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Doping transition of doped ZnO nanorods measured by Kelvin probe force microscopy
Thin Solid Films 520 (2012) 4622–4625
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Doping transition of doped ZnO nanorods measured by Kelvin probe force microscopy Chu Van Ben a, Hak Dong Cho b, Tae Won Kang b, Woochul Yang b,⁎ a b
Academy of Nano and Information Technology, Dongguk University, 3-26 Pil-dong, Jung-gu, Seoul 100-715, Republic of Korea Department of Physics and Quantum-Functional Semiconductor Reseach Center, Dongguk University, 3-26 Pil-dong, Jung-gu, Seoul 100-715, Republic of Korea
a r t i c l e
i n f o
Available online 29 October 2011 Keywords: ZnO nanorod Kelvin probe force microscopy Vapor phase transport Surface potential Surface state
a b s t r a c t We have investigated the doping transition of one-dimensional (1-D) doped-ZnO nanorods with Kelvin probe force microscopy (KPFM). Vertically aligned (undoped, As-doped, and undoped/As-doped homojunction) ZnO nanorods were grown on Si (111) substrates without any catalyst by vapor phase transport. Individual ZnO nanorods are removed from the substrates and transferred onto thin Au films grown on Si substrates. The morphology and surface potentials of the nanorods were measured simultaneously by the KPFM. For the homo-junction nanorods with ~ 250 nm in diameter, the KPFM image shows localization of the doping transition along the nanorods. The measured Kelvin signal (surface potential) across the junction induces the work function difference between the undoped and the As-doped region of ~85 meV. Also, the work function of As-doped nanorods is ~ 95 meV higher than that of intrinsically undoped nanorods grown in similar conditions. These consistent results indicate that the KPFM is reliable to determine the localization of the doping transition in 1-D structures. © 2011 Elsevier B.V. All rights reserved.
1. Introduction One dimensional (1-D) ZnO nanostructures are promising candidates for future electronic and optoelectronic devices such as light emitting diodes, lasers, photodetectors, field effect transistors, and solar cells due to a wide and direct bandgap semiconductor and a large exciton binding energy of 60 meV at room temperature. Nevertheless, it is extremely difficult to grow stable and reproducible p-type doped ZnO because of low solubility of p-type dopants and self-compensation of the existing abundant n-type impurities in intrinsic ZnO. Thus, many researchers have developed effective methods to achieve the p-type ZnO. Even though several groups have recently reported fabrication of p-type ZnO with N, P, As, and Ag dopants [1–4], 1-D ZnO nanostructures with p-type doping were rarely reported [5]. In addition, it is difficult to recognize doping type and concentration of 1-D ZnO nanostructures with ordinary methods such as Hall measurement, secondary ion mass spectroscopy, and others microscopy techniques because of narrow and localized distribution of charges and dopants in 1-D ZnO nanostructures. On the contrary, Kelvin probe force microscopy (KPFM) [6,7], which is one of techniques in built-in atomic force microscopy (AFM), is an adequate tool to simultaneously measure the electronic and the structural properties of 1-D nanostructures of various materials with high spatial resolution. The interaction between the conductive
⁎ Corresponding author. Tel.: + 82 2 2260 3444; fax: + 82 2 2278 4519. E-mail address: [email protected] (W. Yang). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.10.129
tip and materials surface in the KPFM measurements allows obtaining local mapping of the electric charge and surface potential distribution with nanoscale resolution. Previously, the KPFM has been employed to measure work function, surface photovoltage, and dopant distribution of various semiconductor nanostructures [8–10]. However, using KPFM to obtain localized electronic properties in compound semiconductor nanorods with different dopants is rare. In this study, we employed the KPFM technique to investigate the local electronic and structural properties of individual doped ZnO nanorods grown by vapor phase transport (VPT). Morphology and Kelvin signal (surface potential) of undoped, As-doped, and undoped/ As-doped homo-junction ZnO nanorods were measured simultaneously. KPFM images clearly displayed the doping contrast of the homo-junction nanorods. The Kelvin signals relative to Au surface led to obtain the work function difference between the undoped and the As-doped regions of the nanorods. In addition, the work function of the doped nanorods was induced from Kelvin signals and the difference between the estimated and KPFM measured work function is discussed in terms of surface state and surface band bending. 2. Experiments Vertically aligned ZnO nanorods were grown on Si (111) substrates by using a VPT method without using any catalyst. Zn (99.9999%) and O2 (99.995%) were utilized as reactants and N2 (99.999%) was used as a carrier gas. For growth of As-doped ZnO nanorods, ZnAs (99.99%) pellets were used as a compound. The growth temperature was
C. Van Ben et al. / Thin Solid Films 520 (2012) 4622–4625
750 °C. The self-assembled undoped/As-doped homojunction nanorods were fabricated by growth of undoped ZnO nanorods and followed by subsequent growth of As-doped ZnO. The surface morphology and size distribution of the ZnO nanorod matrix were characterized by scanning electron microsopy (SEM). The SEM image (Fig. 1) shows that the grown homo-junction nanorods are vertically well aligned along the c-axis with uniform diameter of ~ 200 nm and average length of ~ 40 μm. To perform the KPFM measurements, the ZnO nanorods need to be on the conducting substrate that provides a work function reference. They were transferred to a 40 nm thick Au film that was grown on a Si (001) substrate by e-beam. The Au film was uniformly flat with an average roughness of ~ 4.0 nm. The bottom parts of the ZnO nanorods were cut from the samples by rubbing the samples with a knife. Then, the nanorods that were cut were dropped onto the substrates. The nanorods were distributed randomly on the Au film and recognized with optical microscopy equipped with the AFM system. A commercial AFM system (Bruker-Nano N8 NEOS) equipped with KPFM module was used for all experiments. For the KPFM measurements, we used conductive Pt-coated Si tips with a radius of less than 25 nm. All measurements were performed in the non-contact mode. The topography and the surface potential (contact potential difference) were detected simultaneously using the amplitude modulation technique with two different frequencies for the topography and the surface \potential. For the topography measurement the tip oscillated at the resonance frequency of the cantilever of 75 kHz. For the surface potential measurements, an AC modulation voltage of ~2 V at the frequency 30 kHz superimposing a DC bias voltage was applied between the sample and the grounded tip. The resulting electrostatic forces acting on the tip were detected by a lock-in technique analyzing local differences in the tip oscillation amplitude. The amplitude signal at the modulation frequency includes the DC bias voltage plus the work function difference between the tip and the sample, which is the contact potential difference (VCPD) between the tip and sample surface, VCPD = (ϕtip - ϕsample)/-e. Here, ϕtip and ϕsample are the work function of the tip and the sample, respectively, and e is the elementary charge. The feedback circuit in the AFM system controls the DC bias voltage to compensate VCPD to make the amplitude signal null. Thus, the compensated DC voltage signal corresponds to the local surface potential of the sample surface. The resulting KPFM image maps the variation of surface potential corresponding to the relative work function on the surface with respect to tip work function.
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3. Results and discussions First, we investigated the intrinsically grown undoped ZnO nanorods and the As-doped nanorods by KPFM. Fig. 2 shows the AFM topography and the KPFM VCPD images of each nanorod transferred on the Au films, respectively. In AFM topography images the shape of the nanorods is shown to be tapered. The diameter of both type nanorods along the axis varied in the range from 25 nm to 300 nm (Fig. 2-a, c). However the brightness of the nanorods in the KPFM VCPD images does not change along the axis (Fig. 2-b, d), which indicates that the surface potential is independent of the diameter variation. The KPFM image is displayed by mapping of the applied DC bias voltage of the feedback loop to maintain a null work function difference (VCPD) between the tip and sample surface. Thus, the brightness variation in the KPFM image reflects the local surface work function difference with respect to the tip work function. The darker region has a smaller VCPD between the tip and the surface region, indicating relatively higher work function than other regions. On the contrary, the brighter region has a relatively lower work function. Accordingly, in Fig. 2-b and d, the brightness variation indicates the VCPD (contact potential) variation from the VCPD between Pt-coated tip and Au substrate ((ϕpt - ϕAu) / e) to the VCPD between tip and ZnO nanorod ((ϕpt - ϕZnO)/ e). Fig. 3 shows VCPD (=(ϕpt - ϕZnO)/e) profile at the varying diameters along the As-doped and undoped nanorods with respect to the work function of Pt, which was extracted from the KPFM images in Fig. 2-b and d, respectively. The average VCPD of undoped ZnO nanorod is ~95 mV higher than that of As-doped ZnO nanorod. This indicates that work function of the As-doped ZnO nanorod is ~95 meV higher than that of the undoped ZnO nanorods. Next, we observed doping transition of the homo-junction ZnO nanorods, in which As-doped ZnO nanorods were grown onto the undoped ZnO nanorod. Fig. 4 shows the 2D morphology and KPFM VCPD images of the homo-junction nanorod on the Au substrate. During the measurement, AFM tip was moved along the rod axis to maintain the same measurement condition. In Fig. 4 (a), the junction nanorod has 19.3 μm in length and a relatively uniform diameter of 260 nm along the nanorod. Also, the topography shows no specific structural variations in the vicinity of the doping transition region. In contrary, in the KPFM signal image (Fig. 4 (b)) the brightness contrast appears obviously near the middle region of the nanorod. The brighter region on the left of the nanorod has a lower work function while the darker region on the right has a higher work function. In comparison with the KPFM measurements of each undoped and As-doped ZnO nanorods
Fig. 1. SEM image of undoped and followed by As-doped ZnO nanorods grown on a Si (111) substrate.
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C. Van Ben et al. / Thin Solid Films 520 (2012) 4622–4625
Fig. 4. (a) Topography image and (b) KPFM image of a homo-junction ZnO nanorod.
Fig. 2. AFM topography (a, c) and KPFM VCPD images (b, d) of As-doped ZnO and undoped ZnO nanorods, respectively.
(Fig. 2), the brighter region corresponds to the undoped ZnO part while the darker region corresponds to the As-doped ZnO part of the homojunction structure. Therefore, we could visualize the doping transition in partially doped ZnO nanorod by the KPFM measurement. In addition, we obtained the surface potential difference (ΔVCPD) of ~85 mV between the undoped and the As-doped regions by extracting the line profile for VCPD along the axis of the nanorod from the KPFM image (Fig. 5). This value is in good agreement with ~95 mV for ΔVCPD between the undoped and As-doped nanorods. Considering the work function of the tip (Pt: 5.6 eV) and the surface (Au: 5.1 eV) [11], we can estimate the work functions of undoped and As-doped ZnO nanorods. The work function of the undoped region is ~245 meV lower while the As-doped region is ~160 meV lower than the work function of the Au substrate. Thus, if we did not consider surface band bending for both regions, we can deduce that the work function of undoped and As-doped region is 4.855 eV and 4.940 eV, respectively. It was reported that ZnO nanorods grown with As-dopant show p-type properties [3,12]. Photoluminescence measurement and current–voltage characteristics of our As-doped ZnO nanorods displayed p-type properties, as will be reported elsewhere. Also, it is known that the as-grown ZnO is an n-type semiconductor for its intrinsic defects. Thus, the homo-junction ZnO nanorods with As-doped and intrinsically undoped regions might be a p-n junction structure. For our homojunction nanorod, the change of VCPD across the junction is expected to be a little lower than ~1.7 V or half the band gap of ZnO. However,
Fig. 3. VCPD at different diameters along the As-doped and the undoped ZnO nanorods extracted from KPFM images in Fig. 2.
the measured VCPD of ~85 mV is much smaller than the expected value. To explain this discrepancy, we can consider two effects; surface electronic structure variation and tip-sample electrostatic interaction in KPFM measurements. Recently S. Vinaji et al. reported a similar KPFM result for p-n junction GaAs nanowires [13]. The measured work function difference between undoped and p-doped GaAs was 80 to 90 meV, which is a similar to our value. Also, D. Theron et al. presented the work function difference ~0.46 eV for n- and p-type GaN films with ~1017 and ~1019 cm− 3 doping concentration [14]. This value is much larger than our measured value. However, Regardless of the band gap and doping level of the compound semiconductors, work function variation from p-type to n-type measured by the KPFM might be much smaller than the expected value. This large difference could be explained by the surface band bending caused by the surface states and surface charges [14–16]. In addition, as S. Vinaji et al. pointed out, it is difficult to directly derive quantitative values of Fermi level and doping concentration in the nanorod from the KPFM data [13]. Also, we could not exclude strong affection on the local surface potential of the nanorods by the electrostatic coupling between the entire probe and the sample to obtain real electrostatic potential under the tip apex in the KPFM measurements [17,18]. Therefore, to obtain quantitative work function of semiconductor 1-D nanostructures by KPFM measurements, it is required to develop a proper model considering detailed tip-sample interaction, and the variation of surface band structures including electron affinity and surface band bending due to surface absorption layer, surface states, and Fermi level pining at the surface.
4. Conclusions In summary, KPFM was employed to characterize the doping transition of individual doped-ZnO nanorods grown by VPT. For the (undoped/As-doped) homo-junction nanorods with ~ 200 nm in
Fig. 5. The line profile along the nanorod axis in the KPFM image of Fig. 4-b.
C. Van Ben et al. / Thin Solid Films 520 (2012) 4622–4625
diameter, the KPFM image showed significant doping contrast along the nanorods. The surface potential of the As-doped regions is ~ 85 mV lower than the undoped region. This is consistent with the value obtained from each undoped and As-doped ZnO nanorods. If we expect the formation of p-type ZnO with As-dopant, this value might be much smaller than the expected value considering a ZnO half bandgap of ~ 1.7 eV. This would be explained by the surface band bending due to surface states and charges in ZnO nanorod. To quantitatively analyze the work function of doped semiconductor 1-D structures by KPFM, it is required to develop a proper model. Even though the surface potential can vary with surface conditions and surface band bending, the KPFM is a unique technique to investigate the doping transition in semiconductor 1-D structures.
Acknowledgments This work was supported by the National Research Foundation (NRF) of Korea grant funded by the Ministry of Education, Science and Technology (MEST) (Nos. 2010-0015844 and 2010-0000751), and by the Leading Foreign Research Institute Recruitment Program through the NRF grant (No. 2010-00218).
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References [1] M. Ding, D. Zhao, B. Yao, B. Li, Zh. Zhang, D. Shen, Appl. Phys. Lett. 98 (2011) 062102. [2] S.J. Kang, Y.H. Joung, J.W. Han, Y.S. Yoon, J. Mater. Sci. Mater. Electron. 22 (2011) 248. [3] C.Y. Zhu, S. Fung, Y.C. Zhong, K.S. Wong, Z. Xie, G. Brauer, W. Anwand, W. Skorupa, C.K. To, B. Yang, C.D. Beling, C.C. Ling, J. Appl. Phys. 106 (2009) 073709. [4] R. Chen, Ch. Zou, J. Bian, A. Sandhu, W. Gao, Nanotechnology 22 (2011) 105706. [5] M.K. Gupta, N. Sinha, B.K. Singh, B. Kuma, Mater. Lett. 64 (2010) 1825. [6] M. Nonnenmacher, M.P. O'Boyle, H.K. Wichramasinghe, Appl. Phys. Lett. 58 (1991) 2921. [7] K.L. Sorokina, A.L. Tolstikhina, Crystallogr. Rep. 49 (2004) 476. [8] S. Sadewasser, Th. Glatzel, M. Rusu, A. Jäger-Waldau, M.Ch. Lux-Steiner, Appl. Phys. Lett. 80 (2002) 2979. [9] W. Bergbauer, T. Lutz, W. Frammelsberger, G. Benstetter, Microelectron. Reliab. 46 (2006) 1736. [10] E. Koren, N. Berkovitch, Y. Rosenwaks, Nano Lett. 10 (2010) 1163. [11] L. Pintilie, I. Vrejoiu, D. Hesse, M. Alexe, J. Appl. Phys. 104 (2008) 114101. [12] W. Lee, M.-C. Jeong, J.-M. Myoung, Appl. Phys. Lett. 85 (2004) 6167. [13] S. Vinaji, A. Lochthofen, W. Mertin, I. Regolin, C. Gutsche, W. Prost, F.J. Tegude, G. Bacher, Nanotechnology 20 (2009) 385702. [14] S. Barbet, R. Aubry, M.-A. Di Frote-Poisson, J.-C. Jacquet, D. Deresmes, T. Melin, D. Theron, Appl. Phys. Lett. 93 (2008) 212107. [15] T. Mizutani, T. Usunami, S. Kishimoto, K. Maezawa, Jpn. J. Appl. Phys. 38 (1999) 4893. [16] S. Saraf, Y. Rosenwaks, Surf. Sci. 574 (2005) L35. [17] D.S.H. Charrier, M. Kemerink, B.E. Smalbrugge, T. de Vries, R.A.J. Janssen, ACS Nano 2 (2008) 622. [18] C. Baumgart, M. Helm, H. Schmidt, Phys. Rev. B 80 (2009) 085305.
Contents lists available at SciVerse ScienceDirect
Thin Solid Films journal homepage: www.elsevier.com/locate/tsf
Doping transition of doped ZnO nanorods measured by Kelvin probe force microscopy Chu Van Ben a, Hak Dong Cho b, Tae Won Kang b, Woochul Yang b,⁎ a b
Academy of Nano and Information Technology, Dongguk University, 3-26 Pil-dong, Jung-gu, Seoul 100-715, Republic of Korea Department of Physics and Quantum-Functional Semiconductor Reseach Center, Dongguk University, 3-26 Pil-dong, Jung-gu, Seoul 100-715, Republic of Korea
a r t i c l e
i n f o
Available online 29 October 2011 Keywords: ZnO nanorod Kelvin probe force microscopy Vapor phase transport Surface potential Surface state
a b s t r a c t We have investigated the doping transition of one-dimensional (1-D) doped-ZnO nanorods with Kelvin probe force microscopy (KPFM). Vertically aligned (undoped, As-doped, and undoped/As-doped homojunction) ZnO nanorods were grown on Si (111) substrates without any catalyst by vapor phase transport. Individual ZnO nanorods are removed from the substrates and transferred onto thin Au films grown on Si substrates. The morphology and surface potentials of the nanorods were measured simultaneously by the KPFM. For the homo-junction nanorods with ~ 250 nm in diameter, the KPFM image shows localization of the doping transition along the nanorods. The measured Kelvin signal (surface potential) across the junction induces the work function difference between the undoped and the As-doped region of ~85 meV. Also, the work function of As-doped nanorods is ~ 95 meV higher than that of intrinsically undoped nanorods grown in similar conditions. These consistent results indicate that the KPFM is reliable to determine the localization of the doping transition in 1-D structures. © 2011 Elsevier B.V. All rights reserved.
1. Introduction One dimensional (1-D) ZnO nanostructures are promising candidates for future electronic and optoelectronic devices such as light emitting diodes, lasers, photodetectors, field effect transistors, and solar cells due to a wide and direct bandgap semiconductor and a large exciton binding energy of 60 meV at room temperature. Nevertheless, it is extremely difficult to grow stable and reproducible p-type doped ZnO because of low solubility of p-type dopants and self-compensation of the existing abundant n-type impurities in intrinsic ZnO. Thus, many researchers have developed effective methods to achieve the p-type ZnO. Even though several groups have recently reported fabrication of p-type ZnO with N, P, As, and Ag dopants [1–4], 1-D ZnO nanostructures with p-type doping were rarely reported [5]. In addition, it is difficult to recognize doping type and concentration of 1-D ZnO nanostructures with ordinary methods such as Hall measurement, secondary ion mass spectroscopy, and others microscopy techniques because of narrow and localized distribution of charges and dopants in 1-D ZnO nanostructures. On the contrary, Kelvin probe force microscopy (KPFM) [6,7], which is one of techniques in built-in atomic force microscopy (AFM), is an adequate tool to simultaneously measure the electronic and the structural properties of 1-D nanostructures of various materials with high spatial resolution. The interaction between the conductive
⁎ Corresponding author. Tel.: + 82 2 2260 3444; fax: + 82 2 2278 4519. E-mail address: [email protected] (W. Yang). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.10.129
tip and materials surface in the KPFM measurements allows obtaining local mapping of the electric charge and surface potential distribution with nanoscale resolution. Previously, the KPFM has been employed to measure work function, surface photovoltage, and dopant distribution of various semiconductor nanostructures [8–10]. However, using KPFM to obtain localized electronic properties in compound semiconductor nanorods with different dopants is rare. In this study, we employed the KPFM technique to investigate the local electronic and structural properties of individual doped ZnO nanorods grown by vapor phase transport (VPT). Morphology and Kelvin signal (surface potential) of undoped, As-doped, and undoped/ As-doped homo-junction ZnO nanorods were measured simultaneously. KPFM images clearly displayed the doping contrast of the homo-junction nanorods. The Kelvin signals relative to Au surface led to obtain the work function difference between the undoped and the As-doped regions of the nanorods. In addition, the work function of the doped nanorods was induced from Kelvin signals and the difference between the estimated and KPFM measured work function is discussed in terms of surface state and surface band bending. 2. Experiments Vertically aligned ZnO nanorods were grown on Si (111) substrates by using a VPT method without using any catalyst. Zn (99.9999%) and O2 (99.995%) were utilized as reactants and N2 (99.999%) was used as a carrier gas. For growth of As-doped ZnO nanorods, ZnAs (99.99%) pellets were used as a compound. The growth temperature was
C. Van Ben et al. / Thin Solid Films 520 (2012) 4622–4625
750 °C. The self-assembled undoped/As-doped homojunction nanorods were fabricated by growth of undoped ZnO nanorods and followed by subsequent growth of As-doped ZnO. The surface morphology and size distribution of the ZnO nanorod matrix were characterized by scanning electron microsopy (SEM). The SEM image (Fig. 1) shows that the grown homo-junction nanorods are vertically well aligned along the c-axis with uniform diameter of ~ 200 nm and average length of ~ 40 μm. To perform the KPFM measurements, the ZnO nanorods need to be on the conducting substrate that provides a work function reference. They were transferred to a 40 nm thick Au film that was grown on a Si (001) substrate by e-beam. The Au film was uniformly flat with an average roughness of ~ 4.0 nm. The bottom parts of the ZnO nanorods were cut from the samples by rubbing the samples with a knife. Then, the nanorods that were cut were dropped onto the substrates. The nanorods were distributed randomly on the Au film and recognized with optical microscopy equipped with the AFM system. A commercial AFM system (Bruker-Nano N8 NEOS) equipped with KPFM module was used for all experiments. For the KPFM measurements, we used conductive Pt-coated Si tips with a radius of less than 25 nm. All measurements were performed in the non-contact mode. The topography and the surface potential (contact potential difference) were detected simultaneously using the amplitude modulation technique with two different frequencies for the topography and the surface \potential. For the topography measurement the tip oscillated at the resonance frequency of the cantilever of 75 kHz. For the surface potential measurements, an AC modulation voltage of ~2 V at the frequency 30 kHz superimposing a DC bias voltage was applied between the sample and the grounded tip. The resulting electrostatic forces acting on the tip were detected by a lock-in technique analyzing local differences in the tip oscillation amplitude. The amplitude signal at the modulation frequency includes the DC bias voltage plus the work function difference between the tip and the sample, which is the contact potential difference (VCPD) between the tip and sample surface, VCPD = (ϕtip - ϕsample)/-e. Here, ϕtip and ϕsample are the work function of the tip and the sample, respectively, and e is the elementary charge. The feedback circuit in the AFM system controls the DC bias voltage to compensate VCPD to make the amplitude signal null. Thus, the compensated DC voltage signal corresponds to the local surface potential of the sample surface. The resulting KPFM image maps the variation of surface potential corresponding to the relative work function on the surface with respect to tip work function.
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3. Results and discussions First, we investigated the intrinsically grown undoped ZnO nanorods and the As-doped nanorods by KPFM. Fig. 2 shows the AFM topography and the KPFM VCPD images of each nanorod transferred on the Au films, respectively. In AFM topography images the shape of the nanorods is shown to be tapered. The diameter of both type nanorods along the axis varied in the range from 25 nm to 300 nm (Fig. 2-a, c). However the brightness of the nanorods in the KPFM VCPD images does not change along the axis (Fig. 2-b, d), which indicates that the surface potential is independent of the diameter variation. The KPFM image is displayed by mapping of the applied DC bias voltage of the feedback loop to maintain a null work function difference (VCPD) between the tip and sample surface. Thus, the brightness variation in the KPFM image reflects the local surface work function difference with respect to the tip work function. The darker region has a smaller VCPD between the tip and the surface region, indicating relatively higher work function than other regions. On the contrary, the brighter region has a relatively lower work function. Accordingly, in Fig. 2-b and d, the brightness variation indicates the VCPD (contact potential) variation from the VCPD between Pt-coated tip and Au substrate ((ϕpt - ϕAu) / e) to the VCPD between tip and ZnO nanorod ((ϕpt - ϕZnO)/ e). Fig. 3 shows VCPD (=(ϕpt - ϕZnO)/e) profile at the varying diameters along the As-doped and undoped nanorods with respect to the work function of Pt, which was extracted from the KPFM images in Fig. 2-b and d, respectively. The average VCPD of undoped ZnO nanorod is ~95 mV higher than that of As-doped ZnO nanorod. This indicates that work function of the As-doped ZnO nanorod is ~95 meV higher than that of the undoped ZnO nanorods. Next, we observed doping transition of the homo-junction ZnO nanorods, in which As-doped ZnO nanorods were grown onto the undoped ZnO nanorod. Fig. 4 shows the 2D morphology and KPFM VCPD images of the homo-junction nanorod on the Au substrate. During the measurement, AFM tip was moved along the rod axis to maintain the same measurement condition. In Fig. 4 (a), the junction nanorod has 19.3 μm in length and a relatively uniform diameter of 260 nm along the nanorod. Also, the topography shows no specific structural variations in the vicinity of the doping transition region. In contrary, in the KPFM signal image (Fig. 4 (b)) the brightness contrast appears obviously near the middle region of the nanorod. The brighter region on the left of the nanorod has a lower work function while the darker region on the right has a higher work function. In comparison with the KPFM measurements of each undoped and As-doped ZnO nanorods
Fig. 1. SEM image of undoped and followed by As-doped ZnO nanorods grown on a Si (111) substrate.
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C. Van Ben et al. / Thin Solid Films 520 (2012) 4622–4625
Fig. 4. (a) Topography image and (b) KPFM image of a homo-junction ZnO nanorod.
Fig. 2. AFM topography (a, c) and KPFM VCPD images (b, d) of As-doped ZnO and undoped ZnO nanorods, respectively.
(Fig. 2), the brighter region corresponds to the undoped ZnO part while the darker region corresponds to the As-doped ZnO part of the homojunction structure. Therefore, we could visualize the doping transition in partially doped ZnO nanorod by the KPFM measurement. In addition, we obtained the surface potential difference (ΔVCPD) of ~85 mV between the undoped and the As-doped regions by extracting the line profile for VCPD along the axis of the nanorod from the KPFM image (Fig. 5). This value is in good agreement with ~95 mV for ΔVCPD between the undoped and As-doped nanorods. Considering the work function of the tip (Pt: 5.6 eV) and the surface (Au: 5.1 eV) [11], we can estimate the work functions of undoped and As-doped ZnO nanorods. The work function of the undoped region is ~245 meV lower while the As-doped region is ~160 meV lower than the work function of the Au substrate. Thus, if we did not consider surface band bending for both regions, we can deduce that the work function of undoped and As-doped region is 4.855 eV and 4.940 eV, respectively. It was reported that ZnO nanorods grown with As-dopant show p-type properties [3,12]. Photoluminescence measurement and current–voltage characteristics of our As-doped ZnO nanorods displayed p-type properties, as will be reported elsewhere. Also, it is known that the as-grown ZnO is an n-type semiconductor for its intrinsic defects. Thus, the homo-junction ZnO nanorods with As-doped and intrinsically undoped regions might be a p-n junction structure. For our homojunction nanorod, the change of VCPD across the junction is expected to be a little lower than ~1.7 V or half the band gap of ZnO. However,
Fig. 3. VCPD at different diameters along the As-doped and the undoped ZnO nanorods extracted from KPFM images in Fig. 2.
the measured VCPD of ~85 mV is much smaller than the expected value. To explain this discrepancy, we can consider two effects; surface electronic structure variation and tip-sample electrostatic interaction in KPFM measurements. Recently S. Vinaji et al. reported a similar KPFM result for p-n junction GaAs nanowires [13]. The measured work function difference between undoped and p-doped GaAs was 80 to 90 meV, which is a similar to our value. Also, D. Theron et al. presented the work function difference ~0.46 eV for n- and p-type GaN films with ~1017 and ~1019 cm− 3 doping concentration [14]. This value is much larger than our measured value. However, Regardless of the band gap and doping level of the compound semiconductors, work function variation from p-type to n-type measured by the KPFM might be much smaller than the expected value. This large difference could be explained by the surface band bending caused by the surface states and surface charges [14–16]. In addition, as S. Vinaji et al. pointed out, it is difficult to directly derive quantitative values of Fermi level and doping concentration in the nanorod from the KPFM data [13]. Also, we could not exclude strong affection on the local surface potential of the nanorods by the electrostatic coupling between the entire probe and the sample to obtain real electrostatic potential under the tip apex in the KPFM measurements [17,18]. Therefore, to obtain quantitative work function of semiconductor 1-D nanostructures by KPFM measurements, it is required to develop a proper model considering detailed tip-sample interaction, and the variation of surface band structures including electron affinity and surface band bending due to surface absorption layer, surface states, and Fermi level pining at the surface.
4. Conclusions In summary, KPFM was employed to characterize the doping transition of individual doped-ZnO nanorods grown by VPT. For the (undoped/As-doped) homo-junction nanorods with ~ 200 nm in
Fig. 5. The line profile along the nanorod axis in the KPFM image of Fig. 4-b.
C. Van Ben et al. / Thin Solid Films 520 (2012) 4622–4625
diameter, the KPFM image showed significant doping contrast along the nanorods. The surface potential of the As-doped regions is ~ 85 mV lower than the undoped region. This is consistent with the value obtained from each undoped and As-doped ZnO nanorods. If we expect the formation of p-type ZnO with As-dopant, this value might be much smaller than the expected value considering a ZnO half bandgap of ~ 1.7 eV. This would be explained by the surface band bending due to surface states and charges in ZnO nanorod. To quantitatively analyze the work function of doped semiconductor 1-D structures by KPFM, it is required to develop a proper model. Even though the surface potential can vary with surface conditions and surface band bending, the KPFM is a unique technique to investigate the doping transition in semiconductor 1-D structures.
Acknowledgments This work was supported by the National Research Foundation (NRF) of Korea grant funded by the Ministry of Education, Science and Technology (MEST) (Nos. 2010-0015844 and 2010-0000751), and by the Leading Foreign Research Institute Recruitment Program through the NRF grant (No. 2010-00218).
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References [1] M. Ding, D. Zhao, B. Yao, B. Li, Zh. Zhang, D. Shen, Appl. Phys. Lett. 98 (2011) 062102. [2] S.J. Kang, Y.H. Joung, J.W. Han, Y.S. Yoon, J. Mater. Sci. Mater. Electron. 22 (2011) 248. [3] C.Y. Zhu, S. Fung, Y.C. Zhong, K.S. Wong, Z. Xie, G. Brauer, W. Anwand, W. Skorupa, C.K. To, B. Yang, C.D. Beling, C.C. Ling, J. Appl. Phys. 106 (2009) 073709. [4] R. Chen, Ch. Zou, J. Bian, A. Sandhu, W. Gao, Nanotechnology 22 (2011) 105706. [5] M.K. Gupta, N. Sinha, B.K. Singh, B. Kuma, Mater. Lett. 64 (2010) 1825. [6] M. Nonnenmacher, M.P. O'Boyle, H.K. Wichramasinghe, Appl. Phys. Lett. 58 (1991) 2921. [7] K.L. Sorokina, A.L. Tolstikhina, Crystallogr. Rep. 49 (2004) 476. [8] S. Sadewasser, Th. Glatzel, M. Rusu, A. Jäger-Waldau, M.Ch. Lux-Steiner, Appl. Phys. Lett. 80 (2002) 2979. [9] W. Bergbauer, T. Lutz, W. Frammelsberger, G. Benstetter, Microelectron. Reliab. 46 (2006) 1736. [10] E. Koren, N. Berkovitch, Y. Rosenwaks, Nano Lett. 10 (2010) 1163. [11] L. Pintilie, I. Vrejoiu, D. Hesse, M. Alexe, J. Appl. Phys. 104 (2008) 114101. [12] W. Lee, M.-C. Jeong, J.-M. Myoung, Appl. Phys. Lett. 85 (2004) 6167. [13] S. Vinaji, A. Lochthofen, W. Mertin, I. Regolin, C. Gutsche, W. Prost, F.J. Tegude, G. Bacher, Nanotechnology 20 (2009) 385702. [14] S. Barbet, R. Aubry, M.-A. Di Frote-Poisson, J.-C. Jacquet, D. Deresmes, T. Melin, D. Theron, Appl. Phys. Lett. 93 (2008) 212107. [15] T. Mizutani, T. Usunami, S. Kishimoto, K. Maezawa, Jpn. J. Appl. Phys. 38 (1999) 4893. [16] S. Saraf, Y. Rosenwaks, Surf. Sci. 574 (2005) L35. [17] D.S.H. Charrier, M. Kemerink, B.E. Smalbrugge, T. de Vries, R.A.J. Janssen, ACS Nano 2 (2008) 622. [18] C. Baumgart, M. Helm, H. Schmidt, Phys. Rev. B 80 (2009) 085305.