The dielectric constant of a material depends on many factors such as temperature, pressure, frequency, and the presence of defects. In the absence of a specific frequency, one could use the static dielectric constant or the high-frequency limit of the dielectric constant, which can be found in literature or calculated theoretically.

It is important to note that the built-in potential is a static measurement, and it may not necessarily reflect the high-frequency limit of the dielectric constant. Therefore, it is possible that the value you used for the dielectric constant is not appropriate for calculating the polarization.

To improve the accuracy of your calculation, you could try the following:

In summary, the choice of dielectric constant for your calculation is crucial in obtaining accurate values for polarization. It is recommended to use literature values or theoretical calculations for the dielectric constant, and to take into account any other factors that could affect the measurement of the built-in potential.

Hope this helps you out.

Thank You.