InAs/GaAs quantum dots (QDs) embedded within InP/GaP nanowires (NWs) were grown on Si substrates by Au-assisted and self-assisted vapor-liquid-solid (VLS) growth using molecular beam epitaxy (MBE). The morphology and structure of the NWs was characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The samples were analysed using photoluminescence (PL) and photocurrent measurements to study the properties of NW-based QDs. The composition of InAs x P 1-x QDs embedded within InP NWs was varied from x = 0.25 to x = 1, demonstrating the tuning of quantum confined energy levels. PL measurements demonstrated an emission peak that shifted towards lower energy levels as the As composition was increased. This result was also observed for QD absorption peaks through wavelength-dependent room temperature photocurrent measurements. InP NWs were successfully passivated with an AlInP shell, which was demonstrated through PL analysis. The growth mechanism of patterned self-assisted GaP NWs on Si was studied through SEM and TEM analysis. It was found that for large V/III flux ratios the Ga seed particle reduced in volume throughout growth, which led to a smaller NW diameter. Conversely, for small V/III flux ratios the Ga seed particle increased in volume throughout growth, resulting in larger NW diameters. The dependence of V/III flux ratio on NW growth was characterized, allowing the tuning of NW diameter. iv GaP NWs with p-i-n junctions were fabricated on a Si substrate with GaAs QDs embedded within the intrinsic region. To the author’s knowledge, this is the first time such a device was demonstrated. The device demonstrated diode characteristics as expected for a p-n junction. Wavelength-dependent photocurrent measurements demonstrated the absorption of light within GaAs QDs, which was collected through electric field dependent tunneling and thermionic emission. The absorption of light extended beyond the bandgap of GaP due to the GaAs QDs. / Thesis / Doctor of Philosophy (PhD)

<p>Molecular Beam Epitaxy (MBE) is a well-established method to grow low-dimensional structures for research applications. MBE has given many contributions to the rapid expanding research-area of nano-technology and will probably continuing doing so. The MBE equipment, dedicated for Silicon/Germanium (Si/Ge) systems, at Karlstads University (Kau) has been studied and started for the first time. In the work of starting the system, all the built in interlocks has been surveyed and connected, and the different subsystems has been tested and evaluated. Service supplies in the form of compressed air, cooling water and electrical power has been connected. The parts of the system, their function and some of the theory behind them are described.</p><p>The theoretical part of this master’s thesis is focused on low-dimensional structures, so-called quantum wells, wires and dots, that all are typical MBE-built structures. Physical effects, and to some extent the technical applications, of these structures are studied and described.</p><p>The experimental part contains the MBE growth of a Si/Ge quantum well (QW) structure and characterisation by Auger Electron Spectroscopy (AES). The structure, consisting of three QW of Si0,8Ge0,2 separated by thicker Si layers, was built at Linköpings University (LiU) and characterised at Chalmers University of Technology (CTH). The result of the characterisation was not the expected since almost no Ge content could be discovered but an extended characterisation may give another result.</p><p>Keywords: Silicon, Germanium, Molecular Beam Epitaxy, MBE, Quantum wells</p> / <p>Molecular Beam Epitaxy (MBE) är en väl etablerad metod när det gäller tillverkning av låg-dimensionella strukturer för forskningsändamål och lämpar sig väl för användning inom det expanderande forskningsområdet nanoteknologi. MBE utrustningen vid Karlstads universitet (Kau), som är avsedd för kisel/germanium (Si/Ge) strukturer, har studerats och startats för första gången. Under studien av systemet har alla inbyggda förreglingar utretts och anslutits och de olika delsystemen har testats och utvärderats. Tryckluft, kylvatten och el har utretts och anslutits. Systemets delar, deras funktion och i viss mån den bakomliggande teorin har studerats.</p><p>Den teoretiska delen av detta arbete är inriktad mot låg-dimensionella strukturer d.v.s. kvant brunnar, kvanttrådar och kvantprickar, som alla är strukturer lämpliga för framställning i MBE processer. De fysikaliska effekterna och i viss mån de tekniska tillämpningarna för dessa strukturer har studerats.</p><p>Den experimentella delen består av MBE tillväxt av en Si/Ge kvantbrunn-struktur och karakterisering m.h.a. Auger Electron Spectroscopy (AES). Tillväxten av strukturen, som består av tre kvantbrunnar av Si0,8Ge0,2 separerade av tjockare Si-lager, utfördes på Linköpings Universitet (LiU) och karakteriseringen utfördes på Chalmers Tekniska Högskola (CTH). Resultatet av karakteriseringen var inte det förväntade då knappast något Ge innehåll kunde detekteras men en utökad undersökning skulle kanske ge ett annat resultat.</p><p>Sökord: Kisel, germanium, Molecular Beam Epitaxy, MBE, kvantbrunn</p>

Molecular Beam Epitaxy (MBE) is a well-established method to grow low-dimensional structures for research applications. MBE has given many contributions to the rapid expanding research-area of nano-technology and will probably continuing doing so. The MBE equipment, dedicated for Silicon/Germanium (Si/Ge) systems, at Karlstads University (Kau) has been studied and started for the first time. In the work of starting the system, all the built in interlocks has been surveyed and connected, and the different subsystems has been tested and evaluated. Service supplies in the form of compressed air, cooling water and electrical power has been connected. The parts of the system, their function and some of the theory behind them are described. The theoretical part of this master’s thesis is focused on low-dimensional structures, so-called quantum wells, wires and dots, that all are typical MBE-built structures. Physical effects, and to some extent the technical applications, of these structures are studied and described. The experimental part contains the MBE growth of a Si/Ge quantum well (QW) structure and characterisation by Auger Electron Spectroscopy (AES). The structure, consisting of three QW of Si0,8Ge0,2 separated by thicker Si layers, was built at Linköpings University (LiU) and characterised at Chalmers University of Technology (CTH). The result of the characterisation was not the expected since almost no Ge content could be discovered but an extended characterisation may give another result. Keywords: Silicon, Germanium, Molecular Beam Epitaxy, MBE, Quantum wells / Molecular Beam Epitaxy (MBE) är en väl etablerad metod när det gäller tillverkning av låg-dimensionella strukturer för forskningsändamål och lämpar sig väl för användning inom det expanderande forskningsområdet nanoteknologi. MBE utrustningen vid Karlstads universitet (Kau), som är avsedd för kisel/germanium (Si/Ge) strukturer, har studerats och startats för första gången. Under studien av systemet har alla inbyggda förreglingar utretts och anslutits och de olika delsystemen har testats och utvärderats. Tryckluft, kylvatten och el har utretts och anslutits. Systemets delar, deras funktion och i viss mån den bakomliggande teorin har studerats. Den teoretiska delen av detta arbete är inriktad mot låg-dimensionella strukturer d.v.s. kvant brunnar, kvanttrådar och kvantprickar, som alla är strukturer lämpliga för framställning i MBE processer. De fysikaliska effekterna och i viss mån de tekniska tillämpningarna för dessa strukturer har studerats. Den experimentella delen består av MBE tillväxt av en Si/Ge kvantbrunn-struktur och karakterisering m.h.a. Auger Electron Spectroscopy (AES). Tillväxten av strukturen, som består av tre kvantbrunnar av Si0,8Ge0,2 separerade av tjockare Si-lager, utfördes på Linköpings Universitet (LiU) och karakteriseringen utfördes på Chalmers Tekniska Högskola (CTH). Resultatet av karakteriseringen var inte det förväntade då knappast något Ge innehåll kunde detekteras men en utökad undersökning skulle kanske ge ett annat resultat. Sökord: Kisel, germanium, Molecular Beam Epitaxy, MBE, kvantbrunn

Since the initial breakthroughs in structural quality and p-type conductivity in GaN during the late 1980s, the group-III nitride material system has attracted an enormous amount of interest because of its properties and applications in both electronics and optoelectronics. Although blue light-emitting diodes have been commercialized based on this success, much less progress has been made in ultraviolet emitters, green emitters, and photovoltaics. This lack of development has been attributed to insufficient structural and electrical material quality, which is directly linked to the growth of the material. The objective of this work is to expand the understanding of III-nitride growth towards the improvement of current device capabilities and the facilitation of novel device designs. Group-III nitride thin films are grown by molecular-beam epitaxy in a pulsed, metal-rich environment. The growths of nitride binaries and ternaries are observed in situ by transient reflection high-energy electron diffraction (RHEED) intensities, which respond to the behavior of atoms on the growing surface. By analyzing and interpreting these RHEED signatures, a comprehensive understanding of nitride thin film growth is obtained. The growth kinetics of unintentionally doped GaN by metal-rich MBE are elucidated, and a novel method of in situ growth rate measurement is discovered. This technique is expanded to InN, highlighting the similarity in molecular-beam epitaxy growth kinetics between III-nitride binaries. The growth of Mg-doped GaN is then explored to increase Mg incorporation and electrical activation. The growth of InxGa1-xN alloys are investigated with the goal of eliminating phase separation, which enables single-phase material for use in photovoltaics. Finally, the growth of unintentionally doped and Mg-doped AlGaN is investigated towards higher efficiency light emitting diodes. These advancements in the understanding of III-nitride growth will address several critical problems and enable devices relying on consistent growth in production, single-phase material, and practical hole concentrations in materials with high carrier activation energies.

Oxide related research has increased as standard oxides reach their operational limits and new classes of devices are imagined that can only be realized through the use of man-made compounds. Many of these devices require high quality films in order to reach their highest potential. Molecular beam epitaxy (MBE) is poised to become a key producer of high quality oxides. One of the most promising oxides is lithium niobate, LiNbO3, which can potentially deliver novel electronic, optic, and hybrid devices not currently possible. Growing lithium niobate using MBE is difficult. Several concepts are presented that will make this task easier. First, high temperature refractory metals can be delivered to the substrate through a novel use of low temperature chloride compounds such as niobium (V) chloride. This chloride chemistry allows low temperature sources to deliver high temperature materials to the substrate. Second, a precision, vapor-phase source and control system is prototyped for these chloride compounds achieving improved flux accuracy and expanding the capability of standard MBEs to support many sources. Chloride sources have high vapor pressures and are sensitive to temperature changes causing flux drift. The vapor-phase source removes the temperature sensitivity and eliminates thermal drifts. Third, a novel method of measuring flux with spontaneous ionzation current has been developed. This design utilizes a low noise design to measure femtoamp currents generated as an evaporant spontaneously ionizes. The measured current with additional predicted data has the potential for directly counting the atoms evaporated and controlling evaporation from a source. The design is sensitive enough to detect outgassing of the cell and cell "spitting" or other non-idealities. Monitoring these non-idealities can help improve other processes by ensuring the cell is fully outgassed and stable. Finally, a miniaturized RF induction cell prototype is shown that can eliminate the need for incandescent filaments in an oxide based MBE. The RF cell has the potential to increase reliability of MBEs for oxide work and achieve higher operating temperatures without the need for densely wound incandescent filaments or electron beam sources.

The objective of the proposed research is to establish the technology for material growth by molecular beam epitaxy (MBE) and fabrication of indium gallium nitride/gallium nitride (InxGa1-xN/GaN) heterojunction solar cells. InxGa1-xN solar cell have the potential to span 90% of the solar spectrum, however there has been no success with high indium (In) incorporation and only limited success with low In incorporation InxGa1-xN. Therefore, this present work focuses on 15 - 30% In incorporation leading to a bandgap value of 2.3 - 2.8 eV. This work will exploit the revision of the indium nitride (InN) bandgap value of 0.68 eV, which expands the range of the optical emission of nitride-based devices from ultraviolet to near infrared regions, by developing transparent InxGa1-xN solar cells outside the visible spectrum. Photovoltaic devices with a bandgap greater than 2.0 eV are attractive because over half the available power in the solar spectrum is above the photon energy of 2.0 eV. The ability of InxGa1-xN materials to optimally span the solar spectrum offers a tantalizing solution for high-efficiency photovoltaics. Using the metal modulated epitaxy (MME) technique in a new, ultra-clean refurbished MBE system, an innovative growth regime is established where In and Ga phase separation is diminished by increasing the growth rate for InxGa1-xN. The MME technique modulates the metal shutters with a fixed duty cycle while maintaining a constant nitrogen flux and proves effective for improving crystal quality and p-type doping. We demonstrate the ability to repeatedly grow high hole concentration Mg-doped GaN films using the MME technique. The highest hole concentration obtained is equal to 4.26 e19 cm-3, resistivity of 0.5 Ω-cm, and mobility of 0.28 cm2/V-s. We have achieved hole concentrations significantly higher than recorded in the literature, proving that our growth parameters and the MME technique is feasible, repeatable, and beneficial. The high hole concentration p-GaN is used as the emitter in our InxGa1-xN solar cell devices.

GaAsBi is a relatively unexplored alloy with interesting features such as a large bandgap reduction for a given lattice mismatch with GaAs substrates and good photoluminescence which make it promising for long wavelength light detection and emission applications. In this research, the molecular beam epitaxy (MBE) method was used to grow epi-layers and hetero-structures. A Vertical-external-cavity surface-emitting-laser (VECSEL) was grown as a part of collaboration with Tampere University in Finland. The process of laser growth promoted the writer’s skills in the growth of hetero-structures and led into an investigation of the effect of growth conditions on GaAsBi optical properties with important results. For instance, when the substrate temperature during growth was reduced from 400°C to 300°C and all other growth conditions were fixed, the Bi concentration in the deposited films increased from 1% to 5% and the photoluminescence (PL) intensity decreased by more than a factor of 1000. This is an indication of the importance of growth temperature in GaAsBi crystal quality. n+/p junctions were grown for the deep level transient spectroscopy (DLTS) experiments in collaboration with Simon Fraser University. The DLTS measurements showed that lowering the GaAsBi growth temperature increases the deep level density by a factor of 10. These deep levels are the source of non-radiative recombination and decrease the PL intensity. The structural properties of GaAsBi were investigated by high resolution x-ray diffraction and polarized PL and revealed long distance atomic arrangement (Cu-Pt ordering) in GaAsBi. The measurements showed that the ordering is more probable at high growth temperature. This can be due to the larger mobility of the atoms on the surface at high growth temperatures that allows them to find the ordered low energy sites. / Graduate

Die vorliegende Arbeit befasst sich mit dem Einfluss einer hyperthermischen Stickstoffionenbestrahlung (Ekin < 25 eV) auf das Galliumnitrid-Schichtwachstum. Dabei wird insbesondere der Einfluss einer Oberflächenrekonstruktion, einer Strukturierung der Oberfläche, einer Zwischenschicht (Pufferschicht) und der Einfluss verschiedener Siliziumsubstratorientierungen auf das epitaktische Wachstum von dünnen Galliumnitrid-Schichten nach einer hyperthermischen Stickstoffionenbestrahlung diskutiert. Ziel war es, möglichst dünne, epitaktische und defektarme Galliumnitrid-Schichten zu erhalten. Für die Charakterisierung der Galliumnitrid-Schichten und der Siliziumsubstrate standen diverse Analysemethoden zur Verfügung. Die kristalline Oberflächenstruktur konnte während des Wachstums mittels Reflexionsbeugung hochenergetischer Elektronen beobachtet werden. Nachfolgend wurde die Oberflächentopografie, die kristalline Struktur und Textur, sowie die optischen Eigenschaften der Galliumnitrid-Schichten mittels Rasterkraftmikroskopie, Röntgenstrahl-Diffraktometrie, hochauflösender Transmissionselektronenmikroskopie und Photolumineszenzspektroskopie untersucht.

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy

published_or_final_version / abstract / Physics / Doctoral / Doctor of Philosophy