Measurements
Electron Spin Resonance and Transient Photocurrent ... - JuSER
Electron Spin Resonance and Transient Photocurrent ... - JuSER
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4.3 ESR Signals and Paramagnetic States in Intrinsic µc-Si:H<br />
Figure 4.6: g-values as a function of IC<br />
RS of material prepared (a) VHF-PECVD and (b)<br />
HWCVD deposited at various substrate temperatures. The values for the VHF-PECVD<br />
material were taken from reference [17].<br />
For the HWCVD material, again the highest N S is always found for material<br />
with the highest crystallinity. Within the crystalline regime, the spin density<br />
decreases considerably with increasing amorphous content for all substrate temperatures<br />
T S . For material dominated by amorphous phase content (IC<br />
RS < 0.4), the<br />
spin density increases, rather than staying constant or decreasing as observed for<br />
the PECVD material. Only for the highest T S of 450 ◦ C the spin density decreases<br />
monotonously, however at very high values of N S > 10 17 cm −3 . Finally, independent<br />
of the particular structure composition, N S increases significantly when the<br />
substrate temperature is increased by up to three orders of magnitude for IC RS ≈ 0.5.<br />
It is important to note that the lowest spin densities of N S = 4 × 10 15 cm −3 are observed<br />
for material prepared at T S = 185 ◦ C and a crystalline volume fraction of<br />
IC<br />
RS<br />
≈ 0.4, and that solar cells prepared under similar conditions have shown maximum<br />
efficiencies of η = 9.4% [162].<br />
The corresponding average g-values are plotted versus I RS in Fig. 4.6. For the<br />
PECVD material shown in Fig. 4.6 (a), the average g-value shifts from a value<br />
of 2.0043 to 2.0051, when going from crystalline to amorphous structure. This<br />
means that with increasing amorphous content the g-value is clearly shifted to<br />
higher values, however without reaching the value of 2.0055 usually found in a-<br />
Si:H. As shown in Fig. 4.6 (b), the g-values for material prepared with HWCVD<br />
are generally higher (note the different scaling of the y-axis in panels (a) and (b)).<br />
The variations as a function of structure are considerably less for the HW-material.<br />
Again, even for material with no contribution of the crystalline phase in the Raman<br />
signal, the g-values do not reach the typical value of 2.0055 found in a-Si:H.<br />
Generally, one observes an increasing g-value for increasing substrate temperature<br />
over the entire range of I RS<br />
C . 45<br />
C