Below are examples of current PhD and MSc projects.
2D electrons in epitaxial strained Germanium quantum wells
Carrier mobility determines how fast a carrier, i.e. electron or hole, can move in a solid material under applied electric field. It is one of the most important material’s parameter, determining its suitability for an application in electronic devices. High mobility and the existence of a reasonably large energy band gap makes any semiconductor material attractive and suitable for applications in large variety of electronic devices including Metal Oxide Semiconductor Field Effect Transistor (MOSFET). This is a core device present in modern Silicon (Si) based Complementary Metal Oxide Semiconductor (CMOS) technology used in the fabrication of Central Processor Units (CPUs), memory and many other devices incorporated in computers and portable electronic devices. Si is one of the most important materials in modern semiconductor technology. However, in order to maintain existing industrial trend in improving performance of continuously scaling down to sub 10 nm FET devices, new materials or device architectures have to be introduced. Germanium (Ge) is a semiconductor material which has high intrinsic 3D hole and electron mobilities among other known semiconductor materials. In particular, its hole mobility is the highest among existing semiconductors. Biaxial compressive strain further enhances the hole mobility in Ge. In practice, this is obtained by epitaxial growth of a compressively strained Ge epilayer, a few nanometers thick, on an underlying standard Si substrate via an intermediate strain-relaxed SiGe buffer. The strain narrows the band gap of Ge and causes the appearance of a QW in the valence band. Holes confined in the strained Ge QW form a two-dimensional hole gas (2DHG) and have an increased mobility due both to their lower effective mass and reduced scattering factors in this material system. These hole mobilities are the highest not only among the group-IV Si and Ge based semiconductors, but also among p-type III-V and II-VI materials. However, existence of 2D Electron Gas in strained Ge QWs has not been demonstrated so far.
This is a unique opportunity to undertake a project at the cutting edge of applied physics and materials science. You will be involved in realization of the 2DEG in strained Ge QWs and research of their basic transport properties (mobility, effective mass, etc). The novel structures will be created by epitaxial growth by an industrial type Chemical Vapour Deposition (CVD) of Ge QW heterostructures on underlying Si substrates. A number of characterization techniques and experimental setups will be used to research electrical, magnetotransport, structural and optical properties of novel materials. The future success of the strained Ge QW heterostructures depends on our ability to understand their transport phenomena and other material properties. Successful outcome from the project would lead to high impact publications in international scientific journals and application of developed materials in a variety of electronic and spintronic devices using capabilities of our academic and industrial collaborators.
Epitaxial Silicon Carbide strain tuning platform on large diameter Si(111) substrates
Silicon carbide (SiC) is a wide band gap semiconductor attractive for high power and high frequency electronic devices, due to its excellent physical properties such as high thermal conductivity, high breakdown field and high saturation velocity. SiC has numerous polytypes, among which 4H- and 6H-SiC are used to fabricate electronic devices. 4H and 6H-SiC films of good crystal quality can be obtained by homoepitaxy at a growth temperature over 1600°C. The largest 4H- and 6H-SiC wafers available commercially are 150 and 100 mm in diameter, respectively, limited by the size of SiC substrates for homoepitaxy. The production cost of SiC is high since the SiC substrate cannot be replaced by the Si wafer for the growth of 4H- and 6H-SiC. Unlike 4H- and 6H-SiC, a 3C-SiC can be heteroepitaxially grown on Si. The heteroepitaxial growth of 3C-SiC on Si has thus attracted attention in the past years due to lower production cost and larger SiC wafer size. Nowadays, Si wafers up to 450 mm diameter are commercially available. The difficulty to grow a heteroepitaxial SiC layer of good quality on Si is due to the large lattice-mismatch (19.7%) and the difference in thermal expansion coefficients (∼8%) between Si and SiC. Among various types of virtual substrates, the high quality 3C-SiC/Si(111) one is an ideal strain tuning platform for subsequent growth of GaN heterostructures and long awaiting realization of low-cost mass produced LEDs, HEMTs and variety of high power electronic devices. Also, it is a substrate for epitaxial growth of various novel 2D materials heterostructures including Graphene.
The project is an exciting opportunity to be involved in development of new concepts in 3C-SiC heteroepitaxial growth for realization of high quality SiC epilayers on large diameter Si(111) substrates. Epitaxial growth of the structures will be developed in-house, using the first among the UK universities, SiC Chemical Vapour Deposition (CVD) and industrial type Si CVD. Materials properties will be researched by variety of in-house cutting edge experimental techniques including Hall effect and resistivity measurements, defects revealing, Transmission Elecron Microscopy (TEM), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM), Fourier Transfer Infrared Spectroscopy (FTIR) and X-ray diffraction. Complex characterization of new epitaxial materials will provide an in-depth analysis and understanding of 3C-SiC heteroepitaxy. Successful outcome from the project would lead to high impact publications in international scientific journals and application of developed materials in a variety of low- and high-power electronic and photonic devices using capabilities of industrial collaborators.
Germanium-Tin-Silicon heterostructures for mid-infrared solid state laser
Epitaxially grown Germanium-Tin-Silicon (GeSnSi) heterostructures are excellent new group-IV semiconductor materials for future realization of monolithic integration of active and passive waveguide components on silicon or silicon on insulator (SOI) substrates. In particular, the Germanium-Tin (GeSn) alloys attract considerable attention because of tuneable band gaps in the mid-infrared and a band gap transition from indirect to direct has been predicted for certain composition ranges. In recent years, the possibility of adjusting lattice parameters and band gaps over a wide range stimulated the research dedicated to Silicon compatible device concepts based on strain engineering and even the combination of Si with III-V based technologies. Within the group-IV photonics, the Si based electrically injected laser diode is a key component that is still missing. GeSn is probably the best-known Sn-containing alloy and for this material the infrared region is most natural for radiative band-to-band emission because the direct band gap of this crystalline alloy is narrower than that of elemental Ge, the semiconductor in which optically pumped Ge-on-Si lasing at 1.6 µm has already been observed.
The project will focus on development of silicon-based group-IV semiconductor injection laser diode in which the GeSn active region has a direct band gap wavelength in the 1.8 to 5.0 µm mid-infrared wavelength range. Epitaxial growth of the structures will be developed in house using an industrial class Reduced Pressure Chemical Vapor Deposition equipment. Materials properties will be researched by variety of in house cutting edge experimental techniques including conductivity, Hall effect, Fourier Transform Infrared Spectroscopy, Photoluminescence, Raman spectroscopy, Transmission Elecron Microscopy, Atomic Force Microscopy, Scanning Electron Microscopy and X-ray diffraction. Devices will be fabricatied and characterized in house and/or in collaboration with national and international research partners using state of the art technologies and techniques. Successful outcome from the project will lead to invention of novel types of photonic devices.
On-axis 4H-SiC homoepitaxial growth by Chemical Vapour Deposition
Silicon carbide (SiC) is an attractive material for developing high-power, high-temperature, and high frequency devices, owing to its superior properties. Recently there has been significant progress in producing such type of SiC devices. The low total power loss and the high switching speed are the driving forces for its introduction to the market. Today yield and performance of the devices, with respect to usable active device area, are mainly dependent on the quality of wafers and epitaxial layers. Although Chemical Vapour Deposition (CVD) has the advantages of the precise control and uniformity of epilayer’s thickness and impurity doping, the quality of the epilayers can be affected by polytype mixing. SiC has numerous polytypes, among which 4H- and 6H-SiC are used to fabricate electronic devices. 4H and 6H-SiC films of reasonable crystal quality can be obtained by homoepitaxy at a growth temperature over 1600°C. The unique SiC properties, superior in comparison to standard semiconductors, can be utilized only when the material is of high quality. Therefore, today most SiC electronic devices are not fabricated directly on wafers prepared from sublimation grown crystals, but on epitaxial layers. To realize high-performance SiC devices, the reduction of crystal defects such as stacking faults, dislocations, and point defects is necessary.
The project will focus on development of on-axis 4H-SiC homoepitaxial growth for high quality SiC epilayers on 4H-SiC substrates. It is one of the most challengining research area and very demanding by manufacturers of SiC wafers and power electronic devices. Epitaxial growth of the structures will be developed in-house, using the first among the UK universities, commercial CVD equipment. Materials properties will be researched by variety of in-house cutting edge experimental techniques including Hall effect and resistivity measurements, defects revealing, Transmission Elecron Microscopy, Atomic Force Microscopy, Scanning Electron Microscopy, Secondary Ion Mass Spectrometry, Fourier Transform Infrared Spectroscopy, Photoluminescence and Raman Spectroscopy. Complex characterization of new epitaxial materials will provide an in-depth analysis and understanding of SiC homoepitaxy. Successful outcome from the project would lead to application of developed materials in a variety of power electronic devices.
Low-dimensional strained Silicon Germanium heterostructures for applications in high efficiency thermoelectric devices
Thermoelectric materials generate electricity from thermal energy using the Seebeck effect to generate a voltage and an electronic current from a temperature difference across the semiconductor. High thermoelectric efficiency ZT requires a semiconductor with high electronic conductivity and low thermal conductivity. At present, however, most thermoelectric devices are based on bulk material properties, limiting the ZT, in part due to the Wiedemann-Franz Law, which linearly relates the electrical and thermal conductivities for heavily doped, metallic semiconductors. Significantly higher ZTs, however, can be obtained for low-dimensional structures using quantum wells, wires or dots. In order to produce practical devices, however, it is necessary for a large number of these quantum well layers to be considered. Ge has a 4.18% larger lattice constant than Si, and therefore, thermoelectric designs using SiGe heterostructures are limited by the critical thicknesses for each heterolayer as well as the complete stack of strained layers. Thus, strained balanced quantum well structures must be utilised, as all practical layer thicknesses for a complete thermoelectric generator are significantly greater than the critical thickness of the heterolayers.
The project will explore novel designs of high Ge content strain balanced SiGe multilayerd heterostructures with very large number of periods above 1000. Thickness of individual SiGe epilayers will be varied from few monolayers up to several nanometers. Thus precise control of the epilayers thicknesses, Ge and dopants contents therein and their reproducibility along growth direction of 1000’s perioads will be essential. Epitaxial growth of the structures will be developed in-house using an industrial class Reduced Pressure Chemical Vapour Deposition equipment. Materials properties will be researched by variety of in-house cutting edge experimental techniques including conductivity, Hall effect, magnetotransport and thermoelectric measurements, Transmission Elecron Microscopy, Atomic Force Microscopy, Scanning Electron Microscopy, Raman spectroscopy and X-ray diffraction. Thermoelectric devices will be fabricatied and characterized in house and/or in collaboration with national and international research partners using state of the art technologies and techniques. Successful outcome from the project would lead to application of developed materials in practicale power generators.
Germanium based superconducting tunnel junction solid state devices for low-temperature cooling applications
Superconducting tunnel junctions (STJs) have been demonstrated in the past to cool platforms which are intended for applications such as quantum computers, sub-Kelvin detectors and STJ detectors. These STJs take advantage of the weak electron-phonon coupling in normal metals and degenerate semiconductors at very low temperatures so that they can filter out the most energetic electrons into a superconductor. The removal of these “hot electrons” will lower the overall temperature of the electrons in the normal metals or degenerate semiconductors which can then be used to cool its phonons, all at the flick of a switch. Most STJs which have been used for this application have been in the form of a Normal metal/Insulator/Superconductor (NIS) junction on an insulating membrane. However, these junctions have a counterpart in the Semiconductor – Superconductor (SmS) Schottky junctions, which allow commercial Silicon and Germanium integrated circuitry (IC) to be integrated into these cooling platforms. We have recently reported on the effectiveness of these cooling platforms using epitaxially grown bulk Silicon and strained Silicon. In contrast to Silicon the Germanium shows a potential to outperform its predecessor.
The aim of this project would be to develop and research Germanium based STJs for cooling applications, with the possibility of placing STJ devices on mirco-electro-mechanical systems (MEMS) structures. It will involve materials structural characterization by variety of in-house cutting edge experimental techniques including Transmission Elecron Microscopy (TEM), Atomic Force Microscopy (AFM), Scanning Electron Microscopy (SEM) and X-ray diffraction; fabrication of micro- and nano-scale devices using in-hose optical and Focused Ion Beam (FIB) lithography, deposition and etching techniques; and low-temperature electrical, Hall effect and thermoelectric measurements of the devices.
Strained Ge quantum well spintronic devices
Spintronics is an emerging technology exploiting both the intrinsic spin of the electron (or hole) and its associated magnetic momentum, in addition to its fundamental electronic charge. Since the discovery of giant magnetoresistance (GMR) by Fert and Grünberg in 1988, for which they were recently awarded the Nobel Prize, research into spintronics has progressed rapidly. Early applications included read heads in magnetic hard disk drives, and more recently magnetic random access memory (MRAM). Progress in applications has always moved hand-in-hand with basic research, and there is still much understanding to be gained of spin interactions in materials. Recently, attention has turned to exploring spin physics in semiconductors and in particular in germanium (Ge). Early results demonstrated spin transport, injection and accumulation at both low and room temperatures. The Nano-Silicon Research group has developed epitaxial growth technology for extremely high 2D hole gas mobility Ge quantum well (QW) heterostructures using both Chemical Vapour Deposition (CVD) and Molecular Beam Epitaxy (MBE) techniques. These are essential materials for research into spin transport in Ge.
The project will offer an opportunity to undertake a cutting edge research in both spintronics and semiconductor physics. Fabrication of a spin-FET and other spintronic devices using optical and e-beam lithography and thin film deposition techniques will be done in house and in collaboration with academic and industrial partners. Changing the magnetic contact material used will alter the degree of spin polarisation. Variation of the injection tunnel barrier material, for example GeO2, Al2O3, MgO or others, will allow the spin transparency of the interface to be tuned and researched. Spin transport measurements as a function of applied magnetic field and temperature using a low temperature probe station (300 – 4K), closed cycle 4He cryostat (300 – 10 K) or closed cycle 3He cryostat (300 – 0.3 K) will be utilized to determine critical spintronic device parameters such as spin diffusion length, spin relaxation time and spin polarisation through analysis and modelling of the data obtained. The future success of spintronics ultimately depends on our ability to precisely control the polarisation of electrons or holes transported within the device structure.
If you are interested in any of these projects or research areas then please contact Dr Maksym Myronov.