Investigation Of Carrier Transport And Recombination In InGaN/GaN Multiple Quantum Wells

GaN-based direct wide band-gap semiconductor materials are being widely used in fabricating light-emitting diodes (LED), laser diodes (LD) and other optoelectronic devices for their impressive photoelectric properties and stable chemical performance. Currently, GaN-based blue/green LEDs have been widely used in large screen display, landscape lighting and other fields. GaN-based blue/ultraviolet LED based high brightness white LED has become the3’Th generation solid state light source. Meanwhile, GaN-based blue/ultraviolet LD also has been widely used in high density optical storage, laser medical and science measurements. As the core part of GaN-based optoelectronic devices, InGaN/GaN multiple quantum wells (MQWs) have been attracted much attention in the field of semiconductor research. Many studies have been made on the growth and physical properties of InGaN/GaN MQWs. At present, the material quality and luminescence efficiency of InGaN/GaN MQWs have been improved greatly; however, the studies of the physical properties of InGaN/GaN MQWs lag behind. The physical mechanism of high luminescence efficiency is still not clearly understood.To reveal the mechanism of high luminescence efficiency of InGaN/GaN MQWs, this dissertation systematically focused on the material growth, structure characterization and optical measurement of InGaN/GaN MQWs. Physical processed of the carrier transport and recombination in InGaN/GaN MQWs are studied in details. The main results are as follows:1. InGaN/GaN MQWs with different barrier thicknesses were grown and characterized by high resolution X-ray diffraction (XRD) and cross-sectional transmission electron microscopy (TEM). The XRD and TEM measurement showed that the interfaces between wells and barriers were abrupt and the entire MQWs region had good periodicity for all three samples.2. The temperature-dependent photoluminescence measurements of InGaN/GaN MQWs with different barrier thicknesses showed that the PL peak energy for all three samples were S shaped with increasing temperature and the temperature of the turning point from blue shift to red shift increased with the barrier thickness increasing. This abnormal emission behavior was owing to the change in the carrier transport and dynamics with increasing temperature due to the compositional inhomogeneity and carrier localization in InGaN/GaN MQWs. Moreover, the localization potential formed by In clusters was deeper for the thicker barrier samples.3. The temperature dependent PL properties of the three samples with different barrier thicknesses were discussed. It was found that there were two kinds of nonradiative recombination processes accounting for the thermal quenching of photoluminescence. The first nonradiative recombination process was induced by thermal emission of the carriers out of the deeper potential. And the other one could be related with the escaping of carriers captured at the localization minima. Meanwhile, it is indicated that the localization potential formed by In clusters was deeper for the thicker barrier samples. This situation was attributed to the redistribution of In-rich clusters during the growth of barrier layers, i.e., clusters with lower In content aggregated into clusters with higher In contents.4. By optimizing the growth parameters of the ITO films, the current spreading layer with low resistivity and high transmittance was obtained, and the EL devices were also fabricated. By comparing the EL properties of the three samples with different barrier thicknesses, it was found that the integrated EL intensity was larger for the samples with thinner barrier with the same injection current. This was mainly attributed to the efficient hole tunneling through barriers and the consequent uniform distribution of carriers in the InGaN/GaN MQWs. Moreover, compared with the samples with thicker barrier, the density of defects was lower for the thinner barrier samples. Therefore, the samples with thinner barrier could confine more carriers and prevented the carriers from being trapped by the nonradiative recombination centers, and finally enhanced the efficiency of radiative recombination.5. According to the PL and EL measurements, the thinner barrier could improve the carrier transport and made the carrier distribution more uniform in InGaN/GaN MQWs. This is beneficial to the devices operating on large current density. Moreover, the carrier recombination process was mainly related to the In-rich clusters and the dislocations around them. The nonradiative recombination centers formed by dislocations resulted in the quenching of the luminescence. The In-rich cluster could confine the carriers from being trapped by the nonradiative recombination centers, and then enhanced the efficiency of radiative recombination.