The major advances in progress reported by delegates at ICNS-4 point to exciting times ahead for nitride semiconductors. Richard Campion and Nicola Stanton report.

The fourth International Conference on Nitride Semiconductors (ICNS-4) was held on 1620 July in Denver, Colorado. With twice the number of delegates (600) than the previous conference in Montpellier, and over 20 invited speakers, ICNS-4 demonstrated that GaN and its alloys have come of age as commercially viable materials. There was strong representation from Europe and Asia, as well as the US, showing the worldwide interest in this topic. Significant progress was reported in the fundamental understanding of growth and defect mechanisms, bulk material growth, and device fabrication and performance. Amongst the applications discussed were high-power LEDs and transistors, blue lasers and exciting flip-chip designs. Whilst AlGaInN dominated the proceedings, there was also work presented on mixed arsenic-nitrides such as AlGaAsN and InGaAsN. FETs, 2DEGs and HBTs Micovic et al. of HRL Laboratories, CA, described significant advances in the growth of AlGaN Schottky HFETs grown by plasma-assisted MBE. They reported a power density of 6.6 W/mm at 20 GHz for devices grown on SiC. Good reproducibility of sheet resistance and effective Schottky barrier thickness was achieved, with low noise operation comparable to state of the art GaAs devices. A device with a short gate design operated up to 140 GHz. HFETs fabricated by MBE on SiC substrates were also described by Webb et al. from the Institute for Microstructural Science in Ottawa. After overcoming the difficulties in SiC surface preparation, low temperature (77 K) mobilities of 11 000 cm2/Vs were recorded, with HFET characteristics showing no current slump. Interestingly both sets of devices showed no light sensitivity and did not require surface passivation. Manfra et al. from a collaborative group including Lucent Technologies, MIT and the University of Columbia reported on the growth of AlGaN/GaN two-dimensional electron gases (2DEGs) by MBE. Low temperature (4 K) electron mobilities of 75 000 cm2/Vs at sheet carrier densities of 1.5 1012/cm2 were measured, and the fractional quantum Hall effect was observed at high fields. This performance demonstrates the vast improvement in growth techniques which have been made over the last couple of years, when state of the art mobilities of such structures were 10 000 cm2/Vs. Makimoto et al. from NTT Basic Research Laboratories described HBT devices based on InGaN/GaN. The transistors showed a current gain () of 20 at a collector current of 3 mA, higher than similar devices based on the AlGaN/GaN system ( 11). The p-type density in the InGaN base was reported to be 1019 cm3 leading to the good device performance. The first reporting of AlN/GaN double barrier resonant tunneling diodes was made by Kikuchi et al. from Sophia University, Tokyo. The devices had a peak to valley ratio of 3:1 at room temperature and were grown by MBE on MOVPE-grown substrates. The characteristics of both double barrier and superlattice (SL) barrier (four monolayers thickness) AlN/GaN devices were discussed, with negative differential resistance observed at 2.4 V and 1.6 V, respectively. UV photodetectors Campbell et al. of the University of Texas discussed two designs for a back illuminated solar-blind photodetector. The first, a PIN type, had a UV to visible light rejection of over three decades and exhibited low noise and low dark current performance. The second, an MSM type that obviates the need for p-doping, exhibited similarly good characteristics and had an external quantum efficiency of 48% at 12 V bias for a wavelength of 262 nm, and a dark current determined to be < 20 fA. LEDs and lasers Koike et al. from Toyoda Gosei, Japan, highlighted the large improvement in the optical efficiency of nitride-based LEDs and lasers that has been achieved since such devices first became commercially available some six years ago. Improving the lifetime of laser diodes for use in consumer electronics such as DVD systems is an obvious goal. The presentation detailed some of the advances in material quality, including a reduction in dislocation density of two orders of magnitude. Since dislocations are known to significantly reduce the lifetime of laser diodes, limiting them is necessary to realize the full commercial potential of nitride-based lasers. Gtz et al. from LumiLeds gave an impressive visual demonstration of high-brightness LEDs. Developments include increasing the conductivity of the n and p layers, engineering the strain in the layers and improving contacting and packaging strategies. The result was a demonstration of a traffic light assembly requiring 12 LEDs, compared to the 200 or so conventional LEDs required in current assemblies. A wall-plug efficiency of 26% was achieved for a wavelength of 428 nm. The fabrication of a deep UV LED was described by Shatalov et al. from the University of South Carolina. They discussed the problems associated with current crowding and the high Al content in the quaternary AlGaInN layers. In an attempt to control the defects in the layer, a pulsed epitaxy technique was devel-oped for layers on sapphire substrates. The total power was determined to be three times higher than similar devices on conventional MOCVD epilayers, the difference being ascribed to a lower defect density in the layer and high quality quaternary barriers. Quaternary InAlGaN UV LEDs emitting at 320 nm were also described by Hirayama et al. of the Institute of Physical and Chemical Research, Japan. The improved UV performance over the traditional AlGaN QW structures was attributed to indium segregation (observed in cathodoluminescence measurements) in the InAlGaN MQWs. By studying the behavior of dislocations in the wings of non-contact laterally overgrown material, Tomiya et al. of Sony have observed a low dislocation density region in the central portion of the wing. They related short laser lifetimes to high dislocation densities and high device power consumption. By carefully positioning the lasers on the low dislocation density regions they have extended the lifetimes to 6000 hours and have To values of 160K. Mukai et al. from Nichia presented a comparison of laser diode characteristics grown by epitaxial layer overgrowth (ELOG) on freestanding GaN substrates and thick GaN templates. It was reported that the application of the ELOG technique was indeed beneficial to device performance, with estimated lifetimes in excess of 15 000 hours achieved under CW, 30 mW operation. New applications Field emission from heavily Si-doped AlN was described by Kobayashi et al. of NTT Basic Research Laboratories. They reported a field emission current density of 0.22 A/cm2 for a 0.4 m, highly Si-doped (1 1021 cm3) AlN layer. This current density is seven times greater than that seen for diamond field emission cathodes and this novel material clearly has potential applications in cold cathodes for displays. Novel applications for FETs based on wurtzite GaN as polar liquid/gas detectors or pressure sensors were reported by Eickhoff et al. of Walter Schottky Institute, Munich. The surface-charge state of the gate region is modified by contact with a polar liquid (e.g. water or acetone) and a suitably biased device can be made to give large voltage swings dependent upon the liquid present. The talk concentrated on the wetting behavior of nitrogen and gallium polarity surfaces. Kneissl et al. from Xerox, Palo Alto, reported a simple ambient excimer laser lift-off process () that gives freestanding devices with ultra smooth back surfaces that can be bonded to another material of choice (e.g. copper/diamond). They reported InGaN MQW lasers with enhanced thermal and optical performance, and proposed face to face emitter/detector designs for lab-on-a-chip fluid spectrometers, as well as the possibility of other applications using laser lift-off, flip-chip technology. Epitaxy and materials Chowdhury et al. of the University of Texas reported on the electrical proper-ties of p-doped AlGaN/GaN SLs with variations in well width, layer numbers and composition grown by low pressure MOCVD. They considered the strain state and coherence of the SLs and made comparisons with similar structures where the SL was replaced by a layer of pure AlGaN of the same composition. They showed that some SLs have higher conductivity than the equivalent thickness of bulk material. By using growth interruption, Sugahara et al. of Oklahoma State University showed that the structural and optical properties of InGaN/GaN MQWs grown on sapphire by MOCVD can be significantly improved. The degree and nature of the improvement is dependent on interruption time and whether the interrupts are before, after, or before and after each InGaN layer. Tanka et al. showed that pre-growth etching of SiC substrates could improve the crystalline and surface quality of AlN films by creating regular stepped terraces on the substrate surface. This allows AlN nucleating on a given step to coalesce with material on adjacent steps in a crystallographically coherent manner. Tang et al. from the Institute for Microstructural Science in Ottawa showed that selective area growth and ELOG are possible with ammonia MBE. This was not previously thought to be feasible. They showed that the morphology of facets at the edges of the lateral growth fronts is strongly dependent upon the crystallographic orientation (). Christen et al. of Otto-von-Guericke University, Magdeburg, presented a beautiful set of images showing a comprehensive scanning cathodoluminescence and micro Raman study of the lateral growth fronts of ELOG GaN grown by MOVPE. They showed that the growth rate and thus the impurity incorporation rate, as well as local strain, are factors which are strongly influenced by the lateral growth direction. By such optimization they have demonstrated very high quality overgrowth coalescence. Stutzmann et al. from the Walter Schottky Institute in Munich described a method for controlled growth of epilayers of mixed polarity separated by inversion domains. This was achieved by growth of a thin buffer layer followed by patterning and subsequent selective etching of the buffer layer. Regrowth over the patterned layer causes controllable growth of epilayers of mixed type. The possibility of fabricating devices exploiting the differing electronic properties of the two types may now be realized. EXAFS studies of InGaN films were reported by Miyajima et al. from Sony. Studies of co-ordination number as a function of indium content revealed good evidence for indium segregation behavior in this alloy. They observed that the co-ordination number of an indium atom s second nearest neighboring indium atom was higher than those estimated from a random GaInN alloy, in which the indium atoms should randomly occupy the cation sublattice. Screw dislocations Using a first-principles total energy approach, Northrop et al. of Xerox, Palo Alto, showed that the likely nature of screw dislocations in GaN is a small hollow core decorated internally with Ga atoms, such cores being strong non-radiative recombination centers. Under very nitrogen-rich growth conditions the core would be depleted of Ga. However, under Ga-rich conditions the core would contain a pair of helices of Ga atoms. Between these two growth conditions, varying degrees of Ga vacancy would appear in the helices. Timely support for this work came from the next presentation by Manfra et al. of Bell Labs, Murray Hill, who have devised a method of scanning current-voltage microscopy based on a biased AFM tip. This beautifully demonstrated that the cores of the hillocks on the surface of GaN grown in Ga-rich MBE are dislocations that are conducting at low bias. Those grown in nitrogen-rich conditions require a high bias before conducting. They also presented structural evidence that the presence of excess gallium leads to structural changes in the dislocation cores and even a nano-droplet of gallium at the top of the dislocation. Conclusions Adding to the oral presentations were a multitude of poster presentations over the course of the conference, making it clear to all who attended that the field is moving forward at a rapid pace. Indeed, what was most apparent in all areas of nitride semiconductor research was an incremental improvement in experimental achievement, and the theoretical background that supports it. These developments are helping to improve the performance of the limited range of commercialized nitride-based devices, and open the way for new classes of device to begin emerging onto the market. The proceedings from ICNS-4 are to be published in two special editions of Physica Status Solidi (Wiley) towards the end of this year. The full technical program of ICNS-4 can be found at: www.mrs.org/meetings/icns-4.