Boron Films Research Articles

Growth of high current density MgB 2 films using ex-situ precursor approach

Superconducting MgB 2 films with T c=39.0 K were prepared using an ex-situ precursor approach. Precursor films of boron, ∼0.6 μm thick, were deposited onto Al 2O 3(1 0 2) substrates by electron beam evaporation; subsequent post-anneal at 890 °C in the presence of MgB 2 and Mg metal produced highly crystalline MgB 2 films. Detailed X-ray diffraction studies indicate that the film is polycrystalline with some degree of c-axis texture. A transport J c of over 4×10 6 A/cm 2 was obtained on MgB 2 films at 25 K and self-field. The higher irreversibility fields, B irr obtained from the transport measurements on MgB 2 films indicate that there is some improvement in flux pinning at lower temperatures. The details of the film growth and transport property measurements are reported.

Superconducting MgB 2 thin films prepared by chemical vapor deposition from diborane

Superconducting MgB 2 thin films have been prepared on Al 2O 3(1 −1 0 2) substrate in a two-step ex situ approach. The precursor boron films were deposited by chemical vapor deposition from B 2H 6 at ∼495 °C, followed by post-annealing in magnesium (Mg) vapor at ∼800 °C. The annealed films have an onset transition temperature of 39 K and a zero resistance temperature of about 36 K, exhibiting a metallic behavior with a residual resistance ratio, RRR=R(300 K)/R(40 K) , of 1.79. X-ray diffraction patterns indicate a highly c-axis-oriented crystal structure perpendicular to the substrate surface. Scanning electron microscopy shows the films consist of well-crystallized MgB 2 grains and smooth surface. Having the advantage to prepare superconducting MgB 2 thin films in large area and compatible with current semiconductor technology, our method may open new perspective for applications of MgB 2 films.

Synthesis and properties of c-axis oriented epitaxial MgB2 thin films

We report the growth and properties of epitaxial MgB2 thin films on (0001) Al2O3 substrates. The MgB2 thin films were prepared by depositing boron films via radio-frequency (rf) magnetron sputtering, followed by a postdeposition anneal at 850 °C in magnesium vapor. X-ray diffraction and cross-sectional transmission electron microscopy reveal that the epitaxial MgB2 films are oriented with their c-axis normal to the (0001) Al2O3 substrate with a 30° rotation in the (0001) plane with respect to the substrate. The critical temperature was found to be 35 K and the anisotropy ratio, Hc2∥/Hc2⊥, was about 3 at 25 K. The critical current densities at 4.2 and 20 K (at 1 T perpendicular magnetic field) are 5×106 and 1×106 A/cm2, respectively. The controlled growth of epitaxial MgB2 thin films opens a new avenue in both understanding superconductivity in MgB2 and technological applications.

A New Method for Preparing Superconducting MgB2 Films fromDiborane

We report on a new preparation method for magnesium diboride (MgB2)films by chemical vapour deposition (CVD) from diborane (B2H6).It is a two-step ex situ approach, with the precursor boron films grownby CVD from B2H6 at 460°C, followed by apost-annealing process in magnesium (Mg) vapour at 830°C. Theprepared MgB2 thin films on Al2O3 polycrystallinesubstrates have an onset transition temperature of 35 Kand a zero-resistance temperature of about 24 K. Well-crystallizedMgB2 grains have clearly been observed in the SEM images andconfirmed by x-ray diffraction analysis. The advantages of the proposedmethod are the feasibility to prepare large-area superconducting filmsand the compatibility with semiconductor technology.

Insights into the Nature of SiH4−BH3Complex: Theoretical Investigation of New Mechanistic Pathways Involving SiH3•and BH4•Radicals

We have investigated a new type of interaction between silane (SiH4) and borane (BH3) using high level ab initio calculations. The SiH4−BH3 complex is found to be extremely stable with the formation of a bridged hydrogen bond between SiH4 and BH3. Hence, it might have a hitherto unknown role in the mechanism of chemical vapor deposition (CVD), which is employed in the fabrication of boron doped silicon semiconductor materials. In an attempt to unravel this role and the underlying reasons responsible for the stability of this complex, we have carried out a detailed analysis based on the structure and molecular orbitals. The results indicate that the binding strength and electronic character of the bridged hydrogen bond in the SiH4−BH3 complex is between those observed in double hydrogen bridged B2H6 and mono-hydrogen bridged anion B2H7-. In contrast to B2H6 and B2H7-, it should be noted that the complex is stabilized by direct strong electrostatic interaction between the positively charged Si atom and the negatively charged B atom as well as by monobridged hydrogen bonding. Furthermore, we also note that the SiH4−BH3 complex would dissociate to form SiH3• and BH4• radicals. The existence of this complex is also inferred from good agreement between the calculated IR spectra and experimentally observed spectra of thin solid films of boron doped silicon semiconductor materials.

Buried superconducting layers comprised of magnesium diboride nanocrystals formed by ion implantation

Superconducting layers of MgB2 were formed on Si substrates using techniques that are widely used and accepted in the semiconductor industry. Mg ions were implanted into boron films deposited on Si or Al2O3 substrates. After a thermal processing step, buried superconducting layers comprised of MgB2 nanocrystals were obtained which exhibit the highest Tc reported so far for MgB2 on silicon (Tconset≈33.6 K, ΔTc=0.5 K, as measured by current transport). These results show that our approach is clearly applicable to the fabrication of superconducting devices that can be operated at much higher temperatures (≈20 K) than the current Nb technology (≈6 K) while their integration with silicon structures remains straight-forward.

Phase transformation of high speed steel after sequential nitrogen and boron high current density ions implantation

The present work continues the study of double high current ion implantation (HCII) of nitrogen and boron into AISI M2 high speed steel. The additional investigations of phase-structure state by conversion electron Mössbauer spectroscopy, glancing X-ray diffraction, Auger electron spectroscopy and tribological properties of steel were carried out. HCII of nitrogen is found to form precipitates of ε-Fe 2+ x (N,C) ( x=0.67, 1) and α″-Fe 16(N,C) 2 carbonitrides and causes strong sputtering of surface. Boron HCII forms boron film (20 nm in thickness) on the surface of steel. The best tribological properties were revealed for the N+B variant of HCII. The 2.2 times improvement of wear-resistance was reached because of surface boron films and carbonitrides enriched sublayer influence.

Preparation of double layer film of boron and carbon by pulsed laser deposition

Double layer films of boron and carbon were prepared by pulsed laser deposition (PLD) on silicon substrates. The film surface morphology was examined by atomic force microscopy (AFM). The chemical composition of the elements, carbon, boron, silicon and oxygen, and bonding state of carbon atoms as a function of the depth from the film surface were analyzed by X-ray photoelectron spectroscopy (XPS) with argon ion sputtering technique. Carbon atoms bonded to boron (CB) were observed as well as carbon atoms bonded to carbon (CC).

Fabrication of MgB 2 superconducting thick films by electrophoresis technique

MgB 2 superconducting thick films of about 40 μm were fabricated successfully by a simple and low-cost approach. Boron films were prepared on Ag/Al 2O 3 substrates using the electrophoresis technique followed by a post-annealing process with excessive magnesium. Resistance measurements of the MgB 2 thick films show T con of 39 K and ΔT c of 0.6 K. The X-ray diffraction analysis reveals that most part of the films consist of c-oriented crystalline MgB 2. The images of surface morphology and cross-section profiles by scanning electron microscopy show well structured grain boundary MgB 2 films and a well-defined compact layer.

Detection of CHx bonds from micro Raman spectroscopy on polycrystalline boron doped diamond electrodes

MicroRaman signals excited at 457.8 nm have been obtained from some areas of heavily boron doped polycrystalline diamond films which have been used for electrochemical reactions. We could identify sp3 CH, CH2, CH3, all with narrow full width at half maximum. From comparisons to IR absorption from the bulk of undoped films and from the (111) and (100) surfaces of IIa crystal, and to Raman signals from the bulk of undoped polycrystalline films, we ascribed our micro Raman signals to CHx from the film surface. The sp3 CH2 and CH3 components appear more stable than the sp3 CH under CW laser exposure. This implies chemical bonding of H with C atoms of the film surface for all the sp3 CHx sites. This difference with the CHx at the surface of IIa crystals might originate from the heavy boron-doping of our films.

Electrical Properties of Boron and Phosphorus Doped Polycrystalline Silicon Germanium Films

Article Electrical Properties of Boron and Phosphorus Doped Polycrystalline Silicon Germanium Films was published on December 1, 2002 in the journal International Journal of Nonlinear Sciences and Numerical Simulation (volume 3, issue 3-4).

Fabrication and Transport Properties of MgB 2 Thin Films and Tunnel Junctions

We report on fabrication and characterization of MgB 2 thin films and tunnel junction structures. The MgB 2 films were prepared on Al 2 O 3 , Si, glass, and plastic foil substrates by either vacuum codeposition of boron and magnesium, or high-temperature magnesium annealing of boron films. The crystalline structure of our films depended directly on the method of preparation. The films prepared by codeposition and postannealed in Ar atmosphere were amorphous with nanocrystal inclusions and were characterized by very smooth surfaces. On the other hand, the boron-precursor films annealed in magnesium vapor were rough, polycrystalline with approximately 1-μm-diameter single-crystal blocks. Because of their surface quality, the amorphous films were used for preparation of point contact junctions and for optical characterization. The point-contact spectra of tested junctions exhibited a two-gap structure. The MgB 2 polycrystalline films was used for bulk transport studies. The best films were characterized by the critical temperature T c of up to 39 K and the current density j c at 4.2 K of about 10 7 A/cm 2 .

Cubic BN formation by ion implantation

Boron nitride was synthesized by nitrogen ion implantation. Boron films were prepared as implantation targets on single-crystal Si(100) substrates by 13.56-MHz radio frequency sputtering. The diatomic single 30-keV nitrogen ions were chosen for implantation. The implantation dose ranged from 1×10 17 ions/cm 2 to 2×10 18 ions/cm 2. The films were characterized using a transmission electron microscope. Several phases of boron nitride were found at the medium implantation dose. At the high dose of 2×10 18 ions/cm 2, the pure c-BN phase was observed. It is believed that the transition from the low ordered phases to c-BN phase occurred during implantation. The films showed good adhesion to the Si substrate.

Effect of nitrogen ion implantation on the microstructural transformation of boron film

The structural transformation by nitrogen ion implantation into boron film as a function of film thickness, implant voltage and nitrogen ion dose was systematically investigated. Boron films were deposited on single crystal Si substrates using the electron beam evaporation of pure metallic boron, the thickness being 200, 500, 1000 and 2000 Å, respectively. The films were then nitrogen ion-implanted at 25-, 50- and 75-kV implant voltages applied to substrate, respectively, for a fixed nitrogen ion dose of 3×10 17 cm −2, and at 1×10 17, 2×10 17 and 3×10 17 cm −2 nitrogen ion doses, respectively, for a fixed implant voltage of 50 kV. The nitrogen ion-implanted boron films were characterized by Fourier transform infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). XPS results showed that the nitrogen ion-implanted boron films were nitrogen-deficient. Boron exists in the form of both metallic boron and boron nitride as indicated by XPS B1s bands. FT-IR spectra revealed that amorphous boron nitride (a-BN) or hexagonal boron nitride (h-BN) formed during nitrogen ion implantation into boron films. As implant voltage and nitrogen ion dose increased, amorphous BN was initially formed, while h-BN occurred at greater implant voltage and nitrogen ion dose. The thinner film led to the formation of h-BN at a relatively low implant voltage and nitrogen ion dose.

Significant reduction of the microwave surface resistance of MgB2 films by surface ion milling

The microwave surface resistance RS of MgB2 films with the zero-resistance temperature of ∼39 K was measured at 8.0–8.5 GHz. The MgB2 films were prepared by deposition of boron films on c-cut sapphire, followed by annealing in a magnesium vapor environment. The RS appeared significantly reduced by ion milling of the as-grown MgB2 film surface, with the observed RS of ∼0.8 mΩ at 24 K for an ion-milled MgB2 film as small as 1/15 of the value for the corresponding as-grown MgB2 film. The reduced RS of the ion-milled MgB2 films is attributed to the effects of the Mg-rich metallic layer existing at the surfaces of the as-grown MgB2 films.

MgB 2 superconducting thin films on Si and Al 2O 3 substrates

Thin films of the superconductor MgB 2 were prepared by three different procedures on sapphire and silicon substrates. Boron thin films, ex situ annealed in magnesium vapour, resulted in textured polycrystalline films with crystal dimensions below about 1 μm, onset critical temperature T c on near 39 K and width of phase transition ΔT⩽1 K. Both, ex situ and in situ annealed co-deposited boron and magnesium thin films on sapphire and silicon substrates give smooth nanocrystalline films. DC properties of nanocrystalline MgB 2 films co-deposited on silicon substrate reached T c on =33 K and zero resistance T c0=27 K, the highest values received until now on Si substrates. In addition, microwave analyses prove the existence of unconnected superconducting parts of the film below 39 K. This result confirms the possibility to synthesize nanocrystalline superconducting MgB 2 thin films on silicon substrate with critical temperature near 39 K, prepared by vacuum co-deposition of boron and magnesium films.

Monitoring Dielectric Thin-Film Production on Product Wafers Using Infrared Emission Spectroscopy

Monitoring of dielectric thin-film production in the microelectronics industry is generally accomplished by depositing a representative film on a monitor wafer and determining the film properties off line. One of the most important dielectric thin films in the manufacture of integrated circuits is borophosphosilicate glass (BPSG). The critical properties of BPSG thin films are the boron content, phosphorus content, and film thickness. We have completed an experimental study that demonstrates that infrared emission spectroscopy coupled with multivariate analysis can be used to simultaneously determine these properties directly from the spectra of product wafers, thus eliminating the need for producing monitor wafers. In addition, infrared emission data can be used to simultaneously determine the film temperature, which is an important film production parameter. The infrared data required to make these determinations can be collected on a time scale that is much faster than the film deposition time, hence infrared emission is an ideal candidate for an in situ process monitor for dielectric thin-film production.

Superconducting MgB2 thin films by pulsed laser deposition

Growth of MgB2 thin films by pulsed laser deposition is examined under ex situ and in situ processing conditions. For the ex situ process, boron films grown by pulsed laser deposition were annealed at 900 °C with excess Mg. For the in situ process, different approaches involving ablation from a stoichiometric target under different growth conditions, as well as multilayer deposition involving interposed Mg layers were examined and analyzed. Magnetic measurements on ex situ processed films show Tc of ∼39 K, while the current best in situ films show a susceptibility transition at ∼22 K.

Plasma-deposited hydrogenated carbon–germanium semiconducting films doped with acceptor and donor centers

Hydrogenated amorphous carbon–germanium (a-Ge X C Y :H) films were doped with acceptors (boron, aluminum and gallium) and donors (phosphorous). The films were prepared by plasma enhanced chemical vapor deposition (PECVD), using an r.f. (13.56 MHz) reactor with a coplanar electrode geometry. Investigations on electrical conductivity and optical absorption were carried out. All used dopants caused a maximal increase in the conductivity of approximately two orders of magnitude. Based on more thorough examinations performed on the boron and phosphorous-doped films, it has been suggested that both acceptor and donor dopands modify only p-type conductivity of the undoped films and any transition to n-type is not observed. Boron creates an additional band of acceptors whereas phosphorous eliminates the oxygen donor centers reducing thereby the compensation of the native acceptors.

Superconducting MgB2 films via precursor postprocessing approach

Superconducting MgB2 films with Tc=38.6 K were prepared using a precursor-deposition, ex situ postprocessing approach. Precursor films of boron, ∼0.5 μm thick, were deposited onto Al2O3 (102) substrates by electron-beam evaporation; a postanneal at 890 °C in the presence of bulk MgB2 and Mg metal produced highly crystalline MgB2 films. X-ray diffraction indicated that the films exhibit some degree of c-axis alignment, but are randomly oriented in plane. Transport current measurements of the superconducting properties show high values of the critical current density and yield an irreversibility line that exceeds that determined by magnetic measurements on bulk polycrystalline materials.