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The band structure for a surface quantum well is shown with the assumption that the piezoelectric field is screened by carriers generated by the excitation source. Without this assumption the diagram is similar, but the tilt of the quantum well at the vacuum interface would be sharper. The bound states for the surface quantum well are also shown.
The surface quantum-well emission as a function of GaN cap layer thickness is shown assuming the piezoelectric filed is screened by the excitation of carriers (solid line), and unscreened (dashed line). Experimentally observed emission energies from the PL spectra are also shown as triangles.
RT PL spectra showing the dependence of the emission energy for different thickness surface quantum wells. In curves (a)–(c), the GaN cap layer is thick enough to confine carriers below the band-gap energy of the AlGaN layer. For curves (d)–(f), the confined energy layer is comparable or greater than the AlGaN band-gap resulting emission near the bulk AlGaN layer emission energy. The emission of the surface quantum well has comparable intensity to the thick GaN layer indicating efficient recombination.
CL spectra taken at for different accelerating voltages with curves (a)–(e) representing 1.0, 1.5, 2.0, 5.0, and , respectively. The spectra have been normalized to the emission from the bulk GaN layer. At lower accelerating voltages, the electrons excite emission near the surface resulting in efficient emission from the surface quantum well. At higher accelerating voltages, the thick interior GaN layer is excited, and emission from the surface quantum well is not as observable.
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