Specific Contact Resistance Values Research Articles

Nonalloyed Al ohmic contacts to MgxZn1−xO

Al nonalloyed ohmic contacts were fabricated and characterized on MgxZn1-xO (0 ≤ × ≤ 0.34) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). Specific contact resistances were evaluated by the transmission line method (TLM). A specific contact resistance of 2.5 × 10-5 Ωcm2 was obtained for Al contact to ZnO with an electron concentration of 1.6 × 1017 cm-3. The current flow mechanism was studied by investigating the dependence of specific contact resistances on electron concentration and on temperature. For Al contact to Mg0.34Zn0.66O, specific contact resistance values are two orders of magnitude larger than that of Al ohmic contacts to ZnO.

Chemical, electrical, and structural properties of Ni/Au contacts on chemical vapor cleaned p-type GaN

Chemical vapor cleaned, Mg-doped, p-type GaN(0001) surfaces and Ni/Au contacts deposited on these surfaces have been studied using several characterization techniques. Stoichiometric surfaces without detectable carbon and an 87% reduction in the surface oxygen to 2±1 at. % were achieved. The binding energies of the Ga 3d and N 1s core level photoelectron peaks were reduced by 0.5±0.1 eV following the chemical vapor clean. The band bending at the clean surface was measured to be 0.8±0.1 eV. As-deposited Ni/Au contacts on chemical vapor cleaned surfaces exhibited significantly less rectification in the low voltage region (<2 V) compared to identical contact structures on conventional HCl treated surfaces. The specific contact resistance of these contacts deposited on chemical vapor cleaned surfaces and subsequently annealed at 450 °C for 30 seconds was 3±2 Ω cm2. Improved ohmic behavior and a specific contact resistance of 4±2 Ω cm2 was obtained for contacts deposited on HCl treated surfaces and annealed using the same schedule. The formation of Au:Ga and Au:Ni solid solutions was observed for contacts on HCl treated surfaces following the 450 °C anneal. There were significantly less interfacial reactions for annealed contacts on chemical vapor cleaned surfaces. The values of specific contact resistance, sheet resistance, and transfer length of the annealed contacts deposited on both chemical vapor cleaned and HCl treated surfaces and measured from room temperature to 140 °C did not change during three successive thermal cycles within this range.

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

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

Characteristics of Au/Ti/p‐diamond ohmic contacts prepared by r.f. sputtering

AbstractLow‐resistance ohmic contacts were fabricated on diamond films by boron ion implantation and subsequent Ti/Au bilayer metallization. The I–V measurements showed that the as‐deposited contacts were ohmic. After annealing at 500 °C for 10 min in a vacuum of 10−4Pa, the I–V characteristics were improved significantly. As a result of annealing, the specific contact resistance (ρC) value, measured by the circular transmission line model (CTLM), decreased from 6.2 × 10−3 to 1.2 × 10−6 Ω·cm2. Compared with the value calculated by the transmission line model (TLM), the ρC value was reduced by more than two orders of magnitude for the annealed contacts. The CTLM should be more accurate in measurement than the TLM. The changes of the ρC value with the operating temperature indicated that the ρC value decreased with increasing operating temperature at low temperatures. Tunnelling was suggested to be the dominant transport mechanism at the metal/diamond interface. The band model was quantified. Copyright © 2001 John Wiley & Sons, Ltd.

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.

Low Resistance Non-Transparent ohmic Pt-contacts on p-GaN

AbstractThe metal – p-GaN junction for low resistance ohmic contacts is still a challenge to be applied in GaN-based opto electronics as well as in power and high frequency devices. Currently, we try to improve the performance of our blue laser diodes. In order to decrease heat generation during device operation it is necessary to ensure as small contact resistances as possible.In this work, we achieved a specific contact resistance value of RC = 1.8 ± 1.7. 10-5 Ωcm2 for Pt-contacts on MOVPE-grown p-GaN. The Pt-layers were deposited by e-beam and thermally assisted vacuum evaporation after a standard cleaning process. For evaluation of Rc we used optimised circular TLM test patterns defined by photolithography. Best contacts were formed by annealing in Nitrogen athmosphere at 500°C.We also investigated the dependence of the contact resistance on the Mg doping concentration. Therefore p-GaN layers with different Mg-concentrations were grown on SiC-substrates and Pt-contacts were processed. For those samples, we investigated the Mg-concentrations, verified by secondary ion mass spectroscopy (SIMS), the hole concentrations and mobilities in dependence of C(Mg), which we obtained from HALL-measurements, and the contact and sheet resistances, measured by circular TLM measurements.The experiments showed that the optimum Mg-concentration for low contact resistances is higher than 2 1019 cm-3 which was found to provide a maximum hole concentration near 7 1017 cm-3. The influence of self-compensation in p-GaN in bulk and near interfaces will be discussed.

New challenges to the modelling and electrical characterisation of ohmic contacts for ULSI devices

Continuing developments in semiconductor process and materials technology have enabled significant reductions to be achieved in the contact resistance R c of devices. This reduction is commonly assessed in terms of the specific contact resistance (SCR) parameter ρ c (Ω cm 2) of the metal–semiconductor interface. Such a reduction in SCR is essential, for as device dimensions decrease, then so also must ρ c and the corresponding contact resistance in order not to compromise the down-scaled ULSI device performance. Thus the ability to accurately model contacts and measure ρ c is essential to ohmic contact development. The cross kelvin resistor (CKR) test structure is commonly used to experimentally measure the Kelvin resistance of an ohmic contact and obtain the specific contact resistance ρ c. The error correction curves generated from computer modelling of the CKR test structure are used to compensate for the semiconductor parasitic resistance, thus giving the SCR value. In this paper the increased difficulty in measuring lower ρ c values, due to trends in technology, is discussed. The challenges presented by the presence of two interfaces in silicided contacts (metal-silicide–silicon) is also discussed. Experimental values of the SCR of an aluminium–titanium silicide interface is determined using multiple CKR test structures.

Al/Ni And Al/Ti Ohmic Contact To P-type SiC Diffused Layer

ABSTRACTOhmic contacts to p-type SiC were fabricated by depositing Al/Ni and Al/Ti followed by high temperature annealing. A p-type layer was fabricated by Al or B diffusion from vapor phase into both p-type and n-type substrates. The thickness of the diffused layer was about 0.1–0.2 μm with surface carrier concentration of about 1.0×1019cm−3. Metal contacts to a p-type substrate with a background doping concentration of 1.2×1018cm−3, without a diffusion layer, were also formed. The values of specific contact resistance obtained by Circular Transmission Line Method (CTLM) and Transfer Length Method (TLM) for the n-type substrate, and by Cox & Strack method for p-type substrate, respectively, varied from 1.3×10−4Ωcm2 to 8.8×10−3 Ωcm2. The results indicate that the specific contact resistance could be significantly reduced by creating a highly doped diffused surface layer.

Effects of Surface and Interlayer Processing Conditions on Selected Ohmic Contact Metallizations for p-type Silicon Carbide

ABSTRACTA comparative study of Pt and Pt/Si contacts to p-type 6H-SiC in terms of various processing conditions and interlayer specifications was performed. Deposition temperature, the thickness of the Si layer, and B-dopant incorporation in the Si were found to significantly affect the specific contact resistivity (SCR) values. In addition, pre-etching of the SiC surface in SF6 + Ar was found to consistently reduce the SCR's. The lowest average SCR values were 3 × 10−5 Ωcm2 for Pt/Si/SiC contacts deposited on pre-etched SiC surfaces (7.0 × 1018 cm−3 doping concentration) and annealed at 1100 °C for 5 min.Aluminum-titanium contacts also showed dependence on the thicknesses of the Al and Ti layers and on the locations of the layers. Differences in both the SCRs and surface morphology are presented.

Reaction Kinetics of Thermally Stable Contact Metallization on 6H-SiC

ABSTRACTThe growth kinetics of thermally stable Ti(100nm)/TaSi2 (200nm)/Pt (300nm) metallization on 6H-SiC was studied after heat treatment in air up to 700°C. Scanning electron microscopy (SEM) of the contact surface morphology reveals a two-dimensional network of features that is attributed to non-uniform oxide growth associated with the multigrain structure of the platinum overlayer. Auger electron spectroscopy (AES) and high-resolution transmission electron microscopy (HRTEM) identified three important reaction zones after initial 30-minute anneal at 600°C in nitrogen. One is the formation of a platinum silicide overlayer resulting from TaSi2 decomposition. The second is titanium silicide formation adjacent to the decomposed TaSi2. The third is pseudo-epitaxial Ti5Si3 at the SiC interface. Specific contact resistance values ranging from 10−4–10−6 Ω-cm2, remained stable after 200 hours at 600°C in air. Activation energies of 1.03eV for platinum silicide oxidation and 1.96eV for Ti5Si3 are obtained from Arrhenius plots.

Modelling the Influence of the Silicon Doping Profile on a Silicide Well Ohmic Contact

AbstractLow values of contact resistance Rc(Ω) and specific contact resistance, ρc(Ωcm2) are essential for ohmic contacts to devices with ULSI dimensions. The incorporation of silicides into contact structures has made a substantial contribution towards this goal. However the modelling and characterisation of these more complex silicide contact structures still requires considerable development. The variation of doping density in the vertical direction will cause a strong local variation in silicon resistivity with depth as well as a pronounced variation in the silicon-silicide barrier layer as a function of depth.In this paper we use a three dimensional (3-D) finite element analysis of sub-micron silicide contacts to illustrate the influence on Rc of the silicon doping profile in the sourcedrain contact regions. In addition, the influence of the doping profile on the current density distribution at the silicon-silicide interface is presented. A comparison of results is made between a uniform doping profile and two ion-implanted profiles. In order to improve the model's accuracy, the often neglected contribution of the metal-silicide barrier (ρca) to Rc is taken into account. Experimental measurements using a Cross Kelvin Resistor (CKR) test structure, have been used to determine ρca (∼3–5 × 10−9Ω.cm2 ) of the aluminium-TiSi2 interface.

Ohmic contacts to n-type and p-type ZnSe

Stable Ohmic contacts are essential for reliable operation of electronic devices. Such contacts have been made to n-type and p-type ZnSe. Au, Pd, Cu and Se for p-type N-doped ZnSe (1×10 17 cm −3) and AuGe, In, Yb and Mg for n-type Cl-doped ZnSe (4.5×10 18 and 1.15×10 19 cm −3) grown by molecular beam epitaxy (MBE) on (100) semi-insulating GaAs substrates have been deposited by thermal evaporation. Annealing techniques at different temperatures, chemical etching and cleaning prior to metallization and reactive ion etching (RIE) in a N 2 plasma and a Ar plasma for p-ZnSe have been studied. The electrical characteristics for the contacts were examined by the current versus voltage curves and the specific contact resistance was determined by use of the transmission line method (TLM). The current transport mechanisms for the Mg/Au contact to n-type ZnSe and the Cu/Au contact to p-type ZnSe have been studied by the current versus voltage for different temperatures ( I– V– T) measurements. In/Au was best for n-type and Cu/Au for the p-type materials. Plasma treatment of the ZnSe surface prior to metallization was proven to lower the contact resistance to p-type ZnSe. The lowest specific contact resistance values of 1.67×10 −1 Ω cm 2 for the Cu/Au contact to p-type ZnSe with a N 2 plasma treatment and 1.04×10 −2 Ω cm 2 for the In/Au contact to n-type ZnSe were achieved. Two different current flow mechanisms were shown for the Cu/Au contact to low doped p-ZnSe (1×10 17 cm −3) and three for Mg/Au contact to highly doped n-ZnSe (1.15×10 19 cm −3). The Cu/Au contact to p-ZnSe and Mg/Au contact to n-ZnSe have been observed to be especially stable and reproducible.

Ohmic contacts formed by electrodeposition and physical vapor deposition on p-GaN

Electrodeposited Pt and sputtered Ni/Pt contacts to p-GaN (p=4.6×1017 cm−3) are reported and compared to sputtered Ni, Pt, and Ni/Au contacts and electron beam and thermally evaporated Ni contacts. Sequential rapid thermal annealing was employed with samples receiving an initial five minute heat treatment of 400 °C followed by 1 min anneals at 500, 600, and 700 °C, all under flowing N2. Plots of current versus voltage for all contacts showed nonlinearity through the origin as deposited and for all annealing conditions. Extracted values of specific contact resistance are thus determined using the measured resistance for a given value of applied current. The lowest contact resistivity was reproducibly provided by the electrodeposited Pt contacts. After a 1 min anneal at 600 °C, a contact resistivity of 1.50×10−2 Ω cm 2 was obtained using the circular transmission line method at a measurement current of 10 mA. Sputtered Ni/Pt contacts provided a contact resistivity of 1.81×10−2 Ω cm2 at 10 mA after a 1 min anneal at 600 °C, while all other metallizations yielded contact resistivities from 3–4×10−2 Ω cm 2. Possible reasons for the lower contact resistivity of the electrodeposited contacts are discussed.

A study of P-type ohmic contacts to InA1As/InGaAs heterostructures

Optical modulators operating at near-infrared wavelengths are of interest for a variety of applications including bi-directional communications and optical interconnects. The fabrication of 1.06 μm and 1.32 μm operating wavelength strained-layer superlattice vertical-cavity optoelectronic modulators requires the formation of a p-type ohmic contact to the InAlAs/InGaAs quarter-wave bottom mirror stack. In this study, BeAu and TiPtAu p-type ohmic contact metallization schemes were evaluated for use on molecular beam epitaxy grown In 0.10Al 0.90As/ In 0.12Ga 0.88As and In 0.32Al 0.68As/In 0.33Ga 0.67As device heterostructures. Recessed and non-recessed transmission line measurement structures were fabricated and evaluated as a function of rapid thermal anneal temperatures over the range of 360–420 °C. Atomic force microscopy was used to determine the surface morphology of each sample for evidence of metal or material degradation. For contacts directly on InGaAs layers, TiPtAu contacts had relatively high specific contact resistance values of π c ∼ 3 × 10 −4 Ω cm 2 and displayed no dependence on the anneal. The BeAu contacts had minimum specific contact resistance values π c ∼ 5 × 10 −7 Ω cm 2 but showed evidence of higher temperatures. Contacts directly made to InAlAs layers had minimum specific contact resistances of π c ∼ 4 × 10 −5 Ω cm 2 and were improved slightly with the addition of a thin GaAs layer.

Au - Be/Ru/Au multilayer metallization as a stable ohmic contact scheme to p-type InP

An attempt has been made to improve the electrical and metallurgical stability of Au - Be alloyed contacts to moderately doped p-type InP by minimizing the thickness of the Au - Be contact layer and using an Ru layer as a new, more effective diffusion barrier between the Au - Be and a thick Au top layer. It was found that the Au - Be contact layer only 40 - 50 nm thick is sufficient to give excellent ohmic contacts with specific contact resistance values as low as and (for and respectively). When subjected to an aging test at for 50 h in gas, the 50 nm Au - Be/50 nm Ru/300 nm Au contacts alloyed at an optimum temperature of about exhibit good thermal stability and no substantial increase in the specific contact resistance. The remarkable metallurgical stability of such contacts was confirmed by secondary neutral mass spectroscopy (SNMS) in-depth profile measurements. A comparison with the previously investigated diffusion barrier metals (such as Cr, Ti, Pt, etc) shows that the Ru layer is a much better barrier against the migration of Au into the InP substrate and, at the same time, it suppresses the out-diffusion of In and P from the semiconductor.

A unique, device-friendly contact system for shallow junction p/n indium phosphide devices

We have discovered what appears to be a unique contact system for use on p-type InP. The new contacts provide low resistance contact to p-InP without the violent metallurgical intermixing that would normally take place between the emitter material and the contact metallization during the contact sintering process. With this new contact system, which is composed of a combination of Ag and Zn, it is possible, for the first time, to make low resistance ohmic contact directly to a shallow junction p/n InP device without destroying the device while doing so. Specific contact resistivity values in the low 10−4 Ω-cm2 range are readily achieved. After a description of the new system, we suggest possible mechanisms to explain the observed behavior.

Low resistance ohmic contact for p-type ZnTe using Au electrode

We report low-resistive Au ohmic contact for a molecular beam epitaxy grown nitrogen doped p-type ZnTe (p-ZnTe:N) layer with high thermal stability, where the p-ZnTe surface is treated by oxygen plasma and HCl solution prior to Au film deposition. C contamination and native oxide layers of ZnTe are removed by oxygen plasma and HCl treatment, respectively, from the surface. As a result, a Te-rich layer is formed on the surface. The annealing temperature dependence of specific contact resistance is investigated. The specific contact resistance of Au ohmic contact for p-ZnTe:N with carrier concentration of 2×1018 cm−3 treated by oxygen plasma and HCl solution reaches as low as 5.8×10−6 Ω cm2 at an annealing temperature of 350 °C. This specific contact resistance value is the same as that of the reported Pd/(Pt)/Au contact for p-ZnTe:N with carrier concentration of 3×1019 cm−3, and clearly shows higher thermal stability.

The role of Ru in improving Schottky and ohmic contacts to InP

Wet chemical passivation of InP was performed in an acid solution that contained Ru ions in solution. Results showed a drastic increase in the Schottky barrier height of Ru contacts from 0.49 eV before passivation to about 0.88 eV after passivation and annealing at 125 ° C. Furthermore, a corresponding decrease in reverse leakage current of more than three orders was also obtained by passivation. The corresponding value for the ideality factor was 1.16. The higher barrier height was related to the XPS results, which showed a decrease in the amount of oxidized indium and phosphorus after passivation. Au/Ru/Au-Ge/Ni/InP ohmic contacts showed remarkable improved surface morphology after rapid thermal annealing, in comparison with the standard Au/Ni/Au-Ge/InP rapid thermal annealed (RTA) ohmic contacts. The Ru-containing contacts also showed a lower minimum specific contact resistance value of 1 × 10 −7 Ω cm 2 , after 400 ° C RTA. From comparative electrical, SEM and Auger electron spectroscopy (AES) investigations, it is clear that Ru has various advantages as a very effective diffusion barrier for ohmic contacts on InP.

Non-alloyed, refractory metal contact optimization with shallow implantations of Zn and Mg

Refractory metal contacts to GaAs show great promise for stability during high temperature processing and for high reliability. In this paper, we report on a study of sputtered W and W-Si contacts to ion-implanted p-GaAs with both Zn and Mg implantations. This study focused on refractory contacts to shallow implanted contact layers that are suitable for devices such as junction field effect transistors and heterojunction bipolar transistors. The very different energy loss mechanisms of Zn and Mg ions result in different levels of implant damage, which is studied by varying the annealing temperatures and measuring the effects on contact and sheet resistances with the transmission line method. For the fabrication schemes investigated, the specific contact resistivity vs. annealing temperature values with implant doses form 1 × 1014 to 5 × 1015 cm-2 are found to vary from non- ohmic to 10-7ω cm2. Low resistance contacts to shallow (<800 Å) implanted layers are achieved.

Contact and via structures with copper interconnects fabricated using dual Damascene technology

A novel submicron process sequence was developed for the fabrication of CoSi/sub 2//n/sup +/-Si, CoSi/sub 2//p/sup +/-Si ohmic contacts and multilevel interconnects with copper as the interconnect/via metal and titanium as the diffusion barrier. SiO/sub 2/ deposited by plasma enhanced chemical vapor deposition (PECVD) using TEOS/O/sub 2/ was planarized by the novel technique of chemical-mechanical polishing (CMP) and served as the dielectric. The recessed copper interconnects in the oxide were formed by chemical-mechanical polishing. (dual Damascene process). Electrical characterization of the ohmic contacts yielded contact resistivity values of 10/sup -6//spl Omega/-cm/sup 2/ or less. A specific contact resistivity value of 1.5/spl times/10/sup -8//spl Omega/-cm/sup 2/ was measured for metal/metal contacts.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>