2, Swansea University, Swansea, , United Kingdom
HfO2 makes a very useful ferroelectric oxide because it is thermodynamically stable in contact with Si, unlike the traditional ferroelectrics PZT and SBT. HfO2 can be brought into the ferroelectric Pca2 phase either by suitable thermal annealing process or by use of alloying or dopants such as group III elements Y, Sc, Al, or N at the O site, or the group IV elements Si, Ge, Zr. Si, Ge, and N could be called efficient dopants in that only a small fraction is needed . However, some of them introduce gap states which will act as traps, or encourage leakage currents, or in the worst case breakdown. Trivalent dopants act by creating O vacancies which lower the mean O coordination and thus stabilise the lower symmetry phase . We recall that previously N and Y were shown to behave beneficially in removing the gap states due to O vacancies from the gap band, but this is true only if they are adjacent to the vacancy [3,4]. We find that ‘efficient’ dopants are less satisfactory in that they will not necessarily be close to the vacancies, so their gap states are not removed. Ge is found to have too small a gap. Si does not introduce gap states.
1 A Toriumi et al, IEDM (2016)
2 J P Goff et al, PRB 59 797 (1999)
3 K Xiong, J Robertson, JAP 99 044105 (2006)
4 D Liu, J Robertson, APL 94 042904 (2009)