Chia Yen Huang

Indium incorporation and emission properties of nonpolar and semipolar InGaN quantum wells

We report indium incorporation properties on various nonpolar and semipolar free-standing GaN substrates. Electroluminescence characterization and x-ray diffraction (XRD) analysis indicate that the semipolar (202¯1¯) and (112¯2) planes have the highest indium incorporation rate among the studied planes. We also show that both indium composition and polarization-related electric fields impact the emission wavelength of the quantum wells (QWs). The different magnitudes and directions of the polarization-related electric fields for each orientation result in different potential profiles for the various semipolar and nonpolar QWs, leading to different emission wavelengths at a given indium composition.

High optical polarization ratio from semipolar (202¯1¯) blue-green InGaN/GaN light-emitting diodes

The optical polarization ratio of spontaneous emission was investigated by electroluminescence measurements for semipolar (202¯1¯) InGaN/GaN light-emitting diodes, covering the blue to green spectral range. Devices fabricated on semipolar (202¯1¯) substrates exhibit polarization ratios ranging from 0.46 at 418 nm to 0.67 at 519 nm. These polarization ratios are significantly higher than those reported on semipolar (202¯1) devices. The valence band energy separation is extracted from spectral measurements and is consistent with the increased polarization ratio and theoretical predictions. Quantum well interdiffusion induced valence band mixing is suggested as a possible explanation for the low experimental value of polarization ratio observed for the (202¯1) devices.

Influence of polarity on carrier transport in semipolar (2021¯) and (202¯1) multiple-quantum-well light-emitting diodes

We investigate the influence of polarity on carrier transport in single-quantum-well and multiple-quantum-well (MQW) light-emitting diodes (LEDs) grown on the semipolar (202¯1) and (2021¯) orientations of free-standing GaN. For semipolar MQW LEDs with the opposite polarity to conventional Ga-polar c-plane LEDs, the polarization-related electric field in the QWs results in an additional energy barrier for carriers to escape the QWs. We show that semipolar (2021¯) MQW LEDs with the same polarity to Ga-polar c-plane LEDs have a more uniform carrier distribution and lower forward voltage than (202¯1) MQW LEDs.

Nonpolar AlGaN-Cladding-Free Blue Laser Diodes with InGaN Waveguiding

We have demonstrated AlGaN-cladding-free m-plane InGaN-based blue laser diodes (LDs) using a novel structure that employs 50-nm-thick n- and p-type InxGa1-xN (x = 5–10%) as waveguiding layers. The thick, high In content InGaN waveguiding layers provided significant refractive index contrast to the GaN cladding layers, thereby eliminating the need for AlGaN cladding. Under pulsed operation, lasing was achieved at 442 nm with a threshold current density of 10 kA/cm2.

Suppressing void defects in long wavelength semipolar (202¯1¯) InGaN quantum wells by growth rate optimization

We report on void defect formation in (202¯1¯) semipolar InGaN quantum wells (QWs) emitting in the green spectral region. Fluorescence and transmission electron microscopy studies indicate that this type of defect is associated with voids with {101¯1}, {101¯0}, and {0001¯} side facets in the QW region. Systematic growth studies show that this defect can be effectively suppressed by reducing the growth rate for the active region. Green light-emitting diodes (LEDs) with reduced active region growth rate showed enhanced power and wavelength performance. The improved LED performance is attributed to the absence of void defects in the active region.

Semipolar (202̄1) Single-Quantum-Well Red Light-Emitting Diodes with a Low Forward Voltage

We have demonstrated the InGaN/GaN single-quantum-well (SQW) red light-emitting diodes (LEDs) grown on the free-standing GaN (201) substrate with a forward voltage as low as 2.8 V at 20 mA. A low p-GaN growth temperature is required to prevent the structure deterioration during the p-GaN growth. The reduction of the forward voltage was observed as the emission wavelength increased in the (201) SQW LEDs, which is attributed to its reversed polarization-related electric field compared to the conventional c-plane LEDs.

Influence of Mg-doped barriers on semipolar (202¯1) multiple-quantum-well green light-emitting diodes

We report the effects of Mg doping in the barriers of semipolar (202¯1) multiple-quantum-well light-emitting diodes (LEDs) with long emission wavelengths (>500 nm). With moderate Mg doping concentrations (3 × 1018–5 × 1018 cm−3) in the barriers, the output power was enhanced compared to those with undoped barriers, which suggests that hole transport in the active region is a limiting factor for device performance. Improved hole injection due to Mg doping in the barriers is demonstrated by dichromatic LED experiments and band diagram simulations. With Mg-doped AlGaN barriers, double-quantum-well LEDs with orange to red emission (λ > 600 nm) were also demonstrated.

Impact of p-GaN Thermal Damage and Barrier Composition on Semipolar Green Laser Diodes

Dark triangle defects (DTDs) are common nonradiative defects in semipolar (202̅1) oriented green quantum wells (QWs), commonly used in green laser diodes (LDs). We show that DTDs do not appear “as-grown,” and DTD size depends strongly on post-QW-growth annealing time and temperature. Using low temperature p-GaN, we prevent catastrophic QW damage and directly compare LDs with GaN and AlGaN containing barriers. The GaN barrier LD exhibited a lasing wavelength of 511 nm, reduced operating voltage, and the lowest threshold current density, likely due to enhanced optical confinement factor and the elimination of very low growth temperature AlGaN in the active region.