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Journal of Electron Spectroscopy and Related Phenomena 114–116 (2001) 415–420www.elsevier.nl / locate /elspec

XPS investigation of the equilibrium segregation of antimony atgermanium surface

*N. TabetSurface Science Laboratory, Physics Department, King Fahd University of Petroleum and Minerals, 31261 Dhahran, Saudi Arabia

Received 8 August 2000; received in revised form 10 October 2000; accepted 10 October 2000

Abstract

X-ray Photoelectron Spectroscopy (XPS) has been used to investigate the antimony segregation at Ge surface. Heattreatments of various durations have been carried out under vacuum at T56008C. Etching Ge surface with Argon ions priorto the heat treament enhances drastically the segregation process. A high value of the antimony diffusion coefficient

210 2(D 5 7 3 10 cm /s) was obtained from the experimental data. A shift of the Ge core levels towards the high bindingenergies was observed as the dopant surface density increases. The shift was related to the band bending that results from ahigh density of surface states. 2001 Elsevier Science B.V. All rights reserved.

Keywords: XPS; Germanium; Surface segregation; Atomic diffusion

1. Introduction and Tersoff in 1989 [4]. The development of theMolecular Beam Epitaxy (MBE) technique to grow

Impurity segregation at surfaces of metals and layered structures such as Si /Ge and the discovery ofalloys has been subject to intensive studies during the surface segregation of some impurities calledthe recent two decades [1,2]. A number of theoretical ‘surfactants’ (surface-active) induced a great deal ofmodels have been developed to understand the interest in surface-impurity interaction in semicon-driving forces that govern the surface segregation ductors [5,6]. However, the survey of the publishedprocess in these materials. The segregation of solute literature shows that there is still an important needatoms at the surface of Ge(0.5%at Sn) alloy has been for experimental data obtained on simple systems tostudied [3]. As far as the theoretical studies are evaluate the theoretical models that describe theconcerned, the first calculation of the equilibrium segregation phenomenon in semiconductors. Dopantssegregation at a semiconductor reconstructed surface, of Group V such as As and Sb are expected tonamely Si–Ge(001)2x1 was performed by Kelires segregate at grain boundaries of Si and Ge [7]. In a

recent study we have observed the surface segrega-tion of antimony during an oxidizing treatment ofgermanium single crystals [8]. In this work, we haveused X-ray Photoelectron Spectroscopy (XPS) toinvestigate the segregation kinetics of Sb under*Tel.: 19-663-860-2443; fax: 19-663-860-2293.

E-mail address: [email protected] (N. Tabet). vacuum at T 5 6008C.

0368-2048/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0368-2048( 00 )00385-6

416 N. Tabet / Journal of Electron Spectroscopy and Related Phenomena 114 –116 (2001) 415 –420

2. Experimental

The germanium samples were cut from a largegrains polycrystalline Sb-doped germanium rod of

17 230.4 V cm resistivity (Sb density . 10 cm ). Thesamples were mechanically polished using adiamond paste down to 1 mm size. Then, they werechemically etched using a (40% HF, 60% H O)2

solution for 3 min, rinsed with distilled water andstored in ethanol till they were introduced into thechamber of the electron spectrometer. The observa-tion of the chemically etched surface of the samplewith an optical microscope did not reveal the pres-ence of any grain boundary. Heat treatments ofvarious durations were carried out at T 5 6008C

26under vacuum (10 mbar) in a heating cell attachedto the electron spectrometer. The XPS spectra wereobtained by using the aluminium anode. The C1sline (E 5 284.5 eV) was used as a reference for theb

charge shift correction. The energy regions corre-sponding to the lines C1s, Sb3d ,Ge2p and Ge3dwere systematically scanned after each treatmentusing a 0.05 eV step size and 20 eV electron passenergy. The quantitative analysis was performed byfitting the XPS spectra using a software based on theLeast Squares Method.

3. Results

Figs. 1a, b and 2 show the evolution of theGe2p , Ge3d lines and Sb3d respectively, after3 / 2

chemical etching followed by successive heat treat-ments of various durations ranging from 5 to 178min. Notice that the Sb3d and Sb3d peaks are5 / 2 3 / 2

barely visible after 1 h treatment (curve 4 on Fig. 2).The broad peak in the 529–536 eV range that isobserved prior to any heat treatment (curve 1, Fig. 2)includes the L M M Auger line of germanium3 23 23

and the O1s line that stems from the oxygen of thecontamination layer. A second series of heat treat-ments has been carried out after bombarding thechemically etched surface of the sample with argonions of 4 keV energy for 15 min. Figs. 3 and 4 show

Fig. 1. Ge2p (a) and Ge3d (b) lines evolution after successive3 / 2the Ge2p and Sb3d lines respectively after succes-3 / 2 treatments at T 5 6008C. Chemically etched surface (1) and aftersive heat treatments of various durations ranging heat treatment of various durations: 5 min. (2), 20 min. (3), 61from 26 to 880 min. Notice that the Sb peaks are min. (4), 91 min. (5), 178 min. (6). Notice the charge shift of theclearly observed after a heat treatment of 26 min peak towards the higher energies.

N. Tabet / Journal of Electron Spectroscopy and Related Phenomena 114 –116 (2001) 415 –420 417

Fig. 3. Evolution of the Ge2p after successive treatments at3 / 2

Fig. 2. Evolution of the Sb lines after the same treatments as in T 5 6008C. Argon etched surface (1) and after heat treatment ofFig. 1. various durations: 26 min. (2), 77 min. (3), 165 min. (4), 261 min.

(5), 381 min. (6), 589 min. (7) and 889 min. (8). Notice thecharge shift of the peak towards the higher energies.

duration. This observation indicates clearly a drasticincrease of the dopant segregation by the argonetching. Furthermore, one can observe a systematic independent of the metal used [10,11]. The width ofshift of the Ge lines towards the higher binding the depletion region near the surface, v decreases as

21 / 2energies (Figs. 1a,b and 3). The XPS line shift can the dopant density, N , increases v~N . Thes dD D

be related to the energy band bending at germanium Fermi level being flat, the distance between thesurface as discussed below. Fermi level and any energy core level decreases as

the distance from the surface decreases, that is thebinding energy decreases (Fig. 5). Considering a

4. Discussion Fermi level pinned at 0.1 eV above the valence band,17 23one obtains v 5 100 nm for N 5 10 cm . ThisD

4.1. Binding energy shift distance is to be compared to the escape depth ofphotoelectrons. Using the expression suggested by

Figs. 1a,b and 3 show a systematic shift (up to 0.3 Seah and Dench [12], one gets l 5 1.01 nm,2p

eV) in the Ge2p and Ge3d lines towards the l 5 2.35 nm and l 5 2.22 nm. The escape depth3 / 2 3d Sb

higher binding energies as the duration of the heat is usually considered as 3l, that is the distance fromtreatment increases. In order to explain this shift, let which 95% of the collected photoelectrons originate.us recall that germanium surface is known to include On can observe that the obtained values of thea high density of surface states that lead to the Fermi escape depth for Ge2p and Ge3d are quite small3 / 2

level pinning at germanium surface [9]. As a result compared to width of the depletion region. Conse-of the Fermi level pinning, for instance, the barrier quently, the binding energies that are measured areheight of Schottky contacts on germanium are quasi referenced to the atoms within the top layers: E .b


E (surface). As the segregation proceeds, the widthb

v becomes comparable or even less than the escapedepth and the measured binding energies shifts to thebulk values which are higher: E . E (Bulk) (Fig.b b

5). Notice that comparable effect has been reportedin Ref. [13] where the authors observed a shift of thecore levels in heavily doped silicon by usingsynchrotron radiation of various wavelengths.

4.2. Segregation kinetics

Assuming that there is no dopant loss from thesurface during the segregation process, the surfaceconcentration of antimony C after a time t, is givens

by [14]:1 / 2Dt Dt

]] ]]C 5 aC 1 2 exp erfc (1)F S D S D Gs b 2 2 2 2a d a d

where C is the bulk concentration of the dopant, Db

is the diffusion coefficient, d is the dopant mono-layer thickness at the surface, usually taken as theatomic size and where a is the ratio of the surfaceconcentration to the bulk concentration at equilib-Fig. 4. Evolution of the Sb lines after the same treatments as inrium (for t → `, C 5 aC ). Fig. 6 shows the timeFig. 3. Notice the appearance of the Sb peaks after 26 min s b

treatment (curve 2). dependence of the ratio I /I 1 I , where I andSb Ge Sb Sb

Fig. 5. Comparison of the surface depletion width to the photoelectron escape depth before and after segregation.

N. Tabet / Journal of Electron Spectroscopy and Related Phenomena 114 –116 (2001) 415 –420 419

our samples include a high density of dislocations7 22(N . 10 cm ) that probably enhances the diffu-

sion process. However, the slow segregation kineticsthat we observed at chemically etched surfacestrongly suggests that the ion etching prior to theheat treatments is the main cause of the accelerationof the dopant segregation. Notice that a similaracceleration of the tin surface segregation has beenobserved in Ge–0.5%Sn alloys [3]. The authorsattributed such acceleration to the creation of fastdiffusion paths as a result of the ion bombardement.It should be pointed out, however, that the presenceof the contamination layer at chemically etchedsurface may also play the role of a diffusion barrierlayer preventing the dopant atoms from reaching thesurface.

In order to probe the Sb gradient across thesegregation region, we have recorded the XPSsignals after successive sputtering cycles of 5 minduration using Ar ions of 3 keV energy. Fig. 7 showsthe decrease of the I /I ratio as the etching timeSb GeFig. 6. Time dependence of the ratio I /I 1 I versus theSb Ge Sb increases. These results have been obtained using atreatment duration. The symbols correspond to the experimentalsample from the first series after 178 min heatdata and the continuous line to the analytical expression given by

Eq. (1).

I are the normalized areas of Sb and Ge XPS linesGe

respectively. The normalized areas have been calcu-lated by means of a dedicated program using alu-minium-Scofield library for the sensitivity factors.We have reported the values of the ratio obtained byusing two different germanium lines, namelyGe2p and Ge3d. The experimental data were fitted3 / 2

using Eq. (1). The atomic concentration C wasb17 22 23taken as 10 /N where N 5 5.3 3 10 cm isGe Ge

the density of germanium atoms in the bulk. One26obtains C 5 2 3 10 . The values of d was takenb

28equal to 3.13310 cm. The best fit of the ex-perimental data was obtained for the following

4 210 2values: a 5 5.5 3 10 , D 5 7 3 10 cm /s. Theaverage value of the diffusion coefficient of Sb ingermanium at T 5 6008C as calculated from the

214 2literature is much smaller (D . 5 3 10 cm /s)[15]. However, Dudko et al. [16], reported thetemperature dependence of Sb diffusion in Ge en-

Fig. 7. Variation of the ratios I /I and I /ISb3d Ge2p Sb3d Ge3d3 / 2hanced by the presence of dislocations. A value versus the sputtering time. The symbols correspond to the211 2D . 2.2 3 10 cm /s at T 5 6008C can be ob- experimental data and the lines to the analytical expression given

tained from their results. It should be pointed out that by Eq. (4).


treatment (see curve 6, on Fig. 2). It is worth shown that etching germanium surface with argonnoticing that the difference between the values ions accelerates drastically the segregation process.obtained using Ge2p and Ge3d lines decreases as A saturation value of the surface concentration of Sb3 / 2

the sputtering time increases. We show in the follow- was reached after a heat treatment of 4 h duration.ing that this result can be related to the difference The analysis of the segregation kinetics suggests abetween the attenuation length l and l . Con- high value of the diffusion coefficient of antimony.2p 3d

sidering a segregation profile given by the Sb atomic A shift of the binding energies of Ge core levels wasconcentration at the depth x, C(x), the XPS collected observed as the dopant concentration at the surfaceintensities are given by: increases. The shift was related to the energy band

` bending across the surface region as a result of a0I xSb high density of surface states.] ]I 5E C(x) exp 2 dx (2)S DSb l lSb Sb

0

`0I xGe] ]I 5E 1 2 C(x) exp 2 dx (3)f g AcknowledgementsS DGe l lGe Ge

0

0 0 The author would like to thank King Fahd Uni-where I and I are the intensities of photoelectronsGe Sbversity of Petroleum and Minerals for its support.emitted from infinitely thick and pure Ge and Sb

samples respectively. It is easy to observe that sincel is much greater than l , the Ge2p signal is3d 2p

expected to be more related to the immediate regionReferencesof the surface where the density of the segregated

dopant is high, i.e. that of germanium, (1 2 C), is[1] P.A. Dowben, A. Miller, Segregation Phenomena, CRClow. Consequently, the ratio I /I is expected toSb Ge

Press, Boca Raton, 1990.be higher when Ge2p line is used. In order to check[2] J. Du Plessis, Surface segregation, diffusion and defects data,this conclusion, one needs to transform the time

solid state phenomenon, Sci. Tech 11 (1990).variable into depth. This correspondence requires a [3] A. Rouabah, J. Bernadini, A. Rolland, Surf. Sci. 315 (1994)calibration that was unfortunately not available. 119.However, we have reported on Fig. 7 the time [4] P.C. Kelires, J. Tersoff, Phys. Rev. Lett. 63 (1989) 1164.dependence that we obtain by assuming a constant [5] W.F. Egelhoff Jr., D.A. Steigerwald, J.Vac. Sci. Technol. A 7

(1989) 2167.sputtering rate v, and a simple exponential form of[6] H.A. van der Vegt, H.M. van Pinxteren, M. Lohmeier, E.the segregation profile C(x) 5 C exp(2bx). By com-0

Vlieg, Phys. Rev. Lett. 22 (1992) 3335.bining Eqs. (2) and (3), one gets:[7] T.A. Arias, J.D. Joannopoulos, Phys. Rev. Lett. 69 (1992)

3330.C0]]] exp(2bx )0 [8] N. Tabet, M. Salim, Appl. Surf. Sci. 134 (1998) 275.tI I 1/l 1 bSb Sb Sb [9] P.Y. Yu, M. Cardona, in: Fundamentals of Semiconductors,] ] ]]]]]]]]5 3 (4)0I CIGe 0 Physics and Materials Properties, Springer-Verlag, Berlin,Ge ]]]l 2 exp(2bx )Ge t 1996, p. Chapter 8.1 /l 1 bGe

[10] A. Thanailakis, D.C. Northrop, Solid State Electr. 16 (1973)where x 5 vt is the depth reached after a sputtering 1383.t

time t. Fig. 7 shows a qualitative agreement with the [11] N. Tabet, C. Monty, Phil. Mag. B 6 (1988) 763.experimental data. A more detailed analysis requires [12] M.P. Seah, W.A. Dench, Surf. Interf. Anal. 1 (1979) 2.

[13] W. Eberhardt, G. Kalkoffen, C. Kunz, D. Aspnes, M.a calibration of the sputtering rate.Cardina, Phys. Stat. Solidi B88 (1978) 135.

[14] C. Lea, M.P. Seah, Phil. Mag. 35 (1977) 213.[15] D. Shaw (Ed.), Atomic Diffusion in Semiconductors, Plenum5. Conclusion

Publishing Company, London, 1973, p. Chapter 5.[16] G.V. Dudko, N.I. Marunina, G.V. Sukhov, D.I. Chere-

The equilibrium segregation of antimony at ger- dnichenko, Sov. Phys. Solid State (English Transl.) 12manium surface was investigated at 6008C. It was (1970) 1016.