In Salatul Thajjud (night prayer), do i do addtional sura after Fatiha or it's not necessary?

Long surahs should be recited after Fatiha in salatu-Tahajjud. The wording in the Quran verses suggests this.

1. And in some parts of the night (also) offer the Salat (prayer) with it (i.e. recite the Qur'an in the prayer), as an additional prayer (tahajjud optional prayer - Nawafil) for you (O Muhammad صلى الله عليه و سلم). It may be that your Lord will raise you to Maqam Mahmud (a station of praise and glory, i.e. the honour of intercession on the Day of Resurrection.) . (Al-Isra, Chapter 17, Verse 79)

2. Verily, the rising by night (for tahajjud prayer) is very hard and most potent and good for governing oneself, and most suitable for (understanding) the Word (of Allah). (Al-Muzzammil, Chapter 73, Verse 6)

3. And during night, prostrate yourself to Him (i.e. the offering of Maghrib and 'Isha' prayers), and glorify Him a long night through (i.e. tahajjud prayer). (Al-Insan, Chapter 76, Verse 26)

The sunnah is to read an additional surah after fatiha.

Additional Surah afer Al.Fathiha is a requirement in all Nafl Salah and first two rakaths of all farz salahs.