Transparent conducting or semiconducting oxides are an important class of materials for (transparent) optoelectronic applications and -by virtue of their wide band gaps -for power electronics. While most of these oxides can be doped n-type only with room-temperature electron mobilities on the order of 100 cm 2 /Vs, p-type oxides are needed for the realization of pn-junction devices but typically suffer from exessively low (< <1 cm 2 /Vs) hole mobilities. Tin monoxide (SnO) is one of the few p-type oxides with a 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 SnO 2 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 Al 2 O 3 (00.1) and Y 2 O 3 -stabilized ZrO 2 (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 function of Sn-to-O-plasma flux ratio and growth temperature. The presence of secondary Sn and SnO x (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 Sn 3 O 4 or Sn as major secondary phases, as well as a fully oxidized SnO 2 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•10 18 to 10 19 cm −3 and 1.0 to 6.0 cm 2 /Vs, respectively, with consistently higher mobility in the single-crystalline films. Temperature-dependent Hall measurements of the single-crystalline films closest to stoichiometric, phase-pure SnO indicate non-degenerate 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 allows us to experimentally estimate the density of states effective hole mass to be in the range of 1 to 8 times ...

The semiconducting oxide β-Gallium Oxide (β-Ga 2 O 3 ) possesses a monoclinic unit cell whose low symmetry generally leads to anisotropic physical properties. For example, its electrical conductivity is generally described by a polar symmetrical tensor of second rank consisting of four independent components. Using van der Pauw measurements in a well-defined square geometry on differently-oriented high-quality bulk samples and the comparison to finite element simulations we precisely determine the ratio of all elements of the β-Ga 2 O 3 3-dimensional electrical conductivity tensor. Despite the structural anisotropy a nearly isotropic conductivity at and above room temperature was found with the principal conductivities deviating from each other by less than 6 % and the off-diagonal element being ≈ 3 % of the diagonal ones. Analysis of the temperature dependence of the anisotropy and mobility of differently doped samples allows us to compare the anisotropy for dominant phonon-scattering to that for dominant ionized-impurity scattering. For both scattering mechanisms, the conductivites along the a and b-direction agree within 2 %. In contrast, the conductivity along c-direction amounts to 0.96× and up to 1.12× that along the b-direction for phonon and ionized impurity scattering, respectively. The determined transport anisotropies are larger than the theoretically predicted effective mass anisotropy, suggesting slightly anisotropic scattering mechanisms. We demonstrate that significantly higher anisotropies can be caused by oriented extended structural defects in the form of low-angle grain boundaries for which we determined energy barriers of multiple 10 meV.

GaP as one of the III–V semiconductors has an indirect band gap in its natural zinc-blend (ZB) crystal phase, limiting its applications in optoelectronics. The atomic arrangements of the ZB GaP, however, can be changed by adding energy to the system, for example, using strain and defects. In such a way, GaP can be crystallized in the wurtzite (WZ) phase with a direct band gap in the yellow–green range and promising new optical properties. GaP nanostructures offer the great possibility to induce strain, and hence, one can expect to obtain the WZ phase by modifying the geometry and dimensionality of GaP. In this work, we present GaP nanowires (NWs) grown on SiO2 substrates by gas-source molecular beam epitaxy. Raman measurements on individual GaP NWs indicate that NWs are poly-type crystal structures with the starting growth of the WZ phase, transforming into the ZB phase, and ending as the WZ phase. Photoluminescence at 9 K from an ensemble of NWs shows emissions at 2.09–2.14 eV, which are related to the direct band gap of the WZ phase and peaks between 2.26 and 2.3 eV due to the ZB phase. The emission of the WZ GaP phase is observable up to 160 K. Cathodoluminescence at 83 K shows directly the emission between 2.09 and 2.14 eV along the single NWs, indicating the presence of the WZ phase. Our results demonstrate the realization of poly-type, ZB, and WZ GaP NWs on SiO2 by gas-source molecular beam epitaxy.

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The transport properties of n- and p-doped AlP layers grown by gas-source molecular beam epitaxy are investigated. n- and p-types of conductivities are achieved using Si and Be with peak room-temperature mobilities of 59.6 cm2/Vs and 65.0 cm2/Vs for electrons and holes, respectively. Si-doping results are then used for the design of n-doped AlP/GaP distributed Bragg reflectors (DBRs) with an ohmic resistance of about 7.5 ± 0.1 Ω. The DBRs are integrated as bottom mirrors in GaP-based light-emitting diodes (LEDs) containing InGaP/GaP quantum dots. The functionality of the LED structure and the influence of the DBRs on the InGaP/GaP electroluminescence spectra are demonstrated.