Article pubs.acs.org/crystal
Ag Nanostructures on GaN (0001): Morphology Evolution Controlled by the Solid State Dewetting of Thin Films and Corresponding Optical Properties Sundar Kunwar,† Mao Sui,† Quanzhen Zhang,† Puran Pandey,† Ming-Yu Li,† and Jihoon Lee*,†,‡ †
College of Electronics and Information, Kwangwoon University, Nowon-gu Seoul 01897, South Korea Institute of Nanoscale Science and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States
‡
S Supporting Information *
ABSTRACT: Silver (Ag) nanostructures have demonstrated the feasibility of being utilized in various optoelectronic, catalytic, biomedical, and sensor devices due to their excellent surface plasmon resonance characteristics. The geometrical structure, spacing, and spatial arrangement of nanostructures are crucial for controlling the properties and device performance. Herein, we demonstrate the fabrication of various configurations of self-assembled Ag nanostructures on GaN (0001) by the systematic control of deposition thickness and annealing duration. The surface morphology evolution is thoroughly investigated, and the corresponding influence on optical properties is probed. The evolution of Ag nanostructures in response to thermal annealing is described based on the dewetting of thin films, Volmer−Weber growth model, coalescence growth, and surface energy minimization mechanism. For the deposition amount variation between 1 and 100 nm, the Ag nanostructures show gradual morphological transitions such as small nanoparticles (NPs) to enlarged NPs between 1 and 7 nm, elongated nanostructures to cluster networks between 10 and 30 nm, and void evolution with layered nanostructures between 40 and 100 nm. In addition, the annealing duration effect has been studied between 0 and 3600 s, where the Ag nanostructures exhibit the evolution of networklike, elongated and isolated irregular shapes, ascribed to Ostwald’s ripening along with Ag sublimation. Furthermore, corresponding Raman, photoluminescence, and reflectance spectra reveal the morphology-dependent behaviors and are discussed based on the phonon, emission band, scattering, absorption, and surface plasmon effect.
1. INTRODUCTION In recent years, tremendous advances have been made in the research on silver (Ag) nanostructures, which has enabled the fabrication of nanostructures with various shapes, sizes, and configurations, applicable in the numerous devices in electronics, optics, catalysis, and sensors.1−16 The embedded nanostructures can demonstrate improved properties such as the increased surface to volume ratio, surface plasmonic resonance, carrier mobility and concentration and large number of active sites and facets, which in turn are essential for the improvement in corresponding applications. Especially, Ag nanoparticles (NPs) are one of the most efficient materials for the electronic and plasmonic related devices due to the electron confinement and localized surface plasmon resonance properties (LSPR).6,9,10 Meanwhile, a wide bandgap (∼3.4 eV) wurtzite structure GaN has been widely applied in the fabrication of light emitting diodes (LEDs), laser diodes, and field-effect transistor (FETs). To realize the improved efficiency, Ag nanostructures with various shapes, sizes, and spacing have been incorporated into the devices. For instance, the optical output power of GaN-based LEDs can be enhanced by the resonance coupling between multiple quantum wells and localized surface plasmons on the Ag NPs.17 At the same time, the enhancement of optical output power was dependent on © XXXX American Chemical Society
the density of Ag nanoparticles. Thus, the systematic morphological characterization together with optical analysis of the Ag nanostructures on GaN (0001) is of great importance to understand their properties based on the unique geometrical configurations, spacing, and density, which has not been reported up to now. In this work, the Ag nanostructures are systematically fabricated on GaN (0001) by the controlled variation of deposition amount and annealing duration. The thermal annealing at the fixed temperature (550 °C) leads to the evolution of Ag nanostructures due to the solid-state dewetting of thin films and three-dimensional (3D) growth driven by the total surface energy minimization mechanism. The results are explained using various theoretical models such as diffusion theory, coalescence mechanism, Rayleigh instability, equilibrium stage, and Ostwald ripening effects.
2. EXPERIMENTAL SECTION 2.1. GaN Substrate Preparation. Initially, n-type 5 μm-thick GaN templates were epitaxially grown on 650 μm-thick sapphires with an off-axis ±0.1° by the Xiamen Powerway Advanced Material Co. Ltd. (PAM-XIAMEN, China). Then, the epitaxial-ready GaN (0001) Received: August 8, 2016 Revised: November 10, 2016
A
DOI: 10.1021/acs.cgd.6b01185 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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wafers were diced into the squares with identical size and degassed in a pulsed laser deposition (PLD) system under 1 × 10−4 Torr at 350 °C for 30 min. After degassing, the surface morphology of the substrates was studied by atomic force microscopy (AFM) scanning as shown in Figure S1a. The smooth surface morphology with the natural atomic steps of GaN surface was obtained within a range of 0.2 nm as reveled by the cross-sectional line profile. Furthermore, the GaN-sapphire configuration of the substrates was confirmed by the characteristic reflectance, Raman, and PL spectra in Figure S1c−e. 2.2. Ag Nanostructures Fabrication. After cleaning of the substrates by degassing, the fabrication of Ag nanostructures was commenced by the deposition of Ag thin film and subsequent annealing well below the melting temperature. First, Ag thin film was deposited on the substrates by the sputtering in the plasma ion coater (COXEM CO.; Ltd. South Korea) at a growth rate of 0.1 nm s−1 with 5 mA ionization under 1 × 10−1 Torr. The samples were prepared with various deposition amounts between 1 and 100 nm for the deposition amount variation series. Also, the fixed 30 nm film thickness samples were prepared for the annealing duration variation series between 0 and 3600 s. The samples were systematically annealed in the PLD chamber at 550 °C under 1 × 10−4 Torr. The target temperature was achieved by a constant increasing rate of 4 °C s−1 and specific dwelling durations were allocated, i.e., 60 s, for the deposition amount variation series and 0 to 3600 s for dwelling time variation series. After annealing at predefined temperature and duration, i.e., 550 °C for 0 to 3600 s, the system temperature was immediately quenched to the ambient. The specific growth parameters and procedures were strictly kept constant using the computer based recipe programs for all the samples in the series to maintain the consistency. 2.3. Characterization. The surface morphology characterization of as-fabricated Ag nanostructures was performed by AFM from the Park Systems Corp. (XE-70, South Korea) under the tapping mode. The NSC16/AIBS tips with a drive frequency of ∼270 kHz were utilized for all samples, which was computer controlled by using the XEP program. The data preparation was carried out by processing the scanning data in terms of the top views, 3D side views, cross-sectional line profiles, and Fourier filter transform (FFT) power spectra using XEI program. Large scale morphologies were investigated by a scanning electron microscope (SEM) from the COXEM (CX-200, South Korea). The elemental analysis was performed by means of spectra and mapping using an energy dispersive X-ray spectroscope (EDS) from the Thermo Fisher Scientific (Noran System 7, USA). For the optical characterizations, Raman, photoluminescence (PL), and reflectance spectra were recorded using a spectrophotometer from the Andor (Shamrock 500i, UK). Specifically, a 532 nm laser for Raman, 266 nm laser for PL, and combined deuterium (UV region) and halogen (VIS and NIR) light source for reflectance measurements were utilized. All the optical characterization was performed in a dark room at ambient condition.
Figure 1. Evolution of silver (Ag) nanoparticles (NPs), wiggly nanostructures and voids on the GaN (0001) by the control of deposition amount from 1 to 100 nm at the annealing condition of 550 °C for 60 s. AFM top-views of 3 × 3 μm2 in (a−d), 5 × 5 μm2 in (e− h), and 10 × 10 μm2 in (i−l). Surface morphology evolution in terms of (m) RMS roughness (Rq) and (n) surface area ratio (SAR).
S2. This is more pronounced for the samples above 15 nm, and the surface consists of round aggregates, which can be due to the natural instability of the thin film deposition under the absence of favorable diffusion and the stronger bonding energy between the Ag atoms. The nanoscale metallic film deposited on the semiconductor substrates at room temperature normally can be unstable and tend to form isolated particles upon providing sufficient thermal energy.18 According to the thermodynamic diffusion theory, the dewetting process can be accelerated when provided with the thermal energy as surface Ag adatoms continuously diffuse and nucleate in order to release the total surface energy, i.e., surface and interface.19−21 As a result, the Ag atoms can be agglomerated, allowing the Ag nanostructure formation driven by the surface free-energy minimization. At the annealing temperature of 550 °C, the spontaneous agglomeration of Ag thin film can be initiated with the nucleation and growth at the random low energy sites such as defects, edges, and grain boundaries.22 Although the constant annealing temperature was applied to all the samples, the dewetting process can be significantly affected by the thickness of Ag film, and thus the final configuration, shape, size, and density of nanostructures were readily varied between 1 and 100 nm. As presented in Figures 1a and 2a, initially, the tiny domeshaped dense Ag NPs were formed. The fabrication of the 3D dome-shaped Ag NPs can be explained based on the Volmer− Weber growth model. During the annealing at 550 °C, the sufficient diffusion and stronger binding energy between Ag adatoms led to the 3D island growth of NPs.23 Moreover, with fixed deposition and annealing condition variation of NPs size can be expected due to the dewetting of Ag films at random
3. RESULTS AND DISCUSSION 3.1. Investigation on Morphology of Ag Nanostructures Based on Deposition Amount Variation. Figure 1 presents the evolution of self-assembled Ag nanostructures on GaN (0001) due to the variable thickness of Ag deposition amount ranging from 1 to 100 nm after annealing at 550 °C for 60 s. The formation of Ag nanostructures can be distinguished in three growth regimes: (i) dome-shaped Ag NPs, (ii) wiggles and network-like nanostructures, and (iii) voids formation. On the basis of the surface morphology and deposition thickness, specifically, the NPs from tiny compact to large isolated between 1 and 7 nm, wiggles nanostructures from isolated to connected between 10 and 30 nm and the highly dense to low density voids between 40 and 100 nm were observed. The detailed analysis with AFM side views, cross-sectional line profiles, and FFT power spectra are shown in Figures 2 and 3. Initially, the surface morphology of deposited Ag thin film became rougher than the bare GaN (0001) as shown in Figure B
DOI: 10.1021/acs.cgd.6b01185 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 3. Formation of wiggly Ag nanostructures and voids on GaN (0001) with the thickness between 20 and 100 nm after fixed annealing at 550 °C for 60 s. AFM side views of 5 × 5 μm2 in (a−c) and 10 × 10 μm2 in (d−f). (a-1)−(f-1) Cross-sectional line profiles showing the height. (a-2)−(f-2) 2-D FFT power spectra of corresponding AFM images.
Figure 2. Transition of Ag NPs to wiggly nanostructures on GaN (0001) based on the various Ag deposition amounts (1−15 nm) annealed at 550 °C for 60 s. AFM side views of 1 × 1 μm2 in (a−d) and 3 × 3 μm2 in (e−f) along with the deposition thickness as labeled. (a-1)−(f-1) Cross-sectional line profiles in reference to the line drawn in images (a−f). (a-2)−(f-2) 2-D Fourier filter transform (FFT) spectra of corresponding AFM images.
average surface height as expressed by the following equation:
position. When the deposition amount was varied from 1 to 7 nm, the size of Ag NPs increased from a few nanometers to several nanometers. The surface energy minimization by isotropic distribution leads the formation of dome configurations of Ag NPs. The surface adatoms concentration was increased with the increased deposition amount, and the Ag NPs absorb the nearby adatoms in order to reduce the surface free energy and gain equilibrium state. Meanwhile, the weak attractive force may also drive the self-assembly of NPs; as a result the low energy configuration can be found. Thus, the dimension of NPs was progressively enhanced in horizontal and vertical directions. Meanwhile, the obvious decrease in the density was observed due to the merging of the adjacent smaller Ag NPs. The AFM side views in Figure 2a,d provide more detailed analysis of the evolution process of tiny to large Ag NPs along with cross-sectional line profiles and 2D-FFT. The average NPs’ height ∼3 at 1 nm to ∼30 at 7 nm was observed from the cross-sectional profiles. In addition, the reducing size of the FFT pattern denoted that the periodicity of the surface height was reduced with the formation of large NPs. Furthermore, the overall surface evolution was investigated by RMS roughness (Rq) and surface area ratio (SAR) as presented in Figure 1, panels m and n respectively. The Rq is the measure of height profile from the mean plane, which represents the
(Rq) =
1 n
n
∑i = 1 yi2 , where yi is profile height at each pixel, and
SAR is the measure of variation between surface area (3D) and geometric area (2D) as expressed by the equation (SAR) = (AT − A S) AT
× 100 [%], where AT is geometric area and As is
surface area. The Rq was incremented from 2.477 to 18.353 nm, whereas SAR increased from 3.419 to 17.750% when the thickness varied between 1 and 7 nm. Such an increment in the surface parameter depicts the enhancement in morphology and dimension of Ag NPs. At 10 nm of deposition amount, the NPs were slightly elongated, and with a further increase in deposition amounts between 15 and 30 nm, they started to merge and as a result connected and branched nanostructures were developed as shown in Figure 1e−h. The elongated and the network-like (wiggly) nanostructure evolution can be attributed to the coalescence phenomenon of the adjacent NPs,24 in which, relatively small NPs can be absorbed to the large ones driven by the surface energy minimization to reach the stable configuration as discussed. Furthermore, due to the large size, dome shape configurations could not be sustained by NPs, which result in the either facet formation or elongated NPs. The AFM side views in Figure 2e,f and Figure 3a,b show the C
DOI: 10.1021/acs.cgd.6b01185 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 4. Elemental analysis of samples with the variation of Ag thickness between 1 and 100 nm, equally annealed at 550 °C and 60 s. (a−c) Energy dispersive X-ray spectroscopy (EDS) spectra showing Ag peaks (Ag Lα1 and Ag Lβ1) for the samples with thickness as denoted by color index. (d) Summary of Ag Lα1 peak counts with respect to the deposition amount. (e−f) SEM images. (e-1)−(f-1) Ag phase, (e-2)−(f-2) Ga phase, and (e3)−(f-3) N phase maps.
detail of merged nanostructure formation and large-scale SEM images depict the overall surface morphology in Figure S4a−d. Similarly, the average height of cross-sectional line profiles was gradually increased with the growth of NPs size, and the overall height distribution was further reduced as presented by FFT power. In the case of Rq and SAR, Rq increased from 10.418 to 42.359 nm, and SAR increased from 11.38 to 12.99% when the thickness was varied between 10 and 30 nm. The Rq was consistently increased due to the enhanced vertical growth of NPs; however, SAR was declined due to the reduced coverage or density of NPs. With the higher thickness of deposition amount, i.e., between 40 and 100 nm, the surface consists of only voids and layered structures. The size and density of voids were consistently decreased with an increase in the deposition amount as shown in Figure 1i,l. The void evolution and layered nanostructures at higher deposition amount can be the consequence of enhanced coalescence and limited diffusion of the thin film. At 550 °C of annealing temperature, the dewetting process can be significantly lowered due to the high thickness of deposited film. The AFM side views together with cross-sectional line profiles, and the FFT power spectra demonstrate the surface modulation, enhanced surface height, and overall height distribution due to the void evolution. The detailed surface
morphology in a large scale is shown by the SEM images in Figure S4e,f and small-scale AFM top views in Figure S3. In addition, the Rq also denoted the increased surface height, whereas SAR was further decreased. The specific value of the Rq and SAR for each sample is listed in Table S1. The elemental composition of samples was investigated by the EDS spectra and phase mapping as shown in Figure 4. The full range EDS spectra of each sample are provided in Figure S7. As shown in Figure 4a−c, the peaks associated with the Ag; Ag Lα1 (at 2.98 keV) and Ag Lβ1 (at 3.14 keV) were consistently increased with the deposition amount as summarized in Figure 4d; i.e., EDS count increases from ∼455 to 11518. Furthermore, the elemental phase mapping was carried out for 75 and 100 nm samples as shown in Figure 4e,f. The Ag phase was constructed from Ag nanostructures, and Ga and N phases were due to the substrate element. It is clearly seen that the traces of Ag nanostructures on Ag phase mapping and SEM images exactly match. Also, the Ga and N phase resembles the voids, i.e., the exposed surface of the substrate in the SEM images. Similarly, for the 30 and 50 nm samples, the elemental phase mapping is presented in Figure S8. On the basis of the deposition thickness variation between 1 and 100 nm, the size, spacing, and configuration of Ag nanostructures on GaN (0001) were significantly modulated at D
DOI: 10.1021/acs.cgd.6b01185 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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Figure 5. (a) Raman spectra of samples with the Ag thickness variation between 1 and 100 nm after annealing at 550 °C for 60 s (excited by 532 nm laser at ambient). The E2 (high) peaks at ∼570 cm−1 for each sample is indicated with the respective deposition amount. (b) Photoluminescence (PL) spectra of samples at room temperature by the 266 nm laser with 20 mW and inset show the emission band of bare and 1 nm samples. (c−e) Summary of Raman peak intensity, peak shift and full width at half-maximum (FWHM) of samples based on the E2 mode. (f−h) Summary of the PL peak intensity, peak shift, and FWHM with respect to the deposition amount.
reveal the three active phonon modes of GaN and sapphire composition. The phonon mode A 1 (g) at 417 cm −1 corresponds to the sapphire, whereas the E2(high) at 569 cm−1 and A1(LO) at 736 cm−1 correspond to the GaN (0001).25,26 To monitor the change in characteristic vibration modes, samples were correspondingly analyzed in terms of intensity, shift, and full width at half-maximum (FWHM) with respect to the most pronounced peak E2(high). As clearly seen in the Raman spectra in Figure 5a and the intensity plot in Figure 5c, between 1 and 100 nm, the intensity was gradually decreased except slightly increased between 10 and 20 nm. The intensity characteristic of samples was dependent on the surface morphology of Ag nanostructures and the average surface coverage. By comparing the surface morphology and the intensity variation, it can be deduced that the intensity was
constant annealing temperature. As described, the dome or semispherical NPs were evolved from small to large size with corresponding decrease density. Meanwhile, with larger Ag NPs, the interparticles spacing became wider. For the elongated Ag NPs and Ag nanoclusters network, the surface coverage was further reduced. With relatively high deposition amounts, voids were fabricated on film and the size and density were significantly reduced. The overall evolution was correlated to the various step of dewetting driven by the energy minimization of thin film. 3.2. Optical Analysis of Ag Nanostructures Based on Deposition Amount Variation. Figure 5a shows the Raman spectra analysis of the samples using a 532 nm excitation laser at 100 mW and room temperature. The full-range Raman spectra for bare GaN (001) and other samples in Figure S5 E
DOI: 10.1021/acs.cgd.6b01185 Cryst. Growth Des. XXXX, XXX, XXX−XXX
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higher for the samples with low Ag surface coverage and vice versa as the incident photon interaction with GaN (0001) was obstructed as well as the laser can be absorbed or scattered by Ag nanostructures. Also, the sharp peak of E2 in each sample denoted that the crystallinity of substrate and the peak was slightly left shifted slightly, i.e., ∼1 cm−1 as shown in Figure 5d, which can be attributed to the stress produced in GaN due to the various Ag nanostructures.25 The results also show that the FWHM was slightly right shifted between 3 and 10 nm and left shifted for other samples, while the change was minor (
