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The growth and annealing of single crystalline ZnO films by low-pressure MOCVD
Journal of Crystal Growth 243 (2002) 151–156
The growth and annealing of single crystalline ZnO films by low-pressure MOCVD Jiandong Ye, Shulin Gu*, Shunmin Zhu, Tong Chen, Liqun Hu, Feng Qin, Rong Zhang, Yi Shi, Youdou Zheng Department of Physics, Nanjing University, Nanjing 210093, China Received 22 March 2002; accepted 17 May 2002 Communicated by H. Ohno
Abstract High-quality c-axis-oriented single-crystal ZnO films have been successfully grown on the (0 0 0 2) sapphire substrate by the low-pressure metal organic chemical vapor deposition technique. The effect of doping and annealing on the optical and structural properties has been investigated by means of X-ray diffraction (XRD), photoluminescence (PL) spectrum and atomic force microscopy (AFM). Annealing at high temperature was found to enhance the intensity of the (0 0 0 2) XRD peak and decrease the c-axis oriented lattice constant. However, the (0 0 0 2) XRD peak for the Ndoped sample shifted to a low degree due to tensile stress possibly caused by nitrogen doping. The green–yellow band emission was observed in the room temperature PL spectrum of the undoped sample while the blue band emission emerged in the PL spectrum of the N-doped one. Low-temperature PL spectrum of the ZnO films was dominated by a sharp bound exciton line. Possible causes to the above differences will be given and discussed. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.Jk Keywords: A1. Atomic force microscopy; A1. Photoluminescence spectrum; A1. X-ray diffraction; B1. Zinc compounds
1. Introduction Blue laser diode is expected to be useful especially for applications in high-density storage and display devices. So far, the most promising known materials for such devices are GaN and related compounds [1]. However, due to the lack of a native substrate for the nitride material system, the electronic properties of GaN have *Corresponding author. Fax: +86-25-3328130. E-mail address: [email protected] (S. Gu).
not met expectations and hampered the development of the detectors, high-power lasers and high-power electronic devices [2]. Recently, another wide band gap material ZnO is attracting much attention as a promising candidate for optoelectric applications in visible and ultraviolet (UV) regions. ZnO has a direct wide band gap of 3.37 eV at room temperature (RT) and wurtzite structure. A large excitonic binding energy (59 meV) and a small Bohr radius (1.8 nm) [3] permit excitonic recombination even at RT. Additionally, due to its high conductance, chemical and thermal stability,
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 4 7 4 - 4
152
J. Ye et al. / Journal of Crystal Growth 243 (2002) 151–156
and high piezoelectric coupling coefficient, ZnO is also used for piezoelectric devices, such as surface acoustic wave (SAW) devices [4] and bulk acoustic wave devices. Investigation has recently been made into their use as transparency conductive films for liquid-crystal devices [5]. However, the viability of any photonic or electronic device technology depends on developing deposition techniques. There are many reports concerning the growth of ZnO by several techniques, such as molecular beam epitaxy (MBE), radio frequency sputtering and metal organic chemical vapor deposition (MOCVD) [6–8] on different substrates. High-quality laser emission [9] was observed in the high-quality ZnO films by a combination of MBE and laser ablation. However, MOCVD is also greatly interesting for achieving devices in commercial level since high deposition rate and high-quality ZnO is possible to be attained, especially operated at low pressure. In this paper, low-pressure MOCVD (LPMOCVD) technique was employed to grow undoped and N-doped single-crystal ZnO films on (0 0 0 2) sapphire substrate. The crystallinity and the surface morphology of ZnO are investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The optical properties of the films have been studied via using photoluminescence (PL). The influence of annealing and the doping on the crystallinity and the optical properties of the samples will also be presented.
2. Experiment ZnO thin films have been grown in a horizontal cool wall reactor by the MOCVD system operated at low pressure (LP-MOCVD). Fig. 1 shows a schematic representation of the reaction chamber. The c-axis oriented films were grown on (0 0 0 2) Al2O3 substrates, using Zn(C2H5)2 as the Zn precursor, and CO2 as the oxygen source. For MOCVD growth of ZnO films, the gas phase reaction will result in particle formation, which will degrade ZnO film quality such as surface morphology and crystallinity. In order to minimize the gas phase reaction, Zn(C2H5)2 and CO2 are introduced into the reactor separately and mixed just above the sample surface. Zn(C2H5)2 bubbler was maintained at the temperature of 141C and at the pressure of 760 Torr. Stoichiometry of the ZnO films was controlled by the supply of oxygen via CO2 flow and the plasma ionization voltage. High-purity H2 (for sample A) or Ar (for sample C) was passed through the DEZn bubbler and saturated with DEZn vapor to the reactor. The growth temperature of sample A was 3601C and sample B was obtained by annealing sample A at 8001C under oxygen ambient for 30 min. In our previous experiments, no difference has been observed on the sample surface and properties when Ar is used to replace H2 as the carrier gas. The sample C was deposited at 3901C using NO as doping gas previously mixed with CO2 gas at the
Fig. 1. Schematic diagram of the low-pressure MOCVD system for ZnO growth.
J. Ye et al. / Journal of Crystal Growth 243 (2002) 151–156
concentration of 0.1%, and then also annealed at 9401C under oxygen ambient for 60 min. The crystallographic characteristics of the films were analyzed by XRD method using CuKa1 radiation (l=0.15405 nm). A Digital Instruments Nanoscope IIIa atomic force microscope was employed to measure the surface morphology of the ZnO films. All AFM images were taken in air using the contacting mode. Several areas were scanned on each sample to get a mean surface roughness for ZnO films. PL was carried out at the low temperature of 4 K and RT, excited by a He–Cd laser (l ¼ 325 nm) or Xe lamp source (excited at 280 nm). The wave number resolution was 1 nm for both cases.
3. Results and discussion Fig. 2 shows the XRD spectra of the samples. The peaks at 2y=34.361, 34.601 and 34.261 correspond to the diffraction from the (0 0 0 2) plane of ZnO films. These three samples all exhibit preferential orientation with the c-axis perpendicular to the substrate surface. Assuming a homogeneous strain across the films, the crystallite size may be estimated from the full-width at half-
maximum (FWHM) of the (0 0 0 2) diffraction peak by the Sherrer equation: D ¼ 0:94l=b cos y; where l (0.15405 nm) is the X-ray wavelength and b is the FWHM in radians. The c-axis lattice constant c can be obtained by the formula: 2d sin y ¼ nl and the biaxial stress was calculated from the measuring parameter c: s ¼ 453:6 109 ððc c0 Þ=c0 Þ[10], where c0 (0.5205 nm) [11] is the strain-free lattice constant. The calculated data were summarized in Table 1. For sample A, the c-axis lattice parameter is 0.5218 nm, larger than c0 and the grain size is small. The tensile force at about 1.13 109 Pa was caused by defects and the large lattice mismatch at about 15% between ZnO and sapphire. Nitrogen doping in the ZnO film was found to increase the tensile force to be about 2.24 109 Pa for the N-doped sample. After annealing, the c-axis lattice parameter of sample B is a slightly shorter than c0 and the compressive stress is induced because of the different thermal expansion coefficients of ZnO and sapphire [12]. Meanwhile, the improvement has been made on the FWHM of samples B and C, indicating the doped and annealed samples are of better quality. Fig. 3 shows the PL spectra of sample B recorded at the low temperature (4 K) and RT
34.26
(c)
25
153
30
35
40
45
50
40
45
50
40
45
50
(b)
CPS
34.6
25
30
35
(a) 34.36
25
30
35
2θ (degree) Fig. 2. XRDs for: (a) undoped ZnO (unannealed); (b) undoped ZnO (annealed) and (c) ZnO:N.
J. Ye et al. / Journal of Crystal Growth 243 (2002) 151–156
154
Table 1 Characteristics of the ZnO samples estimated from XRD patterns
3
FWHM ( 10 radian) Grain size (nm) c-lattice constant (nm) Stress ( 109 Pa)
(a) 4K
A
B
C
3.11 48.74 0.5218 1.13
2.89 52.48 0.5180 2.17
2.89 52.43 0.5233 2.24
(b) Room Temp.
0 D X
(b)
PL Intensity (a.u.)
Sample
(a)
360
400
440
480
520
360
400
440
480
520
Wavelength (nm)
Fig. 4. PL spectra of annealed ZnO films at RT. (a) Non-doped ZnO film; (b) N-doped ZnO film.
PL Intensity (a.u.)
DAP
370
350
400
450
500
380
550
600
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. PL spectra of ZnO films measured at 4 K and RT. The UV band emission lines are shown in the inset.
(300 K). At 4 K, the spontaneous emission spectrum is dominated by two peaks which are magnified and shown in the inset of Fig. 3(a). The peak at 370 nm, denoted by D0X [3,13], can be assigned to donor-bound exciton recombination and the peak at 377 nm was labeled as donor– acceptor pair (DAP) emission. In ZnO, many interstitial Zn atoms act as intrinsic native donor [3] and participate in DAP emission. Additionally, deep level emission is hardly observed in the PL spectra at low temperature. At RT, the boundexciton and the DAP emission change over to the free-exciton recombination shown in Fig. 3(b). The peak of UV emission broadened and decreased, and had a small Stokes shift. This is due to the excitons localized by the film in homogeneities caused by the strain fields that result from the large lattice mismatch [14] between ZnO and the sapphire. Meanwhile, the deep level emission (green–yellow band) is related to oxygen vacancies
or interstitial Zn atoms, which both form the donor levels [15]. Comparing Fig. 4(a) and (b), we can find the blue band emission was enhanced in intensity with the green–yellow emission vanished. This may be ascribed to the irradiative transition between shallow acceptor and the conducting band (CB). According to theoretical prediction, N would work as a shallow acceptor [16] in ZnO crystal. Whereas, the FWHM of the free-exciton emissions were about 20 and 17 nm for the undoped and doped ZnO films, respectively. The narrower FWHM implies that the N-doped ZnO annealed at high temperature is of better optical properties. AFM measurements were performed to study the differences on the surface morphology between samples A and B. The images are shown in Fig. 5 over a scale of 500 nm 500 nm. We can see that the film was deposited in a column-by-column growth process and the grain size became larger with the augments of annealing temperature. This agrees with the result shown in XRD. Meanwhile, the mean surface roughness is about 2.47 and 3.84 nm, respectively. This is related to the integration of the grains. Fujimura et al. [17] suggested that the surface energy density of the (0 0 0 2) orientation is the lowest in the ZnO crystal. At high temperature, the atoms have enough diffuse activation energy to occupy the correct site in the crystal lattice and grains with
J. Ye et al. / Journal of Crystal Growth 243 (2002) 151–156
155
Fig. 5. AFM images for non-doped ZnO films grown on sapphire substrate. (a) The unannealed sample and (b) the sample annealed at 8001C.
the lower surface energy will become larger at high temperature. Then the growth orientation develops into one crystallographic direction of the low surface energy, leading to the improvement of ZnO crystallinity.
4. Conclusion (1) High-quality single-crystal undoped and N-doped ZnO films have been successfully deposited on (0 0 0 2) sapphire substrate by LP-MOCVD. (2) From a comparative analysis of XRD and AFM results, annealing at high temperature under oxygen ambient can decrease the defects such as oxygen vacancies in the films and boost the integration of the grains. (3) The PL spectrum at 4 K was dominated by D0X and DAP emission lines. At RT, the green–yellow band emission was ascribed to the native defects such as oxygen vacancies while the blue band emission was caused by nitrogen dopants that act as the shallow acceptors in ZnO films. The narrow UV band emission implied the improvement of ZnO crystalline by annealing at high temperature.
Acknowledgements Support by Special funds for Major state Basic Research Project of China (Project No. G20000683), National Natural Science Foundation of China (Project No. 60136020) and Project of High Technology Research and Development of China are greatly appreciated.
References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [2] J.E. Nause, Cermet Inc, III-V Reviews, Vol. 12, No. 4, 1999. [3] Y. Sun, J.B. Ketterson, G.K.L. Wong, Appl. Phys. Lett. 77 (2000) 2322. [4] T. Yamamoto, T. Shiosaki, A. Kawabata, J. Appl. Phys. 51 (1980) 3113. [5] T. Minami, H. Nanto, S. Takata, Jpn. J. Appl. Phys. 23 (1984) 280. [6] J.D. Larson, D.K. Winslow, L.T. Zitelli, IEEE Trans. Sonics Ultrason SU 19 (1972) 18. [7] T. Hata, T. Minamikawa, E. Noda, et al., Jpn. J. Appl. Phys. 18 (Suppl.) (1979) 219. [8] M. Minakata, N. Chubachi, Y. Kikuchi, Jpn. J. Appl. Phys. 12 (1973) 474. [9] M. Kawasaki, A. Ohtomo, I. Ohkubo, et al., Mater. Sci. Eng. B 56 (1998) 239. [10] W. Taeg Lim, C. Hyo Lee, Thin Solid Films 353 (1999) 12.
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J. Ye et al. / Journal of Crystal Growth 243 (2002) 151–156
[11] M.K. Puchert, P.Y. Timbrell, R.N. Lamb, J. Vac. Sci. Technol. A 14 (1996) 2220. [12] R.R. Reeber, J. Appl. Phys. 21 (1970) 5063. [13] K. Thonke, Th. Gruber, N. Teofilov, et al., Physica B 308–310 (2001) 945. [14] D.M. Bagnall, Y.F. Chen, Z. Zhu, et al., Appl. Phys. Lett. 73 (1998) 1038.
[15] Mathew Joseph, Hitoshi Tabata, Tomoji Kawai, Jpn. J. Appl. Phys. 38 (1999) 1205. [16] Y. Sato, S. Sato, Thin Solid Films 281–282 (1996) 445. [17] N. Fujimura, T. Nishihara, S. Goto, et al., J. Crystal Growth 130 (1993) 269.
The growth and annealing of single crystalline ZnO films by low-pressure MOCVD Jiandong Ye, Shulin Gu*, Shunmin Zhu, Tong Chen, Liqun Hu, Feng Qin, Rong Zhang, Yi Shi, Youdou Zheng Department of Physics, Nanjing University, Nanjing 210093, China Received 22 March 2002; accepted 17 May 2002 Communicated by H. Ohno
Abstract High-quality c-axis-oriented single-crystal ZnO films have been successfully grown on the (0 0 0 2) sapphire substrate by the low-pressure metal organic chemical vapor deposition technique. The effect of doping and annealing on the optical and structural properties has been investigated by means of X-ray diffraction (XRD), photoluminescence (PL) spectrum and atomic force microscopy (AFM). Annealing at high temperature was found to enhance the intensity of the (0 0 0 2) XRD peak and decrease the c-axis oriented lattice constant. However, the (0 0 0 2) XRD peak for the Ndoped sample shifted to a low degree due to tensile stress possibly caused by nitrogen doping. The green–yellow band emission was observed in the room temperature PL spectrum of the undoped sample while the blue band emission emerged in the PL spectrum of the N-doped one. Low-temperature PL spectrum of the ZnO films was dominated by a sharp bound exciton line. Possible causes to the above differences will be given and discussed. r 2002 Elsevier Science B.V. All rights reserved. PACS: 68.55.Jk Keywords: A1. Atomic force microscopy; A1. Photoluminescence spectrum; A1. X-ray diffraction; B1. Zinc compounds
1. Introduction Blue laser diode is expected to be useful especially for applications in high-density storage and display devices. So far, the most promising known materials for such devices are GaN and related compounds [1]. However, due to the lack of a native substrate for the nitride material system, the electronic properties of GaN have *Corresponding author. Fax: +86-25-3328130. E-mail address: [email protected] (S. Gu).
not met expectations and hampered the development of the detectors, high-power lasers and high-power electronic devices [2]. Recently, another wide band gap material ZnO is attracting much attention as a promising candidate for optoelectric applications in visible and ultraviolet (UV) regions. ZnO has a direct wide band gap of 3.37 eV at room temperature (RT) and wurtzite structure. A large excitonic binding energy (59 meV) and a small Bohr radius (1.8 nm) [3] permit excitonic recombination even at RT. Additionally, due to its high conductance, chemical and thermal stability,
0022-0248/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 1 4 7 4 - 4
152
J. Ye et al. / Journal of Crystal Growth 243 (2002) 151–156
and high piezoelectric coupling coefficient, ZnO is also used for piezoelectric devices, such as surface acoustic wave (SAW) devices [4] and bulk acoustic wave devices. Investigation has recently been made into their use as transparency conductive films for liquid-crystal devices [5]. However, the viability of any photonic or electronic device technology depends on developing deposition techniques. There are many reports concerning the growth of ZnO by several techniques, such as molecular beam epitaxy (MBE), radio frequency sputtering and metal organic chemical vapor deposition (MOCVD) [6–8] on different substrates. High-quality laser emission [9] was observed in the high-quality ZnO films by a combination of MBE and laser ablation. However, MOCVD is also greatly interesting for achieving devices in commercial level since high deposition rate and high-quality ZnO is possible to be attained, especially operated at low pressure. In this paper, low-pressure MOCVD (LPMOCVD) technique was employed to grow undoped and N-doped single-crystal ZnO films on (0 0 0 2) sapphire substrate. The crystallinity and the surface morphology of ZnO are investigated by X-ray diffraction (XRD) and atomic force microscopy (AFM). The optical properties of the films have been studied via using photoluminescence (PL). The influence of annealing and the doping on the crystallinity and the optical properties of the samples will also be presented.
2. Experiment ZnO thin films have been grown in a horizontal cool wall reactor by the MOCVD system operated at low pressure (LP-MOCVD). Fig. 1 shows a schematic representation of the reaction chamber. The c-axis oriented films were grown on (0 0 0 2) Al2O3 substrates, using Zn(C2H5)2 as the Zn precursor, and CO2 as the oxygen source. For MOCVD growth of ZnO films, the gas phase reaction will result in particle formation, which will degrade ZnO film quality such as surface morphology and crystallinity. In order to minimize the gas phase reaction, Zn(C2H5)2 and CO2 are introduced into the reactor separately and mixed just above the sample surface. Zn(C2H5)2 bubbler was maintained at the temperature of 141C and at the pressure of 760 Torr. Stoichiometry of the ZnO films was controlled by the supply of oxygen via CO2 flow and the plasma ionization voltage. High-purity H2 (for sample A) or Ar (for sample C) was passed through the DEZn bubbler and saturated with DEZn vapor to the reactor. The growth temperature of sample A was 3601C and sample B was obtained by annealing sample A at 8001C under oxygen ambient for 30 min. In our previous experiments, no difference has been observed on the sample surface and properties when Ar is used to replace H2 as the carrier gas. The sample C was deposited at 3901C using NO as doping gas previously mixed with CO2 gas at the
Fig. 1. Schematic diagram of the low-pressure MOCVD system for ZnO growth.
J. Ye et al. / Journal of Crystal Growth 243 (2002) 151–156
concentration of 0.1%, and then also annealed at 9401C under oxygen ambient for 60 min. The crystallographic characteristics of the films were analyzed by XRD method using CuKa1 radiation (l=0.15405 nm). A Digital Instruments Nanoscope IIIa atomic force microscope was employed to measure the surface morphology of the ZnO films. All AFM images were taken in air using the contacting mode. Several areas were scanned on each sample to get a mean surface roughness for ZnO films. PL was carried out at the low temperature of 4 K and RT, excited by a He–Cd laser (l ¼ 325 nm) or Xe lamp source (excited at 280 nm). The wave number resolution was 1 nm for both cases.
3. Results and discussion Fig. 2 shows the XRD spectra of the samples. The peaks at 2y=34.361, 34.601 and 34.261 correspond to the diffraction from the (0 0 0 2) plane of ZnO films. These three samples all exhibit preferential orientation with the c-axis perpendicular to the substrate surface. Assuming a homogeneous strain across the films, the crystallite size may be estimated from the full-width at half-
maximum (FWHM) of the (0 0 0 2) diffraction peak by the Sherrer equation: D ¼ 0:94l=b cos y; where l (0.15405 nm) is the X-ray wavelength and b is the FWHM in radians. The c-axis lattice constant c can be obtained by the formula: 2d sin y ¼ nl and the biaxial stress was calculated from the measuring parameter c: s ¼ 453:6 109 ððc c0 Þ=c0 Þ[10], where c0 (0.5205 nm) [11] is the strain-free lattice constant. The calculated data were summarized in Table 1. For sample A, the c-axis lattice parameter is 0.5218 nm, larger than c0 and the grain size is small. The tensile force at about 1.13 109 Pa was caused by defects and the large lattice mismatch at about 15% between ZnO and sapphire. Nitrogen doping in the ZnO film was found to increase the tensile force to be about 2.24 109 Pa for the N-doped sample. After annealing, the c-axis lattice parameter of sample B is a slightly shorter than c0 and the compressive stress is induced because of the different thermal expansion coefficients of ZnO and sapphire [12]. Meanwhile, the improvement has been made on the FWHM of samples B and C, indicating the doped and annealed samples are of better quality. Fig. 3 shows the PL spectra of sample B recorded at the low temperature (4 K) and RT
34.26
(c)
25
153
30
35
40
45
50
40
45
50
40
45
50
(b)
CPS
34.6
25
30
35
(a) 34.36
25
30
35
2θ (degree) Fig. 2. XRDs for: (a) undoped ZnO (unannealed); (b) undoped ZnO (annealed) and (c) ZnO:N.
J. Ye et al. / Journal of Crystal Growth 243 (2002) 151–156
154
Table 1 Characteristics of the ZnO samples estimated from XRD patterns
3
FWHM ( 10 radian) Grain size (nm) c-lattice constant (nm) Stress ( 109 Pa)
(a) 4K
A
B
C
3.11 48.74 0.5218 1.13
2.89 52.48 0.5180 2.17
2.89 52.43 0.5233 2.24
(b) Room Temp.
0 D X
(b)
PL Intensity (a.u.)
Sample
(a)
360
400
440
480
520
360
400
440
480
520
Wavelength (nm)
Fig. 4. PL spectra of annealed ZnO films at RT. (a) Non-doped ZnO film; (b) N-doped ZnO film.
PL Intensity (a.u.)
DAP
370
350
400
450
500
380
550
600
350
400
450
500
550
600
Wavelength (nm)
Fig. 3. PL spectra of ZnO films measured at 4 K and RT. The UV band emission lines are shown in the inset.
(300 K). At 4 K, the spontaneous emission spectrum is dominated by two peaks which are magnified and shown in the inset of Fig. 3(a). The peak at 370 nm, denoted by D0X [3,13], can be assigned to donor-bound exciton recombination and the peak at 377 nm was labeled as donor– acceptor pair (DAP) emission. In ZnO, many interstitial Zn atoms act as intrinsic native donor [3] and participate in DAP emission. Additionally, deep level emission is hardly observed in the PL spectra at low temperature. At RT, the boundexciton and the DAP emission change over to the free-exciton recombination shown in Fig. 3(b). The peak of UV emission broadened and decreased, and had a small Stokes shift. This is due to the excitons localized by the film in homogeneities caused by the strain fields that result from the large lattice mismatch [14] between ZnO and the sapphire. Meanwhile, the deep level emission (green–yellow band) is related to oxygen vacancies
or interstitial Zn atoms, which both form the donor levels [15]. Comparing Fig. 4(a) and (b), we can find the blue band emission was enhanced in intensity with the green–yellow emission vanished. This may be ascribed to the irradiative transition between shallow acceptor and the conducting band (CB). According to theoretical prediction, N would work as a shallow acceptor [16] in ZnO crystal. Whereas, the FWHM of the free-exciton emissions were about 20 and 17 nm for the undoped and doped ZnO films, respectively. The narrower FWHM implies that the N-doped ZnO annealed at high temperature is of better optical properties. AFM measurements were performed to study the differences on the surface morphology between samples A and B. The images are shown in Fig. 5 over a scale of 500 nm 500 nm. We can see that the film was deposited in a column-by-column growth process and the grain size became larger with the augments of annealing temperature. This agrees with the result shown in XRD. Meanwhile, the mean surface roughness is about 2.47 and 3.84 nm, respectively. This is related to the integration of the grains. Fujimura et al. [17] suggested that the surface energy density of the (0 0 0 2) orientation is the lowest in the ZnO crystal. At high temperature, the atoms have enough diffuse activation energy to occupy the correct site in the crystal lattice and grains with
J. Ye et al. / Journal of Crystal Growth 243 (2002) 151–156
155
Fig. 5. AFM images for non-doped ZnO films grown on sapphire substrate. (a) The unannealed sample and (b) the sample annealed at 8001C.
the lower surface energy will become larger at high temperature. Then the growth orientation develops into one crystallographic direction of the low surface energy, leading to the improvement of ZnO crystallinity.
4. Conclusion (1) High-quality single-crystal undoped and N-doped ZnO films have been successfully deposited on (0 0 0 2) sapphire substrate by LP-MOCVD. (2) From a comparative analysis of XRD and AFM results, annealing at high temperature under oxygen ambient can decrease the defects such as oxygen vacancies in the films and boost the integration of the grains. (3) The PL spectrum at 4 K was dominated by D0X and DAP emission lines. At RT, the green–yellow band emission was ascribed to the native defects such as oxygen vacancies while the blue band emission was caused by nitrogen dopants that act as the shallow acceptors in ZnO films. The narrow UV band emission implied the improvement of ZnO crystalline by annealing at high temperature.
Acknowledgements Support by Special funds for Major state Basic Research Project of China (Project No. G20000683), National Natural Science Foundation of China (Project No. 60136020) and Project of High Technology Research and Development of China are greatly appreciated.
References [1] S. Nakamura, G. Fasol, The Blue Laser Diode, Springer, Berlin, 1997. [2] J.E. Nause, Cermet Inc, III-V Reviews, Vol. 12, No. 4, 1999. [3] Y. Sun, J.B. Ketterson, G.K.L. Wong, Appl. Phys. Lett. 77 (2000) 2322. [4] T. Yamamoto, T. Shiosaki, A. Kawabata, J. Appl. Phys. 51 (1980) 3113. [5] T. Minami, H. Nanto, S. Takata, Jpn. J. Appl. Phys. 23 (1984) 280. [6] J.D. Larson, D.K. Winslow, L.T. Zitelli, IEEE Trans. Sonics Ultrason SU 19 (1972) 18. [7] T. Hata, T. Minamikawa, E. Noda, et al., Jpn. J. Appl. Phys. 18 (Suppl.) (1979) 219. [8] M. Minakata, N. Chubachi, Y. Kikuchi, Jpn. J. Appl. Phys. 12 (1973) 474. [9] M. Kawasaki, A. Ohtomo, I. Ohkubo, et al., Mater. Sci. Eng. B 56 (1998) 239. [10] W. Taeg Lim, C. Hyo Lee, Thin Solid Films 353 (1999) 12.
156
J. Ye et al. / Journal of Crystal Growth 243 (2002) 151–156
[11] M.K. Puchert, P.Y. Timbrell, R.N. Lamb, J. Vac. Sci. Technol. A 14 (1996) 2220. [12] R.R. Reeber, J. Appl. Phys. 21 (1970) 5063. [13] K. Thonke, Th. Gruber, N. Teofilov, et al., Physica B 308–310 (2001) 945. [14] D.M. Bagnall, Y.F. Chen, Z. Zhu, et al., Appl. Phys. Lett. 73 (1998) 1038.
[15] Mathew Joseph, Hitoshi Tabata, Tomoji Kawai, Jpn. J. Appl. Phys. 38 (1999) 1205. [16] Y. Sato, S. Sato, Thin Solid Films 281–282 (1996) 445. [17] N. Fujimura, T. Nishihara, S. Goto, et al., J. Crystal Growth 130 (1993) 269.