16 Journal of Mineral, Metal and Material Engineering, 2020, 6, 16-20

E-ISSN: 2414-2115/20 © 2020 Scientific Array

Annealing Effect on the Evolution of Nanocomposite Growth of the Sputtered Silicon Rich Nitride with Underlined Tantalum Metal Thin Film

Chen-Kuei Chung* and Cheng-Han Li

Department of Mechanical Engineering, and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan701, Taiwan

Abstract: Chemical vapor deposition and furnace annealing up to more than 1000 °C were conventionally used to form the nanocomposite film of silicon nanocrystals (nc-Si) embedded in a dielectric matrix of SiNx or SiOx. Here, we investigate the novel bi-layer silicon rich nitride (SRN)/ underlined tantalum (Ta) thin films deposited by sputtering at room temperature (RT) and followed by the furnace annealing in air at low temperatures of 500-700 °C for nanocomposite formation. The effect of annealing temperature on the evolution of microstructure and photoluminescence (PL) of the annealed SRN/Ta nanocomposite films was studied. XRD data show that the as-deposited SRN film is amorphous at RT, the TaOx suboxide formed at 500 °C and the distinct nc-Si and Ta2O5 nanocrystals formed at 700 °C. The evolution of microstructure is linked to the PL behavior. The strong broad PL spectrum covering wavelengths of 400-750 nm at RT was observed at 500 °C annealing and greatly enhanced at 700 °C annealing as no PL was found before annealing. The relationship between the annealing temperature, microstructure and PL behavior of the annealed SRN/Ta nanocomposite films is discussed.

Keywords: Silicon-rich nitride, Metal, Thin film, Microstructure, Photoluminescence, Annealing.

1. INTRODUCTION

Silicon nanocrystals (nc-Si) attract considerable interest due to a significant transformation of optical to electrical properties in materials due to quantum confinement effect [1,2]. The nanocomposite materials of nc-Si embedded in a dielectric material have attracted great attention in recent decades because of the potential applications to the Si optoelectronic and photovoltaics devices. It merits the compatibility with the existing manufacturing infrastructure for Si integrated circuits [3]. Among different dielectric materials, silicon nitride is the most addressed as a host matrix for nc-Si [4,5]. Bright photoluminescence (PL) in a broad wavelength range at room temperature originates from quantum confinement effect [1,2], luminescence center [6-8] and surface chemistry effect [9,10]. The laser annealed silicon-rich nitride thin films produced two-band PL behavior from the nc-Si embedded in an amorphous SiOxNy matrix [11]. In recent years, some types of research on the metal oxide together with silicon nanostructure [12,13] are committed to improving the light-emitting properties of nc-Si containing thin film. In this regard, the high-k dielectric such as Ta2O5 is considered as promising gate dielectrics due to the lower equivalent oxide thickness. Also, nc-Si embedded in such a high-k host

*Address correspondence to this author at the Department of Mechanical Engineering, and Center for Micro/Nano Science and Technology, National Cheng Kung University, Tainan701, Taiwan; Tel: +886-6-2757575-62111; Fax: +886-6-2352973; E-mail: ckchung@mail.ncku.edu.tw

offer a more extensive application for non-volatile memories due to the higher performance of the corresponding devices [14,15]. From a photonic application viewpoint, Ta2O5 has been considered as a promising candidate material for optical communication [16]. In addition, Ta2O5 has a high dielectric constant (~50-70), high refractive index (~2.125) and an optical band gap as large as 4.4 eV. Frequently, fully oxidized Ta2O5 might help and enhance the visible light emission, since oxygen vacancies produced the deep-level emission between 2.1~2.7 eV the conduction band [17]. Therefore, it is of interest to investigate the microstructure and PL behavior of Ta2O5 embedded in a SiNx matrix.

In this work, the bi-layer SRN/Ta thin films were synthesized by magnetron sputtering and furnace annealing at 500-700 °C in the air for the growth of nanocomposite films made of nc-Si and Ta2O5 nanocrystals in an amorphous SiOxNy matrix. The adjustable broad luminescence band in the range of 400-750 nm is concerned with the annealing temperature. The evolution of microstructure, bonding configuration and PL characteristics of the annealed SRN/Ta nanocomposite films was studied.

2. EXPERIMENTAL PROCEDURES

The p-Si(100) substrates were ultrasonically cleaned by a solution of acetone, isopropanol, and alcohol solvent and finally rinsed in DI water. The sputtering system vacuum chamber was evacuated

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down to 4×10-4 Pa. The bi-layer SRN/Ta with an underlined Ta thin film were deposited by magnetron sputtering using argon gases from the SRN target (made of Si3N4/Si of 80/20 wt%, 99.995% purity, 50 mm diameter) and Ta target (99.995% purity, 50 mm diameter). The SRN target was employed by radio frequency power while Ta target by direct current power. Sputtering was performed at a constant cathode power of 5 W/cm2 and the work pressure is around 9×10-1 Pa. The substrate holder was rotated at 35 rpm (revolution per minute) to improve the thin films deposition uniformity and it is not biased (Us) at room temperature without any cooling or heating. The SRN/Ta thickness of each layer was 200 and 300 nm measured by a surface profiler. After deposition, the samples were annealed in the air for one h at 500°C and 700°C to study the effect of annealing temperature on the evolution of microstructure and PL behavior in the annealed SRN/Ta nanocomposite film.

The crystalline properties of the samples were characterized by grazing incidence X-ray Diffractometer (GIXRD, D/MAX2500, RIGAKU, Japan) with an incident angle of 2° using Cu Kα radiation (0.15418 nm). The local atomic environments and bonding configuration bonding in the films were examined by Fourier transform infrared spectrometer (FTIR, Thermo Nicolet, USA). The morphology of films was examined using high-resolution field emission scanning electron microscope (HR-FESEM, JSM-6700, JEOL, Japan). The room-temperature (RT) PL spectra of the annealed SRN/Ta films were excited by a 50 Mw He-Cd laser (Kimmon, Japan) with a wavelength of 325 nm and measured from 380 to 800 nm through the lens, mirrors, grating, photomultiplier and charge-coupled device components.

3. RESULTS AND DISCUSSION

Figure 1 shows the GIXRD patterns of SRN/Ta films deposited at room temperature (RT) and annealed at 500°C and 700°C, respectively. The possible crystalline Si, Ta, and Ta-O compounds in JCPDS numbers are Si (No.26-1484), α-Ta (No.04-0788) and β-Ta2O5 (No.25-09022), respectively. For the sample deposited at RT, a broad peak with a large full width half maximum (FWHM) is observed in the films; it is characteristic of quasi-amorphous microstructure which consists of nanocrystalline grains of α-Ta embedded in an amorphous matrix [18]. There is no diffraction peak of Si observed in the as-deposited SRN films due to amorphous state at RT. As the temperature increases to 500 °C, the α-Ta phase has three sharp diffracted

peaks at 2θ of 38°, 55° and 69° which are diffracted from (110), (200) and (211) planes, respectively. The transition from α-Ta phase to β-Ta2O5 polycrystalline microstructure occurs at 700 °C. Besides, the peaks of at 28°, 47° and 56° are also observed corresponding to the formation of crystalline Si phase (111), (220) and (311) planes due to higher annealing energy.

Figure 1: GIXRD patterns of the SRN/Ta films deposited at room temperature and annealed at 500°C and 700°C.

Figure 2 shows the FTIR absorption spectra measured in the range of 400–2000 cm−1 and corrected using the blank substrate. Four main absorption bands are observed around 1100, 850, 600 and 450 cm−1, which can be assigned to Si-O-Si stretching, Si-N asymmetric stretching, Ta-O stretching and Si-N breathing vibration modes, respectively [19]. Although the nc-Si formed at 700°C is measured by GIXRD, but no Si-Si bonding mode is shown because the Si-Si bonds are insensitive to FTIR absorption from the lack of dipole momentum [19]. However, the band located at about 450 cm−1 for the as-deposited and annealed samples is assigned to Si-N breathing vibration modes, which confirm the existence of Si-N film. Also, the Si-O-Si stretching band was also observed at 1100 cm−1 for the as-deposited and annealed samples due to residual oxygen content into the surface of films in the sputter and heating in the air. The Ta-O stretching and Si-N asymmetric stretching band at 600 cm−1 and 850 cm−1 is not observed in the as-deposited sample, but it appears in the films annealed at 500-700°C. It indicates that the reaction of (Ta, O) and (Si, N) to form Ta-O and Si-N bonding at 500-700°C and N or Si dangling bonds at the interface are diminished and passivated. The amount of mixed Ta-O, Si-O and Si-N bonds and nc-Si in the films may greatly affect the PL behavior [20].

18 Journal of Mineral, Metal and Material Engineering, 2020, Vol. 6 Chung and Li

Figure 2: FTIR spectra of SRN/Ta films deposited at room temperature and annealed at 500°C and 700°C.

Figures 3a-c show the FESEM micrographs of the SRN/Ta film at RT and annealed at 500 and 700 °C, respectively. Lots of particles appear on the film at RT and the higher density and bigger particles are formed on the films at 500°C and 700°C. In addition, the bigger particles are formed by lots of tiny nanoparticles. From the results of GIXRD, it implies that the nanoparticles in SEM may consist of smaller nanocrystals [21]. That is, the SRN/Ta film could be made of a composite phase nc-Si in np-Si or in an amorphous matrix after annealing at higher temperatures of 500-700°C. The evolution of the annealed SRN/Ta microstructure and bonding configuration can influence the PL behavior.

Figure 4 shows the PL spectra of the films deposited at RT and post-annealed at 500 and 700°C, respectively. It is noted that no PL could be observed from the RT due to the amorphous matrix and insufficient annealing condition, respectively. The PL spectrum of 500 °C and 700 °C samples can be deconvoluted by auto curve-fitting of Gaussian function into three bands of UV-blue at 430 nm (2.88 eV), green-yellow at 530 nm (2.33 eV) emission and yellow-orange emission at 590 nm (2.1 eV), respectively. The intensity of this band also increases with annealing temperature. The broad visible luminescence from the annealed SRN/Ta nanocomposite films probably originates from the formation of nanocomposite structures which consists of unsatisfied states (dangling bonds) or interface states or plenty of oxygen vacancy in imperfections, located states related to the mixed Si-O and Si-N bonds in Ta-Si-N:O layer and nc-Si embedded in the amorphous matrix. The UV-blue at 430 nm (2.88 eV) emission intensity is increased with increasing temperature due to the unsatisfied states

being decreased and diminished at the interface. In other words, the higher annealing temperature can reduce the non-radiative defect states [22]. The green-yellow at 530 nm (2.33 eV) emission is not shifted, and its intensity increases with increasing temperature. It is from the radiative recombination in the localized states relate to mixed Si-O and Si-N bonds. Huang et al. [20] reported that the PL intensity increases with increasing oxidation time in an oxygen atmosphere at 100 °C due to the increase of Si-O-Si bonds observed by FTIR. The yellow-orange emission at 590 nm (2.1 eV) is not shifted, and intensity also enhanced. It probably originates from the formation of nc-Si. Increasing annealing temperature provides higher thermal energy

Figure 3: HRFSEM micrographs of the SRN/Ta films deposited on the Si (100) substrate (a) at RT, and post annealed at temperatures of (b) 500°C and (c) 700°C.

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which is beneficial for the formation of a higher density of nc-Si [18]. Also, when the annealing temperature is over 500°C, the Ta2O5 phase is formed as shown in GIXRD, FTIR and XPS. The enhanced oxygen deficiency in the film might increase the visible light emission, since the first and second ionization energies of the O-vacancy double donor are ~1.2 and 2.1-2.7 eV below the conduction band [17]. Thus the phase transition from TaOx suboxides into Ta2O5 should contribute to the increase of this green-yellow and yellow-orange emission with deep-level emission from plenty of oxygen vacancy. The visible PL of nc-Si embedded in a wide band-gap matrix such as a-SiOx or a-SiNx has also attracted much attention these years. They all attributed nc-Si provides high-quality recombination centers for the photogenerated carriers in the a-SiOx or a-SiNx matrix which plays a vital role in the luminescence process. Compared with these previous studies, we propose the annealed bi-layer SRN/Ta for the nanocomposite formation could promote SRN structure luminous. The great PL was probably due to the oxygen vacancy in the films, which was greatly enhanced by further oxidation into Ta2O5 by thermal annealing in the air.

Figure 4: Room-temperature PL spectra of Gaussian functions curve fitting of SRN/Ta films at RT and annealed at 500°C and 700°C.

4. CONCLUSION

The nc-Si/SiNx nanocomposite films were traditionally synthesized by chemical vapor deposition and followed by furnace annealing at a temperature up to 1000°C or more. Here, we investigate the bi-layer SRN/Ta films deposited by magnetron sputtering at RT and followed by furnace annealing in air at 500-700°C for the novel nanocomposite formation. The evolution of microstructure and PL behavior of the annealed

SRN/Ta films is significantly influenced by annealing temperature and examined by XRD, FTIR, HRSEM and PL spectra, respectively. No diffraction peak of Si is observed because the as-deposited SRN film is amorphous by sputtering at RT. Then the distinct nc-Si and Ta2O5 nanocrystals are formed at 700°C for the transition from α-Ta phase to β-Ta2O5 polycrystalline microstructure while the α-Ta phase with three sharp diffracted peaks are formed at 500°C. No PL is found in the as-deposited SRN/Ta films. In contrast, the broad PL emission covering wavelength of 400-750 nm is observed at 500 °C annealing and greatly enhanced at 700°C annealing. The broad visible band can be deconvoluted into three Gaussian-fitted bands of UV-blue (~430 nm), green-yellow (~530 nm) and yellow-orange (~590 nm) emissions, corresponding to the emission origins from the unsatisfied states in imperfections of interface, located states related to the mixed Si-O or Si-N bonds, and nc-Si embedded in a amorphous matrix, respectively. The broad PL is attributed to the nc-Si and Ta2O5 nanocrystals embedded in an amorphous SiOxNy matrix.

ACKNOWLEDGEMENTS

This work is partially sponsored by Ministry of Science and Technology, Taiwan, under contract No MOST 106-2221-E-006-101-MY3.

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Received on 19-07-2020 Accepted on 12-08-2020 Published on 19-08-2020 DOI: https://doi.org/10.31437/2414-2115.2020.06.3 © 2020 Chung and Li; Licensee Scientific Array. This is an open access article licensed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/3.0/) which permits unrestricted, non-commercial use, distribution and reproduction in any medium, provided the work is properly cited.