Aimeric Courville

Polar and semipolar GaN/Al0.5Ga0.5N nanostructures for UV light emitters

AlxGa1−xN-based ultra-violet (UV) light emitting diodes (LEDs) are seen as the best solution for the replacement of traditional mercury lamp technology. By adjusting the Al concentration, a large emission spectrum range from 360 nm (GaN) down to 200 nm (AlN) can be covered. Owing to the large density of defects typically present in AlxGa1−xN materials usually grown on sapphire substrates, LED efficiencies still need to be improved. Taking advantage of the 3D carrier confinement, quantum dots (QDs) are among the solutions currently under investigation to improve the performances of UV LEDs. The objectives of this work are to present and discuss the morphological and optical properties of GaN nanostructures grown by molecular beam epitaxy on the (0 0 0 1) and the (11–22) orientations of Al0.5Ga0.5N. In particular, the dependence of the morphological properties of the nanostructures on the growth conditions and the surface orientation will be presented. The optical characteristics as a function of the nanostructure design (size, shape and dimensionality) will also be shown and discussed. The electroluminescence characteristics of a first series of QD-based GaN/Al0.5Ga0.5N LEDs grown on the polar (0 0 0 1) plane will be investigated.

GaN/Al0.5Ga0.5N (11-22) semipolar nanostructures: A way to get high luminescence efficiency in the near ultraviolet range

The epitaxial growth of GaN/Al0.5Ga0.5N (11-22) semipolar nanostructures and their structural and optical properties are reported. The nanostructure formation results from a strain induced growth process (Stransky-Krastanov-like growth mode). Atomic force microscopy measurements show that depending on the amount of deposited GaN, the nanostructure shape evolves from an island shape to a string shape aligned along the [1-100] direction. Transmission electron microscopy experiments reveal that (11-20) and (11-23) lateral facets are formed, making with the (11-22) growth plane an angle of 32° and 12°, respectively, and giving a very asymmetric nanostructure shape. Photoluminescence (PL) experiments as a function of the excitation power and temperature show that the internal electric field is very low compared to the case of GaN/Al0.5Ga0.5N (0001) polar quantum dots (QDs). As a consequence, the PL emission is strongly shifted towards the UV range compared to polar QDs and the full width at half maximum of the...

Photo-induced droop in blue to red light emitting InGaN/GaN single quantum wells structures

The variation of the internal quantum efficiency (IQE) of single InGaN quantum well structures emitting from blue to red is studied as a function of the excitation power density and the temperature. By changing the well width, the indium content, and adding a strain compensation AlGaN layer, we could tune the intrinsic radiative recombination rate by changing the quantum confined Stark effect, and we could modify the carrier localization. Strong quantum confined Stark effect and carrier localization induce an increase in the carrier density and then favor Auger non-radiative recombination in the high excitation range. In such high excitation conditions with efficient Auger recombination, the variation of the IQE with the photo-excitation density P is ruled by a universal power law independent of the design: IQE = IQEMAX – a log10P with a close to 1/3. The temperature dependences of the different recombination mechanisms are determined. At low temperature, both quantum confined Stark effect and carrier localization trigger electron-electron repulsions and therefore the onset of the Auger effect. The increase in the value of coefficient C with changing temperature reveals indirect Auger recombination that relates to the interactions of the carriers with other phonons than the longitudinal optical one.

Impact of sapphire nitridation on formation of Al-polar inversion domains in N-polar AlN epitaxial layers

In this work, we study the basic processes during the initial stages of growth which control polarity in N-polar AlN films grown on c-plane sapphire substrates by metalorganic chemical vapor deposition. More specifically, we study the morphology and atomic structure of the films as dependent on nitridation conditions, i.e., duration and temperature, by atomic force microscopy, high resolution transmission electron microscopy ,and high resolution high-angle annular dark field scanning transmission electron microscopy. Our experimental results show that beyond a critical temperature of 1000 °C in addition to an omnipresent two-dimensional aluminum-oxynitride layer, three-dimensional Al-polar AlN islands form. While the aluminum-oxynitride layer is unstable under high temperature growth conditions and results in N-polar films, Al-polar islands are stable and induce Al-polar columnar inversion domains in the N-polar AlN films. Appropriate nitridation conditions (approximately 10 minutes at T = 850 °C–950 °C) ...

Investigation of Al y Ga1− y N/Al0.5Ga0.5N quantum dot properties for the design of ultraviolet emitters

Self-assembled Al y Ga1− y N quantum dots (QDs), with y = 0 and 0.1, have been grown by molecular beam epitaxy on Al0.5Ga0.5N(0001) oriented layers using sapphire substrates. The QD formation has been followed in situ by reflection high energy electron diffraction (RHEED). A two- to three-dimensional (2D–3D) transition of the layer morphology is observed, characterized by a change of the RHEED pattern from streaky lines to Bragg spots. High QD densities, from 1010 up to near 1012 cm−2, have been obtained. By decreasing the GaN QD size and incorporating Al inside the QDs, a strong variation in the photoluminescence (PL) emission has been observed, enabling to cover a large spectral range from near UV (3 eV) to UV-B (3.95 eV). By combining temperature-dependent and time-resolved PL measurements, the internal quantum efficiency of the QDs has been determined at both low and high temperatures as a function of the PL energy.

Efficient second harmonic generation in low-loss planar GaN waveguides

We demonstrate low-loss GaN/AlGaN planar waveguides grown by molecular beam epitaxy on sapphire substrates. By using a proper AlGaN cladding layer and reducing surface roughness we reach <1dB/cm propagation losses at 633nm. These low propagation losses allow an efficient second harmonic generation using modal phase matching between a TM0 pump at 1260nm and a TM2 second harmonic at 630nm. A maximal power conversion of 2% is realized with an efficiency of 0.15%·W-1cm-2. We provide a modelling that demonstrates broadband features of GaN/AlGaN platform by showing second harmonic wavelengths tunability from the visible up to the near-infrared spectral region. We discuss drawbacks of modal phase matching and propose a novel solution which allows a drastic improvement of modal overlaps with the help of a planar polarity inversion. This new approach is compatible with low propagation losses and may allow as high as 100%·W-1cm-2 conversion efficiencies in the future.

The microstructure, local indium composition and photoluminescence in green‐emitting InGaN/GaN quantum wells

In this work, we analyse the microstructure and local chemical composition of green‐emitting InxGa1–xN/GaN quantum well (QW) heterostructures in correlation with their emission properties. Two samples of high structural quality grown by metalorganic vapour phase epitaxy (MOVPE) with a nominal composition of x = 0.15 and 0.18 indium are discussed. The local indium composition is quantitatively evaluated by comparing scanning transmission electron microscopy (STEM) images to simulations and the local indium concentration is extracted from intensity measurements. The calculations point out that the measured indium fluctuations may be correlated to the large width and intensity decrease of the PL emission peak.

Growth of GaN nanostructures with polar and semipolar orientations for the fabrication of UV LEDs

(Al,Ga)N light emitting diodes (LEDs), emitting over a large spectral range from 360 nm (GaN) down to 210 nm (AlN), have been successfully fabricated over the last decade. Clear advantages compared to the traditional mercury lamp technology (e.g. compactness, low-power operation, lifetime) have been demonstrated. However, LED efficiencies still need to be improved. The main problems are related to the structural quality and the p-type doping efficiency of (Al,Ga)N. Among the current approaches, GaN nanostructures, which confine carriers along both the growth direction and the growth plane, are seen as a solution for improving the radiative recombination efficiency by strongly reducing the impact of surrounding defects. Our approach, based on a 2D - 3D growth mode transition in molecular beam epitaxy, can lead to the spontaneous formation of GaN nanostructures on (Al,Ga)N over a broad range of Al compositions. Furthermore, the versatility of the process makes it possible to fabricate nanostructures on both (0001) oriented “polar” and (11 2 2) oriented “semipolar” materials. We show that the change in the crystal orientation has a strong impact on the morphological and optical properties of the nanostructures. The influence of growth conditions are also investigated by combining microscopy (SEM, TEM) and photoluminescence techniques. Finally, their potential as UV emitters will be discussed and the performances of GaN / (Al,Ga)N nanostructure-based LED demonstrators are presented.

Intentional polarity conversion of AlN epitaxial layers by oxygen

Nitride materials (AlN, GaN, InN and their alloys) are commonly used in optoelectronics, high-power and high-frequency electronics. Polarity is the essential characteristic of these materials: when grown along c-direction, the films may exhibit either N- or metal-polar surface, which strongly influences their physical properties. The possibility to manipulate the polarity during growth allows to establish unique polarity in nitride thin films and nanowires for existing applications but also opens up new opportunities for device applications, e.g., in non-linear optics. In this work, we show that the polarity of an AlN film can intentionally be inverted by applying an oxygen plasma. We anneal an initially mixed-polar AlN film, grown on sapphire substrate by metal-organic vapor phase epitaxy (MOVPE), with an oxygen plasma in a molecular beam epitaxy (MBE) chamber; then, back in MOVPE, we deposit a 200 nm thick AlN film on top of the oxygen-treated surface. Analysis by high-resolution probe-corrected scanning transmission electron microscopy (STEM) imaging and electron energy-loss spectroscopy (EELS) evidences a switch of the N-polar domains to metal polarity. The polarity inversion is mediated through the formation of a thin AlxOyNz layer on the surface of the initial mixed polar film, induced by the oxygen annealing.

Selective area sublimation of GaN for top-down fabrication of nanostructures (Conference Presentation)

A fraction of a SiNx mono-layer is formed on a GaN layer by exposing the surface to a Si flux. When the sample is heated under vacuum at high temperature (900°C), we observe the sublimation of GaN in the regions uncovered by the thermally resistant SiNx mask. This selective area sublimation (SAS) process can be used for the formation of nanopyramids and nanowires with a diameter down to 4 nm. Also, if InGaN quantum wells are included in the structures before sublimation, InGaN quantum disks with quasi identical sizes in the 3 dimensions of space can be formed using SAS.