$ .ajax在IE中不起作用

Do anyone know what is going on here? i wrote this code to call a php file to generate an image then display it out on the screen. Somehow this working fine for Mozilla and Chrome, but not in IE. Anyone know what is it going wrong for me code?

var src = $.ajax({
url: "em_captcha.php",
async: false
}).responseText;

$("#imgCaptcha").html(src); 

what if you try:

$.ajax({
   url : "em_captcha.php",
   async : false,
   success : function(src){
      $("#imgCaptcha").html(src); 
    }
});

The caching causing this issue, I have solved this issue by adding in the following code into the ajax:

cache: false,