SURFACE AND INTERFACE ANALYSISSurf. Interface Anal. 26, 872È875 (1998)

Deposition and Growth of on SurfacesAg Si(111)Studied by Optical Second-harmonic Generation

Kjeld Pedersen,1* Corinne Tomas1 and Per Morgen21 Institute of Physics, Aalborg University, 103, 9220 Aalborg DenmarkPontoppidanstr…de ^st,2 Physics Department, Odense University, Campusvej 55, 5230 Odense M, Denmark

Optical second-harmonic generation has been used to follow in situ the growth of thin Ag Ðlms deposited on Si(111)under various substrate conditions and temperatures. This method is particularly sensitive to the microscopic struc-ture of the deposited Ag Ðlms. Thus, island formation is found to enhance the second-harmonic signal, and stronglytemperature-dependent island formations were indeed observed at lower Ag coverages. For a Ðlm thickness above25 monolayers, identical Ðlm roughness characteristics were found for growth temperatures between 200 and 450 Kand for di†erent types of substrate reconstruction. A layer-by-layer growth mode can be obtained for low substratetemperatures or for surface passivation by oxygen or hydrogen. In such cases the second-harmonic signal oscillates.The period of oscillation is 10 monolayers. These oscillations are due to the formation of quantum-well states in theÐlm. 1998 John Wiley & Sons, Ltd.(

KEYWORDS: second-harmonic generation, surface roughness ; silicon ; silver

INTRODUCTION

The growth of thin metal Ðlms on semiconductor sur-faces has been investigated with most of the techniquesavailable for surface studies.1 However, very few tech-niques allow direct, continuous monitoring of thegrowth process during deposition. In that respectoptical techniques, such as second-harmonic generation(SHG), are advantageous. The surface and interfacesensitivity of SHG from centrosymmetric materials,which lack bulk dipole non-linearities, has been demon-strated for a large number of systems.2 In connectionwith thin-Ðlm growth, the access to buried interfaces isalso particularly interesting. The thin Ðlm may act as aquantum well (QW) of uniformly expanding width if itgrows in a perfect layer-by-layer mode. Previous experi-ments on Au Ðlms on Co3 and Si4 demonstrate that thisleads to oscillating SHG with coverage.

Most of the metal/semiconductor systems investi-gated do not normally grow in a layer-by-layer modeduring deposition. In such cases variations in Ðlm uni-formity within the probed area will cause loss of thequantum-well signature of the deposited metal. On theother hand, Ðlm roughness may lead to local Ðeldenhancements due to plasmon resonances, in particularfor Ag due to its low bulk plasma frequency. For so-called island Ðlms on glass, such e†ects can lead toorders of magnitude enhancements of SHG comparedto a Ñat surface, 5h7 but the e†ect of island formationon crystalline substrates has not yet been studied.

* Correspondence to : K. Pedersen, Institute of Physics, AalborgUniversity, 103, 9220 Aalborg Denmark.Pontoppidanstr…de ^st,E-mail : il3kp=physics.auc.dk

The Si(111)/Ag system is very well studied anddetailed information about the variation in growthmodes is available from studies using for instance, scan-ning tunelling microscopy, (STM),8h10 reÑection high-energy electron di†raction (RHEED),11,12 low-energyion scattering,13,14 and linear optics.15 On the clean7 ] 7 surface the Ðlm grows in a two-dimensional layer-by-layer mode at low temperature and in a three-dimensional island mode at higher temperatures.8,9 Thegrowth mode has also been shown to depend on condi-tions of the substrate, such as reconstruction with andwithout foreign atoms.12,13 Although SHG by now isestablished as a surface-sensitive technique, only a fewinvestigations of the growth of thicker Ðlms exist. It istherefore interesting to use the rather Ðrm and detailedknowledge about the Si/Ag system to test the applica-tion of SHG to monitor and discriminate between thevarious growth processes. By recording SHG duringgrowth at di†erent substrate temperatures, conclusionsabout the sensitivity of SHG to island formation can bereached. In addition, the results provide a comparisonof the morphology of Ðlms growth on substrates withvarious surface structures.

EXPERIMENTAL RESULTS

The Si samples were cut from 1 mm thick n-typeP-doped wafers with a resistivity of 4.5 ) É cm. X-raydi†raction showed that the wafer surfaces were (111)-oriented within ^0.3¡. Clean surfaces with negligibletraces of impurities and sharp 7 ] 7 low-energy electrondi†raction (LEED) patterns were obtained by a purelyresistive heating procedure. Silver was deposited from

CCC 0142È2421/98/120872È04 $17.50 Received 15 January 1998( 1998 John Wiley & Sons, Ltd. Accepted 8 June 1998

Ag ON Si(111) STUDIED BY SECOND-HARMONIC GENERATION 873

an electron beam evaporator. A quartz oscillator mea-sured the evaporation rate before and after deposition.The sample temperature at and below room tem-perature (RT) was measured with a thermocouple con-nected to the sample holder. Higher temperatures,where the sample was resistively heated, were deter-mined by a pyrometer. During the evaporation the pres-sure stayed below 5 ] 10~10 mbar. The SHGmeasurements were performed with a Q-switchedNd :YAG laser delivering 10 ns pulses at a 20 Hz repeti-tion rate with a wavelength of 1064 nm. The pulseenergy was below 10 mJ in a 0.25 cm2 spot to keep thetemperature rise during the measurements at a fewdegrees and thus with negligible e†ects on the growthprocess.2 The second-harmonic signal was detected by aphotomultiplier tube connected to gated electronics andwas recorded continuously at a 60¡ angle to the normalof the sample during Ag deposition.

In the experiments presented here the growth condi-tions have been varied by using di†erent temperaturesand surface conditions of the Si(111) sample. Figure 1shows recordings of the p-polarized second-harmonicsignal generated by p-polarized pump light vs. Agcoverage at di†erent substrate temperatures. For alltemperatures a common level and form of the signal isreached as the coverage approaches 30 monolayers(ML), while the signal variation before this stagedepends on temperature.

Figure 2 shows the s-p polarized signal as a functionof coverage recorded at RT and at 450 K. The signalsinitially decay, due to the removal of dangling bonds,16and then grow to temperature-dependent maxima at 4ML. A low level is reached after 10 ML and the signalsstay at this level for higher coverage. The SHG in thisconÐguration is due to the response tensor element s

zxx(2)

that describes the polarizability perpendicular to thesurface (z) induced by a pump Ðeld parallel (x) to thesurface. Silver island Ðlms on glass substrates havealready been shown to give strong SHG in the s-p pol-arized mode.7 The results in Fig. 2 therefore indicatethat islands are formed during deposition of the Ðrst few

Figure 1. Second-harmonic generation (SHG) with p-p polariza-tions as a function of Ag coverage recorded at different substratetemperatures.

Figure 2. Coverage dependence of s-p polarized SHG recordedat room temperature and at 450 K.

MLs and a more compact Ðlm is created above 10 MLon Si(111).

Figure 3 shows results for Ag growth on 1 ] 1 : Hand reconstructed substrates. HydrogenJ3 ] J3 : Agtermination was achieved by back-Ðlling the chamberwith to a pressure of 105 mbar for 15 min while theH2sample was facing a hot W-Ðlament. After this treat-ment LEED showed a clear 1] 1 pattern with notraces of reconstruction. Deposition on the 1 ] 1 : Hsurface at 570 K leads to a high maximum in p-ppolarized SHG at 4 ML followed by a second maxi-mum at 10 ML. The 4 ML maximum is also present forthe substrate but absent for growthJ3 ] J3 : Agon the hydrogen-terminated surface at RT.

Deposition on the 7 ] 7 surface at low temperatures(Fig. 1) and on the hydrogen-terminated surface at RT

Figure 3. Second-harmonic generation (SHG) as a function ofcoverage on a hydrogen-terminated substrate at 570 K recordedfor p-p and p-s polarizations. The p-p signals obtained for roomtemperature growth on the 1 Ã1 : H and the surfacesJ3ÃJ3 : Agare also shown.

( 1998 John Wiley & Sons, Ltd. Surf. Interface Anal. 26, 872È875 (1998)

874 K. PEDERSEN ET AL .

Figure 4. Comparison of oscillations in p-p polarized SHG withcoverage recorded for three different substrate conditions. The 10ML oscillation period is indicated.

(Fig. 3) clearly leads to oscillations in SHG, with cover-age and absence of a maximum at 4 ML. Figure 4shows these results together with results obtained forRT growth on a 7 ] 7 substrate exposed to 60 Loxygen. All signals rapidly increase during deposition ofthe Ðrst ML and then tend to oscillate within a 10 MLperiod.

DISCUSSION

Two interesting e†ects will be discussed in the follow-ing : namely, enhancement of SHG due to island forma-tion, and the oscillations in SHG with coverage in thelayer-by-layer growth situation.

Island formation

An enhancement of SHG due to plasmons has beeninvestigated by Aussenegg et al.7 for Ðlms consisting ofAg particles on a glass substrate. The particle sizeincreased with average mass thickness until a compactÐlm was reached at 12 nm. The p-p polarized SHGshowed a maximum enhancement of D10 compared tothe compact Ðlm when the average mass thickness was6 nm, corresponding to 10È15 nm spherical islands. Thes-p polarized signal showed a maximum for the sameparticle size and vanished for the compact Ðlm.

Although the enhancement mechanism for Ag on Si isexpected to be of the same type as for the glass sub-strate, the maximum (at 4 ML) and the compact Ðlmregion are reached at much lower mass thickness for theSi substrate. In their STM investigations Meyer et al8and Park et al9 both found that the average island sizeincreased with temperature and with coverage. Park etal9 found an average radius of 4 nm after RT growth of1.5 ML Ag on the 7 ] 7 surface. Because this radius

grows with coverage, the sizes at the signal maxima arecomparable for glass and Si substrates. The islands onSi are therefore thinner than on glass. Moreover,because comparable enhancements are obtained for thetwo substrates, the particle size along the surface seemsto be important for the enhancement. The enhancementdecreases after the maximum when the deposited Agcoalesces into a compact Ðlm, because this removes theplasmon resonance.

Further information about the Ðlm morphologycomes from the oscillations in SHG with coverage.Oscillating SHG vs. Ðlm thickness has previously beenobserved and discussed by Kirilyuk et al.3 for Au andCu Ðlms grown on Co(100), where a period of 14 MLwas observed. The SHG from Au Ðlms on Si(111) alsoshows this oscillation period.4 Kirilyuk et al.3 foundthat the non-linear response of each of the two Ðlminterfaces varied with a 7 ML period, in agreement withlinear optical results, but interference between the twocontributions resulted in doubling of the oscillationperiod for SHG. Quantum-well states in thin metalÐlms on metal substrates have been observed in photo-emission and inverse photoemission. Ortega et al.17found oscillations with a period of 5 ML in the densityof states at the Fermi level for Ag on Fe(100). Consider-ing the period doubling, the 10 ML period presentlyobserved in SHG agrees with the photoemission results.

The maximum near 1 ML in the curves on Fig. 4 canalso be caused by quantum-well states. At least twoe†ects can lead to the irregular distance betweenmaxima, i.e. 6È8 ML at low coverage and 10 ML athigher coverage : the shape of the islands and therebythe ratio between island height and coverage maychange with coverage, which also explains the di†erenceamong the substrates in Fig. 4 ; and the appearance ofperiod doubling giving rise to the 10 ML periodrequires that the two Ðlm interfaces be fully developed,which is not the case for low coverage. The presentSHG results therefore demonstrate that quantum-wellstates can be observed also with the Ag/Si(111) system.

The SHG oscillation amplitude should be highest forperfect two-dimensional growth and should decreasedue to Ðlm roughness. An island Ðlm can also give oscil-lations if the islands are Ñat and have little variation inheight. This is the case for the signals in Fig. 1, whichdo tend to oscillate even at 450 K, although roughnessenhancement is observed. The variation in island heightmust therefore be less than the 5 ML period for theresponse of the individual Ðlm interfaces.

Substrate surface modiÐcations

ModiÐcations of the substrate surface by introductionof foreign atoms can change the growth mode of thinÐlms. Hydrogen termination in particular has attractedmuch attention as a useful modiÐcation of Si surfaces.The presence of hydrogen on the surface prevents theformation of the structure for growth atJ3 ] J3 : Aghigh temperatures, which otherwise would lead to theformation of very thick islands.13 After growth at 570 Kthe Ag Ðlm showed a Ag(111) LEED pattern and rota-tionally anisotropic SHG (cf. the p-s signal on Fig. 3).This shows that an ordered structure is formed, inagreement with the results of Sumitomo et al.,13 who

Surf. Interface Anal. 26, 872È875 (1998) ( 1998 John Wiley & Sons, Ltd.

Ag ON Si(111) STUDIED BY SECOND-HARMONIC GENERATION 875

found epitaxial growth of a single domain type underthese conditions. In all other cases studied here, Aggrowth did not lead to observable LEED patterns orrotationally anisotropic SHG.

The growth at 570 K on the hydrogen-terminatedsurface leads to enhancement of SHG and thus islandsizes comparable to those found on the 7] 7 surface.However, an additional structure, which could be oscil-lations due to quantum-well states, is clearly seen on the1 ] 1 :H substrate but is barely observable on the 7 ] 7substrate. It is thus seen that the islands on the 1 ] 1 : Hsubstrate grow with temperature and enhance SHG butthe islands must be Ñat with little variation in height inorder to produce clear quantum-well e†ects.

Figure 3 shows that reconstruction into a J3surface does not improve the conditions for] J3 : Ag

growth of Ag, in contrast to what was observed for Auon this surface.18 Exposure to on the other hand,O2 ,leads to clear quantum-well oscillations (cf. Fig. 4). Itmay at Ðrst sight be surprising that the disorderedsurface produced by oxygen adsorption improves thegrowth characteristics compared to the clean surface.However, recent x-ray di†raction experiments19 haveshown that the interface between thick Ag Ðlms andSi(111) has a modiÐed 7] 7 or 1] 1 structure, depend-ing on the temperature, while there was no indication ofthe reconstruction. Oxygen exposure at RTJ3 ] J3does not entirely remove the 7] 7 reconstruction,although it is somewhat disordered. The oxygenatedsurface, contrary to the surface, thus has aJ3 ] J3structure compatible with the preferred Ag/Si(111) inter-face structure. The lower enhancement of SHG compa-

rable to the clean 7 ] 7 surface may be the result of alower di†usion rate along the surface, and thus smallerislands, due to the inserted oxygen in the surface.

CONCLUSIONS

The present work demonstrates how recording of SHGduring deposition provides information about the Ðlmmorphology on a monatomic height scale. Two e†ectsof electron conÐnement dominate the steps of change ofSHG from that of the Si(111) surface(s) to SHG of pure,compact Ag : enhancement due to plasmon resonancevarying with island size ; and quantum-well e†ectsvarying with Ðlm thickness. The surface conditions arefound to be of little importance when the purpose is togrow compact Ðlms of [25 ML thick. At low coverage,not only hydrogen termination but also passivation byoxygen improves the Ðlm quality. It is concluded thatthe 7 ] 7 and 1 ] 1 :H structures lead to better thinÐlms than the structure because theyJ3 ] J3 : Agmatch the preferred Ag/Si(111) interface structurebetter. Finally, quantum-well states are found in AgÐlms, also in cases where the Ðlm forms islands.

Acknowledgements

This work has been supported through grants from Ib Hen-DirektÔrriksens Fond, det Obelske Familiefond (the C. W. Obel Foundation),the EU Human Capital and Mobility Project and The DanishResearch Council for the Natural Sciences.

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( 1998 John Wiley & Sons, Ltd. Surf. Interface Anal. 26, 872È875 (1998)