The main idea of this paper is inspired by a paper written together by my advisor Dr. Hang, Dr. Huang of the Industrial Technology Research Institute, and Dr. Chao of Institute of Applied Mechanics of National Taiwan University[quant-ph/0506153 v1,2005]. After some mathematical calculations we can extend the WKB approximation to treat position-dependent effective mass problem (PDEM). Then we did simulation on a model PDEM problem to compare the well-know closed form solution and the extended WKB approximation. We demonstrated that the extended WKB approximation not only can obtain the eigenvalues very accurately, but also is very useful to estimate the distribution of the wave function. We also found the modulation on the oscillations of wave function under PDEM by the extended WKB approximation.

In doctoral dissertation, results of investigation on zinc oxide based semiconductor layers and heterostructures for application in optoelectronics by spectroscopy methods are presented. High quality zinc oxide layers and heterostructures were characterized optically. That is ZnO, ZnO:Ga and MgZnO epitaxial layers grown by molecular beam epitaxy technique, ZnO:N layers grown by magnetron sputtering method, CdZnO/ZnO quantum wells structures for light-emitting diodes grown on GaN layers by combined molecular beam epitaxy and metalorganic chemical vapor deposition techniques. In this work, new data on dynamics and interaction of photoexcited carriers in zinc oxide based structures were acquired. It was shown, that the contribution of impurities bound excitons in the inelastic interaction of free excitons influences the position of luminescence band of the interaction in spectrum. In MgZnO epitaxial layers, the localization of carriers in the field of two different localization centers is described. Due to the localization the radiative recombination efficiency increases significantly and it has importance for application in optoelectronic devices. Investigation of the CdZnO/ZnO quantum wells structures for green spectral range light-emitting diodes showed that low radiative recombination efficiency at room temperature is mainly due to weak carrier localization effect, which is responsible for high efficiency of conventional InGaN/GaN quantum wells structures. / Daktaro disertacijoje pristatomi cinko oksido pagrindu užaugintų puslaidininkinių sluoksnių ir jų įvairialyčių darinių, skirtų taikymams optoelektronikoje, tyrimų rezultatai spektroskopijos metodais. Optiškai charakterizuoti aukštos kokybės cinko oksido sluoksniai ir jų įvairialyčiai dariniai: ZnO, ZnO:Ga ir MgZnO epitaksiniai sluoksniai, auginti molekulinės epitaksijos būdu, ZnO:N sluoksniai, auginti magnetroninio dulkinimo būdu, šviesos diodams paruošti CdZnO/ZnO kvantinių duobių dariniai ant GaN sluoksnių, auginti apjungiant molekulinės epitaksijos ir cheminio metaloorganinio junginio nusodinimo iš dujinės fazės auginimo metodus. Šiame darbe surinkta naujų duomenų apie fotosužadintų krūvininkų dinamiką ir sąveiką cinko oksido dariniuose. Nustatyta, kad netamprioje laisvų eksitonų sąveikoje dalyvaujantys prie priemaišų pririšti eksitonai įtakoja sąveikos liuminescencijos juostos padėtį spektre. Aprašyta krūvininkų lokalizacija MgZnO epitaksiniuose sluoksniuose dviejų skirtungų lokalizacijos centrų lauke. Dėl to žymiai padidėja spindulinės rekombinacijos efektyvumas, kas turi didelės svarbos taikymams optoelektronikos prietaisams. CdZnO/ZnO kvantinių duobių darinių, skirtų žalios spektro srities šviesos diodams, tyrimai parodė, kad mažas spindulinės rekombinacijos efektyvumas kambario temperatūroje yra dėl ženkliai silpnesnio krūvninkų lokalizacijos efekto, kuris užtikrina didelį efektyvumą įprastuose InGaN/GaN kvantinių duobių dariniuose.

Daktaro disertacijoje pristatomi cinko oksido pagrindu užaugintų puslaidininkinių sluoksnių ir jų įvairialyčių darinių, skirtų taikymams optoelektronikoje, tyrimų rezultatai spektroskopijos metodais. Optiškai charakterizuoti aukštos kokybės cinko oksido sluoksniai ir jų įvairialyčiai dariniai: ZnO, ZnO:Ga ir MgZnO epitaksiniai sluoksniai, auginti molekulinės epitaksijos būdu, ZnO:N sluoksniai, auginti magnetroninio dulkinimo būdu, šviesos diodams paruošti CdZnO/ZnO kvantinių duobių dariniai ant GaN sluoksnių, auginti apjungiant molekulinės epitaksijos ir cheminio metaloorganinio junginio nusodinimo iš dujinės fazės auginimo metodus. Šiame darbe surinkta naujų duomenų apie fotosužadintų krūvininkų dinamiką ir sąveiką cinko oksido dariniuose. Nustatyta, kad netamprioje laisvų eksitonų sąveikoje dalyvaujantys prie priemaišų pririšti eksitonai įtakoja sąveikos liuminescencijos juostos padėtį spektre. Aprašyta krūvininkų lokalizacija MgZnO epitaksiniuose sluoksniuose dviejų skirtungų lokalizacijos centrų lauke. Dėl to žymiai padidėja spindulinės rekombinacijos efektyvumas, kas turi didelės svarbos taikymams optoelektronikos prietaisams. CdZnO/ZnO kvantinių duobių darinių, skirtų žalios spektro srities šviesos diodams, tyrimai parodė, kad mažas spindulinės rekombinacijos efektyvumas kambario temperatūroje yra dėl ženkliai silpnesnio krūvninkų lokalizacijos efekto, kuris užtikrina didelį efektyvumą įprastuose InGaN/GaN kvantinių duobių dariniuose. / In doctoral dissertation, results of investigation on zinc oxide based semiconductor layers and heterostructures for application in optoelectronics by spectroscopy methods are presented. High quality zinc oxide layers and heterostructures were characterized optically. That is ZnO, ZnO:Ga and MgZnO epitaxial layers grown by molecular beam epitaxy technique, ZnO:N layers grown by magnetron sputtering method, CdZnO/ZnO quantum wells structures for light-emitting diodes grown on GaN layers by combined molecular beam epitaxy and metalorganic chemical vapor deposition techniques. In this work, new data on dynamics and interaction of photoexcited carriers in zinc oxide based structures were acquired. It was shown, that the contribution of impurities bound excitons in the inelastic interaction of free excitons influences the position of luminescence band of the interaction in spectrum. In MgZnO epitaxial layers, the localization of carriers in the field of two different localization centers is described. Due to the localization the radiative recombination efficiency increases significantly and it has importance for application in optoelectronic devices. Investigation of the CdZnO/ZnO quantum wells structures for green spectral range light-emitting diodes showed that low radiative recombination efficiency at room temperature is mainly due to weak carrier localization effect, which is responsible for high efficiency of conventional InGaN/GaN quantum wells structures.

The family of semiconducting 2H-phase group-VI transition metal dichalcogenides (TMDs) have been suggested to be promising candidates for hosting optically accessible spin qubits due to their desirable optical and electrical properties, however, experimental progress towards this goal has been impeded by the difficulties associated with the fabrication of clean structures with quality contacts. In this thesis, we present the complex process for obtaining functional contacts to two particular TMDs, molybdenum disulfide (MoS2) and tungsten diselenide (WSe2), from which we use as the foundation for the fabrication of three important gate defined quantum structures: quantum dots, a charge detector, and a long 1D channel. These structures all play an important role in furthering the understanding of these materials and are the building blocks for achieving functional spin qubits. More precisely, we investigate the contact resistances associated with various cleaning procedures and contact architectures and report a recipe that results in an ultra-low contact resistance even at cryogenic temperatures. We then demonstrate electrical control of hole quantum dots, the host of the spin qubit, in gated heterostructure devices based on monolayer WSe2 and study its properties. With a similar structure, we demonstrate that a gate-defined nano-constriction is sensitive to the charge occupation of a nearby quantum dot and is therefore suitable to be used as a charge sensor, a valuable component of elaborate quantum circuits. Finally, we demonstrate the realization of a gate-defined quantum confined 1D channel in a high mobility monolayer WSe2 sample and observe an anomalous conductance quantization in units of e2/h. These results pave the way for the development of quantum devices based on electrostatically confined quantum dots defined in semiconducting TMDs and push forward our understanding of their electronic properties.

Le photovoltaïque (PV) s’est imposé comme un acteur majeur de l’énergie. L’innovation dans ce domaine passera sans doute par le PV à haut rendement sur des couches minces flexibles et légères permettant son déploiement dans les applications mobiles. Cette thèse étudie le développement de cellules solaires III-V à structures quantiques visant des concepts PV hauts rendements tels les cellules solaires à bande intermédiaire (IBSC). Ces IBSC se sont montrés limités du fait de l’échappement thermique des porteurs à température ambiante ainsi que la faible absorption optique sous le gap. Nous avons évalué la topologie, le mécanisme d’échappement thermique, la structure quantique ainsi que l’absorption de boites quantiques en In(Ga)As dans un matériau hôte en Al0.2GaAs à grand gap. Nous avons aussi caractérisé de manière quantitative comment opère ce système et avons amélioré son design optique. Sous une forte irradiation, nous avons mis en évidence l’apparition d’une population de porteurs chauds dans les boites quantiques. Par ailleurs, l’effet d’absorption sequentielle à deux photons (S-TPA) a été démontré. Nous avons observé une augmentation de ce S-TPA d’un facteur x5-10 grâce à du management de la lumière réalisé notamment avec des cavités de Fabry-Pérot. Des nanostructures périodiques ont aussi été fabriquées dans le cas de cellules solaires à multi-puits quantiques par l’utilisation de lithographie en nanoimpression. Dans l’ensemble cette étude vise à discuter la possibilité de réaliser des cellules solaires à porteurs chauds assistés d’une bande intermédiaire et améliorées par un management optique afin d’ouvrir la voie pour des cellules à hauts rendements. / In the past decade, photovoltaics (PV) has become a key player for the future of worldwide energy generation. Innovation in PV is likely to rely on high efficiency PV with flexible and lightweight thin films to enable PV deployement for mobile applications. In the framework of the Japanese-French laboratory “NextPV”, this thesis investigates the development of III-V quantum structured solar cells to explore high-efficiency photovoltaic concepts especially intermediate band solar cells (IBSC). Quantum structured IBSC have proven to be limited by thermal escape at room temperature and by low subbandgap light absorption. Following a consistent approach, we evaluate the topology, thermal escape mechanism, quantum structure and optical absorption of In(Ga)As quantum dots in a wide gap Al0.2GaAs host material. We also characterize quantitatively the device operation and improve the optical design. For a high irradiation, we evidence a hot carrier population in the quantum dots. At the same time, sequential two-photon absorption (S-TPA) is demonstrated both optically and electrically. We also show that S-TPA for both subbandgap transitions can be enhanced by a factor x5-10 with light management techniques, for example by implementation of Fabry-Perot cavities with the different epitaxial transfer methods that we developed. More advanced periodical nanostructures were also fabricated in the case of multi-quantum well solar cells using nanoimprint lithography techniques. Overall we discuss the possibility of realizing intermediate-band-assisted hotcarrier solar cells with light management to open the path for high-efficiency quantum structured IBSC.

GaP ist ein Halbleiter mit einer großen Bandlückenenergie und infolgedessen transparent im größten Teil der sichtbaren Wellenlängen. GaP hat außerdem die kleinste Gitterfehlanpasung zu Si (weniger als 0.4%). Das macht GaP ein interessantes Material für monolithische Integration zu III–V Lichtsender auf Si. Diese Arbeit ist eine Untersuchung über die strukturellen und optische Eigenschaften von (Al, Ga) (As, P) Heterostrukturen auf GaP (001) -Substrat aufgewachsen. Die Einflüsse des PH3 Fluss und Wachstumstemperatur untersucht auf dem Kristallqualität und Oberflächenqualität von AlGaP/GaP Heterostructure. Experimentelle Ergebnisse deuten darauf hin, dass eine Wachstumstemperatur von 490 oC und ein geknackter (engl. cracked) PH3 Fluss von 2.7 sccm zur besten AlGaP Qualität und gleichzeitig zur guten GaP Qualität führen. Um die ineffiziente Lichtemission von GaP zu überwinden wurde GaAs in der GaP-Matrix gewachsen. Die Entstehung der Quantenpunkte wurde durch die 3.7% Gitterfehlanpassung zwischen GaAs und GaP für GaAs Nenndicke über 1,2 ML. Die optischen Messungen zeigen zwei Peaks im Bereich von 1,7 bis 2,1 eV und die Lumineszenz auf Raumtemperatur für 2,7 und 3,6 ML-Proben. Die hohe Energieemission wird der indirekten Rekombination in den dünnen Quantentröge oder kleine gespannte Quantenpunkte zurückzuführen, Während die niedrige Energie Emission ist aufgrund der direkten Elektron-Loch- Rekombination in der entspannten Quantenpunkte. Die Wirkung von Al wird untersucht auf die energetische Bandausrichtung und auf die elektronische Struktur der (Al,Ga)As Quantenstrukturen. Die optische Spektren zeigten einen blaue Verschiebung (engl. blue shift) mit wachsendem Al-Inhalt und die höchste missionsenergie für die (Al,Ga)As/GaP- Heterostruktur war 2.17 eV die zum indirekten Typ-II-Rekombination zusammenhängt. / Transparency of GaP due to the large indirect bandgap energy and its small lattice mismatch with Si make GaP an interesting candidate for optoelectronic devices in visible wavelength. This thesis is an investigation on the structural and optical characteristics of (Al,Ga)(As,P) heterostructures grown on GaP (001) substrates. The influences of the PH3 flux and growth temperature are studied on the crystal and surface quality of AlGaP/GaP heterostructure. The results indicate the narrow growth window of PH3 = 2.7 sccm and growth temperature = 490oC as the optimized conditions. To overcome the inefficient light emission of indirect GaP, direct bandgap GaAs was grown as the quantum structures in the GaP matrix. The QD formation is driven by the 3.7% lattice mismatch between GaAs and GaP for GaAs nominal thickness above 1.2 ML. The optical measurements show two peaks in the range of 1.7 to 2.1 eV and the luminescence up to room temperature for 2.7 and 3.6 ML samples. The high energy emission is attributed to indirect carrier recombination in the thin quantum wells or small strained quantum dots, whereas the low energy red emission is due to the direct electron-hole recombination in the relaxed quantum dots. The influence of the Al content on the band alignment and electronic structure of (Al,Ga)As quantum structures is studied. The optical spectra illustrate the blueshift of the radiative emission with increasing the Al content and the highest emission energy of 2.17 eV is observed for the (Al,Ga)As/GaP system that is related to the indirect type-II radiative recombination.