S. Vézian

Molecular Beam Epitaxy of Group‐III Nitrides on Silicon Substrates: Growth, Properties and Device Applications

We report on the growth and properties of GaN films grown on Si(111) substrates by molecular beam epitaxy using ammonia. The properties of the layers show that our growth procedure is very efficient in order to overcome the difficulties encountered during the growth of nitrides on silicon substrates: first, no nitridation of the silicon substrate is observed at the interface between the AIN buffer laver and the silicon surface: second. there is no Si autodoping coming from the substrate and resistive undoped GaN layers are obtained; and, also, strain balance engineering allows one to grow thick GaN epilayers (up to 3 mum) without formation of cracks. The optical, structural and electrical properties of these films are studied. In order to evaluate the potentialities of III-V nitrides grown on silicon substrates, we have grown heterostructures to realize light emitting diodes (LEDs), photodetectors and high electron mobility transistors (HEMTs).

Defect characterization in ZnO layers grown by plasma-enhanced molecular-beam epitaxy on (0001) sapphire substrates

The structural properties of high-quality (0001)ZnO/Al2O3 films grown by plasma-enhanced molecular-beam epitaxy are investigated by x-ray diffraction and transmission electron microscopy. The only defects encountered are threading dislocations with a density of 1010–4×1010 cm−2. Most numerous dislocations are pure-edge dislocations (Burgers vector of 1/3〈1120〉), which accommodate slight in-plane misorientations between subgrains. The oxygen polarity of these films is also established.

GaN grown on Si(111) substrate: From two-dimensional growth to quantum well assessment

We report on the epitaxial growth of high quality GaN films on Si(111) substrates by molecular beam epitaxy using ammonia. The surface morphology and crystallinity of thick undoped GaN films are characterized by reflection high-energy electron diffraction (RHEED), scanning electron microscopy, and x-ray diffraction. Films having compact morphologies and flat surfaces are obtained and RHEED intensity oscillations are demonstrated for GaN and (Al, Ga)N alloys indicating two-dimensional growth. This has been applied to the growth of AlGaN/GaN quantum well (QW) structures. Low-temperature photoluminescence (PL) spectra of GaN are dominated by a strong and narrow (full width at half maximum=5 meV) band edge luminescence intensity at 3.471 eV assigned to donor bound exciton recombination. PL properties of AlGaN/GaN QW are also very similar to those obtained on equivalent structures grown on sapphire.

Direct growth of few-layer graphene on 6H-SiC and 3C-SiC/Si via propane chemical vapor deposition

We propose to grow graphene on SiC by a direct carbon feeding through propane flow in a chemical vapor deposition reactor. X-ray photoemission and low energy electron diffraction show that propane allows to grow few-layer graphene (FLG) on 6H-SiC(0001). Surprisingly, FLG grown on (0001) face presents a rotational disorder similar to that observed for FLG obtained by annealing on (000–1) face. Thanks to a reduced growth temperature with respect to the classical SiC annealing method, we have also grown FLG/3C-SiC/Si(111) in a single growth sequence. This opens the way for large-scale production of graphene-based devices on silicon substrate.

Long wavelength GaInNAs/GaAs quantum-well heterostructures grown by solid-source molecular-beam epitaxy

We have investigated as-grown Ga1−xInxNyAs1−y/GaAs quantum-well heterostructures (QWHs) prepared by solid-source molecular-beam epitaxy (SS-MBE). We show that the QWH properties appear to depend strongly on the growth technique and that SS-MBE emerges as a technique of choice for growing these QWHs. We demonstrate photoluminescence emission at wavelength as long as 1.43 μm at 295 K, and up to 1.68 μm at 10 K. This shows that development of 1.55 μm optoelectronics based on the Ga1−xInxNyAs1−y/GaAs materials system may now be reasonably thought of.

Spectroscopy of excitons, bound excitons and impurities in h-ZnO epilayers

Photoluminescence (PL) and selective photoluminescence (SPL) experiments have been carried out to analyse the neutral donor bound exciton spectra of nominally undoped wurtzite ZnO epilayers and to study the electronic levels introduced by arsenic doping. The investigated samples were grown by molecular beam epitaxy on various substrates. The PL spectra of the undoped heterostructure layers exhibit little or no detectable deep emissions and are dominated by donor bound exciton recombination lines with distinct emission energies. The existence of residual strain distributions in the samples make possible the observation of resonant sharp lines due to the recombination of the bound excitons selectively created in their excited states. It is being shown that the first excited state spectra of the bound excitons consist of rotational states of the hole belonging to the (A) valence band. The PL spectra of ZnO: As samples present a new excitonic line as well as donor-acceptor pair emissions. The arsenic impurity is revealed to generate a shallow acceptor level in ZnO, an estimate of its binding energy is given.

Growth modes and microstructures of ZnO layers deposited by plasma-assisted molecular-beam epitaxy on (0001) sapphire

Transmission electron microscopy and high resolution x-ray diffraction are used to characterize defects in ZnO layers grown by plasma-assisted molecular-beam epitaxy on (0001) sapphire. Two- and three-dimensional types of growth modes are described and the observed mosaic structure is analyzed in each case. It is found that two-dimensional layers exhibit a roughness as low as 6 nm. Their subdomains have small lateral coherence lengths and a mean in-plane misorientation of ±0.4°, leading to an important dislocation density of 1–4×1010 cm−2. On the contrary, it is demonstrated that, through numerous interactions between dislocations, the three-dimensional growth mode leads to a better structural quality with a larger lateral coherence length and a smaller in-plane mosaic spread of ±0.07°. The total dislocation density is consequently reduced by 1 order of magnitude down to 3–5×109 cm−2 and the radical modification of the structure results in a change of the dislocation distribution. Our results thus demonst...

On the polarity of GaN micro- and nanowires epitaxially grown on sapphire (0001) and Si(111) substrates by metal organic vapor phase epitaxy and ammonia-molecular beam epitaxy

The polarity of GaN micro- and nanowires grown epitaxially by metal organic vapor phase epitaxy on sapphire substrates and by molecular-beam epitaxy, using ammonia as a nitrogen source, on sapphire and silicon substrates has been investigated. On Al2O3(0001), whatever the growth technique employed, the GaN wires show a mixture of Ga and N polarities. On Si(111), the wires grown by ammonia-molecular beam epitaxy are almost entirely Ga-polar (around 90%) and do not show inversion domains. These results can be understood in terms of the growth conditions employed during the nucleation stage.

Effects of pressure, temperature, and hydrogen during graphene growth on SiC(0001) using propane-hydrogen chemical vapor deposition

Graphene growth from a propane flow in a hydrogen environment (propane-hydrogen chemical vapor deposition (CVD)) on SiC differentiates from other growth methods in that it offers the possibility to obtain various graphene structures on the Si-face depending on growth conditions. The different structures include the (6√3 × 6√3)-R30° reconstruction of the graphene/SiC interface, which is commonly observed on the Si-face, but also the rotational disorder which is generally observed on the C-face. In this work, growth mechanisms leading to the formation of the different structures are studied and discussed. For that purpose, we have grown graphene on SiC(0001) (Si-face) using propane-hydrogen CVD at various pressure and temperature and studied these samples extensively by means of low energy electron diffraction and atomic force microscopy. Pressure and temperature conditions leading to the formation of the different structures are identified and plotted in a pressure-temperature diagram. This diagram, togeth...

Role of magnetic polarons in ferromagnetic GdN

We report an interplay between magnetism and charge transport in the ferromagnetic semiconductor GdN, pointing to the formation of magnetic polarons centred on nitrogen vacancies. The scenario goes some way to resolving a long-standing disagreement between the measured and predicted Curie temperature in GdN. It further constitutes an extension of concepts that relate closely to the behaviour of ferromagnetic semiconductors generally, and EuO in particular.