Yuanqing Chen

Observation of electronic Raman scattering from Mg-doped wurtzite GaN

Electronic Raman scattering experiments have been carried out on both molecular beam epitaxy and metal-organic chemical vapor deposition-grown Mg-doped wurtzite GaN samples. Aside from the expected Raman lines, a broad structure (full width at half maximum≅15 cm−1) observed at around 841 cm−1 is attributed to the electronic Raman scattering from neutral Mg impurities in Mg-doped GaN. Our experimental results demonstrate that the energy between the ground and first excited states of Mg impurities in wurtzite GaN is about one-half of its binding energy.

Fabrication of PZT/CuO composite films and their photovoltaic properties

AbstractThe existence of the Schottky barriers at the top and bottom electrodes of the ferroelectric thin film sandwich structure makes it difficult to separate and collect electron-hole pairs, thus limiting the enhancement of the photocurrent. In this paper, Pb(Zr,Ti)O3 (PZT) and composite structure of PZT/CuO films are prepared by a sol-gel method and their photovoltaic properties have been investigated. It is found that the PZT/CuO films show a short circuit photocurrent density (JSC) enhanced by nearly 6 times and power conversion efficiency (PCE) increased by six-fold when compared to those of the PZT film. The increase of photovoltaic response is due to the internal electric field of PZT/CuO p–n junction, which plays an important role in driving the photo-generated carriers. The Ohmic contact between the interfaces of LNO/PZT and CuO/Pt also reduce the resistance of the transportation of photogenerated carriers. Furthermore, the JSC of PZT/CuO film are observed to be 0.03 and 0.013 mA/cm2 after upward poling and downward poling, respectively, indicating that the photocurrent can be modulated by the direction of the polarization electric field. The photovoltaic effect of composite films and its potential mechanism are also explored. This work provides an efficient approach to develop ferroelectric film based on photovoltaic devices. It is found that the PZT/CuO films show a short circuit photocurrent density (JSC) enhanced by nearly 6 times and power conversion efficiency (PCE) increased by six-fold when compared to those of the PZT film. The increase of photovoltaic response is due to the internal electric field of PZT/CuO p–n junction which plays an important role in driving the photogenerated carriers.HighlightsPZT/CuO composite films are prepared using sol-gel methodThe photocurrent is increased when PZT is combined with CuOThe transport mechanism of the photoelectrons in PZT/CuO composite films is explored

Effect of La/Zr ratio in the precursor solution on the properties of La2Zr2O7 and CeO2/La2Zr2O7 films

La2Zr2O7 (LZO) films and CeO2/LZO composite films were prepared on the biaxially textured NiW tapes using metal organic deposition method. Effect of molar ratio of La/Zr in the precursor solution on the texture and surface morphology of LZO films and composite CeO2/LZO films was investigated systemically. It was found that the content of lanthanum played an important role in the orientation control of the LZO film. Finally, the LZO film with ratio of La/Zr = 1.1/1 was obtained with the best properties as compared to others. And highly textured LZO-11 and CeO2/LZO-11 films with smooth surface were obtained on the NiW tape. YBa2Cu3O7-x (YBCO) films with enhanced critical current density (Jc) were obtained on the optimized CeO2/LZO buffer layer.Graphical AbstractThe LZO films were prepared from different LZO solutions with different molar ratio of La/Zr, and the effects of the La/Zr ratio on the texture and surface morphology of LZO films were systemically investigated. The results confirmed that the content of lanthanum in solution was closely related to the orientation control of the LZO film. When the ratio of La/Zr was smaller than La/Zr = 1, the pure (l00)-oriented phase was not achieved. When that was larger than La/Zr = 1.2, in spite of the pure (l00)-oriented phase, the more lanthanum content increased the lattice parameter of the LZO films. In summary, the ratio of La/Zr = 1.1 was the optimized ratio for the fabrication of LZO film due to the best texture and surface morphology.

Picosecond Raman scattering studies of nonequilibrium electron distributions and energy-loss rate in InxGa1−xAs1−yNy

Nonequilibrium electron distributions and energy-loss rate in a metal–organic chemical-vapordeposition-grown InxGa1−xAs1−yNy (x=0.03 and y=0.01) epilayer on a GaAs substrate have been studied by picosecond Raman spectroscopy. It is demonstrated that for electron density n≅1018 cm−3, electron distributions can be described very well by Fermi–Dirac distributions with electron temperatures substantially higher than the lattice temperature. From the measurement of electron temperature as a function of the pulse width of the excitation laser, the energy-loss rate in InxGa1−xAs1−yNy is estimated to be 64 meV/ps. These experimental results are compared with those of GaAs.

Ultraviolet-assisted direct patterning and low-temperature formation of flexible ZrO2 resistive switching arrays on PET/ITO substrates.

We demonstrate a low-cost and facile photochemical solution method to prepare the ZrO2 resistive switching arrays as memristive units on flexible PET/ITO substrates. ZrO2 solution sensitive to UV light of 337 nm was synthesized using zirconium n-butyl alcohol as the precursor, and benzoylacetone as the complexing agent. After the dip-coated ZrO2 gel films were irradiated through a mask under the UV lamp (with wavelength of 325-365 nm) at room temperature and rinsed in ethanol, the ZrO2 gel arrays were obtained on PET/ITO substrates. Subsequently, the ZrO2 gel arrays were irradiated by deep UV light of 254 and 185 nm at 150 °C, resulting in the amorphous ZrO2 memristive micro-arrays. The ZrO2 units on flexible PET/ITO substrates exhibited excellent memristive properties. A high ratio of 104 of on-state and off-state resistance was obtained. The resistive switching behavior of the flexible device remained stable after being bent for 103 times. The device showed stable flexibility up to a minimum bending diameter of 1.25 cm.

Resistive Switching Characteristics of Flexible TiO 2 Thin Film Fabricated by Deep Ultraviolet Photochemical Solution Method

A novel ultraviolet photochemical method was used to prepare TiO2 resistive-switching films. Amorphous TiO2 films were formed on flexible indium-tin oxide (ITO) coated polyethylene terephthalate (PET) substrates by deep ultraviolet irradiation at 150 °C. A Pt/TiO2/ITO/PET device was then fabricated to investigate bipolar resistive switching of the films for potential application in non-volatile memories. The ratio of on-state to off-state currents was measured, and a good value of 1000 was obtained. The retention and switch-cycling characteristics of the device were investigated for different bending radii. The resistive switching behavior of the flexible device remained stable after 600 cycles of electrical switching and 1000 cycles of bending.

Polarization dependent ferroelectric photovoltaic effects in BFTO/CuO thin films

Bi5FeTi3O15 (BFTO) and BFTO/CuO films were deposited by a sol-gel technique, which exhibited macroscopic ferroelectric properties. It was found that the BFTO/CuO films showed a short circuit photocurrent density (Jsc) enhanced by nearly 10 times and power conversion efficiency increased by 13-fold compared to those of the BFTO film. The significant increase in the photovoltaic (PV) response may be attributed to the p-n junction internal electric field acting as the driving force of photogenerated carriers. Furthermore, both BFTO and BFTO/CuO films indicated a switchable PV response in both polarities. The open circuit voltage (Voc) and Jsc for BFTO and BFTO/CuO were observed to be −0.59 V and +43.88 μA/cm2 and −0.23 V and +123.16 μA/cm2, respectively, after upward poling, whereas after downward poling, +0.11 V and −6.26 μA/cm2 and +0.17 V and −83.21 μA/cm2 for BFTO and BFTO/CuO were observed, respectively. The switchable PV responses were explained by the ferroelectric depolarization field, whose directio...

Resistive switching IGZO micro-arrays realized through UV assisted photochemical solution method

AbstractA low-cost and facile two-step UV photolysis method was used to prepare the InGaZnO (IGZO) resistive switching films and micro-arrays. Using benzoylacetone (BzAc) as the complexing agent, we synthesized a unique IGZO solution sensitive to the UV light of 328 nm. We found that high-performance resistive switching IGZO amorphous films could be obtained by UV irradiation (325~365 nm) at room temperature (first step), and further exposure to deep UV light of 185 and 254 nm at 150 °C (second step). We found that the first and the second steps of the UV soaking play different roles in the formation process of amorphous IGZO films. Combined with the rinsing process, patterned IGZO micro-arrays acting as memristive units were also obtained using this two-step UV photolysis process. The IGZO micro-arrays with a high ratio of 104 of on-state and off-state resistance were obtained. We synthesized a unique IGZO solution sensitive to the UV light of 328 nm. Using a UV irradiation (325~365 nm) technique at room temperature, IGZO gel film arrays were obtained. Further exposure to deep UV light of 185 and 254 nm at 150 °C made the IGZO arrays exhibit excellent resistive switching properties.HighlightsA novel two-step photolysis solution method was developed to prepare IGZO arrays.The UV irradiation under light of 325~365 nm produced the IGZO gel film arrays.The irradiation under deep UV light enhanced the resistive switching properties of the IGZO films.