SiC Doping Research Articles

Nanoscale-SiC doping for enhancing Jc and Hc2 in superconducting MgB2

The effect of nanoscale-SiC doping of MgB2 was investigated in comparison with undoped, clean-limit, and Mg-vapor-exposed samples using transport and magnetic measurements. It was found that there are two distinguishable but related mechanisms that control the critical current-density-field Jc(H) behavior: increase of upper critical field Hc2 and improvement of flux pinning. There is a clear correlation between the critical temperature Tc, the resistivity ρ, the residual resistivity ratio RRR=R(300K)∕R(40K), the irreversibility field H*, and the alloying state in the samples. The Hc2 is about the same within the measured field range for both the Mg-vapor-treated and the SiC-doped samples. However, the Jc(H) for the latter is higher than the former in a high-field regime by an order of magnitude. Mg vapor treatment induced intrinsic scattering and contributed to an increase in Hc2. SiC doping, on the other hand, introduced many nanoscale precipitates and disorder at B and Mg sites, provoking an increase of ρ(40K) from 1μΩcm (RRR=15) for the clean-limit sample to 300μΩcm (RRR=1.75) for the SiC-doped sample, leading to significant enhancement of both Hc2 and H* with only a minor effect on Tc. Electron energy-loss spectroscope and transmission electron microscope analysis revealed impurity phases: Mg2Si, MgO, MgB4, BOx, SixByOz, and BC at a scale below 10nm and an extensive domain structure of 2–4-nm domains in the doped sample, which serve as strong pinning centers.

Nitrogen Doping of Epitaxial SiC: Experimental Evidence of the Re-Incorporation of Etched Nitrogen during Growth

Nitrogen Doping of Epitaxial SiC: Experimental Evidence of the Re-Incorporation of Etched Nitrogen during Growth

Critical Current Densities of Powder-In-Tube (PIT)-Processed MgB<SUB>2</SUB> Tapes

We fabricated MgB2 tapes by a powder-in-tube method. Reacted MgB2 powder (ex situ method) or Mg(MgH2)+B mixed powder (in situ method) were used as the starting powder. We used stainless steel, carbon steel, Cu-Ni, and Cu tubes for the ex situ method. Jc and the anisotropy in Jc with respect to the field orientation of ex situ-processed tape rapidly increased with increasing mechanical hardness of the tube. Annealing after cold rolling significantly enhanced the Jc values for the stainless steel and carbon steel tubes. For the in situ process, pure Fe tubes were used as a sheath material. The superconducting properties, such as Tc, Jc, and Birr, of in situ-processed tapes were sensitive to oxygen contamination during heat treatment. Heat treatment under a closed Ar gas atmosphere was effective in suppressing the oxidation of the tapes and enhancing the Jc values. The Jc value at 4.2 K of the in situ-processed tape was significantly enhanced by SiC doping. At 20 K, on the other hand, little Jc increase was observed due to the degradation of Tc by SiC doping. Typical Jc values of 108 A/m2 at 4.2 K in 12 T and 1.4 × 108 A/m2 at 20 K in 4 T were obtained for in situ-processed tapes heat-treated under a closed Ar gas atmosphere. Birr of in situ-processed tape was much higher than that of the as-rolled ex situ-processed tape.

Implantation-induced defects in silicon carbide

The electrical activation of N and P donors in C/N-, Si/N-, C/P- and Si/P-coimplanted and subsequently annealed 4H–SiC samples is investigated by Hall effect. It turns out that doping by coimplantation, which is a process far apart from thermal equilibrium, is different from doping of SiC during growth by the chemical vapor deposition, which is largely governed by the site-competition effect. In case of N and Si coimplantation, the formation of thermally stable and electrically neutral VSi(VC)4 complexes is proposed to serve as a noticeable sink for N atoms. By illumination with photons of different energy, the electrical properties of the dominating intrinsic-related defect centers (E1/E2−/+-, Z1/Z2−/+- and Z1/Z2(3C)) are investigated in 6H–, 4H– and 3C–SiC by deep level transient spectroscopy. In 4H– and 6H–SiC, negative-U-centers are observed. The optical ionization energy of the Z1/Z2−/0-center in 4H–SiC is determined.

Effect of sample size on the magnetic critical current density in nano-SiC dopedMgB2superconductors

The effect of sample size on the critical current density and the flux pinning of pure and SiC doped ${\mathrm{MgB}}_{2}$ bulk samples has been investigated. At high fields a systematic degradation of magnetic ${J}_{c}$ and ${H}_{\mathrm{irr}}$ was observed as the sample size decreased. However, ${J}_{c}$ remarkably increased on decreasing the sample volume at low magnetic fields below 1 T. The SiC doped samples show less sample size effect than the pure samples, indicating a larger $n$ factor and therefore a stronger pinning effect due to SiC doping. ${H}_{\mathrm{irr}}$ was observed to decrease as a logarithmic function of the sample volume, and the zero field ${J}_{c}$ can be fitted as an exponential decay function.

Superconductivity, critical current density, and flux pinning in MgB2−x(SiC)x/2 superconductor after SiC nanoparticle doping

We investigated the effect of SiC nanoparticle doping on the crystal lattice structure, critical temperature Tc, critical current density Jc, and flux pinning in MgB2 superconductor. A series of MgB2−x(SiC)x/2 samples with x=0–1.0 were fabricated using an in situ reaction process. The contraction of the lattice and depression of Tc with increasing SiC doping level remained rather small most likely due to the counterbalancing effect of Si and C co-doping. The high level Si and C co-doping allowed the creation of intragrain defects and highly dispersed nanoinclusions within the grains which can act as effective pinning centers for vortices, improving Jc behavior as a function of the applied magnetic field. The enhanced pinning is mainly attributable to the substitution-induced defects and local structure fluctuations within grains. A pinning mechanism is proposed to account for different contributions of different defects in MgB2−x(SiC)x/2 superconductors.

Effect of SiO2 and SiC doping on the powder-in-tube processed MgB2 tapes

Effect of SiO2 and SiC nano-powder doping was investigated for the powder-in-tube processed MgB2/Fe tapes. Mg or MgH2 powder was used as the Mg source of starting materials, and heat treatment was carried out at 600 °C for 1 h. These heat treatment conditions of lower temperature and shorter heating time are advantageous from the aspect of practical production processes. MgH2 powder improved the connection of MgB2 grains and prevented oxidation of MgB2. SiC and SiO2 doping greatly enhanced the critical current density (JC) values of the tapes prepared with Mg + B powder. However, only the SiC doping was effective in enhancing JC values for MgH2 + B powder. SiC doping decreased magnetic field sensitivity of JC, while SiO2 doping did not change the field dependence of JC. The SiC doped tape showed transport JC value of about 6 500 A cm−2 at 4.2 K and in the magnetic field of 12 T. The irreversibility field increased from 17 T to 23 T by the SiC doping.

Effect of grain size and doping level of sic on the superconductivity and critical current density in MgB/sub 2/ superconductor

SiC doped MgB/sub 2/ polycrystalline samples were fabricated by in-situ reaction using different grain sizes (20 nm, 100 nm, and 37 /spl mu/m) of SiC and different doping levels (0, 8, 10, 12, 15 wt%). Phases, microstructures, superconductivity, critical current density and flux pinning have been systematically investigated using XRD, SEM, TEM, and magnetic measurements. Results show that grain sizes of the starting precursors of SiC have a strong effect on the critical current density and its field dependence. The smaller the SiC grains are, the better the J/sub c/ field performance is. Significant enhancement of J/sub c/ and the irreversibility field H/sub irr/ were revealed for all the SiC doped MgB/sub 2/ with additions up to 15 wt%. A J/sub c/ as high as 20,000 A/cm/sup 2/ in 8 Tesla at 5 K was achieved for the sample doped with 10 wt% SiC with a grain size of 20 nm. Results indicate that the nano-inclusions and substitution inside MgB/sub 2/ are responsible for the enhancement of flux pinning.

Transport critical current density in Fe-sheathed nano-SiC doped MgB/sub 2/ wires

The nano-SiC doped MgB2/Fe wires were fabricated using a powder-in-tube method and an in-situ reaction process. The depression of Tc with increasing SiC doping level remained rather small due to the counterbalanced effect of Si and C co-doping. The high level SiC co-doping allowed creation of the intra-grain defects and nano-inclusions, which act as effective pinning centers, resulting in a substantial enhancement in the Jc(H) performance. The transport Jc for all the wires is comparable to the magnetic Jc at higher fields despite the low density of the samples and percolative nature of current. The transport Ic for the 10wt% SiC doped MgB2/Fe reached 660A at 5K and 4.5T (Jc = 133,000A/cm2) and 540A at 20K and 2T (Jc = 108,000A/cm2). The transport Jc for the 10wt% SiC doped MgB2 wire is more than an order of magnitude higher than for the state-the-art Fe-sheathed MgB2 wire reported to date at 5K and 10T and 20K and 5T respectively. There is a plenty of room for further improvement in Jc as the density of the current samples is only 50%.

“Some like it shallower” – p‐type doping in SiC

AbstractThe usual p‐type dopants of SiC, B, and Al, do not produce really shallow levels. In fact, boron can give rise to a secondary very deep acceptor level as well. The picture is complicated by hydrogen which is readily incorporated during in‐growth doping into p‐type material and can passivate both dopants but to a different degree. First principle calculations are reported regarding the interaction of hydrogen with B and Al in SiC. The results explain why hydrogen is incorporated in much higher amounts into B‐doped than into Al‐doped samples, and also reveal the influences of hydrogen on boron to produce the shallower acceptor. It will be shown that hydrogen incorporation during growth does not influence Al. Finally an Al–N–Al complex is proposed as a shallower acceptor in SiC.

Enhancement of the critical current density and flux pinning of MgB2 superconductor by nanoparticle SiC doping

Doping of MgB2 by nano-SiC and its potential for the improvement of flux pinning were studied for MgB2−x(SiC)x/2 with x=0, 0.2, and 0.3 and for 10 wt % nano-SiC-doped MgB2 samples. Cosubstitution of B by Si and C counterbalanced the effects of single-element doping, decreasing Tc by only 1.5 K, introducing intragrain pinning centers effective at high fields and temperatures, and significantly enhancing Jc and Hirr. Compared to the undoped sample, Jc for the 10 wt % doped sample increased by a factor of 32 at 5 K and 8 T, 42 at 20 K and 5 T, and 14 at 30 K and 2 T. At 20 K and 2 T, the Jc for the doped sample was 2.4×105 A/cm2, which is comparable to Jc values for the best Ag/Bi-2223 tapes. At 20 K and 4 T, Jc was twice as high as for the best MgB2 thin films and an order of magnitude higher than for the best Fe/MgB2 tapes. The magnetic Jc is consistent with the transport Jc which remains at 20 000 A/cm2 even at 10 T and 5 K for the doped sample, an order of magnitude higher than the undoped one. Because of such high performance, it is anticipated that the future MgB2 conductors will be made using a formula of MgBxSiyCz instead of pure MgB2.

Impurity-Controlled Dopant Activation - The Role of Hydrogen in p-Type Doping of SiC

Hydrogen is a natural contaminant of SiC growth processes, and may influence the doping efficiency. Hydrogen incorporation proportional to that of boron was observed during CVD growth while the amount of hydrogen was two orders of magnitude less than the aluminum concentration. Passivation by complex formation with hydrogen has been proven both for Al and B. The experimentally observed reactivation energy of these complexes differ by 0.9 eV. Our ab initio supercell calculations in 4H-SiC indicate, that in the absence of hydrogen, boron is incorporated as isolated substitutional and prefers the carbon site, while under typical CVD conditions boron is incorporated together with hydrogen (in equal amounts), favoring the silicon site. Therefore, the presence of H is advantageous for the activation of B as a shallow acceptor. In contrast to boron, aluminum is incorporated independently of the presence of hydrogen as isolated substitutional at the silicon site. The calculated difference between the dissociation of the stable dopant plus hydrogen complexes agrees very well with experiments. Vibration frequencies for the dopant complexes have been also calculated.

P-Type doping of SiC by high dose Al implantation—problems and progress

The development of optimized processes for p-type doping of SiC by ion implantation and subsequent annealing is a remaining challenge to SiC-device technology. Al is a promising acceptor in SiC. Compared to B it has a shallower acceptor level and a stronger tendency to occupy atomic sites in the Si sublattice which makes Al more suitable for the production of heavily doped, low resistivity layers. However, also in the case of Al very high acceptor concentrations (>10 19 cm −3) are necessary to obtain SiC layers with low resistivities (<1 Ω cm). The physical consequences of such high impurity concentrations in SiC for the annealing of implantation damage and the electrical activation will be discussed. A survey of the results of several implantation and annealing schemes is presented.

Formation of ohmic contacts to α-SiC and their impact on devices

The history of power electronic semiconductor devices is reviewed that leads to a discussion of new materials. The wide bandgap and good thermal conductivity of SiC permit its use at high temperatures, and the high electric field breakdown is favorable for high power devices. However, there remain a number of key problems with SiC semiconductor technology that must be solved before reliable production can begin. Difficulties with the insulator-to-SiC interface, SiC epitaxial material, passivation, doping, and stability of contacts drive viable device structures. One issue, the formation of electrical contacts, is described in more detail. In particular, the impact of SiC preparation procedures, surface roughness, and deposition conditions are discussed, e.g., variations in the chemical cleaning process affect the removal of overlayers and/or the roughneing of the SiC surface. Studies have shown that this cleaning or etching step can in turn affect device performance. At present, fabrication of the best quality contacts is interlinked with other material problems, and even to obtain adequately low resistivities, constraints are placed on the processing of devices.

Vanadium doping of 4H SiC from a solid source: Photoluminescence investigation

Photoluminescence (PL) spectroscopy was used to monitor the efficiency of vanadium doping from a solid vanadium nitride source during CVD growth of 4H silicon carbide. More than an order of magnitude increase of vanadium related infrared photoluminescence in comparison to undoped samples was observed. By correcting the spectra for the contribution from the substrate, vanadium was shown to be incorporated in the epitaxial layer during CVD and was responsible for the observed differences in photoluminescence signal. Possible mechanisms of the vanadium transport to the growing layer and its incorporation are discussed.

Radiation Defects and Doping of SiC with Phosphorous by Nuclear Transmutation Doping (NTD)

Radiation Defects and Doping of SiC with Phosphorous by Nuclear Transmutation Doping (NTD)

Selective Doping of 4H-SiC by Aluminum/Boron Co-diffusion

ABSTRACTBased upon graphite mask, selective aluminum/boron doping of SiC by thermal diffusion has been successfully realized in a temperature range of 1800 to 2100°C. Secondary ion mass spectrometry (SIMS) was used to identify the doping profiles, which showed very high aluminum concentration (5×1019 cm−3) near the surface and linearly graded boron profile up to several micrometers in depth. Hall-effect measurement was also employed to obtain the carrier concentration, which showed more than 1019 cm−3 carrier concentration at room temperature. Cathodoluminescence (CL) image clearly illustrated the locally diffused pattern. In addition, planar p-n diodes based upon this technique were fabricated and current-voltage (I-V) characteristics were measured. Excellent rectification property has been obtained. Built-in voltage of 2.9 V in the formed p-n junction was obtained by capacitance-voltage (C-V) measurement.

Indium doping of amorphous SiC:H films prepared by reactive magnetron co-sputtering

Hydrogenated amorphous silicon-carbon was doped with indium. The films were prepared by reactive magnetron sputtering, using composite targets of silicon and indium in a methane-argon gas mixture. The doping effects of indium on the optical and optoelectronic properties of the films were investigated. The film composition, including the concentration of hydrogen bonded to silicon depends on fractional indium content z. The incorporation of indium in the range of z below 10 −2 yields a compensation of n-type conduction in the undoped film, i.e. the dark conductivity decrease and its activation energy shows a maximum without changing the optical band gap. In the large- z range above 10 −2, the films exhibit alloying behavior as shown by a rapid decrease in the optical band gap, and a quenching of photoconductivity effect together with an increase in the dark conductivity.

Ion Implantation Doping in SiC and its Device Applications

The latest ion-implantation results on SiC are presented. The authors have performed nitrogen and phosphorus (N/P) co-implantations to obtain very high n-type carrier concentrations, Si and C bombardments for compensating n-type SiC, and V-implantation for compensating p-type SiC. They have also performed N and Al implantations directly into V-doped semi-insulating 6H-SiC substrates. Vertical p-n junction diodes were made by selective area N, P, and N/P implantations into p-type epitaxial layers grown on 6H-SiC substrates.

Some effects of phosphorus doping in SiC:H films prepared using ECR-CVD

This paper reports the deposition of hydrogenated silicon carbide (SiC:H) films using the electron cyclotron resonance chemical vapour deposition (ECR-CVD) technique. Using this technique, SiC:H films were prepared from a mixture of methane, silane and hydrogen, with phosphine as the doping gas. The effects of changing the phosphine fraction on the optical bandgap, activation energy and conductivity were investigated in films deposited at two different microwave powers of 150 and 600 W, respectively. The results showed that the deposition conditions strongly influence the structural, optical and electrical properties of the SiC:H films. In films deposited at low and high microwave powers, phosphorus doping has the effect of enhancing the formation of the silicon microcrystalline phase. In films which contain a strong silicon microcrystalline phase, the optical bandgap remained relatively constant while the conductivity increased rapidly followed by saturation. Correspondingly, the activation energy decreased and saturated suggesting an effect due to dopant saturation. The results showed that good phosphorus doping efficiency can be obtained in SiC:H films deposited at high microwave power.