The implications of spontaneous polarization effects for carrier transport measurements in GaN

In wurtzite-phase GaN, AIN and InN, the dimensions of the crystallographic unit cell are distorted from the ideal c:a ratio of root 8/3 This produces a net dipole moment across the cell, with a resultant internal electric field in excess of 1 MV cm(-1), and corresponding polarization charges of +/-3-8 x 10(-6) C cm(-2) on the two surfaces of the epitaxial layer. The effect of this field on the carrier distribution within a film of doped GaN is considered, and shown to produce accumulation and inversion layers of free carriers at opposite surfaces. Theoretical expressions are derived for the effective carrier density and mobility of such films obtained from Hall measurements, and compared with characteristic experimental results. Qualitatively consistent behaviour is observed in some, but not all, samples, but quantitative agreement is generally lacking, and possible explanations for this are considered.