How can I get leading zeros in my PKey?

SQL/PostgreSQL newb here ... I would like to have a primary key in a
table where the key is numeric, but has a retains leading zeros. The
pkey values should start at:

00000001

and increase from there. Is there a data type that will do this for me?
Or should I be storing this as text instead? Or should the leading zeros
be handled outside the database itself, perhaps by the interface?

Thanks,
Mark