Growth rates for Si1−xGex films (0≤x≤0.19) have been measured between 610–750 °C using low pressure H2/SiH4/GeH4 mixtures and at different temperatures these rates show different dependencies on composition x. A model attributes this complex behavior to competition between an increasing rate for desorption of surface hydrogen and a decreasing sticking probability for the reactive hydrides as x increases. The latter effect is explicitly reported for the first time. GeH4 is found to be ∼4.7 times more reactive than SiH4, and relative surface hydrogen coverages on Si and Si0.87Ge0.13 films measured by secondary ion mass spectroscopy are compared with the model.
The temperature dependence of cathodoluminescence intensity and decay time in the range I0~176 is reported for both nominally pure and Ce 3+doped yttrium aluminum garnet (YAG) crystals. Luminescence from characteristic lattice defect levels and from the 2D excited state of the Ce 3 + impurity have both been investigated. Three lattice defect levels have been detected. Recombination is dominated by the shallowest centers at low temperatures but, at higher temperatures, thermalization effects reduce the net capture rates at the competitive defect centers. As a result of thermalization in the doped crystal the efficiency of energy transfer to Ce s+ is increased, but only at the expense of introducing a long afterglow into the Ce 8+ decay near room temperature. The temperature-dependent interactions between different centers in the lattice are adequately described by a simple kinetic model. Each center is characterized by three rate processes and by a therrealization activation energy which is derived from the decay time data. The experimental results are in good agreement with preliminary studies already reported by the authors.
The concentration-dependence of cathodoluminescence efficiency for the rare earth (RE) activators Ce 3+, Eu 3+, Gd 3+, or Tb ~+ in YAG is compared at 300 ~ and 25~ and extends earlier work by the authors. Competitive recombination processes in the garnet phosphors include recombination at bulk lattice defect centers detected by near-u.v, luminescence and recombination through some shunt pathway dominant at 300~The experimental data are interpreted using a kinetic model, and except for the activator Gd 3 +, can be fitted using reasonable parameters. The model includes a specific mechanism for activator excitation (a bound exciton state) and the effects of impurity scattering on the rate of recombination at lattice defect centers. It is shown that one of the most important factors governing the general shape of the efficiency vs. concentration curve is the activator thermalization factor, x. This in turn depends on the depth of the impurity potential and the rate of energy transfer in the impurity-exciton complex. The general theory as developed is useful in rationalizing earlier measurements of efficiency vs. concentration for RE activators in a number of oxide and oxysulfide lattices.
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