Theoretical Simulation Of Polarization Effect In GaN LED

The LED (Light Emitting Diode) has come into use in various fields due to its advantages including long lifetime, high integration and low driving voltage etc., and it has the potential to become the new generation of lighting power.For the wide band gap GaN based materials, their luminescence spectrum covers the range from deep ultraviolet to infrared, which makes them have vaster potential of development and broader prospects of application than other semiconductor materials.GaN based LED was born in 1994 followed by the sharp development of GaN based materials. The GaN LED has very low quantum efficiency at high current density injection, though it has been produced in quantity at factories. The probable reason is that the effects of spontaneous polarization and piezoelectric polarization, both of which are the nature properties ofⅢ-nitride materials, bend the energy band of multiple quantum wells, leading to the so called Quantum-Confined Stark effect. Subsequently, in the LED the electron leak current gets larger as the driving current grows, which lowers the quantum efficiency and does great damage to LED at high current density.In my thesis, after modeling the LED device and setting the corresponding parameters, I systematically conducted the numerical simulation and completely analyzed the results, such as internal quantum efficiency, spontaneous emission rate and light output power, then I changed and improved the conventional LED device by optimizing the structure and materials of the LED, and finally we solved the problem of quantum efficiency droop at high carrier injection and enhanced the output power effectively.The main contents are as follows:Chapter 1, the development of the three generations of semiconductors is introduced briefly, as well as the application of semiconductors in different areas, my research background and the feasibility of my job.Chapter 2, the related physical models and basic theories are discussed in detail, including the kp perturbation method, energy band models of zinc-blende and wurtzite models deduced from kp method, quantum well model, and mechanism of the polarization effect in wurtzite structure ofⅢ-nitride materials.Chapter 3, the essential parameters and the detailed epitaxial structure of GaN LED are illustrated. Subsequently, from the simulation results, the data of energy band, carrier density distribution, spontaneous emission rate distribution, quantum efficiency and output power are selected and analyzed step by step. At last, a series of LED structures with different polarization charge are simulated and the impacts of polarization on the whole LED device are concluded.Chapter 4, in the former part of this chapter, the latest solutions to the quantum efficiency droop of GaN LED raised by other research groups are listed and discussed. In the latter part, our research on GaN LED droop phenomenon especially some novel methods of lifting the quantum efficiency are represented, including 1) utilizing the Al component graded electron blocking layer (EBL) to reduce polarization effect between the EBL and space layer, and vanishing the 2 dimension electron gas at the interface between that two layers; 2) designing a graded height barrier MQW structure using InGaN materials with different In component, increasing the hole carrier injection and spontaneous emission rate distribution; 3) adopting the asymmetrical quantum barriers to improve the effective height of quantum barriers, minimizing the electron leak current and settling the problem of efficiency droop at large current injection.Chapter 5, the entire research job and innovations are concluded and some existing questions are listed, also the prospects of LED theoretical simulation are forecasted.