Structural Evolution During Formation and Filling of Self-patterned Nanoholes on GaAs (100) Surfaces
© to the authors 2008
Received: 13 October 2008
Accepted: 17 October 2008
Published: 4 November 2008
Nanohole formation on an AlAs/GaAs superlattice gives insight to both the “drilling” effect of Ga droplets on AlAs as compared to GaAs and the hole-filling process. The shape and depth of the nanoholes formed on GaAs (100) substrates has been studied by the cross-section transmission electron microscopy. The Ga droplets “drill” through the AlAs layer at a much slower rate than through GaAs due to differences in activation energy. Refill of the nanohole results in elongated GaAs mounds along the [01−1] direction. As a result of capillarity-induced diffusion, GaAs favors growth inside the nanoholes, which provides the possibility to fabricate GaAs and AlAs nanostructures.
Over the past decade, there has been much emphasis directed toward semiconductor nanostructures [1–4]. One area of epitaxial growth that has attracted attention is that of droplet epitaxy for lattice-matched systems [5, 6]. High-temperature droplet epitaxy is a versatile technique: with the right growth conditions, an intriguing range of nanostructures such as nanoholes, double ring-like structures, and quantum dot (QD) molecules can be obtained [7–12]. Among the many nanostructures fabricated by this method, one interesting structure is the nanohole formed on GaAs (100) substrates.
Many types of top-down patterning processes have been investigated as an alternative approach to utilizing strain fields as the driving force for growing strained QDs as occurs in the Stranski-Krastanow (SK) growth method [13, 14]. Thus, a range of processes, including electron-beam lithography (EBL), X-ray lithography, extreme-ultraviolet (EUV) lithography, and nanoelectrode lithography [15–18], have been employed for developing surface templates in an effort to obtain lateral patterning of nanostructures. Consequently, the patterned nanostructures are limited in size by lithographic features as well as the presence of defects that result from these processes . The self-limiting inverted-pyramid templating approach proposed by Biasiol and co-authors  requires an additional ex situ processing step, but it results in better nanostructure uniformity, since it overcomes the intrinsic randomness of the nucleation process while maintaining high crystal quality at the interface.
For this investigation, all samples were grown on semi-insulating GaAs (100) substrates by molecular beam epitaxy (MBE) in a Riber 32. Following oxide desorption and growth of a 0.5 μm GaAs buffer layer at 600 °C, the substrate was cooled down to 500 °C. Sample A was grown by depositing 20 periods (6/6 nm) of AlAs/GaAs, followed by 20 monolayers of Ga droplets, with a growth interruption (GI) and the arsenic valve fully closed for 1 min and 20 s. The sample was then annealed with exposure to the Ga droplets under As flux 1.0 × 10−6 Torr for 100 s before deposition of the AlAs/GaAs superlattice. The growth for sample B was performed in the reverse sequence to that of sample A.
The samples were removed from the MBE system and analyzed using atomic force microscopy (AFM) and transmission electron microscopy (TEM). The specimens were prepared for electron microscope examination in cross-sectional geometry using standard thinning procedures. Cross-section samples were sliced mechanically, polished, dimpled, and then ion-milled to perforation using a 4.0-keV argon ion beam. Bright-field and high-resolution electron micrographs were recorded with a JEOL JEM-4000EX high-resolution electron microscope operated at 400 keV. Cross-section samples were tilted to a -type projection so that the ML planes would be aligned parallel to the incident electron beam direction.
The AFM results of Fig. 1a shows that nanoholes on the AlAs/GaAs superlattices are “square” shaped with an average diameter,l = 91 nm and depth,d = 11.6 nm with an anisotropic lobe ofh = 10.7 nm along the [01−1] direction and 8.1 nm along the  direction. In contrast, Fig. 1b shows that the surface of sample B consists of GaAs mounds that are greatly elongated along the [01−1] direction, which is attributed to higher diffusion in that direction. The mounds are 4.5 nm in height, 1 μm in the [01−1] direction, and 200 nm in the  direction. The depth of the nanohole beneath the superlattice is 15.3 nm and the diameter,l = 44.8 nm.
In terms of nanohole formation, the u-shaped hole appears to contain thicker material at the bottom compared to the sides, as seen at the bottom of the superlattice in Fig. 2a and atop the superlattice in Fig. 3. This appearance is most likely due to the anisotropy in the growth rates along different crystallographic directions as well as capillarity-induced diffusion of the different underlying materials, with lower diffusion length . In fact, in the absence of capillarity-induced fluxes, growth-rate anisotropy becomes the dominant mechanism, thus suppressing the bottom of the nanohole and yielding high-surface curvature. However, for this case, the growth rate anisotropy does not favor growth on the sidewalls of the self-patterned nonplanar surfaces, and lateral surface fluxes of adatoms are not driven to the bottom of the nanoholes, as reported in Ref. . Further, the expansion in the nanohole width seems to be due to the lower growth rate on the sidewall (see Fig. 3) since growth rate depends on flux in MBE growth . In addition, the self-limiting width of the nanoholes seems to be determined mainly by the arsenic desorption rate at the interface between the Ga droplets and the GaAs (100) surface, as well as the dependence of capillarity fluxes on the adatom diffusion length as a function of the growth conditions .
In summary, we have demonstrated the structural evolution from nanohole formation to GaAs mounds. During the re-fill process, a reasonably uniform superlattice is maintained although the regions of the lattice directly above the holes are higher than the regions in between the holes. During nanohole formation, the bottoms of the holes are wide due to balance between the lower growth rate on the sidewall and the capillarity-induced diffusion. More important, we have demonstrated that Ga droplets do in fact etch AlAs but do so very slowly. Furthermore, as the refill process proceeds, the respective layer thicknesses approach the deposited thickness of 6 nm.
The authors acknowledge the financial support of the NSF through Grant No. DMR-0520550 and the ONR through Grant No. N00014-00-1-0506. We also acknowledge the use of facilities in the John M. Cowley Center for High-Resolution Electron Micrograph at Arizona State University.
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