Concentration Of Nonradiative Defects Research Articles

The effect of thermal annealing on the optical properties of Mg-doped zincblende GaN epilayers

The effects of thermal annealing on the optical properties of Mg-doped cubic zincblende GaN epilayers grown by metalorganic chemical vapor deposition on 3C-SiC/Si (001) substrates are investigated. The photoluminescence spectra show near band edge features and a blue luminescence band that depend on Mg concentration, temperature, and excitation power density. Annealing the sample in a N2 atmosphere causes the intensity of the blue band to increase by a factor of 5. Power dependent photoluminescence measurements show a reduction in the laser excitation density required for saturation of the blue band after annealing, indicating an increase in the recombination lifetime. Time decay measurements confirm this increase, which is attributed to a reduction in the concentration of non-radiative defects after annealing. The results presented here are compared to those reported previously for Mg-doped hexagonal wurtzite GaN.

Positron annihilation and cathodoluminescence study on inductively coupled plasma etched GaN

AbstractEtching damage induced by inductively coupled plasma (ICP) with a mixture of Cl and BCl gases and the effect of subsequent annealing in Nwere studied through positron annihilation and cathodoluminescence (CL) for Mg‐doped and Si‐doped GaN. The changes in the concentration of nonradiative defects and in the shape of band bending caused by ICP etching or subsequent annealing were studied by comparing the CL intensities and the diffusion lengths of implanted positrons, respectively. For Mg‐doped GaN, CL spectra revealed that etching inactivated the Mg acceptors at the near‐surface region. After annealing at 900 °C, the Mg acceptors were reactivated. For Si‐doped GaN, etching introduced neutral vacancy‐type defects, and after annealing at 900 °C, the etching‐induced defects almost disappeared. However, there were residual nonradiative defects on the top surface that were associated with a change in the surface's Fermi level position.

Contact light-emitting diodes based on vertical ZnO nanorods

We report vertical contact light-emitting diodes (VCLEDs), that are based on heterojunctions formed by using the point contacts of n-ZnO nanorods (NRs) to the p-type semiconductor substrate and that are fabricated using a new approach to the formation of LEDs (Appl. Phys. Lett. 98, 093110 (2011)). A p-type GaN film grown on a sapphire substrate was used to form n-ZnO NRs/p-GaN VCLEDs on a large area of about 4 cm2. The VCLEDs emitted a pure blue electroluminescence with high efficiency. Electroluminescence at 470 nm, which is visible to the naked eye, started at small current of about 50 μA and is attributed to the good optical properties of the structurallyperfect heterojunctions in the point contacts. The VCLED configuration allows the creation of ZnO/p-GaN nano-LEDs of high density and high-quality with a greatly reduced concentration of nonradiative defects in the active regions. The VCLEDs showed the high brightness of light required for active matrix displays and general solid-state lighting.

Antibacterial activity of ZnO nanoparticles under ambient illumination — The effect of nanoparticle properties

ZnO nanomaterials exhibit prominent antibacterial activity even without UV illumination. Their antibacterial activity was attributed to different mechanisms, such as production of reactive oxygen species (ROS) and/or release of zinc ions in solution. Here we examined the relationship between the properties of eight different ZnO nanoparticles, their production of ROS, zinc ion release and antibacterial activity. We found that while the particles exhibited differences in antibacterial activity, there was no correlation with ROS production and zinc ion release. However, ROS production was inversely correlated with the concentration of nonradiative defects indicating that the recombination losses of photogenerated carriers can negatively affect ROS production.

Defect mapping in full-size multi-crystalline Si wafers

Application of scanning techniques is strongly required for in-line/off-line diagnostics of semiconductors with spatial inhomogeneity of electronic properties due to uneven defect distribution. The defect mapping allows performing an initial screening and follow-up local analyses in large-scale materials and devices for electronic applications. This approach is illustrated on multi-crystalline silicon (mc-Si) wafers for solar cells. Four different mapping techniques were applied concurrently to full-size as-grown and processed mc-Si. They include the mapping of (1) minority carrier lifetime, (2) room-temperature photoluminescence, (3) dislocation density, and (4) temperature dependent EBIC and cathodoluminescence. Analyses revealed a clear correlation between the defect PL band intensity with the maximum at about 0.8 eV and concentration of the dislocations contaminated by impurities and defects. The 0.8 eV band has a distinctive location at wafer areas with reduced minority carrier lifetime, i.e. regions with enhanced recombination activity of defects. It is demonstrated that the concentration of the 0.8 eV defects has analytical relation to the concentration of non-radiative defects in dislocated regions. The model of these defects is discussed.

Hydrogenation of strain engineered InAs/In x Ga 1− x As quantum dots

InxGa1−xAs/InAs/InxGa1−xAs structures have been grown by atomic layer molecular beam epitaxy on top of a GaAs buffer and substrate. In these structures, the thickness d and/or the In composition x of the lower InxGa1−xAs confining layer control the strain in self-assembled quantum dots. This strain engineering has allowed achieving emission energies as low as 1.5 μm at low T, with a rapid quenching of the photoluminescence (PL) signal at high T. Hydrogen irradiation of these structures leads to an increase in the PL efficiency, higher in samples with higher x, with a blue shift in the peak energy. A higher concentration of non radiative defects in confining layers richer in indium is responsible for the observed PL quenching, more than an increased thermal escape of carriers toward the InxGa1−xAs barriers. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Optical properties of GaN with Ga and N polarity

AbstractWe compared photoluminescence (PL) and cross-sectional transmission electron microscopy (TEM) characteristics of GaN samples with Ga and N polarities grown by molecular beam epitaxy (MBE) on sapphire substrates. Ga-polar films grown at low temperature typically have very smooth surfaces, which are extremely difficult to etch with acids or bases. In contrast, the N-polar films have rougher surfaces and can be easily etched in hot H3PO4 or KOH. The quality of the X-ray diffraction spectra is also much better in case of Ga-polar films. Surprisingly, PL efficiency is always much higher in the N-polar GaN, yet the features and shape of the PL spectra are comparable for both polarities. We concluded that, despite the excellent quality of the surface, MBE-grown Ga-polar GaN layers contain higher concentration of nonradiative defects. From the analyses of cross-sectional TEM investigations, we have found that Ga-polar films have high density of threading dislocations (5x109 cm-2) and low density of inversion domains (1x107 cm-2). For N-polar GaN the situation is the reverse: the density of dislocations and inversion domains are 5x108 and ~1x1011 cm-2, respectively. One of the important conclusions derived from the combined PL and TEM study is that inversion domains do not seem to affect the radiative efficiency very adversly, whereas dislocations reduce it significantly.

Photoluminescence Investigations of High Purity Mbe-Grown Sil−xGex/Si Single Quantum Wells

AbstractSil-xGex/Si(100) single quantum well samples were grown by MBE with x varying from 18.5% to 36%, well widths Lz from 1.1 nm to 175 nm, and growth temperatures To in the range from 350°C to 750°C. We studied the photoluminescence (PL) properties in detail using Fourier transform spectroscopy. In most of the samples, the dominant and phononresolved SiGe band edge PL exhibits exclusively free exciton luminescence even at sample temperatures as low as 2 K. From the absence of bound excitons we conclude that the background concentration of shallow impurities is low in our SiGe layers. Variation of the growth temperature of the Si cap layer strongly influences the efficiency of the SiGe band edge PL at higher temperatures, whereas the PL efficiency is less affected at T=4 K. This indicates a reduction of the concentration of non-radiative defects in the cap layer when it is grown at higher temperatures. A broad PL band 150 meV below the SiGe bandgap, supposed to be typical for MBE-grown material, is weakly observed in only a few samples. The strength of the broad PL band does not unambiguously depend on the strain energy density in the SiGe layer as recently suggested.