Use of Au/Te/Ni films for ohmic contact to GaAs

Ohmic contacts to n-type GaAs are usually fabricated by alloying AuGe/Ni films on GaAs. Ge acts as a donor to GaAs for fabrication of the ohmic contact. In an attempt to replace Ge, which is an amphoteric impurity, with a group VI element to improve on the ohmic contact resistivity, experiments were done with AuTe and AuTe/Ni contacts. A very low resistivity of ∼5 × 10^-7Ω.cm²was obtained by alloying 1700 Å of AuTe film with 300 Å of nickel on top at 510°C. This is the lowest contact resistivity obtained with any material other than AuGe/Ni on n-type GaAs.     

Nonalloyed ohmic contacts to n-type GaAs by pulsed ruby laser diffused tin from tin-silica film

Nonalloyed ohmic contacts were formed on n⁺diffused layers on GaAs. The n⁺layers were formed on semi-insulating substrates by depositing a layer of tin-silica film and irradiating by ruby laser alone without thermal diffusion. Vacuum-evaporated AuGe-Ni contacts display low specific contact resistance ≃1.8 × 10^-6Ω.cm², without alloying.     

Low-resistance ohmic contacts to p-type GaAs using Zn/Pd/Au metallization

We have fabricated the low resistance ohmic contacts to p-type GaAs. Specific contact resistances as low as 7 × 10^-7Ω.cm²have been obtained for contacts prepared by heat treating Zn/Pd/Au metallizations deposited on p-type epitaxial GaAs layers with an acceptor concentration of 1.5 × 10¹⁹cm^-3. These contacts are reproducible, simple to fabricate, exhibit excellent adhesion, and have a uniformly smooth surface morphology.     

An (Al,Ga)As/GaAs heterostructure bipolar transistor with nonalloyed graded-gap ohmic contacts to the base and emitter

Graded regions of n-(Ga,In)As and p-Ga(As,Sb) were incorporated side-by-side as emitter and base contacts, respectively, into an n-p-n (Al,Ga)As/GaAs heterostructure bipolar transistor (HBT). The process involved two separate molecular beam epitaxy (MBE) growths, leading to base contact regions that were self-aligned to the emitter mesas. The devices could be easily probed with pressure contacts even prior to any metallization, and excellent characteristics were obtained after final metallization. Contact resistivities of 5 × 10^-7and 3 × 10^-6Ω.cm²were measured for n- and p-type graded-gap ohmic contact structures, respectively.     

Al—Ge ohmic contacts to n-type GaAs

Ohmic contacts have been fabricated to n-type GaAs with an alloy of AL-Ge which has an eutectic temperature of 424°C with 53- weight percent Germanium. The lowest contact resistance of 1.4 × 10^-6Ω.cm²was measured with a transfer length transmission line structure. The application to GaAs integrated-circuit design is demonstrated.     

Improved uniformity of contact resistance in GaAs MESFET usingPd/Ge/Ti/Au ohmic contacts

Improved contact resistance uniformity, with a low resistance on high-low doped GaAs MESFET, was demonstrated using a Pd/Ge/Ti/Au ohmic contact. The lowest contact resistivity obtained was 2.8×10^-6 Ω-cm². The average value and standard deviation (ΔRc) of the contact resistance (Rc) were 0.73 and 0.07 Ω-mm, respectively, which were more uniform than those for AuGe/Ni contacts with an average Rc of 0.77 Ω-mm and ΔRc of 0.16 Ω-mm. The improved uniformity was attributed to the uniform penetration of the ohmic junction into the buried high-doped channel layer by solid-state reactions, resulting in the improved uniformity of device performance     

HEMT with nonalloyed ohmic contacts using n+-InGaAs cap layer

We have investigated nonalloyed ohmic contacts on HEMT's using a highly conductive n⁺-InGaAs layer. The minimum specific contact resistance obtained was 4.8 × 10^-7Ω.cm², and the IV characteristics were equal to or better than those of conventional HEMT's with alloyed ohmic contacts. The maximum transconductances of a nonalloyed ohmic HEMT were 240 mS/mm at 300K and 340 mS/mm at 88K for a gate length of 1.1 µm. We conclude that it is not necessary for HEMT's with two-dimensional electron gas (2DEG) channels to have alloyed ohmic contacts, because the tunneling conduction is significant at the n-GaAs/n-AlGaAs/undoped GaAs double heterojunction.     

Low emitter resistance GaAs based HBT's without InGaAs caps

Low emitter resistance is demonstrated for AlGaAs/GaAs heterojunction bipolar transistors using Pd/Ge contacts on a GaAs contact layer. The contact resistivity to 2-10×10¹⁸ cm ^-3 n-type GaAs is 4-1×10^-7 Ω-cm² . These are comparable to contact resistivities obtained with non-alloyed contacts on InGaAs layers. The non-spiking Pd/Ge contact demonstrates thermal stability and area independent resistivity suitable for scaled devices. The substitution of Pd/Ge for AuGe/Ni GaAs emitter and collector contacts reduced by an order of magnitude the emitter-base offset voltage at high current densities and increased f_t by more than 15% with significantly improved uniformity for devices with 2 and 2.6 μm wide emitters having lengths two, four and six times the width     

Hot-plate alloying for ohmic contacts to GaAs

The fabrication and optimization of ohmic contacts to GaAs prepared by heating the wafers on a hot plate is described. The method offers high throughput and is production adaptable. The apparatus consists of a hot plate constructed of a heat pipe with a high surface temperature uniformity (±2°C over 4-in diameter) located inside a glove box with an ambient controlled to 5 percent H2/95 percent N2forming gas. Specific resistance and morphology of AuGe/Ni/Au contacts were characterized as a function of hot-plate surface temperature. At the optimum alloy temperature, specific resistances of less than 10^-6Ω-cm²were obtained repeatably for VPE GaAs with active layers of n ∼ 10¹⁷cm^-3and thicknesses of ∼0.4 µm as well as for VPE GaAs with an n⁺surface layer greater than 10¹⁸cm^-3. The contact surface morphologies were smoothest for those alloyed at temperatures below the optimum, and, for those alloyed at or above the optimum, they were increasingly less smooth for increasingly higher alloy temperatures. A general discussion of this method's potential application to high-volume GaAs processing is given.     

Sintered ohmic contacts to n-and p-type GaAs

Previously, all known ohmic contacts to n-GaAs have involved a so-called "alloying" procedure which consists of melting a Au-Ge eutectic or Sn-based alloy films on GaAs. We describe here a new contact metallization scheme consisting of Pd/Ge/n-GaAs which requires sintering rather than melting in order to produce ohmic contacts. The sintering is done at temperatures ranging from 350°C, 15 min to 500°C, 2 h depending on the doping level of n-GaAs (10¹⁸-10¹⁶cm^-3. For n-10¹⁶cm^-3GaAs, a specific contact resistance of 3 × 10^-4Ω.cm²was achieved. Sintering leads to the formation of some PdGe and intermetallics associated with the Pd/GaAs interaction, namely, PdAs2and PdGa. Ohmic behavior is attributed to a combination of the doping action of Ge (as donor) and fast in-diffusion kinetics of Pd. Sintered contacts to n⁺and p⁺GaAs (ND.A∼ 10¹⁸cm^-8) made by Au, Pt, and Ti Were also investigated for ohmic behavior. Each of these three metals was at least partially effective in forming ohmic contacts to p⁺GaAs; the degree of effectiveness increases on going from Ti to Au to Pt. It is proposed that a reasonable guideline to follow When searching for ohmic contacts is Xm≳ Xdwhere Xmis the width of the metallurgical junction and Xdis the ideal depletion layer width. This condition should favor ohmic contacts by promoting a micro 3-dimensional current flow at the conductor-semiconductor interface and thereby maximizing field-emission probability.     

Schottky barriers and ohmic contacts on n-type InP based compound semiconductors for microwave FET's

InP, and In0.73Ga0.27As0.6P0.4, and In0.53Ga0.47As lattice matched to InP are of special importance as active FET channel materials because of the high electron velocity and/or high electron mobility they offer. Using a AuGe/Ni/Au metallization system, specific contact resistances of 5 × 10^-7Ω . cm², 8 × 10^-7Ω . cm², and 5.8 × 10^-6Ω cm²were obtained for ohmic contacts on In0.53Ga0.47As, InP, and In0.89Ga0.11As0.24P0.76, respectively. Leakage currents of 10 µA at 7-V reverse bias were observed for 1 × 200-µm gates on InP. and In0.89Ga0.11As0.24P0.76FET's having a SiO2film about 50 Å thick under the gate. A thin SiO2layer underneath the gate improved the Schottky-gate I-V characteristics, but thick oxides severely degraded the microwave performance of the FET's. These excellent ohmic contacts and Schottky barriers resulted in a maximum insertion gain of 15 dB at 8 GHz and a noise figure of 2.5 dB with 8-dB gain at 7 GHz for the InP deviees. For 1.15-eV InxGa1-xAsyP1-yFET's, the resulting gain was 9 dB at 8 GHz.     

Ohmic contacts to GaAs lasers using ion-beam technology

The ion-beam sputtering technique was used to deposit PtTi contacts to p-type GaAs. With the use of an annealing process, ohmic behavior was eminently enhanced. Specific contact resistance was measured and AES utilized for structural analysis. Specific contact resistance down to 2.4 × 10^-5ω.cm²was achieved. This low specific contact resistance is to our knowledge the lowest reported on p-type GaAs, and is comparable with state-of-the-art PtSi and Cr contacts reported on Si. The contacts also exhibited very stable characteristics. Furthermore, this contacting process, was applied to double heterostructure (DH)-GaAs stripe lasers with excellent results. These results clearly demonstrate the advantage of this process, making it very suitable for industrial applications.     

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.     

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.     

A high-performance InGaAs/InAlAs double-heterojunction bipolartransistor with nonalloyed n+-InAs cap layer on InP(n) grownby molecular beam epitaxy

An InGaAs/InAlAs double-heterojunction bipolar transistor (DHBT) on InP(n) grown by molecular-beam epitaxy (MBE) that exhibits high DC performance is discussed. An n⁺-InAs emitter cap layer was used for nonalloyed contacts in the structure and specific contact resistances of 1.8×10^-7 and 6.0×10^-6 Ω-cm² were measured for the nonalloyed emitter and base contacts, respectively. Since no high-temperature annealing is necessary, excellent contact surface morphology on thinner base devices can easily be obtained. In devices with 50×50-μm² emitter area, common-emitter current gains as high as 1500 were achieved at a collector current density of 2.7×10³ A/cm² . The current gain increased up to 2000 for alloyed devices     

The use of refractory metal and electron-beam sintering to reduce contact resistance for VLSI

Low contact resistance for metal on silicon is particularly important for VLSI where contact dimensions become ≤1 µm. This paper reports on e-beam sintering of refractory metal contacts on implanted n⁺- and p⁺-silicon layers. With this technique, we have obtained contact resistivities as low as 1.5 × 10^-7Ω. cm²and 1.2 × 10^-7Ω . cm²for n⁺and p⁺contacts, respectively. These values are the lowest contact resistivities which have been achieved experimentally to date. We found no measurable metal-silicon interdiffusion when e-beam sintering was used. Electron-beam-induced MOS damage, including neutral traps, can be removed by a forming gas anneal.     

Low-resistance ohmic contacts to AlGaAs/GaAs and In0.52Al0.48As/In0.53Ga0.47As modulation-doped structures obtained by halogen lamp annealing

The successful application of short-term halogen lamp annealing to form ohmic contacts to AlGaAs/GaAs and In0.52Al0.48As/ In0.53Ga0.47As modulation-doped structures is demonstrated. Use of Ti in the electron-beam evaporated metallization scheme and a two-step annealing cycle give contacts with reproducibly good electrical and morphological characteristics. Minimum values of specific contact resistancerho_{c} = 4.0 times 10^{-7}and6.0 times 10^{-7}Ω.cm²for AlGaAs/GaAs and In0.52Al0.48As/In0.53Ga0.47As, respectively, are measured. Corresponding values of the transfer resistance Rcare 0.12 ± 0.02 and 0.18 ± 0.05 Ω.mm. These values are the lowest achieved with lamp annealing and are comparable to the best obtained with transient furnace annealing.     

Source—Drain contact resistance in CMOS with self-aligned TiSi2

The contact resistance between TiSi2and n⁺-p⁺source-drain in CMOS is studied for a variety of junction profiles and silicide thicknesses. It is shown that the measured contact resistance is consistent with the transmission-line model for electrically long contacts. The contact contribution to the total device series resistance can be significant if excessive silicon is consumed during silicide formation. Contact resistivities of 3 × 10^-7and 1 × 10^-6Ω . cm²can be obtained for 0.15-0.20-µm-deep arsenic and boron junctions, respectively, if the interface doping concentration is kept at 1 × 10²⁰/cm³. Furthermore, low-temperature measurements show that the contact resistivity is nearly constant from 300 to 77 K, as would be expected from a tunneling-dominated current transport at the TiSi2-n⁺and TiSi2-P⁺interfaces.     

Novel Cu/Cr/Ge/Pd Ohmic Contacts on Highly Doped n-GaAs

The thermal stability of the Cu/Cr/Ge/Pd/n⁺-GaAs contact structure was evaluated. In this structure, a thin 40 nm layer of chromium was deposited as a diffusion barrier to block copper diffusion into GaAs. After thermal annealing at 350°C, the specific contact resistance of the copper-based ohmic contact Cu/Cr/Ge/Pd was measured to be (5.1 ± 0.6) × 10⁻⁷ Ω cm². Diffusion behaviors of these films at different annealing temperatures were characterized by metal sheet resistance, X-ray diffraction data, Auger electron spectroscopy, and transmission electron microscopy. The Cu/Cr/Ge/Pd contact structure was very stable after 350°C annealing. However, after 400°C annealing, the reaction of copper with the underlying layers started to occur and formed Cu₃Ga, Cu₃As, Cu₉Ga₄, and Ge₃Cu phases due to interfacial instability and copper diffusion.     

Solid-phase epitaxial Pd/Ge ohmic contacts to In1-xGaxAsyP1-y/InP

A Pd/Ge metallization to InGaAsP/InP semiconductors, formed with solid-phase epitaxy (SPE) technique, has been investigated in this study. With this method, ohmic contacts with low specific contact resistance (rho_{c} approx 2.3 times 10^{-6}Ω.cm²) have been achieved on p-type In0.53Ga0.47As(p approx 1.8 times 10^{19}/cm³). The same contact scheme also gives low specific contact resistance (rho_{c} approx 6 times 10^{-7}Ω.cm²) on n-type In0.53Ga0.47As (n approx 1.0 times 10^{19}/cm³). Excellent surface morphology is observed in all the samples, and the contacts do not deteriorate for at least 4 h at temperatures between 300 and 500°C.