Semiconductor–metal transition in GaAs nanowires under high pressure

We investigate the structural phase transitions and electronic properties of GaAs nanowires under high pressure by using synchrotron x-ray diffraction and infrared reflectance spectroscopy methods up to 26.2 GPa at room temperature.The zinc-blende to orthorhombic phase transition was observed at around 20.0 GPa.In the same pressure range, pressureinduced metallization of GaAs nanowires was confirmed by infrared reflectance spectra.The metallization originates from the zinc-blende to orthorhombic phase transition.Decompression results demonstrated that the phase transition from zincblende to orthorhombic and the pressure-induced metallization are reversible.Compared to bulk materials, GaAs nanowires show larger bulk modulus and enhanced transition pressure due to the size effects and high surface energy.     

Fabrication of crystalline selenium microwire

A method of fabricating selenium(Se) microwire is demonstrated.A multimaterial fiber with amorphous selenium(a-Se) core and multicomponent phosphate glass cladding is drawn by using a conventional optical fiber drawing technique.Then the a-Se core of the fiber is crystallized by a post thermal process at 150 ℃.After the multicomponent phosphate glass cladding is stripped from the multimaterial fiber by marinating the fiber in HF acid solution,a crystalline selenium(c-Se)microwire with high uniformity and smooth surface is obtained.Based on microstructure measurements,the c-Se microwire is identified to consist of most hexagonal state particles and very few trigonal state whiskers.The good photoconduction property of c-Se microwire with high quality and longer continuous length makes it possible to apply to functional devices and arrays.     

In situ X-ray absorption fine structure detection of reversible photoinduced anisotropy in amorphous selenium

Photoinduced anisotropy in amorphous selenium ( a-Se) has been studied by in situ x-ray absorption fine structure spectroscopy. It is found that upon irradiation with linearly polarized light the chainlike fragments in a-Se are oriented with their c-axis perpendicular to the polarization plane of the inducing light. This chain orientation is reversible, correlates with optical anisotropy, and acts as a prelude to photoinduced anisotropic crystallization. This optically controlled reorientation of molecular fragments in a solid can be a basis for future novel devices.     

Semiconductor-metal transition in selenium under shock compression

Phase transitions in selenium are studied by time-resolved measurements of the electrical conductivity under shock compression at a pressure of up to 32 GPa. The pressure dependence of the electrical conductivity (σ(P)) has two portions: a sharp increase at P < 21 GPa and a plateau at P > 21 GPa. The experimental data and the temperature estimates indicate that, at P < 21 GPa, selenium is in the semiconductor state. The energy gap of semiconducting selenium decreases substantially under compression. At P > 21 GPa, the electrical conductivity saturates at ～10⁴ Ω^?1 cm^?1. Such a high value of the electrical conductivity shows the effective semiconductor-metal transition taking place in shock-compressed selenium. Experiments with samples having different initial densities demonstrate the effect of temperature on the phase transition. For example, powdered selenium experiences the transition at a lower shock pressure than solid selenium. Comparison of the temperature estimates with the phase diagram of selenium shows that powdered selenium metallizes in a shock wave as a result of melting. The most plausible mechanism behind the shock-induced semiconductor-metal transition in solid selenium is melting or the transition in the solid phase. Under shock compression, the metallic phase arises without a noticeable time delay. After relief, the metallic phase persists for a time, delaying the reverse transition.     

InSe的高压电输运性质研究

高压下对InSe样品进行原位电阻率和霍尔效应测量. 电阻率测量结果显示, 样品在5–6 GPa区间呈现金属特性, 在12 GPa 的压力下发生由斜六方体层状结构到立方岩盐矿的结构相变, 且具有金属特性. 霍尔效应测量结果显示, 样品在6.6 GPa由p型半导体转变成n型半导体, 电阻率随着压力的升高而逐渐下降是由于载流子浓度升高引起的. 关键词： InSe 高压 电阻率 霍尔效应     

高压下LiB3O5晶体-非晶体相变的研究

 采用X射线粉末衍射和同步辐射能散X射线衍射技术，对LiB₃O₅（LBO）晶体进行了压致非晶化相变的研究，压力达45.1 GPa。实验结果表明，LBO存在两个中间相变，压力分别为5 GPa和15.8 GPa。而当压力增加到25~31.6 GPa时，LBO出现非晶态，这个非晶化相变是不可逆的。     

超高压下CsBr的结构与相变

 采用金刚石对顶压砧高压装置（DAC）、同步辐射X光源和能散法，对CsBr粉末样品进行了原位高压X光衍射实验，最高压力达115 GPa。观测到在53 GPa左右压力下，CsBr的最强衍射峰(110)劈裂成两个峰，标志了简单立方结构向四方结构的转变；在0至最高压力范围内（相应于V/V₀为1至0.463）测量了晶轴比c/a；在115 GPa内未观测到样品的金属化现象。     

高压下钡的硫化物的结构相变和光学性质研究(英文)

利用基于密度泛函的第一性原理,计算了高压下钡的硫化物(BaS、BaSe和BaTe)的结构相变和光学性质。计算结果表明,这些化合物的压致结构相变是从NaCl型结构转变为CsCl型结构;对于结构转变压力和金属化转变压力,BaS为8.57 GPa和45.4 GPa,BaSe为7.44 GPa和36.5 GPa,BaTe则分别为5.67 GPa和16.7 GPa。光学性质计算结果显示:随着压力的增加,静态介电常数ε0不断增加,介电常数虚部ε2的峰值向高能方向移动(蓝移)。     

Electroded avalanche amorphous selenium (a-Se) photosensor

Although avalanche amorphous selenium (a-Se) is a very promising photoconductor for a variety of imaging applications, it is currently restricted to applications with electron beam readout in vacuum pick-up tube called a High-gain Avalanche Rushing Photoconductor (HARP). The electron beam readout is compatible with high definition television (HDTV) applications, but for use in solid-state medical imaging devices it should be replaced by an electronic readout with a two-dimensional array of metal pixel electrodes. However, due to the high electric field required for avalanche multiplication, it is a technological challenge to avoid possible dielectric breakdown at the edges, where electric field experiences local enhancement. It has been shown recently that this problem can be overcome by the use of a Resistive Interface Layer (RIL) deposited between a-Se and the metal electrode, however, at that time, at a sacrifice in transport properties.Here we show that optimization of RIL deposition technique allows for electroded avalanche a-Se with transport properties and time performance previously not achievable with any other a-Se structures. We have demonstrated this by detailed analysis of transport properties performed by Time-of-Flight (TOF) technique. Our results showed that a stable gain of 200 is reached at 104 V/μm for a 15-μm thick a-Se layer, which is the maximum theoretical gain for this thickness. We conclude that RIL is an enabling technology for practical implementation of solid-state avalanche a-Se image sensors.     

高压下碳纳米管的X射线衍射研究

 用同步辐射原位高压能散X射线衍射技术,对碳纳米管进行了结构和物性的研究,压力达50.7 GPa。在室温常压下,碳纳米管的结构和石墨的hcp结构相似,其(002)衍射线的面间距为d₀₀₂=0.340 4 nm,(100)衍射线的面间距为d₁₀₀=0.211 6 nm。从高压X射线衍射实验看到,当压力升到8 GPa以上时,(002)线变宽变弱,碳纳米管部分非晶化。而当压力从10 GPa或20 GPa卸压至零时,(002)线部分恢复。但当压力升高至最高压力50.7 GPa时,碳纳米管完全非晶化,而且这个非晶化相变是不可逆的。用Birch-Murnaghan方程拟合实验数据,得到体弹模量为K₀=(54.3±3.2)GPa（当K′₀=4.0时）。     

Surface grating formation and erasing on a-Se films

Surface relief formation at holographic recording on amorphous selenium films was demonstrated and investigated. The presence of this optical phase modulation component is essential for ensuring significant, stable and erasable optical recording in a-Se films at 290–320 K temperatures, where conventional photodarkening was known as insignificant and unstable. Photocrystallization can only be observed in super-exposed a-Se films at the given experimental conditions of hologram recording. Erasing behavior of surface relief gratings under heat treatment was also investigated in order to reveal further details of the mechanism. Photoinduced structural transformations within the amorphous phase, connected to local ordering under the condition of light-induced fluidity, are proposed as an explanation for the relief formation and erasing. The observed reversible optical recording process may be useful for the various optoelectronic applications of photoconductive a-Se layers. Received: 12 June 2000 / Accepted: 6 June 2001 / Published online: 30 August 2001     

Metallic liquid hydrogen and likely Al<Subscript>2</Subscript>O<Subscript>3</Subscript> metallic glass

Dynamic compression has been used to synthesize liquid metallic hydrogen at 140 GPa (1.4 million bar) and experimental data and theory predict Al₂O₃ might be a metallic glass at ∼ 300 GPa. The mechanism of metallization in both cases is probably a Mott-like transition. The strength of sapphire causes shock dissipation to be split differently in the strong solid and soft fluid. Once the 4.5-eV H-H and Al-O bonds are broken at sufficiently high pressures in liquid H₂ and in sapphire (single-crystal Al₂O₃), electrons are delocalized, which leads to formation of energy bands in fluid H and probably in amorphous Al₂O₃. The high strength of sapphire causes shock dissipation to be absorbed primarily in entropy up to ∼400 GPa, which also causes the 300-K isotherm and Hugoniot to be virtually coincident in this pressure range. Above ∼400 GPa shock dissipation must go primarily into temperature, which is observed experimentally as a rapid increase in shock pressure above ∼400 GPa. The metallization of glassy Al₂O₃, if verified, is expected to be general in strong oxide insulators. Implications for Super Earths are discussed.     

硼酸锂系列晶体的高压拉曼散射研究

本文进行了硼酸锂系列晶体的高压拉曼散射及其压致相变的研究。对于三硼酸锂（ＬｉＢ３Ｏ５），我们发现在５．０ＧＰａ有一可逆的晶态到晶态的相变，在２７．０ＧＰａ有一不可逆的晶态到非晶态的相变。二硼酸锂（Ｌｉ２Ｂ４Ｏ７）不可逆压致非晶相变发生在３２．０ＧＰａ附近。对于一硼酸锂，我们研究了０—５５．８ＧＰａ范围内的高压拉曼光谱，只在２．０ＧＰａ发现了一个晶态到晶态的相变，但未发现不可逆压致非晶化现象。在硼酸锂系列晶体中，不可逆压致非晶化的压力随Ｌｉ２Ｏ的含量的增加而升高。硼酸锂晶体中Ｌｉ２Ｏ的含量越高，压致非晶化越不容易发生，这与熔体急冷法制备硼酸锂玻璃的规律是一致的。     

CdSe在高压下的电学性质及相变

 利用在金刚石压砧上集成的微电路,原位测量了CdSe多晶粉末在温度为300~450 K、压力达到23 GPa时电阻率随温度和压力的变化关系。实验结果表明：在加压过程中,电阻率在2.6 GPa压力时出现的异常改变,对应着CdSe从纤锌矿向岩盐矿结构的转变,而在6.0、9.8、17.0 GPa等压力处出现的电阻率异常,则是由CdSe中的电子结构的变化所引起的;在卸压过程中,只在约14.0和3.0 GPa压力下观察到了两个电阻率异常点。通过对电阻率随压力变化曲线的模拟,得出了CdSe高压相的带隙随压力的变化关系,据此预测CdSe金属化的压力应在70~100 GPa之间。变温实验结果表明,在实验的温度和压力范围内,CdSe的电阻率均随温度的增加而升高。     

Solidification of selenium melt under pressure up to 6.5 GPa

Abstract

High purity selenium samples were melted under high pressure (≤6.4 GPa) and quenched at various rates ranging from 2 K s^?1 to 500 K s^?1 and the recovered material was examined by X-ray diffraction and electron microscopy. In the entire range of pressure and cooling rate, the melt was found to solidify into a polycrystalline aggregate of the trigonal phase of selenium. The samples obtained by slow cooling of the melt at 6.4 GPa contain, in addition to crystalline phase, regions which appear to be amorphous.     

Experimental and theoretical evidence for pressure-induced metallization in FeO with rocksalt-type structure

Electrical conductivity of FeO was measured up to 141 GPa and 2480 K in a laser-heated diamond-anvil cell. The results show that rock-salt (B1) type structured FeO metallizes at around 70 GPa and 1900 K without any structural phase transition. We computed fully self-consistently the electronic structure and the electrical conductivity of B1 FeO as a function of pressure and temperature, and found that although insulating as expected at ambient condition, B1 FeO metallizes at high temperatures, consistent with experiments. The observed metallization is related to spin crossover.     

Frequency-resolved photocurrent studies of a-Se

 Optically modulated photocurrent response of amorphous selenium (a-Se) thin films was measured between 20 K and 295 K, by using the in-quadrature frequency-resolved spectroscopy method. The results show that the modulated photocurrent depends on external parameters such as excitation light intensity and temperature, giving information about recombination kinetics. Received, 28 July 1996 /Accepted: 20 September 1996     

高压下锗化镁的金属化相变研究

锗化镁是一种窄带半导体,压力作用可以使锗化镁导带底与价带顶的能隙变小.本文基于第一性原理计算了锗化镁在高压下的能带结构以及反萤石相(常压稳定相)和反氯铅矿相(高压相)的焓值,发现在7.5 GPa时反萤石结构锗化镁导带底与价带顶的能隙闭合,预示着半导体相转变为金属相,计算结果还预测在11.0 GPa时锗化镁发生从反萤石结构到反氯铅矿结构的相变.实验研究方面,本文采用长条形压砧在连续加压条件下测量了锗化镁高压下的电阻变化,采用金刚石对顶压砧测量了锗化镁的高压原位拉曼光谱,发现在8.7 GPa锗化镁的电阻出现不连续变化,9.8 GPa以上锗化镁的拉曼振动峰消失.由于金属相的自由电子浓度高会阻碍激发光进入样品,进而引起拉曼振动峰消失,因此我们推测锗化镁在9.8 GPa转变为金属相.     

Dark current suppression of amorphous selenium based photosensors by the ZnO hole blocking layer

To study the influence of defects in the hole blocking layer (HBL) on the dark current of amorphous selenium (a-Se) based photosensors, we prepared ZnO thin films by reactive sputter deposition (RSD) for the use as the HBL of the photosensors. The ZnO HBL layers prepared with different oxygen flow rates were characterized by X-ray photoelectron spectroscopy, Raman scattering analysis and photoluminescence, indicating that the density of oxygen vacancies in the ZnO thin films is significantly affected by the oxygen flow rate. The deep level transient spectroscopy measurement reveals two hole trap levels present in the RSD deposited ZnO thin films; one is at 0.94 eV and the other at 0.24 eV above the valence band edge. The electrical performance of the a-Se photosensor is largely influenced by the amount of oxygen vacancies in the ZnO thin film. The a-Se photosensor with the ZnO HBL of the most oxygen vacancies has the lowest dark current and demonstrates the highest breakdown field.     

高压下CaF_2结构相变和光学性质的第一性原理计算

利用基于密度泛函理论的第一性原理方法,计算了在压力作用下CaF2的结构相变和光学性质。结果证实了CaF2的压致结构转变的顺序是从氟石结构(空间群Fm3m)转变到PbCl2型结构(空间群Pnma),然后继续转变为Ni2In型结构(空间群P63/mmc)。在Fm3m和Pnma两种结构中,电子带隙随着压力的增加而增加,而在P63/mmc结构中,带隙随着压力的增加开始下降。实验结果显示,直到210 GPa,CaF2没有发生由绝缘体到金属的转变。据此推测,CaF2的金属化压力高于300 GPa。还讨论了压力对CaF2光学性质的影响。