Changes in the electronic structure of the Si surface as a result of ion implantation

. The composition, structure, and physicochemical properties of the surface and near-surface layers of silicon doped with low-energy (E 0 <5 keV) Ba + and O 2+ ions have been studied using a set of photoelectron and secondary electron spectroscopy methods. It has been established that in the process of ion implantation, chemical bonds are formed between the atoms of the alloying element and the matrix, the width of the energy bands and the density of electronic states in the bands change.


Introduction
The parameters of most types of micro-and emission electronics devices depend to a certain extent on the properties of the surface and near-surface layers of a solid. To purposefully change the structure and properties of the surface layers, a controlled amount of atoms of other elements is often entered into the sample, which can be entered both from pure elements and in the form of their compounds that are present in various minerals [1]. Various methods are known for entering elements into interstructural layers of materials [2], but one of the most effective methods for introducing impurities into thin surface layers is low-energy ion implantation. Our recent studies [3][4][5][6][7][8][9][10] have shown that the low-energy ion implantation method is an effective means of creating nanosized phases and layers on the surface and in the near-surface region of materials of various nature. At present, the composition, structure, and properties of silicon-based films of various thicknesses obtained by various methods are well studied [11][12][13][14][15][16]. Some information has also been obtained on the change in the position of the maxima of the density of electronic states of silicon upon implantation of Ba+ and Na+ ions [17][18].
The main goal of this work was study the composition and electronic structure of surface and subsurface layers of p-type silicondoped by implantation of Ba+ ions with low energy (0.5 keV).

Methods
The experimental and measurement technique are described in detail in [19,20]. The design of the device made it possible to study the state and properties of near-surface layers of solids under high vacuum conditions P ≈5⋅10-7 Pa by photoelectron and secondary electron spectroscopy (PhES and SES) in combination with various methods of technological processing (ion and electron bombardment, heating, laser annealing).The use of multigrid quasi-spherical analyzers with a sector collector made it possible to study the spectra of electrons emitted from the sample surface both in the entire inverse hemisphere and in the solid angle. PhES were taken at photon energy ћν≤10.8 eV. The presence of polycrystalline palladium in the device, along with silicon, made it possible to determine the position of the Fermi level in pure and ion-doped silicon.The latter was determined from the gap between the high-energy PhES edges measured for pure palladium and silicon doped with Ba+ ions with different doses and energies.
The concentration distribution profiles of the main and impurity atoms in ion-doped silicon were determined on a standard LAS-2200 setup. The surface layers were etched with Ar + ions with an energy of 3000 eV at an angle of 15° to the sample surface. The etching rate was approximately 4 Å•min -1 .

Results and discussion
Implantation with Ba + and O2 + ions was mainly carried out at a dose of D≈5-6•10 16 cm -2 , corresponding to saturation, since a further increase does not lead to a noticeable change in the composition and properties of the near-surface region of silicon. At such a dose, the surface layers of silicon are completely disordered [20].On fig.1 shows the PhES of pure silicon and silicon doped with Ba + and O2 + ions with E0=0.5 keV and D=6•10 16 cm -2 . The abscissa shows the binding energy of electrons (the energy is measured from the Fermi level of palladium), and the energy distribution curves (EDCs) of photoelectrons are normalized so that the area under the curve is proportional to the quantum yield.  At low energies of photon (≤10-12 eV), it can be assumed that the structure of the EDCs approximately reflects the density of states of the electrons in the valence band. It can be seen from fig.1 that the structures of EDCs obtained after implantation of O2 + and Ba + ions differ sharply. Based on these spectra, the main parameters of the energy bands of the surface of the studied samples were determined [19] (Table 1). * Ф -the value of the photoelectronic work function, φ -thermionic work function, δsthe position of the Fermi level, Vs -the bending of the band, ϰ -the electron affinity, Eg-the band gap.When determining the values of Eg, the data of elastic electron reflection spectroscopy were also used.
Let us dwell on a qualitative analysis of the data shown in figs. 1 and in the table. In the photoelectron spectrum of pure silicon (Fig. 1, curve 1), three distinct features are observed at Ebind=-1.5; -2.3 and -4.5 eV, corresponding to the photoemission of electrons from the maxima of the density of states of valence electrons. After the implantation of Ba + ions, the following changes occur in the spectrum: the position and shape of the peaks change, new features appear, the width of the EDCs and the quantum yield of photoelectrons increase. The results of Auger electron spectroscopy (AES) show that the main part (80-85%) of the implanted barium atoms enters into a chemical bond with silicon atoms and forms barium silicide.This, apparently, leads to a change in the energy parameters of the surface and straightening of the bending of the energy bands. In the formation of chemical compounds, barium atoms are electron donors, and silicon atoms are acceptors.
Due to the partial transfer of valence electrons from barium to silicon, the redistribution of electronic states occurs in the valence band of silicon.Barium atoms that do not enter into chemical bonding with silicon atoms form impurity levels of the donor type in the semiconductor band gap. At high doses, splitting of the donor levels occurs, and a narrow band filled with electrons is formed near the bottom of the conduction band. This explains the appearance of a new high-energy peak in the PhES (Fig. 1, curve 2) and a sharp shift of ЕF towards the bottom of the conduction band.
In the case of implantation of O2 + ions, silicon oxide is predominantly formed in a nearsurface layer 15-20 Å thick. As a result, the band gap increases and ϰ decreases. In this case, due to the presence of uncompensated oxygen and silicon atoms, a number of impurity levels are formed in the band gap of the oxide.
On Fig.2 shows the dependences of the first derivative of the electron elastic reflection coefficient R with respect to the energy -dR/dEp(Ep) for the samples under study in the region of low energies of primary electrons Ep and near the core level L2,3 of silicon. It is known, that in the Ep≤25-30 eV region, there are some maxima of the -dR/dEp(Ep) dependence of silicon. These changes for different types of ions have a different character.
For example, the energy of Si plasmons after the implantation of Na and Ba ions decreases, and after the implantation of oxygen it increases. Based on the analysis of the photoelectron spectrum and the -dR/dEp(Ep) dependence, it is possible to construct an energy diagram of the surface of the samples under study. In this work, the density of states of electrons in the valence band is first determined, then the maxima of the density of free states are found from the energies of the maxima of the lowenergy part of the -dR/dEp curve, and the energy positions of the maxima in the lower part of the valence band are determined from the high-energy peaks of this curve. The surface energy diagrams for pure Si and Si doped with Ba + and O2 + ions constructed in a similar way are shown in fig. 3. The energy gap ∆Ес between the maxima of the density of filled and free states corresponds to the position of the maxima of the -dR/dEp(Ep) curve.
The vertical dashed lines correspond to the maxima of the density of electronic states of the valence band.
All the above results refer to surface layers no thicker than 5-6 Å. Therefore, it is of particular interest to study changes in the composition and electronic structure of deeper layers of an ion-doped sample.
On fig. 4 shows the concentration profiles of the distribution of Ba over the depth of Si doped with Ba + ions and E0=0.5 keV at a saturation dose. The main parameters of the energy bands and the proposed type of the chemical bond BamSin for some fixed depths are shown in the table 2 to fig. 4. It can be seen that Ba atoms up to a depth of 25-30 Å are distributed almost uniformly. In these layers, the concentration of barium reaches 50-55 at.% and mainly barium monosilicide is formed.  In the range d≈30-70 Å, the barium concentration first rapidly and then relatively slowly decreases.Consequently, there is a change in the electronic structure: the position of EF changes, the values of Ф, φ, ϰ and Еg increase. In this case, the main features characteristic of undoped silicon are established in the spectrum of photoelectrons. A comprehensive analysis of the SES and PhES spectra showed that the energy diagrams of the ion-doped sample for d≥70 Å do not differ significantly from those for undoped polycrystalline silicon.

Conclusion
It was established that changes in the parameters of the energy bands were appeared during ion implantation and led to a change in the quantum yield of silicon photoelectrons. The observed changes are explained both by the formation of a chemical bond between the atoms of the alloying element and silicon, and by the appearance of new electronic states in the band gap due to the presence of unbound atoms of the implanted element.
In silicon doped with Ba + ions with Е0=0.5 keV with a high dose in the near-surface region with a thickness of 20-25 Å, an almost homogenous distribution of matrix and dopant atoms is formed. There is a transition layer between this layer and the undoped region, the thickness of which is ~35-40 Å. The parameters of the energy bands of the transition layer change with increasing depth.