Effect of growth temperature and V/III-ratio on the surface morphology of MOVPE-grown cubic zincblende GaN

The influence of growth temperature and V/III-ratio on the surface morphology of (001) cubic zincblende GaN epilayers during metal organic vapour phase epitaxy growth has been investigated using atomic force microscopy and transmission electron microscopy. The zincblende phase purity as determined by X-ray diffraction was found to be above 98% for most GaN epilayers studied. As the growth temperature was increased from 850 °C to 910 °C and as the V/III-ratio was separately increased from 38 to 300, surface features were found to be elongated in the [1-10] direction, and the ratio of the length to width of such surface features was found to increase. Faceting was observed at V/III-ratios below 38 and above 300, which in the latter case was accompanied by a reduction of the zincblende phase purity. An explanation for these morphological trends is proposed based on effects such as the reduced symmetry of the top monolayer of the (001)-oriented zincblende GaN lattice, diffusion of Ga and N adatoms on such a surface, and the relative energies of the crystal facets.We would like to thank Innovate UK for the financial support within the Energy Catalyst Round 2 - Early Stage Feasibility scheme (Ref. 132135) and Energy Catalyst Round 4 - Mid Stage Feasibility scheme (Ref. 102766). We acknowledge the support of EPSRC through grant no. EP/M010589/1 and grant no. EP/R01146X/1. DJW would like to thank the support of EPSRC through grant no. EP/N01202X/1

Vertical leakage mechanism in GaN on Si high electron mobility transistor buffer layers

Control of leakage currents in the buffer layers of GaN based transistors on Si substrates is vital for the demonstration of high performance devices. Here, we show that the growth conditions during the metal organic chemical vapour deposition growth of the graded AlGaN strain relief layers (SRLs) can significantly influence the vertical leakage. Using scanning capacitance microscopy, secondary ion mass spectrometry, and transmission electron microscopy, we investigate the origins of leakage paths and show that they result from the preferential incorporation of oxygen impurities on the side wall facets of the inverted hexagonal pyramidal pits which can occur during the growth of the graded AlGaN SRL. We also show that when 2D growth of the AlGaN SRL is maintained a significant increase in the breakdown voltage can be achieved even in much thinner buffer layer structures. These results demonstrate the importance of controlling the morphology of the high electron mobility transistor buffer layer as even at a very low density the leakage paths identified would provide leakage paths in large area devices.This work was funded by the Engineering and Physical Sciences Research Council under Grant Code Nos. EP/K014471/1 and EP/N01202X/1 and the European Research Council under the European Community's Seventh Framework Programme Grant Agreement No. 279361 (MACONS)

Nano-cathodoluminescence reveals the effect of electron damage on the optical properties of nitride optoelectronics and the damage threshold

Nano-cathodoluminescence (Nano-CL) reveals optical emission from individual InGaN quantum wells for applications in optoelectronic devices. We show the luminescent intensity decays over time with exposure to the electron beam for energies between 80 and 200 keV. Measurements of the CL intensity over time show an exponential decline in intensity, which we propose is due to the formation of nitrogen Frenkel defects. The measured CL damage decreases with reductions in the electron accelerating voltage and we suggest that the electron induced structural damage may be suppressed below the proposed damage threshold. The electron beam induced damage leads to a non-radiative region that extends over the measured minority carrier diffusion length. Nano-CL may thus serve as a powerful technique to study III-nitride optoelectronics.This work was carried out with the support of the United Kingdom Engineering and Physical Sciences Research Council under Grant Nos. EP/NO17927/1 and EP/J003603/1. R. Oliver acknowledges funding from the European Research Council under the European Community's Seventh Framework Programme (FP7/2007-2013) ERC grant agreement number 279361 (MACONS) and the from the Royal Academy of Engineers/Leverhulme Trust senior research fellowship

Enhancement mode operation in AlInN/GaN (MIS)HEMTs on Si substrates using a fluorine implant

We have demonstrated enhancement mode operation of AlInN/GaN (MIS)HEMTs on Si substrates using the fluorine treatment technique. The plasma RF power and treatment time was optimized to prevent the penetration of the fluorine into the channel region to maintain high channel conductivity and transconductance. An analysis of the threshold voltage was carried out which defined the requirement for the fluorine sheet concentration to exceed the charge at the dielectric/AlInN interface to achieve an increase in the positive threshold voltage after deposition of the dielectric. This illustrates the importance of control of both the plasma conditions and the interfacial charge for a reproducible threshold voltage. A positive threshold voltage of +3 V was achieved with a maximum drain current of 367 mA mm−1 at a forward gate bias of 10 V.The authors acknowledge financial support from the Engineering and Physics Sciences Research Council (EPSRC) under EP/K014471/1 (Silicon Compatible GaN Power Electronics)

X-ray diffraction analysis of cubic zincblende III-nitrides

Solving the green gap problem is a key challenge for the development of future LED-based light systems. A promising approach to achieve higher LED efficiencies in the green spectral region is the growth of III-nitrides in the cubic zincblende phase. However, the metastability of zincblende GaN along with the crystal growth process often lead to a phase mixture with the wurtzite phase, high mosaicity, high densities of extended defects and point defects, and strain, which can all impair the performance of light emitting devices. X-ray diffraction (XRD) is the main characterization technique to analyze these device-relevant structural properties, as it is very cheap in comparison to other techniques and enables fast feedback times. In this review, we will describe and apply various XRD techniques to identify the phase purity in predominantly zincblende GaN thin films, to analyze their mosaicity, strain state, and wafer curvature. The different techniques will be illustrated on samples grown by metalorganic vapor phase epitaxy on pieces of 4'' SiC/Si wafers. We will discuss possible issues, which may arise during experimentation, and provide a critical view on the common theories.We would like to thank Anvil Semiconductors Ltd. for providing 3C-SiC on Si templates for our experiments, and Innovate UK for financial support within the Energy Catalyst Round 2—Early Stage Feasibility scheme (Ref. 132135): 'To demonstrate the potential to make low cost, high efficiency LEDs using 3C-SiC substrates'. S-L Sahonta and M J Kappers would also like to acknowledge the support of EPSRC through platform grant no. EP/M010589/1: 'Beyond Blue: New Horizons in Nitrides'. D J Wallis would like to acknowledge the support of EPSRC through grant no. EP/N01202X/1

All-GaN Integrated Cascode Heterojunction Field Effect Transistors

All-GaN integrated cascode heterojunction field effect transistors were designed and fabricated for power switching applications. A threshold voltage of +2 V was achieved using a fluorine treatment and a metal-insulator-semiconductor gate structure on the enhancement mode part. The cascode device exhibited an output current of 300 mA/mm by matching the current drivability of both enhancement and depletion mode parts. The optimisation was achieved by shifting the threshold voltage of the depletion mode section to a more negative value with the addition of a dielectric layer under the gate. The switching performance of the cascode was compared to the equivalent GaN enhancement-mode-only device by measuring the hard switching speed at 200 V under an inductive load in a double pulse tester. For the first time, we demonstrate the switching speed advantage of the cascode over equivalent GaN enhancement-mode-only devices, due to the reduced Miller-effect and the unique switching mechanisms. These observations suggest that practical power switches at high power and high switching frequency will benefit as part of an integrated cascode configuration.This work was funded by the Engineering and Physical Sciences Research Council (EPSRC), United Kingdom, under EP/K014471/1 (Silicon Compatible GaN Power Electronics)

Novel GaN-based vertical heterostructure field effect transistor structures using crystallographic KOH etching and overgrowth

A novel V-groove vertical heterostructure field effect transistor structure is proposed using semi-polar (11-22) GaN. A crystallographic potassium hydroxide self-limiting wet etching technique was developed to enable a damage-free V-groove etching process. An AlGaN/GaN HFET structure was successfully regrown by molecular beam epitaxy on the V-groove surface. A smooth AlGaN/GaN interface was achieved which is an essential requirement for the formation of a high mobility channel.This work was funded by the Engineering and Physical Sciences Research Council (EPSRC), United Kingdom, under EP/K014471/1 (Silicon Compatible GaN Power Electronics)

A concept for major incident triage: full-scaled simulation feasibility study

<p>Abstract</p> <p>Background</p> <p>Efficient management of major incidents involves triage, treatment and transport. In the absence of a standardised interdisciplinary major incident management approach, the Norwegian Air Ambulance Foundation developed Interdisciplinary Emergency Service Cooperation Course (TAS). The TAS-program was established in 1998 and by 2009, approximately 15 500 emergency service professionals have participated in one of more than 500 no-cost courses. The TAS-triage concept is based on the established triage Sieve and Paediatric Triage Tape models but modified with slap-wrap reflective triage tags and paediatric triage stretchers. We evaluated the feasibility and accuracy of the TAS-triage concept in full-scale simulated major incidents.</p> <p>Methods</p> <p>The learners participated in two standardised bus crash simulations: without and with competence of TAS-triage and access to TAS-triage equipment. The instructors calculated triage accuracy and measured time consumption while the learners participated in a self-reported before-after study. Each question was scored on a 7-point Likert scale with points labelled "Did not work" (1) through "Worked excellent" (7).</p> <p>Results</p> <p>Among the 93 (85%) participating emergency service professionals, 48% confirmed the existence of a major incident triage system in their service, whereas 27% had access to triage tags. The simulations without TAS-triage resulted in a mean over- and undertriage of 12%. When TAS-Triage was used, no mistriage was found. The average time from "scene secured to all patients triaged" was 22 minutes (range 15-32) without TAS-triage vs. 10 minutes (range 5-21) with TAS-triage. The participants replied to "How did interdisciplinary cooperation of triage work?" with mean 4,9 (95% CI 4,7-5,2) before the course vs. mean 5,8 (95% CI 5,6-6,0) after the course, p < 0,001.</p> <p>Conclusions</p> <p>Our modified triage Sieve tool is feasible, time-efficient and accurate in allocating priority during simulated bus accidents and may serve as a candidate for a future national standard for major incident triage.</p

Characterization of p-GaN1−x_{1−x} Asx_{x}/n-GaN PN junction diodes

The structural properties and electrical conduction mechanisms of p-type amorphous GaN$_{1−x}$ As$_{x}$ /n-type crystalline GaN PN junction diodes are presented. A hole concentration of 8.5 × 10$^{19}$ cm$^{-3}$ is achieved which allows a specific contact resistance of 1.3 × 10$^{-4}$ Ω cm$^{2}$. An increased gallium beam equivalent pressure during growth produces reduced resistivity but can result in the formation of a polycrystalline structure. The conduction mechanism is found to be influenced by the crystallinity of the structure. Temperature dependent current voltage characteristics at low forward bias (<0.35 V) show that conduction is recombination dominated in the amorphous structure whereas a transition from tunneling to recombination is observed in the polycrystalline structure. At higher bias, the currents are space charge limited due to the low carrier density in the n-type region. In reverse bias, tunneling current dominates at low bias (<0.3 V) and recombination current becomes dominant at higher reverse bias.This work was undertaken with support from the EPSRC (EP/K014471/1)