Yoshinobu Kawaguchi

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.

Blue Laser Diodes Fabricated on m-Plane GaN Substrates

Blue laser diodes (LDs) were fabricated on m-plane oriented GaN substrates by atmospheric-pressure metalorganic chemical vapor deposition. Typical threshold current for stimulated emission at a wavelength λ of 463 nm was 69 mA. Blueshift of the spontaneous emission peak with increasing injection current was examined in LDs fabricated on m- and c-plane GaN substrates. Blueshifts for the m-plane LD (λ=463 nm) and the c-plane LD (λ=454 nm) with an injection current density just below threshold were about 10 and 26 nm, respectively. These results confirm that the blueshift in quantum-wells fabricated on m-plane oriented substrates is smaller than on c-plane oriented substrates due to the absence of polarization-induced electric fields.

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.

Green Semipolar (202̄1̄) InGaN Light-Emitting Diodes with Small Wavelength Shift and Narrow Spectral Linewidth

We study the optical spectral properties for green semipolar (20) and (201) light-emitting diode (LED) with same indium compositions. Compared to (201) devices, the fabricated (20) micro-LED (~0.005 mm2) showed negligible blue shift and smaller full width at half maximum (FWHM) up to extremely high current densities (10,000 A/cm2). Theoretical simulation indicates that the (20) InGaN quantum well (QW) has reduced polarization-related effects due to combined effects of electric field cancelling and Coulomb screening effect. In addition, the packaged device performance for small-area (~0.144 mm2) semipolar green (20) and (201) LEDs were also discussed. The green (20) LED showed smaller wavelength shift and narrower FWHM than green LEDs fabricated on other planes.

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.

Highly reliable 500 mW laser diodes with epitaxially grown AlON coating for high-density optical storage

Highly reliable operation of 405 nm laser diodes for high-density optical storage was demonstrated. Introduction of epitaxially grown AlON layer between the front facet and normal coating layer was shown to be effective to suppress catastrophic optical damage at the laser facet. Stable operation in excess of 1000 h was confirmed at an output power of 500 mW in a pulsed-mode at a case temperature of 80 °C.Highly reliable operation of 405 nm laser diodes for high-density optical storage was demonstrated. Introduction of epitaxially grown AlON layer between the front facet and normal coating layer was shown to be effective to suppress catastrophic optical damage at the laser facet. Stable operation in excess of 1000 h was confirmed at an output power of 500 mW in a pulsed-mode at a case temperature of 80 °C.

Electric Field Induced Carrier Sweep-Out in Tandem InGaN Multi-Quantum-Well Self-Pulsating Laser Diodes

Mechanisms of carrier sweep-out in tandem InGaN multiple-quantum-well self-pulsating laser diodes were investigated. Laser diodes showed self-pulsating characteristics without significant change in the light output–current (I–L) characteristics when an electric field high enough was established in the saturable absorber by the applied reverse bias. Improvements in the design of the band-energy profile allowed a substantial reduction in the bias required for self-pulsating operation. These results indicate that carrier lifetime can be controlled by the electric field in the saturable absorber and that band-energy profile engineering is effective for the reduction of carrier lifetime.