Study of surface segregation and surface tension of SnGa solution using AES and sessile drop methods

surface

science

Surface Science 338 (1995) 279-282

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Study of surface segregation and surface tension of Sn-Ga solution using AES and sessile drop methods O.G. Ashkotov

*, M.V. Zdravomislov

Department of Physics, Kabardino-Balkarian State Uniuersity, Nalchik, Russian Federation Received 18 January 1995; accepted for publication

24 April 1995

Abstract Using Auger electron spectroscopy with cylinder mirror analyzer the equilibrium surface concentrations of Sn-Ga liquid solution were measured as function of solution composition and temperature. An excess of Sn at the surface Sn-Ga solutions was found in comparison with the volume concentration. Surface tension and surface concentrations was measured in situ between the melting point and 773 K for melts Ga, Sn and Sn-Ga. Keywords: Auger electron spectroscopy;

Gallium; Liquid surfaces; Tin

Surface tension of liquids has been studied for many years by methods including “small” and “large” drop, bubble pressure, capillary and sessile drop. By these methods surface tension measurements are performed without a control of surface impurities. Usually the contaminations arise at room temperature (in air) and remain under vacuum condition at melting [l]. The surface tension data are usually obtained in a vacuum of the order of lop6 Torr with a liquid nitrogen cold trap. Under such conditions it is impossible to remove completely the impurities covering the drop. In this paper we report studies of surface composition and surface tension made “in situ” in ultrahigh vacuum by Auger electron spectroscopy (AES) and the sessile drop method @MD). The AES and SMD data were obtained using an Auger spectrometer [I] equipped by a single-pass

* Corresponding

author.

0039-6028/9.5/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDI 0039-6028(95)00547-l

cylindrical mirror analyzer (CMA) having an energy resolution of i\ E/E = 0.1%. The Auger spectra were excited by a 3 keV electron coaxial beam parallel to the CMA axis with currents 3-5 PA being typically employed for these experiments. The corresponding data have been analyzed using the first derivative spectra. Electron energy distribution N(E) measured in the usual way by applying a sinusoidal modulation F = 1.5 kHz, 1-2 V ptp to the outer cylinder of CMA and recording the first harmonic of the output current. The data to be reported were obtained using high-purity Sn and Ga (99,999 at%) and their alloys. The Sn-Ga alloys containing 5.2, 12.1, 17.9, 24.1, 30.3, 37.4, 48.1, 71.8, 79.5, 94.6 at% Ga were prepared in our laboratory inside an AES spectrometer chamber. A liquid drop of a diameter of about 10 mm, was sited in a horizontal ceramic cup (Fig. 1). The ceramic cup was clamped to the sample during heating. Electrical contact with the drops was ensured with a

280

O.G. Ashkotou, M. V. Zdrauomislo~/Surface

Fig. 1. Ultra-high-vacuum chamber for liquid AES measurements. (1) sample holder; (2) energy analyzer (CMA); (3,4) viewers; (5) pressure manometer; (6) to sorption, sublimation and ion pumps; (7) manipulator; (8) sample; (9) ceramic cup; (10) heater; (11) holder; (12) heat screen; (13) reflector; (14) axle of manipulator; (15) cathode; (16) thermocouple.

fine W wire being insulating in high temperature regime. The proper temperatures have been monitored with a chromel-alumel thermocouple spot welded to the top of the cup. Surface cleaning was made by 0.6 keV Arf sputtering at 5 X 10e5 Torr of argon. The typical current densities was about 15 PA/cm’. The ion gun bombarded the drop until characteristic AES signals from the metallic surface were observed. After an additional ion etching during the time interval of about lo-60 min, residual gas was removed by an ion pump. After 1 min of pumping the pressure in the vacuum chamber reached 10m9 Torr. The Ar+ bombardment was used for the liquid drops. Before melting, the sample was ion etched until the Auger analysis showed that the surface was clean. Because of the orientation of the ion beam, approximately one-fourth of the drop could not be etched. After melting, contaminations became visible presumably due to the migration of the unsputtered native oxide across the cleaned portion of the drop. The ion gun was again turned on until eventually the molten surface remained free of contaminants even after the ion beam was removed. Before experi,ments started the temperature was lowered a step further to the next value, then maintained for about 1 h to ensure thermal homogeneity of the drop before taking the AES spectra and photographing the drop, which results from the balance between gravity and surface tension. The surface tension in

Science 338 (199.5) 279-282

this work was determined using the sessile drop technique. This method is based on a comparison of the profile of a liquid drop with the profile calculated by solving the Laplace equation. The comparison was done in a traditional Bashfort-Adams [2] procedure. After the cleaning procedure the surface tension of pure liquid tin and gallium was obtained between the melting point and 773 K. The surface tension of pure tin can be represented by the equation, as, = 645 - O.l(Z’- T,,) mJ mP2. The surface tension of gallium has a,, = 860 mJ mP2 at the melting point and a temperature dependence da/dT = -0.16 mJ me2 K-‘. Equilibrium surface compositions of solid and liquid Sn-Ga alloys of bulk composition were examined by AES. After bakeout at 773 K for 24 h, the major contaminants as deduced from AES were C(272 eV> and O(520 eV> on all samples. The impurities could be reduced both in the solid and in the liquid state by ion bombardment. For surface quantitative analysis of alloys Sn-Ga we consider the Auger peaks of Sn(Ma,N4,5N4,5 -428 eV) and Ga(M,,M,,,M,,, -55 eV and -1070 eV) [3]. It is remarkable [4], Lr,zM,,,M,,, that soft electrons of 55 eV originate from surface

20

40

60

Fig. 2. Surface segregation (T = 773 K) in Sn-Ga alloys, calculated with matrix effects (curves (1) and (211, curve (3) with correction factors, applying elemental sensitivity factors (curve (4)). (X,,, X&-bulk and surface concentration of Sn.)

O.G. Ashkotou, M.V. Zdraoomislou/Surface

Science 338 (1995) 279-282

281

are the atomic densities; A is the Auger electron escape depth, p the coefficient of attenuation of the primary beam and r is the backscattering factor. Index 1, 2 and superscript 0 refer to the component alloys and pure element, respectively. Mean free path of electrons for high and low energy Ga Auger transitions are 1.17 and 0.215 nm, respectively, and for Sn, 0.782 nm. The considered back-scattering factors were: Ga - L 1,2M,,,M,,,

a

67-3

4?3

t

Fig. 3. The temperature dependence of liquid alloy surface compositions for alloys 5.2 at% Ga (curve (l)), 12.1 at% Ga (curve (2)), 37.4 at% Ga (curve (3)), 30.3 at% Ga (curve (4)).

and subsurface layers, while the harder electrons of 1070 eV come from deeper layers (about 1.17 nm). The escape depth for an electron of energy E was given by Penn [5] as: h=E/[a(ln

E+b)],

-1.2, Sn - M,,,N,,& -0.35, Ga - Mz,M,,,M,,, -0.52. Calculations show an increase of pi with increasing atomic number Z for our metals; they range from 0.7 to 0.99. Fig. 2 shows surface segregation as a function of bulk concentration at T = 773 K. The calculated concentrations including matrix effects correspond to high energy (curve 1) and low energy (curve 2) Ga Auger transitions. Curve 3 has been calculated assuming that pi = pi’), ri = do) and N{” = Ni’). Also, in this figure are reported the values of surface concentration calculated applying elemental sensitivity factors (curve 4). In all cases Sn was strongly segregated at the surface. Fig. 3 shows the temperature dependence of the surface compositions of Sn-Ga alloys above their melting point. The values of dX:/dT have a nega-

(1)

where a and b depend on the electron concentrations. Under the same experimental conditions, a ratio of the peak-to-peak heights of the Auger lines -428 eV) and Ga(L,,,M,,,M,,, Sn(M.&,,N,,, 1070 eV) was equal to 1: 10. The ratio Zo,/Z,, (if 1o, is the low energy peak of Ga) is 1: 18. For a high energy Ga line (Lr2M2,aM2,a) the same ratio is l/9. The Auger intensity peak height ratios were converted into surface concentrations using a method described in Ref. [5]. The determination of alloy surface concentrations from AES data has been performed using several approximations. The surface concentrations of Sn and Ga were calculated from the Auger measurements using:

x;=

1 1+

z,zp)N,(O)A~ypp

+ rp))h,(p,

r,z,(O’NjO’h;O)( p$O) + r,(a))h,(p,

where I are the Auger intensities

+

-’

rl)

+ r*)

1

’

(4 of the elements;

N

Fig. 4. Surface tension 0(X,,) of liquid Sn-Ga (T = 773 K). (1) Present data; (2) calculated by, see Ref. [6]; (3) from Ref. [7]. (U surface tension, Xs, bulk concentration Sn.)

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O.G. Ashkotoc, M.V. Zdrauomislov /Surface

tive sign and fall with increasing Xs,. At 30 at% Ga, dXT/dT = 0. There was no evidence of any systematic drift in concentrations due to evaporated of components. The straight lines through the data on Fig. 3 are least-squares linear fits. Fig. 4 shows the bulk concentration dependence of the surface tension determined by the large drop method for liquid (T = 773 K) alloys Sn-Ga (curve 1). Each data point was determined with an accuracy 3%. Curve 2 shows the surface tension of binary alloys Sn-Ga varying with Sn content calculated through differential expression [6], using the surface concentration data from our AES experiments. One can see that our values of surface tension are in good agreement with the calculated data. Curve 3 represents the surface

Science 338 (1995) 279-282

tension after Ref. [7]. However, measured values of surface tension are higher than those obtained in Ref. [71.

References 111O.G. Ashkotov, A.A. Shehzukhov, Doklady AN USSR 274, 6 (1984) 1427.

Dl F.D. Bashfort, J.E. Adams, An Attempt to Test Theories of Capillary Action (Cambridge,

1883).

[31 L.E. Devis, N.C. Donald, P.W. Palmberg, Handbook of Auger Electron Spectroscopy

(Phys. Electron. Ind., 1976) 253.

[41 O.G. Ashkotov and A.A. Shebzukhov, Surface 10 (1982) 101. [51 D.R. Penn, Electron. Spectrosc. 9 (1976) 29. 161A.I. Rusanov, J. Chem. (1967) 327. [71 A.G. Nalgiev, K.I. Ibragimov, Zh. Fiz. Khim. 48 (1974) 1289.