Ti/WSi/Ni ohmic contact to n-type SiCN

Ti/WSi/Ni contact to n-type SiCN was investigated using the circular transmission line method. Current-voltage characteristics, X-ray diffraction and X-ray photoelectron spectroscopy were used to characterize the contacts before and after annealing. It is shown that the conducting behavior of the contacts is dependent on the annealing temperature. After annealing at 900 ℃ or above, ohmic contacts with specific contact resistivity were achieved. The 1000-℃-annealed contact exhibits the lowest specific contact of 3.07 ×10-5 Ω·cm2. The formation of ohmic contact with low specific contact resistivity was discussed.     

采用Ni/Ti/Ni多层金属在N型4H-SiC衬底改善粘附性和界面的欧姆接触

The Ni/Ti/Ni multilayer ohmic contact properties on a 4H-SiC substrate and improved adhesion with the Ti/Au overlayer have been investigated. The best specific contact resistivity of 3.16 × 10^-5 Ω.cm^2 was obtained at 1050 ℃. Compared with Ni/SiC ohmic contact, the adhesion between Ni/Ti/Ni/SiC and the Ti/Au overlayer was greatly improved and the physical mechanism under this behavior was analyzed by using Raman spectroscopy and X-ray energy dispersive spectroscopy （EDS） measurement. It is shown that a Ti-carbide and Ni-silicide compound exist at the surface and there is no graphitic carbon at the surface of the Ni/Ti/Ni structure by Raman spectroscopy, while a large amount of graphitic carbon appears at the surface of the Ni/SiC structure, which results in its bad adhesion. Moreover, the interface of the Ni/Ti/Ni/SiC is improved compared to the interface of Ni/SiC.     

The thermal stability of ohmic contact to n-type InGaAs layer

The thermal stability of ohmic contact to n-type InGaAs layer is investigated. When Ni/Ge/Au is used as the contact metal, the characteristics of the ohmic contact are degraded after thermal treatment. The specific contact resistance of (Ni/Ge/Au)-InGaAs ohmic contact after annealing at 450°C is about 15 times larger than that of as-deposited sample. This is due to the decomposition of InGaAs and the interdiffusion of Ga and Au. A new phase of Au₄ln appears after annealing at 300°C. While in the case of Ti/Pt/Au, Au does not penetrate into the InGaAs layer as revealed by secondary ion mass spectroscopy. The specific contact resistance of (Ti/Pt/Au)-InGaAs ohmic contact after annealing at 450°C is eight times larger than that of as-deposited sample. Therefore, the thermal stability of (Ti/Pt/Au)-InGaAs ohmic contact is better than that of (Ni/Ge/Au)InGaAs ohmic contact.     

Low-resistance Ti/Al ohmic contact on undoped ZnO

We report a low-resistance ohmic contact on undoped ZnO using a promising contact scheme of Ti/Al. Specific-contact resistivity, as low as 9.0 × 10^?7 ωcm², was obtained from the Ti (300 Å)/Al (3,000 Å) contact annealed at 300°C. It was found that TiO was produced, and the atomic ratio of Zn/O was dramatically increased after annealing at 300°C. This provides the evidence that a number of oxygen vacancies, acting as donors for electrons, were produced below the contact. This leads to the increase of electron concentration via the reduction of contact resistivity.     

Role of titanium in Ti/Ni ohmic contact on n-type 6H-SiC

A set of Ti/Ni metallizations with different thickness of the underlying titanium layer was prepared on 6H-SiC together with structures that contained only pure Ti and Ni. Samples were gradually annealed at 750-1150 °C. Structures Ti(2)/Ni(50) and Ti(100)/Ni(50) showed the lowest contact resistivity, 2 × 10⁻⁴ Ω cm² in both cases. For the Ti(2)/Ni(50) structure, low contact resistivity was reached most likely due to reduction of surface oxides on SiC by the thin titanium layer. In the Ti(100)/Ni(50) structure, the titanium layer prevents diffusion of nickel towards SiC and there is a layer containing mainly TiC at the interface with silicon carbide.     

Simultaneous formation of p- and n-type ohmic contacts to 4H-SiC using the ternary Ni/Ti/Al system

Fabrication procedures for silicon carbide power metal oxide semiconductor field effect transistors (MOSFETs) can be improved through simultaneous formation (i.e., same contact materials and one step annealing) of ohmic contacts on both the p-well and n-source regions. We have succeeded with the simultaneous formation of the ohmic contacts for p- and n-type SiC semiconductors by examining ternary Ni/Ti/Al materials with various compositions, where a slash symbol “/” indicates the deposition sequence starting with Ni. The Ni(20 nm)/Ti(50 nm)/Al(50 nm) combination provided specific contact resistances of 2 × 10⁻³ Ω-cm² and 2 × 10⁻⁴ Ω-cm² for p- and n-type SiC, respectively, after annealing at 800°C for 30 min, where the doping level of Al in the SiC substrate was 4.5 × 10¹⁸ cm⁻³ and the level of N was 1.0 × 10¹⁹ cm⁻³.     

关于石墨烯上Ge/Au/Ni/Au欧姆接触的对比研究

Superior graphene-metal contacts can improve the performance of graphene devices. We report on an experimental demonstration of Ge/Au/Ni/Au-based ohmic contact on graphene. The transfer length method （TLM） is adopted to measure the resistivity of graphene-metal contacts. We designed a process flow, which can avoid residual photoresist at the interface of metal and graphene. Additionally, rapid thermal annealing （RTA） at different temperatures as a post-processing method is studied to improve graphene-metal contact. The results reveal that the contact resistivity of graphene and Ge/Au/Ni/Au can reach 10^-5 Ω· cm^2 after RTA, and that 350 ℃ is optimum annealing temperature for the contact of graphene-Ge/Au/Ni/Au. This paper provides guidance for fabrication and applications of graphene devices.     

Microstructural and electrical investigation of Ni/Au ohmic contact on p-type GaN

Electrical properties of Ni/Au ohmic contacts on p-type GaN were interpreted with the change of microstructure observed under transmission electron microscopy. The contact resistivity was decreased from 1.3×10⁻² to 6.1×10⁻⁴ Ωcm² after annealing at 600°C. The reduction is due to the dissolution of Ga atoms into Au−Ni solid solution formed during annealing, via the generation of Ga vacancies. Thus, net concentration of holes increased below the contact, resulting in the reduction of contact resistivity. At 800°C, N atoms decomposed; reacted with Ni, and forming cubic Ni₄N. Consequently, N vacancies, acting as donors in GaN, were generated below the contact, leading to the increase of contact resistivity to 3.8×10⁻² Ωcm².     

Au/Pt/AuGe/Ni中Pt扩散阻挡层对n-GaAs欧姆接触的影响

The multi-layer metals of Ni/Au Ge/Pt/Au with a Pt diffusion barrier layer of ohmic contact to n-GaAs were studied. The surface morphology and ohmic contact resistivity of multi-layer metals were characterized, with and without the Pt diffusion barrier layer for comparison. The SEM and EDS measurements show the Pt diffusion barrier layer can block the interdiffusion of atoms in multi-layer metals, and improve the surface morphology.The TLM results show that the samples with a Pt diffusion barrier layer have uniform ohmic contact resistance,indicating that the Pt diffusion barrier layer can increase the repetition and uniformity of ohmic contact to n-GaAs,and improve the thermal stability and reliability of GaAs-based devices.     

Ta/Au ohmic contacts to n-type ZnO

Ta/Au ohmic contacts are fabricated on n-type ZnO (∼1 × 10¹⁷ cm⁻³) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). After growth and metallization, the samples are annealed at 300°C and 500°C for 30 sec in nitrogen ambient. The specific contact resistance is measured to be 3.2×10⁻⁴ Ωcm² for the as-deposited samples. It reduces to 5.4×10⁻⁶ Ωcm² after annealing at 300°C for 30 sec without significant surface morphology degradation. When the sample is annealed at 500°C for 30 sec, the specific contact resistance increases to 3.3 × 10⁻⁵ Ωcm². The layer structures no longer exist due to strong Au and Ta in-diffusion and O out-diffusion. The contact surface becomes rough and textured.     

金属Al与半导体Ge欧姆接触的制备和表征

基于圆形传输线模型,通过测试样品的比接 触电阻率和电流-电压(I-V)特性曲线,分析 对比了Al与Si基上外延生长的p型Ge、n型Ge和n型Si的接触特性。实验结果发现,由于金 属与Ge材料接触存在强烈的费米钉效应,导致金属与n型Ge接触有高的接触电阻,难实 现低的比接触电阻率;而Al与p型Ge在掺杂浓度为4.2×10¹⁸ cm－3时,并且经过退火,比接 触电阻率能达到4.0×10－7 Ω·cm²;Al与n型Ge和n型Si接 触电极相比,后者可形成良好的 欧姆接触,其比接触电阻率较n型Ge接触降低了1个量级,经合金化处理后的Al/n⁺Si接触 电阻率能达到5.21×10－5 Ω·cm²,达到了制作高性能Ge 光电器件的要求。     

Ni/Pd-based ohmic contacts to p-GaN through p-InGaN/p+-GaN contacting layers

Specific contact resistance \begin{document}$ {\rho }_{\mathrm{c}} $\end{document} to p-GaN was measured for various structures of Ni/Pd-based metals and thin (20–30 nm thick) p-InGaN/p⁺-GaN contacting layers. The effects of surface chemical treatment and annealing temperature were examined. The optimal annealing temperature was determined to be 550 °C, above which the sheet resistance of the samples degraded considerably, suggesting that undesirable alloying had occurred. Pd-containing metal showed ~35% lower \begin{document}$ {\rho }_{\mathrm{c}} $\end{document} compared to that of single Ni. Very thin (2–3.5 nm thick) p-InGaN contacting layers grown on 20–25 nm thick p⁺-GaN layers exhibited one to two orders of magnitude smaller values of \begin{document}$ {\rho }_{\mathrm{c}} $\end{document} compared to that of p⁺-GaN without p-InGaN. The current density dependence of \begin{document}$ {\rho }_{\mathrm{c}} $\end{document}, which is indicative of nonlinearity in current-voltage relation, was also examined. The lowest \begin{document}$ {\rho }_{\mathrm{c}} $\end{document} achieved through this study was 4.9 × 10^–5 Ω·cm² @ J = 3.4 kA/cm².     

Study of contact resistivity, mechanical integrity, and thermal stability of Ti/Al and Ta/Al ohmic contacts to n-type GaN

The annealing conditions and contact resistivities of Ta/Al ohmic contacts to n-type GaN are reported for the first time. The high temperature stability and mechanical integrity of Ti/Al and Ta/Al contacts have been investigated. Ta/Al (35 nm/115 nm) contacts to n-type GaN became ohmic after annealing for 3 min at 500°C or for 15 s at 600°C. A minimum contact resistivity of 5×10⁻⁶Ω cm² was measured after contacts were repatterned with an Al layer to reduce the effect of a high metal sheet resistance. Ti/Al and Ta/Al contacts encapsulated under vacuum in quartz tubes showed a significant increase in contact resistivity after aging for five days at 600°C. Cross section transmission electron microscopy micrographs and electrical measurements of aged samples indicate that the increased contact resistivity is primarily the result of degradation of the metal layers. Minimal reactions at the metal/GaN interface of aged samples were observed.     

p型GaN欧姆接触的研究进展

宽带隙的GaN作为半导体领域研究的热点之一,近年来发展得很快.p型GaN的欧姆接触问题一直阻碍高温大功率GaN基器件的研制.本文讨论了金属化方案的选择、表面预处理和合金化处理等几个问题,回顾了近年来p型GaN欧姆接触的研究进展.     

Reliability of AllnAs/lnGaAs/lnP HEMT with WSi ohmic contacts

We have investigated the degradation mechanism of Al_0.48In_0.52As/In_0.53Ga_0.47As/ InP high electron mobility transistors (HEMTs) using WSi ohmic electrodes. Cross-sectional transmission electron microscopy (TEM) observation and en-ergy dispersive x-ray (EDX) analysis reveal impurities diffusion of gate electrode (titanium: Ti) and fluorine (F) in the AlInAs layer after a high temperature (T_a = 170°C operating life test for 500 h. The decrease of drain current (I_ds) during life test shows linear dependence on square root of aging time. It suggests that the degradation is controlled by a diffusion mechanism. Hence, the estimated degradation mechanism of this device is related with decrease of carrier concentration in the epitaxial layer by these diffused impurities. On the other hand, TEM and EDX show no degradation of WSi/InGaAs interface after aging. Therefore, the WSi electrode for this type of HEMT demonstrates excellent high stability under the accelerated operating life test.     

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.     

AlGaN/GaN HEMT器件欧姆接触多步退火工艺的分析与优化

利用Ti/Al/Ni/Au的多步退火工艺能够在 AlGaN/GaN HEMT上制作出低接触电阻,表面形貌平整的欧姆接触。本文通过实验对多步退火工艺的机制进行了详细地分析并对多步退火工艺进行了优化。实验结果显示出调整不同的退火的时间和温度在接触电阻和表面形貌的优化中起到了至关重要的作用。取得高质量的欧姆接触的关键是通过调整退火的时间和温度以平衡金属与金属之间,金属与半导体之间的各种反应的速率。我们利用优化后多步退火工艺在未掺杂AlGaN/GaN结构上制作出了低接触电阻的欧姆接触,其比接触电阻率为3.22?10-7Ω.cm2。     

金-锗系统欧姆接触制备研究

对于 Ga As MESFET和以 Ga As或 In P为衬底的 PHEMT的欧姆接触制备 ,虽均采用 Au-Ge- Ni系统 ,但其合金条件却因材料特性各异而不同。     

Improved ohmic contact to n-type 4H and 6H-SiC using nichrome

Results are reported for ohmic contacts formed on n-type 4H and 6H-SiC using nichrome (80/20 weight percent Ni/Cr). The electrical characteristics of these NiCr contacts are similar to those of contacts formed on 6H-SiC using pure Ni (∼1×10⁻⁵Ω-cm² for moderately doped material), but the contacts exhibit significant improvement with regard to physical stability. Composite Au/NiCr contacts exhibit good stability during long-term anneals (∼2500 h) at 300°C without the requirement of a diffusion barrier layer between the NiCr ohmic contact layer and the Au cap layer. In addition, the use of NiCr results in success rates near 100% for direct wire bonding to the Au cap layers. Characterization of the contacts by Auger electron spectroscopy, Rutherford backscattering spectroscopy, and transmission electron microscopy provides an explanation for the observed behavior.     

Low-thermal-budget Au-free ohmic contact to an ultrathin barrier AlGaN/GaN heterostructure utilizing a micro-patterned ohmic recess

A pre-ohmic micro-patterned recess process,is utilized to fabricate Ti/Al/Ti/TiN ohmic contact to an ultrathin-barrier(UTB) AlGaN/GaN heterostructure,featuring a significantly reduced ohmic contact resistivity of 0.56 Ω.mm at an alloy temperat-ure of 550 ℃.The sheet resistances increase with the temperature following a power law with the index of +2.58,while the spe-cific contact resistivity decreases with the temperature.The contact mechanism can be well described by thermionic field emis-sion (TFE).The extracted Schottky barrier height and electron concentration are 0.31 eV and 5.52 × 1018 cm-3,which suggests an intimate contact between ohmic metal and the UTB-AlGaN as well as GaN buffer.A good correlation between ohmic trans-fer length and the micro-pattern size is revealed,though in-depth investigation is needed.A preliminary CMOS-process-compat-ible metal-insulator-semiconductor high-mobility transistor (MIS-HEMT) was fabricated with the proposed Au-free ohmic con-tact technique.