Ti Contacts Research Articles

Unveiling interface engineering dynamics between Ti and Ga2O3 nanowire

In this study, we investigate the interfacial reactions between Ga2O3 nanowires (NWs) and Ti contacts during annealing processes through in-situ transmission electron microscopy (TEM) observations. A thin Ga3Ti2 intermetallic compound was observed to form at the Ti/Ga2O3 interface, coinciding with a transition in the NW device’s contact behavior from Schottky to ohmic upon annealing at 470 °C. Utilizing in-situ TEM, we monitored the evolution of the Ga3Ti2 layer, revealing Ga atoms as the primary diffusing species within the Ga2O3 NW, evidenced by asymmetric diffusion at the interface. Ga3Ti2 intermetallic compound effectively reducing the energy barrier between Ga2O3 NW and Ti electrode, contributing to linear electrical transport characteristics. This investigation underscores the significance of interface engineering and offers valuable insights for future electronic applications utilizing Ga2O3 NWs.

Effect of Thickness and Thermal Treatment on the Electrical Performance of 2D MoS2 Monolayer and Multilayer Field-Effect Transistors

We deposited high-quality molybdenum disulfide (MoS2) monolayer and multilayer crystals on SiO2/Si substrates, by means of a chemical vapor deposition (CVD) process at atmospheric pressure. Notably, NaCl salt was used as component of the precursors to assist the growth of MoS2 crystals, which were intended for use as the active channel layer in the fabrication of field-effect transistors (FETs). The resulting MoS2 crystals from this CVD process were analyzed by optical, scanning electron, and atomic force microscopies, and by Raman and photoluminescence spectroscopies. The optical images and the micrographs obtained by SEM revealed the formation of dispersed MoS2 crystals with a triangular shape all over the SiO2 surface. The thickness of the MoS2 crystals, analyzed by atomic force microscopy, showed minimum values of around 0.7 nm, confirming the formation of monolayers. Additionally, multilayers with larger thickness were also identified. The Raman and photoluminescence spectra of the MoS2 crystals corroborated the formation of single and multiple layers. The fabrication of the FET back-SiO2 -gate configuration was made by depositing patterned source and drain Ti contacts on the dispersed MoS2 crystals to achieve the Ti/MoS2/SiO2/Si layer stacks. MoS2-based FETs with one and three layers were assembled and their electrical response analyzed by I–V output and transfer curves showing the typical characteristics of an n-type semiconductor channel operating in depletion mode. The electrical performance parameters of the devices, such as mobility and threshold voltage, were also determined from this analysis. Finally, to enhance their electrical response, the MoS2-based devices were thermally annealed at 200 °C for 30 min in Ar atmosphere. The increase in the mobility of the device was 176% compared to the device before the treatment.

Ta/Al/CuW low temperature ohmic contacts for GaN-on-Si HEMT

We proposed a low temperature Au-free ohmic contacts of GaN-on-Si HEMT with the Ta/Al/CuW metal stack. The CuW was deposited by using the dual-target magnetron sputter deposition method. The annealing conditions and recess depth of ohmic area were systematically investigated. By utilizing the Ta/Al/CuW structure, an improved contact characteristic (0.49 Ω·mm) is obtained following annealing at 550 °C for 10 min in vacuum, with the recess depth of 30 nm(±2 nm). This performance surpasses that of Ta/Al/W Au-free contacts (1.07 Ω·mm). Furthermore, both the Ta/Al/CuW ohmic contacts (RMS = 6.3 nm) and the Ta/Al/W ohmic contacts (RMS = 6.0 nm) exhibit smooth surface morphology. Compared to Ti contact layer, Ta demonstrates superior performance in low temperature contact and breakdown test. Amorphous Ta layer can effectively suppress Cu diffusion. The GaN-on-Si HEMT was also fabricated based on Ta/Al/CuW Au-free ohmic contacts, exhibiting excellent DC characteristics.

Asymmetric contact-induced selective doping of CVD-grown bilayer WS2 and its application in high-performance photodetection with an ultralow dark current.

Two-dimensional (2D) transition metal dichalcogenides (TMDs) are excellent candidates for high-performance optoelectronics due to their high carrier mobility, air stability and strong optical absorption. However, photodetectors made with monolayer TMDs often exhibit a high dark current, and thus, there is a scope for further improvement. Herein, we developed a 2D bilayer tungsten disulfide (WS2) based photodetector (PD) with asymmetric contacts that exhibits an exceptionally low dark current and high specific detectivity. High-quality and large-area monolayer and bilayer WS2 flakes were synthesized using a thermal chemical vapor deposition system. Compared to conventional symmetric contact electrodes, utilizing metal electrodes with higher and lower work functions relative to bilayer WS2 aids in achieving asymmetric lateral doping in the WS2 flakes. This doping asymmetry was confirmed through the photoluminescence spectral profile and Raman mapping analysis. With the asymmetric contacts on bilayer WS2, we find evidence of selective doping of electrons and holes near the Ti and Au contacts, respectively, while the WS2 region away from the contacts remains intrinsic. When compared with the symmetric contact case, the dark current in the WS2 PD with asymmetric (Au, Ti) contact decreases by an order of magnitude under reverse bias with a concomitant increase in the photocurrent, resulting in an improved on/off ratio of ∼105 and overall improved device performance under identical illumination conditions. We explained this improved performance based on the energy band alignment showing a unidirectional charge flow under light illumination. Our results indicate that the planar device structure and compatibility with current nanofabrication technologies can facilitate its integration into advanced chips for futuristic low-power optoelectronic and nanophotonic applications.

Tunneling current through non-alloyed metal/heavily-doped SiC interfaces

In this paper, tunneling phenomena at metal/heavily-doped SiC Schottky (non-sintered) interfaces are reviewed, and a design guideline for low-resistance non-alloyed SiC ohmic contacts is proposed. Carrier transport at Schottky contacts on heavily-doped SiC epitaxial layers is quantitatively described by direct tunneling (DT) including both of the thermionic field emission (TFE) and field emission (FE) when the doping density (Nd) is higher than mid-1017cm−3. As for p-type SiC, tunneling of holes in the split-off band is the dominant conduction mechanism due to its light effective mass (0.21m0). Phosphorus ion (P+) implantation makes the interface tunneling current larger by several orders of magnitude compared with that at contacts on epitaxial layers with almost the same Nd, which is plausibly explained by trap-assisted tunneling (TAT) through implantation-induced defect levels. The contact resistivity (ρc) at non-alloyed ohmic contacts formed on heavily P+-implanted SiC (Nd>4×1018cm−3) is significantly reduced thanks to the contribution of TAT especially when Nd is lower than about 1020cm−3, while ρc decreases with increasing Nd according to the DT theory in a higher Nd range (>1020cm−3). At a very high Nd of 2×1020cm−3, an extremely low ρc of 1–2×10−7Ωcm2 is demonstrated for non-alloyed Mg and Ti contacts. Through the experiment and calculation of tunneling current, a model to predict ρc at non-alloyed ohmic contacts taking account of the contributions of TAT and DT is proposed, and a design guideline for low-resistance non-alloyed ohmic contacts is presented as follows; a very low ρc (<10−6Ωcm2) is achievable without sintering by lowering the interface barrier height (≤1eV) and utilizing high-dose ion implantation (≥3×1019cm−3).

Effect of thermal oxidation on the dry sliding friction and wear behaviour of CP-Ti on CP-Ti tribopairs

Thermal oxidation (TO) has proven to be a cost-effective and efficient technique to engineer the surfaces of titanium and its alloys to achieve enhanced surface properties. The benefits of TO treatment in enhancing the tribological properties of titanium have been demonstrated by many investigators. However, most of the reported tribological studies have been based on the contact between a TO treated titanium specimen and a counter-body made of other materials, mainly ceramics, steels and polymers. Very few studies have been reported on the friction and wear behaviour of TO treated titanium sliding against TO treated titanium. In this work, the effect of thermal oxidation on the dry sliding friction and wear behaviour of commercially pure Ti (CP-Ti) on CP-Ti tribopairs was investigated under loading conditions ranging from elastic contact to plastic contact. Comparisons were made among three contact pairs: (1) untreated Ti on untreated Ti (Ti–Ti), (2) untreated Ti on TO treated Ti (Ti-TO) and (3) TO treated Ti on TO treated Ti (TO-TO). The results show that the TO-TO contact pair presents an ideal material combination to achieve the best tribological performance in terms of low friction and superior wear resistance. On the other hand, the Ti–Ti pair presents the worst combination in terms of tribological performance. While the Ti-TO pair performs better than the Ti–Ti pair tribologically, it is not as good as the TO-TO pair. It is essential to thermally oxidize both specimens in order to achieve optimal tribological performance. It is the oxide layer-on-oxide layer contact that imparts the excellent tribological performance. Failure of the oxide layer in one of the contact bodies can lead to high and unstable friction and increased wear from both contacting bodies. The tribological performance of the three contact pairs and the failure mechanism of the oxide layer are discussed in the paper. The results of this work suggest that the TO treated Ti on TO treated Ti contact pair would have potential tribological applications in engineering.

Control of Carrier Polarity and Schottky Barriers in Layered PdSe2 Field-Effect Transistors

Palladium diselenide (PdSe2) has been discovered as an intriguing two-dimensional (2D) semiconductor for its unique pentagonal crystalline structure, electrical and optical anisotropy, thickness-modulated band gap, robust air stability, and high carrier mobility, demonstrating great potential in field-effect transistors (FETs), photodetectors, and thermoelectric devices. Controlling the carrier polarity and electrical contact Schottky barriers is of great significance for realizing high-performance PdSe2 optoelectronic devices. Here, by combining work-function measurement and electrical transport, we observe a thickness-modulated carrier polarity transition in layered PdSe2 FETs from n-type unipolar to intrinsic ambipolar and p-type unipolar behavior by simply varying the number of layers of PdSe2. Due to the weak Fermi-level pinning in few-layer PdSe2, n-type and ambipolar FETs are achieved by contacting PdSe2 to Sc, Ti, and Pd electrodes, respectively. The Schottky barrier heights are determined by temperature-dependent transport, showing a 15 meV barrier for the Sc contact at the electron side and 62 and 93 meV barriers for Ti and Pd contacts, both at the hole side, respectively. We further demonstrate ozone-treatment-induced hole doping in PdSe2, which effectively converts the PdSe2 FETs from n-type to p-type. The doping is robust in the ambient environment over 3 months. In-plane homojunction on a PdSe2 flake is realized by selective ozone doping, demonstrating typical diode rectifying behavior. The ability to control the carrier polarity and Schottky barriers in layered PdSe2 makes this 2D semiconductor highly feasible for complementary metal-oxide semiconductor device integration and high-performance photodetectors based on atomically thin p–n junctions.

Modulation of Contact Resistance of Dual-Gated MoS2 FETs Using Fermi-Level Pinning-Free Antimony Semi-Metal Contacts.

Achieving low contact resistance (RC ) is one of the major challenges in producing 2D FETs for future CMOS technology applications. In this work, the electrical characteristics for semimetal (Sb) and normal metal (Ti) contacted MoS2 devices are systematically analyzed as a function of top and bottom gate-voltages (VTG and VBG ). The semimetal contacts not only significantly reduce RC but also induce a strong dependence of RC on VTG , in sharp contrast to Ti contacts thatonly modulate RC by varying VBG . The anomalous behavior is attributedto the strongly modulated pseudo-junction resistance (Rjun ) by VTG , resulting from weak Fermi level pinning (FLP) of Sb contacts. In contrast, the resistances under both metallic contacts remain unchanged by VTG as metal screens the electric field from the applied VTG . Technology computer aided design simulations further confirm the contribution of VTG to Rjun , which improves overall RC of Sb-contacted MoS2 devices. Consequently, the Sb contact has a distinctive merit in dual-gated (DG) device structure, as it greatly reduces RC and enables effective gate control by both VBG and VTG . The results offer new insight into the development of DG 2D FETs with enhanced contact properties realized by using semimetals.

Minimization of the electrical contact resistance in thin-film thermoelectric device

High electrical contact resistance refrains the performance of thin-film thermoelectric devices at the demonstrative level. Here, an additional Ti contact layer is developed to minimize the electrical contact resistance to ∼4.8 Ω in an as-assembled thin-film device with 50 pairs of p–n junctions. A detailed interface characterization demonstrates that the low electrical contact resistance should be mainly attributed to the partial epitaxial growth of Bi2Te3-based thin-film materials. Correspondingly, the superlow electrical contact resistance facilitates the applicability of the out-of-plane thin-film device and results in an ultrahigh surface output power density of ∼81 μW cm−2 at a low temperature difference of 5 K. This study illustrates the Ti contact layer that strengthens the contact between Cu electrodes and Bi2Te3-based thermoelectric thin films mainly through partial epitaxial growth and contributes to high-performance thin-film thermoelectric devices.

Enhanced Method of Schottky Barrier Diodes Performance Assessment

An elaborate characterization of Si Schottky diodes, fabricated with Ti and Mo contacts, is presented. Thermal treatment in forming gas is performed in order to improve the electrical performance of the fabricated samples. X-ray diffraction measurements s

Vertical van der Waals heterojunction diodes comprising 2D semiconductors on 3D β-Ga2O3.

Wide bandgap semiconductors such as gallium oxide (Ga2O3) have attracted much attention for their use in next-generation high-power electronics. Although single-crystal Ga2O3 substrates can be routinely grown from melt along various orientations, the influence of such orientations has been seldom reported. Further, making rectifying p-n diodes from Ga2O3 has been difficult due to lack of p-type doping. In this study, we fabricated and optimized 2D/3D vertical diodes on β-Ga2O3 by varying the following three factors: substrate planar orientation, choice of 2D material and metal contacts. The quality of our devices was validated using high-temperature dependent measurements, atomic-force microscopy (AFM) techniques and technology computer-aided design (TCAD) simulations. Our findings suggest that 2D/3D β-Ga2O3 vertical heterojunctions are optimized by substrate planar orientation (-201), combined with 2D WS2 exfoliated layers and Ti contacts, and show record rectification ratios (>106) concurrently with ON-Current density (>103 A cm-2) for application in power rectifiers.

Extraordinarily Weak Temperature Dependence of the Drain Current in Small‐Molecule Schottky‐Contact‐Controlled Transistors through Active‐Layer and Contact Interplay

AbstractLow saturation voltages and extremely high intrinsic gain can be achieved in contact‐controlled thin‐film transistors (TFTs) with staggered device architecture, enabled by the energy barrier introduced at the source contact. The resulting device, the source‐gated transistor (SGT), is limited in its usefulness by the high temperature dependence of the drain current induced by the source energy barrier. Here, the interaction between the thermal characteristics of the source contact and the semiconductor to show drastically reduced temperature dependence for SGTs based on organic semiconductors (OSGTs) is exploited. This extraordinarily weak temperature dependence of the drain current is observed regardless of the height of the source energy barrier (27.8% in OSGTs with Ti contacts compared to 22.1% when using Au contacts, over a 34 K range). The reduction in mobility of the semiconductor offsets an increase in thermionic‐field emission of charge carriers at the source. This is a first for SGTs and provides a route to removing one of the last hurdles to their wider adoption. The OSGTs with Ti contacts also demonstrate: drain‐current saturation at very low drain‐source voltages (saturation factor of 0.22); noteworthy stability after 70 days; and minimal drain‐current variation with channel length or illumination.

(Invited) Nanoelectronics Based on Assembled High-Density and High-Semiconducting-Purity Carbon Nanotube Films

Carbon nanotubes hold great promise for high-performance electronics but also face significant challenges in terms of assembly and integration. On one hand, aligned carbon nanotubes are proposed as an alternative to III-V semiconductor technologies in radio frequency (RF) applications because of their high linearity as amplifiers and compatibility with CMOS electronics. We will first report high-performance RF transistors with operation frequencies beyond 100 GHz. These devices are built upon high-density (~50 nanotubes / micron) and high semiconducting purity (> 99.99%) aligned single-wall carbon nanotube films assembled at wafer scale. With gate length ~110 nm and T-shaped gate to reduce the gate charging resistance, the devices showed an extrinsic cutoff frequency and maximum oscillation frequency of over 100 GHz. The performance surpasses the 90 GHz cutoff frequency of radio-frequency CMOS transistors with gate length of 100 nm and is close to the performance of GaAs technology. [1] On the other hand, Carbon nanotubes are ideal candidates for beyond-silicon nanoelectronics because of their high mobility and low-cost processing; however, n-type transistors based on assembled aligned nanotubes has not been reported yet. Fabrication of n-type behavior field effect transistors (FETs) based on assembled aligned CNT arrays is needed for advanced CNT electronics. We will report a scalable process to make n-type transistors based on assembled aligned CNT arrays. Air-stable and high-performance n-type CNT FETs are achieved with high yield by combining atomic layer deposition dielectric and Ti contacts with gold overcoating, which are stable in air and widely used for III-V semiconductors. We also systematically studied the contribution of metal contacts and atomic layer deposition passivation in determining the transistor polarity. [2] Based on these experimental results, we report the successful demonstration of complementary metal-oxide-semiconductor inverters with good performance, which paves the way to realizing the promising future of carbon nanotube nanoelectronics.[1] “Wafer-scalable, aligned carbon nanotube transistors operating at frequencies of over 100 GHz”, C. Rutherglen, A. A. Kane, P. F. Marsh, T. A. Cain, B. I. Hassan, M. R. AlShareef, C. Zhou and K. Galatsis, Nature Electronics, volume 2, pages 530–539, 2019.[2] “Air-Stable n-Type Transistors based on Assembled Aligned Carbon Nanotube Arrays and Their Application in CMOS Electronics”, Z. Li, K. R. Jinkins, D. Cui, M. Chen, Z. Zhao, M. S. Arnold and C. Zhou, Nano Res. (2021). https://doi.org/10.1007/s12274-021-3567-9.

Ab initio simulations of metal contacts for graphene-based devices

The precise atomic structure of a metal contact significantly affects the performance of nanoscale electronic devices. We use an accurate, DFT-based non-equilibrium Green’s function method to evaluate various metal contacts with graphene or graphene nanoribbons. For surface metal contacts not chemically bound to graphene, Ti contacts have lower resistance than those of Au, Ca, Ir, Pt, and Sr. However, as an edge contact, Ti has larger resistance than Au. Bridging O atoms at Ti and Au edge contacts lowers the transmission by over 30%.

Large-Area, High-Specific-Power Schottky-Junction Photovoltaics from CVD-Grown Monolayer MoS2.

The deployment of two-dimensional (2D) materials for solar energy conversion requires scalable large-area devices. Here, we present the design, modeling, fabrication, and characterization of monolayer MoS2-based lateral Schottky-junction photovoltaic (PV) devices grown by using chemical vapor deposition (CVD). The device design consists of asymmetric Ti and Pt metal contacts with a work function offset to enable charge separation. These early stage devices show repeatable performance under 1 sun illumination, with VOC of 160 mV, JSC of 0.01 mA/cm2, power conversion efficiency of 0.0005%, and specific power of 1.58 kW/kg. An optoelectronic model for this device is developed and validated with experimental results. This model is used to understand loss mechanisms and project optimized device designs. The model predicts that a 2D PV device with ∼70 kW/kg of specific power can be achieved with minimum optimization to the current devices. By increasing the thickness of the absorber layer, we can achieve even higher performance devices. Finally, a 25 mm2 area solar cell made with a 0.65 nm thick MoS2 monolayer is demonstrated, showing VOC of 210 mV under 1 sun illumination. This is the first demonstration of a large-area PV device made with CVD-grown scalable 2D materials.

Barrier properties and current conduction mechanism for metal contacts to lightly and highly doped p-type 4H-SiC

The barrier properties of Ti, Ni and Pt contact to lightly (9 × 1016 cm−3) and highly (9 × 1018 cm−3) doped p-type 4H-SiC were investigated. It is found that the barrier heights and ideality factors estimated from the thermionic emission model for the lightly doped samples are non-ideal and abnormally temperature dependent. The anomalies have been successfully explained in terms of both the pinch-off model and the Gaussian distribution of inhomogeneous barrier heights. In addition, the evaluated homogeneous barrier heights are reasonably close to the average barrier heights from capacitance–voltage measurements. For the highly doped samples, thermionic field emission (TFE) is found to be the dominant carrier transport mechanism. The barrier heights estimated from the TFE model are temperature independent. If the barrier inhomogeneities and tunneling effects are considered, the experimental results of the samples are in well agreement with the theoretical calculations.

Spray deposited thin uniform NiO/Spiro-OMeTAD composite hole transport layer with top carbon nanotube electrode

Thin Spiro-OMeTAD and NiO nanoparticles layers, as well as their composite layer was formed by layer-by-layer spray deposition as hole-transport layer (HTL), with followed carbon nanotubes (CNT) deposition to form Ti/TiO2/HTL/CNT structures. Layers’ uniformity was estimated by Raman intensity maps, AFM and current-voltage characteristics of the CNT layer and between CNT and Ti contacts. The possibility of formation of thin, less than 100 nm, pinhole-free uniform composite NiO/Spiro-OMeTAD layer by spray-deposition was shown, which manifests itself as continuous HTL even after top CNT layer deposition.

Investigation of Ni/Al/Ti Ohmic Contact on N-Type 4H-SiC

In order to understand the contribution of various metals in the formation of ohmic contacts, Ni/Al/Ti ohmic contacts on n-type 4H-SiC in terms of a different annealing temperature and Ti composition are investigated, which is more difficult to form than p-type ohmic contact. The formation of the Ni/Al/Ti metal alloy system is much more sensitive to metal composition than annealing conditions. With the increase of metal composition, the contact with a high Ti content yields a lower specific contact resistivity compared with the low Ti contact. The annealed surface morphology and phase resultants were examined by scanning electron microscopy (SEM) and atomic force microscope (AFM), respectively. With the increase of Ti components, the surface morphology of the samples becomes more uniform and smoother, while the surface roughness remains unchanged. It implies that Ti metal can not only reduce the ohmic resistance, but also protect the surface of the sample and maintain the roughness.

Fabrication of Large-Area Molybdenum Disulfide Device Arrays Using Graphene/Ti Contacts.

Two-dimensional (2D) molybdenum disulfide (MoS2) is the most mature material in 2D material fields owing to its relatively high mobility and scalability. Such noticeable properties enable it to realize practical electronic and optoelectronic applications. However, contact engineering for large-area MoS2 films has not yet been established, although contact property is directly associated to the device performance. Herein, we introduce graphene-interlayered Ti contacts (graphene/Ti) into large-area MoS2 device arrays using a wet-transfer method. We achieve MoS2 devices with superior electrical and photoelectrical properties using graphene/Ti contacts, with a field-effect mobility of 18.3 cm2/V∙s, on/off current ratio of 3 × 107, responsivity of 850 A/W, and detectivity of 2 × 1012 Jones. This outstanding performance is attributable to a reduction in the Schottky barrier height of the resultant devices, which arises from the decreased work function of graphene induced by the charge transfer from Ti. Our research offers a direction toward large-scale electronic and optoelectronic applications based on 2D materials.

Study of Ti contacts to corundum α-Ga2O3

We present a study of the electrical, structural and chemical properties of Ti contacts on atomic layer deposited α-Ga2O3 film. Ti forms an ohmic contact with α-Ga2O3. The contact performance is highly dependent on the post-evaporation annealing temperature, where an improved conductivity is obtained when annealing at 450°C, and a strong degradation when annealing at higher temperatures. Structural and chemical characterisation by transmission electron microscopy techniques reveal that the electrical improvement or degradation of the contact upon annealing can be attributed to oxidation of the Ti metallic layer by the Ga2O3 film in combination with the possibility for Ti diffusion into the Au layer. The results highlight that the grain boundaries and inclusions in the Ga2O3 film provide fast diffusion pathways for this reaction, leaving the α-Ga2O3 crystallites relatively unaffected—this result differs from previous reports conducted on β-Ga2O3. This study underlines the necessity for a phase-specific and growth method-specific study of contacts on Ga2O3 devices.