Reduction In Specific Contact Resistance Research Articles

Reduction of specific contact resistance between the conducting polymer PEDOT:PSS and a metal electrode by addition of a second solvent during film formation and a post-surface treatment

The contact resistance at conducting polymer-electrode junctions could substantially affect device performance because electrical currents or signals generated in the polymer devices must be transferred in their final form through the junctions to the connected load or devices. We report here the specific contact resistance between the conducting polymer poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS) and a metal electrode as measured by a transmission line technique. Toward developing printed electronics that do not require vacuum processes, we focus on the specific contact-resistance, rc, between PEDOT:PSS films and metal pastes. Addition of a second solvent such as ethylene glycol (EG), poly-ethylene glycol (PEG), and dimethyl sulfoxide (DMSO) to PEDOT:PSS solution for film formation induced a significant reduction of rc together with an enhancement of electrical conductivity. A surface treatment using EG or DMSO on electrode areas of the film also decreased rc at the PEDOT:PSS-Ag paste junction to the value at the PEDOT:PSS-Ag junction formed by electron-beam evaporation. The mechanism of reduction of rc is discussed based on morphology of the PEDOT:PSS film.

Reduction of Specific Contact Resistance on n-Type Implanted 4H-SiC Through Argon Inductively Coupled Plasma Treatment and Post-Metal Deposition Annealing

The effects of Ar inductively coupled plasma (ICP) treatment on the Ohmic contact on n+-implanted SiC were investigated in this letter. The effects of ICP treatment were negligible on the Ni-silicide contact, because the treated surface layer was consumed during silicide formation. However, for the Ti contact, as the annealing temperature increased, the ion-bombardment-induced damage layer transformed to a uniform amorphous C-rich layer, and the specific contact resistance of the ICP-treated Ti contact decreased to $8.3\times 10^{-7}~\Omega$ -cm2 after 600 °C annealing. This is the lowest value achieved at such a low process temperature. Thus, appropriate ICP treatment and annealing can be used to form low-resistivity Ohmic contacts with a lower thermal budget that are comparable with Ni-based silicide Ohmic contacts.

Low resistance indium tin oxide contact to n-GaAs nanowires

Indium tin oxide (ITO) was deposited by RF sputtering on n-GaAs nanowires grown by the Au-assisted vapor–liquid–solid process in a molecular beam epitaxy (MBE) system. The ITO formed an Ohmic contact with n-doped (n = 8 × 1018 cm−3) GaAs nanowires with a specific contact resistance of <1.41 Ω cm2. Insertion of a 25 nm thick indium layer between 500 nm thick ITO and the GaAs nanowires resulted in a reduction of specific contact resistance to <0.13 Ω cm2 after annealing at 400 °C for 30 s. The In/ITO film had an average transmittance of 89% from 400 to 900 nm and a sheet resistance of 13 Ω/□, which is well suited for nanowire-based optoelectronic applications.

Copper-Metalized GaAs pHEMT with Cu/Ge Ohmic Contacts

The fully Cu-metalized GaAs pHEMT using developed Cu/Ge-based ohmic contacts and T-gate Ti/Mo/Cu with length of the 150 nm has been successfully fabricated for the high-frequency applications. The fabricated Cu-metalized pHEMT has a maximum drain current of 360 mA/mm, an off-state gate-drain breakdown of 7 V, and a transconductance peak of 320 mS/mm at Vds=3 V. The maximum stable gain value was about 15 dB at frequency 10 GHz. The current gain cutoff frequency of the copper-metalized device is about 60 GHz at Vds=3 V, and maximum frequency of oscillations is beyond 100 GHz. This work investigated in detail the formation of Cu/Ge ohmic contacts to n-GaAs with an atomic hydrogen preannealing step. It was shown that after the first preliminary annealing is carried out in a flow of atomic hydrogen with a flow density of atoms of 1013/1016 at. cm2 s−1 a reduction in specific contact resistance by 2/2.5 times is observed. The reduction in specific contact resistance is apparently caused by the action of the hydrogen atoms which minimise the rate of the oxidizing reactions and activate solid phase reactions forming the ohmic contact during the thermal treatment process.

Evaluation of Specific Contact Resistance of Al, Ti, and Ni Contacts to N Ion Implanted 3C-SiC(100)

The specific contact resistance of Al, Ti and Ni ohmic contacts to N+ implanted 3C-SiC(100) has been investigated by means of TLM method. The p-type epitaxial layer grown on n+ substrate is multiply implanted with N ions with energy ranging from 15 to 120 keV at a total dose of 1.4×1015 cm-2 at room temperature and is subsequently annealed by RF annealer at a temperature of 1400 oC for 10 min in Ar gas flow, resulting in the sheet resistance of 130 0/sq. The deposited Al layer on the annealed sample shows the extremely low specific contact resistance of about 1×10-7 0cm2. The ohmic contacts of Ti and Ni also show the specific contact resistance of 5×10-6 and 2×10-5 0cm2, respectively. The obtained specific contact resistance is proportional to the Schottky barrier height of metal cotact to n-type 3C-SiC. The annealing of Ni ohmic contact above 600 oC results in the considerable reduction of specific contact resistance due to the silicidation of Ni.

Minimization of specific contact resistance in multiple gate NFETs by selective epitaxial growth of Si in the HDD regions

High parasitic S/D resistance is a major obstacle in realizing future generations of CMOS technologies using multiple gate FETs with narrow fins. Reduction of specific contact resistance by selective epitaxial growth of Si in heavily doped S/D regions of a multiple gate FET helps with achieving low S/D resistance. This paper addresses integration of low temperature selective epitaxial growth process into multiple gate FET processing. Our experimental results show more than 30% reduction in the parasitic S/D resistance for 16-nm selective epitaxial growth of Si in the heavily doped S/D regions of multiple gate NFETs with less than 20-nm wide fins. A follow up of this work with HfO 2-TiN gate stack shows more than 20% improvement in the drive current at a constant I OFF for 40-nm selective epitaxial growth of Si in the heavily doped S/D regions of multiple gate FETs.

Characterization of Pd/Ni/Au ohmic contacts on p-GaN

A low-resistance ohmic contact to Mg-doped p-type GaN grown by metal-organic chemical vapor deposition (MOCVD) with a carrier concentration of 2 × 10 17 cm −3 using Pd/Ni/Au metallization was formed. An anneal at 500 °C for 1 min in a flowing N 2 ambient produced an excellent ohmic contact with a specific contact resistivity as low as 2.4 × 10 −5 Ω cm 2. X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) depth profiles of Pd/Ni/Au contacts annealed at 500 °C demonstrated a strong correlation between Ni and Pd interdiffusion toward the GaN surface and a reduction in specific contact resistance. The low contact resistance is attributed to the reduction of the native oxide by Ni diffusion along with the formation of Pd and Ni related-gallide phases at the p-GaN surface region.

Tungsten silicide contacts to polycrystalline silicon and silicon–germanium alloys

Silicon–germanium alloy layers will be employed in the source–drain engineering of future MOS transistors. The use of this technology offers advantages in reducing series resistance and decreasing junction depth resulting in reduction in punch-through and SCE problems. The contact resistance of metal or metal silicides to the raised source–drain material is a serious issue at sub-micron dimensions and must be minimised. In this work, tungsten silicide produced by chemical vapour deposition has been investigated as a contact metallization scheme to both boron and phosphorus doped polycrystalline Si 1− x Ge x , with 0 ≤ x ≤ 0.3. Cross bridge Kelvin resistor (CKBR) structures were fabricated incorporating CVD WSi 2 and polycrystalline SiGe. Tungsten silicide contacts to control polysilicon CKBR structures have been shown to be of high quality with specific contact resistance ρ c values 3 × 10 −7 ohm cm 2 and 6 × 10 −7 ohm cm 2 obtained to boron and phosphorus implanted samples respectively. The SiGe CKBR structures show that the inclusion of Ge yields a reduction in ρ c for both dopant types. The boron doped SiGe exhibits a reduction in ρ c from 3 × 10 −7 to 5 × 10 −8 ohm cm 2 as Ge fraction is increased from 0 to 0.3. The reduction in ρ c has been shown to be due to (i) the lowering of the tungsten silicide Schottky barrier height to p-type SiGe resulting from the energy band gap reduction, and (ii) increased activation of the implanted boron with increased Ge fraction. The phosphorus implanted samples show less sensitivity of ρ c to Ge fraction with a lowest value in this work of 3 × 10 −7 ohm cm 2 for a Ge fraction of 0.3. The reduction in specific contact resistance to the phosphorus implanted samples has been shown to be due to increased dopant activation alone.

Contact Resistance Measurement of Bonded Copper Interconnects for Three-Dimensional Integration Technology

A novel test structure for contact resistance measurement of bonded copper interconnects in three-dimensional integration technology is proposed and fabricated. This test structure requires a simple fabrication process and eliminates the possibility of measurement errors due to misalignment during bonding. Specific contact resistances of bonding interfaces with different interconnect sizes of approximately 10/sup -8/ /spl Omega/-cm/sup 2/ are measured. A reduction in specific contact resistance is obtained by longer anneal time. The specific contact resistance of bonded interconnects with longer anneal time does not change with interconnect sizes.

Effects of Surface Treatment on the Electrical Properties of Ohmic Contacts to (In)GaN for High Performance Optical Devices

We investigate the effects of surface treatment on the Ohmic properties of the contacts to p-GaN and n-(In)GaN. I–V measurements show that for all the samples, the two-step surface treatment causes a large reduction in specific contact resistance. The I–V and XPS results show that for the Pt contacts to p-GaN, the two-step treatment using a BOE solution results in a reduction of SBH due to the effective removal of native oxide and the shift of the surface Fermi level toward the valence band edge. It is also shown that the two-step treatment using a HCl solution is very effective in improving the electrical and thermal properties of the Ti/Al contacts to n-(In)GaN. These results indicate that the two-step surface treatment can be a promising processing technique for optical devices.

Common n- and p-Contact for Serial Wiring of Quaternary Antimonide Monolithic Interconnected Module Thermophotovoltaic Devices

A common contact to both n and p Ga-based antimonide materials has been developed for serial wiring of quaternary monolithic interconnected module thermophotovoltaic devices. The contact consists of a multilayer stack of Pd/Ge/Au/Ti/Au. Circular transmission line model measurements yield a specific contact resistance as low as for contacts to n-GaSb and for contacts to p-GaSb without annealing. Further reduction in specific contact resistance can be obtained for contacts annealed in nitrogen ambient for 10 s at 250-300°C. Cell isolation and thermophotovoltaic diodes fabricated using the five-layer metallization for both the p- and n-type contact exhibit current-voltage characteristics equivalent to diodes formed using multilayer metallization for the n-type contact and standard Ti/Au for the p-contact. While annealing improves contact properties, excessive annealing can lead to diffusion of the metallization into the semiconductor and lead to device degradation. © 2002 The Electrochemical Society. All rights reserved.

Novel type of ohmic contacts to p-doped GaN using polarization fields in thin InxGa1−xN capping layers

Thin p-doped InGaN layers on p-doped GaN were successfully used to demonstrate a new type of low-resistance ohmic contact. A significant reduction of specific contact resistance can be achieved by increasing the free-hole concentration and the probability for hole tunneling through the Schottky barrier as a consequence of polarization-induced band bending. As obtained from the transmission-line method, the specific contact resistances of Ni (10 nm)/Au (30 nm) contacts deposited on InGaN capping layers were 1.2 × 10-2 Ωcm2 and 6 × 10-3 cm2 for capping layer thicknesses of 20 nm and 2 nm, respectively.

Effect of alcohol-based sulfur treatment on Pt Ohmic contacts to p-type GaN

The effects of an alcohol-based (NH4)2S solution [t-C4H9OH+(NH4)2S] treatment on Pt Ohmic contacts to p-type GaN are presented. The specific contact resistance decreased by three orders of magnitude from 2.56×10−2 to 4.71×10−5 Ω cm2 as a result of surface treatment using an alcohol-based (NH4)2S solution compared to that of the untreated sample. The O 1s and Pt 4f core-level peaks in the x-ray photoemission spectra showed that the alcohol-based (NH4)2S treatment was effective in removing of the surface oxide layer. Compared to the untreated sample, the alcohol-based (NH4)2S-treated sample showed a Ga 2p core-level peak which was shifted toward the valence-band edge by 0.25 eV, indicating that the surface Fermi level was shifted toward the valence-band edge. These results suggest that the surface barrier height for hole injection from Pt metal to p-type GaN can be lowered by the surface treatment, thus resulting in a drastic reduction in specific contact resistance.

Reactivation of Mg acceptor in Mg-doped GaN by nitrogen plasma treatment

Mg-doped GaN films, grown by metalorganic chemical vapor deposition, were treated with a nitrogen plasma after a conventional rapid thermal annealing (RTA). The conductivity of the p-type GaN film was greatly enhanced by nitrogen plasma treatment, and exhibited a higher sheet hole concentration as well as lower sheet resistance than the RTA samples. A photoluminescence (PL) band which peaked at 3.27 eV was new, and a band at 2.95 eV was markedly attenuated in the plasma treated samples. PL measurements suggest that self-compensation in a Mg-doped GaN caused by the nitrogen vacancies is effectively reduced by the nitrogen plasma treatment, leading to an enhanced p-type conductivity. In addition, the plasma-treated sample revealed a drastic reduction in specific contact resistance by three orders of magnitude, compared with the RTA samples.

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.

Contact Structure for a Superconducting Field Effect Transistor Using SrTiO3/YBa2Cu3O7- x Films

A contact structure for a superconducting field effect transistor (SuFET) was developed using a sequentially-deposited SrTiO3 (STO)/YBa2Cu3O7-x (YBCO) bilayer. A buried contact structure was proposed to contact the edge of the ultrathin YBCO layer of the STO/YBCO bilayer, which was used as a channel layer of a SuFET. To reduce the contact resistance, focused ion beam (FIB) etching of the bilayer and in situ noble metal deposition were adopted. Specific contact resistance was reduced to 5×10-8 Ω cm2 for a 30-nm-thick YBCO film without annealing. Furthermore, annealing in ozone caused a reduction of specific contact resistance to 3×10-8 Ω cm2 for a 20-nm-thick YBCO film. This structure is useful to shrink the device dimensions and to improve the modulation characteristics of a SuFET.

Pd/Zn/Pd/Au ohmic contacts to p-type In0.47Ga0.53As/InP

The electrical properties of Pd/Zn/Pd/Au ohmic contacts to p-type In0.47Ga0.53As/InP have been investigated as a function of the ratio of the interfacial Pd and Zn layers and the annealing treatment. For as-deposited contacts, the presence of an increasing thickness of interfacial Zn and Pd to ∼300 Å in the metallization resulted in a reduction in specific contact resistance, ρc, to a low value of 1.2×10−5 Ω cm2. Annealing of all of the contact configurations except the Zn=0 and 20 Å structures produced a reduction in ρc to a minimum value of 7.5×10−6 Ω cm2 at 500 °C. A critical thickness of the Zn≥50 Å and Pd≥100 Å interfacial layers was required in order to produce a significant reduction in ρc during annealing. These results have been interpreted in terms of the reaction between Pd and In0.47Ga0.53As and an associated doping at the near surface region by Zn atoms. Annealing of the contacts at temperatures of ≥450 °C resulted in significant intermixing of the metal layers and the In0.47Ga0.53As as revealed by Rutherford backscattering spectrometry and Auger depth profiling.

Reduction of specific contact resistance on GaInAsP by rapid thermal process anneal

An order of magnitude improvement in the specific contact resistance of gold-based ohmic contacts to p-type GaInAsP (Eg=1.13 eV) is reported. A novel technique using a silicon susceptor has been employed in a rapid thermal processor. A direct comparison between gold-based contacts annealed in a conventional furnace and the rapid thermal processor indicated a specific contact resistance of 4.2×10−5 and 4.1×10−6 Ω cm2, respectively. Auger electron spectroscopy, in depth profile mode, revealed two different metallurgical profiles for the conventional furnace and the rapid thermal processor. The rapid thermal processor was successfully implemented in a p-i-n optical detector process resulting in a reduction in the device series resistance and improved performance.