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1. Effect of Electron Injection on Minority Carrier Transport in 10 MeV Proton Irradiated β-Ga ₂ O ₃ Schottky Rectifiers

   https://doi.org/10.1149/2162-8777/ab902b  

   Modak, Sushrut ; Chernyak, Leonid ; Khodorov, Sergey ; Lubomirsky, Igor ; Ruzin, Arie ; Xian, Minghan ; Ren, Fan ; Pearton, Stephen J. ( May 2020 , ECS Journal of Solid State Science and Technology)

   We report the effect of extended duration electron beam exposure on the minority carrier transport properties of 10 MeV proton irradiated (fluence ∼10¹⁴cm⁻²) Si-dopedβ-Ga₂O₃Schottky rectifiers. The diffusion length (L) of minority carriers is found to decrease with temperature from 330 nm at 21 °C to 289 nm at 120 °C, with an activation energy of ∼26 meV. This energy corresponds to the presence of shallow Si trap-levels. Extended duration electron beam exposure enhancesLfrom 330 nm to 726 nm at room temperature. The rate of increase forLis lower with increased temperature, with an activation energy of 43 meV. Finally, a brief comparison of the effect of electron injection on proton irradiated, alpha-particle irradiated and a reference Si-dopedβ-Ga₂O₃Schottky rectifiers is presented.
2. Variable temperature probing of minority carrier transport and optical properties in p -Ga ₂ O ₃

   https://doi.org/10.1063/5.0086449  

   Modak, Sushrut ; Chernyak, Leonid ; Schulte, Alfons ; Sartel, Corinne ; Sallet, Vincent ; Dumont, Yves ; Chikoidze, Ekaterine ; Xia, Xinyi ; Ren, Fan ; Pearton, Stephen J. ; et al ( March 2022 , APL Materials)

   Electron beam-induced current in the temperature range from 304 to 404 K was employed to measure the minority carrier diffusion length in metal–organic chemical vapor deposition-grown p-Ga 2 O 3 thin films with two different concentrations of majority carriers. The diffusion length of electrons exhibited a decrease with increasing temperature. In addition, the cathodoluminescence emission spectrum identified optical signatures of the acceptor levels associated with the V Ga − –V O ++ complex. The activation energies for the diffusion length decrease and quenching of cathodoluminescence emission with increasing temperature were ascribed to the thermal de-trapping of electrons from V Ga − –V O ++ defect complexes.
3. Improved thin film quality and photoluminescence of N-doped epitaxial germanium-on-silicon using MOCVD

   https://doi.org/10.1364/OME.10.000001  

   Zhou, Guangnan ; Cuervo Covian, Alejandra V. ; Lee, Kwang Hong ; Han, Han ; Tan, Chuan Seng ; Liu, Jifeng ; Xia, Guangrui ( December 2019 , Optical Materials Express)

   This paper addresses one of the key issues in the scientific community of Si photonics: thin-film quality and the light emission properties of band-engineered n⁺Germanium-on-Silicon (Ge-on-Si). Compared to the traditional delta doping approach, which was utilized in the first electrically-pumped Ge-on-Si lasers, we offer an n⁺Ge-on-Si thin film with better material quality and higher carrier injection efficiency grown by metal-organic chemical vapor deposition (MOCVD). The impacts of thermal cycle annealing and Si substrate offcut on the thin film quality were investigated, including surface roughness, strain, threading dislocation density, Si-Ge interdiffusion, and dopant diffusion. It was revealed that: 1) MOCVD overcomes the outdiffision issue of n-type dopants by having the dopant peaks at the bottom of the Ge films; 2) the characterization of the light emission properties of these MOCVD n⁺Ge-on-Si samples (1.0 × 10¹⁹cm⁻³doped) compared to delta-doped ultra-high vacuum chemical vapor deposition (UHVCVD) Ge, showing comparable photoluminescence (PL) spectral intensity at 1/4 of the doping level; 3) Detailed PL spectral analyses showed that population inversion from the direct gap transition has been achieved, and the injected electron density in the direct Γ valley is comparable to that of the delta-doped sample even though the n-type doping level is 75% less; and 4)more »Experimental evidences that Si-Ge interdiffusion has a much larger impact on PL intensity than threading dislocation density in the range of 10⁸-10⁹/cm³. These results indicate that MOCVD n⁺Ge is very promising to reduce the threshold of Ge gain media on Si notably.

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4. Transport and trap states in proton irradiated ultra-thick κ-Ga2O3

   https://doi.org/10.1116/6.0002673  

   Polyakov, A. Y. ; Nikolaev, V. I. ; Pechnikov, A. I. ; Yakimov, E. B. ; Lagov, P. B. ; Shchemerov, I. V. ; Vasilev, A. A. ; Kochkova, A. I. ; Chernykh, A. V. ; Lee, In-Hwan ; et al ( May 2023 , Journal of Vacuum Science & Technology A)

   Changes induced by irradiation with 1.1 MeV protons in the transport properties and deep trap spectra of thick (>80 μm) undoped κ-Ga2O3 layers grown on sapphire are reported. Prior to irradiation, the films had a donor concentration of ∼1015 cm−3, with the two dominant donors having ionization energies of 0.25 and 0.15 eV, respectively. The main electron traps were located at Ec−0.7 eV. Deep acceptor spectra measured by capacitance-voltage profiling under illumination showed optical ionization thresholds near 2, 2.8, and 3.4 eV. The diffusion length of nonequilibrium charge carriers for ɛ-Ga2O3 was 70 ± 5 nm prior to irradiation. After irradiation with 1.1 MeV protons to a fluence of 1014 cm−2, there was total depletion of mobile charge carriers in the top 4.5 μm of the film, close to the estimated proton range. The carrier removal rate was 10–20 cm−1, a factor of 5–10 lower than in β-Ga2O3, while the concentration of deep acceptors in the lower half of the bandgap and the diffusion length showed no significant change.
5. Intrinsic electronic conductivity of individual atomically resolved amyloid crystals reveals micrometer-long hole hopping via tyrosines

   https://doi.org/10.1073/pnas.2014139118  

   Shipps, Catharine ; Kelly, H. Ray ; Dahl, Peter J. ; Yi, Sophia M. ; Vu, Dennis ; Boyer, David ; Glynn, Calina ; Sawaya, Michael R. ; Eisenberg, David ; Batista, Victor S. ; et al ( December 2020 , Proceedings of the National Academy of Sciences)

   Proteins are commonly known to transfer electrons over distances limited to a few nanometers. However, many biological processes require electron transport over far longer distances. For example, soil and sediment bacteria transport electrons, over hundreds of micrometers to even centimeters, via putative filamentous proteins rich in aromatic residues. However, measurements of true protein conductivity have been hampered by artifacts due to large contact resistances between proteins and electrodes. Using individual amyloid protein crystals with atomic-resolution structures as a model system, we perform contact-free measurements of intrinsic electronic conductivity using a four-electrode approach. We find hole transport through micrometer-long stacked tyrosines at physiologically relevant potentials. Notably, the transport rate through tyrosines (10⁵s⁻¹) is comparable to cytochromes. Our studies therefore show that amyloid proteins can efficiently transport charges, under ordinary thermal conditions, without any need for redox-active metal cofactors, large driving force, or photosensitizers to generate a high oxidation state for charge injection. By measuring conductivity as a function of molecular length, voltage, and temperature, while eliminating the dominant contribution of contact resistances, we show that a multistep hopping mechanism (composed of multiple tunneling steps), not single-step tunneling, explains the measured conductivity. Combined experimental and computational studies reveal that proton-coupled electron transfermore »confers conductivity; both the energetics of the proton acceptor, a neighboring glutamine, and its proximity to tyrosine influence the hole transport rate through a proton rocking mechanism. Surprisingly, conductivity increases 200-fold upon cooling due to higher availability of the proton acceptor by increased hydrogen bonding.

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