Page Output To Text.txt "&" Becomes Undesired Text

I use the code below to output the string to txt and it works. However, for characters like "&" it output as "amp" etc which is undesirable. How could I fix it.

// Output the text file to the stream
Response.Clear();
Response.AddHeader("content-disposition",
"attachment;filename=FileName.txt");
Response.Charset = "";
Response.Cache.SetCacheability(HttpCacheability.NoCache);
Response.ContentType = "application/vnd.text";
System.IO.StringWriter stringWrite =
new System.IO.StringWriter();
System.Web.UI.HtmlTextWriter htmlWrite =
new HtmlTextWriter(stringWrite);
Response.Write(str.ToString());
Response.End();