Mössbauer effect isomer shift of Fe57 in silicon and germanium

J. Phys.

Chem. Solids

Pergamon Press 1962. Vol. 23, pp. 1111-1118.

MOSSBAUER

EFFECT

SILICON

AND

P. C. NOREMt Bell Telephone

ISOMER

Laboratories, (Received

Printed in Great Britain.

SHIFT OF

Fes7 IN

GERMANIUM*

and G. IL WERTHJZIM Incorporated, 22 January

Murray Hill, New Jersey 1962)

Abetract-The Miissbauer spectra of Fes7 in n- and p-type silicon and germanium have been studied. In silicon, iron produced by the decay of cobalt has a dominant, unsplit absorption at -0.0012 f OX@03 cm/set doppler velocity relative to potassium ferrocyanide. No difference in isomer shift has been observed-between rr- and p-type silicon, indicating that the iron tested was electricallv inactive. The snectrum of substitutional Fe57 has been identified with an absorption at +0*054 i 0.004 cm/set showing less than 3 Mc/sec electric quadrupole splitting. The spectrum of iron produced by the decay of cobalt in germanium is a doublet with an isomer shift of + 0.036 f 0.001 cm/set and with a quadrupole splitting of 5 Mc/sec. It is assumed that this splitting is due to the asymmetrical positions of the Fes and Fel- atoms in the germanium lattice. No differences in isomer shift or quadrupole splitting were observed between iron in rz- and $-type germanium, indicating that the iron is electrically inactive.

INTRODUCTION

RECENT studies of the Mijssbauer effect(l) of the 14*4-keV gamma ray of Fe57(s) in various chemical environments have shown that there is a small shift in emission energy, the chemical(s) or isomer@) shift, which measures the density of the electronic wave functions at the nucleus. This shift has been shown to be a sensitive function of the configuration of the outer 3d and 4s electrons. The question naturally arises whether this effect is sufficiently sensitive to detect the difference in electron configuration between atoms situated in strongly doped n-type and those in strongly doped p-type semiconductors. Interstitial iron in silicon has been shown to possess a donor level near the center of the forbidden band.(s) In heavily doped n-type silicon one would therefore expect to find the iron in a neutral state, while in heavily doped p-type one would expect to find it in a 1+ charge state. For a Mossbauer experiment it would be most advantageous to use Fesv-doped silicon as the * This work was supported in part by the Wright Air Development Division of the U.S. Air Force. t Now at the University of Chicago,

Chicago,

Illinois. 1111

absorber for the 14*4-keV gamma rays produced by the decay of Co57 in a metallic source. The transmission through the silicon absorber would be measured as a function of the relative velocities of source and absorber. In order to perform this experiment, an absorber with an area1 density of Fe57 of about 101s atoms/cm2 is required. Since the maximum solubility of iron in silicon is 101s atoms/ems(5) it is clear that the absorber would be too thick to be useful. As a result one is forced to perform the experiment by studying gamma rays from iron produced by the decay of Co57 in a silicon crystal. This complicates the interpretation of result because the electrical properties of iron and cobalt in silicon are quite different although they are similar in most other respects. Both have solid solubilities near 101s cm-s, diffuse rapidly, and are thought to be interstitial diffusers,(s) but while iron may remain interstitial and exhibit donor action on quenching, cobalt always remains inactive. The electrical activity of iron has been shown to depend critically on the quenching rate. Electrically active iron is produced only by a rapid quench. This iron acts as a donor which may exhibit either of two levels depending on ageing.

1112

P. C. NOREM

and

It is not known whether the electron-capture decay of inactive cobalt will lead to the electrically active form of iron or to an inactive one. A further complication arises from the effect of the electron-capture decay on the electronic shell of the atom. Some recent work(7) on the effects of the electron-capture decay of Co57 on the charge state of the Fe57 atom makes it clear that the effects of the decay may be substantial. The iron atom is created with an empty 1s orbital and in the process of de-excitation may lose electrons by Auger effect. The time-constant for recapture of these outer electrons, in the present case, can be estimated from 7 = (ma,n)-1 where 12s is the electron density in electrons/ems, en is the electron capture cross section in ems, and v is the thermal velocity of electrons. The capture cross section for Fel+ in silicon is estimated to be lo-14 cma.@) The recapture time is then approximately lo-11 set in heavily doped n-type silicon, indicating that charge equilibrium is reached before emission of the Mijssbauer gamma ray. The behavior of transition metal ions in germanium is quite different. The electrically active ions which are produced when germanium is quenched from high temperature are substitutional.@) Iron has been shown to possess two acceptor levels, one located 0.34 eV above the valence band and the other 0.27 eV below the conduction band.(ls) These levels correspond to 0, l-, and 2charge states of the iron atom. The electrical behavior of cobalt here is quite similar to that of iron. It is also a double acceptor with energy levels located not far from those of iron. The decay of cobalt will therefore produce an iron atom which should not differ from iron introduced directly. (The recoils associated with the neutrino of the electron-capture process or with the first gamma in Fe57 are insufficient to displace the iron atom.) The possibility does exist here, however, that the iron atom will not reach its thermal equilibrium charge state before emission of the M6ssbauer gamma ray. The neutral iron atom has an electron capture cross section of 10-15 cm2,(12) and, by the formula previously given, would have a lifetime of about lo-10 set in 0.01 n-type germanium. The electron capture cross section for singly negative iron is probably less than 10-1s ems and the lifetime is then greater than that of

G. K. WERTHEIM the first excited nuclear state of Fes7. Thus there is a possibility that the 2- charge state of iron in germanium is not formed, though the land 0 states should both be produced. The behavior of donor electrons has been quite thoroughly studied only for cases in which the potential well is shallow enough to produce an electron wave function large compared to the lattice constant. For deep impurities such as iron or cobalt the electron is quite localized and may be expected to have a significant effect on the isomer shift, but in this case the potential function is sufficiently complicated to preclude calculations of wave functions. It is not possible, therefore, to obtain a reliable estimate of the magnitude expected for the isomer shift for the various charge states in silicon or germanium. Such effects are, however, more likely to be measurable in silicon than in germanium, since the electron given up by iron in silicon has 3d character while the electrons added to iron in germanium would probably be of 4p character. The latter should produce an insignificant change in the charge density at the nucleus and therefore may not be readily observable in the Mossbauer experiment. EXPERIMENTAL The samples of silicon were prepared by reducing cobalt chloride at 500°C on the surface of vacuum floating zone refined silicon and diffising the Co into the crystal in a hydrogen atmosphere, first at 900°C for 15 min and then at 1250°C for 1 hr. The surface of the silicon was prepared by grinding with emery and Carborundum and lapping with AlsOs abrasive. Some samples were also etched in CP4 before diffusion. The germanium samples were prepared by reduction of CoCla in hydrogen, followed by diffusion at 800°C for 1 hr. The surfaces of some crystals were etched in HF after diffusion, but this had no noticeable effect on line shape. The simultaneous diffusion of copper and cobalt was made by drying, reducing in Hs, and diffusing at 1250°C a solution of COW& and CuSO4 placed on the surface. Two millionths of an inch of copper was also evaporated on one previously cobalt-diffused sample which was subsequently diffused at 1250°C. The radioactive samples were placed in a copper

MC)SSBAUER

EFFECT

ISOMER

SHIFT

OF

Fe57 IN

SILICON

1.

A spectrum characteristic of Fe57 in silicon is shown in Fig. 1. The primary absorption located near zero doppler velocity is dominant in all samples of silicon regardless of type. No shift greater than 0.0003 cm/set has been observed between 0.01 Q-cm n- and 0.16 !&cm p-type samples at room temperature. Data taken at 78” and 4°K differ only with respect to the exact hyperfine position of the line. No magnetic structure was observed or expected since these

0.01

I

I -0.04

I -0.02 VELOCITY,

1113

Silicon

n-TYPE CLEANLY

I I -O*lO-0.08-0.06

GERMANIUM

RESULTS AND DISCUSSION

sample holder which was screwed to the bottom of a vacuum dewar. This arrangement permitted the sample to be cooled to 78°K (with liquid nitrogen) and to 4°K (with liquid helium). Data were taken in transmission, using an absorber made of isotopically enriched potassium ferrocyanide and equipment previously described.(ls) The absorber was moved by a dual voice coil loudspeaker driven to perform double parabolic motion at 3.5 cps. The 14*4-keV gamma rays were detected by a scintillation counter spectrometer set for the interval from 10 to 20 keV.

30 -0.12

AND

0

0.02

I 0.04

I 0.06

L?-CM SILICON PREPARED

I 0.08

I 0.10

I 12

CM/SEC

FIG. 1. Resonant absorption of the 14*4-keV gamma ray of Fe5’ produced by the decay of Co57 in 0.01 &cm n-type silicon (clean preparations at 1250°C).

The output pulses of the single-channel analyzer of the spectrometer were modulated by the voltage from the velocity sensing coil of the loudspeaker to produce pulses with amplitude proportional to the instantaneous velocity of the absorber. These pulses were then fed into a 256channel analyzer to produce the velocity spectrum. The more detailed isomer shift data on Fe57 in silicon were taken with a machine in which the absorber is moved at constant velocity backward and forward over a fixed distance but at variable frequencies. Pulses were fed into separate counters for positive and negative velocities and printed out on tape after a preset time.

experiments are done without an applied magnetic field so that the plus and minus magnetic substates remain degenerate. The principal difficulty in the determination of the exact position of the line arises from the distorting effect of the second absorption at O-054 + 0.004 cm/set. This is apparent in the analysis of curve shapes in Fig. 2 made by subtracting a symmetrical absorption from the observed spectrum. The second absorption has a tail with a significant amplitude and slope at -0.001 cm/set which is capable of shifting the apparent center of the line. The most reliable value, -0*0012 + 0.003 cm/set, was obtained for the isomer shift

1114

P.

42

C.

NOREM

and

G.

K.

WERTHEIM

x I03

40 iii 5 38 I’ $36 5 34 s

n - TYPE SILICON DIRTILY PREPARED 32 I -0.10

30 I -0.12

I I I -0~08-0~08-0~04-0~02

I

I 0

VELOCITY,

I 0.02

I 0.08

I 0.10

CM/SEC

FIG. 2. Same as Fig. 1 but “dirty”

i:

I 0.08

I 0.04

preparation.

0.16 P-TYPE

30 -

Ji? -CM SILICON

E 1

r 5 s

I

29

22

x1o4

21

-

20

-

19 ‘ -GO20

I

I

I

I

I

I

n - TYPE I -0.016

-0.012

-0.008-0.004 VELOCITY,

0

0.004

WO8

0012

I

SILICON

OOl6

01

CM/SEC

Frc. 3. Comparison of the isomer shift in n-type and p-type silicon of low resistivity.

I 0.12

MC)SSBAUER

EFFECT

ISOMER

SHIFT

OF Fe57 IN SILICON

from measurements made on samples with nearly symmetric curves, Fig. 3, using the constant velocity Miissbauer equipment. Neutral interstitial iron in silicon has an outer electronic structure of 3&3.(14) The isomer shift expected for this configuration is 0.17 cm/set.(4) The observed isomer shift is much more consistent with the configuration of metallic iron. This discrepancy between the isomer shift and the known electronic configuration may indicate a considerable distortion of the 3d electron wave functions or, more likely, the presence of a different type of iron in this experiment.

42

d

40

5 I

38

< F 5 8

AND

GERMANIUM

1115

that this type of iron is produced by the decay of electrically inactive Co57. The second line at O-054 + 0.004 cm/set is of variable intensity, but is most prominent in “dirty” samples, Fig. 2. It is enhanced by the simultaneous diffusion of copper and cobalt, Fig. 4, a process known to force other transition metal atoms into substitutional sites,(la) indicating that it is due to substitutional cobalt. The two samples which were prepared with large amounts of copper both displayed prominent second absorptions. The one in which CuSO4 was used showed asecond absorption as large as the primary absorption. The second

x 103

36 O*OOl P-TYPE 34

32 1 -0.12

O-CM SILICON

DOPED WITH I -0.10

I 0.08

I -0.06

I 0.04

I 0.02

I 0

VELOCITY,

I o-02

I 004

I 0.06

I 13.08

I 0.10

COPPER I 0.12

0.14

CM/SEC

FIG. 4. Same as Fig. 1 but prepared with copper.

The absence of a measurable isomer shift between iron in n- and in p-type silicon and the absence of hyperfine structure at low temperature also favor the second interpretation. The expected difference in energy between n-type and p-type is large, since the neutral atom has structure 3d* and the singly ionized atom has structure 3d7. Assuming no distortion of the 3d wave function there should be an isomer shift of -0.03 cm/set between n- and p-type silicon. (On the assumption of 3d74s structure for the neutral atom an isomer shift of O-16 cm/set would be expected.) These shifts are quite large and would be easily measurable. Their absence indicates that the expected charge states of iron have not been produced and suggest that the iron is electrically inactive. It is known that such iron can be produced by insufficiently fast quenching and it is quite likely

line is also enhanced in samples diffused near 9OO”C, ‘Fig. 5, suggesting that there is a temperature-dependent equilibrium between interstitial and substitutional cobalt in silicon. It is clear that this second absorption is not due to precipitation of metallic cobalt, since a 0.03 in. silicon sample, prepared in contact with a O-1 mil electrolytically refined cobalt foil, did not show an enhanced second absorption. Transition metal ions can also be forced into substitutional sites by electron bombardment.(l5) However, 101s 1 MeV electrons per cm2 produced no change in a 1200 L&n-r p-type high purity vacuum floating zone silicon sample, nor was any change apparent in a 0.01 n-type sample bombarded with 2 x 1017 electrons/cm at room temperature. It is likely that this disparity between vacancies created by high-energy electrons and by

1116

P. C. NOREM

and

precipitating copper is due to a difference in the state of aggregation of the cobalt at the temperature at which the vacancies are produced, i.e., it is possible that while vacancies can convert isolated interstitial atoms into substitutional ones, they are not able to convert precipitated atoms, which probably are the dominant species at room temperature. Additional evidence that the line at 0.054 cm/set is due to substitutional iron lies in the similarity of the isomer shift to that of iron in germanium

36

G.

K. WERTHEIM

bulk of the material.(ls) Removal of the high concentration surface layer had only a very small effect on the spectrum of the sample, indicating that while the cobalt may diffuse by two different mechanisms, the lattice sites occupied after quenching from high temperature are similar in both cases. 2. Germanium The spectrum of Fe57 in germanium, Fig. 6, shows an isomer shift of 0.036 + 0.001 cmisec and

x 103

3 $ 33 5 <

32

r 5 31 s

0~01 n-TYPE DIFFUSED

30

28 -012

-010

-0*08-0*06-DO4

-0.02

0

VELOCITY,

0.02

004

006

0.08

J2-CM SILICON AT 900%

0.10

042

C 4

CM/SEC

FIG. 5. Same as Fig. 1 but diffused at 900°C.

where the electrically active species is believed to an electric quadrupole splitting of 5 Mcisec. No be substitutional.@) The spectrum of substitu- variation of line shape or position greater than the tional iron in silicon was never sufficiently well limits of error was found between 0.02 I- and resolved to obtain a reliable measure of the 0.02 p-type pulled germanium. The absence of a quadrupole splitting. However, it is clear that the shift is consistent with the assumption that the substitutional absorption in silicon is split by not extra electrons added to the neutral iron atom have predominantly 4p character but an alternate more than 3 Mcisec. A determination of the distribution of the Co57 explanation is given below. in a sample O-028 in. thick, diffused for 1 hr at The electric quadrupole interactions observed 125O”C, indicated that fifty per cent of the activity for substitutional iron are not diflicult to explain is contained in a surface film less than O-3 mil even though the germanium lattice is cubic and deep, but that more than forty per cent of the the lattice point symmetry is Z3m. In order for activity had diffised deeper than 1.5 mils into the quadrupole splitting to occur, it is sufficient for

Mt)SSBAUER

EFFECT

ISOMER

SHIFT

OF

the iron atom to be slightly displaced from the lattice site. The structure of the incomplete bonding between Fe0 and Fel- and the germanium probably takes the form of sp and spz respectively, which could serve to produce the requisite shift in a manner similar to that found for Nil-.(lv) This does not explain, however, why the quadrupole splitting is the same in n- and p-type germanium of low resistivity. It is also significant that no change was noted in the Mossbauer spectrum of iron in germanium after 6 days of vacuum annealing at 500°C. This

Fe57 IN

SILICON

AND

GERMANIUM

1117

attributed to substitutional iron. The isomer shifts are different from those expected for iron in 3da configuration. No difference in isomer shift of the principal line greater than 0*0003 cn+ec has been observed between n- and p-type silicon of low resistivity. It is likely, therefore, that the iron formed is precipitated and electrically inactive. The measured isomer shift is compatible with the 3d74s configuration of metallic iron. The spectrum of substitutional iron in germanium shows a doublet at O-036 + 0.001 cm/set with a splitting of 5 Mc/sec. No difference in

GERMANIUM

-0~10-0~06

-0.06

-0~04-0~02

0

0.02

VELOCITY,

0.04

0.06

0.06

0.10

0.12

0

4

CM /SEC

FIG. 6. Resonant absorption of the 14.4 keV gamma ray of Fe57 produced by the decay of Co5’ in 0.02 n-cm p-type germanium.

is in contrast to the work of GLINCHUK,MISELIUK and FORTUNATOVAWwho found that cobalt becomes electrically inactive during this process. These results all suggest that the cobalt diffused into the germanium, and the iron produced by its decay, are present both in an electrically active and in an inactive form. Measurements of lifetime and carrier concentration are sensitive only to the electrically active form; the Mijssbauer effect detects both. The present work indicates that major fraction of the cobalt diffised into germanium in these experiments is electrically inactive. CONCLUSION The Miissbauer spectrum of Fe57 in silicon shows a strong emission line from electrically inactive iron at -0.0012 + 0.0003 cm/set. An additional line at +0*054 + 0.004 cm/set is

isomer shift or splitting was observed between rrand p-type germanium of low resistivity indicating that the iron studied here was similarly inactive. authors are indebted to F. G. ALLEN for a discussion of MGssbauer effect results obtained with Fesr atoms deposited on clean silicon surfaces, and to D. N. E. BUCHANANand F. R. EYLER for assistance with various aspects of the sample ‘preparation and measurement.

Acknowledgements-The

REFERENCES L., 2. Physik 151, 124 (1958); Naturwissenschaften 45. 538 (19581: 2. Naturf..

1. M~~SSBAUERR.

14a, 211 (1959): . ’ ” 2. POUND R. V. and REBKA G. A. Jr., Phys. Rev. Letters 3,554 (1959) ; SCHIFFJZRJ. P. and MARSHALL W., ibid. 3, 556 (1959). 3. KISTNER 0. C. and SUNYAR A. W., Phys. Rm.

Letters 4, 412 (1960).

1118

P. C. NOREM

and

4. WALL L. R., WRRTHEIM G. K. and JACCARINOV., Phys. Rev. Letters 6. 98 (1961). 5. COtiNS C. B. and C&so& R. ‘O., Phys.Rev. 108, 1409(1957). 6. WEI L. Y., J. Phys. Chm. Solids 18, 162 (1961) see aho SMMITS F. M., Ergebn. Exakt. Natww. 31, 167 (1959). 7. WElITIiEIM G. K., Phys. Reu. 124, 764 (1961). 8. LAX M., Phys. Reu. 119,1502(1960). 9. See for example the &rod&oG paragraph of WOODBURG H. H. and Tnm W. W.. Phvs. Rev. 105,84 (1957). 10. TYLER W. W. and WOODBURY H. H., Phys. Reu. %, 874 (1954).

G.

K. WERTHEIM

11. TYLBRW. W., NEWMANR, and WOODBURYH. H., Phys. Rev. 97, 669 (1955). 12. GL~&~K K. D:, M&n&E. G. and FORTIJNATOVA N. N.. Ukrain. Fia. Zh. 4.208 119591. 13. WERTHB~M G. K.. J. [email protected]. I& llb5 (1961). 14. WOODBIJRYH. H: -and -LIJD+IO G; W., Phys. Rev. 117. 102 (19601: LUDWIG G. W. and WOODBURY H. k., Piys. I&. Letters 5, 98 (1960). 15. WOODBURY H. H. and Lu~wrc G. W., Phys. Rev. Letters 5, 96 (1960). 16. This finding is similar to some reported in Ref. 6. 17. LUDWIG G. W. and WOODBURY H. H., Phys. Rev. 113, 1014 (1959).