In GaN/AlGaN heterostructures, a spatially-dependent polarization, P(r), arises from two sources: (1) the spontaneous polarization, P s , due to the difference in electronegativities between GaN/AlGaN that leads to the formation of molecular dipole fields , and (2) the piezoelectric polarization, P p , due to the lattice mismatch at the epitaxially grown GaN/AlGaN interface that induces strain during thermal expansion. In both cases, a non-zero charge density emerges at the GaN/AlGaN interface due to the discontinuity in P = P s + P p . For GaN/AlGaN crystalline systems, the spontaneous polarization can be written P s = P s ẑ, where ẑ denotes a unit vector in the [0001]-direction. From classical electrostatics, it follows that the interfacial charge due to spontaneous polarization at the GaN/Al x Ga 1-x N interface is given by σ s = -∇ • P = (P GaN s -P Al x Ga 1-x N s ) cos ϕ, where ϕ is the interfacial angle with respect to the [0001]-direction (a typographical error in the expression for σ s occurs in Ref. ). Due to this angular dependence with respect to the crystallographic axes, the interfaces in the hexagonal cross-section are all nonpolar, whereas the triangular crosssection has one polar and two semi-polar faces, as shown in Fig. . Using Vegard's law, the spontaneous polarization for Al x Ga 1-x N satisfies P Al . The specific values used for Al 0.3 Ga 0.7 N (x = 0.3) give |σ s | = 0.0156 C/m 2 for the polar interface (i.e., ϕ = 0), where a positive/negative interfacial charge occurs in the Ga-/N-face orientation, respectively. Conversely, the charge density on the two semi-polar interfaces at ϕ = 2π/3 and ϕ = 4π/3 yields |σ s /2| = 0.0078 C/m 2 , where the positive/negative interfacial charge occurs on the N-/Ga-face orientation, respectively.