The valence band alignment at ultrathin SiO 2 / Si interfaces

High resolution x-ray photoelectron spectroscopy has been used to determine the valence band alignment at ultrathin SiO2/Si interfaces. In the oxide thickness range 1.6–4.4 nm the constant band-offset values of 4.49 and 4.43 eV have been obtained for the dry SiO2/Si~100! and the wet SiO2/Si~100! interfaces, respectively. The valence band alignment of dry SiO2/Si~111! ~4.36 eV! is slightly smaller than the case of the dry SiO2/Si~100! interface. © 1997 American Institute of Physics. @S0021-8979~97!00203-X#

Ultralarge scale integration ͑ULSI͒ of metal-oxidesemiconductor ͑MOS͒ devices will need reliable gate oxides thinner than 5.0 nm in the near future.In order to predict the tunneling leakage current through the ultrathin gate oxides, an accurate description of the energy band profile at the ultrathin SiO 2 /Si interfaces or a precise knowledge on the value of the valence band alignment and the conduction band barrier height is required. 1Although the energy band profile of thick SiO 2 /Si interfaces has been determined by an internal photoemission technique, 2 reliable values for the electron or hole barrier height at ultrathin SiO 2 /Si interfaces remain an unresolved issue.Among the various attempts that have been made in the past to gain a comprehensive understanding of the barrier height or the energy band profile for ultrathin SiO 2 /Si interfaces, [3][4][5][6][7][8] the consistent picture of the energy band profile has not yet been drawn.Horiguchi and Yoshino 3 have reported that the barrier height for SiO 2 /Si͑100͒ decreases when the oxide thickness becomes thinner than 3.1 nm.On the other hand, by using an electron-beam-assisted scanning tunneling microscopy technique, Heike et al. 4 have concluded that the barrier height at the SiO 2 /Si͑100͒ interfaces keeps a constant value of 2.7 eV in the oxide thickness range 1.8-4.5 nm.Grunthaner and Grunthaner 9 have measured valence band spectra of ϳ6-nm-thick SiO 2 thermally grown on Si͑111͒ by using x-ray photoelectron spectroscopy and found the valence band offset of 4.5 eV.Also, Himpsel et al. 10 have obtained a valence band alignment of 4.3 eV for the SiO 2 /Si͑100͒ interface by using the Si 2 p core level spectrum.Thus well established values of the valence band alignment or the barrier height at ultrathin SiO 2 /Si interfaces are not available.
The purpose of our study is to directly determine the magnitude of the valence band alignment or the hole barrier height at the ultrathin SiO 2 /Si interface and derive a value for the conduction band barrier height by using a measured SiO 2 band gap, based on the valence band density of states ͑VB-DOS͒ for ultrathin gate oxides ͑below 5.0 nm͒ thermally grown on Si͑100͒ and Si͑111͒ surfaces by employing highresolution x-ray photoelectron spectroscopy ͑XPS͒.
Ultrathin gate oxides were grown at 1000 °C in a 2% dry O 2 gas diluted with N 2 or at 850 °C in wet ambient.Hydrogen-terminated p-type Si͑100͒ substrates ͑10 ⍀ cm͒ were obtained by modified RCA cleaning with a low concentration of NH 4 OH followed by a chemical treatment in a 1% HF solution.The high-resolution XPS measurements were performed with an ESCA-300 ͑Scienta Instruments AB͒, using monochromatic Al K␣ radiation ͑1486.6 eV͒ with an acceptance angle of 3.3°.The base pressure during the measurements was maintained in the 10 Ϫ10 Torr range.The Si 2 p, O 1s, and C 1s core level peaks were measured at photoelectron take-off angles of ϭ35°, 60°, and 90°, and the valence band spectra were acquired at a take-off angle of 35°.The thicknesses of the ultrathin oxides were evaluated from angle-resolved XPS measurements of the Si 2p corelevel by assuming that the Si 2 p photoelectron escape depths in Si and SiO 2 are 2.7 and 3.4 nm, respectively. 11he deconvolution of the Si 2 p core-level peak indicates that the Si 4ϩ 2 p 3/2 peak arising from SiO 2 shifts towards higher binding energies and O 1s also exhibits a similar shift when the oxide thickness increases.The observed peak shift of the Si 0 2 p 3/2 signal from bulk Si is at most 40 meV, whereas the maximum energy shift of Si 4ϩ 2 p 3/2 reaches a value of 333 meV, a factor of 8 larger.A similar result is obtained also for the binding energy of the O 1s core level peak where the maximum energy shift is very close to that of the Si 4ϩ 2 p 3/2 peak.Both dependencies reflect a differential charging effect for core-level peaks originating from the oxide layer and bulk Si.Photoelectrons emitted from the bulk Si are compensated by electrons supplied from the sample holder, while those originated in the ultrathin oxide cannot be fully compensated by tunneling electrons from the substrate.This effect leads to the formation of a positive charge in the oxide layer and the corresponding band bending of SiO 2 .Detailed treatment of this effect has been described in Ref. 12. Once the differential-charging-induced shift for each of the Si 2p components is identified, it is possible to determine the valence band density of states ͑VBDOS͒ with an exact energy scale for ultrathin SiO 2 /Si interfaces as a function of the oxide thickness by means of the XPS valence band spectra.The binding energy corresponding to the O 1s or Si 4ϩ 2 p 3/2 peak for the thinnest ͑1.6 nm͒ oxide is taken as the energy reference for all other oxides since the charging effect is minimum or negligible for this oxide.Therefore, the oxide valence band edge energies have been corrected by shifting each spectrum by the value given by the difference between the respective O 1s or Si 4ϩ 2p 3/2 peak position and that of the 1.6 nm oxide to correct the oxide charging contribution.
The VBDOS measured for the 1.6-, 2.2-, 2.8-, and 3.5nm-thick dry SiO 2 /Si͑100͒ are shown in Fig. 1.The VBDOS of the H-terminated Si͑100͒ and a 40-nm-thick dry SiO 2 are also displayed as references in the same figure, showing that the VBDOS of the various ultrathin dry SiO 2 /Si͑100͒ can be considered as a linear combination of the Si͑100͒ and SiO 2 VBDOS spectra.The method employed here to obtain the ultrathin SiO 2 VBDOS depicted of any Si substrate influence consists of subtracting the Si substrate VBDOS component, which has been measured from a hydrogen-terminated p-Si͑100͒ sample, from the measured SiO 2 /Si interface VB-DOS.
The ultrathin and thick dry oxide VBDOS show nearly identical spectra as shown in Fig. 2. Note that the top of the valence band of the various ultrathin oxides coincides with that of the thick oxide when the differential charging effect of oxides is carefully corrected.The difference between the energy position of the top of the ultrathin SiO 2 valence band and the top of the Si͑100͒ reference valence band was determined by the method described in Ref. 9, yielding the valence band alignment, which is a constant value of 4.49 eV regardless of the oxide thickness from 1.6 to 3.5 nm.An SiO 2 band gap of 8.90 eV as recently obtained for ultrathin gate oxides by the analysis of the O 1s plasmon loss peak 13 can yield the corresponding conduction band barrier height of 3.29 eV for ultrathin dry SiO 2 /Si͑100͒ interfaces.In the case of wet oxides we have obtained similar VBDOS with a constant valence band alignment value of 4.43 eV regardless of the oxide thickness from 2.5 to 4.4 nm and a corresponding electron barrier height of 3.35 eV.Thus determined valence band alignment and the conduction band barrier height of dry SiO 2 /Si͑100͒ have been used to calculate the tunneling current through 3-5-nm-thick gate oxides.It is demonstrated that the measured tunnel current density versus oxide voltage characteristic agrees well with the calculated results. 1 The values found for the valence band alignment and the conduction band barrier height are summarized in Table I for the various ultrathin SiO 2 /Si interfaces.The reported value of the conduction band barrier height for thick SiO 2 /Si͑100͒ is known to be 3.25 eV ͑see Ref. 14͒ that is close to the value obtained for ultrathin SiO 2 /Si͑100͒.It is interesting to note that the value of the valence band alignment for dry SiO 2 /Si͑100͒ is 0.13 eV larger than dry SiO 2 /Si͑111͒.Although this difference is not far from the experimental error bar of 0.1 eV, this reproducible result implies that the band alignment depends slightly on the silicon surface orientation.A possible mechanism to explain this might be the existence of strained Si-O-Si bonds near the SiO 2 /Si interface.In fact, it is shown that the LO-phonon peak of ultrathin SiO 2 on Si͑111͒ exhibits a larger redshift than that for SiO 2 /Si͑100͒. 15he redshift is largest at the interface and becomes small in the oxide layer within 2 nm from the interface.Since the redshift is explained by compressive stress in the oxide, the higher stress is existing in the SiO 2 /Si͑111͒ interface as compared to SiO 2 /Si͑100͒.Such orientation dependent stress in the interface might modify the interface dipole moment for the SiO 2 /Si͑111͒ and SiO 2 /Si͑100͒ boundaries.
In summary, the VBDOS of ultrathin oxides grown on Si is found to be nearly identical to that of thick SiO FIG. 1. Valence band density of states for ultrathin dry SiO 2 /Si͑100͒ interfaces obtained by high-resolution XPS.The VBDOS for a H-terminated Si surface and a 40.0-nm-thick dry SiO 2 layer are also displayed for comparison.The charging effect has been corrected for all spectra.

TABLE I .
Valence band alignment or hole barrier height and electron barrier height for the various ultrathin SiO 2 /Si interfaces.