Analysis of PTCDA/ITO Surface and Interface Using X-ray Photoelectron Spectroscopy and Atomic Force Microscopy

利用X射线光电子能谱(XPS)对苝四甲酸二酐[3,4,9,10-perylenetetracarboxylic dianhydride(PTCDA)]/铟锡氧化物(ITO)表面和界面进行了研究.用原子力显微镜(AFM)对PTCDA/ITO样品的表面形貌进行了分析.XPS表明,在原始表面的C1s精细谱存在两个主谱峰和一个伴峰,主谱峰分别由结合能为284.6eV的苝环中的C原子和结合能为288.7eV的酸酐基团中的C原子激发;而结合能为290.4eV的伴峰的存在,说明发生了来源于ITO膜中的氧对C原子的氧化现象.O原子在C=O键和C-O-C键的结合能分别为531.5和533.4eV.在界面处,C1s谱中较高结合能峰消失,且峰值向低结合能方向发生0.2eV的化学位移;O1s谱向低结合能方向发生1.5eV的化学位移.由此可以推断,在界面处PTCDA与ITO的结合是PTCDA中的苝环与ITO中的In空位的结合.AFM的结果显示,PTCDA薄膜为岛状结构,岛的直径约为100～300nm,表面起伏约为14nm.相邻两层PTC-DA分子由于存在离域大π键而交叠和PTCDA分子中的苝环与ITO的In空位的紧密结合是最终导致PTCDA岛状结构形成的原因.     

Al组分对AlxGa1-xN/GaN异质结构中二维电子气输运性质的影响

在低温和强磁场下,通过磁输运测量研究了不同Al组分调制掺杂AlxGa1-xN/GaN异质结二维电子气(2DEG)的磁电阻振荡现象.观察到低Al组分异质结中的2DEG有较低的浓度和较高的迁移率.     

AlGaN/GaN异质结构中二维电子气的高温输运性质

采用高温Hall测量仪对一个全应变和一个部分应变弛豫的AlGaN/GaN异质结构中2DEG的高温输运特性进行了研究,温度变化范围从室温到680K.研究结果表明:在高温段2DEG的迁移率主要受LO声子散射限制; 在室温,异质界面处的非均匀压电极化场对2DEG迁移率的散射也是一个主要的散射机制.同时,计算结果显示,随着温度升高,更多的电子跃迁到更高的子带,在更高的子带,其波函数逐渐扩展到AlGaN层内部以及GaN体内更深的位置,导致LO声子散射的屏蔽效应减弱且来自AlGaN层内的合金无序散射增强.     

Al_xGa_(1-x)N/GaN异质结构中二维电子气的自旋性质

Ⅲ族氮化物材料有很长的电子自旋弛豫时间以及很高的居里温度,成为近年来半导体自旋电子学研究的重要材料体系之一。介绍了目前两种最主要的研究AlxGa1-xN/GaN异质结构中二维电子气(2DEG)自旋性质的物理效应:磁电阻的舒伯尼科夫-德哈斯拍频振荡和弱反局域效应,回顾了AlxGa1-xN/GaN异质结构中2DEG自旋性质的研究进展。AlxGa1-xN/GaN异质结构材料中有很强的极化电场,诱导产生很高浓度的2DEG,能够产生相当大能量的自旋分裂,并且这种自旋分裂可以被栅压所调控,因此在自旋场效应晶体管方面有很好的应用前景。然而要实现GaN基自旋电子学器件的应用,GaN中自旋注入效率是目前所面临的问题。     

Polarization-Engineered Enhancement-Mode High-Electron-Mobility Transistors Using Quaternary AlInGaN Barrier Layers

Group III nitride heterostructures with low polarization difference recently moved into the focus of research for realization of enhancement-mode (e-mode) transistors. Quaternary AlInGaN layers as barriers in GaN-based high-electron-mobility transistors (HEMTs) offer the possibility to perform polarization engineering, which allows control of the threshold voltage over a wide range from negative to positive values by changing the composition and strain state of the barrier. Tensile-strained AlInGaN layers with high Al contents generate high two-dimensional electron gas (2DEG) densities, due to the large spontaneous polarization and the contributing piezoelectric polarization. To lower the 2DEG density for e-mode HEMT operation, the polarization difference between the barrier and the GaN buffer has to be reduced. Here, two different concepts are discussed. The first is to generate compressive strain with layers having high In contents in order to induce a positive piezoelectric polarization compensating the large negative spontaneous polarization. Another novel approach is a lattice-matched Ga-rich AlInGaN/GaN heterostructure with low spontaneous polarization and improved crystal quality as strain-related effects are eliminated. Both concepts for e-mode HEMTs are presented and compared in terms of electrical performance and structural properties.     

The influence of composition and unintentional doping on the two-dimensional electron gas density in AlGaN/GaN heterostructures

We have studied the influence of Al content, AlGaN layer thickness, and unintentional background doping by oxygen on the two-dimensional electron gas (2DEG) density in AlGaN/GaN heterostructures. Hall measurements were made on samples grown with molecular beam epitaxy. The 2DEG densities in the range 2–3×10¹³ cm^?2 were measured. A one-dimensional Schrödinger-Poisson model was used to describe the heterostructure. The calculations gave two-dimensional electron densities in accordance with measured values. The electron density is very sensitive to the Al concentration in the AlGaN layer, whereas the sensitivity to layer thickness is small. Our simulations also showed that the two-dimensional concentration increased 50% when the free-carrier concentration changed from 10¹⁵ cm^?3 to 10¹⁸ cm^?3. The relation between donor concentration and free-carrier concentration was found to agree when using oxygen ionization energy as a parameter.     

Electrical Spin Injection into the 2D Electron Gas in AlN/GaN Heterostructures with Ultrathin AlN Tunnel Barrier

The spin injection into 2D electron gas (2DEG) in AlN/GaN heterostructures is studied by magneto-transport measurements. An ultrathin AlN layer at the hetero-interface acts as a barrier to form high-quality 2DEG in the triangular quantum well and a tunneling barrier for the spin injection to overcome the conductance mismatch issue. In this study, Hanle signals and inversed Hanle signals are observed, proving that the spin injection is achieved in the 2DEG in the AlN/GaN heterostructure rather than in the interfacial states. The spin-relaxation time in 2DEG at 8 K is found to be as long as 860 ps, which almost keeps constant with bias and decreases with increasing temperature. The spin-relaxation process is illustrated as Rashba spin-orbit coupling dominated D'yakonov Perel’ mechanisms above 8 K. These results show the promising potential of 2DEG in AlN/GaN heterostructures for spin field-effect transistor applications.     

AlGaN/GaN/AlGaN双异质结材料生长及性质研究

基于能带理论设计并利用MOCVD技术在76.2 mm蓝宝石衬底上生长了不同GaN沟道层厚度的AlGaN/GaN/AlGaN双异质结材料.室温霍尔测试结果表明:双异质结材料的二维电子气面密度随沟道层厚度增加有所升高并趋于饱和;二维电子气迁移率则随沟道厚度增加明显升高.200 nm厚GaN沟道的双异质结材料方块电阻平均值3...     

Electron mobility in two-dimensional electron gas in AIGaN/GaN heterostructures and in bulk GaN

We report on temperature dependencies of the electron mobility in the two-dimensional electron gas (2DEG) in AIGaN/GaN heterostructures and in doped bulk GaN. Calculations and experimental data show that the polar optical scattering and ionized impurity scattering are the two dominant scattering mechanisms in bulk GaN for temperatures between 77 and 500K. In the 2DEG in AIGaN/GaN heterostructures, the piezoelectric scattering also plays an important role. Even for doped GaN, with a significant concentration of ionized impurities, a large volume electron concentration in the 2DEG significantly enhances the electron mobility, and the mobility values close to 1700 cm²/Vs may be obtained in the GaN 2DEG at room temperature. The maximum measured Hall mobility at 80K is nearly 5000 cm²/Vs compared to approximately 1200 cm²/Vs in a bulk GaN layer. With a change in temperature from 300 to 80K, the 2DEG in our samples changes from nondegenerate and weakly degenerate to degenerate. Therefore, in order to interpret the experimental data, we propose a new interpolation formula for low field mobility limited by the ionized impurity scattering. This formula is valid for an arbitrary degree of the electron gas degeneracy. Based on our theory, we show that the mobility enhancement in the 2DEG is related to a much higher volume electron concentration in the 2DEG, and, hence, to a more effective screening.     

AlGaN/GaN HEMT 2DEG电致耦合模型的研究

传统计算AlGaN/GaN异质结二维电子气(2DEG)的方法是根据Hooke定律计算c轴方向压应变与拉应变,然后利用压电模量计算出不依赖于栅压的2DEG,称为非耦合模型计算.提出了一种电致耦合模型来计算2DEG,在计算过程中考虑到弛豫度与附加电场对材料压电效应的影响,结果发现,当Al组分x=0.30时,压电极化电荷密度低于传统方法的计算值,两种模型的计算值相差7.17%,由此可见,当电场作用于材料时,材料产生逆压电效应,最终导致压电极化电荷密度降低.     

High-temperature transport properties of 2DEG in AlGaN/GaN heterostructures

Transport properties of the two-dimensional electron gas (2DEG) in fully strained and partially strain-relaxed Al_0.22Ga_0.78N/GaN heterostructures at temperatures from 300 to 680 K have been investigated by Hall effect measurements. The 2DEG mobility was found to decrease rapidly with increasing temperature at the initial stage and then decrease slowly as temperature is further increased. Those features indicate strongly that the 2DEG mobility is primarily limited by LO phonon scattering processes at high temperatures. Meanwhile, the calculated results show that more electrons transfer to the higher-order sub-bands with increasing temperature, and hence the effect of screening on LO phonon scattering is weakened and the alloy scattering of the AlGaN layer on the 2DEG becomes stronger. Thus variation of 2DEG occupation in different sub-bands with increasing temperature also decreases mobility of the 2DEG.     

Study of internal electric fields in AlGaAs/GaAs two-dimensional electron gas heterostructures

Modulation-doped GaAs/AlGaAs heterostructures have been studied by photoreflectance spectroscopy. The spectra at room temperature show Franz–Keldysh oscillations associated to the substrate–buffer layer interface. The built-in electric field magnitude calculated from these oscillations is related with the two-dimensional electron gas (2DEG) mobility. In addition we observed two signals associated to the GaAs capping layer and to the 2DEG, respectively.     

InAlN/GaN HEMTs: a first insight into technological optimization

High-electron mobility transistors (HEMTs) were fabricated from heterostructures consisting of undoped In/sub 0.2/Al/sub 0.8/N barrier and GaN channel layers grown by metal-organic vapor phase epitaxy on (0001) sapphire substrates. The polarization-induced two-dimensional electron gas (2DEG) density and mobility at the In/sub 0.2/Al/sub 0.8/N/GaN heterojunction were 2/spl times/10/sup 13/ cm/sup -2/ and 260 cm/sup 2/V/sup -1/s/sup -1/, respectively. A tradeoff was determined for the annealing temperature of Ti/Al/Ni/Au ohmic contacts in order to achieve a low contact resistance (/spl rho//sub C/=2.4/spl times/10/sup -5/ /spl Omega//spl middot/cm/sup 2/) without degradation of the channels sheet resistance. Schottky barrier heights were 0.63 and 0.84 eV for Ni- and Pt-based contacts, respectively. The obtained dc parameters of 1-/spl mu/m gate-length HEMT were 0.64 A/mm drain current at V/sub GS/=3 V and 122 mS/mm transconductance, respectively. An HEMT analytical model was used to identify the effects of various material and device parameters on the InAlN/GaN HEMT performance. It is concluded that the increase in the channel mobility is urgently needed in order to benefit from the high 2DEG density.     

Mobility spectrum analysis of anisotropic electron transport in N-polar GaN/AlGaN heterostructures on vicinal sapphire substrates

In this work, we present results of a study of anisotropic two-dimensional electron gas (2DEG) transport in N-polar GaN/AlGaN heterostructures grown on slightly mis-oriented sapphire substrates. High-resolution mobility spectrum analysis of magnetic-field dependent Hall-effect and resistivity indicate an isotropic 2DEG sheet carrier density, yet significant anisotropy was observed in carrier mobility. A single electron species with a narrow mobility distribution was found to be responsible for conduction parallel to the multi-atomic steps resulting from growth on the vicinal substrates; whilst in the perpendicular direction two distinct electrons peaks are evident at T ? 150 K, which merge near room temperature. The linewidth of the mobility distributions for transport in the perpendicular direction was found to be significantly broader than that of the single electron in the parallel direction. The broader mobility distribution and the lower average mobility for the 2DEG in the perpendicular direction are attributable to interface roughness scattering associated with the GaN/AlGaN interfacial steps.     

Influence of annealed ohmic contact metals on electron mobility of strained AlGaN/GaN heterostructures

The influence of annealed ohmic contact metals on the electron mobility of a two dimensional electron gas (2DEG) is investigated on ungated AlGaN/GaN heterostructures and AlGaN/GaN heterostructure field effect transistors (AlGaN/GaN HFETs). Current-voltage (I-V) characteristics for ungated AlGaN/GaN heterostructures and capacitance-voltage (C-V) characteristics for AlGaN/GaN HFETs are obtained, and the electron mobility for the ungated AlGaN/GaN heterostructure is calculated. It is found that the electron mobility of the 2DEG for the ungated AlGaN/GaN heterostructure is decreased by more than 50% compared with the electron mobility of Hall measurements. We propose that defects are introduced into the AlGaN barrier layer and the strain of the AlGaN barrier layer is changed during the annealing process of the source and drain, causing the decrease in the electron mobility.     

Influence of RIE-induced damage on luminescence and electron transport properties of AlGaAs-GaAs heterostructures

Reactive ion etching induced damage was systematically studied by photoluminescence (PL), cathodoluminescence (CL) and electronic microwave absorption in GaAs/AlGaAs multiple quantum well (MQW) and two-dimensional electron gas (2DEG) heterostructures. Using QW’s of differing widths at various depths, PL and CL characterization of the individual quantum wells allowed a depth sensitive detection of RIE induced damage. Etching was done with CC1₂F₂ at constant pressure and exposure time, while the bias voltage was successively increased from 55 to 320 V. A remarkable degradation in PL-intensity was observed for the topmost 1 nm QW located 30 nm beneath the surface, even at the lowest etch bias voltage. In 2DEG heterostructure samples investigated electrically, both mobility and carrier concentration of the 2DEG were seen to be strongly reduced. After illumination however, the initial values were almost completely restored, indicating that RIE damage predominantly reduces the electron supply efficiency of the AlGaAs barrier, whereas the 2DEG channel itself is not severely degraded even at the highest etch bias voltage.     

GaAs基InAs/AlSb二维电子气结构的生长优化

采用分子束外延设备 (MBE) , 外延生长了InAs/AlSb二维电子气结构样品.样品制备过程中, 通过优化AlGaSb缓冲层厚度和InAs/AlSb界面厚度、改变AlSb隔离层厚度, 分别对比了材料二维电子气特性的变化, 并在隔离层厚度为5nm时, 获得了室温电子迁移率为20500cm2/V·s, 面电荷密度为2.0×1012/cm2的InAs/AlSb二维电子气结构样品, 为InAs/AlSb高电子迁移率晶体管的研究和制备提供了参考依据.     

AlGaN/GaN HFET的2DEG和电流崩塌研究(Ⅰ)

从不同的视角回顾和研究了A1GaN/GaN HFET的二维电子气(2DEG)和电流崩塌问题.阐述了非掺杂的AIGaN/GaN异质结界面存在2DEG的原动力是极化效应,电子来源是AlGaN上的施主表面态.2DEG浓度与AlGaN/GaN界面导带不连续性、AlGaN层厚和Al组分有密切关系.揭示了AlGaN/GaN HFET的2DEG电荷涨落受控于表面、界面和缓冲层中的各种缺陷及外加应力,表面空穴陷阱形成的虚栅对输入信号有旁路和延迟作用,它们导致高频及微波状态下的电流崩塌.指出由于构成电流崩塌因素的复杂性,各种不同的抑制电流崩塌方法都存在不足,因此实现该器件大功率密度和高可靠性还有很长的路要走.     

Influence of Ni Schottky contact thickness on two-dimensional electron-gas sheet carrier concentration of strained AI0.3Ga0.7N/GaN heterostructures

Ni/Au Schottky contacts with thicknesses of either 50(A)/50(A) or 600 (A)/2000(A) were deposited on strained Al0.3Ga0.7N/GaN heterostructures.Using the measured C-V curves and Ⅰ-Ⅴ characteristics at room temperature,the calculated density of the two-dimensional electron-gas (2DEG) of the 600(A)/2000(A) thick Ni/Au Schottky contact is about 9.13 x 1012 cm-2 and that of the 50(A)/50(A) thick Ni/Au Schottky contact is only about 4.77 ×1012 cm-2.The saturated current increases from 60.88 to 86.34 mA at a bias of 20 V as the thickness of the Ni/Au Schottky contact increases from 50(A)/50(A)to 600(A)/2000(A).By self-consistently solving Schrodinger's and Poisson's equations,the polarization charge sheet density of the two samples was calculated,and the calculated results show that the polarization in the AIGaN barrier layer for the thick Ni/Au Schottky contact is stronger than the thin one.Thus,we attribute the results to the increascd biaxial tensile stress in the Al0.3Ga0.7N barrier layer induced by the 600(A)/2000(A)thick Ni/Au Schottky contact.