Prediction of the relative intensities of Auger lines in mica

Volume 41A, number 1

PHYSICS LETTERS

28 August 1972

PREDICTION OF THE RELATIVE INTENSITIES OF AUGER LINES IN MICA P. STAIB Max-Planck-Institut fur Plasmaphysik, EURA TOM A ssociation, 8046 Garching, Germany Received 3 July 1972 Experimental values for the relative intensities of the KLL Auger lines of 0, Al and Si and of the LMM lines of K are compared with theoretical values calculated from the electron impact ionisation cross section of Drawin at primary energies of E~= I keV and 2.5 keV.

A major difficulty encountered in comparing theoretical and experimental values of the intensities of Auger lines is the indeterminable number of atoms

~ 2)

Cross section (cm 10.17

Silicon IL

[1,21. We overcome this difficulty by analysing a cleaved mica involved in the sample experimental for whichAuger the number electronofyield the atomic layers under the cleaved surface [3, 4] namely: 1. potassium 2. oxygen (3), 3. silicon (~) and aluminium The(i),number parentheses various atoms is (i). known from theinsuccession of mdicates the relative number of atoms in each layer. The Auger spectrum is acquired from a four.grid retarding field analyser by way of a lock-in-amplifier in a multichannel analyser working in multiscaling mode. It is numerically integrated in order to restore the true shape of the Auger lines and the background is subtracted from this distribution. This method, to be described in a later paper, is accurate (5% error for the relative Auger intensities). The computed intensities are not essentially different from the relative peak to peak amplitudes of the lines for energies < 1 keV. In a first approximation, these intensities are expected to be proportional to the product Na of the number of atoms and the electron impact ionisation cross section a for the electron shalls concerned, a is calculated by the semi.empirical formula of Drawin [51with

23)

a

~

18

~

io 10-19

Carbon (K) _________

I ~o-zo Aluminiurn(K( 5

~ Si (K)

______________

0

.0

2,0

3,0 Primary Energy Ikey)

l:jg. 1.

is expected to represent the relative cross sections. The values are indicated in fig. 1 and are normalized

f

1 = f2 = 1. The number ~ of electron capable of leading to an Auger process is = 2 for the K shell and ~L = 6 (not 8) for the L shell of potassium, be. cuase we did not observe any L1MM, but only L23MM lines. Values of a are depicted in fig. 1 for primary energies up to 3 keV. In order to compare the a values with experimental intensities, we form for each line the quotient I/N that

to the a value of oxygen. Our values are indicated at a primary energy of 2.5 keV together with the values of Poppa [6] at I keV. For the latter the intensities are the peak to peak amplitudes. The one value indicated for potassium (upper dot) refers to measurements on fresh cleaved surface and the second value (lower dot) to measurement after some 30 minutes of electron bombardment leading to a depletion of the K layer. 3

Volume 41A, number 1

PHYSICS LETTERS

The agreement between predicted and measured values is surprisingly good. This indicated that the values of backscattering and escape probability do not strongly depend on the position and jonisation energy of the different atoms. Therefore the product No can be used to predict, in a first approximation, the relative intensities of Auger lines of chemical elements situated in the topmost layer and having Auger electron energies greater than about 100 eV.

4

28 August 1972

References [II HE. Bishop and J.(’. Riviere, J. Appl. Phys. 40(1969) 1740. [21 J.H. Neave, C.T. Foxon and B.A. Joyce, Surf. Set. 29 (1972)411. 131 K. MUller and C.C. Chang, Surf. Sci. 14 (1969) 39. 141 K. MUller and CC. Chang, Sruf. Sci. 8 (1968) 455. 151 H.W. Drawin, Z. Physik 164 (1961) 513.

161

H. Poppa and GA. Elliot. Surf. Set. 24 (1971) 149.