| Abstract [eng] |
III-group nitride alloys are widely used in various nitride-based photonics devices, such as light emitting diodes (LEDs), laser diodes (LDs), etc. One of the most common type of active layers in these devices are InGaN/GaN multiple quantum wells (MQWs). Internal quantum efficiency of InGaN/GaN quantum structures can reach as high as 90% (in blue LEDs). Further improvements of light emission efficiency of these devices are currently limited by various factors, such as compositional fluctuations, internal electric fields and threading dislocations. In case of electric fields, the non-polar InGaN MQWs can be used in order to decrease the QCSE. In case of compositional fluctuations, the Indium containing underlayer can be used to reach more homogeneous In distribution. However, most of InGaN structures still suffer from large dislocation and point defect densities, which affect luminescence properties. Thus, this work was dedicated to get a better understanding of defect types and their effect on luminescence properties in InGaN alloys. The high-resolution Scanning Electron Microscopy (SEM) as well as spatially- and time- resolved Cathodoluminescence (CL) Spectroscopy were employed to investigate the optical and structural properties of three sets of InGaN structures. These three sets contained MQWS and layers of: (i) various polarities; (ii) In content; (iii) different substrates and underlayers. SEM allowed to evaluate defect density at different sample areas, while CL analysis was convenient to get an insight into material properties, such as bandgap, CL intensity distribution, and carrier dynamics with high spatial and temporal resolutions. So, the microscopic characterization of studied samples let us hypothesize that dislocation densities are not the main factor describing the emission efficiency in InGaN alloys. Study showed that CL intensity of InGaN/GaN MQWs is directly related with luminescence decay time. In case of polar structures this can be attributed to local variations of v-pits defects density; is case of non-polar structures luminescence is affected by point defects density fluctuations. Moreover, research results show that InGaN/GaN superlattice used as underlayer can decrease point defects density in QW structures. |
