To cite plm in publications use:

Croissant Y and Millo G (2008). “Panel Data Econometrics in R: The plm Package.” Journal of Statistical Software, 27(2), pp. 1–43. doi: 10.18637/jss.v027.i02.

For the covariance matrix estimators in plm, please cite:

Millo G (2017). “Robust Standard Error Estimators for Panel Models: A Unifying Approach.” Journal of Statistical Software, 82(3), pp. 1–27. doi: 10.18637/jss.v082.i03.

Corresponding BibTeX entries:

  @Article{,
    title = {Panel Data Econometrics in {R}: The {plm} Package},
    author = {Yves Croissant and Giovanni Millo},
    journal = {Journal of Statistical Software},
    year = {2008},
    volume = {27},
    number = {2},
    pages = {1--43},
    doi = {10.18637/jss.v027.i02},
  }
  @Article{,
    title = {Robust Standard Error Estimators for Panel Models: A
      Unifying Approach},
    author = {Giovanni Millo},
    journal = {Journal of Statistical Software},
    year = {2017},
    volume = {82},
    number = {3},
    pages = {1--27},
    doi = {10.18637/jss.v082.i03},
  }