- Email: [email protected]
The growth and properties of ZnO film on Si(1 1 1) substrate with an AlN buffer by AP-MOCVD
ARTICLE IN PRESS
Journal of Luminescence 122–123 (2007) 905–907 www.elsevier.com/locate/jlumin
The growth and properties of ZnO film on Si(1 1 1) substrate with an AlN buffer by AP-MOCVD Fengyi Jiang, Changda Zheng, Li Wang, Wenqing Fang, Yong Pu, Jiangnan Dai Education Ministry Research Center for Luminescence Materials and Devices, Nanchang University, 330047 PR China Available online 15 March 2006
Abstract A thin AlN buffer layer was used to grow ZnO thin film on Si(1 1 1) substrate by atmospheric pressure MOCVD to protect the substrate from being oxidized and to eliminate the mismatch between the epilayer and the substrate. Double crystal X-ray diffraction results indicate that high-crystallinity ZnO film has been obtained. The full-width of half-maximum (FWHM) of ZnO (0 0 0 2) and ZnO ð1 0 1¯ 2Þ o-rocking curve peaks are 46000 and 110500 , respectively. The crack density Of ZnO surface is 20 strip/cm by optical microscope graph determination. In situ laser reflectance trace shows that a quasi-two-dimension growth mode was obtained when the film growth rate is up to 4.3 mm/h. Free exciton emission and bound exciton emission accompanied by their longitudinal optical phonon replicas can be observed from the photoluminescence spectrum at 10 K. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 81.15.Gh; 68.55.a Keywords: ZnO/AlN/Si film; AP-MOCVD; In situ monitor; XRD; Photoluminescence
1. Introduction ZnO has received much attention because of its possible applications in optoelectronic devices in the ultraviolet wavelength region [1]. In most studies, sapphire is widely used as substrates. Since sapphire is a hard, electrically insulting and poorly thermal conducting material, which would introduce great complexity and trouble to the device fabrication processes and device applications. Silicon substrate with its good electrical and thermal conductivity, low cost, high crystal quality and availability of large size may be a good candidate substrate to solve all these problems. However, because of the oxidization of silicon substrate by oxygen source and the large lattice and thermal mismatch between ZnO and Si, direct growth of ZnO film on Si is extremely difficult and often results in amorphous or polycrystalline film. In the last few years, various buffer layers have been used, including metal films [2,3], ZnS [4], GaN [5]. Low-temperature AlN film, introduced as buffer layer, has played a key role in Corresponding author. Tel./fax: +86 791 8304441.
E-mail address: [email protected] (F. Jiang). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.322
successfully growing of high-quality GaN MQW LED epilayer on Si [6]. So, high-quality ZnO film may also be expected to grow on Si substrate with an AlN buffer layer. Tiwari [7] firstly reported the application of AlN as the buffer between ZnO layer and silicon substrate. Jin [8] has reported the use of AlN buffer layer to the growth of preferred orientation ZnO film on silicon. But, the crystal quality of the film was not reported in detail. We have reported the successful growth of ZnO film on Si substrate by inducing a 10 A˚ initial Al layer [3]. In this work, we would investigate the effect of the thin AlN buffer layer on the properties of ZnO film on Si(1 1 1).
2. Experimental ZnO thin film was grown on Si(1 1 1) substrate by atmospheric pressure MOCVD (AP-MOCVD). The growth procedure progressed in two steps: 20 nm AlN buffer layer was firstly deposited on 200 Si(1 1 1) substrate at 1100 1C by a low-pressure MOCVD system with Thomas Swan closecoupled showerhead (CCS) reactor. TMAl and Ammonia were used as Al and N precursors, respectively. Afer that,
ARTICLE IN PRESS F. Jiang et al. / Journal of Luminescence 122–123 (2007) 905–907
the AlN/Si(1 1 1) template was transferred to a home made AP-MOCVD to grow ZnO epilayer: a 60 nm ZnO buffer layer was firstly deposited at 300 1C, and in situ annealed at 850 1C for 20 min. Then, ZnO epilayer was grown at 680 1C for 30 min. Zn and O sources were DEZn and H2O, respectively. The properties of the film were studied by laser in situ monitor system, interference microscopy (Olympus BX51), X-ray diffractometer (Bede D1 System) and photoluminescence (PL) spectrum (excited by He–Cd laser 325 nm line).
20.0 LT-ZnO Buffer Growth 17.5
Reflectance /%
906
15.0 12.5 Buffer Annealling 10.0 7.5 5.0
3. Results and discussion
ZnO Epitaxyer Growth
Growth Ending
2.5
FWHM=1105" Intensity /a.u.
1200
-2000-1500-1000 -500
0
500 1000 1500 2000
∆ω/arcsec
800 400
AlN(0004)
1600
FWHM=460"
ZnO(0004)
(a)
(0002) (10-12)
AlN(0002)
Intensity /cps
8000 7500 2000
Si(111)
8500
ZnO(0002)
Fig. 1 shows the result of XRD 2y/o scan of the ZnO film. Only diffraction peaks of Si(1 1 1) plane, ZnO(0 0 0 l) index planes and AlN (0 0 0 l) index planes (including the (0 0 0 2) and (0 0 0 4) planes) appear in this curve, which proves the film is strongly c-oriented. The lattice constant c of ZnO film is 0.5195 nm calculated from the peak position spacing of (0 0 0 2) and (0 0 0 4) planes using Bragg’s law. This value is small compared to the bulk value of 0.5207 nm, which indicates the film is under compression stress along c direction and under tensile stress along a direction. The stress is mainly caused by thermal mismatch between the ZnO epilayer and Si substrate. The double crystal X-ray diffraction (DCXRD) o-rocking curves of symmetry ZnO (0 0 0 2) plane and skew symmetry ZnO ð1 0 1¯ 2Þ plane are showed by the inset graph of Fig. 1. Fullwidth of half-maximum (FWHM) of ZnO (0 0 0 2) is only 46000 , which is less than the minimum value of 72000 for ZnO films grown on 1.3 mm thick GaN layers on Si(1 1 1) substrate reported by Oleynik [5], to our knowledge. This means that AlN buffer layer has improved the crystallinity and reduced the mosaic structure of ZnO epilayer. The FWHM of ZnO ð1 0 1¯ 2Þ peak is 110500 . To our knowledge, this is the first report of the ZnO film on silicon substrate having the skew symmetry plane o-rocking curve result.
0 20 25 30 35 40 45 50 55 60 65 70 75 80 2θ/deg Fig. 1. XRD 2y/o scanning curve for ZnO/AlN/Si film, the inset graph is the DCXRD rocking curve of symmetry ZnO(0 0 0 2) and skew symmetry ZnOð1 0 1¯ 2Þ planes.
0
500
1000
1500
2000
2500
3000
3500
Growth Time /s Fig. 2. In situ laser reflectance trace of ZnO film growth process.
Fig. 3. Optical microscope surface morphology of ZnO film.
Fig. 2 shows the laser (l ¼ 635 nm) in situ monitor interference trace of ZnO growth. The fact that the regular interference periods appear in the trace indicates that a quasi-two-dimension growth mode has been realized. The swing shrinking of the trace may be resulted from the increase of surface roughness. Calculated from the trace, thickness of ZnO film is 2.14 mm and growth rate is 4.3 mm/ h. The high growth rate is greatly in favor of the future industrialization of ZnO semiconductor film. Fig. 3 shows the microscopic image of the film. Straight crack lines can be observed from the image and they are parallel to each other or intersect at angle 601 or 1201. Determined from the direction of the cracks and
ZnO planes, the crack lines are along the ZnO 1 1 2¯ 0 directions. These cracks probably originated from the difference in thermal expansion coefficient between ZnO layer (4.75 106 K1) and silicon substrate (2.62 106 K1). Usually, the crack density would become larger with increase in the film thickness and the crystal quality.
ARTICLE IN PRESS F. Jiang et al. / Journal of Luminescence 122–123 (2007) 905–907
100000 3.353
RT
3.370
10000 PL Intensity /a.u.
10K
3.317 1000
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Photon Energy /eV
3.211 100
10 3.05
3.15
is the 2LO phonon replica of free exciton. The 1LO phonon replica of free exciton may be submerged in the peaks of 3.282 and 3.317 eV. Only 3.290 eV ultraviolet near-band-edge emission appears in the RT PL spectrum and the deep-level associative emission is not observed.
3.282
3.229
4. Conclusions
3.140
3.10
907
3.20 3.25 3.30 3.35 Photon Energy /eV
3.40
3.45
Fig. 4. 10 K PL spectrum of ZnO film; the inset graph is RT PL spectrum.
The crack density of ZnO film with thickness of 2.14 mm and high crystal quality (oð0 0 0 2Þ ¼ 46000 ) is only 20 strip/cm, which is less than that reported by Kawamo [2]. We deem that the successful growth of high crystallinity and few cracks ZnO film on Si should be attributed to the introduction of AlN buffer. Firstly, AlN can be easily nucleated on silicon surface [6] and this protects the substrate from being oxidized. Secondly, the thermal expansion coefficient of AlN (4.20 106 K1) is situated between that of ZnO and Si, which can also eliminate the thermal mismatch to a certain extent. Lastly, lattice constant a of AlN is less than that of ZnO, this lattice mismatch would introduce compressive stress to ZnO film. All these are in favour of improving the qualities of ZnO on Si substrate. Fig. 4 shows the 10 K and room temperature (RT) PL spectra. The strong peak at 3.353 eV and a weak peak at 3.370 eV are due to the bound exciton and free exciton emission. The emission peaks at 3.282, 3.211, 3.140 eV are attributed to the 1, 2 and 3 LO (longitudinal optical), phonon replicas of bound exciton from the spacing of 72 meV [9]. The peak at 3.317 eV is the two-electron satellite (TES) peak of bound exciton. The peak at 3.228 eV
ZnO film with high crystallinity and few cracks was successfully grown on Si(1 1 1) substrate by introducing a 20 nm thick AlN buffer layer. The FWHMs of ZnO (0 0 0 2) and ZnOð1 0 1¯ 2Þ DCXRD o-rocking curve peaks are 46000 and 110500 , respectively. The crack density is 20 strip/cm. Quasi-two-dimension growth mode has been realized and the growth rate is up to 4.3 mm/h. Free exciton emission and bound exciton emission accompanied by their LO phonon replicas are also observed from the PL spectrum at 10 K. All the results indicate that AlN buffer layer is an effective route to obtain high-quality ZnO film on silicon substrate grown by MOCVD. Acknowledgments The work was supported by 863-Project and Electronic Development Foundation in China. References [1] Z.K. Tang, G.K.L. Wang, P. Yu, Appl. Phys. Lett. 72 (1998) 3270. [2] N. Kawamoto, M. Fujita, T. Tasumi, Jpn. J. Appl. Phys. 42 (2003) 7209. [3] Y. Chen, F. Jiang, L. Wang, J. Cryst. Growth 275 (2005) 486. [4] Y.Z. Yoo, T. Sekiguchi, T. Chikyow, Appl. Phys. Lett. 84 (2004) 502. [5] N. Oleynik, A. Dadgar, J. Blasing, Jpn. J. Appl. Phys. 42 (2003) 7474. [6] A. Watanabe, T. Takeuch, K. Hirosawa, J. Cryst. Growth 128 (1993) 391. [7] A. Tiwari, M. Park, C. Jin, H. Wang, J. Mater. Res. 17 (2002) 2480. [8] C. Jin, R. Narayan, A. Tiwari, Mat. Sci. Eng. B 117 (2005) 348. [9] C. Klingshirn, phys. stat. sol. B 71 (1975) 547.
Journal of Luminescence 122–123 (2007) 905–907 www.elsevier.com/locate/jlumin
The growth and properties of ZnO film on Si(1 1 1) substrate with an AlN buffer by AP-MOCVD Fengyi Jiang, Changda Zheng, Li Wang, Wenqing Fang, Yong Pu, Jiangnan Dai Education Ministry Research Center for Luminescence Materials and Devices, Nanchang University, 330047 PR China Available online 15 March 2006
Abstract A thin AlN buffer layer was used to grow ZnO thin film on Si(1 1 1) substrate by atmospheric pressure MOCVD to protect the substrate from being oxidized and to eliminate the mismatch between the epilayer and the substrate. Double crystal X-ray diffraction results indicate that high-crystallinity ZnO film has been obtained. The full-width of half-maximum (FWHM) of ZnO (0 0 0 2) and ZnO ð1 0 1¯ 2Þ o-rocking curve peaks are 46000 and 110500 , respectively. The crack density Of ZnO surface is 20 strip/cm by optical microscope graph determination. In situ laser reflectance trace shows that a quasi-two-dimension growth mode was obtained when the film growth rate is up to 4.3 mm/h. Free exciton emission and bound exciton emission accompanied by their longitudinal optical phonon replicas can be observed from the photoluminescence spectrum at 10 K. r 2006 Elsevier B.V. All rights reserved. PACS: 78.55.Et; 81.15.Gh; 68.55.a Keywords: ZnO/AlN/Si film; AP-MOCVD; In situ monitor; XRD; Photoluminescence
1. Introduction ZnO has received much attention because of its possible applications in optoelectronic devices in the ultraviolet wavelength region [1]. In most studies, sapphire is widely used as substrates. Since sapphire is a hard, electrically insulting and poorly thermal conducting material, which would introduce great complexity and trouble to the device fabrication processes and device applications. Silicon substrate with its good electrical and thermal conductivity, low cost, high crystal quality and availability of large size may be a good candidate substrate to solve all these problems. However, because of the oxidization of silicon substrate by oxygen source and the large lattice and thermal mismatch between ZnO and Si, direct growth of ZnO film on Si is extremely difficult and often results in amorphous or polycrystalline film. In the last few years, various buffer layers have been used, including metal films [2,3], ZnS [4], GaN [5]. Low-temperature AlN film, introduced as buffer layer, has played a key role in Corresponding author. Tel./fax: +86 791 8304441.
E-mail address: [email protected] (F. Jiang). 0022-2313/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jlumin.2006.01.322
successfully growing of high-quality GaN MQW LED epilayer on Si [6]. So, high-quality ZnO film may also be expected to grow on Si substrate with an AlN buffer layer. Tiwari [7] firstly reported the application of AlN as the buffer between ZnO layer and silicon substrate. Jin [8] has reported the use of AlN buffer layer to the growth of preferred orientation ZnO film on silicon. But, the crystal quality of the film was not reported in detail. We have reported the successful growth of ZnO film on Si substrate by inducing a 10 A˚ initial Al layer [3]. In this work, we would investigate the effect of the thin AlN buffer layer on the properties of ZnO film on Si(1 1 1).
2. Experimental ZnO thin film was grown on Si(1 1 1) substrate by atmospheric pressure MOCVD (AP-MOCVD). The growth procedure progressed in two steps: 20 nm AlN buffer layer was firstly deposited on 200 Si(1 1 1) substrate at 1100 1C by a low-pressure MOCVD system with Thomas Swan closecoupled showerhead (CCS) reactor. TMAl and Ammonia were used as Al and N precursors, respectively. Afer that,
ARTICLE IN PRESS F. Jiang et al. / Journal of Luminescence 122–123 (2007) 905–907
the AlN/Si(1 1 1) template was transferred to a home made AP-MOCVD to grow ZnO epilayer: a 60 nm ZnO buffer layer was firstly deposited at 300 1C, and in situ annealed at 850 1C for 20 min. Then, ZnO epilayer was grown at 680 1C for 30 min. Zn and O sources were DEZn and H2O, respectively. The properties of the film were studied by laser in situ monitor system, interference microscopy (Olympus BX51), X-ray diffractometer (Bede D1 System) and photoluminescence (PL) spectrum (excited by He–Cd laser 325 nm line).
20.0 LT-ZnO Buffer Growth 17.5
Reflectance /%
906
15.0 12.5 Buffer Annealling 10.0 7.5 5.0
3. Results and discussion
ZnO Epitaxyer Growth
Growth Ending
2.5
FWHM=1105" Intensity /a.u.
1200
-2000-1500-1000 -500
0
500 1000 1500 2000
∆ω/arcsec
800 400
AlN(0004)
1600
FWHM=460"
ZnO(0004)
(a)
(0002) (10-12)
AlN(0002)
Intensity /cps
8000 7500 2000
Si(111)
8500
ZnO(0002)
Fig. 1 shows the result of XRD 2y/o scan of the ZnO film. Only diffraction peaks of Si(1 1 1) plane, ZnO(0 0 0 l) index planes and AlN (0 0 0 l) index planes (including the (0 0 0 2) and (0 0 0 4) planes) appear in this curve, which proves the film is strongly c-oriented. The lattice constant c of ZnO film is 0.5195 nm calculated from the peak position spacing of (0 0 0 2) and (0 0 0 4) planes using Bragg’s law. This value is small compared to the bulk value of 0.5207 nm, which indicates the film is under compression stress along c direction and under tensile stress along a direction. The stress is mainly caused by thermal mismatch between the ZnO epilayer and Si substrate. The double crystal X-ray diffraction (DCXRD) o-rocking curves of symmetry ZnO (0 0 0 2) plane and skew symmetry ZnO ð1 0 1¯ 2Þ plane are showed by the inset graph of Fig. 1. Fullwidth of half-maximum (FWHM) of ZnO (0 0 0 2) is only 46000 , which is less than the minimum value of 72000 for ZnO films grown on 1.3 mm thick GaN layers on Si(1 1 1) substrate reported by Oleynik [5], to our knowledge. This means that AlN buffer layer has improved the crystallinity and reduced the mosaic structure of ZnO epilayer. The FWHM of ZnO ð1 0 1¯ 2Þ peak is 110500 . To our knowledge, this is the first report of the ZnO film on silicon substrate having the skew symmetry plane o-rocking curve result.
0 20 25 30 35 40 45 50 55 60 65 70 75 80 2θ/deg Fig. 1. XRD 2y/o scanning curve for ZnO/AlN/Si film, the inset graph is the DCXRD rocking curve of symmetry ZnO(0 0 0 2) and skew symmetry ZnOð1 0 1¯ 2Þ planes.
0
500
1000
1500
2000
2500
3000
3500
Growth Time /s Fig. 2. In situ laser reflectance trace of ZnO film growth process.
Fig. 3. Optical microscope surface morphology of ZnO film.
Fig. 2 shows the laser (l ¼ 635 nm) in situ monitor interference trace of ZnO growth. The fact that the regular interference periods appear in the trace indicates that a quasi-two-dimension growth mode has been realized. The swing shrinking of the trace may be resulted from the increase of surface roughness. Calculated from the trace, thickness of ZnO film is 2.14 mm and growth rate is 4.3 mm/ h. The high growth rate is greatly in favor of the future industrialization of ZnO semiconductor film. Fig. 3 shows the microscopic image of the film. Straight crack lines can be observed from the image and they are parallel to each other or intersect at angle 601 or 1201. Determined from the direction of the cracks and
ZnO planes, the crack lines are along the ZnO 1 1 2¯ 0 directions. These cracks probably originated from the difference in thermal expansion coefficient between ZnO layer (4.75 106 K1) and silicon substrate (2.62 106 K1). Usually, the crack density would become larger with increase in the film thickness and the crystal quality.
ARTICLE IN PRESS F. Jiang et al. / Journal of Luminescence 122–123 (2007) 905–907
100000 3.353
RT
3.370
10000 PL Intensity /a.u.
10K
3.317 1000
2.0 2.2 2.4 2.6 2.8 3.0 3.2 3.4 3.6 Photon Energy /eV
3.211 100
10 3.05
3.15
is the 2LO phonon replica of free exciton. The 1LO phonon replica of free exciton may be submerged in the peaks of 3.282 and 3.317 eV. Only 3.290 eV ultraviolet near-band-edge emission appears in the RT PL spectrum and the deep-level associative emission is not observed.
3.282
3.229
4. Conclusions
3.140
3.10
907
3.20 3.25 3.30 3.35 Photon Energy /eV
3.40
3.45
Fig. 4. 10 K PL spectrum of ZnO film; the inset graph is RT PL spectrum.
The crack density of ZnO film with thickness of 2.14 mm and high crystal quality (oð0 0 0 2Þ ¼ 46000 ) is only 20 strip/cm, which is less than that reported by Kawamo [2]. We deem that the successful growth of high crystallinity and few cracks ZnO film on Si should be attributed to the introduction of AlN buffer. Firstly, AlN can be easily nucleated on silicon surface [6] and this protects the substrate from being oxidized. Secondly, the thermal expansion coefficient of AlN (4.20 106 K1) is situated between that of ZnO and Si, which can also eliminate the thermal mismatch to a certain extent. Lastly, lattice constant a of AlN is less than that of ZnO, this lattice mismatch would introduce compressive stress to ZnO film. All these are in favour of improving the qualities of ZnO on Si substrate. Fig. 4 shows the 10 K and room temperature (RT) PL spectra. The strong peak at 3.353 eV and a weak peak at 3.370 eV are due to the bound exciton and free exciton emission. The emission peaks at 3.282, 3.211, 3.140 eV are attributed to the 1, 2 and 3 LO (longitudinal optical), phonon replicas of bound exciton from the spacing of 72 meV [9]. The peak at 3.317 eV is the two-electron satellite (TES) peak of bound exciton. The peak at 3.228 eV
ZnO film with high crystallinity and few cracks was successfully grown on Si(1 1 1) substrate by introducing a 20 nm thick AlN buffer layer. The FWHMs of ZnO (0 0 0 2) and ZnOð1 0 1¯ 2Þ DCXRD o-rocking curve peaks are 46000 and 110500 , respectively. The crack density is 20 strip/cm. Quasi-two-dimension growth mode has been realized and the growth rate is up to 4.3 mm/h. Free exciton emission and bound exciton emission accompanied by their LO phonon replicas are also observed from the PL spectrum at 10 K. All the results indicate that AlN buffer layer is an effective route to obtain high-quality ZnO film on silicon substrate grown by MOCVD. Acknowledgments The work was supported by 863-Project and Electronic Development Foundation in China. References [1] Z.K. Tang, G.K.L. Wang, P. Yu, Appl. Phys. Lett. 72 (1998) 3270. [2] N. Kawamoto, M. Fujita, T. Tasumi, Jpn. J. Appl. Phys. 42 (2003) 7209. [3] Y. Chen, F. Jiang, L. Wang, J. Cryst. Growth 275 (2005) 486. [4] Y.Z. Yoo, T. Sekiguchi, T. Chikyow, Appl. Phys. Lett. 84 (2004) 502. [5] N. Oleynik, A. Dadgar, J. Blasing, Jpn. J. Appl. Phys. 42 (2003) 7474. [6] A. Watanabe, T. Takeuch, K. Hirosawa, J. Cryst. Growth 128 (1993) 391. [7] A. Tiwari, M. Park, C. Jin, H. Wang, J. Mater. Res. 17 (2002) 2480. [8] C. Jin, R. Narayan, A. Tiwari, Mat. Sci. Eng. B 117 (2005) 348. [9] C. Klingshirn, phys. stat. sol. B 71 (1975) 547.