Preparation of NbAs Single Crystal by the Seed Growth Process

: A Weyl semimetal is a novel crystal with low-energy electronic excitations that behave as Weyl fermions. It has received worldwide interest and was believed to have opened the next era of condensed matter physics after graphene and three-dimensional topological insulators. However, it is not easy to obtain a single large-size crystal because there are many nucleations in the preparation process. Here, a bottom-seed CVT growth method is proposed in this paper, and we acquired the large-size, high-quality NbAs single crystals up to 5  4  4 mm 3 finally. X-ray diffraction and STEM confirmed that they are tetragonal NbAs, which the key is to use seed crystal in vertical growth furnace. Notably, the photoelectric properties of the crystal are obtained under the existing conditions, which paves the way for the result provides clues to its light response mechanism. It shows that NbAs have stronger absorption and more energy


Introduction
Weyl semimetals (WSM) have a topologically non-mediocre energy band structure and play an essential role in the study of fundamental physical properties that is a threedimensional analog of graphene, in which the conduction and valence bands cross near the Fermi energy [1][2][3] . The band-crossing point, the so-called Weyl point, acts as a magnetic monopole (a singular point of Berry curvature) in momentum space and always comes in pairs [4,5] . This peculiar phenomenon makes these materials exhibit many unique properties, such as Fermi arc [3] , ultra-high mobility [6] , negative magnetoresistance caused by chiral anomalies [7] , and broadband light response in the mid-infrared range [8] .
NbAs is one of the first-class Weyl semimetal (TaAs, TaP, NbAs, NbP), which crystallizes in a body-centered tetragonal unit cell with lattice constants a = 3.452 Å, c = 11.679 Å, and the space group is I41md(C Sadly, there are always many nucleations in this process during the growth of NbAs. We believe there are two reasons. On the one hand, the transmission capacity of the raw material atmosphere is limited. On the other hand, the quartz tube's axial and radial temperature gradients are large [13,14] . As a result, the environment of raw material atmosphere during crystal growth is uneven and unstable, and the continuity of crystal growth cannot be maintained. Finally, small polycrystalline grains are obtained, which is very unfavorable to the growth of large-size crystals. This paper provides a bottom-seed crystal growth process, which solves the above problems. Firstly, the vertical CVT process growth method is used, the raw materials were placed at the bottom of the quartz tube [15] . In this case, the raw material concentration at the bottom of the quartz tube will be much greater than that at the top, which is conducive to the growth of crystals at the bottom of the quartz tube when the transport distance of gas is significantly shortened. Secondly, the volume of the growth zone used for vertical growth will be much smaller than of the horizontal CVT method, thus reducing the axial and radial temperature gradients in the growth zone and further ensuring the stability and continuity of crystal growth. In addition, the crystal with the exposed surface of (001) is added as the seed crystal, which provides a growth site for the raw material atmosphere and reduces the barrier of crystal growth. So we can think these methods are the key to obtaining large-size and high-quality crystals [16,17] .

Synthesis of NbAs crystal
As we all know, exploring an appropriate growth process is the key to obtaining large crystals. In this work, we reported a bottom-seed CVT growth method, and Figure 1(b) shows the experimental configurations. In a typical run, Nb foil (99.99%), As (99.99%), and I2 (99.99%) with the molar ratio of 1:1.05:0.05 were selected as raw materials, loaded in a 45 ml quartz tube, which was 14 cm in length and 2 cm in inner diameter. Meanwhile, we selected the grain of NbAs with classy crystal orientation (here, the {001} facets was selected as the growth section) as the seed, then it is added into the quartz tube together with the raw materials.
The quartz tube was initially filled with argon and sealed quickly to avoid the loss of iodine and arsenic when evacuated to a pressure below 410 -4 Pa. Nb powder, Nb foil has a larger specific surface area and few defects, which is conducive to acquiring large crystals [10] . In addition, Saini et al. [18] found that As must be excessive if we want to obtain the best crystal growth effect that even for the growth of metal-rich crystals in the study of the Pd-As system. Therefore, here we add a slight excess of As to achieve the growth of NbAs single crystal. As for I2, it plays a catalytic and transport role that is accord to the kinetic analysis of I2 in the growth of TaAs crystal by Li [11] et al. And to avoid the explosion of quartz tube caused by the vapor pressure generated when I2 and As sublimated to gaseous, the amount of I2 selected is 1 mg/ml here.
Afterward, the quartz tube was heated gradually from room temperature to 1000 ℃ over 72 h, keep it for 30 days in a high-temperature environment of 1050 ℃ and finally naturally cooled down to room temperature (Although the reaction time is longer, after comparing the growth results many times, a long-term heat preservation process is essential). Through the above work, we obtained NbAs single crystals with good crystallinity, with the size of up to 433 mm 3 in three-dimension (as the Figure 1(c) shown). However, we have not seen giant NbAs single-crystal reported at present.

Device Fabrication
NbAs devices are produced by dry transfer technology under argon filling conditions. With the help of an optical microscope, the tape was utilized to fix the surface of the NbAs crystal. After determining the position of the sample, use thermal evaporation to deposit Cr/Au (10 nm/70 nm) as a contact electrode. Semiconductor parameter analyzer (Keithley, 4200-SCS) and standard probe station are earmarks for electronic and photoelectric measurement. The optical response of the device was measured using a laser with adjustable power and incident wavelength.

Structure Analysis
Niobium arsenide, NbAs, crystallizes in a body-centered tetragonal Bravai lattice, space group I41md, point group C4v. Our X-ray diffraction (XRD) obtains lattice constants of a = 3.45 Å and c = 11.68 Å, consist with earlier crystallographic studies. As shown in Figure 2(a), the upper black line in the diagram is the XRD of NbAs-powder (obtained by grinding NbAs-crystals), which is completely consistent with the data in PDF No. 17-0896 (blue line in the figure). In addition, we checked the main peak positions and marked the corresponding crystal planes. Among them, few noncorresponding crystal planes belong to polyarsenides (such as NbAs2) [19,20] , and their existence is temporarily unavoidable in the growth of this kind of crystal. The result shows that the grown NbAs have high purity and few miscellaneous items.
Note that the seed of NbAs, its exposed surface is mainly {001} facets, and the onedimension length is about 1.5mm. These faces have relatively larger lattice-plane-space and, therefore, a slower growth rate and thus are found more frequently than others in the as-grown crystals [11] . Nevertheless, when we use these seed crystals for further growth, they can provide a basis for the growth of other plans in the NbAs crystal so that other poorly developed planes can be displayed.

Material Characterization
As a widely used and powerful characterization method, Raman spectroscopy can preliminarily understand the phono information of synthetic crystals [21] . We characterized the NbAs crystals by Raman spectroscopy, shown in Figure 2(b) of bulk NbAs crystals, which mainly displayed three prominent Raman peaks in the range of 180-330 cm -1 . According to the previous Raman spectra of NbAs [22,23] , we marked the vibration modes corresponding to the three peak positions in Figure 2(b), which are B1(1) mode at 234 cm -1 , B1(2) mode at 252cm -1 , and A1 mode at 272cm -1 respectively. The spectral characterization results are consistent with the theoretical data [24,3] , indicating the excellent crystalline quality of NbAs crystals.
A typical XPS (X-ray photoelectron spectroscopy) spectrum of NbAs is shown in Figures 2(c) and (d). The experimental data were labeled by consulting the literature and comparing the database, i.e., Nb 3d5/2 at 203.1 eV and Nb 3d3/2 at 205.8 eV. The pair has an energy splitting of ~2.7 eV [22] , which is consistent with our expectation for the spin-orbitsplit 3d5/2 and 3d3/2 levels of Nb core levels. Similarly, the XPS data of As at the low binding energy end is consistent with the reported, indicating that the 41.5 eV belongs to the 3d orbit of As.
As a means of characterizing crystal quality and structure, atomic morphology characterization can more intuitively and accurately reflect the specific information of Nb and As atoms in NbAs crystals. Figure 3(a) is a typical STEM image that shows clear lattice stripes, and Figure 3(b) is a low-power SEM image of bulk NbAs placed on an ultra-thin carbon film. The test results say that the spacing between (001) and (010) is about 3.4 Å, which is consistent with the data in the literature. At the same time, the diffraction points of (110) and (001) are also in the Fast Fourier Transform (FFT) diagram in Figure 3(a). Similarly, the structure of these facets was drawn in Figures 3(d) and 3(e). In the recent research of NbAs crystals, the coexistence of hexagonal and tetragonal phases was discussed. The observation or characterization of NbAs single crystal on the atomic scale shows that the crystal is not a single phase structure [9] . In our experiments, the coexistence phase problem was overcome by improving the bottom-seed method and synthesizing the high-quality NbAs crystals with a single-phase structure, highlighting our work's significance. The EDS (energy dispersion spectrum) is shown in Figure 3(c), in which the uniform distribution of Nb and As atoms can be seen. The illustration also indicated that Nb: As is close to 1:1. This information proved the high quality of our crystals on the micro-scale.

Photoelectric response detection of NbAs crystal
As for the photoelectric response of NbAs, the photodetection of TaAs can be used as a reference [25] . Inspired by TaAs broadband photodetection and the inherent limitations of the experimental environment, we only investigated a photo-detecting prototype made by Weyl semimetal NbAs in a visible light region (550 nm-800 nm) at room temperature.
After evaporation, the tape is carefully torn off, and the area covered by the tape becomes the device channel of the unevaporated electrode. The optical microscope photo of the device is shown in Figure 4(a). The dark current (I) is measured under an applied voltage (V) that varies from -0.1 to 0.1 V. The I-V curve is shown in Figure 4(b). The linearity of the curve indicates that the Schottky barrier at the contact interface between the TaAs sample and the Cu electrode is shallow. Therefore, the incident light is usually projected on the (001) surface, as shown in the red circle marked area in Figure 4(a). NbAs absorb photons, then causes electron transitions and generate carriers, leading to the generation of photocurrent, the difference between current under light and dark. In order to form a good comparison with TaAs, we also measured the time-resolved light response of the NbAs device at a source-drain voltage of 0.1mV. We only list the test results under 550 nm and 800 nm lasers, as shown in Figure 4(c-d). It can be seen that the photoresponse current of NbAs is equivalent to that of TaAs (under laser conditions, the response current is in the μA level), and the switching ratio is close to 5, but the bright and dark current is relatively poor under stable conditions. We believe that, compared to the photoelectric detection of TaAs, we have not adopted data optimization processing to demonstrate the intrinsic properties of NbAs better. In addition, because our laser has poor stability at 10mW (if the laser intensity is too small, the photoresponse current of the order of μA will be more difficult to distinguish), can not reach the TaAs photoelectric detection conditions (stable 20mW laser intensity output), so the photoresponse current is unstable. Although we cannot reach the test conditions of TaAs, the photoresponse current of the same magnitude under the same source and drain voltage conditions indicates the research potential of NbAs in wide-band photoresponse detection [25] . The photoelectric responsivity (R) and detection rate (D*) can be defined as [26][27][28][29] ph I R PA = (1) Among them, ph I , P, and A represent the photocurrent, incident optical power density, and the effective irradiation area of the detector.
Where, R, S, q, and Idark respectively represent the responsivity, the effective irradiation area of the heterojunction, the primary charge, and the dark current. Figure 5(a-b) lists the device's R and D* related information in the range of 550-800 nm (corresponding to the incident photon energy of 1.5 to 2.25 eV) when the incident power is fixed at 10 mW. It can be seen that R and D* do not change significantly with the incident photon energy. Except for the high responsivity at 550 nm, the responsivity is stable at 2 mA/W (D* close to 10 7 Jones). This result is close to the reported value of TaAs in the visible light band (R = 0.4~0.7 mA/W, D* = 4~7×10 8 Jones) [25] . When the incident laser wavelength is 550 nm, R and D* reached 16mA/W and 1.1×10 8 Jones, respectively. This result provides clues to its light response mechanism. It shows that NbAs have stronger absorption and more energy scattering events at short wavelengths, which helps to achieve a more significant light response. In addition, more incredible energy is transferred from the absorbed photons to the electrons excited on the short-wavelength side.
It is worth noting that, according to reports [25] , TaAs have a broad-spectrum light response from visible range to the long-infrared range at room temperature. As for NbAs, we only conducted preliminary tests on its light response in the visible light range, and its broadband light response at room temperature still needs more in-depth theoretical and experimental research. The relationship between the detection rate D* of NbAs devices and the energy of incident photons.

Conclusions
In summary, the one-dimensional size of the NbAs single crystal grown by the CVT method using the bottom seed method can reach 4 mm, and the three-dimensional size can reach 4×3×3 mm 3 . The quality of the product has been confirmed by powder XRD and Raman methods. More importantly, the atomic-level morphology characterization results show that we have overcome the multi-phase coexistence in the past growth process. XPS data explains that the surface state of NbAs crystals will be affected by oxygen and lead to some changes, as specific results need to be further studied and discussed. In addition, we prepared a prototype of the NbAs photodetector in the visible light range and discussed the relationship between R and D* with the energy of incident photons. The result is equivalent to the report of the TaAs photodetector in the visible light range [30] . Our improved growth method provides reference and helps grow difficult-to-synthesize materials such as Weyl semimetal. At the same time, the synthesis of high-quality crystals guarantees the subsequent electrical properties research and photoelectric performance detection of this type of Weyl semimetal.