AlGaN/GaN High Electron Mobility Transistors on Sapphires with f_(max) of 100GHz

AIGaN/GaN high electron mobility transistors grown on sapphire substrates with a 0.3μm gate length and 100μm gate width are fabricated. The device reveals a drain current saturation density of 0.85A/mm at a gate voltage of 0V and a peak transconductance of 225mS/mm. The unity current gain cutoff frequency and maximum frequency of oscillation are obtained as 45 and 100GHz,respectively. The output power density and gain are 1.8W/mm and 9.5dB at 4GHz,and 1.12W/mm and 11.5dB at 8GHz.

High performance micromachining of sapphire by laser induced plasma assisted ablation(LIPAA)using GHz burst mode femtosecond pulses

GHz burst-mode femtosecond(fs)laser,which emits a series of pulse trains with extremely short intervals of several hundred picoseconds,provides distinct characteristics in materials processing as compared with the conventional irradiation scheme of fs laser(single-pulse mode).In this paper,we take advantage of the moderate pulse interval of 205 ps(4.88 GHz)in the burst pulse for high-quality and high-efficiency micromachining of single crystalline sapphire by laser induced plasma assisted ablation(LIPAA).Specifically,the preceding pulses in the burst generate plasma by ablation of copper placed behind the sapphire substrate,which interacts with the subsequent pulses to induce ablation at the rear surface of sapphire substrates.As a result,not only the ablation quality but also the ablation efficiency and the fabrication resolution are greatly improved compared to the other schemes including single-pulse mode fs laser direct ablation,single-pulse mode fs-LIPAA,and nanosecond-LIPAA.

The Mamfe Basin in the southern Benue trough extension between Cameroon and Nigeria:Gem origin and problem of lithostratigraphic nomenclature and marine transgression

The Mamfe Basin has been the subject of many studies,but some debates persist,especially concerning the stratigraphic nomenclature,corundum origin,and marine transgression.The aims of this work are(1)to propose a new lithostratigraphic nomenclature of the Mamfe Basin formation based on new field observations,(2)to determine the source rock distribution and the origin of gem deposits and(3)to correlate the Cameroon section of the Mamfe Basin with the Nigerian deposits.The main results show that the name Manyu River Group is more appropriate as the Manyu River crosses all the facies in the Mamfe sedimentary Basin belonging to the Manyu Division.According to the facies analysis,the age of deposition,the mineralogic and geochemical data such as the V vs.Al2O3andΣREE vs.Al_(2)O_(3),TiO_(2)MgO,Na_(2)O,P_(2)O_(5),and CaO diagrams,this Group is composed of at least five Formations,including four Cretaceous Formations,from bottom to top:the Etoko Formation(alluvial and fluvial channel to fluvio-lacustrine deposits),the Nfaitok Formation(lagoonal deposits),the Bachuo Ntai Formation(fluvial braided channels or fluvio-deltaic environment)and the Eyumojock/NsanaragatiFormation(fluvio-lacustrine deposits)and the new Cenozoic Formation named Bakebe Formation(fluvio-lacustrine deposits).The gem minerals such as corundum,rutile,or tourmaline in the Cretaceous deposits of the Mamfe Basin are mainly detrital as indicated by the presence of worn shapes and fragments of these minerals.The presence of sapphire in the AlboCenomanian deposits indicates a Precambrian age of the felsic source rock,likely the plutonic rocks such as granite or pegmatite as indicated by the abundance of tourmaline and high LREE/HREE ratios(14.81–34.29)and slightly negative and positive Eu anomalies(0.85–1.15).This marine incursion in the Mamfe Basin was probably from West Nigeria,according to the geographic location of the Mamfe Basin and the general palaeogeographic evolution of the Benue Trough.

Transporting Cold Atoms towards a GaN-on-Sapphire Chip via an Optical Conveyor Belt

Trapped atoms on photonic structures inspire many novel quantum devices for quantum information processing and quantum sensing.Here,we demonstrate a hybrid photonic-atom chip platform based on a Ga N-onsapphire chip and the transport of an ensemble of atoms from free space towards the chip with an optical conveyor belts.Due to our platform’s complete optical accessibility and careful control of atomic motion near the chip with a conveyor belt,successful atomic transport towards the chip is made possible.The maximum transport efficiency of atoms is about 50%with a transport distance of 500μm.Our results open up a new route toward the efficient loading of cold atoms into the evanescent-field trap formed by the photonic integrated circuits,which promises strong and controllable interactions between single atoms and single photons.

Evolution of microstructure, stress and dislocation of AlN thick film on nanopatterned sapphire substrates by hydride vapor phase epitaxy

A crack-free AlN film with 4.5 μm thickness was grown on a 2-inch hole-type nano-patterned sapphire substrates(NPSSs) by hydride vapor phase epitaxy(HVPE). The coalescence, stress evolution, and dislocation annihilation mechanisms in the AlN layer have been investigated. The large voids located on the pattern region were caused by the undesirable parasitic crystallites grown on the sidewalls of the nano-pattern in the early growth stage. The coalescence of the c-plane AlN was hindered by these three-fold crystallites and the special triangle void appeared. The cross-sectional Raman line scan was used to characterize the change of stress with film thickness, which corresponds to the characteristics of different growth stages of AlN. Threading dislocations(TDs) mainly originate from the boundary between misaligned crystallites and the c-plane AlN and the coalescence of two adjacent c-plane AlN crystals, rather than the interface between sapphire and AlN.

差分吸收激光雷达测量NO_2浓度的实验研究

介绍了差分吸收激光雷达测量大气中物质成份及浓度的测量原理,并利用差分吸收激光雷达测量了3.5 km内NO2的浓度分布,测量结果表明,使用双通道可调谐的Ti:Sapphire激光器的一组波长(448.2nm和446.8 nm)可以获得比较精确的NO2的浓度随距离的分布。

Thermal Stresses and Cracks During the Growth of Large-sized Sapphire with SAPMAC Method

The finite-element method has been used to study the thermal stress distribution in large-sized sapphire crystals grown with the sapphire growth technique with micro-pulling and shoulder-expanding at cooled center （SAPMAC） method. A critical defect model has been established to explain the growth and propagation of cracks during the sapphire growing process. It is demonstrated that the stress field depends on the growth rate, the ambient temperature and the crystallizing direction. High stresses always exist near the growth interfaces, at the shoulder-expanding locations, the tailing locations and the sites where the diameters undergo sharp changes. The maximum stresses always occur at the interface of seeds and crystals. Cracks often form in the critical defect region and spread in the m-planes and a-planes under applied tensile stresses during crystal growth. The experimental results have verified that with the improved system of crystal growth and well-controlled techniques, the large-sized sapphire crystals of high quality can be grown due to absence of cracks.

铁电晶体铌酸钾锂的倍频性能研究

用 cw- Ti:sapphire激光对不同组成的熔体中生长得到的铌酸钾锂晶体进行了倍频性能研究 .实验结果表明 ,只有当晶体中 L i离子含量达到一定值时 ,铌酸钾锂晶体才具有非线性光学效应 ;晶体中 L i离子含量越高 ,倍频性能越好 .L i2 O浓度为 2 6 mol%的熔体中生长得到的铌酸钾锂晶体的倍频实验的结果表明 ,该晶体能对 82 0～96 0 nm的近红外 cw- Ti:sapphire激光倍频实现蓝绿光输出 ,具有较好的倍频性能 .

利用氩获得795nm飞秒激光在静态气室中的高次谐波

报道了 12 0fs/ 795nm的Ti∶sapphire飞秒激光在静态气体靶室中利用稀有气体氩作为非线性介质的高次谐波辐射 ,在静态靶室中获得高次谐波辐射 ,较常用的气体喷嘴具有结构简单 ,实验容易操作等优点 ,实验中在 4 2mJ和 7.

0.25μm Gate-Length AlGaN/GaN Power HEMTs on Sapphire with f_T of 77GHz

MOCVD-grown 0.25μm gate-length AlGaN/GaN high electron mobility transistors （HEMTs） are fabricated on sapphire substrates. A peak extrinsic transconductance of 250mS/mm and a unity current gain cutoff frequency （fT） of 77GHz are obtained for a 0.25μm gate-length single finger device. These power devices exhibit a maximum drain current density as high as 1.07A/mm. On-chip testing yielded a continuous-wave output power of 27. 04dBm at 8GHz with an associated power-added efficiency of 26. 5% for an 80 × 10μm device.

Decarbonization and Decolorization of Large Sapphire Crystals Grown by the Temperature Gradient Technique

A crystalline sapphire （Al2O3） boule （Ф10 × 80mm^3） grown by the temperature gradient technique （TGT） is a bit colored due to carbon volatilization from the graphite heater at high temperatures and the absorption of transitional metal inclusions in the raw material. The sapphire becomes colorless and transparent after decolorization and decarbonization in successive annealings in air and hydrogen at high temperatures. The quality, optical transmissivity,and homogeneity of the sapphire are remarkably improved.

Cascode Connected AlGaN/GaN Microwave HEMTs on Sapphire Substrates

Fabrication and characteristics of cascade connected AlGaN/GaN HEMTs grown on sapphire substrates are reported.The circuit employs a common source device,which has a gate length of 0.8μm cascode connected to a 1μm common gate device.The second gate bias will not only remarkably affect saturated current and transconductance,but also realize power gain control.Cascode device exhibits a slight lower of f T,a less feedback,a largely greater of maximum available gain and a higher impedance compare to that of common source device.

静态气体靶室中氖的高次谐波辐射

利用0.5TW的商用飞秒激光器,研究了超短脉冲强激光场中高次谐波辐射,报道了120 fs/795 nm 的Ti:sapphire飞秒激光在静态气体靶室中利用稀有气体氖作为非线性介质的5-17次高次谐波辐射,实验中重点观察了单脉冲能量为38.5 mJ线偏振激光驱动下谐波辐射谱随气压变化情况.

Effect of thickness on the microstructure of GaN films on Al_2 O_3 (0001) by laser molecular beam epitaxy

Heteroepitaxial GaN films are grown on sapphire （0001） substrates using laser molecular beam epitaxy. The growth processes are in-situ monitored by reflection high energy electron diffraction. It is revealed that the growth mode of GaN transformed from three-dimensional （3D） island mode to two-dimensional （2D） layer-by-layer mode with the increase of thickness. This paper investigates the interfacial strain relaxation of GaN films by analysing their diffraction patterns. Calculation shows that the strain is completely relaxed when the thickness reaches 15 nm. The surface morphology evolution indicates that island merging and reduction of the island-edge barrier provide an effective way to make GaN films follow a 2D layer-by-layer growth mode. The ll0-nm GaN films with a 2D growth mode have smooth regular hexagonal shapes. The X-ray diffraction indicates that thickness has a significant effect on the crystallized quality of GaN thin films.

新一代哈龙替代灭火剂

介绍了新的 Novec1 2 30灭火剂以及 Sapphire灭火系统 ,分析了当前哈龙替代物的现状及其缺陷。提供了Novec 1 2 30在分子结构、物理化学性能、环境污染、毒性、灭火性能以及应用等方面的数据 ,并论证了这个新型灭火剂的无环境污染 ,对人体安全 ,以及优良的灭火性能等优点。由于克服了氟代烷类灭火剂所具有的温室效应和影响气候等问题 。

LASER LIFT-OFF OF GaN THIN FILMS FROM SAPPHIRE SUBSTRATES

Gallium Nitride film was successfully separated from sapphire substrate by laser radiation. The absorption of the 248 nm radiation by the GaN at the interface results in rapid thermal decomposition of interfacial layer, yielding metallic Ga and N2 gas. The substrate can be easily removed by heating above the Ga melting point (29°C). X-ray diffraction, atomic force microscopy and photoluminescence of GaN before and after lift-off process have been performed, which demonstrated that the separation and transfer process do not alter the structural quality of the GaN films. And further discussions on the threshold energy and crack-free strategies of laser lift-off process have also been presented.

Laser machining of transparent brittle materials: from machining strategies to applications

Transparent brittle materials such as glass and sapphire are widely concerned and applied in consumer electronics, optoelectronic devices, etc. due to their excellent physical and chemical stability and good transparency. Growing research attention has been paid to developing novel methods for high-precision and high-quality machining of transparent brittle materials in the past few decades. Among the various techniques, laser machining has been proved to be an effective and flexible way to process all kinds of transparent brittle materials. In this review, a series of laser machining methods, e.g. laser full cutting, laser scribing, laser stealth dicing, laser filament, laser induced backside dry etching (LIBDE), and laser induced backside wet etching (LIBWE) are summarized. Additionally, applications of these techniques in micromachining, drilling and cutting, and patterning are introduced in detail. Current challenges and future prospects in this field are also discussed.

High-Rate Growth and Nitrogen Distribution in Homoepitaxial Chemical Vapour Deposited Single-crystal Diamond

High rate （〉 50 μm/h） growth of homoepitaxial single-crystal diamond （SCD） is carried out by microwave plasma chemical vapour deposition （MPCVD） with added nitrogen in the reactant gases of methane and hydrogen, using a polyerystalline-CVD-diamond-film-made seed holder. Photolumineseenee results indicate that the nitrogen concentration is spatially inhomogeneous in a large scale, either on the top surface or in the bulk of those as-grown SCDs. The presence of N-distribution is attributed to the facts： （i） a difference in N-incorporation efficiency and （ii） N-diffusion, resulting from the local growth temperatures changed during the high-rate deposition process. In addition, the formed nitrogen-vacancy eentres play a crucial role in N-diffusion through the growing crystal. Based on the N-distribution observed in the as-grown crystals, we propose a simple method to distinguish natural diamonds and man-made CVD SCDs. Finally, the disappearance of void defect on the top surface of SCDs is discussed to be related to a filling-in mechanism.

β-Ga_2O_3 thin film grown on sapphire substrate by plasmaassisted molecular beam epitaxy

Monoclinic gallium oxide(Ga_2O_3) has been grown on(0001) sapphire(Al_2O_3) substrate by plasma-assisted molecular beam epitaxy(PA-MBE). The epitaxial relationship has been confirmed to be [010]( 2ˉ01) β-Ga_2O_3||[ 011ˉ0](0001)Al_2O_3 via in-situ reflection high energy electron diffraction(RHEED) monitoring and ex-situ X-ray diffraction(XRD) measurement. Crystalline quality is improved and surface becomes flatter with increasing growth temperature, with a best full width at half maximum(FWHM) of XRD ω-rocking curve of( 2ˉ01) plane and root mean square(RMS) roughness of 0.68° and 2.04 nm for the sample grown at 730 °C,respectively. Room temperature cathodoluminescence measurement shows an emission at ～417 nm, which is most likely originated from recombination of donor–acceptor pair(DAP).

Anisotropic brittle-ductile transition of monocrystalline sapphire during orthogonal cutting and nanoindentation experiments

Single-crystal sapphire is utilized as a high-performance engineering material,especially in extreme and harsh environments.However,due to its extreme hardness and brittleness,the machinability of sapphire is still a challenge.By means of nanoindentation and plunge-cut experiments,the anisotropic brittle-ductile transition of the prismatic M-plane and rhombohedral R-plane is examined by analyzing crack morphologies and the critical depth-of-cut(CDC).The experimental results of the nanoindentation tests are correlated to the plunge-cut experiment.Both the prism plane and the rhombohedral crystal plane exhibit a two-fold symmetry of ductility with various crack patterns along the machined grooves.The direction-dependent plasticity of the hexagonal sapphire crystal is mainly connected to a twinning process accompanied by slip dislocation.