Growth of Ultrasmooth Silver Thin Films Deposited with a Germanium Nucleation Layer
Junce ZhangDavid M. FryaufMatthew GarrettV. J. LogeeswaranAtsuhito SawabeMaidul IslamNobuhiko P. Kobayashi
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Growth of Ultrasmooth Silver Thin Films Deposited with a Germanium Nucleation Layer Junce Zhang 1 , David M. Fryauf 1 , Matthew Garrett 1 , VJ Logeeswaran 2 , Atsuhito Sawabe 3 , M. Saif Islam 2 , Nobuhiko P. Kobayashi 1 1) Nanostructured Energy Conversion Technology and Research (NECTAR), Department of Electrical Engineering 1) Baskin School of Engineering, University of California Santa Cruz, Santa Cruz, CA, U.S.A. 2) Department of Electrical & Computer Engineering, University of California at Davis, One Shields Avenue, Davis, California 95616, United States 3) Department of Electrical Engineering and Electronics, College of Science and Engineering, Aoyama Gakuin University, 5-10-1 Fuchinobe, Chuo-ku, Sagamihara-shi, Kanagawa 252-5258, Japan XRD, GIXRD, and XRR Ultrasmooth Ag Introduction Silver (Ag) thin films have been the most frequently employed material for innovative applications in a wide range of fields including nanoplasmonics, solar energy, optical waveguides, OLEDs, and superlenses. The conventional methods of depositing Ag thin films on insulators by e-beam evaporation, chemical vapor deposition, ion-beam sputtering, rf/dc sputtering, electroless plating, and pulsed laser deposition tend to proceed in Volmer−Weber growth mode, which often results in rough Ag surface morphologies. Such rough surface morphology combined with large grain size leads to a significant loss that severely lowers the performance and yield of, for instance, plasmonic devices and metamaterials. The structural properties of optically thin (15 nm) Ag films deposited on SiO2/Si(100) substrates with a germanium (Ge) nucleation layer were studied. The surface roughness of Ag thin films was found to decrease significantly by inserting a Ge nucleation layer with a thickness in the range of 1 to 2 nm (i.e., smoothing mode). However, as the Ge nucleation layer thickness increased beyond 2 nm, the surface roughness increased (roughing mode). Experiment Si(100) covered with a ∼3 nm native oxide (SiO2) layer was used as the substrate. The SiO2/Si substrates were treated in a cleaning bath of H2SO4/H2O2 (3:1), rinsed with deionized water, and dried with nitrogen (N2). Subsequently, Ge and Ag were sequentially deposited onto the substrates, without breaking vacuum, in an electron-beam evaporation system (CHA Mark 50 ISS). The evaporation chamber was held at a base pressure of ∼1 μTorr and ambient temperature during the deposition. The deposition rates of Ge and Ag were 0.01 and 0.1 nm/ s, respectively. The thickness of Ge was varied (0, 1, 2, 5, and 15 nm) while the thickness of Ag was fixed at 15 nm. . The morphological and crystallographical characteristics of Ag thin films with different Ge nucleation layer thicknesses were assessed by cross-sectional transmission electron microscopy (XTEM), reflection high-energy electron diffraction (RHEED), X-ray diffractometry (XRD), grazing incidence X-ray diffractometry (GIXRD), X-ray reflection (XRR), and Fourier transform infrared spectroscopy (FTIR). Figure 3. Grazing incidence X-ray diffraction (GIXRD) of five Ag/Ge/ SiO2/Si samples with different Ge thicknesses. The total intensity (i.e., the sum of the four peak intensities) decreases and reaches a minima as dGe thickness increases from 0 to 2 nm, suggesting that the polycrystalline Ag film with dGe = 0 nm becomes quasi-amorphous with dGe = 2 nm. Figure 4. X-ray diffraction (XRD) of five Ag/Ge/SiO2/Si samples with different Ge thicknesses. The sample without the Ge layer has the smallest FWHM of the Ag(111) peak at 38.2°, indicating the largest crystallite size in these five samples. The samples with a thin Ge layer (1−5 nm) exhibit broad Ag(111) peaks manifesting in the Ag film containing smaller grains. With thicker Ge layers (5−15 nm), the crystallite sizes increased again, leading to a rougher Ag surface. XTEM and RHEED Figure 1. XTEM images of Ag/Ge/SiO2/Si for various thicknesses of Ge at two different magnification settings: (a, b) with no Ge nucleation layer, (c, d) with a 2 nm Ge nucleation layer, and (e, f) with a 15 nm Ge nucleation layer. The scale bar for (a, c, e) is 50 nm and for (b, d, f) is 5 nm. Figure 2. RHEED patterns of Ag/Ge/SiO2/Si stacks for near-optimal Ge thickness: (a) with 0 nm Ge, (b) with 0.5 nm Ge, (c) with 1 nm Ge, and (d) with 2 nm Ge. The RHEED pattern of a sample without a Ge layer shows many distinct Laue rings, indicative of a polycrystalline Ag surface. As the thickness of Ge progresses from 0.5 to 1 to 2 nm, the rings became more diffuse, which is evidence of a smoother Ag surface. X-Ray Summary Figure 5. X-ray reflection (XRR) of five Ag/Ge/SiO2/Si samples with different Ge thicknesses (left). The XRR spectrum of the sample without a Ge layer exhibits a damped oscillation amplitude, indicating the presence of a rough surface. For samples with 1 and 2 nm Ge layers, the oscillations in the reflected X-ray intensity show rather persistent and consistent oscillation in reciprocal space, indicating the presence of much smoother surfaces. With an increased Ge thickness to 5 nm and then 15 nm, the oscillation becomes less persistent, suggesting that the surface roughness increases with a Ge layer thicker than 2 nm. The experimental XRR profiles were fitted by varying the thickness and interfacial roughness for the Ag/Ge/SiO2 stacks by employing a genetic algorithm model. The obtained Rsurf of Ag surfaces with different Ge thicknesses are shown (right). FTIR of SAMs Self-assembled monolayers (SAMs) of CH3-terminated alkanethiolate (CH3-(CH2)17-SH) were formed by immersion in an ethanol solution containing the alkanethiolate (molar concentration of 0.01 M) for extended times (>24 h) at room temperature to obtain a strong hydrophobic surface on Ag thin films deposited with and without a 2 nm Ge nucleation layer. FTIR spectra provide indirect but highly relevant proof of the smoothing role of a 2 nm Ge nucleation layer. The sample with a 2 nm Ge layer resulted in stronger absorbance at all four peaks and a narrower a-CH2 peak, suggestive of a more orderly arrangement of alkanethiolate complexes, which was presumably caused by a smoother Ag surface established on the 2 nm Ge layer. SAMs Table 1. FWHM and corresponding average crystallite sizes from XRD and relative intensity of the peaks associated with each lattice plane from GIXRD for the five samples XRD samples 1.15 nm Ag/SiO2/Si 2.15 nm Ag/1 nm Ge/SiO2/Si 3.15 nm Ag/2 nm Ge/SiO2/Si 4.15 nm Ag/5 nm Ge/SiO2/Si 5.15 nm Ag/15 nm Ge/SiO2/Si GIXRD FWHM crystallite 111) (degree) size (nm) (38.2°) (44.3°) (64.3°) Ag Ge SiO2 Figure 6. Illustration of (left) SAMs disorderly adhering to Ag/SiO2 surface and (right) SAMs with greater order and alignment on Ag/Ge/SiO2 surface. Illustrations (not drawn to scale) represent the assumption that smooth Ag on Ge wetting layer allows SAMs to adhere more orderly and yield stronger FTIR vibrational mode signals. Figure 7. FTIR of two silver thin film samples with an alkanethiolate SAM coating. The blue plot shows SAM/Ag/SiO2 absorbance, and the red plot shows SAM/Ag/2 nm Ge/SiO2 absorbance. In both cases, the Ag nominal thickness was 15 nm. The stronger absorbances and narrower a-CH2 peak at 2916.806 cm−1 show a more ordered arrangement of alkanethiolate complexes, indicative of a less rough surface in the sample with a 2 nm Ge layer.Cite
Abstract Low‐temperature (350 °C) crystallization of amorphous Ge films on SiO 2 was investigated using Al‐induced layer exchange (ALILE) process. Thicknesses of Ge and catalytic Al layers were varied in the range of 30‐300 nm, which strongly influenced the ALILE growth morphology. Based on the study, the Ge thickness was adjusted to 40 nm while the Al thickness was adjusted 50 nm. This sample satisfied both of the surface coverage of polycrystalline‐Ge and the annihilation of randomly oriented Ge regions. Moreover, the enhancement of the heterogeneous Ge nucleation improved the (111) orientation and the grain size. As a result, the area fraction of the (111)‐orientation reached as high as 97% and the average grain size as large as 70‐μm diameters. This (111)‐oriented Ge layer with large‐grains promises to be the high‐quality epitaxial template for various functional materials to achieve next‐generation devices. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)
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Abstract Au layer thickness dependence (9–34 nm) of Ge crystallization in the metal-induced layer exchange process has been investigated. It has been found that Ge crystals are (111) oriented when the Au layer is as thin as 9 nm, whereas crystal grains are randomly oriented when the Au layer is as thick as 34 nm. The difference is discussed in terms of the difference in the position of nucleation sites of Ge crystals.
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Al-induced crystallization yields the larger grain and (111)-orientation planes of poly-Ge thin film grown on SiO2 substrate, the (111)-orientation planes of poly-Ge thin film grown on SiO2 substrate are very important for the superior performance electronics and solar cells. We discussed the 50 nm thickness poly-Ge thin film grown on SiO2 substrate by Alinduced crystallization focusing on the lower annealing temperature and the diffusion control interlayer between Ge and Al thin film. The (111)-orientation planes ratio of poly-Ge thin film achieve as high as 90% by merging the lower annealing temperature (325℃) and the GeOx diffusion control interlayer. Moreover, we find the lack of defects on poly-Ge thin film surface and the larger average grains size of poly-Ge thin film over 12 μm were demonstrated by electron backscatter diffraction measurement. Our results turn on the feasibility of fabricating electronic and optical device with poly-Ge thin film grown on SiO2 substrate.
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The radio frequency magnetron sputtering method is used to prepare well-dispersed pyramidal-shaped Ge nanoislands embedded in amorphous SiO2sublayers of various thicknesses. The estimated size and number density of Ge nanoislands in SiO2sublayer thicknesses beyond 30 nm are approximately 15 nm and 1011cm 2, respectively. Atomic force microscopy(AFM) reveals root mean square(RMS) roughness sensitivity as the SiO2sublayer thickness varies from 30 to 40 nm. The formation of nanoislands with high aspect ratios is attributed to the higher rate of surface reactions between Ge adatoms and nucleated Ge islands than reactions associated with SiO2and Ge. The Ge nanoisland polyorientation on SiO2(50-nm thickness) is revealed by X-ray difraction(XRD) patterns. Photoluminescence(PL) peaks of 2.9 and 1.65 eV observed at room temperature(RT) are attributed to the radiative recombination of electrons and holes from the Ge nanoislands/SiO2and SiO2/Si interfaces, respectively. The mean island sizes are determined by ftting the experimental Raman profle to two models, namely, the phonon confnement model and the size distribution combined with phonon confnement model. The latter model yields the best ft to the experimental data. We confrm that SiO2matrix thickness variations play a signifcant role in the formation of Ge nanoislands mediated via the minimization of interfacial and strain energies.
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The initial stage of Ge heteroepitaxy on a Si(100)-2×1 surface has been investigated by low-energy electron diffraction (LEED) and Auger-electron spectroscopy (AES). The growth mode of the Ge films was studied by measuring the decrease in the Si(LVV) AES line at 92 eV with an increase in the Ge overlayer thickness. The Ge films deposited at room temperature exhibit layer-by-layer growth up to at least six monolayers. When the substrate is heated up to 350 °C, the growth mode is characterized by the Stranski–Krastanov type; i.e., the first three monolayers of growth is followed by island formation. Although these characteristics of the growth mechanism are similar to the case of Ge on Si(111)-7×7 surfaces, annealing behavior of the Ge films suggests that the bond strength between Ge and Si is stronger on Si(100) than on Si(111) surfaces. In contrast to the case of Ge on Si(111) surfaces, where the original 7×7 superstructure of the Si surface is replaced by a new 5×5 pattern at about two-monolayer coverage of Ge, the original 2×1 LEED pattern is not strongly disturbed up to about 1–2 monolayers of Ge. In addition to the detailed study on the initial stage of heteroepitaxial growth, we observed that thick Ge films deposited onto Si(100) surfaces held at 350 and 470 °C display a sharp 2×1 LEED pattern and demonstrate a single-crystal growth of a Ge(100) face on the Si(100) surface. This is further supported by a measurement of the x-ray diffraction pattern of the Ge films.
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The formation of a crystalline composite Ag:Si material with Ag nanoparticles by low-energy (E = 30 keV) high-dose (D = 1.5 × 1017 ion cm−2) Ag+ implantation into a monocrystalline c-Si substrate followed by nanosecond pulsed laser annealing (PLA) is demonstrated. Compared to traditional thermal annealing, PLA allows us to perform local heating of the sample both for its depth and area, and eliminate implantation-induced defects more efficiently, due to rapid liquid-phase recrystallization. Moreover, dopant diffusion during a nanosecond laser pulse is mainly limited by the molten region, where the dopant diffusion coefficient is several orders of magnitude higher than in the solid state. During PLA by a ruby laser (λ = 0.694 µm), the optical probing of the irradiated zone at λ = 1.064 µm with registration of time-dependent reflectivity R(t) was carried out. By scanning electron microscopy, it was established that Ag+ implantation leads to the creation of a thin amorphous Ag:Si layer of porous structure, containing Ag nanoparticles with sizes of 10–30 nm. PLA with energy density W = 1.2–1.8 J cm−2 results in the melting of the implanted layer (d ~ 60 nm) and the topmost layers of the c-Si substrate (d < 400 nm), followed by the rapid recrystallization of the Si matrix containing Ag nanoparticles with dominate sizes of 5–15 nm and some fraction of larger particles of 40–60 nm. Energy dispersive x-ray (EDX) spectroscopy did not show a noticeable change of Ag atomic concentration in the implanted layer after PLA. Spectral dependence R(λ) of Ag:Si layers showed the partial recovery of c-Si bands with maxima at 275 and 365 nm with simultaneous weakening of plasmon band for Ag nanoparticles in Si at 835 nm.
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