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Improved microstructural properties of a ZnO thin film using a buffer layer in-situ annealed in argon ambient
Thin Solid Films 515 (2007) 6721 – 6725 www.elsevier.com/locate/tsf
Improved microstructural properties of a ZnO thin film using a buffer layer in-situ annealed in argon ambient Dong Jun Park a , Jeong Yong Lee a , Tae Eun Park b , Young Yi Kim c , Hyung Koun Cho c,⁎ a
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373–1 Guseong-dong, Yuseong-gu, Daejeon 305–701, South Korea b Department of Materials Science and Engineering, Dong-A University, 840 Hadan-dong, Saha-gu, Busan, 604–714, South Korea c School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do, 440–746, South Korea Received 7 February 2006; received in revised form 18 December 2006; accepted 31 January 2007 Available online 7 February 2007
Abstract ZnO films with improved crystallinity were grown on a Si (111) substrate by a two-step growth process using low-temperature ZnO buffer layers. The effect of the ambient gas during the temperature elevation and the in-situ thermal annealing after the growth of the low-temperature buffer layers on the optical and structural properties of the films was investigated by X-ray diffraction (XRD), photoluminescence, and transmission electron microscopy. The use of argon as the ambient gas during the thermal treatment of the buffer layer leads to the enhancement of the (0002) diffraction peak intensity at 2θ ∼ 34.4° and the reduction of the full width at half maximum value in the XRD rocking curve, which means that well-defined and c-axis oriented ZnO film was obtained. The relationship between the thickness of the SiO2 layer between the ZnO buffer layers and Si substrates and the structural and optical properties of the ZnO films is discussed. © 2007 Elsevier B.V. All rights reserved. PACS: 68.37.Lp; 68.55.Jk; 61.72.Nn; 61.43.Bn Keywords: ZnO films; Buffer layer; RF sputter; Si substrate; TEM
1. Introduction Zinc oxide (ZnO) with a wurtzite structure has many useful properties, such as a wide band gap (3.37 eV) and a high exciton binding energy (∼ 60 meV), which give it a more efficient exciton emission at room temperature compared with other wide-band-gap materials, such as GaN (24 meV). For this reason, the use of ZnO for ultraviolet optoelectronic devices has spurred considerable interest [1]. There are many different methods that can be used to produce ZnO films, such as molecular beam epitaxy [2], pulsed laser deposition [3], atomic layer deposition [4], plasma enhanced chemical vapor deposition [5], metal–organic chemical vapor deposition [6], and magnetron sputtering [7]. Among these, magnetron sputtering is the preferred method because it ⁎ Corresponding author. Tel.: +82 31 290 7364; fax: +82 31 290 7410. E-mail address: [email protected] (H.K. Cho). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.01.047
can be conducted at a temperature lower than the other methods [8]. Among the various substrate materials for ZnO, Si is a particularly promising candidate. The use of a Si substrate to deposit high quality ZnO films not only allows for the effective integration of optoelectronic devices with Si IC technology, but also represents a material which is cheaper and easier to cleave than other materials, such as sapphire. The commercialization of ZnO, however, has been derailed by a number of problems, such as the deterioration of the crystalline quality, due to the prevalence of grain boundaries owing to the formation of polycrystalline films with a preferred orientation along the c-axis, as well as the difficulty in producing p-type ZnO films. The two-step growth technique using the low-temperature deposition of a thin buffer layer prior to the ZnO film growth is often employed for the growth of highly mismatched heteroepitaxy layers like GaN-based materials [9]. In the two-step growth process, the crystalline properties of the ZnO films that
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Table 1 The growth conditions of the samples used in this experiment Sample
Thickness (Å)
Tg (°C)
PL FWHM (meV)
XRD FWHM (°)
RMS (nm)
Annealing condition
A B
3000 300 2700 300 2700
600 400 600 400 600
103 146
5.1 3.1
15.1 21.0
None O2,15 min
101
2.5
13.4
Ar,15 min
C
grow on the thin buffer layer depend to a large extent on the initially grown layer. However, most of the previous works on the use of low-temperature buffer layers for the growth of highquality ZnO were mainly focused on the growth temperature and thickness using oxygen ambient gas [10–12]. In this paper, argon was used as the ambient gas instead of oxygen during the thermal treatments, and its effect on the structural and optical properties of the ZnO film was investigated. 2. Experimental procedure ZnO thin films were deposited on Si (111) substrates using a radio frequency (RF) magnetron-sputtering system. The Si substrates were cleaned with a solution of trichloroethylene, acetone, and methanol using ultrasonor. They were then rinsed with de-ionized water. A sintered ZnO (99.999%, 4-in.) target was used for the growth of the ZnO films. The base pressure of the chamber was 0.667 × 10− 4 Pa, and the working pressure with both oxygen and argon gases was 0.267 Pa. Prior to the ZnO growth, pre-sputtering was conducted with an RF power of 50 W for 30 min. It is well known that the use of ZnO buffer layers grown at a low temperature and of the in-situ annealing process conducted at a high temperature improves the crystallinity and emission properties of ZnO films. To relieve the large-misfit strain between the Si substrates and the ZnO films, low-temperature
Fig. 1. X-ray diffraction patterns of ZnO films grown on buffer layer annealed in different ambient gases.
ZnO buffer layers (400 °C) were grown as seed crystals, followed by the growth of ZnO films at 600 °C. Pure oxygen with high purity was used as the ambient gas for the growth of the ZnO buffer layers and films. The growth temperature was increased from 400 °C to 600 °C after the growth of the ZnO buffer layers in order to subject them to thermal treatment at 600 °C prior to the growth of the ZnO films. Most of the previous works on the ZnO films grown by magnetron sputtering and the thermal treatment of ZnO were conducted in an oxygen atmosphere for the purpose of suppressing the oxygen vacancies [10–12]. To investigate the effect of the ambient gas, argon and oxygen were used respectively during the ramping-up process from 400 °C to 600 °C and during the in-situ annealing process for 15 min at 600 °C. The detailed growth conditions of the samples used in this experiment are shown in Table 1. The growth was interrupted during the thermal treatment by extinguishing the plasma between the buffer layer and the ZnO film. X-ray diffraction (XRD) (RIGAKU, D/MAX-RC) analysis was carried out to evaluate the structural properties of the samples. A monochromatic X-ray beam with Cu Kα(λ = 1.5418 Å) radiation was used for the XRD analysis. The transmission electron microscopy (TEM) samples were mechanically polished and completed through ion milling at 3.5 kV using Ar+ ions. A TEM (JEOL, JEM-3010 operated at 300 KV) equipped with energy-dispersive X-ray (EDX)
Fig. 2. The cross-sectional TEM images obtained from (a) sample A, (b) sample B, and (c) sample C to compare the grain size, and the corresponding diffraction patterns of ZnO/Si shown on the right.
D.J. Park et al. / Thin Solid Films 515 (2007) 6721–6725
Fig. 3. HRTEM images obtained from the ZnO/Si interfaces of (a) sample A, (b) sample B, and (c) sample C.
spectrometer with a beam broadening size of less than 10 nm was used to characterize the structures and to determine the chemical composition of the ZnO thin films. The root-mean-square (RMS) surface roughness of the ZnO films was measured by atomic force microscopy (AFM). The photoluminescence (PL) was measured under excitation with a 10 mW He–Cd laser operated at 325 nm.
affects that of the subsequently deposited high-temperature ZnO film by acting as a template and, as such, a dense ZnO film with a smooth surface can be grown. The grain size could be evaluated using the Scherrer formula from the measured full width at half maximum (FWHM) values of the (0002) XRD rocking curve, in which the broadening of the FWHM values indicated the small grain size [13]. However, the grain size of sample C, which showed a reduced FWHM of the XRD rocking curve (indicated in Table 1), is smaller than those of samples A and B, as shown Fig. 2. The smaller grain size and stronger (0002) diffraction peak of sample C indicate that the (0002) planes within each grain in sample C are stacked parallel to those within other grains, and that the grains are mainly rotated along the c-axis. This was confirmed by the diffraction pattern obtained from sample C using TEM, showing the small tilting angle of the 0002 diffraction spot compared to those of samples A and B, as shown in Fig. 2. These results are consistent with the XRD results, indicating that the tilt of the columnar structures is related to the degradation of the crystalline quality in the ZnO films. To analyze the microstructural properties of the films, such as the stacking and rotation of the (0002) planes, high-resolution TEM (HRTEM) images were obtained from sample A (without a buffer layer), sample B (with a buffer layer annealed in oxygen ambient), and sample C (with a buffer layer annealed in argon ambient), as shown in Fig. 3, with the incident electron beam parallel to the [112¯0]ZnO direction. The image of sample A in Fig. 3(a) shows grains that are irregularly shaped and that are ¯00] direction, near the ZnO/Si interface tilted towards the [11 region, while in the case of sample B, a regular shaped columnar
3. Results and discussion To investigate the crystalline quality of the ZnO films deposited on the buffer layers annealed under different ambient gases, XRD measurement was conducted. A preferred orientation along the c-axis perpendicular to the substrate surface was clearly observed in all of the samples, as shown in Fig. 1. A higher intensity of the (0002) diffraction peak was detected in samples B and C, in which in-situ annealing of the buffer layers was conducted. This reveals that the two-step growth technique using the low-temperature buffer layers prior to the ZnO film growth improves the crystalline structure and enhances the caxis preferred orientation of the ZnO films, regardless of the ambient gas type. The strongest (0002) diffraction peak appeared in sample C, which implies that argon is more suitable as an ambient gas than oxygen during the thermal treatment. Fig. 2 (a), (b), and (c) shows the cross-sectional bright-field TEM images of samples A, B, and C, respectively. All these samples showed c-axis oriented columnar structures. While the surface morphology of samples A and B was a little bumpy [Fig. 2(a) and (b)], a flat and smooth surface was obtained in sample C, whose buffer layer was annealed in argon ambient [Fig. 2(c)]. These results are in good agreement with the data obtained by atomic force microscopy, as indicated in Table 1. This change in the surface morphology means that the enhancement of the crystalline quality of the buffer layer
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Fig. 4. EDX data of (a) sample A and (b) sample C.
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structure is observed. It could be considered that the zinc or oxygen atoms have sufficient atomic energy to occupy the stable sites in the buffer layer during the in-situ annealing and, consequently, grains with a regular shape were grown in sample B, as shown Fig. 3(b). In the case of sample C, more parallel (0002) planes with reduced tilting along the c-axis and better crystalline quality were observed. A SiO2 layer, which formed between the ZnO films and Si substrate due to the oxidation of the Si substrate or diffusion of Si into the ZnO film, was observed in all three samples. A Previous study reported that the thickness of the SiO2 layer increased with increasing annealing time and relatively high annealing temperature in oxygen ambient [14,15]. However, in our experiment, the thickness of the SiO2 layer was decreased in sample C (annealed in argon ambient), which showed enhanced ZnO crystalline properties. It means that there is a correlation between the thickness of the SiO2 layer and the crystalline properties of the ZnO. That is, in the case of sample C, it would be expected that the Si substrate exerts a stronger influence on the c-axis preferred orientation of the ZnO film, due to the reduced distance between the film and substrate. EDX was used to investigate the depth profile of the elemental composition from the ZnO/Si interface to the surface. As shown in Fig. 4(a), the atomic concentration of Zn decreases gradually from the SiO2 interface to the surface for sample A (without a buffer layer), implying the presence of excess Zn in the buffer region. However, there is little variation of the Zn content in sample C, indicating that the stoichiometric composition was obtained throughout the ZnO films. The reduced thickness of SiO2 observed in sample C can be ascribed to the diffusion of the oxygen atoms from the SiO2 layer into the ZnO layer. That is, the departure of the buffer layer from the stoichiometric composition toward a predominance of Zn led to the diffusion of oxygen atoms from SiO2 to the ZnO films to form a stoichiometric ZnO layer during the in-situ annealing. In the case of sample B, oxygen atoms diffused into the ZnO film from the surface as well as the SiO2 layer, due to the thermal treatment in oxygen ambient. Therefore, the thickness of the SiO2 layer in sample B was not different from that in sample A. These results indicate that the in-situ annealing in argon ambient reduced the excess Zn atoms in the buffer layer, and led to the enhancement of the crystalline characteristics and stoichiometry of the subsequently grown ZnO films.
Fig. 6. Room temperature PL spectra for samples A, B, and C. The inset shows the integrated UV emission intensity of samples A, B, and C.
In order to investigate the crystalline properties of the thermally treated buffer layer, 30 nm-thick buffer layers were grown and annealed in oxygen and argon ambient, respectively. HRTEM analysis was performed and the results are shown in Fig. 5. While the buffer layer annealed in oxygen ambient shows relatively poor crystalline characteristics with a thick SiO2 layer (3.7 nm), the use of the buffer layer annealed in argon ambient [Fig. 5(b)] resulted in excellent crystallinity with a reduced SiO2 layer thickness (1.8 nm). This provides evidence that the diffusion of oxygen atoms from the SiO2 layer leads to the stoichiometric composition of the buffer layer, resulting in better crystalline quality. Furthermore, the crystalline characteristics of the buffer layer are consistent with those of the subsequently grown ZnO films according to the XRD results [Fig. 1] and TEM analysis [Fig. 5]. Therefore, we found that the effect of the low-temperature buffer layer was not confined to the adjacent region, but extended into the whole ZnO film. Fig. 6 shows the room temperature PL spectra for samples A, B, and C. While both the insertion of the low-temperature buffer layer and the in-situ annealing process using oxygen ambient (Sample B) leads to better crystalline quality, the UV emission efficiency does not improve significantly. However, the thermal treatment in argon ambient resulted in the enhancement of the integrated UV emission intensity, which indicated better emission characteristics of the exciton emission, as shown in Fig. 6. Therefore, the use of argon as the ambient gas simultaneously improves the structural and optical properties of the film required for the fabrication of UV optical devices. Consequently, the growth of the ZnO thin film utilizing argon as the ambient gas during the thermal treatment improved the structural properties of the buffer layer, thereby modifying the crystallinity of the ZnO films. In addition, all the samples grown recently in our group exhibit the same trends. 4. Summary
Fig. 5. High-resolution TEM images of 30 nm-thick buffer layers thermally treated with different ambient gases: (a) oxygen ambient; (b) argon ambient.
In order to produce ZnO films with improved crystallinity, low-temperature ZnO buffer layers were grown as seed crystals. The effects of different ambient gases during the thermal treatments on the crystalline properties of the ZnO film were
D.J. Park et al. / Thin Solid Films 515 (2007) 6721–6725
investigated, and a large improvement in the quality of the ZnO films was achieved when argon was used as the ambient gas during the in-situ annealing. The results of XRD and TEM indicated that the tilt of the columnar structures was related to the degradation of the crystalline quality in the ZnO films. The reduced thickness of the oxide at the interface, which is associated with enhanced crystalline properties, was observed in the ZnO films grown on the buffer layer annealed in argon ambient, due to the diffusion of oxygen atoms during the thermal treatment. Acknowledgments This work was supported by Grant No. R-11–2000–086– 0000–0 from the Center of Excellent Program of the Korea Science and Engineering Foundation and Ministry of Science and Technology and supported by Grant No. R01–2006–000– 10027–0 from the Basic Research Program of the Korea Science & Engineering Foundation. References [1] D.G. Thomas, J. Phys. Chem. Solids 15 (1960) 86. [2] K. Iwata, P. Fons, S. Niki, A. Yamada, K. Matsubara, K. Nakahara, T. Tanabe, H. Takasu, J. Cryst. Growth 214/215 (2000) 50.
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[3] H. Kim, J.S. Horwitz, S.B. Qadri, D.B. Chrisey, Thin Solid Films 420 (2002) 107. [4] A. Yamada, B. Sang, M. Konagi, Appl. Surf. Sci. 112 (1997) 216. [5] B.S. Li, Y.C. Liu, D.Z. Shen, J.Y. Zhang, Y.M. Lu, X.Q. Fan, J. Cryst. Growth 249 (2003) 179. [6] J. Ye, S. Gu, S. Zhu, T. Chen, L. Hu, F. Qin, R. Zhang, Y. Shi, Y. Zheng, J. Cryst. Growth 243 (2002) 151. [7] R. Ondo-Ndong, F. Pascal-Delannoy, A. Boyer, A. Giani, A. Foucaran, Mater. Sci. Eng., B, Solid-State Mater. Adv. Technol. 97 (2003) 68. [8] Ü. Özgür, Ya.I. Alivov, C. Liu, A. Teke, M.A. Reshchikov, S. Doğan, V. Avrutin, S.-J. Cho, H. Morkoç, J. Appl. Phys. 98 (2005) 041301. [9] F. Schulze, A. Dadgar, J. Blasing, A. Krost, Appl. Phys. Lett. 84 (2004) 4747. [10] S.H. Jeong, I.S. Kim, J.K. Kim, B.T. Lee, J. Cryst. Growth 264 (2004) 327. [11] J.F. Yan, Y.M. Lu, Y.C. Liu, H.W. Liang, B.H. Li, D.Z. Shen, J.Y. Zhang, X.W. Fan, J. Cryst. Growth 266 (2004) 505. [12] Y.S. Jung, O. Kononenko, J.S. Kim, W.K. Choi, J. Cryst. Growth 274 (2005) 418. [13] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd edition, Prentice Hall, New Jersey, 2001, p. 170. [14] A. Miyake, H. Kominami, T. Aoki, N. Azuma, Y. Nakanish, Y. hatanaka, Phys. Status Solidi, B Basic Res. 229 (2002) 829. [15] M.N. Jung, J.Y. Lee, S.W. Park, H.S. Kim, J. Chang, J. Cryst. Growth 283 (2005) 384.
Improved microstructural properties of a ZnO thin film using a buffer layer in-situ annealed in argon ambient Dong Jun Park a , Jeong Yong Lee a , Tae Eun Park b , Young Yi Kim c , Hyung Koun Cho c,⁎ a
Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 373–1 Guseong-dong, Yuseong-gu, Daejeon 305–701, South Korea b Department of Materials Science and Engineering, Dong-A University, 840 Hadan-dong, Saha-gu, Busan, 604–714, South Korea c School of Advanced Materials Science and Engineering, Sungkyunkwan University, 300 Cheoncheon-dong, Jangan-gu, Suwon, Gyeonggi-do, 440–746, South Korea Received 7 February 2006; received in revised form 18 December 2006; accepted 31 January 2007 Available online 7 February 2007
Abstract ZnO films with improved crystallinity were grown on a Si (111) substrate by a two-step growth process using low-temperature ZnO buffer layers. The effect of the ambient gas during the temperature elevation and the in-situ thermal annealing after the growth of the low-temperature buffer layers on the optical and structural properties of the films was investigated by X-ray diffraction (XRD), photoluminescence, and transmission electron microscopy. The use of argon as the ambient gas during the thermal treatment of the buffer layer leads to the enhancement of the (0002) diffraction peak intensity at 2θ ∼ 34.4° and the reduction of the full width at half maximum value in the XRD rocking curve, which means that well-defined and c-axis oriented ZnO film was obtained. The relationship between the thickness of the SiO2 layer between the ZnO buffer layers and Si substrates and the structural and optical properties of the ZnO films is discussed. © 2007 Elsevier B.V. All rights reserved. PACS: 68.37.Lp; 68.55.Jk; 61.72.Nn; 61.43.Bn Keywords: ZnO films; Buffer layer; RF sputter; Si substrate; TEM
1. Introduction Zinc oxide (ZnO) with a wurtzite structure has many useful properties, such as a wide band gap (3.37 eV) and a high exciton binding energy (∼ 60 meV), which give it a more efficient exciton emission at room temperature compared with other wide-band-gap materials, such as GaN (24 meV). For this reason, the use of ZnO for ultraviolet optoelectronic devices has spurred considerable interest [1]. There are many different methods that can be used to produce ZnO films, such as molecular beam epitaxy [2], pulsed laser deposition [3], atomic layer deposition [4], plasma enhanced chemical vapor deposition [5], metal–organic chemical vapor deposition [6], and magnetron sputtering [7]. Among these, magnetron sputtering is the preferred method because it ⁎ Corresponding author. Tel.: +82 31 290 7364; fax: +82 31 290 7410. E-mail address: [email protected] (H.K. Cho). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.01.047
can be conducted at a temperature lower than the other methods [8]. Among the various substrate materials for ZnO, Si is a particularly promising candidate. The use of a Si substrate to deposit high quality ZnO films not only allows for the effective integration of optoelectronic devices with Si IC technology, but also represents a material which is cheaper and easier to cleave than other materials, such as sapphire. The commercialization of ZnO, however, has been derailed by a number of problems, such as the deterioration of the crystalline quality, due to the prevalence of grain boundaries owing to the formation of polycrystalline films with a preferred orientation along the c-axis, as well as the difficulty in producing p-type ZnO films. The two-step growth technique using the low-temperature deposition of a thin buffer layer prior to the ZnO film growth is often employed for the growth of highly mismatched heteroepitaxy layers like GaN-based materials [9]. In the two-step growth process, the crystalline properties of the ZnO films that
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Table 1 The growth conditions of the samples used in this experiment Sample
Thickness (Å)
Tg (°C)
PL FWHM (meV)
XRD FWHM (°)
RMS (nm)
Annealing condition
A B
3000 300 2700 300 2700
600 400 600 400 600
103 146
5.1 3.1
15.1 21.0
None O2,15 min
101
2.5
13.4
Ar,15 min
C
grow on the thin buffer layer depend to a large extent on the initially grown layer. However, most of the previous works on the use of low-temperature buffer layers for the growth of highquality ZnO were mainly focused on the growth temperature and thickness using oxygen ambient gas [10–12]. In this paper, argon was used as the ambient gas instead of oxygen during the thermal treatments, and its effect on the structural and optical properties of the ZnO film was investigated. 2. Experimental procedure ZnO thin films were deposited on Si (111) substrates using a radio frequency (RF) magnetron-sputtering system. The Si substrates were cleaned with a solution of trichloroethylene, acetone, and methanol using ultrasonor. They were then rinsed with de-ionized water. A sintered ZnO (99.999%, 4-in.) target was used for the growth of the ZnO films. The base pressure of the chamber was 0.667 × 10− 4 Pa, and the working pressure with both oxygen and argon gases was 0.267 Pa. Prior to the ZnO growth, pre-sputtering was conducted with an RF power of 50 W for 30 min. It is well known that the use of ZnO buffer layers grown at a low temperature and of the in-situ annealing process conducted at a high temperature improves the crystallinity and emission properties of ZnO films. To relieve the large-misfit strain between the Si substrates and the ZnO films, low-temperature
Fig. 1. X-ray diffraction patterns of ZnO films grown on buffer layer annealed in different ambient gases.
ZnO buffer layers (400 °C) were grown as seed crystals, followed by the growth of ZnO films at 600 °C. Pure oxygen with high purity was used as the ambient gas for the growth of the ZnO buffer layers and films. The growth temperature was increased from 400 °C to 600 °C after the growth of the ZnO buffer layers in order to subject them to thermal treatment at 600 °C prior to the growth of the ZnO films. Most of the previous works on the ZnO films grown by magnetron sputtering and the thermal treatment of ZnO were conducted in an oxygen atmosphere for the purpose of suppressing the oxygen vacancies [10–12]. To investigate the effect of the ambient gas, argon and oxygen were used respectively during the ramping-up process from 400 °C to 600 °C and during the in-situ annealing process for 15 min at 600 °C. The detailed growth conditions of the samples used in this experiment are shown in Table 1. The growth was interrupted during the thermal treatment by extinguishing the plasma between the buffer layer and the ZnO film. X-ray diffraction (XRD) (RIGAKU, D/MAX-RC) analysis was carried out to evaluate the structural properties of the samples. A monochromatic X-ray beam with Cu Kα(λ = 1.5418 Å) radiation was used for the XRD analysis. The transmission electron microscopy (TEM) samples were mechanically polished and completed through ion milling at 3.5 kV using Ar+ ions. A TEM (JEOL, JEM-3010 operated at 300 KV) equipped with energy-dispersive X-ray (EDX)
Fig. 2. The cross-sectional TEM images obtained from (a) sample A, (b) sample B, and (c) sample C to compare the grain size, and the corresponding diffraction patterns of ZnO/Si shown on the right.
D.J. Park et al. / Thin Solid Films 515 (2007) 6721–6725
Fig. 3. HRTEM images obtained from the ZnO/Si interfaces of (a) sample A, (b) sample B, and (c) sample C.
spectrometer with a beam broadening size of less than 10 nm was used to characterize the structures and to determine the chemical composition of the ZnO thin films. The root-mean-square (RMS) surface roughness of the ZnO films was measured by atomic force microscopy (AFM). The photoluminescence (PL) was measured under excitation with a 10 mW He–Cd laser operated at 325 nm.
affects that of the subsequently deposited high-temperature ZnO film by acting as a template and, as such, a dense ZnO film with a smooth surface can be grown. The grain size could be evaluated using the Scherrer formula from the measured full width at half maximum (FWHM) values of the (0002) XRD rocking curve, in which the broadening of the FWHM values indicated the small grain size [13]. However, the grain size of sample C, which showed a reduced FWHM of the XRD rocking curve (indicated in Table 1), is smaller than those of samples A and B, as shown Fig. 2. The smaller grain size and stronger (0002) diffraction peak of sample C indicate that the (0002) planes within each grain in sample C are stacked parallel to those within other grains, and that the grains are mainly rotated along the c-axis. This was confirmed by the diffraction pattern obtained from sample C using TEM, showing the small tilting angle of the 0002 diffraction spot compared to those of samples A and B, as shown in Fig. 2. These results are consistent with the XRD results, indicating that the tilt of the columnar structures is related to the degradation of the crystalline quality in the ZnO films. To analyze the microstructural properties of the films, such as the stacking and rotation of the (0002) planes, high-resolution TEM (HRTEM) images were obtained from sample A (without a buffer layer), sample B (with a buffer layer annealed in oxygen ambient), and sample C (with a buffer layer annealed in argon ambient), as shown in Fig. 3, with the incident electron beam parallel to the [112¯0]ZnO direction. The image of sample A in Fig. 3(a) shows grains that are irregularly shaped and that are ¯00] direction, near the ZnO/Si interface tilted towards the [11 region, while in the case of sample B, a regular shaped columnar
3. Results and discussion To investigate the crystalline quality of the ZnO films deposited on the buffer layers annealed under different ambient gases, XRD measurement was conducted. A preferred orientation along the c-axis perpendicular to the substrate surface was clearly observed in all of the samples, as shown in Fig. 1. A higher intensity of the (0002) diffraction peak was detected in samples B and C, in which in-situ annealing of the buffer layers was conducted. This reveals that the two-step growth technique using the low-temperature buffer layers prior to the ZnO film growth improves the crystalline structure and enhances the caxis preferred orientation of the ZnO films, regardless of the ambient gas type. The strongest (0002) diffraction peak appeared in sample C, which implies that argon is more suitable as an ambient gas than oxygen during the thermal treatment. Fig. 2 (a), (b), and (c) shows the cross-sectional bright-field TEM images of samples A, B, and C, respectively. All these samples showed c-axis oriented columnar structures. While the surface morphology of samples A and B was a little bumpy [Fig. 2(a) and (b)], a flat and smooth surface was obtained in sample C, whose buffer layer was annealed in argon ambient [Fig. 2(c)]. These results are in good agreement with the data obtained by atomic force microscopy, as indicated in Table 1. This change in the surface morphology means that the enhancement of the crystalline quality of the buffer layer
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Fig. 4. EDX data of (a) sample A and (b) sample C.
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structure is observed. It could be considered that the zinc or oxygen atoms have sufficient atomic energy to occupy the stable sites in the buffer layer during the in-situ annealing and, consequently, grains with a regular shape were grown in sample B, as shown Fig. 3(b). In the case of sample C, more parallel (0002) planes with reduced tilting along the c-axis and better crystalline quality were observed. A SiO2 layer, which formed between the ZnO films and Si substrate due to the oxidation of the Si substrate or diffusion of Si into the ZnO film, was observed in all three samples. A Previous study reported that the thickness of the SiO2 layer increased with increasing annealing time and relatively high annealing temperature in oxygen ambient [14,15]. However, in our experiment, the thickness of the SiO2 layer was decreased in sample C (annealed in argon ambient), which showed enhanced ZnO crystalline properties. It means that there is a correlation between the thickness of the SiO2 layer and the crystalline properties of the ZnO. That is, in the case of sample C, it would be expected that the Si substrate exerts a stronger influence on the c-axis preferred orientation of the ZnO film, due to the reduced distance between the film and substrate. EDX was used to investigate the depth profile of the elemental composition from the ZnO/Si interface to the surface. As shown in Fig. 4(a), the atomic concentration of Zn decreases gradually from the SiO2 interface to the surface for sample A (without a buffer layer), implying the presence of excess Zn in the buffer region. However, there is little variation of the Zn content in sample C, indicating that the stoichiometric composition was obtained throughout the ZnO films. The reduced thickness of SiO2 observed in sample C can be ascribed to the diffusion of the oxygen atoms from the SiO2 layer into the ZnO layer. That is, the departure of the buffer layer from the stoichiometric composition toward a predominance of Zn led to the diffusion of oxygen atoms from SiO2 to the ZnO films to form a stoichiometric ZnO layer during the in-situ annealing. In the case of sample B, oxygen atoms diffused into the ZnO film from the surface as well as the SiO2 layer, due to the thermal treatment in oxygen ambient. Therefore, the thickness of the SiO2 layer in sample B was not different from that in sample A. These results indicate that the in-situ annealing in argon ambient reduced the excess Zn atoms in the buffer layer, and led to the enhancement of the crystalline characteristics and stoichiometry of the subsequently grown ZnO films.
Fig. 6. Room temperature PL spectra for samples A, B, and C. The inset shows the integrated UV emission intensity of samples A, B, and C.
In order to investigate the crystalline properties of the thermally treated buffer layer, 30 nm-thick buffer layers were grown and annealed in oxygen and argon ambient, respectively. HRTEM analysis was performed and the results are shown in Fig. 5. While the buffer layer annealed in oxygen ambient shows relatively poor crystalline characteristics with a thick SiO2 layer (3.7 nm), the use of the buffer layer annealed in argon ambient [Fig. 5(b)] resulted in excellent crystallinity with a reduced SiO2 layer thickness (1.8 nm). This provides evidence that the diffusion of oxygen atoms from the SiO2 layer leads to the stoichiometric composition of the buffer layer, resulting in better crystalline quality. Furthermore, the crystalline characteristics of the buffer layer are consistent with those of the subsequently grown ZnO films according to the XRD results [Fig. 1] and TEM analysis [Fig. 5]. Therefore, we found that the effect of the low-temperature buffer layer was not confined to the adjacent region, but extended into the whole ZnO film. Fig. 6 shows the room temperature PL spectra for samples A, B, and C. While both the insertion of the low-temperature buffer layer and the in-situ annealing process using oxygen ambient (Sample B) leads to better crystalline quality, the UV emission efficiency does not improve significantly. However, the thermal treatment in argon ambient resulted in the enhancement of the integrated UV emission intensity, which indicated better emission characteristics of the exciton emission, as shown in Fig. 6. Therefore, the use of argon as the ambient gas simultaneously improves the structural and optical properties of the film required for the fabrication of UV optical devices. Consequently, the growth of the ZnO thin film utilizing argon as the ambient gas during the thermal treatment improved the structural properties of the buffer layer, thereby modifying the crystallinity of the ZnO films. In addition, all the samples grown recently in our group exhibit the same trends. 4. Summary
Fig. 5. High-resolution TEM images of 30 nm-thick buffer layers thermally treated with different ambient gases: (a) oxygen ambient; (b) argon ambient.
In order to produce ZnO films with improved crystallinity, low-temperature ZnO buffer layers were grown as seed crystals. The effects of different ambient gases during the thermal treatments on the crystalline properties of the ZnO film were
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investigated, and a large improvement in the quality of the ZnO films was achieved when argon was used as the ambient gas during the in-situ annealing. The results of XRD and TEM indicated that the tilt of the columnar structures was related to the degradation of the crystalline quality in the ZnO films. The reduced thickness of the oxide at the interface, which is associated with enhanced crystalline properties, was observed in the ZnO films grown on the buffer layer annealed in argon ambient, due to the diffusion of oxygen atoms during the thermal treatment. Acknowledgments This work was supported by Grant No. R-11–2000–086– 0000–0 from the Center of Excellent Program of the Korea Science and Engineering Foundation and Ministry of Science and Technology and supported by Grant No. R01–2006–000– 10027–0 from the Basic Research Program of the Korea Science & Engineering Foundation. References [1] D.G. Thomas, J. Phys. Chem. Solids 15 (1960) 86. [2] K. Iwata, P. Fons, S. Niki, A. Yamada, K. Matsubara, K. Nakahara, T. Tanabe, H. Takasu, J. Cryst. Growth 214/215 (2000) 50.
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