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Structural and optical properties of Zn1-xMgxO thin films deposited by ultrasonic spray pyrolysis
Thin Solid Films 492 (2005) 248 – 252 www.elsevier.com/locate/tsf
Structural and optical properties of Zn1-x Mgx O thin films deposited by ultrasonic spray pyrolysis Xia Zhang, Xiao Min Li *, Tong Lai Chen, Ji Ming Bian, Can Yun Zhang State Key Laboratory of Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 21 October 2004; received in revised form 25 May 2005; accepted 30 June 2005 Available online 1 August 2005
Abstract The structural and optical properties of Zn1x Mgx O films deposited by ultrasonic spray pyrolysis were studied. All the Zn1x Mgx O thin films maintained the ZnO wurtzite structure and had no impurity phase, even for the Mg content up to x = 0.27. The optical properties were characterized by transmittance, absorption spectroscopy and photoluminescence measurements. For all the films, the average transmission in the visible wavelength region (k = 400 – 800 nm) was over 85%, and the absorption edge shifted to a shorter wavelength as the Mg content increased. The optical energy band gap of Zn1x Mgx O thin films, measured from transmittance spectra, could be controlled between 3.29 and 3.58 eV by adjusting Mg contents. The photoluminescence emission peaks of Zn0.94Mg0.06O and Zn0.73Mg0.27O thin films were located at 369 and 349 nm, showing an evident blue-shift with Mg content increase. The room temperature absorption and photoluminescence properties of the films were also discussed. D 2005 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 61.14.Hg; 81.15.Fg Keywords: Zinc magnesium oxide; Spray pyrolysis; Optical properties; X-ray diffraction
1. Introduction ZnO is a II – IV wide band gap semiconductor used in optical and electronic device applications such as solar cells, transparent conducting electrodes for displays, chemical sensors, varistors, modulators, light emitters diodes, laser diodes and ultraviolet (UV) lasers [1 –3]. In the context of optoelectronics, its direct band gap (3.3 eV) and large exciton binding energy (60 meV), leading to a lower threshold, are favorable for efficient operation of optical devices. Short wavelength devices based on ZnO have become even more interesting, greatly inspired by the demonstrations of the ultraviolet laser at both low and room temperatures [4,5]. For fabricating a double heterostucture laser diode using a ZnO active layer, two key techniques are p-type doping and band gap engineering in semiconductors. Very recent reports of p-type doping in our * Corresponding author. Tel.: +86 21 52412441; fax: +86 21 52413122. E-mail address: [email protected] (X.M. Li). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.06.088
groups [6,7], aided by theoretical work, are indicative of the bright future of light-emitting diodes, laser diodes even working at different wavebands such as deep ultraviolet and visible. The light-emitting diodes working at short wavelength can considerably maximize the storage density [8], which is an efficient way to handle the enormous information and particularly sought for mass data visualization such as compact disk read only memory and digital video disk. On the other hand, band gap devices based on ZnO/ZnMgO superlattices or quantum wells can confine both excitons and photons in the low dimensions, making the stimulated exciton-related emission process more efficient [9]. Consequently, modulation of the band gap while keeping the crystal structure similar to that of ZnO is very essential in terms of the aftermentioned two points. This promotes the extensive search of doping other ions into the ZnO lattice. Recently, Zn1x Mgx O has found great interest, because of its wider band gap compared to pure ZnO and possibility of band gap modulation through adjusting Mg contents. The former provides an opportunity
X. Zhang et al. / Thin Solid Films 492 (2005) 248 – 252
for fabrication of light-emitting diodes at short wavelength. The latter implies that Zn1x Mgx O can act as a suitable layer in ZnO/ZnMgO superlattices or quantum wells. In this letter, we concentrate on the Zn1x Mgx O films for purpose of exploring its potential applications in ultraviolet optoelectronics. ZnO has a wurtzite hexagonal structure, while MgO has a NaCl-type cubic structure. In spite of a large structural dissimilarity between them, the similarity in ionic ˚ ) and Zn2+ (0.60 A ˚ ) allows radius between Mg2+ (0.57 A some replacement in either structure. Researchers have reported that wide gap (5.0 eV), cubic-phase, metastable Zn1x Mgx O thin films can be grown with Mg incorporation greater than 50 at.% [10]. Since Zn1x Mgx O containing MgO over 4 at.% is in a thermodynamically metastable state [11], this result indicates that the solubility limit of Mg in ZnO depends on growth mechanisms as well as growth conditions. However, current research on the growth of Zn1x Mgx O is restricted to pulsed-laser-deposition [10], molecular-beam epitaxy [12] and rf-magnetron sputtering [13]. Despite the potential for the realization of epitaxial growth of Zn1x Mgx O films, these methods might have disadvantages in mass production, due to their high cost and low throughout. Comparing to those methods, the ultrasonic spray pyrolysis deposition technique is known to have the distinct advantages of low costs, process simplicity, feasibility of composition control, ease of doping, and thus is an excellent technique for the growth of Zn1x Mgx O films [6,7]. In this letter, the ultrasonic spray pyrolysis deposition method was employed to fabricate Zn1x Mgx O films and their structural, optical and photoluminescence properties were studied.
249
400 -C during the film growth according to previously optimized results. The thickness of films was measured to be approximately 200 nm. The crystal structure and crystallinity of as-grown Zn1x Mgx O films were investigated by X-ray diffraction (XRD) using a D/MAX-2550 V diffraction with Cu Ka ˚ ). Then the lattice parameters were radiation (1.5418 A estimated from XRD patterns. X-ray photoelectron spectroscopy (Micro-lab 310 F, MgK a source) was employed to determine the Mg contents in Zn1x Mgx O films. The optical properties of Zn1x Mgx O films were characterized by photoluminescence (PL), transmittance and absorption measurements. PL measurements were carried out at room temperature using a He – Cd laser as a light source at an excitation wavelength of 325 nm. The room temperature transmittance and absorption measurements were both performed using an UV – Vis – NIR spectrophotometer (Shimadzu), with a blank glass as the reference and a deuterium lamp as light source.
3. Results and discussion The XRD patterns (Fig. 1) reveal that all the films are composed of wurtzite-type ZnO phase without any impurity phase (e.g., NaCl-type MgO phase). The maximum Mg content x = 0.27 is significantly larger than the thermodynamic solubility limit (x = 0.04) [11]. Polycrystalline films exhibit three main peaks, which correspond to (002), (101), (102) planes of ZnO wurtzite phase. We
2. Experimental details Zn1x Mgx O films with different Mg contents were deposited by ultrasonic spray pyrolysis under ambient atm osp he re. Two ki nds o f aqu eo us so lutio ns, Zn(CH 3 COO) 2I2H 2 O (AR) and Mg(CH 3 COO)2 I4H 2 O (AR), were chosen as the sources of zinc and magnesium respectively. In order to obtain Zn1x Mgx O films with different Mg contents (x ranging from 0 to 0.27), the concentration of Zn2+ was controlled at 0.45, 0.40, 0.35, 0.30, 0.25 mol/l and the Mg2+ was correspondingly adjusted to 0.05, 0.10, 0.15, 0.20, 0.25 mol/l. For comparison, the deposition parameters were the same for the series of Zn1x Mgx O films. High purity N2 gas was used as carrier gas. Single-crystal Si (100) wafers and quartz glass were used as substrates. Before loading into the growth chamber, Si wafers were thoroughly cleaned for 2 min in a mixed solution of HF: distilled water : ethanol in a volume ratio of 1 : 1 : 10 to remove the surface native oxide layer. The aerosol of precursor solution was generated using a commercial ultrasonic nebulizer, and transported onto the surface of substrate. The substrate temperature was set at
Fig. 1. XRD patterns of Znx Mg1x O films: x = 0.06, 0.12, 0.19, 0.24, 0.27. The peaks of Si wafers is indicated by ‘‘*’’.
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X. Zhang et al. / Thin Solid Films 492 (2005) 248 – 252
Fig. 2. Mg content dependences of the a- and c-axis lattice parameters in the Znx Mg1x O films.
observe that the intensity of (002) peaks becomes weaker, whereas (101) peaks become more intense as the Mg content increases, which is consistent with another report [14]. For all the Zn1x Mgx O films, the angle position of the (002) peak moves toward greater values with increasing Mg content, which indicates that Zn2+ ions are successfully substituted by Mg2+ in the ZnO lattice. The a- and c-axis lengths determined by XRD are plotted as a function of Mg content in Fig. 2. The a-axis lengths monotonically increase when increasing Mg content from 0.06 to 0.27, while the c-axis lengths monotonically decrease. Fig. 3 shows the UV/visible transmittance spectra of Zn1x Mgx O thin films deposited on glass with different Mg contents. The transmittance spectra of the films can be analysed as follows. (1) The excitonic nature of the films is clearly apparent in the spectra. Because the exciton binding energy is almost the same as ZnO (å 60 meV) in the Zn1x Mgx O thin films, the exciton peak remains present for all alloy compositions with increasing Mg concentration. (2) Oscillations are observed in the transmittance spectra, which
Fig. 3. Transmittance spectra of Znx Mg1x O films measured at room temperature.
Fig. 4. a 2 vs. photon energy (hr) characteristics measured at room temperature for Zn0.94Mg0.06O, Zn0.88Mg0.12O, Zn0.81Mg0.19O, Zn0.76 Mg0.24O, Zn0.73Mg0.27O films. The absorption edge shifts toward higher energy with increasing Mg content. The insert shows the relationship between bang gap energy and Mg content.
correspond to multi-reflexions at the film-air and filmsubstrate interfaces. (3) For all films, the average transmittance in the visible wavelength region (k = 400 –800 nm) is greater than 85%. (4) The slope of the absorption edge are softened and there is an obvious shift of the absorption edge to shorter wavelengths with increasing Mg content. In the high energy spectral range, where the film is strongly
Fig. 5. Photoluminescene (solid lines) and absorption spectra (dotted lines) of Znx Mg1x O films with different Mg contents: x = 0.06, 0.12, 0.19, 0.24, 0.27. The alloy films show a luminescence peak at a slightly lower energy than the absorption edge. Note that the FWHM (Full width half Maximum) were significantly broadened with increasing Mg concentration. The inset on right top corner shows the detailed luminescence peak position.
X. Zhang et al. / Thin Solid Films 492 (2005) 248 – 252
Fig. 6. The energy gap variations vs. photoluminescence peaks location.
absorbent, the absorption coefficient a of the films can be calculated from the transmittance using the relationship 1 a ¼ lnT ; t where T is the transmittance, and t is the film thickness. It is well known that the absorption coefficient a for allowed direct transitions at a given photon energy hr can be expressed as 1=2 a ;A4 hr Eg ; where A* is a function of the refractive index and hole/ electron effective masses. The characteristics of a 2 vs. hr (photon energy) were plotted for evaluating the band gap (E g) values (Fig. 4). As can be seen clearly, E g values increase from 3.29 to 3.58 eV with x values ranging from 0.06 to 0.27. In other words, the optical energy band gap of Zn1x Mgx O thin films become wider as Mg content increases and can be precisely controlled between 3.29 and 3.58 eV. This blue-shift of the band gap energy measured on a film with x = 0.19 coincides with that reported for Zn0.81Mg0.19O thin films deposited by rfmagnetron sputtering [13]. The band gap energy of films apparently varies linearly with the Mg content according to the relation E g (Zn1x Mgx O) = 1.36608 (Mg content) + 3.20957 eV (the insert in Fig. 4). Therefore the present results clearly indicate that Mg2+ can be doped in the ZnO lattice by ultrasonic spray pyrolysis. Photoluminescence and absorption spectra are shown in Fig. 5 for x ranging from 0.06 to 0.27. It can be seen that all the alloy films exhibit a luminescence peak at a slightly lower energy than the absorption edge. This phenomenon is probably related to a bound exciton emission line (I) [15], i.e., the recombination of excitons trapped in shallow impurity levels. Increasing the Mg content yields a blue-shift of the photoluminescence peak energy, which is consistent with that of the energy band gap deduced from transmission spectra, as shown in Fig. 6. The emission peak is tuned from
251
369 to 349 nm when increasing x from 0 to 0.27. Thus, Zn1x Mgx O films are not only promising ultraviolet lightemitting materials, but also can be considered as active layers for band gap engineering such as superlattices or quantum wells. The photoluminescence of our films was surprisingly bright. In the photoluminescence spectra, all the films exhibit one peak located in the ultraviolet region, which originates from the excitonic near-band-gap emission. Besides, the Zn0.73Mg0.27O film shows an additional weak green emission peak, which is presumably due to the intrinsic defects such as interstitial Zn or O vacancies. This green photoluminescence indirectly indicates that there is a subtle deterioration in the crystallinity of highly Mg-doped ZnO film, which might be due to strain induced by the Mg substitution on Zn2+ sites. Inset of Fig. 5 also indicates that when x increased the luminescence peaks were broadened. This broadening presumably results from fluctuations in the compound, where localized excitons experience a different Coulomb potential depending on the local concentration and/or arrangement of the substituting elements [14 – 16]. However, compared with other III – V semiconductors, Zn1x Mgx O films exhibit somewhat larger broadening behavior. This is possibly due to that the excitons in ZnO ˚ , and therefore are more have a small Bohr radius of 18 A sensitive to local inhomogeneity, then largely affected by local (atomic-scale) fluctuations related to Mg content.
4. Conclusions To explore applications in ultraviolet optoelectronics, we prepared Zn1x Mgx O films using ultrasonic spray pyrolysis method and found that the band gap of ZnO could be widen with increasing Mg content. With increasing Mg content, the optical band gap and photoluminescence peak could be tuned to the wider energy while maintaining high crystallinity and without inducing significant change of the lattice constants. The results imply that Zn1x Mgx O films can be considered as active layers for band gap engineering such as Zn1x Mgx O-based superlattices or quantum wells.
Acknowledgements This work was supported by the Ministry of Science and Technology of P.R. China through 973 National Nature Science Foundation under Grant 2002CB613306.
References [1] R.F. Service, Science 276 (1997) 895. [2] W.W. Wenas, A. Yamada, K. Takahashi, J. Appl. Phys. 70 (1991) 7119. [3] H. Ohta, K. Kawamura, M. Orita, M. Hirano, N. Sarukura, H. Hosono, Appl. Phys. Lett. 77 (2000) 475. [4] D.C. Look, Mater. Sci. Eng., B 80 (2001) 383.
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[5] D.C. Reynolds, D.C. Look, B. Jogai, Solid State Commun. 99 (1996) 873. [6] J.M. Bian, X.M. Li, X.D. Gao, W.D. Yu, L.D. Chen, Appl. Phys. Lett. 84 (2004) 541. [7] J.M. Bian, X.M. Li, L.D. Chen, Q. Yao, Chem. Phys. Lett. 393 (2004) 256. [8] K. Watanabe, T. Taniguchi, H. Kanda, Nat. Matters 3 (2004) 404. [9] A. Ohtomo, K. Tamura, M. Kawasaki, T. Makino, Y. Segawa, Z.K. Tang, G.K.L. Wong, Y. Matsumoto, H. Koinuma, Appl. Phys. Lett. 77 (2000) 2204. [10] S. Choopun, R.D. Vispute, W. Yang, R.P. Sharma, T. Venkatesan, H. Shen, Appl. Phys. Lett. 80 (2002) 1529.
[11] E.R. Segnit, A.E. Holland, J. Am. Ceram. Soc. 48 (1965) 412. [12] K. Ogata, K. Koike, T. Tanite, T. Komuro, F. Yan, S. Sasa, M. Inoue, M. Yano, J. Cryst. Growth 251 (2003) 623. [13] M. Takashi, N. Takayuki, N. Shiro, H. Takakura, Y. Hamakawa, Thin Solid Films 372 (2000) 173. [14] D.X. Zhao, Y.C. Liu, D.Z. Shen, Y.M. Lu, J.Y. Zhang, X.W. Fan, J. Appl. Phys. 90 (2001) 5561. [15] A. Ohtomo, M. Kawasaki, T. Koida, K. Masubuchi, H. Koinuma, Y. Sakurai, Y. Yoshida, T. Yasuda, Y. Segawa, Appl. Phys. Lett. 72 (1998) 2466. [16] W.I. Park, G.C. Yi, H.M. Jang, Appl. Phys. Lett. 79 (2001) 2022.
Structural and optical properties of Zn1-x Mgx O thin films deposited by ultrasonic spray pyrolysis Xia Zhang, Xiao Min Li *, Tong Lai Chen, Ji Ming Bian, Can Yun Zhang State Key Laboratory of Performance Ceramics and Superfine Microstructures, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, PR China Received 21 October 2004; received in revised form 25 May 2005; accepted 30 June 2005 Available online 1 August 2005
Abstract The structural and optical properties of Zn1x Mgx O films deposited by ultrasonic spray pyrolysis were studied. All the Zn1x Mgx O thin films maintained the ZnO wurtzite structure and had no impurity phase, even for the Mg content up to x = 0.27. The optical properties were characterized by transmittance, absorption spectroscopy and photoluminescence measurements. For all the films, the average transmission in the visible wavelength region (k = 400 – 800 nm) was over 85%, and the absorption edge shifted to a shorter wavelength as the Mg content increased. The optical energy band gap of Zn1x Mgx O thin films, measured from transmittance spectra, could be controlled between 3.29 and 3.58 eV by adjusting Mg contents. The photoluminescence emission peaks of Zn0.94Mg0.06O and Zn0.73Mg0.27O thin films were located at 369 and 349 nm, showing an evident blue-shift with Mg content increase. The room temperature absorption and photoluminescence properties of the films were also discussed. D 2005 Elsevier B.V. All rights reserved. PACS: 68.55.Jk; 61.14.Hg; 81.15.Fg Keywords: Zinc magnesium oxide; Spray pyrolysis; Optical properties; X-ray diffraction
1. Introduction ZnO is a II – IV wide band gap semiconductor used in optical and electronic device applications such as solar cells, transparent conducting electrodes for displays, chemical sensors, varistors, modulators, light emitters diodes, laser diodes and ultraviolet (UV) lasers [1 –3]. In the context of optoelectronics, its direct band gap (3.3 eV) and large exciton binding energy (60 meV), leading to a lower threshold, are favorable for efficient operation of optical devices. Short wavelength devices based on ZnO have become even more interesting, greatly inspired by the demonstrations of the ultraviolet laser at both low and room temperatures [4,5]. For fabricating a double heterostucture laser diode using a ZnO active layer, two key techniques are p-type doping and band gap engineering in semiconductors. Very recent reports of p-type doping in our * Corresponding author. Tel.: +86 21 52412441; fax: +86 21 52413122. E-mail address: [email protected] (X.M. Li). 0040-6090/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2005.06.088
groups [6,7], aided by theoretical work, are indicative of the bright future of light-emitting diodes, laser diodes even working at different wavebands such as deep ultraviolet and visible. The light-emitting diodes working at short wavelength can considerably maximize the storage density [8], which is an efficient way to handle the enormous information and particularly sought for mass data visualization such as compact disk read only memory and digital video disk. On the other hand, band gap devices based on ZnO/ZnMgO superlattices or quantum wells can confine both excitons and photons in the low dimensions, making the stimulated exciton-related emission process more efficient [9]. Consequently, modulation of the band gap while keeping the crystal structure similar to that of ZnO is very essential in terms of the aftermentioned two points. This promotes the extensive search of doping other ions into the ZnO lattice. Recently, Zn1x Mgx O has found great interest, because of its wider band gap compared to pure ZnO and possibility of band gap modulation through adjusting Mg contents. The former provides an opportunity
X. Zhang et al. / Thin Solid Films 492 (2005) 248 – 252
for fabrication of light-emitting diodes at short wavelength. The latter implies that Zn1x Mgx O can act as a suitable layer in ZnO/ZnMgO superlattices or quantum wells. In this letter, we concentrate on the Zn1x Mgx O films for purpose of exploring its potential applications in ultraviolet optoelectronics. ZnO has a wurtzite hexagonal structure, while MgO has a NaCl-type cubic structure. In spite of a large structural dissimilarity between them, the similarity in ionic ˚ ) and Zn2+ (0.60 A ˚ ) allows radius between Mg2+ (0.57 A some replacement in either structure. Researchers have reported that wide gap (5.0 eV), cubic-phase, metastable Zn1x Mgx O thin films can be grown with Mg incorporation greater than 50 at.% [10]. Since Zn1x Mgx O containing MgO over 4 at.% is in a thermodynamically metastable state [11], this result indicates that the solubility limit of Mg in ZnO depends on growth mechanisms as well as growth conditions. However, current research on the growth of Zn1x Mgx O is restricted to pulsed-laser-deposition [10], molecular-beam epitaxy [12] and rf-magnetron sputtering [13]. Despite the potential for the realization of epitaxial growth of Zn1x Mgx O films, these methods might have disadvantages in mass production, due to their high cost and low throughout. Comparing to those methods, the ultrasonic spray pyrolysis deposition technique is known to have the distinct advantages of low costs, process simplicity, feasibility of composition control, ease of doping, and thus is an excellent technique for the growth of Zn1x Mgx O films [6,7]. In this letter, the ultrasonic spray pyrolysis deposition method was employed to fabricate Zn1x Mgx O films and their structural, optical and photoluminescence properties were studied.
249
400 -C during the film growth according to previously optimized results. The thickness of films was measured to be approximately 200 nm. The crystal structure and crystallinity of as-grown Zn1x Mgx O films were investigated by X-ray diffraction (XRD) using a D/MAX-2550 V diffraction with Cu Ka ˚ ). Then the lattice parameters were radiation (1.5418 A estimated from XRD patterns. X-ray photoelectron spectroscopy (Micro-lab 310 F, MgK a source) was employed to determine the Mg contents in Zn1x Mgx O films. The optical properties of Zn1x Mgx O films were characterized by photoluminescence (PL), transmittance and absorption measurements. PL measurements were carried out at room temperature using a He – Cd laser as a light source at an excitation wavelength of 325 nm. The room temperature transmittance and absorption measurements were both performed using an UV – Vis – NIR spectrophotometer (Shimadzu), with a blank glass as the reference and a deuterium lamp as light source.
3. Results and discussion The XRD patterns (Fig. 1) reveal that all the films are composed of wurtzite-type ZnO phase without any impurity phase (e.g., NaCl-type MgO phase). The maximum Mg content x = 0.27 is significantly larger than the thermodynamic solubility limit (x = 0.04) [11]. Polycrystalline films exhibit three main peaks, which correspond to (002), (101), (102) planes of ZnO wurtzite phase. We
2. Experimental details Zn1x Mgx O films with different Mg contents were deposited by ultrasonic spray pyrolysis under ambient atm osp he re. Two ki nds o f aqu eo us so lutio ns, Zn(CH 3 COO) 2I2H 2 O (AR) and Mg(CH 3 COO)2 I4H 2 O (AR), were chosen as the sources of zinc and magnesium respectively. In order to obtain Zn1x Mgx O films with different Mg contents (x ranging from 0 to 0.27), the concentration of Zn2+ was controlled at 0.45, 0.40, 0.35, 0.30, 0.25 mol/l and the Mg2+ was correspondingly adjusted to 0.05, 0.10, 0.15, 0.20, 0.25 mol/l. For comparison, the deposition parameters were the same for the series of Zn1x Mgx O films. High purity N2 gas was used as carrier gas. Single-crystal Si (100) wafers and quartz glass were used as substrates. Before loading into the growth chamber, Si wafers were thoroughly cleaned for 2 min in a mixed solution of HF: distilled water : ethanol in a volume ratio of 1 : 1 : 10 to remove the surface native oxide layer. The aerosol of precursor solution was generated using a commercial ultrasonic nebulizer, and transported onto the surface of substrate. The substrate temperature was set at
Fig. 1. XRD patterns of Znx Mg1x O films: x = 0.06, 0.12, 0.19, 0.24, 0.27. The peaks of Si wafers is indicated by ‘‘*’’.
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X. Zhang et al. / Thin Solid Films 492 (2005) 248 – 252
Fig. 2. Mg content dependences of the a- and c-axis lattice parameters in the Znx Mg1x O films.
observe that the intensity of (002) peaks becomes weaker, whereas (101) peaks become more intense as the Mg content increases, which is consistent with another report [14]. For all the Zn1x Mgx O films, the angle position of the (002) peak moves toward greater values with increasing Mg content, which indicates that Zn2+ ions are successfully substituted by Mg2+ in the ZnO lattice. The a- and c-axis lengths determined by XRD are plotted as a function of Mg content in Fig. 2. The a-axis lengths monotonically increase when increasing Mg content from 0.06 to 0.27, while the c-axis lengths monotonically decrease. Fig. 3 shows the UV/visible transmittance spectra of Zn1x Mgx O thin films deposited on glass with different Mg contents. The transmittance spectra of the films can be analysed as follows. (1) The excitonic nature of the films is clearly apparent in the spectra. Because the exciton binding energy is almost the same as ZnO (å 60 meV) in the Zn1x Mgx O thin films, the exciton peak remains present for all alloy compositions with increasing Mg concentration. (2) Oscillations are observed in the transmittance spectra, which
Fig. 3. Transmittance spectra of Znx Mg1x O films measured at room temperature.
Fig. 4. a 2 vs. photon energy (hr) characteristics measured at room temperature for Zn0.94Mg0.06O, Zn0.88Mg0.12O, Zn0.81Mg0.19O, Zn0.76 Mg0.24O, Zn0.73Mg0.27O films. The absorption edge shifts toward higher energy with increasing Mg content. The insert shows the relationship between bang gap energy and Mg content.
correspond to multi-reflexions at the film-air and filmsubstrate interfaces. (3) For all films, the average transmittance in the visible wavelength region (k = 400 –800 nm) is greater than 85%. (4) The slope of the absorption edge are softened and there is an obvious shift of the absorption edge to shorter wavelengths with increasing Mg content. In the high energy spectral range, where the film is strongly
Fig. 5. Photoluminescene (solid lines) and absorption spectra (dotted lines) of Znx Mg1x O films with different Mg contents: x = 0.06, 0.12, 0.19, 0.24, 0.27. The alloy films show a luminescence peak at a slightly lower energy than the absorption edge. Note that the FWHM (Full width half Maximum) were significantly broadened with increasing Mg concentration. The inset on right top corner shows the detailed luminescence peak position.
X. Zhang et al. / Thin Solid Films 492 (2005) 248 – 252
Fig. 6. The energy gap variations vs. photoluminescence peaks location.
absorbent, the absorption coefficient a of the films can be calculated from the transmittance using the relationship 1 a ¼ lnT ; t where T is the transmittance, and t is the film thickness. It is well known that the absorption coefficient a for allowed direct transitions at a given photon energy hr can be expressed as 1=2 a ;A4 hr Eg ; where A* is a function of the refractive index and hole/ electron effective masses. The characteristics of a 2 vs. hr (photon energy) were plotted for evaluating the band gap (E g) values (Fig. 4). As can be seen clearly, E g values increase from 3.29 to 3.58 eV with x values ranging from 0.06 to 0.27. In other words, the optical energy band gap of Zn1x Mgx O thin films become wider as Mg content increases and can be precisely controlled between 3.29 and 3.58 eV. This blue-shift of the band gap energy measured on a film with x = 0.19 coincides with that reported for Zn0.81Mg0.19O thin films deposited by rfmagnetron sputtering [13]. The band gap energy of films apparently varies linearly with the Mg content according to the relation E g (Zn1x Mgx O) = 1.36608 (Mg content) + 3.20957 eV (the insert in Fig. 4). Therefore the present results clearly indicate that Mg2+ can be doped in the ZnO lattice by ultrasonic spray pyrolysis. Photoluminescence and absorption spectra are shown in Fig. 5 for x ranging from 0.06 to 0.27. It can be seen that all the alloy films exhibit a luminescence peak at a slightly lower energy than the absorption edge. This phenomenon is probably related to a bound exciton emission line (I) [15], i.e., the recombination of excitons trapped in shallow impurity levels. Increasing the Mg content yields a blue-shift of the photoluminescence peak energy, which is consistent with that of the energy band gap deduced from transmission spectra, as shown in Fig. 6. The emission peak is tuned from
251
369 to 349 nm when increasing x from 0 to 0.27. Thus, Zn1x Mgx O films are not only promising ultraviolet lightemitting materials, but also can be considered as active layers for band gap engineering such as superlattices or quantum wells. The photoluminescence of our films was surprisingly bright. In the photoluminescence spectra, all the films exhibit one peak located in the ultraviolet region, which originates from the excitonic near-band-gap emission. Besides, the Zn0.73Mg0.27O film shows an additional weak green emission peak, which is presumably due to the intrinsic defects such as interstitial Zn or O vacancies. This green photoluminescence indirectly indicates that there is a subtle deterioration in the crystallinity of highly Mg-doped ZnO film, which might be due to strain induced by the Mg substitution on Zn2+ sites. Inset of Fig. 5 also indicates that when x increased the luminescence peaks were broadened. This broadening presumably results from fluctuations in the compound, where localized excitons experience a different Coulomb potential depending on the local concentration and/or arrangement of the substituting elements [14 – 16]. However, compared with other III – V semiconductors, Zn1x Mgx O films exhibit somewhat larger broadening behavior. This is possibly due to that the excitons in ZnO ˚ , and therefore are more have a small Bohr radius of 18 A sensitive to local inhomogeneity, then largely affected by local (atomic-scale) fluctuations related to Mg content.
4. Conclusions To explore applications in ultraviolet optoelectronics, we prepared Zn1x Mgx O films using ultrasonic spray pyrolysis method and found that the band gap of ZnO could be widen with increasing Mg content. With increasing Mg content, the optical band gap and photoluminescence peak could be tuned to the wider energy while maintaining high crystallinity and without inducing significant change of the lattice constants. The results imply that Zn1x Mgx O films can be considered as active layers for band gap engineering such as Zn1x Mgx O-based superlattices or quantum wells.
Acknowledgements This work was supported by the Ministry of Science and Technology of P.R. China through 973 National Nature Science Foundation under Grant 2002CB613306.
References [1] R.F. Service, Science 276 (1997) 895. [2] W.W. Wenas, A. Yamada, K. Takahashi, J. Appl. Phys. 70 (1991) 7119. [3] H. Ohta, K. Kawamura, M. Orita, M. Hirano, N. Sarukura, H. Hosono, Appl. Phys. Lett. 77 (2000) 475. [4] D.C. Look, Mater. Sci. Eng., B 80 (2001) 383.
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