Qiao Zheng

Plasma enhanced multistate storage capability of single ZnO nanowire based memory

Multiple-state storage (MSS) is common for resistive random access memory, but the effects of plasma treatment on the MSS and the switching properties have been scarcely investigated. We have demonstrated a stable four-state storage capability of single zinc oxide nanowire (ZnO NW) treated by argon plasma. The electrical switching is attributed to the electron trapping and detrapping from the oxygen vacancies (Vos). The MSS relates to the electrical-thermal induced distribution of the Vos which determines electron transport behavior to show different resistance states. Additionally, programming (set and reset) voltages decrease with plasma treatment due to the thickness modulation of the interface barrier.

Resistive Switching of Plasma-Treated Zinc Oxide Nanowires for Resistive Random Access Memory.

ZnO nanowires (NWs) were grown on Si(100) substrates at 975 °C by a vapor-liquid-solid method with ~2 nm and ~4 nm gold thin films as catalysts, followed by an argon plasma treatment for the as-grown ZnO NWs. A single ZnO NW–based memory cell with a Ti/ZnO/Ti structure was then fabricated to investigate the effects of plasma treatment on the resistive switching. The plasma treatment improves the homogeneity and reproducibility of the resistive switching of the ZnO NWs, and it also reduces the switching (set and reset) voltages with less fluctuations, which would be associated with the increased density of oxygen vacancies to facilitate the resistive switching as well as to average out the stochastic movement of individual oxygen vacancies. Additionally, a single ZnO NW–based memory cell with self-rectification could also be obtained, if the inhomogeneous plasma treatment is applied to the two Ti/ZnO contacts. The plasma-induced oxygen vacancy disabling the rectification capability at one of the Ti/ZnO contacts is believed to be responsible for the self-rectification in the memory cell.

Tuning of Rashba/Dresselhaus Spin Splittings by Inserting Ultra-Thin InAs Layers at Interfaces in Insulating GaAs/AlGaAs Quantum Wells

The ratio of Rashba and Dresselhaus spin splittings of the (001)-grown GaAs/AlGaAs quantum wells (QWs), investigated by the spin photocurrent spectra induced by circular photogalvanic effect (CPGE) at inter-band excitation, has been effectively tuned by changing the well width of QWs and by inserting a one-monolayer-thick InAs layer at interfaces of GaAs/AlGaAs QWs. Reflectance difference spectroscopy (RDS) is also employed to study the interface asymmetry of the QWs, whose results are in good agreement with that obtained by CPGE measurements. It is demonstrated that the inserted ultra-thin InAs layers will not only introduce structure inversion asymmetry (SIA), but also result in additional interface inversion asymmetry (IIA), whose effect is much stronger in QWs with smaller well width. It is also found that the inserted InAs layer brings in larger SIA than IIA. The origins of the additional SIA and IIA introduced by the inserted ultra-thin InAs layer have been discussed.

Ultralow switching current in HfOx/ZnO bilayer with tunable switching power enabled by plasma treatment

Decreasing switching power of a memory cell to meet demands of further downsizing is feasible with several methods. However, effects of plasma treatment on switching current and power are scarcely investigated. We therefore replaced traditional single storage layer with a HfOx/ZnO bilayer and also treated its interface with argon plasma. The switching current could be suppressed to μA due to a Schottky barrier at the HfOx/ZnO interface. Additionally, argon plasma treatment on the interface enables tunability of switching power and current, which is attributed to the tunable barrier height with the absorbed oxygen species introduced by plasma treatment.

Temperature dependence of spin photocurrent spectra induced by Rashba- and Dresselhaus-type circular photogalvanic effect at inter-band excitation in InGaAs/AlGaAs quantum wells

Spin photocurrent spectra induced by Rashba- and Dresselhaus-type circular photogalvanic effect (CPGE) at inter-band excitation have been experimentally investigated in InGaAs/AlGaAs quantum wells at a temperature range of 80 to 290 K. It is found that, the sign of Rashba-type current reverses at low temperatures, while that of Dresselhaus-type remains unchanged. The temperature dependence of ratio of Rashba and Dresselhaus spin-orbit coupling parameters, increasing from -6.7 to 17.9, is obtained, and the possible reasons are discussed. We also develop a model to extract the Rashba-type effective electric field at different temperatures. It is demonstrated that excitonic effect will significantly influence the Rashba-type CPGE, while it has little effect on Dresselhaus-type CPGE.

Effect of deposited temperatures of the buffer layer on the band offset of CZTS/In2S3heterostructure and its solar cell performance

The effect of the deposition temperature of the buffer layer In2S3 on the band alignment of CZTS/In2S3 heterostructures and the solar cell performance have been investigated. The In2S3 films are prepared by thermal evaporation method at temperatures of 30, 100, 150, and 200 °C, respectively. By using x-ray photoelectron spectroscopy (XPS), the valence band offsets (VBO) are determined to be , , , and eV for the CZTS/In2S3 heterostructures deposited at 30, 100, 150, and 200 °C, respectively, and the corresponding conduction band offsets (CBO) are found to be , , , and eV, respectively. The XPS study also reveals that inter-diffusion of In and Cu occurs at the interface of the heterostructures, which is especially serious at 200 °C leading to large amount of interface defects or the formation of CuInS2 phase at the interface. The CZTS solar cell with the buffer layer In2S3 deposited at 150 °C shows the best performance due to the proper CBO value at the heterostructure interface and the improved crystal quality of In2S3 film induced by the appropriate deposition temperature. The device prepared at 100 °C presents the poorest performance owing to too high a value of CBO. It is demonstrated that the deposition temperature is a crucial parameter to control the quality of the solar cells.

Tuning of in-plane optical anisotropy by inserting ultra-thin InAs layer at interfaces in (001)-grown GaAs/AlGaAs quantum wells

The in-plane optical anisotropy (IPOA) in (001)-grown GaAs/AlGaAs quantum wells (QWs) with different well widths varying from 2 nm to 8 nm has been studied by reflectance difference spectroscopy. Ultra-thin InAs layers with thickness ranging from 0.5 monolayer (ML) to 1.5 ML have been inserted at GaAs/AlGaAs interfaces to tune the asymmetry in the QWs. It is demonstrated that the IPOA can be accurately tailored by the thickness of the inserted ultra-thin InAs layer at the interfaces. Strain-induced IPOA has also been extracted by using a stress apparatus. We find that the intensity of the strain-induced IPOA decreases with the thickness of the inserted InAs layer, while that of the interface-induced IPOA increases with the thickness of the InAs layer. Theoretical calculations based on 6 band k ⋅ p theory have been carried out, and good agreements with experimental results are obtained. Our results demonstrate that, the IPOA of the QWs can be greatly and effectively tuned by inserting an ultra-thin InAs layer with different thicknesses at the interfaces of QWs, which does not significantly influence the transition energies and the transition probability of QWs.

Spin photocurrent spectra induced by Rashba- and Dresselhaus-type circular photogalvanic effect at inter-band excitation in InGaAs/GaAs/AlGaAs step quantum wells

Spin photocurrent spectra induced by Rashba- and Dresselhaus-type circular photogalvanic effect (CPGE) at inter-band excitation have been experimentally investigated in InGaAs/GaAs/AlGaAs step quantum wells (QWs) at room temperature. The Rashba- and Dresselhaus-induced CPGE spectra are quite similar with each other during the spectral region corresponding to the transition of the excitonic state 1H1E (the first valence subband of heavy hole to the first conduction subband of electrons). The ratio of Rashba- and Dresselhaus-induced CPGE current for the transition 1H1E is estimated to be 8.8±0.1, much larger than that obtained in symmetric QWs (4.95). Compared to symmetric QWs, the reduced well width enhances the Dresselhaus-type spin splitting, but the Rashba-type spin splitting increases more rapidly in the step QWs. Since the degree of the segregation effect of indium atoms and the intensity of build-in field in the step QWs are comparable to those in symmetric QWs, as proved by reflectance difference and photoreflectance spectra, respectively, the larger Rashba-type spin splitting is mainly induced by the additional interface introduced by step structures.

Efficient (Cu1−xAgx)2ZnSn(S,Se)4 solar cells on flexible Mo foils

Cation substitution plays a crucial role in improving the efficiency of Cu2ZnSn(S,Se)4 (CZTSSe) solar cells. In this work, we report a significant efficiency enhancement of flexible CZTSSe solar cells on Mo foils by partial substitution of Cu+ with Ag+. It is found that the band gap (Eg) of (Cu1−xAgx)2ZnSn(S,Se)4 (CAZTSSe) thin films can be adjusted by doping with Ag with x from 0 to 6%, and the minimum Eg is achieved with x = 5%. We also found that Ag doping can obviously increase the average grain size of the CAZTSSe absorber from 0.4 to 1.1 μm. Additionally, the depletion width (Wd) at the heterojunction interface of CAZTSSe/CdS is found to be improved. As a result, the open-circuit voltage deficit (Voc,def) is gradually decreased, and the band tailing is suppressed. Benefiting from the enhanced open-circuit voltage (Voc), the power conversion efficiency (PCE) is successfully enhanced from 4.34% (x = 0) to 6.24% (x = 4%), and the Voc,def decreases from 915 to 848 mV.

Observation of Extrinsic Photo-Induced Inverse Spin Hall Effect in a GaAs/AlGaAs Two-Dimensional Electron Gas

The inverse spin Hall effect induced by circularly polarized light has been observed in a GaAs/AlGaAs two-dimensional electron gas. The spin transverse force has been determined by fitting the photo-induced inverse spin Hall effect (PISHE) current to a theoretical model. The PISHE current is also measured at different light power and different light spot profiles, and all the measurement results are in good agreement with the theoretical calculations. We also measure the PISHE current at different temperatures (i.e., from 77 to 300 K). The temperature dependence of the PISHE current indicates that the extrinsic mechanism plays a dominant role, which is further confirmed by the weak dependence of the PISHE current on the crystal orientation of the sample.