Hafnia-Based Luminescent Insulator for Phosphor Applications

Hafnia-based Luminescent Insulator for Phosphor Applications

L. Khomenkovaa,b, Y.-T.Ana, C. Labbéa, X.Portiera, and F. Gourbilleaua

a CIMAP/CEA/CNRS/Ensicaen/UCBN, 6 Boulevard Maréchal Juin, Caen 14050, France b V.Lashkaryov Institute of Semiconductor Physics, 45 Pr.Nauky, Kyiv 03028, Ukraine

Pure and doped hafnia-based films were fabricated by RF magnetron sputtering and their microstructure, optical and luminescent properties were investigated versus annealing treatment. It was observed a phase separation in Si-rich-HfO2 films at 900-1100°C resulted in the formation of HfO2 and SiO2 phases, as well as pure silicon clusters. A light emission in the orange-red spectral range was observed from Si-rich-HfO2 samples contrary to their pure counterparts. The correlation of the peak position of red photoluminescence with silicon cluster sizes allowed gives the evidence of the carrier recombination in silicon clusters. The comparison of Er-doped-HfO2 and Er-doped Si-rich-HfO2 films showed an enhancement of rare-earth emission efficiency due to an effective energy transfer from Si-clusters.

Introduction

Nowadays, hafnia-based insulators are mainly considered as alternative gate dielectrics towards silicon oxide in complementary metal-oxide semiconductor technology (1). This latter has already reached its physical limit of 0.7 nm and its further downscaling is out of interest due to high leakage current (2). Ultrathin hafnia-based films are proposed to replace SiO2 in CMOS devices owing its high dielectric constant and wide band gap. We showed recently that among different dopants, silicon plays the most important role to improve the thermal stability of ultrathin films at high temperature annealing (3,4) and, thus, Si-doped hafnia can be successfully used for nanomemory applications (5).

Apart from these, hafnia and zirconia are excellent materials for optical applications due to their hardness, high refractive index (almost 2.1 at 1.95 eV), high optical transparency in the ultraviolet-infrared spectral range, wide optical bandgap (~ 5.8 eV) and low phonon cut-off energy (~about 780 cm-1) offered low probability of phonon assisted relaxation.

In spite of twin properties of zirconia and hafnia, the most attention was paid to optical applications of zirconia-based materials. The intrinsic luminescence of ZrO2 and HfO2 was demonstrated in the 4.2-4.35 eV and 4.2-4.4 eV spectral range, respectively, (6,7). It was ascribed to self-trapped exciton, but it is almost quenched at room temperature. An additional emission in the 2.5-3.5 eV spectral range was also observed (6) and considered as that originated from different types of oxygen vacancies with trapped electrons.

The promised results were demonstrated by zirconia doped by trivalent rare-earth ions. The luminescence of Pr3+- (8,9), Tb3+- (10,11), Eu3+- (12), Sm3+- (13,14), Er3+- (15) doped ZrO2 (bulk, powders, sol-gel and nanocrystals) have been investigated showed the possibility to achieve simultaneous blue, green and red emission as well as infrared one (8-15). Among different rare-earth trivalent elements, the Er3+ ion is one of the most

ECS Transactions, 45 (5) 119-128 (2012)10.1149/1.3700418 © The Electrochemical Society

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popular due to its radiative transitions in the green (4S3/2 →4I15/2) and the infrared (4I13/2 →4I15/2) being extensively used as an eye-safe source in atmosphere, laser radar, medical and surgery (4I11/2 →4I13/2) (16,17).

It is known that absorption cross-section of rare-earth ions is about 10-18-10-20 cm-2 for 4f-4f transitions and about 10-12 cm-2, for 4f-5d ones. However, they are located in UV and vacuum UV region, restricting their use for many applications. Thus, more effective excitation of 4f-4f transitions required a host mediated excitation via energy transfer. This approach has been widely used for the rare-earth doped silica with embedded Si-nanoclusters (Si-ncs). A lot of efforts were concentrated to obtain efficient emission from Er3+-doped SiO2 with embedded Si-ncs (18-23). The Si-ncs incorporation allows increasing of absorption cross-section of Er3+ ions from 10-21 cm-2 (24) up to 10-16 cm-2 (22,23,25,26) due to effective energy transfer from Si-ncs towards Er3+ions. For this purpose, visible broad-band excitation was used offering safely applications of these materials. Thus, based on analogy with Si-ncs embedded in SiO2 host, one can suppose that an incorporation of Si-ncs in hafnia matrix can allow achieving an effective Er3+ excitation in visible spectral range

The optical studies of HfO2-based materials are not numerous. An application of HfO2 films as optical coatings was shown in (25,26). The Eu3+ - and Tb3+-doped HfO2 samples were investigated in (27,28), however, there is still no clear understanding of the energy transfer mechanism from hafnia host towards rare-earth ions.

It is worth to note that Si-rich-HfO2 materials are considered mainly for microelectronic applications (1,2,5,29); only few studies on Er-doped SiO2-HfO2 materials were presented (27,30). However, the mechanism of the Er3+ ion excitation was not reported. Moreover, any formation of Si-ncs in hafnia was not also observed.

The various fabrication techniques have been used to synthesize hafnia-based materials. They are generally divided into solution and gas-phase processes. The latter includes both chemical and physical vapor deposition methods (1,29). However, among them, RF magnetron sputtering was not widely addressed.

Recently, we showed that RF magnetron sputtering approach has some benefit for the fabrication of hafnium silicate materials with homogeneous chemical composition in a wide thickness range (4). In the present paper, we study a formation of Si nanoclusters (Si-ncs) in hafnia-based host via annealing treatment. The effect of surrounded host on the emission of Er3+ ions at room temperature is also investigated. The observed light emitting properties of hafnium silicate materials demonstrate their potential photonic applications.

Experiment

The layers were grown on B-doped (100) Si wafers with a resistivity of 5-15 Ω·cm. The substrates were submitted to standard RCA cleaning, dipped in a diluted hydrofluoric solution (10%), dried in N2, and immediately transferred to the vacuum chamber of the deposition setup. Doped hafnia layers were fabricated by RF magnetron sputtering of pure HfO2 target (99.9%) topped by Si, Er2O3 or Nd2O3 calibrated chips. The surface ratio of each type of chips was estimated as a ratio of the surface of all corresponding chips to the total surface of the HfO2 target. It was 12% for each type of chips used. The deposition was performed in pure argon plasma with a RF power density value of 0.74 W/cm2. The substrate temperature was kept at 100°C, the total plasma pressure and substrate-cathode distance were fixed at 0.04 mbar and 57 mm, respectively. An annealing of the samples was performed in a conventional furnace at different

ECS Transactions, 45 (5) 119-128 (2012)


temperatures (TA) ranging from 800 to 1100 °C and durations, tA, varying between 10 to 60 min in nitrogen flow.

Several techniques were used to analyze the layer properties. Their chemical composition was studied by means of Fourier Transmission Infra-Red (FTIR) spectroscopy. The spectra were recorded in the range of 600–4000 cm−1 using a Nicolet Nexus spectrometer under normal and Brewster incidence angle (65°). Raman scattering spectra were investigated using a dispersive Raman spectrometer equipped with a CCD camera and a laser source at 532 nm. The PL emission was achieved using the 476 nm, 488 nm lines of an Ar+-ion laser. PL spectra were recorded with a fast Hamamatsu photomultiplier after dispersion of the light through a Jobin–Yvon TRIAX 180 monochromator linked with a SRS lock-in amplifier (SP830 DPS) referenced to the chopping light frequency of 73 Hz. Jobin–Yvon Fluorolog3-22 setup equipped with an Xe lamp as excitation source and R928 photomultiplayer tube was used for measurements of PL excitation spectra in the 200-800 nm spectral range. The samples were observed by conventional (CTEM) and high resolution transmission electron microscopy (HR-TEM) using a FEG 2010 JEOL instrument, operated at 200 kV. Cross-sectional specimens were prepared for TEM observation by the standard procedure involving grinding, dimpling and Ar+ ion beam thinning until electron transparency. Image processing was done with the commercial Digital micrograph GATAN software.

Results and discussion

Properties of Si-rich HfO2 materials versus annealing treatment

FTIR and Raman scattering studies. The evolution of chemical composition upon an annealing treatment was investigated to get an insight into the phase separation process. FTIR spectrum of as-deposited sample (AD) represents broad band in the range of 700-1200 cm-1 with a maximum at ~1000-1030 cm−1 which corresponds to the Si-O-Hf stretching vibration mode. This latter is close to another one observed in the range of 460-700 cm-1 due to Hf-O vibrations (Fig.1). An annealing at TA=800-900 °C during tA=30 min leads to an increase the both Si-O-Hf band intensity and spectral shift towards higher wavenumbers up to 1070-1080 cm-1 (Fig.1a).

Further TA increase up to 1000 °C results in an appearance of several peaks at about 1240 cm-1, 1090 cm-1 and 650 cm-1. This is an evidence of a phase separation process and a formation of silica and hafnia phases (3,4). However, the presence of a shoulder at about 950 cm−1 confirms that this process is not completed yet and HfSiO phase is still present in the annealed films (Fig.1). A treatment at TA=1100 °C causes the narrowing of all vibration bands and the appearance of well-defined peaks at 630 cm-1 and 770 cm-1 due to crystallized hafnia phase (3,4). The peaks at 820 cm−1, 1090 cm-1 and 1250 cm-1 (Fig.1a) correspond to LO2-TO2, TO3 and LO3 vibration modes of pure silica, respectively (3).

To investigate further the evolution of the microstructure of Si-rich HfO2 layers, Raman scattering spectra were obtained for the same samples deposited on quartz substrate (Fig.1b). This latter was used to exclude the contribution of Si bulk phonon band at 521 cm-1. As one can see, as-deposited samples demonstrate only broad band with a maximum at about 499 cm−1. This peak position shifts towards 502 cm-1 at TA=800°C due to the formation of silicon phase whereas the slight decrease in the intensity is due to an increase of sample transparency (Fig.1b).



Figure 1. FTIR (a) and Raman scattering (b) spectra of Si-rich-HfO2 samples measured at 65° angle of the incident light for as-deposited (AD) and annealed at 800°C, 900°C, 1000°C and 1100°C during 30 min in nitrogen flow. Corresponding Raman scattering spectrum for quartz substrate is also shown in (b). Peak position of bulk Si phonon at 521 cm-1 is shown by dashed line.

An annealing at higher temperatures (TA=900°C) results in the continuous shift of Raman peak position up to 516 cm-1. Its intensity increase is attributed to Si nanoclusters growth. Their sizes was estimated of about 3.7-4.0 nm using the model described in (31). The further TA increase up to 1000-1100°C results in significant rise of the Raman signal intensity as well as in the appearance of well-defined peaks at 455cm-1, 506 cm-1 and 523 cm-1. These latter correspond to crystallized hafnia phase. It is possible that upon such a thermal treatment the Si-ncs formation is more pronounced (similar to the case of Si-ncs formation in SiO2 host). However, the corresponding Raman scattering signal is hardly observed due to its possible overlapping with the signal from hafnia host (Fig.1b).

It is worth to note that the comparison of Si-rich-HfO2 samples with the Si-rich-SiO2 ones grown with the same conditions (5,32) suggests that the formation of Si-ncs in high-k host requires lower thermal budget in comparison with that of Si-rich-SiO2 system; the crystallization of these Si-ncs in SiO2 matrix, fabricated with such deposition technique, occurs usually upon an annealing at 1000-1100°C and results in the bright photoluminescence, PL, (32,33). Thus, one can suppose that the formation of Si-ncs in HfO2-based host can be accompanied by an appearance of light emission in visible spectral range similar to the case of Si-rich-SiO2 materials (5,32,33).

Light emission properties of pure and Si-rich-hafnia. As-deposited pure HfO2 sample

demonstrates a weak PL emission with a maximum at about 630 nm under excitation by 488 nm light (Fig.2a). An annealing at TA=800°C and tA=30min results in its significant



decrease being not detectable at higher annealing temperatures. This can be due to a densification of pure HfO2 films causes by their crystallization (4). It is worth to note that formation of self-trapped excitons cannot be ruled out in this case; however, they can be hardly excited with a 488-nm-light wavelength since they require an ultraviolet excitation (7).

PL emission of Si-rich-HfO2 samples appeared only after annealing. Normalized PL spectra of these samples versus annealing treatment are presented in Fig.2a. It is seen two broad maxima in the visible spectral range at 580 nm and 720 nm (TA=800°C, tA=30min). The comparison of pure and Si-rich HfO2 films shows clear that Si-incorporation results in the appearance of the orange-red emission upon annealing treatment.

Although the interference process can give some contribution to the PL spectra, nevertheless some tendency of PL behavior can be easy seen from Fig.2a. Thus, the TA increase up to 1000°C at tA=30min results in the “blue” shift of PL spectrum. Meanwhile, the tA increase from 10 min to 60 min at TA=900°C leads the “red” shift of PL band. This behavior of PL band was found to be similar to that observed for Si-rich-SiO2 materials (33,34) and can be explained by the variation of Si-ncs sizes. The brightest PL emission was achieved upon an anneal treatment at TA=900°C during tA=60 min (Fig.2b). Note that annealing at TA=1100°C results in the PL quenching.

Figure 2. (a) PL spectra of pure and Si-rich-HfO2 samples. Annealing conditions are mentioned in the figure; the spectrum for as-deposited pure HfO2 sample is also presented for comparison. PL for as-deposited Si-rich-HfO2 samples was undetectable; (b) Evolution of PL intensity of Si-rich-HfO2 samples, detected at the “red” PL peak position (680-720 nm), versus an annealing temperature; tA=30 and 60 min. Excitation wavelength is 488 nm.

The evolution of PL intensity shows that the optimal TA value to achieve light emission from Si-ncs embedded in HfO2 host is around 900°C. To obtain more information about the microstructure, the cross-sections of sample annealed at TA=900°C and tA=60 min were prepared for TEM analysis.

High-resolution TEM study. Figure 3 presents a high-resolution TEM image of the sample annealed at 900°C for 60 min and shows a phase separation in the film volume. Easily observed use dark grey and white regions corresponding to HfO2 and SiO2 phases. Besides, light grey regions separated both phases can correspond to either pure Si phase



(or SiOx phase with low amount of oxygen), or HfSiOx phase, considering the relatively large volume of this phase. From the comparison of FTIR (Fig.1a) and Raman scattering spectra (Fig.1b) one can suppose that the contribution of silicon-rich phase exceeds that from HfSiOx.

Figure 3. Cross-section TEM image of the Si-rich-HfO2 layer annealed at 900°C for 60 min. The dark grey regions correspond to HfO2 phase, white bright regions are SiO2 phase, and grey light regions correspond either to pure Si or SiOx regions with high Si content.

It is interesting to notice that the diameters of light-grey regions do not exceed in most cases 5 nm (Fig.3). This is in agreement with the average size of Si-ncs estimated from Raman scattering spectra (Fig.1b). However, any sequences of crystalline planes were not revealed during TEM observation that is also confirmed by SAED data (Fig.3, inset). This latter demonstrates rather amorphous than nanocrystalline nature of the annealed sample. One of the reasons can be a non-completed phase separation process. In this case, not only the HfO2 and SiO2 phases are formed, but also SiOx (with a high Si content). Besides a HfSiOx phase can be observed since it is stable at 900°C (4). On the other hand, the annealing temperature such as 900°C can be too low to cause the crystallization of Si-rich phase. This is also in the agreement with the Raman scattering data that give a value of 516 cm-1 for the Si peak for TA=900°C instead of 521 cm-1 (Fig.1b). The presence of amorphous Si-ncs was also confirmed by atomic-probe tomography study for the similar samples (35).

Light emission properties of Er3+ doped hafnia-based materials.

As it was mentioned above, in the Si-rich-HfO2 materials doped by rare-earth ions the effective emission of these latter can be achieved either with resonant excitation or via energy transfer from Si-ncs or oxide defects. Moreover, additional information about Si-



ncs formation, excitation mechanism of rare-earth PL as well as about crystalline nature of the hafnia-based host can be also obtained.

PL spectra of the samples. Figure 4a represents the comparison of PL spectra of Er-doped HfO2 and Er-doped Si-rich-HfO2 films annealed at 900°C during 60 min. As we showed above, this annealing treatment provided the brightest PL of Er-free Si-rich-HfO2 samples and quenched PL of pure HfO2 materials.

PL spectra were measured in the 630-700-nm and in the 1.4-1.7 μm spectral ranges, corresponded to 4F9/2 →4I15/2 (Fig.4a) and 4I13/2→4I15/2 (Fig.4b) radiative transitions of Er3+ ions. In the former case, PL was excited with 532-nm excitation light (4I15/2→4S3/2 absorption of Er3+ ions), whereas in the latter case, the 488-nm- (4I15/2→4F7/2 absorption of Er3+ ions) and 476-nm-illumination (“non-resonant” with the Er3+ ions energy levels) were used.

Since pure HfO2 does not emit under any mentioned excitation (476 nm, 488 nm and 532 nm), this means that, if any oxygen deficient centers can be present in pure HfO2 films, they are not excited by this illumination wavelengths. Thus, well-defined PL peaks observed for Er-doped HfO2 films, corresponding to 4F9/2 →4I15/2 relaxations (Fig.4a) and 4I13/2→4I15/2 transitions (Fig.4b), are due to direct excitation of Er3+ ions. This latter is confirmed by very low PL intensity (as a noise level) observed under “non-resonant” 476-nm-illumination.

Figure 4. (a) PL spectra versus Er and Si incorporation in HfO2 matrix for the samples annealed at 900°C during 30 min; (b) PL spectra of Er-doped HfO2 (Er-HfO2) and Er-doped-Si-rich-HfO2 (Er-HfSiOx) films versus annealing conditions and excitation parameters. These latter are mentioned for each PL spectrum in the figures.

Earlier we showed the formation of monoclinic HfO2 phase in the pure HfO2 samples submitted to an annealing at 900°C for 15 min (3,4). This crystallization becomes more pronounced at longer annealing treatment. The shapes of PL spectra of Er-doped HfO2



samples (Fig.4a,b) confirm crystallized environment of Er3+ ions. However, the host crystallization is not completed; the broad “baseline” is still visible in 1.54-µm PL spectrum (Fig. 4b). It is known that Er3+ ions are used for the stabilization of amorphous nature of HfO2 matrix, especially for the case of microelectronic application (36). This can be a reason of the presence of some amorphous HfO2 regions surrounded Er3+ ions.

As we showed above, the orange-red emission was observed from Er-free Si-rich-HfO2 samples annealed at 900°C for 30 min and was associated with Si-ncs (Fig.2). However, it differs from that of Er-doped Si-rich-HfO2 films in the 630-700nm spectral range (Fig. 4a). This latter is narrower corresponding to 4F9/2 →4I15/2 transitions. Its shape is an evidence of the amorphous nature of Er3+ surrounded host that was already confirmed by TEM observation (Fig.3). Another evidence can be seen from the shape of PL band corresponded to 4I13/2→4I15/2 transitions (Fig.4b) that is similar to that of Er-doped Si-rich-SiO2 materials (18-22). Since 1.54-μm PL emission from Er-doped Si-rich-HfO2 films is efficient under non-resonant 476-nm illumination (Fig.4b), one can conclude that the energy transfer from Si-ncs towards Er3+ ions takes place. It is worth to note that the sharp PL peaks in the 1.4-1.7 μm spectral range appear after sample annealing at 1100°C (Fig.4b) caused by a crystallization of HfO2 phase (4).

Similar PL properties were demonstrated for the Er-doped Si-rich-ZrO2 films (37,38). The Er3+ PL emission under non-resonant excitation was obtained and ascribed to energy transfer from Si-ncs towards rare-earth ions. However, any evidence on Si-ncs formation in such materials was not presented (37).

Thus, based on the correlation of mictrostructure evolution with a variation of optical and luminescent properties of Er-doped Si-rich-HfO2 films upon an annealing treatment, one can conclude that the Er3+ related light emission under non-resonant excitation is due to the formation of Si-ncs upon an annealing treatment. These Si-ncs were found to be amorphous, being effective sensitizers of the Er3+ ions.

Conclusions

In the present study, the properties of RF magnetron sputtered hafnia-based films were investigated by means of TEM, FTIR, Raman scattering and PL techniques. It was observed that high temperature annealing governs a phase separation process and the formation of silica and hafnia phases, as well as silicon phase. The appearance of a PL emission in the visible-near-infrared spectral range occurred. The evolution of the PL peak position was found to be correlated with Si-ncs sizes. The properties of Si-doped-HfO2 films are compared with those of their counterparts doped by Er3+ ions. The investigation of the effect of annealing treatment on luminescent properties revealed that the enhancement of Er3+ PL emission occurs due to an effective energy transfer from Si-NPs towards RE ions. Since hafnia-based materials have high density and are very sensitive to high-energy excitation, our results offer multifunctional applications of doped hafnia films, such as luminescent materials for traditional phosphors applications.

Acknowledgments

This work is supported by French National Agency (ANR) through Nanoscience, Nanotechnology Program (NOMAD project n°ANR-07-NANO-022-02 and DAPHNES project n°ANR-08-NANO-005) and the Conseil Regional de Basse Normandie through CPER project - Nanoscience axe (2007-2013).



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Hafnia-Based Luminescent Insulator for Phosphor Applications

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