Nanostructured N-polar GaN surfaces and their wetting behaviors

1.1 1. Introduction Gallium nitride (GaN) is a direct and wide band gap (Eg = 3.4 eV) semiconductor with extensive applications in optoelectronic, high-power and high-frequency devices. Recently, chemical sensors and biosensors based on GaN have attracted great attention [1,2] due to the newly discovered superior biocompatibility [3,4] and the feasibility of covalent functionalization with biomolecular recognition elements [5,6]. As a result, tuning of GaN surface properties, including the wetting behaviors, received some attention due to their potentials for efficient regulation of cell growth [7] and in vivo detection of biomolecules. Wurtzite GaN is a typical polar material with two polar surfaces, N-polar face and Ga-polar face, which exhibit distinct chemical properties [8,9]. For example, N-polar GaN sensors exhibit greater responsivities to dilute concentrations of hydrogen [10], water and acetone [11] than Ga-polar devices, while Ga-polar GaN is more suitable for oxygen detection due to the formation of Ga–O bonds [12]. Although the wetting behaviors of Ga-polar GaN have been studied [13,14], similar investigation on N-polar GaN is absent. Moreover, the carbon face of silicon carbide exhibits a water contact angle (CA) greater than that of the silicon face [15] due to polarity induced interaction with polar liquid. As a result, it is necessary to study the wetting behaviors of 1

N-polar GaN due to its potential application as sensor for polar molecules and fluidcontrollable devices. In this article, the wetting behavior of nanostructured N-polar GaN was explored. The effects of Au nanoparticles (NPs) and surface modification with lauric acid (C12H24O2) were also studied.

1.2 2. Experimental The epi-ready N-polar GaN samples of n-type were purchased from Nanowin, China, with a background carrier concentration of ~ 1017/cm3 and a dislocation density of ~ 106/cm2. The samples were grown by Metal Organic Chemical Vapor Deposition (MOCVD) and were obtained after separated from sapphire substrates by laser lift-off. The size is 5 mm × 5 mm, with a thickness of 0.5 mm. After supersonically washed with acetone, ethanol and deionized water in sequence, each for 15 min the samples were immersed into the etchant that consists of 0.5 M KOH and 0.1 M K2S2O8 for 50 min under UV illumination by a 400 W mercury lamp with a power density of 6 mW/cm2. Au NPs were prepared on the etched samples as a secondary structure by a sputtering post annealing process. The sputtering process was accomplished in an ETD 2000 sputter coating system at ~ 6 mA for 10 s, 30 s and 50 s, respectively. After that, the samples were annealed at 300°C for 1 h. The samples were modified by dip coating in 0.01 M lauric (C12H24O2) ethanol solution for 30 min and dried at 60°C for 8 h. For convenience, the samples before and after etching are named N-blank, N-etched, and the samples coated with Au NPs for 10 s, 30 s and 50 s are named N-Au-I, N-Au-II and N-Au-III, respectively. A blank sample coated with Au NPs for 50 s is called N-blank-Au. In order to ensure the reproducibility of our experiments, each experiment was repeated at least 3 times on 3 identical samples, which show a CA difference less than 5°. The morphologies of the nanostructured N-polar GaN were examined using a scanning electron microscope (SEM, Hitachi S-4800, Japan). The CAs of sessile water drop (2.5 μL) and the contact angle hysteresis (CAHs) were measured on a CA meter (JY-PHb, China) at ambient temperature. Finally, the CA of a sample is obtained by averaging the values obtained in 5 measurements. The transmission spectra were measured using a spectrophotometer (UV1901PC, China). The roughness of samples were obtained by atomic force microscopy (AFM, Veeco Dimension Icon, USA).

1.3 3. Results and Discussion Figure 1a shows the optical image of N-polar GaN before and after etching. The etching process turns the GaN from transparent to white due to enhanced backward scattering. As shown in Figure 1b, the etching process greatly reduces the transmittance of N-etched and N-Au-III, in contrast with N-blank characterized by a sharp increase of transmittance for wavelength greater than 365 nm. Note that there is a transmittance minimum at 550 nm for N-Au-III due to excitation of the localized surface plasmon resonances (LSPRs) of Au NPS.

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Figure 2a reveals that photochemical etching results in formation of conical nanostructures with an average cone size of ~ 600 nm and a density of ~ 5.5×108/cm2. A magnified image in Figure 2b shows the smooth conical side wall which is different from the pyramid nanostructures formed in KOH etchant [16]. Figure 2c shows that Au NPs with average size of ~ 30 nm were formed after the sputtering post annealing process, resulted from a combination of coalescence and Ostwald ripening [17,18]. Figure 2d shows a schematic representation for the photochemical etching process [19]. UV illumination excites the valence band electrons to the conduction band to form electronhole pairs. The holes are highly active and oxidize GaN to form Ga2O3 soluble in a basic solution, while the electrons in the conduction band are consumed either directly by S2O82- or by radicals (SO4-· and OH·) formation after photolysis of S2O82-. The shape of the conical nanostructures is determined by the ratio of three dissolution rates [20], 1) the etch rate vn normal to the surface in flat areas free from a hillock, 2) the lateral etch rate vt parallel to the surface, and 3) the rate vp that describes the dissolution (or polishing) of the hillock. Both vn and vp are determined by temperatures and surface energies of crystallographic planes while vp is also a function of the composition of the etchant. When vp << vn, the slope of a hillock is given by vn/vt and a pyramid is formed. However, when vp is large, a conical structure with a slope given by (vn - vp)/vt will be formed. As the radium of the water drop is much less than the capillary length (cl), which has been determined to be 2.7 mm for water by cl = (γlv/ρg)1/2, with γlv being the liquid surface tension and ρ the density, the effect of gravity is negligible [21]. Figure 3a and 3d show the CAs for N-blank before (~ 68.3°) and after (~ 95.2°) modification with lauric acid, respectively. The intrinsic CA is smaller than the CA (~ 83.7°) measured on epitaxial GaN film [22] possibly due to different surface roughness and strong affinity of hydrogen with N-polar surfaces [23]. Modification with lauric acid results in a hydrophobic transition. After preparation of Au NPs, the CAs before and after laruic acid modification are ~ 62.7° and ~ 98.8°, respectively. The CA values measured in the case of etched GaN before and after surface modification are of ~ 53.5° and ~ 110.4°, respectively. The samples coated with Au NPs exhibit CAs of ~ 42.1° and ~ 129.5° before and after surface modification, respectively. The increased CA of N-Au-III results from a combined effect of the conical structures and the Au NPs. Figure 4 shows the CAHs of the samples before and after modification with lauric acid. The water droplets adhere to the substrate tightly without slipping when the substrate is vertically placed. The largest difference between the advancing CA and the receding CA can reach 50.0° for the sample coated with Au NPs for 50 s (Figure 4f). The CAH reflects the surface adhesion. A surface with high CA and CAH is often observed on biological surfaces in nature, such as the Chinese Kaffir lily, red rose and sunflower [24,25].This kind of surface is suitable to serve as ``mechanical hand'' in fluid-controllable devices for lossless liquid transfer [26]. 3

The theories commonly used for the wetting behavior of a liquid droplet on a solid substrate are the Wenzel and Cassie models. The Wenzel model [27] assumes that the liquid completely fills the grooves of a rough surface where they contact. The relationship between the apparent CA θ* and the intrinsic CA θ can be described by cos(θ*) = β·cos(θ) = β·(γsv - γls)/γlv, where β is the roughness ratio, γsv, γls and γlv are the solid–gas, liquid–solid and liquid–gas surface tensions, respectively. As β > 1, the hydrophilicity or hydrophobicity of a surface that follows the Wenzel model will be amplified. In contrast, a surface that follows the Cassie model [28] will have vapor pockets trapped underneath the liquid. Unlike the Wenzel case, even if the intrinsic CA is < 90°, the apparent CA can still be enlarged due to trapped vapor pockets. Notably, the adhesion behaviors for surfaces that follow the Wenzel and Cassie models are quite different. The Wenzel mode allows liquid to impregnate into roughened surface, resulting in increased adhesive force [29]. The Cassie model will ease liquid slipping due to reduced pinning by a cushion of air [22]. The Root-Mean-Square (RMS) roughnesses of sample N-blank, N-etched and N-Au-III obtained from AFM images (Figure S1) were 0.265 nm, 24.6 nm and 39.2 nm, respectively. Note that the Au NPs can further increase the roughness of etched surface. We can see from Figure 3 that a larger RMS roughness will result in a more hydrophilic/hydrophobic surface before/after modification with lauric acid. Besides, the CAH measurement (Figure 4) reveals a high adhesive surface. Both the CA and the CAH measurements indicate the wetting behavior of nanostructured N-polar GaN follows the Wenzel model [27]. Figure 5a shows the CAs of etched N-polar GaN after deposition of Au NPs and modification with lauric acid. The average CAs for N-blank, N-etched, N-Au-I, N-Au-II, and N-Au-III are ~ 98.3°, ~ 110.3°, ~ 113.9°, ~ 119.3° and ~ 128.0°, respectively. The results further prove the wetting behavior follows the Wenzel model as the CA increases with the surface roughness increasing. However, a sputtering time longer than 50 s will result in reduced surface roughness due to coalescence of Au NPs, as shown in Figure S2, which results in a drop of the CA. For a droplet on a tilt substrate, the tilt angle α when the droplet sliding down can be determined by the maximum of CAH as [30] mg·sin(α)/w = γlv˖[cos(θR) – cos(θA)], where θA and θR are the advancing and receding CAs, respectively, mg is the gravity force, w is the contact length of the droplet. For a droplet to adhere tightly to a surface, lower droplet weight and larger difference between the advancing and receding CAs are necessary. As shown in Figure 5b, the static friction γH of a tightly adhered water droplet on nanostructured N-polar GaN is estimated by [31] cos(θR,A) = β·(cos(θ) ± γH/γlv). As γlv = 72 mJ/m2, γH is calculated to be ~ 15 mJ/m2. This value is equal to ~ 75 μN for the measured area, much larger than the adhesive force (~ 60 μN) for other surfaces [32,33]. It can be used as a ‘‘mechanical hand’’ in lossless liquid transfer. The high static friction force may result from dipolar interaction between water molecules and the N-polar surfaces besides the Van der Waals’ force [34]. We 4

expect this principle is general and can be extended to polar liquid transfer and polar molecule detection.

1.4 4. Conclusions The wetting behavior of nanostructured N-polar GaN was studied for the first time. It is confirmed that the wettability follows the Wenzel model. The CAs can change from 42.1° to 129.5° by nanostructures and surface modification. We also find that the CAH is very significant such that water droplets will tightly adhere to the surface even when the GaN samples were vertically placed. The static friction of the nanostructured GaN was estimated to be ~ 15 mJ/m2. We hope that this investigation will aid on the design and fabrication of GaN based sensor for polar molecules and fluid-controllable device. Acknowledgements The authors thank National Natural Science Foundation of China (NSFC) (Grant No. 51472143, 91233122, 91123007), the Fundamental Research Funds of Shandong University (Grant No. 2014JC032 and 2014YQ003), SRF for ROCS, State Education Ministry, and National Basic Research Program of China (973 Program) (Grant No. 2009CB930503) for financial support. References [1] G. Steinhoff, M. Hermann, W. J. Schaff, L. F. Eastman, M. Stutzmann, M. Eickhoff,;1; pH response of GaN surfaces and its application for pH-sensitive field-effect transistors, Appl. Phys. Lett. 83 (2013) 177-179. [2] A. Kamińska, I. DzięRcielewski, J. L. Weyher, J. Waluk, S. Gawinkowski, V. Sashuk, M. Fiałkowski, M. Sawicka, T. Suski, S. Porowski, R. Hołyst,;1; Highly reproducible, stable and multiply regenerated surface-enhanced Raman scattering substrate for biomedical applications, J. Mater. Chem. 21 (2011) 8662-8669. [3] I. Cimalla, F. Will, K. Tonisch, M. Niebelsch€utz, V. Cimalla, V. Lebedev, G. Kittler, M. Himmerlich, S. Krischok, J. A. Schaefer, M. Gebinoga, A. Schober, T. Friedrich, O. Ambacher,;1; AlGaN/GaN biosensor—effect of device processing steps on the surface properties and biocompatibility, Sensor Actuators B 123 (2007) 740-748. [4] S. A. Jewett, M. S. Makowski, B. Andrews, M. J. Manfra, A. Ivani-sevic,;1; Gallium nitride is biocompatible and non-toxic before and after functionalization with peptides, Acta. Biomater. 8 (2012) 728-733. [5] R. Stine, B. S. Simpkins, S. P. Mulvaney, L. J. Whitman, C. R. Tamanaha,;1; Formation of amine groups on the surface of GaN: A method for direct biofunctionalization, Appl. Surf. Sci. 256 (2010) 4171-4175. [6] X. Xu, V. Jindal, F. Shahedipour-Sandvik, M. Bergkvist, N. C. Cady,;1; Direct immobilization and hybridization of DNA on group III nitride Semiconductors, Appl. Surf. Sci. 255 (2009) 5905-5909.

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Figure 1. Optical properties of nanostructured N-polar GaN. (a) Optical images of N-polar GaN before and after wet photochemical etching; (b) the optical transmittance of N-blank, N-etched and N-Au-III. The inset is an enlarged view for N-etched and NAu-III.

Figure 2. Morphologies of nanostructured N-polar GaN. (a) SEM image of etched GaN showing conical nanostructure; (b) a magnified image of (a); (c) SEM image

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of etched GaN decorated with Au NPs; (d) schematic of the wet photochemical etching mechanism.

Figure 3. Static CA measurements on nanostructured N-polar GaN surfaces before (a-d) and after (e-h) surface modification with lauric acid for (a,e) N-blank, (b,f) N-blank-Au, (c,g) N-etched and (d,h) N-Au-III, respectively.

Figure 4. Dynamic CAH measurements on nanostructured N-polar GaN surfaces before (a-d) and after (e-f) surface modification with lauric acid for (a,e) Nblank, (b,f) N-blank-Au, (c,g) N-etched and (d,h) N-Au-III, respectively.

Figure 5. (a) Measured CAs of nanostructured N-polar GaN coated with Au NPs for different sputtering time. Insets are optical images of the water droplets; (b) a schematic of the static friction (γH) and the concomitant CAH corresponding to Figure 4c and 4f. TDENDOFDOCTD

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