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Polarized luminescence of an aggregate defect centre in Al2O3
L. Welch, A. Hughes, G. Pells
To cite this version:
L. Welch, A. Hughes, G. Pells. Polarized luminescence of an aggregate defect centre in Al2O3. Journal de Physique Colloques, 1980, 41 (C6), pp.C6-533-C6-536. �10.1051/jphyscol:19806140�. �jpa-00220050�
Polarized luminescence of an aggregate defect centre in A1
20
3 L. S. Welch, A. E. Hughes and G. P. PellsMaterials Development Division, Atomic Energy Research Establishment, Harwell, Oxon, UK
Abstract. — The dichroic nature of the optical absorption and luminescence bands of a defect centre in fast neutron and proton irradiated a-Al203 have been studied at 4 K. The investigation centred on an absorption band at 358 nm and a mirror image emission band at 379 nm. These bands show resolved vibronic structure with a zero-phonon line at 368.4 nm and are due to allowed electric dipole transitions. Plane polarized light experiments show that the transition dipole moment is set at 40° to the c-axis of the crystal and we suggest that it originates at a defect centre which consists of two next nearest neighbour oxygen ion vacancies which have trapped one or more electrons.
1. Introduction. — Exposure of crystalline <x-Al203
to fast particle or ionizing radiation effects marked changes in the optical properties of the material.
The numerous optical absorption bands which may be induced in a-Al203 by neutron, proton or electron irradiation, or ion-implantation, have been reported in previous papers [1 -8]. Three of the higher energy absorption bands have been allocated to specific defect centres, the 205 nm (6 eV) band to the F centre and the 227 nm (5.4 eV) and 256 nm (4.8 eV) bands to the F+ centre.
The anisotropic nature of the optical absorption properties of a-Al203 has been reported by Mitchell et al. [5]. The present paper describes the results of plane polarized light experiments on the 358 nm (3.46 eV) absorption band and its associated mirror image luminescence band at 379 nm (3.27 eV). The theoretical and experimental results are discussed and a defect ceatre based on two adjacent oxygen ion vacancies assigned to the 358 nm absorption band.
2. Experimental details. — 2.1 CRYSTALS.—Single crystals of a-Al203, grown using the Verneuil tech- nique, were obtained from Imperial College, London, and Salford Electrical Instruments Ltd. The optical absorption and luminescence properties of all the as- grown crystals were essentially the same. Analysis showed that only Si (200 ppm) and Ca (16 ppm (Salford) and 52 ppm (Imperial College)) were present at levels exceeding 20 ppm.
Suitably sized samples were diamond cut to within
1.5° of the desired planes after first orienting the crystals using a Laue back reflection technique. The samples were then polished to 1 urn and annealed at 1100 °C overnight in vacuum. A sample of dimensions 3.5 x 3.5 x 5 mm was cut along the (0001) and (lOlO) planes and irradiated with fast neutrons at 60 °C in the Herald reactor at AWRE Aldermaston to a dose of 1018 neutrons c m- 2. Samples cut to dimensions 1.5 x 6 x 7 mm were proton irradiated at 50 °C in the 6 MeV Van de Graaff accelerator at Harwell to doses up to 3 x 1018 protons c m- 2. The proton energy was increased from 1 MeV to 3 MeV in equal dose steps of 0.1 MeV to avoid a build-up of hydrogen in one layer of the material. The 6 x 7 mm face of the proton irradiated samples was parallel to the (1010) plane.
2.2 OPTICAL STUDIES. — For all studies reported here the samples were held at a temperature of 4 K in an Oxford Instruments C F 104 continuous gas flow cryostat. Absorption measurements were made using a Cary 14 spectrophotometer.
To obtain photoluminescence emission spectra an excitation wavelength was selected by a 365 nm Oriel thin film interference filter tilted at an angle of 20°
to the collimated beam of a WOTAN high pressure mercury lamp Type HBO W/2. The emission from the sample was taken off at right angles to the excita- tion direction, and processed by a Hilger Monospek 1000 monochromator in combination with a cooled EMI 9659 QB extended S20 photomultiplier and JOURNAL D E PHYSIQUE Colloque C 6 , supplément au n° 7, Tome 4 1 , Juillet 1980, page C6-533
Résumé. — Cette étude, à 4 K, porte sur le caractère dichroique des bandes d'absorption et de luminescence d'un défaut présent dans l'alumine a - A l203 irradiée aux neutrons rapides ou aux protons. Elle a été centrée sur une bande d'absorption située à 358 nm et sur la bande d'émission qui lui est symétrique à 379 nm. Ces bandes possèdent une structure vibronique résolue avec une raie à zéro phonon à 368,4 nm et sont dues à des transitions dipolaires électriques autorisées. Les expériences en polarisation rectiligne montrent que le dipôle induisant la transition fait un angle de 40° avec l'axe c du cristal ; nous proposons qu'il provient d'un défaut constitué par deux lacunes d'oxygène en position de second voisin et ayant piégé un ou plusieurs électrons.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19806140
C6-534 L. S. WELCH. A. E. HUGHES AND G. P. PELLS
Harwell 2000 series photon counting system. The emission spectrum was displayed on an Oxford Instruments 3000 series chart recorder.
Polarization of the incident and emitted light was achieved by the insertion of Polaroid HNP'B pola- rizers (for incident wavelengths above 280 nm) and Polacoat 105 UV polarizers (for wavelengths below 280 nm) into the appropriate light path.
3. Results. - 3.1 OPTICAL ABSORPTION STUDIES. -
The optical absorption spectra of the neutron irra- diated sample are shown in figure 1. The background
I I I I I I
300 LO0 5 0 0 6 0 0
Wavelength rnml
-
01 ! I I I I
ZOO 3 0 0 LOO 5 0 0 6 0 0 7 0 0
W A V E L E N G T H I n m l
-
Fig. 2. - Absorption spectra at 4 K of an cc-Al,O, crystal proton irradiated to a dose of 3 x 1018 protons cm-'. Curve A is the background absorption of the crystal before irradiation. Curves B (s) and C (a) refer to the same polarizations as in figure 1.
irradiated sample is too broad and weak to be seen in figure 2. We suggest that the broadening has occurred because of internal stresses created in the crystal by the proton irradiated layers.
The vibronic frequency tiw derived from the separation of the zero-phonon line and the phonon-assisted sidebands of the 358 nm band is
260 & 30 cm-l. Using this value the Huang-Rhys
factor S derived from the energy separation Sfio
Fig. I . - Absorption spectra at 4 R of an u-A120, crystal neutron between the band centroid and the zero-phonon line
irradiated to a dose of 1018 neutrons cm-'. Curve A is the back- is found to be 3.7 5 0.6. This value agrees with
ground absorption spectrum of the crystal before irradiation. that derived from the fractional intensity in the
curve B (s)
;
absorption spectrum for JZ//[O001] and 1 [ I O ~ O ] . zero-phonon line, which should be e-'Curve C (a) : absorption spectrum for E 1 [0001] and 1 [1010].
3 .2 PHOTOLUMINESCENCE STUDIES. - Excitation of spectrum of the crystal before irradiation is included.
The light beam was incident normal to the [ I O ~ O ] direction and plane polarized with the E vector either parallel (curve B (71)) or perpendicular (curve C (a)) to the [0001] c-axis of the crystal. There are absorption bands at 358 nm (3.46 eV) and 450 nm (2.75 eV) in spectra C and B and a 302 nm (4.1 eV) band in spec- trum B. Bands at shorter wavelengths are not recorded due to the high optical densities which also prevent observation of the 302 nm band in spectrum C. The 358 nm band has strong dichroic properties and also a sharp, strong zero-phonon line at 368,.4 _f 0.1 nm with a halfwidth of 0.2
+
0.05 nm.Figure 2 shows the polarized absorption spectra of the proton irradiated sample held in the same orien- tation as the neutron irradiated crystal of figure 1.
Again the background spectrum is included. The lower optical densities allow a larger number of bands to be recorded compared with the neutron irradiated sample.
The more prominent absorption bands are the 256 nm (4.8 eV), 302 nm, 332 nm (3.7 eV) and 358 nm bands. The dichroic nature of the optical absorption properties has been reported by Mitchell et al. 151.
Unlike figure 1 the zero-phonon line of the proton
neutron or proton irradiated samples by light in the 358 nm absorption band region results in photo- emission at 379 nm and 468 nm. We shall limit our discu'ssion to the 379 nm emission band.
W o v e l e n g t h I n m I
LOO 3 9 0 3 8 0 3 7 0 LOO 3 9 0 3 8 0 3 7 0
[ '
}
NEUTRON IRRADIATEDFig. 3. - Emission spectra of LX-A~~OB measured at 4 K : a ) neu- tron irradiated crystal, dose 10" n.cm-I, 6) proton irradiated crystal, dose 3 x 1018 protons cm-'. Note reversal of wavelength scale with respect to figures 1 and 2.
POLARIZED LUMINESCENCE OF AN AGGREGATE DEFECT CENTRE IN A1203 C6-535
Figure 3a shows the unpolarized 379 nm band from the neutron irradiated sample. The band possesses strong vibronic structure and is the mir- ror image of the 358
nm
absorption band. Its zero- phonon line peaks at 368.4 f 0.1 nm and has a halfwidth of about 0.15 nm. The vibronic fre- quency is 237+
25 cm-' and the Huang-Rhys factor 3.9 f 0.6. These agree well with values mea- sured from the absorption band(8
3 . l).The unpolarized 379 nm emission band of the proton irradiated sample (Fig. 3b) has a split zero- phonon line due to stresses produced by the proton damaged layers. The vibronic frequency is estimated as 230
k
20 cm-' and the Huang-Rhys factor as 3.9+
0.6. Both values agree well with those deduced from the neutron irradiated sample.Polarized luminescence experiments have been undertaken to understand the nature of the defect responsible for the emission band. The neutron irradiated sample was orie~ted so that the excitation path was parallel to the [1010] direction and the emis- sion path to the [0001] direction (Fig. 4a). In the case of the proton irradiated sample the excitation path is also parallel to the [10i0] direction but the emission path is parallel to the [1210] direction (Fig. 4b). The polarizers
were placed in the excitation and emission paths and could be oriented to produce four configurations for each sample as defined in figures 4a and b. Table I lists the configurations used and the corresponding theoretical and experimental results. There are large differences in the intensity of the 379 nrn emission band as the polarization conditions are changed and a discussion of the results is given in section 4.
4. Discussion.
-
The polarization data are used to deduce the orientation of the transition dipole moment of the defect centre. The anisotropic absorption ratio A represents the ratio p,,,,
: p,,,,
where ,u is theoptical absorption coefficient. From figures 1 and 2 A was measured. For a defect whose transition dipole moment is at an angle 8 to the c-axis, it is easily shown that [9]
The value of 8 is calculated to be 40.2
+
2.00 and39.7 f 2.00 for the neutron and proton irradiated samples respectively.
In order to apply the results from the polarized luminescence experiments it is essential to study the distorted hexagonal lattice of E-AI2O3. Projections along the [1210] and [0001] directions are given in figures 5a and b. Aluminium ions are represented by points and oxygen ions by circles. The line joining two next nearest neighbour oxygen ions from adja-
0 Above plone o l paper Below plone of paper
O r ~ e n t a t ~ o n o f n e u t r o n ~ r r a d ~ a t e d
a - AI2 O3 s a m p l e
O n g n t a t ~ o n of I10101 t y p e p r o t o n
~ r r a d i a t e d a - AI2O3 S a m p l e v r o j e ~ o o n oleng lhe 112101 dlrectlo"
0 Above plone of poper C! Below plane of paper
Fig. 4. - Geometrical arrangement for polarized luminescence la1 I b l
measurements on : a) neutron irradiated a-Al,O,, b) proton Flg. 5. - Projection of the posibon_s of oxygen ions surrounding irradiated a-Al,03. The arrows and symbols refer to the direction an A13+ ion in u-A120, on : a) the (1210) plane, b) the (0001) plane.
of the electricvectors of the exciting and emitted light. : A13+ ion, 0 : 0'-ion above plane, ::> : 0'- ion below plane.
Table I . - Polarization data for the 379 nm emission band.
dose 1018 neutrons cm-2 [ I O ~ O ] type . dose 3 x 10" protons cm-2
379 nm emission band 379 nm emission band
Polarization t) = 40.20 Polarizat~on 0 = 39.70
configuration Theoretical
+
2.0° Experimental configuration Theoretical _+ 2.0° Experimental- - - - - - - -
ni ni 0.75 sin4 0 3 2 9
+
0.3 HL n k 2 cos4 t) 17 5 11+
1nt uj 0.25 sin4 0 1 1.0
+
0.3 Tk uj sin2 0 cos2 0 5.8,
0.7 3.3+
0.5uk ni cos2 0 sin2 0 5.6
+
0.7 5.3+
0.3 ui nli sin2 0 cos2 0 5.8+
0.7 5.4 0.7OR uj cos2 0 sin2 €J 5.6 f 0.7 5.0
+
0.3 ui uj 0.25 sin4 0 1 .O 1.0 & 0.2C6-536 L. S. WELCH, A. E. HUGHES A N D G. P. PELLS
cent (0001) planes makes an angle of 3 3 O with thee-axis (see Fig. 5a). This is fairly close to the experimental value of IV 40° obtained from the absorption data and leads us to suggest that the origin of the 358 nm absorption band is a defect centre of two such next nearest neighbour oxygen vacancies which have trapped one or more electrons. The lines joining the oxygen ions from adjacent (0001) planes which are drawn in figures 5a and b represent all orientations for such a defect, the angle r j being defined as in figure 5b. The data obtained from the polarized luminescence measurements are summarized in table I alongside the theoretical predictions for transition dipoles at an angle 19 to the c-axis in the orientations
given in figure 5b.
To calculate the theoretical luminescence intensi- ties the six dipole vectors d, (where n = 1-6) were resolved into their component vectors in the [1Zl0] (i), [loio] (j) and [0001] (k) directions. The theoretical values in table I are calculated from the expression for polarized luminescence intensity
where
e, : unit polarization vector of exciting light, e, : unit polarization vector of emitted light.
The unit vectors e, and ee are put equal to i, j or k according to the appropriate polarization combina- tion depicted in figure 4.
Note that the angle
4
cancels out from the theore- tical predictions in table I. The good agreement between the theoretical and experimental data in table I shows that our model is consistent with both the absorption and luminescence results. The agree- ment is better for the neutron irradiated sample.Discrepancies in the proton irradiated data are pro- bably due to the large stresses introduced into the crystal by the stepped irradiation technique which results in platelets of damaged a-A1203 separated by relatively undamaged regions.
5. Conclusions. - We have adopted the simplest model which allows good agreement between the theoretical and experimental results. This model is a defect which has a transition dipole moment at about 400 to the c-axis of the crystal and consists of a pair of next nearest neighbour oxygen vacancies with one or more trapped electrons.
References
LEE, K. H. and CRAWFORD, J. N. Jr., Phys Rev. B 15 (1977) 4065.
LEE, K. H., HOLMBERG, G. E. and CRAWFORD, J. H. Jr., Phys.
Status Solidi (a) 39 (1977) 669.
- - TURNER, T. J. and CRAWFORD, J. H. Jr., Phys. Rev. B 13 (1976) 1735.
[4] EVANS, B. D., HENDRICKS, H. D., BAZZARRE, F. D. and BUNCH, J. M., in Ion Implantation in Semiconductors edited by Chernow F., Borders J. A. and Brice D. K.
(Plenum, New York) 1977, 265.
[5] MITCHELL, E. W. J., RIGDEN, J. D. and TOWNSEND, P. W., Phil. Mag. 5 (1960) 1013.
[6] ARNOLD, G. W. in Proceedings of the First Topical Meeting on the Technology of Controlled Nuclear Fusion, San Diego, Calzx (Nat. Techn. Info. Service, Washington, D.C., 1974) Vo!. I1 590.
[7] ARNOLD, G. W., KREFFT, G. B. and NORRIS, C. B., Appl. Phys.
Lett. 25 (1974) 540.
[8] EVANS, B. D. and STAPELBROEK, M., Phys. Rev. B 18 (1978) 7089.
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