DYNAMICS OF MULTI-PHOTON PROCESSES IN SEMICONDUCTOR HETEROSTRUCTURES

One-dimensional excitons in V-shaped quantum wires

By means of a tight-binding approach, we have studied the influence of barrier states on the conduction band of the T- and V-shaped quantum wires. For AlAs/GaAs superlattices or pure AlAs barriers, $X$-related states induce strong modifications in the wire conduction band that cannot be accounted for by a simple effective-mass approximation. With respect to quantum wells, we show that the quasi-one-dimensional confinement of quantum wires enhances valley mixing and leads to a reduction in the energy of lower lying states.

Group III-nitrides have been considered a promising choice for the realization of optoelectronic devices since 1970. Since the first demonstration of the high-brightness blue light-emitting diodes (LEDs) by Shuji Nakamura and coworkers, the fabrication of highly efficient white LEDs has passed successful developments. A serious physical issue still remained, which prevents their use for high power and highly efficient LEDs: the drop of external quantum efficiency (EQE) of III-nitride LEDs when increasing the driving current, the so-called ''efficiency droop'' problem. In order to have a fast expansion to the lighting market, the cost-per-lumen of packaged LEDs must rapidly decrease. This indeed demands for having LED chips operating with high EQE under the high current operation. Besides the industrial interest of III-nitrides, owing to their large direct bandgap, they feature some interesting optical properties such as large exciton binding energy and large oscillator strength of excitons. However, when the carrier density raised in a semiconductor, a transition should occur from an insulating state consisting of a gas of excitons to a conductive electron-hole plasma, that is called the Mott transition. This crossover can drastically affect the optical and electrical characteristics of semiconductors and may, for instance, drive the transition from a polariton laser to a vertical cavity surface-emitting laser. More interestingly, even if biexcitons are frequently seen to dominate the emission of III-nitride heterostructures when the density is raised, no clear experimental report is available on the role of biexcitons in the Mott transition in a two-dimensional (2D) nanostructure. In the first part, the emission properties of high-quality GaN/AlGaN single quantum wells (QWs) at high-carrier densities are examined. They are of crucial importance to provide a deeper insight into the operating conditions of III-nitride based lasers and LEDs, as well as the transition from strong to weak coupling regime of exciton-polaritons in semiconductor microcavities. Employing the same technique then to investigate some InGaN/GaN QWs, the droop signature was investigated comprehensively in both polar and non-polar QWs. Having an accurate estimation of the carrier density in the QWs, the contribution of several non-radiative processes were quantitatively examined. These experiments can indeed provide a deeper insight on the physical phenomena responsible for the efficiency droop in III-nitride LEDs, and can stimulate several theoretical and experimental on this subject. The second part focuses on the transport mechanisms of excitons in ZnO and III-nitride based nano-structures. The transverse movement of donor-bound excitons in a purely bent ZnO microwire as a function of temperature have been investigated, owing to the high spatial, spectral and temporal resolutions of our original time resolved cathodoluminescence system. The movement mechanism was modeled by a hopping process of excitons. Our results pave the way to new experiments allowing to reveal the physics at play at the nanoscale in different materials. However, in case of highly disordered systems, one should take into account more complex considerations, as in the case of InGaN core-shell QW structures studied in the last chapter, the large energy fluctuations prevent excitons to move along the energy gradient at low temperatures.

Capture, relaxation and recombination in quantum dots

Exciton-polarons in self-assembling helical aggregates: relating optical properties to supramolecular structure

The strong light-matter interaction in the atomically thin transition metal dichalcogenides (TMDCs) has allowed the use of optical spectroscopy to investigate these materials in great depth. It has been shown that optoelectronic properties of ultrathin TMDCs are remarkably different from their bulk counterparts. Among them, this dissertation focuses on ultrathin MoTe2 (molybdenum ditelluride) and ReS2 (rhenium disulfide). We first introduce the fundamental properties of the two material systems, MoTe2 and ReS2, investigated in this dissertation. Specific experimental methods for optical spectroscopy of 2D materials, 2D sample preparation, and related optics calculations are presented. Absorption and photoluminescence measurements are applied to demonstrate that semiconducting MoTe2, an indirect band gap bulk material, acquires a direct band gap in the monolayer limit. Furthermore, strain-tuned optical spectroscopy on MoTe2 shows that tensile strain can significantly redshift its optical gap and partially suppress the intervalley exciton-phonon scattering. This suppression results in a narrowing of the near-band excitonic transitions. We also discuss the effect of strain on the transport properties of MoTe2 due to this reduction in scattering. We investigate monolayer ReS2 as a TMDC system exhibiting strong in-plane anisotropy. These properties are explored by polarization-resolved spectroscopy. We show how the accessible optical properties vary with optical polarization. We find that the near-band excitons in ultrathin ReS2, absorb and emit light along specific polarizations. We also show that purely non-contact, optical techniques can determine the crystallographic orientation of ReS2.

In this dissertation, a series of polythiophene copolymers have been synthesized to study to photovoltaic characteristics. First of all, we have used Grignard metathesis polymerization to successfully synthesize a series of regioregular polythiophene copolymers that contain electron-withdrawing and conjugated phenanthrenyl-imidazole moieties as side chains. The introduction of the phenanthrenyl-imidazole moieties onto the side chains of the regioregular polythiophenes increased their conjugation lengths and thermal stabilities and altered their band gap structures. The band gap energies, determined from the onset of optical absorption, could be tuned from 1.89 eV to 1.77 eV by controlling the number of phenanthrenyl-imidazole moieties in the copolymers. Moreover, the observed quenching in the photoluminescence of these copolymers increases with the number of phenanthrenyl-imidazole moieties in the copolymers, owing to the fast deactivation of the excited state by the electron-transfer reaction. Both the lowered bandgap and fast charge transfer contribute to the much higher external quantum efficiency of the poly(3-octylthiophene)-side-chain-tethered phenanthrenyl-imidazole than that of pure poly(3-octylthiophene), leading to much higher short circuit current density. In particular, the short circuit current densities of the device containing the copolymer having 80 mol % phenanthrenyl-imidazole, P82, improved to 14.2 mA/cm2 from 8.7 mA/cm2 for the device of pure poly(3-octylthiophene), P00, an increase of 62%. In addition, the maximum power conversion efficiency improves to 2.80% for P82 from 1.22% for P00 (pure P3OT). Second, intramolecular donor–acceptor structures prepared by binding conjugated octylphenanthrenyl-imidazole moieties covalently onto the side chains of regioregular poly(3-hexylthiophene)s exhibit lowered bandgaps and enhanced electron transfer. For instance, conjugating 90 mol% octylphenanthrenyl-imidazole moieties onto poly(3-hexylthiophene) chains reduced the optical bandgap from 1.91 to 1.80 eV, and the electron transfer probability was at least twice than that of pure poly(3-hexylthiophene) when blended with [6,6]-phenyl-C61-butyric acid methyl ester. The lowered bandgap and the fast charge transfer both contribute to the much higher external quantum efficiencies—and, thus, much higher short-circuit current densities—for the copolymers presenting octylphenanthrenyl-imidazole moieties, relative to those of pure poly(3-hexylthiophene)s. In particular, the short-circuit current density of a device containing the copolymer presenting 90 mol% octylphenanthrenyl-imidazole moieties improved to 13.7 mA/cm2 from 8.3 mA/cm2 for the device containing pure poly(3-hexylthiophene)—an increase of 65%. In addition, the maximum power conversion efficiency was 3.45% for the copolymer presenting 90 mol% octylphenanthrenyl-imidazole moieties. Finally, PHPIT, a new kind of intramolecular D–A side-chain-tethered hexylphenanthrenyl-imidazole polythiophene has been synthesized. The visible light absorption of the PHPIT/PCBM blend is enhanced by the presence of the electron-withdrawing hexylphenanthrenyl-imidazole. The EQE of the device was maximized when the PHPIT/PCBM blend experienced annealing at 120 °C for 30 min. The more-balanced electron and hole mobilities and the enhanced visible and internal light absorptions in the devices consisting of annealed PHPIT/PCBM blends both contributed to a much higher short-circuit current density, which in turn led to a power conversion efficiency as high as 4.1%, despite the fact that PHPIT is only comprised of ca. 20 repeating units.

Optical absorption coefficients of un-implanted and implanted HgCdTe have been measured in a range of temperatures and compositions. The index of refraction for photon energies larger than bandgap was obtained. With the measured index of refraction, large values of the optical absorption coefficient were extracted from measured transmission spectra. The obtained optical absorption coefficient agrees very well with the Kane’s model. The measured optical absorption coefficient at the bandgap linearly depends on temperature. Cutoff photon energy for highly n-type doped HgCdTe resulted from ion implantation of boron at 150 keV energy and with 1 × 1015 cm−2 dose is larger than that for the un-implanted HgCdTe. The magnitude of the shift is consistent with the prediction of the theory of Moss-Burstein shift, using measured doping profile.

This study reports on fundamental processes in Quantum-Dot Solids, after light absorption. Transient Absorption and Time-resolved Photoluminescence spectrocopy reveal the dynamics of charge transfer and charge trapping processes. Typically, both occur on a picosecond time scale and compete with each other. We find that the efficiency of these processes depends on the Fermi level in the Quantum-Dot Solid. The latter can be controlled electrochemically, via charge injection into the Quantum-Dot Solid, using a potentiostat. The presented findings aid the rational design of opto-electronic devices based on Quantum Dots, such as solar cells or LEDs.

Objectives: In this article, a new procedure for increasing the light penetration depth in a tissue is studied and simulated. It has been reported that the most important problem in biomedical optical imaging relates to the light penetration depth, and so this makes a dramatic restriction on its applications. In the optical imaging method, the detection of the backscattered photons from a deep tumor is rarely done or is done with a low efficiency; it is because of the high absorption and scattering losses. Methods: Unlike the common methods (using a high energy laser for deep penetration) by engineering the nanoparticles’ optical properties such as their anisotropy, absorption, and scattering efficiency, which are distributed into a tissue, the detected photons amplitude can be manipulated. In other words, by engineering the nanoparticle plasmon properties and their effect on the dye molecules’ quantum yield, fluorescence emission and more importantly influence on the scattering direction, the light penetration depth is dramatically increased. Results: The modeling results (Monte-Carlo statistical method) illustrate that the detected photons dramatically increased which is on order of 4 mm. So, this method can fix the light penetration problems in the optical imaging system. Conclusions: Finally, the original idea of this study attributes to the indirect and transient manipulation of the optical properties of the tissue through the nanoparticles plasmon properties engineering. Moreover, by engineering plasmonic nanoparticles, maybe, the penetration depth can be enhanced which means that we can easily send light into a soft tissue and get its back scattering.

Supplemental Document

In this review, we discuss first some of the recent works to reveal properties of conventional ZnO/ZnMgO QWs grown c-axis oriented. This will include the properties of the quantum confined Stark effect (QCSE) that results from the internal electric field in the unit cell. We will then discuss various unconventional QW growths, including non-polar ZnO QWs, graded barrier QWs and double QWs. We finish with a review of current progress towards light emitting devices based on ZnO/ZnMgO QWs. ZnO has been a material of interest for over 50 years; however, the ability to grow high-quality epilayers of ZnO and ZnO-based ternary systems[1–4 has led to renewed interest over the past decade in ZnO for device applications. The demand for optoelectronic devices in the blue-UV region of the electromagnetic spectrum has been well established and ZnO possesses several properties that are superior to GaN for many applications [5, 6]. The large exciton binding energy of ∼ 60 meV suggests excitonic emission that is very efficient above room temperature, leading to great potential for light emitting devices. The large piezoelectric and pyroelectric coefficients suggest potential for applications as piezoelectric sensors or actuators. Other advantages of ZnO are its comparatively low growth temperatures [5], low optical power threshold for lasing [7], radiation hardness [8, 9] and biocompatibility [10]. ZnO may also find application as a transparent conductive oxide [11] to replace indium-tin-oxide (ITO) in photovoltaic applications because it remains transparent even when doped above the level of degeneracy [12]. This ability to grow high-quality epilayers and thin films rapidly led to the development of ZnO-based quantum wells (QWs). The two main attractions of developing QWs are the tunability offered for the transition energy, and the increased exciton binding energy, which typically leads to increased oscillator strength and greater efficiency for light emitting devices. The two most common types of ZnO-based QWs are Zno/ZnMgo and ZnO/ZnCdO-based systems, where alloying with Mg leads to an increase in the band gap and with Cd leads to a reduction in the band gap. In this chapter, we will focus on the more common ZnO/ZnMgO QWs. The band gap of Znx − 1Mg x O is given by 3. 37 + 2. 51x eV [13], which together with a conduction to valence band offset ratio of approximately 70:30 leads to type I confinement in \({\mathrm{ZnO/Zn}}_{x-1}{\mathrm{Mg}}_{x}\mathrm{O}\) QWs. The value of x is limited to less than 0.43 [13] in these systems as above this value phase separation tends to occur. Nevertheless, even with values significantly less than 0.43, strong confinement is obtained. In such ZnO/ZnMgO QWs, exciton binding energies up to 120 meV have been reported [14]. The biexciton binding energy is also enhanced in QWs going from 15 meV in bulk ZnO to ∼ 25 meV in ZnO/ZnMgO QWs [15–17]. With biexciton binding energies greater than the thermal energy, it is possible that biexcitons may play a major role in the optical properties at room temperature. Since the first ZnO epitaxial layers were grown, high-quality ZnO quantum wells have been grown by several different methods, including molecular beam epitaxy (MBE), metal-organic vapour phase epitaxy (MOVPE) and pulsed laser deposition (PLD). The growth in almost all cases is c-axis oriented despite growth on a range of single crystal substrates, including sapphire, ZnO and Si oriented along various crystal planes.KeywordsTransition EnergyPulse Laser DepositionExciton Binding EnergyPotential ProfileInternal Electric FieldThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Single-grain Silicon Technology for Large Area X-ray Imaging

Performance of the structure AlxGa1-xAs1-yNy/Ge for solar cell applications

Quantum confinements such as quantum wells, wires, and dots posses several advantages for next-generation solar cells. In this study, we present results on quantum confinement in PbS-ZnS quantum wells deposited by Atomic Layer Deposition (ALD). Materials selection criteria are presented with a focus on the properties of the well and barrier material. PbS quantum wells embedded in thin ZnS barrier layers are shown to demonstrate quantum confinement effects through scanning tunneling microscopy (STM). The band gap of the PbS films has been varied from 0.4–1.0 eV by varying the number of ALD cycles. The bandgap variation with film thickness is well matched to results predicted by effective-mass theory.