Generate many random numbers

I need to generate 4 million unique random numbers between 1 and 1 billion of 9 digits. Fill with zeros until 9 digits.

My script works (but slow) for eq. 400000 numbers. But not for 4 millions

I need the numbers in a text file. Its fine to just CTRL+S the output.

Is there any ways to optimize the memory/performance?

function zeroPad(num, places) {
          var zero = places - num.toString().length + 1;
          return Array(+(zero > 0 && zero)).join("0") + num;
        }

        var arr = []
        while(arr.length < 4000000){
            var randomnumber = Math.ceil(Math.random()*100000000)
            if(arr.indexOf(randomnumber) > -1) continue;
            arr[arr.length] = randomnumber;
        }
        for( i=0; i<arr.length; i++ )
        {
            document.write(zeroPad(arr[i],9) + '<br />');
        }