T. Moudakir

Nanometer-scale, quantitative composition mappings of InGaN layers from a combination of scanning transmission electron microscopy and energy dispersive x-ray spectroscopy

Using elastic scattering theory we show that a small set of energy dispersive x-ray spectroscopy (EDX) measurements is sufficient to experimentally evaluate the scattering function of electrons in high-angle annular dark field scanning transmission microscopy (HAADF-STEM). We then demonstrate how to use this function to transform qualitative HAADF-STEM images of InGaN layers into precise, quantitative chemical maps of the indium composition. The maps obtained in this way combine the resolution of HAADF-STEM and the chemical precision of EDX. We illustrate the potential of such chemical maps by using them to investigate nanometer-scale fluctuations in the indium composition and their impact on the growth of epitaxial InGaN layers.

Distributed Bragg reflectors based on diluted boron-based BAlN alloys for deep ultraviolet optoelectronic applications

Highly reflective deep UV distributed Bragg reflectors (DBRs) based on the BAlN material system have been grown by metalorganic vapour phase epitaxy on AlN template substrates. These structures make use of the transparency of BAlN in the deep UV and the high refractive index contrast between BAlN and AlN, which has been demonstrated to exceed 0.27 at 280 nm. 18-pair BAlN/AlN DBRs showed experimental peak reflectivity of 82% at 311 nm and a stop-bandwidth of 20 nm. At 282 nm, a 24-pair BAlN/AlN DBR structure is demonstrated with experimental peak reflectivity of 60% and stop-bandwidth of 16 nm.

Characteristics of the surface microstructures in thick InGaN layers on GaN

This paper focuses on a comparative study of optical, morphological, microstructural and microcompositional properties of typical InGaN samples which exhibit V-defects but also two additional surface defects features, referred to as inclusion#1 (Ic1) and inclusion#2 (Ic2). HR-XRD, AFM, SEM, STEM and EDX are used to characterize such defects. Furthermore, hyperspectral mapping, spot mode and depth-resolved CL measurements provided useful informations on the optical emission properties and microstructure. The main characteristic of Ic1 luminescence peak is a decrease in intensity and no obvious shift in the CL peak position when going from the outside to the middle of such defect. More interesting was Ic2 which is shown to be local 3D top surface In-rich InGaN domains embedded in an homogeneous InGaN matrix. In fact, this study pointed out that close to the interface GaN/InGaN, it exists a 30 nm thick fully strained InGaN layer with constant indium incorporation. As the growth proceeds spatial fluctuation of the In content is observed and local In-rich 3D domains are shown to emerge systematically around threading dislocations terminations.

Solar blind metal-semiconductor-metal ultraviolet photodetectors using quasi-alloy of BGaN/GaN superlattices

Large internal gains that can be obtained in wide band gap semiconductors-based (GaN and ZnO types) Schottky and/or metal-semiconductor-metal photodetectors are generally accompanied by large dark current and time response. We show that, using quasi-alloy of BGaN/GaN superlattices as the active layer, the dark current can be lowered while maintaining high internal gain (up to 3 × 104) for optical power in the nW range and low time response (few tens of ns) for optical power in the W range. Furthermore, the boron incorporation allows the tuning of the cutoff wavelength.

Dual-purpose BGaN layers on performance of nitride-based high electron mobility transistors

A GaN/ultrathin BGaN/GaN heterojunction is used in AlGaN/GaN high electron mobility transistors (HEMTs) to provide an electrostatic barrier to electrons and to improve the confinement of the 2-dimensional electron gas. BGaN back-barrier layers limit leakage in the GaN buffer thanks to two effects: a polarization-induced band discontinuity and a resistive barrier originating from excellent insulation properties of BGaN. Compared to conventional AlGaN/GaN HEMTs, structures grown with BGaN back-barrier showed a significant improvement of static performances, transport properties, and trapping effects involving a limited current collapse in dynamic regime. A DC maximum current increase of 58.7% was observed.

Design, Fabrication, and Characterization of Near-Milliwatt-Power RCLEDs Emitting at 390 nm

We report on the realization and first demonstration of CW near-milliwatt-power emission at λ = 390 nm from resonant-cavity light-emitting diode (RCLED) on GaN templates. The vertical cavity consists of a bottom AlGaN/GaN distributed Bragg reflector and a top dielectric SiO2/ZrO2 mirror enclosing a GaInN/GaN multiple-quantum-well active layer. RCLEDs with total optical output of about 600 μW at an injection current of 20 mA were achieved before packaging, taking account of current growth and processing considerations. Dislocations generated during the growth of the RCLED structure seem to be affecting the mean light output. This can be further improved by the use of high-quality low-dislocation-density GaN templates or freestanding GaN substrates.

Novel process for direct bonding of GaN onto glass substrates using sacrificial ZnO template layers to chemically lift-off GaN from c-sapphire

GaN was grown on ZnO-buffered c-sapphire (c-Al2O3) substrates by Metal Organic Vapor Phase Epitaxy. The ZnO then served as a sacrificial release layer, allowing chemical lift-off of the GaN from the c-Al2O3 substrate via selective wet etching of the ZnO. The GaN was subsequently direct-wafer-bonded onto a glass substrate. X-Ray Diffraction, Scanning Electron Microscopy, Energy Dispersive X-ray microanalysis, Room Temperature Photoluminescence & optical microscopy confirmed bonding of several mm2 of crack-free wurtzite GaN films onto a soda lime glass microscope slide with no obvious deterioration of the GaN morphology. Using such an approach, InGaN based devices can be lifted-off expensive single crystal substrates and bonded onto supports with a better cost-performance profile. Moreover, the approach offers the possibility of reclaiming and reusing the substrate.

AlGaN-based MQWs grown on a thick relaxed AlGaN buffer on AlN templates emitting at 285 nm

We report on the growth of Al0.57Ga0.43N/Al0.38Ga0.63N MQWs grown on a relaxed Al0.58Ga0.42N buffer on AlN template by Metal Organic Vapor Phase Epitaxy. The MQW structure is designed so that the strain in the quantum wells induced by their lattice mismatch with barriers is sufficient to enhance TE polarized emission (E-field ⊥ c). A 630-nm thick relaxed Al0.58Ga0.42N buffer grown on AlN template serves as a pseudo-substrate to release the strain in the barriers and to avoid related defects or composition fluctuation in the active region. Thin (< 2 nm) quantum wells allow preservation of the overlapping of electron and hole wavefunctions considering the strong quantum-confined Stark effect in AlGaN-based MQW structures. Scanning transmission electron microscopy (STEM) coupled to energy-dispersive X-ray spectroscopy (EDX) analysis is used to optimize the growth conditions and to determine the composition of wells and barriers. Optical characterizations of the grown structure reveal a well-defined band-edge emission peak at 285 nm. Based on macro-transmission measurements and simulations, the absorption coefficient of the wells is estimated to be 3 × 105 cm−1 (E-field ⊥ c), attesting that the oscillator strength is preserved for these AlGaN MQWs with high Al content, which is promising for efficient surface-emitting devices in the deep ultra-violet (DUV) region.

Wafer-scale epitaxial lift-off of optoelectronic grade GaN from a GaN substrate using a sacrificial ZnO interlayer

Full 2 inch GaN epilayers were lifted off GaN and c-sapphire substrates by preferential chemical dissolution of sacrificial ZnO underlayers. Modification of the standard epitaxial lift-off (ELO) process by supporting the wax host with a glass substrate proved key in enabling full wafer scale-up. Scanning electron microscopy and x-ray diffraction confirmed that intact epitaxial GaN had been transferred to the glass host. Depth-resolved cathodoluminescence (CL) analysis of the bottom surface of the lifted-off GaN layer revealed strong near-band-edge (3.33 eV) emission indicating a superior optical quality for the GaN which was lifted off the GaN substrate. This modified ELO approach demonstrates that previous theories proposing that wax host curling was necessary to keep the ELO etch channel open do not apply to the GaN/ZnO system. The unprecedented full wafer transfer of epitaxial GaN to an alternative support by ELO offers the perspective of accelerating industrial adoption of the expensive GaN substrate through cost-reducing recycling.

Link between crystal quality and electrical properties of metalorganic vapour phase epitaxy InxGa1−xN thin films

We report on the crystal quality of metalorganic vapour phase epitaxy-grown InGaN with indium content ranging from 0% to 20%. Absorbance measurements are fit to a model including band tails and a defect represented as a Brendel oscillator (R. Brendel, Appl. Phys. A 50, 587, 1990). Band tail absorbance, corresponding to contorted bonds, increases with increased In content. Above 10% of In, the presence of another defect, the concentration of which increases with In content, has been correlated with x-ray diffraction and Raman. We suggest that this defect corresponds to nitrogen vacancies, in agreement with a reported model for GaN.