Hyung Gyoo Lee

MANY-BODY EFFECTS ON MODULATION-DOPED INAS/GAAS QUANTUM DOTS

The excitation intensity dependent photoluminescence spectra of various modulation-doped InAs/GaAs quantum dots exhibit the band filling and band-gap renormalization. From the time-resolved photoluminescence spectra, in addition to the interaction of the carriers in quantum dots with the remote ionized impurities in a GaAs barrier, the screening due to the two-dimensional charges is found to mainly affect the carrier lifetime in the modulation-doped quantum dots.

Single-Electron-Based Flexible Multivalued Exclusive- or Logic Gate

By using two symmetrical sidewall gates, we implemented a Si-based single-electron exclusive- OR (XOR) gate and reported on the first flexible multivalued (MV) functionality. A grayscale contour plot of the output voltages displays alternating high/low values as a function of two single-electron transistor (SET) input voltages. Their voltage transfer characteristics display typical XOR or XNOR gate function depending on input voltages for binary, MV, and binary-MV mixed-modes. This flexible two-input XOR gate, combined with the previously reported NAND/NOR gates, provide three basic arithmetic blocks for the SET-based MV logic gate family.

Structural defects in the growth of multiple periods of InAs quantum dots on a GaAs substrate

Microstructural observations on 20 periods of InAs quantum dots on a GaAs substrate grown by molecular beam epitaxy system were carried out by using high resolution transmission electron microscopy. The spherical cap-shaped InAs quantum dots were formed in a self-organized fashion, dot over dot, along the growth direction. However, two types of anomalities were found in the growth of these superlattice structures. One is the stoppage of quantum dot formation after 4 or 5 layers have been deposited. The morphology of the quantum dots was rather flat and faceted and a black and white contrast layer has appeared in the dot structure. The other type was a volcano-like defect which was grown vertically along the growth direction with a size of about 120 nm in diameter and about 400 nm in spacing. Inside the defect, black and white contrast layers have been formed along the [110] direction at the bottom of the epilayer and then changed to the [111] direction as the growth continued to the top layer.

Effect of native defects on electrical and optical properties of undoped polycrystalline GaN

Polycrystalline GaN (poly-GaN) films were grown on silica glass substrates by metal-organic chemical vapor deposition. An n-type conductivity in all the undoped poly-GaN films was observed and background impurities such as silicon and carbon were detected. The Si impurity was found to be diffused from the silica substrate and thought to be a dominant source of the n-type conductivity. It was also assumed that the C impurity as a compensating acceptor slightly affected the background electron concentration. It was thought that deep level emissions around 2.2 eV were associated with gallium vacancies and carbon impurity complexes at grain boundaries of the poly-GaN.

Time-resolved spectroscopy of InAs quantum dots using one-side modulation-doping technique: renormalization and screening

We present the optical properties of modulation-doped InAs/GaAs quantum dots (QDs). Bandgap renormalization is clearly observed from a red shift of the emission peaks on the QDs with various modulation-doping concentrations, demonstrating that exchange interactions of electrons within single states have no effect on renormalization. Screening due to the electrons at modulation-doped layer as well as the photo-generated carriers in GaAs matrix plays an important role in reducing the radiative recombination rate.

Carrier relaxation of modulation-doped InAs/GaAs quantum dots

Abstract We present the carrier relaxation of modulation-doped InAs/GaAs quantum dots depending on the excitation wavelength and modulation-doping concentration by using the time-resolved spectroscopy. At the excitation below GaAs band gap, the relaxation processes become very slow, implying the observation of phonon bottleneck effects. On the other hand, at excitation far above GaAs band gap, phonon bottleneck effects are broken down due to Auger processes. Increasing modulation-doping concentration, Coulomb scattering between electrons in GaAs-doped layer and carriers in InAs quantum dots makes the relaxation times become fast.

Optimization of Reverse Engineering Processes for Cu Interconnected Devices

Reverse engineering of semiconductor devices utilizes delayering processes, in order to identify how the interconnection lines are stacked over transistor gates. Cu metal has been used in recent fabrication technologies, and de-processes becomes more difficult with the shrinking device dimensions. In this article, reverse engineering technologies to reveal the Cu interconnection lines and Cu via-plugs embedded in dielectric layers are investigated. Stacked dielectric layers are removed by CF4 plasma etching, then the exposed planar Cu metal lines and via-plugs are selectively delineated by wet chemical solution, instead of the commonly used plasma-based dry etch. As a result, we have been successful in extracting the layouts of multiple layers within a system IC, and this technique can be applicable to other logic IC, analog IC, and CMOS IC, etc.

Magneto-optical spectroscopy of modulation-doped GaAs/AlGaAs asymmetric coupled double quantum wells

Abstract We have presented the magneto-optical studies of modulation-doped GaAs/AlGaAs asymmetric coupled double quantum wells. The transition energies of two asymmetric quantum wells are in good agreement with those obtained from a self-consistent calculation. The effective mass of the first subband electron is found to be larger than that of the second subband electron. This means that, by virtue of the band bending effects, the first electron subband originates from a narrow well and the second from a wide well. The magnetic-field dependence of Landau-level transitions between the second electron subband and the first heavy-hole subband shows an enhancement of an excitonic emission near the Fermi edge.