两步镀膜Ti/Al/Ti/Au的n型GaN欧姆接触研究

报道了一种可靠稳定且低接触电阻的n型GaN欧姆接触。首先在掺硅的n型GaN(3×1018cm-3)蒸镀Ti(30nm)/Al(500nm),然后在氮气环境530℃合金化3min,最后蒸镀Ti(100nm)/Au(1000nm)用于保护Al层不被氧化。该接触电极有良好的欧姆接触特性,比接触电阻率为8.8×10-5Ωcm2,表面平坦、稳定、易焊线,可应用于制作高性能的GaN器件.     

Au/Ti/W/Ti与n-GaAs欧姆接触的特性研究

用磁控溅射系统和快速合金化法制备了Au/Ti/W/Ti多层金属和n-GaAs材料的欧姆接触,用传输线法对其比接触电阻进行了测试,并利用俄歇电子能谱(AES)和X射线衍射图谱(XRD)对接触的微观结构进行了研究。结果表明该接触在700℃时比接触电阻为1.5×10~(-4)Ω·cm~2,快速合金化后呈现欧姆特性可能与接触界面处生成的TiAs相有关。     

Si基GaN上的欧姆接触

研究了Ｓｉ基ＧａＮ上的欧姆接触,对在不同的合金化条件下铝（Ａｌ）和钛铝铂金（Ｔｉ／Ａｌ／Ｐｔ／Ａｕ）接触在不同的合金下的性质作了详尽的分析。Ａｌ／ＧａＮ在４５０℃氮气气氛退火３ｍｉｎ取得最好的欧姆接触率７．５＊１０＾－３Ω．ｃｍ＾２,而Ｔｉ／Ａｌ／Ｐｔ／Ａｕ／ＧａＮ接触在６５０℃氮气气氛退火２０ｓ取得最好的欧姆接触８．４＊１０＾－５Ω．ｃｍ＾２,而且Ｔｉ／Ａｌ／Ｐｔ／Ａｕ／ＧａＮ接触有较好的热稳定     

N-polar GaN上的欧姆接触:材料制备、金属化方案与机理

氮极性(N-polar)GaN与镓极性(Ga-polar)GaN极性相反,且具有较高的表面化学活性,使其在光电子、微电子及传感器等领域逐渐受到关注。文章结合一些相关研究报道,综述了N-polar GaN上欧姆接触的研究进展。首先对N-polar GaN材料的制备进行了分析,随后对N-polar GaN的欧姆接触电极的金属化方案及欧姆接触机理等内容进行了综合讨论,以期为实际N-polar GaN欧姆接触研究提供一些参考。     

Ti/Al/Ni/Au与n型GaN的欧姆接触研究

通过电流 电压 (I V)特性和传输线方法 (TLM )测量研究在n型GaN上淀积Ti/Al/Ni/Au电极形成欧姆接触的机制。Ni/Au作为Ti/Al的覆盖层起了阻止Ti ,Al,Au的互扩散及抗接触层氧化的作用。在 4 0 0℃到 90 0℃范围内 ,Ti/Al/Ni/Au与n型GaN的接触电阻随温度升高先略有上升 ,到 5 0 0℃以后单调下降。而表面形貌却在合金温度高于6 0 0℃以后随温度升高逐步变差。通过两步合金法得到了n GaN上Ti/Al/Ni/Au形成的接触电阻低达 9.6 5× 1 0 - 7Ωcm2 。最后还对两步合金法形成n GaN欧姆接触的机制进行了讨论。     

SiC上Ti/Ni/Au多层金属的欧姆接触特性

采用Ti/Ni/Au多层金属在高掺杂n型4H-SiC外延层上制作了欧姆接触测试图形,通过传输线法(Transmission Line Method,TLM)测量得到的最小比接触电阻为1.4×10-5Ω·cm2,经500℃N2老化后接触电阻大约有一个数量级的增加并保持稳定.     

Ni/Au、Pd/Au与p-AlGaN材料的接触特性

在AlGaNpin型日盲紫外探测器结构中的p-AlGaN层上生长了Ni/Au和Pd/Au,并在600～850℃温度下进行快速热退火,测量其退火前后传输线模型中各金属接触间的电学性质。实验发现,Ni/Au与Pd/Au在p-AlGaN上表现出了不同的接触性能。为了更好的说明金属与p-AlGaN材料接触之间在退火后电流的变化,还测量了p-AlGaN材料裸片两点之间I-V曲线在退火前后的变化。实验表明,比起Ni/Au来,Pd/Au在p-AlGaN材料上制备欧姆接触具有一定的优势,并在文中进行了分析。     

GaN基器件中的欧姆接触

GaN材料以其优良的光电性质,已成为制造发光器件和高温大功率器件的最有前途的材料。欧姆接触是制备GaN基器件的关键技术之一。着重论述了在n-GaN和p-GaN上制备欧姆接触的研究现状。     

表面处理对n-GaN上无合金化Ti/Al欧姆接触的作用

本文研究了表面处理对n-GaN上无合金化的Ti/Al电极起的作用,比较了(NH4)2Sx和CH3CSNH2两种不同的表面处理方法.在用CH3CSHN2/NH4OH溶液处理过的样品上制作的无合金化的Ti/Al电极,可得到较低的(4.85～5.65)×10-4Ω·cm2的接触电阻率,而且材料的发光特性也有明显提高.     

Zn+注入对p-GaN/Ni/Au欧姆接触特性的影响

首次采用离子注入工艺研究金属电极和p-GaN的欧姆接触特性.Zn为Ⅱ族元素,可以提高p-GaN表面的栽流子浓度,对p-GaN/Ni/Au(5/10nm)界面处进行Zn+注入.经Zn+注入后的样品在空气氛围中快速热退火处理5min,以减少离子注入带来的晶格损伤.研究发现Zn+注入改善了p-GaN/Ni/Au的欧姆接触特性,接触电阻率ρc从10-1Ω·cm2数量级降低到10-3Ω·cm2数量级.研究了不同Zn+注入剂量(5×1015、1×1016、5×1016cm-2)对接触电阻率的影响,在注入剂量为1×1016cm-2、300℃下退火得到最优的接触电阻率为1.45×10-3Ω·cm2.用扫描电子显微镜观察了离子注入前后的表面形貌变化,探讨了接触电阻率改变的内在机制.     

Thermal stability of N-polar n-type Ohmic contact for GaN-based light emitting diode on Si substrate

Thermal stability of N-polar n-type Ohmic contact for GaN light emitting diode (LED) on Si substrate was investigated. Al/Ti/Au were deposited as the contacts on the N-polar n-type GaN with and without AlN buffer layer on the surface, respectively, and both contacts exhibited Ohmic behaviors. The samples with AlN showed excellent Ohmic contact thermal stability when annealed below 700 °C, while the samples without AlN experienced serious degradation on electrical properties after being annealed in the temperature range of 250-600 °C. After the process of aging at 30 mA (155 A/cm²) and room temperature for 1000 h, operating voltage increase less than 0.05 V for LEDs with AlN but more than 0.45 V for LEDs without AlN. Therefore, we conclude that the existence of AlN buffer layer is a key of forming high stable Ohmic contact for GaN-based vertical structure LED on Si substrate.     

Magnetic field effect on the escape rate in Nb/AlOx/Nb josephson junctions

We studied the escape rate of Nb/AlO_x/Nb Josephson junctions in zero and finite magnetic fields. The escape rate was determined from the distribution of the critical currents and the experimental data were fit to the theoretical model to determine effective temperatures, which govern the thermal activation over the energy barrier. The effective temperatures were found to depend strongly on the magnetic-field modulated critical current. We discuss a possible cause for the magnetic field dependence of the escape rate in terms of non-uniform current distribution in the junctions.     

Application of titanium oxide nanotube films containing gold nanoparticles for the electroanalytical determination of ascorbic acid

Au/TiO₂/Ti electrodes have been prepared by galvanic deposition of gold particles on TiO₂ nanotube substrates. Titanium oxide nanotubes are fabricated by anodizing titanium foil in a Dimethyl Sulfoxide electrolyte containing fluoride. The scanning electron microscopy results indicated that gold particles are homogeneously deposited on the surface of TiO₂ nanotubes. The TiO₂ layers consist of individual tubes of about 40-80 nm diameters. The electro-catalytic behavior of Au/TiO₂/Ti and flat gold electrodes for the ascorbic acid electro-oxidation was studied by cyclic voltammetry. The results showed that the flat gold electrode is not suitable for the oxidation of ascorbic acid. However, the Au/TiO₂/Ti electrodes are shown to possess catalytic activity toward the oxidation reaction. Catalytic oxidation peak current showed a linear dependence on the ascorbic acid concentration and a linear calibration curve is obtained in the concentration range of 1-5 mM of ascorbic acid. Also, determination of ascorbic acid in real samples was evaluated. The obtained results were found to be satisfactory. Finally the effects of interference on the detection of ascorbic acid were investigated.     

Characterization of sputter deposited Au/Ni/Al multilayers on Si substrates

Multilayered Au/Ni/Al thin film metallization deposited by DC sputtering on n⁺Si substrates has been investigated. AES depth profiling was performed to reveal the concentration depth profiles of the Au/Ni/Al multilayers before and after annealing at different temperatures in the range 623-723 K. It was found that Ni aluminide layers begin to form during heat treatments at temperatures above 623 K. In addition to this process, Ni was found to diffuse significantly through the Au layer and segregates at the surface, proportionally to the increased annealing temperature. Consequently, the Ni oxidation process was found to take place thus causing the degradation of electrical contact. On the other hand Ni diffuses faster as well toward the Si/Al interface. No contamination traces at interfaces were observed. Electrical measurements of the metallized diode forward characteristics showed minor influence of the metallization heat treatment on the series resistance. Degradations were observed only in the reverse characteristics if the annealing was performed above 723 K.     

Sputter deposited nanocrystalline Ni-25Al alloy thin films and Ni/Ni3Al multilayers

Thin films of nominal composition Ni-25at%Al have been sputter deposited from a target of the intermetallic compound Ni₃Al at different substrate deposition temperatures. The film deposited on an unheated substrate exhibited a strongly textured columnar growth morphology and consisted of a mixture of metastable phases. Nanoindentation studies carried out on this film exhibited a strong strain hardening tendency. In contrast, the film deposited at 200 °C exhibited a recrystallized non-textured microstructure consisting of grains of a partially ordered Ni₃Al phase. At higher deposition temperatures (∼400 °C), larger grains of the bulk equilibrium, long-range ordered, Ll₂ Ni₃Al phase were observed in the film. Unlike the film deposited on an unheated substrate, the films deposited at elevated temperatures did not exhibit any dependence of the hardness on the indentation depth and, consequently no strain hardening. The average hardness of the film deposited at 200 °C was higher than the one deposited at 400 °C. In addition to monolithic Ni-25Al thin films, multilayered Ni/Ni3Al thin films were also deposited. Multilayers deposited non-epitaxially on unheated substrates exhibited a strong {111} fiber texture while those deposited epitaxially on (001) NaCl exhibited a {001} texture. Free-standing multilayers of both types of preferred orientations as well as of different layer thicknesses were deformed in tension untill fracture. Interestingly, the {111} oriented multilayers failed primarily by a brittle fracture while the {001} multilayers exhibited features of ductile fracture.     

Towards All Electrical Spin Injection and Detection in GaAs in a Lateral Geometry

Herein we discuss our approach to realizing all electrical spin injection and detection in GaAs. We propose a lateral geometry, with two ferromagnetic electrodes crossing an n-doped GaAs channel. AlO _x tunnel barriers are to be used in order to overcome the impedance mismatch and different widths of the two electrodes ensure different coercive fields. We present a detailed theoretical analysis of the expected magnetoresistance. Differences in behavior between lateral and vertical devices, the influence of the applied bias (electric field), and opportunities offered by different measurement geometries were explored. The MBE grown wafer consisted of 100 nm Al_0.3Ga_0.7As, acting as confinement layer, 100 nm n-doped (4 × l0¹⁷ cm⁻³) GaAs, 3 nm n⁺⁺ GaAs (10²¹ cm⁻³), to suppress Schottky barrier formation, and 1.5 nm Al. The Al was oxidized naturally in order to obtain tunnel barriers. By making use of in-situ shadow masks, a 0.1 mm wide channel is defined by covering the rest of the sample by insulating SiO₂, followed by deposition of Ta bonding pads. Finally, 500 and 1000 nm wide CoFe electrodes crossing the GaAs channel are obtained by e-beam lithography and sputtering. We show that the I–V characteristics of the CoFe/AlO _x /GaAs interface are consistent with tunneling as the main injection mechanism. However, the resistance-area (5 × 10⁹ Ω μm²) of our barriers is too high compared to the GaAs conductance (50 Ω square resistance) leading to a strong suppression of magnetoresistance. Further experiments are in progress toward optimizing barrier and channel impedance matching.     

采用SHS/QP技术制备TiC-xNi金属陶瓷——I.燃烧合成过程研究

SHS/QP技术是一种新的材料制备方法。本文分析了组成、稀释剂等对TiC-xNi体系的SHS过程和结构的影响。TiC-20mol%Ni的SHS过程激活能为166kJ/mol,预示着其反应过程为溶解-析晶机理控制。采用CFQ法,分析了TiC-20mol%Ni在SHS过程中结构形成的机理