Hydrogen migration and electronic carriers in a-Si:H

The relationship between electronic carriers and hydrogen migration in a-Si:H was investigated by using secondary-ion-mass spectrometry to measure deuterium-diffusion profiles in the intrinsic (i) layer of p-i-n a-Si:H photodiodes. The carrier concentration in the i layer was controlled by varying either the temperature, or the illumination intensity, or the bias applied to the devices. It is demonstrated that hydrogen migration in a-Si:H is controlled by an electronic mechanism, and (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. The trap density under these conditions is close to the dangling-bond density, suggesting that the migration length is determined by capture into these defects. Possible mechanisms for the interaction between hydrogen migration, carriers, and defects are discussed.