Guiju Zhang

Polarized GaN-based LED with an integrated multi-layer subwavelength structure.

A novel type of GaN-based LED with a highly polarized output using an integrated multi-layer subwavelength grating structure is proposed. Characteristics of both optical transmission and polarization extinction ratio of the polarized GaN-based LED with three different multi-layer subwavelength structures are investigated. It is found that both TM transmission (T(TM)) and the extinction ratio(ER) of the LED output can be effectively enhanced by incorporating a dielectric transition layer between the metal grating and GaN substrate with a lower refractive index than that of the GaN substrate. Flat sensitivity of the T(TM) on the period, duty cycle of the metallic grating, and the wide range of operating wavelength have been achieved in contrast to the conventional sensitive behavior in single-layer metallic grating. Up to 0.75 high duty cycle of the metallic grating can be employed to achieve >60dB ER while T(TM) maintains higher than ~90%, which breaks the conventional limit of T(TM) and ER being always a pair of trade-off parameters. Typical optimized multilayer structures in terms of material, thickness, grating periods and duty cycle using MgF(2) and ZnS, respectively, as the transition layers are obtained. The results provide guidance in designing, optimizing and fabricating the novel integrated GaN-based and polarized photonic devices.

Mechanism on Effect of Surface Plasmons Coupling with InGaN/GaN Quantum Wells: Enhancement and Suppression of Photoluminescence Intensity

The photoluminescence (PL) intensity enhancement and suppression mechanism on surface plasmons (SPs) coupling with InGaN/GaN quantum wells (QWs) have been systematically studied. The SP-QW coupling behaviors in the areas of GaN cap layer coated with silver thin film were compared at different temperatures and excitation powers. It is found that the internal quantum efficiency (IQE) of the light emitting diodes (LEDs) varies with temperature and excitation power, which in turn results in anomalous emission enhancement and suppression tendency related to SP-QW coupling. The observation is explained by the balance between the extraction efficiency of SPs and the IQE of LEDs.

Polarization Properties of Metal-Dielectric Composite Subwavelength Gratings on GaN Substrate

A novel type of GaN substrate for light emitting devices with a highly polarized output using metal-dielectric composite subwavelength grating structure is proposed. Characteristics of both optical transmission and polarization extinction ratio of the polarized GaN-based LED with a double-layer grating structures are investigated. It is found that both TM transmission (TTM) and the extinction ratio(ER) of the LED output can be effectively enhanced by incorporating a dielectric transition grating between the metal grating and GaN substrate with a lower refractive index than that of the GaN substrate. Flat sensitivity of the TTM on the period, duty cycle of the grating, and the wide range of operating wavelength have been achieved. Up to 0.7 high duty cycle of the grating can be employed to achieve ER > 56dB while TTM maintains higher than ~93%. The results provide guidance in designing, optimizing and fabricating the novel integrated GaN-based and polarized photonic devices.

In Situ Spectroscopic Ellipsometry Monitoring GaN Nucleation Layer Growth and Annealing Behavior in MOVPE

In-situ spectroscopic ellipsometry (SE) was applied to metal-organic vapor phase epitaxial (MOVPE) growth of GaN. The effects of MOVPE reactor pressures on the growth modes of GaN buffer layer were investigated by SE with the assistance of ex-situ scanning electron microscopy (SEM). We further demonstrated the on-line control of the GaN growth mode in MOVPE, which established a feasibility to grow high-quality GaN layers on sapphire substrate at the reactor pressure as low as 20 Torr. SE is known to be sensitive on the small changes in the effective optical constants of the growth front surface, which is caused by many factors such as surface stoichiometry, atomic level roughness, surface atomic reconstruction, the nature and density of the absorbed species etc (6). Therefore, measurements of the pseudodielectric functions are suitable for in situ monitoring and optimization of the growth. In the present work, in-situ SE was used to study the GaN buffer layer growth under various reactor pressures. With the assistance of ex-situ scanning electron microscopy (SEM), we investigated the associated morphologies and microstructure evolutions during deposition and temperature ramping of the GaN buffer layers, and the growth mode variation of GaN buffer layer under different reactor pressures were clarified.