The Tail of Localized States in the Band Gap of the Quantum Well in the In[sub 0.2]Ga[sub 0.8]N∕GaN System and Its Effect on the Laser-Excited Photoluminescence Spectrum

Abstract

It is established experimentally that the peak in the photoluminescence spectrum of the In0.2Ga0.8N/GaN heterostructure with a quantum well shifts by ∼150 meV as the power density of a nitrogen laser used for excitation is increased from 10 to 1000 kW/cm2. The large blue shift is interpreted as a manifestation of the tail of the density of localized states in the band gap of the quantum well. It is shown that, in the model of the ideal quantum well in which there is no tail of localized gap states, it is impossible to account for the large blue shift. A phenomenological expression is suggested for the density of states. The expression involves the adjustable parameter, namely, the Urbach energy that characterizes the tails of the density of localized states. By this means, both the long-wavelength edge of the luminescence spectrum and the large blue shift can be described. The quasi-Fermi level of photoelectrons depends on the pump intensity and is selected to fit each experimental curve. A qualitative agreement between the theoretical and experimental spectra is obtained, demonstrating that the model is adequate. From the condition for electrical neutrality, the surface carrier concentration is determined. It is found to be several orders of magnitude higher than the pyroelectric concentration in narrow quantum wells.