多层金属-n型4H-SiC的欧姆接触

研究了热退火条件下Au/Ti/Ni-4H-SiC欧姆接触形成机制.通过950 ℃下的快速热退火形成的最低欧姆接触电阻为2.765×10-6 Ω·cm2.SIMS分析表明退火过程中NiSi化合物的形成会带来SiC内部多余C原子的溢出,并在接触面上与Ti形成间隙化合物TiC.这一过程造成接触表面存在由大量C空位形成的缺陷层从而增强了表面间接隧穿.通过界面能带结构图直观地解释了欧姆接触在热退火条件下的形成机制.     

SiC表面氢化研究

该文提出6H-SiC ( 0001)/SiO2间过渡层的概念和过渡层结构,通过分析过渡层与HF溶液的反应机理,建立湿化学处理中的SiC表面氢化模型。模型以氢钝化SiC表面悬挂键,降低SiC表面的界面态密度,消除了费米能级钉扎,获得理想的SiC表面。将此模型用于SiC/金属接触的SiC表面处理,在100℃以下制备了理想因子n=1.2~1.25的肖特基结和比接触电阻＝510-3 cm2~710-3 cm2的SiC欧姆接触,其优点在于不仅避免了欧姆接触800~1200℃的高温合金,而且改善了肖特基接触的电学特性。SiC表面模型与实验结果吻合较好。     

表面氢化对SiC/金属接触的作用机理

提出 Si C表面氢化模型。以氢饱和 Si C表面悬挂键 ,减少界面态 ,从而制备理想的金属 /半导体接触 ,并将此模型用于 Si C器件表面处理 ,其优点在于避免了欧姆接触 80 0～ 1 2 0 0°C的高温合金 ,且肖特基接触整流特性较好。在 1 0 0°C以下制备了比接触电阻 ρc=5～ 8× 1 0 - 3Ω·cm2的 Si C欧姆接触和理想因子 n=1 .2 5～ 1 .3的肖特基结。与欧姆接触采用 95 0°C高温合金制备的 Si C肖特基二极管比较表明 ,表面氢化处理不仅能避免高温合金 ,降低工艺难度 ,而且能改善器件的电学特性。     

Ti and Ti/Sb ohmic contacts on n-type 6H-SiC

Ohmic contacts consisting of Ti and Ti/Sb were prepared on silicon and carbon face of 6H-SiC with different doping densities. Ni contacts were used as reference. All structures were gradually annealed at different temperatures. Specific contact resistance was measured and morphology was monitored after each annealing. It was found that the same metallization has different properties on different 6H-SiC polar faces. Sb addition to Ti contact helped to reach lower values of specific contact resistance on Si-faces of substrates. On C-faces of substrates, pure titanium contacts were comparable or better than Ti/Sb. Annealing at 960 °C and 1065 °C caused contact morphology deterioration and surface spreading of Ti and Ti/Sb contacts. Ni contacts kept good morphology at all annealing temperatures and had the best values of specific contact resistance after annealing at 960 °C and 1065 °C. Using XPS profiling only small amount of free carbon was found in Ti-based contacts.     

GaN紫外探测器的Ti/Al接触的电学特性

在非故意掺杂的和掺Si的GaN薄膜上蒸镀Ti(24nm)/Al(nm)薄膜,氮气环境下400～800℃范围内进行退火。实验结果表明,在非故意掺杂的样品上,随退火温度的升高,肖特基势垒高度下降,理想因子升高,表面状况逐渐变差,600℃退火形成较低接触电阻的欧姆接触,比接触电阻率为3.03×10-4Ωcm2,而载流子浓度为5.88×1018cm-3的掺Si的样品未退火就形成欧姆接触,比接触电阻可达到4.03×10-4Ωcm2。     

Laser Annealed and Thermal Annealed Refractory Ohmic Contacts to GaAs

Ohmic contacts to n-type GaAs have been developed for high-temperature device applications up to 300°C. Refractory metallizations were used with epitaxial Ge layers to form the contacts TiW/Ge/GaAs, Ta/Ge/GaAs, Mo/Ge/GaAs, and Ni/Ge/GaAs. Contacts with high dose Si or Se ion implantation (1012 to 1014/cm2) of the Ge/GaAs interface were also investigated. The purpose of this work was to develop refractory ohmic contacts with low specific-contact resistance (~10-6 ?cm2 on 1 x 1017cm-3GaAs) which are free of imperfections, resulting in a uniform n+ doping layer. The contacts were fabricated on epitaxial GaAs layers (n = 2 x 1016 to 2 x 1017 cm-3) grown on n+ ( 2 x 1018 cm-3) or semi-insulating GaAs (at strates. Ohmic contact was formed by both thermal annealing ( at temperatures up to 700°C) and laser annealing (pulsed Ruby). Examination of the Ge/GaAs interface revealed Ge migration into GaAs to form an n+layer. Under optimum laser anneal conditions, the specific contact resistance was in the range 1-5 x 10-6 ?-cm2 (on 2 x 1017cm-3GaAs). Thermally annealed TiW/Ge had a contact resitivity of 1 x 10-6 ? cm2 on 1 x 1017 cm-3 GaAs under optimum anneal conditions. The contacts also showed improved thermal stability over conventional Ni/AuGe contacts at temperatures above 300°C.     

Electrical characterization of annealed Ti/TiN/Pt contacts onN-type 6H-SiC epilayer

We report results of the electrical characteristics of in vacuo deposited Ti/TiN/Pt contact metallization on n-type 6H-SiC epilayer as function of impurity concentration in the range of 3.3×10¹⁷ cm^-3 to 1.9×10¹⁹ cm^-3. The as-deposited contacts are rectifying, except for the highly doped sample. Only the lesser doped remains rectifying after samples are annealed at 1000°C between 0.5 and 1 min in argon. Bulk contact resistance ranging from factors of 10^-5 to 10^-4 Ω-cm² and Schottky barrier height in the range of 0.54-0.84 eV are obtained. Adhesion problems associated with metal deposition on pre-processed titanium is not observed, leading to excellent mechanical stability. Auger electron spectroscopy (AES) reveals the out diffusion of Ti-Si and agglomeration of Ti-C species at the epilayer surface. The contact resistance remains appreciably stable after treatment in air at 650°C for 65 h. The drop in SBH and the resulting stable contact resistance is proposed to be associated with the thermal activation of TiC diffusion barrier layer on the 6H-SiC epilayer during annealing     

High temperature stability of chromium boride ohmic contacts to p-type 6H-SiC

Ohmic contacts have been fabricated on p-type 6H-SiC using CrB₂. Two hundred nanometer thick films were sputter-deposited on substrates of doping concentration 1.3×10¹⁹ cm⁻³ in a system with a base pressure of 3×10⁻⁷ Torr. Specific contact resistances were measured using the linear transmission line method, and the physical properties of the contacts were examined using Rutherford backscattering spectrometry, x-ray photoelectron spectroscopy, and transmission electron microscopy. The as-deposited CrB₂ contacts exhibited rectifying characteristics and contained oxygen as a major contaminant. Ohmic behavior with linear current-voltage characteristics was observed following short anneals at 1100°C for 2 min at a pressure of 5×10⁻⁷ Torr. The oxygen in the CrB₂ films was removed by the annealing process, and the lowest value of the specific contact resistance (r_c) measured at room temperature was 8.2×10⁻⁵ Ω-cm². Longer anneals at 1100°C for 3.5 h and 1200°C for 2 h reduced the room temperature values of r to 1.4×10⁻⁵ Ω-cm². A thin reaction region has been identified at the CrB₂/SiC interface; however, the interface remains essentially stable. Thermal stressing at 300°C in vacuum for over 2200 h produced only a slight increase in the specific contact resistance. The low value of the specific contact resistance and the excellent high temperature stability of the CrB₂/SiC interface make this contact a candidate for high power/high temperature SiC device applications.     

Structural properties of alloyed Ti/Al/Ti/Au and Ti/Al/Mo/Au ohmic contacts to AlGaN/GaN

Two alloyed ohmic contact structures for AlGaN/GaN–Ti/Al/Ti/Au and Ti/Al/Mo/Au were studied. Both structures were optimized for minimum ohmic contact resistance. Structures grown on sapphire and SiC substrates were used to investigate structural properties of ohmic contacts to AlGaN/GaN. Ohmic contacts to AlGaN/GaN on SiC showed higher contact resistance values compared to contacts to AlGaN/GaN on sapphire. Ohmic contact metals were etched on samples after annealing. The alloyed interface was studied with backside illumination under an optical microscope. Alloyed inclusions associated with threading dislocations were observed on the surface. For the AlGaN/GaN on SiC sample the inclusion density was an order of magnitude lower than for the sample on sapphire. Conductive atomic force microscopy with carbon nanotube tip was used to investigate topography and conductivity profile of the surface after ohmic contact metal removal by etching.     

Thermal stability of Re Schottky contacts to 6H-SiC

The thermal stability of a 100-nm thick sputter-deposited Re film as contact to 6H-SiC was studied by backscattering spectrometry and by measurements of the forward current-voltage (I-V) characteristic. The initial Schottky barrier height of 0.71 eV and ideality factor of 1.6 change after 2 h of annealing in vacuum at 700/spl deg/C to 1.04 eV and 1.1, respectively. They remain stable after annealing for additional 2 h at that same temperature. The initial change is attributed to a recovery of sputter damage in the SiC. The observed stability of the Schottky barrier is attributed to the thermodynamic stability of Re with SiC, as confirmed by the unchanging backscattering depth profiles. After annealing at 900/spl deg/C, the Schottky barrier becomes unstable although no interaction between the Re film and the SiC substrate is detectable in the depth profiles.     

Investigation of Ti/Al and TiN/Al thin films as the stable ohmic contact for p-type 4H-SiC

Interfacial reactions, surface morphology, and current-voltage (I-V) characteristics of Ti/Al/4H-SiC and TiN/Al/4H-SiC were studied before and after high-temperature annealing. It was observed that surface smoothness of the samples was not significantly affected by the heat treatment at up to 900°C, in contrast to the case of Al/SiC. Transmission electron microscopy (TEM) observation of the Ti(TiN)/Al/SiC interface showed that Al layer reacted with the SiC substrate at 900°C and formed an Al-Si-(Ti)-C compound at the metal/SiC interface, which is similar to the case of the Al/SiC interface. The I-V measurement showed reasonable ohmic properties for the Ti/Al films, indicating that the films can be used to stabilize the Al/SiC contact by protecting the Al layer from the potential oxidation and evaporation problem, while maintaining proper contact properties.     

Interface observation in Au/Ni/p-GaN studied by HREM and energy-filtering TEM

The contact resistance of Au/Ni/p-GaN ohmic contacts for different annealing conditions was measured. This was then correlated with microstructure, including phase distribution, observed by high-resolution electron microscopy combined with energy-filtering imaging. A contact resistance of 2.22 x 10(-4) ohms cm2 for Au/Ni contacts to p-GaN after annealing at 500 degrees C for 5 min in air ambient was obtained. NiO layers were identified at the interface and upper area of annealed Ni/Au/p-GaN for air ambient. In addition, an Au layer was found at the interface of p-GaN due to a reversal reaction during annealing. Identification of the observed phases is discussed, along with possible formation mechanisms for the ohmic contacts in the Au/Ni/p-GaN system.     

Ohmic contacts to n-type GaN using Pd/Al metallization

Ohmic contacts to n-type GaN grown by metalorganic chemical vapor deposition were formed using a Pd/Al-based metallization. Ohmic contact resistances and specific contact resistances were investigated as a function of rapid thermal annealing temperature, Pd interlayer thickness, and annealing time. As-deposited Pd/AI was found to produce rectifying contacts while the metallization exhibited ohmic characteristics after annealing at temperatures as low as 400°C. A minimum contact resistance of 0.9 ohm-mm (specific contact resistance = 1.2 × 10⁻⁵ ohm-cm²) was obtained upon annealing at 650°C for 30 s. For comparison, Al and Ti/Al contacts were also investigated. Auger electron spectroscopy, secondary ion mass spectrometry, and x-ray diffraction were used to investigate metallurgical reactions.     

Study of the Formation Mechanism of Cu/Ge/Pd Ohmic Contact to <Emphasis Type="Italic">n</Emphasis>-Type InGaAs

This study investigates electrical characteristics and the formation mechanism of the Cu/Ge/Pd Ohmic contact to n-type InGaAs. After annealing the contact at 250°C for 20 min, Cu₃Ge and Pd₁₂Ga₅As₂ compounds formed and Ge diffused into the InGaAs layer, achieving a heavily doped InGaAs layer with a low contact resistivity of 1 × 10⁻⁶ Ω cm². Thermal stability tests were performed on the Cu/Ge/Pd Ohmic contact to InGaAs after Ohmic contact formation, showing no obvious degradation after a 72 h reliability test at 250°C. The results indicate excellent electrical characteristics and thermal stability using Cu/Ge/Pd as an Ohmic contact metal to an n-InGaAs layer.     

Ti/Al/Ni/Au金属体系在N-polar GaN上的欧姆接触特性

利用金属有机化合物化学气相淀积(MOCVD)在SiC衬底上外延生长了N-polar GaN材料,采用传输线模型(TLM)分析了Ti/Al/Ni/Au金属体系在N-polar GaN上的欧姆接触特性.结果表明,Ti/Al/Ni/Au (20/60/10/50 nm)在N-polar GaN上可形成比接触电阻率为2.2×10-3Ω·cm2的非合金欧姆接触,当退火温度升至200℃,比接触电阻率降为1.44×10-3 Ω·cm2,随着退火温度的进一步上升,Ga原子外逸导致欧姆接触退化为肖特基接触.     

SiO2/4H-SiC界面氮化退火

通过1 300℃高温干氧热氧化法在n型4H-SiC外延片上生长了厚度为60 nm的SiO2栅氧化层.为了开发适合于生长低界面态密度和高沟道载流子迁移率的SiC MOSFET器件产品的栅极氧化层退火条件,研究了不同退火条件下的SiO2/SiC界面电学特性参数.制作了MOS电容和横向MOSFET器件,通过表征SiO2栅氧化层C-V特性和MOSFET器件I-V特性,提取平带电压、C-V磁滞电压、SiO2/SiC界面态密度和载流子沟道迁移率等电学参数.实验结果表明,干氧氧化形成SiO2栅氧化层后,在1 300℃通入N2退火30 min,随后在相同温度下进行NO退火120 min,为最佳栅极氧化层退火条件,此时,SiO2/SiC界面态密度能够降低至2.07×1012 cm-2·eV-1@0.2 eV,SiC MOSFET沟道载流子迁移率达到17 cm2·V-1·s-1.     

高能量钒注入n型4H-SiC的欧姆接触形成

借助二次离子质谱法分析了注入的钒离子在碳化硅中的分布.即使经过1650℃的高温退火,钒在碳化硅中的再扩散也不显著.退火并没有导致明显的钒向碳化硅表面扩散形成堆积的现象,由于缺少钒的补偿作用,表面薄层的自由载流子浓度保持不变.采用线性传输线模型测量了钒注入n型4H-SiC上的Ni基接触电阻,在1050℃下,在氮、氢混合气体中退火10min,形成的最低比接触电阻为4.4×10-3Ω·cm2.金属化退火后的XRD分析结果表明,镍、碳化硅界面处形成了Ni2Si和石墨相.观测到的石墨相是由于退火导致C原子外扩散并堆积形成,同时在碳化硅表面形成C空位.C空位可以提高有效载流子浓度,降低势垒高度并减小耗尽层宽度,对最终形成欧姆接触起到了关键作用.     

Ir Diffusion Barriers in Ni/Au Ohmic Contacts to <Emphasis Type="Italic">p</Emphasis>-Type CuCrO<Subscript>2</Subscript>

The use of Ir diffusion barriers in Ni/Au-based Ohmic contacts to p-type CuCrO₂ layers was investigated. A specific contact resistance of ~5 × 10⁻⁴ Ω cm² was achieved after annealing at 500°C for the Ir-containing contacts, and the contacts were rectifying for lower anneal temperatures. In this case, the contact resistance was basically independent of the measurement temperature, indicating that tunneling is the dominant transport mechanism in the contacts. The morphology for the Ir-containing contacts was still smooth at 500°C although Auger electron spectroscopy depth profiling showed that some of the nickel had diffused to the surface and had oxidized. Contacts annealed at 800°C showed that some copper and most of the nickel had diffused to the surface and oxidized. The presence of the Ir diffusion barrier does increase the thermal stability of the contacts by ∼200°C compared to conventional Ni/Au contacts. By contrast, the use of other materials such as TaN, ZrN, and W₂B₅ as the diffusion barrier led to poorer thermal stability, with the contact resistance increasing sharply above 400°C.     

Low resistance bilayer Nd/Al ohmic contacts on n-type GaN

A bilayer Nd/Al metallization structure has been deposited onto low pressure organometallic vapor phase epitaxy grown n-type GaN ( 1 × 10¹⁸ cm⁻³) by electron-beam evaporation. Ohmic metal contacts were patterned photolithographically for standard transmission line measurement, and then thermally annealed at temperatures ranging from 200 to 350°C and from 500 to 650°C using conventional thermal annealing (CTA) and rapid thermal annealing (RTA), respectively. The lowest values for the specify contact resistivity of 9.8 × 10⁻⁶ Ω−cm² and 8 × 10⁻⁶ Ω−cm² were obtained using Nd/Al metallization with CTA of 250°C for 5 min and RTA of 600°C for 30 s. Examination of the surface morphology using atomic force microscopy as a function of annealing temperature revealed that the surface roughness was strongly influenced by conventional thermal annealing, it was smooth in the temperature range from 550 to 650°C for rapid thermal annealing. Auger electron spectroscopy depth profiling was employed to investigate the metallurgy and interdiffusion of contact formation.     

Au/Pt/Ti/Ni ohmic contacts to p-ZnTe

Ohmic contacts of Au/Pd/Ti/Ni to p-ZnTe show a minimum specific contact resistance of 10^-6 Ωcm² for a p-type doping level of 3×10¹⁹ cm^-3 and at an annealing temperature of 300°C. The Ni and Ti layers are very effective in improving the electrical properties of these contact