Optical spectroscopy of excitonic states in zinc diarsenide

The luminescence and transmission of zinc diarsenide single crystals near the fundamental absorption edge have been investigated in the temperature range 4.2–300 K. Intense luminescence and absorption lines at 1.0384, 1.0488, and 1.0507 eV, referring to the ground state (n=1) and excited states (n=2, n=3) of a free exciton were observed at low temperatures. The free-exciton binding energy was found to be ∼13.9 meV on the basis of the hydrogen-like model and the direct band gap was found to be 1.0523, 1.0459, and 0.9795 eV at 4.2, 78, and 300 K, respectively. Fiz. Tekh. Poluprovodn. 31, 1029–1032 (September 1997)     

CuInSe_2纳米晶体的制备研究

以三乙醇胺为溶剂,通过易于实现的非注入法(noninjection)合成铜铟硒(CuInSe2)纳米晶体。采用SEM、XRD、XPS和EDX分析了所制CuInSe2纳米晶体的组分与结构。结果表明,合成的CuInSe2纳米晶体为类球形,尺寸均匀,具有单一相结构并且接近化学计量比。该纳米晶体的能带隙为(1.03±0.03)eV,对可见光吸收良好,符合光伏应用的要求。     

Lattice vibrations in CuInSe2 crystals

Infrared reflection spectra of CuInSe₂ crystals are investigated for the polarizations E∥c and E⊥c. Contours of the reflection spectra are calculated and phonon parameters and dielectric constants are determined. The effective charges of the Cu, In, and Se ions in these materials are determined. The two-phonon absorption spectra in CuInSe₂ are investigated and the absorption bands are identified from the selection rules. Fiz. Tekh. Poluprovodn. 31, 329–334 (March 1997)     

The lattice constants of CuInSe2

The lattice parameters of stoichiometric CuInSe2 and of several related defect structures grown from off-stoichiometric melt compositions are reported. For CuInSe₂ a = 5.814 ± .003 A and c = 11.63 ± 0.04 A corresponding to c/a = 2.000 ± 0.008. The current JPDS data for CuInSe₂ represent an average of the lattice constants of off-stoichiometric defect structures within its homogeneity range.     

Valley dynamics of different excitonic states in monolayer WSe2 grown by molecular beam epitaxy

Monolayer transition-metal dichalcogenides possess rich excitonic physics and unique valley-contrasting optical selection rule, and offer a great platform for long spin/valley lifetime engineering and the associated spin/valleytronics exploration. Using two-color time-resolved Kerr rotation and time-resolved reflectivity spectroscopy, we investigate the spin/valley dynamics of different excitonic states in monolayer WSe₂ grown by molecular beam epitaxy. With fine tuning of the photon energy of both pump and probe beams, the valley relaxation process for the neutral excitons and trions is found to be remarkably different—their characteristic spin/valley lifetimes vary from picoseconds to nanoseconds, respectively. The observed long trion spin lifetime of > 2.0 ns is discussed to be associated with the dark trion states, which is evidenced by the photon-energy dependent valley polarization relaxation. Our results also reveal that valley depolarization for these different excitonic states is intimately connected with the strong Coulomb interaction when the optical excitation energy is above the exciton resonance.     

Cathodoluminescence of p-n-p microstructures in CuInSe2 crystals

Microstructures in p-CuInSe₂ single crystals tailored by the strong electric field have been studied using the method of local (d⩽1 μm) cathodoluminescence (CL). The shortest-wavelength radiation (ℏω=1.023 eV) has been observed from the n-type layer and longer-wavelength radiation (ℏω=1.006 eV)—from the p-type regions. An analysis of the cathodoluminescence spectra has allowed us to attribute the experimental features to optical transitions associated with donor and acceptor levels of V _Cu, V _Se, and Cu_i point defects in the crystal. Test measurements of EBIC, the C–V characteristics, and the DLT spectra confirm the cathodoluminescence data and reveal additional features of the p-n-p microstructures. Fiz. Tekh. Poluprovodn. 31, 114–119 (January 1997)     

The properties of CuInSe2-MnSe crystals

The composition and the impurity back-ground levels of (CuInSe₂)_1-x (MnSe)_2x alloy crystals are characterized by ion microprobe analyses. Room temperature photoreflectance (PR) and photoconductivity (PC) measurements in the temperature range 10 K ⪯ T ⪯ 300 K reveal changes in the point defect chemistry and a shift of the band gap with increasingx towards higher energies. Also, a deep luminescence feature at 0.89 eV is observed in Mn doped crystals, in addition to the contributions at 0.90 and 0.94 eV that appear in both doped and nominally undoped CuInSe₂. X-ray diffraction studies show that the chalcopyrite structure is retained to at leastx = 0.25.     

Electrical properties and luminescence of CuInSe2

We have studied the electrical properties and luminescence spectra of melt-grown CuInSe₂ single crystals and the doping of the crystals with intrinsic defects and extrinsic impurities. The preparation and the properties of p-n junctions are also discussed. We find that the low temperature photoluminescence spectra of crystals as-grown or annealed between 400 and 700 C are characteristic of the conductivity type. At 77 K, p-type crystals emit in a band peaked at 1.00 eV (type A spectrum) whereas the emission of n-type crystals peaks at 0.93 eV (type B spectrum). Type A and type B spectra can be interchanged by alternate anneals in minimum or maximum Se pressure. Type B emission dominates the electroluminescence spectrum of p-n junctions. These features are explained by the appearing and disappearing of a donor level as a consequence of annealing in vacuum or in Se atmosphere. We find that Zn and Cd act as donors. Crystals doped with these impurities have electron concentrations above 10¹⁸ cm⁻³. Zn-dopped samples exhibit a very broad recombination band below the bandgap in photoluminescence both at 77 and 300 K.     

Structural and electronic properties of CuInSe2

Structural investigation on monocrystalline CuInSe₂ samples has been made. From the single crystal results, the space group of CuInSe₂ was confirmed to be Iˉ42d and the crystal solidification direction was investigated. Compositional uniformity of the ingots was established by EPMA and it was found that the indium concentration was greater than that for copper. Systematic annealing experiments were carried out in vacuum at different temperatures (as low as 160° C) and for different times. Large variation in resistivity was observed after the annealing treatment. P-type samples were found to convert to n-type after the heat-treatments.     

Phase composition of CuInSe2 in different annealing process

CuInSe₂ was synthesized by a low cost, non-vacuum hydrothermal solution method using copper chloride, indium chloride and Se powder as raw materials. The reaction schemes of CuInSe₂ in different annealing processes were investigated by in-situ X-ray diffraction measurements. Its phase composition, crystal structure and morphology properties of CuInSe₂ were studied. The results show that CuInSe₂ has chalcopyrite crystal structure and it remains stable below the temperature of 773 K, but it decomposes to CuSe and InSe at temperature above 773 K in vacuum annealing. While in oxygen annealing, CuInSe₂ is oxidized to CuO, In₂O₃ and SeO₂ at the temperature of 523 K. Therefore, the temperature of selenization or annealing must be lower than 773 K in order to reduce the amount of CuSe and InSe in large production scale. The results were demonstrated by energy dispersive spectrometer as well. The process of its reaction mechanism was discussed based on the experimental data.     

Optical frequency synthesis spectroscopy

In order to measure the super narrow spectral features of cooled atoms and ions, in the optical region, optical frequency synthesis (OFS) techniques rather than wavelength techniques must be used. It is anticipated that many of these resonances will be in the optical region of the spectrum, and this paper will address the state-of-the-art of the measurements of frequencies in that region. Two recent optical frequency measurements of iodine transitions in the visible will be described as well as recent improvements in fabricating the point-contact diode used in these measurements.     

Irregular shifts of excitonic states in reflectivity and PL spectra of ZnSe crystals under uniaxial stress

Splittings and shifts of excitonic states in ZnSe crystals under uniaxial stress along [100], [111] and [110] directions were studied by spatially resolved spectroscopy of both the reflectivity and the photoluminescence (PL) at 5 K. These methods enabled us to check the local strains in different surface microareas of deformed crystals and as a result to reveal the irregular free and bound exciton state shifts related to the influence of the strain nonuniformity and to the presence of structural imperfections (twins) in the crystals. It is shown that elastic deformation of twinned crystals along directions not exactly [111] gives rise to a doublet structure for all the bound exciton lines in the PL spectra. New values of deformation potentials for ZnSe crystals were deduced. The anisotropic influence of the uniaxial stress on the free and bound exciton state splittings was determined.     

两种制备多晶铜铟硒薄膜方法的比较

采用真空顺序蒸发铜铟金属预置层后真空硒化退火的方法(硒化法),以及真空三元叠层蒸发后氮气气氛退火的方法(叠层法)分别制备了太阳电池吸收层材料CuInSe2薄膜.通过X射线衍射、扫描电子显微镜、能量色散X射线分析技术等分析手段对薄膜进行了表征.结果表明:两种方法制备的薄膜形貌都比较致密均匀,晶粒直径分别约1.5 μm和约1 μm.组分分析表明所制薄膜均为富铜CIS.硒化法制备的CIS薄膜具有单一的黄铜矿相结构;而叠层法制备的薄膜含有少量杂相,如β-In2Se3等.因此硒化法制备的薄膜更适于作为太阳能吸收层材料.     

Changes in the opto-electronic properties of CuInSe2 following ion implantation

The development of efficient thin-film solar cells based on CuInSe₂ absorber layers has encouraged fundamental research on both thin films and single crystals of this chalcopyrite semiconducting compound. The resistance to radiation and ion bombardment is of technical importance particularly for a material which could find future applications in space photovoltaic power systems. In this paper results are described for an ion implantation study using CuInSe₂ single crystal substrates. Oxygen, helium and neon implantations have produced significant changes in surface resistivity and photoconductivity. Also the near-surface regions ofn-type crystals have been type-converted top-type following ion implantation. It is apparent that the ion implantation process creates defects which affect surface state densities and recombination probabilities. In the case of oxygen there is an additional doping effect caused either by the introduction of acceptor states or by the reduction of the existing donor state population. Following implantation there appears to be an overall decrease in carrier recombination at the surface which leads to an enhanced photoconductive response.     

CuInSe2纳米颗粒膜的磁控溅射制备和特性分析

采用Cu,In,Se三元扇形复合靶,在玻璃基片上用射频磁控反应溅射技术制备CuInSe2(CIS)纳米颗粒膜.CIS颗粒的大小可通过改变溅射功率、基片温度和膜厚来调节.测量并讨论了所制备的CIS纳米颗粒膜的内部晶相结构、电阻率、导电类型以及光吸收等性质.     

Photovoltaic effect in a p-type CuInSe2/green leaf heterojunction

Photosensitive p-type CuInSe₂/green leaf heterojunctions are fabricated. The photocurrent polarization indicatrix, as well as the spectral dependences of the quantum efficiencies for photoconversion and of the natural photopleochroism of the heterojunctions, are measured. The polarization dependence of the photosensitivity suggests that the upper valence band in CuInSe₂ is of type G₇. A window effect is observed in the sensitivity ratio and these heterojunctions may find applications as photoconverters for the intensity and polarization of light. Fiz. Tekh. Poluprovodn. 31, 193–196 (February 1997)     

Lattice vibrations of sequentially evaporated CuInSe2 by Raman microspectrometry

Copper Indium Diselenide (CuInSe₂) thin films of different atomic concentrations were prepared using sequential elemental evaporation. Raman analysis has been carried out to determine the structure and the different vibration modes of CuInSe₂ thin film. The analysis reveals that CuInSe₂ films possess chalcopyrite structure with preferential orientation along (112) plane. Vibration modes such as A₁, B₁, B₂ and E were observed in the Raman spectra. It has been observed that the intensity of the A₁ mode is directly proportional to the crystallinity. Also it has been noted that, the intensity of all the vibration modes are varied with respect to the compositions. The Raman spectra of the CuInSe₂ films were interpreted and incorporated with XRD and EDAX.     

Photosensitivity of structures produced by heat treatment of CuInSe2 in different media

Photosensitive structures were produced by heat-treatment of polycrystalline p-and n-CuInSe₂ substrates in vacuum and in air at temperatures near 500 °C. The spectral dependences of the photosensitivity of two types of structures in natural and linearly polarized light were investigated and analyzed. The photosensitivity of the best structures reached 16 mA/W at T=300 K. The laws of polarization photosensitivity of such structures were determined and discussed in relation to the fabrication conditions of the structures. It was concluded that there is a new possibility of using polarized photoelectric spectroscopy for diagnostics of phase interactions in complex semiconductors and for optimizing the technology for producing photoconversion structures. Fiz. Tekh. Poluprovodn. 33, 954–958 (August 1999)     

Optical crosstalk-induced switching between bistable soliton states

It is shown that the controlled use of optical crosstalk can induce switching between bistable soliton states in a fiber whose intensity-dependent refractive index is described by the linear plus smooth step model. The closely related phenomena of soliton fission, soliton fusion, radiation stripping, and the soliton transparency-extinction transition are also examined. The evanescently coupled model equations use are discussed, and numerical results are given. It is stressed that the general features of the crosstalk-induced switching and related phenomena do not depend on the specific nonlinearity (the LSS model) chosen     

量子阱态光电子谱研究的理论模型

近年来，光电子谱被广泛应用于薄膜中量子阱态的研究。为解释薄膜中分立的量子阱态和薄膜的光电子谱间的关系，发展了一些理论模型。介绍了近自由电子模型、相位积累模型和电子干涉模型等关于薄膜中量子阱态的理论模型，并用它们解释了量子阱态在光电子谱中峰位和线宽．线宽由准粒子寿命的倒数Γ以及电子在表面和界面反射系数的和R决定，电子波矢κ以及电子在表面和界面相位移之和Φ决定了峰的位置。