Anne Ponchet

Molecular beam epitaxy and properties of GaAsBi/GaAs quantum wells grown by molecular beam epitaxy: effect of thermal annealing

We have grown GaAsBi quantum wells by molecular beam epitaxy. We have studied the properties of a 7% Bi GaAsBi quantum well and their variation with thermal annealing. High-resolution X-ray diffraction, secondary ion mass spectrometry, and transmission electron microscopy have been employed to get some insight into its structural properties. Stationary and time-resolved photoluminescence shows that the quantum well emission, peaking at 1.23 μm at room temperature, can be improved by a rapid annealing at 650°C, while the use of a higher annealing temperature leads to emission degradation and blue-shifting due to the activation of non-radiative centers and bismuth diffusion from the quantum well.

Elastic strains at interfaces in InAs/AlSb multilayer structures for quantum cascade lasers

InAs/AlSb multilayers similar to those used in quantum cascade lasers have been grown by molecular beam epitaxy on (001) InAs substrates. Elastic strain is investigated by high resolution transmission electron microscopy. Thin interfacial regions with lattice distortions significantly different from the strain of the AlSb layers themselves are revealed from the geometrical phase analysis. Strain profiles are qualitatively compared to the chemical contrast of high angle annular dark field images obtained by scanning transmission electron microscopy. The strain and chemical profiles are correlated with the growth sequences used to form the interfaces. Tensile strained AlAs-like interfaces tend to form predominantly due to the high thermal stability of AlAs. Strongly asymmetric interfaces, AlAs-rich and (Al, In)Sb, respectively, can also be achieved by using appropriate growth sequences.

Monolithic Integration of Diluted-Nitride III–V-N Compounds on Silicon Substrates: Toward the III–V/Si Concentrated Photovoltaics

Abstract GaAsPN semiconductors are promising material for the development of high-efficiency tandem solar cells on silicon substrates. GaAsPN diluted-nitride alloy is studied as the top-junction material due to its perfect lattice matching with the Si substrate and its ideal bandgap energy allowing a perfect current matching with the Si bottom cell. The GaP/Si interface is also studied in order to obtain defect-free GaP/Si pseudo-substrates suitable for the subsequent GaAsPN top junctions growth. Result shows that a double-step growth procedure suppresses most of the microtwins and a bi-stepped Si buffer can be grown, suitable to reduce the anti-phase domains density. We also review our recent progress in materials development of the GaAsPN alloy and our recent studies of all the different building blocks toward the development of a PIN solar cell. GaAsPN alloy with energy bandgap around 1.8 eV, lattice matched with the Si substrate, has been achieved. This alloy displays efficient photoluminescence at room temperature and good light absorption. An early-stage GaAsPN PIN solar cell prototype has been grown on a GaP(001) substrate. The external quantum efficiency and the I–V curve show that carriers have been extracted from the GaAsPN alloy absorber, with an open-circuit voltage above 1 eV, however a low short-circuit current density obtained suggests that GaAsPN structural properties need further optimization. Considering all the pathways for improvement, the 2.25% efficiency and IQE around 35% obtained under AM1.5G is however promising, therefore validating our approach for obtaining a lattice-matched dual-junction solar cell on silicon substrate.

Abrupt GaP/Si hetero-interface using bistepped Si buffer

We evidence the influence of the quality of the starting Si surface on the III-V/Si interface abruptness and on the formation of defects during the growth of III-V/Si heterogeneous crystal, using high resolution transmission electron microscopy and scanning transmission electron microscopy. GaP layers were grown by molecular beam epitaxy on vicinal Si (001). The strong effect of the Si substrate chemical preparation is first demonstrated by studying structural properties of both Si homoepitaxial layer and GaP/Si heterostructure. It is then shown that choosing adequate chemical preparation conditions and subsequent III-V regrowth conditions enables the quasi-suppression of micro-twins in the epilayer. Finally, the abruptness of GaP/Si interface is found to be very sensitive to the Si chemical preparation and is improved by the use of a bistepped Si buffer prior to III-V overgrowth.

Antiphase domain tailoring for combination of modal and 4¯ -quasi-phase matching in gallium phosphide microdisks

We propose a novel phase-matching scheme in GaP whispering-gallery-mode microdisks grown on Si substrate combining modal and 4¯ -quasi-phase-matching for second-harmonic-generation. The technique consists in unlocking parity-forbidden processes by tailoring the antiphase domain distribution in the GaP layer. Our proposal can be used to overcome the limitations of form birefringence phase-matching and 4¯ -quasi-phase-matching using high order whispering-gallery-modes. The high frequency conversion efficiency of this new scheme demonstrates the competitiveness of nonlinear photonic devices monolithically integrated on silicon.

Formation of strained interfaces in AlSb/InAs multilayers grown by molecular beam epitaxy for quantum cascade lasers

Structural and chemical properties of InAs/AlSb interfaces have been studied by transmission electron microscopy. InAs/AlSb multilayers were grown by molecular beam epitaxy with different growth sequences at interfaces. The out-of-plane strain, determined using high resolution microscopy and geometrical phase analysis, has been related to the chemical composition of the interfaces analyzed by high angle annular dark field imaging. Considering the local strain and chemistry, we estimated the interface composition and discussed the mechanisms of interface formation for the different growth sequences. In particular, we found that the formation of the tensile AlAs-type interface is spontaneously favored due to its high thermal stability compared to the InSb-type interface. We also showed that the interface composition could be tuned using an appropriate growth sequence.

Highly strained AlAs-type interfaces in InAs/AlSb heterostructures

Spontaneously formed Al-As type interfaces of the InAs/AlSb system grown by molecular beam epitaxy for quantum cascade lasers were investigated by atomic resolution scanning transmission electron microscopy. Experimental strain profiles were compared to those coming from a model structure. High negative out-of-plane strains with the same order of magnitude as perfect Al-As interfaces were observed. The effects of the geometrical phase analysis used for strain determination were evidenced and discussed in the case of abrupt and huge variations of both atomic composition and bond length as observed in these interfaces. Intensity profiles performed on the same images confirmed that changes of chemical composition are the source of high strain fields at interfaces. The results show that spontaneously assembled interfaces are not perfect but extend over 2 or 3 monolayers.

Sb surfactant mediated growth of InAs/AlAs0.56Sb0.44 strained quantum well for intersubband absorption at 1.55 μm

Surfactant mediated growth of strained InAs/AlAs0.56 Sb 0.44 quantum wells on InP (001) substrate is investigated. X ray diffraction and transmission electron microscopy analysis reveal that the supply of antimony on InAs surface delays the 2D to 3D growth transition and allows the growth of thick InAs/AlAsSb quantum wells. Quantum well as thick as 7 ML, without defect was achieved by Sb surfactant mediated growth. Further high resolution transmission electron microscopy measurement and geometric phase analysis show that InAs/AlAsSb interfaces are not abrupt. At InAs on AlAsSb interface, the formation of a layer presenting lattice parameter lower than InP leads to a tensile stress. From energetic consideration, the formation of As rich AlAsSb layer at interface is deduced. At AlAsSb on InAs interface, a compressive layer is formed. The impact on optical properties and the chemical composition of this layer are discussed from microscopic analysis and photoluminescence experiments.

Achievement of InSb Quantum Dots on InP(100) Substrates

The formation of InSb quantum dots within a GaAs0:51Sb0:49 matrix lattice matched to InP is investigated. We show that the deposit of InSb on GaAs0:51Sb0:49 alloy surface, allows the achievement of a high density of InSb islands without dislocations. A strong dissolution of InSb quantum dots occurs during the capping with a GaAsSb layer. Reflection high energy electron diffraction analysis shows that InSb island dissolution occurs during the growth interruption under As and Sb. We propose a procedure based on the deposit of a thin GaSb capping layer on top of InSb islands to prevent the As/Sb exchange. Optical properties are investigated using photoluminescence. Electronic properties are discussed within an improved tight-binding model.

Impact of antiphase boundaries on non-linear frequency conversion in GaP/Si microdisks

GaP is a promising candidate for the development of highly integrated photonic functions on silicon, because of its quasi lattice-matching with Si and its interesting χ(2) non-linear properties. We here present the realization of GaP-based microdisks on Si substrates, and discuss their optical properties. Especially, emphasis is given on the impact of crystal antiphase boundaries generated during the heteroepitaxial growth on second-harmonic generation.