Two decades after the isolation of graphene, hybrid van der Waals (vdW) heterostructures, combining 2D materials with conventional semiconductors, have garnered significant attention for their potential to create novel functionalities on established platforms. Among 2D materials, semiconducting transition-metal dichalcogenides (TMDs) stand out thanks to their sizeable bandgap and strong light-matter interactions, making them ideal for optoelectronics. III-nitride semiconductors are well-established in optoelectronics thanks to their excellent material properties. Hybrid 2D-TMD/III-nitride vdW heterostructures have been proposed for numerous applications, but the surface effects and near-field interactions at the vdW interface remain underexplored.
This thesis investigates the optical properties of vdW heterostructures combining 2D TMDs and III-nitride quantum wells (QWs), focusing on the interactions between TMD monolayers (MLs) and surface III-nitride QWs, controlled by the thickness of the QW top barrier.
The optical properties of surface GaN/AlGaN QWs with varying top barrier thicknesses are examined using photoluminescence (PL) and cathodoluminescence (CL) to explore the impact of surface states on QW emission. A method is developed to model QW emission intensity as a function of top barrier thickness. ML and few-layer MoS2 samples are then transferred onto the QWs. By analysing the CL signals from these vdW heterostructures as a function of MoS2 thickness and QW top barrier thickness, we assess the absorption properties of the MoS2 layers. Despite absorption, ML MoS2 effectively passivates intrinsic defects at the III-nitride surface, significantly enhancing the QW emission at room temperature. However, at low temperatures, this passivation effect is overshadowed by a strong interaction between ML MoS2 and the QW, leading to a reduction in QW emission intensity. This interaction increases as the ML-QW distance decreases, likely driven by charge transfer via carrier tunnelling across the top barrier or Förster resonance energy transfer (FRET), a near-field dipole-dipole coupling.
To investigate this further, a new set of samples is fabricated combining surface InGaN/GaN QWs with ML MoSe2. PL measurements of the surface InGaN/GaN QWs show that in the absence of a top barrier, the QW emission is highly sensitive to environmental changes, highlighting the impact of surface states. In contrast, QWs with a top barrier of a few nanometres are protected from these effects. Time-resolved PL measurements of the MoSe2/InGaN QW vdW heterostructures reveal that ML MoSe2 induces a fast non-radiative decay in the surface QWs. This effect is more pronounced for QWs closer to the vdW interface and is associated with high-energy states in the QWs. The temperature dependence of this effect is consistent with carrier localisation in the QW, though it diminishes near room temperature, possibly due to competing phenomena within the heterostructures. This interaction could also be related to FRET or charge transfer from the InGaN QW to the MoSe2 ML.
This thesis demonstrates a proof-of-concept for analysing surface effects in surface III-nitride QWs and their interactions with 2D material coatings in vdW heterostructures. These findings have potential applications in the development of micro- and nanoscale optoelectronic devices, and in creating novel optoelectronic systems based on these hybrid heterostructures.