Yves Lacroix

High-Performance 348 nm AlGaN/GaN-Based Ultraviolet-Light-Emitting Diode with a SiN Buffer Layer.

A 348 nm ultraviolet-light-emitting diode (UV-LED) based on an AlGaN/GaN single quantum well (SQW) with a high optical power is reported. In this structure, a thin SiN buffer is introduced before the growth of a conventional low-temperature GaN buffer layer. Such a buffer layer can dramatically reduce the density of threading dislocation as we have previously reported. Since the optical performance of UV-LED is generally known to be sensitive to the density of threading dislocations, unlike the InGaN/GaN- based blue LED, our UV-LED has a higher optical power than that of a similar structure but without a SiN buffer layer. Since our new buffer technology is much easier than the so-called epitaxial lateral overgrowth (ELO) or pendeo-epitaxy method, it is highly recommended for use in the fabrication of GaN-based optical devices, particularly AlGaN/GaN-based UV-LED.

PHOTOLUMINESCENCE PROPERTIES OF SODIUM INCORPORATED IN CUINSE2 THIN FILMS

CuInSe2 films with Cu/In ratios of m=0.83–0.99 have been deposited on glass substrates by the co-evaporation method using a bi-layer process. The effects of sodium in these films have been investigated by comparing the electrical and optical properties of the films with and without the Na2Se incorporation. Two donor-acceptor pair emissions, [D, A]α at 0.89–0.94 eV and [D, A]β at 0.84–0.87 eV, were typically observed in the photoluminescence spectra. The relative intensity of [D, A]α to [D, A]β was found to depend strongly on both the Cu/In ratio and Na incorporation. For the films with m=0.94, [D, A]α was predominantly observed regardless of Na incorporation. For m≤0.90, [D, A]β became dominant for the films without Na, while [D, A]α remained as the dominant emission for the films with Na. A correlation between the prominence of [D, A]α and resistivities below 2×103 Ω cm was found, suggesting a reduced compensation by suppressed Se vacancy donors due to the presence of Na.

Effect of Oxygen on the Activation of Mg Acceptor in GaN Epilayers Grown by Metalorganic Chemical Vapor Deposition

The effect of oxygen mixed in nitrogen on p-type activation in Mg-doped GaN epilayers grown by metalorganic chemical vapor deposition (MOCVD) was investigated. The samples annealed in N2/O2 (1%) ambient exhibited the best electrical properties with respect to hole concentration. SIMS data suggested that oxygen reacted with hydrogen present in the Mg-doped GaN samples during the thermal annealing process, thereby enhancing the activation of Mg acceptors.

Role of Potential Barriers in Epitaxial Layers of Semi-Insulating GaN Layers

The electrical properties of semi-insulating GaN were investigated by dc and microwave techniques. The different epitaxial GaN layers were grown by Metal Organic Chemical Vapour Deposition on a Al2O3 (0001) substrate. Space charge effects along and perpendicular to the direction of layer growth were proposed to explain the observed peculiarities of the dark current and photocurrent. From the temporal and temperature dependences of characteristic time constants in the photoconductivity decay, the details of non-equilibrium states were analysed in the semi-insulating GaN. The well-resolved alpha-particle spectra obtained demonstrates the promise of semi-insulating GaN for ionising radiation detection.

Influence of Inversion Domains on Formation of V-Shaped Pits in GaN Films.

The influence of inversion domains (IDs) on the formation of V-shaped pits in GaN films grown by metalorganic chemical vapor deposition has been investigated using transmission electron microscopy. It was found that IDs can induce V-shaped pits which have a large size distribution. Upon introducing InGaN/GaN multiple-quantum wells to observe various growth stages, our results showed that the origin of these ID-induced pits may be a delayed formation of the IDs during island-island coalescence at the initial stage of film growth, thus we need not adopt a general explanation based on the different growth rates for the two opposite polarities.

Characterization of GaN films etched using reactive ion etching technique by secondary ion mass spectrometry

Abstract We investigated GaN films etched by using reactive ion etching (RIE) technique to fabricate the GaN-based devices. The samples were grown on sapphire substrate by metal organic chemical vapor deposition (MOCVD), and Ti/Al contacts were formed on n -GaN surfaces after etching processes. The effects of the kinds of reactive gases were evaluated by secondary ion mass spectrometry (SIMS). The results showed that in the sample etched using BCl 3 gas, the signal from boron contaminations was strongly detected at the interface between the contact metal and n -GaN, and we found that additional etching in Cl 2 plasma after etching with BCl 3 gas was essential to make a good contact.

Cracking of GaN on sapphire from etch-process-induced nonuniformity in residual thermal stress

An experiment was performed to explain the appearance of cracks along mesa structures during the processing of GaN device layers grown on sapphire substrates. Micro-Raman spectroscopy was used to measure the position-dependent stress in the GaN layer. We show evidence that the stress at the interface with the substrate may be larger along the mesa structures than that of the as-grown layer, and that this increase in stress can be enough to induce cracks along mesa structures during processing. We report on the formation of cracks that propagate guided by the nonuniformity of the stress induced by the formation of mesa structures in the GaN layer, independent of crystal direction. The understanding of cracking mechanisms has implications in GaN-based device structures that require heteroepitaxial growth of layers with different lattice size and thermal expansion coefficients.