Yves Monteil

Infrared kinetic study of ultrathin SiC buffer layers grown on Si(100) by reactive chemical vapour deposition

Abstract A kinetic study was carried out on the growth of an SiC buffer layer on Si(100) by reactive chemical vapour deposition. Experiments were performed at temperatures in the range 1150–1300 °C for 1 to 45 min using C 3 H 8 and H 2 as gas reactants. Infrared transmittance spectrometry was used for accurate film thickness determination (down to 1.2 nm). The growth profiles as a function of time show a four-step mechanism involving the rapid formation of an SiC “thermal layer” by coalescence of SiC islands. The thickness increases by Si out-diffusion through this layer until a critical thickness, controlled by the temperature and Si etching, is reached. Only the initial values of the diffusion profiles can be fitted by Ficks second law. The hypothesis of silicon etching by H 2 is confirmed by thermodynamic calculations. The etching activation energy is E e = 4.4 eV. The temperature dependence of the resulting diffusion coefficients gives an apparent diffusion activation energy of E d = 4.5 eV. The close agreement between these two activation energies illustrates the competition between the two mechanisms deduced from the growth profiles.

Growth and characterization of high quality BxGa1−xAs/GaAs(0 0 1) epilayers

In a first step towards the growth of BInGaAs, we have grown and characterized the B x Ga 1-x As/GaAs ternary compound with boron composition up to x = 0.06 on GaAs(0 0 1) vicinal substrates. The incorporation behavior of boron has been studied as a function of growth temperature, diborane flux, gallium precursor and carrier gas (hydrogen and nitrogen). A maximum for boron incorporation (x 0.04-0.06) is found at 550-600°C depending on the precursor and the carrier gas. The epilayers have good crystalline quality as measured by X-ray rocking curve of the (004) diffraction peak (full-width at half-maximum of 38 arcsec for x = 0.035). However, the surface morphology is very sensitive to the diborane supersaturation in the gas phase. At high diborane flow rate, the surface appears as though it is covered in dust. A low surface roughness of 0.4 nm was measured by atomic force microcopy (AFM) in the best growth conditions. AFM images also show a cross-hatch pattern for the highest boron composition.

A comprehensive study of SiC growth processes in a VPE reactor

Abstract We performed an experimental study of the effect of the gas phase composition on the growth mechanism of 3C-SiC on Si(100) by atmospheric-pressure vapour phase epitaxy at 1350°C. Silane and propane diluted in hydrogen were used as precursors for the growth. We demonstrate the existence of an equilibrium partial pressure of carbon above the growing surface, which ensures a mirror-like morphology. For too low a carbon partial pressure (C/Si ratio in the gas phase lower than 2.7 with a growth rate of 3 μm h −1 ), the layer morphology and crystalline quality quickly degrade. For too high a carbon partial pressure (C/Si ratio higher than 5 with the same growth rate), SiC clusters form on the growing layers. We propose a mechanism of formation for these clusters taking into account the interactions between the C and Si species in the hot boundary layer.

Hexamethyldisilane/propane versus silane/propane precursors: application to the growth of high-quality 3C–SiC on Si

From a comparative evaluation of hexamethyldisilane (HMDS) and silane–propane (SP) precursor systems, it is shown that HMDS needs a small addition of propane to deposit heteroepitaxial layers of 3C–SiC on Si with superior crystalline properties. In this case, propane compensates for the secondary reactions induced by hydrogen reacting with carbon. Using atmospheric pressure chemical vapour deposition conditions, the new system (HMDS–propane) demonstrates several advantages. It is safer to handle than SP and allows a higher growth rate (up to 7 µm h−1 at 1350 °C) without any degradation of the layer morphology. However, when lowering the deposition temperature, HMDS is revealed to be more stable than silane. This is in contrast to most standard beliefs but explains why a high temperature (~1350 °C) is always necessary to grow high-quality material using HMDS.

On the growth of 4H–SiC by low-temperature liquid phase epitaxy in Al rich Al–Si melts

Abstract The growth of 4H–SiC by low-temperature liquid phase epitaxy was studied in Al–Si melts. The temperature ranged from 1000°C to 1200°C. Some problems, which were sources of non-homogeneity of the growth or low reproducibility of the process, were identified and reviewed: (1) local delayed wetting of the seed by the melt, (2) morphological (3) presence of alumina particles on the liquid, (4) high reactivity of the melt with graphite at temperature above 1200°C, (5) formation of crystallites on the surface upon cooling. The solutions proposed to avoid or limit these problems are: (1) deposition of a Si layer before the growth, (2) careful backside gluing, (3) a two-step procedure involving the pre-dipping in the melt of a graphite rod on which alumina particles agglomerate, (4) growth at temperature lower than 1200°C. No solution was found to avoid the crystallites formation upon cooling.

Structural properties of undoped and doped cubic GaN grown on SiC(001)

Transmission electron microscopy and x-ray diffraction measurements reveal the presence of stacking faults (SFs) in undoped cubic GaN thin layers. We demonstrate the importance of the defects in the interfacial region of the films by showing that the SFs act as nucleation sites for precipitates of residual impurities such as C and Si present in the GaN layers grown on SiC(001) substrates. We used the imaging secondary ion mass spectroscopy technique to locate these impurities. The systematic decrease of the SF density as a function of the layer thickness is explained by an annihilation mechanism. Finally, the effects of usual dopants on the structural properties of GaN layers are discussed. It is shown that Mg has a tendency to incorporate out of the Ga site by forming Mg precipitates for a concentration higher than 1019 cm−3 in contrast with the results found for heavily Si doped layers.

Raman investigations of reaction process in MOVPE

Although metalorganic vapour phase epitaxy (MOVPE) has been developed to a large extent, very little is known on reaction mechanism occuring in the MOVPE reactor. A non intrusive non destructive method is needed to investigate the flow dynamics and the reaction taking place. Raman spectroscopy has proved to be a useful tool to establish the temperature distribution from the rotation modes of H2. The nature and concentration of the gaseous species can also be determined. Temperature distribution inside a standard vertical MOVPE reactor shows a huge gradient and a singularity near the interface, the thermal boundary layer δT. Using He as a carrier gas does not change significantly the temperature pattern but N2 gives a completely different temperature distribution: for the same flow velocities δT is much narrower. Experiments in closed tubes on mixtures TMG-AsH3 pointed out the existence of a solid complex between TMG and AsH3 in which an As-Ga bond is identified. Thickness and composition uniformity in the GaAlAs layer are correlated with the slight radial temperature gradient of H2 observed in a vertical reactor above the susceptor.

Radiative N-localized recombination and confinement in GaAsN/GaAs epilayers and quantum well structures

Abstract The photoluminescence (PL) properties of GaAsN/GaAs epilayers and single quantum wells (QWs) have been investigated as a function of the excitation density and the sample temperature (10–300 K). At low temperatures, the PL spectra were sensitive to the excitation density for epilayers and QWs. For both structures, a blue shift of the PL peak is noted with increasing the excitation power. In contrast, the temperature dependence shows different behaviors for the bulk epilayers and for the quantum wells structures. An S-shape of the PL peak energy versus temperature has been observed for the GaAsN/GaAs epilayer while the QWs peak energy decreases monotically with the sample temperature and could be fitted by conventional Varshni’s law. This behavior is due to the exciton localization effect which is induced by the local fluctuation of nitrogen concentration.

Surface segregation of boron in BxGa1-xAs/GaAs epilayers studied by x-ray photoelectron spectroscopy and atomic force microscopy

The behavior of boron incorporation into GaAs has been studied by x-ray photoelectron spectroscopy, x-ray diffraction, and atomic force microscopy. As the boron content of the film was increased, both the characteristic peak for the B 1s core level at 188 eV and As Auger transition (260 eV) could be detected by XPS. At 550–600 °C, single crystalline films could only be grown for x⩽0.06. Upon increasing the diborane flux in the gas phase, the film stoichiometry and the boron surface composition evolved rapidly towards a boron-rich subarsenide compound. This trend is followed by a clear degradation of the surface morphology and an increase in the surface roughness. A surface segregation of boron is suggested due to the high diborane vapor supersaturation needed during growth.

Optical properties of InP/InAlAs/InP grown by MOCVD on (100) substrates: influence of V/III molar ratio

Abstract We report on the optical study of double heterostructures InP/InAlAs/InP grown on (100) substrates at various V/III molar ratios, grown by metal-organic chemical vapor phase epitaxy (MOCVD). Photoluminescence experiments have been performed to investigate the optical transitions in these samples. In order to identify the optical transition at inverted interface (InP grown on InAlAs) at 1.25 eV, previously assigned to a type II interface band structure, a study based on transfer matrix technique under the envelope function approximation has been performed. Taking into account the formation of InAsxP1−x thin graded layer located at the inverted interface (between InP and InAlAs), a simulation indicates that this recombination originates in fact from a type II interface band structure between electrons confined on InAsP and holes located on top of the InAlAs valence band. Detailed studies of photoluminescence spectroscopy (PL) properties of InAlAs epilayer at low temperature indicate that the PL spectrum, recorded at low temperature (12 K) on (100) substrate sample with V/III=50, shows double effects: the presence of a clustering and of a natural superlattice (SL) due to composition modulation. For other samples with different V/III, the composition modulation disappears so that only a clustering effect remains.