In this work we outline our multiscale approach for modeling electronic, optical and transport properties of III-N-based heterostructures and light emitting diodes (LEDs). We discuss our framework for connecting atomistic tight-binding theory and continuum-based calculations and how finite element and finite volume meshes are generated for this purpose. Utilizing this framework we present an initial comparison of the electronic structure of an (In,Ga)N quantum well carried out within tight-binding theory and a single band effective mass approximation. We show that for virtual crystal approximation studies, a very good agreement between tight-binding and effectivemass model results is achieved. However, for random alloy fluctuations noticeable deviations in the electronic ground and excited states are found when comparing the two methods. In addition to these electronic structure calculations, we present first LED device calculations, using a drift-diffusion model.
To harness the advanced fabrication capabilities and high yields of the electronics industry for photonics, monolithic growth and CMOS compatibility are required. One promising candidate which fulfils these conditions is GeSn. Introducing Sn lowers the energy of the direct Γ valley relative to the indirect L valley. The movement of the conduction band valleys with Sn concentration is critical for the design of efficient devices; however, a large discrepancy exists in the literature for the Sn concentration at which GeSn becomes a direct band gap. We investigate the bandgap character of GeSn using hydrostatic pressure which reversibility modifies the bandstructure. In this work we determine the movement of the band-edge under pressure using photocurrent measurements. For a pure Ge sample, the movement of the band-edge is dominated by the indirect L valley with a measured pressure coefficient of 4.26±0.05 meV/kbar. With increasing Sn concentration there is evidence of band mixing effects, with values of 9.4±0.3 meV/kbar and 11.1±0.2 meV/kbar measured for 6% and 8% Sn samples. For a 10% Sn sample the pressure coefficient of 13±0.5 meV/kbar is close to the movement of the direct bandgap of Ge, indicating predominately direct Γ-like character for this GeSn alloy. This further suggests a gradual transition from indirect to direct like behaviour in the alloy as also evidenced from theoretical calculations. The implications of this in terms of optimising device performance will be discussed in further detail at the conference.
We report on blue and green light-emitting-diodes (LEDs) grown on (11-22)-GaN templates. The templates were created
by overgrowth on structured r-plane sapphire substrates. Low defect density, 100 mm diameter GaN templates were
obtained by metal organic vapour phase epitaxy (VPE) and hydride VPE techniques. Chemical-mechanical polishing
was used to obtain smooth surfaces for the subsequent growth of LED structures. Ohmic contacts to the p-type GaN
were obtained despite the lower activated acceptor levels. The LEDs show excellent output power and fast carrier
dynamics. Freestanding LEDs have been obtained by use of laser-lift-off. The work is the result of collaboration under
the European Union funded ALIGHT project.
We present a detailed analysis of wave function localization effects in InxGa1−xN alloys and quantum wells. Our work is based on density functional theory to analyze the impact of isolated and clustered In atoms on the wave function localization characteristics in InxGa1−xN alloys. We address the electronic structure of In0.25Ga0.25N/GaN quantum wells by means of an atomistic tight-binding model. Random alloy fluctuations in the quantum well region and well-width fluctuations are explicitly taken into account. The tight-binding model includes strain and built-in field fluctuations arising from the random In distribution. Our density functional theory study reveals increasing hole wave function localization effects when an increasing number of In atoms share the same N atom. We find that these effects are less pronounced for the electrons. Our tight-binding analysis of In0.25Ga0.27N/GaN quantum wells also reflects this behavior, revealing strong hole localization effects arising from the random In atom distribution. We also show that the excited hole states are strongly localized over an energy range of approximately 50 meV from the top of the valence band. For the quantum wells considered here we observe that well-width fluctuations lead to electron wave function localization effects.