The electron transport within the wide energy gap compound semiconductors gallium nitride and zinc oxide

In this thesis, the electron transport that occurs within two wide energy gap semiconductors, gallium nitride and zinc oxide, is considered. Electron transport within gallium arsenide is also examined, albeit primarily for benchmarking purposes. The over-arching goal of this thesis is to provide the materials community with tools for analysis and optimization to be used when evaluating the consequences of transient electron transport within these compound semiconductors. Providing fresh insights into the character of the electron transport within zinc oxide, with particular focus on the device implications, is another aim of this analysis. Initially, Monte Carlo electron transport simulation results are used for a comparative analysis of the transient electron transport that occurs within bulk zinc-blende gallium arsenide and bulk wurtzite gallium nitride. It is found that for both materials the electron drift velocity and the average electron energy field-dependent settling times are strongly correlated and that the electric field resulting in the shortest electron transit-time is a function of channel length. Then, the applicability of the semi-analytical approach of Shur in evaluating the transient electron transport response within gallium arsenide, gallium nitride, and zinc oxide is critically examined. In particular, a comparison with Monte Carlo results is performed in order to establish the utility of this approach as a tool in studying the transient electron transport response. Next, a Monte Carlo analysis of the electron transport within bulk wurtzite zinc oxide is performed. The applied electric field strength that ensures the minimum electron time-to-transit across a given channel length is determined. These results are then used in order to provide an upper bound on the potential performance of zinc oxide based devices. Finally, the utility of the semi-analytical approach of Shur, for the purposes of device design optimization, is considered for the specific case of bulk wurtzite ZnO. It is found that the results produced through the semi-analytical approach of Shur are, in many cases, imperceptibly different from those of the Monte Carlo simulations. This adds to the allure of the semi-analytical approach as a versatile tool for transient electron transport analyzes and device design

Optical and transport properties of GaN and its lattice matched alloys

The study of carrier dynamics in wide band gap semiconductors is of great importance for UV detectors and emitters which are expected to be the building blocks for optoelectronic applications and high voltage electronics. On the experimental side, the progress made in the past two decades in generating subpicosecond laser pulses, resulted in numerous experiments that gave insight into the carrier dynamics in semiconductors. From the theoretical standpoint, the study of carrier interactions together with robust simulation methods, such as Monte-Carlo, provided great progress toward explaining the experimental results. These studies immensely improve our understanding of time scales of carrier recombination, relaxation and transport in semiconductor materials and devices which lead to optimizing the operation of optoelectronic devices, more specifically, emitters and detectors. Wide band gap materials having high breakdown field, wide band gap energy and high saturation velocity are among the most important semiconductors employed in the active layer of LEDs and lasers. GaN , its alloys, and ZnO are among the most important materials in semiconductor devices. Moreover, the use of lattice matched layers based on InAlN or InAlGaN is an alternative design approach which could mitigate the effect of polarization and enable growing thicker layers due to the higher structural quality. We first perform the study of carrier dynamics generated by ultrafast laser pulses in bulk GaN and ZnO materials to investigate the temperature dependent luminescence rise time. The obtained results are compared to the experimental results which show an excellent agreement. In this work, we use Monte Carlo method to evaluate the distribution of carriers considering the interaction of carriers with other carriers and also with polar optical phonons in the system. Considering the ongoing research about the advantages of lattice matched nitride based material systems, we also studied the properties of GaN layers lattice matched to InAlN and InAlGaN. As an application, we utilized the GaN/InAlGaN material system to study the carrier dynamics in Quantum Cascade Lasers. Furthermore, due to the superior properties of GaN which makes it an excellent candidate in power electronic applications, we also design and simulate an advanced vertical trench power MOSFET using drift diffusion and Monte Carlo models and characterize the performance of the device

Modelling silver thin film growth on zinc oxide

Ag thin film growth on ZnO substrates has been investigated theoretically using multi-timescale simulation methods. The models are based on an atomistic approach where the interactions between atoms are treated classically using a mixture of fixed and variable charge potential energy functions. After some preliminary tests it was found that existing fixed charge potential functions were unreliable for surface growth simulations. This resulted in the development of a ReaxFF variable charge potential fitted to Ag/ZnO surface interactions. Ab initio models of simple crystal structures and surface configurations were used for potential fitting and testing. The dynamic interaction of the Ag atoms with the ZnO surface was first investigated using single point depositions, via molecular dynamics, whereby the Ag impacted various points on an irreducible symmetry zone of the ZnO surface at a range of energies. This enabled the determination of the relative numbers of atoms that could penetrate, reflect or bond to the surface as a function of incident energy. The results showed that at an energy of up to 10 eV, most atoms deposited adsorbed on top of the surface layer. The second part of the dynamic interaction involved a multi-timescale technique whereby molecular dynamics (MD) was used in the initial stages followed by an adaptive kinetic Monte Carlo (AKMC) approach to model the diffusion over the surface between impacts. An impact energy of 3 eV was chosen for this investigation. Ag was grown on various ZnO surfaces including perfect polar, O-deficient and surfaces with step edges. Initial growth suggests that Ag prefers to be spread out across a perfect surface until large clusters are forced to form. After further first layer growth, subsequent Ag atoms begin to deposit on the existing Ag clusters and are unlikely to join the first layer. Ag island formation (as mentioned within the literature) can then occur via this growth mechanism. O-deficient regions of ZnO surfaces result in unfavourable Ag adsorption sites and cause cluster formation to occur away from O-vacancies. In contrast, ZnO step edges attract deposited Ag atoms and result in the migration of surface Ag atoms to under-coordinated O atoms in the step edge. Various improvements have been made to the existing methodology in which transitions are determined. A new method for determining defects within a system, by considering the coordination number of atoms, is shown to increase the number of transitions found during single ended search methods such as the relaxation and translation (RAT) algorithm. A super-basin approach based on the mean rate method is also introduced as a method of accelerating a simulation when small energy barriers dominate. This method effectively combines states connected by small energy barriers into a single large basin and calculates the mean time to escape such basin. To accelerate growth simulations further and allow larger systems to be considered, a lattice based adaptive kinetic Monte Carlo (LatAKMC) method is developed. As off-lattice AKMC and MD results suggest Ag resides in highly symmetric adsorption sites and that low energy deposition events lead to no penetrating Ag atoms or surface deformation, the on-lattice based approach is used to grow Ag on larger perfect polar ZnO surfaces. Results from the LatAKMC approach agree with off-lattice AKMC findings and predict Ag island formation. Critical island sizes of Ag on ZnO are also approximated using a mean rate approach. Single Ag atoms are placed above an existing Ag cluster and all transition states are treated as belonging to a single large super-basin . Results indicate that small Ag clusters on the perfect ZnO surface grow in the surface plane until a critical island size of around 500 atoms is reached. Once a critical island size is reached, multiple Ag ad-atoms will deposit on the island before existing Ag atoms join the cluster layer and hence islands will grow upwards. A marked difference is seen for second layer critical island sizes; second layer Ag islands are predicted to be two orders of magnitude smaller (< 7 atoms). This analysis suggests that Ag on ZnO (000 ̄1) may exhibit Stranski-Krastanov (layer plus island) growth

Simulations of High Mobility AlGaN/GaN Field Effect Transistors. Mobility and Quantum Effects

Introduction to GaN and GaN-based HEMT. Mobility in HEMT and implementation of GaN mobility model in Sentaurus simulator. Quantum effects in HEMT and simultations of different back barriers (InGaN and AlGaN

Modelling zinc oxide thin-film growth

Photovoltaics have a significant role in the solution of energy supply and energy security. Research on photovoltaic devices and their production processes has been carried out for decades. The transparent conducting oxide layer, in the photovoltaic solar cell, composed of aluminium doped zinc oxide, is produced through deposition techniques. By modelling these depositions using classical molecular dynamics, a better understanding on the short term kinetics occurring on the growing surface has been achieved. Compared to the molecular dynamics, the employment of the adaptive kinetic Monte Carlo method enabled such surface growth dynamics simulation to reach much longer time scale. Parallelised transition searching was carried out in an on-the-fly manner without lattice approximation or predefined events table. The simulation techniques allowed deposition conditions to be easily changed, such as deposition energy, deposition rate, substrate temperature, plasma pressure, etc. Therefore, in this project three main deposition techniques were modelled including evaporation (thermal and assisted electron beam), reactive magnetron sputtering and pulsed laser depositions. ZnO as a covalent compound with many uses in semiconductors was investigated in its most energy favourable wurtzite configuration. The O-terminated surface was used as the substrate for the growth simulation. Evaporation deposition at room temperature (300 K) with a stoichiometric distribution of deposition species produced incomplete new layers. Holes were observed existing for long times in each layer. Also, stacking faults were formed during the low-energy (1 eV) growth through evaporation. The reactive sputtering depositions were more capable of getting rid of these holes structures and diminished these stacking faults through high energy bombardments but could also break these desirable crystalline structure during the growth. However, single deposition results with high energies showed that the ZnO lattice presented good capacity of self-healing after energetic impacts. Additionally, such self-healing effects were seen for substrate surface during thin film growth by the sputtering depositions. These facts shed some light on that the sputtering technique is the method of choice for ZnO thin film depositions during industrial production. Simulation results of pulsed laser deposition with separated Zn and O species showed the thin films were grown in porous structures as the O-terminated surface could be severely damaged by Zn atoms during the very short pulse window (10 microseconds). An important growth mechanism with ZnO dimer deposited on the O-terminated polar surface was the coupling of these single ZnO dimers, forming highly mobile strings along the surface and thus quenching its dipole moments, whilst the isolated single ZnO dimers were hardly of this mobility. Such strings were the building blocks for the fabrication occurring on the surface resulting in new layers. Last but not least, a reactive force field for modelling Al doped ZnO was fitted. DFT calculations showed that the Al atoms on the surface were likely to replace Zn atoms in their lattice sites for more energy favourable structures. Al on the ZnO surfaces, structures with Al in the bulk as well as configurations with Al interstitials were used to train the force field to reproduce favourable surface binding sites, cohesive energies and lattice dimensions. The combination scheme of MD and the AKMC allowed simulation work to reach over experimentally realistic time scale. Therefore, crucial mechanisms occurring during the growth could be precisely understood and investigated on an atomistic level. It has been shown from the simulation results that certain types of deposition play significant roles in the quality of resultant thin films and surface morphology, thus providing insight to the optimal deposition conditions for growing complete crystalline ZnO layers

A comprehensive review of ZnO materials and devices

The semiconductor ZnO has gained substantial interest in the research community in part because of its large exciton binding energy (60 meV) which could lead to lasing action based on exciton recombination even above room temperature. Even though research focusing on ZnO goes back many decades, the renewed interest is fueled by availability of high-quality substrates and reports of p-type conduction and ferromagnetic behavior when doped with transitions metals, both of which remain controversial. It is this renewed interest in ZnO which forms the basis of this review. As mentioned already, ZnO is not new to the semiconductor field, with studies of its lattice parameter dating back to 1935 by Bunn [Proc. Phys. Soc. London 47, 836 (1935)], studies of its vibrational properties with Raman scattering in 1966 by Damen et al. [Phys. Rev.142, 570 (1966)], detailed optical studies in 1954 by Mollwo [Z. Angew. Phys.6, 257 (1954)], and its growth by chemical-vapor transport in 1970 by Galli and Coker [Appl. Phys. Lett.16, 439 (1970)]. In terms of devices, Au Schottky barriers in 1965 by Mead [Phys. Lett.18, 218 (1965)], demonstration of light-emitting diodes (1967) by Drapak [Semiconductors 2, 624 (1968)], in which Cu2O was used as the p-type material, metal-insulator-semiconductor structures (1974) by Minami et al. [Jpn. J. Appl. Phys.13, 1475 (1974)], ZnO∕ZnSe n-p junctions (1975) by Tsurkan et al. [Semiconductors 6, 1183 (1975)], and Al∕Au Ohmic contacts by Brillson [J. Vac. Sci. Technol.15, 1378 (1978)] were attained. The main obstacle to the development of ZnO has been the lack of reproducible and low-resistivity p-type ZnO, as recently discussed by Look and Claflin [Phys. Status Solidi B241, 624 (2004)]. While ZnO already has many industrial applications owing to its piezoelectric properties and band gap in the near ultraviolet, its applications to optoelectronic devices has not yet materialized due chiefly to the lack of p-type epitaxial layers. Very high quality what used to be called whiskers and platelets, the nomenclature for which gave way to nanostructures of late, have been prepared early on and used to deduce much of the principal properties of this material, particularly in terms of optical processes. The suggestion of attainment of p-type conductivity in the last few years has rekindled the long-time, albeit dormant, fervor of exploiting this material for optoelectronic applications. The attraction can simply be attributed to the large exciton binding energy of 60 meV of ZnO potentially paving the way for efficient room-temperature exciton-based emitters, and sharp transitions facilitating very low threshold semiconductor lasers. The field is also fueled by theoretical predictions and perhaps experimental confirmation of ferromagnetism at room temperature for potential spintronics applications. This review gives an in-depth discussion of the mechanical, chemical, electrical, and optical properties of ZnO in addition to the technological issues such as growth, defects, p-type doping, band-gap engineering, devices, and nanostructures

Spectroscopy of single photon emitting defects in Gallium Nitride and Diamond

University of Technology Sydney. Faculty of Science.A single photon is among the few quantum mechanical systems that are finding applications in myriad fields. The applications include serving as building blocks for the ongoing endeavour to realise faster computers and secure communication technologies. As a result, a variety of platforms are being inspected to generate single photons on-demand. Point defects and complexes in wide bandgap semiconductors such as nitrogen-vacancy (NV) and silicon-vacancy (SiV) centres in diamond, carbon antisite in Silicon Carbide (SiC), etcetera, are shown to be reliable room temperature (RT), single photon emitters (SPEs). Despite reports of several defect based SPEs in diamond and other semiconductors, the exploration continues to find ideal sources for applications. The central part of this work also focuses on the discovery and characterisation of novel SPE in the device fabrication friendly material- Gallium Nitride (GaN). The other important aspect in the study of SPEs is the method by which emitters are excited. While optical technique via laser excitation is the standard approach, electrically excited single photon generation is highly desirable for large-scale nanophotonic applications. The second part of the work investigates electrically driven fluorescence from SiV ensemble in diamond, whose properties so far, were only investigated using optical excitations. Therefore, the thesis consists of two main parts. First, the discovery as well as study of a new family of SPEs in GaN via optical excitation is covered. The second part features electrically driven characterisation of SiV centre in diamond. The RT stable, SPEs are discovered in GaN films using a confocal microscope. The emitters are off-resonantly excited using a continuous wave (cw) laser of wavelength 532 nm. The centre of wavelength in the emission spectra spans a wide range of from around 600 nm to 780 nm. Also, a significant portion of the emission comes from the characteristic, narrow zero-phonon lines (ZPLs) with the mean cryogenic and RT Full Width at Half Maximum (FWHM) of around 0.3 nm and 5 nm, respectively. The nature of the defect responsible for the emission is studied experimentally via temperature resolved spectroscopy as well as numerical modelling giving a strong indication that the emitter is a defect localised near cubic inclusions. Absorption and emission polarisation properties from the SPEs in GaN is studied in detail via polarization-resolved spectroscopy. High degree of linear, emission polarisation is observed with an average visibility of more than 90 %. The absorption polarisation measurement shows that individual emitters may have different dipole orientation. In addition, brightness measurements from several of the SPEs in GaN show the average maximum intensity of around 427 kCounts/s placing the emitters among the brightest reported so far. A three-level model describes the transition kinetics of the SPEs successfully which explains some of the observed properties of the emitters such as photon statistics. A small number of the SPEs in GaN show unusual photo-induced blinking. This blinking is shown to be due to a permanent change in the transition kinetics of the emitters when exposed to a laser power above a certain threshold. This is evidenced by the change in the transition kinetics observed before and after blinking of SPEs. Combining long-time autocorrelation measurement and photon statistics analysis, numerical values for power-dependent blinking behaviours are determined. The second major result in this work is the first electrically driven luminescence from the negative charge state of Silicon-Vacancy (SiV⁻). The result was directly obtained by measuring photoluminescence (PL) and electroluminescence (EL) spectra from SiV⁻ ensemble located in PIN diamond diode. The defect was incorporated into the diode via ion implantation. Further characterisation shows that the saturation behaviour under excess carrier injection yields similar results with when the defect is pumped optically by lasers. Finally, charge state switching between the negative and neutral states of the defect was also attempted by using reverse-biased PL elucidating transition dynamics of SiV centres in diamond. This work, therefore, reports new findings in the spectroscopic studies of defect based single photon emission. Furthermore, it provides detailed photophysical studies which may serve as a benchmark for future investigation of SPEs in GaN for multiple applications. The results provide new platform as well as alternative excitation approach for the application of defect based SPEs in nanophotonics

Improving the efficiency of computation of free energy differences

There has been a recent focus on investigating the properties of semi-conductors at the nanoscale as it is well known that the band-gap of semi-conducting materials is altered due to quantum confinement effects. The potential to fine-tune a material's properties based solely on particle size has raised significant interest both in experimental and computational studies. Zinc sulfide is one of the most studied metal sulfide semi-conductor minerals, due to its potential technological applications.Computational studies of the structural and thermodynamic properties of zinc sulfide nanoparticles and bulk structures have been performed throughout this work. A variety of computational methods have been employed, including molecular dynamics, lattice dynamics, first principles calculations, and free energy techniques, such as metadynamics and free energy perturbation. The thermodynamic stability of zinc sulfide nanoparticles as a function of size and shape has been studied. Investigation of the phase space of these systems required the use of enhanced sampling methods. The metadynamics method was specifically utilised to explore as many structures as possible in combination with extensive simulations. The use of first principles methods for these exploratory simulations was found to be prohibitively expensive, and so force field methods were primarily utilised. Throughout this investigation several force fields were used to compare and contrast their accuracy, while first principles calculations were performed, where possible, to assist in the interpretation and validation of the results.In the present study, two different collective variables, the trace of the inertia tensor and the Steinhardt bond order parameters, have been implemented and their performance in metadynamics compared. The trace of the inertia tensor was found to be useful for exploring clusters of small sizes, while the Q4 Steinhardt parameter, which describes the crystalline order of a solid, is more applicable to larger clusters. Both of these metadynamics studies resulted in clusters displaying zeolite structural motifs, including the zeolite framework `BCT'. This led us to investigate more thoroughly the stability of different zinc sulfide zeolite analogues, thereby highlighting the strengths and weaknesses of all the force fields employed. Many force fields are found to be unable to accurately represent the order of stability for bulk polymorphs.First principles calculations also highlighted that the BCT phase is less stable than either of the bulk polymorphs of zinc sulfide, in contrast to the order of stability obtained by force fields lacking a torsional term, both from literature and the rigid ion model developed during the current study. The larger nanoparticles cleaved from wurtzite exhibited internal strain upon relaxation. A new hypothetical zeolite framework was constructed from the distorted core of these clusters, and was found to possess structural similarities with the `APC' framework. The APC framework is composed of double crankshaft-chains with ”ABCABC…” stacking, while the hypothetical framework identified is formed by the same composite building unit with `ABAB: : : ' type stacking. For all the force fields used the new hypothetical framework was lower in energy than the APC framework, but higher in energy than sphalerite, wurtzite or the BCT phase.Free energy differences between small ZnS clusters in vacuum were calculated using the path variable technique, and also using static methods within the quasi-harmonic approximation. Similar values were obtained using both of these methods, validating the path collective variables used with metadynamics as an effective means of obtaining free energy differences for clusters in vacuum.In addition to clusters in vacuum, a number of studies of ZnS clusters in water were also performed. Both force field and first principles studies were employed to validate the ZnS-water interactions used for the binding energies of water to small clusters. As a further validation, the free energies of solvation of Zn2+ and S2?? in aqueous solution were calculated. The free energy of solvation for the sulfide anion was found to be close to the experimental value, while the parameters for Zn2+-water were found to require substantial modification as the solvation free energy was in error by 500 kJ/mol. While newly derived ZnS-water parameters may prove to be superior for describing ZnS clusters in bulk water, a repetition of the binding energy calculations for individual water molecules bound to ZnS clusters gave energies 2-3 times greater than those obtained via first principles methods and using the five other force fields investigated. These results highlight the issues present when attempting to transfer a model fitted in a certain way to a different application. In particular, the many-body and polarisation effects present when modelling water need to be considered when parameterising ZnS-water interactions

Modelling thin film growth over realistic time scales

Energy security and supply is a key problem for the UK in the coming years. Photovoltaics have an important role to play in this. In order for demand to be met, continued research into new materials and methods of production is necessary. By modelling deposition techniques using classical molecular dynamics (MD), an atomistic scale understanding can be obtained. Combining this with long time scale dynamics (LTSD) techniques allows us to also model diffusion and surface growth over realistic time scales. The LTSD technique applied throughout this project is an on-the-fly Kinetic Monte Carlo (otf-KMC) method, which determines diffusion pathways and barriers, in parallel, with no prior knowledge of the involved transitions. These simulation techniques allow parameters such as deposition energy, substrate bias and plasma pressure to be easily changed to gain understanding of their effects. During this project, growth via industrial scale deposition techniques has been simulated, including evaporation (thermal and electron beam), ion-beam assisted evaporation and reactive magnetron sputtering. Metal thin films, of interest due to their uses in reflectors in concentrator photovoltaics, electrical conductors in the monolithic interconnect processes and back contacts, were investigated using otf-KMC. Ag and Al film growth was simulated for around 0.3 seconds of real time. It was found that Ag has the ability to grow smooth surfaces, using several mechanisms including multiple-atom concerted motion, exchange mechanisms, and damage and repair mechanisms. Ag (111) and (100) surfaces grew dense, complete and crystalline films when sputtering was simulated, however, evaporation deposition produced incomplete layers. The inclusion of Ar in the ion-beam assisted evaporation of Ag (111) aided growth by transferring extra energy to the surface allowing increased diffusion and atomic mixing. Al (111) and (100), however, show different patterns. Growth by evaporation deposition and magnetron sputtering actually produced very similar results. The inclusion of the ion-beam assist on the (111) surface actually damaged the film, producing subsurface Ar clusters where Al atoms were displaced, creating voids throughout the film. Otf-KMC methods enabled the investigation of specific mechanisms allowing film growth and a very important transition enabling the smooth and complete Al film growth was found to be the Ehrlich-Schwoebel (ES) barrier. The ES barrier involves an atom dropping off a step edge of an island and this barrier was found to be much smaller for the Al surfaces, therefore allowing the more complete growth from both evaporation and sputtering. Metal oxides are also of great interest in the photovoltaic industry. The rutile TiO$_2$ (110) surface was investigated using single point depositions, high temperature MD and otf-KMC. Otf-KMC enabled the simulation for up to 9 seconds of real time, totally inaccessible using traditional simulation methods. Results concluded that the evaporation deposition process produced a void filled, incomplete structure, even with the use of a low energy ion-beam assist, this material is of interest for dye-sensitised solar cells where a dye is injected into the voids. Sputtering, however, produced dense and crystalline film, which is much more applicable to anti-reflective coatings where a crystalline structure is required. Mechanisms which enabled crystalline rutile to form were also investigated, highlighting Ti interstitial annealing in the presence of an O rich surface as an important rutile growth mechanism. ZnO, an inorganic compound with many uses including transparent conductive oxides, is investigated in the most stable wurtzite phase. The O-terminated (000$\bar{1}$) polar surface was used as the substrate for otf-KMC growth simulations, where around 1 second of real time was simulated. Evaporation deposition of a stoichiometric distribution of deposition species was found to produce the best quality film, however, a phase boundary was observed where an area of zinc blende forms within the wurtzite. Sputtering resulted in a denser, more complete and crystalline structure due to the higher deposition energy of arriving species, similar to the TiO$_2$ results. Post-annealing at 770K did not allow complete recrystallisation, resulting in films with stacking faults where monolayers formed in the zinc blende phase. Annealing at 920K, however, in some cases enabled the complete recrystallisation of films back into the wurtzite structure. Although, the higher annealing temperature did not always enable recrystallisation and in some cases both wurtzite and zinc blende phases existed in the same layer, resulting in a phase boundary. An important mechanism for the nucleation of ZnO growth was found to be the formation and vibration of Zn$_x$O$_y$ strings on the surface, which after hundreds of milliseconds formed the desired hexagonal structure. Combining MD and otf-KMC enabled the simulation of systems over very large time scales which were previously totally inaccessible. Key mechanisms occurring during the growth of metals and metal oxides were investigated, providing a much more precise understanding of how growth occurs. It is clear from the work that the deposition technique used plays a significant role on the resulting film quality and surface morphology and we are now able to provide an insight into the optimum conditions under which complete, crystalline layers can form

On the use and development of advanced computational techniques to determine the properties and behaviour of metastable materials

This thesis contains discussions and results pertaining to three distinct pieces of work, all related by an underlying theme: the use and development of computational techniques to discover and characterise novel metastable materials. Zinc oxide is a cheap and abundant material with many potential uses in the electronics and Optics industries. However, its wurtzite ground state structure gives rise to a number of undesirable properties. Thus, knowledge of how to stabilise more useful metastable phases is desirable. To that end, the mechanism of the pressure-induced phase transition between the zincblende and rocksalt polymorphs of the compound was deduced using transition path sampling. Following this, a novel technique combining TPS methods with metadynamics was applied to classify the free-energy landscape relevant to the transition pathway. This provided further information relating to the transition that would have been impossible to determine using path based analyses alone. Water ice exhibits a wealth of structural polymorphism, with at least eighteen phases known to experiment and many more configurations predicted. However, a true understanding of the transition pathways that link these structures remains elusive. Using both metadynamics techniques and a novel procedure known as rotational shooting, attempts to deduce pathways between different phases of ice have been made. The results presented include successful transformations between two crystalline phases of ice and several amorphous phases, as well as the possible elucidation of a novel ice polymorph. Crystal structure prediction remains a challenge in materials science. Using a random structure search technique, eight novel allotropes of carbon and three novel high-pressure polymorphs of zinc oxide have been found and subsequently characterised using density functional theory. Each of the materials displays its own unique array of properties, demonstrating both the variety exhibited by polymorphs of the same material and the ability of random structure prediction techniques to predict such dissimilar materials