In recent years, GaN nanorods are emerging as a very promising novel route toward devices for nano-optoelectronics and nano-photonics. In particular, core-shell light emitting devices are thought to be a breakthrough development in solid state lighting, nanorod based LEDs have many potential advantages as compared to their 2 D thin film counterparts. In this paper, we review the recent developments of GaN nanorod growth, characterization, and related device applications based on GaN nanorods. The initial work on GaN nanorod growth focused on catalyst-assisted and catalyst-free statistical growth. The growth condition and growth mechanisms were extensively investigated and discussed. Doping of GaN nanorods, especially p-doping, was found to significantly influence the morphology of GaN nanorods. The large surface of 3 D GaN nanorods induces new optical and electrical properties, which normally can be neglected in layered structures. Recently, more controlled selective area growth of GaN nanorods was realized using patterned substrates both by metalorganic chemical vapor deposition (MOCVD) and by molecular beam epitaxy (MBE). Advanced structures, for example, photonic crystals and DBRs are meanwhile integrated in GaN nanorod structures. Based on the work of growth and characterization of GaN nanorods, GaN nanoLEDs were reported by several groups with different growth and processing methods. Core/shell nanoLED structures were also demonstrated, which could be potentially useful for future high efficient LED structures. In this paper, we will discuss recent developments in GaN nanorod technology, focusing on the potential advantages, but also discussing problems and open questions, which may impose obstacles during the future development of a GaN nanorod based LED technology.

GaN based nanorods for solid state lighting

This paper reviews the recent activities for normally-off GaN-based gate injection transistors (GITs) on Si substrates and their application to inverters. Epitaxial growth of the AlGaN/GaN heterostructures with good crystallinity over 200-mm Si substrates with eliminated bowing enables low-cost fabrication of GaN devices with high breakdown voltages. A novel normally-off GaN transistor called as GIT is proposed in which hole injection from the p-type AlGaN gate increases the drain current with low on-state resistance by conductivity modulation. The low on-state resistance in GaN-based devices greatly helps to increase the efficiency of power switching systems. A GaN-based three-phase inverter successfully drives a motor with high efficiency of 99.3% at a high output power of 1500 W. The presented GaN-based devices are expected to greatly help saving energy in the future as an indispensable power switching system.

GaN on Si Technologies for Power Switching Devices

The advance of graphene-based nanoelectronics has been hampered due to the difficulty in producing single- or few-layer graphene over large areas. We report a simple, scalable, and cost-efficient method to prepare graphene using methane-based CVD on nickel films deposited over complete Si/SiO2 wafers. By using highly diluted methane, single- and few-layer graphene were obtained, as confirmed by micro-Raman spectroscopy. In addition, a transfer technique has been applied to transfer the graphene film to target substrates via nickel etching. FETs based on the graphene films transferred to Si/SiO2 substrates revealed a weak p-type gate dependence, while transferring of the graphene films to glass substrate allowed its characterization as transparent conductive films, exhibiting transmittance of 80% in the visible wavelength range.

Synthesis, Transfer, and Devices of Single- and Few-Layer Graphene by Chemical Vapor Deposition

This letter presents details of high-performance enhancement-mode GaN MIS high-electron-mobility transistor (MIS-HEMT) devices. Devices with an n-GaN/i-AlN/n-GaN triple cap layer, a recessed-gate structure, and high- k gate dielectrics show high drain current and complete enhancement-mode operation. The maximum drain current and threshold voltage (V th) are 800 mA/mm and +3 V, respectively. These results indicate that a recessed AlGaN/GaN MIS-HEMT with the triple cap could be a promising new technology for future device applications.

Enhancement-Mode GaN MIS-HEMTs With n-GaN/i-AlN/n-GaN Triple Cap Layer and High- $k$ Gate Dielectrics

It has been several years since InGaN/GaN light-emitting diodes (LEDs) on nonpolar and semipolar orientations were first demonstrated. Prominent performance and inherent potential of these crystallographic orientations have been revealed as bulk-GaN substrates of arbitrary orientations became available for epitaxial device growth. At this point in time, we intend to survey the progress made to date and prospect the future requirements for further device improvements. The discussion begins with a historical background: how nonpolar/semipolar orientations were introduced to III-nitride LEDs and why they are beneficial. The discussion then provides information on elementary crystallography and piezoelectricity in addition to the electronic band structure of wurtzite crystals. Later in this paper, LED reports are collected to develop comprehensive knowledge of the past research efforts and trends. Nonpolar and semipolar orientations provide not only high LED performances, e.g., optical output power and wavelength ranges, but also unique functions, e.g., polarized light emission, which will explore new fields of applications.

Nonpolar and Semipolar III-Nitride Light-Emitting Diodes: Achievements and Challenges

Smooth, high quality N-polar GaN films were realized by metal organic chemical vapor deposition (MOCVD) through growth on misoriented (0001) sapphire substrates and the development of a high temperature nucleation process. Misorientation angles from 0.5° to 4° toward the a and the m plane of the sapphire substrate were investigated. Whereas GaN films grown on substrates with a misorientation angle of only 0.5° or 1° exhibited high densities of hexagonal surface features as commonly observed for N-polar GaN films grown by MOCVD, smooth GaN layers were obtained on sapphire substrates with misorientation angles of 2° or larger. In addition, the structural and optical properties of the GaN films significantly improved with increasing misorientation angle, as evaluated by high resolution x-ray diffraction, atomic force microscopy, transmission electron microscopy, and photoluminescence measurements. The properties of GaN layers grown on (0001) sapphire with a misorientation of 4° were comparable to Ga-polar GaN films grown in the same reactor.

Influence of the substrate misorientation on the properties of N-polar GaN films grown by metal organic chemical vapor deposition

Impurity incorporation in heteroepitaxial N-face and Ga-face GaN films grown by metalorganic chemical vapor deposition

The properties of N-polar InGaN∕GaN multiquantum wells (MQWs) grown by metal-organic chemical vapor deposition were investigated. Samples grown under optimized conditions exhibited distinct quantum well related emission, smooth surfaces, and abrupt interfaces as evaluated by room temperature photoluminescence, atomic force microscopy, and x-ray diffraction. Enhanced incorporation of indium into N-polar compared to Ga-polar MQW samples was observed for MQWs simultaneously deposited onto the (0001) and (0001¯) GaN-on-sapphire base layers using trimethylindium-to-trimethylgallium-flow-ratios larger than 1.2 during growth. Necessary adjustments of the growth procedure for N polar in comparison with Ga-polar MQWs are described.

Growth and characterization of N-polar InGaN/GaN multiquantum wells

GaN nanopillar and nanostripe arrays with embedded InGaN∕GaN multi-quantum wells (MQWs) were fabricated by holographic lithography and subsequent reactive ion etching. Etch related damage of the nanostructures was successfully healed through annealing in NH3∕N2 mixtures under optimized conditions. The nanopatterned samples exhibited enhanced luminescence in comparison to the planar wafers. X-ray reciprocal space maps recorded around the asymmetric (101¯5) reflection revealed that the MQWs in both nanopillars and nanostripes relaxed after nanopatterning and adopted a larger in-plane lattice constant than the underlying GaN layer. The pillar relaxation process had no measurable effect on the Stokes shift typically observed in MQWs on c-plane GaN, as evaluated by excitation power dependent photoluminescence (PL) measurements. Angular-resolved PL measurements revealed the extraction of guided modes from the nanopillar arrays.

Optical and structural properties of GaN nanopillar and nanostripe arrays with embedded InGaN∕GaN multi-quantum wells

Smooth N-polar GaN/AlxGa1−xN/GaN heterostructures with a different Al mole fraction were grown by metalorganic chemical vapor deposition on (0001) sapphire substrates with a misorientation angle of 4° toward the a-sapphire plane. The sheet electron density of the two-dimensional electron gas (2DEG), which formed at the upper GaN/AlxGa1−xN interface increased with an increasing Al-mole fraction in the AlxGa1−xN layer and increasing silicon modulation doping, similar to the observations for Ga-polar heterostructures. The transport properties of the 2DEG, however, were anisotropic. The growth on vicinal substrates led to the formation of well ordered multiatomic steps during AlxGa1−xN growth and the sheet resistance of the 2DEG parallel to the steps was about 25% lower than the resistance measured in the perpendicular direction. The fabricated devices exhibited a drain-source current, IDS, of 0.9 A/mm at a gate-source voltage +1 V. At a drain-source voltage of 10 V and IDS=300 mA/mm, current-gain and maximum...

N. Fichtenbaum

Papers

Influence of the substrate misorientation on the properties of N-polar GaN films grown by metal organic chemical vapor deposition

Impurity incorporation in heteroepitaxial N-face and Ga-face GaN films grown by metalorganic chemical vapor deposition

Growth and characterization of N-polar InGaN/GaN multiquantum wells

Optical and structural properties of GaN nanopillar and nanostripe arrays with embedded InGaN∕GaN multi-quantum wells

Properties of N-polar AlGaN/GaN heterostructures and field effect transistors grown by metalorganic chemical vapor deposition