T. Tsuruoka

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.

Microscopic identification of dopant atoms in Mn-doped GaAs layers

Using cross-sectional scanning tunneling microscopy (XSTM), we have identified the dopant atoms in Mn-doped GaAs layers grown at 400 °C by molecular-beam epitaxy. The Mn-dopant atoms appeared as diffuse light areas superimposed on the background of As atomic rows in the STM images. The Mn acceptor concentration deduced from the STM images agreed well with the hole concentration determined by Hall measurements. No As antisite and associated defects were observed. These results indicate that Mn atoms are incorporated into the GaAs layer as electrically activated acceptors.

Local electronic structures of GaMnAs observed by cross-sectional scanning tunneling microscopy

Using cross-sectional scanning tunneling microscopy (STM), we have investigated the local electronic properties of molecular-beam epitaxy grown GaMnAs layers on a p-GaAs substrate. The STM image shows light and dark areas with the average size on the order of nm. From conductance spectra measured with the STM, the bandgap of the GaMnAs is estimated to be 1.23±0.05u2009eV. An apparent conductance within the bandgap indicates the presence of hole states in the valence band, which are induced by Mn acceptors. A conductance peak at 0.7 eV above the valence band edge can be identified with electron tunneling into the ionization levels of As antisites.

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.