E-beam Co-evaporation Research Articles

Electronic and thermal properties of stoichiometric Pt3Si films grown by co-evaporation

A stoichiometric Pt3Si thin-film phase has been stabilized by electron beam (e-beam) co-evaporation of controlled Pt and Si fluxes onto several different substrates. This Pt3Si phase has high electrical conductivity (≥ 106 S/m) and is inaccessible via the more conventional solid-state thermal reaction of a Pt film/Si substrate diffusion couple, a method that is ubiquitous for forming Pt-silicide films (PtSi and Pt2Si) in the semiconductor industry. The Pt3Si films exhibit finely grained columnar morphologies when grown at ambient temperature but develop a larger granular morphology at deposition temperatures exceeding 200 °C and have an electrical conductivity of ~ 1 × 106 S/m independent of substrate type and film thickness. For films deposited above 400 °C, film agglomeration and grain boundary grooving during fabrication leads to a decrease in overall film conductivity. Pt3Si films are less conductive than Pt2Si films, yet have a higher DOS population near the Fermi level as measured by valence band photoemission spectra in agreement with DOS calculations, which suggests a lower electron mobility in the Pt3Si phase. In situ high-temperature X-ray diffraction (XRD) studies indicate that a polymorphic phase transition between a monoclinic-Pt3Si and an orthorhombic-Pt3Si phase occurs ca. 225 °C. Also, the melting temperature of Pt3Si films was measured by in situ XRD to be ca. 920 °C; after cooling from melt, a small subset of Pt3Si grains resolidify with a crystallographic texture strongly oriented in the direction of the film normal.

Synthesis and thermal stability of Pt3Si, Pt2Si, and PtSi films grown by e-beam co-evaporation

Platinum silicide thin films formed via solid–solid reaction of Pt–Si interfaces play an important role as interconnects in microelectronics. This “self-aligned” silicide formation, however, is diffusion-limited and only Pt2Si and PtSi phases form. Also, their usefulness in electronic applications at high temperatures is limited due to agglomeration effects. Here, e-beam co-evaporation is presented as a means of fabricating well-defined films of Pt3Si as well as the Pt2Si and PtSi phases familiar to conventional, self-aligned silicide technology. Pt–Si films with a range of compositions (silicon atomic fractions of XSi = 0.00–0.77) were grown on r-sapphire substrates by co-deposition of independently controlled Pt and Si evaporant fluxes at 400 °C. Phase, morphology, and electronic properties were analyzed upon deposition and after vacuum annealing for 48 h at 1000 °C. As-deposited films with XSi < 0.70 are polycrystalline and at higher Si concentrations are fully amorphous. Valence spectra from Pt, Pt2Si, and PtSi films confirm previous reports and a valence spectrum is presented for the stoichiometric Pt3Si phase; these data provide experimental support for recently proposed revisions to the standard transition metal–silicon bonding model. As-deposited films, nominally 200 nm thick, are electrically conductive in the range 0.1–4.0 × 106 S/m and remain so after vacuum annealing for 48 h at 1000 °C. The phase, morphological, and electrical stabilities are attributed to the finely grained as-deposited morphologies of co-evaporated Pt silicide films, which hinder agglomeration at 1000 °C for much greater times than do morphologies of traditional, self-aligned silicides grown via solid-state reaction.

Comparison of PtSi Films Grown by Solid-State Reaction and by E-Beam Co-Evaporation: Thermal Stability in Air at 1000 °C

ABSTRACTTwo different methods were used to synthesize 200 nm thick single-phase, orthorhombic-PtSi films: (i) e-beam co-evaporation (EBC) of Pt and Si onto r-sapphire substrates and (ii) solid-state reaction (SSR) of sputtered Pt films on Si (100) wafers. Morphology, electrical conductivity, and crystalline structure were characterized for as-grown films and for films annealed in air at 1000 °C via scanning electron microscopy (SEM), 4-pt conductivity measurements, andin situX-ray diffraction (XRD). As-grown EBC films exhibit columnar grain morphology and slight (101) crystalline texture, while SSR films exhibit granular morphology with many voids and a strong (002) texture. Above 600 °C, EBC PtSi films rapidly oxidize to form crystalline Pt3Si and amorphous SiO2phases. Around 1000 °C, the Pt3Si phase melts and c-Pt grains nucleate. After air annealing for 6 h at 1000 °C, room-temperature XRD shows that the oxidized EBC films consist of Pt3Si and Pt phases within a SiO2matrix and become electrically insulating. SSR films initially form with a (002) o-PtSi orientation and above 900 °C they recrystallize to preferred (101) texture and exhibit an unchanged electrical conductivity and a stable film morphology during 48 h of air annealing at 1000 °C. Separate oxidation mechanisms are proposed for the two film types.

Influence of composition and multilayer architecture on electrical conductivity of high temperature Pt-alloy films

Abstract A multilayer film architecture has been investigated as a means to stabilize the morphology and electrical conductivity of Pt-based nanocomposite films that are exposed to temperatures up to 1200 °C in air. The multilayer architecture consists of a 150 nm thick conducting Pt-alloy electrode inserted between a 10 nm thick adhesion layer and a 50 nm thick protective capping layer on sapphire or langasite substrates. The Pt-alloy films were fabricated by e-beam co-evaporation of a Pt or Pt 83 Rh 17 source and a range of different alloying elements (Co, Ni, Cr, Rh, Ta, Ti, Nb, Al, Sn) or oxides (HfO 2 , ZrO 2 , Nb 2 O 3 , Y 2 O 3 , RuO 2 , CeO 2 , NiO, CoO, MnO 2 ). Second phase inclusions or interlayers within the Pt-alloy films provide grain boundary pinning and hinder agglomeration of Pt grains, resulting in films that retain conductivities > 1 × 10 6 S/m after annealing in air. Pt–Al 2 O 3 , Pt–HfO 2 , and nanolaminate Pt–Ni/Pt–Zr films performed the best, remaining conductive after annealing to 1050–1150 °C. For Pt–Rh/HfO 2 films, adhesion layers of Ni, Zr, Y, and CeO x yielded the highest film stability temperature. Capping layers of ZrO 2 or Y 2 O 3 did not significantly improve the film stability. Electrically conducting films that remain stable above 1000 °C in oxidizing high temperature environments have potential applications as electrodes in a wide range of technologies, including microwave acoustic sensors, MEMS devices, and fuel cells.

Electrically stable nanocomposite thin films formed by oxidation of Pt-ZrB2 nanolaminate templates

Platinum films of thickness &amp;lt;100 nm undergo agglomeration at temperatures exceeding 700 °C and lose electrically conductive pathways, making them unsuitable as stable films in high temperature electronic devices. To prevent agglomeration and create films that remain conductive above 1000 °C, a series of nanolaminate Pt-ZrB2 films comprised of Pt and ZrB2 were deposited to a total thickness of 200 nm at ambient temperature onto sapphire substrates using e-beam coevaporation. Annealing the nanolaminate films above 700 °C in air causes the ZrB2 and Pt layers to intermix, resulting in a nanocomposite Pt-ZrB2 film architecture. At temperatures between 1000 and 1300 °C in air, both monoclinic and tetragonal ZrO2 nanocrystallites are formed as indicated by x-ray diffraction, and they serve to hinder agglomeration of the Pt phase and allow for a stable network of conductive Pt-rich grains. Film electrical conductivity measurements acquired with a four-point probe indicate that the Pt-ZrB2 nanocomposite films retain conductivities in the 105–106 S/m range from 1000 to 1300 °C, and that the overall conductivity and stability depends on the Pt-ZrB2 layer thickness ratio. A nanolaminate film comprised of five 30 nm Pt/10 nm ZrB2 bilayers remained conductive after 1 h at 1300 °C. The use of a 100 nm thick amorphous Al2O3 protective capping layer grown by atomic layer deposition helps to limit oxygen diffusion into the Pt-ZrB2 films, but the films show regions of cracking and delamination of the Al2O3 layer due to stress caused by thermal expansion mismatch.

Novel co-evaporation approach for the growth of Sb doped SnO2 nanowires

Antimony doped tin oxide (Sb:SnO2) nanowires were grown by thermal and e-beam assisted co-evaporation of Sb and Sn in the presence of oxygen at a low substrate temperature of 450°C. The field emission scanning electron microscopy study revealed that the nanowires had a length and diameter of 2–4µm and 20–60nm respectively. Transmission electron microscopy study revealed the single crystalline nature of the nanowires; energy dispersive X-ray spectroscopy (EDS) and EDS mapping on the nanowires confirmed the presence of Sb doping in the nanowires. UV light detection study on the doped SnO2 nanowire films exhibited fast response and recovery time compared to undoped SnO2 nanowire films. This is an innovative and simple method to grow doped SnO2 nanowires.

Electrochromic Nb-doped WO3 films: Effects of post annealing

The Nb-doped WO3 films were deposited by e-beam co-evaporation method using ceramic WO3 targets and metal Nb slugs. The films were analyzed by glancing incident angle X-ray diffraction (GIAXRD), UV/visible spectrophotometer, electrochemical cyclic voltammetry, X-ray photoelectron spectroscopy (XPS). The as-prepared film is brown and amorphous in structure. The film has low transmission in optical visible region. The XPS results indicate that the as-deposited film is non-stoichiometric. By applying a negative potential, the as-deposited film does not show obvious electrochromic effect. However, the electrochromic properties of Nb-doped WO3 films are improved by post annealing treatment at 350, 400, and 450°C in oxygen atmosphere. The Nb-doped WO3 films transform into crystalline structure and become transparent after post annealing treatment. The energy band gap, optical modulation, and color efficiency increase with annealing temperature.

Multi-Walled Carbon Nanotube-Doped Tungsten Oxide Thin Films for Hydrogen Gas Sensing

In this work we have fabricated hydrogen gas sensors based on undoped and 1 wt% multi-walled carbon nanotube (MWCNT)-doped tungsten oxide (WO3) thin films by means of the powder mixing and electron beam (E-beam) evaporation technique. Hydrogen sensing properties of the thin films have been investigated at different operating temperatures and gas concentrations ranging from 100 ppm to 50,000 ppm. The results indicate that the MWCNT-doped WO3 thin film exhibits high sensitivity and selectivity to hydrogen. Thus, MWCNT doping based on E-beam co-evaporation was shown to be an effective means of preparing hydrogen gas sensors with enhanced sensing and reduced operating temperatures. Creation of nanochannels and formation of p-n heterojunctions were proposed as the sensing mechanism underlying the enhanced hydrogen sensitivity of this hybridized gas sensor. To our best knowledge, this is the first report on a MWCNT-doped WO3 hydrogen sensor prepared by the E-beam method.

${\rm SmBa}_{2}{\rm Cu}_{3}{\rm O}_{7-\delta}$ Coated Conductors on the IBAD-MgO Substrate Fabricated by the Ex-situ Conversion Process

We investigated the ex-situ conversion process of amorphous precursor films for SmBa2Cu3O7-delta (SmBCO) coated conductors (CC). Amorphous precursor films were first deposited on the LaMnO3/homoepi-MgO/ IBAD-MgO/Y2O3/Al2O3/Hastelloy substrate by e-beam co-evaporation and subsequently crystallized to the superconducting phase at various post-annealing temperatures in two different reduced oxygen pressures. In 100 ppm of O2 in Ar, c-axis oriented SmBCO grains with minor a-axis oriented grains were grown at 825degC whereas polycrystalline SmBCO film with some a -axis oriented grains was formed at 800degC. In the temperature region of 825 ~ 850degC, protruded spots were observed on the sample surface. In 6 ppm of O2 in Ar, SmBCO film composed of a-axis oriented SmBCO grains with partial c-axis oriented grains were obtained at 700degC but other phases were formed above 725degC. The SmBCO sample grown at 700degC exhibited Tc ,zero value of 90.2 K. However, its Jc value at 77 K was negligibly small due to the formation of a-axis oriented grains.

Engineering of FePt nanoparticles by e-beam co-evaporation

Fe50Pt50 nanoparticles were deposited on thermally oxidized Si substrates byelectron-beam co-evaporation of Fe and Pt, at substrate temperaturesTs between300 and 700 °C. The co-deposition led to the formation of drop-like, coalesced nanoparticles,chain-like structures or continuous films, the morphology being dependent onTs or the nominalthickness of the layer, f. The nanoparticles have a mean diameterDp between 3 and 45 nm, which increases with increasingf. The degree of crystallization in the ordered face centred tetragonal (fct) phase of thesamples depends strongly on the growth conditions and increases with increasingTs andf.Nanoparticles with a higher proportion of the fct phase exhibit higher coercivity, with a maximum value of∼10.3 kOe (for thespecimens prepared at 600 °C with f = 8.5 nm). Conversely, samples with a high proportion of the cubic phase are eithersuperparamagnetic or ferromagnetically soft. The thermal annealing performed on selectedsamples resulted in structural transformation as well as magnetic hardening that depended onf and Dp.

R&D of RABiTS-based coated conductors: Conversion of ex situ YBCO superconductor using a novel pulsed electron-beam deposited precursor

Pulsed electron deposition (PED), an ablation-based film growth technique similar to pulsed-laser deposition, is a relatively new method for the physical vapor deposition (PVD) of films. In the area of PVD ex situ superconductor precursor deposition, PED is potentially simpler and more versatile than its traditional e-beam co-evaporation counterpart. We have deposited near-stoichiometric YF3–BaF2–CuO PED precursors of various thicknesses in moving configuration on RABiTS substrates. Critical current densities (Jc) as high as 1.6MA/cm2 have been achieved on these precursors converted at standard slow rates of roughly 1Å/s, which has previously been necessary for high performance PVD precursors. More importantly, these as-deposited PED precursors were found to be able to tolerate aggressive conversion conditions. Consequently, simultaneous attainment of high conversion rate of nearly 8Å/s and high Jc of 1MA/cm2 has been realized.

The growth of YBCO films with high critical current at reduced pressures using theBaF2 ex situ process

The growth of 0.9–1.0 µm thick Y Ba2Cu3O7−δ (YBCO) films on biaxially textured Ni–3 at.% W (NiW) substrates using theBaF2 ex situ process was investigated at reduced pressures. By varying the water vapour pressure(PH2O), Y–BaF2–Cu–O (YBFCO) precursor films deposited by e-beam co-evaporation were converted at a reduced total pressure(Ptotal) of 50–55 Torr andconversion temperature (TS) of 740 °C for a wet conversiontime (tW) of 60 min. Criticalcurrent density (JC) values greater than 1 MA cm−2 for the thick YBCO films were obtained under the condition of varyingPH2O from low pressure to 10 Torr. The transition temperatures(TC) of the sampleswere over 90 K with ΔTC = 1.8–2.5 K. Pre-heat treatment of the precursor films onCeO2/Y SZ/Y2O3/Ni/NiW substratesunder an O2 atmosphere condition before the conversion resulted in smooth surfaces without largesecondary phase particles embedded in the films.

Phase evolution of Ba2YCu3O6+x films during the BaF2 process

High-temperature x-ray diffraction and transmission electron microscopywere used to study the phase evolution and reaction kinetics during theex situ BaF2 conversionprocess to form Ba2YCu3O6+x (Y-213). For this study, precursor films with compositions corresponding to Y-213 were deposited on singlecrystal SrTiO3 substrates using an e-beam co-evaporation technique. Two different precursors,(BaF2+Y +Cu)and (BaF2+Y F3+Cu), were used, and the phase evolution in the resulting series of samples was compared. For the(BaF2+Y +Cu) films, at intermediate stages of the conversion process, theamorphous precursor partially crystallized into a mixture of aBaF2-like oxyfluoride phase (, where BF denotes BaF2) andCuO. At temperatures close to 700 °C, Y2Cu2O5 anda BaF2-related superstructure (, , ) were observed. For this series of films, the ex situ process involved severalhydration/oxidation reactions which ultimately transformed the precursor into the Y-213phase. The overall reaction scheme can be represented as: , where Ba (OxFy) representsboth the BaF2-likeoxyfluoride and the BaF2-related superstructure phases. The growth of the Y-213 phase and the consumption ofBaF2-like phases were approximately linear in time. The resulting Y-213 films were predominantlyc-axis textured, along with smaller volume fractions ofa-axis textured and randomly oriented regions. For the(BaF2+Y F3+Cu) films investigated, we observed a similar evolution of theBa (OxFy) phaseand the BaF2-related superstructure, along with crystallization of CuO; however,Y2Cu2O5(s) was notobserved. The Y F3 component remained amorphous (from x-ray diffraction results) and reacted withBa (OxFy), CuO andH2O to form Y-213and Y2O3. The presenceof unreacted Ba (OxFy),Y2O3 and CuO in the filmafter prolonged (>2 h) annealing at 735 °C indicates incomplete Y-213 conversion. We conclude that the(BaF2+Y F3+Cu) precursor was lesssuitable relative to (BaF2+Y +Cu) under the present processing conditions.

The thermal stability of tungsten/silicon multilayered nanostructures

The influence of excimer laser processing on the thermal stability, especially on the surface roughness evolution of W 1− x Si x /Si multilayers (MLs), is studied. MLs with the compositions x=0 (W/Si) and x=0.33, 0.5 and 0.66 were evaporated by e-beam evaporation and co-evaporation onto Si substrates. The samples were irradiated by XeCl laser pulses at fluences F=0.075–0.6 J cm −2 and N=1 or 100 pulses. Then, they were studied by atomic force microscopy, thus completing previous X-ray scattering analyses. It was found that for x≥0.5 the clusters are formed at the MLs surface even in the as-deposited state. Sources of surface roughness are the laser melting itself, Si crystallization and shrinking of the volume connected with the formation of tungsten silicides.

Optical and magneto-optical properties of (CoFe)x(HfO21)1−x magnetic granular films

Granular films of composition (CoFe)x(HfO2)1−x with continuous variation of the Co0.5Fe0.5 volume fraction in the range 0.1&amp;lt;x&amp;lt;1 have been prepared by e-beam co-evaporation. The optical properties of the granular films have been determined from transmission and reflection spectra in the 300–1100 nm region. The optical characteristics of the films as a function of x were also determined by the ellipsometrical method at λ=632.8 and 830 nm. The Faraday rotation in the films was determined at λ=632.8 nm. We find that the transmission and reflection spectra and the dielectric function have anomalies near the percolation threshold at x≈0.45. The spectral dispersion of Faraday effects has a peak in the region of the plasma frequency. The Faraday effect is greatly enhanced near xp. We also find that the extraordinary Hall resistivity, measured at room temperature and at 8 kOe of applied field, increases dramatically from 0.2 to about 600 μΩ cm near the percolation threshold. These films also possess large magnetoresistance, whose values are about 4% in maximum for x=0.35–0.45. The calculated optical and magneto-optical parameters and the extraordinary Hall resistivity, described in the framework of the effective medium approximation, are in qualitative agreement with experimental data.

Study of optical and magneto-optical properties of CoFe–HfO 2 granular magnetic films

Granular films of composition (CoFe) x (HfO 2) 1− x with continuous variation of the Co 0.5Fe 0.5 volume fraction in the range 0.1< x<0.9 have been prepared by e-beam co-evaporation. The optical properties of the thin granular composite films have been determined from transmission and reflection spectra in the 300–1100 nm region. The optical characteristics of the granular films as a function of x were also determined by the ellipsometrical method at λ=632.8 nm. We find that the transmission and reflection spectra and the dielectrical function have anomalies near the percolation threshold at x p ≈0.45. The Faraday rotation in the films was determined at λ=632.8 nm. The spectral dispersion of Faraday effects have peak in the region of the plasmon frequency. The Faraday effect is greatly enhanced near x p . The calculated optical and magneto-optical parameters, described in the framework of the effective medium approximation, are in qualitative agreement with experimental data.

Coercivity in amorphous Tb–Fe alloys

The perpendicular magnetic anisotropy, coercive field, and initial magnetization curves of amorphous TbxFe1−x (x between 0.15 and 0.32) were measured at room temperature for samples prepared under a wide variety of preparation conditions, including both e-beam co-evaporation and dc magnetron cosputtering. The effect of growth temperature, annealing, and thickness were investigated. The perpendicular magnetic anisotropy shows little dependence on substrate type, sample thickness, or details of the deposition such as sputtering or e-beam evaporation, but is strongly dependent on growth temperature, increasing with increasing growth temperature up to nearly 300 °C. Coercivity on the other hand is extremely dependent on microstructure, and hence, on details of preparation, substrate type, and thickness. It is much larger in evaporated films than in similarly prepared dc magnetron sputtered films. Normalized coercivity decreases monotonically with increasing growth temperature. The dominant mechanism appears to be domain wall pinning in the bulk of the film due to columnar microstructure. High growth temperature stabilizes the material against subsequent annealing which tends to eliminate the anisotropy and, more slowly, the coercivity.

Influence of growth temperature and composition on structural disorder in YBCO thin films

Structural disorder has been measured using Raman spectroscopy for a series of films, which were grown by e-beam co-evaporation. The growth temperature and the cation stoichiometry of YBCO are essential parameters controlling the formation of structural disorder. Higher growth temperatures and higher Y fluxes produced more perfectly ordered films. Less spatial variation occurs in films with lower amounts of structural disorder.

Influence of co-evaporation technique on the structural and magnetic properties of CoCu granular films

Structural, magnetic and transport properties of granular Co x Cu 1 − x ( x = 0.11–0.45) films prepared by e-beam co-evaporation, were studied. Scanning tunneling microscopy showed the surface crystallites having an elongated shape. Magnetization and FMR data indicated a large in-plane uniaxial anisotropy in all the samples. A correlation between the orientation of the surface crystallites and the anisotropy easy axis has been found. Magnetoresistance in the fields up to 0.5 T is strongly different for transverse and longitudinal geometries.

Specific heat of amorphous rare-earth–transition-metal films

Microcalorimeters have been used to measure the temperature dependence of the specific heat ${C}_{p}(T)$ of amorphous ${R}_{x}{\mathrm{Fe}}_{100\ensuremath{-}x}$ $(R=\mathrm{Gd},\phantom{\rule{0ex}{0ex}}\mathrm{Tb})$ thin films prepared by both sputtering and e-beam coevaporation. $a\ensuremath{-}{\mathrm{Tb}}_{x}{\mathrm{Fe}}_{100\ensuremath{-}x}$ films possess large randomly oriented local magnetic anisotropy and large exchange coupling; they are considered random-anisotropy magnets. By varying growth temperature and by annealing, films of the same composition but with very different macroscopic anisotropy constant ${K}_{u}$ were prepared and studied. ${K}_{u}$ reflects the degree of nonrandomness in the local anisotropy axis directions. $a\ensuremath{-}{\mathrm{Gd}}_{x}{\mathrm{Fe}}_{100\ensuremath{-}x}$ films possess negligible local and macroscopic anisotropy. All samples show a relatively sharp peak in ${C}_{p}(T)$ at the Curie temperature ${T}_{c}$ determined by magnetization measurements, indicative of a phase transition, independent of the magnitude of ${K}_{u}.$ Effective critical exponents of $\ensuremath{\alpha}={\ensuremath{\alpha}}^{\ensuremath{'}}=\ensuremath{-}0.6$ to -0.7 and a critical amplitude ratio of 1.5--2.5 are measured for reduced temperatures down to 0.02. Nearly all possible magnetic entropy is developed below ${T}_{c},$ unlike what is seen in spin glasses. Increased growth or annealing temperature causes a small but systematic increase in ${T}_{c},$ in the inverse high-field susceptibility \ensuremath{\chi} and in the homogeneity of the sample; ${K}_{u}$ by contrast increases with growth temperature, but decreases with annealing.