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.

A quantum computer can outperform classical computers for certain tasks. The general challenge to realize a quantum computer is to solve decoherence, which is due to coupling of a quantum state with the local environment. One possible way to overcome decoherence is to use a topological quantum state. Topologically protected states are expected to have long coherence times. In a topological quantum computer, the information is carried by nonlocal Majorana states. Such states can be engineered in a semiconductor nanowire, which has strong spin–orbit interactions, coupled to a superconductor. The first signatures of Majorana states have been observed recently. In order to substantiate the existence of Majorana states and use them as qubits, an exchange—or braiding—operation of these states has to be performed. The current challenge is to improve the quality of the materials and interfaces. Recent progress toward improved Majorana signals using materials science advances is reviewed in this article.

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).

To achieve the first demonstration of non-polar a-plane gallium nitride (GaN) epitaxy on (0 1 0) gallium oxide substrates by metal organic chemical vapor deposition (MOCVD), a low temperature AlGaN nucleation layer was engineered. Specific low temperature AlGaN growth parameters were necessary because the gallium oxide substrate begins to decompose at ∼600 °C in the ambient of H2. To achieve a smooth GaN epitaxial surface, low V/III molar ratio, and low pressure were required. To characterize the GaN film, AFM along with an orientation-dependent crystal tilt mosaic study by X-ray diffraction was performed. We effectively reduced threading dislocation density by applying in situ SiN interlayers grown by MOCVD. The oxygen contamination in the GaN film was found to originate from the substrate decomposition during GaN growth and can be reduced more than 10 times by using GaN buffer layer grown under N2 ambient.

Metal Organic Decomposition (MOD)-made BaTiO3 (BT) thin films were prepared for Resistive Random Access Memory (ReRAM) under various annealing conditions and investigated for improving the properties of bipolar-type resistive switching, focusing on the relation between oxygen vacancies and the behavior of resistive hysteresis. BT thin films with both pre- and final- annealing in nitrogen showed the resistive hysteresis of bipolar-type switching with current ON/OFF ratios of 2 orders of magnitude for both bias polarities. Finally they showed the endurance property with the 106 switching cycles. It was suggested that oxygen vacancies near the oxide surface (both interfaces at metal electrode/oxide and between layer-by-layered oxide layers) are increased by N2 annealing and enhanced the interface-type resistive switching. Pre-annealing in N2 was also found to be very effective to improve endurance properties, implying that not only the electrode/oxide interface but also the middle part of the film would contribute the interface-type mechanism.

InAlN films and InAlN/GaN high electron mobility transistor (HEMT) structures were demonstrated on 150mm <111> Si using Veeco’s Propel single wafer metal-organic chemical vapor deposition (MOCVD) system. Smooth surfaces with root mean square (rms) roughness of 0.68 nm were observed in a 5x5 μm2 atomic force microscope (AFM) scan. X-ray diffraction (XRD) analysis shows well-defined layer peaks and fringes, indicating good structural quality and abrupt layer interfaces. Thickness uniformity of InAlN is 0.87%, 1σ, for a 7-point XRD measurement across the 150 mm wafer. Secondary ion mass spectrometry (SIMS) analysis confirms the uniform indium depth profile and the presence of abrupt layer interfaces. Negligible Ga (< 100 ppm, atomic) incorporation was detected in the InAlN bulk film. Film sheet resistance of 230Ω/sq, charge of 2.1×1013/cm2, and mobility of 1270 cm2/V.s were measured on a prototypical InAlN/GaN HEMT structure comprising a 10 nm-thick, 17% indium, InAlN barrier.

In this work, we present a detailed investigation of the growth of palladium-seeded GaAs nanowires. Nanowires grown on GaAs (111)B substrates consist of three different morphologies, denoted as curly (containing multiple kinks), inclined (relative to the substrate, such as 〈001〉), and vertical. We show that the relative yield of the different types is controllable by a combination of V/III ratio and temperature, where vertical and inclined nanowires are promoted by a high temperature and low V/III ratio. These growth conditions are expected to promote a higher Ga incorporation into the Pd particle, which is confirmed by energy dispersive x-ray analysis. We propose that the observed relationship between particle composition and nanowire morphology may be related to the particle phase, with liquid particles promoting straight nanowire growth. In addition, particles at the tips of nanowires are sometimes observed to be smaller than the initial particle size, suggesting that Pd has been lost during the growth process. Finally, we demonstrate the importance of initial particle size-control to interpret diameter changes after growth.

Using scanning tunneling microscopy (STM) we observed atomic scale interference patterns on quasi-freestanding WSe2 islands grown on top of graphene. The bias-independent double atomic size periodicity of these patterns and the sharp Brillouin zone edge revealed by 2D STM Fourier analysis indicate formation of optical phonon standing waves due to scattering on intercalating defects supporting these islands. Standing wave patterns of both synchronized and non-synchronized optical phonons, corresponding to resonant and non-resonant phonon scattering regimes, were experimentally observed. We also found the symmetry breaking effect for individual phonon wave packets, one of the unique features distinguishing phonon standing waves. We show that vibrational and electronic anharmonicities are responsible for STM detection of these patterns. A significant contribution to the interference contrast arises from quantum zero-point oscillations.

In this work, we report a comparative investigation of InxGa1-xN (SL) and InxGa1-xN/GaN (MQW) structures with an indium content equivalent to x=10%. Both structures are grown on (0001) sapphire substrates using MOCVD and MBE growth techniques. Optical properties are evaluated for samples using PL characteristics. Critical differences between the resulting epitaxy are observed. Microstructures have been assessed in terms of crystalline quality, density of dislocations and surface morphology. We have focused our study towards the fabrication of vertical PIN photodiodes. The technological process has been optimized as a function of the material structure. From the optical and electrical characteristics, this study demonstrates the benefit of InGaN/GaN MQW grown by MOCVD in comparison with MBE for high speed optoelectronic applications.

In situ curvature measurements were used to compare the stress evolution of GaN films grown directly on 6H-SiC via a two-step temperature growth to films grown with an AlN buffer layer. The two-step temperature growth consisted of an initial low-temperature and a main high-temperature GaN layer. In the case of GaN grown directly on 6H-SiC, the high-temperature layer initiated growth under compressive stress which transitioned to tensile stress. Films grown directly on 6H-SiC exhibited a reduction in the threading dislocation (TD) density and an improvement in the surface roughness compared to growth on the AlN buffer layer. Furthermore, transmission electron microscopy of the GaN grown directly on 6H-SiC revealed predominant (a + c)-type TD along with basal plane stacking faults and $\left\{ {11\bar 20} \right\}$ prismatic stacking faults. Channeling cracks were observed in the GaN film when the AlN buffer layer was not utilized. This was attributed to tensile stress induced from the thermal expansion coefficient mismatch.

Self-catalyzed growth of position-defined InP nanowires (NWs) was investigated on SiO2-mask-pattered Si substrates using metalorganic vapor-phase epitaxy. Using low growth temperatures and high group-III flow rates, pyramidal InP NWs were formed vertically on the mask openings. The diameter and tapering of the InP NWs were successfully controlled by the introduction of HCl and H2S gases during the NW growth. In addition, crystal growth of radial InP/InAsP/InP quantum wells on the sidewall of the InP NWs was performed on Si substrates.

Metal organic chemical vapor deposition, as well as material and basic device properties of nitride-based high electron mobility transistor structures on (111) silicon substrates varying in diameter from 4 to 8 inch were studied using in-situ and ex-situ characterization techniques. All substrates used for the growth of the nitride structures in this study were of SEMI standard thicknesses. The total thickness of the nitride structures was in the range of 1.5 – 5 µm. It is reported that nitride structures can be grown on 4, 6 and 8 inch diameter substrates with very similar post-growth wafer shape, material and device characteristics. It is also shown that their crystal quality, 2DEG transport properties and isolation blocking voltages can be improved by increasing nitride structure thickness while maintaining post-growth wafer bow and warp less than 50 µm. The maximum thickness of nitride structures that can be successfully grown on 8 inch diameter SEMI standard substrates seems to be limited to about 4.5 µm due to plastic deformation of Si. Blocking voltages of more than 700 V were achieved using 4.5 µm thick nitride-based high electron mobility transistor structures grown on 8 inch Si substrate.

An inline metal organic chemical vapor deposition system was used to deposit tin sulfide at temperatures >500 °C. Tetramethyltin was used as the tin source and diethyldisulfide as the sulfur source. An overhead injector configuration was used delivering both precursors directly over the substrate. The tin and sulfur precursors were premixed before injection to improve chemical reaction in the gas phase. Growth temperatures 500 – 540 °C were employed producing films with approximate 1:1 stoichiometry of Sn and S detected by energy dispersive x-ray spectroscopy. X-ray diffraction showed there to be mixed phases with Sn2S3 present with SnS.

Our nation discards more than 50% of the total input energy as waste heat in various industrial processes such as metal refining, heat engines, and cooling. If we could harness a small fraction of the waste heat through the use of thermoelectric (TE) devices while satisfying the economic demands of cost versus performance, then TE power generation could bring substantial positive impacts to our society in the forms of reduced carbon emissions and additional energy. To increase the unit-less figure of merit, ZT, single-crystal semiconductor nanowires have been extensively studied as a building block for advanced TE devices because of their predicted large reduction in thermal conductivity and large increase in power factor. In contrast, polycrystalline bulk semiconductors also indicate their potential in improving overall efficiency of thermal-to-electric conversion despite their large number of grain boundaries. To further our goal of developing practical and economical TE devices, we designed a material platform that combines nanowires and polycrystalline semiconductors which are integrated on a metallic surface. We will assess the potential of polycrystalline group III-V compound semiconductor nanowires grown on low-cost copper sheets that have ideal electrical/thermal properties for TE devices. We chose indium phosphide (InP) from group III-V compound semiconductors because of its inherent characteristics of having low surface states density in comparison to others, which is expected to be important for polycrystalline nanowires that contain numerous grain boundaries. Using metal organic chemical vapor deposition (MOCVD) polycrystalline InP nanowires were grown in three-dimensional networks in which electrical charges and heat travel under the influence of their characteristic scattering mechanisms over a distance much longer than the mean length of the constituent nanowires. We studied the growth mechanisms of polycrystalline InP nanowires on copper surfaces by analyzing their chemical, optical, and structural properties in comparison to those of single-crystal InP nanowires formed on single-crystal surfaces. We also assessed the potential of polycrystalline InP nanowires on copper surfaces as a TE material by modeling based on finite-element analysis to obtain physical insights of three-dimensional networks made of polycrystalline InP nanowires. Our discussion will focus on the synthesis of polycrystalline InP nanowires on copper surfaces and structural properties of the nanowires analyzed by transmission electron microscopy that provides insight into possible nucleation mechanisms, growth mechanisms, and the nature of grain boundaries of the nanowires.

Effect of oxygen to nickel molar ratio (O2/Ni) on the crystallinity of atmospheric pressure metal organic chemical vapor deposition (APMOCVD) grown NiO at 500°C is reported. X-ray diffraction (XRD) analysis including grazing incident angle θ of 0.6°, θ-2θ, ɸ and rocking curve scan are employed for crystallographic characterization. Furthermore, surface roughness is studied by atomic force microscopy (AFM). No evidence of diffraction peaks in X-ray grazing incident angle measurement confirms that all the grown NiO films are well oriented along a certain direction. θ-2θ scan results further indicate that the samples are highly oriented only along [111] direction on (0001) sapphire substrates. The analysis of full width at half maximum (FWHM) of rocking curve scan of (111) plane shows that higher O2/Ni ratio results in better crystallinity. The best crystallinity is achieved with FWHM as low as 0.106° at (111) rocking curve scan corresponding to 82.57nm grain size. AFM measurement shows that NiO films grown with higher O2/Ni ratio have smoother surface morphology.

Multi-junction III-V solar cells are based on a triple-junction design that employs a 1eV bottom junction grown on the GaAs substrate with a GaAs middle junction and a lattice-matched InGaP top junction. There are two possible approaches implementing the triple-junction design. The first approach is to utilize lattice-matched dilute nitride materials such as InGaAsN(Sb) and the second approach is to utilize lattice-mismatched InGaAs employing a metamorphic buffer layer (MBL). Both approaches have a potential to achieve high performance triple-junction solar cells. A record efficiency of 43.5% was achieved from multi-junction solar cells using the first approach [1] and the solar cells using the second approach yielded an efficiency of 41.1% [2]. We studied carrier dynamics and defects in bulk 1eV InGaAsNSb materials and InGaAs layers with MBL grown by MOVPE for multi-junction solar cells.

Nowadays, the nature of the non radiative recombination centres in ZnO is a matter of controversy; they have been related to extended defects, zinc vacancy complexes, and surface defects, among other possible candidates. We present herein the optical characterization of catalyst free ZnO nanorods grown by atmospheric MOCVD by microRaman and cathodoluminescence spectroscopies. The correlation between the defect related Raman modes and the cathodoluminescence emission along the nanorods permits to establish a relation between the non radiative recombination centers and the defects responsible for the local Raman modes, which have been related to Zn interstitial complexes.

We present a detailed study carried out on oxide buffer layers grown by Metal-Organic Decomposition (MOD) on metallic substrates for YBa2Cu3O7-x (YBCO) coated conductor applications. Precursor solutions have been made starting from acetates or pentanedionates and characterized by means of Differential Scanning Calorimetry (DSC) and Thermogravimetric (TG) analyses coupled with Fourier Transform Infra-Red spectroscopy (FT-IR). Thin buffer layers have been grown by spin-coating on Ni-5at.%W substrates. X-ray diffraction spectra (XRD), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM) have been employed in order to optimize buffer layers in terms of film microstructure and surface quality, with the final aims of producing a suitable template for YBCO growth. It will be shown that the optimization of the recrystallization process can lead to high quality buffer layer allowing the growth of YBCO films showing good superconductive properties.

Blue light-emitting diodes (LED's), utilizing InGaN-based multi-quantum well (MQW) active regions deposited by organometallic chemical vapor epitaxy (OMVPE), are one of the fundamental building-blocks for current solid-state lighting applications. Studies [1,2] have previously been conducted to explore the optical and physical properties of the active MQW's over a variety of different OMVPE growth conditions. However, the conclusions of these papers have often been contradictory, possibly due to a limited data set or lack of understanding of the fundamental fluid dynamics and gas-phase chemistry that occurs during the deposition process.

Multi-quantum well structures grown over a range of pressures from typical low-pressure production processes at 200 Torr, up to near-atmospheric growth conditions at 700 Torr, have been investigated in this study. At all growth pressures, clear trends of gas-phase chemical reactions are observed for increased gas residence times (lower gas speeds from the injector flange and lower rotation rates) and increased V/III ratios (higher NH3 flows).

Confocal microscopy, excitation-dependent PL (PLE), and time-resolved photo-luminescence (TRPL) have been employed on these MQW structures to investigate the carrier lifetime characteristics. Confocal emission images show spatially-separated bright and dark regions. The bright regions are red-shifted in wavelength relative to the dark regions, suggesting microscopic spatial localization of high indium content regions. As the growth pressure and gas residence times are reduced, a larger difference in band-gap between bright and dark regions, longer lifetimes, and higher average PL intensities can be obtained, indicating that higher optical quality material can be realized. Optimized MQW's grown at high pressure exhibit higher PLE slope intensities and IQE characteristics than lower pressure samples. Results on simple LED structures indicate that the improvement in MQW optical quality at high pressures translates to higher output power at a 110 A/cm2 injection current density.

We present the results of our systematic investigation of the RE dependency of superconductivity in the parent compounds T’-RE2CuO4 (RE = Pr, Nd, Sm, Eu, Gd, and Tb). Superconducting samples were prepared by metal organic decomposition (MOD). A stringent control of synthesis- and post-annealing-conditions is required to obtain superconducting samples. Superconductivity with a transition temperature (Tconset) ≥ 30 K is achieved for RE = Pr and Nd. By contrast, Tconset is at highest 20 K for RE = Gd. Our results indicate that the induction of superconductivity into T’-RE2CuO4 cuprates strongly depends on the RE3+ ionic size. This trend is discussed from the viewpoint of RE-dependent thermodynamic stability of T’-RE2CuO4. For smaller RE3+ ions, the thermodynamic boundary conditions become tighter.