Structural , electronic transport and magnetoresistance of a 142 nm lead telluride nanowire synthesized using stress-induced growth

In this study, structurally uniform single crystalline PbTe nanowires (NWs) were synthesized using a stress-induced growth. Selected-area electron diffraction patterns show that the PbTe NWs were grown along the [100] direction. The electrical conductivity σ of a NW with 142 nm in diameter exhibited a semiconducting behavior at 50–300 K. An enhancement of electrical conductivity σ up to 2383 S m−1 at 300 K is much higher than σ [0.44–1526 S m−1, Chen et al., Appl. Phys. Lett. 103, p023115, (2013)] in previous studies. The room temperature magnetoresistance of the 142 nm NW was ∼0.8% at B = 2 T, which is considerably higher than that [0.2% at B = 2 T, Ovsyannikov et al., Sol. State Comm. 126, 373, (2003)] of the PbTe bulk reported.


I. INTRODUCTION
PbTe is a semiconductor with an energy band gap of 0.32 eV at 300 K.7][8] PbTe doped with transition elements is perspective material for applications in spintronics.Electrical properties in such manner obtained materials strongly depend on the type and concentration of the added dopants. 9Various phenomena have been observed in doped A IV B VI compounds, which not only show the potential application of the doped semiconductors but also indicate that numerous properties are unique to these compounds. 10Magnetoresistance (MR) measurements provide an informative technique for characterizing NWs because they yield detailed information about the geometry of the Fermi surface 11,12 and localization effects in the NWs. 13,14 he presence of the peak in the MR of NWs requires a high quality crystal with long mean free paths of carrier along the NW axis, so that most scattering events occur at the wire boundary instead of at a grain boundary, at impurities sites, or at defect sites within the nanowire.We previously synthesized PbTe NWs from a PbTe thin film on a SiO 2 /Si substrate using a stress-induced method, 15 the technique provide high-quality single crystals of NWs.Thus these high qualities crystalline NWs are adequate for the study which has not a Authors to whom correspondence should be addressed: E-mail: dediamada@phys.sinica.edu.tw;E-mail: cheny2@phys.sinica.been reported yet.In this paper we present structural, electronic transport and MR measurements of a single crystal PbTe NW.

II. EXPERIMENTAL
PbTe NWs were synthesized using stress-induced growth, which is a practical method for growing NWs with a high aspect ratio and high crystallinity. 15As-received Pb (Alfa Aesar, -200 mesh, 99.9%) and Te (Alfa Aesar, -325 mesh, 99.99%) powders were first mixed at a specific ratio 1:1, and melted at 1000 • C for 4 h in a vacuumed quartz tube below 5 × 10 −6 Torr.The molten compound was slowly cooled to room temperature in the furnace.Subsequently, a disk cut from the compound was served as the target for the pulsed laser deposition.Single-crystal SiO 2 /Si (100) wafers (e-light Tech.Inc.; SiO 2 thickness, 413-434 nm; diameter, 100 ± 0.5 mm) with double-side polishing were cut into 1.5 × 1.5 cm 2 squares as substrates, which were cleaned using acetone, isopropyl alcohol, deionized water in an ultrasonic bath for 10 min before being dried with N 2 stream.The PbTe films were prepared in an ArF excimer laser (Lamda Physik LPXpro 210) and deposited onto substrates in a vacuum system with a base pressure of 5.0 × 10 −7 Torr.The PbTe thin films were grown with deposition rate at 0.22 Å/s, which the excimer laser was applied at 140 mJ (frequency, 10 Hz) for 15 min at room temperature.The substrate was rotated at approximately 10 rpm, and the thickness of the films was 20 nm.The films were sealed in a vacuumed quartz tube below 5 × 10 −6 Torr, annealed at 450 • C for 5 d followed by furnace cool to room temperature.During the annealing process, the NWs were grown from the film by compressive stress release due to the difference of the thermal expansion coefficient between the PbTe film and SiO 2 /Si substrate.

III. RESULTS AND DISCUSSIONS
Figures 1(a)-1(c) respectively show the X-ray diffraction (XRD) patterns of the PbTe for annealed thin film, un-annealed thin film and bulk.XRD spectra analysis was conducted for phase identification using a powder XRD (X'Pert PRO-PANalytical, CuKα radiation) from 2θ = 20 • to 65 • .The Figures 1(a) and 1(b) show that the preferential orientation (h00) of a PbTe film.All of the diffraction peaks were indexed to the face-centered cubic (FCC) PbTe structure with the space group Fm-3m (225), as referenced from ICSD no.98-009-0653 (Fig. 1(d)).With these data, the values of measurements.Figure 3(c) shows the temperature dependence of ρ and σ for a 142-nm NW that exhibits a semiconducting behavior.The σ at near room temperature was 2383 S m −1 , which was lower than that of the PbTe bulk (65789.47S m −1 ). 8 It is likely caused by the difference in the carrier concentration or mobilities values.[17] All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported license.See: http://creativecommons.org/licenses/by/3.0/Downloaded to IP: 140.109.103.227On: Wed, 14 May 2014 02:53:04 The MR was measured with the applied magnetic field B perpendicular to the longitudinal axis.In this arrangement, the electric current was perpendicular to B. The value of MR was calculated using the formula M R = R H −R 0 R 0 × 100 %, where R H and R 0 represent the resistance R with and without a magnetic field, respectively (Figs. 4(a) and 4(b)).The dependence of perpendicular MR up to B = ± 8 T of the 142 nm NW is parabolic for all measurement temperatures.The obtained positive MR for the 142-nm NW at 150 K is approximately 7% at B = 8 T, which is about 2.24 times higher than that at room temperature, whereas the room temperature MR of the 142 nm NW is ∼0.8% at B = 2 T, which is considerably higher than that of the PbTe bulk reported (MR = 0.2% at B = 2 T). 18For the measurement at 325 K, the MR curve exhibits relatively similar behaviour to that of the measurement at 300 K.According to Al'tshuler et al., 19 the sign and magnitude of MR and the temperature and field dependence characteristics of semiconductors are determined by multiple factors, including band structure, spin-and inter-valley relaxation mechanisms, sample dimensionality, interactions between impurities and electrons, and the scattering of electrons within the wire boundaries.

IV. SUMMARY
This study reports the structural characteristics of a single-crystal PbTe NW with 142-nm diameter that was synthesized using stress-induced growth.][17] Furthermore the magnetoresistance of the NW at 150 K was approximately 7% when the applied magnetic field was ± 8 T perpendiculars to the longitudinal axis, which was 2.24 times higher than that obtained at room temperature.No previous magnetoresistance study has reported this result for single-crystal PbTe NW.

FIG. 2 .
FIG. 2. (a) SEM image (top view) of the PbTe NWs grown from the surface of the PbTe thin film.(b) The distance between the crystal faces is 0.33 nm, indicating the growth direction along [200].(c) The SAED pattern at the [001] zone axis.(d) EDS spectrum of a PbTe NW.

FIG. 4 .
FIG. 4. (a) Electrical resistance and (b) magnetoresistance of the PbTe NW (d = 142 nm) as a function of the magnetic field at 325 K (green dot), 300 K (blue dot), and 150 K (red dot).