H.W. Shim

Low temperature growth and photoluminescence of well-aligned zinc oxide nanowires

Abstract Well-aligned single-crystalline zinc oxide (ZnO) nanowires with high density were successfully synthesized on nickel monoxide (NiO) catalyzed alumina substrate through a simple metal–vapor deposition method at an extremely low temperature (450 °C). The single-crystalline ZnO nanowires had a hexagonal wurzite structure and diameters of about 55 nm, and lengths up to 2.6 μm. The photoluminescence spectra under excitation 325 nm showed a ultra-violet (UV) emission at 3.26 eV and a green emission at 2.44 eV. The UV emission and green emission bands were attributed to near band-edge transition and radial combination of a singly ionized oxygen vacancy with a photo-induced hole, respectively.

Anomalous photoluminescence from 3C-SiC grown on Si(111) by rapid thermal chemical vapor deposition

Single crystal 3C-SiC epitaxial films are grown on Si(111) surfaces using tetramethylsilane by rapid thermal chemical vapor deposition. Strong blue/green photoluminescence (PL) was observed at room temperature from the free films of SiC prepared by etching the Si substrate. The main PL peak energy varies from 2.1 to 2.4 eV with full widths at half-maximum between 450 and 500 meV, depending on the growth condition, excitation wavelength and excitation light intensity. A weak peak at 3.0 eV also appeared. The infrared (IR) spectra of free films of SiC exhibit modes associated with CH and OH groups. We also compared PL characteristics of free films of SiC with those from porous SiC produced by anodization of SiC/Si to determine the origin of the PL. Porous SiC shows a PL peak centered at 1.9 eV, different from those in SiC. From the analysis of the IR spectra and scanning electron microscopic images, we tentatively suggest that the origin of the PL from free films of SiC might be associated with an OH group ...

Efficient blue light-emitting diodes with InGaN/GaN triangular shaped multiple quantum wells

InGaN/GaN triangular shaped multiple quantum wells (QWs) grown by grading In composition with time were adopted as an active layer of blue light-emitting diodes (LEDs). Compared to the LEDs with conventional rectangular QW structures, the triangular QW LEDs showed a higher intensity and a narrower linewidth of electrical luminescence (EL), a lower operation voltage, and a stronger light-output power. EL spectra of the triangular-QW-based LEDs also showed that the peak energy is nearly independent of the injection current and temperature, indicating that the triangular QW LED is more efficient and stable than the rectangular one.

Influence of the quantum-well shape on the light emission characteristics of InGaN/GaN quantum-well structures and light-emitting diodes

Structural and optical properties of various shapes of quantum wells (QWs), including rectangular, triangular, trapezoidal, and polygonal ones are investigated. Photoluminescence (PL) measurements show that the highest light emission efficiency and the best reproducibility in the intensity and wavelength are obtained from trapezoidal QWs. The temperature dependence of PL spectra indicates the more localized nature of excitons in the trapezoidal QWs. A plan-view transmission electron microscopy shows that quantum dots (QDs) are formed inside the dislocation loop in trapezoidal QWs. The distribution of QDs in size and composition becomes more uniform with trapezoidal QWs than with rectangular QWs, leading to superior light-emission characteristics. It is suggested that QD engineering and dislocation control are possible, to some extent, by the modulation of the QW shape in InGaN/GaN-based light-emitting devices.

Two different behavior patterns of photoluminescence depending on Mg doping rate in p-type GaN epilayers

Mg-doped p-type GaN epilayers grown by metalorganic chemical vapor deposition exhibit two different types of photoluminescence (PL) characteristics depending on Mg doping rate; the existence of critical Mg concentration is found where both electrical and optical characteristics show an abrupt change in their behavior. In samples with relatively low Mg concentration, a band edge emission, a peak associated with shallow donor-acceptor pair recombination, and a 2.8 eV blue band emission appear in the PL spectrum. Intensities of all three PL peaks decrease with increasing annealing temperature; the blue band quenches most rapidly. In contrast, in samples grown with high ratio of Mg/Ga flow rate, only a strong blue emission band is observed and the intensity of the blue band increases with the annealing temperature suggesting a different origin from the blue band of lightly doped samples. From the annealing temperature dependence of the blue band intensity, we speculate that the emission mechanism of the blue ...

Photoluminescence characterization of biaxial tensile strained GaAs

Heteroepitaxial GaAs layers were grown on Si (001) substrates by metalorganic chemical vapor deposition. The tetragonal distortion induced by the lattice and the thermal expansion coefficient mismatches gives substantial effects on the acceptor energy level as well as the valence band structure. The biaxial tensile strain in GaAs layers is investigated using low-temperature photoluminescence. The origins of intrinsic exciton lines and carbon-related extrinsic lines observed in the photoluminescence spectra are identified by the two-band model. It is also found that the binding energy of the carbon acceptor is reduced as biaxial tensile strain increases.

Growth of GaN nanowires on Si substrate using Ni catalyst in vertical chemical vapor deposition reactor

GaN nanowires were successfully grown on Ni-coated Si substrate by direct reaction of gallium with ammonia gas in a home-made vertical tubular chemical vapor deposition reactor. The growth of GaN nanowires was uniformly observed across the Si substrate surface, but the density and average diameter of the nanowires varied along the position of the substrate surface. At the position of 5 cm above Ga source surface, the growth of GaN crystal grains was observed with few nanowires. The length of the nanowires reaches several micrometers. The clear lattice fringes in HRTEM image revealed the growth of good quality hexagonal single-crystal GaN nanowires. Photoluminescence of the GaN nanowires showed a strong band edge emission at the energy position of /spl sim/3.4 eV with negligible deep level yellow emission. Field emission characteristics of the GaN nanowires showed that the turn-on field of GaN nanowires was /spl sim/7.4 V//spl mu/m with a field enhancing factor /spl beta/ of /spl sim/555. The catalytic growth mechanism of the GaN nanowires was discussed on the basis of experimental results in this work.

Optical investigation of InGaAsGaAs heterointerfaces grown by metalorganic chemical vapor deposition

Photoluminescence (PL) measurements were carried out on strained In0.2Ga0.8AsGaAs quantum well structures. By analyzing the transition energies and linewidths as functions of temperature as well as the excitation intensity, we found that well width fluctuations are up to six monolayers depending upon the sample structure. PL peak energies and linewidths vary with temperature and the excitation intensity due to the change of the distribution ratio of energy levels generated by the structural disorder at the interfaces.

Substrate imposed stress-strain effect on photoluminescence in hydrogenated amorphous silicon alloys

Hydrogenated amorphous silicon films were deposited on the unstrained and strained Si substrates by an electron cyclotron resonance plasma source. The photoluminescence spectra show that emission energies are different from each other. The redshift of photoluminescence induced by the biaxial tensile stress is increased with decreasing the temperature in the range of 0.11–0.17 eV. The stress effect also enhances the quantum efficiency significantly. The pseudomorphic growth of Si on a relaxed Si0.75Ge0.25 (100) surface provides the strain energy of about 0.17 eV. These comparable results indicate that the shift of emission energy is attributed to the stress effect perturbing the polysilane structure.

Catalytic growth and characterization of ZnO nano-needles

ZnO nanostructures were synthesized at 500/spl deg/C over nickel catalyst supported on Si substrate in a homemade vertical chemical vapor deposition (CVD) reactor. The ZnO nanostructures had a needle-like morphology that the diameter of structures decreased linearly from the bottom to the top. The diameters of the ZnO nano-needles normally ranged from 20-100 nm and the lengths were in the range of 2-3 /spl mu/m. The nano-needles had a single wurtzite structure. The clear lattice fringes in HRTEM image also revealed the growth of good quality hexagonal single-crystal ZnO. The photoluminescence (PL) spectra of the nano-needles excited with a ultra-violet (UV) emission of 3.3 eV and a weak green emission at 2.4 eV at room temperature. Field emission characteristics of the ZnO nanowires showed that the turn-on field of ZnO nanowires was /spl sim/6.10 V//spl mu/m with a field enhancing factor /spl beta/ of /spl sim/1099. The catalytic growth mechanism of the ZnO nanowires was discussed on the basis of experimental results in this work.