Low-resistance Tunnel Junctions Research Articles

Low Resistance Tunnel Junctions for Efficient Electrically Pumped Nanolasers

Extremely compact nanoscale devices such as electrically pumped nanolasers are difficult to operate at room temperature due to the high electrical resistance inherent to small cavities. As a consequence, large voltages are necessary to reach the lasing threshold, which generates heat and reduces device efficiency. The poor heat sinking of small devices makes matters worse, dramatically reducing the laser efficiency. Instead of looking for solutions to dissipate heat from small structures more efficiently, designing nanolasers to produce less heat in the first place is an important goal. Here we propose and theoretically analyze the effect of adding an InGaAsP tunnel junction for efficient carrier injection in metallo-dielectric nanolasers. With our theoretical model we show that the device resistance is reduced by a factor of ∼6.5. The applied voltage at the room temperature lasing threshold is reduced from 3.05 to 1.35 V, a reduction of 69% in heat generation, whereas the Q-factor and gain threshold of the cavity are not degraded.

High power and high efficiency 1.8-kW pulsed diode laser bar

There is a great interest in the development of high-power, high-efficiency, and low-cost quasicontinuous wave (QCW) diode laser bars and arrays for pumping solid state lasers. We report on the development of kW-class 88x-nm diode laser bars that are based on a bipolar cascade design, in which multiple lasers are epitaxially grown in electrical series on a single substrate and separated by low-resistance tunnel junctions with resistance as low as 8.0×10−6 Ω-cm2. QCW power of 630 W was demonstrated in a 3-mm-wide minibar with 3-mm cavity length. Peak efficiency of 61% was measured with 200 μs and 14 Hz pulses, at 10°C. Further power scaling was demonstrated in a 1-cm-wide bar with 3-mm cavity length, where a record peak power of 1.8 kW was measured at 1-kA drive current. Ongoing work for further power scaling includes development of triple-junction diode laser bars.

GaN-based three-junction cascaded light-emitting diode with low-resistance InGaN tunnel junctions

We report on the cascading of blue light-emitting diodes (LEDs) up to three junctions using low-resistance InGaN tunnel junctions (TJs). At a forward current density of 10 A/cm2, triple- and dual-junction LEDs operated at 9.09 and 6.07 V with total differential resistances of 6.31 × 10−2 and 4.16 × 10−2 Ω cm2, respectively. A significant increase in output power was observed from the triple-junction LED compared to the dual one, showing that all LED layers contribute to the luminescence output. Enabling high brightness at low current, cascaded LEDs can circumvent efficiency droop mechanism.

Tunneling-based carrier regeneration in cascaded GaN light emitting diodes to overcome efficiency droop

In this work, we demonstrate visible wavelength transparent GaN/GdN/GaN tunnel junction interconnects to cascade multiple p-n junctions with low resistance (5.7 × 10−4 Ω × cm2). We model a device structure using cascaded light emitting diodes (LEDs) with tunnel junction-based carrier regeneration to create high-power LEDs operating at low current density. Experimental LED characteristics are used to model the use of these low resistance tunnel junctions in cascaded multiple active region LEDs that can effectively provide high power by operating under low current and high forward voltage. The adoption of cascaded LED structures can enable high power LEDs while maintaining high efficiency.

GdN Nanoisland-Based GaN Tunnel Junctions

Tunnel junctions could have a great impact on gallium nitride and aluminum nitride-based devices such as light-emitting diodes and lasers by overcoming critical challenges related to hole injection and p-contacts. This paper demonstrates the use of GdN nanoislands to enhance interband tunneling and hole injection into GaN p-n junctions by several orders of magnitude, resulting in low tunnel junction specific resistivity (1.3 × 10(-3) Ω-cm(2)) compared to the previous results in wide band gap semiconductors. Tunnel injection of holes was confirmed by low-temperature operation of GaN p-n junction with a tunneling contact layer, and strong electroluminescence down to 20 K. The low tunnel junction resistance combined with low optical absorption loss in GdN is very promising for incorporation in GaN-based light emitters.

Josephson effect and nonequilibrium superconductivity in superconducting tunnel structures

Nonequilibrium superconductivity induced by tunnel current injection of quasiparticles is studied. It is found that an instability in the form of a negative voltage jump in the oscillator current-voltage characteristic (CVC), which leads to an inhomogeneous state, as well as the spatial structure of the inhomogeneous state are very sensitive to low magnetic fields. The shape of the CVC of low-resistance tunnel junctions for bias voltages V ≈ 2Δ/e depends strongly on the junction dimensions and barrier transparency. These results are interpreted in terms of Josephson vortices (fluxons) in a tunnel oscillator. Studies of the nonequilibrium phenomena, with the Josephson properties of low-resistance tunnel structures taken into account, reveal a number of new effects, such as nonequilibrium suppression of the energy gap at bias voltages V < 2Δ/e, the possible existence of an entire series of instabilities of the nonequilibrium superconducting state during tunnel injection, and inhomogeneity in the tunnel injector effect.

III-V junctions on silicon substrates using BeTe buffer layers for solar cell applications

Epitaxial growth of AlGaAs–GaAs quantum well junctions on beryllium telluride layers on silicon was investigated as an approach toward multijunction solar cells. BeTe maintained a well-ordered, planar growth mode on silicon, while it relaxed the lattice constant for subsequent AlGaAs growth. AlGaAs on BeTe growth initiation using solid-phase epitaxy beginning from near room temperature was required to maintain planar growth, attributed to surface energy effects. AlGaAs on BeTe films required several hundred angstrom thickness low temperature growth to be stable against roughening with further growth at normal growth conditions. I-V measurements showed that the n+-Si∕As∕p+-BeTe junction formed a low resistance tunnel junction. The photocurrent of the AlGaAs–GaAs quantum well p-i-n junction under one sun condition was as high as JSC=7.9mA∕cm2 with VOC=600mV. Junctions grown on BeTe∕Si substrates were compared to similar junctions grown on GaAs substrates.

InGaN Thin Films Grown by ENABLE and MBE Techniques on Silicon Substrates

ABSTRACTThe prospect of developing electronic and optoelectronic devices, including solar cells, that utilize the wide range of energy gaps of InGaN has led to a considerable research interest in the electronic and optical properties of InN and In-rich nitride alloys. Recently, significant progress has been achieved in the growth and doping of InGaN over the entire composition range. In this paper we present structural, optical, and electrical characterization results from InGaN films grown on Si (111) wafers. The films were grown over a large composition range by both molecular beam epitaxy (MBE) and the newly developed “energetic neutral atomic-beam lithography & epitaxy” (ENABLE) techniques. ENABLE utilizes a collimated beam of ∼2 eV nitrogen atoms as the active species which are reacted with thermally evaporated Ga and In metals. The technique provides a larger N atom flux compared to MBE and reduces the need for high substrate temperatures, making isothermal growth over the entire InGaN alloy composition range possible. Electrical characteristics of the junctions between n- and p-type InGaN films and n- and p-type Si substrates were measured and compared with theoretical predictions based on the band edge alignment between those two materials. The predicted existence of a low resistance tunnel junction between p-type Si and n-type InGaN was experimentally confirmed.

Effect of the bias current on the magnetic field switching in micrometer AlOx-based tunnel junctions

We have examined the relationship between switching characteristics and the tunnel magnetoresistance under finite bias in microscopic low resistance (RA<30Ωμm2) tunnel junctions with AlOx barriers. By switching the free layer with a magnetic field applied in plane along the magnetically easy axis, it is observed that the coercive field depends strongly on the bias current. Steps on the R-H curve due to nonuniform magnetization reversal can be attributed to the in-plane domain wall pinning effect, and it is thought to account for the diminished junction resistance changes upon the application of perpendicular-to-plane spin-polarized currents.

Low resistance tunnel junctions with type-II heterostructures

We propose tunnel junctions with type-II heterostructures to reduce the electrical resistance of vertical-cavity surface-emitting lasers. We fabricated the type-II tunnel junctions on GaAs substrates, which consist of highly C-doped GaAsSb on the p side and highly Si-doped InGaAs on the n side. We achieved an extremely low specific resistance of 4×10−6Ωcm2, which corresponded to a resistance of 20Ω for an aperture 5μm in diameter. The specific resistance of the type-II tunnel junction was about 40% smaller than that of the type-I tunnel junction.

Magnetization switching by spin-polarized current in low-resistance magnetic tunnel junction with MgO [001] barrier

Current-induced magnetization switching (CIMS) was demonstrated in low resistance magnetic tunnel junctions (MTJs) with thin MgO [001] barrier. The resistance change by CIMS was more than 100%, which is much larger than the previous report in Al-O based MTJs. The switching current density was about 2/spl times/10/sup 7/ A/cm/sup 2/, which was comparable with that reported values in metallic multilayers.

Low resistance tunnel junctions with remote plasma underoxidized thick barriers

Low resistance tunnel junctions suitable for >200Gb∕inch2 read heads require R×A<1Ωμm2 and TMR>10%, usually achieved by natural oxidation with tAl<0.7nm barriers. This paper shows that as-deposited junctions with competitive electrical and magnetic properties can be produced starting from 0.9nm Al barriers and remote plasma oxidation in ion beam-deposited stacks using Co73.8Fe16.2B10 electrodes. TMR∼20% for R×A∼2–15Ωμm2 is obtained, while in the R×A∼40–140Ωμm2 range TMR can reach 40%–45%, in as-deposited samples. A limited number of junctions exhibits considerably lower R×A values with respect to the average while keeping similar MR (down to 0.44Ωμm2 with 20% and down to 2.2Ωμm2 with 51%).

Low-resistance tunnel junctions on GaAs substrates using GaInNAs

Using molecular-beam epitaxy, we have fabricated p-n tunnel junctions lattice matched to GaAs substrates that consist of highly C-doped Ga0.93In0.07As on the p side, and highly Si-doped Ga0.93In0.07N0.02As0.98 on the n side. The introduction of N on the n side of the tunnel junction: (1) lowers the conduction band edge while leaving the valence band edge unchanged, (2) allows the introduction of In to further lower the band gap while remaining lattice matched to GaAs, and (3) increases the maximum Si donor activation that can be achieved. All three of these effects increase the tunneling probability for carriers across the junction and decrease the specific resistance. We estimate the active C acceptors on the p side to be 1.5×1020 cm−3 and the active Si donors on the n side to be 1.8×1019 cm−3. Because of the mutual passivation effect between N and Si atoms, the Si doping level was increased to 1.0×1020 cm−3 to achieve this active net donor concentration. The specific resistance of test tunnel junctions has been measured to be <1×10−5 Ω cm2 corresponding to <0.1 V drop at a current density of 10 kA/cm2. These results are a substantial improvement over corresponding test structures without N on the n side of the junction. The experimental results are in good agreement with model calculations.

Current-induced switching in low resistance magnetic tunnel junctions

Current-induced magnetization switching (CIMS) in low resistance tunnel junctions is reported at critical current densities of 1.9×106 A/cm2 for tunnel junction areas ranging from 2 to 3 μm2, and junction resistances from 6 to 20 Ω μm2. Typical tunnel magnetic resistance values for these junctions range from 15% to 21% (measured in an external magnetic field) and 10% to 14% resistance changes are obtained by CIMS. Micromagnetic simulation indicates that vortex fields and spin transfer effects cannot fully account for the observed current-induced switching. Although able to explain the observed transition from a parallel or antiparallel state to a vortex state, it fails to explain the switch back to the original state, at a comparable but symmetrical critical current density.

Continuous thin barriers for low-resistance spin-dependent tunnel junctions

The occurrence of pinholes in thin barrier low-resistance junctions degrades the TMR signal and increases the coupling field between pinned and free layers. The tunnel junction coupling field (Hf), junction resistance and TMR signal dependence on the barrier thickness was studied for various types of barriers (HfOx,HfAlOx,ZrAlOx,AlOx). Micromagnetic simulation was employed to simulate the coupling field versus pinhole density. From the coupling field results, HfOx makes the thinnest continuous barriers, followed by doped HfAlOx and ZrAlOx, and then AlOx. HfAlOx and ZrAlOx offer the best compromise between low resistance (1–5 Ω μm2) and reasonable TMR (12%–14%). Pure HfOx can be made with R×A products of 0.4 Ω μm2 but the TMR does not exceed 5.5%.

Low-resistance spin-dependent tunnel junctions with HfAlO/sub x/ barriers for high-density recording-head application

Spin-dependent tunnel junctions with the structure (Ta 70 /spl Aring//NiFe 70 /spl Aring//MnIr 80 /spl Aring//CoFe 35 /spl Aring//HfAlO/sub x//CoFe 35 /spl Aring//NiFe 40 /spl Aring//TiW(N) 150 /spl Aring/) were fabricated on top of 600-/spl Aring/-thick ion-beam-smoothed low-resistance Al electrodes. HfAlO/sub x/ barriers were formed by natural oxidation (5 min at 1 torr in pure O/sub 2/) of 5-/spl Aring/-thick (2-/spl Aring/ Hf+3-/spl Aring/ Al) films or 6-/spl Aring/-thick (2-/spl Aring/ Hf+4-/spl Aring/ Al) films. Resistance/spl times/area (R/spl times/A) products of 0.65 /spl Omega//spl times//spl mu//sup 2/ and 2.1 /spl Omega//spl times//spl mu/m/sup 2/ were achieved with 9.5% and 13.5% tunnel magnetoresistance signal (TMR), respectively. Current inhomogeneity effects on the measured (R/spl times/A) products and TMR values were calculated in particular for junctions with resistance below 1 /spl Omega//spl times//spl mu/m/sup 2/. Transmission electron microscopy indicates that HfAlO/sub x/ forms a continuous amorphous barrier that follows conformally the topography of the bottom electrode. X-ray photoelectron spectroscopy analysis indicates that 2.5% metallic Hf is left inside the barrier closer to the bottom electrode. These low-resistance tunnel junctions are attractive for read-head applications at recording densities above 100 Gbit/in/sup 2/.

Low resistance spin-dependent tunnel junctions with ZrAlOx barriers

Spin-dependent tunnel junctions with ZrAlOx barriers were fabricated with low resistance×area product 4 Ω×μm2, and tunnel magnetoresistance of 15.2%. Barrier fabrication was done by natural oxidation (5 min, at oxidation pressures ranging from 0.5 to 10 Torr). The junctions were deposited on top of 600 Å thick, ion beam smoothed, low resistance, Al electrodes. X-ray photoelectron spectroscopy analysis indicates the presence of AlOx, ZrO2, some remnant metallic Zr, but no metallic Al in the as-deposited barriers. High resolution transmission electron microscopy indicates that ZrAlOx forms an amorphous barrier that is smoother than pure crystalline ZrOx or pure amorphous AlOx barriers. These low resistance tunnel junctions are attractive for read head applications above 100 Gbit/in2 where competitive signal to noise ratios imply resistance×area product below a few Ω×μm2, and tunneling magnetoresonance signals near or above 20%.

Low-resistance spin-dependent tunnel junctions with ZrAlOx barriers

Spin-dependent tunnel junctions with ZrAlOx barriers were formed by natural oxidation (5 min at 10 Torr) of 7-Å-thick Zr–Al films. Resistance×area products of 6 Ω μm2 were achieved with a 15.3% tunnel magnetoresistance (TMR) signal. Bottom-pinned (MnIr) junctions were deposited on top of 600-Å-thick, ion-beam-smoothed, low-resistance, Al electrodes. Effective average barrier height and thickness are 0.28 eV and 8.2 Å, respectively, and breakdown voltage is 0.41 V for 1 μm2 junctions. The TMR signal decreases by half at a bias voltage of 210 mV. Junction TMR decreases for anneals above 250 °C. High-resolution transmission electron microscopy indicates that ZrAlOx forms an amorphous barrier that is smoother than pure crystalline ZrOx or pure amorphous AlOx barriers. These low-resistance tunnel junctions are attractive for read head applications above 100 Gbit/in.2 where competitive signal-to-noise ratios imply resistance×area products of few Ω μm2 and TMR signals near or above 20%.

Low-resistance InGa(Al)As Tunnel Junctions for Long Wavelength Vertical-cavity Surface-emitting Lasers

A new method is proposed for a significant reduction of series resistance and device heating in long wavelength vertical-cavity surface-emitting lasers (VCSELs) based on InP. Our technique involves a twofold epitaxial growth with a buried low-resistance (3×10-6 Ωcm2) tunnel junction. The substitution of high-resistive p-type confining layers by low-resistive n-type material results in total resistances smaller 100 Ω even for the case of non-conducting dielectric mirrors. With the application of buried tunnel junctions in a VCSEL-structure, we could demonstrate small electrical series resistance (U < 1 V at 3 kAcm-2) and effective current confinement simultaneously. Furthermore, because of the laterally varying cavity length, an effective lateral waveguiding effect occurs.

Epitaxially stacked GaAs/GaAlAs lasers using a low-resistance tunnel junction

Low-resistivity Ga(In)As tunnel junctions have been used to interconnect epitaxially two separate confinement heterostructure (SCH) InGaAs/GaAlAs laser structures. Laser structures consist of a 0.2 μm GaAs optical cavity containing three Ga 0.88In 0.12As strained quantum wells sandwiched within two 1.5 μm thick Al 0.45Ga 0.55As n- and p-type cladding layers. P( I) characteristic exhibits two different laser thresholds: one at 420 A cm −2 and another at 760 A cm −2. A reference laser sample shows a threshold current density of about 400 A cm −2. The reason for the higher threshold current value of the top laser is unclear at present. Details of both growth aspects of heavy doped tunnel diodes and optimum design of stacked laser diode structures are presented.