Extending device performance in photonic devices using piezoelectric properties

This study focuses on the influence of epi-layer strain and piezoelectric effects in asymmetric GaInAs/GaAlAs action regions that potentially lead to intra-cavity frequency mixing. The theoretical limits for conduction and valence band offsets in lattice-matched semiconductor structures have resulted in the deployment of non-traditional approaches such as strain compensation to extend wavelength in intersubband devices, where strain limits are related to misfit dislocation generation. Strain and piezoelectric effects have been studied and verified using select photonic device designs. Metrics under this effort also included dipole strength, oscillator strength, and offset of energy transitions, which are strongly correlated with induced piezoelectric effects. Unique photonic designs were simulated, modeled, and then fabricated using solid-source molecular beam epitaxy into photonic devices. The initial designs produce lambda wavelength, and the introduction of the piezoelectric effect resulted in lambda/2 wavelength. More importantly, this work demonstrates that the theoretical cutoff wavelength in intersubband lasers can be overcome

Advanced Photonic Sciences

The new emerging field of photonics has significantly attracted the interest of many societies, professionals and researchers around the world. The great importance of this field is due to its applicability and possible utilization in almost all scientific and industrial areas. This book presents some advanced research topics in photonics. It consists of 16 chapters organized into three sections: Integrated Photonics, Photonic Materials and Photonic Applications. It can be said that this book is a good contribution for paving the way for further innovations in photonic technology. The chapters have been written and reviewed by well-experienced researchers in their fields. In their contributions they demonstrated the most profound knowledge and expertise for interested individuals in this expanding field. The book will be a good reference for experienced professionals, academics and researchers as well as young researchers only starting their carrier in this field

A review of the electrical properties of semiconductor nanowires: Insights gained from terahertz conductivity spectroscopy

Accurately measuring and controlling the electrical properties of semiconductor nanowires is of paramount importance in the development of novel nanowire-based devices. In light of this, terahertz (THz) conductivity spectroscopy has emerged as an ideal non-contact technique for probing nanowire electrical conductivity and is showing tremendous value in the targeted development of nanowire devices. THz spectroscopic measurements of nanowires enable charge carrier lifetimes, mobilities, dopant concentrations and surface recombination velocities to be measured with high accuracy and high throughput in a contact-free fashion. This review spans seminal and recent studies of the electronic properties of nanowires using THz spectroscopy. A didactic description of THz time-domain spectroscopy, optical pump–THz probe spectroscopy, and their application to nanowires is included. We review a variety of technologically important nanowire materials, including GaAs, InAs, InP, GaN and InN nanowires, Si and Ge nanowires, ZnO nanowires, nanowire heterostructures, doped nanowires and modulation-doped nanowires. Finally, we discuss how THz measurements are guiding the development of nanowire-based devices, with the example of single-nanowire photoconductive THz receivers.The authors gratefully acknowledge EPSRC (UK) for research funding. H J Joyce gratefully acknowledges the Royal Commission for the Exhibition of 1851 for her research fellowship.This is the final version of the article. It first appeared from IOP via https://doi.org/10.1088/0268-1242/31/10/10300

Fabrication and characterization of a Magnetic Tunnel Transistor with an epitaxial spin valve by the shadow mask technique

This work is concerned with the development of a fabrication method for Magnetic Tunnel Transistor (MTT) having an epitaxial spin valve. Over the years, this device has been used for the study of spin polarized hot electron transport in thin films. Further, the high on/off ratio that the use of thin magnetic films enables makes this device interesting for industrial application. One of the main limitations of this device however is the low ratio between the injected current and the collected current. In this work, use of epitaxial layers has been made so as to decrease the scattering probability of hot electrons, thereby increasing the aforementioned ratio. In order to ensure a high fabrication throughput as well as reduce the amount of lithography induced defects, a shadow mask based deposition method has been successfully developed. This thesis first describes the theoretical framework of the spin polarized hot electron transport in the MTT and investigates using a Monte Carlo algorithm the influence of structural defects, bandstructure as well temperature on the different characteristic parameters of the MTT. The fabrication method is subsequently describes and finally the experimental results are discussed in light of the theoretical prediction of the model as well as of the previous experimental reports on MTTs

BEEM imaging and spectroscopy of buried structures in semiconductors

Ballistic Electron Emission Microscopy (BEEM) has been shown to be a powerful tool for nanometer-scale characterization of the spatial and electronic properties of semiconductor structures. In this article, we will discuss general aspects of BEEM experiment and theory in true ballistic and quasi-ballistic hot carrier transport. We will review the current state and recent progress in the use of the BEEM imaging and spectroscopy to study metal-semiconductor and metal-insulator-semiconductor interfaces, buried semiconductor heterojunctions and novel quantum objects. Various theoretical BEEM models are discussed, and their ability to describe BEEM experiments is examined. Special attention is drawn to the role of the electron scattering in the metal base layer, at the metal-semiconductor interface and in the semiconductor heterostructure on BEEM spectra

Optical and Electrical Analysis of ZnO/ZnTe Micropillar Solar Cells

The prime focus of the energy-research community in recent times has been replacing fossil fuels with renewable energy. Therefore, photovoltaic research areas are rapidly expanding in this era. The purpose of this work is to compare three different structural ZnO/ZnTe solar cell types (planar, axial micropillar and radial micropillar). The best optical and electrical performance has been obtained by the radial junction (core-shell) ZnO/ZnTe micropillar solar cell due to its pillar structure and radial junction. The unique advantage of the radial junction micropillar is that the angle of the incident light and the carrier collection is orthogonal. Therefore, the pillar can be long enough to absorb 90% of the incident light. We explored the effect of dimension of the pillar (height, pitch and diameter) on the optical and electrical performance of the ZnO/ZnTe core-shell micropillar structure. An exploration of height in the range between 1.5 μm to 4 μm was studied. The results demonstrated that increasing the pillar height increases both optical and electrical performance of the device. Pitch value between 0.2 μm ~ 0.6 μm was explored. Both the minimum pitch value (0.2 μm) and maximum pitch value (0.6 μm) presented the worst performance for the device. In addition, the shell thickness (ZnTe) between 70 nm ~ 130 nm was studied. A shell thickness of 70 nm showed promising results in terms of optical and electrical performance. The effect of doping concentration on the electrical performance and auger recombination rate of the core-shell ZnO/ZnTe solar cell has been studied in this work

Transient carrier and lattice dynamics in photo-excited semiconductors studied by femtosecond spectroscopic ellipsometry

This work investigates transient optical properties of semiconductors and the underlying carrier and lattice dynamics after intense pulsed optical excitation. To this aim, the ex- perimental technique of pump-probe spectroscopic ellipsometry and the corresponding experimental setup is introduced first. The pump-probe scheme yields sub-picosecond temporal resolution while the spectroscopic ellipsometry measurement allows direct ex- cess to the complex-valued optical response, that means real and imaginary part of the dielectric function. The functionality of the experimental setup as well as technical de- tails, capabilities and limitations are discussed. First measurements are demonstrated on the prototypical wide-bandgap semiconductor ZnO and the classical semiconductors Ge, Si and InP. Furthermore, the full dielectric function tensor of optically anisotropic materials can be obtained from ellipsometry measurements, if suitable orientations of the material are measured and collectively analyzed. This capability will be demonstrated for the uniaxial material ZnO. Upon optical excitation, the transient occupation of electronic states is varied which leads to a redistribution of the spectral weight of absorption. This embodies the com- bined intricate effects of inter- and intra-band transitions, carrier scattering with the heated lattice as well as many-body effects such as band-gap renormalization, carrier screening and Pauli blocking. The contributions of these effects are disentangled by means of line-shape analysis of the dielectric function. For ZnO, we additionally find a strong influence of the polar electron-phonon interaction on the dielectric function that are framed as hot-phonon effects in the literature. They exemplify the importance of the lattice in the relaxation process of photo-excited semiconductors. The experimental dielectric functions will be compared to theoretical results from first-principles calcu- lation taking excitonic effects and the photo-excited carriers at elevated temperatures into account. The transient carrier dynamics are additionally supported by simula- tions of the transient carrier and lattice temperature. Moreover, spatial information on the transient carrier dynamics was obtained from pump-probe imaging ellipsometry on ZnO under similar excitation conditions. Here, the photo-excitation enables a delicate interplay between diffusion and ballistic propagation of the carriers, that leads to a non- homogeneous lateral carrier profile. This spatial modulation of the carrier density and subsequently the optical properties challenges the standard assumption of homogeneous lateral excitation in the analysis of pump-probe experiments.:Introduction 1 1 Measurement of transient optical properties 5 1.1 Light polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 Optical properties and ellipsometry . . . . . . . . . . . . . . . . . . . . . 6 1.3 Transient optical properties and time-resolved ellipsometry . . . . . . . . 7 1.4 Broadband femtosecond spectroscopic ellipsometry . . . . . . . . . . . . 10 2 Transient charge-carrier and lattice dynamics in photo-excited semicon- ductors 12 2.1 Four regimes of carrier relaxation . . . . . . . . . . . . . . . . . . . . . . 12 2.1.1 Hot-phonon effects in photo-excited wide-bandgap semiconductors 16 2.2 Effects of high carrier density on optical properties . . . . . . . . . . . . 17 2.3 Transient dielectric functions of ZnO . . . . . . . . . . . . . . . . . . . . 21 2.3.1 Ultrafast dynamics of hot charge carriers in an oxide semiconduc- tor probed by femtosecond spectroscopic ellipsometry . . . . . . . 21 2.3.2 Transient birefringence and dichroism in ZnO studied with fs-time- resolved spectroscopic ellipsometry . . . . . . . . . . . . . . . . . 22 2.3.3 Femtosecond-time-resolved imaging of the dielectric function of ZnO in the visible to near-IR spectral range . . . . . . . . . . . . 23 2.4 Transient dielectric functions of Ge, Si and InP . . . . . . . . . . . . . . 24 2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3 Summary and outlook 29 Bibliography 32 Cumulated Publications 52 Symbols and abbreviations 54 Danksagung 56 Zusammenfassung nach §11 (4) der Promotionsordnung 58In dieser Arbeit werden die transienten optischen Eigenschaften von Halbleitern nach gepulster optischer Anregung und die zugrundeliegende Prozesse der Ladungsträgerund Kristallgitterdynamik untersucht. Zu diesem Zwecke wird die experimentelle Methode der femtosekunden-zeitaufgelösten spektroskopische Ellipsometrie eingeführt. Das Pump-Probe-Messschema gewährt eine zeitliche Aufösung von weniger als einer Pikosekunde während es die spektroskopischen Ellipsometrie ermöglicht, direkten Zugang zur komplex-wertigen optischen Antwortfunktion auf eine eintreffende elektromagnetische Welle das heißt Real- und Imaginärteil der dielektrischen Funktion (DF) in einem breiten Spektralbereich zu erhalten. Zu Beginn wird der Messaufbau der zeitaufgelösten spektroskopischen Ellipsometrie vorgestellt. Seine Funktionalität wird durch Untersuchungen am prototypischen weitbandlückigen Halbleiter ZnO und den klassischen Halbleitern Ge, Si und InP demonstriert. Weiterhin können richtungsund polarisationsabhängige optischen Eigenschaften bestimmt werden, wenn entsprechende Orientierungen der Probe gemessen und simultan modelliert werden. Diese Fähigkeit wird ebenfalls an ZnO demonstriert, da es aufgrund seiner hexagonalen Kristallstruktur anisotrope optische Eigenschaften aufweist. Die intensive optische Anregung der Halbleiter bewirkt eine zeitweilige Umverteilung der Besetzung der elektronischen Zustände, welche sich in einer deutlich veränderten Linienform der DF widerspiegelt. Verantwortlich dafür sind unter anderem elektronische Interund Intra-Band-Übergänge und Streuprozesse mit dem aufgeheizten Gitter sowie verschiedene Vielteilcheneffekte wie Bandlückrenormierung, Abschirmung der Ladungsträger und das Pauli-Prinzip. Die Beiträge dieser Effekte können mittels geeigneter Linienformanalyse der DF näher untersucht werden. Am Beispiel von ZnO wird auch die starke Wechselwirkung der Elektronen mit dem aufgeheizten Gitter und deren Auswirkungen auf die DF gezeigt. Die experimentelle DF wird mit theoretischen Berechnungen verglichen, wobei bei exzitonische Effekte und die hohe Überschussenergie der Ladungsträger berücksichtigt werden. Zusätzlich erklären Simulationen der transienten Ladungsträgerund Gittertemperatur den Verlauf der Relaxation der Ladungsträger. Weiterhin werden Information über die räumliche Ausbreitung der Ladungsträger nach optischer Anregung mittels abbildender zeitaufgelöster Ellipsometrie an ZnO gewonnen. Hierbei wird ein komplexes Zwischenspiel zwischen Diffusion und ballistischer Propagation der Ladungsträger beobachtet, welches zu einer ringförmigen Verteilung der Ladungsträger führt.:Introduction 1 1 Measurement of transient optical properties 5 1.1 Light polarization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 1.2 Optical properties and ellipsometry . . . . . . . . . . . . . . . . . . . . . 6 1.3 Transient optical properties and time-resolved ellipsometry . . . . . . . . 7 1.4 Broadband femtosecond spectroscopic ellipsometry . . . . . . . . . . . . 10 2 Transient charge-carrier and lattice dynamics in photo-excited semicon- ductors 12 2.1 Four regimes of carrier relaxation . . . . . . . . . . . . . . . . . . . . . . 12 2.1.1 Hot-phonon effects in photo-excited wide-bandgap semiconductors 16 2.2 Effects of high carrier density on optical properties . . . . . . . . . . . . 17 2.3 Transient dielectric functions of ZnO . . . . . . . . . . . . . . . . . . . . 21 2.3.1 Ultrafast dynamics of hot charge carriers in an oxide semiconduc- tor probed by femtosecond spectroscopic ellipsometry . . . . . . . 21 2.3.2 Transient birefringence and dichroism in ZnO studied with fs-time- resolved spectroscopic ellipsometry . . . . . . . . . . . . . . . . . 22 2.3.3 Femtosecond-time-resolved imaging of the dielectric function of ZnO in the visible to near-IR spectral range . . . . . . . . . . . . 23 2.4 Transient dielectric functions of Ge, Si and InP . . . . . . . . . . . . . . 24 2.5 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25 3 Summary and outlook 29 Bibliography 32 Cumulated Publications 52 Symbols and abbreviations 54 Danksagung 56 Zusammenfassung nach §11 (4) der Promotionsordnung 5

High efficiency and high frequency resonant tunneling diode sources

Terahertz (THz) technology has been generating a lot of interest due to the numerous potential applications for systems working in this previously unexplored frequency range. THz radiation has unique properties suited for high capacity communication systems and non-invasive, non-ionizing properties that when coupled with a fairly good spatial resolution are unparalleled in its sensing capabilities for use in biomedical, industrial and security fields. However, in order to achieve this potential, effective and efficient ways of generating THz radiation are required. Devices which exhibit negative differential resistance (NDR) in their current-voltage (I – V) characteristics can be used for the generation of these radio frequency (RF) signals. Among them, the resonant tunnelling diode (RTD) is considered to be one of the most promising solid-state sources for millimeter and submillimeter wave radiation, which can operate at room temperature. However, the main limitations of RTD oscillators are producing high output power and increasing the DC-to-RF conversion efficiency. Although oscillation frequencies of up to 1.98 THz have been already reported, the output power is in the range of micro-Watts and conversion efficiencies are under 1 %. This thesis describes the systematic work done on the design, fabrication, and characterization of RTD-based oscillators in monolithic microwave/millimeter-wave integrated circuits (MMIC) that can produce high output power and have a high conversion efficiency at the same time. At the device level, parasitic oscillations caused by the biasing line inductance when the diode is biased in the NDR region prevents accurate characterization and compromises the maximum RF power output. In order to stabilise the NDR devices, a common method is the use of a suitable resistor connected across the device, to make the differential resistance in the NDR region positive. However, this approach severely hinders the diode’s performance in terms of DC-to-RF conversion efficiency. In this work, a new DC bias decoupling circuit topology has been developed to enable accurate, direct measurements of the device’s NDR characteristic and when implemented in an oscillator design provides over a 10-fold improvement in DC-to-RF conversion efficiency. The proposed method can be adapted for higher frequency and higher power devices and could have a major impact with regards to the adoption of RTD technology, especially for portable devices where power consumption must be taken into consideration. RF and DC characterization of the device were used in the realization on an accurate large-signal model of the RTD. S-parameter measurements were used to determine an accurate small-signal model for the device’s capacitance and inductance, while the extracted DC characteristics where used to replicate the I-V characteristics. The model is able to replicate the non-stable behavior of RTD devices when biased in the NDR region and the RF characteristics seen in oscillator circuits. It is expected that the developed model will serve in future optimization processes of RTD devices in millimeter and submillimeter wave applications. Finally, a wireless data transmission link operating in the Ka-band (26.5 GHz – – 40 GHz) using two RTDs operating as a transmitter and receiver is presented in this thesis. Wireless error-free data transfer of up to 2 gigabits per second (Gbit/s) was achieved at a transmission distance of 15 cm. In summary, this work makes important contributions to the accurate characterization, and modeling of RTDs and demonstrates the feasibility of this technology for use in future portable wireless communication systems and imaging setups