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Sci. Technol. Adv. Mater. 9 (2008) 044204 (8pp) doi:10.1088/1468-6996/9/4/044204

Superconductivity in carrier-dopedsilicon carbideTakahiro Muranaka1, Yoshitake Kikuchi1, Taku Yoshizawa1,Naoki Shirakawa2 and Jun Akimitsu1

1 Department of Physics and Mathematics, Aoyama-Gakuin University, 5-10-1 Fuchinobe, Sagamihara,Kanagawa 229-8558, Japan2 National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Umezono, Tsukuba,Ibaraki 305-8578, Japan

E-mail: muranaka@phys.aoyama.ac.jp

Received 8 December 2008Accepted for publication 12 December 2008Published 28 January 2009Online at stacks.iop.org/STAM/9/044204

AbstractWe report growth and characterization of heavily boron-doped 3C-SiC and 6H-SiC andAl-doped 3C-SiC. Both 3C-SiC:B and 6H-SiC:B reveal type-I superconductivity with acritical temperature Tc = 1.5 K. On the other hand, Al-doped 3C-SiC (3C-SiC:Al) showstype-II superconductivity with Tc = 1.4 K. Both SiC:Al and SiC:B exhibit zero resistivity anddiamagnetic susceptibility below Tc with effective hole-carrier concentration n higher than1020 cm−3. We interpret the different superconducting behavior in carrier-doped p-typesemiconductors SiC:Al, SiC:B, Si:B and C:B in terms of the different ionization energies oftheir acceptors.

Keywords: boron-doped SiC, Al-doped SiC, hexagonal and cubic SiC, type-I superconductor,type-II superconductor

(Some figures in this article are in colour only in the electronic version)

1. Introduction

The recently discovered superconductivity in B-dopeddiamond (C:B) by Ekimov et al [1], subsequent enhancementof the critical temperature Tc in thin films to 11 K by Takanoet al [2, 3], and observation of superconductivity in B-dopedSi (Si:B) by Bustarret et al [4] have stimulated renewedinterest in the low-carrier-density superconductivity of dopedsemiconductors. Many experimental and theoretical studieshave focused on the origin of the metallic nature responsiblefor superconductivity in diamond. Investigations continueto clarify whether it originates from the holes at the topof the diamond valence band or from the boron impurityband formed above the valence band [5–15]. In particular, ahigher-Tc superconductivity in C:B has been suggested froma theoretical model by a Japanese group [16–18], where bondstransform into bands by carrier doping of a semiconductor. Inan extension of research on C:B and Si:B, our group focusedon SiC. Silicon carbide has many stable polytypes, including

cubic zinc-blende, hexagonal and rhombohedral polytypes. Inthe cubic zinc-blende structure, labeled as 3C-SiC or β-SiC,Si and C occupy ordered sites in a diamond framework.In hexagonal polytypes nH-SiC and rhombohedral polytypesnR-SiC, generally referred to as α-SiC, nSi-C bilayersconsisting of C and Si layers stack in the primitive unit cell.The lattice structures of the 3C-SiC and 6H-SiC phases, whichare most important for this paper, are presented in figure 1.

Undoped SiC is a wide-band-gap semiconductorwith a band gap of ∼2–3 eV depending on the crystalmodification. By carrier doping, we successfully inducedtype-I superconductivity in boron-doped SiC (SiC:B) witha boron doping level of ∼1.06–1.91 × 1021 cm−3 [19].In the previously studied samples [19], both 3C-SiC:Band 6H-SiC:B phases were identified by powder x-raydiffraction (PXRD), and thus it was unclear which phase wassuperconducting.

SiC opens a new gateway to the physics of thenew fascinating family of superconductors originating

1468-6996/08/044204+08$30.00 1 © 2008 National Institute for Materials Science Printed in the UK

Sci. Technol. Adv. Mater. 9 (2008) 044204 T Muranaka et al

⟨111⟩SiC

⟨001⟩

SiC

(a) (b)

Figure 1. (a) Unit cell of cubic 3C-SiC. (b) Four unit cells ofhexagonal 6H–SiC. For the drawings the software Vesta wasused [20].

from wide-gap semiconductors. SiC lightly doped withdonors or acceptors was intensively studied for N, P,B, Al, etc, introduced by ion implantation or thermaldiffusion. A natural question arises whether SiC exhibitssuperconductivity when doped by other elements. Insemiconductors, an insulator-to-metal transition takes placewhen the dopant-induced carrier concentration increasesa certain level. Moreover, superconductivity has beenexperimentally achieved in some semiconductors at very lowtemperatures [21–24] following the theoretical predictionsby Gurevich et al [25] and Cohen [26]. Recently, thesemiconductor-to-metal transition has been realized in n-typeN-doped 4H-SiC with carrier concentration above 1019 cm−3

but without a report of superconductivity [27]; whereasp-type Al-doped 4H-SiC with Al concentration nAl = 8.7 ×

1020 cm−3 showed metallic behavior and a small drop around7 K in the temperature dependence of sheet resistance [28].The authors mention that this drop may indicate the onset of asuperconducting transition.

Here, we report superconductivity of B-doped3C-SiC and 6H-SiC polytypes and of Al-doped 3C-SiC.Furthermore, we show that the superconductivity inhole-doped semiconductors can be interpreted in termsof the ionization energies of their acceptors.

2. Experimental details

Samples were prepared by substitutional reaction sinteringfrom submicron 3C-SiC (β form) powder (WAKO, meanparticle size 0.27 µm), 6H-SiC (α form) powder (99%, RAREMETALLIC), Si powder (99.99%), amorphous boron (99%),and Al powder (99.99%). The powders were mixed in certainproportions (typically SiC/Si/B or SiC/Si/Al with molarratios 4 : 1 : 0.3–1.0), ground thoroughly, and pressed intopellets. They were then placed in an alumina boat and sinteredin a pre-evacuated tube furnace at about 1600 C for 12 h,with heating and cooling rates of 200 C per hour, in aflow of Ar/H2 gas mixture (50 ml/50 ml per minute). Wehave avoided the phase transition from 3C-SiC to 6H-SiCduring the sintering, which occurs above 1700 C [29]. Theas-prepared samples were cut into slices and the surfacelayers were removed before the transport measurements. An

electron-probe microanalyzer with a wavelength-dispersivespectrometer (EPMA-WDS, JEOL JXA8200S) was employedfor a quantitative elemental analysis. PXRD patterns werecollected with CuKα radiation, using a conventional x-rayspectrometer equipped with a graphite monochromator(MultiFlex, RIGAKU), in the range 5 6 2θ 6 80 at 0.02

step. For dc magnetic susceptibility and magnetizationmeasurements in the temperature range 0.45–1.8 K, weemployed a commercial SQUID magnetometer (MPMSR2Quantum Design Co, Ltd) with a i-Helium3 3He refrigerator.Electrical resistivity was measured with a conventionalfour-probe technique. The Hall coefficient was measuredusing a commercial system (PPMS Quantum Design Co, Ltd)in a five-probe ac mode, scanning the magnetic field from −5to 5 tesla.

3. Superconductivity in B-doped SiC

The sintered 3C-SiC:B and 6H-SiC:B samples had blackceramic-like appearance. From typical PXRD patterns, themain phases in the 3C-SiC:B and 6H-SiC:B samples areindexed as the cubic zinc-blende 3C-SiC and the hexagonal6H-SiC phases, respectively. In each sample, unreacted Siis detected. For the major 3C-SiC phase, the refined latticeparameter a increased after sintering, but only by ∼0.1% from4.3575(3) to 4.3618(4) Å. For the major 6H-SiC phase, therefined lattice parameter c also increased after sintering by∼0.2% from 15.06 to 15.09 Å. On the other hand, the refinedlattice parameter a did not change within the experimentalaccuracy. Note that the lattice parameter a of commercial6H-SiC is 3.064 Å. We expect that the presence of liquidsilicon facilitates the boron diffusion, due to the much fastermass transport compared to the solid sintering, and enhancesthe boron substitution efficiency. The lattice parameter ofsilicon did not change after the sintering, indicating that littleboron was doped into silicon.

The atomic sizes of boron and carbon are comparable,but are much smaller than that of silicon. Therefore, theminute change of the lattice parameters suggests that boronsubstitutes at the carbon site in these samples. We expectthe boron substitution to be enhanced with a slow growthof crystal grains, and indeed, by the present syntheticmethod, the samples had much higher boron doping levelscompared to the previous reports [30–33]. EPMA analysiswith area-scans (within 50 × 50 µm2) at various positionsoccasionally detected less than 0.05 at.% of Fe, togetherwith Si, C and B, but no other elements, in all samples.From Hall measurements at room temperature, the effectivehole concentrations n are deduced as ∼1.91 × 1021 cm−3 for3C-SiC:B and ∼2.53 × 1020 cm−3 for 6H-SiC:B.

As shown in figure 2, the magnetic susceptibility of3C-SiC:B and 6H-SiC:B in a magnetic field of 1 Oe (zerofield cooling process) significantly decreases at ∼1.4 K,suggesting the occurrence of superconductivity. The estimatedsuperconducting volume fraction at 0.45 K exceeds 100%because of the shielding effect. Figure 3 shows magnetizationdependences versus magnetic field (M–H) of 3C-SiC:B and6H-SiC, exhibiting the type-I superconducting behavior. In

2

Sci. Technol. Adv. Mater. 9 (2008) 044204 T Muranaka et al

–0.04

–0.03

–0.02

–0.01

0

0.01

0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8 2.0

g–1)

ume( ytilibitpecsuS

Temperature (K)

3C-SiC:B

6H-SiC:B

Figure 2. Temperature dependence of dc magnetic susceptibility in3C-SiC:B and 6H-SiC.

both compounds, the onset field of magnetization shows ahysteresis with about 30 Oe width at the lowest temperature.From the M–H curves, we estimate the critical field Hc as100 Oe. The hysteresis width and Hc value are consistent withthe previous report on SiC:B [19].

Figure 4 presents the temperature dependence of theelectrical resistivity in 3C-SiC:B and 6H-SiC:B. Bothsamples show metallic conductivity reflecting the highdoping level. However, 3C-SiC:B exhibits a much smallerresistivity and almost linear temperature dependence athigh temperatures due to the even higher boron dopingconcentration. The slope dρ/dT is always positive and itstemperature dependence is linear for 3C-SiC:B, whereas abroad feature is observed at around 150 K for 6H-SiC:Bsuggesting a weak carrier localization or a contribution fromnon-metallic grain boundaries. The residual resistivity ratio

RRR (ρ300K /ρ5K ) is 11.4 and 4.7 for 3C-SiC:B and 6H-SiC:B,respectively. The inset of figure 4 shows an expandedview of the low-temperature data. Below 1.5 K (3C-SiC:B)and 1.4 K (6H-SiC:B), the resistivity drop indicates aclear superconducting transition and corresponds to thetransition temperature observed in the magnetic susceptibilitymeasurements. We note that despite the large difference in theresidual resistivity, Tc is nearly the same and the transitionwidth is less than 0.2 K for both samples.

The phase diagram in the magnetic field–temperature(H–T) plane for both samples was determined using resistivitymeasurements. They involved varying the temperature(heating and cooling) at different magnetic fields (T-scan infigure 5), and varying the magnetic fields (increasing anddecreasing) at different temperatures (H-scan in figure 6). Themeasurements were performed upon cooling and warming atconstant magnetic field (0–80 Oe with a step 20 Oe) as well asupon increasing or decreasing the magnetic field (0–200 Oe)at constant temperature (0.35–1.0 K).

The temperature dependence of the in-field resistivitywas measured as follows: the temperature was set above Tc.It was then reduced down to 350 mK (‘field-cooling (FC)run’) and subsequently increased above Tc (‘warming afterFC run’). Figure 5 shows the resistivity for H = 0, 20, 40,60 and 80 Oe. In zero field, only one transition is found atTc, whereas in finite fields, a large supercooling effect isobserved in both SiC polytypes. We obtained the same curvesindependent of the chosen sweep rate, which guarantees thatthe observed supercooling behavior is an intrinsic propertyof the sample and that the superconducting transition inboth polytypes is of the first order under finite fields. Asshown in figure 6, the hysteresis was also observed in themagnetic field dependence of the resistivity. Above 130 Oe in3C-SiC:B and above 120 Oe in 6H-SiC:B, we did not find anysign of superconductivity. The observations of the (i) in-fieldhysteresis, (ii) absence of a hysteresis in a zero field, and (iii)very small value of the critical field give strong evidence fortype-I superconductivity in both SiC polytypes.

–2.0

–1.5

–1.0

–0.5

0

0.5

1.0

1.5

2.0

0 50 100 150 200

(a) 3C-SiC:B

T=0.46 KT=0.8KT=1.0 K

g–1)

ume( noitazitenga

M

g–1)

ume( noitazitenga

M

Magnetic field (Oe)

–0.2

–0.15

–0.1

–0.05

0

0.05

0.1

0.15

0.2

0 50 100 150 200

(b) 6H-SiC:B

T=0.45 KT=0.8 KT=1.0 K

Magnetic field (Oe)

Figure 3. Magnetization versus magnetic field in (a) 3C-SiC:B and (b) 6H-SiC.

3

Sci. Technol. Adv. Mater. 9 (2008) 044204 T Muranaka et al

0

1×10–4

2×10–4

3×10–4

0

5×10–3

1×10–2

0 50 100 150 200 250 300

ρ(3

C-S

iC:B

) (Ω

cm

)

ρ(6

H-S

iC:B

) (Ω

cm

)

Temperature (K)

3C-SiC:B

6H-SiC:B

0

5×10–6

1×10–5

1.5×10–5

2×10–5

2.5×10–5

3×10–5

0

5×10–4

1×10–3

1.5×10–3

2×10–3

0 0.5 1.0 1.5 2.0 2.5 3.0

ρ (Ω

cm

) ρ (Ω

cm)

T (K)

3C-SiC:B

6H-SiC:B

Figure 4. Temperature dependence of resistivity in 3C-SiC:B and6H-SiC:B. The inset magnifies the region near Tc.

The H–T phase diagram deduced from the resistivitydata is shown in figure 7. We determined Tc(H) from theonset of superconductivity in T-scan and H-scan both forthe field-cooling run (normal-to-superconducting transition)and the subsequent warming run (superconducting-to-normaltransition). Applying the conventional formula Hc(T ) =

Hc(0)[1 − (T/Tc(0))α], we obtained a good fitting of thewarming data with α ∼2.0 for 3C-SiC:B and α ∼1.6 for6H-SiC:B. We identify this transition with the thermodynamiccritical field Hc(T ); Hc(0) = (132 ± 3) Oe for 3C-SiC:B and(125 ± 5) Oe for 6H-SiC:B, respectively. The same procedureapplied to the cooling data yields α ∼1.6 in both SiCpolytypes. We identify this transition as the upper limit ofthe intrinsic supercooling limit. The corresponding transitionfields are denoted as Hsc (the subscript ‘sc’ stands for

supercooling) with an estimated Hsc(0) = (102 ± 6) Oe for3C-SiC:B and (94 ± 3) Oe for 6H-SiC:B, respectively.

Applying the Ginzburg–Landau (GL) theory of type-Isuperconductivity to these data, one can estimate an upperlimit of the GL parameter κ from the difference of thecritical fields obtained by a field-cooling run and a subsequentwarming run [34, 35]: κ(0)6 Hsc(0)/(1.695

√2Hc(0)). This

yields κ ∼0.32 for 3C-SiC:B and 0.31 for 6H-SiC:B, inagreement with the analysis of the Hall effect and the specificheat data given above, and therefore supports the type-Inature of superconductivity in SiC:B. Note that the valueof κ is below 0.41, which is required in a model based onsupercooling instead of superheating [35–37].

4. Superconductivity in Al-doped SiC

The sintered 3C-SiC:Al sample had black ceramic-likeappearance. From a typical PXRD pattern, three phasesincluding 3C-SiC, unreacted Si and Al were identified. Themain phase in 3C-SiC:Al sample is indexed as the cubiczinc-blende 3C-SiC. For the main phase 3C-SiC, the refinedlattice parameter a increased after sintering by ∼0.1% from4.338(5) to 4.342(4) Å. Because the atomic sizes of Al andSi are comparable, but are much bigger than the atomic sizeof C, the minute change of the lattice parameters suggeststhat Al substitutes Si sites in 3C-SiC. The lattice parameterof Si did not change within the experimental resolution,indicating that little Al was doped into Si. We expect that thepresence of liquid Si facilitates the Al diffusion due to themuch faster mass transport compared to solid sintering, andenhances the Al-substitution efficiency. This also enhancesthe C site substitution by Si. EPMA analysis with area-scans(within 50 × 50 µm2) at various positions detected Si, Cand Al and no other elements. From room-temperature Hallmeasurements, the effective hole concentration was deducedas n ∼(3.86–7.06) × 1020 cm−3.

0

1 × 10–5

2 × 10–5

0 0.5 1.0 1.5 2.0

0 Oe10 Oe20 Oe40 Oe60 Oe80 Oe

( ytivitsiseR

Ω)

mc

Temperature (K)

(a) 3C-SiC:B

0

5 × 10–4

1 × 10–3

1.5 × 10–3

0 0.5 1.0 1.5 2.0

0 Oe10 Oe20 Oe40 Oe60 Oe80 Oe

( ytivitsiseR

Ω)

mc

Temperature (K)

(b) 6H-SiC:B

Figure 5. Temperature dependence of resistivity under different magnetic fields (T-scan) in (a) 3C-SiC:B and (b) 6H-SiC:B.

4

Sci. Technol. Adv. Mater. 9 (2008) 044204 T Muranaka et al

0

5 × 10–6

1 × 10–5

1.5 × 10–5

2 × 10–5

0 50 100 150 200

0.35 K0.4 K0.5 K0.6 K0.7 K0.8 K0.9 K1.0 K

( ytivitsiseR

Ω)

mc

Magnetic field (Oe)

(a) 3C-SiC:B

0

5 × 10–4

1 × 10–3

1.5 × 10–3

0 50 100 150 200

0.35 K0.4 K0.5 K0.6 K0.7 K0.8 K0.9 K1.0 K

( ytivitsiseR

Ω)

mc

Magnetic field (Oe)

(b) 6H-SiC:B

Figure 6. Magnetic field dependence of resistivity (H-scan) in (a) 3C-SiC:B and (b) 6H-SiC:B.

0

20

40

60

80

100

120

140

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Hsc

Hc

H)e

O(

T (K)

(a) 3C-SiC:B

0

20

40

60

80

100

120

140

0 0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6

Hsc

Hc

H)e

O(

T (K)

(b) 6H-SiC:B

Figure 7. H–T phase diagram for (a) 3C-SiC:B and (b) 6H-SiC:B, determined from the onset of superconductivity in T-scan and H-scan ofresistivity.

As shown in figure 8, the magnetic susceptibility ofSiC:Al in a magnetic field of 1 Oe (zero field cooling process)significantly decreases at 1.4 K, suggesting the occurrenceof superconductivity. The estimated superconducting volumefraction at 0.45 K exceeds 100% because of the shieldingeffect. The inset of figure 8 shows magnetization versusmagnetic field (M–H) curves, which are typical for a type-IIsuperconductor. These M–H curves and the value of Tc

exclude a possibility that superconductivity is caused by animpurity, such as Al (Tc = 1.175 K). From the M–H curves,we estimate the lower critical field Hc1 as 53 Oe.

In figure 9, the resistivity of 3C-SiC:Al in zero magneticfield shows metallic behavior from room temperature,reflecting the high level carrier doping and a sharp drop at1.5 K and zero resistivity below 1.35 K (inset in figure 9). Theresidual resistivity (ρ0) and the residual resistivity ratio (RRR)

are 0.746 m cm and 5.3, respectively. The ρ0 value is higherand the RRR value is lower than those of 3C-SiC:B (60 µ cmand 10) [19], but the small transition width (<0.15 K) revealsgood quality of superconducting transition in SiC:Al. Amongour samples sintered with other nominal compositions, ρ0

and RRR showed minor changes, however, the Tc did notvary. The phase diagram in the magnetic field-temperature(H–T) plane was deduced from resistivity measurements.They involved varying the temperature (heating and cooling)at different magnetic fields (T-scan in figure 10(a)), andincreasing and decreasing the magnetic fields at differenttemperatures (H-scan in figure 10(b)). Neither hysteresis norsupercooling behavior were observed in T-scan and H-scan,which is consistent with the transition to superconductivityunder magnetic fields being of second order. Thus, a type-IIsuperconductivity occurs in 3C-SiC:Al, in contrast with the

5

Sci. Technol. Adv. Mater. 9 (2008) 044204 T Muranaka et al

–0.07

–0.06

–0.05

–0.04

–0.03

–0.02

–0.01

0

0 0.5 1.0 1.5 2.0

Z.F.C

g–1

)u

me( ytilibitpecsuS

Temperature (K)

H = 1 Oe

–0.8

–0.6

–0.4

–0.2

0

0.2

0.4

0.6

0.8

0 50 100 150 200 250 300

g–1)

ume( noitazitenga

M

Magnetic field (Oe)

T=0.45 KHdown

Hup

Figure 8. Temperature dependence of dc magnetic susceptibility inSiC:Al. The inset shows magnetization versus magnetic field atT = 0.45 K.

type-I superconductivity in 3C-SiC:B. Figure 11 presents theTc and the upper critical field Hc2, determined from the onsetof resistivity, and the irreversibility field Hirr, determinedfrom zero resistivity. Applying the conventional formulaHc2(T ) = Hc2(0)[1 − (T/Tc)

2], we plot the dependencesHc2(T ) with Hc2(0) = 370 ± 3 Oe and the Hirr(T) withHirr(0) = 140 ± 4 Oe.

Table 1 lists preliminary specific heat coefficients γn [38];basic normal-state parameters: Fermi wave number kF,effective mass m∗, Fermi velocity vF, mean free path l; aswell as superconducting state parameters: penetration depthλ, coherence length ξ and Ginzburg–Landau parameter κGL.Those parameters were estimated as described in [39]. Forcomparison, the previously reported parameters for 3C-SiC:B,C:B and Si:B are added. A small but larger than 1/

√2 value

of κGL ∼1.8 gives an evidence for type-II superconductivity.

0

1

2

3

4

5

0 50 100 150 200 250 300

m( ytivitsiseR

Ω c

m)

Temperature (K)

0

0.2

0.4

0.6

0.8

1.0

0.5 1.0 1.5 2.0

ρm(

Ω c

m)

T (K)

Figure 9. Temperature dependence of resistivity in 3C-SiC:Al. Theinset magnifies the region near Tc.

Our results also imply that 3C-SiC:Al, similar to SiC:B, is adirty-limit superconductor because ξ(0) l.

5. Discussion

Here, we introduce the ionization energies EA of the acceptorsfor qualitative understanding of superconductivity in dopedsemiconductors. Theoretically, the superconductivity in dopedsemiconductors can be interpreted in terms of degree ofrandomness caused by the impurity band, which is related toEA [17, 18, 42]. When EA is large (acceptor level is deep),the randomness caused by the impurity band is large. TheEA values in p-type semiconductors [40, 41, 43] are shown

0

0.2

0.4

0.6

0.8

1.0

1.2

0 0.5 1.0 1.5 2.0

ρ /ρ 0

T (K)

(a) T-scans

0 Oe

300 Oe

200 Oe100 Oe

0

0.2

0.4

0.6

0.8

1.0

1.2

0 50 100 150 200 250 300 350 400

0.35 K0.4 K0.5 K0.6 K0.7 K0.8 K0.9 K1.0 K

ρ/ρ 0

H (Oe)

(b) H-scans

Figure 10. (a) Temperature dependence of resistivity normalized by ρ0 at different applied magnetic fields (0–400 Oe with a 20 Oe step) at0.35–2 K (T-scan). (b) Magnetic field dependence of resistivity normalized by ρ0 (0.35–1.0 K) at 0–400 Oe (H-scan).

6

Sci. Technol. Adv. Mater. 9 (2008) 044204 T Muranaka et al

Table 1. Parameters of the studied SiC:Al (this work and [38]), SiC:B [39], C:B [1, 5] and Si:B [4] samples.

SiC:Al SiC:B C:B Si:B

n (cm−3) 7.06 × 1020 1.91 × 1021 1.80 × 1021 2.80 × 1021

γn (mJ molK−2) 0.35 0.294 0.113 –ρ0 (m cm) 0.746 0.06 2.5 0.13RRR 5.3 10.0 0.9 1.2Tc(onset) (K) 1.5 1.45 4.50 0.35Hc(0) (Oe) – 115 – –Hsc(0) (Oe) – 80 – –Hc2(0) (Oe) 370 – 4.2 × 104 4000kF (nm−1) 2.8 3.8 3.8 –m∗ (mel) 2.0 1.2 1.7 –vF(m s−1) 1.6 × 105 3.8 × 105 – –l (nm) 2.2 14 0.34 –ξ(0) (nm) 148 360 80(9) (20)λ(0) (nm) 281 130 160 –κGL 1.8 0.35 2(18) –

0

100

200

300

400

500

0 0.5 1.0 1.5 2.0

H)e

O(

T (K)

Hc2

(T)=Hc2

(0)(1-T/Tc)2

Hc2

Hirr

Figure 11. Temperature dependence of the upper critical field Hc2

and of the irreversibility field Hirr in 3C-SiC:Al.

in table 2. The EA of 3C-SiC:Al is almost same with thatof 3C-SiC:B (type-I superconductor) and smaller than that ofC:B (type-II). Type-II superconductivity in C:B reveals carrierlocalization before the superconducting transition [1–3], and ξ

may be reduced by randomness caused by the dopant (80 nm).On the other hand, type-I superconductivity of 3C-SiC:B doesnot reveal such carrier localization [19], and ξ is not reducedby randomness (ξ ∼360 nm). Superconductivity in SiC:Aldoes not reveal carrier localization, similar to SiC:B, and theκGL value is small (∼1.8), but larger than 1/

√2. Therefore,

from EA value, we consider that the superconductivity in3C-SiC:Al is at the border between type I and type II.

In terms of EA, Si:B must reveal type-I superconductivitybecause EA has the smallest and its randomness due to thedopant must be small, according to theory. In the previousreport by Bustarret et al [4], B was introduced to Si thinfilms using UV laser doping technique. Such B-doped Sithin films may include defects or disorder. Because type-I

Table 2. Ionization energies EA of acceptors in p-typesemiconductors, as well as ξ , κGL and the superconductivity (SC)type in doped semiconductors.

EA(eV) ξ(0)(nm) κGL SC type Reference

B in Si 0.045 20 – type-II [4]B in C 0.37 80 2(18) type-II [1–3]Al in SiC ∼0.25 148 1.8 type-II This workB in SiC ∼0.29 360 0.35 type-I [19]

superconductivity can be hidden and change to type-IIbehavior by disorder or impurities, type-II superconductivityin Si:B might be influenced by the crystal defects or disorder.

From the same point of view, type-II superconductivityin SiC:Al with a small κGL value may be essentially a type-Isuperconductivity hidden by crystal defects or impurities.In preliminary ac susceptibility results (T-scan and H-scan),small hysteresis between cooling and warming process wasobserved (1T ∼0.02 K at H = 100 Oe, 1H ∼19 Oe at T =

0.26 K) [38]. Concentration of crystal defects and impuritiesshould be reduced in SiC:Al for a more detailed study of thesuperconductivity.

6. Summary

We successfully synthesized boron-doped SiC (3C- and6H-SiC polytypes) and Al-doped 3C-SiC by a substitutionalsintering method and characterized bulk superconductivity inthose materials. Both 3C-SiC:B and 6H-SiC:B reveal type-Isuperconductivity at Tc = 1.5 K. On the other hand, Al-doped3C-SiC (3C-SiC:Al) reveals type-II superconductivity atTc = 1.4 K. Using this observation, the superconductivity incarrier-doped semiconductors can be interpreted in termsof the depth of acceptor levels in p-type semiconductors.One may only speculate on the physical reasons behind thisdifference. One possibility could be a much different Fermivelocity vF in the case of SiC:B due to a different shape ofthe Fermi surface. Further experimental and theoretical workis clearly required.

7

Sci. Technol. Adv. Mater. 9 (2008) 044204 T Muranaka et al

Acknowledgments

We would like to acknowledge Dr M Kriener, ProfessorY Maeno (Kyoto University) and Dr Y Yanase (TokyoUniversity) for helpful discussions. This work wassupported by ‘High-Tech Research Center’ Project forPrivate Universities from the Ministry of Education, Culture,Sports, Science and Technology (MEXT), Grant-in-Aid forYoung Scientists (B) (No. 20740202) and a Grand-in-Aid forScientific Research on Priority Area (no. 16076212) fromMEXT.

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