Low-resistance and thermally stable Pd/Ru ohmic contacts to p-type GaN

We report on low-resistance and thermally stable Pd/Ru ohmic contacts to surface-treated p-GaN (3 × 10¹⁷ cm⁻³). It is shown that annealing at 500°C for 2 min in a N₂ ambient improves ohmic contact properties. Specific contact resistance is measured to be 9.2(±0.2) × 10⁻⁴ and 2.4(±0.2) × 10⁻⁵ Ωcm² for the as-deposited and annealed samples, respectively. Atomic force microscopy results show that the surfaces of both the contacts are remarkably smooth with a root-mean-square (rms) roughness of about 0.6 nm. The current-voltage-temperature (I-V-T) and calculation results indicate that, for the as-deposited contact, thermionic field emission is dominant, while for the annealed contact, field emission dominates the current flow.     

Effects of annealing in an oxygen ambient on electrical properties of ohmic contacts to p-type GaN

Effects of annealing ambient of an oxygen and nitrogen mixed gas on the electrical properties were studied for Au-based ohmic contacts (NiAu, CoAu, CuAu, PdAu, and PtAu) to p-type GaN. Addition of oxygen to the nitrogen gas reduced the specific contact resistances (ρ_c) and resitivities of the p-GaN epilayers (ρ_s) of the contacts after annealing at temperatures of 500–600°C. The microstructural analysis at the p-GaN/metal interfaces did not detect the heterostructural intermediate semiconductor layer at the GaN/metal interfaces. The reason for reduction of both the ρc and ρ_s values by the oxygen gas addition was believed to be due to formation of the p-GaN epilayer with high hole concentrations, caused by removal of hydrogen atoms which bonded with Mg atoms.     

Electrical characteristics of thermally stable Ru and Ru/Au ohmic contacts to surface-treated p-type GaN

The electrical and thermal properties of Ru and Ru/Au ohmic contacts on two-step-surface-treated p-GaN have been investigated using current-voltage (I–V) measurements and Auger electron spectroscopy. It is shown that annealing at 700°C for 2 min in a flowing N₂ atmosphere improves the I–V characteristics of the contacts. For example, the annealed Ru and Ru/Au schemes produce a specific contact resistance of 3.4 (±0.9)×10⁻³ and 1.2 (±1.1)×10⁻³ Ωcm², respectively. It is also shown that annealing results in a large reduction (by ∼100 meV) in the Schottky barrier heights of the Ru and Ru/Au contacts, compared to the as-deposited ones. The electrical properties of the two-step-surface-treated Ru/Au contacts are compared with those of the conventionally treated contacts.     

Formation of ohmic contacts to p-ZnTe

Formation and temperature stability of sputter deposited gold ohmic contacts to molecular beam epitaxially grown p-type ZnTe (doped with nitrogen to a free hole concentration of ≈3 × 1018 cm³) have been studied using current-voltage (I-V), Auger electron spectroscopy, secondary ion mass spectrometry, and optical and scanning electron microscopy. The I-V characteristics of ≈1500Å Au/p-ZnTe contacts were measured as-deposited and after heat treatments at 150, 200, 250, and 350°C for 15 min intervals up to 90 min. As deposited, the contacts were poor Schottky contacts, but became ohmic after 15 min at all temperatures. There was an increased resistance at t>15 min for T≤250°C, and a very large resistance increase upon heat treatment for all times at 350°C. The interface between the metallization and ZnTe was initially very planar, and remained planar upon formation of the ohmic contact. Upon heating at T>250°C, Au diffused into ZnTe. The ohmic behavior of the Au/p-ZnTe contacts is attributed to this diffusion which created a highly doped near-surface region in the ZnTe. Microscopy showed that Au also migrated across the ZnTe surface forming an extended reaction zone (≈100 μm) around the dot contact at T≥250°C.     

A comparison between different ohmic contacts for ZnO thin films

There are several metals that form ohmic contacts for ZnO thin films, such as copper, aluminum and silver. The aim of this work is to make a comparison between these ohmic contacts. To achieve this purpose,polycrystalline ZnO thin films were prepared by the spray pyrolysis technique, and characterized by the I–V measurements at room temperature. Two strips of each metal were thermally evaporated on the surface of the film and measurements were first recorded in the dark and room light, then in the dark before and after annealing for Al,which was found to be the best in the set. Films with aluminum contacts gave the smallest resistivity, best ohmicity and they are slightly affected by light as required. On the other hand, copper was found to be the worst, and films with copper contacts gave the largest resistivity, worst ohmicity and they are the most affected by light. Annealing improved the aluminum contacts due to alloying and doping.     

磷注入4H-SiC（0001）的欧姆接触特性

摘要: 研究了不同剂量的磷注入4H–SiC层经高温退火处理后的电学特性。通过在1650 ?C 退火30分钟激活了注入的磷离子。TLM及Hall法测试的结果表明,尽管随着磷注入浓度的增加而产生的位错环使得Ni/SiC界面处势垒增高,仍形成了良好的镍欧姆接触,表面接触电阻率为1.30?10-6Ω.cm2。磷注入层电阻率随着磷掺杂浓度的升高而减小。通过对不同注入剂量下电子浓度随温度的变化曲线进行测试,结果表明在200–500K的温度范围内,磷注入层的电子浓度与温度基本无关。     

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.     

A study of the electrical characteristics of various metals on p-type GaN for ohmic contacts

We study the electrical characteristics (current vs voltage, I/V) of Co, In, Mg, Mn, Ni, and Zn each with an Au overlayer to determine their usefulness as ohmic contact metals to p-type GaN. For all the metals, none of the I/V relationships are completely linear even after annealing. At a fixed voltage of 3V Co, In, Ni, and Zn have comparable current levels, whereas Mg and Mn are almost an order of magnitude less. Due to the various mechanisms by which the metals may form ohmic contacts, we further examine the metals in multilayer combinations in an attempt to reduce contact resistance. Three p-type GaN wafers with carrier concentrations of 1.2 × 10¹⁷, 1.5 × 10¹⁷ and 4.7 × 10¹⁷ cm⁻³ are used with Ni/Au metallizations as a common standard for comparison. The lowest average specific contact resistance obtained in this study is with Co/Au at 0.0081 ohm-cm². In addition to comparing magnitudes of contact resistances, thermal aging studies of the metal contacts are performed from 300 to 700°C for 6 h periods to determine its effect on their electrical stability. In this test, Ni/Au is found to be the most electrically invariant with thermal aging prior to failure. However, the temperature at which it fails occurred sooner than that for many of the other metallizations examined (e.g., Co/Au, In/Au, and Zn/Au). The temperature for failure is arbitrarily defined to be the temperature that the contact resistance degrades to twice its pre-thermal-aging contact resistance.     

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.     

Interface annealing characterization of Ti/Al/Au ohmic contacts to p-type 4H-SiC

Ti/Al/Au ohmic contacts to p-type 4H-SiC in terms of a different annealing time and Ti composition are reported. At 1050℃, proper increase in annealing time plays a critical role in the Schottky to ohmic contact conversion. With the optimized annealing time, the contact with a high Ti content yields a lower specific contact resistivity(ρ_c) of 6.4×10^-5 Ω·cm² compared with the low-Ti contact. The annealed surface morphology and phase resultants were examined by scanning electron microscopy(SEM) and X-ray diffraction(XRD), respectively. For the better ohmic contact, element distribution and chemical states were qualitatively identified by X-ray photoelectron spectroscopy(XPS) depth analysis. In particular, the presence of C and a Si-related phase was discussed and associated with the change in the surface status of the as-grown epilayer of 4H-SiC during annealing. The results reveal that the out-diffused C and Si atoms, with an approximate atomic ratio of 1:1 in the contact layer, can combine to form an amorphous Si-C state. The polycrystalline graphite instead of an unreacted C cluster in the whole alloyed structure and an extra nanosize graphite flake on the outermost surface of the annealed contact were confirmed by Raman spectroscopy.     

Mechanisms for the formation of low temperature,non-alloyed Au-Ge ohmic contacts to n-GaAs

Low temperature, non-alloyed Au-Ge contact formation ton-GaAs is a multi-step pro-cess. During the first 5 min of annealing at 320° C the Au and Ge segregate into regions a few microns in size and extend over the entire thickness of the metal layer and sig-nificant in-diffusion of the Au and Ge and out-diffusion of the Ga and As occurs. This intermixing reduces the barrier height from 0.75 to 0.40 eV. The contact does not show ohmic behavior until it has been annealed for 3 hr. During this time Ge continues to in-diffuse but at a slower rate than it did initially. The rate of Ge in-diffusion is en-hanced by the presence of Au since samples containing less Au require longer anneals to show ohmic behavior and have higher specific contact resistances. The presence of excess As, which is prevented from evaporating by a Si₃N₄ cap has the opposite effect since capped layers have higher specific contact resistances. Au-Ge phases appear after approximately 3 hr of annealing, therefore, Au-Ge phases cannot be responsible for the reduction in barrier height. The interface morphology is smooth, differing from that of pure Au and alloyed contacts that often contain spiking of the metals into the semi-conductor. The orientation relationship for the Au grains differs from that of pure Au. Work performedat U.S. Army ETDL, Fort Monmouth, NJ 07703. Work performed at U.S.Army ETDL, Fort Monmouth, NJ 07703.     

Comparison between TiAl and TiAlNiAu ohmic contacts to n-type GaN

We report a comparison between Ti/Al and Ti/Al/Ni/Au ohmic contacts in terms of contact resistivity, thermal stability, depth profile, and surface morphology. The metals were deposited by conventional electron-beam evaporation, and then annealed at 900°C for 30 s in a N₂ atmosphere. The lowest value for the specific contact resistivity was obtained using Ti/Al/Ni/Au metallization. Ti/Al/Ni/Au contacts showed no increase in contact resistivity after aging for five days at 600°C in an air atmosphere. Examination of the surface morphology using atomic force microscopy revealed that the surface roughness was clearly better in the case of Ti/Al/Ni/Au contacts. X-ray photoelectron spectroscopy was also employed and gave primary results of Ti/Al/Ni/Au contact formation.     

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.     

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.     

Novel type of ohmic contacts to p-doped GaN using polarization fields in thin InxGa1−xN capping layers

Thin p-doped InGaN layers on p-doped GaN were successfully used to demonstrate a new type of low-resistance ohmic contact. A significant reduction of specific contact resistance can be achieved by increasing the free-hole concentration and the probability for hole tunneling through the Schottky barrier as a consequence of polarization-induced band bending. As obtained from the transmission-line method, the specific contact resistances of Ni (10 nm)/Au (30 nm) contacts deposited on InGaN capping layers were 1.2×10⁻² Ωcm² and 6×10⁻² Ωcm² for capping layer thicknesses of 20 nm and 2 nm, respectively.     

The effects of reactive ion etching-induced damage on the characteristics of ohmic contacts to n-Type GaN

The effects of reactive ion etching n-GaN surfaces with both SiCl₄ and Ar plasmas have been investigated using transmission line measurements. The measurements were made from ohmic contacts consisting of Al (as-deposited) and Ti/Al (as-deposited and rapid thermal annealed). The contact resistance, specific contact resistance, and sheet resistance were investigated as functions of the dc plasma self-bias voltage and etch time. The contact resistance extracted from contacts fabricated on surfaces etched with SiCl₄ was found to be improved over the unetched samples for all conditions investigated. Dry etching the surface with Ar severely degraded the contact resistance over the unetched sample except at the lower self-bias voltages. Rapid thermal annealing of etched samples prior to Al deposition was found to be effective in removing some of the reactive ion etching/SiCl₄-induced damage.     

Correlation between the electrical properties and the interfacial microstructures of TiAl-based ohmic contacts to p-type 4H-SiC

In order to understand a mechanism of TiAl-based ohmic contact formation for p-type 4H-SiC, the electrical properties and microstructures of Ti/Al and Ni/Ti/Al contacts, which provided the specific contact resistances of approximately 2×10⁻⁵ Ω-cm² and 7×10⁻⁵ Ω-cm² after annealing at 1000°C and 800°C, respectively, were investigated using x-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Ternary Ti₃SiC₂ carbide layers were observed to grow on the SiC surfaces in both the Ti/Al and the Ni/Ti/Al contacts when the contacts yielded low resistance. The Ti₃SiC₂ carbide layers with hexagonal structures had an epitaxial orientation relationship with the 4H-SiC substrates. The (0001)-oriented terraces were observed periodically at the interfaces between the carbide layers and the SiC, and the terraces were atomically flat. We believed the Ti₃SiC₂ carbide layers primarily reduced the high Schottky barrier height at the contact metal/p-SiC interface down to about 0.3 eV, and, thus, low contact resistances were obtained for p-type TiAl-based ohmic contacts.     

Effects of Si interlayer conditions on platinum ohmic contacts for p-type silicon carbide

A study of Pt ohmic contacts with Si interlayers on p-type SiC (7.0×10¹⁸ cm⁻³) was performed as a function of the Si interlayer thickness, deposition temperature, and dopant incorporation. All contacts were ohmic after annealing at 1100°C for 5 min in vacuum. The use of a Si layer was found to decrease the specific contact resistance (SCR) relative to Pt contacts that did not contain Si, regardless of the deposition conditions used in this study. The SCR values were reduced further by three independent effects: the deposition of the Si layer at 500°C, the incorporation of B in the layer, and the design of the Pt:Si layer thicknesses in a 1:1 atomic ratio. By combining all of these effects, the lowest average SCR values (2.89×10⁻⁴ Ω cm²) were obtained. After annealing for 5 min at 1100°C, x-ray diffraction of the contacts with the 1:1 Pt:Si ratio showed a single phase of PtSi. Analyses by cross-sectional transmission electron microscopy revealed no reaction of the films with the SiC substrate. The electrical characteristics of these contacts were stable after annealing at 400°C and 600°C for 96 h and 60 h, respectively. These results are in contrast to those observed for pure Pt contacts and for contacts containing a higher Pt:Si ratio.     

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.     

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。