Optoelectronic Devices Based on Monolayer Transition Metal Dichalcogenides

Monolayer transition metal dichalcogenides (TMD) such as molybdenum disulfide (MoS2), tungsten diselenide (WSe2) and tungsten disulfide (WS2), have shown in the last years remarkable physical properties. These direct badgap three-atoms thick monolayers, with a broken inversion symmetry, present a unique coupling between the spin and valley degrees of freedom originated from the relativistic spin¿orbit interaction. Together with their mechanical flexibility, result in materials promising for flexible, transparent and low power electronic, optoelectronic and spin/valleytronic applications. In this thesis, we investigate optoelectronic and spin/valleytronic devices based on monolayer MoS2, the most studied monolayer from the TMD familly, with 1.9 eV direct band gap and 6.5 Å thick. We show a monolayer MoS2 photodetector with 100,000-fold improved photoresponsivity from previous monolayer MoS2 phototransistors, first time reported entire junction light emitting diodes (LED) and solar cells based on monolayer MoS2/p-type silicon heterojunctions, which can work as an avalanche photodiodes too, and first spin-valley tunable light emitting diode made with monolayer MoS2/monolayer WSe2. Utilizing high-quality monolayer MoS2, we achieved a broad spectral range phototransistor with photoresponsivity of 880 A/W and low noise equivalent power (NEP) of 1.8 x 10^-15 W/Hz^1/2. Afterwards, we used the two-dimensional (2D) n-type monolayer MoS2 combined with three-dimensional (3D) p-type silicon to build vertical p-n junctions. The entire junction area of our 2D/3D heterostructures emitted light with a low emission threshold power density of 3.2 W/cm^2 and spectrum related to the direct band gap of monolayer MoS2. The heterojunction diode could operate as a solar cell with an external quantum efficiency (EQE) of 4.4% and a broad spectral response. With the use of large area chemical vapor deposition (CVD) grown monolayer MoS2, we scaled up the manufacturing process and showed the capability of these heterostructures to work as avalanche photodiodes (APD) with a multiplication exceeding 1000 for -10 V. Finally, we combined monolayer MoS2 with WSe2 or WS2 and made 2D/2D heterojunctions able to emit light with different characteristic spectrums related to the type of heterojunction. Monolayer WS2/monolayer MoS2 showed one of the emission peaks in the green region of the visible spectrum while all heterojunctions showed peaks in the red region. Spin-polarized charge carriers were injected across the Schottky barrier between a ferromagnetic electrode and the monolayer WSe2 of a monolayer WSe2/monolayer MoS2 heterostructure, resulting in valley polarization due to spin-valley locking. The degree of spin/valley polarization was controlled by a magnetic field between a polarization of ± 20%. A slope of 0.47 ± 0.08 meV/T was also seen related to the valley Zeeman effect.

Spin-valley optoelectronics based on two-dimensional materials

Dmitrii Unuchek

Two-dimensional (2D) materials, in particular graphene and transition metal dichalcogenides (TMDC), have attracted great scientific interest over the last decade, revealing exceptional mechanical, electrical and optical properties. Owing to their layered nature, subnanometer-thick single-layer forms of these crystals can be chemically grown or mechanically isolated, representing an ultimately scaled-down platform for further miniaturization of electronic devices. Indeed, the large family of 2D materials contains hundreds of members, including metals, semiconductors, insulators, and others, covering the whole spectrum of potential applications. Availability of all necessary building blocks for the construction of all-2D solid-state devices makes this platform ideal for fabrication of ultrathin, transparent and flexible devices. Extensively investigated graphene has demonstrated excellent electrical properties. However, the absence of a bandgap is an obstacle for its interaction with light and therefore limits applications in optics. In this aspect, atomically thin TMDC semiconductors appear to be an alternative, more promising platform, as they possess a direct band gap in the visible range in the monolayer form. Despite being only three atoms thick, these semiconductors demonstrate exceptionally large light absorption, efficient light emission, and strong light-matter interaction, attracting justified interest from the optics community. We will focus on their potential applications for optoelectronics in Chapter 4. Even more, broken inversion symmetry and strong spin-orbit coupling in single-layer TMDC crystals reveal a new quantum number, the so-called valley index. Indeed, this unique degree of freedom of charge carriers is known for some materials since the 1970s. However, in the case of 2D semiconductors, spin-valley locking mechanism and valley contrasting optical selection rules open new ways for addressing, manipulation and sensing of this pseudospin, opening the whole new field of valleytronics. Due to the large splitting of the valence band, spin and valley degree of freedom become locked in these atomically thin materials. We employ this unique feature in Chapter 5 for indirect injection of spins into graphene by pumping valley polarized carriers in an adjacent TMDC monolayer. Being gapless, graphene does not allow direct optical injection of spins. Another striking feature of 2D materials is their unique ability to be stacked in vertical heterostructures with strong electrical coupling between layers. The weak van der Waals force, which keeps components together, provides freedom in the assembly process, so that a lattice mismatch or a twist angle becomes unimportant in the first approximation. This provides a novel platform for harvesting complementary properties of the constituent materials, allowing the engineering of new artificial meta-materials as we demonstrate in Chapter 6. Furthermore, synergistic effects in van der Waals heterostructures enable completely new properties and phenomena, which do not exist in the single materials, with twist angle being an important knob for tuning these effects. We observe and exploit such novel phenomena arising in heterostructures of 2D semiconductors in the last chapter of this thesis. On the way to the realization of practical optoelectronic and valleytronic applications, this thesis studies fundamental aspects of rich spin-valley physics of atomically thin semiconductors

EPFL2019

Polarization properties of semiconductor quantum dot and dash lasers

Philipp Ridha

Self-assembled quantum-dots (QDs) represent a distributed ensemble of zero dimensional structures, each presenting a near-singular density of states. The QD size, shape, areal density and optical properties depend on growth parameters, such as growth temperature, growth rate and amount of InAs deposed. QDs are very suitable for optoelectronic device applications as during the epitaxial QD growth the phase transition relieves the strain elastically without introducing defects. Embedded in the active medium of a semiconductor laser diode QD's unique properties lead to improved and often novel characteristics as compared to bulk or quantum-well (QW) devices. In particular In(Ga)As QD lasers on GaAs substrates are of largest interest as the spectral range of their emission wavelength reaches further into the infrared than that of QW lasers of the same material system. The emission wavelength of strained InGaAs QW lasers is limited to about 1150nm, whereas in In(Ga)As QD lasers an emission wavelength beyond 1.3µm is feasible. Such kind of QD lasers performed – as recently reported – ultralow threshold current densities combined with decreased temperature sensitivity, relatively high modulation bandwith, low chirp, which has been already predicted many years ago. Beside quantum dot's application in semiconductor lasers, it has been suggested to use QDs also for the optical active region in so-called semiconductor optical amplifier (SOA). So far, experimentally it was possible to demonstrate in QD SOAs a much faster gain recovery time due to more effective carrier re-filling from excited states than in quantum well structures. Also a smaller noise figure ratio compared to QWs was predicted in QD SOA, but has not been experimentally demonstrated yet. However, two major constraints related to the limit of the high-frequency modulation of QD lasers and the QD SOA's polarization sensitivity are required to be first resolved, before it is possible to implement both technologies replacing QW devices in optical broad telecommunications devices. The polarization properties of semiconductor quantum dot/-dash lasers and amplifiers have been addressed in this PhD thesis. Within this research work – studying systematically several quantum dot and -dash heterostructure of different material families – two major goals have been identified: (1) characterization of the optical polarization properties of quantum dot/-dash structures embedded in the active region of semiconductor optical amplifiers and (2) realization of polarization-insensitive quantum dot based SOAs. Moreover, this thesis focuses on the understanding of the underlying physics being responsible for the QD's polarization and the dependence of the optical gain on the QD's aspect ratio and compositional material contrast. In terms of the experimental work broad area laser devices have been fabricated by applying different clean room processing methods like photolithography, metal deposition, wet and plasma etching. Through a subsequent device characterization using optoelectronic measurement techniques the polarization characteristics of dots and dashes have been accessed. From broad area laser device characterization – measuring the polarization-resolved edge-emitted electroluminescence (EL) – the polarization properties of standard Stranski-Krastanov (SK), closely- and columnar stacked InAs/GaAs quantum dots as well of closely-stacked InAs/InP quantum dots and columnar-stacked InAs/InP quantum dashes – in total 32 devices – have been evaluated. The comparison of the transversal magnetic (TM) versus -electric (TE) integrated EL signal for above heterostructures provided very promising results in order to achieve polarization-independent QD SOA: Starting from standard SK QDs – emitting dominantly in TE mode – it was possible first to enhance the TM EL signal compared to TE by epitaxial shape-engineering of the dot/dash structure of the QD's aspect ratio and the compositional material contrast in columnar-stacked InAs/GaAs (InAs/InP) QDs (QDashes), and then inverting the polarization direction from dominant TE towards dominant TM. The latter polarization transition has been also confirmed by photovoltage spectroscopy (PVS), and as well from room-temperature TM-lasing demonstration – both carried out by our research partner from the Cardiff University, Wales (UK). Introducing the well-known segment contact method for gain measurements, which is based on the analysis of the amplified spontaneous emission (ASE) generated by cavities of different length, it was possible to access both the TM and TE gain of such columnar-stacked InAs/GaAs quantum dots. The work was motivated and funded by the European Union project "Zero Order Dimension based Industrial components Applied to teleCommunications" (ZODIAC) within the sixth framework program. Very fruitful collaborations in order to exchange scientific ideas/know-how and achieving joint results have been established in a pan-European and multicultural inspired environment between four industrial partners – Alcatel Thales III-V Lab (France), Bookham (GB), Innolume – GmbH (Germany), Nanoplus Nanosystems and Technologies-GmbH (Germany) – and three national research institutes – Laboratory for Photonics and Nanostructures (France), The Tyndall National Institute (Irland), Saint-Petersburg Physics and Technology Centre for Research and Education of the Russian Academy of Science (Russia) – and three Universities: École Polytechnique Fédérale de Lausanne (Switzerland), Politechnika Wroclawska (Poland), Würzburg University (Germany). Besides this European ZODIAC project it was possible to establish with the Cardiff University, Wales (UK) a very constructive collaboration.

EPFL2008

Stretchable and Highly Bendable Thin Film Transistors Based on Atomically Thin Chemical Vapor Deposited Molybdenum Disulfide

Yen-Cheng Kung

Two-dimensional (2D) materials have received tremendous research attention recently, as they possess peculiar physical properties in their monolayer and few-layer forms, which further lead to novel applications. A wide range of 2D materials covers insulators, metals, superconductors and semiconductors with various bandgap, giving a fresh platform to reinvent conventional electronic and optoelectronic devices with new advantages. Molybdenum disulfide (MoS2), belonging to the group of transition metal dichalcogenides (TMDCs), has been widely studied for its particular physical properties and potential applications as a leading example among 2D semiconductors. In this thesis, we focus on the applications ofMoS2 in microelectronic and micro-electromechanical devices. The MoS2 we studied is grown specifically by chemical vapor deposition, which offers the highest potential in large area growth. Utilizing the flexible nature such as high in-plane fracture strain and low bending stiffness of atomically thinMoS2, we developed stretchable thin film transistors (TFTs) on polydimethylsiloxane (PDMS) and highly bendable thin film transistors on polyimide. The fabrication process is ameliorated and designed to provide good chemical and temperature compatibility to the polymer substrates. Electrical performance of the flexible devices under different degrees of bending and stretching has been studied. The stretchableMoS2 TFTs could sustain up to 4% of uniaxial tensile strain maintaining device function, and the failure occurs at 5% of strain due to the fracture in gate dielectric layer. The highly bendableMoS2 TFTs with parylene-C gate dielectric layer exhibit tolerance to the smallest bending radius of 0.625 mm. Different encapsulation layers such as HfO2, Cytop and SU8 have been implemented on the bendable devices to improve the performance represented by the increase in field effect mobility, reduction in hysteresis and better endurance to mechanical deformation. The resulting flexibleMoS2 TFTs show field effect mobility of 21.4 cm^2V^-1s^-1 and a large on-off ratio of 106 in air. The SU8 encapsulation layer is further demonstrated as controllable n-type doping strategy with low temperature process, which is ideal for flexible MoS2 devices as it additionally exhibits good stability in air and water.

EPFL2018