利用同步辐射光电子能谱技术测量ZnO/PbTe异质结的能带带阶

异质结结构界面的能带带阶是一个非常重要的参数,该参数的精确确定直接影响异质结的光电性质研究以及异质结在光电器件上的应用.利用同步辐射光电子能谱技术测量了ZnO/PbTe异质结结构的能带带阶.测量得到该异质结价带带阶为2.56 eV,导带带阶为0.49 eV,是一个典型的类型I的能带排列.利用变厚度扫描的测量方法发现,ZnO/PbTe界面存在两种键,分别是Pb—O键(低结合能)和Pb—Te键(高结合能).在ZnO/PbTe异质结界面的能带排列中导带带阶较小,而价带带阶较大,这一能带结构有利于PbTe中的激发电子输运到ZnO导电层中.该类结构在新型太阳电池、中红外探测器、激光器等器件中具有潜在的应用价值.     

PbTe/CdTe量子点的光学增益

PbTe/CdTe量子点是一类新型异系低维结构材料,实验发现具有强的室温中红外光致发光现象.为研究这一材料体系的发光特性,建立了理论模型,计算了PbTe/CdTe量子点的光学跃迁和增益.模型基于k·p包络波函数方法并考虑了PbTe能带结构的各向异性.分析了量子点光学增益与量子点尺寸、注入载流子浓度的关系.结果表明,当注入载流子浓度在(0.3—3)×10¹⁸cm^-3范围时,尺寸为15—20nm的量子点可以产生 关键词： PbTe/CdTe量子点 光学增益 铅盐矿半导体     

Structural and electronic properties of PbTe (rocksalt)/CdTe (zinc-blende) interfaces

We study interfaces between highly ionic crystals with different crystal structure by first-principles total-energy calculations in the repeated-slab approximation and compare the results with experimental data extracted from high-resolution transmission electron micrographs. The non-polar (1 1 0) interface between PbTe (rocksalt) and CdTe (zinc-blende) crystals gives rise to a lateral spatial offset between the two crystal halves. At the polar (1 0 0) interfaces a strong variation of the interface extent with respect to the cation or anion termination is observed. Furthermore, we calculate band offsets and projected interface band-structures for PbTe/CdTe interfaces. The results are discussed versus the interface orientation.     

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.     

The In2O3/CdTe interface: A possible contact for thin film solar cells?

Transparent conductive In₂O₃ films were deposited by reactive evaporation of In and analyzed in-situ with photoelectron spectroscopy. The interface formation of In₂O₃ with evaporated CdTe has been investigated using the same technique. A valence band offset ΔE_VB=2.1±0.1 eV is determined, resulting in a negligible conduction band offset. However, In₂O₃ will not provide an Ohmic contact to n-CdTe, due to the Fermi level position at the interface.     

Valence band offset of ZnO/BaTiO3 heterojunction measured by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy has been used to measure the valence band offset of the ZnO/BaTiO₃ heterojunction grown by metal-organic chemical vapor deposition. The valence band offset (VBO) is determined to be 0.48±0.09 eV, and the conduction band offset (CBO) is deduced to be about 0.75 eV using the band gap of 3.1 eV for bulk BaTiO₃. It indicates that a type-II band alignment forms at the interface, in which the valence and conduction bands of ZnO are concomitantly higher than those of BaTiO₃. The accurate determination of VBO and CBO is important for use of semiconductor/ferroelectric heterojunction multifunctional devices.     

First-principles study of Cu2ZnSnS4 and the related band offsets for photovoltaic applications

First-principles calculations of the band offsets between Cu(2)ZnSnS(4) (CZTS) and XS (X = Cd, Zn) are performed. While the interface dipole contribution for the band offsets is calculated using the Perdew-Burke-Ernzerhof functional, the Heyd-Scuseria-Ernzerhof hybrid functional is employed to introduce the quasiparticle corrections to the band offsets. The calculated conduction band offset between CZTS and CdS is 0.2 eV, validating CdS for the buffer layer of the CZTS solar cell. The small conduction band offset stems from the band gap narrowing of CdS under the interface strain caused by the lattice misfit with CZTS. A large valence band offset over 0.9 eV between CZTS and ZnS indicates that precipitated ZnS is regarded as an inactive insulator phase in CZTS absorbers.     

Two-carrier transition in radiative recombination of two-dimensional electrons in GaAs/AlGaAs

We observed photoluminescence (PL) and photoluminescence excitation (PLE) spectra due to shake-up processes of recombination of two-dimensional electrons and free excitons in a modulation-doped GaAs quantum well at He temperatures. One of the processes is that when an electron recombines with a hole, another electron is excited from the conduction band in GaAs to that in AlGaAs. The other process is that a hole is excited from an acceptor level or the valence band in GaAs to the valence band in AlGaAs during recombination. The electron process is observed in both PL and PLE spectra while the hole process only in the PL spectra. The excitation-intensity dependence of the peak intensity of hole-excited PL is almost quadratic, indicating three-carrier process in the shake-up process. The band offsets of the conduction and valence bands are estimated to be 220 and 146 meV, respectively.     

Experimental determination of valence band offset at PbTe/Ge(1 0 0) interface by synchrotron radiation photoelectron spectroscopy

The band offset at the interface of PbTe/Ge (1 0 0) heterojunction was studied by the synchrotron radiation photoelectron spectroscopy. A valence band offset of ΔE_V = 0.07 ± 0.05 eV, and a conduction band offset of ΔE_C = 0.27 ± 0.05 eV are concluded. The experimental determination of the band offset for the PbTe/Ge interface should be beneficial for the heterojunction to be applied in new optoelectronic and electronic devices.     

PbTe/CdTe量子阱光学性质的研究

PbTe/CdTe量子阱是一类新型异系低维结构材料,实验观察到具有强的室温中红外光致发光现象.建立了理论模型,计算了PbTe/CdTe量子阱的自发辐射率和光学增益.模型中量子阱分立能级的计算采用k·p包络波函数方法和有限深势阱近似,考虑了PbTe能带结构的各项异性和阱层中应变对能级的影响.计算了PbTe/CdTe量子阱自发辐射谱与带间弛豫和注入载流子浓度间的依赖关系,计算结果与实验观察到的光致发光峰相符合.自发辐射谱线峰位随着注入载流子浓度的增加而出现蓝移,当载流子浓度从2×1017cm-3增加到2.8×1018cm-3,基态发射峰从372 meV蓝移到397 meV,而第一激发态发射峰蓝移量为15 meV.上述蓝移现象是由载流子与载流子及载流子与声子间的相互作用引起的.与PbTe体材料相比.PbTe/CdTe量子阱结构具有更高的增益强度(提高近15倍)和更宽的增益区,因而该体系可能是实现室温连续工作的中红外激光器的理想材料.     

Si/ZnS极性界面能带偏移的同步辐射光电子能谱研究

用同步辐射光电子能谱测量了Si/ZnS（111）及（100）异质结的价带偏移ΔE_v．对于Si/ZnS（111）及（100）两界面，ΔE_v的实验结果均为（1.9±0.1）eV，与已有理论预期值相当符合，但与Maierhofer所报告的ZnS/Si（111）异质结测量结果之间则存在明显差别．该实验结果表明对于Si/ZnS极性界面，互逆性规则（commutativity rule）可能不成立，就此进行了讨论． 关键词：     

The role of CdCl2 treatments and annealing in the formation of sintered CdTe nanocrystal solar cells

Solution processing of CdTe nanocrystals is a promising approach for the fabrication of low cost photovoltaic devices at low process temperatures. However despite the inherent advantages of this approach, the nanoscale physical mechanisms underlying the fabrication process are not well understood. Here we demonstrate that chemical treatment of CdTe nanoparticles leads to the production of a deep valence band edge. However, the removal of ligands also introduces sizable quantities of excess oxygen into the films in the form of TeO₂, as proved by XPS. Thermal annealing of the CdTe films in air removes this excess oxygen and the TeO₂ signal. Annealing further increases the energy of the valence band edge, decreases the energy of the conduction band edge, and is responsible for the p-type conductivity observed in NC CdTe films resulting in enhanced photovoltaic performance. Importantly, the presence of oxygen during the annealing step is crucial because it leads to an increase in hole concentrations needed for high performance devices.     

Far-infrared spin—flip resonance in PbTe

We present experimental values of the transverse and longitudinal components of the g-tensor for both conduction and valence bands in PbTe. These result from our observation of far-infrared spin resonance in low concentration epitaxial films and apply very near the band edges.     

Valence band offset of MgO/TiO2 (rutile) heterojunction measured by X-ray photoelectron spectroscopy

The valence band offset (VBO) of MgO/TiO₂ (rutile) heterojunction has been directly measured by X-ray photoelectron spectroscopy. The VBO of the heterojunction is determined to be 1.6 ± 0.3 eV and the conduction band offset (CBO) is deduced to be 3.2 ± 0.3 eV, indicating that the heterojunction exhibits a type-I band alignment. These large values are sufficient for MgO to act as tunneling barriers in TiO₂ based devices. The accurate determination of the valence and conduction band offsets is important for use of MgO as a buffer layer in TiO₂ based field-effect transistors and dye-sensitized solar cells.     

Coupled ultrathin InAs layers in GaAs as a tool for the determination of band offsets

We have determined the band offsets at the highly strained InAs/GaAs heterointerface by photoluminescence excitation (PLE) measurements of the symmetric and antisymmetric states in two coupled ultrathin InAs layers embedded in a GaAs matrix. The conduction band offset ΔE_ccould be separated from the valence band offsets, since in a 32 monolayer (ML) barrier sample, the splitting between the heavy-hole exciton transitions is solely determined by ΔE_c. Knowing ΔE_c, the heavy-hole (hh) and light-hole (lh) band offsets ΔE_hhand ΔE_lhcould subsequently be determined from the coupling-induced shift and splitting in samples with a 16, 8 and 4 ML barrier. We find a conduction band offset of 535 meV, a conduction band offset ratio ofQ_c= 0.58 and a strain induced splitting between the hh and lh subbands of 160 meV.     

The electronic bandstructure of CdO by the augmented plane wave method

An augmented plane wave calculation of the energy bands of CdO has been performed using two ionicities (0 and ± 1). Energy bands are found to be sensitive to the degree of ionicity assumed.The band structure for ionicity 0 is like that of a metal and the Fermi energy lies inside the conduction band whereas for ionicity ±1, the band structure is like that of a semiconductor and the Fermi energy lies in the Γ-Σ gap just above the valence band maxima at Σ₄.     

PbTe(001)表面原子几何结构和电子结构的第一性原理计算

用第一性原理的密度泛函理论计算了PbTe（001）表面的几何结构和电子结构.计算结果表明：PbTe（001）表面不发生重构，但表面几层原子表现出明显的振荡弛豫现象，其中第一、第二层间距减小4.5%，第二、第三层间距增加2.0%，并且表面层原子出现褶皱.表面带隙在X 点，带隙变宽，在基本带隙中不引入新的表面态，而导带底和价带顶附近等多处出现新的表 面共振态；弛豫后费米面处态密度很低，所以表面结构很稳定. 关键词： 密度泛函理论 表面几何结构 表面电子结构 PbTe     

Energy band alignment of MgO (111)/ZnO (0002) heterojunction determined by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to measure the energy discontinuity in the MgO (111)/ZnO (0002) heterostructure. The valence band offset (VBO) was determined to be 1.22±0.23 eV and a type-I heterojunction with a conduction band offset (CBO) of 3.24±0.23 eV was obtained. The discrepancy of VBO values between MgO/ZnO and ZnO/MgO heterojunctions was mainly attributed to the internal electric field induced by spontaneous polarization effect in ZnO layer.     

Band offsets at ZnO/SiC heterojunction: Heterointerface in band alignment

Pulsed laser deposited ZnO layers on 6H-SiC substrates showed the six-fold symmetry, indicating a two-dimensional epitaxial growth mode. X-ray photoelectron spectroscopy was employed to study the valence band discontinuity and interface formation in the ZnO/6H-SiC heterojunction. The valence band offset was measured to be 1.38 ± 0.28 eV, leading to a conduction band offset value of 1.01 ± 0.28 eV. The resulting band lineup in epitaxial ZnO/6H-SiC heterojunction is determined to be of staggered-type alignment.     

Band offsets and properties ofSi1 − xGex/Si material systems

Following the upsurge in the study ofSi_1 − xGe_x /Si material systems for high-speed applications, we calculate the band offsets using reformulated tight-binding methods. The calculated value of 0.78 eV for the valence band offset (VBO) of pure substances is in excellent agreement with recent experimental measurements. The VBO for alloy interfaces is 0.78 x. We apply the VBO and conduction band offsets (CBO) to shift in photoluminescence and electron confinement in microstructures respectively. The calculated VBO is strongly dependent on the precursor flux ratio.