DSpace at VNU: Band-edge photoluminescence in nanocrystalline ZnO : In films prepared by electrostatic spray deposition

Applied Surface Science 252 (2006) 2770–2775 www.elsevier.com/locate/apsusc Band-edge photoluminescence in nanocrystalline ZnO:In films prepared by electrostatic spray deposition Dam Hieu Chi a,b, Le Thi Thanh Binh a, Nguyen Thanh Binh a, Le Duy Khanh a, Nguyen Ngoc Long a,* b a Faculty of Physics, Hanoi University of Science, 334 Nguyen Trai, Thanh Xuan, Hanoi, Vietnam Center for Strategic Development of Science and Technology, Japan Advanced Institute of Science and Technology, 1-1, Asahidai, Tatsunokuchi, Ishikawa, Japan Received February 2005; received in revised form April 2005; accepted 14 April 2005 Available online 13 June 2005 Abstract ZnO:In films are successfully prepared by using the electrostatic spray deposition technique X-ray diffraction indicates that ˚ and c ¼ 5:209 A ˚ the ZnO:In films have a polycrystalline hexagonal wurtzite structure with lattice parameters a ¼ 3:267 A Photoluminescence properties of the films are investigated in the temperature range of 11.6–300 K, showing strong luminescence in the whole range of temperature The temperature dependence of the photoluminescence are carried out with full profile fitting of spectra, which clearly shows that the ultraviolet (UV) emission in In-doped ZnO films at low temperature are attributed to emission of a neutral donor-bound exciton (DX) and recombination of donor–acceptor pairs (DAP), while the UV emission at room temperature originates from radiative transition of an electron bound on a donor to the valence band # 2005 Elsevier B.V All rights reserved PACS: 78.55.Et; 81.05.Dz; 81.15.Rs Keywords: Semiconductors; Chemical synthesis; Luminescence Introduction Zinc oxide (ZnO) has a direct and wide bandgap of 3.37 eV with a large exciton binding energy of 60 meV Owing to the strong exciton binding energy, ZnO is recoginzed as a promising photonic material in the UV region ZnO-based films have been actively * Corresponding author E-mail address: ngoclong@yahoo.com (N.N Long) studied because of their potential application, including transparent conductive contacts in solar cells, gas sensors, varistors, luminescent material, ultraviolet (UV) light-emitting devices, and UV lasers ZnO films have been grown by many methods, such as r.f magnetron sputtering [1], metal organic chemical vapor deposition (MOCVD) [2], plasma-assisted molecular beam epitaxy (P-MBE) [3], pulsed laser deposition [4], sol–gel [5], spray pyrolysis [6], and electrostatic spray deposition [7] Among these 0169-4332/$ – see front matter # 2005 Elsevier B.V All rights reserved doi:10.1016/j.apsusc.2005.04.011 D.H Chi et al / Applied Surface Science 252 (2006) 2770–2775 methods, electrostatic spray deposition is a simple, economic and efficient method for preparing ZnO films The inclusion of dopants, such as Al [8], In [9], and Ga [10] has further improved the opto-electrical properties of ZnO films Because of a strong exciton binding energy, ZnO is expected to exhibit emissions based on excitonic mechanisms: free exciton [11–13], bound exciton [11–13], exciton–exciton scattering [11,12], biexciton [13], and electron hole plasma [12] In addition, the emission of ZnO also contains radiative transitions related to impurities, such as the free-to-bound transition [13–15] and donor– acceptor transition [15] In this paper, we present our success in fabrication of In-doped ZnO films using the electrostatic spray deposition technique The photoluminescence spectra of the derived films are measured in the temperature range of 11.6–300 K The photoluminescence of the near band-edge is focused, concerning the origin of emission lines in the ultraviolet region at room temperature Studies of the temperature dependence of the photoluminescence are carried out carefully with full profile fitting of spectra The UV emission in Indoped ZnO films at low temperature are found to be attributed to emission of a neutral donor-bound exciton (DX), recombination of donor–acceptor pairs (DAP) On the other hand, the UV emission at room temperature is found to originate from radiative transition of an electron bound on a donor to the valence band Experiment Zinc acetate (Zn(CH3COO)2 ˙ 2H2O) was dissolved in a mixed solvent consisting of double distilled water and isopropyl alcohol in proportion 2:3 A solution of 0.2 M zinc acetate was prepared A few drops of chlorine acid were added to the solution to dissolve some precipitates of zinc hydroxide resulting from the reaction of the isopropyl alcohol with the zinc acetate A suitable amount of indium acetate was put into the solution for preparation of the In-doped zinc oxide films The doping ratio In=Zn for the films varied from to at.% The films were fabricated by using the electrostatic spray deposition (ESD) technique, which are suitable for the fabrication of thin films and 2771 nanoparticles The basis of the ESD technique is the aerosol generation of liquid precursors The working liquid is forced to flow through a small metall nozzle which is subjected to an electric field, then leave the outlet of the nozzle in form of droplets with extremely small size This aerosol is subsequently directed towards a heated substrate in order to form thin films The system consists of three functionally distinct components: the ESD spray unit, the spray processing chamber, and the deposition unit The ESD spray unit consists of a metal capillary containing the working liquid, which is to be electrostatically sprayed The metal capillary is connected to a high-voltage power supply, therefore, its nozzle works as a charge-injection electrode The apparatus is similar to that in [16] and has been described elsewhere [7] A high DC voltage fixed at 18 kV was applied between a substrate and a metal capillary nozzle The distance from the nozzle to the substrate was kept at 25 cm Substrates were 20  20  0:1 mm3 glass plates The substrate temperature Ts during deposition could be varied from 220 to 400  C The crystal structure of the ZnO:In films was analyzed using a SIMENS D5005 X-ray diffractometer The composition of the films was determined by an energy dispersive X-ray (EDX) spectrometer (EDS, OXFORD ISIS 300) attached to the JEOL-JSM 5410 LV scanning electron microscope Photoluminescence (PL) spectra were measured at temperatures ranging from 11 up to 300 K using a Fluorolog FL3-22 Jobin Yvon Spex USA spectrofluorometer with a Xenon lamp of 450 W as an excitation source Results and discussion Fig shows a typical X-ray diffraction (XRD) pattern of ZnO:In films deposited at Ts ¼ 350  C substrate temperature The XRD pattern clearly indicates that the ZnO:In films possess a polycrystalline ˚, hexagonal wurtzite crystal structure (a ¼ 3:267 A ˚ c ¼ 5:209 A) with no preferred orientation In order to evaluate the mean grain size of the films, we adopted Scherrer’s formula [17]: d¼ 0:9l B cos u (1) 2772 D.H Chi et al / Applied Surface Science 252 (2006) 2770–2775 Fig X-ray diffraction pattern of ZnO:In (3 at.%) film, Ts ¼ 350  C where l is the X-ray wavelength, u the Bragg diffraction angle, and B the linewidth at half-maximum of (1 1) peak around 36.25  The mean grain size of the films was determined to be about 14 nm EDX measurements were carried out in order to examine the elemental composition of the ZnO:In films Fig shows the EDX spectrum, in which characteristic X-ray peaks originating from Zn, O, In, Cl, and Ti are observed Traces of Cl and Ti in the EDX spectrum are due to the impurity of the precursory solution The EDX measurements indicated that the atomic ratio of In/Zn in the films is lower than that in the initial spray solution The indium content of the films was found to be $ at.% The undoped ZnO films show high sheet resistance, higher than 20 MV/square Doping with indium remarkably decreases the sheet resistance; the sheet resistance of ZnO film doped with at.% In is about Fig EDX spectrum of ZnO:In (3 at.%) film, Ts ¼ 350  C 10 kV/square The doped indium atoms, behaving as donor impurities in ZnO films [1], induce a decrease in the electrical resistivity of the films The typical PL spectrum at 11.6 K of the ZnO:In films exhibits two sharp lines at 3.375 and 3.322 eV (in following discussions, we use these values as indications of these emission lines), and a broad line peaked at 3.228 eV in the UV region (Fig 3) A weak broad green emission band (not shown in the figure) appears at 2.46 eV This green band is usually associated with singly ionized oxygen vacancies [18] In this paper, we focus our attention only on the line group in the UV region, concerning the origin these emission lines As will be shown later, the strong sharp line at 3.375 eV can be interpreted as a neutral donor-bound exciton (denoted by DX) The line at 3.322 eV is attributed to the recombination of bound charge carriers on impurities with free carriers in the allowed bands (BF) The broad line at 3.228 eV is due to donor–acceptor pairs (DAP) In order to investigate the origins of emission lines, the PL spectra were measured in the temperature range from 11.6 to 300 K Data analysis, including determination of peak position and peak integrated intensity, was carried out by a routine of peak least Fig Temperature dependence of the PL spectra from ZnO:In (3 at.%) film, Ts ¼ 350  C, measured with excitation wavelength of 300 nm D.H Chi et al / Applied Surface Science 252 (2006) 2770–2775 2773 square fitting (reliability factors > 99%), using symmetric Pearson VII functions [19]: pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 21=m p f xị ẳ Iwh p Gmị f1 ỵ 21=m 1ịx2 gm Gm 0:5Þ (2) where I is intensity, wh full width at half maximum, m peak flack parameter, and G function is defined as: Z xlÀ1 eÀx dx; l > 0: (3) Glị ẳ It is evident from Fig that the intensity of the sharp line observed at 3.375 eV is decreased rapidly and is shifted slightly to the low-energy side with increasing measuring temperature At 120 K, this line becomes much weaker than the line at 3.322 eV (Fig 3, inset) While the line at 3.375 eV decreases rapidly and almost disappears at 150 K, the line at 3.322 eV is still maintained up to room temperature This line is dominant and located at 3.214 eV at 300 K The wide line that peaks at 3.228 eV is considerably shifted to the low-energy side with increasing temperature At 80 K, this line is located at 3.034 eV, that is, the shift in energy is about 194 meV Under such conditions, a new line is exhibited at 3.242 eV The temperature dependence of the PL-integrated intensity can be expressed by the equation [20]: ITị ẳ I0 ỵ A exp ðÀE=kB TÞ (4) where E is the activation energy of the thermal quenching process, kB Boltzmann constant, I0 the emission intensity at K, and A a constant Fig shows the temperature dependence of the PL integrated intensity of the line at 3.375 eV with a constant A ¼ 90:45 and E ¼ 13:06 meV This value of the activation energy of the thermal quenching process is close to the value of the binding energy of the exciton bound to the neutral donors, experimentally obtained to be 12 meV [11] Similar analysis for the line at at 3.322 eV with a constant A ¼ 284:54 and E ¼ 41:45 meV was carried out The strong sharp line located at 3.375 eV with linewidth of 12 meV can be attributed to neutral donor-bound exciton (DX) Fig PL integrated intensity for the neutral donor-bound exciton (DX) and the bound-to-free transition (BF) vs temperatures from Fig The solid triangles and the open squares represent the experimental data for (DX) and (BF), respectively; solid line and dotted line are best-fit profiles for corresponding experimental data using Eq (4) Assuming that the peak position of donor-bound exciton emission varies with temperature as the energy band gap, we tried to fit the observed temperature dependence to Varshnis semiempirical formula [21]: ETị ẳ E0ị aT T ỵb (5) where E(0), a and b are fitting parameters As can be seen in Fig 5, the experimental values for the (DX) line fit rather well to the Varshni’s curve with fitting parameters: a ¼ À9:8  10À4 eV/K, b ¼ À791 K, and E0ị ẳ 3:375 eV The emission line, which is encountered around 3.322 eV, has been observed by other authors [6,11,15,22,23] Opinions in the literature vary as to the origin of this Some authors interpret this line as a neutral acceptor-bound exciton [11,22], others as a two-electron transition of an exciton bound to a neutral donor [6,23] or donor–acceptor pairs [15] In our opinion, the line at 3.322 eV in our sample cannot be interpreted as a two-electron transition of an exciton bound to a neutral donor, because this line still remains at the temperatures at which the bound 2774 D.H Chi et al / Applied Surface Science 252 (2006) 2770–2775 exciton line almost disappears This line cannot be attributed to an exciton bound to a neutral acceptor either, because of the low binding energy for this complex In the case of the exciton bound to a neutral acceptor, the excitons are thermally detached from these centers even at low temperatures, which is contrary to our experimental results The line at 3.322 eV, with a linewidth of around 13 meV, as narrow as the bound exciton line (whose linewidth ¼ 12 meV) is not due to donor–acceptor pairs Within the hypothesis based on donor–acceptor pairs, the transition energy must strongly depend on the donor–acceptor distance and the position of the corresponding peak must be dependent on excitation intensity However, no excitation dependence is found for the line of 3.322 eV in our samples For the origin of this emission line, we believe that the line at 3.322 eV probably corresponds to the recombination of a free carrier with a carrier bound on a defect In our case, the sample is a n-type semiconductor, so it is more likely that an electron bound on a donor recombines with a free hole in the valence band (BF) In that case, the peak position of the BF emission should vary with temperature more slowly than the energy band gap does, as may be seen Fig Temperature dependence of the peak position for (DX) (solid triangles) and (BF) (open squares) Curves (dotted line and solid line) are calculated using Varshni’s formula with a ¼ 9:8 104 eV/K, b ẳ 791 K, and E0ị ¼ 3:375 eV, 3.322 eV for (DX) and (BF) lines, respectively in Fig The free-to-bound radiative transition has been observed in ZnO both at low temperatures and at room temperature by other authors [13–15] The very broad line peaked at 3.228 eV and shifted to the low-energy side with increasing temperature is interpreted as a donor–acceptor pairs (DAP) emission DAP emission energy is described as: hn ¼ Eg À EA ED ỵ q2 er (6) where Eg is the band gap, EA and ED are the acceptor and donor binding energy, respectively, q is the electrical charge of the acceptor and donor ions, e is the dielectric constant, and r is the donor–acceptor distance With increasing temperature, carriers on donor– acceptor pairs with small distance r are released into the band, which results in extinguishing the high-energy side of DAP emission line, and the line is shifted to the low-energy side as observed in our experiment A new line is revealed at 3.249 eV after the broad DAP line is shifted to the low-energy side with increasing temperature This line is assigned to a LOphonon replica of the BF line, because the energy distance between them is 71 meV, equal to the energy of an LO-phonon (Fig 6) Fig Peak separation for PL spectra of ZnO:In (3 at.%) film at 90 K Dots are raw data, solid line represents the best-fit profile, and dotted line are calculated results for each peak D.H Chi et al / Applied Surface Science 252 (2006) 2770–2775 Conclusions In conclusion, nanocrystalline ZnO:In films have been successfully prepared by using an electrostatic spray pyrolysis technique Doping with indium decreased the sheet resistance of ZnO films The obtained ZnO:In films exhibited strong luminescence in the temperature range from 11.6 K to room temperature The neutral donor-bound exciton and donor– acceptor pair’s emission lines were clearly observed at low temperatures, while the radiative transition of electrons from shallow donors to valence band was observed both at low temperatures and at room temperature Acknowledgements This work was partly supported by Natural Science Council, Ministry of Science and Technology of Vietnam and the 21st COE project from Japan Advanced Institute of Science and Technology References [1] K Tominaga, N Umezu, I Mori, T Ushiro, T Moriga, I Nakabayashi, J Vac Sci Technol A16 (1998) 1213 [2] C.R Gorla, N.W Emanetoglu, S Liang, W.E Mayo, Y Lu, M Wraback, H Shen, J Appl Phys 85 (1999) 2595 [3] H.J Ko, Y.F Chen, T Yao, K Miyajima, A Yamamoto, T Goto, Appl Phys Lett 77 (2000) 537 2775 [4] E Holmelund, J Schou, S Tougaard, N.B Larsen, Appl Surf Sci 197–198 (2002) 467 [5] G.K Paul, S.K Sen, Mater Lett 57 (2002) 742 [6] S.A Studenikin, M Cocivera, W Kellner, H Pascher, J Lumin 91 (2000) 223 [7] M Koyano, P.Q Bao, L Thi Thanh Binh, L Hong Ha, N Ngoc Long, 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Conclusions In conclusion, nanocrystalline ZnO :In films have been successfully prepared by using an electrostatic spray pyrolysis technique Doping with indium decreased the sheet resistance of ZnO films