Highly-doped Si:P (HDSiP) epitaxially grown thin films with P concentrations above 10²¹ cm^-3 are being investigated for transistor applications due to their ability to achieve record-setting low nMOS contact resistance junctions suitable for the ITRS 7 nm technology node. Efficient dopant activation of these HDSiP layers with negligible diffusion have been reported by using an Applied Materials Astra^TM millisecond laser dynamic surface anneal (DSA). This work investigates the stability of the dopant activation in these HDSiP layers during subsequent post-DSA thermal processing via Hall effect measurements. The corresponding structural changes in the HDSiP films are investigated using cross-sectional transmission electron microscopy (TEM). 40 nm thick epitaxial layers were grown with chemical doping concentrations between 0.3 – 4.0 x 10²¹ cm^-3. Subsequent DSA treatment at 1200 °C leads to electrical dopant activation of 1 x 10²¹ cm^-3. Subsequent low temperature annealing between 700 °C and 900 °C for times up to 30 minutes resulted in significant electrical deactivation. TEM examination shows these films are defect free after DSA treatment but for higher concentrations, extended defect formation in the epitaxial film was observed. These extended defects appear to be in the form of dislocation loops and may be the byproduct of point defect release during deactivation. The mechanism involved in deactivation will be discussed.