Poster presented at the The Seventh Yugoslav Materials Research Society Conference - YUCOMAT 2005, Herceg Novi, Crna Gora, September 12-16, 2005

Ivetić, Tamara

Nikolić, Maria Vesna

Urošević, Dragan

Young, D.

English

Serbian Academy of Science and Arts Digital Archive (DAIS)

Photoacoustic Properties of Thin Film Zinc-Stannate T. Ivetić1a, M. V. Nikolić2b, D. L. Young3c, D. Vasiljević-Radović4d, D. Urošević5e 1Institute of Technical Sciences of SASA, Knez Mihailova 35, 11000 Beograd, Serbia and Montenegro 2Center for Multidisciplinary Studies of the University of Belgrade, Kneza Višeslava 1, 11000 Beograd, Serbia and Montenegro 3National Renewable Energy Laboratory, Golden, Colorado 80401, USA 4Institute of Microelectronics and Single Crystals, Njegoševa 12, 11000 Beograd, Serbia and Montenegro 5Mathematical Institute, SASA, Knez Mihailova 35, 11000 Beograd, Serbia and Montenegro atamara@itn.sanu.ac.yu, bmaria@mi.sanu.ac.yu, cdavid_young@nrel.gov, ddana@nanosys.ihtm.bg.ac.yu, edraganu@turing.mi.sanu.ac.yuAbstract. Thin films of single-phase zinc-stannate (Zn2 SnO4 ) were grown by rf magnetron sputtering onto glass substrates. Transmission in the visible range was measured allowing determination of the energy gap and thickness of analyzed thin film samples using interference fringes. The photoacoustic phase and amplitude spectra of all samples were measured as a function of the laser beam modulating frequency using a transmission detection configuration. Fitting of experimental data enabled calculation of thermal diffusivity, the coefficient of minority carrier diffusion, their mobility and lifetime. 10 20 30 40 50 60 7005101520unknown(440)(511)(331)(400)(222)(311)(220)(111)I (c/s)2θ(ο)λCuKαFig. 1 XRD pattern of a typical Zn2 SnO4 thin film sample after annealingResults and DiscussionFig. 1 shows the X-ray diffraction pattern of a typical Zn2 SnO4 thin film sample after annealing. It was polycrystalline with peaks characteristic for spinel Zn2 SnO4 and one small unknown peak at 4.321 A. The quality of the obtained sample surface is shown in fig. 2 giving typical AFM images of the analyzed Zn2 SnO4 thin films. The images reveal “grains” with an average diameter of about 100nm. Fig. 2 AFM image of a Zn2 SnO4 thin film 600 nm thick a) plan-view, b) 2DTransmission results of a typical Zn2 SnO4 thin film sample after annealing are given in fig. 3. Film thickness and index of refraction were calculated using the interference fringes observed on fig. 3. The refractive index nf of the thin film was first calculated using the following equation [1]:(1)where na is the refractive index of air, ng is the refractive index of glass and nf is the refractive index of the film,(2)where Tmax and Tmin are the maximum and minimum of the transmittance versus the wavelength given in fig. 3.2/12222222244(22/)(⎥⎥⎦⎤⎢⎢⎣⎡ −+++++= gaxgagaxgagaf nnTnnnnTnnnnnminmaxminmaxTTTTTx−=300 400 500 600 700 800020406080100transmittance [%]wavelength [nm]Fig. 3 Transmission vs. wavelength for a Zn2 O4 thin film 600 nm thickTable 1 Calculated parameters for Zn2 SnO4 thin films Sample thickness DT (m2/s) D (m2/s) τ (s) α (m-1) sg (m/s) sb (m/s) μ (cm2/Vs) E (eV)600 nm 0.1⋅10-5 0.50⋅10-4 4⋅10-2 682 0.2⋅10-5 0.5⋅10-10 20 0.381316 nm 0.11⋅10-5 0.63⋅10-4 2⋅10-2 11957 0.5⋅10-4 0.2⋅10-11 25 0.385The value obtained for the refractive index was nf =1.94. The film thickness for each sample was calculated using the value obtained for the refractive index and the classical equation for interference fringes:2dnf = mλ (3)where d is the film thickness, m is the fringe order and λ is the wavelength. The values obtained were in the range of 316-600 nm (the sample given in fig. 3).The direct band gap for each sample was calculated from the absorption edge. It read to be 0.325 nm for the sample 600 nm thick (fig. 3). The energy gap was calculated to be: Eg =1.24/0.325 = 3.81 eV. The values obtained for two film thicknesses are given in table 1. It is well known that the optical direct energy gap for Zn2 SnO4 is about 3.35 eV. In our case the higher value of the energy gap is the consequence of a strong Burstein-Moss shift. The free carrier concentration of a sample with a film thickness of 643 nm was determined in [2] as 3.3⋅1019 cm3, so we expect this value to be higher for the thinner film with a higher energy gap.10 100020406080100 experimental fittedPA phase [o]frequency [Hz]10 1000.010.1 experimental fittedPA amplitudefrequency [Hz]Fig. 4 Phase (a) and amplitude (b) experimental and theoretical diagrams for Zn2 SnO4 thin film 600 nm thicka)b)The experimental PA phase and amplitude diagrams were then fitted with the theoretically calculated PA signals for Zn2 SnO4 thin films (fig. 4). A fitting procedure described in detail in [5] was then used to calculate thermal parameters of Zn2 SnO4 thin films. The calculated values of the thermal parameters (DT – the thermal diffusivity, τ - the excess carrier lifetime, D – the diffusion coefficient of the minority free carriers, α - the optical absorption coefficient, sg and sb are the front and rear surface recombination velocities) for two different thicknesses of Zn2 SnO4 thin films are given in table 1. One can see that the values obtained for thermal diffusivity are very similar for both film thicknesses. The mobility of minority free carriers for the analyzed Zn2 SnO4 films were determined using the values calculated for the diffusion coefficient of the minority free carriers (D) and the following relation:(4)where k is the Boltzmann coefficient, e is the electron charge and T is the temperature, as the majority carrier effects are negligible for the cases when n>>p or p>>n according to the van Roosbroech ambipolar equation for the ambipolar diffusion coefficient [6]:(5)where p and n are the electron and hole concentration and Dn and Dp are the electron and hole diffusion coefficient, respectively. Thus, in this case n>>p and D*=Dp =D.  eTkD μ=pn DnDpnpD//*++=The values obtained for the minority carrier mobility for two film thicknesses are given in table 1. One can see that mobility was slightly higher for the thinner film. Both obtained values are in accordance with the ones calculated in [2] for different film thicknesses using the four-coefficient method for determining transport properties and the transport theory. Low mobility values and carrier concentrations are responsible for the high resistivities of Zn2 SnO4 films. According to [2] incomplete crystallization of thin-film samples reflected in a larger full width at half maximum of XRD intensity peaks of thin films compared to the bulk sputter target could account for low mobility values. Minority carrier mobility is a parameter whose values could be useful for characterizing solar cell materials, as the minority carriers in absorber materials can be indicators of device performance.Fig. 4 shows the phase (a) and amplitude (b) of the photoacoustic signal vs. the modulating frequency for a typical Zn2 SnO4 thin film sample 600 nm thick. A theoretical analysis of the experimental results of photoacoustic measurements was per- formed using the method described in detail in [3] based on the Rosencwaig-Gersho thermal piston model [4].AcknowledgementsThe authors would like to express their gratitude to Prof. P. M. Nikolić for many helpful conversations and S. Đurić for the X-ray measurements. This research was performed within projects 1832 and 6150 financed by the Ministry of Science and Environmental Protection of the Republic of Serbia.References1. S. Venkatachalam, D. Mangalaraj, Sa. K. Narayandass, K. Kim, J.Yi, Physica B: Condens. Matter, 358 (2005), 27-352. D. L. Young, H. Montinho, Y. Yan, T. J. Coutts, J. Appl. Phys. 92 (2002) 310-3193. P. M. Nikolić, M. V. Nikolić, D. Luković, S. Savić, M. M. Ristić, Zeitschrift fur Metallkunde, 95 (2004), 147-1504. A. Rosencwaig, A. Gersho, J. Appl. Phys. 47 (1976) 64-695. D. M. Todorović, P. M. Nikolić, Opt. Eng. (1997), 432-4456. E. S. Yang, Fundamentals of semiconductor devices, Mc-Graw Hill, 1978YUCOMAT 2005, September 12-16, Herceg Novi

Photoacousic Properties of Thin Film Zinc-Stannate

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Photoacousic Properties of Thin Film Zinc-Stannate

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