This dissertation includes work done on graphene and related materials, examining their electronic and topological properties using first-principles methods. Ab-initio computational methods, like density functional theory (DFT), have become increasingly popular in condensed matter and material science. Motivated by the search for novel materials that would help us devise fast, low-power, post-CMOS transistors, we explore the properties of some of these promising materials. We begin by studying graphene and its interaction with dielectric oxides. Graphene has recently inspired a flurry of research activity due to its interesting electronic and mechanical properties. For the device community, graphene's high charge carrier mobility and continuous gap tunability can have immense use in novel transistors. In Chapter 3 we examine the properties of graphene placed on two oxides, namely quartz and alumina. We find that oxygen-terminated quartz is a useful oxide for the purpose of graphene based FETs. Inspired by a recent surge of interest in topological insulators, we then explore the topological properties of two-dimensional materials. We conduct a theoretical study to examine the relationship between crystal space group symmetry and the electric polarization of a two-dimensional crystal. We show that the presence of symmetry restricts the polarization values to a small number of distinct groups. There groups in turn are topologically inequivalent, making polarization a topological index. We also conduct density functional theory calculations to obtain actual polarization values of materials belonging to C3 symmetry and show that our results are consistent with our theoretical analysis. Finally we prove that any transformation from one class of polarization to another is a topological phase transition. In Chapter 5 we use density functional theory to examine the electronic properties of graphene intercalation compounds. Bilayer pseudospin field effect transistor (BiSFET) has been proposed as an interesting low-power, efficient post-CMOS switch. In order to implement this device we need bilayer graphene with reduced interlayer interaction. One way of achieving that is by inserting foreign molecules between the layers, a process which is called intercalation. In this chapter we examine the electronic properties of bilayer graphene intercalated with iodine monochloride and iodine monobromide molecules. We find that intercalation of graphene indeed makes it promising for the implementation of BiSFET, by reducing interlayer interaction. As an interesting side problem, we also use hybrid, more extensive approaches in DFT, to examine the electronic and optical properties of dilute nitrides. Dilute nitrides are highly promising and interesting materials for the purposes of optoelectronic applications. Together, we hope this work helps in elucidating the electronic properties of promising material systems as well as act as a guide for experimentalists. / text

abstract: III-V multijunction solar cells have demonstrated record efficiencies with the best device currently at 46 % under concentration. Dilute nitride materials such as GaInNAsSb have been identified as a prime choice for the development of high efficiency, monolithic and lattice-matched multijunction solar cells as they can be lattice-matched to both GaAs and Ge substrates. These types of cells have demonstrated efficiencies of 44% for terrestrial concentrators, and with their upright configuration, they are a direct drop-in product for today’s space and concentrator solar panels. The work presented in this dissertation has focused on the development of relatively novel dilute nitride antimonide (GaNAsSb) materials and solar cells using plasma-assisted molecular beam epitaxy, along with the modeling and characterization of single- and multijunction solar cells. Nitrogen-free ternary compounds such as GaInAs and GaAsSb were investigated first in order to understand their structural and optical properties prior to introducing nitrogen. The formation of extended defects and the resulting strain relaxation in these lattice-mismatched materials is investigated through extensive structural characterization. Temperature- and power-dependent photoluminescence revealed an inhomogeneous distribution of Sb in GaAsSb films, leading to carrier localization effects at low temperatures. Tuning of the growth parameters was shown to suppress these Sb-induced localized states. The introduction of nitrogen was then considered and the growth process was optimized to obtain high quality GaNAsSb films lattice-matched to GaAs. Near 1-eV single-junction GaNAsSb solar cells were produced. The best devices used a p-n heterojunction configuration and demonstrated a current density of 20.8 mA/cm2, a fill factor of 64 % and an open-circuit voltage of 0.39 V, corresponding to a bandgap-voltage offset of 0.57 V, comparable with the state-of-the-art for this type of solar cells. Post-growth annealing was found to be essential to improve Voc but was also found to degrade the material quality of the top layers. Alternatives are discussed to improve this process. Unintentional high background doping was identified as the main factor limiting the device performance. The use of Bi-surfactant mediated growth is proposed for the first time for this material system to reduce this background doping and preliminary results are presented. / Dissertation/Thesis / Doctoral Dissertation Electrical Engineering 2017

[pt] Se faz um estudo detalhado sobre o crescimento de GaInNAs pela técnica de metalorganic vapor phase epitaxy (MOVPE) com o objetivo de diminuir a contaminação residual do carbono no material. Para isso se pesquisa a influência dos precursores assim como dos diferentes parâmetros de crescimento na morfologia, na contaminação residual e na incorporação de diferentes elementos nos materiais. A temperatura, a taxa de crescimento, a razão As/III, o conteúdo do nitrogênio e os diferentes precursores são os parâmetros alterados de uma amostra para a outra. Como alguns precursores não foram usados antes para este material semicondutor, inicialmente apenas o GaAs foi examinado, passando posteriormente aos materiais ternários GaInAs e GaNAs, para finalmente obter o GaInNAs. Através da caracterização das amostras obtém-se a qualidade dos materiais assim como a contaminação residual do carbono. São utilizadas técnicas de difração de raios-X de alta resolução (HR-XRD), microscopia de força atômica (AFM), perfil eletroquímico de capacitância-tensão (ECV), espectrometria de massa de íons secundários (SIMS), fotoluminescência (PL) e técnicas in-situ próprias do reator MOVPE para avaliar o efeito dos parâmetros de crescimento epitaxial sobre a qualidade das estruturas obtidas, assim como sobre a incorporação dos diferentes elementos nos materiais. O crescimento do GaInNAs no reator CRIUS foi bem sucedido com algumas combinações dos precursores. Se confirmou que o alto nível do carbono está relacionado à quantidade do nitrogênio no cristal e que o carbono é fornecido pelos grupos metil dos precursores, principalmente pelo TMGa, seguido do TMIn. Todas as medidas para reduzir a incorporação residual do carbono foram bem sucedidas. O uso de precursores do grupo III sem grupo metil reduz significativamente a dopagem residual do carbono. Finalmente foram crescidas células solares para avaliar o material no dispositivo. / [en] A detailed investigation the GaInNAs growth by metalorganc vapor phase epitaxy (MOVPE) is performed in order to reduce the carbon background in the material. For this, the precursors , as well as the different growth parameters influence on crystal morphology, carbon background and the incorporation of different elements on the semiconductor material, are investigated. The temperature, the growth rate, the As/III ratio, the nitrogen content, and the different precursors were varied from one sample to another. Particularly, since some precursors were never used for this semiconductor material, initially, only GaAs was examined, later the GaInAs and the GaNAs ternary materials were grown, to finally obtain the GaInNAs. The samples characterization was used to assess materials quality, as well as the carbon background incorporation. Different characterization techniques such as High-Resolution X-Ray Diffraction (HR-XRD), Atomic Force Microscopy (AFM), Electrochemical Capacitance-Voltage (ECV) and In-Situ measurements were used to evaluate the effect of the epitaxial growth parameters on the quality of the obtained structures, as well as on the different elements incorporation in the semiconductor material. GaInNAs was successfully grown in the MOVPE reactor with particular precursors combinations. The relation between the high carbon level and the nitrogen amount in the crystal was confirmed, as well as the fact that carbon is supplied by methyl-groups of the precursors, especially TMGa, followed by TMIn. All measures to reduce carbon background incorporation were successful. The use of methyl group free III precursors significantly reduces the carbon background incorporation. Finally, solar cells were grown to evaluate the material in the device.

Das Thema dieser Dissertation ist das MBE-Wachstum und die Charakterisierung von Ga(As,N) und (In,Ga)(As,N). Die Arbeit beginnt mit der Optimierung des Wachstums von Ga(As,N). Aufgrund der hohen Mischbarkeitslücke von GaN in GaAs verursacht der Einbau von Stickstoff in GaAs eine strukturelle Degradation, die von der Substrattemperatur, der Stickstoffkonzentration und der Quantentopfdicke abhängt. Ein weiteres Problem bezüglich des Wachstums von Ga(As,N) sind Punktdefekte, die einen schädlichen Einfluß auf optische Eigenschaften haben. Eine thermische Behandlung verringert die Konzentration dieser Punktdefekte. Dies geht mit einer Steigerung der Photolumineszenz-Intensität einher. Punktdefekte sind zum Beispiel Stickstoff-Dimere, die sich in Gallium- oder Arsen-Vakanzen einbauen. Eine thermische Behandlung bei hohen Temperaturen bewirkt jedoch eine strukturelle Degradation im Ga(As,N)-Materialsystem, die sich in einer Abnahme der Photolumineszenz-Intensität manifestiert. Es wird gezeigt, daß die Temperatur der thermischen Behandlung, die die höchste Photolumineszenz-Ausbeute erzielt, von der Stickstoffkonzentration abhängt. Bezüglich des Wachstums von (In,Ga)(As,N) verursacht die Mischbarkeitslücke von (In,Ga)N in (In,Ga)As ebenfalls eine strukturelle Degradation. Auch im quaternären Materialsystem ist eine thermische Behandlung essentiell für die Verbesserung optischer Eigenschaften. Es wird außerdem gezeigt, daß die thermische Behandlung von (In,Ga)As eine Indiumdiffusion verursacht, die durch den Einbau von Stickstoff gestoppt wird. Die Charakterisierung von (In,Ga)(As,N) kantenemittierenden Lasern zeigt Emissionen bei Wellenlängen bis zu 1366 nm. Mit dem Einbau von Stickstoff ist ein Anstieg der Schwellstromdichte und ein Abfall der Emissionsleistung verbunden. / This dissertation deals with the MBE growth and characterization of Ga(As,N) and (In,Ga)(As,N). The work commences with the optimization of the Ga(As,N) growth. Owing to a large miscibility gap of GaN in GaAs, the incorporation of nitrogen into GaAs causes a structural degradation that is dependent on the substrate temperature, the nitrogen concentration, and the quantum well thickness. Another problem related to the growth of Ga(As,N) are point defects that have a detrimental influence on optical properties. A thermal treatment of Ga(As,N) reduces the concentration of these point defects. This leads to a substantial improvement of optical properties. We will show that nitrogen split interstitials that incorporate into gallium and arsenic vacancies may be attributed to these point defects. A thermal treatment of Ga(As,N) at high temperatures, on the contrary, results in a creation of extended defects which are detrimental to optical properties. We will show that the temperature of the thermal treatment that yields the highest photoluminescence intensity is nitrogen concentration-dependent. The growth of (In,Ga)(As,N) is similar with respect to Ga(As,N). Again, one has to face a high miscibility gap of (In,Ga)N in (In,Ga)As that results in a structural degradation. A thermal treatment of (In,Ga)(As,N) is also beneficial for improving optical properties. We will show that a thermal treatment of (In,Ga)As results in an indium diffusion that is suppressed by the incorporation of nitrogen. The characterization of (In,Ga)(As,N) edge emitting lasers shows emission at wavelengths up to 1366 nm. With higher nitrogen concentrations, there is a strong increase of the threshold current density and a decrease of the output power.

This thesis work aims at a better understanding of magneto-optical properties of dilute nitrides and II-VI diluted magnetic semiconductor quantum structures. The thesis is divided into two parts. The first part gives an introduction of the research fields, together with a brief summary of the scientific results included in the thesis. The second part consists of seven scientific articles that present the main findings of the thesis work. Below is a short summary of the thesis. Dilute nitrides have been of great scientific interest since their development in the early 1990s, because of their unusual fundamental physical properties as well as their potential for device applications. Incorporation of a small amount of N in conventional Ga(In)As or Ga(In)P semiconductors leads to dramatic modifications in both electronic and optical properties of the materials. This makes the dilute nitrides ideally suited for novel optoelectronic devices such as light emitting devices for fiber-optic communications, highly efficient visible light emitting devices, multi-junction solar cells, etc. In addition, diluted nitrides open a window for combining Si-based electronics with III-V compounds-based optoelectronics on Si wafers, promising for novel optoelectronic integrated circuits. Full exploration and optimization of this new material system in device applications requires a detailed understanding of their physical properties. Papers I and II report detailed studies of effects of post-growth rapid thermal annealing (RTA) and growth conditions (i.e. presence of N ions, N2 flow, growth temperature and In alloying) on the formation of grown-in defects in Ga(In)NP. High N2 flow and bombardment of impinging N ions on grown sample surface is found to facilitate formation of defects, such as Ga interstitial (Gai) related defects, revealed by optically detected magnetic resonance (ODMR). These defects act as competing carrier recombination centers, which efficiently decrease photoluminescence (PL) intensity. Incorporation of a small amount of In (e.g. 5.1%) in GaNP seems to play a minor role in the formation of the defects. In GaInNP with 45% of In, on the other hand, the defects were found to be abundant. Effect of RTA on the defects is found to depend on initial configurations of Gai related defects formed during the growth. In Paper III, the first identification of an interfacial defect at a heterojunction between two semiconductors (i.e. GaP/GaNP) is presented. The interface nature of the defect is clearly manifested by the observation of ODMR lines originating from only two out of four equivalent <111> orientations. Based on its resolved hyperfine interaction between an unpaired electronic spin (S=1/2) and a nuclear spin (I=1/2), the defect is concluded to involve a P atom at its core with a defect/impurity partner along a <111> direction. Defect formation is shown to be facilitated by N ion bombardment. In Paper IV, the effects of post-growth hydrogenation on the efficiency of the nonradiative (NR) recombination centers in GaNP are studied. Based on the ODMR results, incorporation of H is found to increase the efficiency of the NR recombination via defects such as Ga interstitials. In Paper V, we report on our results from a systematic study of layered structures containing an InGaNAs/GaAs quantum well, by the optically detected cyclotron resonance (ODCR) technique. By monitoring PL emissions from various layers, the predominant ODCR peak is shown to be related to electrons in GaAs/AlAs superlattices. This demonstrates the role of the SL as an escape route for the carriers confined within the InGaNAs/GaAs single quantum well. The last two papers are within a relatively new field of spintronics which utilizes not only the charge (as in conventional electronics) but also the quantum mechanical property of spin of the electron. Spintronics offers a pathway towards integration of information storage, processing and communications into a single technology. Spintronics also promises advantages over conventional charge-based electronics since spin can be manipulated on a much shorter time scale and at lower cost of energy. Success of semiconductor-based spintronics relies on our ability to inject spin polarized electrons or holes into semiconductors, spin transport with minimum loss and reliable spin detection. In Papers VI and VII, we study the efficiency and mechanism for carrier/exciton and spin injection from a diluted magnetic semiconductor (DMS) ZnMnSe quantum well into nonmagnetic CdSe quantum dots (QD’s) by means of spin-polarized magneto PL combined with tunable laser spectroscopy. By means of a detailed rate equation analysis presented in Paper VI, the injected spin polarization is deduced to be about 32%, decreasing from 100% before the injection. The observed spin loss is shown to occur during the spin injection process. In Paper VII, we present evidence that energy transfer is the dominant mechanism for carrier/exciton injection from the DMS to the QD’s. This is based on the fact that carrier/exciton injection efficiency is independent of the width of the ZnSe tunneling barrier inserted between the DMS and QD’s. In sharp contrast, spin injection efficiency is found to be largely suppressed in the structures with wide barriers, pointing towards increasing spin loss.