SnO thin films were fabricated by electron beam evaporation on (100) Si and $c$ - and $r$ -plane ${\text{Al}}_2{\text{O}}_3$ substrates. The films grown at $25°\text{C}$ are nanocrystalline, while the films grown at $600°\text{C}$ are epitaxial on $r$ -plane ${\text{Al}}_2{\text{O}}_3$ and (001) textured on Si and $c$ -plane ${\text{Al}}_2{\text{O}}_3$ . The SnO films have an optical band gap of 2.82–2.97 eV and $p$ -type conductivity, according to Hall measurements, with resistivities of $0.5–110\Omega \text{cm}$ , hole concentrations of ${10}^{17}–{10}^{19}{\text{cm}}^{-3}$ , and Hall mobilities of $0.1–2.6{\text{cm}}^2/\text{Vs}$ . The $p$ -type conductivity, which appears to correlate with ${\text{V}}_{\text{Sn}}$ , can be enhanced via Y- and Sb-doping. Defect complexes of ${\text{Sb}}_{\text{Sn}}-2{\text{V}}_{\text{Sn}}$ are suggested to be the acceptors in Sb-(or Y-) doped SnO films.

Amorphous SnO thin films were prepared on quartz and 190 nm <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>SiO</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>/</mml:mo><mml:mtext>Si</mml:mtext><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mn>001</mml:mn></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math>SiO2/Si(001) substrates by electron beam evaporation. X-ray diffraction results reveal that amorphous SnO transforms into polycrystalline <mml:math display="inline"><mml:mrow><mml:mi>α</mml:mi><mml:mtext>-SnO</mml:mtext></mml:mrow></mml:math>α-SnO (tetragonal litharge structure) after rapid thermal annealing in Ar ambient at <mml:math display="inline"><mml:mrow><mml:mn>350</mml:mn><mml:mo>–</mml:mo><mml:mn>400</mml:mn><mml:mo>°</mml:mo><mml:mi mathvariant="normal">C</mml:mi></mml:mrow></mml:math>350–400°C and starts decomposing into <mml:math display="inline"><mml:mrow><mml:mi>o</mml:mi><mml:msub><mml:mrow><mml:mtext>-SnO</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>o-SnO2 (orthorhombic structure) with the expulsion of Sn atoms at <mml:math display="inline"><mml:mrow><mml:mn>450</mml:mn><mml:mo>–</mml:mo><mml:mn>500</mml:mn><mml:mo>°</mml:mo><mml:mi mathvariant="normal">C</mml:mi></mml:mrow></mml:math>450–500°C . The optical properties were characterized via spectroscopic ellipsometry. The polycrystalline SnO thin films have a higher refractive index <mml:math display="inline"><mml:mi>n</mml:mi></mml:math>n and a narrower bandgap <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>E</mml:mi><mml:mi mathvariant="normal">g</mml:mi></mml:msub></mml:mrow></mml:math>Eg than the amorphous ones, which is due to the polarizability enhancement in the crystallization process. Moreover, the relationship between <mml:math display="inline"><mml:mi>n</mml:mi></mml:math>n and <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>E</mml:mi><mml:mi mathvariant="normal">g</mml:mi></mml:msub></mml:mrow></mml:math>Eg of the amorphous and polycrystalline SnO thin films can be explained by the “Moss rule” law, and a decreasing trend in <mml:math display="inline"><mml:mi>n</mml:mi></mml:math>n was verified with the transformation from SnO to <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>SnO</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>SnO2 . Bottom-gate-type thin film transistors (TFTs) employing polycrystalline SnO channels on the <mml:math display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mtext>SiO</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msub><mml:mo>/</mml:mo><mml:mtext>Si</mml:mtext><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mn>001</mml:mn></mml:mrow><mml:mo>)</mml:mo></mml:mrow></mml:mrow></mml:math>SiO2/Si(001) substrates exhibit p-type field-effect transistor characteristics. The optimum field-effect mobilities <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>μ</mml:mi><mml:mrow><mml:mtext>sat</mml:mtext></mml:mrow></mml:msub></mml:mrow></mml:math>μsat and <mml:math display="inline"><mml:mrow><mml:msub><mml:mi>μ</mml:mi><mml:mrow><mml:mtext>lin</mml:mtext></mml:mrow></mml:msub></mml:mrow></mml:math>μlin are 0.46 and <mml:math display="inline"><mml:mrow><mml:mn>0.87</mml:mn><mml:mspace width="0.5em"></mml:mspace><mml:msup><mml:mrow><mml:mtext>cm</mml:mtext></mml:mrow><mml:mrow><mml:mn>2</mml:mn></mml:mrow></mml:msup><mml:mspace width="0.5em"></mml:mspace><mml:msup><mml:mi mathvariant="normal">V</mml:mi><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup><mml:mspace width="0.5em"></mml:mspace><mml:msup><mml:mi mathvariant="normal">s</mml:mi><mml:mrow><mml:mo>−</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:msup></mml:mrow></mml:math>0.87cm2V−1s−1 , respectively, which are the same order of magnitude as those reported for epitaxial SnO TFTs.

Memristive devices have been widely employed to emulate biological synaptic behavior. In these cases, the memristive switching generally originates from electrical field induced ion migration or Joule heating induced phase change. In this letter, the Ti/ZnO/Pt structure was found to show memristive switching ascribed to a carrier trapping/detrapping of the trap sites (e.g., oxygen vacancies or zinc interstitials) in ZnO. The carrier trapping/detrapping level can be controllably adjusted by regulating the current compliance level or voltage amplitude. Multi-level conductance states can, therefore, be realized in such memristive device. The spike-timing-dependent plasticity, an important Hebbian learning rule, has been implemented in this type of synaptic device. Compared with filamentary-type memristive devices, purely electronic memristors have potential to reduce their energy consumption and work more stably and reliably, since no structural distortion occurs.