P-type silicon

The silicon diode (photodiode) detector consists of a strip of p-type silicon on the surface of a silicon chip (n-type silicon). By application of a biasing potential with the silicon chip connected to the positive pole of the biasing source, electrons and holes are caused to move away from the p-n junction. This creates a depletion region in the neighbourhood of the junction which in effect becomes a capacitor. When light strikes the surface of the chip, free... 

In recent years further concepts have been developed for the construction of polymer-based diodes, requiring either two conjugated polymers (PA and poly(A-methyl-pyrrole) 2 > or poly(A-methylpyrrole in a p-type silicon wafer solid-state field-effect transistor By modifying the transistor switching, these electronic devices can also be employed as pH-sensitive chemical sensors or as hydrogen or oxygen sensors 221) in aqueous solutions. Recently a PPy alcohol sensor has also been reported 222). 

Laser D, Bard AJ (1976) Semiconductor Electrodes. IV. Electrochemical behavior of n- and p-type silicon electrodes in acetonitrile solutions. J Phys Chem 80 459 66... 

Dominey RN, Lewis NS, Bruce JA, Bookbinder DC, Wrighton MS (1982) Improvement of photoelectrochemical hydrogen generation by surface modification of p-type silicon semiconductor photo-cathodes. J Am Chem Soc 104 467 82... 

The fact that pure silicon is not a conductor but is "doped to make it a good semi-conductor might seem odd, particularly since considerable effort goes into obtciining very pure silicon. Nonetheless, the undoped silicon is not conductive. This allows one to form discrete areas of n-type and p-type silicon on the same wafer and to position these in conjunction with each other in a manner so that a current amplifying device results. An example of such a device is shown in the following diagram ... 

Nakato, Y., Yano, H., Nishiura, S., Ueda, T., and Tsubomura, H., Hydrogen photoevolution at p-type silicon electrodes coated with discontinuous metal layers, ]. Electroanal. Chem. Interfacial Electrochem., 228,97,1987. 

For completeness it should be mentioned that the passivation of gold, presumably via the same AuH complex, has also been studied in p-type silicon, where it is the donor rather than the acceptor level of gold that is active (Hansen et al., 1984). Though no profiles were reported in this work, apparent hydrogen diffusion coefficients inferred by these authors are of the same order as the Pearton (1985) points of Fig. 16 at temperatures 110°C and below. 

It was discovered early that the penetration of hydrogen from a plasma source into a substrate of p-type silicon is greatly reduced if the substrate is covered by a thin layer of strongly n-type material (Pankove et al., 1985 ... 

Similar hydrogen-acceptor pairing effects are observed. An explanation of the observed neutralization of acceptors in Si. B has been proposed by Pantelides (1987), assuming that hydrogen acts as a deep compensating donor in p-type silicon. Formation of hydrogen-acceptor pairs would follow compensation as a result of the coulombic attraction between... 

Such dissolution reactions usually contain several steps and are complicated. An important example is silicon. In aqueous solutions this is generally covered by an oxide film that inhibits currents and hence corrosion. However, in HF solutions it remains oxide free, and p-type silicon dissolves readily under accumulation conditions. This reaction involves two holes and two protons, the final product is Si(IV), but the details are not understood. A simpler example is the photodissolution of n-type CdS, which follows the overall reaction ... 

Thus, the n-type and p-type silicon become the negative pole and the positive pole respectively of the solar cell (Figure 11.4). The change in potential energy of the electrons and holes brought about by photon absorption means that the Fermi levels in the n- and p-type silicon become separated. 

Figure 4.22 Schematic diagram of a field effect transistor. The silicon-silicon dioxide system exhibits good semiconductor characteristics for use in FETs. The free charge carrier concentration, and hence the conductivity, of silicon can be increased by doping with impurities such as boron. This results in p-type silicon, the p describing the presence of excess positive mobile charges present. Silicon can also be doped with other impurities to form n-type silicon with an excess of negative mobile charges.

Let us now consider the charge state of the electrode. The emitter is positively biased. A p-type silicon electrode is therefore under forward conditions. If the logarithm of the current for a forward biased Schottky diode is plotted against the applied potential (Tafel plot) a linear dependency with 59 meV per current decade is observed for moderately doped Si. The same dependency of 1EB on VEB is observed at a silicon electrode in HF for current densities between OCP and the first current peak at JPS, as shown in Fig. 3.3 [Gal, Otl]. Note that the slope in Fig. 3.3 becomes less steep for highly doped substrates, which is also observed for highly doped Schottky diodes. This, and the fact that no electrons are detected at the collector, indicates that the emitter-base interface is under depletion. This interpretation is sup-... 

Fig. 3.4 The electrochemical l-V characteristic of n-type and p-type silicon electrodes of (100) and (111) orientation in 40% aqueous KOH solution at 60°C in the dark. The potential is scanned from cathodic to anodic at a sweep rate ofl mVs"1. Redrawn from results of [Sm6],...

In the cathodic regime the silicon atoms of the electrode do not participate in the chemical reaction. Therefore, an n-type or a strongly illuminated p-type silicon electrode behave like a noble metal electrode and hydrogen evolution or metal plating reactions are observed. For the case of an aqueous electrolyte free of metal ions the main reaction is electrochemical hydrogen evolution according to ... 

It is shown that the rate-limiting step in the photoelectrochemical evolution of hydrogen in an HF electrolyte is linearly dependent on the excess electron concentration at the surface of the p-type silicon electrode. The rate of this step does not depend on the electrode potential and the H+ concentration in the solution, but is sensitive to the surface pretreatment [Sell]. The plateau in the I-V curve, slightly... 

Fig. 5.4 Voltage-time curve for a p-type silicon electrode anodized galvanostatically at 0.1 mA cm"2 in 10% acetic acid. Silicon electrodes were removed from the electrolyte after various anodization times (filled circles) and the thickness of the anodic oxide was measured by ellipsometry (open circles). The curvature of the sample was monitored in situ and is plotted as the value of stress times oxide thickness (filled triangles). The bar graph below the V(t) curve shows a proposed formation mechanism. Galvanostatically a...

Fig. 5.11 The frequency/of potentiostatic electrochemical oscillations at a p-type silicon electrode in aqueous HF solutions is plotted versus the concentration cF and the average current density J.

An electric field in the semiconductor may also produce passivation, as depicted in Fig. 6.1c. In semiconductors the concentration of free charge carriers is smaller by orders of magnitude than in metals. This permits the existence of extended space charges. The concept of pore formation due to an SCR as a passivating layer is supported by the fact that n-type, as well as p-type, silicon electrodes are under depletion in the pore formation regime [Ro3]. In addition a correlation between SCR width and pore density in the macroporous and the mesoporous regime is observed, as shown in Fig. 6.10 [Thl, Th2, Zh3, Le8]. 

QC in silicon structures requires dimensions of a few nanometers and is therefore proposed to be responsible for the formation of microporous films on Si electrodes, as discussed in Chapter 7. QC is independent of doping and is often found as a superposition to pore formation by SCR effects. Only for p-type silicon electrodes of doping densities of 1016-1017 cm-3 is no formation of SCR-related pores observed upon anodization in aqueous HF. This substrate doping regime is therefore best suited for formation of purely microporous layers. 

Fig. 7.20 Luminescence intensity and peak position versus RTO processing temperature for PS samples grown on p-type silicon substrates (A 1 Q cm, B 1 Q cm, C 0.07 12 cm). Note the anti-correlation of the PL intensity and of the ESR signal (taken for sample series A). After [Pel],...

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