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The AlGaN/GaN heterostructure in cross section: (a) TEM image along a 〈110〉 projection showing microtwins typical in cubic GaN epitaxy; the  growth direction is indicated by an arrow. (b) Thickness and (c) phase profiles obtained from an electron hologram corresponding to the outlined region in (a).
Potential energy profile obtained by dividing the phase and amplitude profiles from the electron hologram. The 2DEG is characterized by a negative curvature in the potential energy profile with a density of 5.1 × 1011/cm2 calculated using Poisson’s equation. A positive curvature in the AlGaN potential energy profile is also observed, indicating a significant ionized donor density.
Cathodoluminescence spectrum of the cubic AlGaN/GaN heterostructure at a temperature of 4 K. The spectrum shows a broad AlGaN emission at 3.95 eV with a FWHM of 176 meV and three separate GaN near-band-edge peaks.
Simulated conduction band profile compared with the electrostatic potential obtained by electron holography. First three electron states are also shown. The n-doping level of AlGaN is set at 2 × 1018/cm3.
Calculated polarization fields as a function of crystal orientation for a cubic Al0.3Ga0.7N/GaN heterojunction. The polar angle is defined with respect to the (001) plane. The polarization value along the growth direction is maximum for a (111) growth plane, while the polarization component on the growth plane is maximum for a (110) growth plane. For a (001) growth plane, both perpendicular and in-plane polarization are zero.
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