Measurement of induced surface charges, contact potentials, and surface states in GaN by electric force microscopy

We have studied molecular beam epitaxy grown GaN films of both polarities u sing electric force microscopy to detect sub 1 mu m regions of charge densi ty variations associated with GaN extended defects. The large piezoelectric coefficients of GaN together with strain introduced by crystalline imperfe ctions produce variations in piezoelectrically induced electric fields arou nd these defects. The consequent spatial rearrangement of charges can be de tected by electrostatic force microscopy and was found to be on the order o f the characteristic Debye length for GaN at our dopant concentration. The electric force microscope signal was also found to be a linear function of the contact potential between the metal coating on the tip and GaN. Electro static analysis yielded a surface state density of 9.4 +/- 0.5 x 10(10) cm( -2) at an energy of 30 mV above the valence band indicating that the GaN su rface is unpinned in this case. (C) 1999 American Institute of Physics. [S0 003-6951(99)01223-1].