XPS and LEISS studies of ion bombarded GaSb, InSb and CdSe surfaces

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s u r f a c e science ELSEVIER

Surface Science 352-354 (1996) 781-787

XPS and LEISS studies of ion bombarded GaSb, InSb and CdSe surfaces W. Yu l, J.L. Sullivan *, S.O. Saied Department of Electronic Engineering and Applied Physics, Aston University, Birmingham, B4 7ET, UK

Received 5 September 1995; accepted for publication 31 October 1995

Abstract Ion bombardment effects in GaSb, InSb and CdSe crystal surfaces were studied with a combination of X-ray photoelectron spectroscopy (XPS) and low energy ion scattering spectroscopy (LEISS). Both XPS and LEISS showed that neglecting surface contamination, the composition of the crystal surfaces prior to ion bombardment were close to their stoichiometric values of 1 : 1. During 3 keV Ar + beam bombardment, XPS analysis showed that the atomic ratios of Sb/Ga, S b / I n and S e / C d in the bombarded surfaces decreased from 1 to 0.71, 0.92 and 0.87, respectively. LEISS results, however, indicated that these ratios increased to 3.33, 1.63 and 1.32, respectively. The variances between XPS and LEISS measurements are attributed to a difference in the sampling depth between the two techniques, and give a clear indication of how ion bombardment changes the surface composition of these compound semiconductors. These changes may be described in terms of bombardment-induced Gibbsian segregation. Keywords" Cadmiun selenide; Gallium antimonide; Indium antimonide; Ion scattering spectroscopy; Sputtering; Surface segregation; X-ray

photoelectron spectroscopy 1. Introduction Ion b o m b a r d m e n t modifications o f surface composition have been observed in many compound systems using various experimental techniques [1-5]. F o r oxides, ion b o m b a r d m e n t generally causes preferential o x y g e n loss, leading to a reduction o f the oxide [3,4]. This phenomenon has been explained in terms o f the mass and surface binding energy effects described b y S i g m u n d ' s linear collision cascade theory [6]. F o r metallic alloys, the surface composition

* Corresponding author. Fax: +44 121 3590156; e-mail: [email protected]. i Current address: Department of Chemistry, The University of Sheffield, Sheffield, $3 7HF, UK.

changes are more likely due to surface segregation and diffusion i n d u c e d / e n h a n c e d by ion b e a m radiation [5,7]. C o m p a r e d to oxides and metallic alloys, studies o f ion bombardment effects on compositional changes in compound semiconductor surfaces are few [1,8,9] and no theoretical model has been established to explain these effects. To understand the ion bombardment effects on compositional changes in compound semiconductor surfaces, several compound semiconductor single crystal surfaces have been examined before and after periods o f ion bombardment, by combining X-ray photoelectron spectroscopy (XPS) and low energy ion scattering spectroscopy (LEISS). In our previous papers, the ion bombardment effects in GeSi and G a A s surfaces were reported [10,11]. In this paper, we presented more experimental results for GaSb,

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w. Yu et al. / Surface Science 352-354 (1996) 781-787

plied by MCP Wafer Technology Ltd. (UK) in the cut and polished state. They were washed in 1,1,1trichloroethane followed by the methanol to remove protective wax. CdSe samples were cut from a CdSe crystal with a diamond saw, the surfaces were then polished using successively finer abrasive paper until a mirror-like surface was obtained. Finally, all samples were ultrasonically washed in acetone.

InSb and CdSe, and attempt to explain these results in terms of Gibbsian surface segregation.

2. Experimental procedure The GaSb and InSb samples were research grade G a S b ( l l 1) and InSb(100) single crystal wafers sup-

(a) GaSb

Sb(ox.) ! . /

i

~

Sb(GaSb)

/ ~ -

39

37

35

......

i~//

33

0°TOA

31

29

27

/~",i

25

23

21

\

19

17

15

BindingEnergy (eV) (b) InSb Sb(ox.)

Sb (InSb)

In (ox.)

0°TOA

In (InSb)

A",

0o OA .........

38

......

36

34

32

30

26

28

24

22

20

18

16

14

BindingEnergy (eV) (c) CdSe Se (CdSe) Cd (CdSe)

Se (Se)

60

58

57

56

/

55

\

54

53

................8OOTOA

52

51

50

14

13

~ _

12

11

Cd (ox.)

10

9

8

7

Binding Energy (eV) Fig. 1. XPS spectra taken at take-off angles of 0 (solid curves) and 80° (dot curves) from the GeSb, InSb and GaSb surfaces before ion bombardment.(a) Sb4d and Ga3d spectra. Sb in GaSb: 31.7 eV (4d5/2) and 33.0 eV (4d3/2), Sb in Sb203:34.2 eV (4d5/2) and 35.5 eV (4d3/2); Ga in GaSb: 18.9 eV, Ga in Ga203:20.2 eV; (b) Sb4d and In4d spectra. In in InSb: 16.8 eV, In in In203: 17.9 eV; Sb in InSb: 31.0 eV (4d5/2) and 32.2 eV (4d3z2), Sb in Sb203:33.9 eV (4d5/2) and 35.1 eV (4,13/2). (c) Se3d and Cd4d spectra. Se in CdSc: 53.4 eV, Se (elemental): 55.1 eV, Se in SeO: 57.9 eV; Cd in CdSe" 10.4 eV, Cd in CdO: 9.3 eV.

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W. Yu et a l . / Surface Science 352-354 (1996) 781-787

lumps on a series of different grade abrasive papers, then ultrasonically washing in methanol. After transferring into the analysis chamber, all standard sample surfaces were cleaned further by prolonged Ar + beam sputtering and examined by both XPS and LEISS under the same experimental conditions.

For accurate XPS and LEISS quantitative analysis, standards of pure Ga, In, Sb, and Cd were prepared by evaporating pure Ga, In, Cd and Se onto polished and solvent cleaned surfaces of titanium discs (diameter of 12 m m × 2 mm) in high vacuum. Pure Sb surfaces were obtained by polishing Sb

2.4 2.2

(a) GaSh

I--X-- Ga(ox)/Ga ~

Sb(ox)/Ga •

Sb/Ga --o- O/Ga I

2

1.8 1.6 1.4 1.2

0 0 0 • 0 0 0 0 0

1

A

O m

0.8

•

•

•

0.6 0.4 0.2 0 2.2 2 r~ C,

1.8 1.6 1.4

..u

I=

1.2 1

.<

0.8 0.6

i

(b) InSb

r

i

i

I--x-- In(ox)/]In ---A--Sb(ox)/l[n •

l

i

Sb/ln 4d --o-- O/In ]

0.4 0.2 t

0

i

t

i

i

i

i

i

r

2.2

CdSe

2

[

•

~Cd --o- O/Ca +

C/CdI

1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 02 0 0

5

10

15

20

25

30

35

40

45

50

Sputter Time (minutes) Fig. 2. XPS measurements of the compositional changes in GaSb, InSb and CdSe surfaces during a period of 50 min Ar + ion bombardment. (a) GaSb; (b) InSb; (c) CdSe.

W. Yu et al. / Surface Science 352-354 (1996) 781-787

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The XPS and LEISS experiments were carried out in a VGS ESCALAB 200D spectrometer. All details of the spectrometer's operating and experimental conditions are the same as those previously described by Yu et al. [10], except that Ne + was used instead of He + in the LEISS experiments. The surface compositions derived from XPS and ISS analyses were calculated using the methods described by Seah [14] and Sullivan et al. [11], respectively. Before ion bombardment, surface structures were analysed by angle-resolved XPS (ARXPS) and LEISS to gain initial information on the elemental distribution in surfaces. For the initial LEISS analysis, a few minutes of sputtering using a 0.5 mA, 2 keV Ne ÷ ion beam was used to remove contamination from

the air exposed surfaces. During this period no measurable damage was observed. After an initial analysis, the surfaces were bombarded by a 1 mA, 3 keV Ar + ion beam for up to 50 min. The compositional changes in the surfaces during ion bombardment were monitored by normal XPS. After bombardment, the surfaces were re-examined using XPS and LEISS.

3. Results and discussions 3.1. XPS analysis 3.1.1. Before Ar + ion beam bombardment GaSb. Ga 3d and Sb 4d spectra taken at TOAs of 0 ° and 80° from the GaSb sample surface prior to ion

before ion b o m b a r d m e n t

Sb

(a) GaSb

- - - after ion b o m b a r d m e n t lil I

I

I

I I

Ga

I

I

(b) InSb

i

/~,

In

t_

/[~

/ l! _

(c) CdSe Se l ~

,J 300

350

400

Cd

./ 450

500

\. 550

600

650

700

Kinetic Energy (eV) Fig. 3. 1 keV Ne+ISS analysis of GaSb, InSb and CdSe surfaces before (solid curves) and after (dotted curves) 3 keV Ar + ion bombardment. (a) GaSb. Sb: 581 eV, Ga: 378 eV; (b) InSb. Sb: 570 eV, In: 542 eV; (c) CdSe. Se: 442 eV, Cd: 528 eV.

w. Yu et al./ Surface Science 352-354 (1996) 781-787

bombardment are shown in Fig. la. The results indicate that the GaSb surface was covered by a very thick oxide overlayer since the intensity of the Ga 4d and Sb 4d lines from the substrate decrease dramatically at 80 ° TOA. This is mainly due to surface oxidation in air [12]. Preferential oxidation of Ga occurs ( s o = 3.5 eV, s 6 a = 1.6 eV and eSb = 1.9 eV), resulting in a higher concentration of Ga than Sb in this overlayer. It may be deduced that there is a Ga-depleted region beneath the overlayer. Quantitative analysis shows that the atomic ratio of S b / G a measured by XPS at TOA of 80 ° was about 0.5, but in the deeper region of the GaSb substrate, the S b / G a atomic ratio measured at 0 ° TOA was about 1.0 corresponding to the stoichiometric value. InSb. In4d and Sb4d spectra taken from the as-received InSb surface are shown in Fig. lb. Similar to GaSb, at 80 ° TOA, the peaks due to In and Sb in the InSb matrix were reduced significantly, indicating a thick oxide overlayer. Quantitative analysis showed that the concentration of Sb20 3 is slightly higher than that of I n 2 0 3 in the contamination layer. ARXPS indicated a stoichiometric InSb sub-surface, with an S b / I n ratio of about 1.0. CdSe. Fig. lc shows the Cd4d and Se3d spectra taken from the CdSe surface before ion bombardment. It can be seen in the figure that the binding energy of Cd in CdO is lower than that o f Cd in CdSe. This unusual feature may be explained by the effects of extra-atomic relaxation [13]. The elemental distribution in the CdSe surface was quite homogeneous, the S e / C d ratios measured at 0 ° TOA was about 1.0. 3.1.2. X P S observations o f compositional changes due to A r + ion bombardment

The compositional changes in GaSb, InSb and CdSe surfaces due to Ar + ion bombardment are shown in Fig. 2. Fig. 2a confirms the previous ARXPS analysis and shows that a thick oxide layer exists on the GaSb surface with G a 2 0 3 as the major component, resulting in the measured atomic ratio of S b / G a greater than 1.0 in the subsurface. This oxide layer was completely removed after about 8 min sputtering. The S b / G a ratios then decreased from 1.0, until after about 25 min the ratio reached a steady state value of 0.71. For InSb, Fig. 2b shows the S b / I n atomic ratios

785

decrease with sputtering time, indicating a slightly Sb-rich thin layer existing between the oxide layer and the InSb crystal surface. The ratios then decrease slightly with the sputter time. After about 10 min sputtering, the S b / I n ratio reached a steady state value of about 0.92. For CdSe, the changes of the S e / C d , O / C d and C / C d atomic ratios in the CdSe surface during ion bombardment are shown in Fig. 3c. The contaminant layers were completely removed after the first 4 - 5 rain of sputtering and the S e / C d ratio decreased within 10 min of sputtering from 1 to a value of 0.87. 3.2. LEISS analysis

LEISS spectra taken from the crystal surfaces before and after 3 keV Ar ion beam bombardment are shown in Fig. 3. GaSb. The Ne + ISS spectrum taken from the GaSb surface before Ar + ion bombardment is given by the solid line in Fig. 3a. The S b / G a ratio calculated from the spectrum was 0.87, indicating that the first atomic layer is slightly Ga enriched. Neglecting experimental errors, this suggests that more Ga oxide is left on the surface. After ion bombardment, the Sb signal at the surface increases significantly. Quantitative analysis shows the S b / G a ratio at the outermost atomic layer of the bombarded GaSb surface reaches 3.33, indicating significant enrichment of Sb in the outermost atomic layer of the GaSb surface. InSb. The Ne + LEISS spectrum taken from the InSb surface prior to Ar ÷ ion bombardment is shown by the solid line in Fig. 3b. By peak fitting, the atomic ratio of S b / I n obtained from the quantitative analysis was about 1.02, which implies a stoichiometric InSb surface. The dotted line in Fig. 3b represents the Ne + LEISS spectrum after Ar ion bombardment. An increase in the Sb peak can be clearly seen, the atomic ratio of S b / I n calculated for the bombarded InSb surface was about 1.63, showing Sb enrichment due to ion bombardment. CdSe. The Ne + LEISS spectrum for the CdSe surface before Ar + bombardment is shown by the solid curve in Fig. 3c. The estimated atomic ratio of Se to Cd was about 0.94, indicating a nearly stoichiometric surface.

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W. Yu et aL / Surface Science 3 5 2 - 3 5 4 (1996) 7 8 1 - 7 8 7

Table 1 Values of "Ym,d ' y / d t , elements [17,18]

Ga In Sb Cd Se

t m and a

for Ga, In, Sb, Cd and Se

Tin

-d3*/dt

tm

(J/M E)

(mJ/M 2- C°)

(C°)

0.784 0.558 0.403 0.666 0.106

0.061 0.089 0.038 0.124 0.02

30 157 631 321 221

the composition o f the outermost atomic layer in the bombarded single crystal surface is determined by [10,111

a ( X 10 - 2 0 M 2)

(xB/xa)* ), [ TBaB ( XB/xA)b exp(--R g z TAaA

7.26 8.79 9.70 2.96 a 12.3 a

a Estimated using ionic radius, considering the large difference in eleelronegativity (ese - 8cd)= 0.7.

The LEISS spectrum taken from the bombarded CdSe surface shows a relative increase in Se peak intensity. The calculated S e / C d atomic ratio in the first atomic layer of the b o m b a r d e d CdSe surface is 1.32, indicating Se enrichment in the first atomic layer of the surface. F r o m the above analyses, it is clear that 3 keV A r + ion bombardment caused compositional changes in GaSb, InSb and CdSe surfaces. The LEISS results, however, contradict XPS results. The sampling depths o f ISS are limited to the first atomic layer, but normal XPS measurements represent an average atomic composition over about 7 0 - 8 0 A [11,14]. The contradiction is clearly due to this difference in sampling depth, the combination of LEISS and XPS analyses gives a clear indication o f the mechanisms responsible for the changes in the surface composition. F o r the explanation o f the observed compositional changes, a model based on the theory of Gibbsian surface segregation has been proposed and used for GeSi and G a A s systems. According to this model,

(1)

where Yi and a i (i = A, B) are surface tension and area o f o n e bulk atom A or B, respectively. The coefficient R is 0.224 and 0.212 for the (100) and (111) faces o f cubic crystals, respectively, assuming the inter-atomic potential between two adjacent atoms estimated from the L e n n a r d - J o n e s potential [15]. The surface tension for a solid at temperature t may be calculated from liquid phase measurements (at melting p o i n 0 using the linear relationship

(2)

Tt = Trn + ( t - - tm) dT dt'

where Ym is the surface tension at the melting point t m. For Ga, In, Sb, Cd and Se, Ym, d T / d t , t m and a ( - 7rr a) [16] are listed in Table 1. Substituting parameters from Table 1 into Eqs. (1) and (2) gives the atomic ratios of S b / G a , S b / I n and C d / S e at the b o m b a r d e d GaSh, InSb and CdSe surfaces, these together with XPS and LEISS results are listed in Table 2. G o o d agreement between the predicted surface composition and the LEISS measurements can be seen clearly. Assuming the number o f B atoms at the outermost layer increases during ion bombardment, the partial sputtering yield for element B will increase. In order to maintain the m i n i m u m surface free energy, there must be a net flux o f B a t o m s moving from the immediate subsurface region. This will cause depletion of B in the subsurface, which is sampled by

Table 2 Comparison of surface composition (XB//XA)observed by XPS and LEISS with those predicted by the model of bombardment-induced Gibbsian surface segregation (BIGSS) A-B XPS LEISS Stoichiometry BIGSS GaSb InSb CdSe

Before IB

After IB

Before IB

After IB

Before IB

After IB

1.08 0.99 0.98

0.71 0.92 0.87

0.87 1.02 0.94

3.33 1.63 1.32

1 1 1

2.74 1.63 1.32

W. Yu et a l . / Surface Science 352-354 (1996) 781-787

XPS. This explains why the compositions measured by LEISS contradict those by XPS. It is therefore likely, from our results, that Gibbsian segregation is responsible for the bombardment-induced compositional changes wrought in these compound semiconductor surfaces.

4. Conclusion Ion bombardment effects in GaSb, InSb and CdSe surfaces have been studied by combining XPS and LEISS techniques. The combination of techniques employed has allowed a more complete description of ion beam effects in the surface regions. Ion bombardment causes significant compositional changes in the surface of these semiconductors, resulting in enrichment at the outermost layer of one component with subsequent depletion of this component in the subsurface. The redistribution in surface composition is likely to be due to ion beam-induced Gibbsian surface segregation.

Acknowledgement The authors wish to thank Aston University for its financial support to this work.

787

References [1] G. Betz and G.K. Wehner, in: Sputtering by Particle Bombardment II, Ed. R. Berish (Springer, Heidelberg, 1983) p. 11. [2] R. Shimizu, Nucl. Iustmm. Methods B 18 (1987) 486. [3] J.B. Malherbe, S. Hofmann and J.M. Sanz Appl. Surf. Sci. 27 (1986) 355. [4] T. Choudhury, S.O. Saied, J.L. Sullivan and A.M. Abbot, J Phys. D: Appl. Phys. 22 (1989) 1185. [5] R. Kelly and A. Oliva, NATO ASI Series E, 112 (1986) 41. [6] P. Sigmund, in: Sputtering by Particle Bombardment I, Ed. R. Berish (Springer, Berlin, 1981) ch. 2. [7] J. Kirschner, Nucl. Instrum. Methods Phys. Res. B 7//8 (1985) 742. [8] J.B. Malherbe, W.O. Barnard, I. le R. Strydom and C.W. Louw, Surf. Interface Anal. 18 (1992) 491. [9] J.B. Malherbe and W.O. Bamard, Surf. Sci. 255 (1991) 309. [10] W. Yu, J.L. Sullivan and S.O. Saied, Surf. Sci. 307-309 (1994) 691. [11] J.L. Sullivan, W. Yu and S.O. Saied, Surf. Interf. Anal. 22 (1994) 515. [12] W.E. Spicer, P.W. Chye, C.M. Garner, I. Lindau and P. Pianetta, Surf. Sci. 86 (1979) 763. [13] S.W. Gaarenstroom and N. Winograd, J. Chem. Phys. 67 (1977) 3500. [14] M.P. Seah, in: Practical Surface Analysis, 2rid ed., Vol. 1, Eds. D. Briggs and M.P. Seah (Wiley, Chichester, 1990). [15] K.N. Tu, J.W. Mayer and L.C. Feldman, in: Electronic Thin Film Science (Macmillan, New York, 1992) p. 170. [16] P. Wyublatt and R.C. Ku, Surf. Sci. 65 (1977) 511. [17] R.D. Lide, Ed., CRC Handbook of Chemistry and Physics 72nd ed. (CRC Press, Boca Raton, FL, 1991) 4.133. [18] S.H. Overbury, P.A. Bertrand and G.A. Somorjai, Chem. Rev. 75 (1975) 547.