Contactless and soft contact electroreflectance of AlGaN/GaN and GaN/AlGaN/GaN field effect transistor heterostructures

Abstract

AlGaN/GaN field effect transistor (FET) structures with a two dimensional electron gas (2DEG) at the interface were investigated by electromodulation spectroscopy [contactless electroreflectance (CER), and soft contact electroreflectance (SCER)]. It has been clearly shown that in contactless mode of electroreflectance (i.e., CER spectroscopy) the 2DEG screens the band bending modulation in GaN layer from the external electromodulation which is generated by the capacitor‐like system. In a result only the optical transition in AlGaN layer and no signal, which could be associated with the optical transition in GaN buffer layer, is observed in CER spectra. This screening is not present in soft contact mode of electroreflectance (i.e., SCER spectroscopy). It has been clearly shown that the soft contact mode of electrorefletance works very well for AlGaN/GaN FET structures with a 2 nm thick GaN cap layer. In this mode of electroreflectance measurements the signal from GaN buffer layer was clearly observed whereas in CER spectra this signal was not detected. In CER spectra only the AlGaN‐related transition and the GaN cap‐related transition were observed. The last transition was observed at the energy of ∼3.65 eV because of quantum confinement in GaN cap layer, i.e., a surface quantum well in fact. (© 2010 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)