Annotation of the results obtained in 2016
In 2016, silicon-doped HfO2 films and based on them test structures were synthesized by ion-beam sputtering deposition (IBSD) and atomic layer deposition (ALD) techniques. Ferroelectric properties of the grown structures were studied. It was found that the ferroelectric phase is not being formed in the films grown by IBSD technique, while the ALD thin HfO2: Si films exhibit ferroelectric properties. Ellipsometry analysis was conducted for synthesized HfO2:Si films to determine the spectral dependences of the refractive index and absorption in the range of 1.13-5.0 eV (250-1100 nm). It was revealed that the optical properties have a characteristic appearance of transparent thin films deposited on absorbent substrates. The analysis showed that the HfO2:Si films can be described by Cauchy dispersion model of a single-layer reflective system. With increasing silica part in the film the dispersion decreases. It was found that the studied HfO2:Si films is SiO2/HfO2 solid solution. It was found that the dominant luminescence band of the HfO2:Si films on a silicon substrate is located near the energy 2.7 eV. This 2.7-eV band corresponds to the oxygen vacancies in HfO2. Other defects and traps in HfO2:Si (if any) are non-radiative recombination centers. Current-voltage characteristics of the structures based on synthesized HfO2:Si were measured. Comparison of experimental data with simulation results showed that the transport in HfO2:Si can be described by phonon-assisted tunneling between traps. However, transport by hopping between oxygen vacancies in HfO2 (Hf-Hf bond, Wt=1.25 eV) is shunted by hopping through a shallow traps with thermal ionization energy of about 0.5 eV (for example, Hf-Si bond or silicon nanoclusters in HfO2), which leads to increased conductivity by 3-5 orders of magnitude compared with ‘pure’ HfO(x) or ferroelectric Hf0.5Zr0.5O2. Quantum-chemical simulations of the electronic structure of HfO2:Si of variable composition with oxygen vacancies and polyvacancies were carried out using the software package Quantum-ESPRESSO in the framework of density functional theory with hybrid exchange-correlation functional B3LYP, in the model of periodic supercells. It was found that all types of neutral oxygen vacancies in crystals Hf(x)Si(1-x)O2 form filled states in the bandgap. 44This is consistent with the published data in scientific literature for SiO2, Hf0.5Si0.5O2 and HfO2. Position of these states in the bandgap and their dispersion law indicate that the oxygen vacancies in Hf(x)Si(1-x)O(y) act as deep charge traps with a strong localization in real space. It has been shown that both positive and negative extra charge in the defective structure is indeed localized around the vacancy. The calculated optical absorption spectra of Hf(x)Si(1-x)O(y), show that the presence of oxygen vacancies leads to a broad spectrum (1.5-2 eV) peak near the fundamental absorption edge. The thermal and optical energy of oxygen vacancy in Hf(x)Si(1-x)O2 were calculated. The technique of quantum-chemical simulations of thin Hf(x)Zr(y)O2 layers of different crystal phases was developed. Using the new designed technique, the electronic structure of thin Hf(x)Zr(y)O2 layers in different crystal phases was studied. It was found that that the defect free bulk Hf0.5Zr0.5O2 crystals is unstable in noncentrosymmetry orthorhombic (ferroelectric) phase. The total energies of ferroelectric and orthorhombic oI-Hf0.5Zr0.5O2 phases are close. Thus, one can conclude that the transitions between these phases are possible at the presence of external factors such as dopants, temperature, and residual tensions lattice. It was found that the dependence of inner tension from the layer thickness is monotonic for monoclinic phase, whereas orthorhombic phase exhibits expressed minimum at a thickness of 25 Å. This indicates forming of thin o-Hf0.5Zr0.5O2 layers is energetically favorable.

 

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Annotation of the results obtained in 2014
In 2014, a laboratory technology of thin conductive films TaN on a silicon substrate, and dielectric films of synthesis by ion-beam sputtering deposition was developed. The stoichiometry (variable x) of HfOx and ZrOx films is controlled during synthesis process. Sheet resistance of TaN films (50 nm), measured by the van der Pauw technique, was in the range of 45-70 Ohms/square. This indicating a good quality of conductive films. The thicknesses of HfOx and ZrOx films were 20-90 nm. For transport measurements Si/HfO2/Ni and Si/ZrO2/Ni MIS structures on n- and p-type silicon substrates were synthesized with oxide layer thickness of 20 nm. The experimentally measured current-voltage characteristics at different temperatures were compared with different models of charge transport in dielectrics. As a result, it was found that the model of isolated Coulomb center ionization (Frenkel model), the model of overlapping Coulomb centers (Model Hill, the Poole law) and the model of multiphonon ionization if isolated traps qualitatively describe the experimental data. However, the values of the fitting parameters in the best qualitative agreement have non-physical values: the trap density is extremely low or the frequency factor is 5-6 orders lower than expected under a given model. At the same time, the model of the phonon-assisted tunneling between traps describes the experimental data both qualitatively and quantitatively. The values of the fitting parameters are consistent with the results of other experiments: trap concentration N~(1-4)×10^19 cm^-3, thermal energy trap Wt=1.25 eV optical energy trap Wopt=2.5 eV. It should be noted that the energy parameters of traps for hafnium and oxides are the same. The results were partly published [DR Islamov, V.A. Gritsenko, C.H. Cheng, A. Chin, Origin of traps and charge transport mechanism in hafnia, Applied Physics Letters 105, 22, 222901 (2014) DOI: 10.1063 / 1.4903169]. Based on results above, we conclude that the Frenkel model does not describe the transport properties of pure ZrO2 and HfO2, as previously mentioned, but model of the phonon-assisted tunneling between traps does. Thus, we can expect that the charge transport properties in the HfxZryO solid solutions will also be described by the model of the phonon-assisted tunneling between traps with Wt=1.25 eV and Wopt=2.5 eV. The current-voltage characteristics of n-Si/ZrOx/Ni and p-Si/ZrOx/Ni structures, measured in depletion mode at various temperatures, show that at low voltages the current is exponentially grows with the voltage increasing, and at moderate and high voltages the current is saturated. The saturation level grows with increasing temperature. These phenomena indicate that in the depletion mode of the silicon substrate injected into ZrOx minority carriers are involved in the charge transport through the oxide. Since the saturation current, and growing of the saturation level with increasing temperature are observed for the structure of n-Si/ZrO2/Ni (minority carriers are holes) as well as for p-Si/ZrO2/Ni (minority carriers are electrons), one can conclude that the charge carriers in ZrO2 are both electrons and holes. I.e. the conductivity if ZrO2 is bipolar. It has been previously identified that the conductivity of hafnium oxide is also bipolar [D. R. Islamov, VA Gritsenko, CH Cheng, and A. Chin, Appl. Phys. Lett. 99, 072109 (2011)]. Thus, one can assume that conductivity of the HfxZryO solid solutions is bipolar too, i.e. the charge carriers are both electrons and holes. For more accurate calculations of the current-voltage characteristics in dielectrics, the bipolar conductivity, space charge and recombination of free electrons with localized holes and free holes with localized electrons should be taken into account. A program for the numerical calculations of the Poisson equation and Shockley-Read-Hall for trapped electrons and holes was developed. The boundary conditions are given by injection of electrons and holes from the semiconductor substrate and a metal contact on the mechanisms of the Fowler-Nordheim and Schottky. Currently the program is under debugging. To identify electronic properties of zirconium oxide independently, we performed a study of photoluminescence excitation spectra and photoluminescence of non-stoichiometric ZrOx films. We also calculated the optical absorption spectrum of oxygen vacancies in the cubic phase of ZrO2. The luminescence spectra of non-stoichiometric zirconium oxide film series with different oxygen vacancies concentrations show the blue luminescence band centered near a 2.7 eV peak. The intensity of the luminescence increases with the oxygen-depletion degree of zirconia (the oxygen vacancies concentration increasing). Calculated from the first principles maximum of ZrO2-with-oxygen-vacancies optical absorption at 5.1 eV is in good agreement with the experimentally observed luminescence excitation peak at 5.2 eV of non-stoichiometric zirconium oxide films. This fact allows to conclude that the defects responsible for the blue photoluminescence in zirconia are oxygen vacancies, as well as for the hafnium oxide [TV Perevalov, V.Sh. Aliev, V.A. Gritsenko et. al, Appl. Phys. Lett. 104, 071904 (2014)]. Thermal trap energy defined from the Stokes shift of the luminescence Wt=(5,2-2,7)/2=1.25 eV is in agreement with the results of transport measurements. In this way, it shows that the defect responsible for transport of zirconia are oxygen vacancies as well as in hafnia. A theoretical quantum-chemical study of the electronic structure of oxygen vacancies in the monoclinic crystalline phase of Hf0,5Zr0,5O2 was carried out. Four types of oxygen vacancies were studied. The difference of these oxygen vacancies was in atomic environment: HfHfZr, ZrZrHf, HfHfZrZr and HfZrHfZr. It was found that the electronic structure of oxygen vacancies in ZrO2, HfO2 and Hf0,5Zr0,5O2 are mostly the same: threefold and fourfold coordinated vacancy forms filled state in band gap of 3.4 eV and 2.7 eV above the valence band, respectively, and regardless of the type of surround atoms. Charge localization energy of different types of oxygen vacancy in ZrO2, HfO2 and Hf0,5Zr0,5O2 was estimated. It was found that the oxygen vacancies of the investigated Hf0,5Zr0,5O2 structures, as in ZrO2 and HfO2, are traps for electrons and holes, and may be involved in the charge transport. It was found that forming of Zr-surrounded oxygen vacancies is the most favorable energetically. Fourfold coordinated oxygen vacancies are formed easier than threefold coordinated ones.

 

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Annotation of the results obtained in 2015
In 2015, test MIS Si/Hf(x)Zr(y)O2/Ni structures of by PVD in Si substrates n- and p-type, and MIM Si/SiO2/TiN/Hf(x)Zr(y)O2/Pt structures by ALD were synthesized for optical (20 nm thick Hf(x)Zr(y)O2, no top electrode) and transport (10 nm thick Hf(x)Zr(y)O2, with top electrode) measurements. XRD analysis showed that as-deposited Hf(x)Zr(y)O2 films were amorphous. After rapid thermal annealing at 400 °C ferroelectric phase was formed in Hf(x)Zr(y)O2 films. The chemical composition of the polycrystalline ALD Hf(x)Zr(y)O2 films was studied by XPS technique using synchrotron radiation with a photon energy of 720 eV for the analysis of the chemical composition and 200 eV to record the spectrum of the valence band. Film stoichiometry x=0.45, y=0.55 was calculated as ratios of the integrated intensities of the main XPS lines of the metal atoms. Found stoichiometry is different from the expected one x=y=0.5. However, obtained values are within the accuracy of the XPS method, which is usually 10%, despite the fact that the determination of the relative composition of Hf/Zr in the film was carried out without taking into account the depth of the photoelectron yield for different elements and transmittance analyzer. Since etching with argon ions of binary oxide HfO2, and ZrO2 leads to the generation high concentration of oxygen vacancies (VO) in the surface layer of the films, at this project stage we tried to deplete oxygen Hf(x)Zr(y)O2 films in a similar manner. However, as a result of Hf(x)Zr(y)O2 etching with Ar+ ions, the XPS lines corresponding to Hf and Zr in the metallic state are not observed, and theoretically predicted XPS peak does not appear above the edge of the valence band. From this it is concluded that the original Hf0.5Zr0.5O2 films are not enriched with metal, and etching with argon ions does not lead to depletion of oxygen. However, the data do not exclude the oxygen vacancies presence in the test Hf0.5Zr0.5O2 films, since XPS technique is insensitive to defects with density less than 0.1%. According to the results of measurements of the stationary photoluminescence (PL) and photoluminescence excitation (PLE) of the polycrystalline Hf0.5Zr0.5O2 film at room temperature, a broad band of the PL with the maximum near 2.7 eV is observed (in the blue region of the spectrum), which is consistent with published data for HfO2 [AA Rastorguev, V.I. Belyi, T.P. Smirnova et al, Phys. Rev. B 76, 235315 (2007)] and ZrO2 [K. Joy, I.J. Berlin, P.B. Nair et al, J. Phys. Chem. of Solids 72. 673 (2011)]. In addition, the PL spectra show a shoulder at an energy of about 3.5 eV. PLE spectrum for the blue PL band has maximums at 4.5 eV, 3.7 eV and 5.2 eV. Since the blue photoluminescence excited by a photon energy 5.2 eV which is lower than Hf0.5Zr0.5O2band gap (5.65 eV according to quantum-chemical simulations), it is concluded that the PL with the energy of 2.7 eV is caused by optical transitions at defects that have electronic states in the gap. The peak with energy of 3.56 eV is exited by quants at 4.4 eV and 4.8 eV and 5.4 eV. An agreement between experimental PLE spectrum with ab ignition calculations of oxygen vacancies in cubic Hf0.5Zr0.5O2 phase shows that PLE band with energy 5.2 eV is due to the presence of oxygen vacancies (which has maximum in the absorption spectrum near 5.1-5.2 eV). So, one can conclude that the blue PL of Hf0.5Zr0.5O5 is due to oxygen vacancies. The presence of appreciable intensity in PLE spectra at energies below 5.2 eV is probably caused by the charge states of oxygen vacancies. This founds allowed us to draw a configuration diagram of optical transitions in the oxygen vacancies, which is the same for HfO2, ZrO2 and Hf0.5Zr0.5O2. Thermal trap energy Wt for oxygen vacancy is estimated using an empirical that it is a half of the Stokes shift of luminescence: Wt = (5.2-2.7)/2 = 1.25 eV. The transport properties of electrons and holes in Hf0.5Zr0.5O2 films with amorphous structure (up to RTA) without ferroelectric properties and crystal structure with ferroelectric properties were studied by measuring current-voltage characteristics. From experiments on transport of minority carriers through the Hf0.5Zr0.5O2 films from n-Si and p-Si substrates it was revealed that charge carriers in Hf0.5Zr0.5O2 can be electrons as well as holes. This indicates that the charge traps are amphoteric, (i.e. the conductivity is bipolar) in amorphous and polycrystalline Hf0.5Zr0.5O2films. A similar result was obtained for HfO2 [D. R. Islamov, VA Gritsenko, CH Cheng, and A. Chin, Appl. Phys. Lett. 99, 072109 (2011)] and ZrO2 [report on 2014]. Current-voltage characteristics of the structures based on amorphous and polycrystalline Hf0.5Zr0.5O2, measured at different temperatures (for enhancement mode MDM structures) show that the current is exponentially dependent on applied voltage. Comparison of experimental data with simulation results showed that the transport in Hf0.5Zr0.5O2 can be described qualitatively by any known model of charge transport in dielectrics: phonon-assisted tunneling between traps [K. A. Nasyrov and V. A. Gritsenko, J. Appl. Phys. 109, 093705 (2011)], multiphonon ionization of neutral traps [S. Makram-Ebeid and M. Lannoo, Phys. Rev. B25, 6406 (1982)], Coulomb center ionization [J. Frenkel, Phys. Rev. 54, 647 (1938)], hopping between overlapping Coulomb centers (Poole law) [R. M. Hill, Philos. Mag. 23, 59 (1971)]. However, the first of the listed model (phonon-assisted tunneling between traps) can describe the experiment quantitatively. Obtained values of thermal and optical trap energies of 1.25 eV and 2.5 eV, respectively, do not depend on the crystal structure and the film producing technique method. Moreover, these values are equal to that obtained for HfO2, and ZrO2, which exhibits the blue luminescence band at 2.7 eV due to the transition to the oxygen vacancies as well (short overview of the HfO2 properties was published in the article [Damir R. Islamov, Vladimir A. Gritsenko, and Timofey V. Perevalov, (Invited) The Influence of Defects on the Electronic Properties of Hafnia, ECS Transactions 69(5), 197-203 (2015), doi: 10.1149/06905.0197ecst]). Taking into account the arguments above, one can conclude that the transport properties of amorphous and crystalline Hf0.5Zr0.5O2 are described by phonon-assisted tunneling between traps, and the traps are oxygen vacancies. It was found that the trap density in the ferroelectric film 3×10^19 cm^-3 is comparable with the value obtained for amorphous films (3-10)×10^19 cm^-3. In this regard, a question of the role of oxygen vacancies in the stabilization of the ferroelectric Hf0.5Zr0.5O2 phase is still open. The results are published in a series of articles [D. R. Islamov, T.V. Perevalov, V.A. Gritsenko, C.H. Cheng, A. Chin, Charge transport in amorphous Hf0.5Zr0.5O2, Applied Physics Letters 106, 102906 (2015), doi: 10.1063 / 1.4914900; D.R. Islamov, A.G. Chernikova, M.G. Kozodaev, A.M. Markeev, T.V. Perevalov, V.A. Gritsenko, M. Orlov, Charge Transport Mechanism in Thin Films of Amorphous and Ferroelectric Hf0.5Zr0.5O2, JETP Letters 102(8), 544-547 (2015), doi: 10.7868 / S0370274X15200126]. Experiments on the charge accumulation in Hf0.5Zr0.5O2 revealed that holes predominantly accumulate in this material, whereas significant accumulation of electrons is not observed. This asymmetry confirms that, despite the bipolar conductivity Hf0.5Zr0.5O2 (i.e. amphoteric trap nature), transport can be described by unipolar model with good accuracy. So, a question of electrons and holes contribution to the conductivity remains open. Quantum-chemical simulations of the electronic structure of oxygen vacancies in the orthorhombic phase noncentrosymmetric o-Hf0.5Zr0.5O2 (space group symmetry Pbc2(1)) were carried out using the software package Quantum-ESPRESSO [P. Giannozzi et al J. Phys .: Condens. Matter 21, 395502 (2009)] in the framework of density functional theory with hybrid exchange-correlation functional B3LYP, in the model of periodic supercells. It is known that noncentrosymmetric orthorhombic Pbc2(1) phase has ferroelectric properties. In o-Hf0.5Zr0.5O2 structure four non-equivalent oxygen atoms are located: two O atoms have fourfold coordination (HHZZ) with different Hf-O and Zr-O bond lengths, but qualitatively with identical electronic structure, and two O atoms with triple coordination characterized immediate surroundings of 2 Hf and 1 Zr atoms (HHZ), and 1 Hf and 2 Zr atoms (HZZ). The sequence of polyvacancy formation (up to 5 oxygen vacancies) in 96 nuclear o-Hf0.5Zr0.5O2 supercell was defined from the principle of the total energy minimizing. The oxygen atoms, which are removed, has one type of HHZZ and are at a significant distance from each other. Forming energy of triply coordinated VO (6.40 eV for HHZ-type and 6.42 eV for HZZ-type) much higher than for fourfold coordinated VO (6.11 eV for HHZZ-type vacancy). Probably, this is the reason why forming of closely spaced VO (i.e. poly-vacancy) in o-Hf0.5Zr0.5O2 is not profitable energetically. This, in turn, may explain why etching with Ar+ ions of the polycrystalline Hf0 .5Zr0.5O2 films does not lead to metal enrichment of the films. On the contrary, for the m-HfO2 and m-ZrO2 formation energies of threefold and fourfold coordinated VO is not so much different from each other, the it is energy profitable to form oxygen vacancies close to each other (there is some clustering of vacancies) and therefore etching with argon ions produced a lot of oxygen vacancies in amorphous films. Estimated band gap of o-Hf0.5Zr0.5O2 Eg = 5.65 eV agrees with published experimental data for HfO2 (Eg = 5.6-5.8 eV) and ZrO2 (Eg = 5.5-5.6 eV). Four-and triply coordinated VO form filled stages in the gap above the valence band at 2.8 eV and 3.3 eV, respectively. Calculation of partial densities of electronic states of the nearest metal atoms to HHZZ VO showed that the largest contribution to the formed defect filled levels provide d electrons of Hf (Hf5d) and Zr (Zr4d) equally. Contribution of s electrons of the metal, and p electrons of oxygen in the order of magnitude smaller. Furthermore, a defect state was identified higher the conduction band edge than about 0.1 eV. This state is formed by d electrons of metal atoms. When an electron or a hole are added into the supercell, this state will move to the band gap. Positions of charged states (~1 eV lower than the conduction band edge) indicate a significant displacement of the atom and the electron density around the defect. So, in an added into the supercell electron localizes itself in an energy well (polaron effect). Localized states in the band gap from the charged defects indicate that any type of oxygen vacancies can capture both electrons and holes. Thermal (Eth) and optical (Eopt) ionization energies, caclucated for different charge states, are more reliable values that can confirm ability of the defects to localize charge carriers. It was found that oxygen vacancy can act as an amphoteric localization center (trap) for charge carriers, i.e., it can capture both electrons and holes. In this mean VO can act in charge transport processes. Calculated Eth and Eopt values for oxygen vacancies with a trapped hole are in good agreement with the values of thermal (Wt) and optical (Wopt) trap energies, obtained from experiments on charge transport. Localization of charge on the oxygen vacancy is confirmed by the spatial distribution of the electron density in VO and its first coordination sphere. The positive charge is distributed approximately evenly between the nearest to VO metal atoms, the negative charge is distributed unevenly, so only a couple of atoms (Hf-Zr) are bounded. Intrinsic defects in solid solution Al(x)Hf(1-x)O(y) films were studied by low temperature time-resolved luminescence spectra technique. Such films are used as model systems of HfO2 layers, doped with Al. It was revealed that such defects are oxygen vacancies near Hf atoms as well as near Al dopant. The results are published in the article [VA Pustovarov, T.P. Smirnova, M.S. Lebedev, V.A. Gritsenko, M. Kirm (2015) Intrinsic and defect related luminescence in double oxide films of Al-Hf-O system under soft X-ray and VUV excitation, Journal of Luminescence 170 (1), 161-167 (2015), doi: 10.1016 /j.jlumin.2015.10.053].

 

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