Mechanisms of enhanced piezoelectric response in nanoporous GaN revealed by quantitative mapping

Electrochemical etching provides a route to introduce nanoscale porosity to GaN and enhance its electromechanical properties. It remains unclear whether this enhancement results from electrostatic effects such as carrier depletion or mechanical effects such as softening. Nonetheless, neither the porosity-dependent piezoresponse nor the influence of porosity on electronic properties have been systematically studied. Herein, we mapped out the conductance and piezoresponse of electrochemically etched GaN with porosity spanning 0%–60% using piezoresponse force microscopy and conductive AFM. It was found that the piezoresponse increased progressively with porosity, first sharply, then moderately, reaching about 7–8 pm/V, compared to the accepted value of 2–3 pm/V for bulk GaN. We attribute this to two effects: an initial establishment of a significant depletion region, followed by mechanical softening of the porous structure, with the latter supported by finite element simulations. Three doping levels were examined; results demonstrated that doping strongly affected etching outcomes but not the piezoresponse magnitude. Since piezoelectricity in semiconductors is fundamentally constrained by free carriers, we studied the local current response of the same samples. The tip-sample conductance was dominated by the pore edges, most likely due to geometrical factors such as sharp edges or enhanced contact area. The I–V characteristics exhibited Schottky diode behavior, indicating that operation under reverse bias conditions is preferable for useful piezoelectric applications of porous GaN. By systematically correlating porosity, piezoresponse and conductance, our work sets a quantitative framework for porous GaN piezoelectric devices.