My understanding is that the electric field across the interface draws the two
materials into proximity that allows a chemical bond to form.  The contacting
force is produced by the applied electric field.

I have been doing anodic bonding, using a piece of coat hanger wire as the
anode. This provides enough force near the center of a Pyrex wafer to start the
bond to the underlying silicon wafer. Within a few minutes the bond spreads to
the edge of the wafers.  I continue applying the heat and voltage for a period
of 4 hours, which supposedly increases the strength of the bond.

If, rather than a point contact, you apply an external mechanical force over the
entire surface to be bonded there is a possibility of trapped air pockets unless
you do the bonding in vacuum.

Concerning bonding to PECVD a-Si, this material usually contains a lot of
hydrogen, which might interfere with bonding.  If so, a high temperature heat
treatment might improve the situation.