The epitaxy of MoO2 on c_plane sapphire substrates is examined. A theoretical approach, based on density functional theory calculations of the strain energy, allowed to predict the preferred layer/substrate epitaxial relationships. To test the result
s of these calculations, MoO2/(001) Al2O3 heterostructures were grown using the chemically_driven isothermal close space vapour transport technique. At the early stages of the growth, two kinds of morphologies were obtained, using the same growth parameters: lying and standing flakes. The composition and morphology, as well as the layer/substrate epitaxial relationships were determined for both kind of morphologies. Experimental epitaxial relationships coincide with those predicted by DFT calculation as the most favourable ones in terms of strain energy. For thicker films, the standing flakes evolve to form an epitaxial porous layer composed by coalesced epitaxial flakes. The interfacial strain between the sapphire substrate and MoO2 enables a self_organization from nanometer to micron scales between separated or coalesced flakes, depending on deposition condition.
Chemically _ driven isothermal close space vapour transport was used to prepare pure MoO2 films which were eventually converted to MoO3 by annealing in air. According to temperature_dependent Raman measurements, the MoO2/MoO3 phase transformation was
found to occur in the 225 _ 350 oC range; no other phases were detected during the transition. A clear change in composition and Raman spectra, as well as noticeable modifications of the band gap and the absorption coefficient confirmed the conversion from MoO2 to MoO3. An extensive characterization of films of both pure phases was carried out. In particular, a procedure was developed to determine the dispersion relation of the refractive index of MoO2 from the shift of the interference fringes the used SiO2/Si substrate. The obtained refractive index was corrected taking into account the porosity calculated from elastic backscattering spectrometry. The Debye temperature and the residual resistivity were extracted from the electrical resistivity temperature dependence using the Bloch _ Gruneisen equation. MoO3 converted samples presented very high resistivity and a typical semiconducting behaviour. They also showed intense and broad luminescence spectra, which were deconvoluted considering several contributions; and its behaviour with temperature was examined. Furthermore, surface photovoltage spectra were taken and the relation of these spectra with the photoluminescence is discussed.
The mechanisms controlling the growth rate and composition of epitaxial CdTe and CdZnTe films were studied. The films were grown by isothermal closed space configuration technique. A GaAs 100 substrate was exposed sequentially to the elemental source
s, Zn, Te, and Cd, in isothermal conditions. While growth of ZnTe followed an atomic layer epitaxy, ALE, regime self regulated at one monolayer per cycle; the CdTe films revealed different growth rates in dependence of the growth parameters,exposure and purge times. Combination of short purge times and larger Cd exposure times led to not self regulated growth regime for CdTe. This is ascribed to large Cd coverages that were dependent on Cd exposure times, following a Brunauer-Emmett and Teller-type adsorption. However, for longer purge times and or short Cd exposure times, an ALE self regulated regime was achieved with 2 ML per cycle. In this sense, the self-regulation of the growth is limited by desorption, instead of absorption, as in the traditional growth technique. Cd atoms substitution by Zn atoms and subsequent evaporation of surface Cd atoms during Zn exposure has been proved. The influence of these facts on the growth and composition of the alloy is discussed.
A detailed reflection high-energy electron diffraction analysis shows relevant features of the lattice parameter relaxation of CdSe thin films grown in a layer-by-layer mode onto ZnSe. In situ investigations of different azimuths show a clear lattice
parameter oscillation in the 110 azimuth. The lattice parameter has a minimum value ~similar to that of ZnSe! during Se exposure steps, and a higher and increasing lattice parameter during Cd exposure steps. The behavior is ascribed to the formation of CdSe islands during Cd exposure steps. The cumulative effect in CdSe exposure steps is considered to be a consequence of a decrease in the island size with the number of cycles. Actual plastic deformation does occur after 5 ML.
Porous silicon layers were embedded with ZnTe using the isothermal close space sublimation technique. The presence of ZnTe was demonstrated using cross-sectional energy dispersive spectroscopy maps. ZnTe embedded samples present intense room temperat
ure photoluminescence along the whole visible range. We ascribe this PL to ZnTe nanocrystals of different sizes grown on the internal pore surface. Such crystals, with different orientations and sizes, were observed in transmission electron microscopy images, while transmission electron diffraction images of the same regions reveal ZnTe characteristic patterns.
A novel procedure, based in a closed space vapor transport (CSVT) configuration, has been devised to grow films or flakes of pure MoO2 in a reductive atmosphere, at relatively low temperature and using MoO3 as the source. In contrast with conventiona
l CSVT technique, in the proposed method a temperature gradient is not required for the growth to take place, which occurs through an intermediate volatile transport species that is produced in the complex reduction reaction of MoO3. An added value of this simple method is the possibility of transforming the MoO2 into MoTe2, one of the most interesting members of the transition metal dichalcogenide family. This is achieved in a sequential process that includes the growth of Mo oxide and its (in-situ) tellurization in two consecutive steps.
