I would suggest you look at these two articles. They are quite technical but explain a lot on low-level.

1. [Peter Gutmann, Data Remanence in Semiconductor Devices][1] 
2. [Peter Gutmann, Secure Deletion of Data from Magnetic and Solid-State Memory][2]

If you are looking also for a counter-measure then I would suggest

3. [TRESOR Runs Encryption Securely Outside RAM][3]


  [1]: https://github.com/arekbulski/Cameleonica/blob/master/documentation/referenced/Gutmann%2C%20Data%20Remanence.pdf
  [2]: https://github.com/arekbulski/Cameleonica/blob/master/documentation/referenced/Gutmann%2C%20Secure%20Deletion.pdf
  [3]: https://github.com/arekbulski/Cameleonica/blob/master/documentation/referenced/Muller%2C%20Tresor%20Runs%20Encryption%20Securely%20Outside%20RAM.pdf