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Structural and photoluminescence properties of Zn0.8Mg0.2O thin films grown on Si substrate by pulsed laser deposition
Thin Solid Films 458 (2004) 161–164
Structural and photoluminescence properties of Zn0.8Mg0.2O thin films grown on Si substrate by pulsed laser deposition Yinzhu Zhanga, Junhui Heb, Zhizhen Yea,*, Lu Zoua, Jingyun Huanga, Liping Zhua, Binghui Zhaoa a
State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China b Department of Physics, Zhejiang University, Hangzhou 310027, PR China
Received 1 May 2003; received in revised form 17 December 2003; accepted 17 December 2003
Abstract Ternary Zn1yxMgxO (0-x-0.42) alloy thin films with complete c-axis orientation have been deposited on p-type Si(100) substrate by pulsed laser deposition. X-Ray diffraction measurements indicate that the hexagonal wurtzite type structure of Zn1yxMgxO can be stabilized up to Mg content x;0.37. The deposition temperature was found to exert evident influence on the crystalline grain size and c-axis orientation of the film. Zn0.8 Mg0.2 O film grown at 650 8C displays the narrowest full width of half maximum value of ;0.198 for (002) reflection peak and the largest grain size (;150 nm in diameter). An apparent blueshift (;40 nm ;0.40 eV) in near band edge (NBE) emission peak has been observed in the room temperature photoluminescence (PL) spectra of Zn0.8Mg0.2O compared with that of ZnO. The ratio (RsINBE yIDL ) of NBE intensity to defect level (DL) peak intensity was found to be as large as 159, indicating that the film is of high crystalline quality and is nearly defect free. High resolution transmission electron microscopy photograph indicates that the c-axis oriented Zn0.8 Mg0.2 O film was grown on an amorphous SiO2 intermediate layer with ;8 nm thickness. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Zinc oxides; Luminescence; Pulsed laser deposition; Silicon substrate
1. Introduction ZnO is a direct wide bandgap (Eg;3.4 eV) semiconductor and has been investigated intensively due to its potential applications for laser diode (LD) and light emitting diode (LED) at ultraviolet or even blue light spectra. More interestingly, its large exciton binding energy (;60 meV) makes the efficient excitonic emission at room temperature possible. For the development of ZnO film based optoelectronic devices, the realization of p-type conductivity ZnO thin films is considered to be a critical issue and has been investigated by various doping techniques w1,2x. Meanwhile, the bandgap modulation of ZnO is a valuable subject as well, for the realization of light-emitting devices operating in a wider wavelength region. In this respect, Ohtomo et al. reported the Zn1yxMgxO alloy film with a tunable bandgap from 3.3 eV (xs0) to 4.0 eV (xs0.33) w3x. Although *Corresponding author. Tel.: q86-571-8795-2124; fax: q86-5718795-2625. E-mail address: [email protected] (Z. Ye).
ZnO is of hexagonal wurtzite-type structure (as3.253 ˚ cs5.213 A) ˚ and MgO is of cubic sodium chloride A, ˚ the similarity in (NaCl) type structure (as4.213 A), ˚ and Mg2q (0.71 A) ˚ ionic radii between Zn2q (0.74 A) allows significant ZnyMg replacement in either structure, while the lattice parameters are kept almost constant. Since the modulation of the bandgap with similar lattice constant is essential for construction of the heterojunction or superlattice to obtain high-performance LD and LED devices, hexagonal Zn1yxMgxO alloy film was found to be a very suitable barrier layer for ZnOy Zn1yxMgxO superlattices w4x and multi-quantum wells w5x. Recently, hexagonal Zn1yxMgxO alloy film with Mg content up to 49 at.% has been reported, and its bandgap was found to be widened to upper limit at approximately ;4.19 eV w6x. Yang et al. reported an UV optoelectronic detector based on Zn1yxMgxO (xs 0.34) thin films deposited on Al2O3 by pulsed laser deposition (PLD) w7x, which display a high UV responsivity of 1200 AyW at 308 nm with a 5 eV bias. Its 90%–10% rise and fall time were 8 ns and 1.4 ms,
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.12.127
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respectively. Recently, Krishnamoorthy et al. observed the resonant action in ZnOyZn0.8Mg0.2O devices with a double barrier structure w8x. Since Zn1yxMgxO alloy film has been found to exhibit a bright UV luminescence at room temperature with excitonic nature, Zn1yxMgxO alloy film therefore may afford the possibility of fabricating novel optoelectronic devices in the UV spectrum. Up to date, a variety of techniques such as PLD w3,9x, molecular beam epitaxy w4x, metal-organic chemical vapor deposition w6x, electrophoresis deposition w10x and r.f. magnetron sputtering w11x have been employed to prepare Zn1yxMgxO alloy films. In this paper, we will report the preparation of Zn1yxMgxO alloy thin films deposited on Si(100) by the PLD technique together with the characterization of the film by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution TEM (HRTEM) and photoluminescence (PL). 2. Experimental details Zn1yxMgxO (xs0, 0.2, 0.37, 0.42) ceramic targets were prepared from ZnO (4N) and MgO (4N) powders and sintered at 1250 8C for 3 h. A KrF excimer laser operating at 248 nm was applied for the deposition of Zn1yxMgxO film. The average pulse energy was 300 mJ, and the duration time per pulse was 45 ns. The laser beam was focused on the target with a spot size of 1=6 mm2, thus average energy density per pulse was estimated to be ;4 Jycm2. The p-type Si(100) substrate was located at a distance of ;4 cm from the target. During deposition, oxygen pressure, pulse frequency and deposition time was maintained at ;10 Pa, 1.5 Hz and 30 min, respectively. The different substrate temperatures such as 500 8C, 550 8C, 600 8C, 650 8C and 700 8C were chosen to investigate the influence on film quality. The Zn0.8Mg0.2O film grown at 650 8C was annealed at 800 8C in vacuum. The growth orientation and microstructure of Zn1yxMgxO alloy films were investigated by XRD (Philips, CuKas0.154056 nm) and HRTEM (Philips CM 20, 200 kV accelerating voltage). The PL spectra of Zn1yxMgxO films were measured at room temperature using a cw He-Cd excimer (IK Series Kimmon Laser, 325 nm, 400 Hz, 50 nW). The cross-sectional specimen for TEM and HRTEM observations was prepared according to the following steps. First, two pieces of Si substrate carrying the Zn0.8Mg0.2O thin film deposited at 650 8C were glued together using epoxy resin with the films face-to-face. Then the sample was cut as a strip of 500 mm of thickness along the (011) plane with a diamond saw. Next the sandwich containing the Zn0.8Mg0.2O film in the chosen direction was carefully mechanically thinned to 50 mm and polished to a final thickness of less than 30 mm. After that the sample was fixed to a copper
Fig. 1. XRD patterns of Zn1yxMgxO films.
loop having a diameter of 3.05 mm, which is a supporter for ion-beam milling. Finally, Ar ion milling was carried out at an angle of 108 at room temperature until a hole was formed in the sample. 3. Results and discussion Fig. 1 shows the XRD patterns of the Zn1yxMgxO films deposited by PLD technique at 650 8C. For the films with x-0.37, only a sharp (002) reflection from hexagonal wurtzite-type structure was observed. Above xs0.37, the cubic MgO phase separation takes place which results in a mixed XRD patterns of hexagonal Zn1yxMgxO and cubic MgO, in agreement with previous reporter w3x. The (002) reflection shifts slightly to larger 2u angle with increasing x, indicating that there is a slight contraction in lattice parameter c due to substitution of smaller Mg2q ion. However, the decrease in lattice parameter is small due to the similarity in the ionic radii of Mg2q and Zn2q. The smallest misfit to ZnO was estimated to be ;0.5% which is comparable with previous reports w6,10,12x. In order to investigate the effects of substrate temperatures, Zn0.8Mg0.2O films were deposited at 500 8C, 550 8C, 600 8C, 650 8C, and 700 8C, respectively. When deposited at temperatures lower or higher than 650 8C, (101) or (102) reflections with small intensity were observed together with dominant (002) reflection as
Y. Zhang et al. / Thin Solid Films 458 (2004) 161–164
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Fig. 3. Cross-section TEM micrograph of Zn0.8Mg0.2O film prepared at 650 8C. A high resolution TEM photograph of the Zn0.8Mg0.2OySiO2ySi interface is presented as an insert. Fig. 2. FWHM and grain size of the Zn0.8Mg0.2O film vs. substrate temperature.
well. Therefore, the films with high c-axis orientation and having a satisfying crystalline quality can be achieved when deposited at 650 8C. Full width of half maximum (FWHM) of (002) reflections and the grain sizes of the Zn0.8Mg0.2O films vs. deposition temperature were shown in Fig. 2. The grain size is evaluated by the Sherrer formula: Gs0.9 lyB cosu, where B represents the value of FWHM. The FWHM value of (002) reflection decreases with increasing deposition temperature up to 650 8C, and turns to increase at higher temperature. In contrast, the grain size varies in an opposite way. The film deposited at 650 8C exhibits the smallest FWHM value (;0.198) and the largest grain size (;150 nm in diameter). It can be concluded that the substrate temperature plays a significant role on the crystalline quality of the Zn1yxMgxO films and the optimized temperature is likely 650 8C. The TEM photograph of cross-section of Zn0.8Mg0.2O film is displayed in Fig. 3. Columnar crystal grains vertical to the substrate directly verify the c-axis orientation of the alloy film. The top of column is not very flat and looks like in triangular shape. The high resolution TEM photograph of the cross-section of Zn0.8Mg0.2O film is shown in the insert. An amorphous SiO2 intermediate layer with a thickness of ;8 nm was observed between the substrate and film, which was formed due to surface thermal oxidation during the PLD deposition. The observed periodic lattice arrangement of
Zn0.8Mg0.2O film and Si substrate indicates high c-axis orientation and single crystal feature in single columnar domain. Fig. 4 shows the room temperature PL spectra of Zn0.8Mg0.2O thin films. The films deposited at 500 8C and 650 8C are designed as No. 1 and No. 2, respectively. The Zn0.8Mg0.2O film deposited at 650 8C (No. 2) exhibits a near-band-edge (NBE) emission peak at
Fig. 4. Room temperature PL spectra of Zn0.8 Mg0.2O thin films grown at 500 8C (No. 1, solid line) and 650 8C (No. 2, dashed line), respectively. The PL spectrum of the annealed sample of No. 2 is also included to investigate the influence of post-annealing treatment (dotted line).
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approximately ;330 nm (3.7 eV) with a blueshift of 40 nm (0.40 eV) from 370 nm (3.3 eV) of ZnO. For the film deposited at 500 8C (No. 1), the NBE emission peak with decreased intensity is located at approximately 337 nm (3.63 eV). The defect level (DL) emission peak (;500 nm) of No.1 was well suppressed, indicating few oxygen vacancies in the Zn1yxMgxO film. Another observed peak at ;405 nm, which likely originates from the zinc interstitial defects w13x, was suppressed greatly for No. 2. These results indicate that the substrate temperature strongly affects the PL property of the Zn1yxMgxO films. Mg content in the film may increase when deposited at higher temperature due to higher vapor pressure of Zn w5x, which results in a larger blue shift and weaker zinc interstitial DL emission. The ratio (INBE yIDL) of NBE emission to DL emission was estimated to be ;159 for the Zn0.8Mg0.2O film deposited at 650 8C, indicating that the film is of high crystalline quality and is nearly defects free w7x. The effect of post-annealing treatment on the Zn0.8Mg0.2O film is also investigated by room temperature PL measurement, as shown in Fig. 4. When the 650 8C as-grown Zn0.8Mg0.2O film was annealed at 800 8C in vacuum for 1 h, the NBE emission peak shifts slightly to 348 nm with its intensity reduced greatly, while the intensity of the DL emission peak increases apparently. Therefore, the annealing at high temperature brings about more oxygen vacancies, which results in the deterioration in crystalline quality and photoluminescent property in agreement with a previous report w14x. 4. Summary To conclude, ternary Zn1yxMgxO (0-x-0.42) alloy films with complete c-axis orientation have been deposited on p-type Si(100) substrate at different temperatures by PLD technique. The hexagonal wurtzite-type structure of Zn1yxMgxO can be stabilized up to Mg content x;0.37. Substrate temperature was found to exert remarkable influence on the crystalline quality and
photoluminescence property of Zn1yxMgxO (0-x0.42) alloy films. Zn0.8Mg0.2O film grown at 650 8C exhibits a narrow FWHM value of ;0.198 for (002) reflection peak and a sharp NBE peak located at ;337 nm with an evident blueshift (;0.40 eV) compared with that of ZnO. The ratio (RsINBE yIDL) of NBE intensity to defect level (DL) peak intensity was found to be as large as 159, indicating that the film is of high crystalline quality and is nearly defect free. Acknowledgments The project is supported by the Special Funds for Major State Basic Research Project G20000683 and the National Natural Science Foundation of China for Key Project (No. 90201038). References w1x M. Joseph, H. Tabata, T. Kawai, Jpn. J. Appl. Phys. 38 (1999) L1205. w2x T. Yamamoto, H. Katayama-Yoshida, Physica B 302y303 (2001) 155. w3x A. Ohtomo, M. Kawasaki, T. Koida, Appl. Phys. Lett. 72 (1998) 2466. w4x A. Ohtomo, M. Kawasaki, Appl. Phys. Lett. 75 (1999) 980. w5x T. Makino, C.H. Chia, N.T. Tuan, H.D. Sun, Appl. Phys. Lett. 77 (2000) 975. w6x W.L. Park, G.-C. Yi, H.M. Jany, Appl. Phys. Lett. 79 (2001) 2022. w7x W. Yang, R.D. Vispute, S. Choopun, Appl. Phys. Lett. 78 (2001) 2787. w8x S. Krishnamoorthy, A.A. Iliadis, A. Inumpudi, Solid-State Electronics 46 (2002) 1633. w9x A.K. Sharma, J. Narayan, J.F. Muth, Appl. Phys. Lett. 75 (1999) 3327. w10x Y. Jin, B. Zhang, S. Yang, Solid State Commun. 119 (2001) 409. w11x T. Minemoto, T. Negami, S. Nishiwaki, Thin Solid Films 372 (2000) 173. w12x J.H. Kang, Y.R. Park, K.J. Kim, Solid State Commun. 115 (2000) 127. w13x B. Lin, Z. Fu, Y. Jia, Appl. Phys. Lett. 79 (2001) 943. w14x V. Srikant, D.R. Clarke, J. Appl. Phys. 81 (1997) 6357.
Structural and photoluminescence properties of Zn0.8Mg0.2O thin films grown on Si substrate by pulsed laser deposition Yinzhu Zhanga, Junhui Heb, Zhizhen Yea,*, Lu Zoua, Jingyun Huanga, Liping Zhua, Binghui Zhaoa a
State Key Laboratory of Silicon Materials, Zhejiang University, Hangzhou 310027, PR China b Department of Physics, Zhejiang University, Hangzhou 310027, PR China
Received 1 May 2003; received in revised form 17 December 2003; accepted 17 December 2003
Abstract Ternary Zn1yxMgxO (0-x-0.42) alloy thin films with complete c-axis orientation have been deposited on p-type Si(100) substrate by pulsed laser deposition. X-Ray diffraction measurements indicate that the hexagonal wurtzite type structure of Zn1yxMgxO can be stabilized up to Mg content x;0.37. The deposition temperature was found to exert evident influence on the crystalline grain size and c-axis orientation of the film. Zn0.8 Mg0.2 O film grown at 650 8C displays the narrowest full width of half maximum value of ;0.198 for (002) reflection peak and the largest grain size (;150 nm in diameter). An apparent blueshift (;40 nm ;0.40 eV) in near band edge (NBE) emission peak has been observed in the room temperature photoluminescence (PL) spectra of Zn0.8Mg0.2O compared with that of ZnO. The ratio (RsINBE yIDL ) of NBE intensity to defect level (DL) peak intensity was found to be as large as 159, indicating that the film is of high crystalline quality and is nearly defect free. High resolution transmission electron microscopy photograph indicates that the c-axis oriented Zn0.8 Mg0.2 O film was grown on an amorphous SiO2 intermediate layer with ;8 nm thickness. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Zinc oxides; Luminescence; Pulsed laser deposition; Silicon substrate
1. Introduction ZnO is a direct wide bandgap (Eg;3.4 eV) semiconductor and has been investigated intensively due to its potential applications for laser diode (LD) and light emitting diode (LED) at ultraviolet or even blue light spectra. More interestingly, its large exciton binding energy (;60 meV) makes the efficient excitonic emission at room temperature possible. For the development of ZnO film based optoelectronic devices, the realization of p-type conductivity ZnO thin films is considered to be a critical issue and has been investigated by various doping techniques w1,2x. Meanwhile, the bandgap modulation of ZnO is a valuable subject as well, for the realization of light-emitting devices operating in a wider wavelength region. In this respect, Ohtomo et al. reported the Zn1yxMgxO alloy film with a tunable bandgap from 3.3 eV (xs0) to 4.0 eV (xs0.33) w3x. Although *Corresponding author. Tel.: q86-571-8795-2124; fax: q86-5718795-2625. E-mail address: [email protected] (Z. Ye).
ZnO is of hexagonal wurtzite-type structure (as3.253 ˚ cs5.213 A) ˚ and MgO is of cubic sodium chloride A, ˚ the similarity in (NaCl) type structure (as4.213 A), ˚ and Mg2q (0.71 A) ˚ ionic radii between Zn2q (0.74 A) allows significant ZnyMg replacement in either structure, while the lattice parameters are kept almost constant. Since the modulation of the bandgap with similar lattice constant is essential for construction of the heterojunction or superlattice to obtain high-performance LD and LED devices, hexagonal Zn1yxMgxO alloy film was found to be a very suitable barrier layer for ZnOy Zn1yxMgxO superlattices w4x and multi-quantum wells w5x. Recently, hexagonal Zn1yxMgxO alloy film with Mg content up to 49 at.% has been reported, and its bandgap was found to be widened to upper limit at approximately ;4.19 eV w6x. Yang et al. reported an UV optoelectronic detector based on Zn1yxMgxO (xs 0.34) thin films deposited on Al2O3 by pulsed laser deposition (PLD) w7x, which display a high UV responsivity of 1200 AyW at 308 nm with a 5 eV bias. Its 90%–10% rise and fall time were 8 ns and 1.4 ms,
0040-6090/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2003.12.127
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respectively. Recently, Krishnamoorthy et al. observed the resonant action in ZnOyZn0.8Mg0.2O devices with a double barrier structure w8x. Since Zn1yxMgxO alloy film has been found to exhibit a bright UV luminescence at room temperature with excitonic nature, Zn1yxMgxO alloy film therefore may afford the possibility of fabricating novel optoelectronic devices in the UV spectrum. Up to date, a variety of techniques such as PLD w3,9x, molecular beam epitaxy w4x, metal-organic chemical vapor deposition w6x, electrophoresis deposition w10x and r.f. magnetron sputtering w11x have been employed to prepare Zn1yxMgxO alloy films. In this paper, we will report the preparation of Zn1yxMgxO alloy thin films deposited on Si(100) by the PLD technique together with the characterization of the film by X-ray diffraction (XRD), transmission electron microscopy (TEM), high resolution TEM (HRTEM) and photoluminescence (PL). 2. Experimental details Zn1yxMgxO (xs0, 0.2, 0.37, 0.42) ceramic targets were prepared from ZnO (4N) and MgO (4N) powders and sintered at 1250 8C for 3 h. A KrF excimer laser operating at 248 nm was applied for the deposition of Zn1yxMgxO film. The average pulse energy was 300 mJ, and the duration time per pulse was 45 ns. The laser beam was focused on the target with a spot size of 1=6 mm2, thus average energy density per pulse was estimated to be ;4 Jycm2. The p-type Si(100) substrate was located at a distance of ;4 cm from the target. During deposition, oxygen pressure, pulse frequency and deposition time was maintained at ;10 Pa, 1.5 Hz and 30 min, respectively. The different substrate temperatures such as 500 8C, 550 8C, 600 8C, 650 8C and 700 8C were chosen to investigate the influence on film quality. The Zn0.8Mg0.2O film grown at 650 8C was annealed at 800 8C in vacuum. The growth orientation and microstructure of Zn1yxMgxO alloy films were investigated by XRD (Philips, CuKas0.154056 nm) and HRTEM (Philips CM 20, 200 kV accelerating voltage). The PL spectra of Zn1yxMgxO films were measured at room temperature using a cw He-Cd excimer (IK Series Kimmon Laser, 325 nm, 400 Hz, 50 nW). The cross-sectional specimen for TEM and HRTEM observations was prepared according to the following steps. First, two pieces of Si substrate carrying the Zn0.8Mg0.2O thin film deposited at 650 8C were glued together using epoxy resin with the films face-to-face. Then the sample was cut as a strip of 500 mm of thickness along the (011) plane with a diamond saw. Next the sandwich containing the Zn0.8Mg0.2O film in the chosen direction was carefully mechanically thinned to 50 mm and polished to a final thickness of less than 30 mm. After that the sample was fixed to a copper
Fig. 1. XRD patterns of Zn1yxMgxO films.
loop having a diameter of 3.05 mm, which is a supporter for ion-beam milling. Finally, Ar ion milling was carried out at an angle of 108 at room temperature until a hole was formed in the sample. 3. Results and discussion Fig. 1 shows the XRD patterns of the Zn1yxMgxO films deposited by PLD technique at 650 8C. For the films with x-0.37, only a sharp (002) reflection from hexagonal wurtzite-type structure was observed. Above xs0.37, the cubic MgO phase separation takes place which results in a mixed XRD patterns of hexagonal Zn1yxMgxO and cubic MgO, in agreement with previous reporter w3x. The (002) reflection shifts slightly to larger 2u angle with increasing x, indicating that there is a slight contraction in lattice parameter c due to substitution of smaller Mg2q ion. However, the decrease in lattice parameter is small due to the similarity in the ionic radii of Mg2q and Zn2q. The smallest misfit to ZnO was estimated to be ;0.5% which is comparable with previous reports w6,10,12x. In order to investigate the effects of substrate temperatures, Zn0.8Mg0.2O films were deposited at 500 8C, 550 8C, 600 8C, 650 8C, and 700 8C, respectively. When deposited at temperatures lower or higher than 650 8C, (101) or (102) reflections with small intensity were observed together with dominant (002) reflection as
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163
Fig. 3. Cross-section TEM micrograph of Zn0.8Mg0.2O film prepared at 650 8C. A high resolution TEM photograph of the Zn0.8Mg0.2OySiO2ySi interface is presented as an insert. Fig. 2. FWHM and grain size of the Zn0.8Mg0.2O film vs. substrate temperature.
well. Therefore, the films with high c-axis orientation and having a satisfying crystalline quality can be achieved when deposited at 650 8C. Full width of half maximum (FWHM) of (002) reflections and the grain sizes of the Zn0.8Mg0.2O films vs. deposition temperature were shown in Fig. 2. The grain size is evaluated by the Sherrer formula: Gs0.9 lyB cosu, where B represents the value of FWHM. The FWHM value of (002) reflection decreases with increasing deposition temperature up to 650 8C, and turns to increase at higher temperature. In contrast, the grain size varies in an opposite way. The film deposited at 650 8C exhibits the smallest FWHM value (;0.198) and the largest grain size (;150 nm in diameter). It can be concluded that the substrate temperature plays a significant role on the crystalline quality of the Zn1yxMgxO films and the optimized temperature is likely 650 8C. The TEM photograph of cross-section of Zn0.8Mg0.2O film is displayed in Fig. 3. Columnar crystal grains vertical to the substrate directly verify the c-axis orientation of the alloy film. The top of column is not very flat and looks like in triangular shape. The high resolution TEM photograph of the cross-section of Zn0.8Mg0.2O film is shown in the insert. An amorphous SiO2 intermediate layer with a thickness of ;8 nm was observed between the substrate and film, which was formed due to surface thermal oxidation during the PLD deposition. The observed periodic lattice arrangement of
Zn0.8Mg0.2O film and Si substrate indicates high c-axis orientation and single crystal feature in single columnar domain. Fig. 4 shows the room temperature PL spectra of Zn0.8Mg0.2O thin films. The films deposited at 500 8C and 650 8C are designed as No. 1 and No. 2, respectively. The Zn0.8Mg0.2O film deposited at 650 8C (No. 2) exhibits a near-band-edge (NBE) emission peak at
Fig. 4. Room temperature PL spectra of Zn0.8 Mg0.2O thin films grown at 500 8C (No. 1, solid line) and 650 8C (No. 2, dashed line), respectively. The PL spectrum of the annealed sample of No. 2 is also included to investigate the influence of post-annealing treatment (dotted line).
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approximately ;330 nm (3.7 eV) with a blueshift of 40 nm (0.40 eV) from 370 nm (3.3 eV) of ZnO. For the film deposited at 500 8C (No. 1), the NBE emission peak with decreased intensity is located at approximately 337 nm (3.63 eV). The defect level (DL) emission peak (;500 nm) of No.1 was well suppressed, indicating few oxygen vacancies in the Zn1yxMgxO film. Another observed peak at ;405 nm, which likely originates from the zinc interstitial defects w13x, was suppressed greatly for No. 2. These results indicate that the substrate temperature strongly affects the PL property of the Zn1yxMgxO films. Mg content in the film may increase when deposited at higher temperature due to higher vapor pressure of Zn w5x, which results in a larger blue shift and weaker zinc interstitial DL emission. The ratio (INBE yIDL) of NBE emission to DL emission was estimated to be ;159 for the Zn0.8Mg0.2O film deposited at 650 8C, indicating that the film is of high crystalline quality and is nearly defects free w7x. The effect of post-annealing treatment on the Zn0.8Mg0.2O film is also investigated by room temperature PL measurement, as shown in Fig. 4. When the 650 8C as-grown Zn0.8Mg0.2O film was annealed at 800 8C in vacuum for 1 h, the NBE emission peak shifts slightly to 348 nm with its intensity reduced greatly, while the intensity of the DL emission peak increases apparently. Therefore, the annealing at high temperature brings about more oxygen vacancies, which results in the deterioration in crystalline quality and photoluminescent property in agreement with a previous report w14x. 4. Summary To conclude, ternary Zn1yxMgxO (0-x-0.42) alloy films with complete c-axis orientation have been deposited on p-type Si(100) substrate at different temperatures by PLD technique. The hexagonal wurtzite-type structure of Zn1yxMgxO can be stabilized up to Mg content x;0.37. Substrate temperature was found to exert remarkable influence on the crystalline quality and
photoluminescence property of Zn1yxMgxO (0-x0.42) alloy films. Zn0.8Mg0.2O film grown at 650 8C exhibits a narrow FWHM value of ;0.198 for (002) reflection peak and a sharp NBE peak located at ;337 nm with an evident blueshift (;0.40 eV) compared with that of ZnO. The ratio (RsINBE yIDL) of NBE intensity to defect level (DL) peak intensity was found to be as large as 159, indicating that the film is of high crystalline quality and is nearly defect free. Acknowledgments The project is supported by the Special Funds for Major State Basic Research Project G20000683 and the National Natural Science Foundation of China for Key Project (No. 90201038). References w1x M. Joseph, H. Tabata, T. Kawai, Jpn. J. Appl. Phys. 38 (1999) L1205. w2x T. Yamamoto, H. Katayama-Yoshida, Physica B 302y303 (2001) 155. w3x A. Ohtomo, M. Kawasaki, T. Koida, Appl. Phys. Lett. 72 (1998) 2466. w4x A. Ohtomo, M. Kawasaki, Appl. Phys. Lett. 75 (1999) 980. w5x T. Makino, C.H. Chia, N.T. Tuan, H.D. Sun, Appl. Phys. Lett. 77 (2000) 975. w6x W.L. Park, G.-C. Yi, H.M. Jany, Appl. Phys. Lett. 79 (2001) 2022. w7x W. Yang, R.D. Vispute, S. Choopun, Appl. Phys. Lett. 78 (2001) 2787. w8x S. Krishnamoorthy, A.A. Iliadis, A. Inumpudi, Solid-State Electronics 46 (2002) 1633. w9x A.K. Sharma, J. Narayan, J.F. Muth, Appl. Phys. Lett. 75 (1999) 3327. w10x Y. Jin, B. Zhang, S. Yang, Solid State Commun. 119 (2001) 409. w11x T. Minemoto, T. Negami, S. Nishiwaki, Thin Solid Films 372 (2000) 173. w12x J.H. Kang, Y.R. Park, K.J. Kim, Solid State Commun. 115 (2000) 127. w13x B. Lin, Z. Fu, Y. Jia, Appl. Phys. Lett. 79 (2001) 943. w14x V. Srikant, D.R. Clarke, J. Appl. Phys. 81 (1997) 6357.