Optoelectronic Devices with III-Nitride Nanostructures and Monolayer Heterostructures

Abstract

The III-Nitrides have emerged as a leading material group for a wide range of optoelectronic applications including but not limited to commercial solid-state lighting, plastic fiber communication, light detection, RF and high-power switching, deep-UV photonics, single photon sources for quantum computing and cryptography, and the study of exciton dynamics. The versatility of these materials allows them to be epitaxially-grown in bulk (3D), planar (2D), nanowire (1D), or quantum dot (0D) form by careful control of the growth conditions. The composition of the Al(Ga)N and In(Ga)N ternaries allows complete coverage of the deep-UV to IR spectrum. These materials can be grown on a wide range of substrates including free-standing GaN, sapphire, SiC, and silicon. Silicon is most economical and of interest for monolithic integration. However the large lattice mismatch ~17% with GaN requires careful strain engineering to prevent propagation of threading dislocations, which are detrimental to device operation. In the present study, Al0.56Ga0.44N/Al0.62Ga0.38N multi-quantum well (MQW) heterostructures grown, in collaboration, on GaN/sapphire templates are optically characterized via photoluminescence techniques. These MQW’s emit ~280nm and can have obvious benefits in deep-UV applications. The interface roughness between the well/barrier layers is measured and modeled. InGaN/GaN dot-in-nanowires (DINWs) with varying In composition covering the green to IR range are grown on silicon and their optical response measured. High-gain photodetectors are then fabricated using green-emitting DINWs. The photoresponse is measured and modeled using diffusion and drift theory. It is found that the primary gain of these detectors is due to the modulation of the conducting cross-section of the nanowires upon photoexcitation. InGaN/GaN self-organized quantum dots (QDs) are attractive for their low built-in polarization fields and defect densities, which translates to obvious benefits for lasers and LEDs (i.e. high T0, low thresholds, decrease in droop, etc.). The ability to grow these QDs on silicon is attractive not just economically, but also for monolithic integration. Here, the growth of device-quality GaN over coalesced GaN nanowires on silicon is demonstrated. Photoluminescence characterization of the green-emitting InGaN/GaN QDs is presented and a LED is demonstrated with a low built-in polarization field ~50kV/cm. These quantum dots are then grown on GaN/sapphire substrates for pump-probe spectroscopy. Excited state absorption and ground state bleaching are observed, which has not been previously reported to our knowledge. The carrier relaxation is analyzed using transient absorption spectroscopy by colleagues at IIT Bombay and the results are discussed. Deep-UV emitting GaN/AlN monolayers are then grown, in collaboration with colleagues, within nanowires on silicon and in planar form as Al(Ga)N on GaN/sapphire for comparison. For the first time, large exciton binding energies ~200meV are measured in these GaN monolayers, in agreement with density functional theory (DFT) and many-body perturbation theory. These structures exhibit strong electron-hole wavefunction overlap and a unique temperature-dependent carrier redistribution that will be discussed. High exciton binding energies are attractive for exciton lasers and polariton devices, microcavities, and for the study of exciton dynamics in GaN to name a few. A deep-UV photodiode with detection from ~200-350nm is demonstrated using these monolayers and its results are presented.