Nuclear Instruments and Methods in Physics Research Bl (1984) 621-627 North-Holland, Amsterdam

621

Section XI, Radiation chemistry

ION IMPLANTATION EFFECTS IN CRYSTALLINE INORGANIC INSULATORS

A. PEREZ

Dkpartemeni de Physique des Matkiaux, Universiie Claude Bernard Lyon I, 69622 Villeurbanne, France

Heavy ion bombardment of inorganic insulators causes complex structural changes and chemical effects, the knowledge of which has been limited until now. The solid phase which is created can incorporate implanted impurities in various charge states. These can

be associated with lattice defects, thus creating a multitude of metastable impurity-defect structures. Thermal annealing usually removes a great deal of damage and leads to the growth of various equilibrium systems via nucleation and diffusion, which can be

predicted on the basis of classical phase diagrams. Using some characteristic examples of implantation in pure ionic materials (LiF and MgO) and ionocovalent materials (TiOs and SiO,), the key-parameters which govern the chemical implantation effects are

discussed: nature of the implanted impurity, local concentration, temperature, nature of target (structure, bonding)) and role of defects (point defects and extended defects). A statistical model for determining the immediate surroundings of implanted ions as a

function of dose, complemented with some considerations of the stability of various impurity-defect structures, is introduced to interpret the experimental results in as-implanted crystals. The annealing behaviours of the implanted materials are presented and compared with those observed in conventionally doped crystals.

1. Introduction

Ion implantation is one of the most promising meth-

ods for modifying the properties of materials. The new

applications in this field cover a wide range of solid

state physics, however in insulating materials we must remark that most of these applications need high dose implantations (2 1016 ions/cm*). This is in contrast to semiconducting materials for which low and medium dose range have been primarily investigated. For exam- ple, it is well known that the presence of iron consider- ably influences the hardness as well as electrical and thermodynamical properties of refractory oxides and ceramics [1,2]. In materials such as lithium niobate (LiNbO,), the significant change of the refractive index after high dose implantation is an attractive method by which to produce waveguide and modulators for use in integrated optics [3]. Another characteristic example is the modification of the magnetic properties of bubble garnets for dense memory device production [4,5]. The main characteristic of a high dose implanted system is the superposition, in the implanted layer, of great con- centrations of defects and implanted impurities. This may cause, in inorganic insulators, some complex struct- ural changes and chemical effects. The solid phase which is created can incorporate implanted impurities in vari- ous charge states, which can be associated with lattice defects thus creating a multitude of metastable impur- ity-Defect structures. Thermal annealing usually re- moves a great deal of damage and leads via nucleation and diffusion to the growth of different precipitates of various equilibrium phases. Therefore, the study of such

0168-583X/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

highly implanted materials needs to reveal defects (point defects, aggregates, extended defects) and implanted atoms (site location, charge states, precipitated phases).

With regard to the defect creation by ion bombard- ment, inorganic insulators can be classified in two kinds: (i) the materials in which defects can be created by electronic processes and (ii) materials which are sensi- tive to the energy deposited in nuclear collisions. The alkali halides are typical of the former group in which electronic excitations in the anionic sublattice lead to very efficient colour centre production [6]. More re- cently a radiolysis mechanism has also been proposed for the defect production in a-quartz [7]. In the second case we can mention the ionic oxides such as MgO [8] and some ionocovalent materials such as TiO, [9], LiNbO, [3], garnets [lo]. . . . Another remark on the defect production by heavy ion bombardment is the possibility that the lattice structure can be destroyed completely and an amorphous material formed. This is observed with ionocovalent materials such as SiO, [7], LiNbO, [3], garnets [4,5] or purely covalent ones such as diamond [ll]. This behavior is in contrast to ionic crystals which are not amorphized even for high dose implantations (2 10” ions/cm*).

Concerning the implanted atoms, it is interesting to characterize their state directly after implantation and after subsequent annealings. In the first case we observe a non equilibrium system in which metastable impur- ity-defect structure or metastable phases can exist. In the second case some internal processes (annealing of defects, diffusion) combined with external ones (role of the annealing procedure and atmosphere) determine the

XI. RADIATION CHEMISTRY

622

Table 1

A. Perez / Ion implanlation effects in crystalline inorganic insulators

High dose ( z 1015 ions/cm*) implantation effects in some crystalline inorganic insulators. OA: optical absorption, EC: electrical conductivity, TDPAC: time differential perturbed angular correlation, RBS: Rutherford backseat&ring, TEM: tran~ssion electron microscopy, CEMS: conversion electron MWbauer spectroscopy, IRS: infrared spectroscopy

Materials Implanted ions

States observed in the as-implanted crystals

Precipitated phases observed after annealings

Applied techniques

Remarks Ref.

12,37

14

29

13

16

13,17

9

1523

LiF Na+, Kf solid solution at 77 K

alkali metal precipitates

OA, EC Insulator-Metal transition in as- implanted crystals at 77 K for a lo- cal cont. of 5% at. In tetragonal with the c-axis II [llo],iF

Indium metal precipitates

annealed in vacuum: iron metal precipitates, annealed in air: ferric compound

alkali metal precipitates

OA, TDPAC In*

Fe“ Fe’+, Fez’(FeF2) and F”

OA, RBS, CEMS

Li; cfc + bee at 800°C Na and K: bee up to 1ooO”c MgFe,O, in epitaxy with MgO

MgO Li *, Na+, K+, Rb+

OA, RBS, TEM

Fe3+, Fe2+ (magnesio-wtistite) and Fe0

annealed in vacuum: iron me- tal precipitates + substitutional Fe*+ annealed in air:

MgFe20, precipitates binary alloys MgsIn or AusMg precipitates

OA, RBS, TEM. CEMS

Fe’

OA, RBS TEM

MgsIn and AusMg hexagonal with the c-axis II [ill],,,

OA, RBS, Exodiffusion of H TEM, IRS and D at 400°C

OA, RBS, TEM

Precipitates in the form of platelets II

t0 [ioilTiO,. Exodiffusion of Ag and Au at 700°C Precipitates in the form of platelets II

to [iO117i0, Pseudo brookite (FeTi,O,) in the form of platelets II t0 p0i]Tj02. Exodiffusion of Fe3+ at 500°C

CO2 dominant for low doses and CO for high doses. Mea- surements in grains and amorphous films

In+, Au+

O-H, O-D bonds correlated with T$ -+ Ti3+ gold or silver metal precipitates

Ti02 H+, D+

Au’, Ag+ Au and Ag metal up to 700°C

Li+, Na+ K+, Rb+

Fe+

Compound precipitates K2Ti03, Rb,TiOs (M,Ti,O,) unidenti- precipitates

fied Fe3+, Fe*+(FeTi,O,), Fe3+ Fe0

OA, RBS, TEM

RBS, TEM; CEMS

15

18

IRS 21,22 SiO, H+, C+ O-H, CO, CO2

A. Perez / Ion implantation effecis in ctystalline inorganic insulators 623

evolution of the system towards the final equilibrium state. However, we must point out that even in a high dose implanted system, in which the local concentration in the implanted layer can be very large, the total quantity of implanted matter remains very low (< 1 pg). Thus for as complete a characterization as possible of the implanted materials, it is generally necessary to associate several well adapted and complementary tech- niques. Unfortunately this has been possible in only a few cases of the various crystalline inorganic insulators which have been implanted. In this context, special attention must be given to ionic crystals because in such materials most of the defects in the anionic and cationic sublattices are well known. Consequently, in this paper, we present only some characteristic examples which allow an introduction of discussions of the key-parame- ters and mechanisms which govern the implantation effects in inorganic insulators: nature of the implanted element with respect to the matrix elements, local con- centration in the implanted zone, temperature and role of defects.

2. Extrinsic phase precipitation phenomena

Various insulating materials have been implanted with protons, rare gas ions and metallic ions in large doses and energy ranges. Some of these results, where the precipitated phases have been identified, are sum- marized in table 1 for three kinds of materials: an alkali halide (LiF) an ionic oxide (MgO) and two different ionocovalent oxides (TiO, and SiO,). Depending on the nature of the implanted impurities with respect to the target elements, two precipitation processes can be ob- served: (i) the implanted impurities precipitate indepen- dently of the matrix elements. This is typically the case for alkali ions implanted in LiF [12] or MgO [13] which form metallic particles. Also indium in LiF [14] or silver and gold in TiO, [15] precipitate in this metallic form. (ii) The implanted impurities precipitate with the matrix elements to form compounds. In MgO this is observed with iron which forms an oxide and a spine1 ferrite (magnesioferrite MgFe,O,) at higher temperature [16]. In the case of indium, a binary alloy with the cations of the matrix is observed (Mg,In) [17]. Compound forma- tion also occurs in TiO, implanted with alkali ions or iron. However we must remark in the particular case of TiO,, that all the precipitates are observed in the form of platelets in the (iO1) planes of the structure [15]. This is a specific effect of ion implantation in TiO, which can explain the characteristic annealing be- haviour observed for various implanted elements in TiO,: an exodiffusion of the implanted impurities which takes place generally in a low temperature range (400 to 6OO’C) due to their presence in extended planar defects [18]. Another interesting phenomenon observed in TiO,

implanted with protons and deuterons is the formation of O-H or O-D bonds [9,19]. This effect is accompa- nied by a reduction of Ti4+ into Ti3+. However the defect structure observed in this case is formed by microtwins [20] which are different from the shear struc- tures observed to accommodate the non-stoichiometry in reduced TiO, crystals. Molecule synthesis has also

been observed in SiO, in which OH, CO and CO, species have been detected after implantations with protons or carbon ions [21,22].

The results summarized in table 1 and commented on above show clearly the role of the nature of the implanted species in determining the extrinsic phase precipitation. In order to interpret these data, Thevenard et al. (231 have suggested that the electronegativities of the implanted elements and the matrix elements be compared. This comparison can explain the formation of some compounds an metallic precipitates in TiO, (K ,TiO, , RbzTiO, , FeTi z0, , Au metal), however several cases remain uninterpreted (alkali metal precipitates in MgO, O-H bond in TiO,, O-H or O-C bonds in SiO,). Another parameter such as the strength of the chemical bond (often known as the bond dissociation energy) for molecules formed by the implanted element and the matrix elements have been considered by Treil- leux [24]. In this case, also, only some phase formations can be explained (alkali metal precipitates in MgO) and it is impossible to obtain a homogeneous description of the observed phenomena in the various matrices under consideration. Thus it is clear that the parameters men- tioned above are not the only ones which govern the chemical effects occurring in the implanted layer and more complex phenomena must be considered in this case, especially the defects (point defects and extended defects), created in great concentration during the im- plantation process, and which can interact with the implanted impurities to form metastable structures. These structures are characteristic of the matrix as shown in TiO, where the precipitation process takes place in extended planar defects [15]. Another example is the compensation of the different charge states of the im- planted ions by point defects in ionic crystals [25-271. This points out the important role of ionic crystals in the fundamental studies of implantation effects com- pared with other materials in which the defect structures are not completely known (TiO,, SiO,. . .).

In addition to the defect structure, the crystalline structure of the matrix plays an important role in the determination of the precipitated phases. This is well illustrated by the formation of alkali metal precipitates in MgO [28]. As mentioned in table 1, the lithium precipitates in MgO exhibit a phase transformation from the fee to the bee structure after annealing around 8OO”C, whereas the sodium and potassium precipitates are observed directly in their normal bee structure. This effect is related to the growth of the precipitates during

XI. RADIATION CHEMISTRY

624 A. Perez / Ion implantation ef/pcts in cry~taU&e inorganic insulators

the annealing procedure. The transformation of a coher- ent phase to an incoherent phase occurs at a critical radius, when the strain energy is greater than the in- terfacial ener8y, in order to minimize the total energy of the precipitate. Applying this concept, a simplified calculation of the critical radius of lithium particles in MgO [28] gives the value of 120 A which is in a good agreement with the measured value (100 to 150 A) when the phase tr~sfo~ation occurs. At this stage, the fee structure imposed by the matrix becomes unstable and the precipitates lose their coherency to lower the strain energy. For sodium and potassium prec$itates the calculated critical radii are very small (5 A and 3 A respectively) [28] which means that these precipitates can never be perfectly coherent in the MgO matrix.

3. Statistical model for the description of the as-implanted system

The results presented and discussed in the previous section show the complexity of a high dose implanted system and consequently the difficulty of applying classical thermodynamic rules to interpret the implanta- tion effects observed. Another possibility consists of considering the implanted system not as a whole but rather at the microscopic scale. Such an idea was sug- gested by Sawicki et al. [X,29] from the results obtained with iron ion implantations in various crystalline in- organic insulators. In this case the conversion electron Mossbauer spectroscopy technique (CEMS) which is a sensitive and powerful method by which to characterize the different states of the implanted ions (charge states, site location and precipitated phases) has been applied over a large dose range. In all the materials investigated (LiF [29], MgO [16], TiO, (IS], Mg,SiO, and garnets (5]), it was found that implantation introduces iron in three charge states: Fe3+, Fe*” and metallic precipitates (Fe’) with the dominant role of Fe3+ at low doses (c lOI ions/err?), Fe2+ at medium doses and metallic iron clusters at the highest doses (= lOi ions/cm2). Such a general behaviour in the different materials mentioned above seems to indicate that the valence state of the implanted ions is only determined by its local environment. At this level we can assume that this environment is a statistical function of the local con- centrations of implanted impurities and defects which are both dependent on the implantation energy and dose. The matrix itself will interfere with its crystalline structure which determines the different site symmetries possible and its defect structure which can interact with the implanted species.

As an example, fig. 1 presents the fractional contri- bution of various iron states measured by CEMS in MgO crystals implanted at room temperature (RT) with 100 keV and 70 keV 57Fe+ ions over a dose range from

Dose [XV ions.cm21 5 a

I 5 t

60- .

Fig. 1. Fractional contribution (R) of various iron states pre- sent in as-implanted Fe-MgO samples (Fe3+, Fe:+, Fe;,+ and metallic iron FeO), estimated from CEMS measurements, and presented versus the concentration of the implanted iron ( X ), a, room temperature data; A, liquid nitrogen temperature data. Curves represent the probability functions calculated from the binomial dist~bution formulas for particular aKangements of iron atoms in an MgO lattice [see $3, eq. (l)].

1015 to 1017 ions/cm* (average local concentration from 0.3 up to 16% of Fe per Mg ) [I@ This figure shows a remarkable concentration dependence of the Fe3’ com- ponent which is significant only for low dose implanted samples and decreases very quickly with the increasing dose. Also a high fraction of Fe’+ ions is a characteris- tic feature of the as-implanted MgO-Fe system, in the range of iron concentration investigated. The total Fe*+ fraction is distributed over two different sites, Fe:+ and Fe::, deduced from the MWbauer spectra 1161. Both Fe*+ fractions increase very sharply in a low dose region (< 3 x 1016 ions/cn&but the dose dependence of the two is different (see,@. 1). The Fe&” component seems to be characteristicof implanted systems when the Massbauer pattern for.&e Fe:+ component is com- parable to that for the magnesium-wiistite solid solu- tion (Mg,_,Fe,O) [16] which corresponds to a random distribution of Fe2+ ions in the MgO matrix. According

A. Perez / Ion implantation effects in crystalline inorganic insulators 625

to the equilibrium phase diagram for the MgO-Fe system [30,31] the magnesio-wtistite is stable at high temperature but decomposes to the mixture of mag- nesio-ferrite and magnesio-wtistite during slow cool- ing. In implanted systems the magnesio-wtistite solu- tion is created without the precipitation of sizeable magnesio-ferrite particles. Concerning the metallic iron aggregates, the relative contribution of this component increases with the dose, approaching 20% in high dose implanted crystals (8 X lOI ions/cm2) [16]. It is to be noted that the metallic particles in the as-implanted samples are always small enough to behave superpara- magnetically down to 4.2 K thus indicating a size of the order of 20 A in diameter [32].

Applying the statistical model suggested above, the possibilities of finding various iron states for an arbi- trary iron concentration x are determined by the bi- nomial distribution:

P,(n,x>= ( 1 ; x”(l -x)-y

where N corresponds to the number of neighbouring atoms which can be replaced by iron and which form complexes of n iron atoms with the iron probe atom. Thus, corresponding probabilities for the presence of isolated iron ions (n = 0), iron dimers (n = l), trimers (n = 2), etc.. . can be evaluated using this formula. We have calculated probability functions of finding 0, 1, 2 etc... of iron atoms [P(O, x), P(1, x), P(2, x0, etc., respectively] in the probe vicinity containing only nearest neighbours ( P,2, twelve Mg atoms) or two, three and several coordination spheres [16]. Among various proba- bility functions, those which give the best simulation of the experimental data are reported in fig. 1. A good agreement between the calculated curves and the experi- mental data indicates on the one hand that various iron states are strongly related to various specific iron arrangements in the MgO lattice and on the other hand the identification of these arrangements is correct.

The Ps4(0, x) function which represents the probabil- ity function of finding no other iron atom in the vicinity containing four cationic coordination spheres (54 ca- tionic sites) fits very well with the dependence of the Fe3+ fraction on iron concentration. One concludes that Fe3+ in implanted MgO can exist only as an isolated impurity so that the presence of another iron impurity at a distance closer than four cationic coordination spheres causes a change of the valence state of iron ions either into Fe’+ or Fe’. It is worth noticing that the exclusion volume around the Fe3+ ion as found from our model (N = 54 cationic sites) is characteristic of the complexes Fe 3+-02- vacancy-02--Fe3+ propoed by Gourdin et al. [25,26]. This is also supported by the study of the defect creation reported in detail in ref. 16 which shows the relation of Fe3+ ions with cationic vacancies.

For Fe:,+, the P,,(l, x) function representing the probability of finding one other iron as a close neighbour (in the first coordination sphere containing 12 cationic sites) fits its evolution well; thus Fe:,+ represents iron dimers. The statistical model accounts also for the con- centration dependence of the metallic aggregate compo- nent deduced from Mbssbauer data [16]. In this case the probability [ P,,( > 2, x)] of aggregating three or more iron atoms in the nearest neighbour cationic sites gives a good description of the concentration dependence of the metallic iron component These small iron clusters could be the nucleation centers for metallic iron pre-

cipitates. Finally the Fe:+ component is described by

the curve corresponding to all other iron configurations other than the three listed above (Fe3+, Fe;: and Fe’).

It is interesting to compare the results for MgO with those obtained in LiF implanted with iron ions under the same conditions [29]. In LiF the observed variation of the number of Fe3+ ions with iron concentration is slower than in MgO and is well reproduced by the P,,(O, x) probability. This indicates that iron may re- main in a Fe3+ state in LiF as long as there is not other iron ion at the nearest cationic site. This is in agreement with the complex structure proposed by Stanek et al. [27] for the local charge compensation of Fe3+ ions in alkali halides. This Fe3+ ion is assumed to be aggre- gated with two alkali metal vacancies (V) distributed in the next nearest neighbour positions (V-Fe3’-V). It is to be noted that the correlations between the evolutions of the two other iron components (Fe2+ and Fe’) and the statistical functions are not as good in LiF com- pared with MgO. However in this case one cannot exclude the precipitation of a ferrous compound such as FeF, observed by Anand [33].

A dose dependence effect of the valence state of the implanted impurities has also been observed in the case of SiO, implanted with carbon ions [21,22]. It has been shown that carbon implantation leads to the preferen- tial synthesis of CO, at low fluences, CO becoming the dominant species at high fluences. It would be interest- ing, in this case, to verify if the molecule synthesis can be explained from a statistical evolution of the local environment of the implanted carbon ions.

4. Annealing behaviour

Some examples of implanted system evolution with thermal treatment are mentioned in table 1. Also some characteristic phenomena observed during the thermal procedures are described in 52. However in this section, a more detailed description of the evolutions of both MgO-Fe and LiF-Fe systems is discussed in order to point out the parameters which govern the annealing behaviours. As an example, the thermal evolution observed with a MgO crystal implanted with 2 X 1Or6

XI. RADIATION CHEMISTRY

626 A. Perez / Ion i~pI~ntation effects in crystalline inorganic insulators

Fe+ ions/cm2 (energy 100 kev) and annealed in air at temperatures up to 1000°C is presented in fig. 2a. For comparison, annealings in vacuum in the same tempera- ture range for a MgO crystal implanted with 1016 Fe’ ions/cm2 (energy 70 keV) are reported in fig. 2b.

Annealing phenomena presented in fig. 2 can be discussed in two temperature regions; the first below 700°C and the second between 700 and 1000°C. An- nealing in the first temperature range is connected with several correlated phenomena observed by the three techniques of characterization employed (opticai ab- sorption, channeling and CEMS) 1161. CEMS measure- ments show the continuous process of oxidation of Fe2+ to Fe3+ which is accompanied by some changes in the local symmetry. At the same time,” no significant rearrangement is observed in the Mg sublattice, but optical absorption spectra show a complete annealing of V-type and F-type centres. It is remarkable that the same behaviour of the Fe 3* fraction is also observed from GEMS spectra measured with samples annealed in vacuum. This could indicate that the oxidation processes in this low temperature range occur without oxygen. In this case the oxidation of Fe2’ can take place subse- quent to the migration of V--type centres and their combination with FZt ions. Fe3+ ions formed by this mechanism are characterized by the same Miissbauer spectra parameters as measured for Fe3+ ions in as-im- planted samples. Thus, the formation of the same kinds of Fe3+-vacancy complexes can be considered. In the high temperature range (800-1000°C) annealing processes in air and in vacuum are drastically different. In the case of annealing in air, one observes the trans- formation of iron into two new ferric states: Fe:+ and Fe: (see fig. 2a). These two states correspond to Fe3+ ions respectively in octahedral and tetrahedral sites. This new arrangement of iron is accompanied by a quasi-complete restoration of the MgO crystal and dif- fusion of iron (profile broadening observed by RBS

technique) (161. This is in agreement with the formation of magnesio-ferrite precipitates (MgFe204). We must remark that the ma~esi~ferrite structure fits very well into the MgO structure. The precipitates are in epitaxy with the MgO matrix and their orientation relationship is [34]:

Pl) M~F~~o~~I~~~)M~o and [~OOIM~F~~O,II[~~~IM~O. In the opposite case, annealing in vacuum at a

temperature higher than 800°C converts all iron to Fe:+ component [161. This is expfained by a total reduction of iron and dilution in the MgO matrix. The same behaviour has been observed in Fe-doped MgO crystals thermally treated in a reducing atmosphere [ 35,361.

A similar behaviour is observed in Fe implanted LiF 1291. Annealing in vacuum results in the growth of metallic iron precipitates, whereas annealing in the pres- ence of oxygen leads to the precipitation of particles of some, still unidentified, ferric compound.

5. Conclusion

Using some characteristic examples, the implanta- tion effects in crystalline inorganic insulators have been described and the parameters and mechanisms which govern these phenomena have been emphasized. The nature of the implanted impu~ty and matrix plays a dominant role in the formation of the phase observed in the as-implanted system. However the difficulty of applying thermodynamic concepts to such complex sys- tems leads us to suggest a statistical model for the description of the local environment of the implanted ions which seems to determine their final state. This model allows us to account for the existence of different valence states of the implanted ions and shows the dominant role of defects for the stabilization of im-

_~_ 100 keV ann. in a; 7

’ ’ ann. in vacuum f

D4006008001000” RT2004006008001000

ANNEALING TEMPERATURE (k) Fig. 2. Evolution of the fractional contribution (It) of various iron states present in iron implanted MgO crystals annealed in air and in vacuum, as a function of the annealing temperature.

A. Perez / Ion implantation effects in crystalline inorganic insulators 627

planted atoms in the lattice. Also the dose dependence with the phase precipitation which generally occurs at high doses can be described. The different stages in the evolution of the implanted system with annealing temperature have been shown with the role of defects and atmosphere. In this case the nucleation and diffu- sion processes which lead to the precipitation and growth of various equilibrium systems can be sometimes predic- ted on the basis of classical phase diagrams. Up to the present, the statistical model has been applied to a limited number of cases and it would be interesting to verify its validity in various other implanted materials. Unfortunately, the lack of knowledge of the defect structures in particular, and very often an incomplete characterization of the different implanted ion states, explains the actual situation.

The author would like to thank P. Thevenard, M. Treilleux and M. Guermazi, of Dept. de Physique des Materiaux, University of Lyon, for fruitful comments and discussions.

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XI. RADIATION CHEMISTRY