Abstract
A detailed photoluminescence (PL), timeโresolved photoluminescence (TRPL), and photoreflectance (PR) analysis of AlGaN/GaN heterostructures grown on different substrates: sapphire and silicon carbide (SiC) is presented in this paper. The properties of high electron mobility transistors (HEMTs) based on AlGaN/GaN structures are strongly dependent on the quality of the Al__~x~__Ga~1โx~N top layer. We have examined a number of samples, grown on sapphire, in which the thicknesses of the Al~0.3~Ga~0.7~N layers vary from 18 to 75 nm. Room temperature PL under pulsed 266ย nm excitation allowed for determination of the Al content in the examined thin AlGaN layers. Timeโresolved PL measured at 1.6ย K showed huge difference in the emission dynamics for different Al~0.3~Ga~0.7~N layer thicknesses. We observed an enormous increase of the emission decay time above the critical thickness of the AlGaN layer. PR spectra (associated with the GaN main layer) measured on AlGaN/GaN systems are discussed in terms of the thickness of the capping layer. The PR modulated by a high power 266ย nm pulsed laser measured on a transistor structure exhibited an additional feature placed between signals related to the GaN and AlGaN layers, respectively. Such a transition is possible to monitor only for the structures of the best quality and is accordingly observable only on samples grown on SiC. (ยฉ 2005 WILEYโVCH Verlag GmbH & Co. KGaA, Weinheim)