Piezoelectric polarization-induced two dimensional electron gases in AlGaN/GaN heteroepitaxial structures: Application for micro-pressure sensors

The wurtzite group-III nitrides exhibit piezoelectric polarization along their c-axis. Differential piezoelectric and spontaneous polarizations in strained AlGaN/GaN heterostructure grown on [0 0 0 1] sapphire substrates induce two-dimensional electron gas (2DEG) at the AlGaN/GaN hetero-interface. By using a simple two-terminal device in a bending configuration, we demonstrate a linear dependence of the 2DEG channel conductance with applied bending strain. A detailed analysis of the elastic strain distribution in the multilayer structure indicates that the applied strain dependence of the conductance is directly proportional to the electron mobility of 2DEG. Thus, the bending test provides a new technique for measuring the electron mobility in this structure. For a mesa-structure device with a partially relaxed applied strain in the top AlGaN layer, the theory further predicts a reversal in the applied strain dependence of the channel conductance for strain relaxation greater than 15% and this prediction is confirmed by the experiment. Finally, the feasibility of fabricating a micro-pressure sensor using a 150 μm diameter thin flexible AlGaN/GaN circular membrane with an interdigitated-fingers device on a (1 1 1) Si substrate is demonstrated. The measured pressure sensitivity is 0.07 mS/bar.