Separated-source techniques

The trimers can be used to prepare AIN and GaN at much lower temperatures than are necessary when separate-source techniques are used. The use of [Me2AlNH2]3 is representative of this process ... 

The basic instrumentation used for spectrometric measurements has already been described in Chapter 7 (p. 277). The natures of sources, monochromators, detectors, and sample cells required for molecular absorption techniques are summarized in Table 9.1. The principal difference between instrumentation for atomic emission and molecular absorption spectrometry is in the need for a separate source of radiation for the latter. In the infrared, visible and ultraviolet regions, white sources are used, i.e. the energy or frequency range of the source covers most or all of the relevant portion of the spectrum. In contrast, nuclear magnetic resonance spectrometers employ a narrow waveband radio-frequency transmitter, a tuned detector and no monochromator. 

The applications generally involve the nitrides as a thin film. However, nanoparticulate materials are emerging as an important form of the material. In some cases (described in the appropriate subsection), a thin film or nanoparticulate material may be obtained from the same precursor molecule simply by changing the deposition conditions. This section on Al and Ga nitrides discusses both separate-source precursor combinations and single-source precursor approaches. The chemistry of these processes rather than the engineering aspects is described. However, the equipment and techniques employed are often the deciding factors in crystalline film growth. 

These techniques often make use of two-stage procedures. Here a separate source is used for the generation of a sufficiently concentrated sample vapor doud, the composition of which is a good reflection of that of the sample, and this doud is lead into a second source. However, several approaches can be used according to the sample properties. 

If solvent extraction may be considered a source technique, derived liquid-liquid separation techniques include configurations in which an extraction solvent is physically immobilized by a coating or impregnation process onto a solid support such as silica, porous resin beads, or foam [13,84—87]. Other derived techniques include membranes of various configurations bulk liquid membranes, supported liquid membranes, emulsion membranes, and polymer-impregnated membranes [88]. Many derived liquid-liquid techniques have been developed, especially for use in analytical applications [13,60,62,64,75,84,85,87]. In each of these derived techniques, the... 

Far better results can be expected from multi-source techniques where each of the constituent metals is heated in a separate crucible. The composition of the deposited film will then be determined by the relative vapour pressures and hence by the relative heating rates applied to the various sources. The heating rates of each of the sources has to be controlled independently. Furthermore, in order to keep the ratio of the vapour pressures constant during the entire evaporation process, one has to be able to correct quickly for any deviation from the desired vapour pressure. 

For ZnO growth, OMVPE/MOVPE technique typically involves the use of metal alkyls, usually dimethyl zinc (CH3)2Zn (DMZn) or diethyl zinc (C2Hs)2Zn (DEZn) in combination with a separate source of oxygen and argon or nitrogen as a carrier gas. 

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