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

ApplyingabicolorcoherenttreatmentofaNdYAGlaserwi thwavelength�1.32� �mand� itssecondharmonicgeneration,� wehaveinvestigatedphotoinducedSHGin Aunanoparticle� depositedZnOnanocrystallinne�(NC)�films.�Wehaveestablishedt hatcoexistenceofthesuch� crystallitesgivessubstantiallybetterresultscompar edtothepureZnONCdepositedontothe��glass� substrate.�Itwasshownthatthevalueofthesecondordersuscept ibilityobtainedforthelarge� incidentanglesofthefundamentalandwritingbeamsisequalto� about�23�pm/V.�Thebetterresults� areobtainedduringthephototreatmentattemperaturesnearthe�60� o CfortheAudopedsamplesand�

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Non-Linear Optical Effects in Au Nanoparticle-Deposited ZnO Nanocrystalline Films

This paper describes metalorganic molecular beam epitaxy (MOMBE) of p-type ZnSe using metal zinc, pre-cracked metalorganic dimethylselenide, and microwave-excited nitrogen plasma as sources. Optical, structural, and electrical properties of the p-type ZnSe layers have been investigated. At present, maximum net acceptor concentration Na-Nd is 3 x 1017 cm-3 without any post-growth annealing. This is the highest acceptor concentration ever reported for MOMBEgrown p-type ZnSe doped with nitrogen plasma, but photoluminescence and deep level transient spectroscopy suggest that acceptors are highly compensated and the reduction of compensating defects is a key to further increase the acceptor concentration.

Growth of P-type ZnSe by metalorganic molecular beam epitaxy using metal Zn and dimethylselenide

A higher zinc composition of illuminated epilayers (Zn0.38Cd0.62S) was employed to form the barrier whereas unilluminated growth sequences were adopted to grow the well (Zn0.30Cd0.70S) in the photoassisted metalorganic vapour phase epitaxy (MOVPE). Time-integrated photoluminescence spectra clearly show two peaks at 437 nm (2.83 eV) and 443 nm (2.80 eV) attributed to the barrier and to the well respectively. The time evolution of these spectra shows an increasing intensity of emission from the well with a time delay due to the confinement and recombination of carriers in the well. Time-resolved measurements have shown a time delay of 60 ps between the barrier and the well. These results indicate the formation of type-I band structure.

Recombination Dynamics in ZnxCd1-xS Single Quantum Well Grown by Photoassisted Metalorganic Vapour Phase Epitaxy by Time-Resolved Photoluminescence Spectroscopy.

1 緒 言 リチウム系酸化物は,リチウムイオン二次電池の電極 材料として広く用いられているが,今後の研究開発が必 須とされる全固体型薄膜リチウムイオン二次電池におい ては,正極・負極・電解質に用いられる多様なリチウム 系酸化物を,安全性・コスト・制御性・生産性がバラン スした技術により成膜することが強く求められている. これを念頭に置くと,化学的気相成膜 (chemical vapor deposition ; CVD) 法が有用と考えられるが,一般にリチ ウム化合物の蒸気圧が低く,CVDの原料として用いるに は配管の加熱など装置構成が複雑になる.また蒸気圧の 高い原料については,安全性の問題もある. われわれは,将来における実用的なリチウムイオン二 次電池の作製を念頭に置き,われわれの研究室で研究開 発を続けてきたミスト CVD法 1), 2)が有用であると考え ている.この成膜技術は,所望の元素を含む原料の水ま たはアルコール溶液を準備し,これに超音波を印加して 発生する微粒子(ミスト)をキャリアガスを用いて成膜 部に送り込むという方法で,CVD法の制御性を持ちなが ら,蒸気圧の高い原料を用いることを要しないという特 徴がある.さらに,非真空の技術であり,Roll-to-Rollプ ロセスにも対応が容易である. そこで本稿では,リチウム系酸化物薄膜の成膜に対し て,ミスト CVD法の適用可能性を調べることを目的と して,基本となる酸化リチウムの成膜から,薄膜リチウ ムイオン二次電池の新しい正極材料として期待されてい るマンガン酸リチウム (LiMn2O4) の成膜実験の結果につ いて報告する. 2 超音波噴霧ミスト CVD法 成膜に用いた装置の概略を Fig. 1に示す.一般の気相 成膜に用いられるのと同様の石英製反応炉を用い,基板 はヒータにより最高 1000°C程度まで加熱可能である.ホッ トプレート状のヒータで基板加熱を行い,ノズルから原 料を基板上に供給するような構造にすれば,3)大面積基板 上への成膜や Roll-to-Rollプロセスにも対応が容易である と考えられる. 周波数 2.4MHzの超音波印加によって得られるミスト の粒径は 2μm程度であり,加湿器やネブライザ等で日常 的に経験するように,このような微小なミストは空気中 に浮遊し,ガス流によって輸送される.実験に用いた成 超音波噴霧ミスト CVD法によるリチウム系酸化物の成膜 井 川 拓 人 金 子 健太郎 藤 田 静 雄 Fabrication of Lithium-Based Oxide Thin Films by Ultrasonic-Assisted Mist CVD Technique

Fabrication of Lithium-Based Oxide Thin Films by Ultrasonic-Assisted Mist CVD Technique

P-type ultra-wide bandgap materials have been desired for gallium oxide electronics. We show that the self-trapped exciton (STE) in α -Ga_2O_3 is a stable state relative to the free exciton from analysis of Urbach tails in absorption spectra. The stable STE formation indicates strong localization of holes, which suggests that α -Ga_2O_3 is disadvantageous as a p-type semiconductor. Instead of doping methods, using p-type material that have the same crystal structure as Ga_2O_3 is effective. An alloy thin film of α -(Rh,Ga)_2O_3 showed clear p-type conductivity with the large positive Hall coefficient. The α -Ir_2O_3 thin films also showed p-type conductivities. The origin of p-type conductivity is attributed to that 5d orbitals of Ir which consists the top of the valence band of α -Ir_2O_3, being located at the upper energy level compared to 2p orbitals of oxygen atoms. Graphical abstract

Shizuo Fujita

Papers

Non-Linear Optical Effects in Au Nanoparticle-Deposited ZnO Nanocrystalline Films

Growth of P-type ZnSe by metalorganic molecular beam epitaxy using metal Zn and dimethylselenide

Recombination Dynamics in ZnxCd1-xS Single Quantum Well Grown by Photoassisted Metalorganic Vapour Phase Epitaxy by Time-Resolved Photoluminescence Spectroscopy.

Fabrication of Lithium-Based Oxide Thin Films by Ultrasonic-Assisted Mist CVD Technique

Novel p-type oxides with corundum structure for gallium oxide electronics