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

Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications

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

http://experimentationlab.berkeley.edu/sites/default/files/AFMImages/butt_AFM.pdf

Force measurements with the atomic force microscope: Technique, interpretation and applications

Wurtzitic ZnO is a wide-bandgap (3.437 eV at 2 K) semiconductor which has many applications, such as piezoelectric transducers, varistors, phosphors, and transparent conducting films. Most of these applications require only polycrystalline material; however, recent successes in producing large-area single crystals have opened up the possibility of producing blue and UV light emitters, and high-temperature, high-power transistors. The main advantages of ZnO as a light emitter are its large exciton binding energy (60 meV), and the existence of well-developed bulk and epitaxial growth processes; for electronic applications, its attractiveness lies in having high breakdown strength and high saturation velocity. Optical UV lasing, at both low and high temperatures, has already been demonstrated, although efficient electrical lasing must await the further development of good, p-type material. ZnO is also much more resistant to radiation damage than are other common semiconductor materials, such as Si, GaAs, CdS, and even GaN; thus, it should be useful for space applications.

Recent Advances in ZnO Materials and Devices

A ZnO diode was fabricated by using a laser-doping technique to form a p-type ZnO layer on an n-type ZnO substrate. A zinc-phosphide compound, used as a phosphorous source, was deposited on the ZnO wafer and subjected to excimer-laser pulses. The current–voltage characteristics showed a diode characteristic between the phosphorous-doped p-layer and the n-type substrate. Moreover, light emission, with a band-edge component, was observed by forward current injection at 110 K.

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ZnO diode fabricated by excimer-laser doping

Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation

A p-type ZnO layer was fabricated by excimer laser doping technique using thermally oxidized ZnO films. Epitaxial ZnO(0001) films were formed by thermal oxidation of epitaxial ZnSe films on Si(111) substrate grown by remote plasma enhanced metal-organic chemical vapor deposition. The p-type ZnO was fabricated by excimer laser irradiation with Sb as a dopant source. A good Ohmic contact was obtained between Sb doped ZnO layer and gold metal electrodes. The Sb doped ZnO layer showed positive Hall coefficient, the resistivity was 8 × 10 -3 Ω cm with a hole mobility of 1.5 cm 2 /Vs and an acceptor concentration of 5 x 10 20 cm -3 as p-type, respectively.

p‐Type ZnO Layer Formation by Excimer Laser Doping

Zn1−xCdxO films in the range of the content x from x=0 to x=1 were grown by remote-plasma-enhanced metal organic chemical vapor deposition The crystal structure of Zn1−xCdxO film changed with increase of the content x from wurtzite structure to rocksalt structure around x=07 The relationship between the cadmium content and axis length in the Zn1−xCdxO films was studied Photoluminescence spectra were observed from the wurtzite Zn1−xCdxO films in the range of 33–18eV at room temperature Stokes’s shift in the restricted composition range was compared with the previous results

Zn1−xCdxO systems with visible band gaps

Zn1-xCdxO films were successfully grown by remote plasma-enhanced metalorganic chemical vapor deposition (RPE-MOCVD). The content ratio of Zn1-xCdxO films was controlled by changing the molar ratio of diethyl zinc (DEZn) to dimethyl cadmium (DMCd). The wurtzite structure of Zn1-xCdxO films was obtained by increasing the Cd content up to x=0.697. The optical-band-gap energy of Zn1-xCdxO films was tuned between 1.85 eV and 3.28 eV at room temperature. The photoluminescence emission of hexagonal Zn1-xCdxO films up to x = 0.697 was observed at room temperature.

Toru Aoki

Papers

ZnO diode fabricated by excimer-laser doping

Preparation of ZnO thin films for high-resolution field emission display by electron beam evaporation

p‐Type ZnO Layer Formation by Excimer Laser Doping

Zn1−xCdxO systems with visible band gaps

Zn1-xCdxO film growth using remote plasma-enhanced metalorganic chemical vapor deposition