Photoluminescence from metastable states in long-range ordered (Al0.5Ga0.5)(0.51)In0.49P

We have investigated photoluminescence of long-range ordered (Al0.5Ga0.5)(0.51)In0.49P grown by metalorganic vapor-phase epitaxy on GaAs(001) and GaAs(115)A. Photoluminescence spectra were measured as functions of temperature and delay time. The fundamental edge luminescence from (Al0.5Ga0.5)(0.51)In0.49P on GaAs(001) shifts to the lower-energy side about 120 meV at 11 K due to the long-range ordering, relative to the luminescence from the Gamma conduction-band valley in the disordered alloy. The Gamma luminescence of (Al0.5Ga0.5)(0.51)In0.49P appears at lower energy than the indirect luminescence from the X valley. These observations give clear evidence that the ordering causes the band gap to change from indirect to direct. In the temperature dependence of the photoluminescence (PL) peak energy, there are three regions divided by a local minimum and a local maximum. Below the temperature showing the local minimum, the temperature dependence of the PL intensity is characteristic of the localization of photoexcited carriers. The origin of the localization is the band-gap fluctuation. The temperature dependence of the PL intensity shows thermal activation between the temperatures showing the local minimum and the local maximum in the PL energy. The activation energy is proportional to the energy shift at the local minimum. In time-resolved PL measurements at low temperatures, we observed a delayed emission due to the electrons trapped at metastable states. The delayed component was only observed below 15 K and the delay time varies with temperature.