Gallium nitride formed by vapour deposition and by conversion from gallium arsenide

Attempts to prepare single crystal gallium nitride in thin films and bulk form are reported. The thin films were prepared by reacting GaCl₃ and NH₃ and depositing on to single crystal silicon carbide substrates. The bulk gallium nitride was prepared by the conversion of single crystals of gallium arsenide using an intermediate oxide phase. The structural perfection of the gallium nitride material thus formed has been assessed using X-ray diffraction and electron diffraction techniques. Both methods of preparation produced single phase gallium nitride exhibiting a high degree of structural disorder.     

Steady-state and transient electron transport within the wide energy gap compound semiconductors gallium nitride and zinc oxide: an updated and critical review

The wide energy gap compound semiconductors, gallium nitride and zinc oxide, are widely recognized as promising materials for novel electronic and optoelectronic device applications. As informed device design requires a firm grasp of the material properties of the underlying electronic materials, the electron transport that occurs within these wide energy gap compound semiconductors has been the focus of considerable study over the years. In an effort to provide some perspective on this rapidly evolving field, in this paper we review analyzes of the electron transport within the wide energy gap compound semiconductors, gallium nitride and zinc oxide. In particular, we discuss the evolution of the field, compare and contrast results determined by different researchers, and survey the current literature. In order to narrow the scope of this review, we will primarily focus on the electron transport within bulk wurtzite gallium nitride, zinc-blende gallium nitride, and wurtzite zinc oxide. The electron transport that occurs within bulk zinc-blende gallium arsenide will also be considered, albeit primarily for bench-marking purposes. Most of our discussion will focus on results obtained from our ensemble semi-classical three-valley Monte Carlo simulations of the electron transport within these materials, our results conforming with state-of-the-art wide energy gap compound semiconductor orthodoxy. A brief tutorial on the Monte Carlo electron transport simulation approach, this approach being used to generate the results presented herein, will also be featured. Steady-state and transient electron transport results are presented. We conclude our discussion by presenting some recent developments on the electron transport within these materials. The wurtzite gallium nitride and zinc-blende gallium arsenide results, being presented in a previous review article of ours (O’Leary et al. in J Mater Sci Mater Electron 17:87, 2006), are also presented herein for the sake of completeness.     

Electronic behavior of calcined material obtained from a gallium-N-phenylene-N hybrid copolymer

Calcination of a gallium-N-phenylene-N hybrid copolymer under an argon atmosphere gave nano-sized gallium nitride-carbon cluster composite material. ESR spectral examinations of the calcined materials reveals the possibility of an electron transfer in the process of gallium nitride → carbon clusters with an oxidation site at gallium nitride particles and a reduction site at carbon clusters. The calcined material was found to reduce methylene blue under visible light irradiation.     

Fabrication of epitaxial III-nitride cantilevers on silicon (1 1 1) substrates

The molecular beam epitaxy of AlGaN/GaN epilayers on silicon (1 1 1) using an aluminum nitride buffer layer, and subsequent fabrication of free standing III-nitride cantilevers on Si(1 1 1) has been investigated. Transmission electron microscopy (TEM) of cross-section samples reveals a columnar structure consisting of the hexagonal gallium nitride polytype. Selected area diffraction indicates an epitaxial relationship between the gallium nitride and silicon substrate which is described by GaN[0 0 0 1]//Si[1 1 1] and GaN(1 1 0 0)//Si(1 1 1). Imaging of the electronic structure of an AlGaN/GaN interface has been investigated by mapping the variation in the plasmon frequency using an electron energy loss spectrometer on a dedicated scanning transmission electron microscope. Cantilevers were fabricated using a combination of etching processes. Nitride etch rates during inductively coupled plasma dry etch processing using a Cl₂/Ar plasma etchant were obtained by monitoring the optical reflectivity of the nitride films in situ. A peak GaN etch rate of 250 nm/min was measured, the etch rate was found to be strongly dependent on the d.c. self-bias. Thin beams of GaN having a length of 7 μm and 0.7 μm thickness, were fabricated and mechanically released from Si(1 1 1) substrates using a combination of two dry ICP etch processes, using Cl₂/Ar and CF₄/Ar/O₂ chemistries, and a potassium hydroxide (KOH) aqueous wet etch.     

Steady-State and Transient Electron Transport Within the III–V Nitride Semiconductors, GaN, AlN, and InN: A Review

The III–V nitride semiconductors, gallium nitride, aluminum nitride, and indium nitride, have, for some time now, been recognized as promising materials for novel electronic and optoelectronic device applications. As informed device design requires a firm grasp of the material properties of the underlying electronic materials, the electron transport that occurs within these III–V nitride semiconductors has been the focus of considerable study over the years. In an effort to provide some perspective on this rapidly evolving field, in this paper we review analyses of the electron transport within the III–V nitride semiconductors, gallium nitride, aluminum nitride, and indium nitride. In particular, we discuss the evolution of the field, compare and contrast results determined by different researchers, and survey the current literature. In order to narrow the scope of this review, we will primarily focus on the electron transport within bulk wurtzite gallium nitride, aluminum nitride, and indium nitride, for this analysis. Most of our discussion will focus on results obtained from our ensemble semi-classical three-valley Monte Carlo simulations of the electron transport within these materials, our results conforming with state-of-the-art III–V nitride semiconductor orthodoxy. A brief tutorial on the Monte Carlo approach will also be featured. Steady-state and transient electron transport results are presented. We conclude our discussion by presenting some recent developments on the electron transport within these materials.     

Amorphous carbon buffer layers for separating free gallium nitride films

The possibility of using amorphous diamond-like carbon (DLC) films for self-separation of gallium nitride (GaN) layers grown by hydride vapor-phase epitaxy has been analyzed. DLC films have been synthesized by plasma-enhanced chemical vapor deposition under low pressure on sapphire (Al₂O₃) substrates with a (0001) crystallographic orientation. The samples have been studied by the methods of Raman scattering and X-ray diffraction analysis. It is shown that thin DLC films affect only slightly the processes of nucleation and growth of gallium nitride films. Notably, the strength of the “GaN film–Al₂O₃” substrate interface decreases, which facilitates separation of the GaN layers.     

Annealing temperature and oxygen-vacancy-dependent variation of lattice strain,band gap and luminescence properties of CeO2 nanoparticles

CeO₂ nanoparticles are annealed in vacuum at 200°C and in air at 200°C, 600°C and 1000°C, respectively. Vacuum-annealed CeO₂ contains high concentration of oxygen vacancies and exhibits very high lattice strain, whereas the corresponding values decrease on air annealing. Oxygen-deficient CeO₂ has redshift in band gap with high Urbach energy. The magnitude of this energy decreases as CeO₂ is annealed in air at 600°C. At 1000°C, thermal disorder increases the Urbach energy. Photoluminescence property of the samples depends on the presence of radiative and non-radiative oxygen vacancy centres. Vacuum-annealed ceria have large numbers of non-radiative oxygen vacancies that act as emission quencher. CeO₂ annealed at 600°C contains requisite amount of oxygen vacancies to show better luminescence property.     

Critical Assessment 23: Gallium nitride-based visible light-emitting diodes

Solid-state lighting based on light-emitting diodes (LEDs) is a technology with the potential to drastically reduce energy usage, made possible by the development of gallium nitride and its alloys. However, the nitride materials family exhibits high defect densities and, in the equilibrium wurtzite crystal phase, large piezo-electric and polarisation fields arising at polar interfaces. These unusual physical properties, coupled with a high degree of carrier localisation in devices emitting visible light, result in ongoing challenges in device development, such as efficiency ‘droop’ (the reduction in efficiency of nitride LEDs with increasing drive current density), the ‘green gap’ (the relatively low efficiency of green emitters in comparison to blue) and the challenge of driving down the cost of LED epitaxy.     

基于碳纳米管模板的氮化镓纳米线束合成

通过金属有机物化学气相沉积方法在碳纳米管模板上生长氮化镓纳米线束.对所生长的纳米结构进行了扫描电镜和X射线能谱分析,结果显示氮化镓纳米晶体可以与碳纳米管形成纳米线束状复合物.纳米线束状复合物直径为100～200 nm,长度为1.5～2.5μm,纳米线的两端呈现尖角状.由于氨气很容易吸附在碳纳米管表面,可知所获得的纳米结构的初始生长机制为碳纳米管的表面氮化.该研究也证明金属有机物化学气相沉积将是用于制造化合物纳米结构材料的一项有效的技术.     

Fabrication of epitaxial III-nitride cantilevers on silicon (1 1 1) substrates

The molecular beam epitaxy of AlGaN/GaN epilayers on silicon (1 1 1) using an aluminum nitride buffer layer, and subsequent fabrication of free standing III-nitride cantilevers on Si(1 1 1) has been investigated. Transmission electron microscopy (TEM) of cross-section samples reveals a columnar structure consisting of the hexagonal gallium nitride polytype. Selected area diffraction indicates an epitaxial relationship between the gallium nitride and silicon substrate which is described by GaN[0 0 0 1]//Si[1 1 1] and GaN(1 1 0 0)//Si(1 1 1). Imaging of the electronic structure of an AlGaN/GaN interface has been investigated by mapping the variation in the plasmon frequency using an electron energy loss spectrometer on a dedicated scanning transmission electron microscope. Cantilevers were fabricated using a combination of etching processes. Nitride etch rates during inductively coupled plasma dry etch processing using a Cl₂/Ar plasma etchant were obtained by monitoring the optical reflectivity of the nitride films in situ. A peak GaN etch rate of 250 nm/min was measured, the etch rate was found to be strongly dependent on the d.c. self-bias. Thin beams of GaN having a length of 7 m and 0.7 m thickness, were fabricated and mechanically released from Si(1 1 1) substrates using a combination of two dry ICP etch processes, using Cl₂/Ar and CF₄/Ar/O₂ chemistries, and a potassium hydroxide (KOH) aqueous wet etch.     

Chemical Vapor Deposition of GaN from Gallium and Ammonium Chloride

Thick polycrystalline gallium nitride films were grown by a two-step chemical vapor deposition process using gallium metal and ammonium chloride as starting reagents, at deposition rates of up to 50 m/h. The deposits were examined by scanning electron microscopy and photoluminescence (PL) spectroscopy. The results demonstrate that the proposed process can be used to grow high-quality, stoichiometric gallium nitride layers with a perfect crystal structure, as evidenced by the high intensity of the 380-nm exciton peak in the room-temperature PL spectra of the films and also by electron-microscopic examination.     

Optimized growth conditions and properties of N-type and insulating GaN

The nucleation and growth of gallium nitride on sapphire have been studied. By an optimization of the different growth parameters, smooth, reproducible, transparent and large area layers of gallium nitride were obtained. Multi-layer structures of zinc doped GaN on N-type GaN have been grown. Devices made on these structures exhibit electroluminescence from orange to dark blue. This luminescence is obtained for voltage as low as 2.4 volts for orange, 2.6 volts for yellow, 2.8 volts for green and 3.2 volts for blue. The external power efficiency is about 10^?3 for dark green and blue light emitting devices.     

Spectral estimation of the energy-band-structure parameters of nanoparticles of potassium polytitanate modified in transition metal salt solutions

An approach to spectral analysis of the energy-band-structure parameters (bandgap width and Urbach energy) of potassium polytitanate (PPT) nanoparticles modified by transition metals is proposed, which can provide a basis for the synthesis of new photocatalytic materials. It is established that the modified PPT samples are characterized by reduced values of the bandgap width and higher values of the Urbach energy as compared to the initial material. Possible mechanisms of this phenomenon are discussed.     

Effects of crystallinity and chemical variation on apparent band-gap shift in polycrystalline indium nitride

The nature of the apparent band-gap shift in polycrystalline indium nitride thin-films, grown by remote-plasma-enhanced chemical vapour deposition at 535 ± 10 °C, has been investigated separately in relation to growth temperature dependent crystallinity and chemical variation. Substrates of sapphire and gallium nitride on sapphire were used to study the effect of a stress-reduced template on indium nitride crystallite quality and apparent band-gap. To mimic surface growth temperature variations two glass substrates of differing thickness and thermal conductivity were intentionally used for the same growth conditions. The samples were characterised using optical transmission, scanning electron microscope, X-ray diffraction, and high-resolution X-ray photoelectron spectroscopy. The results indicate that the apparent band-gap shift in polycrystalline narrow band-gap indium nitride thin-films is not primarily determined by the quality of indium nitride crystallites but rather it is associated with growth temperature dependent chemical variations in the films.     

Electron transport within the wurtzite and zinc-blende phases of gallium nitride and indium nitride

Wide energy gap semiconductors are broadly recognized as promising materials for novel electronic and opto-electronic device applications. As informed device design requires a firm grasp on the material properties of the underlying electronic materials, the electron transport that occurs within the wide energy gap semiconductors has been the focus of considerable study over the years. In an effort to provide some perspective on this rapidly evolving and burgeoning field of research, we review analyzes of the electron transport within some wide energy gap semiconductors of current interest in this paper. In order to narrow the scope of this review, we will primarily focus on the electron transport that occurs within the wurtzite and zinc-blende phases of gallium nitride and indium nitride in this review, these materials being of great current interest to the wide energy gap semiconductor community; indium nitride, while not a wide energy gap semiconductor in of itself, is included as it is often alloyed with other wide energy gap semiconductors, the resultant alloy often being a wide energy gap semiconductor itself. The electron transport that occurs within zinc-blende gallium arsenide will also be considered, albeit primarily for bench-marking purposes. Most of our discussion will focus on results obtained from our ensemble semi-classical three-valley Monte Carlo simulations of the electron transport within these materials, our results conforming with state-of-the-art wide energy gap semiconductor orthodoxy. A brief tutorial on the Monte Carlo electron transport simulation approach, this approach being used to generate the results presented herein, will also be provided. Steady-state and transient electron transport results are presented. The evolution of the field, a survey of the current literature, and some applications for the results presented herein, will also be featured. We conclude our review by presenting some recent developments on the electron transport within these materials. This review is the latest in a series of reviews that have been published on the electron transport processes that occur within the class of wide energy semiconductor materials. The results and references have been updated to include the latest developments in this rapidly evolving field of study.     

Deposition and characteristics of GaN films on Ni metal substrate by ECR-PEMOCVD

Gallium nitride (GaN) films were deposited on Ni metal substrate using electron cyclotron resonance plasma-enhanced metal organic chemical vapor deposition system. With this approach, highly c-oriented GaN films with smooth surface were obtained at an extremely low temperature of ~480 °C. The trimethyl gallium (TMGa) flux dependent structural, morphological, and optical characteristics of GaN films were investigated by X-ray diffraction analysis, reflection high energy electron diffraction, atomic force microscopy and photoluminescence analysis. The results indicate that it is feasible to deposit GaN films on Ni metal substrate under the proper deposition procedures. The high quality GaN films with high c-axis orientation and strong ultraviolet emission peak are successfully achieved under the optimized TMGa flux of 1.2 sccm. The GaN/Ni structure has great potential for the development of high power devices with excellent heat dissipation.     

Nanoporous gallium nitride square microtubes

Porous gallium nitride microtubes were self-fabricated from gallium nitride submicron irregular structures. The microtubes were of square cross-section. Electron diffractions indicated that the microtubes were composed of zincblende gallium nitride. Electron energy-loss spectrum and photoluminescence spectrum of the microtubes were collected and compared with that of single crystals.     

Metalorganic molecular beam epitaxy of GaN and Al(Ga)N on GaAs(001) studied using laser reflectometry and reflectance anisotropy spectroscopy

We report the growth of GaN and AlGaN films on GaAs (0 0 1) substrates in the temperature range 400–800 °C by metalorganic molecular beam epitaxy. An r.f. plasma nitrogen source was used in conjunction with triethylgallium and ethyl-dimethylamine-alane group III sources. Growth was initiated using either a low temperature AlN buffer layer or a graded arsenide-nitride buffer layer. The growth was monitored in real time using in-situ laser reflectometry. The temperature dependence of growth rates for the nitride layers are compared with their arsenide analogs. The relative growth rate of gallium nitride/gallium arsenide from triethylgallium was found to be in the range 54–60%, the Ga incorporation rates are closely comparable when the higher density of GaN is taken into account. The range of growth temperatures for gallium nitride extends to higher temperatures compared with gallium arsenide probably due to lower evaporation rates of Ga bound to the nitride surface. Reflection anisotropy spectra indicate that atomic nitrogen readily reacts with the GaAs (0 0 1)-c (4 × 4) As-stabilized surface at temperatures as low as 400 °C but without the gross faceting that has been observed at higher temperatures.     

Substrates for wide bandgap nitrides

Different substrates for gallium nitride growth are discussed. The commercially relevant substrates, silicon carbide and sapphire, and the two most promising alternatives, silicon and gallium nitride, are compared in terms of suitability for epitaxial processes and in their effects on devices. An estimation on future market success is given.     

Studies of electron trapping in GaN doped with carbon

Irradiation by the beam of the scanning electron microscope is shown to induce a systematic decay of the cathodoluminescence intensity in gallium nitride semiconductor doped with carbon. This decay is accompanied by increased electronic carrier diffusion length, indicating that electron irradiation results in the increase of carrier lifetime. Temperature-dependent cathodoluminescence measurements yielded activation energy for irradiation-induced effect of 210 meV. This observation is consistent with trapping of non-equilibrium electrons on deep, non-ionized carbon levels.