public function indexAction() {
        $this->loadLayout();
        // Add out block to content to render
        $this->_addContent($this->getLayout()->createBlock('codename/adminhtml_blockname'));
        // Say Magento Load ExtJs
        $this->getLayout()->getBlock('head')->setCanLoadExtJs(true);
        $this->renderLayout();
}

<script type="text/javascript">
var tree;
var defaultLoadTreeParams = {
		animate:false,
		enableDD:false,
		containerScroll: true,
		rootUIProvider: Ext.tree.CheckboxNodeUI,
		selModel:new Ext.tree.CheckNodeMultiSelectionModel(),
		rootVisible:false
}
/** 
 * Function for render new Tree. 
 * @param requestUrl url for get tree information
 */
function renderNewTree(requestUrl) {
  if (!requestUrl) {
     requestUrl = 'treesample';
  }
  defaultLoadTreeParams['loader'] = new Ext.tree.CustomUITreeLoader({
     dataUrl: requestUrl,
     baseAttr: {
        uiProvider: Ext.tree.CheckboxNodeUI
     }
  });
 
  if (tree) {
     // Clear prev tree version
     tree.purgeListeners();
     tree.el.dom.innerHTML = '';
  }
 
   tree = new Ext.tree.TreePanel('tree-div', defaultLoadTreeParams);
 
   root = new Ext.tree.AsyncTreeNode({
      text: 'root',
      draggable:false,
      id:'source',
      uiProvider: Ext.tree.CheckboxNodeUI
   });
   tree.setRootNode(root);
   tree.render();
   root.expand(false, false, function() {
      if (root.childNodes.length>0) {
        // Tree Successfull loaded (root node has child).
        //  Show tree container
        $("tree-div").show($("tree-div"));
        root.firstChild.expand(false);
      } else {
         // Root node don't have child elements
         // Hide tree container and show some error 
         $("tree-div").hide($("tree-div"));
         alert("Empty Tree!");
      }
  });
}
</script>

   <div id="tree-div"></div>

$categoryList = array(
       0 => array(
           "text" => "Sample",
            "id" => 123123,
            "depth" => 0,
            "leaf" => false,
            "children" => array(
                    0 => array(
                       "text" => "Sample2",
                        "id" => 1231233,
                        "depth" => 1,
                        "leaf" => true
                    )
                )
            )
);
echo json_encode($categoryList);