Quantum information technologies require reliable sources of correlated/entangled photons. To realize and use these technologies in real life rather than just in laboratory, high efficiency and stable sources of entangled photons are needed. This work considers the design of various semiconductor heterostructures, based on Al$_x$Ga$_{1-x}$As/GaAs, that rely on intersubband transitions in the conduction or in the valence band, to deliver the second order nonlinear process, known as spontaneous parametric down-conversion (SPDC). The second-order SPDC can produce Bell state entangled photons. Second harmonic generation (SHG) is a well known and broadly discussed process, and using the fact that SHG is the reverse process of SPDC, initial studies of SHG are used to support the validity and accuracy of the methodology employed for nonlinear susceptibility calculations. In designing the heterostructures, genetic optimization is used to reduce the computational cost in finding the best structure. The heterostructures designed by considering the intersubband transitions in the conduction band constitute good sources of spectrally entangled photons. The efficiency of the process is estimated and the Schmidt number calculation shows that the structure can produce twin photons with a reasonably good degree of entanglement. Alternatively, using intersubband transitions in the valence band can deliver the polarization entangled photons, which cannot be achieved with conduction intersubband transitions. The genetic optimization is again used to design the best structures for this purpose, and the efficiency of the process is also calculated. We then extend our work to multiparticle entangled states, also known as Greenberger-Horne-Zeilinger (GHZ) states, by considering the third order nonlinear SPDC (TOSPDC) process in designing the heterostructures for this purpose. This designed structure can be a good candidate as a direct TOSPDC source, since the second order nonlinearity is here suppressed by considering symmetric structures only. The efficiency of the process is also calculated and discussed in the thesis.

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Fast pulse methods have been used to study the drift mobility of both electrons and holes in pure CdS crystals, obtained from different sources, over the temperature range 500°K - 77°K. An electron pulse of 5 nsec. duration generates free carriers near the top electrode, and measurements of the transit time in a pulsed applied field lead to a value for the drift mobility. The method is sensitive to the injection of additional carriers and the choice of electrodes is discussed. The experimental method provides the first definite evidence for hole transport in CdS. The hole drift mobility Mh at room temperature has a mean value of 15 cm2/v/sec and the hole lifetime lies between 1 X 10-7 and 3 X10-7 sec. An attempt is made to explain the temperature dependence of Mh by a model involving transport in the upper two valence bands of CdS. The mobility and effective mass ratios in these bands are estimated to be 50 and 3 respectively. The presence of about 1018 cm-3 charged defects has to be assumed. The model is used to interpret some optical properties of CdS. At room temperature the electron drift mobility Me is 275 cm 2/v/sec in agreement with recent Hall mobility and other measurements. Below about 220°K the temperature dependence of Me begins to differ significantly from that of the Hall mobility and the charge transport is predominantly controlled by shallow trapping states which may be connected with non-stoichiometric defects. In the temperature range where both Me and Mh are largely controlled by lattice scattering, none of the known theories can predict the experimental curves. It is concluded, however, that nonpolar scattering by the acoustic modes must be considered to be predominant over most of this temperature range.

The purpose of this thesis is to investigate the initial stages of the growth of heteroepitaxial films on Si substrates. Two prototype systems were chosen for this research: first is Ge/Si(001), where the two species have similar chemical properties; second is CaF2/Si(111), in which the ionic epitaxial film and substrate have similar crystal structures. Both are strained heteroepitaxial films because of their lattice mismatch. These systems have attracted much attention largely due to various promising applications in micro-electronics and fundamental interest in the basic studies of heteroepitaxy. The Metropolis Monte Carlo method is used for this research. For Ge/Si, because of short range interaction forces between atoms, the CELL method is developed and applied to this research. Results have shown that the such a method is fast, efficient and is easily adapted to study all other systems with short range interaction forces; the other method, called the BIG JUMP, is also developed. The results have shown that the BIG JUMP method is particularly useful in generating equilibrium or metastable configurations. The initial stage of MBE growth of Ge on Si(001) was studied using MC method combined with CELL method, or both CELL and BIG JUMP methods. It was found that at least an (8 x 8 x 8) computational cell with six layers that were allowed to move was needed for the simulations. 2% acceptance of MC moves was found to lead a quicker energy minimisation process. This result implies that the energy minimisation process involves big jumps of atoms, corresponding to atom diffusion on a real surface. The energy map of a Ge atom on Si(001)(2 x 1) was calculated and compared with ab initio calculation. An exchange mechanism of a Ge adatom with a Si atom of the substrate was found. This mechanism can be used in understanding the ordered structure observed during the initial stage of MBE growth of Ge on a Si(001) substrate surface.

The behaviour of hydrogen in crystalline and amorphous semiconductors has been simulated using the positive muon as a microscopic probe. Measurements have been made on a wide range of materials: polycrystalline and amorphous silicon, both doped and intrinsic; polycrystalline and amorphous germanium; crystalline CaAs and GaP; and selenium-based glassy chalcogenides. Using the techniques of SR (Muon Spin Resonance/Rotation/Relaxation/Repolarisation), the evolution of the spin polarisation of the muon is monitored. In semiconductors and insulators, the muon is bonded to an electron to form muonium (Mu=+e-) and it is this species that behaves as an analogue of isolated atomic hydrogen. The muon spin polarisation is greatly influenced by the local environment, and its study has yielded information on the sites occupied by the muon/muonium, via determinations of diamagnetic fractions and hyperfine parameters. The technique of muon spin repolarisation has been used for the first time in conjunction with recently derived theoretical expressions to extract this information. Preliminary measurements using the technique of muon spin resonance on a newly developed facility at the Rutherford Appleton Laboratories has also been made.

The High-luminosity upgrade of the LHC will deliver the dramatic increase in luminosity required for precision measurements and to probe Beyond the Standard Model theories. At the same time, it will present unprecedented challenges in terms of pileup and radiation degradation. The CMS experiment is set for an extensive upgrade campaign, which includes the replacement of the current Tracker with another all-silicon detector with improved performance and reduced mass. One of the most ambitious aspects of the future Tracker will be the ability to identify high transverse momentum track candidates at every bunch crossing and with very low latency, in order to include tracking information at the L1 hardware trigger stage, a critical and effective step to achieve triggers with high purity and low threshold. This thesis presents the development and the testing of the CMS Binary Chip 2 (CBC2), a prototype Application Specific Integrated Circuit (ASIC) for the binary front-end readout of silicon strip detectors modules in the Outer Tracker, which also integrates the logic necessary to identify high transverse momentum candidates by correlating hits from two silicon strip detectors, separated by a few millimetres. The design exploits the relation between the transverse momentum and the curvature in the trajectory of charged particles subject to the large magnetic field of CMS. The logic which follows the analogue amplification and binary conversion rejects clusters wider than a programmable maximum number of adjacent strips, compensates for the geometrical offset in the alignment of the module, and correlates the hits between the two sensor layers. Data are stored in a memory buffer before being transferred to an additional buffer stage and being serially read-out upon receipt of a Level 1 trigger. The CBC2 has been subject to extensive testing since its production in January 2013: this work reports the results of electrical characterization, of the total ionizing dose irradiation tests, and the performance of a prototype module instrumented with CBC2 in realistic conditions in a beam test. The latter is the first experimental demonstration of the Pt-selection principle central to the future of CMS. Several total-ionizing-dose tests highlighted no functional issue, but observed significant excess static current for doses < 1 Mrad. The source of the excess was traced to static leakage current in the memory pipeline, and is believed to be a consequence of the high instantaneous dose delivered by the x-ray setup. Nevertheless, a new SRAM layout aimed at removing the leakage path was proposed for the CBC3. The results of single event upset testing of the chip are also reported, two of the three distinct memory circuits used in the chip were proven to meet the expected robustness, while the third will be replaced in the next iteration of the chip. Finally, the next version of the ASIC is presented, highlighting the additional features of the final prototype, such as half-strip resolution, additional trigger logic functionality, longer trigger latency and higher rate, and fully synchronous stub readout.

Recently, the use of an electric field (E-field) to control the magnetic properties of thin magnetic films has drawn intensive interest due to their important potential applications such as magnetoelectric random access memory (MERAM) devices and magnetoelectric (ME) sensor. In this thesis, the work first includes a study of the strain-mediated ME coupling strength manipulation by either changing ferromagnetic layer thickness (30-100 nm) or inserting a thin Ti buffer layer (0-10 nm). A large remanence ratio (Mr/Ms) tunability of 95% has been demonstrated in the 65 nm CoFe/PMN-PT heterostructure, corresponding to a giant ME constant (α) of 2.5 × 10-6 s/m, when an external E-field of 9 kV/cm was applied. Also, a record high remanence ratio (Mr/Ms) tunability of 100% has been demonstrated in the 50 nm CoFe/8 nm Ti/PMN-PT heterostructure, corresponding to a large ME constant α of 2.1 × 10-6 s/m, when the E-field of 16 kV/cm was applied. Furthermore, the E-field induced magnetic response was repeatable and quick even after 30 repeats were made. Secondly, a study of non-volatile magnetization change has been demonstrated in the 65 nm CoFe/24 nm Metglas/PMN-PT. In this heterostructure, the E-field created two new non-volatile remanence states, although the as-grown magnetic anisotropy was altered permanently, when the E-field between -6 kV/cm to +6 kV/cm was applied. Based on giant magnetoresistance (GMR) or anisotropic magnetoresistance (AMR), the MERAM memory cell was proposed for the fast, low-power and high-density information storage.

Gallium Nitride (GaN) has been proven to be a very suitable material for advanced power electronics on account of its outstanding material properties. Today, researchers are exploring GaN-based high electron mobility transistors (HEMTs) for conventional as well as high-end solutions in the range of 600 – 1200 V. However, thermal and power density limitations have impeded the achievement of the peak operational capability of AlGaN/GaN HEMTs. GaN-on-Diamond technology has proven to be a feasible solution to reduce thermal resistance and increase power density of AlGaN/GaN HEMTs for RF applications. The work presented in this thesis is focused on the realisation of high-voltage GaN-on-Diamond power semiconductor devices. This goal was achieved through extensive numerical simulations applied to device design, fabrication, and characterisation. The fabricated devices include conventional AlGaN/GaN HEMT design in circular and linear form with and without field plate engineering. The circular GaN-on-Diamond HEMTs with gate width of ~ 430 μm, gate length of 3 μm, gate-to-drain separation of 17 μm and source field plate length of 3 μm have shown breakdown voltage of ~ 1.1 kV. In this work a new concept of normally-off optically-controlled AlGaN/GaN-based power semiconductor device is proposed. A simulation study has been carried out in order to explore the DC characteristics, switching characteristics, breakdown voltage, and current gain of these novel devices. The typical structure comprises a 20 nm of undoped Al0.23Ga0.77N barrier layer, a 1.1 μm undoped-GaN buffer layer and a p-doped region (to locally deplete the electron channel and ensure a normally-off operation). The simulation study shows that the gain and the breakdown voltage of the device are highly dependent on the depth of the p-doped region. At a particular depth of the p-doped region of 500 nm the gain of the device is 970 (at light intensity of 7 W/cm2) and the breakdown voltage is ~ 350 V. The rise and fall times of the device is found to be 0.4 μsec and 0.3 μsec respectively. The simulation results show a significant potential of the proposed structure for high-frequency and high-power applications.

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No description available.