Characterization and In-situ Monitoring of Sub-stoichiometric Adjustable Tc Titanium Nitride Growth

The structural and electrical properties of Ti-N films deposited by reactive sputtering depend on their growth parameters, in particular the Ar:N2 gas ratio. We show that the nitrogen percentage changes the crystallographic phase of the film progressively from pure \alpha-Ti, through an \alpha-Ti phase with interstitial nitrogen, to stoichiometric Ti2N, and through a substoichiometric TiNX to stoichiometric TiN. These changes also affect the superconducting transition temperature, Tc, allowing, the superconducting properties to be tailored for specific applications. After decreasing from a Tc of 0.4 K for pure Ti down to below 50 mK at the Ti2N point, the Tc then increases rapidly up to nearly 5 K over a narrow range of nitrogen incorporation. This very sharp increase of Tc makes it difficult to control the properties of the film from wafer-to-wafer as well as across a given wafer to within acceptable margins for device fabrication. Here we show that the nitrogen composition and hence the superconductive properties are related to, and can be determined by, spectroscopic ellipsometry. Therefore, this technique may be used for process control and wafer screening prior to investing time in processing devices

Etch Induced Microwave Losses in Titanium Nitride Superconducting Resonators

We have investigated the correlation between the microwave loss and patterning method for coplanar waveguide titanium nitride resonators fabricated on Si wafers. Three different methods were investigated: fluorine- and chlorine-based reactive ion etches and an argon-ion mill. At high microwave probe powers the reactive etched resonators showed low internal loss, whereas the ion-milled samples showed dramatically higher loss. At single-photon powers we found that the fluorine-etched resonators exhibited substantially lower loss than the chlorine-etched ones. We interpret the results by use of numerically calculated filling factors and find that the silicon surface exhibits a higher loss when chlorine-etched than when fluorine-etched. We also find from microscopy that re-deposition of silicon onto the photoresist and side walls is the probable cause for the high loss observed for the ion-milled resonator

Proximity-Coupled Ti/TiN Multilayers for use in Kinetic Inductance Detectors

We apply the superconducting proximity effect in TiN/Ti multi-layer films to tune the critical temperature, Tc, to within 10 mK with high uniformity (less than 15 mK spread) across a 75 mm wafer. Reproducible Tc's are obtained from 0.8 - 2.5 K. These films had high resistivities, > 100 uOhm-cm and internal quality factors for resonators in the GHz range on the order of 100k and higher. Both trilayers of TiN/Ti/TiN and thicker superlattice films were prepared, demonstrating a highly controlled process for films over a wide thickness range. Detectors were fabricated and showed single photon resolution at 1550 nm. The high uniformity and controllability coupled with the high quality factor, kinetic inductance, and inertness of TiN make these films ideal for use in frequency multiplexed kinetic inductance detectors and other potential applications such as nanowire detectors, transition edge sensors and associated quantum information applications

Coherence in a transmon qubit with epitaxial tunnel junctions

We developed transmon qubits based on epitaxial tunnel junctions and interdigitated capacitors. This multileveled qubit, patterned by use of all-optical lithography, is a step towards scalable qubits with a high integration density. The relaxation time T1 is .72-.86mu sec and the ensemble dephasing time T2 is slightly larger than T1. The dephasing time T2 (1.36mu sec) is nearly energy-relaxation-limited. Qubit spectroscopy yields weaker level splitting than observed in qubits with amorphous barriers in equivalent-size junctions. The qubit's inferred microwave loss closely matches the weighted losses of the individual elements (junction, wiring dielectric, and interdigitated capacitor), determined by independent resonator measurements

Sub-micrometer epitaxial Josephson junctions for quantum circuits

We present a fabrication scheme and testing results for epitaxial sub-micrometer Josephson junctions. The junctions are made using a high-temperature (1170 K) "via process" yielding junctions as small as 0.8 mu m in diameter by use of optical lithography. Sapphire (Al2O3) tunnel-barriers are grown on an epitaxial Re/Ti multilayer base-electrode. We have fabricated devices with both Re and Al top electrodes. While room-temperature (295 K) resistance versus area data are favorable for both types of top electrodes, the low-temperature (50 mK) data show that junctions with the Al top electrode have a much higher subgap resistance. The microwave loss properties of the junctions have been measured by use of superconducting Josephson junction qubits. The results show that high subgap resistance correlates to improved qubit performance

Dielectric loss of boron-based dielectrics on niobium resonators

Advanced solid-state quantum bits (qubits) are likely to require a variety of dielectrics for wiring crossovers, substrates, and Josephson junctions. Microwave superconducting resonators are an excellent tool for measuring the internal dielectric loss of materials. We report the dielectric loss of boron-based dielectric films using a microwave coplanar waveguide (CPW) resonator with heterostructure geometry. Power-dependent internal quality factors of magnetron-sputtered boron carbide ( B4C ) and boron nitride (BN) were measured and are compared to silicon oxide ( SiO2 ), a common material used in wiring crossovers. The internal dielectric loss due to two-level systems for B4C , and BN is less than silicon dioxide ( SiO2 ), which demonstrates the existence of low-loss sputtered materials. We also found that niobium (Nb) CPW resonators suffer a decrease in internal quality factor after deposition of B4C at temperatures above 150 ∘C . This result is consistent with the idea that the oxidation of the surface of the superconducting metal can contribute to loss in a device

The structure of the CoS\u3csub\u3e2\u3c/sub\u3e (100)-(1 × 1) surface

Quantitative low-energy electron diffraction (LEED) has been used to determine the structure of the cubic CoS2 (100)-(1 × 1) surface. The clearly favored structural model from the LEED analysis is the 1S-terminated (1 × 1) surface, in which the S–S dimer is intact and the terminal surface layer retains a complete S–Co–S sandwich structure. The surface S atoms move outwards towards the vacuum while the subsurface Co atoms move towards the bulk, by approximately 0.03 and 0.11 Å, respectively. In addition, the S atoms in the third sublayer relax outwards by about 0.12 Å, thus providing an indication of a stronger S–S dimer bond and a denser surface region. The complete atomic coordinates of the S–Co–S surface layers are determined in this analysis

The structure of the CoS\u3csub\u3e2\u3c/sub\u3e (100)-(1 × 1) surface

Quantitative low-energy electron diffraction (LEED) has been used to determine the structure of the cubic CoS2 (100)-(1 × 1) surface. The clearly favored structural model from the LEED analysis is the 1S-terminated (1 × 1) surface, in which the S–S dimer is intact and the terminal surface layer retains a complete S–Co–S sandwich structure. The surface S atoms move outwards towards the vacuum while the subsurface Co atoms move towards the bulk, by approximately 0.03 and 0.11 Å, respectively. In addition, the S atoms in the third sublayer relax outwards by about 0.12 Å, thus providing an indication of a stronger S–S dimer bond and a denser surface region. The complete atomic coordinates of the S–Co–S surface layers are determined in this analysis