Fast growth of smooth AlN in a 3 x 2 showerhead-type vertical flow MOVPE reactor

The conditions required for a high growth rate of AlN in a 3 × 2″ showerhead-type vertical flow metalorganic vapor phase epitaxy (MOVPE) reactor are studied. It is found that at the standard growth conditions (low V/III, 50 mbar, 1110 °C, H2), the growth rate linearly increases with the trimethylaluminium (TMAl) flow rate until about 280 μmol min−1 with some drop of precursor utilization efficiency at higher pressures. While the pre-reaction of TMAl with NH3 at 140 μmol min−1 of TMAl is still not a major issue, it is not possible, however, to maintain a smooth AlN surface morphology during this “fast” growth. To suppress the surface morphology deterioration, the growth pressure requires optimization. An increase of the growth pressure, to 75 mbar, is found to be critical to grow 20+ μm of smooth AlN at a rate of about 3.6 μm h−1 on bulk AlN substrates

InAlN-based LEDs emitting in the near-UV region

Fully functional InAlN-based ultraviolet LEDs emitting at 340–350 nm were demonstrated for the first time; detailed electrical and optical characterization is presented and discussed. Results from the measurements at pulsed conditions are in agreement with the attribution of the dominant electroluminescence peak to near-band-edge emission. The composition of the AlGaN barriers was chosen to give the same internal polarization field as that of the InAlN wells. A simulation study of this polarization-matched heterostructure shows a significant increase in the electron-hole overlap integral if compared with a standard AlGaN/AlGaN active region having the same level of carrier confinement. Limitations and problems of these preliminary devices are also presented, and possible future work aimed at increasing their efficiency is discussed

Epitaxial lateral overgrowth of AlN on self-assembled patterned nanorods

We report an inexpensive nanoscale patterning process for epitaxial lateral overgrowth (ELOG) in AlN layers grown by metal organic vapour phase epitaxy (MOVPE) on sapphire. The pattern was produced by an inductively coupled plasma etch using a self-assembled monolayer of silica spheres on AlN as the lithographic mask. The resulting uniform 1 [small mu ]m length rod structure across a wafer showed a massive reduction in threading dislocations (TDs) when annealed at 1100 [degree]C. Overgrowing homoepitaxial AlN on top of the nanorods, at a temperature of 1100 [degree]C, produced a crack free coalesced film with approximately 4 [small mu ]m of growth, which is formed at a much lower temperature compared to that typically required for microscale ELOG. The improved crystal quality, in terms of TD reduction, of the AlN above the rods was determined by detailed weak beam (WB) electron microscopy studies and showed that the threading dislocation density (TDD) was greatly reduced, by approximately two orders of magnitude in the case for edge-type dislocations. In situ reflectance measurements during the overgrowth allowed for thickness coalescence to be estimated along with wafer curvature changes. The in situ measurements also confirmed that tensile strain built up at a much slower rate in the ELOG AlN layer compared to that of AlN prepared directly on sapphire

The effect of a varied NH3 flux on growth of AlN interlayers for InAlN/GaN heterostructures

The effects of AlN interlayer growth conditions on InAlN/AlN/GaN heterostructures are investigated, with interlayers imaged as they would appear prior to InAlN barrier layer deposition using surface atomic force microscopy scans undertaken immediately after growth. Surface morphologies and subsequent heterostructure conductivity suggested minimum on-resistance can be achieved by balancing the underlying GaN channel decomposition and interfacial roughening when deciding AlN interlayer growth parameters on a sapphire substrate of a given miscut. (C) 2013 AIP Publishing LLC. (DOI: 10.1063/1.4818645

Significant contribution from impurity-band transport to the room temperature conductivity of silicon-doped AlGaN

Silicon-doped n-type (0 0 0 1) AlGaN materials with 60% and 85% AlN content were studied close to the doping condition that gives the lowest resistivity (Si/III ratios in the ranges 2.8–34  ×  10−5 and 1.3–6.6  ×  10−5, respectively). Temperature-dependent conductivity and Hall-effect measurements showed that, apart from the diffusion-like transport in the conduction band, a significant amount of the conductivity was due to phonon-assisted hopping among localized states in the impurity band, which became almost completely degenerate in the most doped sample of the Al0.6Ga0.4N series. In the doping range explored, impurity-band transport was not only dominant at low temperature, but also significant at room-temperature, with contributions to the total conductivity up to 46% for the most conductive sample. We show that, as a consequence of this fact, the measurements of Hall carrier concentration and Hall mobility using the usual single-channel approach are not reliable, even at high temperatures. We propose a simple method to separate the contributions of the two channels. Our model, although only approximate, can be used to gain insight into the doping mechanism: particularly it shows that the room-temperature free-electron concentration in the conduction band of the Al0.6Ga0.4N material reaches its maximum at about 1.6  ×  1018 cm−3, well below the value that would have been obtained with the standard single-channel analysis of the data. This maximum is already achieved at dopant concentrations lower than the one that gives the best conductivity. However, further increase of the doping levels are required to enhance the impurity-band channel, with concentrations of the carriers participating in this type of transport that increase from 2.1  ×  1018 cm−3 up to 4.3  ×  1018 cm−3. For the Al0.85Ga0.15N, even though it was not possible to estimate the actual carrier concentrations, our measurements suggest that a significant impurity-band channel is present also in this material

Ultra-high-density arrays of defect-free AlN nanorods: a "space-filling" approach

Nanostructured semiconductors have a clear potential for improved optoelectronic devices, such as high-efficiency light-emitting diodes (LEDs). However, most arrays of semiconductor nanorods suffer from having relatively low densities (or “fill factors”) and a high degree of nonuniformity, especially when produced by self-organized growth. Ideally an array of nanorods for an optoelectronic emitter should have a fill factor close to 100%, with uniform rod diameter and height. In this article we present a “space-filling” approach for forming defect-free arrays of AlN nanorods, whereby the separation between each rod can be controlled to 5 nm due to a self-limiting process. These arrays of pyramidal-topped AlN nanorods formed over wafer-scale areas by metal organic chemical vapor deposition provide a defect-free semipolar top surface, for potential optoelectronic device applications with the highest reported fill factor at 98%

Composition dependence of photoluminescence properties of InxAl1-xN/AlGaN quantum wells

A series of InAlN/AlGaN five quantum well (QW) heterostructures was prepared by metal-organic vapour phase epitaxy to investigate their photoluminescence (PL) properties as a function of indium content in QWs at aluminium content in barriers fixed at 59%. In addition to the expected redshift of the emission spectrum, a strong rise of PL efficiency was observed with increasing indium content from 12.5 to 18%. Use of a higher indium content leads to a further redshift but also to a sudden and sharp degradation of PL efficiency. Reasons for the observed behaviour are discussed in detail, which raise the possibility of a transition to a type II band lineup in the InAlN-AlGaN system

Bandgap and refractive index estimates of InAlN and related nitrides across their full composition ranges

III-Nitride bandgap and refractive index data are of direct relevance for the design of (In, Ga, Al)N-based photonic and electronic devices. The bandgaps and bandgap bowing parameters of III-nitrides across the full composition range are reviewed with a special emphasis on InxAl1−xN, where less consensus was reached in the literature previously. Considering the available InAlN data, including those recently reported for low indium contents, empirical formulae for InAlN bandgap and bandgap bowing parameter are proposed. Applying the generalised bandgap data, the refractive index dispersion data available in the literature for III-N alloys is fitted using the Adachi model. For this purpose, a formalism involving a parabolic dependence of the Adachi parameters on the dimensionless bandgap ξEg=(Eg,AxB1−xN−Eg,BN)/(Eg,AN−Eg,BN) of the corresponding ternary alloys is used rather than one directly invoking the alloy composition

Highly-ordered growth of solution-processable ZnO for thin film transistors

We demonstrate that crystalline, epitaxial-like and highly ordered ZnO thin films and quasi-superlattice structures can be achieved from a precursor liquid at relatively low temperature via spin-coating. The synthesised films are smooth, stoichiometric ZnO with controllable thickness. An iterative layer-by-layer coating schematic is employed to demonstrate the effects of film thickness on structure, morphology as well as the surface and internal defects. Characterisation of the crystallinity, morphology, O-vacancy formation, stoichiometry, surface roughness and thickness variation was determined through X-ray diffraction, scanning and transmission electron and atomic force microscopy, X-ray photoelectron and photoluminescence spectroscopy. We demonstrate that iterative spin-coating of deposited ZnO films results in a transition in crystal texture with increasing thickness (number of layers) from the [ ] m-plane to the [ ] c-plane. The films attain a c-axis preferential orientation, with no other crystalline peaks present. Results show that the film’s surface morphology was very smooth, with average rms roughness <0.15 nm. Examination of these films also shows the consistency of the surface composition and defect level while highlighting the effect of temperature and cumulative annealing condition on the internal defect concentration

GaN Nanowire Schottky Barrier Diodes

A new concept of vertical gallium nitride (GaN) Schottky barrier diode based on nanowire (NW) structures and the principle of dielectric REduced SURface Field (RESURF) is proposed in this paper. High-threading dislocation density in GaN epitaxy grown on foreign substrates has hindered the development and commercialization of vertical GaN power devices. The proposed NW structure, previously explored for LEDs offers an opportunity to reduce defect density and fabricate low cost vertical GaN power devices on silicon (Si) substrates. In this paper, we investigate the static characteristics of high-voltage GaN NW Schottky diodes using 3-D TCAD device simulation. The NW architecture theoretically achieves blocking voltages upward of 700 V with very low specific on-resistance. Two different methods of device fabrication are discussed. Preliminary experimental results are reported on device samples fabricated using one of the proposed methods. The fabricated Schottky diodes exhibit a breakdown voltage of around 100 V and no signs of current collapse. Although more work is needed to further explore the nano-GaN concept, the preliminary results indicate that superior tradeoff between the breakdown voltage and specific on-resistance can be achieved, all on a vertical architecture and a foreign substrate. The proposed NW approach has the potential to deliver low cost reliable GaN power devices, circumventing the limitations of today's high electron mobility transistors (HEMTs) technology and vertical GaN on GaN devices