The polarization of elliptic silicon grains evaluated by fixed *NDB* and stark energy shifts for levels in electronic structure by external electromagnetic field.

## Abstract

Local electric fields are appeared in dielectric and semiconductors due to the destruction of symmetry, creating the vacancies, point defects and chemical impurities in material. By increasing in external electric field value there are numerous structural changes will be generated. Point defects in silicon films were characterized by using electron-paramagnetic resonance spectroscopy and laser picoseconds spectroscopy. Chemical bonding properties was investigated by means of Fourier-transformed infrared spectroscopy. The possible mechanism of phase destruction was proposed.

### Keywords

- local field
- point defect
- dangling bonds
- Raman spectroscopy
- nanocrystals
- second-harmonic generation
- silicon films

## 1. Introduction

Local electric fields are appeared in dielectric and semiconductors due to the destruction of symmetry, creating the vacancies, point defects and chemical impurities in material. By increasing in external electric field value there are numerous structural changes will be generated. Some of them will produce such great local fields that will destroy all material or change its physical properties. The studying the nature of local electric fields will open new tendency in electronic device producing, from one side, and, help to change materials’ properties according to our needs, from another side.

Description of local electromagnetic fields is a continuously durable through the all history of physics and was began with publication of first articles written by Maxwell Garnett which were devoted to colors in metal glasses and metallic films [1], Lorentz [2], and later in works of Brugeman [3] was developed by Edmund Stoner from University of Leeds [4] and Osborn from Naval Research Laboratory [5]. For a complicated medium such as the binary system with components A and B the dielectric function can be estimated as following [5]:

In a case that one component is included in another dielectric component and polarized media with averaged value of polarizability of _{.}

Structural properties of material may be strictly different as for surface and thin films, as for nanostructures such as clusters or nanocrystals, as for bulk material. However, it is obviously that most amounts of media in the universe is nanostructured or even in nanocrystal phase. For example, interplanetary dust was observed charged coupled devices (CCD) detectors and infrared space telescope [6]. They observed a cometary coma of Hale-Bopp comet. The dust destruction in space (Cygnus Loop supernova) was observed by using a Spitzer Telescope tuned in infrared (IR) range from 22 to 36 μm [7] and shows us the chemical properties, such as dust chemical compositions. These observations show the great fraction of silicon in all space dust. Space dust destruction and ion formation was studied by Mann and Czechovsky [8], which results from model calculations in silicate grains, carbon and ice grains. Grain destruction in a supernova remnant shock wave was investigated by astronomers of Harvard University [9]. It was observed by Spitzer telescope in IR 24 μm range of wave length. The case of impact of nanoscopic dust grain with solar wind of spacecraft already was estimated by using the dimensionless parameter equals to ratio between Debye length and radius of dust cloud spherical shell with radius R [10]. It was shown that the dust particle with mass 10^{−20} kg produces by impact 10^{7} charged particles.

From the other physical scientific trends we have an observations of local field by a nonlinear spectroscopic experiments with nonmaterial and nanocomposites, particularly, semiconductors. By using semiconductor materials have been made numerous types of devices, such as electronic devices and photon detectors, integrated circuits and thin film transistors, optoelectronic devices and others. Every time when the device is developed the problem of reproducibility of its work regimes and durability of their realization is appeared. The solution of this problem is very important for device manufacturing, and it depends on properties of used active semiconductor materials. The electrical properties often are not so transparent due to slightly nonlinear behavior of their characteristics. Figure 1 shows the current-voltage and resistance-voltage characteristics for two silicon films prepared by plasma-enhanced chemical vapor deposition technique with gas mixture of silane diluted by hydrogen and silicon tetra fluoride gas: amorphous and nanocrystallized [11]. It is seen, than their current-voltage characteristics are similar in this voltage range, but resistance-voltage characteristics are strictly different. Such difference can be explained by the disorder of amorphous phase and generating of numerous point defects by applying external electrical field. It is clear, that the voltage is varied in the range from the −10 to 10 V. Hydrogenated amorphous silicon was widely used in last decades in electronics. In recent years the nanocrystalline silicon are studying for many technological applications. The structural transformation from crystal to disordered materials, however, is investigated very poor, mainly resulted in Staebler-Wronski photo-stimulated effect. However, the electric field applied to the nanostructured silicon thin film gives the new possibility to change structural order. Such kind of structural transformation is caused because of there are numerous defects inside the silicon film.

The anomalous characteristic of resistance-voltage can be explained by random distributed the point defects inside the amorphous film along with the hydrogen atoms, and existing the dipoles Si-O which turn to compensate the external electric field. But, for the nanocrystalline silicon film, the point defects are incorporated into silicon nanostructured net and cannot move freely, because there is a stabile electric characteristic for nanocrystalline silicon film, and anomalous for amorphous.

The other new area of scientific interests is crystal-amorphous phase transformations by applying electric fields and role of local fields in phase transition from order to disorder. Because, it is important to investigate the point defects which can be responsible for local electrical fields generation in polarized media, such as dielectric silicon oxide media or semiconductor thin film of silicon. The main role plays here the silicon-oxide bonding in side thin film of silicon. Si-O dipoles play a dramatic role in crystal phase destruction by applying electric fields. The induced dipole moment by applied electric field can be written in the following form [12]:

_{0} is the dipole polarizability. However, the Hamiltonian of semiconductor cluster can be surely expressed by using donor and acceptor states in bulk material [13]: *H*_{0} is the Hamiltonian pure semiconductor for electron matrix elements of transitions between own conductive and valence bands, but *HD* and *HA* are the exchange energies matrix elements due to the donor and acceptor states, and for them it is clear be presented the following expressions:

_{x}, from one side, and their ratio between covalent and ion fractions of inter atomic bond are not so homogeneous, from the other side. Because, there is a necessity of detail investigation of nanoscopic nature of local fields appearance and mechanisms of crystal-amorphous phases’ transformations.

The present work is devoted to the nature of local field appearance in silicon nanoscopic material and role point defects in phase transformation of material.

## 2. Experimental researches of field-assisted destruction of silicon nanocrystals

Nonlinear polarization associated with the phonons can be written as

Raman effect is result from the interaction of an electromagnetic field and optical phonon mode. The vibration wave

The microcrystal wave function is a superposition of Eigen functions with

the normalized first-order correlation function

For a microcrystalline and nanocrystalline silicon with sizes of crystals L if the weight function is Gaussian the first-order Raman spectrum is following [14]: ^{5} cm^{−2} and B = 10^{5} cm^{−2} [15]:

where local field factor can be written as

where *q* is a vector of inverted lattice, *2π/a*, a is a lattice constant. If the value *λ0* is the bond length Si‐Si in bulk silicon, *λ1* is the weak bond resulted the silicon-oxygen surrounding. By the value of density of bonds is *NSiO = 21021* cm^{−3} and for crystalline silicon the density of silicon bonds N_{Si−Si} = 510^{22} cm^{−3} the lattice constants are following: *a2 = 0.98 a0* аnd *a1 = 0.996 a0*.

For the silicon nanocrystalline and microcrystalline films the phonons can be generated in crystals by laser field or annealing. The wave of deformations can be generated by picoseconds laser pulse [16]. The acousto-electric effect was observed in n-type germanium [17]. The electric field which was appeared by ultrasound waves can be estimated by using the formula *E* is an acousto-electric field, τ is a relaxation time

The nanocrystalline film was made by me using CVD method of silane diluted by hydrogen (gaze flow rates ratio is 1:10) at low temperature of substrate (80°C). The RF power was 20 W. Working pressure was 0.2 Torr. The crystalline volume fraction was 66%. The crystal orientations for nanocrystals were determined by means of X-ray diffraction technique (111) and their average size was 24 nm. The thickness of silicon film was more than 300 nm. Figure 5 shows the changes in Raman scattering spectral data by applying the external electric field. It is seen, that there is phase destruction by the relatively high voltage. It is assumed that the nanocrystals which have grain boundary with oxygen atoms incorporated into silicon were destroyed in their crystal structure by Si-O dipoles reorientations caused by applied field. The initial crystal orientation was (111). The incorporated oxygen atoms are adsorbed in determined places. Their position results the appearance of numerous dangling bonds which are multiplied by the electric field and create the deep cracks in crystals. The crystal order is damaged along the axis that is perpendicular to (111). According to the Raman data for SiO_{2} [18] the Raman spectrum of SiO_{2} has the variation modes D1 (at 490 cm^{−1}) with defects and activation energies 0.14 eV and pure mode *w1* of Si‐O‐Si bridge. The sum dipole moment consists of dipole moment that is created due to the ellipsoidal shape and because of surface charges are appeared by silicon net deformation due to the oxygen incorporation in silicon *SiO* or hydrogen termination of dangling bonds and creation the *SiH* bonds: *NSiM,* δ*SiM* are the density of the bonds and partial charge for SiM bond (O or H), respectively (Table 1).

NDB,cm^{−3} | P ellipsoid, Debye | ΔE = μE_{ext}, μeVby the E _{ext} = 10^{6} V/m | ΔE = μE_{ext}, μeVby the E _{ext} = 10^{7} V/m |
---|---|---|---|

10^{17} | 1.1 | 20 | 200 |

10^{18} | 10.8 | 200 | 2000 |

It is necessary to note that the fractal structure of several kinds of nanocrystals may cause the dramatically changes (four orders of magnitude) in intensity of Raman scattering due to existing of plasmon resonance into the gaps between the fractals [19]. The Raman intensity by these conditions can be expressed as

where α is a polarizability and σ is a local conductivity of a fractal structure.

In addition, the light irradiation of amorphous silicon film causes the point defects generating and, mainly for amorphous hydrogenated silicon films, causes the appearance new dihydride configurations: (H‐Si Si‐H)2(H‐Si Si‐H) and SiH_{2} [20]. The two atoms of hydrogen in the SiH_{2} unit show an average proton separation of 2.39 Å. Because, for the hydrogenated silicon nanocrystalline films under influence of applying the external electric field the hydrogen diffusion increases and polysilane chains are created, surely.

For poly-Si films with nanocrystals the values of densities of SiO and SiH bonds varies in wide range from 10^{19} to 10^{21} cm^{−3}. The density of bonds were estimated for the poly-Si films prepared by using PECVD as following: for Si-Si bonding the density of bonds is equals to 510^{22} cm^{−3}, but densities of SiO and SiH bonds are 10^{21} cm^{−3}. In these films there is an oxygen contamination on the 2% level. The S/V ratio is 1.25%. I suppose that all the oxygen is concentrated around crystals in their grains boundaries. By these values of densities the dipole moments causes by surface charges can be estimated as *PSiO* = 0.04 D and *PSiH* = 0.015 D. By applying external voltage it is clear that all Si-O dipoles move to compensate external field and destroy the crystal structure. Because, by applied electric field I observed the crystal phase destruction and *SiOx* creation by using the Raman scattering data which correspond to the results reported in work [21].

## 3. Matrix Hamiltonian by small perturbation of Si‐Si‐Si bridge

For very small nanocrystals with sufficient ration S/V the mechanism of three elements simultaneous interaction is important for precise calculations. The energy shift due to the stress appearance for crystal orientation (111) is less than 0.14 eV for the vacancy-oxygen (VO) complex by a stress 0.3 GPa. The Hamiltonian of such system of n atoms as for example,‐Si‐Si‐Si‐ and, particularly the Hamiltonian of interaction between atoms with indexes k−1 and k + 1 can be explained in matrix form is given by using the operators of creation and elimination of boson particles, such as phonons: *Si1* and *Si3* atoms without oxygen) can be written as

Here, the matrix elements

*ωphonon = ω0 ± ξ,* where *ω0* dispersion curve without applied field (for energy 0.14 eV the estimated additional frequency is less than 3.4 × 10^{13} Hz, and additional frequency ξ= *μ=dQ* is dipole moment of nanocrystal, *E* is field, *d* is the size of crystal, *Q* is charge (Figure 3).

## 4. Model of polaron state in silicon nanocrystals

The probability of changing the polarization state from one to another can be described by using Golden rule of Fermi

where

where V is a nanocrystal volume (Figure 4).

The second-harmonic generation is forbidden for center symmetric crystal such as bulk silicon because the sum dipole moment is zero, but is possible due to the surface breaking symmetry and quadruple terms contributions. The opposite situation is for nanostructured oxidized silicon film, the surface area for a great amount of nanocrystals is significant, the breaking symmetry is permanent and lateral isotropic. The oxygen atoms with concentration up to the values of 10^{20}–10^{21} cm^{−3} show the sharp increase in SHG by increase in polarization properties of material, that have its properties as silicon nanocrystals, as silicon oxide inclusions. Figures 5 and 6 illustrate the SHG spectra of radiation reflected from silicon films.

The reflected SHG response was measured by using the radiation of optical parametric oscillator/amplifier pumped by the third harmonic (355 nm) of a Q-switched Nd: YAG laser (Spectra-Physics, MOPO 730) at a 10 Hz repetition rate with spectral range between 440 and 1700 nm. The bandwidth of radiation is 0.3 cm^{−1}. The SHG response was detected by a photomultiplier tube and gated electronics with an average of 100 pulses. The linear polarized radiation was focused on the surface of the sample at the angle 45° and detected SHG signal was observed at the angle 45°, too. Such optical scheme arrangement was useful for surface contributors’ detection from the silicon surface (111). The diameter of irradiated spot was 0.5 mm. The energy of the primary laser beam was 4 mJ. The second-harmonic intensity can be written as

where the L(ω, ρ) value is a local field factor of film with crystalline volume fraction equals to ρ = 70%.

The SHG intensity as a function of the average grain size in poly-Si films, with crystalline volume fraction 70%, is presented in Figures 5 and 6 where

is normalized SHG signal, *εc* and *εa* are dielectric functions of crystalline and amorphous silicon, respectively. For sphere depolarization factor Λ is equal to 1/3, β is the Lorentz constant (for homogeneous spherical surrounding β =1/3). For calculations the following estimated values were λ_{w} *=* 1064 nm, λ_{2w} = 532 nm, ε*c’(*w*) =* 13*,* ε*c”(*w*) =* 0.03*,* ε*a’(*w*) = 12,* ε*a”(*w*) =* 0.23*,* ε*c’(*2w*) =* 18*,* ε*c”(*2w*)* = 0.5, ε*a’(*2w*)* = 18, ε*a”(*2w*)* = 7.5. Resonance spectra consist of two sharp peaks. The peak at 1.6 eV is caused by SHG response due to *E0’= Γ21−Γ15* transition. We suppose that the second peak can be recognized as SHG response due to *E1 = L2’−L1* transition in silicon nanocrystallites.

The contribution of point defects as deviations of local fields and external applied electric field for a phonon generation in silicon nanocrystalline can be described by using the perturbation theory. The model Hamiltonian’s matrix for two-level system including the point defect as small perturbation *ε* that causes the violence of phonon energies of system *E1* (for unperturbed state) and *E2* (perturbed state):

The changes in Eigen values from the *E1* and *E2* by the field *E = 0* to the new values of energies are following

It is assumed, that the field value *ε* determined as linear combinations of external applied field and all local deviations due to film structural disorder. We assume that there is no strict disorder media, but some small disorder is determined. _{;} ^{−} can be evaluated by using the energy of their interaction of atoms with dipole Si‐O, that has its polarized charge 0.2 е:

From the other side, for drift of particle, such as hydrogen atom, by driving forces in condensed matter can be expressed by using formula for force *HM* is an enthalpy or energy of transport by heat, *u*:

## 5. Modular group translation model for crystal phase destruction by applied electric field

I propose the modular group translation (MGT) model for crystal phase destruction by applied electric field for explanation the Raman data which are on Figure 7 and show the dramatic changes in silicon crystal phase related spectral component at 520 cm^{−1} due to the applying electric field.

It is assumed that the electric fields of external field and local polarized field can be written as *Nj* is a number of neighbor atoms,

For analysis of deformation by applying the external electrical field it is clear to use ratio between free energy components for different bonding and directions:

Accordingly, the ratio between the polarization charges for two dipoles inside the electric field is given by

The first dipole is devoted to the description of electrical polarization properties of Si‐Si bond, but the second – Si‐O bond. By substituting the expression for Si‐O polarization charge in expression for free energy we can easily to obtain the following expression

The relation between the deformation values along the 1 and 2 axis can be used for analysis the translations consequences of modular group.

*p,q* describe the translation result applied field, but the second pair *r, s* causes the translation due to the local and external fields interaction with Si‐Si dipoles. Crystal phase destruction can be generated by using significant values of electric field combining with sufficient density of Si‐O dipoles inside the silicon film (Figure 10).

Transformations which create the low dimensional model structure are illustrated on Figures 11 and 12 and are explained as combination of modular group transformation of two-dimensional nanocrystal and Knop triangular transformations into Kantor dust set or fractal structure with low dimension. The surface and interface point defects and impurities cause the local electric fields which can generate by applying external field great values of field.

## 6. Possible scenario of nanocrystal destruction: from bulk silicon to Kantor dust

Model of phase destruction by modular group substitution [26] which consists of arc series and Knop transformation of two-dimension area under arc through the triangular decomposition [27] to one dimensional structure. The down picture illustrates the creation of Kantor dust by dividing the triangular angle on the top and neglecting the area of triangle in the middle of primary triangle area. Such nonlinear triangular transformations can be caused by a point defects and impurities which were included in bulk silicon net of nanocrystal.

For such transformation the equations for triangular quantities X_{N} = 2X_{N + 1} and for areas S_{N + 1} = 0.38*S_{N}. The estimated value of Hausdorf dimension for such mathematical set is

## 7. Classical and quantum mechanical models of charge and current densities

We have to propose the new model that is more suitable to explain electric properties of nanometrical scale media with strong anisotropic and non-homogeneous properties (see Figure 12). It will be necessary to describe the further possibility to design new nanoelectronic devices based on quantum conductivity properties and atomic scale sizes.

For current density of homogeneous media with charge density ρ in classical theory we usually use the formula

For ρ value of non-homogeneous anisotropic media it is possible to use the expression:

It is clear, that such approach is approximate and can be applicable to study the electric properties of point defects.

Nonlinear polarization concludes as linear as nonlinear terms:

It is supposed that currents which was created due to the electromagnetic field of second harmonic generation and induced in nanocrystals dominate in surface layers and grain boundaries. Because, such currents can be explained by the first term in equation and relate to absorption and emission of photons. Equation (17) can be written in suitable form:

Because, the surface current can play a significant role in nanostructured silicon film and the current density can be written as following J:

By a symmetrical form of wave functions *Ес-0.17* eV.

For two energetic levels (A and B) which are situated closed to each other the expression for the current density is following:

The values of currents for two energy states А and В are different due to the difference in their energies, and their occupations are also varied because they depends on Boltzmann distribution for unperturbed case, and for the laser excitation of carriers they distributed according to the Gauss distribution. Because, the current of charges depends strictly on energetic location of defects levels which are closed to the bottom of conductivity band of silicon. Therefore, the nonzero current is appeared because the field of SHG is applied in silicon nanocrystals. The current spectrum has a resonant energetic peak by the electron energy became equal to the energy of defect level:

The dipoles-field interaction causes the appearance of oscillations on frequencies *а* and b as a levels’ widths are following ^{3}, where R is a size of area of surface SHG assisted currents are generated. Therefore, there is a surface current in nanocrystals which are generated by applying the SHG fields. Such currents are caused by repolarization of pairs of atoms of silicon by their distribution of the surfaces of all nanocrystals irradiated by laser light. By this irradiation the times of repolarization are much more than period of oscillation of electromagnetic field of laser radiation, because there is absorption of radiation.

The free energy for nanocrystal with volume V can be expressed as following:

The estimated value of free energy to destroy the silicon crystal phase is following *F* < 1.23 *10^{−4} J/cm^{2}. There are various preliminary states between stabile Si‐Si bonds and broken bonds. The bond length distortion is varied from 0–15% of initial bond length. There are minima in potential energy for hydrogen which are varied from 1.3 eV to 2.3 eV. The Coulomb force driving the migration of charged impurity or defect can be written as *Z* is an effective charge of defect, *E* is applied electric field. According to [28] the force due to the appeared interstitial atom is written as an Eshelby formula *Tr(σ) = σ11 + σ22+ σ33*, σ is a stress, *r0* is a radius of point defect. By this way it is clear that the electric field necessary to make stress can be evaluated as following:

## 8. MGT model and mechanism of crystal phase destruction by local fields

Free energy is determined as energy

For estimation the ratio between the energy of deformation and weak bond length which can be appeared by applying the external electric field we can use the Einstein relation for relation between the drift velocity and applying force *N* is quantity of atoms that was locations were deformed, *d* is bond length of polarized silicon. The *N* value can be evaluated as following

where *NH* is the density of silicon-hydrogen bonds, *n* is a number of neighbor interstitial places of location, *ν* is frequency of vibrations in interstitial position, *a* is a lattice constant, ^{14} cm^{−3} by the melting temperature. Therefore, the diffusion as interstitial as dopant diffusion plays mainly if we observe only the hydrogen diffusion with energy of its activation 1.5 eV [32] and investigate the hydrogen moving through the sites with different potential caused by variation in electric charges.

Analytical solution of system of differential equations results in the following expression for density of dangling bonds

*W2* and *W3* as *W2* = 0.001* *W3*, or W_{2} ➔0, the coefficients A, B, C have view.

It is known the model of defects generating by light irradiation in amorphous silicon which was proposed by the scientists of Ames Laboratory [33] which calculated the evolution of density of dangling bonds according to their proposed model. It is seen, that the evolution which was shown in Figure 14 has the same increasing tendency as evolution stimulated by light irradiation. By the values

By the ratio between rates

By the ratio between rates