Reduction Of Threshold Current Density Research Articles

Threshold current density reduction of strained AlInGaAs quantum-well laser

In the last decades, researchers have tried to implement novel optoelectronic devices with new semiconductor material compounds. There are few competitors in the race for more reliable, more efficient, pump sources for solid-state lasers. These diodes should operate at high power, intense brightness, and with low threshold current. The strained AlInGaAs-GaAs quantum-well (QW) laser is a promising candidate. In this study we optimized the growth parameters of strained AlInGaAs quantum wells using a model for linewidth broadening of photoluminescence, which was extended for the first time to handle quaternary alloys. This model enables us to identify the dominant contributions to the broadening. As a result of our growth parameters optimization technique, low threshold current density of simple broad-area lasers has been obtained, indicating a superior material quality. Moreover, we have studied for the first time the effect of indium and aluminum content and QW width on the threshold current density of quaternary AlInGaAs QW lasers. As a result of these studies the lowest known threshold current density for AlInGaAs on GaAs single QW broad-area laser has been achieved.

III-N based short-wavelength LEDs, LUCO-LEDs, and lasers

Results are presented on the effect of using GaN templates with homogeneously reduced defect density on the performance of violet- und UV-emitting (AlGaIn)N LEDs, as well as on the use of such LED chips as pump light sources in three-phosphor white luminescence conversion LEDs (LUCO-LEDs). For LED chips emitting at 385 nm, an improvement in electroluminescence efficiency by a factor of two was found for growth on 8 x 10 7 cm -2 defect density templates compared to direct growth on sapphire. Further we report on the fabrication and on-wafer testing of violet-emitting ridge waveguide (AlGaIn)N quantum well diode lasers with etched laser facets, grown on sapphire using conventional low-temperature GaN nucleation layer technology as well as on the above low defect density GaN templates. In on-wafer pulsed-mode operation a 35% reduction in threshold current density was achieved for the latter, resulting in a minimum injected power at threshold of 0.9 W for 500 μm x 2 μm ridge laser diodes with uncoated facets.

Comparative study of [formula omitted] and [formula omitted] InGaAs single quantum well laser diodes

In this work, a series of identical In x Ga 1− x As/Al y Ga 1− y As single quantum well laser diodes, grown on (1 0 0) and (1 1 1) B GaAs substrates, have been thoroughly studied. For all samples, clear evidence of reduced threshold current densities in the (1 1 1) B substrate has been observed in electroluminescence spectra at 17 and 300 K . Modelling of the devices, based on a self-consistent solution of Schrödinger–Poisson's equations, was utilised in order to reproduce the experimental results. The model incorporates strain and piezoelectric effects on the quantum well states, free carrier screening, overlap integral computation, and optical gain calculation. The underlying mechanism, that explains the threshold reduction observed in the (1 1 1) B laser diodes, is discussed based on the results of the modelling.

Comparative study of (1 0 0) and (1 1 1)B InGaAs single quantum well laser diodes

In this work, a series of identical In x Ga 1− x As/Al y Ga 1− y As single quantum well laser diodes, grown on (1 0 0) and (1 1 1) B GaAs substrates, have been thoroughly studied. For all samples, clear evidence of reduced threshold current densities in the (1 1 1) B substrate has been observed in electroluminescence spectra at 17 and 300 K . Modelling of the devices, based on a self-consistent solution of Schrödinger–Poisson's equations, was utilised in order to reproduce the experimental results. The model incorporates strain and piezoelectric effects on the quantum well states, free carrier screening, overlap integral computation, and optical gain calculation. The underlying mechanism, that explains the threshold reduction observed in the (1 1 1) B laser diodes, is discussed based on the results of the modelling.

High-performance 1.5 [micro sign]m GaInNAsSb lasers grown on GaAs

Substantially reduced threshold current density and improved efficiency in long-wavelength (>1.4 µm) GaAs-based lasers are reported. A 20×1220 µm as-cleaved device showed a room temperature continuous-wave threshold current density of 580 A/cm2, external efficiency of 53%, and 200 mW peak output power at 1.5 µm. The pulsed threshold current density was 450 A/cm2 with 1145 mW peak output power.

Semiconductor quantum dots - A new class of gain materials for advanced optoelectronics

Since the invention of semiconductor lasers, huge improvements in device performance have been achieved, and a large variety of specialized designs for different applications were conceived. Two major steps have played a key role in the improvement of device properties. The first step was the application of semiconductor heterostructures that allowed the separate optimization of optical and carrier confinement. The second step was the introduction of quantum films, also called quantum wells, in the carrier recombination zone (started in the 1980s). This permitted a strong reduction of threshold current density due to an increased density of states at the laser energy. This effect of increased density of states is related to the partial discretization of the allowed energy states of carriers, i.e., electrons and holes, and is based on quantum mechanical principles. One major advantage of quantum-dot structures results from the full three-dimensional carrier confinement on a nanometer scale. Therefore, a semiconductor quantum dots, InAs dots embedded in GaAs, behave like non- or weakly interacting single atoms. In addition, the realization of device-quality quantum dot structures became possible by the introduction of self-organized growth. Both, molecular beam epitaxy (MBE) and metal organic vapor phase epitaxy (MOVPE) techniques, which are capable of the controlled deposition of a fraction of an atomic monolayer, can be used.

Improved large optical cavity design for [formula omitted] (Al)GaAs quantum cascade lasers

We have developed an improved waveguide design for 10.6 μm (Al)GaAs quantum cascade lasers based on a large optical cavity. With the optical confinement based on the free-carrier plasma effect we accurately evaluate the complex dielectric constant as a function of doping densities of the GaAs waveguide layers. Free-carrier absorption losses and confinement factors are simulated and compared with experimental results. An improved design was obtained by varying the doping profile of the large optical cavity resulting in a 30% reduced threshold current density of 3.4 kA cm −2 at 77 K heatsink temperature.

Morphological and optical properties of InGaN laser diodes on laterally overgrown GaN

The morphological evolution of InGaN laser diodes on laterally overgrown GaN on sapphire was investigated to understand the effect of threading dislocations (TDs) on the morphology and subsequent device performance. Screw-component TDs were found to strongly influence the surface morphology and growth mechanism of InGaN-based laser diodes. Spiral growth about mixed character TDs (Burgers vector=a+c) resulted in growth hillocks for laser structures grown directly on sapphire as well as laser structures grown on lateral epitaxially overgrown (LEO) GaN on sapphire. The dislocation distribution dominated the size of growth hillocks. Small spirals, with typical heights of 15 Å and diameters of 1.5 μm, were formed on laser structures grown on sapphire due to the uniformly distributed mixed character TDs, but large spiral hillocks, with heights of 65 Å and diameters up to 30 μm, developed on the LEO GaN due to the larger spacing between mixed character TDs. The spirals exhibited steps oriented in crystallographic directions, showing the six-fold symmetry of the hexagonal crystal. Spirals that formed during the growth of the active region did not diminish with subsequent high-temperature growth. Even with larger spirals, the lasers on LEO GaN exhibited reduced threshold current densities as compared to lasers grown on sapphire.

Scaling behavior of bipolar cascade VCSELs

We present theoretical and experimental results on the scaling properties of bipolar cascade vertical-cavity surface-emitting lasers with respect to active region stacking and out-coupling mirror reflectivity. The measurement results at around 980-nm wavelength confirm the theoretically expected behavior of an almost linear increase in slope efficiency and a reduction in threshold current density with the number of stages. A three-stage laser shows a differential quantum efficiency of 128% in continuous-wave operation at room temperature. For the threshold current density, we obtain a decrease from 3.75 kA/cm/sup 2/ for the conventional VCSEL to about 1.2 kA/cm/sup 2/ for a three-stage device. Such multi-diode cascade VCSELs are highly interesting for optical links, analog systems, and high-power applications.

Low threshold high efficiency MBE grown GaInAs/(Al)GaAs quantum dot lasers emitting at 980 nm

Using solid source molecular beam epitaxy, quantum dot lasers with a single active layer have been fabricated. The influence of the waveguide design on basic device properties has been investigated. Through the use of a large vertical cavity a significant reduction in threshold current density could be achieved. Slope efficiencies of more than 1 W/A and absorption levels as low as 2.6 cm −1 could be realized. First high power devices with wall-plug efficiencies of up to 50% were fabricated, demonstrating the high quality of these quantum dot lasers.

Strained layer (111)B GaAs/InGaAs single quantum well lasers and the dependence of their characteristics upon indium composition

We have studied a series of (111) oriented GaAs–InGaAs single quantum well, broad area lasers with active regions containing a range of indium concentrations, x, in order to understand the advantages and limitations of pseudomorphic strain. For x⩽0.3, both an increased emission wavelength and reduced threshold current were observed with increasing x. The predominant cause of the wavelength increase is the reduction in bulk InGaAs band gap. The reduction in threshold current is attributed mainly to the reduced in-plane density of states caused by the strain induced lifting of the heavy and light hole degeneracy at the valence band edge. For x>0.3, we see a marked deterioration in laser performance. However, we believe that this deterioration is not directly associated with strain relaxation at layer thicknesses beyond the critical value. Rather, imperfections in the AlGaAs cladding layers appear to seed the formation of dislocations within the strained regions. Within the limitation of strain relaxation, we observed monomode continuous wave emission at room temperature at wavelengths up to 1.072 μm and with threshold current densities as low as 74 A/cm2. The differential gain of 1.45×10−15 cm2 is around four times higher than measured on unstrained GaAs/AlGaAs single quantum well lasers. Like the reduction in threshold current density, this relatively high value is attributed to the strain induced reduction in the in-plane, heavy hole effective mass.

Optical, electrical and thermal analysis for GaN semiconductor lasers

AbstractDevice characteristics of GaN‐based semiconductor lasers have been analysed by numerical calculation. Anti‐guide‐like behaviour of the waveguide mode appears in InGaN laser structures where the cladding layers have insufficient thickness. Such phenomena are peculiar to laser structures with GaN material systems. The layer structure should be designed so as to suppress the anti‐guide mode. Carrier overflow is also a significant problem for GaN lasers. A high acceptor concentration for the p‐type cladding layer and the reduction of the active layer volume have a large effect on the reduction of such carrier overflow. In regard to the temperature characteristics, one of the restricting factors is the use of sapphire substrates which have relatively high thermal resistivity. The maximum operation temperature has been calculated based on the analysis for the thermal resistance of the device. It was shown that reduction in the stripe width as well as reduction in threshold current density and operation voltage is very important for improving the temperature characteristics of GaN semiconductor lasers. Copyright © 2001 John Wiley & Sons, Ltd.

Improved characteristics of InGaN multiple-quantum-well laser diodes grown on laterally epitaxially overgrown GaN on sapphire

InGaN multiple-quantum-well laser diodes have been fabricated on fully coalesced laterally epitaxially overgrown (LEO) GaN on sapphire. The laterally overgrown “wing” regions as well as the coalescence fronts contained few or no threading dislocations. Laser diodes fabricated on these low-dislocation-density regions showed a reduction in threshold current density from 10 to 4.8 kA/cm2 compared to those on conventional planar GaN on sapphire. The internal quantum efficiency also improved from 3% for laser diodes on conventional GaN on sapphire to 22% for laser diodes on LEO GaN on sapphire.

Improved Characteristics of InGaN Multi-Quantum-Well Laser Diodes Grown on Laterally Epitaxially Overgrown GaN on Sapphire

InGaN multi-quantum-well laser diodes have been fabricated on fully-coalesced laterally epitaxially overgrown (LEO) GaN on sapphire. The laterally overgrown ‘wing’ regions as well as the coalescence fronts contained few or no threading dislocations. Laser diodes fabricated on the low-dislocation-density wing regions showed a reduction in threshold current density from 8 kA/cm2 to 3.7 kA/cm2 compared the those on the high-dislocation ‘window’ regions. Laser diodes also showed a two-fold reduction in threshold current density when comparing those on the wing regions to those fabricated on conventional planar GaN on sapphire. The internal quantum efficiency also improved from 3% for laser diodes on conventional GaN on sapphire to 22% for laser diodes on LEO GaN on sapphire.

Annealing of the deep recombination center in GaInP/AlGaInP quantum wells grown by solid-source molecular beam epitaxy

Time resolved photoluminescence and deep level transient spectroscopy have been used to monitor the effect of rapid thermal annealing on bulk GaInP and GaInP/AlGaInP quantum wells grown by solid source molecular beam epitaxy similar to those used in 650 nm range lasers. Following rapid thermal annealing at temperatures up to 875 °C, reductions in the concentration of several deep level traps are observed. Correlation of these data with photoluminescent intensity and lifetime measurements indicate that the defect labeled N3, 0.83 eV below the conduction band, is the dominant recombination center. The combination of these two transient spectroscopy measurement techniques is therefore not only able to measure the change in deep level concentration, but also to correlate this change with improved carrier lifetimes and, ultimately, reduced threshold current densities in quantum well lasers. There is also evidence to suggest that this same defect, possibly a phosphorous vacancy or a related complex, plays an important role in other GaInP based devices.

Molecular beam epitaxy (MBE) growth of composite (In,Al)As/(In,Ga)As vertically coupled quantum dots and their application in injection lasers

Injection lasers based on self-organized (In,Ga)As/(Al,Ga)As quantum dots (QD) suffer from the gain saturation due to the limited amount of QD states participating in lasing. In the present work, we demonstrate the direct increase in the areal density of (In,Ga)As QDs. We used an array of (In,Al)As QDs demonstrating considerably higher density than Al-free QDs as nucleation centers for the (In,Ga)As QD formation. Finally, composite vertically coupled (In,Al)As/(In,Ga)As QDs with increased areal density are formed, which is confirmed by photoluminescence and TEM. Using the denser array of (In,Al)As/(In,Ga)As QDs in the active region of injection laser leads to the increase in modal gain, reduction in threshold current density at high mirror loss, and increase in maximum output power.

Thermal Analysis for GaN Laser Diodes

Improvement in temperature characteristics of GaN laser diodes is important for realizing reliable devices operating at high temperatures. Thermal characteristics of GaN lasers have been analyzed by using a three-dimensional thermal conduction model. Maximum operation temperature has also been calculated. It was shown that the reduction in the stripe width as well as the reduction in threshold current density and operation voltage is very important for improving the temperature characteristics of GaN laser diodes.

Improved Characteristics of InGaN Multi-Quantum-Well Laser Diodes Grown on Laterally Epitaxially Overgrown GaN on Sapphire

AbstractInGaN multi-quantum-well laser diodes have been fabricated on fully-coalesced laterally epitaxially overgrown (LEO) GaN on sapphire. The laterally overgrown ‘wing’ regions as well as the coalescence fronts contained few or no threading dislocations. Laser diodes fabricated on the low-dislocation-density wing regions showed a reduction in threshold current density from 8 kA/cm2 to 3.7 kA/cm2 compared the those on the high-dislocation ‘window’ regions. Laser diodes also showed a twofold reduction in threshold current density when comparing those on the wing regions to those fabricated on conventional planar GaN on sapphire. The internal quantum efficiency also improved from 3% for laser diodes on conventional GaN on sapphire to 22% for laser diodes on LEO GaN on sapphire.

Reduced threshold current densities of (GaIn)(NAs)/GaAs single quantum well lasers for emission wavelengths in the range 1.28 – 1.38 [micro sign

(GaIn)(NAs)/GaAs single quantum well (SQW) broad area lasers with emission wavelengths at 1.28 and 1.38 µm at room temperature on GaAs substrates have been realised by applying optimised low-temperature metal organic vapour phase epitaxy (MOVPE) using the group V sources 1,1-dimethylhydrazine (UDMHy) in combination with tertiarybutylarsine (TBAs). Record-low threshold current densities of 0.8 and 2.2 kA/cm2, together with high differential efficiencies of 0.18 and 0.16 W/A per facet, are obtained for 800 µm long broad area lasers emitting at 1.28 and 1.38 µm, respectively.

Optimization of p-dopant profiles for GaInAsP MQW laser structures in MOMBE

For industrial device fabrication gaseous dopant sources are preferred in metalorganic growth technologies. Both in metalorganic vapor phase epitaxy (MOVPE) and metalorganic molecular beam epitaxy (MOMBE/CBE) diethylzinc is used. For doping of InP based ridge waveguide laser structures the critical parts are the active region with GaInAsP confinement layers, MQW layers and the p-type region with InP spacer layer, GaInAsP etch stop layer and InP cladding layer. Growing Zn doped InP in between GaInAsP layers leads to a significant Zn diffusion into the adjacent GaInAsP layers. This effect is much more pronounced in MOMBE than in MOVPE. It is demonstrated how this effect is reduced by insertion of additional intermediate layers with appropriate dopant concentration, and by compensating Si co-doping in the spacer and adjacent layers taking advantage of the Fermi level effect. Applying these techniques, reduced threshold current densities and improved high temperature performance can be obtained. The results are discussed by models of dopant diffusion in MOMBE in comparison to MOVPE.