Allowing for current spreading in semiconductors during measurements of the contact resistivity of ohmic contacts

A modification of the contact-area pattern with radial geometry, which has certain advantages in determining the contact resistivity of ohmic contacts (ρ _c) fabricated on substrates and low-resistance semiconductor layers, is proposed. Different variants of its application for both the transmission line method (TLM) and methods based on a numerical calculation of the resistance of the semiconductor with allowance for current spreading are considered. It is shown that the transmission line method makes it possible to obtain an upper estimate of the contact resistivity on substrates. The errors of such estimates are also calculated as a function of the parameters of the semiconductor and the contact. The TLM estimate is a good first approximation for determining the exact value of ρ _c by numerically calculating the resistance of the semiconductor. The results obtained are used to study the contact resistivity of Ni-based ohmic contacts on n-6H-SiC substrates. Fiz. Tekh. Poluprovodn. 32, 832–837 (July 1998)     

Improved Ni ohmic contact on n-type 4H-SiC

This paper presents the structural, chemical and electronic properties of Al/Ni/ Al-layers evaporated on 4H silicon carbide and then annealed at 1000°C for 5 min. The structure was investigated before and after annealing by transmission electron spectroscopy from cross-sectional specimens. With x-ray photoelectron spectroscopy, both element distribution and bonding energies were followed during sputtering through the alloyed metal-semiconductor contact. Voids are found in both annealed Ni/4H-SiC and Al/Ni/Al/4H-SiC contact layers, though closer to the metal-semiconductor interface in the former case. The first aluminum-layer is believed to prevent voids to be formed at the interface and also to reduce the oxide on the semiconductor surface. The contact was found to be ohmic with a specific contact resistance ρ_c - 1.8 × 10⁻⁵ Ωcm² which is more than three times lower ρ_c than for the ordinary Ni/4H-SiC contact prepared in the same way.     

Pd-Ge contact to n-GaAs with the TiW diffusion barrier

Pd-Ge based ohmic contact to n-GaAs with a TiW diffusion barrier was investigated. Electrical analysis as well as Auger electron spectroscopy and the scanning electron microscopy were used to study the contact after it was subjected to different furnace and rapid thermal annealing and different aging steps. All analyses show that TiW can act as a good barrier metal for the Au/Ge/Pd/n-GaAs contact system. A value of 1.45 × 10⁻⁶ Ω-cm² for the specific contact resistance was obtained for the Au/TiW/Ge/Pd/n-GaAs contact after it was rapid thermally annealed at 425°C for 90 s. It can withstand a thermal aging at 350°C for 40 h with its ρ_c increasing to 2.94 × 10⁻⁶Ω-cm² and for an aging at 410°C for 40 h with its ρ_c increasing to 1.38 × 10⁻⁵ Ω-cm².     

A transmission electron microscopy study of microstructure evolution with increasing anneal temperature in Ti/Al ohmic contacts to n-GaN

Al(200 nm)/Ti(20 nm)/n-GaN contacts have been studied using transmission electron microscopy (TEM) and the resulting microstructures correlated with the observed variation in specific contact resistance (ρ_c). A minimum ρ_c value of 7×10⁻⁷ Ωcm² was obtained after annealing at 550°C for 1 min in argon. Bulk metal and interfacial phases have been characterized, and explanations for the observed electrical behavior are proposed. A transition from TiN to AlN at the interface occurs between 650°C and 700°C.     

Effect of microwave irradiation on the resistance of Au-TiB x -Ge-Au-n-n +-n ++-GaAs(InP) ohmic contacts

Temperature dependences of the contact resistivity ρ _c of Au-TiB _x -Ge-Au-n-n ⁺-n ⁺⁺(GaAs)-InP ohmic contacts before and after short-term (10 s) microwave treatment have been studied both experimentally and theoretically. It is shown that ρ _c can decrease after microwave treatment in the entire temperature range of ρ _c measurements (100–400 K). Good agreement between the theoretical and experimental ρ _c (T) curves is attained and interpreted on the assumption that the dislocation density in the semiconductor near-surface region is varied as a result of microwave radiation.     

Effects of annealing in an oxygen ambient on electrical properties of ohmic contacts to p-type GaN

Effects of annealing ambient of an oxygen and nitrogen mixed gas on the electrical properties were studied for Au-based ohmic contacts (NiAu, CoAu, CuAu, PdAu, and PtAu) to p-type GaN. Addition of oxygen to the nitrogen gas reduced the specific contact resistances (ρ_c) and resitivities of the p-GaN epilayers (ρ_s) of the contacts after annealing at temperatures of 500–600°C. The microstructural analysis at the p-GaN/metal interfaces did not detect the heterostructural intermediate semiconductor layer at the GaN/metal interfaces. The reason for reduction of both the ρc and ρ_s values by the oxygen gas addition was believed to be due to formation of the p-GaN epilayer with high hole concentrations, caused by removal of hydrogen atoms which bonded with Mg atoms.     

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 cm²) 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.     

A decrease in ohmic losses and an increase in power in GaSb photovoltaic converters

The transmission-line model with radial and rectangular geometry of contact pads has been used to study the contact systems Cr-Au, Cr-Au-Ag-Au, Ti-Pt-Au, Pt-Ti-Pt-Au, Pt-Au, Ti-Au, Ti-Pt-Ag, Ti-Pt-Ag-Au, and Pt-Ag deposited on the p-GaSb surface by the methods of magnetron sputtering and resistive evaporation. It is established that the contact systems Ti-Pt-Ag-Au and Ti-Pt-Ag exhibit the smallest values of the specific contact resistance (ρ _c ≤ 10⁻⁶ Ω cm²), which makes it possible to use these systems in fabrication of photovoltaic converters generating photocurrents with densities as high as 15 A/cm².     

Modeling and measurement of contact resistances

This paper presents a generalized model of ohmic contacts and a unified approach for the accurate extraction of specific contact resistivity (ρc) for ohmic contacts from measured contact resistance using the cross bridge Kelvin resistor, the contact end resistor, and the tranmsission line tap resistor test structures. A general three-dimensional (3-D) model of the contacts has been developed from the first principles and has been reduced to 2-D, 1-D, and 0-D (one lump) models with the necessary approximations. It is shown that the conventional I-D models overestimate the value of ρcbecause of the parasitic resistance due to 2-D current flow around the periphery of the contact window. Using 2-D simulations, we have accurately modeled the current crowding effects and have extracted accurate values of ρcindependent of contact size and the test structure type. A theory of scaling of contacts has been developed and is applied to commonly used structures. A universal set of curves has been derived for each particular contact resistance test structure and, given the geometry of the structure, these allow accurate determination of ρc, Without the actual use of the 2-D simulator. Experimental and theoretical accuracy of the three test structures has been compared. Accurate values of ρcfor various contact materials to n⁺and ρ⁺Si have been determined. The data confirm that in the past researchers have overestimated ρc, and that ρcwill not limit device performance even with submicrometer design rules.     

Very low resistance ohmic contacts to n-GaN

Ohmic contacts with low resistance are fabricated on n-GaN films using Al/Ti bilayer metallization. GaN films used are 0.3 μm thick layers with carrier concentrations of 1 × 10¹⁹ cm⁻³ grown on the c-plane sapphire by ion-removed electron cyclotron resonance molecular beam epitaxy. The lowest value for the specific contact resistivity (ρ_c) of 1.2×10⁻⁸ Ω·cm² was obtained with furnace annealing at 500°C for 60 min. This result shows the effectiveness of high carrier concentration GaN layers and the low temperature annealing for the realization of low resistance ohmic contacts. Sputtering Auger electron spectroscopy analysis reveals that Al diffuses into Ti layer and comes into contact with the GaN surface.     

Low resistance, thermally stable Au/Pt/Ti/WSiN ohmic contacts on n+-InGaAs/n-GaAs layer systems

The electrical properties of the ohmic contact systems Au/Pt/Ti/WSiN and Au/Pt/Ti to n⁺-InGaAs/GaAs layers grown by metalorganic vapor phase epitaxy were investigated and compared to each other. The thermal stability properties of these contact systems were characterized by accelerated stress tests at elevated temperatures and by complementary thin film x-ray diffraction analysis to evaluate the microstructural properties of degraded and nondegraded structures. The goal of these efforts was to develop stable, homogeneous emitter contacts for power heterojunction bipolar transistors. It was found that for both contact systems the best (specific) contact resistance R_c (ρ _c) is about 0.05 Ωmm (2 × 10⁻⁷ Ωcm²) in the as-deposited state. Au/Pt/Ti/WSiN contacts show no degradation after aging at 400°C for more than 20 h. This is in contrast to standard Au/Pt/Ti contacts which significantly degrade even after short time annealing at 400°C. The good long-time stability of the Au/Pt/Ti/WSiN system is related to the advantageous properties of the reactively sputtered WSiN barrier layer.     

Pd/Zn/Pd/Au ohmic contacts to ρ-Type InP

Low resistance ohmic contacts (ρ_c = 7 x 10^-5 Ω-cm ² ) have been fabricated to Zn-doped p-type InP using an annealed Pd/Zn/Pd/Au metallization. Palladium reacts with InP at low temperatures to form a Pd₂InP ternary phase, which is initially amorphous but crystallizes and grows epitaxially on InP. Zinc reacts with some of the overlying Pd to form PdZn (≅250° C), which decomposes at 400-425° C to form PdP₂, freeing up Zn to diffuse into Au as well as InP. The contact resistance reaches a minimum as the decomposition reaction takes place. The resultant ohmic contact is laterally uniform and consists of epitaxial Pd₂InP adjacent to InP, followed by a thin layer of PdP₂ and then the outer Au layer. Further annealing leads to a breakdown of the contact structure,i.e. decomposition of Pd₂InP, and an increase in contact resistance.     

An accurate method to extract specific contact resistivity using cross-bridge Kelvin resistors

The cross-bridge Kelvin resistor structure is used to extract true interfacial specific contact resistivity (ρc). Two-dimensional (2-D) simulations demonstrate that the sublinear behavior of the measured contact resistance versus contact area on a log-log plot is due to current crowding around the contact which results from the contact window size being smaller than the diffusion width. The effect is more pronounced for low values of ρc. Excellent agreement has been found between the simulations and measured data of contact resistances. An accurate value of ρchas been extracted for the case of PtSi to n⁺polysilicon contacts.     

The sidewall resistor—A novel test structure to reliably extract specific contact resistivity

Conventional contact resistance extraction structures suffer inaccuracies when measuring small values of contact resistivity which are sensitive to small perturbations in device fabrication. This is because two-dimensional current crowding around the periphery of the contact window introduces parasitic resistance components which are not accounted for by the one-dimensional model and which can be much larger than the contact resistance component for low values of specific contact resistivity ρc. Unfortunately, there is no direct method to isolate these parasitics, which makes the extraction of ρcusing the conventional test structures difficult. In this paper, a novel test device, the sidewall resistor, which utilizes the vertical sidewall dimensions, is demonstrated to circumvent these difficulties. The proposed structure has the advantage that a simple one-dimensional (1-D) extraction technique can still be used. The specific contact resistivity between pure Al and N+ polycrystalline Si is accurately extracted with the value of 7.4 × 10^-8Ω.cm². The sidewall resistor is then used to extract the contact resistivity between pure Al and WSi2which turns out to be less than 5 × 10^-9Ω.cm².     

Temperature dependence of contact resistance for Au-Ti-Pd<Subscript>2</Subscript>Si-<Emphasis Type="Italic">n</Emphasis><Superscript>+</Superscript>-Si ohmic contacts subjected to microwave irradiation

Based on a theoretical analysis of the temperature dependence of the contact resistance R _c for an Au-Ti-Pd₂Si-n ⁺-Si ohmic contact, a current-transfer mechanism explaining the experimentally observed increase in R _c in the temperature range 100–380 K is proposed. It is shown that microwave treatment of such contacts results in a decrease in the spread of R _c over the wafer and a decrease in the value of R _c whilst retaining an increase in R _c in the temperatures range 100–380 K.     

Ohmic contact formation on inductively coupled plasma etched 4H-silicon carbide

We report on the investigation of ohmic contact formation using sputtered titanium-tungsten contacts on an inductively coupled plasma (ICP) etch-damaged 4H-SiC surface. Transfer length method (TLM) measurements were performed to characterize how ICP-etch damage affects the performance of ohmic contacts to silicon carbide. In order to recover etch damage, high-temperature oxidation (1250°C for 1 h) was evaluated for one of the samples. Some of the etch damage was recovered, but it did not fully recover the etch damage for the sample etched with medium platen power (60 W). From our TLM measurements, the specific contact resistance (ρ _C of sputtered titanium tungsten on highly doped n⁺-type 4H-SiC epilayers with a doping of 1.1×10¹⁹ cm⁻³ for the unetched reference sample, 30-W etched, and 60-W etched with and without sacrificial oxidation was as low as 3.8×10⁻⁵ Ωcm², 3.3×10⁻⁵ Ωcm², 2.3×10⁻⁴ Ωcm², and 1.3×10⁻³ Ωcm², respectively. We found that the low-power (30 W) ICP-etching process did not affect the formation of ohmic contacts, and we did not observe any difference between the unetched and the 30-W etched sample from our TLM measurements, having the same value of the ρ _C. However, medium-platen-power (60 W) ICP etching showed significant influence on the ohmic contact formation. We found that the specific contact resistance is highly related to the surface roughness and quality of the metals, and the lower, specific contact resistance is due to smoother and denser ohmic contacts.     

Interdiffusion of the p-InP with Au−Zn,Ti/Au,Pd/Au,Ti/Pd/Au at interface and their electrical properties

 In this report,the interdiffusion between the p-InP with Au-Zn,Ti/Au,Pd/Au and Ti/Pd/Au at interface have been investigated by Auger electron spectroscopy and electron spectroscopy for chemical analysis.The surface morphology for the heat treatment are observed with scanning electron microscopy. It is found that the indiffusion of Au is easier than that of Pd and Ti and the outdiffusion of In is easier than that of P.The combination state of In and Au is formed during the heat treatment of p-InP/Au-Zn. The effects of the alloying temperature and time on the specific contacts resistance of p-InP/Au-Zn system are studided.The low specific contact resistance,p_c=2.4-2.7×10~(-4)Ω-cm~2,is obtained when alloying at 450℃ for 2 min or at 350℃ for 30 min. These results indicate that the specific contact resistance strongly depend on the“interdiffusion degree”.The Zn in Au-Zn ahoy distributes onto the most surface layer of p-InP/Au-Zn system during evaporation process and heat treatment.It may be one of the reasons for the higher specific contacts resistance.     

Effect of nitridation on the electrical properties of W:Ti resistors

The electrical properties of tungsten-titanium (W:Ti) thin film resistors sputtered in an argon-nitrogen atmosphere were investigated. The resistivity ρ and the thermal coefficient of resistivity α were calculated as a function of film thickness and nitrogen content. A bulk resistivity of 70±4 μΩ-cm and the mean free path λ_o), of 0.8±0.1 μm were obtained for samples sputtered without nitrogen. The authors believe this to be the first report for the value of the λ_o in sputtered W:Ti. By appropriately controlling the nitrogen content during sputtering, it is possible to vary the value of α from positive to negative. It was found that α decreases with the nitrogen content and is zero at 0.5% N₂. This added degree of freedom in controlling α allows the integrated circuit designer to compensate the thermal effects within a circuit by customizing the resistor parameters without significant layout modifications.     

Material properties of Pb1-xSnxSe epilayers on Si and their correlation with the performance of infrared photodiodes

Hall mobilities and resistance area products R_oA of infrared diodes in epitaxial Pb_1-xSn_xSe layers on CaF₂ covered Si(111) substrates were correlated with threading dislocation densities p. The low temperature saturation Hall mobilities were entirely determined by p and proportional to their mean spacing 1/ √ρ. For the photodiodes, the R₀A values at low temperatures were inversely propor-tional to ρ. A model where each dislocation in the active area of the diodes causes a shunt resistance correctly describes the results, the value of this resistance for a single dislocation is 1.2 GΩ for PbSe at 85K. The dislocation densities were in the 2 × 10⁷ to 5 × 10⁸cm^-2 range for the 3-4 μm thick as-grown layers. Higher R₀A values are obtainable by lowering these densities by thermal annealing, which sweeps the threading ends of the misfit dislocations to the edges of the sample.     

Magnetoresistance and magnetization studies of the layered magnanite system La<Subscript>1.2</Subscript>Ca<Subscript>1.8</Subscript>Mn<Subscript>2−x</Subscript>Ru<Subscript>x</Subscript>O<Subscript>7</Subscript> (0 ≤ x ≤ 1)

We report here the results of magnetotransport and electrical resistivity (ρ) measurements in the temperature range of 4.2–320 K and in the presence of magnetic fields up to 10 T on the Ru-doped, bilayered manganite system, La_1.2Ca_1.8Mn_2−xRu_xO₇ (0≤x≤1). We find that the Ru doping affects the magnetotransport properties considerably. The ρ versus H data were analyzed by fitting the data to the power-law equation, ρ = ρ₀ − αHⁿ. The isothermal magnetoresistance (MR) versus H curves taken up to ± 10 T are highly symmetrical, and their curvature changes from concave up to concave down as the temperature increases. The MR, defined as [ρ(H) − ρ(0)]/ρ(0), is found to increase with Ru doping from 58% to 64% up to x=0.1 and to decrease to 45% for the x=1 sample at 10 K. Analysis of the ρ-T data below 30 K shows that, at low temperature, the system behaves like a disordered metal.