Ming Lun Lee

Emission mechanism of mixed-color InGaN/GaN multi-quantum-well light-emitting diodes

In this study, GaN-based light-emitting diodes (LEDs) were designed with a multi-quantum-well active region, including a yellow-green and a blue quantum well in each period. Photoluminescence (PL) and electroluminescence (EL) measurements revealed two emission bands (at λ~450 and 560 nm) originating from the two well regions. The ratio of blue to yellow-green emission intensities changes with the excitation intensity. In EL, the intensity of the blue emission peak exceeds that of the yellow-green emission peak when a low DC current (I40 mA) is applied. However, when a high pulsed current is applied (I100 mA) to the LEDs, the intensity of the yellow-green band exceeds that of the blue band, because of the competition between carrier tunneling and band-to-band recombination.

GaN-Based LEDs Output Power Improved by Textured GaN/Sapphire Interface Using In Situ

In this study, we demonstrate an in situ roughening technique at the GaN/sapphire interface in GaN-based LEDs using a silane treatment (SiH4 treatment) process that forms a thin SiNx layer with nanometer-sized holes on the sapphire surface that behave like a patterned sapphire substrate. A plurality of voids at the GaN/sapphire interface is observed according to the transmission electron microscopy analysis. With a 20 mA current injection, the results indicate that the typical output power of LEDs grown with and without the SiH4 treatment process are approximately 18.0 and 15.6 mW, respectively. In other words, the output power can be enhanced by 15% with the use of the SiH4 treatment process. The enhancement of output power is mainly due to light scattering at the naturally textured GaN/sapphire interface, which can lead to a higher escape probability for the photons emitted from the active layer in an LED.

\hbox{SiH}_{\bf 4}

InGaN/GaN-based solar cells with vertical-conduction feature on silicon substrates were fabricated by wafer bonding technique. The vertical solar cells with a metal reflector sandwiched between the GaN-based epitaxial layers and the Si substrate could increase the effective thickness of the absorption layer. Given that the thermally resistive sapphire substrates were replaced by the Si substrate with high thermal conductivity, the solar cells did not show degradation in power conversion efficiency (PCE) even when the solar concentrations were increased to 300 suns. The open circuit voltage increased from 1.90 V to 2.15 V and the fill factor increased from 0.55 to 0.58 when the concentrations were increased from 1 sun to 300 suns. With the 300-sun illumination, the PCE was enhanced by approximately 33% compared with the 1-sun illumination.

Treatment Process During Epitaxial Growth

GaN-based light-emitting diodes (LEDs) grown on sapphire with ex situ AlN nucleation layer prepared by radio-frequency sputtering were investigated. GaN-based epitaxial layers grown on the Ar-implanted AlN/sapphire (AIAS) substrates exhibited selective growth and subsequent lateral growth due to the difference of lattice constants between the implanted and implantation-free regions. Consequently, air voids over the implanted regions were formed around the GaN/AlN/sapphire interfaces. We proposed the growth mechanisms of the GaN layer on the AIAS substrates and characterized the LEDs with embedded air voids. With a 20 mA current injection, experimental results indicate that the light output power of LEDs grown on the AIAS substrates was enhanced by 25% compared with those of conventional LEDs. This enhancement can be attributed to the light scattering at the GaN/air void interfaces to increase the light extraction efficiency of the LEDs.

Vertical InGaN-based green-band solar cells operating under high solar concentration up to 300 suns

This paper investigates GaN-based blue light-emitting diodes (LEDs) grown on sapphire substrates with selective-area Ar-ion implantation. The GaN-based epitaxial layers grown on the Ar-implanted sapphire substrates (Ar-ISS) exhibited selective growth and subsequent lateral growth because of different lattice constants between the implantation and implantation-free regions. As a result, air voids were formed at the GaN/sapphire interface, above the implanted regions and below the active layers of LEDs. We proposed the GaN layer growth mechanisms on the Ar-ISS, and characterized the LEDs with embedded air voids at the GaN/sapphire interface. Using a 20-mA current injection, the light output of the experimental LEDs was found to be 15% greater than that of conventional LEDs. This enhancement can be attributed to the light scattering at the textured GaN/air void interfaces, which increases the probability of photons escaping from the LEDs.

Gallium nitride-based light-emitting diodes with embedded air voids grown on Ar-implanted AlN/sapphire substrate

The band-engineered structure design for electron-blocking layer (EBL) and hole-blocking layer (HBL) in AlxGa1-xN-based ultraviolet light-emitting diodes (UV LEDs) is performed and analyzed theoretically. Simulation results show that the severe polarization effect is efficiently mitigated and the downward-bended band profile of the EBL is improved when the EBL is designed with a graded-composition and multiquantum barrier (GMQB) structure. As a result, the capabilities of both electron confinement and hole injection, and also the light output power are promoted. On the contrary, for the HBL, the design of composition graded and/or multiquantum barrier structures reduces the effective potential barrier height for holes in the valence band and, consequently, causes a considerable hole overflow. The UV LED, thus, exhibits superior optical performance when the LED structure is simultaneously designed with a GMQB EBL and a bulk HBL.

Improved Output Power of GaN-based Blue LEDs by Forming Air Voids on Ar-Implanted Sapphire Substrate

We present a trichromatic GaN-based light-emitting diode (LED) that emits near-ultraviolet (n-UV) blue and green peaks combined with red phosphor to generate white light with a low correlated color temperature (CCT) and high color rendering index (CRI). The LED structure, blue and green unipolar InGaN/GaN multiple quantum wells (MQWs) stacked with a top p-i-n structure containing an InGaN/GaN MQW emitting n-UV light, was grown epitaxially on a single substrate. The trichromatic LED chips feature a vertical conduction structure on a silicon substrate fabricated through wafer bonding and laser lift-off techniques. The blue and green InGaN/GaN MQWs were pumped with n-UV light to re-emit low-energy photons when the LEDs were electrically driven with a forward current. The emission spectrum included three peaks at approximately 405, 468, and 537 nm. Furthermore, the trichromatic LED chips were combined with red phosphor to generate white light with a CCT and CRI of approximately 2900 and 92, respectively.

Design of Hole-Blocking and Electron-Blocking Layers in Al x Ga 1– x N-Based UV Light-Emitting Diodes

GaN-based solar cells with Mn-doped absorption layer grown by metal-organic vapor-phase epitaxy were investigated. The transmittance spectrum and the spectral response showed the presence of an Mn-related band absorption property. Power-dependent, dual-light excitation, and lock-in amplifier techniques were performed to confirm if the two-photon absorption process occurred in the solar cells with Mn-doped GaN absorption layer. Although a slight decrease in an open circuit voltage was observed, a prominent increase in the short circuit current density resulted in a significant enhancement of the overall conversion efficiency. Under one-sun air mass 1.5u2009G standard testing condition, the conversion efficiency of Mn-doped solar cells can be enhanced by a magnitude of 5 times compared with the cells without Mn-doped absorption layer.

Warm-white light-emitting diode with high color rendering index fabricated by combining trichromatic InGaN emitter with single red phosphor.

Al0.25Ga0.75N ultraviolet (UV) p-i-n photodiodes (PDs) with top and inverted p-layer structure are created by selective-area growth on a GaN template layer. All PDs exhibit typical zero bias peak responsivity at 310 nm and the UV-to-visible (310/400 nm) spectral rejection ratio is greater than three orders of magnitude. In contrast to conventional AlGaN-based p-i-n photodiodes, the inverted devices with low-resistivity n-type AlGaN top-contact layers exhibit a nearly flat spectral response at the short wavelength regions (210-310 nm). The proposed device structure can achieve solar-blind, AlGaN/GaN-based p-i-n PDs with high-aluminum content and reduced tensile strain due to the lattice mismatch between AlGaN and GaN layers.

Improved conversion efficiency of GaN-based solar cells with Mn-doped absorption layer

The performance of GaN-based flip-chip light-emitting diodes (LEDs) with embedded air voids grown on a selective-area Ar-implanted sapphire (SAS) substrate was demonstrated in this letter. The GaN-based epitaxial layers grown on Ar-implanted regions exhibited lower growth rates compared with those grown on implantation-free regions. Accordingly, air voids formed over the implanted regions after merging laterally grown GaN facet fronts. The light-output power of LEDs grown on SAS was greater than that of LEDs grown on implantation-free sapphire substrates. The output power of LEDs grown on SAS was enhanced by 20% at an injection current of 700 mA. The increase in output power was mainly attributed to the scattering of light around the air voids, which increased the probability of photons escaping from the LEDs.