An understanding of the aging and oxidation of the (0001) surface of Bi2Se3 is critical to a comprehensive physical picture of its topologically protected surface states. Here, the authors contribute new experimental observations about the aging and oxidation process. The authors find that surface aging in ambient conditions occurs in two major steps. Within 2 h of exfoliation, a series of ∼3.2 Å high islands are observed by atomic force microscopy over approximately 10% of the surface. Subsequently, patch growth stops, and oxidation begins after the 2 h and continues until one quintuple layer has been oxidized. X-ray photoelectron spectroscopy shows no sign of oxidation before ∼120 min of exposure to air, and the oxygen 1 s peak, as well as oxidized Se 3d and Bi 4d peaks, are clearly present after ∼190 min of ambient exposure. Variable angle spectroscopic ellipsometry indicates that the oxidation of a full quintuple layer occurs on the time scale of days. These results are in good agreement with the time dependent changes observed in the surface crystal structure by second harmonic generation. In addition to providing the ability to nondestructively measure oxide on the surface of Bi2Se3 crystals, ellipsometry can be used to identify the thickness of Bi2Se3 flakes. With these methods, the authors have constructed a consistent, experimentally based model of aging process at the surface of Bi2Se3.
The Aspect Ratio Trapping technique has been extensively evaluated for improving the quality of III-V heteroepitaxial films grown on Si, due to the potential for terminating defects at the sidewalls of SiO2 patterned trenches that enclose the growth region. However, defects propagating along the trench direction cannot be effectively confined with this technique. We studied the effect of the trench bottom geometry on the density of defects of GaAs fins, grown by metal-organic chemical vapor deposition on 300 mm Si (001) wafers inside narrow (<90 nm wide) trenches. Plan view and cross sectional Scanning Electron Microscopy and Transmission Electron Microscopy, together with High Resolution X-Ray Diffraction, were used to evaluate the crystal quality of GaAs. The prevalent defects that reach the top surface of GaAs fins are {111} twin planes propagating along the trench direction. The lowest density of twin planes, ∼8 × 108 cm−2, was achieved on “V” shaped bottom trenches, where GaAs nucleation occurs only on {111} Si planes, minimizing the interfacial energy and preventing the formation of antiphase boundaries.
Qubit information processors are increasing in footprint but currently rely on e-beam lithography for patterning the required Josephson junctions (JJs). Advanced optical lithography is an alternative patterning method, and we report on the development of transmon qubits patterned solely with optical lithography. The lithography uses 193 nm wavelength exposure and 300-mm large silicon wafers. Qubits and arrays of evaluation JJs were patterned with process control which resulted in narrow feature distributions: a standard deviation of 0.78% for a 220 nm linewidth pattern realized across over half the width of the wafers. Room temperature evaluation found a 2.8 − 3.6% standard deviation in JJ resistance in completed chips. The qubits used aluminum and titanium nitride films on silicon substrates without substantial silicon etching. T1 times of the qubits were extracted at 26 µs -27 µs, indicating a low level of material-based qubit defects. This study shows that large wafer optical lithography on silicon is adequate for high-quality transmon qubits, and shows a promising path for improving many-qubit processors.
The electrodeposition of Cu-Ag alloys was studied as a possible application for interconnect technology, where Cu-Ag alloys may be less susceptible to electromigration than Cu alone. The presence of chloride in state-of-the-art copper plating electrolytes limited the solubility of Ag. However, pulse-plating approach enabled a wide range of Cu-Ag alloy compositions at substantial chloride concentration levels. The deposition of Ag was driven by the displacement reactions between the metallic copper and ionic silver during the off-time. Measured alloy compositions were consistent with theoretical estimates at various electrolyte concentrations, electrode rotation speeds, pulse frequencies and duty cycles. However, organic additives decreased incorporation of Ag into the alloy. It was also discovered that CuSO 4 · 5H 2 O from a number of major chemical suppliers contained Ag as an impurity. The roughness of the films was significant when produced by pulsed plating, but was shown to be substantially reduced in the presence of a leveling agent. Additionally, the concentration of chloride in the electrolyte was shown to significantly affect surface quality of the deposited Cu-Ag thin films. With the continuing miniaturization of microelectronics, electromigration effects in copper interconnect systems are becoming a major factor in determining device lifetime and reliability.1-3 Accordingly, there is a need for interconnect materials with improved electromigration resistance, while maintaining adequate electrical resistivity. It has been shown that co-deposits of Cu with small amount of other metals (such as Ag, Sn, Co, and Mg) can potentially mitigate both electro-and stress-migration.4-8 However, the resistivity of the interconnect increases with the addition of foreign metals. The International Technology Roadmap of Semiconductors (ITRS) has set the resistivity criterion at ρ < 2.2 μ · cm, 9 only slightly above the bulk resistivity of pure copper at 1.68 μ · cm. Alloying copper with silver (ρ Ag = 1.59 μ · cm) was shown to increase the resistivity of electrochemically deposited Cu-Ag films the least when compared to other copper alloys.10 For Ag content between 0.17 and 3.2 wt% the resistivity ranges 1.8 to 3.1 μ · cm.10 For this reason Cu-Ag alloys, especially at the lower Ag weight percentages, are potential candidates for the fabrication of interconnects in microelectronic devices.Acidic copper-sulfate electrolytes containing chloride have been successfully applied for many years to the electrochemical fabrication of copper interconnects. [11][12][13] Chloride in these electrolytes is known to be one of the critical constituents enabling defect-free filling of surface features.14-20 The main challenge for depositing silver from copper-plating electrolytes, which contain about 50 ppm of chloride, is the low solubility of silver in the presence of chloride ions (the solubility product of AgCl in water at 25• C is 1.8×10 We demonstrate that the application of a pulsating current instead of a direct current permits ...
Impurities incorporated into copper during electrochemical deposition have long been postulated to inhibit grain growth in narrow interconnects. In this article we examine an alternative possible origin for the microstructural differences observed between narrow and wide lines. Specifically, the iPVD Cu seed layers in wide (1 μm) and narrow (70 nm) lines differ significantly in microstructure and texture, as analyzed by transmission electron microscopy and electron diffraction. The seed in the wide lines has a polycrystalline microstructure similar to that seen in blanket films, with strong 111 texture in the field and sidewalls, and 112 texture at the bottom of the trench. Although the field between the 70 nm lines is also strongly 111-textured, the seed on the sidewalls and bottom of the trenches are largely untextured and, in fact, not entirely crystalline. This results in a higher energy, metastable microstructure which can contribute an additional driving force for the rapid nucleation and recrystallization of polycrystalline copper in the trenches before the overburden can transform and force the creation of bamboo grains in the lines.
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