Dominant Photoluminescence Band Research Articles

Effects of irradiation of ZnO/CdS/Cu2ZnSnSe4/Mo/glass solar cells by 10 MeV electrons on photoluminescence spectra

Solar cells with the structure ZnO/CdS/Cu2ZnSnSe4/Mo/glass were studied by photoluminescence (PL) before and after irradiation with a dose of 1.8 × 1015 cm−2 and then 5.4 × 1015 cm−2 of 10 MeV electrons carried out at 77 K in liquid nitrogen bath. The low temperature PL spectra before irradiation revealed two bands, a broad and asymmetrical dominant band at 0.94 eV from the CZTSe layer and a lower intensity high energy band (HEB) at 1.3 eV, generated by defects in the CdS buffer layer. Analysis of the excitation intensity and temperature dependencies suggested that the dominant band is free-to-bound (FB): the recombination of free electrons with holes localised at acceptors whose energy levels are affected by potential fluctuations of the valence band due to high concentrations of randomly distributed charged defects. Irradiation did not induce any new band in the examined spectral range (from 0.5 μm to 1.65 μm) but reduced the intensity of both bands in the PL spectra measured at 77 K without warming the cells. The higher the dose the greater was the reduction. After this the cells were warmed to 300 K and moved to a variable temperature cryostat to measure temperature dependencies of the PL spectra. After irradiation the red shift rate of the FB band with temperature rise was found to increase. Electrons displace atoms in the lattice creating primary defects: interstitials and vacancies. These defects recombine during and shortly after irradiation forming secondary defect complexes which work as deep non-radiative traps of charge carriers reducing the PL intensity and increasing the rate of the temperature red shift. Irradiation did not affect the mean depth of the band tails estimated from the shape of the low energy side of the dominant PL band.

Magnesium acceptor in gallium nitride. I. Photoluminescence from Mg-doped GaN

Defect-related photoluminescence (PL) is analyzed in detail for $n$-type, $p$-type, and semi-insulating Mg-doped GaN grown by different techniques. The ultraviolet luminescence (UVL) band is the dominant PL band in conductive $n$-type and $p$-type GaN:Mg samples grown by hydride vapor phase epitaxy (HVPE) and molecular beam epitaxy. The UVL band in undoped and Mg-doped GaN samples is attributed to the shallow $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ acceptor with the ionization energy of 223 meV. In semi-insulating GaN:Mg samples, very large shifts of the UVL band (up to 0.6 eV) are observed with variation of temperature or excitation intensity. The shifts are attributed to diagonal transitions, likely due to potential fluctuations or near-surface band bending. The blue luminescence $(\mathrm{B}{\mathrm{L}}_{\mathrm{Mg}})$ band is observed only in GaN:Mg samples grown by HVPE or metalorganic chemical vapor deposition when the concentration of Mg exceeds ${10}^{19}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}3}$. The $\mathrm{B}{\mathrm{L}}_{\mathrm{Mg}}$ band is attributed to electron transitions from an unknown deep donor to the shallow $\mathrm{M}{\mathrm{g}}_{\mathrm{Ga}}$ acceptor. Basic properties of the observed PL are explained with a phenomenological model.

HVPE GaN with Low Concentration of Point Defects for Power Electronics

ABSTRACTWe have studied photoluminescence (PL) from undoped GaN films grown by HVPE technique on sapphire. Several defect-related PL bands are observed in the low-temperature PL spectrum. The concentrations of the defects responsible for these PL bands are determined from the dependence of PL intensity on excitation intensity. The RL band with a maximum at 1.8 eV is often the dominant PL band in HVPE GaN. It is caused by an unknown defect with the concentration of up to ∼1017 cm-3. The concentrations of defects responsible for other defect-related PL bands rarely exceed 1015 cm-3.

The Origin of a 650 nm Photoluminescence Band in Rubrene

Commonly observed variations in photoluminescence (PL) spectra of crystalline organic semiconductors, including the appearance or enhancement of certain PL bands, are shown to originate from a small amount of structural disorder (e.g., amorphous inclusions embedded in a crystal), rather than be necessarily related to chemical impurities or material oxidation. For instance, in rubrene, a minute amount of such disorder can lead to the appearance of a dominant PL band at 650 nm as a result of triplet excitons captured and fused at these sites, with a subsequent emission from the amorphous phase.

Effect of annealing on conductivity behavior of undoped zinc oxide prepared by rf magnetron sputtering

An undoped ZnO thin film with high resistivity was prepared by rf magnetron sputtering on the quartz substrates using a ZnO (99.999% pure) as target and high pure Ar as sputtering gas. Upon annealing in a temperature range from 513 to 923 K under 10 −3 Pa, the conductive properties of the film change from high resistivity, to n-type, then to p-type and finally to n-type with increasing annealing temperature and the p-type ZnO film was fabricated near 863 K reproducibly. Temperature-dependent photoluminescence (PL) of the p-type ZnO shows a dominant PL band at 3.072 eV at low temperature, which is related to Zn vacancy (V Zn) acceptor. The intensity of the 3.072 eV band decreases with increasing temperature, implying increment of amount of the hole induced by V Zn. X-ray photoelectron spectroscopy (XPS) and room temperature PL measurements indicate that the undoped ZnO is Zn-rich and has oxygen vacancy (V o) and interstitial Zn (Zn i) donor defects. The amount of V o and Zn i donors changes with the annealing temperature. The intrinsic p-type conduction of the undoped ZnO film is ascribed to that the V Zn acceptor concentration can compensate V o and Zn i donor concentration.

Optical detection of electron paramagnetic resonance in room-temperature electron-irradiated ZnO

The dominant defect observed in the photoluminescence (PL) of room-temperature electron-irradiated ZnO by optical detection of electron paramagnetic resonance (ODEPR) is determined to be the positively charged oxygen vacancy $({V}_{\mathrm{O}}^{+})$. Its spectrum, labeled L3, was previously observed in a $4.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ in situ irradiation study [Yu. V. Gorelkinskii and G. D. Watkins, Phys. Rev. B 69, 115212 (2004)], but it was thought there not to be stable at room temperature and was not identified. Here it is found to be stable to $400\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, where it disappears. It is observed as a competing process (negative signal) to the dominant PL band produced by the irradiation at $\ensuremath{\sim}700\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, but is positive in a weaker band at $\ensuremath{\sim}600\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$. Models are presented for its electrical level position in the gap to explain the results. Two other ODEPR signals are also detected, one of which is tentatively identified as also associated with the oxygen vacancy.

Deep level photoluminescence commonly present in undoped Czochralski grown GaAs

Photoluminescence (PL) under the below band-gap excitation with the 1.32-μm line of a yttrium aluminum garnet laser has been measured for undoped, semi-insulating Czochralski grown GaAs crystals. A Gaussian band with a peak at 0.67 eV and a half-width of 120 meV has been found to appear commonly, regardless of the groups of crystals classified according to their dominant PL band measured under the above band-gap excitation. The occurrence of the PL fatigue effect, the positive correlation of the PL intensity with the dislocation density, and the spectral shape indicate that the main deep donor EL2 is involved in the transition responsible for the 0.67-eV band.