Chapter 4: Optimization of doped a-Si:H thin films
4.2 Results and discussion
4.2.3 Series (III): Substrate temperature (T s =90-150°C) variation of a-
4.2.2.5 Conductivity
The dark (σd) and photo (σph) conductivity measurements were carried out at room temperature (RT) in coplanar geometry. Figure 4.12 shows the variation of σd and σph for films deposited at different MFR. The σd of films is found to be more than 4 orders of magnitude higher than intrinsic a-Si:H deposited at the same temperature (Chapter 3, section 3.4.7). However a gradual fall in conductivity with increase in MFR may be attributed to increase in the void density due to increase in structural disorder in films deposited at higher MFR.
Figure 4.12: Room temperature dark (σd) and photo (σph) conductivity of a-Si:H(p) films deposited at different MFR.
4.2.3 Series (III): Substrate temperature (Ts=90-150°C) variation of a-Si:H(n) thin films
Chapter 2. One of the deconvoluted Raman spectrum corresponding to Ts at 150oC is shown in Figures 4.13(b). The TO mode peak position, FWHM of TO mode, ΔθB and ITA/ITO values for these films are listed in the Table 4.7.
Figure 4.13: (a) Raman spectra for the films prepared with varying Ts. (b) Deconvoluted Raman spectra for Ts = 150°C.
It was observed that, TO mode peak position did not vary much with variation in Ts. The ΔθB
(°) values of a-Si:H(n) films at different Ts were found to be in the range of 7.43 ± 1.62o to 7.68± 1.74o. The substrate temperature had not much effect on the TO mode peak position, ΔθB and ITA/ITO values of a-Si:H(n) films. However, film deposited at Ts 150oC showed lower ΔθB =7.43o and ITA/ITO =0.31 indicating an improvement in SRO and MRO at higher Ts.
Table 4.7: The TO mode peak position, FWHM of TO mode, ΔθB (°) and ITA/ITO values of a-Si:H(n) thin films deposited at different Ts on corning 1737 glass substrate.
Substrate temperature (°C)
TO mode peak Position (cm-1)
FWHM (cm-1)
ΔθB (o)
ITA/ITO
90 476 60.56 7.59 0.39
110 477 61.08 7.68 0.32
130 477 61.09 7.68 0.32
150 478 59.59 7.43 0.31
4.2.3.2 Spectroscopic ellipsometry (SE)
Figure 4.14 (a and b) show the SE experimental (open) and fitted (solid line) data of the pseudo dielectric function of real <ε1> and imaginary <ε2> of a-Si:H(n) thin films deposited at different Ts. To deduce optical parameters, experiential data is fitted with T-L model by constructing a two-layer model [6]. The T-L model details have been discussed in Chapter 2.
In <2> spectra, <2>max is observed around ~3.5 eV, which is a characteristic feature of the direct optical transition in amorphous silicon structure [6].
Figure 4.14: Plot of experimental (open) and fitted (solid line) data of (a) real dielectric function
<1> and (b) imaginary dielectric function <2>against photon energy at different Ts.
The fitted parameters of T-L model for a-Si:H(n) at different Ts are listed in Table 4.8. The thickness of rough surface layer is slightly increased with substrate temperature. It can be seen that amplitude factor (A) has also slightly increased with increase in Ts, which indicates that denser film formation takes place at higher Ts. The broadening parameter (C) did not change much with increase in Ts. However, slightly lower C value was observed at Ts=150oC indicating that these films have less fraction of Si-H2 bonds.
Table 4.8: The T-L model fitted parameters: rough surface layer thickness (nm), thickness of bulk layer (nm), amplitude factor A (eV), resonance energy E0 (eV), broadening parameter C (eV), optical band gap Eg (eV) and constant ε1(∞) from ellipsometry for a-Si:H(n) films at different Ts.
Ts (o)
Thickness of rough surface layer (nm)
bulk layer thickness
(nm)
A (eV)
E0
(eV)
C (eV)
Eg (eV)
ε1(∞)
90 0.86±0.04 65±4 185.21±2.15 3.53±0.01 2.69±0.01 1.78±0.01 1.32 110 1.03±0.07 73±6 192.85±1.02 3.51±0.01 2.66±0.01 1.78±0.01 1.38 130 1.56±0.11 86±2 192.03±2.54 3.47±0.01 2.66±0.01 1.80±0.01 1.14 150 1.82±0.26 102±7 198.54±7.26 3.45±0.01 2.62±0.01 1.81±0.01 0.89
4.2.3.3 UV-Vis-NIR
Figure 4.15(a) show the UV-Vis-NIR transmission spectra of a-Si:H(n) films at different Ts. In the transmission spectra, there were not sufficient interference fringes to apply
Swanepoel’s method for the determination of film thickness [9]. Hence, the thicknesses of these films were estimated using surface profilometer. Figure 4.15(b) show the thickness of a-Si:H(n) at different Ts.
Figure 4.15: (a) UV-Vis-NIR transmission spectra of a-Si:H(n) films at different Ts (b) Thickness and band gap of a-Si:H(n) films estimated from different measurements at different Ts.
It was observed that the thickness of the films is in the range of 63±05-110±06 nm. Thickness of the films increases with increase in Ts since the increase in the sticking probability of precursor radicals adsorbed on the growing surface of a-Si:H is more in comparison with that of rate of surface recombination reactions [35]. The Eg of the films was calculated from Tauc’s plot [10]. The Eg of the films at different Ts is in the range of 1.78±0.01-1.82±0.01 eV.
The Eg was slightly higher (0.1 eV) at high Ts. The thickness values estimated from profilometry and and Eg values estimated from Tauc plot are in good agreement with SE results.
4.2.3.4 Conductivity
In order to study electronic transport properties of a-Si:H(n) films, dark (σd) and photo conductivity (σph) measurements were performed in coplanar geometry at room temperature.
Figure 4.16(a) shows the variation of σd and σph at different Ts. The σd of a-Si:H(n) films are found to be more than 6 orders of magnitude higher than intrinsic a-Si:H films (chapter 3, section 3.2.1.6). These results suggest that films are doped efficiently by adding phosphine during deposition. Figure 4.16(b) shows the temperature dependence conductivity of film deposited at Ts = 150 oC. The activation energy is found to be 0.33 eV, indicating that Fermi level is closer to the conduction band. The dark conductivity and activation energy values are comparable with other reports [36, 37].
Figure 4.16: (a) Room temperature dark (σd) and photo (σph) conductivity of a-Si:H(n) films at different Ts. (b) Temperature dependent conductivity of a-Si:H(n) film at Ts=150oC.