Device Engineering for Internal Quantum Efficiency Enhancement and Efficiency Droop Issue in III-Nitride Light-Emitting Diodes

Abstract

Over the past few decades, III-Nitride semiconductors have found the tremendous impacts in solid state lighting, power electronics, photovoltaics and thermoelectrics. In particular, III-nitride based light-emitting diodes (LEDs) with long lifetime and eco-friendliness are fundamentally redefining the concepts of light generation due to the superior material properties of direct bandgap, efficient light emission and robustness. The industry of LED based solid state lighting is fulfilling the potential of reducing the 20% of the total US energy consumed by lighting to half of this usage. However, several major obstacles are still hindering the further development of LEDs for general illuminations. They include efficiency droop phenomenon at high operating current, low efficiency in green spectrum, and low extraction efficiency due to the large difference in refractive index. The report will present both experimental and theoretical works on III-nitride semiconductor materials and devices for solid state lighting, including 1) novel barrier design for efficiency-droop suppression, 2) novel active region design for radiative efficiency enhancement, and 3) fabrication of ultrahigh density and highly uniform III-nitride based quantum dots (QDs) for high efficiency optoelectronics and photovoltaic cells. In addition to the three main topics, a new topic on the p-type III-nitrides doping sensitivity will be investigated in the latter part of this report.Firstly, the use of large bandgap thin barrier layers surrounding the InGaN QWs in LEDs will be proposed for efficiency droop suppression. The efficiency of LED devices suffers from reduction at high current injection, which is referred as efficiency droop phenomenon. Although the origin is still inconclusive up till now, the carrier leakage issue is widely considered as one of the major reasons. The increased effective barrier heights from the use of a thin (d \u3c 2 nm) lattice-matched AlGaInN barriers are shown to improve current injection efficiency and internal quantum efficiency. The optimization of epitaxial conditions of lattice-matched AlInN material has been carried out by metal-organic chemical vapor deposition (MOCVD) for the fabrication of InGaN QW LEDs with the insertion of AlInN thin barrier. The device characterizations of cathodoluminescence and electroluminescence show the great potential of the InGaN-AlInN design in addressing the efficiency droop issue at high current density. Secondly, the staggered InGaN QW and InGaN-delta-InN QW are investigated for the high efficiency LEDs emitting at green or longer emission spectrum region to provide solutions for greengap challenge. The introduction of energy local minima in QW region by the novel structures of staggered InGaN QWs enables the spatial shift of electron and hole wavefunction towards the center of active region. Therefore, the approach leads to the enhancement of electron-hole wavefunction overlap and thus the radiative recombination rate and optical gain. The analysis of InGaN-delta-InN QW LED with the potential of effectively extending the emission wavelength without sacrificing the radiative recombination rates will also be presented. Thirdly, the sensitivity study of the doping levels of p-type layers in InGaN/GaN MQW LEDs will be discussed for industrial application. Due to the difficulty in activating the acceptor magnesium in III-nitrides, thermal annealing process is employed to increase the hole concentration in p-type semiconductors. The uniform temperature distributions in the annealing chambers will lead to non-uniformity in p-type doping levels. The effect of doping levels on LED device performance will be examined, and the doping sensitivity of light output power and internal quantum efficiency will be investigated in this report. The results will provide guidance for the parameter optimization of the fabrication process for commercial product line to increase the yield.Fourthly, the growths of ultra-high density and highly uniform InGaN QDs on GaN/ sapphire template as an important alternative active region for high-efficiency optoelectronic devices will be discussed. The growths of ultra-high density and highly uniform InGaN QDs by employing selective area epitaxy were realized on nanopatterned GaN template fabricated by diblock copolymer lithography. It results in well-defined QD density in the range of 8x1010 cm-2, which represents the highest QD density reported for nitride-based QDs. In comparison, the InGaN QD density by the prevailing Stranski-Krastanow (S-K) growth mode is around mid 109 cm-2 with non-uniformity in dot sizes and distributions. The availability of highly-uniform and ultra-high density InGaN QDs formed by this approach has significant and transformational impacts on developing high-efficiency light-emitting diodes for solid state lighting, ultra-low threshold current density visible diode lasers, and intermediate-band nitride-based solar cells