The implications of spontaneous polarization effects for carrier transportmeasurements 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 hel d 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 con sidered, and shown to produce accumulation and inversion layers of free car riers at opposite surfaces. Theoretical expressions are derived for the eff ective carrier density and mobility of such films obtained from Hall measur ements, and compared with characteristic experimental results. Qualitativel y consistent behaviour is observed in some, but not all, samples, but quant itative agreement is generally lacking, and possible explanations for this are considered.