Address for Correspondence

Size: px

Start display at page:

Download "Address for Correspondence"

Norman Neal
6 years ago
Views:

Research Paper EFFECT OF ALUMINUM DOPING ON THE PROPERTIES OF SPRAY DEPOSITED COPPER SULFIDE (Cu 2 S) THIN FILMS S Jahan* 1, M N H Liton 2, M K R Khan 3 and M Mozibur Rahman 3 Address for

2 Lecturer, Department of Physics, Begum Rokeya University, Rangpur. 3 Professor, Department of Physics, Rajshahi University, Bangladesh.

1 Research Paper EFFECT OF ALUMINUM DOPING ON THE PROPERTIES OF SPRAY DEPOSITED COPPER SULFIDE (Cu 2 S) THIN FILMS S Jahan* 1, M N H Liton 2, M K R Khan 3 and M Mozibur Rahman 3 Address for Correspondence 1 * Research Student, Department of Materials Science and Engineering, Rajshahi University. Phone: , Fax: , sharmin.mse@gmail.com. 2 Lecturer, Department of Physics, Begum Rokeya University, Rangpur. 3 Professor, Department of Physics, Rajshahi University, Bangladesh. ABSTRACT Undoped and aluminum doped copper sulfide (Cu 2 S) thin films were prepared by spray pyrolysis technique on glass substrates at 325 C. Prepared films are characterized by optical and electrical properties. From the optical study, it is seen that, undoped and Al doped Cu 2 S thin films are good absorber materials (α > 10 5 order) at visible region. From the variation of absorption co-efficient with photon energy it is seen that, there is a valley of absorption co-efficient at photon energy ~ 2.0 ev. The reflectance of undoped Cu 2 S films is slightly high (~14%), which decreases (3-4%) with increasing Al-doping concentration. Doping of Al (up to 5 %) in Cu 2 S film tunes optical band gap from 2.45 ev to 2.72 ev. Refractive index, n (~2.0) and dielectric constant, ε 1 (~ 4.25) for undoped Cu 2 S films decrease with increasing Al-doping concentration. From Hall measurement, it is seen that, the measured value of Hall constant (R H ), is positive both for undoped and Al-doped Cu 2 S thin films, which indicates that the Cu 2 S thin films are p-type semiconductors with carrier concentration of the order of to (cm -3 ). The Hall mobility (µ H ) of Cu 2 S films is very low and nearly equal unity. From the resistivity data it is seen that the resistivity of Cu 2 S thin films increases with increasing Al concentration. Temperature dependence resistivity shows metallic behavior at low temperature that is its resisitivity increases with increasing temperature and reaches a maximum value at ( K) and then decreases with increasing temperature like a semiconductor, which indicates phase changes at this temperature range. From the measurement of activation energy, two types of conduction mechanisms are observed. At low temperature region, it may be due to impurity scattering and at high temperature region it may be due to free band transition of semiconductor. KEYWORDS: Copper sulfide; Spray pyrolysis; Optical properties; Electrical properties. 1. INTRODUCTION The Cu 2 S films is a binary p-type semiconducting materials. Its band gap varies from 1.2 ev to 2.5 ev [1,2]. The study of Cu 2 S thin films has received considerable attention in recent years due to its numerous technological applications in the achievement of solar cell, in photochemical conversion of solar energy, as solar absorber coating, as selective radiation filters on architectural windows, as solar control in warm climates, as electro conductive coating deposited on organic polymers [3,4,5,6,7,8,9], non volatile memory device [10,11], gas sensing applications [12] and lithium ion batteries [13]. Various processes for preparing Cu 2 S thin films have been investigated. These include chemical bath deposition (CVD) [14], photo chemical deposition [15], metal organic chemical vapor deposition (MOCVD) [16], sputtering [17], spray pyrolysis [18] etc. Cu 2 S thin films are technologically attractive p-type material; so far there is no information of doping effect on the properties of Cu 2 S thin films. In the present investigation, Al doped Cu 2 S thin films have been deposited on glass substrate by simple spray pyrolysis technique. The effect of Al doping on the electrical and optical properties of Cu 2 S thin films has been studied. 2 EXPERIMENTAL The films are deposited by simple homemade spray pyrolysis method. Details are described elsewhere [19]. The spray solution consisted of 1:1 mixture (by volume) of solutions of pure (99.99%) copper (II) acetate at 0.1M (used as a source of copper ions) and thiourea at 0.1M (as source of sulfur ions). Aluminum nitrate was used as a dopant source with doping concentration 1,2,3,4 and 5 (mole %) in the starting solution. The substrate temperature was at 325 C. The solution flow rate was 0.3 ml/min. The deposition time was ~10 minutes which produce copper sulfide thin film of thickness 150±5 nm. The possible chemical reaction during spray deposition may be the followings: (CH 3 COO) 2 Cu.H 2 O + SC(NH 2 ) 2 + Al(NO 3 ) 3.9H 2 O Cu 2 S:Al + CO 2 + CH 4 + NO +steam. Cleaned optically flat microscopic glass slide was used as substrate. The thickness of the films is measured by Newton s rings method. The electrical current and voltage are measured by conventional electrometer. The temperature was measured by copper-copper constantan thermocouple. Hall voltage and resistivity are measured by van der pauw method [20]. The magnetic field used for the study of Hall effect experiment was provided by Newport instruments Ltd., England. 3 RESULTS AND DISCUSSION 3.1Optical characterization Transmittance Optical study of undoped and Al-doped 1(mol)%, 2 (mol)%, 3 (mol)%, 4 (mol)% and 5 (mol)%) copper sulfide (Cu 2 S) thin films were carried out in the wavelength range of 380 to 1100 nm at room temperature. Transmittance spectra for undoped and Al-doped Cu 2 S thin films as a function of wavelength are shown in figure 1. Figure 1. Variation of optical transmittance (T), with wavelength for undoped and Al-doped Cu 2 S films

From the figure 1, it is seen that, transmittance in the low wavelength region is very low up to 400 nm and then it increases sharply and reaches maximum (600 ~ 650 nm) and then decreases rapidly,

It is also observed that, the transmittance increases with increasing doping concentration of Al and maximum transmittance (~ 45%) is observed for 5% Al-doped Cu 2 S films and minimum transmittance

2 Reflectance Reflectance spectra for undoped and for 1 to 5 % Al doped Cu 2 S films as a function of wavelength in the spectral range of 380 to1100 nm are shown in fig. 2. with increasing photon energy.

2 From the figure 1, it is seen that, transmittance in the low wavelength region is very low up to 400 nm and then it increases sharply and reaches maximum (600 ~ 650 nm) and then decreases rapidly, become flats at near infrared region. The variation of transmittance with wavelength ( ) is similar to the reported values [21]. It is also observed that, the transmittance increases with increasing doping concentration of Al and maximum transmittance (~ 45%) is observed for 5% Al-doped Cu 2 S films and minimum transmittance (~ 29%) of undoped Cu 2 S films is observed Reflectance Reflectance spectra for undoped and for 1 to 5 % Al doped Cu 2 S films as a function of wavelength in the spectral range of 380 to1100 nm are shown in fig. 2. with increasing photon energy. It is also seen from the figure that, α decreases slightly with increasing doping concentration of Al and this result may be due to the increasing of optical band gap with Al doping [22]. This result indicates that Cu 2 S films are good absorber at visible region and whose absorbance slightly decreases with Al doping. Similar result of Cu 2 S films was obtained by Shinde et al. [1] and Mehdi et al. [21] Optical band gap The optical band gaps are estimated by the following relation; (αhν) = A (E g hν) n (2) Where A is the proportionality factor and n is the index depending on the types of transition. For direct allowed transition, n = 1/2. Variation of (αhν) 2 (direct allowed transition) with hν for undoped and Al-doped Cu 2 S films are shown in figure 4. The band gaps are obtained from the intercepts on energy axis, after extrapolation of the straight-line section of (αhν) 2 vs. hν curve. From the figure, it is seen that, optical band gap of Cu 2 S sample increases with increasing Al-doping concentration. Figure 2. Variation of optical reflectance (R), with wavelength of light for undoped and Al-doped Cu 2 S films From this figure, it is seen that, reflectance of undoped Cu 2 S films are slightly high (7-14%) and it decreases with increasing Al doping concentration and reaches to minimum value ~ 3%. It is also observed that, reflectance of Cu 2 S films is not constant but varies with wavelength and maximum value attains at nm wavelength ranges for each case Absorption coefficient The absorption co-efficient (α) was calculated using the relation, α = 1/t ln [(1/T] (1) where t is the thickness and T is the transmittance of the films. The variation of α with photon energy for undoped and Al-doped Cu 2 S films is shown in fig. 3. Figure 3. Variation of absorption co-efficient (α), with photon energy, hν for undoped and Al-doped Cu 2 S films From the figure 3, it is seen that, the absorption coefficient (α) is high value (~ x 10 5 order) at lower energy region and decreases to minimum value at photon energy of ~2.0 ev and then increases rapidly Fig 4. Variation of (αhν) 2 with photon energy, hν for undoped and Al-doped Cu 2 S films The increase of band gaps may be happen due to reducing band tail by Al doping in Cu 2 S films and it varies from 2.45 to 2.72 ev, but these variations are not systematic. This slight anomaly may be due to slight variation of films thickness. The estimated values of optical band gaps are presented in table 1. The increasing band gap of Cu 2 S films with Al doping can be explained as follows: Since the p-type behavior of Cu 2 S may happen due to vacancy of Cu site in the lattice. The doping of Al atom in Cu 2 S lattice replaces Cu + ion by Al 3+ ion and for each replacement of Cu ion by Al ion extra two electrons become free and as a result, p-type carrier concentration is reduced and hence reduces band tail and increases optical band gap. Table 1: Direct band gap of undoped and Al-doped Cu 2 S films. Doping concentration of Al (mole)% Direct band gap (ev) Refractive index The variation of refractive index with photon energy for undoped and Al-doped Cu 2 S films for 1 to 5 % (mol) of Al concentration is shown in figure 5. From the figure, it is seen that, the value of refractive index n of all the samples (doped and undoped) increases

$It is also observed that, in the mentioned photon energy range (1.7-2.4 ev), the refractive index decreases with increasing Al doping concentration.$ $Here, it should be noted that, in the inter band region, the most important factor affecting the refractive index (n), is the concentration of voids.$ As a conclusion, the difference between the optical properties of the films may be explained by the influence of voids and surface roughness [23].

As a conclusion, the difference between the optical properties of the films may be explained by the influence of voids and surface roughness [23].

The room temperature resistivity of undoped Cu 2 S film is the order of ~ x 10-4 -cm and whose value increases with Al-doping concentrations, and it becomes of the order of ~ x 10-2 for Al-doping

3 with the increase of photon energy and reaches the maximum values at photon energy of ~ 2.04 ev and then it decreases with the increase of photon energy (except 3% Al-doping). It is also observed that, in the mentioned photon energy range ( ev), the refractive index decreases with increasing Al doping concentration. Here, it should be noted that, in the inter band region, the most important factor affecting the refractive index (n), is the concentration of voids. For films with more voids, the value of n in this region is lower. As a conclusion, the difference between the optical properties of the films may be explained by the influence of voids and surface roughness [23]. The variation of room temperature resistivity of Cu 2 S with Al doping concentration is shown in figure 7. The room temperature resistivity of undoped Cu 2 S film is the order of ~ x cm and whose value increases with Al-doping concentrations, and it becomes of the order of ~ x 10-2 for Al-doping concentrations. Since the doping of Al atom in Cu 2 S lattice replaces Cu + ion by Al 3+ ion and for each replacement of Cu ion by Al ion extra two electrons become free and as a result, p-type carrier concentration is reduced and hence reduces the free carriers and increases its resistivity. Fig 5. Variation of refractive index (n), with photon energy for undoped and Al-doped Cu 2 S films Dielectric constant The real (ε 1 ) and imaginary (ε 2 ) part of dielectric constants are determined by the following equations, ε 1 = n 2 k 2 & ε 2 = 2nk.. (3) The real part of dielectric constant generally relates to dispersion and imaginary part to loss factor [24]. Variation of real part of dielectric constant (є 1 ), with photon energy for undoped and Al-doped Cu 2 S films is shown in figure 6. From this figure, it is observed that, the value of є 1 initially increases with increase of photon energy and reaches a maximum value (~ 4.23) and then decreases with the increase of photon energy (except 3% Al-doped Cu 2 S films) as the same fashion of refractive index. The figure also shows that, the dielectric constant ε 1 decreases with increasing Al doping concentrations. Fig 6. Variation of real part of dielectric constant, є 1 with photon energy for undoped and Al-doped Cu 2 S films 3.2 Electrical measurement Resistivity of the Cu 2 S thin films The resistivity of undoped and Al-doped Cu 2 S thin films was measured by Van der Pauw s method [20]. Figure 7. Variation of resistivity of Cu 2 S with Al-doping concentration at room temperature The variation of resistivity with temperature of undoped Cu 2 S film is shown in figure 8. Figure 8. Variation of resistivity (ρ), with temperature of undoped Cu 2 S film From figure 8, it is seen that, the resistivity of undoped Cu 2 S film rapidly increases with temperature up to ~353K behaves like a metal, and after that, the resistivity sharply decreases with increasing temperature, like a semiconductor. So, two types of conduction mechanisms are observed from this figure. This variation of resistivity with temperature indicates the occurrence of phase changes at temperature ~ 355 C. The increase of resistivity up to transition temperature may be due to impurity scattering. The variation of resistivity with temperature of Al-doped Cu 2 S films for 1 5 % Al doping concentration is shown in figure 9. From figure 9, it is seen that, resistivity of Al-doped Cu 2 S films increases with temperature like undoped Cu 2 S films and reaches the maximum values (at 375K 342 K) for different doping concentration of Al and after that it decreases with increasing temperature like a semiconductor. From this figure it is also seen the transition temperature for each film is not strongly depend on Al doping concentrations. Since the transition not only depends on only Al concentration but also on the environment of crystal formation. It is assumed

that at some critical temperature, the change of phases occurs and then the samples behave like normal semiconductor and its resistivity decreases with increasing temperature due to free band

S films 3.2.2 Activation energy of Cu 2 S films Conductivity of undoped Cu 2 S and Al-doped Cu 2 S thin films are calculated from the resistivity data.

Variation of conductivity (σ), with temperature of undoped Cu 2 S film Figure 11. Variation of conductivity (σ), with temperature of Al-doped Cu 2 S films.

4 that at some critical temperature, the change of phases occurs and then the samples behave like normal semiconductor and its resistivity decreases with increasing temperature due to free band transition. Figure 9. Variation of resistivity (ρ), with temperature of Al-doped Cu 2 S films Activation energy of Cu 2 S films Conductivity of undoped Cu 2 S and Al-doped Cu 2 S thin films are calculated from the resistivity data. Figure 10 and 11 shows the plot of conductivity σ vs. temperature, T for undoped and Al-doped Cu 2 S thin films. Figure 10. Variation of conductivity (σ), with temperature of undoped Cu 2 S film Figure 11. Variation of conductivity (σ), with temperature of Al-doped Cu 2 S films. Figure shows two types of conduction mechanisms. At low temperature region, the conduction may be due to impurity scattering and at high temperature the conduction mechanism may be due to free band transition of semiconductor. The electrical conductivity for the films can be expressed by the usual relation, = 0 exp [- ( E/2kT)] (4) where E is the activation energy and k is the Boltzmann constant and T is the temperature. The activation energy E of the undoped and Aldoped Cu 2 S thin films can be calculated by the relation, E = - 2K[ ln /(1/T)] (5) The calculated activation energy E, is shown in table 2. Table 2. The activation energy, E for Cu 2 S thin films. Doping concentration (mole% Al) Activation energy, E 1(eV) Activation energy, E 2 (ev) Hall measurement The Hall measurement was done at room temperature and at constant magnetic field of KG. The value of Hall constants (R H ), Hall mobility (μ H ) and carrier concentration of undoped and Al-doped Cu 2 S thin films are determined. Hall constant, (R H ) both for undoped and Al-doped Cu 2 S thin films are positive and of the value of (cm 3 /con). And this implies that, undoped and Al-doped Cu 2 S films are p- type material and the majority charge carrier is hole. Hall mobility of Cu 2 S thin films are very low and nearly equal to unity. The carrier concentration of the films are the order of (cm -3 ) and it decreases with increasing Al doping concentration. 4 CONCLUSIONS Undoped and Al-doped Cu 2 S thin films were deposited onto glass substrate at a temperature 325⁰C by a locally developed spray pyrolysis method. The optical and electrical properties of undoped and Aldoped Cu 2 S thin films were studied. From the optical study it is seen that, undoped and Al-doped Cu 2 S thin films are good absorber in the visible spectral range and whose absorption coefficient decreases with Al doping. The reflectance of Cu 2 S films decreases with the increase of Al-doping concentration. The measured direct band gap energy for undoped Cu 2 S is 2.45 ev and this value increases to 2.72 ev with Al-doping concentration upto 5 %. Hall constant, (R H ) both for undoped and Al-doped Cu 2 S films is positive, which indicates that the undoped and Al-doped Cu 2 S thin films are p-type semiconductor with carrier concentration (cm -3 ). Resistivity was measured at a temperature range of 303 K to 398 K and it is observed that, in this temperature range, there are two types of conduction mechanisms and change of phases are occurred at the temperature range K for different doping concentration of Al. 5 ACKNOWLEDGEMENTS One of the author greatly acknowledged to the Ministry of Science & Technology, Govt. Peoples Republic of Bangladesh for giving financial assistance to carry out the research. REFERENCES 1. M.S Shinde, P.B Ahirrao, I.J Patil and RS Patil, Indian Joural of Pure and Applied Physics, vol. 50, sep 2012, pp S.V. Bagul, S.D. Chavhan and R. Sharma, J. Phys. Chem, solids, 2007, 68, D.M. Mattox and RR Sowell, J. Vac. Sci. Technol. 11 (1974) 793.

5 4. S.B. Gadgil, R Thangaraj, J.V. Lyer, A.K. Sharma, B.K. Gupta and O.P. Agnihotori, Sol. Energ. Matter, 5 (1981) P.K. Nair and M.T.S Nair, J. Appl. Phys., 24 (1991) P.J Sebastian, O. Gomez-Daza, J. Campos, L Banos, and P.K. Nair, Sol. Energ. Matter. Sol. C.32 (1994) S. Lindross, A Arnold and M Leskela, Appl. Surf. Sci. 158 (2000) F. Li, T. Kong, W.T.Bi, D.C.Li, and X.T. Huang, Appl. Surf. Sci. 255 (2009) J. Liu and D.F. Xue, J.cryst. Growth, 311 (2009) T. Sakamoto, H. Sunamura, H. Kawaura, T. Hasegewa, T. Nakayama and M. Aono, Appl. Phys., lett., 82 ( L. Chen, Y.D. Xia, X.F. Liang, K.B. Yin, J. Yin, Z G. Liu and Y. Chen, Appl. Phys. Lett., 91 (2007) A. A. Sagade, R. Sharma and I. Sulaniya, J. Appl. Phys. 105 (2009) L. Zhao, F. Tao, Z. Quan, X Zhou, Y. Yuan and J. Hu, Matter Lett. 68 (2012) P. K. Nair, M.T.S Nair, J. Campos and L.E. Sansoyex, Sol. Cells, 22(1987) J. Podder, R. Kobayashi and M. Ichimura, Thin Solid Films, 427 (2005) R. Nomura, K. Miyawaki, T. Toyosaki and H. Matsuda, Chem. Vap. Depos. 2(1996) X. B. He, A. Polity, D. I. Osterreicher, D. Pfisterer, R. Gregor, B. K. Meyer and M. Hard, Physica B: Condensed Matter, (2001) S.Wang, W. Wand, Z. H. Lu., Matter. Sci. Eng. B. 103 (2003) M. K. M. Khan, M. A. Rahman, M. Shahajahan, M. M. Rahman, M. A. Hakim, D. K. Saha, J. U. Khan, Curr. Appl. Phys. 10 (2010) Van-der Pauw, Philips Tech. Rdch. 20 (1958) A. Mehdi, E, Hosein and M. Mohammad, Bull, Matter, Sci. 35 (50 (2012) Mahadi H.S, Issam M.I, Mohan Rao G., J.Electron Devices, 13 (2012) AA Ziabari, FE Ghodsi,, J Mater Sci;Mater Electron (2012). 24. GA Kumar, J Thomas, N George, B.A Kumar, P Radhakri, V.P,N Shnan Phys Chem Glass 41 (2000) 89.

Deposition and Characterization of p-cu 2 O Thin Films

SUST Journal of Science and Technology, Vol., No. 6, 1; P:1-7 Deposition and Characterization of p- O Thin Films (Submitted: July 18, 1; Accepted for Publication: November 9, 1) M. Rasadujjaman 1*, M.