ZnO nanostructures for optoelectronics: Material properties and device applications

p-Type ZnO materials: Theory, growth, properties and devices

Abstract In the past 10 years, ZnO as a semiconductor has attracted considerable attention due to its unique properties, such as high electron mobility, wide and direct band gap and large exciton binding energy. ZnO has been considered a promising material for optoelectronic device applications, and the fabrications of high quality p-type ZnO and p–n junction are the key steps to realize these applications. However, the reliable p-type doping of the material remains a major challenge because of the self-compensation from native donor defects (V O and Zn i ) and/or hydrogen incorporation. Considerable efforts have been made to obtain p-type ZnO by doping different elements with various techniques. Remarkable progresses have been achieved, both theoretically and experimentally. In this paper, we discuss p-type ZnO materials: theory, growth, properties and devices, comprehensively. We first discuss the native defects in ZnO. Among the native defects in ZnO, V Zn and O i act as acceptors. We then present the theory of p-type doping in ZnO, and summarize the growth techniques for p-type ZnO and the properties of p-type ZnO materials. Theoretically, the principles of selection of p-type dopant, codoping method and X Zn –2V Zn acceptor model are introduced. Experimentally, besides the intrinsic p-type ZnO grown at O-rich ambient, p-type ZnO (MgZnO) materials have been prepared by various techniques using Group-I, IV and V elements. We pay a special attention to the band gap of p-type ZnO by band-gap engineering and room temperature ferromagnetism observed in p-type ZnO. Finally, we summarize the devices based on p-type ZnO materials.

RaoP-Type ZnO Materials: Theory, Growth, Properties, and Devices

GalliumNitridebasedhighelectronmobilitytransistors(HEMTs)areattractiveforuseinhighpowerandhighfrequencyapplications, with higher breakdown voltages and two dimensional electron gas (2DEG) density compared to their GaAs counterparts. Specific applications for nitride HEMTs include air, land and satellite based communications and phased array radar. Highly efficient GaNbased blue light emitting diodes (LEDs) employ AlGaN and InGaN alloys with different compositions integrated into heterojunctions and quantum wells. The realization of these blue LEDs has led to white light sources, in which a blue LED is used to excite a phosphor material; light is then emitted in the yellow spectral range, which, combined with the blue light, appears as white. Alternatively, multiple LEDs of red, green and blue can be used together. Both of these technologies are used in high-efficiency white electroluminescent light sources. These light sources are efficient and long-lived and are therefore replacing incandescent and fluorescent lamps for general lighting purposes. Since lighting represents 20‐30% of electrical energy consumption, and because GaN white light LEDs require ten times less energy than ordinary light bulbs, the use of efficient blue LEDs leads to significant energy savings. GaN-based devices are more radiation hard than their Si and GaAs counterparts due to the high bond strength in III-nitride materials. The response of GaN to radiation damage is a function of radiation type, dose and energy, as well as the carrier density, impurity content and dislocation density in the GaN. The latter can act as sinks for created defects and parameters such as the carrier removal rate due to trapping of carriers into radiation-induced defects depends on the crystal growth method used to grow the GaN layers. The growth method has a clear effect on radiation response beyond the carrier type and radiation source. We review data on the radiation resistance of AlGaN/GaN and InAlN/GaN HEMTs and GaN‐based LEDs to different types of ionizing radiation, and discuss ion stopping mechanisms. The primary energy levels introduced by different forms of radiation, carrier removal rates and role of existing defects in GaN are discussed. The carrier removal rates are a function of initial carrier concentration and dose but not of dose rate or hydrogen concentration in the nitride material grown by Metal Organic Chemical Vapor Deposition. Proton and electron irradiation damage in HEMTs creates positive threshold voltage shifts due to a decrease in the two dimensional electron gas concentration resulting from electron trapping at defect sites, as well as a decrease in carrier mobility and degradation of drain current and transconductance. State-of-art simulators now provide accurate predictions for the observed changes in radiation-damaged HEMT performance. Neutron irradiation creates more extended damage regions and at high doses leads to Fermi level pinning while 60 Co γ-ray irradiation leads to much smaller changes in HEMT drain current relative to the other forms of radiation. In InGaN/GaN blue LEDs irradiated with protons at fluences near 10 14 cm −2 or electrons at fluences near 10 16 cm −2 , both current-voltage and light output-current characteristics are degraded with increasing proton dose. The optical performance of the LEDs is more sensitive to the proton or electron irradiation than that of the corresponding electrical performances. © The Author(s) 2015. Published by ECS. This is an open access article distributed under the terms of the Creative Commons Attribution 4.0 License (CC BY, http://creativecommons.org/licenses/by/4.0/), which permits unrestricted reuse of the work in any

https://iopscience.iop.org/article/10.1149/2.0251602jss/pdf

Review—Ionizing Radiation Damage Effects on GaN Devices

New developments in theoretical studies of defects and impurities in III-Nitrides as pertinent to compensation and recombination in these materials are discussed. New results on experimental studies on defect states of Si, O, Mg, C, Fe in GaN, InGaN, and AlGaN are surveyed. Deep electron and hole traps data reported for GaN and AlGaN are critically assessed. The role of deep defects in trapping in AlGaN/GaN, InAlN/GaN structures and transistors and in degradation of transistor parameters during electrical stress tests and after irradiation is discussed. The recent data on deep traps influence on luminescent efficiency and degradation of characteristics of III-Nitride light emitting devices and laser diodes are reviewed.

Deep traps in GaN-based structures as affecting the performance of GaN devices

Current transport mechanisms and persistent photoconductivity effects were studied in nitrogen-doped ZnO films grown by molecular beam epitaxy having p-type or n-type conductivity at 25°C. In both types of samples the current flow is determined by the n-type channels surrounded by higher resistivity regions. The persistent photoconductivity wave form is reasonably described by the stretched-exponents-type expression, with only a slight temperature dependence of the characteristic decay time. The persistent photocurrent decay process is greatly accelerated by infrared illumination (threshold energy of the photons ∼1.4eV). The results suggest that the Fermi level in the higher resistivity regions is pinned near Ev+1.9±0.1eV and the height of the potential barrier for electrons in the n-type channels is around 1.4±0.1eV.

Persistent photoconductivity in p-type ZnO(N) grown by molecular beam epitaxy

Vibrational modes were studied in n-type and p-type ZnMgO films doped with P (Mg composition of 7at.%) grown by pulsed laser deposition on sapphire. The characteristic phonon frequencies were deduced from the analysis of IR reflectance measured by Fourier-transform spectroscopy. From comparison with similarly grown ZnO (P) films, Mg incorporation reduced the frequency of TO phonons by 14.5cm−1 and introduced two Mg related modes near 530 and 969cm−1. The first is likely to belong to the local vibrational mode of substitutional Mg, and the second is tentatively attributed to strongly lattice relaxed off-center Mg atoms. In addition, it was found that Mg incorporation triggers the formation of two phonon bands with characteristic frequencies of 501 and 634cm−1 that most likely belong to lattice defects.

Lattice vibrational properties of ZnMgO grown by pulsed laser deposition

Electrical properties and deep trap spectra are reported for MgZnO(P) films grown by pulsed laser deposition on undoped n-ZnO substrates. The as-grown films are n type with a “bulk” donor concentration of ∼1018cm−3 and have a compensated high resistivity layer near the surface. Deep trap spectra in these films are dominated by electron traps with an activation energy of 0.3eV, hole traps with an activation energy of 0.14eV, and some unidentified electron traps with a barrier for capture of electrons. After annealing in oxygen at 800°C the MgZnO(P) becomes p type, with the dominant hole traps having an activation energy of 0.2eV. The space charge region of the formed p-n junction is mainly located in the n-ZnO substrate. The main hole traps in this part of the heterojunction have activation energies of 0.14 and 0.84eV, while the main electron traps have activation energies of 0.15 and 0.3eV.

Annealing effects on electrical properties of MgZnO films grown by pulsed laser deposition

AlGaN/GaN high electron mobility transistors (HEMTs) with similar active layers structures were grown on SiC or sapphire substrates using different buffer layer structures, including GaN of different thickness (1 or 2 μm) or composite AlGaN/GaN buffers. The highest density of hole traps was observed in the buffer on sapphire, while the lowest density of hole traps was obtained in the thick (2 μm) GaN buffer on SiC. The reverse leakage currents in HEMTs were lower in the devices grown on SiC substrates and the on-off ratios improved by two orders of magnitude for thicker GaN buffers or composite AlGaN/GaN buffers compared to a standard 1 μm GaN buffer. The maximum drain-source currents and tranconductances were all larger for the devices on SiC compared to sapphire.

Effect of buffer layer structure on electrical and structural properties of AlGaN/GaN high electron mobility transistors

The electrical properties, microcathodoluminescence spectra, and persistent photoconductivity observed for as-grown and annealed ZnO(P) and Zn 0.93 Mg 0.07 O(P) films grown by pulsed laser deposition on sapphire substrates are reported. As-grown ZnO(P) had n-type conductivity with the Fermi level pinned near E c -0.035 eV. Annealing in Ar at progressively higher temperatures of 850, 900, and 950°C shifted the Fermi level first to E c -0.07 eV and then to E v + (0.1-0.13) eV, the latter level being most likely the acceptor level of P. In ZnMgO(P) the residual donors and the P acceptors are slightly deeper than in ZnO, respectively, 0.08 and 0.19 eV. The persistent photoconductivity observed was due to centers located above E v + 0.5 eV and having a barrier for capture of electrons of 0.45 eV. These centers are tentatively associated with off-center P atoms located on Zn sites.

A. I. Belogorokhov

Papers

Persistent photoconductivity in p-type ZnO(N) grown by molecular beam epitaxy

Lattice vibrational properties of ZnMgO grown by pulsed laser deposition

Annealing effects on electrical properties of MgZnO films grown by pulsed laser deposition

Effect of buffer layer structure on electrical and structural properties of AlGaN/GaN high electron mobility transistors

Electrical Properties of ZnO ( P ) and ZnMgO ( P ) Films Grown by Pulsed Laser Deposition