Something

function busqueda_alumnos()
{
	var matricula = document.getElementById("matricula").value;
	var capa=document.getElementById("resultado");
	var ajax=nuevoAjax();
	capa.innerHTML="Cargando...";
	ajax.open("POST", "d.php", true);
	ajax.onreadystatechange=function()
	{
		if (ajax.readyState==4)
		{
			capa.innerHTML=ajax.responseText;
		}
	}
	ajax.setRequestHeader("Content-Type", "application/x-www-form-urlencoded");
	ajax.send("matricula="+matricula);

}