Native Oxide Barriers Research Articles

Plated copper electrodes for two-terminal perovskite/silicon tandem solar cells

Electroplated copper electrodes are manufactured for the first time on 22.5 cm2 two-terminal perovskite/silicon tandem solar cells. This study demonstrates that a 10 nm thin atomic layer deposited (ALD) Al2O3 masking layer on ITO enables the tandem cells to withstand the chemistry of the wet chemical metallization process. Our approach uses a screen-printed Ag seed-layer on ITO for homogeneous electroplating current distribution and a photoconversion efficiency up to ≈ 15% is reached. To avoid the use of any metal seed-layer on ITO, we also demonstrate a light-induced Cu plating (LIP) process. This further validates the presence of an electrical contact between the Cu electrode and both perovskite and Si cells. Finally, progress with the transfer of the native oxide barrier layer for selective electroplating (NOBLE) metallization to bifacial perovskite/silicon tandem solar cells is presented.

Native Oxide Barrier Layer for Selective Electroplated Metallization of Silicon Heterojunction Solar Cells

The metallization of silicon heterojunction (SHJ) solar cells by electroplating of highly conductive copper onto a multifunctional patterned metal layer stack is demonstrated. The approach features several advantages: low temperature processing, high metal conductivity of plated copper, no organic making, and low material costs (almost Ag‐free). A PVD layer stack of copper and aluminum is deposited onto the cell subsequently to TCO deposition. The aluminum layer is patterned with a printed etchant and its native oxide on the remaining areas inhibits plating. The full area aluminum layer while electroplating supports plating current distribution and allows homogeneous plating height distributions over the cell. The NOBLE (native oxide barrier layer for selective electroplating) approach allows reaching a first encouraging SHJ solar cell efficiency of 20.2% with low contact resistivity.

Plasmon-induced nanoscale quantised conductance filaments

Plasmon-induced phenomena have recently attracted considerable attention. At the same time, relatively little research has been conducted on electrochemistry mediated by plasmon excitations. Here we report plasmon-induced formation of nanoscale quantized conductance filaments within metal-insulator-metal heterostructures. Plasmon-enhanced electromagnetic fields in an array of gold nanodots provide a straightforward means of forming conductive CrOx bridges across a thin native chromium oxide barrier between the nanodots and an underlying metallic Cr layer. The existence of these nanoscale conducting filaments is verified by transmission electron microscopy and contact resistance measurements. Their conductance was interrogated optically, revealing quantised relative transmission of light through the heterostructures across a wavelength range of 1–12 μm. Such plasmon-induced electrochemical processes open up new possibilities for the development of scalable devices governed by light.

Tunneling investigation of the electron scattering effect on the momentum-dependent energy gap distribution in MgB2

We have studied the effect of electron scattering on the momentum-dependent energy gap distributions in MgB2 by measuring planar tunnel junctions made from epitaxial MgB2 thin films on different substrates, of different thicknesses, with different junction interface properties, and different counter-electrode materials. The phonon features in the tunneling spectra indicated that the native oxide barrier is mainly MgO with estimated barrier thickness ranging from 1.1 to 1.5 nm and the average barrier height from 1.7 to 2.6 eV. When tunneling into the ab-plane occurred in very clean films, both the π (∼1.8 meV) and σ (∼7.2 meV) gaps were observed with fine structures in the conductance peaks, indicating a distribution of gap values due to anisotropic electron-phonon interaction. The σ gap was enhanced (∼7.9 meV) in MgB2 thin films on SiC substrates which had Tc values over 40 K due to epitaxial tensile strain. As the MgB2 film thickness decreased from 100 nm to 33 nm, the π gap distribution range was narrowed from ∼1.7 meV to ∼1.4 meV, and the fine peak structures disappeared due to the reduced electron mean free path limited by the film thickness. The fine peak structures were also gradually smeared out when the junction resistance increased. The results show that the electron scattering, either from the MgB2 film or the junction interface or barrier, can smear out the gap distribution structures in the tunneling spectra. Deterioration of the MgB2 film surface was also found to cause an increase in the π gap value, likely due to an enhancement of interband scattering.

Characterization of Josephson and quasi-particle currents inMgB2/MgB2 and Pb/Pb contact junctions

The electrical properties of Josephson junctions formed by pressing two oxidizedMgB2 thinfilms together were measured. This fabrication method allowed us to characterize the properties ofMgB2 junctions with native or thermal oxide barriers without exposing the barrier and bottomelectrode to the high temperature deposition process required to synthesize the topMgB2 electrode in the conventional trilayer junctions. These junctions, with electrodeTcs of ∼ 32 K, have criticalcurrents up to ∼ 25 K, broadened energy gaps at over 30 K, subharmonic gap structures at moderate voltages (∼0.6–2.0 mV) and Fiskemodes at low voltage (<0.6 mV). We show that this method can be used to quantify the barrier properties and theextent of suppression of the superconductor order parameter at the surface. Our resultssuggest that the near interface superconducting order parameter is reduced at both nativeand thermal oxide surfaces, with a greater reduction at the thermal oxide surface. Forcomparison, the same experimental equipment and protocol were used to investigate Pbcontact junctions. The junctions containing a thermal oxide barrier were found to haveelectrical properties similar to those of multiply connected junctions modulated by theirself-field, as initially observed by Clarke and Fulton in a SLUG (superconducting lowinductance undulatory galvanometer) (Clarke and Fulton 1969 J. Appl. Phys. 404470).

All magnesium diboride Josephson junctions with MgO and native oxide barriers

We present results on all-MgB2 tunnel junctions, where the tunnel barrier is deposited MgO or native-oxide of base electrode. For the junctions with MgO, the hysteretic I-V curve resembles a conventional underdamped Josephson junction characteristic with critical current-resistance product nearly independent of the junction area. The dependence of the critical current with temperature up to 20 K agrees with the [Ambegaokar and Baratoff, Phys. Rev. Lett. 10, 486 (1963)] expression. For the junctions with native-oxide, conductance at low bias exhibits subgap features while at high bias reveals thick barriers. As a result no supercurrent was observed in the latter, despite the presence of superconducting-gaps to over 30 K.

Electroluminescence from single nanowires by tunnel injection: an experimentalstudy

We present a hybrid light-emitting diode structure composed of an n-type gallium nitridenanowire on a p-type silicon substrate in which current is injected along the lengthof the nanowire. The device emits ultraviolet light under both bias polarities.Tunnel injection of holes from the p-type substrate (under forward bias) and fromthe metal (under reverse bias) through thin native oxide barriers consistentlyexplains the observed electroluminescence behaviour. This work shows that thestandard p–n junction model is generally not applicable to this kind of devicestructure.

H/D Isotope Effects in LaNi4.5Mn0.5 Electrodes

The effect of isotopic substitution of by on the electrochemical cycling properties as well as on the electrochemical and solid-gas isotherms of intermetallic compound has been investigated. Contrary to what is observed in the solid-gas isotherms, significant isotope effects have been found in the electrochemical isotherms. This is due to different electrolyte activities and standard electrode potentials. An empirical method has been developed for converting electrode potentials into equivalent and pressures based on the Nernst equation. Hydrogen and deuterium equilibrium pressures obtained by this method agree with those obtained by solid-gas reactions. Isotope effects in the rate of transfer across the electrode surface have also been observed. The activation period of the electrode is much longer in electrolyte. This effect is ascribed to slower permeation rates of atoms as compared to atoms through the native oxide barriers at the electrode surface. Moreover, higher charge-transfer overpotential has been observed for than for atoms.

Mg B 2 tunnel junctions with native or thermal oxide barriers

Mg B 2 tunnel junctions (MgB2/barrier/MgB2) were fabricated using a native oxide grown on the bottom MgB2 film as the tunnel barrier. Such barriers therefore survive the deposition of the second electrode at 300°C, even over junction areas of ∼1mm2. Studies of such junctions and those of the type MgB2/native or thermal oxide/metal (Pb, Au, or Ag) show that tunnel barriers grown on MgB2 exhibit a wide range of barrier heights and widths.

Barrier characteristic in Nb/Ni planar tunnel junctions

Fabrication of native Nb oxide barriers and their influence on the tunneling behavior of Nb/Ni planar junctions are investigated. Each junction film electrode was grown by magnetron sputtering technique and two methods were used to obtain the barrier. In the first method, a NbxOy oxide layer was formed at room conditions on top of Nb. In the second one, a saturated water vapor atmosphere is used to obtain the NbxOy layer. Samples prepared on these ways show different tunneling characteristics related with changes in the barrier. Characterization of the barrier was done by low-angle X-ray diffraction, atomic force microscopy and X-ray photoelectron spectroscopy.

Superconducting and spin-dependent tunneling using native oxide barriers on CoFe thin films

Superconducting tunneling and magnetoresistance were exhibited in junctions with native oxide barriers grown by exposing thin films of ${\mathrm{Co}}_{50}{\mathrm{Fe}}_{50}$ to ambient atmosphere. When Pb was used as a counterelectrode, $I\ensuremath{-}V$ curves at 4.2 K reflected the presence of the superconducting energy gap of Pb. Conductance measurements at 1.5 K revealed clear longitudinal and transverse phonon structure from the superconducting Pb as evidence of single-step, elastic tunneling. Junctions with Co counterelectrodes showed a maximum tunneling magnetoresistance of 2.5% at room temperature and 6% at 77 K. These are the largest values reported to date in spin-dependent tunnel junctions using a native oxide barrier. Junction performance depended strongly on the deposition conditions for the top ferromagnetic layer.

The grain growth blocking effect of polycrystalline silicon film by thin native silicon oxide barrier during the excimer laser recrystallization

A method to control the recrystallization depth of amorphous silicon (a-Si) film during the excimer laser annealing is presented. By irradiating the XeCl excimer laser on the triple film structure of a-Si/thin native silicon oxide (∼20 Å)/thick a-Si layer, only the upper a-Si film is recrystallized, whereas the lower thick a-Si film remains amorphous. The thin native silicon oxide layer blocks the grain growth and prevents the upper grains from growing into the lower a-Si. As a result, the thin oxide film sharply defines the boundary between polycrystalline silicon (poly-Si) and the a-Si layer. The poly-Si/a-Si double layer is useful for the fabrication of high-performance poly-Si thin film transistors.

Angle-resolved tunneling study on the anisotropic gap of Bi-2212 superconducting whisker

A tunneling study was performed on the c-axis direction of the Bi-2212 superconducting whiskers with the superconducting transition temperature of 72 K. The contact junction method was adopted to fabricate the nanometer scale tunnel junction by using a native oxide barrier on the whisker surface. We carried out the angle-resolved tunneling measurements at 7.1 K by rotating the probe whisker over the sample whisker. We find that the superconducting gap energy of Bi-2212 superconductor is strongly dependent on the rotating angle, reflecting the d-wave symmetry of the Cooper pair wave function.

Superconducting tunneling on NbN thin films

Superconducting tunnel junctions were fabricated on reactively sputtered NbN thin films. The NbN films were homogeneous and of single cubic phase; their transition temperatures were relatively high (14.0–17.3 K). Native oxide barriers and gold counter electrodes completed the junctions. Results ofdV/dI versusV measurements located the maximum energy gap near 3.30 meV, yielding a strong coupling parameter of above 4.5. Acoustic phonons were clearly observed ind 2 I/dV 2 versusV measurements at around 22.5 and 27.5 meV.

New fabrication technique and electronic tunneling studies of NbN

We have fabricated NbN films using a new dual ion-beam sputter deposition technique. In this technique, one ion beam sputters Nb from a target, while the second ion beam directly bombards the growing film. The T c of these NbN films is typically 11–12 K when deposited on near-room temperature substrates. Preliminary results on tunneling through a native oxide barrier yield a superconducting energy gap for NbN of 1.9 meV, for which 2 Δ/ kT c = 3.9. We have also studied oxidized overlayers of Al and Ta to produce an artificial oxide tunneling barrier. Conductance data indicate that the native oxide barrier is broad with a low barrier height (0.2–0.3 eV), whereas the oxidized Al overlayer has a higher (0.5–1.2 eV) and narrower barrier. The quality of the I–V curves for native oxide and for artificial oxide barriers is comparable.

Superconducting tunneling on VN x films

Superconducting tunneling junctions were fabricated on reactively sputtered VN x films with Al2O3, SiO2, or native oxide barriers. Results of thed2I/dV2 versusV measurements reveal a phonon structure that agrees well with that obtained in neutron scattering experiments.

Nb and Nb-based A15 compound tunnel junctions fabricated using a new CF&amp;lt;inf&amp;gt;4&amp;lt;/inf&amp;gt;cleaning process

A new surface cleaning technique, the CF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> Cleaning Process (CFCP), is proposed for the fabrication of Nb and Nb-based Al5 compound tunnel junctions with native oxide barriers. The effects of fabrication conditions and these superconducting material properties on the characteristics of these junctions with Pb or Pb alloy counterelectrodes are investigated. High-quality junctions with a well-defined gap, no knee and very low excess conductance are successfully fabricated through the conventional lift-off technique for Al5 compounds (Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Al, Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ge and Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Si) as well as Nb. Arrays of 100 series-connected 2 μmφ Nb/Pb-Bi Junctions are fabricated with the standard deviation (σ) of critical current (J <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</inf> = 15.1 KA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ) equal to 2.5 %. The CFCP mechanism that contributes to the formation of such good junctions is studied using in-situ X-ray photoelectron spectroscopy (XPS). Chemical shifts of the Nb 3d levels on the CF <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</inf> plasma-cleaned Nb surfaces indicate the presence of an Nb-F layer. These XPS spectra show that the tunnel barrier is composed of mixed Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> O <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</inf> and Nb-FO compounds.

Selective niobium anodization process for fabricating Josephson tunnel junctions

A novel process for fabricating refractory sperconducting tunnel junctions is described, which is useful with both deposited and native oxide barriers. The distinguishing feature of the method is that the entire superconductor-barrier-superconductor sandwich is formed before the patterning of any layer. Isolated Josephson junctions are then formed by anodizing through the upper electrode, while the devices themselves are protected by a photoresist mask. Using this process, Nb-Si:H-Nb junctions have been fabricated, whose product of critical current and subgap resistance exceeds 10 mV and whose critical current density varies by about 50% over a 2-in. diameter wafer.