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Studies the Ni/Wsi/Ti/Pt Ohmic contacts to n-SiC as a function of annealing temperatures up to 1000 degree centigrade. Characteristics of the contact-SiC interface at temperatures between 950 and 1000 degree centigrade; Exhibition of minimal contact expansion; Maintenance of the desirable electrical properties associated with Ni contacts.

The valence band offsets in rf-sputtered Indium Tin Oxide (ITO)/single crystal β-Ga 2 O 3 (ITO/Ga 2 O 3 ) heterostructures were measured with X-Ray Photoelectron Spectroscopy using the Kraut method. The bandgaps of the component materials in the heterostructure were determined by Reflection Electron Energy Loss Spectroscopy as 4.6 eV for Ga 2 O 3 and 3.5 eV for ITO. The valence band offset was determined to be −0.78 ± 0.30 eV, while the conduction band offset was determined to be −0.32 ± 0.13 eV. The ITO/Ga 2 O 3 system has a nested gap (type I) alignment. The use of a thin layer of ITO between a metal and the Ga 2 O 3 is an attractive approach for reducing contact resistance on Ga 2 O 3 -based power electronic devices and solar-blind photodetectors. [ABSTRACT FROM AUTHOR]

AZO interlayers between n-Ga2O3 and Ti/Au metallization significantly enhance Ohmic contact formation after annealing at 300°C.Without the presence of the AZO, similar anneals produce only rectifying current-voltage characteristics. Transmission Line Measurements of the Au/Ti/AZO/Ga2O3 stacks showed the specific contact resistance and transfer resistance decreased sharply from as-deposited values with annealing. The minimum contact resistance and specific contact resistance of 0.42 -mm and 2.82×10-5 Ω-cm2 were achieved after a relatively low temperature 400°C annealing. The conduction band offset between AZO and Ga2O3 is 0.79 eV and provides a favorable pathway for improved electron transport across this interface. [ABSTRACT FROM AUTHOR]

Ti/Al/Ni/Au Ohmic contact metallization on InAlN/GaN heterostructures both with and without a thin GaN cap layer was annealed at different temperatures. The minimum transfer resistance for the contacts of 0.65 Ω mm (specific contact resistivity of 2×10-5 Ω cm2) was achieved after 800 °C annealing for structures without the GaN cap, while those with the cap exhibited their lowest resistance at higher temperatures. The contact morphology showed considerable roughening by 750 °C but the carrier mobility was stable until annealing temperatures of 850 °C. Diffuse scattering experiments showed that the morphological roughness of the InAlN/GaN interface increased as a result of annealing at these temperatures and the data were consistent with outdiffusion of Ga into the InAlN. Unpassivated high electron mobility transistors with a gate dimension of 0.7×180 μm2 were fabricated using these contacts and showed a maximum drain current of 1.3 A/mm and an extrinsic transconductance of 366 mS/mm. The presence of the GaN cap increased the effective barrier height of Ni/Au Schottky contacts from 0.91 to 1.01 eV on the heterostructure. [ABSTRACT FROM AUTHOR]

Low resistance Ohmic contacts using Ti/Au metallization on n-type amorphous indium zinc oxide (IZO) (n∼5×1020 cm-3) deposited on paper substrates are reported. The minimum specific contact resistivity of 8×10-7 Ω cm2 was achieved on IZO films both as-deposited and for annealing temperatures of up to 125 °C. The contact resistance increased to 4×10-6 Ω cm2 at 175 °C. The sheet resistance was found to vary from 24 to 17 Ω/sq, and the transfer resistance was ∼0.045 Ω mm for the as-deposited and low temperature annealed samples. The contact resistance was independent of measurement temperature, indicating that field emission plays a dominant role in the current transport. Such Ohmic contacts achieved with little or no annealing are important for paper based electronics requiring low temperature processing (<200 °C). [ABSTRACT FROM AUTHOR]

There is interest in developing more stable contacts to a variety of GaN-based devices. In this paper, we give two examples of devices that show improved thermal stability when boride or Ir diffusion barriers are employed in ohmic-contact stacks. AlGaN/GaN high electron mobility transistors (HEMTs) were fabricated with Ti/Al/X/Ti/Au source-drain ohmic (where X is or Ir) contacts and were subjected to long-term annealing at 350 . By comparison with companion devices with conventional Ti/Al/Ni/Au ohmic contacts, the HEMTs with boride-based ohmic metal showed superior stability of both source-drain current and transconductance after 25 days of aging at 350 . The gate current for standard HEMTs increases during aging, and the standard ohmic contacts eventually fail by shorting to the gate contact. Similarly, InGaN/GaN multiple quantum well light-emitting diodes (MQW-LEDs) were fabricated with either Ni/Au//Ti/Au or Ni/Au/Ir/Au p-ohmic contacts. Both of these contacts showed superior long-term thermal stability compared to LEDs with conventional Ni/Au contacts. [ABSTRACT FROM PUBLISHER]

The use of Ir diffusion barriers in Ni/Au-based Ohmic contacts to p-type CuCrO2 layers was investigated. A specific contact resistance of ~5 x 10-4 Ω cm² was achieved after annealing at 500°C for the Ir-containing contacts, and the contacts were rectifying for lower anneal temperatures. In this case, the contact resistance was basically independent of the measurement temperature, indicating that tunneling is the dominant transport mechanism in the contacts. The morphology for the Ir-containing contacts was still smooth at 500°C although Auger electron spectroscopy depth profiling showed that some of the nickel had diffused to the surface and had oxidized. Contacts annealed at 800°C showed that some copper and most of the nickel had diffused to the surface and oxidized. The presence of the Ir diffusion barrier does increase the thermal stability of the contacts by ~200°C compared to conventional Ni/Au contacts. By contrast, the use of other materials such as TaN, ZrN, and W2B5 as the diffusion barrier led to poorer thermal stability, with the contact resistance increasing sharply above 400°C. [ABSTRACT FROM AUTHOR]

Ir/Au Schottky contacts and Ti/Al/Jr/Au ohmic contacts on n-type GaN were investigated as a function of annealing temperature and compared to their more common Ni-based counterparts. The Jr/Au ohmic contacts on n-type GaN with n ≈ 1017 cm-3 exhibited barrier heights of 0.55 eV after annealing at 700°C and displayed less intermixing of the contact metals compared to Ni/Au. A minimum specific contact resistance of 1.6 x 10-6 Ωcm-2 was obtained for the ohmic contacts on n-type GaN with n ≈ 1018 cm-3 after annealing at 900°C. The measurement temperature dependence of contact resistance was similar for both Ti/Al/Ir/Au and Ti/Al/Ni/Au, suggesting the same transport mechanism was present in both types of contacts. The Ir-based ohmic contacts displayed superior thermal aging characteristics at 350°C. Auger electron spectroscopy showed that Ir is superior to Ni as a diffusion barrier at these moderate temperatures. [ABSTRACT FROM AUTHOR]

The effect of cryogenic temperatures during metal deposition on the contact properties of Pd, Pt, Ti, and Ni on bulk single-crystal n-type ZnO has been investigated. Deposition at both room and low temperature produced contacts with Ohmic characteristics for Ti and Ni metallizations. By sharp contrast, both Pd and Pt contacts showed rectifying characteristics after deposition with barrier heights between 0.37 eV and 0.69 eV. Changes in contact behavior were measured on Pd to anneal temperatures of ∼300 °C, showing an increase in barrier height along with a decrease in ideality factor with increasing annealing temperature. This difference with annealing temperature is in sharp contrast to previous results for Au contacts to ZnO. There were no differences in near-surface stoichiometry for the different deposition temperatures; however, low temperature contacts demonstrated some peeling/ cracking for Pt and Pd. [ABSTRACT FROM AUTHOR]

AlGaN/GaN High Electron Mobility Transistors (HEMTs) were fabricated with Ti/Al/TiB2/Ti/Au source/drain Ohmic contacts and a variety of gate metal schemes (Pt/Au, Ni/Au, Pt/TiB2/Au or Ni/TiB2/Au) and subjected to long-term annealing at 350°C. By comparison with companion devices with conventional Ti/Al/Pt/Au Ohmic contacts and Pt/Au gate contacts, the HEMTs with boride-based Ohmic metal and either Pt/Au, Ni/Au or Ni/TiB2/Au gate metal showed superior stability of both source-drain current and transconductance after 25 days aging at 350°C. [ABSTRACT FROM AUTHOR]

The contact characteristics on bulk single-crystal n-type ZnO of an Ir/Au metallization scheme deposited by sputtering are reported as a function of annealing temperature in the range 200–1000°C (N2 ambient). The contacts exhibited ohmic behavior for all temperatures and show a minimum specific contact resistivity of 3.6 × 10-5 Κ cm² after a 1000°C anneal. The contacts transition to rectifying behavior after annealing above 1100°C, coincident with a degraded surface morphology including agglomeration of Ir to the surface and heavy intermixing of the Ir and Au. The Ir contacts exhibit higher thermal stability but poorer specific contact resistivity than conventional Ti/Au metal stacks on bulk n-type ZnO. The contacts showed very little change in resistance after extended aging (30 days) at 350°C. Annealing under O2 ambient led to an increase in contact resistivity by orders of magnitude. [ABSTRACT FROM AUTHOR]

Three different metal borides (TiB2, CrB2, and W2B5) were examined for the use in Ti/Al/boride/Ti/Au Ohmic contacts on n-type GaN and the reliability compared to the more usual Ti/Al/Ni/Au metal scheme. The minimum specific contact resistance obtained was in the range of 10-5 Ω cm2 with CrB2 and W2B5 and approximately an order of magnitude lower with TiB2. In all cases, the minimum contact resistance is achieved after annealing in the range of 700–900 °C. The main current transport mechanism in the contacts after this annealing is tunneling as determined by the absence of any significant measurement temperature dependence to the contact resistance. The TiB2 and CrB2 contacts retain smooth morphology even after annealing at 1000 °C. Auger electron spectroscopy depth profiling indicated that the formation of an interfacial TiNX layer is likely responsible for the Ohmic nature of the contact after annealing. All three boride-based contacts show lower contact resistance than Ti/Al/Ni/Au after extended aging at 350 °C. [ABSTRACT FROM AUTHOR]

Process technology of high-speed implant-apertured index-guide lateral-current-injection top dielectric-mirror quantum-well 850-nm vertical cavity surface-emitting lasers (VCSELs) has been developed. Oxygen and helium implantation for aperture definition and extrinsic capacitance reduction, dielectric mirror formation, p- and n-ohmic contact formation, VCSEL resistance, and thermal analysis were investigated. Employing this technology, GaAs/AlGaAs-based 850-nm VCSELs with small signal modulation bandwidths up to 11.5 Gb/s and an eye diagram generated at 12 Gb/s by a pseudorandom bit sequence of 231-1 were achieved. The bit-error rates were below 10-13. The threshold current is as low as 0.8 mA for 7-μm-diameter current apertures and typical slope efficiencies of 0.45-0.5 mA/mW were obtained. [ABSTRACT FROM PUBLISHER]

The use of Pd coatings on multiple InN nanobelts is shown to enhance their sensitivity for hydrogen sensing at hundreds of ppm level at 25 °C. Without the metal coating to catalyze dissociation of the hydrogen molecules, the InN nanobelts with Ohmic contacts at either end showed no detectable change in current when exposed to hydrogen under the same conditions. Moreover, the Pd-coated InN showed no response to CO2, C2H6, NH3, and O2 (all in N2 ambient). The relative resistance change in the Pd-coated sensors was not linearly dependent on the hydrogen concentration at dilute levels, i.e., 8% at 100 ppm H2 and 9.5% at 1000 ppm H2. The recovery characteristics of the sensors at room temperature after hydrogen sensing were also examined and ∼50% of the initial InN resistance was recovered 10 min after sensor exposure to air. At higher temperatures, larger resistance changes and faster response and recovery were obtained. Pd-coated InN nanobelt sensors displayed much higher relative response than Pt-coated sensors. [ABSTRACT FROM AUTHOR]

Ohmic contacts to p-type GaN were fabricated using W2B and CrB2 as diffusion barriers for a traditional Ni/Au contact scheme. The annealing temperature dependence (25–1000 °C) of contact resistance and the thermal aging characteristics at 200 °C were examined. A minimum contact resistance of ∼2×10-4 Ω cm2 was achieved after annealing at 700 °C for 60 s. These contacts also showed excellent stability as a function of aging at 200 °C. Auger electron depth profiles reveal a large degree of intermixing at the GaN interface between Ni and Au. [ABSTRACT FROM AUTHOR]

The use of TiB2-based Ohmic contacts on Pt-gate AlGaN/GaN heterostructure diode hydrogen sensors is shown to provide very stable operation for detection of 1% H2 in air under field conditions where temperature is allowed to vary. In contrast, the use of more conventional Ti/Al/Pt/Au Ohmic contacts led to higher background variations in current that affect the ultimate detection threshold of the sensors. Combined with a differential pair geometry that compares current from an active diode with Pt-gate contact and a passive diode with Ti/Au gate, the more stable TiB2-based Ohmic contacts reduce false alarms due to ambient temperature changes. The diodes exhibit a change in forward current of more than 1 mA at 1.5 V when 1% H2 is introduced into an air ambient. [ABSTRACT FROM AUTHOR]

InGaN/GaN multiple quantum well light-emitting diodes (MQW-LEDs) were fabricated with either Ni/Au/TiB2/Ti/Au or Ni/Au/Ir/Au p-Ohmic contacts and annealed at 200 and 350 °C for 45 days. By comparison with companion devices with conventional Ni/Au Ohmic contacts fabricated on the same wafer, MQW-LEDs with TiB2- and Ir-based Ohmic metallization schemes showed superior long-term thermal stability as judged by the change in turn-on voltage, leakage current, and output power, a promising result for applications where reliable operation at high temperature is required. [ABSTRACT FROM AUTHOR]

Ohmic contacts to p-GaN using a Ni/Au/X/Ti/Au metallization scheme, where X is TaN, TiN, or ZrN, are reported. The dependence of the contact properties on annealing temperature (25–1000 °C) in N2 is examined. For annealing temperatures greater than 500 °C, the contacts display Ohmic characteristics and reach a minimum of about 2×10-4 Ω cm2 after annealing at 700 °C for 60 s in a N2 ambient. The specific contact resistance is stable on annealing up to at least 1000 °C. However, at high temperatures the morphology of the contacts are very rough and there is a large degree of intermixing between the metallic layers. The thermal stability of these contacts are superior as compared to conventional Ni/Au, which display poor characteristics at annealing temperatures greater than 500 °C. [ABSTRACT FROM AUTHOR]

Ti/Al/Ni/Au and Ti/Al/TiB2/Ti/Au Ohmic contacts on n-type InN were investigated as a function of annealing temperature. A minimum specific contact resistance of 1.6×10-6 Ω cm-2 was obtained for the TiB2-based Ohmic contacts on n-type GaN with n∼1018 cm-3 after annealing at 900 °C. The minimum value with the Ni-based metallization was higher (6×10-6 Ω cm2). The measurement temperature dependence of contact resistance was similar for both Ti/Al/TiB2/Ti/Au and Ti/Al/Ni/Au, suggesting that the same transport mechanism was present in both types of contacts. The TiB2-based Ohmic contacts displayed superior thermal stability, suggesting that it is a superior diffusion barrier at these temperatures than Ni. [ABSTRACT FROM AUTHOR]

Ohmic contact formation on p-type Mg-doped CuCrO2 layers grown by pulsed-laser deposition was investigated. While the current-voltage characteristics from Ti/Au contacts showed back-to-back Schottky behavior, a specific contact resistance of ∼1×10-4 Ω cm2 was achieved by using Ni instead of Ti. The contact resistivity was fairly independent of measurement temperature, suggesting that tunneling is the dominant transport mechanism. The contact resistance remained practically constant upon annealing in the 100–400 °C range. Above 500 °C, the morphology became rough and the contact showed rectifying behavior. This degradation resulted from both the out-diffusion of oxygen and the in-diffusion of Ni and Au in CuCrO2. [ABSTRACT FROM AUTHOR]

The specific contact resistivity and chemical intermixing of indium-tin-oxide (ITO)/Ti/Au Ohmic contacts on n-type bulk ZnO substrates are reported as a function of annealing temperature up to 450 °C. A minimum contact resistivity of 4.6×10-6 Ω cm2 was obtained at 50 °C and the value remained <10-5 Ω cm2 for all temperatures up to 450 °C, suggesting that the Ti is an effective diffusion barrier which prevents formation of Au–In solid solutions. Optical microscopy of the contacts shows roughening of the morphology after annealing at 450 °C. Indium outdiffusion to the surface of the contact stack is significant by 350 °C, and Zn outdiffusion is evident by 450 °C. Both the In and underlying Ti become oxidized after these higher temperature treatments. [ABSTRACT FROM AUTHOR]

Ti/Au Ohmic contacts on heavily Al-doped (n∼1019 cm-3) n-ZnO produce low specific contact resistivity of 2.4×10-7 Ω cm2 in the as-deposited condition and extremely low minimum values of 6×10-8 Ω cm2 after annealing at 300 °C.The contact resistance is independent of measurement temperature after low temperature anneals, suggesting that tunneling is the dominant transport mechanism in the contacts. The contact morphology roughens after annealing at 150 °C and Auger electron spectroscopy depth profiling shows Zn outdiffusion through the metal and intermixing of Au and Ti. However, the morphology does not significantly worsen after anneals at 450 °C. This metallization scheme looks very attractive for the n-electrode of ZnO-based light-emitting diode structures. [ABSTRACT FROM AUTHOR]

The annealing temperature (25–800 °C) dependence of ohmic contact characteristics on p-GaN using a W2B/Ti/Au metallization scheme deposited by sputtering are reported. The contacts are rectifying in the as-deposited condition but become ohmic for annealing at >=500 °C. A minimum specific contact resistivity of 1.7×10-3 Ω cm-2 was obtained after annealing at 800 °C for 60 s. Higher annealing temperatures produced sharp increases in the resistivity of the GaN and irreproducible contact properties. However, the contact morphology was similar over the entire annealing range used here. Auger electron spectroscopy profiling showed the onset of Ti out-diffusion through the Au at 500 °C. By 800 °C the Ti was almost completely removed to the surface, where it became oxidized. These boride-based contacts have superior thermal stability to the more common Ni/Au, whose morphology degrades significantly above 500 °C. [ABSTRACT FROM AUTHOR]

A comparison of Ti/Au and Ti/Al/Pt/Au Ohmic contacts on n-type Zn0.05Cd0.95O layers grown on ZnO buffer layers on GaN/sapphire templates showed a minimum contact resistivity of 2.3×10-4 Ω cm2 at 500 °C anneal temperature for Ti/Al/Pt/Au and 1.6×10-4 Ω cm2 at 450 °C for Ti/Al. The morphology of the Ti/Al/Pt/Au contacts showed much better thermal stability and remained smooth until at least 450 °C, whereas the Ti/Au contacts show a reacted appearance after 350 °C anneals. Auger electron spectroscopy depth profiling of the contact schemes as a function of anneal temperature suggests that the formation of TiOx phases that induce oxygen vacancies in the ZnCdO are responsible for the improved contact resistance after annealing in both types of metal schemes. [ABSTRACT FROM AUTHOR]

Electron-beam-deposited Ti/Au ohmic contacts on undoped (n∼1017 cm-3) bulk ZnO substrates exhibited as-deposited specific contact resistivity of 3×10-4 Ω cm2, regardless of the polarity (Zn face or O face) of the ZnO substrate. The annealing environment (air or N2) also had no significant effect on contact properties. The specific contact resistivity slightly decreased after annealing at 300 °C but started to increase above 350 °C. The measurement temperature dependence of specific contact resistivity revealed that the dominant current transport mechanism is field emission even in the moderately doped ZnO. As the annealing temperature increased, some voids were observed on the metal surface, possibly due to reaction of Ti/Au metallization and the evaporation of the oxygen from the ZnO substrate. [ABSTRACT FROM AUTHOR]

The transport properties of single InN nanowires grown by thermal catalytic chemical vapor deposition were measured as a function of both length/square of radius ratio and temperature. The resistivity of the n-type InN nanowires with diameter >100 nm was measured by the transmission line method and the value was on the order of 4×10-4 Ω cm. The specific contact resistivity for unalloyed Pd/Ti/Pt/Au ohmic contacts was near 1.09×10-7 Ω cm2. The temperature dependence of resistance showed a positive temperature coefficient and a functional form characteristic of metallic conduction in the InN. [ABSTRACT FROM AUTHOR]

Both electron-beam-deposited Au and Au/Ni/Au are found to produce low-resistance Ohmic contacts (ρC in the range 2.5×10-5–7.6×10-6 Ω cm2) to p-type ZnMgO (p∼1016 cm-3) on sapphire substrates after annealing at 600 °C. The as-deposited contacts are rectifying in both cases. For the Au contact, the annealing produces a small amount of outdiffusion of Zn to the surface of the contact, whereas with the Au/Ni/Au, both Ni and Zn are found on the surface. In both cases, the formation of Zn vacancies may play a role in increasing the near-surface hole concentration in the ZnMgO. [ABSTRACT FROM AUTHOR]

Electron-beam deposited Ni/Au is found to produce ohmic contacts to p-type ZnMgO (p∼1016 cm-3) after annealing in the range 300–450 °C. A minimum specific contact resistance of ∼10-2 Ω cm2 at room temperature and of 4×10-5 Ω cm2 at 473 K from circular transmission line measurements was obtained after annealing at 350 °C for 1 min in air. Higher anneal temperatures produced higher contact resistances and reaction of the contact metallurgy with diffusion of Ni to the surface of the Au layer where it became oxidized. The effective barrier height obtained from the measurement temperature dependence of the specific contact resistance, assuming thermionic emission, showed a value of 0.39±0.01 eV, much lower than the theoretical value of ∼2.4 eV. The difference in theoretical and experimental values is a clear indication of the strong role played by surface states, as reported earlier for Au Schottky contacts on ZnO. [ABSTRACT FROM AUTHOR]

Single ZnO nanorods with Ohmic contacts at either end exhibit large changes in current upon exposing the surface region to polar liquids introduced through an integrated microchannel. The polar nature of the electrolyte introduced led to a change of surface charges on the nanorod, producing a change in surface potential at the semiconductor/liquid interface. The nanorods exhibit a linear change in conductance between pH 2 and 12 of 8.5 nS/pH in the dark and 20 nS/pH when illuminated with ultraviolet (365 nm) light. The nanorods show stable operation with a resolution of ∼0.1 pH over the entire pH range. The results indicate that ZnO nanorods may have applications in integrated chemical, gas, and fluid monitoring sensors. [ABSTRACT FROM AUTHOR]

Single ZnO nanorods with diameters of ∼130 nm were grown on Au-coated Al2O3 substrates by catalyst-driven molecular beam epitaxy. Individual nanorods were removed from the substrate and placed between Ohmic contact pads and the current–voltage characteristics measured as a function of temperature and gas ambient. In the temperature range from 25 to 150 °C, the resistivity of nanorods treated in H2 at 400 °C prior to measurement showed an activation energy of 0.089±0.02 eV and was insensitive to the ambient used (C2H4,N2O,O2 or 10% H2 in N2). By sharp contrast, the conductivity of nanorods not treated in H2 was sensitive to trace concentrations of gases in the measurement ambient even at room temperature, demonstrating their potential as gas sensors. [ABSTRACT FROM AUTHOR]

Ohmic and Schottky contacts to p-type Zn[sub 0.9]Mg[sub 0.1]O are reported. The lowest specific contact resistivity of 3×10[sup -3] Ω cm[sup 2] was obtained for Ti/Au annealed at 600 °C for 30 s. Ni/Au was less thermally stable and showed severe degradation of contact morphology at this annealing temperature. Both Pt and Ti with Au overlayers showed rectifying characteristics on p-ZnMgO, with barrier heights of ∼0.55–0.56 eV and ideality factors of ∼1.9. Comparison of these results with the same metals on n-type ZnO indicates that high surface state densities play a significant role in determining the effective barrier height. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Measurement of the electrical characteristics of 250 devices on the same 2 in. diameter wafer shows that Ti/Al/Ir/Au Ohmic contacts on AlGaN/GaN high electron mobility transistors (HEMTs) have lower average specific contact resistance after annealing at 850 °C for 30 s (4.6×10[sup -5] Ω cm[sup 2]) compared to more standard Ti/Al/Ni/Au contacts (2×10[sup -4] Ω cm[sup 2]). HEMTs with these Ir-based contacts also show average interdevice isolation currents approximately a factor of 2 lower, higher peak transconductance (134 mS/mm compared to 121 mS/mm), and higher device breakdown voltage (31 V compared to 23 V) than the devices with Ni-based contacts. This Ir-based contact metallurgy looks promising for applications requiring extended thermal stability of the HEMTs. © 2004 American Institute of Physics. [ABSTRACT FROM AUTHOR]

A comparison was made of specific contact resistivity and morphology of Ti/Al/Pt/WSi/Ti/Au and Ti/Al/Pt/W/Ti/Au ohmic contacts to AlGaN/GaN heterostructures relative to the standard Ti/Al/Pt/Au metallization. The W- and WSi-based contacts show comparable specific resistivities to that of the standard contact on similar layer structures, reaching minimum values of ∼10[sup -5] Ω cm[sup 2] after annealing in the range 850–900 °C. However, the W- and WSi-based contacts exhibit much smoother surface morphologies, even after 950 °C annealing. For example, the root-mean-square roughness of the Ti/Al/Pt/WSi/Ti/Au contact annealed at 950 °C was unchanged from the as-deposited values whereas the Ti/Al/Pt/Au contact shows significant deterioration of the morphology under these conditions. The improved thermal stability of the W- and WSi[sub x]-based contacts is important for maintaining edge acuity during high-temperature operation. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The effects of the initial Mg reactivation annealing temperature, surface cleaning treatments, and contact annealing conditions on the specific contact resistance of Ni/Au ohmic contacts on p-GaN are reported. The lowest contact resistances were obtained for 900 °C activation annealing and cleaning steps that reduced the native oxide thickness (i.e., KOH rinsing). Removal of this interfacial oxide reduced the barrier for hole transport, providing a contact resistance of 9×10[sup -4] Ω cm[sup 2] for Ni/Au metallization annealed at 500 °C. The use of a ten period p-Al[sub 0.1]Ga[sub 0.9]N(Mg)/GaN(Mg) superlattice with individual layer thickness 50 Å led to a specific contact resistance of 9×10[sup -5] Ω cm[sup 2] under the same conditions. © 2001 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Reports on a thermal stability study of WSi[sub 0.79] contacts on Te-doped InP. Growth of the epitaxial InP layers on Fe-doped semi-insulating InP substrates by metalorganic molecular beam epitaxy; Use of tri-isopropylindium-di-isopropyltellurium as the Te precursor; Measurement of the specific contact resistivity.

Reports on a low contact resistance indium-based system of obtaining ohmic contacts to p-type gallium arsenide. Observation of excellent morphology and transfer resistance after annealing; Action of silver (Au) as a diffusion barrier to prevent Au spikes which will degrade the performance of the thin base layer heterojunction bipolar transistor.

Studies an in situ argon ion mill clean step prior to ohmic metal deposition developed to improve the uniformity of contact parameters and reduce the contact resistance. Native oxide regrowth of molecular beam epitaxy grown gallium arsenide and AlGaAs layers in vacuum chambers after ion mill cleaning; Oxide layers identified as the cause of problems in the formation of ohmic contacts.

Examines the use of InN grown by metalorganic molecular beam epitaxy for Ohmic contacts on GaAs/AlGaAs heterojunction bipolar transistors. Samples grown in an Intervac Gen II gas-source system on GaAs substrates; Demonstration of the total resistance of abrupt and graded contact structures as a function of the pad spacing; Drawback of the InN-based contacts.

Describes low resistance ohmic contacts on nitrogen ion bombarded indium phosphide (InP). Use of acceleration voltages of 100-300 V and exposure times of 3-1 minutes; Removal of InP native oxide and production of a shallow disordered donor layer; Identification of electron diffraction patterns.

Demonstrates a low contact resistance of nonalloyed titanium/platinum/gold metallization on n-type indium nitride. Investigation on the thermal stability of the ohmic contacts; Measurement of the low contact resistivity; Degradation of the contact morphology; Inquiry on the current transport mechanisms of metals.

Abstract: Ohmic contacts on n-GaN using a novel Ti/Al/ZrB2/Ti/Au metallization scheme were studied using contact resistance, scanning electron microscopy and Auger electron spectroscopy (AES) measurements. A minimum specific contact resistivity of 3×10−6 Ωcm2 was achieved at an annealing temperature of 700°C. The lowest contact resistance was obtained for 60s anneals. The contact resistance was essentially independent of measurement temperature, indicating that field emission plays a dominant role in the current transport. The Ti and Al in the contact stack began to outdiffuse to the surface at temperatures of ∼500°C, while at 1000°C the B also began to migrate to the surface. By this latter temperature, AES showed almost complete intermixing of the metallization even though the contact morphology was still smooth. The boride appears susceptible to getting of residual water vapor during sputter deposition. [Copyright &y& Elsevier]

Abstract: The annealing temperature dependence of contact resistance and layer stability of ZrB2/Ti/Au and Ni/Au/ZrB2/Ti/Au Ohmic contacts on p-GaN is reported. The as-deposited contacts are rectifying and transition to Ohmic behavior for annealing at ≥750°C, a significant improvement in thermal stability compared to the conventional Ni/Au Ohmic contact on p-GaN, which is stable only to <600°C. A minimum specific contact resistance of ∼2×10−3 Ωcm−2 was obtained for the ZrB2/Ti/Au after annealing at 800°C while for Ni/Au/ZrB2/Ti/Au the minimum value was 10−4 Ωcm−2 at 900°C. Auger Electron Spectroscopy profiling showed significant Ti, Ni and Zr out diffusion at 750°C in the Ni/Au/ZrB2/Ti/Au while the Ti and Zr intermix at 900°C in the ZrB2/Ti/Au. These boride-based contacts show promise for contacts to p-GaN in high temperature applications. [Copyright &y& Elsevier]

Abstract: The specific contact resistivity and chemical intermixing of Ti/Au and Ti/Al/Pt/Au Ohmic contacts on n-type Zn0.05Cd0.95O layers grown on ZnO buffer layers on GaN/sapphire templates is reported as a function of annealing temperature in the range 200–600°C. A minimum contact resistivity of 2.3×10−4 Ωcm2 was obtained at 500°C for Ti/Al/Pt/Au and 1.6×10−4 Ωcm2 was obtained at 450°C for Ti/Al. These values also correspond to the minima in transfer resistance for the contacts. The Ti/Al/Pt/Au contacts show far smoother morphologies after annealing even at 600°C, whereas the Ti/Au contacts show a reacted appearance after 350°C anneals. In the former case, Pt and Al outdiffusion is significant at 450°C, whereas in the latter case the onset of Ti and Zn outdiffusion is evident at the same temperature. The improvement in contact resistance with annealing is suggested to occur through formation of TiO x phases that induce oxygen vacancies in the ZnCdO. [Copyright &y& Elsevier]

Abstract: Ohmic contact formation on n-GaN using a novel Ti/Al/W2B/Ti/Au metallization scheme was studied using contact resistance, scanning electron microscopy and Auger electron spectroscopy measurements. A minimum specific contact resistivity of 7×10−6 Ωcm2 was achieved at an annealing temperature of 800°C. The contact resistance was essentially independent of measurement temperature, indicating that field emission plays a dominant role in the current transport .The Ti began to outdiffuse to the surface at temperatures of ∼500°C, while at 800°C the Al also began to intermix within the contact. By 1000°C, the contact showed a reacted appearance and AES showed almost complete intermixing of the metallization. The contact resistance showed excellent stability for extended periods at 200°C, which simulates the type of device operating temperature that might be expected for operation of GaN-based power electronic devices. [Copyright &y& Elsevier]

Abstract: N-polar and Ga-polar GaN grown on c-plane sapphire were used to fabricate platinum deposited Schottky contacts for hydrogen sensing at room temperature. After exposure to hydrogen, Ga-polar GaN Schottky barrier reduced by 3–4meV, while the N-polar GaN Schottky contacts became fully Ohmic. The N-polar GaN Schottky diodes showed stronger and faster response to 4% hydrogen than that of Ga-polar Schottky diodes. The abrupt current increase from N-polar GaN Schottky exposure to hydrogen was attributed to the high reactivity of the N-face surface termination. The surface termination dominates the sensitivity and response time of the hydrogen sensors made of GaN Schottky diodes. [Copyright &y& Elsevier]

Abstract: Ohmic contacts to p-type CuCrO2 using Ni/Au/CrB2/Ti/Au contact metallurgy are reported. The samples were annealed in the 200–700°C range for 60s in flowing oxygen ambient. A minimum specific contact resistance of ∼2×10−5 Ωcm2 was obtained after annealing at 400°C. Further increase in the annealing temperature (>400°C) resulted in the degradation of contact resistance. Auger Electron Spectroscopy (AES) depth profiling showed that out-diffusion of Ti to the surface of the contact stacks was evident by 400°C, followed by Cr at higher temperature. The CrB2 diffusion barrier decreases the specific contact resistance by almost two orders of magnitude relative to Ni/Au alone. [Copyright &y& Elsevier]

Abstract: Ir-based electrical contacts to p-type GaN have been fabricated and characterized. Both GaN//Ni/Au/Ir/Au and GaN//Ni/Ir/Au contact structures were deposited, however, only the former produced Ohmic current–voltage characteristics. At an anneal temperature of 500°C, the Ni/Au/Ir/Au contact had a specific contact resistance of ∼2×10−4 Ωcm2, comparable or superior to conventional Ni/Au contacts that are less thermally stable. Anneal temperatures above 500°C caused the Ir-based contact to fail. Auger electron spectroscopy was used to obtain depth profiles of both types of contacts at a variety of temperatures in order to provide insight into the mechanism of Ohmic formation as well as potential reasons for failure. A comparison to other metallization schemes on p-GaN is also given. [Copyright &y& Elsevier]

Abstract: There is a strong interest in developing thermally stable metallization schemes for ZnO and boride-based contact stacks are expected to have potential because of their thermodynamic stability. The contact characteristics on bulk single-crystal n-ZnO of a ZrB2/Pt/Au metallization scheme deposited by sputtering are reported as a function of annealing temperature in the range 300–800°C. The contacts were rectifying for anneal temperatures <500°C but exhibited Ohmic behavior at higher temperatures and exhibit a minimum specific contact resistivity of 9×10−3 Ωcm after 700°C anneals. The contact stack reverts to rectifying behavior after annealing above 800°C, coincident with a degraded surface morphology and intermixing of the Au, Pt and ZrB2. The boride-based contacts exhibit higher thermal stability but poorer specific contact resistivity than conventional Ti/Au metal stacks on ZnO. [Copyright &y& Elsevier]

Abstract: The use of a TiB2 diffusion barrier for Ni/Au contacts on p-GaN is reported. The annealing temperature (25–950°C) dependence of ohmic contact characteristics using a Ni/Au/TiB2/Ti/Au metallization scheme deposited by sputtering were investigated by contact resistance measurements and auger electron spectroscopy (AES). The as-deposited contacts are rectifying and transition to ohmic behavior for annealing at ≥500°C . A minimum specific contact resistivity of ∼3×10−4 Ωcm−2 was obtained after annealing over a broad range of temperatures (800–950°C for 60s). The contact morphology became considerably rougher at the higher end of this temperature range. AES profiling showed significant Ti and Ni outdiffusion through the TiB2 at 800°C. By 900°C the Ti was almost completely removed to the surface, where it became oxidized. Use of the TiB2 diffusion barrier produces superior thermal stability compared to the more common Ni/Au, whose morphology degrades significantly above 500°C. [Copyright &y& Elsevier]