Development of III-V Semiconductor Materials for Thermophotovoltaic Cells

Thermophotovoltaic energy conversion (TPV) is concerned with generation of power from heat sources. Multiple types of TPV systems have been developed so far; unfortunately, they all suffer from high losses and low overall efficiencies, usually only around 1%. Their performances could be greatly enhanced by high efficiency converter cells, development of which is the main concern of the work presented in this thesis. The first part focuses on research into materials suitable for fabrication of TPV cells. Low bandgap III-V and group IV semiconductors such as GaInAs, InAsP or GeSn were investigated. Then the thesis describes the model used to simulate behaviour of TPV cells under different illumination conditions. The results show that best performances are achieved for cells bandgap matched to the emission of the radiator. Maximum theoretical efficiency of 27% has been predicted for cells with 0.43 eV bandgaps and a light trapping architecture operating with a source at 1800 K delivering 500 kW.m-2 of power. The chapter on modelling is followed by detailed description of growth, fabrication and characterisation of GaInAs TPV devices. Quality of the grown material, its morphology and composition have been evaluated and then the processing steps for contacts deposition have been briefly explained. They are followed by a discussion of optical and electrical measurements for the fabricated devices. The last chapter describes details of growth and characterisation of InAs nanowires. Using nanostructures such as nanowires rather than bulk materials has significant advantages. Nanowires can be grown on virtually any substrate, which allows for integration with Si for CMOS-compatible devices. The improvement of optical properties of InAs nanowires has been the chief objective of this part of the thesis. Through a series of photoluminescence measurements, it has been demonstrated that capping the core-InAs nanowires with an InP passivation layer increases the photoluminescence emission up to ten times

Ten-fold enhancement of InAs nanowire photoluminescence emission with an InP passivation layer

In this letter, we demonstrate that a significant improvement of optical performance of InAs nanowires can be achieved by capping the core InAs nanowires with a thin InP shell, which successfully passivates the surface states reducing the rate of non-radiative recombination. The improvements have been confirmed by detailed photoluminescence measurements, which showed up to ten-fold increase in the intensity of room-temperature photoluminescence from the capped InAs/InP nanowires compared to the sample with core-only InAs nanowires. Moreover, the nanowires exhibit high stability of total photoluminescence emission strength across temperature range from 10 to 300 K as a result of strong quantum confinement. These findings could be the key to successful implementation of InAs nanowires into optoelectronic devices

Growth of Pure Zinc-Blende GaAs(P) Core-Shell Nanowires with Highly Regular Morphology

The growth of self-catalyzed core–shell nanowires (NWs) is investigated systematically using GaAs(P) NWs. The defects in the core NW are found to be detrimental for the shell growth. These defects are effectively eliminated by introducing beryllium (Be) doping during the NW core growth and hence forming Be–Ga alloy droplets that can effectively suppress the WZ nucleation and facilitate the droplet consumption. Shells with pure zinc-blende crystal quality and highly regular morphology are successfully grown on the defect-free NW cores and demonstrated an enhancement of one order of magnitude for room-temperature emission compared to that of the defective shells. These results provide useful information on guiding the growth of high-quality shell, which can greatly enhance the NW device performance

Low threading dislocation density and antiphase boundary free GaAs epitaxially grown on on-axis Si (001) substrates

The interactions between 1D defect threading dislocations and 2D defect antiphase boundaries and antiphase boundary annihilation in III–V materials on Si heteroepitaxy growth are revealed

Photoelectrochemical water oxidation of GaP 1−x Sb x with a direct band gap of 1.65 eV for full spectrum solar energy harvesting

International audienceHydrogen produced using artificial photosynthesis, i.e. solar splitting of water, is a promising energy alternative to fossil fuels. Efficient solar water splitting demands a suitable band gap to absorb near full spectrum solar energy and a photoelectrode that is stable in strongly alkaline or acidic electrolytes. In this work, we demonstrate for the first time, a perfectly relaxed GaP0.67Sb0.33 monocrystalline alloy grown on a silicon substrate with a direct band gap of 1.65 eV by molecular beam epitaxy (MBE) without any evidence of chemical disorder. Under one Sun illumination, the GaP0.67Sb0.33 photoanode with a 20 nm TiO2 protective layer and 8 nm Ni co-catalyst layer shows a photocurrent density of 4.82 mA cm−2 at 1.23 V and an onset potential of 0.35 V versus the reversible hydrogen electrode (RHE) in 1.0 M KOH (pH = 14) aqueous solution. The photoanode yields an incident-photon-to-current efficiency (IPCE) of 67.1% over the visible range between wavelengths 400 nm to 650 nm. Moreover, the GaP0.67Sb0.33 photoanode was stable over 5 h without degradation of the photocurrent under strong alkaline conditions under continuous illumination at 1 V versus RHE. Importantly, the direct integration of the 1.65 eV GaP0.67 Sb0.33 on 1.1 eV silicon may pave the way for an ideal tandem photoelectrochemical system with a theoretical solar to hydrogen efficiency of 27%

The Epitaxial Growth and Unique Morphology of InAs Quantum Dots Embedded in a Ge Matrix

In this work, we investigated the epitaxial growth of InAs quantum dots (QDs) on Ge substrates. By varying the growth parameters of growth temperature, deposition thickness and growth rate of InAs, a high density of 1.2 ×1011 cm-2 self-assembled InAs QDs were successfully epitaxially grown on Ge substrates by solid-source molecular beam epitaxy (MBE) and capped by Ge layers. Pyramidal- and polyhedral-shaped InAs QDs embedded in Ge matrices were revealed, which are distinct from the lens- or truncated pyramid-shape dots in InAs/GaAs or InAs/Si systems. Moreover, with 200 nm Ge capping layer, one third of the embedded QDs are found with ellipse and hexagonal nanovoids with sizes of 7 – 9 nm, which is observed for the first time for InAs QDs embedded in a Ge matrix to the best of our knowledge. These results provide a new possibility of integrating InAs QD devices on Group-IV platforms for Si photonics

Kemnitz’ conjecture revisited

AbstractA conjecture of Kemnitz remained open for some 20 years: each sequence of 4n-3 lattice points in the plane has a subsequence of length n whose centroid is a lattice point. It was solved independently by Reiher and di Fiore in the autumn of 2003. A refined and more general version of Kemnitz’ conjecture is proved in this note. The main result is about sequences of lengths between 3p-2 and 4p-3 in the additive group of integer pairs modulo p, for the essential case of an odd prime p. We derive structural information related to their zero sums, implying a variant of the original conjecture for each of the lengths mentioned. The approach is combinatorial

All-MBE grown InAs/GaAs quantum dot lasers with thin Ge buffer layer on Si substrates

A high-performance III–V quantum-dot (QD) laser monolithically grown on Si is one of the most promising candidates for commercially viable Si-based lasers. Great efforts have been made to overcome the challenges due to the heteroepitaxial growth, including threading dislocations and anti-phase boundaries, by growing a more than 2 µm thick III–V buffer layer. However, this relatively thick III–V buffer layer causes the formation of thermal cracks in III–V epi-layers, and hence a low yield of Si-based optoelectronic devices. In this paper, we demonstrate a usage of thin Ge buffer layer to replace the initial part of GaAs buffer layer on Si to reduce the overall thickness of the structure, while maintaining a low density of defects in III–V layers and hence the performance of the InAs/GaAs QD laser. A very high operating temperature of 130 °C has been demonstrated for an InAs/GaAs QD laser by this approach

Imaging 3D nanostructure of III-V on Si via cross-section SPM: quantum wells and nanowires - defects, polarity, local charges

Merging unique performance of compound semiconductor (CS) III-V materials in optoelectronics, high frequency and power devices with mature Si manufacturing is a holy grail of modern semiconductor technology. The difference between lattice constants, processing, and chemistry are just a few major challenges to be resolved. With practically non-existing methods for studying nanoscale physical properties of these buried structures, we developed a new concept for fast and efficient 3D nanoscale resolution quantitative mapping of physical properties of CS materials and devices. We combine novel nano-sectioning using variable energy Ar ion beam targeted at the edge of the sample to create a perfectly flat oblique near-atomic flat section through all layers of interest, and the material sensitive scanning probe microscopy (SPM), to reveal 3D morphology, composition, strain and crystalline quality via local physical properties – mechanical and piezoelectric moduli, nanoscale heat conductance, workfunction and electrical conductivity. We can observe the propagation of antiphase domains (APD) from the GaAs-Si interface through the 3D structure, reporting for the first time APD effect on electronic properties of multiple quantum wells that are electrically short the structure evident on charge distribution nanomaps. In GaN nanowires, we directly observe NW/Si substrate interface, and unexpectedly find the in-NWs domains of the opposite polarity via piezoelectric moduli maps. The novel paradigm will make a disruptive change on how 3D structure and physical properties of CS and microelectronics materials and devices are currently studied