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.     

Simultaneous formation of p- and n-type ohmic contacts to 4H-SiC using the ternary Ni/Ti/Al system

Fabrication procedures for silicon carbide power metal oxide semiconductor field effect transistors (MOSFETs) can be improved through simultaneous formation (i.e., same contact materials and one step annealing) of ohmic contacts on both the p-well and n-source regions. We have succeeded with the simultaneous formation of the ohmic contacts for p- and n-type SiC semiconductors by examining ternary Ni/Ti/Al materials with various compositions, where a slash symbol “/” indicates the deposition sequence starting with Ni. The Ni(20 nm)/Ti(50 nm)/Al(50 nm) combination provided specific contact resistances of 2 × 10⁻³ Ω-cm² and 2 × 10⁻⁴ Ω-cm² for p- and n-type SiC, respectively, after annealing at 800°C for 30 min, where the doping level of Al in the SiC substrate was 4.5 × 10¹⁸ cm⁻³ and the level of N was 1.0 × 10¹⁹ cm⁻³.     

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.     

Cu3Ge ohmic contacts to n-type GaAs

We show that Cu-Ge alloys prepared by depositing sequentially Cu and Ge layers onto GaAs substrates at room temperature followed by annealing at 400°C form a low-resistance ohmic contact to n-type GaAs over a wide range of Ge concentration that extends from 20 to 40 at.%. A contact resistivity of (4-6) x 10^-7 Ω cm² is obtained on n-type GaAs with doping concentrations of～ 1 x 10¹⁷ cm^-3. The contact resistivity is affected only slightly by varying the Ge concentration in the range studied and is not influenced by the deposition sequence of the Cu and Ge layers. In addition, the contacts are electrically stable during annealing at 450°C after contact formation. Structure and properties of Cu-Ge contact layers having lower and higher Ge concentrations from the stoichiometric Cu₃Ge composition are compared. High-resolution transmission electron microscopy and x-ray diffractometry have been used to study the ordering in the ε₁-Cu₃Ge (average lattice parameters: a₀= 5.30Å, b₀= 4.20Å, c₀= 4.56Å) which is responsible for orthorhombic distortion of the parent hexagonal ζ-phase. The results suggest that the formation of theξ and ε-Cu₃Ge phases creates a highly doped n⁺-GaAs surface layer which leads to the low contact resistivity.     

Ohmic contacts to single-crystalline 3C-SiC films for extreme-environment MEMS applications

This paper describes the ohmic contacts to single-crystalline 3C-SiC thin films heteroepitaxially grown on Si (0 0 1) wafers. In this work, a TiW (titanium-tungsten) film was deposited as a contact material by RF magnetron sputter and annealed through the vacuum and rapid thermal anneal (RTA) process. Contact resistivity between the TiW film and the n-type 3C-SiC substrate was measured by the circular transmission line model (C-TLM) method. The contact phases and interface of the TiW/3C-SiC were evaluated with X-ray diffraction (XRD), scanning electron microscope (SEM) and Auger electron spectroscopy (AES) depth-profiles. The TiW film annealed at 1000 °C for 45 s with the RTA plays an important role in the formation of ohmic contact with the 3C-SiC film and the contact resistance is less than 4.62×10⁻⁴ Ω cm². Moreover, the inter-diffusion at the TiW/3C-SiC interface was not generated during, before and after annealing, and was kept in a stable state. Therefore, the ohmic contact formation technology of single-crystalline 3C-SiC films by using the TiW film is very suitable for high-temperature micro-electro-mechanical system (MEMS) applications.     

Development of Al-free ohmic contact to n-GaN

We have investigated the electrical properties and interfacial reactions of the Si/Ti-based ohmic contacts to Si-doped n-GaN grown by metal organic chemical vapor deposition and the electrical properties were related to the material reactions. Si/Ti contact system was selected because Ti silicides have a low work function comparable to Al and also Si was used widely as an n-type dopant. As the annealing temperature increased, the specific contact resistance of Si/Ti-based ohmic contacts decreased and showed minimum contact decreased and showed minimum contact resistance as low as 3.86 10^?6 cm² after annealing at 900°C for 3 min under N₂ ambient. Our experimental results show that the ohmic behavior of Si/Ti-based contact, were attributed to the low barrier height of Ti-silicide/GaN interface, which was formed through the interfacial reaction between Si and Ti layers. In order to clarify the current conduction mechanism of Si/Ti-based contact, temperature dependent contact resistance measurement was carried out for Au(1000 Å)/Ti(400 Å)/Si(1500 Å)/Ti(150 Å) contact system after annealing at 700°C for 3 min. The contact resistance of Si/Ti-based ohmic contact decreased exponentially with the measuring temperature and so it can be concluded that current flows over the low barrier height by thermionic emission.     

高Al组分n-AlGaN的Ti/Al/Ti/Au欧姆接触

采用Ti/Al/Ti/Au多层金属电极对高Al组分n-AlxGa1-xN(x=0.6)欧姆接触的制备进行了研究,通过优化Ti接触层厚度以及合金退火条件,获得了较低的比接触电阻率(5.67×10-5Ω.cm2)。研究证实,Ti接触层厚度对欧姆接触特性有着重要影响,同时发现,高低温两步退火方式之所以能够改善欧姆接触特性的本质是与Al3Ti及TiN各自的生成条件直接相关,即低温利于生成Al3Ti,高温利于生成TiN,而这对n型欧姆接触的有效形成至关重要。     

Nanopatterned Contacts to GaN

The effect of nanoscale patterning using a self-organized porous anodic alumina (PAA) mask on the electrical properties of ohmic and Schottky contacts to n-GaN was investigated with the aim of evaluating this approach as a method for reducing the specific contact resistance of ohmic contacts to GaN. The electrical characteristics of contacts to these nanopatterned GaN samples were compared with contacts to planar, chemically prepared (“as-grown”) GaN samples and reactive ion etched (RIE) GaN films without any patterning. The specific contact resistivities to unintentionally doped n-GaN using a Ti/Al bilayer metallization were determined to be 7.4 × 10⁻³ Ω cm² for the RIE sample and 7.0 × 10⁻⁴ Ω cm² for the PAA patterned sample. Schottky metal contacts with Pt and Ni were prepared on the three samples to validate the effects of RIE and nanopatterning on electrical behavior. The effective barrier height was decreased and the reverse current was increased significantly in the PAA patterned sample. The radius of curvature of the nanoscale corrugation in the patterned interface was smaller than the depletion width. The reduction of the depletion width at sharp corners enhanced the local tunneling current, reducing the specific contact resistivity and decreasing the effective barrier height. These results suggest that nanopatterning with PAA on GaN can significantly lower the contact resistance.     

Investigation of Au/Ti/Al ohmic contact to N-type 4H–SiC

In this study, investigation on Au/Ti/Al ohmic contact to n-type 4H–SiC and its thermal stability are reported. Specific contact resistances (SCRs) in the range of 10⁻⁴–10⁻⁶ Ω cm², and the best SCR as low as 2.8 × 10⁻⁶ Ω cm² has been generally achieved after rapid thermal annealing in Ar for 5 min at 800 °C and above. About 1–2 order(s) of magnitude improvement in SCR as compared to those Al/Ti series ohmic systems in n-SiC reported in literature is obtained. XRD analysis shows that the low resistance contact would be attributed to the formation of titanium silicides (TiSi₂ and TiSi) and Ti₃SiC₂ at the metal/n-SiC interface after thermal annealing. The Au/Ti/Al ohmic contact is thermally stable during thermal aging treatment in Ar at temperature in the 100–500 °C range for 20 h.     

Ohmic contacts to n-type GaSb and n-type GalnAsSb

An investigation with the objective of improving n-type ohmic contacts to GaSb-based devices is described. This study involves a series of n-GaInAsSb and n-GaSb samples with varying doping, grown on semi-insulating (SI) GaAs substrates. These samples were fabricated into mesa-etched, transfer-length method (TLM) structures, and the specific-contact resistivity and sheet resistance of these layers as a function of majority electron concentration were measured. Extremely low specific-contact resistivities of about 2 × 10⁻⁶ Ω-cm² and sheet resistances of about 4 Ω/▭ are found for n-type GaInAsSb doped at about 3 × 10¹⁸ cm⁻³.     

Characterization of polycrystalline 3C-SiC thin film diodes for extreme environment applications

This paper describes the fabrication and characteristics of polycrystalline (poly) 3C-SiC thin film diodes for extreme environment applications, in which the poly 3C-SiC thin film was deposited onto oxidized Si wafers by APCVD using HMDS as a precursor. In this work, the optimized growth temperature and HMDS flow rate were 1100 °C and 8 sccm, respectively. A Schottky diode with a Au, Al/poly 3C-SiC/SiO₂/Si(n-type) structure was fabricated and its threshold voltage (V_d), breakdown voltage, thickness of depletion layer, and doping concentration (N_D) values were measured as 0.84 V, over 140 V, 61 nm, and 2.7 × 10¹⁹ cm³, respectively. To produce good ohmic contact, Al/3C-SiC were annealed at 300, 400, and 500 °C for 30 min under a vacuum of 5.0 × 10⁻⁶ Torr. The obtained p-n junction diode fabricated by poly 3C-SiC had similar characteristics to a single 3C-SiC p-n junction diode.     

Nonalloyed Al ohmic contacts to MgxZn1−xO

Al nonalloyed ohmic contacts were fabricated and characterized on Mg_xZn_1−xO (0≤×≤0.34) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). Specific contact resistances were evaluated by the transmission line method (TLM). A specific contact resistance of 2.5×10⁻⁵ Ωcm² was obtained for Al contact to ZnO with an electron concentration of 1.6×10¹⁷ cm⁻³. The current flow mechanism was studied by investigating the dependence of specific contact resistances on electron concentration and on temperature. For Al contact to Mg_0.34Zn_0.66O, specific contact resistance values are two orders of magnitude larger than that of Al ohmic contacts to ZnO.     

Thermally stable Ge/Cu/Ti ohmic contacts to n-type GaN

The performance of a novel Ge/Cu/Ti metallization scheme on n-type GaN has been investigated for obtaining thermally and electrically stable low-resistance ohmic contacts. Isochronal (2 min.) anneals in the 600–740°C temperature range and isothermal (690°C) anneals for 2–10 min. duration were performed in inert atmosphere. For the 690°C isothermal schedule, ohmic behavior was observed after annealing for 3 min. or longer with a lowest contact resistivity of 9.1 × 10⁻⁵ Ωcm² after the 10 min. anneal for a net donor doping concentration of 9.2 × 10¹⁷ cm^−Ω3. Mean roughness (R_a) for anneals at 690°C was almost constant at around 5 nm, up to an annealing duration of 10 min., which indicates a good thermal stability of the contact scheme.     

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.     

High-efficiency (49%) and high-power photovoltaic cells based on gallium antimonide

High-efficiency GaSb-based photovoltaic cells designed for conversion of high-power laser radiation and infrared radiation of emitters heated by concentrated solar radiation are fabricated and studied. The maximum efficiency of conversion of the radiation with λ = 1680 nm was 49% at the photocurrent density of 50–100 A/cm² for the fabricated photovoltaic cells. The methods for reducing the losses at ohmic contacts to p-and n-GaSb are investigated. The minimum values of the specific resistance, (1–3) × 10^?6 Θ cm², of contact to p-GaSb with the doping level of 10²⁰ cm^?3 were obtained using the Ti/Pt/Au contact system. The minimum values of the specific contact resistance were (1–3) × 10^?6 Θ cm² in the case of n-GaSb with the doping level of 2 × 10¹⁸ cm^?3 if the Au(Ge)/Ni/Au and Au/Ni/Au contact systems are used.     

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.     

Vanadium-based ohmic contacts to n-type Al<Subscript>0.6</Subscript>Ga<Subscript>0.4</Subscript>N

Four vanadium-based contacts to n-type Al_0.6Ga_0.4N were compared in this work. Both V/Al/Pd/Au and V/Al/V/Au contacts with optimized layer thicknesses provided lower specific-contact resistances than did the previously reported V/Al/Pt/Au ohmic contact. Specific contact resistances of the V/Al/Pd/Au (15 nm/85 nm/20 nm/95 nm) and V/Al/V/Au (15 nm/85 nm/20 nm/95 nm) contacts were 3×10⁻⁶ Ω·cm² and 4×10⁻⁶ Ω·cm², respectively. On the other hand, an analogous V/Al/Mo/Au contact never became ohmic, even after it was annealed at 900°C for 30 sec. Compared to the V/Al/Pd/Au contact, the V/Al/V/Au contact required a less severe annealing condition (30 sec at 700°C instead of 850°C). The V/Al/V/Au contact also provided a smoother surface, with a root-mean-square (RMS) roughness of 39 nm.     

Ohmic contacts to n-type GaN using Pd/Al metallization

Ohmic contacts to n-type GaN grown by metalorganic chemical vapor deposition were formed using a Pd/Al-based metallization. Ohmic contact resistances and specific contact resistances were investigated as a function of rapid thermal annealing temperature, Pd interlayer thickness, and annealing time. As-deposited Pd/AI was found to produce rectifying contacts while the metallization exhibited ohmic characteristics after annealing at temperatures as low as 400°C. A minimum contact resistance of 0.9 ohm-mm (specific contact resistance = 1.2 × 10⁻⁵ ohm-cm²) was obtained upon annealing at 650°C for 30 s. For comparison, Al and Ti/Al contacts were also investigated. Auger electron spectroscopy, secondary ion mass spectrometry, and x-ray diffraction were used to investigate metallurgical reactions.     

Heavily Aluminum-Doped Epitaxial Layers for Ohmic Contact Formation to <Emphasis Type="Italic">p</Emphasis>-Type 4H-SiC Produced by Low-Temperature Homoepitaxial Growth

In this work, heavily aluminum (Al)-doped layers for ohmic contact formation to p-type SiC were produced by utilizing the high efficiency of Al incorporation during the epitaxial growth at low temperature, previously demonstrated by the authors’ group. The low-temperature halo-carbon epitaxial growth technique with in situ trimethylaluminum (TMA) doping was used. Nearly featureless epilayer morphology with an Al atomic concentration exceeding 3 × 10²⁰ cm⁻³ was obtained after growth at 1300°C with a growth rate of 1.5 μm/h. Nickel transfer length method (TLM) contacts with a thin adhesion layer of titanium (Ti) were formed. Even prior to contact annealing, the as-deposited metal contacts were almost completely ohmic, with a specific contact resistance of 2 × 10⁻² Ω cm². The specific contact resistance was reduced to 6 × 10⁻⁵ Ω cm² by employing a conventional rapid thermal anneal (RTA) at 750°C. Resistivity of the epitaxial layers better than 0.01 Ω cm was measured for an Al atomic concentration of 2.7 × 10²⁰ cm⁻³.