25 research outputs found
Isotropic plasma-thermal atomic layer etching of superconducting TiN films using sequential exposures of molecular oxygen and SFH plasma
Microwave loss in superconducting titanium nitride (TiN) films is attributed
to two-level systems in various interfaces arising in part from oxidation and
microfabrication-induced damage. Atomic layer etching (ALE) is an emerging
subtractive fabrication method which is capable of etching with Angstrom-scale
etch depth control and potentially less damage. However, while ALE processes
for TiN have been reported, they either employ HF vapor, incurring practical
complications; or the etch rate lacks the desired control. Further, the
superconducting characteristics of the etched films have not been
characterized. Here, we report an isotropic plasma-thermal TiN ALE process
consisting of sequential exposures to molecular oxygen and an SF/H
plasma. For certain ratios of SF:H flow rates, we observe selective
etching of TiO over TiN, enabling self-limiting etching within a cycle.
Etch rates were measured to vary from 1.1 \r{A}/cycle at 150 C to 3.2
\r{A}/cycle at 350 C using ex-situ ellipsometry. We demonstrate that
the superconducting critical temperature of the etched film does not decrease
beyond that expected from the decrease in film thickness, highlighting the
low-damage nature of the process. These findings have relevance for
applications of TiN in microwave kinetic inductance detectors and
superconducting qubits.Comment: 17 pages, 7 figure
Ultrathin superconducting TaCxN1−x films prepared by plasma-enhanced atomic layer deposition with ion-energy control
This work demonstrates that plasma-enhanced atomic layer deposition (PEALD) with substrate biasing enables the preparation of ultrathin superconducting TaCxN1−x films. By comparing with films grown without substrate biasing, the enhanced ion energies yield a hundredfold reduction in room-temperature resistivity: a comparably low value of 217 μΩ cm is obtained for a 40 nm film. The ion-energy control enables tuning of the composition, counteracts oxygen impurity incorporation, and promotes a larger grain size. Correspondingly, the critical temperature of superconductivity (Tc) displays clear ion-energy dependence. With optimized ion energies, a consistently high Tc around 7 K is measured down to 11 nm film thickness. These results demonstrate the high ultrathin-film quality achievable through PEALD combined with substrate biasing. This process is particularly promising for the fabrication of low-loss superconducting quantum devices
Innovative remote plasma source for atomic layer deposition for GaN devices
High-quality dielectric films could enable GaN normally off high-electron-mobility transistors (HEMTs). Plasma atomic layer deposition (ALD) is known to allow for controlled high-quality thin-film deposition, and in order to not exceed energy and flux levels leading to device damage, the plasma used should preferably be remote for many applications. This article outlines ion energy flux distribution functions and flux levels for a new remote plasma ALD system, Oxford Instruments Atomfab™, which includes an innovative, RF-driven, remote plasma source. The source design is optimized for ALD for GaN HEMTs for substrates up to 200 mm in diameter and allows for Al2O3 ALD cycles of less than 1 s. Modest ion energies of <50 eV and very low ion flux levels of <1013 cm−2 s−1 were found at low-damage conditions. The ion flux can be increased to the high 1014 cm−2 s−1 range if desired for other applications. Using low-damage conditions, fast ALD saturation behavior and good uniformity were demonstrated for Al2O3. For films of 20 nm thickness, a breakdown voltage value of 8.9 MV/cm was obtained and the Al2O3 films were demonstrated to be suitable for GaN HEMT devices where the combination with plasma pretreatment and postdeposition anneals resulted in the best device parameters
Optical modeling of plasma-deposited ZnO films: Electron scattering at different length scales
Plasma atomic layer deposition
Plasma atomic layer deposition (ALD) is optimized through modulation of the gas residence time during an excited species phase, wherein activated reactant is supplied such as from a plasma. Reduced residence time increases the quality of the deposited layer, such as reducing wet etch rates, increasing index of refraction and/or reducing impurities in the layer. For example, dielectric layers, particularly silicon nitride films, formed from such optimized plasma ALD processes have low levels of impurities remaining from the silicon precursor.</p
Plasma atomic layer deposition
Plasma atomic layer deposition (ALD) is optimized through modulation of the gas residence time during an excited species phase, wherein activated reactant is supplied such as from a plasma. Reduced residence time increases the quality of the deposited layer, such as reducing wet etch rates, increasing index of refraction and/or reducing impurities in the layer. For example, dielectric layers, particularly silicon nitride films, formed from such optimized plasma ALD processes have low levels of impurities remaining from the silicon precursor.</p
Electron Scattering and Doping Mechanisms in Solid-Phase-Crystallized In<sub>2</sub>O<sub>3</sub>:H Prepared by Atomic Layer Deposition
Hydrogen-doped indium oxide (In<sub>2</sub>O<sub>3</sub>:H) has
recently emerged as an enabling transparent conductive oxide for solar
cells, in particular for silicon heterojunction solar cells because
its high electron mobility (>100 cm<sup>2</sup>/(V s)) allows for
a simultaneously high electrical conductivity and optical transparency.
Here, we report on high-quality In<sub>2</sub>O<sub>3</sub>:H prepared
by a low-temperature atomic layer deposition (ALD) process and present
insights into the doping mechanism and the electron scattering processes
that limit the carrier mobility in such films. The process consists
of ALD of amorphous In<sub>2</sub>O<sub>3</sub>:H at 100 °C and
subsequent solid-phase crystallization at 150–200 °C to
obtain large-grained polycrystalline In<sub>2</sub>O<sub>3</sub>:H
films. The changes in optoelectronic properties upon crystallization
have been monitored both electrically by Hall measurements and optically
by analysis of the Drude response. After crystallization, an excellent
carrier mobility of 128 ± 4 cm<sup>2</sup>/(V s) can be obtained
at a carrier density of 1.8 × 10<sup>20</sup> cm<sup>–3</sup>, irrespective of the annealing temperature. Temperature-dependent
Hall measurements have revealed that electron scattering is dominated
by unavoidable phonon and ionized impurity scattering from singly
charged H-donors. Extrinsic defect scattering related to material
quality such as grain boundary and neutral impurity scattering was
found to be negligible in crystallized films indicating that the carrier
mobility is maximized. Furthermore, by comparison of the absolute
H-concentration and the carrier density in crystallized films, it
is deduced that <4% of the incorporated H is an active dopant in
crystallized films. Therefore, it can be concluded that inactive H
atoms do not (significantly) contribute to defect scattering, which
potentially explains why In<sub>2</sub>O<sub>3</sub>:H films are capable
of achieving a much higher carrier mobility than conventional In<sub>2</sub>O<sub>3</sub>:Sn (ITO)
Atomic layer deposition of high-mobility hydrogen-doped zinc oxide
In this work, atomic layer deposition (ALD) has been employed to prepare high-mobility H-doped zinc oxide (ZnO:H) films. Hydrogen doping was achieved by interleaving the ZnO ALD cycles with H2 plasma treatments. It has been shown that doping with H2 plasma offers key advantages over traditional doping by Al and B, and enables a high mobility value up to 47 cm2/Vs and a resistivity of 1.8 mΩcm. By proper choice of a deposition regime where there is a strong competition between film growth and film etching by the H2 plasma treatment, a strongly enhanced grain size and hence increased carrier mobility with respect to undoped ZnO can be obtained. The successful incorporation of a significant amount of H from the H2 plasma has been demonstrated, and insights into the mobility-limiting scatter mechanisms have been obtained from temperature-dependent Hall measurements. A comparison with conventional TCOs has been made in terms of optoelectronic properties, and it has been shown that high-mobility ZnO:H has potential for use in various configurations of silicon heterojunction solar cells and silicon-perovskite tandem cells