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1. Metalorganic chemical vapor deposition of (100) β-Ga ₂ O ₃ on on-axis Ga ₂ O ₃ substrates

   https://doi.org/10.1116/6.0002179  

   Meng, Lingyu; Bhuiyan, A F; Feng, Zixuan; Huang, Hsien-Lien; Hwang, Jinwoo; Zhao, Hongping (December 2022, Journal of Vacuum Science & Technology A)

   Metalorganic chemical vapor deposition (MOCVD) growths of β-Ga 2 O 3 on on-axis (100) Ga 2 O 3 substrates are comprehensively investigated. Key MOCVD growth parameters including growth temperature, pressure, group VI/III molar flow rate ratio, and carrier gas flow rate are mapped. The dependence of the growth conditions is correlated with surface morphology, growth rate, and electron transport properties of the MOCVD grown (100) β-Ga 2 O 3 thin films. Lower shroud gas (argon) flow is found to enhance the surface smoothness with higher room temperature (RT) electron Hall mobility. The growth rate of the films decreases but with an increase of electron mobility as the VI/III molar flow rate ratio increases. Although no significant variation on the surface morphologies is observed at different growth temperatures, the general trend of electron Hall mobilities are found to increase with increasing growth temperature. The growth rates reduce significantly with uniform surface morphologies as the chamber pressure increases. By tuning the silane flow rate, the controllable carrier concentration of (100) β-Ga 2 O 3 thin films between low-10 17  cm −3 and low-10 18  cm −3 was achieved. Under optimized growth condition, an (100) β-Ga 2 O 3 thin film with RMS roughness value of 1.64 nm and a RT mobility of 24 cm 2 /Vs at a carrier concentration of 7.0 × 10 17  cm −3 are demonstrated. The mobilities are primarily limited by the twin lamellae and stacking faults defects generated from the growth interface. Atomic resolution scanning transmission electron microscopy reveals the formation of twin boundary defects in the films, resulting in the degradation of crystalline quality. Results from this work provide fundamental understanding of the MOCVD epitaxy of (100) β-Ga 2 O 3 on on-axis Ga 2 O 3 substrates and the dependence of the material properties on growth conditions. The limitation of electron transport properties of the (100) β-Ga 2 O 3 thin films below 25 cm 2 /Vs is attributed to the formation of incoherent boundaries (twin lamellae) and stacking faults grown along the on-axis (100) crystal orientation. 

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2. Coexistence of vitreous and crystalline phases of H ₂ O at ambient temperature

   https://doi.org/10.1073/pnas.2117281119  

   Shargh, Ali K.; Picard, Aude; Hrubiak, Rostislav; Zhang, Dongzhou; Hemley, Russell J.; Deemyad, Shanti; Abdolrahim, Niaz; Saffarian, Saveez (July 2022, Proceedings of the National Academy of Sciences)

   Formation of vitreous ice during rapid compression of water at room temperature is important for biology and the study of biological systems. Here, we show that Raman spectra of rapidly compressed water at greater than 1 GPa at room temperature exhibits the signature of high-density amorphous ice, whereas the X-ray diffraction (XRD) pattern is dominated by crystalline ice VI. To resolve this apparent contradiction, we used molecular dynamics simulations to calculate full vibrational spectra and diffraction patterns of mixtures of vitreous ice and ice VI, including embedded interfaces between the two phases. We show quantitatively that Raman spectra, which probe the local polarizability with respect to atomic displacements, are dominated by the vitreous phase, whereas a small amount of the crystalline component is readily apparent by XRD. The results of our combined experimental and theoretical studies have implications for detecting vitreous phases of water, survival of biological systems under extreme conditions, and biological imaging. The results provide additional insight into the stable and metastable phases of H 2 O as a function of pressure and temperature, as well as of other materials undergoing pressure-induced amorphization and other metastable transitions. 

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3. Doping and compensation in heavily Mg doped Al-rich AlGaN films

   https://doi.org/10.1063/5.0082992  

   Bagheri, Pegah; Klump, Andrew; Washiyama, Shun; Hayden Breckenridge, M.; Kim, Ji Hyun; Guan, Yan; Khachariya, Dolar; Quiñones-García, Cristyan; Sarkar, Biplab; Rathkanthiwar, Shashwat; et al (February 2022, Applied Physics Letters)

   Record low resistivities of 10 and 30 Ω cm and room-temperature free hole concentrations as high as 3 × 10¹⁸ cm⁻³were achieved in bulk doping of Mg in Al_0.6Ga_0.4N films grown on AlN single crystalline wafer and sapphire. The highly conductive films exhibited a low ionization energy of 50 meV and impurity band conduction. Both high Mg concentration (>2 × 10¹⁹cm⁻³) and low compensation were required to achieve impurity band conduction and high p-type conductivity. The formation of V_N-related compensators was actively suppressed by chemical potential control during the deposition process. This work overcomes previous limitations in p-type aluminum gallium nitride (p-AlGaN) and offers a technologically viable solution to high p-conductivity in AlGaN and AlN. 

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4. Carrier density control in Cu ₂ HgGeTe ₄ and discovery of Hg ₂ GeTe ₄ via phase boundary mapping

   https://doi.org/10.1039/c8ta10332a  

   Ortiz, Brenden R.; Gordiz, Kiarash; Gomes, Lídia C.; Braden, Tara; Adamczyk, Jesse M.; Qu, Jiaxing; Ertekin, Elif; Toberer, Eric S. (January 2019, Journal of Materials Chemistry A)

   The optimization and application of new functional materials depends critically on our ability to manipulate the charge carrier density. Despite predictions of good n-type thermoelectric performance in the quaternary telluride diamond-like semiconductors ( e.g. Cu 2 HgGeTe 4 ), our prior experimental survey indicates that the materials exhibit degenerate p-type carrier densities (>10 20 h + cm −3 ) and resist extrinsic n-type doping. In this work, we apply the technique of phase boundary mapping to the Cu 2 HgGeTe 4 system. We begin by creating the quaternary phase diagram through a mixture of literature meta-analysis and experimental synthesis, discovering a new material (Hg 2 GeTe 4 ) in the process. We subsequently find that Hg 2 GeTe 4 and Cu 2 HgGeTe 4 share a full solid solution. An unusual affinity for Cu Hg and Hg Cu formation within Cu 2 HgGeTe 4 leads to a relatively complex phase diagram, rich with off-stoichiometry. Through subsequent probing of the fourteen pertinent composition-invariant points formed by the single-phase region, we achieve carrier density control ranging from degenerate (>10 21 h + cm −3 ) to non-degenerate (<10 17 h + cm −3 ) via manipulation of native defect formation. Furthermore, this work extends the concept of phase boundary mapping into the realm of solid solutions and clearly demonstrates the efficacy of the technique as a powerful experimental tool within complex systems. 

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5. Resistance drift of metastable amorphous and crystalline fcc GeSbTe memory devices

   Silva, H.; Noor, N.; Tripathi, S.; Carter, C. B. (March 2019, APS March Meeting)

   Phase‐change memory is an emerging technology that utilizes the electrical resistivity contrast between the amorphous and crystalline phases of chalcogenide glasses to store data. The most commonly used material for PCM has been GeSbTe (GST), which has metastable amorphous and crystalline fcc phases and a stable crystalline hcp phase [1]. One difficulty with the implementation of PCM is the upward resistance drift of the metastable amorphous and crystalline fcc phases. We are using electrical characterization together with transmission electron microscopy and finite‐element electrothermal simulations [2] to study the physical mechanisms that give rise to the electrical resistance drift of GST cells. 

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