Refractory metal boride ohmic contacts to P-type 6H-SiC

Ohmic contacts have been fabricated on p-type 6H-SiC (1.3×10¹⁹ cm⁻³) using CrB₂, W₂B, and TiB₂. The boride layers (∼100–200 nm) were sputter-deposited in a system with a base pressure of 3×10⁻⁷ Torr. Specific contact resistances were measured using the linear transmission line method, and the physical properties of the contacts were examined using Rutherford backscattering spectrometry. All as-deposited contacts exhibited rectifying characteristics. Ohmic behavior was observed following short anneals (2–10 min) at 1100°C and 5×10⁻⁷ Torr. Current-voltage characteristics were linear for CrB₂ and W₂B and quasi-linear for TiB₂. The lowest values of the specific contact resistance (r_c in Ω-cm²) measured at room temperature for CrB₂ and W₂B were 8.2×10⁻⁵ and 5.8×10⁻⁵, respectively. The specific contact resistance for TiB₂ was not determined accurately. Longer anneals (30 min for W₂B and 90 min for CrB₂) reduced the room temperature values of r_c to 6.1×10⁻⁵ for W₂B and 1.9×10⁻⁵ for CrB₂. Backscattering spectra revealed substantial concentrations of oxygen in all as-deposited boride films. The short anneal cycle removed the oxygen in the CrB₂ films and reduced the concentration substantially in the W₂B films; however, annealing had no affect on the oxygen concentration in the TiB₂ films. The CrB₂/SiC interface remained stable during annealing; i.e., Si and carbon were not observed in the boride layers after annealing. In contrast, W₂B and TiB₂ reacted with the SiC epilayers, and after annealing, Si and carbon were observed at the surface of each boride layer.     

Correlation between the electrical properties and the interfacial microstructures of TiAl-based ohmic contacts to p-type 4H-SiC

In order to understand a mechanism of TiAl-based ohmic contact formation for p-type 4H-SiC, the electrical properties and microstructures of Ti/Al and Ni/Ti/Al contacts, which provided the specific contact resistances of approximately 2×10⁻⁵ Ω-cm² and 7×10⁻⁵ Ω-cm² after annealing at 1000°C and 800°C, respectively, were investigated using x-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Ternary Ti₃SiC₂ carbide layers were observed to grow on the SiC surfaces in both the Ti/Al and the Ni/Ti/Al contacts when the contacts yielded low resistance. The Ti₃SiC₂ carbide layers with hexagonal structures had an epitaxial orientation relationship with the 4H-SiC substrates. The (0001)-oriented terraces were observed periodically at the interfaces between the carbide layers and the SiC, and the terraces were atomically flat. We believed the Ti₃SiC₂ carbide layers primarily reduced the high Schottky barrier height at the contact metal/p-SiC interface down to about 0.3 eV, and, thus, low contact resistances were obtained for p-type TiAl-based ohmic contacts.     

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.     

Effect of Si or Al interface layers on the properties of Ta and Mo contacts to p-type SiC

We present our results on the role of Si or Al interface layers on the structure and electrical properties of tantalum and molybdenum contacts to p-type 6H-SiC. Thin films of Ta or Mo were deposited on p-type SiC with and without p-doped Si or Al interface layers. The Ta/p-SiC, Ta/p-Si/p-SiC, Ta/Al/p-SiC, Mo/p-SiC, and Mo/Al/p-SiC structures were annealed at high temperatures up to 1200°C using the rapid thermal annealing process, in Ar-H₂ or N₂-H₂ ambient. X-ray diffraction analysis showed TaSi₂ in both Ta/p-SiC and Ta/p-Si/p-SiC structures annealed in Ar-H₂ ambient. For the N₂-H₂ ambient anneal tantalum nitride (TaN) was formed in Ta/p-SiC and Ta/Al/p-SiC, and TaN plus TaSi₂ in Ta/p-Si/p-SiC. While there was evidence of interaction between Mo and Si or Al no intermetallic phases were observed. Electrical measurements revealed that both TaN in Ta/p-SiC and TaN + TaSi₂ in Ta/p-Si/p-SiC structures made ohmic contacts, with specific contact resistances of about 2.13 × 10⁻³ and 1.47 × 10⁻¹ Ω-cm², respectively. The specific contact resistance for Ta/Al and Mo/Al layers on p-SiC decreases with increasing temperature and varies with anneal ambient. The values calculated for Ta/Al/p-SiC and Mo/Al/p-SiC were about 4.22 × 10⁻⁴ at 1100°C and 4.5 × 10⁻⁵ Ω-cm² at 1200°C, respectively. The heavy surface doping provided by Al in Ta/Al/p-SiC and Mo/Al/p-SiC is responsible for the low specific contact resistance.     

Ohmic contact formation in palladium-based metallizations to n-Type InP

Two Pd-based metallizations have been systematically studied, i.e., Au/Ge/Pd and Pd/Ge contacts to n-type InP, in an attempt to better understand the role of the metallization constituents in forming ohmic contacts. Ohmic contacts were obtained with minimum specific resistances of 2.5 × 10⁻⁶ Ω-cm² and 4.2 × 10⁻⁶ Ω-cm² for the Au/Ge/Pd and the Pd/Ge contacts, respectively. The annealing regime for ohmic contact formation is 300-375°C for the Au/Ge/Pd/InP system and 350-450°C for the Pd/GelnP system. Palladium, in both cases, reacts with InP to form an amorphous layer and then an epitaxial layer at low temperatures, providing good metallization adhesion to InP substrates and improved contact morphology. Ohmic contact formation in both contacts is attributed to Ge doping, based on the solid state reaction-driven decomposition of an epitaxial layer at the metallization/InP interface, producing a very thin, heavily doped InP layer. Gold appears to be responsible for the difference in contact resistance in the two systems. It is postulated that Au reacts strongly with In to form Au-In compounds, creating additional In site vacancies in the InP surface region (relative to the Au-free metallization), thereby enhancing Ge doping of the InP surface and lowering the contact resistance. Both contacts degrade and ultimately become Schottky barriers again if over annealed, due to consumption of additional InP, which destroys the heavily doped InP layer.     

Thermal Stability of Nitride-Based Diffusion Barriers for Ohmic Contacts to <Emphasis Type="Italic">n</Emphasis>-GaN

The use of TaN, TiN, and ZrN diffusion barriers for Ti/Al-based contacts on n-GaN (n ∼ 3 × 10¹⁷ cm⁻³) is reported. The annealing temperature (600–1,000°C) dependence of the Ohmic contact characteristics using a Ti/Al/X/Ti/Au metallization scheme, where X is TaN, TiN, or ZrN, deposited by sputtering was investigated by contact resistance measurements and Auger electron spectroscopy (AES). The as-deposited contacts were rectifying and transitioned to Ohmic behavior for annealing at ≥600°C. A minimum specific contact resistivity of ∼6 × 10⁻⁵ Ω-cm⁻² was obtained after annealing over a broad range of temperatures (600–900°C for 60 s), comparable to that achieved using a conventional Ti/Al/Pt/Au scheme on the same samples. The contact morphology became considerably rougher at the high end of the annealing range. The long-term reliability of the contacts at 350°C was examined; each contact structure showed an increase in contact resistance by a factor of three to four over 24 days at 350°C in air. AES profiling showed that the aging had little effect on the contact structure of the nitride stacks.     

Annealed AuGe based Ohmic contacts on InP with ion milling prior to metallization

Ohmic contacts have been fabricated onn-type InP with an alloyed AuGe based metallurgy that involved ion milling prior to metallization. Minimum values for contact resistance and specific resistance of 0.015Ω-mm and 3.2 × 10⁻⁸ Ω-cm², respectively, were found with an annealing temperature in the range, 440–480° C. Addition of Ni to the contact metallurgy improves the wetting characteristics of the AuGe and lowers the contact resistance. It is proposed that ion milling prior to metallization results in a reactive metal-semiconductor interface and low contact resistance values for samples with and without Ni.     

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.     

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.     

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.     

Low resistive ohmic contact formation on surface treated-n-GaN alloyed at low temperature

Ohmic contacts to n-type GaN with low contact resistance were developed by (NH₄)₂S_x and KOH+(NH₄)₂S_x surface treatments prior to Ti/Al metal deposition. The lowest specific contact resistance of 3.0×10⁻⁶ Ω-cm² was obtained for Ti/Al contacts in an (NH₄)₂S_x-treated GaN layer alloyed at 300°C for 3 min. To obtain the lowest specific contact resistance for a low temperature alloy, the (NH₄)₂S_x treatment conditions for both (NH₄)₂S_x and KOH+(NH₄)₂S_x-treated n-GaN layers have been investigated and the mechanism for ohmic formation in low temperature alloys analyzed.     

High-quality schottky and ohmic contacts in planar 4H-SiC metal semiconductor field-effect transistors and device performance

We have fabricated planar 4H-SiC, metal-semiconductor field-effect transistors (MESFETs) with high-quality metal/SiC contacts. To eliminate potential damage to the gate region caused by etching and simplify the device fabrication process, gate Schottky contacts were formed without any recess gate etching, and an ideality factor of 1.03 was obtained for these gate contacts. The interface state density between the contact metal and SiC was 5.7×10¹² cm⁻²eV⁻¹, which was found from the relationship between the barrier height and the metal work function. These results indicate that the interface was well controlled. Thus, a transconductance of 30 mS/mm was achieved with a 3-μm gate length as the performance figure of these MESFETs with high-quality metal/SiC contacts. Also, a low ohmic contact resistance of 1.2×10⁻⁶ Θcm² was obtained for the source and drain ohmic contacts by using ion implantation.     

Properties of electrical contacts on bulk and epitaxial n-type ZnO

The electrical properties of several metal contacts to n-type ZnO (0001) were studied. The ZnO samples consisted of bulk single-crystal material, epitaxial layers on sapphire grown by molecular beam epitaxy (MBE), and polycrystalline thin films on sapphire obtained by pulsed laser deposition (PLD). Ohmic and rectifying contacts were observed dependent upon both the metal material and the ZnO surface. Ohmic contacts were characterized using the circular transmission line method (c-TLM), where contact resistivity was found to be in the range of 10⁻⁴−10⁻⁵ Ω-cm². Schottky behavior was observed using Ag contacts exhibiting varying leakage current and breakdown voltage dependent on the polarity of the ZnO surface.     

Pressure electrical contact improved by carbon black paste

Pressure and pressureless electrical contacts were evaluated by measuring the contact electrical resistivity between copper mating surfaces. Pressure electrical contacts with a contact resistivity of 2×10⁻⁵ Ω·cm² have been attained using a carbon black paste of a thickness of less than 25 μm as the interface material. In contrast, a pressureless contact with silver paint as the interface material exhibits a higher resistivity of 3×10⁻⁵ Ω·cm² or above. A pressureless contact with colloidal graphite as the interface material exhibits the same high contact resistivity (1×10⁻⁴ Ω·cm²) as a pressure contact without any interface material. On the other hand, pressureless contacts involving solder and silver epoxy exhibit lower contact resistivity than carbon black pressure contacts.     

A technique to reduce the contact resistance to 4H-silicon carbide using germanium implantation

The effects of implanted Ge on the resistance of nickel-metal contacts to n-type and p-type 4H-SiC are reported. The Ge was implanted with an energy of 346 keV and a dose of 1.7×10¹⁶ cm⁻², and the wafer was annealed up to 1700°C for 30 min. Contact resistance measurements using the transfer length method (TLM) were performed on etched mesas of n-type and p-type 4H-SiC, with and without the Ge. For the annealed-Ni metal contacts, the Ge lowered the specific contact resistivity from 5.3×10⁻⁴ Ωcm² to 6.0×10⁻⁵ Ωcm² for n-type SiC and from 1.2×10⁻³ Ωcm² to 8.3×10⁻⁵ Ωcm² for p-type SiC. For the as-deposited (unannealed) Ni, the Ge produced ohmic contacts, whereas the contacts without Ge were rectifying. These results suggest that the addition of Ge can be an important process step to reduce the contact resistance for SiC-device applications.     

Ta/Au ohmic contacts to n-type ZnO

Ta/Au ohmic contacts are fabricated on n-type ZnO (∼1 × 10¹⁷ cm⁻³) epilayers, which were grown on R-plane sapphire substrates by metal organic chemical vapor deposition (MOCVD). After growth and metallization, the samples are annealed at 300°C and 500°C for 30 sec in nitrogen ambient. The specific contact resistance is measured to be 3.2×10⁻⁴ Ωcm² for the as-deposited samples. It reduces to 5.4×10⁻⁶ Ωcm² after annealing at 300°C for 30 sec without significant surface morphology degradation. When the sample is annealed at 500°C for 30 sec, the specific contact resistance increases to 3.3 × 10⁻⁵ Ωcm². The layer structures no longer exist due to strong Au and Ta in-diffusion and O out-diffusion. The contact surface becomes rough and textured.     

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.     

Effect of epilayer characteristics and processing conditions on the thermally oxidized SiO2/SiC interface

The optimization of the SiO₂/SiC interface is critical for the development of SiC MOS devices. We investigate the effects of several variables spanning both epilayer attributes and processing conditions relative to our control oxidation process. Varying the shallow vicinal angle of the wafer does not affect the interface. There is a definite degradation of the interface as the epilayer doping density is increased. Sacrificial oxidation appears to reduce the number of border traps in the final oxide. Fluorine annealing has no effect on the interface quality. A low temperature (950°C) re-oxidation, which follows a bulk oxide growth at 1150°C, reduces D _it to the mid-10¹⁰ cm⁻²eV⁻¹ range near midgap and Q_f to a reacord low 5×10¹¹ cm⁻².     

Low resistance pd/ge ohmic contacts to epitaxially lifted-off n-type GaAs film

A low resistance PdGe nonalloyed ohmic contact has been successfully formed to epitaxially lifted-off n-type GaAs films. The contact is made by lifting off partially metallized n-type GaAs films using the epitaxial lift-off method and bonding them to metallized Si substrates by natural intermolecular Van Der Waals forces. Low temperature sintering (200°C) of this contact results in metallurgical bonding and formation of the ohmic contact. We have measured specific contact resistances of 5 × 10⁻⁵ Ω-cm₂ which is almost half the value obtained for pure Pd contacts. Germanium forms a degenerately doped heterojunction interfacial layer to GaAs. Our experimental results show that germanium diffuses to the interface and acts as a dopant layer to n-GaAs film surface. Therefore, for epitaxially lifted-off n-type GaAs films, PdGe is a low resistance ohmic metal contact to use.