Illumination porous silicon

Porous silicon (PS) is a material that is formed by anodic dissolution of silicon in HF solutions. The formation of PS was first reported in the late 1950s in studies on electropolishing of silicon. Since then, particularly after 1990 when luminescence of PS was discovered, numerous investigations have been undertaken. These investigations have revealed that PS has extremely rich morphological features with properties that are very different from those of silicon and the formation process of PS is a very complex function of many factors such as HF concentration, type of silicon, current density, and illumination intensity. 

A. M. Thonisson, M. G. Berger, R. Arens-Fisher, O. Gliick, M. Kruger, and H. Liith, Illumination-assisted formation of porous silicon. Thin Solid Films 276, 21, 19%. 

Fig. 16.4. Photoluminescent porous silicon patterned with chemically bound dodecenyl groups. Illumination of the substrate with UV light (365 nm) induces orange photoluminescence from the unfunctionalized (hydride-...

Salonen J, Lehto VP, Bjorkqvist M, Laine E (1999a) A role of illumination during etching to porous silicon oxidation. Appl Phys Lett 75(6) 826-828... 

Control over silicon particle shape, porosity, and polydispersity could provide structural control of color of powders in the future. Of relevance here are the so-called silicon colloids made by bottom-up routes. Porous silicon microspheres of 0.5-5 5 pm diameter scattered yellow, orange, and red colors when under white light illumination (Fenollosa et al. 2010). 

Table 2 EL characteristics of most devices ineluding a single porous silicon layer. D and L mean diat anodization was eondueted in the dark and under illumination, respeetively. EQE and EPE are external quantum effieieney and external power effieieney, respeetively ...

This kind of pore can be obtained under a variety of conditions and with differing morphologies (see chapter Routes of Formation for Porous Silicon ). In this review, we focus on electrochem-ically etched macropores. The key parameters are the electroljde type (aqueous (aqu), organic (org), oxidant (ox)) the HF concentration, the surfactant, the Si doping type and level (n, n", p, p" ), and in some cases the illumination (backside illumination (bsi) or frontside illumination (fsi)). Detailed reviews regarding their formation are available (F6U et al. 2002 Lehmann 2005 Chazalviel and Ozanam 2005 Lehmann 2002 and handbook chapter Porous Silicon Formation by Anodisation ). 

Koyama H (2004) In-plane refractive-index anisotropy in porous silicon layers induced by polarized illumination during electrochemical etching. J Appl Phys 96(7) 3716-3720 Kiinzner N et al (2001) Giant birefringence in anisotropically nanostructured silicon. Opt Lett 26(16) 1265-1267... 

Porous silicon is a promising template for the preparation of metal nanostructures by eleetroehem-ical deposition. Because porous siheon is a semiconduetive porous electrode, eleetroehemieal deposition of metals oeeurs not only at the bottom of pores but also on the pore wall and pore openings. Thus, the control of electrochemical deposition within porous siheon has been a challenging issue. Eleetroehemieal deposition on porous siheon is influenced by illumination condihons. Metal deposition on porous siheon is possible by displacement deposihon. Many studies have reported on electrochemical deposition of metal for prachcal appheations. In this chapter, electrodeposition under polarization is firstly reviewed. Secondly, displacement deposition on porous siheon is explained. Finally, the microscopic structure formation by electrodeposition on porous siheon is summarized. 

Fig. 2 Energy band diagram for n-Si immersed in pure HF solution (a, b) and those in HF/KIO3 solution (c, d). In (b), porous silicon (PS) is formed stably on the back side in opposition to the illmninated surface. In (d), PS is formed only on the illuminated surface...

Xu YK, Adachi S (2008) Properties of light-emitting porous silicon formed by stain etching in HF/KIO3 solution under light illumination. J Appl Phys 103 103512-1-6 Yamamoto N, Takai H (2000) Visible luminescenee from photo-chemically etched sihcon. Thin Solid Films 359 184-187... 

To prepare porous silicon microarrays in the Laurell group, 380 pm-thick <100> silicon wafers were utilized. The p-type boron-doped wafer with a resistivity of 10-15 Qcm was mounted in an in-house-made electrochemical etch cell. The porosification was made in an electrolyte solution of HF/DMF 1 10, and the wafer was illuminated by a 100 W halogen lamp at a distance of 10 cm. A current density of 2 mA/cm was applied for 1 h. The macropores in the porous layer produced for protein microarrays typically measured 0.5-1.5 pm with a fine side branching network of nanopores (Ressine et al. 2005). 

Porous silicon has been under evaluation as both a photosensitizer and a photothermal agent against eancer. Photodynamie therapy approaches exploit the ability of mesoporous silicon to generate reactive oxygen speeies under illumination photothermal therapy exploits its ability to absorb near-infrared light and the low thermal conductivity. Both the in vitro and in vivo data acquired to date are reviewed. 

Porous silicon layers with a porosity of 60% on n-type (lll)Si substrates were prepared by anodic etching under white illumination. Silver/porous-silicon/Si and metal Ag/Si structures were fabricated by evaporation of thin metal films onto the porous silicon or Si surface, respectively. The diffusion annealing of structures was carried out in air at 100-250C. Examination of the Ag concentration distribution in porous silicon layers and monocrystalline Si substrates was performed by successive removal of thin layers and measurement of the energy dispersive X-ray fluorescence intensity of AgK( peaks. The effective diffusion coefficients were described by ... 

Figure 7.5. Experimental configuration for the DIOS-MS experiments, (a) Four porous silicon plates are placed on a MALDI plate. Each of the porous silicon plates contains photopattemed spots or grids prepared through illumination of -type silicon with a 300-S tungsten filament through a mask and an 50 reducing lens, (b) Ibe silicon-based laser desorption/ionization process, in which the sample is placed on the porous silicon plate and allowed to dry, followed by laser-induced desorption/ionization mass spectrometry, (c) Cross section of porous silicon, and the surface functionalities after hydrosilylation R represents phenyl or alkyl chains. (Reproduced with permission from Wei, J. Buriak, J. Siuzdak, G. Desorption/ionization mass spectrometry on porous silicon. Nature 1999, 399, 243-246.)...

A schematic view of the cold cathode fabrication process is shown in Fig. 10.18. The cold cathode is fabricated by low pressure chemical vapor deposition (LPCVD) of 1.5 pm of non-doped polysilicon on a silicon wafer or a metallized glass substrate. The topmost micrometer of polysilicon is then anodized (10 mA cnT2, 30 s) in ethanoic HF under illumination. This results in a porous layer with inclusions of larger silicon crystallites, due to faster pore formation along grain boundaries. After anodization the porous layer is oxidized (700 °C, 60 min) and a semi-transparent (10 nm) gold film is deposited as a top electrode. 

The effect of illumination on pore formation in n-type silicon has been studied by a number of groups [117, 118]. In general, photogenerated holes appear to make the porous structure similar to the porous layers formed in p-type silicon. The structure of porous layers as a function of depth formed under illumination is strongly dependent on wavelength and whether frontside or backside illumination is used. 

The anodic polarization curves of p-Si or strongly illuminated n-Si in fluoride solutions are typically characterized hy two peak currents, Ji and J2, and two plateau currents, J3 and A as shown in Fig. 5.2. At anodic potentials up to that at J], the electrode behavior is characterized hy an exponential dependence of current on potential and by the uneven dissolution of silicon surface leading to the formation of porous... 

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