利用晶体结构工程提升GeSe化合物热电性能的研究

在热电研究领域, Ge Se是一种二维层状结构具有较大带隙的半导体,本征载流子浓度低,热电性能差.在本工作中,采用熔融淬火结合放电等离子活化烧结工艺制备了一系列的Ge Se_1–x Te_x (x=0, 0.05, 0.15, 0.25,0.35, 0.45)多晶样品,研究了Te含量对Ge Se化合物物相结构和热电输运性能的影响规律.结果表明:随着Te含量的增加, Ge Se的晶体结构逐渐由正交相向菱方相转变,使得材料的带隙降低,载流子浓度和迁移率同步增加;同时,晶体对称性的提高增加了化合物的能带简并度,有效提高了载流子有效质量.在这些因素的共同作用下,菱方相Ge Se的功率因子比正交相Ge Se提高约2—3个数量级.此外,菱方相Ge Se具有丰富的阳离子空位缺陷以及铁电特性所导致的声子软化现象,这导致其晶格热导率比正交相Ge Se降低近60%.当Te含量为0.45时,样品在573 K取得最大热电优值ZT为0.75,是本征Ge Se样品的19倍.晶体结构工程是提升Ge Se化合物热电性能的有效途径.     

Observation of new Raman lines in GeSe and SnSe at low temperatures

Polarized Raman spectra are investigated in GeSe and SnSe at low temperatures. New Raman lines which can not be expected by a group theoretical analysis for the known crystal structure of the orthorhombic D¹⁶_2h have been observed typically in the (a, a) and (b, b) polarization configurations. With decreasing temperature, three lines in GeSe grow weakly at 89,201 and strongly at 226 cm^-1 below 150 K, accompanied by enhancement of layer breathing mode (175 cm^-1 at 273 K) intensity. One line in SnSe grows weakly at 193 cm^-1 below 50 K. The appearance of the new Raman lines as well as a resistivity anomaly suggests a novel structural phase transition in this system.     

Thermally induced effects in amorphous GeSe2 and GeSe films studied by ultraviolet photoelectron spectroscopy

During thermal annealing, the changes in the valence states of amorphous GeSe₂ and GeSe films deposited on the substrates held at 77 K were observed by UPS. The experimental results are explained in terms of the four (Ge)-two (Se) fold coordination for amorphous GeSe₂ and the three(Ge)-three(Se) fold one for amorphous GeSe.     

Defect physics of the quasi-two-dimensional photovoltaic semiconductor GeSe

GeSe has recently emerged as a photovoltaic absorber material due to its attractive optical and electrical properties as well as earth abundancy and low toxicity. However, the efficiency of GeSe thin-film solar cells (TFSCs) is still low compared to the Shockley-Queisser limit. Point defects are believed to play important roles in the electrical and optical properties of GeSe thin films. Here, we perform first-principles calculations to study the defect characteristics of GeSe. Our results demonstrate that no matter under the Ge-rich or Se-rich condition, the Fermi level is always located near the valence band edge, leading to the p-type conductivity of undoped samples. Under Se-rich condition, the Ge vacancy (V_Ge) has the lowest formation energy, with a (0/2-) charge-state transition level at 0.22 eV above the valence band edge. The high density (above 10¹⁷ cm^-3) and shallow level of V_Ge imply that it is the p-type origin of GeSe. Under Se-rich growth condition, Se_i has a low formation energy in the neutral state, but it does not introduce any defect level in the band gap, suggesting that it neither contributes to electrical conductivity nor induces non-radiative recombination. In addition, Ge_i introduces a deep charge-state transition level, making it a possible recombination center. Therefore, we propose that the Se-rich condition should be adopted to fabricate high-efficiency GeSe solar cells.     

提高单层GeSe对H2O气体传感性能的第一性原理研究

采用第一性原理计算方法,通过在单层GeSe上施加双轴向应变、外加电场、掺Ag等途径来探索提高单层GeSe对H₂O分子传感性能的有效方法,并从微观角度阐明内在机理.计算结果表明,-1.0 V/?的外加电场能有效降低H₂O分子在单层GeSe的吸附能并使二者之间的电荷转移量增加11倍,显著提高了单层GeSe对H₂O分子的响应速度和敏感性.研究结果为进一步设计并制成二维GeSe基湿度传感器提供了理论依据.     

GeTe和GeSe 分子在外电场下的特性研究

对Ge原子采用6-311++G^**基函数,Te和Se原子采用SDB-cc-pVTZ基函数,利用密度泛函理论的局域自旋密度近似方法优化得到了GeTe和GeSe分子的稳定构型,并计算了外电场作用下GeTe和GeSe基态分子的平衡核间距、总能量、最高已占据分子轨道能量E_H、最低未占分子轨道能量E_L、能隙、谐振频率和红外谱强度. 在上述计算的基础上利用单激发组态相互作用-局域自旋密度近似方法研究了GeTe和GeSe分子在外电场下的激发特性. 结果表明:随着正向电场强度的增大,分子核间距逐渐增大,分子总能量逐渐降低,谐振频率逐渐减小,红外谱强度则逐渐增大. 在0-2.0569×10¹⁰ V·m^-1的电场范围内,GeTe分子的E_H 均高于GeSe分子的E_H;随着正向电场的增大,GeTe与GeSe的E_H差逐渐变大,GeTe的E_L低于GeSe的E_L,它们的E_L均随正向电场的增大而增大. 无外场时,GeTe分子的能隙比GeSe分子的能隙要小;在外电场反向增大的过程中, GeTe和GeSe的分子能隙始终减小. 外电场的大小和方向对GeTe和GeSe分子的激发能、振子强度及跃迁的波长均有较大影响. 关键词： GeTe GeSe 外电场 激发态     

The structure of amorphous GeSe and GeTe

We present the results of electronic structure calculations on model 3:3 and 4:2 co-ordinated networks of amorphous GeSe and GeTe. The calculations show that a- GeTe has a predominantly 4:2 chemically ordered structure and suggest that GeSe may be 3:3 co-ordinated. They emphasize how it is often easier to discriminate between different models on the basis of electronic structure rather than other information.     

新型GeSe2-In2Se3-AgI玻璃性能研究

采用传统熔融-淬冷法制备了一系列新型(100-x)(4GeSe2-In2Se3)-xAgI(x=20,30,40mol%)硫系玻璃样品.利用X射线衍射分析、差热分析、可见-近红外吸收光谱、红外透过光谱、喇曼分析等技术手段研究了该玻璃系统的组成、结构、热稳定性和光学特性等.利用Tauc方程计算出了样品的间接带隙;测试了部分样品在不同升温速率下的差示扫描量热曲线,并采用Kissinger法计算了玻璃样品的析晶活化能.X射线衍射数据表明,该玻璃体系在较宽的组分范围内有良好的非晶特性,成玻范围较宽;差热分析和析晶动力学研究表明,玻璃样品70(4GeSe2-In2Se3)-30AgI具有较好的热稳定性(ΔT=114℃)和较高的活化能(Ea=320.4kJ/mol).随着AgI含量的增加,玻璃的短波吸收限蓝移,并且光学带隙有增大的趋势.此外,红外透过光谱分析表明该玻璃体系具有良好的红外透过性能,其红外截止波长不会随着AgI含量的增加而发生明显变化,皆为16μm左右.     

Infrared spectra of some matrix-isolated germanium,tin, and lead chalcogenides

Infrared spectra are reported for GeS, GeSe, GeTe, SnS, SnSe, PbS and PbSe in argon and nitrogen matrices at 12°K. The absorptions due to the matrix-isolated diatomic IV–VI molecules show considerable isotopic structure. The dimer absorptions are treated in terms of a D_2h planar model and indicate that the dimers have nearly square geometries and that the bond stretching force constants in the dimers are about half those of the diatomic molecules.     

二维GeSe纳米片五族和七族原子掺杂的受主和施主杂质态

采用第一性原理的计算方法研究GeSe纳米片结构掺杂V和VII族元素对其电子结构、形成能和跃迁能级的影响.结果表明：无论是掺杂V族还是VII族元素,体系的形成能均随杂质半径的增加而增加.V族元素掺杂体系的跃迁能级随杂质原子半径的增加而降低,而VII元素掺杂的体系却随杂质原子半径的增加而增加.其中,F、Cl、Br和I的掺杂为n型施主浅能级杂质,而N、P和As掺杂为p型受体深能级杂质.为相关的实验研究提供了理论参考.     

Optical and photoelectric properties of GeSe in the energy range 0·5–1·5 eV

It is possible to deduce from the optical and photoelectric measurements of a GeSe single crystal that the absorption of light near the absorption edge corresponds to the indirect forbidden transitions as may be proved by the dependenceK (E-E _i )³. The energy gap isE _g= =(1·10±0·02) eV at room temperature and does not depend on the crystal orientation. The photoconductivity of a GeSe single crystal is relatively high. The interpretation of the photoconductivity measurements confirms the conclusions drawn from optical measurements. The paper is dedicated to the memory of Dr. A. Vaíek, Professor of the J. E. Purkyn University in Brno, who died on November 16, 1966.In conclusion the author would like to express his gratitude to Dr. F. Kosek from the Technical University in Pardubice for the GeSe single crystals and to Dr. J. Kubna from the Department of Solid State Physics, Faculty of Science, Purkyn University in Brno, for the X-ray analysis of the crystals.     

Structural,infra-red and optical measurements in some IV-VI alloy semiconductors

Single crystal alloy specimens of (GeSe)_x(SnSe)_1-x and (GeS)_x(SnS)_1-x have been prepared. The solubility limits, absorption edges, temperature coefficients of the energy gap and far infra-red reflectivities are investigated for the first time in these materials.     

Dc current transport behavior in amorphous GeSe films

The dc electric conduction behavior of amorphous GeSe films sandwiched between the Pt bottom electrode and various counter electrodes (Pt, Cr, and Ti) was examined in a voltage as well as a time domain. The time domain investigation identified time-dependent resistance change and relaxation. The voltage domain analysis revealed that the current transport in these metal-insulator-metal junctions is probably attributed to bulk-limited band-conduction due to delocalized charge carriers.     

Raman scattering in SnSe

Zone center optical phonons in SnSe were investigated using Raman scattering data obtained here and previously reported far infrared reflectivity measurements. The values of the ratio of the pairs of interlayer force constants were calculated te be between 3·3 and 4·5. Thus it can be concluded that SnS is a layer like semiconductor but less layer like than GeS, GeSe or SnS.     

Effect of pressure on phonon spectra and elastic properties of orthorhombic GeSe

The pressure dependences of the phonon frequencies of the Brillouin zone center and also the elastic constants of the GeSe compound have been calculated by the density functional theory method using the ABINIT program package. The results have been compared with the available data of theoretical calculations and measurements of the pressure dependences of the Raman frequency. The calculations have demonstrated that the compound undergoes a continuous phase transition from the simple orthorhombic to body-centered orthorhombic lattice at a pressure about 29 GPa.     

Defects in a disordered world: the structure of glassy GeSe2

The full set of partial structure factors for the prototypical network glass GeSe2 was measured using the method of isotopic substitution in neutron diffraction. The basic building block of the network is the Ge(Se(1/2))(4) tetrahedron in which 34(5)% of the Ge reside in edge-sharing configurations. The intrinsic chemical order of the glass is, however, broken with a maximum of 25(5)% Ge and 20(5)% Se being involved in homopolar bonds at distances of 2.42(2) and 2.32(2) A, respectively.     

Network rigidity in GeSe2 glass at high pressure

Acoustic measurements using synchrotron radiation have been performed on glassy GeSe2 up to pressures of 9.6 GPa. A minimum observed in the shear-wave velocity, associated anomalous behavior in Poisson's ratio, and discontinuities in elastic moduli at 4 GPa are indicative of a gradual structural transition in the glass. This is attributed to a network rigidity minimum originating from a competition between two densification mechanisms. At pressures up to 3 GPa, a conversion from edge- to corner-sharing tetrahedra results in a more flexible network. This is contrasted by a gradual increase in coordination number with pressure, which leads to an overall stiffening of the glass.     

GaSe：AgGaSe_2晶体的光学性能及应用（英文）

从GaSe∶AgGaSe₂熔体(质量掺杂浓度为10%)中生长的非线性光学晶体ε-GaSe∶Ag晶体(质量掺杂浓度≤0.04%)是一种非中心对称晶体,可用于相位匹配频率转换。Ag的掺入使GaSe晶体的显微硬度提高了30%,从而使其可以在任意方向上进行切割和抛光。本文研究了GaSe∶AgGaSe₂晶体在可见、中红外及太赫兹波段的光学性能。实验证明:GaSe∶AgGaSe₂晶体的吸收系数是纯GaSe晶体的2倍,其CO₂激光倍频效率是ZnGeP₂晶体的1.7倍。     

Determination of shallow core level spectra in selected compound semiconductors

Core level spectra of the M shells of Ga, Ge, As, and Se and of the L shell of S have been obtained from X-ray photoemission measurements on GaAs, GeSe, and GeS. Broadening contributions from the achromatic source, the analyzer momentum window, and the extrinsic losses experienced by the photoemitted electrons in traversing the solid, as well as satellite lines due to K_3,4 emission, are removed by deconvolution of the data with a measured electron backscatter spectrum convoluted with a source function. The results are compared with theory where available.     

Far infrared absorption of a-GeSe

Far infrared absorption measurements have been made on a series of a-Ge_xSe_1?x films with near equiatomic compositions. The only sharp feature is an absorption line at 260cm^-1, and the compositional dependance of this feature is correctly described by a model in which the Ge(Se) atoms are 4(2) fold co-ordinated. There is no evidence in our amorphous samples of any absorption band in the frequency range (160?190cm^?1) of the major IR active modes of 3:3 fold co-ordinated crystalline GeSe.