laravel error cannot convert to string

I am using Laravel 5.2 and have a problem.

My code is;

$sts = STSMember::find($member_id)->join('rating', 's_t_s_members.member_id', '=', 'rating.member_id');

But I get the following error

Object of class Illuminate\Database\Eloquent\Builder could not be converted to string'.

How to get member_id from two table?

via Chebli Mohamed