- Email: [email protected]
Low temperature preparation of ZnO by a nearby vaporizing chemical vapor deposition method
ARTICLE IN PRESS
Journal of Crystal Growth 268 (2004) 174–177
Low temperature preparation of ZnO by a nearby vaporizing chemical vapor deposition method Junichi Nishino*, Yoshio Nosaka Department of Chemistry, Nagaoka University of Technology, 1603-1 Kamitomioka-machi, Nagaoka-shi, Niigata 940-2188, Japan Received 28 January 2004; accepted 1 May 2004
Communicated by D.P. Norton
Abstract ZnO films were prepared by a nearby vaporizing chemical vapor deposition (CVD) method where the substrate locates over the surface of source material, bis(2,4-pentanedionato)zinc, with the separation distance of 2.5 mm. Crystalline ZnO was easily prepared by this CVD method at low substrate temperatures ranging from 100 C to 300 C. The highest preferred orientation to the c-axis was obtained at the substrate temperature of 225 C. r 2004 Elsevier B.V. All rights reserved. PACS: 52.75.R; 81.15.G; 81.15; 81.10 Keywords: A1. Crystal structure; A1. X-ray diffraction; A3. Chemical vapor deposition; B1. Oxides; B1. Zinc compounds; B2. Semiconducting II–VI materials
1. Introduction Zinc oxide has been utilized widely in the field of scientific and technological applications. For example, highly oriented ZnO films are exploited as piezoelectric devices in acoustic range, and as optical waveguides and laser oscillation devices in photonic range [1]. Although ZnO film is one of the important materials, there exist few chemical *Corresponding author. Tel.: +81-258479314; fax: +81258479300. E-mail address: [email protected] (J. Nishino).
vapor deposition (CVD) sources. Alkyl metal compounds such as dimethyl zinc (DMZ) and diethyl zinc (DEZ) [2–6] are only materials for conventional low temperature CVD processes, but these compounds have a difficulty in treatment owing to their ignitable properties. bis(2,4-pentanedionato)zinc was investigated previously by the present authors as a promising candidate for the CVD source material without ignitability [7]. However, a high substrate temperature of ca. 550 C is necessary for this compound to fabricate ZnO films of c-axis preferred orientation by a conventional atmospheric CVD apparatus. Recently, we have reported a new modification of a
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.05.006
ARTICLE IN PRESS J. Nishino, Y. Nosaka / Journal of Crystal Growth 268 (2004) 174–177
CVD method [8], in which the vaporization of bis(2,4-pentanedionato)zinc takes place by radiant heat from the substrate. When, the separation between substrate to the source material (D) was 5.0 mm, the substrate temperature to obtain ZnO crystalline ranged from 300 C to 600 C. This deposition technique named nearby vaporizing CVD (NV-CVD) brings several advantages: low cost, energy saving, and easy control with few experimental parameters. Since no carrier gas is used, the present experimental setup is regarded as the simplest CVD apparatus in the world. The lower substrate temperature has an advantage to select a wide range of substrate such as plastic films. The present report concerns the formation of ZnO by this CVD method, where the substrate temperature could be lower down to 100 C by closing the separation (D) to be 2.5 mm.
2. Experimental procedure The schematic diagram of the apparatus of the NV-CVD in atmospheric pressure is shown in Fig. 1. Table 1 is a list of the preparation conditions. The substrate was heated by a hot Al susceptor with a resistance cartridge heater. We have added ceramics heat-insulators to improve the heating efficiency. The substrate temperature was sensed by a thermocouple inside the Al susceptor. A source material, bis(2,4-pentandionato)zinc
Fig. 1. Schematic view of NV-CVD apparatus.
175
Table 1 Preparation conditions Source material Substrate Dish of source material Substrate temperature (Ts ) Distance between the substrate surface and the surface of the source material (D)
bis(2,4-Pentanedionato)zinc [Zn(C5H7O2)2] Fused quartz Ceramic 100–300 C 2.5 mm
[Zn(C5H7O2)2], was synthesized according to the literature [9]. To control the intensity of radiant heat from the nearby substrate, the distance between the substrate and the surface of source material (D) was adjusted by using a micrometer. The surface morphology of the film was observed by using a scanning electron microscope (SEM; JEOL JSM-5310LVB). The structure of the films was characterized by X-ray diffraction (XRD). The absorbance of the zinc oxide deposited was measured with a Shimadzu UV-3150 spectrophotometer at room temperature in the wavelength ranging from 190 to 800 nm.
3. Results and discussion Fig. 2 shows the vaporization rate of Zn(C5H7O2)2 and the deposition rate of the ZnO films as a function of the substrate temperature (Ts ) with the separation (D) of 2.5 mm. The deposition rate increased almost exponentially from 0.0087 to 0.37 nm s1 with increasing the substrate temperature from 100 C to 300 C. This exponential dependence of the deposition rate on the temperature can be explained by the exponential dependence of vapor pressure as demonstrated in Fig. 2 by the vaporization rate from 4.7 109 to 1.1 106 mol s1 with the increase of the substrate temperature from 100 C to 300 C. Fig. 3 shows the XRD patterns of ZnO films deposited at various substrate temperatures (Ts ) ranging from 100 C to 300 C. The orientation preferred to the c-axis of ZnO was obtained in the temperature range of 100–300 C, because zinc oxide shows a tendency in nature to be
ARTICLE IN PRESS 176
J. Nishino, Y. Nosaka / Journal of Crystal Growth 268 (2004) 174–177
Fig. 2. Dependence of substrate temperature on the vaporization rate of Zn(C5H7O2)2 and the deposition rate of ZnO films.
Fig. 3. XRD patterns of ZnO films deposited at various substrate temperatures (Ts ) ranging from 100 C to 300 C.
preferentially oriented in the c-axis. It is noticeable that even at Ts ¼ 100 C the growth of c-axis oriented ZnO on fused quartz substrate is obtainable by using bis(2,4-pentandionato)zinc as a source material. The explanation of the mechanism of the ZnO growth in such a low substrate temperature can be explained that the source material reacts with the water vapor in air, but the detail of the reaction is unclear at this moment and further study is underway to elucidate the mechanism. Fig. 4 shows the crystallite size calculated from XRD at 002 peak of ZnO for the films fabricated
Fig. 4. Relationship between the substrate temperature and the crystallite size of ZnO.
on a fused quartz plate at various substrate temperatures. The crystallite size increased from about 25 to 43 nm as substrate temperature (Ts ) increased from 100 C to 225 C and decreased at Ts above 225 C. These changes in the crystallite size correspond with that of the crystallinity of the ZnO observed as the peak intensity in the XRD patterns. At low temperature below Ts =225 C, the low crystallinity, which results in small crystallite size, may be explained by the shortage of thermal energy for crystallization. Above Ts =225 C, when the separation distance (D) became large, the crystallinity increased. Then the decrease of the crystallite size observed at the higher temperature in Fig. 4 likely originates from the luck of oxygen gas diffused into the space between the source surface and the substrate. Fig. 5 shows the relationship between the substrate temperature and full-width at halfmaximum [FWHM (Dy50 )] of the rocking curves for 002 peaks of ZnO films. The highest preferred orientation to the c-axis was obtained under the condition that Ts was 225 C. This optimal temperature is lower by 325 C than that obtained in the previous report [7] where the conventional CVD method was used. The characteristics of the present CVD method that needs no carrier gas can lower the process temperature for c-axis preferred orientation. Fig. 6 shows the surface morphology of the ZnO films prepared on the fused quartz substrate at
ARTICLE IN PRESS J. Nishino, Y. Nosaka / Journal of Crystal Growth 268 (2004) 174–177
Fig. 5. Relationship between the substrate temperature and FWHM (Dy50 ) of the rocking curves for 002 peaks of ZnO films.
177
Fig. 7. Absorption spectra of ZnO films on fused quartz substrate obtained at Ts ¼ 100 C and 225 C.
4. Conclusions In conclusion, by using a newly developed nearby vaporizing CVD method, we demonstrated the growth of ZnO on a fused quartz substrate at low temperatures of 100–300 C by employing bis(2,4-pentanedionate)zinc as a source material. The highest preferred orientation to the c-axis and the best crystallinity were obtained under the condition that the distance between substrate and source surface (D) was 2.5 mm and the substrate temperature (Ts ) was 225 C. It is noticeable that the growth of ZnO on a fused quartz substrate can be obtained at an extremely low substrate temperature of 100 C. Fig. 6. Surface morphology of the ZnO films prepared on the fused quartz substrate at Ts ¼ 150 C, 200 C, and 250 C.
References
various temperatures. The surface of the film consisted of unsharped small grains at 100 C and sharped fine grain at 150 C. When the substrate temperature was 250 C, the big grain with a corner angle of 120 was composed by the primary grains of about 0.3 mm. Fig. 7 shows the absorption spectra of the films deposited on fused quartz substrate at Ts =100 C and 225 C. The formation of ZnO semiconductor can also be confirmed by the sharp increase in the absorbance at 380 nm.
[1] M. Kawasaki, A. Ohtomo, I. Ohkubo, H. Koinuma, Z.K. Tang, P. Yu, G.K.L. Wong, B.P. Zhang, Y. Segawa, Mater. Sci. Eng. B56 (1998) 239. [2] P. Souletie, B.W. Wessels, J. Mater. Res. 3 (1988) 740. [3] A.P. Roth, D.F. Williams, J. Appl. Phys. 52 (1981) 6685. [4] A.P. Roth, D.F. Williams, J. Electrochem. Soc. 128 (1981) 2684. [5] W.W. Wenas, A. Yamada, M. Konagai, K. Takahashi, Jpn. J. Appl. Phys. 30 (1991) L441. [6] Z. Fu, B. Lin, J. Zu, Thin Solid Films 402 (2002) 302. [7] K. Kamata, J. Nishino, S. Ohshio, K. Maruyama, M. Ohtuki, J. Am. Ceram. Soc. 77 (1994) 505. [8] J. Nishino, Y. Nosaka, J. Mater. Res. 18 (2003) 2029. [9] G. Rudolph, M.C. Henry, Inorg. Synth. X (1967) 74.
Journal of Crystal Growth 268 (2004) 174–177
Low temperature preparation of ZnO by a nearby vaporizing chemical vapor deposition method Junichi Nishino*, Yoshio Nosaka Department of Chemistry, Nagaoka University of Technology, 1603-1 Kamitomioka-machi, Nagaoka-shi, Niigata 940-2188, Japan Received 28 January 2004; accepted 1 May 2004
Communicated by D.P. Norton
Abstract ZnO films were prepared by a nearby vaporizing chemical vapor deposition (CVD) method where the substrate locates over the surface of source material, bis(2,4-pentanedionato)zinc, with the separation distance of 2.5 mm. Crystalline ZnO was easily prepared by this CVD method at low substrate temperatures ranging from 100 C to 300 C. The highest preferred orientation to the c-axis was obtained at the substrate temperature of 225 C. r 2004 Elsevier B.V. All rights reserved. PACS: 52.75.R; 81.15.G; 81.15; 81.10 Keywords: A1. Crystal structure; A1. X-ray diffraction; A3. Chemical vapor deposition; B1. Oxides; B1. Zinc compounds; B2. Semiconducting II–VI materials
1. Introduction Zinc oxide has been utilized widely in the field of scientific and technological applications. For example, highly oriented ZnO films are exploited as piezoelectric devices in acoustic range, and as optical waveguides and laser oscillation devices in photonic range [1]. Although ZnO film is one of the important materials, there exist few chemical *Corresponding author. Tel.: +81-258479314; fax: +81258479300. E-mail address: [email protected] (J. Nishino).
vapor deposition (CVD) sources. Alkyl metal compounds such as dimethyl zinc (DMZ) and diethyl zinc (DEZ) [2–6] are only materials for conventional low temperature CVD processes, but these compounds have a difficulty in treatment owing to their ignitable properties. bis(2,4-pentanedionato)zinc was investigated previously by the present authors as a promising candidate for the CVD source material without ignitability [7]. However, a high substrate temperature of ca. 550 C is necessary for this compound to fabricate ZnO films of c-axis preferred orientation by a conventional atmospheric CVD apparatus. Recently, we have reported a new modification of a
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.05.006
ARTICLE IN PRESS J. Nishino, Y. Nosaka / Journal of Crystal Growth 268 (2004) 174–177
CVD method [8], in which the vaporization of bis(2,4-pentanedionato)zinc takes place by radiant heat from the substrate. When, the separation between substrate to the source material (D) was 5.0 mm, the substrate temperature to obtain ZnO crystalline ranged from 300 C to 600 C. This deposition technique named nearby vaporizing CVD (NV-CVD) brings several advantages: low cost, energy saving, and easy control with few experimental parameters. Since no carrier gas is used, the present experimental setup is regarded as the simplest CVD apparatus in the world. The lower substrate temperature has an advantage to select a wide range of substrate such as plastic films. The present report concerns the formation of ZnO by this CVD method, where the substrate temperature could be lower down to 100 C by closing the separation (D) to be 2.5 mm.
2. Experimental procedure The schematic diagram of the apparatus of the NV-CVD in atmospheric pressure is shown in Fig. 1. Table 1 is a list of the preparation conditions. The substrate was heated by a hot Al susceptor with a resistance cartridge heater. We have added ceramics heat-insulators to improve the heating efficiency. The substrate temperature was sensed by a thermocouple inside the Al susceptor. A source material, bis(2,4-pentandionato)zinc
Fig. 1. Schematic view of NV-CVD apparatus.
175
Table 1 Preparation conditions Source material Substrate Dish of source material Substrate temperature (Ts ) Distance between the substrate surface and the surface of the source material (D)
bis(2,4-Pentanedionato)zinc [Zn(C5H7O2)2] Fused quartz Ceramic 100–300 C 2.5 mm
[Zn(C5H7O2)2], was synthesized according to the literature [9]. To control the intensity of radiant heat from the nearby substrate, the distance between the substrate and the surface of source material (D) was adjusted by using a micrometer. The surface morphology of the film was observed by using a scanning electron microscope (SEM; JEOL JSM-5310LVB). The structure of the films was characterized by X-ray diffraction (XRD). The absorbance of the zinc oxide deposited was measured with a Shimadzu UV-3150 spectrophotometer at room temperature in the wavelength ranging from 190 to 800 nm.
3. Results and discussion Fig. 2 shows the vaporization rate of Zn(C5H7O2)2 and the deposition rate of the ZnO films as a function of the substrate temperature (Ts ) with the separation (D) of 2.5 mm. The deposition rate increased almost exponentially from 0.0087 to 0.37 nm s1 with increasing the substrate temperature from 100 C to 300 C. This exponential dependence of the deposition rate on the temperature can be explained by the exponential dependence of vapor pressure as demonstrated in Fig. 2 by the vaporization rate from 4.7 109 to 1.1 106 mol s1 with the increase of the substrate temperature from 100 C to 300 C. Fig. 3 shows the XRD patterns of ZnO films deposited at various substrate temperatures (Ts ) ranging from 100 C to 300 C. The orientation preferred to the c-axis of ZnO was obtained in the temperature range of 100–300 C, because zinc oxide shows a tendency in nature to be
ARTICLE IN PRESS 176
J. Nishino, Y. Nosaka / Journal of Crystal Growth 268 (2004) 174–177
Fig. 2. Dependence of substrate temperature on the vaporization rate of Zn(C5H7O2)2 and the deposition rate of ZnO films.
Fig. 3. XRD patterns of ZnO films deposited at various substrate temperatures (Ts ) ranging from 100 C to 300 C.
preferentially oriented in the c-axis. It is noticeable that even at Ts ¼ 100 C the growth of c-axis oriented ZnO on fused quartz substrate is obtainable by using bis(2,4-pentandionato)zinc as a source material. The explanation of the mechanism of the ZnO growth in such a low substrate temperature can be explained that the source material reacts with the water vapor in air, but the detail of the reaction is unclear at this moment and further study is underway to elucidate the mechanism. Fig. 4 shows the crystallite size calculated from XRD at 002 peak of ZnO for the films fabricated
Fig. 4. Relationship between the substrate temperature and the crystallite size of ZnO.
on a fused quartz plate at various substrate temperatures. The crystallite size increased from about 25 to 43 nm as substrate temperature (Ts ) increased from 100 C to 225 C and decreased at Ts above 225 C. These changes in the crystallite size correspond with that of the crystallinity of the ZnO observed as the peak intensity in the XRD patterns. At low temperature below Ts =225 C, the low crystallinity, which results in small crystallite size, may be explained by the shortage of thermal energy for crystallization. Above Ts =225 C, when the separation distance (D) became large, the crystallinity increased. Then the decrease of the crystallite size observed at the higher temperature in Fig. 4 likely originates from the luck of oxygen gas diffused into the space between the source surface and the substrate. Fig. 5 shows the relationship between the substrate temperature and full-width at halfmaximum [FWHM (Dy50 )] of the rocking curves for 002 peaks of ZnO films. The highest preferred orientation to the c-axis was obtained under the condition that Ts was 225 C. This optimal temperature is lower by 325 C than that obtained in the previous report [7] where the conventional CVD method was used. The characteristics of the present CVD method that needs no carrier gas can lower the process temperature for c-axis preferred orientation. Fig. 6 shows the surface morphology of the ZnO films prepared on the fused quartz substrate at
ARTICLE IN PRESS J. Nishino, Y. Nosaka / Journal of Crystal Growth 268 (2004) 174–177
Fig. 5. Relationship between the substrate temperature and FWHM (Dy50 ) of the rocking curves for 002 peaks of ZnO films.
177
Fig. 7. Absorption spectra of ZnO films on fused quartz substrate obtained at Ts ¼ 100 C and 225 C.
4. Conclusions In conclusion, by using a newly developed nearby vaporizing CVD method, we demonstrated the growth of ZnO on a fused quartz substrate at low temperatures of 100–300 C by employing bis(2,4-pentanedionate)zinc as a source material. The highest preferred orientation to the c-axis and the best crystallinity were obtained under the condition that the distance between substrate and source surface (D) was 2.5 mm and the substrate temperature (Ts ) was 225 C. It is noticeable that the growth of ZnO on a fused quartz substrate can be obtained at an extremely low substrate temperature of 100 C. Fig. 6. Surface morphology of the ZnO films prepared on the fused quartz substrate at Ts ¼ 150 C, 200 C, and 250 C.
References
various temperatures. The surface of the film consisted of unsharped small grains at 100 C and sharped fine grain at 150 C. When the substrate temperature was 250 C, the big grain with a corner angle of 120 was composed by the primary grains of about 0.3 mm. Fig. 7 shows the absorption spectra of the films deposited on fused quartz substrate at Ts =100 C and 225 C. The formation of ZnO semiconductor can also be confirmed by the sharp increase in the absorbance at 380 nm.
[1] M. Kawasaki, A. Ohtomo, I. Ohkubo, H. Koinuma, Z.K. Tang, P. Yu, G.K.L. Wong, B.P. Zhang, Y. Segawa, Mater. Sci. Eng. B56 (1998) 239. [2] P. Souletie, B.W. Wessels, J. Mater. Res. 3 (1988) 740. [3] A.P. Roth, D.F. Williams, J. Appl. Phys. 52 (1981) 6685. [4] A.P. Roth, D.F. Williams, J. Electrochem. Soc. 128 (1981) 2684. [5] W.W. Wenas, A. Yamada, M. Konagai, K. Takahashi, Jpn. J. Appl. Phys. 30 (1991) L441. [6] Z. Fu, B. Lin, J. Zu, Thin Solid Films 402 (2002) 302. [7] K. Kamata, J. Nishino, S. Ohshio, K. Maruyama, M. Ohtuki, J. Am. Ceram. Soc. 77 (1994) 505. [8] J. Nishino, Y. Nosaka, J. Mater. Res. 18 (2003) 2029. [9] G. Rudolph, M.C. Henry, Inorg. Synth. X (1967) 74.