An initial investigation of the microstructure of Ti/Pd/Au ohmic contact structures for gaas microwave devices applications

A preliminary investigation of both as-deposited and annealed titanium (75 nm), palladium (75 nm), gold (400 nm), ohmic contacts to thinp ⁺-GaAs layers, was carried out using a combination of transmission electron microscopy, energy dispersive x-ray analysis, secondary ion mass spectroscopy and electrical measurements. The annealed contacts showed limited interaction between the metallization and the semiconductor with a metal penetration depth of only 2 nm for a 4 minute anneal at 380° C. The contacts were found to remain layered after annealing. The layers consisted of a uniform upper layer of large a Au(Ga) grains, a central, non-uniform layer containing small Pd-rich grains and a lower uniform layer of almost pure Ti. Preliminary SIMS studies suggested Zn dopant outdiffusion from the epilayer into the metal layer and this may have important implications for the electrical properties of these contacts.     

Au/Ge/Ni ohmic contacts to n-Type InP

The relationship between the electrical properties and microstructure for annealed Au/Ge/Ni contacts to n-type InP, with an initial doping level of 10¹⁷ cm^-3, have been studied. Metal layers were deposited by electron beam evaporation in the following sequence: 25 nm Ni, 50 nm Ge, and 40 nm Au. Annealing was done in a nitrogen atmosphere at 250-400‡C. The onset of ohmic behavior at 325‡C corresponded to the decomposition of a ternary Ni-In-P phase at the InP surface and the subsequent formation of Ni₂P plus Au₁₀In₃, producing a lower barrier height at the InP interface. This reaction was driven by the inward diffusion of Au and outward diffusion of In. Further annealing, up to 400‡C, resulted in a decrease in contact resistance, which corresponded to the formation of NiP and Au₉ln₄ from Ni₂P and Au₁₀In₃,respectively, with some Ge doping of InP also likely. A minimum contact resistance of 10^-7 Ω-cm² was achieved with a 10 s anneal at 400‡C.     

Long-term thermal stability of Ti/Al/Mo/Au ohmic contacts on n-GaN

Improved performance of the ohmic contacts on n-GaN has been demonstrated with the use of MoAu as the capping layer on TiAl metallization. Contact resistance as low as 0.13 Θ-mm was achieved in these ohmic contacts when annealed at 850°C for 30 sec. We have studied the long-term thermal stability of these contacts at 500°C, 600°C, 750°C, and 850°C, respectively. The Ti/Al/Mo/Au metallization forms low contact-resistance ohmic contacts on n-GaN that are stable at 500°C and 600°C after 25 h of thermal treatment. The ohmic-contact performance degrades after 10 h of thermal treatment at 750°C, while the contacts exhibit nonlinear current-voltage (I-V) characteristics after 1 h of thermal treatment at 850°C with the formation of oxide on the surface of the contacts accompanied by surface discoloration. The intermetallic reactions taking place in the contacts during the long-term thermal treatments were studied using Auger electron spectroscopy (AES), and the surface morphology was characterized using atomic force microscopy (AFM).     

难熔金属与n-GaAs的欧姆接触特性

用磁控溅射系统和快速合金化法制备了Mo/W/Ti/Au多层金属和n-GaAs材料的欧姆接触,在溅射金属层之前分别用HCl溶液和(NH4)2S溶液对n-GaAs材料的表面进行处理．用传输线法对比接触电阻进行了测试,并利用俄歇电子能谱（AES）、X射线衍射图谱（XRD）对接触的微观结构进行了分析．结果表明,用(NH4)2S溶液对n-GaAs材料表面进行处理后,比接触电阻最小;在700℃快速合金化后获得最低的比接触电阻,约为4.5×10－6Ω·cm2．这是由于(NH4)2S溶液钝化处理后降低了GaAs的表面态密度,消除了费米能级钉扎效应,从而改善了难熔金属与GaAs的接触特性．     

Low resistance pd/ge ohmic contacts to epitaxially lifted-off n-type GaAs film

A low resistance PdGe nonalloyed ohmic contact has been successfully formed to epitaxially lifted-off n-type GaAs films. The contact is made by lifting off partially metallized n-type GaAs films using the epitaxial lift-off method and bonding them to metallized Si substrates by natural intermolecular Van Der Waals forces. Low temperature sintering (200°C) of this contact results in metallurgical bonding and formation of the ohmic contact. We have measured specific contact resistances of 5 × 10⁻⁵ Ω-cm₂ which is almost half the value obtained for pure Pd contacts. Germanium forms a degenerately doped heterojunction interfacial layer to GaAs. Our experimental results show that germanium diffuses to the interface and acts as a dopant layer to n-GaAs film surface. Therefore, for epitaxially lifted-off n-type GaAs films, PdGe is a low resistance ohmic metal contact to use.     

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.     

Microstructural analysis of the Ge/Pd(Zn) ohmic contact to p-InP

The interfacial microstructural analysis of the Ge/Pd(Zn) ohmic contact to p-InP, based on the solid-phase regrowth principle, is reported. Typical contact resistivities of low 10⁻⁴ to low 10⁻⁵ Ω-cm² can be obtained for this contact scheme annealed at temperatures higher than 400°C. Cross-sectional transmission electron microscopy, energy dispersive x-ray composition mapping, and con-verge beam electron diffraction were utilized in this study for the interfacial microstructure analysis. The solid-phase regrowth process has been confirmed in this contact system on InP. Precipitates of trapped materials during solid phase regrowth have also been observed. The correlation between the electrical and microstructural properties is addressed.     

The ohmic contact formation mechanism of Au/Pt/Pd ohmic contact to p-ZnTe

The ohmic contact formation mechanism and the role of Pt layer of Au(500Å) Pt(500Å)/Pd(100Å) ohmic contact to p-ZnTe were investigated. The specific contact resistance of Au/Pt/Pd contact depended strongly on the annealing temperature. As the annealing temperature increased, the specific contact resistance decreased and reached a minimum value of 6×10^?6 Θcm² at 200°C. From the Hall measurement, the hole concentration increased with the annealing temperature and reached a maximum value of 2.3×10¹⁹ cm^?3 at 300°C. The Schottky barrier height decreased with the increase of annealing temperature and reached a minimum value of 0.34 eV at 200°C and it was due to the interfacial reaction of Pd and ZnTe. Therefore, the decrease of contact resistance was due to the increase of doping concentration as well as the decrease of Schottky barrier height by the interfacial reaction of Pd ZnTe. The specific contact resistances of Au Pd, Au/Pt/Pd and Au/Mo/Pd as a function of annealing time was investigated to clarify the role of Pt layer.     

Microstructural analysis of a Au/Pt/Pd/Zn ohmic contact to an AlGaAs/GaAs heterojunction bipolar transistor

Gold-based ohmic contacts, incorporating Pt, Pd, and Zn layers, to AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have been characterized using transmission electron microscopy (TEM). The metallization was deposited onto a 30 nm graded emitter layer of n-type Al_xGa_1−xAs, which was on a 30 nm emitter layer of n-type Al_0.3Ga_0.7As, with the aim of contacting the underlying 80 nm thick graded base layer of p-type Al_xGa_1−xAs. Metal layers were deposited sequentially using electron beam evaporation and the resultant metallizations were annealed at temperatures ranging from 250-500°C for up to several minutes. A minimum contact resistance of ≈8.5 × 10⁻⁷ Ω-cm² was achieved, which corresponded to the decomposition of ternary phases at the metallization/semiconductor interface, to binary phases, i.e., PdGa and PtAs₂. Long term stability tests were done on the optimum contacts. Anneals at 270°C for up to four weeks in duration produced virtually no change in microstructure, with the exception of some outward diffusion of Ga and As.     

Effect of annealing temperature on Ti/Al/Ni/Au ohmic contacts on undoped AlN films

The Ti/Al/Ni/Au metals were deposited on undoped AlN films by electron beam evaporation. The influence of annealing temperature on the properties of contacts was investigated. When the annealing temperatures were between 800 and 950℃, the AlN-Ti/Al/Ni/Au contacts became ohmic contacts and the resistance decreased with the increase of annealing temperature. A lowest specific contacts resistance of 0.379 Ω·cm² was obtained for the sample annealed at 950℃. In this work, we confirmed that the formation mechanism of ohmic contacts on AlN was due to the formation of Al-Au, Au-Ti and Al-Ni alloys, and reduction of the specific contacts resistance could originate from the formation of Au₂Ti and AlAu₂ alloys. This result provided a possibility for the preparation of AlN-based high-frequency, high-power devices and deep ultraviolet devices.     

Low resistance Pd/Zn/Pd Au ohmic contacts to P-type gaas

Many optoelectronic devices require contacts top-doped epitaxial layers. To achieve low contact resistance, the semiconductor has to be doped to high levels. Thep-dopants most commonly used are Be, Mg, and Zn. The contacts were formed by the sequential e-beam evaporation of 10 nm Pd, ≤5 nm Zn, 20 nm Pd and 40 nm Au layers onto a 0.2 μm thick Be-doped (5 × 10¹⁸ cm) GaAs layer grown by MBE. The minimum contact resistance of 0.04Ω-mm (≤1 × 10⁻⁷ Ω-cm²), as measured using the transmission line method, was obtained for contacts annealed at 500° C for 30s. These are the lowest contact resistance values reported to date for alloyed contacts top-GaAs.     

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。     

NiGe-based ohmic contacts to n-type GaAs

Refractory NiGe ohmic contacts which have excellent thermal stability and smooth surface have been developed. To apply these contacts to the future very large scale integration GaAs devices, reduction of the contact resistance (R_c) of the NiGe contacts is mandatory. In the present paper, in order to obtain a guideline for the R_c reduction, the formation mechanism of the NiGe contacts was investigated. The NiGe contacts were found to have two different ohmic contact formation mechanisms. These mechanisms suggested that facilitation of heavy doping at the GaAs surface and/or in the Ge layer was very effective to reduce the R_c values of the NiGe contacts. Experimentally, the R_c reduction was demonstrated by adding a small amount of third elements. Direct doping elements (Sn, Sb, and Te) and indirect doping elements (Pd, Pt, Au, Ag, and Cu) were chosen as the third elements. In additon, the effect of addition of In, which forms alow barrier layer between metal and GaAs, was investigated. The contact resistance of these NiGe-based contacts were close to 0.3 Ω mm, and they provided smooth surface and shallow reaction depth. Finally, the NiGe-based contacts were compared with the conventional AuGeNi contact.     

Study on the failure temperature of Ti/Pt/Au and Pt5Si2-Ti/Pt/Au metallization systems

The Ti/Pt/Au metallization system has an advantage of resisting KOH or TMAH solution etching. To form a good ohmic contact, the Ti/Pt/Au metallization system must be alloyed at 400℃. However, the process temperatures of typical MEMS packaging technologies, such as anodic bonding, glass solder bonding and eutectic bonding, generally exceed 400℃. It is puzzling if the Ti/Pt/Au system is destroyed during the subsequent packaging process. In the present work, the resistance of doped polysilicon resistors contacted by the Ti/Pt/Au metallization system that have undergone different temperatures and time are measured. The experimental results show that the ohmic contacts will be destroyed if heated to 500℃. But if a 20 nm Pt film is sputtered on heavily doped polysilicon and alloyed at 700℃ before sputtering Ti/Pt/Au films, the Pt₅Si₂-Ti/Pt/Au metallization system has a higher service temperature of 500℃, which exceeds process temperatures of most typical MEMS packaging technologies.     

Low resistance, thermally stable Au/Pt/Ti/WSiN ohmic contacts on n+-InGaAs/n-GaAs layer systems

The electrical properties of the ohmic contact systems Au/Pt/Ti/WSiN and Au/Pt/Ti to n⁺-InGaAs/GaAs layers grown by metalorganic vapor phase epitaxy were investigated and compared to each other. The thermal stability properties of these contact systems were characterized by accelerated stress tests at elevated temperatures and by complementary thin film x-ray diffraction analysis to evaluate the microstructural properties of degraded and nondegraded structures. The goal of these efforts was to develop stable, homogeneous emitter contacts for power heterojunction bipolar transistors. It was found that for both contact systems the best (specific) contact resistance R_c (ρ _c) is about 0.05 Ωmm (2 × 10⁻⁷ Ωcm²) in the as-deposited state. Au/Pt/Ti/WSiN contacts show no degradation after aging at 400°C for more than 20 h. This is in contrast to standard Au/Pt/Ti contacts which significantly degrade even after short time annealing at 400°C. The good long-time stability of the Au/Pt/Ti/WSiN system is related to the advantageous properties of the reactively sputtered WSiN barrier layer.     

Microstructural analysis of NiInGe ohmic contacts for n-type GaAs

NiInGe ohmic contact materials, which are attractive to use in future GaAs devices, were previously developed in our laboratories. Although the NiInGe contacts provided low contact resistances of about 0.3 Ω-mm and excellent thermal stability, further reduction of the contact resistance (R_C) of the NiInGe contacts was mandatory to use these contacts in submicron devices. In this paper, the microstructural parameters, which influence the R_C values, were investigated by correlating the R_C values with the microstructure at the interface between the contact materials and the GaAs substrate. The R_C values of the NiInGe contacts were found to depend strongly on the volume fraction and the In concentration (x) of the In_xGa_1−xAs compound semiconductor layers, which were formed at the metal/GaAs interface. Both the volume fraction and the In concentration of the In_xGa_1−xAs layers were found to depend on the thickness of the In layer used in the NiInGe contact and the annealing temperature to form the ohmic contact. A R_C value of 0.18 Ω-mm was obtained for the Ni (18 nm)/In (13 nm)/Ge (30 nm) contact (where a slash “/” indicates the deposition sequence) after annealing at temperature of 650°C for 5 sec.     

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.     

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.     

Pd-Ge contact to n-GaAs with the TiW diffusion barrier

Pd-Ge based ohmic contact to n-GaAs with a TiW diffusion barrier was investigated. Electrical analysis as well as Auger electron spectroscopy and the scanning electron microscopy were used to study the contact after it was subjected to different furnace and rapid thermal annealing and different aging steps. All analyses show that TiW can act as a good barrier metal for the Au/Ge/Pd/n-GaAs contact system. A value of 1.45 × 10⁻⁶ Ω-cm² for the specific contact resistance was obtained for the Au/TiW/Ge/Pd/n-GaAs contact after it was rapid thermally annealed at 425°C for 90 s. It can withstand a thermal aging at 350°C for 40 h with its ρ_c increasing to 2.94 × 10⁻⁶Ω-cm² and for an aging at 410°C for 40 h with its ρ_c increasing to 1.38 × 10⁻⁵ Ω-cm².     

Thermally stable Ge/Cu/Ti ohmic contacts to n-type GaN

The performance of a novel Ge/Cu/Ti metallization scheme on n-type GaN has been investigated for obtaining thermally and electrically stable low-resistance ohmic contacts. Isochronal (2 min.) anneals in the 600–740°C temperature range and isothermal (690°C) anneals for 2–10 min. duration were performed in inert atmosphere. For the 690°C isothermal schedule, ohmic behavior was observed after annealing for 3 min. or longer with a lowest contact resistivity of 9.1 × 10⁻⁵ Ωcm² after the 10 min. anneal for a net donor doping concentration of 9.2 × 10¹⁷ cm^−Ω3. Mean roughness (R_a) for anneals at 690°C was almost constant at around 5 nm, up to an annealing duration of 10 min., which indicates a good thermal stability of the contact scheme.