Articles you may be interested inEffect of quantum confinement on shallow acceptor transitions in δ-doped GaAs/AlAs multiple-quantum wells Appl. Phys. Lett. 84, 735 (2004); 10.1063/1.1644912Effect of quantum-well confinement on acceptor state lifetime in δ-doped GaAs/AlAs multiple quantum wells Appl.

We present a detailed study of the magnetic behavior of iron-oxide (γ-Fe 2 O 3 and Fe 3 O 4 ) nanoparticles constituents of ferrofluids (FF's) with average particle sizes = 2.5 and 10 nm. The particles were dispersed in the frozen liquid carrier (pure FF) and in a frozen lyotropic liquid crystalline matrix in the nematic phase or ferronematic (FN) (ferrolyomesophase). Both FF and FN phases displayed superparamagnetic (SPM) behaviour at room temperature, with blocking temperatures T B ~ 10 and 100 K for = 2.5 and 10 nm, respectively. Dynamic ac susceptibility measurements showed a thermally activated Néel-Brown dependence of the blocking temperature with applied frequency. Our results show that dipolar interactions are small, but non-negligible, as compared to the single-particle energy barriers from magnetic anisotropy. From the fit of ac susceptibility we calculated the effective magnetic anisotropy constant K eff for 2.5 nm maghemite particles. Although interparticle interactions present in highly diluted samples do not appreciably modify the dynamic magnetic behavior of isolated particles, the calculated magnetic anisotropy were abut one order of magnitude larger that the bulk materials, suggesting the existence of large surface anisotropy. Using the thermally activated model to fit the dynamic data yielded effective energy barriers E a = 3.5x10 -21 J. From these data, we obtained K eff = 422 kJ/m 3 for the single-particle effective magnetic anisotropy.

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Room-temperature and low-temperature (1.7-K) photoluminescence (PL) characteristics of heteroepitaxial ZnSe layers on GaAs which are doped with Ga by either conventional (bulk) or planar doping techniques are described. Low-temperature PL peaks at 2.27 and 2.0 eV involving deep acceptor levels are introduced by Ga doping, as well as newly reported shallow acceptor levels with binding energies of approximately 68 and 85 meV. The behavior of these peaks and the excitonic transitions is studied as a function of Ga-doping level and, for the case of the planar-doped layers, surface stoichiometry during doping. The exciton peaks exhibit substantially greater broadening for doping on Zn-rich surfaces than for Se-rich surfaces, corresponding to the higher carrier concentrations observed by electrical measurements in the former case. The deep acceptor levels are found to be incorporated to a lesser degree for doping on Zn-rich surfaces, while the incorporation of the 85-meV acceptor is enhanced in this case. The net electrical compensation is evidently dominated by the behavior of the deep levels. The results are explained by assuming that both deep levels involve donor-Zn vacancy complexes, whose formation is suppressed by the excess Zn flux supplied during the planar-doping process. A comparison of the exciton spectra of nominally undoped and bulk Ga-doped samples is used to demonstrate that the residual donor in the undoped material is Ga.

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