Hybrid QM/MM Embedded Cluster Study on Defects in Gallium Nitride

Author:Xie Zi Juan

Supervisor:sui yu


Degree Year:2019





GaN is a fundamental wide band gap semiconductor and has been the focus of condensed matter physics and material physics research.Intentionally doping in GaN can lead to n-or p-type conductivity,as well as various photoluminescence in different wavelength bands including visible and untraviolet range.Therefore,it is widely used in light emitting diodes and laser diodes and has huge scientific and application value.The semiconducting behaviour and optoelectronic response of gallium nitride is governed by point defect processes,which,despite many years of research,remain poorly understood.The key difficulty in the description of the dominant charged defects is determining a consistent position of the corresponding defect levels,which is difficult to derive using standard supercell calculations.Besides,charged defects polarise a host dielectric material with long-range forces that strongly affect the outcome of defect simulations.Our mehod of hybrid QM/MM embedded cluster provides direct access to a common zero of the electrostatic potential for all point defects in all charge states and excels at describing polarisation due to local charges in a crystal,which is beneficial to defect calculations in wide-bandgap semiconductors.Using this method,the electrical and luminescent properties of intensionally or unintensionally doped n-type GaN were investigated.The description of shallow-level defects in semiconductors remains a difficulty,which is seen in calculations of n-type dopant Si and O in GaN.Many spectroscopical data indicate that Si and O doping can introduce shallow defect levels in GaN which trap charge carriers.However,to date there have been only a small number of theoretical studies on the subject which do not support the presence of such traps.We have showed the diffuse character of O and Si and developed a strategy to describe diffuse state in the framework of QM/MM embedded cluster method.The calculated defect levels of Si and O are close to the experimental values.In intensionally and unintensionally doped n-GaN samples there is always a yellow luminescence(YL)band,often accompanied by green and/or red luminescence bands to form a broad band.The origin of this yellow band is still in debate of both experimental and theretical research.We study Ga vacancy complexing with O and find its formation energy largely reduced in n-type GaN by complexing with O,while its thermodynamic and optical transition levels remain within the band gap.We introduce diffuse state model of charge carriers and find that Ga vacancy and O complex can give rise to yellow as well as green and red peaks depending on the initial state of the hole carriers that make the transition.Our results can explain the multiple bands observed in experiments.Besides YL,a plurality of photoluminescence peaks in the ultraviolet(UV)and visible range can be observed in unintentionally doped GaN,and which level of which defect give rise to wich photoluminescence peak is still not clear.The origin of the n-type character of as-grown GaN is still in debate,so as the difficulty of p-type doping of GaN.We thus study native defects in GaN by calculating their structures,defect levels,formation energies,concentrations and optical properties.N vacancies are the most thermodynamically favourable native defects in GaN,which contribute to the n-type character of as-grown GaN but are not the main source,a result that is consistent with experiment.Our calculations show no native point defects can form thermodynamically stable acceptor states.GaN can be easily doped n-type,but,in equilibrium conditions at moderate temperatures acceptor dopants will be compensated by N vacancies and no significant hole concentrations will be observed,indicating non-equilibrium processes must dominate in p-type GaN.We identify spectroscopic signatures of native defects in the infrared,visible and ultraviolet luminescence ranges and complementary spectroscopies.Crucially,we calculate the effective-mass-like-state levels associated with electrons and holes bound in diffuse orbitals.These levels may be accessible in competition with more strongly-localised states in luminescence processes and allow the attribution of the observed 3.46 and 3.27 eV UV peaks in a broad range of GaN samples to the presence of N vacancies.