Hydrogen effect on nanostructural features of nanocrystalline silicon thin films deposited at 200 °C by PECVD

Journal of Non-Crystalline Solids 385 (2014) 17–23

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Hydrogen effect on nanostructural features of nanocrystalline silicon thin films deposited at 200°C by PECVD Atif Mossad Ali a,b,⁎, Hikaru Kobayashi c a b c

Department of Physics, Faculty of Science, King Khalid University, Abha, Saudi Arabia Department of Physics, Faculty of Science, Assiut University, Assiut 71516, Egypt Institute of Scientific and Industrial Research, Osaka University, Japan

a r t i c l e

i n f o

Article history: Received 16 July 2013 Received in revised form 18 October 2013 Available online xxxx Keywords: Nanocrystalline silicon; Oscillator strength; Photoluminescence; Stress; Etching, cleaning and coverage mechanism

a b s t r a c t In this work, we reported hydrogen effect on nanostructural features of nanocrystalline silicon (nc-Si) thin films by using a plasma-enhanced chemical vapor deposition (PECVD) method at low deposition temperatures. Structural and optical properties have been investigated by means of X-ray diffraction, Raman scattering, photoluminescence (PL), and oscillator strength measurements. In addition, mechanical property was also investigated by stress measurements. It is shown that with an increase of the hydrogen content, R = [H2]/[SiH4], the crystalline volume fraction, the average crystallite size, bδN, and the intensity of the 1.75–1.85 eV PL band were increased, while the optical band-gap, Eopt g , the peak energy and the intensity of the 2.15–2.25 eV PL band were decreased. The oscillator strength and Eopt g were increased when bδN decreased. The remarkable result obtained from PL measurements showed that amorphous silicon (a-Si) exhibits a PL peak at 1.65 eV and also possesses a wider band-gap than that of bulk Si. The stress in the nc-Si films is slightly compressive, which is favored for application of nc-Si to device technology. Moreover, addition of hydrogen-related molecules to the feed gases under low deposition temperature plays an important role, such as etching, cleaning, and coverage on the growing surface, in the crystallization of nc-Si. © 2013 Elsevier B.V. All rights reserved.

1. Introduction Currently, increasing interest has been shown towards fabrication and characterization of nanocrystalline silicon (nc-Si). The expected and promising superior properties of nc-Si compared to amorphous silicon (a-Si) such as better stability, high charge carrier mobility, higher absorption in the near-infrared wavelength range, quantum effects and single electron effects are useful for optoelectronic devices overcoming the limitation of miniaturizing the present a-Si transistorbased devices and very large scale integrated devices in future [1–13]. In addition, silicon-based quantum devices are more promising than the compound semiconductor counterpart since it can make use of processing technologies already developed for Si in ultra large-scale integrated circuits [1–13]. Moreover, the study of nc-Si is also interesting from the viewpoint of basic science, and it may answer the following: how very small crystalline Si is formed, what is property like, and what the structure of a-Si is. In addition, nc-Si thin films have been developed as an absorber layer of thin film Si solar cells due to their striking advantages over a-Si [14,15]. A high short circuit photocurrent density and suppression of light induced degradation are the main advantages. Because the optical absorption coefficient of nc-Si films in ⁎ Corresponding author at: Department of Physics, Faculty of Science, King Khalid University, Abha, Saudi Arabia. Tel.: +966 17 241 7099; fax: +966 17 241 8319. E-mail address: [email protected] (A.M. Ali). 0022-3093/$ – see front matter © 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jnoncrysol.2013.10.019

the visible region is much lower than that of a-Si, so the absorber layer should be sufficiently thick (≥2 μm) to absorb solar radiation without increasing the resistance of the device. For deposition of high quality nc-Si films for solar cell used at high growth rates, two conditions of plasma are essential: a low ion energy and a high density of electrons with energy above a certain level. Therefore, an increase in the nucleation rate to decrease crystallite size became a key technique. Plasma-enhanced chemical vapor deposition (PECVD) technique seems to be an effective for this purpose. Various growth processes based on the PECVD technique have been studied for production of nc-Si which exhibits photoluminescence (PL) even at room temperature (RT). Although much effort has been devoted to various aspects of porous silicon (PS) and nc-Si, the origin of the light emission is not clear. Veprek et al. [16] prepared nc-Si:H films by hydrogen-diluted-silane plasma and by chemical transport of Si in hydrogen plasma. However, similar to PS [17,18], these films exhibit an increase in PL intensity only after post-processing treatments such as oxidation and annealing in forming gas (H2–N2). The deposition method of nc-Si films utilizing H2 diluted SiH4, commonly employed in production of nanocrystalline, microcrystalline or polycrystalline Si [19,20], has also been reported by Liu et al. [21]. They obtained nc-Si films at temperatures lower than 200 °C and observed visible PL at 1.82 eV [19–21]. Furthermore, Toyama et al. [22] have obtained nc-Si by using the PECVD technique for deposition of boron-doped microcrystalline silicon (μc-Si) and subsequent electrochemical treatment in an HF aqueous solution. Alternatively, high

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helium dilution instead of hydrogen in silane plasmas [23] has been used for production of PL-active silicon films, which are amorphous with an optical energy gap of 2.1 eV and PL has been ascribed to the presence of (SiH2)n groups. In addition, Cicala et al. [24] have obtained nc-Si films by PECVD providing PL at RT. In the PECVD method, the structural, optical, and electrical properties of nc-Si films are greatly influenced by hydrogen atoms; hydrogen and hydrogen-related bonds are critically associated with surface passivation [25], etching reaction [26], and modification of Si―Si networks [19]. H2 dilution technique has been widely used for fabrication of nanocrystalline silicon, but the following important questions are not clarified yet; what is a direct cause of crystallization, and how can H2 contribute to the formation mechanism [27]. In addition, it has been pointed out that H radicals eliminate H2 from the growing silicon surface, leading to relaxation of strain in the Si network generated during crystallization of Si films [28]. In our recent study on nc-Si thin films which fabricated by using PECVD method, two PL peaks were observed at 1.7–1.75 and 2.2– 2.3 eV [29]. We found that the deposition temperature, and also the hydrogen flow rate during deposition, the crystallite size decreased, while the optical band-gap energy and the PL energy were increased. We also observed that the film structure changed from amorphous to nanocrystalline by an increase in the silane flow rate, [SiH4], and the intensity of the vibrational peak due to Si―H decreased with [SiH4] [30]. In the light of these references, we have investigated effects of the R = [H2]/[SiH4] ratio on the mechanism of the nc-Si formation at low temperatures and the nanostructure of the thin films in the present study. The aim of this work is also to clarify the effect of R = [H2]/[SiH4] during growth on structural features of the nc-Si thin films. Since the fabricated thin films consist of amorphous and nanocrystalline Si regions, PL peaks arising from both amorphous and nanocrystalline Si phases have been observed. The results showed hydrogen-containing molecules greatly contribute to crystallization of nc-Si. 2. Experimental About 0.4 μm nc-Si thin films were deposited on glass, fused quartz, and (100) Si substrates by rf glow discharge (at 13.56MHz) decomposition of SiH4–H2 mixtures in an inductively coupled reactor inserted in an electric furnace. The samples were deposited on a 0.3-mm-thick glass (Corning 7059) substrates for measurements of X-ray diffraction (XRD) patterns and Raman scattering spectra, on 0.3-mm-thick Si(100) (Ntype, high resistivity 1000–3000 Ω cm) substrates for measurements of Fourier transform infrared (FT-IR) spectra, and on 0.3-mm-thick fused quartz substrates for photoluminescence (PL) and stress measurements. The flux of SiH4 was kept at 0.5 sccm, while the flux of H2 was varied in the range between 4 and 22 sccm. We used deposition temperature, Td, of 200 °C, the rf power supply of 15 W and the total pressure of 0.1 Torr for every deposition. The structural properties were investigated by measuring the XRD (SHIMADZU XD-D1) patterns with a slit width of 0.1mm. The XRD intensity was estimated from the integrated area of the peak with a given texture. The average crystallite size, bδN, in the film was estimated from the half-value width of the XRD peak by means of the Scherrer's formula [31].

from 100 to 300 mW was used as the PL excitation source. The IR vibration spectra, using a Fourier-transform spectrometer (JASCO FT/IR-610), were measured at a normal light incidence and under vacuum conditions. In this case, a bare Si wafer being similar to the substrates was used for the reference in the range of 400–4000 cm−1. Atomic force microscopy (AFM) was used to measure the surface morphology of the nc-Si thin films. Tapping mode AFM experiments were performed on a Solver Next AFM microscope (Molecular Devices and Tools for Nano Technology (NT-MDT Co)). The stress was measured at RT. When nc-Si film is deposited on one side of quartz substrate, the stress in nc-Si thin film will result in bending of substrate (Fig. 1), The stress value, σ, was estimated from changes in the curvature of the quartz substrate/thin film having a rectangular shape, using Stoney's formula [35]: Y Q b2 δ  ; σ¼
3 1−υQ tL2

where YQ is Young's modulus of quartz substrate (YQ = 8.3 × 1010 N/m2 and υQ = 0.17 [36]), b and L are the substrate thickness and length, respectively, υQ is the Poisson ratio for the quartz substrate, δ is the change in curvature of the thin film/quartz substrate and t is the film thickness. 4. Results and discussion Fig. 2 shows the Raman spectra for the nc-Si thin films deposited at various ratios of R = [H2]/[SiH4]. As shown in this diagram, the films with R = [H2]/[SiH4] higher than 2 exhibited a sharp peak, and a shoulder peak appeared at around 520 and around 500–515 cm−1, which were due to the transverse optical (TO) mode of the crystalline phase and very small crystallites, respectively. And at R = [H2]/[SiH4] = 2, the film exhibited the broad Raman peak at 480 cm−1 due to TO-like modes in an amorphous phase. No crystalline phase was observed at R = [H2]/[SiH4] = 2 because of a low migration rate and/or an increase in the deposition rate [37]. In addition, the intensity of the 520 cm−1 component increased with increasing R = [H2]/[SiH4]. Deconvolution of the Raman spectra for the nc-Si films gives information on the crystalline volume fraction and the average crystallite size. In addition, the Raman peak positions depended on R = [H2]/[SiH4]. Such Raman peak shifts would be related to a change in the stress of the films. Here, a positive Raman peak shift is indicative of an increase in the compressive stress or a decrease in the tensile stress, as we will discuss later. Fig. 3 shows the peak frequency of the Raman signal arising from the crystalline Si (c-Si) phase for the nc-Si films, as a function of R = [H2]/ [SiH4]. With an increase in R = [H2]/[SiH4], the peak frequency of the

3. Characterization The crystalline volume fraction, ρ, was estimated from the Raman spectra using procedure proposed by Tsu et al. [32], Kim et al. [33] and Gope et al. [34]. The Raman spectra for the nc-Si films consisted of a narrow line near 520cm−1 arising from a crystalline phase and a broad line at around 480 cm−1 arising from an amorphous phase. The ρ value was estimated from the ratio of the Raman integrated intensity for the crystalline component to the total intensity, taking the Raman crosssections into account. PL was analyzed using a Jobin Yvon RAMANOR HG 2S spectrometer coupled with a cold photo-multiplier tube (Hamamatsu Photonics R649S). The 488-nm Ar-ion laser with power ranging

ð1Þ

Fig. 1. Change in the curvature of the quartz substrate/thin film.

A.M. Ali, H. Kobayashi / Journal of Non-Crystalline Solids 385 (2014) 17–23

19

-0.5

R=[H2/SiH4]=11

Stress (108 N/m2)

Raman Intensity (a.u.)

0.0

8 6 3 2 400

-1.0

-1.5

-2.0 450

500

550

600

0

Raman Shift (cm-1)

4

8

12

R = [H2/SiH4]

Fig. 2. Raman spectra for the nc-Si thin films with various R = [H2]/[SiH4] values.

Raman signal increased in contrast to the previous work [34]. The blue shift of the Raman peak and an increase in the Raman peak intensity are likely to result from a reduction in the bδN and an increase in the ρ values. Such a shift is usually explained by the quantum confinement effect [38,39]. In order to examine the effect of strain on the crystallization process, the stress in the films was measured. The observed stress, σ, is expressed by the sum of two different components. The first one is the stress, σt, due to the thermal expansion mismatch between the film and the substrate. The second one is the intrinsic stress, σi, caused by the growth process of the film. Fig. 4 shows the dependence of σ on R = [H2]/[SiH4]. It is evident that the stress changed to weak compressive stress (−2.1 × 108 to −0.5 × 108 N/m2) with increasing R = [H2]/ [SiH4]. Under the conditions of R = [H2]/[SiH4] = 2–11 and low Td of 200 °C, the observed weak compressive stresses rather than tensile stresses are desired in order to avoid the occurrence of cracks from application viewpoint of these nc-Si thin films to device technology. As shown in Figs. 3 and 4, the peak position of the Raman peak showed a blue shift with an increase of R = [H2]/[SiH4] in the structural change region. In addition, the compressive stress in nc-Si thin films is likely to be higher for higher R = [H2]/[SiH4], and the blue shift of the phonon bands may be related to an increase of the compressive stress. Fig. 5 shows the ρ values for the nc-Si thin films as a function of R = [H2]/[SiH4]. The ρ value increased with R = [H2]/[SiH4]. This result is consistent with a previously reported result [40] that hydrogen atoms on the growing surface increase a diffusion coefficient of the precursors, resulting in the formation of nc-Si. In other words, the presence of hydrogen atoms on the growing surface and/or in the surface region is

Fig. 4. Compressive stress observed for the nc-Si thin films, as a function of the R = [H2]/[SiH4] values.

needed to form crystalline structure. Based on the results shown in Figs. 2 and 5, the full width at half maximum (FWHM) of the Raman peak at 520 cm−1 due to a crystalline phase decreased with R = [H2]/ [SiH4]. This decrease in the FWHM reflects to a decrease in random strain. This result also indicates that the crystalline quality is improved with an increase in R = [H2]/[SiH4]. In other words, the increase in the ρ and the decrease in the FWHM with increasing R = [H2]/[SiH4] indicate that the crystalline quality is improved when R = [H2]/[SiH4] increased. Fig. 6. shows the XRD spectra for the nc-Si thin films deposited at different R=[H2]/[SiH4] values. As shown in this diagram, the XRD spectra from the (111) plane is observed. In addition, the intensity of the (111) texture increases by increasing the R = [H2]/[SiH4] except that the sample was deposited at R = [H2]/[SiH4] = 2, which is amorphous. Fig. 7 shows the average crystallite size, bδ(111)N, of the nc-Si thin films, obtained from the b111N XRD peak, as a function of R=[H2]/[SiH4]. For the nc-Si films used in the present work, only this b111N diffraction peak was observed, indicating that a dominant texture of crystallites exhibited the (111) orientation, as shown in Fig. 6. In our recent work [30], the following deposition conditions were used: the rf power supply and the deposition temperature during the film deposition were maintained at 30 W and 200 °C, respectively. The flow rates of SiF4 ([SiF4]) and H2 were fixed at 0.2 sccm and 15 sccm, respectively, and [SiH4] was varied from 0.0 to 0.7 sccm. In this work [30], the half width of the XRD peak (b111N texture) varied only slightly, indicating that bδN does not change appreciably with [SiH4]. On the other hand, the most surprising

80

60

520.5

ρ (%)

Peak Frequancy (cm-1)

521.0

40

520.0 20

519.5 0

0 4

8

12

R = [H2/SiH4] Fig. 3. Peak frequency of the Raman signal arising from crystalline Si phases for the nc-Si thin films, as a function of the R = [H2]/[SiH4] values.

0

4

8

12

R = [H2/SiH4] Fig. 5. Crystalline volume fraction, ρ, for the nc-Si thin films, as a function of the R = [H2]/ [SiH4] values.

A.M. Ali, H. Kobayashi / Journal of Non-Crystalline Solids 385 (2014) 17–23

XRD Relative Intensity (a.u.)

20

R=[H2/SiH4]=11

6

2 20

30

40

50

2 θ (deg.) Fig. 6. XRD spectra for nc-Si thin films with different R = [H2]/[SiH4] values.

feature of the results shown in Fig. 7 of the present work is that the bδN values increased with increasing R = [H2]/[SiH4], in contrast to the previous works [33,40–42]. Accordingly, for the films obtained by changing R = [H2]/[SiH4], it is found that ρ and bδN values increased as shown in Figs. 6 and 7 with a decrease in the FWHM of the Raman peak at around 520 cm−1 which arise from a crystalline phase, resulting from a decrease in random strain. Namely, a decrease in the FWHM values caused by increasing R = [H2]/[SiH4] corresponds to an increase in the average crystallite size and a crystalline volume fraction. To understand the mechanism of the crystalline growth by PECVD depending on R = [H2]/[SiH4], the following effects should be considered: i) the effects of chemical etching and cleaning, ii) the coverage of hydrogen on the growing surface of the films, to remove weak bonds and impurities, and iii) surface migration of adsorbates. It has been reported that the etching rate by hydrogen radicals decreases with increasing the deposition temperature [33,43–46]. Therefore, addition of hydrogen-related molecules to the feed gases at low Td plays an important role in crystallization of nc-Si thin films. It is found from Figs. 6 and 7 that the bδN and ρ values increased with increasing R = [H2]/[SiH4] at low deposition temperature of 200 °C. Thus, we suggest that the above mechanisms are very important for the formation of the crystalline structure. On the other hand, it is reported that the change in ρ with [SiH4] may arise from the formation of b111N textured grains [30]. Moreover, the increase in ρ with [SiH4] would be due to an increase in the density of b111N grains [30]. The surface morphology of the nc-Si thin films prepared at different R = [H2]/[SiH4] has been measured by atomic force microscopy (Fig. 8). It can be seen from Fig. 8(a) [2 μm × 2 μm] that the surface is flat,

<δ (111)> (nm)

20

15 Fig. 8. The AFM images of the nc-Si thin films. (a) The AFM of sample deposited at R = [H2]/[SiH4] = 2, (b) the AFM of sample deposited at R = [H2]/[SiH4] = 6, and (c) the AFM of sample deposited at R = [H2]/[SiH4] = 11.

10

5

0 0

4

8

12

R = [H2/SiH4] Fig. 7. Average crystallite size, bδ(111)N, of the nc-Si thin films, as a function of the R = [H2]/[SiH4] values.

corresponding to the amorphous tissue in good agreement with the result obtained from Raman and XRD results (see Figs. 2 and 6). On the other hand, it can be seen from Fig. 8(b) [2 μm × 2 μm] and Fig. 8(c) [2 μm × 2 μm] that the average grain size values increased with increasing R = [H2]/[SiH4], in good agreement with XRD results (Fig. 7). In addition, the shape of the grains on the surface is spherical and the nanocrystallites of the Si are distributed nearly uniformly over the

A.M. Ali, H. Kobayashi / Journal of Non-Crystalline Solids 385 (2014) 17–23

Transmittance (r.u.)

R=[H2/SiH4]=11

6 2

500

1000 1500 2000 2500 3000 3500 4000

Wavenumber (cm-1) Fig. 9. The infrared-absorption spectra over the range 400–4000 cm−1, for the nc-Si thin films with different R = [H2]/[SiH4] values.

surface and hence are suitable for integration in device structure. It is therefore expected that grown thin films could be used as protective coatings in the device. Fig. 9 shows the FTIR transmission spectra of the nc-Si thin films deposited at different R = [H2]/[SiH4] values. The absorption bands observed at around 645 and 2100 cm−1 are assigned to the rocking/ wagging ((Si―H)wag), and stretching ((Si―H)str) absorption, respectively [47–49]. The peaks at 805 and 1040 cm−1 are assigned to the bending ((Si―O)ben) and stretching ((Si―O)str) modes of Si―O―Si vibration, respectively [47–49]. The absorption band at 865 cm−1 is assigned to the bending (i.e. “scissors”) vibration of hydrogen in the dihydride configuration in nc-Si thin film. The hydrogen content for the nc-Si films deduced from integration of the infrared absorption band at 2100 cm−1 is shown in Fig. 10. It is clearly seen that hydrogen content increased with increasing R = [H2]/[SiH4]. We found a correlation between the increase in the dominant hydrogen content values and the increase in bδ(111)N values with R = [H2]/[SiH4], as shown in Figs. 7 and 10. Thus, we suggested that nc-Si thin films consist of small crystalline silicon particles surrounded by hydrogen atoms, which are bonded in dihydride form (SiH2 molecules). In addition, we expect that the volume fraction of amorphous regions decreases with an increase in R = [H2]/[SiH4], in agreement with the increase in ρ (Fig. 5). This is consistent with the results shown in Figs. 2, 5 and 7. In addition, the increase in R = [H2]/[SiH4] appears to increase the density of hydrogen atoms in the films in contrast to the previous work [41,50]. We have found that the luminescence, optical, and structural properties of nc-Si are very sensitive to plasma conditions such as the gas flow rate, gas composition, Td, gas pressure, and rf power in the SiH4/

21

H2 system. The PL spectra for the nc-Si thin films are shown in Fig. 11. The measured ensemble spectra exhibited two dependent peaks on R = [H2]/[SiH4] and located at ~1.75–1.85eV and ~2.15–2.25 eV, respectively. These energies exceed the band-gap energy for bulk Si which has an indirect band-gap of 1.12 eV and therefore, does not give luminescence in the visible range. The reason for no PL in c-Si is the fact that the involvement of phonons makes radiative recombination a secondorder process [51]. The behavior of the intensities of the two PL peaks is reverse; in other words, the intensity of the first peak increases and the intensity of the second peak decreases with increasing R = [H2]/ [SiH4]. In our previous work [29], the following deposition conditions were used: The rf power, gas pressure, and [SiH4] were 20 W, 0.3 Torr, and 0.6 sccm, respectively. The flow rate of SiF4 was 0.38 sccm, [H2] was 25 or 46 sccm and Td was varied from 95 to 250 °C. In this work [29], we observed two separated PL bands: one is a relatively strong PL band with peak energy at around 1.7–1.75 eV and the other is a weak band at around 2.2–2.3 eV. In addition, the first one was the main peak and the second one appeared only at low Td. Moreover, we found that the peak energy of the 1.7–1.75-eV PL band shifted as Td or [H2] changed [29]. On the other hand, in our recent paper [30], the PL spectra showed three peaks at around 2.1–2.25, 1.77–1.80 eV, and 2.36 eV. The first one is the main and strong peak, the second and third peaks were weak, in contrast with the previous work [29]. No PL was observed for the film deposited at [SiH4] b2 sccm, which was amorphous. Therefore, we suggested that an a-Si phase is not responsible for the observed luminescence in this work [30]. A remarkable feature shown in Fig. 11 of the present work is that aSi, which is a direct transition semiconductor, has a PL peak energy located at 1.65 eV and also has a higher band-gap energy than that of bulk Si. It is well known that PL peak widths depend on the crystallite size, i.e., the smaller the crystallite size, the broader PL peaks. This result suggested that the peak broadening was caused by the crystallite size distribution. As seen in Fig. 11, the increase in the PL intensity of the first peak is found to correspond well with an increase in the hydrogen content and the crystalline volume fraction (see Figs. 5 and 10). Therefore, this band may originate from PL centers associated with SiHrelated bonds. However, as shown in Fig. 11, the intensity of the 2.15–2.25 eV band decreased with increasing R = [H2]/[SiH4] which is in good correspondence to decrease in the PL intensity. Moreover, the 2.15–2.25 eV peak showed a red shift with increasing R = [H2]/[SiH4], corresponding well to increasing bδN. These results suggest that peak energy is determined by bδN in the nc-Si thin films causing a quantum-size effect. Nc-Si thin films with small crystallites and corresponding oscillator strength value will give PL at a given energy. The red shift of the 2.15–2.25 eV band, as well as the decrease in the PL intensity of the 1.7–1.8 eV band, is attributed to the increase in oscillator

30

R=3

PL Intensity (a.u.)

H Content (1021 cm-3)

R=11 25

20

15

R=6

R=2

10 1.6 0

4

8

12

1.8

2.0

2.2

2.4

Photon Energy (eV)

R = [H2/SiH4] Fig. 10. Hydrogen content in the nc-Si thin films, as a function of the R = [H2]/[SiH4] values.

Fig. 11. Photoluminescence (PL) spectra for the nc-Si thin films deposited at various R = [H2]/[SiH4] values, as a function of the photon energy.

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2.3

Oscillator Strength

4.2x10-6

Eg

opt

(eV)

2.2

2.1

2.0 0.04

4.0x10-6 3.8x10-6 3.6x10-6 3.4x10-6 3.2x10-6 10

0.05

0.06

0.07

0.08

0.09

12

0.10

14

16

18

20

22

<δ> (nm)

1/<δ> (nm-1) Fig. 14. Oscillator strength, fosc, as a function of the average crystallite size, bδN. Fig. 12. Optical band-gap, Egopt, as a function of the average crystallite size, bδN.

that assuming spherical silicon clusters, oscillator strength increased with decreasing δ according to Eq. (3): strength, caused by the increase in the bδN value. Therefore, this relationship supports attribution of the 2.15–2.25 eV band to nanometersized crystallites in the nc-Si thin films with the quantum-size effect, as mentioned above. The most prominent feature of the quantum confinement effect is widening of the band-gap from that of the bulk Si crystal. The electronic band structure of nc-Si with several nanometers is considerably different from that of the bulk Si crystal due to the quantum confinement effects of electrons, holes and phonons [49]. We suggest that further research work is necessary for better understanding of the mechanisms responsible for the origin of PL. The optical band-gap, Eopt g , which was determined from the Tauc plot ((αhυ)1/2-vs-(hυ −Eopt g )) using the optical absorption coefficient, α, observed at a photon energy of hυ, increased with a decrease in the crystallite size, bδN, (Fig. 12). As shown in Fig. 12, nc-Si thin films possess a very large Eopt g . The increase in the optical band-gap with decreasing bδN is partly attributable to incorporation of Si―H bonds whose bond energy is much higher than the Si―Si bond energy [52]. It should be noted that the band-gap energy of hydrogenated a-Si increases with the incorporated hydrogen concentration. The dependence of the oscillator strength on the optical band-gap according to Eq. (2) [51] is shown in Fig. 13. h i h i −5 −8 opt opt f osc Eg ðeVÞ ¼ 1:4  10 þ 5:80  10 exp Eg ðeVÞ=0:332 ð2Þ where fosc is the oscillator strength. As shown in Fig. 13, the oscillator strength increased with increasing Eopt g , which was attributable to stronger confinement in small crystallites, i.e., the quantum size effect. In addition, to confirm the dependence of the oscillator strength on bδN. Khurgin et al. [53], Trwoga et al. [54], and Meier et al. [51] reported

f osc ½ðbδN ðnmÞ ¼ 1:4  10 h  10

−6

−6

þ  1:7 i 1:39 exp 11:24=bδN ðnmÞ

ð3Þ

The size dependence according to Eq. (3) is plotted in Fig. 14. As shown in Fig. 14, the oscillator strength increases with decreasing bδN, in good agreement with previous works [51,53,54]. 5. Conclusions Four hundred nm-thick nc-Si thin films were deposited at 200 °C under various R = [H2]/[SiH4] ratios ranging between 2 and 11, by the plasma-enhanced chemical vapor deposition technique. The hydrogen content which calculated from dihydride (SiH2) and caused the 2100 cm−1 band, the average crystallite size bδN and the crystalline volume fraction, ρ, value increased with R = [H2]/[SiH4]. Based on these results, close correlation was observed between ρ, bδN and the hydrogen content. The PL spectra showed two peaks at approximately 1.75–1.85 eV and 2.15–2.25 eV. However, the intensity and position of the 2.15–2.25-eV band decreased with increasing R = [H2]/[SiH4]. We also observed the PL peak at 1.65 eV for amorphous samples deposited at R = [H2]/[SiH4] = 2. The optical band-gap, the PL peak energy of 2.15–2.25 eV, and the oscillator strength increased with decreasing bδN. Acknowledgment Financial support by King Abdulaziz City for Science and Technology under grant number: 08-NAN153-7 is gratefully acknowledged. References

Oscillator Strength

7.0x10-5 6.5x10-5 6.0x10-5 5.5x10-5 5.0x10-5 4.5x10-5 4.0x10-5

2.05

2.10

2.15 opt

Eg

2.20

2.25

2.30

(eV)

Fig. 13. Oscillator strength, fosc, as a function of the optical band-gap, Eopt g .

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