Electromigration effects in power MESFET rectifying and ohmic contacts

Both gate and source/drain electromigration are significant failure mechanisms in power MESFTs. The correlation between electromigration effects due to high current density and measured electrical degradation is investigated in devices of different technologies. A safety zone of operation for ohmic contact electromigration is defined.     

Low resistance contacts to p-type cadmium telluride

A survey of metal contacts to p-type CdTe has revealed that the best contact is one prepared by vacuum deposition of CuAu onto a surface that has been etched with K₂Cr₂O₇:H₂SO₄ . The contact resistivity on 0.5 ohm-cm p-type CdTe is between 0.1 and 0.5 ohm-cm² . The contact resistivity does increase if subjected to heat treatment, but is stable in air at room temperature for at least several months. Contacts of CuAu and Au on a chromate etched surface were compared in detail with similar metal contacts on a methanol-bromine etched surface through measurements of current vs. voltage as a function of temperature, photovoltaic response, and Auger analysis. CuAu contacts on a chromate etched surface are describable in terms of a thermally assisted tunneling model with a barrier height of about 0.60 eV. Au contacts on either a chromate etched or a methanol-bromine etched surface can be described equally well by a thermally assisted tunneling model or by a thermionic emission model. In either case the barrier height is about 0.1 eV larger on the methanol-bromine etched surface than on the chromate etched surface. Auger analysis indicates that chromate etching produces a tellurium rich surface that plays a key role in determining subsequent contact properties.     

A unified simulation of Schottky and ohmic contacts

The Schottky contact is an important consideration in the development of semiconductor devices. This paper shows that a practical Schottky contact model is available for a unified device simulation of Schottky and ohmic contacts. The present model includes the thermionic emission at the metal/semiconductor interface and the spatially distributed tunneling calculated at each semiconductor around the interface. Simulation results of rectifying characteristics of Schottky barrier diodes (SBD's) and resistances under high impurity concentration conditions are reasonable, compared with measurements. As examples of application to actual devices, the influence of the contact resistance on salicided MOSFETs with source/drain extension and the immunity of Schottky barrier tunnel transistors (SBTTs) from the short-channel effect (SCE) are demonstrated     

A model of ohmic contacts to semiconductors

A unified and detailed model of both thermionic and tunneling ohmic contacts to semiconductors is described. The model takes into account the actual profile of the energy barrier as it is determined by the difference between the metal work function and the semiconductor electron affinity, by the ionized impurities in the semiconductor, by the interfacial quantum electric dipole and by the classical and quantum penetration of the charge carriers into the depletion layer. The current, above and below the energy barrier peak, is computed by using the Kemble generalized transmission coefficient, by taking into account the anisotropy of the effective masses as well as their dependence on impurity-concentration, and by employing the Fermi-Dirac statistics. Good agreement between theoretical and experimental results is observed. The model enables the design of ohmic contacts to semiconductors, i.e. the determination of the semiconductor impurity concentration required to achieve pre-assigned features.     

Radiation-induced degradation of ohmic contacts

Ohmic contacts can be severely degraded by the introduction of small amounts of radiation damage at the metal-semiconductor interface. This can lead to failure of GaAs transferred-electron devices; a safe limit for proton bombardment of ohmic contacts is proposed.     

A self-consistent characterization methodology for Schottky-barrierdiodes and ohmic contacts

Based on the simple interfacial-layer theory, the extraction methods for the interface parameters of the metal-semiconductor contact have been developed and applied to characterize both the Schottky-barrier diodes and the ohmic contacts in a self-consistent manner. It has been shown that the physical parameters at the metal-semiconductor interface can be extracted from the I-V characteristics of the Schottky-barrier diodes and the degradation of the thermal-equilibrium barrier height due to the thermal cycle can be directly modeled in terms of the extracted interface parameters. Besides, using the extracted parameters, the specified surface-treatment process can be evaluated by the extracted thermal-equilibrium barrier height, and thus the strongly process-dependent specific contact resistivity ρ_c of the ohmic contacts can be theoretically calculated by a modified tunneling model considering the impurity band. Furthermore, by comparing the simulated results and the measured ρ _c data deduced from the Al and Ti contacts on both doping types of the Si-substrate, satisfactory agreements have been obtained     

Passivation of ohmic contacts to GaAs

A film of silicon dioxide was used to passivate Au-Ge-Ni metallization during the alloying cycle. Ohmic contacts prepared in this way had smooth surfaces, better edge definition, and more uniform electrical characteristics. Also the time-temperature alloying cycle for fabricating suitable contacts was significantly broadened. These improvements are most likely due to the prevention of arsenic loss during the alloying procedure.     

Formation of ohmic contacts to p-ZnTe

Formation and temperature stability of sputter deposited gold ohmic contacts to molecular beam epitaxially grown p-type ZnTe (doped with nitrogen to a free hole concentration of ≈3 × 1018 cm³) have been studied using current-voltage (I-V), Auger electron spectroscopy, secondary ion mass spectrometry, and optical and scanning electron microscopy. The I-V characteristics of ≈1500Å Au/p-ZnTe contacts were measured as-deposited and after heat treatments at 150, 200, 250, and 350°C for 15 min intervals up to 90 min. As deposited, the contacts were poor Schottky contacts, but became ohmic after 15 min at all temperatures. There was an increased resistance at t>15 min for T≤250°C, and a very large resistance increase upon heat treatment for all times at 350°C. The interface between the metallization and ZnTe was initially very planar, and remained planar upon formation of the ohmic contact. Upon heating at T>250°C, Au diffused into ZnTe. The ohmic behavior of the Au/p-ZnTe contacts is attributed to this diffusion which created a highly doped near-surface region in the ZnTe. Microscopy showed that Au also migrated across the ZnTe surface forming an extended reaction zone (≈100 μm) around the dot contact at T≥250°C.     

Measurement of low resistive ohmic contacts on semiconductors

Macroscopic analog models of planar contacts to semiconductor layers were made and equipotential lines underneath the contact were traced. Voltage drop and current density across the interfacial layer of such contacts were determined and compared to theoretically calculated values. The extended transmission line model (TLM) is used to describe the measurements and a reasonable limit for its application to the measurement of ρcisrho_{c}/(rho_{s} . h) > 0.2; forrho_{c}/(rho_{s} . h) le 0.2the model of Overmeyer appears to be applicable.     

Carbon structural transitions and ohmic contacts on 4H-SiC

The structural properties of sputtered carbon films on SiC are investigated using X-ray photoelectron spectroscopy (XPS) and Raman scattering. The as-deposited films are amorphous with an sp²/sp³ ratio of 1. The sp² carbon structures gradually increase with increasing temperatures and consist of amorphous aromatic-like carbon, polyene-like carbon, and nano-size graphite flakes. Schottky contacts on carbon/SiC are converted to ohmic contacts after annealing. The concentration of nano-graphitic flakes relative to the aromatic-like and polyene-like carbon increases nearly linearly with annealing temperature. Stacked graphitic structures are not observed. The specific contact resistivities are at 10⁻³–10⁻⁴Ωcm² on the carbon/SiC after annealing from 1050°C to 1350°C.     

A review of the theory and technology for ohmic contacts to group III–V compound semiconductors

The technology for ohmic contacts to group III–V compound semiconductors is reviewed in this paper. The basic principles of current transport in metal-semiconductor (Schottky barrier) contacts are presented first. The modes of current transport considered are thermionic emission over the barrier, and tunneling through the barrier due to thermionic-field or field emission. Special attention is devoted to the parameters of temperature and doping concentration which determine the dominant mode of conduction. As the primary mode of conduction changes from thermionic emission dominated to tunneling dominated, the current-voltage behavior of the contact changes from rectifying to ohmic in character. The experimental techniques for fabricating ohmic contacts to III–V compound semiconductors are then described. Contact problems as they pertain to specific device applications are considered. Finally, present difficulties with contacts to mixed III–V crystals are discussed.     

Characteristics of AuGeNi ohmic contacts to GaAs

We have studied AuGeNi ohmic contacts to n-type MBE grown GaAs epitaxial-layer with doping in the (10¹⁶?10¹⁹) cm^?3 range, and found several new effects: (a) Contact resistivity exhibit a weak dependence on carrier concentration (much weaker than 1/N_D depencence); (b) We find evidence for a high resistivity layer under the contact at least several thousands angstroms deep, which dominate the contact resistance in most cases; (c) We find a peripheral zone around the contact, about 1 μm wide which differs chemically from the GaAs epi-layer; (d) SIMS analysis reveals a deep diffusion into the GaAs of Ni and Ge; (e) Correlation between density of GeNi clusters in the contact and the contact resistivity are found; (f) Temperature measurements justify that tunneling is responsible for the ohmic contact. We discuss also the validity of the transmission line method and the commonly accepted model of the contact.     

Selective formation of ohmic contacts to n-GaAs

A new process is presented for ohmic contact realisation to n-GaAs. The process enables selective formation of ohmic contacts showing ohmic characteristics without an alloying process. This is realised through the deposition of a heavily doped n-Ge layer on an n-GaAs layer. It is possible to grow a Ge layer on GaAs without deposition on SiO2 which is used as a mask. A Ge layer with doping in excess of 1019 cm?3 can be obtined, using GeH4, PH3 and H2 as source gases. This doping level is sufficiently high to exhibit nonalloyed ohmic characteristics.     

Cadmium telluride growth on patterned substrates for mercury cadmium telluride infrared detectors

Patterned silicon-on-insulator (SOI) substrates have been proposed to grow low defect density CdTe. The CdTe epilayers so obtained will enable the growth of good-quality mercury cadmium telluride (HgCdTe) layers subsequently. This would increase the scope for better performance of infrared detectors fabricated on the HgCdTe epilayers. A background of our previous work on metal organic chemical vapor deposition (MOCVD) of GaN on nanopatterned alternative silicon substrates has been presented. Residual stress measurements were made on the GaN epilayers by spatially resolved Raman spectroscopy, which shows reduced strain in the epilayer growth on the patterned substrates. A theoretical approach of strain in planar substrates, compliant substrates, and patterned compliant substrates is presented with detailed plots.     

Charge-carrier exclusion and accumulation intensified by ohmic contacts

The effect of the high generation-recombination power of an ohmic contact (S contact) on exclusion-accumulation processes in structures with an antiblocking contact (asymmetric structures of the type p ⁺-p-S) was studied theoretically and experimentally. It is shown that, in contrast to the ordinarily studied symmetric structure p ⁺-p-p ⁺, the asymmetric structures, which is being studied, forms a region of accumulation or exclusion, depending on the direction of the current. The nonequilibrium carrier density in the accumulation layer is much higher while the exclusion region is longer than in the symmetric structure. A density n 100 times higher than the equilibrium value n ₀ was obtained experimentally for Ge at 300 K. The length of the exclusion region reached 96% of the sample length. Applications of structures with antiblocking and ohmic contacts based on narrow-gap materials are proposed. Fiz. Tekh. Poluprovodn. 32, 634–637 (May 1998)     

Electroluminescence of graded-gap structures with blocking and ohmic contacts

Special features of the electroluminescence of uniformly doped graded-gap structures with blocking and ohmic contacts are studied theoretically. Analytical expressions for the spectral and integrated electroluminescence intensities are derived and analyzed in the case of a constant gradient of the band gap and a strong absorption of light. It is shown that the presence of the blocking contact on the wide-gap face of the graded-gap structure leads to an increase in the efficiency of the negative and, especially, positive electroluminescence compared to the case of a graded-gap structure with two ohmic contacts. The recombination radiation of a graded-gap structure with the blocking contact at the narrow-gap face features in the majority of cases a sign-alternating spectral dependence; a broad band of negative electroluminescence is prevalent in this dependence in the region of high currents. absorption.     

Effect of ohmic contacts on buffer leakage of GaN transistors

The effect of ohmic contacts on the buffer leakage of GaN transistors is presented. The buffer leakage for AlGaN/GaN high-electron mobility transistors and GaN MESFETs grown on the same underlying buffer was observed to be different. Controlled experiments show that the increased buffer leakage is due to the nature of the alloyed ohmic contacts and can be minimized if they are screened by the Si doping or by the two-dimensional electron gas.     

Series resistance of silicided ohmic contacts for nanoelectronics

The electrical behavior of shallow silicide-to-silicon ohmic contacts has been evaluated for lateral dimensions to 50 nm. Trench-isolated series resistance test structures were fabricated using TiSi₂ self-aligned contact technology. Resistance measurements have been performed as a function of contact size in the temperature range from 100 to 300 K and analyzed using multidimensional resistor ladder network models. Well-behaved small-volume ohmic contacts have been achieved. Large-area contact characteristics can be maintained to the smallest sizes. Multidimensional current flow has little effect on the measured resistances. Small lateral dimensional variations are responsible for the higher than predicted series resistance for the smallest sizes. The implications on nanoelectronic devices and circuits are quantified     

Optimization of AuGe-Ni-Au ohmic contacts for GaAs MOSFETs

GaAs-based metal-oxide-semiconductor field-effect transistors (MOSFETs) are promising devices for high-speed and high-power applications. One important factor influencing the performance of a GaAs MOSFET is the characteristics of ohmic contacts at the drain and source terminals. In this paper, AuGe-Ni-Au metal contacts fabricated on a thin (930 /spl Aring/) and lightly doped (4/spl times/10/sup 17/ cm/sup -3/) n-type GaAs MOSFET channel layer were studied. The effects of controllable processing factors such as the AuGe thickness, the Ni/AuGe thickness ratio, alloy temperature, and alloy time to the characteristics of the ohmic contacts were analyzed. Contact qualities including specific contact resistance, contact uniformity, and surface morphology were optimized by controlling these processing factors. Using the optimized process conditions, a specific contact resistance of 5.6/spl times/10/sup -6/ /spl Omega//spl middot/cm/sup 2/ was achieved. The deviation of contact resistance and surface roughness were improved to 1.5% and 84 /spl Aring/, respectively. Using the improved ohmic contacts, high-performance GaAs MOSFETs (2 /spl mu/m/spl times/100 /spl mu/m) with a large drain current density (350 mA/mm) and a high transconductance (90 mS/mm) were fabricated.     

Metrological aspects of measuring resistance of ohmic contacts

Modern methods of measuring resistance of ohmic contacts are considered in the review: Cocks-Strack, transmission line, Kelvin, boundary probing. Examples of their application are provided and errors of measuring the contact resistivity are calculated. The most accurate measuring methods are the Kelvin method and the boundary probing method, while the least accurate one is the Cocks-Strack method. Requirements and possibilities of their application are specified.