Multiscale simulation of electronic & optical properties of (Al,Ga)N-based light emitting devices
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Date
2026-02-25
Authors
Finn, Robert
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University College Cork
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Abstract
Wurtzite III-Nitride semiconductor materials, such as AlN, GaN, InN and their alloys, all possess direct band gaps and can, in principle, emit light from the deep ultraviolet wavelength region up to the infrared part of the spectrum. However, experimental studies have shown that the efficiency of III-N devices in these extreme wavelength windows are very low. Although there has been extensive research done on these materials, (Al,Ga)N-based alloys used in ultraviolet-emitting devices remain mostly inefficient. In order to improve these device efficiencies, a strong theoretical guidance is required. However, experimental studies in the literature have revealed that a multitude of factors are at play, making the modelling of these devices extremely challenging.
This thesis aims to develop a multiscale simulation framework that captures the intrinsic features of (Al,Ga)N-based heterostructures and devices.
To do so, in a first step an atomistic tight-binding model has been developed to understand the impact of alloy disorder and connected carrier localisation effects, a feature seen to be important in experimental studies, on the electronic and optical properties of quantum wells. In a second step this electronic structure model is connected with a carrier transport drift-diffusion framework that incorporates quantum corrections via the localisation landscape theory, while addressing the differences in the valence band ordering between AlN and GaN via a hybrid effective mass approach. In general this multiscale approach allows us to gain insight into fundamental electronic structure properties of (Al,Ga)N alloys and heterostructures, their optical properties and important information on carrier transport and recombination properties of deep UV LEDs.
Our results indicate that random alloy fluctuations are sufficient to induce carrier localisation across a range of (Al,Ga)N quantum wells, most noticeably for the holes, while electrons may also be localised at higher Al contents in the wells. Furthermore, we find that these localisation effects are stronger with increasing quantum well width, due to the corresponding increase in the built-in potential drop across the quantum well, as reflected by increasing Urbach tail energies. Under high carrier densities, this potential drop reduces due to carrier density screening, resulting in a noticeable decrease in observed carrier localisation effects, indicated by a decrease in Urbach tail energies. Additionally, we investigate the degree of optical polarisation in wells of transverse electric and transverse magnetic emission. There we find that carrier localisation can enhance the transverse electric component of emission, particularly for quantum wells where the dominant emission is transverse magnetic.
Finally, we perform quantum corrected multiscale drift-diffusion calculations on deep UV (Al,Ga)N-based light emitting diodes. We find that alloy fluctuations result in percolative pathways that enhance carrier transport through the disordered alloy. Additionally, carrier localisation can significantly increase the carrier density locally inside the active region of the device, which in turn enhance both the radiative and non-radiative Auger-Meitner recombination rates. These effects lead to a more asymmetric distribution of the carriers across a multi-quantum well structure compared to a simulation that neglects alloy fluctuations, and a noticeable increase in electron leakage. Our results also show that effective carrier injection into the active region of a deep UV light emitting diode requires p-type doping of the electron blocking layer.
Overall, these findings indicate the importance of accounting for alloy fluctuations in wurtzite (Al,Ga)N in order to accurately simulate the electronic, optical and carrier transport properties of (Al,Ga)N-based devices.
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Keywords
III-nitrides , Carrier localisation , Light emitting diodes , Drift-diffusion , Alloy disorder
Citation
Finn, R. 2026. Multiscale simulation of electronic & optical properties of (Al,Ga)N-based light emitting devices. PhD Thesis, University College Cork.