SiC Buffered Si Substrates Research Articles

Comparison of critical properties in MgB2 nanometer films prepared on SiC/Si substrate

Critical superconducting properties in 100nm MgB2 film are investigated and examined in comparison with our previous results in 10nm and 50nm films. MgB2 films were prepared by sequential evaporation of boron and magnesium on SiC-buffered Si substrate followed by in situ annealing. The amount of supplied boron was controlled so as to result in the required MgB2 film thickness with excess Mg top layer. The superconducting transition temperature Tc is estimated to be 33.1K from the onset of AC diamagnetic susceptibility. The upper critical field Hc2 is also estimated from the AC susceptibility. Their temperature dependences are almost linear with |dHc2/dT|∼5.6kOe/K for the parallel magnetic field and |dHc2/dT]~3.9kOe/K for the perpendicular field. These values are larger than those in our previous 50nm film (Tc of 34.5K) in spite of a little lower transition temperature. The critical current density Jc and irreversibility field Hirr are estimated from DC magnetization hysteresis. Scaling behavior of Hirr ~ [1 – (T/Tc)2]n is examined in comparision to our previous studies in 10nm and 50nm films with different critical exponents n.

The distribution of elements in sequentially prepared MgB2 on SiC buffered Si substrate and possible pinning mechanisms

MgB2 thin films are prepared by sequential evaporation of boron and magnesium bilayers on SiC buffered Si substrates followed by an in situ annealing. Precursor Mg–B bilayers are deposited by electron beam evaporation at room temperature. The amount of B is varied so as to result in different thickness (15nm and 50nm) of stoichiometric MgB2 final film after an in situ reaction with the excess Mg top layer in the vacuum. We show the distribution of the elements through the film.X-ray photoelectron spectroscopy analyses have shown that carbon is not free in the films (except the surface of the film) and silicon is in the compound form, too. In the case of the 15nm thick films we see a strong interdiffusion of the elements (C, B) and we observe a suppression of TC of the film to 20K. We register different slope of the HC2(T) dependence – the lowest temperature value of HC2 for the 15nm thick film exceeds the one for the 50nm thick film in spite of lower TC. We suppose that δl pinning mechanism is dominant for the 15nm thick film.

Examination of the Scaling Behavior of Critical Properties in MgB2/SiC/Si Thin Films

Abstract Scaling behaviors of critical current densities in MgB2 thin films are investigated with different film thickness. MgB2 films were grown on SiC buffered Si substrate by sequential evaporation of boron and magnesium. The amount of supplied boron was controlled so as to result in MgB2 film thickness of 50 nm or 10 nm with excess magnesium. The critical current density J c and irreversibility field H irr were estimated from magnetic field dependence of DC magnetization hystereses. Variation of J c is analyzed against temperature at each value of constant magnetic field. For the 50 nm film J c could scale with [1 − ( T / T c)2] m’ with critical exponents m’ from 4 to 8. However, variation of m’ is interpreted as only in appearance and as due to low-field approximation assumed in this simplified scaling. More comprehensive scaling formula for reduced critical current density has been applied in wide range of temperature and field, and good fitting to our experimental data has been obtained over 10 orders of magnitude. On the other hand, for the 10 nm film, experimental data of the reduced current density indicated a kink in the middle temperature and field range, and fitting with the formula was poor, inferring weak pinning. Various scaling behaviors of J c and H irr are examined in relation to film quality, pinning strength and nature of superconductivity.

Investigation of critical properties in MgB2/SiC/Si thin films prepared under varied conditions

Critical magnetic properties of MgB2 thin films grown on SiC/Si substrate are investigated with different preparation conditions. MgB2 films were prepared by sequential evaporation of boron and magnesium on SiC buffered Si substrate. The substrate temperature and the amount of supplied boron was varied and influence of varied conditions is investigated. The maximum superconducting transition temperature obtained is 34.5 K. The upper critical field Hc2 is estimated from the onset of AC diamagnetic susceptibility. The temperature dependence is generally linear, being fitted with |dHc2/dT| around 3 ∼ 7 kOe/K depending on the substrate temperature during in situ annealing as well as the magnetic field direction. The critical current density is evaluated from DC magnetization hysteresis to be more than 106 A/cm2 at lower temperatures. Irreversibility field scales with [1 — (T/Tc)2]n with critical exponents n of 3 ∼ 7. Correlations among critical properties, crystal qualities and sample preparation conditions are discussed.

Investigation of critical behaviour of MgB2 thin films on SiC/Si substrate

Critical magnetic behaviour of MgB2 thin films grown on SiC-buffered Si substrate was investigated in comparison with MgB2 films on NbN-buffered Si substrate. MgB2 films were prepared by sequential evaporation of boron and excess magnesium followed by in situ annealing in an Ar atmosphere. The upper critical field Hc2 estimated from the onset of AC diamagnetic susceptibility was larger in films with SiC buffer than those with NbN buffer. The temperature dependence was almost linear with |dHc2/dT| up to 8 kOe/K, especially under parallel magnetic field to the film surface. Although the critical current density evaluated from DC magnetization hysteresis approached 1 MA/cm2 at the lowest temperatures, the maximum value was smaller than that in MgB2/NbN/Si. The irreversibility field also inferred effects of weak links, possibly due to some impurity phases.

Nanogranular MgB2 thin films on SiC buffered Si substrates prepared by an in situ method

MgB2 thin films were deposited on SiC buffered Si substrates by sequential electronbeam evaporation of a B–Mg bilayer followed by in situ annealing. Theapplication of a SiC buffer layer enables the maximum annealing temperature of830 °C to be reached. Transmission electron microscopy analysis confirms the growth of a nanogranularMgB2 film and thepresence of a Mg2Si compound at the surface of the film. The 150–200 nm thickfilms show a maximum zero resistance critical temperatureTC0 above 37 K and acritical current density JC∼106 A cm−2 at 11 K.

Investigation of lower critical fields of MgB2 thin films on SiC/Si substrate with parallel magnetic fields

Lower critical fields Hc1 of MgB2 thin films grown on SiC-buffered Si substrate were investigated with magnetic fields applied parallel to the film surface. MgB2 films were prepared by sequential evaporation of boron and magnesium on SiC/Si substrate. The amount of supplied boron was adjusted so as to result in 200nm stoichiometric MgB2 film after reaction with the excess Mg top layer. In order to estimate Hc1, the film was zero-field cooled, and the initial (virgin) magnetization was measured with increasing field. Square root of magnetic deviation from the perfect diamagnetism was plotted against the applied field, and Hc1 was evaluated from extrapolation analyses. The temperature variation can be fitted by the linear dependence with the temperature derivative |dHc1∥/dT| of 2.4Oe/K in MgB2/SiC/Si which is smaller than that of 5.2Oe/K in MgB2/NbN/Si. The difference was examined in relation to the Ginzburg–Landau (GL) parameter, which was estimated to be about 86 with the SiC buffer under the parallel field.