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

Corundum-structured gallium oxide (-Ga2O3) on sapphire substrates is advantageous in views of band gap engineering and low-cost, but heterogeneous phases and rotation domains tend to be included owing to the heteroepitaxy. The insertion of -(AlxGa1-x)2O3 alloy buffer layers was effective for their elimination. This paper showed the elimination of heterogeneous phases (especially the -Ga2O3 phase) and rotational domains, which have been often seen in -Ga2O3 thin films on c-plane sapphire substrates with the use of -(AlxGa1-x)2O3 buffer layers. These results are attributed to the stress reduction in -Ga2O3 grown on the underlying layer. This was done without modifying the growth conditions, that is, without narrowing the windows of growth conditions. This result also suggests the important role of any buffer layers on sapphire substrates in order for further improvements of crystal quality of -Ga2O3, leading to evolution of a variety of Ga2O3-based devices on low-cost and large-area on sapphire substrates.

Synthesis of High-Quality α-Ga 2 O 3 thin films on Sapphire Substrates with Introduction of Buffer Layers

Dynamics of dense excitonic systems in ZnSe-based single quantum wells

Optical properties of organic multilayer structures with type-I and type-II energy lineups with respect to the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) energy levels were investigated and compared. The fast energy transfer followed by carrier confinement in the material with lower HOMO-LUMO energy difference was proved for the structures with type-I energy lineup, while the tendency of separation of excited states, i.e., electrons and holes generated in the excited molecules, was demonstrated for those with type-II. These results indicate that the luminescence dynamics of organic multilayer structures are well discussed in terms of HOMO and LUMO energy lineups similarly to valence and conduction band energies in inorganic semiconductors.

Luminescence Dynamics of Alq3-Based Multilayer Structures in Terms of HOMO and LUMO Energy Discontinuity

Gallium oxide is receiving increased attention as a next-generation material for power devices having both low switching loss and high breakdown voltage. In this paper, we described the growth of Ga2O3 on Si substrates using mist chemical vapor deposition (CVD) technique. Firstly, amorphous Ga2O3 and polycrystalline Ga2O3 thin films were obtained on Si(111) substrates accompanying with rough surface conditions and low crystallinity. Secondly, <001> oriented κ-Ga2O3 thin films were grown on n-type Si(100) and Si(111) substrates by introducing ZnO buffer layers to avoid influence from surface oxide layers of Si substrates. The RMS value of κ-Ga2O3 was smaller than the values of amorphous Ga2O3 and polycrystalline Ga2O3 on Si(111). These results suggested that the ZnO buffer layer on Si played significant roles in crystallinity and improved planarity of κ-Ga2O3. 

Growth of Ga2O3 Thin Films on Si Substrates by Mist CVD Technique

ZnCdSe/ZnSe multiple quantum well (MQW) transmission and reflection photomodulators operating at room temperature were fabricated employing quantum-confined Stark effect on the exciton absorption. Samples were grown on p-type GaAs substrates by MBE with an i-Zn0.87Cd0.13Se/ZnSe MQW heterostructure sandwiched by a ZnSe p-n junction. The transmission modulator was constructed with a Zn0.87Cd0.13Se/ZnSe MQW glued onto a piece of ITO film-covered glass with silver paste and epoxy. To avoid absorption in GaAs substrates, a window with a diameter of about 2 mm was opened using a selective etch. For the reflective use an Al mirror was deposited on the glass back surface, the device then operates in reflection with the light to be modulated making a double pass through the active quantum well region, thereby increasing the modulation amplitude. Measurement results are given in this paper for transmission, reflection, differential transmission, differential absorption, and differential reflection as a function of the incident photon wavelength and the applied field.© (1996) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

Shizuo Fujita

Papers

Synthesis of High-Quality α-Ga 2 O 3 thin films on Sapphire Substrates with Introduction of Buffer Layers

Dynamics of dense excitonic systems in ZnSe-based single quantum wells

Luminescence Dynamics of Alq3-Based Multilayer Structures in Terms of HOMO and LUMO Energy Discontinuity

Growth of Ga2O3 Thin Films on Si Substrates by Mist CVD Technique

Zn1-xCdxSe/ZnSe multiple quantum well photomodulators