Interdiffusion of the p-InP with Au−Zn,Ti/Au,Pd/Au,Ti/Pd/Au at interface and their electrical properties

 In this report,the interdiffusion between the p-InP with Au-Zn,Ti/Au,Pd/Au and Ti/Pd/Au at interface have been investigated by Auger electron spectroscopy and electron spectroscopy for chemical analysis.The surface morphology for the heat treatment are observed with scanning electron microscopy. It is found that the indiffusion of Au is easier than that of Pd and Ti and the outdiffusion of In is easier than that of P.The combination state of In and Au is formed during the heat treatment of p-InP/Au-Zn. The effects of the alloying temperature and time on the specific contacts resistance of p-InP/Au-Zn system are studided.The low specific contact resistance,p_c=2.4-2.7×10~(-4)Ω-cm~2,is obtained when alloying at 450℃ for 2 min or at 350℃ for 30 min. These results indicate that the specific contact resistance strongly depend on the“interdiffusion degree”.The Zn in Au-Zn ahoy distributes onto the most surface layer of p-InP/Au-Zn system during evaporation process and heat treatment.It may be one of the reasons for the higher specific contacts resistance.     

Pd/Sb(Zn) and Pd/Ge(Zn) ohmic contacts on p-type indium gallium arsenide: The employment of the solid phase regrowth principle to achieve optimum electrical and metallurgical properties

The development of two metallizations based on the solid-phase regrowth principle is presented, namely Pd/Sb(Zn) and Pd/Ge(Zn) on moderately doped In_0.53Ga_0.47As (p=4×10¹⁸ cm⁻³). Contact resistivities of 2–3×10⁻⁷ and 6–7×10⁻⁷ Ωcm², respectively, have been achieved, where both systems exhibit an effective contact reaction depth of zero and a Zn diffusion depth below 50 nm. Exhibiting resistivities equivalent to the lowest values of Au-based systems in this doping range, especially Pd/Sb(Zn) contacts are superior to them concerning metallurgical stability and contact penetration. Both metallizations have been successfully applied for contacting the base layer of InP/In_0.53Ga_0.47As heterojunction bipolar transistors.     

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.     

Microstructural analysis of a Au/Pt/Pd/Zn ohmic contact to an AlGaAs/GaAs heterojunction bipolar transistor

Gold-based ohmic contacts, incorporating Pt, Pd, and Zn layers, to AIGaAs/GaAs heterojunction bipolar transistors (HBTs) have been characterized using transmission electron microscopy (TEM). The metallization was deposited onto a 30 nm graded emitter layer of n-type Al_xGa_1−xAs, which was on a 30 nm emitter layer of n-type Al_0.3Ga_0.7As, with the aim of contacting the underlying 80 nm thick graded base layer of p-type Al_xGa_1−xAs. Metal layers were deposited sequentially using electron beam evaporation and the resultant metallizations were annealed at temperatures ranging from 250-500°C for up to several minutes. A minimum contact resistance of ≈8.5 × 10⁻⁷ Ω-cm² was achieved, which corresponded to the decomposition of ternary phases at the metallization/semiconductor interface, to binary phases, i.e., PdGa and PtAs₂. Long term stability tests were done on the optimum contacts. Anneals at 270°C for up to four weeks in duration produced virtually no change in microstructure, with the exception of some outward diffusion of Ga and As.     

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.     

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.     

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.     

Novel Cu/Cr/Ge/Pd Ohmic Contacts on Highly Doped n-GaAs

The thermal stability of the Cu/Cr/Ge/Pd/n⁺-GaAs contact structure was evaluated. In this structure, a thin 40 nm layer of chromium was deposited as a diffusion barrier to block copper diffusion into GaAs. After thermal annealing at 350°C, the specific contact resistance of the copper-based ohmic contact Cu/Cr/Ge/Pd was measured to be (5.1 ± 0.6) × 10⁻⁷ Ω cm². Diffusion behaviors of these films at different annealing temperatures were characterized by metal sheet resistance, X-ray diffraction data, Auger electron spectroscopy, and transmission electron microscopy. The Cu/Cr/Ge/Pd contact structure was very stable after 350°C annealing. However, after 400°C annealing, the reaction of copper with the underlying layers started to occur and formed Cu₃Ga, Cu₃As, Cu₉Ga₄, and Ge₃Cu phases due to interfacial instability and copper diffusion.     

Low-resistance ohmic contacts to p-type GaAs using Zn/Pd/Au metallization

We have fabricated the low resistance ohmic contacts to p-type GaAs. Specific contact resistances as low as 7 × 10^-7Ω.cm²have been obtained for contacts prepared by heat treating Zn/Pd/Au metallizations deposited on p-type epitaxial GaAs layers with an acceptor concentration of 1.5 × 10¹⁹cm^-3. These contacts are reproducible, simple to fabricate, exhibit excellent adhesion, and have a uniformly smooth surface morphology.     

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.     

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.     

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.     

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.     

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.     

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.     

High-speed InP/lnGaAs heterojunction bipolar transistor utilizing nonalloyed contacts on n+-InP contacting layers

We have demonstrated a high-speed InP/lnGaAs heterojunction bipolar transistor with nonalloyed TiPtAu contacts on n⁺-InP emitter and collector contacting layers. The use of SiBr₄ as a silicon doping source enabled the formation of low resistance (p_c <2 × 10⁻⁶Ω. cm²), nonalloyed TiPtAu contacts to the heavily doped (n = 2 × 10¹⁹ cm⁻³) InP contacting layers. A device with a 3 × 10 Μm² emitter contact exhibited excellent dc characteristics and had ƒ_T = ƒ_max = 107 GHz. Emitter and collector resistances are compared to a device with InGaAs contacting layers.     

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.     

Simultaneous Formation of Ni/Al Ohmic Contacts to Both n- and p-Type 4H-SiC

The fabrication procedure for silicon carbide power metal oxide semiconductor field-effect transistors can be improved through simultaneous formation (i.e., using the same contact materials and a one-step annealing process) of ohmic contacts on both the n-source and p-well regions. We have succeeded in the simultaneous formation of Ni/Al ohmic contacts to n- and p-type SiC after annealing at 1000°C for 5 min in an ultrahigh vacuum. Ohmic contacts to n-type SiC were found when the Al-layer thickness was less than about 6 nm, while ohmic contacts to p-type SiC were observed for an Al-layer thickness greater than about 5 nm. Only the contacts with an Al-layer thickness in the range of 5 nm to 6 nm exhibited ohmic behavior to both n- and p-type SiC, with a specific contact resistance of 1.8 × 10⁻⁴ Ω cm² and 1.2 × 10⁻² Ω cm² for n- and p-type SiC, respectively. An about 100-nm-thick contact layer was uniformly formed on the SiC substrate, and polycrystalline δ-Ni₂Si(Al) grains were formed at the contact/SiC interface. In the samples that exhibited ohmic behavior to both n- and p-type SiC, the distribution of the Al/Ni ratios in the δ-Ni₂Si(Al) grains was larger than that observed for any of the samples that showed ohmic behavior to either n- or p-type SiC. Furthermore, the grain size of the δ-Ni₂Si(Al) grains in the samples showing ohmic behavior to both n- and p-type SiC was smaller than the grains in any of the samples that showed ohmic behavior to either n- or p-type SiC. Thus, the large distribution in the Al/Ni ratios and a fine microstructure were found to be characteristic of the ohmic contacts to both n- and p-type SiC. Grains with a low Al concentration correspond to ohmic contacts to n-type SiC, while grains with a high Al concentration correspond to ohmic contacts to p-type SiC.     

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.     

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.