Femtosecond pump-probe characterization of high-pressure grown Al x Ga 1-x N single crystals

Abstract

We report our experimental studies on the time-resolved dynamics of conduction electrons in high quality Al x Ga 1-x N single crystals. The Al x Ga 1-x N synthesis with the Al content of up to 90% was performed using a solution growth technique in a high nitrogen pressure system, maintained for several days under the high (up to 15 kBar) gas pressure and at very high (up to 1800°C) temperatures. The resulting crystals were colorless (fully transparent to visible light) platelets up to ~1 mm 2 in size, and exhibited a hexagonal Würtzite structure. Our optical measurements were performed at room temperature, using a two-color (ultraviolet and near-infrared), femtosecond, pump-probe spectroscopy, by invoking a two-photon, volume absorption and scanning the normalized transient differential transmitivitty ( T/T) signal of the probe beam. The observed T/T signal was a superposition of a femtosecond, two-photon-absorption (TPA) correlation transient and a conventional optical pump-probe photoresponse signal with a rise time of ~1 ps and decay of ~75 ps. We have found that the TPA process was quite efficient with the absorption coefficient 0.337 cm/GW at 380 nm, and enabled us to directly determine that the optical bandgap of Al x Ga 1-x N (x = 0.86) was 5.81 +/-0.01 eV. The latter value is in the good agreement with the one deduced from the x-ray diffraction measurements.