Couches minces d'un absorbeur à grand gap (Cu(In,Ga)S2)

Ce travail s'inscrit dans le domaine des cellules solaires photovoltaïques de la filière CuInSe2 et plus particulièrement des cellules ayant un absorbeur à grand gap (1,6 -1,7 eV). Cu(In,Ga)S2 a été choisi pour former cet absorbeur, notamment pour la facilité du dopage (type n ou type p). Dans une première étape, la croissance et l'étude fondamentale de couches minces de Cu(In,Ga)S2 ont été réalisées. La méthode de croissance est basée sur une technique faible coût de transport à courte distance (CSVT), que nous avons mis au point au cours de ce travail. Les conditions d'obtention des couches minces ont été déterminées et discutées, notamment en termes de porosité de la source, de la température de substrat, de la pression d'argon (0,1 à 1 bar).... Des morphologies différentes ont été obtenues, suivant les conditions expérimentales.This present work concerns the CuInSe2-type solar cells having a wide-gap absorber (1,6 -1,7 eV). Cu(In,Ga)S2 was selected to form this absorber, mainly for its easy doping (p or n-type). As a first step, a systematic and fundamental study of the growth and characterization of thin films was carried out. The aim of this work was to grow thin film absorbers having suitable morphologies, quasi-stoichiometric compositions and various electrical resistivities, by means of a new low cost, vacuum free and potentially large area method based on the close-spaced vapor transport (CSVT) principle. The deposition conditions were determined and discussed, particularly in terms of source porosity, substrate temperature, argon pressure (0,1 à 1 bar).... Different morphologies were observed, according to the experimental conditions.PERPIGNAN-BU Sciences (661362101) / SudocSudocFranceF

A study of BGaN back-barriers for AlGaN/GaN HEMTs

We study the use of a BGaN back-barrier layer in the GaN buffer of AlyGa1−yN/GaN highelectron mobility transistors to improve confinement of carriers in the 2D electron gas region. Unlike InGaN back-barrier designs, whose polarization-induced sheet charges form an electrostatic barrier at the backbarrier/ buffer interface, BGaN back-barrier designs create an electrostatic barrier at the channel/backbarrier interface. This can result in carrier confinement to sub-15 nm thickness, even when the channel is 30 nm wide. Although polarization sheet charges due to the BGaN back-barrier form a secondary well at the back-barrier/buffer interface, increasing the thickness of the back-barrier may move the secondary well so that it no longer interacts with the primary channel

Asymmetrical Design of AlGaN/GaN Distributed Bragg Reflectors for Near-UV Optoelectronic Applications

International audienceAlGaN/GaN materials family is one of the best solution to achieve near-UV high reflective Bragg mirrors on GaN substrates. However, the large lattice mismatch occuring between AlGaN and GaN can lead to relaxed structures by the way of cracks which affect the DBR performances. In this work, asymmetrical designs were investigated for the modeling of fully-strained AlGaN/GaN distributed Bragg Reflectors. Such designs should allow to obtain stacks with crack-free surfaces well-adapted for the regrowth of efficient additional monocrystalline layers. First, the critical thickness of MOVPE-grown AlGaN on GaN templates was experimentally determined and modeled. Then, several AlGaN/GaN mirrors with various Al molar fractions and asymmetry factors were simulated demonstrating that non relaxed DBRs could be obtained with adequate parameters. Finally, it has also been shown that there is a best suited Al molar fraction in AlGaN for each DBR centering wavelength

Theoretical analysis of the influence of defect parameters on photovoltaic performances of composition graded InGaN solar cells

International audienceIn this paper, we have used simulations to evaluate the impact of the distribution of electrically active defects on the photovoltaic performances of InGaN-based solar cell. The simulations were carried out using Silvaco's ATLAS software. We have modeled a P-GaN/Grad-InGaN/i-In0.53Ga0.47N/Grad-InGaN/N-ZnO where Grad-InGaN corresponds to an InGaN layer with a graded composition. This layer is inserted to eliminate the band discontinuities at the interface between InGaN and the GaN and ZnO layers. The defects were modeled through the introduction of band tails and a Gaussian distribution of defects in i-InGaN material. We have evaluated the influence of band tail widths as well as the parameters of the Gaussian distribution (i.e. defect density, mean position and standard deviation) on the short-circuit current, the open-circuit voltage and the fill-factor (efficiency) of the solar cell. These results have allowed us to identify key structural parameters useful for the optimization of InGaN solar cells, as well as to give realistic estimates of the performances of such cells

Bandgap energy bowing parameter of strained and relaxed InGaN layers,

International audienceThis paper focuses on the determination of the bandgap energy bowing parameter of strained and relaxed InxGa1−xN layers. Samples are grown by metal organic vapor phase epitaxy on GaN template substrate for indium compositions in the range of 0<x<0.25. The bangap emission energy is characterized by cathodoluminescence and the indium composition as well as the strain state are deduced from high resolution X-ray diffraction measurements. The experimental variation of the bangap emission energy with indium content can be described by the standard quadratic equation, fitted using a relative least square method and qualified with a chi square test. Our approach leads to values of the bandgap energy bowing parameter equal to 2.87±0.20eV and 1.32±0.28eV for relaxed and strained layers (determined for the first time since the revision of the InN bandgap energy in 2002), respectively. The corresponding modified Vegard’s laws describe accurately the indium content dependence of the bandgap emission energy in InGaN alloy and for the whole range of indium content. Finally, as an example of application, 3D mapping of indium content in a thick InGaN layer is deduced from bandgap energy measurements using cathodoluminescence and a corresponding hyperspectral map

Ohmic and Schottky contacts for BGaN based UV photo- and α,β detectors

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Asymmetrical design for non-relaxed near-UV AlGaN/GaN distributed Bragg reflectors

International audienceTowards the development of high efficient GaN-based Vertical Cavity devices, the fabrication of cracks-free high reflective semiconductor mirrors is still an issue. For near-UV operating devices, one of the best solution is the use of AlGaN/GaN materials family. With a relatively high Al molar fraction in AlGaN, a large enough index contrast can be achieved to fabricate high reflectivity mirrors. However, the lattice mismatch between AlGaN and GaN increases with the Al molar fraction and induces a lot of cracks in the structure which affect its optical and electrical properties. Moreover, for a regrowth of an active layer on the top of the mirror, it is necessary to suppress crack generations to achieve a smooth surface. In this work, asymmetrical designs were investigated for the modeling of fully-strained AlGaN/GaN distributed Bragg Reflectors with crack-free surfaces. First, the critical thickness of MOVPE-grown AlGaN on GaN-on-sapphire templates was experimentally determined and modeled. Then, several AlGaN/GaN mirrors with various Al molar fractions and asymmetry factors were simulated demonstrating that non relaxed DBRs could be obtained with adequate parameters. Finally, it has also been shown that there is a best suited Al molar fraction in AlGaN for each DBR centering wavelength