• PDF / 355,219 Bytes
• 6 Pages / 414.72 x 648 pts Page_size
• 12 Downloads / 215 Views

DOWNLOAD

REPORT

---

ABSTRACT Hydrogenated amorphous silicon thick films deposited by dc glow discharge on molybdenum substrates were annealed by a pulsed Nd:glass laser. Mass spectrometry showed hydrogen remaining in all the laser annealed films. The amount of hydrogen remaining decreased with decreasing scan rate. The hydrogen evolved upon heating at 365 oC and mainly at 658 °C before laser annealing, but at 365, 575 (mainly) and 645 oC after laser annealing, indicating weakening of the silicon-hydrogen bonding after laser annealing. The presence of hydrogen inhibited crystallization, as indicated by Raman scattering. The photo and dark conductivity of the film increased by one and three orders of magnitude respectively with increasing laser energy density up to 12 J/cm 2 at a fixed scan rate. This means that the photoresponse was decreased with laser annealing, in spite of the associated increase in crystallinity. This photoresponse decrease is attributed to the hydrogen evolution. INTRODUCTION Polycrystalline silicon film supported on a substrate is a promising candidate for the development of large area low cost solar cells [1]. This can be achieved by first depositing hydrogenated amorphous silicon (a-Si:H) at low temperatures where unwanted impurity diffusion and thermal stress problems can be eliminated and then crystallizing the a-Si:H to polycrystalline silicon by a thermal treatment. However a major limitation in polycrystalline silicon is the high density of recombination centers at the grain boundaries and this degrades the electrical properties of the device. The role of hydrogen in polycrystalline silicon is likely to be similar to that in amorphous silicon, which is the passivation of the dangling bonds [2]. However, thermal treatment of a-Si:H by furnace annealing results in the evolution of hydrogen from the film. Subsequent hydrogenation of polycrystalline silicon by a plasma treatment results in the passivation of the grain boundaries [3]. An alternate way is to crystallize the a-Si:H film in a short duration such that the out diffusion of hydrogen can be minimized. Therefore, it has been suggested that laser crystallization of a-Si:H could serve this purpose [4]. Previous work in this area [5] have shown improved electrical properties. Prior studies had focussed only on laser annealing of glow discharge a-Si:H thin films deposited on insulating substrates (c-Si or glass) [6-8], where the film thickness was not greater than 2 pim. However, the minimum thickness necessary for a polycrystalline silicon film solar cell is 5 pm. To our knowledge there has been no work done on laser 719 Mat. Res. Soc. Symp. Proc. Vol. 321. ©1994 Materials Research Society

crystallization of thick a-Si:H films. For such a purpose, only a laser in the infrared region (such as a Nd:YAG, Nd:glass or a CO 2 laser) could be used. The objective of this work is to study the crystallization and detect the presence and form of hydrogen in the dc glow discharge deposited intrinsic a-Si:H thick film on a molybdenum substrate after being annealed b