Thermal stress reduction of GaAs epitaxial growth on V-groove patterned Si substrates

We investigate the thermal stresses for GaAs layers grown on V-groove patterned Si substrates by the finite-element method. The results show that the thermal stress distribution near the interface in a patterned substrate is nonuniform,which is far different from that in a planar substrate. Comparing with the planar substrate, the thermal stress is significantly reduced for the Ga As layer on the patterned substrate. The effects of the width of the V-groove, the thickness, and the width of the SiO₂ mask on the thermal stress are studied. It is found that the SiO₂ mask and V-groove play a crucial role in the stress of the Ga As layer on Si substrate. The results indicate that when the width of V-groove is 50 nm, the width and the thickness of the SiO₂ mask are both 100 nm, the Ga As layer is subjected to the minimum stress. Furthermore,Comparing with the planar substrate, the average stress of the Ga As epitaxial layer in the growth window region of the patterned substrate is reduced by 90%. These findings are useful in the optimal designing of growing high-quality Ga As films on patterned Si substrates.     

Experimental evaluation of interface states during time-dependent dielectric breakdown of GaN-based MIS-HEMTs with LPCVD-SiN_x gate dielectric

We experimentally evaluated the interface state density of Ga N MIS-HEMTs during time-dependent dielectric breakdown(TDDB). Under a high forward gate bias stress, newly increased traps generate both at the Si Nx/Al Ga N interface and the Si Nx bulk, resulting in the voltage shift and the increase of the voltage hysteresis. When prolonging the stress duration, the defects density generated in the Si Nx dielectric becomes dominating, which drastically increases the gate leakage current and causes the catastrophic failure. After recovery by UV light illumination, the negative shift in threshold voltage(compared with the fresh one) confirms the accumulation of positive charge at the Si Nx/Al Ga N interface and/or in Si Nx bulk, which is possibly ascribed to the broken bonds after long-term stress. These results experimentally confirm the role of defects in the TDDB of Ga N-based MIS-HEMTs.     

Temperature dependence of twinning stress – Analogy between Cu–Ni–Al and Ni–Mn–Ga shape memory single crystals

Abstract

To obtain a direct non-magnetic analogy to Ni–Mn–Ga 10M martensite with highly mobile twin boundaries, we present the recalculation of twinning systems in Cu–Ni–Al martensite. In this approach, the twinning planes denoted as Type I, Type II and compound have similar orientations for both alloys (Ni–Mn–Ga and Cu–Ni–Al). In Cu–Ni–Al, compound twinning exhibits the twinning stress of 1 to 2 MPa comparable to twining stress of Ni–Mn–Ga. In contrast Type II twinning stress of Cu–Ni–Al is approximately 20 MPa, i.e. much higher than twinning stress for Type II in Ni–Mn–Ga (0.1 to 0.3 MPa). Similarly to Ni–Mn–Ga, the twinning stress of Type II in Cu–Ni–Al is temperature independent. Moreover, no temperature dependence was found also for compound twinning in Cu–Ni–Al.     

Temperature-dependent photoluminescence spectra of GaN epitaxial layer grown on Si(111) substrate

In this paper, the temperature-dependent photoluminescence(PL) properties of Ga N grown on Si(111) substrate are studied. The main emission peaks of Ga N films grown on Si(111) are investigated and compared with those grown on sapphire substrates. The positions of free and bound exciton luminescence peaks, i.e., FX A and D0 X peaks, of Ga N films grown on Si(111) substrates undergo red shifts compared with those grown on sapphire. This is attributed to the fact that the Ga N films grown on sapphire are under the action of compressive stress, while those grown on Si(111) substrate are subjected to tensile stress. Furthermore, the positions of these peaks may be additionally shifted due to different stress conditions in the real sample growth. The emission peaks due to stacking faults are found in Ga N films grown on Si(111) and an S-shaped temperature dependence of PL spectra can be observed, owing to the influence of the quantum well(QW) emission by the localized states near the conduction band gap edge and the temperature-dependent distribution of the photo-generated carriers.     

Electronic and optical properties in ZnO:Ga thin films induced by substrate stress

The effects of biaxial stress in ZnO:Ga thin films on different substrates, e.g., sapphire(0001), quartz, Si(001), and glass have been investigated by X-ray diffraction, atomic force microscopy, and electrical transport and ellipsometric measurements. A strong dependence of orientation, crystallite size, transport, and electronic properties upon the substrate-induced stress has been found. The structural properties indicate that a tensile stress exists in epitaxial ZnO:Ga films grown on sapphire, Si, and quartz, while a compressive stress appears in films grown on glass. The resistivity of the films decreased with increasing biaxial stress, which is inversely proportional to the product of the carrier concentration and Hall mobility. The refractive index n was found to decrease with increasing biaxial stress, while the optical band gap E₀ increased with stress. These behaviors are attributed to lattice contraction and the increase in the carrier concentration that is induced by the stress. Our experimental data suggest that the mechanism of substrate-induced stress is important for understanding the properties of ZnO:Ga thin films and for the fabrication of devices which use these materials.     

Electrical properties of p-type GaP schottky barrier under stress

The effect of stress on a Schottky barrier height at a metal-semiconductor interface is investigated for metal-p-type GaP contacts. The diodes are fabricated by evaporating metals (Ag, Au) on polar (111)Ga and (1̄1̄1̄)P surfaces. Stress is applied to the diodes by bending the crystal wafers attached to the cantilever. The variation of the barrier height with stress is determined from the measurements of the current-voltage characteristics under stress. The barrier height decreases under compressive stress parallel to the interface and increases under tensile stress. The change in barrier height on the (111)Ga surface is greater than that on the (1̄1̄1̄)P surface. These experimental results are discussed from the point of both the piezoelectricity and the change in band gap caused by stress.     

Reaction of sputtered Pt films on GaAs

Pt/GaAs interface reaction has been investigated between 250–500°C. Schottky barrier Impatt diodes for microwave applications with this structure have an operating temperature of ～250°C and the solid state reaction at the interface can lead to degradation of the device. Using RF sputtered Pt films on (100) GaAs, the reaction has been investigated with the help of X-ray diffraction, Auger spectroscopy, Rutherford backscattering and electrical resistivity measurements.The reaction starts by a rapid diffusion and dissolution of Ga in Pt and is slowed down by the formation of PtAs₂ at the interface which acts as a barrier for further Ga diffusion and reaction. Five different Pt-Ga phases and PtAs₂ were identified at various stages of the reaction. The final reaction products are PtAs₂ and GaPt. The reaction becomes self limited (up to 500°C) for Pt films thicker than ～2000 Å, probably due to extremely slow diffusion of Ga through the interfacial PtAs₂. Electrical resistivity data seem to indicate that Pt₃Ga is more resistive than PtGa which is the final equilibrium Pt-Ga compound observed in this reaction.     

Effects of temperature-induced stress on the structural,electrical, and optical properties of ZnO:Ga thin films grown on Si substrates

Crystalline ZnO:Ga thin films with highly preferential c-axis oriented crystals were prepared on Si(001) substrates at different temperatures using the reactive magnetron sputtering technique. Effects of temperature-induced stress in ZnO:Ga films were investigated using X-ray diffraction (XRD), atomic force microscopy (AFM), electrical transport, and spectroscopic ellipsometry measurements. XRD results showed that the films were highly c-axis (out-of-plane) oriented and crystallinity improved with growth temperature. The residual compressive stress in films grown at low temperature relaxes with substrate temperature and becomes tensile stress with further increases in growth temperature. Resistivity of the films decreases with increasing stress, while the carrier concentration and mobility increase as the stress increases. The mechanism of the stress-dependent bandgap of ZnO:Ga films grown at different temperatures is suggested in the present work.     

AlGaN插入层对InAlN/AlGaN/GaN异质结散射机制的影响

InAlN/AlN/GaN异质结中,名义上的AlN插入层实为Ga含量很高的AlGaN层, Al, Ga摩尔百分比决定了电子波函数与隧穿几率,因此影响与InAlN/AlGaN势垒层有关的散射机制.本文通过求解薛定谔-泊松方程与输运方程,研究了AlGaN层Al摩尔百分含量对InAlN组分不均匀导致的子带能级波动散射、导带波动散射以及合金无序散射三种散射机制的影响.结果显示:当Al含量由0增大到1,子带能级波动散射强度与合金无序散射强度先增大后减小,导带波动散射强度单调减小;在Al含量为0.1附近的小组分范围内,合金无序散射是限制迁移率的主要散射机制,该组分范围之外,子带能级波动散射是限制迁移率的主要散射机制;当Al摩尔百分含量超过0.52,三种散射机制共同限制的迁移率超过无插入层结构的迁移率, AlGaN层显示出对迁移率的提升作用.     

Hot and solid gallium clusters: too small to melt

A novel multicollision induced dissociation scheme is employed to determine the energy content for mass-selected gallium cluster ions as a function of their temperature. Measurements were performed for Ga(+)(n) (n=17 39, and 40) over a 90-720 K temperature range. For Ga+39 and Ga+40 a broad maximum in the heat capacity-a signature of a melting transition for a small cluster-occurs at around 550 K. Thus small gallium clusters melt at substantially above the 302.9 K melting point of bulk gallium, in conflict with expectations that they will remain liquid to below 150 K. No melting transition is observed for Ga+17.     

Silicon doped with gallium photoconductors: Effect of uniaxial stress and detector development

Conclusion The evolution of the spectral response of Si:Ga photoconductors with uniaxial stress is studied. The cut-off wavenumber is lowered by 14% at a stress of 0.6 GPa, in a spectral range where strong absorption by the lattice occurs. Unstressed Si:Ga detectors have been developed for the AROME instrument focal plane. This experiment was devoted to measure the diffuse near infrared emission of our Galaxy in order to estimate the power radiated in particular spectral bands characteristic to Polycyclic Aromatic Hydrocarbons. The recent transmediterranean balloon flight gave extensive data which are now being processed.     

Stress corrosion cracking of alloy C-22 and Ti Gr-12 using double-cantilever-beam technique

Susceptibility to stress corrosion cracking (SCC) of two candidate alloys for the inner container of the multi-barrier nuclear waste package was evaluated by using wedge-loaded precracked double-cantilever-beam (DCB) specimens in deaerated acidic brine (pH≈2.70) at 90°C. Materials tested included Alloy C-22 and Ti Grade-12. Duplicate samples of each material were loaded at different initial stress intensity factor (K_I) values ranging between 22 and 43 MPa√m. Both metallography and the compliance methods were used to determine the final crack length. The final stress intensity for SCC (K_f) was computed from the measured final wedge load and the average crack length. The results indicate that, in general, the final crack lengths measured by metallography and compliance agreed well with one another, thus, providing very similar K_f values. The alloy C-22 showed higher susceptibility to SCC than Ti Grade-12 in terms of the average crack growth. Fractographic evaluation by scanning electron microscopy (SEM) of broken DCB specimens revealed three distinct regions showing the characteristics of fatigue precrack, SCC, and fast fracture.     

基于粘结界面模型的三维裂纹扩展研究

根据界面断裂的特点,将粘结界面模型应用于有限元,采用非线性显式动力学算法,编写粘结-体积单元计算程序.其中体积和粘结单元分别采用四面体和三棱柱单元;粘结单元本构关系选择双线性内聚力-位移曲线,避免了界面完全断裂后发生穿透;以界面相对位移的形式定义线性损伤准则;以Benzeggagh-Kenane模式的扩展准则确定界面失效位移;以Turon模式的初始损伤准则确定界面启裂位移.该程序能够实现混合加载模式下材料界面断裂问题的三维数值模拟.用所编写的程序(CVFEM)分别对Ⅰ,Ⅱ,Ⅲ型裂纹扩展问题进行数值模拟,并且与Abaqus6.7计算结果进行对比.     

Giant magnetostriction in rapidly quenched Fe–Ga?

Substituting Fe by nonmagnetic Ga causes a dramatic increase of the magnetostriction. The reason for this effect is related to structure and also due to softening of the elastic properties. Of special interest is that in literature “giant” magnetostriction values (up to 2100 ppm) for rapidly quenched Fe–Ga (15–20% Ga) ribbons were reported. In this work, careful investigations using a strain gauge method as well as a capacitance cell were performed. Especially for the case applying an external field perpendicular to the ribbon plane, it is demonstrated that bending effects can occur and they are difficult to avoid without introducing any stress into the sample. This effect leads to large signals in the strain gauge of more than ±3000 ppm, which sign depends on the occurrence of strain or stress. Experiments on a 25-μm-thin Fe foil leads to similar results. Avoiding bending by gluing ribbons or thin foils or splat-cooled thin pure Ni on a thin plastic plate, gave magnetostriction values close to those of polycrystalline bulk materials.     

Ga0.51In0.49P的MOCVD生长特性研究

本文报导了用国产常压MOCVD系统生长GaInP材料.研究GaInP的生长特性,用光致发光、扫描电镜、X光双晶衍射对材料进行表征.发现Ⅴ/Ⅲ比对GaInP的光学特性有很大影响.在温度680℃,Ⅴ/Ⅲ比为90的条件下得到Ga0.51In0.49P外延层的光致发光谱的半高宽为26meV,这是目前报导的最好结果.Ga0.51In0.49P的本征载流子浓度为1.1×1015cm-3,晶格失配为4×10-4.     

Uniaxial strain-dependent magnetic and electronic properties of (Ga,Mn)As nanowires

Variations in magnetic and electronic properties as a function of uniaxial strain in wurtzite(Ga,Mn)As nanowires(NWs) grown along the [0001] direction were investigated based on density functional theory(DFT). We found that(Ga,Mn)As NWs are half-metal, and the ferromagnetic state is their stable ground state. The magnetism of the NWs is significantly affected by the strain and by the substituent position of Mn impurities. By examining charge densities near the Fermi level, we found that strain can regulate the conductive region of the NWs. More interestingly, the size of spin-down band gap of the NWs is tunable by adjusting uniaxial stress, and the NWs can be converted from indirect to direct band gap under tension.     

Preparation of Cu（In,Ga）Se2 Thin Film Solar Cells by Selenization of Metallic Precursors in an Ar Atmosphere

Cu（In, Ga）Se2 thin films are deposited on Mo-coated glass substrates by Se vapour selenization of sputtered metallic precursors in the atmosphere of Ar gas flow under a pressure of about 10 Pa. The in situ heat treatment of as-grown precursor leads to the formation of a better alloy. During selenization, the growth of CuInSe2 phase preferably proceeds through Se-poor phases as CuSe and InSe at relatively low substrate temperature of 250℃, due to the absence of In2Se3 at intermediate stage at low reactor pressure. Subsequently, the Cu（In,Ga）Se2 phase is produced by the reactive diffusion of CuInSe2 with a Se-poor GaSe phase at high temperature of up to 560℃. The final film exhibits smooth surface and large grain size. The absorber is used to fabricate a glass/Mo/Cu（In, Ga）Se2/CdS/ZnO cell with the total-area efficiency of about 7%. The low open-circuit voltage value of the cell fabricated should result from the nonuniform distribution of In and Ga in the absorber, due to the diffusion-controlled reaction during the phase formation. The films, as well as devices, are characterized.     

Projectile interactions in granular impact cratering

We present evidence for the interactions between a ball and the container boundaries, as well as between two balls, that are mediated by the granular medium during impact cratering. The presence of the bottom boundary affects the final penetration depth only for low drop heights with shallow filling, in which case, surprisingly, the penetration becomes deeper. By contrast the presence of the sidewall causes less penetration and also an effective repulsion. Repulsion is also found for two balls dropped side by side.     

Gax In1-xP缓冲层组分对InP自组装形貌影响的研究

应用缓冲层对自组装结构的作用能Er和自组装结构表面能E8 的协同作用分析了InP自组装结构在GaxIn1-xP缓冲层表面的形貌变化.计算发现缓冲层组分影响自组装结构的形貌.随着缓冲层与InP自组装结构之间应力的增加,InP岛倾向于拉长.理论计算还发现随着自组装结构体积的增大,自组装结构也随之拉长.而且缓冲层的参数决定了自组装结构最小能量状态时的体积大小.应用金属有机物化学气相沉积技术在GaAs衬底上生长了不同的InP/GaInP体系,并对实验得到的自组装体系形貌进行了分析.实验结果证实了以上的理论分析.     

Millimetric response of stressed gallium doped germanium photoconductor

Conclusion Conventional Ge:Ga photoconductors presents a residual responsivity at millimetric wavelengths, well below the cut-off wavelength due to the gallium impurity ionization energy. At constant bias voltage, this millimetric responsivity is greatly enhanced when a strong uniaxial stress is applied. We suggest that this response is mainly due to a change in the free hole lifetime when the hole gas is heated by the absorbed microwave photons. This photoconductive process has a shorter response time than the InSb Putley mechanism. Thus optimization of this millimetric Ge:Ga detector could find an application for example for heterodyne detection with larger spectral width than InSb.