Studies of hydrogen adsorption on silicon (100) surfaces by means of time-of-flight type electron stimulated desorption spectroscopy (TOF-ESD)

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Studies of hydrogen adsorption on silicon (100) surfaces by means of time-of-flight type electron stimulated desorption spectroskpy (TOF-ESD) Kazuyuki

Ueda, Shinji

Kodama

and Akemi Takano,

Department

of Applied

Physics, Osaka University,

Suita,

Osaka 565, Japan received

4

October

1991 and accepted

29 January

7992

The termination with hydrogen of a silicon surface for the epitaxial gro wth becomes an important technique. The adsorption processes of atomic hydrogen on Si(700) surfaces have been studied by means of a time-of-flight type electron stimulated desorption spectroscopy (TOF-ESD). The ES0 has a fairly high sensitivity for the yield of H’ against atomic hydrogen exposure. A characteristic of hydrogen adsorption reveals different modes for different surface orientations. An energy distribution curve of desorbed ion was also obtained from the TOF spectrum. Over 7000 Langmuir exposures of atomic hydrogen gave the structural change from the (2 x 7) to (7 x 7).

1. Introduction In industrial technology there are many important processes involving the treatment of silicon surfaces, e.g. the growing of high quality oxide thin films, and the growing of metallic thin films in atomic layer epitaxy, and so on. To produce an inactive surface of silicon for the device fabrication process, the silicon surfaces are often stabilized by atomic hydrogen prior to molecular beam epitaxy’,2. Many studies reported so far have investigated phenomena which are due to the interaction between a substrate and hydrogen, but where there has been no direct detection of hydrogen on the surface except via high resolution electron energy loss spectroscopy (HREELS)3 and infrared reflection spectroscopy (ir)“. Sakurai and Hagstrum have described the surface reconstruction of (7 x 7) to (7 x 1) on atomic hydrogen adsorbed Si( 111) surfaces5. In temperature programmed desorption (TPD), the existence of two adsorption states of hydrogen on Si surfaces has been described by many authors” *. Several adsorption modes have been investigated by high resolution electron energy loss spectroscopy (HREELS)‘,’ “. Theoretical predictions are also in good agreement with experimental results ’ 3 ’ ‘. However, there still remain plenty of problems concerning quantitative analysis, and the understanding of hydrogen behavior on the solid surface. The precise role of hydrogen is little known due to the difficulty of hydrogen detection. Those phenomena due to hydrogen adsorption which have been so far investigated experimentally include surface reconstruction, electronic density of states, and Si-H bond oscillation. There have been several pioneering studies of hydrogen detection on semiconductor and metal surfaces using Rutherford backscattering (RBS) 16.” which is less surface sensitive, and electron stimulated desorption (ESD) by Lichtmann et al’* and Johnson et al 19.In some adsorption systems, because the ESD yield of the proton is very small, dynamic measurements of the

progress of hydrogen adsorption could not be performed in real time. Here,we propose a dynamic measurement of the interaction of the silicon surface with H by TOF-ESD, applying the method to different pretreatments of silicon surfaces. We will describe the adsorption characteristics of hydrogen on different surfaces of the Si(100) plane which is used widely in the fabrication of electronic devices. 2. Experimental Detailed descriptions of the experimental apparatus have appeared in previous reports”,“. Briefly, Figure 1 shows schematically the apparatus of our experimental set-up. The TOF-ESD apparatus is a stainless steel chamber of volume about 100 litres. The base pressure of the experimental chamber is normally 1 x lo- lo torr after 8 h baking out, and the working pressure is 3-5 x 1O- ’ O torr. The primary electron beam was pulsed with about 150 ns width, and an energy of 300 eV. The incident angle of the electron beam is 45 with respect to the sur-

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Figure 1. Schematic drawing of experimental apparatus for TOF-ESD. Primary electron energy is 300 eV, pulse width 150 ns with duty cycle of 400 Hz. Specimen bias is variable from 0 to 90 V.

795

K Ueda et al: Hydrogen adsorption fitcc normal of the specimen. The collection angle of dcsorbcd ions by a microchannel plate (MU’) with an cffcctivc diamctcr of 40 mm is along the surface normal. The flight path of ions bctwcen the specimen and detector is 150 mm. Spccimcns arc cut to a size of 13 x 4 x 0.3 mm’ from Si( 100) tlat wafers (within kO.5 ) which arc p- and n-type with 3-5 Qcm. While the vicinal surl’ace is cut at about 4 from the surface normal towards the [I IO] axis. Each specimen was positioned on the Vuriain manipulator. supported by a molybdenum holder III order to apply resistive heating by a dc current. The specimen was initially outgasscd in uhv at about 800 C. fashcd at 1300 C fat 30 s. followed by annealing at 1000 and X00 C for ;I few minutes. and gradual cooling to room temperature. The cleanliness of the surface was confirmed by observing a sharp RHEED pattern without any Sic pattern at a glancing angle. For about 20 min no ESD signal was obscrvcd at room temperature after clcaninp the spccimcn in uhv. A positive spccimcn bias gives a high detection sensitivity due to ion acceleration. reducing the neutralization probability ol desorbing positive ions. and also concentrates the angular distribution, etc. Changes of dctcction sensitivity are shown in Figurc 2 which wcrc obtained by changing the spccimcn bias in the range Icro to 90 V. for the proton ESD signal. In the figure ;I signal P is associated with an X-ray intensity gcncratcd b> incldent clcctrons at the spccimcn surface. Normally it is taken ;I\ the starting time of the desorbed ions. In the prcssurc range 01 about 5 x 10 ‘(’ torr. the proton signal appears about 20 min after cleaning as a result of adsorption of residual wntcr vapor or hydrogen on the specimen. Thus, small oxygen signals in the low energy side at C’.s= 90 V in Figure 2 are likely to be impurities on the surface accumulated overnight. Hydrogen gas was introduced into the uhv chamber through a variable leak valve via the liquid nitrogen cold trap for a dcsircd period at room tempcraturc. During hydrogen exposure. a tungsten hot tilamcnl (7000 C) was used to make atomic hydrogen in the chamber. Impurity gases made up less than about 2% of the signal intcnsit) 01 ;I quadrupolc mass-spcctrometcr (Q-MAS). After each exposure. hydrogen gas was quickly evacuated to uhv bq 2 400 I s ’ noble gas ion pump. and subscqucntly Tot;-ESD

mcasurcment was performed with ;L spccimcn bias of ‘8) V and with zero spccimcn bias at the end of the cxpcrimcntal run. This mcasurcmcnt was useful in converting the TOF spcctrum into an energy distribution curve. A structural change due to hydrogen corrosion was also observed by RHEED aftct1000 L-H exposure as has been reported previously”. 3. Experimental

Since the ESD signal associated with molecular hydrogen 15 dominnnt in a hydrogen atmosphere in the 10 ” torr range. in order to avoid signals from the gas phase prior to ESD mcasurcment. the introduced hydrogen was removed to give uhv conditions immcdiatcly after the dcsircd exposure. Figure 3(a) shows proton yields against hydrogen cxposurc cxprcssed in Langmuirs (I x IO ” lorr 5 ‘) for the Si(lOO)-(2 x I), /J-lypc flat surfacc (within +O.S ). An inset shows a scrics ofTOF spectra obtain& at a specimen bias of 90 V after each cxposurc. Since thcrc were slight changes in electron gun emission current and MCP gain due to hydrogen adsorption. the ESD signal intensities wcrc normalized by each P-peak intensity. which is proportional tn the number of incident electrons. Figure 3(b) shows the kinetic cncrgy distribution curve (KEDC) ofthedesorbed protons which ha5 hccn calculated from the spectrum obtained at the end of the cxpozurc (1160 L). As rcportcd previousI> h) L;cda 1’1 C/I“. the SI( IO(J)-(2 x I I

k (a)

0 FLIGHT

(b)

2

4 TIME

6 ,“,us

Figure 2. A series of TOF spectrum changing High1 Lime dcpcnding on the spccinlcn bias on H:Si( 100) system. The higher biases. the larger S/N lYlli0.

796

results

0

500 1000 l%EcoTm H7 EXPOSURE/‘L

KINETIC

ENERGY

/eV

yield LSatonG hydrogen exposure in Langmu~r ( I x IO torr s ‘) Ihr Si( 100) /j-type fat surface (+0.5 ). The primzq electron cncrpy is 300 eV and specimen bias 90 V. The inset reveals original spectra against hydrogen exposure. (h) A KEDC obtained at the end of the‘ cxpcrilnental run which corresponds to 2 160 L-H and convcrtcd numerically f’rom the TOF spectrum. Figure 3. (a) tl

K Ueda et al: Hydrogen

adsorption

surface allows adsorption of molecular hydrogen even at room temperature. Here, the step-wise adsorption of the flat surface shows a linear increase of proton yield against exposure as shown in Figure 4(a). In this case the tungsten filament and the Q-MAS filament were turned off during the experiment, and the ESD measurement was performed under the same conditions as that of Figure 3(a). In Figure 3(a), the ESD yield shows a peak at about 500 L and then gradually decreases with increasing exposure. In the case of the experimental run without the hot filament [Figure 4(a)], the H+ yield increases linearly at least until an exposure of 2160 L. This is a large difference compared with Si(l11)7 x 7 surfaces, and other experiments (e.g. Ibach and Rowe”). At the end of the final exposure in an experimental run, a spectrum obtained without the specimen bias was used to convert our results to a kinetic energy distribution curve (KEDC) form. Figure 4(b) shows the KEDC of desorbed protons obtained from a molecular hydrogen adsorbed surface. No essential difference exists between Figures 3(b) and 4(b) in KEDC except in the intensity. As described above, the proton yield is very small if the specimen bias is zero, and in such cases no smoothed spectrum was obtained due to low S/N ratio, and the converted KEDC is not smoothed. Specimens of n-type Si(lO0) were also studied in the same manner as described above. The results are almost the same except that peaks in the KEDC are somewhat higher energy than for the n-type case. The desorption characteristic of protons for vicinal surfaces of the p-type is shown in Figure 5. This is very

12 a > + (a)

(b)

4 FLIGHTTIME /us I 0

I

I

500 1000 1500 2000

KINETIC

ENERGY

/f?v

Figure 5. (a) ESD yield of H+ vs molecular hydrogen exposure for Si( 100) 2 x I,p-type flat surface. (b) KEDC of H+ after 2160 L-H.

different from the previous results in Figures imental data are easily reproducible.

3 and 4. Exper-

4. Discussion

(a)

z3._

(b)

H2

EXPOSURE/L

I

I

I

I

I

’

KINETIC

ENERGY /eV

Figure 4. (a) ESD yield of H+ vs exposure of molecular hydrogen without

W-hot filament; and (b) KEDC of H+ obtained after the final exposure.

Figure 3(a) reveals adsorption behavior characteristic of stepwise adsorption of atomic hydrogen produced by the tungsten hot filament for the flat surface of the p-type Si( 100) 2 x 1, where the proton yield has a peak at about 500 L and then gradually decreases with increasing hydrogen exposure. Though it is not yet possible to make quantitative analysis in ESD measurements, this tendency is very similar to the HREELS results for hydrogen on Si(ll1) described by Kobayashi et al ‘O. According to their description, coverages of hydrogen are B - 0.9 at 100 L and 0 > 1 at 500 L, respectively. In this region t-SiH(tangential), SiH, and SiH, are formed. At 0 > 1, the energy loss peak of 1.6 eV corresponding surface state transition (dangling bond surface states) is completely extinguished. The great difference between results with and without the hot filament is attributed to structural change by atomic hydrogen on Si( 100) 2 x 1 to the 1 x 1 structure as described previously by Kato et al 24. Such a structural change may be induced by hydrogen corrosion. Gates et al ’ state that in the H/Si(lOO) system several kinds of hydrides (SiH,, Si,H,, Si,H, etc.) have been observed by TPD, though in our TPD system they were not observed due to lower sensitivity. Therefore, over about 1 ML coverage of hydrogen, corrosion areas on the surface have many defects which act as neutralization centers for desorbed ions and they result in a decrease of proton yield. The above description 797

K Ueda et al: Hydrogen adsorptlon is supported by the fact that a structural change from 2 x I to 1 x 1 has been observed by RHEED after 1000 L cxposurc. A vicinal surface of Si( 100) has different hydrogen adsorption characteristics independent of dopant material. In adsorption experiments with the tungsten hot filament. the proton yield increased quickly in the initial stages and then saturated at a final value. The structural change from 2 x I to 1 x I was also obscrvcd in RHEED. In the sense of epitaxial growth. commercially available silicon wafers which are normally vicinal surfaces are suitable for the hydrogen mediated epitaxy. In contrast, in the hydrogen adsorption experiment without the hot filament, the proton yield increased linearly with the exposure. and further. no structural change was observed in RHEED. Since the yield depends on the coverage. these facts suggest the proton might be excited directly from adsorbed molecular hydrogen. It is considered that molecular hydrogen on the silicon surface has no strong interaction associated with electric charge and does not change the surface electronic state which retains a low neutralization probability for dcsorbed protons. Within the accuracy ofour experiments. no essential difference of ESD yield was observed between n- and p-type Si( 100) Ilat surfaces. In the case of p-type silicon the ESD yield of protons has a peak at about 500 L and decreases immediately. This fact suggests that the surface was covered by monolayer hydrogen before 500 L exposure, over the monolayer of hydrogen the hydride surface changes to be metallic-like. This is supported bq Souris (‘/ ~1~’ who have reported that the w:ork function decreased 0.34 eV after atomic hydrogen adsorption on the Si( 100) surface. This fact suggests that the reneutralization probability- for desorption goes up and leads to a lower ESD yield. Since. after the final exposure in each experimental run. the cncrgy distribution curves of dcsorbed ions wcrc convcrtcd from TOF spectra numerically, thcrc are some errors on the low energy side, below 2 cV. this region contains noise and signals due to kery slow velocity ions. It is tentatively considered for each distribution that thcrc are roughly three components. namely. their peaks arc about I .5. 3.5 and 6 eV. respectively. These peak values may correspond to the ground states of different adsorption modes of hydrogen. According to the experimental results threshold is at about by .Avouris et nl 7h. the H’ dcsorption 21 eV for H/Si( 100). H;Si( I I I) and NH ?!Si( 100) systems. This observed threshold is higher than any substrate single-particle excitations. Thus. they have concluded that H’ desorption from Si occurs via a multi-electron excitation mechanism which directly places two holes in the SiH bonding orbital and gives the repulsive state to be Hi dcsorption. Yasuc and Ichimiya have supported the above mechanism”. In our preliminary experiment, as a threshold of a proton is slightly different for difrercnt substrate conditions, details associated with the excitation mechanism of the HiSi system will be published in relation to the threshold and energy distribution curves. In conclusion for atomic hydrogen adsorption a hydrogen termination for Si(lO0) is more eflective for the vicinal surf&c

798

than the flat surface. Over IO00 L exposure of atomic hydrogen the superstructure disappeared changing to I x I structure. however. the superstructure remained with molecular hydrogen exposure. The energy distribution curve of desorbcd ions shows a broad peak. however, at the beginning of adsorption the peak tends to remain in the lower energy side in comparison with the end ot exposure.

Acknowledgements

authors M,ish to thank Professor R Shimizu lor helpful suggestions during this work. One of the authors (KU) wishes to thank Dr P Cumpson (NPL. UK) who carefully I-cad the manuscript and gave comments prior to publication. This work was supported in part by a Grant-in-Aid for Scientific Research on Priority Area (Functional Materials) No 032050X1 and also for Dcvclopmcntal Scientific Research (B) No 02555005 by the Ministry of Education. Science and Culture The

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