2D semiconductors based on transition metal dichalcogenides are highly promising for ultrathin photodetectors due to their thickness in the nanometer range and their exceptional light absorption properties. To enable efficient separation of optically generated electron-hole pairs heterostructures have to be implemented, which are usually prepared by poorly controlled mechanical steps such as exfoliation, transfer and stacking processes that prevent industrial upscaling. Here, semitransparent photodetectors in the mm2 range based on MoS2/WS2 heterostructures are presented that are realized without any transfer step by a scalable metal-organic chemical vapor deposition process on a sapphire substrate in a continuous growth run. The heterostructure device exhibits a responsivity, which is enhanced by about 5-6 orders of magnitude with respect to reference devices based on either MoS2 or WS2 monolayers only. The large gain enhancement is attributed to efficient charge carrier separation at the MoS2/WS2 heterointerface combined with hole trapping, leading to an improved electron transport in the heterostructure under illumination. © 2021 The Author(s).

Strong covalent in-plane bonds and a tiny thickness in the nanometer range make two-dimensional (2D) materials ideally suited for flexible electronic or optoelectronic applications. Despite this exciting perspective, only a few prototypes of such flexible devices—photodetectors and transistors—have been reported until now. The first large-area flexible light-emitting device (LED) based on 2D materials is realized by integrating a transition metal dichalcogenide (TMDC) monolayer synthesized by metal organic chemical vapor deposition (MOCVD) into a p–n architecture on conductive polymer foil. This flexible LED demonstrates homogeneous red light emission from a few square millimeter area in a scalable design. Uniquely, the electroluminescence can be tuned over 30 meV simply by bending the devices, i.e., by applying a defined strain. This approach combines the flexibility of organic semiconductor device concepts with the durability of inorganic semiconductor technology. © 2020 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Most publications on (opto)electronic devices based on 2D materials rely on single monolayers embedded in classical 3D semiconductors, dielectrics and metals. However, heterostructures of different 2D materials can be employed to tailor the performance of the 2D components by reduced defect densities, carrier or exciton transfer processes and improved stability. This translates to additional and unique degrees of freedom for novel device design. The nearly infinite number of potential combinations of 2D layers allows for many fascinating applications. Unlike mechanical stacking, metal-organic vapour phase epitaxy (MOVPE) can potentially provide large-scale highly homogeneous 2D layer stacks with clean and sharp interfaces. Here, we demonstrate the direct successive MOVPE of MoS2/WS2 and WS2/MoS2 heterostructures on 2 sapphire (0001) substrates. Furthermore, the first deposition of large-scale MoS2/graphene and WS2/graphene heterostructures using only MOVPE is presented and the influence of growth time on nucleation of WS2 on graphene is analysed. Copyright © 2020 Materials Research Society.

Transition metal dichalcogenides (TMDC) have become attractive candidates for 2D electronics and optoelectronics. While several concepts for light emitting devices have been reported, many of them realized using exfoliated TMDC flakes of micrometer size, only few approaches tackle the challenge of upscaling to relevant device sizes. We demonstrate a light emitting diode based on WS2 monolayers in a scalable design. The devices are fabricated by combining two industrially relevant deposition processes in a vertical p-n architecture: Metal organic CVD (MOCVD) is used to realize the optically active WS2 monolayers, while ZnO deposited by spatial atomic layer deposition (sALD) is employed as an electron injection layer on the cathode side. Organic layers spin-coated on an ITO covered glass substrate provide hole injection and transport. The resulting devices exhibit rectifying behavior and red electroluminescence from an area of 6 mm2. © COPYRIGHT SPIE. Downloading of the abstract is permitted for personal use only.

The 2D transition metal dichalcogenide (TMDC) tungsten disulfide (WS2) has attracted great interest due to its unique properties and prospects for future (opto)electronics. However, compared to molybdenum disulfide (MoS2), the development of a reproducible and scalable deposition process for 2D WS2 has not advanced very far yet. Here, we report on the systematic investigation of 2D WS2 growth on hydrogen (H2)-desorbed sapphire (0001) substrates using a hydrogen sulfide (H2S)-free metal-organic vapor phase epitaxy (MOVPE) process in a commercial AIXTRON planetary hot-wall reactor in 10 × 2" configuration. Tungsten hexacarbonyl (WCO, 99.9 %) and di-tert-butyl sulfide (DTBS, 99.9999 %) were used as MO sources, nitrogen (N2) was selected as carrier gas for the deposition processes (standard growth time 10 h). In an initial study, the impact of growth temperature on nucleation and growth was investigated and an optimal value of 820 °C was found. The influence of the WCO flow on lateral growth was investigated. The aim was to maximize the edge length of triangular crystals as well as the total surface coverage. Extending gradually the growth time up to 20 h at optimized WCO flow conditions yields fully coalesced WS2 samples without parasitic carbon-related Raman peaks and with only sparse bilayer nucleation. After substrate removal, a fully coalesced WS2 film was implemented into a light-emitting device showing intense red electroluminescence (EL). © Materials Research Society 2018.

Transition metal dichalcogenides (TMDCs) represent a novel and sustainable material basis for ultrathin optoelectronic devices. Although various approaches toward light-emitting devices, e.g., based on exfoliated or chemical vapor deposited (CVD) TMDC monolayers, have been reported, they all suffer from limited scalability and reproducibility required for industrial fabrication. Here, we demonstrate a light-emitting device in a scalable approach by embedding metal-organic (MO-)CVD WS2 monolayers into a vertical p-i-n device architecture using organic and inorganic injection layers. Red electroluminescence is emitted from an active area of 6 mm2 starting already at a driving voltage of about 2.5 V. © 2019 American Chemical Society.

2D semiconductors represent an exciting new material class with great potential for optoelectronic devices. In particular, WS 2 monolayers are promising candidates for light-emitting devices (LEDs) due to their direct band gap with efficient recombination in the red spectral range. Here, we present a novel LED architecture by embedding exfoliated WS 2 monolayer flakes into a vertical p-n layout using organic p- and inorganic n-supporting layers. Laser lithography was applied to define the current path perpendicular to the WS 2 flake. The devices exhibit rectifying behavior and emit room temperature electroluminescence with luminance up to 50 cd m -2 in the red spectral range. © The Royal Society of Chemistry 2019.

Fabrication of transition metal dichalcogenides (TMDCs) via metalorganic chemical vapor deposition (MOCVD) represents one of the most attractive routes to large-scale 2D material layers. Although good homogeneity and electrical conductance have been reported recently, the relation between growth parameters and photoluminescence (PL) intensity - one of the most important parameters for optoelectronic applications - has not yet been discussed for MOCVD TMDCs. In this work, MoS2 is grown via MOCVD on sapphire (0001) substrates using molybdenum hexacarbonyl (Mo(CO)6, MCO) and di-tert-butyl sulphide as precursor materials. A prebake step under H2 atmosphere combined with a reduced MCO precursor flow increases the crystal grain size by one order of magnitude and strongly enhances PL intensity with a clear correlation to the grain size. A decrease of the linewidth of both Raman resonances and PL spectra down to full width at half maxima of 3.2 cm-1 for the E 2g Raman mode and 60 meV for the overall PL spectrum indicate a reduced defect density at optimized growth conditions. © 2018 IOP Publishing Ltd.

The influence of the main growth parameters on the growth mechanism and film formation processes during metalorganic vapor-phase epitaxy (MOVPE) of two-dimensional MoS2 on sapphire (0001) have been investigated. Deposition was performed using molybdenum hexacarbonyl and di-tert-butyl sulfide as metalorganic precursors in a horizontal hot-wall MOVPE reactor from AIXTRON. The structural properties of the MoS2 films were analyzed by atomic force microscopy, scanning electron microscopy, and Raman spectroscopy. It was found that a substrate prebake step prior to growth reduced the nucleation density of the polycrystalline film. Simultaneously, the size of the MoS2 domains increased and the formation of parasitic carbonaceous film was suppressed. Additionally, the influence of growth parameters such as reactor pressure and surface temperature is discussed. An upper limit for these parameters was found, beyond which strong parasitic deposition or incorporation of carbon into MoS2 took place. This carbon contamination became significant at reactor pressure above 100 hPa and temperature above 900°C. © 2017, The Minerals, Metals & Materials Society.

Two-dimensional (2D) transition metal dichalogenides exhibit a unique band structure: In contrast to many direct-gap classical semiconductors, their band-gap minimum is not at the center of the Brillouin zone, but at finite values of the k vector. We report on clear indications that this momentum mismatch fundamentally influences the carrier transfer between a 2D WS2 crystal and a three-dimensional (3D) GaN layer: Populating different local band extrema of the WS2 in k space by selective laser excitation leads to a pronounced difference in the WS2 photoluminescence signal. These findings may be of high importance for future 2D-3D semiconductor devices. © 2017 American Physical Society.

The direct deposition of the 2D transition metal dichalcogenide MoS2 via metal-organic vapour phase epitaxy (MOVPE) is investigated. Growth is performed in a commercial AIXTRON horizontal hot-wall reactor. Molybdenum hexacarbonyl (MCO) and Di-tert-butyl sulfide (DTBS) are used as metal-organic precursors for molybdenum and sulfur, respectively. The successful deposition of MoS2 is demonstrated via Raman spectroscopy on various substrates such as sapphire and Si as well as AlN and GaN templates. The influence of growth time on the evolution of layer morphology is investigated. Variation of carrier gas reveals that a pure nitrogen growth atmosphere and a growth temperature of 750C improve layer quality. Additionally, a post-deposition annealing process of the grown samples is examined. It is shown that annealing in a pure nitrogen atmosphere at temperatures between 650C and 750C strongly increases the Raman intensities. © 2016 Elsevier B.V.

We demonstrate gate control of the carrier distribution in k-space in bilayer and monolayer MoS2 devices, probed by microphotoluminescence spectroscopy on a contacted single flake. The characteristic emission lines of the neutral and the negatively charged exciton act as a sensor for electron depletion/agglomeration via gate voltage. Gate-induced carrier depletion enhances the sensitivity to defects in monolayers, whereas in bilayers the indirect transition becomes more weight. The specific band structure of bilayers results in a thermal dissociation of trions at 200 K and above, in contrast to monolayers where trion emission is observed up to room temperature. We show that these findings are a consequence of a bias-driven redistribution of charge carriers between the different band minima. © 2015 American Physical Society.

High temperature operation of an electrically driven single photon emitter based on a single epitaxial quantum dot is reported. CdSe/ZnSSe/MgS quantum dots are embedded into a p-i-n diode architecture providing almost background free excitonic and biexcitonic electroluminescence from individual quantum dots through apertures in the top contacts. Clear antibunching with g 2(τ = 0) = 0.28 ± 0.20 can be tracked up to T = 200 K, representing the highest temperature for electrically triggered single photon emission from a single quantum dot device. © 2014 AIP Publishing LLC.

We demonstrate electrically driven single photon emission from a CdSe/ZnSSe/MgS single quantum dot embedded in a resonant cavity light emitting diode. Patterned Pd/Au top contacts are used to inject the charge carriers locally. Single quantum dot electroluminescence is obtained at T = 4 K from nanoapertures lithographically defined in the top contacts. At low current densities, antibunching with a value of g(2)(0) = 0.16 is achieved. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We report on the charging behavior of a single self-assembled InGaAs quantum dot with unpolarized and spin-polarized electrons under direct current (DC) and high-frequency biasing. The tunnel coupling of the quantum dot to a spin-polarized electron reservoir leads to characteristic voltage dependence of the polarization of the neutral and the negatively charged exciton emissions in a magnetic field under DC biasing conditions. Via high-frequency adaptation of the device, electrical control of the charge state of the single quantum dot in the gigahertz regime is achieved. A technique for optical preparation of single holes and subsequent electrical charging via high-frequency voltage pulses allows for an ultrafast injection and readout of spin-polarized electrons on a subnanosecond timescale. © 2012 American Physical Society.

ZnO has a high potential for use in light-emitting devices in the visible and UV spectral range. One of the main challenges in an electrically driven device is the low energy of the valence band and, consequently, the difficult injection of holes. Here, we present an approach combining naturally n-type ZnO nanocrystals with intentionally p-doped Si nanoparticles in a solution-processable nanoparticle heterojunction multilayer. The heterojunction device exhibits an efficiency, that is more than one order of magnitude enhanced compared with the ZnO reference device. White electroluminescence with color rendering indices up to 98 is obtained. © 2012 The Japan Society of Applied Physics.

Single photon emission from an epitaxially grown quantum dot at room temperature is presented. CdSe/ZnSSe quantum dots are embedded into MgS barriers, providing dominant radiative recombination up to 300 K. Under continuous wave optical excitation, the autocorrelation function g (2)(t) exhibits a sharp dip at (t = 0) with g (2)(0) = 0.16 ± 0.15 at T = 300 K, revealing excellent suppression of multiphoton emission even at room temperature. © 2012 American Institute of Physics.

We present an approach to electrical control of the spin polarization in a diluted magnetic semiconductor (DMS) structure. A variable magnetic field induced by a micro-scale current loop magnetizes the Mn 2+ ions in a CdMnTe/CdMgTe DMS quantum well, which via the sp-d exchange interaction polarizes photo-generated electron-hole pairs confined in the well. A maximum spin polarization degree of ±8.5% is obtained at 4.2 K without external magnetic field. The current-induced magnetic field and the current-generated heating of the spin system are quantitatively extracted by micro magneto-luminescence measurements. © 2009 Springer Science+Business Media, LLC.

The recombination dynamics and its temperature dependence are studied in detail in epitaxially grown CdSe/Zn(S,Se) quantum dots (QDs) with additional wide-band MgS barriers. Such design allows to preserve a very high quantum yield and to track the QD recombination dynamics up to room temperature. At low temperatures, a fast initial decay ~0.6 ns is observed which is followed by a long decay with a time constant ~25 ns. The fast initial decay disappears with increasing temperature and above 100K only a single-exponential decay is observed with a decay time τ of 1.2-1.5 ns, which is weakly dependent on temperature up to 300 K. A two-level model including bright and dark exciton states and temperature dependent spin-flip between them describes well the experimental findings. According to the model, the long decay tail results from a thermally activated population of the bright exciton state from the energetically lowest dark state. © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We demonstrate dominant radiative recombination in high-quality self-assembled quantum dots all the way up to room temperature. This allows a proof of the theoretically predicted characteristic doubling of the radiative recombination time with increasing temperature, entirely caused by fine-structure effects. Over the whole temperature range, the transient decay of the photoluminescence can be described by temperature-independent radiative decay rates of dark and bright excitons, respectively, taking into account a thermally activated interplay between dark and bright states and dark-exciton accumulation. © 2010 The American Physical Society.

We studied in details the recombination dynamics and its temperature dependence in epitaxially grown neutral CdSe/ZnSSe quantum dots with additional wide-band gap MgS barriers. Such design allows to preserve a very high quantum yield and track the radiative recombination dynamics up to room temperature. A fast initial decay of ~0.6 ns followed by a slow decay with a time constant ~30-50 ns is observed at low temperature T < 50 K. The fast decay gradually disappears with increasing temperature while the slow decay shortens and above 100 K predominantly a single-exponential decay is observed with a time constant ~1. 3 ns, which is weekly temperature dependent up to 300 K. To explain the experimental findings, a two-level model which includes bright and dark exciton states and a temperature dependent spin-flip between them is considered. According to the model, it is a thermal activation of the dark exciton to the bright state and its consequent radiative recombination that results in the long decay tail at low temperature. The doubling of the decay time at high temperatures manifests a thermal equilibrium between the dark and bright excitons. © 2010 Pleiades Publishing, Ltd.

We report on ultrafast control of the charge state of a single InGaAs quantum dot in a charge-tunable p-i-n diode structure. Focused ion beam etching is employed to decrease the capacitance of the device to enable radio frequency operation. A time-resolved photoluminescence technique is demonstrated that allows monitoring the charge state with a time-resolution which is limited only by the radiative lifetime of the charged and neutral exciton, respectively. Experimental data show that the charge state can be manipulated on time scales shorter than the radiative lifetime of approximately 1.4 ns. © 2010 American Institute of Physics.