Synthesis and properties of thallium nitride films

Abstract Thallium nitride, the last of nitrides in A iii B v subgroup of nitrides, has been synthesized in a film form by DC reactive sputtering of metal thallium in nitrogen. Transmission electron microscopy (TEM) and electron diffraction investigations of as-deposited films showed that the TlN crystallizes in wurtzite type structure with a = 0.368 ± 0.001 A, c = 0.601 ± 0.002 A and c/a = 1.633 lattice parameters. Studies of optical and electrical properties of as-deposited TlN films allowed to establish that the thallium nitride is highly degenerated direct band semiconductor with E g  = 1.5 eV, ρ  = 1.2·10 −3  Ω cm resistivity and Hall coefficient, R H  ≈ −3.7·10 −9  m 3 ·qul −1 . TEM investigations of samples spent different time intervals open air revealed that TlN easily oxidizes by its oxidation in accordance with TlN + O 2 →TlN + Tl 2 O→TlN + Tl 2 O + Tl 2 O 3 →Tl 2 O 3 +Tl 2 O→Tl 2 O 3 scheme without formation of intermediate oxynitrides. The reason for this effect is that the Tl N bonding is much weaker compared to Tl O bonding. However, TlN films may be conserved without oxidation in neutral environment like vacuum or nitrogen gas. It is supposed that appearance of unusual microstructure and growth mechanism of TlN films on initial growth stages is a result of that the film-forming species are the TlN molecules and molecular clusters rather than atoms. The change of the growth from coagulation to normal coalescence mode, revealed for thick (>100 nm) films, occurs due to increase of the surface temperature during deposition.