Microscopy Images High-Density Quantum Dots

Gary Boas

Scientists at Seoul National University in Korea and at the University of Tokyo have developed a technique with which they can perform high-resolution photoluminescence microscopy of high-density InAs/GaAs quantum dots. The work promises to advance the development of devices based on quantum dots as well as to add to the basic understanding of the structures themselves.

Researchers in Korea and Japan have employed near-field optical photoluminescence microscopy to image high-density samples of InAs/GaAs quantum dots. Here, the colored bars indicate the scale between selected peak emission energies for individual dots in a scanning area of 250 x 250 nm. Courtesy of Wonho Jhe. Reprinted with permission of Applied Physics Letters.

Self-assembled semiconductor quantum dots -- and, particularly, their optical properties -- are increasingly the focus of research. Near-field scanning optical microscopy has been deployed to perform high-resolution photoluminescence microscopy of quantum dots. The small interdot distances, however, have led researchers to use samples with dot densities of less than one per square micron for imaging. Optical applications, such as quantum dot lasers, typically employ densities 100 times greater or more.

The researchers used a Coherent Ti:sapphire laser with a 200-nm-diameter aperture in their setup. The choice of aperture size involved a trade-off between spatial resolution and throughput efficiency. Specifically, the aperture had to be small enough to resolve single dots, but a diameter of less than 100 nm would have resulted in a loss of transmission efficiency.

Coupled to a single-mode optical fiber, the laser illuminated the sample through a gold-coated, chemically etched fiber tip, which provided a sufficiently small excitation and detection spot size. The same fiber collected the resulting photoluminescence signal, which was then dispersed by a 0.3-m single monochromator and detected by a liquid-nitrogen-cooled CCD camera. Because individual dots have different emission energies -- the result of inhomogeneous broadening -- the CCD camera took the complete spectrum at each point in the scanning area. Using this setup, the researchers resolved six InAs/GaAs quantum dots in a scanning area of 250 x 250 nm, including two that were only 56 nm apart.

There are questions to be addressed before the technique can find application outside the laboratory. A better understanding of interactions among dots and the dynamic properties of individual dots is needed, said Wonho Jhe, a professor of physics at Seoul and one of the authors of the study.

It also will be important to be able to perform photoluminescence microscopy at room temperature. Efficient photoluminescence microscopy in the near-IR at room temperature will require detection systems with better spatial resolution and higher detection efficiency.