Crystal Symmetry and Polarized Luminescence on Nonpolar ZnO

We introduce excitonic polarized photoluminescence (PL) of nonpolar ZnO layers and related quantum well (QW) structures in terms of crystal symmetries and lattice distortions. Polarized PL characters are attributed to in-plane anisotropic strains in the host, which are fully demonstrated on A-plane ZnO. Theoretical evaluations propose that in-plane compressive strains induced in ZnO layers play an important role in obtaining highly polarized optical properties. We experimentally achieve polarized PL responses in strain-controlled A-plane ZnO layers. Furthermore, we find interesting relationship between polarization degree of PL and in-plane anisotropic strains. Finally, highly polarized PL at room temperature is obtained by controlling well width in Cd0.06ZnO0.94O/ZnO QWs as a consequence of change in crystal symmetry from C6v to C2v at interfaces between Cd0.06Zn0.94O well and ZnO barrier layers in the QW samples

Optical Properties of Wurtzite InN and Related Alloys

In dieser Arbeit werden die optischen Eigenschaften von Wurtzitstruktur InN und verwandten ternären InGaN und AlInN, sowie quaternären AlInGaN Legierungen untersucht. Der Schwerpunkt wird auf die Charakterisierung mittels spektroskopischer Ellipsometrie gelegt. Die auf Si(111) Substraten gewachsenen InN-Proben und die Kohlstoff dotierten InN-Proben sind im Spektralbereich vom mittleren Infrarot bis hin zum Vakuum-Ultraviolett untersucht worden. Die Elektronenkonzentration für die InN-Proben wird durch selbstkonsistentes Lösen (der Ellipsometriedaten Analyse im Infrarotbereich und der Anpassung des Absorption Ansatz) bestimmt. Die intrinsische spannungsfreie Bandlücke für InN Proben wird unter Berücksichtigung von Vielteilcheneffekten wie der Bandlückenrenormierung und der Burstein-Moss-Verschiebung, sowie dem Einfluss der Verzerrung für die Bandlücke bestimmt. Die k*p-Methode wird verwendet, um die Verschiebung der Bandlücke (beeinflusst durch Verzerrung) zu berechnen. Es wird demonstriert, dass eine Erhöhung des Kohlenstofftetrabromid (CBr4) Drucks während des Wachstumsprozess, die Elektronenkonzentration in den InN-Proben erhöht. Die Indium-verwandten Legierungen wurden im Spektralbereich des nahen Infrarot bis zum Vakuum-Ultraviolett untersucht. Das analytische Modell, der dielektrichen Funktion im Spektralbereich 1-10 eV, für die Indium-verwandte Legierungen wird präsentiert. Durch die Anpassung des analytischen Modells an die experimentellen dielektrischen Funktionen, werden die Bandlücke und die Übergangsenergien im Hochenergie-Bereich evaluiert. Die Bowing-Parameter der spannungsfreien Bandlücke für die ternären Systeme InGaN und AlInN werden bestimmt. Es wird demonstriert, dass der Bowing-Parameter für AlInN von der Komposition der Legierung abhängig ist. Die Kenntnis von Bowing-Parametern für die ternären Legierungen ermöglicht die Entwicklung einer empirischen Gleichung, zur Berechnung der Bandlücke in quaternären Legierungen. Alle experimentell durch Ellipsometrie bestimten Bandlücken der untersuchten Legierungen werden durch ab-initio Daten unterstützt.In this work, optical properties of wurtzite structure InN and related ternary InGaN and AlInN, as well as quaternary AlInGaN alloys were investigated. The spectroscopic ellipsometer was used as the main characterization tool for the analysis of the optical properties. The InN samples grown on Si(111) substrates, as well as carbon doped InN samples were investigated from mid-infrared up to vacuum-ultraviolet spectral range. The electron concentration for InN samples were evaluated by solving a self-consistent problem that includes the IR-SE ellipsometry data analysis and the imaginary dielectric function around the band gap calculation. The intrinsic strain-free band-gap was estimated after taking into consideration a band-gap renormalization and Burstein-Moss shift, as well as a strain influence on the band gap. The k*p method was used to calculate the strain induced band-gap shift. From the analysis, it was shown that for the carbon doped InN samples the electron concentration increases linearly by increasing the CBr4 dopant pressure during the MBE growth process. The In-related alloys were investigated from near-infrared up to vacuum-ultraviolet spectral range. The analytical model of the dielectric function in the spectral range 1-10 eV was presented. From the fit of the analytical model to the experimental dielectric functions, the band gaps and high-energy inter-band transitions were estimated. The strain-free band-gap bowing parameters for ternary InGaN and AlInN alloys were obtained. It was demonstrated, that the bowing parameter for AlInN is composition dependent. With the knowledge of the bowing parameters of ternary alloys, it was possible to develop an empirical equation that allows to estimate the band gap for a quaternary AlInGaN alloy. All experimentally obtained band gaps are in good agreement with the ab-initio calculated values

Dielectric Function Tensor (1.5 eV to 9.0 eV), Anisotropy, and Band to Band Transitions of Monoclinic \u3cem\u3eβ\u3c/em\u3e-(Al\u3cem\u3e\u3csub\u3ex\u3c/sub\u3e\u3c/em\u3eGa\u3csub\u3e1–\u3cem\u3ex\u3c/em\u3e\u3c/sub\u3e)\u3csub\u3e2\u3c/sub\u3eO\u3csub\u3e3\u3c/sub\u3e (x ≤ 0.21) Films

A set of monoclinic β-(AlxGa1–x)2O3 films coherently grown by plasma-assisted molecular beam epitaxy onto (010)-oriented β-Ga2O3 substrates for compositions x ≤ 0.21 is investigated by generalized spectroscopic ellipsometry at room temperature in the spectral range of 1.5 eV–9.0 eV. We present the composition dependence of the excitonic and band to band transition energy parameters using a previously described eigendielectric summation approach for β-Ga2O3 from the study by Mock et al. All energies shift to a shorter wavelength with the increasing Al content in accordance with the much larger fundamental band to band transition energies of Al2O3 regardless of crystal symmetry. The observed increase in the lowest band to band transition energy is in excellent agreement with recent theoretical predictions. The most important observation is that charge confinement in heterostructures will strongly depend on the growth condition due to the strongly anisotropic properties of the band to band transitions

Propagating Polaritons in III-Nitride Slab Waveguides

We report on III-nitride waveguides with c-plane GaN/AlGaN quantum wells in the strong light-matter coupling regime supporting propagating polaritons. They feature a normal mode splitting as large as 60 meV at low temperatures thanks to the large overlap between the optical mode and the active region, a polariton decay length up to 100 $\mu$m for photon-like polaritons and lifetime of 1-2 ps; with the latter values being essentially limited by residual absorption occurring in the waveguide. The fully lattice-matched nature of the structure allows for very low disorder and high in-plane homogeneity; an important asset for the realization of polaritonic integrated circuits that could support nonlinear polariton wavepackets up to room temperature thanks to the large exciton binding energy of 40 meV

Optical and Collective Properties of Excitons in 2D Semiconductors

We study the properties of excitons in 2D semiconductors (2DSC) by numerically solving the Schr\ {o}dinger equation for an interacting electron and hole in the effective mass approximation, then calculating optical properties such as the transition energies, oscillator strengths, and absorption coefficients. Our theoretical approach allows us to consider both direct excitons in monolayer (ML) 2DSC and spatially indirect excitons in heterostructures (HS) consisting of two 2DSC MLs separated by few-layer insulating hexagonal boron nitride (h-BN). In particular, we study indirect excitons in TMDC HS, namely MoS2, MoSe2, WS2, and WSe2; both direct and indirect excitons in the buckled 2D allotropes of silicon, germanium, and tin, known as silicene, germanene, and stanene respectively, or collectively as the Xenes; and both direct and indirect excitons in the anisotropic 2DSC phosphorene, the 2D allotrope of black phosphorus. Our study of indirect excitons in TMDC/h-BN HS was one of the first to study the dependence of the properties of spatially indirect excitons in 2DSC HS with respect to the interlayer separation. When considering excitons in the Xenes, we focused on the dependence of the excitonic properties on the magnitude of an external electric field oriented perpendicular to the Xene monolayer(s), which can be used to tune the band gap of the Xenes in-situ, thereby changing the charge carrier effective mass and thus the properties of the excitons themselves. Interestingly, our results for excitons in the Xenes indicate that freestanding ML Xenes may in fact be excitonic insulators in their ground states, that is, when there is zero external electric field. Furthermore, we predict, based on our results, that the freestanding ML Xenes should undergo a phase transition from the excitonic insulator state to a semiconducting state as the external electric field is increased beyond some critical value which is unique to each material. Lastly, our results show that the anisotropic exciton reduced mass, inherited from the anisotropic effective masses of electrons and holes in phosphorene, causes significant deviations in the eigenstates compared to the isotropic 2D model used for TMDCs and Xenes, and that furthermore, this anisotropy leads to enhanced (suppressed) optical absorption compared to the isotropic exciton, under linearly polarized excitations along the in-plane crystal axes with relatively smaller (larger) charge carrier effective masses. In addition, we were able to extend our theoretical framework to consider both exciton-photon and exciton-exciton interactions in a weakly interacting Bose gas of excitons, thereby allowing for the study of exciton-polaritons in an optical microcavity. Using this extended framework, we calculate the Rabi splitting between upper and lower polaritons in a model microcavity, as well as the critical temperature for the Berezinskii-Kosterlitz-Thouless (BKT) phase transition of a weakly interacting Bose gas of lower polaritons. In particular, we applied these methods to study polaritons in the ML Xenes, once again focusing on the dependence of these quantities on the magnitude of the external electric field. Based on our calculations, we predict that, assuming a particular type of open microcavity which maximizes the exciton-photon interaction strength, both freestanding ML silicene and ML silicene encapsulated by h-BN should support polaritons with relatively large Rabi splittings whose BKT critical temperature is greater than room temperature, such that it should be possible to achieve room-temperature superfluidity of polaritons in these materials for a particular range of values of the external electric field

First-Principles Calculations of Optoelectronic and Transport Properties of Materials for Energy Applications.

Modern semiconductor technology and nanoengineering techniques enable rapid development of new materials for energy applications such as photovoltaics, solid- state lighting, and thermoelectric devices. Yet as materials engineering capabilities become increasingly refined, the space of controllable properties becomes increasingly large and complex. Selecting the most promising materials and parameters to focus on represents a significant challenge. We approach this challenge by applying state-of-the-art predictive first-principles calculation methods to guide research and development of materials for energy applications. This work describes our first-principles investigations of nanostructured group-III-nitrides for solid-state lighting applications and bulk titanium dioxides for thermoelectric applications. We demonstrate several remarkable properties of nanostructured group-III-nitrides. In InN nanowires with diameters on the order of 1 nm, we predict that quantum confinement shifts optical emission into the visible range at 2.3 to 2.5 eV (green to cyan) and results in a large exciton binding energy of 1.4 eV. These findings offer a new approach to addressing the ”green-gap” problem of low efficiency in solid-state lighting devices emitting in this part of the spectrum. In ultra-thin GaN-AlN quantum wells, we show how to adjust the well and barrier thicknesses for tuning the optical gap in the deep ultraviolet range between 3.85 and 5.23 eV. Furthermore, we predict that quantum confinement in ultra-thin GaN wells results in large exciton binding energies between 80 and 210 meV and enhances radiative recombination by reducing the exciton lifetime to as short as approximately 1 ns at room temperature. These findings highlight the capability of quantum-confined group-III-nitrides to improve the efficiency and utility of visible and ultraviolet solid-state light emitters. Additionally, we calculate the n-type thermoelectric transport properties of the naturally occurring rutile, anatase, and brookite polymorphs of TiO2 and predict optimal temperatures and free-carrier concentrations for thermoelectric energy conversion. We also predict a theoretical limit on the figure of merit ZT of 0.93 in the rutile polymorph, demonstrating that TiO2 can potentially achieve thermoelectric energy conversion efficiency comparable to that of commercialized thermoelectrics.PhDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/116701/1/bayerl_1.pd

Thickness-Dependent Differential Reflectance Spectra of Monolayer and Few-Layer MoS2, MoSe2, WS2 and WSe2

The research field of two dimensional (2D) materials strongly relies on optical microscopy characterization tools to identify atomically thin materials and to determine their number of layers. Moreover, optical microscopy-based techniques opened the door to study the optical properties of these nanomaterials. We presented a comprehensive study of the differential reflectance spectra of 2D semiconducting transition metal dichalcogenides (TMDCs), MoS2, MoSe2, WS2, and WSe2, with thickness ranging from one layer up to six layers. We analyzed the thickness-dependent energy of the different excitonic features, indicating the change in the band structure of the different TMDC materials with the number of layers. Our work provided a route to employ differential reflectance spectroscopy for determining the number of layers of MoS2, MoSe2, WS2, and WSe2.Comment: Main text (3 Figures) and Supp. Info. (23 Figures

Physics of Polariton Condensates in GaN-based Planar Microcavities

Since its prediction in 1996 by Imamoğlu and coworkers, the use of a non-equilibrium polariton condensate to produce an intense coherent light source referred to as a polariton laser has attracted a lot of interest in the whole physics community as it should allow the realization of ultralow threshold coherent light-emitting devices due to the release of the Bernard-Duraffourg condition. Excitons-polaritons, admixed particles resulting from the strong coupling between a cavity photon and an exciton, are the eigenmodes of a strongly coupled microcavity and exhibit a very light effective mass at the center of the Brillouin zone (105 times lighter that a free electron) inherited from the cavity photon. In the present work, we are interested in III-nitride based microcavities embedding GaN quantum wells in the active region. Thanks to the stability of the excitons at room temperature in this system and a large oscillator strength, polariton condensation has been observed up to 340K under optical excitation, paving the way toward the realization of the first electrically injected polariton laser. The goal of the present study is to provide a detailed analysis of the system properties accounting for nitride specificities, to describe the mechanisms leading to the formation of polariton condensates and to give the key elements for the optimization of devices relying on polariton nonlinearities. For this purpose, a Fourier-imaging setup allowing for the simultaneous monitoring of real space and far-field energy dispersions was carefully designed to operate in the UV spectral range in order to probe the sample emission at various temperatures. The first main result of this thesis is the establishment of the complete polariton phase diagram of our multiple quantum well-based GaN microcavity, which provides a comprehensive tool to favor or inhibit the condensation threshold by adjusting the microcavity parameters. The condensation is shown to be governed either by the kinetics or by the thermodynamics depending on the strength of the interactions. As polaritons are half-light, half-matter particles, the mechanisms leading to the nonlinear threshold are totally different from those of a conventional semiconductor laser. In particular, the possibility to tune the interactions in the system by changing the photonic fraction of the polaritons or the lattice temperature allows discriminating between different relaxation regimes. Then the spin of the polariton condensate is discussed. It is shown that the dimensionality of the system plays a major role in the polarization state of the emitted light. In particular above threshold, for a bulk microcavity, the polarization is randomly oriented whereas for a GaN multiple quantum well based microcavity, the polarization is pinned by the system anisotropy originating from the static disorder. With increasing pumping power, a depinning of the polarization is observed resulting in a progressive decrease in the polarization degree of the emitted light. These two results are well accounted for by a stochastic model of the condensate formation taking into account the in-plane anisotropy caused by the stationary photonic disorder, the self-induced Larmor precession of the condensate pseudospin and the interplay between energy and polarization relaxation rates. In the last part of this work, the case of nonpolar m-plane GaN based microcavities is addressed. In these structures, the optical axis lies in the plane of the cavity leading to a twofold anisotropy: the birefringence is responsible for the anisotropy of the cavity mode and the distribution of the exciton oscillator strength causes different coupling constants between light and matter along the two orthogonal directions. In such structures different selection rules and optical constants for light polarization perpendicular and parallel to the optical axis can lead to the coexistence of weak and strong coupling regimes with a transition to nonlinear emissions