- Email: [email protected]
Vapour phase transport growth of ZnO layers and nanostructures
Superlattices and Microstructures 42 (2007) 33–39 www.elsevier.com/locate/superlattices
Vapour phase transport growth of ZnO layers and nanostructures A. Bakin ∗ , A. Che Mofor, A. El-Shaer, A. Waag Institute of Semiconductor Technology, Technical University Braunschweig, Hans-Sommer-Straße 66, D-38106 Braunschweig, Germany Available online 11 June 2007
Abstract We have developed a novel advanced VPT set-up. ZnO layers and nanorods were grown employing a specially designed horizontal vapour transport system with elemental sources at relatively low temperatures without catalysis. We employed 6N elemental Zn carried by nitrogen and 99.995% oxygen as reactants. Sapphire, SiC, bulk ZnO and ZnO epitaxial layers were implemented as substrates for ZnO growth. Growth temperatures ranged from 500 to 900 ◦ C. Reactor pressures were from 5 mbar to atmospheric pressure. We employed x-ray diffractometry, optical microscopy, scanning electron microscopy and atomic force microscopy to investigate the obtained ZnO samples and the influence of different growth parameters on the ZnO homo- and heteroepitaxial growth and to optimise the set of growth parameters either for both epitaxial layers and nanostructures. We also show that the quality of the VPT grown ZnO epitaxial layers on sapphire can be even higher (evaluated from FWHM of the XRD rocking curves) than the MBE grown ones used as epiwafers for VPT growth. High quality ZnO layers with extremely narrow FWHM of the (0002) rocking curve of 3800 are fabricated employing our VPT approach. c 2007 Elsevier Ltd. All rights reserved.
Keywords: ZnO; Epitaxy; Vapour phase transport
1. Introduction ZnO-based semiconductors are expected to provide a great potential for optoelectronics in the ultraviolet (UV) and blue spectral range, transparent electronics, and magnetoelectronics operating at room temperature. An additional benefit of ZnO is the possibility to fabricate ZnO nanopillars by self-organisation, with a high degree of c-axis orientation. The defect density in the nanopillars remains extremely low due to the small footprint of nanopillar on the substrate, ∗ Corresponding author. Tel.: +49 531391 3779; fax: +49 531391 5844.
E-mail address: [email protected] (A. Bakin). c 2007 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2007.04.067
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A. Bakin et al. / Superlattices and Microstructures 42 (2007) 33–39
Fig. 1. Schematic view of the advanced horizontal vapour phase transport set-up.
providing the possibility to use a wide range of different substrates necessary for certain device applications or low cost materials for mass production. ZnO-based materials have an additional advantage as opposed to other semiconductor materials like GaAs in that surface oxidation is obviously not an issue. Significant progress in ZnO homo- and heteroepitaxy technology has been observed in recent years but further improvement of ZnO epitaxial layer quality is still necessary to achieve reproducible p-type doping and to realize reliable ZnO-based devices. The implementation of gas phase growth techniques typically facilitates the fabrication of the best quality bulk crystals and epitaxial layers as well as device heterostructures, which cannot be realized with liquid phase growth. Molecular Beam Epitaxy (MBE), Vapour Phase Transport (VPT) and Chemical Vapour Deposition (CVD) are the most promising techniques for producing ZnO. Conventional CVD or MOCVD methods employ Zn halides [1] or metal–organic material sources, such as DeZn [2–5]. VPT is the low cost method, which provides the possibility of manufacturing both nanostructures and epitaxial layers (and even bulk crystals) of high quality and purity on a massproduction scale. Growth of ZnO nanostructures using metal catalysts (mostly Au) and graphite at relatively high temperatures (usually above 1000 ◦ C, which is typically above the eutectic melting point of the metal catalyst) has been reported [1,2,6–8]. In some cases carbon is used to catalyse the ZnO gas phase growth [9]. These methods as well as the CVD technique are associated with impurities that may not be detected by conventional crystal characterisation methods like transmission electron microscopy and x-ray diffractometry but will significantly influence the electrical and optical properties of the fabricated material. In the present paper we report on the growth of ZnO layers and nanorods by employing a specially designed horizontal vapour transport system with elemental sources at relatively low temperatures without catalysis or metal–organic sources. 2. Experimental We have developed a novel home-made advanced horizontal VPT set-up providing ZnO growth at temperatures from up to 1000 ◦ C and reactor pressures from 1 mbar to 1 atmosphere. We employed 6N (99.9999%) elemental Zn carried by an inert gas (for instance N2 obtained by evaporating liquid nitrogen) and 99.995% oxygen as the initial sources. Sapphire, SiC and ZnO epitaxial wafers were employed as substrates for ZnO growth. The use of SiC along with low cost sapphire for ZnO nanorod growth is due to the additional advantages, which can be provided by SiC for certain device applications like electrical conductivity, stability in harsh environments (radiation, chemical, heat etc.) and also lower lattice mismatch with ZnO. A schematic view of the reactor of our computer-controlled VPT set-up is presented in Fig. 1. A three-zone automated
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resistant oven is employed to provide the necessary temperature distribution (Tsource , Tsubstrate ) in the quartz reactor. The temperatures are measured using a Pt–PtRh thermoelement inside the reactor. The reactor pressure control is provided by the specially designed vacuum system and the gas flows (oxygen, inert gases) are provided by a gas distribution system with mass flow and pressure controllers similar to commercial MOCVD systems. Zn shots are placed in a quartz source connected to a carrier gas (for instance N2 ) line (cf. Fig. 1). Zn is heated up to the necessary Tsource (typically 420–490 ◦ C) and the vapour is transported to the substrate by the inert gas flow. Dopants can be introduced into the reactor through a separate line with the separate quartz source and separate carrier gas (cf. Fig. 1). The reactor can be pumped out to 1 mbar. Typical reactor pressures are from 5 to 25 mbar. In order to avoid unwanted pre-reactions before reaching the growth area around the substrate, oxygen was introduced through a separate line with the quartz outlet directly to the substrate. Al2 O3 , SiC, ZnO epitaxial layers, and even plastic and glass were implemented as substrates for ZnO growth. (11–20)Al2 O3 substrates were cleaned by boiling separately for 10 min in acetone and iso-propanol, while SiC substrates were treated in a 1:1 mixture of boiling H2 O2 and H2 SO4 for 5 min and later in an ammonium fluoride etching mixture to get rid of the native oxide. The MBE-grown ZnO/sapphire epiwafers were implemented “as-grown” without additional chemical cleaning. The ZnO layers and nanorods were grown at temperatures from 450 to 900 ◦ C. We have also developed ZnO growth on plastic and glass wafers for transparent and flexible electronics. In this case the growth temperatures were down to 200 ◦ C [12]. Optical microscopy and atomic force microscopy (AFM) were used to investigate the surface of the substrates and obtained ZnO samples. A Cambridge Instruments scanning electron microscopy (SEM) system was employed to evaluate the size, length and density of the obtained nanorods. A Phillips High-resolution x-ray diffractometry (XRD) system was used (without a slit) to obtain rocking curves of the as-grown ZnO layers and nanorods as well as of the initial substrates. 3. Results and discussion We have realised the vapour transport fabrication of ZnO nanopillars, leading to homogeneous, well aligned, c-axis oriented nanopillar systems. Typical diameters of the nanopillars are between 50 and 900 nm and they are up to 10 µm long and density of 2 × 109 cm−2 . SEM investigations of the ZnO nanopillars grown under different conditions showed that the increase in growth temperature from 650 up to 800 ◦ C for the same source flows, reactor pressure and source temperature led to a drastic reduction in the size and a significant increase in the length and density of the nanostructures. In some cases we observed that the tip of the rods continue to grow into tiny whiskers. SEM images of ZnO nanopillars manufactured on c-Al2 O3 wafer and on 6H-SiC wafer grown at 800 ◦ C are shown in Fig. 2(a) and Fig. 2(b) respectively. The more rapid growth of nanopillars on silicon carbide wafer along certain lines is caused by the wafer surface damage. AFM image of the surface of a commercial 6H-SiC wafer revealing scratches caused by mechanical polishing is shown in Fig. 2(c). Surface damaged layer can be removed by etching SiC wafers in hydrogen [10]. Growth of ZnO nanopillars on SiC wafers after hydrogen etching leads to formation of homogeneous nanopillar arrays. X-ray diffraction rocking curves with a full width at half maximum (FWHM) of 82800 were obtained for the grown nanopillar arrays (cf. Fig. 5(a)) showing an extremely good c-axis alignment for such several µm long nanopillars. Also, room temperature photoluminescence
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Fig. 2. SEM images of the VPT ZnO nanopillars grown on: (a) sapphire wafer; (b) SiC wafer; and (c) AFM image of the surface of a commercial SiC wafer revealing scratches caused by mechanical polishing.
Fig. 3. SEM images of ZnO grown on ZnO/sapphire epiwafers at different growth conditions: (a) nanopillars grown at 550 ◦ C growth temperature, Zn carrier gas flow 70 ml/min and 10 mbar reactor pressure; (b) pyramid-shaped structures obtained at 500 ◦ C, Zn carrier gas flow 60 ml/min and 8 mbar reactor pressure and (c) epitaxial layer after improvements at 500 ◦ C growth temperature, Zn carrier gas flow 60 ml/min and 5 mbar reactor pressure.
peaks of high intensity and FWHM of 90 meV were measured for the fabricated ZnO nanorods [11]. We also developed the growth of ZnO layers using our VPT approach. The initial experiments on ZnO layer growth directly on c-sapphire led to columnar growth of low-quality layers. The growth temperature was then reduced in relation to nanopillar growth temperatures. Nevertheless we did not obtain acceptable quality of the ZnO layers directly on c-sapphire. As a next step we employed a 6 nm low-temperature (LT) ZnO buffer grown with MBE on c-sapphire with intermediate MgO buffer layer [13,14]. In this case we obtained, under optimised conditions, quite reasonable results providing a FWHM of (0002) ZnO XRD rocking curve of 498.600 (cf. Fig. 5(b)). This value is comparable to the best results reported for ZnO layers grown on csapphire obtained by conventional MOCVD technique. We proceeded to improve the ZnO VPT growth and employed 300 nm ZnO/sapphire epiwafer grown with our MBE system. The details of the MBE growth are described elsewhere [13,14]. The initial experiments on these epiwafers were performed at 550 ◦ C, with a reactor pressure 10 mbar, Zn source temperature 460 ◦ C, oxygen and nitrogen flows of 55 and 70 sscm. This set of parameters resulted in nanopillar growth (cf. Fig. 3(a)). The suppression of three dimensional growth was targeted by a successive reduction in the growth rate. At the same oxygen flow (55 ml/min) the pressure was reduced to 8 mbar, the nitrogen carrier gas flow to 60 sscm and the growth temperature to 500 ◦ C. The result was predominantly broader pyramidal structures, indicating a transition from nanorod growth to layer growth (cf. Fig. 3(b)). Further decrease in the reactor pressure to 5 mbar led to growth of smooth ZnO layer (cf. Fig. 3(c)). Fig. 4 presents SEM images of high-quality smooth surface of the MBE 300 nm ZnO/sapphire epiwafer (cf. Fig. 3(a))
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Fig. 4. SEM images of: (a) high-quality smooth surface of the MBE ZnO/sapphire epiwafer; (b) ZnO layer grown in optimized conditions at 500 ◦ C growth temperature and 5 mbar reactor pressure. AFM images of the surfaces of the initial 300 nm ZnO/sapphire epiwafer (c) and of the surface of the consequently VPT grown ZnO layer (d) and (e). Estimated values of the surface roughness rms are 1.3 nm, 2.3 nm and 6.1 nm correspondingly.
Fig. 5. (0002) ZnO XRD rocking curves of: (a) ZnO nanorods on (11–20)Al2 O3 substrate showing full width at half maximum of 82800 ; (b) ZnO layer grown on 6 nm LT ZnO buffer grown with MBE on sapphire epiwafer showing FWHM of 498.600 ; and (c) the initial 300 nm ZnO/sapphire epiwafer showing FWHM of 4200 ; (d) ZnO layer grown on 300 nm ZnO/sapphire epiwafer (cf. Fig. 5(c)) showing full width at half maximum of 3800 .
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and of the VPT ZnO layer grown in optimized conditions at 500 ◦ C and 5 mbar reactor pressure and the corresponding AFM images of the surfaces of the initial 300 nm ZnO/sapphire epiwafer (cf. Fig. 3(c)) and of the surface of the subsequently VPT-grown ZnO layer (cf. Fig. 3(d)). Estimated values of the root mean square (rms) surface roughness are 1.3 nm and 2.3 nm correspondingly. We have to mention here that in this case of the initial experiments, we did not use the smoothest epiwafer that is producible with our MBE ZnO growth process. The best rms values which we obtained for the MBE grown ZnO/sapphire epiwafers are 0.26 nm [13,14]. So, there is further possibility to improve the surface roughness of the ZnO layer grown with the VPT system by employing better ZnO epiwafers. Sometimes, rare lone standing nanopillars were observed on the VPT-grown ZnO samples (cf. Fig. 3(e)). The origin of these nanopillars is not yet quite clear. In the case of MBE growth we observed such lone standing defects due to deviation from the optimised II/VI ratio in the gas phase, namely if we grew layers in slightly Zn-rich conditions. We can also suppose that the nanopillars originate from such rare nano-pyramidal defects which could be eventually present in some places on the surface of the initial epiwafer. (0002) ZnO XRD rocking curves of the initial 2” 300 nm ZnO/sapphire MBE epiwafer (showing FWHM of 4200 ) and of the ZnO layer grown on this epiwafer (showing full width at half maximum of 3800 ) are presented in figure (c) and (d), respectively. As can be seen from the XRD measurements, we have realised high growth rate of the VPT ZnO layer (about 2 µm/h) and at the same time slightly improved the quality of the initial MBE grown epitaxial layer on the sapphire substrate. 4. Conclusion ZnO thin films and ZnO nanostructures were fabricated using a home-made advanced vapour transport system with elemental sources. Al2 O3 , SiC and ZnO epiwafers implemented as substrates for this novel advanced VPT ZnO growth. In the latter case VPT growth technique was combined with MBE technology. The VPT fabrication of ZnO nanopillars, leading to well aligned, c-axis oriented nanopillars with excellent quality and purity is demonstrated. We have successfully grown ZnO nanorods directly on 6H-SiC and (11–20)Al2 O3 . Our approach is advantageous in that sources of undesired impurities are tremendously reduced, since elemental material sources are used and the growth is catalyst-free. The crystal and hence optical qualities of the rods are excellent. The nanopillars should be the basis for future nano-LED and UV detector structures, providing a potential of 109 –1010 devices/cm2 . The possibility to employ several fabrication techniques is of special importance for the realization of unique device structures. MBE grown ZnO/sapphire epiwafers were implemented for further high quality VPT ZnO layers’ fabrication. The quality of the VPT grown ZnO epitaxial layers on sapphire can be even higher (evaluated from FWHM of the XRD rocking curves) compared to the MBE grown ones used as the epiwafers for VPT growth. A FWHM of the (0002) rocking curve of 3800 was obtained for the VPT grown ZnO layers. More so, the growth rate is several times higher than for MBE (up to 2 µm/h), and growth takes place at relatively low temperatures and pressure in a simple system that can be applied to mass production. Acknowledgements The authors thank M. Karsten, K.-H. Lachmund and D. R¨ummler for their technical support and the Deutsche Forschungsgemeinschaft for financial support through project number DFGWA870/7 DFG-WA870/8 and EU project “NANDOS”.
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References [1] C.X. Xu, X.W. Sun, Z.L. Dong, M.B. Yu, T.D. My, X.H. Zhang, S.J. Chua, T.J. White, Nanotechnology 15 (2004) 839–842. [2] Xiang Liu, Xiaohua Wu, Hui Cao, R.P.H. Chang, J. Appl. Phys. 95 (6) (2003) 3141. [3] B.P. Zhang, N.T. Binh, K. Wakatsuki, Y. Segawa, Y. Kashiwaba, K. Haga, Nanotechnology 15, S382–S388. [4] W.I. Park, G.C. Yi, M. Kim, S.J. Pennycook, Adv. Mater. 14 (24) (2002) 1841. [5] Jih-Jen Wu, Sai-Chiang Liu, Adv. Mater. 14 (3) (2002) 215–218. [6] D. Banerjee, J. Rybczynski, J.Y. Huang, D.Z. Wang, K. Kempa, Z.F. Ren, Appl. Phys. A 80 (2004) 749. [7] J.C. Johnson, H. Yan, R.D. Schaller, L.H. Haber, R.J. Saykally, P. Yang, J. Phys. Chem B. 105 (46) (2001) 11387. [8] R.S. Wagner, W.C. Ellis, Appl. Phys. Lett. 4 (5) (1964) 89–90. [9] D. Banerjee, J.Y. Lao, D.Z. Wang, J.Y. Huang, Z.F. Ren, Appl. Phys. Lett. 83 (10) (2003) 2061. [10] C. Hallin, A.S. Bakin, F. Owman, P. Martensson, O. Kordina, E. Janzen, The 6th International Conf. on Silicon Carbide and Related Materials-1995, ICSCRM ’95, Kyoto, Japan, Inst. Phys. Conf. Ser. No 142, pp. 613–616 (Chapter 3). [11] A.C. Mofor, A.S. Bakin, A. Elshaer, D. Fuhrmann, F. Bertram, A. Hangleiter, J. Christen, A. Waag, Phys. Status Solidi C 3 (4) (2006) 1046. [12] A.C. Mofor, A. Bakin, B. Postels, M. Suleiman, A. Elshaer, A. Waag, Thin Solid Films (2007). DOI: http://dx.doi.org/10.1016/j.tsf.2007.05.045. [13] A. El Shaer, A. Bakin, A. Che Mofor, J. Bl¨asing, A. Krost, J. Stoimenos, B. P´ecz, M. Kreye, M. Heuken, A. Waag, Phys. Status Solidi B 243 (2006) 768. [14] A. Bakin, A. El-Shaer, A. Che Mofor, M. Kreye, A. Waag, F. Bertram, J. Christen, J. Stoimenos, J. Crystal Growth 287 (2006) 7.
Vapour phase transport growth of ZnO layers and nanostructures A. Bakin ∗ , A. Che Mofor, A. El-Shaer, A. Waag Institute of Semiconductor Technology, Technical University Braunschweig, Hans-Sommer-Straße 66, D-38106 Braunschweig, Germany Available online 11 June 2007
Abstract We have developed a novel advanced VPT set-up. ZnO layers and nanorods were grown employing a specially designed horizontal vapour transport system with elemental sources at relatively low temperatures without catalysis. We employed 6N elemental Zn carried by nitrogen and 99.995% oxygen as reactants. Sapphire, SiC, bulk ZnO and ZnO epitaxial layers were implemented as substrates for ZnO growth. Growth temperatures ranged from 500 to 900 ◦ C. Reactor pressures were from 5 mbar to atmospheric pressure. We employed x-ray diffractometry, optical microscopy, scanning electron microscopy and atomic force microscopy to investigate the obtained ZnO samples and the influence of different growth parameters on the ZnO homo- and heteroepitaxial growth and to optimise the set of growth parameters either for both epitaxial layers and nanostructures. We also show that the quality of the VPT grown ZnO epitaxial layers on sapphire can be even higher (evaluated from FWHM of the XRD rocking curves) than the MBE grown ones used as epiwafers for VPT growth. High quality ZnO layers with extremely narrow FWHM of the (0002) rocking curve of 3800 are fabricated employing our VPT approach. c 2007 Elsevier Ltd. All rights reserved.
Keywords: ZnO; Epitaxy; Vapour phase transport
1. Introduction ZnO-based semiconductors are expected to provide a great potential for optoelectronics in the ultraviolet (UV) and blue spectral range, transparent electronics, and magnetoelectronics operating at room temperature. An additional benefit of ZnO is the possibility to fabricate ZnO nanopillars by self-organisation, with a high degree of c-axis orientation. The defect density in the nanopillars remains extremely low due to the small footprint of nanopillar on the substrate, ∗ Corresponding author. Tel.: +49 531391 3779; fax: +49 531391 5844.
E-mail address: [email protected] (A. Bakin). c 2007 Elsevier Ltd. All rights reserved. 0749-6036/$ - see front matter doi:10.1016/j.spmi.2007.04.067
34
A. Bakin et al. / Superlattices and Microstructures 42 (2007) 33–39
Fig. 1. Schematic view of the advanced horizontal vapour phase transport set-up.
providing the possibility to use a wide range of different substrates necessary for certain device applications or low cost materials for mass production. ZnO-based materials have an additional advantage as opposed to other semiconductor materials like GaAs in that surface oxidation is obviously not an issue. Significant progress in ZnO homo- and heteroepitaxy technology has been observed in recent years but further improvement of ZnO epitaxial layer quality is still necessary to achieve reproducible p-type doping and to realize reliable ZnO-based devices. The implementation of gas phase growth techniques typically facilitates the fabrication of the best quality bulk crystals and epitaxial layers as well as device heterostructures, which cannot be realized with liquid phase growth. Molecular Beam Epitaxy (MBE), Vapour Phase Transport (VPT) and Chemical Vapour Deposition (CVD) are the most promising techniques for producing ZnO. Conventional CVD or MOCVD methods employ Zn halides [1] or metal–organic material sources, such as DeZn [2–5]. VPT is the low cost method, which provides the possibility of manufacturing both nanostructures and epitaxial layers (and even bulk crystals) of high quality and purity on a massproduction scale. Growth of ZnO nanostructures using metal catalysts (mostly Au) and graphite at relatively high temperatures (usually above 1000 ◦ C, which is typically above the eutectic melting point of the metal catalyst) has been reported [1,2,6–8]. In some cases carbon is used to catalyse the ZnO gas phase growth [9]. These methods as well as the CVD technique are associated with impurities that may not be detected by conventional crystal characterisation methods like transmission electron microscopy and x-ray diffractometry but will significantly influence the electrical and optical properties of the fabricated material. In the present paper we report on the growth of ZnO layers and nanorods by employing a specially designed horizontal vapour transport system with elemental sources at relatively low temperatures without catalysis or metal–organic sources. 2. Experimental We have developed a novel home-made advanced horizontal VPT set-up providing ZnO growth at temperatures from up to 1000 ◦ C and reactor pressures from 1 mbar to 1 atmosphere. We employed 6N (99.9999%) elemental Zn carried by an inert gas (for instance N2 obtained by evaporating liquid nitrogen) and 99.995% oxygen as the initial sources. Sapphire, SiC and ZnO epitaxial wafers were employed as substrates for ZnO growth. The use of SiC along with low cost sapphire for ZnO nanorod growth is due to the additional advantages, which can be provided by SiC for certain device applications like electrical conductivity, stability in harsh environments (radiation, chemical, heat etc.) and also lower lattice mismatch with ZnO. A schematic view of the reactor of our computer-controlled VPT set-up is presented in Fig. 1. A three-zone automated
A. Bakin et al. / Superlattices and Microstructures 42 (2007) 33–39
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resistant oven is employed to provide the necessary temperature distribution (Tsource , Tsubstrate ) in the quartz reactor. The temperatures are measured using a Pt–PtRh thermoelement inside the reactor. The reactor pressure control is provided by the specially designed vacuum system and the gas flows (oxygen, inert gases) are provided by a gas distribution system with mass flow and pressure controllers similar to commercial MOCVD systems. Zn shots are placed in a quartz source connected to a carrier gas (for instance N2 ) line (cf. Fig. 1). Zn is heated up to the necessary Tsource (typically 420–490 ◦ C) and the vapour is transported to the substrate by the inert gas flow. Dopants can be introduced into the reactor through a separate line with the separate quartz source and separate carrier gas (cf. Fig. 1). The reactor can be pumped out to 1 mbar. Typical reactor pressures are from 5 to 25 mbar. In order to avoid unwanted pre-reactions before reaching the growth area around the substrate, oxygen was introduced through a separate line with the quartz outlet directly to the substrate. Al2 O3 , SiC, ZnO epitaxial layers, and even plastic and glass were implemented as substrates for ZnO growth. (11–20)Al2 O3 substrates were cleaned by boiling separately for 10 min in acetone and iso-propanol, while SiC substrates were treated in a 1:1 mixture of boiling H2 O2 and H2 SO4 for 5 min and later in an ammonium fluoride etching mixture to get rid of the native oxide. The MBE-grown ZnO/sapphire epiwafers were implemented “as-grown” without additional chemical cleaning. The ZnO layers and nanorods were grown at temperatures from 450 to 900 ◦ C. We have also developed ZnO growth on plastic and glass wafers for transparent and flexible electronics. In this case the growth temperatures were down to 200 ◦ C [12]. Optical microscopy and atomic force microscopy (AFM) were used to investigate the surface of the substrates and obtained ZnO samples. A Cambridge Instruments scanning electron microscopy (SEM) system was employed to evaluate the size, length and density of the obtained nanorods. A Phillips High-resolution x-ray diffractometry (XRD) system was used (without a slit) to obtain rocking curves of the as-grown ZnO layers and nanorods as well as of the initial substrates. 3. Results and discussion We have realised the vapour transport fabrication of ZnO nanopillars, leading to homogeneous, well aligned, c-axis oriented nanopillar systems. Typical diameters of the nanopillars are between 50 and 900 nm and they are up to 10 µm long and density of 2 × 109 cm−2 . SEM investigations of the ZnO nanopillars grown under different conditions showed that the increase in growth temperature from 650 up to 800 ◦ C for the same source flows, reactor pressure and source temperature led to a drastic reduction in the size and a significant increase in the length and density of the nanostructures. In some cases we observed that the tip of the rods continue to grow into tiny whiskers. SEM images of ZnO nanopillars manufactured on c-Al2 O3 wafer and on 6H-SiC wafer grown at 800 ◦ C are shown in Fig. 2(a) and Fig. 2(b) respectively. The more rapid growth of nanopillars on silicon carbide wafer along certain lines is caused by the wafer surface damage. AFM image of the surface of a commercial 6H-SiC wafer revealing scratches caused by mechanical polishing is shown in Fig. 2(c). Surface damaged layer can be removed by etching SiC wafers in hydrogen [10]. Growth of ZnO nanopillars on SiC wafers after hydrogen etching leads to formation of homogeneous nanopillar arrays. X-ray diffraction rocking curves with a full width at half maximum (FWHM) of 82800 were obtained for the grown nanopillar arrays (cf. Fig. 5(a)) showing an extremely good c-axis alignment for such several µm long nanopillars. Also, room temperature photoluminescence
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Fig. 2. SEM images of the VPT ZnO nanopillars grown on: (a) sapphire wafer; (b) SiC wafer; and (c) AFM image of the surface of a commercial SiC wafer revealing scratches caused by mechanical polishing.
Fig. 3. SEM images of ZnO grown on ZnO/sapphire epiwafers at different growth conditions: (a) nanopillars grown at 550 ◦ C growth temperature, Zn carrier gas flow 70 ml/min and 10 mbar reactor pressure; (b) pyramid-shaped structures obtained at 500 ◦ C, Zn carrier gas flow 60 ml/min and 8 mbar reactor pressure and (c) epitaxial layer after improvements at 500 ◦ C growth temperature, Zn carrier gas flow 60 ml/min and 5 mbar reactor pressure.
peaks of high intensity and FWHM of 90 meV were measured for the fabricated ZnO nanorods [11]. We also developed the growth of ZnO layers using our VPT approach. The initial experiments on ZnO layer growth directly on c-sapphire led to columnar growth of low-quality layers. The growth temperature was then reduced in relation to nanopillar growth temperatures. Nevertheless we did not obtain acceptable quality of the ZnO layers directly on c-sapphire. As a next step we employed a 6 nm low-temperature (LT) ZnO buffer grown with MBE on c-sapphire with intermediate MgO buffer layer [13,14]. In this case we obtained, under optimised conditions, quite reasonable results providing a FWHM of (0002) ZnO XRD rocking curve of 498.600 (cf. Fig. 5(b)). This value is comparable to the best results reported for ZnO layers grown on csapphire obtained by conventional MOCVD technique. We proceeded to improve the ZnO VPT growth and employed 300 nm ZnO/sapphire epiwafer grown with our MBE system. The details of the MBE growth are described elsewhere [13,14]. The initial experiments on these epiwafers were performed at 550 ◦ C, with a reactor pressure 10 mbar, Zn source temperature 460 ◦ C, oxygen and nitrogen flows of 55 and 70 sscm. This set of parameters resulted in nanopillar growth (cf. Fig. 3(a)). The suppression of three dimensional growth was targeted by a successive reduction in the growth rate. At the same oxygen flow (55 ml/min) the pressure was reduced to 8 mbar, the nitrogen carrier gas flow to 60 sscm and the growth temperature to 500 ◦ C. The result was predominantly broader pyramidal structures, indicating a transition from nanorod growth to layer growth (cf. Fig. 3(b)). Further decrease in the reactor pressure to 5 mbar led to growth of smooth ZnO layer (cf. Fig. 3(c)). Fig. 4 presents SEM images of high-quality smooth surface of the MBE 300 nm ZnO/sapphire epiwafer (cf. Fig. 3(a))
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Fig. 4. SEM images of: (a) high-quality smooth surface of the MBE ZnO/sapphire epiwafer; (b) ZnO layer grown in optimized conditions at 500 ◦ C growth temperature and 5 mbar reactor pressure. AFM images of the surfaces of the initial 300 nm ZnO/sapphire epiwafer (c) and of the surface of the consequently VPT grown ZnO layer (d) and (e). Estimated values of the surface roughness rms are 1.3 nm, 2.3 nm and 6.1 nm correspondingly.
Fig. 5. (0002) ZnO XRD rocking curves of: (a) ZnO nanorods on (11–20)Al2 O3 substrate showing full width at half maximum of 82800 ; (b) ZnO layer grown on 6 nm LT ZnO buffer grown with MBE on sapphire epiwafer showing FWHM of 498.600 ; and (c) the initial 300 nm ZnO/sapphire epiwafer showing FWHM of 4200 ; (d) ZnO layer grown on 300 nm ZnO/sapphire epiwafer (cf. Fig. 5(c)) showing full width at half maximum of 3800 .
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and of the VPT ZnO layer grown in optimized conditions at 500 ◦ C and 5 mbar reactor pressure and the corresponding AFM images of the surfaces of the initial 300 nm ZnO/sapphire epiwafer (cf. Fig. 3(c)) and of the surface of the subsequently VPT-grown ZnO layer (cf. Fig. 3(d)). Estimated values of the root mean square (rms) surface roughness are 1.3 nm and 2.3 nm correspondingly. We have to mention here that in this case of the initial experiments, we did not use the smoothest epiwafer that is producible with our MBE ZnO growth process. The best rms values which we obtained for the MBE grown ZnO/sapphire epiwafers are 0.26 nm [13,14]. So, there is further possibility to improve the surface roughness of the ZnO layer grown with the VPT system by employing better ZnO epiwafers. Sometimes, rare lone standing nanopillars were observed on the VPT-grown ZnO samples (cf. Fig. 3(e)). The origin of these nanopillars is not yet quite clear. In the case of MBE growth we observed such lone standing defects due to deviation from the optimised II/VI ratio in the gas phase, namely if we grew layers in slightly Zn-rich conditions. We can also suppose that the nanopillars originate from such rare nano-pyramidal defects which could be eventually present in some places on the surface of the initial epiwafer. (0002) ZnO XRD rocking curves of the initial 2” 300 nm ZnO/sapphire MBE epiwafer (showing FWHM of 4200 ) and of the ZnO layer grown on this epiwafer (showing full width at half maximum of 3800 ) are presented in figure (c) and (d), respectively. As can be seen from the XRD measurements, we have realised high growth rate of the VPT ZnO layer (about 2 µm/h) and at the same time slightly improved the quality of the initial MBE grown epitaxial layer on the sapphire substrate. 4. Conclusion ZnO thin films and ZnO nanostructures were fabricated using a home-made advanced vapour transport system with elemental sources. Al2 O3 , SiC and ZnO epiwafers implemented as substrates for this novel advanced VPT ZnO growth. In the latter case VPT growth technique was combined with MBE technology. The VPT fabrication of ZnO nanopillars, leading to well aligned, c-axis oriented nanopillars with excellent quality and purity is demonstrated. We have successfully grown ZnO nanorods directly on 6H-SiC and (11–20)Al2 O3 . Our approach is advantageous in that sources of undesired impurities are tremendously reduced, since elemental material sources are used and the growth is catalyst-free. The crystal and hence optical qualities of the rods are excellent. The nanopillars should be the basis for future nano-LED and UV detector structures, providing a potential of 109 –1010 devices/cm2 . The possibility to employ several fabrication techniques is of special importance for the realization of unique device structures. MBE grown ZnO/sapphire epiwafers were implemented for further high quality VPT ZnO layers’ fabrication. The quality of the VPT grown ZnO epitaxial layers on sapphire can be even higher (evaluated from FWHM of the XRD rocking curves) compared to the MBE grown ones used as the epiwafers for VPT growth. A FWHM of the (0002) rocking curve of 3800 was obtained for the VPT grown ZnO layers. More so, the growth rate is several times higher than for MBE (up to 2 µm/h), and growth takes place at relatively low temperatures and pressure in a simple system that can be applied to mass production. Acknowledgements The authors thank M. Karsten, K.-H. Lachmund and D. R¨ummler for their technical support and the Deutsche Forschungsgemeinschaft for financial support through project number DFGWA870/7 DFG-WA870/8 and EU project “NANDOS”.
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