PL Images Research Articles

Defect characterization of Si-doped GaN films by a scanning near-field optical microscope-induced photoluminescence

We developed a tapping-mode-scanning near-field optical microscope to measure near-field photoluminescence (SNOM-PL) with nanometer spatial resolution using an ultraviolet laser, and we measured the defect distribution of a Si-doped GaN film. The obtained result was compared with one measured by high spatial resolution cathodoluminescence (CL) spectroscopy. Some dark spots with an average period of 100–300nm were observed in the image of the peak intensity of near-field PL at about 362nm, measured with a pyramidical cantilever, and in a plan-view CL image at the same wavelength. The near-field PL image agreed well with the plan-view CL image. The threading dislocations in GaN films were found to act as nonradiative recombination centers not only for band-edge emission but also for yellow luminescence around 580nm. Furthermore, it was found that free carrier concentration decreased near the threading dislocations. The SNOM-PL we developed has at least a spatial resolution of about 100nm. The SNOM-PL could potentially be applied to the estimation at a nanometer scale of defects in semiconductor films without requiring a vacuum environment.

Nanoscale photoluminescence mapping for MOVPE InN films using scanning near-field optical microscopy (SNOM)

This paper reports for the first time the scanning near-field optical microscopy (SNOM) analysis of MOVPE InN. A near-field PL spectrum and its intensity mapping for MOVPE InN are obtained successfully at room temperature. The near-field PL spectrum has a smaller FWHM and a little higher peak energy compared with the conventional macroscopic PL spectrum. Near-field PL images are used to know the effects of GaN buffer layer on in-plain optical uniformity in MOVPE InN. A large non-uniformity is seen in the image for the sample grown without GaN buffer. Compared with the film grown without buffer, the film grown with a GaN buffer has a better uniformity. Although the use of buffer improves the apparent in-plane uniformity, a fine structure is found in both the PL and topographic images. The fine structure seems to be related to the small grains of InN grown on the GaN buffer composed of small grains.

Effects of localization on the excitonic linewidth in partially orderedGaxIn1−xPalloys

Spontaneous ordering in the ternary alloy GaxIn1−xP naturally leads to a reduction in alloy scattering and thus a reduction in the excitonic linewidth with increasing order parameter h. Concomitantly, the effect of localized states associated with various defects tends to increase the exciton linewidth, and it was observed that for hu0.45, the linewidth actually increases with order parameter [Phys. Rev. B 61, 9910 (2000)]. This was attributed to the increase in native defects which occurs for higher order parameters due to limitations in current growth technology. We show, using spatially resolved PL imaging with confocal and near-field microscopy, that this trend is reproduced in all orderedGaxIn1−xP samples, within the micro-ensemble formed by the collection of all spectra in a spatially and spectrally resolved PL image, directly imaging the effect of exciton localization. This reversed trend is a result of the interplay between the effects of localization and spontaneous ordering, and the spatial averaging which occurs when examining areas many times larger than the exciton Bohr radius. Agreement with the trend observed in macro-PL is obtained via micro-ensemble averaging of each data set, and by examination of the spatial correlation of exciton linewidths. The implications of these results on the origin of the exciton linewidth in ordered alloys and the effect of ordering on the spatial distribution of exciton linewidths are discussed.

Plasmon mode imaging of single gold nanorods.

We have investigated two-photon-induced photoluminescence images and spectra of single gold nanorods by using an apertured scanning near-field optical microscope. The observed PL spectrum of single gold nanorod can be explained by the radiative recombination of the electron-hole pair near the X and L symmetry points. PL images reveal characteristic features reflecting an eigenfunction of a specific plasmon mode as well as electric field distributions around the nanorod.

Leakage current induced anomalies in the photoluminescence of GaAs coupled quantum wells

We have performed photoluminescence experiments on GaAs/AlGaAs coupled quantum wells as a function of bias voltage and photoexcitation intensity. Spatially indirect recombination appears at higher electric fields for weak photoexcitation instead of the spatially direct one which is dominant at zero bias voltage. However, for strong photoexcitation with photon energy larger than the band gap of the barrier layers, the spatially direct recombination remains dominant over a wide range of bias voltage. Under this condition, the exciton ring emission is observed in the PL image at positive biases for which a large leakage current flows through the device. These results suggest that the 2DEGs induced by the large leakage current is important for interpreting the behavior of the ring emission. (C) 2004 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.

Sub-μm scale photoluminescence images of wide bandgap semiconductors by cryogenic scanning optical microscope

Photoluminescence images with a sub-μm spatial resolution were obtained at 15 K using a newly developed cryogenic scanning microscope with both high throughput and simplicity of measurement procedure. The objective was put in a sample chamber to reduce thermal conduction from the objective to the sample. The spatial resolution at 15 K was measured to be 300 nm at a wavelength of 488 nm, which is almost equal to the diffraction limit. The microscope demonstrated μm-scale defective regions emitting a PL signal at 3.40 eV in GaN grown on SiC. At an elongated surface defect in a 4H–SiC homoepitaxial layer, the PL image ascribed to excitons bound to neutral nitrogen atoms showed a dark line coincident with the defect, indicating that non-radiative recombination dominates over the excitonic emission at the surface defect.

Microphotoluminescence characterization of cleaved edge overgrowth T-shaped InxGa1−xAs quantum wires

Microphotoluminescence (micro-PL) characterization was performed for T-shaped InxGa1−xAs quantum wires (T-QWRs) fabricated by the cleaved edge overgrowth method with molecular beam epitaxy. The spatial distribution of optical properties in InxGa1−xAs T-QWRs was examined by means of PL intensity imaging and scanning micro-PL spectroscopy with about 1 μm spatial resolution. In the successfully fabricated 3.5-nm-scale In0.17Ga0.83As T-QWRs with Al0.3Ga0.7As barriers, uniform PL image and PL spectra from QWRs along the wire direction were observed, which indicates spatially uniform and high quality QWRs were formed. The effective lateral confinement energy of one-dimensional excitons was found to be 34 meV, showing the usefulness of InxGa1−xAs as a well material in T-QWR structures. On the other hand, in the unsuccessfully fabricated 4-nm-scale In0.09Ga0.91As T-QWRs with Al0.3Ga0.7As barriers, PL image and spectra were inhomogeneous. These results demonstrate the importance of flat cleaved surface and optimized overgrowth condition to fabricate uniform T-QWRs.

E-vector sensitivity patterns in the goldfish optic tectum

1. Spike responses of single optic tectal units to plane polarized light have been recorded with extracellular electrodes in the goldfish. Responses to a series of eight 500 msec flashes were summed and the quantitative effects ofe-vector direction studied over 4–5 log units of intensity with white light and narrow spectral bands of equal quantal content at 460, 540 and 620 nm. 2. As previously reported all or nearly all tectal cells tested (115) were sensitive to the direction of the stimulus' polarization plane. Control experiments with a thin high order birefringent retarder next to the cornea and acting as a pseudo depolarizer provided further proof that thee-vector discrimination found in the optic tectum (as well as the previously reported oriented behavior to polarized light in fish) depends on an intraocular analyzer. 3. Systematic study of tectal cells has been made at various depths and in all directly accessible areas. No localized or differential effects of recording depth one-vector discrimination have been found. However, both directions of maximum response and degree of polarized light sensitivity (PLS) show distinctive patterns over the tectal area. 4. On the basis of established tectal projection maps these data show that preferred retinale-vector directions are tangentially arranged around the eye axis when the stimulus is axial. Distribution of this angular sensitivity seems continuous without discrete channels favoring particular polarization planes. 5. Sensitivity (determined from intensity response curves) is minimal in the center of the retina for an axial stimulus and increases peripherally out to 50–60° or more off axis. Sensitivity ratios of 4–6 are common and much larger ones have been occasionally recorded. 6. Shifting the direction of stimulus from axial to 45° and 60° upward from the axis proved that retinal patterns of preferrede-vector directions and sensitivities are symmetrical relative to the beam axis rather than the anatomical eye axis. Therefore individual tectal cells and before them their retinal receptor elements show different directions ofe-vector preference and different PLS ratios depending on the stimulus configuration. 7. Intensity response curves determined with the red, green and blue narrow spectral bands were essentially superimposable. Therefore, we have no evidence that there is any special interaction of λ and PLS. 8. The effects of stimulus intensity and retinal adaptation on the IR curves do not indicate any particular selective effects. Substantial polarized light sensitivity was present in both the light adapted and dark adapted state. PLS ratios in various cases were (1) about the same at different intensity levels or (2) greater at low than high intensities or (3) greater at high than low intensities. 9. We conclude that linearly polarized light evokes a large entoptic image in the goldfish eye. Two opposite light sectors perpendicular to thee-vector alternate with two dark sectors parallel to thee-vector. Both Haidinger's brushes and Boehm's brushes in human vision show some similarities but also some important differences. The goldfish PL image is achromatic, weak or absent axially and strong peripherally. Also the contrast between the sectors does not depend on movement of the stimulus on the retina to prevent fading as it does in both of the other phenomena. 10. The most likely mechanism for the observed PLS in the goldfish eye is either differential scattering intraocularly or oblique entry of light into the receptor outer segments. Yet present data prevent a firm choice between them.