Effect of a halogen-based precursor on dopant incorporation in 3C-SiC film epitaxy

Silicon carbide thin films were synthesised by vapour phase epitaxy technique on silicon substrates using silane and propane as precursors. Methyltrichlorosilane (MTS) was added, and nitrogen was used as dopant precursor. Samples with different doping concentrations were obtained varying the nitrogen flow during the growth. Doping level for each sample was assessed using Raman technique, and a correlation between dopant flow and doping level was confirmed. The influence of MTS on nitrogen incorporation is analysed and discussed: the introduction of MTS increases the growth rate and increases the doping level. We exclude a direct doping effect by the MTS, but we think that it promotes the incorporation of nitrogen doping species. The crystalline quality of the as-grown films was evaluated using X-ray diffraction, assessing the good crystalline quality even in samples obtained using high growth rates and high doping level.

Combinatorial High-Vacuum Chemical Vapor Deposition of Lithium Niobate Thin Films

Ali Dabirian

Lithium niobate is a key material for photonics with applications in optical modulators and light frequency converters. Present lithium niobate optical devices are realized in bulk lithium niobate single crystals while interests in higher integration and also better performance derives efforts towards high quality epitaxial lithium niobate thin films. In the past, epitaxial lithium niobate thin films obtained using chemical vapor deposition (CVD), pulsed-laser deposition and crystal-ion slicing. Among these methods CVD is the one that can provide large area growth. Optimizing films properties stays an issue in CVD because in CVD of multi-element oxides using multiple precursors the stoichiometry of the film does not reflect the precursor ratios and additionally it varies with precursors total flux values. In this thesis optimization and discovery of multi-element oxide thin films were performed in a high-vacuum CVD reactor that is capable of performing combinatorial experiments. Using this reactor epitaxial lithium niobate thin films were obtained on sapphire {001g} and lithium tantalate {001} substrates with rocking curve full-width at half-maximum (FWHM) values of 0.03° and 0.024° respectively. The epitaxial growth of lithium niobate films was confirmed from X-ray diffraction (XRD) measurements and transmission-electron microscope (TEM) observations. Niobium tetra-ethoxy-di-methy-amino-ethoxide (Nb(OEt)4(dmae)) and lithium tert-butoxide (Li(OBut)) were used as precursors. Optically induced inhomogeneities, also known as optical damage, in lithium niobate limit its usage for high laser intensity applications. Lithium niobate single crystals doped with hafnium showed improved optical damage thresholds. Therefore combinatorial HVCVD experiments were conducted and textured {001} hafnium-doped lithium niobate films, with about 6 [mol%] hafnium, were obtained on sapphire f001g. Although the crystalline quality of the film is not high (rocking curve FWHM 0.8°) its Raman spectrum is still quite close to lithium niobate single crystal and epitaxial lithium niobate films on sapphire. Hafnium tert-butoxide (Hf(OBut)4) was used as precursor for hafnium. In the HVCVD reactor, a precursor transport system was applied such that spatial control of the precursor impinging rate on the substrate can be tailored. The precursor flux distribution that was mostly used in this thesis was a linear gradient with ratio of three across a 150 mm diameter circular area. In fact combinatorial experiments were performed by producing a linear gradient for individual precursors over the substrate with a certain angle between the gradient directions. The linear gradient feature was used to perform efficiently the systematic study of single precursors providing the benefit that one deposition at certain substrate temperature gives information about deposition kinetics for a vast range of precursor flux values. A systematic study of the Nb(OEt)4(dmae) precursor was performed using this feature and chemical-reaction-limited, mass-transport-limited, and desorption-limited regimes of CVD process were identified. The possibility of screening a large range of precursor flux values led us to discovering a CVD regime in which deposition rate is decreased by raising the precursor flux. A model was proposed for interaction of gas with surfaces that can explain all the different CVD regimes observed. The precursor molecules in HVCVD reactor reach the substrate with nearly no gasphase collision. Based on this fact a lift-off process was proposed for large-area in-situ patterning of multi-element oxide thin films.

EPFL2010

Crystalline Order in Compound Semiconductors in Nanoscale Structures and Thin Films

Mahdi Zamani

Semiconductors materials and devices are essential building blocks for many of the technologies deeply embedded in modern life. Improving the performance of semiconductor devices requires a deeper understanding of the fundamental mechanisms controlling the crystalline structure of the semiconductor materials. This thesis focuses on two compound semiconductor systems in the form of ðð3ð2 thin films and ðºððŽð nanowires and tries to establish methods for minimizing the defects in these structures by the careful control of order at the atomic scale. In the first part of this work, we have focused on Zn3 P2 thin films. The combination of suitable material properties and the abundancy of its constituting elements makes this platform a promising candidate for large-scale and scalable photovoltaic applications. Despite this, Zn3 P2 is not adopted by the solar industry due to the many technical challenges in the growth of this material. In this work, by careful control of the state of the interface between the thin films and the substrates, we have provided a method for the growth of thick and monocrystalline Zn3 P2 thin films with superior quality. This is crucial for the realization of any successful photovoltaic cell based on this material. Although molecular beam epitaxy is used in this study, the process provided is generalizable and could also be exploited for the other growth methods. A host of different characterization techniques, including electron microscopy and spectroscopy, Raman spectroscopy, photoluminescence spectroscopy and x-ray diffraction are used to assess the different aspects of the thin films. It is observed that monocrystalline films have better optical properties compared to polycrystalline thin films, making them more suitable for photovoltaic applications. In the second part of the thesis, we have focused on improving the crystalline quality of ðºððŽð nanowires. Similar to other III-V semiconductors, ðºððŽð hosts an internal electrical dipole known as polarity. Arsenide and phosphide nanowires often grow along (111)ðµ direction, which implies the termination of (111) bilayer by group V elements. However, this configuration is often defective and results in polytypism. On the other hand, (111)ðŽ growth, which refers to group III termination, results in a crystalline structure that is largely free of defects. However, growth in this polarity is rarely reported. In this work, we try to understand the reason for this elusiveness and the superior crystalline quality associated with A-polar growth. To do that, we have established a new framework that explicitly focuses on the atomic structure and order of liquid-solid interface in nanowires grown by vapor-liquid-solid method. The atomic structure of this interface has been ignored up to now in the literature due to the technical challenges for investigating such fragile atomic order. Here, we employ a combination of experimental observations via electron microscopy and spectroscopy and machine-learning-based molecular dynamics simulations that were developed by external collaborators. The results of this study provide a unique insight into the fundamental aspects of nanowire growth. Key words: compound semiconductors, photovoltaic cells, molecular beam epitaxy, thin films, nanowires, growth polarity, liquid-solid interface, liquid ordering

EPFL2021

Crystalline Order in Compound Semiconductors in Nanoscale Structures and Thin Films

Mahdi Zamani

EPFL2021

Combinatorial High-Vacuum Chemical Vapor Deposition of Lithium Niobate Thin Films

Ali Dabirian

EPFL2010