Quantum Dots Aim to Be Smart Pixels

Dr. James P. Smith

Scientists at the University of California reported the development of a “smart pixel” device based on quantum dots that can collect, store and retrieve optical information in a single efficient unit.

Optical information processing typically requires three technologies: a CCD that collects the image or pattern; a computer that stores and interprets the digital output of the detector; and LEDs in a display that convert the signal back into visual information.

The researchers fabricated the smart pixels by molecular beam epitaxy. They inserted an InAs layer of self-assembled quantum dots into a GaAs/AlGaAs metal insulator semiconductor. The InAs contains an ultrahigh density of quantum dots, about 1010/cm². The lens-shaped dots vary in size; their average height is about 4 nm, and their average diameter is 30 to 40 nm.

The mismatch in the crystal structures of InAs and GaAs/AlGaAs produces a strain in the materials grown over a quantum dot, creating a “strain-induced quantum dot” in the GaAs quantum-well layer. In this way, the group created coupled pairs of quantum dots and strain-induced quantum dots separated by a potential barrier.

An incoming photon generates an electron-hole pair, which is stored in adjacent, strain-coupled quantum dots. After a few seconds, an applied bias voltage allows the hole to tunnel to the electron, and their recombination produces a photon. The right image is an electromicrograph of the structure diagrammed at left.

Pierre M. Petroff, the leader of the team, explained that an incoming photon creates an electron-hole pair at the point of impact on the smart pixel device that the adjacent layers of a quantum dot can store. The dots are very efficient at localizing these carriers, so they can keep the pairs confined and 10 nm apart for several seconds.

When a bias voltage is applied between the quantum dots, the electron-hole pair recombines, which then releases a photon from the same point where the original photon impacted the surface. The researchers demonstrated the effect by exposing the device to pulses from an argon-ion laser and making it re-emit up to 3 s later.

Room-temperature operation

Although the smart pixels are promising, problems must be solved. Most importantly, the present device was tested at temperatures below 5 K, but Petroff hopes a GaN/AlN structure eventually will yield a device that works at room temperature. Also, the introduction of optical microcavities and photonic bandgap structures could preferentially control the direction of the photons.

The researchers described the process in the Dec. 17, 1999, issue of Science.