Ag system

~i~i~i~ii!iiii~ii~iii~iii~i~!~i~i~iiiiii applied surface science ELSEVIER

Applied Surface Science 123/124 (1998) 476-479

Interface structure analysis of Si/(H)/Ag system S. Iida *, M. Koike, H. Banshoya, T. Yamauchi Dept. of Electrical Engineering and Electronics, Osaka Sangyo Unicersity, 3-1-1 Nakagaito, Daito, Osaka 574, Japan

Abstract A comparison of Schottky barrier heights was carried out between samples of an H-terminated S i - A g and samples having thermal desorption of H from the Si-Ag. The barrier height of the latter samples increased more than that of the former except in the case where the Ag ion was deposited at an acceleration voltage of 1500 V, The experimental analysis suggests that the Ag ion accelerated at a high voltage bonded between the surface atom and the back bond of Si, suffered little degradation by the presence of H, and made a good S i - A g interface. © 1998 Elsevier Science B,V. Keywords: Schonky barrier; Adatom; Back bond; H-termination; Interface

1. Introduction The crystalline structure of Ag film formed on an H-terminated Si surface had been reported by Oura et al. [ 1]. They found that the Ag films changed from a flat or island structure depending on the presence of H on the Si substrate. To fabricate a flat thin film, the cluster ion beam deposition method was proposed [2]. This method is known to fabricate a good crystalline film on a substrate, although the method has disadvantages in that the mechanism of ion source is complex and the ionization ratio is unstable. As already reported, by using an advanced ion beam deposition method [3], we fabricated Ag and Cu films on Si substrates and examined the changes in Schottky barrier height [4]. As an extension of that study, we express here new experimental results. We measured the Schottky barrier heights (SBH) of Ag

* Corresponding author. Tel.: +81-720-75-3001; fax: +81720-70-8189; e-mail: [email protected].

thin film, that was fabricated by the ion beam deposition method on H-terminated Si substrates and analyzed the bonding positions of H and Ag on the Si surface by RHEED (reflection high energy electron diffraction). Then H was thermally desorped from the Si substrate, and the SBH change was compared with that of the H-terminated Si substrate. As an ex situ study, we conducted film crystal structure analysis by XRD (X-ray diffraction), and surface structure observation by STM (scanning tunneling microscopy). These studies were used with in situ study results to analyze the film structure and A g - S i interface conditions.

2. Experiments A substrate was cut from a single crystal of a p-type Si(111)7 X 7 wafer with resistivity of about 3 1~ cm. Then the substrate was washed in a 1% HF solution. After treatment, it was boiled in ultra-high purity water to form a flat surface [5]. Next, it was installed in the main UHV vessel (1 X 10 -9 Torr)

0169-4332/98/$19.00 © 1998 Elsevier Science B.V. All rights reserved. PII S0 169-4332(97)00526-6

477

s. lida et al./ Applied SurJace Science 123/124 (1998) 476-479

via a sub-chamber and heated at 1250°C for several minutes by direct current. By such treatment, the surface showed a well defined RHEED pattern of a 7 X 7 crystal structure at RT (room temperature). Then H was terminated on the substrate by using a W-filament that was heated at 1800°C. By inspecting the RHEED pattern, we observed that H terminated on the Si surface (Si/H). Then Ag film was fabricated on the S i / H surface by the ion beam deposition method to make the S i / H / A g system. Ag was deposited by both the conventional Ag vapor deposition and Ag ion beam deposition. When Ag was ionized, the acceleration voltage (I~doc) changed from 0 V to 1500 V. During deposition, the film structure was examined by RHEED pattern analysis. o At the film thickness of 200 A, the deposition was stopped and the SBH was obtained by I - V measurement. After measurement, the sample was transported to the sub-chamber, and the sample holder in the main chamber was heated to 420°C to desorp H from it. After confirming that the H density decreased to the background state, temperature was decreased to RT, and the sample was returned to the holder and heated at 350°C to desorp H on the sample. The density of H in the vessel was monitored by a quadrupole mass spectrometer. We call such a sample the S i / ( H ) / A g system when H has been desorped. We measured the SBH of the sample at RT. Then we took the sample from the vessel and carried out XRD and STM measurements.

We report here that for form a flat film on an H terminated Si surface, an ion's kinetic energy is also important. (1) Normal deposition and rg~cc range of V,~c~ < 250 V. (a) S i / H / A g system. As shown in Fig. 1, the SBH was larger than that of the S i / ( H ) / A g . At Edc~ = 0 V, the SHB was 0.87 eV and decreased to 0.78 eV when ~a~ was larger than 250 V. As shown in Fig. 2(a), the Ag film's crystal structure obtained by XRD was a (111) structure, and the crystallization did not significantly vary with the Ea~ value. However, the surface roughness obtained by STM, was the poorest in comparison to the samples manufactured by l~c~ values higher than 250 V. This reason was considered that the kinetic energy of the Ag ion being too small to make a flat surface. Moreover, the ion's weak kinetic energy was not sufficient to improve the interface between H-Si and Ag. From RHEED analysis, we observed that Ag bonded to the Si adatom in the same way as H. From these results, the SBH did not decrease. This means that the H on Si worked to fabricate a good Ag film crystallization but interfered with the decreasing of the SBH. In Fig. 1, at V~ = 0, a discrepancy between SBH's for the samples of ionization and no ionization had occurred by the charging effect or not. The effect is evident at the initial stage of the film growth.

i

3. Discussion '~'

The obtained results showed that the SBH, RHEED pattern and Ag film surface structure were influenced by the presence of terminated H on the Si surface. In addition, it was found that the value of ~c~ was also important for improving the SBH and the Ag film crystallization. From the RHEED analysis, H terminated on the Si surface was considered to be bonded on the Si adatom by taking a monolayer structure. Several studies have reported [6-8] that H strongly bonded with a metal and worked to rearrange the atoms on the metal's surface. These papers, however, did not report the effect of metal ion deposition having acceleration energy on the H-terminated Si surface.

.... 81/H/Ag S[/(H)/Ag

- -

",

.~

@Si/H/Ag

No Ionization

•

OSi/(HI/Ag NO Ionization

= 0.8

i

°"o

5do

ldoo

Ion Acceleration Voltage Vacc

5'oo iV]

Fig. 1. Change in Schottky barrier height depending on ion acceleration voltage. Two kinds of samples are shown: fabricated by conventional deposition (no ionization) and by ionization deposition,

478

S. lida et al. /Applied Surfitce Science 123 / 124 (1998) 476-479 Ag(11 i)

roughness changed greatly and increased more than that of S i / H / A g . This was considered due to the fact that since the Ag film's thickness was only 200 o A, the interface's roughness made by Si and Ag mismatch appeared directly on the surface. Therefore, we decided that Si and Ag made a direct bonding after H desorption. The evidence suggests that the role of H on the Si worked to alleviate the mismatch between Si and Ag. (2) ~,~ range of 250 < F ~ < 1500 V. (a) S i / H / A g system. As expressed above, the film's crystallization has a poor correlation to SBH, but the surface became flatter with increasing V~,~c.A sample of such a surface is shown in Fig. 3(a). This surface is much flatter than the film manufactured at ~,~. < 250 V, and the SBH decreased to about 0.77 eV from 0.85 eV. However, as expressed in the next section, there was the possibility that the kinetic energy of the Ag ion left some distortion on the interface between H - S t and Ag. We also inspected the Ag bonded to the Si adatom in the same way as done in (1)-(a). (b) S i / ( H ) / A g system. As shown in Fig. 2(b), the film's crystallization did not significantly change; however, from Fig. 3(b) it can be seen that the surface condition became poorer than that of the S i / H / A g . We assume this is due to the distortion still remaining at the Ag and Si interface, which is furthermore believed to be the cause of the SBH increase to higher than 0.80 eV. A similar effect was suggested for the inhomogeneity at the interface of A g / S i ( l l l ) [9]. The distortion was assumed to be

e00( / / ~ i

400

O 200

0 2 0 (degree)

(a) Si/H/Ag

2 O

(degree)

(b) Si/(H)/Ag

Fig. 2. Crystalline change in the Ag film on the Si substrate obtained by XRD. (a) Si/H/Ag, with H terminated; (b) Si/(H)/Ag, with H desorpecl. New crystalline (200) and (220) appeared after H desorption.

(b) S i / ( H ) / A g system. After H was desorped, the SBH decreased to about 0.77 eV from 0.87 eV. This was considered to be caused by direct bonding of Ag and St. This assumption was made on the basis of the following observations. By comparing Fig. 1 and Fig. 2 we found that throughout all V~ value, there were no correlations between change in SBH and film crystallization, After H desorption, the film's crystal changed to a slightly complex structure. However, their ordinate did not correspond to the change in SBH. On the other hand, the surface

o

AIJ

Wtt

mJNV/

/

~

nm

40

o

(a)

Si/H/Ag

20

(b)

Si/(H)/Ag

Fig. 3. Surface structure of the Ag film observed by STM. The fihn was fabricated at V~ = 500 V. (a) Si/H/Ag system; (b) Si/(H)/Ag system.

S. lida et al. / Applied Surjace Science 123 / 124 (1998) 476-479

479

a direct bonding that caused the SBH to decrease and the kinetic energy was used to form a good crystallization with the flat surface. Accordingly, the SBH was not little influenced by the presence of H on Si.

4. Conclusions

(a)

Si / H/Ag, Vacc = 500V

(b)

Si/H/Ag, Vacc= 1500V

Pig. 4. RHEED photos of film thickness at 10 A manufacturedat (a) V~c = 500 V; (b) V~c = 1500 V. Note streaked lines appeared

The SBH between Si and Ag was controlled by the presence of H on the Si. This H worked to form a flat and well crystallized Ag film on the Si and decreased the SBH when the Ag ion was accelerated in the range of 250 V < V~cc< 1500 V. Therefore, these characteristics changed after the desorption of H from the samples. When the Ag ion was accelerated at V,~cc = 1500 V, Ag bonded to between the surface atom and the back bond of Si, as well as the Si adatom and these characteristics made little change to the termination of H on the Si.

Acknowledgements The authors wish to thank Mr. M. Ashimura and the our laboratory staff.

in (b). much bigger than in the case of the samples manufactured by V,~ < 250 V. The RHEED pattern also changed after H desorption, and many streaked lines were observed. (c) Vd~c range of Vac~ = 1500 V. There was no difference in SBH between the S i / H / A g and S i / ( H ) / A g systems; their values were about 0.79 eV. As shown in Fig. 4, the RHEED patterns differed at the initial stage of Ag deposition. From the analysis of the RHEED pattern, when Ag was accelerated at V,d~ = 1500 V, it bonded to between the surface atom and the back bond of Si, as well as the adatom of Si. This means that some Ag and Si made

References [1] K. Oura, M. Naito, F. Shoji, Microbeam Anal. 2 (1993) 130. [2] I. Yamada, G.H. Takaoka, Jpn. J. Appl. Phys. 32 (1993) 2121. [3] S. lida, T. Nakamura, Y. Ashimura, T. Shindo, Jpn. J. Appl. Phys. 34 (1995) 4920. [4] S. Iida, T. Shindo, S. Matsuura, Y. Ashimura, Nucl. Instr. Meth. Phys. Res. B 21 (1997) 162. [5] S. Watanabe, N. Nakayama, T. Ito, Appl. Phys. Lett. 59 (1991) 1458. [6] R. Imbihl, M.P. Cox. G. Ertl, J. Chem. Phys. 84 (1986) 3519. [7] W. Moritz, D. Bunsenges,Phys. Chem. 90 (1988) 184. [8] P. Feulner, D. Menzel, Surf. Sci. 154 (1985) 382. [9] H.H. Weitering, J.P. Sullivan, R.J. Carolissen, R. PerezSandoz, W.G. Graham, R.T. Tung, J. Appl. Phys. 79 (1996) 7820.