The most widely used ALD precursors are monodentate ligated metal chlorides, alkyls, cyclopentadienyls, alkoxides and amides due to their high reactivity and volatility. While sufficient volatility can be achieved for most precursors, there is always a trade-off between reactivity and thermal stability. Strong M-Cl, M-C or M-O bonds provide precursors with high thermal stability but make it challenging to completely remove their ligands from the deposited precursor at low temperatures, leading to high impurity levels in deposited films. Polarised M-N bonds are more reactive, making it easier to remove their surface ligands also at low temperatures and thus useful for a broader range of materials formed with less thermodynamic driving force. However, the low thermal stability of the deposited surface species leads to impurities in the film and thus sets upper limits for the deposition temperatures. This has led to the use of M-N bonded bidentate ligands, which form two bonds to the metal centre, thereby improving the thermal stability of the precursor and its surface species.