Senior Researcher, Atomic Electronics Group, Nanomaterials Laboratory, National Institute for Materials Science (NIMS)
Study of the optical and electronic properties of individual semiconductor nanostructures using scanning tunneling microscopy
With recent developments in crystal growth and micro-processing techniques, semiconductor nanostructures, such as quantum wires and dots, on the order of 10-20 nm can be easily fabricated. With conventional methods, like photoluminescence (PL), it was difficult to determine the properties of individual structures, because only the averaged properties of an ensemble of nanostructures with size and shape distributions could be measured. Using the tip of a scanning tunneling microscope (STM) as the local carrier injection source, we have investigated the properties of individual semiconductor nanostructures by analyzing spectroscopically the light emitted by radiative recombination of the injected carriers just under the tip.
On a cleaved (110) surface of AlGaAs/GaAs multi-quantum well (QW) structures that were grown on GaAs(100) substrates, individual GaAs well layers were identified by cross-sectional STM. By measuring the light emission intensity from the respective wells by injecting tunneling electrons as a function of the tip position along the [001] direction, two distinct decay lengths were determined with a precision of a few nanometers. To understand the physical origin of the obtained decay lengths, we have performed Monte Carlo simulations, in which the quantum size effect and the relevant boundary conditions were taken into account. As a result, it was found that the two decay lengths correspond to the thermalization length and the diffusion length of the injected electrons. From the detailed comparison between the experiment results and simulation, important parameters, such as the mechanism dominating the electron transport in nanostructures and the interface recombination rates, were estimated.
By mapping the light emission spectra from self-assembled InAs/AlGaAs quantum dots (QDs), we have also measured the light intensity images of single dots. The emission linewidth of individual dots was much narrower than the PL linewidth for ensembled QDs. Thus, a significant portion of the broad linewidth of the PL peak arises from the inhomogeneous emission energies of individual dots. The spatial resolution of the light intensity images was estimated to be about 25 nm. Moreover, we found that the emission linewidth of individual dots increases with the increase of Al content. These results show that local light emission spectroscopy, based on STM, is a powerful tool for investigating the optical properties of individual semiconductor nanostructures.
