Polarization Engineering in Gallium-Nitride-based Light-Emitting Diodes

In this dissertation, we present Ga1-xInxN LEDs with Ga1--yInyN QBs (x > y), rather than conventional GaN QBs, to reduce the polarization mismatch between the Ga1-xInxN QWs and the Ga1--yInyN QBs. The electroluminescence spectra of the GaInN/GaInN LED show much less blueshift with increasing injection current as compared to the GaInN/GaN reference LED. The forward voltage is reduced in the GaInN/GaInN LED due to reduced energy barriers for electron and hole transport in the active region. In addition, the pit density in the GaInN/GaInN MQW is much reduced and correlated to a reduced reverse leakage current in the GaInN/GaInN LED. Furthermore, the light-output power and external quantum efficiency of the GaInN/GaInN LED are much improved over the GaInN/GaN reference LED. These results are attributed to the reduction of polarization mismatch and the reduction of lattice mismatch in the MQW active region by using GaInN QBs.;In contrast to GaAs- and InP-based compound semiconductors for which a stable growth technology has been developed by using homoepitaxial buffer layers grown on (001) lattice-matched bulk substrates, GaN-based materials have been developed on heteroepitaxial c-plane (0001) sapphire substrates. Also, typical ternary alloy materials such as GaInN and AlGaN in GaInN-based LEDs have a rather large lattice mismatch to GaN. Threading dislocations (TDs) are caused by the mismatch of lattice constant and thermal expansion coefficient between the epitaxial layers and substrates. In GaInNbased LED structures, TD densities in the mid 108 cm-2 range are common, unless specialized techniques, such as lateral epitaxial overgrowth, and thick GaN growth by hydride vapor-phase epitaxy, are employed. Therefore, one of the challenges for GaInNbased LEDs is the control of lattice-mismatch-induced dislocations as well as their impact on the performance of the devices. Despite the impressive progress in LED performance, further understanding of the relation between the TD density and LED performance is necessary for continued development of LED technology. It is well known that the TD density influences the internal quantum efficiency of ultraviolet (UV) LEDs and laser diodes. The performance of UV LEDs and LDs is very sensitive to the TD density in the epitaxial structures. Although blue LEDs on sapphire substrates show a relatively low sensitivity to TDs, it is still worthwhile to investigate the effect of TD density on LED characteristics especially for high-power and high-efficiency LEDs.;In this dissertation, we investigate how strain and threading dislocation density in templates affects the efficiency of GaInN-based LEDs that are pseudomorphically grown on the templates. The templates consist of GaN layers grown on either an AlN or a GaN buffer layer. The template based on the AlN buffer shows higher compressive strain and lower threading dislocation density compared to the template based on the GaN buffer. We simulate the LED structures grown pseudomorphically on the templates in order to investigate the effect of strain in the template on the polarization field and efficiency for the LED structures. We show that the strain in the templates significantly affects the polarization field in the LED structure. Higher compressive strain in the template increases the polarization sheet charge density at the interface between the GaInN quantum well and the GaN quantum barrier grown on the template. However, the higher compressive strain also decreases the polarization sheet charge density at the interface between GaN spacer and AlGaN electron blocking layer. Therefore, the effects of the piezoelectric polarization fields in the multiple quantum well and electron blocking layer due to strain in the templates are counteractive with respect to the electron leakage. As evidence of higher piezoelectric polarization field in QW, a larger spectral shift of the emission band is observed for the LEDs grown on the template having higher compressive strain, when increasing the injection current. Finally, we find that the LEDs grown on the template based on the AlN buffers having high compressive strain shows enhanced light-output power and external quantum efficiency. This enhancement can be attributed to the effect of reduced threading dislocation density. (Abstract shortened by UMI.)...