Effects of Chlorine on Dopant Activation in a-Si:H
- PDF / 374,968 Bytes
- 6 Pages / 382.5 x 615.6 pts Page_size
- 60 Downloads / 238 Views
Sill4 and 100nm DCS Layer FF Vo• (V) J. (mA/cm 2) .39 .661 5.21 .30 .645 3.88 .39 .647 6.00 .47 .765 6.26 .59 .766 9.84
409 Mat. Res. Soc. Symp. Proc. Vol. 557 ©1999 Materials Research Society
i • gded i
p-Sisic
"
I
•
i-layer (150nm total) nSi
actual structure
n-Si
150nm DCS / Si- 4 i-layer (original) 50nm Sill4 /100nm DCS
50nm DCS / 50nm Si4/ 50rnm DCS
depicted at right M3 100rnm DCS / 50nm Sil4 (optimal)
Z]
150nm SiH 4 (control)
Fig. 1 shows the cell structure on the left with a depiction of the i-layer on the right. The five cells grown are detailed in Ref. [7]. EXPERIMENTS Film Growth To investigate this loss of V., we grew two phosphorus doped films, one from a source gas having a small amount of DCS and one without DCS. These films were grown under identical deposition conditions except for the addition of dichlorosilane (Tbb• = 300'C, deposition pressure 500 mTorr (62 Pa), DC excitation, power density -75-100mW/cm2, 50 or 47 sccm Sll4, 0 or 3 sccm SiC12H2, and 1% PI-I3 in 1-12at 6 sccm). Adding the DCS tends to lower the applied voltage, therefore lowering the power slightly, but the deposition rates were unchanged at -4 - 5 A/s. Co-planar thermally evaporated aluminum electrodes 7.5 mm long with a gap of 0.5 mm were used to determine the electrical conductivity and its thermal activation energy in the range between room temperature and 110TC. Optical transmission spectra were evaluated to determine the film thickness and absorption coefficient, from which the Tauc gap was calculated. From these measurements we observed that C1 reduces the n-type conductivity of a-Si:H from 2. 10-3 S/cm to 8.10-6 S/cm. This confirms a previous result for phosphorus-doped films deposited from H2 and SiC14 [8]. Other groups depositing from dichlorosilane did not observe this effect, probably because their gas phase concentration of PH3 was significantly higher than ours, and because their films are microcrystalline rather than amorphous [4, 9, 10]. We hypothesize that in the solar cells described above, when the Cl in the i-layer comes in contact with the n-layer, the presence of the Cl reduces the n-layer conductivity and thus decreases V. In addition to the phosphorus-doped films, two series (13.56 MHz RF and DC) of borondoped films were also deposited under the same conditions, except for the use of 1% B2H6 in H2 (varied from 0.3sccm to 50sccm) instead of PH3 and a substrate temperature of 260TC. Fig. 2 shows the variation in dark conductivity for the DC and RF deposited films as a function of the gas phase concentration of diborane. The corresponding activation energies and Tauc gaps are shown in Fig. 3. From these Fig.s we see that the chlorinated films have higher conductivity except at the highest gas phase concentration of diborane, where the conductivities are the same. We further note that the conductivity enhancement with C1 is more noticeable for the DC deposited films.
410
Room Temperature Dark Conductivity 0.01 ---
o.dc-silane -dc-DCS
--rfsi-an i10-4 .=o..• 0---10.---4 rf-silane AA-
-_-----
Data Loading...
