Thin Tunnel Barrier Research Articles

Enhanced quantum efficiency in polymer electroluminescence devices by inserting a tunneling barrier formed by Langmuir–Blodgett films

Quantum efficiency in a polymer electroluminescence device is significantly improved by inserting a thin insulating layer with the thickness of tunneling range. Four times higher quantum efficiency was obtained without the increase of the threshold voltage. Poly(methyl methacrylate) Langmuir–Blodgett films were used as the thin tunneling barrier. The enhancement may result from the lowering of the effective barrier height for electron injection while increasing the effective barrier for hole injection. The effects improve the balanced injection of electrons and holes into the light-emitting devices.

Optimization of the slow-mode plasmon polariton in light-emitting tunnel junctions

Light emitted from metal/oxide/metal tunnel junctions can originate from the slow-mode surface plasmon polariton supported in the oxide interface region. The effective radiative decay of this mode is constrained by competition with heavy intrinsic damping and by the need to scatter from very small scale surface roughness; the latter requirement arises from the mode’s low phase velocity and the usual momentum conservation condition in the scattering process. Computational analysis of conventional devices shows that the desirable goals of decreased intrinsic damping and increased phase velocity are influenced, in order of priority, by the thickness and dielectric function of the oxide layer, the type of metal chosen for each conducting electrode, and temperature. Realizable devices supporting an optimized slow-mode plasmon polariton are suggested. Essentially these consist of thin metal electrodes separated by a dielectric layer which acts as a very thin (a few nm) electron tunneling barrier but a relatively thick (several 10’s of nm) optically lossless region.

Transmission electron microscopy and atomic force microscopy analysis of Nb-Al-AlOx-Nb superconducting tunnel junction detectors

The performance of photon detectors based on superconducting tunnel junctions are related to their current - voltage (I-V) curve characteristics and, ultimately, to the quality of the thin tunnel barriers (of order 1 nm) which separate the two superconducting thin films. Both the optimization of the spectroscopic performance of these detectors and the development of a reproducible and high yield fabrication route, require a better understanding of barrier quality and growth techniques. Transmission electron microscopy (TEM) and atomic force microscopy (AFM) provide valuable tools for the investigation of the barrier region and for the control of the quality of the different thin films and related interfaces. In this paper, the results of a TEM and AFM evaluation of Nb-Al-AlOx-Nb tunnel junctions are reported, together with their interpretation on the basis of the I-V curve performance at low temperature (T≥0.3 K). Thickness disuniformities of the Al plus AlOx overlayer and evidence of barrier defects have been found, which may place constraints on the spectroscopic performance of such devices. Through the use of TEM it has also been possible to confirm the epitaxial nature of the Nb base electrode. The junction counter electrode however appears to be polycrystalline, with a columnar morphology and an average grain width of 40 nm. The overall structure of the various layers may well place constraints on the tunneling characteristics of the device.

Tunnel barrier growth dynamics of Nb/AlO xAl/Nb and Nb/A1N xAl/Nb Josephson junctions

The growth of very thin aluminum oxide and aluminum nitride tunnel barriers on top of an about 7 nm aluminum layer is deduced by measurements of the reflectivity change of a laser beam due to the decrease of the aluminum laser thickness. Within the first seconds of the process thermal aluminum oxide grows much faster than aluminum nitride in a nitrogen plasma. For both barrier types the reflectivity change can be correlated with the Josephson current density of the finished junctions. In a semi-logarithmic scale the current density versus reflectivity change can be approximated by a straight line up to 20 kA/cm 2. High current densities with AIN x seem to be more easily controllable than with AlO x .

Suppression of the Josephson effect by quantum fluctuations in the fractional quantum Hall state.

We use the Chern-Simons gauge theory for the fractional quantum Hall effect (FQHE) to calculate the Josephson current for two weakly coupled two-dimensional electron gas (2DEG) subsystems separated by a thin tunnel barrier, for the primary fractional quantum Hall states [ital v]=1/(2[ital k]+1) ([ital k]=0,1,2,...), at zero temperature. We find that the Josephson current amplitude is mainly suppressed by the long-range quantum fluctuations of the Chern-Simons gauge field, by a factor [proportional to]exp[l brace][[minus](2[ital k]+1)/4[pi]]([ital L]/[ital l][sub 0])[r brace] where [ital L] is the system size, and [ital l][sub 0] is of the order of the magnetic length. Thus it is unlikely that the Josephson effect can be observed for weakly coupled 2DEG subsystems, despite the strong analogy between the FQHE in the Chern-Simons formulation and two-dimensional superconductivity.

Comparison of anodization spectroscopy with SIMS and RBS measurements for the characterization of Nb/AlAlO x/NB Josephson junction structures

The thin tunneling barrier in a Nb/AlAlO x /Nb was characterized by anodization spectroscopy, SIMS and RBS. From the anodization spectroscopy we can get the film thickness from the voltage span and, most importantly, also the sharpness of the Nb/AlO x and Al/Nb interfaces around the tunneling barrier. Comparison of the anodization profiles with SIMS depth profiles and RBS spectra shows that the anodization spectroscopy can be used to characterize the junction quality.

Tunneling spectroscopy of a quantum resonator

A tunneling current across a thin wall of a narrow-wide-narrow (NWN) cavity region in an electron waveguide is calculated. A lead is attached to the cavity region at right angle through a thin tunneling barrier to drain the current. The transmission coefficient through the NWN waveguide shows successive dips due to resonances through quasibound states in the cavity region as the length of the cavity region or the Fermi energy of an incident electron is varied. The transmission coefficient leaking out the tunneling barrier shows peaks that line up with the dips in the forward transmission probability, demonstrating that a tunneling spectroscopy of the quasibound state levels is provided through sweeping the Fermi energy.

Transport asymmetry and photovoltaic response in (AlGa)As/AlAs/GaAs/(AlGa)As single-barrier quantum-well infrared detectors

We have studied the wavelength and electric-field characteristics of intersubband photodetectors where a thin AlAs tunnel barrier was introduced between one side of the GaAs quantum well and thicker layers of (AlGa)As. The photoresponse of these structures has an extremely broad (3–11 μm) spectral range and there is a preferential escape direction of the photoexcited carriers towards one direction of the quantum wells giving rise to photovoltaic detector behavior. We found evidence that this transport asymmetry is not only caused by the asymmetric potential distribution, but that interface scattering processes are involved. These scattering processes act differently on the photocurrent and the dark current, which is useful for further optimization of quantum-well infrared detectors.

Observation of resonant electron capture in AlGaAs/GaAs quantum well structures

We demonstrate for the first time the experimental evidence for the resonant electron capture in AlGaAs/GaAs quantum wells. The sample used contains thin AlAs tunnel barriers on both sides of the GaAs well layer to enhance the quantum-mechanical reflectivity of the electron. It was found that the capture efficiency of photogeneruted electrons in the AlGaAs barrier into the quantized state of the well is strongly dependent on the well width and increases under the resonant condition where the well width is such that the highest quantized state in the well coincides with the conduction-band edge of the AlGaAs barrier. The enhanced capture efficiency under the resonant condition is ascribed to the increased probability that the electron in the continuum state at the barrier band-edge is found in the well.

Tunneling spectroscopy of an electron waveguide.

On an AlGaAs/GaAs heterostructure, we have implemented a unique ``leaky'' electron waveguide with a thin tunneling barrier as one of its confining boundaries. The current flowing through the waveguide as well as the 1D to 2D tunneling current leaking out the thin side barrier were independently measured as the electron density in the waveguide was modulated. The tunneling current shows very strong oscillations lined up with the 2${\mathit{e}}^{2}$/h conductance steps for the waveguide current. A proposed explanation is that the oscillations are a result of the 1D density of states sweeping through the Fermi level.

Observation of excitonic effects on electroabsorption in coupled quantum wells.

An experimental study was made on the electroabsorption of asymmetric GaAs double quantum wells separated by thin AlAs tunnel barriers to better understand the optical properties in the coupled systems. It was clearly shown that the field-dependent absorption characteristics, i.e., the anticrossing properties, were not fully explained by the predictions for the optical transitions between the quantized electronic levels but better interpreted by taking account of the contribution of the exciton binding energies, i.e, in terms of tunneling between the excitonic states in the adjacent wells.

Guided wave study of langmuir-blodgett films of eicosanoic acid

The attenuated total reflection technique with a thin metal film tunnel barrier has been used to couple radiation into waveguides made of eicosanoic acid. Through the use of a pyramidal geometry and both TE and TM polarisation the full optical dielectric tensor of Langmuir-Blodgett multilayers of this material has been characterised for a range of deposition pressures. Results show that the mean molecular axis within the multilayers tilts over as the surface pressure used during film deposition is decreased.

Evaluation of AlOx barrier thickness in Nb Josephson junctions using anodization profiles

Using anodization profiles, we have analyzed the thin AlOxAl tunneling barrier in a Nb/AlOxAl/Nb Josephson junction. We measured the voltage width at the AlOxAl barrier in the profiles and found that it is closely related to the AlOxAl thickness. We proposed a way to evaluate this thickness from the voltage width. The anodization profile is useful in diagnosing the 4.2 K critical current density of Josephson junctions even at room temperature.

Characterization of Nb/AlOx-Al/Nb Josephson junctions by anodization profiles

The thin tunneling barrier in a Nb/AlOx-Al/Nb Josephson junction was characterized by anodization profiles. Our studies focused on Nb/AlOx and Al/Nb interfaces in Nb/AlOx-Al/Nb junctions made by varying certain process parameters. The interface quality was checked by the anodization profiles and was greatly affected by film thickness, layer sequence, annealing treatment, and the existence of a thin oxide. By comparing the profiles with the junction characteristics measured at 4.2 K, we confirmed that the interface sharpness in the profiles is related to the junction quality. The anodization profile is a useful tool to diagnose the tunneling barrier interfaces in Nb/AlOx-Al/Nb Josephson junctions, especially at room temperature during fabrication.

Characterization of Nb/AlO/sub x/-Al/Nb junction structures by anodization spectroscopy

The thin tunneling barrier in a Nb/AlO/sub x/-Al/Nb Josephson junction was characterized by anodization spectroscopy. Nb/AlO/sub x/ and Al/Nb interfaces made by varying certain process parameters were examined. The interface quality is greatly affected by film thickness, layer sequence, annealing, and existence of a thin oxide. It is concluded that anodization spectroscopy is a useful technique to diagnose the tunneling barrier in the Nb/AlO/sub x/-Al/Nb Josephson junctions during fabrication processes. >

Negative transconductance via gating of the quantum well subbands in a resonant tunneling transistor

Operation of a new resonant tunneling transistor is reported in the AlGaAs/GaAs material system. The device contains an undoped quantum well collector separated from a heavily doped emitter by a thin tunnel barrier. The collector is gated and the gate field controls resonant tunneling characteristics ‘‘from behind’’ via a combination of the generalized Stark effect and the quantum capacitance effect. The common-collector characteristics show negative differential resistance at a fixed gate bias and negative transconductance at a fixed emitter bias. Excellent agreement is found between the measured and calculated shifts of the peaks of the current-voltage characteristics.

Dynamics of tunneling to and from small metal particles.

The charge-transfer process between a small metal particle and a nearby electrode is investigated with electron tunneling experiments. The small particles are produced as an island film embedded in a capacitor, separated from one plate by a thin tunnel barrier and from the other by a thick insulator. The device properties reflect an ensemble average of independent particle-electrode tunneling systems. A significant property of the small-particle capacitor is that the potential change e/c associated with the transfer of a single electron is a non-negligible quantity. The experiments investigate the dynamics of the charge transfer process between particle and electrode by measuring the device capacitance C and dissipation constant D as a function of frequency \ensuremath{\omega}, temperature T, and ac voltage amplitude V. Experiments are carried out in the regime kT/e, V<e/c so that at most one electron transfer to or from a particle is induced during a cycle of the applied voltage. A model for the suppression of the tunneling rate by the Coulomb charging energy is presented which predicts C and D should scale as \ensuremath{\omega}/T in the zero-voltage limit and \ensuremath{\omega}/V in the zero-temperature limit. Experimental results verify these scaling laws. It is shown that at low temperature and voltages, a quantum size effect may cause the results to deviate from these scaling laws. Experimental results are presented which qualitatively support this prediction. Additional experiments investigate the transport between a small particle and a superconductor.

Interface states and impurities in MOS structures with very thin tunneling barriers

Electron tunneling spectroscopy was used to investigate MOS junctions with very thin silicon oxide or silicon oxynitride layers (2–5 nm) as tunneling barriers. For the tunneling measurements at 4.2 K highly degenerate P-doped (3×10 20 cm -3) Si substrates, oxidized in dry oxygen at 600°C were used. Silicon oxynitride layers were prepared by plasma nitridation in an NH 3 discharge. As gate electrodes evaporated films of Al, Au or Pb were utilized. Changes in the tunneling conductivity were attributed to changes in the density of interface states, caused by hydrogen annealing or by high field stress. The results indicate a correlation between the generation of interface states and the removal of Si-H configurations. Vibrational modes of phonons, dopants and impurities were detected by inelastic electron tunneling spectroscopy.

Delta- (°-) doping in MBE-grown GaAs: Concept and device application

The implementation of °-function-like doping profiles during molecular beam epitaxy (MBE) of GaAs by using Si donors and Be acceptors is employed to generate V-shaped potential wells with a quasi-two-dimensional electron (hole) gas and to create a new GaAs sawtooth doping superlattice with strongly reduced energy gap. Three examples for application of °-doped n-type GaAs are given; (i) as electron supplying layer in selectively doped asymmetric GaAs/Al x Ga 1− x As quantum well heterostructures, (ii) as thin tunneling barrier for non-alloyed ohmic contacts on n-GaAs, and (iii) as Schottky gate °-doped field effect transistor. GaAs sawtooth doping superlattices are attractive for application in photonic devices due to the significant red-shift of their luminescence.

Thin Semiconducting Films as Tunneling Barriers

We have investigated several semiconductors to determine their suitability for use as thin film quantum mechanical tunneling barriers for experiments with superconductors. We find that amorphous carbon films are particularly suitable, as carbon has a negligible miscibility with most metals and it forms continuous films at low thicknesses.