Photonic crystals in biology

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Poster Session, Tuesday, June 15 Theme A1 - B702 Resonant donor states in quantum well Arnold Abramov 1 * 1 Department of Applied Mathematics, Donbass State Engineering, Academy, Kramatorsk 84313, Ukraine Abstract-A method of calculation of donor impurity states in quantum well is developed. The used techniques have made it possible to find the binding energy both of ground and excited impurity states attached to each QW subband. The positions of resonant states in 2D continuum are determined as poles of corresponding wave functions. As result of such approach the identification of resonant states in 2D continuum is avoided without introducing special criterions. The calculated dependences of binding energies versus impurity position are presented for various width of Si/Si1-xGex quantum. It is known, that the incorporation of small amounts of impurities in semiconductor led to occurrence of additional (impurity) states in band structure. In case of a complex band structure consisting of several subbands impurity states arise under each of them. For the lower subband they are in the forbidden gap and are localized. Impurity states for overlying subbands are on a background of a continuum, and become quasilocal. The existence of such states was experimentally confirmed in bulk and nanostructures. To describe them theoretically it is necessary to solve Shredinger equation for multiband model. To solve this problem a method of expansion the unknown electron wave function (WF) on plane waves basis has been developed in [1]. Then the problem is reduced to a task of determination of eigen vectors (envelope functions) and eigen values (binding energy) for rather large matrixes. For characteristic sizes of the matrix >1000 (more than thousand) most of the solutions are in the continuum. And it is necessary to introduce special criterion to identificate (resonant) impurity states among the obtained set of solutions. As such one the peaks on energy dependence of wave function were used. However, in vicinity of RS the wave function has a pole and use of explicit energy dependence of WF is not correct. Thus, the main difficulties of the calculations are in need of processing large matrices, as well as the correct and easy identification of RS. Our method allow to overcome these problems. WF of impurity states (both localized and resonant) are determined from set of coupled integral equations, which first were introduced in [2]. Proposed techniqes consist in replacement the integrals by the finite sums and followed conversion from set of integral equations to the equation in matrix form. Unlike the works [1,3] energy here are not eigenvalue of matrix, but is parameter on which matrix elements depends. The positions of resonant impurity states correspond to poles of WF, which are defined by equating a determinant of the matrix to zero. Our method allows easy to include in calculations external fields - electric, magnetic. Other advantages include no necessity for explicit knowledge of the band structure, and also the absence of Coulombs divergence problem. The efficiency of the method is demonstrated on the analysis of the impurity state depending on the position of the impurity center: at the moving impurity atom away the middle of the QW impurity state can be transformed from a resonant state to localized. This fact is of interest for further theoretical study, and may also have practical applications in the creating an optical device based on intracenter transitions. Besides, resonant impurity states can play a determining role in creation of a so-called inverted distributions, which can lead to novel optical devices in the far-infrared (or terahertz) range. Therefore efficient and correct calculation of RS, in addition to theoretical, also has a practical importance for the analysis and development of a theory of the experimentally observed lasing effect. This work was partially supported by by Ukranian Ministry of Education and Science. We thank Dr. H.Najafov for fruitful discussions. *Corresponding author: qulaser@gmail.com [1] A. Blom, M. A. Odnoblyudov, I. N. Yassievich, K.-A. Chao, Phys. Rev. B 68, 165338 (2003) [2] B.Vinter, Phys.Rev. B 26, 6808 (1982) [3] A.A.Abramov, C.-H. Lin, C.W. Liu, Int. J. of Nanoscience 7, No. 4/5, 181 (2008) 6th Nanoscience and Nanotechnology Conference, zmir, 2010 371
Poster Session, Tuesday, June 15 Theme A1 - B702 Electrical, structural and optical properties of spray deposited SnO 2 and SnO 2 :F thin films Demet Tatar 1* , Güven Turgut 1 , Erdal Sönmez 1 , Bahattin Düzgün 1 , and Mehmet Ertugrul 2 ,1 K. K. Education Faculty, Department of Physics, Ataturk University, Erzurum 25240, Turkey 2 Engineering Faculty, Department of Electric-Electronic, Ataturk University, Erzurum 25240, Turkey Abstract—The undoped and fluorine doped thin films are synthesized by using cost-effective spray pyrolysis technique. The dependence optical structural and electrical properties of SnO 2 films, on the concentration of fluorine is reported. Optical absorption, X-ray diffraction, scanning electron microscope (SEM) and Hall effect studies have been performed on SnO 2 :F (FTO) films coated on glass substrates.X-ray diffraction pattern reveals the presence of cassiterite structure with (200) preferential orientation for FTO films. The roughness of the films changed from 22,52 to 15,52 nm. Atomic force microscopy (AFM) study reveals the surface of FTO to be made of nanocrystalline particles. The electrical study reveals that the films exhibit n-type electrical conductivity. The 20 wt% F doped film has a minimum resistivity of 1,29.10 -4 Ωcm, carrier density of 8,48.10 18 cm -3 and mobility of 568 cm 2 V -1 s -1 . The sprayed FTO film having minimum resistance of 12,46 Ω/cm 2 and a good transparency in the visible, these films are useful as conducting layers in electrochromic and photovoltaic devices and also as the passive counter electrode. The undoped stoichiometric SnO 2 films have very high electrical resistivity because of their low intrinsic carrier density and mobility [1]. Therefore the challenge is to prepare non-stoichiometric doped thin films. The conductivity of weakly nonstoichiometric tin oxide films is supposed to be due to doubly ionized vacancies serving as donors [2]. Dopants as antimony, Sb, indium, In, and fluorine, F, are frequently used. The fluorine-doped tin oxide (FTO), being an n-type, wide band gap semiconductor (≥3 eV) with special properties, high transmittance in the visible range and high reflectance in the infrared, excellent electrical conductivity, greater carrier mobility and good mechanical stability are used in different devices like solar cells as transparent, protective electrodes [3], flat panel collectors as spectral selective windows, sensors for detection of gases, sodium lamps, gas sensors, and varistors [4–6]. FTO films have been prepared by various techniques, such as chemical vapour deposition, metalorganic deposition, rf sputtering, sol–gel, and spray pyrolysis [7–9]. Spray pyrolysis is used to prepare films because of its simplicity and commercial viability [10,11]. Moreover, the spray pyrolysis technique is well suited for the preparation of doped tin oxide thin films because of it is ease to adding various doping materials, controlling the texture via various deposition temperatures and mass production capability for uniform large area coatings. The prime aim of this work is to produce low thickness with high transmission, low resistance and highly conducting F doped tin oxide thin films with higher figure of merit by costeffective chemical spray pyrolysis technique and study their optical, structural, electrical and optoelectrical properties. and X-ray rocking curve with CuK α radiation. The surface morphology of the FTO thin films were observed by an atomic force microscopy (AFM) (by products nanomagnetic-inst). The electrical studies were carried out by Hall measurements in van der Pauw configuration. The visible transmission spectra of FTO films were measured using UV– spectrometer . (a) (b) Figure 2. AFM images of samples, a) SnO 2 (TO), b) SnO 2:F (FTO) The physical properties of the spray pyrolyzed tin oxide and FTO thin film deposited at 420˚C from SnCl 2 .2H 2 O precursor have been presented. The transmittance enhancement of these films is due to the well-crystallized film and the pinhole free surface. X-ray diffraction pattern reveals the presence of cassiterite structure with (200) preferential orientation for FTO film. The roughness of the films changed from 22,52 to 15,52 nm. The AFM analysis showed that improved morphological structural of the films. The FTO film deposited on 420°C revealed the minimum resistivity of about 1,29.10 −4 Ω.cm and high transmittance in the visible band. This high conductivity and transparency of FTO film suggest that these films are likely to be useful as electrical contacts in various electronic and energy harvest applications. *Corresponding author: demettatar@atauni.edu.tr Figure 1. X-ray diffraction pattern of samples Figure 1. shows the X-ray difraction (XRD) patterns of the FTO films. XRD studies were made with a X-ray diffraction [1] A.V. Moholkar, et. al. Applied Surface Science 255, 9358–9364 (2009). [2] Z.M. Jarzebski, J.P. Marton, J. Electrochem. Soc. 123, 2 (2000). [3] S. Colen, Thin Solid Films 77, 127 (1981). [4] P.S. Patil, et. al. Thin Solid Films 437, 34 (2003). [5] A. Dima, et. al. Thin Solid Films 427, 427 (2003). [6] D.S. Lee, et. al. Thin Solid Films 416, 271 (2002). [7] J. Kane, H.P. Schweizer, J. Electrochem. Soc. 123, 270 (1976). [8] T.N. Blanton, M. Lelental, Mater. Res. Bull. 29, 537 (1994). [9] K.Y. Rajpure, et. al. Mater. Chem. Phys. 64, 184 (2000). [10] P.S. Patil, Mater Chem Phys. 59, 158 (1999). [11] E. Elangovan, K. Ramamurthi, J. Optoelect. Adv. Mater. 5, 45 (2003). 6th Nanoscience and Nanotechnology Conference, zmir, 2010 372
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