Mechanism investigation of NiO<Subscript>x</Subscript> in Au/Ni/p-type GaN ohmic contacts annealed in air

Recently, Au/Ni/p-type GaN ohmic contacts annealed in an air ambient have been widely investigated. However, to obtain a low specific-contact resistance, the annealing window is limited. In this study, to understand the oxidation function of metallic Ni, the Au/Ni/p-type GaN structure was annealed in an air ambient for 10 min at various temperatures. Using x-ray photoelectron spectroscopy (XPS) analysis, the metallic Ni was oxidized into NiO and NiO_1.3 compositions at annealing temperatures of 500°C and 600°C, respectively. However, metallic Ni still existed on the interface of the Ni/p-type GaN annealed at 400°C. The associated barrier heights of 0.42 eV, 0.21 eV, and 0.31 eV were obtained with p-type GaN for the Ni, NiO, and NiO_1.3 contacts, respectively. The hole concentrations of p-type NiO and p-type NiO_1.3 were 2.6×10¹⁶ cm⁻³ and 2.0×10¹⁸ cm⁻³, respectively. The lower hole concentration of the p-type NiO would lead to reducing the valence-band bending of the p-type GaN, as well as the barrier height for holes crossing from the p-type NiO to the p-type GaN. The formation of NiO was thus an important issue for lowering the specific-contact resistance of the Au/Ni/p-type GaN ohmic contacts annealed in an air ambient.     

Low-resistance and thermally stable Pd/Ru ohmic contacts to p-type GaN

We report on low-resistance and thermally stable Pd/Ru ohmic contacts to surface-treated p-GaN (3 × 10¹⁷ cm⁻³). It is shown that annealing at 500°C for 2 min in a N₂ ambient improves ohmic contact properties. Specific contact resistance is measured to be 9.2(±0.2) × 10⁻⁴ and 2.4(±0.2) × 10⁻⁵ Ωcm² for the as-deposited and annealed samples, respectively. Atomic force microscopy results show that the surfaces of both the contacts are remarkably smooth with a root-mean-square (rms) roughness of about 0.6 nm. The current-voltage-temperature (I-V-T) and calculation results indicate that, for the as-deposited contact, thermionic field emission is dominant, while for the annealed contact, field emission dominates the current flow.     

Temperature-Dependent Studies of Si-Implanted Al<Subscript>0.33</Subscript>Ga<Subscript>0.67</Subscript>N with Different Annealing Temperatures and Times

Electrical activation studies were carried out on Si-implanted Al_0.33Ga_0.67N as a function of ion dose, annealing temperature, and annealing time. The samples were implanted at room temperature with Si ions at 200 keV in doses ranging from 1 × 10¹⁴ cm⁻² to 1 × 10¹⁵ cm⁻², and subsequently proximity-cap annealed from 1150°C to 1350°C for 20 min to 60 min in a nitrogen environment. One hundred percent electrical activation efficiency was obtained for Al_0.33Ga_0.67N samples implanted with a dose of 1 × 10¹⁵ cm⁻² after annealing at either 1200°C for 40 min or at 1300°C for 20 min. The samples implanted with doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² exhibited significant activations of 74% and 90% after annealing for 20 min at 1300°C and 1350°C, respectively. The mobility increased as the annealing temperature increased from 1150°C to 1350°C, showing peak mobilities of 80 cm²/V s, 64 cm²/V s, and 61 cm²/V s for doses of 1 × 10¹⁴ cm⁻², 5 × 10¹⁴ cm⁻², and 1 × 10¹⁵ cm⁻², respectively. Temperature-dependent Hall-effect measurements showed that most of the implanted layers were degenerately doped. Cathodoluminescence measurements for all samples exhibited a sharp neutral donor-bound exciton peak at 4.08 eV, indicating excellent recovery of damage caused by ion implantation.     

Tellurium-rich growth and laser fabrication of lead-tin-telluride (Pb1−xSnxTe: 0.06<x<0.08)

Single crystals of Pb_1−x Sn_x Te (0.06<x<0.08) have been grown by using an ingot-nucleation technique from a Te-rich source. The as-grown crystals have a p-type carrier concentration around 10¹⁹ cm⁻³ and dislocation density as low as 10³ cm⁻². Diode lasers fabricated from these crystals have contact resistances of 2×10⁻⁵ Ω-cm² and a single-mode single-ended output power of 750 μW at heat sink temperatures around 15 K.     

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.     

Effects of Annealing in a Partially Reducing Atmosphere on Sputtered Al-Doped ZnO Thin Films

Aluminum-doped ZnO (AZO) films have been deposited by room-temperature radio frequency (RF) magnetron sputtering onto fused-quartz substrates using a ZnO:Al₂O₃ (98:2 wt.%) target. A post-deposition anneal in an N₂ flow has been shown to improve the electrical and optical properties of as-deposited AZO. All films were polycrystalline and exhibited a hexagonal wurtzite structure with the c-axis oriented perpendicular to the substrate. An increase in the estimated crystallite size was observed after annealing. Electrical resistivity was reduced from 3.7 × 10⁻³ Ω cm for as-deposited layers to 7.1 × 10⁻⁴ Ω cm for annealed films. An average optical transmittance >85% for annealed films was routinely measured. X-ray photoelectron spectroscopy measurements indicated that the surfaces of all layers investigated were oxygen deficient, and the density of oxygen vacancies was found to increase following the anneal.     

Microstructural and electrical investigation of Ni/Au ohmic contact on p-type GaN

Electrical properties of Ni/Au ohmic contacts on p-type GaN were interpreted with the change of microstructure observed under transmission electron microscopy. The contact resistivity was decreased from 1.3×10⁻² to 6.1×10⁻⁴ Ωcm² after annealing at 600°C. The reduction is due to the dissolution of Ga atoms into Au−Ni solid solution formed during annealing, via the generation of Ga vacancies. Thus, net concentration of holes increased below the contact, resulting in the reduction of contact resistivity. At 800°C, N atoms decomposed; reacted with Ni, and forming cubic Ni₄N. Consequently, N vacancies, acting as donors in GaN, were generated below the contact, leading to the increase of contact resistivity to 3.8×10⁻² Ωcm².     

Electrical characteristics of thermally stable Ru and Ru/Au ohmic contacts to surface-treated p-type GaN

The electrical and thermal properties of Ru and Ru/Au ohmic contacts on two-step-surface-treated p-GaN have been investigated using current-voltage (I–V) measurements and Auger electron spectroscopy. It is shown that annealing at 700°C for 2 min in a flowing N₂ atmosphere improves the I–V characteristics of the contacts. For example, the annealed Ru and Ru/Au schemes produce a specific contact resistance of 3.4 (±0.9)×10⁻³ and 1.2 (±1.1)×10⁻³ Ωcm², respectively. It is also shown that annealing results in a large reduction (by ∼100 meV) in the Schottky barrier heights of the Ru and Ru/Au contacts, compared to the as-deposited ones. The electrical properties of the two-step-surface-treated Ru/Au contacts are compared with those of the conventionally treated contacts.     

Nearly Perfect Electrical Activation Efficiencies from Silicon-Implanted Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Ga<Subscript>1−<Emphasis Type="Italic">x</Emphasis></Subscript>N with High Aluminum Mole Fraction

Electrical activation studies of Al _x Ga_1−x N (x = 0.45 and 0.51) implanted with Si for n-type conductivity have been made as a function of ion dose and anneal temperature. Silicon ions were implanted at 200 keV with doses ranging from 1 × 10¹⁴ cm⁻² to 1 × 10¹⁵ cm⁻² at room temperature. The samples were subsequently annealed from 1150°C to 1350°C for 20 min in a nitrogen environment. Nearly 100% electrical activation efficiency was successfully obtained for the Si-implanted Al_0.45Ga_0.55N samples after annealing at 1350°C for doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² and at 1200°C for a dose of 1 × 10¹⁵ cm⁻², and for the Al_0.51Ga_0.49N implanted with silicon doses of 1 × 10¹⁴ cm⁻² and 5 × 10¹⁴ cm⁻² after annealing at 1300°C. The highest room-temperature mobility obtained was 61 cm²/V s and 55 cm²/V s for the low-dose implanted Al_0.45Ga_0.55N and Al_0.51Ga_0.49N, respectively, after annealing at 1350°C for 20 min. These results show unprecedented activation efficiencies for Al _x Ga_1−x N with high Al mole fractions and provide suitable annealing conditions for Al _x Ga_1−x N-based device applications.     

Nickel Ohmic Contacts to N-Implanted (0001) 4H-SiC

Samples for transmission line model (TLM) and Hall measurements were fabricated on (0001) 4H-SiC implanted with nitrogen at 1 × 10¹⁸ cm⁻³, 4 × 10¹⁸ cm⁻³, 1 × 10¹⁹ cm⁻³, 4 × 10¹⁹ cm⁻³, and 1 × 10²⁰ cm⁻³. Following high-temperature activation, the activation percentage dropped from ~90% to ~20%, and the Hall mobility decreased from ~100 cm²/V · s to ~20 cm²/V · s as the implant concentration increased from 1 × 10¹⁸ cm⁻³ to 1 × 10²⁰ cm⁻³. The specific contact resistance as a function of Hall concentration is compared with published data for Ni contacts to epitaxial layers. The specific contact resistance as a function of activation temperature was also studied for two fixed implant concentrations of 5 × 10¹⁸ cm⁻³ and 1 × 10²⁰ cm⁻³.     

Investigation of Thermal Stability and Degradation Mechanisms in Ni-Based Ohmic Contacts to <Emphasis Type="Italic">n</Emphasis>-Type SiC for High-Temperature Gas Sensors

We investigated the thermal stability of Pt/TaSi _x /Ni/SiC ohmic contacts, which have been implemented in SiC-based gas sensors developed for applications in diesel engines and power plants. The contacts remained ohmic on lightly doped n-type (~1 × 10¹⁶ cm⁻³) 4H-SiC for over 1000 h in air at 300°C. Although a gradual increase in specific contact resistance from 3.4 × 10⁻⁴ Ω cm² to 2.80 × 10⁻³ Ω cm² was observed, the values appeared to stabilize after ~800 h of heating in air at 300°C. The contacts heated at 500°C and 600°C, however, showed larger increases in specific contact resistance followed by nonohmic behavior after 240 h and 36 h, respectively. Concentration profiles from Auger electron spectroscopy and electron energy-loss spectroscopy show that loss of ohmic behavior occurs when the entire tantalum silicide layer has oxidized.     

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.     

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.     

Low resistance bilayer Nd/Al ohmic contacts on n-type GaN

A bilayer Nd/Al metallization structure has been deposited onto low pressure organometallic vapor phase epitaxy grown n-type GaN ( 1 × 10¹⁸ cm⁻³) by electron-beam evaporation. Ohmic metal contacts were patterned photolithographically for standard transmission line measurement, and then thermally annealed at temperatures ranging from 200 to 350°C and from 500 to 650°C using conventional thermal annealing (CTA) and rapid thermal annealing (RTA), respectively. The lowest values for the specify contact resistivity of 9.8 × 10⁻⁶ Ω−cm² and 8 × 10⁻⁶ Ω−cm² were obtained using Nd/Al metallization with CTA of 250°C for 5 min and RTA of 600°C for 30 s. Examination of the surface morphology using atomic force microscopy as a function of annealing temperature revealed that the surface roughness was strongly influenced by conventional thermal annealing, it was smooth in the temperature range from 550 to 650°C for rapid thermal annealing. Auger electron spectroscopy depth profiling was employed to investigate the metallurgy and interdiffusion of contact formation.     

High-Dose Phosphorus-Implanted 4H-SiC: Microwave and Conventional Post-Implantation Annealing at Temperatures ≥1700°C

Semi-insulating 4H-SiC ⟨0001⟩ wafers have been phosphorus ion implanted at 500°C to obtain phosphorus box depth profiles with dopant concentration from 5 × 10¹⁹ cm⁻³ to 8 × 10²⁰ cm⁻³. These samples have been annealed by microwave and conventional inductively heated systems in the temperature range 1700°C to 2050°C. Resistivity, Hall electron density, and Hall mobility of the phosphorus-implanted and annealed 4H-SiC layers have been measured in the temperature range from room temperature to 450°C. The high-resolution x-ray diffraction and rocking curve of both virgin and processed 4H-SiC samples have been analyzed to obtain the sample crystal quality up to about 3 μm depth from the wafer surface. For both increasing implanted phosphorus concentration and increasing post-implantation annealing temperature the implanted material resistivity decreases to an asymptotic value of about 1.5 × 10⁻³ Ω cm. Increasing the implanted phosphorus concentration and post-implantation annealing temperature beyond 4 × 10²⁰ cm⁻³ and 2000°C, respectively, does not bring any apparent benefit with respect to the minimum obtainable resistivity. Sheet resistance and sheet electron density increase with increasing measurement temperature. Electron density saturates at 1.5 × 10²⁰ cm⁻³ for implanted phosphorus plateau values ≥4 × 10²⁰ cm⁻³, irrespective of the post-implantation annealing method. Implantation produces an increase of the lattice parameter in the bulk 4H-SiC underneath the phosphorus-implanted layer. Microwave and conventional annealing produce a further increase of the lattice parameter in such a depth region and an equivalent recovered lattice in the phosphorus-implanted layers.     

Indium-tin oxide/Si contacts with In- and Sn-diffusion barriers in polycrystalline Si thin-film transistor liquid-crystal displays

Indium and tin were used as the diffusion barrier between indium-tin oxide (ITO) and polycrystalline-silicon layers to reduce the contact resistance. The ITO/Si contacts may be adopted in thin-film transistor liquid-crystal displays (TFT-LCD) to reduce the number of fabrication steps. With In and Sn layers, contact-resistance values of 5 × 10⁻³−4×10⁻³ Ωcm² were obtained. These values were higher than those of the conventional ITO/Mo/Al/Si contacts (3×10⁻⁵−4 × 10⁻⁴ Ωcm²) but lower than the values obtained from ITO/Si contacts (about 1×10⁻² Ωcm²). The Sn was stable after annealing, but In diffused into Si and lost its function as the diffusion barrier.     

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.     

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.     

Novel type of ohmic contacts to p-doped GaN using polarization fields in thin InxGa1−xN capping layers

Thin p-doped InGaN layers on p-doped GaN were successfully used to demonstrate a new type of low-resistance ohmic contact. A significant reduction of specific contact resistance can be achieved by increasing the free-hole concentration and the probability for hole tunneling through the Schottky barrier as a consequence of polarization-induced band bending. As obtained from the transmission-line method, the specific contact resistances of Ni (10 nm)/Au (30 nm) contacts deposited on InGaN capping layers were 1.2×10⁻² Ωcm² and 6×10⁻² Ωcm² for capping layer thicknesses of 20 nm and 2 nm, respectively.     

High temperature stability of chromium boride ohmic contacts to p-type 6H-SiC

Ohmic contacts have been fabricated on p-type 6H-SiC using CrB₂. Two hundred nanometer thick films were sputter-deposited on substrates of doping concentration 1.3×10¹⁹ cm⁻³ 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, x-ray photoelectron spectroscopy, and transmission electron microscopy. The as-deposited CrB₂ contacts exhibited rectifying characteristics and contained oxygen as a major contaminant. Ohmic behavior with linear current-voltage characteristics was observed following short anneals at 1100°C for 2 min at a pressure of 5×10⁻⁷ Torr. The oxygen in the CrB₂ films was removed by the annealing process, and the lowest value of the specific contact resistance (r_c) measured at room temperature was 8.2×10⁻⁵ Ω-cm². Longer anneals at 1100°C for 3.5 h and 1200°C for 2 h reduced the room temperature values of r to 1.4×10⁻⁵ Ω-cm². A thin reaction region has been identified at the CrB₂/SiC interface; however, the interface remains essentially stable. Thermal stressing at 300°C in vacuum for over 2200 h produced only a slight increase in the specific contact resistance. The low value of the specific contact resistance and the excellent high temperature stability of the CrB₂/SiC interface make this contact a candidate for high power/high temperature SiC device applications.