Band Edge Luminescence Intensity Research Articles

Photoluminescence of dislocations in plastically deformed GaN

Fresh (a/3) [ 1 1 2 ¯ 0 ] dislocations on the ( 1 1 ¯ 0 0 ) prismatic plane were introduced into GaN bulk crystals by plastic deformation at 950 °C. Photoluminescence (PL) studies at 11 K showed that the near band edge (3.48 eV) luminescence intensity decreased drastically in the deformed GaN, attributed to the introduction of a high-density of non-radiative recombination centers during plastic deformation. The yellow band (2.22 eV) luminescence decreased by plastic deformation, while several luminescence bands centered at 1.79, 1.92, 2.4 and 2.8 eV developed. The dependence of PL features on deformation and annealing indicates that yellow luminescence is not related to the native structure of dislocations in GaN.

Cathodoluminescence study of plastically deformed bulk ZnO single crystal

Vickers microindentation of bulk ( 0 0 0 1 ¯ ) ZnO single crystal has been studied by means of a scanning microscope using cathodoluminescence (CL) monochromatic imaging and spectral modes. A main feature of the sample's deformation is the propagation of the latter away from the indenter contact diameter, in well-defined directions, characterizing the hexagonal crystal systems. An attempt is made to understand plastic deformation mechanism, which is responsible for the observed deformation behavior. The deformation-produced dislocations act as nonradiative recombination centers, which is confirmed by the absence of any additional emission band and the diminution of the band edge luminescence intensity in the Cl spectra after micro-indentation.

Temporal profiles of excitons in surface‐modified CdS and CdMnS quantum dots prepared by a reverse‐micelle method

We have investigated surface-modification effects on photoluminescence properties of CdS and CdMnS quantum dots (QDs) prepared by a reverse-micelle method from the viewpoint of photoluminescence dynamics. By the surface modification, the band-edge luminescence intensity is markedly increased in CdS QDs. The surface modification also causes drastic changes of decay profiles of the band-edge luminescence. The Mn2+ luminescence originated from the intra-3d shell transition of Mn2+ is clearly observed in CdMnS QDs, and its intensity is also increased by the surface modification. Furthermore, the decay time of the band-edge luminescence in surface-modified CdMnS QDs is faster than that in CdS QDs, which is due to the energy transfer from excitons to Mn2+. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Metastability in the excitonic luminescence of electron‐irradiated GaN

Metastability under focused 267 nm laser beam is observed at low temperature in the excitonic luminescence of GaN after irradiation with 0.42 MeV electrons. The oxygen-bound exciton intensity, the total band edge luminescence intensity, and the luminescence decay lifetime of free and bound excitons all increase with laser exposure time. The rate of reaching the stable state depends on the average laser power. The observed metastable behavior is explained by the carrier trapping or recombination-enhanced dissociation of non-radiative defect complexes related to Ni. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Dry etching of ZnO films and plasma-induced damage to optical properties

To study the effects of plasma chemistries on etch characteristics and plasma-induced damage to the optical properties, dry etching of ZnO films has been carried out using inductively coupled plasmas of Cl2/Ar, Cl2/H2/Ar, and CH4/H2/Ar. The CH4/H2/Ar chemistry showed a faster etch rate and a better surface morphology than the Cl2-based chemistries. Etched samples in all chemistries showed a substantial decrease in the PL intensity of band-edge luminescence mainly due to the plasma-induced damage. The CH4/H2/Ar chemistry showed the least degradation of the optical properties.

Studies of deep centers in dilute GaAsN and InGaAsN films grown by molecular beam epitaxy

Deep levels spectra DLTS, 77 K photoluminescence (PL) spectra and photosensitivity were measured for GaAsN and InGaAsN films with low N and In concentration grown by molecular beam epitaxy and in GaAs films grown on GaAsN buffer. It is shown that the bandedge luminescence intensity is greatly decreased in GaAsN, GaAs/GaAsN and particularly in InGaAsN structures compared to the homoepitaxial GaAs. Comparison of the DLTS and PL spectra strongly suggests that the main recombination center in such films is the EL3-like electron trap whose concentration greatly increases upon In and N incorporation into the solid solution. Based on published results the trap is associated with substitutional oxygen on As site and the results are discussed in view of such possible assignment.

Optical properties and defects in GaAsN and InGaAsN films and quantum well structures

Photoluminescence and microcathodoluminescence spectra of thick-film GaAsN and InGaAsN structures and GaAs/InGaAsN, AlGaAs/InGaAsN quantum wells (QWs) were studied for InGaAsN layers with low nitrogen concentration of 0.35–0.5%. It is shown that in thick-film structures the bandedge luminescence intensity is strongly decreased in the row homoepitaxial GaAs, GaAsN on GaAs buffer, GaAsN, GaAs on GaAsN buffer, InGaAsN which correlates with the increasing concentration of electron traps with activation energy 0.53–0.55 eV. The type of defect bands in the thick-film structures was found to strongly depend on composition of the layers. For the GaAs/InGaAsN QW structures the intensity of luminescence was found to be more than an order of magnitude higher than in InGaAsN single films.

Effects of proton implantation on electrical and recombination properties of n-GaN

Effects of implantation with 150 keV protons and 350 keV deuterium ions on deep levels spectra and electrical and recombination properties of n-GaN samples were studied for fluences between 8×10 13 and 5×10 15 cm −2. Implantation led to substantial decrease of the electron concentration so that after the highest dose used, the samples became highly resistive with the Fermi level pinned near 0.3–0.4 eV from the conduction band edge. We also observed a strong degradation of the minority carriers diffusion length and a very strong decrease of the band edge luminescence intensity in the heavily implanted samples. Deep levels spectra measurements showed that proton implantation led to introduction of deep electron traps with activation energies near 0.2, 0.3, 0.45, 0.75 and 0.95 eV and of deep hole traps with activation energies of 0.25, 0.6 and 0.9 eV. The results strongly indicate that among these traps, the 0.75 eV electron traps and/or the 0.6 eV hole traps are most likely responsible for the lifetime degradation. Strong recovery of the electric conductivity of the implanted samples was observed starting with annealing temperatures near 250°C, whereas no recovery of the luminescent characteristics could be observed for annealing temperatures as high as 700°C. We compare the results with previously published data on proton and He implantation of GaN and discuss possible implications of our findings for practical use in GaN devices isolation.

Photoluminescence Intensity and Spectral Distribution of GaN Films on SiC Substrates — The Dependence on Dislocation Density and Structure

GaN samples were grown having threading dislocation density in the range of 5.5 × 108 to 5 × 1010 cm—2. The decrease of the band edge luminescence which occurs as the dislocation density increases is measured and numerically simulated with minority carrier diffusion length L = 90 nm. The emitted band edge luminescence intensity and wavelength has a spatial distribution which depends on the dislocation structure in the crystal. An observed trend toward lower energy of the band edge peak as the dislocation density increases indicates that some relaxation of the SiC/GaN interfacial misfit stress occurs through the defects.

High resolution reactive ion etching of GaN and etch-induced effects

We have developed a process using electron beam lithography and reactive ion etching for the high resolution pattern transfer of GaN. 150 nm dots have been fabricated in GaN successfully. Photoluminescence, scanning electron microscopy, and x-ray photoelectron spectroscopy have been employed to compare the damage inflicted on the GaN surfaces after SF6 and Ar plasma exposures. Near-band-edge luminescence analysis indicates the existence of a higher concentration of donors on the top 100 nm of the GaN surface after Ar as supposed to SF6 plasma exposure. An order of magnitude decrease in the ratio of the yellow to the band-edge luminescence intensity is found in the samples subjected to lower ion energies. Formation of pits is observed on the substrate surfaces after plasma treatment. Nitrogen deficient surfaces limited to the top few monolayers, as well as defect propagation down to 100 nm, exist in our plasma exposed GaN samples.

GaN grown on Si(111) substrate: From two-dimensional growth to quantum well assessment

We report on the epitaxial growth of high quality GaN films on Si(111) substrates by molecular beam epitaxy using ammonia. The surface morphology and crystallinity of thick undoped GaN films are characterized by reflection high-energy electron diffraction (RHEED), scanning electron microscopy, and x-ray diffraction. Films having compact morphologies and flat surfaces are obtained and RHEED intensity oscillations are demonstrated for GaN and (Al, Ga)N alloys indicating two-dimensional growth. This has been applied to the growth of AlGaN/GaN quantum well (QW) structures. Low-temperature photoluminescence (PL) spectra of GaN are dominated by a strong and narrow (full width at half maximum=5 meV) band edge luminescence intensity at 3.471 eV assigned to donor bound exciton recombination. PL properties of AlGaN/GaN QW are also very similar to those obtained on equivalent structures grown on sapphire.

Evidence of compensating centers as origin of yellow luminescence in GaN

The dependence of the near-band edge and the yellow luminescence in n-type GaN grown by organometallic vapor-phase epitaxy is investigated as a function of doping concentration. The band edge and yellow luminescence intensity increase as the doping concentration is increased. However, the band-edge-to-yellow luminescence ratio does not change significantly as the doping concentration is increased by two orders of magnitude. A theoretical model based on rate equations is developed for the band-edge-to-yellow intensity ratio. Analysis of the experimental data in terms of the model reveals that the concentration of the level causing the yellow luminescence increases linearly with doping concentration. This dependence shows that the yellow luminescence is due to a compensating center.

Depth-Resolved and Excitation Power Dependent Cathodo-Luminescence of MBE Grown Cubic GaN Epilayers

AbstractWe present a detailed investigation of the cathodoluminescence (CL) of MBEgrown cubic GaN (c-GaN) epitaxial layers. Under high excitation conditions the room temperature CL-spectrum is dominated by a single excitonic emission band at about 3.21 eV with a full width at half maximum of 55 meV. Under low electron beam excitation a second, deep emission band at 2.4 eV is observable. A strong variation of the ratio between the deep luminescence intensity and the near band edge luminescence intensity ratio is observed with increasing excitation intensity. Using a simple model based on bimolecular rate equations the concentration of defects involved in this transition is estimated to be about 1015 cm−3. CL-measurements with varying excitation intensity reveal that these recombination levels have only minor influence on the performance of high injection optoelectronic devices like laser diodes. Depth resolved measurements and CL-images from a cleaved c-GaN/GaAs reveal that the deep recombination centers are homogeneously distributed within the epilayer, however the concentration of nonradiative recombination centers is increasing towards the GaN/GaAs interface.

Morphology and photoluminescence improvements from high-temperature rapid thermal annealing of GaN

Rapid thermal annealing of GaN in an Ar or N2 ambient up to 1100 °C is shown to improve surface morphology and photoluminescence intensity. For both ambients the average rms surface roughness as determined by atomic force microscopy decreases from ∼4 nm on the as-grown material to ∼1 nm after a 1100 °C anneal. The band-edge luminescence intensity was increased by a factor of 4 after a 1100 °C anneal in a N2 ambient and a factor of 2 for annealing at 1100 °C in an Ar ambient as compared to as-grown material. The 1100 °C anneal improves the ratio of band edge to deep-level luminescence and also reduces the electron concentration and mobility. The reduction in mobility can be explained in terms of a two-band conduction mechanism where defect band conduction dominates at the lower carrier densities or an increase in the free-carrier compensation ratio.

Photoluminescence study of highly mismatched In0.53Ga0.47As epilayers grown on InP-coated GaAs substrates

Photoluminescence measurement is used to study the properties of In0.53Ga0.47As epilayers grown on InP-coated GaAs substrates. Several structures are designed to test the qualities of the epilayers. It is shown that device-quality In0.53Ga0.47As on a GaAs substrate can be achieved. The effects of hydrogenation using a photochemical-vapor-deposition system are studied, and an enhancement of the band-edge luminescence intensity by a factor of 4 is observed. The resulting intensity even exceeds that of the film grown directly on a lattice-matched InP substrate. In addition, the emission coming from lattice defects is greatly reduced after hydrogenation. These results not only further support the potential application of using binary buffer layer concepts in large lattice-mismatched systems, but also demonstrate that the photochemical-vapor-deposition system is a useful tool for injecting hydrogen into InGaAs epilayers.

Investigation of the growth technique dependence on the optical properties of Si 1-xGe x alloy layers

The discrepancy between the measured photoluminescence spectra of Si 1- x Ge x strained alloy layers originating from molecular beam epitaxy (MBE) and chemical vapor deposition (CVD) processes is investigated. Sharp excitonic band edge luminescences are observed for films by both growth techniques. For certain films grown by MBE, the photoluminescence spectra contain an additional broad alloy band luminescence at low energy. Deuterium passivation of the MBE layers is shown to annihilate the broad band luminescence and to increase the band edge luminescence intensity. These results are compared with the more intense band edge luminescence signals from CVD layers for which abundant hydrogen radicals are usually present during the deposition.

Oxygen Doping of GaAs During Omvpe Controlled Introduction of Impurity Complexes

AbstractGaAs epilayers have been grown by Organo-Metallic Vapor Phase Epitaxy using dimethylaluminum methoxide as a dopant source. This compound contains a strong aluminumoxygen bond which is thought to remain intact during low temperature deposition and result in the incorporation of Al-O as a complex. Incorporation of aluminum and oxygen was investigated by Secondary Ion Mass Spectroscopy as a function of growth conditions: growth temperature, growth rate, V/III ratio, reactor pressure and dopant mole fraction. High doping levels up to 1020 cm−3 (for both oxygen and aluminum) were achieved without degradation of surface morphology andlor precipitation of a second phase. Oxygen concentration is lower than that of aluminum for all investigated growth conditions but at low deposition temperatures oxygen/aluminum ratios approach 1, indicating that Al-O is incorporated as a pair. Infrared absorption measurements in the 600-1200 cm−1 range did not detect well known isolated oxygen localized vibrational modes (LVM). Also in layers grown at low temperatures the intensity of isolated aluminum LVM at 362 cm−1 is much smaller than the concentration obtained by SIMS. Both observations prove that oxygen not only is incorporated as an Al-O pair but remains bonded in the bulk of the layer. Low temperature photoluminescence measurements indicate that the A1-O complex is electrically active in GaAs, forms a deep level within the GaAs band gap, and serves as an efficient non-radiative recombination center. Near band edge luminescence intensity correlates well incorporation of oxygen. The Al-O pairs act as deep acceptors in GaAs and cause the compensation of shallow tellurium donors.

The effect of isolated dislocations on substrate and device properties in low-dislocation czochralski GaAs

Czochralski GaAs grown with In incorporated into the melt has large regions with fewer than 100 cm-2 dislocations. We have examined the effect of these dislocations on substrate and device properties. Infrared transmission images reveal dark filaments of high EL2 concentration a few tens of microns in diameter surrounding dislocations, Cathodo and photoluminescence images show orders of magnitude contrast in band-edge luminescence intensity near dislocations. Single dislocations appear to be surrounded by bright rings ˜200 μm in diameter in luminescence images, with dark spots 50 to 75 μm across centered on the dislocation. More complex luminescence structures with larger dark regions (˜150 μ across) and central bright spots are centered on small dislocation clusters. Differences in lifetime of photogenerated electrons or holes are the most likely cause of the luminescence contrast. Anneals typical of our post-implant processing substantially lower the luminescence contrast, suggesting the defect lowering the lifetime is removed by annealing. This may partially explain why we do not observe any effect of dislocation proximity on the properties of devices made in the material, in spite of the enormous luminescence contrast observed near dislocations.