Low noise “ohmic” contacts on n-type silicon

A technique is given for preparation of low noise ohmic contacts on n-type silicon using silver-silicon eutectic. It allows reaching electric fields up to 20 kV cm¹ in a wide range of resistivities and lattice temperatures (300–6°K). Interface chemical composition and structure is studied using scanning electron microscopy and X-ray analysis. Contact resistivity ?_c is studied versus bulk resistivity ? and lattice temperature T. High field conductivity shows that ?_c is not sufficient to characterize ohmic contacts. It is shown that the contact noise (j²/f) law is valid up to 500 MHz and that noise measurements are much more sensitive to contact behavior than first order coefficients. Contact noise may be important at intermediate bias but is proved to be negligible both at very low and very high bias. Moreover it has been possible to define a contact quality factor which provides a quantitative characterization of contact behavior.     

Ohmic contacts to low-resistivity ZnS: Preparation,noise properties and nature of contact resistance

The results of simultaneous investigations of noise characteristics and contact resistance of different contacts to low-resistivity (ρ_b = 1–10 ohm cm) ZnS single crystals including so-called ohmic contacts with a contact resistance ρ_c = 0.1–0.5 ohm cm² are described and analyzed. A conclusion is drawn that the difficulties in obtaining ohmic contacts to low-resistivity ZnS are connected with the presence on the semiconductor contact surface of acceptor centers filled by electrons which are responsible for the increase of band bending at the semiconductor surface. In the case of ohmic contacts the contact barrier is lowered significantly owing to formation of the ionized donor centers in the contact region; electrons penetrate such barriers by tunneling.     

Mechanism of current flow in a Au-Ti-Al-Ti-n +-GaN ohmic contact in the temperature range of 4.2–300 K

The temperature dependence of the contact resistivity ρ _c (T) of Au-Ti-Al-Ti-n ⁺-GaN ohmic contacts is studied experimentally and substantiated theoretically in the temperature range T = 4.2–300 K. It is shown that the saturation portion of ρ _c (T) is observed in the low-temperature measurement region (4.2–50 K). As the temperature increases, ρ _c decreases by the exponential law. The experimental and calculated dependences ρ _c (T) are in agreement. The obtained results make it possible to conclude the field nature of the current transfer for the saturation region of ρ _c (T) and the thermal-field one, for the exponential region.     

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.     

Metal-N-type semiconductor ohmic contact with a shallow N+ surface layer

Most papers covering metal-semiconductor ohmic contact theory which have been published up to date consider systems with homogeneous impurity concentration in the semiconductor. However, there are techniques of ohmic contact formation on nondegenerate semiconductor where only a very shallow surface layer is impurity enriched. In this paper a model of such contacts is proposed and a simple approximate analytical expression for the specific resistivity is derived. If the impurity concentration in the surface layer is very high, the contact specific resistivity is essentially proportional to N_B^?1, N_B being the semiconductor substrate impurity concentration. To make a good ohmic contact, it is sufficient that the width of the heavily doped surface layer be equal to the equilibrium contact depletion region width. Any further enlargement of the enriched layer practically does not influence the total sample resistance due to the dominant share of the semiconductor body resistance. Experimental results confirm these conclusions qualitatively.     

Effect of thermal treatment on the properties of YBa2Cu3O7-x thin films with multilayer Al/Cr/Yb metals as ohmic contact electrodes

Detailed studies were conducted on YBa₂Cu₃O_7-x thin films with multilayer Al/Cr/Yb metals as ohmic contact electrodes. Ytterbium not only provided an excellent ohmic contact to YBa₂Cu₃O_7-x thin films, but also improved the zero resistance temperature after thermal treatment of the contacts. Superconducting thin films with zero resistance temperature of around 90 K with extremely sharp resistivity transitions, contact current density greater than 1.3 x 10³A/cm², and specific contact resistivity as low as 10^-9 ohm · cm² at 77 K were achieved on yttrium stabilized ZrO₂ substrates. This occurred after contact deposition and thermal treatment at 450° C in pure oxygen for 20 min.     

Low Resistance Contacts to p-CulnSe2 and p-CdTe Crystals

Room temperature formation of ohmic contacts by electroplating gold on chemically treated surfaces of p-CuInSe₂ and p-CdTe single crystals is reported. The effect of Br₂/methanol and KOH+KCN+H₂O treatments prior to plating was analyzed in the case of CuInSe₂. It is shown that the former treatment yields better ohmic contacts, with lower contact resistance, than the latter. While annealing these contacts made them highly non-ohmic, the method gives reasonably ohmic contacts on surfaces, that were purposely oxidized prior to contact preparation. In the case of p-CdTe stable, low resistance ohmic contacts were obtained at room temperature by electrochemical diffusion of Hg from solution, prior to gold plating. The treatment forms a highly degenerated p^+- HgCdTe layer. The contacts, which have a very low contact to bulk resistivity ratio, were further improved by vacuum annealing.     

Electrical properties of inhomogeneous SiC MIS structures

Current-voltage characteristics of metal contacts on 6H-SiC with a thin (5–20Å) oxide layer have been measured in the temperature range 300 to 1000K. The contacts were investigated in both H₂ and O₂-atmospheres. As the SiC surface was nonideal due to pin holes and other defects generated during the growth process, it was necessary to treat the Schottky contacts as inhomogeneous contacts. The inhomogeneity explains the nonideal current-voltage behavior of the contacts such as ideality factors much larger than unity and voltage dependent ideality factors. It was found that some metals gave Schottky contacts in the entire temperature range, while other metals were ohmic at higher temperatures. Several different contact metals were investigated: Al, Ti, TaSi_x, and Pd were found to be ohmic at high temperatures, while Pt, Pt+Cr, Ni, Cr and another TaSi_x contact were found to behave like Schottky contacts in the entire temperature range. This is a preliminary investigation of the electrical characteristics of different metals that could be useful for high temperature gas sensor purposes.     

An optimal annealing technique for ohmic contacts to ion-implanted n-layers in semi-insulating indium phosphide

An optimal annealing process was developed for sintering AuGe ohmic contacts to ion-implanted semi-insulating InP substrates. Contacts were annealed using a standard furnace, graphite strip heater and a lamp annealer. Alloying at 375°C for 3 min was found to be most suitable for achieving good contact morphology and lowest contact resistivity. Of the three techniques, the lamp annealing technique was found to give the best results when contacts were annealed under a SiO₂ cap. Contact resistivity as low as 8 × 10⁻⁶ cm² was obtained for ion-implanted n⁺ layers in semi-insulating InP.     

Au and non-Au based rare earth metal-silicide ohmic contacts to p-InGaAs

The aim of this study is to improve the electrical properties of ohmic contacts that plays crucial role on the performance of optoelectronic devices such as laser diodes (LDs), light emitting diodes (LEDs) and photodetectors (PDs). The conventional (Pd/Ir/Au, Ti/Pt/Au and Pt/Ti/Pt/Au), Au and non-Au based rare earth metal-silicide ohmic contacts (Gd/Si/Ti/Au, Gd/Si/Pt/Au and Gd/Si/Pt) to p-InGaAs were investigated and compared each other. To calculate the specific contact resistivities the Transmission Line Model (TLM) was used. Minimum specific contact resistivity of the conventional contacts was found as 0.111 × 10⁻⁶ Ω cm² for Pt/Ti/Pt/Au contact at 400 °C annealing temperature. For the rare earth metal-silicide ohmic contacts, the non-Au based Gd/Si/Pt has the minimum value of 4.410 × 10⁻⁶ Ω cm² at 300 °C annealing temperature. As a result, non-Au based Gd/Si/Pt contact shows the best ohmic contact behavior at a relatively low annealing temperature among the rare earth metal-silicide ohmic contacts. Although the Au based conventional ohmic contacts are thermally stable and have lower noise in electronic circuits, by using the non-Au based rare earth metal-silicide ohmic contacts may overcome the problems of Au-based ohmic contacts such as higher cost, poorer reliability, weaker thermal stability, and the device degradation due to relatively higher alloying temperatures. To the best of our knowledge, the Au and non-Au based rare earth metal-silicide (GdSi_x) ohmic contacts to p-InGaAs have been proposed for the first time.     

Effect of microwave treatment on current flow mechanisms in Au-TiB<Subscript>x</Subscript>-Al-Ti-n<Superscript>+</Superscript>-n-n<Superscript>+</Superscript>-GaN-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> ohmic contacts

The temperature dependences of the contact resistivity ρ_c of Au-TiB_x Al-Ti-n⁺-n-n⁺-GaN-Al₂O₃ ohmic contacts have been studied before and after microwave treatment followed by nine-nonth room-temperature sample storage. The temperature dependences of ρ_c of initial samples were measured twice. The first measurement showed the temperature dependence typical of ohmic contacts; the repeated measurement in the temperature region above 270 K showed a ρ_c increase caused by metallic conductivity. After microwave treatment, the metallic conductivity in the ohmic contact is not observed. This is presumably associated with local heating of metal Ga inclusions under microwave irradiation and the formation, due to high chemical activity of liquid gallium, of compounds of it with other metallization components. In this case, the temperature dependence of ρ_c is controlled by ordinary charge transport mechanisms. After nine-nonth room-temperature storage, the temperature dependence of ?c is described by the tunneling mechanism of charge transport.     

Formation peculiarities and properties of ohmic contacts to n-GaN(AlN) and artificial diamond

The paper considers ohmic contacts of Au-TiB _x -Al-Ti-n-GaN, Au-Pd-Ti-Pd-n-AlN and Au-Pd-Ti-n-C to the promising for use in microelectronics wide-gap semiconductors. Ohmic contact formation takes place after sequential layering of metal with further fast thermal processing, which leads to solid-phase reactions between the semiconductor and metal. It is shown that the use of X-ray amorphous TiB _x layer in ohmic contact as the diffusion barrier allows for creating thermal stability contacts up to T = 900 °C. Current flow in the considered ohmic contacts is described using a model with current flow along metal shunts considering diffusion limitation on the charge carrier supply.     

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)     

Specific resistivity of a metal-n GaAs OHMIC contact with an intermediate N+ layer

The specific contact resistivity ρ_c of a metal-n GaAs structure, over a wide range of carrier concentrations in the intermediate layer (5 × 10¹⁸–5 × 10²⁰ cm^?3) and in the substrate (10¹⁵?10¹⁷ cm^?3) is calculated. The results which are presented graphically demonstrate the dependence of ρ_c on metal-semiconductor barrier height from 0.2 eV–0.8 eV. A comparison with the measured data of an alloyed AuGeNi-n GaAs system, suggests that the calculations corresponding to a concentration of 5 × 10¹⁸ cm^?3 in the layer and barrier height of 0.4 eV give the best fit based on the experimental observations. It is hoped that the results can be used as a guideline in developing ohmic contacts of ultra low resistance values.     

Formation mechanism of contact resistance to III–N heterostructures with a high dislocation density

The temperature dependences of the contact resistance ρ _c (T) of ohmic Pd-Ti-Pd-Au contacts to n-GaN and n-AlN wide-gap semiconductors with a high dislocation density are studied. The dependences ρ _c (T) for both contacts contain portions of exponential decrease ρ _c (T) and very weak dependence ρ _c (T) at higher temperatures. Furthermore, a plateau portion ρ _c (T) is observed in the low-temperature region for the Au-Pd-Ti-Pd-n-GaN contact. This portion appears only after rapid thermal annealing (RTA). In principle, the appearance of the plateau portion can be associated with preliminary heavy doping of the near-contact region with a shallow donor impurity and with doping during contact fabrication as a result of RTA, if the contact-forming layer contains a material that is a shallow donor in III–N. The dependences obtained are not explained by existing charge-transport mechanisms. Probable mechanisms explaining the experimental dependences ρ _c (T) for ohmic contacts to n-GaN and n-AlN are proposed.     

Improvedn-type GaAs ohmic contacts compatible with a chlorine-based dry-etch process

We report the use of a Mo barrier layer within Ni/Au-Ge based ohmic contacts to GaAs for eliminating an etch stop reaction that occurs during Cl-based dry etching of heterojunction bipolar transistors. With conventional Ni/Au-Ge/Ag ohmic contacts, chlorinecontaining discharges produce a passivating layer of AgCl on the semiconductor surface, preventing further etching. This layer is absent when the Ag in the contact is replaced with Mo. The Mo has several advantages over other diffusion barrier layers and yields contacts with excellent adhesion, smooth morphology, and sharp edge definition. The average contact resistivity of these contacts ton ⁺-GaAs(n = 6 × 10¹⁸ cm^-3) was 0.074 ohm-mm, which is lower than the typical contact resistivity of conventional Ni/Au-Ge/ Ag metallization (0.11 ohm-mm).     

Technological Fabrication Features of Microwave Device with Schottky Barriers

At present, research and development of heterojunctions are conducted in the directions of searching for new compositions and technological regimes for the creation of ohmic and barrier transitions for gallium arsenide. The transition to silver-based metallization, which has large thermal and electrical conductivity comparing with gold and a relatively low diffusion coefficient to gallium arsenide, should improve the technical characteristics of the devices. One of the most important technological operations in the formation of Schottky ohmic contacts and barriers is thermal annealing. Silver to gallium arsenide contacts are made in vacuum by the method of thermal evaporation. The deposition and thermal treatment regimes for creating ohmic contacts of Ag–Ge–In/n–n⁺ GaAs with specific contact resistance ρ_c = (5...7)+10^–5 Ω.cm² are developed. The influence of the substrate temperature during the silver deposition and the annealing temperature on the height of the Schottky barrier Ag/n–n⁺ GaAs, the injection coefficient γ and the nonideality factor η is established.     

Study of contact resistivity, mechanical integrity, and thermal stability of Ti/Al and Ta/Al ohmic contacts to n-type GaN

The annealing conditions and contact resistivities of Ta/Al ohmic contacts to n-type GaN are reported for the first time. The high temperature stability and mechanical integrity of Ti/Al and Ta/Al contacts have been investigated. Ta/Al (35 nm/115 nm) contacts to n-type GaN became ohmic after annealing for 3 min at 500°C or for 15 s at 600°C. A minimum contact resistivity of 5×10⁻⁶Ω cm² was measured after contacts were repatterned with an Al layer to reduce the effect of a high metal sheet resistance. Ti/Al and Ta/Al contacts encapsulated under vacuum in quartz tubes showed a significant increase in contact resistivity after aging for five days at 600°C. Cross section transmission electron microscopy micrographs and electrical measurements of aged samples indicate that the increased contact resistivity is primarily the result of degradation of the metal layers. Minimal reactions at the metal/GaN interface of aged samples were observed.     

Comparison between TiAl and TiAlNiAu ohmic contacts to n-type GaN

We report a comparison between Ti/Al and Ti/Al/Ni/Au ohmic contacts in terms of contact resistivity, thermal stability, depth profile, and surface morphology. The metals were deposited by conventional electron-beam evaporation, and then annealed at 900°C for 30 s in a N₂ atmosphere. The lowest value for the specific contact resistivity was obtained using Ti/Al/Ni/Au metallization. Ti/Al/Ni/Au contacts showed no increase in contact resistivity after aging for five days at 600°C in an air atmosphere. Examination of the surface morphology using atomic force microscopy revealed that the surface roughness was clearly better in the case of Ti/Al/Ni/Au contacts. X-ray photoelectron spectroscopy was also employed and gave primary results of Ti/Al/Ni/Au contact formation.     

Microstructure and chemistry of Cu-Ge ohmic contact layers to GaAs

We report systematic studies of microstructure and chemistry of Cu-Ge alloyed ohmic contacts to n-GaAs with very low specific contact resistivity ((4-6) x 10^-7 Ωcm² for n∼l x 10¹⁷cm^-3). Using transmission electron microscopy, x-ray microanalysis, and secondary ions mass spectroscopy, we investigated chemistry of phase formation, crystal structure, and mechanism of ohmic contact formation in Cu-Ge alloyed layers with Ge concentration in the range of 0–40 at.%. Layers with Ge deficiency to form ζ-phase (average composition Cu₅Ge) reveal the formation of a nonuniform intermediate layer of hexagonal -Cu₃As phase which grows epitaxially on Ga₁₁₁ planes of GaAs. In this case, released Ga diffuses out and dissolves in the alloyed layer stabilizing ζ-phase, which is formed in the structures with average Ge concentration as low as 5 at.%. Unique properties of the contact layers, namely low specific contact resistivity, high thermal stability, interface sharpness, and high contact layer uniformity are related to the formation of an ordered orthorhombic ε₁ Cu₃Ge phase. In the alloyed layer with Ge concentration >25 at.%, no phases due to the chemical reactions with GaAs in the interface region were found demonstrating the chemical inertness of the ε₁Cu₃Ge ordered phase with respect to GaAs. This results in sharp interfaces and uniform chemical composition, the characteristics needed for superior contacts.