Myung-Jae Kim

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...

Strain distribution and interface modulation of highly lattice-mismatched InN/GaN heterostructure nanowires

The structural properties of InN/GaN heterostructure nanowires (NWs) were studied using transmission electron microscope techniques to determine strain behavior. A great quantity of the misfit strain between InN and GaN was relaxed through the introduction of misfit dislocations along the interface. Geometric phase analysis revealed a strain-concentration phenomenon in the strain map of the out-of-plane components and a gradual lattice recovery in that of the in-plane components over the InN/GaN interface. Interface structures that were modulated at the atomic-scale were observed in several InN/GaN heterostructure NWs. Complex strain distributions were identified in both InN and GaN.

Strain effects in and crystal structures of self-assembled InAs/GaAs quantum dots

The strain effects in and the crystal structures of self-assembled InAs/GaAs quantum dots (QDs) were investigated by using transmission electron microscopy (TEM). The in-plane lattice constant of the InAs QDs was larger than that of the GaAs substrate, and the vertical lattice constant of the InAs QDs was smaller than that of the InAs bulk. The variation of the lattice constant for the InAs QD originated from the strain effect. A schematic diagram of a strained InAs QD based on the TEM results, indicative of the strain distribution around the QD, is presented.

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.

Simultaneous existence and atomic arrangement of CuPt-type and CuAu-I type ordered structures near ZnTe/ZnSe heterointerfaces

Selected area electron diffraction pattern and high-resolution transmission electron microscopy measurements on a ZnTe/ZnSe heterointerface grown on a GaAs(001) substrate showed two structures of the CuPtB-type ordering structures, one for each direction of the doublet periodicity on the {111} lattice planes along the [110] axis, and superstructure spots related to CuAu-I-type ordering. Auger electron spectroscopy measurements showed that the Se atoms were interdiffused into the ZnTe thin film and that the diffused Se atoms formed a ZnSexTe1−x layer, which might be related to the coexistence of the two types of ordered structures. The coexisting behavior of the two ordered structures are discussed. The present results can help improve the understanding of the formation mechanism and the coexisting behaviors of the two ordered structure near the ZnTe/ZnSe heterointerface.

Microstructural and interband transition properties of vertically stacked InAs/GaAs self-assembled quantum dots embedded in modulation-doped heterostructures

The transmission electron microscopy image and selected area electron diffraction pattern showed that self-assembled InAs quantum-dot (QD) arrays embedded in GaAs barriers were periodically inserted in an Al0.25Ga0.75As/GaAs heterostructure. The temperature-dependent photoluminescence spectra of the InAs/GaAs quantum dots embedded in modulation-doped heterostructures showed interband transitions from the first-excited electronic subband to the first-excited heavy-hole subband together with those from the ground subband to the ground heavy-hole band (E1–HH1) while the spectra of the InAs/GaAs QDs alone showed only the peak related to the (E1–HH1) transitions.

Microstructural and interband transition studies of multiple stacked layers of Si-doped InAs self-assembled quantum dot arrays inserted into GaAs barriers

Microstructural and optical properties of Si-doped InAs quantum dot (QD) arrays inserted into undoped GaAs barriers have been investigated by using energy dispersive X-ray fluorescence (EDX), transmission electron microscopy (TEM), and photoluminescence (PL) measurements. The EDX pattern, the TEM image, and the selected area electron diffraction pattern showed that self-assembled Si-doped InAs vertically stacked QD arrays were embedded in the GaAs barriers. The temperature-dependent PL spectra showed that the peaks corresponding to the interband transitions of the InAs QDs shifted to the low-energy side with increasing temperature and that the distribution of carriers in the InAs QDs varied with changing temperature. These results indicate that Si-doped InAs QD arrays inserted into GaAs barriers hold promise for potential applications in optoelectronic devices.

Lattice mismatch and atomic structure studies on InxGa1-xAs/InyAl1-yAs coupled double-step quantum wells

Abstract Lattice mismatch and atomic structure studies of In x Ga 1− x As/In y Al 1− y As coupled double-step quantum wells have been performed by transmission electron microscopy (TEM) and electron-diffraction pattern measurements. The high-resolution TEM image of the In x Ga 1− x As/In y Al 1− y As coupled double-step quantum well showed that two sets of a 60-A In 0.65 Ga 0.35 As deep quantum well and a 60-A In 0.53 Ga 0.47 As shallow step quantum wells bounded by two thick In 0.52 Al 0.48 As barriers are separated by a 38-A In 0.52 Al 0.48 As embedded potential barrier. The selective-area electron-diffraction pattern obtained from TEM measurements on the In x Ga 1− x As/In y Al 1− y As double-step quantum well showed that In x Ga 1− x As active layers were grown pseudomorphologically on the InP buffer layer. The value of the lattice mismatch between the In 0.53 Ga 0.47 As and the In 0.65 Ga 0.35 As layers obtained from the high-resolution TEM measurements is 1.6%. A possible crystal structure for the In x Ga 1− x As/In y Al 1− y As coupled double-step quantum well is presented based on the TEM results. These results can help improve understanding of the structural properties for the applications of strained In x Ga 1− x As/In y Al 1− y As coupled double-step quantum wells in new kind of the optoelectronic devices.