Electronic properties heterojunctions

The main advantages that compound semiconductor electronic devices hold over their siUcon counterparts He in the properties of electron transport, excellent heterojunction capabiUties, and semi-insulating substrates, which can help minimise parasitic capacitances that can negatively impact device performance. The abiUty to integrate materials with different band gaps and electronic properties by epitaxy has made it possible to develop advanced devices in compound semiconductors. The hole transport in compound semiconductors is poorer and more similar to siUcon. Eor this reason the majority of products and research has been in n-ty e or electron-based devices. 

Nanometer-Sized Electronic Devices The possible use of carbon nanotubes in nanoelectronics has aroused considerable interest. Dramatic recent advances have fueled speculation that nanotubes (SWNTs) will be useful for downsizing circuit dimensions. Because of their unique electronic properties, SWNTs can be interfaced with other materials to form novel heterostructures [156]. The simplest device one can imagine with carbon nanotubes is that involving a bend or a kink, arising from the presence of a diametrically opposite pentagon-heptagon pair. The resultant junction connects two nanotubes of different chirality and hence of different electronic structure, leading to the realization of an intramolecular device. Such a device in SWNTs is found to behave like a diode rectifier [157]. Silicon nanowire-carbon nanotube heterojunctions do indeed exhibit a rectification behavior [158]. 

Electronic and optical properties of structured thin films and heterojunctions are treated in Section 6.4. Since non-optimal electronic properties are to be accommodated in the new cell design, the transport properties to be discussed will naturally be those of defect-rich materials. 

The unique structure and electronic properties of CNTs provide a tremendous potential for construction of CNTs and MOX hybrid materials in the field of gas-sensing applications. Advantages for mixing CNTs in metal oxides for gas sensors are the reduction of operating temperature and enhancement of sensitivity and selectivity due to the amplification effects of p-n heterojunctions with the gas reaction, formation of nanochannels for gas diffusion, high specific surface area, and increase of charge carrier on the surface. As a result of these advantages, the hybrid CNT/metal oxide gas sensor may be used instead of the popular commercial metal oxide gas sensors (such as TGS gas sensors) in the near future. 

A film of electroactive polymer [Os(bpy)2(vpy)2p, sandwiched between Pt and Au electrodes, becomes conductive when both the electrode potentials have appropriate values such that the mixed valent state, Os(III)/(II), is generated. Such arrangements are used in two-terminal and three-terminal diodes Solid-state organic heterojunctions utilizing two conducting polymers [PA and poly(A-methylpyrrole) junctions] were reported The electronic properties can be modified chemically. A field-effect transistor was fabricated utilizing chemically prepared poly(A-methyl-pyrrole)... 

Being still one of the most-studied redox dyes, MPs continuously attract attention because of their functional universality, chemical stability, ease of chemical modification, and great potential of tuning their chemical and electronic properties. MPs have intensively been studied with respect to their promising application for conversion of solar energy into electric energy in SCs. MPs are mostly used in three types of SCs. These include DSSCs, supramolecular SCs (SMSCs), and bulk heterojunction solar cells (BHJ SCs). 

Understanding of electronic properties is necessary for selection of suitable polymer matrix and nanofillers for the fabrication of electronic devices. In many cases, p-n heterojunctions play an important role both in modern electronic applications and in understanding of other SC devices. 

The electrical and electronic characteristics of heterojunctions, e.g., GaAs/Ga Ali-xAs, are of fundamental importance in electronic and opto-electronic devices. Experimentally it is quite difficult to gain detailed atomistic information on the properties of the buried interfaces of a heterojunction. Therefore, first-principles electronic structure calculations play an important role to provide insight into the quantities such as the charge density distribution and the electrostatic potential across heterojunctions. Calculations thus give an understanding of the relationship between chemical composition, crystallographic structures, and the electronic properties. 

Structural, optical, and electronic properties of n-Si/n-BP and p-Si/n-BP heterojunctions have been investigated by Goossens et al. (77,78). Impedance spectroscopy has been used to obtain Mott-Schottky (MS) plots of (Csc) versus DC bias, V (Fig. 24). The slope of the MS plot was positive and concordant with an effective donor density No - Na of about 5 X 10 cm for all studied samples. For crystalline CVD layers of BP (100), the flat-band potential was -0.55 V versus SCE at pH 4.6 and was observed to show a Nernstian -60 mV/pH dependence. 

A Goossens, EM Kelder, RJM Beeren, CJG Bartels, J Schoonman. Structural, optical and electronic properties of sUicon/boron phosphide heterojunction photoelecirodes. Ber Bunsenges Phys Chem 95 503, 1991. 

Another class of heterojunction solar cells are CuInSei-based devices, formed from p-n junctions with CdS thin films. CuInSei is a ternary compound with a band gap of 2.4 eV and is stable as a chalcopyrite or sphalerite structure.Chalcopyrite (lattice constant a = 0.5789 and c = 1.162 A) is stable at room temperature up to 810 °C. The band gap is direct and approximately 1.02 eV at room temperature, with an absorption coefficient above 5 x 10" cm . On substitution of Ga for In or S for Se, the band gap increases up to 1.68 eV. For high efficiency devices, band gaps between 1.20 eV and 1.25 eV are used with [Ga]/[In- -Ga] ratios between 25% and 30%. Different stoichiometries give rise to different intrinsic defects and hence electronic properties, e.g. Cu and In (acceptor) vacancies (excess Se) give rise to p-type character and Se vacancies lead to n-type material (for solar cells a slightly Cu-deficient material is used). 

Blase, X., CharUer, J.C., De Vita, A., Car, R. Structural and electronic properties of composite BxCyNz nanotubes and heterojunctions. Appl. Rhys. A 68,293-300 (1999)... 

Figure 6.13 Electron and hole mobilities in Sii. Gex alloys. Solid lines and figures refer to electron properties while dashed lines and points represent hole behaviors. Figure redrawn with permission from Van der Walle, C.B. SiGe heterojunctions and band offsets, in Kasper, Erich, and Lyutovich, Klara, editors. Properties of Silicon Germanium and SiGe.Carbon. London INSPEC, 2000, p. 149. Copyright 2000, The Institution of Engineering Technology.

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