Theory of carrier transport in III-Nitride based heterostructures
University College Cork
Wurtzite III-nitride materials and their alloys have attracted significant interest for solid state lighting applications. This is due to the direct band gaps of InN, GaN, and AlN crystals, which span a wide range of emission wavelengths. Due to the importance of these systems, the goal of optimising device performance has been an extremely active field of research. An important aspect of this is the development of improved modeling techniques. More recently, an emphasis has been put on understanding the impact the disordered alloy microstructure has on the electronic structure, however models focusing on transport properties are less mature. This is in part due to the challenges of connecting a random alloy description of the underlying microstructure with transport models. This thesis addresses this difficult problem by developing and utilizing different simulation frameworks, focusing on transport properties of (In,Ga)N/GaN quantum well systems. More specifically, the non-equilibrium Green's function (NEGF) formalism has been employed to study ballistic transport in a fully quantum mechanical setting. This builds on a tight-binding description of the electronic structure which ensures an atomistic description of the alloy is achieved. Our results indicate that while the alloy microstructure is of secondary importance for electrons, the transmission of holes is strongly perturbed by the presence of disorder. This is attributed to the breakdown of the translational symmetry of the system, which opens up new channels not present when fluctuations in local alloy content are neglected (using a virtual crystal approximation). Moreover, we have developed a new semi-classical multi-scale drift-diffusion model. This allows simulation of full devices due to a reduced computational demand compared to the NEGF formalism, while still keeping a microscopic resolution and accounting for important quantum corrections. The starting point is again the tight-binding model, which is used as a foundation to describe the alloy microstructure: A 3-dimensional energy landscape is extracted which includes an atomistic description of alloy fluctuations, local strain, and local polarization. This can be used as a confining potential for electrons and holes, and quantum corrections can be included in a numerically efficient manner via the recently developed localization landscape theory. This landscape, including or excluding quantum corrections, is used to study both uni-polar electron and hole transport. Our results show that, when quantum corrections are accounted for, the virtual crystal approximation is again a good approximation for electron transport, whereas hole transport is reduced due to carrier localization effects in the quantum well region. Finally this framework is extended to a p-i-n junction, where carrier (and thus recombination) distribution across a multi-quantum well system is studied. This system allows for a comparison between our in-house model and a commercial software package. Without including disorder in the alloy microstructure both schemes fail to reproduce literature experimental results. However, the situation changes when the random alloy microstructure is accounted for using our newly developed approach: The predicted behaviour is consistent with literature experimental results, without changing any other simulation parameters. These results highlight the importance of the treatment of the alloy microstructure in simulations, and indicate that our developed framework is an ideal starting point for modeling III-N systems to understand fundamental properties and guide device design.
Nitride , Transport , Alloy fluctuations , Numerical simulation , Light emitting diode
O'Donovan, M. J. O. 2023. Theory of carrier transport in III-Nitride based heterostructures. PhD Thesis, University College Cork.