The role of phase impurities and lattice defects on the electron dynamics and photochemistry of CuFeO2 solar photocathodes

CuFeO2 is a promising photocathode for H2 evolution and CO2 reduction reactions.To better understand the complex defect chemistry and role of impurity phases in this material and their effect on the photochemical performance,we employ visible light transient absorption spectroscopy and density functional theory(DFT)calculations to investigate the electron dynamics in electrochemically deposited Cu-Fe oxide thin films.Kinetic analysis of carrier lifetime shows a fast,sub-ps contribution to relaxation followed by persistence of a Iong-lived state to time delays greater than 2 ns.Increasing amplitude of the Iong-lived state is shown to correlate with the rate of fast initial relaxation,and this is explained in terms of a competition between charge carrier trapping and charge separation.Charge separation in CuFeO2 occurs via hole thermalizati on from O 2p to Cu 3d vale nee band states leadi ng to segregatio n of electr ons and holes across layers in the CuFeO2 lattice.Correlation between transient absorption measurements and DFT calculations suggest that Cu vacancies enhanee photochemical performance by facilitating charge separation kinetics.In contrast,O interstitials are predicted to switch the relative positions of O 2p and Cu 3d vale nee band states,which would in hibit charge separatio n by in ter-band hole thermal izatio n.Fin ally,we find no evide nee for electron in jecti on from CuFeO2 to CuO suggest!ng that charge separati on at this heterostructure in terface does not play a role in the carrier lifetime or photochemical performance of the catalysts studied here.

Series resistance influence on performance of waveguide-type germanium photodetectors on silicon

We investigate influences of series resistances on the performance of 1.55 μm waveguide-type germanium photodetectors（Ge-PDs） on a silicon-on-insulator substrate. The current-voltage characteristics, responsivities,saturation photo-current characteristics, electrical reflection coefficients, and photodetection frequency responses of Ge-PDs, having different series resistances, are measured, and their equivalent circuit models are established. By analyzing the resulting circuit model parameters, we determine how much Ge-PD series resistances influence Ge-PD saturation photo-currents and photodetection bandwidth. These results should be of great use for optimization of Ge-PD fabrication processes and device parameters for target applications.

Large-signal SPICE model for depletion-type silicon ring modulators

We present an accurate, easy-to-use large-signal SPICE circuit model for depletion-type silicon ring modulators(Si RMs). Our model includes both the electrical and optical characteristics of the Si RM and consists of circuit elements whose values change depending on modulation voltages. The accuracy of our model is confirmed by comparing the SPICE simulation results of 25 Gb/s non-return-to-zero(NRZ) modulation with the measurement. The model is used for performance optimization of monolithically integrated Si photonic NRZ and pulse-amplitudemodulation 4 transmitters in the standard SPICE circuit design environment.

Silicon electronic-photonic integrated 25 Gb/s ring modulator transmitter with a built-in temperature controller

We demonstrate a silicon electronic–photonic integrated 25 Gb/s nonreturn-to-zero transmitter that includes driver circuits,depletion-type Si ring modulator,Ge photodetector,temperature sensor,on-chip heater,and temperature controller,all monolithically integrated on a 0.25μm photonic BiCMOS technology platform.The integrated transmitter successfully provides stable and optimal 25 Gb/s modulation characteristics against external temperature fluctuation.