Investigations on the Crystallographic Orientation Induced Surface

Subscriber access provided by HOWARD UNIV

Article

Investigations on the Crystallographic Orientation Induced Surface Morphology Evolution of ZnO Thin Films and Their Wettability and Conductivity Chung-Hua Chao, Po-Wei Chi, and Da-Hua Wei J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b01573 • Publication Date (Web): 31 Mar 2016 Downloaded from http://pubs.acs.org on April 4, 2016

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

The Journal of Physical Chemistry C is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 48

The Journal of Physical Chemistry

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 ACS Paragon Plus Environment



Page 2 of 48

Page 3 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


ACS Paragon Plus Environment



Page 4 of 48

Page 5 of 48





Page 6 of 48

Page 7 of 48





Page 8 of 48

Page 9 of 48




1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Investigations on the Crystallographic Orientation Induced Surface Morphology Evolution of ZnO Thin Films and Their Wettability and Conductivity Chung-Hua Chao, Po-Wei Chi, and Da-Hua Wei* Institute of Manufacturing Technology ＆ Department of Mechanical Engineering, National Taipei University of Technology (TAIPEI TECH), Taipei 10608, Taiwan Abstract The intrinsic zinc oxide (ZnO) thin films with controllable crystallographic orientation have been synthesized on silicon(100) substrates using plasma enhanced chemical vapor deposition (PECVD) system without any buffer layer. Based on X-ray diffraction (XRD) results, the evolution of crystallographic orientation of ZnO thin films from polar c-plane ̅ 0) coexist, to nonpolar m-plane and a-plane (0002), polar c-plane and nonpolar m-plane (101 ̅ 0) coexist was achieved by a simple factor of controlling synthesized temperature. The (112 plane-view morphological images exhibited that the surface texture and grain shape of ZnO thin films could have evolved from hexagonal to stripe-like grains when the ZnO crystallographic orientation changed from perpendicular to parallel to the substrate. The characterization analysis indicated that the zinc precursor [diethylzinc (DEZn), Zn(C2H5)2] played a key role on the crystallographic orientation evolution of ZnO thin films during the early stage of the growth process, because DEZn not only can be served as Zn precursor but also employed as passivating agent to influence the crystal growth under different synthesized temperatures. Room-temperature Hall effect measurement showed that intrinsic ZnO thin film with stripe-like grains possessed the lowest value of resistivity ~7.11 × 102 Ω cm, which had an estimated carrier concentration and mobility of about 5.73 × 10 14 cm-3 and 15.34 cm2/V s, respectively. The water contact angle (CA) measurement was also provided to determine the surface wettability and surface free energy of ZnO thin films, indicating that CA could be adjusted via different crystallographic orientation of ZnO thin film.

*Author to whom correspondence should be addressed; [email protected] Phone: 886-227712171#2022.


Page 10 of 48

Page 11 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


1. Introduction ZnO and its compound have attracted much attention in recent years due to some outstanding advantages such as wide direct band gap (3.37 eV), large binding energy (60 meV) over III-nitride semiconductors, non-toxic and abundant raw materials.1-3 Because of these unique properties, ZnO has been widely used in the light emitting diodes (LED), fieldeffect transistors (FET), solar cells, and photodetectors.4-7 Since the material and corresponding mechanical properties of the ZnO depend on their crystallographic orientation due to different surface atomic chemistry construction and electrical and optical anisotropic properties,8 to manipulate the crystallographic orientation of ZnO is an essential requirement for various commercial applications. For instance, the preferred orientation of polar c-plane (0002) ZnO thin film is usually required as a buffer layer to grow vertically well-aligned ZnO nanowires for high-performance devices such as the field-effect transistor (FET).9 Moreover, Sinha et al. synthesized high-quality single-crystal c-axis ZnO nanowire arrays to develop an ultrafast and reversible gas sensor.10 Cheng et al. used dimensional tailoring hydrothermal method to synthesize functional ZnO nanowire arrays, indicating that the ZnO nanostructures can be used in many commercial applications.11 In contrast, nonpolar m-plane ̅ 0) and a-plane (112 ̅ 0) ZnO crystals are employed for the potential applications such as (101 high-efficiency LED and humidity sensor.12-13 For example, Bail et. al. developed a ZnObased nonpolar light emitting diode showing distinct UV-blue EL emissions and with a low turn-on voltage of 3 V.14 It is well known that anisotropic c-axis [0001] orientation is the preferred growth direction for ZnO on most currently available substrates due to the thermodynamically favorable growth direction of the ZnO wurtzite structure.15 However, wurtzite ZnO structure is polarized along the c-axis, which is induced by the spontaneous and piezoelectric polarizations effects (i.e., quantum confined Stark effect), resulting in the wave functions of electrons and holes are separated in the multiquantum well (MQW) structure.16 This



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

phenomenon will greatly limit the luminous efficiency of the device and lead to an undesirable red shift in the emission spectra.17 Therefore, how to control crystallographic orientation has become an important issue for fabrication of nonpolar plane, which is needed for ZnO crystal to overcome such problem of low light-emitting quantum efficiency. Nowadays, the polar and nonpolar ZnO thin films are mainly formed onto small lattice mismatch substrates to improve crystal quality, for example, c-plane ZnO on AlN/csapphire,18 a-plane ZnO on r-sapphire,19 and m-plane ZnO on LiGaO2(100).20 However, as for the consideration of the cost and formation size of above single crystal substrates, it is not suitable for practical industrial applications. Hence, the simple way for the fabrication of high quality polar and nonpolar ZnO thin films on the silicon substrates is urgently necessary. On the other hand, a vast number of studies are recently in progress to synthesize polar and nonpolar ZnO nanostructures by intentionally introducing surface catalyst or passivating agent.21-22 The above methods also exist some problems such as complex synthesis procedures and chemical reactions. In this presented work, therefore, we report a simple method that the ZnO thin films with controllable crystallographic orientation for polar and nonpolar planes can be effectively manipulated by employing plasma enhanced chemical vapor deposition (PECVD) system with a key factor of synthesized temperature directly onto silicon substrates. The relevant crystal structures, surface morphologies, optical and electrical properties of intrinsic ZnO thin films with different crystallographic orientations have been investigated. We also discussed the fundamental synthesis and growth mechanism for interpreting the evolution of the crystallographic orientation. Besides, the wettability of water is one of the significant properties for solid surface. Hydrophobic surface (water contact angle > 90o) can be used in many applications of selfcleaning surface23, anti-corrosion24, and so on. It has been known that the wettability is governed by both chemical composition and surface geometrical structure. 25 Either reduction of surface energy by surface modification or change of the surface morphology can promote


Page 12 of 48

Page 13 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the hydrophobicity of the solid surface. Although ZnO has been widely investigated the wettability with different nanostructures, our research work focused on the evolution of wettability variation originated from different crystallographic orientations of ZnO films was seldom reported. Therefore, we discussed the effect of crystallographic orientation on the water contact angle variation based on a new viewpoint of grain shape.

2. Experimental Procedure The synthesis of ZnO thin films was performed by plasma enhanced chemical vapor deposition (PECVD) system. Prior to the synthesis of ZnO thin films, single-crystal Si(100) substrates with 10 mm × 10 mm in size were cleaned in acetone, alcohol and isopropanol by ultrasonic cleaner for 35 min, and then introduced into PECVD synthesis chamber. For the plasma-synthesized process of ZnO thin films on Si(100) substrates without using any buffer layer, the carbon dioxide (CO2) and diethylzinc [DEZn, Zn(C2H5)2] were used as precursors for supplying oxygen and zinc sources in this method, respectively, and the gas flow rate ratio of CO2 to DEZn was fixed at 3:1.7 The synthesized temperatures of ZnO phase were set as 350 oC, 450 oC and 550 oC and these thin films were named as Z350, Z450 and Z550, respectively. The working pressure and radio-frequency (RF) power were maintained at 6 Torr and 70 W during the synthesis for all ZnO samples. The detail experimental process can be found in our previous work.26 The crystal quality and orientation of ZnO thin films were confirmed by X-ray diffraction (XRD) using Cu Kα radiation (λ = 1.54 Å ) and high-resolution transmission electron microscopy (HRTEM). The RF plasma was monitored by the in-situ optical emission spectroscopy (OES) in order to analyze the plasma chemical composition during the plasma-synthesized process, which could be used to explain the precursor sticking probability. The surface morphologies of the ZnO thin films were investigated by field emission scanning electron microscope (FE-SEM). Photoluminescence (PL) was carried out



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

at room temperature to study the optical properties of the ZnO thin films using a He-Cd laser (325 nm). The electrical properties were studied by Hall measurements by using van der Pauw configuration. The measurements were implemented at a current of 1 μA with a magnetic field of 0.55 T, and fixed at a temperature of 300K. The wettability of ZnO thin films was recorded by water contact angle using a contact-angle goniometer.

3. Results and discussion 3.1 Crystal stricture, morphology and optical properties Figure 1 shows the typical θ-2θ XRD patterns for the ZnO thin films synthesized on Si(100) substrates with different synthesized temperatures ranged from 350 to 550 oC. For all ZnO thin films, the diffraction peaks are matched well with the standard diffraction pattern of wurtzite ZnO phase (JCPDS-36-1451). The XRD patterns obviously reveal that the crystallographic orientation of the ZnO thin films strongly depends on the synthesized temperature in the plasma-synthesized process due to the fact that the DEZn as Zn precursor can be decomposed into different chemical compositions under different temperatures, which will further influence the crystal growth as shown in Figure 5 and 6. When the synthesized temperature was set at 350 oC (Z350), only the polar (0002) diffraction peak located at 34.3o can be observed except the diffraction peaks from the silicon substrate, indicating that this sample was only with a preferred orientation of polar c-plane. While with increasing the synthesized temperature to 450 oC (Z450), the ZnO thin film was dominated ̅ 0) and polar (0002) orientations located at 31.6o and 34.3o, respectively. by nonpolar (101 Finally, the synthesized temperature of ZnO thin film was increased up to 550 oC (Z550), ̅ 0) but the crystallographic orientation belong to nonpolar plane was not only appeared (101 ̅ 0) oriented crystallite located at 56.4o existed due to the thermal mismatch strain. also (112 These results obviously indicate that the ZnO grains possess a crystallographic orientation of polar c-axis perpendicular to the silicon substrate surface in the thin film synthesized at


Page 14 of 48

Page 15 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


350 oC, and then the ZnO are composed of a mixture of grains with polar c-axis perpendicular and parallel to the substrate at 450 oC. All c-axis grains are parallel to the substrate completely as the synthesized temperature increased to 550 oC. In other words, mand a-axes oriented ZnO crystals lie parallel to the substrates at 350 oC, and then rise up at a critical temperature of 550 oC. Plan-view SEM images for the Z350, Z450, and Z550 are shown in Figures 2(a)-2(c), and Figures 2(d)-2(f) are the corresponding cross-section images. From Figures 2(a) and 2(d), the plane-view and cross-sectional SEM images of Z350 clearly show a high density of hexagonal grains with columnar structures marked by red dotted lines and green rectangles, respectively. The diameter of the nanograins has a basically uniform distribution about 40 ± 8.3 nm as shown in the inset of Figure 2(a). Figure 2(b) exhibits that the Z450 surface possesses both hexagonal (marked by red dotted lines) and stripe-like grains (marked by green rectangles), and the average grain sizes for hexagonal and stripe-like grains are about 37 ± 3.8 nm and 7 ± 3.3 nm, respectively, as shown in insets of Figure 2(b). The cross-sectional image for Z450 shown in Figure 2(e) illustrates the ambiguous nanostructure including quasi- columnar and hexagonal grains. While the Z550 shows all of the grains with a stripe-like shape (marked by green rectangles) and with an average grain size of about 6.8 ± 4.5 nm, which is a typical feature observed in nonpolar ZnO as shown in Figure 2(c).27 Interestingly, the obvious nanograins with hexagonal shape (marked by red lines) could be observed from the cross-sectional view SEM image as shown in Figure 2 (f). From the above SEM images, it can be seen that the ZnO thin films revealed various surface morphologies with different crystallographic orientations, which was consistent with XRD results. The diagrammatic sketch for c-, m- and a-plane ZnO wurtzite structures are shown in Figures 2(g)-2(i), respectively. From these sketches, it is easy to claim and understand the evolution of morphology variation on textured ZnO nanocrystals. Further crystalline structure characterization for the plasma-synthesized ZnO thin films



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

were confirmed by TEM as shown in Figure 3. All the cross-sectional TEM samples were prepared by using a focused ion beam (FIB) lift-out method and with a 40 nm cap Pt protection layer pre-deposited on the samples to prevent charging. Figures 3(a), 3(b) and 3(c) are high-resolution TEM (HRTEM) images for the Z350, Z450 and Z550, respectively. The lattice spacing (0.261 nm) for Z350 and the lattice spacing (0.284 nm) for Z550 match well ̅ 0) basal planes belong to wurtzite-type ZnO with the corresponding (0002) plane and (101 ̅ 0) and (0002) phase, whereas Z450 exhibits coexistence of crystal orientations along (101 planes, providing the confirmation of varied and tunable crystallographic orientation consisted with the XRD analysis results. In order to confirm the structural-defect-related optical properties, room temperature PL spectra for Z350, Z450, and Z550 was investigated as shown in Figure 4. It is known that PL emission band of ZnO phase can mainly divide into two parts. One is in UV region, named as near-band-edge (NBE) emission, which is attributed to the recombination of freeexciton. The other one is in visible region, named as deep-level (DL) emission, which is formed by the impurities and various defects in the film such as zinc interstitials and oxygen vacancies.28 From the PL spectra, it is found that Z350 and Z550 exhibit a strong NBE emission band and a negligible DL emission band, indicating pure polar and nonpolar ZnO thin films with great optical and crystalline quality. The Z450 with board DL emission is presumably attributed to the ambiguous crystallographic orientation between polar and nonpolar plane, leading to plenty of defects and impurities in the grain boundaries. On the other hand, it is also found that the NBE emission has a blueshift to lower wavelength with increasing synthesized temperature. This is due to the strong residual anisotropic strain in nonpolar ZnO phase that can induce a significant blueshift of the NBE emission compared with that in strain-free polar ZnO.29-30

3.2 Crystal growth mechanism


Page 16 of 48

Page 17 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


In general, ZnO columnar nanostructures with c-axis crystallographic orientation are most commonly obtained in several nano-synthesized methods including wet and physicalchemical vapor deposition techniques. A lot of models have been developed to explain crystal growth mechanism including preferential nucleation, sticking probability on each plane, surface diffusion of adatoms and growth rate anisotropy of the different crystallographic plane.31-35 Here, we deduce a growth mechanism to interpret the crystallographic orientation evolution of ZnO thin film depending on the synthesized temperature, which is on the basis of the DEZn as a precursor that could be decomposed into different chemical species such as metal zinc, ethyl groups and its fragments under different synthesized temperatures and then further influence the nucleation and growth of grains. 36 For verifying and better understanding this growth mechanism in our case, the in-situ OES analysis was used to monitor plasma chemical composition during the synthesis process as shown in Figure 5, and Figure 6 depicted a schematic drawing for the proposed growth mechanisms. At the beginning stage of the nucleation for all samples, the ZnO nuclei are formed on the substrates through combining the zinc and oxygen molecules from each precursor. Since the nucleation occurs onto non-epitaxial substrate, the ZnO grains normally nucleated and grew on the non-epitaxial substrate surface with random orientations (see schematics in Figure 6). Nevertheless, the films still appear specific crystallographic orientation in our case, ̅ 0) and (0002) at 450 oC, and (101 ̅ 0) and (112 ̅ 0) at 550 oC, i.e., (0002) at 350 oC, (101 respectively. The evolutionary selection theory based on kinetic growth aspects is the most frequently adopted to interpret this behavior.37 The evolutionary selection depends on the growth rate anisotropy of the different crystallographic orientations of a material, indicating that the preferential nucleation of dynamic grains with fastest growth direction will envelop the other oriented grains, and then finally dominate the film texture. According to the consideration of surface energy with different crystallographic planes reported by Fujimura



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

̅ 0) and a-plane et al,38 the calculated surface energy values for c-plane (0002), m-plane (101 ̅ 0) are 1.6 J/m2, 3.4 J/m2 and 2.0 J/m2, respectively. Therefore, the direction with fastest (112 growth rate is along the c-axis.39 Our experimental results agree well with this theoretical calculation for the Z350. However, the crystallographic orientation was changed with increasing the synthesized temperature. This behavior could be interpreted by a viewpoint of a unique characteristic of the ZnO (0002) polar surface. ZnO crystal grown along the caxis has alternative zinc ion layers with oxygen ion layers, which cause polarization in [0001] direction. Generally, the zinc ions possess positive charge (Zn2+) and the oxygen ions possess negative charge (O2-). It can be considered as a Zn/O/Zn/O… stacking sequences of the hexagonal type AbBaA… along its polar c-axis.40 Consequently, this polar plane growth can be suppressed by intruding passivating agent. Owing to this unique characteristic, many techniques including solution and vapor routes have been used to develop diverse ZnO nanostructures.41-43 In our case, the DEZn has been effectively not only used as zinc precursor but also employed as a passivating agent to control the crystal growth. The in-situ OES analysis of the RF plasma was carried out during the plasma synthesis process because OES was a great tool to monitor and determine the plasma chemical composition. As shown in Figure 5, all the thin films exhibit the main Zn emission peaks at the wavelength around 475 nm and another obvious emission peak at 634 nm, which are determined as O2 species.44 When the synthesized temperature is at 350 oC (Z350), zinc from DEZn and oxygen from CO2 will react in the chamber simultaneously without other impurities and then the formation of ZnO compound phase as depicted in the nucleation stage of Z350 illustrated in Figure 6. Interestingly, OES emission peaks located at around ranging from 320 to 430 nm became stronger when the synthesized temperatures rose to 450 and 550 oC (Z450 and Z550). These broad and complicated emission peaks could be denoted as the ion fragment region, which were originated from decomposition species of DEZn. The DEZn not only could be dissociated into metal zinc but also decomposed into some ethyl group fragments including


Page 18 of 48

Page 19 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


anions and cations such as CH3−, CH3CH2− and CO+.36 As a result, when the synthesized temperature increased to 450 oC, the dissociated anions and cations could adhere onto the positive zinc ions layer or negative oxygen ions layer along the c-axis as depicted in the nucleation stage of Z450 illustrated in Figure 6. Consequently, the c-axis direction growth ̅ 0} or {21 ̅1 ̅ 0} was partially suppressed, resulting in forcing the crystal to grow in the {011 direction as depicted in the growth stage of Z450 illustrated in Figure 6. Hence, the Z450 ̅ 0) and (0002), respectively. Furthermore, the showed mixed crystallographic planes of (101 DEZn decomposed into more anions and cations when the synthesized temperature increased to 550 oC according to OES analysis, indicating that the c-axis growth direction could be ̅ 0) and (112 ̅ 0) completely suppressed. Therefore, the Z550 exhibits pure nonpolar (101 oriented crystallites as sketched at the nucleation and growth stage of Z550 illustrated in Figure 6.

3.3 Electrical property For evaluating the impact of crystallographic orientation induced surface morphology evolution on the electrical transport properties of the intrinsic ZnO thin films, Hall effect measurement was carried out as a function of synthesized temperature for the ZnO thin films. The Ohmic contacts were achieved by soldering indium (In) metal ball to connect the four corners of the samples. Acceptable linear current-voltage curves were obtained, indicating a good Ohmic contact was formed between the intrinsic ZnO and In interfaces. The results of the Hall effect measurement are shown in Figure 7. As expected, all of the intrinsic ZnO thin films show n-type conduction behavior due to inherent defects, which will generate shallow donor levels in the ZnO.45-46 A shown in Figure 7, it can be clearly observed that the carrier concentration (n) and Hall mobility (μ) have an inverse relationship with the resistivity (ρ) of the ZnO thin films. According to the following equation 1: 𝜌 =

1

(1)

𝑛𝜇𝑒



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

it can be understood to explain the variations of resistivity. The carrier concentration obviously decreased from ~2.75 × 1015 to ~7.64 × 1014 cm-3 with increasing synthesized temperature from 350 to 450 oC, and then decreased to the minimum ~5.73 × 1014 cm-3 with further increasing temperature up to 550 oC. However, the resistivity of the ZnO thin films first increased from ~2.2 × 103 to ~4.5 × 103 Ω cm, and then sharply decreased to ~7.1 × 102 Ω cm with the increment of synthesized temperature. This is because the mobility of the thin film slightly increased from ~1.02 to 1.81 cm2/V s with increasing synthesized temperature from 350 to 450 oC, and then dramatically increased to ~15.34 cm 2/V s when the pure nonpolar ZnO phase formed at 550 oC. The free charge carriers of intrinsic ZnO are mainly originated from some native defects like oxygen vacancy, which will create the shallow donor levels to upgrade carrier density. A decrease in carrier concentration with different crystallographic orientations in the intrinsic ZnO could be attributed to two reasons: (i) the elimination of the oxygen vacancy and (ii) the chemisorption of oxygen molecules into the grains and then annihilated shallow donor levels.47-48 In order to manifest the influence of the oxygen-related defects and chemisorption of oxygen molecules, the surface chemical properties of ZnO films with different synthesized temperatures have been investigated by X-ray photoelectron spectroscopy (XPS). The XPS spectra of the O 1s core level region and the compositions of O 1s peaks for all ZnO films are shown and listed in Figure S1 and Table 1, respectively. The O 1s peak can be generally decomposed into three subspectral components centered at ~530.16 ± 0.09 eV (OL), 531.5 ± 0.12 eV (OV), and 532.15 ± 0.12 eV (OC), respectively. The OL is associated with the O-Zn bonds in the hexagonal wurtzite ZnO crystal structure. The OV is attributed to the oxygen vacancy such as O2- ions in oxygendeficient regions while OC is usually related to the chemisorbed oxygen on the surface. 49 According to the relative percentages from the OL, OV, and OC components in the ZnO film with three crystallographic orientations, the Z450 contains the most plentiful OV and OC components due to the polar and nonpolar crystal structure coexist. Even though the Z450


Page 20 of 48

Page 21 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


contains a lot of defects from oxygen vacancy, which will provide more charge carriers, the relatively large chemisorbed oxygen will neutralize the charge carries,50 resulting in a reduction of carrier concentration. The fewest carrier concentration of Z550 is due to the lowest OV accompanying with the second highest OC component. In contrast, the Z350 is with 9.25 % OV and the lowest 6.1 % OC content components, indicating that the Z350 possesses the highest carrier concentration. On the other hand, the mobility increased significantly with increasing the synthesized temperature presumably due to the reduction of carrier scattering or elongation of electron mean free path originated from different grain shapes.51-52 Some research works have been reported that the larger grain size could increase the Hall mobility due to fewer grain boundaries.53-54 The mobility is usually limited by several scattering mechanisms like neutral impurity scattering, ionized impurity scattering, grain boundary scattering, and dislocations. However, in our case, we found that even though the ZnO grain size obviously reduces in the thin films accompanied with oriented crystallite evolution from polar to nonpolar, the Hall mobility contrarily increases drastically. Considering the crystallographic orientation and surface morphology of the ZnO thin films as revealed in Figures 2, it can be concluded that the highest Hall mobility is attributed to the good structural continuity with dense stripe-like morphology and high crystallinity with ̅ 0) and (112 ̅ 0) on the outmost surface of the ZnO thin film. Therefore, we nonpolar (101 deduce that the Z550 with nonpolar outmost surface and stripe-like grains can reduce the carrier scattering and elongate the electron mean free path. Similar behavior has also been observed with reference to published article.55

3.4 Surface wettability Wettability is an important property of the interface between solid and liquid. According to Young model for wettability, contact angle on a flat surface (𝜃𝑓𝑙𝑎𝑡 ) can be correlated to three interfacial free energies at the solid-liquid (𝛾𝑠𝑙 ), solid-vapor (𝛾𝑠𝑣 ) and



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 48

liquid-vapor (𝛾𝑙𝑣 ) interfaces. The contact angle in this model is described as equation 2: cos 𝜃𝑓𝑙𝑎𝑡 = (𝛾𝑠𝑣 − 𝛾𝑠𝑙 )/𝛾𝑙𝑣

(2)

However, it does not consider the role of the surface roughness. It is well accepted that the surface roughness and the surface free energy play the key roles in the wettability. Therefore, the Wenzel and Cassie-Baxter models have been extended and developed to interpret surface wettability. For high surface energy materials, the Wenzel model assumed that the water drop completely penetrates into the entire rough surface, not any air pocket remains in the surface. 56

Under the above conditions, the contact angle is expressed as equation 3:

cos 𝜃𝐶𝐴 = 𝛾 cos 𝜃𝑓𝑙𝑎𝑡

(3)

where 𝜃𝐶𝐴 is the contact angle on solid with a rough surface. 𝛾 is the surface roughness factor defined as a ratio of the actual surface area to its horizontal projection. In contrast, for low surface energy materials, the Cassie-Baxter model introduced that the surface roughness can create air pockets, leading to the liquid does not completely penetrate into entire surface.57 Therefore, the existence of air pockets in the interstices of ZnO grains will partially support the water drop, thus resulting in enhancement of the hydrophobicity. The contact angle in this state is described as equation 4: cos 𝜃𝐶𝐴 = 𝜑𝑠 (cos 𝜃𝑓𝑙𝑎𝑡 + 1) − 1

(4)

where φs is the area fractions of the liquid-solid interface. It can be understood that θCA increased with a reducing area fraction of the liquid-solid interface (φs). This is in agreement with the fact that surface hydrophobicity is enhanced when more air pockets were formed between the liquid and solid interfaces. For evaluating the surface free energy for different crystallographic orientations of ZnO thin films and selecting a suitable model to explain the surface wettability in our case as shown in Fig. 8, we used contact-angle goniometer and combined with Girifalco-GoodFowkes (GGF) theory to calculate the surface energy of ZnO thin films.58-59 The wettability images for the Z350, Z450, and Z550 were measured and recorded as shown in Figures 8(a),


Page 23 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(b), and (c). The measured values of surface water contact angle (CA) for the Z350, Z450, and Z550 are 113.66o, 105.49o and 110.1o, respectively, indicating that all the ZnO samples show the hydrophobic surface. According to GGF theory, the dispersive force or van der Waals force across the interface existed between the water drop and solid surface is critical interaction, which can be expressed as equation 5: 𝛾𝑠𝑙 = 𝛾𝑠 + 𝛾𝑙 − 2(𝛾𝑠𝑑 𝛾𝑙𝑑 )

1⁄ 2

(5)

where 𝛾sd and 𝛾ld are the dispersive portions of the surface tension for the solid and liquid surface, respectively. Then combining Young equation into Eq. (5), and employing nonpolar liquid deionized water (72.8 mJ/m2) as a testing liquid, 𝛾ld is equal to γ𝑙 , the GGF equation can therefore be rewritten as equation 6: 𝛾𝑠𝑑 =

𝛾𝑙 (cos 𝜃𝑓𝑙𝑎𝑡 +1)

(6)

4

where 𝛾sd is the surface free energy of the calculated samples. The calculated surface free energy for Z350, Z450 and Z550 are 10.88, 12.6 and 11.28 mJ/m2 as shown in Figure 8(d). The ZnO thin films possess low surface energy, and the tendency of surface energy is opposite to the contact angle. Therefore, the Cassie-Baxter model is chosen to interpret the variation of ZnO contact angle with different crystallographic orientations. A vast number of articles have been reported that the wetting behavior of c-axis ZnO is hydrophobic attributed to the high aspect ratio and rough surface, which will create more air pockets to enhance the hydrophobicity.60-61 Some research groups also investigated the reversible wettability of c-axis ZnO nanostructures by various methods of surface modification and UV-light irradiation.62-64 However, there are still few reports to investigate the surface wettability of the c-, m-plane coexist and m-, a-plane coexist in ZnO thin films. Here, we propose a model to explain the wettability behavior of polar and nonpolar coexisting ZnO thin films. As shown in Figure 8, the water CA of the c-, m-plane coexisting ZnO thin film (Z450) is slightly lower than that of pure c-axis ZnO (Z350). On the other hand, the water



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 48

CA of m-, a-plane coexisting ZnO (Z550) is slightly large than Z450, but lower than Z350. We believe that the m-plane ZnO nanograins play a critical role to reduce the water CA, while the a-plane ZnO can increase it contrarily. The schematic diagrams for water drop onto m- and a-plane ZnO nanograins surface are shown in Figure 9. Because the hexagonal ZnO grain can be equally divided into six regular triangles, then the Dm-plane can be given by equation 7: 𝐷𝑚−𝑝𝑙𝑎𝑛𝑒 = 𝑆 + 𝑆 = 2𝑆

(7)

where D and S are defined as distance from left to right side and side length of ZnO grains, respectively. The Da-plane can be given by equation 8: 𝐷𝑎−𝑝𝑙𝑎𝑛𝑒 =

√3 𝑆 2

+

√3 𝑆 2

= √3𝑆

(8)

If the length and height of ZnO grain for a- and m-plane are ideally supposed equal, the Daplane

will be slightly smaller than the Dm-plane. Based on our provided model for calculation,

the number of air pockets existed in interstices among a-plane grains is more than that of mplane grains within a linear dimension per unit length, indicating that the water CA could be increased for the case of a-plane grains. From the viewpoint of the grain shape, the outermost nanograin of m-plane ZnO is with a flat plane, whereas a-plane ZnO is with a wedge angle plane as depicted in Figures 2(h) and (i). When the water drop is on the surface of m-plane ZnO grains, large parts of the flat plane will contact the water drop, leading to decrease the water CA due to the higher surface energy state between the liquid and solid interface. Actually, the a-plane ZnO grain is equal to turn 30o to the m-plane grain as shown in Figure 9. The wedge angle plane will oppositely create the lower surface energy state in the liquidsolid interface comparing with m-plane grains, which encourages the water drop to tend to hydrophobicity. Therefore, we conclude that if c-plane and m-plane ZnO grains are coexistence on the ZnO thin film surface, even the c-plane grain can provide a rough terrace to produce the hydrophobic surface, whereas the m-plane grain will decrease the water CA due to the high surface energy state and fewer air pockets. In contrast, if a-plane grains are


Page 25 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


formed into m-plane matrix, the low surface energy state and more air pockets will enhance the hydrophobicity.

4. Conclusions In summary, the crystallographic orientation induced surface morphology evolution of PECVD synthesized ZnO thin films have been explored and reported. The c-plane, c-, mplane coexistence and m-, a-plane coexisting ZnO thin films were formed at the synthesized temperature of 350 oC, 450 oC and 550 oC, respectively. It is shown that the DEZn plays a crucial role to manipulate the crystallographic orientation of ZnO thin film. The DEZn not only can be used as zinc precursor under all synthesized temperatures to react with oxygen from CO2 to form ZnO phase, but also can be employed simultaneously as a passivating agent under 450 oC and 550 oC to suppress crystal growth direction along perpendicular to the substrate. On the other hand, the crystal growth direction was forcibly changed from perpendicular to parallel to substrate under high temperature. Room-temperature Hall effect measurement showed that the Z550 possessed the lowest value of resistivity accompanying with the highest Hall mobility, indicating that the stripe-like grains can reduce the carrier scattering and elongate the electron mean free path. Combining with the water contact angle (CA) measurement and Girifalco-Good-Fowkes (GGF) theory calculation, all ZnO thin films exhibit low surface energy state resulting in the hydrophobic behavior. Due to the mand a-plane ZnO crystal reveal dissimilar air pockets density and outermost plane, the CA could be affected slightly via different crystallographic orientation of ZnO thin film. In consideration of the low cost and straightforward preparation of ZnO thin films on commercial Si-based substrates, our presented work is of great approach in many academic fields and potential industrial applications.

Acknowledgment



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

The authors acknowledge financial support of the main research projects of the Ministry of Science and Technology (MOST) under Grant Nos. 104-2221-E-027-006 and 104-2731-M027-001.

Supporting Information XPS spectra of the O 1s core level region for all ZnO thin films. The asymmetric feature observed in the O 1s region could be fitted by three subspectral components corresponding to O-Zn bonds in the hexagonal wurtzite ZnO crystal structure (OL, 530.16 ± 0.09 eV), oxygen vacancy (OV, 531.5 ± 0.12 eV), and surface chemisorbed oxygen (OC, 532.15 ± 0.12 eV).


Page 26 of 48

Page 27 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


References (1) Sohn, J. I.; Hong, W. K.; Lee, S.; Lee, S.; Ku, J.; Park, Y. J.; Hong, J.; Hwang, S.; Park, K. H.; Warner, J. H.; et al. Surface Energy-Mediated Construction of Anisotropic Semiconductor Wires with Selective Crystallographic Polarity. Sci Rep 2014, 4, 5680. (2) Al-Kutubi, H.; Dikhtiarenko, A.; Zafarani, H. R.; Sudhölter, E. J. R.; Gascon, J.; Rassaei, L. Facile Formation of ZIF-8 Thin Films on ZnO Nanorods. Crystengcomm 2015, 17, 5360-5364. (3) Ramadan, A. J.; Rochford, L. A.; Ryan, M. P.; Jones, T. S.; Heutz, S. The Influence of Polar (0001) Zinc Oxide (ZnO) on the Structure and Morphology of Vanadyl Phthalocyanine (VOPc). RSC Adv. 2015, 5, 65949-65952. (4) Long, H.; Li, S.; Mo, X.; Wang, H.; Huang, H.; Chen, Z.; Liu, Y.; Fang, G. Electroluminescence from ZnO-Nanorod-Based Double Heterostructured LightEmitting Diodes. Appl. Phys. Lett. 2013, 103, 123504. (5) Han, C.; Xiang, D.; Zheng, M.; Lin, J.; Zhong, J. Sow, C. H.; Chen, W. Tuning the Properties of ZnO Nanowires Field Effect Transistors via Surface Functionalization. Nanotechnology 2015, 26, 095202. (6) Neuthe, K.; Bittner, F.; Stiemke, F.; Ziem, B.; Du, J.; Zellner, M.; Wark, M.; Schubert, T.; Haag, R. Phosphonic Acid Anchored Ruthenium Complexes for ZnO-Based DyeSensitized Solar Cells. Dyes Pigment. 2014, 104, 24-33. (7) Chao, C. H.; Chen, M. Y.; Lin, C. R.; Yu, Y. C.; Yao, Y. D.; Wei, D. H. Postannealing



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Effect at Various Gas Ambients on Ohmic Contacts of Pt/ZnO Nanobilayers toward Ultraviolet Photodetectors. Int. J. Photoenergy 2013, 2013, 372869. (8) Birkholz, M.; Selle, B.; Fenske, F.; Fuhs, W. Structure-Function Relationship between Preferred Orientation of Crystallites and Electrical Resistivity in Thin Polycrystalline ZnO:Al Films. Phys. Rev. B 2003, 68, 205414. (9) Ng, H. T.; Han, J.; Yamada, T.; Nguyen, P.; Chen, Y. P.; Meyyappan, M. Single Crystal Nanowire Vertical Surround-Gate Field-Effect Transistor. Nano Lett. 2004, 4, 12471252. (10) Sinha, M.; Mahapatra, R.; Mondal, B.; Maruyama, T.; Ghosh, R. Ultrafast and Reversible Gas-Sensing of ZnO Nanowire Arrays Grown by Hydrothermal Technique. J. Phys. Chem. C 2016, 120, 3019-3025. (11) Cheng, J. J.; Nicaise, S. M.; Berggren, K. K.; Gradečak, S. Dimensional Tailoring of Hydrothermally Grown Zinc Oxide Nanowire Arrays. Nano Lett. 2016, 16, 753-759. (12) Wang, T.; Wu, H.; Zheng, H.; Wang, J. B.; Wang, Z.; Chen, C.; Xu, Y.; Liu, C. Nonpolar Light Emitting Diodes of M-Plane ZnO on C-Plane GaN with the Al2O3 Interlayer. Appl. Phys. Lett. 2013, 102, 141912. (13) Biswas, P.; Kundu, S.; Banerji, P.; Bhunia, S. Super Rapid Response of Humidity Sensor Based on MOCVD Grown ZnO Nanotips Array. Sens. Actuator B-Chem. 2013, 178, 331-338. (14) Baik, K. H.; Kim, H.; Kim, J.; Jung, S.; Jang, S. Nonpolar Light Emitting Diode with


Page 28 of 48

Page 29 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Sharp Near-Ultraviolet Emissions Using Hydrothermally Grown ZnO on p-GaN. Appl. Phys. Lett. 2013, 103, 091107. (15) Umar, A.; Ribeiro, C.; Al-Hajry, A.; Masuda, Y.; Hahn, Y. B. Growth of Highly C-AxisOriented ZnO Nanorods on ZnO/Glass Substrate: Growth Mechanism, Structural, and Optical Properties. J. Phys. Chem. C 2009, 113, 14715-14720. (16) Noguera, C. Polar Oxide Surface. J. Phys.:Condens. Matter 2000, 12, R367. (17) Park, S. H.; Ahn, D. Spontaneous and Piezoelectric Effects in Wurtzite ZnO/MgZnO Quantum Well Lasers. Appl. Phys. Lett. 2005, 87, 253509. (18) Xiong, H.; Dai, J. N.; Hui, X.; Fang, Y. Y.; Tian, W.; Fu, D. X.; Chen, C. Q.; Li, M.; He, Y. Effects of the AlN Buffer Layer Thickness on the Properties of ZnO Films Grown on C-Sapphire Substrate by Pulsed Laser Deposition. J. Alloy. Compd. 2013, 554, 104109. (19) Lai, C. M.; Huang, Y. E.; Kou, K. Y.; Chen, C. H.; Tu, L. W.; Feng, S. W. Experimental and Theoretical Study of Polarized Photoluminescence Caused by Anisotropic Strain Relaxation in Nonpolar A-Plane Textured ZnO Grown by a Low-Pressure Chemical Vapor Deposition. Appl. Phys. Lett. 2015, 107, 022110. (20) Lee, C. Y.; Chen, C.; Chang, L.; Chou, M. M. C. Growth of Nonpolar ZnO Films on (100) β-LiGaO2 Substrate by Molecular Beam Epitaxy. J. Cryst. Growth 2014, 407 1116. (21) Alenezi, M. R.; Alshammari, A. S.; Jayawardena, K. D. G. I.; Beliatis, M. J.; Henley, S. J.; Silva, S. R. P. Role of the Exposed Polar Facets in the Performance of Thermally



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

and UV Activated ZnO Nanostructured Gas Sensors. J. Phys. Chem. C 2013, 117, 17850-17858. (22) Soundarrajan, P.; Sethuraman, K. Interface Energy Barrier Tailoring the Morphological Structure Evolution from ZnO Nano/Micro Rod Arrays to Microcrystalline Thin Films by Mn Doping. RSC Adv. 2015, 5, 44222-44233. (23) Guo, Z.; Liu, W.; Su, B. L. Superhydrophobic Surface: From Natural to Biomimetic to Functional. J. Colloid Interface Sci. 2011, 353, 335-355. (24) Bertoldo, M.; Coltelli, M. B.; Miraglia, L.; Narducci, P.; Bronco, S. Surface Energy Inducing Asymmetric Phase Distribution in Films of a Bynary Polymeric Blend. Polymer 2005, 46, 11311-11321. (25) Quéré, D. Wetting and Roughness. Ann. Rev. Mater. Res. 2008, 38, 71-99. (26) Chao, C. H.; Wei, D. H. Synthesis and Characterization of High C-Axis ZnO Thin Films by Plasma Enhanced Chemical Vapor Deposition System and Its UV Photodetector Application. J. Vis. Exp. 2015, 104, e53097. (27) Zhang, L. Q.; Lu, B.: Lu, Y. H.; Ye, Z. Z.; Lu, J. G.; Pan, X. H.; Huang, J. Y. Non-Polar P-Type Zn0.94Mn0.05Na0.01O Texture: Growth Mechanism and Codoping Effect. J. Appl. Phys. 2013, 113, 083513. (28) Shan, F. K.; Liu, G. X.; Lee, W. J.; Lee, G. H.; Kim, I. S.; Shin, B. C. Aging Effect and Origin of Deep-Level Emission in ZnO Thin Film Deposited by Pulsed Laser Deposition. Appl. Phys. Lett. 2005, 86, 221910. ACS Paragon Plus Environment

Page 30 of 48

Page 31 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(29) Koida, T.; Chichibu, S. F.; Uedono, A.; Sota, T.; Tsukazaki, A.; Kawasaki, M. Radiative ̅ 0) and Polar (0001 ̅ ) and Polar and Nonradiative Excitonic Transitions in Nonpolar (112 (0001) ZnO Epilayers. Appl. Phys. Lett. 2004, 84, 1079. (30) Ghosh, S.; Waltereit, P.; Brandt, O.; Grahn, H. T.; Ploog, K. H. Electronic Band Structure of Wurtzite GaN Under Biaxial Strain in the M Plane Investigated with Photoreflectance Spectroscopy. Phys. Rev. B 2002, 65, 075202. (31) Fanni, L.; Aebersold, B. A.; Alexander, D. T. L.; Ding, L.; Morales M.; Nicolay, S.; Ballif, C. C-Texture Versus A-Texture Low Pressure Metalogranic Chemical Vapor Deposition ZnO Films: Lower Resistivity Despite Smaller Grain Size. Thin Solid Film 2014, 565, 1-6. (32) Qin, N.; Xiang, Q.; Zhao, H.; Zhang, J.; Xu, J. Evolution of ZnO Microstructures from Hexagonal Disk to Prismoid, Prism and Pyramid and Their Crystal Facet-Dependent Gas Sensing Properties. Crystengcomm 2014, 16, 7062-7073. (33) Sundara Venkatesh, P.; Jeganathan, K. Investigations on the Morphological Evolution of Zinc Oxide Nanostructures and Their Optical Properties. Crystengcomm 2014, 16, 7426-7433. (34) Wang, M.; Jiang, L.; Wang, Y.; Kim, E. J.; Hahn, S. H. Growth Mechanism of Preferred Crystallite Orientation in Transparent Conducting ZnO:In Thin Films. J. Am. Ceram. Soc. 2015, 98, 3022-3028. (35) Morita, T.; Ueno, S.; Tokunaga, T.; Hosono, E.; Oaki, Y.; Imai, H.; Matsuda, H.; Zhou, ACS Paragon Plus Environment


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

H.; Hagiwara, M.; Fujihara, S. Fabrication of Transparent ZnO Thick Film with Unusual Orientation by the Chemical Bath Deposition. Cryst. Growth Des. 2015, 15, 3150-3156. (36) Rueter, M. A.; Vohs, J. M. The Surface Reactions of Ethyl Groups on Si(100) Formed via Dissociation of Adsorbed Diethylzinc. Surf. Sci. 1992, 262, 42-50. (37) van der Drift, A. Evolutionary Selection, A Principle Growth Orientation in VapourDeposited Layer. Phillips Res. Rep. 1967, 22, 267-288. (38) Fujimura, N.; Nishihara, T.; Goto, S.; Xu, J.; Ito, T. Control of Preferred Orientation for ZnOx Films: Control of Self-Texture. J. Cryst. Growth 1993, 130, 269-279. (39) Umar, A.; Hahn, Y. B. Aligned Hexagonal Coaxial-Shaped ZnO Nanocolumns on Steel Alloy by Thermal Evaporation. Appl. Phys. Lett. 2006, 88, 173120. (40) Gao, P. X.; Wang, Z. L. Substrate Atomic-Termination-Induced Anisotropic Growth of ZnO Nanowires/Nanorods by the VLS Process. J. Phys. Chem. B 2004, 108, 7534-7537. (41) Harish, S.; Navaneethan, M.; Archana, J.; Silambarasan, A.; Ponnusamy, S.; Muthamizhchelvan, C.; Hayalawa,Y. Controlled Synthesis of Organic Ligand Passivated ZnO Nanostructures and Their Photocatalytic Activity under Visible Light Irradiation. Dalton Trans. 2015, 44, 10490-10498. (42) Kavitha, M. K.; Jinesh, K. B.; Philip, R.; Gopinath, P.; John, H. Defect Engineering in ZnO Nanocones for Visible Photoconductivity and Nonlinear Absorption. Phys. Chem. Chem. Phys. 2014, 16, 25093-25100. ACS Paragon Plus Environment

Page 32 of 48

Page 33 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(43) Li, F.; Ding, Y.; Gao, P.; Xin, X.; Wang, Z. L.; Single-Crystal Hexagonal Disks and Rings of ZnO: Low-Temperature, Large-Scale Synthesis and Growth Mechanism. Angew. Chem. 2004, 116, 5350-5354. (44) Pearse, R. W. B.; Lichtenberg, A. J. The Identification of Molecular Spectra. Chapman and Hall Ltd, London, 1976, 4th ed. (45) Niu W.; Xu, H.; Guo, Y.; Li, Y.; Ye, Z.; Zhu, L. The Effect of Sulfur on the Electrical Properties of S and N Co-Doped ZnO Thin Films: Experiment and First-Principles Calculations. Phys. Chem. Chem. Phys. 2015, 17, 16705-16708. (46) Dhara, S.; Giri, P. K. Stable P-Type Conductivity and Enhanced Photoconductivity from Nitrogen-Doped Annealed ZnO Thin Film. Thin Solid Film 2012, 520, 5000-5006. (47) Panda, S. K.; Jacob, C. Preparation of Transparent ZnO Thin Film and Their Application in UV Sensor Devices. Solid-State Electron. 2012, 73, 44-50. (48) Jin, B. J.; Lee, S. Y. Violet and UV Luminescence Emitted from ZnO Thin Films Grown on Sapphire by Pulsed Laser Deposition. Thin Solid Film 2000, 366, 107-110. (49) Park, C.; Lee, J.; So, H. M.; Chang, W. S. An Ultrafast Response Grating Structural ZnO Photodetector with Back-to-Back Schottky Barriers Produced by Hydrothermal Growth. J. Mater. Chem. C 2015, 3, 2737-2743. (50) Soci, C.; Zhang, A.; Xiang, B.; Dayeh, S. A.; Aplin, D. P. R.; Park, J.; Bai, X. Y.; Lo, Y. H.; Wang, D. ZnO Nanowire UV Photodetectors with High Internal Gain. Nano Lett. 2007, 7, 1003-1009.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

(51) Soumahoro, I.; Colis, S.; Schmerber, G.; Leuvrey, C.; Barre, S.; Ulhap-Bouillet, C.; Muller, D.; Abd-lefdil, M.; Hassanain, N.; Petersen, J.; et al. Structural, Optical, Spectroscopic and Electrical Properties of Mo-Doped ZnO Thin Films Grown by Radio Frequency Magnetron Sputtering. Thin Solid Films 2014, 566, 61-69. (52) Tang, K.; Gu, S.; Liu, J.; Ye, J.; Zhu, S.; Zheng, Y. Effects of Indium Doping on the Crystallographic, Morphological, Electrical, and Optical Properties of Highly Crystalline ZnO Thin Films. J. Alloy. Compd. 2015, 653, 643-648. (53) Shinde, S. S.; Shinde, P. S.; Bhosale, C. H.; Rajpure, K. Y. Optoelectronic Properties of Sprayed Transparent and Conducting Indium Doped Zinc Oxide Thin Films. J. Phys. D: Appl. Phys. 2008, 41, 105109. (54) Brandt, M.; von Wenckstern, H.; Schmidt, H.; Rahm, A.; Biehne, G.; Benndorf, G.; Hochmuth, H.; Lorenz, M.; Meinecke, C.; Butz, T.; et al. High Electron Mobility of Phosphorous-Doped Homoepitaxial ZnO Thin Films Grown by Pulsed-Laser Deposition. J. Appl. Phys. 2008, 104, 013708. (55) Ding, P.; Pan, X.; Huang, J.; Lu, B.; Zhang, H.; Chen, W.; Ye, Z. Growth of P-Type APlane ZnO Thin Films on R-Plane Sapphire Substrates by Plasma-Assisted Molecular Beam Epitaxy. Mater. Lett. 2012, 71, 18-20. (56) Wenzel, R. N. Resistance of Solid Surfaces to Wetting by Water. Ind. Eng. Chem. 1936, 28, 988-994. (57) Cassie, A. B. D.; Baxter, S. Wettability of Porous Surfaces. Trans. Faraday Soc. 1944, 40, 546-551. (58) Żenkiewicz, M. Methods for the Calculation of Surface Free Energy of Solids. J. Achiev. Mater. Manuf. Eng. 2007, 24, 137-145. (59) Chi, P. W.; Wei, D. H.; Wu, S. H.; Chen, Y. Y.; Yao, Y. D. Photoluminescence and Wettability Control of NiFe/ZnO Heterostructures Bilayer Films. RSC Adv. 2015, 5,


Page 34 of 48

Page 35 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


96705-96713. (60) Molaei, R.; Bayati, M. R.; Alipour, H. M.; Estrich, N. A.; Narayan, J. Nanosecond Laser Switching of Surface Wettability and Epitaxial Integration of C-Axis ZnO Thin Films with Si(111) Substrates. J. Phys.:Condens. Matter 2014, 26, 015004. (61) Hu, J.; Sun, Y.; Zhang, W.; Gao, F.; Li, P.; Jiang, D.; Chen, Y. Fabrication of Hierarchical Structures with ZnO Nanowires on Micropillars by UV Soft Imprinting and Hydrothermal Growth for a Controlled Morphology and Wettability. Appl. Surf. Sci. 2014, 317, 545-551. (62) Feng, X.; Feng, L; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. Reversible Super-Hydrophobicity to Super-Hydrophilicity Transition of Aligned ZnO Nanorods Films. J. Am. Chem. Soc. 2004, 126, 62-63. (63) Wu, X.; Zheng, L.; Wu, D. Fabrication of Superhydrophobic Surfaces from Microstructured ZnO-Based Surfaces via a Wet-Chemical Route. Langmuir 2005, 21, 2665-2667. (64) Kwak, G.; Seol, M.; Tak, Y.; Yong, K. Superhydrophobic ZnO Nanowire Surface: Chemical Modification and Effects of UV Irradiation. J. Phys. Chem. C 2009, 113, 12085-12089.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 36 of 48

Table 1. Relative percentages from the OL, OV and OC components in the Z350, Z450, and Z550. The compositions of O 1s peak for all ZnO films obtained from XPS spectra.

Film code Z350 Z450 Z550

OL

OV

OC

Relative percentage (%)



84.65 66.97 82.99

9.25 14.78 2.24

6.1 18.25 14.77


Page 37 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figures Captions: Fig. 1. (Color online) XRD patterns for the Z350, Z450, and Z550 synthesized onto Si (100) substrates. Fig.2. (Color online) Plane-view FE-SEM images for the (a) Z350, (b) Z450, and (c) Z550, the insets show the corresponding histograms for grain size average distribution, respectively. (d), (e), and (f) are the cross-sectional SEM images for the Z350, Z450, and Z550, respectively. (g), (h), (i) are the schematic diagrams for c-, m- and a-plane ZnO wurtzite structure, respectively. Fig. 3. (Color online) HRTEM images for (a) Z350, (b) Z450, and (c) Z550, respectively. Fig. 4. (Color online) Room temperature PL spectra for the Z350, Z450, and Z550. Fig. 5. (Color online) In-situ OES spectra for the RF plasma during ZnO thin films synthesized at 350 oC, 450 oC and 550 oC. Fig. 6. (Color online) Schematic diagrams of the proposed mechanism of crystallographic orientation evolution for Z350, Z450 and Z550. Fig. 7. (Color online) Resistivity, Hall mobility and carrier concentration values for Z350, Z450 and Z550. Fig. 8. (Color online) Water contact angle images for (a) Z350, (b) Z450, and (c) Z550. (d) The water contact angle and surface free energy of ZnO thin films as a function of synthesized temperature. Fig. 9. (Color online) Schematic diagrams for water drop deposited onto surface of (a) mplane ZnO grains and (b) a-plane ZnO grains. (c) and (d) are schematic diagrams in lateral view of hexagonal grain with m- and a- orientation, respectively.



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig. 1. (color online)


Page 38 of 48

Page 39 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60





1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 40 of 48

Page 41 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60





1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 42 of 48

Page 43 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fig .6. (color online)



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Fig .7. (color online)


Page 44 of 48

Page 45 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60





1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



Page 46 of 48

Page 47 of 48

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC



1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47


Page 48 of 48

Investigations on the Crystallographic Orientation Induced Surface

Recommend Documents