is it possible to generate unique values for a primary key in sql server/MSSQL

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SQL-Server

hi i am presently using identity function on columns to generate values but i donot find it good . is there any function in sql sever which converts numbers in strings so that i can assign to column . i mean i want to add value such that c1 or say c102 or anything like that in primary key column.

is there any way to generate an unique value for primary key column.any functions/stored procedures or triggers

also i am using triggers to add values from old tables to new tables when ever there is update/insert or delete on old table . i am using following syntax but i am geeting error so please help

my syntax is

create trigger <triggername> on <tablename>
after insert 
as begin
  insert into triggertable values(:old.tablecolumn1,:old.tablecolumn2,.....)
end
</tablename></triggername>

i am getting error for above syntax such as "error at : or old.tablecolumn" is not permitted .