(Invited) Phosphor-Free III-Nitride Nanowire White Light Emitting Diodes: Challenges and Prospects

Abstract

Group III-nitride nanowire heterostructures have been intensively studied as an emerging platform for future solid-state lighting and full-color display. Compared to the conventional GaN based planar light emitting diodes (LEDs), due to the effective lateral stress relaxation, III-nitride nanowires offer several distinct advantages including greatly reduced dislocation densities, polarization fields, and quantum-confined Stark effect [1-3], promising high efficiency full-color LEDs [4, 5]. However, several factors have been identified as the limiting process for further improving the nanowire LED quantum efficiency and light output power, which include the lack of carrier confinement in the active region, poor hole transport [6], electron overflow [7], and the nonradiative recombination along the wire lateral surfaces [8]. The quantum efficiency of nanowire LEDs generally exhibits a very slow rise with injection current and the peak quantum efficiency occurs at very high injection current (>100 A/cm2), which is much higher than the commonly reported values of conventional InGaN/GaN quantum well blue LEDs (in range of ~ 10-20 A/cm2) [4, 9]. Due to the highly effective lateral stress relaxation associated with the large surface areas, it is expected that both the polarization fields and dislocation densities can be significantly reduced in nanowire structures. However, the presence of large surface states and defects can contribute significantly to the carrier loss in nanowire LEDs. As a consequence, currently reported nanowire LEDs show relatively low output power, which is often in the range of μW, or lower [4]. Several approaches have been reported for the enhanced performance of III-nitride nanowire LED devices including usage of dots/disks in-a-wire to enhance carrier confinement. The poor hole transport and the electron overflow can be significantly improved by employing the p-type modulation doping technique and the usage of electron blocking layer, respectively [4, 7]. Such devices can exhibit relatively high internal quantum efficiency of ~57% and stable emission characteristics with increasing current. Most importantly, by reducing nonradiative surface recombination, the device performance including light output power and quantum efficiency can be significantly improved. Several surface passivation techniques have been employed including dielectric material and large bandgap energy semiconductors. However, passivating the InGaN/GaN nanowire structure with an in-situ grown AlGaN shell shows a record high output power which is more than 100 times stronger than that of nanowire white LEDs without using an AlGaN shell [10]. Moreover, a new self-organized InGaN/AlGaN dot-in-a-wire core-shell white LED heterostructure will also be presented. Multiple AlGaN shell layers are spontaneously formed during the growth of the InGaN/AlGaN quantum dot active region. Due to the drastically reduced nonradiative surface recombination, such core-shell nanowire structures exhibit significantly increased carrier lifetime (from ~ 0.3 ns to ~ 4.5 ns) and massively enhanced photoluminescence intensity, compared to that of nanowire LED without using core-shell structures. Strong white-light emission was recorded for those core-shell nanowire LEDs with an output power of >5 mW (at ~ 60 A/cm2), and a color rendering index of ~ 95 with stable white light emission for injection current from 50 mA to 500 mA, with the x and yvalues in the ranges of ~ 0.35 ‒ 0.36 and 0.37 ‒ 0.38, respectively. The significantly enhanced carrier lifetime is attributed to the greatly reduced nonradiative surface recombination and the effective lateral confinement offered by the large bandgap AlGaN shell. Such unique core-shell nanowire heterostructures, with controllable carrier dynamics, will significantly advance the achievement of a high power nanowire LEDs and also the development of broad range of nanowire photonic devices, including lasers, solar cells, and photodetectors. This presentation will cover current status of nanowire LED research, challenges and approaches to enhance nanowire LED performance. The progress being made to develop high efficiency phosphor-free nanowire LEDs for future solid-state lighting and full color display will also be presented.