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- Bergmann, Michael Alexander, 1989, et al.
(författare)
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Electrochemical etching of AlGaN for the realization of thin-film devices
- 2019
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Ingår i: Applied Physics Letters. - : AIP Publishing. - 0003-6951 .- 1077-3118. ; 115:18, s. 182103-
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Tidskriftsartikel (refereegranskat)abstract
- Heterogeneously integrated AlGaN epitaxial layers will be essential for future optical and electrical devices like thin-film flip-chip ultraviolet (UV) light-emitting diodes, UV vertical-cavity surface-emitting lasers, and high-electron mobility transistors on efficient heat sinks. Such AlGaN-membranes will also enable flexible and micromechanical devices. However, to develop a method to separate the AlGaN-device membranes from the substrate has proven to be challenging, in particular, for high-quality device materials, which require the use of a lattice-matched AlGaN sacrificial layer. We demonstrate an electrochemical etching method by which it is possible to achieve complete lateral etching of an AlGaN sacrificial layer with up to 50% Al-content. The influence of etching voltage and the Al-content of the sacrificial layer on the etching process is investigated. The etched N-polar surface shows the same macroscopic topography as that of the as-grown epitaxial structure, and the root-mean square roughness is 3.5 nm for 1 µm x 1 µm scan areas. Separated device layers have a well-defined thickness and smooth etched surfaces. Transferred multi-quantum-well structures were fabricated and investigated by time-resolved photoluminescence measurements. The quantum wells showed no sign of degradation caused by the thin-film process.
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| 2. |
- Jafari, Sonia, et al.
(författare)
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QM/MM study of the stereospecific proton exchange of glutathiohydroxyacetone by glyoxalase I
- 2019
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Ingår i: Results in Chemistry. - : Elsevier BV. - 2211-7156. ; 1
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Tidskriftsartikel (refereegranskat)abstract
- We have performed quantum mechanics (QM), molecular mechanics (MM) and hybrid QM/MM calculations to study the stereospecific proton exchange of glutathiohydroxyacetone (HOC-SG) by glyoxalase I (GlxI). We did the QM/MM calculations with a large QM system (246 atoms) to investigate the proton-exchange mechanism. Moreover, single-point big-QM energies with 1303 atoms in the big QM system and 22,412 atoms in the MM sys- tem were used to compare the energy difference of the stationary structures. GlxI catalyzes the exchange of the pro-S, but not the pro-R hydroxymethyl proton of HOC-SG with a deuterium from the D2O solvent. Classical mo- lecular dynamics simulations with different protonation states of Glu99, Glu172 and HOC-SG led to the determi- nation of most stable species (Glu-172 is protonated and the alcoholic oxygen of HOC-SG is deprotonated). The QM/MM results showed that before binding of HOC-SG, both active-site glutamates are charged, whereas HOC- SG is protonated. When HOC-SG binds, its alcoholic proton (HO) can point toward either Glu-99 or Glu-172. How- ever, if the substrate binds so that HO is directed toward Glu-99, it is not transferred, whereas if it is directed to- ward Glu-172, the latter abstracts HO. The results showed that transferring HO to the glutamates from the reactant states is the key step to make the proton exchange reaction possible. Our calculations show that order of basicity of the glutamates and HOC-SG inside the enzyme is: Glu-172 N HOC-SG N Glu-99. The calculations allow us to propose a reaction mechanism for the stereospecific proton exchange of HOC-SG by GlxI with an over- all barrier of 14.1 kcal/mol.
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