Illumination backside

An interesting question is whether such well-ordered pore arrays can also be produced in other semiconductors than Si by the same electrochemical etching process. Conversion of the macropore formation process active for n-type silicon electrodes on other semiconductors is unlikely, because their minority carrier diffusion length is usually not large enough to enable holes to diffuse from the illuminated backside to the front. The macropore formation process active in p-type silicon or the mesopore formation mechanisms, however, involve no minority carrier diffusion and it therefore seems likely that these mechanisms also apply to other semiconductor electrodes. 

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 ). 

B backside illumination 4 MeCN acetonitrile MP macropores of two-layer PS d eth ethanol,... 

The diffusion length of electronic grade silicon wafers is about 0.5 mm and therefore in the order of the wafer thickness. Illumination of the backside of a silicon electrode may, as a result, influence the electrochemistry at the front side, as discussed in Section 10.3. 

Providing the backside with an electrolyte contact, which is forward biased or alternatively reverse biased and intensely illuminated. 

For the p-type substrate a significant number of electrons are collected at the backside, as shown in the top part of Fig. 3.2. This is true not only for the illuminated p-type electrode but also if the electrode is kept in the dark, which indicates that electrons are injected during the tetravalent dissolution reaction. In the regime of oscillations the electron injection current is found to oscillate, too [CalO]. 

Fig. 9.8 SEM micrograph of the surface, cross-section and a 45° bevel of an anodized n-type silicon sample (5 2 cm, (100), 6% HF) showing a random pattern of macropores. Pore initiation was enhanced by applying 10 V bias in the first minute of anodization followed by 149 min at 3 V. The current density was kept constant (10 mA cm-2) by adjusting the backside illumination. After [Le9].

A lower limit of bias is given by the onset of unstable macropore formation. This is shown in Fig. 9.13, which shows the pore morphology and the corresponding formation conditions in the current density-voltage plot The current density J is held constant by the intensity of the backside illumination, so that the influence of the applied bias can be studied independently. At -0.4 V, versus a platinum wire as a pseudoreference electrode, the current density is constant over the... 

A local variation in porosity can be produced by an inhomogeneous illumination intensity. However, any image projected on the backside of the wafer generates a smoothed-out current density distribution on the frontside, because of random diffusion of the charge carriers in the bulk. This problem can be reduced if thin wafers or illumination from the frontside is used. However, sharp lateral changes in porosity cannot be achieved. 

Paulose M, Shankar K, Varghese OK, Mor GK, Hardin B, Grimes CA (2006) Backside illuminated dye-sensitized solar cells based on titania nanotube array electrodes. Nanotechnol 17 1446-1448... 

A simple and practical way to achieve the field enhancement is to use backside illumination of a dielectric plate, for instance a cover glass, in a standard DLW geometry with an oil-immersion focusing lens. According to the Fresnel formulas for the right angle incidence (0, = 0°), the coefficients of the in-plane ( ) polarized amphtudes of transmitted and reflected electric fields are, respectively ... 

A method for processing a backside illuminated detector assembly, comprising the use of a temporary substrate on which a detector wafer is attached, is presented in EP-A-0171801. 

A backside illuminated imager is presented in JP-A-63273365. The amount of cross-talk between adjacent photodiodes, which have been separated by grooves, is reduced by forming an infrared blocking film in the groove regions. 

The efficiency for backside illumination (bi-facial cells) is limited because the carriers are generated outside the field zone in proximity to the poorly passivated contact (poor blue response). Further optimization of absorber thickness, diffusion length, and contact passivation appears to be feasible. 

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. 

As has already been established for fluorescence emission fiom individual nanoapertures, it is advantageous to perform detection fiom the backside of the substrate in order to minimize the contribution of background fluorescence. For experimental simplicity, all measurements were made in the trans-illumination geometry. 

In principle, electrochemical photovoltaic cells (also known as regenerative solar cells) can be constructed very simply. They consist of a semiconductor electrode, an electrolyte containing a redox system and a counter electrode, as illustrated in the lower part of Fig. 2.30. The energy scheme for a cell with an -type semiconductor and a redox system with a standard potential close to the valence band is shown in the upper part of Fig. 2.30. On illumination of such a cell, holes are created and transferred to the reduced species of the redox couple. The electrons reach the ohmic contact on the backside of the semiconductor, traverse the external circuit, do useful work, and reach the counter electrode, at which they reduce the oxidised component... 

An example of a two-electrode PECS that does not need a membrane is polycrystaUine Cd(Se,Te) in an alkaline Sn VSn electrolyte (G. Hodes, unpublished results). Menezes et al. (1977) reported that acidic Sn VSn" electrolyte stabilised CdTe photoelectrodes to some extent. In experiments using polycrystalline Cd(Se,Te) on an exposed (on the back side) Ti substrate, this electrolyte was found to stabilise the photoanode somewhat, but not very efficiently. However, use of alkaline Sn VSn" resulted in considerably improved stability (although still inferior to polysulphide). On illumination, metallic Sn deposited on the exposed backside (the Ti substrate) of the photoelectrode. In the dark, this Sn discharged (current flow in the... 

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