Z-contrast imaging, using a high-angle annular dark field detector, can be used to characterise III-V heterostructures. GaAs/AlAs heterostructures were grown using MBE and prepared for TEM using a cross-sectional method. SuperSTEM 1 was used to investigate both the GaAs-on-AlAs and the AlAs-on-GaAs interfaces as a function of specimen thickness. The analysis of the images showed that the apparent interface widths varied with thickness in an unexpected manner. The measured GaAs-on-AlAs interface widths remained constant with thickness while the AlAs-on-GaAs interface widths increased. Furthermore, the apparent width of the GaAs layer increased with increasing thickness. The actual interfacial width can be a result of either surface stepping during MBE growth or inter-diffusion of the Type-3 atoms. To assist the interpretation of these results, a series of interfacial models were created and explored using a modified version of the frozen phonon multislice simulation. The models consisted of terraced, vicinal and diffused interfaces. The model results indicate that a diffuse interface can be used to describe the characteristics observed in the experimental images. However, probe scattering from the interfacial region can be counter intuitive. A systematic study of these effects is presented outlining complications that can occur when interpreting interfacial structures using HAADF imaging.

The ever increasing demand for higher power devices at higher frequencies has prompted much research recently into the aluminium nitride/gallium nitride high electron mobility transistors (AlN/GaN HEMTs) in response to theoretical predictions of higher performance devices. Despite having superior material properties such as higher two-dimensional electron gas (2DEG) densities and larger breakdown field as compared to the conventional aluminium gallium nitride (AlGaN)/GaN HEMTs, the AlN/GaN devices suffer from surface sensitivity, high leakage currents and high Ohmic contact resistances. Having very thin AlN barrier layer of ∼ 3 nm makes the epilayers very sensitive to liquids coming in contact with the surface. Exposure to any chemical solutions during device processing degrades the surface properties, resulting in poor device performance. To overcome the problems, a protective layer is employed during fabrication of AlN/GaN-based devices. However, in the presence of the protective/passivation layers, formation of low Ohmic resistance source and drain contact becomes even more difficult. In this work, thermally grown aluminium oxide (Al2O3) was used as a gate di- electric and surface passivation for AlN/GaN metal-oxide-semiconductor (MOS)-HEMTs. Most importantly, the Al2O3 acts as a protection layer during device processing. The developed technique allows for a simple and effective wet etching optimisation using 16H3PO4:HNO3:2H2O solution to remove Al from the Ohmic contact regions prior to the formation of Al2O3 and Ohmic metallisation. Low Ohmic contact resistance (0.76Ω.mm) as well as low sheet resistance (318Ω/square) were obtained after optimisation. Significant reduction in the gate leakage currents was observed when employing an additional layer of thermally grown Al2O3 on the mesa sidewalls, particularly in the region where the gate metallisation overlaps with the exposed channel edge. A high peak current ∼1.5 A/mm at VGS=+3 V and a current-gain cutoff frequency, fT , and maximum oscillation frequency, fMAX , of 50 GHz and 40 GHz, respectively, were obtained for a device with 0.2 μm gate length and 100 μm gate width. The measured breakdown voltage, VBR, of a two-finger MOS-HEMT with 0.5μm gate length and 100 μm gate width was 58 V. Additionally, an approach based on an accurate estimate of all the small-signal equivalent circuit elements followed by optimisation of these to get the actual element values was also developed for AlN/GaN MOS-HEMTs. The extracted element values provide feedback for further device process optimisation. The achieved results indicate the suitability of thermally grown Al2O3 for AlN/GaN-based MOS-HEMT technology for future high frequency power applications.

This thesis reports on Semiconductor Optical Amplifiers (SOAs) and their use in optical communication systems; in particular improving the reach of Passive Optical Networks (PON). Following a comprehensive overview of the components of optical communication systems a PON is introduced and the standard of Gigabit-PON (GPON) explained. The concept of extending the reach of GPON through the introduction of amplification is presented and the business drivers of the telecommunication operators detailed. The physics of SOAs are described followed by the parameters used to characterise them. Carrier dynamics of SOAs are explained and the methods of measurement of the carrier dynamics are detailed including the spectrogram technique. This method simultaneously measures the gain and phase recovery which is desirable for applications in long range telecommunications which require unchirped signals with a fast response for both gain and phase. Parameters of commercially available SOAs are compared with the requirements to extend the reach of PONs. Following this the fabrication tolerances for SOAs insensitive to polarisation dependent gain (PDG) are modelled. Results from SOA modelling showed that the greatest contributing factor to PDG variation was the active region thickness error. In the context of bulk production this requires a realistic tolerance of ~10nm to maintain PDG of ~1dB. A polarisation insensitive high gain SOA is designed and experimentally measured. This SOA is measured in the context of GPON and shown to extend the reach of the current standard by a record margin of 28dB. The limitation of the improvement is attributed to gain modulation sourced intersymbol interference (the patterning effect). The patterning effect has been reported in literature to be reduced through the introduction of SOAs with an active region made from quantum dot (Qdot) material. A comparative study of the gain and phase recovery time and alpha factor of various dimensional SOAs is presented. Using the spectrogram method it is shown that reducing the power and increasing the bias of the SOA can reduce the carrier recovery time. A Qdot active region SOAs is shown to considerably reduce the gain recovery time compared to a bulk SOA of similar length. The active region of the Qdot SOA alludes to a faster carrier recovery time which could be beneficial to extend the reach of PONs without patterning. However as these are more difficult to fabricate in mass production it is unknown if they are a viable solution on a commercial scale. In the context of GPON a low alpha factor is desired for minimizing chirp and phase nonlinearities during amplification of short pulses. An alpha factor study is presented and the Qdot SOA was measured to have the lowest alpha factor which could be beneficial for reducing chirp in 10G-PON.

No description available.

No description available.

Dilute nitride alloys of III–V semiconductors, and transparent conducting group-II oxides may both be categorised as highly mismatched compounds. The small size and high values of electronegativity of nitrogen and oxygen (see figure), compared to the substituted anion, in dilute nitrides, and the cation, in transparent conducting oxides, give rise to striking properties in these materials. The dilute nitride alloys GaNSb, InNSb, and GaInNSb, grown by molecular beam epitaxy, have been studied. Infrared absorption measurements of GaNSb are presented, showing the divergence of transitions from the valence band to E− and E+ conduction bands with increasing nitrogen incorporation. The fitting of the positions of the valence band to E+ transitions gives a value of 2.6 eV for the coupling parameter in this material. A reduction in the bandgap of InNSb from that of InSb is shown by modelling the competing effects of Moss-Burstein band filling and bandgap renormalisation. Finally, bandstructure calculations of the quaternary material GaInNSb, with dilute incorporations of nitrogen and indium, show that the material is suitable for the exploitation of the 8–14 μm atmospheric transmission window. Structural characterisation of GaInNSb shows that this material can be grown lattice matched to GaSb with nitrogen and indium incorporations of 1.8 and 8.4 per cent, respectively. The conducting oxide CdO, grown by metal-organic vapour-phase epitaxy, has also been studied. Analysis and simulation of infrared reflectance data, including conduction band non-parabolicity and Moss-Burstein band filling, reveal bandgap and band-edge effective mass values of 2.16 eV and 0.21 m0, respectively. In addition, high energy 4He+ ion irradiation was used to stabilise the Fermi level in CdO. Carrier statistics calculations were performed and the charge neutrality level was found to be 2.52 eV with respect to the "-point valence band maximum, corresponding to 0.36 eV above the conduction band minimum. The location of the charge neutrality level within the conduction band explains the propensity for high unintentional n-type doping, and the high conductivity observed in CdO.

The effect on the electron transport of the confinement of the electrons to a narrow channel in GaAs/GaAlAs heterojunctions and Si MOSFETs is reflected in quantities like the thermopower (S) which is sensitive to the transport of both heat and charge. The calculations described here confirm that in these systems S is dominated by phonondrag (Sg) at temperatures (T) around 1-10K and reveals more sensitivity than previously imagined. Simple models and the Boltzmann transport formalism have been investigated. The formalism enhances the predictions of the simple models and reproduces the simple S. formulae in appropriate limits. Amplification of S9 in quasi-2D arises from the loss of the momentum conservation constraint across the channel at small widths b. Earlier calculations were numerically inaccurate and greatly overestimate -S9 by ignoring screening. An effective multi-subband screening dielectric function is defined which reduces to the single subband approximation at small b and low electron density (n). Nondegeneracy has also been included. It is an important consideration despite the low temperatures of most of the data. The treatment of electron confinement has been improved and the temperature dependence of the polarizability investigated. It is unimportant in the current experimental systems but significant at lower n and higher T. The piezoelectric scattering mechanism has been introduced and dominates S. in the heterojunction for T <1K. A dominant 2D wavevector component has been defined for the phonon population at given T which is very helpful in understanding S9. A correction for the energy dependence of the electron relaxation-time is necessary and demonstrates the dependence of S. upon the dominant electron scattering mechanism. The calculations of S. in the quantum-limit and boundary scattering regime now explain the measured S in heterojunctions and peaks in -Sg/T3 in the MOSFET up to an accuracy better than 10% without adjustable parameters.

A few percent of nitrogen (N) or bimuth (Bi) incorporation in GaAs compound semiconductors have proved to lower significantly its bandgap. This unusual bandgap reduction is of interest for numerous applications such as long wave-length lasers, solar cells etc. However, the addition of these impurity atoms also introduces defect levels in the bandgap of the host materials. These can have severe implications on the material's quality, for example they can decrease the lifetime of the charge carriers and degrade the optical efficiency. In this work, deep levels traps were investigated in silicon-doped GaAsN epitaxial layers containing N concentrations from 0.2% to 1.2% grown by molecular beam epitaxy (MBE) on n+ GaAs substrates using DLTS and high resolution Laplace DLTS techniques. In addition, a further investigation was carried out to study the effect of annealing and hydrogenation treatments on the defects present in the as-grown layers. Several deep levels were detected in the as-grown GaAsN samples. These were identified with previously reported (SiGa-NAs), EL6 (Ga vacancies-related complex), (N-As)As, EL3 (off-centre substitutional oxygen in As sites) and EL2-like (anti site AsGa) defect levels. It was found that, depending upon the N concentration, heat treatment has a different effect on the traps. For samples with N = 0.2 - 0.4 %, some defects were annihilated and no generation of new defects was observed. In the case of samples with N = 0.8- 1.2 % the annealing results in both generation of new traps and elimination of some existing traps. In general, it was found that hydrogenation of the as-grown GaAsN epilayers passivates most of the deep levels. However, for the samples with N = 0.8%, although hydrogen passivates some of the defects and reduce the concentration of others it also creates new defects which are suspected to be hydrogen-related complexes. (100) and (311)B GaAsBi layers grown by molecular beam epitaxy under various arsenic overpressures have been investigated using optical and structural techniques. The optimised Bi incorporation was found to occur near stoichiometric conditions. The incorporation of Bi into the GaAsBi alloy, as determined by high resolution X-ray diffraction (HRXRD), is sizably larger in the (311)B epilayers than in (100) epilayers. HRXRD reveals 4% Bi-incorporation in (311)B and 3% in (100) GaAs orientations. The conventional optical transmission results confirmed that the bandgap of the (311)B epilayer is around 90 meV lower than that corresponding to (100) sample. This measurement provide further evidence that Bi incorporates more in (311)B than in (100) surfaces. The low temperature post-growth heat treatment of GaAsBi alloys reveals an improvement in the structural and optical properties of these materials. A substantial increase in photoluminescence signal infers a large reduction of defects.

The prospect of a new generation of electronic devices based on the fundamental quantum property of angular momentum, known as spin, has led to the rapidly developing field of spintronics. It is envisioned that these advanced devices will have significant advantages over traditional charge based electronics in properties such as speed, power consumption and long coherence times. By combining the properties of magnetics with that of semiconductors, the novel class of materials known as dilute magnetic semiconductors (DMSs) are considered a promising system for exhibiting spintronic functionality. These materials are created by using molecular beam epitaxy (MBE) to incorporate into traditional semiconductors a quantity of transition metal atoms sufficient that ferromagnetism is exhibited. The most widely studied DMS is (Ga,Mn)As which has well characterised behaviour and can be processed using standard III-V fabrication techniques, thus providing an excellent basis for further study. In this research the properties of (Ga,Mn)As based systems are studied as the material dimensions are reduced to nanometre length scales. Three complementary approaches are used for this purpose. The first is to use ultra-high-resolution electron-beam lithography to construct devices. By being able to selectively remove material, laterally patterned structures can have sizes as small as 10 nm. The second approach is to exploit the atomic layer growth of MBE to allow the construction of epilayers and heterostructures with well defined vertical compositions. Thirdly, a theoretical k.p kinetic-exchange model allows the simulation of multilayer structures and an exploration of the parameter spaces available in such materials. Two systems are considered: lateral nanoconstricted magnetic tunnel junctions and vertically defined magnetic superlattices. The nanoconstrictions are analysed using low temperature magnetotransport techniques and novel anisotropic magnetoresistance (MR) effects are measured. Primarily, tunnelling anisotropic magnetoresistance (TAMR) is observed, demonstrating that it is a generic property of ferromagnetic tunnel devices and is therefore of wide interest for other spintronic systems. Secondarily, anisotropic switching behaviour is observed and is interpreted as Coulomb blockade anisotropic magnetoresistance (CBAMR). Additionally, the significance of the processing stages and material properties are highlighted. The magnetic superlattices are firstly considered on a theoretical basis in order to determine structural parameters in which a new MR effect might be observed. This effect derives from the interlayer exchange coupling (IEC) between the magnetic layers which can either be in parallel or opposed orientations. Based on the calculations, samples are measured using low temperature magnetotransport and magnetometry techniques in order to explore the possibility of some of the dramatic properties predicted in magnetic superlattice structures.

This thesis describes the growth and characterisation of four different III-V semiconductor materials. Growth was primarily performed by molecular beam epitaxy, while characterisation, which was largely structural, was carried out mainly using X-ray difraction and atomic force microscopy. Growth of low temperature(LT)GaAs was undertaken to investigate whether a phase transition accompanies the structural transition which occurs when GaAs is grown at temperatures below ~ 150C. It was found however, that LT GaAs remains zinc-blende, albeit with a signicant degree of disorder. Migration enhanced epitaxy was subsequently used to grow LT GaAs, resulting in single crystalline GaAs at growth temperatures down to 115C. The possibility of using AlN as a source for nitrogen, in the growth of GaAs based dilute nitrides was explored. No conclusive evidence has been presented to suggest that small amounts of nitrogen were incorporated into the GaAs lattice. The potential for ScN to be used as a buffer layer/interlayer to reduce the defect density in cubic GaN (c-GaN) was investigated. It was found that ScN grows on c-GaN(0 0 1)/GaAs(0 0 1) in a (1 1 1) orientation, leading to overgrowth of GaN occurring in the hexagonal phase. If the ScN interlayer was sufficiently thin (<3nm), then overgrowth of GaN was cubic, but no evidence of a reduction in stacking fault density was observed. Growth of ScN on GaAs(0 0 1) was also found to result mainly in a (1 1 1) orientation, but films were of poor quality. Growth of ScN on ScAs(0 0 1) was subsequently explored. ScN was found to grow in a (0 0 1) orientation, with both smoother surfaces and improved material quality than ScN(1 1 1). Growth of GaN atop ScN(0 0 1) was found to be c-GaN(0 0 1), but insuffcient studies have been carried out to determine the effect on material quality. During the growth of InGaN, it was found that unmounted substrates lead to large temperatures rises (>100C) for In rich compositions. Modelling heat absorption due to bandgap, phonon and plasmon absorption showed that this is due primarily to the large number of free carriers and not to the narrow bandgap (wrt substrate). The preliminary doping of In0.8Ga0.2N with Mn was investigated. The amount of Mn that can be incorporated without causing a signicant reduction in film quality was found to increase with decreasing growth temperature.