EPITAXIAL GROWTH OF THE TOPOLOGICAL SEMIMETAL PtAl

This collaborative research project explores the promise of a new compound conductor PtAl for narrow high conductivity interconnects. PtAl is a topological semimetal with a cubic chiral structure with the P2$_1$3 space group. It is theoretically predicted to exhibit topologically protected surface states which lead to scattering-free electron conduction, resulting in an inverted resistivity size effect at small dimensions.

PtAl thin films are deposited by ultra-high vacuum magnetron sputtering onto Al$_2$O$_3$(0001), Al$_2$O$_3$(11 ̅02), Al$_2$O$_3$(112 ̅0) and MgO(001) substrates at T$_s$ = 400-1000 °C. Growth on Al$_2$O$_3$(11 ̅02) at Ts = 1000 °C leads to epitaxial PtAl(001) layers with a resistivity $\rho$ that decreases from 115 - 67 $\mu\Omega$·cm with increasing thickness $d$ = 13 – 109 nm. The resistivity also decreases for deviations from stoichiometry which lead to Pt$_5$Al$_3$ and Pt$_2$Al$_3$ impurity phases for Pt and Al rich conditions, respectively. PtAl deposition at T$_s$ = 700 °C leads to a thickness-dependent crystalline orientation, with predominantly PtAl 001 layers for $d\geq$ 26 nm but PtAl 210 orientation for $d\geq$ 13 nm, and a decreasing $\rho$ = 140 – 92 $\mu\Omega$·cm for $d$ = 13 – 57 nm. $In situ$ vacuum annealing at 850 °C of layers deposited at 400 °C causes a transition from 210 to 001 oriented films with a measured $\rho$ = 220 $\mu\Omega$·cm for $d$ = 3 nm.