Properties of Amorphous Silicon Thin Film Transistors with Phosphorous-Doped Hydrogenated Microcrystalline Silicon
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deposited. Type B (Etch-Back, E/B) is formed by depositing SiN. gate dielectric, a-Si:H active layer, and n÷*/.c-Si:H on top of the gate line metal. After active etching, the S/D metal is patterned. Finally, the ITT channel is defined by removing the n'layer.
TFT A E/S
Drain
iNx
Passi. SiNx n\-4 C-Si
GATE
ITO
a-Svritki)
GATE
INSULATOR
TFT B
Fig. 1 Cross Section View of Two types of a-Si:H TFrs. Type A is the etch stopper type and the type B is the back channel etch type TFT. RESULTS 1. Properties of n/ U c-Si:H Fig. 2(A) and (B) show the RF-power dependence of Raman spectra for SiHS(H 2+ Sil 4) ratios of 2.0% and 1.4% each. Typical Raman peak of amorphous state is 480cm 1 and crystalline state is 520cm1.
(A) SiH Ha: 25/1000 500W
400W *,
200W 4i
*
S400
450
500
580
600 100W 600
400
450
500
550
600
650
Raman Shift(cm" 1 )
Fig. 2 Raman scattering spectra for various deposition RF-power.
908
Fig. 2(A) shows all amorphous states regardless of RF power where the hydrogen ratio is relatively small. But for higher hydrogen ratio and for the RF-power of higher than 300 W, the Raman peaks shift toward 520cm-' which shows the formation of crystalline state as in Fig. 2(B). This suggests that the formation of Atc-Si:H film requires high RF-power as well as high hydrogen dilution. In Fig. 2(B) the 520cm-' peak is very broad. This suggests that the film is formed by the mixture of microcrystalline and amorphous states. Figure 3 shows the deposition time dependence of film thickness and resistivity. The deposition thickness is linear to deposition time, but the resistivity which should be the intrinsic property of n+igc-Si:H film is not constant with respect to deposition time (film thickness). For very thin film the resistivity is much higher than thick film which is almost constant. This suggests the existence of the transition layer at the beginning of the film formation. [5] Since the interface between a-Si:H and source/drain metal is critical for good ohmic contact formation the transition layer should be removed. More work needs to be done to solve this problem. Theoretically, large contact resistance between data line and aSi:H reduces the on-current of TFT. By replacing n'a-Si:H to n' ,c-Si:H, contact resistance is reduced and the TFT on-current is increased effectively. [6] 0.30
1200 1100
0.25
1000 #A I-U)
0.20 900 800 700 0.10
U,
600 0.05 500 400
rt
-I
8
12
16
20
Deposition Time
.00
24
(min.)
Fig. 3 Thickness & resistivity vs Depo-Time of the film deposited at 2750C Properties of the present n+ / c-Si:H and the conventional n'a-Si:H films are shown in Table I. The resistivity of n+/-tc-Si:H film is about three orders of magnitude lower than that of n'a-Si:H as shown in Table I. It suggests that the fraction of electrically active donors in n+ ic-Si:H film is larger than that in n'a-Si:H film. Therefore n+/.tc-Si:H is better for ohmic contact layer than n'a-Si.
909
Table I. Properties of n+1c-Si:H and n'a-Si:H.
n+/u c-Si:H
n'a-Si:H
Thickness (A)
900
500
Sheet Resista
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