Abderraouf Boucherif x Lift-off process of III-V multijunction solar cells by using porous Ge sacrificial layers 1,2 1,2 1,2 Abderraouf Boucherif , Vincent Aimez , Richard Arès Institut interdisciplinaire d’innovation technologique (3IT), Université de Sherbrooke, Sherbrooke, Québec, Canada 2 Laboratoire Nanotechnologies Nanosystèmes (LN2)- CNRS UMI-3463, Université de Sherbrooke, Sherbrooke, Québec, Canada 1 Abstract Mono-crystalline germanium (Ge) is the most widely used substrate in concentrated photovoltaics (CPV) high efficiency multijunction solar cells (MJSC). Increasing demand on MJSC would undoubtedly lead to a shortage of this rare material, resulting in a dramatic increase in the Ge wafer price, not to mention that today, the cost of the Ge substrate already represents a substantial share of the total cell cost. Typical MJSC process uses > 140 µm thick Ge wafers as substrates, whereas a few microns would be sufficient for the bottom cell to match the photogenerated currents at the top and middle subcells. Moreover, using a thin Ge film rather than a thick substrate would reduce electrical and thermal resistances, the weight of the cell and increase the efficiency of the Ge subcell. In order to separate the active region of the MJSC from its original substrate, we propose a layer 1 transfer process based on the simple electrochemical porosification of Ge . A similar process has been already successfully applied to crystalline silicon thin film solar cells, yielding cells with performances very similar to crystalline cells that are manufactured on standard thick Si wafers2. A schematic representation of the process is shown in fig. 1; first, a double porosity layer is formed on top of a p-type Ge substrate in HF based electrolyte; the topmost layer has a low porosity whereas the buried layer is highly porous. After ultra high vaccuum (UHV) annealing at high temperature, the porous top layer transforms into quasi-monocristalline germanium (QMG) film and will serve later as seed for epitaxial deposition of III-V top and middle subcells. On the other hand, the buried layer forms a film with large lateral voids that weaken the interface between the QMG film and the substrate, creating a so called “separation layer”. After epitaxial deposition of the III-V MJSC device is done, the cell is bonded to a low cost host substrate, and separated from the original Ge substrate, which is then ready to be re-used in another manufacturing cycle and could potentially yield a large number of MJSC solar cells, as the active region of the Ge subcell is only a few microns thick. The synthesis and transformation during annealing of the mesoporous germanium double layer are key steps in this process. In this presentation, we show the results of electrochemical etching of Ge in HF electrolyte to form the double porosity layers, and its morphology transformation during UHV annealing at high temperature which is due to surface diffusion at constant volume (see Fig.2.). The monocrystallinity of the layers and their suitability as seed layers for epitaxy of GaAs is confirmed by X-ray diffraction rocking curves. Book of Abstracts Next Generation Solar 2014 – Photovoltaics Canada – Fifth National Scientific Conference May 14 to 16, 2014 in Montréal, Québec Fig. 1. Process ﬂow for thin monocrystalline Ge seed formation: (a) formation of double porosity layer; (b) annealing to create a monocrystalline Ge seed layers and a separation layer; (c) epitaxial growth of solar cell device layers; (d) layer transfer to host substrate and Ge substrate re-use. GaAs 1 µm QMG Fig. 2. Cross-sectional SEM view of the heterostructure consisting of GaAs on sintered porous Ge layers during the separation step. References (1) Boucherif, A.; Beaudin, G.; Aimez, V.; Arès, R. Mesoporous Germanium Morphology Transformation for Lift-off Process and Substrate Re-Use. Appl. Phys. Lett. 2013, 102, 011915. (2) Petermann, J.; Zielke, D. 19% efficient and 43 um thick Crystalline Si Solar Cell from Layer Transfer Using Porous Silicon. Prog. Photovoltaics Res. Appl. 2012, 20, 1–5. Book of Abstracts Next Generation Solar 2014 – Photovoltaics Canada – Fifth National Scientific Conference May 14 to 16, 2014 in Montréal, Québec