Ordered Ga nanoparticles embedded in GaAs islands on patterned Si substrate

Metal nanoparticles (NPs) present unique optical properties, which are very different from those of bulk material. The localized surface plasmon resonance of these particles results in strong optical scattering and a strongly enhanced optical near-field around the particle [1]. Recently, metal nanoparticles have been investigated as a possible way to improve the performance of thin-film solar cells [2]. Metal NPs embedded in a semiconductor material act as antennas for the incident light and store energy in the localized surface plasmon resonance. The strong near-field absorption enhancement can be used to reduce the thickness of a thin film solar cell without a reduction of optical absorption. Here we show that, by the combination of substrate patterning and droplet epitaxy [3], it is possible to obtain the fabrication of and ordered and controlled array of embedded Ga nanoparticles in a semiconductor matrix. A Si(001) wafer patterned with regular arrays of half micron inverted pyramid pits was used as substrate for the subsequent fabrication of GaAs islands in the droplet epitaxy (DE) growth mode. DE separates Ga deposition, used for the formation of an ensemble of localized Ga reservoirs on the surface, from the As supply, necessary to crystallize the droplets into GaAs nanostructures. Figure 1(a) shows a SEM image taken before pre-growth cleaning on a typical array of {111} pits, etched into a Si(001) substrate surface. Figure 1(b) is taken on the same sample after DE growth, showing that a dot is present at the base of each pit. The capture of the Ga droplet by the inverted pits is caused by capillarity forces. The occupancy ratio of the pits by Ga droplets, obtained from several similar images, is 80%. The chemical and structural quality of the GaAs nanoislands at the bottom of pits was studied by means of set of complementary characterization techniques. In particular cross-sectional TEM was employed in order to extract the exact shapes and composition of the islands. Figure 2 (a) shows such a cross sectional TEM image along a [110] direction through a tip of a pit after the Ga droplet crystallization process. The original shape of the pit is visible as a {111} inverted pyramid. Figures 2 (b) and (c) report the EDS maps of the characteristic x-ray Kα emissions of Ga and As, respectively. Strongly different concentration profiles of Ga and As are easily observable. Ga fills uniformly the whole space occupied by the GaAs island from the tip of the pit upto the island surface. Arsenic is detected only in limited crust close at the island surface. Extracting the EDS spectrum from narrower areas along the vertical axis of the pit shows that the As:Ga ratio nearly approaches 1 at the very top of the As map whereas it strongly decreases on going deeper in the island. The inner part of the island is constituted by a pure Ga NP. This NP is then fully embedded within a semiconductor matrix, constituted by the pit Si sidewalls and the GaAs crust which is formed during the crystallization process with As. Monte Carlo simulations (MC) examining nanostructural evolution in the Ga droplet nucleated in the pit during the crystallization process support the observed phenomenology. [1] C. F. Bohren and D. R. Huffman, Absorption and Scattering of Light by Small Particles (Wiley, 2008). [2] H. R. Stuart and D. G. Hall, Appl. Phys. Lett. 69, 2327 (1996). [3] N. Koguchi and K. Ishige, Japanese Journal of Applied Physics 32, 20522058 (1993).