Jul 3, 2009
Nano LEDs printed on silicon
Patterned quantum-dot based light emitting diodes (LEDs) fabricated on silicon have applications in nanophotonics, optical micro/nanoelectromechanical systems (MEMS/NEMS) and biomedical sensing and imaging. Control of both the thickness and the area of the nanoparticles during deposition can be achieved via microcontact printing.
A silicon-based light source represents a new path towards integrated, compact and mass-producible microsystems for advanced computing, networking and sensing. Unfortunately, silicon is an indirect bandgap material and therefore is an inefficient light emitter. To address the problem, researchers at the University of Texas at Austin, US, have used quantum dots (QDs) as the light emitter in an inorganic light-emitting device with silicon as the hole-transporting layer. QDs such as CdSe:ZnS (core shell configuration) are size tunable and hence wavelength tunable. Advantages such as narrow bandwidth and high luminescence quantum yield make them suitable for LED applications.
Patterned deposition of QD monolayers was demonstrated on a silicon substrate using an efficient and silicon compatible micro-contact printing (or "stamping") technique. Then sputtering of ZnO:SnO2 created an amorphous electron transport film on top of the QD layer. A thin transparent metal cathode was deposited using e-beam evaporation to observe light emission. A look-up table was created in the computer to correlate the fluorescence with the atomic force microscopy measurement of the thickness of the nanoparticle thin films. Transmission electron micrographs were taken to determine the uniformity of the particles on stamping. Electrical injection into the inorganic transport layers resulted in light emission. Multicolor LEDs were demonstrated by loading the patterned PDMS stamp with particles of different sizes (and therefore different emission wavelengths).
This research is likely to open up many exciting opportunities in novel optoelectronic applications including near-field microscopy beyond the diffraction limit, MEMS-based medical endoscopes for sub-cellular imaging, and compact light-on-chip biosensors and biochips.
About the author
The work was performed at the University of Texas at Austin, US, and supported by the National Science Foundation (NSF). Ashwini Gopal is a PhD student in the Department of Electrical Engineering at the University of Texas at Austin. Her current research interests include optical MEMS, quantum-dot-based light-emitting devices and nano-opto-mechanical systems at the Zhang research group. Dr Kazunori Hoshino is a postdoctoral researcher at the Department of Biomedical Engineering. His research interests are MEMS and optical MEMS, biologically inspired microvisual systems, and nanoscale light-emitting devices and nanophotonics. Sunmin Kim is an undergraduate senior student in the Zhang research Group. She was a NSF NNIN funded REU (research experience for undergraduate) student. Prof. Xiaojing (John) Zhang is currently an assistant professor at the Department of Biomedical Engineering and the Microelectronics Research Center. His research interests include the integration of photonics with micro-electro-mechanical systems and micro-fluidic devices for in vivo imaging, near-field microscopy and sensing, in particular, toward investigating cellular processes that are critical to development and disease. The US National Science Foundation supported this work.