M. C. Wood

The influence of Mg doping on the materials properties of Ba1−xSrxTiO3 thin films for tunable device applications

Abstract We have investigated the structural, microstructural, interfacial, and surface morphological properties of Ba 0.60 Sr 0.40 TiO 3 thin films Mg doped from 0 to 20 mol%. A strong correlation was observed between the films materials properties and the prior determined dielectric and insulating characteristics as a function of Mg doping. Non-textured polycrystalline films with a dense microstructure and abrupt film-Pt electrode interface were obtained after annealing at 750°C for 30 min. Single phase solid solution films were achieved at Mg doping levels up to 5 mol%, while multiphased films were obtained for Mg doping levels of 20 mol%. Decreases in the films dielectric constant, dielectric loss, tunability and leakage current characteristics were paralleled by a reduction in grain size as a function of increasing Mg dopant concentration. Our results suggest that Mg doping serves to limit grain growth and is thereby responsible for lowering the dielectric constant from 450 to 205. It is suggested that Mg behaves as an acceptor-type dopant at the grain boundary and is responsible for the doped films low dielectric loss and good leakage current characteristics. Examination of the performance-property trade-offs advocates the 5-mol% Mg doped BST film to be an excellent choice for tunable microwave device applications.

Structure-property relationships in pure and acceptor-doped Ba1-xSrxTiO3 thin films for tunable microwave device applications

The influence of low concentration (1 mol %) Mg doping on the structural, microstructural, surface morphological, and dielectric properties of Ba1−xSrxTiO3 (BST) thin films has been measured and analyzed. The films were fabricated on MgO and Pt–Si substrates via the metalorganic solution deposition technique using carboxylate–alkoxide precursors and postdeposition annealed at 800 °C (film/MgO substrates) and 750 °C (film/Pt–Si substrates). The structure, microstructure, surface morphology, and film/substrate compositional quality were analyzed by glancing angle x-ray diffraction, field emission scanning microscopy, atomic force microscopy, and Auger electron spectroscopy studies. Dielectric properties of unpatterned films were measured at 10 GHz using a coupled, tuned split dielectric resonator system, and at 100 kHz using metal–insulator–metal capacitors. The Mg-doped BST films exhibited improved dielectric and insulating properties compared to the undoped Ba0.6Sr0.4TiO3 thin films. The improved dielectr...

Chemical Analysis of Graphite/Electrolyte Interface Formed in LiBOB-Based Electrolytes

To understand the source of thermal stability of LiBOB-based electrolyte in lithium-ion cells as well as its unique ability to stabilize graphitic anodes even in the strongly exfoliating solvent propylene carbonate (PC), the solid electrolyte interface on graphite formed by LiBOB-based electrolyte was investigated by X-ray photoelectron spectroscopy. Preliminary results show that, due to the BOB anion presence, the content of semicarbonate-like components in the graphite/electrolyte interface increases significantly, as indicated by the conspicuous peak located at 289 eV. These components, believed to originate from the oxalato moiety of the anion, are mainly responsible for the protection of graphitic anodes, either at elevated temperatures or in the presence of PC.

Electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au multilayer Ohmic contacts to n-type GaN

The electrical, thermal, and microstructural characteristics of Ti/Al/Ti/Au (30 nm/100 nm/30 nm/30 nm) multilayer Ohmic contacts to n-GaN (doping level 5×1017 cm−3) were studied. The lowest contact resistivity derived from the annealed contact was ρS=3.0×10−6Ω cm2. The contacts were robust and showed high-thermal stability. X-ray diffraction and Auger electron spectroscopy studies were made to investigate the microstructure of the annealed contacts. The key to the success of the contact was the Ti layers placed on both sides of the Al layer. Upon annealing, there occurred both in-diffusion and out-diffusion of the Ti layer in intimate contact with the GaN film. The in-diffusion of this led to the formation of TiN, while the out-diffusion of this led to the formation of Ti–Al alloys. The second Ti layer also in-diffused and out-diffused during annealing. However, due to the presence of Au, the out-diffusion was marginalized, and the in-diffusion was higher than the out-diffusion. The in-diffusion led to th...

Improved Ni based composite Ohmic contact to n-SiC for high temperature and high power device applications

Ni/WSi/Ti/Pt Ohmic contacts to n-SiC were investigated as a function of annealing temperatures up to 1000 °C. Annealing at temperatures between 950 and 1000 °C yielded excellent Ohmic behavior. At these temperatures the contact–SiC interface was smooth, defect free, and characterized by a narrow Ni2Si reaction region. The annealed contacts possessed atomically smooth surface morphologies and exhibited minimal contact expansion. The residual carbon, resultant from SiC decompositon and reaction with Ni to form Ni2Si, was constrained by reaction with the WSi and Ti layers forming carbide phases of W and Ti spatially distant from the metal semiconductor interface. Our results demonstrate that the Ni/WSi/Ti/Pt composite Ohmic contact maintains the desirable electrical properties associated with Ni contacts and possesses excellent interfacial, compositional, and surface properties which are required for reliable high power and high temperature device operation.

Evaluation of Ta2O5 as a buffer layer film for integration of microwave tunable Ba1-xSrxTiO3 based thin films with silicon substrates

Successful integration of Ba1−xSrxTiO3 (BST) based thin films with affordable Si substrates has a potential significant commercial impact as the demand for high-frequency tunable devices intensifies. This work evaluates the material properties of postdeposition annealed Ta2O5 thin films fabricated via the metalorganic solution technique for use as a passive buffer layer between BST and Si. The microstructure, surface morphology, and interfacial properties of the Ta2O5 film deposited on n+-Si and PtSi substrates was evaluated via field-emission scanning electron microscopy, Rutherford backscattering spectrometry, Auger electron spectroscopy, and atomic force microscopy. The presence of a transition interaction zone between the Ta2O5 film and Si substrate was detected after annealing at temperatures >600 °C. The width of the interaction zone increased with increasing annealing temperature reaching a maximum of 29 nm at 750 °C. The width of the interaction zone was also found to be dependent on the Ta2O5 fil...

AlN as an encapsulate for annealing SiC

AlN films grown by either organometallic vapor phase epitaxy (OMVPE) or pulsed laser deposition (PLD) can be used to encapsulate SiC when heated in an argon atmosphere at temperatures at least as high as 1600 °C for times at least as long as 30 min. The coverage of the AlN remains complete and the AlN/SiC interface remains abrupt as determined by Auger electron spectroscopy. However, considerable atomic movement occurs in the AlN at 1600 °C, and holes can form in it as the film agglomerates if there are large variations in the film thickness. Also, the SiC polytype near the surface can in some instances be changed possibly by the stress generated by the epitaxial AlN film. Using x-ray diffraction measurements, we also found that, during the 1600 °C anneal, grains with nonbasal plane orientations tended to grow at the expense of those with basal plane orientations in the OMVPE films, whereas grains with only the basal plane orientation tended to grow in the PLD films. However, there is no indication that t...

High-transparency Ni/Au bilayer contacts to n-type GaN

A unique metallization scheme has been developed for obtaining both Schottky and low-resistance Ohmic contacts to n-GaN. It has been demonstrated that the same metallization can be used to make both Schottky and Ohmic contacts to n-GaN using a Ni/Au bilayer composite with Ni in contact to GaN. Using this metallization, contacts with a specific contact resistivity, ρs, as low as 6.9×10−6 Ω cm2 for a doping level of 5.0×1017 cm−3 was obtained after annealing the sample for 10 s at 800 °C in a rapid thermal annealer. The presence of only (111)Au and (111)Ni peaks in the x-ray diffraction (XRD) pattern of as-deposited samples indicates that both metals participate to form epitaxial or highly textured layers on the basal GaN plane. When the contact layer is annealed, Au and Ni react with GaN creating interfacial phases. Both XRD and transmission electron microscopy confirm that Ni3Ga and Ni2Ga3 intermetallic phases together with Au and Ni based face-centered-cubic solid solutions, are formed during annealing. ...

Annealing ion implanted SiC with an AlN cap

Abstract An AlN cap was used to try to prevent the preferential evaporation of Si during the high temperature anneals required to activate N implanted into a SiC substrate. The process was essentially successful as the electrical measurements showed that the resistivity continued to decrease with increasing annealing temperatures up to 1600°C and times up to 120 min. The changes were, however, marginal when compared to a 1500°C, 30 min anneal suggesting that this anneal would be sufficient to activate most of the N implants. There is evidence for a small amount of Si being lost near the surface. This could occur where the AlN pulled away locally from the SiC wafer; this effect was stronger for patterned substrates where stress concentrations can occur at steps. For the most part, however, the SiC surface retained its integrity even during the process of removing the AlN film with a hot KOH etch. Also, there was no evidence that Al from the AlN contaminated the N implanted region by diffusing in during the anneals. The surface of the AlN retained its integrity during the anneal although topographical changes suggested that considerable atomic motion had occurred. This coincided with the formation of an amorphous AlN layer in the film.

Approach to optimizing n-SiC Ohmic contacts by replacing the original contacts with a second metal

Nickel (Ni) contacts to n-type silicon carbide (n-SiC) have good electrical properties, but the physical contact, and therefore the reliability, is poor. An approach is described for using the good electrical properties of Ni Ohmic contacts, while using another metal for its superior topological, mechanical, thermal, or chemical properties for the chosen application. In the present work, we show that once the Ni contact has been annealed, forming nickel silicides, it can be etched off and replaced by a second metal chosen for its desired properties. This second metal displays an as-deposited contact resistance as low as the original annealed Ni contact, indicating that the critical feature responsible for Ohmic contact formation has not been removed when the silicide phases were etched off. Not only does this approach provide more flexibility for optimizing the contact for a given application but it also provides some insight into the Ohmic contact formation mechanism.