Since nitrides in wurtzite structure exhibits large spontaneous and piezoelectric polarization it is relatively easy to obtain 2D-electron gas in GaN/AlGaN structures. Electric field can be controlled by external bias and optical pumping, so it is possible to investigate quantum wells (QWs) interacting with 2D-gas in controlled, significant electric field. The investigated structure was grown by MOCVD on sapphire substrate. On the thick Si doped GaN layer there were grown several intentionally

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... ndoped layers. First 150 nm GaN layer, then AlGaN barrier (with 15% of Al) and InGaN QW (with 10% of In). The InGaN QW was covered by the AlGaN layer (with 5% of Al) and finally with the AlGaN barrier (15% of Al). The whole structure was covered with very thin GaN cap. Semitransparent Schottky contact made by evaporation of gold gave possibility of external electric field application. Diodes with QWs of width from 2 nm to 3 nm were obtained. The width of QWs in different parts of the wafer was controlled by transmission electron microscopy. The structure was investigated by electro-reflectance (ER), photo-luminescence (PL) and photo-current (PC) measurements at room temperature. PL was measured also at 4 K. The energy of photoluminescence peak of the InGaN QW changed with the width of QW from 2.93 to 3.15 eV, at room temperature. The lines were about 80 meV broad. PL energy of the InGaN QW changed clearly also with applied bias. For one of the diodes, at 4 K, it shifted fast from 2.97 eV (U = +0.8 V) to 3.03 eV (U = -0.5 V). Stronger reversed bias caused only small shift. Electro-reflectance signal from AlGaN cavity was visible at energy above 3.5 eV. This line changed its phase at bias -3 V. The effect was probably due to change of direction of electric field in this layer. Signal from the AlGaN barriers showed clear Franz-Keldysh oscillations. They period increased with increasing reverse bias (electric field). Franz-Keldysh oscillations from undoped GaN layer were observed only for reverse bias stronger than -6 V. It means that at weaker bias, electric field in GaN was screened by 2D electron gas present at GaN/AlGaN interface. InGaN QW was visible in electro-reflectance spectrum at energy about 3.1 eV. The position of the line slightly shifted into higher energies with external bias applied in reverse direction. Current generated by illumination (in PC measurements) had different signs depending on photon energy. For example it was positive at hν = 3.2 eV (QW excitation), negative at 3.42eV (GaN), positive at 3.5 eV (intermediate AlGaN layer) and back negative at 3.7 eV (AlGaN barrier). Changes of QW energy with bias were observed. They were most significant (0.1 eV) in the widest (3 nm) QW. These experiments gave as possibility of detailed analysis of high (up to 1 MV/cm) electric field influence on the quantum well properties.