In oxide materials, defects often presents and are responsible for specific optical and electrical proprieties. These defects play a significant role in the radiative emission.
The luminescence can be used as an indicator of the presence of different intrinsic and extrinsic defects in insulator or semiconductor material. The intrinsic defects in MgO were the subject of our first study (Kadri et al., 2005).
The motivation for the present research was to determine the extrinsic (impurities) type defects Cr, Ni, Fe, Ca and K present in our sample MgO single crystal. The measurements were carried out using thermoluminescence (TL) and absorbance techniques. The deconvolution method has been used in the determination with precision the emission bands related to these defects.
MATERIALS AND METHODS
In the literature review of different irradiation and MgO studies aiming to determine extrinsic defects (impurities) such as Cr, Ni, Fe, K and Ca contained in our sample we find the following:
Concerning the chromium impurity, Jiménez et al. (1985) found
that the large red emission band at 730 nm (1.7 eV) lies in the region which
characterizes the Cr3+, this emission is due to the capture of holes
released from traps, in his quality assessment of MgO substrate materials, Flecher
and Leach (1995) found the presence of several peaks in the red region between
696 and 740 nm, which is also confirmed widely in the documentation that substitional
Cr3+ ions are responsible for the red peaks. Kawaguchi (2001) also
attributes the 730 nm band to the Cr3+ impurity, Chao (1971) reported
a red band which peaks at about 700 nm with vibrational sidebands lines in thermo
luminescence spectrum of MgO and assigned the band to Cr3+ impurity,
Kantorovich et al. (2001) considers that substitutional Cr3+
in MgO occupies a Mg site, at small concentration, most chromium ions are in
octahedral (cubic) symmetry sites without neighboring vacancies and are mostly
responsible for a broad 2.00 eV absorption band and a sharp 1.776 eV (700 nm)
peak ( the R- line), Karner et al. (2001) in their neutron-irradiated
MgO single crystals study, found two kinds of luminescence bands one of which
belongs to the red region and is apparently due to Cr3+ in sites
with different symmetry, the R-line is situated at 1.776 eV.
Concerning the Nickel impurity, observable peaks in the visible region have been revealed for four distinct excitation (emissions) 476, 481, 486 and 490 nm in Flecher and Leach (1995), these results correlate very well with the TL data of Ralph and Townsend (1970) for substitutional Ni2+ ions.
As far as the iron impurity is concerned there is some controversy about its
role, some suggest that the red emission band is ascribed to it as well as chromium
Ziniker et al. (1972) and Clement and Hodgson (1984), other authors attribute
Fe3+ emission to the ultra violet region Jiménez et al.
(1995), a small band of 340 nm (3.65 eV) in the 320-380 nm region occurs and
is possibly related to Fe3+ emission due to hole capture by Fe2+
ions. Acceptor impurities such as Li, Na, K from group I are known to give rise
to rather deep traps for holes and the luminescence bands due to their presence
are observed at energies less than 6 eV (greater than 207 nm), the monovalent
substitutional impurity K+ belongs to acceptors and are rather find
on surface because of their relatively big size Molnar (2000), emitting in the
ultra violet region. Also if it is assumed that the 6.9 eV (180 nm) band is
due to Ca2+ impurities, we must admit that at concentrations as small
as several ppm practically all excitons decay at Ca2+ ions, a situation
not very likely to take place, Rachko and Valbis (1979).
Thermoluminscence and absorption measurements were carried out at University of Science and Technology of Oran (Algeria), Electronic Microscopy and Material Science Laboratory (2006). The sample analysed in this work is a single crystal MgO polished from Soekawa chemicals Japan. The dimensions of the sample are 10x10x1 mm3. It is known that the MgO crystal have the NaCl structure with a cubic-face-centred (cfc) Bravais lattice with a cubic lattice constant of 4.21 Å. A micro probe analysis of this crystal showed an impurity content of Cr, Ni, Fe, Ca and K in small quantities. Firstly the sample was irradiated by an UV Hg source lamp (4.8 eV for 10 min in air) at low temperature -100°C (170°K).
Secondly the crystal was thermally heated in a furnace to a temperature between 170 and 500°K. Thermoluminescence (TL) emission is detected through a Spectrograph CP 200 Jobin Yvon connected to a CCD 3000 (coupled charge device) cooled to 150°K, with wavelength bands of 250-1200 nm for grating 133 g mm1 and 180-1000 nm for grating 200 g mm1.
The TL spectrum were recorded and analysed with Spectramax software, by means of the Fourier self-deconvolution method which synthetically narrows the effective trace bandwidth features, this aids identifying the principal bands, it can also be useful for more accurate determination of the number of peaks in a trace region, the band positions and areas.
RESULTS AND DISCUSSION
The TL curves recorded from -100°C (170°K) to room temperature showed that the spectral intensity decreases, whereas in the second heating that is from room temperature up to 230°C (500°K) we noticed that the spectral intensity increases up to a maximum limit at 110°C (380°K) then start decreasing with increasing temperature (Kadri et al., 2005).
In TL experimental analysis of extrinsic defects (impurities: Cr, Ni, Fe, K and Ca) in MgO single crystal and by considering the different irradiation techniques mentioned in the previous literature review, we considered a single spectrum at -40°C ( 233°K) as shown in Fig. 1 to be deconvoluted (Fig. 2), three emission bands appears 694, 715 and 737 nm, where the 715 nm (1.73 eV) is the dominant peak and is attributed to Cr3+ its TL- intensity spectra is represented in Fig. 3 where it can be seen that intensity behaves differently with regards to temperature varying from -100°C to ambient and from ambient to 230°C, whereas the two other emissions are assumed vibrational side bands.
A close look at the 280-500 nm region where the Ni and Fe impurities are located according to the literature, only weak intensity signals were found what made impurity identification rather difficult, deconvolution method applied to the spectrum band maximum temperature of 110°C (Fig. 4) has been used to determine the emission bands.
||Spectrum recorded at -40°C (233°K) (grating 133g mm1)|
||Sub-region 670-770 nm deconvolution of the spectrum (Fig. 1)|
||TL-intensity curve versus temperature of the 715 nm band (Cr)|
recorded at 110°C (380°K), region of interest (280-500 nm) (grating
133 g mm1)|
465-500 nm deconvolution of the spectrum (Fig. 4)|
Two sub-regions has been identified 465-500 nm (Fig. 5) and 310-375 nm (Fig. 6) responsible of Ni and Fe emissions, respectively.
In the first sub-region we recorded a dominant emission at 473 nm (2.62 eV) which was two assigned to Ni2+, its TL-intensity spectra is shown in Fig. 7, in the second deconvolution region two emissions were recorded 330 and 350 nm, the most probable emission according to what was already achieved in previously mentioned works in the literature corresponding to Fe3+ is the 330 nm (3.75 eV) band, its TL-intensity spectra is shown in Fig. 8.
A second recorded spectrum at a maximum temperature of 110°C with a grating
of 200 g mm1 was chosen to have a closer look at the 180-260 nm
region (Fig. 9), again and by the mean of the deconvolution
method we found four peaks 190, 210, 233 and 243 nm (Fig. 10),
the 190 nm ( 6.52 eV) emission band is attributed to Ca2+ not to
excitons since that the energy is less than 6.9 eV as was proposed by Rachko
and Valbis (1979), for the TL-intensity (Fig. 11), the other
three emissions are localised in the energy region less than 6 eV (greater than
207 nm) where the luminescence bands of K+ are likely to occur.
310-375 nm deconvolution of the spectrum (Fig. 4)|
spectra versus temperature of the 473 nm band (Ni)|
It can easily noticed that results obtained in the present work agree to a
very high extent with results obtained elsewhere (see literature review) and
come also to confirm some other undertaken works, even though the analysis techniques
might be different.
spectra versus temperature of the 330 nm band (Fe)|
recorded at 110°C (380°K), region of interest (170-400 nm) (grating
200 g mm1)|
180-260 nm deconvolution of the spectrum (Fig. 9)|
curve versus temperature of the 190 nm band (Ca)|
In this study the thermoluminescence (TL) combined with absorbance and deconvolution
method has been used to determine the extrinsic defects of MgO single crystal
previously irradiated by UV (4.8 eV), the analysis of results indicates emission
bands for Cr3+ at 715 nm, Ni2+ at 473 nm, Fe3+
at 330 nm, Ca3+at 190 nm and for K+ three emission bands
are localised 210, 223 and 243 nm one of which is attributed to this impurity.
Important results for the evolution of the characteristics and properties of
magnesium oxide were presented. These experimental results are in agreement
with the prediction of the standard insulator MgO. The deconvolution method
has enabled us to determine a certain number of identified and non identified
emission bands regardless of the intensity of the recorded spectra. We intend,
in near future work to undergo much deeper investigations in order to attribute
these non identified emissions to defects which may be contained in the sample.