Doped crystals

Aziz R A 1984 Interatomic potentials for rare-gases pure and mixed interactions Inert Gases Potentials, Dynamics and Energy Transfer in Doped Crystals ed M L Klein (Berlin Springer) oh 2, pp 5-86... 

Dubost H 1984 Speotrosoopy of vibrationai and rotationai ieveis of diatomio moieouies in rare-gas orystais Inert Gases. Potentials, Dynamics, and Energy Transfer in Doped Crystals (Springer Ser. Chem. Phys. 34) ed M L Kiein (Beriin Springer) pp 145-256... 

This reaction is occasionally used for doping crystals uniformly after they have been grown. The process is called transmutation doping (37). 

The other species observed in irradiated Fe(CO)5-doped crystals of Cr(CO)6 also showed coupling to 57Fe, to a unique 13C, and to four other carbons. However, in this case g, AFe, and AC1 have only one matrix axis in common (that corresponding to the third component of each matrix listed in Table 4.10). 

Sometimes it is possible to dope crystals with impurities which act as electron or hole traps, but if powder e.s.r. spectra suffice, it is convenient to use specific solvents to encourage either specific electron-capture or hole-capture by dilute solutions of suitable compounds. 

The electrical contact with the bulk of the doped crystal is made through a very heavily doped layer, to reduce the height of the Schottky barrier between the bulk and the metal of the external contact (Au). The charge carriers cross this layer by tunnel effect. 

Significant changes in electronic properties of a solid can result from composition variation. The examples chosen to illustrate this will mainly be drawn from oxides as these have been studied in most detail. In this chapter, pure (single-phase) solids will be described—intrinsic conductivity—while in the following chapter impurities and doping—that is, extrinsic conductivity—will be considered. Note that the principles described below apply equally well to doped crystals—the division into two chapters is a matter of convenience. 

As in the previous chapter, most work has been carried out on oxides, and these figure prominently here. As the literature on oxides alone is not only vast but is also rapidly increasing, this chapter focuses upon a number of representative structure types to explain the broad principles upon which the defect chemistry depends. However, despite considerable research, the defect chemistry and physics of doped crystals is still open to considerable uncertainty, and even well-investigated simple oxides such as lithium-doped nickel oxide, Li Nij- O, appear to have more complex defect structures than thought some years ago. 

In this section we are concerned with the properties of intrinsic Schottky and Frenkel disorder in pure ionic conducting crystals and with the same systems doped with aliovalent cations. As already remarked in Section I, the properties of uni-univalent crystals, e.g. sodium choride and silver bromide which contain Schottky and cationic Frenkel disorder respectively, doped with divalent cation impurities are of particular interest. At low concentrations the impurity is incorporated substitutionally together with an additional cation vacancy to preserve electrical neutrality. At sufficiently low temperatures the concentration of intrinsic defects in a doped crystal is negligible compared with the concentration of added defects. We shall first mention briefly the theoretical methods used for such systems and then review the use of the cluster formalism. 

We consider first the activity coefficients. The contribution of the defects, N cation vacancies and N divalent ions, to the Gibbs free energy of the doped crystal is... 

Suppose that you are going to develop an ultraviolet-emitting phosphor based on a trivalent lanthanide rare earth ion doped crystal. If you want this phosphor... 

A Gd + doped crystal is illuminated with a pulsed light source, so that the l7/2 excited state of this ion is populated by absorbing 1 mJ of energy per incident pulse. Determine the heat delivered to the crystal per excitation pulse if the nonradiative rate from this state is 10 s The fluorescence lifetime of the l7/2 state is 30 /xs. 

Hirota used doped crystals to observe weak Ti-<- So absorption spectra by phosphorescence excitation spectroscopy. Triplet excitons of the host are formed by direct light absorption. The guest molecules, chosen to have lower triplet energy, act as traps and emit guest phosphorescence. 

High-purity semi-insulating (SI) SiC material has the highest reported thermal conductivity with a value of 4.9 W/(cm-K). Lower values are measured for the doped crystals but they are all above 4 W/(cm-K) at room temperature [10]. 

Now, 2-inch SI wafers without vanadium doping are sold commercially from several sources. Vanadium doped crystals will rapidly disappear and soon only be used as displays in museums or as book rests. 

An example of exciplex formation in the solid state may be afforded by perylene doped crystals of pyrene which emit a green structureless fluorescence in addition to the blue and orange-red excimer bands of pyrene and perylene, respectively. Hochstrasser112 has shown that the energy of the emitting species is consistent with that of a charge transfer complex of pyrene and perylene molecules in a bimolecular unit of the pyrene lattice. 

The material YCaA104 is of interest for solid state lasers. The orientational dependence of the HFEPR spectra at 250 GHz was measured32 for an iron-doped crystal using a goniometer in a quasi-optical spectrometer at 253 K The spectra of Fe3+, S — 5/2, revealed the existence of two magnetically inequivalent sites of roughly equal concentration. Only the site with its z-axis along the c-axis of the crystal was studied in detail and was found to have a 0-factor close to 1.99 and a ZFS of about 29 GHz. 

The electron mobilities at 296 and 420°K are given for several Cr-doped and -doped samples in Table II. The data for the Cr-doped crystals should be considered less accurate since a mixed-conductivity analysis was necessary in most cases (Look, 1980). However, the temperature dependences are not unlike those of conductive GaAs samples with similar impurity concentrations (1016—1018 cm-3). At least two of the crystals (MA 287/80 and MOR 56/76) appeared to be inhomogeneous, as evidenced by nonlinear Arrhenius plots. However, it is doubtful that the bulk of the data require a percolation-type conduction mechanism to be operative, as has been suggested (Robert et ai,... 

Some of these effects described above are illustrated in Fig. 13, which shows B 2 versus / , and B 2 versus —RJAR plots for two SI GaAs samples, one doped, and the other Cr doped. The -doped crystal has a resistivity of... 

The excitation spectrum of neodymium in the doubly doped crystal having 1 per cent chromium and 1.3 per cent neodymium clearly shows chromium bands. From a detailed study of the chromium and neodymium lifetimes, Kiss and Duncan show that energy transfer takes place from the chromium 2E state. This is contrary to the conclusion drawn by Murphy and co-workers on LaA103. 

When both neodymium and chromium are in the host together, the chromium lifetime is shortened to 3.5 msec at 77°K. The time dependence of the neodymium fluorescence is complex in the doubly doped crystal. It consists of a fast part with a lifetime of 240 /xsec and a second part with a longer time constant 3.5 msec. 

In direct contrast to the behavior of the singly doped europium samples, the decay curves for the doubly doped crystals cannot be characterized by a simple exponential behavior. It appears, however, that such behavior is approached asymptotically. 

Pollack also observes that the transition probabilities in his erbium-doped crystal change with concentration. He ascribes the decrease in emission intensity of certain transitions to reductions in radiative-transition... 

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