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Register a new userUnderstanding Leakage Currents through $Al_2O_3$ on $SrTiO_3$
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Leakage currents through insulators received continuous attention for decades, owing to their importance for a wide range of technologies, and interest in their fundamental mechanisms. This work investigates the leakage currents through atomic layer deposited (ALD) $Al_2O_3$, grown on $SrTiO_3$. This combination is not only a key building block of oxide electronics, but also a clean system for studying the leakage mechanisms without interfacial layers that form on most of the conventional bottom electrodes. We show how tiny differences in the deposition process can have a dramatic effect on the leakage behavior. Detailed analysis of the leakage behavior rules out Fowler-Nordheim tunneling (FNT) and thermionic emission, and leaves the trap-related mechanisms of trap-assisted tunneling (TAT) and Poole-Frenkel as the likely mechanisms. After annealing the sample in air, the currents are reduced, which is ascribed to transition from trap-based mechanism to FNT, due to the elimination of the traps. The dramatic role of the assumptions regarding the flat-band voltage used for analysis is critically discussed, and the sensitivity of the extracted parameters on this magnitude is quantitatively described. We show that field effect devices based on structures similar to those described here, should be able to modulate $>10^{13} cm^{-2}$ electrons. These results provide general guidelines for reducing and analyzing leakage currents in insulators, and highlight some of the possible approaches and pitfalls in their analysis.
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Transition metal dichalcogenide (TMD) materials have emerged as promising candidates for thin film solar cells due to their wide bandgap range across the visible wavelengths, high absorption coefficient and ease of integration with both arbitrary substrates as well as conventional semiconductor technologies. However, reported TMD-based solar cells suffer from relatively low external quantum efficiencies (EQE) and low open circuit voltage due to unoptimized design and device fabrication. This paper studies $Pt/WSe_2$ vertical Schottky junction solar cells with various $WSe_2$ thicknesses in order to find the optimum absorber thickness.Also, we show that the photovoltaic performance can be improved via $Al_2O_3$ passivation which increases the EQE by up to 29.5% at 410 nm wavelength incident light. The overall resulting short circuit current improves through antireflection coating, surface doping, and surface trap passivation effects. Thanks to the ${Al_2O_3}$ coating, this work demonstrates a device with open circuit voltage ($V_{OC}$) of 380 mV and short circuit current density ($J_{SC}$) of 10.7 $mA/cm^2$. Finally, the impact of Schottky barrier height inhomogeneity at the $Pt/WSe_2$ contact is investigated as a source of open circuit voltage lowering in these devices
Pyroelectric energy converter is a functional capacitor using pyroelectric material as the dielectric layer. Utilizing the first-order phase transformation of the material, the pyroelectric device can generate adequate electricity within small temperature fluctuations. However, most pyroelectric capacitors are leaking during energy conversion. In this paper, we analyze the thermodynamics of pyroelectric energy conversion with consideration of the electric leakage. Our thermodynamic model is verified by experiments using three phase-transforming ferroelectric materials with different pyroelectric properties and leakage behaviors. We demonstrate that the impact of leakage for electric generation is prominent, and sometimes may be confused with the actual power generation by pyroelectricity. We discover an ideal material candidate, (Ba,Ca)(Ti,Zr,Ce)O$_3$, which exhibits large pyroelectric current and extremely low leakage current. The pyroelectric converter made of this material generates 1.95 $mu$A/cm$^2$ pyroelectric current density and 0.2 J/cm$^3$ pyroelectric work density even after 1389 thermodynamic conversion cycles.
Defects are generally regarded to have negative impact on carrier recombination, charge-transport and ion migration in perovskite materials, which thus lower the efficiency and stability of perovskite optoelectronic devices. Meanwhile, lots of efforts which focused on minimizing defects have greatly promoted the application of perovskite materials. Then, can defects be positive in perovskite optoelectronic devices? Herein, relying on in-depth understanding of defect-associated effects in semiconductors, trapping of photo-generated carriers by defects is applied to enlarge photoconductive gain in perovskite photodetector. The record photoconductive gain, gain-bandwidth product and detection limit were achieved in the photodetector. Exceeding the general concept that defects are harmful, we identify a new view that the defects can be positive in perovskite optoelectronic devices.
This novel work investigates the influence of the inspection system acceleration on the leakage signal in magnetic flux leakage type of non-destructive testing. The research is addressed both through designed experiments and simulations. The results showed that the leakage signal, represented by using peak to peak value, decreases between 20% and 30% under acceleration. The simulation results indicated that the main reason for the decrease is due to the difference in the distortion of the magnetic field for cases with and without acceleration, which is the result of the different eddy current distributions in the specimen. The findings will help to allow the optimisation of the MFL system to ensure the main defect features can be measured accurately during the machine acceleration. It also shows the importance of conducting measurements at constant velocity, wherever possible.
We have investigated in detail the growth dynamics of gold nanorods with various aspect ratios in different surrounding environments. Surprisingly, a blue shift in the temporal evolution of colloidal gold nanorods in aqueous medium has been observed during the growth of nanorods by UV visible absorption spectroscopy. The longitudinal surface plasmon resonance peak evolves as soon as the nanorods start to grow from spheres, and the system undergoes a blue shift in the absorption spectra. Although a red-shift is expected as a natural phenomenon during the growth process of all nanosystems, our blue shift observation is regarded as a consequence of competition between the parameters of growth solution and actual growth of nanorods. The growth of nanorods contributes to the red-shift which is hidden under the dominating contribution of the growth solution responsible for the observed massive blue shift.
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