Laima Trinkler

Luminescent properties of epitaxial Cu2O thin films were studied in 10-300 K temperature range and compared with luminescent properties of Cu2O single crystals. Cu2O thin films were deposited epitaxially by the electrodeposition method on either Cu or Ag substrates at different processing parameters, which determined the epitaxial orientation relationships and encapsulated surface. Cu2O (100) and (111) single crystal samples were cut from a crystal rod grown by the floating zone method. Luminescence spectra of thin films contain the same emission bands as single crystals around 720, 810 and 910 nm, characterizing VO2+, VO+ and VCu defects, correspondingly. Additional emission bands, whose origin is under discussion, are observed around 650-680 nm, while the exciton features are negligibly small. The relative mutual contribution of the emission bands varies depending on the particular thin film sample. Existence of the domains of crystallites with different orientation determines the polarization of luminescence. PL of both Cu2O thin films and single crystals is characterized with negative thermal quenching in the low temperature region, the reason of this phenomenon is discussed.

We have studied luminescence of LiGaO ₂ , Al ₂ O ₃ -Ga and Al ₂ O ₃ -Li crystals in order to reveal the nature of luminescence centres and mechanisms in these crystals. In Al ₂ O ₃ -Ga presence of Ga impurities determines occurrence of the 280 nm emission band, which demonstrates intra-centre character in photoluminescence and recombination character under X-ray irradiation. In Al ₂ O ₃ -Li crystal lithium induced luminescence is presented with the 326 nm band, which has a recombination character. Basing on spectral similarity of the main luminescence bands in pure LiGaO ₂ crystal with the dopant-induced emission bands in Al ₂ O ₃ , and on peculiarities of the X-ray induced thermoluminescence, the adjustment of the previous luminescence interpretation is done. It is proposed that the donor-acceptor pairs with random separation distribution responsible for the 280 nm emission are represented with gallium Ga (plus an electron) and O (plus a hole) pairs, while the donor-acceptor pairs, producing the 330 nm emission band contain a lithium ion, presumably in the interstitial position Li _i ⁰ , and a neighbouring oxygen ion with a caught hole.

Photoluminescence (PL) and afterglow luminescence (AGL) produced by UV laser irradiation with varied intensity were studied in AlN ceramics in the 10–300 K temperature range. Luminescence spectra of AlN ceramics contain the UV–blue band built up of two components – the UV (3.18 eV) and Blue (2.58 eV) bands, which are presumably ascribed to recombination luminescence involving oxygen‐related centers in the bulk and on the surface of AlN, correspondingly. It was found that position of the emission band maximum of AlN ceramics depends on such factors as excitation density, temperature, and delay time after excitation ceasing – in the case of AGL. In PL spectra, the excitation density growth produces the blue shift of the UV emission band peak, and temperature growth from 10 to 300 K causes first the blue and then the red shift. The most characteristic feature of the AGL is the red shift of the resultant UV–blue peak occurring with growth of delay time after excitation ceasing. Explanations of these phenomena are proposed basing on properties of radiative donor–acceptor pair recombination in semiconductors and varied relative contribution of the UV and Blue components to build‐up of the resultant UV–blue band.

The excitation energy dependence and temperature dependence of photoluminescence from boron nitride nanotubes and hexagonal BN powder samples are reported. The results are discussed within a model attributing the broad 3.2 eV luminescence from these samples to self-trapped excitons in the low-dimensional structures of BN nanotubes and of nano-arch surface reconstructions on h-BN sheet edge faces in powder. An empirical model accounting for the unusual combination of excitation and temperature dependence of photoluminescence seen in these measurements is suggested. For the model to be consistent with the hypothesis of self-trapped excitons on BN nanotubes, it may be necessary to show that the cores of multiwall nanotubes are selectively probed by light tuned below the h-BN exciton.

AlN ceramics, untreated and subjected to oxygen ion implantation, are studied for potential application in TL dosimetry of UV light. Based on photoluminescence and thermoluminescence measurements there are shown advantages of the selection of the 480 emission band for TL signal recording instead of the previously used 400 nm band: higher intensity of the TL signal, lower fading rate of the accumulated TL signal, excitation region (270‐340 nm) corresponding to UV‐B range, monitoring of which is actual in UV dosimetry.