K. P. O’Donnell

EXCITON LOCALIZATION AND THE STOKES' SHIFT IN INGAN EPILAYERS

We report a comparative study of the emission and absorption spectra of a range of commercial InGaN light-emitting diodes and high-quality epilayers. A working definition of the form of the absorption edge for alloys is proposed, which allows a unique definition of the Stokes’ shift. A linear dependence of the Stokes’ shift on the emission peak energy is then demonstrated for InGaN using experimental spectra of both diode and epilayer samples, supplemented by data from the literature. In addition, the broadening of the absorption edge is shown to increase as the emission peak energy decreases. These results are discussed in terms of the localization of excitons at highly indium-rich quantum dots within a phase-segregated alloy.

Luminescence decay in disordered low‐dimensional semiconductors

The luminescence decay of excitons in disordered low‐dimensional semiconductors with quantum confinement is shown experimentally to be characterized by a nonexponential profile and an absence of spectral diffusion. We are able to describe this luminescence as a hopping‐assisted recombination using the correlation function approach to nondispersive transport developed by H. Scher, M. F. Shlesinger, and J. T. Bendler [Phys. Today 41, 26 (1991)]. We suggest a simple derivation of analytical functions which accurately describe the anomalous luminescence decays of disordered II‐VI superlattices and of porous silicon, and show that this model includes exponential and Kohlrausch [Pogg. Ann. Phys. 119, 352 (1863)] (stretched‐exponential) relaxations as special cases.

Selectively excited photoluminescence from Eu- implanted GaN

The intensity of Eu-related luminescence from ion-implanted GaN with a 10nm thick AlN cap, both grown epitaxially by metal organic chemical vapor deposition (MOCVD) is increased markedly by high-temperature annealing at 1300°C. Photoluminescence (PL) and PL excitation (PLE) studies reveal a variety of Eu centers with different excitation mechanisms. High-resolution PL spectra at low temperature clearly show that emission lines ascribed to D05-F27 (∼622nm), D05-F37 (∼664nm), and D05-F17 (∼602nm) transitions each consist of several peaks. PL excitation spectra of the spectrally resolved components of the D05-F27 multiplet contain contributions from above-bandedge absorption by the GaN host, a GaN exciton absorption at 356nm, and a broad subedge absorption band centred at ∼385nm. Marked differences in the shape of the D05-F27 PL multiplet are demonstrated by selective excitation via the continuum/exciton states and the below gap absorption band. The four strongest lines of the multiplet are shown to consist ...

Optical energies of AlInN epilayers

Optical energy gaps are measured for high-quality Al1−xInxN-on-GaN epilayers with a range of compositions around the lattice match point using photoluminescence and photoluminescence excitation spectroscopy. These data are combined with structural data to determine the compositional dependence of emission and absorption energies. The trend indicates a very large bowing parameter of ≈6eV and differences with earlier reports are discussed. Very large Stokes’ shifts of 0.4–0.8eV are observed in the composition range 0.13<x<0.24, increasing approximately linearly with InN fraction despite the change of sign of the piezoelectric field.

Interpretation of double x-ray diffraction peaks from InGaN layers

The presence of two, or more, x-ray diffraction (XRD) peaks from an InGaN epilayer is sometimes regarded as an indicator of phase segregation. Nevertheless, detailed characterization of an InGaN/GaN bilayer by a combination of XRD and Rutherford backscattering spectrometry (RBS) shows that splitting of the XRD peak may be completely unrelated to phase decomposition. Wurtzite InGaN/GaN layers were grown in a commercial reactor. An XRD reciprocal space map performed on the (105) plane shows that one component of the partially resolved InGaN double peak is practically aligned with that of the GaN buffer, indicating that part of the layer is pseudomorphic to the GaN template. The other XRD component is shown to have the same indium content as the pseudomorphic component, from a consideration of the effect of strain on the c- and a-lattice constants. The composition deduced from XRD measurements is confirmed by RBS. Depth-resolving RBS channeling angular scans also show that the region closer to the GaN/InGaN ...

Morphology of luminescent GaN films grown by molecular beam epitaxy

GaN thin films were grown by molecular beam epitaxy on sapphire substrates. Scanning electron (SE) and atomic force microscopies reveal that on a typical film an assembly of oriented hexagonal microcrystallites rises above a background of polycrystalline or amorphous material. Cathodoluminescence (CL) spectra of the films feature bright UV exciton peaks and a broad green emission band. We identify the exciton peaks as those of the wurtzite form of GaN. A comparison of SE and CL micrographs of the same sample area shows that the luminescence emanates almost entirely from the hexagonal crystallites.

Photoluminescence of localized excitons in pulsed-laser-deposited GaN

Continuous-wave photoluminescence (PL) and time-resolved photoluminescence of gallium nitride layers grown by pulsed laser deposition are compared. The temperature dependence of the photoluminescence decay time and the PL-integrated intensity allows a determination of radiative and nonradiative time constants of GaN. We find that luminescence peaks centered at 3.360 and 3.305 eV at low temperature can be attributed to recombination of excitons localized at extended defects. The photoluminescence radiative lifetime at room temperature is on the order of tens of ns.

Lattice site location of optical centers in GaN:Eu light emitting diode material grown by organometallic vapor phase epitaxy

Eu-doped GaN was grown by organometallic vapor phase epitaxy at temperatures from 900 to 1100 °C. Eu incorporation is influenced by temperature with the highest concentration found for growth at 1000 °C. In all samples, Eu is incorporated entirely on substitutional Ga sites with a slight displacement which is highest (∼0.2 A) in the sample grown at 900 °C and mainly directed along the c-axis. The major optical Eu3+ centers are identical for in situ doped and ion-implanted samples after high temperature and pressure annealing. The dominant Eu3+ luminescence lines are attributed to isolated, substitutional Eu.

Optical properties of Si nanocrystals prepared by magnetron sputtering

Visible photoluminescence (PL), optical transmission, and Raman scattering spectra of Si nanocrystals embedded in SiO2 matrix have been studied. Films were deposited by rf magnetron sputtering of compound SiO2/Si targets. Films containing silicon in the form of nanocrystals clusters show intense luminescence at room temperature. Films containing only amorphous Si clusters show little or no PL. Annealing in vacuum or in specific atmosphere leads to strong change of the PL spectrum and its intensity. Results are explained by strong influence of the Si/SiO2 interface state on integral PL emission.

Optical absorption of ZnSe‐ZnS strained layer superlattices

A study of the absorption spectra of excitons in ZnSe‐ZnS strained layer superlattices (SLS) with well widths ranging from 0.6 to 7.6 nm is reported. The n=1 heavy hole (hh) and light hole (1h) exciton absorptions are clearly resolved for all samples even near room temperature. A theoretical estimation of the n=1 hh exciton peak energy, which takes account of strain, quantum confinement of free carriers, and exciton binding energy enhancement by reduced dimensionality, is in excellent agreement with the experimental results. The variations in absorption linewidth and energy shift between absorption and emission band peaks, as a function of quantum well width, have also been measured: the experimental results provide evidence that the origin of the so‐called ‘‘Stokes’ shift’’ lies in Anderson localization due to monolayer fluctuations in the well width. The temperature dependence of the exciton peak energy and its linewidth are interpreted in terms of electron‐phonon and exciton‐phonon interactions.