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

The interest in nanoscale materials stems from the fact that new properties are acquired at this length scale and, equally important, that these properties * To whom correspondence should be addressed. Phone, 404-8940292; fax, 404-894-0294; e-mail, mostafa.el-sayed@ chemistry.gatech.edu. † Case Western Reserve UniversitysMillis 2258. ‡ Phone, 216-368-5918; fax, 216-368-3006; e-mail, burda@case.edu. § Georgia Institute of Technology. 1025 Chem. Rev. 2005, 105, 1025−1102

Chemistry and properties of nanocrystals of different shapes.

In the past ten years we have witnessed a revival of, and subsequent rapid expansion in, the research on zinc oxide (ZnO) as a semiconductor. Being initially considered as a substrate for GaN and related alloys, the availability of high-quality large bulk single crystals, the strong luminescence demonstrated in optically pumped lasers and the prospects of gaining control over its electrical conductivity have led a large number of groups to turn their research for electronic and photonic devices to ZnO in its own right. The high electron mobility, high thermal conductivity, wide and direct band gap and large exciton binding energy make ZnO suitable for a wide range of devices, including transparent thin-film transistors, photodetectors, light-emitting diodes and laser diodes that operate in the blue and ultraviolet region of the spectrum. In spite of the recent rapid developments, controlling the electrical conductivity of ZnO has remained a major challenge. While a number of research groups have reported achieving p-type ZnO, there are still problems concerning the reproducibility of the results and the stability of the p-type conductivity. Even the cause of the commonly observed unintentional n-type conductivity in as-grown ZnO is still under debate. One approach to address these issues consists of growing high-quality single crystalline bulk and thin films in which the concentrations of impurities and intrinsic defects are controlled. In this review we discuss the status of ZnO as a semiconductor. We first discuss the growth of bulk and epitaxial films, growth conditions and their influence on the incorporation of native defects and impurities. We then present the theory of doping and native defects in ZnO based on density-functional calculations, discussing the stability and electronic structure of native point defects and impurities and their influence on the electrical conductivity and optical properties of ZnO. We pay special attention to the possible causes of the unintentional n-type conductivity, emphasize the role of impurities, critically review the current status of p-type doping and address possible routes to controlling the electrical conductivity in ZnO. Finally, we discuss band-gap engineering using MgZnO and CdZnO alloys.

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Fundamentals of zinc oxide as a semiconductor

Atomic layer deposition(ALD), a chemical vapor deposition technique based on sequential self-terminating gas–solid reactions, has for about four decades been applied for manufacturing conformal inorganic material layers with thickness down to the nanometer range. Despite the numerous successful applications of material growth by ALD, many physicochemical processes that control ALD growth are not yet sufficiently understood. To increase understanding of ALD processes, overviews are needed not only of the existing ALD processes and their applications, but also of the knowledge of the surface chemistry of specific ALD processes. This work aims to start the overviews on specific ALD processes by reviewing the experimental information available on the surface chemistry of the trimethylaluminum/water process. This process is generally known as a rather ideal ALD process, and plenty of information is available on its surface chemistry. This in-depth summary of the surface chemistry of one representative ALD process aims also to provide a view on the current status of understanding the surface chemistry of ALD, in general. The review starts by describing the basic characteristics of ALD, discussing the history of ALD—including the question who made the first ALD experiments—and giving an overview of the two-reactant ALD processes investigated to date. Second, the basic concepts related to the surface chemistry of ALD are described from a generic viewpoint applicable to all ALD processes based on compound reactants. This description includes physicochemical requirements for self-terminating reactions,reaction kinetics, typical chemisorption mechanisms, factors causing saturation, reasons for growth of less than a monolayer per cycle, effect of the temperature and number of cycles on the growth per cycle (GPC), and the growth mode. A comparison is made of three models available for estimating the sterically allowed value of GPC in ALD. Third, the experimental information on the surface chemistry in the trimethylaluminum/water ALD process are reviewed using the concepts developed in the second part of this review. The results are reviewed critically, with an aim to combine the information obtained in different types of investigations, such as growth experiments on flat substrates and reaction chemistry investigation on high-surface-area materials. Although the surface chemistry of the trimethylaluminum/water ALD process is rather well understood, systematic investigations of the reaction kinetics and the growth mode on different substrates are still missing. The last part of the review is devoted to discussing issues which may hamper surface chemistry investigations of ALD, such as problematic historical assumptions, nonstandard terminology, and the effect of experimental conditions on the surface chemistry of ALD. I hope that this review can help the newcomer get acquainted with the exciting and challenging field of surface chemistry of ALD and can serve as a useful guide for the specialist towards the fifth decade of ALD research.

Surface chemistry of atomic layer deposition: A case study for the trimethylaluminum/water process

Since the successful demonstration of a blue light-emitting diode (LED)1, potential materials for making short-wavelength LEDs and diode lasers have been attracting increasing interest as the demands for display, illumination and information storage grow2,3,4. Zinc oxide has substantial advantages including large exciton binding energy, as demonstrated by efficient excitonic lasing on optical excitation5,6. Several groups have postulated the use of p-type ZnO doped with nitrogen, arsenic or phosphorus7,8,9,10, and even p–n junctions11,12,13. However, the choice of dopant and growth technique remains controversial and the reliability of p-type ZnO is still under debate14. If ZnO is ever to produce long-lasting and robust devices, the quality of epitaxial layers has to be improved as has been the protocol in other compound semiconductors15. Here we report high-quality undoped films with electron mobility exceeding that in the bulk. We have used a new technique to fabricate p-type ZnO reproducibly. Violet electroluminescence from homostructural p–i–n junctions is demonstrated at room-temperature.

Repeated temperature modulation epitaxy for p-type doping and light-emitting diode based on ZnO

Basic concepts of ultrasonic-assisted mist chemical vapor deposition (CVD), and its applications to the fabrication of II-oxide and related oxide films are described. Fairly good thin film properties and device performance, compared to those achieved by conventional vacuum-based processes, encourage the mist CVD as a simple, inexpensive, energy-saving and non-vacuum process, opening new and wide application of oxide materials for novel devices supporting the future environment. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Ultrasonic-assisted mist chemical vapor deposition of II-oxide and related oxide compounds

As a safe, simple, environmental-friendly, and cost-e ective growth technique of oxide materials, we have developed the mist chemical vapor deposition method. To grow ZnO, a water and/or alcohol solution of zinc-compounds (for example, zinc acetate) is used as the source, and micronsized aerosol or mist particles formed by ultrasonic atomization are supplied to the reaction area with a carrier gas. The ZnO thin lms grown on glass substrates exhibited c-axis orientation under the selected growth conditions. The transparency in the visible region was higher than 90 %, the room temperature photoluminescence showed near band edge emission without noticeable deep level emissions, and the surface root-mean-square roughness was 7.5 nm despite the polycrystalline structure. These results are satisfactory for optical applications. The minimum resistivity, however, by gallium doping was 1.1 10 3 cm, which needs further progress by enlarging the grain size. The growth of other oxide thin lms, for example, MgO and CdO, as well as ZnO nanorods was reported, suggesting potential of wide applications of this growth technique to various oxide thin lms and nanostructures with the friendliness to environment.

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Mist CVD Growth of ZnO-Based Thin Films and Nanostructures

Conductivity and its temperature dependence, conduction type, and field-effect mobility in metal (In or Sb)-doped C60 thin films are investigated together with those of undoped films. All electrical measurements have been conducted without exposure to air after deposition, in order to minimize the degradation of films due to incorporation of oxygen. For films with both dopants, (1) the conductivity is several orders of magnitude higher than that of undoped films, (2) the conductivity follows a semiconductor-like temperature dependence with the activation energy of 0.10-0.17 eV, which is much lower than that of the undoped films, 0.51 eV, (3) the conduction is n-type, and (4) the field-effect mobility is 0.03-0.04cm2/Vs. Enhancement of conductivity in metal-doped C60 films is attributed to the increase of both carrier concentration and mobility.

Semiconductor-like carrier conduction and its field-effect mobility in metal-doped C60 thin films

Fluorinated silicon nitride films are deposited by the plasma-reaction of SiF4–N2–H2 gas mixture. The fluorine content in the film is ~25 at%. Although hydrogen is incorporated in the form of N–H bonds, the bondings remain stable up to 640°C. The films have relatively high resistivity (7×1016 Ωcm at 2 MV/cm), high breakdown strength (10 MV/cm), and low deep trap density.

Plasma-Enhanced Chemical Vapor Deposition of Fluorinated Silicon Nitride

The surface structure of α-Ga2O3(0001) grown on an α-Al2O3(0001) substrate by mist chemical vapor deposition was studied by coaxial impact-collision ion scattering spectroscopy (CAICISS) and atomic force microscopy (AFM). The minimum step height observed in the AFM image was 0.21 ± 0.01 nm, coinciding with the height of three atomic layers of α-Ga2O3(0001). It was revealed by CAICISS analysis that the surface of α-Ga2O3(0001) is terminated by a Ga layer followed by an O layer, which is consistent with the surface termination of α-Al2O3(0001). A structural model taking surface relaxation into account was also constructed by fitting the simulated curve for the azimuth angle dependence of the Ga intensity to the experimental dependence. The resultant structural model is similar to the model of an α-Al2O3(0001) surface, which indicates analogous behavior in corundum crystals.

Shizuo Fujita

Papers

Ultrasonic-assisted mist chemical vapor deposition of II-oxide and related oxide compounds

Mist CVD Growth of ZnO-Based Thin Films and Nanostructures

Semiconductor-like carrier conduction and its field-effect mobility in metal-doped C60 thin films

Plasma-Enhanced Chemical Vapor Deposition of Fluorinated Silicon Nitride

Surface termination structure of α-Ga2O3 film grown by mist chemical vapor deposition