In this work, we demonstrate two-junction UV LEDs enabled by transparent tunnel junctions. Low voltage-drop tunnel junctions were realized in Al0.3Ga0.7N layers through a combination of high doping and compositional grading. Capacitance and current–voltage measurements confirmed the operation of two junctions in series. The voltage drop of the two-junction LED was 2.1 times that of an equivalent single-junction LED, and the two-junction LED had higher external quantum efficiency (147%) than the single junction.more » « less
- Award ID(s):
- NSF-PAR ID:
- Publisher / Repository:
- DOI PREFIX: 10.35848
- Date Published:
- Journal Name:
- Applied Physics Express
- Medium: X Size: Article No. 082001
- ["Article No. 082001"]
- Sponsoring Org:
- National Science Foundation
More Like this
Ultra-violet light emitting diodes (UV-LEDs) and lasers based on the III-Nitride material system are very promising since they enable compact, safe, and efficient solid-state sources of UV light for a range of applications. The primary challenges for UV LEDs are related to the poor conductivity of p-AlGaN layers and the low light extraction efficiency of LED structures. Tunnel junction-based UV LEDs provide a distinct and unique pathway to eliminate several challenges associated with UV LEDs1-4. In this work, we present for the first time, a reversed-polarization (p-down) AlGaN based UV-LED utilizing bottom tunnel junction (BTJ) design. We show that compositional grading enables us to achieve the lowest reported voltage drop of 1.1 V at 20 A/cm2 among transparent AlGaN based tunnel junctions at this Al-composition. Compared to conventional LED design, a p-down structure offers lower voltage drop because the depletion barrier for both holes and electrons is lower due to polarization fields aligning with the depletion field. Furthermore, the bottom tunnel junction also allows us to use polarization grading to realize better p- and n-type doping to improve tunneling transport. The epitaxial structure of the UV-LED was grown by plasma-assisted molecular beam epitaxy (PAMBE) on metal-organic chemical vapor deposition (MOCVD)-grown n-type Al0.3Ga0.7N templates. The transparent TJ was grown using graded n++-Al0.3Ga0.7N→ n++-Al0.4Ga0.6N (Si=3×1020 cm-3) and graded p++-Al0.4Ga0.6N →p++-Al0.3Ga0.7N (Mg=1×1020 cm-3) to take advantage of induced 3D polarization charges. The high number of charges at the tunnel junction region leads to lower depletion width and efficient hole injection to the p-type layer. The UV LED active region consists of three 2.5 nm Al0.2Ga0.8N quantum wells and 7 nm Al0.3Ga0.6N quantum barriers followed by 12 nm of p- Al0.46Ga0.64N electron blocking layer (EBL). The active region was grown on top of the tunnel junction. A similar LED with p-up configuration was also grown to compare the electrical performance. The surface morphology examined by atomic force microscopy (AFM) shows smooth growth features with a surface roughness of 1.9 nm. The dendritic features on the surface are characteristic of high Si doping on the surface. The composition of each layer was extracted from the scan by high resolution x-ray diffraction (HR-XRD). The electrical characteristics of a device show a voltage drop of 4.9 V at 20 A/cm2, which corresponds to a tunnel junction voltage drop of ~ 1.1 V. This is the best lowest voltage for transparent 30% AlGaN tunnel junctions to-date and is comparable with the lowest voltage drop reported previously on non-transparent (InGaN-based) tunnel junctions at similar Al mole fraction AlGaN. On-wafer electroluminescence measurements on patterned light-emitting diodes showed single peak emission wavelength of 325 nm at 100 A/cm2 which corresponds to Al0.2Ga0.8N, confirming that efficient hole injection was achieved within the structure. The device exhibits a wavelength shift from 330 nm to 325 nm with increasing current densities from 10A/cm2 to 100A/cm2. In summary, we have demonstrated a fully transparent bottom AlGaN homojunction tunnel junction that enables p-down reversed polarization ultraviolet light emitting diodes, and has very low voltage drop at the tunnel junction. This work could enable new flexibility in the design of future III-Nitride ultraviolet LEDs and lasers.more » « less
Ultra-violet (UV) light emitting diodes operating at 339 nm using transparent interband tunnel junctions are reported. Tunneling-based ultraviolet light emitting diodes were grown by plasma-assisted molecular beam epitaxy on 30% Al-content AlGaN layers. A low tunnel junction voltage drop is obtained through the use of compositionally graded n and p-type layers in the tunnel junction, which enhance hole density and tunneling rates. The transparent tunnel junction-based UV LED reported here show a low voltage drop of 5.55 V at 20 A/cm2 and an on-wafer external quantum efficiency of 1.02% at 80 A/cm2.more » « less
The single-molecule magnet (SMM) is demonstrated here to transform conventional magnetic tunnel junctions (MTJ), a memory device used in present-day computers, into solar cells. For the first time, we demonstrated an electronic spin-dependent solar cell effect on an SMM-transformed MTJ under illumination from unpolarized white light. We patterned cross-junction-shaped devices forming a CoFeB/MgO/CoFeB-based MTJ. The MgO barrier thickness at the intersection between the two exposed junction edges was less than the SMM extent, which enabled the SMM molecules to serve as channels to conduct spin-dependent transport. The SMM channels yielded a region of long-range magnetic ordering around these engineered molecular junctions. Our SMM possessed a hexanuclear [Mn6(μ3-O)2(H2N-sao)6(6-atha)2(EtOH)6] [H2N-saoH = salicylamidoxime, 6-atha = 6-acetylthiohexanoate] complex and thiols end groups to form bonds with metal films. SMM-doped MTJs were shown to exhibit a solar cell effect and yielded ≈ 80 mV open-circuit voltage and ≈ 10 mA/cm2 saturation current density under illumination from one sun equivalent radiation dose. A room temperature Kelvin Probe AFM (KPAFM) study provided direct evidence that the SMM transformed the electronic properties of the MTJ's electrodes over a lateral area in excess of several thousand times larger in extent than the area spanned by the molecular junctions themselves. The decisive factor in observing this spin photovoltaic effect is the formation of SMM spin channels between the two different ferromagnetic electrodes, which in turn is able to catalyze the long-range transformation in each electrode around the junction area.more » « less
Metal-molecule-metal junctions based on alkane thiol (C n T) and oligophenylene thiol (OPT n ) self-assembled monolayers (SAMs) and Au electrodes are expected to exhibit similar electrical asymmetry, as both junctions have one chemisorbed Au–S contact and one physisorbed, van der Waals contact. Asymmetry is quantified by the current rectification ratio RR apparent in the current–voltage ( I – V ) characteristics. Here we show that RR < 1 for C n T and RR > 1 for OPT n junctions, in contrast to expectation, and further, that RR behaves very differently for C n T and OPT n junctions under mechanical extension using the conducting probe atomic force microscopy (CP-AFM) testbed. The analysis presented in this paper, which leverages results from the previously validated single level model and ab initio quantum chemical calculations, allows us to explain the puzzling experimental findings for C n T and OPT n in terms of different current rectification mechanisms. Specifically, in C n T-based junctions the Stark effect creates the HOMO level shifting necessary for rectification, while for OPT n junctions the level shift arises from position-dependent coupling of the HOMO wavefunction with the junction electrostatic potential profile. On the basis of these mechanisms, our quantum chemical calculations allow quantitative description of the impact of mechanical deformation on the measured current rectification. Additionally, our analysis, matched to experiment, facilitates direct estimation of the impact of intramolecular electrostatic screening on the junction potential profile. Overall, our examination of current rectification in benchmark molecular tunnel junctions illuminates key physical mechanisms at play in single step tunneling through molecules, and demonstrates the quantitative agreement that can be obtained between experiment and theory in these systems.more » « less
Gap junctions formed by different connexins are expressed throughout the body and harbour unique channel properties that have not been fully defined mechanistically.
Recent structural studies by cryo‐electron microscopy have produced high‐resolution models of the related but functionally distinct lens connexins (Cx50 and Cx46) captured in a stable open state, opening the door for structure–function comparison.
Here, we conducted comparative molecular dynamics simulation and electrophysiology studies to dissect the isoform‐specific differences in Cx46 and Cx50 intercellular channel function.
We show that key determinants Cx46 and Cx50 gap junction channel open stability and unitary conductance are shaped by structural and dynamic features of their N‐terminal domains, in particular the residue at the 9th position and differences in hydrophobic anchoring sites.
The results of this study establish the open state Cx46/50 structural models as archetypes for structure–function studies targeted at elucidating the mechanism of gap junction channels and the molecular basis of disease‐causing variants.
Connexins form intercellular communication channels, known as gap junctions (GJs), that facilitate diverse physiological roles, from long‐range electrical and chemical coupling to coordinating development and nutrient exchange. GJs formed by different connexin isoforms harbour unique channel properties that have not been fully defined mechanistically. Recent structural studies on Cx46 and Cx50 defined a novel and stable open state and implicated the amino‐terminal (NT) domain as a major contributor for isoform‐specific functional differences between these closely related lens connexins. To better understand these differences, we constructed models corresponding to wildtype Cx50 and Cx46 GJs, NT domain swapped chimeras, and point variants at the 9th residue for comparative molecular dynamics (MD) simulation and electrophysiology studies. All constructs formed functional GJ channels, except the chimeric Cx46‐50NT variant, which correlated with an introduced steric clash and increased dynamical behaviour (instability) of the NT domain observed by MD simulation. Single channel conductance correlated well with free‐energy landscapes predicted by MD, but resulted in a surprisingly greater degree of effect. Additionally, we observed significant effects on transjunctional voltage‐dependent gating (
Vjgating) and/or open state dwell times induced by the designed NT domain variants. Together, these studies indicate intra‐ and inter‐subunit interactions involving both hydrophobic and charged residues within the NT domains of Cx46 and Cx50 play important roles in defining GJ open state stability and single channel conductance, and establish the open state Cx46/50 structural models as archetypes for structure–function studies targeted at elucidating GJ channel mechanisms and the molecular basis of cataract‐linked connexin variants.