The Influence of Superlattice on the Internal Quantum Efficiency of LED Structures with InGaN / GaN Quantum Wells

Abstract

epitaxy. The structures were grown on the planar and profiled sapphire substrates with the (0001) orientation. Two groups of samples were investigated. The LED-structures of the group QB were grown on the planar sapphire substrates and were composed of a low-temperature nucleation layer, a 2 µm-thick buffer i-GaN layer, a 2 µm-thick n-GaN layer, an active region consisting of 5 InGaN quantum wells with the thickness 2.5–3 nm and GaN barrier layers with the thickness 3–15 nm (an InN content in the InхGa1–хN solid solution was 10–15%), and a 140 nm-thick p-GaN layer. In addition, in the buffer region of some samples of the group QB, a short-period InGaN / GaN superlattice with the 1 nmthick layers was grown. The samples of the LQB group grown on the planar and profiled sapphire substrates consisted of a low-temperature nucleation layer, a 4 µm-thick buffer i-GaN layer, a 2 µm-thick n-GaN layer, an active region consisting of 10 InGaN quantum wells with the thickness 3 nm and GaN barrier layers with the thickness 15 nm, and a 140 nm-thick p-GaN layer. In the samples of the LQB group, the thickness of the last barrier GaN layer, located between the active region and the p-GaN layer, varied in the range 5–15 nm. The grown samples were studied by the methods of electro- and photoluminescence. For the electroluminescence (EL) measurements, indium contacts were fused into the samples. The measurements were carried out at room temperature at low currents (up to 5mA) in a pulsed mode. The external quantum efficiency (EQE) was estimated from the slope of the photodetector current–current through the sample dependence. Photoluminescence (PL) spectra were measured in the temperature range 10–300 K using an optical cryostat. A pulsed YAG-laser with an average power of 35 mW (1 kHz, pulse duration – 10 ns, wavelength – 355 nm) was used as an excitation source. To determine the internal quantum efficiency (IQE), an integrated PL intensity was measured as a function of the excitation radiation intensity [3, 4].