Evolution of Defective State of Aluminum Oxide Irradiated with Chromium Ions after Annealing in Oxygen Environment

The characteristics of interband and exponential optical absorption of leucosapphire and polycrystalline corundum (polycor) after irradiation with chromium ions and subsequent annealing in vacuum at 300–1800 K and in air at 300–800 K are studied. Contributions of defects with different thermal and chemical stability into optical parameters were established. The effect of intrinsic radiation defects, of substitutional defects and of complexes on base of oxygen and defects on formation the focal point in absorption spectra owing to fulfilment of the Urbach rule was determined. Heating in air of strongly defective material synthesized in surface layers of alumina by the ion–heat modification influences the characteristics of defects and the electronic structure of band gap negligibly.


Introduction
The application of irradiation with ions for the modification of properties of dielectric materials stimulates studying the radiation damage processes and the changes in their electronic structure [1][2][3][4][5][6]. Postimplantation thermal annealing of materials is a necessary stage of modification of their properties [2,4,5]. Thermal annealing in vacuum and in reducing atmosphere stabilizes the properties of material owing to coagulation of implanted elements into clusters and formation of new phases, having characteristics different from those for pristine matrix [2,4]. Thermal annealing in oxygencontaining environment influences the electronic structure of material owing to realization oxidative and restoration reactions with participation of defects and oxygen and formation of oxygen-containing complexes (OCCs) [3,4]. Localized states (LSs) induced by radiation defects (RDs) well manifest in leucosapphire and polycrystalline corundum (polycor), which contain a low concentration of impurities [1][2][3]5]. The implantation of ions with different ability to substitute for lattice atoms of Al 2 O 3 and to form the solid solutions allows evaluating of their role in changing of properties [1][2][3][4][5][6].
RDs and complexes on their base induce in the band gap (BG) of Al 2 O 3 the complicated spectrum of LSs [1][2][3]5]. The study of the produced nonequilibrium state in dielectrics requires approaches using generalized microscopic parameters characterizing structural disordering of the system of a whole [7,8]. In the analysis of radiation-induced disorder is taken into account the principle of equivalence of static and dynamic components of its [7,8]. That allows determining quantitatively the contribution of the atomic disordering into properties change. The parameters of induced disordering, calculated from absorption spectra with using of Urbach rule, depend on concentration of RDs [5][6][7] and stoichiometric and phase-composition of compounds [8].
The aim of this paper is to study the characteristics of the optical absorption of leucosapphire and polycor after irradiation with chromium ions Cr n+ and subsequent annealing in vacuum and in air, to determine the thermal and chemical stability of the defects of different nature and to determine the degree of their influence on the optical properties.

Experimental
Leucosapphire and polycor plates were irradiated with Cr n+ ions in the frequency pulse mode (the ion energy was 50-150 keV, the fluence was 10 16 -210 17 cm -2 , and the current density in the pulse was 10 -2 -10 -3 A/cm 2 ). Annealing after irradiation was performed in the temperature range T ann1 =300-1800 K at the residual pressure P≤10 Pa and in air in the temperature range T ann2 =300-800 K at atmospheric pressure P=10 5 Pa. Choice of the Cr n+ ions for irradiation was caused by high probability for synthesis of hard solutions on base of these ions in the surface layers of alumina [2,[4][5][6]. The high concentration of RDs allows considering this solution as disordered.
The spectral dependence of absorption coefficient (h) calculated from the diffusion reflection spectra according to [2,5,6] was approximated by the Urbach rule of general form (1) where E U is the Urbach energy which depend on the total dynamic and static disorder in the lattice [7,8]. The pre-exponential factor  00 and the characteristic energy h 0 are the parameters of the Urbach focus [7,8]. For the energy ranges (h) in which the Urbach rule is satisfied we calculated E U and the pre-exponential factor  0 from the simplified equation (2) To evaluate the applicability of the concept of Urbach focus we calculated the dependence  0 (E U ) taking into account (1) and (2) according to equation where parameters  00 and h 0 were determined from the spectra (h, , T ann1, 2 ). Isoabsorption optical gap E g' was calculated from spectra for a number fixed values of '. Value E g corresponds to energy of transitions h for execution of equality (h)=', 7, 8. Values E g and E U were compared with dependence E g (E U ), calculated with using of parameters h 0 and  00 according to equation [7,8] The interrelation between the absorption by the LSs produced by RDs and the interband absorption was verified using the approximation of the (h) in the ranges (h) by the power-like law where m=1/2 and 2 correspond to the direct and indirect interband allowed transitions across the optical gaps E' g and E'' g [5]. We determined the contribution of local levels of RDs into the absorption by using the expansion of the (h) in terms of elementary components in the Gaussian form.

Result and discussion
The charge state of the substitutional defects Cr Al n+ (n=2-4) and intrinsic RDs (anion and cation vacancies V O and V Al , as well as interstitial defect Al i ) is determined by  of ions and vary after annealing in vacuum, that reflects on the absorption spectra (figures 1 and 2). The interband absorption in the ranges (h)=2.4-4.2 and 3.4-4.8 eV is caused by electronic transitions in clusters Cr Al n+ …Cr Al n+ which were formed after annealing at T ann1 =300-1300 K. The complexes on the base Cr Al n+ and intrinsic RDs (T ann1 =1300-1600 K) determine the exponential absorption with participation of their LSs in ranges (h)=1.5-3.4 eV and 3.1-3.9 eV and the interband absorption in ranges  (1) and after irradiation with Cr n+ ions and after annealing in vacuum at T ann1 =1800 K (2) and in air at T ann2 =500 K (3) and 800 K (4). The fluence is =10 17 cm -2 .
The change of absorption parameters in range h=1.5-4.0 eV after annealing at T ann2 =500-800 K is caused by exchanging by electrons between the energetic levels of substitutional defects, the levels of divacancies F 2 0…n+ (their LSs were identified in [1,2,9]) and acceptor levels created by OCCs (≥4.0 eV). The recharge of divacancies F 2 n+ →F 0 →F + , which stimulated by the sequential acts  The change in the structure of LSs causes the intersection of the linear sections of the spectra (h), where relation (2) is valid in the high energy range. The convergence region (is denoted by a rectangle in figures 1, 2) is located at h  .8±0.08 eV (  800±90 cm -1 ) in leucosapphire (figure 1) and at h  3.95±0.15 eV(  8500±500 cm -1 ) in polycor (figure 2) and can be interpreted as a focal point of generalized Urbach rule, just as in [6][7][8]. Annealing in air supplements the criteria of Urbach rule (1) for irradiated Al 2 O 3 (similar points were fixed at h  .4 eV for Cr n+ and Ti n+ ions and at .2 eV for Si n+ ions) [5,6]. The similarity of focal points for ions with different ability to substitution of cation of lattice is caused by existence of interrelation between the kinetic of RDs accumulation and parameters of the exponential absorption [1][2][3][4][5]6]. The fan-like character of curves (h) persists after adsorption of oxygen in the surface layers of alumina (figures 1 and 2). The separation of focal points for single and polycrystals occurs (figures 1 and 2). At that, the low-energy shift of focal point on ~ 0.3 eV was fixed in polycor. The focal point in polycor at h  3.95 eV is situated between the levels induced by the interstitials defect Al i 0(+) (=4.1 eV [2,9]) and divacancy F 2 0 (=3.6 eV [2,5,9]). This point is connected with boundary of deep LSs in BG of irradiated alumina [2,8]. Focal point in leucosapphire at h  4.8 eV is neared to energies of F + -center [9,10] and of defect Cr Al 3+ (=4.7-4.8 eV [5,6,10]). Besides, value h  3.95 eV is neared to value E g =4.3 eV in films Al 2 O 3 containing the amorphous and ordered phases [11][12][13]. Biographical defects have a stabilized effect on parameters of RDs (figure 2). Contribution of composition disorder into total disorder of alumina is 15-20 % according [6]. Influence of distortion of stoichiometry is negligible. The basic component of disorder in alumina lattice is the RDs formation, as show the optical parameters (figures 1-4). Other criteria of this approach is accordance of dependence between parameters  0 and E U , which calculated from the spectra, with dependence  0 =f(E U ), which calculated according equation (3) with using of focal point coordinates h 0 =3.95 eV,  00 =8500 cm -1 ( figure 5). The values E U and  0 until and after impact of oxygen are united into common array for leucosapphire and polycor ( figure 5). The values  0 situated lower the curve 2 correspond to spectra of alumina after ≤5·10 16 cm -2 , and values  0 situated higher the curve 2 -to spectra of polycor after ≥10 17 cm -2 . The basic contribution to this dependency give the clusters Cr Al n+ …Cr Al n+ and complexes on the base of RDs. Isoabsorption optical gap E g and optical gaps E' , '' g for interband direct (ranges '(h)=3.3-4.0 and 4.0-5.3 eV) and indirect transitions ('(h)=1.5-4.0 and 3.0-5.1 eV) are changed in correlation with E U changing ( figure 4). This relationship has a general character for materials containing the high concentration of defects [6][7][8]. The straight lines 4-7 in figure 4 are calculated according to equation (4) (at value  00 =8500 cm -1 and value pairs h 0 =3.40 eV, '=500 cm -1 (4); 4.1 eV, 2300 cm -1 (5); 4.8 eV, 3200 cm -1 (6); 5.3 eV, 3500 cm -1 (7)) limit the arrays 1-3 of the experimental values E' , '' g and E U . At that, values E g and E U obtained before and after impact of the oxygen on alumina surface are united in the common array ( figure 4). Values of gap for direct transitions which were obtained after T ann2 =500-800 K are excluded from this picture ( figure 4, array 3). This caused by influence of the OCCs on narrowing of the BG of alumina. At that, value h 0 =3.4 eV is neared to energies of F 2 0 (=3.6 eV [2,9]), and h 0 =4.1 eV is neared to focal points 3.95 eV in spectra of polycor (figures 2 and 4). Values h 0 =4.8 and 5.3 eV coincide with energy of bands of F + -center (=4.8 and 5.4 eV [9,10]) and of defects Cr Al 3+ (=4.7-4.8 eV [5,6,9,10]). Relations between the parameters of the exponential and interband absorption allowed us to conclude, that accumulation of OCCs has a weak effect on the electronic structure of new strongly defective material which was formed by irradiation with ions and by subsequent annealing.
Analysis of spectra (h) by expansion on the elementary components of Gaussian form with the centers at energies  i allowed us to estimate the influence of interaction between the oxygen and RDs, which have local energetic levels, on the total disorder induced by ions in the lattice of alumina ( figure  6). The intensity of bands with centers at  i =1.7 eV in leucosapphire and at 1.5-1.6 eV in polycor caused by the substitutional defects Cr Al 3+ is decreased in 1.5-4 times owing to adsorption of oxygen.  . Absorption spectrum (h) of leucosapphire after irradiation with Cr n+ ions at fluence =10 17 cm -2 and subsequent annealings in vacuum at T ann1 =1800 K and in air at T ann2 =800 K and its decomposition. Stability of centers to oxidation is increased as growth of ions fluence and temperature of vacuum annealing from T ann1 =1470 to 1800 K (figure 2). The weakening of bands induced by electronic transitions in the defects Cr Al 3+ was caused by recharging Cr Al 3+ →Cr Al 2+, 4+ , as shows the relations between concentration N i of defects Cr Al n+ with different n. Divacancies F 2 0 and clusters of the intrinsic defects Al i (0,+) …F 2,3 n+ (n=2-4) (were identified in [2,5,9]) have influence on the recharging of defects also. The processes of recharging of defects are predominate according to scheme F + →F 0 , Cr Al 2,4+ →Cr Al 3+ and F 2 + →F 2 0 , as show the absorption spectra. The stability of the bands induced by RDs to oxidation is higher in polycor in compare with leucosapphire owing to influence on optical absorption of boundaries between the structural fragments in the polycrystals (figures 1, 2 and 6). The band at  i =2.15 eV induced by substitutional defects Cr Al 2+…4+ and band at  i =3.0 eV connected with the cation vacancy V Al (0,-) in leucosapphire disappear after annealing in air. Annealing at T ann2 =500-800 K promotes the redistribution of absorption in polycor in the bands of F + -centers from the band with center at  i =4.8 eV to  i =5.3 eV (figure 2). The new band with  i =4.4 eV appears in leucosapphire and this band is caused by forming of OCCs on the base of V O , Al i and O 2 (figure 6). At that, bands induced by F + -and Al i -centers disappear. We cannot exclude the influence of the electronic transitions (at =3.7-3.9 eV) between the LSs of compounds (Cr 2 O 3 ) x (Al 2 O 3 ) y [5] on the absorption processes in substitutional defects and divacancies.

Conclusion
Annealing in air of alumina after irradiation with chromium ions and subsequent annealing in vacuum changes characteristics of the energetic levels induced by the RDs and complexes on their base. This is reflected in parameters of the exponential and interband absorption and in their interconnection. The charge state of substitutional defects and intrinsic RDs and degree of their influence on properties are determined by fluence of ions and by annealing temperature in vacuum and in air. Annealing in air of irradiated alumina stabilizes the structure of defects and their energetic characteristics. Fulfillment of criteria of generalized Urbach rule is supplemented owing to realization of oxidation and reduction reactions between the defects and oxygen molecules. Accumulation of OCCs with participation of RDs changes negligibly the electronic structure of irradiated alumina with small decreasing of maximal optical gap from 4.5 to 4.0 eV.