B. Berzina

Investigation of Oxygen‐Related Luminescence Centres in AlN Ceramics

The structure of oxygen-related luminescence centres in nominally undoped and Y 2 O 3 doped AlN ceramics was investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optically-detected EPR. The photoluminescence-detected EPR lines having g-values of 1.990 and 2.008 were assigned to a recombination between neighbouring donor and acceptor pairs. The two EPR lines at g = 1.987 and 2.003 detected via the recombination luminescence in the afterglow are thought to be due to a recombination between the same, but distant donor and acceptor pairs. The donor was previously speculated to be an electron trapped on an oxygen impurity which substitutes for a nitrogen on a regular lattice site. The defect structure of the acceptor was established by ENDOR to be a hole trapped on an O N -v Al complex (ON oxygen on a regular N site, v Al Al vacancy). The measured superhyperfine interaction is caused by two equivalent 27 Al nuclei both with a = ±29.6 MHz and one 27 Al nucleus with a = ±27.0 MHz.

Luminescence properties of wurtzite AlN nanotips

The optical properties of aluminum nitride nanotips (AlNNTs) synthesized via vapor transport and condensation process have been studied by cathodoluminescence, photoluminescence (PL), thermoluminescence (TL), and UV absorption measurements. Two defect related transitions around 2.1 and 3.4eV and an excitonic feature at 6.2eV were identified. Compared to the AlN macropowders, the AlNNTs showed a blueshift (+0.2eV) of the ∼3.2eV peak. Analysis of both PL and TL excitation measurements indicated the existence of subband gap multiple energy levels in AlNNTs. A significant TL intensity even at 145°C suggests possible ultraviolet detector and dosimetric applications of these AlNNTs.

Luminescence mechanisms of oxygen-related defects in AlN

Spectral characteristics of native oxygen-related defects existing in the crystalline lattice of AlN were studied. Features of photoluminescence observed under exposure to ultraviolet light together with those of the photostimulated luminescence testify the recombination character of luminescence. The mechanism of luminescence of oxygen-related defects is proposed.

Spectral characterization of bulk and nanostructured aluminum nitride

Spectral characteristics including photoluminescence (PL) spectra and its excitation spectra for different AlN materials (AlN ceramics, macro size powder and nanostructured forms such as nanopowder, nanorods and nanotips) were investigated at room temperature. Besides the well known UV-blue (around 400 nm) and red (600 nm) luminescence, the 480 nm band was also observed as an asymmetric long-wavelength shoulder of the UV-blue PL band. This band can be related to the luminescence of some kind of surface defects, probably also including the oxygen-related defects. The mechanisms of recombination luminescence and excitation of the UV-blue luminescence caused by the oxygen-related defects were investigated. It was found that the same PL band is characteristic for different AlN materials mentioned above; however, in the nanostructured materials (nanorods, nanotips and nanopowder) the intensity of UV-blue PL is remarkable lower than in the bulk material (ceramics). In the case of nanostructured AlN materials, excitation of the oxygen-related defect is mainly realized through the energy transfer from the host material (electron/hole or exciton processes) to the defects and this mechanism prevails over the mechanism of direct defect excitation.

Luminescence properties of AlN nanostructures revealed under UV light irradiation

The luminescence properties of the AlN nanostructures – nanorods and nanotips -revealed under the UV irradiation are similar to those of the AlN ceramics. Presumably they are induced by the recombination processes in the oxygen-related centers. All the studied luminescence processes (photoluminescence, thermoluminescence and optically stimulated luminescence) in the nanostructures occur mainly through the host lattice excitation. That may be explained by the smaller concentration of the defect centers and more perfect structure of the host lattice of the nanostructures compared to the ceramics. The small mutual differences revealed in the spectra and TL curves of the AlN nanotips and nanorods possibly arise due to the modifications of the host lattice structure and variations of the defect concentration during production of the samples.

Luminescence Properties of AlN Ceramics and Its Potential Application for Solid State Dosimetry

Aluminum nitride (AlN) is a wide band material (Eg = 6.2 eV) with a wurtzite structure. It has already found practical application in microelectronics as substrate, insulator and packaging material due to combination of the uppermost qualities, such as high thermal conductivity, good dielectric properties and thermal expansion coefficient comparable with that of silicon. There are different modifications of the material used for application and investigation purposes: single crystal, powder, nanostructures, thin films and so on. AlN ceramics is one of them, it is easy to produce and to handle, thereat in the form of ceramics AlN maintains the main properties of the material. The objective of the study is investigation of luminescence properties of AlN ceramics including elucidation of luminescence centers and mechanisms as well as estimation of potential application of the material in the field of solid state dosimetry. This chapter is based on our papers devoted to luminescent and dosimetric properties of AlN ceramics published in 1998-2009 and quite recent measurements performed at low temperatures.

Spectral properties of AIN ceramics

Spectral properties of oxygen-related defects are studied in AIN ceramics at room temperatures. Original results concerning the photoluminescence under ultraviolet irradiation are obtained; they include the excitation spectrum and irradiation dose effects. The ultraviolet light energy storage and its release under irradiation with visible or infrared light in the form of the photostimulated luminescence has been observed in AIN ceramics. The properties of the photostimulated luminescence such as creation, emission and stimulation spectra are reported. For the explanation of the experimental results the mechanism of the recombination luminescence involving the oxygen-related defect is proposed.

Use of AlN ceramics in ultraviolet radiation dosimetry

AlN-Y2O3 ceramics is studied as a material for application in the area of ultraviolet radiation (UVR) dosimetry. The properties of optically stimulated luminescence (OSL) and thermally stimulated luminescence (TL) revealed by AlN ceramics are characterized and considered for practical application. A special attention is devoted to studies of the spectral properties of the material, including stimulated luminescence. Spectral properties of the material make it potentially suitable for dosimetric application both in UV-C region (200-290 nm), where it has the maximum sensitivity, and in UV-B+UV-A (290-350 nm) region, where the spectral behavior of its sensitivity coincides rather well with that of the human skin.

Magnetic resonance investigations of oxygen-related luminescence centres in AlN ceramics

Abstract The structure of oxygen-related luminescence centres in nominally undoped and Y2O3-doped AIN ceramics were investigated by electron paramagnetic resonance (EPR), electron nuclear double resonance (ENDOR) and optically-detected EPR. The photoluminescence-detected EPR lines having g values of 1.990 and 2.008 were attributed to neighbouring donor and acceptor pairs causing the recombination luminescence excited in the ultraviolet. The two EPR lines at g = 1.987 and g = 2.003, detected via the recombination luminescence in the afterglow, are thought to be due to a recombination between the same, but more distant donor and acceptor pairs. The donor is supposed to be an electron trapped at an oxygen impurity which substitutes for a nitrogen (ON)−. The defect structure of the acceptor was established by ENDOR to be a hole trapped at an ON-VAl complex (VAl = Al vacancy).

Self-trapped exciton formation through photo-induced recombination of F and H centers in alkali iodides

The photo-induced conversion of the primary F, H center pairs into self-trapped excitons have been proposed and studied in alkali iodides.