Optical Properties of Intrinsic and Shallow Impurity‐Doped Silicon Nanocrystals

Abstract

The electronic band structure of silicon (Si) crystals is significantly modified when the size is reduced to below the exciton Bohr radius ($4.9 nm) of bulk Si crystals due to the quantum size effect. The quantum size effect manifests itself as a high-energy shift of the luminescence band and an enhancement of the spontaneous emission rate. Furthermore, enlargement of the singlet-triplet splitting energy of exciton states with decreasing size has been demonstrated [1-4]. The modified band structure opens up new application of Si nanocrystals in the fields where bulk Si crystal has not been involved. Besides well-known high-efficiency visible photoluminescence (PL), one of the most exotic new feature of Si nanocrystals is their ability to generate a kind of active oxygen species called singlet oxygen by energy transfer from excitons to oxygen molecules (O 2 ) adsorbed onto the surface of Si nanocrystals [5]. This feature is due to the molecule-like energy structure of Si nanocrystals and is a direct consequence of the quantum size effects. Similarly, efficient photosensitization of rare earth ions by Si nanocrystals has been reported and this phenomenon is expected to lead to the development of an efficient planar waveguide-type optical amplifier operating at the optical telecommunication wavelength. Since almost all new features of Si nanocrystals arise from the modified electronic band structure, its deep understanding is indispensable to explore new nano-Si-based research fields and devices. Fortunately, the energy structure of Si nanocrystals is almost clarified, at least qualitatively, by detailed optical spectroscopy. In Section 3.2, we briefly summarize fundamental optical properties of intrinsic Si nanocrystals [1][2][3][4][5][6][7][8].The properties of Si nanocrystals can be controlled by the size, shape, surface termination, and so on. An additional freedom of material design can be introduced by impurity doping. The introduction of shallow impurities in a semiconductor significantly modifies its optical and electrical transport properties. Actually, a precise control of an impurity profile is key to achieve desirable functions in almost all kinds Silicon Nanocrystals: Fundamentals, Synthesis and Applications. Edited by Lorenzo Pavesi and Rasit Turan

Doped and codoped silicon nanocrystals: The role of surfaces and interfaces

“…The dangling bonds at the surface of the Si-NCs are usually passivated with H atoms, which excludes the occurrence of effects induced by the presence of surface dangling bond states and surface reconstructions. Although Si-NCs are usually oxidized, there are numerous experiments where the dangling bonds at the surface are passivated with H [10]. Moreover, as pointed out by several authors, in H-terminated Si-NCs HOMO and LUMO wavefunctions don't contain contributions from H atoms [62,64,93,139,140,141].…”

“…Reprinted with permission from Ref. [10] . 5 The possibility to exploit the size, shape and surface termination [30] of the Si-NCs togheter with their low toxicity and good biocompatibility has prompted their application in several broad fields such as microeletronics [2,31,32], photonics [33,34], non-linear optics [35,36,37], secure communications [36], photovoltaics, in the so-called third-generation solar cells, solar fuels [3,38,39,40,41,42,43], thermoelectrics [44,45] and biomedicine [46,47,48,49,50].…”

“…Among the different Si-based nanostructures, Si-NCs have attracted, from the beginning, much interest because they exhibit very bright visible PL (see Fig. 1), which is tunable with respect to the dimension of the Si-NCs [10], and sample dependent high quantum yields, ranging from 1% up to 60%. The quantum yields can be enhanced by careful surface passivation and by avoiding oxygen contamination during all stages of the Si-NC preparation [11].…”

Doping Silicon Nanocrystals with Boron and Phosphorus

“…Since the technological importance of Si NCs largely originates from their efficient light emission at room temperature [9,39,40], the effect of B and P doping on the optical properties of Si NCs is clearly the central research topic. Fujii has devoted one review to this topic based on the related work published before 2009 [41]. The current review deals with the progress in the research on the B and P doping of Si NCs with a focus on improved theoretical understanding.…”