Single-photon emitters operating at room temperature could be used in on-chip quantum communication applications, and as a source of “flying” qubits for quantum computing. Quantum dots (QDs) made from the III-nitride materials could be perfect in such devices thanks to their unique properties that include the fact that they are highly stable (both chemically and at high temperatures) and have a large breakdown voltage, and because they can emit photons over a wide range of light wavelengths from the ultraviolet to the infrared parts of the electromagnetic spectrum.

The problem is that, until now, no-one had ever seen single-photon emission from these materials at room temperature because sample quality was poor. “And although researchers have observed room-temperature single-photon emission from other nanostructures, such as the colour centres in diamond, this is the first time that they have been seen emanating from a QD that has been fabricated at a pre-defined location," team member Mark Holmes told “Indeed, previous studies relied on structures that had formed at random locations on a substrate.

The Tokyo researchers, led by QD pioneer Yasuhiko Arakawa, fabricated their devices in their clean room using a process called selective-area metal-organic chemical vapour deposition. The researchers grew the quantum dots on sapphire substrates covered with a 25 nm layer of aluminium nitride. This process included sputtering a 25 nm deep silicon dioxide layer onto the substrate surface, and then creating arrays of apertures (25 nm in diameter) using electron-beam lithography and reactive-ion etching. These apertures then house gallium-nitride nanowires and quantum dots, which were grown separately.

Next, the team grew a layer of AlxGa1–xN around the nanowires to form a core−shell type structure, onto which they grew a GaN QD thanks to a short eight-second GaN growth step. Finally, they capped the structures with a layer of AlGaN. The process allows the researchers to control where each QD ends up on the substrate since the linear (XY axis) position of every dot depends on where each nanowire is located in the first place and its distance from the substrate (Z axis) is defined by the nanowire height (which is around 700 nm).

Arakawa and colleagues then measured the light emission properties of their quantum dots by exciting them with a pulsed laser beam and detecting the light that came out.

It's official: GaN QD emits single photon per excitation light pulse - and at 300 K

In theory, a single-photon quantum emitter should emit a single photon per excitation light pulse. To test this, the researchers thus split the light emitted into two paths and, using two separate detectors, measured the time elapsed between the light pulses recorded at each detector. “For a pure single photon source, we should not see photons at both detectors simultaneously – something we verified in an experiment for the first time for this kind of GaN site-controlled nanowire quantum dot,” said Holmes.

More importantly, the observations hold up even when the quantum dot is at room temperature, he adds. “We believe this is because we are using small GaN dots whose position we can control accurately. Such dots are less contaminated spectrally, which means that we can still detect them at such high temperatures.”

Single-photon emitters in general are often touted as being ideal for quantum cryptography applications, but we reckon that the devices we have made will be more suited to on-chip communications for quantum information processors, he says.

“We are now busy looking at ways to measure the operating speeds of our devices,” said Holmes. “We are also trying to make them work by injecting current into them rather than exciting them with a laser.”

The current work is detailed in Nano Lett. DOI: 10.1021/nl404400d.

Further reading

Quantum dot nanowire emits light (Feb 2007)
Elongated quantum dots emit via multiexcitons (Jul 2012)
Hybrid structures show improved light emission (Feb 2012)
Simulation tool reveals new origin for silicon QD emission (May 2009)
InP quantum dots grown on silicon emit single photons (Jul 2012)