In-situ Study of the Growth, Composition and Morphology of III-V Semiconductor Nanowires

It is widely known that nanoparticle seeded growth of III-V semiconductor nanowires often occurs via the vapor-liquid-solid mechanism. However, conventional growth of nanowires is carried out in closed systems, where mostof the details and dynamics of the growth are impossible to follow. Since all analysis is typically carried out after growth completion and transfer, only the trends in the produced growth series give hints to the processes occurringat the nanoscale.In this thesis, the growth of Au-seeded III-V semiconductor nanowires has been studied in-situ, during growth, by means of environmental transmission electron microscopy. The supply of growth species in the form of precursormolecules directly into the microscope column provides a unique opportunity to follow the nanowire growth while it is occurring, through all techniques commonly available in a transmission electron microscope. This means that nanoscale growth dynamics can be studied in real time under static or changing growth conditions, and changes in crystal structure, composition and morphology can be revealed.The present investigation highlights the relation between the liquid nanoparticle and the solid nanowire. For GaSb nanowires, I show that the nanoparticle can swell by alloying to various extents with Ga, which will influence the formed nanowire diameter. I also investigate the compositional relationship between the Au-based nanoparticles and growing ternary InGaAs nanowires, where it is established that the solidified composition is dependent on both kinetics and thermodynamics. One of the main conclusions is that a high concentration of indium in the nanoparticle is needed in order to form indium rich solid InGaAs. The in-situ investigations reveal that nanowire growth is dynamic, and the layer-by-layer growth process consists of separate steps of material collection (incubation) and solidification (step-flow). I present how the layer-by-layer growth is affected by changes in the precursor flows for GaSb nanowires, and how the formation of defects in GaAs affects growth rate and can influence the growth behavior. Finally, I discuss the multilayer growth phenomenon in InGaAs nanowires, where multiple layers nucleate and grow simultaneously at the liquid-solid interface. In many cases the results are compared to simulations and models, which can be used to provide a more detailed understanding about the factors influencing the growth. The results presented in this thesis provide fundamental insight into the growth of nanostructures and are expected to be useful in the continued pursuit of atomic scale control

Growth of InAsSb and GaAsSb Nanowires

Att kunna ändra egenskaperna hos nanotrådar till specifika applikationer är viktigt om nanotrådar ska kunna inkorporeras i elektronikindustrin. Bandgapet är en sådan egenskap som för ternära III-V halvledare kan ändras om "ordning" uppstår i materialet. Med ordning menas här att de ingående komponenterna spontant lägger sig periodiskt längs med en kristallografisk riktning. Ett steg mot att uppnå ordning är att få nanotrådarna att växa i den kristallografiska riktningen 001, som har setts vara fördelaktig för ordning. I det här projektet har förmågan av InAsSb och GaAsSb nanotrådar att växa i 001 på InAs (001) respektive GaAs (001) substrat, utforskats. Nanotrådarna har växts från Au, AgAu och Ag partiklar med det slutgiltiga målet att uppnå ordning. Transmissionselektronmikroskopi har använts för att bestämma kristallstruktur, växtriktning och för att se om någon ordning har uppstått i de färdiga trådarna. Vertikal växt uppnåddes för InAsSb trådar från både Au och AgAu partiklar, men ordning observerades inte.The ability to tailor the properties of nanowires exactly to fit a specific application is important if nanowires are to be useful and incorporated into the electronics industry. The bandgap is one such property, which, for ternary III-V semiconductors, can be altered by the presence of ordering. In this sense, ordering is referred to as a spontaneous modulation of the ingoing components along certain crystallographic directions. A step towards achieving ordering is to make nanowires to grow in the 001 direction, which has been shown to promote it. In this project, the possibility to grow InAsSb and GaAsSb nanowires in the 001 direction on InAs (001) and GaAs (001) substrates, respectively, is investigated. The nanowires are grown from Au, AgAu and Ag particles with the final goal of achieving ordering. Transmission electron microscopy is used to determine the crystal structure, growth direction and presence of ordering in the final nanowires. Ultimately, growth of Au and AgAu seeded InAsSb wires in the 001 direction was achieved, but no ordering was observed.Alla elektroniska prylar i vår vardag fungerar tack vare att man använder halvledarmaterial. Länge har man kunnat göra halvledarkomponenterna mindre och mindre, vilket i sin tur har gjort datorer mindre och effektivare. Snart kan man dock inte fortsätta göra saker mindre, så därför behövs andra tillvägagångssätt. En lösning är att bygga på höjden i så kallade nanotrådar. Nanotrådar är små och avlånga kristaller som kan användas i elektriska och optiska komponenter. I labbet kan man med specifika metoder växa trådarna, ungefär som plantor, men man måste välja olika parametrar rätt för att det ska bli bra. I det här projektet har viktiga steg mot integrationen av nanotrådar i olika applikationer tagits

Diameter Control of GaSb Nanowires Revealed by In Situ Environmental Transmission Electron Microscopy

Several nanowire properties are strongly dependent on their diameter, which is notoriously difficult to control for III–Sb nanowires compared with other III–V nanowires. Herein environmental transmission electron microscopy is utilized to study the growth of Au nanoparticle seeded GaSb nanowires in situ. In this study, the real time changes to morphology and nanoparticle composition as a result of precursor V/III ratio are investigated. For a wide range of the growth parameters, it is observed that decreasing the V/III ratio increases the nanoparticle volume through Ga accumulation in the nanoparticle. The increase in nanoparticle volume in turn forces the nanowire diameter to expand. The effect of the V/III ratio on diameter allows the engineering of diameter modulated nanowires, where the modulation persisted after the growth. Lastly, this study demonstrates the observed trends can be reproduced in a conventional ex situ system, highlighting the transferability and importance of the results obtained in situ

Time-resolved compositional mapping during in situ TEM studies

In situ studies using transmission electron microscopy (TEM) can provide insights to how properties, structures and compositions of nanostructures are affected and evolving when exerted to heat or chemical exposure. While high-resolved imaging can be obtained continuously, at video-framerates of hundreds of frames per second (fps), compositional analysis struggles with time resolution due to the long acquisition times for a reliable analysis. This especially holds true when performing mapping (correlated spatial and compositional information). Hence, transient changes are difficult to resolve using mapping. In this work, the time-resolution of sequential mapping using scanning TEM (STEM) and energy dispersive spectroscopy (EDS) is improved by acquiring spectrum images during short times and filtering the spectroscopic data. The suggested algorithm uses regularization to smooth and prevent overfitting (known from compressed sensing) to fit model spectra to the data. The algorithm is applied on simulations as well as acquisitions of catalyzed crystal growth (nanowires), performed in situ in a specialized environmental TEM (ETEM). The results show the improved temporal resolution, where the compositional progression of the different regions of the nanostructure is revealed, here with a time-resolution as low as 16 s compared to the minutes usually needed for similar analysis

Observation of the Multilayer Growth Mode in Ternary InGaAs Nanowires

Au-seeded semiconductor nanowires have classically been considered to only grow in a layer-by-layer growth mode, where individual layers nucleate and grow one at a time with an incubation step in between. Recent in situ investigations have shown that there are circumstances where binary semiconductor nanowires grow in a multilayer fashion, creating a stack of incomplete layers at the interface between a nanoparticle and a nanowire. In the current investigation, the growth behavior in ternary InGaAs nanowires has been analyzed in situ, using environmental transmission electron microscopy. The investigation has revealed that multilayer growth also occurs for ternary nanowires and appears to be more common than in the binary case. In addition, the size of the multilayer stacks observed is much larger than what has been reported previously. The investigation details the implications of multilayers for the overall growth of the nanowires, as well as the surrounding conditions under which it has manifested. We show that multilayer growth is highly dynamic, where the stack of layers regularly changes size by transporting material between the growing layers. Another observation is that multilayer growth can be initiated in conjunction with the formation of crystallographic defects and compositional changes. In addition, the role that multilayers can have in behaviors such as growth failure and kinking, sometimes observed when creating heterostructures between GaAs and InAs ex situ, is discussed. The prevalence of multilayer growth in this ternary material system implies that, in order to fully understand and accurately predict the growth of nanowires of complex composition and structure, multilayer growth has to be considered

Compositional Correlation between the Nanoparticle and the Growing Au-Assisted InxGa1-xAs Nanowire

The nanowire geometry is favorable for the growth of ternary semiconductor materials, because the composition and properties can be tuned freely without substrate lattice matching. To achieve precise control of the composition in ternary semiconductor nanowires, a deeper understanding of the growth is required. One unknown aspect of seeded nanowire growth is how the composition of the catalyst nanoparticle affects the resulting composition of the growing nanowire. We report the first in situ measurements of the nanoparticle and InxGa1-xAs nanowire compositional relationship using an environmental transmission electron microscopy setup. The compositions were measured and correlated during growth, via X-ray energy dispersive spectroscopy. Contrary to predictions from thermodynamic models, the experimental results do not show a miscibility gap. Therefore, we construct a kinetic model that better predicts the compositional trends by suppressing the miscibility gap. The findings imply that compositional control of InxGa1-xAs nanowires is possible across the entire compositional range

Direct Observations of Twin Formation Dynamics in Binary Semiconductors

With the increased demand for controlled deterministic growth of III–V semiconductors at the nanoscale, the impact and interest of understanding defect formation and crystal structure switching becomes increasingly important. Vapor–liquid–solid (VLS) growth of semiconductor nanocrystals is an important mechanism for controlling and studying the formation of individual crystal layers and stacking defects. Using in situ studies, combining atomic resolution of transmission electron microscopy and controlled VLS crystal growth using metal organic chemical vapor deposition, we investigate the simplest achievable change in atomic layer stacking–single twinned layers formed in GaAs. Using Au-assisted GaAs nanowires of various diameters, we study the formation of individual layers with atomic resolution to reveal the growth difference in forming a twin defect. We determine that the formation of a twinned layer occurs significantly more slowly than that of a normal crystal layer. To understand this, we conduct thermodynamic modeling and determine that the propagation of a twin is limited by the energy cost of forming the twin interface. Finally, we determine that the slower propagation of twinned layers increases the probability of additional layers nucleating, such that multiple layers grow simultaneously. This observation challenges the current understanding that continuous uniform epitaxial growth, especially in the case of liquid-metal assisted nanowires, proceeds one single layer at a time and that its progression is limited by the nucleation rate