Shaofei Zhang

p-Type modulation doped InGaN/GaN dot-in-a-wire white-light-emitting diodes monolithically grown on Si(111).

Full-color, catalyst-free InGaN/GaN dot-in-a-wire light-emitting diodes (LEDs) were monolithically grown on Si(111) by molecular beam epitaxy, with the emission characteristics controlled by the dot properties in a single epitaxial growth step. With the use of p-type modulation doping in the dot-in-a-wire heterostructures, we have demonstrated the most efficient phosphor-free white LEDs ever reported, which exhibit an internal quantum efficiency of ∼56.8%, nearly unaltered CIE chromaticity coordinates with increasing injection current, and virtually zero efficiency droop at current densities up to ∼640 A/cm(2). The remarkable performance is attributed to the superior three-dimensional carrier confinement provided by the electronically coupled dot-in-a-wire heterostructures, the nearly defect- and strain-free GaN nanowires, and the significantly enhanced hole transport due to the p-type modulation doping.

Controlling Electron Overflow in Phosphor-Free InGaN/GaN Nanowire White Light-Emitting Diodes

We have investigated for the first time the impact of electron overflow on the performance of nanowire light-emitting diodes (LEDs) operating in the entire visible spectral range, wherein intrinsic white light emission is achieved from self-organized InGaN quantum dots embedded in defect-free GaN nanowires on a single chip. Through detailed temperature-dependent electroluminescence and simulation studies, it is revealed that electron leakage out of the device active region is primarily responsible for efficiency degradation in such nanowire devices, which in conjunction with the presence of nonradiative surface recombination largely determines the unique emission characteristics of nanowire light-emitting diodes. We have further demonstrated that electron overflow in nanowire LEDs can be effectively prevented with the incorporation of a p-doped AlGaN electron blocking layer, leading to the achievement of phosphor-free white light-emitting diodes that can exhibit for the first time virtually zero efficiency droop for injection currents up to ~2200 A/cm(2). This study also provides unambiguous evidence that Auger recombination is not the primary mechanism responsible for efficiency droop in GaN-based nanowire light-emitting diodes.

Breaking the Carrier Injection Bottleneck of Phosphor-Free Nanowire White Light-Emitting Diodes

We have examined the carrier injection process of axial nanowire light-emitting diode (LED) structures and identified that poor carrier injection efficiency, due to the large surface recombination, is the primary cause for the extremely low output power of phosphor-free nanowire white LEDs. We have further developed InGaN/GaN/AlGaN dot-in-a-wire core-shell white LEDs on Si substrate, which can break the carrier injection efficiency bottleneck, leading to a massive enhancement in the output power. At room temperature, the devices can exhibit an output power of ~1.5 mW, which is more than 2 orders of magnitude stronger than nanowire LEDs without shell coverage. Additionally, such phosphor-free nanowire white LEDs can deliver an unprecedentedly high color rendering index of ~92-98 in both the warm and cool white regions, with the color rendering capability approaching that of an ideal light source, i.e. a blackbody.

Full-color InGaN/GaN dot-in-a-wire light emitting diodes on silicon

We report on the achievement of a new class of nanowire light emitting diodes (LEDs), incorporating InGaN/GaN dot-in-a-wire nanoscale heterostructures grown directly on Si(111) substrates. Strong emission across nearly the entire visible wavelength range can be realized by varying the dot composition. Moreover, we have demonstrated phosphor-free white LEDs by controlling the indium content in the dots in a single epitaxial growth step. Such devices can exhibit relatively high internal quantum efficiency (>20%) and no apparent efficiency droop for current densities up to ~ 200 A cm(-2).

On the Carrier Injection Efficiency and Thermal Property of InGaN/GaN Axial Nanowire Light Emitting Diodes

We have investigated the impact of surface recombination on the effective carrier injection efficiency and the Joule heating of axial InGaN/GaN nanowire light-emitting diodes (LEDs). The results reveal that the carrier injection efficiency of such devices is extremely low (<;10%), due to the severe carrier loss through nonradiative surface recombination. It is further observed that the thermal resistance of typical nanowire LEDs is comparable with, or lower than that of their planar counterparts, in spite of the reduced thermal conductivity of nanowires. The poor carrier injection efficiency, however, leads to significantly elevated junction temperatures for nanowire LEDs. We have further demonstrated, both theoretically and experimentally, that the carrier injection efficiency can be significantly improved in p-doped nanowires, due to the downward surface band bending, and in InGaN/GaN/AlGaN dot-in-a-wire core-shell nanoscale heterostructures, due to the superior carrier confinement offered by the large bandgap AlGaN shell. This paper offers important insight for the design and epitaxial growth of high-performance nanowire LEDs.

High-Efficiency InGaN/GaN Dot-in-a-Wire Red Light-Emitting Diodes

We report on the achievement of high-performance InGaN/GaN dot-in-a-wire red light-emitting diodes on Si(111) substrates. Owing to the superior 3-D carrier confinement offered by the self-organized dot-in-a-wire heterostructures, the devices exhibit relatively high (~18%-32%) internal quantum efficiency at room temperature. Moreover, no efficiency droop was observed for injection current up to ~480A/cm2 under pulsed biasing conditions. We have also demonstrated that, by controlling the inhomogeneous broadening of the dot-in-a-wire heterostructures, the devices can exhibit relatively stable emission characteristics with increasing current.

On the efficiency droop of top-down etched InGaN/GaN nanorod light emitting diodes under optical pumping

The optical performance of top-down etched InGaN/GaN nanorod light emitting diodes (LEDs) was studied using temperature variable photoluminescence spectroscopy with a 405 nm pump laser. Efficiency droop is measured from such nanorod structures, which is further enhanced with decreasing temperature. Through detailed rate equation analysis of the temperature-dependent carrier distribution and modeling of the quantum efficiency, this unique phenomenon can be largely explained by the interplay and dynamics between carrier radiative recombination in localized states and nonradiative recombination via surface states/defects.

Tunnel injection InGaN/GaN dot-in-a-wire white-light-emitting diodes

We report on the demonstration of InGaN/GaN dot-in-a-wire tunnel injection white-light-emitting diodes on Si, wherein electrons and holes are injected into the quantum dot active region through two separate InGaN injector wells. Significantly reduced electron overflow is realized without the use of any Al-containing electron blocking layer. Moreover, the InGaN hole injector well, which is positioned between the quantum dot active region and p-GaN, can harvest electrons that have escaped through the near-surface region of nanowires.

Impact of Surface Recombination on the Performance of Phosphor-Free InGaN/GaN Nanowire White Light Emitting Diodes

We show that the performance of InGaN/GaN axial nanowire LEDs is largely limited by the poor carrier injection efficiency. We have further demonstrated high performance phosphor-free white LEDs using InGaN/GaN/AlGaN dot-in-a-wire core-shell heterostructures.

Molecular beam epitaxial growth and characterization of InGaN/GaN dot-in-a-wire nanoscale heterostructures: toward ultrahigh efficiency phosphor-free white light emitting diodes

One of the grand challenges for future solid state lighting is the development of high efficiency, phosphor-free white light emitting diodes (LEDs). In this context, we have investigated the molecular beam epitaxial growth and characterization of nanowire LEDs on Si, wherein intrinsic white-light emission is achieved by incorporating selforganized InGaN quantum dots in defect-free GaN nanowires on a single chip. We have further demonstrated that, with the incorporation of p-type modulation doping and AlGaN electron blocking layer, InGaN/GaN dot-in-a-wire white LEDs can exhibit nearly zero efficiency droop and significantly enhanced internal quantum efficiency (up to ~57%) at room-temperature.