Yes so many workarounds I found over Net, thank Google. None of them worked! I'm gonna list them here:
1- Using data-rel attribute, setting value to external
2- Installing cordova InAppBrowser plugin as well as adding onclick attribute onclick="window.open('http://www.google.com', '_system'); return false;" to the respective a tag
3- Checking whether or not cordova.js, cordova_plugins.js are existed & included in relevant html file
They were the most suggested ideas I found on Stackoverflow like this one and the other forums. Yet it's opened within app window!
I appreciate your help.