We have studied the effect that structural defects and chemical impurities have on the electron mobility in GaN films grown in a production scale metalorganic chemical vapor depositon system. Structural defects such as dislocations, stacking faults, twins, and amorphous regions in the buffer layer have been examined. In general, we have found that the structural defects are not the primary contributor to low mobility. However, there is one type of defect Cnanopipe") that may be an important indirect contributor to mobility degradation by acting as a conduit into the film interior for impurities such as carbon and oxygen. We have also investigated the role that the principal impurities play in determining the electrical performance. Of particular concern was the presence of carbon resulting from an incomplete dissociation of trimethylgallium precursor gas. Also present in the films were traces of oxygen, hydrogen, and aluminum, while heavy metals such as iron, chromium, and molybdenum were detected at or near the resolution limit of secondary ion mass spectrometry analysis. We present evidence for compensation by carbon at low carrier concentrations, which would help to explain the anomalous mobility behavior in GaN.
We report a single frequency lasing phenomenon with a narrow linewidth of ~3 kHz in cascaded fiber that is composed of three types of low-loss communication fibers. The Rayleigh scattering of the Brillouin Stokes light created in the middle fiber section along both directions is enhanced by the other two fiber sections. When the Brillouin gain of the middle fiber exceeds the effective loss of the Brillouin stokes light in a roundtrip, a narrow linewidth lasing is observed on the top of the Brillouin spectrum line of the middle fiber. To the best of our knowledge, it is the first report on Rayleigh scattering-assisted Brillouin lasing with single frequency and narrow linewidth in cascaded low-loss communication fibers.
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