Temperature-dependent thermal conductivity and diffusivity of a Mg-doped insulating β\beta-Ga2O3\mathrm{Ga_2O_3} single crystal along [100], [010] and [001]

The monoclinic crystal structure of $\beta$-$\mathrm{Ga_2O_3}$ leads to significant anisotropy of the thermal properties. The 2$\omega$-method is used to measure the thermal diffusivity $D$ in [010] and [001] direction respectively and to determine the thermal conductivity values $\lambda$ of the [100], [010] and [001] direction from the same insulating Mg doped $\beta$-$\mathrm{Ga_2O_3}$ single crystal. We detect a temperature independent anisotropy factor of both the thermal diffusivity and conductivity values of $D_{[010]}/D_{[001]}=\lambda_{[010]}/\lambda_{[001]}=1.4\pm 0.1$. The temperature-dependence is in accord with phonon-phonon-Umklapp scattering processes from 300 K down to 150 K. Below 150 K point-defect-scattering lowers the estimated phonon-phonon-Umklapp-scattering values.Comment: 11 pages, 8 figure

Growth Techniques for Bulk ZnO and Related Compounds

ZnO bulk crystals can be grown by several methods. 1) From the gas phase, usually by chemical vapor transport. Such CVT crystals may have high chemical purity, as the growth is performed without contact to foreign material. The crystallographic quality is often very high (free growth). 2) From melt fluxes such as alkaline hydroxides or other oxides (MoO3, V2O5, P2O5, PbO) and salts (PbCl2, PbF2). Melt fluxes offer the possibility to grow bulk ZnO under mild conditions (<1000 deg. C, atmospheric pressure), but the crystals always contain traces of solvent. The limited purity is a severe drawback, especially for electronic applications. 3) From hydrothermal fluxes, usually alkaline (KOH, LiOH) aqueous solutions beyond the critical point. Due to the amphoteric character of ZnO, the supercritical bases can dissolve it up to several per cent of mass. The technical requirements for this growth technology are generally hard, but this did not hinder its development as the basic technique for the growth of {\alpha}-quartz, and meanwhile also of zinc oxide, during the last decades. 4) From pure melts, which is the preferred technology for numerous substances applied whenever possible, e.g. for the growth of silicon, gallium arsenide, sapphire, YAG. The benefits of melt growth are (i) the high growth rate and (ii) the absence of solvent related impurities. In the case of ZnO, however, it is difficult to find container materials that are compatible from the thermal (fusion point Tf = 1975 deg. C) and chemical (required oxygen partial pressure) point of view. Either cold crucible (skull melting) or Bridgman (with reactive atmosphere) techniques were shown to overcome the problems that are inherent to melt growth. Reactive atmospheres allow to grow not only bulk ZnO single crystals, but also other TCOs such as {\beta}-Ga2O3 and In2O3.Comment: 10 pages, 7 figures, talk on MRS Fall 2011 Bosto

Thermal conductivity, diffusivity and specific heat capacity of as-grown, degenerate single-crystalline ZnGa2O4

This work provides the first experimental determination of the low-temperature thermal properties for novel highly pure single-crystalline ZnGa2O4. The temperature dependence of the thermal conductivity, diffusivity and specific heat capacity of as-grown, degenerated ZnGa2O4 single crystals is measured using the 2ω-method between T = 27 K and room temperature. At room temperature the thermal diffusivity is D ≈ 6.9 · 10−6 m2s, the thermal conductivity is λ ≈ 22.9 W mK−1 and the specific heat capacity is CV ≈ 537 J kgK−1. The thermal conductivity increases with decreasing temperatures due to reduced phonon-phonon Umklapp scattering down to T = 50 K. For lower temperatures the thermal conductivity is limited by boundary scattering.Deutsche Forschungsgemeinschafthttps://doi.org/10.13039/501100001659Peer Reviewe

Offcut-related step-flow and growth rate enhancement during (100) β\beta-Ga2O3 homoepitaxy by metal-exchange catalyzed molecular beam epitaxy (MEXCAT-MBE)

In this work we investigate the growth of $\beta$-Ga2O3 homoepitaxial layers on top of (100) oriented substrates via indium-assisted metal exchange catalyzed molecular beam epitaxy (MEXCAT-MBE) which have exhibited prohibitively low growth rates by non-catalyzed MBE in the past. We demonstrate that the proper tuning of the MEXCAT growth parameters and the choice of a proper substrate offcut allow for the deposition of thin films with high structural quality via step-flow growth mechanism at relatively high growth rates for $\beta$-Ga2O3 homoepitaxy (i.e., around 1.5 nm/min, $\approx$45% incorporation of the incoming Ga flux), making MBE growth on this orientation feasible. Moreover, through the employment of the investigated four different (100) substrate offcuts along the [00-1] direction (i.e., 0$^\circ$, 2$^\circ$, 4$^\circ$, 6$^\circ$) we give experimental evidence on the fundamental role of the (-201) step edges as nucleation sites for growth of (100)-oriented Ga2O3 films by MBE