MEDIOS DE PROMOCIÓN

Figure 10 illustrates the wide variety of defects introduced in a crystalline target during ion irradiation. Whereas, many of the defects are eliminated by dynamic annealing process during bombardment, many are trapped to leave the sample in a thermodynamic metastable state. Upon annealing, thermal activated processes (e.g. diffusion and phase changes) drive the system toward the equilibrium state. Regions rendered amorphous by irradiation, recrystallize at elevated temperatures.

Bull and Page suggested a quantitative model that indicates the activation energy for annealing of the amorphous material during irradiation (dynamic annealing) is very much lower than might be expected for post-implantation thermal annealing of the same material [193]. There have been many studies on post-implantation annealing recovery of disorder, implanted species distribution, and phase formation. Two cases pertain to this proposed study:

1) Annealing of damaged but crystalline structures

2) Recrystallization and epitaxial regrowth of implanted amorphous Al2O3

Figure 10 Schematic illustration of the evolution during ion irradiation of a solid target processes. The grey shaded processes correspond to long term effects. After Zhang and

Whitlow [194]

2.4.1 Annealing of α-Al2O3 irradiated with iron followed by oxygen

McHargue et al. irradiated c-cut sapphire single crystals with 160 keV Fe+ ions to a fluence of 4×1016 Fe/cm2, followed by 54 keV O+ ions to a fluence of 6×1016 O/cm2.

The samples were annealed for one hour at 500, 800 and 1200 ◦C in an oxidizing or a reducing (Ar-4% H2) atmosphere. The residual damage (disorder) was determined by RBC-C spectroscopy. All samples retained about 90% of the disorder after 1 hour at 500

◦

C in both annealing atmospheres. The retained disorder (20-25%) was similar for the conditions annealed in the oxidizing atmosphere at 800 ◦C but significantly more (~53%) for the sample irradiated with iron only. The sample irradiated only with iron also retained more disorder after annealing at 1200 ◦C in both atmospheres (Figure 11). Hence, the irradiation of iron plus oxygen accelerated the recovery process(s) at temperatures above 500 ᵒC in both annealing atmospheres [195]. Implanted specious may migrate toward the surface during recrystallization but under certain conditions, the implant can be trapped at defects or create a second phase. Mouritz et al studied annealing of low melting point metals implanted into sapphire in an argon atmosphere. They found the retained concentration of these metals such as Zn, In, and Pb is independent of both implanted fluence and phase diagram consideration [174].

2.4.2 Annealing of pre-amorphized Al2O3 produced by ion-beam irradiation White et al summarized the studies on annealing samples in an inert (Ar) atmosphere that have an amorphous surface layer produced by irradiation with aluminum ions followed by oxygen ions at 77 K [129]. The amorphous layer first transforms to γ-Al2O3 containing subgrains. Further annealing grows the subgrains of γ-Al2O3 and the

α-Al2O3 sub-region epitaxially grows into the γ-phase and eventually converts the entire

same sequence during annealing samples irradiated with iron at 77 K. They followed the redistribution and precipitation of the implantation in both reducing and oxidizing atmospheres [196]. White et al reported that the presence of Fe in the amorphous region increases the kinetics of crystallization significantly but does not change the nature of the crystallization behavior [129].

Figure 11 Annealing effect in Fe-implanted sapphire a) annealing in air b) annealing in Ar-4% H2 atmosphere [195]

Romana et al., produced an amorphous surface layer on c-axis α-Al2O3 irradiated

with 180 keV Sn+ ions to a fluence of 4×1014 Sn/cm2 at room temperature. During annealing in a reducing atmosphere, the recrystallization occurred in the same manner (amorphous→γ-Al2O3→α-Al2O3) as described above at temperature above 1233 K. In

this case, precipitates of SnAl2O3 formed at 1233 K. Thermal annealing at 1373 K

resulted in the loss of 97% of the implanted Sn. The start of recrystallization of the amorphous region was delayed to 1373 K in the oxidizing atmosphere. In this case, the surface layer consisted of SnO2 coherent with γ-Al2O3. The SnO2 precipitates appear to

stabilize the γ structure [94].

Annealing of a Sn implanted sample into sapphire has been also reported by Xiang et al. Tetragonal Sn nanoparticles of ∼15 nm diameter in Al2O3 produced by direct

Sn irradiation at room temperature oxidized after thermal annealing at 1000◦C in oxygen. The irradiation-induced amorphous region, recrystallized and the Sn nanoparticles turned into SnO2 nanoparticles with an average diameter of ∼30 nm [197].

Marques et al studied amorphization of a c-cut Al2O3 sample by irradiation with

150 keV Ti+ ions to a fluence of 5×1016 Ti/cm2. Annealing in oxidizing environment led to complete recovery of the amorphous region and segregation of Ti (in form of Ti3O5) to

the surface. In the reducing atmosphere the competition between Al and Ti favors the formation of titanium oxide to lower the energy. In the amorphous region, a Ti rich layer and a depletion of Al were detected [172].

Although the disorder as determined from RBS-C measurements reaches the random value and the TEM examinations detect no coherently scattering regions in the amorphous layer, the extended loss fine structure (EXELFS) indicates short-range order (SRO) to be present [159]. This SRO is similar to crystalline α-Al2O3 in Fe-implanted

amorphous layers but similar to crystalline γ-Al2O3 in stoichiometrically (Al+O)

irradiated samples. This SRO appears to influence the kinetics of the recrystallization but not the path.

2.4.3 Annealing of zirconium implanted sapphire

There are only a few studies about the effect of annealing in zirconium implanted in sapphire. The thermal annealing behavior of irradiation with 150 keV with Zr+ ions to a fluence of 2×1016 Zr/cm2 in α-Al2O3 has been investigated by Naramoto et al.

Annealing in an oxidizing atmosphere to temperatures as high as 1600 ᵒC showed that there is little if any redistribution in depth of Zr [168].

2.5 Doping effects on the kinetics of solid-phase epitaxial growth of amorphous →γ