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Schematic cross-section of studied structures. The layer thicknesses and compositions are specified in Table I .
Polarization-induced p-type doping densities as a function of the distance at which the material is graded from Al0.3Ga0.7N to GaN or GaN to In0.15Ga0.85N (with Ga-face growth). If the direction of the grading changes or the growth is performed N-face, the sign of the polarization doping changes and results in n-type doping.
Band diagram of structure (a) A, (b) B, (c) C, and (d) D at zero bias voltage (left panel) and at the operating point where the maximum IQE takes place (right panel). Note that in (b) and (d) the n- and p-type regions are on different sides than in (a) and (c). Ec is the conduction band edge, EF is the equilibrium Fermi level, Ev is the valence band edge, EFn is the conduction band quasi-Fermi level, and EFp is the valence band quasi-Fermi level. The voltage and current density are written in the figures, and the 5 nm thick AR is marked with a gray background.
Band diagram of structure (a) E (b) F, and (c) G of Table I at zero bias voltage (left panel) and at the bias voltage where the maximum IQE takes place (right panel). Inset in (a) shows the region with the grading from GaN to InGaN, the InGaN AR, and the grading back from InGaN to GaN with no impurities.
(a) Effective bandgaps (i.e., energy differences between the ground states of electrons and holes) in the structures with QW ARs as a function of current density. The dependence of the effective bandgap on the input current reproduces a typical QCSE for III-N QW devices. 26 (b) Overlap factor in GaN and InGaN QWs as a function of the electric field in the QW. The inset in (b) shows the electron and hole wavefunctions together with the band edges in structure C at its maximum IQE.
IQE of (a) structures A-D and (b) structures E-G of Table I . Polarization-doped structures B and E have a significantly larger injection current corresponding to their peak IQE than their impurity-doped counterparts.
Layer thicknesses and compositions of the studied structures (refer to Fig. 1 ): A and B are polarization-doped Al0.3Ga0.7N/GaN structures, C and D are impurity-doped Al0.3Ga0.7N/GaN structures, E is a partly polarization-doped GaN/In0.15Ga0.85N structure, and F and G are impurity-doped GaN/In0.15Ga0.85N structures. The indicated layer thicknesses are shared by structures A-D and E-G. Note that structures B and E do not contain a well-defined QW since the necessary carrier confinement is provided by the grading.
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