Crystal Growth and Photoluminescence Properties of Reactive CVD

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Crystal Growth and Photoluminescence Properties of Reactive CVD-Derived Monoclinic Hafnium Dioxide Victor V. Lozanov, Natalya Baklanova, Vladimir Shayapov, and Alexey Sergeevich Berezin Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.6b00824 • Publication Date (Web): 09 Aug 2016 Downloaded from http://pubs.acs.org on August 9, 2016

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Crystal Growth & Design

Crystal Growth and Photoluminescence Properties of Reactive CVD-Derived Monoclinic Hafnium Dioxide Victor V. Lozanov1)*, Natalya I. Baklanova1), Vladimir R. Shayapov2), Alexey S. Berezin2) 1)

Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze st. 18, Novosibirsk, 630128 Russian Federation

2)

Nikolaev Institute of Inorganic Chemistry SB RAS, Lavrentiev ave. 3, Novosibirsk, 630090 Russian Federation

Abstract: Here we report the first synthesis of large-size (up to 2 cm), transparent high-purity monoclinic HfO2 single crystals by reactive chemical vapor deposition (RCVD) using CF4 as a transport agent at 1000°C. The single crystals were comprehensively characterized in terms of their phase and elemental composition, as well as morphology by the modern analytical techniques. Thermodynamic modelling of the Hf – C – Si – O – F heterogeneous system was undertaken to understand in detail the chemical equilibria that occur in this transport system. Based on modelling results, it was shown that HfO2 formation occurs through HfOF2 decomposition. Morphological peculiarities of RCVD-derived HfO2 single crystals were studied by optical and scanning electron microscopy. Vicinal hills and numerous growth steps were observed on the crystal surfaces. The HfO2 single crystals exhibit strong and broad emission under UV excitation. The fit of the broad optical emission into spectral components allowed us to identify the nature of emission and assign it to the intrinsic defects of crystals. The successful synthesis of plate-like high-purity monoclinic HfO2 single crystals may provide some insight into the design of optical or other devices.

Corresponding author: PhD Victor V. Lozanov, ISSC SB RAS Kutateladze st. 18 Novosibirsk 630128 Russian Federation, Phone: +7(383)233-24-10*1132, Fax: +7(383)3322847, E-mail: [email protected] www.solid.nsc.ru

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Crystal Growth and Photoluminescence Properties of Reactive CVD-Derived Monoclinic Hafnium Dioxide Victor V. Lozanov1)*, Natalya I. Baklanova1), Vladimir R. Shayapov2), Alexey S. Berezin2) 1)

Institute of Solid State Chemistry and Mechanochemistry SB RAS, Kutateladze st. 18, Novosibirsk, 630128 Russia

2)

Nikolaev Institute of Inorganic Chemistry SB RAS, Lavrentiev ave. 3, Novosibirsk, 630090 Russia Corresponding author: [email protected] (Victor Lozanov)

Abstract: Here we report the first synthesis of plate-like (up to 2 cm), transparent high-purity monoclinic HfO2 single crystals by reactive chemical vapor deposition (RCVD) using CF4 as a transport agent at 1000C. The single crystals were comprehensively characterized in terms of their phase and elemental composition, as well as morphology by the modern analytical techniques. Thermodynamic modelling of the Hf – C – Si – O – F heterogeneous system was undertaken to understand in detail the chemical equilibria that occur in this transport system. Based on modelling results, it was shown that HfO2 formation occurs through HfOF2 decomposition. Morphological peculiarities of RCVD-derived HfO2 single crystals were studied by optical and scanning electron microscopy. Vicinal hills and numerous growth steps were observed on the crystal surfaces. The HfO2 single crystals exhibit strong and broad emission under UV excitation. The fit of the broad optical emission into spectral components allowed us to identify the nature of emission and assign it to the intrinsic defects of crystals. The successful synthesis of plate-like high-purity monoclinic HfO2 single crystals may provide some insight into the design of optical or other devices.

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1. Introduction Hafnium dioxide is widely adopted by the semiconductor industry for the fabrication of state-of-the-art complimentary metal-oxide semiconductor devices due to its superior gate oxide capacitance values, when compared to traditional materials such as SiO2.1–4 HfO2 is also recognized as the most stable and reliable candidate in the field of the fully transparent resistive memory (TRRAM) devices due to excellent transparency, resistive switching capability, and environmental stability.5–7 HfO2 TRRAM shows reliable performance under harsh conditions, such as high oxygen partial pressure, high moisture, exposure to corrosive agents, and proton irradiation. 3 During the recent years, the perspective of applying hafnium dioxide has geared up in various fields of medical science such as neutron detection, bioimplants, biosensors, radiotherapy etc.8–15 Indeed, hafnium oxide has chemical stability, pH sensitivity, high dielectric constant. Also, its isoelectric point is 7.011 which is very close to physiological pH value. Recently, it was shown that hafnium dioxide surfaces can be successfully used for phosphatedependent, oriented immobilization of DNA. Authors proposed that there is a secondary interaction between the exposed nucleobases of single-stranded DNA and the surface. The lattice spacing of monoclinic hafnium dioxide matches the base-to-base pitch of DNA. Because of this secondary DNA immobilization mechanism, monoclinic HfO2 could impede DNA hybridization or cause nonspecific surface interaction, which is desirable for biosensing applications. Another example of HfO2 application in the area of biomedicine is the detection of dsDNA by translocation through solid HfO2 nanopores in multilayered membranes.10,13 It was shown that the nanopores of crystallized HfO2, which were formed under exposure of the membrane to a focused electron beam, reveal improved hydrophilicity, compared with asdeposited HfO2 film and, as a consequence, improved sensitivity for the detection of small nucleic acid oligomers.10,13



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More recently, hafnium dioxide has been considered as a luminescent material16 Intrinsic luminescence without the employment of rare-earth activator impurities has been observed in HfO2, lying in the UV-blue region of the spectrum, which has been ascribed to the presence of point defects, such as oxygen vacancies, in the lattice. The relevance of these results lies in the opportunity to obtain new and lower-cost solutions for the realization of phosphor-converted white light-emitting diodes, which are considered as an indispensable solid-state light source for the next generation lighting industry and display systems.17,18 Despite numerous needs in HfO2 single crystals, in particular, monoclinic HfO2, the fabrication of single crystals remains a real challenge since several years. This is mainly due to a very high melting point of HfO2. There are several approaches to form hafnium dioxide. The first of them is based on the deposition of hafnium dioxide from aqueous solution.19–21 According to this approach19, aqueous solution of KOH was drop-by-drop added into aqueous solution of hafnium tetrachloride and stirred. Hydrated hafnium dioxide was deposited as white powder. This approach is a multistage one, and it is necessary to wash the product several times until its purity would be satisfactory. Another disadvantage of this approach is the formation of powdered product, but not a single crystal. In addition, the formed product can contain some water. The synthesis of powdered monoclinic HfO2 was described by many researchers.22-24 Pinna et al. reported about synthesis of monoclinic HfO2 via solvolysis using hafnium ethoxide.23 Lauria et al.24 and Rauwel et al.22 studied the interaction of hafnium tert-butoxide (Hf(OtBu)4) precursor and benzyl alcohol. The synthesis was carried out in the inert atmosphere in autoclave at 300C for 2 days. The resulting milky suspension was centrifuged; the precipitate was thoroughly washed and subsequently dried in the air at 60C. The drawbacks of this method include the usage of expensive and air- and moisture-sensitive reagents, the necessity to conduct the process in a glove box with the low oxygen concentration and low moisture, hardly


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controllable synthesis parameters, the usage of toxic solvents, multiple steps, and, finally, the formation of monoclinic HfO2 as nanoparticles but not as single crystals. Hafnium dioxide can be obtained by ALD (atomic layer deposition) method at 300500С using Hf(NO3)4 or HfCl4 and oxidative reagents, such as O2, O3, H2O or H2O2.25,26 This method allows obtaining polycrystalline monoclinic HfO2 as a film on silicon substrate. Singlecrystal HfO2 was grown by Arashi27 from a PbF2 flux using a platinum crucible within the temperature region where the monoclinic phase is stable. The PbF2 melt containing 7 mol% HfO2 was kept at 1050°C for 5 h, after that, it was slowly cooled to 500°C. To remove the single-crystal HfO2 from the PbF2 flux, the platinum crucible was immersed in hot diluted H2SO4 solution. The obtained single crystals are not larger than 2.5 mm in size. The drawbacks of the method are multiple steps, expensive equipment, and small sizes of single crystals. As one of the most convenient approach to grow the crystals of refractory compound such as hafnium dioxide, the reactive chemical vapor deposition or chemical transport through the gas phase was proposed by Schäfer28, Binnewies et al.29 and us.30 There was a great number of successful attempts to synthesize single crystals of various compounds, including those with high melting point.29 The purpose of this work was threefold: (i) to study the formation of monoclinic hafnium dioxide using the chemical transport reactions, (ii) to characterize the product comprehensively, and (iii) to study the photoluminescence of HfO2 single crystal. 2. Experimental 2.1. Initial substances and Crystal Growth Conditions Hafnium cuttings (GOST 22517-77, content of Hf not less than 99.8%(wt.)), tetrafluoromethane (R14, TU 301-14-78-92, vol. fraction of CF4 no less than 99.3%), Grafoil (graphite tape, TU 5728-003-93978201-2008, carbon content not less than 99.5%), were used as the initial substances.



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Hafnium cuttings were tightly packed in a grafoil cylinder with a small circular hole. The length of cylinder was about 100 mm long. The cylinder was placed into a quartz ampoule (TU 5932-014-00288679-01). The quartz ampoule was heated under vacuum to T = 1273 K for 5 hours, with the total pressure in the system being 5∙10-2 Torr to remove the moisture traces from quartz walls of reactor. After cooling, gaseous CF4 was injected into the ampoule to the total pressure 200 Torr. The ampoule was sealed, placed in a vertical furnace and heated at 1273 K for 48 hours. During crystal growth, the whole ampoule was kept at a constant temperature of 1273 K. Total pressure at this temperature was approximately 850 Torr.

2.2. Material Characterization The X-ray powder diffraction (XRD) analysis was performed with a D8 Advance diffractometer (Bruker, Germany) using a Cu Kα radiation (λ1 = 1.54056 Å, λ2 = 1.54439 Å). The diffraction patterns were analyzed with DIFFRACplus software using the ICDD PDF-2 database (2008). The single-crystal X-ray diffraction was measured with a DUO diffractometer (Bruker, Germany) using a Mo Kα radiation (λ = 0.7107 Å). The XRD pattern was analysed with APEX II software package. Single crystals with size about 250 μm were used for XRD measurement. The observations of the microstructure of HfO2 crystals and examination of the local chemical composition were conducted with the scanning electron microscopes MIRA 3 LMU, (TESCAN, Czech Republic) and TM-1000 (Hitachi Ltd., Japan) coupled with energy dispersive X-ray spectroscopy instruments INCA Energy 450 XMax 80 and Swift-TM (Oxford Instruments Analytical Ltd. GB), respectively. Preliminary, the samples were coated with a conducting nanosized chromium layer (MIRA 3 LMU). The variation coefficient characterizing the repeatability of a single determination is found to be ~ 1% for EDS within the compositional range of the main components (C > 10%). Images in polarized light were obtained using light microscopes (Carl Zeiss, Germany and POLAM L-213, LOMO, Russia). 6 ACS Paragon Plus Environment

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The FT-Raman spectra of hafnium dioxide crystal plates were recorded using a Bruker RFS 100/S spectrometer equipped with a Nd-YAG laser operating at the excitation wavelength of 1064 nm. The laser output was 100 mW. For each spectrum, 100 scans were accumulated. Micro Raman spectra of as-grown crystals were recorded using a Triplemate, SPEX spectrometer equipped with a CCD spectrometric detector cooled by liquid nitrogen, and a microscope attachment for back scattering geometry in the region of 40-1700 cm-1. The 488 nm radiation from an argon laser was used for spectral excitation. All measurements were carried out using a laser power of 5 mW on sample surface. The laser beam was focused with an optical objective on a spot 2µm in diameter. Because the trace impurities can play an important role in the performance of single crystals used in different applications, the determination of the concentrations of trace elements in crystals is of special interest. The trace analysis of the synthesized HfO2 single crystals was carried out by laser ionization mass spectrometry (LIMS).31 The analysis of HfO2 crystals was carried out using a double-focusing mass spectrometer combined with a laser ion source (EMAL-2) in the shared research centre in Nikolaev Institute of Inorganic Chemistry SB RAS. A Nd-YAG laser was used with a wavelength of 1064 nm, pulse width of 10-20 ns, laser energy per pulse 1-10 mJ and frequency 10-100 Hz. Due to laser power density 109 – 1010 W/cm2, the composition of the laser plasma corresponds to the elemental composition of the target material.31 Crushed HfO2 single crystal mixed with high-purity bismuth oxide powder was used as a target. High-purity bismuth oxide (99.9999%) was synthesized in Nikolaev Institute of Inorganic Chemistry SB RAS.32,33 UV-VIS spectroscopy (UV-3101 PC, Shimadzu, Japan) was used to measure the optical transmittance of m-HfO2 plate in the 180 – 800 nm spectral range. Corrected photoluminescence (PL) spectra were recorded on a Fluorolog 3 spectrometer (Horiba Jobin Yvon) with a cooled PC177CE-010 photon detection module equipped with R2658 photomultiplier. The real-time correction of spectra was performed with the consideration of detector sensitivity, intensity of 7 ACS Paragon Plus Environment


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ozone free xenon bulb (450W) and distortions caused by monochromators. Spectra were taken at room temperature in the air. The analysis of spectra was performed with Fityk 0.9.8 software.34 Voigt profiles were applied to the peak-fitted determination.35,36

3. Results and Discussions 3.1. Thermodynamic modelling of the Hf – C – Si – O – F system. The thermodynamic modelling is a convenient way to understand in details the chemical equilibria that occur in the multicomponent transport systems. Thermodynamic calculations can provide indications of favorable conditions concerning the temperature and pressure. Besides, the modelling provides information about the composition of the equilibrium gas phase, which is often not possible to extract from direct experimental measurements. The thermodynamic modelling of the Hf – C – Si – O – F system was performed within the 1100 – 1600 K temperature (T) range with 100 K interval and within the total pressure (P) range of 1 – 1000 Torr, with the interval of one order of magnitude. For more clarity, all pressure units in this work will be presented in Torr in order to facilitate the comparison of thermodynamic results with experimental ones. It was assumed that all fluorine-containing species were in vapour phase only. Therefore, the lower temperature for calculation was chosen as 1100 K. This is the temperature at which all lower hafnium fluorides are gaseous. The upper temperature value is a common temperature of quartz material operation. The following species in vapour phase were taken into attention, namely, HfFn (n = 1 – 4), SiFm (m = 1 – 4), CFx (x = 1 – 4), C2Fy (y = 1 – 6), SiOF2, HfOF, HfOF2, SiO, SiO2, HfO, HfO2, Sik (k = 1 – 3), Ci (i = 1 – 5), Oj (j = 1 - 3), OF, OF2, F, F2, SiC, SiC2, Si2C, CO, CO2, C3O2, C2O, COF, COF2, Hf in the vapor phase. Graphite, Hf, HfC, SiO2, HfO2, HfSiO4, Hf2Si, HfSi, HfSi2, SiC and Si were considered as solid phases. It was proposed that all compounds are stoichiometric ones. The SiF4:CO2 ratio in


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the vapor phase was given as 1:1, solid hafnium was in a tenfold molar excess relative to solid SiO2. Also it was proposed that equilibrium is established very quickly. Two sets of thermodynamic equilibria between the vapour and solid phases, namely, in zones of SiO2 disposition and metallic hafnium, respectively, were calculated. The solids in equilibrium with the vapour phase in “hafnium zone” are HfO2, HfSi, HfC and metallic hafnium; in “SiO2 zone” there are HfO2, HfSiO4 and Si. At the first glance, involving the silicon phase seems to be unexpected. However, there are some evidences that in the Hf – Si – O system the silicon phase can co-exist with HfO2 and HfSiO4 at 760 Torr within the 543 – 1600 K temperature range.37 Modelling was based on minimization of the Gibbs free energy function and was performed using the “Applied program for physical and chemical equilibria” coupled with the “Database on properties of materials for electronics” (DB SMET, Nikolaev Institute of Inorganic Chemistry SB RAS).38 For thermodynamic calculations, the data presented in39–41 as well as in42 (for hafnium fluorides and hafnium oxyfluorides) were used. The results of thermodynamic calculations of the vapour phase composition for the “hafnium zone” are presented in Fig. 1. One can note that all vapour species can be conventionally divided into three groups. The first group is composed of those species the partial pressures of which are less than 10-8 Torr at each P and T values. These species include Si2, Si3, SiO2, HfO2, HfO, OF2, OF, F2, Si2C, SiC, SiC2, C3O2, C2O, CO2, CO, COF, COF2, CFx, C2Fy, Ci, F, Hf, Oj, SiOF2. These vapor species will not be taken into account under consideration of the chemical transport in the Hf – C – Si – O – F system. The other vapour species group is represented by only those components the partial pressures of which are above 10-8 Torr for each P and T value in the 1100 – 1600 K temperature range and in the total pressure range of 1 – 1000 Torr. It was proved that just only hafnium fluorides (HfFn, n = 1 – 4) meet this demand. The intermediate group is represented by Si, SiO, SiFm, HfOF, HfOF2 vapour species. Their partial 9 ACS Paragon Plus Environment


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pressures can be higher than 10-8 Torr only for some P and T values, but at these P and T these partial pressures are always lower than the HfFm partial pressures. Thus, according to thermodynamic calculations of the Hf – C – Si – O – F system, the main components of the vapour phase in “hafnium zone” are hafnium fluorides, hafnium tetrafluoride being a dominating compound (Fig.1). The solubility of hafnium in the vapour phase increases with increasing temperature and decreasing total pressure, which means an increase in the lower fluoride content in the vapour phase.43 At a total pressure of 1000 Torr and temperature 1300 K (these conditions are favorable from practice point of view) the composition is  98.93% (vol.) HfF4,  1.07% HfF3, 310-5% HfF2, 410-6% HfF. The rest vapour species can be neglected.

Figure 1. The major vapour species in the “hafnium zone”. Each curved surface area presents a set of the calculated values of partial pressures of given vapour species in dependence on T and P. Let us imagine that the vapour phase from “hafnium zone” is transferred into “SiO2 zone”. As was mentioned above, the composition of the vapour phase in the “hafnium zone” was represented mainly by HfFn (99.999 % (vol.)). In the “SiO2 zone”, the vapour phase composition is drastically changed. The solid HfO2, HfSiO4 and Si phases are formed and new composition is 10 ACS Paragon Plus Environment

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established, which is in equilibrium with these solid phases. The calculations showed that the main constituents in the vapour phase in this case will be SiF4, SiF3, SiF2, SiO, HfF4, HfF3, and SiOF2 (Fig.2). The partial pressure of each component listed above is more than 10-8 Torr. One can note several distinguishing features of the vapour phase in the “SiO2 zone”, namely, (i) the appearance of Si-containing species, silicon tetrafluoride being the dominant compound; (ii) the content of HfFn was significantly decreased compared with that in the “hafnium zone”; (iii) the appearance of Si – O – containing species in noticeable quantities. Again, at a total pressure 1000 Torr and temperature 1300 K, the vapour phase composition is represented by  97.15%(vol.) SiF4, 2.80% HfF4, 0.04% SiF2, 910-4 % HfF3, 310-4 % SiF3, 310-4 % SiO, 2105

% SiOF2. One can note that the SiO and SiOF2 contents increased up to 6 – 8 orders compared

with the equilibrium vapour phase over metallic hafnium.

Figure 2. The vapour phase composition (major species) in “SiO2 zone”. Each curved surface area presents a set of the calculated values of partial pressures of given vapour species in dependence on T and P. As was shown by thermodynamic modelling, at P = 1000 Torr and T = 1300 K the solid phase which is in equilibrium with the vapor phase will be HfO2, HfSiO4, and Si mixture, the 11 ACS Paragon Plus Environment


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HfO2:HfSiO4:Si molar ratio being 342:9:1. With a decrease total pressure to 1 Torr, the equilibrium solid phases will be HfO2 + HfSiO4 (195:1) at T = 1400 K and single HfO2 phase at T = 1500 – 1600 K. According to the results of thermodynamic modelling, the formation of gaseous HfOF2 and SiOF2 occurs in the “SiO2 zone”. It could be proposed that these vapour species are generated by reaction (1). Further, HfOF2 is decomposed with the formation of solid HfO2 according to reaction (2) HfF4(g) + SiO2(s) ↔ HfOF2(g) + SiOF2(g)

(1)

2HfOF2(g) ↔ HfO2(s) + HfF4(g)

(2)

The equilibrium constant Kp for reaction (2) can be estimated in accordance with formula (3). Here, [HfF4] and [HfOF2] are the corresponding partial pressures at given T and P and were calculated in this work (Fig.1 and 2).

(3) The obtained Kp values are shown as a ln Kp – T dependence in Fig. 3. The obtained results demonstrate that the rate of the direct reaction is 1011 – 1022 times higher than the rate of the reverse reaction. Therefore, relying on the results of thermodynamic modelling it could be proposed that HfO2 is mainly formed via reaction (2).


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Figure 3. The dependence of ln Kp for reaction (2) on temperature, and the fitting curve. 3.2. Monoclinic hafnium dioxide single crystal growth 3.2.1. Phase and elemental composition of products In our experiments, hafnium cuttings changed color and got bright grey. In addition, turnings became very brittle. The experimental observations may serve as evidence that the reactions with hafnium metal takes place. The solid products were readily grounded in agate mortar. According to the X-Ray diffraction data, HfSi is present as a main phase together with HfC. In addition to the mentioned compounds, the traces of cubic HfO2 phase were also detected in X-Ray diffraction patterns (Fig.4). No any diffraction peaks belonging to metallic hafnium were found. The obtained experimental results are in good accordance with theoretical modelling. Indeed, the formation of HfSi can be readily explained by the reaction of SiFm gaseous species with metallic hafnium, and the formation of HfC by the reaction of metallic hafnium with gaseous CF4. Cubic hafnium dioxide is not a thermodynamically stable phase under the experimental conditions of this work. It appears to be the product of the interaction of HfSi or HfC with oxygen-containing components of the vapour phase. One can note that the 13 ACS Paragon Plus Environment


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formation of cubic hafnium dioxide at temperatures as low as 1000C was reported by many researchers.44–46

Figure 4. X-Ray diffraction data of the products formed on metal hafnium. The walls of quartz ampoule were coated with a white fine crystalline deposit. A thorough inspection of the surface of the quartz ampoule showed that the deposit was grown into the walls of the ampoule (Fig. 5 a). As follows from XRD analysis, the phase composition of the deposit corresponds to monoclinic HfO2 and HfSiO4 phases (Fig. 5 b). One can emphasize that the formation of hafnon (HfSiO4) together with HfO2 in the “SiO2 zone” was predicted by thermodynamic modelling.


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Figure 5. SEM image and X-Ray diffraction pattern of the deposit grown into the quartz glass. Over the white fine-crystalline deposit a great number of transparent well-developed plate-like large-size crystals were formed. The sizes of some crystals reached up to 2 cm in length, and the thickness was about 120 – 150 μm. To our knowledge, this is the largest reported monoclinic HfO2 single crystal. Earlier, cubic hafnium dioxide single crystal with remarkable sizes (about 2 cm) was grown by skull melting method.47 As an example, the plate-like transparent crystals joining one another lying over a coloured picture is shown in Figure 6.

Figure 6. Photograph of the transparent plate-like HfO2 crystal over the coloured picture The XRD pattern of finely ground powder derived from the as-grown crystals is consistent with ICDD-PDF No. 34-0104, indicating that they are monoclinic crushed HfO2 crystals (Fig. 7 a). No any other hafnium-containing phases were detected. The XRD pattern of the as-grown transparent crystals has a distinct feature, namely, some diffraction peaks are systematically extinct. Therefore, it could be proposed that the as-grown crystals belong to textured monoclinic HfO2 phase. As follows from XRD pattern, the crystallographic planes are oriented perpendicular to the [001] direction (Fig. 7 b). The single-crystal X-ray diffraction pattern presented in Fig.7 c demonstrates that the crystal has continuous structure and [001] orientation.



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Figure 7. The XRD patterns of HfO2: (a) - of crushed plate-like crystals; (b) - as-grown plate-like crystals; (c) - Laue diffraction pattern obtained at 30 kV, 50 mA. Green circles are positions of hk3 reflections of m-HfO2. Due to the fact that the Raman spectrum of monoclinic HfO2 is sharply different from that of tetragonal and cubic HfO2, Raman spectroscopy can be a very useful tool for the identification of hafnium dioxide phases in addition to X-Ray analysis.48 Therefore, as-grown crystals were also identified by Raman spectroscopy. The obtained Raman spectra are presented in Figure 8. The Raman spectra in the 100 – 1500 cm-1 spectral region are presented in Supporting information. The number and positions of all Raman peaks in the spectra taken at  = 1064 and 488 nm are in a good agreement with literature data for the monoclinic modification.20,27,49 Based on the results of two independent analytical methods, one can conclude that the obtained crystals present monoclinic hafnium dioxide phase as a single phase.



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Figure 8. Raman spectra of as-grown plate-like HfO2 single crystal, taken at wavelengths 1064 and 488 nm. The energy dispersive X-ray analysis taken from different single crystals showed that they are of very high purity. Only oxygen and hafnium elements were found. No traces of fluorine or silicon were detected within the method limitations. Taking into attention the variation coefficient for EDS ( ~ 1%)50, one can conclude from the data presented in Table 1 that the Hf/O ratio is very close to the stoichiometric one (15.2 % (wt.) oxygen and 84.8 % (wt.) hafnium). Table 1. EDS results of different areas of plate-like HfO2 single crystals

Spectrum

O, % (wt.)

Hf, % (wt.)

1

14.9 ± 0.2

85.1 ± 0.2

2

14.9 ± 0.2

85.1 ± 0.2

3

14.6 ± 0.2

85.4 ± 0.2

4

14.5 ± 0.2

85.5 ± 0.2

5

14.5 ± 0.2

85.5 ± 0.2

6

14.4 ± 0.2

85.6 ± 0.2 18


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According to LIMS analysis data, in the synthesized HfO2 single crystals, the following elements are presented as impurities: C (7∙10-4 %), F (2∙10-3 %), Na (2∙10-3 %), Mg (1∙10-3 %), Si (5∙10-3 %(wt.)). Na and Mg impurities could originate from the quartz ampoule. Other elements (C, F, and Si) can be trapped from the vapour phase during the growth of HfO2 single crystals. According to data51, the Si - F stretching mode can be expected at  945 cm-1 in Raman spectrum. However, no any Raman peaks corresponding to the stretching Si - F mode were detected for HfO2 crystals prepared by RCVD in this work (Supporting information). Such discrepancy between Raman and LIMS analysis data can be explained by higher sensitivity of the last. It follows from LIMS analysis data that the synthesized HfO2 single crystals are not less than 99.99% (wt.) purity; hence, they can be regarded as very high pure ones.

3.2.2. The morphology of monoclinic HfO2 crystals As-grown plate-like m-HfO2 crystals present an excellent sample for the study of the peculiarities of crystal growth. As seen from Fig. 9 a, hafnium dioxide crystals have welldeveloped facets of habitus predominantly with vicinal surfaces (Fig. 9 b). No evidence of the screw dislocations that give start to the spiral growth was detected on single HfO2 crystals under investigation in this work. Hence, it could be proposed that the crystal face is growing on the account of two-dimensional nuclei that are formed on it, and the crystal growth appears to occur through the attachment of atoms to the pre-existing steps.52 As a rule, the process takes place at significant critical saturations. Investigation of the vicinal surfaces showed the growth patterns are of the miscellaneous type, namely, a part of the generating lines of vicinal hills are straight ones, while the others are of rounded shapes (Fig.9 b). This can be a consequence of the fact that the crystal growth conditions favour neither polygonal nor circular growth patterns. One can note that usually the 19 ACS Paragon Plus Environment


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steps run parallel to the z direction, although in some cases the deviation of the steps from their regular positions can be detected (Fig. 9 c). The reason can be related with the presence of impurities, such as C, Si, F or others which were determined by LIMS analysis. In addition to the above-mentioned peculiarities, the appearance of the so-called kinematic waves on vicinal surfaces can be detected too (Fig. 9 d). According to the modern view on crystal growth52,53, both step meandering and kinematic waves are the consequences of crystal growth instability. Both these instabilities can arise in the case when the surface is driven out of equilibrium.


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Figure 9. SEM images of HfO2 crystals: a – a survey view of HfO2 plate-like crystals; b – vicinal surface inclined down in the x direction. Steps run parallel to the z direction; c – the deviation of the steps from their regular position; d – step bunching. On SEM images the orientation of the crystal surface is [001].



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Figure 10 a – c. Photos of m-HfO2 crystal growth steps taken in polarized light. The orientation of the crystal surface is [001]. The study of hafnium dioxide crystals with optical microscopy in polarized light showed that they are anisotropic ones. The waves of step bunching on the vicinal surface of crystals can be especially well distinguished in polarized visible light, as shown on Fig. 10 a, b. They are evident as crossing steps and dark spots in polarized light. Twins can be observed on some optical (Fig.10 c) and SEM (Fig. 11 a, b) images.


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Figure 11 a, b. SEM images of twins

Concluding the consideration of the peculiarities of monoclinic hafnium oxide crystals to be grown under RCVD conditions, one can note that they display a surprising variety of morphological features. The reasons of the observable features are not always clear. More experiments are necessary in order to shed further light upon their origin in HfO2 crystals grown by RCVD method.

3.3. UV-Vis and Photoluminescence (PL) studies To understand the nature of defects in HfO2 single crystals and to determine the energy gap, the UV-Vis transmission and photoluminescence (PL) spectra of m-HfO2 plate-like crystals were investigated in this work. From XRD experiments, it was obtained that the direction of hafnium dioxide plate surface is [001]. It is known that the lowest barrier of the three-fold coordinated oxygen vacancy hopping is that along the [001] direction.54 Oxygen vacancies migrate on the surfaces which are normal to [001] direction. The UV-Vis transmission spectrum of m-HfO2 single crystal in the [001] direction is presented in Fig.12. As one can see from Fig. 12, a rather low transmission for crystal is observed. This can be explained by light scattering arising from the imperfection of the crystal 23 ACS Paragon Plus Environment


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surface, namely, presence of twins and steps. The drop of transmittance in the 210 – 230 nm range is due to the energy gap.

Figure 12. UV-Vis transmission spectrum of m-HfO2 plates in the [001] direction. To determine the energy gap, the transmission spectrum was transformed using Tauc’s relation.55 The Tauc’s relation is

(4) where α is the absorption coefficient, hν is the photon energy, Eg is the gap energy, n is the constant depending on the band gap type, namely, 1/2 – direct allowed, 2 – indirect allowed, 3/2 – direct forbidden, 3 – indirect forbidden band gap.55–58 Coefficient α was obtained from relation 5, where d is the thickness of plate-like crystal or film and T is transmittance.59

(5) It was stated previously that hafnium dioxide has direct allowed and indirect allowed band gaps.60–62 Taking into account these data, two dependences, namely, (αhν)2 on hν (Fig. 13 a) and (αhν)1/2 on hν (Fig. 13 b) were plotted using the experimental data of this work. The linear


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approximation of both dependences with the subsequent extrapolation of linear fits up to y = 0 allows us to determine both the direct and indirect allowed gap energies.

Figure 13. The Tauc’ plots: a – the dependence of (αhν)2 on hν for direct allowed energy gap; b – the dependence of (αhν)1/2 on hν for indirect allowed energy gap. The direct allowed energy gap was determined to be equal to 5.89 eV and the indirect allowed energy gap to be equal to 5.54 eV. One can note that the value of direct allowed energy gap is close to that reported by different authors for monoclinic HfO2.60–62 The indirect allowed energy gap value is also in good correspondence with data reported elsewhere.60–62 25 ACS Paragon Plus Environment


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The PL spectra of monoclinic HfO2 single crystal taken at the excitation wavelengths of 280 nm (4.43 eV) and 240 nm (5.17 eV) are presented as a function of energy in Fig. 14 a, b, respectively. The PL emission spectra measured in wider spectral range (1.3 – 4.3 eV) are shown in Supporting information. The photos of HfO2 single crystal excited by the same energy are shown in Fig.15 a, b. One can see that both PL spectra have a broad complex emission ranging from 1.6 to 3.1 eV. All the PL spectra were deconvolved using the Voigt function. Each spectrum taken at the energy of 4.43 eV and 5.17 eV can be fitted by four Voigt functions. For excitation with the energy 4.43 eV, the components have maxima at 2.98 eV, 2.69 eV, 2.38 eV, and 1.95 eV, the highest intensity emission band being at 2.69 eV. For excitation with the energy of 5.17 eV, the components have maxima at 2.62 eV, 2.20 eV, 1.96 eV и 1.76 eV. The emission band with the highest intensity is shifted to the red region and appears at 1.96 eV.

Figure 14. Dependences of the optical emissions of HfO2 single crystals excited by 4.43 eV (a) and 5.17 eV (b). Components (blue lines) obtained by Voigt function fitting are shown together with experimental curves (black).


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Figure 15. Optical images of HfO2 crystal upon excitation with 4.43 eV (a) and 5.17 eV (b) illumination. The most intense PL band (2.69 eV) appears in the blue region of the Vis spectrum (a) and the PL band (1.96 eV) appears in the red region of the Vis spectrum.63 The excitation spectra of the HfO2 single crystal for 2.69 eV and 1.96 eV emission bands are presented in Fig. 16 a, b, respectively. One can see that the intensity increases monotonously within the whole interval of energies to be tested in this work for emission energy 1.96 eV (Fig.16 b). A sharp increase in intensity suggests that the maximum of excitation could exist beyond the value of 5.4 eV and can be ascribed to the ionization energy of interstitial oxygen ions.64 Analogously, the intensity is increased for the emission energy of 2.69 eV, however, the dependence has a local maximum at 4.33 eV (Fig. 16 a) that corresponds to the ionization energy of threefold- and fourfold-coordinated oxygen vacancies.64



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Figure 16. Excitation spectra of HfO2 single crystals for emission energy 2.69 eV (a) and 1.96 eV (b). The analysis of the optical emission data of this work and a comparison of them with the literature data obtained by other investigators allows us to find a correlation between optical emission and the defects in HfO2 crystals.5,16,64,66–71 The experimental data of this work and published previously are listed in Table 2. It was stated that the defects that can exist in monoclinic hafnium dioxide are vacancies and interstitial atoms and ions.64,65 However, the calculated energy of hafnium vacancy formation is higher than that for oxygen vacancy formation. Based on DFT calculations, Foster et.al.64 stated that the presence of hafnium vacancies does not introduce any additional level in the gap. Broqvist et al.66, Rastorguev et al.5, Perevalov et al.67 and Chuang et al.68 experimentally showed that the bands of 2.2 – 2.8 eV can be associated with oxygen vacancies. Because the experimental values of emission band energies obtained in this work are in good agreement with those calculated by Foster et al.64 for monoclinic HfO2 and experimentally obtained by the other researchers for monoclinic HfO2 powder,5,16,66–71 it was proposed that they are associated with oxygen vacancies. The origination of the PL bands in the 1.5 – 2.0 eV emission range in spectra of hafnium dioxide and the other oxides was discussed in works.72, 73 It was proposed that these bands can be associated with interstitial oxygen ions and atoms. These defects can be both own defects of the 28 ACS Paragon Plus Environment

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crystal structure and induced by the oxygen photoadsorption. Earlier, it was shown that under E > 3 eV excitation the formation of O– or O2– adsorbed ions and interstitials can take place in metal oxides.74 After photoabsorption at 4.66 eV, O– ions disintegrate, forming either 1D or 3P state of oxygen with a partition of 1D:3P = 0.32.75 O(1D) is the excited state of atomic oxygen and transition into the O(3P) ground state is accompanied by emission in red region (1.96 eV).76 Table 2. Comparison of calculated64 and experimental (this work) emission band energies (eV) for different type of defects in monoclinic HfO2. Here, VO×, VO• and VO•• are uncharged, singly and double positively charged oxygen vacancies, respectively; Oi′ is singly negatively charged oxygen interstitial ion (Kröger-Vink notations); O(1D) and O(3P) are singlet and triplet (ground) states of oxygen atom, respectively. Experimental Eex = 4.43 eV Eex = 5.17 eV

Energy, eV

Calculated Assignment of emission bands

2.98

-

2.92-2.93

hole affinity of threefold-coordinated VO× and/or electron affinity of threefoldcoordinated VO••

2.69

2.62

2.75-2.76

hole affinity and/or electron affinity of threefold-coordinated VO•

2.38

2.20

2.42

hole affinity of fourfold-coordinated VO× and VO•

1.95

1.96

1.96a

O(1D) → O(3P) transition

1.76 1.73 Experimental data, Sergienko et al.76

hole affinity of Oi′

a

4. Conclusions Here we report the synthesis of plate-like (up to 2 cm linear size), transparent high-purity monoclinic HfO2 single crystals. Hafnium dioxide crystals were produced by reactive chemical vapor deposition in a quartz ampoule using CF4 as a transport agent at temperature as low as 1000C. Thermodynamic modelling of the Hf – C – Si – O – F heterogeneous system in the wide 29 ACS Paragon Plus Environment


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temperature and pressure ranges was undertaken to understand in detail the chemical equilibria that occur in this transport system. Based on the modelling results, it was shown that HfO2 formation occurs through the decomposition of HfOF2. The results of modelling are in good agreement with experimental results and help to explain the formation of different solid products in the course of chemical transport through the gas phase. The HfO2 single crystals were comprehensively characterized by the modern analytical techniques in terms of their phase and elemental composition, as well as morphology, and it was confirmed that the obtained single crystals are of very high purity. The morphological peculiarities of the RCVD-derived HfO2 single crystals were studied by optical and scanning electron microscopy. Vicinal hills and numerous growth steps, as well as step meandering and kinematic waves were observed on the crystal surfaces, which may be a consequence of the instability of crystal growth. It was shown that HfO2 crystals exhibit strong and broad emission under UV excitation. The fit of the broad optical emission into spectral components allowed us to identify the nature of the emission and assign it to intrinsic defects of crystals. The approach based on reactive CVD and successfully used in this work for the synthesis of plate-like high-purity monoclinic HfO2 single crystals can be recommended for the lowtemperature synthesis of other high-melting oxide single crystals. The results obtained on the synthesis and photoluminescence properties of HfO2 single-crystals will be of interest for researchers in various modern application areas, such as biosensors, white light-emitting diodes, microelectronics etc. Acknowledgement The authors are thankful to PhD N.V. Bulina (ISSC SB RAS) for XRD measurements, Dr.Sci. A.A. Sidelnikov (ISSC SB RAS) for optical microscopy analysis, PhD A.T. Titov (IGM SB RAS) for SEM/EDS analysis and D.Sci. I.Yu. Prosanov (ISSC SB RAS) for Raman spectroscopy measurements. The authors are grateful to the shared research centre in Nikolaev 30 ACS Paragon Plus Environment

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Institute of Inorganic Chemistry SB RAS for Raman spectroscopy measurements, single-crystal XRD measurements and LIMS analysis. This work was supported by the Project of RAS # I-38. Notes The authors declare no competing financial interests Supporting information is available free of charge via the Internet at http://pubs.acs.org/.



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(44) Aarik, J.; Aidla, A.; Mändar, H.; Sammelselg, V.; Uustare, T. J. Cryst. Growth 2000, 220, 105–113 (45) Xie, Y.; Ma, Z.; Su, Yu.; Liu, Y.; Liu, L.; Zhao, H.; Zhou, J.; Zhang, Zh.; Li, J.; Xie, E. J. Mater. Res. 2011, 26 (1), 50–54 (46) Matović, B.; Babić, B.; Bučevac, D.; Čebela, M.; Maksimović, V.; Pantić, J.; Miljković, M. Ceram. Int. 2013, 39, 719–723 (47) Kurosawa, Sh.; Futami, Y.; Kochurikhin, V.V.; Borik, M.A.; Yokota, Y.; Yanagida, T.; Yoshikawa, A. Key Eng. Mater. 2012, 508, 81-86 (48) Quintard, P.E.; Barberis, P.; Mirgorodsky, A.P.; Merle-Mejean, T. J. Am. Ceram. Soc. 2002, 85 (7), 1745–1749 (49) Kim, B.-K.; Hamaguchi, H. Mater. Res. Bull. 1997, 32 (10), 1367–1370 (50) Lavrent’ev, Yu.G.; Karmanov, N.S.; Usova, L.V. Russian Geology and Geophysics. 2015, 56, 1154–1161 (51) Chiodini, N.; Lauria, A.; Lorenzi, R.; Brovelli, S.; Meinardi, F.; Paleari, A. Chem. Mater. 2012, 24, 677–681 (52) Misbah, Ch.; Pierre-Louis, O.; Saito, Yu. Rev. Mod. Phys. 2010, 82, 981–1040 (53) Sovremennaya Kristallographiya (Modern Crystallography); Vainstein, B.K., Chernov, A.A., Shuvalov, L.A., Eds.; Nauka: Moscow 1979; Vols 1-4 (in Russian) (54) Wang, Zh.; Yu, H.; Su, H. Sci. Rep. 2013, 3, 3246 (55) Eliziário, S.A.; Cavalcante, L.S.; Sczancoski, J.C.; Pizani, P.S.; Varela, J.A.; Espinosa, J.W.M.; Longo, E. Nanoscale Res. Lett. 2009, 4, 1371–1379 (56) Amorphous and liquid semiconductors; Tauc, J., Ed.; Plenum Publishing Company Ltd.: New York, 1974 (57) Agekyan, V.T. Phys. Status Solidi A 1977, 43, 11–42



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(58) Amitharaj, P.M.; Seiler, D.G. In Handbook of Optics, 2nd Ed.; Brass, M., Stryland, E.W.V., Williams, D.R., Wolfe W.L., Eds.; McGraw-Hill, Inc.: New York, 1995; Vol. 2, Chapter 36. (59) Haarindraprasad, R.; Hashim, U.; Gopinath, S.C.B.; Kashif, M.; Veeradasan, P.; Balakrishnan, S.R.; Foo, K.L.; Poopalan, P. PLoS ONE 2015, 10 (7), e0132755 // http://dx.doi.org/10.1371/journal.pone.0132755 (60) Rauwel, P.; Rauwel. E. In Microscopy: advances in scientific research and education; Mendez-Vilas, A., Ed.; Formatex Research Center: Badajoz, Spain 2014; Vol. 2, pp. 875– 886 (61) Jiang, H.; Gomez-Abal, R.I.; Rinke, P.; Scheffler, M. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 81, 085119 (62) Aarik, J.; Mändar, H.; Kirm, M.; Pung, L. Thin Solid Films 2004, 466, 41–47 (63) Bruno, T.J.; Svoronos, P.D.N. CRC Handbook of Fundamental Spectroscopic Correlation Charts; CRC Press: Boca Raton, FL 2006 (64) Foster, A.S.; Gejo, F.L.; Shluger, A.L.; Nieminen, R.M. Phys. Rev. B: Condens. Matter Mater. Phys. 2002, 65, 174117 (65) Kofstad, P.; Ruzicka, D.J. J. Electrochem. Soc. 1963, 110 (3), 181-184 (66) Broqvist, P.; Pasquarello, A. Appl. Phys. Lett. 2006, 89, 262904 (67) Perevalov, T.V.; Aliev, V.Sh.; Gritsenko, V.A.; Saraev, A.A.; Kaichev, V.V.; Ivanova, E.V.; Zamoryanskaya, M.V. Appl. Phys. Lett. 2014, 104, 071904 (68) Chuang, Sh.-H.; Lin, H.-C.; Chen, Ch.-Hs. J. Alloys Compd. 2012, 534, 42–46 (69) Ciapponi, A.; Wagner, F.R.; Palmier, S.; Natoli, J.-Y.; Gallais, L. J. Lumin. 2009, 129, 1786–1789 (70) Modreanu, M.; Monaghan, S.; Povey, I.M.; Cherkaoui, K.; Hurley, P.K.; Androulidaki, M. Microelectron. Eng. 2011, 88, 1499–1502


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Captions Figure 1. The major vapour species in the “hafnium zone”. Each curved surface area presents a set of the calculated values of partial pressures of given vapour species in dependence on T and P. Figure 2. The vapour phase composition (major species) in the “SiO2 zone”. Each curved surface area presents a set of the calculated values of partial pressures of given vapour species in dependence on T and P. Figure 3. The dependence of ln Kp for reaction (2) on temperature and the fitting curve. Figure 4. X-Ray diffraction data of the products formed on metal hafnium Figure 5. SEM image and X-Ray diffraction pattern of deposit grown into the quartz glass Figure 6. Photograph of the transparent plate-like crystal over the coloured picture Figure 7. The XRD patterns of HfO2: (a) - of crushed plate-like crystals; (b) - as-grown plate-like crystals; c - Laue diffraction pattern obtained at 30 kV, 50 mA. Green circles are positions of hk3 reflections of m-HfO2. Figure 8. Raman spectra of as-grown plate-like HfO2 single crystal, taken at wavelengths 1064 and 488 nm. Figure 9 a – d. SEM images of HfO2 crystals: a – a survey view of HfO2 plate-like crystals; b – vicinal surface inclined down in the x direction. Steps run parallel to the z direction; c – the deviation of the steps from their regular position; d – step bunching. On SEM images the orientation of the crystal surface is [001]. Figure 10 a – c. Photos of m-HfO2 crystal growth steps taken in polarized light. The orientation of the crystal surface is [001]. Figure 11 a, b. SEM images of twins Figure 12. UV-Vis transmission spectrum of m-HfO2 plates in the [001] direction Figure 13. The Tauc plots: a – the dependence of (αhν)2 on hν for direct allowed energy gap; b – the dependence of (αhν)1/2 on hν for indirect allowed energy gap. 38 ACS Paragon Plus Environment

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Figure 14 a, b. Dependences of the optical emissions of HfO2 single crystals excited by 4.43 eV (a) and 5.17 eV (b). Components (blue lines) obtained by Voigt function fitting are shown together with experimental curves (black). Figure 15 a, b. Optical images of HfO2 crystal upon excitation with 4.43 eV (a) and 5.17 eV (b) illumination. The most intense PL band (2.69 eV) appears in the blue region of the Vis spectrum (a) and the PL band (1.96 eV) appears in the red region of the Vis spectrum.63 Figure 16 a, b. Excitation spectra of HfO2 single crystals for emission energy 2.69 eV (a) and 1.96 eV (b).



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For Table of Contents Use Only

Large-size, transparent monoclinic HfO2 single crystals of high-purity were synthesized by reactive chemical vapor deposition using CF4 as a transport agent at a temperature as low as 1273K. Thermodynamic modelling of the Hf – C – Si – O – F heterogeneous system was undertaken to understand the chemical equilibria in this transport system. The HfO2 single crystals exhibit strong emission under UV excitation.


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Figure for Cover Page Abstract: Here we report the f 72x98mm (96 x 96 DPI)



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For Table of Contents Use Only Large-size, transparent monocl 223x141mm (96 x 96 DPI)


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Figure 2. The vapour phase composition (major species) in “SiO2 zone”. Each curved surface area presents a set of the calculated values of partial pressures of given vapour species in dependence on T and P. The calculations showed that t 251x167mm (96 x 96 DPI)


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Figure 3. The dependence of ln Kp for reaction (2) on temperature, and the fitting curve. The obtained Kp values are sho 143x114mm (96 x 96 DPI)



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Figure 4. X-Ray diffraction data of the products formed on metal hafnium. In addition to the mentioned c 349x269mm (96 x 96 DPI)


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Figure 5. SEM image and X-Ray diffraction pattern of the deposit grown into the quartz glass. A thorough inspection of the s 59x56mm (96 x 96 DPI)



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Figure 5. SEM image and X-Ray diffraction pattern of the deposit grown into the quartz glass. As follows from XRD analysis, 349x270mm (96 x 96 DPI)


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Figure 6. Photograph of the transparent plate-like HfO2 crystal over the coloured picture As an example, the plate-like 186x126mm (96 x 96 DPI)



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Figure 7. The XRD patterns of HfO2: (a) - of crushed plate-like crystals; (b) - as-grown plate-like crystals; (c) - Laue diffraction pattern obtained at 30 kV, 50 mA. Green circles are positions of hk3 reflections of mHfO2. Indicating that they are monoc 345x272mm (96 x 96 DPI)


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Figure 7. The XRD of crushed (a) and as-grown (b) plate-like crystals. As follows from XRD pattern, t 345x272mm (96 x 96 DPI)



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Figure 7. The XRD patterns of HfO2: (a) - of crushed plate-like crystals; (b) - as-grown plate-like crystals; (c) - Laue diffraction pattern obtained at 30 kV, 50 mA. Green circles are positions of hk3 reflections of mHfO2. The single-crystal X-ray diffr 66x65mm (96 x 96 DPI)


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Figure 8. Raman spectra of as-grown plate-like HfO2 single crystal, taken at wavelengths 1064 and 488 nm. The obtained Raman spectra are 344x287mm (96 x 96 DPI)



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Figure 9. SEM images of HfO2 crystals: a – a survey view of HfO2 plate-like crystals; b – vicinal surface inclined down in the x direction. Steps run parallel to the z direction; c – the deviation of the steps from their regular position; d – step bunching. As seen from Fig. 9 a, hafnium 139x139mm (96 x 96 DPI)


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Figure 9. SEM images of HfO2 crystals: a – a survey view of HfO2 plate-like crystals; b – vicinal surface inclined down in the x direction. Steps run parallel to the z direction; c – the deviation of the steps from their regular position; d – step bunching. a part of the generating lines 146x123mm (96 x 96 DPI)



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Figure 9. SEM images of HfO2 crystals: a – a survey view of HfO2 plate-like crystals; b – vicinal surface inclined down in the x direction. Steps run parallel to the z direction; c – the deviation of the steps from their regular position; d – step bunching. One can note that usually the 140x116mm (96 x 96 DPI)


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Figure 9. SEM images of HfO2 crystals: a – a survey view of HfO2 plate-like crystals; b – vicinal surface inclined down in the x direction. Steps run parallel to the z direction; c – the deviation of the steps from their regular position; d – step bunching. In addition to the above-menti 160x159mm (96 x 96 DPI)



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Figure 10 a – c. Photos of m-HfO2 crystal growth steps taken in polarized light The waves of step bunching on 512x339mm (96 x 96 DPI)


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Figure 10 a – c. Photos of m-HfO2 crystal growth steps taken in polarized light The waves of step bunching on 312x264mm (96 x 96 DPI)



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Figure 10 a – c. Photos of m-HfO2 crystal growth steps taken in polarized light Twins can be observed on some 214x234mm (96 x 96 DPI)


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Figure 11 a, b. SEM images of twins and SEM (Fig. 11 a, b) images. 218x146mm (96 x 96 DPI)



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Figure 11 a, b. SEM images of twins and SEM (Fig. 11 a, b) images. 115x111mm (96 x 96 DPI)


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Figure 12. UV-Vis transmission spectrum of m-HfO2 plates in the [001] direction. The UV-Vis transmission spectr 219x164mm (96 x 96 DPI)



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Figure 13. The Tauc’ plots: a – the dependence of (αhν)2 on hν for direct allowed energy gap; b – the dependence of (αhν)1/2 on hν for indirect allowed energy gap. Taking into account these data 206x165mm (96 x 96 DPI)


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Figure 13. The Tauc’ plots: a – the dependence of (αhν)2 on hν for direct allowed energy gap; b – the dependence of (αhν)1/2 on hν for indirect allowed energy gap. Taking into account these data 209x168mm (96 x 96 DPI)



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Figure 14. Dependences of the optical emissions of HfO2 single crystals excited by 4.43 eV (a) and 5.17 eV (b). Components (blue lines) obtained by Voigt function fitting are shown together with experimental curves (black). The PL spectra of monoclinic H 337x271mm (96 x 96 DPI)


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Figure 14. Dependences of the optical emissions of HfO2 single crystals excited by 4.43 eV (a) and 5.17 eV (b). Components (blue lines) obtained by Voigt function fitting are shown together with experimental curves (black). The PL spectra of monoclinic H 339x271mm (96 x 96 DPI)



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Figure 15. Optical images of HfO2 crystal upon excitation with 4.43 eV (a) and 5.17 eV (b) illumination. The most intense PL band (2.69 eV) appears in the blue region of the Vis spectrum (a) and the PL band (1.96 eV) appears in the red region of the Vis spectrum.57 The photos of HfO2 single crys 51x64mm (96 x 96 DPI)


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Figure 15. Optical images of HfO2 crystal upon excitation with 4.43 eV (a) and 5.17 eV (b) illumination. The most intense PL band (2.69 eV) appears in the blue region of the Vis spectrum (a) and the PL band (1.96 eV) appears in the red region of the Vis spectrum.57 The photos of HfO2 single crys 53x64mm (96 x 96 DPI)



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Figure 16. Excitation spectra of HfO2 single crystals for emission energy 2.69 eV (a) and 1.96 eV (b). Analogously, the intensity is 351x273mm (96 x 96 DPI)


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Figure 16. Excitation spectra of HfO2 single crystals for emission energy 2.69 eV (a) and 1.96 eV (b). One can see that the intensity 353x270mm (96 x 96 DPI)


Crystal Growth and Photoluminescence Properties of Reactive CVD

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