Piezoresistivity of Al_xGa_1-xN layers and Al_xGa_1-xN/GaN heterostructures

The piezoresistivity of wurtzite Al_xGa_1-xN layers with different Al contents and electron concentrations grown by plasma induced molecular beam epitaxy is investigated. A strong increase of the piezoresistivity with increasing Al content and decreasing carrier density is observed. The corresponding piezoresistive gauge factor is negative and its absolute value increases from 3.5 to 25.8 if the Al concentration is increased from x=0 to 0.35. The dependence of the piezoresistive effect on the free electron concentration in the epitaxial layers is calculated by a model which compares the strain induced piezoelectric field to an externally applied gate voltage in field effect transistors. In addition, the strain dependence of the channel conductivity in pseudomorphic, undoped AlGaN/GaN high electron mobility transistors containing a polarization induced two-dimensional electron gas close to the heterointerface, is measured. A remarkably high gauge factor of -85 is observed for these devices.