A hydride vapor phase epitaxy, (HVPE) was designed to grow nonpolar GaN. LiAlO2 single crystal grown by Czochralski (Cz) method in our lab was used as the substrate. The X-ray diffraction and scanning electron microscopy were used to study the GaN epilayer¡¦s orientation and surface morphology. At the first part, we used the c-plane sapphire as substrate and make sure that our HVPE reactive system is working. And the second part, we used LiAlO2 substrate to grow non-polar GaN substrate. After the growth, GaN was separated from LiAlO2 substrate and become free-standing. We found that as-grown GaN has both c-plane (0001) and m-plane (10-10) orientations. After some improvements, we got a nonpolar GaN substrate. But the m-plane (10-10) GaN grains are random. The photo-luminescence¡]PL¡^showed that the light emissive quality of these two GaN thick film are well. We will keep improving the design of our HVPE.

The group III nitride semiconductor grown on c-plane sapphire by radio frequency plasma assisted molecular beam epitaxy has been studied. To archive good quality GaN thin film, nitridation and low temperature AlN buffer layer were applied to overcome the issue of lattice mismatch. Low temperature and long period nitridation process shows better improved of optical properties and crystal quality of GaN film. Buffer layer grown with slightly Ga-rich, substrate temperature at 600¢J, for 20 minutes leads to better GaN thin film. High substrate temperature and sufficient nitrogen to gallium ratio are two important factors to control the growth of the good quality GaN epilayer. We have grown M-plane GaN films with self-assembled c-plane GaN nanopillars on a £^-LiAlO2 substrate by plasma-assisted molecular-beam epitaxy. The diameters of the basal plane of the nanopillars are about 200 to 900 nm and the height is up to 600 nm. The formation of self-assembled c-plane GaN nanopillars is through nucleation on hexagonal anionic bases of £^- LiAlO2. Dislocation defects were observed and analyzed by transmission electron microscopy. From the experimental results, we developed a mechanism underlying the simultaneous growth of three-dimensional c-plane nanopillars and two dimensional M-plane films on a £^-LiAlO2 substrate.

Thesis (M. Phil.)--Hong Kong Baptist University, 2006. / Thesis submitted to the Dept. of Chinese Language and Literature. Includes bibliographical references (leaves 123-126).

Ces dernières années, la consommation énergétique n’a cessé de s’accroitre. Pour contrer cela, les systèmes de conversion électrique se doivent d’être plus performants, d’améliorer les rendements tout en limitant l’encombrement des systèmes, réduisant ainsi les matières premières utilisées. Parmi les systèmes de conversion efficaces, le correcteur du facteur de puissance et sa diode "boost" assurent un rendement de conversion d’environ 95 %. Ces travaux de thèse s’intéressent à la réalisation de ce type de diode en utilisant un matériau différent du traditionnel silicium, le nitrure de gallium (GaN), ce qui pourrait permettre d’atteindre des rendements de conversion supérieurs à 98 %. Le GaN est épitaxié sur Si et des diodes Schottky sont réalisées à sa surface. Des protocoles expérimentaux ont été mis en place en vue d’optimiser leurs performances, notamment la réduction des densités de courant de fuite. / These past few years, energy consumption has continued to increase. To counter this, the power conversion systems need to be more efficient, to improve yields while limiting systems volume, which reduce raw materials. Among efficient conversion systems, the power factor corrector and its "boost diode" provide a conversion efficiency of about 95%. This work concerned with the realization of this type of diode using a different material from the traditional silicon, the gallium nitride (GaN), which could allow higher conversion efficiencies of 98% to be achieved. GaN is grown up on Si and Schottky diodes are made on its surface. Experimental protocols were implemented in order to optimize their performance, especially the reduction of leakage current densities. Two research areas were defined: surface treatments and the realization of the guard ring. Our results showed that the different surface preparations either by wet or plasma can reduce leakage current.

Wide bandgap devices are the future of this dynamically changing technological world. Considering Gallium Nitride (GaN) and Silicon Carbide (SiC), GaN has exceptional characteristics that will likely allow it to proliferate greatly in the area of low-and-mid-power power electronic converters. One of the current challenges in this context to completely utilize GaN are the reliability issues, especially false turn-on events, which is a main focus of this thesis. False turn-on due to its momentary short circuit capability deteriorates the converter performance. Furthermore, simple and accurate modeling of GaN device losses is critical to help electronic designers optimize converter designs. This thesis focuses on three contributions to help reduce false turn-on events and improve GaN modeling efforts. First, this work uniquely investigates the optimal pulse-width-modulation (PWM) scheme to balance efficiency and false turn-on. The experimental results lead to a recommendation to use a larger negative bias than is currently recommended by device manufacturers. Secondly, the work proposes a new simplified switching loss model with high accuracy that can be used with different gate drive circuits (including negative gate bias voltages) to make it more useful for power electronics design engineers as a tool. And thirdly, since the main contributor to false turn-on events are the parasitic inductances in the switch and on the PCB, this work proposes a new parasitic inductance measurement methodology which can be implemented using only simple laboratory instruments. / Thesis / Doctor of Engineering (DEng) / Wide bandgap devices are the future of this dynamically changing technological world. Considering Gallium Nitride (GaN) and Silicon Carbide (SiC), GaN has exceptional characteristics that will likely allow it to proliferate greatly in the area of low-and-mid-power power electronic converters. One of the current challenges in this context to completely utilize GaN are the reliability issues, especially false turn-on events, which is a main focus of this thesis. False turn-on due to its momentary short circuit capability deteriorates the converter performance. Furthermore, simple and accurate modeling of GaN device losses is critical to help electronic designers optimize converter designs. This thesis focuses on three contributions to help reduce false turn-on events and improve GaN modeling efforts. First, this work uniquely investigates the optimal pulse-width-modulation (PWM) scheme to balance efficiency and false turn-on. The experimental results lead to a recommendation to use a larger negative bias than is currently recommended by device manufacturers. Secondly, the work proposes a new simplified switching loss model with high accuracy that can be used with different gate drive circuits (including negative gate bias voltages) to make it more useful for power electronics design engineers as a tool. And thirdly, since the main contributor to false turn-on events are the parasitic inductances in the switch and on the PCB, this work proposes a new parasitic inductance measurement methodology which can be implemented using only simple laboratory instruments.

No description available.

GaN-based HFETs demonstrate ubiquitous high power and high frequency performance and attract tremendous research efforts. Even though significant advances have been achieved, there still exist some critical issues needed to be investigated and solved. In particular, high defect densities due to inhomogeneous growth and operation under high power conditions bring many unique problems which are not so critical in the conventional Si and GaAs materials systems. In order to reduce the defect density and heat dissipation of GaN-based HFETs, research work on the realization of GaN-based HFETs on bulk GaN substrate has been carried out and the key problems have been identified and solved. Hot phonon scattering is the bottleneck which limits the enhancement of electron velocity in the GaN 2DEG channel. It is found that the plasmon-phonon coupling is the mechanism for converting of hot phonons into high group velocity acoustic phonons. In order to push more electrons into the GaN 2DEG channel in the plasmon-phonon coupling regime and to further reduce the hot phonon lifetime, a novel AlGaN/GaN dual channel HFET structure has been proposed. The growth, fabrication and characterization of such a AlGaN/GaN dual channel HFET structure has been carried out. Conventionally GaN-based light emitting diodes and laser diodes are grown and fabricated using the c-plane III-nitride expitaxy layers. In c-plane III-nitride epi-layers, the polarization-induced electric field introduces spatial separation of electron and hole wave functions in quantum wells (QW)s used LEDs and laser diodes LDs and degrades quantum efficiency. As well, blueshift in the emission wavelength becomes inevitable with increasing injection current unless very thin QWs are employed. The use of nonpolar orientations, namely, m-plane or a-plane GaN, would solve this problem. So far, m-plane GaN has been obtained on LiAlO2 (100), m-plane SiC substrates, and m-plane bulk GaN, which all have limited availability and/or high cost. Silicon substrates are very attractive for the growth of GaN due to their high quality, good thermal conductivity, low cost, availability in large size, and ease with which they can be selectively removed before packaging for better light extraction and heat transfer when needed To realize the low cost and improve the internal quantum efficiency of GaN based light emitting diodes, the process for m-plane GaN growth on Si (112) substrates has been studied and optimized. The continuous m-plane GaN film is successfully grown on Si (112) substrates.

Disertacijoje pristatomi didelio krūvininkų tankio dinamikos tyrimai skirtingo defektiškumo GaN, InGaN bei deimanto sluoksniuose atlikti naudojant šviesa indukuotų dinaminių gardelių metodą. Naudojant eksperimentinius rezultatus gautus skirtingo žadinimo intensyvumo, temperatūros sąlygomis bei pasitelkus skaitmeninį krūvininkų dinamikos modeliavimą nustatomi dominuojantys krūvininkų rekombinacijos bei sklaidos mechanizmai tirtuose plačiatarpiuose puslaidininkiuose. Parodoma, kad dislokacijų sąlygota nespindulinė rekombinacija dominuoja GaN bandiniuose su dislokacijų tankiu didesniu nei 1E8 cm-2, esant mažesniems dislokacijų tankiams krūvininkų gyvavimo trukmę lemia bimolekulinė rekombinacija. Nustatyta, kad eksperimentiškai stebėtas krūvininkų difuzijos koeficiento didėjimas GaN yra sukeltas Fermi slėgio išsigimusioje krūvininkų plazmoje. Krūvininkų rekombinacijos trukmių bei azoto priemaišų kiekio koreliacija sintiniuose deimantuose parodė, kad su azotu susietos priemaišos yra pagrindiniai rekombinacijos centrai Tai pat darbe pristatoma nauja šviesa indukuotų dinaminių gardelių eksperimento schema su holografiniu pluoštelio dalikliu, leidžianti supaprastinti eksperimentą bei įgalinanti heterodininį difrakcijos signalo detektavimą. Parodoma, kad fazės skirtumas tarp signalo ir foninės šviesos gali būti kontroliuojamas keičiant holografinio daliklio padėtį išilgai jo gardelės vektoriaus krypties. / The investigation of high density carrier dynamics in GaN, InGaN, and diamond samples with different defect density by light induced transient grating technique is presented in the thesis. The experimental studies on numerous samples, grown at different conditions, combined with extensive measurements in a wide range of carrier densities (1E16-1E20 cm-3) and temperature (9-300K) is used to identify the interplay of radiative and nonradiative recombination mechanisms, to determine carrier lifetime, diffusion coefficient, and diffusion length. It is shown that dislocation governed carrier recombination is a dominant recombination channel in GaN samples with dislocation density above 1E8 cm-2, otherwise, the bimolecular recombination dominates at high carrier densities. Experimentally observed increase of carrier diffusivity in GaN is caused by Fermi pressure at degenerate carrier plasma. The correlation between the carrier lifetime and concentration of nitrogen defects points out that nitrogen-related defects act as the main centers of nonradiative recombination in synthetic diamonds. A novel heterodyne detection scheme for LITG technique is presented. The heterodyning is achieved by coherently mixing the picosecond pulses of diffracted and scattered light. It is shown that a phase difference between theses fields can be controlled by moving holographic beam splitter along its grating vector.

III-nitrides, diamonds are extremely promising wide band gap semiconductor materials for optoelectronics and high temperature, high power electronics. Therefore, there is huge scientific interest in investigation electrical and optical properties of these materials. The light induced transient grating technique (LITG) is very suitable for exploration of carrier dynamics which governed by fundamental and defect related properties of materials. The main goals of the thesis were gain a new knowledge on carrier dynamics in wide bandgap semiconductors (namely GaN, InGaN, and diamonds) by using and developing light induced transient grating technique. The experimental studies on numerous samples, grown at different conditions, combined with extensive measurements in a wide range of carrier densities (1016-1020 cm-3) and temperature (9-300K) was targeted to identify the interplay of radiative an nonradiative recombination mechanisms, to determine carrier lifetime dependence on the excess carrier density, to explain the carrier diffusion coefficient dependence on excitation intensity, to find the optimal materials growth conditions. A novel heterodyne detection scheme for LITG technique was presented. The heterodyning was achieved by coherently mixing the picosecond pulses of diffracted and scattered light. A phase difference between theses fields was controlled by moving holographic beam splitter (HBS) along its grating vector. LITG signal decay kinetics, recorded at two HBS... [to full text] / III grupės nitridai bei deimantai tai platų draustinės energijos tarpą turintys puslaidininkiai, pasižymintys unikaliomis medžiagos savybėmis ir turintys didelį potencialą aukštų temperatūrų, didelių galių, opto/elektroniniams taikymams. Todėl šių medžiagų elektrinės bei optinės savybės pastaruoju metu yra intensyviai tiriamos. Šviesa indukuotų dinaminių gardelių (ŠIDG) metodas labai tinka tyrinėti krūvininkų dinamiką, kuri yra nulemta fundamentinių bei defektinių medžiagos savybių. Pagrindiniai darbo tikslai buvo gauti naujų žinių apie krūvininkų dinamiką plačiatarpiuose puslaidininkiuose (GaN, InGaN bei deimantuose) naudojat bei plėtojant šviesa indukuotų gardelių metodiką. Ištirti didelio nepusiausvirųjų krūvininkų tankio rekombinacijos ir difuzijos ypatumus skirtingo defektiškumo GaN, InGaN sluoksniuose bei sintetiniuose deimantuose. Skaitmeniškai modeliuojant krūvininkų dinamiką nustatyti dominuojančius krūvininkų rekombinacijos mechanizmus bei krūvininkų gyvavimo trukmes, difuzijos koeficientus ir nuotolius. Darbe pristatoma nauja ŠIDG eksperimento schema su holografiniu pluoštelio dalikliu, leidžianti supaprastinti eksperimentą. Ši schema taip pat įgalino heterodininį difrakcijos signalo detektavimą. Parodoma, kad fazės skirtumas tarp signalo ir foninės šviesos gali būti kontroliuojamas keičiant holografinio daliklio padėtį išilgai jo gardelės vektoriaus krypties. Ištyrus didelį kiekį GaN sluoksnių, užaugintų skirtingomis technologijomis bei pasižyminčiu skirtingu... [toliau žr. visą tekstą]

Investigations made on new materials and their structures for production of particle detectors based on semi-insulating SiC and GaN comprise the technological and applied importance of this study. Innovations in defect control technology, especially, in recognition of extended defects and percolative carrier transport in heavily irradiated detector structures are considered and applied for scientific implementations. These investigations have been performed within a framework of CERN rd50 project. Irradiation by 24 GeV protons varying fluence up to 1016 cm-2 deteriorates rectifying properties of the 4H-SiC particle detectors. Different isotopes produced in 4H-SiC during irradiation by protons have been revealed by gamma spectroscopy. In the non-irradiated GaN material the temperature-dependent variations of leakage current have been unveiled to be caused by the carrier mobility temperature changes. Activation energy values have been extracted for proton radiation induced deep centres in the GaN detectors by thermally stimulated current spectroscopy as well as the isotopes and long-living radio-nuclides have been identified by gamma spectroscopy. In the Si detectors, irradiated by reactor neutrons, the photo-activation energy values have been determined for the deep levels located below the mid-gap by photo-ionisation spectroscopy while isochronal anneals enhance the density of the acceptor-type vacancy-related defects. / Disertacijoje išanalizuoti gilieji centrai didžiavaržiuose Si, SiC ir GaN dariniuose, sietini su didelės energijos protonų bei neutronų spinduliuote sudarytais defektais, atskleistos radiacinių defektų transformacijos po iškaitinimų, didelių energijų spinduliuotės įtaka krūvio pernašai ir pagavai medžiagose, tinkamose jonizuojančiosios spinduliuotės detektoriams, tiriamiems pagal Europos branduolinių tyrimų centro (CERN) projektus. 4H-SiC dariniuose, apšvitintuose 24 GeV/c protonais, išanalizuota elektrinių charakteristikų kaita. Iš šiluma skatinamųjų srovių spektrų nustatytos šiluminės aktyvacijos energijų vertės. Taip pat 4H-SiC dariniuose, apšvitintuose protonų įtėkiais, siekiančiais 1016 cm-2, įvertintas skirtingų spinduliuote sukurtų izotopų kiekis. Neapšvitintose GaN dariniuose nustatyta, kad medžiagos elektrinio laidumo parametrų kaitą nulemia krūvininkų judrio kitimas. Apšvitintuose neutronais GaN dariniuose šiluma skatinamųjų srovių spektroskopijos būdu buvo nustatyti dominuojančių defektų lygmenys. Aptikta, kad po apšvitos 24 GeV/c protonų įtėkiais, siekiančiais 1016 cm-2, GaN susidarė 7Be, 22Na ir kiti ilgaamžiai radionuklidai, kurių atominis skaičius A<70, bei žymiai pakito spinduliuotės detektorių krūvio pernašos savybės. Didžiavaržio silicio detektoriuose po apšvitos reaktoriaus neutronais susidarė visa eilė radiacinių defektų, kuriems priskirtinų giliųjų centrų parametrai buvo įvertinti fotojonizacijos spektroskopijos ir tamsinės srovės temperatūrinių kitimų... [toliau žr. visą tekstą]