Wafer-scale MoS2 with water-vapor assisted showerhead MOCVD

Among numerous thin film synthesis methods, metalorganic chemical vapor deposition performed in a showerhead reactor is the most promising one for broad use in scalable and commercially adaptable two-dimensional material synthesis processes. Adapting the most efficient monolayer growth methodologies from tube-furnace systems to vertical-showerhead geometries allows us to overcome the intrinsic process limitations and improve the overall monolayer yield quality. Here, we demonstrate large-area, monolayer molybdenum disulphide growth by combining gas-phase precursor supply with unique tube-furnace approaches of utilizing sodium molybdate pre-seeding solution spincoated on a substrate along with water vapor added during the growth step. The engineered process yields a high-quality, 4-inch scale monolayer film on sapphire wafers. The monolayer growth coverage, average crystal size and defect density were evaluated using Raman and photoluminescence spectroscopy, X-ray photoelectron spectroscopy, scanning electron microscopy and scanning transmission electron microscopy imaging. Our findings provide a direct step forward toward developing a reproducible and large-scale MoS2 synthesis with commercial showerhead reactors.

Selective light induced chemical vapour deposition of titanium dioxide thin films

Estelle Wagner

Light Induced Chemical Vapour Deposition (LICVD) of titanium dioxide thin films is studied in this work. It is shown that this technique enables to deposit locally and selectively a chosen crystalline phase with a precise controlled thickness at low substrate temperature, allowing even the use of polymer substrates. A home made LICVD reactor was set up, consisting of a main chamber in which the substrate was placed on a temperature controlled plate and could be irradiated perpendicularly through a window by a 250 ns long pulse XeCl excimer laser (308 nm) with a controlled fluence (energy per surface area per pulse). A liquid precursor (titanium tetra-isopropoxide) was brought by an oxygen carrier gas flow in the chamber and a nitrogen flow was added to prevent deposition on the window. The total pressure in the chamber was maintained at 10 mbar. Projection of a mask image on the substrate was realized. Titanium dioxide films could be deposited locally by this technique with a thickness ranging from few nanometers to several micrometers. Deposits were realized at substrate temperatures as low as room temperature and with fluences between 1 and 400 mJ/cm2. The deposited films were characterized by profilometry, Atomic Force Microscopy, optical microscopy, Scanning Electron Microscopy, Transmission Electron Microscopy, X-ray diffraction, Raman spectroscopy, X-ray Photoelectron Spectroscopy, UV-vis spectroscopy and ellipsometry. A detailed and systematic study of the influence of some selected process parameters (substrate temperature and irradiation dose principally) on the deposition rate and material properties was realized on glass and silicon substrates. An empirical equation of the growth rate as a function of the different experimental parameters was derived and was tested successfully to obtain a precise control of the deposited thickness (a precision better than 10 nm was demonstrated for films below 300 nm), with growth rates up to 300 nm/min. From this equation, a deposition mechanism involving both a photolytic reaction and a thermal contribution of the substrate temperature was proposed. The deposited material was shown to consist of TiO2 with a possible very low additional carbon contamination. Depending on the deposition conditions, amorphous, anatase or rutile material was deposited. One parameter explaining the variation of the crystalline state of the material was the variation of the laser induced temperature rise which was simulated theoretically in parallel. Surface temperature rises in the order of several hundreds of degrees were calculated during the laser pulse, varying with the substrate, the fluence and the film thickness. The optical properties of the deposited material vary in a correlated way with the crystalline state of the material, and high indexes of refraction (up to 2.6 at 633 nm) were measured. Selective deposition in the irradiated area was studied. Provided substrate temperature was kept below 150°C, no thermal deposition was observed, and deposits were strictly located in the irradiated area. The edge resolution (around 10 micrometers) was demonstrated to be due to the optical aberrations of the mask projection set up. No effect of substrate temperature and irradiation conditions on the resolution could be evidenced, but resolution decreased with increasing deposited thickness due to optical effects. Thanks to the low substrate temperature required for the deposition process, deposition on thermo- fragile substrates (such as polymers, in particular PMMA) was demonstrated. However, in the case of polymer substrates, the fluence had to be kept below 15 mJ/cm2 not to damage or ablate the polymers, resulting in very low growth rates (in the order of 10-3 nm/pulse). In these conditions, only amorphous films with about 10% of carbon contamination could be obtained. Additionally, films were shown to crack above a certain deposited thickness, which is attributed to thermal effects. As a matter of fact, calculations showed a high temperature rise up to 70°C of the polymer/oxide interface. Deposition on polymers coated with transparent conductive oxide was also demonstrated.

EPFL2003

Characterization of a-plane grown GaN on sapphire substrates by electron microscopy

Yadira Arroyo Rojas Dasilva

Gallium nitride (GaN) is one of the most interesting materials for devices applications such as blue light emitting diodes, laser diodes and high power and high temperature electronic applications, because of its large band gap (3.39 eV). Several growth techniques including metal organic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE) and hybrid vapor phase epitaxy (HVPE) are employed to synthesize high quality GaN. Sapphire, that is available in several orientations, is the common substrate to grow GaN. The GaN that grown on c-(0001) sapphire, has a wurtzite structure and suffers of spontaneous polarization effects. These effects create internal electric fields that affect the operation and the efficiency of the devices. This work is part of a large effort to grow defects free GaN layers in non-polar geometries undertaken at LASPE-IQEP-EPFL. Among the non-polar geometries, the a-(1120) GaN geometry has been selected. The specimens were produced using the HVPE and Epitaxial lateral Overgrowth (ELO) techniques and were fully characterized mainly by electron microscopy observations. Despite the ELO improvements and smaller lattice mismatch in a-GaN (≈1.1%) than c-GaN (≈16%), a high density of dislocations and stacking faults are still observed. They are results of the lattice mismatch and the difference of the thermal expansion coefficients between the components. The reduction of defect density due to ELO has been found to be approximately two orders of magnitude for dislocations (1 × 1010 cm-2 to 3 × 108 cm-2) and for stacking faults (1 × 106 cm-1 to 4 × 104 cm-1). In the ELO window areas, the threading dislocations have screw and mixed characters with Burgers vector b→ = 1/3 [1120] and b→ = 1/3 [1123] respectively. In the overgrown ELO areas, the dislocations have different characters such as screw character with b→ = 1/3 [1120], edge character with b→ = 1/3 [2110] and partial dislocations. Different types of stacking faults have been observed in the a-GaN film: basal (BSF) I1 and I2, prismatic (PSF) (1120) and (1010). The I2 BSF is bordered by two Shockley partial dislocations of Burgers vector b→ = 1/3 < 1010 > and its energy γ has been calculated: 50 ergs/cm2. Prismatic stacking faults, R→ = 1/2 [1101] are located at the end of the I1- BSFs, where stair rod dislocations are formed at the intersection of the two faults. In order to know the influence of these defects on the optical properties, cathodoluminescence (CL) and microphotoluminescence (µ-PL) experiments were performed. These observations demonstrate that the optical properties are affected by the high density of stacking faults. The CL spectra show 4 different emission peaks where the dominant emission at 3.42 eV is attributed to the I1 BSFs. In (µ-PL spectra, 4 emission peaks are also observed, where the dominant one at 3.44 eV in the window area of the ELO mask is also attributed to the SFs and that at 3.49 eV in the overgrown mask area is the typical NBE transition. This shows that locally the GaN film is of good quality. A single GaN quantum well (SQW) intercalated by two Al1-xGaxN layer were grown on HVPE-ELO templates by MBE. The structural characterization of six specimens have shown a high pits density on the sample surface (1 × 1012 cm-2) associated with the threading dislocations of screw character ending at the bottom of the pits. Moreover, the pits created at the intersection, near the surface, of several dislocations groups have been observed. A high density of BSFs (1 × 105 to 1 × 106 cm-1) was found in the samples, which are formed at Al1-xGaxN/GaN interface and propagated towards the specimen surface. We also observed that these stacking faults affect the optical properties of the SQW-GaN. Our results have shown that the inhomogeneity of the optical properties of a-(1120) GaN layers is associated with specific structural defects, in particular BSFs. These results provide guidelines to design new growth procedures aimed at improving the overall quality of such GaN films.

EPFL2009

Nanocomposites of carbon nanotubes embedded in a (Ti,Al)N coated film

Philippe Spätig, Christian Wäckerlin

Titanium aluminum nitride ((Ti,Al)N) thin films are widely used as hard protective coating materials. Due to their remarkable tensile strength and elastic modulus carbon nanotubes (CNTs) are an ideal candidate as reinforcing components in such coatings. In this work (Ti,Al)N thin film were deposited on steel substrate by reactive magnetron sputtering and CNTs were grown on top by chemical vapor deposition (CVD). A second (Ti,Al)N film covered the CNTs in order to achieve a nanocomposite structure similar to reinforced concrete. The chemical evolution of the substrate, structure, morphology and growth of CNTs were analyzed by X-ray photoelectron spectroscopy and scanning electron microscopy. It was possible to achieve homogeneous dispersion of the CNTs within the matrix by adjusting the CVD time. Mechanical testing of the adherence and nanohardness of the (Ti,Al)N/CNT/(Ti,Al)N composite coating were performed by scratch and nanoindentation tests. The improvement of mechanical properties by CNT integration resulted in an increase of the hardness and of Young's modulus. (C) 2012 Elsevier B.V. All rights reserved.