D. U. Lee

Microstructural, electrical, and optical properties of SnO2 nanocrystalline thin films grown on InP (100) substrates for applications as gas sensor devices

SnO2 thin films were grown on p-InP (100) substrates by using radio-frequency magnetron sputtering at low temperature. Transmission electron microscopy (TEM) and electron diffraction pattern measurements showed that these SnO2 thin films were nanocrystalline. 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 1.62×1016 cm−3, respectively. Raman scattering measurements showed that the grain sizes of the nanocrystalline films were below 10 nm, which was in reasonable agreement with the result obtained from the high-resolution TEM measurements. Photoluminescence measurements showed a broad peak below the band-to-band emission. These results can help improve the understanding of SnO2 nanocrystalline films grown on p-InP (100) substrates for applications in high-sensitivity gas sensors.

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 200 °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 1.67×1016 cm−3, respectively, and the current–voltage curve indi...

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.

Magnetotransport, magneto-optical, and electronic subband studies in InxGa1−xAs/InyAl1−xAs modulation-doped strained double quantum wells

Magnetotransport and magneto-optical measurements on InxGa1−xAs/InyAl1−yAs modulation-doped strained double quantum wells with a 100 A In0.8Ga0.2As well and a 100 A In0.53Ga0.47As quantum wells separated by a 35 A In0.25Ga0.75As potential barrier were carried out to investigate the electrical and the optical properties of the electron gas in the quantum wells. The Shubnikov-de Haas measurements at 1.5 K demonstrated clearly the occupation of three subbands in the quantum wells by a two-dimensional electron gas. The electron effective masses determined from the slopes of the cyclotron resonance peak energies as a function of the magnetic field were 0.06171 and 0.05228me. The electronic subband energies, the subband energy wavefunctions, and the Fermi energy in the quantum wells were self-consistently calculated a transfer matrix method taking into account the exchange-correlation effect, the strain effect, and the nonparabolicity effect, and the results of the cyclotron resonance measurements qualitatively...

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.

Existence of a CuAu–I-type ordered structure in lattice-mismatched InxGa1−xAs/InyAl1−yAs multiple quantum wells

Selected area electron diffraction pattern (SADP) and transmission electron microscopy (TEM) measurements were carried out to investigate the ordered structures in lattice-mismatched InxGa1−xAs/InyAl1−yAs multiple quantum wells (MQWs). The SADP showed two sets of extra spots with asymmetrical intensity, and the high-resolution TEM image showed doublet periodicity in the contrast of the (001) lattice planes. The results of the SADP and the TEM measurements showed that a CuAu–I-type ordered structure was observed near the lattice-mismatched InxGa1−xAs/InyAl1−yAs heterointerfaces. This CuAu–I-type ordered structure had an antiphase boundary in the periodically regular InxGa1−xAs/InyAl1−yAs lattice-mismatched region. The existence of a CuAu–I-type ordered structure in InxGa1−xAs/InyAl1−yAs MQWs might originate from the lattice mismatch between the InxGa1−xAs and the InyAl1−yAs layers. These results provide important information on the microstructural properties for improving operating efficiencies in long-wav...

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.