n-Hg0.80Mg0.20Te界面积累层中二维电子气的输运特性研究

通过变磁场霍耳测量研究了MBE生长的Hg_0.80Mg_0.20Te薄膜在15 —250K温度范围内的输运特性.采用迁移率谱(MS)和多载流子拟合过程(MCF)相结合的方法对 实验数据进行了分析,由该方法获得的结果和Shubnikov de Hass(SdH)振荡测量的结果都证 明材料中存在二维(2D)电子和三维(3D)电子.其中2D电子主要来自于Hg_1-xMgxTe-CdTe的界面积累层或Hg_1-x关键词： 变磁场霍耳测量 界面积累层 二维电子气 1-xMg_xTe')" href="#">Hg_1-xMg_xTe     

AlGaN/AlN/GaN结构中二维电子气的输运特性

对使用金属有机物汽相沉积法生长的AlGaN/AlN/GaN结构进行的变温霍尔测量，测量结果指出在AlN/GaN界面处有二维电子气存在且迁移率和浓度在2K时分别达到了1.4×10⁴cm²·V^-1·s^-1和9.3×10¹²cm^-2，且在200K到2K范围内二维电子气的浓度基本不变，变磁场霍尔测量发现只有一种载流子(电子)参与导电.在2K温度下，观察到量子霍尔效应，Shubnikov-de Haas (SdH) 振荡在磁场约为3T时出现，证明了此结构呈现了典型的二维电子气行为.通过实验数据对二维电子气散射过程的半定量分析，推出量子散射时间为0.23ps，比以往报道的AlGaN/GaN结构中的散射时间长，说明引入AlN层可以有效减小合金散射，进一步的推断分析发现低温下以小角度散射占主导地位.     

AlN插入层对AlxGa1-xN/GaN界面电子散射的影响

本文研究AlN作为Al_xGa_1-xN/GaN插入层引起的电子输运性质的变化, 考虑了Al_xGa_1-xN和AlN势垒层的自发极化、压电极化对Al_xGa_1-xN/AlN/GaN双异质结高电子迁移率晶体管(HEMT)中极化电荷面密度、二维电子气(2DEG) 浓度的影响, 分析了AlN厚度与界面粗糙度散射和合金无序散射的关系; 结果表明, 2DEG 浓度、界面粗糙度散射和合金无序散射依赖于AlN层厚度, 插入一层1–3 nm薄的AlN层, 可以明显提高电子迁移率.     

不同散射机理对 Al2O3/InxGa1-xAs nMOSFET 反型沟道电子迁移率的影响

通过考虑体散射、界面电荷的库仑散射以及 Al₂O₃/In_xGa_1-xAs 界面粗糙散射等主要散射机理, 建立了以 Al₂O₃为栅介质In_xGa_1-xAs n 沟金属-氧化物-半导体场效应晶体管 (nMOSFETs) 反型沟道电子迁移率模型, 模拟结果与实验数据有好的符合. 利用该模型分析表明, 在低至中等有效电场下, 电子迁移率主要受界面电荷库仑散射的影响; 而在强场下, 电子迁移率则取决于界面粗糙度散射. 降低界面态密度, 减小 Al₂O₃/In_xGa_1-xAs 界面粗糙度, 适当提高In含量并控制沟道掺杂在合适值是提高 InGaAs nMOSFETs 反型沟道电子迁移率的主要途径. 关键词： InGaAs MOSFET 反型沟道电子迁移率 散射机理     

晶格匹配InAlN/GaN和InAlN/AlN/GaN材料二维电子气输运特性研究

文章研究了InAlN/GaN和引入AlN界面插入层形成的InAlN/AlN/GaN材料的输运性质. 样品均在蓝宝石上以脉冲金属有机物化学气相淀积法生长,霍尔迁移率变温特性具有典型的二维电子气(2DEG)特征. 综合各种散射机理包括声学形变势散射、压电散射、极性光学声子散射、位错散射、合金无序散射和界面粗糙度散射,理论分析了温度对迁移率的影响,发现室温下两种材料中2DEG支配性的散射机理都是极性光学波散射和界面粗糙度散射;AlN插入层对InAlN/GaN材料迁移率的改善作用一方面是免除2DEG的合金无序散射,另外还显著改善异质界面,抑制了界面粗糙度散射. 考虑到2DEG密度也是影响其迁移率的重要因素,结合实验数据给出了晶格匹配InAlN/GaN和InAlN/AlN/GaN材料的2DEG迁移率随电子密度变化的理论上限. 关键词： InAlN/GaN 二维电子气 迁移率     

p-H_(1-x)Cd_xTe材料中轻空穴的性质研究

通过变磁场霍尔测量方法 ,采用由迁移率谱和多载流子拟合过程相结合的混合电导法 ,在 1 2 - 30 0K范围内 ,获得了两块分子束外延 (MBE)生长的p Hg1-xCdxTe(x =0 2 2 4)样品中的轻、重空穴以及体电子、表面电子的浓度和迁移率 .此外 ,在实验中 ,还直接观察到了轻空穴对电导张量分量的贡献 .实验值不仅具有明确的物理意义 ,而且有助于红外探测器模型的建立 .     

AlGaN/GaN高电子迁移率晶体管中二维电子气的极化光学声子散射

Al Ga N/Ga N界面处的二维电子气迁移率是描述高电子迁移率晶体管特性的一个重要参数,极化光学声子散射是高温时限制二维电子气迁移率的主要散射机制.本文对极化光学声子散射进行计算,结果表明在二维电子气浓度为6×10~(11)—1×10~(13) cm~(–2),温度为200—400 K范围内,极化光学声子散射因素决定的迁移率随温度的变化近似为μPO=AT-α(α=3.5);由于Ga N中光学声子能量较大,吸收声子对迁移率的影响远大于发射声子的影响.进一步讨论了极化光学声子散射因素决定的迁移率随光学声子能量变化的趋势,表明增加极化光学声子能量可提高二维电子气的室温迁移率.     

Hg_(1-x)Cd_xTe材料变温特性的实验研究

对碲镉汞(Hg1-xCdxTe)材料的电学特性进行了变温实验研究.通过变温(80~300 K)霍尔效应测量,研究了Hg1-xCdxTe材料的霍尔系数RH、电导率σ和霍尔迁移率μH的温度依赖关系.结果表明,Hg1-xCdxTe晶体在低温下为p型导电,而在室温下为n型导电.     

GdVO_4∶Eu~(3+)发光的温度效应

研究了GdVO4 ∶Eu3 + 在高压汞灯的 3 1 3和 3 65nm激发下室温以上 ( 3 0 0～ 60 0K)发光的温度依赖关系。发现来自5D0 的发射强度随温度的升高而显著增强 ,直到 60 0K也未见饱和。其中5D0 →7F2 的 61 9nm发射在60 0K温度时的强度是室温下的 2 0多倍。我们认为Eu3 + 的电荷迁移态作为中间态是造成其发光增强的根本原因。激发过程中 ,先激发到Eu3 + 本身的5DJ(J=1 ,2 ,3… )激发态 ,然后在温度的作用下上升到电荷迁移态(CTS) ,温度升高时传递几率显著增强 ,并按5D3 ,5D2 ,5D1依次使被激发的电子转入CTS态 ,从电荷迁移态直接弛豫传递给5D0 态 ,由于5D0 态电子数不断增多 ,致使来自5D0 的发射随温度升高而增强。     

K(5P)与H_2的反应碰撞和电子-振动能量转移

K(5P)与H2反应生成KH(v″=0-3)振动态,测量了各振动态的转动分布,转动玻尔兹曼温度为455K,而振动温度为1604K,这个接近池温的转动温度和很高的振动温度是共线碰撞机制的有力证据.利用高分辨率瞬时吸收技术得到各振动能级上转动态的布居分布,从而得到反应碰撞转移速率系数,对于v″=0、1、2、3,分别为(3.45±0.86)×10-13、(1.35±0.34)×10-13、(6.28±1.57)×10-14和(2.35±0.59)×10-14cm3s-1.同时研究了K(5P)-H2的电子-振动能量转移,利用相干反斯托克斯拉曼散射(CARS)探测H2的振动态分布.扫描CARS谱发现v=1、2、3上有布居.由CARS峰值得到H2(0,1)、(1,1)、(2,1)、(3,1)和(3,3)布居之比.H2(0,1)布居由450K的转动分布得到,因而得到(1,1)、(2,1)、(3,1)和(3,3)态的布居,从而获得K(5P)-H2(1,1)、(2,1)、(3,1)和(3,3)的电子-振转速率系数分别是(1.1±0.3)×10-13、(9.3±2.5)×10-14、(4.2±1.1)×10-14和(3.8±1.0)×10-14cm3s-1.     

The effect of interface roughness scattering on low field mobility of 2D electron gas in GaN/AlGaN heterostructure

We report the results of our experimental and theoretical studies concerning the temperature dependence of electron mobility in a two dimensional electron gas (2DEG) confined at the GaN/AlGaN interface. Experimental mobility of about at 3.8 K remains almost constant up to lattice temperature , it then decreases rapidly down to about at . The results are discussed using a theoretical model that takes into account the most important scattering mechanisms contributing to determine the mobility of electrons in 2DEG. We show that the polar optical phonon scattering is the dominant mechanism at high temperatures and the acoustic deformation potential and piezoelectric scatterings are dominant at the intermediate temperatures. At low temperatures, the Hall mobility is confined by both the interface roughness (IFR) and ionised impurity scattering. The correlation length (Λ) and lateral size (Δ) of roughness at the GaN/AlGaN heterointerface have been determined by fitting best to our low-temperature experimental data.     

Convergence of conduction bands as a means of enhancing thermoelectric performance of n-type Mg2Si(1-x)Sn(x) solid solutions

Mg(2)Si and Mg(2)Sn are indirect band gap semiconductors with two low-lying conduction bands (the lower mass and higher mass bands) that have their respective band edges reversed in the two compounds. Consequently, for some composition x, Mg(2)Si(1-x)Sn(x) solid solutions must display a convergence in energy of the two conduction bands. Since Mg(2)Si(1-x)Sn(x) solid solutions are among the most prospective of the novel thermoelectric materials, we aim on exploring the influence of such a band convergence (valley degeneracy) on the Seebeck coefficient and thermoelectric properties in a series of Mg(2)Si(1-x)Sn(x) solid solutions uniformly doped with Sb. Transport measurements carried out from 4 to 800 K reveal a progressively increasing Seebeck coefficient that peaks at x=0.7. At this concentration the thermoelectric figure of merit ZT reaches exceptionally large values of 1.3 near 700 K. Our first principles calculations confirm that at the Sn content x≈0.7 the two conduction bands coincide in energy. We explain the high Seebeck coefficient and ZT values as originating from an enhanced density-of-states effective mass brought about by the increased valley degeneracy as the two conduction bands cross over. We corroborate the increase in the density-of-states effective mass by measurements of the low temperature specific heat. The research suggests that striving to achieve band degeneracy by means of compositional variations is an effective strategy for enhancing the thermoelectric properties of these materials.     

非晶La_(1-x)Nd_xP_5O_(14)的拉曼散射研究

本文利用拉曼散射对非晶 La_(1-X)Nd_XP_5O_(14)及其晶化过程进行了研究。拉曼散射表明在非晶La_(1-X)Nd_XP_5O_(14)中仍保留着晶态 La_(1-X)Nd_XP_5O_(14)中具有的 PO_4~(3-)四面体结构单元。非晶La_(1-X)Nd_XP_5O_4在不同温度下晶化,其拉曼谱线的频率不同,这一现象可用应力模型给予解释。     

Electron mobility in a modulation doped AlGaN/GaN quantum well

We consider a two dimensional electron gas confined to a modulation doped AlGaN/GaN quantum well and study the dependence of low field mobility on various parameters such as composition, well width, remote impurity and interface roughness as a function of temperature. GaN is assumed to be in the zincblende structure. Acoustic and optical phonon, ionized remote impurity and interface roughness scatterings are taken into account in mobility calculations. The scattering rates are calculated using the self-consistently calculated wave functions obtained from the numerical solution of Poisson and Schr?dinger equations. Also found from the self-consistent solutions are the potential profile at the junction, the energy levels in the well and electron concentrations in each level. Ensemble Monte Carlo method is used to find the drift velocities of the two dimensional electrons along the interface under an applied field. The mobility of two dimensional electrons is obtained from the drift velocity of electrons. It is found that while remote impurity scattering is very effective for small values of spacer layer and doping concentrations, increasing Al concentration reduces the mobility of electrons. The effect of surface roughness, on the other hand, on mobility is almost independent of well width. The results of our simulations are compatible with the existing experimental data.     

Interplay between crystal field splitting and Kondo effect in CeNi(9)Ge(4-x)Si(x)

The pseudo-ternary solid solution CeNi(9)Ge(4-x)Si(x) (0?≤?x?≤?4) has been investigated by means of x-ray diffraction, magnetic susceptibility, specific heat, electrical resistivity, thermopower and inelastic neutron scattering studies. The isoelectronic substitution of germanium by silicon atoms causes a dramatic change of the relative strength of competing Kondo, RKKY and crystal field (CF) energy scales. The strongest effect is the continuous elevation of the Kondo temperature T(K) from approximately 3.5?K for CeNi(9)Ge(4) to about 70?K for CeNi(9)Si(4). This increase of the Kondo temperature is attended by a change of the CF level scheme of the Ce ions. The interplay of the different energy scales results in an incipient reduction of the ground state degeneracy from an effectively fourfold degenerate non-magnetic Kondo ground state with unusual non-Fermi-liquid features of CeNi(9)Ge(4) to a lower one, followed by an increase towards a sixfold, fully degenerate ground state multiplet in CeNi(9)Si(4) (T(K)?～?Δ(CF)).     

Influence of temperature on strain-induced polarization Coulomb field scattering in AlN/GaN heterostructure field-effect transistors

Electron mobility scattering mechanism in AlN/GaN heterostuctures is investigated by temperature-dependent Hall measurement, and it is found that longitudinal optical phonon scattering dominates electron mobility near room temperature while the interface roughness scattering becomes the dominant carrier scattering mechanism at low temperatures(～100 K).Based on measured current–voltage characteristics of prepared rectangular AlN/GaN heterostructure field-effect transistor under different temperatures, the temperature-dependent variation of electron mobility under different gate biases is investigated. The polarization Coulomb field(PCF) scattering is found to become an important carrier scattering mechanism after device processing under different temperatures. Moreover, it is found that the PCF scattering is not generated from the thermal stresses, but from the piezoelectric contribution induced by the electrical field in the thin AlN barrier layer. This is attributed to the large lattice mismatch between the extreme thinner AlN barrier layer and GaN, giving rise to a stronger converse piezoelectric effect.     

Evolution of magnetic properties in the normal spinel solid solution Mg(1-x)Cu(x)Cr2O4

We examine the evolution of magnetic properties in the normal spinel oxides Mg(1-x)Cu(x)Cr2O4 using magnetization and heat capacity measurements. The end-member compounds of the solid solution series have been studied in some detail because of their very interesting magnetic behavior. MgCr2O4 is a highly frustrated system that undergoes a first-order structural transition at its antiferromagnetic ordering temperature. CuCr2O4 is tetragonal at room temperature as a result of Jahn-Teller active tetrahedral Cu2+ and undergoes a magnetic transition at 135 K. Substitution of magnetic cations for diamagnetic Mg2+ on the tetrahedral A site in the compositional series Mg(1-x)Cu(x)Cr2O4 dramatically affects magnetic behavior. In the composition range 0 ≤ x ≤ ≈0.3, the compounds are antiferromagnetic. A sharp peak observed at 12.5 K in the heat capacity of MgCr2O4 corresponding to a magnetically driven first-order structural transition is suppressed even for small x. Uncompensated magnetism--with open magnetization loops--develops for samples in the x range ≈0.43 ≤ x ≤ 1. Multiple magnetic ordering temperatures and large coercive fields emerge in the intermediate composition range 0.43 ≤ x ≤ 0.47. The Néel temperature increases with increasing x across the series while the value of the Curie-Weiss Θ(CW) decreases. A magnetic temperature-composition phase diagram of the solid solution series is presented.     

Effects of absorbed hydrogen on the electronic properties of (Zr2Fe)(1-x)H(x) metallic glasses

The electrical conductivity (σ) of hydrogen doped (Zr(2)Fe)(1-x)H(x) metallic glasses has been measured in the temperature range from 290 down to 5 K. The decrease of the room temperature conductivity and the increase of its temperature coefficient are explained as consequences of increased disorder due to hydrogen doping. σ(T) for (Zr(2)Fe)(1-x)H(x) metallic glasses at low temperatures decreases with the increase of temperature, forming a minimum at T(min), before it starts a monotonic increase with increasing temperature. Both the functional forms and the magnitudes of the observed σ(T) are interpreted in terms of weak localization, electron-electron interaction and spin-fluctuation effects. Our results reveal that the electron-phonon scattering rate varies with the square of temperature from low temperatures up to 100 K and changes behaviour to a linear form at higher temperatures. At low temperatures, the minimum in σ(T) is shifted to higher temperatures, which is ascribed to the increase of the screening parameter of the Coulomb interaction F* associated with the enhancement of the spin fluctuations arising from the increase of the hydrogen doping. The spin-orbit scattering rate and the electron diffusion constant are reduced by hydrogen doping.     

The apparent Dingle temperature in Zn−Mn,Zn−Cr,and Zn−Fe

The apparent Dingle temperature of the 3rd zone needles in several alloys of Zn?Mn, Zn?Cr and Zn?Fe has been determined from the absolute amplitude of the de Haas-van Alphen effect over the temperature range 1.6 K to 4.2 K and the field range 0 to 9 kG. The results show clearly that potential scatteringnot spin scattering dominates in all three systems. In addition, a “cross-over” behaviour in the de Haas-van Alphen amplitudes results from the scattering of the magnetic moments of the impurity spin.