Saphir | EPFL Graph Search

vignette|redresse|Star of India ("Étoile de l'Inde"), New-York, musée américain d'histoire naturelle. Il présente un astérisme remarquable. Le saphir est une pierre précieuse. C'est une variété gemme du corindon pouvant présenter de multiples couleurs, sauf la couleur rouge (le corindon rouge est uniquement appelé rubis). On doit à Jacques Louis de Bournon d’avoir montré que le saphir (gemme orientale) et le corindon (spath adamantin) ne font qu’un. Étymologie Selon Ferdinand Hœfer, On trouve également des saphirs de couleur rose, verte, jaune, violette et d'autres qui sont incolores ou avec d'autres tons. La variété bleue est la plus connue. La teinte rose-violacé ou rose-orangé, appelée saphir « Padparadja » ou « Padparadscha » (traduit du cinghalais par « fleur de lotus »), est la plus rare et la plus recherchée. Structure Les saphirs sont constitués de cristaux d'oxyde d'aluminium (Al2O3) contenant des impuretés (oxydes) en traces qui leur donnent leur co

Non-polar GaN epilayers and heterostructures for photonic applications

Tiankai Zhu

This PhD thesis describes the experimental study of wurtzite III-nitride semiconductors grown on non-polar crystal orientations, namely (1120) a- and (1010) m-planes. Hindered by poor material quality, they were not as extensively investigated as the polar basal (0001) c-plane. Nevertheless, the renewed interest in this topic has been gaining momentum since 2000, thanks to the improvement in material quality. The motivation is to overcome the quantum-confined Stark effect (QCSE) in heterostructures grown on c-plane, which is induced by the intrinsic spontaneous and piezoelectric polarization field. QCSE weakens the performance of c-plane optoelectronic devices such as light emitting diodes (LEDs) and laser diodes and it can be avoided by growing these devices on non-polar planes. In addition, non-polar LEDs are also featured by the spontaneously polarized light emission and thus have a large potential for liquid crystal display backlighting application. This thesis includes two aspects: (1) Since previous study in our laboratory was lacking before this work, the first target was to establish the heteroepitaxial growth procedures for a- and m-plane GaN layers on r- and m-plane sapphire substrates, respectively, by hydride vapor phase epitaxy (HVPE). Then, smooth planar templates have been obtained. Particularly, m-plane GaN has been demonstrated on m-plane sapphire. Further analysis revealed that actually three orientations (two semi-polar and one non-polar) can be grown on m-plane sapphire through the modification of sapphire surface chemistry and/or growth procedure. Nevertheless, the material quality of these non-polar templates is inferior to that of c-plane as they are characterized by the presence of high-density stacking faults and threading dislocations. To reduce the defect density, epitaxial lateral overgrowth (ELO) technique has been applied. As a result, material quality close to that of planar c-plane was achieved in GaN layers overgrown on the mask areas. Overall, the experimental results revealed peculiar characteristics of non-polar orientation, induced by the intrinsic anisotropy of wurtzite structure parallel and perpendicular to the basal plane. It also lays the foundation for further improvement in material quality. (2) Based on these aforementioned results, the next stage was to investigate the heterostructures grown on those HVPE ELO templates. GaN/AlGaN single quantum wells (QWs) and AlN/GaN distributed Bragg reflectors (DBR) have been grown by NH3-source molecular beam epitaxy (MBE). The experiments confirm that QWs are free from QCSE and their optical properties are comparable to those grown on planar c-plane templates. Furthermore, nitride-based microcavities (MCs) have been demonstrated on a-plane ELO templates. Their optical properties are encouraging and open the way for more sophisticated structure aimed at polariton physics. In conclusion, non-polar a- and m-plane GaN planar and ELO templates have been obtained through epitaxy on r- and m-plane sapphire substrates, respectively, by HVPE. Nanostructures such as QWs, DBRs, and MCs have been grown on non-polar HVPE GaN ELO templates by MBE.

EPFL2009

Wafer-scale MoS2 with water-vapor assisted showerhead MOCVD

Hyungoo Ji, Andras Kis, Michal Daniel Macha, Aleksandra Radenovic, Mukeshchand Thakur, Mukesh Kumar Tripathi, Yanfei Zhao

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.

ROYAL SOC CHEMISTRY2022

Stress control in GaN/sapphire templates for the fabrication of crack-free thick layers

Nicolas Grandjean, Marc Ilegems, Denis Martin

The reduction of the tensile stress contained in GaN layers grown oil sapphire by metalorganic vapor phase epitaxy (MOVPE) is achieved using a low density of initial GaN crystallites. The template layers exhibit a significant reduction in dislocation density, down to 5 x 10(7) cm(-2) in similar to 6 mu m thick MOVPE GaN templates. The grain size and their density are controlled during the low temperature nucleation step of GaN on sapphire. Subsequently, very thick (up to similar to 200 mu m) crack-free GaN layers were Successfully grown by hydride vapor phase epitaxy (HVPE). A direct correlation between the stress state of the GaN/sapphire template prepared by MOVPE and the critical thickness for crack appearance during HVPE growth thickening is presented. (c) 2005 Elsevier B.V. All rights reserved.

2006

Non-polar GaN epilayers and heterostructures for photonic applications

Tiankai Zhu

EPFL2009