2D mouse/touch controlled turret problems

I am quite new to unity, just started messing about with it about a week ago, and have looked at many tutorials. So I'm trying to make a 2d game for andriod, which involves using a touch controlled turret to shoot down enemies falling from the skies. I of course got stuck right away, I figured out a way for the turret barrel to point where the player the player taps, and moves, but once it goes from right to left the barrel rotates 180 on the y axis, and no matter where I look, I can't figure out why it does this, or how to fix it.

Here is my code:

 private var ray : Ray;
 private var rayCastHit : RaycastHit;
 var aPosition = Vector3(1, 1, 1);
 
 
 function Update()
 {
   if(Input.GetMouseButton(0))
   {
     ray = Camera.main.ScreenPointToRay (Input.mousePosition);
     
     if (Physics.Raycast (ray, rayCastHit))
     {
       aPosition = Vector3(rayCastHit.point.x, rayCastHit.point.y, this.transform.position.z);
       transform.LookAt(aPosition);
     }
   }
 }