On converting the UFT-8 xml to Unicode in Powershell, $encoding attribute value is showing bigEndianUnicode in the output xml, I want UTF-16 there

Getting this line in output file after converting UTF-8 to Unicode

<?xml version="1.0" encoding="bigEndianUnicode"?>

But I need below line in the xml

<?xml version="1.0" encoding="UTF-16"?>