InGaN-based Laser Research Articles

Analysis of Diffusion-Related Gradual Degradation of InGaN-Based Laser Diodes

This report reveals that diffusion of hydrogen induces gradual degradation in InGaN-based laser diodes (LDs). The increase in nonradiative recombination centers (NRCs) in the LDs has been attributed to diffusion-related phenomena. Factors other than NRCs, such as the threshold carrier density Nth, can increase threshold current Ith. Those factors, however, were not fully investigated. Moreover, the diffusant responsible for the degradation of the LDs has not been univocally identified yet. To separately evaluate the roles of NRCs and Nth in increasing Ith, this report analyzes the stress-induced variation of nonradiative recombination lifetime τnr and lasing wavelength λl. It is revealed that the density of NRCs increases at the first stage of gradual degradation, followed by a rise in Nth. In addition, this report proposes a novel model for the time-variation of 1/τnr to investigate the diffusion-related degradation. By using this model, we extrapolate the value of the diffusion coefficient of diffusants involved in the degradation in InGaN-based LDs. The proposed analysis methods and obtained results are useful for understanding the physics of LD degradation.

Numerical analysis of indirect Auger transitions in InGaN

Indirect phonon-assisted Auger recombination mechanisms in bulk InGaN are investigated in the framework of perturbation theory, using first-principles phonon spectral density functions and electronic structures obtained by nonlocal empirical pseudopotential calculations. Nonpolar carrier-phonon interactions are treated within the rigid pseudoion framework, thus avoiding the introduction of empirical deformation potentials. The calculated indirect Auger coefficients exhibit a weak temperature dependence and dominate over direct processes for alloy compositions corresponding to the entire visible spectrum. The present results suggest that indirect Auger processes may be relevant in the operation of InGaN-based light-emitting diodes and lasers, at least in the yellow-green spectral region.

Investigation of the radiative efficiency and threshold in InGaN laser diodes under the influence of efficiency droop

Based on the rate equation model of semiconductor lasers, the radiative efficiency and threshold current density of InGaN-based blue laser diodes (LDs) are theoretically investigated, including the effect of efficiency droop in the InGaN quantum wells. The peak point of the radiative efficiency versus current density relation is used as the parameter of the rate equation analysis. The threshold current density of InGaN blue LDs is found to depend strongly on the maximum radiative efficiency at low current density, implying that improving the maximum efficiency is important to maintain a high radiative efficiency at a large current density and to achieve a low-threshold lasing action under the influence of efficiency droop.

Improvement of hole injection and electron overflow by a tapered AlGaN electron blocking layer in InGaN-based blue laser diodes

We studied the influence of a tapered AlGaN electron blocking layer (EBL) with step-graded aluminum composition on hole injection and electron overflow effects in InGaN-based laser diodes (LDs) theoretically. Schrödinger-Poisson self-consistent method together with transfer matrix method was applied to calculate carrier distribution and transport properties for both electrons and holes in tapered EBL and conventional EBL. The results indicate that the new structure favors the tunneling of low energy holes from the p-side to the active region. Meanwhile, more uniform carrier distribution and better balance between electrons and holes are obtained for the tapered structure by proper modification of band diagrams. An advanced device simulation shows the elimination of electron overflow even at a current of 180 mA in the LD with tapered EBL. Decrease of threshold current density from 2.0 kA/cm2 to 1.6 kA/cm2 is benefited from the more uniform local gain profile.

Catastrophic-Optical-Damage-Free InGaN Laser Diodes With Epitaxially Formed Window Structure

High-power InGaN-based laser diodes (LDs) are expected as light-sources of various high-power applications. The catastrophic-optical-damage (COD) is a major failure mechanism to limit the output power, where optical absorption at a light-emitting facet causes an irreversible damage to the LDs. In this paper, a window structure in InGaN-based LDs to suppress the COD is demonstrated for the first time. The structure is formed by the metal organic chemical vapor deposition growth over a recess placed besides of the ridge-waveguide. The In composition in the quantum well which is corresponding to the bandgap energy is reduced by controlling the local tilt angle at the sidewall of the recess. Thus, the formed window structure eliminates undesired optical absorption at the cleaved facet so that extremely high light output power over 2 W in narrow-stripe ridge-waveguide InGaN-based blue-violet LDs is achieved without any CODs.

Investigation of the deep level involved in InGaN laser degradation by deep level transient spectroscopy

This paper reports an extensive analysis of the properties of the deep level responsible for the degradation of InGaN-based laser diodes. The analysis is based on combined optical measurements and Deep-Level Transient Spectroscopy (DLTS) investigation. Results indicate that stress induces a significant increase in threshold current of the devices, which is strongly correlated to the increase in the concentration of a deep level (DL) detected by DLTS. The DL involved in the degradation process is located 0.35–0.45 eV below the conduction band. 2D simulation indicates that degradation occurs within the quantum-well region.

Degradation of InGaN lasers: Role of non-radiative recombination and injection efficiency

With this paper we propose a detailed study of the gradual degradation of InGaN-based laser diodes and Light-Emitting Diodes submitted to electro-thermal stress. The two device structures have been processed from the same epitaxial wafer. Our purpose is to compare the behavior of the two devices by means of electro-optical measurements, electroluminescence characterization, and near field emission measurements. We demonstrate that: (i) stress induces a decrease in the optical power and an increase in threshold current; (ii) the two failure modes are strictly linked each other, and correlated to the increase in defect-related current components, indicating a substantial increase in defect density during degradation; (iii) analysis of characteristic temperature and near-field emission measurements do not indicate any strong variation of injection efficiency nor current confinement of the devices.

Optical bistability in InGaN-based multisection laser diodes

Optical bistability is observed in cw-operating InGaN-based laser diodes including a saturable absorber (SA) section. The dependence of the light-current hysteresis on the SA section length and reverse bias (VSA) has been studied. An analytical approach is developed to estimate the carrier lifetime τa in the SA section from the measurements of the hysteresis width, which leads to τa=1.9 ns at zero bias. τa is found to decrease rapidly for higher reverse biases and a minimum of τa=0.4 ns is interpolated for flatband conditions. We explain the dependence of the carrier lifetime on VSA via the modification of the quantum-confined Stark effect.

Effects of a step-graded AlxGa1−xN electron blocking layer in InGaN-based laser diodes

A step-graded AlxGa1-xN electron blocking layer (EBL) is studied on InGaN-based laser diodes (LDs). Its efficacy on device performance is investigated with respect to stimulated emission properties, internal quantum efficiency, internal loss, and temperature-dependent characteristics. When compared to a LD structure with an abrupt Al0.18Ga0.82N EBL design, the LD with the step-graded AlxGa1-xN EBL design demonstrates lower threshold current density and higher slope efficiency. The threshold current density is reduced from 4.6 kA/cm2 to 2.5 kA/cm2 under pulsed-current operation and the corresponding slope efficiency is increased from 0.72 W/A to 1.03 W/A. The insertion of the step-graded AlxGa1−xN EBL leads to a dramatic enhancement in internal quantum efficiency from 0.60 to 0.92, while internal loss keeps at 9 ∼ 10 cm−1. The temperature-dependent measurement also shows that the step-graded AlxGa1−xN EBL can improve the thermal stability with reduced red-shift from 0.05 nm/K to 0.034 nm/K. This simple yet efficient structural design change provides an effective way to achieve high-performance InGaN-based LDs with higher optical-output power and lower electric-power consumption.

III-Nitride Optoelectronic Devices: From Ultraviolet Toward Terahertz

We review Ill-Nitride optoelectronic device technologies with an emphasis on recent breakthroughs. We start with a brief summary of historical accomplishments and then report the state of the art in three key spectral regimes as follows: 1) Ultraviolet (AIGaN-based avalanche photodiodes, single photon detectors, focal plane arrays, and light emitting diodes); 2) Visible (InGaN-based solid state lighting, lasers, and solar cells); and 3) Near-, mid-infrared, and terahertz (AIGaN/GaN-based gap-engineered intersubband devices). We also describe future trends in Ill-Nitride optoelectronic devices.

Degradation of InGaN-based laser diodes analyzed by means of electrical and optical measurements

In this paper we present a detailed analysis of the degradation of InGaN-based laser diodes carried out by means of electrical and optical techniques. The study is based on the comparison between the degradation kinetics of laser diodes and light-emitting diode (LED)-like samples, i.e., devices with the same epitaxial structure as the lasers, but with no ridge and facets. Results described in the following indicate that degradation of lasers and LED-like samples is due to the same mechanism, possibly involving the generation of point defects within the active region of the devices. Furthermore, since degradation occurs both in lasers and in LED-like samples (i.e., structures with no current confinement), results suggest that degradation of lasers is not correlated with the geometry of the devices, nor to worsening of current confinement under the ridge.

High-brightness Phosphor-conversion White Light Source Using InGaN Blue Laser Diode

A phosphor-conversion white light source is demonstrated using an InGaN-based blue laser diode (LD) and a yellow-emitting phosphor excited by the blue LD. The photometric and colorimetric properties of this blue-LD-based white light source are characterized. When injection current of the LD is 100 mA, luminous flux and luminous efficiency of the white light are found to be over 5 lm and 10 lm/W, respectively. When injection current is >90 mA, luminance is estimated to be larger than 10 Mcd/<TEX>$cm^2$</TEX>. In addition, color characteristics of the white light such as chromaticity coordinates, a correlated color temperature, and a color rendering index are found to be quite stable as current and temperature of the LD varies. The demonstrated LD-based white light source is expected to be used in high-brightness illumination applications with good color stability.

Room-Temperature Continuous-Wave Operation of InGaN-Based Blue-Violet Laser Diodes with a Lifetime of 15.6 Hours

We report our recent progress of investigations on InGaN-based blue-violet laser diodes (LDs). The room-temperature (RT) cw operation lifetime of LDs has extended to longer than 15.6 h. The LD structure was grown on a c-plane free-standing (FS) GaN substrate by metal organic chemical vapor deposition (MOCVD). The typical threshold current and voltage of LD under RT cw operation are 78 mA and 6.8 V, respectively. The experimental analysis of degradation of LD performances suggests that after aging treatment, the increase of series resistance and threshold current can be mainly attributed to the deterioration of p-type ohmic contact and the decrease of internal quantum efficiency of multiple quantum well (MQW), respectively.

Emission characteristics of GaN-based blue lasers including a lattice matched Al0.83In0.17N optical blocking layer for improved optical beam quality

We demonstrate room-temperature continuous-wave operation of 425 nm InGaN-based blue laser diodes including a thin Al0.83In0.17N optical blocking layer. Structures are grown on c-plane GaN freestanding substrates with a lattice-matched AlInN layer positioned below an Al0.07Ga0.93N bottom cladding. Such devices are compared to standard InGaN based lasers looking at threshold current density, electrical characteristics, and far-field emission. Threshold current densities of 4 kA/cm2 and slope efficiencies of ∼0.6 W/A (for uncoated facets) have been achieved in AlInN containing devices. Lower mode leakage in the substrate is highlighted by a better transversal far-field pattern resulting in an improved beam quality.

InGaN-based true green laser diodes on novel semi-polar [formula omitted] GaN substrates

The crystal quality and emission characteristics of InGaN-based laser diodes (LDs) with lattice-matched quaternary InAlGaN cladding layers on novel semi-polar { 2 0 2 ¯ 1 } plane GaN substrates were investigated. Highly homogeneous InGaN quantum wells (QWs) with a suppressed piezoelectric field can be realized on the { 2 0 2 ¯ 1 } plane even in the green spectral region. Optical polarization measurements revealed that [ 1 ¯ 0 1 4 ] oriented stripes are advantageous for LDs on { 2 0 2 ¯ 1 } planes. Gain-guided LDs on the novel { 2 0 2 ¯ 1 } plane exhibited the longest lasing wavelength at 533.6 nm under pulsed operation. Continuous-wave (cw) operation at the lasing wavelength of 523.3 nm at 115 mA was also demonstrated. The I th , J th , V th , and the slope efficiency were 110 mA, 9.2 kA/cm 2, 7.4 V, and 0.04 W/A, respectively. These results indicate that the green LDs on semi-polar { 2 0 2 ¯ 1 } plane GaN substrates are promising candidates for laser display applications.

Structural Defects and Degradation Phenomena in High-Power Pure-Blue InGaN-Based Laser Diodes

To improve the lifetime of high-power pure-blue InGaN-based laser diodes, the need to reduce the number of newly created structural defects in active regions, consisting of multiple quantum well structures, is inevitable. We first report on detailed structural analyses of these new types of defects and discuss their formation mechanisms and reduction methodologies. We then fabricated laser diodes with current-injection free regions near the laser facets and confirm that this is an effective method for the suppression of degradation by catastrophic optical damage. Based on the analyses of aged devices by using fluorescence microscopy, we also discuss the degradation mechanisms of GaN-based laser diodes.

Expansion of emission band based on energy transfer in diode-pumped microcavity dye lasers

We demonstrate energy-transfer microcavity dye lasers pumped by an InGaN-based violet laser diode. We obtain the oscillation wavelength range from 500 to 610 nm using a combination of Coumarin dyes as a donor and Pyrromethene and Rhodamine dyes as acceptors. The operational lifetime of the mixed Coumarin 540A (C540A)/Pyrromethene 567 laser was improved by 14 times compared with the C540A laser. We dope a singlet oxygen quencher into the mixed C540A/Rhodamine 6 G laser, resulting in an improvement in the operational lifetime by 10 times compared with the laser without the quencher.

Reliability evaluation for Blu-Ray laser diodes

With this paper we describe an extensive analysis of the reliability of InGaN-based laser diodes, emitting at 405 nm. These devices have excellent characteristics for application in the next-generation optical data storage systems. The analysis aims at describing the degradation process, as well as at investigating the role of current in determining the degradation rate. The results obtained within this paper suggest that the degradation of the laser diodes is correlated to the increase in the non-radiative recombination rate, with subsequent worsening of the optical properties of the devices. Furthermore, our findings support the hypothesis that current is the main driving force for degradation, while temperature and optical power play only a limited role in determining the degradation kinetics.

Optical waveguide simulations for the optimization of InGaN-based green laser diodes

Two-dimensional optical waveguide mode simulations have been employed to investigate the optimized device structures for ridge-waveguide (Al, In, Ga) N-based green (520nm) laser diodes (LDs). The effects of thicknesses, alloy compositions, and doping densities of each epitaxially grown layers as well as ridge geometries on optical confinement factors (Γ) and waveguide absorption (α) were comprehensively surveyed. InyGa1−yN (y=0.07–0.1) guiding layers (GLs) with thickness more than 50nm were effective for realizing high Γ and low α. To minimize the absorption by the anode metal, p-cladding layer (p-CL) was required to be more than 500nm. At the same time, low index insulator such as SiO2 was preferable for the narrow ridge, where the thickness at the sidewall had to be more than 60nm. We also found that InGaN barriers layers between the quantum wells (QWs) were superior to GaN barriers to increase Γ and reduce α. Moreover, a thicker last barrier between the topmost QW and the electron blocking layer was also effective to reduce α. Regarding the effect of Mg doping concentration on the absorption, the reduction in Mg in the p-CL and the p-GL was significant to reduce α. Generally, it was confirmed the design for typical 405nm LDs can be applied for 520nm LD with the inclusion of InGaN GLs and barriers for the QWs.

Laser diodes go green

Researchers at Nichia Corporation have demonstrated green InGaN-based lasers grown on c-plane sapphire, with lifetimes capable of supporting commercial applications.