Green Vertical-Cavity Surface-Emitting Lasers Based on InGaN Quantum Dots and Short Cavity.

Room temperature low threshold lasing of green GaN-based vertical cavity surface emitting laser (VCSEL) was demonstrated under continuous wave (CW) operation. By using self-formed InGaN quantum dots (QDs) as the active region, the VCSEL emitting at 524.0 nm has a threshold current density of 51.97 A cm-2, the lowest ever reported. The QD epitaxial wafer featured with a high IQE of 69.94% and the δ-function-like density of states plays an important role in achieving low threshold current. Besides, a short cavity of the device (~ 4.0 λ) is vital to enhance the spontaneous emission coupling factor to 0.094, increase the gain coefficient factor, and decrease the optical loss. To improve heat dissipation, AlN layer was used as the current confinement layer and electroplated copper plate was used to replace metal bonding. The results provide important guidance to achieving high performance GaN-based VCSELs.

Dual-wavelength switching in InGaN quantum dot micro-cavity light-emitting diodes.

Dual-wavelength switchable emission has been demonstrated in InGaN quantum dot (QD) micro-cavity light-emitting diodes (MCLEDs). By simply modulating the injected current levels, the output of the device can be dynamically tuned between the two distinct cavity modes at 498.5 and 541.7 nm, exhibiting deterministic mode switching in the green spectral range. Owing to the microcavity effect, high spectral purity with a narrow linewidth of 0.21 nm was obtained. According to the experimental and theoretical results, it can be concluded that the dual-wavelength switching for the investigated MCLEDs is ascribed to the broad and tunable gain of a thin InGaN QD active region, together with the mode selection and enhancement effect of the cavity. To provide additional guidelines for controllable dual-wavelength switchable operation in nitride-based light-emitting devices, detailed design and fabrication strategies are discussed. This work presents an effective method to achieve mode switching for practical applications such as multi-wavelength optical recording, frequency mixing, flip-flop and optical switches.

Reduction of Polarization Field Strength in Fully Strained c-Plane InGaN/(In)GaN Multiple Quantum Wells Grown by MOCVD.

The polarization fields in c-plane InGaN/(In)GaN multiple quantum well (MQW) structures grown on sapphire substrate by metal-organic chemical vapor deposition are investigated in this paper. The indium composition in the quantum wells varies from 14.8 to 26.5% for different samples. The photoluminescence wavelengths are calculated theoretically by fully considering the related effects and compared with the measured wavelengths. It is found that when the indium content is lower than 17.3%, the measured wavelengths agree well with the theoretical values. However, when the indium content is higher than 17.3%, the measured ones are much shorter than the calculation results. This discrepancy is attributed to the reduced polarization field in the MQWs. For the MQWs with lower indium content, 100% theoretical polarization can be maintained, while, when the indium content is higher, the polarization field decreases significantly. The polarization field can be weakened down to 23% of the theoretical value when the indium content is 26.5%. Strain relaxation is excluded as the origin of the polarization reduction because there is no sign of lattice relaxation in the structures, judging by the X-ray diffraction reciprocal space mapping. The possible causes of the polarization reduction are discussed.