Recently, fluorescent point defects in silicon have been explored as promising candidates for single photon sources, which may pave the way towards the integration of quantum photonic devices with existing silicon-based electronic platforms. However, the current processes for creating such defects are complex, and commonly require one or two implantation steps. In this work, we have demonstrated implantation-free methods for obtaining G and W-centers in commercial silicon-on-insulator substrates using femtosecond laser annealing. We also demonstrate an enhancement of the color centers’ optical properties by coupling them with photonic structures. For example, we have shown an improvement in emission directivity for G centers by embedding them into silicon Mie resonators fabricated by dewetting, achieving an extraction efficiency exceeding 60% with standard numerical apertures. We will also address the control of emission polarization by embedding color centers in photonic crystals.
The realization of integrated active optical systems is crucial for the use of components such as amplifiers, lasers or photodetectors on a chip. Alumina (Al2O3) doped with erbium is a promising material for the realization of various active functions. Indeed, the low propagation losses of Al2O3, the rare earth compatibility and its wide transmission band makes Al2O3 suitable for a wide range of applications. Nevertheless, current methods for producing such waveguides are often costly and difficult, requiring complex and potentially loss-making processing steps like etching. In this context, Pulsed Laser Deposited (PLD) combined with lift-off is a relevant method for avoiding etchings. The process is composed of three main steps: photolithography, PLD and lift-off. In this work, we present how the different steps have to be optimized to make suitable waveguides for light propagation. Notably, photolithography needs a precise cross-section profile to obtain smooth sidewall, to ease lift-off and get high resolution patterning. For PLD, SEM images showed the importance of plume directivity and orientation in the PLD chamber to achieve a good control of the waveguide shapes. Finally, we also have shown that the Erbium photoluminescence is dependent on the annealing temperature. These results highlight the essential parameters which need to be precisely controlled to achieve accurate microstructures by liftoff processing performed in PLD layers, paving the way for the demonstration of low-loss waveguides and fully integrated erbium laser without etching.
KEYWORDS: Nanowires, Solar cells, Silicon, Electron beams, Microscopy, Doping, Photovoltaics, Multijunction solar cells, Group III-V semiconductors, Nanolithography
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.
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