Effect of Au thickness on the evolution of self-assembled Au droplets on GaAs (111)A and (100)
© Li et al.; licensee Springer. 2014
Received: 17 July 2014
Accepted: 14 August 2014
Published: 20 August 2014
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© Li et al.; licensee Springer. 2014
Received: 17 July 2014
Accepted: 14 August 2014
Published: 20 August 2014
In this paper, we report the effect of Au thickness on the self-assembled Au droplets on GaAs (111)A and (100). The evolution of Au droplets on GaAs (111)A and (100) with the increased Au thickness progress in the Volmer-Weber growth mode results in distinctive 3-D islands. Under an identical growth condition, depending on the thickness of Au deposition, the self-assembled Au droplets show different size and density distributions, while the average height is increased by approximately 420% and the diameter is increased by approximately 830%, indicating a preferential lateral expansion. Au droplets show an opposite evolution trend: the increased size along with the decreased density as a function of the Au thickness. Also, the density shifts on the orders of over two magnitude between 4.23 × 1010 and 1.16 × 108 cm−2 over the thickness range tested. At relatively thinner thicknesses below 4 nm, the self-assembled Au droplets sensitively respond to the thickness variation, evidenced by the sharper slopes of dimensions and density plots. The results are systematically analyzed and discussed in terms of atomic force microscopy (AFM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), cross-sectional surface line profiles, and Fourier filter transform (FFT) power spectra.
Self-assembled metallic droplets have been attracting considerable attention due to their outstanding physical and optoelectronic properties such as an improved optical absorption at their localized surface plasmon resonance (LSPR) frequency, the shift of wavelengths and the local heating, etc. through the interactions with quantum and nanostructures and thus have found various applications with diverse semiconductors. For example, self-assembled droplets can act as a nanoscale surface drilling medium for the fabrication of ‘nanoholes’ using the droplet etching technique [1–4]. Quantum dots have then been demonstrated around the nanoholes . Also, metallic droplets have been successfully utilized in the fabrications of various quantum- and nanostructures such as quantum rings [6–9], quantum dots [10–12], and nanowires (NWs)  through ‘droplet epitaxy’ following the successful fabrication of homo-epitaxial GaAs nanocrystals on a GaAs substrate . In addition, Au droplets have been adapted as catalysts for the fabrication of diverse NWs via various epitaxial approaches and have attracted extensive interest due to their unique properties such as surface plasmonic resonance, biosensing, quantum size effect, and biology [15–18]. Moreover, given the wide range of substrates and vapor phase materials utilized, Au droplets can be successfully utilized in the fabrication of various NWs and many elements utilized can diffuse into catalyst gold droplets based on the vapor-liquid-solid (VLS) mechanism during the fabrication of NWs [19–27]. For example, Si, Ge, GaN, GaAs, and InAs-InSb NWs have been successfully synthesized by molecular beam epitaxy, chemical beam epitaxy, pulsed laser deposition, and chemical vapor deposition [28–30]. In the VLS-based growth, from the supersaturated catalyst alloy droplets, the nucleation and growth of NWs can occur at the L-S interface due to a much higher sticking probability. Therefore, the design of NWs including diameter, length, configuration, and density is originally determined by that of the Au droplet catalysts. Consequently, the study of the behavior of Au droplets on various surfaces becomes an essential step to accomplish desired NW synthesis; however, to date, the systematic study of the control of Au droplets on GaAs is still deficient. Therefore, in this study, we investigate the effect of systematic thickness variation on self-assembled Au droplets on GaAs (111)A and (100).
In this study, the fabrication of Au droplets was carried out on GaAs (111)A and semi-insulting (100) substrates in a pulsed laser deposition (PLD) system. The substrates used were epi-ready with an off-axis of ±0.1° from the American Xtal Technology (AXT, Inc., Fremont, CA, USA). Samples were initially indium bonded on an Inconel holder and degassed at 350°C for 30 min under 1 × 10−4 Torr in order to remove the contaminants. With the aim of investigating the effect of the Au thickness on the self-assembled Au droplets, various thicknesses of gold films were deposited at a growth rate of 0.5 Å/s with the ionization current of 3 mA as a function of time. The growth rate was calibrated by the XRD measurement. Gold films 2, 2.5, 3, 4, 6, 9, 12, and 20 nm thick were systematically deposited on GaAs (111)A and (100) at the same time in an ion-coater chamber under 1 × 10−1 Torr. Subsequently, substrate temperature (Tsub) was ramped up to the target temperature of 550°C for an annealing process at a ramp rate of 1.83°C/s. The ramping was operated by a computer-controlled recipe in a PLD system, and the pressure was maintained below 1 × 10−4 Torr during the annealing process. To ensure the uniformity of Au droplets after annealing for 150 s, the Tsub was immediately quenched down to minimize the Ostwald ripening [30–32]. Subsequent to the fabrication of the self-assembled Au droplets, an atomic force microscope (AFM) was utilized for the characterization of surface morphology under the non-contact (tapping) mode with the AFM tips (NSC16/AIBS, μmasch). The Al-coated tips were between 20 and 25 μm in length with a radius of the curvature of less than 10 nm. The tip had a spring constant of approximately 40 N/m and a resonant frequency of approximately 170 kHz. The convolution of tips more sensitively affects the lateral measurement when measuring objects with high aspect ratios as well as high density in general. Thus, to minimize the tip effect and maintain consistency of the analysis, the same type of tips from a single batch were utilized for the characterization of Au droplets. The XEI software (Park Systems, Suwon, South Korea, and Santa Clara, CA, USA) was utilized for the analysis of the acquired data including AFM images, cross-sectional surface line profiles, and Fourier filter transform (FFT) power spectra. The acquired AFM images were processed by flattening along the x and y directions to improve the image quality. FFT power spectrum is generated by converting the height information from the spatial domain to the frequency domain using Fourier filter transform. Different colors represent different frequency intensities of height; thus, height distribution with directionality of nanostructures can be determined by the color distribution. For larger area surface characterization, a scanning electron microscope (SEM) under vacuum was utilized. The elemental analysis was performed using an energy-dispersive X-ray spectroscopy (EDS) system in vacuum with the spectral mode (Thermo Fisher Noran System 7, Pittsburgh, PA, USA).
In this experiment, with the increased thicknesses, the Au droplets persistently developed into 3-D islands with the dimensional increase including the height and diameter along with the decrease in density. This can be explained based on the Volmer-Weber mode . After the nucleation, due to the weaker binding energy between surface and Au adatoms (EI) than the binding energy between Au adatoms (EA), Au atoms have a tendency to form 3-D islands rather than a layer (EA > EI). The size expansion of Au droplets with increased thicknesses can also be seen with a variety of metal droplets on various surfaces [32–38]. As is well known, the diffusion length (LD) can be expressed as , where DS is the diffusion coefficient and t is the residence time of the atoms. The DS is a direct function of the surface temperature. In this case, as the annealing temperature (TA) was fixed for all samples, an identical LD can be expected. Meanwhile, in a thermodynamic system, a larger surface area is preferred with the nanostructures in order to reduce the surface energy. Thus, with the presence of additional Au atoms within the fixed LD, droplets tend to absorb near the Au adatoms to increase the surface area, until reaching equilibrium provided with the condition of EA > EI. Therefore, with the increased thicknesses with a favorable diffusion, Au droplets can keep expanding in size with the accompanying decrease in density when thickness was increased. Au droplets on polystyrene, polymethyl methacrylate , Si , and TiO2 were reported to grow initially in the Volmer-Weber mode; however, Au droplets began to coalesce and even form a layer when the critical thickness was reached. The critical radius (<RC>) [41, 42] can be expressed as , where γ is the surface free energy, Ω is the Au atomic volume, and DC is the critical amount. As can be seen, the < RC > is a direct function of Ω and DC, and thus, while other parameters are fixed, we can expect a direct increase of < RC > with the thickness increase. For example, Au droplets on Si (111)  evolved based on the coalescence mode growth with the increased thickness and began to show an early stage of coalescence mode at a thickness as low as 5 nm and showed a significant coalescence at approximately 10 nm. With the thickness of 20 nm on Si (111), the Au droplets almost formed into a layer. However, perhaps due to the strong dominance of the Volmer-Weber mode in this experiment on GaAs (111)A, the coalescence mode did not occur and the self-assembled Au droplets persistently developed into 3-D islands with the increased thicknesses.
In conclusion, the evolution of self-assembled Au droplets on GaAs (111)A and (100) with a systematic variation of the Au thickness (thickness) between 2 and 20 nm has been investigated and the results were analyzed using AFM, surface line profiles, FFT spectra, SEM, and EDS data. The self-assembled Au droplets were fabricated based on the Volmer-Weber growth mode on GaAs (111)A and (100), resulting in distinctive 3-D islands, and the average dimension including height and diameter of the self-assembled Au droplets was gradually increased. While, the average density was progressively decreased along with the increased thicknesses on both GaAs (111)A and (100). The binding energy between the Au atoms is greater than that between the Au and surface atoms (EA > EI); Therefore, the growth (even with the increased thickness) resulted in the formation of 3-D islands rather than a layer. At relatively lower thicknesses below 6 nm, Au droplets responded more sensitively in terms of the size and density evolution, shown by the sharper slopes of the size and density plots, which was also demonstrated by the sharply increased Rq. The evolution of self-assembled Au droplets depending on the surface index showed quite similar behavior in terms of the size and density evolution. This can be due to the minor index effect when the diffusion length is fixed by the fixed annealing temperature; it could also be due to the excessive degree of change in the size and density of Au droplets. This result can be promising in various related nanostructure fabrications: quantum size effect, nanowires, biosensing, catalysis, study on the improvement of the localized surface plasmonic resonance, etc. on GaAs (111)A and (100) surfaces.
This work was supported by the National Research Foundation (NRF) of Korea (no. 2011–0030821 and 2013R1A1A1007118). This research was in part supported by the research grant of Kwangwoon University in 2014.
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