Investigation of electronic structure of the phosphorus–doped Si and SiO2:Si quantum dots by XPS and HREELS
methods.
A.I. Kovalev1*, D.L. Wainstein1, D.I. Tetelbaum2, W. Hornig3,
Yu. N. Kucherehko4
1
Surface Phenomena Researches Group (SPRG), 9/23,2nd Baumanskaya str., CNIICHERMET, 107005, Moscow, Russia, e-mail:
sprg@mail.sitek.ru
2
Physico–Technical Research Institute, 23/3, Gagarin pr. 603950, Niznii Novgorod, Russia
3
B-P-E International Dr. Hornig GmbH, Foehrenstrasse 51, 90542 Eckental, Germany
4
Institute for Metal Physics, UA-03142, Kiev, Ukraine
Abstract
The system of the nanocrystals of Si in the SiO2 matrix (SiO2:Si) attracts a great attention due to its ability of the luminescence in
visible and near-IR range of spectrum. The influence of the P ion doping was investigated for electronic structure of Si single
crystal and SiO2:Si nanocomposite. The P doping of SiO2 implanted with Si+ and post–annealed at T=1000 oC (2 hours) results in
the enhancement of the PL peak connected with the Si nanocrystals. Due to low concentration of Si nanocrystals in SiO2 matrix,
the peculiarities of P influence on Si electronic structure were investigated on model samples (Si single crystals with ion doping
by P). The determination of the chemical state of P impurity and electronic structure of the P doped Si and (SiO2:Si) quantum
dots was carried out using XPS and high resolution electrons energy loss spectroscopy (HREELS). Experimental density of the
states (DOS) in valence band and conduction band of SiO2:Si composite are in good accordance with calculation of the local
electronic structure around small Si inclusions in the SiO2 matrix by means of the LMTO (linear muffin-tin orbital) method.
Keywords: Silicon, Quantum dots, Electronic structure, XPS, HREELS
1. Introduction
The system of Si nanocrystals embedded into SiO2 matrix is very promising one for many modern applications in the electronic
industry due to its compatibility with ordinary silicon technology as well as due to the outlooks for light-emitting devices in the
optoelectronics. In this system the strong visible and near-infrared photoluminescence has been revealed [1, 2] that is believed to
be related to silicon nanocrystals. Light emission with the energy exceeding the band gap of bulk silicon has been reported in
nanocrystals of a few-nanometer size [3, 4]. However, the mechanism of luminescence is still controversial, because Si
nanocrystals show various optical characteristics and their optical properties are sensitive to the surface chemistry of Si
nanocrystals. The phosphorus doping increases the luminescence of SiO2:Si nanocomposite. On this reason the investigation of P
doping influence on electronic structure of quantum dots and determination of P distribution between SiO2 matrix and Si
nanoparticles had the most importance.
2. Methods
The present research was carried out on electron spectrometer ESCALAB MK2 (VG) using Al K X-ray monocromated source
(h=1486.6 eV) and low energy electrons source EMU-50 (Ep=7.0 eV). The single crystal samples of Si(100) thin plates
10х10х2 mm in initial state, after ion doping by P, or after oxidation and simultaneous doping by P and Si were investigated. The
thermally grown SiO2 films on silicon were used as the target for Si implantation.
The oxidized Si samples with oxide film thickness about 0.6 m were subjected to ion implantation by the Si+ ions (1017 cm-2) at
accelerating voltage 150 kV. After ion implantation the samples were annealed at 1000 оC during 2 hours, and doped by P ions
with total dose about 31016 cm-2 and annealed at 1000 оC during 0.5 hours. The annealing was done in N2 atmosphere. After
implantation of silicon in SiO2 and annealing of samples the nanocrystals of Si (quantum dots) were formed in surface layers.
The specimens were cleaned by Ar ions before investigations in the preparation chamber of spectrometer at pressure near 110-6
Torr, accelerating voltage 8 kV, ion beam 100 A. The investigations in work chamber of spectrometer were carried out at
vacuum 110-10 Torr. Due to low intensity of valence band spectra the recording speed was 0.04 eV/sec with 24 scans. The
spectrometer was adjusted using gold reference sample. The physico-technical conditions for electronic spectra acquisition were
selected in the way to provide energy resolution of the spectrometer better than 0.6 eV. The high resolution energy losses spectra
were recorded at speed 0.002 eV/sec, 5 scans. The half-width of the elastically scattered electrons line was 35 meV.
The energy band structures of the systems considered in the present work have been calculated by means of the self-consistent
LMTO (linear muffin-tin orbitals) method in the atomic-sphere approximation.
Fig.1
Fig. 4
Fig. 2
Fig. 3
3. Results and discussion
3.1. Investigation of core levels
Fig. 1 shows the electron spectra of inner levels Si 2s of silicon standard (1) and P 2p of red phosphorus (2) that were used as
reference lines. One can see that binding energy of Si 2s electrons was 153.0 eV, and binding energy of P 2p electrons in free
and oxidized states was 132.0 and 136.8 eV, correspondingly. The line 3 on Fig. 1 shows Si 2s and P 2p photoeletron spectra for
oxidized silicon after its doping by Si and P. The splitting of Si 2s and P 2p is observed due to presence of silicon nanocrystals in
the SiO2 matrix and redistribution of phosphorus between matrix and nanocrystals.
Good resolution of the P 2p doublet allows to clearly determine positions of overlapped peaks. Chemical shift of P 2p line in the
Si:SiO2 (P) nanocomposite is -1.3 eV for P in free state and +3.8 eV for oxidized P. These values are 1.0 eV smaller comparing
to ones for red phosphorus reference sample. So one can suggest that implanted P is redistributed at annealing between matrix
and Si quantum dots, and it presents in both phases. Most probably free P forms the solid solution in Si nanocrystals, and P bond
with oxygen forms segregation on "nanocrystal—matrix" interface.
Due to low concentration of quantum dots in SiO2 and correspondingly low signal the modeling samples of the P doped Si single
crystal were used for investigations of electronic structure peculiarities. Fig. 2 present electron spectra of the Si sample valence
band in initial state and after ion doping by P. The characteristic peaks are observed at -3, -7, and -10 eV, and correspond to 3p (0
- 4 eV), s+p (4 - 8 eV), and 3s (8 - 12 eV) states of Si. At P doping one can observe increasing of density of states near Fermi
level and at -10 eV. This distinguish is shown on differential spectrum (Fig. 2, bottom spectrum) obtained by subtraction of
spectrum of ion doped Si from spectrum of Si in initial state. In this case phosphorus increases concentration of electrons at 3s
and 3p levels of Si forming substitution solution.
3.2. Investigation of valence band
The electronic structure of silicon valence band was modeled using following initial data. The silicon crystal has a cubic lattice
consisting of two face-centered cubic sublattices shifted along [111] direction (diamond structure, space symmetry group O 7h).
The cubic lattice constant is equal to a = 0.5431 nm, the interatomic distance is d = 0.2352 nm. The nearest surrounding of each
Si atom is a tetrahedron of 4 atoms from the other sublattice. The primitive cell for this structure contains two atoms, whereas the
cubic unit cell contains 8 Si atoms.
The electronic band structure of the Si crystal is well known now and described in detail elsewhere. Therefore we do not give a
general discussion of our calculated results (figure 3), they are in very good agreement with results, obtained by means of other
calculation methods [5].
Phosphorus impurity in the bulk of the Si leads to perturbations of the electron states at the Si atoms surrounding the point defect.
This effect is decreased with increasing of distances from P atom. Phosphorus doping increases the DOS at –10…–11 eV, nearest
of Fermi level at 1.25 eV. The big s and p contributions to the electron charge density in the sphere of the P impurity in the bulk
indicate a local increasing of the charge density around this point defect.
But there is need to note that main effect produced by the point defect is the occupied donor state just below the conduction-band
bottom, that is spread up to the second Si neighbors (see dark area on DOS of Si (layer 6) in Fig. 3).
The calculation of valence band structure of the P doped Si allowed us to determine total charge of P atom and its energy levels
distribution, and gave possibility to interpret the electronic structure changes observed in experiment. Accounting the
experimentally determined chemical shift of the P 2p level (–1.3 eV), ion types of bonds and atomic radii of P and Si, one can
determine approximately the effective charge change on the P atom equal to 0.43 eV. Phosphorus as a donor changes the local
electronic structure of Si. The calculation of valence electron charges in atomic spheres of P and of the neighbouring Si allows to
explain the experimentally determined charge change. The partial charge change of P impurity in Si is the following: Dq s = 0.14;
Dqp = 0.24; Dqd = 0.04; Dqf = 0.01. The increasing of charged state of P results to growth of DOS in valence and conducting
bands. One can see that most changes are observed in 3s and 3p levels that corresponds well to observed increasing of DOS in
valence band (Fig. 2).
Fig. 5
Fig. 6
3.3. Investigation of intraband transitions by HREELS
Figs. 4 show (from the top to bottom) the high resolution energy losses spectra (HREELS) from (100) surface of Si single crystal
surface in initial state (1), after P doping (2), and differential spectrum obtained by subtracting of spectrum 1 from spectrum 2.
The subtraction of spectra allowed to remove structure of spectra tied with inelastic losses, and to amplify input of P doping in
the picture of characteristic losses. According to these results, the intensities of some peaks of characteristic losses are increased
at P doping. The excitation of electronic states in this low energy range is the resonance one with excitation of surface states and
intrazone transitions. The types of excited electron transitions were determined accounting energy positions of points with
maximal integral DOS in the Brillouin zone (in valence and conductivity bands) that were calculated by LMTO method. The
values close to these ones were obtained also in [6 - 10].
This good agreement of experimental and calculated data allows to make the following conclusions. In silicon, at ’25 the valence
band splitting takes place into the light hole band ’25(l) and heavy hole band ’25(h). Second electron states (SES) for the Si
(100) face include of two groups levels. The first SES group lies in the forbidden band has density maxima at the 0.92, 0.705,
0.430 and 0.275 eV energies. The second group of the surface states is located in the valence band and consists of the surface
resonance near the valence band top –0.165, the surface state band with the E = –0.365 energy and the local surface state –0.515
eV. As seen from Table 1,
P addition increases the electron transitions: (i) between the integral density-of-state maxima in the valence band and conduction
band; (ii) from the surface states lying in the valence band to unfilled surface states and to the conduction band; (iii) from the
valence band top to surface states located in the forbidden band; (iv) between the surface states lying in the forbidden band. The
results obtained agree well with experimental and theoretically data [6 - 10].
4. Conclusions
It is established that one can observe some change of chemical state of P and Si after P implantation. According to electron
spectroscopy data, the decrease of effective charge on P atoms in solid solution leads to increasing of DOS in valence band. Sand p-electrons are playing the most role in this change. The phosphorus impurity in the bulk of the Si leads to perturbations of
the electron states at the Si atoms surrounding the point defect. The main effect produced by the point defect is the occupied
donor state just below the conduction-band bottom, that is spread up to the second Si neighbours. Phosphorus doping increases
the electron transitions: (i) between the integral density-of-state maxima in the valence band and conduction band; (ii) from the
surface states lying in the valence band to unfilled surface states and to the conduction band; (iii) from the valence band top to
surface states located in the forbidden band; (iv) between the surface states lying in the forbidden band.
P doping in the SiO2:Si system dissolves in Si nanocrystals and on the Si-SiO2 interface changing the electronic structure of
quantum dots.
Acknowlegement
The research was supported by INTAS grant 00-0064.
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Captions to figures.
Figure 1. Comparison of XPS spectra of Si single crystal (1), red phosphorus (2) and SiO 2 layer on Si after Si ion implantation, P
ion alloying, and annealing at 1000 oC (3).
Figure 2. Valence band of Si and Si ion alloyed by P (top) and affect of P ion doping on DOS of Si (differential spectrum,
bottom).
Figure 3. The calculated local DOS at the phosphorus impurity in the middle of
the Si slab, and at the Si atoms that are its nearest and second neighbors. Shaded
area indicates the donor impurity state produced by the P impurity.
Figure 4. HREEL spectra of Si (1) and Si ion alloyed by P (2) and the differential spectrum (3)
Table 1. Interpreting of electrons transitions on energy losses spectra from Si (100) surface after P ion doping.
Excited electron transition
Transition energy (eV) experiment
Mean energy loss values (eV) (Fig. 2)
Sr – Ev
 0.0 [8 - 10]
0.165
S3 – ’25(h) (p-type)
0.35-0.40 [6, 7]
0.365
’25(h) – S2
0.34 [9]; 0.36 [6, 7]
0.275
S2 – S1
0.49 [9]
0.430
S4 – ’25(h)
0.56 [6, 7]
0.515
’25(I) – S1
0.74 [6, 7]
0.705
S2 – 1(1)
0.98 [6, 7]
0.920