Surface structure and polarization effects in GaN thin films as studied by Electric Force Microscopy

Stress induced piezoelectric fields in III-nitride semiconductors induce charge at the surface that is comparable to the free carrier concentration intended by doping. These fields, coupled with mixed Ga-polarity and N-polarity within the film, are detrimental to device performance due to an additional scattering caused by a reversal of the electric field normal to the surface. Consequently, investigation of the polarization effects is important and timely as nitride semiconductor devices are being contemplated for system applications. By imaging the surface potential by electric force microscopy (EFM), both a qualitative and a quantitative measure of polarization induced charge distribution across the surface could be obtained. In this vein, we undertook an EFM study in a series of samples. EFM images of as-grown and progressively etched samples not only led to the identification and imaging of inversion domains, but also showed the evolution of different polarities.