Offering Nafl Salah at ‘Ishraq’ time (a few minutes after sunrise) as well as during mid morning (Duha) are both reported by Sayyiduna ‘Ali (radiyallahu’anhu) as the habit of Rasulullah (sallallahu’alayhi wasallam).
(Sunan Tirmidhi, Hadith: 598-599)
Imam Tirmidhi (rahimahullah) has declared this Hadith as sound (hasan) and he has quoted Imam Ishaq ibn Rahuyah to have also authenticated it.
The Muhaddithun generally refer to both as Salatud Duha, while the Sufis call the earlier salah: ‘Salatul Ishraq’
(Fathul Mulhim, vol.4 pg.634 and Taqrir Bukhari of Shaykh Zakariyya -rahimahullah)
The time for Salatud Duha commences from after sunrise till before zawal (mid-day). It is best to offer Salatud Duha at the middle point of these 2 times. (mid-morning)
(Fathul Mulhim, vol.4 pg.634)
The salat one offers after sunrise is also called Salatul Ishraq.
Allah Ta’ala says: ‘And glorify the praises of your Lord in the evening and at the time of Ishraq’
(Surah Sad, verse: 18)
And Allah Ta’ala Knows best.