D. C. Choo

Single-electron transistors operating at room temperature, fabricated utilizing nanocrystals created by focused-ion beam

A focused-ion-beam (FIB) technique utilizing both lithographic and nanoparticle formation processes has been introduced to fabricate a single-electron transistor (SET) that can operate at room temperature. The results for the drain current as a function of the gate voltage at different source voltages at room temperature clearly show Coulomb oscillations indicative of Coulomb-blockade effects. These results indicate that SETs operating at room temperature, fabricated utilizing the FIB technique, hold promise for potential applications in ultra-high-density memory devices.

Structural, electrical, and optical properties of SnO2 nanocrystalline thin films grown on p-InSb (111) substrates

SnO2 thin films were grown on p-InSb (111) substrates by radio-frequency magnetron sputtering at low temperature. Atomic force microscopy images showed that the root mean square of the average surface roughness of the SnO2 films grown on the InSb (111) substrates with an Ar/O2 flow rate of 0.667 and at a temperature of 200u200a°C had a minimum value of 2.71 nm, and x-ray diffraction and transmission electron microscopy (TEM) measurements showed that these SnO2 thin films were polycrystalline. Auger electron spectroscopy and bright-field TEM measurements showed that the SnO2/p-InSb(111) heterointerface was relatively abrupt. High-resolution TEM measurements revealed that the SnO2 films were nanocrystalline and that the grain sizes of the nanocystalline films were below 6.8 nm. The capacitance–voltage measurements at room temperature showed that the type and the carrier concentration of the nominally undoped SnO2 film were n type and approximately 1.67×1016u200acm−3, respectively, and the current–voltage curve indi...

Enhancement of the activation energy in coupled CdTe/ZnTe quantum dots and quantum-well structures with a ZnTe thin separation barrier

Temperature-dependent photoluminescence measurements showed that the activation energy of CdTe/ZnTe quantum dots (QDs) coupled with a quantum well is much larger than that of the QDs alone, This behavior is attributed to the tunneling of carriers via a thin separation layer from the quantum well to the QDs. The present observations can help improve understanding of the enhancement of the activation energy in coupled CdTe/ZnTe nanostructures.

Coalescence and electron activation energy in CdTe/ZnTe nanostructures

Atomic force microscopy (AFM) and photoluminescence (PL) measurements were carried out to investigate the coalescence and electron activation energy in CdTe/ZnTe nanostructures. The results of the AFM images show that uniform CdTe quantum dots (QDs) are formed and that the transformation from CdTe QDs to CdTe quantum wires is caused by the coalescence. The excitonic peaks corresponding to the transition from the ground electronic subband to the ground heavy-hole band in the CdTe/ZnTe QDs shifted to higher energy in comparison with those of the CdTe/ZnTe quantum wires. The activation energy of the electrons confined in the CdTe QDs, as obtained from the temperature-dependent PL spectra, was higher than those in CdTe quantum wells and quantum wires. The present results can help to improve the understanding of coalescence and electron activation energy in CdTe/ZnTe nanostructures.

Formation mechanism of CdTe self-assembled quantum dots embedded into ZnTe barriers

Photoluminescence spectra showed that the formation mechanism for CdTe layers grown on ZnTe thin films changed from a two-dimensional mode to a three-dimensional mode with increasing submonolayer CdTe layer thickness, and the temperature-dependent PL spectra indicated that the activation energy of CdTe quantum dots is larger than that of CdTe single quantum wells. The formation mechanism for the CdTe QDs is in reasonable agreement with a Stranski–Krastanov growth mode.

Dependence of the InAs size distribution on the stacked layer number for vertically stacked InAs/GaAs quantum dots

Abstract Atomic force microscope (AFM), transmission electron microscopy (TEM), and photoluminescence measurements were carried out to investigate the dependence of the InAs quantum dot size distribution on the stacked layer number for vertically stacked InAs/GaAs quantum dots (QDs) grown on (0xa00xa01) GaAs substrates. AFM and TEM images showed that the size of the QDs increased with increase in the stacked layer number up to the deposition time of 20xa0s. However, the size distribution uniformity of the QDs was improved with increase in the stacked layer number when the deposition time and the stacking layer of the InAs QDs gradually decreased. These results can help in an improved understanding of the control of sizes of QDs in InAs/GaAs QD arrays.

Microstructural and optical properties of InAs/GaAs quantum dots embedded in modulation-doped AlxGa1−xAs/GaAs heterostructures

The microstructural and the optical properties of InAs/GaAs quantum-dot (QD) arrays inserted into undoped GaAs barriers embedded in an AlxGa1−xAs/GaAs were investigated by using transmission electron microscopy (TEM) and photoluminescence (PL) measurements. The TEM images and the selected-area electron diffraction patterns showed that vertically stacked InAs QD self-assembled arrays were embedded in the GaAs barriers. The temperature-dependent PL spectra showed that the peak corresponding to the interband transitions from the ground electronic subband to the ground heavy-hole band of the InAs QDs shifted to lower energy with increasing temperature. The PL intensity of the InAs dots was significantly enhanced by the modulation-doped AlxGa1−xAs/GaAs heterostructure, and the thermal activation energy of the InAs dots was decreased by the addition of the modulation-doped AlxGa1−xAs/GaAs heterostructure. The present results can help to improve the understanding of the microstructural and the optical properties...

Atomic arrangements and formation mechanisms of the CuPt-type ordered structure in CdxZn1−xTe epilayers grown on GaAs substrates

Selected area electron diffraction pattern (SADP) and transmission electron microscopy (TEM) measurements were carried out to investigate the ordered structure in CdxZn1−xTe epitaxial layers grown on (001) GaAs substrates. The SADP showed to sets of superstructure reflections with symmetrical intensities, and the corresponding high-resolution TEM image showed a doublet periodicity in the contrast of the {111} lattice planes. The results of the SADP and the TEM measurements showed the existence of a CuPt-type ordered structured in the CdxZn1−xTe epitaxial layers. This CuPt-type ordered structure had two different variants with an antiphase boundary existing between the two variants. The formation of a CuPt-type ordered structure in a CdxZn1−xTe epitaxial layer might originate from the minimization of the strain relaxation energy in the reconstructed GaAs (001) surface. A possible atomic arrangement of and a formation mechanism for the CuPt-type ordered structure in the CdxZn1−xTe epitaxial layer are presen...

Optical parameters in SnO2 nanocrystalline textured films grown on p-InSb (111) substrates

Abstract Spectroscopic ellipsometry and photoluminescence (PL) measurements on SnO2 nanocrystalline textured films grown on p-InSb (111) substrates by using radio-frequency magnetron sputtering at low temperature were carried out to investigate the dependence of the optical parameters on the SnO2 thin film thickness. As the SnO2 film thickness increases, while the energy gap of the SnO2 film decreases, its refractive index increases. The PL spectra show that the broad peaks corresponding to the donor–acceptor pair transitions are dominant and that the peak positions change with the SnO2 film thickness. These results can help improve understanding for the application of SnO2 nanocrystalline thin films grown on p-InSb (111) substrates in potential optoelectronic devices based on InSb substrates.

The dependence of the carrier density and the mobility on the spacer-layer thickness in modulation-doped AlxGa1−xAs/InyGa1−yAs/GaAs asymmetric single quantum wells

The variation of the electron carrier occupation and the mobility in the subband as a function of the spacer layer thickness in modulation-doped AlxGa1−xAs/InyGa1−yAs/GaAs strained single quantum wells was investigated by Shubnikov-de Haas (S-dH) and Van der Pauw Hall-effect measurements. The results of the fast Fourier transform (FFT) for the S-dH data and those of the Hall-effect data showed that the magnitude of the electron carrier density in the sub-band increased as the spacer layer became thinner, and the increase in the carrier density with decreasing spacer-layer thickness resulted from an increase in the distance between the Fermi energy level and the top of the depletion layer. The full width at half maxima of the FFT results for the S-dH data and of the results of the Hall-effect measurements indicated that the value of the electron mobility increased as the spacer-layer thickness increased, and the increase in the electron mobility originated from a decrease in the Coulomb interaction between the ionized donors and the electrons. The electronic sub-band energies, corresponding wave functions, and the Fermi energies in the InyGa1−yAs quantum wells were calculated by a self-consistent method taking into account exchange-correlation effects together with the strain and nonparabolicity effects. These present results can help to improve the understanding for the application of AlxGa1−xAs/InyGa1−yAs/GaAs strained single quantum wells in electronic devices such as high-frequency and high-speed field-effect transistors.