Problem Solving: Create Scalar-valued Function

Environment: SQL server 2005 and Win7
Tools: MS SQL Management studio 2008 R2
Problem: How to create scalar-valued function, which generates randomly ID Numbers. Those Id number must be 9 char in length and don't exist in the dbo.Table_people nor dbo.Table_ssn
Code:

Create Function keygen_pn_id (@get Char(1)) 
Returns char(9) AS 
Begin IF (@get=0) 

DECLARE @new_nbr CHAR(9), @len INT 
SELECT @new_nbr = ltrim(rtrim(convert(CHAR(9),convert(int,(RAND()*1000000000)))))
-- check if the generated ID number whether exists on both tables. If does not exist then assign new_nbr to pn_id otherwise skip and generate a new number. 
If not exists (Select pn_id from dbo.Table_people where pn_id=@new_nbr) and (select pn_id from Table_ssn where pn_id=@newnbr)
Return @new_nbr 
END