Abstract
Recent topics of application studies on porous silicon (PS) are reviewed here with a focus on the emissive properties of visible light, quasiballistic hot electrons, and acoustic wave. By exposing PS in solvents to pulse laser, size-controlled nc-Si dot colloids can be formed through fragmentation of the PS layer with a considerably higher yield than the conventional techniques such as laser ablation of bulk silicon and sol-gel precursor process. Fabricated colloidal samples show strong visible photoluminescence (~40% in quantum efficiency in the red band). This provides an energy- and cost-effective route for production of nc-Si quantum dots. A multiple-tunneling transport mode through nc-Si dot chain induces efficient quasiballistic hot electron emission from an nc-Si diode. Both the efficiency and the output electron energy dispersion are remarkably improved by using monolayer graphene as a surface electrode. Being a relatively low operating voltage device compatible with silicon planar fabrication process, the emitter is applicable to mask-less parallel lithography under an active matrix drive. It has been demonstrated that the integrated 100 × 100 emitter array is useful for multibeam lithography and that the selected emission pattern is delineated with little distortion. Highly reducing activity of emitted electrons is applicable to liquid-phase thin film deposition of metals (Cu) and semiconductors (Si, Ge, and SiGe). Due to an extremely low thermal conductivity and volumetric heat capacity of nc-Si layer, on the other hand, thermo-acoustic conversion is enhanced to a practical level. A temperature fluctuation produced at the surface of nc-Si layer is quickly transferred into air, and then an acoustic wave is emitted without any mechanical vibrations. The non-resonant and broad-band emissivity with low harmonic distortions makes it possible to use the emitter for generating audible sound under a full digital drive and reproducing complicated ultrasonic communication calls between mice.
Highlights
As the scaling of integrated silicon devices approaches 10 nm or below, precise control of the physical and chemical properties of silicon becomes very important
Making use of the non-resonant and broad-band emissivity with no harmonic distortions, possible applications have been pursued to audible compact speaker under a full digital drive, probing source for 3-dimentional object sensing in air, acoustic pressure generator for noncontact actuation, directivity control under phased array configuration, loud speaker, noise cancellation, thermoacoustic tomography, and thermoacoustic sound projector (Koshida, 2017c; Aliev et al, 2018; Bobinger et al, 2018; Julius et al, 2018; Liu et al, 2018; Song et al, 2018)
Including photonic visible luminescence, emerging functions of nanostructured Porous silicon (PS) has extended to electronics, biometrics, biomedicine, acoustics, thermology, and energetics
Summary
As the scaling of integrated silicon devices approaches 10 nm or below, precise control of the physical and chemical properties of silicon becomes very important. By using the silicon nanoparticles prepared by this sol-gel precursor process, solution-based multicolor light emitting diodes having high external quantum efficiencies ∼1.1% were demonstrated (Figure 4) (Maier-Flaig et al, 2013) with combining the size separation technique (Mastronardi et al, 2011). Kusová demonstrated the formation of yellow emitting organically capped nc-Si dots prepared by combining the sonification of porous layer and subsequent photo-assisted hydrosilylation treatments (Figure 5) (Kusová et al, 2010) These colloidal nanoparticles exhibit the nanosecond PL decay, due to electron-hole direct gap recombination induced by the crystalline strain and resultant modification of the electronic band structure (Kusová et al, 2014). The emission efficiency η, defined as the ratio of the emission current density to the diode current density, depends on the nanostructure arrangement of nc-Si dots, quality of interfacial tunneling oxide, and the surface electrode material
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