Indian Journal of Pure & Applied Physics Vol. 48, February 2010, pp. 127-132

Synthesis and characterization of thin films by chemical bath deposition technique

R H Bari & L A Patil* Nanomaterials Research Laboratory, P G Department of Physics, Pratap College, Amalner 425 401 *E-mail: [email protected] Received 20 June 2008; revised 24 July 2009; accepted 10 November 2009

Thin films of bismuth selenide were prepared by chemical bath deposition technique onto glass at 55°C. The deposition parameters such as time, temperature of deposition and pH of the solution were optimized. The set of films having different elemental compositions was prepared by varying Bi/Se ratio from 0.93 to 2.03. The composition, morphology, structure, optical absorption and electrical conductivity of the films were studied. Characterization includes X-ray diffraction (XRD), scanning electron microscopy (SEM), atomic force microscopy (AFM), energy dispersive X-ray analysis (EDAX), absorption spectroscopy and electrical conductivity. Keywords: Bismuth selenide, Thin films, Chemical bath deposition, Deposition parameters

1 Introduction method. The deposition bath was prepared in 100 ml In recent years, the synthesis of thin beaker by addition of 0.1 M bismuth nitrate solution. films by chemical deposition of colloidal Appropriate amount of TEA was added to this has been studied. Compounds of the solution. Sodium selenosulphate (Na2SeSO3) solution column V-B (As, Sb, Bi) and VI-B (S, Se, Te) was then added to the bath. Ammonium hydroxide elements in the periodic table with chemical formula solution was used to increase the pH of the solution. V-B VI-B m2 n3 basically have the structure of antimony The pH of the solution bath was set about 10 and the sulphide which are the most important material for the temperature of the bath was maintained at 55°C. After applications in photosensitivity, photoconductivity a deposition period of one hour, substrates were and thermoelectric power. Bi2Se3, Sb2Se3 compounds removed from the bath and washed well with distilled belong to this class of family. The Bi2Se3 is a narrow water and dried. Both sides of the glass substrate were band gap semiconductor. Its calculated band gap1 is coated with thin films. Films with different Bi and Se approximately 0.24 eV with the measured value2 compositions were prepared by changing volume of between 0.2-0.3eV. The study of bismuth (III) precursor solutions of Bi and Se. The Bi2Se3 films selenide thin film is motivated from its suitable were annealed in vacuum at 375°C for 5 min and kept optical and electrical properties for construction of in decicator. optical and photosensitive devices, modern Various characterization techniques such as XRD, thermoelectric, Hall effect magnetometer, high optical spectroscopy, scanning electron microscopy, frequency power sensor-thermopliers wide band atomic force microscopy etc. were employed to study radiation detectors and humidity sensors using the the films. The structural properties of thin films were 3-6 Seeback and Peltier effects . investigated by X-ray diffraction (XRD) using CuKα Thin films of bismuth (III) selenide have been (λ=1.5418 Å) radiation. The optical absorption prepared using various techniques such as studies of the films were carried out using Hitachi 7 electrodeposition , successive ionic layer adsorption U-2000 spectrophotometer. The elemental analysis of 8 and reaction (SILAR), metal organic chemical the films was carried out using an energy dispersive 9 vapour deposition (MOCVD) and chemical bath spectrometer (EDS) JEOL, JED-2300, and scanning 10-11 deposition techniques . electron microscopic (SEM) studies were carried out using JEOL 6300(LA). An AFM Nanoscope (Model- 2 Experimental Details NSE, Serial no-245) digital instrument with a silicon Bismuth triselenide films of different Bi/Se ratios nitride cantilever was used to probe different portions were prepared by simple chemical bath deposition of the film surface in contact mode AFM. 128 INDIAN J PURE & APPL PHYS, VOL 48, FEBRUARY 2010

3 Results and Discussion composition of the Bi2Se3 films from EDAX. temperature. Table 1 presents the elemental 3.1 Structural studies Theoretically expected stoichiometric composition of To identify the material, the Bi2Se3 films were Bi2Se3 (in terms of at %) is Bi = 40% and Se = 60%. analyzed with the help of X-ray diffraction technique. The films are non-stoichiometric in (Table 1). Figure 1 shows the X-ray diffractograms of sample 4, Among the four samples, the elemental composition which was nearer to the stoichiometric composition in of sample 4 is nearer to stoichiometry. 2θ range between 20-80°. The observed peaks are approximately matching with standard JCPDS data. 3.3 Microstructural studies The planes (104), (015), (0012), (110) correspond to The surface morphology of the synthesized bismuth Bi2Se3 (JCPDS file no.33-214). selenide films were studied by using scanning electron microscopy (SEM). Figure 2(a,b,c,d) shows 3.2 Elemental analysis by EDAX SEM images of the as-synthesized Bi2Se3 films with The quantitative elemental analysis of the as- different Bi/Se ratios. The images clearly demonstrate deposited Bi2Se3 films was carried out at room nearly spherical grains with fine threads on them. It is

seen that the substrate is well covered by the Bi2Se3. As Bi/Se ratio decreases, the process of agglomeration becomes more prominent. The average

Table 1 — Elemental composition of Bi2Se3 films

Sample No. Bi (at%) Se(at%) Bi/Se (at%)

1 67.65 32.35 2.09 2 60.40 39.54 1.53

3 55.07 44.93 1.22 Fig. 1 — XRD of the sample 4 48.25 51.75 0.93

Fig. 2 — SEM images of the Bi2Se3 films with different Bi/Se ratios (a)=2.09, (b) 1.53, (c) 1.22 (d)=0.93 BARI & PATIL: SYNTHESIS AND CHARACTERIZATION OF BISMUTH SELENIDE THIN FILMS 129

grain sizes calculated from the SEM images are agglomeration of particles in most of the cases as presented in Table 2. The average grain size goes on evident from the 2D micrographs. The root mean decreasing with the increase in at % of bismuth in the square value of the surface roughness of the film from film composition. Larger the concentrations of different areas of the film is calculated. It was bismuth, smaller the grains. For films with higher observed that the surface roughness of the film is concentration of bismuth ions, the growth occurs with 20.60 nm/1 μm*1 μm. This observation infers that the multiple nucleation centers resulting in lower grain film surface is smooth. size, while for lower concentration of bismuth ions; comparatively lower nucleation centers give higher 3.5 Optical studies An optical absorption study of Bi Se films was grain size. The amount of feed material available in 2 3 the reaction vessel is constant for a particular reaction. carried out in the wavelength (λ) range 300-1100 nm If the same material would be divided on the larger at room temperature. The variation of absorbance number of nucleation centers, the grain would not with the wavelength (λ) is shown in Fig. 4(a). The grow larger but remains smaller12-14. band gap energies of the samples were calculated from the absorption edges of the spectra. The values 3.4 Surface morphology of band gap energies go on increasing with the The surface morphology of the Bi2Se3 thin film was increase in at % of Bi in the film composition. It may analyzed using atomic force microscopy (AFM) be due to smaller at % of Se in the composition (say techniques. Figure 3 shows the AFM pictures of the sample 1), smaller the at % of Se, smaller would be film having Bi/Se ratio 0.93 (sample 4). The surface is the possibility of formation of localized levels in the well covered with Bi2Se3 grains with uneven spherical forbidden gap. grains. At right hand side of the image, intensity strip Figure 4 (b) shows the variation of optical band gap is shown which indicates the height of the surface energy with the variation of Bi/Se ratio. The values of grains along Z-axis (Fig. 3). AFM reveals the granular band gap energy go on increasing with the increase in nature of particles. There would have been Bi/Se ratio (Fig. 4b). Table 2 summarizes the effect of variation of Bi/Se Table 2 — Dependence of band gap energies and grain size on ratio on band gap energy and grain size. Bi/Se ratio 3.6 Electrical studies Sample No. Bi/Se (at %) Grain Size Band gap Electrical conductivity of the Bi2Se3 thin films was (µm) energy (eV) measured using dc two-probe method in the 1 2.09 0.16 2.69 temperature range 308-423 K. Figure 5 shows the 2 1.53 0.28 2.53 variation in the logarithm of conductivity with an 3 1.22 0.35 2.43 inverse of temperature for various compositions of 4 0.93 0.39 2.34 Bi2Se3.

Fig. 3 — AFM pictures of the sample 130 INDIAN J PURE & APPL PHYS, VOL 48, FEBRUARY 2010

Fig. 4(a) — Optical absorbance versus wavelength (λ) Fig. 5 — Variation of electrical conductivity with inverse of temperature

Fig. 6 — Variation of electrical conductivity versus Bi/Se ratio at temperature 313 K

increase in Bi/Se ratio, may be due to higher at % of Bi. Bismuth is metallic in nature which could induce Fig. 4(b) — Variation of band gap energy versus Bi/Se metal ratio higher conductivity. Table 3 presents the variation of electrical conductivity with Bi/Se ratio at temperature The conductivity increases with increase in 313 K. temperature indicating the semiconducting nature of The activation energies were calculated from the Bi2Se3 thin films (Fig. 5). Moreover, conductivity slope of the graphs of logarithm of conductivity increases with increase in Bi/Se ratio in the film versus inverse of temperature at high and low as shown in Fig. 6. Increase in conductivity with temperature range and are presented in Table 3. The BARI & PATIL: SYNTHESIS AND CHARACTERIZATION OF BISMUTH SELENIDE THIN FILMS 131

activation energy goes on decreasing with increase of In a semiconductor, temperature gradient yields the Bi/Se ratio as shown in Fig. 7. Activation energy is thermoelectric effect, in which phonons travel related to conductivity of the samples. Higher the preferentially from the hot end to cold end due to conductivity, lower is the activation energy15. Charge electron-phonon interactions. During TEP carriers need lower energy of activation in case of measurements, thermal gradient established changes materials having higher conductivity. of the density of charged defect state by capturing electrons and holes. The motion of the electrons and Table 3 — Dependence of conductivity and activation energies on holes can take place through the process of diffusion. Bi/Se Figure 8 shows the variation of the thermoelectric Sample Bi/Se Conductivity Activation Activation power with temperature in the range 300-400 K. −1 No. (at %) (σ) (ΩCm) energy (eV) energy (eV) Thermoelectric power is negative throughout the (at 313K) (For temp. (For temp. range range temperature range, suggesting that the samples are 308-348K) 348-423K) n-type. It is seen that TEP increases with increase in temperature linearly. Similar results were observed by 1 2.09 1.50E-17 0.54 0.17 Pawar and Bhosale16. 2 .53 9.28E-18 0.56 0.18 3 1.22 8.00E-18 0.59 0.20 4 0.93 2.34E-18 0.69 0.21 4 Discussion The Bi2Se3 thin films were obtained from an aqueous alkaline bath consisting of Bi salt in complex form and selenide ions. The deposition process is based on slow release of the Bi3+ and Se2− ions in the solution, which then condense on the substrate as an ion-by-ion basis. The formation of Bi2Se3 occurs when the ionic product of Bi3+ and Se2− exceeds the product of Bi2Se3. In the present case, the formation of Bi2Se3 involves hydrolysis of sodium selenosulphate, which release selenide ions into the bath.

Fig. 7 — Variation of activation energy at (a) high temperature (T= 308-348 K) and (b) low temperature (T= 348-423 K) with Fig. 8 — Temperature dependence of thermoelectric power of Bi/Se ratio various Bi/Se samples 132 INDIAN J PURE & APPL PHYS, VOL 48, FEBRUARY 2010

In an alkaline medium sodium selenosulphate bismuth selenide, thermoelectric power was negative hydrolyses to give Se2− ions as: throughout the temperature range, suggesting that the samples were n-type. Also TEP was observed to be − − Na2SeSO3 + OH ↔ Na2SO4 + HSe …(1) increasing with increase in temperature.

− − 2− HSe + OH ↔ H2O +Se …(2) Acknowledgement

From Eqs (1) and (2), the equilibrium constant of The authors are thankful to Head, Department of HSe- is predominant in the solution. The Physics and Principal, Pratap College, Amalner, for concentration of Se2- ions can be increased by providing laboratory facilities for this work. Thanks addition of the excessive hydroxide ions to facilitate to Principal, G D M Arts, M D Science and K R N forward reaction. Commerce College, Jamner, for his encouragement. The authors are grateful to Dr Ajay Gupta, Center 3+ 3+ [BiN(CH2-CH2-OH)3] →Bi +N(CH2-CH2-OH)3 Director, IUC, Indore, for giving the consent for triethanolamine …(3) completing the part of this work at consortium. Mr R H Bari acknowledges University Grants The reactions given in Eqs (2) and (3) reveal that Commission, Western Region, Pune, for the award of the Bi3+ and Se2− ions will condense on as an ion by the teacher fellowship under 10th plan. ion basis on the glass substrate as: References 3+ − 2[BiN(CH2-CH2-OH)3] +3Na2SeSO3+6OH →Bi2Se3 1 Mishra S K, Satpathy S & Tepsen O, J Phys Condens Mat, 9 +3Na2SO4 +2[N(CH2-CH2-OH)3]+3H2O …(4) (1997) 461. 2 Mooser E & Pearson W , Phys Rev B, 101 (1956) 492. 5 Conclusions 3 Giani, Bayaz A Al, Foucaran A, Pascal-Delannoy F& Boyer A, J Cryst Growth, 236 (2002) 217. It is concluded that bismuth selenide thin films 4 Bhattacharya R N & Pramanik P, J Electrochem Soc, 129 were successfully deposited onto glass substrate by (1982) 332. simple chemical bath deposition technique. The films 5 Bates C W, England L, Appl Lett, 14 (1996) 390. were uniform and had good adherence to the 6 Hyun D B, Hwang J S, Oh T S, Shim J D & Kolmotes N V, J substrate. The EDAX of the films indicated that the Phys Chem Solids, 59 (1998) 1039. 7 Torane A P, Lokhande C D , Patil P S & Bhosale C H, Mater films were non-stoichiometric and the XRD of the Chem Phys, 55 (1998) 51. bismuth selenide film confirmed the formation of 8 Sankpal B R & Lokhande C D , Mater Chem & Phys, 73 Bi2Se3 phase. The process of agglomeration becomes (2002) 151. more and more prominent with the decrease of Bi/Se 9 Bayaz A Al , Giani A , Foucaran A , Pascal-Delannoy F & Boyer A , Thin Solid Films, 441 (2003) 1. ratio. It was observed that the surface roughness of 10 Sankpal B R , Pathan H M & Lokhande C D, Indian J Pure the typical bismuth selenide film is 20.60 nm/1 μm × & Appl Phys, 40 (2002) 331. 1 μm. This observation inferred that the film surface 11 ljana Pejova, Ivan Grozdanov & Atanas Tanusevski, Mater is smooth. The values of band gap energy of Bi Se Chem & Phys, 83 (2004) 245. 2 3 12 innott M J The solid states for engineers (John Wiley ,New were increasing with the increase in Bi/Se ratio. The York) (1958) pp 97-103. increase in the conductivity increases with the 13 Patil L A, Wani P A, Saraf K B & Wagh M S, Cryst Res increase in temperature indicated the semiconducting Techno, 33 (1998) 233. 14 Patil L A & Wani P A, Cryst Res Technol, 36 (2001) 371. nature of Bi2Se3 thin films. Also conductivity was observed to increase with increase in Bi/Se ratio in the 15 Bari R H, Patil L A & Patil P P, Bull Mater Sci, 29 (2006) 529. film. The activation energy was decreasing with the 16 Pawar S H & Bhosale P N , Mater Chem & Phys, 11 (1984) increase of Bi/Se ratio in bismuth selenide. For 461.