The record in photovoltaic conversion efficiency is detained by multi-junction solar cells based on III-V semiconductors. However, the wide adoption of these devices is hindered by their high production cost, to a large extent due to the expensive III-V substrates. As an alternative, a hybrid geometry has been proposed [LaPierre JAP 2011], which combines a 2D Si bottom cell with a III-V nanowire top cell in a tandem device. This approach, which may reach theoretical efficiencies of approx. 34%, requires smaller amounts of expensive III-V materials compared to conventional III-V tandem cells and benefits from the nanowire light trapping effects.
In this work, we report the fabrication and nanoscale characterization of two types of nanostructures for solar cells: radial GaAlAs and axial GaAsP p-n junction nanowires. Nanowires are grown by gallium-assisted molecular beam epitaxy using Be and Si as doping sources. The composition (probed by EDX and cathodoluminescence) was adjusted to tune the bandgap toward the optimal value for a III-V-on-Si tandem cell (approx. 1.7 eV). Local I-V characteristics and electron beam induced current (EBIC) microscopy under different biases are used to probe the electrical properties and the generation pattern of individual nanowires. For radial junction nanowires, EBIC mappings revealed a homogeneous collection of carriers on the entire nanowire length. For axial junction nanowires, the doping concentrations and the minority carrier diffusion lengths were extracted from the EBIC generation profiles. The effect of an epitaxial GaP passivating shell on the optical and generation properties was assessed.
We report on design, simulation and fabrication of ultimate and compact 3D close-geometries optical microcavities.
These are based on the extension of the so-called 2.5D nanophotonic approach where a quasi 3D control of the photons
has been soon demonstrated by our group. A tight control of photons, spectrally and spatially, in a small air region
inside a circular regular pattern of high index material-based nanopillars is demonstrated when adjusting the number of
pillars, their diameters and the diameter of the pillar-circle. Bottom-up approach based on InP nanowires grown by
molecular beam epitaxy and top-down approach based on high aspect ratio anisotropic etching have been developed for
fabricating these optical microcavities.
The ground and excited state luminescent transitions in self-organized InAs/InP(001) quantum islands (QIs) grown in two different matrices (In0.52Al0.48As and InP), have been studied by cw photoluminescence (PL) and time resolved photoluminescence (TRPL). PL excitation (PLE) shows that the multi-component PL spectrum measured for the InAs/InAlAs QIs is associated to ground and related excited state transitions of QIs having monolayer-height fluctuation whereas for InAs/InP QIs the multi-component PL spectrum is only due to one ground state and their related excited states. This attribution is confirmed by the recombination life times measured by TRPL which are in the 1.2-1.4 ns range for the ground state transitions and in the 90-600 ps range for the excited state transitions.
We demonstrate NIR (1.8 micrometer - 2.3 micrometer) resonant photo-detectors based on inter-band (Ecl- Ehhl) absorption in strain compensated, indium rich, InGaAs quantum wells (QW). Extremely low room temperature dark current densities are achieved by reduction of the active layer thickness combined with low defect density of the pseudomorphic strain compensated QWs. The weak absorption of the QW is enhanced by embedding the quantum well into a vertical resonant cavity. We present the experimental results for a demonstrator designed for a wavelength of 2 micrometer. The device, based on a single In0.83Ga0.17As quantum well and tensile strained barriers for strain compensation, exhibits a selectivity of 9 nm and 18% quantum efficiency. InP/InGaAs and Si/SiO2 material systems are used for the bottom and top distributed Bragg reflectors (DBR) of the cavity, with 20 pairs and 2 pairs respectively. The semiconductor structure is grown by MOCVD. The top Si/SiO2 DBR is deposited after fabrication of p-i-n planar photodiodes. Typical dark current densities are lower than 10-7 A/cm2 at -2 V bias. Conditions for extension of the operating wavelength up to 2.3 micrometer have been obtained experimentally using InAs/GaAs superlattice deposition to increase the thickness of the strained QW. A prospective tunable detector based on an actuable micro-machined air cavity and air/InP bottom DBR is proposed.
We have studied actuable suspended beams that form vertical optical resonant cavities. They are fabricated by sacrificial layers etching techniques on InP. They can be used as tunable optical filters for telecommunication applications. In this paper, we report optical characterizations of those devices. We have used micro reflectivity and optical profilometer measurements to make those characterizations. We have developed a micro- reflectivity experimental setup which uses the confocal principle. We have shown the deformation of the suspended beams and the variation of the reflectivity response induced by electrostatic actuation of the devices.
The design and the fabrication of vertical InP-based micro- opto-electro-mechanical devices are reported. These are based on micromachined III-V semiconductor structures realized by selective removal of adapted sacrificial layers in order to produce Fabry-Perot resonant microcavity. Continuous wavelength running of 50nm around 1.55 micrometers for a 15 volt bias actuation has been demonstrated. Resonant peak full width at half maximum of about 10 nm at 1.5 micrometers has been performed on a InP/air gap multilayered interferometric filter. The integration of absorbing layers inside the cavity will allow us to realize resonant cavity enhanced photonic devices with thinner, and therefore faster, photodetector structures with high quantum efficiencies.
Microoptoelectromechanical (MOEMS) systems with InP based micromechanics are proposed for devices with wide tuning ranges in the optical wavelengths where InP optoelectronics are normally used. To evaluate if these InP based micromechanical structures may be strong enough the mechanical strengths of surface micromachined epitaxial InP micro beams are evaluated. Reactive ion etching (RIE) with CH4:H2:H2Ar is used to structure the beams. A sacrificial InGaAs layer is below the InP microstructures and selectively etched by HCl:H2O2:H2O in ratios 1:1:10 to release the InP beams. Sublimation of tert-butanol is used to dry the micro structures. The RIE conditions are shown to be of large importance, since the induced surface defects are here the dominant reasons for fracture. Bending strength values up to 3.1 GPa were measured, i.e. much higher than for the strongest construction steel. Weibull statistics show that it is possible to make micromachines for typical MOEMS applications with acceptable loss in yield due to fracture probability, i.e. with a fracture probability of 0.0001 for 100 MPa maximum bending stress.
In this work, we propose solutions based on engineering of III-V heterostructures to develop new types of semiconductor magnetic sensors. These micro-Hall sensors use the properties of a 2D electron gas and the benefit of pseudomorphic material, in which both the alloy composition and the built-in strain offer additional degrees of freedom for band structure tailoring, to exhibit high magnetic sensitivity, good linearity, low temperature coefficient and high resolution. With the growth optimization which is described, two pseudomorphic In0.75Ga0.25As/In0.52Al0.48As heterostructures were grown on a semi- insulating InP substrate by molecular beam epitaxy. To understand better the influence of the heterostructure design on its electronic properties, a model involving the self-consistent solution of the Poisson and Schrodinger equations using the Fermi-Dirac statistics has been developed. These results have been used to optimize the structure design. A magnetic sensitivity of 346 V/AT with a temperature coefficient of -230 ppm/ degree(s)C between -80 degree(s)C and 85 degree(s)C has been obtained. The device show good linearity against magnetic field and also against the supply current. High signal-to-noise ratios corresponding to minimal magnetic field of 350 nT/Hz1/2 at 100 Hz and 120 nT/Hz1/2 at 1 kHz have been measured.
In this paper we report experimental results on InGaAs/InAlAs single quantum wells (SQW) obtained by photoreflectance (PR) between 5 K and 450 K. In the first part of the paper we focus on the evolution of the broadening parameter of E1H1 in the lattice matched 5 nm well width sample, E1H1 and E2H2 in the lattice matched 25 nm SQW. From this study we derive information about the relative influence of interface roughness, alloy scattering, and electron phonon interactions. In the second part we apply the PR technique to the study of quantum wells near the surface in which we observe an increase of the broadening parameter. These studies show the great interest of PR technique for the qualification of materials and for the surface probe.