Impurity levels of iron-group ions in TiO2(II)

Photocurrent measurements on oxidized, single-crystal TiO₂ undoped and doped with V, Cr, Mn or Fe were made both before and after exposure to white light from a tungsten lamp; they revealed a band of states centered at 2.0 eV below the conduction-band edge E_c that appears to be the band of surface states active in the photoelectrolysis of water. They also revealed a band of bulk states extending from near the valence-band edge E_v to about 1.9 eV below E_c. The ESR signals from V⁴⁺, Cr³⁺ and Fe³⁺ ions in oxidized samples and from Mn³⁺ ions in reduced samples were measured before and after exposure to white and monochromatic light; changes in the ESR signal intensities allow placement of the V⁴⁺: d¹ level about 2.1 eV below E_c, the Cr³⁺: d³ level about 2.7 eV belo E_c, the Fe³⁺ level at or just below the valence band edge E_v which is 3.0 eV below E_c, the Fe²⁺ level approximately at E_c, the Mn³⁺ level about 1.9 eV below E_c. Comparison of experiments on oxidized and reduced samples suggests that the bulk states are associated with cation vacancies, and measurements with polarized light gave anisotropies that are qualitatively interpretable with a cation-vacancy model for the oxidized samples.     

掺杂非晶氧化硅薄膜中三元化合态与电子结构的第一性原理计算

基于密度泛函理论和分子动力学方法,研究了ITO-SiO_x(In,Sn)/n-Si异质结光伏器件中非晶SiO_x层的氧化态和电子结构.计算结果表明:具有钝化隧穿功能的超薄(2 nm)非晶SiO_x层,是由In,Sn,O,Si四种元素相互扩散形成的,其中In,Sn元素在SiO_x网格中以In-O-Si和Sn-O-Si成键态存在,形成了三元化合物.In和Sn的掺杂不仅在SiO_x的带隙中分别引入了E_v+4.60 eV和E_v+4.0 eV两个电子能级,还产生了与In离子相关的浅掺杂受主能级(E_v+0.3 eV).这些量子态一方面使SiO_x的性能得到改善,在n-Si表面形成与反型层相衔接的p-型宽禁带"准半导体",减少了载流子的复合,促进了内建电场的建立.另一方面有效地降低了异质结势垒高度,增强了ITO-SiO_x(In,Sn)/n-Si光伏器件中光生非平衡载流子的传输概率,促进了填充因子的提升(72%).     

薄膜硅/晶体硅异质结电池中本征硅薄膜钝化层的性质及光发射谱研究

本文采用甚高频等离子体化学气相沉积技术 (VHF-PECVD) 制备薄膜硅/晶体硅异质结电池中的本征硅薄膜钝化层, 光发射谱 (OES) 测量技术研究了硅薄膜沉积过程中等离子体发光谱随时间的变化. 结果表明: 在实验优化条件下等离子体发光谱很快达到稳定 (大约25 s), 并且SiH^*/H_α^* 的比值随时间变化较小, 避免了生长过程中硅薄膜结构的不均匀性, 这主要是SiH₄没有完全耗尽避免了SiH₄的反向扩散. 进一步研究了沉积参数对稳态发光谱和硅薄膜性质的影响, 结果表明: 随着硅烷浓度增加, H_α^*峰强度减小, SiH^*峰强度增加, 薄膜从微晶转变成非晶, 非晶硅薄膜钝化效果好; 随着沉积气压增大, H_α^*和 SiH^*峰强度先增加后减小, 高气压下H_α^*和 SiH^*峰强度下降主要是反应前驱物的聚合形成高聚合物, 不利于形成高质量的硅薄膜, 因此钝化效果下降; 随着反应功率密度增加, H_α^*和 SiH^*峰强度增大, 当功率密度为150 mW/cm² 趋于饱和, 硅薄膜的致密度和钝化效果也开始下降, 50 mW/cm²的低功率密度下硅薄膜钝化效果差可能是由于原子H 浓度低, 不能完全钝化单晶硅表面的悬挂键. 关键词： 薄膜硅 异质结 光发射谱 钝化     

Band alignment of Ga2O3/6H-SiC heterojunction

A high-quality Ga₂O₃ thin film is deposited on an SiC substrate to form a heterojunction structure. The band alignment of the Ga₂O₃/6H-SiC heterojunction is studied by using synchrotron radiation photoelectron spectroscopy. The energy band diagram of the Ga₂O₃/6H-SiC heterojunction is obtained by analysing the binding energies of Ga 3d and Si 2p at the surface and the interface of the heterojunction. The valence band offset is experimentally determined to be 2.8 eV and the conduction band offset is calculated to be 0.89 eV, which indicate a type-II band alignment. This provides useful guidance for the application of Ga₂O₃/6H-SiC electronic devices.     

Luminescence excitation and its relation to the structures of natural crystalline diamond and diamond-like films

The luminescence excitation spectra (LES) of natural crystalline diamond and amorphous diamond-like films are measured at energies from 4 to 20 eV at the LOCUS station for luminescence and optical studies of the Kurchatov Institute synchrotron radiation source. Two sharp peaks at 5.5 and 10.5 eV are observed in the LES of diamond, while the LES of the film demonstrate only one peak at 10.5 eV. For crystalline diamond, the peaks can be explained within the framework of the diamond band structure and the processes of inelastic scattering of photoexcited holes. Dark-field electron microscope images of the diamond-like film show that, in spite of its amorphous structure, this film contains regions of coherent electron scattering whose dimensions are on the order of 2–5 nm, which is sufficient to describe the film LES using a model for crystalline diamond. The absence of the first peak in the film LES is related to the fact that the absorption coefficient of diamond is very small at 5.5 eV and radiation is only slightly absorbed by the thin film.     

Characteristics of cylindrical surrounding-gate GaAs_xSb_(1-x)/In_yGa_(1-y)As heterojunction tunneling field-effect transistors

A Ⅲ-Ⅴ heterojunction tunneling field-effect transistor(TFET) can enhance the on-state current effectively,and GaAs_xSb_1_x/In_yGa_1_yAs heterojunction exhibits better performance with the adjustable band alignment by modulating the alloy composition.In this paper,the performance of the cylindrical surrounding-gate GaAs_xSb_1_x/In_yGa_1_yAs heterojunction TFET with gate-drain underlap is investigated by numerical simulation.We validate that reducing drain doping concentration and increasing gate-drain underlap could be effective ways to reduce the off-state current and subthreshold swing(SS),while increasing source doping concentration and adjusting the composition of GaAs_xSb_1_xIn_yGa_1_yAs can improve the on-state current.In addition,the resonant TFET based on GaAs_xSb_1_x/In_yGa_1_yAs is also studied,and the result shows that the minimum and average of SS reach 11 mV/decade and 20 mV/decade for five decades of drain current,respectively,and is much superior to the conventional TFET.     

Photoelectron spectroscopy measurements of the valence band structures of polymerized thin films of C60 and La0.1C60

The electronic valence band structures of polymerized thin films of C₆₀ and La_0.1C₆₀ have been studied by using ultra-violet photoelectron spectroscopy. Additionally, the films have been characterized by using Raman spectroscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. The valence band of the C₆₀ film shows major peaks at binding energies of 2.6, 7.2, 10.3, and 12.6 eV. In the case of the doped film, we observe (i) an additional peak with a binding energy of 13.7 eV, (ii) evidence for redistribution of the density of electronic states due to hybridization between the 5d orbitals of La and the C₆₀ cage, and (iii) significantly higher density of the electronic states near the Fermi energy. The valence band spectra of the doped film are in good agreement with recent results of the density functional theory that support strong hybridization between the d-valence orbitals of La and the C₆₀ cage.     

基于氧化镍背接触缓冲层碲化镉薄膜太阳电池的研究

采用电子束蒸发法制备了NiO薄膜,并对其作为碲化镉薄膜太阳电池背接触缓冲层材料进行了相关研究.NiO缓冲层的加入使得碲化镉太阳电池开路电压显著增大.通过X射线光电子能谱测试得到的NiO/CdTe界面能带图表明NiO和CdTe的能带匹配度很好.NiO是宽禁带P型半导体材料,在电池背接触处形成背场,减少了电子在背表面处的复合,从而提高电池开路电压.通过优化NiO薄膜厚度,制备得到转换效率为12.2%、开路电压为789 mV的碲化镉太阳电池.研究证实NiO是用来制备高转换效率、高稳定性碲化镉薄膜太阳电池的一种极有前景的缓冲层材料.     

(Cu,N)共掺杂TiO2/MoS2异质结的电子和光学性能:杂化泛函HSE06

在密度泛函理论的基础上,系统地研究了Cu/N（共）掺杂的TiO₂/MoS₂异质结体系的几何结构、电子结构和光学性质.计算发现,TiO₂/MoS₂异质结的带隙相比于纯的TiO₂（101）表面明显变小,Cu/N（共）掺杂TiO₂/MoS₂异质结体系的禁带宽度也明显地减小,这导致光子激发能量的降低和光吸收能力的提高.通过计算Cu/N（共）掺杂TiO₂/MoS₂的差分电荷密度,发现光生电子与空穴积累在掺杂后的TiO₂（101）表面和单层MoS₂之间,这表明掺杂杂质体系可以有效地抑制光生电子-空穴对的复合.此外,我们计算了在不同压力下TiO₂/MoS₂异质结的几何、电子和光学性质,发现适当增加压力可以有效提高异质结的光吸收性能.本文结果表明,Cu/N（共）掺杂TiO₂/MoS₂异质结和对TiO₂/MoS₂异质结加压都能有效地提高材料的光学性能.     

电极间距对μc-Si_(1-x)Ge_x:H薄膜结构特性的影响

采用射频等离子体增强化学气相沉积(RF-PECVD)技术,使用SiH4加GeH4的反应气源组合生长微晶硅锗(μc-Si1-x Gex:H)薄膜.研究了电极间距对μc-Si1-x Gex:H薄膜结构特性的影响.发现薄膜中的Ge含量随电极间距的降低逐渐增加.当电极间距降至7 mm时,μc-Si1-x Gex:H薄膜具有较大的晶粒尺寸并呈现较强的(220)择优取向,同时具有较低的微结构因子.通过薄膜结构特性的变化分析了反应气源的分解状态,认为Ge含量的提高主要是SiH4的分解率降低所导致的.在较窄的电极间距(7 mm)下,等离子体中GeH3基团的比例较大,增强了Ge前驱物的扩散能力,使μc-Si1-x Gex:H薄膜的质量得到提高.     

Ge20Sb15Se65薄膜的热致光学特性变化研究

采用磁控溅射法制备了Ge₂₀Sb₁₅Se₆₅薄膜, 研究热处理温度(150—400 ℃)对薄膜光学特性的影响. 通过分光光度计、X射线衍射仪、显微拉曼光谱仪对热处理前后薄膜样品 的光学特性和微观结构进行了表征, 并根据Swanepoel方法以及Tauc公式分别计算了薄膜折射率色散曲线和光学带隙等参数. 结果表明当退火温度(T_a)小于薄膜的玻璃转化温度(T_g)时,薄膜的光学带隙(E_g^opt)随着退火温度的增加由1.845 eV上升至1.932 eV, 而折射率由2.61降至2.54; 当退火温度大于薄膜的玻璃转化温度时,薄膜的光学带隙随退火温度的增加由1.932 eV降至1.822 eV, 折射率则由2.54增至2.71. 最后利用Mott和Davis提出的非晶材料由非晶到晶态的结构转变模型对结果进行了解释, 并通过薄膜XRD和Raman光谱进一步验证了结构变化是薄膜热致变化的重要原因. 关键词： 20Sb₁₅Se₆₅薄膜')" href="#">Ge₂₀Sb₁₅Se₆₅薄膜 热处理 光学带隙 折射率     

A new intrinsic defect in amorphous SiO2: Twofold coordinated silicon

The well known optical absorption band at 5.03 eV (the “B₂ band”) and luminescence band at 4.3 eV in amorphous SiO₂ are due to singlet-to-singlet transitions, while the luminescence band at 2.65 eV - due to triplet-to-singlet transitions in a silicon-related intrinsic defect. This defect occurs both in the bulk and on the surface. Luminescence polarization data indicate C_2v symmetry. The most probable model for this center is a silicon atom with only two neighboring oxygens. Such defects form a separate class of valence alternation defects, characteristic for amorphous materials having atoms in tetrahedral coordination.     

Valence band offset of GaN/diamond heterojunction measured by X-ray photoelectron spectroscopy

XPS was used to measure the energy discontinuity in the GaN/diamond heterostructure. The valence band offset (VBO) was determined to be 0.38 ± 0.15 eV and a type-II heterojunction with a conduction band offset (CBO) of 2.43 ± 0.15 eV was obtained.     

新型硒化锑材料及其光伏器件研究进展

硒化锑(Sb₂Se₃)是一种二元单相化合物, 原料储量大、毒性低、价格便宜; 同时其禁带宽度合适(～1.15 eV), 吸光系数大(>10⁵ cm^-1), 长晶温度低, 非常适合制作新型低成本低毒的薄膜太阳能电池, 理论光电转换效率可达30%以上. 目前文献报道的Sb₂Se₃薄膜太阳能电池效率已达3.7%, 初步证明了Sb₂Se₃材料在薄膜太阳能电池应用方面的巨大潜力. 本文综述了近年来Sb₂Se₃太阳能电池的研究进展, 着重介绍了Sb₂Se₃的材料特性和薄膜制备及相关理论研究, 阐述了不同结构电池器件的研究进展, 并对其发展趋势进行了展望.     

VO_2薄膜Vis-NIR及NIR-MIR椭圆偏振光谱分析

采用射频磁控溅射在石英玻璃基底上反应溅射制备单斜相(M相)VO_2薄膜.利用V-VASE和IR-VASE椭圆偏振仪及变温附件分别在0.5—3.5 eV(350—2500 nm)和0.083—0.87 eV(1400—15000 nm)入射光能量范围内对相变前后的VO_2薄膜进行光谱测试,运用逐点拟合的方式,并通过薄膜的吸收峰的特征,在0.5—3.5 eV范围内添加3个Lorentz谐振子色散模型和0.083—0.87 eV范围内添加4个Gaussion振子模型对低温态半导体态的薄膜椭偏参数进行拟合,再对高温金属态的薄膜添加7个Lorentz谐振子色散模型对进行椭偏参数的拟合,得到了较为理想的拟合结果.结果发现:半导体态的VO_2薄膜的折射率在近红外-中红外基本保持在最大值3.27不变,且消光系数k在此波段接近于零,这是由于半导体态薄膜在可见光-近红外光范围内的吸收主要是自由载流子吸收,而半导体态薄膜的d//轨道内的电子态密度较小.高温金属态的VO_2薄膜的折射率n在近红外-中红外波段具有明显的增大趋势,且在入射光能量为0.45 eV时大于半导体态的折射率;消光系数k在近红外波段迅速增大,原因是在0.5—1.62 eV范围内,能带内的自由载流子浓度增加及电子在V_(3d)能带内发生带内的跃迁吸收,使k值迅速增加;当能量小于0.5 eV时k值变化平缓,是由于薄膜内自由载流子浓度和电子跃迁率趋于稳定所致.     

Dielectric and piezoelectric properties of(110) oriented Pb(Zr_(1-x)Ti_x)O_3 thin films

A phenomenological Landau–Devonshire theory is developed to investigate the ferroelectric, dielectric, and piezoelectric properties of(110) oriented Pb(Zr_(1-x)Ti_x)O_3(x = 0.4, 0.5, 0.6, and 0.7) thin films. At room temperature, the tetragonal a_1 phase, the orthorhombic a_2c phase, the triclinic γ_1 phase, and the triclinic γ_2 phase are stable. The appearance of the negative polarization component P_2 in the a_2c phase and the γ_1 phase is attributed to the nonlinear coupling terms in the thermodynamic potential. The γ phase of the Pb(Zr_(1-x)Ti_x)O_3 thin films has better dielectric and piezoelectric properties than the a_2c phase and the a_1 phase. The largest dielectric and piezoelectric coefficients are obtained in the Pb(Zr_(0.5)Ti_(0.5))O_3 thin film. The piezoelectric coefficient of 110–150 pm/V is obtained in the(110) oriented Pb(Zr_(0.5)Ti_(0.5))O_3 thin film, and the Pb(Zr_(0.3)Ti_(0.7))O_3 thin film has the remnant polarization and relative dielectric constant of 50 μC/cm~2 and 100, respectively,which are in agreement with the experimental measurements reported in the literature.     

Doping effect on the structure and physical properties of quasi-one-dimensional compounds Ba_9Co_3(Se_(1-x)S_x)15(x=0–0.2)

A series of samples of Ba_9Co_3(Se_(1-x)S_x)15 (x=0,0.05,0.1,0.15,0.2) with quasi-one-dimensional (1D) structure were successfully synthesized under high-temperature and high-pressure conditions.The influence of partial substitution of S for Se on the structure,electronic transport,and magnetic properties of Ba_9Co_3(Se_(1-x)S_x)15 has been investigated in detail.The x-ray diffraction data shows that the lattice constant decreases linearly with increasing S-doping level,which follows the Vegrad’s law.The doped S atoms preferentially occupy the site of Se atoms in CoSe_6 octahedron.Physical properties measurements indicate that all the samples of Ba_9Co_3(Se_(1-x)S_x)15 are semiconducting and display spin glass behavior.As the replacement of Se by smaller size S,although the inter-chain distance decreases,the electronic hopping between CoSe/S_6 chains is weakened and leads to an increase of band gap from 0.75 e V to 0.86 e V,since the S-3p electrons are more localized than Se-4p ones.Ba_9Co_3(Se_(1-x)S_x)15 exhibits 1D conducting chain characteristic.     

Epitaxial Bi2Sr2CuOy thin films as p-type transparent conductors

Development of p-type transparent conducting thin films is tireless due to the trade-off issue between optical transparency and conductivity. The rarely concerned low normal state resistance makes Bi-based superconducting cuprates the potential hole-type transparent conductors, which have been realized in Bi₂Sr₂CaCu₂O_y thin films. In this study, epitaxial superconducting Bi₂Sr₂CuO_y and Bi₂Sr_1.8Nd_0.2CuO_y thin films with superior normal state conductivity are proposed as p-type transparent conductors. It is found that the Bi₂Sr_1.8Nd_0.2CuO_y thin film with thickness 15 nm shows an average visible transmittance of 65% and room-temperature sheet resistance of 650 Ω/sq. The results further demonstrate that Bi-based cuprate superconductors can be regarded as potential p-type transparent conductors for future optoelectronic applications.     

Band offsets and electronic properties of the Ga2O3/FTO heterojunction via transfer of free-standing Ga2O3 onto FTO/glass

The determination of band offsets is crucial in the optimization of Ga₂O₃-based devices, since the band alignment types could determine the operations of devices due to the restriction of carrier transport across the heterogeneous interfaces. In this work, the band offsets of the Ga₂O₃/FTO heterojunction are studied using x-ray photoelectron spectroscopy (XPS) based on Kraut's method, which suggests a staggered type-Ⅱ alignment with a conduction band offset (ΔE_C) of 1.66 eV and a valence band offset (ΔE_V) of -2.41 eV. Furthermore, the electronic properties of the Ga₂O₃/FTO heterostructure are also measured, both in the dark and under ultraviolet (UV) illuminated conditions (254 nm UV light). Overall, this work can provide meaningful guidance for the design and construction of oxide hetero-structured devices based on wide-bandgap semiconducting Ga₂O₃.     

Sub-stochiometric MoOx by radio-frequency magnetron sputtering as hole-selective passivating contacts for silicon heterojunction solar cells

The silicon heterojunction (SHJ) solar cell has long been considered as one of the most promising candidates for the next-generation PV market. Transition metal oxides (TMOs) show good carrier selectivity when combined with c-Si solar cells. This has led to the rapid demonstration of the remarkable potential of TMOs (especially MoO_x) with high work function to replace the p-type a-Si:H emitting layer. MoO_x can induce a strong inversion layer on the interface of n-type c-Si, which is beneficial to the extraction and conduction of holes. In this paper, the radio-frequency (RF) magnetron sputtering is used to deposit MoO_x films. The optical, electrical and structural properties of MoO_x films are measured and analyzed, with focus on the inherent compositions and work function. Then the MoO_x films are applied into SHJ solar cells. When the MoO_x works as a buffer layer between ITO/p-a-Si:H interface in the reference SHJ solar cell, a conversion efficiency of 19.1% can be obtained. When the MoO_x is used as a hole transport layer (HTL), the device indicates a desirable conversion efficiency of 17.5%. To the best of our knowledge, this current efficiency is the highest one for the MoO_x film as HTL by RF sputtering.