Credit: WILEY

It is widely believed that the fabrication of foldable and bendable optoelectronics requires technologies such as organic polymer semiconductors or colloidal quantum dots, both of which can be deposited onto plastic substrates using solution-processing techniques.

It now turns out that foldable optoelectronics can also be made from conventional inorganic semiconductors such as GaAs and InP, thanks to a new innovative fabrication approach developed by John Rogers and co-workers at the University of Illinois at Urbana–Champaign and Northwestern University in the USA (Adv. Mater. doi:10.1002/adma.201000591; 2010). Until recently, the deposition of such compound semiconductors onto a plastic substrate was thought to be impossible owing to their incompatibility with semiconductor fabrication technologies such as molecular beam epitaxy and metal organic chemical vapour deposition.

The researchers used an etch-and-release scheme to fabricate red (∼675 nm) LEDs based on conventional GaAs–InGaP semiconductors on a thin sacrificial layer of AlGaAs deposited on a GaAs wafer. The clever part of the scheme is that the sacrificial AlGaAs layer can be etched away after fabrication, which allows the active semiconductor forming the LED to be transferred onto a thin plastic substrate. The researchers say that their approach produces brighter and more efficient LEDs than organic light emitters, while still enjoying the benefits of a flexible plastic substrate. A rather unexpected outcome is the extreme flexibility of the LEDs, which allegedly permits a bending radius as small as 0.7 mm — appreciably lower than the previous record of several millimetres using organic LED technology.

Fabrication of the device starts with the growth of microscale LEDs, each measuring 100 μm × 100 μm × 2.523 μm, using the etch-and-release scheme. The LEDs are then removed and transfer-printed as arrays onto a substrate of polyethylene terephthalate coated with polyurethane. An epoxy coating is applied, etching is performed and ohmic metals are then deposited to define the n- and p-electrodes of the microscale LEDs. Electrical interconnection lines are formed through photolithographic patterning. The final encapsulation process involves applying epoxy of varying thickness to different regions. The thickness of the encapsulation layer is incremented in steps during deposition to reduce the mechanical strain on the LED quantum wells when the substrate is folded or flexed.

It is suggested that the development of these highly flexible LED arrays, which have superior optical performance and mechanical properties to organic LEDs, may prove useful for creating flexible display technologies.