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The interfacial properties of a planar SnO/κ-Ga2O3p–n heterojunction have been investigated by capacitance–voltage (C–V) measurements following a methodological approach that allows consideration of significant combined series resistance and parallel leakage effects. Single-frequency measurements were carried out in both series- and parallel-model measurement configurations and then compared to the dual-frequency approach, which permits us to evaluate the depletion capacitance of diode independently of leakage conductance and series resistance. It was found that in the bias region, where the dissipation factor was low enough, they give the same results and provide reliable experimental C–Vdata. The doping profile extracted from the C–Vdata shows a nonuniformity at the junction interface that was attributed to a depletion of subsurface net donors at the n-side of the diode. This attribution was corroborated by doping profiles and carrier distributions in the n and p sides of the heterojunction obtained from the simulation of the measured C–Vdata by the Synopsys Sentaurus-TCAD suite. Hall effect measurements and Hg-probe C–Vinvestigation on single κ-Ga2O3layers, either as-grown or submitted to thermal treatments, support the hypothesis of the subsurface donor reduction during the SnO deposition. This study can shed light on the subsurface doping density variation in κ-Ga2O3due to high-temperature treatment. The investigation of the SnO/κ-Ga2O3heterointerface provides useful hints for the fabrication of diodes based on κ-Ga2O3. The methodological approach presented here is of general interest for reliable characterization of planar diodes.

The crystal structure and ferroelectric properties of ε-Ga2O3deposited by low-temperature MOCVD on (0001)-sapphire were investigated by single-crystal X-ray diffraction and the dynamic hysteresis measurement technique. A thorough investigation of this relatively unknown polymorph of Ga2O3showed that it is composed of layers of both octahedrally and tetrahedrally coordinated Ga3+sites, which appear to be occupied with a 66% probability. The refinement of the crystal structure in the noncentrosymmetric space group P63mcpointed out the presence of uncompensated electrical dipoles suggesting ferroelectric properties, which were finally demonstrated by independent measurements of the ferroelectric hysteresis. A clear epitaxial relation is observed with respect to the c-oriented sapphire substrate, with the Ga2O3[10–10] direction being parallel to the Al2O3direction [11–20], yielding a lattice mismatch of about 4.1%.

In this paper, we focus on the growth of β- and ε/κ-Ga2O3thin films via metal–organic vapor phase epitaxy on c-plane sapphire using water and trimethyl-gallium at temperatures between 610 and 650 °C. Using these precursors, the monoclinic β-phase is usually obtained only at temperatures higher than 700 °C. We show here, for the first time, that both β- and ε-Ga2O3can also be obtained by tuning the growth rate of the film, that is, by controlling the supersaturation of the vapor phase. The experimental findings are discussed in the framework of classical nucleation theory and Ostwald’s step rule, showing the interplay of thermodynamic (related to different chemical potentials for the metastable ε/κ phase and the stable β phase) and kinetic effects (mainly related to different surface energy barriers for nuclei of different crystallographic phases/planes). The experimental conditions that permit the nucleation and growth of the desired Ga2O3polymorph are identified and thoroughly explained, giving to this work a fundamental as well as a technological relevance.

In this work, the conduction mechanisms across novel contacts to epitaxial films of pure phase epsilon-Ga2O3(ε-Ga2O3) were investigated. Different structures made by sputtered metal and oxide thin films were tested as electrical contacts. I-V characteristics show heterogeneous behaviors, revealing different conduction mechanisms according to the applied bias. The results are interesting as they offer a viable method to obtain ohmic contacts on ε-Ga2O3, which is less studied than other gallium oxide polymorphs but may find application in new electronic and optoelectronic devices. The newly developed ohmic contacts allow to fabricate simple test devices and assess the potential of this material.

The lattice parameters of monoclinic ?-Ga2O3were determined by powder diffraction in a wide temperature range. The experiments provide a quantitative evaluation of the thermal expansion of the unit cell. It was observed that the expansion of the monoclinic cell is not isotropic. These data are of great technological interest for the deposition of ?-Ga2O3films on foreign substrates as well as for the deposition of different materials (such as GaN) on Ga2O3substrates.