Abstract
We present an optical study on three Al~x~ Ga~1โx~ N/GaN high electron mobility transistor structures, which were grown by metalorganic chemical vapor deposition (MOCVD) on sapphire substrates, with different Al compositions using contactless electroreflectance (CER), piezoreflectance (PzR) and surface photovoltage spectroscopy (SPS). The Al composition x can be determined from the band gap energy of Al~x~ Ga~1โx~ N layer observed in CER and PzR spectra. The determination of the builtโin electric field strength in the barrier layer (F) has been done by the analysis of FranzโKeldysh oscillations (FKOs) above the Al~x~ Ga ~1โx~ N band gap in CER spectra. A broad feature appeared at energies above the band gap of GaN was observed in each CER, PzR and SPS spectrum. We believe that it is related to the twoโdimensional electron gas (2DEG). The energy of the broad 2DEG minimum roughly corresponds to the Fermi energy plus the band gap. The Hallโeffect measurements have also been carried out in van der Pauw geometry at room temperature to reveal the sheet carrier density (n ~s~). For the sample with lowest alloy composition (x = 0.07) the analysis of the FKOs yields F = 316 kV/cm at room temperature. When the temperature was cooled down to 50 K the electric field strength decreases to 188 kV/cm. This result can be explained by the change of piezoelectric polarization. (ยฉ 2006 WILEYโVCH Verlag GmbH & Co. KGaA, Weinheim)