Electric field distribution in GaN/AlGaN/GaN heterostructures with two-dimensional electron and hole gas

Ga-face GaN/AlGaN/GaN heterostructures with different cap thicknesses are investigated by electroreflectance spectroscopy (ER). The voltage dependent electric field strengths of the barrier and cap layers are determined. The AlGaN electric field amounts of up to -2.6 MV/cm, whereas the GaN electric field is always below 700 kV/cm. The two electric fields have opposite signs. Characteristic features in the voltage maps of the ER spectra are assigned to the formation/depletion of a two-dimensional electron gas below and a two-dimensional hole gas above the AlGaN barrier. Between -6.5 and 0 V, both carrier gases coexist.