Y. Ohizumi

Light emission spectra of AlGaAs/GaAs multiquantum wells induced by scanning tunneling microscope

We have investigated the scanning-tunneling-microscope light emission (STM-LE) spectra of p-Al0.4Ga0.6As/p-GaAs multiquantum wells. The injection current level was kept as low as 0.1–0.5 nA to ensure that the sample is not damaged by the tunneling current. This is the current level ordinarily used for taking STM images. The peak energy of the emission shifts to the high energy side with decreasing well widths. A corresponding peak shift behavior was also observed in the photoluminescence (PL) spectra for the same samples. From comparisons of the STM-LE and the PL spectra, we find that although there is a difference in the excitation process, the final recombination process is identical in both cases.

Light emission spectra of individual GaAs quantum wells induced by scanning tunneling microscope

We have investigated the light emission from individual single GaAs quantum wells of cleaved (110) AlGaAs/GaAs heterostructures, using the scanning tunneling microscope tip as a local injection source of minority carriers. Single emission peaks were observed to shift to the high-energy side with decreasing well width. The emission peaks are assigned to the transition between n=1 single-quantum-well electron and heavy-hole states of the respective wells.

Light intensity imaging of single InAs quantum dots using scanning tunneling microscope

Light intensity images of self-assembled p-type InAs quantum dots (QDs) embedded in Al0.6Ga0.4As were measured by injecting electrons from the tip of a scanning tunneling microscope at room temperature. Bright round features appeared in the images for different photon energies. The light emission spectrum measured over each bright feature showed a single emission peak with different peak energy. By comparing the emission peak energies with the transition energies calculated for pyramidal shaped QD structures, we found that the observed bright features correspond to individual InAs QDs.

Electron transport in the barriers of AlGaAs/GaAs quantum well structures observed by scanning-tunneling-microscope light-emission spectroscopy

Using scanning-tunneling-microscope light-emission (STM-LE) spectroscopy, we have investigated the transport properties of minority carriers in p-Al0.3Ga0.7As/p-GaAs quantum well (QW) structures. The optical measurements were performed on a cleaved (110) surface at room temperature. The STM-LE spectra were measured by injecting hot electrons from the STM tip positioned at different distances from the QWs. The emission intensity from individual wells as a function of the tip-well distance was found to decay with two distinct decay constants.

Formation of misfit dislocations in GaAs/InGaAs multiquantum wells observed by photoluminescence microscopy

We have investigated the formation mechanism of misfit dislocations of GaAs/InGaAs multiquantum well structures by means of photoluminescence (PL) microscopy using the scanning near-field optical microscope. In the PL images, dark lines appeared along both [110] and [110] directions. From comparison with the surface topographic images, we found that these dark lines correspond to misfit dislocations, which give rise to nonradiative recombination centers in the InGaAs well. The density of dark lines in the 〈110〉 directions as a function of the total layer thickness shows the existence of two critical layer thicknesses for the formation of misfit dislocations. The two distinct critical thicknesses are explained in terms of the modified J. W. Matthews and A. E. Blakeslee, [J. Cryst. Growth 27, 118 (1974)] model in which a lattice frictional force proportional to the In mole fraction is taken into account.

Characterization of surface nanostructures by STM light emission: individual GaAs/AlGaAs quantum wells

By spectroscopically analyzing the light emitted by specific nanostructures under the scanning tunneling microscope tip (scanning tunneling microscopy light emission spectroscopy: STM-LES), we have investigated the electronic and optical properties of individual quantum wells (QWs) of p-type AlGaAs/GaAs layered structures. Atomic resolution was obtained on the cleaved (110) surface that shows the cross-sections of QWs, and the emission spectra from individual wells were measured by injecting electrons from the STM tip into them. Each individual well emits a spectrum that is consistent with the electronic transitions for the appropriate well width and also with the photoluminescence (PL) spectra. Furthermore, the diffusion length of minority carriers were estimated in real space by injecting electrons at different distances from a given well, and by observing the change in the emission intensity.

Ground-state interband transition of individual self-assembled InAs/Al0.6Ga0.4As quantum dots observed by scanning-tunneling-microscope light-emission spectroscopy

We have investigated the optical transitions in individual self-assembled InAs/Al0.6Ga0.4As quantum dots (QDs) by means of scanning-tunneling-microscope (STM) light-emission spectroscopy. Localized bright features were observed in the spectrally resolved light intensity images measured by injecting electrons from the STM tip. The light emission spectra measured over the bright features showed single emission peaks having different peak energies with linewidths of 30–45 meV. By comparing these results with atomic-force-microscope images and photoluminescence (PL) spectra, we have identified the bright features with the ground-state interband transition from individual InAs QDs. The emission peak energies were compared with the transition energies calculated for pyramidal-shaped QD structures, based on a single-band and constant-confining-potential model. A reasonable agreement was obtained between the experimental and calculated results. The emission linewidth of individual dots is much narrower than the l...

Diffusion process of electrons injected from STM tip into AlGaAs/GaAs quantum wells

We have investigated the electron diffusion process in Al0.3Ga0.7As/GaAs quantum well (QW) structures by means of scanning tunneling microscope light emission (STM-LE) spectroscopy. The optical measurements were performed on a cleaved (1 1 0) surface at room temperature. The STM-LE spectra were measured by injecting hot electrons from the tip positioned at different distances from the QWs. The emission intensity from individual wells as a function of the tip-well distance was found to decay with two distinct decay constants. From comparison with Monte Carlo simulations for hot electron relaxation, we found that the intervalley scattering from the G valley to the L and X valleys has the most significant effect on the diffusion process of the injected electrons. # 2002 Elsevier Science B.V. All rights reserved.

STM light emission spectroscopy of individual quantum wells: measurement of transport parameters in real space

By spectroscopically analyzing the light emitted from the tip-sample gap of the scanning tunneling microscope (STM), we have investigated the carrier transport as well as the luminescence properties of AlGaAs/GaAs quantum wells (QWs). The emission intensity form a target well was measured as a function of the tip position on a cleaved (110) surface of the QW structures. The thermalization length and the diffusion length of the injected electrons were determined in real space.

Determination of Electron Transport Parameters in Al0.3Ga0.7As by Scanning‐Tunneling‐Microscope Light‐Emission Spectroscopy

We have determined the electron transport parameters in AlGaAs in real space, using a scanning tunneling microscope (STM). The electrons were injected from the STM tip into the cleaved (110) surface of AlGaAs/GaAs quantum well structures at different distances from the target well. Then by measuring the light emission intensity from the target well, two distinct decay constants were found. From comparison between the experimental results and Monte Carlo simulations, we have identified the two decay constants with the thermalization length and the diffusion lengths of the injected electrons.