Long-wavelength type-II InAs/GaSb superlattice infrared photodetectors

Type-II superlattices (T2SLs) have demonstrated great potential for long-wavelength infrared (LWIR) photodetectors but have yet to achieve their theoretical performance levels. In this research project, the capability at Cardiff University for simulation, fabrication, and characterisation of LWIR T2SLs is established and improved with the ultimate aim of improving the current performance levels of LWIR T2SL detectors. The strategies for improving performance, described herein, include improving the design of the superlattice (SL) period, optimisation of the fabrication process, and monolithic integration on GaAs substrates. 8 band k·p simulations are used for band structure modelling, which enables device design optimisation and informs analysis of experimental results. Molecular beam epitaxy (MBE) is used for growth of experimental T2SL reference wafers/samples, which employ novel SL period designs, GaAs substrates, or both. The material quality of these wafers is then investigated by x-ray diffraction (XRD), atomic force microscopy (AFM), and transmission electron microscopy (TEM). Band structure information is obtained through photoluminescence (PL) measurements. Diodes were fabricated using a standard photolithography process which was later modified to incorporate experimental improvements. The diode performance of the T2SLs was then tested in a cryogenic probe station. Various strategies for improving device performance were tested and evaluated. By comparing different combinations of superlattice layer thicknesses, it was observed that the novel 12 ML InAs/4 ML GaSb superlattice structure was better suited to high-temperature operation than conventional designs. A comparison of the structural, optical, and device characteristics of LWIR T2SLs on GaSb and GaAs substrates was made to investigate the feasibility of LWIR T2SLs on a low-cost GaAs platform. It was observed that, despite using an interfacial misfit (IMF) array between the GaSb buffer and GaAs substrate, a material degradation due to the heteroepitaxial growth led to a corresponding degradation in optical and device performance. However, the reported work in this thesis is mere preliminary demonstration. There is a scope for improvement by involving different buffer layers, dislocation filters, and interfacial schemes of heteroepitaxy. The reasons for and nature of this material degradation were studied in detail using TEM. This thesis also reports intermediate results from the development of a dry etching process for T2SLs are reported. Scanning electron micrographs show this process significantly improves the sidewall verticality and fill factor compared to wet etching. I-V measurements indicate the performance of dry-etched devices is degraded by roughly two orders of magnitude at low temperatures compared to wet etched reference samples but is comparable at high temperatures

Recent trends in 8-14 µm type-II superlattice infrared detectors

Type-II superlattices (T2SLs) hold enormous potential for next-generation, 8 – 14μm long-wavelength infrared (LWIR) detectors for use at high operating temperature (HOT). The inherit flexibility of the material system has enabled the incorporation of unipolar barriers to eliminate generation-recombination currents and enhance device performance. In addition to suppressed Auger recombination and tunneling currents, this has led to sustained research interest in this material system over the past several decades. For these reasons they are theoretically predicted to outperform the current state-of-the-art Mercury Cadmium Telluride (MCT) detectors. This review provides an overview of LWIR T2SL detectors and highlights some recent developments towards HOT applications. Recent studies on the minority carrier lifetime and diffusion length of T2SLs are examined to appraise the extent to which they limit the performance of HOT LWIR T2SL detectors. Strategies for mitigating these limitations are also explicated

Flexbility of Ga-containing type-II superlattice for long-wavelength infrared detection

In this paper, the flexibility of long-wavelength Type-II InAs/GaSb superlattice (Ga-containing SL) is explored and investigated from the growth to the device performance. First, several samples with different SL period composition and thickness are grown by molecular beam epitaxy. Nearly strain-compensated SLs on GaSb exhibiting an energy band gap between 105 to 169 meV at 77K are obtained. Second, from electronic band structure calculation, material parameters are extracted and compared for the different grown SLs. Finally, two p-i-n device structures with different SL periods are grown and their electrical performance compared. Our investigation shows that an alternative SL design could potentially be used to improve the device performance of diffusion-limited devices for long-wavelength infrared detection

High-performance mid-wavelength InAs avalanche photodiode using AlAs013Sb087 as the multiplication layer

We report on a high-performance mid-wavelength infrared avalanche photodetector (APD) with separate absorption and multiplication regions. InAs is used as the absorber material and high-bandgap AlAs0.13Sb0.87 is used as the multiplication material. At room temperature, the APD’s peak response wavelength is 3.27 μm, and the 50% cutoff wavelength is 3.5 μm. The avalanche gain reaches 13.1 and the responsivity is 8.09 A/W at 3.27 μm when the applied reverse bias voltage is 14.6 V. The measured peak detectivit

Authoritarianism in the Living Room: Everyday Disciplines, Senses, and Morality in Taiwan’s Military Villages

With the nationalist government – Kuomintang (KMT) – retreating from mainland China in 1949, some 600,000 military personnel relocated to Taiwan. The military seized former Japanese colonial properties and built its own settlements, establishing temporary military dependents’ villages called juancun (眷村). When the prospect of counter-attacking the mainland vanished, the KMT had to face the reality of settling permanently in Taiwan. How, then, did the KMT’s authoritarian power enter the everyday lives of its own support group? In this article I will focus on the coercive elements of KMT authoritarianism, which permeated these military villages in Taiwan. I will look at the coercive mechanisms through the analytical lens of Foucauldian discipline. I argue that disciplinary techniques such as surveillance, disciplining of the body and the senses, as well as the creation of morality regimes played an important role in the cooptation of village residents into KMT authoritarianism by normalising and naturalising it

Resolving the inner parsec of the blazar J1924-2914 with the Event Horizon Telescope

The blazar J1924-2914 is a primary Event Horizon Telescope (EHT) calibrator for the Galactic Center's black hole Sagittarius A*. Here we present the first total and linearly polarized intensity images of this source obtained with the unprecedented 20 $\mu$as resolution of the EHT. J1924-2914 is a very compact flat-spectrum radio source with strong optical variability and polarization. In April 2017 the source was observed quasi-simultaneously with the EHT (April 5-11), the Global Millimeter VLBI Array (April 3), and the Very Long Baseline Array (April 28), giving a novel view of the source at four observing frequencies, 230, 86, 8.7, and 2.3 GHz. These observations probe jet properties from the subparsec to 100-parsec scales. We combine the multi-frequency images of J1924-2914 to study the source morphology. We find that the jet exhibits a characteristic bending, with a gradual clockwise rotation of the jet projected position angle of about 90 degrees between 2.3 and 230 GHz. Linearly polarized intensity images of J1924-2914 with the extremely fine resolution of the EHT provide evidence for ordered toroidal magnetic fields in the blazar compact core

Monitoring the Morphology of M87* in 2009-2017 with the Event Horizon Telescope

The Event Horizon Telescope (EHT) has recently delivered the first resolved images of M87*, the supermassive black hole in the center of the M87 galaxy. These images were produced using 230 GHz observations performed in 2017 April. Additional observations are required to investigate the persistence of the primary image feature—a ring with azimuthal brightness asymmetry—and to quantify the image variability on event horizon scales. To address this need, we analyze M87* data collected with prototype EHT arrays in 2009, 2011, 2012, and 2013. While these observations do not contain enough information to produce images, they are sufficient to constrain simple geometric models. We develop a modeling approach based on the framework utilized for the 2017 EHT data analysis and validate our procedures using synthetic data. Applying the same approach to the observational data sets, we find the M87* morphology in 2009-2017 to be consistent with a persistent asymmetric ring of ∼40 μas diameter. The position angle of the peak intensity varies in time. In particular, we find a significant difference between the position angle measured in 2013 and 2017. These variations are in broad agreement with predictions of a subset of general relativistic magnetohydrodynamic simulations. We show that quantifying the variability across multiple observational epochs has the potential to constrain the physical properties of the source, such as the accretion state or the black hole spin

Copy Number Variation in Intron 1 of SOX5 Causes the Pea-comb Phenotype in Chickens

Pea-comb is a dominant mutation in chickens that drastically reduces the size of the comb and wattles. It is an adaptive trait in cold climates as it reduces heat loss and makes the chicken less susceptible to frost lesions. Here we report that Pea-comb is caused by a massive amplification of a duplicated sequence located near evolutionary conserved non-coding sequences in intron 1 of the gene encoding the SOX5 transcription factor. This must be the causative mutation since all other polymorphisms associated with the Pea-comb allele were excluded by genetic analysis. SOX5 controls cell fate and differentiation and is essential for skeletal development, chondrocyte differentiation, and extracellular matrix production. Immunostaining in early embryos demonstrated that Pea-comb is associated with ectopic expression of SOX5 in mesenchymal cells located just beneath the surface ectoderm where the comb and wattles will subsequently develop. The results imply that the duplication expansion interferes with the regulation of SOX5 expression during the differentiation of cells crucial for the development of comb and wattles. The study provides novel insight into the nature of mutations that contribute to phenotypic evolution and is the first description of a spontaneous and fully viable mutation in this developmentally important gene

Nanotopographical induction of osteogenesis through adhesion, bone morphogenic protein cosignaling, and regulation of microRNAs

It is emerging that nanotopographical information can be used to induce osteogenesis from mesenchymal stromal cells from the bone marrow and it is hoped that this nanoscale bioactivity can be utilized to engineer next generation implants. However, the osteogenic mechanism of surfaces is currently poorly understood. In this report, we investigate mechanism and implicate bone morphogenic protein (BMP) in up-regulation of RUNX2 and show that RUNX2 and its regulatory miRNAs are BMP sensitive. Our data demonstrates that osteogenic nanotopography promotes co-localization of intergrins and BMP2 receptors in order to enhance osteogenic activity and that vitronectin is important in this interface. This provides insight that topographical regulation of adhesion can have effects on signaling cascades outside of cytoskeletal signaling and that adhesions can have roles in augmenting BMP signaling