Automatic RADAR Target Recognition System at THz Frequency Band. A Review

The development of technology for communication in the THz frequency band has seen rapid progress recently. Due to the wider bandwidth a THz frequency RADAR provides the possibility of higher precision imaging compared to conventional RADARs. A high resolution RADAR operating at THz frequency can be used for automatically detecting and segmenting concealed objects. Recent advancements in THz circuit integration have opened up a wide range of possibilities for on chip applications, like of security and surveillance. The development of various sources and detectors for generation and detection of THz frequency has been driven by other techniques such as spectroscopy, imaging and impulse ranging. One of the central vision of this type of security system aims at ambient intelligence: the computation and communication carried out intelligently. The need for higher mobility with limited size and power consumption has led to development of nanotechnology based THz generators. In addition to this some of the soft computing tools are used for detection of radar target automatically based on some algorithms named as ANN, RNN, Neuro-Fuzzy and Genetic algorithms. This review article includes UWB radar for THz signal, its characteristics and application, Nanotechnology for THz generation and issues related to ATR

The Growth and Characterization of InN Quantum Dots Using Droplets Epitaxy in MBE

III-nitride semiconductor material systems offer great potential for next-generation optoelectronic devices due to their direct bandgaps, which vary from 0.7 eV (InN) to 3.5 eV (GaN) to 6.2 eV (AlN), as well as their other unique properties. InN has gained much less attention than GaN and AlN within this family of semiconductors due to its complicated low-temperature growth. However, the prediction that an InN quantum well on GaN can become a two-dimensional (2D) topological insulator has resulted in expanding the research interest in InN. At the same time, this renewed interest has begun to reveal that the formation of an appropriate 2D InN film is difficult at best and physically forbidden by strain at worst. This has shifted the focus on InN to 3D nanostructures in an attempt to achieve similar novel affects. However, this shifted focus has revealed a challenging landscape for the study of the growth of these 3D nanostructures. This research has focused on investigating the growth of InN quantum dots (QDs) by droplet epitaxy (DE) using radio frequency plasma-assisted molecular beam epitaxy (MBE) in order to discover and learn to control the growth kinetics of this novel system. The QD growth kinetics from the formation of liquid In droplets to the crystallization of InN QDs was studied with a focus on the effects of ambient nitrogen and substrate type and temperature. The substrates studied were c-plane sapphire and (0001) GaN, while the temperature varied from nearly room temperature to ~400 °C. The growth quality, dot density, diameter, and height of In droplets as well as InN QDs were investigated utilizing reflection high-energy electron diffraction (RHEED), X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM), and Scanning Electron Microscopy (SEM). The droplet formation was determined to follow well known principles of nucleation theory with ripening. By analyzing the areal density of nanostructures as functions of temperature, the corresponding activation energies for surface diffusion where determined. The resulting activation energy for In surface diffusion on sapphire was found to be 0.62 ± 0.07 eV in ultra-high vacuum, ~10-10 Torr, and 0.57 ± 0.08 eV in ambient N2, ~10-5 Torr. For the InN QDs on GaN, the resulting activation energy for In surface diffusion on GaN was found to be 0.23 ± 0.03 eV. In addition, it was found that by analyzing the density of crystallized QDs, following the droplet formation under ambient N2, a very close activation energy of 0.25 ± 0.1 eV was found

Surface modification of III-V nanostructures studied by low-temperature scanning tunneling microscopy

In the past decade, driven by the demand for materials with high performance for next-generation semiconductor devices (e.g., for quantum computing), the exploration of III-V semiconductor materials and the design of improved devices based on these materials has extended to the nanometer scale, with several highlights in the studies of quantum wells, quantum dots, and nanowires (NW) in recent years. On the path of seeking smaller scale devices, the lateral scale is usually limited by the spatial resolution of the lithographic processes. Now, the challenge lies in the combination of semiconductor nanoscale structure with the desired electronic properties. Scaling down material synthesis to crystalline structures of only few atoms in size and precisely positioned in device configuration has not been realized so far. Moreover, the compatibility for large-scale industrial device processing is also challenging.In this dissertation, I present the surface characterization and studies of the modification of nanostructures on III-V semiconductor surfaces, with the techniques of low temperature scanning tunneling microscopy/spectroscopy (LT-STM/S) and X-ray photoelectron spectroscopy (XPS). Two main topics are Bi incorporation in GaAs (and InAs) surfaces and self-driven formation of nanostructures with atomic-scale precision. Different zinc blende and wurtzite crystal planes have been investigated, including the {11-20}- type facet which for GaAs and InAs uniquely exists on the side walls of NWs and nanoplatelets. The utilization of the tailored facets of NWs as templates for Bi-induced nanostructure formation has been explored as well. Bi-introduced low-dimensional nanostructures and exotic electronic states in III-V semiconductor systems have been investigated. The covalent bonds of Bi atoms in the self-formed Bi nanostructures on III-V substrates can vary depending on the substrate template and preparation condition, such as the Ga-Bi bonds in the 1D chain and 2D island nanostructures on Wz{11-20}-type facets on GaAs NWs. The possibility of tuning the self-formed III-V:Bi nanostructures in a more controllable way has been explored in this thesis. A significant high coverage of Bi on III-V semiconductor surface has been achieved. The observed variable bandgap and Bi-induced surface states are promising for applications in surface bandgap engineering and quantum technology components

The Growth and Characterization of InN Quantum Dots Using Droplets Epitaxy in MBE

III-nitride semiconductor material systems offer great potential for next-generation optoelectronic devices due to their direct bandgaps, which vary from 0.7 eV (InN) to 3.5 eV (GaN) to 6.2 eV (AlN), as well as their other unique properties. InN has gained much less attention than GaN and AlN within this family of semiconductors due to its complicated low-temperature growth. However, the prediction that an InN quantum well on GaN can become a two-dimensional (2D) topological insulator has resulted in expanding the research interest in InN. At the same time, this renewed interest has begun to reveal that the formation of an appropriate 2D InN film is difficult at best and physically forbidden by strain at worst. This has shifted the focus on InN to 3D nanostructures in an attempt to achieve similar novel affects. However, this shifted focus has revealed a challenging landscape for the study of the growth of these 3D nanostructures. This research has focused on investigating the growth of InN quantum dots (QDs) by droplet epitaxy (DE) using radio frequency plasma-assisted molecular beam epitaxy (MBE) in order to discover and learn to control the growth kinetics of this novel system. The QD growth kinetics from the formation of liquid In droplets to the crystallization of InN QDs was studied with a focus on the effects of ambient nitrogen and substrate type and temperature. The substrates studied were c-plane sapphire and (0001) GaN, while the temperature varied from nearly room temperature to ~400 °C. The growth quality, dot density, diameter, and height of In droplets as well as InN QDs were investigated utilizing reflection high-energy electron diffraction (RHEED), X-Ray Diffraction (XRD), Atomic Force Microscopy (AFM), and Scanning Electron Microscopy (SEM). The droplet formation was determined to follow well known principles of nucleation theory with ripening. By analyzing the areal density of nanostructures as functions of temperature, the corresponding activation energies for surface diffusion where determined. The resulting activation energy for In surface diffusion on sapphire was found to be 0.62 ± 0.07 eV in ultra-high vacuum, ~10-10 Torr, and 0.57 ± 0.08 eV in ambient N2, ~10-5 Torr. For the InN QDs on GaN, the resulting activation energy for In surface diffusion on GaN was found to be 0.23 ± 0.03 eV. In addition, it was found that by analyzing the density of crystallized QDs, following the droplet formation under ambient N2, a very close activation energy of 0.25 ± 0.1 eV was found

Multispectral plasmon enhanced quantum dots in a well infrared photodetectors

Infrared detectors in 3-5 μm and 8-12 μm regions are extensively used for applications in remote sensing, target detection and medical diagnostics. Detectors using intersubband transitions in the quantum dots in a well (DWELL) system for infrared detection have gained prominence recently, owing to their ability to detect normally incident light, bicolor detection and use of mature III-V technology. In this dissertation, two aspects of DWELL detectors that make them suitable for third generation infrared systems are discussed: 1) High temperature operation, 2) Multispectral detection. There are two parts to this dissertation. In the first part, an alternate structure with an improved operating temperature and thicker active region is presented. Traditionally, DWELL detectors use InAs quantum dots embedded in In0.15Ga0.85As wells with GaAs barriers. Intersubband transitions in the conduction band of this system result in infrared detection. InAs quantum dots are grown using self assembly on a GaAs substrate for this system. The strain of the quantum dots and the In0.15Ga0.85As well limits the thickness of the active region. An improved design that minimizes the strain in growth of DWELL active region is discussed. By minimizing the amount of In0.15Ga0.85As in the quantum well, a lower strain per DWELL active region stack is achieved. This design consists of InAs dots in In0.15Ga0.85As/GaAs wells, forming dots-in-a-double-well (DDWELL) is presented. Optimization using PL and AFM is discussed. Detectors fabricated using DDWELL design show an operating temperature of 140 K and a background limited performance at 77 K. A peak detectivity of 6.7x1010 cm.Hz/W was observed for a wavelength of 8.7 μm. In the second part of this dissertation, multispectral and polarization detectors using DWELL absorbers are discussed. Integration of a subwavelength metallic pattern with the detector results in coupling of surface plasmons excited at the metal- semiconductor interface with DWELL active regions. Simulations indicate the presence of several modes of absorption, which can be tuned by changing the pitch of the pattern. Enhancement of absorption is predicted for the detector. Experimental demonstration show spectral tuning in MWIR and LWIR regions and a peak absorption enhancement of 4.9x. By breaking the symmetry of the fabricated pattern, we can extract a polarization dependent response, as shown from device measurements. The technique used is detector agnostic, simple and can easily be transferred to focal plane arrays (FPA). Integrating plasmonic structures on detectors using low noise DDWELL active regions can provide a higher operating temperature and high absorption. The origin of resonant peaks in multispectral DWELL detectors is examined. Use of surface patterns that selectively excite different types of modes, with absorbers of different thicknesses, show the presence of enhancement mechanisms in these devices. A 2.2x enhancement is measured from waveguide modes and 4.9x enhancement is observed from plasmon modes. Finally, a pathway of integration with FPA and integration with other infrared technologies is discussed

RESISTIVE SWITCHING CHARACTERISTICS OF NANOSTRUCTURED AND SOLUTION-PROCESSED COMPLEX OXIDE ASSEMBLIES

Miniaturization of conventional nonvolatile (NVM) memory devices is rapidly approaching the physical limitations of the constituent materials. An emerging random access memory (RAM), nanoscale resistive RAM (RRAM), has the potential to replace conventional nonvolatile memory and could foster novel type of computing due to its fast switching speed, high scalability, and low power consumption. RRAM, or memristors, represent a class of two terminal devices comprising an insulating layer, such as a metal oxide, sandwiched between two terminal electrodes that exhibits two or more distinct resistance states that depend on the history of the applied bias. While the sudden resistance reduction into a conductive state in metal oxide insulators has been known for almost 50 years, the fundamental resistive switching mechanism is a complex phenomenon that is still long-debated, complex process. Further improvements to existing memristor performance require a complete understanding of memristive properties under various operation conditions. Additional technical issues also remain, such as the development of facile, low-cost fabrication methods as an alternative to expensive, ultra-high vacuum (UHV) deposition methods. This collection of work explores resistive switching within metal oxide-based memristive material assemblies by analyzing the fundamental physical insulating material properties. Chapter 3 aims to translate the utility and simplicity of the highly ordered anodic aluminum oxide (AAO) template structure to complex, yet more functional (memristive) materials. Functional oxides possessing ordered, scalable nanoporous arrays and nanocapacitor arrays over a large area is of interest to both the fields of next-generation electronics and energy storing/harvesting devices. Here their switching performance will be evaluated using conductive atomic force microscopy (C-AFM). Chapter 4 demonstrates a convective self-assembly fabrication method that effectively enables the synthesis of a low-cost solution processed memristor comprising binary oxide and perovskite ABO3 nanocrystals of varying diameter. Chapter 5 systematically compares the influence of inter-nanoparticle distance on the threshold switching SET voltage of hafnium oxide (HfO2) memristors. Utilizing shorter phosphonic acid ligands with higher binding affinity on the nanocrystal surface enabled a record-low SET voltage to be achieved. Chapter 6 extends the scope to the fine tuning of solution processed memristors with two types of perovskites nanocrystals. The primary advantage of nanocrystal memristors is the ability to draw from additional degrees of freedom by tuning the constituent nanocrystal material properties. Recent advancement of solution phase techniques enables a high degree of controllability over the nanocrystal size and structure. Thus, this work found in this dissertation aims to understand and decouple the effects of the geometric size and substitutional nanocrystal parameters on resistive switching

Surfaces and interfaces of low dimensional III-V semiconductor devices

The demand for fast and energy efficient (opto-)electronic applications needs high mobility semiconductor materials, such as InAs with a very high electron mobility and GaSb with a very high hole mobility. Beyond the material itself, also an innovative device geometry is needed, for example, the gate-all-around geometry that provides higher efficiency and electrostatic control for computational units. Vertically or laterally grown nanowires and nanosheets are excellent candidates for realizing such beneficial device geometries. The logic operations and charge transport could be realized in different device architectures, such as the concepts of tunnelFETs instead of classical FETs or new neuromorphic hardware instead of complementary metal-oxide-semiconductor (CMOS).With both the excellent functional properties of III-V materials and the flexibility of nanostructuring into 1D nanowires and 2D nanosheets, III-V semiconductors could be the stars for next-generation applications. For example, lateral grown InxGa1−xAs nanowires have a high spin-orbit coupling and moderate bandgap promising for quantum computing devices. GaSb nanowires are excellent high-speed p-channels for III-V CMOS, and InAs/InP nanowires have an energy barrier in the axial direction which can be used for photovoltaic and sensor applications. Due to the high surface-to-bulk ratio of nanowires and nanosheets, their surface condition becomes the key to the device performance. In this work, III-V nanowire and nanosheet devices are studied with an emphasis on surfaces and interfaces, using a wide range of characterization methods. The dissertation explores the fabrication of novel nano-devices and the characterization of their surface chemistry, topography, electronic properties, electrical transport and interaction with photons. The characterization techniques include scanning tunneling microscopy/spectroscopy (STM/S) for atomic level topography and electronic properties. Development of a Scanning gate microscopy (SGM) system with additional single-mode focused lasers for simultaniously probing influence of static and optical fields. Synchrotron based X-ray techniques, mainly X-ray photoelectron spectroscopy (XPS) is used for evaluating surface chemistry. Surface treatment processes, e.g., ultra-high vacuum (UHV) annealing, digital etchants, atomic hydrogen cleaning, and atomic layer deposition (ALD), are applied and the resulting surface chemistry, structure and electronic properties measured. Beyond studying the surface properties, we also investigate the device efficiency and performance down to the nanometer scale. Therefore, we perform measurements to monitor the device while the local gate and/or a focused light interact with the device.In conclusion, in this thesis the surfaces and interfaces of low-dimensional materials for future device applications are studied using many different characterization methods. It is the hope that the thesis will assist in the progress toward novel devices and improve the energy efficiency and performance of devices. Both the method development and the results give relevant contributions opening for future quantum technologies and (opto)electronics

Dreams of Molecular Beams: Indium Gallium Arsenide Tensile-Strained Quantum Dots and Advances Towards Dynamic Quantum Dots (Moleculare Radiorum Somnia: Indii Gallii Arsenicus Tensa Quanta Puncta et ad Dinamicae Quantae Puntae Progressus)

Through the operation of a molecular beam epitaxy (MBE) machine, I worked on developing the homoepitaxy of high quality InAs with a (111)A crystallographic orientation. By tuning substrate temperature, we obtained a transition from a 2D island growth mode to step- ow growth. Optimized MBE parameters (substrate temperature = 500 °C, growth rate = 0.12 ML/s and V/III ratio ⩾ 40) lead to growth of extremely smooth InAs(111)A films, free from hillocks and other 3D surface imperfections. We see a correlation between InAs surface smoothness and optical quality, as measured by photoluminescence spectroscopy. This work establishes InAs(111)A as a platform for future research into other materials from the 6.1 Å family of semiconductors grown with a (111) orientation. Continuing this work, we also have determined a reproducible set of growth conditions for the self-assembly of tensile-strained In1-xGaxAs quantum dot nanostructures on InAs(111)A surfaces. During molecular beam epitaxy, In1-xGaxAs islands form spontaneously on InAs(111)A when the Ga content x ≥ 50 %. We analyze the structure and composition of InGaAs/InAs(111) samples using atomic force microscopy, transmission electron microscopy, electron energy loss spectroscopy, and photoluminescence spectroscopy. We demonstrate control over the size and areal density of the islands as a function of In1-xGaxAs coverage, In1-xGaxAs composition, and substrate temperature. Furthermore, we also present a study aimed to determining the growth conditions of In1-xGaxAs self-assembled tensile-strained QDs on GaSb(111)A surfaces. From previous work we determined that a larger band gap barrier was necessary to ensure the confinement of charge carriers in the InGaAs nanostructures. Through a series of temperature, V/III ratio, and growth rate we determined the best parameters for GaSb(111) homoepitaxy. We then studied the nucleation of optimal-morphology In1-xGaxAsQDs by locking the compositions at In0.5Ga0.5As, studying the critical pause for group V element transition and V/III ratio prior and post QD growth. Several photoluminescence techniques are employed to determine the light emission properties of these structures. Finally, we did preliminary studies on how to achieve the dynamic lateral confinement of charge carriers in 2D and 3D using near-THz surface acoustic phonon pulses in polar semiconductors. Using the acousto-electrical effect, we measure the degree to which surface acoustic waves (SAWs) confine electrons and holes limiting the number of recombination processes. Applications for this technological development include the external modulation of lateral confinement size in the SAWs and subsequent photon emission wavelength, as well as potential quantum logical gate design using acoustic pulses to drive electrons in a circuit

Cellular Automata

Modelling and simulation are disciplines of major importance for science and engineering. There is no science without models, and simulation has nowadays become a very useful tool, sometimes unavoidable, for development of both science and engineering. The main attractive feature of cellular automata is that, in spite of their conceptual simplicity which allows an easiness of implementation for computer simulation, as a detailed and complete mathematical analysis in principle, they are able to exhibit a wide variety of amazingly complex behaviour. This feature of cellular automata has attracted the researchers' attention from a wide variety of divergent fields of the exact disciplines of science and engineering, but also of the social sciences, and sometimes beyond. The collective complex behaviour of numerous systems, which emerge from the interaction of a multitude of simple individuals, is being conveniently modelled and simulated with cellular automata for very different purposes. In this book, a number of innovative applications of cellular automata models in the fields of Quantum Computing, Materials Science, Cryptography and Coding, and Robotics and Image Processing are presented