Superconductivity of InN

Article abstract of DOI:10.1002/pssa.200563510

The crystal structure of InN is wurtzite and its interplanar spacing of (1011) is almost the same as that of tetragonal In(101). The crystallographic similarity produces many problems to solve about the electronic properties of semiconducting InN. Above all, there is a controversy over the possibility that In layers in InN that contains poly-crystalline phase couple by tunneling and exhibit no substantial depression of their superconducting transition temperature from the bulk In value. Here we present a superconductor to insulator transition in highly disordered InN with grains having a (1011) plane parallel to sapphire (0001).

Full text of DOI:10.1002/pssa.200563510

phys. stat. sol. (a) 203, No. 1, 80–84 (2006) / DOI 10.1002/pssa.200563510
Superconductivity of InN
*
, 1
T. Inushima , N. Kato , T. Takenobu , and M. Motokawa
1
2
2
1
Department of Electronics, Tokai University, 1117 Kitakaname, Hiratsuka 259-1292, Japan
2
Institute for Materials Research, Tohoku University, Katahira, Sendai 980-8577, Japan
Received 22 August 2005, revised 21 November 2005, accepted 21 November 2005
Published online 23 December 2005
PACS 72.80.Ey, 74.20.–z, 74.25.Fy
The crystal structure of InN is wurtzite and its interplanar spacing of (1011) is almost the same as that of
tetragonal In(101). The crystallographic similarity produces many problems to solve about the electronic
properties of semiconducting InN. Above all, there is a controversy over the possibility that In layers in
InN that contains poly-crystalline phase couple by tunneling and exhibit no substantial depression of their
superconducting transition temperature from the bulk In value. Here we present a superconductor to insu-
lator transition in highly disordered InN with grains having a (1011) plane parallel to sapphire (0001).
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
1 Introduction
When we reported our observation of superconductivity of InN in 2000 for the first time, we did not
discuss its mechanism because we could not regard the superconductivity as a fundamental property of
InN [1]. At that time it was well known that crystal growth of InN was very difficult and a minor change
of the growth condition would easily produce a metal–In droplet in InN. In the succeeding experiments,
however, we made clear that neither metal–In films nor the involvement of metal–In droplets were the
cause of the superconductivity [2]. We then considered that there was an optimum carrier density for the
occurrence of the superconductivity. The scenario was as follows: when the lattice imperfection becomes
large enough, but not too large, to produce high carrier density, a metal–In network would form in InN
and produce superconductivity originating from the metal In; when the carrier density becomes small
enough to produce a clear Fermi surface of a degenerate semiconductor, the superconductivity vanishes
[3]. This scenario, however, was found to be wrong in the recent experiments done at Grenoble High
Magnetic Field Lab., where InN films with a clear Fermi surface became superconductors [4].
Still, it is obvious that InN with a small carrier density and with a well-defined Fermi surface shows
superconductivity, and that neither the electron density nor the crystal quality is the main cause of the
superconductivity. We also know that the superconductivity is of the second kind and that the upper-
critical field determined from the magnetic field dependence of zero-resistance temperature is a cross-
over from glassy-vortex lattice to liquid-vortex phases due to the thermal and quantum fluctuations of the
vortex, which suggests that the coherent length is short and the penetration length is long. Neither the
surface electron accumulation layer nor the metal–In precipitation has any contribution to the supercon-
ductivity.
Recently it was reported that the XRD diffraction feature at 2θ ≈ 33°, which had been attributed to
metal In(101), i.e., the presence of metallic In clusters, was found to be an InN(1011) Bragg spot. Ma-
leyre et al. showed that MOCVD grown powdered InN did not contain a metal In droplet and that the
Bragg intensity of (1011) was similar to that of (0002) [5]. Yu et al. made clear that InN grown directly
*
Corresponding author: e-mail: inushima@keyaki.cc.u-tokai.ac.jp, Phone: +81 463 1211, Fax: +81 463 58 8320
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Original
Paper
phys. stat. sol. (a) 203, No. 1 (2006)
81
on a sapphire substrate without buffer layers involved a polycrystalline phase and that a (1011) spot was
observed at 33° [6]. The spot at 33° is often observed in MOCVD grown InN, and we used it in the pre-
vious papers as an index of the crystal degradation, where the superconducting transition temperature T
c
showed no relation to the intensity ratio of In(101)/InN(0002). If we regard this spot as an index of the
involvement of the polycrystalline phase or disorder in InN, we will have a new understanding of the
physical properties of superconducting InN.
At present it is not easy to evaluate the involvement of disorder in the InN with high mobility and low
carrier concentration and hence it is difficult to determine its contribution to the occurrence of the super-
conductivity in such InN. On the other hand, when the InN contains disorder as a poly-crystalline phase,
the magnitude of disorder can be evaluated by detecting the intensity of the (1011) reflection. Hence in
this report we present the relationship between the superconducting resistivity change and the involve-
ment of polycrystalline phase using the InN with high carrier concentration, and propose a possible me-
chanism of the superconductivity of InN, assuming In layers coupled by tunneling, and exhibiting no
substantial depression of their superconducting transition temperature from the bulk value.
2 Experimental details
InN films were grown on sapphire (0001) substrates using a conventional MOCVD method. The growth
temperature was 600 to 615 °C, the reaction pressure was 600 Torr, and the source gases were NH and
3
TMI. Nitrogen was used as the carrier gas. The growth of InN on sapphire was preceded by 10 min of
nitridation at 1000 °C under an NH flow rate of 1000 sccm. The grown film thickness was ∼350 nm.
3
The obtained InN had a mobility (µ ) from 30 to 100 cm2 V–1 s–1 and a carrier density (n ) from
e
e
3 × 1020 cm–3 to 7 × 1020 cm–3. The InN films had a hexagonal structure with its c-axis perpendicular to
the sapphire (0001) plane. The orientation of InN [1210] was parallel to Al O [1010]. The sample con-
3
2
3
ditions are listed in Table 1. The samples were cooled to 0.5 K in a He cryostat equipped with a 15 T
superconducting magnet. The magnetoresistance was measured as a function of field B and the angle
between the direction of B and the crystal c-axis. The resistance of the samples was measured by a con-
ventional dc 4-probe method. The measurement current along [1010] was 0.1 mA.
In this experiment we used two types of buffer layers; one was an amorphous double-buffer layer of
10 nm-thick InN and 10 nm-thick GaN grown at 550 °C on nitrided sapphire (0001) (no. 680, no. 692),
and the other was a crystalline buffer layer grown at 600 °C (no. 753, no. 757).
High-resolution X-ray diffraction and reciprocal space mapping were performed using a Philips high
resolution diffractometer with Cu Kα radiation. The samples investigated had InN(1011) reflection. As
1
is given in the previous report [7], when the sample has a strong (1011) reflection, the reciprocal space
mapping of (1011) is round and isolated, indicating that there are grains or involvement of a secondary
phase in InN with their c axis tilted so that (1011) is parallel to the sapphire (0001) plane (denoted as
(1011) grain). The degree of the grain involvement was estimated from the X-ray Bragg intensity ratio of
InN(1011) to Al O (0002), which is given in Table 1. The difference between no. 753 and no. 757 and
2
3
between no. 692 and no. 680 are their growth temperatures. As is seen in Table 1, a slight difference in
growth temperature affects the electrical characteristics considerably, especially superconducting proper-
ties.
Table 1 Summary of sample conditions. Growth temperature is given by T . Involvement of grains with
s
the InN(1011) plane parallel to sapphire (0001) is estimated by the X-ray intensity ratio of
InN(1011)/Al O (0006).
2
3
sample
680
613
692
616
753
613
757
610
T (°C)
s
n (1020 cm–3)
µ (cm2 V–1 s–1)
InN(1011)/Al O (0006) (×10–3)
2.9
102
4.3
4.6
57
2.1
4.5
66
0.5
6.8
28
0.6
e
e
2
3
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© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

82
T. Inushima et al.: Superconductivity of InN
Fig. 1 (online colour at: www.pss-a.com) XRD meas-
urements of the InN films in θ –2θ configuration. Peaks
∼
at 2θ = 31.56° are InN(0002), 33.10° InN(1011), and
–
∼
–
34.8° the GaN(0002) buffer layer.
3 Results and discussion
Figure 1 shows the XRD scan of the samples investigated. Samples no. 680 and no. 692 do not have crys-
talline buffer layers, and therefore they have a clear InN(1011) reflection around 33°. Judging from the
intensity ratio of InN(1011) to sapphire (0006), no. 680 contains the highest amount of (1011) grains.
Samples no. 753 and 757 also have a broad peak around 33°, but their intensity ratios are lower than those
of the samples with amorphous buffer layers. Sample no. 753 has the lowest amount of (1011) grain.
Figure 2 shows the temperature dependence of the resistivity of the samples shown in Fig. 1. The
vertical scale is normalized by the resistivity at 4.2 K. All of the samples show a resistivity anomaly
around 3.4 K, which is the superconducting transition temperature of bulk In. Sample no. 680 changes
into an insulator. Previously we reported that InN with a carrier density of 1.8 × 1018 cm–3 showed insu-
lating behavior due to the carrier localization [8]. As for sample no. 680, its resistivity increases below
3.4 K and the ratio of the resistivity change below 3.4 K is more than one order of magnitude higher than
that observed in InN with the carrier localization. Sample no. 692, whose n is similar to that of no. 680,
e
starts to show a small resistivity decrease at 3.4 K, and shows superconducting transition at 1.5 K and
reaches zero resistance at 0.5 K. As for InN with a crystalline buffer layer, it shows a clear superconduct-
ing transition at 3.4 K. When a sample contains more (1011) grains (no. 757), its transition starts at 3.4
K, which is very steep and clear. On the other hand, no. 753 starts to show a gradual decrease of resist-
ance around 3 K and reaches zero resistance at 1.8 K, the same as no. 757.
Figure 3 shows the magnetoresistance of the four samples at 0.5 K at B ⊥ ab-plane. Melting field
of the glassy-vortex solid of InN (resistance vanishing field) is 0.3 T for no. 753, 0.2 T for no. 757 and
Fig. 2 (online colour at: www.pss-a.com) Tem-
peratur dependence of the resistivity of InN. The
vertical scale is normalized by the value at 4.2 K.
Fig. 3 (online colour at: www.pss-a.com) Magnetic
field dependence of the resistivity of InN. The vertical
scale is normalized by the value at 4.2 K. The field is
applied perpendicular to the ab-plane and to the current.
© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
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phys. stat. sol. (a) 203, No. 1 (2006)
83
0.01 T for no. 692, much larger values than 0.023 T, the critical field of metal In, which is a type I super-
conductor. The resistivity of no. 680 returns to its initial value at B = 0.3 T. The magnetic field depend-
ence of the resistivity is similar to that observed in the temperature dependence of the resistivity shown
in Fig. 2. Hence it can be said that the resistivity anomaly of no. 680 originates in the superconductivity
of InN and that a superconductor-insulator transition occurs at 3.4 K.
Since P. W. Anderson predicted the behavior of the superconductivity of disordered metallic systems,
where localization did not have a significant contribution to the BCS superconductivity in dirty metals,
but reduced the T and finally changed the superconductor into insulator [9], the superconductor–
c
insulator transition has been extensively studied in various systems. Disordered metal–In films have
been investigated as a good candidate to understand this mechanism, and the transition is understood as
the metal In films being coupled by Josephson junction, exhibiting no substantial depression of their
superconducting transition mechanism from the bulk In [10]. When InN has a carrier concentration lower
than 1018 cm–3 and shows superocnducting transition, its resistivity change is smooth as is observed in
no. 753, though it occurs at lower temperatures [4]. When the disorder (involvement of (1011) grains)
increases (no. 757), the temperature of the resistivity change increases to 3.4 K and the change is not so
smooth as in no. 753 because of the trapping of the Cooper-pairs. When the disorder becomes even
stronger, the Cooper-pairs produced by the superconductiving transition are trapped by the disorder more
strongly (no. 692), and finally, InN changes into an insulator, where the Cooper-pairs are localized by the
strong disorder in InN. When the field is applied, the Cooper-pairs are more easily dissolved into elec-
trons and the resistivity returns to its initial value in reverse, from no. 680 to no. 753 as is seen in Fig. 3.
The superconductivity of InN, therefore, is very sensitive to the presence of the (1011) grains within.
The interplanar spacing of InN(1011) is 2.704 Å and that of metal In(101) is 2.712 Å at room tem-
perature. According to Paszkowicz et al., the spacing of In(101) becomes 2.706 Å at 105 K [11], so both
spacings are almost the same. As was reported before, InN has a very sharp lower E phonon at 87 cm–1,
2
which indicates that the out-of-phase vibration of In atoms parallel to sapphire (0001) has a long lifetime
even when n is larger than 1020 cm–3 [1]. Moreover, the A (LO) phonon and free carriers in InN couple
e
1
non-linearly, where Fano interference between the zone-center LO phonon and quasicontinuum elec-
tronic state along c-axis is prominent [12]. Judging from these results, when the grains having a (1011)
plane parallel to sapphire (0001) are involved, the crystal structure of InN is regarded as having a two-
dimensional In-layer structure intercalated by nitrogen layers along [1011], or along c-axis. The localiza-
tion anisotropy observed in the ab-plane [8] is an evidence to support this consideration. If it is so, we
can understand the superconductivity of InN in terms of the metal In layers coupled by Josephson junc-
tion along [1011]. In this case the carrier density of InN has no relation to the superconducting transition.
When the disorder becomes stronger, or the involvement of grains becomes larger, the Cooper-pairs are
localized and finally InN changes into an insulator. In this scenario the carrier concentration is the meas-
ure of the disorder and we do not need to consider the contribution of the In-droplet to the occurrence of
the superconductivity.
4 Conclusion
We reported that there is a superconductor to insulator transition in highly disordered InN. From the
relationship between the superconductivity and the involvement of (1011) grains, we suggested that
when the grains having a (1011) plane parallel to sapphire (0001) are involved, the crystal structure of
InN is regarded as having a two-dimensional In-layer structure intercalated by nitrogen layers along
[1011], or along c-axis because the interplanar spacing of InN(1011) is almost the same as that of metal
In(101). It may be concluded that the superconductivity of InN is caused by the metal In layers coupled
by Josephson junction along [1011].
Acknowledgements The author (T. I.) is grateful to the Ministry of Education, Science, Sports, and Culture of
Japan for the Grant-in-Aid for Scientific Research No. 17560293, and to the Nippon Sheet Glass Foundation for the
financial supports.
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T. Inushima et al.: Superconductivity of InN
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