光電所所訊

A method for enhancing the favored transverse-electric-polarized emission of an AlGaN deep-ultraviolet quantum well

Professor C. C. Yang’s Lab.

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所楊志忠教授

An AlGaN quantum well (QW) structure of a deep-ultraviolet (UV) light-emitting diode (LED) needs to be well designed for controlling its band structure such that the heavy-hole (HH) band edge becomes lower than the split-off (SO) band edge and hence the transverse-electric (TE) polarization dominates the emission for achieving a higher light extraction efficiency. Here, we report the discovery of un-intentionally formed high-Al AlGaN nano-layers right above and below such a QW and their effects on the QW for changing the relative energy levels of the HH and SO bands. The comparison between the results of simulation study and polarization-resolved photoluminescence measurement confirms that the high-Al layers (HALs) represent the key to the observation of the dominating TE-polarized emission. By applying a stress onto a sample along its c-axis to produce a tensile strain in the c-plane for counteracting the HAL effects in changing the band structure, we can further understand the effectiveness of the HALs. The formation of the HALs is attributed to the hydrogen back-etching of Ga atoms during the temperature transition from quantum barrier growth into QW growth and vice versa. The Al filling in the etched vacancies results in the formation of an HAL. This discovery brings us with a simple method for enhancing the favored TE-polarized emission in an AlGaN deep-UV QW LED.

Fig. 1 (upper) TEM image to show the HALs (dark stripes). (lower) Geometric phase analysis image.

Fig. 2 Schematic drawings to show the formation mechanism of the HALs.

Surface plasmon coupling for suppressing p-GaN absorption and TM-polarized emission in a deep-UV light-emitting diode

Professor Y. W. Kiang’s Laboratory

Graduate Institute of Photonics and Optoelectronics, National Taiwan University

臺灣大學光電所江衍偉教授

The radiated power enhancement (suppression) of an in- (out-of-) plane-oriented radiating dipole at a desired emission wavelength in the deep-ultraviolet (UV) range when it is coupled with a surface plasmon (SP) resonance mode induced on a nearby Al nanoparticle (NP) is demonstrated. Also, it is found that the enhanced radiated power propagates mainly in the direction from the Al NP toward the dipole. Such SP coupling behaviors can be used for suppressing the transverse-magnetic (TM)-polarized emission, enhancing the transverse-electric (TE)-polarized emission, and reducing the UV absorption of the p-GaN layer in an AlGaN-based deep-UV light-emitting diode by embedding a sphere-like Al NP in its p-AlGaN layer.

Fig. 1. (a)-(d) Schematic demonstrations of simulation structures A-D.

Fig. 2. Spectra of the normalized radiated power in structures B-D when an x-dipole is used (d = 50 nm). The vertical arrow marks 270 nm.

Fig. 3. Similar to Fig. 2 except when a z-dipole is used (d = 50 nm).