Formation of compounds by metalloid ion implantation in iron

Nuclear Instruments and Methods 209/210 (1983) 249-257 North-Holland Publishing C o m p a n y

249

F O R M A T I O N OF C O M P O U N D S BY M E T A L L O I D I O N I M P L A N T A T I O N IN IRON K. H O H M U T H , B. R A U S C H E N B A C H , A. K O L I T S C H and E. R I C H T E R Akademie der Wissenschaften der DDR, Zentralinstitut fiir Kernforschung Rossendorf, 8051 Dresden, Postfach 19, DDR

An investigation by high-voltage transmission electron microscopy and selected area diffraction of implanted iron layers is discussed. Phase transformations occur in thin films of iron after implantation with boron, carbon, nitrogen, silicon and phosphorus ions with energies of 30 to 60 keV in the dose range of 1 × 1016 to 1 × 10 TM i o n s / c m ~. Implantation of B +, Si ÷ or P+ ions in iron produced an amorphous phase. A criterion is proposed to parametrize the occurrence of amorphous metal-metalloid alloys after implantation. Crystalline stable and metastable phases in iron can be produced by B +, C + and N ÷ ions bombardment. It is possible to give "state diagrams" of iron phases after metalloid ion implantation in the pretended dose range and temperature range. The phase transformation of the metastable to stable phase is discussed. The influence of implantation temperature is demonstrated in an example of formation of martensite.

1. Introduction Ion implantation can be used to modify surface layers of metals. For example, the interactions between ion beams and metal targets can lead to improvement of wear resistance [1] and microhardness [2], increase in the average fatigue lifetime [3] and corrosion resistance [4]. Generally, the cause of the improvement of these properties is considered to be connected with the evaluation of micro-alloys, the formation of crystal defects and the interaction between precipitations and lattice defects within the near-surface region in the metal. Ion implantation in metals can result in the formation of metastable and equilibrium phases. Basic investigations of ion implantation metallurgy are necessary to use this technique most effectively. In this paper we will firstly use the implantation to prepare amorphous surface layer alloys in iron. We report on microstructural investigations of iron implanted with boron, nitrogen and carbon ions and subsequent annealing. Different amorphous, metastable and stable crystalline phases can be prepared by ion implantation. The formation of martensite is induced by the implantation of N + and C+.

10 - 2 Pa. Thickness of the layers was varied between 50 and 2000 nm. The iron films were implanted at room temperature with B +, C +, Si +, N + and P+ with energies of 30 or 50 keV in the dose range of 1 × 1016 to 1 × 10 Is i o n s / c m 2. Implantation at higher temperatures (50 to 600°C) was realized with a special sample holder. After ion implantation the iron films were investigated by means of a high-voltage electron microscope (HVEM) using the transmission technique and selected area diffraction (SAD). With special annealing equipment in the HVEM it was possible to attain a defined tempering and annealing regime of the implanted metal film to carry out an in situ observation during the the/'mal process. The heating velocity was varied between 10 and 35 Kmin - I and the maximum temperature was 700°C. The scattering curves were determined by calibrated electron diffraction patterns. The crystal structure was analysed on the basis of the experimental scattering curves.

3. Results and discussion

3.1. Amorphous iron-metalloid phases formed by implantation

2. Experimental procedure Polycrystalline iron films were deposited on freshly cleaved NaC1 or KC1 in a high vacuum of 0167-5087/83/0000

High dose implantation of B + boron and P+ at room temperature in iron [5,6] and in stainless steel [7] can produce non-crystalline phases. We

0000/$03.00 © 1983 North-Holland

III. NEW PHASES

250

K. Hohmuth et al. / Metalloid ion implantation in iron

Fig. 1. Transmission electron diffraction patterns obtained from a) 1×10 Is boron ions cm 2 implanted in iron, b) 1× 1017 phosphorus ions c m - 2 implanted in iron.

found amorphous surface layers in iron after implantation of B +, P+ and Si + [8,9]. Figs. la and b show electron diffraction patterns of iron implanted with 1 × 10 I~ B + / c m 2 (a) and 1 X l017 P + / c m 2 (b) at room temperature. The diffuse pattern systems are typical for the amorphous state. To get information on the atomic structure of the amorphous layers the scattering curve must be measured, i.e. the dependence of the scattering intensity on the scattering angle parameter (4~r sin 0 / h ) . Fig. 2 shows the scattering intensities of B +, Si + and P+ implanted iron. In table 1 the ascertained peak positions of the scattering parameter and their ratios are compared with some values found in the literature. The discussion of the scattering curves and the accepted theoretical concepts on the atomic structure of amorphous metal metalloid systems show that the structure of amorphous alloys produced by ion implantation is compatible with models of dense random packing of hard spheres (DRPHSmodel) [10] and the model of the random arrangement of trigonal prisms by Gaskell [11]. Not all implantation of metalloid ions result in an amorphous state (for example N ÷ ions in Fe, see sect. 3.3.). It is possible to make a prediction of the occurrence of amorphous metal-metalloid compounds developed on the embedment criterion for

the formation of crystalline metal-metalloid alloys by H~igg [12]. If the ratio of the radii of the covalent metalloid atoms and the metal atoms $1

Z k

~ Z s-i'.-Fe vO*-20

40

Fe 60

SCATTERING ANGLE PARAMETER [nm'lJ Fig. 2.Diffracted intensity as a function of the scattering angle parameter s (s = 4 7 r s i n 0 / ~ ) for polycrystalline iron films implanted with 50 keV silicon, phosphorus or boron ions.

K. Hohmuth et al. / Metalloid ion implantation in iron

251

Table 1 Peak positions and their ratios for iron after metalloid ion i m p l a n t a t i o n Implantation

Peak positions [nm

i]

Peak ratio

S1

S2

S3

$2/S

B + Fe P+ Fe Si + Fe

30.5 30.6 31.0

52.5 52.0 53.3

61.0

1.72 1.70 1.72

B÷ P+ B÷

30.0 30.7 30.2

54.0 52.2 50.7

-

Fe Fe st. steel

-

E L.j

Mo~,4t tnau li' to

::lF

~ ',VcU /,

¢r ,l~tollit~

0.12

.4,-

/o~+

$3/S

1

2.0 -

-

[8] [8] [5] [5] [7]

tion collected from the literature and our own investigations [8,9] confirm that no exception to this rule exists. This criterion based on structural considerations is completed by a criterion based on electronic base considerations [8]. Hafner [13] has shown that a higher negative enthalpy of formation has a great influence on the tendency for formation of glass. According to Miedema et al. [14] the enthalpy formation consists of a negative contribution A,~* corresponding to the chemical potential difference and a positive contribution Anw~ corresponding to the difference of the electron densities at the boundary of the Wigner Seitz cell. The enthalpy of formation is

As

In 0

o

Si

~p

0

J

/

O.fO

I

1.72 1.70 1.70

R > 0.59 a simple embedment of the metalloid atoms in the crystal lattice of metals is not possible. The occurrence of structural rearrangement processes would be necessary but these cannot take place in the short time of 10 10 to 10-~2 s (lifetime of the spikes). The amorphous structure initiated by implantation is "frozen". Because no stability for amorphous compounds was found above the radius ratio of R = 0.88, amorphous metal-metalloid alloys only exist in the interval 0.59 < R < 0.88 [8]. In fig. 3 known amorphous metal-metalloid alloys produced by ion implantacut~Co~

Ref.

/

where e is the elementary charge and P and Q are constants. Since for the existence of a compound A Hf = 0 must be at least given, it results from eq.

(1)

J

,q

7

Adp*/An,~ = ~ / Q / p e .

Q .,4

(2)

~B

"~ 0.08

+/

~C crystali~

AF/wt i •

J 0.06 i

0.70

In fig. 4 a large number of metal-metalloid combinations are represented dependent on A~* and

Q13 METAL

i

i

i

0.15 ,4 TOMIC R A D I U S

i

/177 [nm]

Fig. 3. The v a r i a t i o n of the ratio, R, between the radius of m e t a l l o i d a t o m s and the radius of metal atoms. S u m m a r y of a m o r p h o u s m e t a l - m e t a l l o i d alloys formed by high dose imp l a n t a t i o n e x p e r i m e n t s ( O a m o r p h o u s phases described in literature, + a m o r p h o u s phases o b t a i n e d in our experiments, crystalline phases o b t a i n e d in our experiments).

It appears that the following equation applies for all amorphous metal-metalloid compounds produced by ion implantation [3]. A~* < 0.75 A I/3 •

(3)

where A~* is in V and Anw,, in 6 × 10 22 electrons cm 3. By this criterion (consisting of a structural and an electronic aspect) it is possible to predict the formation of amorphous metal metalloid alloys after ion implantation. III. N E W P H A S E S

252

K. Hohmuth et al. / Metalloid ion implantation in iron Z~Fe

• ,, u 5.

_J

<

z 1.5

+

LU

FO Q_

8oride Phos{ hide %lLiclde NitFide Arsenlde (.urbide

.J

~M

+2t

~Hf

0 ti

1.0

°h~PHt ~W

,o ,,,~,

• t,

oT,

l~

uJ

T (O UO

× CJ

~Pe

+ T,

<

~Au --Tt Ozr*Zr oAu ~ ~1,4o

o~

+ N~

/

~

.~. I

+ ePf

• TO

'~

5V

*,n 1/3

B~

O/hop r os,B,

O~

I

OTo

e,,

0.5

g

"~'j "0' ~ ~] C , ' ~ ~'~?SV An 1/3.0x'5v °~] 1

t t

°co

" ~

1

!

.

~

!

.~] C3 0

DCu~/ @X,

0.6

2~

l

1.0

i

i

M°°I

-- "~I

12

1A

I

I

J

i

1.6

ly

[)/FH[-HUVCE OF t-LECTRON DENSITY AT THE 80(./NDARY OF THE W/GN{ R-.SEIT/ CEt wS

Fig. 4. The difference in chemical potential (or work function or electronegativity) versus difference of electron density at the boundary of the Wigner Seitz cell. The large squares agree with the amorphous metal-metalloid alloy described in literature a n d / o r obtained in our experiments.

3.2. Iron implanted with boron ions

Only few results are known on boron implantation into iron in the literature [5,8,9,15,16]. Two

examples of electron diffraction patterns and the corresponding photometer plots are shown in figs. 5 and 6 after implantation of iron with boron ions at different doses and after thermal treat-

(t10) - FeaB ~ 1

( 0 2 0 ) - Fez B

I i

(.o~-re --

--

(2fl) - Fe, B

- - - j - -

(310l -Fe~B (200}Fe

- -

lfO~ B*/cm ~

(220)-Fe

F~

E - 5O keV

X

\

T - 250 °C

i

i

i

i

i

20

~0

60

80

100

i

•

t20

SCATTERING ANGLE PARAMETER [nm4J

Fig. 5. Transmission electron diffraction pattern and corresponding photometer scan obtained from 1x implanted in iron and post annealed at 250°C.

1016 boron

ions c m 2

253

K. H o h m u t h et al. / Metalloid ion implantation in iron

( l f O } - Fe

~

(020)-Fe e3B

r

(201} - F% B (201}-

-

- -

(#22)-Fe~8

--

(200)-Fe

/1 - -

(213l -Fe, B

IJ~ I - ~

(322)-Fe

III

I I - -

, ~

~/

/1

( , , z o ) - Fe

f(330)-

/ - -

\1 h

- -

I #OrtB*/cm'~F~

( 3 ~ 5 ) - Fe, B

-

E- 50 ke V

Fe

~'(411)-Fe

(4,~5) -Fe3B

T - 4-00°C i

i

i

A

2O

~0

60

80

L

-

700

120

SCATTERING ANGLE P A R A M E T E R [rim'12

Fig. 6. Transmission electron diffraction pattern and corresponding photometer scan obtained from 1× 1017 boron ions cm 2 implanted in iron and post annealed at 400°C.

ment. Fig. 5 shows a crystalline phase as a result of a 1 × 1016 B + / c m 2 implantation. With this implantation the stable Fe2B compound was always observed in the investigated temperature range in addition to the c~-ferrite phase of iron. This crystal phase has a tetragonal body-centered Bravais lattice of the structure type CuAI 2 (a = 0.5109 nm, c = 0.4249 nm). The boron atoms are embedded in the tetragonal lattice of the iron atoms (embedding structure) and therefore they are relatively isolated from each other. This stable crystal phase is also formed after the thermal recrystallization process of the amorphous iron boride after implantation with doses greater than 3 t o 5 × 1016 B + / c m 2. The recrystallization process generally takes place via different metastable states (MS I, MS II . . . . ). For example an iron foil implanted with 1 × 10 ~v B + / c m 2 recrystallizes via the metastable phase F%B to Fe2B. Fig. 6 shows such an implanted layer after annealing to 400°C. The metastable iron boride Fe3B possesses an orthorhombic Bravais lattice of the structure type F%C (a = 0.5433 nm, b = 0.6656 nm, c - 0.4454 nm). At high implantation doses ( > 5 × 1017 B + / c m 2) and high temperatures ( > 300°C) an additional metastable crystalline phase (MS II) is inserted in the recrystallization process. According

to Herold and KOster [17] the chemical composition corresponds to Fe4B (probably F%3B6) and it has a face-centered cubic structure of the type Cr23C6 (a = 1.0670 nm). Depending on the implantation dose the metastable phase Fe4B is transformed in Fe3B at 4 0 0 . . . 5 5 0 ° C . For a detailed analysis of the crystalline iron boride phases see ref. 15. Recently, a spinodal decomposi-

500

amOrphou3

100

=

1016

2

5

amor phou3

7017

2

5

IMPLANTATION DOSE

1018 [ c m -2]

Fig. 7. Analyzed iron boride phases in dependence on the temperature and implantation dose (a-FeB amorphous iron boride). Ill. N E W PHASES

254

K. Hohmuth et al. / Metalloid ion implantation in iron

generally recrystallizes via the metastable FesB to Fe 2B. Only at very high doses a second metastable phase Fe4B is produced. Summarizing, the following transformations can be established: 1 × 1016-4 X 1016: Fe2B 4 X 1 0 1 6 - 7 . 5 X 1017:

a-FeB s°°-35°°~" FesB 350 450°c~~FezB 7 . 5 × 1 0 1 7 - 1 )< 1 0 i s :

/ 2°-35°°~"Fe 3B Fig. 8. HVEM-micrograph of iron irradiated with 5 × 1017 boron ions cm 2 and post annealed at 450°C.

tion was observed at implantation of approximately 1 × 10 Is B + / c m 2 [18]. Analysing the phase dependence on temperature and implantation dose it is possible to give a "state diagram" of the system i r o n - b o r o n after implantation (fig. 7). It shows that at low doses ( < 5 × 10 ~6 B + / c m 2) the stable crystalline Fe2B is produced. The direct connection to the dose range investigated by Takagi et al. [16] was observed. The amorphous phase appears at room temperature above implantation dose of 3 to 6 × 1016 B + / c m 2 [8,9]. This phase

a-FeB \320-350°C

Fe4B 40o 55°°c~_Fe3 B + Fe2B

a-FeB is the amorphous iron boride phase. Fig. 8 shows an example of the morphology of a boride phase. Implantation at high dose and temperature results in metastable Fe 3B. This phase appears in some cases with a plate-shaped bearing. 3. 3. Iron implanted with nitrogen ions The formation of nitrides on the surface of pure iron as a result of nitrogen implantation with high doses has not been as much investigated. M6ss-

----

(1tO) -- ~"

"%o;/--" F

--- f12o) - ~" -

WVI~

-

(oo0

I

V I

--

c2oo) I -(0'i3)-~

VI'i "~,111 'VIII

E.

50

~'

r~j - e r,2Jo -(222)

r,

'

-

key

r . 150"C

"~

|

!

i

i

I

i

2O

LO

60

aO

100

120

•

SCATTERING ANGLE PARAMETER [,~m-lJ

Fig, 9. Transmission electron diffraction pattern and corresponding photometer scan obtained from 1 x 10 TM nitrogen ions cm 2 implanted in iron and post annealed at 150°C.

255

K. Hohmuth et aL / Metalloid ion implantation in iron

500

300

~, ~ ~ "

lO0

7016

2

5

,

7017 2 5 70/8 IMPLANTATION DOSE [cm -2]

Fig. 10. Analyzed iron nitride phases in dependence on the temperature and implantation dose.

bauer electron backscattering was used to analyse iron nitrides in iron and steels [19-21]. According to Longworth and Hartley [19] nitrides begin to form at doses greater than 2 x 1017 N + / c m 2 which has been interpreted as y'-Fe4N. At 4 x 1017 N + / c m 2, in addition to y'-Fe4N, E-iron nitride compound also appears and at 6 x 1017 N + / c m 2 they decrease. Recently, we have used electron area diffraction to analyse surface layers of iron after nitrogen implantation [22]. In no case an amorphous phase was established. In fig. 9 an example is represented for an electron diffraction pattern after implantation of 1 × 1018 N + / c m 2 and a thermal post-treatment at 150°C. It appears that the metastable a"-Fel6N2 and the hexagonal close-packed ¢-Fe2N(1 ~) acts in addition to c(-martensite and in some cases to the y-austenite solid solution. With increase of temperature the martensite decomposes ( - 1 8 0 ° C ) and a"-Fe16N 2 dissolves (275°C). After this decomposition process ~,'-Fe4N can be found but this is relatively unstable at these high implantation doses and it can no longer be detected at 350°C. On the contrary y'-Fe4N could be found from 450 to 525°C at doses from 2.5 x 1016 to 5 x 1016 N + / c m 2. At high temperatures and high doses the known transformation of the hexagonal E-nitride into orthorhombic ep-nitride could be obtained. A detailed discussion of crystal phases is given in [22]. By means of a great number of these phases

Fig. 11. HVEM-micrograph of iron irradiated with 1 × 10 TM nitrogen ions cm 2 and post annealed at 250°C.

analyses the representation given in fig. 10 could be set up. Between the different phases in this "state diagram" junction regions are placed in which the phases exist side by side. Investigations on the morphology of nitrogen implanted iron show clear differences between the different phases. Fig. 11 shows a TEM-micrograph after implantation of 1 x 1018 N + / c m 2 in iron and heating to 250°C. The typical triangular habit of the c-nitride phase can be clearly recognized. Beside this the austenite solid solution and the Fej6N2-phase (dark regions) also exist. The narrow plates of y'-nitride can already be partially recognized. A thermal treatment for longer times ( > 10 min to 6 h) yields changes not only of the dimension of the precipitations but also of the existence range of the single phases [22].

3.4. Iron implanted with carbon ions The high dose implantation of carbon ions has the same importance for the modification of mechanical properties of iron and steel as nitrogen ion implantation. Also in the case of carbon ion implantation in iron a "state diagram" can be ascertained. In accordance with Pavlov et al. [23] and Vogel [24] the stable crystalline phase cementite Fe3C was shown to exist at higher temperatures. Martensite could also be found at lower temperatures as Vogel found [22]. Here the influence of the implantation temperature is worth discussing. Implantation of 5 x 1017 C + / c m 2 at room temperature produces a marten!11. NEW PHASES

256

K. Hohmuth et al. / Metalloid ion implantation in iron

Fig. 12. HVEM-micrograph of iron irradiated with 5 × 1017 carbon ions cm -2 at room temperature (a) and at 550°C (b).

site phase (fig. 12a). It appears to be nearly exclusively lath martensite. An implantation of 5 × 1017 C + / c m 2 at 550°C shows a completely different morphology (fig. 12b). Cementite is formed. This is the only equilibrium alloy in the F e - C system. This Fe3C phase has been formed at implantation temperatures of about 300°C. However, investigations show a great part of implanted carbon is at interstitial positions of iron at room temperature [25]. At higher temperatures the carbon is increasingly built in substitutionally into the iron lattice. In this case different metastable phases are produced between martensite and stable trigonal prisms of cementite, such as the Hfigg-carbide FesC 2 and the Eckstrom-Adcock carbide Fe7C 3 [22]. 3.5. General discussion

A penetrating ion leads to the formation of vacancies and displaces host atoms from their lattice positions. The complete system of displaced atoms, lattice atoms, vacancies and implanted atoms relax to a configuration of smaller energy if the implanted ion comes to rest. Therefore it is possible to achieve by implantation amorphous metal alloys, crystalline metastable phases and supersaturated solid solutions with a solute concentration greatly exceeding the equilibrium solid solubility [26]. Consequently ion implantation is basically a non-equilibrium process, i.e. the laws of equilibrium thermodynamics are not applicable. For example the Gibbs phase rule is violated, solid

solubility can be exceeded and precipitation phenomena may be observed which differ from those occurring under more conventional metallurgical conditions. A metastable phase is characterized by a relative minimum of the free energy when plotted versus a generalized structural coordinate describing the configuration of the atomic arrangement. The process of the transformation of metastable state to the equilibrium state, i.e. the relaxation process, is combined with a change of their configuration. The relaxation strongly depends on the ambient temperature and concentration of defects and implanted atoms. Change of configuration can be considered as spontaneous fluctuations of structure [27]. It is likely that this process occurs via a vacancy diffusion mechanism [26]. At higher temperatures precipitation can happen and one may find new metastable phases which are not present in the equilibrium phase diagram [28]. Many experiments have demonstrated the possibility of manufacturing metastable phases by implantation in metals [5,8,15,19,22]. For example the implantation of boron or nitrogen ions in iron shows metastable (amorphous or crystalline) phases. Already a small change of temperature leads to structural fluctuations and therefore to phase transformations. Generally the experiments show that the phase transformations turn out to be in the direction of a stable equilibrium phase, i.e. strive for the minimum of free energy. It is shown that the formation of a metastable phase strongly depends on the concentration of implanted ions and temperature. We can describe the formation of metastable alloys using the concept of a rapidly collapsing spike. Recently Carter [29] has proposed a shock wave model which the energy dissipation in the spike regards as an extension of a shock wave. Short pressure and temperature increase initiates among other effects the phase transformation. There is an analytical treatment of this idea based on the Hugoniot-Rankine equation in ref. [30]. The authors are grateful to Prof. H. Bethge and Dr. G. K~stner, Institut for Festk/Srperphysik und Elektronenmikroskopie Halle, for the possibility of carrying out the investigations on the high-voltage electron microscope and Dr. J. SchiSneich, J. Altmann and J. Schneider, Zentralinstitut for Kernforschung Rossendorf, for help in the implantation.

K. Hohmuth et al. / Metalloid ion implantation in iron

References [1] N.E.W. Hartley, in Treatise on materials science and technology, ed., J.K. Hirvonen, vol. 18 (Academic Press, New York 1980) p. 321. [2] R.A. Kant, J.K. Hirvonen, A.R. K n u d s o n and J.S. Wollam, Thin Solid Films 63 (1979) 27. [3] W.W. Hu, C.R. Clayton, H. Hermann, J.K. Hirvonen and R.A. Kant, Rad. Effects 49 (1980) 71. [4] A.H. AI-Saffar, V. Ashworth, W.A. Grant and R.P.M. Procter, Corros, Sci. 18 (1978) 687. [5] A. Ali, W.A. Grant and D.J. Grundy, Phil. Mag. B 37 (1978) 353. [6] P.J. Grundy, A. Ali, C.E. Christodoulides and W.A. Grant, Thin Solid Films 58 (1979) 253. [7] W.A. Grant, J.L. Whitton and J.S. Williams, Rad. Effects 49 (1980) 65. [8] B. Rauschenbach and K. Hohmuth, Phys. Stat. Sol. (a) 72 (1982) 667. [9] K. H o h m u t h and B. Rauschenbach, Proc. XIV. Conf. metals, Dresden (Akad. Wiss. DDR, 1981) p. 136. [10] D.E. Polk, Scripta Met. 4 (1970) 117 and Acta Met. 20 (1972) 485. [11] P.H. Gaskell, J. Non-Cryst. Solids 32 (1979) 207. [12] G. H~igg, Z. phys. Chem. B 12 (1931) 3. [13] J. Hafner, Phys. Rev. B 21 (1980) 406 and Proc. XIV. Conf. Metals, Dresden (Akad. Wiss. DDR, 1981) p. 235. [14] R. Miedema, R. Boom and F.R. DeBoer, J. Less Comm. Metals 41 (1975) 283.

257

[15] A. Kolitsch, B. Rauschenbach and E. Richter, submitted to Rad. Effects. [16] T. Takagi, I. Yamada and H. Kimura, Proc. IV. Int. Conf. on Ion implantation in semiconductors and other materials, Osaka (1974) (Plenum Press, New York, 1975). [17] U. Herold and U. K~ster, Z. Metallkde. 69 (1978) 326. [18] B. Rauschenbach, E. Richter and K. Hohmuth, Phys. Stat. Sol. (a) 65 (1981) K 103. [19] G. Longworth and N.E.W. Hartley, Thin Solid Films 48 (1978) 95. [20] G. Principi, P. Matteazzi, E. Ramous and G. Longworth, J. Mat. Sci. 15 (1980) 2665. [21] M. Carbucicchio, L. Bordoni and S. Tosto, J. Appl. Phys. 52 (1981) 4589. [22] B. Rauschenbach and A. Kolitsch, to be published. [23] P.V, Pavlov, E.I. Zorin, D.I. Tetelbaum, G.M. Ryzhkov and A.V. Pavlov, Phys. Stat. Sol. 19 (1973) 373. [24] F.L. Vogel, Thin Solid Films 27 (1975) 369. [25] L.C. Feldman, E.N. Kaufmann, J.M. Poate and W.M. Augustyniak in Ion implantation in semiconductors and other materials, ed., B.L. Cowder (Plenum Publ. Corp., New York, 1974) p. 491. [26] S.M. Myers, J. Vac. Sci. Technol. 15 (1978) 1650. [27] J.A. Borders, Ann. Rev. Mater. Sci. 9 (1979) 313. [28] E.N. K a u f m a n n and L. Buene, Nucl. Instr. and Meth. 182/183 (1981) 327. [29] G. Carter, Rad. Eff. Lett. 43 (1979) 193 and 50 (1980) 105. [30] B. Rauschenbach and K. Hohmuth, Phys. Star. Sol. (a) to be published in 75 (1983).

III. N E W PHASES