Thermal stability study of n-type and p-type ohmic contacts simultaneously formed on 4H-SiC

In this paper, various annealing conditions using Al-based (Ti/Al/Ti/Au=70nm/100nm /5nm/120nm) and Ni-based (Ti/Ni/Ti/Au=20nm/90nm/5nm/120nm) metal contacts to n-type and p-type ion-implanted 4H-SiC epi layers have been studied in the effort to optimize simultaneous ohmic contact formation with the lowest specific contact resistance (SCR) values. Values of 1.091×10 -4 Ω∙cm 2 and 1.158×10 -5 Ω∙cm 2 were achieved using Al-based Ohmic metal contacts for p-type and n-type 4H-SiC, respectively, at an annealing temperature of 950°C and under vacuum for 90 sec. Ohmic formation mechanisms were analyzed using the X-Ray Diffraction (XRD) surface analysis method, indicating Ti 3 SiC 2 alloys to be the key intermediate layer formed at SiC/Ti interface, responsible for Ohmic properties to p-type SiC. The paper summarizes the metal process combinations possible for the formation of Ohmic contacts to both n-type and p-type 4H-SiC, offering various options in either using the same metal materials and/or common annealing conditions.

Ohmic contacts are a necessary part of microelectronic devices. In this chapter, we review the materials used for ohmic contacts and their interfacial reactions and structures on n- and p-type SiC, discuss material selection for ohmic-contact formation on SiC and the mechanism of formation of such contacts, and summarize recent advances. The metals for ohmic contacts on SiC are classified by group as the periodic table, and the relationship between the chemical reactivity of the metal with SiC and ohmic-contact formation is presented. Most metals in group VIII are excellent for ohmic contacts on n-type SiC, and only silicides and carbon are formed. A group IIIA metal, Al, is the most effective material for ohmic-contact formation on p-type SiC, and forms carbides and Si on SiC. The metals in groups IVB—VIIB generally show good thermal stability on SiC, and both silicides and carbides are formed. Various compounds, including silicides, carbides, nitrides, borides, and composites, for ohmic contacts on SiC are summarized. The recent advances in improved contacts, in our understanding of the mechanism, and in forming ohmic contacts with high thermal stability are also summarized. The role of nanosize graphitic structures in ohmic contacts and the mechanism based on catalytic graphitization are presented. Finally, challenging issues such as interfacial structural characterization on the nanometer and atomic scales and high-stability ohmic contacts in oxidizing environments are discussed.

Ohmic contacts comprised of the layer sequence of W/WC/TaC/SiC showed excellent thermal stability after testing at 600 °C for 1000 h. No degradation of the specific contact resistance nor a reaction of the film with the SiC substrate was observed. From Auger electron spectroscopy depth profiles, it was found that W and WC reacted to form W2C on TaC after annealing. After annealing for several hundred hours at 1000 °C, the specific contact resistance of the W/WC/TaC/SiC contacts displayed noticeable degradation. According to secondary ion mass spectrometry (SIMS) analysis, after annealing at 1000 °C for 600 h, small but measurable changes in the electrical characteristics were associated with O incorporation at the interface between TaC and SiC. Investigation of the W/WC/TaC/SiC interface by transmission electron microscopy (TEM) indicated that a reaction between the W and WC had occurred, but there was no observed reaction with the SiC substrate. After annealing for 1000 h, substantial changes in the chemistry, the microstructure, the specific contact resistance, and the spreading resistance of the contacts were observed. To understand the mechanisms associated with the degradation of the electrical properties, SIMS and TEM analyses were performed on samples annealed for 600, 700, 800, 900, and 1000 h at 1000 °C. These findings indicated that both oxidation and metallurgical reactions played important roles in the degradation of the electrical properties.

Electrical properties of pseudo‐vertical Schottky barrier diodes (pVSBDs) prepared on (113)–oriented boron‐doped diamond (BDD) layers using ruthenium (Ru) for both the ohmic and Schottky contacts are investigated. First, Ru ohmic contacts are evaporated on homoepitaxial BDD layers with different resistivity, and their specific contact resistance is measured using circular transfer length method structures after annealing at various temperatures up to 750 °C. Then, pVSBD structures are fabricated on the boron‐doped bilayer consisting of a lower, heavily boron‐doped layer ensuring an ohmic contact and an upper, lightly doped layer providing a rectifying Schottky contact. After necessary mesa etching, both contacts are formed by the Ru evaporation. The results show that Ru forms a stable ohmic contact with very low contact resistance (10−5–10−6 Ω cm2) when deposited on BDD layers with metallic conductivity. It also provides an acceptable Schottky contact on low‐doped (113) homoepitaxial BDD. Both contacts, which are made simultaneously, realize pVSBDs with low on‐state resistance and low forward voltage drop. However, the lower barrier of the ruthenium contacts results in higher leakage. Ru pVSBDs thus show a lower rectification ratio, higher leakage, and a worse ideality factor compared to analogical pVSBDs using molybdenum contacts.

The electrical and thermal characteristics of three ohmic contact structures: (i) in situ deposited epitaxial Al on delta -doped GaAs, (ii) evaporated Au-Ge on delta -doped GaAs (iii) evaporated Au-Ge on GaAs were compared with each other. The I-V characteristics of the non-alloyed Au-Ge and the epitaxial Al, as-deposited, on delta -doped GaAs were linear, whereas the non-alloyed Au-Ge contact on GaAs showed a nonlinear I-V characteristic, as expected. The specific contact resistivity ( rho c) of the non-alloyed Au-Ge contacts on delta -doped GaAs was 1.4*10-6 Omega cm2, whereas it was 9*10-7 Omega cm2 for the epitaxial Al contacts on delta -doped GaAs, which is the lowest ever reported for a pure metal on delta -doped GaAs. No value of the specific contact resistivity ( rho c) could be obtained for the Au-Ge contacts on GaAs because of the nonlinearity of the I-V characteristics. After an alloying process at 450 degrees C all samples showed linear I-V characteristics. The specific contact resistivity of all samples varied inversely with the n+ doping concentration up to 5*1017 cm-3. Above this concentration the specific contact resistivity of the Au-Ge and epitaxial Al contacts on delta -doped GaAs was constant and hardly changed after alloying. Our results confirm that the well known double-barrier (metal-n+-n) model is valid for the ohmic contact on delta -doped GaAs.

We investigated the electrical contact characteristics of indium tin oxide (ITO)/doped hydrogenated amorphous silicon (a-Si:H) junctions. For efficient collection of photo-generated carriers, photovoltaic and photodetector devices require good ohmic contacts with transparent electrodes. The amorphous-Si thin films were sputter deposited on ITO coated glass substrates. As-deposited p-type a-Si:H on ITO formed nearly ohmic type contacts and further annealing did not improve the contact characteristics. On the other hand, as-deposited n-type a-Si:H on ITO formed an ohmic contact, while further annealing resulted in a Schottky type contact. The ITO contact with p-type silicon semiconductor is a robust ohmic contact for Si based optoelectronic devices.

A study has examined the effect of mixing methods on the thermal stability and detonation characteristics of ammonium nitrate (AN) and sodium chloride (NaCl) mixtures. NaCl was mixed with AN by two methods. The thermal stability, detonation velocity and structural properties were investigated by differential scanning calorimetry (DSC), measurement of detonation velocity and X‐ray diffraction (XRD). For the mechanical mixing method, in all tested scope of proportions of NaCl in the mixtures, activation energies increase when the proportion of NaCl increases; for solution mixing method, the activation energies decrease first and then start to increase as the proportion of NaCl increases. The detonation velocity of AN‐NaCl mixtures prepared by two mixing methods also showed different results. The results indicate that the mixing methods significantly affect the thermal stability and detonation characteristics of AN.

Silicon carbide is a wide band-gap semiconductor material which exists in more than 170 polytypes. In this work, heteropolytype epitaxy was used to decrease the specific contact resistance of ohmic contacts to the 6H-SiC polytype (band gap Eg∼3.0 eV). High quality ohmic contacts were produced by metallizing and annealing a thin cap layer of 3C-SiC (Eg∼2.3 eV) grown by chemical vapor deposition on either n- or p-type 6H-SiC. The measured specific contact resistance (rc) of the ohmic contacts to n-type 6H-SiC was found to be less than 1.7×10−5 and 2×10−5 Ω cm2 for contacts to p-type 6H-SiC.

Pd/Ge/Pd/Ti/Au and Pd/Si/Pd/Ti/Au ohmic contacts to n-type InGaAs were investigated for applications to AlGaAs/GaAs HBT emitter ohmic contacts. In the Pd/Ge/Pd/Ti/Au ohmic contact, a minimum specific contact resistivity of 1.1×10−6 Ωcm2 was achieved by annealing at 425°C/10s, but the ohmic performance was slightly degraded with increasing annealing temperature due to the reaction between the ohmic contact materials and the InGaAs substrate. However, a non-spiking planar interface and relatively good ohmic contact (high-10−6 Ωcm2) were maintained after annealing at 450°C/10s. In the Pd/Si/Pd/Ti/Au ohmic contact, in spite of the lower barrier height of the metal-InGaAs junction, as-deposited contacts showed non-ohmic behaviors due to the presence of the insulating Si layer. However, the specific contact resistivity decreased remarkably to 4.3×10−7 Ωcm2 by annealing at 425°C/10s. Minimum specific contact resistivity of 3.9×10−7 Ωcm2 was achieved by annealing at 400°C/20s. RF performance of the AlGaAs/GaAs HBT was also examined by employing the Pd/Ge/Pd/Ti/Au and Pd/Si/Pd/Ti/Au systems as emitter ohmic contacts. Cutoff frequencies were 65.0 GHz and 74.4 GHz, respectively, and maximum oscillation frequencies were 51.3 GHz and 52.5 GHz, respectively, indicating very successful high frequency operations.

To improve the performances of ohmic contacts for GaN devices, a novel multilayer Ti/Al-based metal scheme (Ti/Al/Ti/Al/Ti/Al/Ti/Al/Ni/Au) on undoped AlGaN/GaN heterostructures was employed. A contact with ?c (specific contact resistance) of 8.74E-07 ?·cm2, Rc of 0.22 ?·mm was demonstrated. Ohmic contacts with the novel metal structure were measured with I-V, SEM, HRTEM to show their properties. The results showed that ohmic contacts with novel structures have better surface morphology and proposed thermal stability than those using conventional Ti/Al/Ni/Au metal scheme, therefore ohmic contacts with novel structures should be better candidates for high power and high frequency GaN devices.

In this Letter, we use first-principles simulations to demonstrate the absence of Fermi-level pinning when graphene is in contact with transition metal dichalcogenides (TMDs). We find that formation of either an ohmic or Schottky contact is possible. Then we show that, due to the shallow density of states around its Fermi level, the work function of graphene can be tuned by ion adsorption. Finally we combine work function tuning of graphene and an ideal contact between graphene and TMDs to propose an ionic barristor design that can tune the work function of graphene with a much wider margin than current barristor designs, achieving a dynamic switching among p-type ohmic contact, Schottky contact, and n-type ohmic contact in one device.

Low-resistance ohmic contacts on boron-doped {113} oriented homoepitaxial diamond layers

We report on the thermal stability and trap characteristics of HfON as a trapping layer in charge trap flash memory device applications. Compared to HfO 2 , HfON prepared by NH 3 annealing of HfO 2 exhibits good interfacial quality, excellent memory properties, and superior thermal stability. The improvement in thermal stability can be explained by the lower interfacial reaction and amorphous state of HfON. The activation energy of a SiO 2 /HfON/Al 2 O 3 device and the trap level of the HfON trapping layer were found to be 1.08 and 1.04 eV, respectively.

This study deals with the electrical characterization of PiN diodes fabricated on a 4°off-axis 4H-SiC n+ substrate with a n- epilayer (1×1016 cm-3 / 10 µm). Optimized p++ epitaxial areas were grown by Vapour-Liquid-Solid (VLS) transport to form p+ emitters localized in etched wells with 1 µm depth. Incorporated Al level in the VLS p++ zones was checked by SIMS (Secondary Ion Mass Spectroscopy), and the doping level was found in the range of 1-3×1020 at.cm-3. Electrical characterizations were performed on these PiN diodes, with 800 nm deposit of aluminium as ohmic contact on p-type SiC. Electrical measurements show a bipolar behaviour, and very high sustainable forward current densities ≥ 3 kA.cm-2, preserving a low leakage current density in reverse bias. These measurements were obtained on structures without any passivation and no edge termination.

We report a new Ni/Pt/Au (20/30/80 nm) metallization scheme to achieve a low ohmic contacts to p-type GaN with a carrier concentration of 9.4 × 1016 cm-3. A Mg-doped GaN layer (0.5 μm) was grown on (0001) sapphire substrate by metalorganic chemical vapor deposition (MOCVD). All metal thin films were deposited on the p-GaN layer in an electron-beam evaporation system. Samples were annealed by a rapid thermal annealing (RTA) process at a range of temperatures from 300 °C to 850 °C under a flowing Ar atmosphere. A circulartransmission line model (c-TLM) was employed to calculate the specific contact resistance, and current-voltage (I-V) data were measured with HP4155A. The Ni/Pt/Au contacts without the annealing process showed nearly rectifying characteristics. The ohmic contacts were formed on the samples annealed at 500 °C for 30 sec and the I-V data showed a linear behavior. The specific contact resistance was 2.1 × 10-2 Ωcm2. However with increasing the annealing temperature above 600 °C, ohmic contacts were again degraded. Auger electron spectroscopy (AES) depth profiles were used to investigate the interfacial reactions between the trilayer and GaN. AES results suggested that Pt plays a significant role in forming ohmic contact as an acceptor at the interface. Atomic force microscope (AFM) also showed that the samples with good ohmic contact have very smooth surface.