Increase In Threshold Current Density Research Articles

Theoretical simulation of quantum cascade lasers based on InGaAs/AlInAs and InGaAs/AlAsSb quantum wells

Quantum cascade lasers based on semiconductor InGaAs/AlInAs and InGaAs/AlAsSb quantum wells exhibit electronic intersubband transitions within conduction bands and attract much attention due to their operation at room-temperature short wavelengths. In this work we performed a theoretical simulation of such quantum cascade lasers and studied the influence of their construction and application conditions on optical behaviour. The intersubband absorption processes in these quanta well structures were studied. Electronic properties and the conduction band edge profiles were simulated as well as the probability densities of the Wannier-Stark states were determined. The simulation results showed that with the rise of the temperature the threshold current density increases which also leads to a decrease in the optical gain. An increase of the applied electric field is accompanied by the optical gain rising and decrease of the threshold current density that results in a blue shift of laser frequency.

Temperature Dependence of the Parameters of 1.55-μm Semiconductor Lasers with Thin Quantum Wells Based on Phosphorus-Free Heterostructures

InGaAs/InGaAlAs laser diodes operating in the 1.55-μm spectral range are studied. It is demonstrated that a certain level of carbon doping (1012 cm–2 per a single quantum well) allows one to reduce the temperature coefficient of variation of the lasing wavelength in such structures and raise the characteristic temperature of threshold current and differential efficiency at temperatures from 16 to ~50°C. These changes are accompanied by an increase in threshold current density and a reduction in differential efficiency.

Design of active region for GaAsP/AlGaAs tensile strain quantum well laser diodes near 800 nm wavelength

As an active region, the tensile strain GaAs1-xPx quantum well plays an important role in the high power semiconductor laser diode with a wavelength of about 800 nm. Accompanied with the improved stability due to the Al-free active region, the GaAs1-xPx quantum well laser also shows a high level of catastrophic optical mirror damage because of the non-absorbing window at the facet, which is formed automatically by the relaxation of the tensile strain GaAs1-xPx material. On the other side, the GaAs1-xPx quantum well laser can provide a transverse magnetic (TM) polarized light source which is important for many solid state laser systems. However, the energy band structure of the tensile strain GaAs1-xPx quantum well is more complicated than that of the compressed or lattice matched quantum well. Although the light hole band is on the top of the heavy hole band for the bulk tensile strain GaAs1-xPx material, the situation may be different from the tensile strain GaAs1-xPx quantum well, in which the first light hole subband lh1 can be either on the top of the first heavy hole subband hh1 or reversed, that will cause the laser to generate either TM or transverse electric (TE) polarized light according to the well structure. So it is meaningful to optimize the tensile strain GaAs1-xPx quantum well structure based on the analysis of the energy band structure. Firstly, according to the 6×6 Luttinger-Kohn theory, the energy band structure of the tensile strain GaAs1-xPx quantum well is calculated by the finite difference method. The relationship between the interband transition energy and the well structure parameters is established. It is found that the well composition x and the well width should increase simultaneously, in order to fix the first subband transition wavelength at about 800 nm. Special attention is paid to the 808 nm quantum well, the valence structures of different well widths are calculated, the detailed analysis of the envelope function shows that the top valence subband is lh1 for wider well width, while it is changed to hh1 for narrower well width. Meanwhile, both the TE and the TM momentum matrix element are calculated as a function of the transverse wave vector for the subband transition from c1 to lh1, lh2, hh1 and hh2, respectively. Further, the threshold optical gains of different well widths are simulated for 808 nm laser diode with the tensile strain GaAs1-xPx quantum well as an active region, the wider well width benefits the TM mode, while the narrower one is favor of TE mode. Finally, according to the threshold carrier density, the relationship between the threshold current density and the well width is analyzed for 808 nm laser diode by considering both the spontaneous and the Auger recombination, an optimum combination of the well width and the well composition exists. For wider well width, the threshold current density will be higher because of the high energy subband carrier filling effect. For narrower well width, the decrease of the optical confinement factor will lead to the increase of threshold current density.

Pressure dependent optical gain in Type-II PbSe/PbS core /shell quantum dots

Pressure induced nonlinear optical properties of Type-II lead based core/shell quantum dot heterostructure are investigated. The pressure dependent exciton binding energies are obtained with the increase of shell thickness for various inner core radii. The exciton interaction energies in a PbSe/PbS core/shell quantum dot inner core radii are found in the presence of hydrostatic pressure using variational method within the single band effective mass approximation. The threshold current density with the hydrostatic pressure and inner dot radii in a PbSe/PbS dot with the constant shell width is obtained. The pressure dependence on peak optical gain with the current density in a PbSe/PbS core/shell quantum dot with the fixed carrier density is found. The pressure related optical gain with the photon energy in a PbSe/PbS core/shell quantum dot is also obtained. The results show that the threshold current density increases by 20% when the pressure increases from 0 GPa to 4 GPa for 20 Å quantum dot radius whereas 16% of increase in threshold current density is observed for 50 Å quantum dot radius for the same increase of pressure values. The peak of the optical gain is observed to increase nonlinearly with the applied hydrostatic pressure and 12% of optical gain is enhanced for every 2 GPa pressure value. The results will be useful for some potential applications in near infrared light sources.

In brief

PAGE 40 A methane-hydrogen dry etching technique has been used to fabricate GaSb-based cascade diode lasers emitting near 3 µm. The process, developed by researchers in the USA, produces a diffraction-limited laser with minimal increase in threshold current density and no voltage penalty when compared with wide ridge multimode lasers. PAGE 36 A performance trade-off was found when examining different methods for inserting microstrip lines into aerospace composite structures. Research from Australia showed that embroidered microstrip lines exhibited better electromagnetic performance, whilst stencil transfer was more resistant to mechanical deformation. Cascade diode lasers fabricated using methane-hydrogen dry etching technique. PAGE 34 Researchers from the UK have designed an easy to control microstrip coupler that enhances power coupling between two microstrips. Two small slot lines are inserted in the ground plane underneath the microstrips, forcing an increased number of shared field lines between strips, thus eliminating the need to narrow the distance between microstrip lines. Microstrip lines stitched into HexPly 914 S-Glass. PAGE 7 A high-performing 3D terahertz (THz) filtering antenna was designed by researchers in China, using a low-temperature co-fired ceramic (LTCC) process. This 3D structure is more compact, has enhanced radiation performance and allows for more design flexibility compared with traditional planar designs. Slot lines underneath microstrips enhance coupling. PAGE 16 Researchers from Korea have developed a dual correlated double sampling scheme (CDS) for low-noise CMOS imagers, without the need for a power-consuming programmable gain amplifier (PGA). This scheme reduces noise by removing sampling error, and reduces power consumption by amplifying the readout channel's output. Photograph of 3D THz filtering antenna. The correlated double sampling scheme can be applied to SAR-based ADCs for CMOS imagers without the use of a PGA.

Enhanced performance of tunable external-cavity 15 μm InAs/InP quantum dot lasers using facet coating

The influence of facet coating on the performance of InAs/InP quantum dot (QD) tunable external-cavity (EC) lasers has been studied. It is shown that antireflection coating of the EC lasers expands the tuning bandwidth from 39 to 50 nm, but accompanied with a great increase in threshold current density. By adding antireflection/high-reflection facet coating, the performance of the EC lasers can be further enhanced. Compared to the laser without any facet coating, the output optical power of the facet coated QD laser increases from 20 to 34 mW under the identical injection current while the slope efficiency increases from 36.4 to 70 mW/A. Moreover, the tunability of the EC laser is extended to 104 nm covering the wavelengths from 1457 to 1561 nm with a relatively low injection current level.

Determination of gain in AlGaN cladding free nitride laser diodes

The optical gain spectra of InGaN-based multiple-quantum-well (MQW) laser diodes (LDs) grown by plasma-assisted molecular beam epitaxy are compared for different emission wavelengths. Two AlGaN cladding free LDs with similar epitaxial structures but with different In compositions in MQW were grown to study the dependence of material gain on lasing wavelength. As the emission wavelength increased from 432 to 458 nm, the differential modal gain decreased from 5.7 to 4.7 cm/kA, and the optical losses increased from 40 to 46 cm−1 resulting in an increase in threshold current density. This dependence is attributed to lower optical mode confinement of LD emitting at longer wavelength. We found a strong decrease of confinement factor with increasing wavelength.

Vertical subwavelength mode confinement in terahertz and mid-infrared quantum cascade lasers

We exploit the modal confinement properties of metal-metal ridge waveguides to investigate the effect of reducing the thickness of the active laser cores in both terahertz and mid-infrared quantum cascade lasers. Devices with active regions over 55 times thinner than the free-space emission wavelength are demonstrated. They show only a modest increase in threshold current density compared with conventional-thickness devices. The limited increase in threshold is possibly due to a parasitic current channel in addition to the radiative current channel. These structures could be useful for the development of ultra-low volume lasers.

Broadband external cavity tunable quantum dot lasers with low injection current density

Broadband grating-coupled external cavity laser, based on InAs/GaAs quantum dots, is achieved. The device has a wavelength tuning range from 1141.6 nm to 1251.7 nm under a low continuous-wave injection current density (458 A/cm(2)). The tunable bandwidth covers consecutively the light emissions from both the ground state and the 1st excited state of quantum dots. The effects of cavity length and antireflection facet coating on device performance are studied. It is shown that antireflection facet coating expands the tuning bandwidth up to ~150 nm, accompanied by an evident increase in threshold current density, which is attributed to the reduced interaction between the light field and the quantum dots in the active region of the device.

Low-Temperature Nonthermal Population of InAs–GaAs Quantum Dots

Measurements of the unamplified spontaneous emission spectra from 80 K to 350 K of a dot ensemble show clear evidence for increased population of higher lying states in the inhomogeneous distribution as the temperature is reduced from 200 K to 80 K, indicating a nonthermal population at low temperature and confirming that the recombination processes are localized in individual dots. These conclusions are supported by modeling an inhomogeneous ensemble of 2 times 106 dots. From simultaneous measurements of optical gain, our data show that the increase in threshold current density with decreasing temperature below about 200 K (which is commonly observed) is due to increased population of higher lying states associated with the transition to nonthermal behavior.

Effect of waveguide sidewall roughness on the threshold current density and slope efficiency of quantum cascade lasers

We report on a study to determine the effect of waveguide sidewall roughness on quantum cascade (QC) laser performance using two two-wavelength heterogeneous QC laser structures, one with emission wavelengths of 7.0μm∕11.2μm, and the other with 8.7μm∕12.0μm. For the range of roughness standard deviation values from about 0.4to1.0μm, for which all four QC lasers were operating, the threshold current density increases by 12%–15% and the slope efficiency decreases by 30%–70% with stronger performance degradation for the shorter wavelength lasers, which is in agreement with a model based on Rayleigh scattering.

DC and Dynamic Characteristics of P-Doped and Tunnel Injection 1.65-$\mu{\hbox {m}}$ InAs Quantum-Dash Lasers Grown on InP (001)

We have studied the characteristics of 1.65-mum InAs self-organized quantum-dash lasers grown on InP (001) substrates, wherein special techniques of p-doping of quantum dashes and tunnel injection are incorporated for the first time. We measured a very large T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> (196 K) in p-doped quantum-dash lasers, accompanied by an increase in threshold current density (J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ~1600 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2 </sup> ), compared to the undoped quantum-dash lasers (T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> =76 K and J <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">th</sub> ~950 A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ). The p-doped lasers exhibit a maximum 3-dB bandwidth of 8 GHz, chirp ~1.0 Aring, and alpha-parameter ~1.0 (measured at subthreshold bias conditions) at a temperature of 278 K. Similar undoped quantum-dash lasers exhibit a 3-dB bandwidth of 6 GHz. A self-consistent model, that includes Auger recombination in quantum dashes, is developed to calculate the threshold current at various temperatures. A comparison of the calculated threshold current and T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> with measured values reveals that Auger recombination in quantum dashes plays a major role in determining the values of threshold current and T <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</sub> in both undoped and p-doped quantum-dash lasers. While p-doping increases the gain and differential gain, the presence of wetting layer states, the relatively large inhomogeneous broadening of quantum dashes, and the substantially increased Auger recombination upon p-doping severely limit the potential benefits. Superior characteristics, including large modulation bandwidth (f <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3 dB</sub> ~12 GHz), near-zero alpha-parameter, and very low chirp (~0.3 Aring), are achieved when the technique of tunnel injection is also utilized

Laser gain properties of AlGaN quantum wells

Laser gain is investigated for AlGaN wurtzite quantum-well structures emitting in the wavelength range from ∼270to340nm. The calculations show that gain properties vary notably with aluminum concentration in the quantum well. The TE gain dominates over the entire spectral range, although an enhancement of TM gain is observed for AlGaN quantum wells with the high aluminum mole fraction. The calculations also predict an increase in threshold current density for the shorter-wavelength lasers.

Effect of the Doping Level of a p-Cladding Layer on the Performance of GaAs–AlGaAs Multiquantum-Well Lasers

CW performance of multiquantum-well (MQW) GaAs-AlGaAs lasers operating near 850 nm at 300 K is investigated as a function of the doping level in the p-cladding layer. Laser structures have been grown by metal-organic chemical vapor deposition using Zn diethyl as a carrier for acceptor dopant. An undoped sublayer (setback) has been introduced to separate the heavily doped part of the cladding from the mode volume. The doping level of zinc ranges from 4/spl times/10/sup 17/ to 6.5/spl times/10/sup 18/cm/sup -3/. Along with this increase of the doping level, the threshold current density increases from 370 to 612 A/cm/sup 2/ whereas the slope efficiency of the laser increases from 0.71 to 1.33 W/A with an improvement of the characteristic temperature constant T/sub 0/ from 150 to 250 K. At the highest level of doping, the internal quantum yield is found to be close to unity and optical losses are as low as 1.36 cm/sup -1/. An increase of the stable single-mode output is also obtained in 3-/spl mu/m-wide ridge-waveguide diodes up to /spl sim/180 mW.

A novel fabrication technique for multiple-wavelength photonic-integrated devices in InGaAs-InGaAsP laser heterostructures

We report the fabrication of multiple wavelength chips in InGaAs-InGaAsP laser structure using a novel ion implantation induced quantum-well (QW) intermixing technique. This technique first consists of using a gray mask photolithography and reactive ion etching process to create a SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> implant mask with variable thickness on the sample. This is followed by a single 360-keV phosphorus ion implantation at a dose of 1×10/sup 14/ cm/sup -2/ at 200/spl deg/C, which creates different amounts of point defects in the sample depending on the local thickness of the SiO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> mask. A subsequent thermal annealing step induces QW intermixing through the diffusion of the point defects across the structure. With this technique, we have successfully fabricated 10-channel multiple wavelength laser diodes, with lasing wavelength spreading over 85 nm (between 1.47 and 1.55 μm), monolithically integrated on a single chip. Only a limited increase of threshold current density of 17% (i.e., from 1.2 to 1.4 kA/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> ), has been observed between the least intermixed and the most intermixed lasers.

Enhanced radiation hardness of quantum dot lasers to high energy proton irradiation

The influence of high energy proton irradiation on the device properties of InGaAs/GaAs quantum dot and quantum well lasers has been investigated. In the regime of spontaneous emission, quantum dot lasers show a much enhanced radiation hardness compared to quantum well lasers, manifested in a smaller increase of threshold current density. However, in the lasing regime the device characteristics are similarly influenced. Internal differential quantum efficiencies are reduced, internal optical losses remain constant.

2.3–2.7µm InGaAsSb/AlGaAsSb Broad-Contact and Single-Mode Ridge-Waveguide SCH-QW Diode Lasers Operating in CW Regime at Room Temperature

AbstractA new approach in the design of (Al)InGaAsSb/GaSb quantum well separate confinement heterostructure (QW-SCH) diode lasers has led to CW room-temperature lasing up to 2.7 gm. To avoid QW material degradation associated with the miscibility gap in the 2.3–2.7 tim wavelength range, we used highly strained, “quasi-ternary” InxGa1−xSbl−yAsy compounds with 0.25&lt;x&lt;0.38 and y&lt;0.07 as the material for QWs. Very low threshold current density (∼300 A/cm2) and high CW output powers (&gt;100 mW) were obtained from broad contact devices operating in the 2.3–2.6 μm wavelength range. From the spontaneous emission measurements we have identified that the Auger process determines the rate of recombination in quantum well active region over the entire temperature range studied (15– 110 'C) for 2.6 gim lasers and only at temperatures higher than 65 'C for 2.3 pim lasers. If Auger recombination dominates, strong temperature dependence of Auger coefficient leads to the rapid increase of threshold current density with temperature (To ∼40 °C). In the range of 15 – 65 °C for 2.3 gim devicesa monomolecular, non-radiative mechanism dominates and To is about 110 °C. In addition, single-mode CW room temperature ridge-waveguide lasers with wavelength of 2.3-2.55 gim have been fabricated for the first time. The lasers display threshold currents around 50 mA with CW output powers of several milliwatts. Since for a certain range of temperatures and currents one of the longitudinal modes dominates in the spectra of the ridge lasers they have been successfully applied forgas spectroscopy.

Molecular beam epitaxy of vertically compact laser-HEMT structures for monolithic integration

Vertically compact Al x Ga 1-x As/GaAs laser structures with Al 0.3 Ga 0.7 As/AlAs SPSL-cladding layers were grown on top of HEMT structures by MBE and investigated with regard to series resistance. We found an exponential dependence of series resistance R s with AlAs and Al 0.3 Ga 0.7 As SPSL-layer thicknesses at ambient temperature. Laser-HEMT structures for monolithic integration were grown and lasers were processed. For 3 x 200 μm 2 3-QW-lasers, suitable for high-frequency performance, threshold currents of 20 mA and series resistances below 12 Ω were obtained. The slope of the linear regression of the I th values as a function of mesa width yielded a low threshold current density j th of 480 A/cm 2 . Reduction of the p-cladding thickness from 600 nm down to 450 nm shows no increase in threshold current density if the p-dopants are kept from diffusing into the active region. This clearly demonstrates that laser structures for monolithic integration can be designed very compact without loosing performance.

MOVPE-grown (AlGa)As double-barrier multiquantum well (DBMQW) laser diode with low vertical beam divergence

We present a (AlGa)As laser diode based on a MOVPE-grown heterostructure modified toward reduction of the vertical (perpendicular to junction plane) light beam divergence. Insertion of thin, wide-gap barrier layers at the interfaces between the MQW active region and the cladding layers allows for separate controlling the carrier and optical confinements. According to theoretical modeling, the antiguiding influence of the barriers on the primary waveguiding properties of a MQW structure causes a weakening of the optical confinement and thereby a reduction of the vertical beam divergence. This occurs however without weakening of the carrier confinement and an excessive increase of threshold current density. As a result, the beam divergence of 17-13° has been experimentally achieved (depending on construction details), for devices manufactured from the heterostructure theoretically expected to give 12°. MOVPE growth conditions of this modified “double-barrier multiquantum well” (DBMQW) heterostructure are described. The light-current characteristics presented show threshold current densities of 0.92-2.1 kA cm−2 for laser cavity lengths of 0.75-0.25 mm, respectively and good quantum efficiencies.

InAs‐GaAs quantum dots: From growth to lasers

AbstractInjection lasers based on InAsGaAs and InGaAsGaAs quantum pyramids (QPs) with a lateral size ranging from 80 to 140 Å, are realized. The structures with relatively small dots (≈80 Å) exhibit properties predicted earlier for quantum dot (QD) lasers such as low threshold current densities (below 100 A cm−2) and ultrahigh characteristic temperatures (T0 = 350 to 425 K). For temperatures of operation above 100 to 130 K T0 decreases and the threshold current density increases (up to 0.95 to 3.3 kA cm−2 at room temperature) due to carrier evaporation from QPs. Larger InAs QPs (≈140 Å) providing better carrier localization exhibit saturation of the ground state emission and enhanced nonradiative recombination rate at high excitation densities. The radiative lifetime shows a weak dependence on the dot size (80 to 140 Å) being close to ≈1.8 to 2 ns, respectively. A significant decrease in radiative lifetime is realized in vertically‐coupled quantum dots formed by a QP shape‐transformation effect. The final arrangement represents a three‐dimensional tetragonal array of InAs islands inserted in a GaAs matrix each composed of several vertically merging InAs parts. The first injection lasing in such an array is achieved.