- Email: [email protected]
Mg2+ substitutions in ZnO–Al2O3 thin films and its effect on the optical absorption spectra of the nanocomposite
Applied Surface Science 253 (2007) 8661–8668 www.elsevier.com/locate/apsusc
Mg2+ substitutions in ZnO–Al2O3 thin films and its effect on the optical absorption spectra of the nanocomposite Soumen Das, Subhadra Chaudhuri ä,* Nanofilm Lab, Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India Received 27 February 2007; received in revised form 26 March 2007; accepted 18 April 2007 Available online 3 May 2007
Abstract ZnO–Al2O3 nanocomposite thin films were prepared by sol–gel technique. The room temperature synthesis was mainly based on the successful peptization of boehmite (AlO(OH)) and Al(OH)3 compounds, so as to use it as matrix to confine ZnO nanoparticles. The relative molar concentrations of xZnO to (1 x) Al2O3 were varied as x = 0.1, 0.2 and 0.5. The optical absorption spectra of the thin films showed intense UV absorption peaks with long tails of variable absorption in the visible region of the spectra. The ZnO–Al2O3 nanocomposites thin films were doped with MgO by varying its molar concentrations as y = 0.05, 0.75, 0.1, 0.125, 0.15 and 0.2 with respect to the ZnO present in the composite. The MgO doped thin films showed suppression of the intense absorption peaks that was previously attained for undoped samples. The disappearance of the absorption peaks was analyzed in terms of the crystalline features and lattice defects in the nanocomposite system. The bulk absorption edge, which is reportedly found at 3.37 eV, was shifted to 5.44 eV (for y = 0.05), 5.63 eV (for y = 0.075) and maximum to 5.77 eV (for y = 0.1). In contrast, beyond the concentration, y = 0.1 the absorption edges were moved to 5.67 eV (for y = 0.125), 5.61 eV (for y = 0.15) and to 5.49 eV (for y = 0.2). This trend was explained in terms of the Burstein–Moss shift of the absorption edges. # 2007 Elsevier B.V. All rights reserved. PACS : 81.20.Fw; 78.66.Hf; 61.72.Vv; 78.66.w Keywords: Sol–gel; Nanocomposite; Thin films; Optical absorption
1. Introduction The wide band gap of ZnO (3.37 eV for the bulk ZnO [1]) can be altered if the particle sizes of the system lie in the nanometric region. This large band gap of ZnO coupled with its large excitonic binding energy (0.060 eV) have made it a potential candidate in flat panel devices, light emitting diodes, LASERs in the ultraviolet region [2,3] and as transparent conducting oxide. Above this, the band gap of ZnO also depends on the size of the crystallites. This phenomenon is called quantum confinement (QC) effect and is analyzed and explained by several authors [4–6]. It is observed that the higher surface to volume ratio of nanocrystals introduces various surface related defects and disorders in the system. So, they are
* Corresponding author. Tel.: +91 33 2473 4971; fax: +91 33 2473 2805. E-mail addresses: [email protected] (S. Das), [email protected] (S. Chaudhuri). ä Deceased. 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.04.072
embedded in insulating matrix to minimize these defects. High band gap materials like MgO, SiO2 or Al2O3 are also used to restrict the growth rate of the nanocrystals and in the process enhance the optical band gap of the system. The quantum confinement effect, and the introduction of Al3+ or Mg2+ ions into the lattice of ZnO are used fruitfully to tune the band gap of ZnO nanocrystals [7–9]. This latter mode of tuning is called the Burstein–Moss Shift, and it is dependent on the diffusibility of the dopant into the crystal lattice [10]. ZnO with MgO (Eg = 7.3–7.7 eV [11]) can produce MgxZn1xO alloy having higher band gap and improved optical and photoluminescence properties [12,13]. The suitability of MgxZn1xO as a multilayer quantum well structure has also been studied in various earlier reports [14,15]. According to the phase diagram of the ZnO–MgO binary system the solubility of MgO in the ZnO lattice is less than 4% in the bulk form [16], but for thin film the solid solubility is as high as 33% [17]. The degree of solubility of Mg in ZnO is found to depend largely on the deposition techniques and on the processing conditions [18–20]. The ionic radius of Mg2+
8662
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
˚ ) is very close to that of Zn2+ (0.72 A ˚ ) [21], so at relatively (0.57 A higher annealing temperatures the Mg ions diffuse into the ZnO lattice, and replace each other. For higher concentration of MgO it has been shown that the MgxZn1xO alloys form a metastable alloy [17]. These metastable phase and the degree of metastability are the influencing factors for the potential applications of MgxZn1xO based devices. On the other hand, the accumulation of MgO on the grain boundaries of ZnO has an influence on the surface states of the nanoparticles [22]. In this article, we have undertaken a comparative study of the UV–vis optical absorption spectra of the single layered undoped and MgO doped ZnO–Al2O3 nanocomposite thin films. We have also concentrated on the shift of the optical absorption edge from its bulk band gap value and the broadening of the optical spectra at the absorption edge. We found out that these phenomena largely depend on the role of the MgO as dopant, its accumulation on the surface of ZnO and its solubility in the nanocomposite.
along with the selected area electron diffraction (SAED) pattern. The composition analysis and the presence of the Mg2+ in the nanocomposite thin films were confirmed by the energy dispersive X-ray analysis (EDAX) attachment to a JEOL JSM J2010 Field emission scanning electron microscope (FESEM). The optical absorption spectra of the products were recorded by an Schimadzu 2401spectrophotometer. 3. Results and discussion 3.1. The TEM study Fig. 1 shows the TEM images of the alumina sol taken on a Cu grid and annealed at 300 8C. The image shows a network
2. Experimental details ZnO–Al2O3 nanocomposites with relative molar concentrations of ZnO to Al2O3 as 10:90, 20:80 and 50:50 were prepared by sol–gel technique. For Al2O3 part, the aqueous solution of aluminum nitrate (Al(NO3)36H2O) was refluxed for 1 h and then treated with NH3 solution (25%). The white precipitate (a mixture of aluminum hydroxide and boehmite) was dissolved in (12:1) volume ratio of ethanol and water. Three cubic centimeter of acetic acid was added to it drop wise under constant stirring. The pH of the sol was measured as 2.75. At the end of 3 h of stirring a completely transparent and viscous sol was obtained. The detail of the preparation of the composite sol and the subsequent deposition of the thin films were reported by the authors elsewhere [23]. At this point we would like to mention that the sol–gel preparation of the alumina was mainly based on the Yoldas method [24], as described in numbers of well-cited papers [25,26]. The main difference in our method from the prescribed one was that we accomplished the entire preparation at room temperature, without resorting to heat treatment during peptization. In fact, heat treatment at 80 8C was a crucial parameter in Yoldas method. In the latter section of this article, the nanocomposites will be denoted by sample SA (10:90), SB (20:80) and SC (50:50). The thin films were annealed at 300, 500 and 700 8C in air. Later, sample SA was doped with MgO at molar concentrations y = 0.05, 0.075, 0.10, 0.125, 0.15 and 0.20 with respect to the molar concentration of ZnO present in the composite. For doping, the required amount of Mg-acetate was mixed directly with the ZnO–Al2O3 sol under stirring. The thin films were dipcoated (5 cm/min) and annealed in air at 300 8C for 25 min. In another case, sample SC was also doped with y = 0.05 MgO and was annealed at 300 and 700 8C. The surface morphology of the thin films was determined by atomic force microscopy (AFM) (Nanoscope IV scanning probe microscope controller). The morphological attributes of the nanocomposite samples were determined by transmission electron microscope (TEM) (JEOL 2010 electron microscope)
Fig. 1. The TEM images of the (a) alumina sol network, (b) the cylindrical fibrillar structure of alumina hydroxide annealed at 300 8C.
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
like build up of the aluminum based hydroxide in the sol. Earlier, Yoldas [24] showed that depending on certain parameters, like the type of alkoxides used, the ratio of alkoxide to water, the rate of hydrolysis, pH standard and the mode of drying there are may be several structural variations of the final alumina gel. The microstructural images in Fig. 1a shows interconnected branch like extension of the hydroxide consisting of closely knit grains forming thick impenetrable structures. These dangling branches contribute to the density, the pore size and the specific area of a dry gel. Colloidal gels obtained from aluminum precursors comprise of two types of oxo-bridges, Al–O–Al and Al–O(OH)–Al(OH). The dry gel in this case forms a random network of neighbouring particles where the proportion of linear linkage between particles varies. The pore presents in the ‘‘cylindrical fibrillar structure’’ also
8663
produces rooms for the guest nanoparticles to get confined and to grow during sintering, as we will observe in Fig. 2. In Fig. 2a and b the ZnO–Al2O3 nanocomposite (sample SA) showed similar branch like structures. That the ZnO nanoparticles are embedded and are randomly dispersed in the Al2O3 matrix can be observed from Fig. 2c. The black dots are agglomerated grains of ZnO nanoparticles and can be well resolved at the inset figure, which shows the high-resolution image of a single ZnO grain consisting of a number of agglomerated nanoparticles. The average size of the ZnO nanoparticles was determined as around 9.0 nm. In Fig. 2d the SAED image shows spots in the diffused diffraction rings indicating the polycrystalline nature of the nanoparticles. In our previous report [23] we have seen that the average particle size was 10.0 nm for sample SB and around 18.0 nm for sample SC
Fig. 2. The TEM images of the ZnO–Al2O3 nanocomposite (a,b) the network like appearance, (c) the ZnO nanoparticles embedded on the amorphous alumina matrix annealed at 300 8C, inset figure shows the ZnO grains consisting of agglomerated ZnO nanoparticles. (d) The SAED images of the composite showing diffraction rings from ZnO nanocrystals.
8664
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
processed under similar conditions. Thus the nanoparticles grow in size when the proportion of the alumina is lesser in the composite. This is because; the presence of larger quantity of ZnO inside the pores of the alumina matrix facilitates the growth to larger grains when treated at different annealing temperatures. 3.2. X-ray diffraction study The crystalline phase of the nanocomposite thin films were studied with the XRD measurements. The XRD spectra of sample SA, SB (annealed at 300–700 8C, not shown in this report) did not show significant difference in appearance from each other, and characteristic peaks either of alumina, or ZnO were absent. As in the subsequent TEM images revealed the polycrystalline nature of the samples, we inferred that the absence of the characteristic peaks was due to the low resolution of the XRD measuring instrument. The characteristic peaks of ZnO are observed for sample SC annealed at 300 and 700 8C as is shown in Fig. 3. The peaks are identified as (1 0 0), (1 0 1) and (2 0 0) [JCPDS-File No. 35-0664]. It is seen that the peak intensity increases at higher annealing temperature and are more prominent in the lower spectra (for films annealed at 700 8C). The average radius of the confined nanoparticle is measured from the highest peak in the spectra and is calculated as 8.2 nm, which is close to the size we obtained for sample SC from TEM (reported in [23]). We would also like to point out that in the XRD spectra of the nanocomposite thin films no peak of Al2O3 is observed. The reasons for this absence may be two. The first is the amorphous nature of the sample. To examine this possibility, the powder form of the alumina gel was annealed at various temperatures and due to this a vast phase transformation is observed as a function of temperature. The observation is shown in Fig. 4. The dry gel at room temperature mainly consisted of boehmite (AlO(OH)) and aluminum hydroxide (Al(OH)3). This is because, Al, with an oxidation number of III, can easily polymerize in a variety of different polycations and constitute various solid phases. In the polycondensation reaction, if the experimental conditions are such that the synthesis temperature is below 80 8C, the structure that is
formed is Al(OH)3. On the other hand if the synthesis temperature is above 80 8C, the structure is similar to that of boehmite, or AlO(OH) [27]. This phase is retained up to a drying temperature of 200 8C for 1 h. At even higher temperatures, constant dehydration or, the removal of chemical water from this gel, left the structure disarrayed and the transition to alumina is stalled up to 800 8C. The XRD spectra in the intermediate steps are more like those for amorphous samples. In the first stage these gel produces a transition alumina at a temperature of 800 8C, where the phase is identified as g-Al2O3. Complete conversion of this g-Al2O3 to a more stable form of a-Al2O3 was attained at 1100 8C. As intermediate phases, the u-Al2O3 and d-Al2O3 phases appear at 1000 8C. To further check the transition from amorphous phase
Fig. 3. The XRD spectra of the ZnO–Al2O3 nanocomposite thin films annealed at 300 8C and 700 8C.
Fig. 5. The XRD spectra of the alumina gel dried at 600 8C and 700 8C for different duration of time. The drying did not result in transition alumina.
Fig. 4. The XRD spectra of the alumina gel dried at different temperatures or 1 h in air medium. The image shows the transition alumina at different temperatures.
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
8665
to g-Al2O3 we further annealed the gel at 600 8C and at 700 8C for different time. The result is shown in Fig. 5. The treatment did not result in transition phase of alumina and the phase remained amorphous. Thus we conclude that the phase transition of the alumina dry gel is mainly dependent on the annealing temperature. So, the nanocomposite thin film annealed at 700 8C may not have resulted in the crystalline phase of alumina and no sign of characteristic alumina peak is observed. Another observation is that alumina thin film drawn on quartz substrate crystallizes only after rapid thermal processing (RTP) at high temperature [28]. This is because during regular annealing the slow diffusion of Si4+ ion in the alumina lattice site breaks the periodicity of the crystal and XRD spectra for this thin film is similar to that coming from amorphous samples.
doped sample (Fig. 6d–f) and y = 0.15 MgO doped sample (Fig. 6g–i) annealed at 300 8C in air. The grain sizes of all the samples are in the nanometric region, and the grains are smooth and regular looking as seen in Fig. 6g–i. The dark and light patches on the surface, which is more in Fig. 6a–c, decrease in the subsequent Fig. 6d–i indicating an increase in the smoothness of the top surfaces. The smoothness of the top surface of the thin film is measured in terms of the roughness, which is a basic parameter indicating the deviation of a surface with respect to a perfect plane. The root mean square roughness R (rms) is defined as
3.3. The AFM study
where Zi is the Z value of each point, Zav the average of the Z values and N is the number of points. Sequentially the rms values of the surfaces are calculated as (i) 2.11, 1.97 and 1.86 nm (undoped SA), (ii) 4.05, 3.16 and 2.01 nm (y = 0.05 MgO doped SA) and (iii) 1.70, 1.45 and 0.99 nm (0.15 MgO doped SA). As the annealing temperature of the thin films was same, thus the smoothness of the films increases with the doping which may be the result of the accumulation of the
The surface topography of the undoped nanocomposite and the MgO doped thin film samples are examined by the atomic force microscopy (AFM). The horizontal sequence of the images shows the surfaces of an area equal to 1.0 mm 1.0 mm, 500 nm 500 nm and 200 nm 200 nm for the undoped samples SA (Fig. 6a–c), and also y = 0.05 MgO
Rrms
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N uX ðZ i Z av Þ2 ¼t N i¼1
(1)
Fig. 6. The surface morphology of the MgO doped ZnO–Al2O3 nanocomposite thin films. The figures show that the surface gets smoother with increasing MgO content.
8666
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
Fig. 7. The representative EDAX spectra show the presence of Mg2+ in the ZnO–Al2O3 nanocomposite thin film.
MgO particles on the surface of ZnO nanograins. A representative EDAX spectrum for the MgO doped ZnO–Al2O3 nanocomposite thin film is shown in Fig. 7. 3.4. UV–vis optical absorption Fig. 8 shows the normalized absorbance spectra of sample SA, SB and SC annealed in the temperature range of 300, 500 and 700 8C. The thickness (t) of these single layered ZnO– Al2O3 nanocomposite thin films is determined as 119 nm. The spectra are normalized in order to compare the intensity of the absorbance peaks obtained for different samples at different synthesis condition. The spectra indicate (a) a long tail with varying absorbance at lower energies, (b) sharpening of the absorption peak with increasing annealing temperature, and (c) broadening of the absorption peak with higher concentration of ZnO in the nanocomposite. In an earlier report by Das et al. [23] the absorbance spectra of ZnO–Al2O3 was discussed in detail. It is evident from the figure that the temperature dependence on the absorption intensity is more prominent for sample SB and SC, whereas for sample SA, the intensities of absorbance for all the three annealing temperature are almost same. In Fig. 9a comparison of the intensity of the absorption peaks is shown for the nanocomposite thin films annealed at 300 8C. It is observed from the figure that the absorbance intensity is largest for sample SA and least for sample SC, whereas for sample SB the intensity is intermediate of the other two. In nanometric dimension the surface to volume ratio is much higher compared to that for larger particles. This large surface area introduces various surface related disorders and defects in the materials. In the host–guest system, for a well-confined nanoparticle these defects are comparatively lesser. The defects and disorders creep into the nanocomposite more rapidly when the confinement effect is weak, this happens when the concentration of the matrix is comparatively lesser. Thus from Fig. 9, we
Fig. 8. The normalized optical absorbance spectra of the ZnO–Al2O3 nanocomposite thin films annealed at 300 8C, 500 8C and 700 8C. (a) sample SA, (b) sample SB and (c) sample SC.
Fig. 9. A comparative study of the normalized absorbance spectra for the composite thin films annealed at 300 8C.
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
8667
Fig. 10. The normalized absorbance spectra of the MgO doped sample SA. The blue shift and the red shift of the absorbance edge are shown in the figure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
may infer that the confinement for sample SA is superior to that for sample SB, and it is less effective for sample SC. We may also conclude that the randomness and disorder are less for sample SA, compared to those for sample SB or, SC [29]. In Fig. 10 the absorption spectra of the MgO doped sample SA are shown. The molar concentrations of MgO were kept at y = 0.05, 0.075, 0.10, 0.125, 0.15 and 0.20 of the total molar concentration of ZnO present in the ZnO–Al2O3 nanocomposite. The doped thin films were annealed at 300 8C. The absorbance spectra show that the high intensity peaks, which are characteristic for undoped sample SA, are reduced in the doped samples. A relative ratios of the intensities of the absorption peak for the MgO doped films and that for sample SA stand at 0.53, 0.45, 0.33, 0.35, 0.47 and 0.49. So, the absorption peaks first decrease and then increase with the MgO concentration in the nanocomposites. It is also observed that the absorption edge of the thin films shifted with the MgO concentration in the nanocomposite. To calculate these shifts, in Fig. 11a we have plotted the (ahn)2 versus hn plots of the MgO doped ZnO–Al2O3 nanocomposite thin films. The band gap was calculated by extrapolating the straight part of the plot to (ahn)2 = 0. The intersection of the straight lines on the energy axis gives the value of the respective band gaps of the composites. The observed absorption edges for y = 0.05, 0.075 and 0.1 were calculated as 5.44, 5.63 and 5.77 eV, respectively. For other concentrations, that is for y = 0.125, 0.15 and 0.20, the band gaps are derived as 5.67, 5.61 and 5.49 eV, respectively. Thus the apparent shifts of the band gap from the bulk band gap value of ZnO (3.37 eV) are put as 2.07, 2.26, 2.4, 2.3, 2.24 and 2.12 eV. Thus we observed that the shift in the obtained band gap with respect to its bulk value first increase to reach a maximum and then decrease with further increase in the MgO concentrations in the composite. This shifting of the ZnO absorption edge may be due to two reasons, (a) an increase in the free carrier concentration due to doping which causes the Burstein–Moss shift [10], or (b) due to the decrease in the nanoparticle sizes. Since we did not observe
Fig. 11. (a) The (ahn)2 vs. hn plots for ZnO–Al2O3 nanocomposite thin films with different molar content of MgO. The extrapolation of straight lines on the energy aixs determines the ban gap of the system. (b) The variation DEabs with n2/3 is shown; the accuracy of the observed band gap and corresponding ionic concentration are determined. The linear fit shows the Burstein–Moss shift of the absorption edges.
significant size reduction after the composite was doped with MgO, we observe that the widening of the band gap for those relatively lower concentrations of MgO in the nanocomposite can be explained by the Burstein–Moss shift. According to the parabolic band theory [30], the absorption edge shift DEabs is given by DEabs ¼
h2 2=3 ½3p2 n Ebgr 2m
(2)
where Ebgr is due to the band-gap-reduction effect. In Fig. 11b we have plotted the variation of DEabs with n2/3 by neglecting the last term and using 1/m = 1/me + 1/mh, where me and mh are the electron and hole effective mass, respectively. The straight line that appears confirms the assumed Burstein–Moss shift with molar concentrations of MgO. The accuracy of the experimentally obtained band gaps calculated in the above method and that of corresponding ionic concentrations are also illustrated in Fig. 11b. The error in DEabs is determined by repeated experimental observations and by considering the overall deviation from the mean of the obtained values. To calculate the error in the ionic concentration (n) of MgO, we have used Eq. (2), along with the error in DEabs. The density of electrons is decreased once the Mg2+ ions are substituted into the Zn2+ ion site in the composites. As a result
8668
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
UV absorption peaks were very intense for undoped nanocomposite though the broadening effect dominating for composite with equal molar concentrations of ZnO and Al2O3. The observed shifting of the absorption edge with MgO doping was ascribed to the Burstein–Moss shift. The effect of the solubility limit of MgO in the composite was put as 10% of the molar concentration of ZnO for smaller grains of ZnO whereas for larger grains it is below 5% of the molar concentration of ZnO. References
Fig. 12. The transmittance spectra of ZnO–Al2O3 nanocomposite thin films of sample SC annealed at 500 8C and 700 8C. The spectra for MgO doped (y = 0.05) films are shown in dotted lines.
of this, an increase in the Fermi-level in the conduction band of degenerate semiconductors leads to widening of the energy bands. On the contrary, the observed decrease in the band gap with the molar concentrations of MgO as y = 0.125, 0.15 and 0.20 may be due to excess Mg atoms in the nanocomposites those are segregated onto the grain boundary. These segregated Mg atoms do not act as dopant and consequently do not assist in the process of band gap widening. It was indicated that the high concentration of extrinsic elements, e.g., Al and Mg, introduces non-equilibrium defects into ZnO films and those defects are the reason for the crystalline degradation and thermal instability of the films [31]. This also causes the observed decrease in the optical absorption coefficient for the MgO doped ZnO–Al2O3 nanocomposites. The effect of MgO doping in sample SC is shown in Fig. 12. The solid lines indicated the transmittance spectra of the undoped thin films annealed at 300 and 700 8C, whereas the dotted lines are those for y = 0.05 MgO doped samples. The spectra reveal that the absorption edges for these thin films moved towards lower values of the band gap with the annealing temperatures for both doped and undoped films, and thus Burstein–Moss shift is not observed in this case. So, in sample SC having higher ZnO concentration, MgO does not act as dopant and is agglomerated on the surface of ZnO nanoparticles as is explained in the previous case. 4. Conclusion The polycrystalline ZnO–Al2O3 nanocomposite was synthesized by sol–gel technique. The sol was doped subsequently with MgO and the surface morphology of the thin films was seen to improve with the concentration of dopant. The observed
[1] C. Bundesmann, M. Schubert, D. Spemann, T. Butz, M. Lorenz, E.M. Kaidashev, M. Grundmann, N. Ashkenov, H. Neumann, G. Wagner, Appl. Phys. Lett. 81 (2002) 2376. [2] K.H. Kim, K.E. Park, D.Y. Ma, J. Appl. Phys. 81 (1997) 7764. [3] A. Vanheusden, C.H. Seager, W.L. Wareen, D.A. Tallant, J. Caruso, M.J. hampden-smith, T.T. Kodas, J. Lumin. 75 (1997) 11. [4] D. Babic´, R. Tsu, R.F. Greene, Phys. Rev. B 45 (1992) 14150. [5] G. Allan, C. Delerue, M. Lannoo, E. Martin, Phys. Rev. B 52 (1995) 11982. [6] L.E. Brus, J. Chem. Phys. 79 (1983) 5566. [7] S.H. Jeong, S. Kho, D. Jung, S.B. Lee, J.H. Boo, Surf. Coat. Technol. 174– 175 (2003) 187. [8] K.C. Park, D.Y. Ma, K.H. Kim, Thin Solid Films 305 (1997) 201. [9] J.G. Ma, Y.C. Liu, C.L. Shao, J.Y. Zhang, Y.M. Lu, D.Z. Shen, X.W. Fan, Phys. Rev. B 71 (2005) 125430. [10] E. Burstaein, Phys. Rev. B 93 (1954) 632. [11] D.M. Roessler, W.C. Walker, Phys. Rev. Lett. 17 (1966) 310. [12] T. Makino, N.T. Tuan, H.D. Sun, C.H. Chia, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura, T. Suemoto, H. Akiyama, M. Baba, S. Saito, T. Tomita, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1979. [13] T. Makino, C.H. Chia, N.T. Tuan, H.D. Sun, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura, H. Koinuma, Appl. Phys. Lett. 77 (2000) 975. [14] V.A. Coleman1, M. Buda1, H.H. Tan1, C. Jagadish1, M.R. Phillips, K. Koike, S. Sasa, M. Inoue, M. Yano, Semicond. Sci. Technol. 21 (2006) L25. [15] C.Y. Liu, B.P. Zhang, N.T. Binh, K. Wakatsuki, Y. Segawa, Thin Solid Films 478 (2005) 30. [16] E.R. Segnit, A.E. Holland, J. Am. Ceram. Soc. 48 (1965) 409. [17] A. Ohotomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466. [18] W.I. Park, G.C. Yi, H.M. Jang, Appl. Phys. Lett. 79 (2001) 2022. [19] N.B. Chen, H.Z. Wu, T.N. Xu, J. Appl. Phys. 97 (2005) 023515. [20] P. Bhattacharya, R.R. Das, R.S. Katiyar, Appl. Phys. Lett. 83 (2003) 2010. [21] R.D. Shannon, Acta Crystallogr. Sect. A: Cryst. Phys. Diffr. Theor. Gen. Crystallogr. 32 (1976) 751. [22] F. Hayashi, T. Uematsu, H. Hashimoto, J. Mater. Sci. 16 (1981) 3227. [23] S. Das, S. Chaudhuri, J. Nanosci. Nanotechnol. 7 (2007) 1. [24] B.E. Yoldas, Ceram. Soc. Bull. 54 (1975) 289. [25] B.E. Yoldas, Ceram. Soc. Bull. 54 (1975) 286. [26] B.E. Yoldas, J. Mater. Sci. 10 (1975) 1856. [27] A.C. Pierre, Introduction to Sol–Gel Processing, Kluwer Academic Publishers, Massachusetts, 1998. [28] N. Bahlawane, T. Watanabe, J. Am. Ceram. Soc. 83 (2000) 2324. [29] G.D. Cody, in: J.J. Pankove (Ed.), Semiconductors and Semimetals, 21B, Academic, 1984 (Chapter 2). [30] E.J. Johnson, in: R.K. Willardson, A.C. Beer (Eds.), Semiconductors and Semimetals, vol. 3, Academic, New York, 1967, p. 154. [31] H. Ryoken, I. Sakaguchi, T. Ohgaki, N. Ohashi, Y. Adachi, H. Haneda, MRS Proceedings 829 (2004), B2.23.
Mg2+ substitutions in ZnO–Al2O3 thin films and its effect on the optical absorption spectra of the nanocomposite Soumen Das, Subhadra Chaudhuri ä,* Nanofilm Lab, Department of Materials Science, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India Received 27 February 2007; received in revised form 26 March 2007; accepted 18 April 2007 Available online 3 May 2007
Abstract ZnO–Al2O3 nanocomposite thin films were prepared by sol–gel technique. The room temperature synthesis was mainly based on the successful peptization of boehmite (AlO(OH)) and Al(OH)3 compounds, so as to use it as matrix to confine ZnO nanoparticles. The relative molar concentrations of xZnO to (1 x) Al2O3 were varied as x = 0.1, 0.2 and 0.5. The optical absorption spectra of the thin films showed intense UV absorption peaks with long tails of variable absorption in the visible region of the spectra. The ZnO–Al2O3 nanocomposites thin films were doped with MgO by varying its molar concentrations as y = 0.05, 0.75, 0.1, 0.125, 0.15 and 0.2 with respect to the ZnO present in the composite. The MgO doped thin films showed suppression of the intense absorption peaks that was previously attained for undoped samples. The disappearance of the absorption peaks was analyzed in terms of the crystalline features and lattice defects in the nanocomposite system. The bulk absorption edge, which is reportedly found at 3.37 eV, was shifted to 5.44 eV (for y = 0.05), 5.63 eV (for y = 0.075) and maximum to 5.77 eV (for y = 0.1). In contrast, beyond the concentration, y = 0.1 the absorption edges were moved to 5.67 eV (for y = 0.125), 5.61 eV (for y = 0.15) and to 5.49 eV (for y = 0.2). This trend was explained in terms of the Burstein–Moss shift of the absorption edges. # 2007 Elsevier B.V. All rights reserved. PACS : 81.20.Fw; 78.66.Hf; 61.72.Vv; 78.66.w Keywords: Sol–gel; Nanocomposite; Thin films; Optical absorption
1. Introduction The wide band gap of ZnO (3.37 eV for the bulk ZnO [1]) can be altered if the particle sizes of the system lie in the nanometric region. This large band gap of ZnO coupled with its large excitonic binding energy (0.060 eV) have made it a potential candidate in flat panel devices, light emitting diodes, LASERs in the ultraviolet region [2,3] and as transparent conducting oxide. Above this, the band gap of ZnO also depends on the size of the crystallites. This phenomenon is called quantum confinement (QC) effect and is analyzed and explained by several authors [4–6]. It is observed that the higher surface to volume ratio of nanocrystals introduces various surface related defects and disorders in the system. So, they are
* Corresponding author. Tel.: +91 33 2473 4971; fax: +91 33 2473 2805. E-mail addresses: [email protected] (S. Das), [email protected] (S. Chaudhuri). ä Deceased. 0169-4332/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsusc.2007.04.072
embedded in insulating matrix to minimize these defects. High band gap materials like MgO, SiO2 or Al2O3 are also used to restrict the growth rate of the nanocrystals and in the process enhance the optical band gap of the system. The quantum confinement effect, and the introduction of Al3+ or Mg2+ ions into the lattice of ZnO are used fruitfully to tune the band gap of ZnO nanocrystals [7–9]. This latter mode of tuning is called the Burstein–Moss Shift, and it is dependent on the diffusibility of the dopant into the crystal lattice [10]. ZnO with MgO (Eg = 7.3–7.7 eV [11]) can produce MgxZn1xO alloy having higher band gap and improved optical and photoluminescence properties [12,13]. The suitability of MgxZn1xO as a multilayer quantum well structure has also been studied in various earlier reports [14,15]. According to the phase diagram of the ZnO–MgO binary system the solubility of MgO in the ZnO lattice is less than 4% in the bulk form [16], but for thin film the solid solubility is as high as 33% [17]. The degree of solubility of Mg in ZnO is found to depend largely on the deposition techniques and on the processing conditions [18–20]. The ionic radius of Mg2+
8662
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
˚ ) is very close to that of Zn2+ (0.72 A ˚ ) [21], so at relatively (0.57 A higher annealing temperatures the Mg ions diffuse into the ZnO lattice, and replace each other. For higher concentration of MgO it has been shown that the MgxZn1xO alloys form a metastable alloy [17]. These metastable phase and the degree of metastability are the influencing factors for the potential applications of MgxZn1xO based devices. On the other hand, the accumulation of MgO on the grain boundaries of ZnO has an influence on the surface states of the nanoparticles [22]. In this article, we have undertaken a comparative study of the UV–vis optical absorption spectra of the single layered undoped and MgO doped ZnO–Al2O3 nanocomposite thin films. We have also concentrated on the shift of the optical absorption edge from its bulk band gap value and the broadening of the optical spectra at the absorption edge. We found out that these phenomena largely depend on the role of the MgO as dopant, its accumulation on the surface of ZnO and its solubility in the nanocomposite.
along with the selected area electron diffraction (SAED) pattern. The composition analysis and the presence of the Mg2+ in the nanocomposite thin films were confirmed by the energy dispersive X-ray analysis (EDAX) attachment to a JEOL JSM J2010 Field emission scanning electron microscope (FESEM). The optical absorption spectra of the products were recorded by an Schimadzu 2401spectrophotometer. 3. Results and discussion 3.1. The TEM study Fig. 1 shows the TEM images of the alumina sol taken on a Cu grid and annealed at 300 8C. The image shows a network
2. Experimental details ZnO–Al2O3 nanocomposites with relative molar concentrations of ZnO to Al2O3 as 10:90, 20:80 and 50:50 were prepared by sol–gel technique. For Al2O3 part, the aqueous solution of aluminum nitrate (Al(NO3)36H2O) was refluxed for 1 h and then treated with NH3 solution (25%). The white precipitate (a mixture of aluminum hydroxide and boehmite) was dissolved in (12:1) volume ratio of ethanol and water. Three cubic centimeter of acetic acid was added to it drop wise under constant stirring. The pH of the sol was measured as 2.75. At the end of 3 h of stirring a completely transparent and viscous sol was obtained. The detail of the preparation of the composite sol and the subsequent deposition of the thin films were reported by the authors elsewhere [23]. At this point we would like to mention that the sol–gel preparation of the alumina was mainly based on the Yoldas method [24], as described in numbers of well-cited papers [25,26]. The main difference in our method from the prescribed one was that we accomplished the entire preparation at room temperature, without resorting to heat treatment during peptization. In fact, heat treatment at 80 8C was a crucial parameter in Yoldas method. In the latter section of this article, the nanocomposites will be denoted by sample SA (10:90), SB (20:80) and SC (50:50). The thin films were annealed at 300, 500 and 700 8C in air. Later, sample SA was doped with MgO at molar concentrations y = 0.05, 0.075, 0.10, 0.125, 0.15 and 0.20 with respect to the molar concentration of ZnO present in the composite. For doping, the required amount of Mg-acetate was mixed directly with the ZnO–Al2O3 sol under stirring. The thin films were dipcoated (5 cm/min) and annealed in air at 300 8C for 25 min. In another case, sample SC was also doped with y = 0.05 MgO and was annealed at 300 and 700 8C. The surface morphology of the thin films was determined by atomic force microscopy (AFM) (Nanoscope IV scanning probe microscope controller). The morphological attributes of the nanocomposite samples were determined by transmission electron microscope (TEM) (JEOL 2010 electron microscope)
Fig. 1. The TEM images of the (a) alumina sol network, (b) the cylindrical fibrillar structure of alumina hydroxide annealed at 300 8C.
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
like build up of the aluminum based hydroxide in the sol. Earlier, Yoldas [24] showed that depending on certain parameters, like the type of alkoxides used, the ratio of alkoxide to water, the rate of hydrolysis, pH standard and the mode of drying there are may be several structural variations of the final alumina gel. The microstructural images in Fig. 1a shows interconnected branch like extension of the hydroxide consisting of closely knit grains forming thick impenetrable structures. These dangling branches contribute to the density, the pore size and the specific area of a dry gel. Colloidal gels obtained from aluminum precursors comprise of two types of oxo-bridges, Al–O–Al and Al–O(OH)–Al(OH). The dry gel in this case forms a random network of neighbouring particles where the proportion of linear linkage between particles varies. The pore presents in the ‘‘cylindrical fibrillar structure’’ also
8663
produces rooms for the guest nanoparticles to get confined and to grow during sintering, as we will observe in Fig. 2. In Fig. 2a and b the ZnO–Al2O3 nanocomposite (sample SA) showed similar branch like structures. That the ZnO nanoparticles are embedded and are randomly dispersed in the Al2O3 matrix can be observed from Fig. 2c. The black dots are agglomerated grains of ZnO nanoparticles and can be well resolved at the inset figure, which shows the high-resolution image of a single ZnO grain consisting of a number of agglomerated nanoparticles. The average size of the ZnO nanoparticles was determined as around 9.0 nm. In Fig. 2d the SAED image shows spots in the diffused diffraction rings indicating the polycrystalline nature of the nanoparticles. In our previous report [23] we have seen that the average particle size was 10.0 nm for sample SB and around 18.0 nm for sample SC
Fig. 2. The TEM images of the ZnO–Al2O3 nanocomposite (a,b) the network like appearance, (c) the ZnO nanoparticles embedded on the amorphous alumina matrix annealed at 300 8C, inset figure shows the ZnO grains consisting of agglomerated ZnO nanoparticles. (d) The SAED images of the composite showing diffraction rings from ZnO nanocrystals.
8664
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
processed under similar conditions. Thus the nanoparticles grow in size when the proportion of the alumina is lesser in the composite. This is because; the presence of larger quantity of ZnO inside the pores of the alumina matrix facilitates the growth to larger grains when treated at different annealing temperatures. 3.2. X-ray diffraction study The crystalline phase of the nanocomposite thin films were studied with the XRD measurements. The XRD spectra of sample SA, SB (annealed at 300–700 8C, not shown in this report) did not show significant difference in appearance from each other, and characteristic peaks either of alumina, or ZnO were absent. As in the subsequent TEM images revealed the polycrystalline nature of the samples, we inferred that the absence of the characteristic peaks was due to the low resolution of the XRD measuring instrument. The characteristic peaks of ZnO are observed for sample SC annealed at 300 and 700 8C as is shown in Fig. 3. The peaks are identified as (1 0 0), (1 0 1) and (2 0 0) [JCPDS-File No. 35-0664]. It is seen that the peak intensity increases at higher annealing temperature and are more prominent in the lower spectra (for films annealed at 700 8C). The average radius of the confined nanoparticle is measured from the highest peak in the spectra and is calculated as 8.2 nm, which is close to the size we obtained for sample SC from TEM (reported in [23]). We would also like to point out that in the XRD spectra of the nanocomposite thin films no peak of Al2O3 is observed. The reasons for this absence may be two. The first is the amorphous nature of the sample. To examine this possibility, the powder form of the alumina gel was annealed at various temperatures and due to this a vast phase transformation is observed as a function of temperature. The observation is shown in Fig. 4. The dry gel at room temperature mainly consisted of boehmite (AlO(OH)) and aluminum hydroxide (Al(OH)3). This is because, Al, with an oxidation number of III, can easily polymerize in a variety of different polycations and constitute various solid phases. In the polycondensation reaction, if the experimental conditions are such that the synthesis temperature is below 80 8C, the structure that is
formed is Al(OH)3. On the other hand if the synthesis temperature is above 80 8C, the structure is similar to that of boehmite, or AlO(OH) [27]. This phase is retained up to a drying temperature of 200 8C for 1 h. At even higher temperatures, constant dehydration or, the removal of chemical water from this gel, left the structure disarrayed and the transition to alumina is stalled up to 800 8C. The XRD spectra in the intermediate steps are more like those for amorphous samples. In the first stage these gel produces a transition alumina at a temperature of 800 8C, where the phase is identified as g-Al2O3. Complete conversion of this g-Al2O3 to a more stable form of a-Al2O3 was attained at 1100 8C. As intermediate phases, the u-Al2O3 and d-Al2O3 phases appear at 1000 8C. To further check the transition from amorphous phase
Fig. 3. The XRD spectra of the ZnO–Al2O3 nanocomposite thin films annealed at 300 8C and 700 8C.
Fig. 5. The XRD spectra of the alumina gel dried at 600 8C and 700 8C for different duration of time. The drying did not result in transition alumina.
Fig. 4. The XRD spectra of the alumina gel dried at different temperatures or 1 h in air medium. The image shows the transition alumina at different temperatures.
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
8665
to g-Al2O3 we further annealed the gel at 600 8C and at 700 8C for different time. The result is shown in Fig. 5. The treatment did not result in transition phase of alumina and the phase remained amorphous. Thus we conclude that the phase transition of the alumina dry gel is mainly dependent on the annealing temperature. So, the nanocomposite thin film annealed at 700 8C may not have resulted in the crystalline phase of alumina and no sign of characteristic alumina peak is observed. Another observation is that alumina thin film drawn on quartz substrate crystallizes only after rapid thermal processing (RTP) at high temperature [28]. This is because during regular annealing the slow diffusion of Si4+ ion in the alumina lattice site breaks the periodicity of the crystal and XRD spectra for this thin film is similar to that coming from amorphous samples.
doped sample (Fig. 6d–f) and y = 0.15 MgO doped sample (Fig. 6g–i) annealed at 300 8C in air. The grain sizes of all the samples are in the nanometric region, and the grains are smooth and regular looking as seen in Fig. 6g–i. The dark and light patches on the surface, which is more in Fig. 6a–c, decrease in the subsequent Fig. 6d–i indicating an increase in the smoothness of the top surfaces. The smoothness of the top surface of the thin film is measured in terms of the roughness, which is a basic parameter indicating the deviation of a surface with respect to a perfect plane. The root mean square roughness R (rms) is defined as
3.3. The AFM study
where Zi is the Z value of each point, Zav the average of the Z values and N is the number of points. Sequentially the rms values of the surfaces are calculated as (i) 2.11, 1.97 and 1.86 nm (undoped SA), (ii) 4.05, 3.16 and 2.01 nm (y = 0.05 MgO doped SA) and (iii) 1.70, 1.45 and 0.99 nm (0.15 MgO doped SA). As the annealing temperature of the thin films was same, thus the smoothness of the films increases with the doping which may be the result of the accumulation of the
The surface topography of the undoped nanocomposite and the MgO doped thin film samples are examined by the atomic force microscopy (AFM). The horizontal sequence of the images shows the surfaces of an area equal to 1.0 mm 1.0 mm, 500 nm 500 nm and 200 nm 200 nm for the undoped samples SA (Fig. 6a–c), and also y = 0.05 MgO
Rrms
vffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi u N uX ðZ i Z av Þ2 ¼t N i¼1
(1)
Fig. 6. The surface morphology of the MgO doped ZnO–Al2O3 nanocomposite thin films. The figures show that the surface gets smoother with increasing MgO content.
8666
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
Fig. 7. The representative EDAX spectra show the presence of Mg2+ in the ZnO–Al2O3 nanocomposite thin film.
MgO particles on the surface of ZnO nanograins. A representative EDAX spectrum for the MgO doped ZnO–Al2O3 nanocomposite thin film is shown in Fig. 7. 3.4. UV–vis optical absorption Fig. 8 shows the normalized absorbance spectra of sample SA, SB and SC annealed in the temperature range of 300, 500 and 700 8C. The thickness (t) of these single layered ZnO– Al2O3 nanocomposite thin films is determined as 119 nm. The spectra are normalized in order to compare the intensity of the absorbance peaks obtained for different samples at different synthesis condition. The spectra indicate (a) a long tail with varying absorbance at lower energies, (b) sharpening of the absorption peak with increasing annealing temperature, and (c) broadening of the absorption peak with higher concentration of ZnO in the nanocomposite. In an earlier report by Das et al. [23] the absorbance spectra of ZnO–Al2O3 was discussed in detail. It is evident from the figure that the temperature dependence on the absorption intensity is more prominent for sample SB and SC, whereas for sample SA, the intensities of absorbance for all the three annealing temperature are almost same. In Fig. 9a comparison of the intensity of the absorption peaks is shown for the nanocomposite thin films annealed at 300 8C. It is observed from the figure that the absorbance intensity is largest for sample SA and least for sample SC, whereas for sample SB the intensity is intermediate of the other two. In nanometric dimension the surface to volume ratio is much higher compared to that for larger particles. This large surface area introduces various surface related disorders and defects in the materials. In the host–guest system, for a well-confined nanoparticle these defects are comparatively lesser. The defects and disorders creep into the nanocomposite more rapidly when the confinement effect is weak, this happens when the concentration of the matrix is comparatively lesser. Thus from Fig. 9, we
Fig. 8. The normalized optical absorbance spectra of the ZnO–Al2O3 nanocomposite thin films annealed at 300 8C, 500 8C and 700 8C. (a) sample SA, (b) sample SB and (c) sample SC.
Fig. 9. A comparative study of the normalized absorbance spectra for the composite thin films annealed at 300 8C.
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
8667
Fig. 10. The normalized absorbance spectra of the MgO doped sample SA. The blue shift and the red shift of the absorbance edge are shown in the figure. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
may infer that the confinement for sample SA is superior to that for sample SB, and it is less effective for sample SC. We may also conclude that the randomness and disorder are less for sample SA, compared to those for sample SB or, SC [29]. In Fig. 10 the absorption spectra of the MgO doped sample SA are shown. The molar concentrations of MgO were kept at y = 0.05, 0.075, 0.10, 0.125, 0.15 and 0.20 of the total molar concentration of ZnO present in the ZnO–Al2O3 nanocomposite. The doped thin films were annealed at 300 8C. The absorbance spectra show that the high intensity peaks, which are characteristic for undoped sample SA, are reduced in the doped samples. A relative ratios of the intensities of the absorption peak for the MgO doped films and that for sample SA stand at 0.53, 0.45, 0.33, 0.35, 0.47 and 0.49. So, the absorption peaks first decrease and then increase with the MgO concentration in the nanocomposites. It is also observed that the absorption edge of the thin films shifted with the MgO concentration in the nanocomposite. To calculate these shifts, in Fig. 11a we have plotted the (ahn)2 versus hn plots of the MgO doped ZnO–Al2O3 nanocomposite thin films. The band gap was calculated by extrapolating the straight part of the plot to (ahn)2 = 0. The intersection of the straight lines on the energy axis gives the value of the respective band gaps of the composites. The observed absorption edges for y = 0.05, 0.075 and 0.1 were calculated as 5.44, 5.63 and 5.77 eV, respectively. For other concentrations, that is for y = 0.125, 0.15 and 0.20, the band gaps are derived as 5.67, 5.61 and 5.49 eV, respectively. Thus the apparent shifts of the band gap from the bulk band gap value of ZnO (3.37 eV) are put as 2.07, 2.26, 2.4, 2.3, 2.24 and 2.12 eV. Thus we observed that the shift in the obtained band gap with respect to its bulk value first increase to reach a maximum and then decrease with further increase in the MgO concentrations in the composite. This shifting of the ZnO absorption edge may be due to two reasons, (a) an increase in the free carrier concentration due to doping which causes the Burstein–Moss shift [10], or (b) due to the decrease in the nanoparticle sizes. Since we did not observe
Fig. 11. (a) The (ahn)2 vs. hn plots for ZnO–Al2O3 nanocomposite thin films with different molar content of MgO. The extrapolation of straight lines on the energy aixs determines the ban gap of the system. (b) The variation DEabs with n2/3 is shown; the accuracy of the observed band gap and corresponding ionic concentration are determined. The linear fit shows the Burstein–Moss shift of the absorption edges.
significant size reduction after the composite was doped with MgO, we observe that the widening of the band gap for those relatively lower concentrations of MgO in the nanocomposite can be explained by the Burstein–Moss shift. According to the parabolic band theory [30], the absorption edge shift DEabs is given by DEabs ¼
h2 2=3 ½3p2 n Ebgr 2m
(2)
where Ebgr is due to the band-gap-reduction effect. In Fig. 11b we have plotted the variation of DEabs with n2/3 by neglecting the last term and using 1/m = 1/me + 1/mh, where me and mh are the electron and hole effective mass, respectively. The straight line that appears confirms the assumed Burstein–Moss shift with molar concentrations of MgO. The accuracy of the experimentally obtained band gaps calculated in the above method and that of corresponding ionic concentrations are also illustrated in Fig. 11b. The error in DEabs is determined by repeated experimental observations and by considering the overall deviation from the mean of the obtained values. To calculate the error in the ionic concentration (n) of MgO, we have used Eq. (2), along with the error in DEabs. The density of electrons is decreased once the Mg2+ ions are substituted into the Zn2+ ion site in the composites. As a result
8668
S. Das, S. Chaudhuri / Applied Surface Science 253 (2007) 8661–8668
UV absorption peaks were very intense for undoped nanocomposite though the broadening effect dominating for composite with equal molar concentrations of ZnO and Al2O3. The observed shifting of the absorption edge with MgO doping was ascribed to the Burstein–Moss shift. The effect of the solubility limit of MgO in the composite was put as 10% of the molar concentration of ZnO for smaller grains of ZnO whereas for larger grains it is below 5% of the molar concentration of ZnO. References
Fig. 12. The transmittance spectra of ZnO–Al2O3 nanocomposite thin films of sample SC annealed at 500 8C and 700 8C. The spectra for MgO doped (y = 0.05) films are shown in dotted lines.
of this, an increase in the Fermi-level in the conduction band of degenerate semiconductors leads to widening of the energy bands. On the contrary, the observed decrease in the band gap with the molar concentrations of MgO as y = 0.125, 0.15 and 0.20 may be due to excess Mg atoms in the nanocomposites those are segregated onto the grain boundary. These segregated Mg atoms do not act as dopant and consequently do not assist in the process of band gap widening. It was indicated that the high concentration of extrinsic elements, e.g., Al and Mg, introduces non-equilibrium defects into ZnO films and those defects are the reason for the crystalline degradation and thermal instability of the films [31]. This also causes the observed decrease in the optical absorption coefficient for the MgO doped ZnO–Al2O3 nanocomposites. The effect of MgO doping in sample SC is shown in Fig. 12. The solid lines indicated the transmittance spectra of the undoped thin films annealed at 300 and 700 8C, whereas the dotted lines are those for y = 0.05 MgO doped samples. The spectra reveal that the absorption edges for these thin films moved towards lower values of the band gap with the annealing temperatures for both doped and undoped films, and thus Burstein–Moss shift is not observed in this case. So, in sample SC having higher ZnO concentration, MgO does not act as dopant and is agglomerated on the surface of ZnO nanoparticles as is explained in the previous case. 4. Conclusion The polycrystalline ZnO–Al2O3 nanocomposite was synthesized by sol–gel technique. The sol was doped subsequently with MgO and the surface morphology of the thin films was seen to improve with the concentration of dopant. The observed
[1] C. Bundesmann, M. Schubert, D. Spemann, T. Butz, M. Lorenz, E.M. Kaidashev, M. Grundmann, N. Ashkenov, H. Neumann, G. Wagner, Appl. Phys. Lett. 81 (2002) 2376. [2] K.H. Kim, K.E. Park, D.Y. Ma, J. Appl. Phys. 81 (1997) 7764. [3] A. Vanheusden, C.H. Seager, W.L. Wareen, D.A. Tallant, J. Caruso, M.J. hampden-smith, T.T. Kodas, J. Lumin. 75 (1997) 11. [4] D. Babic´, R. Tsu, R.F. Greene, Phys. Rev. B 45 (1992) 14150. [5] G. Allan, C. Delerue, M. Lannoo, E. Martin, Phys. Rev. B 52 (1995) 11982. [6] L.E. Brus, J. Chem. Phys. 79 (1983) 5566. [7] S.H. Jeong, S. Kho, D. Jung, S.B. Lee, J.H. Boo, Surf. Coat. Technol. 174– 175 (2003) 187. [8] K.C. Park, D.Y. Ma, K.H. Kim, Thin Solid Films 305 (1997) 201. [9] J.G. Ma, Y.C. Liu, C.L. Shao, J.Y. Zhang, Y.M. Lu, D.Z. Shen, X.W. Fan, Phys. Rev. B 71 (2005) 125430. [10] E. Burstaein, Phys. Rev. B 93 (1954) 632. [11] D.M. Roessler, W.C. Walker, Phys. Rev. Lett. 17 (1966) 310. [12] T. Makino, N.T. Tuan, H.D. Sun, C.H. Chia, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura, T. Suemoto, H. Akiyama, M. Baba, S. Saito, T. Tomita, H. Koinuma, Appl. Phys. Lett. 78 (2001) 1979. [13] T. Makino, C.H. Chia, N.T. Tuan, H.D. Sun, Y. Segawa, M. Kawasaki, A. Ohtomo, K. Tamura, H. Koinuma, Appl. Phys. Lett. 77 (2000) 975. [14] V.A. Coleman1, M. Buda1, H.H. Tan1, C. Jagadish1, M.R. Phillips, K. Koike, S. Sasa, M. Inoue, M. Yano, Semicond. Sci. Technol. 21 (2006) L25. [15] C.Y. Liu, B.P. Zhang, N.T. Binh, K. Wakatsuki, Y. Segawa, Thin Solid Films 478 (2005) 30. [16] E.R. Segnit, A.E. Holland, J. Am. Ceram. Soc. 48 (1965) 409. [17] A. Ohotomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466. [18] W.I. Park, G.C. Yi, H.M. Jang, Appl. Phys. Lett. 79 (2001) 2022. [19] N.B. Chen, H.Z. Wu, T.N. Xu, J. Appl. Phys. 97 (2005) 023515. [20] P. Bhattacharya, R.R. Das, R.S. Katiyar, Appl. Phys. Lett. 83 (2003) 2010. [21] R.D. Shannon, Acta Crystallogr. Sect. A: Cryst. Phys. Diffr. Theor. Gen. Crystallogr. 32 (1976) 751. [22] F. Hayashi, T. Uematsu, H. Hashimoto, J. Mater. Sci. 16 (1981) 3227. [23] S. Das, S. Chaudhuri, J. Nanosci. Nanotechnol. 7 (2007) 1. [24] B.E. Yoldas, Ceram. Soc. Bull. 54 (1975) 289. [25] B.E. Yoldas, Ceram. Soc. Bull. 54 (1975) 286. [26] B.E. Yoldas, J. Mater. Sci. 10 (1975) 1856. [27] A.C. Pierre, Introduction to Sol–Gel Processing, Kluwer Academic Publishers, Massachusetts, 1998. [28] N. Bahlawane, T. Watanabe, J. Am. Ceram. Soc. 83 (2000) 2324. [29] G.D. Cody, in: J.J. Pankove (Ed.), Semiconductors and Semimetals, 21B, Academic, 1984 (Chapter 2). [30] E.J. Johnson, in: R.K. Willardson, A.C. Beer (Eds.), Semiconductors and Semimetals, vol. 3, Academic, New York, 1967, p. 154. [31] H. Ryoken, I. Sakaguchi, T. Ohgaki, N. Ohashi, Y. Adachi, H. Haneda, MRS Proceedings 829 (2004), B2.23.