Semiconductors II-VI compounds

Hydrides are an important group of precursors that are used to deposit single elements such as boron or silicon. As described in Ch. 4, they are also used in conjunction with metallo-organics to form III-V and II-VI semiconductor compounds as shown in the following examples ]... 

The m-V and II-VI semiconductor compounds have excellent optical properties and are the most important group of optoelectronic materials, which are all produced by CVD for many optoelectronic applications. The properties of these materials and their CVD reactions are reviewed in Ch. 12, Secs. 3.0 and 4.0 and Ch. 13, Sec. 6.0. It is possible to tailor the bandgap, by the proper combination of these materials, to suit any given application (See Fig. 13.2 of Ch. 13). 

Nanotubes and nanowires of II—VI semiconductor compounds such as CdS and CdSe have been obtained by a soft chemical route involving surfactant-assisted synthesis.28,75 For CdSe nanotubes, the metal oxide was reacted with the selenidizing reagent in the presence of a surfactant such as Triton 1G0X. Substantial amounts of nanotubes were obtained by this method (Fig. 25a and b). Annealing of the as-prepared nano-... 

The progress in development of optoelectronics demands the further investigation of new materials and new effects being a basis for the production of various optoelectronic systems. A special interest is focuses on II-VI semiconductor compounds containing 3d transition metals as impurities. Due to the unique combination of physical properties ZnO crystals are potential candidates for numerous industrial applications. 

As can be seen, II-VI semiconductor compounds can be used as sensing materials in all types of gas sensors, including chemiresistors, SAW, heterojunction based, and optical. They can be applied to surface functionalizing and composites forming as well. The application of II-VI saniconductor compounds in quantum dots-based gas sensors will be discussed in Chap. 5 (Vol. 2). Operating characteristics of several Il-VI-based gas sensors are shown in Figs. 5.18 and 5.19. 

It is important that the sensitivity of Il-VI-based gas sensors of the conductometric type for various semiconductors changes according to the value of S from Fig. 5.17. For example, in the couple CdS-CdSe, CdS-based sensors had much better sensitivity to vapors of water, ethanol, ammonia, acetone, and iodine (Nesheva et al. 2006). We also need to note that aU regularities established for metal oxides work in Il-VI-based gas sensors as well (Lantto and Golovanov 1995). This means that for better sensitivity, II-VI semiconductor compounds should be porous with small both the grain size... 

PBE dendrons coordinate to the surface of II-VI semiconductor nanocrystals (e.g., CdSe [33] and CdSe/ZnS core/shell structure [34, 35]) to modulate the photoluminescence of the nanocrystals [32]. Trioctylphosphine oxide (TOPO)-capped II-VI semiconductor nanocrystals of several-nanometers diameter have been synthesized by a pyrolysis reaction of organometallics in TOPO [33-35]. The capping ligand (TOPO) can be replaced by stronger ligands such as thiol compounds [36], suggesting that dendrons bearing sulfur atom(s) at the focal point replace TOPO as well. 

As a final comment on terminology, we note that elemental semiconductors are formed from a single element, e.g., Si or Ge, whereas compound semiconductors are formed from two binary), three ternary), four quaternary), or, rarely, more elements. Semiconductor alloys refer to solid solutions where either one anion or one cation can substitute for another, or possibly two or more such substitutions can occur for a binary semiconductor AB a simple alloy with C would be represented as Ai CjcB. Semiconductors are often classified by the group numbers in the periodic table. Thus, for example, I-VII semiconductors include Cul and AgBr, II-VI semiconductors include ZnS, CdTe, and HgTe, III-V semiconductors include GaAs, GaN, InP, and InSb, and IVx-VIv semiconductors include PbSe and Sn02. Fundamental physical properties are compiled in a recent handbook [22]. 

Dale L. Martin, Molecular Beam Epitaxy of IV-VI Compound Heterojunctions Robert L. Gunshor, Leslie A. Kolodziejski, Arto V. Nurmikko, and Nobuo Otsuka, Molecular Beam Epitaxy of II-VI Semiconductor Microstructures... 

The next five chapters deal with deposition of specific groups of semiconductors. In Chapter 4, II-VI Semiconductors, all the sulphides, selenides, and (what little there is on) tellurides of cadmium (most of the chapter), zinc (a substantial part), and mercury (a small part). (Oxides are left to a later chapter.) This chapter is, understandably, a large one, due mainly to the large amount of work carried out on CdS and to a lesser extent on CdSe. Chapter 5, PbS and PbSe, provides a separate forum for PbS and PbSe, which provided much of the focus for CD in earlier years. The remaining sulphides and selenides are covered in Chapter 6, Other Sulphides and Selenides. There are many of these compounds, thus, this is a correspondingly large chapter. Chapter 7, Oxides and Other Semiconductors, is devoted mainly to oxides and some hydroxides, as well as to miscellaneous semiconductors that have only been scantily studied (elemental selenium and silver halides). These previous chapters have been limited to binary semiconductors, made up of two elements (with the exception of elemental Se). Chapter 8, Ternary Semiconductors, extends this list to semiconductors composed of three elements, whether two different metals (most of the studies) or two different chalcogens. 

Correlations between catalytic activity and a variety of bulk properties of semiconductors have been reported (i) the average band gap of III-V and II-VI semiconductors and activity towards hydrogenation of isopropanol (ii) enthalpy of oxides and their activity towards oxidation of propylene and (iii) number of d-electrons (and crystal field stabilization energy) or 3rf-metal oxides and their activity towards N2O decomposition. The last correlation, due to Dowden (1972), is important since it provides a connection between heterogeneous catalysis and coordination chemistry of transition-metal compounds. A correlation between the catalytic activity of transition-metal sulphides towards hydrodesulphurization of aromatic compounds and the position of the transition metal in the periodic table has been made by Whittingham ... 

Organometallic compounds are particularly important for the production of III-V and II-VI semiconductors, in a process known as metal-organic chemical vapor... 

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