PSGCAS Search: A Journal of Science and Technology Volume : 2 No. : 1

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1 PREPARATION AND STUDY OF BISMUTH SELENIDE THIN FILMS BY ELECTRON BEAM GUN EVAPORATION Krishnendu G.*, Venkatapathy R., Sureshkumar M., Rahman J. and Balasundaram O N Department of Physics, PSG College of Arts and Science, Coimbatore , Tamilnadu, INDIA. *Corresponding author: krishnenduu.g@gmail.com ABSTRACT Bismuth Selenide (Bi 2 Se 3 ) thin films were deposited on glass substrates by employing the technique of electron beam gun evaporation. The chamber was evacuated to a pre-deposition vacuum of 5x10-6 mbar. The substrate temperature was maintained at room temperature (29 o C). A potential of 4.5kV was applied to the electrode and a target current of 10mA was passed. High purity (99.999%) Bi 2 Se 3 chucks were used as a source material and they were kept in a Graphite crucible. The ejected beam of electrons was bent by the magnetic field and it was allowed to evaporate the material inside the crucible. The thickness of the films was found to be 250nm & 300nm. The XRD studies conforms rhombohedral and orthorombic structures. The resistivity decreases as thickness increases but magneto resistance increases with increasing thickness. Optical studies conforms direct and allowed transitions. Keywords: Bismuth Selenide, XRD studies, Bi 2 Se 3 chucks INTRODUCTION Bismuth Selenide with a molecular formula Bi 2 Se 3 appears in a dull gray color. Bi 2 Se 3 is often used in thermoelectric generators. Topologically protected surface states have been observed in Bi 2 Se 3 which is the subject of ongoing scientific research. The material conducts electricity only on its surface while its interior remains insulating, an unexpected property with unknown potential for fundamental research and practical applications. Since surface atoms dominate the structure of Bi 2 Se 3, it is an exceptionally good electrical conductor as good as gold. Unlike gold, however, Bi 2 Se 3 is transparent to infrared light, which we know as heat. Besides, like Bismuth Telluride (Bi 2 Te 3 ), it is very popular with researchers who use the surface insulating properties of these materials to develop highperformance transistors and memory devices [1, 2]. MATERIALS AND METHODS The electron beam gun unit consists of the following important accessories: Electron Gun and Power Supply, Sample Holder (Electron Gun Source), Magnetic Field Arrangement, Pumping System and the Vacuum Chamber, Cooling System and Electronic Safety Circuits. Evaporation involves two basic processes (i) a hot source material evaporates (ii) condenses on the substrate. The advantages of electron beam gun evaporation over thermal evaporation is the evaporated vapour cannot become contaminated because the focused electron beam strikes only the evaporant source material in the crucible. The crucible is cooled so efficiently that its surface never gets warm enough to react with the evaporant material. Deposition of Bismuth Selenide High purity (99.999%) Bi 2 Se 3 chucks were used as a source material and they were kept in a Graphite crucible. Films were deposited onto ultrasonically cleaned glass substrates at room temperature at a base pressure of 9 x10-6 mbar. The substrates were closed from the gun by means of a movable shutter. The entire setup was closed from the atmosphere using a Quartz Bell Jar. The thin films were deposited on a glass substrates fixed at three different positions in a sample holder. The time duration for the deposition of thin films was maintained between 90 and 180 seconds with fixed current 34

2 Krishnendu G., Venkatapathy R., Sureshkumar M., Rahman J. and Balasundaram O N of 20mA. The thickness of the films was measured using a multiple beam interferometer (MBI) technique by forming Fizeau fringes. Characterization X-Ray Diffraction X-ray crystallography is used to determine the structure, i.e. how the atoms pack together in the crystalline state, inter atomic distance and angle etc. X-ray diffraction is one of the most important characterization tools used in materials science. The X-ray diffraction experiment requires an X-ray source, the sample under investigation and a detector to pick up the diffracted X-rays. Samples are mounted on a diffractometer coupled with a machine that emits a beam of X-rays. The X- rays are diffracted at every possible orientation of 2θ due to their interaction with the electrons in the crystal, and the pattern of diffraction is recorded on film or more recently chargecoupled device detectors and scanned into a computer. Successive images are recorded as a crystal is rotated within the X-ray beam. Field Emission Scanning Electron Microscopy A field-emission cathode in the electron gun of a scanning electron microscope provides narrower probing beams at low as well as high electron energy, resulting in both improved spatial resolution and minimized sample charging and damage. Hall Effect Studies Hall Effect measurement is one of the principal motivators leading to a better understanding to be a useful technique for characterizing the electrical transport properties of metals and semiconductors [3]. Ultraviolet Visible Spectroscopy This technique is complementary to fluorescence spectroscopy, in that fluorescence deals with transitions from the excited state to the ground state, while absorption measures transitions from the ground state to the excited state. A spectrophotometer can be a doublebeam instrument; the light is split into two beams before it reaches the sample. One beam is used as the reference; the other beam passes through the sample. RESULTS AND DISCUSSION Structural Analysis X-ray diffraction pattern of the as-deposited Bi 2 Se 3 thin films via electron beam gun evaporation technique for two different thicknesses (300 nm and 250 nm) is shown in the Energy-Dispersive X-Ray Spectroscopy Energy-dispersive X-ray spectroscopy (EDS, EDX, or XEDS) is an analytical technique used for the elemental analysis or chemical characterization of a sample. It relies on the investigation of an interaction of some source of X-ray excitation and a sample. Its characterization capabilities are due in large part to the fundamental principle that each element has a unique atomic structure allowing unique set of peaks on its X-ray spectrum. Fig. 1 XRD pattern of Bi 2 Se 3 thin films The strongest peaks were obtained at diffraction angle (2θ) equal to 27.98, 29.80, corresponding to the reflection plane of (112) (313) (301) and 27.85, 23.37, corresponding to the reflection plane of (112) (301) (214) proving the crystalline nature of the film. The as-deposited Bi 2 Se 3 thin film shows two phases of Bi 2 Se 3 : rhombohedral 35

3 Preparation and Study of Bismuth Selenide Thin Films by Electron Beam Gun Evaporation. and orthorhombic phases for the thickness 300 nm and 250 nm respectively. The reflections from the plane (112), (313), (301) are in agreement with the standard JCPDS data (card No; ) and from (112), (301), (214) are in agreement with the standard JCPDS data (card No; ). Also, the obtained values confirm the formation of rhombohedral and orthorhombic Bi 2 Se 3 thin film [4]. From the results it can be concluded that the crystal phase of Bi 2 Se 3 thin films depends on the thickness of the film. [5] For an increase in thickness from 250 nm to 300 nm crystallinity was found to increase with increase in intensity and decrease in full width half maximum (FWHM) [6]. Table 1 Structural parameters of the as-deposited Bi 2 Se 3 films Film Thickness (nm) Intensity (Counts) FWHM (deg) Grain size (nm) Micro strain Dislocation Density x m The micro-strain, dislocation density decreased with the increase in thickness. This may be due to the movement of interstitial Bi atoms from its grain boundary to the crystallites, which may lead to reduction in the concentration of lattice imperfections [7]. The grain size is found to increase with thickness. Fig. 2 shows EDS images of the as-deposited Bi 2 Se 3 film. The quantitative analysis by energy dispersive X-ray analysis was performed for the as-deposited Bi 2 Se 3 thin film sample at different points. The EDS spectrum reveals that the as-deposited films are rich in bismuth concentration [8]. It is reported that for the films with higher concentration of bismuth ions results in lower grain size [9]. This conclusion is found to be in accordance with the obtained XRD results. The presence of carbon and oxygen ad species is due to the surface hydrocarbon contaminants and moisture uptake [6, 10]. Table 2 EDS-analysis of the as-deposited Bi 2 Se 3 thin film Element AT. % Bi ~60 Se ~5 Fig. 2 EDS images of Bi 2 Se 3 thin film 36

4 Krishnendu G., Venkatapathy R., Sureshkumar M., Rahman J. and Balasundaram O N Optical Properties Fig. 3 shows the optical absorption spectra of the as-deposited films, prepared at room temperature. The thickness of the film was 300nm. The spectra are dominated by absorption peak at 330 nm [11] nm %T nm nm Absorbance(a.u) Wave length(nm) Fig. 3 Absorption Vs wavelength for the as-deposited Bi 2 Se 3 thin films λ nm Fig.4 Transmission Vs wavelength for the as-deposited Bi 2 Se 3 thin films The optical band gap energy (Eg) of the asdeposited Bi 2 Se 3 thin film was obtained on the basis of recorded absorption spectra from 200 to 800 nm range using double beam spectrophotometer. Fig. 4 shows the transmittance spectra, obtained at room 37

5 Krishnendu G., Venkatapathy R., Sureshkumar M., Rahman J. and Balasundaram O N. temperature for as-deposited film sample of three different thickness 300 nm, 280 nm and 250 nm. From the figure, it was observed that the transmittance decreases as the thickness of the film increases. For all the three thickness of the films the transmittance showed a steep increase from 200 nm to 250 nm and then showed a steep decrease from 250 nm to 300 nm. The transmittance was constant from 300 nm for all the three thickness. The maximum transmittance was obtained at a wavelength of about 250 nm, near ultraviolet range that ranges from 200 nm to 380 nm for all the three thickness. The minimum transmittance was obtained at a wavelength from about 300 nm. The absorption coefficient was calculated using Lambert law was found to be m -1. The variation of (αhυ) 2 verses hυ for Bi 2 Se 3 thin films deposited in three different thicknesses is shown in figures (5, 6 & 7). From the figure it was observed that the optical absorption coefficient α is a function of energy hυ. The optical absorption coefficient is found to be greater than 10 6 supporting the allowed direct band transition of the material. This value of absorption coefficient finds best use for optical recording devices. [7,12] The optical band gap energy is estimated by extrapolating the linear portion near the onset of absorption edge to the energy axis and the values were found to be.90 ev, 0.92eV, 0.93 ev for 300 nm, 280nm, and 250 nm thicknesses respectively. (ahn) 2 (10 12 ev 2 m -3 (αhn) 2 (10 12 ev 2 m -3 ) 6.00E E E E E E E hn(ev) Fig. 6 Variation of (αhυ) 2 verses hυ for Bi 2 Se 3 thin with thickness 280 nm 1.20E E E E E E E hn(ev) 6.00E+014 Fig. 7 Variation of (αhυ) 2 verses hυ for Bi 2 Se 3 thin films with thickness 250 nm (ahn) 2 (10 12 ev 2 m -3 ) 4.00E E E hn(ev) Fig. 5 Variation of (αhυ) 2 verses hυ for Bi 2 Se 3 thin films with thickness 300 nm films Even though different thickness of the thin film can provide varied results of optical band gap, the comparison in this case does not show any unique variation. This could be due to the difference in preparative conditions of deposition techniques [12]. Electrical Properties Electrical properties of the Bi 2 Se 3 thin films were measured using Hall Effect studies for three different thicknesses. The figures below 38

6 Preparation and Study of Bismuth Selenide Thin Films by Electron Beam Gun Evaporation shows the variation of mobility (µ), resistivity (ρ), sheet resistance (Ns), hall coefficient (Rh), conductivity (σ) with respect to different film thickness of the as-deposited Bi 2 Se 3 thin films. grain-growth effect in thicker films. Fig. 10 shows the dependence of resistivity (ρ) of the as-deposited Bi 2 Se 3 films on thickness. The resistivity decreased as thickness increased, which is consistent with earlier [6]. Mobility(µ)(cm 2 /Vs) Resistivity(ρ)(οhm cm) 2.5x x x x x x x x Thickness(nm) 1.7x10-3 Thickness(nm) Fig. 8 Variation of mobility (µ) with thickness Fig. 10 Variation of resistivity (ρ) with thickness Sheet resistance(ns)(/cm 2 ) 1x x x x x x10 16 Magneto resistance (delta-r) (m 2 /c) Thickness(nm) Fig. 9 Variation of sheet resistance (Ns) with thickness Figures 8 & 9 show that the mobility (µ) increases considerably and sheet resistance (Ns) decreases when film thickness increases. This mobility increase could also be caused by reduced grain boundary scattering from the Thickness (nm) Fig. 11 Variation of magneto resistance (delta-r) with thickness Fig. 11 shows the variation of magneto resistance (delta-r) with thickness. Delta-R is found to decrease with the decreasing film thickness. The as-deposited Bi 2 Se 3 thin film exhibits a transverse magneto resistance (delta-r) that is positive at a thickness of 39

7 Krishnendu G., Venkatapathy R., Sureshkumar M., Rahman J. and Balasundaram O N. 300nm and becomes negative at lower thickness. Fig. 13 shows the dependence of hall coefficient (Rh) on thickness of the asdeposited Bi 2 Se 3 thin films. The hall coefficient shows almost constant values till about 280 nm and then shows a steep decrease with the increasing thickness to 300 nm. This shows the semiconducting nature of the film. The parameters may also be affected by intrinsic internal quality of the films deposited at various thicknesses [13]. Hall coefficient (Rh)(m 2 /C) 2.5x x x x x x10-3 Thickness (nm) Fig. 12 Variation of hall coefficient (Rh) with thickness conductivity(σ) (1/ 1/ohmcm) Thickness(nm) Fig. 13 Variation of conductivity (σ) with thickness Fig. 13 shows that the conductivity (σ) increases with increase in thickness. Increase in conductivity with thickness may also be due to the higher percentage of Bi. This conclusion is in accordance with the obtained XRD and EDS results. Bi is metallic in nature which could induce higher conductivity [6] CONCLUSION Bismuth Selenide (Bi 2 Se 3 ) thin films were deposited on glass substrates by employing the technique of electron Beam Gun. The base pressure was maintained at 9 x 10 6 mbar during the deposition. The glass substrates were fixed at three different positions in a sample holder. The time duration for the deposition of thin films was maintained at 90 and 180 seconds with fixed current of 20mA. The thickness of the films was measured using a multiple beam interferometer technique (MBI) by forming Fizeau fringes. A Field Emission Scanning Electron Microscopy (FE- SEM) (Sigma/v) was used to measure the composition of the films. Electrical properties and optical were performed on as-deposited Bi 2 Se 3 films of different thicknesses using Hall Effect studies and ultra-violet spectroscopic technique respectively. The X-ray diffraction (XRD) studies reveals that the as-prepared films are crystalline and rhombohedral and orthorhombic nature for the thicknesses 300 nm and 250 nm respectively. From the optical studies the possible optical transitions in these films were found to be direct and allowed. The transmittance was found to decrease as the thickness of the film increases. From the electrical studies various parameters were calculated and discussed. REFERENCES 1. Goswami A., Thin Film Fundamentals, New Delhi: New Age International Private Limited publishers. 2. Milton Ohring, The Material Science of Thin Films. San Diego: Academic Press. 3. Pengra. D. B., Stoltenberg J., Van Dyck R. and Vilches O., Hall Effect

8 4. Jinggeng Zhao., Haozhe Liu., Lars Ehm., Dawei Dong., Zhiqiang Chen and Genda Gu., High-pressure phase transitions. amorphization, and crystallization behaviors in Bi2Se3. J. Phys.: Condens. Matter Lijian Meng, Hui Meng, Wenjie Gong and Wei Liu and Zhidong Zhang, Growth and characterization of Bi2Se3 thin films by pulsed laser deposition using alloy target.. Thin Solid Films. 6. Phuoc Huu Le., Kaung Hsiung Wu., Chih Wei Luo. and Jihperng Leu., Growth and characterization of topological insulator bi2se3 thin films on srtio3 using pulsed laser deposition. Thin solid films. 7. Manjulavalli T E., Balasubramanian T. and Nataraj D., Structural and Optical Properties of Thermally Evaporated Bi2se3 Thin Film. Chalcogenide Letters. Vol. 5. No Patil N S., Sargar A M., Mane S R and Bhosal P N., Growth mechanism and characterisation of chemically grown Sb doped Bi2Se3 thin films. Applied Surface Science. 9. Bari R H. and Patil L A., Synthesis and characterization of Bismuth Selenide thin films by chemical bath deposition technique. Indian Journal of Pure and Applied Physics. Vol Lokhande C D., Sankapal B R., Mane R S., Pathan H M.., Muler M., Giersig M., Tributsch H and Ganeshan V., Structural characterization of chemically deposited Bi 2 S 3 & Bi 2 Se 3 thin films. Applied surface science Subramanian S. and Pathinettam Padiyan D., Effect of structural. electrical and optical properties of electrodeposited bismuth selenide thin films in polyaniline aqueous medium. Materials Chemistry and Physics Saji Augustine, Ampili S., Jeung Ku Kang and Elizabeth Matha., Structural, electrical and optical properties of Bi 2 Se 3 & Bi 2 Se (3-x) Te x thin films. Materials Research Bulletin Namrata Bansal., Yong Seung Kim., Matthew Brahlek., Eliav Edrey and Seongshik OH., 2010.Thicknessindependent transport channels in topological insulator Bi 2 Se 3 thin films. 41