Porous Silicon Layers

Interferometry on porous silicon The average refractive index of the porous silicon layer is affected by analyte adsorption, resulting in a shift of the Fabry Perot fringes 6,18... 

To understand the electrochemical behavior of silicon, however, the formation and the properties of anodic oxides are important The formation of an anodic oxide on silicon electrodes in HF and HF-free electrolytes will therefore be discussed in detail in this chapter. The formation of native and chemical oxides is closely related to the electrochemical formation process and will be reviewed briefly. The anodic oxidation of porous silicon layers is closely related to the morphology and the luminescent properties of this material and is therefore discussed in Section 7.6. 

Electropolishing is well established as a simple, in situ method to separate porous silicon layers from the silicon electrode. By switching the anodic current density from values below JPS to a value above JPS, the PS film is separated at its interface to the bulk electrode. The flatness of a PS surface separated by electropolishing is sufficient for optical applications, as shown in Fig. 10.10. 

NHE OCP ONO OPS PCD PDS PL PLE PMMA PP PP PS PSG PSL PTFE PVC PVDF normal hydrogen electrode (= SHE) open circuit potential oxide-nitride-oxide dielectric oxidized porous silicon photoconductive decay photothermal displacement spectroscopy photoluminescence photoluminescence excitation spectroscopy polymethyl methacrylate passivation potential polypropylene porous silicon phosphosilicate glass porous silicon layer polytetrafluoroethylene polyvinyl chloride polyvinylidene fluoride... 

Another way to use silicon wafers as DLs was presented by Meyers and Maynard [77]. They developed a micro-PEMFC based on a bilayer design in which both the anode and the cathode current collectors were made out of conductive silicon wafers. Each of fhese componenfs had a series of microchannels formed on one of their surfaces, allowing fhe hydrogen and oxygen to flow through them. Before the charmels were machined, a layer of porous silicon was formed on top of the Si wafers and fhen fhe silicon material beneath the porous layer was electropolished away to form fhe channels. After the wafers were machined, the CEs were added to the surfaces. In this cell, the actual diffusion layers were the porous silicon layers located on top of the channels because they let the gases diffuse fhrough fhem toward the active sites near the membrane. 

In a similar design, DArrigo et al. [78] also used porous silicon as the DL in a fuel cell. The porous silicon was deposited by chemical vapor deposition (CVD) on top of a silicon wafer that already had microgrooves machined on it. Then, catalytic particles were deposited on top of fhe porous silicon layer. Unfortunately, no performance-related data indicating whether the cell was acceptable or not were published for fhis design. 

Kim SJ, Jeon BH, Choi KS (1999) Improvement of the sensitivity by UV light in alcohol sensors using porous silicon layer. In CAS 99 Proceedings of the international semiconductor conference, Sinaia, Romania, 2 475 78... 

SorU B, Garcia M, Benhida A et al (1999) Porous silicon layer used as a humidity sensor. In Proceedings of the european matter conference E-MRS spring meeting. Symposium 1 micro-crystalline and nanocrystaUine semiconductors, 1-8... 

On semiconductors light emission is induced by injection of electrons into the conduction band and subsequent band-to-band radiative recombination with holes (Fig. 38a). The process is reminiscent of electroluminescence or cathodolumines-cence and works with p-type substrates only (at n-type specimens no hole is available at the surface). Tunnel biases of 1.5-2 V are necessary in the case of GaAs, for instance. Figure 38b is a photon map of a GaAlAs/GaAs multiquantum well obtained by Alvarado et al. [140], The white stripes are regions where photons are emitted and correspond to the GaAs layers. The lateral resolution is about 1 nm and is limited by the diffusion distance of minority carriers. In Sec. 5.1 we have seen an example of the application of this technique in the case of porous silicon layers. 

Historically, the first reports of porous silicon layers were by Uhlir [59] and Turner [60]. These authors reported on the electropolishing of silicon and noted that under certain conditions a porous layer was formed at the silicon surface. The first models for porous layer formation assumed that the layer was formed on the silicon substrate by a deposition process thought to involve the reduction of divalent silicon to amorphous Si via a disproportionation reaction in solution [61]. Subsequently, Theunissen [62] showed that the porous structure was the result of a selective etching process within the silicon, contradicting the silicon deposition model. 

Although the dissolution process results in the formation of a porous structure, electrode impedance measurements [72, 73] have shown that the etching process is not limited by mass transport, even for thick porous silicon layers [74]. Figure 12 shows a plot of potential as a function of time during pore formation in p-Si(lOO),... 

Although most work related to pore formation in silicon has involved electrochemical etching of silicon in HF solutions, porous silicon layers have also been formed by chemical etching and by spark erosion in vacuum. 

The formation of porous layers in silicon by chemical etching in HF/HNO3 solutions was first reported at about the same time as electrochemical etching [101-104]. These so-called stain etch films are characterized by rough or porous surfaces, typically < 500 A in thickness. Recent work has shown that these stain etch films exhibit strong visible photoluminescence, similar to the emission observed from electrochemically etched porous silicon layers. 

Internal reflection infrared spectra measured in situ during etching of silicon in HF solutions exhibit characteristic Si - H modes, although the Si - H spectrum is broad because of interaction of the surface Si-H groups with the electrolyte. No electrochemical or chemical intermediate species have been detected [112]. Infrared spectra of porous silicon layers after drying reveal characteristic Si-H and Si-H2 peaks similar to the spectra obtained for hydrogen on Si(lOO) 2x1 surfaces [112]. 

Porous silicon structures have been studied in order to provide combustion and explosion this material. The combustion process has been observed in the porous silicon layers formed by anodization if the specific area is more than 100 mVcm We have also found that the combustion intensity increases with porous silicon specific area and if the latter is larger than 200 mVcm the explosion process occurs. The time response of explosion development is in the microsecond range. 

It is well known that the fundamental distinctions between combustion and explosion are the value of the time response and presence of a shock wave. Thus, the combustion time response is in millisecond and second ranges, while the time typical of explosion development is within the microsecond range. Fig. 3 shows the fragments of fast oxidation process in the porous silicon layer formed on p-type silicon wafer. The time response of flash development is in the microsecond range (Fig. 3a,b,c). That confirms the explosion nature of the investigating oxidation process. Also the sound accompaniment of this process was similar to the gunshot, testifying presence of the shockwave. The flash is appeared to be the fireball. 

Fast oxidation process in a way of combustion and, in some cases, of explosion in porous silicon films has been observed at pore wall thickness less than 10 nm. The increasing of porous specific area results in an enhancement of combustion and explosion intensity. The explosion process has been observed at the specific area more than 200 m /cm. Thus combustion and explosion processes in the porous silicon layers can be attributed to nanoscale phenomena. 

A DNA biosensor based on various porous silicon layers was fabricated using an oxidized microcavity resonator design developed by Chan et al. [37], the porous sihcon containing silicon nanocrystals that can luminescence efficiently in the visible. 

Deviation of 60 mV/decade can be seen in Table 5.3 under different conditions. In addition to the potential distribution in the two double layers, there are two other possible causes for the deviations. The first is possible potential drops in other parts of the electrical circuit, e.g., in the electrolyte and semiconductor. The second possibility is the change of effective surface area due to the formation of a porous silicon layer during the course of i-V curve measurement. In addition, if the reaction is controlled by a process involving the Helmholtz layer, the apparent Tafel slope may be smaller than the 60 mV/decade as would be expected from the formula, B = kTI23anq, because the effective dissolution valence n is not a constant with respect to potential but varies from 2 to 3 in the exponential region. 

M. Yamana, N. Kashiwazaki, A. Kinoshita, T. Nakano, M. Yamamoto, and C. W. Walton, Porous silicon oxide formation by the electrochemical treatment of a porous silicon layer, J. Electrochem. Soc. 137, 2925,1990. 

T. Unagami, Formation mechanism of porous silicon layer by anodization in HF solution, J. Electrochem. Soc. 127, 476, 1980. 

R. Herino, G. Romchil, K. Boala, and C. Bertrand, Porosity and pore size distribution of porous silicon layers, J. Electrochem. Soc. 134, 1994, 1987. 

T. Uganami and M. Seki, Structure of porous silicon layer and heat treatment effect, J. Electrochem. Soc. 125, 1339, 1978. 

T. C. Teng, An investigation of the application of porous silicon layer to the dielectric isolation of integrated circuits, J. Electrochem. Soc. 126, 870, 1979. 

S. F. Chuang, S. D. Collins, and R. L. Smith, Preferred crystallographic directions of pore propagation in porous silicon layers. The Technical Digest of the Solid State Sensor and Actuator Workshop, Hilton Head Island, SC, June 6-9 (IEEE, New York), p. 151, 1988. 

K. Barla, R. Herino, and G. Bomchil, Stress in oxidized porous silicon layers, J. Appl. Phys. 59,439, 1986. 

A. Bsiesy, F. Gaspard, R. Herino, M. Ligeon, F. Muller, and J. C. Oberhn, Anodic oxidation of porous silicon layers formed on lightly p-doped substrates, J. Electrochem. Soc. 138, 3450, 1991. 

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