Design and demonstration of ultra-wide bandgap AlGaN tunnel junctions

Abstract

Ultra violet light emitting diodes (UV LEDs) face critical limitations in both the injection efficiency and light extraction efficiency due to the resistive and absorbing p-type contact layers. In this work, we investigate the design and application of polarization engineered tunnel junctions for ultra-wide bandgap AlGaN (Al mole fraction > 50%) materials towards highly efficient UV LEDs. We demonstrate that polarization-induced 3D charge is beneficial in reducing tunneling barriers especially for high composition AlGaN tunnel junctions. The design of graded tunnel junction structures could lead to low tunneling resistance below 10 -3 Ω cm 2 and low voltage consumption below 1 V (at 1 kA/cm 2 ) for high composition AlGaN tunnel junctions. Experimental demonstration of 292 nm emission was achieved through non-equilibrium hole injection into wide bandgap materials with bandgap energy larger than 4.7 eV, and detailed modeling of tunnel junctions shows that they can be engineered to have low resistance, and can enable efficient emitters in the UV-C wavelength range.a) Authors to whom correspondence should be addressed. Electronic mail: zhang.3789@osu.edu, rajan@ece.osu.edu 2The large range of direct bandgaps in the III-Nitride system makes these semiconductors uniquely suitable for achieving ultra violet emission over a wide wavelength range down to 210 nm. III-Nitride ultra-violet light emitting diodes (UV LEDs) have a large variety of applications including water purification and air disinfection, and they provide distinct advantages over traditional gas-based lamps. We propose an approach to realize highly efficient hole injection based on interband tunneling. In this approach, a transparent n-type AlGaN layer is connected to the p-AlGaN layer using an interband tunnel junction ( Fig. 1(b)). Non-equilibrium tunneling injection of holes overcomes limits imposed by thermal ionization of deep acceptors. 9,10 At the same time, it leads to efficient light extraction since it eliminates absorbing p-type top contact layers. 11Making efficient interband p-n tunnel junctions is challenging for ultra-wide bandgap AlGaN materials due to the large band gap, wide depletion barrier, and doping limitations. In this work, we show that nanoscale heterostructure and polarization engineering of tunnel junctions can enable highly efficient tunneling even for ultra-wide bandgap materials where ordinary dopant-based homojunctions would have very low tunneling current density. In this letter, we discuss the design of AlGaN tunnel junctions over the entire Al composition range. We then experimentally demonstrate highly efficient tunnel junctions for AlGaN with Al composition greater than 50%. was used for tunneling probability calculation, and tunneling current was calculated by considering all possible contributions by carriers with different energies. Bandtail states and bandgap narrowing effect due to heavy impurity doping are not included in the simulations, though they could lead to increase in the tunneling probability. ...

Numerical Investigations on the n+‐GaN/AlGaN/p+‐GaN Tunnel Junction for III‐Nitride UV Light‐Emitting Diodes

“…Here, it is worth defining the polarization tunnel junction in which the polarization induced electric field in the polarization tunnel junction is along the same direction as the built‐in electric field, and thus the coupled electric field possesses the enhanced intensity. Zhang et al also demonstrate the polarization tunnel junction by studying the [0001] oriented n‐AlGaN/InGaN/p‐AlGaN . Recently, we have proposed and reported a n + ‐GaN/AlGaN/p + ‐GaN based tunnel junction for ultraviolet (UV) LEDs for which the n + ‐GaN/AlGaN/p + ‐GaN tunnel junction comprises a thin AlGaN layer to avoid absorbing the 380 nm UV photons .…”

Tunnel-injected sub-260 nm ultraviolet light emitting diodes

“…10,11,13 Simulations further showed that efficient interband tunneling could be achieved for high Al content AlGaN by using a compositionally graded tunnel junction structure. 12 It demonstrated the feasibility of achieving tunneling hole injection into deep UV LEDs.…”

“…The structure was grown by N 2 plasma assisted molecular beam epitaxy (MBE) on Si-doped metal-polar Al 0.72 Ga 0.28 N template with a threading dislocation density of 3×10 9 cm -2 . [10][11][12][13] The template was grown on sapphire substrate using metal-organic chemical vapor deposition (MOCVD). The MBE growth was layer.…”

Tunnel-injected sub 290 nm ultra-violet light emitting diodes with 2.8% external quantum efficiency

“…Another benefit of the tunnel-injected UV LED structure is that holes are injected through interband tunneling, which resists the influence of the extremely low thermal activation rate of Mg acceptors in AlGaN. 17,18 This is critical for deep UV LEDs, as the thermally activated hole concentration decreases significantly with the increasing Al content in the p-AlGaN layer. While conventional UV LEDs have encountered a substantial efficiency reduction when lowering the emission wavelength in the deep UV wavelength range, the tunnel-injected UV LED structure could potently solve the problem and lead to efficient deep UV emitters.…”