Plasma-assisted molecular beam epitaxy of SnO(001) films: Metastability, hole transport properties, Seebeck coefficient, and effective hole mass
Among semiconducting materials transparent semiconducting oxides have gained increasing attention within the last decade. While most of these oxides can be only doped n -type with room-temperature electron mobilities on the order of 100 cm2V-1s-1 , p -type oxides are needed for the realization of p n -junction devices but typically suffer from excessively low (≪1 cm2V-1s-1 ) hole mobilities. Tin monoxide (SnO) is one of the few p -type oxides with higher hole mobility, yet is currently lacking a well-established understanding of its hole transport properties. Moreover, growth of SnO is complicated by its metastability with respect to Sn O2 and Sn, requiring epitaxy for the realization of single crystalline material typically required for high-end applications. Here, we give a comprehensive account on the epitaxial growth of SnO, its (meta)stability, and its thermoelectric transport properties in the context of the present literature. Textured and single-crystalline, unintentionally doped p -type SnO(001) films are grown on Al2O3 (00.1) and Y2O3 -stabilized Zr O2(001 ) , respectively, by plasma-assisted molecular beam epitaxy, and the epitaxial relations are determined. The metastability of this semiconducting oxide is addressed theoretically through an equilibrium phase diagram. Experimentally, the related SnO growth window is rapidly determined by an in situ growth kinetics study as a function of Sn-to-O-plasma flux ratio and growth temperature. The presence of secondary Sn and Sn Ox(1 <x ≤2 ) phases is comprehensively studied by x-ray diffraction, Raman spectroscopy, scanning electron microscopy, and x-ray photoelectron spectroscopy, indicating the presence of Sn3O4 or Sn as major secondary phases, as well as a fully oxidized Sn O2 film surface. The hole transport properties, Seebeck coefficient, and density-of-states effective mass are determined and critically discussed in the context of the present literature on SnO, considering its strongly anisotropic effective hole mass: Hall measurements of our films reveal room-temperature hole concentrations and mobilities in the range of 2 ×1018 to 1019 cm-3 and 1.0 to 6.0 cm2V-1s-1 , respectively, with consistently higher mobility in the single-crystalline films. Temperature-dependent Hall measurements of the single-crystalline films indicate nondegenerate band transport by free holes (rather than hopping transport) with dominant polar optical phonon scattering at room temperature. Taking into account the impact of acceptor band formation and the apparent activation of the hole concentration by 40-53 meV, we assign tin vacancies rather than their complexes with hydrogen as the unintentional acceptor. The room-temperature Seebeck coefficient in our films confirms p -type conductivity by band transport. Its combination with the hole concentration and model scattering parameters allows us to experimentally estimate the density of states effective hole mass to be in the range of 1 to 8 times the free electron mass.