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

Plasma CVD silicon nitride films are produced from SiH4–N2 gas mixture with a constant flow ratio SiH4/N2=0.1. When the pressure decreases and the substrate temperature increases, the Si/N composition approaches the stoichiometric value 0.75. As compared with the film deposited from SiH4–NH3 reaction, the present ammonia-free silicon nitride contains little N–H bonds and less hydrogen, and can be expected to have higher atom concentration and tight bondings.

Composition and Chemical Bonds in Silicon Nitride by SiH4-N2 Gas Mixture Plasma CVD

The successful growth of Zn1-xCdxSySe1-y (ZnCdSSe) quaternary alloys by metalorganic molecular beam epitaxy (MOMBE) is reported. Layer-by-layer growth was confirmed for the first time by monitoring the intensity oscillation of a specular beam in a reflection high-energy diffraction (RHEED) pattern. ZnCdSSe layers coherently grown on GaAs exhibited good crystallographic and optical properties.

Metalorganic Molecular Beam Epitaxy of Zn1-xCdxSySe1-y Quaternary Alloys on GaAs Substrate

(111)-oriented Zn3N2 thin films were grown on a-plane (1120) sapphire substrates by plasma-assisted molecular beam epitaxy. Zn3N2/sapphire exhibits orientational relationships of out-of-plane ∥[1120] and in-plane almost ∥[0001] and ∥[0001]. These specific orientations result from similar surface configurations of N atoms in Zn3N2 and Al atoms in sapphire at the interface. Under the optimum growth conditions, the film showed a full width at half maximum of 185 arcsec for an X-ray (222) diffraction rocking curve and a root-mean-square roughness of 8.5 A in an atomic force microscope image. These results suggest that a-plane sapphire is a suitable heteroepitaxial substrate for high-quality Zn3N2 thin films.

(111)-Oriented Zn3N2 growth on a-plane sapphire substrates by molecular beam epitaxy

We report the growth and optical properties of a new strained-layer wide-band-gap single-quantum well (SQW): Zn1-yCdySe/ZnSxSe1-x. A SQW structure with four ZnCdSe wells of 1, 4, 8 and 12 monolayers (ML5) was grown on a GaAs substrate by metalorganic molecular beam epitaxy (MOMBE). The corresponding full widths at half-maximum (FWHMs) of 4.2 K photoluminescence peaks are 15.8, 18.2, 18.4 and 15.0 meV, respectively. These values are considerably small in spite of the high strain in well layers (about 2%), which are attributed to the well-defined structures of the SQW as confirmed by reflection high-energy electron diffraction (RHEED) in situ observation.

https://iopscience.iop.org/article/10.1143/JJAP.30.L555/pdf

Growth and optical properties of novel wide-band-gap strained-layer single quantum wells : Zn1-yCdySe/ZnSxSe1-x

We investigated the effects of (NH4)2Sx-pretreatment of GaAs substrates on the properties of ZnSe epilayers and heterointerfaces in pseudomorphic ZnSe on GaAs by MOMBE. Compared to the pretreatment of GaAs substrates by a conventional method where thermal etching is employed as a cleaning process, (NH4)2Sx-pretreatment resulted in (i) two-dimensional growth from an earlier stage of growth, (ii) better optical properties of pseudomorphic ZnSe epilayers, and (iii) better heterointerfaces. Thus, the interface state density near an energy level of 0.18 eV from the top of the valence band is estimated to be about 5×1011 cm-2eV-1, and the density near the midgap is below the lower detection limit of Terman's method without post-annealing. These features show that this pretreatment technique is very promising for the fabrication of high-quality heterojunction devices.

Shizuo Fujita

Papers

Composition and Chemical Bonds in Silicon Nitride by SiH4-N2 Gas Mixture Plasma CVD

Metalorganic Molecular Beam Epitaxy of Zn1-xCdxSySe1-y Quaternary Alloys on GaAs Substrate

(111)-Oriented Zn3N2 growth on a-plane sapphire substrates by molecular beam epitaxy

Growth and optical properties of novel wide-band-gap strained-layer single quantum wells : Zn1-yCdySe/ZnSxSe1-x

Effects of (NH4)2Sx-Pretreatment of GaAs Surfaces on Properties of Epilayers and Heterointerfaces in Pseudomorphic ZnSe/GaAs Gown by MOMBE