Thin Ti Interlayer Research Articles

Towards Controlled Transfer of (001) β-Ga2O3 to (0001) 4H-SiC Substrates

We demonstrate successful blistering of He-implanted (001) β-Ga2O3, bonding to (0001) 4H-SiC, and initial results towards large-area transfer of (001) β-Ga2O3 to 4H-SiC. Compatible with large-scale processing, exfoliation is an important step in controlled-thickness transfer of films for heterogeneous integration. Furthermore, integration of β-Ga2O3 to high thermal conductivity materials will be crucial for thermal management of β-Ga2O3-based power devices. Two-inch (001) β-Ga2O3 wafers were implanted with He+ at an energy of 160 keV and a dose of 5×1016 cm-2, which are the same implant parameters used previously to successfully exfoliate (010) β-Ga2O3.1 Strain fringes were observed after the implant which corresponded to a maximum strain of ~1.7%, which is higher than the ~1% strain when implanting (010) β-Ga2O31 likely due to the larger Poisson’s ratio of (001) β-Ga2O3.2 The implanted substrates were then bonded to (0001) 4H-SiC at room temperature using a ~5 nm thin Ti interlayer to assist with the bond. Both unbonded implanted substrates and the bonded structures were first annealed at 200 °C for 10 hours to simultaneously initiate He bubble nucleation at the implanted projected range (~0.68 μm) and strengthen the bond. Then, even after annealing at 500 °C for 10 hours to initiate He bubble growth, the (001) β-Ga2O3 did not transfer to the (0001) 4H-SiC. Unlike what was observed for (010) β-Ga2O3,1 blistering did not even occur at this temperature. Annealing at 800 °C for up to 12 hours resulted in ~10 μm blisters for the unbonded substrates, which is typically an indication that large wafer-area transfer can be achieved if bonding was done prior to annealing.3 However, the β-Ga2O3 substrate did not wafer split from the bonded structure, and instead only small area transfers up to ~200 μm were achieved. Only ~7% of the total bonded area transferred while the entire structure remained bonded. Strategies to improve transfer will be presented, including initiating a cold split prior to exfoliation and refined annealing strategies to improve He bubble nucleation and larger bubble growth at the projected range. These are promising results towards achieving large wafer-scale (001) β-Ga2O3 composite wafers, and when combined with subsurface damage-free chemical mechanical polishing,4 these composite wafers would be suitable for subsequent devices and/or epitaxial growth.The authors would like to acknowledge the support from the Office of Naval Research through a MURI program, grant No. N00014-18-1-2429. This research was performed while M.E.L. and J.S.L. held an NRC Research Associateship award at the U.S. Naval Research Laboratory.References M.E. Liao, et al., ECS J. Solid State Sci. Technol., 8(11), P673 (2019).K. Adachi, et al., J. Appl. Phys., 124, 085102 (2018).M. Bruel, et al., Jpn. J. Appl. Phys., 36, 1636 (1997).M.E. Liao, et al., J. Vac. Sci. Technol. A, 41, 013205 (2023). Figure 1

Strong Mo/Cu interfacial bonding facilitated by consumable Ti interlayer

It is challenging to construct interfacial metallurgical bonding in Mo/Cu immiscible system because of the lack of mutual solubility. Here it was demonstrated that such problem can be effectively circumvented and robust Mo/Cu interfacial bonding can be achieved by diffusion bonding at 800 °C using a thin Ti interlayer. At the initial stage, bonding was made by forming MoTi solid solution (s.s) and TiCu intermetallic compounds (IMCs). The presence of IMCs was detrimental to joint integrity, leading to poor bonding strength. Prolonged bonding duration resulted in dilution of Ti and progressive elimination of TiCu IMCs. Ultimately, IMCs free joint exhibiting excellent interfacial bonding strength comparable with Cu base metal properties was obtained at bonding duration of 60 min.

Sintering temperature effect on microstructure, mechanical and electrical properties of multi-layered ZrB2-based ceramics with thin Ti interlayer

Laminated ZrB2-SiCw ceramics with a thin Ti interlayer were synthesized via spark plasma sintering at varying temperatures. The effect of sintering temperature on the interlayer morphology, phase composition, and mechanical properties of laminated ZrB2-SiCw/Ti ceramics was assessed. With increasing sintering temperature from 1600 ℃ to 1800 ℃, element diffusion between the matrix and the interlayer gradually increased. The green-body ductile Ti gradually transformed into a multiphase mixture with increasing hardness at the interlayer, shortening the crack propagation path. The toughening mechanisms changed from delamination to deflection, leading to a decrease in fracture toughness from 15.30 ± 0.72 to 11.21 ± 0.45 MPa m1/2. Compared to monolithic ZrB2-SiCw ceramics, the introduction of multiple toughening mechanisms significantly improved the toughness of laminated ceramics with a small loss in strength. The electrical conductivity under parallel and perpendicular directions decreased with the decrease in residual Ti, with an important effect on electromagnetic effectiveness, reduced from 61.5 to 45.1 dB.

Efficient n+p-Si photocathodes for solar H2 production catalyzed by Co-W-S and stabilized by Ti buffer layer

Photoelectrochemical (PEC) water splitting is regarded as a promising route to produce sustainable hydrogen fuel using sunlight and water as sole inputs. Si is one of the most promising potential materials for PEC photocathode, while developing highly active non-precious catalysts and stable semiconductor/catalyst interface is critical to bring solar water splitting into reality. Herein, we reported a cheap and effective strategy based on a wet chemical method to integrate WS2 and Co-doped WS2 (Co-W-S) onto n+p-Si as noble metal−free catalysts for H2 production. Co-W-S/n+p-Si photocathode exhibited much better PEC performance, resulting from the Co-W-S catalyst which has more electrochemically active sites and better electrical conductivity. A thin Ti interlayer between Co-W-S and Si was inserted to further optimize the PEC performance, especially the stability of the photocathode. As a result, an onset potential of 0.36 V vs. RHE, a photocurrent of 30.4 mA/cm2 at 0 V vs. RHE and an energy conversion efficiency of 4.0% were obtained under simulated AM1.5 G illumination, along with a long-term stability for 6 days of continuous PEC reaction.

Investigations on the Cosputtered ITO-ZnO Transparent Electrode Ohmic Contacts to n-GaN

Transparent indium tin oxide (ITO) and cosputtered ITO-zinc oxide (ZnO) films’ contacts to an n-GaN epilayer were investigated. Both of these electrodes’ contact to the n-GaN epilayer showed Schottky behavior, although the contact resistance of the ITO-ZnO/n-GaN system was lower than that of the ITO/n-GaN system. By placing a thin Ti interlayer between the ITO-ZnO/n-GaN interface, nonalloyed ohmic contact was achieved. The inset Ti interlayer was both beneficial both for enhancing the outdiffusion of the nitrogen atoms at the surface of the n-GaN and suppressing the indiffusion of oxygen atoms from the surface of the ITO-ZnO to n-GaN. The figure-of-merit (FOM), evaluated from the specific contact resistance and optical property of the Ti/ITO-ZnO system’s contact to the n-GaN epilayer, was optimized further at an adequate thickness of the Ti interlayer.

Formation of High-Oriented Epitaxial Ni(Si0.8Ge0.2) on Strained Si0.8Ge0.2/Si(100) via Adding a Thin Ti Interlayer

Ultrathin titanium (Ti) interlayer is introduced to the formation of nickel germanosilicide on strained Si0.8Ge0.2/Si(001). After rapid thermal annealing, a uniform flattened NiSi0.8Ge0.2 layer is finally formed. The incorporation of Ti increases the transition temperature from the Ni-rich germanosilicide phase to the mono-germanosilicide phase, and improves the surface and interface roughnesses of Ni(Si0.8Ge0.2). The achievement of the highly-oriented (010) Ni(Si0.8Ge0.2) on (001) Si0.8Ge0.2 is contributed to the balanced silicidation velocity for the existence of Ti interlayer.

Plasma Enhanced-Chemical Vapour Deposition of Scuff-Resistant Hydrogenated Amorphous Carbon Coatings on C100 Steel

Hydrogenated amorphous carbon coatings, deposited by low pressure plasma to minimize the wear of C100 steel components, were optimized and characterized. In order to ensure good adhesion of the films to the steel surface, a thin Ti interlayer was deposited, by magnetron sputtering, before the plasma deposition. The chemical characterization of the deposits was performed by means of RAMAN, XPS, RBS and ERDA analysis, while nanoindentation, nanoscratch and nanowear tests allowed to estimating the tribomechanical properties of the deposits, with the aim of evaluating their scuff-resistance. It was found that the optimized plasma deposited hydrogenated amorphous carbon coatings were well adherent to C100 steel and increased more than 70% its surface hardness.

Simultaneous Formation of Ohmic Contacts on p +- and n +-4H-SiC Using a Ti/Ni Bilayer

In this work, Ti/Ni bilayer contacts were fabricated on both p+- and n+-4H-SiC formed by ion implantation, and the effects of the Ti interlayer on the contact resistance and interfacial microstructure were studied. Adoption of a thin (10 nm) Ti interlayer resulted in specific contact resistance of 4.8 μΩ cm2 and 1.3 mΩ cm2 on n+- and p+-4H-SiC, respectively, comparable to the values for contacts using only Ni. Moreover, contacts using Ti/Ni provide a flat and uniform interface between Ni2Si and SiC, whereas discontinuous, agglomerated Ni2Si islands are formed without the use of a Ti interlayer. In addition, the Ti interlayer was demonstrated to effectively dissociate the thin oxide film on SiC, which is advantageous for low-resistance, reliable ohmic contact formation. In summary, use of a Ti/Ni bilayer is a promising solution for one-step formation of ohmic contacts on both p+- and n+-4H-SiC, being especially suitable for SiC n-channel metal-oxide-semiconductor field-effect transistor (nMOSFET) fabrication.

Ar + H2 plasma etching for improved adhesion of PVD coatings on steel substrates

Ar + H2 plasma cleaning has been described for the surface modification of the steel substrates, which removes oxides and other contaminants from substrate surface effectively leading to a better adhesion of the physical vapor deposited (PVD) coatings. Approximately 1.1–1.3 μm thick TiAlN coatings were deposited on plasma treated (Ar and Ar + H2) and untreated mild steel (MS) substrates. A mechanism has been put forward to explain the effect of plasma treatment on the substrate surface based upon the data obtained from X-ray photoelectron spectroscopy (XPS), field-emission scanning electron microscopy (FESEM) and atomic force microscopy (AFM). The XPS measurements on untreated and Ar + H2 plasma etched MS substrates indicated that the untreated substrate surface mainly consisted of Fe3O4, whereas, after etching the concentration of oxides decreased considerably. The FESEM and the AFM results showed changes in the surface morphology and an increase in the substrate roughness as a result of Ar + H2 plasma etching. Removal of oxide/contaminants, formation of coarser surface and increased substrate surface roughness as a result of Ar + H2 plasma etching facilitate good mechanical interlocking at the substrate surface, leading to a better adhesion of the deposited PVD coatings. The adhesion of TiAlN coating could be increased further by incorporating a very thin Ti interlayer.

Effects of Co, Ti Interlayer, and Post-Annealing on the Adhesion Property between TiN Coatings and WC-Co Substrate

In order to improve adhesion strength between TiN coatings and WC-Co substrate, two kinds of interlayer of Co and Ti were pre-deposited before TiN coating process. Thickness and postannealing effects of each interlayer on the adhesion property were investigated through scratch test in this work. Introduction of thin Ti interlayer largely enhanced the adhesion strength between TiN coatings and WC-Co substrate in scratch test. The critical load, Lc2 increased from 64 N without Ti interlayer up to 130N with Ti one of ~ 0.15 thickness. However, post-annealing of TiN/Ti/WCCo system at high temperature of 600 reduced the critical load again. The Ti interlayer caused a deficit of Co content in WC-Co substrate during annealing through diffusion of Co element into Ti interlayer. The reduction of critical load after post-annealing was believed due to diminution in mechanical properties of the substrate derived from the Co deficit in WC-Co substrate. On the other hand, introduction of thin Co interlayer of ~ 0.027 thickness also increased the critical load up to 84 N and improved failure mode, but did not reduce the critical load even after annealing, rather increased it. And, any Co deficit of the substrate was not found after annealing for TiN/Co/WC-Co system.

Fabrication of photocatalytic heat-mirror with TiO 2/TiN/TiO 2 stacked layers

The photocatalytic heat-mirror based on TiO 2/TiN/TiO 2 stacked layers is prepared by reactive magnetron sputtering on quartz substrates under substrate-heating condition. We find that the addition of a thin Ti interlayer between the TiN and the outer TiO 2 layers drastically improves the heat-insulating performance. This type of stacked layer also exhibits higher photocatalytic activity for decomposition of acetaldehyde gas, compared with a TiO 2 single layer. The optical property of the TiN in TiO 2/TiN/TiO 2 stacked layers is the key not only revealing excellent heat-insulating effect but also improving the photocatalytic performance of the outer TiO 2 layers in the stacked layers.

Neutron reflection studies of titanium segregation to metal–ceramic interfaces

Titanium is an extremely reactive element which is known to promote the wetting of ceramics by liquid metals. The wetting process is believed to be controlled by the segregation of a very thin layer of Ti to the metal/ceramic interface where it undergoes a localised redox reaction with the ceramic. This hypothesis is consistent with cross-sectional microanalysis studies of metal–ceramic interfaces where thin layers of substoichiometric titanium oxides have been identified. The study of titanium segregation is ideally suited to neutron reflection because of the negative scattering length of the dominant isotope. In this study we have characterised the chemical composition of three interfaces between metals and a sapphire (Al 2O 3) single crystal using specular neutron reflection. The interfaces are: Sn/sapphire, Sn/sapphire containing a thin (≈20 nm) Ti interlayer introduced by physical vapour deposition (PVD), and an interface between sapphire and a commercial Ag-Cu-Ti alloy used to braze ceramics. We have evaluated the neutron reflection data collected from the CRISP reflectometer at the ISIS facility using a multilayer model of the interface. The results indicate that the 20 nm evaporated layer cannot be distinguished from interface roughness while the interface between sapphire and the commercial braze alloy contains a thin (≈70 nm) titanium suboxide layer.

In-Situ X-Ray Diffraction and Resistivity Analysis of CoSi2 Phase Formation with and without a Ti Interlayer at Rapid Thermal Annealing Rates

AbstractIt has been demonstrated, using synchrotron radiation, that at rapid thermal annealing rates (3°C/s) the formation of CoSi2 shifts to higher temperatures when a thin Ti interlayer is placed between Co and polycrystalline Si. It has also been shown that the Ti interlayer reduces the temperature range between the start of CoSi formation and CoSi2 formation (i.e. the range over which CoSi is present). 13 nm of Co deposited by physical vapor deposition on polycrystalline Si with and without either a 2 nm or 3.4 nm interlayer of Ti was analyzed in-situ by monitoring x-ray diffraction (XRD) peak intensity as a function of temperature using monochromatic radiation from a synchrotron beam line and by monitoring resistivity as a function of temperature in a rapid thermal annealing (RTA) system. The XRD analysis indicates that the phase formation proceeds from CoSi to CoSi2 in a temperature range that decreases from about 200°C to 140°C to 115°C with pure Co, Co/2 nm Ti and Co/3.4 nm Ti films respectively. The onset of the CoSi formation increases by about 135°C and 160° for Co/ 2 nm Ti and Co/3.4 nm Ti compared to pure Co. The CoSi temperature range decreases from about 75°C in pure Co to less than 50°C in Co/Ti. In-situ RTA resistance along with in-situ XRD analysis indicates that the onset formation temperatures for CoSi are about 440°C, 575°C and 600°C and the temperatures for the completion of CoSi2 formation are about 640°C, 715°C and 715°C for Co, Co / 2 nm Ti and Co / 3.4 nm Ti films respectively. The results are consistent with the Ti interlayer acting as a diffusion barrier during the initial stages of the Co-Si reaction.

Multilevel Contact System with a Thin Silicide Reaction Controlling Interlayer

ABSTRACTTaking advantage of controllable interdiffusion and reaction processes by using a 50 nm thin Ti interlayer an annealing step in the temperature range from 450 to 475°C transforms the TiNiAg layer sequence on silicon into a stable final state, whereby a contamination free, homogeneous nanoscale NiSi contact layer arises with low values of the contact resistance. Intercalating a thin Ti layer the nickel silicide growth rate is lowered by 2 orders of magnitude. Low sheet resistance and a good bondability are preserved by the remaining Ni-Ag top layer sequence which does not interact during the contact formation process.

Reactive interdiffusion at metal–CdS and metal–CdSe interfaces

The initial stages of room temperature interface formation for reactive and unreactive metals on II–VI single crystal semiconductors have been studied using UPS, XPS, AES, Kelvin work function, and photoinduced band bending measurements. For reactive metals we see evidence for extended reacted interface formation with new chemical structure and electronic properties. For example, Cu on CdS or CdSe forms a thick (∠250 A) reacted layer as shown by the appearance of dissociated Cd and Cu–chalcogen alloy formation. Pre‐evaporation of a thin (10 A) Ti interlayer dramatically modulates many aspects of subsequent interdiffusion, including junction abruptness and stability, stoichiometry of semiconductor out‐diffusion, and the nature of intermetallic compounds that are formed. In contrast, In (unreactive) shows no evidence for chemical reaction although substantial in‐diffusion is indicated. These microscopic parameters of interface formation are related to macroscopic Schottky barrier formation.