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.     

Calculations and measurements of contact resistance of semi-transparent Ni/Pd contacts to p-GaN

Calculations of specific contact resistance as a function of doping and barrier height were performed for p-type GaN. These calculations took into account two valence bands, each with different effective masses, and show that at low doping, the heavy hole band accounts for most of the conduction, whereas at heavier doping, the light hole band dominates conduction. These calculations also indicate the barrier height for typical contacts to p-GaN is between 0.75 eV and 1 eV. Specific contact resistance measurements were made for oxidized Ni/Au, Pd, and oxidized Ni/Pd ohmic contact metal schemes to p-GaN. The Ni/Pd contact had the lowest specific contact resistance, 6×10⁻⁴ Ω cm². Auger sputter depth profile analysis showed some Ni diffused away from the GaN surface to the contact surface with the bulk of the Pd located in between two areas of Ni. Both Ni and Pd interdiffused with the GaN at the semiconductor surface. The majority of the oxygen observed was with the Ni as NiO. Angle-resolved-x-ray photoelectron spectroscopy (AR-XPS) analyses showed the formation of predominantly NiO and PdO species, with higher Ni and Pd oxides at the contact surface.     

Interface observation in Au/Ni/p-GaN studied by HREM and energy-filtering TEM

The contact resistance of Au/Ni/p-GaN ohmic contacts for different annealing conditions was measured. This was then correlated with microstructure, including phase distribution, observed by high-resolution electron microscopy combined with energy-filtering imaging. A contact resistance of 2.22 x 10(-4) ohms cm2 for Au/Ni contacts to p-GaN after annealing at 500 degrees C for 5 min in air ambient was obtained. NiO layers were identified at the interface and upper area of annealed Ni/Au/p-GaN for air ambient. In addition, an Au layer was found at the interface of p-GaN due to a reversal reaction during annealing. Identification of the observed phases is discussed, along with possible formation mechanisms for the ohmic contacts in the Au/Ni/p-GaN system.     

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.     

Ir Diffusion Barriers in Ni/Au Ohmic Contacts to <Emphasis Type="Italic">p</Emphasis>-Type CuCrO<Subscript>2</Subscript>

The use of Ir diffusion barriers in Ni/Au-based Ohmic contacts to p-type CuCrO₂ layers was investigated. A specific contact resistance of ~5 × 10⁻⁴ Ω cm² was achieved after annealing at 500°C for the Ir-containing contacts, and the contacts were rectifying for lower anneal temperatures. In this case, the contact resistance was basically independent of the measurement temperature, indicating that tunneling is the dominant transport mechanism in the contacts. The morphology for the Ir-containing contacts was still smooth at 500°C although Auger electron spectroscopy depth profiling showed that some of the nickel had diffused to the surface and had oxidized. Contacts annealed at 800°C showed that some copper and most of the nickel had diffused to the surface and oxidized. The presence of the Ir diffusion barrier does increase the thermal stability of the contacts by ∼200°C compared to conventional Ni/Au contacts. By contrast, the use of other materials such as TaN, ZrN, and W₂B₅ as the diffusion barrier led to poorer thermal stability, with the contact resistance increasing sharply above 400°C.     

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.     

氧化对 GaN 基LED透明电极接触特性的影响

研究了热退火对InGaN/GaN 多量子阱LED的Ni/Au-p-GaN欧姆接触的影响.发现在空气和 N2气氛中交替地进行热退火的过程中Ni/Au接触特性显示出可逆现象. Ni/Au-p-GaN接触的串联电阻在空气中随合金化时间逐渐减小,在随后的 N2 中的热退火后会使该串联电阻增加,但在空气中再次热退火能使接触特性得到恢复.同时对Ni/Au-p-GaN 接触在空气中合金化过程中的层反转的成因进行了讨论.     

Electrical and structural properties of low-resistance Ti/Al/Re/Au ohmic contacts to n-type GaN

Titanium (15 nm)/aluminum (60 nm)/rhenium (20 nm)/gold (50 nm) ohmic contacts to moderately doped n-type GaN (4.07×10¹⁸ cm⁻³) have been investigated as a function of annealing temperature. It is shown that the current-voltage (I–V) characteristics of the contacts are improved upon annealing at temperatures in the range of 550–750°C. Specific contact resistance as low as 1.3 × 10⁻⁶ Ωcm² is obtained after annealing at 750°C for 1 min in a nitrogen ambient. X-ray photoemission spectroscopy (XPS) results show that the Ga 2p core level for the sample annealed at 750°C shifts toward the high binding side by 0.71 eV compared with that of the as-deposited one. It is also shown that the contact does not seriously suffer from thermal degradation even when annealed at 750°C for 30 min. Based on Auger electron spectroscopy (AES), glancing angle x-ray diffraction (GXRD), and XPS results, possible explanations for the annealing-induced improvement of the ohmic behavior are described and discussed.     

A study of the electrical characteristics of various metals on p-type GaN for ohmic contacts

We study the electrical characteristics (current vs voltage, I/V) of Co, In, Mg, Mn, Ni, and Zn each with an Au overlayer to determine their usefulness as ohmic contact metals to p-type GaN. For all the metals, none of the I/V relationships are completely linear even after annealing. At a fixed voltage of 3V Co, In, Ni, and Zn have comparable current levels, whereas Mg and Mn are almost an order of magnitude less. Due to the various mechanisms by which the metals may form ohmic contacts, we further examine the metals in multilayer combinations in an attempt to reduce contact resistance. Three p-type GaN wafers with carrier concentrations of 1.2 × 10¹⁷, 1.5 × 10¹⁷ and 4.7 × 10¹⁷ cm⁻³ are used with Ni/Au metallizations as a common standard for comparison. The lowest average specific contact resistance obtained in this study is with Co/Au at 0.0081 ohm-cm². In addition to comparing magnitudes of contact resistances, thermal aging studies of the metal contacts are performed from 300 to 700°C for 6 h periods to determine its effect on their electrical stability. In this test, Ni/Au is found to be the most electrically invariant with thermal aging prior to failure. However, the temperature at which it fails occurred sooner than that for many of the other metallizations examined (e.g., Co/Au, In/Au, and Zn/Au). The temperature for failure is arbitrarily defined to be the temperature that the contact resistance degrades to twice its pre-thermal-aging contact resistance.     

The behavior of Ni/Au contacts under rapid thermal annealing in GaN device structures

The effect of rapid thermal annealing (RTA) on Ni/Au contacts on P-type GaN was investigated in terms of surface morphology and diffusion depth of metallic species. Ni/Au contacts were evaporated on the P-type 0.5 μm thick top layer of a GaN P/N homojunction. Optical micrographs revealed that the contact morphology degrades when annealed above 800°C for 1 min. At the same time, both Ni and Au atoms strongly diffuse in the P-type layer and even can reach the junction for a 1 min long annealing at 900°C, therefore making the junction structure unoperable. This behavior was evidenced using the Auger voltage contrast (AVC) technique.     

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 Electrical Properties of Low Temperature Processed Ohmic Contacts to p‐Type GaN

With Ni/Au and Pd/Au metal schemes and low temperature processing, we formed low resistance stable Ohmic contacts to p‐type GaN. Our investigation was preceded by conventional cleaning, followed by treatment in boiling HNO₃:HCl (1:3). Metallization was by thermally evaporating 30 nm Ni/15 nm Au or 25 nm Pd/15 nm Au. After heat treatment in O₂ + N₂ at various temperatures, the contacts were subsequently cooled in liquid nitrogen. Cryogenic cooling following heat treatment at 600 ·C decreased the specific contact resistance from 9.84·10^?4 Ωcm² to 2.65·10^?4 Ωcm² for the Ni/Au contacts, while this increased it from 1.80·10^?4 Ωcm² to 3.34·10^?4 Ωcm² for the Pd/Au contacts. The Ni/Au contacts showed slightly higher specific contact resistance than the Pd/Au contacts, although they were more stable than the Pd contacts. X‐ray photoelectron spectroscopy depth profiling showed the Ni contacts to be NiO followed by Au at the interface for the Ni/Au contacts, whereas the Pd/Au contacts exhibited a Pd:Au solid solution. The contacts quenched in liquid nitrogen following sintering were much more uniform under atomic force microscopy examination and gave a 3 times lower contact resistance with the Ni/Au design. Current‐voltage‐temperature analysis revealed that conduction was predominantly by thermionic field emission.     

Pd/Zn/Pd/Au ohmic contacts to ρ-Type InP

Low resistance ohmic contacts (ρ_c = 7 x 10^-5 Ω-cm ² ) have been fabricated to Zn-doped p-type InP using an annealed Pd/Zn/Pd/Au metallization. Palladium reacts with InP at low temperatures to form a Pd₂InP ternary phase, which is initially amorphous but crystallizes and grows epitaxially on InP. Zinc reacts with some of the overlying Pd to form PdZn (≅250° C), which decomposes at 400-425° C to form PdP₂, freeing up Zn to diffuse into Au as well as InP. The contact resistance reaches a minimum as the decomposition reaction takes place. The resultant ohmic contact is laterally uniform and consists of epitaxial Pd₂InP adjacent to InP, followed by a thin layer of PdP₂ and then the outer Au layer. Further annealing leads to a breakdown of the contact structure,i.e. decomposition of Pd₂InP, and an increase in contact resistance.     

Thermally stable Nb and Nb/Au ohmic contacts to p-GaN

We have investigated Nb single and Nb/Au metallization schemes for the formation of thermally stable ohmic contacts to p-GaN. It is shown that the asdeposited Nb and Nb/Au contacts exhibit rectifying behavior. However, both the contacts produce ohmic characteristics when annealed at 850°C. Measurements show that the 850°C Nb/Au and Nb contacts yield a specific contact resistance of 1.9×10⁻⁸ and 2×10⁻² ωcm², respectively. Schottky barrier heights are found to decrease with increasing annealing temperature. A comparison of the XRD and electrical results shows that the formation of gallide phases such as Ga-Nb and Ga-Au compounds, play a role in forming ohmic contacts. Atomic force microscopy results show that the surface morphology of the Nb contacts is fairly stable up to 850°C, while the Nb/Au contacts are slightly degraded upon annealing at 850°C.     

Iridium-based semi-transparent current spreading layer on short-period-superlattice (SPS) tunneling contact of InGaN/GaN LEDs

Iridium-containing and Ni(4 nm)/Au(6 nm) films were evaporated separately on the n⁺-InGaN–GaN short-period-superlattice (SPS) structure of light-emitting diodes (LEDs). The collective deposition of iridium and other metals as an ohmic contact induces the formation of highly transparent IrO₂, which helps to enhance the light output and decrease the series resistance of LEDs. By comparing different metal films used as current spreading contact layer, Ir/Ni film annealed at 500 °C for 20 min in O₂ ambient renders devices with lowest turn-on voltage at 20 mA and highest luminous intensity. Moreover, we also analyzed films using atomic force microscopy (AFM) with an emphasis on studying how the surface quality of Ir/Ni and Ni/Au films influences the current spreading and luminosity of LEDs.     

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.     

Influence of Surface Treatment and Annealing Temperature on the Formation of Low-Resistance Au/Ni Ohmic Contacts to <Emphasis Type="Italic">p</Emphasis>-GaN

We have studied the influence of surface treatment and annealing temperature on the specific contact resistance of Au/Ni ohmic contacts to p-GaN with hole concentrations in the range of 10¹⁶ cm⁻³ to 10¹⁸ cm⁻³. The sample with a hole concentration of 1 × 10¹⁸ cm⁻³, treated with the surface treatment HCl:H₂O = 3:1 solution and annealed at 500°C in a 90% N₂ and 10% O₂ atmosphere, yielded the lowest specific contact resistance of ~4 × 10⁻⁵ Ω cm² and ~2 × 10⁻⁷ Ω cm² at room temperature and at 150°C, respectively. To investigate the roles of interdiffusion between layer interfaces and the formation of NiO and nickel gallides, we examined the metallization stacks before and after annealing using high-resolution x-ray diffraction. We conclude that the nickel-gallide formation and the deterioration of the NiO layer are together responsible for the large deviation in contact resistances observed for samples annealed at various temperatures.     

InGaN/GaN LEDs with a Si-doped InGaN/GaN short-period superlattice tunneling contact layer

Nitride-based light-emitting diodes (LEDs) with Si-doped n⁺-In_0.23Ga_0.77N/GaN short-period superlattice (SPS) tunneling contact top layer were fabricated. It was found that although the measured specific-contact resistance is around 1 × 10⁻² Ω-cm² for samples with an SPS tunneling contact layer, the measured specific-contact resistance is around 1.5×10⁰ Ω-cm² for samples without an SPS tunneling contact layer. Furthermore, it was found that one could lower the LED-operation voltage from 3.75 V to 3.4 V by introducing the SPS structure. It was also found that the LED-operation voltage is almost independent of the CP₂Mg flow rate when we grow the underneath p-type GaN layer. The LED-output intensity was also found to be larger for samples with the SPS structure.     

Improved Long-Term Thermal Stability At 350°C Of TiB2–Based Ohmic Contacts On AlGaN/GaN High Electron Mobility Transistors

AlGaN/GaN High Electron Mobility Transistors (HEMTs) were fabricated with Ti/Al/TiB₂/Ti/Au source/drain Ohmic contacts and a variety of gate metal schemes (Pt/Au, Ni/Au, Pt/TiB₂/Au or Ni/TiB₂/Au) and subjected to long-term annealing at 350°C. By comparison with companion devices with conventional Ti/Al/Pt/Au Ohmic contacts and Pt/Au gate contacts, the HEMTs with boride-based Ohmic metal and either Pt/Au, Ni/Au or Ni/TiB₂/Au gate metal showed superior stability of both source-drain current and transconductance after 25 days aging at 350°C.