Antimonide-based superlattice membranes for infrared applications

Semiconductor membranes offer an interesting materials and device development platform due to their ability to integrate dissimilar materials through a print, stamp and transfer process. There is a lot of interest in integrating antimonide based type-II superlattices (T2SL) onto inexpensive substrates, such as Si, to not only undertake fundamental studies into the optical, electronic and structural properties of the superlattices but also to fabricate wafer-level infrared (IR) photodetectors. An effective approach to transfer type-II superlattice membranes (T2SL-M) onto alternate substrates is based on membrane release from the native GaSb substrate followed by the transfer to a new host substrate. In this work, I have transferred InAs/GaSb and InAs/InAsSb T2SLs with different in-plane geometries from a GaSb substrate to a variety of host substrates, including Si, polydimethylsiloxane and metal coated surfaces. Electron microscopy shows structural integrity of transferred membranes with thicknesses ranging from 100 nm to 2.5 µm and lateral sizes from 24x24 µm2 v to 1x1 cm2 . Atomic force and electron microscopy reveal the excellent quality of the membrane interface with the new host. The crystalline structure of the membrane is not altered by the fabrication process, and minimal elastic relaxation occurs during the release step, as demonstrated by X-ray diffraction and mechanical modeling. I have also used the antimonide superlattice membranes to realize wafer level infrared detectors on silicon substrates without using conventional Indium-bump hybridization. In this approach, PIN superlattices are grown on top of a 60 nm Al0.6Ga0.4Sb sacrificial layer on a GaSb host substrate. Following the growth, I have transferred the individual pixels using an epitaxial lift-off technique, which consists of a wet-etch to undercut the pixels followed by a dry-stamp process to transfer the pixels to a silicon substrate prepared with a gold layer. I have done structural and optical characterization of the transferred pixels using an optical microscope, scanning electron microscopy, and photo luminescence. The interface between the transferred pixels and the new substrate is abrupt, and no significant degradation in the optical quality is observed. Next, I have fabricated an indium-bump-free membrane detector using this approach. Spectral response measurements and the current-voltage characteristics of an infrared photodetector, based on the InAs/InAsSb superlattice, bonded to Si demonstrates the functionality of transferred membranes in the infrared range. The performance of the membrane detector is compared to a control detector using the as-grown epitaxial material. The proposed approach to fabricate Indium-bump free detectors could pave the way for wafer-level integration of photonic detectors on silicon substrates, which could dramatically reduce the cost of these detectors. Since the release of T2SL-M is achieved using a high etch selectivity between the active region and the Al-containing sacrificial layers, a poor selectivity between the sacrificial layers and a variety of T2SL active regions will result in significant damage to the active layer of an IR detector. I have developed a novel two-step etching process to protect the T2SL-M while the sacrificial layer is etched away. In this process, both the top surface and the sidewalls of the membrane are coated with a hard-baked polymer film (i.e., photoresist), and therefore they are unexposed to the chemical etchant. For Al and Ga containing compounds, with no membrane isolation, this process leads to rough sidewalls which are expected to increase surface recombination in the membrane and therefore increases the dark current density of an IR detector. I have quantified this effect by characterizing T2SL IR detectors fabricated on isolated and non-isolated mesas. A comparative analysis of the dark current density measured for the two devices signify the effect of having exposed sidewalls during membrane release. These experimental results are consistent with theoretical calculations which show a relative enhancement of surface recombination with increasing roughness of the membrane sidewalls. The development of these Sb based T2SL membranes opens up new exciting prospects for material science studies and device architecture integration

Multispectral Metamaterial Detectors for Smart Imaging

The ability to produce a high quality infrared image has significantly improved since its initial development in the 1950s. The first generation consisted of a single pixel that required a two-dimensional raster scan to produce an image. The second generation comprised of a linear array of pixels that required a mechanical sweep to produce an image. The third generation utilizes a two-dimensional array of pixels to eliminate the need for a mechanical sweep. Third generation imaging technology incorporates pixels with single color or broadband sensitivity, which results in \u27black and white\u27 images. The research presented in this dissertation focuses on the development of 4th generation infrared detectors for the realization of a new generation of infrared focal plane array. To achieve this goal, we investigate metamaterials to realize multicolor detectors with enhanced quantum efficiency for similar function to a human retina. The key idea is to engineer the pixel such that it not only has the ability to sense multimodal data such as color, polarization, dynamic range and phase but also the intelligence to transmit a reduced data set to the central processing unit (neurophotonics). In this dissertation, we utilize both a quantum well infrared photodetector (QWIP) and interband cascade detector (ICD) hybridized with a metamaterial absorber for enhanced multicolor sensitivity in the infrared regime. Through this work, along with some design lessons throughout this iterative process, we design, fabricate and demonstrate the first deep-subwavelength multispectral infrared detector using an ultra-thin type-II superlattice (T2-SL) detector coupled with a metamaterial absorber with 7X enhanced quantum efficiency. We also identify useful versus non-useful absorption through a combination of absolute absorption and quantum efficiency measurements. In addition to these research efforts, we also demonstrate a dynamic multicolor metamaterial in the terahertz regime with electronically tunable frequency and gain for the first time. Utilizing an electronically tunable metamaterial, one can design an imaging system that can take multiple spectral responses within one frame for the classification of objects based on their spectral fingerprint.\u2

Spintronics: Fundamentals and applications

Spintronics, or spin electronics, involves the study of active control and manipulation of spin degrees of freedom in solid-state systems. This article reviews the current status of this subject, including both recent advances and well-established results. The primary focus is on the basic physical principles underlying the generation of carrier spin polarization, spin dynamics, and spin-polarized transport in semiconductors and metals. Spin transport differs from charge transport in that spin is a nonconserved quantity in solids due to spin-orbit and hyperfine coupling. The authors discuss in detail spin decoherence mechanisms in metals and semiconductors. Various theories of spin injection and spin-polarized transport are applied to hybrid structures relevant to spin-based devices and fundamental studies of materials properties. Experimental work is reviewed with the emphasis on projected applications, in which external electric and magnetic fields and illumination by light will be used to control spin and charge dynamics to create new functionalities not feasible or ineffective with conventional electronics.Comment: invited review, 36 figures, 900+ references; minor stylistic changes from the published versio

Innovative Long Wavelength Infrared Detector Workshop Proceedings

The focus of the workshop was on innovative long wavelength (lambda less than 17 microns) infrared (LWIR) detectors with the potential of meeting future NASA and DoD long-duration space application needs. Requirements are for focal plane arrays which operate near 65K using active refrigeration with mission lifetimes of five to ten years. The workshop addressed innovative concepts, new material systems, novel device physics, and current progress in relation to benchmark technology. It also provided a forum for discussion of performance characterization, producibility, reliability, and fundamental limitations of device physics. It covered the status of the incumbent HgCdTe technology, which shows encouraging progress towards LWIR arrays, and provided a snapshot of research and development in several new contender technologies

ULTRARAM™:Design, Modelling, Fabrication and Testing of Ultra-low-power III-V Memory Devices and Arrays

In this thesis, a novel memory based on III-V compound semiconductors is studied, both theoretically and experimentally, with the aim of developing a technology with superior performance capabilities to established and emerging rival memories. This technology is known as ULTRARAM™. The memory concept is based on quantum resonant tunnelling through InAs/AlSb heterostructures, which are engineered to only allow electron tunnelling at precise energy alignment(s) when a bias is applied. The memory device features a floating gate (FG) as the storage medium, where electrons that tunnel through the InAs/AlSb heterostructure are confined in the FG to define the memory logic (0 or 1). The large conduction band offset of the InAs/AlSb heterojunction (2.1 eV) keeps electrons in the FG indefinitely, constituting a non-volatile logic state. Electrons can be removed from the FG via a similar resonant tunnelling process by reversing the voltage polarity. This concept shares similarities with flash memory, however the resonant tunnelling mechanism provides ultra-low-power, low-voltage, high-endurance and high-speed switching capability. The quantum tunnelling junction is studied in detail using the non-equilibrium Green’s function (NEGF) method. Then, Poisson-Schrödinger simulations are used to design a high-contrast readout procedure for the memory using the unusual type-III band-offset of the InAs/GaSb heterojunction. With the theoretical groundwork for the technology laid out, the memory performance is modelled and a high-density ULTRARAM™ memory architecture is proposed for random-access memory applications. Later, NEGF calculations are used for a detailed study of the process tolerances in the tunnelling region required for ULTRARAM™ large-scale wafer manufacture. Using interfacial misfit array growth techniques, III-V layers (InAs, AlSb and GaSb) for ULTRARAM™ were successfully implemented on both GaAs and Si substrates. Single devices and 2×2 arrays were then fabricated using a top-down processing approach. The memories demonstrated outstanding memory performance on both substrate materials at 10, 20 and 50 µm gate lengths at room temperature. Non-volatile switching was obtained with ≤ 2.5 V pulses, corresponding to a switching energy per unit area that is lower than DRAM and flash by factors of 100 and 1000 respectively. Memory logic was retained for over 24 hours whilst undergoing over 10^6 readout operations. Analysis of the retention data suggests a storage time exceeding 1000 years. Devices showed promising durability results, enduring over 10^7 cycles without degradation, at least two orders of magnitude improvement over flash memory. Switching of the cell’s logic was possible at 500 µs pulse durations for a 20 µm gate length, suggesting a subns switching time if scaled to modern-day feature sizes. The proposed half-voltage architecture is shown to operate in principle, where the memory state is preserved during a disturbance test of > 10^5 half-cycles. With regard to the device physics, these findings point towards ULTRARAM™ as a universal memory candidate. The path towards future commercial viability relies on process development for aggressive device and array-size scaling and implementation on larger Si wafe

Accurate quantum transport modelling and epitaxial structure design of high-speed and high-power In0.53Ga0.47As/AlAs double-barrier resonant tunnelling diodes for 300-GHz oscillator sources

Terahertz (THz) wave technology is envisioned as an appealing and conceivable solution in the context of several potential high-impact applications, including sixth generation (6G) and beyond consumer-oriented ultra-broadband multi-gigabit wireless data-links, as well as highresolution imaging, radar, and spectroscopy apparatuses employable in biomedicine, industrial processes, security/defence, and material science. Despite the technological challenges posed by the THz gap, recent scientific advancements suggest the practical viability of THz systems. However, the development of transmitters (Tx) and receivers (Rx) based on compact semiconductor devices operating at THz frequencies is urgently demanded to meet the performance requirements calling from emerging THz applications. Although several are the promising candidates, including high-speed III-V transistors and photo-diodes, resonant tunnelling diode (RTD) technology offers a compact and high performance option in many practical scenarios. However, the main weakness of the technology is currently represented by the low output power capability of RTD THz Tx, which is mainly caused by the underdeveloped and non-optimal device, as well as circuit, design implementation approaches. Indeed, indium phosphide (InP) RTD devices can nowadays deliver only up to around 1 mW of radio-frequency (RF) power at around 300 GHz. In the context of THz wireless data-links, this severely impacts the Tx performance, limiting communication distance and data transfer capabilities which, at the current time, are of the order of few tens of gigabit per second below around 1 m. However, recent research studies suggest that several milliwatt of output power are required to achieve bit-rate capabilities of several tens of gigabits per second and beyond, and to reach several metres of communication distance in common operating conditions. Currently, the shortterm target is set to 5−10 mW of output power at around 300 GHz carrier waves, which would allow bit-rates in excess of 100 Gb/s, as well as wireless communications well above 5 m distance, in first-stage short-range scenarios. In order to reach it, maximisation of the RTD highfrequency RF power capability is of utmost importance. Despite that, reliable epitaxial structure design approaches, as well as accurate physical-based numerical simulation tools, aimed at RF power maximisation in the 300 GHz-band are lacking at the current time. This work aims at proposing practical solutions to address the aforementioned issues. First, a physical-based simulation methodology was developed to accurately and reliably simulate the static current-voltage (IV ) characteristic of indium gallium arsenide/aluminium arsenide (In-GaAs/AlAs) double-barrier RTD devices. The approach relies on the non-equilibrium Green’s function (NEGF) formalism implemented in Silvaco Atlas technology computer-aided design (TCAD) simulation package, requires low computational budget, and allows to correctly model In0.53Ga0.47As/AlAs RTD devices, which are pseudomorphically-grown on lattice-matched to InP substrates, and are commonly employed in oscillators working at around 300 GHz. By selecting the appropriate physical models, and by retrieving the correct materials parameters, together with a suitable discretisation of the associated heterostructure spatial domain through finite-elements, it is shown, by comparing simulation data with experimental results, that the developed numerical approach can reliably compute several quantities of interest that characterise the DC IV curve negative differential resistance (NDR) region, including peak current, peak voltage, and voltage swing, all of which are key parameters in RTD oscillator design. The demonstrated simulation approach was then used to study the impact of epitaxial structure design parameters, including those characterising the double-barrier quantum well, as well as emitter and collector regions, on the electrical properties of the RTD device. In particular, a comprehensive simulation analysis was conducted, and the retrieved output trends discussed based on the heterostructure band diagram, transmission coefficient energy spectrum, charge distribution, and DC current-density voltage (JV) curve. General design guidelines aimed at enhancing the RTD device maximum RF power gain capability are then deduced and discussed. To validate the proposed epitaxial design approach, an In0.53Ga0.47As/AlAs double-barrier RTD epitaxial structure providing several milliwatt of RF power was designed by employing the developed simulation methodology, and experimentally-investigated through the microfabrication of RTD devices and subsequent high-frequency characterisation up to 110 GHz. The analysis, which included fabrication optimisation, reveals an expected RF power performance of up to around 5 mW and 10 mW at 300 GHz for 25 μm2 and 49 μm2-large RTD devices, respectively, which is up to five times higher compared to the current state-of-the-art. Finally, in order to prove the practical employability of the proposed RTDs in oscillator circuits realised employing low-cost photo-lithography, both coplanar waveguide and microstrip inductive stubs are designed through a full three-dimensional electromagnetic simulation analysis. In summary, this work makes and important contribution to the rapidly evolving field of THz RTD technology, and demonstrates the practical feasibility of 300-GHz high-power RTD devices realisation, which will underpin the future development of Tx systems capable of the power levels required in the forthcoming THz applications

Accurate characterisation of Resonant Tunnelling Diodes for high-frequency applications

Recent scientific advancements regarding the generation and detection of terahertz (THz) radiation have led to a rapid increase in research interest in this frequency band in the context of its numerous potential applications including high-speed wireless communications, biomedical diagnostics, security screening and material science. Various proposed solutions have been investigated in the effort to bridge this relatively unexplored region of the electromagnetic spectrum, and thus exploit its untapped potential. Among them, the resonant tunnelling diode (RTD) has been demonstrated as the fastest electronic device with its room temperature operation extending into the THz range. The RTD exhibits a negative differential resistance (NDR) region in its I-V characteristics, with this feature being key to its capabilities. Even though the unique capabilities of RTD devices have been experimentally proven in the realisation of compact NDR oscillators and detectors, with fundamental frequencies of about 2 THz, and high-sensitivity detectors up to 0.83 THz, the reliable design procedures and methodologies of RTD-based circuits are yet to be fully developed. In this regard, significant effort has been devoted primarily to the accurate theoretical description of the high-frequency behaviour of RTDs, using various small-signal equivalent circuit models. However, many of these models have had either limited or no experimental validation, and so a robust and reliable RTD device model is desirable. The aim of this thesis is to describe a systematic approach regarding the design, fabrication and characterisation of RTD devices, providing a universal methodology to accurately determine their radio-frequency (RF) behaviour, and so this way enable a consistent integrated circuit design procedure for high-frequency circuits. A significant challenge in the modelling of RTD devices is represented by the presence of parasitic bias oscillations within the NDR region. This has been identified as one of the main restricting factors with regards to the accurate high-frequency characterisation of this operating region. The common approach to overcoming this limitation is through a stabilising technique comprising of an external shunt-resistor network. This approach has been successfully demonstrated to suppress bias oscillations in RTD-based circuits which require operation within the NDR region. However, the introduction of the additional circuit component associated with this method increases the complexity of the de-embedding procedure of the extrinsic parasitic elements, rendering the overall device characterisation generally difficult at high-frequencies. In this work, a novel on-wafer bond-pad and shunt resistor network de-embedding technique was developed in order to facilitate the characterisation of RTDs throughout the complete bias range, without limitation to device sizing or frequency, under a stable operating regime. The procedure was demonstrated to accurately determine the circuit high-frequency behaviour of the RTD device from S-parameter measurements up to 110 GHz. The universal nature of this procedure allows it to be easily adapted to accommodate higher complexity stabilising networks configuration or different bond-pad geometries. Furthermore, the de-embedding method has also enabled the development of a novel quasi-analytical procedure for high accuracy extraction of the device equivalent circuit parameters, which is expected to provide a strong experimental foundation for the further establishment of a universal RTD RF model. The applicability of the developed high-frequency model, which can be easily scaled for various device sizes, together with the measured RTD I-V characteristics was further demonstrated in the development of a non-linear model, which was integrated in a commercial simulator, the Advanced Design Systems (ADS) software from Keysight Technologies. From an application perspective, the model was used in the design of an RTD as a square-law detector for high-frequency data transmission systems. The simulated detector performance was validated experimentally using an RTD-based transmitter in the W-band (75 – 110 GHz) up to 4 Gbps (error free transmission: BER < 10-10 in a waveguide connection), and in the Ka-band (26.5 – 50 GHz) up to 2.4 Gbps (error free transmission in a wireless data link), which demonstrated the accuracy of the developed RTD modelling approach. Lastly, a sensitivity analysis of the RTD-based detector within the Ka-band showed a superior RTD performance over commercially available solutions, with a peak (corrected) detector responsivity of 13.48 kV/W, which is a factor of >6 better compared to commercially available Schottky barrier diode (SBD) detectors

Nanometer Probing of Operating Nano-Photonic Devices

The external performance of quantum optoelectronic devices is governed by the three-dimensional profiles of electric potentials determined by the distribution of charge carriers (electrons and holes) within the active regions of the devices. Charge carrier dynamics play a vital role in active photonic quantum/nano devices, such as electrically-pumped semiconductor lasers. As an example, in quantum cascade lasers (QCLs) the Electric Field Domain (EFD) hypothesis posits that the potential distribution might be simultaneously spatially non-uniform and temporally unstable. Until now, there are no experimental means of probing the inner potential profile directly and as a result the mechanisms responsible for sub-par device performance of QCLs remain the subject of speculation. Another example is interband cascade lasers (ICLs), in which the distribution of gain-providing charge carrier governs the operation and performance of the devices, but has not been experimentally measured prior to this study. This work presents a systematic experimental study of gain-providing charge carrier distribution in a lasing interband cascade laser and electric potential distribution in THz QCLs. The unique charge carrier distribution profile in the quantum-well active region is quantitatively measured at nanometer scales by using the non-invasive scanning voltage microscopy (SVM) technique. Experimental results clearly confirm the accumulation and spatial segregation of holes and electrons in the core of the ICL device. The measurement also shows that the charge carrier density is essentially clamped in the presence of stimulated emission in ICLs, thus conclusively differentiating the lasing from non-lasing devices. The SVM technique has been applied to lasing THz QCLs to verify the hypothesis of electric field domains in semiconductor quantum structures. The experimental results reveal that the multi-quantum-well active region is divided into multiple sections having distinctly different electric fields. The electric field across these serially-stacked quantum cascade modules are observed not to continuously increase in proportion to the gradual increase of the applied device bias, but rather jumps between discrete values related to tunneling resonances. Also in the THz QCLs the progression of the observed EFDs are carefully probed. Experimental evidences reveal that an incremental change in device bias leads to a hopping-style shift in the EFD boundary – the higher electric field domain expands at least one module each step at the expense of the lower field domain within the active region. The SVM findings in THz QCLs indicate the importance of quantum active region design for intrinsically more uniform and stable electric field profiles. The two showcase study examples demonstrate that the cryogenic-temperature SVM is an enabling technique, being able to measure and resolve nanometer scale features non-destructively on operating devices. This experimental approach allows directly mapping the electric field distribution as well as the charge carrier distribution inside operating semiconductor quantum devices at nanometer scales, thus connecting the inner workings with the external measures of the devices. The experimental approach is expected to facilitate a deeper understanding of fundamental processes that are governing the operation and performance of a wide range of nanoelectronic and nanophotonic devices.4 month