Au Ohmic Contacts Research Articles

Study of Ti/Au, Ti/Al/Au, and Ti/Al/Ni/Au ohmic contacts to n -GaN

The contacts of Ti/Au, Ti/Al/Au, and Ti/Al/Ni/Au films deposited on n-GaN were studied by current–voltage (I–V) and transmission-line-method measurements. The effect of annealing temperature on specific contact resistivity has been investigated by changing the annealing temperature from 400 to 900 °C. Ti/Al/Au and Ti/Al/Ni/Au films were superior to the bilayer (Ti/Au) in ohmic contact characteristics and thermal stability. The Ti/Al/Ni/Au composite showed the best thermal stability due to the fact that Ni plays a more important role than the alloy of Ti/Al in preventing the interdiffusion of Ti, Al, and Au. The lowest contact resistivity (∼10-7Ω cm2) to n-GaN was obtained for the Ti/Al/Ni/Au sample by short-time/high-temperature annealing. The formation mechanism of ohmic contacts to n-GaN is also discussed.

Fermi-level pinning at the metal/p-type CuGaS2 interfaces

A Schottky contact to p-type CuGaS2 that showed an extremely high rectification ratio of approximately 500 was realized using a Cu electrode on a HF/HNO3-treated surface, as well as an excellent Au ohmic contact on a HF-etched surface. The effective Schottky barrier height of 0.9 eV was obtained from the current–voltage and capacitance–voltage characteristics. The value was smaller by 1.1 eV than that calculated from the values of the work function of Cu and electron affinity of CuGaS2. The results indicated a surface pinning of the Fermi level to certain acceptor-type gap states below the midgap.

Preparation of ohmic n-type cubic boron nitride contacts

Ohmic electrodes in the form of n-type (Si-doped) cubic boron nitride (c-BN) bulkcrystals were fabricated by utilizing a covering technique, depositing Ti(10nm)/Mo/(20 nm)/Pt–Au(200 nm) ohmic contact metal on both the sides of thec-BN substrate. The size of the specimen electrode was 100 × 100 μ m2 on one sideand 300 × 300 μm2 on the other side. Measurements on the specimen were made using a speciallymade device. Linear current–voltage characteristics were obtained. It isconsidered that the contact between the Ti-and Si-doped c-BN was ohmic.

Ohmic contacts to Al-rich n -AlGaN

Ti/Al/Pt/Au and V/Al/Pt/Au ohmic contacts were studied on n-Al0.44Ga0.56N and n-Al0.6Ga0.4N. Optimised V/Al/Pt/Au contacts provided lower resistance than did Ti/Al/Pt/Au contacts, which are commonly used on less Al-rich n-AlGaN. Specific contact resistances of 1×10−5 Ω · cm2 and 4×10−5 were achieved on n-Al0.44Ga0.56N and n-Al0.6Ga0.4N, respectively.

GaN p–n junction diode formed by Si ion implantation into p-GaN

28Si + implantation into Mg-doped GaN, followed by thermal annealing in N 2 was performed to achieve n +-GaN layers. The carrier concentrations of the films changed from 3×10 17 (p-type) to 5×10 19 cm −3 (n-type) when the Si-implanted p-type GaN was properly annealed. Specific contact resistance ( ρ c) of Ti/Al/Pt/Au Ohmic contact to n-GaN, formed by 28Si + implantation into p-type GaN, was also evaluated by transmission line model. It was found that we could achieve a ρ c value as low as 1.5×10 −6 Ω cm 2 when the metal contact was alloyed in N 2 ambience at 600 °C. Si-implanted GaN p–n junction light-emitting diodes were also fabricated. Electroluminescence measurements showed that two emission peaks at around 385 and 420 nm were observed, which could be attributed to the near band-edge transition and donor-to-acceptor transition, respectively.

High temperature characteristics of Ti/Al and Cr/Al ohmic contacts to n-type GaN

The thermal stability of the Ti/Al, Ti/Al/Ni/Au, Cr/Al and Cr/Al/Ni/Au ohmic contact metallizations on n-GaN were investigated. Bilayers of Ti/Al and Cr/Al, with and without Ni/Au overlayers, were deposited on n-GaN using an e-beam deposition method and ohmic contacts formed by a vacuum annealing technique. For both the Ti/Al and Ti/Al/Ni/Au minimum specific contact resistivities of ∼ 1×10 −5 Ω cm 2 were achieved after vacuum annealing at 1100 °C for 2 min. Minimum specific resistivities of similarly formed Cr/Al and Cr/Al/Ni/Au ohmic contacts were 3.8×10 −5 and 2.3×10 −5 Ω cm 2 respectively, obtained at 700 and 950 °C respectively. The above mentioned contacts were exposed to long term aging at temperatures of 300, 400 and 500 °C, for periods up to 100 h and their electrical and morphological characteristics were monitored at the various stages of the aging process. The Ti/Al and Cr/Al/Ni/Au proved to be the most stable of the four metallizations in terms of both electrical performance and surface morphology. The Cr/Al had shown stable morphological stability, but displayed the poorest electrical performance characteristics as a result of the aging process.

Optimisation of the Ti/Al/Ni/Au ohmic contact on AlGaN/GaN FET structures

We present a systematic approach to reduce the resistance of ohmic contacts on AlGaN/GaN FET structures. We have optimised the Ti/Al/Ni/Au contact with respect to the metal composition and annealing conditions. Our optimised contact has a very low contact resistance of 0.2 Ω mm (7.3×10 −7 Ω cm 2), a good reproducibility and an excellent line definition, making this contact very suitable for use in AlGaN/GaN FETs with short gate-source distances. This contact resistance on FET material is among the best values reported. The approach presented here can be applied to other metal schemes like Ti/Al/Pt/Au or Ti/Al/Ti/Au.

Effect of Various Pre-Treatments on Ti/Al/Ti/Au Ohmic Contacts for AlGaN/GaN HFET Devices

The effect of a number of different treatments on the contact resistance for Ti/Al/Ti/Au (30/80/120/55 nm) contacts to Al0.3Ga0.7N (20 nm)/GaN HFET structures were investigated. The structures were grown by MBE. Neither preannealing of the surface at temperatures up to 1100 °C in N2, nor etching of the surface using chemically assisted ion beam etching (CAIBE) was effective in reducing the contact resistance. Both light CAIBE etching (≈5 nm) to partially remove the AlGaN layer, and deeper etching (≈50 nm and 100 nm) to remove the AlGaN and contact the GaN channel layer directly, resulted in a substantial increase in the measured contact resistance. The only treatment that was found to decrease the contact resistance was a brief wet etch in K2S2O8/KOH solution under 254 nm UV irradiation. A resist stabilization treatment was developed so that the process was self-aligned. The contact resistance in Ω mm was decreased by a factor of 1.7 for the optimum etch time. Scanning electron microscopy showed that the etching resulted in the formation of discrete holes perforating the AlGaN layer.

Effect of Various Pre-Treatments on Ti/Al/Ti/Au Ohmic Contacts for AlGaN/GaN HFET Devices

The effect of a number of different treatments on the contact resistance for Ti/Al/Ti/Au (30/80/120/ 55 nm) contacts to Al 0.3 Ga 0.7 N (20 nm)/GaN HFET structures were investigated. The structures were grown by MBE. Neither preannealing of the surface at temperatures up to 1100 °C in N 2 , nor etching of the surface using chemically assisted ion beam etching (CAIBE) was effective in reducing the contact resistance. Both light CAIBE etching (5 nm) to partially remove the AlGaN layer, and deeper etching (50 nm and 100 nm) to remove the AlGaN and contact the GaN channel layer directly, resulted in a substantial increase in the measured contact resistance. The only treatment that was found to decrease the contact resistance was a brief wet etch in K 2 S 2 O 8 /KOH solution under 254 nm UV irradiation. A resist stabilization treatment was developed so that the process was self-aligned. The contact resistance in Ω mm was decreased by a factor of 1.7 for the optimum etch time. Scanning electron microscopy showed that the etching resulted in the formation of discrete holes perforating the AlGaN layer.

Ti/Al/Pt/Au and Al ohmic contacts on Si-substrated GaN

Al and Ti/Al/Pt/Au ohmic contacts on GaN epitaxial layers were studied. The epilayers were grown on Si (111) substrates by low-pressure metalorganic chemical vapor deposition. Al/GaN contacts achieved a minimum contact resistivity of 7.5×10−3 Ω cm2 after annealing in N2 ambient at 450 °C for 3 min. Further annealing degraded the contacts. Ti/Al/Pt/Au and GaN contacts achieved a minimum contact resistivity of 8.4×10−5 Ω cm2 after annealing in N2 at 650 °C for 20 s. The Ti/Al/Pt/Au contacts on GaN showed a better thermal stability than Al/GaN contacts. After annealing at 600 °C for 30 min. they were still ohmic contacts. The mechanisms for ohmic contact formation of Ti/Al/Pt/Au contacts were also analyzed.

Reliable Ti/Al and Ti/Al/Ni/Au ohmic contacts to n-type GaN formed by vacuum annealing

In this work, we investigated Ti/Al and Ti/Al/Ni/Au ohmic contact metallizations to n-GaN which were formed by a vacuum annealing method. Our effort was focused on minimizing the oxidation of the Al, which limits the performance of the Al-based contacts. We have also investigated the effect of the Ni/Au overlayers on the performance and morphological characteristics of the contacts. Vacuum annealing was found to require temperatures similar to those used in halogen lamp rapid thermal annealing for forming ohmic contacts on n-type GaN, but with minimal oxidation of the Al surface. It was found that the Ni/Au overlayer on the Ti/Al reduces the specific contact resistivity values slightly and at the same time causes an upward shift, by ∼200 °C, in the contact formation temperature. For the Ti/Al and Ti/Al/Ni/Au on n-GaN doped at 1.1×1018 cm−3, a minimum specific contact resistivity of 1.2×10−5 Ω cm2 was achieved after 950 and 1100 °C anneals, respectively. Auger electron spectroscopic studies revealed that obtaining optimum ohmic contact resistance requires the penetration of Al through the Ti layer to reach the GaN surface in addition to N outdiffusing from the GaN surface and into the metal layers. Atomic force microscopy investigations indicated that the introduction of the Ni/Au overlayer on the Ti/Al had the effect of increasing the surface roughness after annealing.

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.

Cr/Al and Cr/Al/Ni/Au ohmic contacts to n-type GaN

In this work we investigate the performance of Cr/Al and Cr/Al/Ni/Au ohmic contacts on n-type GaN. Annealing of the contacts was achieved by using a low temperature conventional quartz tube furnace in an Ar ambient and a new vacuum annealing technique using a tungsten strip heater. Low specific contact resistivity (ρc) metallizations were achieved with furnace annealing at considerably lower temperatures (550–600 °C) than those typically required for GaN contacts by halogen lamp rapid thermal annealing (∼900 °C). Vacuum annealing was found to require temperatures similar to those used in halogen lamp rapid thermal annealing for forming ohmic contacts on n-type GaN, but with minimal oxidation of the Al surface. For the Cr/Al bilayer on GaN with n doping of 1018 cm−3, minimum specific contact resistivities of 1.6×10−4 Ω cm2 and 2.3×10−5 Ωcm2 were achieved for furnace annealing and vacuum annealing, respectively. Our experiments showed that, when Cr was used as a contact material, the simultaneous presence of Cr and Al was necessary in order to obtain the best possible ohmic properties. Furthermore, the Cr/Al contacts maintained good stability at elevated temperatures. The Cr/Al/Ni/Au system offers lower ρc values and even greater temperature stability than the Cr/Al system when annealed in the 800–1200 °C temperature range. Atomic force microscopy investigations indicated that the introduction of the Ni/Au overlayer had the effect of increasing the surface roughness after annealing.

The Influence of Pt in a Ti—Al–Pt–Au Ohmic Contact on n-Type GaN

A number of alternative metallisation has been used to obtain low ohmic contact resistance to n-type GaN. Of these, the most common is the bi-metal alloy of Ti–Al. Typical alloying temperatures used for this process are in the vicinity of 900 °C. We report that the addition of Pt and Au to the ohmic metal provides specific contact resistivity of the order of 4 × 10—5 Ω cm2 at temperatures in the vicinity of 700 °C. This makes the alloying step less demanding on both the material and the processing equipment. The paper also presents the effects of alloying temperature and the influence of the formation of an Al–Pt phase on the quality of the ohmic contact.

The Influence of Pt in a Ti—Al–Pt–Au Ohmic Contact on n-Type GaN

A number of alternative metallisation has been used to obtain low ohmic contact resistance to n-type GaN. Of these, the most common is the bi-metal alloy of Ti–Al. Typical alloying temperatures used for this process are in the vicinity of 900 °C. We report that the addition of Pt and Au to the ohmic metal provides specific contact resistivity of the order of 4 × 10—5 Ω cm2 at temperatures in the vicinity of 700 °C. This makes the alloying step less demanding on both the material and the processing equipment. The paper also presents the effects of alloying temperature and the influence of the formation of an Al–Pt phase on the quality of the ohmic contact.

AlGaN/GaN HFETs with New Ohmic and Schottky Contacts for Thermal Stability up to 400 °C

Results on the thermal stability of AlxGa1—xN/GaN Heterostructure Field-Effect Transistors (HFETs) are presented. The epilayer structures were grown by MOVPE on sapphire with Al mole fractions in the AlGaN barrier layers of 20 and 25%, respectively. Different metallizations for ohmic and Schottky contacts were employed and their influence on the thermal stability of HFETs was investigated. Transistors with standard Ti/Al based ohmic and Pt based Schottky contacts showed rapid degradation after aging at 400 °C. Using a WSiN diffusion barrier on top of the Ti/Al/Ti/Au ohmic contact the morphology could be strongly improved with smooth contact surfaces and well defined contact edges. The ohmic contact resistance of this system is thermally stable and exhibits a value of 0.77 Ωmm after aging at 400 °C. Transistors with WSiN/Au Schottky gate contacts already showed improved aging behaviour while HFETs with Ir/Au Schottky contacts and 25% Al mole fraction in the barrier layer demonstrated long term stability at 400 °C with the highest Schottky barrier height and lowest gate leakage current.

Ohmic contacts in AlSb/InAs high electron mobility transistors for low-voltage operation

We report on the fabrication and characteristics of Pd/Pt/Au ohmic contacts that have been used in AlSb/InAs high electron mobility transistors (HEMTs) with low access resistance. The metalization exhibits minimal in-diffusion and a contact resistance of 0.08 Ω mm after a 175 °C hot-plate heat treatment. By comparison, AuGe/Ni/Pt/Au ohmic contact metalizations formed using a 300 °C rapid thermal anneal exhibit a contact resistance of 0.11 Ω mm, but with considerable Au in-diffusion. Using the Pd/Pt/Au contact, 0.6 μm gate-length AlSb/InAs HEMTs exhibit a low-field source-drain resistance of 0.47 Ω mm and a transconductance above 1 S/mm. After removal of the gate bonding pad capacitance from an equivalent circuit, an fTLg product of 38 GHz μm is obtained at VDS=0.4 V. HEMTs with a 60 nm gate length exhibit a low-field source-drain resistance of 0.35 Ω mm and a measured fT of 90 GHz at a drain voltage of only 100 mV. These fTLg and fT values are the highest reported for any field effect transistor at these drain voltages.

The Role of Ni in the Formation of Low Resistance Ni–Ge–Au Ohmic Contacts to n+ GaAs Heterostructures

Nickel is a commonly used wetting agent in alloyed Au–Ge ohmic contacts to n-GaAs, resulting in uniformity improvements to the morphology and contact resistance. In order to study the role of Ni in Ni–Ge–Au alloys, we have fabricated samples with varying Ni content and characterized them using electron microbeam techniques. Our data indicate the amount of Ni in the alloy affects the microstructure and composition, the morphology of the metal/GaAs interface, and the amount of GaAs consumed during the alloy reaction. Also, the dopant distribution into the GaAs is heterogeneous depending on the alloy microstructure.

Au/Ge-based Ohmic contact to an AlGaAs/InGaAs pseudomorphic high electron mobility transistor with an undoped cap layer

The effect of existence of undoped GaAs/AlGaAs cap layers on Ohmic contact resistivity in an AlGaAs/InGaAs pseudomorphic high electron mobility transistor was investigated. The Au/Ge/Ni/Au Ohmic contact formed on the undoped GaAs cap layer yields the lowest contact resistivity of 3.8×10−6 Ω cm2. Meanwhile, the contact resistivity increases a little to 5.0×10−6 Ω cm2 for the contacts formed on an n-Al0.23Ga0.77As layer exposed by removing the undoped cap layer. Both contact resistivities are comparable to those obtained using the n+-GaAs cap layer. The good Ohmic contacts obtained independent of removal of the undoped cap layer are due to the formation of the interfacial compounds, Au2Al and Au2Ga, during annealing. The interfacial compounds penetrate deep into the buried InGaAs channel, resulting in direct contacts to the channel.

Improved uniformity of contact resistance in GaAs MESFET using Pd/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/spl times/10/sup -6/ /spl Omega/-cm/sup 2/. The average value and standard deviation (/spl Delta/Rc) of the contact resistance (Rc) were 0.73 and 0.07 /spl Omega/-mm, respectively, which were more uniform than those for AuGe/Ni contacts with an average Rc of 0.77 /spl Omega/-mm and /spl Delta/Rc of 0.16 /spl Omega/-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.