Ohmic Contact Formation Mechanism Research Articles

The dichotomous role of oxygen in the ohmic contact formation on p-type GaN grown by MBE and MOCVD

Molecular beam epitaxy (MBE) is often considered as an ideal equipment for growing low-resistance p-type GaN (p-GaN). However, the Ohmic contact formation mechanism of p-GaN grown by MBE is still unclear. In this work, MOCVD and MBE-grown p-GaN were prepared for contrastive analysis. We reported for the first time the different variation tendency in their electrical properties after annealing in the same ambient. Surface analyzing instruments were used to investigate the interfacial properties of p-GaN. Experimental results revealed that the oxygen in the annealing ambient had a negative effect on the Ohmic contact formation of MBE p-GaN, and the physical mechanism underlying it was figured out. It is proved that the annealing conditions for preparing Ohmic contacts on MOCVD p-GaN is not suitable for that on MBE p-GaN. To further reduce the effect of oxygen, an optimized Ohmic contact scheme for MBE p-GaN based on the Ni-Pd contact layer was proposed, which successfully reduced the specific contact resistivity (ρc) by two orders, to 8.6 × 10−5 Ω•cm2, compared with the conventional Pd/Pt/Au system. This research provides valuable guidance for device fabrication based on GaN grown by MBE and lays an essential foundation for preparing optoelectronic devices.

Low resistance ohmic contact of multi-metallic Mo/Al/Au stack with ultra-wide bandgap Ga2O3 thin film with post-annealing and its in-depth interface studies for next-generation high-power devices

Ultra-wide band gap materials like β-Ga2O3 is considered as a potential candidate for power devices owing to their high breakdown voltage in contrast to the conventional semiconductors. The performance and the figure-of-merit for power devices critically depend on resistance offered by ohmic contact of metal with ultra-wide gap materials. However, high-quality ohmic contacts with Ga2O3 presents a significant challenge due to Fermi-level pinning and formation of Schottky barriers for the majority of metal contacts. Here, we investigated the electrical characteristics and interfacial reactions of Au/Al/Mo metal stack with β-Ga2O3 thin film at various annealing temperatures to demonstrate the ohmic contact formation mechanism. In-depth surface-sensitive XPS and UPS are employed to study the interface of metallic stack/Ga2O3 thin film. Our results indicate that post-annealing at high temperatures led to the intermixing of the metallic stack and results in the formation of binary intermetallic compounds of Al and Au. The intermetallic compound diffuses into the Ga2O3 layer and resulting in the creation of an oxygen-deficiency near the interface and helping to achieve contact resistance of 7.15 × 10−6 Ω cm2. This study establishes a robust foundation for the expanded utilization of Ga2O3 in power electronics and optoelectronics devices, emphasizing the vital role of interface engineering.

Recent Progress in Source/Drain Ohmic Contact with β-Ga2O3

β-Ga2O3, with excellent bandgap, breakdown field, and thermal stability properties, is considered to be one of the most promising candidates for power devices including field-effect transistors (FETs) and for other applications such as Schottky barrier diodes (SBDs) and solar-blind ultraviolet photodetectors. Ohmic contact is one of the key steps in the β-Ga2O3 device fabrication process for power applications. Ohmic contact techniques have been developed in recent years, and they are summarized in this review. First, the basic theory of metal–semiconductor contact is introduced. After that, the representative literature related to Ohmic contact with β-Ga2O3 is summarized and analyzed, including the electrical properties, interface microstructure, Ohmic contact formation mechanism, and contact reliability. In addition, the promising alternative schemes, including novel annealing techniques and Au-free contact materials, which are compatible with the CMOS process, are discussed. This review will help our theoretical understanding of Ohmic contact in β-Ga2O3 devices as well as the development trends of Ohmic contact schemes.

4H-SiC Ohmic contacts formation by MoS2 layer intercalation: A first-principles study

Due to the difficulty of forming a low Schottky barrier at the interface of a metal/SiC contact, preparing Ohmic contacts is still a key technical problem in developing SiC devices. In this paper, the effects of MoS2 intercalation on the interface properties of metal/SiC (Al, Ag, Ti, Au, and Mg) systems were investigated by first-principles calculation. The calculations show that all the metal/SiC contacts exhibit p-type Schottky contacts with strong Fermi level pinning (FLP) at the interfaces. After inserting a layer of MoS2, the Schottky barrier heights are significantly reduced. All the metal/MoS2/SiC systems are tuned to be n-type Ohmic contacts. By calculating and analyzing electron localization functions, projected band structure, partial density of states, and planar-averaged charge density difference, the Ohmic contact formation mechanism may be due to the saturation of dangling bonds of the SiC surface, the reduction in metal-induced gap states, the formation of interface dipole layer, and the shift of FLP position to the interface of metal/MoS2.

Very-Low Resistance Contact to 2D Electron Gas by Annealing Induced Penetration Without Spikes Using TaAl/Au on Non-Recessed i-AlGaN/GaN

A novel ohmic contact formation mechanism is revealed for the annealed Ta0.83 Al0.17/Au stacks on non-recessed i-AlGaN/GaN. It is demonstrated that the contact metal alloy mainly composed by Au penetrates the AlGaN layer without apparent Ta-related solid phase reactions, establishing a spike-free contact to the 2D electron gas. A low contact resistivity of $0.14\Omega ~\cdot $ mm (4.24E- $7~\Omega \cdot $ cm2) is then obtained after 900 °C/60s annealing.

Band-energy estimation on silicon cap annealed 4H-SiC surface using hard X-ray photoelectron spectroscopy

Silicon-cap annealing (SiCA) emerged as a promising silicidation-less ohmic contact formation method that can solve the crucial reliability limitation of ohmic contacts formed with metals; this limitation was due to carbon aggregation introduced during silicidation annealing. However, no previous study for a complete understanding of SiCA effects on the metal/SiC exists. In this study, the band-energy state of silicidation-less ohmic contacts formed by SiCA-SiC is directly estimated using hard X-ray photoelectron spectroscopy (HAXPES). The results show that Si-dot formation on the SiC surface reduces the contacts resistivity, and ohmic contact behavior is still observed even after Si-dot removal. A peak position analysis of Si 1 s orbit using HAXPES shows a clear increase in the band energy under various SiC surface conditions. Particularly, the Al/SiCA-SiC sample shows a peak shift of 0.765 eV. This strong potential barrier lowering the derived formation of the thin-depletion layer and low potential barrier on Al/SiCA-SiC junction. Moreover, the observations made using HAXPES, and transmission electron microscopy, suggest that the modification of the outer-most surface layer plays an essential role in the ohmic contact formation. These results provide insights on the ohmic contact formation mechanism for wide-band-gap semiconductor materials.

Ohmic contact formation mechanisms of TiN film on 4H–SiC

The atomic structure, interfacial charge distribution, bonding nature, and interfacial electronic states of a 4H–SiC/TiN interface are systematically investigated to understand the Ohmic contact formation mechanisms of TiN to 4H–SiC. The experiment results clearly demonstrate that the well-arranged TiN (111)-oriented lattice planes are parallel to the (0001) SiC-oriented substrate, which is in line with the XRD results. In addition, the interface is coherent without any secondary phase layers, amorphous layers, or transition regions, which confirms the direct contact of TiN to SiC at the atomic scale, exhibiting a linear current–voltage relationship. Quantitatively, first-principle calculations reveal that the Schottky barrier height (SBH) is as low as 0.03 eV and that the band gap nearly vanishes at the interface, indicating an excellent Ohmic contact of TiN to 4H–SiC. Furthermore, the SBH is significantly reduced through the interfacial charge polarization effect and strong coupling of interfacial electronic states, enhancing the quantum electron transport. The present results provide insight into the complicated electronic effects of the Ohmic contact interface and indicate that TiN is a promising SiC Ohmic contact material for advanced next-generation power device applications.

Ohmic contact formation mechanism of Ge-doped 6H-SiC

Abstract We present a comprehensive investigation of mechanism of ohmic contact characteristic in Ge-doped 6H-SiC single crystals. Raman mapping and Rutherford backscattering spectrometry reveal high crystalline quality of Ge-doped 6H-SiC and the Ge as a substitutional impurity. Deep-level transient spectroscopy (DLTS) spectrum was measured. Its Arrhenius fitting shows two shallow energy levels. They are below the conductive band bottom 0.298 eV and 0.323 eV respectively. The first-principles calculations indicate that the no new levels are in ( Ge ) Si configuration, but ( Ge ) C can induce deep levels in 6H-SiC. Thus, the shallow energy levels are caused by native defects which arise in a process of crystal growth. The two characteristics shallow energy levels, the small trap cross sections and positions near to conductive band bottom, can emit electron under effect of an applied field. In addition, the Ge Si bonds are relatively easily to break for generating electrons to jump into conductive band bottom though the shallow energy levels.

Investigation of the Mechanism for Ohmic Contact Formation in Ti/Al/Ni/Au Contacts to β-Ga2O3 Nanobelt Field-Effect Transistors.

The issue of contacts between the electrode and channel layer is crucial for wide-bandgap semiconductors, especially the β-Ga2O3 due to its ultra-large bandgap (4.6-4.9 eV). It affects the device performance greatly and thus needs special attention. In this work, the high-performance β-Ga2O3 nanobelt field-effect transistors with Ohmic contact between multilayer metal stack Ti/Al/Ni/Au (30/120/50/50 nm) and unintentionally doped β-Ga2O3 channel substrate have been fabricated. The formation mechanism of Ohmic contacts to β-Ga2O3 under different annealing temperatures in an N2 ambient is systematically investigated by X-ray photoelectron spectroscopy. It is revealed that the oxygen vacancies at the interface of β-Ga2O3/intermetallic compounds formed during rapid thermal annealing are believed to induce the good Ohmic contacts with low resistance. The contact resistance (Rc) between electrodes and unintentionally doped β-Ga2O3 reduces to ∼9.3 Ω mm after annealing. This work points to the importance of contact engineering for future improved β-Ga2O3 device performance and lays a solid foundation for the wider application of β-Ga2O3 in electronics and optoelectronics.

Features of the Formation of Ohmic Contacts to n+-InN

We report about a study of the formation and current transport mechanism of ohmic contacts to n+-InN with electron concentrations of 2×1018, 8×1018, and 4×1019 cm−3. Pd/Ti/Au ohmic contacts are formed by the proposed approach of simultaneous magnetron metal deposition and in-situ temperature annealing, which allows obtaining a low contact resistivity (4.20±2.67)×10−6 Ohm· cm2. The additional rapid thermal annealing in the temperature interval 350–400 ∘C is used to improve further contact characteristics. Optimal parameters of the temperature treatment are determined by statistic methods. As for the current transport mechanism, the unusual growing temperature behavior of contact resistivity is observed in the wide temperature range 4.2–380K for each doping level of InN films. The mechanism of thermionic current flow explains the current transport through metal shunts, which is associated with the conducting dislocations. The extracted density of conducting metal shunts has a good agreement with experimental values of the screw and edge dislocation densities experimentally obtained by high-resolution X-ray diffraction. Additionally, from the obtained contact resistivity temperature dependences, we can argue about the metal, which penetrates dislocations and forms shunts.

Role of thin Ti layer in formation mechanism of low temperature-annealed Ti/Al-based ohmic contact on AlGaN/GaN heterostructure

To obtain Ti/Al-based ohmic contacts on AlGaN/GaN heterostructures with a low resistance, it is crucial to understand their formation mechanism. Such a mechanism is still unclear, especially when the annealing temperature is as low as 500 °C and the Ti layer is thin, typically with a thickness of 2.7 nm. We investigated the sputtered Ti/Al/W ohmic contacts on the AlGaN/GaN heterostructure on-Si grown by metal–organic chemical vapor deposition, using scanning transmission electron microscopy, energy dispersive x-ray spectrometry, and x-ray diffraction. The microstructural analysis of the samples revealed that as-deposited Ti removes oxygen from the AlGaN surface before annealing; such phenomenon represents the capital mechanism of oxygen-removal in a low temperature annealing process. After annealing, Ti0.5Al0.5N and possibly TiGa3 compounds are formed, and these formations suggest that AlGaN is decomposed by Ti. As a result, the interface between metal alloy and AlGaN turns into an intact and intimate one. Furthermore, we found that the contact resistance correlates with the Al/O atomic ratio at the metal/AlGaN interface, and the Al/O ratio becomes maximum when the Ti thickness is 2.7 nm. From the findings, an optimal formation mechanism of ohmic contacts is proposed.

Effect of annealing temperature on Ti/Al/Ni/Au ohmic contacts on undoped AlN films

The Ti/Al/Ni/Au metals were deposited on undoped AlN films by electron beam evaporation. The influence of annealing temperature on the properties of contacts was investigated. When the annealing temperatures were between 800 and 950 °C, the AlN–Ti/Al/Ni/Au contacts became ohmic contacts and the resistance decreased with the increase of annealing temperature. A lowest specific contacts resistance of 0.379 Ω·cm 2 was obtained for the sample annealed at 950 °C. In this work, we confirmed that the formation mechanism of ohmic contacts on AlN was due to the formation of Al–Au, Au–Ti and Al–Ni alloys, and reduction of the specific contacts resistance could originate from the formation of Au 2Ti and AlAu2 alloys. This result provided a possibility for the preparation of AlN-based high-frequency, high-power devices and deep ultraviolet devices.

A Thermally Stable NiZn/Ta/Ni Scheme to Replace AuBe/Au Contacts in High-Efficiency AlGaInP-Based Light-Emitting Diodes

We developed NiZn/(Ta/)Ni ohmic contacts to replace expensive AuBe/Au contacts commonly used in high-efficiency AlGaInP-based light-emitting diodes (LEDs), and compared the electrical properties of the two contact types. Unlike the AuBe/Au (130 nm/100 nm) contact, the NiZn/Ta/Ni (130 nm/20 nm/100 nm) contact shows improved electrical properties after being annealed at 500°C, with a contact resistivity of 5.2 × 10−6 Ω cm2. LEDs with the NiZn/Ta/Ni contact exhibited a 4.4% higher output power (at 250 mW) than LEDs with the AuBe/Au contact. In contrast to the trend for the AuBe/Au contact, the Ga 2p core level for the NiZn/Ta/Ni contact shifted toward lower binding energies after being annealed at 500°C. Auger electron spectroscopy (AES) depth profiles showed that annealing the AuBe/Au samples caused the outdiffusion of both Be and P atoms into the metal contact, whereas in the NiZn/Ta/Ni samples, Zn atoms indiffused into the GaP layer. The annealing-induced electrical degradation and ohmic contact formation mechanisms are described and discussed on the basis of the results of x-ray photoemission spectroscopy and AES.

Formation mechanism of gold-based and gold-free ohmic contacts to AlGaN/GaN heterostructure field effect transistors

To better understand the formation mechanism of ohmic contacts to GaN-based heterostructure field effect transistors, we have compared in detail Ti/Al/Ti/Au and Ti/Al/Ti/TiN contacts. Transmission electron microscopy and electron dispersive X-ray spectroscopy revealed that following anneal, TiN islands penetrated through the AlGaN barrier, as already well known, in the gold-based ohmic contacts but not in the gold-free contacts. We hence conclude that gold facilitates the formation of the TiN islands and propose that the role of gold is extraction of gallium from the semiconductor, providing a gallium depleted region for TiN island formation. For the case of the gold-free contacts, a 8 nm thick semi continuous TiN layer was formed following 900 °C anneal. A 2 nm thick TiN layer was observed in the as deposited samples and remained intact after anneal up to 825 °C. The different ohmic contact formation mechanism of gold-based and gold-free contacts is also manifested by our finding that a discontinuous AlN nitride spacer layer between the barrier and the bulk may lead to non-uniformity in contact behavior across the wafer in the case of gold-free contacts. For gold-based contacts, ohmic contact behavior was uniform across the wafer.

Impact of Ti/Al atomic ratio on the formation mechanism of non-recessed Au-free Ohmic contacts on AlGaN/GaN heterostructures

The formation mechanism of non-recessed Au–free Ohmic contacts on the AlGaN/GaN heterostructures is investigated for various Ti/Al atomic ratios (Al–rich versus Ti–rich) and annealing temperatures ranging from 500 to 950 °C. It is shown that Ti/Al atomic ratio is the key parameter defining the optimum annealing temperature for Ohmic contact formation. Ti–rich contacts processed at high temperature result in low contact resistance ∼0.7 Ω mm, better to those obtained at low temperature or with Al–rich metal stacks. The variation of the contact resistance with Ti/Al atomic ratio and annealing temperature is correlated with the intermetallic phase changes and interfacial reaction. Depending on the Ti/Al atomic ratio, two distinct mechanisms can be distinguished. For a small quantity of Ti (e.g., Al–rich contacts), Ohmic contact formation is done through a weak interfacial reaction which is nonexistent at high temperature due to the degradation of the metal morphology. However, for a quantity of Ti higher than 25 at. % (e.g., Ti–rich contacts), the agglomeration is delayed by 200 °C as compared to Al–rich contacts, and optimal contacts are formed at high temperature through a strong interfacial reaction.

Ohmic contacts to Gallium Nitride materials

In this review article, a comprehensive study of the mechanisms of Ohmic contact formation on GaN-based materials is presented. After a brief introduction on the physics of Ohmic contacts, a resume of the most important results obtained in literature is reported for each of the systems taken in consideration (n-type GaN, p-type GaN and AlGaN/GaN heterostructures). The optimal metallization schemes and processing conditions to obtain low resistance Ohmic contacts are presented, discussing the role of the single metals composing the stack and the modification induced by the thermal annealing, either on the metal layers or at the interface with GaN. Physical insights on the mechanism of Ohmic contact formation have been gained by correlating the temperature dependence of the electrical parameters with a morphological/structural analysis of the interface. In the case of the AlGaN/GaN systems, the influence of the heterostructure parameters on the Ohmic contacts has been taken into account adapting the classical thermionic field emission model to the presence of the two dimensional electron gas (2DEG). Finally, the state of the art of “Au-free” metallization to AlGaN/GaN heterostructures is also presented, being this latter a relevant topic for the integration of GaN technology on large scale Silicon devices fabs.

Ni-Based Ohmic Contacts ton-Type 4H-SiC: The Formation Mechanism and Thermal Stability

The fabrication of low-resistance and thermal stable ohmic contacts is important for realization of reliable SiC devices. For then-type SiC, Ni-based metallization is most commonly used for Schottky and ohmic contacts. Many experimental studies have been performed in order to understand the mechanism of ohmic contact formation and different models were proposed to explain the Schottky to ohmic transition for Ni/SiC contacts. In the present review, we summarize the last key results on the matter and post open questions concerning the unclear issues of ohmic contacts ton-type SiC. Analysis of the literature data and our own experimental observations have led to the conclusion that the annealing at high temperature leads to the preferential orientation of silicide at the heterointerface (0001)SiC//(013)δ-Ni2Si. Moreover, we may conclude that onlyδ-Ni2Si grains play a key role in determining electrical transport properties at the contact/SiC interface. Finally, we show that the diffusion barriers with free diffusion path microstructure can improve thermal stability of metal-SiC ohmic contacts for high-temperature electronics.

Improvement of Metal-Graphene Ohmic Contact Resistance in Bilayer Epitaxial Graphene Devices**Supported by the National Natural Science Foundation of China under Grant No 61306006.

We report on an improved metal-graphene ohmic contact in bilayer epitaxial graphene on a SiC substrate with contact resistance below 0.1 ω·mm. Monolayer and bilayer epitaxial graphenes are prepared on a 4H-SiC substrate in this work. Their contact resistances are measured by a transfer length method. An improved photoresist-free device fabrication method is used and is compared with the conventional device fabrication method. Compared with the monolayer graphene, the contact resistance Rc of bilayer graphene improves from an average of 0.24 ω·mm to 0.1 ω·mm. Ohmic contact formation mechanism analysis by Landauer's approach reveals that the obtained low ohmic contact resistance in bilayer epitaxial graphene is due to their high carrier density, high carrier transmission probability, and p-type doping introduced by contact metal Au.

Low specific contact resistance on epitaxial p-type 4H-SiC with a step-bunching surface**Project supported by the Key Specific Projects of Ministry of Education of China (Grant No. 625010101), the National Natural Science Foundation of China (Grant No. 61234006), the Natural Science Foundation of ShaanXi Province, China (Grant No. 2013JQ8012), the Doctoral Fund of Ministry of Education of China (Grant No. 20130203120017),

This paper reports the performances of Ti/Al based ohmic contacts fabricated on highly doped p-type 4H-SiC epitaxial layer which has a severe step-bunching surface. Different contact schemes are investigated based on the Al:Ti composition with no more than 50 at.% Al. The specific contact resistance (SCR) is obtained to be as low as 2.6 × 10−6 Ω·cm2 for the bilayered Ti(100 nm)/Al(100 nm) contact treated with 3 min rapid thermal annealing (RTA) at 1000 °C. The microstructure analyses examined by physical and chemical characterization techniques reveal an alloy-assisted ohmic contact formation mechanism, i.e., a high degree of alloying plays a decisive role in forming the interfacial ternary Ti3SiC2 dominating the ohmic behavior of the Ti/Al based contact. Furthermore, a globally covered Ti3SiC2 layer with (0001)-oriented texture can be formed, regardless of the surface step bunching as well as its structural evolution during the metallization annealing.

Effects of annealing temperature on the structure and electrical properties of tungsten contacts to n-type silicon carbide

Abstract Tungsten contacts deposited on n-type 4H-SiC substrates were annealed at temperatures from 700 to 1400 °C and the phase composition of the contact layers was analyzed by TOF-SIMS technique, X-ray diffraction method and scanning electron microscopy. W 5 Si 3 and WC were characterized as the main crystalline phases formed in the contact layer after annealing at 1400 °C and crystallite size was calculated as equal to 74 nm and 73 nm, respectively. Measurements of current–voltage characteristics showed that ohmic character of the contact was obtained after annealing at the highest applied temperature 1400 °C while I – V characteristics of 1200 °C annealed contact showed limited deviation from linearity. The specific contact resistivities equal to 2.42 × 10 −5 Ω cm 2 and 2.27 × 10 −5 Ω cm 2 were achieved for 1200 and 1400 °C annealed contacts, respectively. The mechanism of ohmic contact formation was proposed.