Thin n/p GaAs Junctions for Novel High-Efficiency Phototransducers Based on a Vertical Epitaxial Heterostructure Archite
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Thin n/p GaAs Junctions for Novel High-Efficiency Phototransducers Based on a Vertical Epitaxial Heterostructure Architecture M.C.A. York,1 F. Proulx,1,2 D. P. Masson,2 A. Jaouad,1 B. Bouzazi,1 R. Arès,1 V. Aimez,1 and S. Fafard1,2,a) 1 Laboratoire Nanotechnologies Nanosystèmes (LN2) – CNRS UMI-3463, Institut Interdisciplinaire d’Innovation Technologique (3IT), Université de Sherbrooke, Sherbrooke, J1K OA5, QC, Canada 2 Azastra Opto Inc., Suite 100, 6090 Longleaf Drive, Ottawa, Ontario, Canada K1W 1G3. a) Electronic mail: [email protected] or [email protected] ABSTRACT Thin GaAs photovoltaic heterostructures are grown by MOCVD with various p-GaAs base thicknesses. The total n/p absorbing thickness is varied systematically. Output voltages up to ~1.155V were obtained for individual n/p junctions at an average illumination intensity of ~8W/cm2. Novel phototransducer devices are then achieved with a vertical epitaxial heterostructure architecture, monolithically integrating 5 or more such thin n/p junctions. Around the design wavelength, the stacked heterostructure design is yielding an optimal external quantum efficiency approaching unity divided by the number of junctions. The modeled and measured conversion efficiencies are exceeding 60%. The photocarrier extraction properties are simulated for different junction thicknesses using a model based on a 3-dimensional (3D) radially-symmetric TCAD implementation of the heterostructures. The study clearly demonstrates that for such thin n/p junctions the photocarrier extraction can still be efficient due to the operation at reduced current densities and higher voltages in heterostructures enhancing electrical power extraction. With the supplementary add-on of a window layer with a reduced sheet resistance for the stacked structure, we demonstrate the possible efficient operation of phototransducers for optical inputs exceeding 150 W/cm2, even for the case of devices designed without gridlines. INTRODUCTION It has been previously observed that thin p-n junctions can be advantageous for obtaining higher open circuit voltages (Voc). Thin p/n junctions have recently been an interesting field of research with good opportunities for reducing the cost and/or increasing the performance of solar cells,1-6 for applications in optoelectronic devices such as phototransducers with ultra-high conversion efficiencies,7-11 or for other microelectronic applications.12 Thinner junctions however have the limitation that they cannot absorb all the input photons due to the restricted cross-section of semiconductor material available. In attempts to circumvent this issue, photon confinement architectures have been studied with the goal of engineering solutions allowing multiple optical passes in such thin layers. Alternatively, vertical arrangements have also been designed to exploit the unique photocarrier properties of such nanoscale thin films. The latter strategies have recently been used effectively8, 13 to avoid the reduction in short-circuit current (Isc) which is otherw
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