Ab-initio study of excited state properties of 2D honeycomb graphene-like Nitrides

Ultrathin GaN layers have been grown on InGaN substrates [3]. Very recently, 2D GaN has been obtained via graphene encapsulation [4]. In this scientific context is placed my Phd Thesis, whose the main aim is to contribute to the determination of the electronic and optical properties of 2D Nitrides, by means a theoretical ab-initio study. As early as 2005 [5] the possibility that III-group nitrides can form single layer honeycomb structures like graphene, was predicted by ab-initio calculations. The stability and the electronic properties of 2D BN, AlN, GaN, and InN have been investigated [68]. However, a better treatment of many-body quasi-particle (QP) e ects, similar to what has been done for 2D BN [6,912], is required in electronic structure calculations. In this work, we systematically investigate the structural and QPelectronic properties of III-N sheet materials, in honeycomb graphene-like geometry, in the framework of the Density Functional Theory (DFT) and Greens function Many Body Perturbation Theory (GW approximation), respectively. To explore the degree of tunability of their properties, we also study In(x)Ga(1 x)N and In(x)Tl(1 x)N alloys and 2d nitride heterostructures. A theoretical understanding of the monolayer optical properties of BN, AlN, GaN, and InN, in particular of their optical absorption and emission properties, could help and focus the experimental activities on 2D nitrides. Excitonic properties have been recently investigated for 2D GaN [13,14] and InN [15], in addition to the activities on the well-known BN layer [11]. Here, we perform a systematic ab-initio study of the optical properties of 2D group-III nitrides solving the Bethe Salpeter equation. We focus on the absorption and emission spectra near the onsets for in- and out-of-plane light polarization. The dominating bound excitons are studied in detail, in particular the inuence of spatial connement e ects and reduced screening of the electron-hole interaction. For the exciton ground state we calculate the binding energy and the excitonic radius. The emission is characterized by the excitonic radiative lifetime. An original element of this thesis is the calculation of the exciton features also with a simple analytical model for excitons in two dimensional systems [16] and the comparison with those obtained full ab-initio. The results con rm that the model represent a valid alternative to obtain a quick qualitative description of the behavior of lowest energy excitons in 2D materials. Finally, we tried to contribute to the research eld of systems made by stacking 2D crystal on top of each other studying the structural, electronic, and optical properties of bilayer indium nitride (b-InN) and of AlN/WS2 heterostructure