Bottom Tunnel Junction Research Articles

Tunneling assisted p-contact free GaN-InGaN green light-emitting diodes

The effects of bottom tunnel junction (TJ) on optoelectronic characteristics of green light-emitting diodes are analyzed numerically. Our proposed LED has enhanced internal quantum efficiency (IQE), radiative recombination rate, and carrier concentration in comparison to reference conventional LED. The IQE increases by 17%, droop is reduced by 28%, and emission intensity is nearly twice as high as than reference design. The improved optical properties of the proposed structure are due to excellent carrier confinement and altered spontaneous polarization in the vicinity of the active region by TJ.

Harnessing III-Nitride Built-In Field in Multi-Quantum Well LEDs.

III-nitrides possess several unique qualities, which allow them to make the world brighter, but their uniqueness is not always beneficial. The uniaxial nature of the wurtzite crystal leads to strikingly large electric polarization fields, which along with the high acceptor ionization energy cause low injection efficiency and uneven carrier distribution for multiple quantum well (QW) light emitting devices. In this work, we explore the carrier distribution in Ga-polar LED in two configurations: standard "p-up" and "p-down", which is accomplished by utilizing a bottom-tunnel junction. This enables the inversion of the sequence of the p and n layers while altering the direction of the current flow with respect to the inherent polarization. To probe the carrier distribution two, color-coded QWs are used in alternating sequences. Our study reveals that for "p-down" devices carrier transport through multiple QWs is limited by the potential barrier at the QW interface, which is in contrast to results for "p-up" structures, where hole mobility is the bottleneck. Moreover, investigated "p-down" LEDs exhibit an extremely low turn-on voltage.

III-Nitride p-down green (520 nm) light emitting diodes with near-ideal voltage drop

We demonstrate p-down green emitting LEDs with low turn-on voltage enabled by efficient tunnel junctions. Due to the polarization field alignment in the (In,Ga)N/GaN interface with the p-down orientation, the electrostatic depletion barrier for electron and hole injection is reduced when compared with the conventional p-up LEDs. A single (In,Ga)N/GaN heterostructure quantum well active region with a GaN homojunction tunnel junction exhibited very low forward operating voltage of 2.42 V at 20 A/cm2 with a peak electroluminescence emission wavelength of 520 nm for current densities above 100 A/cm2. The bottom tunnel junction with minimal voltage drop enabled excellent hole injection into the bottom p-GaN layer.

Bottom tunnel junction-based blue LED with a thin Ge-doped current spreading layer

In this work, we present a GaN-based blue LED construction utilizing bottom tunnel junction (TJ) grown by plasma-assisted molecular beam epitaxy. The setup allows for N-polar-like built-in field alignment while being grown on a Ga-polar substrate. In this study, we present an efficient bottom TJ LED in which the distance between the quantum well and device surface is only 25 nm. This is achieved by the utilization of an n-type current spreading layer consisting of 20 nm thick In0.02Ga0.98N with a Ge doping concentration of 7 × 1019 cm−3. Heavily Ge and Mg doped bottom TJs allowed achieving a low LED turn-on voltage of 2.75 V at 20 A/cm2 and a differential resistivity of 4.7 × 10−4 Ω cm2 at 1 kA/cm2. The device maintained high crystal quality and smooth morphology, which allows for its use as a light emitting platform for further integration. Although the p-up reference LED exhibits lower resistivity at high current, its luminous efficiency is lower than for bottom TJ LEDs.

Nitride light-emitting diodes for cryogenic temperatures.

A novel approach to fabricate efficient nitride light-emitting diodes (LEDs) grown on gallium polar surface operating at cryogenic temperatures is presented. We investigate and compare LEDs with standard construction with structures where p-n junction field is inverted through the use of bottom tunnel junction (BTJ). BTJ LEDs show improved turn on voltage, reduced parasitic recombination and increased quantum efficiency at cryogenic temperatures. This is achieved by moving to low resistivity n-type contacts and nitrogen polar-like built-in field with respect to current flow. It inhibits the electron overflow past quantum wells and improves hole injection even at T=12K. Therefore, as cryogenic light sources, BTJ LEDs offer significantly enhanced performance over standard LEDs.

Bottom tunnel junction blue light-emitting field-effect transistors

A recent thrust toward efficient modulated light emitters for use in Li-Fi communications has sparked renewed interest in visible III-N InGaN light-emitting diodes (LEDs). With their high external quantum efficiencies, blue InGaN LEDs are ideal components for such devices. We report a method for achieving voltage-controlled gate-modulated light emission using monolithic integration of fin- and nanowire-n–i–n vertical FETs with bottom-tunnel junction planar blue InGaN LEDs. This method takes advantage of the improved performance of bottom-tunnel junction LEDs over their top-tunnel junction counterparts, while allowing for strong gate control on a low-cross-sectional area fin or wire without sacrificing the LED active area as in lateral integration designs. Electrical modulation of five orders and an order of magnitude of optical modulation are achieved in the device.

Nitride LEDs and Lasers with Buried Tunnel Junctions

Traditionally, Nitride semiconductors have suffered from poor p-type conductivity, requiring Mg activation by removing hydrogen from grown layers either through thermal annealing or electron irradiation. This requirement restricts the growth of buried p-type layers. Here, we report structures obtained using a Hydrogen-free growth technique – plasma assisted molecular beam epitaxy. Using this method, top and bottom tunnel junctions are realized for top and bottom contacts to traditional Ga-polar devices. Advantages of using both constructions are discussed. The efficiency of the bottom-tunnel junction design is presented through realization of a stable laser diode operating at room temperature. Further work needed to improve tunnel junction performance as well as optical mode confinement to fully benefit from this design is outlined.

Polarization control in nitride quantum well light emitters enabled by bottom tunnel-junctions

The frozen internal polarization-induced electric fields due to broken inversion symmetry in all conventional blue and green nitride semiconductor light-emitting semiconductor quantum well heterostructures point in a direction opposite to what is desired for efficient flow of electrons and holes. This state of affairs has persisted because of the desire to have p-type hole injectors on top of the quantum well active region. Because of the internal polarization fields in nitride heterostructures, there exist four permutations of doping and polarization for the realization of such light emitters. Which permutation is the most desirable for efficient light emission? In this work, we answer this question by demonstrating a fundamentally new approach toward efficient light emission with “bottom-tunnel junctions.” The bottom-tunnel junction design aligns the polarization fields in the desired direction in the quantum well while simultaneously eliminating the need for p-type contacts and allowing efficient current spreading. By preventing electron overshoot past quantum wells, it disables carrier recombination in undesired regions of the quantized heterostructures and opens up the possibility for new geometries of integrating and stacking multiple light emitters.

Properties of stacked NbN tunnel junctions

Stacks of up to three tunnel junctions were fabricated using the NbN-MgO technique. Different preparation methods were tested, and two gave favorable results. In the first one, the whole stack is prepared in situ and structured afterwards by lift-off and reactive and argon ion etching. The authors investigated the resulting I-V (current-voltage) characteristics of stacks of 8.3 mV. Since it was not possible to establish electrical connections to the intermediate electrodes by this method, a second one was applied in which each NbN/MgO layer is prepared in a separate step and structured by lift-off. Here the I-V characteristics, the interaction between the tunnel junctions, and their RF properties were investigated. Shapiro steps and photon-assisted tunneling were observed in the I-V characteristics of a receiver junction, while the bottom tunnel junctions were used as microwave generators. >