Strong correlation of the growth mode and electrical properties of BiCuSeO single crystals with growth temperature

Abstract

In this paper, BiCuSeO single crystals are successfully grown by the flux method under different growth temperature (690 °C, 730 °C and 775 °C). The crystal surface morphology, microstructure, chemical composition and electrical property are systematically characterized. By changing the growth temperature, the growth mechanism evolution, from dislocation-driven spiral growth mode to two-dimensional layer-by-layer one, is observed due to the different growth supersaturation. Simultaneously, temperature-dependent resistance substantiates that the electrical property changes from semiconductor to metal. Chemical analysis proves that BiCuSeO crystals grown on higher temperatures (730 °C and 775 °C) are slightly non-stoichiometric. The present results demonstrate the possibility to modulate crystal morphology and electrical property of BiCuSeO by controlling supersaturation. This method may be applicable to similar compounds (BiCuOCh (Ch=S, Te)). 6 four-probe technique on a physical property measurement system (PPMS, Quantum Design Inc.). Results and discussion Characteristics of the as-grown BiCuSeO CrystalsThe inset of Fig. 2(a) shows a photograph of the BiCuSeO single crystal synthesized at 730 o C.One can see that the thin sheet-shape crystals, with the typical sizes of 2.5×2×0.05 mm 3 , show metallic luster. As depicted in Fig. 2(a), all reflections in the X-ray diffraction (XRD) result of BiCuSeO single crystal can be indexed as (00l). It confirms that the thin foil samples were grown along ab-plane. The Laue pattern further proves that the axis perpendicular to BiCuSeO crystal surface belongs to c-axis (see inset of Fig. 2(a)). It proves again that the growth direction of BiCuSeO is along ab-plane. SEM images of the as-grown BiCuSeO crystal and the corresponding element mapping of Bi, Cu and Se are presented in Fig. 2(b). The distribution of Bi, Cu and Se in the grown sample is homogeneous. It strongly suggests that the growth is homogeneous. The XRD patterns of BiCuSeO single crystal powders under different growth temperatures (690 °C, 730 °C and 775 °C) are shown in Fig. 2(c). All Bragg peaks can be indexed to the ZrSiCuAs-type structure and no peaks of impurity phase are detected within the instrument detection limit. The XRD pattern and Rietveld refinement of BiCuSeO-775 powders are shown in Fig. 2(d).Comparing these results, we substantiate that the experimental data (black circle) are in good agreement with the calculations (red line). The lattice parameters of this sample are a=b=3.9821Å and c=8.8744Å, similar to previous results. 12 The unit-cell parameters extracted from the Rietveld refinement are summarized in Table 1. It is demonstrated that the lattice constants a, b and c decrease obviously as the growth temperature increasing. As is well known, the BiCuSeO crystal structure (shown in the inset of Fig. 2(c)) is formed by the alternative stacking of (Cu 2 Se 2 ) 2− layers

Growth of c‐Axis‐Oriented BiCuSeO Thin Films Directly on Si Wafers

“…Figure shows that the room temperature ρ ab and S ab of the film are about 14 mΩ cm and 210 μV/K, respectively, which are comparable to those of high‐quality epitaxial thin‐film samples grown on SrTiO 3 single‐crystal substrate . Moreover, the room temperature ρ ab of the present film is found to be much lower than those obtained from polycrystalline bulks (~0.1–40Ω·cm) as well as the recently reported value for single‐crystal samples (~90–770 mΩ·cm) . The very low resistivity of the thin‐film sample in comparison with the single crystals remains unclear now as most data, including the carrier concentration n and the carrier mobility μ , are still not available for single‐crystal samples .…”

“…28 Moreover, the room temperature q ab of the present film is found to be much lower than those obtained from polycrystalline bulks (~0.1-40ΩÁcm) [10][11][12][13][14]23 as well as the recently reported value for single-crystal samples (~90-770 mΩÁcm). 29 The very low resistivity of the thin-film sample in comparison with the single crystals remains unclear now as most data, including the carrier concentration n and the carrier mobility l, are still not available for single-crystal samples. 18,29 When compared with the BiCuSeO polycrystalline bulk samples, the reduced resistivity of the thin film can be partially ascribed to the c-axis-orientated nature of the film.…”

“…29 The very low resistivity of the thin-film sample in comparison with the single crystals remains unclear now as most data, including the carrier concentration n and the carrier mobility l, are still not available for single-crystal samples. 18,29 When compared with the BiCuSeO polycrystalline bulk samples, the reduced resistivity of the thin film can be partially ascribed to the c-axis-orientated nature of the film. In addition, the high carrier concentration of the film (n~6.6 9 10 19 cm À3 at 300 K), which is nearly an order of magnitude larger than those reported in most polycrystalline bulks, [5][6][7][10][11][12] should be also responsible for its decreased resistivity.…”

Promising Thermoelectric Bulk Materials with 2D Structures

“…[112] To date, only flakes of BiCuSeO single crystals from a NaCl/KCl flux have been reported. [113] As an alternative, the increase in the degree of orientation along the in plane direction can partially achieve this advantage. [52] Figure 6b www.advmat.de www.advancedsciencenews.com backscattered diffraction (EBSD) detector.…”

BiCuSeO Thermoelectrics: An Update on Recent Progress and Perspective

“…They successfully synthesized BiCuSeO single crystals through a flux method at 963 K, 1003 K and 1048 K, respectively. BiCuSeO crystals with a typical size of 2.5 × 2 × 0.05 mm 3 were obtained [45]. Meanwhile, Wu et al [46] synthesized Bi 1− x Pb x CuSeO film with high texture orientation through the pulsed laser deposition technique on the SrTiO 3 (001) substrates, a high power factor of ~1.2 mW·m −1 ·K −2 at 673 K was realized.…”