No abstract

The majority of a GaN light-emitting diode (LED) is released from its sapphire substrate through selective-area laser lift-off to form a freely suspended light emitter. By virtue of being suspended in air without supporting substrates, the ultrathin crystalline and crack-free film possesses flexibility and bendability. The free-standing LEDs benefit from significant relaxation of strain, evident from red-shifting of the E2(high) phonon frequencies as measured by Raman spectroscopy toward those of strain-free free-standing GaN substrates. The phonon frequencies remain invariant upon bending of the film; this indicates that the properties of the flexible device will not be dependent on the bending curvatures. The observation of pronounced spectral blue-shifts from the photoluminescence (PL) spectrum from the flexible regions further confirms the occurrence of strain relaxation in the quantum wells. Being free-standing and thus lacking a direct heat-sinking pathway, emissions from the different regions of the suspended film can be affected by thermal effects to different extents, which are investigated by long-wave infrared thermometry. Heat accumulation is determined to be most severe at the far end of the flexible stripe at higher currents, leading to reduced efficiencies and electroluminescence (EL) spectral red-shifts. Based on this architecture, a monolithic 3 × 4 dot-matrix microdisplay prototype is demonstrated, comprising three adjacent flexible stripe emitters with four individually addressable pixels on each stripe. This proof-of-concept demonstration opens up new opportunities for GaN optoelectronics for a wide range of flexible display and visual applications.

This paper reviews the recent progress in the design and fabrication strategies of III-nitride semiconductor microdisk lasers on sapphire or Si substrates. Involving more accurate lithography methods in the fabrication boosts quality factors (Q-factors) due to the improved optical confinement of whispering-gallery modes (WGMs) at the rim of the microdisks with smooth sidewalls and perfect circularity. Quantum dots (QDs) as gain medium within the microdisks are investigated and promising for the demonstration of microlasers with super-high Q-factors and ultra-low thresholds. And these QD-cavity systems are also facilitating the research of cavity quantum electrodynamics (QED) in the strong coupling regime and cavity-enhanced single photon emission (SPE) for quantum computing. In this paper, we also report successful fabrication of transferred GaN free-standing microdisks with InGaN quantum wells (QWs) on conductive substrates, which achieves a significant step forward to realize electrically pumped high-quality microdisk lasers on target substrates. Strategies of applying flexible transparent electrodes such as graphene and metallic nanowires are potential effective solutions for realizing current-injection of nitride-based microdisks.ß 2015 WILEYÀVCH Verlag GmbH & Co. KGaA, Weinheim 1 Introduction Over past few decades, group IIInitride compound semiconductors have been extensively studied and have emerged as indispensable materials for a wide range of optoelectronic devices. With a wide and direct energy bandgap, emissions from III-nitride semiconductors can cover the wavelength range from deep ultraviolet to the near-infrared, making them ideal candidates for incoherent and coherent light sources [1][2][3][4]. Moreover, as epitaxial growth techniques of III-nitride materials become mature, high quality quantum structures such as quantum wells (QWs) and quantum dots (QDs) can be readily achieved, greatly improving the emission efficiency via quantum confinement of the carriers within a small active region so that the possibility of radiative recombination increases, serving as gain medium for highly efficient light emitters [5][6][7]. In addition, the high binding energies of excitons in these wide bandgap materials motivate the research on photon-exciton (or polariton) dynamics and its coherent state polariton lasing in microcavities [8][9][10][11]. In fact, the research of nitride-based microcavity lasers has attracted much attention in past decades for both physical insight of lasing mechanisms in both the weak and strong coupling regimes and their widespread applications [12]. Until now,

Optically pumped multi-mode whispering-gallery mode (WGM) lasing has been observed in optically free-standing InGaN/GaN quantum well microdisks at room temperature. The ∼6.6  μm optically isolated microdisks are patterned by microsphere lithography on GaN thin-films prepared by laser lift-off of the sapphire substrate, enabling superior optical confinement. The modes are determined to be of second order according to simulations. The lasing threshold is found to be ∼9.06  mJ/cm2, with a quality factor Q of ∼770 evaluated from the dominant mode at λ=430.2  nm, and a free space range (FSR) of 3.17 to 2.16 nm, which is mode-dependent.

No abstract

No abstract

No abstract

We demonstrate hybrid white light emitters by employing Gallium Nitride (GaN)-based light-emitting diodes (LEDs) as blue emitters and aggregation-induced emission (AIE) organic molecule 4,7-Bis[4-(1,2,2triphenylvinyl)phenyl]benzo-2,1,3-thiadiazole (BTPETD) as downconverted yellow emitters. The fabricated device shows CIE coordinates of (0.32, 0.33) and 45.4% optical extraction efficiency from BTPETD layer with pump leakage of 17.3%. At driving current of 30 mA, the highest luminous efficacy of 123.8 lm/W was obtained for the hybrid devices which corresponded a luminance of 1.4 × 10 5 cd/m 2 . Our work indicates that formation of aggregates from AIE materials such as BTPETD is able to greatly enhance the device extraction efficiency and improve the hybrid white LED performance.