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Photoluminescence from porous textured ZnO films grown by chemical bath deposition
Author’s Accepted Manuscript Photoluminescence from porous textured ZnO films grown by chemical bath deposition S.S. Kurbanov, H.C. Jeon, Z. Sh. Shaymardanov, R.Y. Rakhimov, T.W. Kang www.elsevier.com/locate/jlumin
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S0022-2313(15)00645-6 http://dx.doi.org/10.1016/j.jlumin.2015.10.063 LUMIN13686
To appear in: Journal of Luminescence Received date: 7 April 2015 Revised date: 29 September 2015 Accepted date: 21 October 2015 Cite this article as: S.S. Kurbanov, H.C. Jeon, Z. Sh. Shaymardanov, R.Y. Rakhimov and T.W. Kang, Photoluminescence from porous textured ZnO films grown by chemical bath deposition, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.10.063 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photoluminescence from porous textured ZnO films grown by chemical bath deposition S. S. Kurbanova,b*, H. C. Jeonb, Z. Sh. Shaymardanova, R. Y. Rakhimova and T. W. Kangb a
Institute of Ion-plasma and laser technologies, Uzbekistan Academy of Sciences, Tashkent
700125, Uzbekistan b
Quantum-functional Semiconductor Research Center, Dongguk Univ.-Seoul, Seoul 100-715,
Republic of Korea Abstract Porous textured ZnO films has been fabricated using the low temperature bath chemical deposition method. The photoluminescence (PL) spectra of the ZnO films show a broad emission band peaked at ~400 nm. The PL exhibits sensitivity to the emission recording conditions. A decrease in temperature leads to separation of the PL into the UV and violet emission bands; at 10 K they are located at 368.4 and 412 nm, respectively. The observed violet emission is attributed to zinc vacancy related defects. The observed sensitivity of the emission to the recording conditions is due to oxygen desorption processes and variations in the width of a depletion region on the porous textured ZnO film surface.
PACS: 78.55.Et, 78.67.Bf Keywords: Photoluminescence, ZnO film 1. INTRODUCTION Zinc oxide (ZnO), has been the center of various studies in the last decade, owing to its unique physical and chemical properties, such as high chemical stability, high electrochemical coupling coefficient, broad range of radiation absorption and high photostability [1,2]. In materials science, zinc oxide is classified as a semiconductor in group II-VI. A wide bandgap (Eg = 3.37 eV at
room temperature), large excitation binding energy (60 meV), and high thermal and mechanical stability at room temperature make it attractive for potential use in electronics, optoelectronics and laser technology. Because of its hardness, rigidity and piezoelectric constant it is an important material in the ceramics industry, while its low toxicity, biocompatibility and biodegradability make it a material of interest for biomedicine and in pro-ecological systems [3]. *
Corresponding author e-mail: [email protected]
2 The variety of structures of nanometric zinc oxide means that ZnO can be classified among new materials with potential applications in many fields of nanotechnology. Zinc oxide can occur in one(1D), two- (2D), and three-dimensional (3D) structures. One-dimensional structures make up the largest group, including nanorods, nanoneedles, nanohelixes, nanosprings and nanorings, nanoribbons, nanotubes, nanobelts, nanowires and nanocombs. Zinc oxide can be obtained in 2D structures, such as nanoplate/nanosheet and nanopellets. Examples of 3D structures of zinc oxide include flower, dandelion, snowflakes, coniferous urchin-like, and various porous structures [4].
Zinc oxide thin films are enjoying a great interest as transparent conductive oxides due to their electro-optical properties, high electro-chemical stability and absence of toxicity. ZnO is an n-type semiconductor and its conductivity is originated, basically, from the ionization of zinc interstitials and oxygen vacancies, which act as donor levels [1]. Unlike non-stoichiometric ZnO films, impurity-doped ZnO films show stable electrical and optical properties. Among the ZnO films doped with group III elements such as barium, aluminum, gallium and indium, aluminum doped zinc oxide (AZO) films show the lowest electrical resistivity and a good optical transmission in the visible and near-infrared regions. At present, the most widely publicized application for ZnO is as ITO replacement for displays and photovoltaic panels, where ZnO could lower costs of transparent conductors [5] Porous materials have potentially large surface areas and a high surface-to-volume ratio; hence, such are useful for a variety of applications such as catalyst, nanosieve filters, and gas sensors [6]. It is expected that application of porous structures will increase efficiency of these devices [6-9]. The photoluminescence (PL) spectra of ZnO nanomaterials are typically composed of two parts: a near-band edge emission at around 380 nm and a defect related deep level emission in the visible range. The visible luminescence of ZnO contains violet-blue (390-460 nm), green (500-520 nm), yellow-orange (560-600 nm) and red (650 nm) bands. The yellow - orange and green emission bands were suggested to relate with the amount of oxygen in samples: the green band is assigned to oxygen vacancy centers, while the yellow-orange band is attributed mostly to interstitial oxygen or complexes formed with oxygen [3]. In contrast to the other emission band the violet band is not frequently observed in ZnO structures. Its origin is controversial and assigned to interstitial zinc [10] or zinc vacancy [11-15]. Depending on synthesis methods, postgrown treatment, excitation conditions and temperature one or another emission band can dominate in the emission spectra [3]. Our studies have shown that porous textured ZnO films could be fabricated on aluminumcontaining substrates and in this case, a growth period is considerably shorter. Raman studies show that the growth structure is ZnO with Al doping. The room temperature (RT)
3 photoluminescence (PL) measurements reveal that the porous textured ZnO films exhibit an emission band at ~400 nm (3.1 eV) and its intensity exhibits sensitivity to the emission recording conditions. A decrease in temperature leads to separation of the spectrum into the UV and violet emission bands; at 10 K they are located at 368.4 and 412 nm, respectively. It is found that the emission bands possess different excitation power dependences; at high excitation power, the UV peak displays a tendency to saturation while the violet emission intensity increases linearly with increasing excitation power. The luminescence mechanisms of the violet PL are discussed on the basis of the obtained results.
2. EXPERIMENT The synthesis of porous textured ZnO films was carried out via a bath chemical deposition method. In a typical procedure, a 0.6586 g zinc acetate dehydrate (Zn(CH3COO)22H2O, (99.999 % purity, Aldrich) was dissolved in a 30 ml methanol solution under vigorous stirring at room temperature, then ultrasonicated for 10 min. Distilled water (10 ml) was added into the solution under stirring. The substrates, previously ultrasonically cleaned in acetone and methanol, were placed in facedown position into the bottom of the conical flask with the prepared solution. Al foil (5×5 mm2) and borosilicate glass with deposited Al film (5×5 mm2) were used as a substrate. Then the conical flask was put into a water tank at constant temperature of 630C for 24 h. After the expiration of the process, it was taken out and cooled to the room temperature. All films obtained were rinsed with methanol and distilled water, and dried at room temperature. The samples were annealed in a tube furnace in ambient air at 4000C for 1 h. PL studies were carried out on a SPEX spectrometer equipped with a 0.75 m grating monochromator using a 50 mW cw He-Cd laser (Kimmon Electric) operating at the wavelength of 325 nm as the excitation source. At the exit slit of the monochromator a cooled photomultiplier tube (Hamamatsu R943-02) was mounted. PL spectra were measured in the back-scattering configuration and record of the PL signal was carried out by using a conventional lock-in technique with a mechanical chopper. In order to vary the excitation intensity, the input power was attenuated by neutral filters. A He diplex system cryostat was used to vary the sample temperature from 10 to 300 K. Morphological characterization a n d chemical composition of films were performed by a high-resolution scanning electron microscope (HRSEM) (XL-30 PHILIPS) and Energy Dispersive X-ray (EDX) spectrometer attached to the HRSEM. Raman spectroscopy was performed at room temperature in backscattering geometry with a Renishaw InVia Raman Microscope system including a frequency-doubled Nd:YAG laser emitting at 532 nm and an edge filter with cut-off at 150 cm-1.
4 3. RESULTS AND DISCUSSION Figure 1 shows plan-view (a) and cross-sectional (b) SEM images of as-grown ZnO film. The images show that the surface morphology of the ZnO film has a unique porous textured structure with advanced voids. The average thickness of the as-grown film is about 500 nm as shown in Figure 1(b). A growth rate of porous textured ZnO films depends on substrates used. In the case of substrates without Al coating the growth rate was very low and in order to obtain the films with thickness of about 500 nm on pure borosilicate glass, more 72 h deposition time was required. Since, it is very likely Al acts as a catalyst to accelerate the film formation process. It should be noted fabrication of the ZnO films with a similar morphology was reported previously, pure and Al doped nest-like porous ZnO structures have been deposited on borosilicate glass substrates [16,17]. Although, the developed nest-like structures were obtained after 72 h of reaction. After annealing in air at 400°C for 1 h the surface morphology of the films is changed: the voids’ size increased due to the shrinkage of the walls, their edges became sharper and many outgrowths arose (Fig. 1(c)). Accordingly, it can be supposed that annealing increased the films porosity and surface area. To determine the purity and composition of the sample, EDX analysis was carried out. The EDX spectrum (not shown) reveals the presence of O, Zn and Al elements. The approximate atomic ratio of Zn, O and Al is 26.24%:69.27%:4.49%. It seems very likely that Al comes from the aluminum substrate. Although the EDX spectrum has suggested the existence of Zn, Al, and O elements in the asdeposited film, this result may not be enough for identification of the synthesized product. Raman scattering is considered to be a valuable technique for fast and nondestructive study of materials. It is one of the most useful methods for gaining insight into the microscopic structural effects of ZnO structures. Figure 2 shows the Raman scattering spectra of as-grown and annealed ZnO films detected in backscattering configuration. The as-grown sample exhibits the relatively strong Raman bands at ~275, 542, 770, 935 and 1082 cm-1 (curve a). The band at ~275 cm-1 was assigned to defect-induced modes of ZnO [18]. Recent the theoretical considerations confirmed this assignment [19]. It was discussed that this band could be related to modes of ZnO, which are Raman-inactive within a perfect crystal. Upon doping-induced defect formation, the translational crystal symmetry can be broken, and Raman-inactive modes may become Raman-active. The Raman band around 541 cm-1 is assigned to the second-order structures, namely it was tentatively suggested to relate to phonons from the region between the and M points of the ZnO Brillouin-zone around 270 cm-1 [20]. The band at ~935cm-1 may originate from acetate
5 groups present in reactants used in the synthesis and are ascribed to the O-C-O symmetric bend and C-C symmetric stretch [21]. The Raman band around 1080 cm-1 is attributed to the [TO+LO]A, L, M mode [22]. Now we cannot identify the band at ~770 cm-1. This band may also originate from acetate groups present in reactants used. The last assumption is confirmed by analyzing the Raman scattering spectrum of the annealed ZnO film (Fig.2 (curve b)). The spectrum exhibits the Raman bands at ~207, 437, 542 and 1082 cm-1. As can be seen the bands at ~275, 770 and 935 cm-1 ascribed to defects and impurities were vanished. New bands, which appear after annealing, are related to ZnO. The band around 207cm-1 is ascribed to the 2TA [22] or 2E2(low) with possible contributions of 2TA at the M point [20]. The peak located at 437 cm-1 can be assigned to the E2(high) mode of nonpolar optical phonons. This is the characteristic peak of hexagonal wurtzite phase. It can exhibit a broadening due to the doping effect. The vanishing of the defect and impurity related bands and appearance of the characteristic Raman bans indicate that the synthesized porous film is ZnO and a high temperature annealing in air improves the film quality. Luminescent methods in combination with structural methods is a sensitive technique for identification of materials. As depicted in Fig. 3, the room temperature (RT) PL spectrum of the as-grown porous textured ZnO film shows a broad band peaked at ~400 nm. There is a week shoulder at the short wavelength part of the band, which could be related to the near-band edge UV emission. From the Lorentzian fitting emission, we can identify that the broadband is composed of two emission bands. These two bands are located at ~385 and ~412 nm (Fig.3 (dashed lines)). The first band could be attributed to the near-band edge (NBE) emission and the second one (violet band) to the defect-induced emission. It is found that annealing in air leads to increase in the PL intensity, but has no effect on the RT spectrum shape. The PL spectra recorded before and after annealing in air at 4000C, are almost the same (Fig.4 (carve 1)). The PL intensity exhibits sensitivity to the emission recording conditions: in vacuum, the intensity grows and it is almost 2.7 times higher than in air. In vacuum the PL peak is red shifted and located at ~409 nm (3.03 eV) (Fig.4 (carve 2)). The red shift of the PL peak implies that in vacuum the violet band intensity increases much more than that of NBE emission band. At the same time PL spectra recording conditions have no effect on the as-grown film’s emission spectrum. With decreasing temperature, the emission intensity of the annealed ZnO film increases and at 10 K the PL intensity stronger 1.8 times than at room temperature. Although the near-band edge UV emission is not distinguished at room temperature, with decreasing temperature its intensity increases more rapidly than intensity of the violet emission and at 10 K a sharp UV peak at 368.4 nm (3.366 eV) appears. Along with increase in the violet emission intensity, the
6 temperature decreasing results in a red shift of its spectral maximum from 409 nm to ~412 nm. At 10 K the PL spectrum of the annealed film consists of two bands – at 368.4 and 412 nm (Fig.4 (curve 3)). In contrast to the annealed film, decrease in temperature does not lead to separation of the untreated film PL spectrum into two bans, although, the emission intensity increases. The UV and violet emission bands exhibit a quite different dependence on excitation power. Figure 5 shows the low temperature (10K) PL spectra of the annealed porous textured ZnO film at different excitation power. With decreasing excitation power, the intensity ratio of the UV emission to the violet emission changes and at low excitation power the UV emission dominates over the violet emission. The inset in Fig. 5 shows a high-energy part of the PL spectra. The PL spectrum obtained under the low laser power of 0.5 mW contains a weak band at ~367.8 nm (3.371 eV), a sharp peak at 368.4 nm (3.366 eV) and a tail of the violet band. The observed bands are attributed to emissions due to free exciton (FX) and neutral-donor bound exciton (D0X) transitions in ZnO, respectively [10]. The full width at half maximum (FWHM) of the neutral-donor bound exciton emission band is found to be 0.83 nm (7.6 meV). This FWHM value is higher than the FWHM values of ZnO thin films grown by using vapor phase methods, such as MBE or MOCVD [3]; however, this value is very small in view of the mild growth conditions and relatively low annealing temperature. It is well known, as the temperature is increased, the neutral-donor bound exciton related peak quenches and thermal dissociation of the bound excitons results in increase of the free excitons intensity. The intensity of free excitons increases up to 40 K, and then decreases, peak maximum shifts to the low-energy region and bandwidth becomes broader as the temperature is increased further. Therefore, at high temperatures the free exciton emission band is not distinguished under the strong violet emission band. We analyzed the excitation power dependence of the UV and violet emission bands intensities (Fig. 6). It is found that the UV emission intensity displays a tendency to saturation at high excitation power. The luminescence intensity I versus excitation power Pex can be expressed as % = !"#$
(1)
Here, η is the constant of proportionality, and the exponent α represents the radiative recombination mechanism. Using eq. (1) to fit the data, we found that α ~ 0.3. Contrary to the UV emission, the violet emission intensity increases linearly with increasing the excitation laser power. The saturation effect of excitonic emission under high power excitation is a rather frequently observed phenomenon. The saturation effects of the excitonic emissions from GaN
7 powder and ZnO nanocrystals excited in ambient air at RT and in vacuum at cryogen temperatures were ascribed to a thermally activated nonradiative process due to laser heating of particles [23,24]. High-power laser illumination can increase the sample surface temperature and high absorption of ZnO of about 105 cm-1 for a He–Cd laser 325 nm line will promote this effect. The RT violet emission bands located in range 390-420 nm (2.95-3.18 eV) have been reported [11-14, 25-27]. Wu et al. observed a cathodoluminescence (CL) band at 405 nm [11] and Chaaya et al. PL band around 400 nm (3.1 eV) [14] from ZnO films deposited by pulsed laser deposition. The emission appears only in the sample with a maximum oxygen pressure and it is assigned to the VZn center. Jeong et al. [12] also reported that the zinc vacancies were responsible for violet emission at 401 nm (3.09 eV) detected from the ZnO films grown by rf magnetron sputtering under oxygen-rich condition. Violet light luminescence located about 413– 424 nm and attributed also to VZn centers has been reported by Fan et al. in ZnO films obtained on quartz glass substrate by the oxidation of Zn films [13]. The intensity of the violet emission increases and the peak position shifts from 424 to 413 nm with increasing oxygen pressures. These results show a real influence of the oxygen concentration on position and intensity of the violent emission band. Violet band from ZnO structures also observed at low temperatures. At 77 K a violet emission band at 402.2 nm (3.08 eV) was recorded from O-implanted bulk ZnO samples and tentatively ascribed to O-antisite OZn [28]. Cao et al. reported violet emission 409 nm (3.03 eV) at 10 K from ZnO nanoneedle arrays, grown on silicon substrate by electrodeposition, which was identified as recombination between the electrons localized at the Zni-shallow donor levels and holes in the valence band [10]. A defect absorption band at 390 nm (3.18 eV) was observed from single-crystal ZnO exposed to 193-nm radiation [15]. This absorption band is 100 meV below the room-temperature band gap of ZnO and tentatively assigned to an electron transfer from the VZn (0/−1) acceptor level to the conduction band minimum. As follows from the above review of the violet emission band reports, the violet emission band was observed from ZnO structures fabricated in oxygen-rich conditions regardless of the methods used: laser or electrodeposition, rf magnetron sputtering, hydrothermal or bath solution. In other words, the main conditions of the violet band appearance is excess of oxygen in the samples investigated. From EDX analyses for the porous textured ZnO films, the atomic ratio of Zn to O is found to be 0.38:1 indicating Zn concentration is much lower than oxygen concentration. In these conditions, the films can include Zn vacancies (VZn) in modest concentrations. In n-type ZnO Zn vacancies have the lowest formation energy among the native point defects and it is energetically favored under oxygen-rich conditions [29]. According to the calculations Zn vacancies are deep acceptors and the transition levels (0/−1) lies at ~0.18 eV
8 [2,30] or ~0.45 eV [31] or ~0.3 eV [29] above the valence band and the transition level (−1/−2) is located at ~0.87 eV [2,30] or ~0.8 eV [31] or 0.7 eV [29] above the valence band. Recently, based on a new theoretical formalism and several different experimental techniques, it has been demonstrate that VZn acts as a compensating center in Ga [32] and Al [33] doped ZnO: the addition of Ga or Al donors in ZnO causes the lattice to form Zn-vacancy acceptors. In this context, it is reasonable to suggest that the violet emission detected from the porous textured ZnO films could be due to the existence of VZn centers. A transition between the conduction band or the shallow donor level and the Zn vacancy acceptor level would give rise to luminescence at 412 nm (3.01 eV), in reasonable agreement with the predicted transition energy. We cannot state at present an origin of the aluminum signal detected by EDX: it is related to substrate or the porous textured ZnO film contains Al atoms. In the latter case, the incorporated Al atoms could create shallow donor levels and stimulate the formation of VZn centers. As usual, the emission bands from ZnO structures as well as free exciton emission undergo a redshift with the increase of temperature due to the temperature-induced band-gap shrinkage. However, the violet emission band position is indifferent to temperature. This behavior of the violet PL band could be explained by two phenomena (namely band gap shrinkage and decreasing the energy interval between the zinc vacancy acceptor level and the valence band with temperature) operating simultaneously. At room and cryogen temperatures, the energy gap between the conduction band and the valence band is considered to be 3.37 eV and 3.43 eV, respectively. Therefore, the zinc vacancy level could be located at 3.37-3.01=0.36 eV and 3.433.01=0.42 eV above the valence band maximum at 300K and 10K, respectively. Hence, with decrease in temperature from 300K to 10K the energy interval between the zinc vacancy acceptor level and the valence band maximum increases by ~0.06 eV. In our opinion, this lowering of the zinc vacancy level could be caused by Jahn–Teller distortion. For the zinc vacancy in the (-1) charge state the Jahn–Teller effect localizes the hole on one of the four nearest neighbor atoms. The neighbor with the hole is expected to relax towards the vacancy, lowering the energy of the system [34]. In ZnO as was estimated by Janotti and Van de Walle, due to the Jahn–Teller effect a transition level between the (-1) and (-2) charge states would be moved away from the valence band and located at around 0.9 eV above the valence band [34]. Thus, at low temperature the Jahn–Teller distortion would give rise the energy difference of 60 meV between RT and 10K positions of the zinc vacancy level. Of cause, this explanation is speculative, additional theoretical and experimental studies should be carried out in future. Regarding the PL spectrum sensitivity to the emission recording conditions, dependence of the PL intensity on the ambient environment has been observed for ZnO thin films [35],
9 nanoparticles [36] and nanorods [37]. This phenomenon was attributed to oxygen desorption process and variation in the width of a depletion region on the surface of the zinc oxide structures. To our knowledge, there have been no previous reports of the violet emission sensitive to the ambient environment. As was indicated above, in vacuum, the RT PL intensity increases 2.7 times and the spectrum undergoes a red shift. This means that the violet band increases more rapidly in intensity than the NBE emission band. Sensitivity of the PL intensity to measurement conditions also indicates that the annealed porous textured ZnO film surface and air molecules should be involved in this process. Obviously, due to the high surface-to-volume ratio of the porous textured ZnO films the surface related effects appear pronouncedly. Zubiaga et al. based on the positron annihilation experiments have shown that in the ZnO film the Zn vacancy concentration decreases when the thickness of the film increases [38]. This result indicates that Zn vacancy centers are located in the surface layer and therefore, can undergo the surface related effects. It is well known when ZnO structure is exposed to air, some oxygen molecules are adsorbed onto the surface. These absorbed oxygen molecules then capture free electrons from the conduction band of ZnO and transform into oxygen ions, which create an electron depletion layer and band bending near the surface of the ZnO in air. Upon UV illumination, electron-hole pairs are generated and the holes migrate to the surface due to the potential slope from the inside to the surface of the ZnO film. The holes reaching the surface neutralize the oxygen ions. This process leads to desorption of oxygen. As a result, negative surface charges are reduced and, therefore, the depletion region width is decreased [36,37,39]. Obviously, the oxygen desorption process effectively takes place in vacuum. The changes in intensities of the NBE and oxygen vacancy defect related emissions caused by ambient environment have been attributed to a decrease in the depletion region width and variations in the concentration ration of the VO0/VO+ centers due to oxygen desorption on the surface of the zinc oxide nanocrystals [36] and nanorods [37]. Based on these points, we can infer that increase in the violet emission intensity is due to decrease in the depletion region width and band bending. These processes lead to recharge the zinc vacancy centers and increase in concentration of the centers responsible for the violet emission. Although, some aspects of the observations reported here including the violet emission origin and indifference of the PL spectrum position to temperature remain unclear, we believe the reported results provide information towards the final clarification of the origins of defect-related emissions in ZnO nanostructures, and extends the optical and electronic applications of nanostructured ZnO. Further investigations of structural and electrical properties of the porous textured ZnO films are in progress and the results will be reported in near future.
10
3. CONCLUSIONS Porous textured ZnO films were fabricated using the low temperature bath chemical deposition method. At room temperature upon UV excitation, the films exhibited a broad emission band peaked at ~400 nm. The PL intensity exhibits sensitivity to the emission recording conditions: in vacuum, the emission intensity grows and its spectrum undergoes redshift. A decrease in temperature results in an appearance of the UV peak and at 10 K the PL spectrum of the annealed porous textured ZnO film consists of two bands – at 368.4 and 412 nm. The low temperature UV and violet emission intensities exhibit a different excitation power dependence. At high excitation power, the UV peak displays a tendency to saturation while the violet emission intensity increases linearly with increasing the excitation laser power. This behavior of the UV peak is ascribed to a laser heating effects. After reviewing the available theoretical and experimental analyses on this subject and relating this to our results, we infer that zinc vacancy related defects are responsible for the observed violet luminescence. The violet emission arises from electron-hole recombination between the conduction band and zinc vacancy defect level. The violet PL band sensitivity to the emission recording conditions is attributed to change of the zinc vacancy charge state and increase in concentration of the emission centers responsible for the violet emission due to photoinduced desorption of surface oxygen and a decrease in the depletion region width. Acknowledgments This work was supported in part by the State Program for Basic Research of Republic of Uzbekistan (F2-FA-F148) and Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No.2014-039452).
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13 Figure captions Figure 1. SEM images of as-grown (plan-view (a) and cross-sectional (b)) and annealed (c) porous textured ZnO films. Figure. 2. Room-temperature Raman spectra of the (a) as-grown and (b) high temperature annealed porous textured ZnO films. Spectra are vertically shifted for more clarity. Figure 3. Room temperature PL spectrum and Lorentzian fitting (dashed lines) of the as-grown porous textured ZnO film. Figure 4. PL spectra of the porous textured ZnO film after annealing in air at 4000C. 1- room temperature PL spectrum recorded in air; 2 - room temperature PL spectrum recorded in vacuum; 3 – PL spectrum at 10K. Figure 5. Low temperature (10K) PL spectra of the annealed porous textured ZnO film recorded at different excitation power. The inside shows a UV part of the spectra. Figure 6. UV and violet emission bands intensities variations of the annealed porous textured ZnO film with excitation power at 10 K.
Highlights
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Porous textured ZnO films were fabricated using a low temperature bath chemical deposition method.
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Under UV excitation, a violet band at 412 nm was observed. The violet band intensity exhibits sensitivity to the emission recording conditions.
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The violet emission arises from electron-hole recombination between the conduction band and zinc vacancy defect level.
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PII: DOI: Reference:
S0022-2313(15)00645-6 http://dx.doi.org/10.1016/j.jlumin.2015.10.063 LUMIN13686
To appear in: Journal of Luminescence Received date: 7 April 2015 Revised date: 29 September 2015 Accepted date: 21 October 2015 Cite this article as: S.S. Kurbanov, H.C. Jeon, Z. Sh. Shaymardanov, R.Y. Rakhimov and T.W. Kang, Photoluminescence from porous textured ZnO films grown by chemical bath deposition, Journal of Luminescence, http://dx.doi.org/10.1016/j.jlumin.2015.10.063 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Photoluminescence from porous textured ZnO films grown by chemical bath deposition S. S. Kurbanova,b*, H. C. Jeonb, Z. Sh. Shaymardanova, R. Y. Rakhimova and T. W. Kangb a
Institute of Ion-plasma and laser technologies, Uzbekistan Academy of Sciences, Tashkent
700125, Uzbekistan b
Quantum-functional Semiconductor Research Center, Dongguk Univ.-Seoul, Seoul 100-715,
Republic of Korea Abstract Porous textured ZnO films has been fabricated using the low temperature bath chemical deposition method. The photoluminescence (PL) spectra of the ZnO films show a broad emission band peaked at ~400 nm. The PL exhibits sensitivity to the emission recording conditions. A decrease in temperature leads to separation of the PL into the UV and violet emission bands; at 10 K they are located at 368.4 and 412 nm, respectively. The observed violet emission is attributed to zinc vacancy related defects. The observed sensitivity of the emission to the recording conditions is due to oxygen desorption processes and variations in the width of a depletion region on the porous textured ZnO film surface.
PACS: 78.55.Et, 78.67.Bf Keywords: Photoluminescence, ZnO film 1. INTRODUCTION Zinc oxide (ZnO), has been the center of various studies in the last decade, owing to its unique physical and chemical properties, such as high chemical stability, high electrochemical coupling coefficient, broad range of radiation absorption and high photostability [1,2]. In materials science, zinc oxide is classified as a semiconductor in group II-VI. A wide bandgap (Eg = 3.37 eV at
room temperature), large excitation binding energy (60 meV), and high thermal and mechanical stability at room temperature make it attractive for potential use in electronics, optoelectronics and laser technology. Because of its hardness, rigidity and piezoelectric constant it is an important material in the ceramics industry, while its low toxicity, biocompatibility and biodegradability make it a material of interest for biomedicine and in pro-ecological systems [3]. *
Corresponding author e-mail: [email protected]
2 The variety of structures of nanometric zinc oxide means that ZnO can be classified among new materials with potential applications in many fields of nanotechnology. Zinc oxide can occur in one(1D), two- (2D), and three-dimensional (3D) structures. One-dimensional structures make up the largest group, including nanorods, nanoneedles, nanohelixes, nanosprings and nanorings, nanoribbons, nanotubes, nanobelts, nanowires and nanocombs. Zinc oxide can be obtained in 2D structures, such as nanoplate/nanosheet and nanopellets. Examples of 3D structures of zinc oxide include flower, dandelion, snowflakes, coniferous urchin-like, and various porous structures [4].
Zinc oxide thin films are enjoying a great interest as transparent conductive oxides due to their electro-optical properties, high electro-chemical stability and absence of toxicity. ZnO is an n-type semiconductor and its conductivity is originated, basically, from the ionization of zinc interstitials and oxygen vacancies, which act as donor levels [1]. Unlike non-stoichiometric ZnO films, impurity-doped ZnO films show stable electrical and optical properties. Among the ZnO films doped with group III elements such as barium, aluminum, gallium and indium, aluminum doped zinc oxide (AZO) films show the lowest electrical resistivity and a good optical transmission in the visible and near-infrared regions. At present, the most widely publicized application for ZnO is as ITO replacement for displays and photovoltaic panels, where ZnO could lower costs of transparent conductors [5] Porous materials have potentially large surface areas and a high surface-to-volume ratio; hence, such are useful for a variety of applications such as catalyst, nanosieve filters, and gas sensors [6]. It is expected that application of porous structures will increase efficiency of these devices [6-9]. The photoluminescence (PL) spectra of ZnO nanomaterials are typically composed of two parts: a near-band edge emission at around 380 nm and a defect related deep level emission in the visible range. The visible luminescence of ZnO contains violet-blue (390-460 nm), green (500-520 nm), yellow-orange (560-600 nm) and red (650 nm) bands. The yellow - orange and green emission bands were suggested to relate with the amount of oxygen in samples: the green band is assigned to oxygen vacancy centers, while the yellow-orange band is attributed mostly to interstitial oxygen or complexes formed with oxygen [3]. In contrast to the other emission band the violet band is not frequently observed in ZnO structures. Its origin is controversial and assigned to interstitial zinc [10] or zinc vacancy [11-15]. Depending on synthesis methods, postgrown treatment, excitation conditions and temperature one or another emission band can dominate in the emission spectra [3]. Our studies have shown that porous textured ZnO films could be fabricated on aluminumcontaining substrates and in this case, a growth period is considerably shorter. Raman studies show that the growth structure is ZnO with Al doping. The room temperature (RT)
3 photoluminescence (PL) measurements reveal that the porous textured ZnO films exhibit an emission band at ~400 nm (3.1 eV) and its intensity exhibits sensitivity to the emission recording conditions. A decrease in temperature leads to separation of the spectrum into the UV and violet emission bands; at 10 K they are located at 368.4 and 412 nm, respectively. It is found that the emission bands possess different excitation power dependences; at high excitation power, the UV peak displays a tendency to saturation while the violet emission intensity increases linearly with increasing excitation power. The luminescence mechanisms of the violet PL are discussed on the basis of the obtained results.
2. EXPERIMENT The synthesis of porous textured ZnO films was carried out via a bath chemical deposition method. In a typical procedure, a 0.6586 g zinc acetate dehydrate (Zn(CH3COO)22H2O, (99.999 % purity, Aldrich) was dissolved in a 30 ml methanol solution under vigorous stirring at room temperature, then ultrasonicated for 10 min. Distilled water (10 ml) was added into the solution under stirring. The substrates, previously ultrasonically cleaned in acetone and methanol, were placed in facedown position into the bottom of the conical flask with the prepared solution. Al foil (5×5 mm2) and borosilicate glass with deposited Al film (5×5 mm2) were used as a substrate. Then the conical flask was put into a water tank at constant temperature of 630C for 24 h. After the expiration of the process, it was taken out and cooled to the room temperature. All films obtained were rinsed with methanol and distilled water, and dried at room temperature. The samples were annealed in a tube furnace in ambient air at 4000C for 1 h. PL studies were carried out on a SPEX spectrometer equipped with a 0.75 m grating monochromator using a 50 mW cw He-Cd laser (Kimmon Electric) operating at the wavelength of 325 nm as the excitation source. At the exit slit of the monochromator a cooled photomultiplier tube (Hamamatsu R943-02) was mounted. PL spectra were measured in the back-scattering configuration and record of the PL signal was carried out by using a conventional lock-in technique with a mechanical chopper. In order to vary the excitation intensity, the input power was attenuated by neutral filters. A He diplex system cryostat was used to vary the sample temperature from 10 to 300 K. Morphological characterization a n d chemical composition of films were performed by a high-resolution scanning electron microscope (HRSEM) (XL-30 PHILIPS) and Energy Dispersive X-ray (EDX) spectrometer attached to the HRSEM. Raman spectroscopy was performed at room temperature in backscattering geometry with a Renishaw InVia Raman Microscope system including a frequency-doubled Nd:YAG laser emitting at 532 nm and an edge filter with cut-off at 150 cm-1.
4 3. RESULTS AND DISCUSSION Figure 1 shows plan-view (a) and cross-sectional (b) SEM images of as-grown ZnO film. The images show that the surface morphology of the ZnO film has a unique porous textured structure with advanced voids. The average thickness of the as-grown film is about 500 nm as shown in Figure 1(b). A growth rate of porous textured ZnO films depends on substrates used. In the case of substrates without Al coating the growth rate was very low and in order to obtain the films with thickness of about 500 nm on pure borosilicate glass, more 72 h deposition time was required. Since, it is very likely Al acts as a catalyst to accelerate the film formation process. It should be noted fabrication of the ZnO films with a similar morphology was reported previously, pure and Al doped nest-like porous ZnO structures have been deposited on borosilicate glass substrates [16,17]. Although, the developed nest-like structures were obtained after 72 h of reaction. After annealing in air at 400°C for 1 h the surface morphology of the films is changed: the voids’ size increased due to the shrinkage of the walls, their edges became sharper and many outgrowths arose (Fig. 1(c)). Accordingly, it can be supposed that annealing increased the films porosity and surface area. To determine the purity and composition of the sample, EDX analysis was carried out. The EDX spectrum (not shown) reveals the presence of O, Zn and Al elements. The approximate atomic ratio of Zn, O and Al is 26.24%:69.27%:4.49%. It seems very likely that Al comes from the aluminum substrate. Although the EDX spectrum has suggested the existence of Zn, Al, and O elements in the asdeposited film, this result may not be enough for identification of the synthesized product. Raman scattering is considered to be a valuable technique for fast and nondestructive study of materials. It is one of the most useful methods for gaining insight into the microscopic structural effects of ZnO structures. Figure 2 shows the Raman scattering spectra of as-grown and annealed ZnO films detected in backscattering configuration. The as-grown sample exhibits the relatively strong Raman bands at ~275, 542, 770, 935 and 1082 cm-1 (curve a). The band at ~275 cm-1 was assigned to defect-induced modes of ZnO [18]. Recent the theoretical considerations confirmed this assignment [19]. It was discussed that this band could be related to modes of ZnO, which are Raman-inactive within a perfect crystal. Upon doping-induced defect formation, the translational crystal symmetry can be broken, and Raman-inactive modes may become Raman-active. The Raman band around 541 cm-1 is assigned to the second-order structures, namely it was tentatively suggested to relate to phonons from the region between the and M points of the ZnO Brillouin-zone around 270 cm-1 [20]. The band at ~935cm-1 may originate from acetate
5 groups present in reactants used in the synthesis and are ascribed to the O-C-O symmetric bend and C-C symmetric stretch [21]. The Raman band around 1080 cm-1 is attributed to the [TO+LO]A, L, M mode [22]. Now we cannot identify the band at ~770 cm-1. This band may also originate from acetate groups present in reactants used. The last assumption is confirmed by analyzing the Raman scattering spectrum of the annealed ZnO film (Fig.2 (curve b)). The spectrum exhibits the Raman bands at ~207, 437, 542 and 1082 cm-1. As can be seen the bands at ~275, 770 and 935 cm-1 ascribed to defects and impurities were vanished. New bands, which appear after annealing, are related to ZnO. The band around 207cm-1 is ascribed to the 2TA [22] or 2E2(low) with possible contributions of 2TA at the M point [20]. The peak located at 437 cm-1 can be assigned to the E2(high) mode of nonpolar optical phonons. This is the characteristic peak of hexagonal wurtzite phase. It can exhibit a broadening due to the doping effect. The vanishing of the defect and impurity related bands and appearance of the characteristic Raman bans indicate that the synthesized porous film is ZnO and a high temperature annealing in air improves the film quality. Luminescent methods in combination with structural methods is a sensitive technique for identification of materials. As depicted in Fig. 3, the room temperature (RT) PL spectrum of the as-grown porous textured ZnO film shows a broad band peaked at ~400 nm. There is a week shoulder at the short wavelength part of the band, which could be related to the near-band edge UV emission. From the Lorentzian fitting emission, we can identify that the broadband is composed of two emission bands. These two bands are located at ~385 and ~412 nm (Fig.3 (dashed lines)). The first band could be attributed to the near-band edge (NBE) emission and the second one (violet band) to the defect-induced emission. It is found that annealing in air leads to increase in the PL intensity, but has no effect on the RT spectrum shape. The PL spectra recorded before and after annealing in air at 4000C, are almost the same (Fig.4 (carve 1)). The PL intensity exhibits sensitivity to the emission recording conditions: in vacuum, the intensity grows and it is almost 2.7 times higher than in air. In vacuum the PL peak is red shifted and located at ~409 nm (3.03 eV) (Fig.4 (carve 2)). The red shift of the PL peak implies that in vacuum the violet band intensity increases much more than that of NBE emission band. At the same time PL spectra recording conditions have no effect on the as-grown film’s emission spectrum. With decreasing temperature, the emission intensity of the annealed ZnO film increases and at 10 K the PL intensity stronger 1.8 times than at room temperature. Although the near-band edge UV emission is not distinguished at room temperature, with decreasing temperature its intensity increases more rapidly than intensity of the violet emission and at 10 K a sharp UV peak at 368.4 nm (3.366 eV) appears. Along with increase in the violet emission intensity, the
6 temperature decreasing results in a red shift of its spectral maximum from 409 nm to ~412 nm. At 10 K the PL spectrum of the annealed film consists of two bands – at 368.4 and 412 nm (Fig.4 (curve 3)). In contrast to the annealed film, decrease in temperature does not lead to separation of the untreated film PL spectrum into two bans, although, the emission intensity increases. The UV and violet emission bands exhibit a quite different dependence on excitation power. Figure 5 shows the low temperature (10K) PL spectra of the annealed porous textured ZnO film at different excitation power. With decreasing excitation power, the intensity ratio of the UV emission to the violet emission changes and at low excitation power the UV emission dominates over the violet emission. The inset in Fig. 5 shows a high-energy part of the PL spectra. The PL spectrum obtained under the low laser power of 0.5 mW contains a weak band at ~367.8 nm (3.371 eV), a sharp peak at 368.4 nm (3.366 eV) and a tail of the violet band. The observed bands are attributed to emissions due to free exciton (FX) and neutral-donor bound exciton (D0X) transitions in ZnO, respectively [10]. The full width at half maximum (FWHM) of the neutral-donor bound exciton emission band is found to be 0.83 nm (7.6 meV). This FWHM value is higher than the FWHM values of ZnO thin films grown by using vapor phase methods, such as MBE or MOCVD [3]; however, this value is very small in view of the mild growth conditions and relatively low annealing temperature. It is well known, as the temperature is increased, the neutral-donor bound exciton related peak quenches and thermal dissociation of the bound excitons results in increase of the free excitons intensity. The intensity of free excitons increases up to 40 K, and then decreases, peak maximum shifts to the low-energy region and bandwidth becomes broader as the temperature is increased further. Therefore, at high temperatures the free exciton emission band is not distinguished under the strong violet emission band. We analyzed the excitation power dependence of the UV and violet emission bands intensities (Fig. 6). It is found that the UV emission intensity displays a tendency to saturation at high excitation power. The luminescence intensity I versus excitation power Pex can be expressed as % = !"#$
(1)
Here, η is the constant of proportionality, and the exponent α represents the radiative recombination mechanism. Using eq. (1) to fit the data, we found that α ~ 0.3. Contrary to the UV emission, the violet emission intensity increases linearly with increasing the excitation laser power. The saturation effect of excitonic emission under high power excitation is a rather frequently observed phenomenon. The saturation effects of the excitonic emissions from GaN
7 powder and ZnO nanocrystals excited in ambient air at RT and in vacuum at cryogen temperatures were ascribed to a thermally activated nonradiative process due to laser heating of particles [23,24]. High-power laser illumination can increase the sample surface temperature and high absorption of ZnO of about 105 cm-1 for a He–Cd laser 325 nm line will promote this effect. The RT violet emission bands located in range 390-420 nm (2.95-3.18 eV) have been reported [11-14, 25-27]. Wu et al. observed a cathodoluminescence (CL) band at 405 nm [11] and Chaaya et al. PL band around 400 nm (3.1 eV) [14] from ZnO films deposited by pulsed laser deposition. The emission appears only in the sample with a maximum oxygen pressure and it is assigned to the VZn center. Jeong et al. [12] also reported that the zinc vacancies were responsible for violet emission at 401 nm (3.09 eV) detected from the ZnO films grown by rf magnetron sputtering under oxygen-rich condition. Violet light luminescence located about 413– 424 nm and attributed also to VZn centers has been reported by Fan et al. in ZnO films obtained on quartz glass substrate by the oxidation of Zn films [13]. The intensity of the violet emission increases and the peak position shifts from 424 to 413 nm with increasing oxygen pressures. These results show a real influence of the oxygen concentration on position and intensity of the violent emission band. Violet band from ZnO structures also observed at low temperatures. At 77 K a violet emission band at 402.2 nm (3.08 eV) was recorded from O-implanted bulk ZnO samples and tentatively ascribed to O-antisite OZn [28]. Cao et al. reported violet emission 409 nm (3.03 eV) at 10 K from ZnO nanoneedle arrays, grown on silicon substrate by electrodeposition, which was identified as recombination between the electrons localized at the Zni-shallow donor levels and holes in the valence band [10]. A defect absorption band at 390 nm (3.18 eV) was observed from single-crystal ZnO exposed to 193-nm radiation [15]. This absorption band is 100 meV below the room-temperature band gap of ZnO and tentatively assigned to an electron transfer from the VZn (0/−1) acceptor level to the conduction band minimum. As follows from the above review of the violet emission band reports, the violet emission band was observed from ZnO structures fabricated in oxygen-rich conditions regardless of the methods used: laser or electrodeposition, rf magnetron sputtering, hydrothermal or bath solution. In other words, the main conditions of the violet band appearance is excess of oxygen in the samples investigated. From EDX analyses for the porous textured ZnO films, the atomic ratio of Zn to O is found to be 0.38:1 indicating Zn concentration is much lower than oxygen concentration. In these conditions, the films can include Zn vacancies (VZn) in modest concentrations. In n-type ZnO Zn vacancies have the lowest formation energy among the native point defects and it is energetically favored under oxygen-rich conditions [29]. According to the calculations Zn vacancies are deep acceptors and the transition levels (0/−1) lies at ~0.18 eV
8 [2,30] or ~0.45 eV [31] or ~0.3 eV [29] above the valence band and the transition level (−1/−2) is located at ~0.87 eV [2,30] or ~0.8 eV [31] or 0.7 eV [29] above the valence band. Recently, based on a new theoretical formalism and several different experimental techniques, it has been demonstrate that VZn acts as a compensating center in Ga [32] and Al [33] doped ZnO: the addition of Ga or Al donors in ZnO causes the lattice to form Zn-vacancy acceptors. In this context, it is reasonable to suggest that the violet emission detected from the porous textured ZnO films could be due to the existence of VZn centers. A transition between the conduction band or the shallow donor level and the Zn vacancy acceptor level would give rise to luminescence at 412 nm (3.01 eV), in reasonable agreement with the predicted transition energy. We cannot state at present an origin of the aluminum signal detected by EDX: it is related to substrate or the porous textured ZnO film contains Al atoms. In the latter case, the incorporated Al atoms could create shallow donor levels and stimulate the formation of VZn centers. As usual, the emission bands from ZnO structures as well as free exciton emission undergo a redshift with the increase of temperature due to the temperature-induced band-gap shrinkage. However, the violet emission band position is indifferent to temperature. This behavior of the violet PL band could be explained by two phenomena (namely band gap shrinkage and decreasing the energy interval between the zinc vacancy acceptor level and the valence band with temperature) operating simultaneously. At room and cryogen temperatures, the energy gap between the conduction band and the valence band is considered to be 3.37 eV and 3.43 eV, respectively. Therefore, the zinc vacancy level could be located at 3.37-3.01=0.36 eV and 3.433.01=0.42 eV above the valence band maximum at 300K and 10K, respectively. Hence, with decrease in temperature from 300K to 10K the energy interval between the zinc vacancy acceptor level and the valence band maximum increases by ~0.06 eV. In our opinion, this lowering of the zinc vacancy level could be caused by Jahn–Teller distortion. For the zinc vacancy in the (-1) charge state the Jahn–Teller effect localizes the hole on one of the four nearest neighbor atoms. The neighbor with the hole is expected to relax towards the vacancy, lowering the energy of the system [34]. In ZnO as was estimated by Janotti and Van de Walle, due to the Jahn–Teller effect a transition level between the (-1) and (-2) charge states would be moved away from the valence band and located at around 0.9 eV above the valence band [34]. Thus, at low temperature the Jahn–Teller distortion would give rise the energy difference of 60 meV between RT and 10K positions of the zinc vacancy level. Of cause, this explanation is speculative, additional theoretical and experimental studies should be carried out in future. Regarding the PL spectrum sensitivity to the emission recording conditions, dependence of the PL intensity on the ambient environment has been observed for ZnO thin films [35],
9 nanoparticles [36] and nanorods [37]. This phenomenon was attributed to oxygen desorption process and variation in the width of a depletion region on the surface of the zinc oxide structures. To our knowledge, there have been no previous reports of the violet emission sensitive to the ambient environment. As was indicated above, in vacuum, the RT PL intensity increases 2.7 times and the spectrum undergoes a red shift. This means that the violet band increases more rapidly in intensity than the NBE emission band. Sensitivity of the PL intensity to measurement conditions also indicates that the annealed porous textured ZnO film surface and air molecules should be involved in this process. Obviously, due to the high surface-to-volume ratio of the porous textured ZnO films the surface related effects appear pronouncedly. Zubiaga et al. based on the positron annihilation experiments have shown that in the ZnO film the Zn vacancy concentration decreases when the thickness of the film increases [38]. This result indicates that Zn vacancy centers are located in the surface layer and therefore, can undergo the surface related effects. It is well known when ZnO structure is exposed to air, some oxygen molecules are adsorbed onto the surface. These absorbed oxygen molecules then capture free electrons from the conduction band of ZnO and transform into oxygen ions, which create an electron depletion layer and band bending near the surface of the ZnO in air. Upon UV illumination, electron-hole pairs are generated and the holes migrate to the surface due to the potential slope from the inside to the surface of the ZnO film. The holes reaching the surface neutralize the oxygen ions. This process leads to desorption of oxygen. As a result, negative surface charges are reduced and, therefore, the depletion region width is decreased [36,37,39]. Obviously, the oxygen desorption process effectively takes place in vacuum. The changes in intensities of the NBE and oxygen vacancy defect related emissions caused by ambient environment have been attributed to a decrease in the depletion region width and variations in the concentration ration of the VO0/VO+ centers due to oxygen desorption on the surface of the zinc oxide nanocrystals [36] and nanorods [37]. Based on these points, we can infer that increase in the violet emission intensity is due to decrease in the depletion region width and band bending. These processes lead to recharge the zinc vacancy centers and increase in concentration of the centers responsible for the violet emission. Although, some aspects of the observations reported here including the violet emission origin and indifference of the PL spectrum position to temperature remain unclear, we believe the reported results provide information towards the final clarification of the origins of defect-related emissions in ZnO nanostructures, and extends the optical and electronic applications of nanostructured ZnO. Further investigations of structural and electrical properties of the porous textured ZnO films are in progress and the results will be reported in near future.
10
3. CONCLUSIONS Porous textured ZnO films were fabricated using the low temperature bath chemical deposition method. At room temperature upon UV excitation, the films exhibited a broad emission band peaked at ~400 nm. The PL intensity exhibits sensitivity to the emission recording conditions: in vacuum, the emission intensity grows and its spectrum undergoes redshift. A decrease in temperature results in an appearance of the UV peak and at 10 K the PL spectrum of the annealed porous textured ZnO film consists of two bands – at 368.4 and 412 nm. The low temperature UV and violet emission intensities exhibit a different excitation power dependence. At high excitation power, the UV peak displays a tendency to saturation while the violet emission intensity increases linearly with increasing the excitation laser power. This behavior of the UV peak is ascribed to a laser heating effects. After reviewing the available theoretical and experimental analyses on this subject and relating this to our results, we infer that zinc vacancy related defects are responsible for the observed violet luminescence. The violet emission arises from electron-hole recombination between the conduction band and zinc vacancy defect level. The violet PL band sensitivity to the emission recording conditions is attributed to change of the zinc vacancy charge state and increase in concentration of the emission centers responsible for the violet emission due to photoinduced desorption of surface oxygen and a decrease in the depletion region width. Acknowledgments This work was supported in part by the State Program for Basic Research of Republic of Uzbekistan (F2-FA-F148) and Leading Foreign Research Institute Recruitment Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST) (No.2014-039452).
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13 Figure captions Figure 1. SEM images of as-grown (plan-view (a) and cross-sectional (b)) and annealed (c) porous textured ZnO films. Figure. 2. Room-temperature Raman spectra of the (a) as-grown and (b) high temperature annealed porous textured ZnO films. Spectra are vertically shifted for more clarity. Figure 3. Room temperature PL spectrum and Lorentzian fitting (dashed lines) of the as-grown porous textured ZnO film. Figure 4. PL spectra of the porous textured ZnO film after annealing in air at 4000C. 1- room temperature PL spectrum recorded in air; 2 - room temperature PL spectrum recorded in vacuum; 3 – PL spectrum at 10K. Figure 5. Low temperature (10K) PL spectra of the annealed porous textured ZnO film recorded at different excitation power. The inside shows a UV part of the spectra. Figure 6. UV and violet emission bands intensities variations of the annealed porous textured ZnO film with excitation power at 10 K.
Highlights
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Porous textured ZnO films were fabricated using a low temperature bath chemical deposition method.
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Under UV excitation, a violet band at 412 nm was observed. The violet band intensity exhibits sensitivity to the emission recording conditions.
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The violet emission arises from electron-hole recombination between the conduction band and zinc vacancy defect level.
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