Is it permissible to look at and read/recite from a physical Quran (or mobile app) during wajib salah in order to recite a surah which is not memorized?

If it is permissible, is it then preferable to recite memorized surahs in order to avoid reading during salat?

If it is impermissible or makruh, then would that still apply to, for example, a new convert who has not memorized any surahs yet but wishes to pray?

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It is permissible but better not to, therefore it is better to recite surahs which one has memorised.

It is necessary to learn Surah Alhamd and ikhlas at least