Avalanche photodetectors are used in telecommunications networks and work by converting relatively weak optical signals into robust electrical pulses. In such devices, an incoming light pulse strikes a semiconductor, freeing a few charge carriers (electron and holes) that are then accelerated by an electric field and in turn impact-ionize (or free) other charge carriers. This process creates an avalanche of carriers that is extracted as the amplified signal.

Scientists have previously shown that shrinking the size of the active avalanche region in the semiconductor results in better performing photodetectors. Nanowires might be ideal building blocks for such devices thanks to their small volume that lowers the capacitance and “dark”, or unwanted leakage, current compared with conventional planar photodetectors.

Incorporating “antennas”

However, the problem is that single nanowire photodetectors absorb less than 1% of the light falling on them, something that reduces the external quantum efficiency of the devices. But, engineering better avalanche gain in nanowires is no easy task because of the large surface-to-volume ratio of these structures that adversely affects carrier transport. A team led by Diana Huffaker has now overcome this inherent limitation by incorporating antenna structures into devices made from gallium arsenide nanowires that can collect light from a larger area and focus it within the nanostructure.

The device consists of a p-doped GaAs light absorber (150 nm wide and 900 nm high) atop a core−shell GaAs p-n junction. The n-GaAs core is 80 nm wide and 800 nm high, while the p-GaAs shell is 35 nm thick surrounding the core. “Our unique architecture allows us to independently engineer photon absorption and carrier multiplication processes, which are thus separated within the nanowire space,” explained team member Pradeep Senanayake. “Through measurements of gain and capacitance in the device, we have shown that photons are absorbed via so-called surface plasmon polariton Bloch wave modes excited by the 3D optical antenna, creating photogenerated carriers within the tip of the nanowire.”

These carriers are funnelled into the thin avalanche multiplication region and the device produces gains of more than 200 at –12 V, he told nanotechweb.org. Such values compare well with those observed in state-of-the art avalanche photodetectors.

Part matter part light

A surface plasmon polariton is a quasiparticle that results from the interaction of light with charge oscillations, or surface plasmons, at the interface between dielectric materials and metals. A surface plasmon polariton has two different components: a charge oscillation created by electrons in the metal and a photon, which is an electromagnetic wave. Surface plasmons have a higher momentum than photons and therefore require grating antenna structures to couple with photons, but the UCLA team has succeeded in engineering the surface plasmon polariton modes in its device so that there are electric field “hot spots” within the nanowire, something that is good for producing small, highly efficient photodetectors.

Scientists believe that surface plasmon polaritons will be important for future photonics devices that exploit light instead of electricity to process information. Such devices would be much faster and use less energy than their electronic counterparts but the strong coupling of polaritons with light will be crucial for the success of this new technology.

Compatible with silicon and III-V substrates

Nanowire avalanche photodetectors have the advantage of being compatible with both III-V or silicon substrates and can be grown on either of these two substrates. This means that the devices could find use as single photon detectors for quantum communication systems, nanophotodiodes for optical interconnects and pixel elements for focal plane arrays, says Senanayake.

The team is now busy looking at how to make the nanowire avalanche photodetectors in a format that is compatible with high-speed devices. The researchers also hope to shift the operating wavelength of their detectors to the technologically important 1.55 µm window. “We also plan to map more accurate 3D doping profiles in our devices and so design 3D avalanche regions,” added Senanayake.

The current work is reported in Nano Letters.