Kai Shek “Clayton” Qwah Graduate Student Researcher, Materials Dept., Speck Group
Hole transport through unipolar p-type InGaN and AlGaN heterostructures
In the field of semiconductor devices, the III-nitride material system, which is mainly made up of Indium Nitride (InN), Gallium Nitride (GaN) and Aluminum Nitride (AlN), has seen a great deal of attention over the past decade. Despite the maturity of this field of research, the physics that govern the behavior of these devices is still poorly understood. For all the devices mentioned, there exist regions called heterojunctions, which can be defined as the interface between two materials of different band gaps. In the case of LEDs, these heterojunctions are typically at the interface of alloy regions, which are the Quantum Well (QWs) layer and the Electron-Blocking Layer (EBL). The QW, comprised of InGaN, is the active region of the LED where electrons and holes recombine to produce light while the EBL, which consists of Aluminum Gallium Nitride, forms a barrier to prevent electron overshoot.
My research involves examining each of these layers and studying the hole transport behavior within these two types of heterostructures to better understand their electrical behavior. However, typical studies use LEDs as test structures, which are bipolar devices and are subject to recombination mechanisms. By using unipolar heterostructures, we can focus solely on the carrier transport within these structures without recombination complicating the analysis of the system, making them ideal test vehicles for theoretical models. My study involves simulating a three-dimensional unipolar p-type heterostructure that incorporates the fluctuations of the alloy composition within the alloy region. This would normally require solving for the wavefunctions of the system via Schrödinger’s equation. However, solving this equation in 3D is a computationally expensive task and could take months to obtain results. By using a mathematical theory called the Localization Landscape theory, we can simplify Schrödinger’s equation and converge to a solution three orders of magnitude faster than current simulation techniques. This allows us to viably give LEDs the full 3-dimensional treatment and obtain band structure information as well as current-voltage characteristics.
Through experimentation and theory, it was found that both undoped InGaN and AlGaN alloys act as barriers to hole transport. According to band-diagram simulations, in the case of undoped AlGaN, this barrier was found to increase with increased thickness. The barrier manifested itself as a voltage penalty in both theoretical and experimental current-voltage data. Furthermore, it was found to be an asymmetric barrier, with the voltage penalty being significantly higher in the reverse bias compared to the forward. This voltage penalty was removed when the AlGaN layer is doped with Mg, which is similar to the EBLs found in LEDs, showing that p-type doping of the EBL was necessary to produce high-efficiency LEDs. As for InGaN, a similar behavior was also found. However, the directionality of the barrier was reversed and there was a larger barrier in the forward compared to the reverse direction. This asymmetry was not found in quantum well structures (which consists of alternating layers of InGaN and GaN) and the barriers in both directions were very similar. This behavior was also seen in both experimental and simulation data and has implications on the behavior of LEDs. This result adds weight to the argument that most of the recombination in LEDs happens at the quantum wells closer to the p-GaN layer, due to both the heavier effective mass of holes as well as the barrier posed by the InGaN alloy.