Hydrogen Dynamics in a-Si:H
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HYDROGEN DYNAMICS IN a-Si:H Paulo V. Santos Max-Planck-Institut fiir Festkbrperforschung, W-7000 Stuttgart 80, Federal Republic of Germany
ABSTRACT The interaction between electronic carriers and hydrogen migration in a-Si:H was investigated by diffusion experiments in the intrinsic (i-) layer of p-i-n a-Si:H photo-diodes. The carrier concentration in the i-layer was controlled by varying the bias applied to the devices. Hydrogen migration (i) is enhanced when the carrier population is increased by illumination and (ii) is suppressed when it is reduced below the thermal equilibrium value by the application of a reverse bias to the diodes. The effect is attributed to the dependence on carrier density of the dissociation rate of hydrogen from Si-H bonds into the diffusion path consisting of interstitial sites. In addition, the migration length in the diffusion path increases under reverse bias. The enhanced migration is associated with a decrease in the effective density of traps for hydrogen in a carrier-depleted layer. Possible mechanisms for the interaction between hydrogen migration, carriers and defects are discussed.
INTRODUCTION Hydrogenated amorphous silicon (a-Si:H) normally contains from 8 to 15 atomic percent of hydrogen atoms, most of which are bonded to silicon in the form of silicon-hydrogen bonds. Hydrogen plays the fundamental role of passivating silicon dangling bond defects and its incorpo15 3 ration in the a-Si:H network reduces the defect density to levels below 5 x 10 ci- in electronic quality material. Hydrogen can migrate relatively rapidiy through the a-Si:H network even at moderate tem0 peratures (e.g., < 300 C).[l] Hydrogen migration may induce structural changes in the silicon network and, therefore, in its electronic properties. In fact, hydrogen motion has been proposed as the microscopic mechanism mediating many metastable phenomena in a-Sh:H such as lightinduced defect formation in undoped a-Si:H [2] (Staebler-Wronski effect[3]) and dopant activation in doped material[4]. Another interesting phenomenon is that hydrogen migration itself is dominated by an electronic mechanism and depends on the density of electronic carriers in the diffusion region. As a consequence, hydrogen diffusion is enhanced when the electronic carrier population is increased in the diffusion region by illumination or by doping.[4, 5] In this paper, the relationship between hydrogen motion and carrier concentration in undoped a-Si:H is investigated by studying hydrogen diffusion in the intrinsic layer of p-i-n a-Si:H photodiodes. The carrier concentration in the diffusion region can be controlled by illumination or by an external bias. It is demonstrated that hydrogen migration is enhanced if the carrier population is increased by illumination or by forward current injection, and is suppressed if the carriers are removed from the diffusion region by applying a reverse bias. In addition, the average migration distances increases under reverse bias. The dependence of hydrogen migration on carrier population is ascr
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