How to make one side circular border of widget with flutter?

Use BorderRadius.only and provide the sides

return Center(
  child: Container(
    height: 100,
    width: 100,
    decoration: BoxDecoration(
      borderRadius: BorderRadius.only(
        topRight: Radius.circular(40),
      ),
      border: Border.all(
        width: 3,
        color: Colors.green,
        style: BorderStyle.solid,
      ),
    ),
    child: Center(
      child: Text(
        "Hello",
      ),
    ),
  ),
);

Output