J. Narayan

III–nitrides: Growth, characterization, and properties

During the last few years the developments in the field of III–nitrides have been spectacular. High quality epitaxial layers can now be grown by MOVPE. Recently good quality epilayers have also been grown by MBE. Considerable work has been done on dislocations, strain, and critical thickness of GaN grown on different substrates. Splitting of valence band by crystal field and by spin-orbit interaction has been calculated and measured. The measured values agree with the calculated values. Effects of strain on the splitting of the valence band and on the optical properties have been studied in detail. Values of band offsets at the heterointerface between several pairs of different nitrides have been determined. Extensive work has been done on the optical and electrical properties. Near band-edge spectra have been measured over a wide range of temperatures. Free and bound exciton peaks have been resolved. Valence band structure has been determined using the PL spectra and compared with the theoretically calcu...

Domain epitaxy: A unified paradigm for thin film growth

We present a unified model for thin film epitaxy where single crystal films with small and large lattice misfits are grown by domain matching epitaxy (DME). The DME involves matching of lattice planes between the film and the substrate having similar crystal symmetry. In this framework, the conventional lattice matching epitaxy becomes a special case where a matching of lattice constants or the same planes is involved with a small misfit of less than 7%–8%. In large lattice mismatch systems, we show that epitaxial growth of thin films is possible by matching of domains where integral multiples of major lattice planes match across the interface. We illustrate this concept with atomic-level details in the TiN/Si(100) with 3/4 matching, the AlN/Si(100)with 4/5 matching, and the ZnO/α−Al2O3(0001) with 6/7 matching of major planes across the film/substrate interface. By varying the domain size, which is equal to intregral multiple of lattice planes, in a periodic fashion, it is possible to accommodate addition...

Optical and structural properties of epitaxial MgxZn1−xO alloys

The optical and structural properties of high-quality single-crystal epitaxial MgZnO films deposited by pulsed-laser deposition were studied. In films with up to ∼36 at. % Mg incorporation, we have observed intense ultraviolet band edge photoluminescence at room temperature and 77 K. The highly efficient photoluminescence is indicative of the excitonic nature of the material. Transmission spectroscopy was used to show that the excitonic structure of the alloys was clearly visible at room temperature. High-resolution transmission electron microscopy, x-ray diffraction, and Rutherford backscattering spectroscopy/ion channeling were used to verify the epitaxial single-crystal quality of the films and characterize the defect content. Post-deposition annealing in oxygen was found to reduce the number of defects and to improve the optical properties of the films. These results indicate that MgZnO alloys have potential applications in a variety of optoelectronic devices.

On the grain size softening in nanocrystalline materials

Materials Science and Engineering Department, North Carolina State University,Raleigh, NC 27695-7907, USA(Received December 17, 1999)(Accepted December 29, 1999)Keywords: Grain boundaries; Thermally activated; Hardness; AtomicIntroductionNanocrystalline materials with grain sizes in the range of 1–100 nm have been found to exhibit noveland often improved physical properties [1–4]. They are thus of considerable interest from bothscientific and technological viewpoints. In the present paper we consider the influence of grain size onthe flow stress (or corresponding hardness) of nanocrystalline materials, with focus on the grain sizesoftening (i.e., the so-called negative or inverse Hall-Petch effect) which has been observed at grainsizes ,;50 nm [5–12].The effect of grain size on the hardness of nanocrystalline materials has been reported to have theform shown schematically in Fig. 1 [5–15]. At the larger grain sizes the hardness increases withdecrease in grain size according to the well-known Hall-Petch relation (with the possibility that theHall-Petch constant k

Excitonic structure and absorption coefficient measurements of ZnO single crystal epitaxial films deposited by pulsed laser deposition

The optical properties of high quality single crystal epitaxial zinc oxide thin films grown by pulsed laser deposition on c-plane sapphire substrates were studied. It was found that annealing the films in oxygen dramatically improved the optical and electrical properties. The absorption coefficient, band gap, and exciton binding energies were determined by transmission measurements and photoluminescence. In both the annealed and the as-deposited films excitonic absorption features were observed at both room temperature and 77 K. In the annealed films the excitonic absorption peaks were substantially sharper and deep level photoluminescence was suppressed.

Zn0.9Co0.1O-based diluted magnetic semiconducting thin films

Here we report a systematic study of structural, optical, and magnetic measurements on epitaxial Zn0.9Co0.1O films grown on c-plane sapphire single crystal, at various temperatures (500–650°C), using pulsed-laser deposition. The main emphasis in this work has been on the correlation of microstructure with properties, specifically with magnetic properties and the fate of cobalt ions into substitutional sites versus precipitates. The reasons for room-temperature ferromagnetism are explored, and convincingly proved to be one of the inherent properties of the material. Most importantly, the presence of nanoclusters of any magnetic phase was ruled out. This was determined by high-resolution transmission electron microscopy, coupled with electron energy loss spectroscopy and STEM-Z (scanning transmission electron microscopy-atomic number) contrast studies.

Epitaxial growth of TiN films on (100) silicon substrates by laser physical vapor deposition

We report epitaxial growth of TiN films having low resistivity on (100) silicon substrates using pulsed laser deposition method. The TiN films were characterized using x‐ray diffraction, Rutherford backscattering, four‐point‐probe ac resistivity, high resolution transmission electron microscopy and scanning electron microscopy techniques and epitaxial relationship was found to be 〈100〉 TiN ∥ 〈100〉 Si. TiN films showed 10%–20% channeling yield. In the plane, four unit cells of TiN match with three unit cells of silicon with less than 4.0% misfit. This domain matching epitaxy provides a new mechanism of epitaxial growth in systems with large lattice misfits. Four‐point‐probe measurements show characteristic metallic behavior of these films as a function of temperature with a typical resistivity of about 15 μΩ cm at room temperature. Implications of low‐resistivity epitaxial TiN in silicon device fabrication are discussed.

Electrical properties of transparent and conducting Ga doped ZnO

In this paper, we report on the metal-semiconductor transition behavior observed in transparent and conducting ZnO:Ga films grown by pulsed-laser deposition. The electrical resistivity measurements were carried out on ZnO films with varying Ga concentration in the temperature range of 14to300K. The electrical properties were correlated with film structure, and detailed structural characterization was performed using x-ray diffraction, transmission electron microscopy, and x-ray photoelectron spectroscopy. The room-temperature resistivity of these films was found to decrease with Ga concentration up to 5% Ga, and then increase. The lowest value of resistivity (1.4×10−4Ωcm) was found at 5% Ga. Temperature dependent resistivity measurements showed a metal-semiconductor transition, which is rationalized by localization of degenerate electrons. A linear variation of conductivity with T below the transition temperature suggests that the degenerate electrons are in a weak-localization regime. It was also found t...

Theoretical model for deposition of superconducting thin films using pulsed laser evaporation technique

We have theoretically and experimentally analyzed the laser‐induced evaporation process for deposition of superconducting thin films from bulk targets. The spatial thickness variations have been found to be significantly different from a conventional thermal deposition process. Unlike a cos θ thickness variation expected from a thermal evaporation process, the laser evaporation process is characterized by a forward‐directed deposit with a sharp variation in its thickness as a function of distance from the center of the deposit. We have studied in detail the interactions of nanosecond excimer laser pulses with bulk YBa2Cu3O7 targets leading to evaporation, plasma formation, and subsequent deposition of thin films. A theoretical model for simulating the pulsed laser evaporation (PLE) process has been developed. This model considers an anisotropic three‐dimensional expansion of the laser‐generated plasma, initially at high temperature and pressure. The forward‐directed nature of laser deposition has been fou...

Metallic conductivity and metal-semiconductor transition in Ga-doped ZnO

This letter reports the metallic conductivity in Ga:ZnO system at room temperature and a metal-semiconductor transition (MST) behavior at low temperatures. Zn0.95Ga0.05O films, deposited by pulsed laser deposition in the pressure range of ∼10−2Torr of oxygen, were found to be crystalline and exhibited degeneracy at room temperature with the electrical resistivity close to 1.4×10−4Ωcm and transmittance >80% in the visible region. Temperature dependent resistivity measurements of these highly conducting and transparent films also showed, for the first time, a MST at ∼170K. Mechanisms responsible for these observations are discussed in the terms of dopant addition and its effect on ionization efficiency of oxygen vacancies.