Dear Kris,
Thank you for you reply.
Please note that i could create a reference to one table from another table without declaring one column as primary key. I declared a column as "unique" and could make a reference to this table from another table successfully as shown below

create table a(aa number unique);
create table b(bb number references a(aa));

So by declaring as "unique not null" also will do the same job. Since 'aa' is declared as unique, a unique index is created by default. So the difference comes only in two constraints vs single constraint.

What difference it makes to database performance when a column declared as single against two constraints

Please reply
Thank You
Akber