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 potentiall...
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|Summary:||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) . In addition, numerous industry branches successfully exploit solid-state light-sources for material processing, characterization, and quality testing . 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 , compact lasers ensure rapid optical communication , and almost every photographer relies on cameras with silicon-based detectors, the so-called charge-coupled-devices or CCDs , only to name a few. Notably, the invention of the latter was honored with the Nobel Prize in Physics in 2009 . 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 . 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. The main hallmark of this semiconductor is the giant band gap reduction with Bi content , unusually large for more typical compound materials . The aim of the studies is the systematic investigation of carrier dynamics influenced by disorder. The measurements are supported by kinetic Monte- Carlo simulations , 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 , offer efficient intra-cavity frequency mixing  combined with frequency stabilization , and are able to operate in a pulsed regime, emitting ultra-short sub-500 fs pulses . 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.|