Formation of uniform GaAs multi-atomic steps with 20–30 nm periodicity and related structures on vicinal (111)B planes by MBE

We studied morphology of GaAs surfaces and the transport properties of two-dimensional electron gas (2DEG) on vicinal (111)B planes. Multi-atomic steps (MASs) are found on the vicinal (111)B facet grown by molecular beam epitaxy, which will affect electron transport on the facet. We also studied how the morphology of GaAs epilayers on vicinal (111)B substrates depends on growth conditions, especially on the As₄ flux. The uniformity of MASs on the substrates have been improved and smooth surfaces were obtained when the GaAs was grown with high As₄ flux, providing step periodicity of 20 nm. The channel resistance of the 2DEG perpendicular to the MASs is reduced drastically with this smooth morphology. These findings are valuable not only for fabricating quantum devices on the (111)B facets but also those on the vicinal (111)B substrates.     

Energy relaxation in GaInAs/AlInAs heterojunctions and GaAs/AlGaAs multiple quantum wells

We have studied electron energy relaxation in GaInAs/AlInAs heterojunctions and GaAs/AlGaAs multiple quantum wells using mobility measurements as a function of electric field and temperature, in the range 3K to 300K. The results in the range 3 to 20K show a power loss rate which is dependent on (T_e − T_l), suggesting that the energy relaxation occurs through acoustic phonon scattering. At electron temperatures greater than 20K, the experimental results are modelled using a standard expression for polar optical phonons. This modelling yields 30meV and 31meV for the polar optical phonon energy in GaAs and InGaAs respectively.     

双轴应变下GaN有效质量的计算及其对迁移率的影响

通过计算双轴应变下氮化镓的电子能带结构,给出了GaN有效质量与应变的变化关系。在弛豫时间近似的条件下,这种关系决定了双轴应变AlGaN/GaN中二维电子气(2DEG)的迁移率的改变。在其他物理参量不变的情况下,这种二维电子气迁移率将随着张应变的增加而增加,并随着压应变的增加而减小。计算结果表明,张应变对2DEG迁移率的影响要比压应变大。此外,GaN有效质量的变化在低温时对迁移率的作用更明显。而在低温低浓度的条件下,迁移率却对有效质量的依赖很小。     

低分子量HTPB聚氨酯弹性体的制备及其性能

为改善高固含量高聚物黏结炸药(PBX)和丁羟推进剂的工艺性能,以低分子量的端羟基聚丁二烯(HTPB)、异佛尔酮二异氰酸酯(IPDI)为主要原料,选用一缩二乙二醇(DEG)为扩链剂,采用二步法制备了聚氨酯弹性体。研究了催化剂用量对浆料黏度的影响,固化参数R及扩链剂用量对HTPB聚氨酯弹性体力学性能的影响。试验结果表明,当催化剂质量分数为0.004%时,适用期可达5 h;R值为1.1,DEG羟基含量占反应总羟基量的60%时,聚氨酯弹性体力学性能较好,拉伸强度达7.60 MPa,断裂伸长率达540.21%。动态力学分析(DMA)测试结果显示,低分子量HTPB聚氨酯弹性体有两个明显的玻璃化转变温度,说明样品存在明显的微相分离结构。     

变温C-V和传输线模型测量研究AlGaN/GaN HEMT温度特性

通过对异质结材料上制作的肖特基结构变温C-V测量和传输线模型变温测量,研究了蓝宝石衬底AlGaN/GaN异质结高电子迁移率晶体管的直流特性在25～200℃之间的变化,分析了载流子浓度分布、沟道方块电阻、欧姆比接触电阻和缓冲层泄漏电流随温度的变化规律.得出了器件饱和电流随温度升高而下降主要由输运特性退化造成,沟道泄漏电流随温度的变化主要由栅泄漏电流引起的结论.同时,证明了GaN缓冲层漏电不是导致器件退化的主要原因.     

聚酯切片中DEG含量对其熔融结晶温度的影响

采用ＤＳＣ的方法观察聚酯切片的熔融结晶行为,发现它与聚酯切片的ＤＥＧ含量有密切的关系,并且具有ＤＥＧ含量越高,熔融结晶温度越低,熔域越宽的特点。     

二甘醇对染色性能的影响

聚酯生产中,生产原料、生产负荷、工艺参数等因素的变化,均有可能对成品中二甘醇(DEG)的含量造成影响,如控制不当,会严重影响纤维的染色均匀性。通过DEG对聚酯染色性能的影响机理及影响DEG含量的因素的分析,提高调控水平,确保DEG含量稳定,避免纺丝染色出现明显色差。     

Deep levels and nonlinear characterization of AlGaN/GaN HEMTs on silicon carbide substrate

Deep levels in AlGaN/GaN high electron mobility transistors (HEMTs) on SiC substrate are known to be responsible for trapping processes like: threshold voltage shift, leakage current, degradation current, kink effect and hysteresis effect. The related deep levels are directly characterized by conductance deep level transient spectroscopy (CDLTS) method. Hereby, we have detected five carrier traps with activation energy ranging from 0.84 to 0.07 eV. In this study, we have revealed the presence of two hole-like traps (HL1 and HL2) observed for the first time by CDLTS with activations energy of 0.40 and 0.84 eV. The localisation and the identification of these traps are presented. Finally, the correlation between the anomalies observed on output and defects is discussed.     

A physics‐based model of DC and microwave characteristics of GaN/AlGaN HEMTs

A physics‐based model of AlGaN/GaN High Electron Mobility Transistor (HEMT) is developed for the analysis of DC and microwave characteristics. Large‐ and small‐signal parameters are calculated for a given device dimensions and operating conditions. Spontaneous and piezoelectric polarizations at the heterointerface and finite effective width of the 2DEG gas have been incorporated in the analysis. The model predicts a maximum drain current of 523 mA/mm and transconductance of 138 mS/mm for a 1 μm × 75 μm device, which are in agreement with the experimental data. © 2007 Wiley Periodicals, Inc. Int J RF and Microwave CAE, 2007.     

GaN-based optoelectronic devices on sapphire and Si substrates

A recessed gate AlGaN/GaN high-electron mobility transistor (HEMT) on sapphire (0 0 0 1), a GaN metal-semiconductor field-effect transistor (MESFET) and an InGaN multiple-quantum well green light-emitting diode (LED) on Si (1 1 1) substrates have been grown by metalorganic chemical vapor deposition. The AlGaN/GaN intermediate layers have been used for the growth of GaN MESFET and LED on Si substrates. A two-dimensional electron gas mobility as high as 9260 cm²/V s with a sheet carrier density of 4.8×10¹² cm⁻² was measured at 4.6 K for the AlGaN/GaN heterostructure on the sapphire substrate. The recessed gate device on sapphire showed a maximum extrinsic transconductance of 146 mS/mm and a drain–source current of 900 mA/mm for the AlGaN/GaN HEMT with a gate length of 2.1 μm at 25°C. The GaN MESFET on Si showed a maximum extrinsic transconductance of 25 mS/mm and a drain–source current of 169 mA/mm with a complete pinch-off for the 2.5-μm-gate length. The LED on Si exhibited an operating voltage of 18 V, a series resistance of 300 Ω, an optical output power of 10 μW and a peak emission wavelength of 505 nm with a full-width at half-maximum of 33 nm at 20 mA drive current.