3.1 Raman spectroscopy
Raman spectroscopy has been used extensively as foremost and most effective approach in analyzing structural evaluation of crystalline and amorphous silicon films [29]. The Raman spectra of different diborane doped films were taken in the wave number ranges from 350 cm− 1 to 650 cm− 1 and are represented in Fig. 2. From the best Lorentzian plot fitting, the spectra could be deconvoluted majorly into three different spectra covers the range 480–520 cm− 1: the narrow crystalline component through a lorentzian peak distribution in the region of ~ 520 cm− 1 was allocate as TO vibrational mode of crystalline silicon or named as typical Raman spectra of crystalline silicon and the related intensities identifies as (Ic). The spectra emerges around ~ 500 cm− 1 defined as intermediate component or attributed as Raman peak of grain boundaries and the linked intensity identifies as (Ib). A broad hump shaped distribution that arises in the region of ~ 470–480 cm− 1 corresponds to TO vibrational mode of amorphous silicon and named as characteristic Raman spectra of amorphous silicon and the associated intensity recognized as (Ia). In calculating percentage of crystalline volume fraction; the intermediate component has to be taking into consideration as inherent portion of crystallinity. From Fig. 2, It was examined that the peak positions change from ~ 480 cm− 1 to wave number 510 cm− 1 with increasing diborane doping. This shift clearly reflects the transition from amorphous to crystalline phase and evidences the dopant induced crystallization in amorphous silicon matrix. From results, it was found that films deposited without doping at FB = (0.00) shows characteristic broad hump at 480 cm− 1 which clearly depict that the deposited film is amorphous in nature. However, the films deposited at FB (0.05) and FB (0.10) shows emergence of sharp crystalline TO peak at 504 and 510 cm− 1 respectively. The Raman peak shifts to side of lower wave number as compared to single crystal silicon peak at 520 cm− 1 suggesting that crystallites of nanometer size dimensions are dispersed in amorphous matrix. It was also observed that increasing the diborane doping favors amorphous silicon growth. This result could be explicated as boron doping effect or atomic hydrogen (H) etching effect. From the known surface diffusion model, it is recognized that the surface reaction kinetics mainly deals with the chemistry of atomic hydrogen. With increasing diborane doping, there might be large probability of configuration of B-H-Si complexes in the material structure. Further, boron related species or radicals remove atomic hydrogen from the depositing surface due to effect of ion-bombardment which might increases the number of dangling bonds on the surface and decrease the crystalline volume fraction or called as atomic boron doping amorphization effect [30].
The Raman peak were deconvoluted into three major peaks using Lorentzian distribution corresponding to crystalline and amorphous phase at around 475 and 501 cm-1 respectively as illustrated in Fig. 3 for the film deposited at FB = 0.05.
The crystalline volume fractions, (XC) of the deposited samples were estimated via the relation:
Where Ic, Ib and Ia defines the integrated Raman intensities analogous to crystalline, grain boundaries and amorphous phase respectively.
The crystallite size in the deposited films can be determined by using the relation:
Where Δw is the shift in nanocrystalline component to single crystal silicon peak at 520 cm-1 also B = 2.24 nm2 cm-1 for silicon.
The size of nano-sized crystallite estimated using above relation is ~ 2 to 4 nm. The crystalline volume fraction at FB = 0.05 is 79% and the crystalline volume fraction calculated in FB = 0.10 is nearly 77%.
3.2 Field Emission Scanning Electron microscopy (FESEM)
To elucidate Raman results, scanning electron microscopy is carried out to examine the micro-structural evolution of amorphous and microcrystalline phase in silicon films. Scanning electron microscopy is an effective technique to investigate the morphological and topographical characteristic of the deposited silicon films.
The micrographs of FESEM images for the boron doped µc/nc-Si:H were shown in Fig. 4. It is clearly visible from the FESEM images that the micro/nanocrystalline grains are dispersed in amorphous matrix. The arrangement of crystallite structure is different at different diborane flow. Since the formation of microcrystalline silicon films initially start with an incubation layer (i.e. named as amorphous region) followed by emergence of crystalline nuclei form within the incubation layer which grows initially like nano-sized grains embedded in amorphous phase, and named as mixed phase structure. Further, these crystallites grew larger and conglomerates grain formation takes place which were named as microcrystalline silicon film.
The growth mechanism in p-type micro/nano crystalline silicon film is analogous to intrinsic micro/nanocrystalline silicon films. The density of atomic hydrogen is lower results in decreasing the diffusion probability of gas precursors on the surface. Therefore, the gas precursors were not able to diffuse deep extent to find energetically favorable positions to form ordered structure on the surface. As a result, few were etched away and the remaining forms amorphous structure i.e. random structure. Consequently, the precursor gases which fall directly on the nuclei simply contribute to micro/nano crystalline growth [31]. From images, it is clearly observed that boron induces crystallization resulting nano-sized crystallites formation at FB = 0.05 and FB = 0.10 respectively. At higher doping ,it might assume that polyhydrides formation takes place resulting amorphization in the silicon network as observed in film deposited at FB = 0.30. The atomic boron may remove; forming other boron related species resulting increased Si-H bonding in the network. Further, these findings are well correlated with Infra-red measurements and termed as boron doping amorphization effect.
3.3 EDX (Energy dispersive X-ray spectroscopy)
EDX is a type of spectroscopy technique used for the elemental analysis, evaluation and chemical characterization of sample. In our Boron doped samples, EDX is one of the important studies. EDX spectra with associate quantities are shown in Fig. 5. With EDX spectra, we confirmed the boron doping and the presence of impurities in our silicon films like Oxygen etc. This is Quantitative analysis to confirm doping quantity in deposited silicon films. From Fig. 5, it is proved that weight % and atomic % increased with boron doping. With increase in boron doping at FB = 0.10 resulting oxygen i.e. associated with post-oxidation decreases with increased silicon content. Whereas, at FB = 0.20, boron and oxygen percentage increases but weight % of silicon decreased.
3.4 Infra-red absorption
The vibrational investigation was carried out using Fourier transform infra-red spectrometer. The Infra-red absorption spectra of deposited films are represented in Fig. 6. The FTIR spectra majorly have three absorption bands i.e. at ~ 630, 885 and 2100 cm− 1 respectively corresponding to different modes named as wagging, bending and stretching respectively [32]. Figure 7 shows broad spectra of boron doped film deposited at FB = 0.05%. The FTIR spectra of boron doped silicon film indicates Si-Si vibrational mode which has properly defined band at ~ 611 cm− 1. This band is related to TO (transversal optical) with TA (transversal acoustic) mode that arises due to monocrystalline silicon substrate [32]. This mode hereby emerges and named as dopant or boron induced crystallization of the a-Si:H. This could be an alternate way to crystallize amorphous silicon. The results are in agreement with the Raman spectroscopy. The Raman spectra in Fig. 2 presented a strong/intense band with peak position centered at ~ 510 cm− 1 i.e. longitudinal optical (LO) mode appears due to c-Si induced with diborane doping. For higher boron doped samples, the strength of this band decreases due to lack of crystallinity. Also, other characteristics of vibration modes shown in Fig. 6 disappear as the diborane doping increases. Additionally, strong absorption band examine nearly at 1060 cm− 1 corresponds to asymmetric stretching vibration mode (Si-O-Si) in the IR spectra. This peak is an strong indication of oxidation effect due to formation of similar to porous microstructures which is a main characteristic for nc-Si:H films. Whereas, the absorption band seems at ~ 2000 cm− 1 corresponds to vibrational stretching mode of different Si-H bonded configurations. Further with increase in doping ,the intensity of absorption band observed at ~ 630 cm− 1 and ~ 2000 cm− 1 increases in the spectra (due to boron induced crystallization effect) then decreases with further doping due to amorphization effect. These consequences indicate that with increase in boron doping there might be shift in hydrogen bonding species from Si-H2, (Si-H2)n to Si-H bonded species or monohydride configurations.
It is already recognized that in a-Si:H ,hydrogen plays vital position in passivation of dangling bonds. This improves the optical and electrical properties due to changes in the morphology. Here, for boron doped samples, it is observed that sample with low boron doping FB = 0.05, has low Si-O intensity which relates with the electrical conductivity. From Fig. 7 we observed a vibrational bond at ~ 611 cm− 1 in all the samples. With slight boron doping the intensity of band increases then decreases with further doping. Moreover, the band observed at ~ 2300 cm− 1 confirmed the nanocrystalline growth via boron dopant in sample as shown in Fig. 7 for the sample FB = 0.05 %.
The FTIR spectra has been deconvoluted and from the integrated intensity of absorption peak observed at ~ 630 cm-1 using the empirical formulae bonded hydrogen content has been calculated [33]
Where α (υ) defined as the absorption coefficient of the film, (Aw) is oscillator strength whose value is equal to 2.1 × 1019 cm− 2, (υ) as wave number in cm− 1 and Nsi = 5 × 1022 cm− 3 termed as material atomic density [33].
From present investigation, it can be concluded that additional band appears at ~ 611cm− 1 is named as boron induced crystallinity mode of vibrational spectra. The Raman spectroscopy is the foremost technique in describing the crystallinity in the material (and best conductivity) which confirmed the boron induced crystallinity effect.
It is observed that initially the peak intensity observed at ~ 630 and 2100 cm− 1 increase with boron doping then after with increasing doping, the absorption peak intensity decreases. Figure 8 illustrates the total hydrogen content and microstructure parameter with different percentage of boron doping.
With slight increase in boron doping, both hydrogen content (CH) and microstructure parameter decreases. However, with further increase in doping percentage both hydrogen content and microstructure parameter increases as observed for FB = 0.30. The increase in hydrogen content may be due to deviation in structure from crystalline to amorphous phase as observed via Raman spectroscopy and XRD measurements as discussed in our previous published article [27]. For FB = 0.30, it was assumed that the atomic hydrogen (H) participates in the formation of Si-H bonds with the dangling bonds on the surface which diffuses in the film and eventually increases the density of SiH3 radicals followed by gas phase reaction i.e.
The hydrogen content (CH) increases at FB = 0.30 can be ascribed to increase in hydrogen add-on next to the defect states which produced because a result of surplus boron dopant atoms that will generate micro void-rich material with increased values of R*.