back button is working after LOgout in asp.net

Hi, Please add the below code on your Log out button event.

1. Session.Clear();  
2. Session.Abandon();  
3. Response.Redirect("logout.html");  

And add the below code on page load event of master page or other pages to check if the session is active or not. 

1. if(Session["username"] == null)  
2. {  
3. Response.Redirect("login.aspx");  
4. }