Flexible circuits are widely seen as desirable for their advantages of light weight, toughness, and their ability to bend and tailor-fit in applications with physical constraints. Three particular devices that could demonstrate the benefit of this technology are light-emitting diodes (LEDs), photovoltaic (PV) cells, and photodetectors. Flexible LEDs can be incorporated in bendable displays, and larger-area PV in particular can greatly benefit from being able to bend over the contours of vehicle bodies, rooftops, or the constantly-flexing realm of backpacks or tactical apparel that incorporate the capacity to charge electronic devices.
Most current flexible device technology, especially that which relates to silicon, utilizes hydrogenated amorphous, microcrystalline, or even nanocrystalline material. Monocrystalline silicon would be advantageous by offering much higher mobility. Previous successful research into this has taken various approaches. One method is to etch away the buried oxide layer of silicon-on-insulator (SOI) wafers, and transfer the top Si device layer to flexible substrates. For this to work, etchant ions must be able to diffuse throughout the thin sandwiched oxide layer from the sides. Unfortunately, this process can become prohibitively slow for large-area samples. We will discuss our work in the transfer of larger-area monocrystalline silicon layers of thickness 3 microns or less onto flexible polymer substrates. Our methodology involves bonding the device layer of SOI to the substrate with an electrically-conducting adhesive, and subsequently removing the donor material. The donor substrate is first mechanically lapped to rapidly remove the majority of material; it is subsequently chemically etched to more gently remove the remaining material, up to the buried-oxide etch stop layer. After HF-driven removal of the exposed oxide face, thin and flexible monocrystalline silicon is left on an electrically-conductive substrate which serves as the backside contact. A demonstration Schottky-barrier diode is made with the transferred silicon, and its electrical and physical characteristics will be discussed. Also demonstrated are flexible monocrystalline GaAsP LEDs fabricated using a similar process. Furthermore, we will discuss the extensibility of this technique to other semiconductor materials for higher solar absorption in PV, improved LEDs, and other electronic devices. NSF-DMR support of this research is gratefully acknowledged.