Polarization sensitive photodetector based on quasi-1D ZrSe3

Xingang Wang; Tao Xiong; Kaiyao Xin; Juehan Yang; Yueyang Liu; Zeping Zhao; Jianguo Liu; Zhongming Wei

doi:10.1088/1674-4926/43/10/102001

Abstract

The in-plane anisotropy of transition metal trichalcogenides (MX₃) has a significant impact on the molding of materials and MX₃ is a perfect choice for polarized photodetectors. In this study, the crystal structure, optical and optoelectronic anisotropy of one kind of quasi-one-dimensional (1D) semiconductors, ZrSe₃, are systematically investigated through experiments and theoretical studies. The ZrSe₃-based photodetector shows impressive wide spectral response from ultraviolet (UV) to near infrared (NIR) and exhibits great optoelectrical properties with photoresponsivity of 11.9 mA·W^-1 and detectivity of ~10⁶ at 532 nm. Moreover, the dichroic ratio of ZrSe₃-based polarized photodetector is around 1.1 at 808 nm. This study suggests that ZrSe₃ has potential in optoelectronic applications and polarization detectors.The in-plane anisotropy of transition metal trichalcogenides (MX₃) has a significant impact on the molding of materials and MX₃ is a perfect choice for polarized photodetectors. In this study, the crystal structure, optical and optoelectronic anisotropy of one kind of quasi-one-dimensional (1D) semiconductors, ZrSe₃, are systematically investigated through experiments and theoretical studies. The ZrSe₃-based photodetector shows impressive wide spectral response from ultraviolet (UV) to near infrared (NIR) and exhibits great optoelectrical properties with photoresponsivity of 11.9 mA·W^-1 and detectivity of ~10⁶ at 532 nm. Moreover, the dichroic ratio of ZrSe₃-based polarized photodetector is around 1.1 at 808 nm. This study suggests that ZrSe₃ has potential in optoelectronic applications and polarization detectors.

Introduction

In recent years, transition metal dichalcogenides (TMDCs) have gained attention due to their tunable optical, optoelectrical and physiochemical properties^[1] that originate from quantum confinement effects and low dimensionality^[2-12]. Meanwhile, layered semiconductor materials and their heterojunctions have potential applications in constructing the next generation of optoelectronic devices^[13-16]. As one member in the TMDCs family, transition metal trichalcogenides (MX₃) have been a source of attention for many years. The weak van der Waals amalgamated structures of MX₃ constitute a distinct family of compounds, where M represents the metal atom from group IV, V or VI and X is chalcogen atom (S, Se or Te)^[17-19]. The linear chain of MX₃ is parallel to the crystallographicb-axis, six chalcogen atoms surround one metal atom. MX₃ bulk materials are wire-like crystals while the counterparts with few layers are nanoribbons with a quasi-1D structure. Previous works have shown that MX₃ enjoy strong in-plane optical and electrical anisotropy properties, due to their structural anisotropy^[20,21].

Nowadays, most polarization sensitive photodetectors that are composed of an optical medium with polarization effect have realized a high extinction ratio through the polarization selection of plasmon microcavities^[22]. However, these polarized photodetectors suffer from several challenges, including complicated fabrication process and high cost. Recently, active materials for polarized photodetector have focused on semiconductors with low symmetry structure^[23-25], such as black phosphorus^[26], TiS₃^[20], GeAs^[27], GeSe^[28]. This work is conducive to the development of polarized photodetectors towards miniaturization and integration. Analogously, quasi-1D ZrSe₃ with low-symmetry is an ideal candidate for polarized photodetectors.

In this work, we prepared high-quality ZrSe₃ nanowires using the chemical vapor transport (CVT) method. The anisotropic optical and optoelectrical properties of ZrSe₃ are confirmed by polarization-resolved optical microscopy (PROM) and polarized optoelectrical measurements. The angularly resolved Raman spectra and polarized light absorption spectroscopy show that ZrSe₃ have in-plane anisotropy. Furthermore, the results of the polarized photodetection imply that ZrSe₃-based photodetectors are sensitive to linearly polarized light.

Results and discussion

Synthesis and structural analysis

High-quality ZrSe₃ nanowires (NWs) were synthesized via the CVT method. Powder Zr (0.45 g) and Se (1.35 g) were mixed in stoichiometric proportions (1 : 3) with 30 mg I₂ as transport agent. The sources were then placed in an ampoule that was placed into a double-zone furnace. The temperatures of the high temperature zone and the low temperature zone are set as 650 °C and 500 °C, respectively. The ampoule was heated to 650 °C/500 °C for 24 hours and kept at this temperature for 3 days. After heating, the furnace was cooled to room temperature in 1 day and the ZrSe₃ NWs had collected on the inner surface of tube. The structure of ZrSe₃ belongs to monoclinic structure witha = 3.788 Å,b = 5.485 Å andc = 10.156 Å, and a symmetry space group of p2₁/m^[29]. Transmission electron microscopy (TEM) is an effective method to characterize crystal microstructure and growth direction.Fig. 1(a) is a low-magnification TEM image of the ZrSe₃ NWs. The high-resolution transmission electron microscopy (HRTEM) image (Fig. 1(b)) was obtained from the marked region ofFig. 1(a). As shown inFig. 1(b), the lattice spacing is 0.38 nm, which represents ( $\bar{1} 02$ ) plane, which indicates that the ZrSe₃ NWs prefer to grow along theb-axis. The obvious diffraction spots in the selected area electron diffraction (SAED) pattern (Fig. 1(c)) indicate that the ZrSe₃ is a single crystal. The diffraction spots agree with (200), ( $\bar{1} 02$ ) and ( $\bar{2} 12$ ) plane, respectively. The elements of Zr and Se are distributed uniformly on the nanowire, as shown in elemental mapping (Fig. S1). The energy-dispersive X-ray spectroscopy (EDX) shows that the elemental stoichiometric ratio is around 1 : 3 (Fig. S2). In addition, the ZrSe₃ NWs were measured by atomic force microscopy (AFM) (Fig. 1(d)), which shows that the thicknesses of ZrSe₃ are about 18 nm. The chemical state can be characterized through the method of X-ray photoelectron spectroscopy (XPS). As shown inFigs. 1(e) and1(f), the binding energies of 179.48 and 182.78 eV are Zr 3d_5/2, 182.28 and 184.88 eV are Zr 3d_3/2^[30]. This observation is similar with the analysis of HfTe₃, in which the Hf was assumed with mixed-valence states^[31]. Therefore, we also speculate that Zr⁴⁺ (the doublets located at 182.78 and 184.88 eV) and Zr²⁺ (the doublets located at 179.48 and 182.28 eV) coexist in the ZrSe₃ NWs. The peak area shows that the ratio of Zr⁴⁺ and Zr²⁺ is approximately 1 : 1.

$(Color online) Characterization of ZrSe3 crystal. (a) Low-magnification transmission electron microscopy (TEM) of the quasi-1D ZrSe3. (b) High-resolution transmission electron microscopy (HRTEM) image of the quasi-1D ZrSe3. (c) Selected area electron diffraction (SAED) pattern of the quasi-1D ZrSe. (d) The AFM image of ZrSe3 crystal. (e) High-resolution spectra of Zr 3d core level. (f) High-resolution spectra of Se 3d core level.$

Figure 1.(Color online) Characterization of ZrSe₃ crystal. (a) Low-magnification transmission electron microscopy (TEM) of the quasi-1D ZrSe₃. (b) High-resolution transmission electron microscopy (HRTEM) image of the quasi-1D ZrSe₃. (c) Selected area electron diffraction (SAED) pattern of the quasi-1D ZrSe. (d) The AFM image of ZrSe₃ crystal. (e) High-resolution spectra of Zr 3d core level. (f) High-resolution spectra of Se 3d core level.

Angle-resolved polarized Raman spectroscopy (ARPRS) is a powerful method to characterize the material structure that can be used to determine the symmetry of chemical bond vibration^[32]. The unit cell of ZrSe₃ includes eight atoms, giving rise to 24 normal vibration modes. The irreducible representations at the center of Brillouin zone: Г = 8A_g+ 4B_g+ 4A_u+ 8B_u. Among these 24 formula units, A_g and B_g modes are Raman active while A_u and B_u are infrared-active.Fig. 2(a) shows the Raman spectra of ZrSe₃ NWs under 532 nm. It is clear that there are four peaks of normal Raman spectra at or near 107.22, 178.87, 234.61 and 301.4 cm⁻¹, which are all assigned to A_g mode^[29]. Furthermore, the polarized Raman spectra was measured in parallel and cross configurations and the ZrSe₃ NW was rotated from 0° to 180°. The contour color maps inFig. 2(b) and2(c) show the obvious anisotropy of chemical bond vibration. The typical Raman peaks of 178.9, 234.61 and 301.4 cm^–1 are extracted and fitted in polar coordinates (Figs. 2(d) and2(e), Fig. S3). The formula of the fitting curve can be expressed as $I \propto$ [e_iRe_s]², wheree_i ande_s are the polarization vectors of incident light and scattered light, respectively (e_i = [cosθ sinθ 0],e_s// = [cosθ sinθ 0]^T,e_s⊥=[–sinθ cosθ 0]^T), andR is the Raman tensor. The Raman tensor of A_g (ZrSe₃ correspond to symmetry space group of p2₁/m) is as follows:

Figure 2.(Color online) (a) Raman spectra under unpolarized and polarized laser (532 nm). (b) Counter maps of angle-resolved Raman spectra under cross configuration. (c) Counter maps of angle-resolved Raman spectra under parallel configuration. (d) Polar plots of angle-resolved and fitted peak intensities of 234.6 cm⁻¹. (e) Polar plots of angle-resolved and fitted peak intensities of 301.4 cm⁻¹.

$A_{g} = [\begin{matrix} a e^{i \cdot φ a} & 0 & d e^{i \cdot φ d} \\ 0 & b e^{i \cdot φ b} & 0 \\ d e^{i \cdot φ d} & 0 & c e^{i \cdot φ c} \end{matrix}] .$

After calculation, the Raman scattering intensities of A_g mode can be expressed as follows:

$A_{g / /} = A \cos^{4} θ + B \sin^{4} θ + C \sin^{2} 2 θ,$

$A_{g ⊥} = A \cos^{2} 2 θ + B \sin^{2} 2 θ + C \sin 4 θ .$

The fitting curves of cross configuration show four-leaf shapes. For parallel configuration, the fitting curves show spindle shapes. All of these results indicate that the structure of ZrSe₃ crystal is anisotropic.

Theoretical calculations

The schematic crystal structure inFig. 3(a) shows infinite ribbons stacked along theb-axis and the layers are combined with each other through van der Waals interaction. Based on the crystal structure, the theoretical calculation of ZrSe₃ is studied. The band structure of ZrSe₃ was calculated through Heyd-Scuseria-Ernzerhof hybrid function method (HSE06)^[33], exhibiting a band gap of 1.05 eV (Fig. 3(b)). For the theoretical calculation, the generalized gradient approximation PBE (GGA-PBE) of correlation functional and exchange were used in the calculation^[34,35]. We then used the hybrid functional (HSE06) method for the accurate calculation of optical property and electronic structure, in which the hybrid functional was mixed with 25% exact Hartree–Fock (HF) exchange^[33]. In addition, a Monkhorst-Pack k–mesh of 6 × 9 × 4 sampling in the full Brillouin zone was used in the calculations^[36]. The integral path of the band structure is Γ → X → S → Y → Γ. The electronic band structure indicates that the valence band maximum (VBM) is located at the X point and the conduction band minimum (CBM) is located at the Γ point. Moreover, the total density of states (DOS) and partial density of states (PDOS) (Fig. S4(a)) were calculated. It is shown that the top of the valence band is mainly composed of Se’s p orbital of Se atom, while the bottom of the conduction band is mainly made up by Zr’s d orbital of the Zr atom. The dielectric constants along thea-axis andb-axis of ZrSe₃ were calculated as shown inFig. 3(c), and the corresponding optical absorbance coefficient (Fig. S4(b)) was calculated theoretically based on the dielectric function^[37]. These results show that there is an obvious difference alonga-axis andb-axis from 250 to 820 nm. The electric dipole transition probabilityR is positively related to the electron-radiation interaction |<c|HeR|v>|², where HeR is Hamiltonian matrix element, |c> and |v> are wave functions of conduction band and valence band^[38]. The transition probability (R_a,R_b) from valence band maximum (VBM) to conduction band minimum (CBM) along different k-points path can be calculated throughR_a = |<c|HeR_a|v>|²/(|<c|HeR_a|v>|² + |<c|HeR_b|v>|²),R_b = |<c|HeR_b|v>|²/(|<c|HeR_a|v>|² + |<c|HeR_b|v>|²). The results ofR_a andR_b are shown inFig. 3(d). Some typical point, such as S and Y, are selected to be analyzed. For the S point, the probabilityR_a is around 0.64 andR_b is around 0.36, which means that the strength of absorption alonga-axis is larger than theb-axis. While for the Y point,R_a is very close to 1, which means that around the Y point the absorption only occurred along thea-axis. The difference strengths of absorption along thea-axis andb-axis lead to the polarization sensitive of ZrSe₃. Furthermore, the polarization sensitivity of ZrSe₃ is influenced by the electron transition of different energy levels and is not limited to the VBM and CBM. This obvious difference of transition probability is based on the charge density distribution alonga andb axes (Figs. 3(e)–3(f)). These results indicate that quasi-1D ZrSe₃ enjoys optical and optoelectric anisotropy.

Figure 3.(Color online) (a) Atomic structure of ZrSe₃ crystal. (b) Band structure of layered ZrSe₃. (c) Calculated real parts and imaginary parts of the dielectric constant alonga-axis andb-axis. (d) TheR_a andR_b of optical transition |v> → |c> along the k-points path. (e, f) Partial charge density of ZrSe₃ at the state of CBM and VBM respectively.

Optical and photoelectrical anisotropy

The optical anisotropy of quasi-1D ZrSe₃ was studied through polarization-resolved optical microscopy (PROM). For the PROM test, ZrSe₃ NWs were exfoliated on the SiO₂/Si substrate, which is isotropic. During the investigation, the direction of polarization analyzer is perpendicular to the linear polarized incident light. It can be seen in Fig. S5 that the images of PROM, which are selected from 0° to 165° by a step of 15° and the brightness of ZrSe₃ express obvious light and dark changes. The reason for this phenomenon is that linearly polarized incident light may be refracted into two components with different vibration directions after passing through the birefringent material^[39]. The result of PROM indicates that quasi-1D ZrSe₃ crystal enjoys obvious anisotropy.

To verify the in-plane optical anisotropy of quasi-1D ZrSe₃, we exfoliated and transferred the ZrSe₃ crystal onto mica. The measured polarization-resolved absorption spectrum is shown in Fig. S6. It is obvious that the maximum absorption appears in the polarized light of 90°, which corresponds to thea-axis. This happens because the incoming photons more effectively interact with thea-axis^[40]. The absorption range covers the entire visible region and part of the near-infrared region, which is nearly coincident with the calculated absorption spectra. To investigate the photoelectric properties of quasi-1D ZrSe₃, the few-layers ZrSe₃ nanowires with a thickness of 18 nm were exfoliated and transferred onto the SiO₂/Si substrate (the thickness of SiO₂ is 300 nm). Because the polarized absorption covered from visible to near-infrared range (360–860 nm), some typical wavelengths (360, 450, 532, 638 and 808 nm) were chosen for the photodetection. As shown inFig. 4(a),Figs. 5(a)–5(c) and Fig. S7, the photocurrent increases with the increase of the light power intensity. Specifically, under 532 nm lumination, the photocurrent increases from 127 to 166.5 pA atV_ds = 5 V with light power intensity from 26.8 to 114.3 mW/cm² (Fig. 4(a)). The relationship of photocurrent and light power intenstiy can be expressed as:I =αP^β, whereα, β are dimensionless constants and I is photocurrent andP is light power intensity. Under the light irradiation, the photocurrent tends to be saturated with stronger light power intensity, which is related to the fact that carriers could be recombianted under the strong irradiation. The results after fitting are shown inFig. 4(b). Corelatively,β = 0.81, 0.15, 0.16, 0.29 and 0.22 corresponding to 360, 450, 532, 638 and 808 nm, respectively. In addition, the other two parameters of the photodetectors are responsivity (R) and detectivity (D*). Responsivity indicates the ability of photodetectors to convert a light signal into an electrical signal, which can be expressed as: $R = Δ I / (P_{λ} S)$ , where ΔI is the difference of photocurrent and dark current,P_λ is power intensity of incident light andS is the effective area of photodetectors. Detectivity (R*) reflects the sensitivity for perceiving weak light, which can be demonstrated as $D^{*} = R S^{1 / 2} / (2 e I_{d})^{1 / 2}$ , whereR is the responsivity,e is unit charge andI_d is dark current. As shown inFig. 4(c), the photodetector based on ZrSe₃ has good performance in the ultraviolet and near-infrared region, especiallyR andD* reach 11.9 mA/W and around 10⁶ Jones at 532 nm. This happens because the short wavelength laser can excite more electrons that are located at different valence bands to the conduction band. The decrease in the ultraviolet region is due to the increased recombination rate of the carriers. As shown in Fig. S8,I–V curves of ZrSe₃-based photodetectors at 360, 450, 638 and 808 nm indicate that the metal-semiconductor contact is an Ohmic contact. Moreover, the rise time is the time from 10% saturated photocurrent to 90% saturated photocurrent and the decay time is the time from 90% saturated photocurrent to 10% saturated photocurrent. The rise time of ZrSe₃-based photodetector is around 2.8 s, while for decay time it is about 30 s. The decay time of ZrSe₃-based photodetector is longer than the rise time because the surface defect states of ZrSe₃ prolong the recombination process of the photogenerated electric-hole pairs. In addition, the short wavelength laser, with larger photon energy, has a higher coupling efficiency leading to thermal heating effect, which results in prolonged response time. The influence of the width of the device is also discussed. The photoelectrical performance of photodetector with different widths (0.7, 1.2, 1.8μm) is measured. The results (Fig. S9) show that the ZrSe₃-based photodetector with appropriate width (1.2μm) has better photoelectrical properties.

Figure 4.(Color online) (a) Time-resolved photoresponse of the ZrSe₃-based photodetector for a bias voltage of 5 V under 532 nm with different light power density. (b) Dependence of the photocurrent on the intensity of incident laser power. (c) The spectral responsivity and detectivity of ZrSe₃-based photodetector. (d) Evolution of the photocurrent with a polarized angle under 532 nm.

Figure 5.(Color online) (a−c) Time-resolved photoresponse of the ZrSe₃-based photodetector for a bias voltage of 5 V under 450, 638 and 808 nm with different light power density respectively. (d–f) Evolution of the photocurrent with polarized angles under 450, 638 and 808 nm, respectively.

Raman spectra, PROM and absorption spectra show that ZrSe₃ NWs possess an obvious crystal structure and optical anisotropy. These results prompt us to expend the investigation from photoelectric detection to polarized photodetection. The linearly polarized light was obtained through the assistance of Glan-Taylor prism, which acts as a polarizer. In addition, we used the half-wave plate to change the direction of the linearly polarized light. The extinction ratio of polarizer (Glan-Taylor prism) reaches 10⁵ : 1, which guarantees the appearance of the linearly polarized light. The drain–source voltage was fitted at 5 V, the direction of linearly polarized light was rotated with a step of 10° or 20°. The photocurrents with the polarized angle from 0° to 360° were extracted and plotted in polar coordinate system (Fig. 4(d) andFigs. 5 (d) and5(e)). The direction of b-axis of ZrSe₃ NWs was defined as 0°. They were then fitted through the equation: $I_{ph} (θ) = I_{pha} \cos^{2} (θ + φ) + I_{phb} \sin^{2} (θ + φ)$ , whereI_pha andI_phb are the photocurrents alonga-axis andb-axis of ZrSe₃ NWs,θ is the polarization angle^[41]. The gourd-shaped fitting curves indicate that the ZrSe₃-based photodetector enjoys polarization-sensitivity and the largest photocurrents are all reached along thea-axis of ZrSe₃ crystal. TheI_pha/I_phb ratios at 450, 532, 638 and 808 nm are 1.02, 1.03, 1.02 and 1.1, respectively. The optoelectrical properties of ZrSe₃-based photodetector are summarized inTable 1. This shows that the quasi-1D ZrSe₃ crystal has application value in the field of optoelectronics.

Table Infomation Is Not Enable

Conclusion

In summary, layered quasi-1D nanowires ZrSe₃ were synthesized with the CVT method. The HRTEM, APRPS, PROM, absorption spectroscopy and polarized optoelectronic investigations show the anisotropy of ZrSe₃ NWs. The ZrSe₃-based photodetector performs a wide photoresponse from UV to NIR with impressive optoelectronic properties, with photoresponsivity of 11.9 mA/W and detectivity of ~10⁶ at 532 nm. It also exhibits polarization-sensitivity due to the anisotropic structure. These results show that quasi-1D ZrSe₃ NWs have the potential in application of polarization photodetector without the help of optical polarizer.

Acknowledgements

This work was financially supported by the National Key Research and Development Program of China (Grant No. 2017YFA0207500), the National Natural Science Foundation of China (Grant No. 62125404, 12004375, 62004193, 62174155), the CAS-JSPS Cooperative Research Project (No. GJHZ2021131), and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB43000000).

Appendix A. Supplementary materials

Supplementary materials to this article can be found online athttps://doi.org/10.1088/1674-4926/43/10/102001.

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