Junzhuan Wang

Raman spectroscopy of (Mn, Co)-codoped ZnO films

Raman spectra of (Mn, Co)-codoped ZnO films were investigated as functions of laser line and temperature. It is shown that the Raman shifts for different phonon modes exhibit redshift with temperature increasing, which can be attributed to the anharmonic effect in the material. Strong resonant Raman spectra of multi-LO phonons were observed as well for the films with the exciting photon energy higher than the direct band gap. Compared with the Raman spectra for ZnO films, the Mn, Co codoping effect on the spectra is revealed by the presence of additional phonon modes at 275 and 642cm−1 and another intensive phonon mode at around 524cm−1. With postannealing, Raman intensity of these phonon modes decrease, owing to the incorporation of oxygen into the films. Origins of the different phonon modes in the spectra are discussed as well.

High‐Performance Monolayer WS2 Field‐Effect Transistors on High‐κ Dielectrics

The combination of high-quality Al2 O3 dielectric and thiol chemistry passivation can effectively reduce the density of interface traps and Coulomb impurities, leading to a significant improvement of the mobility and a transition of the charge transport from the insulating to the metallic regime. A record high mobility of 83 cm(2) V(-1) s(-1) (337 cm(2) V(-1) s(-1) ) is reached at room temperature (low temperature) for monolayer WS2 . A theoretical model for electron transport is also developed.

Ferroelectric properties of La and Zr substituted Bi4Ti3O12 thin films

Thin films of A-site substituted, B-site substituted, and both A- and B-sites cosubstituted Bi4Ti3O12 (BTO) by La3+ and Zr4+, i.e., Bi3.25La0.75Ti3O12 (BLT), Bi4Ti2.8Zr0.2O12 (BTZ), and Bi3.25La0.75Ti2.8Zr0.2O12 (BLTZ), were fabricated on Pt/Ti/SiO2/Si substrates by pulsed laser deposition. Structures of the films are investigated by x-ray diffraction, Raman spectroscopy, and scanning electron microscopy. Compared to the well known BLT films, both the BTZ and BLTZ films have larger remanent polarization (Pr) but smaller coercive field (Ec). It is shown experimentally that the oxygen vacancy is the predominant factor determining ferroelectric fatigue. The effects of substitution on structural and ferroelectric properties of BTO are discussed in detail. As a result, the A- and B-sites cosubstitution might be one of the promising ways to improve ferroelectric properties of BTO.

1.54μm photoluminescence emission and oxygen vacancy as sensitizer in Er-doped HfO2 films

In this letter, we report on the characteristics of 1.54μm photoluminescence emission of Er-doped HfO2 films synthesized by pulsed laser deposition and ion implantation. An efficient emission at 1.54μm in the annealed HfO2 films has been observed under a broad band excitation from 400nm to a higher energy at room temperature. X-ray diffraction and electron paramagnetic resonant measurements were used to analyze the correlation between the optical properties and microstructures. An energy transfer mechanism is proposed that the O vacancy in the bulk acts as an effective sensitizer for the neighboring Er ions and greatly enhances the 1.54μm band emission. These results lay an important basis for the future silicon-based integrated optoelectronic device applications of Er-doped HfO2 films.

Ultrafast Solar‐Blind Ultraviolet Detection by Inorganic Perovskite CsPbX3 Quantum Dots Radial Junction Architecture

Inorganic CsPbX3 (X = Cl, Br, I, or hybrid among them) perovskite quantum dots (IPQDs) are promising building blocks for exploring high performance optoelectronic applications. In this work, the authors report a new hybrid structure that marries CsPbX3 IPQDs to silicon nanowires (SiNWs) radial junction structures to achieve ultrafast and highly sensitive ultraviolet (UV) detection in solar-blind spectrum. A compact and uniform deployment of CsPbX3 IPQDs upon the sidewall of low-reflective 3D radial junctions enables a strong light field excitation and efficient down-conversion of the ultraviolet incidences, which are directly tailored into emission bands optimized for a rapid photodetection in surrounding ultrathin radial p-i-n junctions. A fast solar-blind UV detection has been demonstrated in this hybrid IPQD-NW detectors, with rise/fall response time scales of 0.48/1.03 ms and a high responsivity of 54 mA W-1 @200 nm (or 32 mA W-1 @270 nm), without the need of any external power supply. These results pave the way toward large area manufacturing of high performance Si-based perovskite UV detectors in a scalable and low-cost procedure.

Understanding Light Harvesting in Radial Junction Amorphous Silicon Thin Film Solar Cells

The radial junction (RJ) architecture has proven beneficial for the design of a new generation of high performance thin film photovoltaics. We herein carry out a comprehensive modeling of the light in-coupling, propagation and absorption profile within RJ thin film cells based on an accurate set of material properties extracted from spectroscopic ellipsometry measurements. This has enabled us to understand and evaluate the impact of varying several key parameters on the light harvesting in radially formed thin film solar cells. We found that the resonance mode absorption and antenna-like light in-coupling behavior in the RJ cell cavity can lead to a unique absorption distribution in the absorber that is very different from the situation expected in a planar thin film cell, and that has to be taken into account in the design of high performance RJ thin film solar cells. When compared to the experimental EQE response of real RJ solar cells, this modeling also provides an insightful and powerful tool to resolve the wavelength-dependent contributions arising from individual RJ units and/or from strong light trapping due to the presence of the RJ cell array.

Charge retention enhancement in stack nanocrystalline-Si based metal-insulator-semiconductor memory structure

Stack nanocrystalline-Si (nc-Si) based metal-insulator-semiconductor memory structure was put forward and fabricated by plasma-enhanced chemical vapor deposition. The capacitance hysteresis, and asymmetric current peaks were observed at room temperature, which confirm the memory effects and results from the nc-Si. The sharper upward current peak and the broader upward current peak for the annealed sample are explained by resonant tunneling of electrons into stack nc-Si and single nc-Si, respectively. The stack nc-Si has better charge-storage ability than single nc-Si, and a model was put forward to explain the retention mechanism of this stack nc-Si based memory structure.

Structural and optical properties of ZnO films on Si substrates using a gamma-Al2O3 buffer layer

High quality ZnO films have been successfully grown on a Si (100) substrate by metal organic chemical vapour deposition with a gamma-Al2O3 buffer. The crystal structure, surface morphology and optical properties of the ZnO films were characterized by x-ray diffraction, Raman spectroscopy, atomic force microscopy and photoluminescence (PL) spectroscopy. The propel-ties of the films with the Al2O3 buffer were improved in comparison with those of as-grown ZnO films. It is shown that the ZnO films with the gamma-Al2O3 buffer grown on Si (100) substrates have a highly-preferential c-axis (0002) orientation, a narrow (0002) peak, smooth surface morphology and better PL spectral properties. This demonstrates that the use of gamma-Al2O3/Si as a ZnO substrate is beneficial for reducing the residual stress for further growth of ZnO films, compared with the growth on bulk Si substrates.

Quality of ZnSe/GaAs epilayers studied by spatial correlation model of Raman scattering

In this letter, the spatial correlation model has been used for interpreting the line shapes of the first‐order longitudinal‐optical phonon Raman spectra of ZnSe/GaAs epitaxial layers. The good agreement between theoretical line shapes and the experiment measurements was illustrated for the samples grown by the molecular beam epitaxy (MBE), hot wall epitaxy (HWE), and hot wall beam epitaxy (HWBE).

The modulation of nicotinic acetylcholine receptors on the neuronal network oscillations in rat hippocampal CA3 area

γ oscillations are associated with higher brain functions such as memory, perception and consciousness. Disruption of γ oscillations occur in various neuro-psychological disorders such as schizophrenia. Nicotinic acetylcholine receptors (nAChR) are highly expressed in the hippocampus, however, little is known about the role on hippocampal persistent γ oscillation. This study examined the effects of nicotine and selective nAChR agonists and antagonists on kainate-induced persistent γ oscillation in rat hippocampal slices. Nicotine enhanced γ oscillation at concentrations of 0.1–10 μM, but reduced it at a higher concentration of 100 μM. The enhancement on γ oscillation can be best mimicked by co-application of α4β2- and α7- nAChR agonist and reduced by a combination of nAChR antagonists, DhβE and MLA. However, these nAChR antagonists failed to block the suppressing role of nicotine on γ. Furthermore, we found that the NMDA receptor antagonist D-AP5 completely blocked the effect of nicotine. These results demonstrate that nicotine modulates γ oscillations via α7 and α4β2 nAChR as well as NMDA activation, suggesting that nAChR activation may have a therapeutic role for the clinical disorder such as schizophrenia, which is known to have impaired γ oscillation and hypo-NMDA receptor function.