Ionic window.open is not working in ios device

window.open('url', '_system');  
//Loads in the system browser 
window.open('url', '_blank');   
//Loads in the InAppBrowser
window.open('url', '_blank', 'location=no');
//Loads in the InAppBrowser with no location bar
window.open('url', '_self');
//Loads in the Cordova web view 

more info: https://wiki.apache.org/cordova/InAppBrowser