Optical spectroscopy of earth-abundant zinc pnictides for photovoltaics

Zinc phosphide (Zn3P2) is an earth-abundant semiconductor with promising properties for applications as an absorber in photovoltaics. To beat the 40 years old record power conversion efficiency of solar cells made with this material, a deep understanding of the compound's electrical, optical, and crystal properties and their interplay with its growth conditions is required. In this thesis, we explore the crystalline and optical properties of zinc phosphide and the closely related zinc arsenide (Zn3As2) by means of Raman and photoluminescence spectroscopy.In the first part of this work, we establish that defect-free zinc arsenide nanosails grown by gold-catalyzed metal-organic vapor phase epitaxy display a metastable crystalline structure. The atomic lattice of these flat structures becomes isostructural to that of zinc phosphide likely due to the nanoscale nature of the system. We also determine that the nanosails are degenerate p-doped semiconductors with pure impurity band conduction at low temperatures.The second part of this thesis is dedicated to establishing a thorough understanding of the dispersion and symmetries of the Raman-active lattice vibrational eigenmodes of zinc phosphide. We unambiguously identify most Raman-active phonons in the lattice and show the zinc- and phosphorus-dominated modes to be separated by a real phonon gap, establishing a reference Raman spectrum for zinc phosphide and its family of isostructural compounds.In the final part of the thesis, we characterize the optical properties of zinc phosphide by photoluminescence of monocrystalline zinc phosphide grown by molecular beam epitaxy. We show that the photogenerated electron-hole pairs experience two main radiative recombination mechanisms. Emission attributable to electronic transitions involving an impurity band is observed, and we show the first measurement by photoluminescence of the crystal field splitting of zinc phosphide.

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

Functional properties of III-V nanowires addressed by Raman spectroscopy

Francesca Amaduzzi

In recent years, semiconductor nanowires have attracted considerable attention as a result of their unique properties and potential applications in many fields. In particular, they can be very attractive materials for certain optoelectronic and electronic devices, such as lasers, detectors and solar cells, which benefit from the photonic properties of nanowires. In order for these future technologies to become a reality, a good understanding of the functional properties of nanowires is fundamental. In this thesis we have investigated the optical and the electrical properties of III-V semiconductors nanowires by the means of Raman spectroscopy. Thanks to its non-destructive and spatial resolution, Raman spectroscopy is a powerful contact-less tool for the characterization of semiconductor nanowires. Raman spectroscopy can provide information about crystallinity, orientation, size and chemical composition. In polar semiconductors it is also possible to characterize the free carriers, through the coupling of plasmons with longitudinal optical modes. Due to the small size and particular morphology of nanowires, the interaction of light can be more complex than in thin films. In particular, the existence of photonic modes alters significantly the corresponding light-matter interaction. In this thesis we exploit the use of photonic modes for the compositional mapping of nanowire core-shell heterostructures and also to circumvent the macroscopic selection rules. In the first part of this thesis, we have performed Raman scattering measurements on GaAs/AlGaAs core/shell nanowires. We have shown that it is possible to select and characterize regions of the structure with different Aluminum content, by performing the measurements at different laser wavelengths. Then, we have shown that the photonic modes can be modified by suspending the nanowires on a trench. We have shown that in this case it is possible to enhance the response of the longitudinal optical phonon mode. We have then applied this configuration for the characterization of the hole concentration on p-type GaAs nanowires in back-scattering geometry. The second part of the thesis focused on the assessment of free carriers by Raman spectroscopy in systems with an expected high electron mobility: GaAs nanowires with a modulation doped structure and InAs(Sb) nanowires. Raman measurements were performed as a function of the temperature on modulation doped GaAs/AlGaAs nanowire. By characterizing the coupling between free carrier and the LO phonons, we have extracted the concentration and the mobility of carriers. We have found that Si donors are almost ionized for a temperature above 50 K. We have shown that the mobility is limited by interface scattering, with values of 400 cm^2/Vs at room temperature and 2700 cm^2/Vs at low temperature. Finally, we investigated the electronic properties of InAs(Sb) nanowires as a function of the temperature. The effect of a dielectric coating on the electronic properties was also studied. We have found an increase of mobility and electron concentration with the antimony content , moving from 5100 cm^2/Vs for InAs nanowires, to 17500 cm^2/Vs for InAsSb with 35% of antimony at 14K. Moreover, we have shown that in the case of InAs electrons are located in the accumulation layer at the surface, while for InAsSb our measurements are consistent with the carriers located in the nanowire core.

EPFL2017

Biological samples studied by optical nanospectroscopy

Johanna Generosi

The different parts of the electromagnetic spectrum result in diverse effects upon interaction with matter: according to the wavelength, the radiation has energy appropriate for the excitation of a specific physical process. X-rays can be used as a tool to analyze the structure of matter since their wavelength is comparable with the interatomic distances. Infrared light is in the spectral region that excites molecular vibrations and is employed to investigate the chemical composition of a material. Visible radiation can study the optical properties of a sample, such as the fluorescence and the absorbance, and provide a chemical fingerprint when the inelastically scattered light is detected. In this thesis work these light sources are used in diverse experimental approaches to study structured biological specimens, resulting in a detailed chemical and physical characterization at the atomic and molecular scale. Conventional spectroscopy is often not enough sensitive and spatially resolved to detect specific elements or domains in a sample. The need of imaging objects on increasingly finer scales and spatially localize specific molecules, brought to combine infrared, visible and Raman spectroscopy with scanning near-field microscopy giving rise to a powerful nanospectroscopic tool used to perform simultaneous topographical measurements and optical/chemical characterizations with subwavelength resolution, overcoming the diffraction limit of light. Our study combines X-ray diffraction and reflectivity with optical nanospectroscopy to investigate the order and clustering of lipid bilayers, the interaction between solid-supported membranes and embedded alamethicin peptides, the optical and chemical properties of hippocampal neuron cells and the trafficking mechanism of specific neuron receptors.