ALBERT

Notes: Two distinct spectra are reported from an optically detected magnetic resonance study of epitaxial films of cubic SiC. The first is a Lorentzian, single line with g=2.0065±0.0015, which is strong in Al-doped SiC. This line is attributed to residual donors. The second spectrum, observed in both Al-doped and undoped samples, is dominated by a pair of exchange-split lines with g=2.0024 and a=0.095 cm−1. Although a definite assignment of this spectrum cannot be made, spectral dependence studies show it is associated with a defect-related luminescence band in the energy range from 1.6 to 1.9 eV.

Notes: Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy of Dy2S3-doped As12Ge33Se55 glasses demonstrate that the broad, below-gap PLE mechanism observed previously for rare-earth emissions in Er- and Pr-doped chalcogenide glasses is a general or universal feature of rare-earth-doped chalcogenide glasses, provided the transition energies of the rare earths are in the correct energy range. The PL spectrum excited in the 815 nm Dy3+ absorption band shows the 1150, 1340, and 1700 nm sequence of Dy3+ transitions expected for Dy-doped chalcogenide glasses. The PLE spectra of the 1340 (6F11/2,6H9/2→6H15/2) and 1700 nm (6H11/2→6H15/2) Dy3+ emissions exhibit broad excitation bands from ∼500 to 1000 nm, upon which the sharp intra F-band absorptions of Dy3+ are superimposed. These broad PLE bands are characterized by an exponentially decreasing slope with decreasing energy in the spectral range below the Urbach edge which is associated with the below-gap, defect- and impurity-induced exponential tails observed in the below-gap absorption spectra of chalcogenide glasses. At high energy, the exponentially rising Urbach absorption edge of the host glass, which leads to competing nonradiative decay mechanisms, eventually dominates the absorption spectrum and imposes an exponentially decreasing slope on the PLE spectra. These features of the broadband PLE have been observed in the PLE spectra of the Er-, Pr- and Dy-doped chalcogenide glasses we have studied. There is a difference in the relative strengths of the broad PLE bands, with the broadband for the 1340 nm PL band being a factor of 3 stronger than that for the 1700 nm PL emission. In addition, there is a shift in the peak energy of the different PLE spectra. Qualitatively speaking, the higher the energy of the luminescence transition, the higher the energy of the corresponding broad PLE peak. Proposed mechanisms for the broadband PLE of rare-earth emissions in chalcogenide glasses are discussed in the context of models for the below-gap, defect- and impurity-induced exponential absorption tails, including the possible role of lattice relaxation associated with charge transfer transitions and the involvement of transition metal impurities. © 1996 American Institute of Physics.

Notes: Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopy carried out on bulk samples of Er:As2S3 glasses demonstrate that Er3+ is incorporated in optically active sites in the glass and gives rise to a broad ∼1500–1600 nm, 4I13/2→4I15/2 emission spectrum similar to those observed in Er-doped oxide glasses. In addition to the expected 980 nm (4I15/2→4I11/2) Er3+ PLE band, the 1500–1600 nm Er3+ PL band in the glass exhibits a remarkably broad PLE spectrum which extends from the As2S3 Urbach absorption edge at ∼550 nm to beyond 1000 nm. This broad PLE band corresponds closely to an exponential PLE band observed in the "band tail'' spectral range for Er-doped Ge33As12Se55 glasses. These unusual PLE spectra indicate that in Er-doped chalcogenide glasses there is an additional broad-band, below gap, extrinsic absorption mechanism which efficiently excites the characteristic 1550 nm, 4I13/2→4I15/2 Er3+ emission band. It is not possible to determine at present whether the Er dopants themselves are responsible for the broad band absorption which excites the Er3+ PL bands, or if photoexcited carriers attributable to absorption by other impurities transfer their energy to the excited states of the Er3+ 4f shells. Microscopic characterization techniques reveal that the Er2S3-doped glasses also contain residual Er2S3 crystallites which give rise to sharp, narrow line 1550, 980, and 810 nm Er3+ optical spectra characteristic of polycrystalline Er2S3. The temperature dependence of the Er2S3 PL and PLE spectra enables the energy levels of the 4I13/2 excited state and 4I15/2 ground state manifolds to be determined. © 1995 American Institute of Physics.

Notes: InGaAs/InP quantum wires with widths ranging from 200 to 40 nm have been fabricated using high-resolution electron-beam lithography and CH4/H2 reactive-ion etching. The excitation intensity dependence of the photoluminescence (PL) energies and line shapes for relatively wide wires (∼100 nm) exhibits the effects of band filling in k space and band-gap renormalization due to many-body effects in dense electron-hole plasmas (EHP). In the narrowest wires studied (∼40 nm), the effects of sidewall surface recombination limit the attainable EHP density. In addition, the results show a blue shift of PL energies when wire width decreases below 100 nm.

Notes: A GaN p-n junction structure was grown on a (0001) sapphire substrate by atmospheric pressure metalorganic chemical vapor deposition using a double buffer layer. The resulting light emitting diode (LED) was further exposed to a low-energy electron beam source. The effect of e-beam exposure on the room-temperature electroluminescence spectra of the LED is reported. It is found that the electroluminescence spectral features change dramatically as a function of the electron-beam exposure time and current density. This is attributed to changes in active Mg concentration. The origin of each electroluminescence band is discussed. © 1996 American Institute of Physics.

Notes: A detailed study of the luminescence properties of GaN layers grown by metalorganic chemical vapor deposition on sapphire substrates with GaN/AlN double buffer layers or GaN single buffer layers was carried out with photoluminescence and cathodoluminescence (CL) spectroscopy. It was discovered that the use of the double buffer layer resulted in an improved surface morphology, but also increased the strain in the samples relative to samples grown on single GaN buffer layers. Free exciton (A exciton), neutral donor-bound exciton, and acceptor-bound exciton photoluminescence peaks were observed for GaN films grown on GaN, AlN, and GaN/AlN buffer layers. Acceptor free-to-bound luminescence was also observed and the thermal activation energy of the acceptors was measured. From these data we are able to determine the acceptor binding energy, EA, to be 231.5 meV and the donor binding energy ED to be 29.5 meV. An exciton peak, the acceptor free-to-bound luminescence, and an unidentified lower energy peak were observed with CL. CL imaging also allowed us to correlate luminescence with surface features of the films. e-beam annealing of both n-type and p-type films was investigated. © 1996 American Institute of Physics.

Notes: We demonstrate the epitaxial lateral overgrowth (ELO) of GaN from narrow stripes with triangular cross sections by atmospheric pressure metal organic chemical vapor deposition, and characterize the optical properties of these stripes at each stage of the growth using spatially resolved cathodoluminescence spectroscopy, wavelength imaging, and line scans. An improvement of the optical quality of the GaN materials grown by the ELO technique is clearly shown by the appearance of a free exciton peak, the enhancement of bandedge emission, and the weakening of the yellow emission. © 1998 American Institute of Physics.

Notes: We report on the successful incorporation of arsenic (As) in GaN during metalorganic chemical vapor deposition (MOCVD). A characteristic room-temperature luminescence band centered around 2.6 eV (480 nm), similar to the peak position of the As ion-implanted GaN, is found to be related to the As impurity in the MOCVD grown GaN:As films. The arsenic incorporation efficiency as a function of experimental conditions and structure is presented. Temperature- and power-dependent cathodoluminescence measurements have been performed to help establish the nature of the As-related peak. © 1998 American Institute of Physics.

Notes: Photoluminescence (PL) and photoluminescence excitation (PLE) spectroscopies have been carried out at 6 K on the 1540 nm 4I13/2→4I15/2 emission of Er3+ in Er-implanted films of GaN grown by metalorganic chemical vapor deposition. The PLE spectra exhibit several broad below-gap absorption bands, which excite three distinct site-selective Er3+ PL spectra. The excitation of two of the site-selective Er PL bands involves optical absorption by defects or background impurities, rather than direct intra-f shell absorption, with subsequent nonradiative transfer of the energy to nearby Er3+ luminescence centers. The characteristics of the PLE spectrum of the third site-selective PL band suggest that an exciton bound at an Er-related trap is involved in the excitation mechanism. © 1997 American Institute of Physics.

Notes: Using a molecular beam epitaxy system equipped with an inductively coupled radio frequency nitrogen plasma source, p-type GaN films were grown on sapphire substrates with no postgrowth treatment. Uniformity of the surface morphology and spatial homogeneity of the luminescence of the films were investigated using scanning electron microscopy and cathodoluminescence (CL) imaging, respectively. By examining the dependence of photoluminescence on the excitation laser power density at 6 and 300 K, three different emissions having different origins were identified. A blue emission at ∼3.25 eV is associated with shallow Mg impurities, while two different lower-energy emissions at ∼2.43 and ∼2.87 eV are associated with deep Mg complexes. The spatial distributions of the shallow and deep Mg impurities dominating the optical properties of the p-type GaN films were also examined along the growth direction by low- and room-temperature CL using an electron beam with a range of penetration depths © 1996 American Institute of Physics.