Jun-Rong Chen

Bio: Jun-Rong Chen is an academic researcher from National Chiao Tung University. The author has contributed to research in topics: Semiconductor laser theory & Light-emitting diode. The author has an hindex of 15, co-authored 32 publications receiving 747 citations. Previous affiliations of Jun-Rong Chen include National Changhua University of Education.

Low efficiency droop in blue-green m-plane InGaN/GaN light emitting diodes

Abstract: We investigated the electroluminescence and relatively external quantum efficiency (EQE) of m-plane InGaN/GaN light emitting diodes (LEDs) emitting at 480 nm to elucidate the droop behaviors in nitride-based LEDs. With increasing the injection current density to 100 A/cm2, the m-plane LEDs exhibit only 13% efficiency droop, whereas conventional c-plane LEDs suffer from efficiency droop at very low injection current density and the EQE of c-plane LEDs decrease to as little as 50% of its maximum value. Our simulation models show that in m-plane LEDs the absence of polarization fields manifest not only the hole distribution more uniform among the wells but also the reduction in electron overflow out of electron blocking layer. These results suggest that the nonuniform distribution of holes and electron leakage current due to strong polarization fields are responsible for the relatively significant efficiency droop of conventional c-plane LEDs.

Room temperature polariton lasing vs. photon lasing in a ZnO-based hybrid microcavity.

Abstract: We report on the room temperature polariton lasing and photon lasing in a ZnO-based hybrid microcavity under optical pumping. A series of experimental studies of the polariton lasing (exciton-photon detunings of δ = -119 meV) in the strong-coupling regime are discussed and compared to a photon lasing (δ = -45 meV) in the weak-coupling regime obtained in the same structure. The measured threshold power density (31.8 kW/cm2) of polariton lasing is one order of magnitude lower than that of the photon lasing (318.2 kW/cm2). In addition, the comparison between polariton lasing and photon lasing is done in terms of the linewidth broadening, blue-shift of the emission peak, and polarization.

Effects of Built-In Polarization and Carrier Overflow on InGaN Quantum-Well Lasers With Electronic Blocking Layers

Abstract: Effects of built-in polarization and carrier overflow on InGaN quantum-well lasers with a ternary AlGaN or a quaternary AlInGaN electronic blocking layer (EBL) have been numerically investigated by employing an advanced device-simulation program. The simulation results indicate that the characteristics of InGaN quantum-well lasers can be improved by using the quaternary AlInGaN EBL. When the aluminum and indium compositions in the AlInGaN EBL are appropriately designed, the built-in charge density at the interface between the InGaN barrier and the AlInGaN EBL can be reduced. Under this circumstance, the electron leakage current and the laser threshold current can obviously be decreased as compared with the laser structure with a conventional AlGaN EBL when the built-in polarization is taken into account in the calculation. Furthermore, the AlInGaN EBL also gives a higher refractive index than the AlGaN EBL, which is a benefit for a higher quantum-well optical confinement factor in laser operations.

Investigation of wavelength-dependent efficiency droop in InGaN light-emitting diodes

Abstract: The physical mechanisms leading to the effi- ciency droop of InGaN/GaN light-emitting diodes (LEDs) are theoretically investigated. We first discuss the effect of Auger recombination loss on efficiency droop by taking dif- ferent Auger coefficients into account. It is found that the Auger recombination process plays a significant nonradia- tive part for carriers at typical LED operation currents when the Auger coefficient is on the order of 10 −30 cm 6 s −1 . Fur- thermore, the InGaN/GaN multiple-quantum-well (MQW) LEDs with varied indium compositions in InGaN quan- tum wells are studied to analyze the wavelength-dependent efficiency droop. The simulation results show that the wavelength-dependent efficiency droop is caused by sev- eral different effects including non-uniform carrier distrib- ution, electron overflow, built-in electrostatic field induced by spontaneous and piezoelectric polarization, and Auger recombination loss. These internal physical mechanisms are the critical factors resulting in the wavelength-dependent ef- ficiency droop in InGaN/GaN MQW LEDs.

Optically Pumped GaN-based Vertical Cavity Surface Emitting Lasers: Technology and Characteristics

Abstract: We review the fabrication technology and performance characteristics of optically pumped GaN-based vertical cavity surface emitting lasers (VCSELs). Two types of VCSELs with different microcavity structures are described. First type of VCSEL has a hybrid microcavity structure that consists of an epitaxially grown AlN/GaN distributed Bragg reflector (DBR), a GaN active layer with InGaN/GaN multiple quantum wells (MQWs), and a Ta2O5/SiO2 dielectric DBR. Second type of VCSEL has a dielectric DBR microcavity structure that has a similar InGaN/GaN MQWs active layer sandwiched in two dielectric DBRs formed by Ta2O5/SiO2 and TiO2/SiO2. Both types of VCSELs achieved laser action under optical pumping at room temperature with emission wavelength of 448 and 414 nm for hybrid DBR VCSEL and dielectric DBR VCSEL, respectively. Both lasers showed narrow emission linewidth with high degree of polarization and large spontaneous emission coupling factors of about 10-2. In addition, a high characteristic temperature of over 240 K was measured, and a distinct spatially inhomogeneous emission pattern was observed.

Bose-Einstein condensation in a gas of sodium atoms

Abstract: Bose-Einstein condensation (BEC) has been observed in a dilute gas of sodium atoms. A Bose-Einstein condensate consists of a macroscopic population of the ground state of the system, and is a coherent state of matter. In an ideal gas, this phase transition is purely quantum-statistical. The study of BEC in weakly interacting systems which can be controlled and observed with precision holds the promise of revealing new macroscopic quantum phenomena that can be understood from first principles.

Light-emitting diodes

Abstract: Light-Emitting Diodes. (Monographs in Electrical and Electronic Engineering.) By A. A. Bergh and P. J. Dean. Pp. viii+591. (Clarendon: Oxford; Oxford University: London, 1976.) £22.

Strong light-matter coupling in two-dimensional atomic crystals

Abstract: Microcavity polaritons—the bosonic quasiparticles that result from strong light–matter coupling—are observed for the first time in a dielectric cavity containing a monolayer of molybdenum disulphide at room temperature.

The road towards polaritonic devices

Abstract: Polaritons are quasiparticles that form in semiconductors when an elementary excitation such as an exciton or a phonon interacts sufficiently strongly with light. In particular, exciton-polaritons have attracted tremendous attention for their unique properties, spanning from an ability to undergo ultra-efficient four-wave mixing to superfluidity in the condensed state. These quasiparticles possess strong intrinsic nonlinearities, while keeping most characteristics of the underlying photons. Here we review the most important features of exciton-polaritons in microcavities, with a particular emphasis on the emerging technological applications, the use of new materials for room-temperature operation, and the possibility of exploiting polaritons for quantum computation and simulation.

Exciton-polaritons in van der Waals heterostructures embedded in tunable microcavities.

Abstract: Layered materials can be assembled vertically to fabricate a new class of van der Waals heterostructures a few atomic layers thick, compatible with a wide range of substrates and optoelectronic device geometries, enabling new strategies for control of light-matter coupling. Here, we incorporate molybdenum diselenide/hexagonal boron nitride (MoSe2/hBN) quantum wells in a tunable optical microcavity. Part-light-part-matter polariton eigenstates are observed as a result of the strong coupling between MoSe2 excitons and cavity photons, evidenced from a clear anticrossing between the neutral exciton and the cavity modes with a splitting of 20 meV for a single MoSe2 monolayer, enhanced to 29 meV in MoSe2/hBN/MoSe2 double-quantum wells. The splitting at resonance provides an estimate of the exciton radiative lifetime of 0.4 ps. Our results pave the way for room-temperature polaritonic devices based on multiple-quantum-well van der Waals heterostructures, where polariton condensation and electrical polariton injection through the incorporation of graphene contacts may be realized.