dc.description.abstract | This dissertation includes the growth and characterization of InGaN/(Al)GaN multiple-quantum-well ultraviolet light-emitting diode (UV LED) structures grown by metalorganic chemical vapor deposition. The main work can be divided into the following four parts.
First, a series of measurements have conducted to investigate the luminescence efficiency behaviors of 400 nm UV LEDs with 5 In0.06Ga0.94N/GaN quantum wells. The emission luminescence efficiency of In0.06Ga0.94N/GaN quantum well near UV LED structures were studied for the purpose to clarify the dominant factor to cause low efficiency of UV LEDs and compared with 470 nm blue LED structures with same periods of quantum well in the active region. It is shown that the behavior of luminescence efficiency at low current density for UV and blue LEDs is different due to the different emission mechanisms in the quantum well. Based on their injection current-dependent characteristics under dc and pulsed operation, it can be concluded that carrier overflow is the dominant factor that affects the quantum efficiency of UV LED before thermal effects take over. The constant external quantum efficiency of the blue or UV LEDs at high current densities rule out the carrier saturation effect unambiguously. It is also experimentally shown that increasing the number of quantum wells from 5 to 10 is found to be effective in the utilization of injected carriers for radiative recombination and hence in improving the luminescence efficiency of the UV LEDs.
Second, the methods of low defect density buffer layer growth, carrier confinement improvement in the quantum wells and p-type AlxGa1-xN cladding layer are introduced for further improving the output power of UV LEDs. Etch pits density is improved by almost one order of magnitude with the use of three SiNx layers as an interlayer in the LED structure. It is also found that adding a little amount of Al into GaN to form a barrier layer does not degrade the crystal quality. Instead, it leads to better carrier confinement in shallower In0.06Ga0.94N quantum well. The luminescence intensity of the UV LED structure with Al0.05Ga0.95N barrier layers is about four-fold higher than that of its counterpart, revealing that the carrier confinement ability and material quality both are significantly improved by introducing a little Al and SiNx interlayers into barrier layers and the n-GaN underlying buffer layer, respectively. The emission intensity of 400 nm UV LED with a graded p-AlxGa1-xN layer is also greatly improved, because of the effective increase in the hole injection effect from p-GaN to the quantum well layers and the increase in the recombination emission rate of electron and hole pairs.
Third, the effects of etching depth and stripe orientation on the structural and optical properties of the GaN epilayer as well as LEDs are investigated. It is found that much better material quality and light output power are obtained when GaN epilayer and LEDs are grown on a 0.9 μm-deep patterned sapphire substrate with stripes along the <1-100>sapphire direction. According to the results of scanning electron microscopy, transmission electron microscopy (TEM), and cathodoluminescence (CL) studies on these samples, the growth modes of GaN epilayer along the <11-20>sapphire and <1-100>sapphire directions are proposed and used to explain the anisotropic material properties between them. Normalized far field emission patterns of the LEDs show that the emitted light are scattered predominantly toward the vertical direction with increasing the groove depth. Their radiation angles, however, are stripe-orientation independent, implying that the light extraction efficiency is almost the same for the LEDs grown on PSSs with stripes along the <11-20>sapphire and <1-100>sapphire directions. Otherwise, the analysis of the internal quantum efficiency (IQE) and light extraction efficiency (LEE) in these two sets of UV LEDs was also investigated. It was also found that the improvement in the output power was mainly due to the enhancement of the LEE and then been changed to the contribution of the IQE by reducing the dislocation density when the LED structure was grown on PSS with stripes along the <11-20>sapphire and <1-100>sapphire directions, respectively. Finally, we can conclude that the maximum contribution of the LEE was about 44.9 % obtained using the 0.9 μm-deep stripe-PSS along either the <11-20>sapphire or <1-100>sapphire direction. However, an improvement of the IQE by as much as 2.785 times higher can be obtained for a LED structure grown on 0.9 μm-deep stripe-PSS along the <1-100>sapphire direction than its counterpart. It is therefore concluded that the superior performance of LED on the <1-100>sapphire stripe is attributed to its lower defect density.
Finally, sapphire substrates with stripe SiON patterns which has adjustable refractive index to match sapphire substrate have been shown effective in improving the output power of light-emitting diodes. The output power of 400 nm UV LEDs grown on the stripe-SIONPSS is increased as much as 60 % compared to the LEDs grown on planar one. In the other work, the estimated values of the IQE and LEE in these LED grown on stripe-SIONPSS and planar substrate are obtained by temperature-dependent PL measurement. The IQE and LEE in the LED grown on stripe-SIONPSS are 28.6 % and 37.62 %, respectively. The increments of the IQE and LEE are about 26.55 % and 26.9 %, respectively, comparing with that of its counterpart. It shows that the power output enhancement of our devices is due not only to the reduction of dislocation density but also to the light extraction contributed by the geometric shape of the SiON stripe patterns. These results indicate that SiON stripe pattern on sapphire substrate is a promising approach to achieve high efficiency LEDs. | en_US |