Growth of GaN films on nitrided GaAs substrates using hot-wire CVD

Epitaxial growth of cubic-type gallium nitride (c-GaN) by hot-wire CVD on GaAs(100) substrates was investigated. Prior to the epitaxial growth, a nitridation layer was formed using ammonia plasma generated by electron cyclotron resonance (ECR). It was found that the crystal phase of the epitaxial layer was predominantly determined by that of the nitrided layer. The best nitridation condition using ECR plasma for the growth of the GaN films with preponderant cubic-type structure was obtained.     

GaN Electronics

An overview is presented of progress in GaN electronic devices for high‐power, high‐temperature applications. The wide bandgaps of the nitride materials, their excellent transport properties, and the availability of heterostructures (e.g., GaN/AlGaN) make them ideal candidates for these applications. In the past few years a wide range of devices have been reported, including heterostructure field effect transistors (HFETs), heterojunction bipolar transistors (HBTs), bipolar junction transistors (BJTs), Schottky and p–i–n rectifiers, and metal–oxide–semiconductor field effect transistors (MOSFETs). Some of the unexpected features of GaN‐based electronics include the ability to use piezoelectrically induced carriers for current transport in heterostructures and the sensitivity of the GaN surface to preferential loss of nitrogen during device processing.     

Changes in the microstructure of GaN layers on sapphire upon annealing at high pressure

The work shows structural changes of GaN layers on sapphire induced by annealing at temperatures up to 1550°C. Such high temperatures could be used due to the application of high nitrogen pressure of up to 16.5 kbar (at 1 bar, GaN decomposes at about 1000°C). After the annealing, the layers were examined using high-resolution X-ray diffraction. It was observed that the annealing at temperatures above 1250°C causes the following changes in the microstructure of the GaN layers on sapphire: (i) a decrease of the out-of-plane tilt mosaicity (X-ray rocking curves of symmetrical reflections become narrower), (ii) an increase of the in-plane twist mosaicity (rocking curves of asymmetrical reflections become broader), (iii) an increase of the thermal strain (the perpendicular lattice parameters increase, the in-plane lattice parameters decrease). The increase of the strain is accompanied by the blue shift of the photoluminescence excitonic peaks. The magnitudes of the changes observed were inversely proportional to the initial tilt mosaicity of the layers.     

化学气相沉积法制备GaN纳米线和纳米棒

采用浸渍法在未抛光的硅衬底上涂抹一层NiCl2薄膜,通过化学气相沉积法(CVD)制备出高质量的GaN纳米线和纳米棒.X射线衍射(XRD)、傅立叶红外吸收光谱(FTIR)、选区电子衍射(SAED)和高分辨透射电子显微镜(HRTEM)的分析结果表明,采用此方法得到了六方纤锌矿结构的GaN单晶纳米线.通过扫描电镜(SEM)观察发现纳米线的形貌,纳米线的直径在50～200nm之间,纳米棒的直径在200～800nm之间.     

AlInN/GaN高电子迁移率晶体管的研究与进展

由于GaN基材料的高温性能好,AlInN/GaN异质界面具有更高的二维电子气密度,因而AlInN/GaN高电子迁移率晶体管(HEMT)具有更高的工作频率和饱和漏电流,以及更强的抗辐射能力,近年来成为微波功率器件和放大电路的研究热点。首先总结了AlInN材料的基本性质,分析了AlInN/GaNHEMT的材料生长和器件结构设计,最后总结了其在高频、大功率方面的最新进展。     

GaN:Eu electroluminescent devices grown by interrupted growth epitaxy

In this paper we report on electroluminescent devices fabricated using Eu-doped GaN films grown by interrupted growth epitaxy (IGE). IGE is a combination of conventional molecular beam epitaxy and migration enhanced epitaxy. It consists of a sequence of ON/OFF cycles of the Ga and Eu beams, while the N₂ plasma is kept constant during the entire growth time. IGE growth of GaN:Eu resulted in significant enhancement in the Eu emission intensity at 620.5 nm. The nitridation of the surface that occurs during the OFF cycle appears to be the dominant process producing the enhancement. Thick dielectric devices fabricated on glass substrates using IGE-grown GaN:Eu have resulted in luminance of ∼1000 cd/m² and luminous efficiency of ∼0.15 lm/W.     

Microstructure and optical properties of GaN films on sapphire substrates

Transmission electron microscopy (TEM), double crystal X-ray diffraction (XRD), photoluminescence (PL) and Raman scattering measurement were applied to study the correlation between the microstructure and material properties of the GaN films grown by light radiation heating metalorganic chemical vapor deposition (LRH-MOCVD), using GaN buffer layer on sapphire substrates. Corresponding to the density of the threading dislocation (TD) increasing approximately one order, the yellow luminescence (YL) intensity was strengthened from negligible to two orders higher than the band-edge emission intensity. The full width of half maximum (FWHM) of GaN (0002) peak of the XRD rocking curve was widened from 11 to 15 min, and in Raman spectra, the width of E₂ mode is broadened from 5 cm⁻¹ to 7 cm⁻¹. A ‘zippers’ structure of GaN buffer layer was discovered by high-resolution electron microscope (HREM).     

The effect of growth temperature on the coaxial InxGa1 − xN/GaN nanowires grown by metalorganic chemical vapor deposition

We report on the growth of coaxial In_xGa_1 − xN/GaN nanowires (NWs) on Si(111) substrates by using pulsed flow metalorganic chemical vapor deposition. The coaxial In_xGa_1 − xN/GaN NWs were grown by a two step process in which the core (GaN) structure was grown at a higher temperature followed by the shell (In_xGa_1 − xN) structure at a lower temperature. Dense and well-oriented coaxial In_xGa_1 − xN/GaN NWs were grown with an average diameter and length of about 300 ± 50 nm and 1.5-2.0 μm, respectively. The coaxial In_xGa_1 − xN/GaN NW was confirmed by cathodoluminescence mapping and high-resolution transmission electron microscopy. It is proposed that the critical dissociation of precursors at an elevated growth temperature can lead to a clear formation of an outer-shell in coaxial In_xGa_1 − xN/GaN NWs.     

Nanosecond pump-and-probe study of wurtzite GaN

We present a nanosecond pump-and-probe optical study of wurtzite GaN under the excitation condition that the carrier-longitudinal optical phonon scattering does not contribute to the energy relaxation process. The delay time after which the isothermal condition is achieved is found to be 50 ns. This comes from both the phonon structure in GaN and the fact that the carrier cooling depends on excess kinetic energies of carriers.     

Structural properties of GaN and related alloys grown by radio-frequency magnetron sputter epitaxy

Single-crystalline layers of GaN and related alloys such as AlGaN and InGaN were grown on Al₂O₃ (0001) substrates by radio-frequency magnetron sputter epitaxy. The crystalline structures of these layers were studied as functions of substrate temperature, N₂ composition ratio in N₂/Ar mixture source gas and gas pressure during the growth. Surface structure of GaN layer depended on Ga/N ratio in flux density, and nitrogen-rich growth condition resulted in pyramid-type facet structure whereas Ga-rich growth produced flat surface. The crystalline quality of GaN layer improved at relatively low N₂ composition ratios, and the GaN layer grown at 30% N₂ condition was transparent and colorless. Al_xGa_1−xN layers with x = 0.06-0.08 and In_xGa_1−xN layers with x = 0.45-0.5, were obtained at 30-40% and 30-50% N₂ composition ratios, respectively. The AlN and InN molar fractions in these layers were considerably different from Al and In molar fractions in starting metal alloys (x = 0.15 in both Al_xGa_1−x and In_xGa_1−x alloys).     

Synthesis of GaN nanowires by CVD method: effect of reaction temperature

GaN nanowires are fabricated on Si substrates by ammoniating Ga₂O₃/NiCl₂ thin films using chemical vapour deposition method. The influence of reaction temperature on microstructure, morphology and optical properties of GaN nanowires is characterised by X-ray diffraction, X-ray photoelectron spectroscopy, Fourier transform infrared spectrophotometer, scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy and photoluminescence. The results demonstrate that the GaN nanowires are single crystalline and exhibit hexagonal wurtzite symmetry. The best crystalline quality was achieved for an reaction temperature of 1150°C for 15?min. The growth process follows vapour–liquid–solid mechanism and Ni plays an important role as the nucleation point and as a catalyst.     

Temperature-dependent electric fields in GaN Schottky diodes studied by electroreflectance

Electroreflectance studies of Pt/GaN Schottky diodes were performed in the temperature range between 5 and 300 K. The data were analysed using the electric field-dependent dielectric function of GaN and a multi-layer formalism. We observed a thermal activation of electric fields underneath the Schottky contact. The results are explained in terms of temperature-dependent ionised impurity concentration by a model with two donor levels.     

Structural and photoluminescence study of thin GaN film grown on silicon substrate by metalorganic chemical vapor deposition

GaN epitaxial layer was grown on Si(111) substrate by metalorganic chemical vapor deposition (MOCVD). The structure consists of 50 nm thick high-temperature grown AlN buffer layer, 150 nm thick AlGaN layer, 30 nm low-temperature grown AlN layer, 300 nm GaN layer, 50 nm AlGaN superlattice layer, followed by 100 nm GaN epitaxial layer. The low-temperature AlN interlayer and AlGaN superlattice layer were inserted as the defect-blocking layers in the MOCVD grown sample to eliminate the dislocations and improve the structural and optical properties of the GaN layer. The dislocation density at the top surface was decreased to ∼ 2.8 × 10⁹/cm². The optical quality was considerably improved. The photoluminescence emission at 3.42-3.45 eV is attributed to the recombination of free hole-to-donor electron. The observed 3.30 eV emission peak is assigned to be donor-acceptor transition with two longitudinal optical phonon side bands. The relationship of the peak energy and the temperature is discussed.     

配合物热分解制备氮化镓纳米线及其生长机理研究

报道了利用VLS生长机制,在NH3气氛中,由配合物GaCl3·NH3在高温下热分解反应成功制备了GaN纳米线,利用X射线衍射(XRD)、高分辨电镜(HRTEM)、X射线能谱(EDS)等分析手段对产物进行了表征,并对氮化镓纳米线的VLS生长机理进行了探讨。     

Two-photon spectroscopy in GaN

Two-photon spectroscopy allows to observe unambigously the free A, B, and C excitons with n=2 in bulk GaN. Their energies allow the precise determination of the three band gaps and of the exciton binding energies.