Difference of anisotropic structures of InAs/GaAs quantum dots between organometallic vapor-phase epitaxy and molecular beam epitaxy

The anisotropies of the baseline in and [1 1 0] of InAs quantum dots (QDs) fabricated by molecular beam epitaxy (MBE) and organometallic vapor-phase epitaxy (OMVPE) are investigated. The structural and optical difference between QDs by MBE and OMVPE are investigated through an atomic force microscopy, a transmission electron microscopy, and a photoluminescence polarization measurement. It is found that the InAs QD structural anisotropy in MBE agrees with the individual growth rate anisotropy. Moreover, it is found that the mixture of the different structural anisotropies is unique in OMVPE at low growth temperature (440°C) and the growth mode is complex. From the photoluminescence polarization measurement, the InAs QD structures which mainly contribute to the optical property are decided by the plus and minus of the polarization degree of the ground state, and it is shown that the baseline anisotropy of the QDs mainly agrees with the growth rate anisotropy.     

Selective area epitaxy of InGaN quantum well triangular microrings with a single type of sidewall facets

Triangular microrings have been formed by selective area epitaxy of GaN and InGaN quantum wells (QWs) on patterned (0 0 0 1) AlN/sapphire. SiO₂ patterns consist of triangular ring openings oriented with edges parallel to two different orientations. InGaN QW microrings with each edge parallel to the 〈1 1? 0 0〉 direction have very rough sidewalls while microrings with each edge parallel to the 〈1 1 2¯ 0〉 direction exhibit well formed and smooth sidewalls as a result of the generation of a single type of {1 1? 0 1} facets on the inner and outer sidewalls. These {1 1? 0 1} facets demonstrate similar cathodoluminescence (CL) spectra that appear to be the superposition of two peaks at photon energies ∼2.5 eV (500 nm) and 2.7 eV (460 nm). Moreover, spatially matched striations are observed in the CL intensity images and surface morphologies of the {1 1? 0 1} sidewall facets. The observed striations are found to be related to subtle surface morphologies of the underlying GaN structures.     

Incorporation behaviors of group V elements in GaAsSbN grown by gas-source molecular-beam epitaxy

We report the incorporation behaviors of As, Sb, and N atoms in GaAsSbN grown by gas-source molecular-beam epitaxy. We found that N atom is more reactive and competitive than Sb atom at the growth temperature ranging from 420 to 450 °C. The increment in Sb beam flux hardly changes the N composition. However, the increment in N flux retards the incorporation of Sb. In addition, the increment in As₂ flux makes the Sb and N compositions decrease at the same rate. Based on these results, we have successfully grown GaAsSbN epilayers lattice-matched to GaAs substrates. The energy gap at room temperature is as low as 0.803 eV. Negative deviation from Vegard's law in lattice constant is observed in these layers.     

Reduction of threading dislocations in migration enhanced epitaxy grown GaN with N-polarity by use of AlN multiple interlayer

High quality GaN layer was obtained by insertion of high temperature grown AlN multiple intermediate layers with migration enhanced epitaxy method by the RF-plasma assisted molecular beam epitaxy on (0 0 01) sapphire substrates. The propagating behaviors of dislocations were studied, using a transmission electron microscope. The results show that the edge dislocations were filtered at the AlN/GaN interfaces. The bending propagation of threading dislocations in GaN above AlN interlayers was confirmed. Thereby, further reduction of dislocations was achieved. Dislocation density being reduced, the drastic increase of electron mobility to 668 cm²/V s was obtained at the carrier density of 9.5×10¹⁶ cm⁻³ in Si doped GaN layer.     

Characterization of heterointerfaces in GaAs/MnAs/MnZn-ferrite structures

We have grown GaAs epitaxial films on MnZn-ferrite substrates using MnAs buffer layers and investigated their heterointerfaces with glazing incidence-angle X-ray reflectivity and X-ray photoelectron spectroscopy. It has been found that the heterointerfaces for this structure are quite abrupt and the roughness at the GaAs/MnAs and MnAs/MnZn-ferrite interfaces are 1.1 and 0.2 nm, respectively. We also found that the diffusion of atoms through the GaAs/MnAs interface into the GaAs film is negligible. These results indicate that the MnAs buffer layer for the GaAs/ferrite structure is chemically stable and promising for the application to the future magnetic electronics.     

The incorporation behavior of arsenic and antimony in GaAsSb/GaAs grown by solid source molecular beam epitaxy

GaAsSb ternary epitaxial layers were grown on GaAs (0 0 1) substrate in various Sb₄/As₂ flux ratios by solid source molecular beam epitaxy. The alloy compositions of GaAs_1−ySb_y were inferred using high-resolution X-ray symmetric (0 0 4) and asymmetric (2 2 4) glance exit diffraction. The non-equilibrium thermodynamic model is used to explain the different incorporation behavior between the Sb₄ and As₂ under the assumption that one incident Sb₄ molecule produces one active Sb₂ molecule. It is inferred that the activation energy of Sb₄ dissociation is about 0.46 eV. The calculated results for the incorporation efficiency of group V are in good agreement with the experimental data.     

Anomalous temperature-dependent photoluminescence characteristic of as-grown GaInNAs/GaAs quantum well grown by solid source molecular beam epitaxy

The photoluminescence (PL) mechanisms of as-grown GaInNAs/GaAs quantum well were investigated by temperature-dependent PL measurements. An anomalous two-segmented trend in the PL peak energy vs. temperature curve was observed, which has higher and lower temperature-dependent characteristics at low temperature (5–80 K) and high temperature (above 80 K), respectively. The low and high-temperature segments were fitted with two separate Varshni fitting curves, namely Fit_low and Fit_high, respectively, as the low-temperature PL mechanism is dominated by localized PL transitions while the high-temperature PL mechanism is dominated by the e1–hh1 PL transition. Further investigation of the PL efficiency vs. 1/kT relationship suggests that the main localized state is located at 34 meV below the e1 state. It is also found that the temperature (80 K) at which the PL full-width at half-maximum changes from linear trend to almost constant trend correlates well with the temperature at which the PL peak energy vs. temperature curve changes from Fit_low to Fit_high.     

Influence of Be on N composition in Be-doped InGaAsN grown by RF plasma-assisted molecular beam epitaxy

Undoped and Be-doped InGaAsN layers were grown on GaAs substrates under the same growth conditions by radio frequency plasma-assisted molecular beam epitaxy. Increased tensile strain (Δa/a=3×10⁻³) was observed for Be-doped InGaAsN layers, compared to undoped InGaAsN layers. The strain is shown to originate from the increase in N composition related to Be incorporation, rather than solely from Be atoms substituting Ga atom sites (Be_Ga). A possible reason is the high Be–N bond strength, which inhibits the loss of N from the growth surface during epitaxial growth, thereby increasing the N composition in the Be-doped InGaAsN layer.     

Growth of embedded ErAs nanorods on (4 1 1)A and (4 1 1)B GaAs by molecular beam epitaxy

The low solubility of Er in GaAs results in the formation of ErAs nanostructures when GaAs is grown with 5–6 at% Er/Ga ratio by molecular beam epitaxy on GaAs surfaces. For growth on the (4 1 1)A GaAs surface, cross-sectional scanning transmission electron microscopy images show the presence of ErAs nanorods embedded in a GaAs matrix extending along the [2 1 1] direction with a spacing of roughly 7 nm and a diameter of roughly 2 nm. Growth on the GaAs (4 1 1)B surface resulted in only nanoparticle formation. Variation of the polarized optical absorption with in-plane polarization angle is consistent with coupling to surface plasmon resonances of the semimetallic nanostructures.     

The role of Sb in the molecular beam epitaxy growth of 1.30–1.55 μm wavelength GaInNAs/GaAs quantum well with high indium content

Sb-assisted GaInNAs/GaAs quantum wells (QWs) with high (42.5%) indium content were investigated systematically. Transmission electron microscopy, reflection high-energy electron diffraction and photoluminescence (PL) measurements reveal that Sb acts as a surfactant to suppress three-dimensional growth. The improvement in the 1.55 μm range is much more apparent than that in the 1.3 μm range, which can be attributed to the difference in N composition. The PL intensity and the full-width at half maximum of the 1.55 μm single-QW were comparable with that of the 1.3 μm QWs.     

Role of vapor-phase diffusion in selective-area MOVPE of InGaN/GaN MQWs

Selective-area growth (SAG) of InGaN/GaN multiple quantum wells (MQWs) was performed by metalorganic vapor phase epitaxy (MOVPE). The layers of a blue light-emitting diode (LED), that includes five InGaN quantum wells, were grown on a patterned GaN template on a sapphire substrate. In order to elucidate the contribution of vapor-phase diffusion of group-III precursors to the in-plane modulation of luminescence wavelength, the width of a stripe selective growth area was 60 μm that is sufficiently larger than the typical surface diffusion length, with the mask width varied stepwise between 30 and 240 μm. The distribution of the luminescence wavelength from the MQWs was measured with cathode luminescence (CL) across the stripe growth area. The peak wavelength ranged between 420 and 500 nm. The peak shifted to longer wavelengths and became broader as the measured point approached to the mask edge. Such a shift in the peak wavelength exhibited parabolic profile in the growth area and the wider mask shifted the entire peak positions to longer wavelengths. These trends clearly indicate that the vapor-phase diffusion play a dominant role in the in-plane modulation of the luminescence wavelength in the SA-MOVPE of InGaN MQWs, when the size of a growth area and/or the mask width exceeds approximately 10 μm.