| Abstract |
The major aim of this doctoral thesis involved the novel development of all-dielectric GaN-based planar microcavity structures for the reduction of polariton lasing threshold at room temperature operation when the device is optically excited, while setting also the fundamentals of subsequent electrically injected polariton lasing structures. The microcavity configuration offers the potential to study the strong interaction of confined light and matter with the same energy and momentum, and hence, allows us to explore the capabilities of a non-trivial physical system and utilize it in future optoelectronic devices and applications. Here, the matter component of polaritons regards electron-hole pairs, known in literature as excitons, which are observed in either inorganic (as in this work) or organic semiconducting materials. If the light-matter interaction is strong enough, which is usually possible when suppressing the overall losses in the system, the distinction of the original states is lost and new quasi-states are formed, thus, sharing properties of both the excitons and the photons. In such case, the system is operating in the so-called "strong coupling regime" and the produced particles are called "polaritons". The spontaneous collective coherence of those particles allows for their condensation at the ground state, referred to as inversionless polariton lasing, while enable other intriguing physical phenomena such as parametric scattering, superfluidity etc. due to their extraordinary composite characteristics of being half-photons/half-excitons along with their quasi-bosonic nature. Nevertheless, the observation of these phenomena remains a challenging task when going to elevated temperatures, while the final goal to reach ambient conditions has troubled many research groups for many years. Apparently, the main reason is the fast decay of the exciton entities due to the finite binding energy in most inorganic semiconductors, which restricts them to "live" only at cryogenic temperatures. Quantum mechanics has shown that confinement of carriers inside quantum wells can enhance the binding energy of an electron-hole pair, as well as the oscillator strength of the optical transition. Previous works on wide bandgap materials, such as GaN, have demonstrated the ability to surpass, at least in part, the previous limitations, since the exciton states possess binding energies which are comparable to kBT at room temperature in the bulk, while they can be much higher when the material is in the form of thin quantum wells. As concerns photon confinement, the necessary configuration involves highly reflective mirrors which in most of cases are made by distributed Bragg reflectors (DBRs) in a planar configuration. These mirrors are basically a stack of alternating high and low refractive index materials that are placed on both sides of the active material.
An important case, of interest in this work, is when dielectric mirrors are used as top and bottom DBRs, due to the increased photon confinement they offer with a reduced number of alternating pairs. Hence, a main goal of this work is to improve further the previous all-dielectric planar microcavity devices, by fabricating GaN-based sub-wavelength films with low roughness, using the photo-electrochemical etching (PEC) technique based on the selective removal of an InGaN layer. The optical design and simulation of the all-dielectric III/V microcavity structures was made with the use of a computer software, in order to achieve the desired resonance between the cavity mode and the exciton state within the active region. The epitaxial growth was made by plasma-assisted molecular beam epitaxy on c-GaN/Sapphire for the polar-oriented structures and on m-plane GaN substrates for the non-polar ones at INAC, CEA. All the samples were studied by a range of techniques to assess their quality and characteristics. Specifically, in Chapter 1 is given the theoretical background of III-nitride materials and principles of polariton physics. In Chapter 2, the study focuses on producing ultra-smooth subwavelength-thin GaN membranes by photo-electrochemically etching an InGaN sacrificial layer, enabling therefore to extract the absorption coefficient of bare membranes, based on μ-transmittance measurements and taking into account the standing wave effects. Chapter 3 describes the utilized methodology to fabricate all-dielectric DBR microcavities made by the oxides SiO2 and Ta205, which illustrate a high refractive-index
contrast, and polar GaN/AlGaN membranes allowing for the observation of well-resolved polariton branches in the reciprocal lattice (k -space) with the characteristic anti-crossing behavior, as well as, remarkable polariton lasing at room temperature with the use of only 4 alternating pairs in the top DBR. In Chapter 4, the same approach was applied to the m-plane orientation in order to produce all-dielectric microcavities with high-quality non-polar GaN/AlGaN membranes for the first time. The investigated state-of-the-art microcavity structure exhibited intriguing polariton characteristics attributed to the in-plane anisotropy while set a new record for the ultra-low polariton lasing threshold due to the elimination of the internal build-in fields. Finally, to overcome limitations in the DBR evaporation based on the large thermal strain acting on thin films when cooled from deposition to room temperature, Chapter 5 introduces a new concept in the development of microcavities with the use of oxide-based " transferrable" DBR membranes (t-DBR) as top mirrors, whose successful operation was confirmed in λ/2 (only oxide) and 3λ/2 (polariton) cavities.
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