From Undulating Si Quantum Wires to Si Quantum Dots: A Model for Porous Silicon

Abstract

Freshly etched porous silicon shows the structure of a crystalline silicon skeleton with a connected undulating-wire morphology; in aged porous silicon samples the presence of Si dots is predominant. In this paper we present, for the first time, ab-initio results of the electronic and optical properties of undulating Si quantum wires, moreover, the transition from Si quantum wires to Si quantum dots is also discussed.Introduction It is now well accepted that the striking luminescence properties of porous silicon (PS) are due to the quantum confinement effects in nanometer-sized silicon crystallites that compose this complex material [1]. A lot of experimental results have actually shown, for a recent review see [1], that PS is characterized by a complicated network of silicon pillar structures of nanometrical dimension crossing each other to form a sponge-like material. In aged samples the cross-section dimensions of the wirelike silicon structures are reduced down to a dot-like situation. From a theoretical point of view fresh structures can be modelled as quantum wires, while heavily aged structures are modelled as dot-like. The modelling of PS both as Si quantum wires and Si quantum dots has been recently reviewed [1±3]. All the calculations show that the electronic states and the optical properties of Si quantum wires and dots are influenced by quantum confinement, symmetry features and passivation regimes. Quantum confinement effects result in an enlargement of the band gap, in a relaxation of the momentum conserving role, and in a size dependence of the absorption and luminescence energy. Nevertheless, no attempt has been made until now to perform calculations which try to simulate the real morphology of PS that is those of undulating wires, the only calculation, to our knowledge, of the electronic properties of an idealised undulating-wire structure is that done by Tigelis et al.[4] 2 , using the effective-mass theory. Moreover, no attempt has been made in order to model the transition from wires to dots. The reason lies in the formidable computational burden that has obliged theoretical calculations to use a simplified model of wires with constant cross section. Here we will present first-principle

Cross-sectional AFM study of etching kinetics of oxidized porous silicon

“…Our results are consistent with the mesoporous morphology of PS [19]. Both the "aged" and UVozone oxidized PS, are consistent with an undulating Si quantum wire morphology [20]. Specifically we show that the remaining Si skeleton after anodization is composed of undulating columns of Si which neck down in regions having quantum-sized dimensions.…”

Ab initiostructural and electronic properties of hydrogenated silicon nanoclusters in the ground and excited state

“…On the other hand, ab initio approaches can be applied to smaller systems with the advantage of being parameter free. Theoretical investigations accounting for microscopic aspects have widely used density-functional theory ͑DFT͒ both within all-electron [7][8][9][10][11] and several pseudopotential schemes ͓local-density approximation ͑LDA͒, [12][13][14][15] time dependent density-functional theory ͑TDDFT͒, 16,17 quantum Monte Carlo techniques, 13,14,18 GW, 18,19 etc.͔. The treatment of excited-state configurations within DFT is still a complex issue, mostly due to well-known problems arising when optical gaps are calculated.…”

Silicon-Based Third Generation Photovoltaics

“…The last one observation is consistent with the explanation for the enhanced energies of QDs in nitride given by Yang et al (Yang et al, 2004), that the reason for it is due to better passivation of Si QDs by nitrogen atoms eliminating the strain at the Si/Si 3 N 4 interface. Influence of interconnections between the QDs on tuning of their bandgap was also studied (Nychyporuk et al, 2009) (Degoli et al, 2000). Indeed, electronic coupling between the neighboring low-dimensional Si nano-objects constituting a complex quantum system must be considered.…”