N-polar InGaN/GaN nanowires: overcoming the efficiency cliff of red-emitting micro-LEDs

A high efficiency, high brightness, and robust micro or sub-microscale red light emitting diode(LED) is an essential, yet missing, component of the emerging virtual reality and future ultrahigh resolution mobile displays.We report, for the first time, to our knowledge, the demonstration of an N-polar In GaN/GaN nanowire submicroscale LED emitting in the red spectrum that can overcome the efficiency cliff of conventional red-emitting micro-LEDs. We show that the emission wavelengths of N-polar In GaN/GaN nanowires can be progressively shifted from yellow to orange and red, which is difficult to achieve for conventional In GaN quantum wells or Ga-polar nanowires. Significantly, the optical emission intensity can be enhanced by more than one order of magnitude by employing an in situ annealing process of the In GaN active region, suggesting significantly reduced defect formation. LEDs with lateral dimensions as small as ~0.75 μm, consisting of approximately five nanowires, were fabricated and characterized, which are the smallest red-emitting LEDs ever reported, to our knowledge. A maximum external quantum efficiency ~1.2% was measured, which is comparable to previously reported conventional quantum well micro-LEDs operating in this wavelength range, while our device sizes are nearly three to five orders of magnitude smaller in surface area.

High-efficiency AlGaN/GaN/AlGaN tunnel junction ultraviolet light-emitting diodes

AlGaN is the material of choice for high-efficiency deep UV light sources,which is the only alternative technology to replace mercury lamps for water purification and disinfection.At present,however,Al Ga N-based mid-and deep UV LEDs exhibit very low efficiency.Here,we report a detailed investigation of the epitaxy and characterization of LEDs utilizing an Al Ga N/Ga N/Al Ga N tunnel junction structure,operating at^265 nm,which have the potential to break the efficiency bottleneck of deep UV photonics.A thin Ga N layer was incorporated between p^+and n^+-Al Ga N to reduce the tunneling barrier.By optimizing the thickness of the Ga N layer and thickness of the top n-Al Ga N contact layer,we demonstrate Al Ga N deep UV LEDs with a maximum external quantum efficiency of 11%and wall-plug efficiency of 7.6%for direct on-wafer measurement.It is also observed that the devices exhibit severe efficiency droop under low current densities,which is explained by the low hole mobility,due to the hole hopping conduction in the Mg impurity band and the resulting electron overflow.

Strain-engineered N-polar InGaN nanowires:towards high-efficiency red LEDs on the micrometer scale

The absence of efficient red-emitting micrometer-scale light emitting diodes(LEDs),i.e.,LEDs with lateral dimensions of 1μm or less is a major barrier to the adoption of microLEDs in virtual/augmented reality.The underlying challenges include the presence of extensive defects and dislocations for indium-rich InGaN quantum wells,strain-induced quantum-confined Stark effect,and etch-induced surface damage during the fabrication of quantum well microLEDs.Here,we demonstrate a new approach to achieve strong red emission(>620 nm)from dislocation-free N-polar InGaN/GaN nanowires that included an InGaN/GaN short-period superlattice underneath the active region to relax strain and incorporate more indium within the InGaN dot active region.The resulting submicrometer-scale devices show red electroluminescence dominantly from an InGaN dot active region at low-to-moderate injection currents.A peak external quantum efficiency and a wall-plug efficiency of 2.2%and1.7%were measured,respectively,which,to the best of our knowledge,are the highest values reported for a submicrometer-scale red LED.This study offers a new path to overcome the efficiency bottleneck of red-emitting microLEDs for a broad range of applications including mobile displays,wearable electronics,biomedical sensing,ultrahigh speed optical interconnect,and virtual/augmented reality.

Impedance Studies of La_2Mo_(2-x)Sn_xO(9-δ) Oxide Ion Conductors

A series of compounds La2Mo2-xSnxO9-6 （x=0-0.3） have been synthesized by solid-state reaction technique. Materials have been characterized by XRD, SEM, DSC and impedance study. In the temperature regime 520℃-590 ℃, the specimens with x〈0.05 have the conductivity higher than La2Mo2O9. Conductivity of Sn-doped compound decreases consistently with increasing Sn-doping, compared to the undoped compound both below and above phase transition, barring the specimens with x〈 0.05, where conductivity values remains almost same as that of undoped specimen in high temperature region. In the intermediate temperature regime （520℃-590℃）, the conductivity of doped compounds increases for x〈0.05 as compared to parent compound. Also, there is no indication of phase stabilization with Sn-doping in this compound even with the highest doping level, x=0.3. Electric modulus analysis suggests that thermally activated oxygen ion hopping mechanism is responsible for the conduction in Sn-doped compound.

Revisiting band structures in^(118)Xe nucleus via in-beamγ-ray spectroscopy

The neutron deficient Xe nuclei with A≈120 are predicted to have strong octupole correlation at low spins.In the present study,an attempt was made to improve upon the level scheme and also to examine the signatures of octupole correlation in^(118)xe via high spin γ-ray spectroscopy.High spin states in^(118)Xe have been populated via the^(93)Nb(^(28)Si,p2n)^(118)Xe fusion-evaporation reaction at a beam energy of 115 MeV provided by the 15 UD pelletron accelerator facility at IUAC,New Delhi.In the experiment,seven newγ-transitions have been found and placed appropriately in the level scheme.A theoretical study using the triaxial projected shell model(TPSM)approach suggests that the first bandcrossing is due to the alignment of two neutrons,and a parallel band tracking the yrast configuration is the γ-band built on the two-quasiparticle state.Enhanced E1 transition rates measured between opposite parity bands involving vh_(11/2)and vd_(5/2)orbitals having△j=△l=3 indicate the presence of octupole correlation in this nucleus.