Wide-band-gap GaN and Ga-rich InGaN alloys, with energy gaps covering the blue and near-ultraviolet parts of the electromagnetic spectrum, are one group of the dominant materials for solid state lighting and lasing technologies and consequently, have been studied very well. Much less effort has been devoted to InN and In-rich InGaN alloys. A major breakthrough in 2002, stemming from much improved quality of InN films grown using molecular beam epitaxy, resulted in the bandgap of InN being revised from 1.9 eV to a much narrower value of 0.64 eV. This finding triggered a worldwide research thrust into the area of narrow-band-gap group-III nitrides. The low value of the InN bandgap provides a basis for a consistent description of the electronic structure of InGaN and InAlN alloys with all compositions. It extends the fundamental bandgap of the group III-nitride alloy system over a wider spectral region, ranging from the near infrared at ∼1.9 μm (0.64 eV for InN) to the ultraviolet at ∼0.36 μm (3.4 eV for GaN...

When group-III nitrides go infrared: New properties and perspectives

20) a-plane] is thoroughly investigated. We find that for the relaxed surfaces of the binary nitrides the difference in electron affinities between m- and a-plane is less than 0.1 eV. The absolute electron affinities are found to strongly depend on the choice of XC functional. However, we find that relative alignments are less sensitive to the choice of XC functional. In particular, we find that relative alignments may be calculated based on Perdew–Becke–Ernzerhof [J. P. Perdew, K. Burke, and M. Ernzerhof, Phys. Rev. Lett. 134, 3865 (1996)] surface calculations with the HSE06 lattice parameters. For InGaN we find that the VBM is a linear function of In content and that the majority of the band-gap bowing is located in the CBM. Based on the calculated electron affinities we predict that InGaN will be suited for water splitting up to 50% In content. © 2011 American Institute of Physics. [doi:10.1063/1.3548872]

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Hybrid functional investigations of band gaps and band alignments for AlN, GaN, InN, and InGaN.

A new catalyst-free method has been developed to grow self-assembled GaN wires on c-plane sapphire substrates by metal-organic vapour phase epitaxy. This approach, based on in situ deposition of thin SiNx layer (~2 nm), enables epitaxial growth of c-oriented wires with 200-1500 nm diameters and a large length/diameter ratio (>100) on c-plane sapphire substrate. The detailed study of the growth mechanisms shows that a combination of key parameters is necessary to obtain vertical growth. In particular, the duration of SiNx deposition prior to the wire growth is critical for controlling the epitaxy with the substrate. The GaN seed nucleation time determines the mean-size diameter and structural quality and a high Si-dopant concentration promotes the vertical growth. Such GaN wires exhibit UV-light emission centered at about 350 nm and a weak yellow band (~550 nm) at low temperature. This approach may be viewed as a fast and reproducible technique to grow GaN wires by MOVPE. Compared to other techniques, it allows studying quite systematically the influence of the growth parameters without being dependent on time consuming ex situ surface preparations like surface patterning used in selective area growth (SAG). The growth of heterostructures, as longitudinal n-u and n-p wires (using Si and Mg dopants) as well as a core-shell InGaN/GaN MQW using the wires as templates has been demonstrated. The growth occurs on the non-polar m-plane facets of the wire and not on c-plane as it is the case on 2D materials with the same sapphire substrate. It gives different piezoelectric contributions to the wires optical properties, which have been studied by cathodo- and photo-luminescence. The fundamental building blocks of wire-based blue LED were demonstrated during this thesis. The realisation of an efficient device still requires a deeper understanding and optimization of the parameters controlling the material growth and an optimization of the electrical contacts.

https://tel.archives-ouvertes.fr/tel-00524190/document

Self-assembled growth of catalyst-free GaN wires by metal–organic vapour phase epitaxy

Sputtered and selenized Sb2Se3 thin-film solar cells with open-circuit voltage exceeding 500 mV

We have investigated the correlated surface electronic and optical properties of [0001]-oriented epitaxial InN nanowires grown directly on silicon. By dramatically improving the epitaxial growth process, we have achieved, for the first time, intrinsic InN both within the bulk and at nonpolar InN surfaces. The near-surface Fermi-level was measured to be ∼0.55 eV above the valence band maximum for undoped InN nanowires, suggesting the absence of surface electron accumulation and Fermi-level pinning. This result is in direct contrast to the problematic degenerate two-dimensional electron gas universally observed on grown surfaces of n-type degenerate InN. We have further demonstrated that the surface charge properties of InN nanowires, including the formation of two-dimensional electron gas and the optical emission characteristics can be precisely tuned through controlled n-type doping. At relatively high doping levels in this study, the near-surface Fermi-level was found to be pinned at ∼0.95–1.3 eV above t...

Tuning the surface charge properties of epitaxial InN nanowires.

Optical, structural investigations and band-gap bowing parameter of GaInN alloys

Since a few years, indium nitride promising properties for device applications have attracted much attention worldwide. Huge efforts are dedicated to optimize indium nitride growth. However, this growth is extremely challenging, in particular using the metal organic vapor phase epitaxy (MOVPE) technique which exhibits very low growth rates. This may explain why most of the samples available in the scientific community, which also present the best electrical properties, were grown by molecular beam epitaxy (MBE). However, up to date, no intrinsic indium nitride layer was obtained. InN epilayer crystalline quality suffers from the lack of lattice matched substrates, leading to the use of double-step growth process. In this paper, we show that InN low lateral growth rate is a limiting parameter for efficient double-step growth process. But, we also report that the use of CBrCl3 during InN growth enhances the lateral growth rate. To improve the growth rate along the c-axis, we investigate alternative precursors for both nitrogen and indium species. We show that ammonia remains the best precursor, but combined with triethylindium, the growth rate increases with optimum crystalline quality. Finally, we compare MOVPE-grown and MBE-grown InN layers in order to understand the difference observed on the electrical properties. We show how thermal annealing can improve the MOVPE-grown InN layers leading to similar electrical properties than reported in MBE-grown samples. The role of ammonia as source of hydrogen is also discussed.

Recent advances in the MOVPE growth of indium nitride

InSe is a promising material in many aspects where the role of excitons is decisive. Here we report the sequential appearance in its luminescence of the exciton, the biexciton, and the P-band of the exciton-exciton scattering while the excitation power increases. The strict energy and momentum conservation rules of the P-band are used to reexamine the exciton binding energy. The new value ≥20 meV is markedly higher than the currently accepted one (14 meV), being however well consistent with the robustness of the excitons up to room temperature. A peak controlled by the Sommerfeld factor is found near the bandgap (~1.36 eV). Our findings supported by theoretical calculations taking into account the anisotropic material parameters question the pure three-dimensional character of the exciton in InSe, assumed up to now. The refined character and parameters of the exciton are of paramount importance for the successful application of InSe in nanophotonics. The optical properties of layered InSe aren’t fully understood. Here, the authors observe the exciton, biexciton, and P-band of exciton-exciton scattering in the photoluminescence spectrum of InSe, and calculate an exciton binding energy value of ≥20 meV, markedly higher than the currently accepted 14 meV.

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InSe as a case between 3D and 2D layered crystals for excitons

Atomic Layer Deposition of zinc oxide for solar cell applications

We extend to any temperature, the sophisticated calculation of the evolution of the 2 K photoluminescence energy of InN proposed by Arnaudov et al. [Phys. Rev. B 69, 115216 (2004)], in view of determining the residual doping of thin films. From the detailed line shape modeling, we extract the full width at half maximum of the photoluminescence line which, in the first order, varies like n0.51 at low temperature. This allows us to propose a handy tool for rapid residual doping evaluation. Last, temperature and inhomogeneous broadening effects are analyzed. Ignoring the latter is shown to lead to an overestimation of the residual doping.

Matthieu Moret

Papers

Optical, structural investigations and band-gap bowing parameter of GaInN alloys

Recent advances in the MOVPE growth of indium nitride

InSe as a case between 3D and 2D layered crystals for excitons

Atomic Layer Deposition of zinc oxide for solar cell applications

The determination of the bulk residual doping in indium nitride films using photoluminescence