Internal Stress and Optical Degradation in a-Si:H Deposited on Glass Substrates
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30 minutes and then cooled to room temperature in 2 hours. Instead of direct measurement of the internal stress, it was estimated by the curvature of a-Si:H films on thin glass substrates due to the difference in the thermal expansion coefficient between a-Si:H and substrates. The curvature radius R or curvature 1/R of the samples was measured as shown in Fig. 1. Parallel laser beams separating d = 3.6 mm each other were reflected on the sample surface with spread angle of AO. Beam separation Al was measured at I = 1.5 m distance. Curvature radius R was calculated by following equations.
splitter He-Ne laser
a) G) L 0 Co
3.6 mm a-Si:H
m
substrate
AO sample Fig. 1 Apparatus to measure the curvature of a-Si:H films on thin glass substrates.
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R = 2J[2d/AO, AO • tanAO = (A/-d)// For electrical measurements, Al coplanar electrodes were evaporated onto the films. In order to introduce SWE, white light (300 mW/cm 2 ) from a Xe lamp through a water filter (thickness: 10 cm) was irradiated on the samples.
4 E
"-3 (D2 ,.)
1
0
200
300
deposition temperature (oc)
RESULTS The curvatures 1/R of aSi:H films on thin glass substrates deposited by CW (0.05 W/cm 2 ) and PM discharge (the same average power density, modulation frequency 1 kHz, duty ratio 5%) are shown as functions
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Fig. 2 Curvature of a-Si:H films on thin glass substrates as functions of deposition temperature for continuous wave (0.05 W/cm 2 ) and pulse modulated discharge (the same average power density, modulation frequency 1 kHz, duty ratio 5%).
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