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.
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Single-electron transistors featuring silicon nitride tunnel barriers prepared by atomic layer deposition
Single electron transistors (SET) featuring metal (Ni) electrodes and silicon nitride dielectric barriers prepared by atomic layer deposition are fabricated and tested. Electrical characterization of the devices reveals electrostatic energy parameters consistent with the parameters of the designed tunnel junctions. In addition, an analysis of temperature dependence of conductance confirms the formation of metal-insulator-metal (MIM) junctions with negligible in-series contribution of any surface native metal oxide. However, the fabricated devices exhibit a very high level of electrical noise, far exceeding the commonly observed shot noise. Experimental investigation reveals the random telegraph signal (RTS) nature of the observed excess noise. The RTS noise in electronic devices is commonly associated with charging of external traps that are electrostatically coupled to the SET island. In the devices under study, however, the defects that result in the observed RTS noise are demonstrated to reside within the tunnel junctions. Our results also indicate the critical importance of interface states and surface preparation for achieving good performance of the SETs fabricated using ALD to form the tunnel barrier.
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- Award ID(s):
- 1207394
- NSF-PAR ID:
- 10018261
- Date Published:
- Journal Name:
- Single-electron transistors featuring silicon nitride tunnel barriers prepared by atomic layer deposition
- Issue:
- 2016 Joint International EUROSOI Workshop and International Conference on Ultima
- Page Range / eLocation ID:
- 32 to 35
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Conference Paper:
- https://doi.org/10.1109/ULIS.2016.7440045
