Molecular Beam Epitaxy (MBE) is an ultrahigh vacuum technique that offers atomic-level control in the epitaxial growth of semiconductors on crystalline substrates. MBE is applied in the epitaxy of III-V semiconductors to create artificial semiconductor materials that combine layers of different semiconductors, i.e. they comprise semiconductor heterostructures. III-V heterostructures provide the means to vary the electronic and optical properties of a semiconductor along its growth axis and to realize the desired spatial variation (profile) for the energies of electrons and holes, the refractive index and absorption coefficient. Such planar heterostructures are used to implement all modern III-V nanoelectronic and optoelectronic devices, with key examples being the High Electron Mobility Transistor (HEMT) and Multiple Quantum Well Laser Diode (MQW-LD). The formation of heterostructures along 2 or 3 dimensions becomes possible by exploiting MBE growth mechanisms and allows to experimentally realize nanostructures of nanowires and quantum dots, respectively. In addition, heteroepitaxy on a foreign substrate enables the realization of semiconductor materials non-existing as bulk crystals, as well as the combination of different semiconductor materials/technologies on the same substrate (monolithic integration). Our III-V MBE related research spans from fundamental studies on growth mechanisms, heterostructure material properties and physics, to the development and optimization of growth processes for various types of III-V semiconductor devices and monolithic integration applications. The work is supported by electro/optical modelling for optimizing the design of the III-V nano-heterostructures, as well as their experimental validation by fabrication and evaluation of semiconductor devices, which includes research on the basic device fabrication processes (e.g. surface passivation).

Recent and current research areas of interest include, but not limited to:

• Polarization induced carriers in III-Nitride heterostructures
• InN thin films, nano-heterostructures and Field-Effect Transistors (FETs)
• AlN/GaN HEMTs for high frequency devices and power switches
• III-Nitride heterostructures on single crystal and polycrystalline diamond
• Monolithic integration of III-Nitrides and III-Arsenides on Si

The semiconductor nanowires are one-dimensional crystals with typical length in the range of micrometers and diameter in the range of nanometers. The geometrical characteristics of semiconductor nanowires result to unique physical and chemical properties for nano electronic, photonic and sensing applications: (a) defect-free lattice-mismatched heterostructures, (b) well defined conductivity nano-channels and (c) high surface area for chemical interactions and efficient light extraction or absorption. Semiconductor nanostructures resembling to nanowires, lying horizontally on a substrate surface, are also the fins (nano-walls). Both the semiconductor fins and nanowires define conductivity nano-channels for Field Effect Transistors (FETs) that can be efficiently modulated by a gate electrode that surrounds the three sides (tri-gate) or all sides (all-around gate) of a fin or nanowire, respectively.

Nanowires based on III-V semiconductors are particularly promising, since many III-V compounds exhibit high electron mobility and drift velocity and efficient light emission and absorption. They can be used in the ultimate nanoelectronic FETs for digital applications, but also in other devices, such as nano Light Emitting Diodes (LEDs), integrated on Si substrates. III-V heterostructures within the nanowires can provide band discontinuities and potential barriers for additional means of carrier confinement and quantum effects’ realization.

Our research aims to develop fundamental understanding and knowhow for the formation, physical properties and applications of III-Nitride (GaN, InN, AlN) semiconductor nanowires and fins. We investigate their formation by both bottom-up and top-down processes. The bottom-up approach is based on spontaneous epitaxial growth of III-N nanowires by Plasma-Assisted Molecular Beam Epitaxy (PAMBE), whilst the top-down approach employs etching of planar semiconductor layers/heterostructures. Substrate nano-patterning techniques are used for selective area formation of III-N nanowires. We develop device nanofabrication technologies with generic application potential, using two challenging platforms:

• GaN vertical nanowire field-effect transistors (NW-FETs)
• GaN fin filed-effect transistors (FinFETs).  

These technologies could be adapted to different GaN transistor applications (digital, RF, power switching), as well as to other types of III-Nitride nanowire devices.

Indium-Gallium-Nitride (InGaN) alloys are of particular importance in advanced optoelectronic and photovoltaic applications due to the direct bandgap spanning the 0.7 to 3.4 eV energy range. However, a number of bottlenecks (thermodynamic immiscibility, large lattice mismatch, complex epitaxial growth) limit the development of high quality material in the entire compositional range. The aim is to study in depth epitaxial growth processes, take advantage of RF-MBE “far from equilibrium” character and develop device quality epitaxial thin films and heterostructures with alloy ternary composition in the entire composition range.

III-Nitride nanostructures pose a set of unique properties (large bandgap differences, strong polarization, specific band alignment to Si and other semiconductors) that open many possibilities for novel functional applications. Our aim is to develop basic technology to enable those applications and explore their characteristics.  Non-traditional epitaxial nanostructures approaches, such as development of III-N quantum dots structures embedded in dielectric materials, patterned epitaxy, as well as, novel heterostructure designs and doping-polarization engineering has been explored to realize devices with new functionalities or enhance device operation. Within this context the possibility of III-N semiconductor QDs floating gates for non-volatile memories with large voltage threshold and 10-years retention and room temperature negative differential resistance characteristics in double barrier RTDs have been demonstrated.

Semiconductor microcavities are nanostructures grown by Molecular Beam Epitaxy, which consist of either GaAs or InGaAs Quantum Wells embedded between two Bragg mirrors forming a planar Fabry-Perot cavity. In such structures, exciton polaritons result from the strong coupling between a resonant optical cavity mode and confined excitons. The bosonic nature of exciton polaritons inherited from their light–matter composition is responsible for observations of stimulated scattering, amplification, condensation and lasing of polaritons. In contrast to conventional lasing, where coherent light emission is driven by stimulated photon emission, polariton lasing is achieved through the stimulated scattering of polaritons and the formation of a condensate without the need for population inversion. Therefore, polariton lasing promises two orders of magnitude lower threshold than conventional photon lasing and a new generation of ultralow threshold laser devices. In addition, polariton condensates have been shown to exhibit similarities to atomic Bose–Einstein condensates (BECs), displaying a spontaneous build-up of long-range spatial coherence and a thermalized distribution within the population. Unlike atomic BECs, polariton condensation can be achieved at high temperatures due to the very small polariton effective mass. Major prerequisite to all the above observations is the ability to fabricate state of the art MBE grown high finesse microcavity structures allowing very long polariton lifetimes capable of supporting efficient relaxations of polaritons to the ground state and formation of condensate. Other important challenges include growth of p-i-n diode microcavities for efficient electrical injection of carriers while preserving finesse of the grown structures.

The purpose is the investigation of SiC nanowires (NWs) for logic and bio applications. The main target for the former is the demonstration of the viability of using SiC NWFETs as an alternative technology to the Si ones. Our previous theoretical studies on the 3C-SiC NWFETs, in various transport regimes have shown that the SiC NWFETs have similar performance to the Si-based ones while they offer the advantage of high temperature operation and eventually efficient heat dissipation. Thus, the use of SiC NWFETs will not degrade the electrical characteristics while it will address main issues in device scaling. Up to now, the potential advantages of SiC-based NWs have not been demonstrated. The main proposed causes were the poor material properties of the grown materials (high density of planar defects and high residual doping) and the basic technology employed for fabricating SiC NWFETs resulting in poor quality interface with gate dielectrics. Top-down NWs have been used for the SiC NWFETs fabrication on the basis of low-doped 3C-SiC material and eliminating, thus, the first reason. The transistors with top-down grown NWs exhibited three orders of magnitude higher current and transconductance values with respect to SiC NWFETs with bottom-up grown NWs. Nevertheless, it was not possible to switch-off the transistors showing the importance of interface with the gate dielectrics. The current effort is to develop the technology for demonstrating Gate-All-Around (GAA) devices with NW diameter lower than 80nm. Towards this aim, two different lithography techniques are employed. E-beam lithography and Nanoimprint lithography.

Semiconductor quantum dots (QDs) are the ultimate sources of “on demand” single photons, entangled photon pairs, and indistinguishable photons, with direct applications in quantum cryptography and quantum information processing schemes. In this direction, piezoelectric (PZ) QDs offer concrete advantages on several practical aspects of single or entangled photon sources. The case of InAs QDs grown on (211)B GaAs substrates is an excellent example of a PZ QD system which is technologically viable, in the sense that one can take full advantage of the available GaAs device technology.

Exciton-polaritons are composite half-photon half-exciton states, existing inside semiconductor microcavities in the so called “strong-coupling” regime. Owing to their composite light-matter nature, polaritons exhibit strong nonlinearities and their behavior is often governed by a distinctive bosonic character, which is responsible for a number of spectacular demonstrations such as stimulated scattering, parametric amplification, condensation, and polariton lasing. From a device point of view, a particularly attractive aspect of polaritons is the possibility to operate a polariton laser at a hundred times lower threshold currents compared to conventional laser diodes. The low-threshold of polariton lasers is due to the fact that their operation is not based on the conventional population inversion lasing scheme, but is rather achieved when a Bose-condensate is formed at the polariton ground state.

The realization of strongly interacting photonic systems is one of the holy grails of quantum optics. Achieving the nonlinear quantum regime in photonics where the single-site effective photon interaction energy is larger than the losses, opens a plethora of interesting phenomena such as photon blockade, photon crystallisation, and opportunities to realize quantum simulators for the study of condensed matter problems such as Mott-insulator to superfluid phase transitions in arrays of optical cavities. In solid-state cavity-polariton systems consisting of a cavity photon and a quantum well exciton, dominant direct exciton interactions originate from short-range exchange terms. These interactions have led to the manifestation of a number of intriguing collective phenomena, including formation of spontaneous coherence. Very recently, photon correlation measurements on strongly confined polaritons have demonstrated weak quantum correlations. Increasing polariton-polariton interaction further is crucial to exploring a new regime of strongly correlated photons. One way to enhance interactions is to engineer polaritonic excitations with a permanent dipole moment. Here, we demonstrate electrical tuning of nonlinearities in exciton-polariton condensates. Enhance polariton-polariton interactions are obtained using wide QWs in an electrically driven MC by exploiting the quantum confined Stark effect (QCSE) to form dipolar polaritons and demonstrate tuning of the exciton-exciton interaction. As a direct consequence of this, we obtain enhancement of the polariton emission in both linear and lasing regimes with a simultaneous reduction of the polariton lasing threshold and shorter polariton condensate formation times due to enhanced exciton scattering. Such controlled enhancement of exciton interactions, are expected to provide the new route to realization of non classical light sources relying on polariton quantum blockade.

Hybrid polarion laser

Organic materials exhibit exceptional room temperature light emitting characteristics and enormous exciton oscillator strength, however, their low charge carrier mobility prevent their use in highperformance applications such as electrically pumped lasers. It has been suggested that combining organic and inorganic semiconductors in a hybrid microcavity, exploiting resonant interactions between these materials would permit to dramatically enhance optical nonlinearities and operation temperature. Here, we study cavity mediated hybridization of GaAs and J-aggregate excitons in the strong coupling regime under electrical injection of carriers as well as polariton lasing at near room temperature under nonresonant optical pumping. Such hybrid organic-inorganic structures, combine many desirable properties such as large exciton Bohr radius, favouring polariton-polariton interaction and relaxation as well as large oscillator strength for room temperature operation. Our experiments pave the way towards realization of hybrid organicinorganic microcavities which utilise the organic component for sustaining high temperature polariton condensation and efficient electrical injection through inorganic structure.

THz polariton bolometer device

Bolometers are well-known for their ability to detect electromagnetic radiation by absorbing energy and measuring associated temperature changes. The main drawback is their limited sensitivity compared to more expensive cryogenically cooled systems. As applications of THz technologies are becoming more common place, the development of future THz detectors, such as antenna-coupled bolometers and graphene-based devices is gaining ever increasing interest. In a seemingly separate area of research, semiconductor microcavities with quantum wells create a fusion of electronic and optical properties in the form of exciton-polaritons. While traditionally studied for their fundamental effects, these quasi-particles have demonstrated a number of properties useful for hybrid electro- optic devices: direct coupling to both electric and optical fields, fast-response times, long coherence lengths/times, and strong nonlinearity as compared to typical nonlinear optical materials. A variety of theoretical studies have also identified polaritons as prominent generators of THz radiation, by making use of the bosonic final state stimulation of THz frequency transitions or oscillations between multiple quantum wells. Here we consider polariton based THz detection scheme. In contrast to conventional bolometer schemes, a polaritonic based bolometer sensor couples THz induced thermal variations to a polariton-based signal rather than an electrical output. The long coherency of polaritons enable an additive effect over the device size leading to increased sensitivity. Based on the above constituents, we realize an exciton-polariton based interferometric device utilizing one-dimensional channels to propagate coherent polariton condensates. We experimentally measure the temperature induced changes in the interference of counter-propagating polariton condensates and show that such a bolometer device can serve as a platform for temperature sensing and consequently be adopted for use as a THz radiation detector.

Förster resonant energy transfer (FRET) is a high efficient non-radiative dipole-dipole mechanism that can be exploited for direct energy transfer between an inorganic system (III-N QW) and an organic system in proximity for lighting and photovoltaic applications. We perform systematic investigation of FRET within a hybrid prototype structure based on a nitride quantum well donor and a light emitting polymer acceptor. Initially, a combination of self-consistent Schrödinger-Poisson modeling, MBE growth runs, and steady-state and time resolved photoluminescence experiments were employed to investigate the influence of a wide structural parameter space of the nitride component that included growth temperature, material composition, and thickness of the well and barrier material. The energy transfer process was subsequent studied in hybrid structures completed with spin-casted overlayers of two polyfluorene materials. Unexplored aspects of the dipole-dipole coupling were probed and correlated to the characteristics of the nitride quantum well and the hybrid interface. Important conclusions of the study have been the beneficial effect of the quantum well confinement on the nitride PL emission and the experimental establishment of the strong correlation between the intrinsic photoluminescence yield of the nitride and the FRET efficiency in the completed hybrid structure. Structure optimization led to demonstration of up to 50% FRET transfer efficiency at room temperature.