Resistivity Changes Induced by Hydrogenation

Heating the B-doped samples above 200°C in vacuum to dissociate the hydrogenated complex results in flat spreading resistance (Rs) at the original bulk value. A second exposure to Hj restores the increased Rs at the surface. These are crucial experiments that demonstrate that hydrogen is involved and that the process is reversible and reproducible. 

In related experiments by Johnson (1985), atomic deuterium was used instead of Hx to neutralize boron in Si. Similar results on spreading resistance were obtained. Furthermore, the distribution profile of D was measured by secondary-ion mass spectrometry (SIMS), as shown in Fig. 4. The distribution profile of D reveals 1) that the penetration depth of D is in good agreement with the resistivity profile and 2) that the D concentration matches the B concentration over most of the compensated region. In another sample, the B was implanted at 200 keV with a dose of 1 x 1014 cm-2, the damage was removed by rapid thermal anneal at 1100°C for 10 sec., and then D was introduced at 150°C for 30 min. As shown in Fig. 5, it is remarkable that the D profile conforms to the B profile. 

Another demonstration that hydrogenation neutralizes acceptors is the decrease in free-carrier absorption observed during infrared transmission experiments by Pankove et al. (1985). 

To find the optimum temperature for boron neutralization by atomic hydrogen, a systematic study was performed in the 40 to 250°C range by 

An Arrhenius plot of the hydrogen penetration depth is shown in Fig. 7. The activation energy derived from the Arrhenius plot is 0.39 eV. This activation energy is believed to be the energy for breaking an Si—H bond near a B atom. Heating above 40°C appears to be necessary to cause the 

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The metal-insulator transition induced by hydrogen causes also changes of the electric resistance of the sample. The physical relation is rather complicated as it depends on the electronic changes as well as hydride growth mechanism and morphology. Therefore, in most cases, the change in resistance behavior is used to determine the onset of hydride formation/ decomposition. Repeating the measurements at various applied pressures/temperatures reveals a van t Hoff plot, similar to DSC-measurements (2.3). 

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