Universality of transport properties of ultrathin oxide films (original) (raw)
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Low-leakage superconducting tunnel junctions with a single-crystal Al 2 O 3 barrier
Superconductor Science and Technology, 2005
We have developed a two-step growth scheme for single-crystal Al 2 O 3 tunnel barriers. The barriers are epitaxially grown on single-crystal rhenium (Re) base electrodes that are grown epitaxially on a sapphire substrate, while polycrystalline Al is used as the top electrode. We show that by first growing an amorphous aluminium (Al) oxide layer at room temperature and crystallizing it at a high temperature in oxygen environment, a morphologically intact single-crystal Al 2 O 3 layer is obtained. Tunnel junctions fabricated from these trilayers show very low subgap leakage current. This single-crystal Al 2 O 3 junction may open a new venue for coherent quantum devices.
Journal of Applied Physics, 2004
Current induced resistance changes were investigated in magnetic tunnel junctions with ultrathin Al-O x barriers. The nonuniformity of the insulator induced a strong coupling between the two magnetic electrodes and no magnetoresistance. However, the current-voltage ͑I-V͒ characteristics at low bias voltages were consistent with a tunnellike behavior. At larger bias voltages, they showed an abrupt change of slope that was reversible for an opposite voltage polarity. The resistance versus current ͑R-I͒ curves exhibited reversible resistance changes that reached over 100%. We interpret this as controlled electromigration in local nanoconstrictions of the barrier.
Universal Distribution of Transparencies in Highly Conductive Nb/AlOx/Nb Junctions
Physical Review Letters, 2000
We report the observation of the universal distribution of transparencies, predicted by Schep and Bauer [Phys. Rev. Lett. {\bf 78}, 3015 (1997)] for dirty sharp interfaces, in uniform Nb/AlO$_x$/Nb junctions with high specific conductance ($10^8$ Ohm$^{-1}$cm$^{-2}$). Experiments used the BCS density of states in superconducting niobium for transparency distribution probing. Experimental results for both the dc I−VI-VI−V curves at magnetic-field-suppressed supercurrent and the Josephson critical current in zero magnetic field coincide remarkably well with calculations based on the multimode theory of multiple Andreev reflections and the Schep-Bauer distribution.
Superconducting triplet pairing in Al/Al$_\mathbf{2} OOO_\mathbf{3} $/Ni/Ga junctions
arXiv (Cornell University), 2021
Ni-Ga bilayers are a versatile platform for exploring the competition between strongly antagonistic ferromagnetic and superconducting phases. We characterize the impact of this competition on the transport properties of highly-ballistic Al/Al 2 O 3 (/EuS)/Ni-Ga tunnel junctions from both experimental and theoretical points of view. While the conductance spectra of junctions comprising Ni (3 nm)-Ga (60 nm) bilayers can be well understood within the framework of earlier results, which associate the emerging main conductance maxima with the junction films' superconducting gaps, thinner Ni (1.6 nm)-Ga (30 nm) bilayers entail completely different physics, and give rise to novel large-bias (when compared to
Journal of Applied Physics, 1997
Measurements of temperature dependence of the return current in high quality Nb/AlO x /Nb Josephson junctions are presented. From the experimental data, we obtain the effective resistance, i.e., the effective dissipation, for the retrapping process, according to the generalized resistively shunted junction model proposed by Chen, Fisher, and Leggett. We present a careful analysis, based on a comparison between the measured temperature dependencies of both the return and the quasiparticle tunneling current. We find that the junction subgap conductance, which includes the quasiparticle and the quasiparticle-pair interference terms, is responsible for the return process. The measurements have been performed on various samples, in a wide range of critical current densities from 50 to 2250 A/cm 2 , covering different damping regimes and spanning over the high and low temperature limits. Junctions with low critical current density show ideal dissipation which makes the return current scale with temperature according to the BCS exponential behavior without flattening out effects. This result may be relevant for the possible use of Nb/AlO x /Nb junctions in macroscopic quantum coherence experiments, which strongly require a very low dissipation.
Subgap current in superconducting tunnel junctions with diffusive electrodes
Physical Review B, 2006
We calculate the subgap current in planar superconducting tunnel junctions with thin-film diffusive leads. It is found that the subharmonic gap structure of the tunnel current scales with an effective tunneling transparency which may exceed the junction transparency by up to two orders of magnitude depending on the junction geometry and the ratio between the coherence length and the elastic scattering length. These results provide an alternative explanation of enhanced values of the subgap current in tunneling experiments often ascribed to imperfection of the insulating layer. We also discuss the effect of finite lifetime of quasiparticles as the possible origin of additional enhancement of multiparticle tunnel currents.
The oxidation state at tunnel junction interfaces
We demonstrate that at the usual 10 −7 torr range of base pressures in the sputtering chamber, X-ray photoelectron spectroscopy shows the existence of a thin AlOx layer at the Nb/Al interface in both Nb/Al-AlOx/Pb tunnel junctions and Nb/Al bilayers. This is due to the time elapsed between the deposition of the Nb and Al bottom layers, even at times as short as 100 s. We also give some direct evidence of the oxidation of the top Pb electrode on the Nb electrode surface. Such oxidation probably occurs at the pinholes of the intermediate Al-AlOx layer of the junctions, as a consequence of the oxidation state at the Nb/Al interface. We therefore suggest that both the base pressure and the time lapse between layer depositions should be carefully controlled in magnetic tunnel junctions. 73.40.Gk; 74.50.+r; 73.50.Jt; 85.30.Mn; 85.70.Kh Ferromagnet/insulator/ferrromagnet (FM/I/FM) magnetic tunnel junctions (MTJs) exhibiting large magnetoresistance (MR) 1 have lately attracted much interest due to their potential applications 2,3 . The performance of the junctions is strongly dependent on the oxidation of the FM electrodes at the FM/I interfaces, as well as on the oxidation state of the barrier, which has to be homogenous and complete. The use of thinner and thinner barriers has reopened the question of how to rule out the presence of pinholes. Rowell and others developed a set of criteria to ascertain that tunneling is the dominant mechanism in junctions with at least one superconducting (S) electrode 4 . Three of these criteria still apply in FM/I/FM structures: (i) an exponential insulator thickness dependence of the conductance, G; (ii) a parabolic voltage dependence of G that can be fitted to the theoretical models 5,6 ; and (iii) a weak insulating-like dependence G(T ).
Direct observation of local hot electron transport through Al[sub 2]O[sub 3] tunnel junctions
Applied Physics Letters, 2002
A modified ballistic electron emission microscopy ͑BEEM͒ technique using local transport of hot electrons through a buried interface, was successfully applied to study the Al 2 O 3 barrier in the Co/Al 2 O 3 /Ru tunnel junction. This technique enabled us to straightforwardly measure an effective barrier height of 1.7 eV and to observe the rise of the barrier height due to continuous current injection into a single point of the junction attributed to charging effects and/or degradation of the barrier structure. Scanning over an area of 510 nmϫ510 nm showed a spatial inhomogenity of the barrier resulting in different dependencies of the BEEM current on the energy of the injected electrons.
Intrinsically damped multilayered (stacked) Nb/Al-AlNx/Nb superconducting tunnel junctions
Applied Superconductivity, 1995
Single and stacked Nb/Al-AlN,/Nb superconducting tunnel junctions with both hysteretic (underdamped) and non-hysteretic (overdamped) current-voltage relationships have been produced utilizing reactively sputtered aluminum nitride tunnel barriers. Standard multilayer deposition and lithographic processing techniques, compatible with existing Nb/Al-AlO,/Nb fabrication techniques, are used in fabrication. The degree of damping in the junctions is controlled through the deposition parameters Critical current dependence on applied magnetic field indicates that the overdamped junctions have a distributed Josephson coupling and are not simple microshorts. The shorter deposition time to grow reactively sputtered AlN, barriers makes this system a promising alternative to fabricate stacked Josephson junctions.