Photon Factory Activity Report 2004 #22 Part B (2005)
Surface and Interface

2C/2002S2-002

Precise determination of band offsets and chemical states in SiN/Si
studied by photoemission spectroscopy and x-ray absorption spectroscopy
Satoshi TOYODA1, Jun OKABAYASHI1, Hiroshi KUMIGASHIRA1, Masaharu OSHIMA1,
2
2
2
2
Guo-Lin LIU , Ziyuan LIU , Kazuto IKEDA , and Koji USUDA
1The University of Tokyo, Bunkyo-ku, Tokyo 113-8656, Japan
2STARC, Kohoku-ku, Kanagawa 222-0033, Japan

Experimental
SiN films with different layer thicknesses were grownby chemical vapor deposition methods using N2-gas
plasma on Si substrates. Photoemission spectroscopy andx-ray absorption spectroscopy (XAS) measurements werecarried out at an undulator beam line BL-2C of thePhoton Factory in High-Energy Accelerator Research
Organization (KEK). The total energy resolution wasabout 0.15 eV using hν = 620 eV photons for Si 2p, N 1s,
and valence-band spectra. The total-electron-yield
method was used for N K-edge XAS measurements.
Results and Discussion
Valence-band spectrum of the 3.7-nm SiN film on theSi substrate normalized by a H-terminated Si spectrum isshown in Fig. 1. By subtracting the valence-band
spectrum of Si from that of the 3.7-nm SiN film, anominal SiN contribution is deduced. The valence-bandoffset (∆Ev) on SiN/Si is determined by the difference
spectrum as 1.6 eV. For the conduction-band offset (∆Ec),
N K-edge XAS is also shown in the same panel of Fig. 1.
Energies in XAS were calibrated as the relative values tothe valence-band maximum (VBM) of Si using the N 1s
binding-energy position since absorption edges appeared*by the excitation from N 1s to N 2p states which werelocated at the conduction-band minimum (CBM) as antibonding states. N K-edge XAS spectra of SiN are notinfluenced by x-ray irradiation effect such as charging
effect, since the N 1s states are also shifted together with*

the N 2p states. First differential spectrum of the N Kedge shows a peak structure, which is applicable to thedetermination of the absorption edge without ambiguity.
SiNThus, as shown in Fig. 1, ∆Ev and band gap (Eg ) for SiN
are deduced as 2.9 and 5.6 eV, respectively.
For the precise determination of the band offsets using
photoemission spectroscopy, time-dependent shifts ofspectra on a few-minute-order scale under x-rayirradiation have to be considered explicitly. The ∆Ev, theSiN∆Ec, and the Eg in SiN are summarized in Table I bytime-dependent Si 2p and N 1s spectra measurements.
The valence-band offsets in SiN/Si excluding thedifferential charging effects are close to the theoreticalvalue [3], suggesting that the x-ray irradiation time is oneof the most important parameters to determine the band
offsets precisely from photoemission spectroscopy.
Valence band hν = 620 eV

Intensity (arb. units)

Introduction
Extensive studies on nitrogen-atom doping into theconventional SiO2 films have been performed as ultra-thin
Si oxinitride (SiOxNy) films, which can protect the boron
penetration, improve the hot-carrier resistance, anddecrease the gate leakage current. In the future device
technology, the nitridation processes of SiO2 have been
excessively developed since reoxidation processes ofhigh-quality Si-nitride (SiN) films have been reported
having a high potential for high-performance 45-nm
technology node [1]. Investigations for a high nitrogenconcentration end, i.e., SiN are required for thedevelopment of the nitride technology. Photoemission
spectroscopy is one of the most powerful techniques todetermine the band offsets on gate insulators on Si [2].
We report on the precise determination of band offsets inSiN/Si excluding the charging effect.

N K-edge XAS

3.7-nm SiN
3.7-nm SiN/Si∆Ev EgSi ∆EcEgSiN

Difference

H-terminated Si
Differential-10-8-6-4-20246810

Energy Relative to VBM of Si (eV)Fig. 1. Valence-band spectrum normalized by a H-terminated Sispectrum and N K-edge XAS spectrum for 3.7-nm SiN film on
Si. Energies in XAS were calibrated to relative values from theVBM of Si. Differentiated XAS spectrum is also shown.
Table I. ∆Ev, ∆Ec, and Eg for several thicknesses of SiN films
are summarized. Values just after x-ray irradiation (t→0) arealso shown.
SiNThickness[nm]

Time (t)[min]∆E v [eV]∆E c [eV]E gSiN [eV]2.1

> 30~01.6
1.7

3.1
3.0

5.7

2.9

> 30~01.6
1.8

2.9
2.7

5.6

3.7

> 30~01.6
1.8

2.9
2.7

5.6

[1] D. Matsushita et al., Symp. VLSI Tech., p172, (2004).
[2] S. Toyoda et al., Appl. Phys. Lett. 83, 5449 (2003).
[3] J. Robertson, J. Vac. Sci. Technol. B 18, 1785 (2000).
1toyoda@sr.t.u-tokyo.ac.jp

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