Houqiang Fu

Optical properties of highly polarized InGaN light-emitting diodes modified by plasmonic metallic grating

We implement finite-difference time-domain (FDTD) method to simulate the optical properties of highly polarized InGaN light emitting diodes (LEDs) coupled with metallic grating structure. The Purcell factor (Fp), light extraction efficiency (LEE), internal quantum efficiency (IQE), external quantum efficiency (EQE), and modulation frequency are calculated for different polarized emissions. Our results show that light polarization has a strong impact on Fp and LEE of LEDs due to their coupling effects with the surface plasmons (SPs) generated by metallic grating. Fp as high as 34 and modulation frequency up to 5.4 GHz are obtained for a simulated LED structure. Furthermore, LEE, IQE and EQE can also be enhanced by tuning the coupling between polarized emission and SPs. These results can serve as guidelines for the design and fabrication of high efficiency and high speed LEDs for the applications of solid-state lighting and visible-light communication.

Crystal orientation dependent intersubband transition in semipolar AlGaN/GaN single quantum well for optoelectronic applications

The optical properties of intersubband transition in a semipolar AlGaN/GaN single quantum well(SQW) are theoretically studied, and the results are compared with polar c-plane and nonpolar m-plane structures. The intersubband transition frequency, dipole matrix elements, and absorption spectra are calculated for SQW on different semipolar planes. It is found that SQW on a certain group of semipolar planes (55° < θ < 90° tilted from c-plane) exhibits low transition frequency and long wavelength response with high absorption quantum efficiency, which is attributed to the weak polarization-related effects. Furthermore, these semipolar SQWs show tunable transition frequency and absorption wavelength with different quantum well thicknesses, and stable device performance can be achieved with changing barrier thickness and Al compositions. All the results indicate that the semipolar AlGaN/GaN quantum wells are promising candidate for the design and fabrication of high performance low frequency and long wavelength optoelectronic devices.

Analysis of low efficiency droop of semipolar InGaN quantum well light-emitting diodes by modified rate equation with weak phase-space filling effect

We study the low efficiency droop characteristics of semipolar InGaN light-emitting diodes (LEDs) using modified rate equation incoporating the phase-space filling (PSF) effect where the results on c-plane LEDs are also obtained and compared. Internal quantum efficiency (IQE) of LEDs was simulated using a modified ABC model with different PSF filling (n0), Shockley-Read-Hall (A), radiative (B), Auger (C) coefficients and different active layer thickness (d), where the PSF effect showed a strong impact on the simulated LED efficiency results. A weaker PSF effect was found for low-droop semipolar LEDs possibly due to small quantum confined Stark effect, short carrier lifetime, and small average carrier density. A very good agreement between experimental data and the theoretical modeling was obtained for low-droop semipolar LEDs with weak PSF effect. These results suggest the low droop performance may be explained by different mechanisms for semipolar LEDs.

Theoretical analysis of modulation doping effects on intersubband transition properties of semipolar AlGaN/GaN quantum well

The effects of modulation doping on the intersubband transition (ISBT) properties of semipolar AlGaN/GaN quantum well (QW) are investigated theoretically using QW doping, barrier doping, and barrier δ-doping schemes at 150 K. Important ISBT parameters such as intersubband transition energies, dipole matrix elements, and absorption spectra are calculated for QW structures on both semipolar ( 20 2 ¯ 1 ) (i.e., with weak polarization) and ( 10 1 ¯ 3 ) (i.e., with strong polarization) planes. For ( 20 2 ¯ 1 ) QW with weak polarization, it is found that high doping concentrations can cause a significant band bowing to the QW structures, which reduce the absorption coefficients and wavelengths. This band bowing effect will become stronger when doping layers are closer to the QW. For ( 10 1 ¯ 3 ) QW with a strong polarization, however, a weak band bowing effect is observed due to the large polarization and large band tilting of ( 10 1 ¯ 3 ) QW. The study shows that modulation doping is a promising method to modi...

Nonpolar and semipolar InGaN/GaN multiple-quantum-well solar cells with improved carrier collection efficiency

We demonstrate the nonpolar and semipolar InGaN/GaN multiple-quantum-well (MQW) solar cells grown on the nonpolar m-plane and semipolar ( 20 2 ¯ 1 ) plane bulk GaN substrates. The optical properties and photovoltaic performance of the nonpolar and semipolar InGaN solar cells were systematically studied, and the results were compared to the conventional polar c-plane devices. The absorption spectra, current density–voltage (J–V) characteristics, external quantum efficiency (EQE), and internal quantum efficiency (IQE) were measured for nonpolar m-plane, semipolar ( 20 2 ¯ 1 ) plane, and polar c-plane InGaN/GaN MQW solar cells. Nonpolar m-plane InGaN/GaN MQW solar cells showed the best performance across all devices, with a high open-circuit voltage of 2.32 V, a low bandgap-voltage offset of 0.59 V, and the highest EQE and IQE. In contrast, the polar c-plane device showed the lowest EQE despite the highest absorption spectra. This huge difference is attributed to the better carrier transport and collection o...

Active tracking system for visible light communication using a GaN-based micro-LED and NRZ-OOK

Visible light communication (VLC) holds the promise of a high-speed wireless network for indoor applications and competes with 5G radio frequency (RF) system. Although the breakthrough of gallium nitride (GaN) based micro-light-emitting-diodes (micro-LEDs) increases the -3dB modulation bandwidth exceptionally from tens of MHz to hundreds of MHz, the light collected onto a fast photo receiver drops dramatically, which determines the signal to noise ratio (SNR) of VLC. To fully implement the practical high data-rate VLC link enabled by a GaN-based micro-LED, it requires focusing optics and a tracking system. In this paper, we demonstrate an active on-chip tracking system for VLC using a GaN-based micro-LED and none-return-to-zero on-off keying (NRZ-OOK). Using this novel technique, the field of view (FOV) was enlarged to 120° and data rates up to 600 Mbps at a bit error rate (BER) of 2.1×10-4 were achieved without manual focusing. This paper demonstrates the establishment of a VLC physical link that shows enhanced communication quality by orders of magnitude, making it optimized for practical communication applications.

Characterizations of nonlinear optical properties on GaN crystals in polar, nonpolar, and semipolar orientations

We report the basic nonlinear optical properties, namely, two-photon absorption coefficient ( β), three-photon absorption coefficient ( γ), and Kerr nonlinear refractive index ( n kerr), of GaN crystals in polar c-plane, nonpolar m-plane, and semipolar ( 20 21 ¯) plane orientations. A typical Z-scan technique was used for the measurement with a femtosecond Ti:S laser from wavelengths of 724 nm to 840 nm. For the two-photon absorption coefficient ( β), similar values were obtained for polar, nonpolar, and semipolar samples, which are characterized to be ∼0.90 cm/GW at 724 nm and ∼0.65 cm/GW at 730 nm for all the three samples. For the Kerr nonlinear refractive index ( n kerr), self-focusing features were observed in this work, which is different from previous reports where self-defocusing features were observed on GaN in the visible and near-UV spectral regions. At 724 nm, n kerr was measured to be ∼2.5 0 × 10 − 14 cm 2 / W for all three samples. Three-photon absorption coefficients ( γ) were also determ...

Ultra-low turn-on voltage and on-resistance vertical GaN-on-GaN Schottky power diodes with high mobility double drift layers

This letter reports the implementation of double-drift-layer (DDL) design into GaN vertical Schottky barrier diodes (SBDs) grown on free-standing GaN substrates. This design balances the trade-off between desirable forward turn-on characteristics and high reverse breakdown capability, providing optimal overall device performances for power switching applications. With a well-controlled metalorganic chemical vapor deposition process, the doping concentration of the top drift layer was reduced, which served to suppress the peak electric field at the metal/GaN interface and increase the breakdown voltages of the SBDs. The bottom drift layer was moderately doped to achieve low on-resistance to reduce power losses. At forward bias, the devices exhibited a record low turn-on voltage of 0.59 V, an ultra-low on-resistance of 1.65 mΩ cm2, a near unity ideality factor of 1.04, a high on/off ratio of ∼1010, and a high electron mobility of 1045.2 cm2/(V s). Detailed comparisons with conventional single-drift-layer (S...

Reliability analysis of InGaN/GaN multi-quantum-well solar cells under thermal stress

We investigate the thermal stability of InGaN solar cells under thermal stress at elevated temperatures from 400 °C to 500 °C. High Resolution X-Ray Diffraction analysis reveals that material quality of InGaN/GaN did not degrade after thermal stress. The external quantum efficiency characteristics of solar cells were well-maintained at all temperatures, which demonstrates the thermal robustness of InGaN materials. Analysis of current density–voltage (J–V) curves shows that the degradation of conversion efficiency of solar cells is mainly caused by the decrease in open-circuit voltage (Voc), while short-circuit current (Jsc) and fill factor remain almost constant. The decrease in Voc after thermal stress is attributed to the compromised metal contacts. Transmission line method results further confirmed that p-type contacts became Schottky-like after thermal stress. The Arrhenius model was employed to estimate the failure lifetime of InGaN solar cells at different temperatures. These results suggest that wh...

Toward ultimate efficiency: progress and prospects on planar and 3D nanostructured nonpolar and semipolar InGaN light-emitting diodes

Nonpolar and semipolar III-nitride-based blue and green light-emitting diodes (LEDs) have been extensively investigated as potential replacements for current polar c-plane LEDs. High-power and low-efficiency-droop blue LEDs have been demonstrated on nonpolar and semipolar planes III-nitride due to the advantages of eliminated or reduced polarization-related electric field and homoepitaxial growth. Semipolar (202¯1) and (202¯1¯) LEDs have contributed to bridging “green gap” (low efficiency in green spectral region) by incorporating high indium compositions, reducing polarization effects, and suppressing defects. Other properties, such as low thermal droop, narrow spectral linewidth, small wavelength shift, and polarized emission, have also been reported for nonpolar and semipolar LEDs. In this paper we review the theoretical background, device performance, material properties, and physical mechanisms for nonpolar and semipolar III-nitride semiconductors and associated blue and green LEDs. The latest progress on topics including efficiency droop, thermal droop, green-gap, and three-dimensional nanostructures is detailed. Future challenges, potential solutions, and applications will also be covered.