Fabrication of Boron Compensated Hydrogenated Amorphous Silicon Films with Significantly Improved Stability Using Plasma
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Microelectronics Research Center, Applied Sciences Complex-I, Iowa State University, 1925 Scholl Road, Ames, Iowa - 50011, USA. ABSTRACT
we have systematically investigated the effects of addition of sub-ppm levels of boron on the stability of a-Si.H films and p-i-n devices, deposited by PE-CVD technique. The films thus produced with appropriate amounts of boron, show a significant improvement in stability, when soaked under both AM1.5 (short-term) as well as 10xsun (long-term) illumination conditions. The opto-electronic properties of the films are quite respectable. It is concluded that boron compensates the native impurities by forming donor-acceptor pairs, which reduces the "fast" defects and hence the initial degradation of the films. It is also speculated that boron may also be improving the short-term stability, by reducing the recombination of light generated electrons and holes, by converting DO into D+ states. The long-term stability appears to get affected by hydrogen dilution which seems to reduce the amount of "slow" defects. As a result of B doping of i-layer, the initial conversion efficiency of the devices decreases. It is presumed that our devices may contain an enhanced level of boron impurity, than expected, making them as worse material and to degrade less. INTRODUCTION
Degradation of hydrogenated amorphous silicon (a-SiH), alloys based on a-Si:H and photovoltaic devices under illumination is a well known phenomenon 1 It is believed that recombination of excess electron-hole pairs generated by illumination creates metastable defects in the bulk of the material 2 . The defect states reduce the mobility-lifetime (ýtt) product of electrons and holes and cause degradation of a-Si:H. The origin of the metastability is not quite known. However, it is generally believed 3-6 that impurities such as C, 0 or N, microvoids due to inhomogeneous growth, weak bonds or a combination of these give rise to metastability which causes the degradation of a-Si:H. A number of techniques, treatments and procedures have been used 7-10 to control the growth kinetics, and thus improve the stability of a-Si:H. Among these, hydrogen dilution is routinely used 5,10 as a standard procedure to obtain highly stable films and devices. Another methodll, 12 of improving the stability of a-Si:H is by compensating and thus reducing the number of donor type native impurities. In the present study, we have also explored this idea and deposited the boron compensated a-Si:H films and devices, using a low pressure plasma enhanced chemical vapor deposition (PE-CVD) technique. We have successfully demonstrated that a deliberate addition of appropriate amounts of B improves the stability of aSi:H. FILM FABRICATION
The boron compensated a-Si:H films were deposited using a radio frequency (13.56MN14z) triode geometry. Prior to deposition, system was evacuated down to a base pressure of Ix10-7 Torr and plasma cleaned. Two sets of films were made under different deposition conditions. It 45 Mat. Res. Soc. Symp. Proc. Vol. 377 0 1995 Materia
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