Reduced Threshold Currents Research Articles

Analysis of Threshold Current Behavior for Bulk and Quantum-Well Germanium Laser Structures

We analyze the optical gain of tensile-strained, n-germanium (n-Ge) material taking bandgap narrowing (BGN) for heavily doped Ge into account. Both the direct bandgap and indirect bandgap are narrowed by 60 meV. Our new modeling explains the wide lasing spectrum of 1520-1700 nm in electrically pumped Ge lasers. The calculated materials gain can reach 1000 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> when the injected carrier density is ~mid-10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> with a combination of 0.25% tensile strain and 4.5×10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">19</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> n-type doping. The threshold current density is estimated to be 0.53 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> for an optimized edge-emitting double-heterojunction Ge device, comparable to bulk III-V lasers. We also review current progress on Ge quantum-well (QW) structures. The threshold current density of most Ge QW structures is similar to bulk Ge. Only tensile-strained QWs show reduced threshold currents.

Colossal resistive switching behavior and its physical mechanism of Pt/p-NiO/n-Mg0.6Zn0.4O/Pt thin films

A heterojunction structure of p-NiO/n-Mg0.6Zn0.4O with an aim to tuning or improving the resistive switching properties was fabricated on Pt/TiO2/SiO2/Si substrates by the sol-gel spin-coating technique. The Pt/NiO/Mg0.6Zn0.4O/Pt heterojunction thin-film device shows excellent resistive switching properties, such as a reduced threshold current of 1 μA for device initiation, a small dispersion of reset voltage ranging from 0.54 to 0.62 V, long retention time and a high resistance ratio of high-resistance state to low-resistance state about six orders of magnitude. These results indicate that the resistive switching properties can be greatly improved by constructing the p-NiO/n-Mg0.6Zn0.4O heterojunction for nonvolatile memory applications. The physical mechanism responsible for colossal resistive switching properties of the heterojunction was analyzed based on interfacial defect effect and formation and rapture of conductive filaments.

Temperature and external-cavity tuning of high-power GaAsP laser diode

Wide-range temperature tuning of 733 nm GaAsP laser diode has been combined with tuning by external grating. Temperature tuning (down to 87 K) yields the 40 nm range (from 733 nm to 693 nm) with reduced threshold currents and external efficiencies above 1 W/A. High power up to 2 W is achieved in the full tuning range. Fine tuning with grating allows for narrower linewidth but the tuning range decreases from 9 nm at 293 K to zero at 130 K. The wavelength range covered by temperature tuning fills the gap between GaAsP quantum-well lasers and InGaP/AlGaInP red lasers. High power lasers operating between 690 nm and 730 nm could be useful in medical applications involving photo-dynamic therapy. (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Continuous wave operation of diode lasers at 3.36μm at 12°C

GaSb-based type-I quantum-well diode lasers emitting at 3.36μm at 12°C with 15mW of continuous wave output power are reported. Devices with two or four InGaAsSb compressively strained quantum wells and AlInGaAsSb quinternary barriers were fabricated and characterized. It was shown that increase in the quantum-well number led to improved laser differential gain and reduced threshold current.

Self-organised quantum dots as dislocation filters: the case of GaAs-based lasers on silicon

The use of multiple layers of self-organised InAs quantum dots as a very effective dislocation filter is demonstrated. In0.5Ga0.5As/GaAs quantum dot lasers grown directly on silicon substrates with the InAs quantum dot buffer layer exhibit substantially reduced threshold currents (Jth∼900 A/cm2), compared to devices grown on silicon without the dot buffer layer (Jth≥1500 A/cm2).

Reduced threshold current of a quantum dot laser in a short period superlattice of indirect-band gap

We propose the idea of making quantum dot lasers by embedding direct-band gap quantum dots in a short period superlattice whose band gap is indirect. This technique reduces the threshold current and its temperature dependence. We show that a higher characteristic-temperature T0 can be achieved in a quantum dot laser with indirect GaAs/AlAs superlattice barriers compared to that with direct GaAs barriers.

Continuous-wave InGaN laser diodes on copper and diamond substrates

InGaN-based optoelectronics were integrated with dissimilar substrate materials using a novel thin-film laser lift-off (LLO) process. The LLO process was employed to integrate InGaN-based laser diodes (LDs) with Cu and diamond substrates. Separation of InGaN-based thin-film devices from their typical sapphire growth substrates was accomplished using a pulsed excimer laser in the ultraviolet regime incident through the transparent substrate. Characterization of the LDs before and after the sapphire substrate removal revealed no measurable degradation in device performance. Reduced threshold currents and increased differential quantum efficiences were measured for LDs on Cu due to a 50% reduction of the thermal impedence. Light output for LDs on Cu was two times greater than comparable LDs on sapphire with a maximum output of 100 mW. Increased light output for LDs on diamond was also measured with a maximum output of 80 mW.

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&amp;gt;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.

Continuous-wave InGaN multiple-quantum-well laser diodes on copper substrates

Continuous-wave (cw) indium-gallium nitride multiple-quantum-well laser diodes (LDs) were transferred from sapphire onto copper substrates using a two-step laser lift-off process. Reduced threshold currents and increased differential quantum efficiencies were measured for LDs on Cu due to a 50% reduction of the thermal impedance. Light output for LDs on Cu was three times greater than comparable LDs on sapphire with a maximum output of 30 mW. CW operation was possible up to heatsink temperatures of 90 °C for LDs on Cu.

Selectively etched undercut apertures in AlAsSb-based VCSELs

Apertures were formed in single-growth, AsSb-based, long-wavelength (1.55 /spl mu/m) vertical-cavity surface-emitting lasers by laterally etching the active region. The materials contrast between the AlAsSb-based mirrors and the AlInGaAs-based active region leads to a high selectivity for the etch, allowing long apertures to be formed with minimal etching of the mirrors. Lasers showing reduced threshold currents and increased efficiencies were demonstrated using these apertures.

Integration of InxGa1−xN Laser Diodes with Dissimilar Substrates by Laser Lift-off

ABSTRACTContinuous-wave (cw) indium-gallium nitride (InGaN) multiple-quantum-well (MQW) laser diodes (LDs) were successfully transferred from sapphire onto copper and diamond substrates using a two-step laser lift-off (LLO) process. Reduced threshold currents and increased differential quantum efficiencies were measured for LDs on Cu due to a 50% reduction of the thermal impedance. Light output for LDs on Cu was three times greater than comparable LDs on sapphire with a maximum output of 30 mW. Increased light output for LDs on diamond were also measure with a maximum output of 80 mW.

Edge-emitting quantum well heterostructure laser diodes with auxiliary native-oxide vertical cavity confinement

Data are presented demonstrating edge-emitting laser diode operation of AlyGa1−yAs– GaAs–InxGa1−xAs quantum well heterostructures modified by the formation of a buried native-oxide distributed Bragg reflecting (DBR) mirror adding vertical confinement to the longitudinal laser cavity. The bottom DBR mirror, combined with the highly reflective top p-contact metallization (Ag), forms a thin broadband vertical cavity. The auxiliary vertical mirrors are tuned to improve the coupling of the spontaneous emission to the longitudinal lasing mode, resulting in reduced threshold currents and modified emission characteristics below threshold.

The properties of MOVPE grown 1.3 μm DFB MQW lasers infilled with semi-insulating InP fabricated on semi-insulating substrates

The use of optoelectronic integrated circuits (OEICs) is now emerging as a practical technology for a variety of applications, particularly in advanced telecommunications. OEICs consist of a range of devices such as lasers, waveguides, modulators, amplifiers, transistors, detectors, etc. fabricated on the same substrate. When a semi-insulating substrate is used, these devices can be electrically isolated by channel etching, resulting in a low capacitance structure with reduced electrical interference between the subcomponents. One of the devices which is particularly advantageous for this type of integration scheme is the distributed feedback (DFB) laser. The laser can be made to function more efficiently by minimizing the current flowing outside the active region. This can be achieved by surrounding the active region with semi-insulating iron doped InP. This work describes for the first time, the MOVPE growth, fabrication, and device characterization of 1.3 um buried heterostructure DFB MQW lasers, which combine the advantages of using both a semi-insulating substrate and a semi-insulating infill region in the same device structure. The potential advantage of this design scheme is improved OEIC performance as a result of, reduced capacitance and electrical crosstalk, enhanced laser output power, higher speed, increased efficiency, wider operating temperature and reduced threshold current. The laser active region consists of 8 x 140 A quantum wells of GalnAsP (λ = 1.3 μm) and 110 Abarriers of GalnAsP (λ= 1.07 μm). Single mode 1.3 urn devices of length 250 μm operating at room temperature produced threshold currents of 8 mA, efficiencies of up to 25%, output powers of 18 mW at 80 mA (pulsed), and a frequency response greater than 12GHz. The parasitic capacitance was estimated to be less than 3 pF.

High-performance λ=1.3 μm InGaAsP-InP strained-layer quantum well lasers

Compressively and tensile strained InGaAsP-InP MQW Fabry-Perot and distributed feedback lasers emitting at 1.3-/spl mu/m wavelength are reported. For both signs of the strain, improved device performance over bulk InGaAsP and lattice-matched InGaAsP-InP MQW lasers was observed. Tensile strained MQW lasers show TM polarized emission, and with one facet high reflectivity (HR) coated the threshold currents are 6.4 and 12 mA at 20 and 60/spl deg/C, respectively. At 100/spl deg/C, over 20-mW output power is obtained from 250-/spl mu/m-cavity length lasers, and HR-coated lasers show minimum thresholds as low as 6.8 mA. Compressively strained InGaAsP-InP MQW lasers show improved differential efficiencies, CW threshold currents as low as 1.3 and 2.5 mA for HR-coated single- and multiple quantum well active layers, respectively, and record CW output powers as high as 380 mW for HR-AR coated devices. For both signs of the strain, strain-compensation applied by oppositely strained barrier and separate confinement layers, results in higher intensity, narrower-linewidth photoluminescence emissions, and reduced threshold currents. Furthermore, the strain compensation is shown to be effective for improving the reliability of strained MQW structures with the quantum wells grown near the critical thickness. Linewidth enhancement factors as low as 2 at the lasing wavelength were measured for both types of strain. Distributed feedback lasers employing either compressively or tensile strained InGaAsP-InP MQW active layers both emit single-mode output powers of over 80 mW and show narrow linewidths of 500 kHz.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Improvement of GaAs/AlGaAs quantum well laser diodes by rapid thermal annealing

Post-growth annealing is shown to improve the laser diode quality of GaAs/AlGaAs graded-index separate confinement heterostructure quantum well laser diode structures grown at a nonoptimal substrate temperature lower than 680°C by molecular beam epitaxy. Reduction by a factor of up to three in the threshold current was accompanied by a reduction in the interface trap density. The reduced threshold current is still higher than that of laser diodes grown at the optimal temperatures which are between 680 and 695°C. The improvement in laser diode performance is ascribed to the reduction of interface nonradiative recombination centers.

Two-dimensional simulation of constricted-mesa InGaAsP/InP buried-heterostructure lasers

A fully two-dimensional self-consistent numerical model of the steady-state behavior of 1.3 /spl mu/m constricted-mesa InGaAsP/InP buried-heterostructure lasers is presented. Devices operating at this wavelength are very temperature sensitive and therefore the model for the first time includes coupled solutions to the thermal as well as the electrical and optical equation sets. The temperature dependence is included in the Fermi-Dirac statistics, bandgaps, mobilities, densities of states, Auger recombination coefficients, intervalence band absorption, optical gain, and thermal conductivities. The lasing mode profiles, carrier distributions, threshold currents, and temperature characteristics are analyzed and good agreement is found with experimental results, including the temperature dependence of the threshold current and the prediction of a break-point temperature. The optimum design parameters are investigated for reduced threshold currents, and the effect of optical loss in the blocking regions on lateral-mode control is analyzed.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Shadow mask MBE for the fabrication of lead chalcogenide buried heterostructure lasers

Laser diodes based on IV–VI-compounds are important sources for tunable infrared radiation in the 3 to 20 μm spectral region. A major application is trace gas analysis of atmospheric pollutants. Fabrication of buried heterostructure (BH) lasers by molecular beam epitaxy (MBE) improves the spectral and farfield emission properties. Using the materials PbSe, PbEuSe, PbSrSe and PbSnSe, BH lasers were fabricated by directly growing the active stripe through a shadow mask with reduced process duration. The shadow masks were prepared by anisotropic chemical etching of Si(100). The lasers show reduced threshold currents when compared to DH lasers, similar to BH lasers made by a photolithographic technique and also improved radiation patterns. Due to their smooth shape and less surface contamination as compared to their photolithographic counterparts, the overgrowth results show a reduced amount of material discontinuities for the shadow mask lasers.

Vertically stacked multiple-quantum-wire semiconductor diode lasers

We report the structure and lasing characteristics of GaAs/AlGaAs vertically stacked multiple-quantum-wire (QWR) semiconductor lasers grown by organometallic chemical vapor deposition on V-grooved substrates. The active region in these lasers consists of three crescent-shaped wires, placed at the center of a single-mode optical waveguide. The higher optical confinement factor, compared to single-QWR structures, leads to reduced threshold currents, as low as 0.6 mA for high-reflection coated devices at room temperature. The lower threshold carrier density results in oscillation at a lower QWR subband as compared to single-QWR laser structures.

Enhanced and inhibited spontaneous emission of free excitons in GaAs quantum wells in a microcavity

Spontaneous emission of the free excitons in GaAs quantum wells in a microcavity is enhanced (x 130) or inhibited (x 1 30 ) by placing excitonic dipoles at either a resonant wavelength and anti-node position or an off-resonant wavelength and node position of the standing-wave vacuum field fluctuations. The resulting spontaneous radiation pattern is highly concentrated into the normal direction for the enhancement case and the spontaneous emission coupling efficiency into a single microcavity resonant mode is estimated to be 0.3. It is expected that semiconductor lasers with substantially reduced threshold currents can be constructed using such a structure.

Ultralow-threshold graded-index separate-confinement single quantum well buried heterostructure (Al,Ga)As lasers with high reflectivity coatings

Unlike conventional semiconductor lasers, single quantum well lasers with high reflectively coatings have dramatically reduced threshold currents as a result of the smaller volume of the (active) quantum well region. A cw threshold current of 0.95 mA was obtained for a buried graded-index separate-confinement heterostructure single quantum well laser with facet reflectivities of ∼70%, a cavity length of 250 μm, and an active region stripe width of 1 μm.