Investigations of GaN surface quantum well in AlGaN∕GaN transistor heterostructures by contactless electroreflectance spectroscopy

Al Ga N ∕ Ga N transistor heterostructures, caped by ∼2nm GaN layer, were investigated by contactless electroreflectance (CER) spectroscopy at room temperature. Below the AlGaN-related transition CER spectra have shown a clear resonance at the energy of ∼3.7eV, i.e., at much higher energy than the GaN band gap energy. The observed feature has been connected with the optical transition within the GaN cap layer. It was concluded that a surface GaN quantum well has been created by the deposition of nominally undoped (or Si-doped) GaN cap layer on AlGaN∕GaN transistor heterostructures.