(Invited) Exploring the Potential of Si and Ge Amorphous Nanostructures for Photonic Applications

Abstract

The optical properties of crystalline IV-groupsemiconductor nanostructures as Si and Ge have been thoroughly investigated in the last years with special emphasis on porous structures, nanoparticles, nanowires and nanolayers. One of the key reasons behind the study of Si and Ge nanostructures lies in the interest of developing low cost photonic devices fully compatible with the state-of-the-art Si technology. Although bulk Si and Ge show inefficient light emission because of their indirect band-gap, strong light emission by nanostructured Si has been observed in the visible/near-infrared (IR) due to quantum confinement effects. In addition, the sensitizing capabilities of Si-based nanostructures for efficient light emission by rare-earth (RE) ions in the IR has been demonstrated in nanocomposite materials codoped with Si nanostructures and RE ions .Remarkably, Ge has also received an increasing attention due to some of its specific properties that can offer advantages with respect to Si due to its larger carrier mobility, lower bandgap and larger exciton Bohr radius that can lead to higher confinement, and a higher sensitivity of the band-gap to nanostructure size variations. On the other hand, amorphous nanostructures have been less studied due to the inherent difficulty to characterize them. Interestingly, it has been shown that they also show tunable optical band-gap properties [2] and are indeed very efficient sensitizers for RE ions [3]. Additionally, they can be produced following a low-cost low-temperature processes and it is expected a lower strain in the nanostructure embedded in the matrix due to their more flexible amorphous structure.In this work, we report our recent results on the design and synthesis of nanocomposite thin films based on Si or Ge amorphous nanostructures embedded in amorphous aluminum oxide (a-Al 2 O 3 ), labeled as a-Si:a-Al 2 O 3 and a-Ge:a-Al 2 O 3 , respectively. The films have been prepared by the non-equilibrium technique of pulsed laser deposition at room temperature in vacuum. The formation of the nanostructures is achieved in-situ by the alternated ablation of semiconductor (Si orGe) and a-Al 2 O 3 targets, with no post-depostion annealing processes. The formation of the semiconductor nanostructures is monitored during deposition by realtime reflectivity measurements. Figure 1 shows a crosssection energy filtered transmission electron microscopy (EFTEM) image of one multilayered a-Si:a-Al 2 O 3 film containing amorphous Si nanostructured layers separated by the embedding a-Al 2 O 3 matrix. The amorphous nature of both the Si nanostructured layers and the embedding a-Al 2 O 3 is evidenced in high resolution transmission electron microscopy images [3].The quantum confinement properties of the a-