Opto-structural and electrical properties of chemically ... - Springer Link

J Mater Sci: Mater Electron (2013) 24:4669–4676 DOI 10.1007/s10854-013-1449-y

Opto-structural and electrical properties of chemically grown Ga doped MoBi2Se5 thin films S. V. Patil • R. M. Mane • N. B. Pawar • S. D. Kharade • S. S. Mali • P. S. Patil • G. L. Agawane • J. H. Kim • P. N. Bhosale

Received: 22 May 2013 / Accepted: 13 August 2013 / Published online: 12 October 2013 Ó Springer Science+Business Media New York 2013

Abstract An arrested precipitation route was developed to obtain gallium doped MoBi2Se5 thin films on substrate support. High purity organometallic complexes of Mo– triethanolamine (Mo–TEA), Bi–triethanolamine (Bi–TEA), Ga–triethanolamine (Ga–TEA) allow to react with sodium selenosulphite (Na2SeSO3) in the presence of sodium dithionite (Na2S2O4) as a reducing agent in an aqueous alkaline reaction bath. As deposited thin films were characterized by X-ray diffraction, which reveals that material is nanocrystalline with mixed phases of rhombohedral (Bi2Se3)-hexagonal (MoSe2)-hexagonal (GaSe) structures. Scanning electron micrographs show the grain of granular morphology decreases with increase in Ga concentration. Energy dispersive x-ray analysis shows presence of Mo, Bi, Ga and Se elements in stoichiometric ratio confirms the chemical formula MoBiGaSe5. Optical absorbance of the films show direct allowed transition in visible region having band gap energy in the range of 1.30–1.47 eV. Thermoelectric power and electrical conductivity measurements S. V. Patil Department of Chemistry, C. T. Bora College, Shirur, Pune, India S. V. Patil R. M. Mane N. B. Pawar S. D. Kharade P. N. Bhosale (&) Materials Research Laboratory, Department of Chemistry, Shivaji University, Kolhapur 416004, India e-mail: [email protected] S. S. Mali P. S. Patil Thin Film Materials Laboratory, Department of Physics, Shivaji University, Kolhapur 416004, India P. S. Patil G. L. Agawane J. H. Kim (&) Department of Materials Science and Engineering, Chonnam National University, Gwangju 500-757, Republic of Korea e-mail: [email protected]

have been carried out for thin film samples in the temperature range 300–500 K and results revealed that n-type semiconducting behavior. It is interesting to note that Ga doping in MoBi2Se5 changes n to p type conductivity.

1 Introduction In recent years synthesis, characterization and applications of mixed metal chalcogenide thin film materials has been of interest to researchers, because of their outstanding photoelectrochemical properties in fabrication of liquid junction photovoltaic cell. The inorganic semiconducting materials of ternary and quaternary chalcogenide compounds belongs to VIB-VA-VIA and VIB-IIIA-VA-VIA have been synthesized in our laboratory because of their tunable band gap energy (Eg) in the range 1.1–2.2 eV and high optical absorption coefficient (104–105 cm-1) [1, 2]. In the last few years solar cells based on CuInSe2, CuInSSe, Cu2ZnSnSe4, CuInGaSe and their pentanary compositions are attracting much attention of the researcher due to high conversion efficiency solar cells (^12 %) [3, 4]. Also it has been observed that the ternary MoBi2Se5, MoBiInSe5 and MoBiCuSe4 based thin film solar cells shows 2–3 % conversion efficiency. The optoelectronic properties of the mixed metal chalcogenide semiconducting materials are critically influenced by stoichiometric composition, defect concentration, structural properties and preparation methods [5–11]. We attempted to develop a facile thin film deposition technique which is based on Oswald’s self organized growth process. The thin film preparation method used in the present investigation is arrested precipitation technique (APT). The APT is simple arresting (complexing) metal ions using organic complexing agent disodium salt of

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ethylenediammineteraacetic acid (EDTA), triethanolamine (TEA), citrate, tartrate, ethylene diamine (EDA). The stable complexes so formed are pH sensitive hence by controlling narrow pH range arrested metal ions are slowly released in the reaction mixture which has more time to react with chalcogen ions and metal chalcogenide molecule so formed is sequentially self undergo organization onto substrate support [12–14]. In the present investigation, we report optostructural and electrical properties of MoBi2-xGaxSe5 thin films prepared by facile APT by variation of composition (x = 0.00–0.10), preparative conditions, particle size, band gap and morphology have been studied. Thin film formation mechanism and its optostructural, electrical properties have been discussed in this paper.

2 Experimental 2.1 Materials In the present investigation, all the chemicals are of the analytical reagent (AR grade) and used without further purification such as ammonium molybdate, ((NH4)6Mo7O24 4H2O), bismuth nitrate (Bi(NO3)35H2O), gallium nitrate (Ga(NO3)3XH2O) complexed with TEA, N(CH2–CH2– OH)3, selenium metal powder (Se), sodium sulphite (Na2SO3), ammonium acetate (CH3COONH4), sodium dithionite (Na2S2O4), ammonia (NH3). All solutions were prepared in de-ionized water. Sodium selenosulphite (Na2SeSO3) solution used as a selenium source for the deposition of MoBi2-xGaxSe5 thin films was prepared by refluxing 6 gm of selenium metal powder with 30 gm of sodium sulphite in 250 mL de-ionized water at elevated temperature of 90 °C for 8 h [15]. 2.2 Synthesis of Ga doped MoBi2Se5 thin films Gallium doped MoBi2Se5 MMC thin films were obtained using APT on ultraclean conducting and non-conducting glass substrates from alkaline aqueous bath. In a typical experiment for the deposition of MoBi2-xGaxSe5 thin films was briefly described as; the metal–TEA complexes were prepared as per our earlier report [13]. In 100 mL reaction container 1 mL of 0.05 M Mo–TEA, 19 mL Bi–TEA complex solution, 20 mL 0.25 M Na2SeSO3 and variable volumes of (x = 0.0, 0.02, 0.04, 0.06, 0.08 and 0.10 mL) 0.05 M Ga–TEA complex solution was added. The pH of the solution was adjusted to 9.8 by using aqueous ammonia and resultant solution was diluted to 60 mL by using de-ionized water. Few drops of ammonium acetate were added to the same solution to maintain constant pH throughout reaction. The details of series are given in Table 1. The preparative

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parameters such as, concentration of precursors, pH, temperature of the bath solution and rate of substrate rotation were initially finalized to obtain quality of thin films. The growth of film involves the reaction among Mo4?, Bi3?, Ga3? and Se2- ions in aqueous solution. At alkaline pH, the Mo–TEA, Bi–TEA, Ga–TEA complexes were slowly dissociated into their ion form. It reacts with Se2- chalcogen ions. When the ionic product (Ip) exceeds solubility product (Sp) it results in condensation of metal ions to form uniform, strongly adherent and pinhole free thin film on substrate support. The schematic presentation for the formation of MoBi2-xGaxSe5 thin films is shown in Fig. 1. 2.3 Characterization techniques The crystal structure of materials were verified by the X-ray diffraction (XRD) analysis [Bruker Axs Model D8 ˚ ) in the Advance X-ray diffractometer] with Cu ka (1.542 A 2h range from 20° to 80°. The surface morphology of the films was studied with the help of scanning electron microscopy [SEM: JEOL-6360A Analytical scanning electron microscope]. The compositional analysis was carried out using energy dispersive X-ray analysis [EDS: JEOL-6360A Analytical scanning electron microscope]. The thickness of films were measured by surface profiler [AMBIOS XP-1]. The optical study was done by using a UV–vis spectrophotometer [UV-1800 Shimadzu, Japan] in wavelength of range 300–1,100 nm. All these characteristics were measured at room temperature. The electrical conductivity and its variation with temperature were measured by the conventional dc two probe method in the temperature range 300–500 K. Type of conductivity of the samples were measured by thermoelectric power (TEP) measurements. TEP unit consisting of two brass blocks one of which was heated and the other was kept cold, a chromel allumel thermocouple was used to measure the temperature difference. Silver paste was used to ensure good electrical and thermal contact between the samples and the two brass blocks. TEP measurements were taken in the range of 300–450 K. Table 1 Preparative parameters for the deposition of MoBi2-x GaxSe5 thin films Sr. No.

Sample id

Composition

1

(a)

MoBi2Se5

2

(b)

MoBi1.98Ga0.02Se5

3

(c)

MoBi1.96Ga0.04Se5

4

(d)

MoBi1.94Ga0.06Se5

5 6

(e) (f)

MoBi1.92Ga0.08Se5 MoBi1.90Ga0.10Se5

rpm is rotations per minute

pH

Temperature

rpm

9.8 ± 0.2

45 °C ± 0.2

45


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Fig. 1 Schematic diagram for deposition of MoBi2-xGaxSe5 thin films

size decreases in the order of a [ b [ c [ d [ e [ f from 48.0 to 12.3 nm.

3 Results and discussion 3.1 XRD analysis

3.2 SEM and EDS analysis In preliminary investigation [15], we have reported hexagonal structure of MoBiGaSe5 but the detailed study of MoBi2-xGaxSe5 thin films show mixed phase crystal structure. Figure 2 show XRD patterns of as deposited MoBi2-xGaxSe5 thin films. The presence of broad and identifiable peaks in the diffractograms suggest that the films are nanocrystalline in nature [16]. The diffraction peaks at 2h values of approximately at 29.77°, 43.75° corresponding to (015), (110) planes of rhombohedral structure of Bi2Se3 phase (JCPDS card no. 12-0732), 38.92° and 54.07° corresponding to (103), (110) planes of hexagonal structure of MoSe2 (JCPDS card no. 77-1715) and 32.37° corresponding to (103) plane of hexagonal structure of GaSe phase (JCPDS card no. 80-2271). The (103) plane is absent in sample (a) at x = 0.00 content of Ga. Mixed phase type crystal structure of MoBi2-xGaxSe5 thin film is observed. The crystallite size ‘D’ of the thin film samples were calculated by Scherrer equation [17]. D¼

0:94k b cos h

Figure 3 shows the SEM micrographs (magnification of 15,0009) of the MoBi2-xGaxSe5 thin films for different compositions. The SEM micrographs of the films deposited for (x = 0.0–0.10) sample (a to f) shows uniform distribution of grains, which covers the surface of glass substrate completely. The dense closely packed grains provide a pinhole free morphology, which would provide better spatial contact between the grains which will be desirable

ð1Þ

where, D is average crystallite size, k is wavelength of the X-ray, b is full width at half maximum (FWHM) and h is Bragg’s angle. Calculations of crystallite size were made at intense (015) peak and values are 48.0, 21.4, 19.4, 16.9, 15.1, 12.3 nm for x = 0.00, 0.02, 0.04, 0.06, 0.08, 0.10 respectively in MoBi2-xGaxSe5 films. It is observed that value of FWHM (b) increases with increase in Ga content in the order of a \ b \ c \ d \ e \ f. Therefore crystallite

Fig. 2 X-ray diffraction patterns of MoBi2-xGaxSe5 thin film sample (a) x = 0.0, (b) x = 0.02, (c) x = 0.04, (d) x = 0.06, (e) x = 0.08 and (f) x = 0.10

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for constructing photoelectrochemical cell [18]. From these micrographs the average grain size was calculated by intercept method using equation [19], G¼

1:5 l mn

ð2Þ

where, G is average grain size, m is the magnification of the micrograph, l is the length of the linear line drawn on the micrograph, n is average number of grains crossed by the line and 1.5 is the parameter assuming spherical grains. The grain sizes decreases from 800 to 153 nm on addition of gallium. These results were confirmed by XRD analysis. These changes in morphology have been observed with increase in gallium content results in increase in the rate of nucleation. The compositional analyses of the films were studied by EDS method. The spectrum reveals the presence of peaks at 2.293, 2.419, 9.241 and 1.379 keV, which confirms the presence of Mo, Bi, Ga and Se elements respectively in the film. The representative EDS spectra of sample (a, c, d and f) are shown in Fig. 4. It is seen that the composition of as deposited MoBi2-xGaxSe5 (x = 0.00–0.10) thin films are closer to stoichiometric ratio. The intensity of Se is weak indicating low atomic percentage. The atomic percentage of Mo and Ga matched with theoretical values actually taken for thin film growth. However atomic percentage of Bi is found to be more than theoretically expected percentage. This may be due to the formation of antisite defects [20] as well as among the elements of the thin films composition,

bismuth is more metallic [19]. The composition of MoBi2-xGaxSe5 (x = 0.00–0.10) thin films is given in Table 2. 3.3 Optical measurement The optical absorption of as deposited nanocrystalline MoBi2-xGaxSe5 thin films were carried out using UV–vis. spectrophotometer in the wavelength range 300–1,100 nm. The variation of optical absorption coefficient with wavelength was further analyzed to find out the nature of electronic transition across the optical band gap. The nature of transition was determined by using the relation [21], n ahm ¼ A hm Eg ð3Þ where, A is a constant, hm is a photon energy, n = 1/2 for a direct gap material, n = 2 for an indirect gap material. The optical absorption coefficient a was calculated for the thin films using the equation, 1 I0 a ¼ log ð4Þ t It where, t is the film thickness, It and I0 are the intensity of transmitted and initial light respectively. Log (Io/It) is the optical density. The thickness (t) of the films were measured by surface profiler and average thickness of films decrease from 633 to 480 nm on addition of gallium content. The optical absorption coefficient is of order 105 cm-1 supporting the direct and

Fig. 3 SEM micrographs of MoBi2-xGaxSe5 thin film sample (a) x = 0.0, (b) x = 0.02, (c) x = 0.04, (d) x = 0.06, (e) x = 0.08 and (f) x = 0.10

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Fig. 4 EDS spectra of MoBi2-xGaxSe5 thin film sample (a) x = 0.0, (c) x = 0.04, (d) x = 0.06 and (f) x = 0.10

Table 2 Different compositions of MoBi2-xGaxSe5 thin films Sr. No.

Sample id

Composition MoBi2-xGaxSe5

Expected percentages

Observed percentages

Mo

Bi

Ga

Se

Mo

Bi

Ga

Se

1

(a)

MoBi2Se5

12.50

25.00

0.00

62.50

13.47

39.22

0.00

47.32

2

(b)

MoBi1.98Ga0.02Se5

12.50

24.75

0.25

62.50

13.51

39.17

0.40

47.92

3

(c)

MoBi1.96Ga0.04Se5

12.50

24.50

0.50

62.50

13.78

39.84

0.60

45.78

4

(d)

MoBi1.94Ga0.06Se5

12.50

24.25

0.75

62.50

13.62

38.88

0.81

44.69

5

(e)

MoBi1.92Ga0.08Se5

12.50

24.00

1.00

62.50

13.52

38.80

1.07

47.61

6

(f)

MoBi1.90Ga0.10Se5

12.50

23.75

1.25

62.50

13.92

38.06

1.57

46.45

allowed transition. Figure 5 shows a plot of (ahm)2 versus hm which shows high absorption coefficient. The extrapolation of straight line to the energy axis gives the values of optical band gap energy. The band gap energies for sample (a) to sample (f) are 1.30, 1.33, 1.37, 1.40, 1.43, 1.47 eV respectively. The band gap energy increases with addition of gallium content. The XRD and SEM characterization revealed that the decrease in crystallite size and grain size respectively which is reflected in optical band gap. This is due to substitution

˚ ) by small ionic radius of large ionic radius of bismuth (1.14 A ˚ of gallium (0.64 A). This facilitates the fast growth of small nanoparticles of MoBiGaSe5. 3.4 Electrical conductivity (EC) analysis The EC (r) was measured as a function of temperature in the range 300–500 K for nanocrystalline thin films of MoBi2-x GaxSe5. The electrical conductivity increases with the

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results in higher electrical conductivities. The Ea for variable gallium content in MoBi2-xGaxSe5 series is shown in Table 3. 3.5 Thermoelectric power (TEP) measurements

Fig. 5 Optical band gap of MoBi2-xGaxSe5 thin film sample (a) x = 0.0, (b) x = 0.02, (c) x = 0.04, (d) x = 0.06, (e) x = 0.08 and (f) x = 0.10

Fig. 6 Electrical conductivity of MoBi2-xGaxSe5 thin film sample (a) x = 0.0, (b) x = 0.02, (c) x = 0.04, (d) x = 0.06, (e) x = 0.08 and (f) x = 0.10

TEP is the most sensitive to any change or distortion of the Fermi energy level in the material. The temperature difference between the ends of sample causes transport of carriers from hot to cold end and it creates electrical field which gives rise to thermal voltage. Thermally generated voltage is directly proportional to the temperature difference created across the semiconductor. The plot of measured voltage versus temperature difference across the sample gives Seebeck coefficient (S) of the film. Plots of Seebeck coefficient (S) verses temperature (K) of MoBi2-xGaxSe5 (x = 0.0, 0.02, 0.04, 0.06, 0.08 and 0.10) thin films in the range 300–450 K is shown in the Fig. 7. The Seebeck coefficient of MoBi2Se5 thin films have negative value indicates n-type material. However after addition of Ga films have quite high positive value indicating dominance of p-type charge carriers [22]. Seebeck coefficient (S) increases more or less smoothly over the measured temperature region. MoBi2Se5 shows smallest S value -41.012 lV/K at 323 K and -6.956 lV/K at 413 K. The Seebeck coefficient (S) increases in the series MoBi2-xGaxSe5 with the increase in Ga content and reaches maximum for x = 0.10 with 57.275 lV/K at 323 K and 80.871 lV/K at 413 K as the Ga3? have less electrons than Bi3?. When the Ga content in MoBi2-x GaxSe5 material increases hole concentration also increases which results in increase in Seebeck coefficient. Seebeck coefficient (S) has been used to evaluate the carrier mobility using relation [23–25]. k Ea TEP ¼ S ¼ þA ð6Þ e kT OR

increasing temperature, exhibiting a semiconducting nature of the MoBi2-xGaxSe5 thin films. However with increase in gallium content the value of electrical conductivity increases because number of charge carriers increases which is further confirmed by thermo electric power measurements. The plot of ln r versus 1/T is shown in Fig. 6. The activation energies were estimated from slopes of graph of ln r versus 1,000/T using relation, r ¼ ro expðEa =kTÞ

ð5Þ

where, r0 represents the pre-exponential factor, Ea is the activation energy for electrical conduction, k is Boltzmann constant and T is Temperature in Kelvin. It is observed that activation energy (Ea) goes on decreasing with the increase in gallium content, which

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TEP ¼

o 3 kn A þ ln½2ð2pme kTÞ2 =nh3 e

ð7Þ

where, k is Boltzmann constant, Ea is activation energy, T is temperature in Kelvin, e is charge of electron, A is a thermoelectric factor, n is electron density, h is Planck’s constant and m*e is the effective mass of the electron. After substitution of various constants in Eq. 7 simplifies to [26] 3 ð8Þ log n ¼ log T 0:005TEP þ 15:719 2 The electron density has been evaluated by using the above equation and was of the order of 1019 cm-3 for the material. The mobility (l) of the charge carriers was determined from the relation,


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Table 3 Activation energy, mobility and carrier concentration for different compositions of MoBi2-xGaxSe5 thin films Sr. No.

Sample id

Composition MoBi2-xGaxSe5

Activation energy (Ea), eV

Mobility (l), cm2/Vs

Carrier concentration (n), cm-3

1

(a)

MoBi2Se5

0.188

2.13 9 10-2

1.653 9 1019

0.171

-3

1.929 9 1019

-3

2.092 9 1019

-4

2.326 9 1019

-4

3.164 9 1019

-4

4.771 9 1019

2

(b)

3

(c)

4

(d)

5

(e)

6

(f)

MoBi1.98Ga0.02Se5 MoBi1.96Ga0.04Se5 MoBi1.94Ga0.06Se5 MoBi1.92Ga0.08Se5 MoBi1.90Ga0.10Se5

9.59 9 10

0.156

1.09 9 10

0.142

4.15 9 10

0.097

1.57 9 10

0.079

1.38 9 10

same time mobility (l) decreases due to scattering of charge carriers. This is shown in Table 3. The plot of mobility and carrier concentration with Gallium content is shown in Fig. 8.

4 Conclusions

Fig. 7 Seebeck coefficient of MoBi2-xGaxSe5 thin film sample (a) x = 0.0, (b) x = 0.02, (c) x = 0.04, (d) x = 0.06, (e) x = 0.08 and (f) x = 0.10

Ga doped MoBi2Se5 thin films were successfully synthesized by APT. The XRD pattern elucidates mixed phases of rhombohedral (Bi2Se3)-hexagonal (MoSe2)-hexagonal (GaSe) crystal structures of the material. Films are chemically stoichiometric having granular morphology. The grain size and crystallite size decrease with Ga content. The absorption coefficient is high which gives band gap in the range of 1.30–1.47 eV. Electrical conductivity increases with temperature that indicates semiconducting behavior. Mobility of MoBi2-xGaxSe5 (x = 0.00–0.10) decreases from 2.13 9 10-2 to 1.38 9 10-4 (cm2/Vs) and carrier concentration increases from 1.653 9 1019 to 4.771 9 1019 (cm-3). In conclusion, the opto-structural and electrical properties of material suggest its usability for fabricating photoelectrochemical solar cell. Acknowledgment One of the authors (SVP) gratefully acknowledges the UGC New Delhi for laboratory facilities in the Department of Chemistry. This work is partially supported by the Human Resource Department of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) Grant funded by the Korean Government Ministry of Knowledge and Economy (No. 20124010203180).

References Fig. 8 Average mobility and carrier concentration of MoBi2-x GaxSe5 thin films

l¼

r ne

ð9Þ

where, n is electron density, r is conductivity and e is the charge of electron. As Ga content in MoBi2-xGaxSe5 increases, carrier concentration (n) increases but at the

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