Influences of InGaN/GaN superlattice thickness on the electronic and optical properties of GaN based blue light-emitting diodes grown on Si substrates

Abstract

GaN based light-emitting diodes (LEDs) are subjected to a large polarization-related built-in electric field in c-plane InGaN multiple quantum well (MQW) during growth, which causes the reduction of emission efficiency. To mitigate the electric field, a superlattice layer with a numerous good characteristics, such as a small thickness, a high crystalline quality, is embedded in the epitaxial structure of LED. However, the effect of the superlattice thickness on the properties of LED is not fully understood. In this paper, two blue-LED MQW thin film structures with different thickness values of InGaN/GaN superlattice inserted between n-GaN and MQW, are grown on Si (111) substrates by metal-organic chemical vapor deposition. Electronic and optical properties of the two kinds of samples are investigated. The obtained results are as follows. 1) Comparing two samples, it is observed that more serious reverse-bias leakage current exists in the one with thicker superlattice; 2) Room temperature electroluminescence (EL) measurement shows that the emission spectrum peak between two samples is blue-shifted to different extents as the injection current increases. With superlattice thickness increasing, the extent to which the peak is blue-shifted decreases. Nevertheless, there is no obvious discrepancy in the EL intensity between two samples with different thickness values at 300 K. In addition, the V-shaped pit characteristics including density and size, and the dislocation densities of two samples are studied by high-resolution X-ray diffraction, scanning electron microscope, and transmission electron microscope. The experimental data reveal that the reason for a tremendously different in reverse-bias leakage current between two samples is that there are larger and more V-pits in the superlattice sample with a large thickness. Whereas, V-pits also act as preferential paths for carriers, resulting in the fact that the thicker superlattice suffers more serious reverse-bias leakage current. According to reciprocal space X-ray diffraction intensity around the asymmetrical (105) for GaN measurement, the relaxed degree of InGaN quantum well on GaN is proportional to the superlattice thickness. On the other hand, it is useful for increasing superlattice thickness to reduce a huge stress in c-plane InGaN. Owing to joint effects of above factors, the EL intensities of the superlattice sample with different thickness values are almost identical. Our results show the functions of superlattice thickness in electronic and optical characteristics. What is more, the conclusions obtained in the present research indicate the practical significance for improving the performances of LED.