Jul 4, 2008
Nanowires light up silicon
Optoelectronic devices on silicon is sometimes denoted as the holy grail of the semiconductor industry. This is partly for the immense opportunities that it brings, but also for the continued evasiveness of its realization. The silicon material itself is a poor light emitter due to its indirect bandgap and other more exotic materials, such as GaAs or InP, are usually used for light emitting devices. While these materials have proven difficult to integrate on silicon with conventional planar technology, the nanowire technology may provide a new opportunity. Nanoscale light sources that can be controlled and integrated on the wafer scale may now be available on silicon. The realization of III-V nanowire LEDs on silicon is a leap towards applications such as low-cost LEDs and on-chip light sources integrated with CMOS technology.
Semiconductor nanowires are one dimensional structures that have been used as building blocks for a variety of nanoscale electronic and photonic devices. In a recent article published in Nanotechnology the authors demonstrate infrared nanoscale LEDs on silicon substrates.
The nanowire LEDs are so called core-shell structures based on III-V semiconductors. Light generation takes place in a GaAs nanowire with an InGaP cladding layer providing the n-contact (see figure). The silicon substrate functions as a template for epitaxial growth and as an electrically integrated p-region for the devices. The use of silicon as a growth template can drastically reduce the cost of producing electronic and photonic devices due to inexpensive substrates and opportunities for large-scale processing. What enables successful integration of LEDs on silicon is the small footprint of the nanowire that accommodates the strain from lattice and thermal expansion coefficient mismatches of the two semiconductor materials.
A major achievement in the present work is the successful use of planar process technologies, which the scientists in Lund have developed for nanowire transistors, for the realization of nanowire LEDs. It was demonstrated, for example, that the position of each individual nanowire could be controlled by lithography. This enables more advanced devices such as LEDs integrated/combined with photonic crystals and nanostructured surfaces for improved light extraction.
The light-generating properties, including EL spectra and radiation pattern, of these nanoscale beacons are reported in the article. The light-generating efficiencies of these novel devices are still substantially below that achieved by planar commercial LEDs, but the integration with silicon and the nanoscale size of these devices may open up for completely new types of applications, say the inventors. III-V nanowires may be the ”bright” future of silicon.
About the author
Patrik Svensson is a research engineer at QuNano. Thomas Mårtensson holds a shared affiliation as research engineer at QuNano and PhD student at Lund University. Johanna Trägårdh recently finished her PhD at Lund University. Christina Larsson is a senior scientist at QuNano. Michael Rask was formerly technical manager at QuNano (now at Svedice AB). Dan Hessman is associate professor (in nano-optics) at Lund University. Lars Samuelson is head of the Nanometer Structure Consortium at Lund University and founder of QuNano AB. Jonas Ohlsson is research director at Qunano AB and co-founder of QuNano AB.