Polycrystalline semiconductor oxide

This chapter presents a quantitative method to determine the photoadsorption capacity of a polycrystalline semiconductor oxide irradiated in liquid-solid system. The determination is performed imder reaction conditions so that it is really indicative of the photoadsorption capacity. The method uses the experimental results obtained in typical batch photoreactivity runs on this ground it has been applied to the following photocatalytic processes carried out in aqueous suspensions (i) oxidation of phenol in the presence of a commercial Ti02 catalyst (Degussa P25) and... 

The percolation model of adsorption response outlined in this section is based on assumption of existence of a broad spread between heights of inter-crystalline energy barriers in polycrystals. This assumption is valid for numerous polycrystalline semiconductors [145, 146] and for oxides of various metals in particular. The latter are characterized by practically stoichiometric content of surface-adjacent layers. It will be shown in the next chapter that these are these oxides that are characterized by chemisorption-caused response in their electrophysical parameters mainly generated by adsorption charging of adsorbent surface [32, 52, 155]. The availability of broad spread in heights of inter-crystalline barriers in above polycrystallites was experimentally proved by various techniques. These are direct measurements of the drop of potentials on probe contacts during mapping microcrystal pattern [145] and the studies of the value of exponential factor of ohmic electric conductivity of the material which was L/l times lower than the expected one in case of identical... 

Absolutely different situation occurs in case of polycrystalline semiconductors obtained under vacuum conditions. Thus, in paper [30] three types of ZnO samples were studied ZnO 1 obtained during heating of carbonates containing zinc in air at 600 C ZnO 2 obtained through vacuum decomposition of carbonate followed by heating at 500°C ZnO 3 powder of spectrally pure zinc oxide. 

This process is obviously a natural scattering process in polycrystalline materials, since polycrystalline films exhibit a high concentration of crystallographic defects, especially dislocations [133,134]. However, this process is rarely used to explain experimental data of carrier transport in polycrystalline semiconductors and especially transparent conducting oxides [88], which is mainly due to the fact that in most works on transport properties of polycrystalline films the density of defects was not determined. Podor [135] investigated bended n-type Ge crystals with a dislocation density around 107 cm 2... 

Hardee, K.L. and A.J. Bard (1977). Semiconductor electrodes X. Photoelectrochemical behavior of several polycrystalline metal oxide electrodes in aqueous solutions. Journal of the Electrochemical Society, 124,215-224. 

Japanese workers have published a paper regarding the study of photoelectro-chemical reactions at a n-type polycrystalline zinc oxide electrode using photoacoustic detection [125], They monitored in situ photoelectrochemical reactions at semiconductor electrodes using photoacoustic techniques. 

Kim YS, Ha S-C, Kim K, Yang H, Chcd S-Y, Kim YT (2005) Room-temperature semiconductor gas sensor based on nonstoichiometiic tungsten oxide nanorod film. Appl Phys Lett 86 213105 Kiriakidis G, Bender M, Katsarakis N, Gagaoudakis E, Hourdakis E, Doulofakis E, Cimalla V (2001) Ozone sensing properties of polycrystalline indium oxide films at room temperature. Physica Status Solid A Appl Res 185(1) 27-32... 

Simple thermocouples are not sensitive enough to detect variations in enzymatic reaction enthalpies. Thermistors and thermopiles can, however, detect such small variations in temperature. Thermistors are mixtures of metallic oxides and polycrystalline semiconductors. The high resistivity values of these materials gives a rapid response time owing to their small size and reduced calorific capacity. TTie resistance R of a thermistor is a function of temperature, T, according to the following expression ... 

Semiconductor films of ZnO used as operational elements are obtained by oxidation at - 500 - 600°C in the jet of purified oxygen of zinc film deposited at vacuum iP 10 Torr) on substrates made of fused quartz with subsequent sintering at - 350°C at high vacuum conditions [34]. As it was concluded in paper [17] the sintered polycrystalline sample obtained in such a manner should not be considered as a set of various separate crystallites touching each other but rather as a monolithic pattern in which microcrystals with diameter of 1-10 pm are linked with each other by bridges with length and thickness of the order of 0,1 pm (see Fig. 2.4). 

In addition, Janczak [26] studied the conductivity property of complex 3 with a polycrystalline sample, and the results show that the conductivity is in the range 2.7 -2.8 x 10-2Q-1cm-1 at room temperature. Very weak temperature dependence of the conductivity and a metallic-like dependence in conductivity are observed in the range 300-15 K. Ibers and co-workers [70] investigated the electrical conductivity of partially oxidized complex 82 with a suitable single crystal and the results indicate its semiconductor nature (Ea = 0.22eV). 

Polycrystalline GaN UV detectors have been realized with 15% quantum efficiency [4], This is about 1 /4 of the quantum efficiency obtained by crystalline devices. Available at a fixed price, however, their increased detection range may well compensate their lack in sensitivity. Furthermore, new semiconductor materials with a matching band gap appear as promising candidates for UV detection if the presumption of the crystallinity is given up. Titanium dioxide, zinc sulfide and zinc oxide have to be mentioned. The opto-electronic properties and also low-cost production processes for these compound semiconductors have already been investigated to some extent for solar cell applications [5]. 

Tan, J., et ah, Metal-Oxide-Semiconductor Capacitors Formed by Oxidation of Polycrystalline Silicon on SiC, Appl. Phys. Lett., Vol. 70, 1997, p. 2280. 

Schorner, R., et al., Enhanced Channel Mobility of 4H-SiC Metal-Oxide-Semiconductor Transistors Fabricated with Standard Polycrystalline Silicon Technology and Gate-Oxide Nitridation, Applied Physics Letters, Vol. 80, No. 22, June 3, 2002, p. 176. 

Plasma etching is widely used in semiconductor device manufacturing to etch patterns in thin layers of polycrystalline silicon often used for metal oxide semiconductor (MOS) device gates and interconnects (see Plasma technology). 

As described in Section 3 of Chapter 4, the stabilization of n-Si electrode by coating with poly(pyrrole) has attracted much attention. The stabilization of a small bandgap n-semiconductor electrode against oxidation is of great value not only to convert visible light into chemical energy, but also to construct liquid-junction solar cells operated under visible irradiation. The poly(pyrrole) film is usually electropolymerized on the semiconductor electrode dipped in the aqueous solution of pyrrole. The remarkable stabilizing effect of poly(pyrrole) film on polycrystalline n-Si is shown in Fig. 22 67). The photocurrent obtained under irradiation in the aqueous solution of... 

Studies of semiconductor films have shown many facets. The properties of epitaxial films have mainly been investigated on Ge and Si, and to a lesser degree on III—V compounds. Much work, lias been done on polycrystalline II-VI films, particularly with regard to the stoichiometry of the deposits, doping and post-deposition treatments, conductivity and carrier mobility, photo-conductance, fluorescence,electroluminescence, and metal-semiconductor junction properties. Among other semiconductors, selenium, tellunum. and a few transition metal oxides have found some interest. 

PVF films may alter the electrochemical response of semiconductor electrodes in useful ways. For example, for potentials more positive than —0.8 V (SCE) only reductions are possible with single crystal or polycrystalline Ti02 in contact with a ferrocene solution (acetonitrile). However, with plasma coated with PVF, ferrocene sites are oxidized in a potential region where n-Ti02 is considered blocked to electron transfer.70 Here two factors are involved obviously the coating of the electrode with an electroactive species, but also alteration of the surface energetics of the semiconductor. 

Fig. 8.8 Model of charge transport and band structure of macroscopic polycrystalline (A) and (B) nano crystalline metal oxide semiconductors (a) in their initial state and (b) after exposure to reducing gas (adapted from Franke et al., 2006)...

This type of material is commonly used in the production of semiconductor devices.57 The silica layer is used as a starting layer for integrated circuit (IC) build-up. IC layer materials range from single crystals and doped polycrystalline silicon, silicon nitride, thermally-grown oxide to vapour deposited or sputtered metal or metal silicide layers. Structural adhesion of the various layers is obtained by the application of organosilanes, such as AEAPTS, APTS and GPTS. 

One of the most widely used materials for the fabrication of modern VLSI circuits is polycrystalline silicon, commonly referred to as polysilicon. It is used for the gate electrode in metal oxide semiconductor (MOS) devices, for the fabrication of high value resistors, for diffusion sources to form shallow junctions, for conduction lines, and for ensuring ohmic contact between crystalline silicon substrates and overlying metallization structures. 

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