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Experimental studies of thorium ions implantation from pulse laser plasma into thin silicon oxide layers

We report the results of experimental studies related to implantation of thorium ions into thin silicon dioxide by pulsed plasma fluxes expansion. Thorium ions were generated by laser ablation from a metal target, and the ionic component of the laser plasma was accelerated in an electric field created by the potential difference (5, 10 and 15 kV) between the ablated target and SiO2/Si(001) sample. Laser ablation system installed inside the vacuum chamber of the electron spectrometer was equipped with YAG:Nd3+ laser having the pulse energy of 100 mJ and time duration of 15 ns in the Q-switched regime. Depth profile of thorium atoms implanted into the 10 nm thick subsurface areas together with their chemical state as well as the band gap of the modified silicon oxide at different conditions of implantation processes were studied by means of X-ray photoelectron spectroscopy (XPS) and Reflected Electron Energy Loss Spectroscopy (REELS) methods. Analysis of chemical composition showed that the modified silicon oxide film contains complex thorium silicates. Depending on local concentration of thorium atoms, the experimentally established band gaps were located in the range of 6.0 - 9.0 eV. Theoretical studies of optical properties of the SiO2 and ThO2 crystalline systems have been performed by ab initio calculations within hybrid functional. Optical properties of the SiO2/ThO2 composite were interpreted on the basis of Bruggeman effective medium approximation. A quantitative assessment of the yield of isomeric nuclei in"hot"laser plasma at the early stages of expansion has been performed. The estimates made with experimental results demonstrated that the laser implantation of thorium ions into the SiO2 matrix can be useful for further research of low-lying isomeric transitions in 229Th isotope with energy of 7.8(0.5) eV.

Low lying isomer state.

Introduction
Currently, wide-band gap materials doped with thorium ions are subject of intensive studies [1,2,3,4,5] This is due to the fact that such materials could serve as the most promising for the study of a unique anomalously low-lying isomeric state of the 229 Th nucleus [6] with the energy of 7.8 ± 0.5 eV [7,8]. However, the measurement uncertainty of ∼ 0.5 eV requires a frequency tuning in the range of ∼ 250 THz that leads to significant impediment to use precision of laser spectroscopy techniques for direct detection of the desired transition. This circumstance, also, is a limiting factor for developing novel devices operating in the vacuum ultraviolet (VUV) range such as, e.g., a laser on a nuclear transition [9], time and frequency references of a new generation [10], etc. Today, the available synchrotron sources may excite the direct energy transitions in the 229 Th nuclei which are accommodated in crystals transparent in the VUV region. Possible candidates are LiCaAlF 6 , LiSrAlF 6 , and CaF 2 [11,12]. However, synthesis of high purity material containing the required amount of the thorium isotope presents a complex and resourceintensive problem [13,14]. The poor availability of a synchrotron radiation source of high intensity with possibility of tuning the lasing energy in the range 7.2 -8.3 eV, also makes it difficult to conduct experiments of this kind.
In this paper we propose an original physical system that allows excitations of the 229 Th nuclei within a wide energy range with significant increase in the fluorescent signal produced as a result of the relaxation of the populated isomeric state. Experimentally, the system is formed by means of the pulsed laser implantation; it consists of an ensemble of the 229 Th ions embedded into the matrix of a broadband dielectric medium (silicon oxide) by means of the pulsed laser implantation. A key feature of the system is that the thorium ions are subject to a large number of inelastic collisions with the neighboring electrons at the early stages of the expansion of the plasma torch when plasma can still be treated as quasi-stationary. Due to the mechanism of Inverse Internal Electronic Conversion (IIC) [15], this process can move the 229 Th nucleus into an excited state. Thus, laser implantation offers a unique opportunity allowing us to combine two important processes: 1 -excitation of the 229 Th isomeric nuclei; 2 -implantation of a wideband dielectric into the matrix. At the same time, the key issue of this work is the study of physicochemical properties of thorium-containing samples obtained during laser implantation. It is important that the band gap should be larger than 7.8 eV, and the number of implanted nuclei should be sufficient to register a VUV photon.
We present an analysis of the electronic structure of thin thorium-containing silicon oxide layers formed by pulsed laser implantation. Using the X-ray Photoelectron Spectroscopy (XPS) and Reflected Electron Energy Loss Spectroscopy (REELS), the chemical environment and the band gap of the formed layers for a different number of implanted thorium ions were extracted. The measured value of the band gap is compared with the calculated value obtained from the first principles. The possibility of using such a system for further studies of a nuclear low-lying isomeric transition in the 229 Th is under discussion.

Experimental setup
Experimental studies were conducted using an XSAM-800 electronic spectrometer (Kratos) in situ equipped with pulsed laser deposition (PLD) [16]. Production of ions, their acceleration in an external electric field followed by the process of implantation are routine applications of PLD method actively utilized in Surface Science [17,18,19]. In particular some details of the PLD technique one can find in Ref. [20] In our experiments thorium was evaporated by laser ablation from a metal target, and the ionic component of the laser plasma was accelerated in an electric field created by the potential difference (5, 10 and 15 kV) between the ablated target and SiO 2 /Si (001) sample. As a substrate for thorium implantation we used a thermally oxidized and highly doped (0.01 Ohm·cm) silicon wafer Si (001) with a 600 nm thick SiO 2 film. The PLD depositions were performed on the freshly cleaned substrate surface SiO 2 /Si(001) under ultra high vacuum (5 × 10 −9 Torr) conditions at room temperature. The PLD system was equipped with YAG:Nd3+ laser (E = 100 mJ) and installed inside the vacuum chamber of the electron spectrometer.
Deposition rate can be varied from 10 13 to 10 15 cm −2 per pulse with time duration of 15 ns in the Q-switched regime.
Immediately after each series of pulses of thorium sputter in an electric field, the Th/SiO 2 /Si (001) sample was transferred into the spectrometer analyzer chamber under the high vacuum where XPS and REELS analyses were performed. To excite photoelectrons we used an X-ray radiation of the AlKα 1,2 line with an energy of hν = 1486.6 eV. The spectrometer was calibrated along the Au4f 7/2 line with the binding energy BE = 84.0 eV.
The band gap width of the substrate and the layers formed on its surface were measured by the REELS method. The electron beam energy was E 0 = 1000 eV, the beam current was I 0 ≈ 30 µA, the scattering angle was equal to ϕ 0 = 125 • ± 20 • (backscattered electrons were collected within the angle ∆ ϕ = ± 20 • ). The energy width of the primary beam electrons was ∆ E ≈ 1.5 eV after reflection.
We note that the study was carried out using a metal target of the 232 Th natural isotope due to the high radioactivity of the 229 Th isotope. This enabled us to reduce the level of radioactivity of the samples by the level allowed to conduct the necessary series of experiments. Since these values are close to the observed binding energy it can be assumed that a thorium chemical bond is formed as a result of implantation which is close to that of thorium for ThSiO 4 in ionicity. In this case the Si2p-line broadens with increasing the Th concentration and shifts toward lower binding energies: from 103.4 eV for SiO 2 to 102.6 eV for a film with a ratio Th/Si = 1. This change in the binding energy of the Si2p line is usually connected with the formation of silicates. In our case it can be due to the formation of a ThSi x O y /SiO 2 /Si -type film structure with an amorphous ThSi x O y film of variable composition [22]. A significant broadening of the Si2p-line may also indicate that thorium silicate as a variable composition presents in the subsurface area.
To obtain depth distribution of implanted thorium atoms, the ion etching XPS method was used in films formed during the deposition with accelerating voltages of 5, 10 and  rather wide spectrum of energy distribution described by the shifted Maxwell-Boltzmann-Coulomb distribution [24]. For completeness we mention the paper [25] specially devoted to detail investigations of thorium ion energy spectra in laser plasma. The Coulomb repulsion and the effect of motion of a plasma bundle as a whole can lead to an energy broadening of the ion beam (W ∼ 1 keV) and to blurring the boundary of thorium ions with energy (E 0 = 10 keV) implanted to a depth of h = 10 nm at a level of several nanometers δh ∼ h×W /E 0 . In Fig. 2, a peculiar manifestation of non-monotonicity occurs in the depth interval of 10 to 15 nm, therefore, the blurring of the implantation depth does not exceed 3 nm, that is consistent with the estimation. Hence, the presence of an exponential decay can not be explained by the broad energy distribution of ions. Apparently, the neutral component, being accelerated only in the early stages of the plasma spread when and to blurring the boundary of thorium ions with energy (E 0 = 10 keV) implanted to a depth of h = 10 nm at a level of several nanometers δh ∼ h×W /E 0 . In Fig. 2, a peculiar manifestation of non-monotonicity occurs in the depth interval of 10 to 15 nm, therefore, the blurring of the implantation depth does not exceed 3 nm, that is consistent with the estimation. Hence, the presence of an exponential decay can not be explained by the broad energy distribution of ions. Apparently, the neutral component, being accelerated only in the early stages of the plasma spread when the particle concentration is close to a solid one, gives rise to a beam of a large number of low-energy particles. This implies that neutral thorium particles arriving at the SiO 2 surface have no sufficient energy to be implanted and therefore are deposited into the upper layer. In addition, one should take into account the effect of droplets and surfaces damage which is inevitable in case of pulsed laser deposition [24,26]. While analyzing the energy band gap of the test samples by the REELS method, such effects can make significant changes. However, when the number of laser pulses is small and the concentration of the deposited thorium atoms does not exceed the concentration of substrate atoms, the influence of surface thorium atoms in the scale range of the depth of the REELS analysis (∼ 3 nm) can be ignored.
The band gap was restored by the REELS technique realized in the XSAM-800 spectrometer chamber in terms of the procedure described in [28]. Spectra of characteristic electron energy loss with energy 1 keV scattered on the surface of the initial SiO 2 film and the same film after Th implantation are shown in Fig. 3. It can be seen that in the process of increasing the amount of implanted thorium the band gap decreases from 9.0 for the original SiO 2 film to 6.5 eV for a film with Th/Si = 1 when the composition becomes close to the compound ThSiO 4 . It is necessary to note that within the energy range up to 30 eV the structure of the observed REELS spectra is determined not only by the losses of electron energies in the excitation of electron interband transitions but also by the excitation of plasmons. For example, as it follows from the SiO 2 spectrum there is the bulk plasmon peak at the energy of 22 eV that agrees with the data [27]. Existence of two arms in the spectrum of SiO 2 at energies of 11 eV and 15 eV can be caused by electron transitions from completed shell corresponding to oxygen 2p-states into unoccupied 3s3p-states of Si [28,29]. We note that energy-loss spectra observed within the energy range of 1-6 eV demonstrate rather small but well pronounced fine structure. The latter may appear due to the contribution of surface states of non-stoichiometric silicon oxide

Ab initio calculations
The multicomponent system consisting of the bulk SiO 2 with the secondary phase of ThO 2 possesses the properties of a heterogeneous material. In studies of light-matter interactions, if the photon wavelength appears to be smaller than the length of ThO 2 interatomic distances, such SiO 2 -ThO 2 composite system can be theoretically considered as the optically homogeneous material. Supposing that the material is formed of N components, the optical properties of the system under consideration can be modelled [32,33] in terms of effective macroscopic dielectric function and refractive index as follows Eq. (1) represents Bruggeman effective medium approximation [33]. The summation over j accounts for the contribution of each component; the parameter f j determines the filling factor, and L j characterizes the shape of the secondary phase. For spherical form L j =1/3. Note that, by using in Eq. (1) spectral distribution of imaginary (ε j2 ) and  [34,35,36] together with the projector augmented-wave (PAW) pseudopotential method [5,6,7,37,38,39]. Exchange and correlation effects have been taken into account within the Perdew-Burke-Ernzerhof (PBE) [40,41] generalizedgradient approximation (f (PBE) GGA) functional [8] or Heyd-Scuseria-Ernzerhofhof hybrid functional (HSE06) (HSE) [9,10,42]. The PAW-PBE pseudopotentials employed in calculations correspond to Th(6s 2 6p 6 6d 2 7s 2 ), Si(3s 2 3p 2 ), and O(2s 2 2p 4 ) valence states.
The calculations were performed with a 8x8x8 k-point mesh, and a plane-wave energy cutoff of 600 eV. Structural relaxation was stopped when the total energy change was less than 10 −8 eV, and the residual forces and pressure were less than 10 −4 eV/A and 0.06 kB, respectively.
The optimized (relaxed) lattice parameters of SiO 2 and ThO 2 have been used for electronic structure calculations within the HSE06 hybrid functional. The band gap estimated as the difference of energy corresponding to the bottommost conduction band and topmost valence band was found to be 5.99 eV for ThO 2 and 9.18 eV for SiO 2 , respectively. Absorption spectra α( w) have been simulated using the calculated ε 2 (w) and ε 1 (w).
From Tauc plot (see Fig. 5a) band gap for SiO 2 and ThO 2 as well as for their composites has been estimated. One estimates the Eg from intersection of slope with abscissa. Here the fill factor that has been used in this work corresponds to the assumption about noninteracting secondary phases. Furthermore, the secondary phases are assumed to possess spherical shape and homogeneous size. Also, the effective medium theory can be applied to the particular system of ThO 2 /SiO 2 when the ThO 2 particle size exceeds 200 nm. As soon as the total number of implanted thorium atoms is not large enough we are obviously allowed to use Effective medium theory only for qualitative estimations. All these limitations contribute on accuracy of the theoretically estimation of the band gaps for the ThO 2 /SiO 2 composite.

Discussion
According conversion IIC [15]. According to this process, plasma electrons populate the ion levels from the states of the continuous spectrum of the energy E, i.e., they change into the states of discrete spectrum of the energy E f .The nucleus is excited by a virtual photon. This is an inverse to the decay process of the isomeric nuclear state through the gamma ray internal electron conversion channel. The IIC cross-section onto a completely unoccupied ion shell takes the following form where Γ c (E is ; f ) is the partial conversion width of the isomeric level upon decay through f -ion shell, λ e is the electron wavelength with energy E res , J F,I are spins of isomeric (final) and initial ground states (unit system is = c = k = 1). The cross section σ IIC has a pronounced resonance character. Only those electrons of the plasma spectrum "work", or result in the excitation of nuclei whose energy E res coincides with the energy difference of the nuclear transition ω N and the absolute value of the binding energy E f on the populated atomic shell within the conversion width of the nuclear state Γ c . The IIC process is most effective at the plasma temperature T which is comparable to or exceeds the nuclear transition energy ω N =7.8 eV several times. Indeed, for these temperature just the states which are important for the internal electronic conversion will be ionised and the number of electrons in the work area of spectrum is relatively high. To achieve such temperatures, it is sufficient to use a laser with an intensity of I ≈ 10 10 W/cm 2 [43]. Now we have a laser offering the pulse energy E L = 100 mJ under the pulse duration τ L ≈ 15 ns. When focusing into a spot of the diameter of d = 10 −2 cm, we obtain the desired intensity.
With the excitation of nuclei in such a plasma of the electron density n e and their energy distribution function f (E), the efficiency of the IIC mechanism can be written as: Integration (incorporation) leads to elimination of the energy delta function from σ IIC .
Then in order of magnitude the efficiency of the mechanism proves to be equal to Γ tot c T f e (E res ) n e τ υ res e , where Γ tot IC is the total IC width of the nuclear level (Γ tot IC = ln 2/7µs [44]), E res is the resonant energy of the electrons determined by the condition (ratio) E res = E is −|E f | ≈ 1.8 эВ matrix can be useful for further research of "nuclear clocks" based on low-lying isomeric transition in 229 Th isotope with energy 7.8 ± 0.5 eV.