Optimization of internal quantum efficiency (IQE) for InGaN quantum wells (QWs) light-emitting diodes (LEDs) is investigated. Staggered InGaN QWs with large electron-hole wavefunction overlap and improved radiative recombination rate are investigated for nitride LEDs application. The effect of interface abruptness in staggered InGaN QWs on radiative recombination rate is studied. Studies show that the less interface abruptness between the InGaN sub-layers will not affect the performance of the staggered InGaN QWs detrimentally. The growths of linearly-shaped staggered InGaN QWs by employing graded growth temperature grading are presented. The effect of current injection efficiency on IQE of InGaN QWs LEDs and other approaches to reduce dislocation in InGaN QWs LEDs are also discussed. The optimization of both radiative efficiency and current injection efficiency in InGaN QWs LEDs are required for achieving high IQE devices emitting in the green spectral regime and longer.

Approaches for high internal quantum efficiency green InGaN light-emitting diodes with large overlap quantum wells

Solid-state lighting (SSL) is now the most efficient source of high color quality white light ever created. Nevertheless, the blue InGaN light-emitting diodes (LEDs) that are the light engine of SSL still have significant performance limitations. Foremost among these is the decrease in efficiency at high input current densities widely known as “efficiency droop.” Efficiency droop limits input power densities, contrary to the desire to produce more photons per unit LED chip area and to make SSL more affordable. Pending a solution to efficiency droop, an alternative device could be a blue laser diode (LD). LDs, operated in stimulated emission, can have high efficiencies at much higher input power densities than LEDs can. In this article, LEDs and LDs for future SSL are explored by comparing: their current state-of-the-art input-power-density-dependent power-conversion efficiencies; potential improvements both in their peak power-conversion efficiencies and in the input power densities at which those efficiencies peak; and their economics for practical SSL.

Comparison between blue lasers and light-emitting diodes for future solid-state lighting

Lasing in pure green region around 520 nm of InGaN based laser diodes (LDs) on semi-polar {2021} free-standing GaN substrates was demonstrated under pulsed operation at room temperature. The longest lasing wavelength reached to 531 nm and typical threshold current density was 8.2 kA/cm2 for 520 nm LDs. Utilization of a novel {2021} plane enabled a fabrication of homogeneous InGaN quantum wells (QWs) even at high In composition, which is exhibited with narrower spectral widths of spontaneous emission from LDs than those on other planes. The high quality InGaN QWs on the {2021} plane advanced the realization of the green LDs.

https://iopscience.iop.org/article/10.1143/APEX.2.082101/pdf

531 nm Green Lasing of InGaN Based Laser Diodes on Semi-Polar {202̄1} Free-Standing GaN Substrates

Room-temperature continuous-wave operation of 520 nm InGaN-based green laser diodes on semi-polar {2021} GaN substrates was demonstrated. A threshold current of 95 mA corresponding to a threshold current density of 7.9 kA/cm2 and a threshold voltage of 9.4 V were achieved by improving the quality of epitaxial layers on {2021} GaN substrates using lattice-matched quaternary InAlGaN cladding layers and also by adopting a ridge-waveguide laser structure.

https://iopscience.iop.org/article/10.1143/APEX.2.092101/pdf

Continuous-Wave Operation of 520 nm Green InGaN-Based Laser Diodes on Semi-Polar {2021} GaN Substrates

We succeeded in developing InGaN-based green laser diodes (LDs) with a wavelength of 515 nm under continuous-wave (cw) operation by improving the growth condition of epitaxial layers and structures of LDs. The LD structures were grown on conventional c-plane free-standing GaN substrates by metal organic chemical vapor deposition (MOCVD). The threshold current and threshold voltage at 515 nm were 53 mA and 5.2 V, respectively. The lifetime of 510–513 nm LDs was estimated to be over 5000 h under cw operation with an optical output power of 5 mW at 25 °C.

https://iopscience.iop.org/article/10.1143/APEX.2.062201/pdf

510–515 nm InGaN-Based Green Laser Diodes on c-Plane GaN Substrate

Based on recent improvements of growth of In-rich InGaN quantum wells with low defect density, we demonstrate current driven InGaN laser diodes at wavelengths as long as 500 nm. The laser structures are grown on c-plane GaN substrate and are processed as broad oxide-insulated stripe laser diodes. We discuss the impact of the piezoelectric field on the emission energy of long wavelength laser diodes for this growth orientation. The combination of low threshold current density of 8.2 kA/cm2 with high slope efficiency of 650 mW/A enables high output powers up to several tens of milliwatts.

500 nm electrically driven InGaN based laser diodes

We demonstrate direct green laser operation from InGaN based diodes at wavelengths as long as 515.9 nm with 50 mW output power in pulse operation. A factor of ∼10 defect reduction for the In-rich InGaN quantum wells based on improvements of the epitaxial growth process and design of the active layers on c-plane GaN-substrates makes it possible to demonstrate laser operation at room temperature. Micrometer-scale photoluminescence mappings and electro-optical measurements confirm the reduction of nonradiative defects in the emitting layers. The 11 μm broad-area gain-guided laser structures were driven in pulse operation to minimize thermal effects and to accurately measure the laser temperature dependence. The threshold current density was ∼9 kA/cm2 and the fitted slope efficiency had a value of ∼130 mW/A for an optical output up to 50 mW.

InGaN laser diodes with 50 mW output power emitting at 515 nm

Quality and thermal stability of thin InGaN films

The challenges of green InGaN lasers are discussed concerning material quality as a function of InGaN composition, quantum well design and piezoelectrical fields Investigations of polar quantum well designs and comparison with simulated non-polar structures demonstrate that the quality of the indium rich layers is more important than the influence of interface charges A high risk of dark spots at high In concentrations of 26–33% is observed Small changes of about 2% of In significant reduce or increase the quantity and size of dark luminescence areas Polar designs are a trade-off between low indium concentrations of 4 nm wide quantum wells and high overlap of electrons and holes in 2 nm narrow designs Furthermore, our single quantum wells have less non-radiative defects than indium rich multi-quantum well structures Optimized active layer designs and the material qualities enable us to get green InGaN lasers on c-plane substrates for cw operation at 515–524 nm and wall plug efficiencies of 39–23% Slope efficiency of 03–04 W/A allows up to now highest optical output power of 50 mW

Pros and cons of green InGaN laser on c-plane GaN

In this paper we investigate the waveguiding (WG) of direct green InGaN laser diodes grown on c-plane GaN substrates. The problem of parasitic modes emerges due to the reduced refractive index difference between the GaN waveguide and AlGaN cladding layers for green compared to blue emitting laser diodes. We discuss several approaches to avoid substrate modes. We investigate different materials and designs for optimized WG of green InGaN laser diodes using a 1D transfer matrix simulation tool.

Désirée Queren

Papers

500 nm electrically driven InGaN based laser diodes

InGaN laser diodes with 50 mW output power emitting at 515 nm

Quality and thermal stability of thin InGaN films

Pros and cons of green InGaN laser on c-plane GaN

Waveguide design of green InGaN laser diodes