The effects of ruby laser irradiation on the electrical and physical properties of coated (Si[3]N[4]) and uncoated semi-insulating GaAs, implanted with selenium ions have been studied. The thermal stability of laser annealed GaAs implanted with tellurium ions has been studied, in the temperature range 200-800°C. Electrical, Rutherford backscattering, electron and optical microscopy measurements coupled with electron microprobe analysis have been used to study the properties of GaAs samples before and after laser irradiation. It has been shown that Si[3]N[4] encapsulants act as a source of n-type doping of the underlying GaAs for Q-switched laser energy densities greater than 0.3 J/cm[2] . Silicon atoms from Si[3]N[4] were shown to be responsible for the doping effect, possibly due to laser induced indiffusion of silicon atoms. It was shown that selenium ions implanted into GaAs to doses less than or equal to 1x10[14]/cm[2] did not become electrically active after Q-switched laser irradiation, with the substrate held at room temperature, most probably due to laser induced compensating defects. However, selenium implants of doses of at least 5x10[13]/cm[2] became electrically active when the substrate was held at elevated temperatures (≥200°C) during laser irradiation. The measured electrical properties which were found to depend on the ion dose and energy, the laser energy density, the number of pulses and the substrate temperature during laser annealing were rather poor. The electron mobilities in all laser annealed samples were lower than predicted by Sze and Irvin. Q-switched laser irradiation of GaAs implanted with tellurium ions, followed by heat treatment at 200°C and above caused a significant reduction in the electron concentration and the extent of the reduction was found to depend on the annealing temperature. Laser induced surface damage was observed, which could be minimised by improving the spatial non-uniformity of energy across the laser beam using a beam diffuser.

The dilute nitrides on GaAs (e.g. Ga(In)NxAs1-x where typically x < 5%) have provided a large amount of interesting physics and led to the development of improved telecommunication lasers. In the last 5 years, novel dilute nitride materials (such as GaN(As)P) have grown in popularity due to applications such as monolithic growth on silicon and improved device characteristics in red-amber-yellow solid state illumination. The Band Anti-Crossing model, using a single weighted nitrogen level (found through photocurrent spectroscopy), is found to successfully predict the band gap energy in GaNP/GaP and GaNAsP/GaP and bulk GaInNAs/GaAs with nitrogen composition, temperature and high hydrostatic pressure. An investigation over a range of host materials found that the coupling parameter can be modelled in terms of the energy separation between the host conduction band minimum and the nitrogen level. GaNAsP/GaP single quantum well lasers are a realistic possibility for an efficient laser source grown on silicon for monolithic optoelectronic integrated circuits. Large threshold current densities of over 0.75kA/cm2 at 80K and 890nm and large temperature variations in the threshold current are shown to be due to non-radiative loss processes. Low characteristic temperatures of the threshold current and differential efficiency (T0=60K and T1=30K at 130K) are found. The sub-linear dependence of the spontaneous emission vs current curve and increasing threshold current with high pressure reveal this loss process is carrier leakage. It is shown that the leakage is not into the X-minima of the GaP barriers, but associated with nitrogen levels in the GaP barriers. If this leakage path can be eliminated, a low room temperature threshold current density should be achieved. (In)GaNP/GaP LEDs are studied for use in red-amber-yellow illumination applications. Through high pressure measurements, it is found that 90 +/-10% of the current flowing through a GaN0.006P0.994 bulk LED is lost to carrier leakage at room temperature and pressure, at a current density of 40A/cm2. When using an In0.14Ga0.86N0.006P0.994 quantum well (of thickness 100 A) active region, the electron potential barrier is increased and carrier leakage accounts for 50+/-15% of the device current density (40A/cm2) at room temperature and pressure. At 100K it was found the leakage reduces to below 10% at current density of 40A/cm2 in all devices. If the leakage path can be reduced, these devices may offer better device characteristics than current red AlInGaP LEDs. Initial investigations on bulk GaInNAs avalanche photodiodes show that with an increase of nitrogen concentration and reduction in band gap, the breakdown voltage increases, consistent with the nitrogen suppressing electron impact ionization as predicted by theory. The pressure coefficient of the breakdown voltage in GaInNAs was found to be larger in magnitude and opposite in sign (dVbd/dP=+5.5+/-0.5 x10-3kbar-1) than the pressure coefficient of the breakdown voltage in GaAs, showing that nitrogen induces a profound change in the carrier scattering mechanisms, which may provide low-noise, high sensitivity detectors at telecommunication wavelengths.

No description available.

This work studies the use of Ga0.12-0.i6In0.88-0.84Sb quantum wells grown by molec­ular beam epitaxy (MBE) on a highly mismatched substrate for use in light emit­ting diodes (LEDs) and lasers emitting in the 3-4 /μm spectral range. Quantum well samples were grown at Lancaster which had abrupt interfaces and showed room temperature photoluminescence (PL) emission between 3.6 and 4.0 /μm. Trans­mission electron microscope (TEM) imaging revealed a very high defect density of more than 10[10]10 cm-2 in the buffer layer and due to this, Shockley-Read-Hall (SRH) recombination was found to dominate the temperature quenching of the PL emission. Despite the structural problems with the material, the PL quenching performance compared well with other materials designed for this spectral range. Modelling of the quantum wells found that a conduction band offset ratio of 80% gave the best agreement with the experimentally determined transition energies. LEDs fabricated at Lancaster emitted in pulsed mode up to room temperature at 3.6 μm and with an efficiency of 34%. At room temperature SRH recombination was found to dominate the total recombination up to 350 mA drive current and this was also reflected in the temperature quenching of the LED output. From the fitting of the temperature dependence of the LED efficiency the SRH recombination centres were calculated to be 30-50 meV from the centre of the band gap. Fitting the room temperature LED emission spectra revealed that the emission comprised of transitions involving the first two heavy hole states as well as holes in the valence band of the barrier. The emission spectra from the edge of the LED mesa contained amplified spontaneous emission modes which were attributed to radial modes formed due to current crowding under the LED top contact. Lasers fabricated by QinetiQ were examined and from the gain spectra the internal loss was found to be -94 cm- 1 . This was attributed to the high defect density in the structure. The devices emitted at 3.2 /μm at 130 K and from po­larisation measurements it was found that the emission was completely polarised in the TE mode corresponding to emission from the heavy hole band. The lasers tested failed to reach operating temperatures above 130 K due to a sharp increase in the threshold current. An analysis of the temperature dependence of the thresh­old current provided evidence for hole current leakage as the cause of the increase in threshold current between 80 and 130 K.

This thesis covers research on low electric conductivity wide band gap semiconductors of the group-III nitride material system. The work presented focussed on using multi-mode scanning electron microscope (SEM) techniques to investigate the luminescence properties and their correlation with surface effects, doping concentration and structure of semiconductor structures. The measurement techniques combined cathodoluminescence (CL) for the characterization of luminescence properties, secondary electron (SE) imaging for imaging of the morphology and wavelength dispersive X-ray (WDX) spectroscopy for compositional analysis. The high spatial resolution of CL and SE-imaging allowed for the investigation of nanometer sized features, whilst environmental SEM allowed the characterisation of low conductivity samples. The investigated AlxGa₁₋xN samples showed a strong dependence on the miscut of the substrate, which was proven to influence the surface morphology and the compositional homogeneity. Studying the influence of the AlxGa₁₋xN sample thickness displayed a reduced strain in the samples with increasing thickness as well as an increasing crystalline quality. The analysis of AlxGa₁₋xN:Si samples showed the incorporation properties of Si in AlxGa₁₋xN, the correlation between defect luminescence, Si concentration and resistivity as well as the influence of threading dislocations on the luminescence properties and incorporation of point defects. The characterization of UV-LED structures demonstrated that a change in the band structure is one of the main reasons for a decreasing output power in AlₓGa₁₋ₓN based UV-LEDs. In addition the dependence of the luminescence properties and crystalline quality of InxAl₁₋xN based UV-LEDs on various growth parameters (e.g. growth temperature, quantum well thickness) was investigated. The study of nanorods revealed the influence of the template on the compositional homogeneity and luminescence of InxAl₁₋xN nanorod LEDs. Furthermore,the influence of optical modes in these structures was studied and found to provide an additional engineering parameter for the design of nanorod LEDs.

This thesis examines the interaction between superconductivity and inhomogeneous ferromagnetism. Through careful engineering of the interface, it is possible to unlock a new spin aligned triplet Cooper pair, which is capable of penetrating and modifying the magnetisation of a ferromagnet in proximity to a singlet, s-wave, BCS, superconductor. This triplet state is the building block for the new class of super-spintronic devices. Two candidate ferromagnetic systems in which to study the spin aligned triplet are considered. Firstly, the rare-earth ferromagnet erbium is fabricated using sputter deposition. Neutron diffraction measurements show the retention of the conical magnetic state in the thin film form for the first time. This conical state makes it an ideal candidate material for triplet Cooper pair generation. Placing erbium next to superconducting niobium has a drastic effect on the critical temperature of the superconductor, causing a suppression and oscillation of Tc with erbium thickness. In addition the remanent state of erbium at a single thickness can be used as a control to switch the niobium from the superconducting state into the normal state. The second system studied is the superconducting spin valve. In this system the inhomogeneity is engineered in a multi-layer structure using exchange biased Co. To study the nature and extent of the triplet Cooper pair in this structure, large scale facility techniques are employed to look for expected changes to the magnetic state of the heterostructure, with the onset of superconductivity. Surprisingly, no observation directly attributable to the triplet Cooper pair was observed. Instead a new type of induced ferromagnetism in a normal metal coupled to the superconducting spin valve was discovered.

No description available.

Transport and ESR measurements have been made on various alloys based on lithium. From the ESR measurements the phase boundaries for some alloys of the lithium rich compositions were determined. These results supplement the available data on phase boundaries. The spin-flip scattering cross sections for several impurities in lithium were measured. These measurements confirm the downwards trend for the cross sections with increase in valence. The need to work in liquid samples to obtain reliable values of cross sections was also emphasised. The transport measurements show for the first time that semiconducting alloys can be based on simple alkali metals. A discussion on the possible experimental evidence for a Mott pseudogap is given.

No description available.

The high electric field properties of GaAs have been investigated using a uniaxial stress apparatus, developed at the University of Surrey, and the hydrostatic pressure equipment at STL. An initial increase of the Gunn effect threshold field with pressure was observed, in contrast to previous measurements which are explained qualitatively in terms of pressure effects on non-ohmic contacts. Comparison of the results with Monte-Carlo calculations suggests that it may be incorrect to ignore the L1c minima as in previous analyses. Indeed, best agreement is obtained with the L1c valleys at 0.38 eV and the X1c valleys at 0.40 eV above the T1c minimum. Uniaxial stress measurements of low-field resistivity and the threshold field also provide evidence that the L1c and X1c valleys are at approximately the same energy. Deformation potentials of the conduction band have been determined. T1c valley, (Ed &deg;-a) = -7(+/-2) eV; X1c valleys, Eux = 21(+/-9)eV and (Edx-a) = -5(+/-5) eV; L1c valleys, EuL = 22 (+/-3) eV and (EdL-a) = -10(+/-3) eV. High field measurements have also been made on InP and InAs. The InP threshold field results agree with preliminary calculations using a 2-level T-L model. The InAs results extend earlier data into the region where the Gunn effect is the dominant mechanism over avalanche breakdown and the T-L sub-band gap is determined as 0.94(+/-.07)eV. The threshold field is found to increase approximately proportional to the effective mass in the range 33-42 kbar and these results are also compared with Monte-Carlo calculations. Impurity levels have been observed both in the forbidden gap and degenerate with the conduction band. Information on their positions is discussed and their effects on high field behaviour considered.