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.     

Thermal stability and data retention of resistive random access memory with HfO_x/ZnO double layers

As an industry accepted storage scheme, hafnium oxide(HfO_x) based resistive random access memory(RRAM)should further improve its thermal stability and data retention for practical applications. We therefore fabricated RRAMs with HfO_x/ZnO double-layer as the storage medium to study their thermal stability as well as data retention. The HfO_x/ZnO double-layer is capable of reversible bipolar switching under ultralow switching current( 3 μA) with a Schottky emission dominant conduction for the high resistance state and a Poole–Frenkel emission governed conduction for the low resistance state. Compared with a drastically increased switching current at 120℃ for the single HfO_x layer RRAM, the HfO_x/ZnO double-layer exhibits excellent thermal stability and maintains neglectful fluctuations in switching current at high temperatures(up to 180℃), which might be attributed to the increased Schottky barrier height to suppress current at high temperatures. Additionally, the HfO_x/ZnO double-layer exhibits 10-year data retention @85℃ that is helpful for the practical applications in RRAMs.     

r面蓝宝石衬底上生长的a面ZnO巨大的平面内光学各向异性

我们在室温下测量采用反射差分光谱测量了r面蓝宝石衬底上生长的a面氧化锌的平面内光学各向异性。由于a面氧化锌为C2v对称性,我们观察到在平行c轴的方向和垂直于c轴的方向存在巨大的平面内光学各向异性。在带隙处观察到了非常尖锐的跃迁振荡信号,这个信号来源于偏振相关的能带移动。激子跃迁产生了尖锐的线形。谱线的拟合和微分光谱给出了偏振相关的带隙能量。垂直于c轴和平行于c轴方向上的各向异性应变产生了巨大的平面内光学各向异性。介电函数模型给出了拟合的结果,估计了带隙附近的偏振度。     

Investigation of reflection anisotropy induced by micropipe defects on the surface of a 4H-SiC single crystal using scanning anisotropy microscopy

Optical reflection anisotropy microscopy mappings of micropipe defects on the surface of a 4H-SiC single crystal are studied by the scanning anisotropy microscopy(SAM) system. The reflection anisotropy(RA) image with a ’butterfly pattern’ is obtained around the micropipes by SAM. The RA image of the edge dislocations is theoretically simulated based on dislocation theory and the photoelastic principle. By comparing with the Raman spectrum, it is verified that the micropipes consist of edge dislo...     

基于有限差分的带间级联结构理论研究

利用前后向有限差分算法,对锑化物带间级联激光器有源区结构InAs/GaSb/AlSb的电子态进行了理论计算与分析。研究了哈密顿量算符序、有效质量参数修正、内界面态对结构能态及波函数的影响。分析表明,导带有效质量参数取正值可以在两种算符序下有效抑制伪解产生,Burt-Foreman算符序下的跃迁能量更为合理。对于内界面采用缓变假设后,其跃迁能量计算值略高于陡变界面下的计算结果,二者的波函数在界面附近差异不明显。     

Giant in-plane optical anisotropy of α-plane ZnO on r-plane sapphire

We have measured the in-plane optical anisotropy （IPOA） of （1120） ZnO （a-plane） on （10]-2） sapphire （r-plane） by reflectance difference spectroscopy （RDS） at room temperature. Giant IPOA has been observed be- tween the light polarized direction parallel and perpendicular to the c axis of ZnO, since the symmetry of a-plane is C2v. A sharp resonance has been observed near the fundamental band gap, which is induced by the polarization- depend band gap shift. The sharp line shape is attributed to the exciton transition. The spectra fitting and differential spectra indicate the polarization-depend band energies. The giant IPOA is possible enhanced by anisotropy strain along and perpendicular to the c axis in the a-plane.     

Investigation of the mode splitting induced by electro-optic birefringence in a vertical-cavity surface-emitting laser by polarized electroluminescence

The mode splitting induced by electro-optic birefringence in a P-I-N InGaAs/GaAs/A1GaAs vertical-cavity surface- emitting laser （VCSEL） has been studied by polarized electroluminescence （EL） at room temperature. The polarized EL spectra with E｜｜[110] and E ｜｜ [150] directions, are extracted for different injected currents. The mode splitting of the two orthogonal polarized modes for a VCSEL device is determined, and its value increases linearly with the increasing injected current due to electro-optic birefringence; This article demonstrates that the polarized EL is a powerful tool to study the mode splitting and polarization anisotropy of a VCSEL device.     

Effect of deposited temperatures of the buffer layer on the band offset of CZTS/In_2S_3 heterostructure and its solar cell performance

The effect of the deposition temperature of the buffer layer In_2S_3 on the band alignment of CZTS/In_2S_3 heterostructures and the solar cell performance have been investigated.The In_2S_3 films are prepared by thermal evaporation method at temperatures of 30,100,150,and 200 ℃,respectively.By using x-ray photoelectron spectroscopy(XPS),the valence band offsets(VBO) are determined to be-0.28 ±0.1,-0.28 ±0.1,-0.34 ±0.1,and-0.42 ±0.1 eV for the CZTS/In_2S_3heterostructures deposited at 30,100,150,and 200 ℃,respectively,and the corresponding conduction band offsets(CBO)are found to be 0.3 ±0.1,0.41 ±0.1,0.22±0.1,and 0.01 ±0.1 eV,respectively.The XPS study also reveals that interdiffusion of In and Cu occurs at the interface of the heterostructures,which is especially serious at 200 ℃ leading to large amount of interface defects or the formation of CuInS_2 phase at the interface.The CZTS solar cell with the buffer layer In_2S_3 deposited at 150 ℃ shows the best performance due to the proper CBO value at the heterostructure interface and the improved crystal quality of In_2S_3 film induced by the appropriate deposition temperature.The device prepared at 100 ℃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.