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.
Depositing thin films is often limited to a specific deposition process by which precursors are transported in a deposition environment. In other words, a deposition environment in which two deposition processes complementary to each other are unified may offer new insights in designing thin film structures. This view motivated us to combine atomic layer deposition (ALD) and magnetron sputtering (SPU) in a single chamber – sputtering atomic layer augmented deposition (SALAD). The SALAD system offers benefits of consistently delivering precursors in ALD and freely choosing chemical elements in SPU. In this paper, the SALAD system is employed to deposit nanocomposites consisting of multiple layers of aluminum oxide deposited by ALD and copper layers deposited by SPU. Distinctive dispersion features seen in optical properties of the nanocomposites are analyzed to reveal the interrelationship between structural properties and electronic properties of the nanocomposites.
A uniform color space in terms of just-noticeable differences (JND) to the human eye is important in industry and image processing, as well as for visually lossless image coding and noise evaluations. The method of this paper builds a non-linear color space, by making use of structural analysis with various JND data sets. Through computer simulation, the color space obtained by this method is shown to be superior to conventional ones such as CIELUV in terms of uniformity, and attained a 3-bit reduction in representing chromaticity of RGB color data.
The substrate temperature (Ts) dependence (350–700 °C) of GaAs and Ga1−y InyAs growth rates was investigated in metalorganic molecular beam epitaxy (MOMBE), using triethylgallium (TEG), trimethylindium (TMI), and solid arsenic (As4) sources. For GaAs growth, four distinct Ts dependent regions are observed, including a weak desorption process (500–650 °C) characteristic of MOMBE, preceding atomic Ga desorption (Ts >650 °C). When adding a TMI flux to grow Ga1−yInyAs, this desorption process was much enhanced up to 550 °C, and then decreased above 550 °C when the In desorption phenomenon takes place. Correlatively, the In alloy composition peaks at 550 °C. The same dependence was observed in Ga1−yInyAs growth using solid In and TEG sources. However, in Ga1−xAlxAs growth using solid Al or triethylaluminum (TEA) and TEG sources, the weak desorption observed in GaAs MOMBE was strongly minimized. From these results, possible growth mechanisms are discussed.
We investigated the minority carrier diffusion length in p- and n-GaN by performing electron-beam-induced current measurements of GaN p–n junction diodes. Minority electron diffusion length in p-GaN strongly depended on the Mg doping concentration for relatively low dislocation density below 108cm−2. It increased from 220to950nm with decreasing Mg doping concentration from 3×1019to4×1018cm−3. For relatively high dislocation density above 109cm−2, it was less than 300nm and independent of the Mg doping concentration. On the other hand, the minority hole diffusion length in n-GaN was shorter than 250nm and less affected by the dislocation density and Si doping concentration. We discuss the doping-concentration and dislocation-density dependence of minority carrier diffusion length.
To realize short wavelength light emitting diodes, nitride quantum structures are studied. Control of the piezoelectric field and thickness design of wurtzite nitride quantum wells are important for band edge emission in the short wavelength region. By reducing the strain between the AlGaN well and the barrier layers of multi-quantum wells, ultraviolet light emitting diodes operating at the wavelength of 346 nm were successfully fabricated.
AlGaN-based ultraviolet light emitting diodes (LED) are characterized. The devices consist of an Alo.00Gao.94N (2 nm) / Alo.12Gao.88N single quantum well active layer sandwiched by Alo.3Gao.7N carrier blocking layers and cladding layers of an Alo.i6Gao.84N (1.5 nm)/Al0.2Gao.8N (1.5 nm) short-period alloy superlattice grown on bulk GaN substrate. Efficient conversion to UV light is confirmed, which indicates a high-quality active layer, effective suppression of carrier overflow, and transparency of conductive cladding layers. Radiative recombination dominantly occurs in the quantum well at injection current density lower than 0.25 kA/cm2. Further dense injection leads to enhancement of the radiative recombination in the barrier layer. This clarifies the importance of the design of the active layer for highpower application. 1Aluminium gallium nitride (AlGaN) alloy is optically direct band gap material and it covers a wide wavelength range of 200-360 nm within ultraviolet (UV) range. Therefore, AlGaN-based light emitting diodes (LEDs) are expected to be the next-generation of ultraviolet (UV) light sources. Such a semiconductor UV light source will replace fluorescent bulbs for lighting, electroluminescence panels and cathode tubes for displays, and gas tubes for spectroscopic excitation applications, such as used in medicine.
