The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60meV) 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. ...

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A comprehensive review of zno materials and devices

We present a review of current research on the optical properties of ZnO nanostructures. We provide a brief introduction to different fabrication methods for various ZnO nanostructures and some general guidelines on how fabrication parameters (temperature, vapor-phase versus solution-phase deposition, etc.) affect their properties. A detailed discussion of photoluminescence, both in the UV region and in the visible spectral range, is provided. In addition, different gain (excitonic versus electron hole plasma) and feedback (random lasing versus individual nanostructures functioning as Fabry-Perot resonators) mechanisms for achieving stimulated emission are described. The factors affecting the achievement of stimulated emission are discussed, and the results of time-resolved studies of stimulated emission are summarized. Then, results of nonlinear optical studies, such as second-harmonic generation, are presented. Optical properties of doped ZnO nanostructures are also discussed, along with a concluding outlook for research into the optical properties of ZnO.

Optical properties of ZnO nanostructures.

Recent progress in processing and properties of ZnO

Industries such as the automotive, aerospace or military, as well as environmental and biological research have promoted the development of ultraviolet (UV) photodetectors capable of operating at high temperatures and in hostile environments. UV-enhanced Si photodiodes are hence giving way to a new generation of UV detectors fabricated from wide-bandgap semiconductors, such as SiC, diamond, III-nitrides, ZnS, ZnO, or ZnSe. This paper provides a general review of latest progresses in wide-bandgap semiconductor photodetectors.

https://iopscience.iop.org/article/10.1088/0268-1242/18/4/201/pdf

Wide-bandgap semiconductor ultraviolet photodetectors

A review is given of recent results in developing improved fabrication processes for ZnO devices with the possible application to UV light emitters, spin functional devices, gas sensors, transparent electronics, and surface acoustic wave devices. There is also interest in integrating ZnO with other wide band-gap semiconductors, such as the AlInGaN system. In this article, we summarize recent progress in controlling n- and p-type doping, materials processing methods, such as ion implantation for doping or isolation, Ohmic and Schottky contact formation, plasma etching, the role of hydrogen in the background n-type conductivity of many ZnO films, and finally, the recent achievement of room-temperature ferromagnetism in transition-metal (Mn or Co)-doped ZnO. This may lead to another class of spintronic devices, in which the spin of the carriers is exploited rather than the charge as in more conventional structures.

Recent advances in processing of ZnO

We report on the electrical characterization of single-crystal ZnO and Au Schottky contacts formed thereon before and after bombarding them with 1.8 MeV protons. From capacitance–voltage measurements, we found that ZnO is remarkably resistant to high-energy proton bombardment and that each incident proton removes about two orders of magnitude less carriers than in GaN. Deep level transient spectroscopy indicates a similar effect: the two electron traps detected are introduced in extremely low rates. One possible interpretation of these results is that the primary radiation-induced defects in ZnO may be unstable at room temperature and anneal out without leaving harmful defects that are responsible for carrier compensation.

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Electrical Characterization of 1.8 MeV Proton-Bombarded ZnO

Gold Schottky-barrier diodes (SBDs) were fabricated on vapor-phase-grown single-crystal ZnO. Deep-level transient spectroscopy, using these SBDs, revealed the presence of four electron traps, the major two having levels at 0.12 eV and 0.57 below the conduction band. Comparison with temperature-dependent Hall measurements suggests that the 0.12 eV level has a temperature activated capture cross section with a capture barrier of about 0.06 eV and that it may significantly contribute to the free-carrier density. Based on the concentrations of defects other than this shallow donor, we conclude that the quality of the vapor-phase-grown ZnO studied here supercedes that of other single-crystal ZnO reported up to now.

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Electrical Characterization of Vapor-Phase-Grown Single-Crystal ZnO

Epitaxial n-GaN was irradiated with 5.4-MeV He ions. Capacitance–voltage (C–V) measurements showed that 5.4-MeV He ions remove free carriers at a rate of 6200±300 cm−1 in the first micron below the surface. Deep level transient spectroscopy (DLTS) revealed that, in addition to the radiation-induced defects previously detected by DLTS at 0.18–0.20 eV below the conduction band, He-ion irradiation introduced two additional prominent defects, ER4 (EC-0.78 eV) and ER5 (EC-0.95 eV) at rates of 1510±300 and 3030±500 cm−1, respectively. Capture cross-section measurements revealed that electron capture kinetics of ER5 is similar to that of a line defect.

Electrical characterization of two deep electron traps introduced in epitaxially grown n-GaN during He-ion irradiation

Using deep-level transient spectroscopy, we have studied the electrical properties of defects introduced in epitaxially grown n-GaN during 2-MeV proton bombardment. The main defects detected, ER2 and ER3, are introduced at rates of 400±150 and 600±100 cm−1, respectively, and have energy levels at 0.16±0.03 and 0.20±0.01 eV, respectively, below the conduction band. A less prominent defect, ER1, with an energy level at 0.13±0.01 eV below the conduction band, is introduced at a rate of 30±10 cm−1. The small capture cross section of ER3 [(8±4)×10−18 cm2] implies that it is in a neutral or negative state when above the Fermi level.

Proton bombardment-induced electron traps in epitaxially grown n-GaN

It is well known that exposure of semiconductor surfaces to energetic particles introduce both optically and electrically active defects. Hydrogen and He-ion implantation has been used in GaN-based microelectronic processes. He-ion implantation produced high resistivity GaN at a fluence that is compatible with photoresist masking techniques. Heavier ion implantation has the added advantage that the depth resolution of the incident ions can be ‘tuned’ for specific applications (lifetime tailoring, etc.). Using deep level transient spectroscopy the defects in as-grown n-GaN as well as those introduced during high energy proton, He-ion and electron bombardment are characterised. Prior to irradiation, four electron defects (EO1–EO3 and EO5) were observed in the as-grown GaN. Two defects ER1 and ER2, not previously observed after electron irradiation, were observed after high energy electron irradiation. He-ion, proton and electron irradiation introduced a defect ER3. Defects ER1 and ER2 were also observed after proton bombardment, whereas two deeper lying defects (ER4 and ER5) were observed after the He-ion bombardment. The electronic properties, introduction rates and the annealing kinetics of the particle induced and as-grown major defects are presented. The influence of defect removal on the Schottky barrier diode properties are also discussed.

S. A. Goodman

Papers

Electrical Characterization of 1.8 MeV Proton-Bombarded ZnO

Electrical Characterization of Vapor-Phase-Grown Single-Crystal ZnO

Electrical characterization of two deep electron traps introduced in epitaxially grown n-GaN during He-ion irradiation

Proton bombardment-induced electron traps in epitaxially grown n-GaN

Radiation induced defects in MOVPE grown n-GaN