MOCVD growth of cubic GaN on 3C-SiC deposited on Si (100) substrates

The growth of cubic GaN on 3C-SiC/Si(100) by metal-organic chemical vapor deposition (MOCVD) under various growth temperatures, thicknesses of 3C-SiC, and V/III ratios was studied. The fractions of cubic and hexagonal phases in the films were estimated from the integrated x-ray diffraction intensities of the cubic (002) and hexagonal (1011) planes. A smooth SiC layer, a high growth temperature, and a moderate V/III ratio are three key factors for the nucleation of the cubic phase and its subsequent growth. Hexagonal GaN with its c-axis perpendicular to the substrate preferentially grows at the low temperature of 750°C. The inclusion of the cubic phase increases with increasing growth temperature. The optimum growth conditions for dominant cubic GaN formation were a growth temperature of 950°C, a 1.5 μm thick SiC layer, and a V/III ratio of 1500. With these growth conditions, a cubic GaN layer with the cubic component of 91% was obtained.     

MOVPE growth of GaAsN: surface study by AFM and optical properties

We report on the growth of GaAs_1−xN_x thin films on GaAs substrates (2° off) by metalorganic vapor-phase epitaxy, in the temperature range 500–600°C. A mixture of N₂ and H₂ was used as the carrier gas. Using dimethylhydrazine as nitrogen source, we incorporated up to 3.5% of nitrogen, at 530°C. The growth condition dependence of nitrogen content was studied, and it reveals a distribution coefficient 350 times lower for nitrogen than for arsine at 530°C. Nitrogen incorporation is controlled by surface kinetics. The evolution of surface morphology has been investigated by atomic force microscopy as a function of the nitrogen composition and of growth temperature. For nitrogen content up to 2%, the GaAsN vicinal surface is characterized by a step–terrace structure with bunched steps, and the step edges straighten when increasing the growth temperature. For higher nitrogen content terraces are no longer observed and, above 3%, widely-spaced cross-hatch lines, characteristic of a partial relaxation of strain in the epilayers, appear. Optical properties were studied by low (7 K) and room-temperature photoluminescence and photoreflectance. As usual for this material, a degradation of optical characteristics is observed with increasing N content along with the evolution of surface morphology.     

Initial stages of GaN/GaAs (100) growth by metalorganic chemical vapor deposition

We have investigated the growth of GaN buffers by metalorganic chemical vapor deposition (MOCVD) on GaAs (100) substrates. Atomic force microscope (AFM) and reflection high-energy electron diffraction (RHEED) were employed to study the dependence of the nucleation on the growth temperature, growth rate, annealing effect, and growth time. A two-step growth sequence must be used to optimize and control the nucleation and the subsequent growth independently. The size and distribution of islands and the thickness of buffer layers have a crucial role on the quality of GaN layers. Based on the experimental results, a model was given to interpret the formation of hexagonal-phase GaN in the cubic-phase GaN layers. Using an optimum buffer layer, the strong near-band emission of cubic GaN with full-width at half maximum (FWHM) value as small as 5.6 nm was observed at room temperature. The background carrier concentration was estimated to be in the range of 10¹³ ∼ 10¹⁴ cm⁻³.     

The incorporation of hydrogen into III-V nitrides during processing

Hydrogen is readily incorporated into GaN and related alloys during wet and dry etching, chemical vapor deposition of dielectric overlayers, boiling in water, and other process steps, in addition to its effects during metalorganic chemical vapor deposition or metalorganic molecular beam epitaxial growth. The hydrogen is bound at defects or impurities and passivates their electrical activity. Reactivation of passivated shallow donors in the nitrides occurs at 450-550°C, but evolution from the crystal requires much higher temperatures (≥800°C for GaN).     

The use of computer controlled group V valved sources for interface type control in InGaSb/InAs strained layer superlattices

The purpose of this research is to demonstrate the necessity of computer controlled valved group V effusion cell sources in the growth of indium gallium antimonide/indium arsenide (InGaSb/InAs). These sources allow enhanced control of the group V flux. This flux control allows the reduction of unwanted cross contamination and complete control of the interface type. For simple structures, this control can be done manually, however, for complicated structures the control must be automated to allow for reproducibility and uniformity. The InGaSb/InAs strained layer superlattice (SLS) is an example of a complicated structure with hundreds of layers that requires interface type control. Arsenic incorporation with typical flux shuttering was found to be a problem in the growth of antimonide layers and limit interface type control. The antimony incorporation was not found to occur for the growth of arsenide layers. In addition, antimony exposure to critical interfaces did not appear to reduce the interface quality. This research demonstrates that the use of computer controlled valve sources is only required for the arsenic source when attempting to create InGaSb/InAs SLS structures.     

基于车辆成组的V2V协同计算卸载策略研究

在车联网中引入V2V计算卸载技术可以缓解当前车载计算卸载热点地区路边单元(RSU)计算资源不足的问题.然而,在计算卸载过程中,服务车辆可能因故障离组或自主选择离开车组.如何返回任务结果并高效地分配计算任务是需要进一步研究的关键问题.提出了一个车组内计算任务分配算法,考虑了可能导致车辆离开车组的因素影响,以及组中每辆车能...     

Ion implantation and rapid thermal processing of Ill-V nitrides

Ion implantation doping and isolation coupled with rapid thermal annealing has played a critical role in the realization of high performance photonic and electronic devices in all mature semiconductor material systems. This is also expected to be the case for the binary III-V nitrides (InN, GaN, and A1N) and their alloys as the epitaxial material quality improves and more advanced device structures are fabricated. In this article, we review the recent developments in implant doping and isolation along with rapid thermal annealing of GaN and the In-containing ternary alloys InGaN and InAlN. In particular, the successful n- and p-type doping of GaN by ion implantation of Si and Mg+P, respectively, and subsequent high temperature rapid thermal anneals in excess of 1000°C is reviewed. In the area of implant isolation, N-implantation has been shown to compensate both n- and p-type GaN, N-, and O-implantation effectively compensates InAlN, and InGaN shows limited compensation with either N- or F-implantation. The effects of rapid thermal annealing on unimplanted material are also presented.     

Structural and optical properties of AlN sputtering deposited on sapphire substrates with various orientations

AlN thin films were deposited on c-,a-and r-plane sapphire substrates by the magnetron sputtering technique.The in-fluence of high-temperature thermal annealing(HTTA)on the structural,optical properties as well as surface stoichiometry were comprehensively investigated.The significant narrowing of the(0002)diffraction peak to as low as 68 arcsec of AlN after HTTA implies a reduction of tilt component inside the AlN thin films,and consequently much-reduced dislocation densities.This is also supported by the appearance of E2(high)Raman peak and better Al-N stoichiometry after HTTA.Furthermore,the in-creased absorption edge after HTTA suggests a reduction of point defects acting as the absorption centers.It is concluded that HTTA is a universal post-treatment technique in improving the crystalline quality of sputtered AlN regardless of sapphire orienta-tion.     

Ammonothermal synthesis of aluminum nitride crystals on group III-nitride templates

Polycrystalline aluminum nitride (AlN) crystals were synthesized using the ammonothermal technique at temperatures between 525°C and 550°C. The growth of AlN was conducted in alkaline conditions with potassium azide (KN₃) as the mineralizer. The growth mechanism was found to be reversegradient soluble, necessitating the placement of the GaN and AlN seeds at a higher temperature than the aluminum metal source. Growth on the GaN seeds varied from 100 to 1500 μm in thickness at a gestation period of 21 days. Additionally, scanning electron micrographs revealed varying microstructure ranging from pointed hexagonal rods, which are approximately 5 μm wide and 20 μm long, to highly densified and contiguous films. Formation of hexagonal AlN was verified using x-ray powder diffraction measurements. Oxygen was detected at 3.7 at.% by inert gas fusion analysis on AlN nucleated on the walls of the autoclave and a qualitative indication of unintentionally incorporated impurities in the AlN grown on the GaN seeds was obtained using energy-dispersive x-ray analysis. Photoluminescence spectroscopy conducted at 20 K revealed a deep-level emission at 3.755 eV due to unintentionally incorporated impurities.     

The effect of organometallic vapor phase epitaxial growth conditions on wurtzite GaN electron transport properties

The growth issues known to effect the quality of GaN organometallic vapor phase epitaxial films are reviewed and the best 300Kmobility vs electron concentration data are discussed. The data probably represent transport properties intrinsic to films grown on sapphire. From the results of Hall measurements, the unintentional donor in high quality GaN films cannot be Si since the donor ionization energy is much larger than that of films intentionally doped with Si (36 vs 26 meV). Electrical properties of a doped channel layer are shown not to be significantly different from those of thick films which implies a viable technology for conducting channel devices. It is argued that 77K Hall measurements are a useful indicator of GaN film quality and a compilation of unintentionally and Si doped data is presented. The 77K data imply that, at least over a limited range, Si-doping does not appreciably change the compensation of the GaN. The 77K data indicate that the low mobilities of films grown at low temperatures are probably not related to dopant impurities.     

不同V/III族元素比对m面GaN薄膜性能的影响

摘要：采用分子束外延（MBE）方法在蓝宝石衬底上外延生长m面GaN薄膜。利用原子力显微镜(AFM)、扫描电子显微镜(SEM)分析了薄膜表面形貌,对比分析结果,发现V/III族元素比从1:80降低到1:90时,外延膜表面均方根粗糙度从13.08nm降低到9.07nm。利用光谱型椭偏仪研究m面GaN薄膜,通过物理模型建立和光谱拟合得到了m面GaN薄膜的厚度、折射率和消光系数。拟合结果显示,GaN样品厚度和理论值一致,且V/III族元素比为1:90时,所得外延膜折射率较低,透射率大。两种测试方法的结果表明,V/III族元素比较小的样品晶体质量高。     

嵌入式软件测试V模型的研究

软件测试是软件质量保证的一个重要环节,对于更为复杂的嵌入式软件测试而言,更需要一个阶段清晰控制好的开发测试模型。文章在分析讨论了已有的模型的基础上,进一步挖掘测试活动之间的并行性,尝试提出一种改进的V模型,该模型将软件测试贯穿软件开发周期的各个阶段,可以尽可能早的发现软件中的错误,同时结合实际的小型软件开发项目——GPRS扩展板通信实验,证明了该模型的正确性和有效性。     

Plasma characteristics and the growth of group III-nitrides by metalorganic molecular beam epitaxy

Improved quality and controllability of growth processes are key issues for the maturation of III-N technologies. One of the most important concerns for the growth of III-N materials in ultra high vacuum is the ability to provide an effective nitrogen flux to the growth surface. This work has sought to correlate radio frequency (rf) plasma parameters and their impact on the growth of GaN by plasma-assisted metalorganic molecular beam epitaxy. Utilizing optical emission spectrometry, the atomic nitrogen production has been optimized as a function of rf power and N₂ flow rate. Growth experiments indicate that the abundance of atomic nitrogen alone does not control growth. Increasing energy per molecule in the rf source, with a constant level of atomic nitrogen, dramatically decreases the GaN growth rate. High levels of atomic nitrogen with a low energy per molecule resulted in restoration of the growth rate to ∼0.5 μm/h.     

亚微米ASIC正向设计中的电源及地考虑

介绍了ＡＳＩＣ设计过程中电源，地压点个数的选择原则；列出了芯片内部电源、地布线宽度的计算方法；讨论了电源，地线在芯片上的走向；最后给出了设计实例。     

The growth and characterization of GaN on sapphire and silicon

MOCVD (metalorganic chemical vapor deposition) of GaN on both silicon and sapphire substrates was studied over the temperature range of 370 to 1050° C. The crystallinity and surface morphology of the films varied with the deposition temperatures. By first depositing an AlN buffer layer, the crystallinity of GaN was improved for low temperature depositions, but little improvement in the surface morphology was observed. On sapphire (0001) substrates, epitaxial layers were produced at a deposition temperature as low as 500° C. With silicon substrates, polycrystalline films were produced which were randomly oriented on the (111) plane and highly oriented on the (100) plane. The surfaces of the films were smooth and specular at low deposition temperatures, but degraded at higher temperatures. The energy band gaps of these films are in the vicinity of 3.4 eV, close to where they are expected. Elemental analysis by Auger electron spectroscopy (AES) showed the films to be stoichiometric with low residual impurity concentrations.     

MOVPE growth and optical properties of gan deposited on c-plane sapphire

We address combined utilization of temperature dependent reflectance, photo-luminescence, and Raman spectroscopy measurements to optimize the struc-tural and electronic properties of GaN epilayers deposited on sapphire. Last, we study residual strain fields in such epilayers.     

Large-signal characterizations of DDR IMPATT devices based on group III-V semiconductors at millimeter-wave and terahertz frequencies

Large-signal (L-S) characterizations of double-drift region (DDR) impact avalanche transit time (IMPATT) devices based on group III-V semiconductors such as wurtzite (Wz) GaN, GaAs and InP have been carried out at both millimeter-wave (mm-wave) and terahertz (THz) frequency bands. A L-S simulation technique based on a non-sinusoidal voltage excitation (NSVE) model developed by the authors has been used to obtain the high frequency properties of the above mentioned devices. The effect of band-to-band tunneling on the L-S properties of the device at different mm-wave and THz frequencies are also investigated. Similar studies are also carried out for DDR IMPATTs based on the most popular semiconductor material, i.e. Si, for the sake of comparison. A comparative study of the devices based on conventional semiconductor materials (i.e. GaAs, InP and Si) with those based on Wz-GaN shows significantly better performance capabilities of the latter at both mm-wave and THz frequencies.     

MBE growth and properties of ZnO on sapphire and SiC substrates

Molecular beam epitaxy (MBE) of ZnO on both sapphire and SiC substrates has been demonstrated. ZnO was used as a buffer layer for the epitaxial growth of GaN. ZnO is a würtzite crystal with a close lattice match (<2% mismatch) to GaN, an energy gap of 3.3 eV at room temperature, a low predicted conduction band offset to both GaN and SiC, and high electron conductivity. ZnO is relatively soft compared to the nitride semiconductors and is expected to act as a compliant buffer layer. Inductively coupled radio frequency plasma sources were used to generate active beams of nitrogen and oxygen for MBE growth. Characterization of the oxygen plasma by optical emission spectroscopy clearly indicated significant dissociation of O₂ into atomic oxygen. Reflected high energy electron diffraction (RHEED) of the ZnO growth surface showed a two-dimensional growth. ZnO layers had n-type carrier concentration of 9 × 10¹⁸ cm⁻³ with an electron mobility of 260 cm²/V-s. Initial I-V measurements displayed ohmic behavior across the SiC/ZnO and the ZnO/GaN heterointerfaces. RHEED of GaN growth by MBE on the ZnO buffer layers also exhibited a two-dimensional growth. We have demonstrated the viability of using ZnO as a buffer layer for the MBE growth of GaN.     

NGN网络测试仪中V5UA及V5.2协议解码的实现

简要介绍了V5UA和V5.2的基本概念,功能,以及它们在NGN中的组网应用方式,并在深入分析V5UA和V5.2消息结构的基础上,实现NGN网络测试仪的VSUA协议的解码、上层协议数据提取、以及V5.2的解码方法.     

Elemental Ferroelectricity and Antiferroelectricity in Group‐V Monolayer

Ferroelectricity is usually found in compound materials composed by different elements. Here, based on first‐principles calculations, spontaneous electric polarization and ferroelectricity in 2D elemental group‐V (As, Sb, and Bi) monolayer with the puckered lattice structure similar to phosphorene is revealed. These are the first example of elemental ferroelectric materials. The polarization is due to the spontaneous lattice distortion with atomic layer buckling and has quite sizable values, comparable or even larger than that recently found in 2D monolayer compound SnTe. Interestingly, for Bi monolayer, apart from the ferroelectric phase, it is found that it can also host an antiferroelectric phase. The Curie temperatures of these elemental materials can be higher than room temperature, making them promising for realizing ultrathin ferroelectric devices of broad interest. A general model is constructed to understand and search for 2D ferroelectric and antiferroelectric materials in future studies.