Graduate Thesis Or Dissertation

An NMR study of bistable defects in CdTe and CdF₂

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  • Bistable properties have been reported in CdF₂ and CdTe systems doped with In and Ga. It is characteristic of these materials that the excited bistable state can be accessed by optical excitation. In this work, pulsed nuclear magnetic resonance (NMR) in conjunction with illumination at low temperatures has been used to study ¹¹³Cd spin-lattice relaxation (1/T₁). In this way bistable effects have been observed. These effects are interpreted in terms of the "negative-U" DX models of bistability that have been put forth for the respective materials. In the context of these models, data concerning the respective DX center's electronic states have been collected. Conclusions are based in part on the complementary nature of cation and anion host NMR. In CdTe:Ga, under illumination at 31 K, a non-persistent, order of magnitude enhancement of the ¹¹³Cd spin-lattice relaxation rate (1/T₁) is observed. After illumination, the rate is persistently enhanced by about 50%. Over the temperature range of 31 K to 200 K, the ¹¹³Cd relaxation rate appears to be due to an activated process attributed to the thermal generation of paramagnetic centers. An activation energy ΔE=15 meV is determined. In CdF₂, the ¹¹³Cd (1/T₁) is persistently enhanced by a factor of two following illumination below 80 K. Above 150 K, the ¹¹³Cd (1/T₁) is strongly temperature dependent. This relaxation is attributed to thermally generated free carriers. This process is ineffective for ¹⁹F relaxation. This activated process yields an activation energy ΔE=185(10)meV. The ¹⁹F nuclei do, however, appear to relax by an activated process. This relaxation is attributed to thermally generated paramagnetic centers and yields an activation energy ΔE=70(10)meV. In CdTe:Ga, ⁶⁹Ga has been observed. It is shown that the observed ⁶⁹Ga resonance is due to Ga situated substitutionally on the Cd sublattice. However, these Ga account for only about 10% of the Ga in the sample. The ⁶⁹Ga (1/T₁) has been measured over the temperature range 200 K to 470 K. These rates are consistent with the T² temperature dependence expected for quadrupolar relaxation due to lattice vibrations.
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