硅表面抗反射纳米周期阵列结构的纳米压印制备与性能研究

硅表面固有的菲涅耳反射, 使得硅基半导体光电器件(如太阳能电池、红外探测器)表面有30%以上的入射光因反射而损失掉, 严重影响着器件的光电转换效率. 寻找一种方法降低硅基表面的反射率, 进而提高器件的效率成为近年来研究的重点.本文基于纳米压印光刻技术, 在2 英寸单晶硅表面制备出周期530 nm, 高240 nm的二维六角截顶抛面纳米柱阵列结构. 反射率的测试表明, 当入射光角度为8° 时, 有纳米结构的硅片相对于无纳米 结构的硅片来讲, 在400到2500 nm波长范围内的反射率有很明显的降低, 其中, 800到2000 nm波段的反射率都小于10%, 在波长1360 nm附近的反射率由31%降低为零. 结合等效介质理论和严格耦合波理论对结果进行了分析和验证. 关键词： 纳米压印 截顶抛物面阵列 抗反射 等效介质理论     

Size dependent charge storage characteristics of MBE grown Ge nanocrystals on surface oxidized Si

We have demonstrated the growth of size controlled Ge nanocrystals by molecular beam epitaxy on oxidized Si for the fabrication of floating gate memory structure. The size and density of the nanocrystals have been controlled by varying the growth temperature. The role of interface states and nanocrystals on the memory characteristics has been studied using frequency dependent conductance-voltage measurements. Superior retention characteristics and an enhanced memory window width have been achieved by replacing SiO₂ with high-k Al₂O₃ as a blocking oxide with a higher barrier height.     

Alternating current surface photovoltage in thermally oxidized chromium-contaminated n-type silicon wafers

We investigated a variation of frequency-dependent alternating current (AC) surface photovoltages (SPVs) in thermally oxidized, chromium-contaminated, n-type silicon (Si) wafers. As previously reported, immediately after rinsing in Cr-contaminated solution, a Cr(OH)₃–Si contact causes a Schottky-barrier-type AC SPV on n-type Si. Upon oxidation at 373 K for 10 min, the Schottky barrier collapses and, with further oxidation, a metal-induced negative oxide charge, due to atomic bridging of (CrOSi)⁻ and/or CrO₂^-\mathrm{CrO}_{2}^{-} networks, definitely grows over time in SiO₂. For samples oxidized at temperatures between 823 and 1023 K for 30 min, the observed AC SPV gives evidence that the metal-induced negative oxide charge causes a strongly inverted state of the Si surface. At oxidation temperatures higher than 1023 K and /or for an oxidation time longer than 60 min, the level height of the AC SPV is reduced, implying that the strongly inverted state changes into a less depleted state, whilst, finally, the AC SPV disappears. In this case, the collapse of the (CrOSi)⁻ and/or CrO₂^-\mathrm{CrO}_{2}^{-} networks is anticipated, with a possible change into Cr₂O₃. The existence of the (CrOSi)⁻ and/or CrO₂^-\mathrm{CrO}_{2}^{-} networks has also been confirmed in p-type Si wafers.     

Effect of self-assembled Ge nanostructures on Si surface electronic properties

Incorporating self-assembled Ge islands on Si surfaces into electronic devices has been suggested as a means of forming small features without fine-scale litho- graphy. For use in electronic devices, the electrical properties of the deposited Ge and their relation to the underlying Si substrate must be known. This report presents the results of a surface photovoltage investigation of the surface energy barrier as increasing amounts of Ge are added to a Si surface by chemical vapor deposition. The results are interpreted in terms of band discontinuities and surface states. The surface barrier increases as a wetting layer is deposited and continues to increase as defect-free islands form. It saturates as the islands grow. As the amount of Ge continues increasing, defects form, and the surface barrier decreases because of the resulting allowed states at the Ge/Si interface. Qualitatively similar behavior is found for Si(001) and Si(111). Covering the Ge with Si reduces the surface-state density and possibly modifies the wetting layer, decreasing the barrier to one more characteristic of Si. Initial hydrogen termination of the surface decreases the active surface-state density. As the H desorbs, the surface barrier increases until it stabilizes as the surface oxidizes. The behavior is briefly correlated with scanning-tunneling spectroscopy data. Received: 13 November 2000 / Accepted: 14 November 2000 / Published online: 23 May 2001     

Si(001)表面稀土金属硅化物纳米结构

稀土金属元素的硅化物在n型硅衬底上具有高电导率和低肖特基势垒的特点,在大规模集成的微电子器件领域具有很好的应用价值.文章系统介绍了在Si(001)表面自组装生长的稀土金属硅化物纳米结构的研究进展,较全面地讨论了退火温度、退火时间以及稀土金属表面覆盖度等生长条件对纳米结构生长的影响作用,并在此基础上分析了纳米线、纳米岛的晶化结构,衬底对纳米结构生长的影响,以及纳米结构的演化过程.搞清楚这些内在的生长机理,有助于人们今后实现可严格控制稀土金属硅化物纳米结构的形貌尺寸和分布的自组装生长.此外,文章还介绍了目前人们对稀土金属硅化物纳米线电学性质的研究进展.     

The structure and properties of the Si nanostructures on an HOPG surface

The morphology, electronic structure, and optical properties of self-assembled silicon nanostructures grown on the surface of Highly Oriented Pyrolytic Graphite (HOPG) by molecular beam epitaxy were studied by ultra high vacuum (UHV) scanning tunneling microscopy (STM) and X-ray photoemission spectroscopy (XPS) in situ, and by Raman spectroscopy ex situ. At coverages of less than 1 monolayer (ML), the formation of monolayered silicon nanoislands with an atomic structure similar to that of graphene was observed.     

Structure and properties of Si nanostructures on highly oriented pyrolitic graphite surface

The morphology, electronic structure, and optical properties of self-assembled silicon nanostructures grown on the surface of highly oriented pyrolitic graphite (HOPG) by molecular beam epitaxy have been studied by ultra-high-vacuum scanning tunneling microscopy and X-ray photoemission spectroscopy in situ, as well as by Raman spectroscopy ex situ. At a coverage of less than one monolayer, formation of monolayer silicon nanoislands with an atomic structure similar to that of graphene has been observed.     

CdTe and PbTe nanostructures on the oxidized pentagonal surface of an icosahedral AlPdMn quasicrystal

By means of congruent evaporation, we have deposited CdTe and PbTe onto the oxidized fivefold-symmetry surface of an icosahedral AlPdMn quasicrystal. This procedure results in the formation of nanocrystals in both cases. While the azimuthal orientations of the crystallites are random, the polar orientations are well defined. The crystalline CdTe and PbTe domains expose their (1 1 1) and (0 0 1) faces, respectively, which are aligned parallel to the pentagonal surface of the quasicrystal. The nanometric size of the domains is not a result of the lattice mismatch between the growing film and the substrate as usually observed in molecular-beam epitaxy, but of the limited size of the oxide domains of the substrate surface.     

Bioinspired Si subwavelength gratings by closely-packed silica nanospheres as etch masks for efficient antireflective surface

We experimentally investigate the antireflective properties of various silicon (Si) subwavelength grating (SWG) structures using closely-packed silica nanospheres monolayers with different sizes as etch masks and a subsequent inductively coupled plasma (ICP) etching, together with theoretical calculations based on a rigorous coupled wave analysis method. The geometric structure of Si SWGs is optimized by changing the size of nanospheres and ICP etching parameters. The antireflective properties depend strongly on the period, height, and shape of the hexagonally ordered SWG structures, especially correlated with ICP etching parameters. For an optimized Si SWG structure with a rounded cone shape, the reflectance is significantly reduced, indicating a low reflectance of <4.4% over a wide wavelength region of 300–1100 nm. From theoretical analysis, the reflectance of rounded cone-shaped Si SWG structures is minimized with a period of ∼300–350 nm and heights of >750 nm, which is reasonably consistent with the experimental results. The angle-dependent antireflection characteristics are also discussed.     

ESR studies of thermally oxidized silicon wafers

We have performed ESR measurements on oxidized (111) silicon wafers and found two ESR centers. One, the previously known P_b center,was found to be a two level interface trap by corona bias studies. The other one, previously known as “damaged silicon” center was found to be a silicon center, probably related to the oxide growth process.     

MBE fabrication of self-assembled Si and metal nanostructures on Si surfaces

Two types of fairly regular distributions of Si nanostructures, of interest as templates to grow spatially controlled ensembles of metal (Co, Fe, Ag, etc.) nanostructures, are presented in this paper. Both of them are achieved by self-assembling processes during Si homoepitaxy. One corresponds to films grown by molecular beam epitaxy (MBE) on Si(0 0 1)-2 × 1 surfaces with low (<1°) miscut angles. In this case, arrays of 3D Si-islands displaying well defined pyramid-like shapes can be obtained, as evidenced by Scanning Force Microscopy (SFM) and Scanning Transmission Electron Microscopy (STEM). Such arrays exhibit strong similarities with those reported for Ge and SiGe islands on Si(0 0 1), and may thus serve as a simpler route to produce ordered distributions of metallic nanodots. On the other hand, on Si(1 1 1)-7 × 7 vicinal substrates misoriented 4° toward the direction, step rearrangement during homoepitaxy permits to produce nanopatterned surfaces, the building-blocks of which are triangular (1 1 1) platforms, with lateral dimensions of hundreds of nanometers, bound by step bunches about 30 nm high. Furthermore, different Ag deposition experiments support this spontaneous patterning on Si(1 1 1) as a promising approach to achieve regular distributions of metallic nanocrystals with an overall homogeneity in sizes, shapes and spacing.     

Optical and electrical properties of thermally oxidized bismuth thin films

Bismuth trioxide (Bi₂O₃) thin films were prepared by dry thermal oxidation of metallic bismuth films deposited by vacuum evaporation. The oxidation process of Bi films consists of a heating from the room temperature to an oxidation temperature (T_o = 673 K), with a temperature rate of 8 K/min; an annealing for 1 h at oxidation temperature and, finally, a cooling to room temperature. The optical transmission and reflection spectra of the films were studied in spectral domains ranged between 300 nm and 1700 nm, for the transmission coefficient, and between 380 nm and 1050 nm for the reflection coefficient, respectively. The thin-film surface structures of the metal/oxide/metal type were used for the study of the static current-voltage (I-U) characteristics. The temperature of the substrate during bismuth deposition strongly influences both the optical and the electrical properties of the oxidized films. For lower values of intensity of electric field (ξ < 5 × 10⁴V/cm), I-U characteristics are ohmic.     

Formation of different periodic nanostructures on semiconductors

The interaction of femtosecond pulses with surfaces of semiconductors accompanied by the appearance of different nanostructures and nanoholes is studied. The formation of nanostructures was observed for semiconductors placed into different media. The effect of different parameters (angle of incidence, light polarization, number of shots, pulse duration, etc.) on the formation of the structures is analyzed. The period of the arising ordered structures could be either on the order of the light wavelength (700–800 nm) or much smaller (180 nm).     

Nanosecond laser-induced reshaping of periodic silicon nanostructures

We demonstrated the use of laser-induced reshaping to produce periodic silicon nanostructures (PSNs) with different geometries. Periodically located silicon nanostructures were preformed by dry etching of a silicon wafer covered with a monolayer of self-assembled polystyrene nanospheres. These PSNs were reshaped under ambient conditions by irradiation with two kinds of nanosecond lasers (532 nm and 355 nm). The effects of the irradiation parameters on the reshaped geometry were systematically investigated. Vertical growth of the irradiated PSNs resulted from the epitaxial deposition of rich silicon vapor during laser irradiation. However, the growth was limited even with higher laser fluence because of the nanoscale structure, the size of which is smaller than the melting depth induced by the nanosecond lasers. The reshaped PSNs displayed reflection spectra that are tunable by varying the characteristics of reshaping-laser input. This method offers a promising approach for the site-selective fabrication of optically tunable 3D nanostructures.     

Memory element based on Si/CaF<Subscript>2</Subscript> periodic nanostructures

A memory element based on a Si/CaF₂ periodic nanostructure is proposed. In this element, information is recorded through charge capture by trap states in a CaF₂ dielectric. The high and low signal levels correspond to the current in the maximum and minimum of the negative differential resistance region, which forms as a result of the resonant tunnel distribution of charge carriers over trap levels in the dielectric. The speed of such logical elements depends on the rate of activation carrier trapping and the rate of tunnel carrier transfer from one state to another. It is shown that both Si/CaF₂-based logical elements and memory elements proposed operate at temperatures from 77 to 300 K, have a switching time of 10⁻¹²–10⁻¹⁰ s, and are compatible with silicon IC technology.     

One-dimensional Ge nanostructures on Si(001) and Si(1 1 10): Dominant role of surface energy

Ge/Si(001) is a prototypical system for investigating three-dimensional island self-assembly owed to the Stranski–Krastanow growth mode. More than twenty years of research have produced an impressive amount of results, together with various theoretical interpretations. It is commonly believed that lattice-mismatch strain relief is the major driving force leading to the formation of these islands. However, a set of recent results on Si(001) and vicinals point out that, under suitable conditions, this is not the case. Indeed, we here review experimental and theoretical results dealing with nanostructures mainly determined by surface-energy minimization. Results are intriguing, as they reveal the existence of magic sizes, show the presence of very peculiar morphologies, such as micron-long wires, and distinguish among attempts to facet the wetting-layer and true SK islands.     

Spherical growth and surface-quasifree vibrations of Si nanocrystallites in Er-doped Si nanostructures

Si-based Er-doped Si nanostructures were fabricated for exploring efficient light emission from Er ions and Si nanocrystallites. High-resolution transmission electron microscopy observations reveal that Si nanocrystallites are spherically embedded in the SiO2 matrix. Energy-dispersive x-ray analysis indicates that the Er centers are distributed at the surfaces of nanocrystallites surrounded by the SiO2 matrix. Low-frequency Raman scattering investigation shows that Lamb's theory can be adopted to exactly calculate the surface vibration frequencies from acoustic phonons confined in spherical Si nanocrystallites and the matrix effects are negligible.     

XPS study of annealing induced effects on surface and interface electronic properties of Si/Ge nanostructures

The present study is focused on the influence of vacuum thermal treatment on surface/interface electronic properties of Si/Ge multilayer structures (MLS) characterized using X-ray photoelectron spectroscopy (XPS) technique. Desired [Si(5 nm)/Ge(5 nm)]_×10 MLS were prepared using electron beam evaporation technique under ultra high vacuum (UHV) conditions. The core-level XPS spectra of as-deposited as well as multilayer samples annealed at different temperatures such as 100 °C, 150 °C and 200 °C for 1 h show substantial reduction in Ge 2p peak integrated intensity, whereas peak intensity of Si 2p remains almost constant. The complete interdiffusion took place after annealing the sample at 200 °C for 5 h as confirmed from depth profiling of annealed MLS. The asymmetric behaviour in intensity patterns of Si and Ge with annealing was attributed to faster interdiffusion of Si into Ge layer. However, another set of experiments on these MLS annealed at 500 °C suggests that interdiffusion can also be studied by annealing the system at higher temperature for relatively shorter time duration.     

A refined Ehrlich-Schwoebel effect on the modification of Si surface nanostructures by post ion milling

A modeling work has been conducted on a phenomenon called post ion milling (PIM), a post-treatment of Ar⁺ ion sputtering to modify nanostructures on solid surface. It was found by experiments that for PIM with a sufficiently low ion flux, both the average dot size and the surface roughness of Si nanodot arrays on Si(1 0 0) decline steadily against milling time. However, the usually adopted Kuramoto-Sivashinsky (KS) model involving the Bradley-Harper (BH) theory failed to explain the experimental results, nor the KS model that combines both the BH and Ehrlich-Schwoebel (ES) effects. We reexamined the ES term in the KS equation, and derived new terms reflecting the ES contribution. With such a modification, the KS model involving both the BH and the refined ES effects finally gave a qualitative explanation to the PIM result.