Shuying Cheng

Polarized Raman scattering of single ZnO nanorod

Polarized Raman scattering measurement on single wurtzite c-plane (001) ZnO nanorod grown by hydrothermal method has been performed at room temperature. The polarization dependence of the intensity of the Raman scattering for the phonon modes A1(TO), E1(TO), and E2high in the ZnO nanorod are obtained. The deviations of polarization-dependent Raman spectroscopy from the prediction of Raman selection rules are observed, which can be attributed to the structure defects in the ZnO nanorod as confirmed by the comparison of the transmission electron microscopy, photoluminescence spectra as well as the polarization dependent Raman signal of the annealed and unannealed ZnO nanorod. The Raman tensor elements of A1(TO) and E1(TO) phonon modes normalized to that of the E2high phonon mode are |a/d|=0.32±0.01,u2009|b/d|=0.49±0.02, and |c/d|=0.23±0.01 for the unannealed ZnO nanorod, and |a/d|=0.33±0.01,u2009|b/d|=0.45±0.01, and |c/d|=0.20±0.01 for the annealed ZnO nanorod, which shows strong anisotropy compared to that of bulk...

Defects and Resistive Switching of Zinc Oxide Nanorods with Copper Addition Grown by Hydrothermal Method

Vertically aligned zinc oxide (ZnO) nanorods (NRs) were hydrothermally synthesized from 0.1xa0M zinc acetate solution on ZnO-seeded Si(100) substrates. ZnO NRs with copper addition were also synthesized by introducing copper acetate into the zinc acetate solution to investigate the effects of copper addition on the growth and resistive switching of the ZnO nanorods. The ZnO NRs had hexagonal wurtzite structure with preferential c-axis orientation. Copper was mainly present as copper oxide (CuO) secondary phase which produces many visible defects, and the lattice fringes of the ZnO NRs are thereby damaged. Copper addition quenches the ultraviolet emission of the ZnO NRs but enhances their green emission. Additionally, copper addition shifts the Zn 2p and O 1s peaks of the x-ray photoelectron spectra towards lower binding energy, which may result from an increase of oxygen vacancies. ZnO NRs with and without copper addition exhibit reversible bipolar resistive switching. The copper addition shrinks the deviations of programming voltages, with a decrease in the minimal set voltage and an increase in the minimal reset voltage, which can probably be attributed to the introduced oxygen vacancies and the copper-related defects.

Temperature dependence of anisotropic mode splitting induced by birefringence in an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser studied by reflectance difference spectroscopy

The temperature dependence of the mode splitting induced by birefringence in an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser has been studied by reflectance difference spectroscopy at different temperatures ranging from 80 to 330 K. The anisotropic broadening width and the anisotropic integrated area of the cavity mode under different temperatures are also determined. The relation between the mode splitting and the birefringence is obtained by theoretical calculation using a Jones matrix approach. The temperature dependence of the energy position of the cavity mode and the quantum well transition are also determined by nearly normal reflectance and photoluminescence, respectively.

Investigation anisotropic mode splitting induced by electro-optic birefringence in an InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser

The mode splitting induced by electro-optic birefringence in an P-I-N InGaAs/GaAs/AlGaAs vertical-cavity surface-emitting laser (VCSEL) has been studied by photocurrent difference spectroscopy (PCDS) at room temperature. The mode splitting, anisotropic broadening width, and the anisotropic integrated area of the two orthogonal polarized modes for a VCSEL device are determined. The mode splitting changes linearly with the injected current, which agree very well with theoretical calculations using a Jones matrix approach. It is demonstrated that the PCDS is a powerful tool to study the cavity anisotropy of a VCSEL device.

Spin-orbit coupling effects on the in-plane optical anisotropy of semiconductor quantum wells

We theoretically study the influence of the spin—orbit coupling (SOC) on the in-plane optical anisotropy (IPOA) induced by in-plane uniaxial strain and interface asymmetry in (001) GaAs/AlGaAs quantum wells (QWs) with different well width. It is found that the SOC has more significant impact on the IPOA for the transition of the first valence subband of heavy hole to the first conduction band (1H1E) than that of 1L1E. The reason has been discussed. The IPOA of (001) InGaAs/InP QWs has been measured by reflectance difference spectroscopy, whose amplitude is about one order larger than that of GaAs/AlGaAs QWs. The anisotropic interface potential parameters of InGaAs/InP QWs are also determined. The influence of the SOC effect on the IPOA of InGaAs/InP QWs when the QWs are under tensile, compressive or zero biaxial strain are also investigated in theory. Our results demonstrate that the SOC has significant effect on the IPOA especially for semiconductor QWs with small well width, and therefore cannot be ignored.

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.

Temperature dependence of photogalvanic effect in GaAs/AlGaAs two-dimensional electron gas at interband and intersubband excitation

The linear (LPGE) and circular photogalvanic effects (CPGE), induced by interband (532u2009nm) and intersubband (1064u2009nm) excitation, have been investigated in a temperature range from 77 to 300u2009K in GaAs/AlGaAs two-dimensional electron gas. The temperature dependences of the CPGE current induced by Rashba and Dresselhaus spin orbit coupling (SOC) under the interband and intersubband excitation are obtained, respectively. It is revealed that the CPGE and LPGE current induced by the intersubband excitation almost increases with increasing temperature, while that induced by interband excitation nearly decreases with increasing temperatures. These phenomena may be attributed to the different variation trends of photo-induced carrier density and SOC with increasing temperatures between interband and intersubband excitation. The evolutions of the ratio of the Rashba and Dresselhaus SOC (RD ratios) and the anisotropic ratio of linear photogalvanic tensors, corresponding to the interband and intersubband excitations...