Cr doping

W. F. Howard, S. H. Lu, W. H. Averill, A. D. Robertson (Covalent Associates, Inc.), Cr-dope LiMn204 in LiPF6 and Li-methidc electrolytes, paper presented at 37th Power Sources Conference, Cherry Hill, NJ, June 1996. 

Let us now turn to Cr -doped titanates. Here it has been proposed that chromium-titanium MMCT transitions are responsible for the brown colors obtained [15, 17,19] ... 

Figure 4.31. (t0-TOF) versus energy spectrum of 295 nm Cr doped FeSi2 on Si measured at 40° with 230 MeV 129Xe ions (Bohne et al. 2000). 

Recent density functional theory (DFT) calculations have been used to calculate the transfer of unpaired electron spin densities to the nearby atoms and analyze the orbitals involved in these mechanisms. For example. Figure 8 shows the spin densities obtained for Cr +-doped LiCo02. The spin densities can be calculated by subtracting the spin-up elec-... 

Cr-doped Ti02 has been widely studied for photoanode application since addition of Cr shifts the optical absorption spectrum towards the visible range, with the caveat that excess chromium doping leads to higher recombination rates. Various synthesis methods have been employed to date. For example Radecka an coworkers used r.f. sputtering to prepare up to 16 at% Cr-doped Ti02 [70]. XRD reveals that at low Cr concentration the... 

Wang D, Zou Z, Ye J (2005) Phtotcatalytic water splitting with the Cr-doped Ba2ln205/ln203 composite oxide semiconductors. Chem Mater 17 3255-3261... 

Emerald, Cr " doped beryl, has a beryl structure with the Cr " impurity ions in highly distorted octahedron sites. The discovery of lasing action in emerald stimulated investigation of its luminescence properties. It was established that its tuning range is approximately 730-810 nm, while luminescence consists of a narrow line at 684 nm and a band peaking at 715 nm with similar decay times of 62 ps. The relative intensities of those line and band are different in a- and 7T-polarized spectra (Fabeni et al. 1991). 

Alexandrite, the common name for Cr-doped chrysoberyl, is a laser material capable of continuously tunable laser output in the 700-800 nm region. It was established that alexandrite is an intermediate crystal field matrix, thus the non-phonon emitting state is coupled to the 72 relaxed state and behaves as a storage level for the latter. The laser-emitted light is strongly polarized due to its biaxial structure and is characterized by a decay time of 260 ps (Fabeni et al. 1991 Schepler 1984 Suchoki et al. 2002). Two pairs of sharp i -lines are detected connected with Cr " in two different structural positions the first near 680 nm with a decay time of approximately 330 ps is connected with mirror site fluorescence and the second at 690 nm with a much longer decay of approximately 44 ms is connected with inversion symmetry sites (Powell et al. 1985). The group of narrow lines between 640 and 660 nm was connected with an anti-Stokes vibronic sideband of the mirror site fluorescence. 

Figure 24 Incident photon energy dependence of the surface charge separation efficiency (h" / photon) measured by PITCS. Squares, circles, triangles, and diamonds represent the results obtained for Cr-implanted rutile, Cr-doped rutile, undoped rutile, and anatase Ti02 films, respectively.

The same is true for the L—S-coupKng parameter A, which can be evaluated from ESR-measurements. For Cr +-doped a-AlsOa the spectroscopic g-factor 8 A ... 

The radiative transitions of the previous descriptions have all been spontaneous Relaxation from the excited state to the ground state and emission of photons occur without external aid. In contrast, a stimulated emission occurs when the half-life of the excited state is relatively long, and relaxation can occur only through the aid of a stimulating photon. In stimulated emission, the emitted photon has the same direction as, and is in phase with, the stimulating photon. The example of Cr +-doped AI2O3 that we utilized earlier for our description of the color of ruby works equally well for a description of stimulated emission. Recall that the presence of chromium in alumina alters the electronic structure, creating a metastable state between the valence and conduction bands. Absorption of a blue-violet photon results in the excitation of an electron from... 

Results are presented in Table I for a typical Cr-doped and a typical... 

Electrical Parameters at 300°K Calculated by Three Different Techniques tor a Typical -Doped and a Typical Cr-Doped SI GaAs Crystal... 

We now turn to the Cr-doped GaAs samples. If we ignore sample MOR 56/76, which showed curvature in the Arrhenius plot, the following mean values are obtained A0 a 0.78 eV and QA a 45. There is more variation in the E0 for Cr-doped samples than for -doped samples, possibly because of contamination in some of the Cr-doped samples. For the Cr acceptor, QK is defined somewhat differently than for the donor case (Eq. 23). Thus, if ND, = 0 in Eq. (21), we get... 

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,... 

Fig. 4. Room temperature optical transmission of two SI GaAs crystals, undoped and Cr-doped, respectively. Sample thickness, 0.473 cm. [From Martin et al. (1979).]...

For Cr-doped GaAs it is usually easy to make n m 107 cm - 3, with light from a typical monochromator (Look, 1977a), and if filling temperature, Eq. (47) shows that dnjdt m 0, i.e., the trap remains filled (ni = Ni). 

The identification of an impurity, defect, or impurity-defect complex by some particular technique must nearly always be accomplished in conjunction with doping experiments. Thus, the well-known, sharp, zero-phonon photoluminescence lines at 0.84 eV in GaAs are almost certainly associated with Cr, as established by Cr-doping experiments (Koschei et al., 1976). However, some care must be taken here. For example, a dominant electron trap (EL2) in -doped GaAs is probably not associated with O, according to recent experiments (Huber et al., 1979). Thus, the doping must be accompanied by a positive identification of the relevant impurity concentration, say by SSMS, or SIMS. These general considerations apply to all the techniques discussed below. 

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... 

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