Wavelength Dependent Anisotropic Photo Sensing Activity of Zirconium Trisulde Crystal

According to the performance requirements, either bulk or nanocrystalline form of the material can be used for different types of device applications. In the present study, zirconium triselenide bulk crystals were grown by direct vapour transport technique. The as-grown crystals and powder have been examined under Carl Zeiss optical microscope and scanning electron microscope for morphological studies which revealed the evolution of crystalline phases of the material by the layered kind of growth mechanism. The energy dispersive analysis of X-ray (EDAX) characterization conrms that no impurity is found in the resulting product except the desired material. To study the effect of different wavelength sources (Blue-470 nm, Green-540 nm, Red-670 nm) on bulk zirconium trisulde photodetectors, pulsed photo response experiment was carried out. The anisotropic behaviour is also revealed using the same sources. Various device parameters like responsivity, sensitivity, detectivity and external quantum eciency (EQE%) were calculated. The highest responsivity and detectivity of 81.7 µA/W and 3.56 × 10 07 Jones were achieved for blue (470 nm) light source respectively.


Introduction
Two-dimensional (2D)-layered materials have emerged as a new class of materials with unusual optical, electrical, mechanical and thermal properties. Owing to their unique properties, they attracted researcher's attention for applications in energy conversion, exible electronics and information technology elds [1][2][3] . Transition Metal Tarichalcogenides (TMTCs) of layered group-IV with the chemical formula of MX 3 , e.g., M = Zr, Ti and X = S, Se, Te, are known to be anisotropic 2D systems 4,5 . These trichalcogenides are often synthesized in thin brous ribbons shape and offer several interesting phenomena originating from their large structural in-plane anisotropy as individual MX 3 layers are made out of moderately interacting 1Dlike chain structures [6][7][8][9] . Among these materials, particularly, transition metal trisul des (TMTS) are receiving attention of researchers because of the number of inherent properties. As the groups (S -S) 2− has the capability of reversibly accepting a pair of electrons 10 , these species are studied as cathode materials in chemical current sources 11 , including ZrS 3 for primary thermal Li-battery as well as semiconductor TiS 3 12 . Some transition metal polysul des, in particular semiconducting MS3, are of interest as catalysts in the production of oxygen (ZrS3) 13 or as gas sensors (TaS3) 14  was cleaned with sulfuric acid, hydrochloric acid and simple water periodically one by one. The bottom part of the ampoule was etched by evaporating hydro uoric acid at 100 °C temperature, to create enough rough surface that provide nucleation points. Further the ampoule was cleaned with simple water and acetone and set to dry in oven at 200 °C. This highly cleaned ampoule was then lled with pure zirconium (99.99 %) and sulfur (99.99%) powder with stoichiometric proportions and sealed at a pressure better than 10 -5 Torr. This sealed ampoule was then inserted into a high temperature dual zone horizontal furnace. The ampoule was set such as the half part is situated in higher temperature zone and other half part in lower temperature zone. The temperature of the furnace was initially raised up to 850 °C for 40 hrs, the reaction time was set for 60hrs at maximum temperature and in last stage the furnace was cool down to room temperature in 80 hrs with slow temperature decrement. The temperature difference between the two zones was maintained 50 °C throughout the cycle. The process yielded needle shaped thin akes with shiny metallic appearance.

Characterization
The as grown compound was analysed quantitatively by performing energy dispersive analysis of X-ray analysis with elemental mapping (Fig. 2). The calculated and observed values of weight% of the constituent elements are mentioned in Table. 1. The scanning electron microscopy of as grown ZrS 3 compound was carried out which is shown in Fig. 3 (c, d). Both the EDAX and SEM analysis were performed using a single instrument named FEG-SEM-450. The grown crystal was analysed under the Carl-Zeiss optical microscope to study the nature of the surface of the as grown ZrS 3 crystal ( Fig. 3 (a, b)).
Further, the grown single crystal was characterized for the study of the photo sensing activity. To study the ability and e ciency of ZrS 3 single crystal, it was analysed under the Carl-Zeiss optical microscope the smooth surface essential for the fabrication of good quality photo detector can be seen here. To fabricate the photo detector, a shiny needle shaped ZrS 3 crystal having area of 0.04 cm 2 was cleaved using scotch tape to get fresh and clean surface. This crystal was then attached on a piece of mica sheet (1 x 1 cm 2 ). Two copper wires were attached at two ends of the crystal to get in plane symmetry (parallel to the basal plane) and one is made at the bottom side of the crystal to get out of plane symmetry (conduction perpendicular to the basal plane) using conductive silver paste (Fig. 1). This mica sheet was further attached on a PCB which was connected to Keithly 4200 SCS. The schematic diagram of prepared device and connections is shown in Fig. 1. Pulse photo response experiment was carried out to study the switching behaviour of ZrS 3 crystal towards three different wavelength sources (Blue-670 nm, Green-540, Red-470 nm) for in plane and out plan contact symmetries.

Material characterization
In the growth process, the temperature was raised gradually in the rst cycle to turn the constituent elements into vapour form from pure metal powder steadily. The second cycle was set for a constant temperature value so that these vaporised materials react with each other properly. In the last cycle, the furnace was cooled down to the room temperature such that material get condensed in crystallin form based on the nucleation points created at the curvy part of the ampoule where etching was done. At the end ne shiny metallic crystals were collected. The elemental investigation of ZrS 3 was carried out using EDAX. Table 1 shows the calculated and observed stoichiometric proportion of the constituent elements i.e. zirconium and sulphur. Fig. 2 shows uniform distribution of Zr and S throughout the area covered by the as grown compound crystal. The observed values are near to the calculated values and the nal product is zirconium rich material as per the EDAX data. The few fresh as grown crystals were then selected for the study under the optical microscope. The optical image of the as grown ZrS 3 crystals are shown in Fig. 3 (a, b) having dimensions in few micrometres is seen and the layers produced during the growth process are clearly been seen. In Fig. 3 (a), the edge of a crystal is shown in which the staking of layers is observed. In Fig. 3 (b) shows the top view of the crystal in which layers are produced with pointed end. The vertically staked layers depicts that the grown material has multilayer structure which is suitable for the anisotropic studies 19 . Fig. 3 (c) and (d) shows the SEM image of as grown compound. The grown compound contains strips having length and width in micrometre range. These strips are nothing but well grown micro crystals of ZrS 3 materials.
In Fig. (d), a monoclinic shaped object is found. In many previously published research articles, it is claimed that ZrS 3 has monoclinic crystal structure. So, this object found in the SEM image indicates in a con rmative way that the grown material has monoclinic structure. Owing to the previously published articles for anisotropy in layered materials, ZrS 3 crystal is also expected to display strong anisotropy. We, therefore, investigated the anisotropic photo response of ZrS 3 crystal. To study the anisotropy of grown ZrS 3 crystal in terms of current-voltage (I-V) and current-time (It) characteristics, two types of contacts were taken: (1) conduction along the basal plane ( to c-axis) and (2) conduction perpendicular to the basal plane ( to c-axis) as shown in Fig. 1. The corresponding (I-V) curves for both con gurations are shown in Fig. 4 (a) Fig. 4 (a) and (c) for both types of contact con gurations. It can be inferred from the graph that ZrS 3 device show good ohmic nature for similar contacts. In both contact con gurations, the current increases upon illuminating the device by different lights, indicating good photo-response of the material. Besides, it is observed that the current along the direction parallel to the basal plan is higher than perpendicular to the basal plan. The dark current along the parallel direction is 447.7 nA, larger than 61.15 nA along the perpendicular direction. The device was illuminated by the blue (470 nm) light, the current rises to 0.448 µA in contacts to c-axis whereas the current rises to 0.061 µA in contacts to c-axis. These values of current inferred the strong anisotropy of ZrS 3 . This anisotropy is also demonstrated well in I-t plots shown in Fig. 4 (b) and (d). Chauhan P. et al. described effect on the current ow mechanism in these kinds of two symmetric ( to c-axis) and anti-symmetric contact( to c-axis) con gurations by demonstrating a model 19 . In consideration of symmetric contact con guration, few upper layers of the multilayer crystal are affected by the light source according to the depth of absorption of incident photons and bias voltage. These are the only layers which may get involved in the dominant conduction. These layers have zirconium and sulphur atoms bonded with covalent bond along with the basal plan. Due to the external bias carrier conduction occur in this case is similar to the case of a planer npn or pnp transistor structure.
The current ow is now con ned and needs less amount of external energy in the case of these few layers bounded by covalent bond as the less amount of resistance faced by the carriers. As we can observe in the optical and SEM images, ZrS 3 crystal has multilayer structure in which number of layers are stacked one upon the other and remain bonded with each other by weak van der Wall's bond. This kind of bonding needed large amount of external energy when carriers have to ow perpendicular to the basal plane i.e. anti-symmetric contact con guration. In this case all the layers of the crystal would take part in carrier transportation along the anti-symmetric con guration. This time carriers would face more resistance compared to the case of symmetric con guration which leads to the fall in the photocurrent.
The detector parameters such as responsivity (R λ ), speci c detectivity (D), sensitivity (S) and external quantum e ciency (EQE%) are evaluated for ZrS 3 using the following equations.
Here, I ph (I ph = I ill -I dark , I ill = current under illuminated condition and I dark = current under dark condition) is the photocurrent, P is the illumination intensity, S is the effective area of the photodetector, h is Planck's constant, c is the speed of light in vacuum, λ is the wavelength of incident radiation, e is the elementary electronic charge (1.6 1019 C) and I dark is dark current.
The I-t characteristics of the device was carried using same sources for both contact con gurations at 1V bias voltage which is shown in Fig. 4  The similar decrements are also observed in the detector parameters shown in Fig. 5. The graphs of the detector parameter i. e. photocurrent, sensitivity, responsivity, detectivity and EQE (%) against wavelength are plotted. The highest responsivity and detectivity of 81.7 µA/W and 3.56 × 10 07 Jones were achieved for blue (470 nm) colour light respectively. It can be noted clearly from the scales of the all plots that the device shows signi cant anisotropy and with the increment of incident light wavelength, all the detector parameters are getting decrease for both types of contact con gurations. So, the ZrS 3 material in bulk crystal form possess good ability to be used in the preparation of the photo sensing device having anisotropic behaviour.

Conclusion
In summary, the ZrS 3 single crystals are successfully synthesized by direct vapour transport technique.
The EDAX characterization con rms that no impurity is found in the grown material and the mapping depicts the uniform distribution of the all-constituent elements. The optical images and SEM images shows that the grown material has layered kind of growth occurred which leads to the uniqueness in the behaviour of many detector parameters. From the study of current-voltage (I-V) and current-time (I-t) characteristics, it is clear that the ZrS 3 material possess strong anisotropy as the device showed good performance for contact con guration made along the basal plane ( to c-axis) compared to the contact con guration made perpendicular to the basal plane ( to c-axis). Due to this uniqueness the material has good candidature for its application in the eld of optoelectronic device applications.