Luminescence spectra and efficiency of GaN-based quantum-well heterostructure light emitting diodes:: Current and voltage dependence

Luminescence spectra and quantum yield in light emitting diodes (LEDs) based on InGaN/AlGaN/GaN heterostructures with multiple quantum wells (MQWs) were studied in the range of currents J = 10(-6)-10(-1) A. Minor spread in the quantum yield at operating currents (+/- 15% at J approximate to 10 mA) was observed in these LEDs, which were fabricated by Hewlett-Packard. The spread is due to differences in the current and voltage dependences of the diode emission intensity, caused by differences in the charged center distribution across the space-charge region (SCR) of the structures and in the role of the tunnel current component at low voltages. In the diodes with a thin (less than or similar to 120 nm) SCR, a tunnel emission band was observed for J less than or similar to 100 muA; the peak energy of this band (h) over bar omega (max) = 1.92-2.05 eV corresponds to the voltage applied. At low currents (J = 0.05-0.5 mA), the spectral position of the main peak (h) over bar omega (max) = 2.35-2.36 eV is independent of the voltage and is determined by the radiative transitions between the localized states. At J > 1 mA, this band shifts with the current (h omega (max) = 2.36-2.52 eV). Its shape corresponds to the model for the occupation of states in the two-dimensional energy band tails, which are caused by the microscopic potential fluctuations. The four parameters in this model are related to the calculated energy band diagram of the MQW structure. (C) 2001 MAIK "Nauka /Interperiodica".