Ion-Beam-Directed Self-Ordering of Ga Nanodroplets on GaAs Surfaces
© Xu et al. 2016
Received: 16 November 2015
Accepted: 5 January 2016
Published: 27 January 2016
Ordered nanodroplet arrays and aligned nanodroplet chains are fabricated using ion-beam-directed self-organization. The morphological evolution of nanodroplets formed on GaAs (100) substrates under ion beam bombardment is characterized by scanning electron microscopy and atomic force microscopy. Ordered Ga nanodroplets are self-assembled under ion beam bombardment at off-normal incidence angles. The uniformity, size, and density of Ga nanodroplets can be tuned by the incident angles of ion beam. The ion beam current also plays a critical role in the self-ordering of Ga nanodroplets, and it is found that the droplets exhibit a similar droplet size but higher density and better uniformity with increasing the ion beam current. In addition, more complex arrangements of nanodroplets are achieved via in situ patterning and ion-beam-directed migration of Ga atoms. Particularly, compared to the destructive formation of nanodroplets through direct ion beam bombardment, the controllable assembly of nanodroplets on intact surfaces can be used as templates for fabrication of ordered semiconductor nanostructures by droplet epitaxy.
Nanodroplets have been intensively investigated to fabricate various III-V nanostructures, including quantum dot molecules, quantum rings, and nanoholes [1–12]. The unique growth protocols of droplet epitaxy have not only enabled fabrication of complex nanostructures but also led to advanced optoelectronic devices [13–19]. However, laterally aligned nanostructures cannot be obtained by using droplet epitaxy. Fabrication of laterally ordered nanodroplets, which can be used as local sources for epitaxy growth, can address such urgent need of ordered nanostructures in this rapidly growing research field of droplet epitaxy. Focused ion beam (FIB) bombardment has been widely used as a surface preparation and nanopatterning technique for fabrication of self-assembled nanostructures such as nanoripples, nanoneedles, nanoholes, and nanodots [20–25]. Ordered nanostructures by ion bombardment are of interest due to their simplicity and ability to be applied to a wide range of materials. For instance, FIB-induced self-assembly of ordered nanostructures has been reported on metals, semiconductors, and insulators [20, 21, 26–32]. In addition, assisted by ion processing, controlled three-dimensional assembly at mesoscopic scale has been demonstrated recently .
Particularly, the recent observation of self-organized nanodroplets on III-V semiconductor substrates has shown a feasible method in forming ordered metallic Ga nanodroplet arrays and opened great opportunities for nanofabrication [34, 35]. Despite the great potential of the ordered Ga nanodroplets in nanomaterial growth, only a few studies have been reported on the evolution and dynamics of nanodroplet self-assembly upon ion beam bombardment. In this paper, the morphological evolution of self-assembled Ga nanodroplets is investigated systematically on the GaAs surface under the influence of focused Ga+ ion beam bombardment with different incident angles and beam currents. The self-assembled Ga nanodroplets exhibit higher uniformity and lateral ordering by increasing the incident angle. In addition to incident angles, the ion beam current also plays a critical role in the surface ordering of nanodroplets. In contrast to previous studies, the increase of ion fluence through higher beam currents resulted a higher density but in a similar size. Moreover, via in situ ion beam patterning, Ga nanodroplets were fabricated on intact surfaces via ion-beam-induced Ga atom migration. Ga nanodroplets arranged in different patterns can be simply engineered by adopting different patterns. Upon exposure to group V molecule beam, such as As2, these ordered nanodroplets can be transferred into various types of nanostructures . The presented results may shed light on the dynamics of nanodroplet self-assembly under high-energy ion beam bombardment and generate the building blocks for ordered nanostructured by droplet epitaxy.
Results and Discussion
Figure 1b–g shows the AFM images of Ga droplets on the GaAs surface after the 10-keV Ga+ bombardment beam for 5 min. The ion beam current was 0.93 nA, and the incident angle was varied from 0° to 55°. After critical fluence of ion bombardment, Ga droplets are formed on the GaAs surfaces. The formation of nanosized Ga droplets under ion beam bombardment is caused by the desorption of As after breaking the Ga-As bonds as well as Ga deposition from the Ga+ ions. Therefore, the mechanism of self-organized Ga droplets can be understood by the FIB sputtering and surface mass transport. First, during high-energy ion bombardment, the GaAs bonds are broken and As atoms are desorbed from the substrate. Although the high-energy ion beam also sputters off Ga atoms from the GaAs substrate, the preferential sputtering of As atoms leaves a Ga-rich surface. The surplus Ga adatoms then start to diffuse and nucleate into nanosized droplets. At the same time, Ga atoms are supplied and deposited to the surface from the ion beam, which also may contribute to the formation of Ga droplets. For example, at the bombardment condition of a beam current of 0.9 nA at an incident angle of 50°, the fluence of Ga+ gains from ion implantation and sputtering is 1.7 × 1017 cm−2 within a 5-min bombardment time. Assuming unity sticking coefficient, the equivalent thickness of material gain from ion beam deposition is in the order of tens of nanometers. At low ion beam incident angles, Ga droplets exhibit random distribution as shown in Fig. 1b–d. On the other hand, short-range lateral ordering of Ga nanodroplets is observed when the ion beam bombardment is performed at higher angles as shown in Fig. 1e–g.
In order to gain further insight of the ordering processes, the 2D-FFT spectra of the Ga nanodroplets are extracted from the AFM images. The insets in Fig. 1b–g show that the 2D-FFT spectra of the Ga nanodroplet samples are obtained with ion bombardment at different incident angles. Again, the 2D-FFT spectra confirm the self-ordering of nanodroplets on the GaAs surface under ion beam bombardment at high incident angles. When ion beam incident angle is 37°, the 2D-FFT spectrum of the nanodroplets starts to show one-dimensional ordering. When the incident angle increases to 50° and 55°, hexagonal patterns are observed in the 2D-FFT spectra, indicating that the formation of nanodroplets aligned in hexagonal lattices. The AFM images also reveal the morphology evolution of nanodroplets with different incident angles, as shown in Fig. 1. At low small incident angles, the droplet sizes are randomly distributed and big dots are presented on the surface. Increasing incident angles, the droplets become smaller and shorter. When the nanodroplets start to order, the size uniformity of the nanodroplets is also improved. The AFM line profiles in Fig. 1 show that the nanodroplets dramatically shrink their sizes while the periodicity is improved. An anisotropic supply and loss of Ga atoms can be used to explain the formation of self-aligned Ga nanodroplet arrays and nanodroplet evolution on the GaAs surface under Ga+ ion beam bombardment at different incident angles . The anisotropic supply and loss of Ga atoms suggest that the profile of energy distribution on the GaAs surface during ion beam bombardment affects nanodroplet ordering. At normal incidence, the distribution of energy on the surface is isotropic and exhibits a circular shape. As a result, the surface diffusion of Ga atoms is random, and Ostwald ripening drives small droplets to merge into bigger and more energetically favorable ones because of their lower surface-to-volume ratio. Therefore, the droplets form randomly with broad size distribution. On the other hand, at off-normal incidence, the energy distribution profile has an anisotropic ellipse contour resulting in directional atom migration on the surface. A consequence of combining the directional atom gain and loss with shadowing and exclusion zone effects is the net supply of Ga atoms between droplets guided by the driving force of the ion beam along the projected beam direction. As a result, the incoming Ga ions can drive the droplets to adjust their location and lead to formation of ordered arrays with sufficient time of ion bombardment. The balance between the Ga gain of droplet from capturing Ga atoms produced at the substrate and the Ga loss resulting from sputtering is attributed to the incident angle-dependent evolution of nanodroplet size.
Ordering of Ga droplets occurs through self-assembly on the GaAs substrate under Ga ion bombardment at off-normal incidence angles. The morphological evolution of the nanodroplets is systematically investigated under various ion beam bombardment conditions. The well-ordered Ga droplets can be obtained with off-normal incidence of high-energy ion beams. The surface arrangement, size, and density of the nanodroplets can be finely tuned by adjusting the ion beam parameters. With increase of incident angle, the Ga droplets exhibit smaller size distribution and smaller height but higher density. With increase in ion fluence by increasing ion beam current, lateral ordering can be also improved with a similar size but higher density. The formation of self-ordered Ga nanodroplets on undamaged GaAs surface has also been achieved by taking advantages of the ion-beam-induced surface migration of Ga atoms. The ordering of Ga nanodroplets can be directed by controlling the ion milling patterns. The method of fabrication of ordered nanodroplets presented in this study provides another essential step towards fabrication of advanced semiconductor nanostructures by using droplet epitaxy.
The authors acknowledge the financial support from the National Basic Research Program (973) of China (2013CB933301) and the National Natural Science Foundation of China through Grant Nos. NSFC-51272038 and NSFC-61474015.
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