Tin oxide films grown by molecular beam epitaxy.
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The growth mechanisms conditioning the structural, optical and electrical properties of SnO2 films grown by Molecular Beam Epitaxy (MBE) was studied in this work. MBE proved to be an extraordinary technique for producing high purity, single crystalline SnO2 thin films, with a precise control over the growth process as evidenced by the atomically abrupt interfaces routinely obtained during this work.
The interplay between the arrival rate of the atoms and their diffusion once adsorbed onto the surface of the substrate was shown to be the key aspect in determining the growth mode and the crystalline quality of the films. This was evidenced by the transitions between polycrystalline, Volmer - Weber and Frank - van der Merwe growth modes observed in RHEED studies on the surface crystallinity of the films and by the AFM and SEM analysis of their surface morphology.
XRD investigations evidenced and improvement in the spacing of the atomic planes with increasing substrate temperature and decreasing cell temperature, associated to higher diffusion of the adatoms and lower arrival rates, respectively. XRR and RHEED studies evidenced that the first atomic layers deposited onto the substrate act as a lattice matching buffer that accommodates the mismatch between the substrate and the film. Consequently, the strain due to the lattice mismatch is distributed and epitaxial growth is observed after five atomic layers have been deposited. These studies were essential for optimising the growth protocols, which allowed obtaining single crystalline SnO2 films of exceptional quality, presenting highly coherent lattice planes with XRD -scan peaks exhibiting less than 0.5º FWHM.
The electrical characteristics of the material were enhanced by means of Sb doping without degradation of the optical and crystalline properties. As a result, tin oxide films with resistivities ranging from 1 mΩ.cm up to 1kΩ.cm and beyond could be controllably and reproducibly obtained.
These films exhibit carrier concentrations between 1016 cm-3 and 1021 cm-3, carrier mobilities as high as 30 cm2 / V.s and plasma edge beyond 14 μm, thus resulting in a material with extraordinary potential for transparent electronics.
The limits for the coexistence of optical transparency and electrical conductivity were explored in these single crystalline SnO2 films. The increase in carrier concentration from 1018 cm-3 to 1021 cm-3 is accompanied by a 40 nm broadening of the transparency window in the UV. This is as a consequence of the blue shift in the optical band gap due to the Fermi level rising inside the conduction band (Burstein-Moss effect). In the IR portion of the spectrum, this increase in carrier concentration produced a reduction in the transparency window larger than 1μm as a result of the blue shift of the plasma frequency with increasing carrier concentration.
Upon UV illumination, the SnO2 films exhibit a sharp increase in the electrical conductivity, followed by a slow recovery of the initial condition after the light source is removed. The increasing recovery time observed with decreasing air pressure, increasing temperature and carrier concentration suggests that a desorption/adsorption mechanism at the surface of the films is responsible for the PPC observed in SnO2. This mechanism produces an increase in the surface potential and the buit-in electric fields that bends the bands upwards and reduces the recombination rate of photo-generated electrons with holes, thus sustaining the enhanced conductivity of the material.
Finally, SnO2-based diodes presenting rectification ratios of up to 6 orders of magnitude and leak currents in the order of pA were obtained in intrinsic and Sb-doped SnO2 films. When combined with the studies on PPC, these results open the door to new exciting technological applications.