Temperature-Dependent Anisotropic Refractive Index in β-Ga2O3: Application in Interferometric Thermometers

An accurate knowledge of the optical properties of β-Ga2O3 is key to developing the full potential of this oxide for photonics applications. In particular, the dependence of these properties on temperature is still being studied. Optical micro- and nanocavities are promising for a wide range of applications. They can be created within microwires and nanowires via distributed Bragg reflectors (DBR), i.e., periodic patterns of the refractive index in dielectric materials, acting as tunable mirrors. In this work, the effect of temperature on the anisotropic refractive index of β-Ga2O3 n(λ,T) was analyzed with ellipsometry in a bulk crystal, and temperature-dependent dispersion relations were obtained, with them being fitted to Sellmeier formalism in the visible range. Micro-photoluminescence (μ-PL) spectroscopy of microcavities that developed within Cr-doped β-Ga2O3 nanowires shows the characteristic thermal shift of red–infrared Fabry–Perot optical resonances when excited with different laser powers. The origin of this shift is mainly related to the variation in the temperature of the refractive index. A comparison of these two experimental results was performed by finite-difference time-domain (FDTD) simulations, considering the exact morphology of the wires and the temperature-dependent, anisotropic refractive index. The shifts caused by temperature variations observed by μ-PL are similar, though slightly larger than those obtained with FDTD when implementing the n(λ,T) obtained with ellipsometry. The thermo-optic coefficient was calculated.This work was supported by MICINN projects (RTI2018-097195-B-I00, RTI2018-096918-B-C41, PID2021-122562NB-I00 and PID2021-123190OB-I00/AEI/10.13039/501100011033/FEDER, UE). The authors acknowledge the financial support of the excellence research network RED2018-102609-T by MINECO. The authors acknowledge the support from the Air Force Office of Scientific Research under Award No. FA8655-20-1-7013 (Program Manager: Ali Sayir). M.A.-O. acknowledges financial support from MICINN (FPU contract No. FPU15/01982) and thanks the Central Research Development Fund (CRDF) of the University of Bremen for funding (ZF04/2021). J.S.M. and J.J. were supported by the Air Force Office of Scientific Research under award number FA9550–21–1–0507, monitored by Dr. Ali Sayir. Any opinions, finding, and conclusions or recommendations expressed in this material are those of the author and do not necessarily reflect the views of the United States Air Force

DOPING AND ALLOYING OF MONOCLINIC β-Ga2O3 GROWN BY CZOCHRALSKI AND VERTICAL GRADIENT FREEZE

With the future of electronics focusing on reduced packing size and efficient high-power applications, ultrawide bandgap semiconductors such as β-Ga2O3 are at the forefront of modern research. The energy gap of an ultrawide bandgap semiconductor is 2 – 4 eV larger than historically common semiconducting materials. This allows for a variety of unintentionally and intentionally introduced defects and dopants to alter the behavior of the matrix material, which must be understood if any device structure is to be constructed. Once understood, engineering of the material can alter fundamental properties. Therefore, this work seeks to further the understanding of bulk single crystal growth of gallium oxide and its resultant optical, electrical, and structural properties while doped or alloyed with a variety of transition metals and metals. Electrical behavior was modulated through doping from n¬-type conduction (e.g., Zr, Hf) and semiconducting (e.g., unintentionally doped, Cr) to heavily insulating (e.g., Zn, Cu, Ni). A novel optical behavior for semiconductors, photodarkening, was also achieved with Cu- and Ni-doped β-Ga2O3. β-Ga2O3 was also studied with respect to alloying (e.g., Al, Sc), specifically to alter the size of the optical bandgap. Of course, through doping, alloying, growth, and post-growth annealing, defects are generated at the surface or in the bulk of the sample which impact electrical and optical behavior, and these were subsequently studied via Fourier transformed infrared spectroscopy as well as positron annihilation spectroscopy. These studies are contributions to the fast-paced and rapidly developing field of gallium oxide, where future work may build upon and functionalize doped or alloyed β-Ga2O3 single crystals, described within this body of work into devices which are fit for the future age of power electronics and opto-electronics

Reduced magnetic disorder at low temperature in Ca3Co2O6 via zinc substitution

The compound Ca3Co2O6 undergoes a transition into a spin-density wave (SDW) state near 24 K. Below ∼10 K, this unstable SDW state coexists with a nearly- degenerate commensurate antiferromagnetic state as well as short-range magnetic order. Clear signatures of this strong magnetic disorder have been observed in the response of entropy to changing magnetic field and temperature. We performed a calorimetry study of Ca3Co2O6 and Ca3Co1.9Zn0.1O6 in order to compare their entropic responses at low temperature. Our results for Ca3Co2O6 reveal that ΔS(T, H) ≡ S(T, H)−S(T, H = 0) increases as either temperature or magnetic field increase. In contrast, ΔS data for Ca3Co1.9Zn0.1O6 were relatively unresponsive to changes in temperature or field, suggesting that Zn substitution may reduce the low-temperature magnetic disorder observed in Ca3Co2O6. These results are discussed within the context of two cases (Ca3Co2O6 under applied pressure and Ca2.75R0.25Co2O6 (R = Dy, Lu)) in which a single magnetic ground state is stabilised

Effect of extended defects on photoluminescence of gallium oxide and aluminum gallium oxide epitaxial films

In this work, a systematic photoluminescence (PL) study on three series of gallium oxide/aluminum gallium oxide films and bulk single crystals is performed including comparing doping, epitaxial substrates, and aluminum concentration. It is observed that blue/green emission intensity strongly correlates with extended structural defects rather than the point defects frequently assumed. Bulk crystals or Si-doped films homoepitaxially grown on (010) β-Ga2O3 yield an intense dominant UV emission, while samples with extended structural defects, such as gallium oxide films grown on either (-201) β-Ga2O3 or sapphire, as well as thick aluminum gallium oxide films grown on either (010) β-Ga2O3 or sapphire, all show a very broad PL spectrum with intense dominant blue/green emission. PL differences between samples and the possible causes of these differences are analyzed. This work expands previous reports that have so far attributed blue and green emissions to point defects and shows that in the case of thin films, extended defects might have a prominent role in emission properties