Nuclear Instruments and Methods 209/210 (1983) 281-287 281 North-Holland Publishing Company

ELECTRICAL CONDUCTIVITY M O D I F I C A T I O N IN IRON I M P L A N T E D MgO

A. PEREZ and J. BERT D~partement de Physique des Mat~riaux, Universit~ Claude Bernard, Lyon L 69622 Villeurbanne, France

G. MAREST Institut de Physique NuclOaire, Universitb Claude Bernard, Lyon L 69622 Villeurbanne, France

B. SAWICKA Institute of Nuclear Physics, 31342 Cracow, Poland

J. SAWICKI Jagiellonian University, 30059 Cracow, Poland

Electrical conductivity measurements of MgO single crystals implanted at room temperature with 100 keV iron ions and doses ranging from 1016 up to 1017 ions. c m - 2 exhibit an insulat ing-conducting transition located around 3 x 1016 ions. cm-2 . Above this critical dose, the conductivity increases rapidly to reach a value of - 100 ~2-I .cm i at 1017 ions-cm -2. The annealing behaviour at temperatures up to 600°C shows a decrease of conductivity and insulating property recovery. The comparison of these results with previous ones on defect characterization by optical absorption and implanted impurity characterization by conversion electron M6ssbauer spectroscopy permits a qualitative interpretation of the electrical conductivity modifications observed.

1. Introduction

Ion implantation as a tool to synthetize new materials or to modify the physical properties of existing materials is a subject of increasing inter- est. In the particular case of refractory oxides such as MgO, several studies of ion implantation effects with various ions in large dose and energy ranges have been performed [1,2]. These studies have allowed us to understand rather well the defect production mechanism which in such ionic oxides takes place in nuclear cascades [1]. There is a difference between ionic oxides and alkali halides in which defects can be produced by electronic excitation in the anionic sublattice [3]. Concerning the phase formation in MgO implanted with high dose (> 1016 ions. cm 2) of metallic ions, three cases have been distinguished [2]: (i) the alkali ions (Li, Na, K, Rb) which form metallic precipitates, (ii) indium and gold which form binary alloys with magnesium, and (iii) iron which forms oxides and a spinel ferrite. However, in the case of iron, if the final state obtained after oxidation by annealing in air is clear, the situation directly after implanta- tion is quite complicated. In this case, previous

studies using conversion electron M6ssbauer spec- troscopy (CEMS) with MgO crystals implanted with STFe+ ions allowed us to characterize the iron states in the implanted layer [4,5]. Depending on the ion dose, iron has been identified in three different states: iron aggregated in small super- paramagnetic metallic precipitates, Fe 2+ in a mag- nesio-wustite solid solution and Fe 3+. Preliminary electrical resistivity measurements with the four tips system commonly used in the case of semicon- ductors allowed us to detect a low resistivity ( - 100 I2. cm) in high dose implanted crystals (1017 ions. cm-2) . From those first results and after taking into account the CEMS results, the idea of a transition from the insulating state to a conducting state referred to changes in local iron concentra- tions in the implanted layer was considered. In order to study this effect, more precise measure- ments of the electrical conductivity using an alter- nating current technique [6] have been performed. The results are presented in this paper. First the study of the dose dependence of the conductivity and next the annealing behaviour up to 600°C are presented.

Other cases of electrical property modifications

0167-5087/83/0000-0000/$03.00 © 1983 North-Holland III. NEW PHASES

282 A. Perez et al. / Electrical conductivity in iron implanted MgO

of insulating materials after ion implantation have also been reported [7,8]. Kalish et al. [7] have shown a transition due to defect production in diamond implanted with a low dose ( - 1014 ion- cm -2) of Sb ions. Davenas et al. [8] observed a non-metal to metal (NMM) transition in LiF crystals implanted at 78 K with alkali ions (Na + and K+). In the case of iron implanted MgO crystals, great interest for the understanding of these phenomena lies in the stability of the system under study at room temperature and in the possi- bility of applying the MiSssbauer spectroscopy, which is a powerful technique for the characteriza- tion of the implanted ions (charge state, site loca- tion, precipitated phases).

2. Experimental procedure

2.1. Sample preparation

MgO samples were cleaved into plates of 20 × 10 × 0.5 mm 3 from a "Spicer" single block (purity 4N). Implantations were performed at room tem- perature with 56Fe+ ions using the 200 keV im- plantor of the "D6par tement de Physique de Mat6riaux". The beam energy was 100 keV and the current density was maintained rather low ( - 1 ~ A. cm-2 ) in order to restrict thermal effects during implantations. The ion beam was scanned to obtain a homogeneous implantation of the whole surface of the samples. The implanted ion doses were in the range from 1016 to 1017 ions • cm -2.

Rutherford backscattering analysis (RBS) using 1.6 MeV alpha-particles were performed after im- plantation in order to check the implanted iron profiles in the MgO crystals. From these measure- ments we deduced a mean range R p --~ 400 .~ and a full width at half maximum of the distribution (fwhm) on the order of 950 A. The experimental R p value corresponds well to the calculated one (Rp ~ 4 5 0 A) [9] but the width is significantly larger (fwhm calculated -- 340 A) [9].

Isochronal thermal annealings (30 min) of the implanted crystals were performed in air at tem- peratures of 300, 400, 500 and 600°C. After each annealing step the iron profiles were examined using the RBS technique. In fact no significant change of the iron distribution was observed under annealing up to the temperature of 600°C. A broadening of the iron profile was noticed only for

annealings at temperatures above 700°C. Optical absorption measurements were per-

formed in the wavelength range of 200-1000 nm, using a Cary 17 double beam spectrophotometer, for the characterization of point defects created in the oxygen sublattice (F, F + centers, band at 250 nm) and in the magnesium sublattice (V centers, band at 540 nm).

2.2. Electrical conductivity measurements

Two silver electrodes were deposited in vacuum on the implanted face of the MgO crystals. The distance between these two electrodes was 6 ram. The electrical contact was ensured by two silver plates maintained on the evaporated electrodes with a convenient pressure. The whole system was mounted in a metallic box for electrical shielding and connected to a "1621 General Radio" capaci- tance bridge. Conductivity measurements were performed at RT in the 10 Hz to 100 kHz frequency range. A very low voltage was applied (20 mV peak to peak) both in order to avoid any electro- chemical decomposition of the sample material and to make the measurements of the impedance in a good linearity conditions. A synchronous de- tection system and a lock-in amplifier were associ- ated with the bridge to ensure low noise measure- ments. The parasitic capacitances and resistances due to wires, contacts, etc. have been taken into account in our experimental data evaluation.

3. Results

3.1. Dose dependence of the electrical conductivity

The admittances of MgO crystals implanted at room temperature with 100 keV iron ions and doses of 1, 2, 3, 4, 5, 6 • 1016 and 1017 ions. cm -2 have been measured in the 10 Hz-100 kHz frequency range. Some examples of the admittance diagrams obtained with these crystals are shown in fig. 1. They consist of two parts: (i) In the low frequency region ( - 10 Hz to 6 kHz) the experimental points are displayed on a curve close to a part of a circle, the center of which is located under the real axis. This part is due to the interfacial relaxation at the contact silver electrode implanted MgO surface [6]. In the case of a perfect series R.C. circuit the diagram should be a circle

A. Perez et al. / Electrical conductivity in iron implanted MgO 283

O A

~ 20 i B •

~, ~ ~ c

"-- ¢~ 10

sO ~ ,/%.6

FJ

/ • 3 0 /

/

/ •20

/ /

~ ' = 1 0

\ \ \

\ \

\ \ '\ \

\ \

3 O

real part of admittance (G, lO9~ -1)

Fig. 1. Some examples of the admittance diagrams obtained for MgO crystals implanted at room temperature with 100 keV iron ions and doses: 2 x 1016 ions-era -2 (A), 4 × 10 ~6 (B) and 6 × 1016 (C). On this last curve, some frequencies are indicated in kHz and the "pseudo circle" which fits the low frequency part of the curve is represented by the broken curve.

centered on the real axis. For our implanted crystal, a Gauss•an distribution of relaxation times could be the source of our curve's differing from a perfect case [10]. For iron implanted MgO, the

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10- 8

10- 9

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10-3 ~ c 0

10 -4

10-5

10-6

1016 L I , I , I i I ~ I

2 4 6 8 10

ion dose ( i o n s . c m -2)

Fig. 2. Conductance and conductivity of implanted MgO crystals as a function of iron ion doses.

'7 A

S

¢-.

-g ,-i

i I

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o • e e • e

me• • • e m i l ~) D

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~0 0 g o

• • • o

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i i i i i , , i i i i . . . . . . i , , , , , , , , 10 2 10 3 10 4 10 5

f r e q u e n c y ( H e r t z )

Fig. 3. log- log plot of the conductance as a function of frequency for MgO crystals implanted at room temperature with 100 keV iron ions and following doses: l016 ions .cm -2 (A), 2 x 1016 (B), 3 × 1016 (C), 4 X 1016 (D), 5 X 1016 ( E ) , 6 x 1016 (F) and l017 (G).

silver electrode is a blocking electrode and this explains why the diagrams start from a nearly zero admittance value at zero frequency (DC). In the case of non-blocking electrodes (magnesium or oxygen) the low frequency part should not exist. (ii) In the high frequency region ( > 10 kHz), the admittance plot becomes an almost vertical line. This part which is typical of a constant parallel resistance and capacitance circuit can be attri- buted to the implanted zone of the MgO crystal. The implanted zone conductance can be de- termined by the intersection of the "pseudo-circle" with the real axis (fig. 1). A non-linear least square fit of the low frequency part of the diagram makes possible the determination of the value of the conductance with good precision.

Fig. 2 presents the conductances of the MgO crystals as a function of implanted ion doses. For doses higher than 3 × 1016 ions • cm -2 an abrupt

Ill . NEW PHASES

284 A. Perez et al. / Electrical conductivity in iron implanted MgO

increase of the conductance is observed. This in- crease is very high with the conductance changing about four orders of magnitude for doses between 3 × 1016 and 1017 ions .cm -2. If we take into

account the sample geometry in our electrical mea- surements and the thickness of the implanted zone ( - 1000 A), we can deduce the electrical conduc- tivity of the implanted zone. The values are also indicated in fig. 2. As seen, the conductivity in- creases from 5 × 10 6 to 10-2 ~2-1.cm ~ in the dose range 3.1016 to 1 × 10 Iv ions. cm 2. Another illustration of the electrical conductivity variation is shown in fig. 3 in which we have plotted the conductance versus frequency on a log-log scale, for all the studied implanted crystals. Three differ- ent regions are clearly seen in this figure: low conductivity domain for 1, 2 and 3 × 1016 ions. cm -2, high conductivity domain for 6 × 1016 and 1 × 10 ~7 ions • cm 2 and an intermediate range for doses 4 and 5 × 1016 ions- cm -2.

4. Thermal annealing behaviour

For the study of the annealing behaviour of the electrical conductivity three samples were chosen,

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O

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I I I 10 2 103 10 4 105

frequency (Hertz) Fig. 4. Log-log plot of the conductance as a function of frequency for the MgO crystal implanted at room temperature with a dose of 6× 1016 iron ions-era -2 (1) and subsequently annealed in air for 30 min at 300°C (2), 400°C (3), 500°C (4) and 600°C (5).

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C,~ 10_7 O~

t . -

8 10 -8

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F

o\ ° ~

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/B

10-111 - I I I I a I I

0 200 4 0 0 600

annealing temperature (°C) Fig. 5. Evolution of the conductance as a function of the annealing temperature for MgO crystals implanted with 100 keV iron ions and with doses of 2×1016 ions-cm -2 (B), 4 x 1016 (D) and 6 x 1016 (F). A, C, E and G are the conduc- tances of the non-annealed samples implanted respectively with 1016ions-cm 2 3×10n6 5 x l 0 1 6 a n d 10 Iv.

namely those implanted with doses of 2, 4 and 6 × 1016 ions-cm -2. The three samples are located in three different characteristic domains discussed before that are visible in figs. 2 and 3. Isochronal annealings (30 min) were performed in air at tem- peratures of 300, 400, 500 and 600°C. Before each annealing step the deposited silver electrodes were carefully removed in order to avoid any diffusion of silver into the implanted zone; new silver de- positions were then performed after each anneal- ing. In fig. 4 are shown the log-log plots of the conductance versus frequency for the 6 × 1016 ions • c m - 2 implanted sample after consecutive anneal- ing steps. A decrease of the conductance in the high frequency range is clearly seen from 300°C up to 600°C. As mentioned in subsect. 3.1 this high frequency region of conductance is related to the characteristics of the implanted zone. Thus the main annealing effect consists in recovering the insulating properties. In contrast, the low frequency

A. Perez et al. / Electrical conductivity in iron implanted MgO 285

conductance, which is connected with interracial effects, does not evolve between 300 and 500°C. The curve after annealing at 600°C shows a marked decrease. Fig. 5 shows the evolution of the conduc- tance deduced from the admittance diagrams, for the three studied samples, as a function of the annealing temperature. For the low dose im- planted crystal (2 x 1016 ions- era-z) , the conduc- tance decreases slightly with temperature; but for the 4 x 1016 and 6 x 1016 i o n s . c m -2 implanted crystals a similar decrease by a factor of fifteen is observed up to 500°C. However, between 500 and 600°C the decrease is much more significant for 6 x 1016 i ons - cm 2 implanted crystal compared with the 4 X 1016 ions. cm -2 implanted one. In conclusion, for samples in all three dose ranges the annealing effects cause the crystals to gradually return to the insulating state, but the kinetics of this evolution depend on the dose range.

5. Discussion and conclusions

The specific effect of iron ion implantation induced modifications of the electrical conductiv- ity in insulating materials such as refractory oxide is an interesting example of the application of ion implantation to material science. This implanted system, stable at room temperature, exhibits a dose-dependent electrical conductivity. It is thus possible to produce implanted MgO crystals with a conductivity, whose value can be well adjusted in a wide range ( - 1 0 -6 to 10 2 ~'~-1 . c m -1 ) . In order to interpret this implantation effect we must point out that this phenomenon takes place in a high dose implantation domain ( > l016 ions. cm -2) with a threshold above which the conduc- tivity increases very rapidly (fig. 2). This threshold, in the case of 100 keV iron ions, corresponds to an ion dose of 3 x 1016 i o n s . c m -2. Taking into account the implanted iron profile determined by RBS measurements (subsect. 2.1), we can deduce that the maximum local concentration of iron in the implanted zone corresponding to the threshold is about 6%. Such a concentration is too small to explain a transition due to a matrix inversion as observed in the case of composite thin layers pre- pared by co-pulverisation of a metal and an in- sulating material. In this case the metal concentra- tion must be of the order of 50% [11]. The species which can be considered in our case of F e / M g O

implanted system are: the defects created with high concentrations in the anionic and cationic sublattices and the implanted impurities in the form of isolated ions or precipitated phases.

Concerning the defects created in MgO by 100 keV iron ions, previous studies using optical absorption [2] have shown that the point defect production increases up to 3 to 4 x 1016 ions. cm-2 , and saturates and decreases at higher doses. At the maximum, the concentrations of defects such as F-type centers and V centers are about 1.5 X 1021 cm -3 (-- 3%) and 2.5 x 1020 cm -3 (--

0.5%), respectively. In addition to point defects, large numbers of extended defects are also present. The transmission electron microscopy observa- tions [12] show an extensive dislocation network in the implanted area and other defects such as inter- stitial loops in which displaced magnesium and oxygen ions are trapped. By considering the de- fects only it would be difficult to explain the electrical conductivity behaviour. However, since the high dose implanted zone consists of the super- position of the defect distribution with the im- planted impurity one, the mechanism involving the implanted impurities plus the defects must cer- tainly be considered. For this purpose a good characterization of the implanted ions is necessary (charge states, site locations, precipitated phases). In the case of iron implanted MgO, recent studies using conversion electron M6ssbauer spectroscopy (CEMS) [4,5] have allowed this characterization. In MgO crystals implanted at RT with 57Fe+ ions in the same dose range (1016 to 8 X 1016 ions. cm -2) and energy (70 and 100 keV), iron has been observed in three different states: superpara- magnetic metallic precipitates ( - 20 ,~ in diame- ter) [4], Fe 2+ in a magnesiowustiste solid solution and Fe 3+ [5]. With increasing ion doses from 1016 up to 8 x 1016. cm 2 the local concentration of both Fe 2+ and metallic iron grew while the con- centration of Fe 3+ remained nearly constant. For the critical dose of 3 X 1016 i ons - cm -2 at which the electrical conductivity transition is observed the CEMS spectrum is composed of about 84% Fe z+, 10% metallic precipitates and 6% Fe 3+. The low Fe 3+ concentration which does not evolve in the considered dose range cannot play a role in the electrical property evolution. These ions are thought to be associated with defects for charge compensation. As for the metallic iron component, if one assumes the precipitates uniformly dis-

III. NEW PHASES

286 A. Perez et al. / Electrical conductivity in iron implanted MgO

lO

>,j 10-2;

° _

~ I " ~ 10-~ U

10-4

10-5

10-6 3

ion dose (x 1016 ions.crn -2)

6 5 4 3 i

~o \

\

\ 4x10 -6 315

(Fe 2~ fraction)-l/3~

Fig. 6. Evolution of the conductivity as a function of (Fe 2+ fraction)- I/3 for MgO crystals implanted at room temperature with 100 keV iron ions and doses ranging from 3 x 1016 up to 1017 ions .cm -2.

tributed in the implanted volume, we can estimate a mean distance between them of the order of 150 ,~ in the 3 × 1016 ions-cm -2 implanted crystal. This distance decreases down to 100 A for a higher dose (8 x 1016 ions. cm 2). Such a distance seems too large to allow for a tunneling effect between the metallic precipitates in order to explain the conductivity measurements. In addition, MgO or LiF crystals implanted in the same dose range ( > 1016 ions .cm -2) with alkali ions, in which metallic precipitates are formed, do not exhibit any increase of the conductivity. Therefore, we have to consider the large iron fraction present in the implanted layer in the form of a magnesio- wustite solid solution. These iron ions can act as hopping centers. The mean distance between two such centers determined from the local concentra- tion of iron in this fraction, for the transition dose 3 X 1016 ions. cm 2 is of the order of 8.6 A. This distance is only two times as high as the MgO

lattice parameter (a = 4.213 A). In addition, in the case of a h,opping process, the logarithm of the conductivity (log o) must be proportional to (-N-1/3), N being the concentration of centers [8]. Using the dose dependence of the Fe 2+ frac- tion deduced from our previous M6ssbauer mea- surements [5], we have plotted in fig. 6 log ~ as a function of (Fe z+ fraction)-1/3. One can see that the increase in the conductivity exhibits a rather well-defined linear dependence on this scale. How- ever, at the present stage of our study it would be difficult to state whether conductivity is due to the magnesio-wustite solid solution only or if metallic precipitates embedded in this medium play a role.

The annealing behaviour of the conductivity indicates that the species responsible for this con- ductivity evolve at low temperature. A decrease of the conductance by a factor of 5 is observed after the first annealing step at 300°C (fig. 5). From the optical absorption and channeling studies [4,5,13] it is known that the defects in MgO are stable at temperatures higher than 300°C. On the other hand, the CEMS studies [4,5] indicate that the main effect of annealing in air is the oxidation of iron. The Fe 3+ component increases while the metallic iron and Fe 2+ components decrease. In the temperature range up to 500°C the decrease of these three components is low. Above 500°C a more profound decrease is observed and at 700°C these components have nearly completely disap- peared in the M6ssbauer spectra. These evolutions can be compared to that of electrical conductivity and we can assume that the oxidation and precipi- tation mechanisms which occur in the samples annealed in air are responsible for the insulating property recovery. For a more complete interpre- tation of the electrical properties of iron implanted MgO some complementary investigations are nec- essary. In particular, photoconductivity measure- ments and temperature dependence of the conduc- tivity would be very helpful. This will be the second stage of our study.

References

[11 B.D. Evans, J. Comas and P.R. Malmberg, Phys. Rev. B6 (1972) 2453.

[2] A. Perez, M. Treilleux, P. Thevenard, G. Abouchacra, G. Marest, L. Fritsch and J. Serughetti, in: Metastable materials formation by ion implantation, eds., S.T. Picraux and W.J. Choyke (North-Holland, Amsterdam, 1982) p. 159.

A. Perez et al. / Electrical conductivity in iron implanted MgO 287

[3] A. Perez, J. Davenas and C.H.S. Dupuy, Nucl. Instr. and Meth. 132 (1976) 219.

[4] A. Perez, J.P. Dupin, O. Massenet, G. Marest and P. Bussiere, Rad. Effects 52 (1980) 127.

[5] G. Marest, A. Perez, B.D. Sawicka, J.A. Sawicki and T. Tyliszczak, to be published.

[6] J. Bert, Thesis, University of Lyon (1978). [7] R. Kalish, T. Bernstein, B. Shapiro and A. Talmi, Rad.

Effects 52 (1980) 153. [8] J. Davenas and C.H.S. Dupuy, Nucl. Instr. and Meth.

182/183 (1981) 753.

[9] G~ Dearnaley, J.H. Freeman, R.S. Nelson and J. Stephen, in: Ion implantation, (North-Holland, Amsterdam, 1973).

[10] K.S. Cole and R.H. Cole, J. Chem. Phys. 9 (1941) 341. [11] R.W. Cohen, G.D. Cody, M.D. Coutts and B. Abeles,

Phys. Rev. B8 (1973) 3689. [12] M. Treilleux, Journal de Microscopie et de Spectroscopie

Electroniques 6 (1981) 399~ [13] A. Perez, M. Treilleux, L. Fritsch and G. Marest, Nucl.

Instr. and Meth. 182/183 (1981) 747.

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