Photoluminescence comparison of SRO-LPCVD films deposited on quartz, polysilicon and silicon substrates

Journal of Luminescence 216 (2019) 116709

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Photoluminescence comparison of SRO-LPCVD films deposited on quartz, polysilicon and silicon substrates

T

H.P. Martínez-Hernándeza,c, J.A. Luna Lópeza, , M. Aceves Mijaresb, K. Monfil Leyvaa, G. García Salgadoa, J.A.D. Hernández-de-la-Luza, A. Luna Floresd, R. Morales-Caporalc, R. Ramírez Amadora, Z.J. Hernández Simóna ⁎

a

Centro de Investigaciones en Dispositivos Semiconductores (CIDS-ICUAP), Benemérita Universidad Autónoma de Puebla (BUAP), San Manuel, Cd. Universitaria, Av. San Claudio y 14 Sur, Edificios IC5 y IC6, 72570, Puebla, Pue., Mexico b Departamento de Electrónica, Instituto Nacional de Astrofísica Óptica y Electrónica (INAOE), Puebla, Mexico c Departamento de Ingeniería Eléctrica y Electrónica, Instituto Tecnológico de Apizaco (ITA), Tlaxcala, Mexico d Facultad de Ingeniería Química, Benemérita Universidad Autónoma de Puebla (BUAP), Puebla, Mexico

ARTICLE INFO

ABSTRACT

Keywords: Photoluminescence Silicon rich oxide LPCVD Polysilicon SRO films Si-ncs

In the present work, photoluminescence spectra of silicon-rich oxide monolayers (SRO10 and SRO25) and bilayers (SRO25/10, SRO10/25) films deposited on quartz, polysilicon on quartz and silicon substrates are compared. Silicon-rich oxide films were deposited using Low Pressure Chemical Vapor Deposition (LPCVD) technique. The films were characterized by Ellipsometry, Fourier Transform Infrared Spectroscopy, Secondary Ion Mass Spectrometry, Optical Transmittance, Photoluminescence, Scanning Electron Microscope, and High-Resolution Transmission Electron Microscopy. The results show that the bilayer films, deposited on polysilicon film on quartz have the stronger photoluminescence, which is indicative that there are different types of defects, Silicon nanocrystals, Silicon nanoparticles, and amorphous silicon nanoparticles that improve the Photoluminescence response. Also, it was observed constructive interferences in the transmittance a spectrum of silicon-rich oxide films/polysilicon films on quartz, due to that their refractive indices are different and they are together. This, when a beam of light crosses the interface between materials with different refraction indexes, its direction of propagation is altered, and the bigger is the difference of its refractive indexes, the greater will be the beam refraction. In all materials known this phenomenon is called as positive refraction. The photoluminescence is related to silicon dioxide defects such as weak oxygen bonds, neutral oxygen vacancy, non-bridging oxygen hole centers, or positively charged oxygen vacancies. Moreover, it is a combined effect where the bandgap energy acquires some direct bandgap properties due to quantum confinement, both effects (defects and quantum confinement) are important to increase the photoluminescence intensity. The average diameters of Silicon nanocrystals were estimated from the band gap energy and also by High-Resolution Transmission Electron Microscopy obtaining an average diameter of 3.8 ± 0.08 nm. Comparative analysis of optical and structural properties was performed on all samples.

1. Introduction Silicon (Si) compatible materials have been investigated in order to overcome its optical limitation and to improve their use in novelty optical and optoelectronic devices as photodetectors, photodiodes, photoconductors, and anti-reflective surfaces among others. Porous silicon, silicon nanowires, silicon oxynitride (SiON), non-stoichiometric silicon oxide, (Silicon Rich Oxide SRO), and other silicon-based nanostructured materials exhibit excellent electrical and optical

⁎

properties [1–6]. The SRO is one of the most promising semiconductor materials for the fabrication of light emission devices compatible with silicon technology. The SRO is a multi-phase material constituted by a mixture of silica (SiO2), off-stoichiometric oxides (SiOx, x < 2) and elemental silicon. Previously, the SRO films have already been deposited by many techniques to produce Si-nanoparticles (Si-nps) such as: Si-implantation [1,7,8], sputtering [6,9,11], Plasma Enhanced Chemical Vapor Deposition (PECVD) [6,13], Hot Filament Chemical Vapor Deposition (HFCVD) [13,16] and Low Pressure Chemical Vapor

Corresponding author. E-mail addresses: [email protected], [email protected] (J.A. Luna López).

https://doi.org/10.1016/j.jlumin.2019.116709 Received 20 February 2019; Received in revised form 15 August 2019; Accepted 16 August 2019 Available online 20 August 2019 0022-2313/ © 2019 Elsevier B.V. All rights reserved.

Journal of Luminescence 216 (2019) 116709

H.P. Martínez-Hernández, et al.

Deposition (LPCVD) [1,2,8,11–13]. In this work, SRO films were obtained using Low Pressure Chemical Vapor Deposition (LPCVD) technique and annealed at 1100 °C for 3 h in nitrogen ambient [1]. This technique allows varying the silicon excess (between 5 and 17% at.) producing high light emission [1,7,8,11–13]. The silicon excess in the SRO-LPVCD films is controlled during the deposit by the partial pressure ratio of reactive gases, R0 = [PN2O]/[PSiH4], where PN2O and PSiH4 stand for the partial pressures of nitrous oxide and silane, respectively. An excess of silicon of up to 17% is obtained with R0 = 3, and stoichiometric oxide is obtained with R0 = 100 [1]. Several methods have been used to improve the photoluminescet emission, such as the deposit of nanometric films as monolayers, bilayers or multilayers [1,7,8,11–13]. However, nowadays no one has reported some study on the photoluminescence emission from SRO films deposited on polysilicon and quartz substrates, such material is compatible with the silicon integrated circuit (IC) technology. In this work, PL of monolayers and bilayers of SRO films deposited on three different substrates: quartz (Q), polysilicon film on quartz (Q/ Pn+) and silicon (Si) is reported, besides such structures are optically analysed, and their photoluminescence (PL) spectra are compared, both before thermal annealing (BTA) and after the thermal annealing (ATA). SRO films deposited on Q, Q/Pn + and Si are monolayers deposited to the fluxes ratios Ro = 10 (SRO10) and Ro = 25 (SRO25), and bilayers with the flux ratios Ro = 25/10 (SRO25/10) and Ro = 10/25 (SRO10/25). The results show that the films deposited on Q/Pn + improve the photoluminescence emission compared to the films deposited on Q or Si. Likewise, these bilayer SRO films improvement the intensity of the FTIR spectra and we can to observe all the vibrational modes related to SiO2. These bilayer SRO films also contains silicon nanocrystals, where its size was estimate by Tauc of 4.12 nm and by PL spectra of 3.72 nm and reaffirmed with HRTEM of 3.95 ± 0.45 nm. PL emission, intensity FTIR and Si-ncs size are due to the thermal annealing applied to the SRO films with which the greatest amount of hydrogen is desorbed, the weak bonds are broken, amorphous silicon nanoislands are formed and silicon nanocrystals obtaining a material with different stoichiometric of a-Si-nanoislands, Si-ncs, SiOx and SiO2.

analysis was carried out to the structure Q/Pn+/SRO25/SRO10/AZO with a TOF-SIMS 5 mass spectrometer IONTOF-GmbH, in a double ion beam regime with an angle of incidence of both beams of 45° with respect to the surface normal. The surface sputtering of the structure was performed by a 2 keV Cs ion beam with an ion current of 300 nA. The Cs beam sputtered a raster of 300 × 300 μm whereas the Bi beam scanned a 100 × 100 μm raster in the center of the Cs sputtered crater. We measured the depth profile of the main elements in our devices (H, O, Al, Si, Zn and P). The positive ions of these elements emitted from this 100 × 100-μm area were mass-separated during their flight in a reflectron-type mass analyzer. In order to avoid charge effects at the surface sample, we used an electron beam. All measurements were performed using the above conditions in ultrahigh vacuum of 4 × 10−10 mbar. Transmission and absorption spectra of SRO films deposited on Q and Q/Pn+ were obtained by using the UV–Vis equipment, model PerkinElmer Lambda 3 b from 200 to 900 nm. PL spectra of SRO films were measured using the Horiba Jobin Yvon Fluoromax-3 spectrofluorometer at room temperature for which an excitation line of 300 nm was used and the emission signal was collected in the range from 400 to 900 nm. 3. Results The average thicknesses and refractive indices for the monolayers and bilayers SRO films are shown in Table 1, the thicknesses were obtained by ellipsometry for the silicon substrates and by profilometry for the quartz substrates. On the other hand, through SEM images were corroborated both results of the ellipsometry and profilometry (thicknesses), as it is shown in Fig. 1. The higher refractive index was obtained for the SRO10 monolayer in comparison with that of the SRO25 one and the bilayers (SRO25/10 and SRO10/25), such results are attributed to the presence of a higher silicon excess [1,7,11–13]. Fourier Transform Infrared Spectroscopy (FTIR) of SRO10, SRO25, SRO25/10 and SRO10/25 films deposited on Si substrate, before and after the thermal annealing, are shown in Fig. 2. In this figure we observe the characteristic vibrational modes of the Si–O–Si bonds, for the SiO2; “Rocking” (R), “Bending”, (B), “Stretching” (S) and “asymmetric stretching” (a-S) in 456, 811, 1078 and 1194 cm−1, respectively [11–13,16]. These spectra show significant differences in their intensities and they also suffer energy shifts in the position of their peaks associated to the characteristic vibrational modes, such events are due to the variations of the SRO film thickness along with the thermal annealing, particularly remarkable differences are exhibited in the vicinity of the Si–O (S) mode, around of 1078 cm−1. The highest intensities in absorbance are observed from the bilayer SRO films such characteristic is attributed to the thickness of films. While in all SRO films with thermal annealing, a change is observed at higher wave numbers in the vibrational modes (R), (S) and (a-S), as can be seen in the inside of Fig. 2, where the vibrational mode S is normalized with respect to the mode associated with the maximum peak of each spectrum. This indicates that there is a phase separation between SiO2 and Si with a greater contribution attributed to the Si–O chemical bonds belonging to the SiO2 atomic structure and also due to the restructuration phase of the material [16,18]. Likewise, the SRO films show a shoulder peaked around of the (a-S) mode, indicating a greater density of Si–O bonds related to a SiOx phase, the chemical composition in this shoulder can be attributed to the three oxidation states with Si atoms tied to 1, 2 or 3 oxygen atoms [16–18]. This causes a change in the refractive index [11–13,16–18], because it is intimately related to the atomic arrangement of the material, and the type of the atomic bond, for this reason such change is due to the oxidation state and restructuration suffered by the SRO films when they are subjected to the thermal annealing at 1100 °C, such thermal treatment leads to changes of the atomic structure of the Silicon Nanoparticles (Si-nps) along with the subspecies of the SiOx phase

2. Methods The SRO monolayer and bilayer films were deposited on substrates of Q, Q/Pn+ and Si (orientation (100), ρ = 1-5 Ω-cm, P-type). The SRO films were deposited in a hot wall LPCVD reactor at 736 °C [1]. The silicon excess was controlled by the partial pressure ratio R0 = [PN20]/ [PSiH4] of nitrous oxide (N2O) and 5% silane (SiH4) in nitrogen (N2) as reactive gases. Trichloroethylene, acetone and deionized water were used to clean quartz (Q) substrate, the silicon substrates were cleaned with the standard RCA method [1,15,16]. After cleaning, the polysilicon was deposited on the Q substrates, where such material was obtained through phosphine deposite by using the LPCVD technique, in this process the SiH4 gas was used as precursor in an atmosphere restricted to a pressure of 1.5 torr and a flux level of 3.4 slpm (standard litre per minute), at 650 °C, for 20 min under these conditions a thickness of 500 nm was obtained. After that, it was made the phosphine doping of polysilicon using different flow levels, namely, 300, 500 and 920 sccm (standard cubic centimeters per minute) of oxygen, phosphine and nitrogen, respectively, for 20 min, at 1000 °C. Re-diffusion was carried out for 15 min at 1000 °C with flow levels at 1000 and 500 sccm of nitrogen and oxygen, respectively. Polysilicon oxidation is carried out with an oxygen flow level at 1000 sccm at 1100 °C, for 20 min. The PSG glass formed was removed with H2O: HF solution in a 7:1 ratio. The technological parameters used to deposit the SRO monolayer and bilayer films are presented in Table 1. Gaertner ellipsometer model L117 was used to obtain the thickness (th) and refractive index (n) such parameters are also shown in Table 1. The FTIR analysis of the SRO films was performed using a BRUKER vector spectrometer, model 22 with a measuring range from 400 to 4000 cm−1. The depth profile 2

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Table 1 Parameters used for SRO films deposit, where the thicknesses and refractive indices are obtained by ellipsometry null technique.

Fig. 2. Absorbance spectra (FTIR) of deposited films on Si substrate considering the BTA and ATA regimens.

Fig. 1. Image of the bilayer structures obtained by Scanning Electron Microscopy. The corresponding structures are as follows: a) Al/Si/SRO10/25-ATA and (b) Q/POLY/SRO25/10-ATA.

On the other hand, the aforementioned results are closely related with the transmittance spectra behavior of the SRO10, SRO25, SRO10/25 and SRO25/10 films, deposited on Q and Q/Pn + substrates as shown in Fig. 4. In this figure, we observe that for the case of the SRO films deposited on Q substrates, in (a) and (b) graphs, both monolayer and bilayer structures, offer a better transmittance than that of the QPn +/SRO structures. We observe in graph (a) that the transmittance curves for the structure of the Q/SRO monolayer type exhibit a higher transmittance than that of the Q/SRO bilayer type, it is an expected result since in the bilayer configuration the presence of the intermediate layer in the structure generates additional absorption effects reducing the transparency of the system. Such characteristic holds for the case of the (b) graph although the thermal annealing causes a clear separation between the monolayer and bilayer transmittance curves. An additional characteristic of these curves is that they exhibit a high transmittance (approximately above 80%) for wavelengths starting from 400 nm (in the BTA regimen) and 300 nm (in the ATA regimen). Their absorption edge of these films after thermal annealing shows a shift-down towards shorter wavelengths (200 nm) for the case of the Q/ SRO films, it means that the energy of the absorption edges increases substantially thereby increasing their band gap energy of the structures.

to form SiO2. It should be noted that when the Bending vibrational mode is present the intensity of the corresponding peak increases too, it implies an increase in the number of Si–O bonds [16,18]. Fig. 3 shows the composition of the Q/Pn+/SRO25/SRO10/AZO structure as a function of erosion depth obtained by Secondary Ion Mass Spectrometry (SIMS). In Fig. 3 (a) are shown the profiles of the composition materials of the BTA films, note that they show a clear evidence of the atomic elements present in the structure, similary in Fig. 3 (b) under the conditions ATA. Note that phosphorus (P2+) is present in both BTA and ATA spectra. The presence of the phosphorus in the ATA films is due to it is incorporated orginally when the Q/Pn + structure is made up as was already described above. The existence of the phosphorus in the film, as has been corroborated, in some extent will modify the photoluminescent intensity of the films because such element when coupled to the structure material may generate a density of radiative electronic states which contribute to ligth emission. The SIMS spectra of all the films are similar to the SIMS spectra of the structure Q/Pn +/SRO25/SRO10/AZO, for this reason we show only the most significant spectra as depicted in Fig. 3. 3

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Fig. 3. SIMS spectra of the Q/Pn+/SRO25/SRO10/AZO structures, (a) BTA, (b) ATA.

Fig. 4. SRO monolayer and bilayer films transmittance spectra deposited on Q and Q/Pn + substrates. a) In the BTA regimen and b) In the ATA regimen.

This fact is intimately related to the phase change in the atomic arrangement of the material structure when it evolves from the amorphous phase to crystalline one. The SRO films grown on Q/Pn + show a transmittance in the range of 800–900 nm with values in the order of 60–40% for the BTA regimen and 35-20% in the ATA one, again the transmittance for the Q/Pn + monolayer type structure is greater than that of the Q/Pn + bilayer type., both effects are due to polysilicon. Besides, the transmittance curves exhibit oscillations as a consequence of the constructive interference of the electromagnetic modes confined within of the films, the shape of such oscillations vary according to as the thicknesses and refractive indices vary in the SRO films as well as the thermal annealing influences such curves. Therefore, the behavior of the transmittance spectra exhibits the strong influence of the geometry factor (thickness of the films), the chemical composition and material phase (refractive index). On the other hand, the polysilicon film thickness for the structures Q/Pn+ is 500 nm and the SRO film thickness ranges from 89 to 180 nm. According to the obtained measurements of the refractive indices of the polysilicon films, they take values of 2.7 and 2.8 [17,22], and for their corresponding SRO films, the refractive indices take the values of 1.5 and 1.7 respectively. As is shown in Fig. 4 the average transmittance percentage for the Q/Pn+/SRO films is ~55%, in the region of 800–900 nm for the case of the BTA regimen with an absorption edge at 400 nm. In the ATA regimen, the average transmittance decreases to 22% in the region of maximum transmittance (800–900 nm) where the absorption edge does not suffer a significant change in respect to that of

the BTA regimen. We suggest that such behavior is due to the presence of silicon nanoclusters (nanocrystals and nano-agglomerates) formed in the SRO single-layer, these nanoclusters are strongly correlated with the binding energy of the atoms forming such nanostructure which in turn determines the electronic configuration and therefore the optical properties so when these nanoclusters are subjected to thermal processes the binding energies are affected generating breakings in the molecular structure such effect brings the transition phase in the composition of the SRO films, this phenomenon modifies the process of absorption (optical properties) as well as the charge transfer process (electrical properties) [33]. Transmittance Spectra also allow us to investigate the absorption properties of the films by means of the absorption coefficient α equation (1) [14].

= 2. 302

A t

(1)

where A is the absorbance, and t is the thickness of the film. The absorption coefficients before and after of thermal annealing, are shown in Fig. 5 for both monolayer and bilayer films in the Q/SRO configuration. According to this figure, with the thermal annealing, the absorption coefficient edge has shifted to major energies from approximately 2.5 eV (BTA) to 3.0 eV (ATA). In particular, the absorbance of the monolayer SRO25 film shows the greatest right-shift in the absorption edge position as well as the lowest intensity because it has low refractive index and lowers excess silicon [13,14], this is also attributed 4

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Fig. 5. Absorption Coefficient of SRO films monolayers and bilayers on quartz substrate. a) BTA and b) ATA.

to small amorphous Si-ncs and quantum dots distribution [17–21]. That is SRO10 and SRO25 films present characteristics of crystalline silicon nanoparticles (c-Si-nps) and amorphous silicon nanoparticles (a-Si-nps). The Tauc-plot equation (2) was used to obtain an estimate of the optical band gap energy of the films [19,22].

( hv )1/ n = C1 (h v

Egopt )

Fig. 6. Tauc graph considering indirect absorption and n = 3. a) BTA and b) ATA.

(2)

where Egopt is the optical band gap energy for a band transition type in the film, α is the absorption coefficient, C1 is a proportionality constant which is independent of the photon energy (C1 ≈ 1) [11–13], ν is the frequency of transition and n characterizes the nature of the transition type. To obtain the value of the approximate optical band gap energy, it was used the Tauc graph, with n = 3 (indirect forbidden transition) [14]. The gaps of the SRO films are shown in Fig. 6. Subsequently, by means of equation (3) [19], the optical band gap energy (Egpot) was used to obtain the diameter of the Si-ncs. These data are shown in Table 2.

Egopt = 1.12 +

3. 73 d1.39

Pn + substrates, in the as-grown status their behavior is similar to that one discussed by Ance et al. [23], in which is found that the Egopt is bigger than the one after annealing, in parallel the Si-ncs size is smaller than the one after annealing. Quantitatively this general tendency in such parameters is shown in Table 2. On the other hand, we have found that our results are consistent with those reported by C. Chiu et al. [24] and C. Ance et al. [23]. Now we analyze the PL spectra corresponding to the SRO monolayers films, such ones are shown in Figs. 7 and 8 and for bilayers in Figs. 9 and 10, both as-grown and after thermal annealing films. The graphs of the PL spectra in subsections (a) and (b), are shown just like they were obtained, wavelength versus PL intensity (a.u.), while in the subsections (c) and (d), these spectra are analyzed according to Y. Wang and P. D. Townsend [31], to obtain in terms of energy the graphs as I (E) dE versus E (eV), with these ones we can develop the deconvolutions, and determine the energy peaks. In these Figs. 9 and 10, we depict several PL spectra generated through varying the Ro ratios taking into account the two conditions BTA and ATA of the SRO films. We point out both the evolution of the PL spectra due to the annealing effect and how the latter modifies the peak intensities and shifts them. As can be seen in the subsections (b) and (d) of Figs. 6–9, the outstanding characteristic exhibited in all PL spectra is related to the presence of the main PL peak in the vicinity of 1.7 eV after the annealing. The thermal annealing affects significantly the PL intensities as well as their shape. We observe from all figures that the behavior of the

(3)

The SRO films grown on Q substrates, before thermal annealing, present an Egopt from 1.51 to 1.8 eV, which is near to the band gap energy of the silicon for indirect transitions 1.17 eV [11–13,22]. After thermal annealing present an Egopt ranging from 1.85 to 2.38 eV, which is closer to the direct optical transition in the bulk silicon 3.4 eV for Γ25′Γ15 [22]. Ance et al. [23] report that an increase of the Egopt has been associated with a reduction in the size of the Si-ncs, due to the transition from an amorphous phase to a crystalline phase in the Si-nps; and they have also reported that for SiOx films, where there is a random combination of Si–O bonds, the minimum of the conduction band is moved to a higher energy level since the valence band maximum does not change. This leads to an increase in the value of the Egopt. In agreement with the previous results, for films deposited on Q/ 5

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Table 2 The optical band gap energies and Si-ncs diameters for SRO films monolayers (SRO10 and SRO25) and bilayers (SRO25/10 and SRO10/25).

PL is also influenced by the type of substrate (Q, Q/Pn+ and Si) used for the deposit of the SRO films. In addition, the deconvolutions associated to each PL spectrum are shown in order to visualize the contribution of the different bands that form the spectrum, where the emission mechanisms that produce the PL phenomenon are found in the highest peaks of the PL spectra. As is known the thermal annealing for the SRO structures is of great importance for the PL intensity and the emission wavelength. For this reason, we list in Table 3 different parameters concerning to percentages of the intensity and peak positions for the SRO films deposited before and after thermal annealing on the three types of substrates, for

their comparison. In this Table 3, we observe that the highest percentages of the total area (AT) in PL intensity are placed in the column corresponding to the thermal treatment films, because they present higher intensities and the spectrum is observed wider and well defined. It is notably observed that the SRO films reduced their PL intensity considerably, in both the blue and red bands in the BTA condition as shown in Fig. 6 (a). It could be due to that the SRO films have higher concentrations of hydrogen available in the SRO film composition and when they are thermally treated at 1100 °C for 3 h, the hydrogen escapes from the film (dehydrogenation effect), this generates that its PL intensity is reduced or disappeared in the blue band. In contrast, in the

Fig. 7. SRO10 monolayer films photoluminescence for the BTA and ATA conditions, considering the three different configurations Q/SRO, Q/Pn+/SRO and Si/SRO, a) and b) graphs show PL spectra for as-grown and thermal annealing films as well as the total area spectra area, c) and d) graphs show the deconvolutions of the spectra and the corresponding diameter size of Si-ncs. 6

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Fig. 8. SRO25 monolayer films Photoluminescence, in their three different configurations like those for SRO10 films in the BTA and ATA conditions. a) SRO films PL spectra in the BTA conditions showing the two band contributions (blue and red), b) The PL spectra in the ATA condition where the red band determines their shape, besides the total area spectra is shown in each curve. Graphs c) and d) exhibit the PL spectra deconvolution curves and the Si-ncs diameter size. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

Fig. 9. SRO25/10 bilayer films photoluminescence, considering the BTA and ATA conditions. a) The PL spectra in the BTA condition and b) The PL spectra in the ATA condition. Additionally, the total area of spectra is shown. c) and d) graphs show the deconvolutions of the PL spectra and the Si-ncs diameter size is also shown. 7

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Fig. 10. SRO10/25 bilayer films Photoluminescence, subjected to the conditions BTA and ATA. a) and b) graphs show the PL spectra in the BTA and ATA conditions besides the spectra total area, c) and d) depict the deconvolutions of the PL spectra and the Si-ncs diameter size associated to each PL peak.

ATA condition, this PL intensity in the red band increases considerably. In addition, by using the optical band gap energy values of the monolayer and bilayer SRO films obtained at the highest peak positions of the PL spectra together with equation (3), we estimate the diameter of the silicon nanocrystal, by example for SRO25 and SRO10 we obtain that Egopt is equal to 1.7eV and 1.73 eV for which the diameter size is of 3.8 nm and 3.6 nm, respectively, all calculated diameters are shown in

Table 4, such diameter values are compared with those obtained from the HRTEM micrographs as shown in Fig. 11. These latter diameters were found using the Digital Micrograph program and the Fourier transform to obtain the reciprocal space, we will discuss this information later. On the other hand, we observe that the PL spectra obtained from the three different substrates before and after thermal annealing are determined basically for three wavelength regions which contribute

Table 3 Position, intensity and percentage of the photoluminescence spectra total area. The results are presented for monolayers (SRO10 and SRO25) and bilayers (SRO25/10 and SRO10/25) for BTA and ATA conditions.

8

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Table 4 Optical band gap Energy (Egopt) results, the PL intensity peaks and diameter of the Si-ncs for the (SRO10 and SRO25) monolayer films and (SRO25/10 and SRO10/25) bilayers films photoluminescence spectra.

to the PL spectra: the first contribution is located at 300–580 nm (4.15–2.14 eV) corresponding to the near ultra violet, violet, blue and green bands [32] as was reported by Flores Gracia et al. [7], for such region the emission mechanisms are attributed to the weak oxygen bonds (WOB≈415 nm) and neutral oxygen vacancies (NOV≈460 nm) (O^Si–Si^O) [1,6–13]; the second contribution lies in the region 580–780 nm corresponding to the yellow, orange and red bands for this case the emission is due to Non Bonding Oxygen Hole Center (NBOHC≈630 nm), and also to the quantum confinement (QC ≈ 760 nm) besides the interaction mechanism between the Si-ncs interface and the oxide matrix [1,16–21] plays an important role in the emission, finally the third contribution is placed between 780 and 1000 nm corresponding to the infrared band generating just one shoulder in the PL spectra in this case emission is ascribed to luminescent centers located at the interface of nc-Si/SiO2 (CLI≈845 nm), all these regions which contribute to the PL spectrum are shown clearly in the insets of each PL spectra. In Figs. 7 and 8 in the subsections (a) and (c) corresponding to the SRO10 and SRO25 monolayer films as grown, we observe two contributions (blue and red bands) which determine the PL spectra, both exhibit low intensity. The SRO10 PL spectra show less intensity than that of the SRO25 ones as can be seen in the insets of each figure. We observe in Fig. 7 (a) that the PL spectra for the SRO10 films deposited on Q and Q/Pn + have a similar behavior in shape and intensity, but they differ from that of the SRO10 film deposited on Silicon substrate which exhibits a lower intensity, however the latter for the case the SRO25 films shows a longer PL intensity as can be seen in Fig. 8 (a). We stress that the PL intensity of the SRO25 films is by far stronger than that of the SRO10 films in both BTA and ATA conditions. On the other hand, from Figs. 7 and 8 in subsections (b) and (d) in the ATA condition, we notice

that the PL spectra show well-defined contributions through their three peaks located at 699 nm, 721 nm and 727 nm for the case of the SRO10 films and at 706 nm, 713 nm and 718 nm for the case of the SRO25 films. It is noteworthy that the best PL intensity is found in the SRO25 structures particularly for the Si/SRO25 films. In regard to the bilayer structures, Figs. 9 and 10 depict the PL spectra of the SRO25/10 and SRO10/25 films as grown (BTA) in subsections (a) and (c) and after thermal treatment (ATA) in subsections (b) and (d). For the SRO25/10 films in the BTA condition, the PL spectra exhibit two main contributions one in the blue band (~428 nm) and other in the red band (~66 nm), however three contributions are observed notably in the SRO25/10 film deposited on the polysilicon substrate (Q/Pn+/SRO) shown in the deconvolution curves of Fig. 9 (c). Furthermore, in the SRO25/10 structure after the thermal annealing, Fig. 9 (b), the red band prevails as the main contribution in the PL spectra for the three configurations, although in the deconvolution curves, we observe two additional contributions for the Q/SRO25/10 structure located at 1.51eV and 3.0eV, Fig. 9 (d). It is noticeable that in this type of structures the PL intensity in the ATA condition is two orders of magnitude strengthened for the three configurations. For the case of the SRO10/25 films, similar behavior is found as to the PL intensity is also strengthened due to thermal effects as shown in Fig. 10 (a) and (b). A few details are outstanding such as the Q/Pn+/SRO films shows the longest PL intensity both in the BTA and ATA conditions, besides according to the deconvolution curves, the three structures contain two main contributions whose peaks are well identified and the energy of such peaks lies in the range from 1.96 eV to 2.9 eV, and by using these energies we calculate the diameters of the corresponding Sincs located in the 1.7nm–2.82 nm range, these Si-ncs are considered as the main luminescence source in the films. 9

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the refractive index of the monolayer of Ro deposited is modified, from previous works [9–11] it is known that the SRO10 possesses the higher silicon concentration and the bigger refractive index, so the refractive index of the SRO monolayer films changes from 1.55 ± 0.01 (BTA) to 1.72 ± 0.01 (ATA), the opposite happens for the bilayer films because their refractive index goes from 1.67 ± 0.01 (BTA) to 1.58 ± 0.02 (ATA). In relation to the thickness film, it is found that it shows a tendency either to decrease with thermal annealing in monolayers or to increase in the case of bilayers in the ATA status. Therefore, the PL that we observe in the monolayers without thermal annealing is of low intensity, due to the refractive index and silicon concentration are bigger. On the other hand, when applying thermal annealing in bilayers, the refractive index, the thickness and the PL intensity increase. The FTIR spectra shown in Fig. 2, without thermal annealing exhibit the vibrational modes Si–O–H, Si–O, O–H, which are part of the defects mentioned in the PL discussion, such spectra allows us to corroborate the existence of these defects. Like in the previous cases when we apply thermal annealing, there is a modification of these FTIR spectra in which we can observe the disappearance of the vibrational modes of flexion and wagging Si–H attributed to hydrogen. Thus, the FTIR spectra modification yields that the biggest PL emission of all these samples corresponds to the Q/Pn+/SRO bilayer. For the SRO films deposited on quartz without thermal annealing, the transmittance is considered greater than 80%, in both monolayers and bilayers, while in the structures Q/Pn+/SRO, the transmittance in the samples without annealing is less than 50%. When we applied thermal annealing the Q/SRO monolayers have a higher transmittance than 80%, meanwhile in samples with Q/Pn+/SRO decreases its transmittance up to 20%. It should be noted that for the Q/SRO samples, without thermal annealing, the absorption edges in each case lie between 350 and 380 nm and with thermal annealing, this absorption edge shifts to the range 200–300 nm. While for the Q/Pn+/SRO samples without thermal annealing, the absorption edge lies between 380 and 420 nm and it is shifted between 420 and 550 nm, after thermal annealing. These behaviors are observed for all samples. In summary, with thermal annealing, the refractive index, and the photoluminescent intensity are increased, and the absorption edge of the transmittance is shifted to longer wavelengths. The BTA Q/SRO films present a small band gap energy when Ro is greater, (the Q/SRO25 film) and when we apply thermal annealing to monolayers and bilayers their band gap energy increases. While, for Q/ Pn+/SRO samples, their band gap energy behavior is contrary to that of the previous case, when they are without thermal annealing, the Ro major leads to a high band gap energy, and with thermal annealing, monolayers and bilayers decrease their gap. Then, when we carried out the analysis of monolayers and bilayers of the Q/SRO samples, it is found that the photoluminescent intensity, transmittance, the absorption edge threshold, band gap energy and refractive index increase with thermal annealing. Respect to the Q/Pn+/SRO samples, it happens that the transmittance without and with thermal treatment decreases, but the absorption edge is shifted to longer wavelengths, as well as the photoluminescent intensity increases substantially, its band gap energy decreases with thermal annealing and it is maintained around 1.7 eV obtaining a Si-ncs whose diameter is of 3.81 nm. According to the previous results, the SRO films photoluminescence is tailored through the substrate type (Q, QPn+, and Si) used for depositing such films. PL spectra of SRO films as grown were multiplied by a 22 factor due to these do not present great PL intensities to be compared with the PL spectra after thermal annealing. Besides, the PL spectra of these SRO films are shown in the insets of Figs. 7–10 (a). In addition, in another inset, we realized the deconvolution of PL spectra, as it is shown in the same Figs. 7–10 (c) and (d), the violet, the red and near-infrared bands are obtained with this deconvolution, and three contributions centered around 400, 600, and 800 nm are the main PL peaks. The emissions around 300 and 688 nm have been related to the oxygen defects which

Fig. 11. Bilayers structures TEM images, a) The Al/Si/SRO10/25-ATA structure and (b) The Q/Pn+/SRO25/10-ATA structure.

In general, both in the BTA and ATA status, the PL intensity exhibited by the SRO10/25 films is longer than that exhibited by the SRO25/10 ones. This remarkable variation in the PL intensity is mainly attributed to the changes in the density of Si–Si, Si–O and Si–O–Si bonds contained in the microstructure of the films which is modified due to thermal effects. According to the above analysis, we have found that the SRO structures which exhibit the best PL intensity correspond to the SRO25/10 and SRO10/25 films deposited on the polysilicon substrate (Q/ Pn+/SRO structures). This is corroborated in Table 3 where the PL intensity peaks are sorted according to the percentages (a.u. Intensity) exhibited in the red band from 100% to 24.7% of the PL intensity spectra total area. Along with this fact, the substrate type used for the deposits of the SRO films plays an important role with respect to the PL intensity of the films. Thus, the polysilicon turned out to be the best suitable substrate, because when comparing the same bilayer film (SRO25/10) deposited on polysilicon, quartz and silicon substrates it was obtained 100, 55.2, and 4.7% of the intensity spectrum total area, respectively. 4. Discussion Through the ellipsometry technique we can quantify by how much 10

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are present in the SRO film [22,31]. Whereas in the region from 700 to 760 nm the effect of quantum confinement along with the interaction of the Si-ncs interface and the oxide matrix is associated [7–11,16]. For this part, the emission of 813 nm is linked to Si-nps within a SiO2 matrix [16]. In these wavelengths, the PL spectra total area percentage was obtained for BTA-SRO films, resulting that the highest percentage was 2.7% and the lowest one was 0.18%, both of which correspond to monolayer films deposited on Silicon, and for the case of the ATA condition, the highest percentage was 100% and the lowest one was 1.6%, both of which correspond to the bilayer films deposited on Q/Pn + and the monolayer films deposited on Q substrate, respectively. All results obtained for the SRO monolayer and bilayers films deposited on Si, Q, and Q/Pn + substrates are shown in Table 3. It is worthwhile to note that the blue emission band almost disappears for most thermally annealed SRO films, as well all PL spectra of annealed films, monolayer and bilayer, present the main emission from ~600 to 850 nm, this has already been previously reported [1,6–11]. Furthermore, the PL intensity increases when decreasing the silicon excess, such is the case for films obtained with a ratio Ro = 25 which present a silicon excess of 7.18+-0.5% [1,7,8,15], and the PL intensity decreases with a silicon excess of 9.42+-0.3% [11–13], corresponding to a ratio Ro = 10. Additionally, after the thermal annealing, the refractive index (n) increases, as is shown in Table 1, being more evident for the film with the greater excess of silicon (Ro = 10). This behavior of the refractive index has already been observed and reported by A. A. González Fernández et al. [25] and A. Akarapu et al. [26], such behavior is attributed to the changes of composition or stoichiometry [26–28] and density/porosity of the film [29,30]. In addition, through FTIR measurements [16], is also observed. There is a shift of the wave number corresponding to the asymmetric stretching vibration of Si–O–Si bonding towards longer wavenumbers [16] and this is related to the increase in the refractive index. On the other hand, it was measured a low intensity (2.7%) of the PL for the case of the monolayers without thermal annealing due to the refractive index and silicon concentration are bigger. Another important result obtained is when applying thermal annealing in bilayers it yields that the refractive index, the thickness, and the PL intensity increase up to 100%. Also, these PL spectra were used to obtain the diameters of the Si-ncs. The PL maximum peak was considered as the optical energy band gap and by using equation (3) was obtained the diameter of the Si-ncs and it was compared with the values of the Si-ncs obtained of the Egopt through Tauc relation, which were corroborated with those obtained experimentally from HRTEM measurements where we observe a good approximation between both results which are listed in Table 4. Therefore, we can infer that the SRO emission measured in monolayers and bilayers, is originated for various factors, namely: oxygen vacancies, Si-ncs, (dimensional quantum confinement), interfacial defects in the oxygen matrix, a-Si and surface states. The better PL emission obtained from the films analyzed corresponds to the SRO25/10 and SRO10/25 bilayers which were grown on Q/Pn + substrates they exhibit a PL intensity peak of 243122 and 185163 (a.u.), respectively and the corresponding Si-ncs size is of the order of 3.8 nm. The improvement of this bilayer film was realized with the film SRO25, which present the best results in the transmittance, FTIR y PL characterizations. This is due to the presence of c-Si-nps and a-Si-nps and other defects, and the contribution of the SRO10 film grown forming the Q/Pn +/SRO25/10 bilayer which presents the best PL features, say, 100% of the intensity spectrum total area. This is due to the contribution of the Si-ncs and defects which confirm the optical transitions (radiative recombination) present in Q/Pn+/SRO25/10 film [31,32]. On the other hand, it is known that when applying the thermal annealing at temperatures greater than 1000 Celsius degree, Phosphorous atoms in the polysilicon can diffuse into the SRO layer as it is shown in Fig. 3, where is observed the phosphorous concentration in the Q/Pn+/SRO25/10/ AZO structure. Besides, it has also been proven that Phosphorous can act as a passivation agent in Silicon nanocrystals by attaching to the

dangling bonds such event may produce an enhancement in the PL intensity of the Q/Pn+/SRO25/10/AZO structure, in our case the passivation effect improved substantially the PL features. 5. Conclusions PL spectra obtained from the SRO10, SRO25, SRO25/10 and SRO10/25 films deposited by the LPCVD technique on the Q, Q/Pn+ and Si substrates were compared. The manufacturing process of the SRO films after thermal annealing modifies their optical, morphological and structural characteristics. Therefore, the thickness for monolayers SRO films is the following: 98.15 ± 8.45 nm BTA and 83.3 ± 7 ATA, for bilayers SRO films the thickness are: 154.35 ± 4.05 for BTA and 160.2 ± 0.9 for ATA. While the refractive index of SRO10 film is 1.69 ± 0.017 BTA and 1.72 ± 0.012 ATA and of SRO25 film is 1.55 ± 0.008 BTA and 1.57 ± 0.032 ATA. In adittion, FTIR spectra shown shifted in the wavenumber and change in the intensity of the absorbance peaks, also the vibrational modes present changes important as hydrogen desorption, defects, Si-ncs and a-Si-nps formations due to thermal annealing. The optical band gap energy Egopt has been associated with a change in the size of the Si-ncs, due to the transition from an amorphous phase to a crystalline phase in the structure SRO films, when is applied ATA. The monolayer films have a Egopt of 1.88 ± 0.22 eV BTA and 3.315 ± 0.495 eV ATA, and bilayer films have a Egopt of 2.115 ± 0.265 eV BTA and 1.72 ± 0.08 eV ATA, obtained by Tauc method. Also, with the maximum peak PL was obtained the Egopt. The monolayer films have a Egopt of 2.915 ± 0.345 eV for BTA and 3.6 ± 0.2 eV for ATA, and bilayer films have a Egopt of 2.8 ± 0.2 eV for BTA and 3.66 ± 0.06 eV for ATA. These results are very important to obtain the Si-ncs size, which was obtained by three methods, two methods used the optical band gap energy (Tauc method and PL) and the other one used the HRTEM technique. It has calculated that the Sincs size for Al/Si/SRO25/10 structure through Tauc, PL and HRTEM is: 3.23 nm, 3.6 nm and 3.8 ± 0.8 nm, respectively. And for Q/Pn +/SRO25/10 structure the Si-ncs size through Tauc, PL and HRTEM is: 4.12 nm, 3.72 nm and 3.95 ± 0.45 nm, respectively. It was observed that the intensity PL from the SRO films is improved due to the use of Q/Pn + substrate, where the Q presents a rugged morphology and together with the phosphorus atoms present in the Pn + that diffuse into the SRO films produce a greater number of defects in these interfaces, as well as, Si-ncs and a-Si-nps formation. In the case of deposited SRO films on Q/Pn+, Phosphorous atoms present in the polysilicon can be diffused into the SRO layer, such atoms passivate the Silicon nanocrystals by attaching to the dangling bonds such fact has contributed to enhance the PL intensity. Nanocrystals on silicon and quartz substrates are unpassivated and the interface defects quench the luminescence. The SIMS characterization showed that phosphorous was diffused into the SRO films and therefore it is present in the Si-ncs and a-Si-nps and other defects, which contributes to increase the PL intensity, especially for the SRO bilayers films. Author contributions statement H. P. Martínez Hernández, designs the experiments and carried out some characterizations besides writing. J. A. Luna López, characterizations, writing-review, M. Aceves Mijares, experiments and review. K. Monfil Leyva. G. García Salgado, R Morales Caporal, R. Ramírez Amador, and J. A. D. Hernández-De-La-Luz, Z. J. Hernández Simón, A. Luna Flores writing-review & editing and critical review. Acknowledgments This work has been partially supported by CONACyT-CB-255062 and VIEP-LULJ-EXC-2019. The authors acknowledge CIDS, CINVESTAV (Dr. René Azomosa. Y. Kudriavtsev y M. Avendaño), INAOE (Pablo 11

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Alarcón, V. Aca Aca, A. Hernandez Flores, A. Itzmoyotl Toxqui and I. Juárez Ramírez) and IFUAP laboratories for their help in the sample's characterizations.

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