High temperature characteristics of AlGaN/GaN high electron mobility transistors

Direct current (DC) and pulsed measurements are performed to determine the degradation mechanisms of AlGaN/GaN high electron mobility transistors (HEMTs) under high temperature. The degradation of the DC characteristics is mainly attributed to the reduction in the density and the mobility of the two-dimensional electron gas (2DEG). The pulsed measurements indicate that the trap assisted tunneling is the dominant gate leakage mechanism in the temperature range of interest. The traps in the barrier layer become active as the temperature increases, which is conducive to the electron tunneling between the gate and the channel. The enhancement of the tunneling results in the weakening of the current collapse effects, as the electrons trapped by the barrier traps can escape more easily at the higher temperature.     

Magnetotransport properties of two-dimensional electron gas in AlGaN/AlN/GaN heterostructures

Magnetotransport measurements are carried out on the AlGaN/AlN/GaN in an SiC heterostructure, which demonstrates the existence of the high-quality two-dimensional electron gas (2DGE) at the AlN/GaN interface. While the carrier concentration reaches 1.32 × 10¹³ cm ^{- 2} and stays relatively unchanged with the decreasing temperature, the mobility of the 2DEG increases to 1.21 × 10⁴ cm²/(V·s) at 2 K. The Shubnikov—de Haas (SdH) oscillations are observed in a magnetic field as low as 2.5 T at 2 K. By the measurements and the analyses of the temperature-dependent SdH oscillations, the effective mass of the 2DEG is determined. The ratio of the transport lifetime to the quantum scattering time is 9 in our sample, indicating that small-angle scattering is predominant.     

Development of 1.3GHz high-T-c rf SQUID

A new high-T_c (HT_c) rf SQUID working at around 1.3GHz has been developed to avoid electromagnetic interference such as growing mobile communication jamming. This new system works in a frequency range from 1.23 to 1.42GHz (centred at 1.3GHz), which is not occupied by commercial communication. The sensor used in the 1.3GHz rf SQUID is made of a HT_c coplanar superconducting resonator and a large-area HT_c superconducting film concentrator. We have achieved in the 1.3GHz HT_c rf SQUID system a minimal flux noise of 2.5×10^{-5}Φ_0/\sqrt{Hz} and a magnetic field sensitivity of 38fT/\sqrt{Hz} in white noise range, respectively. The effective area of the concentrator fabricated on a 15×15mm^2 substrate is 1.35mm^2. It is shown that the 1.3GHz rf SQUID system has a high field sensitivity. Design and implementation of 1.3GHz HT_c rf SQUID offers a promising direction of rf SQUID development for higher working frequency ranges.     

退火对EBE,IBS和ALD沉积HfO2薄膜的抗激光损伤性能影响

结合自身实验条件采用电子束蒸发(EBE)、离子束溅射(IBS)和原子层沉积(ALD)三种工艺制备了HfO2薄膜,对其进行退火实验,采用1064 nm Nd:YAG激光测定了即时沉积和退火后各HfO2薄膜的抗激光损伤能力。研究发现,ALD HfO2薄膜的激光损伤阈值最高,EBE HfO2薄膜次之,IBS HfO2薄膜的损伤阈值最低;300℃退火对各工艺薄膜抗激光损伤能力的影响均为负面,500℃退火则会显著降低ALD HfO2薄膜的抗激光损伤能力。     

超导量子干涉仪计算机自动控制系统

提出了计算机控制的射频超导量子干涉器自动测量、控制系统的实现 ;并对系统工作原理、硬件电路和软件编制的设计思想做了简要介绍。     

重力坝坝基沿软弱结构面滑动失稳的非线性机制

对于存在单组软弱结构面的坝基深层抗滑问题,考虑了坝基软弱结构面介质在不同环境因素条件下的复杂弹脆性(应变硬化)和应变软化属性,用突变论建立了坝基沿单软弱结构面滑动的尖点突变模型。通过建立的模型分析,发现坝基滑动失稳主要取决于软弱面上复杂弹脆性介质和应变软化介质在应变软化拐点处的刚度比k、材料的均匀性指标m及几何—力学参数ξ;并给出了发生突变失稳的充分力学判据,指出传统的刚体极限平衡分析法存在一定的缺陷。指出库水水位上升还存在一种降低介质材料刚度比k的新机制,从理论上阐明,坝基滑动突变失稳的发生相应于水位峰值,在时间上可能具有滞后性;而工程处理措施的实施除提高坝基软弱结构面抗滑能力外,还具有一种增大材料刚度比k的新效应,使得滑动失稳更不易发生。     

High-Temperature Superconducting YBa2Cu3O7-δ Josephson Junction Fabricated with a Focused Helium Ion Beam

As a newly developed method for fabricating Josephson junctions, a focused helium ion beam has the advantage of producing reliable and reproducible junctions. We fabricated Josephson junctions with a focused helium ion beam on our 50 nm YBa₂Cu₃O_7-δ（YBCO） thin films. We focused on the junction with irradiation doses ranging from 100 to 300 ions/nm and demonstrated that the junction barrier can be modulated by the ion dose and that within this dose range, the junct...     

高超声速类HTV2模型全目标电磁散射特性实验研究

针对临近空间高超声速飞行器目标探测与识别研究的需求,开展了高超声速飞行器非均匀等离子体电磁散射特性模拟测量研究.利用弹道靶设备发射高超声速类HTV2模型形成模拟的超高速复杂外形目标,弹道靶高精度阴影成像系统和雷达测量系统分别测量高超声速类HTV2模型姿态、全目标C波段/X波段电磁散射特性,获得了不同实验条件下模型全目标雷达散射截面积(RCS)等实验数据.研究结果表明:在不同实验状态下,包覆等离子体鞘套的高超声速类HTV2模型同一测量波段的RCS差别超过1个数量级,模型姿态角对包覆等离子体鞘套的高超声速类HTV2模型RCS影响较大,最大相差1个多数量级;在给定的实验条件下,模型尾迹C波段RCS远小于包覆等离子体鞘套的模型RCS,模型尾迹X波段RCS显著增强;高超声速类HTV2模型全目标C波段电磁散射能量主要分布在模型及其绕流区域, X波段电磁散射能量主要分布在模型及其绕流区域和等离子体尾迹区域.根据弹道靶实验条件,开展了包覆等离子体鞘套的高超声速类HTV2模型电磁散射特性数值仿真,仿真结果与实验结果之间的最大误差小于4 d B,验证了本文提出的非均匀等离子体包覆目标电磁散射特性建模方法的...     

渐变折射率Ta_2O_5/SiO_2薄膜多脉冲激光损伤特性

采用离子束溅射(IBS)的方式,制备了1064nm高反射Ta2O5/SiO2渐变折射率光学薄膜。对其光学性能和在基频多脉冲下抗损伤性能进行了分析。通过渐变折射率的设计方式,很好地抑制了边带波纹,增加了1064nm反射率。通过对损伤阈值的分析发现,随着脉冲个数的增加,损伤阈值下降明显;但是在20个脉冲数后,损伤阈值(维持在22J/cm2左右)几乎保持不变直到100个脉冲数。通过Leica显微镜对损伤形貌的观察,发现损伤诱因是薄膜表面的节瘤缺陷。通过扫描电镜(SEM)以及聚集离子束(FIB)对薄膜表面以及断面的观察,证实了薄膜的损伤起源于薄膜表面的节瘤缺陷。进一步研究得出,渐变折射率薄膜在基频光单脉冲下损伤主要是由初始节瘤缺陷引起的,在后续多脉冲激光辐照下初始节瘤缺陷引起烧蚀坑的面积扩大扫过薄膜上的其他节瘤缺陷,引起了其他节瘤缺陷的喷射使损伤加剧,造成损伤的"累积效应"。     

混合物理化学气相沉积法制备MgB_2单晶纳米晶片的研究

利用混合物理化学气相沉积法在石墨衬底上制备出了晶形为六角结构、厚度不同、径向尺寸不一的Mg B2单晶纳米晶片.利用纳米定向转移技术将此晶片转移到了碳支持膜铜网上,以便对其精细结构等物性进行表征.电输运测量和磁性测量结果都表明晶片具有超导电性:T c onset=38K,T c(0)=33K.扫描电子显微镜图像表明,晶片表面平整、厚度分布在几个纳米到200 nm之间,宽度从几微米到上百微米;高分辨透射电镜图像显示出晶片具有周期性晶格条纹.选区电子衍射数据与Mg B2已有的单晶衍射数据相符.这些测量结果证实了其确为高质量单晶Mg B2超导纳米晶片.本文不仅提出了一种全新的制备单晶Mg B2的方法,也观察到了纳米尺度Mg B2单晶的零电阻现象,为后续的磁通钉扎、纳米力学性能等领域的深入研究提供了合适的素材.