Experimentalists have observed the predicted half-integer quantum Hall effect using the topological insulator BiSbTeSe2, which exhibits topological surface states at room temperature, with each surface contributing a half quantum of Hall conductance.

/pdf/observation-of-topological-surface-state-quantum-hall-effect-2w1c0tnh9i.pdf

Observation of topological surface state quantum Hall effect in an intrinsic three-dimensional topological insulator

Gallium nitride (GaN) light-emitting-diode (LED) technology has been the revolution in modern lighting. In the last decade, a huge global market of efficient, long-lasting, and ubiquitous white light sources has developed around the inception of the Nobel-prize-winning blue GaN LEDs. Today, GaN optoelectronics is developing beyond solid-state lighting, leading to new and innovative devices, e.g., for microdisplays, being the core technology for future augmented reality and visualization, as well as point light sources for optical excitation in communications, imaging, and sensing. This explosion of applications is driven by two main directions: the ability to produce very small GaN LEDs (micro-LEDs and nano-LEDs) with high efficiency and across large areas, in combination with the possibility to merge optoelectronic-grade GaN micro-LEDs with silicon microelectronics in a hybrid approach. GaN LED technology is now even spreading into the realm of display technology, which has been occupied by organic LEDs and liquid crystal displays for decades. In this review, the technological transition toward GaN micro- and nanodevices beyond lighting is discussed including an up-to-date overview on the state of the art.

/pdf/beyond-solid-state-lighting-miniaturization-hybrid-8yfeg7uoig.pdf

Beyond solid-state lighting: Miniaturization, hybrid integration, and applications of GaN nano- and micro-LEDs

Three-dimensional (3D) topological Dirac semimetals (TDSs) are a recently proposed state of quantum matter that have attracted increasing attention in physics and materials science. A 3D TDS is not only a bulk analogue of graphene; it also exhibits non-trivial topology in its electronic structure that shares similarities with topological insulators. Moreover, a TDS can potentially be driven into other exotic phases (such as Weyl semimetals, axion insulators and topological superconductors), making it a unique parent compound for the study of these states and the phase transitions between them. Here, by performing angle-resolved photoemission spectroscopy, we directly observe a pair of 3D Dirac fermions in Cd3As2, proving that it is a model 3D TDS. Compared with other 3D TDSs, for example, β-cristobalite BiO2 (ref. 3) and Na3Bi (refs 4, 5), Cd3As2 is stable and has much higher Fermi velocities. Furthermore, by in situ doping we have been able to tune its Fermi energy, making it a flexible platform for exploring exotic physical phenomena.

https://absimage.aps.org/image/MAR15/MWS_MAR15-2014-003558.pdf

ARPES study on a stable three-dimensional topological Dirac semimetal Cd3As2

This perspective provides an overview of early developments, current status, and remaining challenges of microLED (μLED) technology, which was first reported in Applied Physics Letters in 2000 [S. X. Jin, J. Li, J. Z. Li, J. Y. Lin and H. X. Jiang, "GaN Microdisk Light Emitting Diodes," Appl. Phys. Lett. 76, 631 (2000)]. Today, microLED is recognized as the ultimate display technology and is one of the fastest-growing technologies in the world as technology giants utilize it on a wide range of products from large flat panel displays and televisions, wearable displays, and virtual reality displays to light sources for the neural interface and optogenetics. It is anticipated that the collective R&D efforts worldwide will bring microLED products not only to the mass consumer electronic markets but also to serve the society on the broadest scale by encompassing sectors in medical/health, energy, transportation, communications, and entertainment.

Development of microLED

Nonpolar and semipolar III-nitride-based blue and green light-emitting diodes (LEDs) have been extensively investigated as potential replacements for current polar c-plane LEDs. High-power and low-efficiency-droop blue LEDs have been demonstrated on nonpolar and semipolar planes III-nitride due to the advantages of eliminated or reduced polarization-related electric field and homoepitaxial growth. Semipolar (202¯1) and (202¯1¯) LEDs have contributed to bridging “green gap” (low efficiency in green spectral region) by incorporating high indium compositions, reducing polarization effects, and suppressing defects. Other properties, such as low thermal droop, narrow spectral linewidth, small wavelength shift, and polarized emission, have also been reported for nonpolar and semipolar LEDs. In this paper we review the theoretical background, device performance, material properties, and physical mechanisms for nonpolar and semipolar III-nitride semiconductors and associated blue and green LEDs. The latest progress on topics including efficiency droop, thermal droop, green-gap, and three-dimensional nanostructures is detailed. Future challenges, potential solutions, and applications will also be covered.

Toward ultimate efficiency: progress and prospects on planar and 3D nanostructured nonpolar and semipolar InGaN light-emitting diodes

A Decade of Nonpolar and Semipolar III-Nitrides: A Review of Successes and Challenges

We demonstrate a high-speed nonpolar ${m}$ -plane InGaN/GaN micro-scale light-emitting diode (LED) with a record electrical −3 dB modulation bandwidth of 1.5 GHz at a current density of 1 kA/cm2. A differential carrier lifetime (DLT) of 200 ps at 1 kA/cm2 was extracted using a rate-equation model. The short DLT is attributed to the high electron–hole wave function overlap in polarization-free nonpolar InGaN/GaN quantum wells, which leads to a higher spontaneous emission rate at low current densities compared to polar $c$ -plane quantum wells. LEDs with improved high-speed performance at low current densities will help to reduce power dissipation and increase efficiency in Gb/s visible-light communication systems.

Nonpolar ${m}$ -Plane InGaN/GaN Micro-Scale Light-Emitting Diode With 1.5 GHz Modulation Bandwidth

Free-standing nonpolar GaN substrates provide an excellent platform for the fabrication of high-speed blue and green light-emitting diodes (LEDs), which are attractive for visible-light communication, plastic optical fiber communication, and short-range under water optical communication. Nonpolar LEDs on free-standing GaN exhibit a large electron-hole wave function overlap, low extended defect density, and favorable thermal properties. Here, we demonstrate high-speed nonpolar InGaN/GaN LEDs with a peak emission wavelength between 455 and 465 nm on free-standing nonpolar GaN substrates. A large frequency modulation bandwidth of 524 MHz is demonstrated at a current density of 10 kA/cm
 2
.

High-Speed Nonpolar InGaN/GaN LEDs for Visible-Light Communication

High-speed InGaN/GaN blue light-emitting diodes (LEDs) are needed for future gigabit-per-second visible-light communication systems. Large LED modulation bandwidths are typically achieved at high current densities, with reports close to 1 GHz bandwidth at current densities ranging from 5 to 10 kA/cm2. However, the internal quantum efficiency (IQE) of InGaN/GaN LEDs is quite low at high current densities due to the well-known efficiency droop phenomenon. Here, we show experimentally that nonpolar and semipolar orientations of GaN enable higher modulation bandwidths at low current densities where the IQE is expected to be higher and power dissipation is lower. We experimentally compare the modulation bandwidth vs. current density for LEDs on nonpolar (101¯0), semipolar (202¯1¯), and polar 0001 orientations. In agreement with wavefunction overlap considerations, the experimental results indicate a higher modulation bandwidth for the nonpolar and semipolar LEDs, especially at relatively low current densities....

Impact of crystal orientation on the modulation bandwidth of InGaN/GaN light-emitting diodes

We present the first demonstration of RF characteristics of electrically injected GaN/InGaN core–shell nanowire-based micro light-emitting diodes (μLEDs) for μLED displays and visible-light communi...

Arman Rashidi

Papers

A Decade of Nonpolar and Semipolar III-Nitrides: A Review of Successes and Challenges

Nonpolar ${m}$ -Plane InGaN/GaN Micro-Scale Light-Emitting Diode With 1.5 GHz Modulation Bandwidth

High-Speed Nonpolar InGaN/GaN LEDs for Visible-Light Communication

Impact of crystal orientation on the modulation bandwidth of InGaN/GaN light-emitting diodes

Electrically Injected GHz-Class GaN/InGaN Core–ShellNanowire-Based μLEDs: Carrier Dynamics and Nanoscale Homogeneity