Buried Heterostructure Devices Research Articles

High-efficiency, high-power mid-infrared quantum cascade lasers [Invited

The step-taper active-region (STA) design concept is implemented for ~5.0 μm-emitting quantum cascade lasers (QCLs) grown by metal-organic chemical vapor deposition (MOCVD). Carrier-leakage suppression yields high characteristic temperatures for the threshold-current density Jth, T0, and for the slope efficiency ηsl, T1: 226 K and 653 K. Resonant-tunneling extraction from the lower level results in miniband-like extraction. In turn, the internal efficiency ηi is found, from a variable mirror-loss study, to be ~77%; thus approaching the ~90% upper limit, when employing only inelastic scattering. Considering interface-roughness and alloy-disorder scattering, the transition efficiency reaches values of ~95%. Then, the injection efficiency is ~81%, and, for λ = 4.6 μm, the wallplug-efficiency ηwp upper limit reaches 41.2%. Results include 4.2 W/A single-facet ηsl and 0.96 kA/cm2Jth values. Buried-heterostructure (BH) QCLs provide single-facet 2.6 W continuous-wave (CW) power and 12% CW ηwp. Optimized 8 μm-emitting, STA-design QCLs provide 2 W/A ηsl, and 1.1 kA/cm2Jth; and BH devices yield single-facet 1 W CW power and 6% CW ηwp.

Low-threshold room-temperature continuous-wave operation of a terahertz difference-frequency quantum cascade laser source

The performance of a room-temperature continuous-wave (CW) terahertz source based on intracavity difference-frequency generation in a mid-infrared (λ ∼ 6.8 µm) quantum cascade laser with a dual-upper-state active region is reported. The fabricated buried heterostructure device, with a two-section buried distributed feedback grating, operates at two mid-infrared wavelengths and demonstrates a terahertz output of 2.92 THz with a very low threshold current density of 0.89 kA/cm2 in pulsed operation. Consequently, despite an epitaxial-side-up mounting configuration, the device achieves CW operation at room temperature in which a low CW threshold current density of 1.3 kA/cm2 is obtained.

Regrowth-free single-mode quantum cascade lasers with power consumption below 1 W

We report on single-mode distributed-feedback quantum cascade lasers emitting at 4.8 μm with continuous-wave threshold power consumption as low as 0.76 W at 20 °C and 0.98 W at 50 °C. Following growth of the laser active region and semiconductor cladding layers by a single molecular beam epitaxy process, devices with 4-μm-wide ridges and vertical sidewall gratings were fabricated using plasma etching and standard dielectric and metal deposition processes. In terms of mode stability, output power, and efficiency, we show that lasers with 1-mm cavity length and high-reflectivity back-facet coatings can match the performance of buried heterostructure devices, but with the advantage of requiring only a single epitaxial growth step.

Semi-insulating InP:Fe for buried-heterostructure strain-compensated quantum-cascade lasers grown by gas-source molecular-beam epitaxy

We describe the realization of buried-heterostructure strain-compensated quantum-cascade lasers that incorporate a very high degree of internal strain and are grown on InP substrates using gas-source molecular-beam epitaxy (GSMBE). The active region of the lasers contains AlAs layers up to 1.6nm thick with 3.7% tensile strain; restricting any post-growth processing to temperatures below 600°C to avoid relaxation. We demonstrate that buried-heterostructure devices can be realized by using GSMBE to over-grow the etched laser ridge with insulating InP:Fe at temperatures low enough to preserve the crystal quality of the strain-compensated active region. Two distinct growth techniques are described, both leading to successful device realization: selective regrowth at 550°C and non-selective regrowth at 470°C. The resulting buried-heterostructure lasers are compared to a reference laser from the same wafer, but with SiO2 insulation; all three have very similar threshold current densities, operational thermal stability, and waveguide losses.

High performance quantum cascade lasers based on three-phonon-resonance design

A quantum cascade laser structure based on three-phonon-resonance design is proposed and demonstrated. Devices, emitting at a wavelength of 9 μm, processed into buried ridge waveguide structures with a 3 mm long, 16 μm wide cavity and a high-reflection (HR) coating have shown peak output powers of 1.2 W, slope efficiencies of 1 W/A, threshold current densities of 1.1 kA/cm2, and high wall-plug efficiency of 6% at 300 K. A 3 mm long, 12 μm wide buried-heterostructure device without a HR coating exhibited continuous wave output power of as high as 65 mW from a single facet at 300 K.

Cross-sectional scanning tunneling microscopy of biased semiconductor lasers

Cross-sectional scanning tunneling microscopy is applied to semiconductor lasers which are biased and producing light. Two device structures are investigated—a double quantum well laser and a buried heterostructure device with two-dimensional growth variation. Scanning tunneling microscopy (STM) images are collected as a function of sample drive bias. Changes that occur in the STM image as a result of powering the sample are observed, which are due to changes in the energy band structure and the carrier concentration in the device as it responds to bias. The observed changes are largely reversible and a model is presented which matches and confirms this behavior. Once these effects are confirmed and decoupled, the technique can be used to study device-specific behavior resulting from physical changes in lasers as they are operated.

Continuous-wave distributed-feedback quantum-cascade lasers on a Peltier cooler

Continuous-wave operation of λ∼9 μm distributed-feedback quantum-cascade lasers is reported up to a temperature of 260 K. Single-frequency emission with a side mode suppression ratio of ⩾27 dB and with a tuning range of 5 cm−1 between 200 and 245 K (a tunability of −0.078 cm−1/K and −0.764 cm−1/W) is obtained for the junction-down mounted buried heterostructure devices. Uncoated lasers display an output power of up to 18 mW at 180 K and still 1 mW at 250 K. Lasers with high-reflection coated facets could be operated up to 260 K.

In situ mesa etching and immediate regrowth in a HVPE reactor for buried heterostructure device fabrication

Mesa etching in a hydride vapour-phase epitaxy (HVPE) reactor has been studied. Etched depth, underetching and shape of the mesas have been analysed as a function of partial pressures of active gases (HCl, PH 3 and InCl), stripe orientation and etching temperature. The experimental results show that the depth and undercut can be etched independently. We propose qualitative mechanisms for etching each of the emerging crystallographic planes ((0 0 1), (1 1 0) and {1 1 1} ). In situ mesa etching with immediate regrowth was applied to the fabrication of buried heterostructure Fabry–Perot lasers. No surface contamination due to exposure to ambient and low process time are advantages of this technique.

Temporally resolved selective regrowth of InP around [110] and [-110] mesas

Temporally resolved selective regrowth of InP around reactive ion etched [110] and [-110] directional mesas is studied by hydride vapor phase epitaxy at the growth temperatures of 600, 650, 685, and 700°C. The regrowth profiles are strikingly different depending upon the mesa orientation. The results are interpreted by invoking the difference in the bonding configurations of these mesas as well as the growth facility in a direction leading to the largest reduction of dangling bonds under the growth conditions. Various emerging planes during regrowth are identified and arehhl planes with initial values of 1/h ≤ 3 but ≥ 3 as the planarization is approached. Initial lateral growth defined as the growth away from the mesa at half of its height in the very first minute is a decreasing function of temperature when plotted as Arrhenius curves. Such a behavior is attributed to the exothermicity of the reaction and to an enhanced pyrolysis of PH3 to P2. The lateral growth rate is much larger than that on the planar substrate. This should be taken into account when regrowth of a doped layer (e.g. InP:Fe or InP:Zn) is carried out to fabricate a buried heterostructure device since the dopant concentration can be very much lower than the one optimized on the planar substrates.

Discrete electroabsorption modulators with enhanced modulation depth

In this paper, we present an investigation into factors that limited the median modulation depth of a batch of packaged discrete waveguide EA modulators to 23 dB at a wavelength of 1.55 /spl mu/m. Results from detailed measurements of the DC absorption and photocurrent spectra are used to show how stray parasitic light can perturb the absorption characteristic and reduce the modulation depth of these high-speed multiple-quantum-well modulators. A novel ridged deeply etched buried heterostructure EA modulator design is presented in which stray light is removed from the immediate vicinity of the guided mode. The key structural difference between these and the previous devices is that they employ a much thicker Fe-doped InP current blocking layer that was grown by atmospheric pressure MOVPE using PCl/sub 3/ for planarization. Detailed measurements of the DC absorption spectra of a packaged ridged deeply etched buried heterostructure device confirm that stray light causes only a minor perturbation on its absorption characteristics. Consequently the new batch of EA modulator modules have a higher median modulation depth of 40 dB, as well as lower fiber-to-fiber insertion losses and picosecond pulse generation capabilities that are very similar to the previous devices which were used in 40 Gb/s optically time division multiplexed experiments.

Strained-layer InGaAs-GaAs-AlGaAs buried-heterostructure quantum-well lasers by three-step selective-area metalorganic chemical vapor deposition

Strained-layer In/sub x/Ga/sub 1-x/As-GaAs-Al/sub y/Ga/sub 1-y/As buried-heterostructure (BH) quantum-well lasers have been fabricated using three-step selective area atmospheric pressure metal-organic chemical vapor deposition. Selective-area epitaxy is used to produce BH lasers involving only GaAs on GaAs regrowth, eliminating the detrimental effects associated with exposed Al/sub y/Ga/sub 1-y/As found in other fabrication methods. Additionally, selective-area epitaxy provides inplane bandgap energy control to fabricate BH devices with different wavelengths on the same wafer. Threshold currents as low as 11 mA are obtained for a 540-/spl mu/m-long, 4-/spl mu/m-wide device with uncoated cleaved facets. The devices operate at room temperature to more than 200 mW/uncoated facet with 40% external differential quantum efficiency. In-plane bandgap energy control results in a wide range of possible laser emission wavelengths for BH lasers grown on the same substrate.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

DFB laser monolithically integrated with an absorption modulator with low residual reflectance and small chirp

Transmission over 150 km of standard fibre at 5 Gbit/s, and over 75 km at 8 Gbit/s, is reported with a monolithically integrated DFB laser and Franz-Keldysh absorption modulator. The modulator facet reflectance is low. The device is stable over a range of operating parameters. The butt-joint, buried-heterostructure device is entirely grown by MOVPE.

Harmonic and phase distortion of analogue amplitude-modulated signals in bulk near travelling-wave semiconductor optical amplifiers

A theoretical model has been formulated using rate equations to analyse the response of a semiconductor optical amplifier to a single analogue amplitude-modulated input signal. The results are found to agree very closely with experimental results obtained from a bulk buried-heterostructure device, and show that signal distortion is significant even for input powers substantially below saturation. The level of distortion is strongly affected by both the saturation power of the device, and the Fabry-Perot resonance arising from residual facet reflectivity. On the basis of good agreement with experimental data, the theoretical model is used to assess a novel method of bias current feedback that reduces the effect of carrier density modulation by the modu lation of the input signal, and greatly improves the harmonic performance of the device. Using this feedback method, the calculated value of signal phase distortion has been reduced by a factor of up to 40, and the harmonic distortion by up to 11 dB.

Regrowth of Semi‐Insulating InP around Etched Mesas Using Hydride Vapor Phase Epitaxy

Hydride vapor phase epitaxy has been used to regrow semi‐insulating around a variety of etched mesa structures used for the fabrication of buried heterostructure devices. Morphological properties of regrowth using this technique include excellent mesa sidewall coverage, no void formation, no mask overgrowth, and good surface morphology in the field regions between mesas. The regrowth morphology is relatively insensitive to parameters such as mesa sidewall shape and the extent of the etch mask overhang. The most striking feature of this hydride regrowth technique is that it is possible to grow very thick (>10 μm) layers without any mask overgrowth, and the resulting morphology in the vicinity of the regrown mesa is well suited for subsequent device processing.

Submilliampere threshold current pseudomorphic InGaAs/AlGaAs buried-heterostructure quantum well lasers grown by molecular beam epitaxy

We report on low threshold current strained InGaAs/AlGaAs single quantum well lasers grown by molecular beam epitaxy. Broad-area threshold current densities of 114 A/cm2 at 990 nm were measured for 1540-μm-long lasers. Threshold currents of 2.4 mA at 950 nm were obtained for an uncoated buried-heterostructure device with a 2-μm-wide stripe and 425-μm-long cavity. With reflective coatings the best device showed 0.9 mA threshold current (L=225 μm). Preliminary modulation measurements show bandwidths up to 5.5 GHz limited by the detector response.

Liquid phase epitaxial growth of buried heterostructure devices

The {111} In plane of InP is shown to be the most difficult surface to obtain a defect free radiative interface by LPE regrowth. Studies of regrowth on this plane are described and it is shown that successful, defect free epitaxy depends upon substrate cleaning procedures, the elimination of furnace contaminants, the correct P overpressure to prevent erosion during the heat-treatment prior to epitaxy and initiation of growth with a super cooled In 1− x Sn x or quaternary melt.

Improved Meltback Procedures for Liquid-Phase-Epitaxial Growth of Planar and Buried Heterostructures

ABSTRACTIn the liquid phase epitaxy on indium phosphide, the substrate,just prior to the growth of the first epitaxial layer, is commonly etched back with an indium melt to remove any thermallydegraded surface and to ensure uniform and consistent wetting.This procedure, however, often produces defects which degrade both planar and buried heterostructure devices. For planar edgeemitters and lasers, the resulting rippled surface morphology degrades device performance by scattering light. For buried heterostructures, the meltback in the regrowth step leads to indium-rich inclusions. The effects of meltback on material quality are presented, and a new multiple meltback procedure which maintains flat surface morphology and eliminates inclusions is described.

CW electrooptical properties of InGaAsP(λ = 1.3 µm) buried-heterostructure lasers

The fabrication procedure, electrical properties, optical-bean characteristics, spectral characteristics, and temperature dependence of emission wavelength and threshold of InGaAsP buried-heterostructure (BH) lasers emitting at 1.3 μm are described. The dimensional requirements for fundamental-transverse mode operation have been determined. BH devices are characterized by low threshold currents, fundamental transverse mode operation, linear light output, and narrow spectral width. For 380 μm long devices threshold currents of 40 mA, slope efficiencies of 18 percent, forward resistance of 5 Ω, and T <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">0</inf> values of 75 K have been attained.