F. Semond

Temperature quenching of photoluminescence intensities in undoped and doped GaN

This work discusses the temperature behavior of the various photoluminescence (PL) transitions observed in undoped, n- and p-doped GaN in the 9–300 K range. Samples grown using different techniques have been assessed. When possible, simple rate equations are used to describe the quenching of the transitions observed, in order to get a better insight on the mechanism involved. In undoped GaN, the temperature dependence of band edge excitonic lines is well described by assuming that the A exciton population is the leading term in the 50–300 K range. The activation energy for free exciton luminescence quenching is of the order of the A rydberg, suggesting that free hole release leads to nonradiative recombination. In slightly p-doped samples, the luminescence is dominated by acceptor related transitions, whose intensity is shown to be governed by free hole release. For high Mg doping, the luminescence at room temperature is dominated by blue PL in the 2.8–2.9 eV range, whose quenching activation energy is in...

From visible to white light emission by GaN quantum dots on Si(111) substrate

GaN quantum dots (QDs) in an AlN matrix have been grown on Si(111) by molecular-beam epitaxy. The growth of GaN deposited at 800 °C on AlN has been investigated in situ by reflection high-energy electron diffraction. It is found that a growth interruption performed at GaN thicknesses larger than three molecular monolayers (8 A) instantaneously leads to the formation of three-dimensional islands. This is used to grow GaN/AlN QDs on Si(111). Depending on their sizes, intense room-temperature photoluminescence is observed from blue to orange. Finally, we demonstrate that stacking of QD planes with properly chosen dot sizes gives rise to white light emission.

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).

High-electron-mobility AlGaN/GaN heterostructures grown on Si(111) by molecular-beam epitaxy

We report on the growth of high-electron-mobility AlGaN/GaN heterostructures on silicon (111) substrates by molecular-beam epitaxy using ammonia as the nitrogen source. Crack-free GaN layers up to 3 μm are obtained. Their optical properties are similar to those commonly obtained for films grown on sapphire, but photoluminescence spectra indicate that GaN on Si(111) is in a tensile strain state which increases with the epitaxial layer thickness. Such uncracked GaN buffer layers grown on Si(111) have been used to achieve undoped AlGaN/GaN heterostructures having electron mobilities exceeding 1600 cm2/V s at room temperature and 7500 cm2/V s at 20 K.

Growth of high quality crack-free AlGaN films on GaN templates using plastic relaxation through buried cracks

A method is presented to achieve thick high quality crack-free AlGaN layers on GaN. This method uses jointly plastic relaxation and lateral growth. In a first step, plastic relaxation by cracking and misfit dislocation introduction is realized. Then the cracks are overgrown to obtain a smooth surface. By this reproducible technique, we grew smooth metal-organic chemical vapor deposition Al0.2Ga0.8N films with a threading dislocation density as low as 5×108 cm−2. This result is the best ever reported for crack-free AlGaN growth over a large area. The control of the complete plastic relaxation opens up perspectives for the realization of high performance devices. In order to explain the mechanisms involved in the full relaxation of the AlGaN/GaN heterostructure, we propose a relaxation scheme and discuss its different steps.

Determination of the refractive indices of AlN, GaN, and AlxGa1−xN grown on (111)Si substrates

The refractive indices of several AlxGa1−xN alloys deposited on silicon are determined by ellipsometry and reflectivity experiments at room temperature. The AlGaN layers are grown on (111)Si substrate by molecular-beam epitaxy on top of an AlN/GaN/AlN buffer in order to reduce the strain of the alloy. The Al composition is deduced from energy dispersive x-ray spectroscopy and photoluminescence experiments. The refractive index n and the extinction coefficient k are determined in the 300–600 nm range. For the transparent region of AlxGa1−xN, the refractive index is given in form of a Sellmeier law.

GaN evaporation in molecular-beam epitaxy environment

GaN(0001) thick layers were grown on c-plane sapphire substrates by molecular-beam epitaxy using NH3. The evaporation of such GaN layers in vacuum was studied as a function of substrate temperature. In situ laser reflectivity was used to quantitatively measure the decomposition rate of the GaN(0001) plane. It is nearly zero below 750 °C, increases rapidly above 800 °C, and reaches 1 μm/h at 850 °C. An activation energy of 3.6 eV is deduced for the thermal decomposition of GaN in vacuum. The evaporation rate as a function of the incident NH3 flux was also investigated for different substrate temperatures. A kinetic model is applied for the interpretation of the experimental results.

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.

Valence band offset of the ZnO/AlN heterojunction determined by x-ray photoemission spectroscopy

The valence band offset of ZnO/AlN heterojunctions is determined by high resolution x-ray photoemission spectroscopy. The valence band of ZnO is found to be 0.43±0.17 eV below that of AlN. Together with the resulting conduction band offset of 3.29±0.20 eV, this indicates that a type-II (staggered) band line up exists at the ZnO/AlN heterojunction. Using the III-nitride band offsets and the transitivity rule, the valence band offsets for ZnO/GaN and ZnO/InN heterojunctions are derived as 1.37 and 1.95 eV, respectively, significantly higher than the previously determined values.

Polariton lasing in a hybrid bulk ZnO microcavity

We demonstrate polariton lasing in a bulk ZnO planar microcavity under non-resonant optical pumping at a small negative detuning (δ ∼ −1/6 the 130 meV vacuum Rabi splitting) and a temperature of 120 K. The strong coupling regime is maintained at lasing threshold since the coherent nonlinear emission from the lower polariton branch occurs at zero in-plane wavevector well below the uncoupled cavity mode. The contribution of multiple localized polariton modes above threshold and the non-thermal polariton statistics show that the system is in a far-from-equilibrium regime, likely related to the moderate photon lifetime and in-plane photonic disorder in the cavity.