Controlling the p-type conductivity of SnO by doping with nitrogen and hydrogen

Abstract

Two series of SnO thin films, one doped with N and one doped with H, were deposited on c-plane sapphire by reactive ion beam sputter deposition starting from growth parameters optimized for stoichiometric SnO. The amounts of dopants incorporated into the SnO:H and SnO:N samples were quantified by secondary ion mass spectroscopy. The influence on the structural and electrical properties of SnO thin films was studied as a function of dopant concentration. In the case of N doping, all N incorporated, probably as NO, are active as the acceptor and exhibit long-term stability. We assign an acceptor activation energy of 100 to 150 meV to NO. However, we observe a change in the film morphology at a critical N concentration of about 7⋅1017cm−3, which deteriorates the structural properties of the films. In the case of SnO:H, the situation is different. We observe an outdiffusion of H after growth, i.e., the samples are not stable in the long term. Nevertheless, all H incorporated up to a H-content of 1019cm−3 seem to be electrically active and exhibit an activation energy between 150 and 250 meV, likely corresponding to Hi. Furthermore, at H contents above 1019cm−3, we observe molecular H2 inside the SnO:H thin films. We conclude that N doping of SnO is better suited for tuning the p-type conductivity of SnO. However, it will be essential to overcome the morphology change observed at the critical N concentration to fully explore the tunability of the p-type conductivity in device applications.

Vacancies and dopants in two-dimensional tin monoxide: An ab initio study

“…Their substitution energetics (see Fig. 4(c)) however reveal that vacancy filling by H2 offers a more stable doping channel, which seems to agree well with experimental observations of p-type conductivity in hydrogen-doped SnO thin films [53]. to-direct bandgap transition is unprecedented in layered tin monoxide, making it especially important if one can be realized through N doping.…”

Plasma-assisted molecular beam epitaxy of SnO(001) films: Metastability, hole transport properties, Seebeck coefficient, and effective hole mass

“…Common challenges for the growth of phase-pure SnO are its metastability with respect to the disproportionation into SnO 2 and Sn, as well as the adjustment of the stoichiometry to prevent the formation of secondary SnO 2 or Sn phases. Note that despite the non-equilibrium nature of thin film growth, equilibrium phase diagrams can provide guidance as discussed next: Firstly, the stability region of SnO at temperatures between 197 and 410 • C (and disproportionation outside this region) rationalizes why most SnO films have been obtained at growth or annealing temperatures in this temperature range; [7][8][9]12,17,20 secondly, during growth by reactive sputtering 9,13 , PLD, 10,18 or MBE 19 the formation of secondary SnO 2 or Sn-phases has been controlled by adjusting the stoichiometry of the source vapor, i.e., the oxygen (background) pressure at fixed flux of SnO x from the source, in qualitative agreement with the equilibrium stoichiometry dependence in the phase diagram. The blue or orange shaded regions as well as colored arrows in Fig.…”

“…5 In fact, hole mobilities between 1 and 5 cm 2 Vs have typically been obtained by Hall measurements of SnO films. 5,[7][8][9][10] More recently, hole mobilities as high as 30, 21, and 19 cm 2 Vs have been reported for polycrystalline SnO bulk ceramics, 11 optimized epitaxial SnO(001) layers, 12 and polycrystalline, mixed SnO+Sn films, 13 respectively. Thus, reasonably high hole mobilities together with a direct bandgap absorption edge arXiv:2007.13448v2 [cond-mat.mtrl-sci] 3 Aug 2020 around 2.6-3.2 eV (and only weak optical absorption by its indirect band gap of 0.6 eV), 5,7,8 fuel the interest in SnO as a p-type semiconducting oxide for transparent thin film transistor applications.…”

“…10 The observed p-type conductivity of unintentionally doped (UID) SnO has been correlated by first-principle calculations with Sn vacancies 14 or their complexes with hydrogen 15 acting as shallow acceptors, whereas oxygen interstitials were predicted to be electrically inactive. 14,15 SnO thin films have been grown by various methods, such as electron-beam evaporation 16 or reactive DC magnetron sputtering, 13 both followed by thermal annealing, reactive ion beam sputter deposition, 9 pulsed laser deposition (PLD) from an oxide target 10,12,17 or a metallic Sn target 18 , and molecular beam epitaxy (MBE). 8,19,20 The largest challenge for the growth of phase pure SnO is its metastability with respect to its stable relatives Sn and SnO 2 .…”

Density functional theory investigation of mechanical and electronic properties of two-dimensional semiconductors

“…Among the different dopant species studied, it is found that N and F can be immersed almost seamlessly within the SnO lattice, rendering them as ideal candidates for O-site substitution with little detriment to the mechanical integrity of the structure. The desirability of N doping has been corroborated in a recent study by Becker et al [231] where nitrogen-doped SnO thin films with p-type conductivity were shown to display exceptional long-term stability over a period of four months. In contrast, the researchers reported rapid outdiffusion and degradation of hydrogen-doped samples over the same time duration, which may be ascribed, at least in part, to differences in mechanical compatibility.…”