There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality.

Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

I and i

Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN...

When group-III nitrides go infrared: New properties and perspectives

J. Y. Tsao,* S. Chowdhury, M. A. Hollis,* D. Jena, N. M. Johnson, K. A. Jones, R. J. Kaplar,* S. Rajan, C. G. Van de Walle, E. Bellotti, C. L. Chua, R. Collazo, M. E. Coltrin, J. A. Cooper, K. R. Evans, S. Graham, T. A. Grotjohn, E. R. Heller, M. Higashiwaki, M. S. Islam, P. W. Juodawlkis, M. A. Khan, A. D. Koehler, J. H. Leach, U. K. Mishra, R. J. Nemanich, R. C. N. Pilawa-Podgurski, J. B. Shealy, Z. Sitar, M. J. Tadjer, A. F. Witulski, M. Wraback, and J. A. Simmons

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Ultrawide-Bandgap Semiconductors: Research Opportunities and Challenges

The modification of surfaces by the deposition of a robust overlayer provides an excellent handle with which to tune the properties of a bulk substrate to those of interest. Such control over the surface properties becomes increasingly important with the continuing efforts at down-sizing the active components in optoelectronic devices, and the corresponding increase in the surface area/volume ratio. Relevant properties to tune include the degree to which a surface is wetted by water or oil. Analogously, for biosensing applications there is an increasing interest in so-called “romantic surfaces”: surfaces that repel all biological entities, apart from one, to which it binds strongly. Such systems require both long lasting and highly specific tuning of the surface properties. This Review presents one approach to obtain robust surface modifications of the surface of oxides, namely the covalent attachment of monolayers.

Covalent surface modification of oxide surfaces.

Au contacts. The B-FTS approach exhibits the unique feature of ultra-rapid growth of ZnO nanotetrapods within few milliseconds and simultaneously in situ bridging electrical contacts. These bridging nanotetrapods were directly integrated on a chip and demonstrated signifi cantly improved performances as a UV photodetector. Comparison of the UV photodetectors performances built from interpenetrating ZnO nano-microstructures fabricated by B-FTS and C-FTS techniques are presented. Fastest response/recovery time constant (≈32 ms) under 365 nm UV light irradiation of B-FTS-made photodetectors (on/off ratio ≈4.5 ◊ 10 3 at 2.4 V) is reported. Different type of nanojunctions formed between neighbor nanowires or nanotetrapods (with ‘arm’ thickness <50 nm) could be the reason for such improved characteristics. The role of nanojunctions in fast UV photodetectors from networked ZnO nanowires and nanotetrapods is discussed. On the basis of the rapid B-FTS fabrication process and fast UV photodetection capabilities, such networked ZnO nanotetrapods can be potential candidates for various nanosensor applications.

Rapid fabrication technique for interpenetrated ZnO nanotetrapod networks for fast UV sensors.

We report heteroepitaxial growth of InN on r-plane sapphire substrates with an AlN nucleation layer and GaN buffer using plasma-assisted molecular-beam epitaxy. The InN film was identified to be nonpolar (1120) a-plane which follows the a-plane GaN buffer. Optical absorption and photoluminescence measurements of this material show that InN has a fundamental band gap of about 0.7 eV, which is also seen for growth on c-plane sapphire. The room-temperature Hall mobility of undoped a-plane InN is around 250 cm2/V s with a carrier concentration around 6×1018 cm−3. We also studied the electrical properties of the a-plane InN as a function of film thickness. In contrast to c-plane InN grown on c-plane sapphire, we did not observe apparent improvement of electrical properties of a-plane InN by growing thicker films.

Growth of a-plane InN on r-plane sapphire with a GaN buffer by molecular-beam epitaxy

Determination of the composition of InxGa1−xN from strain measurements

Ozone sensors are of great demand for monitoring high-concentration ozone for industrial applications and low-concentration ozone for protecting people's health, although commercial ozone sensors are limited in their detection range. In this work, it is demonstrated that compact energy-saving photostimulated ozone sensors based on indium oxide nanoparticles can detect ozone with a dynamical range over four orders of magnitude at room temperature. The photostimulated ozone sensor shows a very low cross response to NO2, CO, and CO2. Furthermore, the sensing signal is very reproducible, and no hysteresis effects were found in repeated measurements.

Photon stimulated sensor based on indium oxide nanoparticles I: Wide-concentration-range ozone monitoring in air

Thin AlOx films were grown on 4H-SiC by plasma-assisted atomic layer deposition (ALD) and plasma assisted electron-beam evaporation at 300˚C. After deposition, the films were annealed in nitrogen at temperatures between 500˚C and 1050˚C. The films were analyzed by X-ray reflectivity (XRR) and atomic force microscopy (AFM) in order to determine film thickness, surface roughness and density of the AlOx layer. No differences were found in the behavior of AlOx films upon annealing for the two different employed deposition techniques. Annealing results in film densification, which is most prominent above the crystallization temperature (800˚C). In addition to the increasing density, a mass loss of ~5% was determined and attributed to the presence of aluminum oxyhydroxide in as deposited films. All changes in film properties after high temperature annealing appear to be independent of the deposition technique.

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Densification of Thin Aluminum Oxide Films by Thermal Treatments

Indium–tin-oxide (ITO) and indium–tin-oxynitride (ITON) thin films have been fabricated by rf-sputtering in plasma containing Ar or a mixture of Ar and N2, respectively. The structural, electrical and optical properties of ITON films were examined and compared with those of ITO films. The microstructure of ITON films was found to be dependent on the nitrogen concentration in the plasma. Increasing the amount of nitrogen in the plasma increased the resistivity and reduced the carrier concentration and mobility of the films. The electrical properties of the ITON films improved after annealing. The absorption edge of the ITON films deposited in pure N2 plasma was shifted towards higher energies and showed reduced infrared reflectance compared to the respective properties of ITO films. The potential of indium–tin-oxynitride films for use as a transparent conductive material for optoelectronic devices is addressed.

Volker Cimalla

Papers

Growth of a-plane InN on r-plane sapphire with a GaN buffer by molecular-beam epitaxy

Determination of the composition of InxGa1−xN from strain measurements

Photon stimulated sensor based on indium oxide nanoparticles I: Wide-concentration-range ozone monitoring in air

Densification of Thin Aluminum Oxide Films by Thermal Treatments

Properties of rf-sputtered indium-tin-oxynitride thin films