A comprehensive review of ZnO materials and devices

Abstract

Undoped and Ga-and Al-doped ZnO films were synthesized using sol-gel and spin coating methods and characterized by X-ray diffraction, high-resolution scanning electron microscopy (SEM), optical spectroscopy and Hall-effect measurements. SEM measurements reveal an average grain size of 20 nm and distinct individual layer structure. Measurable conductivity was not detected in the unprocessed films; however , annealing in hydrogen or zinc environment induced significant conductivity (∼10 −2 Ω.cm) in most films. Positron annihilation spectroscopy measurements provided strong evidence that the significant enhancement in conductivity was due to hydrogen passivation of Zn vacancy related defects or elimination of Zn vacancies by Zn interstitials which suppress their role as deep acceptors. Hydrogen passivation of cation vacancies is shown to play an important role in tuning the electrical conductivity of ZnO, similar to its role in passivation of defects at the Si/SiO2 interface that has been essential for the successful development of complementary metal-oxide-semiconductor (CMOS) devices. By comparison with hydrogen effect on other oxides, we suggest that hydrogen may play a universal role in oxides passivating cation vacancies and modifying their electronic properties. © 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution (CC BY) license

Aspect Ratio Dependence of the Elastic Properties of ZnO Nanobelts

“…The following values were used in this study: V 1 ) 0.27, E 1 ) 169 GPa for the silicon tip, and V 2 ) 0.3, an average value calculated using the elastic constants tabulated in ref 18. The contact radius a is given by the Hertz model:…”

ZnO Nanowire UV Photodetectors with High Internal Gain

“…In the past decade, the demonstration of a large variety of functional ZnO nanowire (NW) devices such as field effect transistors, 2,3 optically pumped lasers, 4,5 and chemical and biological sensors 6 have aroused growing interest in this material. 7 In particular, ZnO NW photodetectors and optical switches have been the subject of extensive investigations. [8][9][10][11][12][13][14][15][16][17][18] Despite the abundant research on NW photoconduction, 19 the two main factors contributing to the high photosensitivity of such nanostructures have been scarcely recognized: (1) the large surface-to-volume ratio and the presence of deep level surface trap states in NWs greatly prolongs the photocarrier lifetime; (2) the reduced dimensionality of the active area in NW devices shortens the carrier transit time.…”

“…Figure 1a shows some typical I-V characteristics of the ZnO NWs in dark and under UV illumination (λ ) 390 nm) at various light intensities; the I-V curves are linear around zero applied bias, which indicates good ohmic behavior of the Ti/Au contacts. 7 The linear increase of the current with applied bias (V) is due to the increase of the carrier drift velocity, hence the reduction of the transit time (T t ) l 2 /µV, where µ is the carrier mobility and l is the separation between the electrodes). The current measured in the NW increases significantly under illumination: by varying the light intensity from 6.3 µW/cm 2 to 40 mW/cm 2 , the current increases from 2 to 5 orders of magnitude, as seen in Figure 1b, where the I-V curves have been redrawn on a natural logarithmic scale.…”

“…41 The gain values obtained are consistently high even in the submillisecond time domain: for instance, G ∼ 2 × 10 6 at ν ) 3 kHz, leading to GB ) 6 GHz. From eq 4, one can infer a carrier transit time of T t ∼ 30 ps, corresponding to a carrier mobility of µ ∼ 270 cm 2 /V‚s for the 2 µm electrode spacing at V ) 5 V. 7 Because the dark current of these NW devices approaches 10 nA at V ) 5 V (Figure 1), we can then estimate the intrinsic carrier concentration to be as low as n ∼ 10 13 cm -3 , which indicates the high crystal quality of the ZnO NWs used in this study and explains the extremely high photoconductive gain observed. 42 Because of oxygen adsorption and desorption at the NW surface, the carrier lifetime and hence the photocurrent intensity are strongly dependent on the ambient gas conditions.…”

Density-Controlled Growth of Aligned ZnO Nanowires Sharing a Common Contact: A Simple, Low-Cost, and Mask-Free Technique for Large-Scale Applications

“…1,2 The attraction of ZnO is the result of its high purity, high crystallinity, wide direct energy band gap (3.37 eV), large excitation binding energy (60 meV), piezoelectricity, and biocompatibility, as well as the divisive nanostructures. 3 Since the successful growth of aligned ZnO nanowires on a single-crystal substrate, [4][5][6] a system that may be very useful for vertical device fabrication has been found. As a result, great interest in acquiring more control over the alignments, including supporting substrates, distribution of nanowires, and density of nanowires, to maximally meet the requirements of nanodevices has been inspired.…”