Construction of the energy band diagrams

Fig. 5.13 illustrates the surface energy band diagrams of the H-terminated Si(111) surface in comparison with the methyl-terminated Si(111) surface after the annealing process. Each fresh Si sample has been first measured and then annealed at ~ 390 °C for 30 min. Only the Si(111) surface modified with CH3 groups has been sustained a second annealing at ~ 430 °C for 30 min. No pronounced changes occur in the surface electronic properties between the two successive annealing in the case of CH3-terminated Si(111) surfaces. Moreover, CH3- and CD3-terminated Si(111) surfaces present the same electronic properties after annealing.

Arbitrary, only the energy band diagrams of H and CD3-terminated Si(111) surfaces have been constructed by following the physical explanations given in Chap. 2.

However, the steps for the determination of the electronic properties shown here have been shortly summarized. First, the work function, Φ, has been determined by the extrapola-tion of the secondary electrons cutoff edge from these Si samples (not shown). Then, the sur-face band bending, eVbb, has been established from the position of the binding energy of the bulk Si 2p3/2 signal (relative to the Fermi energy) acquired from the curve fitting of the Si 2p core level emission spectra recorded at hν = 650 eV (bulk sensitive condition). For this deter-mination, the binding energy of the Si 2p signal in the bulk with respect to the valence band maximum is assumed to be 98.74 eV,^[40] and knowing the doping of the Si wafer, the position of the Fermi energy has been established. Moreover, taking into account the energy band gap of the Si (Eg = 1.12 eV) and the electron affinity of the bulk silicon as χSi = 4.05 eV,^[41] the energy band diagram can be constructed and thereby, the electron affinity, χ, and the surface dipole, δ, as well can be then easily determined. The parameters obtained from these measure-ments are reported in Tab. 5.3.

From the electronic properties determined, a trend has been observed for the modified Si samples measured before and after annealing, respectively. For the fresh samples, the position of the bulk Si 2p3/2 emission seems to shift to the lower binding energies from H-, to CH3 and CD3-terminated Si(111) surfaces. This demeanor reveals then also a decrease of the band bending which goes to the flat band conditions direction from H- to methyl-terminated Si(111) surfaces. However, the CD3-terminated Si(111) surface seems to possess the better chemical passivated Si surface because the lower band bending (0.25 eV) occurs as compared to the other modified Si surfaces (0.42 to 0.48 eV). In the ideal case for a surface, the flat band conditions appear when every surface states of a surface have been totally passivated chemically and then no band bending occurs. Moreover, the work function, the electron affinity and the surface dipole are also decreasing from the H-, to CH3- and CD3-terminated

Si(111) surfaces, respectively. The corresponding Φ found for these modified Si surfaces are 4.40, 4.05, and 4.07 eV, respectively, which is in compliance with previous studies.^[88,117] The work function reveals a higher barrier for the electrons of slightly above ~ 0.33 eV in the case of H-terminated Si(111) surfaces as compared to methyl-terminated Si surfaces. A negative surface dipole has been found for each Si sample: – 0.09, – 0.48, and – 0.58 eV for H-, CH3-, and CD3-terminated Si(111) surfaces, respectively. Here, a difference about 0.10 eV is ob-served between the two methylated Si surfaces, which is in the range of the uncertainty error for the determination of these parameters. However, this could also certainly arise from the presence of different adventitious species still present on the Si surfaces (i.e., more Br atoms on CD3-modified Si(111) surfaces, for instance). After annealing these samples, H-terminated Si(111) surfaces show no prominent change in the band bending. However, an important increase in the work function (+ 0.18 eV), electron affinity (+ 0.11 eV), and surface dipole (+ 0.11 eV) is observed.^[118] In the case of CH3-terminated Si(111) surfaces, no tremendous difference appears between the first and the second annealing step. However, in comparison to the fresh CH3-terminated Si surfaces a tendency towards the flat band conditions is observed (– 0.07 eV), while for the electron affinity and the surface dipole only a small increase ap-peared (+ 0.03 eV after the second annealing). An increase of + 0.10 eV for the work function is already observed after the first annealing step. In the case of CD3-terminated Si(111) sur-faces, here again, an increase in the work function, electron affinity, and surface dipole occurs in relation to the fresh modified Si sample. In that case, higher increase of the band bending by + 0.07 eV is observed. The change in band bending, surface dipole and work function are probably due to the desorbtion of the adventitious Br atoms with the removal of the remnant aliphatic carbons from the Si surface. However, the electronic properties of the two type (CH3

and CD3) of methylated Si(111) surfaces after the annealing step are well correlated. An error of about ± 0.02 eV is observed for both annealed surfaces, except the work function where a difference of about ± 0.10 eV appears. However, this difference could also arise from the un-certainty error from the value determined for the work function.

According to the C 1s and Si 2p core level emissions recorded, these two annealed methyl-ated Si surfaces have shown similar line shapes and behaviors which are well correlmethyl-ated here with the surface electronic properties deduced from the SXPS measurements.

pSi(111)−H

Fig. 5.13: Energy band diagrams of H- and CD3-terminated Si(111) surfaces after annealing at 390 °C for 30 min.

Tab. 5.3: Surface electronic properties of H-, and CH3-, and CD3-terminated Si(111) surfaces before and after annealing as determined from SXPS measurements.

R=

(annealed) 99.42 0.48 4.58 4.07 0.02

–CH3 99.36 0.42 4.05 3.57 -0.48

(annealed) 99.27 0.33 4.24 3.61 -0.44 incertitude ± 0.05 ± 0.05 ± 0.10 ± 0.15 ± 0.15

1^st annealing: 390 °C for 30 min; 2^nd annealing: 430 °C for 30 min; Si 2p3/2 at hν = 650 eV.