Table of contents

The aligned growth of III-nitride nanowires, along with results providing insights into the nanowire properties obtained using electrical, optical and structural characterization techniques, are discussed. A new "top-down" approach for fabricating ordered arrays of high quality GaN-based nanorods with controllable height, pitch and diameter is also presented, along with results from preliminary LEDs grown on these nanorod arrays. Additionally, a novel application of aligned nanowire arrays as strain-relief templates for the growth of high quality GaN is demonstrated.

We report on fabrication of ultra-thin GaN membranes of nanometer scale thickness, by using the concept of surface charge lithography based on low-energy ion treatment of the sample surface with subsequent photoelectrochemical etching. The membranes prove to be transparent to both electrons and UV radiation, emit mainly yellow cathodoluminescence, and exhibit electrical conductivity. Successful fabrication of nanometer-thin membranes opens unique possibilities for exploration of two dimensional GaN-based structures predicted to be ferromagnetic with defect-induced half-metallic configuration which is of peculiar importance for spintronics applications.

Electrolyte contacts were used to investigate the properties of pure and N-doped titania nanotubes (ntTiO₂). Mott-Schottky analysis gave a flat-band potential E_fb = -0.57V vs. Ag/AgCl for pure ntTiO₂ and E_fb = -0.22V vs. Ag/AgCl for N-doped ntTiO₂. The charge carrier density was N_D = 6.7 10²⁰ cm^-3 for pure ntTiO₂ and N_D = 3.9 10²⁰ for N-doped ntTiO₂. This investigation also allowed estimation of the apparent diffusion coefficient of H⁺ in ntTiO₂. Following the Randles-Sevcik method, the effective proton diffusion coefficient in ntTiO₂ is (2±1)•10^-11 cm²s^-1 while in N-doped ntTiO₂ it is (4±1)•10^-11 cm²s^-1. Using the Warburg diffusion element determined by electrochemical impedance spectroscopy, the proton diffusion coefficient is (2±1)•10^-11 cm² s^-1 for pure ntTiO₂ while for nitrogen-doped ntTiO₂ a value of (7±3)•10^-11 cm² s^-1 is found. These values are consistent with those of the Randles-Sevcik method.

In this study, composite electroplating technique is used to fabricate the diamond-added copper heat spreader for UV LED applications. Thermal dissipation characteristic and optical performance are improved as the composite diamond-added copper heat spreader adoption. The low thermal resistance of 18.4 K/W with UV LED using diamond-added copper heat spreader was measured. Surface temperature of UV LED using the diamond-added copper heat spreader is 50.42 oC at 500 mA injecting current lower than pure copper heat spreader of 59.9 oC and original heat spreader of 90.4 oC. The thermal diffusivity of the diamond-added copper is 0.7179 cm2/s measurement by laser flash method. Output power and power efficiency of UV LEDs are also enhanced to 71.81 mW and 4.32 %, respectively, at 400 mA injection current. The optimal structure design and materials fabrication will be discussed.

We report on the molecular beam epitaxial growth, fabrication, and characterization of InGaN/GaN dot-in-a-wire white light emitting diodes (LEDs) grown on Si(111). By varying the In compositions in the dot layers, we have demonstrated strong white light emission. The dot-in-a-wire LEDs exhibit a relatively high internal quantum efficiency of ~ 36.7% and virtually zero efficiency droop for current densities up to ~ 200 A/cm2 at room temperature, which are attributed to the superior 3-dimensional carrier confinement provided by the dots and to the use of nearly defect and strain-free GaN nanowires.

InGaN-based solar cells with p-InGaN/i-InGaN/n-GaN double heterojunction structure have been fabricated and characterized in our study. Two kinds of sapphire substrate, conventional sapphire substrate and pattern sapphire substrate, were used for epitaxial growth of the heterojunction structure. Both the solar cells grown on conventional sapphire substrate and pattern sapphire substrate demonstrated high open-circuit voltage (VOC) of 2.05 V and 2.08 V, respectively. However, short-circuit current density (JSC) of solar cell grown on pattern sapphire substrate shown an improvement of 27.6% when comparing to its grown on conventional sapphire substrate. Such an enhancement could be contributed to the increase of effective light absorption path due to the incident light scattering from textured surface on pattern sapphire substrate.

A 460 nm InGaN/GaN blue light-emitting diode (LED) with a self-textured oxide mask (STOM) structure was fabricated and demonstrated. The design of the STOM on the GaN/sapphire substrate could be used to reduce the threading dislocation density in the epitaxial template and enhance the light extraction efficiency via the light scattering or deflection from the corrugated STOM. Under an injection current of 20 mA, the forward voltage of the STOM-LED and conventional LED (C-LED) was nearly identical at 3.41 V. Moreover, the leakage current of the STOM-LED was lower than the C-LED. Furthermore, the light output power of the STOM-LED was approximately 43% higher (at 20 mA) than the C-LED. This significant improvement was attributed to the enhanced light extraction via the STOM mask.

In this paper we propose that under Ga-rich conditions the growth of III-Nitride semiconductors by plasma-assisted MBE takes place though saturation of the metallic Ga covering the surface of the growing film with nitrogen, alloy constituents and impurities. According to this model the growth process is a liquid phase epitaxy (LPE) rather than vapor phase epitaxy. Experimental evidence is presented in support of this model. Such include, for example, that the growth rate of GaN does not decrease at temperatures in excess to 800 ºC, a result attributed to higher solubility of active nitrogen in Ga at higher temperatures. Also the metallic Ga in the surface of the film controls the incorporation of impurities and is responsible for the introduction of compositional inhomogeneities and thus band structure potential fluctuations in AlGaN due to lateral statistical fluctuations of the thickness of Ga in the surface.

The effect of growth parameters on stress in thick GaN films grown on sapphire by HVPE method was investigated. We have found two modes of growth with different growth stress. Films grown in one mode have rough surfaces and low stress. The second mode leads to smooth surfaces but the films contain many cracks due to high growth stress. A combination of these modes allows growth of films without cracks and with smooth surfaces.

Cu-alloyed GaN epilayers were prepared by plasma assisted molecular beam epitaxy with Cu-to-Ga beam equivalent pressure ratio of 1.2 to 4.8 %. Islands enriched with Cu were found on the GaN epitaxial layers grown in a Ga-rich environment. The islands are composed of a Cu9Ga4 intermetallic phase and GaN between which an orientation relationship was identified as [111]Cu9Ga4//[1 10]GaN and (10)Cu9Ga4//(0001)GaN. X-ray spectroscopy analyses indicated that the 1.2 % and 4.8 % samples contains 0.10 0.02 wt.%Cu and 0.04 0.03 wt.%Cu, respectively.

GaN thick films, grown on specially patterned 2" sapphire substrates by HVPE methods have lower bowing and are less susceptible to fracture then ones, grown on unpatterned substrates under the same growth conditions. Numerical calculation shows good agreement with experiments. Such substrates could be an alternative to expensive GaN wafers sliced from GaN boules.

The aim of this work is to give an overview on 3C-SiC growth on Si substrates. Starting from the reasons why SiC is considered such an interesting innovative material, with a survey of application already demonstrated, we will present data explaining the most important issues in this hetero-epitaxy system and how the chemical vapor deposition process influences the resulting 3C-SiC film properties. 3C-SiC crystal structure is strongly dependent on the process parameters within the reaction chamber during growth as well as the substrate surface properties. Part of this work is then focused on the main crystallographic defects characterizing the 3C-SiC/Si system and on the resulting wafer bow due to the large misfit between the materials. Defects and wafer bow, are a direct consequence of the large stress generated at the interface. The work closes discussing the encouraging improvements in 3C-SiC crystal quality obtained by the introduction of compliant Si substrates.

An application of an optical characterization technique able to evaluate the quality of 4H-SiC epitaxial layers is here proposed. By using high power density UV optical pumping it was possible to stress 4H-SiC epitaxial layers after the CVD growth process and verify the generation and evolution of Single Shockley faults across the interface through the epitaxial layer without the fabrication of bipolar junctions. Thanks to this characterization method, it is possible to choose a good CVD process for the growth of epitaxial layers.

In this article, helped by finite element simulations, we show that, properly designed, planar-rotator microstructures can be used to simultaneously determine the uniform and gradient residual stresses in thin films in the limit of linear residual stress form. TEM characterization studies on the defect formation and propagation as a function of 3C-SiC/Si thickness revealed that the linear stress approximation in such a hetero-epitaxial thin film is wrong. With finite element modeling four different stress relationships were studied and compared. This study shows that the new approximation forms of the total residual stress function result in a better fit of the experimental data and reduces the disagreement between theory and experiments.

The growth behavior of ZnO epilayer on (100) LiGaO2 (LGO) substrates by chemical vapor deposition was studied. At 650oC or above, (10 0)-oriented ZnO epilayers were obtained. The epitaxial deposition of ZnO is, however, in competition with the volatilization of Li from the substrate surface. A discrete layer composed of ZnGa2O4 spinel nanocrystals is formed on the substrate surface. The (10 0)-oriented ZnO grains, which nucleate epitaxially on the LGO substrate, have thus to grow laterally over the spinel nanocrystals to form a continuous epilayer.

A co-dopand approach was used to investigate influence of gallium-nitrogen co-doping on the microstructure and electrical parameters of ZnO. ZnO:Ga:N thin films were deposited by rf diode sputtering at varying nitrogen content (0÷100%) in Ar/N2 gas. ZnO:Ga films (0% N2) showed minimum resistivity of 0.12 Ωcm, electron concentration of 2.5x1019 cm-3 and mobility of 2 cm2/Vs. A hole concentration of 2.6x1018 cm-3, a mobility of 2 cm2/Vs and a resistivity of 1.5 Ωcm in ZnO:Ga:N resulted from the deposition with 100% N2 in Ar/N2 gas. XRD patterns revealed a profound impact of Ga-N co-doping on film orientation. The estimated crystallite size varied from 234 to 41 nm, depending on the N2 content. TEM images of the co-doped films along with the corresponding selected area diffraction pattern indicated a polycrystalline, columnar layer with a c-axis preferred orientation. AFM images demonstrated the different crystalline structure and grain formation depending on nitrogen content.

ZnO layers having variable percentage of Zn-contents were prepared to ascertain the role of Zn-interstitial defect in the ultraviolet (UV) emission from ZnO with the help of photoluminescence (PL) measurements. The typical PL spectrum performed at room temperature displayed a dominant UV line at 3.28 eV and a visible line centered at 2.25 eV. A detailed investigation reveals that the UV emission is due to bound exciton transition-Zn-interstitial transition. The UV intensity due to Zn-rich sample was almost double as compared to that of the sample with lower Zn-contents. The results obtained from the energy dispersive x-ray spectrum (EDAX) and Raman spectroscopy strengthened the PL results.

Rapid Thermal Processing has been evaluated as an alternative to the conventional furnace process for gate oxide formation of SiC MOSFETs. We show that the growth of the SiO2 films in a RTP chamber is orders of magnitude faster than in a conventional furnace. As well as being fast, this innovative oxidation method produces a significant improvement of MOSFET performances. Indeed, we demonstrate that combining the beneficial effect of in-situ surface preparation by H2 anneal with the one of N2O oxidation, the channel mobility increases and the electrical stability with respect to constant bias stress at low-field is improved.

In the high-k/SiO2 stacked dielectric MOS capacitor, the electrons captured by the defects associated with the oxygen vacancies in the dielectric may affect the trap assist tunneling current of the device. In this work, high bandgap material was utilized as substrate for its considerable interface states which are important to enhance the effect of trapped charges on the tunneling current. It was found that the electrons captured in high-k/SiO2 interface layer were crucial to block the current conduction path. Two-state current behavior was clearly observed by this mechanism.

This study has provided considerable insight into the impact of device down scaling on the characteristics of RF devices. This type of RF devices, featuring a thin p-layer based on a semi-insulating substrate, is free from the unwanted parasitic effects resulting from traditional conducting substrates. We fabricated 4H-SiC RF MOSFETs with fT/fMAX of 0.7/1.5 GHz and, in so doing, identified the key issues associated with short channel effects, influencing device mobility and the RF characteristics of RF MOSFETs on a semi-insulating substrate.

We have investigated the effects of SiO2 passivation on the oxygen annealed AlGaN/GaN HEMTs. DC and pulsed I-V characteristics are analyzed to investigate the variation of trap level caused by SiO2 passivation and oxygen annealing. Both of oxygen annealing and SiO2 passivation are found to be effective methods to suppress the surface leakage current and increase the breakdown voltage of the AlGaN/GaN HEMTs. After an oxygen annealing, the leakage current is decreased from 621 μA/mm to 1.7 nA/mm when -5 V of VGS and -50 V of VDS are applied. However, the leakage current is increased to 5.7 μA/mm after SiO2 passivation on the oxygen annealed AlGaN/GaN HEMTs. The electron trapping through the deep trap is found to be dominant mechanism of the suppressed leakage current of the AlGaN/GaN HEMTs due to the low probability of the de-trapping from the deep trap to conduction band. SiO2 passivation suppresses the electron trapping through deep trap, which is produced by oxygen annealing. The breakdown voltage of conventional device without any treatment is 180 V. After oxygen annealing, the breakdown voltage is increased to 830 V while the AlGaN/GaN HEMT employing both of SiO2 passivation and oxygen annealing is 650 V.

The design, fabrication and device characteristics of a vertically configured nanodiamond vacuum field emission transistor array are reported. The device is fabricated by a simple dual-mask process, involving mold transfer technique, growth of chemical vapor deposited nanodiamond and formation of self-aligning gate structure from a silicon-on-insulator (SOI) substrate. The gate controlled emission current modulation was achieved at a relatively low gate turn-on voltage of 25 V. The device demonstrates transistor characteristics with high emission anode current of 160 uA and negligible gate intercepted current of less than 3 uA at gate voltage of 34 V. This nanodiamond VFET promises potential applications in vacuum microelectronics, including vacuum integrated circuits.

Thin film transistors (TFTs) with a ZnO channel layer deposited by atmospheric pressure plasma jet (APPJ) were demonstrated. ZnO channel layers were fabricated with a non-vacuum and low-temperature process (100°). The effects of different carrier gases (nitrogen gas and compressed dry air) and channel thickness on the characteristics of ZnO TFTs were investigated. Reactive oxygen species can effectively repair the oxygen vacancies result in a low leakage current. By reducing the channel thickness, the undesired source to drain current flow can be eliminated. By using CDA as a carrier gas and reducing the channel thickness, a subthreshold swing of 3.75 V/decade, a field-effect mobility of 3.49 cm2/Vs and a Ion/Ioff current ratio of 4.08×107 were obtained