN substrate. Room temperature electroluminescence (EL) of the LED demonstrates strong broadband white emission spectrum consisting of predominating peak centred at 560 nm and relatively weak violet–blue emission peak at 450 nm under forward bias. The broadband EL emission covering the whole visible spectrum has been attributed to the large surface area and high surface states of ZnO NTs produced during the etching process. In addition, comparison of the EL emission colour quality shows that ZnO nanotubes have much better quality than that of the ZnO nanorods. The colour-rendering index of the white light obtained from the nanotubes was 87, while the nanorods-based LED emit yellowish colour

Influence of helium-ion bombardment on the optical properties of ZnO nanorods/p-GaN light-emitting diodes

Light-emitting diodes (LEDs) based on zinc oxide (ZnO) nanorods grown by vapor-liquid-solid catalytic growth method were irradiated with 2-MeV helium (He+) ions. The fabricated LEDs were irradiated with fluencies of approximately 2 × 1013 ions/cm2 and approximately 4 × 1013 ions/cm2. Scanning electron microscopy images showed that the morphology of the irradiated samples is not changed. The as-grown and He+-irradiated LEDs showed rectifying behavior with the same I-V characteristics. Photoluminescence (PL) measurements showed that there is a blue shift of approximately 0.0347 and 0.082 eV in the near-band emission (free exciton) and green emission of the irradiated ZnO nanorods, respectively. It was also observed that the PL intensity of the near-band emission was decreased after irradiation of the samples. The electroluminescence (EL) measurements of the fabricated LEDs showed that there is a blue shift of 0.125 eV in the broad green emission after irradiation and the EL intensity of violet emission approximately centered at 398 nm nearly disappeared after irradiations. The color-rendering properties show a small decrease in the color-rendering indices of 3% after 2 MeV He+ ions irradiation