Optical properties of Cl-doped ZnSe epilayers grown on GaAs substrates

We have determined the optical properties of a series of Cl-doped ZnSe epilayers grown on GaAs substrates using ellipsometry and prism coupling. Initially, the carrier concentrations were determined using Hall measurements for samples between 6.30×10¹⁶ cm⁻³ and 9.50×10¹⁸ cm⁻³. Using a variable angle spectroscopic ellipsometer in the energy range between 0.7 eV and 6.5 eV, we then obtained experimental spectra for each of the samples. By incorporating a three-layer model to simulate the experimental data, we determined the complex dielectric functions for these Cl-doped ZnSe epilayers. In order to facilitate this modeling procedure, we have complemented the ellipsometric results with prism coupling experiments that measured the film thickness and the index of refraction (at discrete wavelengths) very precisely. For the fundamental band gap, we observe a blue shift with respect to the doping concentration, which can be explained by the Burstein-Moss effect. In addition, we have determined the critical point parameters related to these specimens by fitting the second derivatives of both the real and the imaginary parts of the dielectric functions. Similar to several doped III-V semiconductors reported thus far, we find that in Cl-doped ZnSe epilayers, both E₁ and E₁ + Δ ₁ red shift, as well as a broadening with respect to the doping concentration.     

ZnSe/ITO thin films:candidate for CdTe solar cell window layer

The crystal structure, electrical and optical properties of ZnSe thin films deposited on an In₂O₃:Sn (ITO) substrate are evaluated for their suitability as the window layer of CdTe thin film solar cells. ZnSe thin films of 80, 90, and 100 nm thickness were deposited by a physical vapor deposition method on Indium tin oxide coated glass substrates. The lattice parameters are increased to 5.834 Å when the film thickness was 100 nm, which is close to that of CdS. The crystallite size is decreased with the increase of film thickness. The optical transmission analysis shows that the energy gap for the sample with the highest thickness has also increased and is very close to 2.7 eV. The photo decay is also studied as a function of ZnSe film thickness.     

Low temperature growth and characterization of ZnSe films grown on GaAs

Metalorganic vapor phase epitaxy (MOVPE) growth experiments of ZnSe on GaAs (100) are described using ditertiary butylselenide as the selenium source. The growth temperature was varied between 300 and 400°C and the growth rate was determined. Below a vapor pressure ratio P_Se/P_Zn of about 5, the selenium is the growth limiting component. The quality of the samples was analyzed by Nomarski microscopy, x-ray diffraction, high resolution transmission electron microscopy, and photoluminescence. Although the selenium-precursor was not specially purified, a sharp excitonic luminescence appears in samples grown at 400°C. The MOVPE growth of ZnSe still suffers from prereactions when H₂Se is used. The optimum growth is above 450°C even with precursors as DMSe or DESe which are less harmful than the hydride. This paper reports results obtained with a novel selenium-precursor: ditertiarybutylselenide.¹     

ZnSe/SiO2薄膜光学常数的椭偏光谱测量

采用溶胶-凝胶工艺与原位生长技术,制备了ZnSe/SiO2复合薄膜。X-射线衍射(XRD)分析表明,ZnSe/SiO2复合薄膜中ZnSe晶体为闪锌矿(立方ZnS)。利用椭偏光谱仪测量了不同ZnSe含量的ZnSe/SiO2复合薄膜的椭偏参数Δ与波长λ的色散关系,采用Maxwell-Garnett(MG)有效介质理论对薄膜的光学常数、厚度、气孔率、ZnSe的浓度进行了计算。结果表明,单层ZnSe/SiO2薄膜厚度在300 nm以上时,随着溶胶体系Zn2 、SeO42-浓度的增加而增大,气孔率在30%左右,ZnSe含量约为溶胶体系中Zn2 、SeO42-浓度的1/2;通过MG有效介质理论的计算表明,可以通过调整旋涂次数及Zn2 、SeO42-浓度来调整薄膜的厚度和ZnSe/SiO2的摩尔比率,可在工艺上控制ZnSe/SiO2复合薄膜光学参数。     

GaAs/InGaAs量子点应变场的TEM研究

运用透射电子显微术(TEM)对由分子束外延(MBE)制备的GaAs／InGaAs多层量子点样品进行观察和分析。利用对量子点周围应变场分布的模拟，定性解释了量子点形貌及其周围发现的凹陷区域。通过对量子点高分辨像显示的晶格错配和化学成分分析研究，解释了所研究样品中量子点尺寸逐层增大的现象。研究结果为量子点材料生长过程中应变场的控制提供了一些思路。     

Si(001)外延ZnSe薄膜界面原子的结合与成键

基于密度泛函理论的第一性原理计算,通过对Si(001)和氮化Si(001)表面单层Zn/Se原子结合的方式,模拟ZnSe外延薄膜的二维生长模式,从单层原子结合能、界面原子电子得失、共价结合成键的角度解释了Zn/Se原子在衬底表面的黏附性问题,阐述了薄膜生长初期界面无定形Se出现等现象,分析了氮化Si(001)表面对薄膜二维均匀生长的作用,结果显示N的引入缓和了Si衬底的非极性共价结合与ZnSe原子间的极性离子键结合之间的异质差异。     

ZnSe纳米材料及界面的制备与表征

利用热蒸发法对ZnSe纳米材料的相控合成进行了研究,制备了多种具有稳定闪锌矿和亚稳纤锌矿结构的ZnSe纳米材料,如ZnSe纳米线、纳米圈、纳米轮及三晶纳米带。同时,在相控合成基础上,通过改变管式炉内反应压强获得了几种同质与异质界面。利用多种透射电子显微学手段对合成的ZnSe纳米材料及相关同质与异质界面进行了深入表征,并对ZnSe纳米材料的生长机理进行了探讨。     

Znse/GaAs超晶格的电子态和芯态激子

本文用半经验紧束缚法计算了ZnSe/GaAs(001)超晶格的能带结构,研究了其能隙与有效质量随层厚的变化.计算了(ZnSe)_5/(GaAs)_5超晶格中与杂质有关的芯态激子,其结果能说明相应异质结中束缚在Ga上的激子峰.本文还提出了该材料中导带底存在界面态.     

双壁碳纳米管的电子显微学研究

本文分析研究了甲酸镍二水合物作为催化剂前驱体,多壁管为原料,采用氢电弧放电法制得的双壁碳纳米管样品中不同管壁数目碳纳米管的分布及双壁碳管直径分布,发现样品中单壁和双壁碳管占主要地位,双壁管的内径主要分布在1.67 nm～3.67 nm之间,外径主要分布在2.33 nm～4.33 nm之间.此外,精确测定了三个双壁碳纳米管的螺旋指数,分别为[(28,3)(29,17)],[(19,8)(29,6)]及[(23,15)(28,22)],其电学性能分别为[半导体性,金属性],[半导体性,半导体性]及[半导体性,金属性].同时,观察了在不同条件下的电子辐照效应.结果表明,经辐照碳管的最内层管壁先发生断裂,其形变速度逐渐变慢,单根碳管易依附于大块物质.在相同的辐照条件下,管壁数目越多碳管越稳定.当管壁数目相同时,直径小的管比直径大的管稳定.高加速电压能够加速碳管的变形.     

Electron microscopy of surface-crater defects on HgCdTe/CdZnTe(211)B epilayers grown by molecular-beam epitaxy

Surface-void defects observed in Hg_1−xCd_xTe (x ∼ 0.2–0.4) alloys grown by molecular-beam epitaxy (MBE) have been investigated using scanning and high-resolution transmission-electron microscopy (HRTEM) as well as atomic force microscopy (AFM). These surface craters, which have been attributed to Hg-deficient growth conditions, were found to originate primarily within the HgCdTe epilayer, rather than at the CdZnTe substrate, and they were associated with the local development of polycrystalline morphology. High-resolution observations established the occurrence of finely spaced HgCdTe/Te intergrowths with semicoherent and incoherent grain boundaries, as well as small HgCdTe inclusions embedded within the Te grains. This study is the first time that high-resolution electron microscopy has been used to investigate this type of defect.     

在ZnO/Si衬底上ZnCdSe/ZnSe多量子阱的生长与光学特性

在经NH3等离子体氮化的Si(100)衬底上。用等离子体增强化学气相淀积(PECVD)的方法生长了ZnO缓冲层，经X射线衍射(XRD)测量，得到了单一取向的ZnO(0002)膜。在此ZnO缓冲层上利用低压金属有机化学气相淀积(LP-MOCVD)方法生长了较高质量的ZnCdSe／ZnSe量子阱。通过不同阱宽的ZnCdSe／ZnSe量子阱生长和测量，得到了多级共振拉曼峰。从发光谱中可见，在1520nm附近有很强的发光，而在未覆盖ZnO的Si衬底上直接生长的ZnCdSe／ZnSe量子阱结构，其光致发光(PL)谱未见发光。可见，在氮化的Si衬底上覆盖ZnO膜生长的ZnCdSe/ZnSe量子阱质量较好。是一种在Si衬底上生长Ⅱ-Ⅵ族化合物半导体材料的有效方法。     

透射电镜电子染色方法的改良

为提高透射电镜生物样品超薄切片的染色效率,在传统染色方法基础上进行改良,使用自制染色装置,在较短时间内完成大批生物样品超薄切片的电子染色。与传统方法相比较省时省药,减少污染机率,电镜观察的超微结构清晰,反差较好。     

Critical thickness determination of InxGa1-xAs/GaAs strained-layer system by transmission electron microscopy

The critical thicknesses of In_xGa_1-xAs/GaAs and GaAs/In_xGa_1-xAs/GaAs strained-layer systems were determined by transmission electron microscopy using the lift-off technique. The onset of misfit dislocation generation has been observed for the first time and the geometries of the misfit dislocations in both uncapped and capped layers correspond to the predicted models. A comparison is given between the predicted and experimental critical thicknesses.     

The effect of high energy electrons during the growth of ZnSe and ZnMgSe by molecular beam epitaxy

Electron irradiation during reflection high-energy electron diffraction is shown to affect the growth of ZnSe and ZnMgSe by molecular beam epitaxy. The high-energy electrons produce an electron stimulated desorption effect during growth of ZnSe which primarily affects adsorbed Se. Se desorption rates under electron irradiation are shown to be significantly larger than thermal desorption rates. Electron irradiation also decreases ZnSe growth rates under Zn-rich conditions. The decrease in growth rate can be suppressed by either growth under Se-rich conditions or by using high-index substrate orientations, in this case (211)B. High-energy electron irradiation does not alter composition during the growth of ZnMgSe.     

Transmission electron microscopy studies of defects in HgCdTe device structures grown by molecular beam epitaxy

Transmission electron microscopy (TEM) was used to evaluate the microstructure of molecular beam epitaxy (MBE) grown (211)B oriented HgCdTe films. TEM analysis of in-situ doped p-on-n and n-p-n device structures will be presented. Under fully optimized growth conditions the substrate-epilayer interface is free of threading dislocations and twins, and a high degree of structural integrity is retained throughout the entire device structure. However, under non-optimal growth conditions that employ high Hg/Te flux ratios, twins can be generated in the p-type layer of p-on-n device structure, resulting in roughness and facetting of the film surface. We propose a mechanism for twin formation that is associated with surface facetting. TEM evaluation of voids, threading dislocations and Te-precipitates in HgCdTe films are also discussed.     

Recent progress on advanced transmission electron microscopy characterization for halide perovskite semiconductors

Halide perovskites are strategically important in the field of energy materials. Along with the rapid development of the materials and related devices, there is an urgent need to understand the structure–property relationship from nanoscale to atomic scale. Much effort has been made in the past few years to overcome the difficulty of imaging limited by electron dose, and to further extend the investigation towards operando conditions. This review is dedicated to recent studies of advanced transmission electron microscopy (TEM) characterizations for halide perovskites. The irradiation damage caused by the interaction of electron beams and perovskites under conventional imaging conditions are first summarized and discussed. Low-dose TEM is then discussed, including electron diffraction and emerging techniques for high-resolution TEM (HRTEM) imaging. Atomic-resolution imaging, defects identification and chemical mapping on halide perovskites are reviewed. Cryo-TEM for halide perovskites is discussed, since it can readily suppress irradiation damage and has been rapidly developed in the past few years. Finally, the applications of in-situ TEM in the degradation study of perovskites under environmental conditions such as heating, biasing, light illumination and humidity are reviewed. More applications of emerging TEM characterizations are foreseen in the coming future, unveiling the structural origin of halide perovskite’s unique properties and degradation mechanism under operando conditions, so to assist the design of a more efficient and robust energy material.     

Cathodoluminescence spectroscopy of deep defect levels at the ZnSe/GaAs interface with a composition-control interface layer

In this work we investigate ZnSe/GaAs heterostructures with an additional 2 nm controlled interfacial layer (CIL) of Se- or Zn-rich composition to modify the band offset. The samples are analyzed as a function of annealing temperature by cathodoluminescence spectroscopy. The as-prepared samples show defect luminescence at ∼ 0.9 eV. With staged annealing at increasing temperatures, both the Zn-rich as well as the Se-rich interfacial layer exhibits luminescence at ∼ 1.9 eV, indicative of defect formation with an onset temperature of ∼400°C. Excitation-dependent spectroscopy provides evidence for defect formation near the interface, which extends into the ZnSe epilayer at higher temperatures. Compared to earlier work, where the threshold temperature for defect formation in bulk samples fabricated under Se-rich growth conditions occurs at temperatures as low as 325°C, the resistance to defect formation has now been improved to that of stoichiometric ZnSe. These results demonstrate that epitaxially grown CILs provide a means to alter ZnSe/GaAs band offsets without degrading the heterojunction’s resistance to defect formation at elevated temperatures.     

Electrical and photo-luminescence properties of ZnSe epitaxial layers grown on GaAs (100) by atmospheric pressure MOVPE: The role of substrate temperature

Epitaxial layers of ZnSe ranging in thickness from 5μm to 30 μm have been grown on GaAs (100) substrates over the temperature range 240° C to 340° C by atmospheric pressure MOVPE employing dimethylzinc and hydrogen selenide. An optimum growth temperature of 280 ± 5° C has been identified and when grown at this temperature the ZnSe epitaxial layers exhibit low resistivity (ρ_{298 K} ≤ 10 ohm · cm), a low compensation ratio (θ_{298 K} = 0.27), a carrier mobility (μ_{298 K}) of 250 ±10 cm²V^-1s^-1) and aren-type (n _{298 K} = 8.0 × 10¹⁴ cm^-3). The ratio of photoluminescence intensity measured at 298K and at 12 K is high (10⁴) and is dominated by a sharp emission due to excitons bound to neutral donors at 2.7956 eV. Mass spectrometric investigations of the chemical reactions occurring inside the reactor in the presence of the GaAs substrate indicate significant surface-controlled reactivity in the region of 280° C.     

In-situ transmission electron microscopy study of the nanoindentation behavior of Al

Nanoindentation is a useful technique for investigating fundamental mechanisms associated with small-volume deformation, processes that are often obscured on coarser scales. Recently, a novel experimental technique of in-situ nanoindentation for the transmission electron microscope (TEM), which provides real-time observations of the mechanisms associated with localized deformation, has been developed. Calibration of the force-displacement-voltage relation and load-frame compliance associated with this instrument allows quantitative force-displacement measurements to be obtained in the manner of traditional indentation experiments. Here, we describe the experimental technique along with methods for quantifying the load-displacement response. Additionally, results from experiments into Al thin films are presented.     

纳米氧化钴电极材料的制备与电子显微学表征

通过模板水热法合成了氧化钴（ CoO）纳米材料。用透射电子显微镜观察发现样品中CoO为岩盐立方结构,以纳米线的形态存在,每根纳米线均由尺寸在30～100 nm的纳米颗粒附着而成。同时,实验产物中也存在一些Co单质。 Co单质以纳米手指的形态存在,且多数具有孪晶结构,孪晶面为｛111｝。 Co孪晶的生成归因于高温高压的反应条件,同时Co单质的存在增强了CoO作为电极材料的导电性能。