Deposition and Characterization of p-cu 2 O Thin Films

Transcription

1 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. Shahjahan, M. K. R. Khan 3 and M. M. Rahman 3 1 Department of Physics, Dhaka University of Engineering and Technology, Gazipur, Bangladesh; Department of Applied Physics and Electronics, Bangabandhu Sheikh Mujibur Rahman Science & Technology University, Gopalganj, Bangladesh; 3 Department of Physics, University of Rajshahi, Rajshahi-65, Bangladesh * rana1phyru@gmail.com Abstract In this work we have studied the p-type cuprous oxide ( O) thin film fabricated with thermal oxidation and spray pyrolysis techniques. Cuprous oxide layer was prepared by thermal oxidation method inside a furnace in the temperature range of 93 C - 11 C. X-ray diffraction studies revealed the formation of single phase cubic O films. The best quality O films were obtained at 97 C. The electrical resistivity of the O layer varies with the variation of temperature and found of the order ~1 3 Ωcm. Such films have also been prepared on glass substrates by spray pyrolysis technique. Optical study showed that the O films were highly absorbing in the visible range of electromagnetic radiation. The absorption coefficient, dielectric susceptibility and optical conductivity are evaluated. Keywords: Cuprous Oxide thin films, Glass substrates, Oxidation method, X-ray diffraction, Spray pyrolysis, Band gap energy, Optical absorbance 1. Introduction Thin film of cuprous oxide ( O) is an interesting well-known p-type semiconductor with a direct band gap of. ev. Furthermore, O is an abundant and economically available material with low toxicity. Even though O is one of the earliest semiconducting materials known to physicists and materials scientists, not much technological advancements has been achieved due to poor conversion efficiencies (%) in solar cell applications [1]. This is due to a very limited amount of work devoted to this semiconductor. Although the crystal structure of cuprous oxide always creates difficulties in the understanding of its electronic conductivity mechanism the deposition and characterization of cuprous oxide thin films via different techniques have attracted considerable attention due to their potential application prospects in solar cells [ 5], magnetic devices [6], catalysis [7,8], photocatalyst [9]. In spite of few studies regarding to the oxidation and spray pyrolysis method, the thermal oxidation and spray pyrolysis method has some merits, such as the easy control of chemical components and fabrication of thin film at a low cost to investigate structure and optical properties of O thin films. In this paper, O films have been deposited by thermal oxidation and describe the structural and electrical properties. Cuprous oxide films have been also deposited by spray pyrolysis method on glass substrate for optical characterization.. Materials and Methods.1. Deposition of O thin film:.1.1. Thermal oxidation The preparation of O film as reported in the literature is done usually by thermal oxidation technique. A high purity copper sheet is heated at an elevated temperature in pure oxygen or in laboratory air. A black CuO is

2 M. Rasadujjaman, M. Shahjahan, M. K. R. Khan and M. M. Rahman formed after a sufficiently long oxidation time, is removed either mechanically or chemically. Also the oxidation process is followed by annealing the sample at lower temperature (8 C) and then stopping the process by quenching in cold water [1]. Several oxidation procedures were developed in the past to oxidize copper in order to obtain O films with particular characteristics. The thermal oxidation procedure we used deviated from the previous one. In this technique, O films were prepared on.8 mm copper sheets of rectangular shape of dimension. cm. cm inside a furnace in the temperature range of (93 11) C. The temperature is raised with a rate of C/min. Properties of p- O strongly depend on the details of the oxidation process. Here no oxygen flow environment was used. It has been suggested that during oxidation, O is formed first and after a sufficient long oxidation time, CuO is formed. The oxidation process includes two steps: 4Cu + O O O + O 4CuO The oxidation temperature was optimized at 97 C, which was found to be the optimum condition to deposit a single phase of O. Since the films were deposited at higher temperature, there is a possibility of creating oxygen vacancies or defects in the film. As a result, resistivity of the film will be increased subsequently charge carrier mobility will be decreased. To reduce resistivity and to maximize the charge carrier mobility post deposition annealing was performed at 5 C for hours Spray pyrolysis technique The spray pyrolysis is a simple and least expensive for the preparation of the films compared with other methods. The deposition of O films was performed by the spray pyrolysis technique. Before deposition, the substrates was initially cleaned with detergent soap in tap water then rinsed with distilled water, acetone and ethyl alcohol and allowed to air dry. Cuprous oxide thin films were deposited on glass substrates using the spray pyrolysis method. The solution was prepared by dissolving appropriate amount of.1m copper (II) chloride (CuCl.H O) and distilled water in a beaker at room temperature with continuous stirring for 1 minutes. Then the solution was sprayed on the heated substrate. Films of different thicknesses were deposited by varying deposition time from 1 to 3 minutes, keeping molar concentration of copper (II) chloride at.1m. Thicknesses of the films measured by Newton s ring method using Na-light, were found to increase with the increase of deposition time... Characterization of O thin film: The crystal structures of the oxidized O films were analyzed with CuKα line from a Shimadzu X-ray diffractometer (XRD-6) and the patterns were recorded in a range of 5 8 (θ). DC electrical resistivity and Hall Effect studies were performed by Van der Pauw s method. Optical properties of the spray deposited O thin films in the wavelength range (38 11) nm were measured using a UV Spectrophotometer (UV-161PC Shimadzu, Japan). 3. Results and Discussion 3.1. Structural studies Figure 1 represents the XRD spectra of typical as-deposited O films (oxidized at 97 C) and annealed O film (annealed at 5 C for hours). The films consists of single phase of O [JCPDS 5-667] depending on the preparation condition [1]. The crystallinity of the films increased after annealing and four XRD peaks at θ = 9.74, 36.56, and appeared, owing to the diffraction of the (11), (111), () and (311) planes of O.

3 Deposition and Characterization of p- O Thin Films 3 Counts, (a.u.) O (11) O (11) O (111) O (111) O () (b) Annealed (a) As-deposited O () O () O (311) O (311) Position, θ (deg.) Fig. 1: X-ray diffraction spectra of typical (a) as-deposited and (b) annealed O film at 5 C for. It is also found that the peak intensity of annealed sample along (111) plane is increased sharply indicating a preferred orientation along this plane. After annealing a new peak is observed at θ = 4.5, which resembles the plane () of O. The full width at half maximum (FWHM) and crystalline sizes of as-deposited and annealed films were calculated from the diffraction peaks using the Debye-Scherrer formula [11], ξ =.9λ β cosθ where, λ is the wavelength of the X-ray used, θ is the Bragg angle and β is the FWHM of a diffraction peak expressed in radians. The cryatallite sizes for as-deposited and annealed samples along (111) planes are estimated to be 18 nm and 4 nm, respectively. The result of XRD patterns indicates that if the oxidation temperature is maintained at 97 C, the pure O films of increased crystallinity can be produced. From the XRD analyses it was concluded that annealing plays a significant role in the microstructure and structural properties of the copper oxide films. 3. Electrical properties DC electrical resistivity measurements were performed in air ambient for freshly deposited films in the temperature range of room temperature (RT) to 53 K by Van der Pauw s method. Resistivity, ρ (ohm-cm) Heating Re-cooling Cooling Re-heating Heating Cooling Re-heating Re-cooling For thickness, t = 9. µm Temperature, T (K) Fig. : Variation of resistivity with temperature for O films of thickness 9. m Resistivity, ρ (ohm-cm) t = 8.5 µm t = 8.7 µm t = 9. µm Temperature, T (K) Fig. 3: Variation of resistivity with temperature for O films of different thickness.

4 4 M. Rasadujjaman, M. Shahjahan, M. K. R. Khan and M. M. Rahman From Fig., it is observed that resistivity shows almost reversible behavior between first heating and cooling cycles as well as reheating-recooling cycles. However, the order of magnitude of resistivity decreased in RT region. This may be due to the fact that during successive heating and cooling cycles the compactness of the films increases and the defect density decreases and due to the removal of metastable phases present if any. Fig. 3 also shows the electrical resistivity of as-deposited O films on Cu-substrates at different film thicknesses and is found to be of the order of 1 3 ohm-cm. The resistivities are in the range of values reported for O thin films prepared by chemical deposition and thermal oxidation methods [1, 1]. The lower resistivity values are required for solar cell applications. Measured resistivity values at 31K for three different thicknesses are presented in table 1 below: Table 1: Variation of resistivity with thickness Thickness (µm) Resistivity (Ωcm) It is observed that the resistivity decreases with increasing temperature and this confirms the semiconducting nature of O. The decrease in the electrical resistivity is considered to be caused by desorption of oxygen from the film surface. It is also observed that the resistivity of O depends on the film thickness. With the increase of film thickness, resistivity is found to be increased in the measured temperature range. Conductivity values are determined for as-deposited O films in the temperature range of room temperature (RT) to 53K. The variation of conductivity with inverse absolute temperature is found to be linear in Fig. 4. The activation energy is estimated for the linear curve using the equation 8 E ac σ = σ exp( ) kt lnσ (mho-cm) t = 8.5 µm t = 8.7 µm t = 9. µm Fig. 4: Variation of lnσ with inverse temperature for O films of different thickness. Measured activation energy values are presented in Table Table : Variation of activation energy with thickness Inverse temperature, 1/T (K -1 ) Thickness (µm) Activation energy (ev) The activation energy is in good agreement with the reported value for O thin film [13] which is ascribed to the hole conduction associated with a single acceptor impurity.

5 Deposition and Characterization of p- O Thin Films 5 From Hall Effect study, the Hall coefficients were found to be 1.61 coul/cm 3 and coul/cm 3 for asdeposited and annealed O films, respectively. The corresponding hole carrier concentrations were found to be of the order of 1 16 cm -3. It was found that in both cases, as-deposited and annealed, O films were p-type in nature. The details processes of electrical conductivity and Hall Effect in cuprous oxide are reported elsewhere [14]. 3.3 Optical properties By fixing the better optical properties of O film for use of solar absorber, we prepared O films by spray pyrolysis technique using different spray time. The as-deposited O films were found to have a very high optical absorption in the visible spectra. It was determined that all films behaved as absorber materials at about 4-8 nm wavelength range and absorbance of between 68% to 98% for the films prepared at different deposition time. The absorbance values of the films decreased sharply at about wavelengths greater than 8 nm because of their transmittance properties as shown in Fig. 5. We also observed that, the transmittance increases in the wavelength range greater than 8 nm. Besides, as seen in Fig. 6, the optical transmissions of the O films were approximately less than 48% in the visible region and these transmission values decreased with deposition time was increased. Absorbance, A (%) A 3 min B 5 min C min D 15 min E 1 min E Wavelength, λ (nm) A B C D Transmittance, T (%) A 3 min B 5 min C min D 15 min E 1 min Wavelength, λ (nm) E D C B A Fig. 5: Variation of absorbance with wavelength for O films on glass substrate. Fig. 6: Variation of transmittance with wavelength for O films on glass substrate. Absorption Coefficient, α x 1 4 (cm -1 ) A 3 min B 5 min C min D 15 min E 1 min A B C D E Photon energy hν (ev) (αhν) x 1 8 (cm -1 ev) t = 15 nm t = 16 nm t = 17 nm t = 18 nm t = 19 nm Photon energy hν (ev) Fig. 7: Variation of absorption coefficient as a function of photon energy for O films of different thicknesses. Fig. 8: Variation of direct band gap with photon energy for O films of different thicknesses.

6 6 M. Rasadujjaman, M. Shahjahan, M. K. R. Khan and M. M. Rahman A typical relationship between the thickness and the absorption co-efficient (α) for the prepared films is shown in Fig. 7. The absorption co-efficient is found to increase with an increase in deposition time or thickness. This indicates a likely dependence of the optical absorbance of the deposited films on the thickness during deposition. In order to better understanding the changes in optical absorbance as a function of deposition parameters, we computed the direct optical band gap, E g values using results based on the optical spectrum. For determination of the optical band gap energy E g ; the method based on the relation α hν = A( hν E g ) was used [15] where n is a number that depends on the nature of the transition. In this case its value was found to be 1 (which corresponds to direct band to band transition) because that value of n yields the best linear graph of (αhν) vs. hυ. Fig. 8 shows (αhν) vs. hυ for the O film. The intersection of the straight line with the hυ-axis determines the optical band gap energy E g. It was found to vary with thickness. Depending on the film thickness (15 nm to 19 nm) of the O films direct band gap varies from 1.9 ev to 1.64 ev. The band gap for film thickness 15 nm is found to be 1.9 ev, which is low given in the earlier report [16]. The direct band gaps obtained for the O films are given in the table-3 below: Table 3: Variation of direct band gap with thickness Thickness, t (nm) Direct band gap, Eg (ev) The studies indicate that the band gap of spray deposited O film is affected by film thickness. Other optical parameters such as absorption coefficient (α ), dielectric susceptibility (χ), and optical conductivity (σ ) of O films are estimated using the relations [17]. 1 (1 R) α = ln T t n / n k = 1+ 4πχ αnc σ = 4π The values of α and χ estimated for a typical O film of thickness 16 nm are cm -1,.4 and respectively at hv = 1.7 ev. The positive susceptibility of the film indicates the paramagnetic nature of the material. The value of σ is s -1 at 1.5 ev. 4. Conclusion In the present work, we studied thin films of cuprous oxide grown by oxidation method. For the best quality of O films, oxidation temperature was optimized to 97 C. Both the as-deposited and annealed films carry the crystalline character of the O phase. The O film has mainly (111) crystalline orientations. The grain size of crystallites was found to be 5 nm for annealed film. The electrical resistivity of the O films varies with the variation of temperature and thickness. The activation energy of the O films of different thicknesses were in the ranges of ( ) ev. Hall effect measurement confirms the p-type nature of the O films. Cuprous oxide thin films with different thicknesses were also prepared on a glass substrate by spray pyrolysis. From optical study it is observed that O films are highly absorbing in the visible range of electromagnetic radiation. The band gap energy decreases with the increasing thickness for O films. It is thought that because of these properties, O thin films can be used as absorber material in photovoltaic applications.

7 Deposition and Characterization of p- O Thin Films 7 Acknowledgements Dr. M. Faruk Hossain, Department of Electrical and Electronic Engineering, University of Toyama, Japan, is gratefully acknowledged for the X-ray diffraction experiment. References [1] A. Mittiga, E. Salza, F. Sarto, M. Tucci, and R. Vasanthi, Appl. Phys. Lett (6). [] S. Ishizuka, S. Kato, Y. Okamoto, T. Sakurai, K. Akimoto, N. Fujiwara and H. Kobayashi, Appl. Surf. Sci., (3). [3] T. Mahalingam, J. S. P. Chitra, J. P. Chu and P. J. Sebastian, Mater. Lett (4). [4] B. P. RaI, Sol. Cells, 5 65 (1988). [5] K. Nakaoka, J. Ueyama and K. Ogura, J. Electro Chem. Soc., 151 C661 (4). [6] R. A. Borzi, S. J. Stewart, R. C. Mercader, G. Punte and F. Garcia, J. Magn. Magn. Mater., (1). [7] A. Kellersohn, E. Knozinger, W. Langel and M. Giersig, Adv. Mater., 65 7 (1995). [8] K. Borgohain, J. B. Singh, M. V. Rama Rao, T. Shripathi and S. Mahamuni Phy. Rev. B (). [9] M. Hara, T. Kondo, M. Komod, S. Ikeda, K. Shinohara, A. Tanaka, J. N. Kondoa and K. Domen, Chem. Commun., (1998). [1] International Centre for Diffraction Data-Copyright JCPDS (1999), cards: JCPDS for O. [11] B. D. Cullity, Elements of X-ray diffraction, Addition-Wesley Publishing Co. Inc, 6 (1967). [1] Y. Hames, S. E. San, Solar Energy, (4). [13] K. Nakaoka and K. Ogura, J. Electrochem. Soc., 149 C579 (). [14] W. Feldman, Physical review, (1943). [15] A. E. Rakhshani, Solid-State Elect., 9 7 (1986). [16] G. P. Pollack, D. Trivich, J. Appl. Phys (19751). [17] T. Mahalingam, J. S. P. Chitra, J. P. Chu, H. Moon, H. J. Kwon, Y. D. Kim, J. Mater Sci: Mater Electron, (6).