Specific Contact Resistance Values Research Articles

A new approach in impurity doping of 4H-SiC using silicidation

Oxidation and silicidation have been found to enhance phosphorus diffusion and incorporation in 4H-SiC. Depth profiling by secondary ion mass spectrometry showed significant concentration of phosphorus in the order of 1018–1019 cm−3 in the near-surface region of 4H-SiC in both oxidation and silicidation-assisted phosphorus-diffused samples. However, silicidation was remarkably more effective than oxidation in promoting phosphorus diffusion, producing comparable phosphorus concentration at even greater depth at a temperature of only 900 °C. Specific contact resistance values of the phosphorus-doped samples confirmed feasibility of this method in ohmic contact fabrication on SiC.

Very low specific contact resistance measurements made on a highly p-type doped 4H-SiC layer selectively grown by vapor-liquid-solid transport

This work reports on the performances of ohmic contacts fabricated on highly p-type doped 4H-SiC epitaxial layer selectively grown by vapor-liquid-solid transport. Due to the very high doping level obtained, the contacts have an ohmic behavior even without any annealing process. Upon variation of annealing temperatures, it was shown that both 500 and 800 °C annealing temperature lead to a minimum value of the Specific Contact Resistance (SCR) down to 1.3×10−6 Ω⋅cm2. However, a large variation of the minimum SCR values has been observed (up to 4×10−4 Ω⋅cm2). Possible sources of this fluctuation have been also discussed in this paper.

Two-contact Circular Test Structure for Determining Specific Contact Resistivity

ABSTRACTAs ohmic contacts decrease in size and approach nanoscale dimensions, accurate electrical characterization is essential, requiring the development of suitable test structures for this task. We present here a new test structure derived from the standard three-contact circular transmission line model (CTLM) [1], for determining the specific contact resistivity of ohmic contacts. This test structure minimizes sources of error which arise from the CTLM by – (i) reducing the number of contacts within one test pattern from three to two, (ii) ensuring the assumption of equipotential metal contacts used in modelling is more easily attained experimentally, and (iii) allowing the fabrication of reduced geometrical dimensions essential for determining low specific contact resistivity values. The analytical expressions are presented and experiment results are undertaken to demonstrate the accuracy of the technique. There are no error corrections required for determining contact parameters using the presented test structure.

Silicided Au/Ni bilayer on p-type [0 0 1] silicon for low contact resistance metallization schemes

Rear contact terminals of integrated circuits have to satisfy electrical and mechanical requirements, such as low specific contact resistance, good adhesion to the substrate and good solderability with external elements. A new metallization scheme, made of sputtered Ni and Au layers, with the addition of a process step needed to ensure nickel silicide formation at low temperature, has been proposed for p-type silicon substrates and investigated in this work. Its electrical and structural properties have been compared with conventional Cr/Ni/Au and Ti/Ni/Au contacts, showing lower specific contact resistance values (@rc), an ohmic behaviour in the explored range of resistivity (i.e. 3m@Wcm<@r<18m@Wcm) despite of the rectifying one of conventional materials, better adhesion with the substrate and limited consumption of nickel and silicon during the reaction process. The proposed metallization scheme provides an effective solution to meet both electrical and mechanical requirements with a single material, with a consequent reduction of logistic and economic effort to realize integrated circuits.

Development of an Extreme High Temperature n-Type Ohmic Contact to Silicon Carbide

We report on the initial demonstration of a tungsten-nickel (75:25 at. %) ohmic contact to silicon carbide (SiC) that performed for up to fifteen hours of heat treatment in argon at 1000 °C. The transfer length method (TLM) test structure was used to evaluate the contacts. Samples showed consistent ohmic behavior with specific contact resistance values averaging 5 x 10-4 Ω-cm2. The development of this contact metallization should allow silicon carbide devices to operate more reliably at the present maximum operating temperature of 600 °C while potentially extending operations to 1000 °C.

Factors controlling the resistance of Ohmic contacts to n-InGaAs

The authors report specific contact resistance (ρc) values for Mo-, Ti-, TiW-, Pd-, and Pt-based Ohmic contacts to n+-In0.86Ga0.14As that are deposited with either collimated sputter or electron-beam deposition. Palladium-based contacts with 5 nm of electron-beam evaporated Pd have a specific contact resistance of 7.6 ± 0.5 × 10−9 Ω cm2, while identical collimated sputter deposited contacts have a specific contact resistance of 4.1 ± 0.4 × 10−9 Ω cm2. Contacts with a very thin evaporated Pd layer (2 nm) or a sputtered Pd/Ti 1/1 nm × 4 multilayer have stable ρc values of 5.4 ± 0.5 × 10−9 and 5.0 ± 0.5 × 10−9 Ω cm2, respectively, after 2 h at 270 °C. The ρc of sputter deposited TiW-based contacts is an order of magnitude lower than for identical evaporated contacts (1.6 ± 0.3 × 10−8 vs 2.6 ± 0.3 × 10−7 Ω cm2) and is stable during annealing, while sputter deposition of Pt-based contacts yields rc values that are half an order of magnitude lower than similar evaporated contacts (4.5 ± 1.0 × 10−9 vs 1.7 ± 0.2 × 10−8 Ω cm2). However, the ρc values of the sputtered Pt contacts approach those of evaporated contacts after annealing. Surface preparations consisting of UV-ozone treatments followed by immersion in buffered oxide etch or ammonium hydroxide yield ρc values that are half an order of magnitude lower than values yielded by similar preparations in which HCl is used for oxide removal. The authors also discuss how to avoid over- and underestimation artifacts that may be encountered in measurements of very low specific contact resistances of Ohmic contacts to semiconductors that have low sheet resistances.

TiAl Ohmic contact on GaN, in situ high or low doped or Si implanted, epitaxially grown on sapphire or silicon

AbstractIn this work, the Ti/Al Ohmic contact quality on n‐type gallium nitride (GaN) films has been studied as a function of different process parameters such as surface cleaning procedure, etching, thickness of the deposited layers or annealing conditions. GaN epilayers, with uniform doping concentration from 1 × 1016 to 5.8 × 1018 at./cm3 were grown on sapphire or silicon substrates using AlN and/or AlGaN buffer layers. Electrical characterizations were made using circular transfer length method (cTLM) patterns with a four‐probe equipment. Specific contact resistance (SCR) was then extracted from current–voltage (I–V) characteristics, for all the process conditions. Contact structures depending on experiment parameters were studied by means of (scanning) transmission electronic microscopy (STEM‐TEM). Our results reveal that process parameters such as surface treatment have a lower impact than annealing temperature or metal thickness and annealing duration. Finally, SCR values of 1 × 10−6 Ω cm2 can be reproducibly achieved. Moreover, good Ohmic contacts have been obtained on etched surfaces or on low‐doped layers implanted with Si. This low value demonstrates a good Ohmic contact and this large parameter process window is of high interest for future device fabrication based on GaN (planar or mesa structures).

An Area-Correction Model for Accurate Extraction of Low Specific Contact Resistance

The parasitic factors that strongly influence the measurement accuracy of cross-bridge Kelvin resistors have been extensively discussed during the last few decades. The minimum value of specific contact resistance that can be accurately extracted has been estimated. In this paper, we present an analytical model to account for the actual current flow across the contact and propose an area-correction method for a reliable extraction of specific contact resistance. The model is experimentally verified for low-resistivity (close-to-ideal) metal-to-metal contacts. The minimum contact resistance is determined by the dimensions of the two-metal stack in the area of contact and sheet resistances of the metals used.

Si&lt;sup&gt;+&lt;/sup&gt; Implantation and Activation in GaN Comparison of GaN on Sapphire and GaN on Silicon

In this paper, we evaluated gallium nitride heteroepitaxially grown on sapphire (GaN/Sa) and grown on silicon (GaN/Si) faced to implantation doping. Si+ was implanted on low doped n-type epilayers in order to create a plateau around 1020at.cm-3. All the samples were capped with a silicon oxide and annealed between 1000°C and 1150°C. The surface quality was evaluated in terms of roughness, pit density and maximum pit diameter using Atomic Force Microscopy (AFM) and Scanning Electron Microscopy (SEM). Finally, the dopant electrical activation was studied with Ti-Al contacts using the circular Transfert Length Method (c-TLM). This study shows that low Specific Contact Resistance (SCR) values of 8x10-5Ω.cm2 and 6x10-6Ω.cm2 are respectively obtained on GaN/Sa sample annealed at 1150°C-2min and on GaN/Si sample annealed at 1150°C-30s, proving that good ohmic contacts are obtained on both materials. Nevertheless, a compromise has to be done between the low SCR values obtained and the GaN surface degradation, observed by AFM and SEM after the different annealing treatments and which could affect the good behaviour of the GaN devices.

Dose Influence on Physical and Electrical Properties of Nitrogen Implantation in 3C-SiC on Si

In this paper, we studied the influence of nitrogen implantation dose on both physical and electrical properties in 3C-SiC grown on Si (100) substrate. Scanning Transmission Electron Microscopy characterizations prove that high dose is responsible for amorphization of the implanted layer and the high defect density after annealing. A high V-shape defect density is still found in the implanted layer after an annealing at 1350°C. By lowering the dose, the layer is less damaged and no amorphization is observed. For the different doses, low Specific Contact Resistances are measured using Ti/Ni contacts. The Specific Contact Resistance value decreases from 8x10-6 Ω.cm2 for the high dose to 3.2x10­6 Ω.cm2 with decreasing the dose. Furthermore, the dopant activation ratio, evaluated by quantitative SSRM measurements, is improved at the same time from 17% (for the high dose) to 60% (for the low dose). This work demonstrates that high activation ratio can be achieved consecutively to a nitrogen implantation at reasonable implantation fluence.

Properties of Al and Pd Contacts on n-type SiC Membranes

ABSTRACTMembranes with dimensions up to 10 mm x 15 mm have been fabricated in epitaxial 3C-SiC/Si wafers. An array of CTLM metal contacts was deposited onto the upper surface of the n-SiC membrane. Both Al/n-SiC and Pd/n-SiC contacts which were formed on the membrane and on the adjacent substrate have shown an ohmic current/ voltage response. Values of specific contact resistance, ρc, were measured directly on the membranes. These results have shown no consistent difference in ρc of the contacts located either on the membrane or off the membrane. The exposure of SiC surfaces to reactive ion etching in CF4 plasma during the fabrication of a membrane has resulted in ρc which was higher by a factor of 103 than with as-grown and KOH etched silicon surfaces.

Structural and electrical characterization of silicided Ni/Au contacts formed at low temperature (&amp;lt;300 °C) on p-type [001] silicon

Silicided Ni/Au contacts with very low contact resistance were realized on p-type [001] silicon at low temperature by ex-situ or, alternatively, by in situ annealing processes. During the ex-situ annealing, performed at 200 °C for 10 s, a uniformly thin (14 nm) Ni2Si layer was formed having an extremely flat interface with silicon thanks to the trans-rotational structure of the silicide. During the in situ annealing, promoted by a sputter etch processing (T &amp;lt; 300 °C), a 44 nm-thick silicide layer was formed as a mixture of trans-rotational NiSi and epitaxial NiSi2, domains. In both cases, using a low thermal budget has guaranteed a limited consumption of silicon during the reaction process and a good adhesion with the substrate avoiding gold contaminations. As a consequence of the presence of trans-rotational domains, wherein a pseudo-epitaxial relationship between the silicide and the silicon lattices is established, an ohmic behavior was observed in a wide range of substrate doping (3.5 × 1018 ÷ 3 × 1019 B/cm3) for both annealing processes (in situ and ex-situ). On the other hand, conventional TiNiAu and CrNiAu contacts showed, in the same range of B doping concentration, a rectifying behavior with systematically higher specific contact resistance values (Rc) compared to those of the Ni silicided contacts.

Si implanted reactivation in GaN grown on sapphire using AlN and oxide cap layers

Gallium nitride (GaN) is a promising material for power electronic devices. Due to GaN sensitivity to high temperature treatments, dopant activation, after ion implant, is one of the major critical steps to be overcome. An annealing cap layer is then mandatory during high temperature treatment to avoid degradations. In this work, cap layers, such as AlN and SiO x , were deposited on Si-implanted N-type GaN. Samples were annealed using both classical (FA) and rapid thermal (RTA) annealing for times ranging from 30 s to 8 h and temperatures from 1000 to 1150 °C. Transmission Electron Microscopy has been done to observe the implanted layer structure. After cap layer removal, samples surface has been investigated through Atomic Force Microscopy measurements. Dopant activity was indirectly evaluated by Specific Contact Resistance (SCR) measurements. This work demonstrates that low SCR value (8.2 × 10 −5 Ω cm 2) with low surface roughness (∼1 nm) can be reached using RTA and an oxide cap layer. However, presence of hexagonal pits in GaN layer is difficult to avoid. Compromise between low SCR with low roughness value and low hexagonal pits density on the GaN surface must be found.

Contact resistivity and Fermi-level pinning in n-type Ge contacts with epitaxial Si-passivation

We report on the study of the electrical and material properties of n-Ge contacts with a thin Si-passivation layer. n-Ge contacts typically show Fermi-level pinning and low dopant activation which results in too large specific contact resistivities of &amp;gt;10−4 Ω cm2 and which impedes applications such as high performance Ge complementary metal-oxide semiconductor technology. In this work a thin in situ doped (1×1020 cm−3 of phosphorus) epitaxial Si-passivation layer is grown selectively on n-Ge, followed by a Ti/TiN stack deposition. The insertion of this thin Si-passivation layer reduces the pinning and activation limitation, achieving specific contact resistivity values of ∼1×10−6 Ω cm2. Physical modeling of the specific contact resistivities was performed, providing the insight validating the underlying explanation of the improvement. Key factors for the contact resistivity lowering are the good alignment of the Si and Ge conduction bands and the higher doping concentration achieved in Si.

Ohmic and Highly Reflective Ag based contacts on p-GaN for Resonant Cavity Light Emitting Diodes

ABSTRACTLow resistance and high reflectance ohmic contacts on p-type GaN were achieved using an Ag-based metallization scheme. The ohmic nature of the contacts can be obtained by annealing the contacts in an O2 ambient. However, Ag based contacts degrade due to agglomeration of Ag when annealed above 400 oC [1]. In this work a Ni (1 nm)/Ag (150 nm)/Pt (50 nm)/Ni (20 nm)/Au (50 nm) metallization stack is investigated to reduce Ag agglomeration. The inclusion of platinum as a diffusion barrier is expected to suppress excess oxygen diffusion into the Ag films thereby preventing Ag agglomeration and can also provide high thermal stability when compared to other metallization schemes. The reflectivity of this kind of metallization scheme is around 85-90 % in the wavelength range of 400-600 nm making it suitable for blue and green LEDs, with a specific contact resistivity value comparable to other well developed contacts to p-GaN.

Low temperature (down to 450 °C) annealed TiAl contacts on N‐type gallium nitride characterized by differential scanning calorimetry

AbstractThis work reports on Differential Scanning Calorimetry (DSC) measurements performed on Ti‐Al metallic layers stacks deposited on n+‐GaN. The aim is to get better understanding of the mechanisms leading to ohmic contact formation during the annealing stage. Two exothermic peaks were found, one below 500 °C and the other one around 660 °C. They can be respectively attributed to Al3Ti and Al2Ti compounds formation. The locations of these peaks provide clear evidence of solid‐solid reactions. Lowest contact resistance is well correlated with the presence of Al3Ti compound, corresponding to Al(200nm)/Ti(50nm) stoichiometric ratio. Subsequently, Al(200 nm)Ti(50 nm) stacks on n+‐GaN were annealed from 400 °C to 650 °C. Specific Contact Resistivity (SCR) values stay in the mid 10‐5 Ω.cm² range for annealing temperatures between 450 °C and 650 °C. Such low‐temperature annealed contacts on n+‐GaN may open new device processing routes, simpler and cheaper, in which Ohmic and Schottky contacts are annealed together. (© 2011 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Interfacial Resistive Properties of Nickel Silicide Thin Films to Doped Silicon

Improved means of electrical access to nanotechnology devices and accurate nanoscale characterization of electrical properties of ultrathin layers constituting such electrical contacts is of utmost interest to nanoelectronics researchers. This paper reports on the characterization of interfacial resistive properties of ohmic contacts to doped silicon, incorporating thin films of nickel silicide. Silicon doping was achieved by carefully designed ion implantation of antimony (for n-type) and boron (for p-type). Cross Kelvin resistor test structures were used to extract the specific contact resistivity (SCR) values for the different ohmic contacts fabricated. SCR values, which are quantitative characteristics of interfacial resistive properties, as low as for contacts to antimony-doped silicon and to boron-doped silicon were estimated. These experimental results, representing the lowest such values measured, were based on a rigorous evaluation technique and verified by finite element modeling.

Interfacial Resistive Properties of Multi-layered Silicon-based Ohmic Contacts

AbstractThis article reports on the characterization of two- and three-layer ohmic contacts comprising of titanium disilicide and nickel silicide. Cross Kelvin resistor test structures were used to extract the specific contact resistivity (SCR) values for the different ohmic contacts fabricated. The SCR of aluminum to titanium silicide (Al-TiSi2) ohmic contacts was evaluated to be as low as 6.0 x 10-10 Ωcm2. Three-layer ohmic contacts were created using aluminum and nickel silicide thin films and doped silicon. SCR values as low as 5.0 x 10-9 Ωcm2 to antimony-doped silicon and 3.5 x 10-9 Ωcm2 to boron-doped silicon were evaluated.

Low Specific Contact Resistance of NiSi and PtSi to Si: Impact of Interface

We recently reported low specific contact resistance values of NiSi and PtSi to silicon. In recent publications, such low values were attributed to dopant segregation. In this work, we tested this hypothesis by spreading resistance probe and secondary ion mass spectroscopy analyses. The observed weak dopant segregation could not explain lowering the contact resistance by orders of magnitude. Interface damage, intentionally created using Ni ion implantation, led to an increase of contact resistance (1–2 orders). We conclude that the low contact resistance is intrinsic to a high-quality silicide-silicon interface and can be obtained without active dopant segregation.

On the high-resistivity contacts to YBa 2Cu 3O 7 thin films, electrical behavior in the regime of high temperatures and high-bias voltages

We prepared in-situ Au contacts on high-quality epitaxial YBa 2Cu 3O 7 (YBCO) films. Very high specific contact resistivity values up to ∼10 −2 Ω cm 2 at 4.2 K were obtained on 12×5 μm 2 contact areas. This resistivity value decreased by two orders of magnitude as the temperature was raised to room temperature. In the temperature range T<200 K, the contacts showed non-ohmic behavior suggesting the presence of a well-defined insulating native Y–Ba–Cu–O barrier between the two electrodes. The electrical transport in this barrier layer was analyzed in the limit of high temperatures and high voltages to follow Mott's variable-range hopping conduction mechanism with physically reasonable parameters describing the localized states in the barrier. The high-resistivity contacts were tested successfully in quasiparticles injection experiments where the critical current I c of the YBCO microbridge could be strongly suppressed on injection of an additional current through the contact into the superconducting channel.