Nuclear Instruments and Methods in Physics Research B32 (1988) 11-22 11 North-Holland. Amsterdam

Section I. Ceramics

ION IMPLANTATION AND ANNEALING OF CRYSTALLINE OXIDES AND CERAMIC MATERIALS ++

C.W. WHITE, L.A. BOATNER, P.S. SKLAD, C.J. McHARGUE, J. RANKIN *, G.C. FARLOW * * and M.J. AZIZ + Oak Ridge Natronal Laboratory, Oak Ridge, TN 37831, USA

The response of several crystalline oxides or ceramic materials to ion implantation and subsequent thermal annealing is described. For both SrTiO, and CaTiOs single crystals, the near-surface region can be turned amorphous by relatively low doses of heavy ions (Pb, 10’5/cm2, 540 keV). During annealing, the amorphous region recrystallizes epitaxlally with the underlying substrate by simple solid-phase epitaxy, and the crystallization kinetics have been determined for both of these materials. In Also,, the amorphous phase of the pure material is produced by a stoichiometric implant at liquid nitrogen temperature. During annealing, the amorphous film crystallizes in the (crystalline) y phase, followed by the transformation of the y to the a phase at a well-defined interface. The kinetics characterizing the growth of a-Also, have been determined. Preliminary results are presented on the effect of impurities (Fe) on the nature and kinetics of the crystallization of amorphous AlaOs.

1. Introduction or crystalline oxides [9-191. For many of the potential applications, it will be necessary to anneal these Ion implantation is being investigated extensively as materials following implantation in order to remove the a method to alter the near-surface properties of a wide radiation damage associated with the implantation pro- range of materials in a manner that is independent of cess. Numerous investigations of ion implantation many of the constraints associated with conventional damage removal or alteration by annealing after im- processing methods. Any element can be injected into a plantation have been carried out for both elemental and solid in a controlled and reproducible manner by ion compound (e.g., GaAs) semiconductors, but there are implantation, and this doping process is also nonequi- only a few reports of similar studies [g-12,16-22] of librium in nature - a feature that often leads to the implanted crystalline oxides. formation of compositions and structures that cannot In the present work, we will discuss the response of be obtained by conventional processing methods. Ion several crystalline oxides to ion implantation damage implantation doping has experienced its greatest success and their behavior during subsequent thermal anneal- by far in the area of semiconductor technology [l], and ing. In the case of SrTiO, and CaTiO,, we find that the technique has also been extensively investigated as a moderate doses of Pb (- 10i5/cm2) implanted at liquid means of altering the physical and chemical properties nitrogen temperature are sufficient to turn the near- of metals [2]. Until recently, however, there were only a surface region amorphous. Thermal annealing at rela- few reported investigations dealing with ion implanta- tively low temperatures (< 500” C) causes the tion of insulating materials [3-81. amorphous region to crystallize epitaxially with the Current activities in the areas of integrated optical underlying substrate. In these two materials, annealing circuits and the tailoring of ceramic properties for takes place by simple solid-phase epitaxy where the specific applications have led to renewed interest in the amorphous + crystal transformation occurs at an inter- use of ion implantation to alter the near-surface optical, face that moves toward the surface [22,23]. The kinetics electrical, or mechanical properties of ceramic materials associated with the solid-phase transformation have been determined for several growth directions in these materials. The crystallization of amorphous Al,O, is * Massachusetts Institute of Technology, Cambridge, MA shown to be more complex than that observed for the 02139, USA. two titanates [21,22]. In cx-A1203, a stoichiometric im- * * Wright State University, Dayton, OH 45435, USA. + Division of Applied Sciences, Harvard University, Cam- plant at liquid nitrogen temperature was used to pro- bridge, MA 02138, USA duce an amorphous film of the pure material on a ++ Research sponsored by the Division of Materials Sciences, crystalline substrate. During annealing, the amorphous US Department of Energy under contrast DE-ACOS- film first crystallizes into the (crystalline) y phase of 84OR21400 with Martin Marietta Energy Systems, Inc. Al,O,, followed by the transformation of y to the (Y

0168-583X/88/$03.50 0 Elsevier Science Publishers B.V. I. CERAMICS (North-Holland Physics Publishing Division) 12 C. W. White et al. / Ion implantation of crystalline oxides and ceramu phase [21]. The kinetics associated with the growth of (Y (KIO) SrTD3 IMPLANTED BY Pb (540 keV, IX ~O’%X?) from y have been determined. Preliminary results are EPITAXIAL RECRYSTALLIZATION presented also that suggest that the transformation kinetics can be changed considerably by the implan- tation of impurities (Fe) into the amorphous film.

Sr 2. Experimental details

kl: Single crystals of CaTiO, and SrTiO, were im- : planted with Pb (250 or 540 keV) to doses of (l-4) X I”----\ : \\ : ----___/’ \ 10i5/cmz at liquid nitrogen temperature. CaTiO, has an \ 1’ orthorhombic structure and crystal faces of both low \..’ symmetry (a- or c-axis oriented, with a = c) and high symmetry (b-axis oriented) were used. SrTiO, has a cubic structure and crystals with faces with a (100) I I I I I I I orientation were used primarily, although limited stud- lb1 ies were also carried out using crystals with a (110) or AWEALED 400 T-30 mn (111) orientation. Following implantation, these crystals 2OQO were annealed in air at temperatures in the range of 270-550 o C for time periods ranging from a few minutes to many hours. For the experiments on Al,O,, single crystals of cy-Al,O, (c-axis oriented) were subjected to a stoichiometric implant (two parts Al, three parts 0, with the ion energies adjusted to give the same pro- jected range) at liquid nitrogen temperature in order to produce the amorphous phase of the pure material on a crystalline a-alumina substrate [20]. In this case, a stoichiometric implant was used initially so that the crystallization behavior of the pure material could be determined. Later, various impurities were implanted at liquid nitrogen temperature in order to form an Fig. 1. Implantation and annealing of (100) SrTiO,. Ion chan- amorphous film that was also doped with the implanted neling results are shown in (a) for the as-implanted state. impurities. These crystals were used to determine the Results obtained after annealing (400°C/30 min) are shown effect of selected impurities on the crystallization behav- in (b). ior. The implanted Al,O, crystals were annealed in flowing Ar at temperatures in the range of 800-1200 o C for time periods ranging from a few minutes to 100 h. is - 1800 A following the implantation. Fig. lb shows Crystals were examined in the as-implanted state ion channeling results obtained after thermal annealing and after thermal annealing using 2-MeV He+ Ruther- at 400 o C for 30 min in air. After annealing, the aligned ford backscattering (RBS) and ion channeling tech- yield from the implanted region is greatly reduced dem- niques. Selected crystals of CaTiO, and Al 203 were also onstrating that the amorphous layer was crystallized by examined by transmission electron microscopy (TEM). annealing and that the recrystallized region is epitaxial with the underlying substrate. Implanted Pb is substitu- tional in the lattice after annealing. The crystallized film 3. Implantation and annealing of SrTiO, is not defect-free, however, because the aligned yield is substantially greater than that from the unimplanted Fig. la shows that the implantation of a modest dose region. Features in the spectrum in both the Sr and Ti of Pb (540 keV, 1 X lO”/cm’) into (100) SrTiO, at sublattice suggest that extended defects, possibly a dis- liquid nitrogen temperature is sufficient to turn the location network, remain in the vicinity of the original near-surface region amorphous. In fig. la, the aligned amorphous/crystal interface after annealing. A positive spectrum in the implanted region reaches the random identification of this network is contingent on a planned value in the Sr sublattice and the step in the aligned TEM investigation. yield from the Ti sublattice is equivalent to the increase Several experiments were carried out using lower in the random spectrum at the energy corresponding to annealing temperatures and/or various annealing times the Ti leading edge. The depth of the amorphous region in order to further investigate the crystallization behav- C. W. White et al. / Ion implantation of crystalline oxldes and ceramics 13

RECRYSTALLIZATION OF Pb (540 keV, 4 x ~O’%xn*) RECRYSTALLIZATION OF Pb IMPLANTED (100) SrTi03 (540 krV, IX ~O’S/ctn2, IMPLANTED SrTi03 ANNEALING AT 302 =X 1 I I I I I I

500 - RAWM ---- AS IMPLANTED -- ANNEALED 302 *C/45 mm

d t.t 4.2 L3 4.4 4.5 1.6 L7 19 (9 ENERGY (MeVI 0 10 20 30 40 50 60 TIME Fig. 2. Partial crystallization of Pb-implanted (100) SrTiO,. Annealing was carried out at 302 o C for 45 min. Fig. 3. Depth crystallized as a function of annealing time (in minutes). A linear rate is observed following an induction period. ior of implanted SrTiO,. Fig. 2 shows the channeling results obtained after annealing implanted SrTiO, at 302°C for 45 min. This annealing treatment results in the period were simply related to the time required to only a partial recrystallization of the amorphous region. bring the sample to the annealing temperature. To ion channeling, the near-surface region appears to Fig. 5 summarizes the crystallization kinetics for still be amorphous, but the annealing treatment has caused the amorphous/crystal interface to advance to- ward the surface by - 800 A. The crystallized region is ANNEALING BEHAVIOR epitaxial with the underlying substrate. These results ((00) SrTi03- Pb(540 keV,~rt0’%m2) suggest the type of layer-by-layer crystallization that is expected when the regrowth occurs by solid-phase epi- I I I I I taxy. In fig. 3, the crystallized depth is plotted versus annealing time at a constant annealing temperature of 302°C. During the initial stages of annealing, there is little motion of the interface. After the initial “induc- tion period”, however, the rate of interface motion becomes linear with a velocity of - 0.91 A/s. This induction period is a property of the crystallization process and is not simply related to the time required to raise the sample to the annealing temperature which is much shorter than the induction period. Fig. 4 summarizes results obtained by annealing Pb-implanted SrTiO, crystals at temperatures of 302, 325, and 350 o C for various times. For each tempera- ture, the depth crystallized is plotted versus the anneal- ing time. In each case, there is a well defined linear rate 0 of growth following an initial induction period. Veloci- 0 10 20 30 40 50 60 ties associated with this linear growth rate are indicated ANNEALING TIME (mn) for the three annealing temperatures. The induction Fig. 4. Summary of annealing results of Pb-implanted SrTiO,. period decreases markedly as the annealing temperature For each temperature the crystallized depth is plotted vs increases. Such behavior is opposite to that expected if annealing time.

I. CERAMICS 14 C. W. White et al. / Ion implantation of crystalhe oxrdes and ceramics

CRYSTALLIZATION OF AMORPHOUS SrT103 amorphous and that thermal annealing at temperatures AMORPHOUS --c CRYSTAL TRANSFORMATION far below the SrTiO, melting point can be used to

T(‘C) crystallize the amorphous region epitaxially with the 350 325 300 275 underlying substrate. Crystallization takes place by solid-phase epitaxy with an activation energy of - 0.77 eV for growth in the (100) direction. It is remarkable that simple layer-by-layer solid-phase epitaxy can be observed in a material as complex as SrTiO,. Solid-phase epitaxy is a well known process in the case of ion-im- planted silicon [24,25], but oxides such as SrTiO, are much more complex since they are ternary compounds. Nevertheless, the results obtained here show that this simple crystallization process is operative even in a complex oxide.

4. Implantation and annealing of CaTiOs

Another titanate that is characterized by a relatively simple annealing behavior is CaTiO,. This crystal has an orthorhombic structure, and fig. 6 shows [21] that I I I I I 0.1 I I I.55 1.65 4.75 4.65 the near-surface region is turned amorphous in the Ti, + xf03(“K-‘) Ca, and 0 sublattice (to a depth of - 1100 A) following the implantation of Pb (250 keV, 4 x 1015/cm2). After Fig. 5. Kinetics of crystallization of Pb-implanted (100) SrTiO,. Growth velocities are the linear rates measured following an annealing at 500°C for one hour, the aligned yield in induction period at each temperature. the implanted region is nearly indistinguishable from that in the unimplanted region. This demonstrates that epitaxial recrystallization is occurring, and that the im- (100) SrTiO, made amorphous by Pb implantation (540 planted Pb is incorporated into a solid solution. Upon keV, 1 x 10r5/cm2) at liquid nitrogen temperature. In heating to higher temperatures, incorporated Pb comes fig. 5, the measured crystallization rate is plotted on a out of solution and forms precipitates which shows the log scale as a function of inverse temperature. The solid solution is metastable. growth velocities in fig. 5 were taken from the linear Annealing at lower temperatures results in only a rates of crystallization that were measured following the partial crystallization of the amorphous CaTiO, layer as induction period (see fig. 4). Assuming that the shown in fig. 7. In this case, the implantation energy amorphous/crystal transformation in SrTiO, is ther- was 540 keV - resulting in an amorphous layer - 1800 mally activated and that the velocity can be represented A thick. Thermal annealing causes the amorphous/ by V(T) = V, e- QiRT, then the data in fig. 5 are con- crystal interface to move - 900 A toward the surface sistent with an activation energy Q = 0.77 eV and a and this effect can be seen from the ion channeling prefactor V0 - 5 X lo-* cm/s. spectra for both the Ti and Ca sublattices in fig. 7. This Limited experiments have also been carried out using behavior indicates that recrystallization also occurs by both (110) and (111) oriented SrTiO, crystals. The solid-phase epitaxy in this material. results obtained for growth in the (110) direction are A cross-section micrograph demonstrating that re- almost indistinguishable from those obtained in the crystallization of amorphous CaTiO, occurs by solid- (100) case for both the growth velocity and induction phase epitaxy, is shown in fig. 8. The original amorphous period. In the (111) case, however, the limited experi- layer (- 1850 A thick) was formed by the implantation ments suggest that growth in this direction is not linear of Pb (540 keV, 1 X 10’5/cm2) at liquid nitrogen tem- with time. Finally, limited experiments were also carried perature. Annealing in situ in the electron microscope at out using higher implanted doses to see whether the - 480 o C resulted in partial recrystallization where the crystallization kinetics depend strongly on the Pb con- interface has advanced toward the surface by - 600 A. centration. The current results show that the crystalliza- The recrystallized layer is epitaxial with the underlying tion kinetics are not strongly affected by the Pb con- substrate and threading dislocations can be observed in centration - at least in the low concentration regime. this region. After annealing, the amorphous/crystal in- The results illustrated in figs. l-5 show that modest terface is very sharp and planar over many thousands of doses of Pb implanted at liquid nitrogen temperature angstroms. This behavior is consistent with the process are sufficient to turn the near-surface region of SrTiO, of solid-phase epitaxy. C. W. White et al. / Ion implantation of crystallineoxides and ceramics 15

EPITAXIAL RECRYSTALLIZATION OF 208Pb(250 keV, 4 X 10’5/Crn2) IMPLANTED CaTiO,

AS IMPLANTED

3500 ? E 3000 a 2 2500 D p 2000

I500

0.6 0.8 t.0 1.2 I.4 1.6 1.8 0.6 0.8 I.0 1.2 1.4 1.6 1.8 ENERGY b&V) ENERGY @VW’) Fig. 6. Implantation and annealing of CaTiOs (high-symmetry face). Ion channeling spectra are shown in the as-implanted state (a) and after annealing at 500 o C/l h in air (b).

The kinetics of crystallization for growth along both fall on a straight line within the experimental uncer- the low-symmetry and high-symmetry (i.e., pseudo- tainty, and activation energies of 1.3 and 3.76 eV are cubic) directions are shown in fig. 9. Measured growth extracted from the slopes of the straight lines for growth velocities in the two directions are plotted on a log scale along the low- and high-symmetry directions respec- versus inverse temperature. On this plot, the velocities tively. The growth is, of course, assumed to be a ther- mally activated process in such a treatment. The data in fig. 9 show clearly that the crystallization kinetics are considerably different for crystal growth in CaTi03 -HIGH SYMMETRY FACE low-symmetry and high-symmetry directions. The data Pb (540 keV, 4 x ~0’5/cm2) IMPLANTED also indicate that the activation energies are consider- ANNEALED 425 OC/i5 HOURS ably different for regrowth along these two directions. We are not aware that an effect of this type has been

2000 reported previously. Studies of solid-phase epitaxy in ion-implanted silicon [24] have shown that the activa- tion energies for growth along the (111) and (100) I “1 0 I 1600 directions are the same, even though the growth rates differ considerably in these two directions. Crystalliza- tion of the amorphous phase in general involves both -- ALIGNED, AS nucleation and growth. In the crystallization of an IMPLANTED -‘- ALIGNED, amorphous film on a single-crystal substrate, one nor- ---- ALIGNED, VIRGIN mally assumes that nucleation sites are provided by the ./ \ underlying substrate and that kinetics (and extracted activation energies) are determined by the growth com- ponent. The results of fig. 9 suggest that there may be a nucleation barrier that must be overcome in order for growth to occur along the high-symmetry direction. Why this should be the case in CaTiO, is not clear, but it has been suggested that the growth of crystals from 3nn___ 1.1 4.2 4.3 4.4 4.5 +.6 other oxide glasses and melts can be limited by the rate ENERGY It&V) of two-dimensional nucleation of monolayers at the Fig. 7. Partial crystallization of Pb-implanted CaTiO, (high- interface [26]. This will occur when there are no “easy” symmetry face). Ion channeling spectra are shown following growth sites on the interface. If this occurs, then the implantation (540 keV, 1 x 10i5/cm2) and after thermal an- crystal growth will be limited by the nucleation of new nealing (425 o C/15 h). crystal monolayers. Such behavior has been proposed

I. CERAMICS 16 C. W. White et al. / Ion implantation of cytalline oxides and ceramics

DARK FIELD

Fig. 8. Cross-section micrographs showing partial crystallization of Pb (540 keV, 1 X 10’5/cm2) implanted CaTiO, (high-symmetry face).

for the growth of B,O, from the melt [26], the growth of 5. Crystallization of amorphous Al24 certain polymers from the melt [27], and the growth of certain metals from the vapor phase [28]. The ap- In contrast to the behavior exhibited by SrTiO, and parently higher activation energy for growth in the CaTiO,, the crystallization of amorphous Al,O, is sig- high-symmetry direction of fig. 9 may imply the need nificantly more complex [21,22,30]. The recrystallization for a similar two-dimensional interfacial nucleation for of Al,O, has been studied by RBS and TEM measure- solid-phase epitaxy in this direction. ments and the results will be summarized here. Our At higher temperatures, the data of fig. 9 indicates work has shown that a thin film of pure amorphous that the curves for the high-symmetry and low-symme- Al,O, on a crystalline cy-Al,O, substrate can be pro- try directions actually cross. This observation seems duced by a stoichiometric implant (two parts Al, three difficult to account for in terms of simple growth mech- parts 0, with ion energies adjusted to give the same anisms. We are currently checking these results and are projected range) at liquid nitrogen temperature [20]. For extending these experiments to higher annealing tem- example, the implantation of Al (4 x 10’6/cm2, 90 keV) peratures. These experiments require a thicker starting and 0 (6 X 1016/cm2, 55 keV) at LN, produces an amorphous layer and higher implantation energies. amorphous layer 1600 A thick of pure Al,O, on a C. W. White et al. / Ion wnplantatron of crystalhneoxldes and ceramics 17

y -+ a transformation is only partially complete. In this RECRYSTALLIZATION OF AMORPHOUS CaTi case, annealing was carried out at 96O’C for 90 min AMORPHOUS - CRYSTAL TRANSITION following a stoichiometric implant that was used to TEMPERATURE (“C) produce the amorphous phase. The TEM cross-section 500 450 400 results and diffraction patterns show that the near I I I surface (region b) is y-A120,. Region (c) is a-Al,O, 0 LOW SYMMETRY FACE - formed by the y + LYtransformation. Region (d) is the 0 HIGH SYMMETRY FACE - underlying a-substrate. The interface separating regions (b) and (c) is planar and relatively sharp, and this is the interface where the y + a transformation occurs. The orientation is such that (111) planes of y are parallel to (0001) planes of a. The position of the y/a interface can be measured using either TEM or RBS-ion channeling. Fig. 12 shows the RBS-channeling results after annealing at 950°C/45 min. The amorphous/crystal interface lies at a depth of - 1600 A following the stoichiometric implant. During annealing, the interface moves - 500 A toward the surface, and the aligned yield in the very near surface does not reach the random value. The structure (in cross section) of the near surface is de- kO-’ picted schematically in fig. 12 and the different regions 4.3 1.4 1.5 are identified in the channeling results from the Al $ (OK-‘) X IO3 sublattice. The interface where the y + a transforma- tion takes place is located at - 1100 A. Fig. 9. Measured crystallization kinetics for amorphous CaTiO,. From measurements such as those in figs. 11 or 12, Results are shown for both low- and high-symmetry directions. the velocity of the y/a interface can be determined. Solid lines through the data refer to the indicated activation From these measurements, the kinetics of the y + a energies, assuming that growth is a thermally activated process.

crystalline a alumina substrate. Crystallization of the CRYSTALLIZATION OF AMORPHOUS AI,01 amorphous film is a relatively complex process that occurs in two stages as illustrated in fig. 10. The first STOCHIOMETRIC step in the annealing process is the conversion of the IMPLANT-LN, amorphous film to the crystalline y phase of Al,O,. This transformation occurs at an interface that moves 1 ANNEAL from the original amorphous/crystal interface region toward the surface as illustrated in fig. 10. Annealing experiments [29] carried out at elevated temperatures in the electron microscope show that the entire amorphous film (- 1600 A thick) can be transformed to the y phase by annealing for a few minutes at a temperature of 800°C. The second step in the annealing process is the conversion of y-Al,O, into a-Alz03. This transfor- mation also takes place at a well-defined interface that moves from the original amorphous/crystal interface region toward the free surface [21]. If the crystal is heated at sufficiently high temperatures for a sufficient length of time, the entire film can be crystallized epi- taxially in the a phase. The y -+ a transformation is more sluggish than the amorphous + y transformation. Approximately 100 h are required at 800°C to trans- form - 800 A of y into the a phase, whereas the entire amorphous film (1600 A) can be transformed into the y Fig. 10. Schematic representation of behavior observed during phase in only a few minutes at 800 ’ C. annealing of an amorphous film of pure Al,O, on an a Fig. 11 shows TEM results from a crystal where the substrate.

I. CERAMICS C. W. Whiie et al. / Ion implantation of crystalline oxides and ceramics

Fig. 11. TEM cross-sectron resuns snowmg tne y to (x phase transition in Al,O,. The amorphous film was formed by a stoichiometric implant at liquid nitrogen temperature, and annealing was carried out at 960 o C/90 min. Region (b) is y-Al,O,, region (c) is (~-Alr0~ formed in the y + a transformation and region (d) is the underlying substrate.

transformation can be determined as shown in fig. 13 in the o-phase. The activation energy we have determined the form of a plot of interface velocity (on a log scale) for the growth component (3.6 ev) is less than the Al-O versus T-‘. Assuming a thermally activated process, the bond energy [36] (4.7 eV) and less than the activation data are consistent with an activation energy of Q - 3.6 energy reported [37] for bulk diffusion of 0 ions either eV for the y + (Ytransformation in pure Al,O,. in either single-crystal cu-Al,O, (6.6 eV) or polycrystal- There have been a number of investigations [31] of line a-Al,O, (4.8 ev). It is also less than the activation the y -+ (Y phase transformation in Al,O,. The more energy reported [38] for bulk diffusion of Al ions in recent determinations [31-351 of the activation energy a-A1203 (5.0 eV). for the y + (Y transformation are in the range of 5.2 to We have observed on some occasions that the u/a: 6.5 eV. Those experiments involved the conversion of interface motion will stop before reaching the surface bulk y-Al,O, to a-A1203 and as such, they involved (after 1200-1600 A of growth). The reasons for this both nucleation and growth of the a-phase, and ad- effect are not clear at this time. This could be related ditionally, the experiments averaged over all crystallo- either to some nonreproducibility in the implantation graphic directions. Our results deal with one crystal process, impurities in the Ar annealing ambient, or orientation only, and the activation energy is that for possibly defects or impurities in the starting crystalline growth alone, since the underlying substrate provides a-substrate. These possibilities are being investigated the nucleation sites. To our knowledge this is the first further and the results will be reported separately. The determination of the activation energy for the growth of results presented here are for crystals where this re- C. W. White et al. / Ion rmplantatron of crystalline oxrdes and ceramics 19

CRYSTALLIZATION OF AMORPHOUS A&O3 STOICHIOMETRIC IMPLANT AT LN2 ANNEALED 960 “C/45 min

0 DEPTH (pm) Al DEPTH (pm) 0.3 0.2 0.1 0 0.3 0.2 0.4 0 V 11’1’1’1 I 1 I ’ I ’ 1

AS IMPLANTED

ANNEALED 960 ‘C/45 min

- RANDOM y 4000 -I ALIGNED, AS IMPLANTED -.- ALIGNED, ANNEALED 500 ----- ALIGNED, VIRGIN

” 0.5 0.6 0.7 0.0 0.9 1.0 1.4 i.2 4.3 4.4 ENERGY (t&V)

Fig. 12. Channeling results demonstrating annealing of Al 203 made amorphous by a stoichiometric implant at LN,. Annealing was carried out at 960 o C/45 min. The structure in the near-surface is depicted schematically and the different regions are identified in the channeling results from the Al sublattice.

tardation was not observed. There are also reports of effect has been reported by others [16]. In the recrystal- other transitional alumina phases in addition to y- lized region, there are few remaining defects as shown alumina. Although we have not been able to identify in the plan view micrograph of fig. 15. This figure shows any of these other phases to date, the possibility re- no evidence of Fe precipitates and very few extended mains that the retardation of the y to (Y transformation defects. The dominant defects remaining are small voids. is due to the formation of another transitional phase. In cross-section micrographs (not shown) these voids lie This possibility is also being explored further. in a band near the projected range of the Fe. The Preliminary experiments have been initiated to in- crystallized near surface is cr-Al,O,, as determined from vestigate the effects of impurities on the nature and diffraction patterns (not shown). kinetics of the crystallization behavior of amorphous The results shown in figs. 14 and 15 are quite differ- Al,O,. For this experiment, Fe (160 keV, 4 x 10’6/cm2) ent from those observed in previous studies of the was implanted into c~-Al,O, at liquid nitrogen tempera- annealing behavior of Fe implanted into Al *03 at room ture. This produced an amorphous layer - 1500 A temperature [39,40]. The implantation of Fe (160 keV, thick. Annealing was carried out in flowing Ar at 800 4 X 10r6/cm2) into o-Al,O, at room temperature will and 96O’C. Fig. 14 shows ion channeling results mea- damage the near surface, but will not produce the sured in the as-implanted state and after annealing amorphous phase. Annealing in either oxidizing or re- (960 o C/67 min). Thermal annealing at this time and ducing environments leads to the formation of precipi- temperature is sufficient to crystallize the entire film tates. In addition, damage is not effectively annealed epitaxially with the underlying substrate. In addition, out until temperatures of 1300-1500 o C are reached. If this treatment redistributes the Fe toward the surface the implant is done at liquid nitrogen temperature in and results in the substitutional incorporation of Fe order to form the amorphous phase, then crystalline into the lattice. The shift in the Fe profile toward the order can be restored to the implanted region and Fe surface suggests a redistribution caused by a recrystalli- can be trapped substitutionally by annealing at temper- zation interface moving toward the surface. A similar atures Q 960°C, as shown in figs, 14 and 15.

I. CERAMICS 20 C. W. White et al. / Ion implantation of crystalline oxides and ceramws

KINETICS OF y-a ANNEALING OF “Fe 060 keV, 4 X ~O’6/,rt2, LN2) TRANSITION IN AI,O, IMPLANTED o -A1203

I I I I I I I I I I I I 1 T (“Cl (000 900 800 1 I I

Fe DEPTH (pm1

++i

0 I I I I I I I I I AI DEPTH (pm1 ‘2oo - 0.3 02 0.4 0 ANNEALED 960 T/58 MN

1000

- RANDOM -- ALIGNED. IMPLANTED ----- ALIGNED. VIRGIN 40-3 L I I I\ 7 a 9 10 !g (OK-Q Fc DEPTH Qml Fig. 13. Temperature dependence of the y/a interface velocity 0.2 0.4 0 I 1 I 1 in Al,O,.

The presence of Fe in the amorphous film increases the crystallization kinetics. By annealing for times at I short as 28 min, the entire film (- 1600 A thick) can be -0.0 09 4.0 4.4 1.2 4.3 (4 45 (6 4.7 ENERGY (Me”) crystallized into the a phase. This implies that the kinetics of the y + a transformation are increased by at Fig. 14. Crystallization of a-Al,O, made amorphous by Fe (160 keV, 4~1O’~/crn’) implantation at LN,. The top shows least a factor of 4 by the presence of Fe. (The amorphous channeling results in the as-implanted state. Channeling results phase of the pure material would crystallize only 300 A after thermal annealing (960 o C/67 min) are shown in the during this time interval.) After annealing at 800°C lower spectrum. (three hours), electron microscopy shows that the film is converted entirely into the y phase, and RBS shows no redistribution of Fe. This means that with Fe in the near-surface region. This is similar to results that have amorphous film, the crystallization behavior remains been observed in ion-implanted semiconductors. In ad- the same, i.e., amorphous + y + a, but the kinetics of dition, annealing at these lower temperatures offers the y/a transformation are increased by the presence of attractive possibilities to form metastable solid solutions Fe. Finally, the redistribution of Fe and its incorpora- by annealing the amorphous film containing impurities tion into lattice sites occurs during the y + a transfor- at temperatures where crystallization can be achieved mation. Fe incorporation probably occurs as a result of but bulk diffusion (and precipitation) is negligible. It is solute trapping (i.e., the growth front moves faster than believed that the Fe case is one such example. Experi- the impurity bulk diffusion rate) in a manner similar to ments are under way at present to carry out similar that observed during solid-phase epitaxy of ion-im- studies with other implanted species. planted silicon [41,42]. We have found that it is much easier to restore crystal quality to the near-surface of Al,O, if one starts 6. Conclusion from the amorphous phase. Excellent annealing results can be achieved at much lower temperatures than those Systematic studies of ion implantation damage and required if one starts with a damaged but crystalline thermal annealing behavior have been carried out for C. W. White et al. / Ion implantation of crystalline oxides and ceramics 21

PI Treatise on Materials Science and Technology, vol. 18, ed. J.K. Hirvonen (Academic Press, New York, 1980); see also numerous articles published in the recent Proc. Int. Conf. on Ion Beam Modification of Materials [Nucl. Instr. and Meth. 182/183 (1981); Nucl. Instr. and Meth. 209/210 (1983); Nucl. Instr. and Meth. B7/8 (1985); Nucl. Instr. and Meth. B19/20 (1987)]. 131 G.W. Arnold, G.B. Krefft and C.B. Norris, Appl. Phys. Lett. 25 (1974) 540. 141 G.B. Krefft, W. Beezhld and E.P. EerNisse, IEEE Trans. Nucl. Sci. NS-22 (1975) 2247. [51 T.F. Luera, J.A. Borders and G.W. Arnold, ref. [l], p. 285. WI G.B. Krefft and E.P. EerNisse, J. Appl. Phys. 49 (1978) 2725. [71 H.M. Naguib, J.F. Singleton, W.A. Grant and G. Carter, J. Mater. Sci. 8 (1973) 1633. PI H.M. Naguibl and R. Kelley, Radiat. Eff. 25 (1975) 1. [91 A.V. Drigo, S. LoRosso, P. Mazzoldi, P.D. Goode and Fig. 15. Plan view micrograph of Fe-implanted Al,O, (LN,) N.E.W. Hartley, Radiat. Eff. 33 (1977) 161. after annealing at 960 ’ C/67 min. WI A. Camera, A.V. Drigo and P. Mazzoldi, Radiat. Eff. 49 (1980) 29. 1111 A. Turos, H.J. Matzke and P. Rabette, Phys. Status Solidi three crystalline oxides, SrTiO,, CaTiO,, and Al 203. A64 (1981) 565. For SrTiO, and CaTiO,, low doses of Pb (< 1 x WI H. Naramoto, C.W. White, J.M. Williams, C.J. Mc- 10i5/cm2) produce an amorphous near-surface region, Hargue, O.W. Holland, M.M. Abraham and B.R. Apple- and thermal annealing at temperatures far below the ton, J. Appl. Phys. 54 (1983) 683. melting point causes the amorphous region to crystallize 1131 C.J. McHargue, C.W. White, B.R. Appleton, G.C. Farlow epitaxially with the underlying substrate. For these two and J.M. Williams, in: Ion Implantation and Ion Beam titanates, crystallization takes place by solid-phase epi- Processing of Materials, eds. G.K. Hubler, O.W. Holland, C.R. Clayton and C.W. White (North-Holland, New York, taxy, and the implanted Pb is incorporated into a sub- 1984) p. 385. stitutional lattice site. In SrTiO,, the activation energy 1141 C.W. White, G.C. Farlow, H. Naramoto, C.J. McHargue for solid-phase epitaxy is measured to be Q - 0.77 eV and B.R. Appleton, in: Defect Properties and Processing for growth in both the (100) and 9110) directions. In of Nonmetallic Materials, eds. J.H. Crawford, Y. Chen CaTiO,, the kinetics of crystallization differ consider- and W.A. Sibley (North-Holland, New York, 1984) p. ably depending upon the growth direction. The results 163. suggest activation energies of 1.3 eV for growth in the 1151 C.J. McHargue, Nucl. Instr. and Meth. B19/20 (1980) low-symmetry direction and 3.76 eV for growth in the 797. high-symmetry direction. In Al,O,, studies have been WI M. Ohkubo, T. Hioko and J. Kawamoto, J. Appl. Phys. 60 carried out to investigate the crystallization of thin films (1986) 1325. P.J. Burnett and T.F. Page, J. Mater. Sci. 19 (1984) 845. of pure amorphous Al,O, on crystalline a-Al,O, sub- [I71 WI C. Buchal, P.R. Ashley and B.R. Appleton, J. Mater. Res. strates. During annealing, the amorphous films convert 2 (1987) 222. first to the crystalline y phase of Al,O,, followed by 1191 R. Kelly, Nucl. Instr. and Meth. 182/183 (1981) 351. the conversion of y-Al,O, to a-Al,O,. An activation PO1 C.W. White, G.C. Farlow, C.J. McHargue, P.S. Sklad, energy of 3.6 eV was determined for the growth of a! M.P. Angelini and B.R. Appleton, Nucl. Instr. and Meth. from y. Finally, preliminary experiments were carried B7/8 (1985) 473. out to investigate the effect of impurities on the behav- WI C.W. White, P.S. Sklad, L.A. Boatner, G.C. Farlow, C.J. ior and kinetics of crystallization of amorphous Al,O,. McHargue, B.C. Sales and M.J. AZ&, Mater. Res. Sot. The addition of Fe to the amorphous film was found to Proc. 60 (1986) 337. increase the rate of the y to a transformation (at WI C.W. White, L.A. Boatner, P.S. Sklad, C.J. McHargue, S.J. Pennycook, G.C. Farlow and J. Rankin, Mater. Res. 960°C) and Fe was incorporated into substitutional Sot. Proc. 74, in press. lattice sites during crystallization. 1231 C.W. White, L.A. Boatner, J. Rankin and M.J. Aziz, Mater. Res. Sot. Proc. 93 (1987) 9. v41 L. Csepregi, E.F. Kennedy, J.W. Mayer and T.W. Sigmon, References J. Appl. Phys. 49 (1978) 3906. v51 J. Narayan, O.W. Holland and B.R. Appleton, J. Vat. Sci. [l] See for example, Ion Implantation in Semiconductors, Technol. Bl (1983) 871. eds. F. Chemov, J.A. Borders and D.K. Brice (Plenum, WI M.J. Aziz, E. Hygren, J.F. Hays and D. Tumbull, J. Appl. New York, 1976). Phys. 57 (1985) 2233.

I. CERAMICS 22 C. W. White et al. / Ion implantation of crystalline oxides and ceramics

[27] J.D. Hoffman, G.T. Davis and J.I. Lauritzen, Jr., in: [35] C.J.P. Steines, D.P.H. Hasselman and R.M. Spriggs, J. Treatise on Solid State Chemistry, vol. 3, ed. N.B. Hannay Am. Ceram. Sot. 54 (1971) 412. (Plenum, New York, 1976) p. 541. [36] Handbook of Chemistry and Physics, 52nd ed. (Chem. [28] J.P. Hirth and G.M. Pund, Progress in Materials Science, Rubber Co., Cleveland, Ohio, 1971) p. F-177. vol. 11, ed. B. Chalmers (Pergamon, New York, 1963) p. [37] Y. Oishi and W.D. Kingery, J. Chem. Phys. 33 (1960) 480. 87. [38] A.E. Paladin0 and W.D. Kingery, J. Chem. Phys. 37 [29] P.S. Sklad, private communication. (1962) 957. [30] G.C. Farlow, P.S. Sklad, C.W. White, C.J. McHargue and [39] G.C. Farlow, C.W. White, C.J. McHargue, P.S. Sklad and B.R. Appleton, Mater. Res. Sot. Proc. 60 (1986) 387. B.R. Appleton, Nucl. Instr. and Meth. B7/8 (1985) 541. [31] For a summary of some of the previous experiments, see [40] C.J. McHargue, G.C. Farlow, P.S. SkIad, C.W. White, A. D.S. Tucker and J.J. Hren, Mater. Res. Sot. Proc. 31 Prez, N. Komilios and G. Marest, Nucl. Instr. and Meth. (1984) 337. B19/20 (1987) 813. [32] H. Schaper and L.L. Van Reijen, Thermochem. Acta 77 [41] S.U. Campisano, E. Rimini, P. Baeri and G. Foti, Appl. (1984) 383. Phys. Lett. 37 (1980) 170; S.U. Campisano, G. Foti, P. [33] S.J. Wilson and J.D. McConnell, J. Sol. Stat. Chem. 34 Baeri, M.G. Grimaldi and E. Rimini, Appl. Phys. Lett. 37 (1980) 315. (1980) 719. [34] B.E. Yoldas, Ceram. Bull. 54 (1975) 286. [42] J.S. Williams and R.G. Elliman, Nucl. Instr. and Meth. 182/183 (1981) 289.