Photoassisted Scanning Tunneling Microscopy Investigation on the

Apr 26, 2012 - Photoassisted Scanning Tunneling Microscopy Investigation on the ZnO(0001)-Zn Surface Treated by Alkaline Solution. Wan-Hsien Lin† ...
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Photoassisted Scanning Tunneling Microscopy Investigation on the ZnO(0001)-Zn Surface Treated by Alkaline Solution Wan-Hsien Lin,† Hikaru Saito,‡ Takashi Nemoto,‡ Hiroki Kurata,*,‡ Mitch M. C. Chou,§ Seiji Isoda,¶ and Jih-Jen Wu*,† †

Department of Chemical Engineering, National Cheng Kung University, Tainan 70101, Taiwan Institute for Chemical Research, Kyoto University, Uji, Kyoto 611-0011, Japan § Department of Materials Science and Opto-electronic Engineering, National Sun Yat-Sen University, Kaohsiung 80424, Taiwan ¶ Institute for Integrated Cell-Material Sciences, Kyoto University, Yoshida Ushinomiya-cho, Sakyo-ku, Kyoto 606-8501, Japan ‡

ABSTRACT: In this study, the surface geometric structures of epitaxial (0001) ZnO films treated by NaOH solution are investigated using photoassisted scanning tunneling microscopy (STM). By illuminating ultraviolet (UV) light on the epitaxial (0001) ZnO film, the tunneling current can be significantly enhanced to construct the well-defined STM images. Polarity identification of the epitaxial (0001) ZnO film by convergent-beam electron diffraction indicates that the epitaxial (0001) ZnO film exhibits the Zn-polar surface. Two types of topographic features, i.e., hexagonal pyramid and flat plane, are observed in the AFM images of the as-grown epitaxial (0001) ZnO film. UV-assisted STM images reveal the anisotropic etching behaviors of the epitaxial (0001) ZnO films in NaOH solution. The faceted and symmetrically layered hexagonal-pyramid feature gets asymmetrical and rounded with increasing etching time. On the other hand, few small hexagonal pits on the as-grown flat ZnO(0001)-Zn surface are developed to asymmetrically hexagonal cavities with flat terraces and steps after NaOH treatments. In addition, triangular reconstruction of the NaOH-treated ZnO(0001)-Zn surface and evidently layerstacking feature on a faceted ZnO surface with a step height resolved in the atomic scale are also demonstrated in ambience using the photoassisted STM.

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The etching mechanisms of both (0001) and (0001̅) surfaces of ZnO crystals in acidic and alkaline solution have been investigated in a few reports.5,10 Owing to the significant difference in electronegativity between Zn and O atoms, the covalent bonds in the ZnO crystal exhibit partial ionic character. The positive and negative paired dangling electrons therefore remain on the Zn- and O-terminated surfaces, respectively. The charged dangling bonds on the surface will attract the opposite charged primary ions in the etchant, such as H3O+ and OH− ions, for further chemical reactions. However, if the primary ions in the etching solution are repelled by the surface dangling bonds due to the same charge polarity, the chemical etching principally takes place at dislocations. As a result, the morphology of Zn- or O-terminated ZnO single crystals developed from acidic and alkaline solutions are very different because of the essentially opposite nature. In the acidic solutions, the ZnO(0001)-Zn surface develops hexagonal pits on the remaining smooth surface whereas the ZnO(0001)-O surface becomes rough because of the formation

INTRODUCTION Because of its wide band gap of 3.37 eV and large exciton binding energy of 60 meV at room temperature, ZnO has attracted much attention as a promising material for optoelectronic and photochemical application.1−4 ZnO possesses a wurtzite structure and grows preferentially along the [0001] direction. Consequently, c-plane ZnO films are conventionally deposited on most substrates. Wet chemical etching is a widely used method to make patterns on the ZnO films either for the fabrication of devices or simply for determining the polarity of film. Acidic and alkaline solutions which provide hydronium (H3O+) and hydroxide (OH−) ions, respectively, have been commonly employed for the wet chemical etchings of ZnO films.5−7 On the other hand, the chemical bath deposition (CBD) of ZnO crystals is achievable in basic aqueous solutions.3,4 In a basic region of pH from 9.0 to 13.0, the crystal structure of ZnO was gradually constructed by dehydration between OH− on the surface of the growing crystals and the OH− ligands of the hydroxyl complexes in aqueous solutions.8 Moreover, it has been suggested that etching of ZnO surface by OH− ions also occurs during CBD growth.9 Accordingly, it is crucial to understand the surface chemistry of the ZnO/etchant interface. © 2012 American Chemical Society

Received: March 1, 2012 Revised: April 25, 2012 Published: April 26, 2012 10664

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Crystalline characterization of the as-grown ZnO film was performed by transmission electron microscopy (TEM, JEOL JEM-2000FS). The TEM specimen used in the present work was prepared by mechanical polishing followed by the Ar ion beam milling. The thickness of as-grown ZnO film was determined by high-angle annular dark-field (HAADF) scanning transmission electron microscopy (JEOL JEM9980TKP1). The polarity of epitaxial (0001) ZnO films was identified by convergent-beam electron diffraction (CBED) using FEI Tecnai F20 TEM. The simulations of the CBED patterns were performed using the web-based electron microscopy application software (Web-EMAPS) developed by University of Illinois at Urbana−Champaign.16 Field-emission scanning electron microscopy (FE-SEM, JEOL JSM-6700F) and the attached energy-dispersive spectrometer (EDS) were employed to investigate the morphology as well as the compositions of ZnO films. Atomic force microscopy (AFM, Innova (di, Bruker AXS)) was performed using the tapping mode. Silicon cantilevers (Nanosensors, PPP-RT-NCHR) with spring constant and typical resonance frequency of ∼42 N m−1 and ∼330 kHz, respectively, were employed for AFM measurement. Scanning tunneling microscopy, (STM, NanoScope IIIa, Multimode (di, Bruker AXS)) was employed to investigate the geometric structures of all ZnO-based samples under UV illumination in ambience. The schematic of UV-assisted STM instrument is shown in Figure 1. The UV source was assembled

of hexagonal pyramids over the surface. On the other hand, hexagonal caves are evolved on the ZnO(0001)-O surface in the alkaline solutions, which is similar to the etching behavior of the ZnO(0001)-Zn surface in acidic solutions. No characteristic structure is present in the scanning electron microscopy (SEM) images of the ZnO(0001)-Zn surfaces etched by NaOH solution.5 Consequently, very few studies reported the geometric structure evolved from alkaline solution in detail. In situ investigation of the ZnO(0001)-Zn surface in alkaline electrolyte by atomic force microscopy (AFM) was demonstrated recently.11 The results suggest that, in addition to being the etchant, OH− ions adsorbed at hcp hollow sites also play a crucial role in the stabilization of the ZnO(0001)-Zn surface in the aqueous solution. Stabilization of the ZnO(0001)-Zn surface has attracted much attention because ZnO is widely used in many applications associated with surface structures, such as catalysis and gas sensing. Three mechanisms have been proposed for the stabilization of Zn-terminated ZnO surface,12 including (1) creation of surface states by charge transfer of negative charges from the O to the Zn face; (2) removal of surface atoms and reconstruction involving triangular surface structures; and (3) adsorbates that stabilize the polar faces of ZnO. Scanning tunneling microscopy (STM) has been commonly used to investigate the stabilization of polar ZnO surfaces.12,13 For the study of surface geometric structure with an atomic-scale resolution, the STM measurements are usually carried out in an ultrahigh vacuum (UHV) system to avoid undesired adsorption of contaminators on the sample surface. In this study, the etching behaviors of epitaxial (0001) ZnO films in NaOH solution are investigated using scanning probe microscopy (SPM). The (0001) ZnO films were grown on the lattice-matched (001) β-LiGaO2 (LGO) substrates using metalorganic chemical vapor deposition (MOCVD).7 The crystal structures and polarity of as-grown ZnO films were first characterized by transmission electron microscopy (TEM) based techniques. The morphology evolution of NaOH-treated (0001) ZnO films with etching periods was investigated by the SPM-based techniques, including AFM and photo (UV)assisted STM. A unique UV-assisted STM is developed in the present work to attain well-defined STM images of the wideband-gap ZnO film in ambience by utilizing the superior photoconductivity of the film. Stabilization of ZnO(0001)-Zn surface after alkaline etching is also investigated in ambience in this study by using the UV-assisted STM.

Figure 1. Schematic diagram of the photoassisted STM instrument.

using commercial UV-LED lamps with the irradiation range of 360−400 nm. The highest intensity in the illuminated spectrum of the UV-LED is at ∼380 nm (Nitride Semiconductors Co., Ltd., NS375L-5RLO). Output power of this UV source is about 33 mW, and the distance between UV-LED and ZnO sample is 1.5 cm during the measurement. All the UV-assisted STM measurements were performed using the constant current mode with bias of +2 V and tunneling current of 5 pA in ambience. A metal shielding was used to avoid the influence of external noise.

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EXPERIMENTAL SECTION Epitaxial polar (0001) ZnO films were grown on (001) LGO substrates by MOCVD in a three-temperature-zone furnace. The LGO bulk crystal were grown using the Czochralski technique with [001] c-axis pulling direction.14 LGO substrates with a size of 1 cm × 1 cm were cleaned by ultrasonic baths of acetone followed by drying in nitrogen. The growth temperature and pressure were maintained at 650 °C and 200 Torr, respectively. Detailed growth condition of the ZnO films has been described elsewhere.7,15 2.5 M NaOH solution was prepared by dissolving commercial NaOH (Sigama-Aldrich, USA, ≥99.8%) into deionized water. The wet chemical etching of the (0001) ZnO film was conducted at a temperature of 40 °C using the 2.5 M NaOH solution. After being etched for a period of time (from 5 to 180 s), ZnO samples were immediately rinsed by deionized water for several times followed by drying in high-purity N2 gas.

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RESULTS AND DISCUSSION Structural characterizations of the as-grown ZnO films were first conducted using TEM-based techniques. Figure 2a shows a typical cross-sectional bright-field TEM image of the ZnO film grown on (001) LGO substrate. ZnO film and LGO substrate can be easily identified from the image by contrast; however, the interface between these two materials is not sharp enough to estimate the thickness of the ZnO film. HAADF-STEM image constructed by the high-angle-scattered electrons17,18 is taken for a better contrast at the interface. Figure 2b demonstrates the HAADF-STEM image of the as-grown ZnO 10665

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Figure 2. Cross-sectional (a) bright-field TEM image and (b) HAADF-STEM image of ZnO film grown on (001) LGO substrate (scale bar = 300 nm in (a) and (b)). (c) Cross-sectional HRTEM image of ZnO film. (scale bar = 2 nm) SAED patterns taken from (d) ZnO film far from the interface and (e) interfacial region between ZnO film and LGO substrate.

film. The region with higher contrast corresponds to ZnO due to its larger average atomic number. On the contrary, the lowcontrast region in Figure 2b is pertaining to the LGO substrate which exhibits smaller average atomic number compared to that of ZnO. The Z-dependent contrast results in a distinct and sharp edge at interface between ZnO film and LGO substrate. The thickness of the ZnO film is estimated to be 250−280 nm from the HAADF-STEM image. The thickness variation is due to the presence of hexagonal pyramids in the as-grown ZnO film, which will be described in the SEM and AFM section afterward. The high-resolution (HR) TEM image of the film taken along the [112̅0]ZnO zone axis is shown in Figure 2c. The d-spacings of the lattice planes parallel to the substrate are estimated to be 0.52 nm, which corresponds to (0001) planes of the wurtzite ZnO structure. In addition, with a d-spacing of 0.28 nm, the lattice planes vertical to the substrate are assigned to be (11̅00)ZnO. Panels d and e of Figure 2 illustrate the selective area electron diffraction (SAED) patterns taken from the regions of bulk ZnO film as well as the interface between ZnO film and LGO substrate, respectively. Only sharp and evident diffraction spots pertaining to the wurtzite ZnO structure appear in Figure 2d, indicating the single-crystalline structure of the as-grown ZnO film with the growth direction of [0001]ZnO vertical to the LGO substrate. Moreover, all diffraction spots observed in the SAED pattern taken from the interface region between ZnO film and the LGO substrate (Figure 2e) are corresponding to the wurtzite ZnO and orthorhombic β-LGO substrate. The perfect superimposition of these two sets of diffraction patterns confirms the fact of small lattice mismatch between the ZnO film and LGO substrate. The in-plane epitaxial relationships between ZnO and LGO are identified as [1120̅ ]ZnO ∥ [020]LGO and [110̅ 0]ZnO] ∥ [200]LGO. The formation of poor ZnGa2O4 crystals at interface between ZnO film and LGO substrate has been demonstrated recently.14 Nevertheless, no additional blurred diffraction spot was observed in Figure 2e, suggesting that there is no significant ZnGa2O4 crystal formed at interface between ZnO film and LGO substrate in the present work. The TEM results indicate that the ZnO film with (0001) orientation ((0001) ZnO) is epitaxially grown on (001) LGO substrate.

The surface morphologies of the as-grown and NaOHtreated ZnO films were examined by SEM, as shown in Figure 3. Figure 3a illustrates that only a few obscure hexagonal

Figure 3. Typical SEM images of (a) as-grown ZnO film, and (b,c) ZnO films after NaOH treatment for 60 and180 s, respectively. (d) Corresponding EDS spectrum of (c) (scale bar = 1 μm).

pyramids are present in the SEM image of the as-grown epitaxial (0001) ZnO films. It indicates the rather smooth surface of the as-grown ZnO film and confirms the small thickness variation of the as-grown ZnO film observed from the HAADF-STEM image in Figure 2b. After being treated by NaOH solution for 60 s, as shown in Figure 3b, the (0001) ZnO film demonstrates an extremely plain feature except for few tiny pits irregularly distributed on the surface. On further extending the etching time to 180 s, no apparent change of surface topography is observed in the SEM image (Figure 3c) but some growing pits compared to that etched for 60 s. The thickness of ZnO film after 180 s etching is around 200−230 nm. SEM-EDS inspection was employed to ensure the existence of ZnO film in the 180 s etched sample. As shown in Figure 3d, two peaks corresponding to Zn and O elements 10666

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Figure 4. AFM images of (a) as-grown ZnO film as well as ZnO films after NaOH treatment for (b) 5 s, (c) 10 s, (d) 20 s, (e) 30 s, (f) 40 s, (g) 50 s, (h) 60 s, and (i) 180 s. The scan size is 5 μm square. The maximum heights (Zmax) of (a−i) are listed in Table 1.

Figure 4a, reveals that in addition to the well-faceted hexagonal pyramids with an average size of ∼500 nm, the regions possessing a flat feature with lower topography are also present in the AFM image. The morphology of the as-grown ZnO film taken using AFM, which shows two different features, is consistent with the SEM image shown in Figure 3a. It confirms again the small thickness variation of the ZnO film presented in HAADF-STEM image (Figure 2b). After the NaOH treatment for 5 s, as shown in Figure 4b, the hexagonal pyramids of the (0001) ZnO film are rounded from the corners. The feature of hexagonal pyramid on the ZnO film gradually disappeared when the etching times were extended from 5 to 30 s, as exhibited in Figure 4b−e. Interestingly, irregular but well-faceted surface structures remain on the NaOH-treated (0001) ZnO film, which is scarcely observed in the SEM images (Figure 3b,c) since the height contrast is much enhanced in AFM. Moreover, as listed in Table 1, the roughness of the films in Figure 4a−e decreases when the etching time is increased. It indicates that the surface of ZnO films can be smoothed by the alkaline-solution treatment. On increasing the etching time over 40 s, the topography of ZnO films look similar; i.e., the irregular but well-faceted surface structures are present, as shown in Figure 4f−i. As demonstrated in Table 1, the roughness of the ZnO film is smaller than 10 nm when the etching time is over 50 s. The smoothest surface of epitaxial (0001) ZnO films can be obtained while the etching time is 50 s (roughness ∼ 5.52 nm). Compared to the as-grown ZnO film, more than 50% reduction of the roughness of ZnO film is achieved by NaOH solution treatment for 50 s.

are present in the EDS spectrum, which indicates the presence of the ZnO film on LGO substrate even after being etched for 180 s. With the Z-axis resolution limitation of the SEM instrument, it is concluded that the epitaxial (0001) ZnO film with hexagonal architecture becomes smoother after the treatment of NaOH solution. To investigate the morphology evolution of NaOH-treated (0001) ZnO films with etch periods in detail, AFM was employed to measure the topography of the ZnO films in this study. AFM images of the as-grown and NaOH-treated (0001) ZnO films (0−180 s) with a scan size of 5 μm are shown in Figure 4. The maximum values of the scale bar for each AFM image (Zmax) as well as the roughness of NaOH-treated ZnO film obtained from Figure 4a−i are listed in Table 1. The topographic image of as-grown (0001) ZnO epitaxial film, Table 1. Zmax and Roughness of As-Grown and NaOHTreated ZnO Films etching time (s)

Zmax (nm)

roughness (nm)

0 5 10 20 30 40 50 60 180

150 150 100 100 100 100 30 60 40

17.8 16.8 15.2 14.6 11.0 12.9 5.52 9.28 6.39 10667

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To discuss the etching behavior of ZnO film in NaOH solution, one should learn which atom (Zn or O) locates on surface and exposes to the etchant. Panels a and b of Figure 5

Figure 6. Band diagrams of ZnO film and Pt/Ir tip as well as the corresponding circuits of STM measurements: (a) before applying voltage, (b) after applying a bias voltage of +2 V to ZnO film, and (c) with UV illumination in case b.

Figure 5. Experimental (exp.) and simulated (sim.) CBED patterns of as-grown ZnO films with (a) flat plane (scale bar = 300 nm) and (b) hexagonal pyramid features (scale bar = 200 nm) taken along the [11̅00]ZnO zone axis.

shown in the figure) to the conduction band of ZnO. However, under this measurement condition, the tunneling current is not stable enough for getting a sharp STM image of the ZnO(0001)-Zn surface. It may be ascribed to the deep traps existing within the ZnO films, which has been confirmed by the green band emission in the CL spectrum of ZnO films grown on LGO substrates.7 As shown in Figure 6b, the tunneling electrons on the conduction band of ZnO film will fill the deep traps rather than transport through the film to the contact. It is a significant loss to the tunneling electrons from tip. The resultant inferior lateral conductivity of the ZnO films is represented by a large resistance in the corresponding circuit of Figure 6b. The tip may even crush into ZnO surface to gain more tunneling current for reaching the set current value during the constant current mode measurement. A sharp STM image of the ZnO(0001)-Zn surface is therefore unachievable. Once UV light is simultaneously illuminated to generate photocarriers, as shown in Figure 6c, the deep traps are occupied by the photoelectrons in advance. The tunneling electrons from tip therefore transport through the ZnO film with a lower resistance, as revealed in the corresponding circuit of Figure 6c, leading to the tunneling current being considerably increased for the construction of topographic feature using the constant current mode. Well-defined STM images of the ZnO(0001)-Zn surface are therefore attainable with the assistance of UV illumination, as shown in Figures 7 and 8. Two types of topographic features of the epitaxial (0001) ZnO film, i.e., hexagonal pyramid and flat plane, are observed in the SEM and AFM images. Therefore, UV-assisted STM were employed to investigate the geometric-structure evolution of these two topographies with etch periods, as shown in Figure 7. The scanning dimension of the STM images is 500 nm square. Moreover, the cartoons of the two etching processes are also illustrated in Figure 7. As revealed in the “0 s” panel of Figure 7a, the well-defined flat terraces and steps are present on the hexagonal pyramid of the as-grown Zn-polar ZnO. It looks that the hexagonal pyramid is layer-by-layer constructed by the gradually size-shrinking hexagonal layers. Each hexagonal layer is composed of two basal planes of (0001) and (0001̅) as well as six pyramidal planes of {101̅1}.22 After being treated by concentrated NaOH solution for 5 s, the sharp and symmetrical hexagonal pyramid is evolved to an asymmetrical feature although faceted layers remained to sustain the layer-by-layer structure, as shown in the “5 s” panel of Figure 7a. When

show the polarity characterizations of two different crystal features of the as-prepared epitaxial (0001) ZnO film, i.e., flat surface and hexagonal pyramid, respectively, by the CBED technique. The experimental CBED patterns which are demonstrated in the “exp.” panels of Figure 5 were taken along the [11̅00]ZnO zone axis with a 200 kV incident electron beam. The simulations of the corresponding CBED patterns were also performed for comparison.16 The simulated CBED patterns are illustrated in the “sim.” panels of the Figure 5. The thicknesses of the TEM specimens used for CBED simulations in Figure 5a, marked by red and yellow frames, and Figure 5b are 150, 140, and 120 nm, respectively. Comparing the experimental and simulated CBED patterns shown in Figure 5, apparently, the epitaxial (0001) ZnO film possesses an outof-plane direction of [0002]ZnO regardless of their surface features. It is therefore concluded that the epitaxial (0001) ZnO film on LGO substrate exhibits the Zn-polar surface.7,19 Since STM can realize higher lateral resolution than AFM, the geometric-structure evolution of NaOH-treated ZnO(0001)-Zn surface with etch periods was further investigated by photoassisted STM. When performing the STM measurements, we found that significantly enhanced images are obtained from the UV-illuminated ZnO film/LGO substrate compared to those attained in the dark. The enhancement of tunneling current due to the UV irradiation is explained by using the band diagrams of the ZnO film and STM tip (Pt/Ir) as well as the corresponding circuits of the measurements, as shown in Figure 6. The Fermi level of the tip (EF,tip = ∼5.3 eV)20 is lower than the conduction band edge of ZnO single crystal (Ec,ZnO = 4.57 eV),21 as shown in Figure 6a. As the STM tip approaches very close to the ZnO/LGO sample without a bias voltage, electrons in the tip are not able to transfer to the conduction band of ZnO (no tunneling current flows from sample to tip) owing to not only the obstacle of the atmosphere but also the energy barrier between the EF,tip and Ec,ZnO. When a bias voltage of +2 V is applied to the ZnO film (the tip is grounded), which provides the potential larger than the energy difference between the EF,tip and Ec,ZnO (∼0.73 eV) as demonstrated in Figure 6b, tunneling currents are detectable. The built field is suggested to be large enough for driving electrons from tip tunneling though both potential barriers of atmosphere and space-charge region on the ZnO surface (not 10668

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Figure 8. High-magnification UV-assisted STM images of ZnO films after NaOH treatments for (a) 20 s (scale bar = 30 nm) and (b) 50 s (scale bar = 32 nm). (c,d) Cross-sectional line profiles of the regions in (b) denoted by blue and green lines, respectively.

to those right on the surface.25 On the other hand, the arriving angles of OH− on the corners and edges are larger than that on the surface of the basal plane. Consequently, etching rate of reaction 1 taking place on the corners and edges will be faster than that on the surface of ZnO basal plane. In addition, half of the six {101̅1} facets of the ZnO hexagonal pyramid, i.e., (1011̅ ), (011̅ 1), and (11̅ 01), are Zn-terminated whereas the rest are O-terminated.26 The partially negative charge state of the dangling bond on the O-terminated surface does not attract hydroxide ions.5 Anisotropic etching behavior therefore takes place on the ZnO hexagonal pyramid, as demonstrated in the “5 s” and “10 s” panels of Figure 7a. On the other hand, few small hexagonal pits appear in the STM image of the as-grown Zn-polar ZnO with plane feature, as shown in the “0 s” panel of Figure 7b. On increasing the etching time, asymmetrical hexagonal cavities with flat terraces and steps are formed as demonstrated in the “20 s” and “50 s” panels of Figure 7b. The presence of hexagonal pits indicates the existence of defects within the epitaxial (0001) ZnO film.27 The etching behavior which is demonstrated in the STM images may be attributed to the aforementioned anisotropic etching of the ZnO pyramidal {101̅1} planes initially occurring from the pristine defects, such as the hexagonal pits. Moreover, it has been reported that the growth rates of different ZnO planes using solution routes are in the order of (0001) > (011̅1̅) > (011̅0) > (0001̅).28 The metastable (0001) face is also preferentially dissolved in the alkaline solution due to its high surface energy, which leads to the formation of ZnO microtubes from ZnO rods.29 Subsequently, we suggest that in addition to initially etching from the pristine defects, preferential chemical dissolution of the metastable (0001) face may also occur when the ZnO(0001)-Zn surface exposes to the NaOH solution. By taking the advantage of UV illumination, stabilization of ZnO(0001)-Zn surface after alkaline etching is also investigated in this study by using the UV-assisted STM. Figure 8a shows a typical high-magnification UV-assisted STM image of the ZnO(0001)-Zn surface after being etched for 20 s. It

Figure 7. UV-assisted STM images of ZnO films with (a) hexagonal pyramid and (b) plane features after NaOH treatments for various periods. The scanning size for all STM images is 500 nm square. The corresponding cartoons are illustrated as well.

extending the etching time to 10 s, the as-grown hexagonal pyramid is apparently rounded as exhibited in the “10 s” panel of Figure 7a. The pH value is believed to play a crucial role in the growth of ZnO crystals in aqueous solutions.8 Phase stability diagrams of ZnO(s)-H2O (Figure 7a in ref 8) reveal that, in the case of pH > 9, Zn2+ prefers to form Zn(OH)2(aq) and Zn(OH)42− in the aqueous solution at various Zn precursor concentrations. Without taking the sophisticated stabilization of ZnO(0001)Zn surface by the adsorption of OH− or H2O23,24 into consideration, we accordingly suggested that the positive dangling bonds of Zn atoms on the ZnO(0001)-Zn surface will preferentially react with OH− ion in the 2.5 M NaOH solution with pH > 13. The following thermodynamically preferred reaction 1 therefore occurs on the ZnO(0001)-Zn surface. ZnO + 2OH− + H 2O → Zn(OH)4 2 −

(1)

Moreover, the Zn atoms locating on the corners and edges of the basal plane possess more positive dangling bonds compared 10669

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NaOH solution. Moreover, apart from successful inspection of triangular reconstruction of the NaOH-treated ZnO(0001)-Zn surface, evidently layer-stacking feature on a faceted ZnO surface with a step height resolved in the atomic scale is also demonstrated in ambience using the UV-assisted STM. The results indicate that the UV-assisted STM is a unique method to attain the well-defined STM images of the wide-band-gap ZnO in ambience.

demonstrates the triangular reconstructions on the edges of terraces. The result is consistent with recently reported STM investigations30 and density function theory calculation23 that the Zn-terminated surface is stabilized by the triangular reconstruction. UV-assisted STM image of the ZnO(0001)Zn surface after being etched for 50 s is demonstrated in Figure 8b. In addition to successful inspection of triangular reconstruction of the ZnO(0001)-Zn surface, the image demonstrates a faceted surface with evidently layer-stacking feature. Intensity line profiles measured from the STM image denoted by the blue and green lines in Figure 8b yield two step heights of ∼2.6 and ∼5.2 Å, which correspond to the one and two double-layer arrangement of Zn and O ions in a hexagonal unit cell, respectively. In the present work, surface geometric structures of the ZnO(0001)-Zn surface evolved by NaOH solution are successfully demonstrated by STM in ambience with the assistance of UV illumination. Well-defined STM images of the illuminated wide-band-gap ZnO films are obtained by utilizing its superior photoconductivity. The results simulate the concept that photoassisted STM is a unique method for the investigation of surface geometric structures of the semiconducting materials. The tunneling current for constructing the STM image of the semiconductor can be significantly enhanced by illuminating with a light of appropriate wavelength to generate photocarriers in the semiconductor. Moreover, the superior photocatalytic properties of ZnO crystal maybe also play a crucial role in the observation of triangular reconstruction of the ZnO(0001)-Zn surface in ambience by the UV-assisted STM. The self-cleaning characteristic of the UV-illuminated ZnO surface31 may facilitate the maintenance of a fresh surface for STM measurement. Accordingly, evidently layer-stacking feature on a faceted ZnO surface with a step height resolved in the atomic scale is successfully demonstrated in ambience using the UV-assisted STM.

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AUTHOR INFORMATION

Corresponding Author

*Tel.: 886-6-2757575 ext. 62694 (J.-J.W.); 81-774-383050 (H.K.). Fax: 886-6-2344496 (J.-J.W.); 81-774-383055 (H.K.). E-mail: [email protected] (J.-J.W.); [email protected]. kyoto-u.ac.jp (H.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Science Council in Taiwan under Contract No. NSC 99-2221-E-006-198-MY3 and NSC 100-2628-E-006-032-MY2 is gratefully acknowledged. W.-H.L. is thankful for the support of “Summer Program, 2011” from the Interchange Association in Japan and the National Science Council in Taiwan.

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REFERENCES

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CONCLUSIONS In this study, the surface geometric structures of epitaxial (0001) ZnO films treated by NaOH solution are investigated using SPMs. The epitaxial (0001) ZnO films are grown on the lattice-matched (001) LGO substrates using MOCVD. The polarity of the epitaxial (0001) ZnO film is identified by CBED, indicating that the epitaxial (0001) ZnO film exhibits the Znpolar surface. Two types of topographic features, i.e., hexagonal pyramid and flat plane, are observed in the AFM images of the as-grown epitaxial (0001) ZnO film. Compared to the as-grown ZnO film, more than 50% reduction of the roughness of ZnO film is achieved by NaOH solution treatment for 50 s (roughness ∼5.52 nm). By illuminating a UV light on the epitaxial (0001) ZnO film, the tunneling current can be significantly enhanced to construct the well-defined STM images. UV-assisted STM images reveal that the faceted and layered hexagonal-pyramid feature is getting rounded although the layered structure is sustained as increasing the etching time. Anisotropic etching behavior occurring on the ZnO hexagonal pyramid is ascribed to the three O-terminated planes of the six {101̅1} facets of ZnO hexagonal pyramid. On the other hand, few small hexagonal pits on the as-grown flat ZnO(0001)-Zn surface are developed to asymmetrical hexagonal cavities with flat terraces and steps when the etching time is increased. In addition to initially etching from the pristine defects, preferential chemical dissolution of metastable (0001) face may also occur when the ZnO(0001)-Zn surface exposes to the 10670

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