Time-resolved photoluminescence spectroscopy of semiconductors for optical applications beyond the visible spectral range

Since the development of the first light-emitting diodes (LEDs) in the early 1960’s [1, 2], opto-electronic technology based on the semiconducting materials evolved rapidly in the last half of the century. Today, barely all aspects of the generation, control, and detection of light are potentially covered by the solid-state semiconductor devices. The reason is a unique combination of flexibility, low-cost fabrication, as well as compact packaging dimensions. In particular, scientific applications profit from the large tunability of the semiconductor diodes and lasers as well as from the high sensitivity of the detectors in a broad spectral range from the ultra-violett (UV) to the infra-red (IR) [4]. In addition, numerous industry branches successfully exploit solid-state light-sources for material processing, characterization, and quality testing [3]. Finally, the semiconductor-based emitters and detectors have already found their way into the everyday’s life. In many cases, the technology is subtly integrated and barely noticable, yet it is often the heart of the respective applications. High-brilliance LEDs provide images for the television projectors [6], compact lasers ensure rapid optical communication [5], and almost every photographer relies on cameras with silicon-based detectors, the so-called charge-coupled-devices or CCDs [7], only to name a few. Notably, the invention of the latter was honored with the Nobel Prize in Physics in 2009 [8]. Still, the journey is far from being over. The ever-increasing need for energy-saving lighting, faster optical communication, as well as for versatile optical sources in the growing field of the bio-physics anticipates and almost demands further technological advance. The research is aimed towards compact and low-cost lasers with high repetition rates in the near-infra-red (NIR) spectral range, bright, more efficient LEDs over the complete visible (VIS) spectrum, as well as strong and tunable lasers emitting in the ultra-violet (UV) wavelength region. In addition, transparent opto-electronic devices as well as the light-emitters on a scale as small as several nanometers are envisioned. To address these challenges, several steps are to be taken. First, a detailed understanding of the fundamental phenomena in semiconductors is required for a proper design of optical devices. The second, equally important procedure is the synthesis and the characterization of novel material systems suited for the desired applications over a broad spectral range. On this basis, semiconductor devices are finally developed and optimized to expoit their respective potential as well as to identify any fundamental restrictions. The work discussed in this thesis is focused on the experimental studies regarding these three steps: (1) investigation of the fundamental effects, (2) characterization of new material systems, and (3) optimization of the semiconductor devices. It goes without saying that only parts of the broad scientific fields are addressed. In all three cases, the experimental technique of choice is photoluminescence (PL) spectroscopy [9]. This method is based on the detection of light emitted by the photo-excited materials. Considering the possibility of spectrally-, temporally- and spatially-resolved measurements, PL spectroscopy remains a flexible and, most-important, a non-destructive probe for the optical response of semiconductors. The thesis is organized as follows. Chapter 2 gives a summary of the PL properties of semiconductors relevant for this work. The first section deals with the intrinsic processes in an ideal direct band gap material, starting with a brief summary of the theoretical background followed by the overview of a typical PL scenario. In the second part of the chapter, the role of the lattice-vibrations, the internal electric fields as well as the influence of the band-structure and the dielectric environment are discussed. Finally, extrinsic PL properties are presented in the third section, focusing on defects and disorder in real materials. In chapter 3, the experimental realization of the spectroscopic studies is discussed. The time-resolved photoluminescence (TRPL) setup is presented, focusing on the applied excitation source, non-linear frequency mixing, and the operation of the streak camera used for the detection. In addition, linear spectroscopy setup for continous-wave (CW) PL and absorption measurements is illustrated. Chapter 4 aims at the study of the interactions between electrons and lattice-vibrations in semiconductor crystals relevant for the proper description of carrier dynamics as well as the heat-transfer processes. The presented discussion covers the experimental studies of many-body effects in phonon-assisted emission of semiconductors due to the carriercarrier Coulomb-interaction [10, 11]. The corresponding theoretical background is discussed in detail in chapter 2. The investigations are focused on the two main questions regarding electron-hole plasma contributions to the phonon-assisted light-matter interaction as well as the impact of Coulomb-correlations on the carrier-phonon scattering. The experiments presented in chapter 5 deal with the characterization of recently synthesizedmaterial systems: ZnO/(ZnMg)O heterostructures, GaN quantumwires (QWires), as well as (GaAs)Bi quantum wells (QWs). The former two materials are designed for potential electro-optical applications in the UV spectral range [12, 13]. TRPL spectroscopy is applied to gain insight as well as a better understanding of the respective carrier relaxation and recombination processes crucial for the device operation. The latter material system, Ga(AsBi), is a possible candidate for light-emitting devices in the NIR, at the telecom wavelengths of 1.3 μm and 1.55 μm[14]. The main hallmark of this semiconductor is the giant band gap reduction with Bi content [22], unusually large for more typical compound materials [15]. The aim of the studies is the systematic investigation of carrier dynamics influenced by disorder. The measurements are supported by kinetic Monte- Carlo simulations [23], providing a quantitative analysis of carrier localization effects. In chapter 6, optimization and characterization studies of semiconductor lasers, based on the well-studied (GaIn)As material system designed for NIR applications, are performed. The device under investigation is the so-called vertical-external-cavity surfaceemitting laser (VECSEL) [16, 17]. This laser perfectly combines the excellent beam quality of surface emitters and the high output power of semiconductor edge-emitting diodelasers. VECSELs are available in a broad spectral range [18], offer efficient intra-cavity frequency mixing [19] combined with frequency stabilization [20], and are able to operate in a pulsed regime, emitting ultra-short sub-500 fs pulses [21]. For the majority of the applications high output power of the device remains crucial. The performance of the laser, however, is typically limited by the heating of the device during the operation. The experiments focus on the study of the thermal properties of a high-power VECSEL. The distribution and removal of the excess heat as well as the optimization of the laser for increased performance are adressed applying different heat-spreading and heat-transfer approaches. Based on these investigations, the possibility for power-scaling is evaluated and the underlying restrictions are analyzed. The latter investigations are performed applying spatially-resolved PL spectroscopy. An experimental setup is designed for monitoring the spatial distribution of heat in the semiconductor structure during laser operation. A brief summary of the experimental findings and the resulting conclusions are given in the chapter 7 in the end of the thesis

The restricted two-body problem in constant curvature spaces

We perform the bifurcation analysis of the Kepler problem on $S^3$ and $L^3$. An analogue of the Delaunay variables is introduced. We investigate the motion of a point mass in the field of the Newtonian center moving along a geodesic on $S^2$ and $L^2$ (the restricted two-body problem). When the curvature is small, the pericenter shift is computed using the perturbation theory. We also present the results of the numerical analysis based on the analogy with the motion of rigid body.Comment: 29 pages, 7 figure

Exciton fine structure splitting and linearly polarized emission in strained transition-metal dichalcogenide monolayers

We study theoretically effects of an anisotropic elastic strain on the exciton energy spectrum fine structure and optical selection rules in atom-thin crystals based on transition-metal dichalcogenides. The presence of strain breaks the chiral selection rules at the $\bm K$-points of the Brillouin zone and makes optical transitions linearly polarized. The orientation of the induced linear polarization is related to the main axes of the strain tensor. Elastic strain provides an additive contribution to the exciton fine structure splitting in agreement with experimental evidence obtained from uniaxially strained WSe$_2$ monolayer. The applied strain also induces momentum-dependent Zeeman splitting. Depending on the strain orientation and magnitude, Dirac points with a linear dispersion can be formed in the exciton energy spectrum. We provide a symmetry analysis of the strain effects and develop a microscopic theory for all relevant strain-induced contributions to the exciton fine structure Hamiltonian.Comment: 12 pages, 5 figure

Exciton propagation and halo formation in two-dimensional materials

The interplay of optics, dynamics and transport is crucial for the design of novel optoelectronic devices, such as photodetectors and solar cells. In this context, transition metal dichalcogenides (TMDs) have received much attention. Here, strongly bound excitons dominate optical excitation, carrier dynamics and diffusion processes. While the first two have been intensively studied, there is a lack of fundamental understanding of non-equilibrium phenomena associated with exciton transport that is of central importance e.g. for high efficiency light harvesting. In this work, we provide microscopic insights into the interplay of exciton propagation and many-particle interactions in TMDs. Based on a fully quantum mechanical approach and in excellent agreement with photoluminescence measurements, we show that Auger recombination and emission of hot phonons act as a heating mechanism giving rise to strong spatial gradients in excitonic temperature. The resulting thermal drift leads to an unconventional exciton diffusion characterized by spatial exciton halos

Exciton-exciton interactions in van der Waals heterobilayers

Exciton-exciton interactions are key to understanding non-linear optical and transport phenomena in van der Waals heterobilayers, which emerged as versatile platforms to study correlated electronic states. We present a combined theory-experiment study of excitonic many-body effects based on first-principle band structures and Coulomb interaction matrix elements. Key to our approach is the explicit treatment of the fermionic substructure of excitons and dynamical screening effects for density-induced energy renormalization and dissipation. We demonstrate that dipolar blue shifts are almost perfectly compensated by many-body effects, mainly by screening-induced self-energy corrections. Moreover, we identify a crossover between attractive and repulsive behavior at elevated exciton densities. Theoretical findings are supported by experimental studies of spectrally-narrow interlayer excitons in atomically-reconstructed, hBN-encapsulated MoSe$_2$/WSe$_2$ heterobilayers. Both theory and experiment show energy renormalization on a scale of a few meV even for high injection densities in the vicinity of the Mott transition. Our results revise the established picture of dipolar repulsion dominating exciton-exciton interactions in van der Waals heterostructures and open up opportunities for their external design

Non-equilibrium diffusion of dark excitons in atomically thin semiconductors

Atomically thin semiconductors provide an excellent platform to study intriguing many-particle physics of tightly-bound excitons. In particular, the properties of tungsten-based transition metal dichalcogenides are determined by a complex manifold of bright and dark exciton states. While dark excitons are known to dominate the relaxation dynamics and low-temperature photoluminescence, their impact on the spatial propagation of excitons has remained elusive. In our joint theory-experiment study, we address this intriguing regime of dark state transport by resolving the spatio-temporal exciton dynamics in hBN-encapsulated WSe2 monolayers after resonant excitation. We find clear evidence of an unconventional, time-dependent diffusion during the first tens of picoseconds, exhibiting strong deviation from the steady-state propagation. Dark exciton states are initially populated by phonon emission from the bright states, resulting in creation of hot (unequilibrated) excitons whose rapid expansion leads to a transient increase of the diffusion coefficient by more than one order of magnitude. These findings are relevant for both fundamental understanding of the spatio-temporal exciton dynamics in atomically thin materials as well as their technological application by enabling rapid diffusion

Dark exciton-exciton annihilation in monolayer WSe2_2

The exceptionally strong Coulomb interaction in semiconducting transition-metal dichalcogenides (TMDs) gives rise to a rich exciton landscape consisting of bright and dark exciton states. At elevated densities, excitons can interact through exciton-exciton annihilation (EEA), an Auger-like recombination process limiting the efficiency of optoelectronic applications. Although EEA is a well-known and particularly important process in atomically thin semiconductors determining exciton lifetimes and affecting transport at elevated densities, its microscopic origin has remained elusive. In this joint theory-experiment study combining microscopic and material-specific theory with time- and temperature-resolved photoluminescence measurements, we demonstrate the key role of dark intervalley states that are found to dominate the EEA rate in monolayer WSe$_2$. We reveal an intriguing, characteristic temperature dependence of Auger scattering in this class of materials with an excellent agreement between theory and experiment. Our study provides microscopic insights into the efficiency of technologically relevant Auger scattering channels within the remarkable exciton landscape of atomically thin semiconductors.Comment: 17 pages, 6 figure