Energy loss of 4He ions in AI2O3 and SiO2

Nuclear Instruments and Methods North-Holland. Amsterdam

in Physics

Research

ENERGY LOSS OF 4He IONS IN Al,03

169

814 (1986) 169-172

AND SiO,

D.C. SANTRY

R.D. WERNER Atomic

Received

Energy of Cunudu

Limited

Re.worch

Compan_v, Chalk

River,

Onrurio

KOJ

IJO

Condo

19 August 1985

From the 4He energy loss measurements in AI, AI,O,. Si and Si02. we have extracted stopping cross sections for “solid” oxygen over the energy range 0.2 to 2 MeV and at 5.48 MeV. Assuming linear additivity of atomic stopping cross sections, values for oxygen are 5.3 to 15% higher in SiO, than in AI,O,, indicating that discrepancies in the Bragg rule for these oxides are clearly not due to phase effects but to chemical binding effects.

1. Introduction

In studying the ranges of natural (Y particles in materials, Bragg and Kleeman [l] found that the stopping power of a compound was approximated by a linear combination of the stopping powers of the constituent elements. For a compound A,B,, the molecular stopping cross section c*,,,~,, = meA + ncB, where
B.V.

necessity of using very thin films which were difficult to characterize as to homogeneity, purity and. thickness. We concluded that thin-film parameters could best be monitored by using self-supported films. Purity, stoichiometry and homogeneity could then be determined by Rutherford backscattering analysis, while thickness could be established by accurate weighing of a precisely measured film area. Having established a consistent method for measuring stopping values of elemental materials [4,5], we choose to examine the validity of stopping additivity by measuring ‘He stopping powers of oxides. Those of Al and Si were chosen since they represent compounds of practical importance to the solid state industry. In addition, our previous studies [4] showed that 4He stopping values (keV per pg cm-‘) were similar for Al and Si. It would be interesting then to note the changes in these values by the addition of oxygen.

2. Experimental The procedures and apparatus used in the present study are essentially the same as those described in our previous pubIications [4,6]; therefore a detailed description of the method need not be repeated here. It is sufficient to recall that a surface barrier particle detector was used to measure the 4He ion energy before and after transmission through self-supported films of known thicknesses. The particle detector was calibrated for energy at 50 keV intervals from 150 to 2700 keV, relative to the Van de Graaff accelerator beam analyzing magnet. which in turn had been calibrated by nuclear threshold reactions.

D.C. Sontry, R.D. Werner / Energy loss of 4He ions in Al_@, and SO,

170

1000

1001

?a

P 5

”

i cn

100

101

0

I

/

/ _, , , / , / , , J , / ,“;-.r-.

I

”

0

500

1000

1500

2000

ENERGY ikeV)

:

Fig. 3. Stopping values of SiO, for 4 He ions. Values for oxygen were obtained by subtracting the contribution due to Si, as determined in our previous work [4].

IO

I

I _ zoo

I

I 100

100

2c CHANNEL

NUMBER

Fig. 1. Rutherford backscattering spectra used in stoichiometry analysis of thin self-supported films of Al,O, and SiO,.

Thin films of Al and Si were prepared by vacuum deposition as before [6] and ranged in weight from 75 to 140 pg cm ~ 2. The oxides of Al and Si were obtained by electron bombardment of alumina and quartz powders, respectively. Films of Al,O, ranged in weight from 55.6

J

/ 0

I

/

/

I 500

I

I

I

/

I loo0

/

/ 8

I

0

I-LLd 1501

to 172.2 pg cm-‘, while SiO, were from 54.1 to 114.0 pg cmW2. Our previous experience had shown that it was very difficult to produce oxide free films of Be, Al, Si, Ti and Ge. It was therefore essential to use Rutherford backscattering analysis to choose films with negligible contributions of oxygen. Spectra of Al and Si films used were shown previously f6]. Since Mitchell et al. [‘7] had observed that stoichiometry was not always observed in the formation of compound layers by evaporation, it was equally important to use Rutherford backscattering to examine the stoichiometry of the oxide films. Fig. 1 shows typical backscattering spectra of the oxide films. Assuming that the Rutherford cross section describes

500

1000

1500

2000

ENERGY (keV) x00

Fig. 4. Comparison Fig. 2. Stopping vaiues of AlaO, for 4He ions. Values for oxygen were obtained by subtracting the contribution due to Al, as determined in our previous work [4].

of stopping cross sections of oxygen for in Al,O, and SiO,. Included are semiempirical values from Ziegler [14j for oxygen as a gas 0 and as a “solid” A.

'He ions are measured

D.C. San~ry, R.D. Werner / Energy loss

the scattering of 2 MeV 4He ions [S], we find that X= 2.96 f 0.06 for Al,O, and Y = 2.02 + 0.04 for SiO,.. Stopping power measurements were made with beams of 4He ions from the CRNL 2.5 MV Van de Graaff accelerator, covering the energy range 0.3 to 2.2 MeV. Additional measurements were made with 5.486 MeV (Y source [9]. The particles from an 24’Am radioactive measurements consisted of determining 4He ion energies before and after transmission through the thin films. By placing eleven different films on a rotating wheel inside the vacuum system, it was possible to make successive measurements on Al, Al,O,, Si and SiO, films without altering any of the 4He beam characteristics or detector electronic parameters. Such an arrangement would be expected to provide better control over any systematic errors.

3. Results and discussions At each accelerator energy, stopping power measurements on Al and Si gave values which were within 3% of the values determined previously [4]. Stopping values for Al,O, were obtained from 205 individual measurements on a total of 10 films. For SiO, the corresponding numbers were 190 on 12 films. A polynomial leastsquares fit to the measured oxide data gave the energy dependence of stopping, shown in fig. 2 for Al,O, and fig. 3 for SiO,. Interpolated values at 50 keV intervals are listed in table 1 and have errors of 5%. Our results compare with other published values for these oxides as follows. For Al,O,, there is agreement to within 5% with values measured by L’Hoir et al. [lo] over the energy range 317 to 1716 keV and to within 4% with the results of Thomas and Fallavier [ll] from 672 to 1988 keV. The data of Thompson and Mackintosh [12] for SiO,, differ from our results by 7 to 14% from 357 to 1661 keV. Our results obtained using 5.486 MeV (Y particles from 24’Am are also listed in table 1. The average of 16 measurements on a total of 10 foils of Al,O,. gave a stopping value of 0.608 f 0.023 keV per pg cmP2. For 12 measurements on 10 foils of SiO,, the average stopping value was 0.636 f 0.026 keV per ug cmm2. The Bragg rule was used to obtain the stopping powers of oxygen in Al ,O, and in SiO,. r(Al,O,) c (Oxxen),4l,0,

- 2e(Al)

=

3 r(Si0,)

c (Ovgen)s,0,

=

- c(Si) 2

3

where c values are in eV per lOI atoms cmm2. Values used for Al and Si stopping were identical to those we published earlier [4].

of "He ions

Table 1 Stopping power values for 4 He Energy

177

in AI,O_+ and SO_,

ions (keV

per pg cm 2, J’

Stopping materials

(kev)

Oxygen in

SiO,

Oxygen in SiO z

200

1.01

0.948

1.10

1.06

250

1.09

0.960

1.16

1.09

300

1.16

1.00

1.24

1.15

350

1.22

1.07

1.30

1.20

400

1.26

1.14

1.35

1.26

450

1.30

1.21

1.39

1.32

500

1.33

1.29

1.42

1.38

550

1.36

1.35

1.45

1.43

600

1.37

1.41

1.46

1.48

650

1.38

1.44

1.48

1.53

700

1.39

1.47

l-.49

1.57

750

1.39

1.48

1.49

1.60

800

1.39

1.49

1.49

1.62

850

1.38

7.51

1.48

1.63

900

1.38

1.50

1.48

1.63

950

1.37

7.49

1.47

1.63

1000

1.36

1.49

1.46

1.63

1050

1.34

1.47

1.44

1.63

1100

1.33

1.47

1.43

1.62

1150

1.32

1.45

1.41

1.60

1200

1.30

1.43

1.39

1.58

1250

1.29

1.41

1.38

1.57

1300

1.28

1.39

1.36

1.54

1350

1.26

1.37

1.34

1.52

1400

1.25

1.36

1.32

1.50

1450

1.23

1.35

1.30

1.47

1500

1.22

1.33

1.29

1.46

1550

1.21

1.32

1.27

1.43

1600

1.20

1.30

1.25

1.41

1650

1.18

1.29

1.24

1.39

1700

1.17

1.28

1.22

1.37

1750

1.16

1.27

1.21

1.35

1800

1.15

1.26

1.19

1.34

1850

1.13

1.24

1.18

1.32

1900

1.12

1.23

1.17

1.31

1950

1.11

1.21

1.16

1.30

2000

1.09

1.18

1.15

1.29

5486

0.608

0.626

0.636

0.681

‘) eV per

1015

atoms cm-‘=

F = 169.29 for AI,O,.

F

x(keV

per pg cmm2)

where

F = 99.759 for SiO, and F = 26.564

for oxygen.

The results for “solid” oxygen are listed in table 1 and plotted in figs. 2 and 3. Fig. 4 compares the stopping values for “solid” oxygen as measured in Al,O, and SiO,. Although absolute stopping values for the element and oxide are each given to 5%, it was felt that relative measurements at each accelerator energy gave oxygen values to 6%. Consequently, the observed 5 to 15% higher values for oxygen in SiO, compared to Al,O,, indicates that there is not a unique set of stopping powers for oxygen as a “solid”. Our results show

the importance of determining the energy dependence of stopping powers, since results obtained at 2 MeV alone would indicate verification of the Bragg rule for oxygen as a gas in SiO,, but oxygen as a “solid” in Al,O,. The energy dependence in fig. 4 indicates that neither case is true and that the linear additivity of elemental stopping values does not produce molecular stopping values for the two oxides studied here. Deviations from the Bragg rule have been attributed to a phase effect, i.e., oxygen values as a gas may differ from oxygen as a solid, or to chemical binding differences in compound formation [Z]. The observed differences between the extracted oxygen values from Al,O, and SiO, clearly indicate an effect due to chemical binding. Whether a phase effect is also present cannot be deduced from our measurements. However Chu et al. [13] have measured the energy loss of He ions in oxygen condensed as a solid and found that the differences in stopping values between solid and gaseous oxygen were relatively small. Any comparison of extracted “solid” oxygen values with those given for elemental oxygen (gas) may not be realistic. It is assumed that elemental oxygen values are half of the measured oxygen molecule (0, gas) values, where chemical binding is neglected. in view of our present work this assumption may not be justified. Our observations on a carefully controlled experiment to check the validity of the Bragg additivity rule are that atomic stopping cross sections required to satisfy the rule may be a function of the molecular

environment. Discrepancies in the rule for Al and Si oxides are not necessarily due to phase effects but to chemical binding effects. In view of these results, the Bragg rule cannot be considered generally valid. References [l] [Z] f3] [4] [5] [6] [7] [8] [9] [lo] [ll] 1121 1131 (141

W.H. Bragg and R. Kleeman. Phiios. Mag. 10 (1905) 318. D.J. Thwaites, Rad. Res. 95 (1983) 495. D. Powers, Act. Chem. Res. 13 (1980) 433. D.C. Santry and R.D. Werner. Nucl. Instr. and Meth. 178 (1980) 523, D.C. Santry and R.D. Werner. Nucl. Instr. and Meth. 178 (1980) 531. D.C. Santry and R.D. Werner. Nucl. Instr. and Meth. 159 (1979) 523. I.V. Mitchell, W. Maenhaut, H. Raemdonck and F. Bodart, Nucl. Instr. and Meth. 197 (1982) 51. J.R. MacDonald, J.A. Davies. T.E. Jackman and L.C. Feldman. J. Appl. Phys. 54 (1983) 1800. D.C. Santry and R.D. Werner, Nucl. Instr. and Meth. Bl (1984) 13. A. L’Hoir, C. Cohen and G. Amsel, Ion Beam Surface Layer Analysis (Plenum Press, New York, 1975) p. 965. J.P. Thomas and M. Fallavier, Nucl. Instr. and Meth. 149 (1978) 169. D.A. Thompson and W.D Mackintosh, J. Appl. Phys. 42 (1971) 3969. W.K. Chu. M. Braun. J.A. Davies, N. Matsunam~ and D.A. Thompson, Nucl. Instr. and Meth. 149 (1978) 115. J.F. Ziegler, He Stopping Powers and Ranges in all Elemental Matter (Pergamon Press. New York, 1978).