How to 100% erase a phone?

[Grey Hawk]

Okay so I use my phone to access work emails as I travel to clients a lot and because of this I have to take confidential/personal files with me. Sadly I think I have to return my phone back to Verizon because it is defective (overheats). How do I make sure with 100% certainty they can't access they files or whoever they send old phones to can't? I deleted the files, but I assume that isn't enough as they may be cached somewhere? Is a factory reset enough? I hear not though because I see programs all over online where you can pull images and texts and such from factory reset android phones.