Quantum-well Heterostructure Lasers Research Articles

Threshold current density in strained layer In/sub x/Ga/sub 1-x/As-GaAs quantum-well heterostructure lasers

The authors consider the transparency carrier density in ideal and practical strained layer In/sub x/Ga/sub 1-x/As-GaAs quantum-well heterostructure lasers. The transparency carrier density in practical structures is then related to transparency current density using realistic values for spontaneous recombination rates. These parameters are incorporated with representative structural parameters into a nonlinear model for gain in a quantum-well laser, in order to provide a complete model for the laser threshold current density in strained layer In/sub x/Ga/sub 1-x/As-GaAs quantum-well heterostructure lasers. These results are then compared and contrasted with experimental laser results from several laboratories. >

Novel etching technique for a buried heterostructure GaInAs/AlGaInAs quantum-well laser diode

A novel etching technique for the 1.5-μm GaInAs/AlGaInAs buried heterostructure quantum-well laser diode is developed. Tartaric acid is used for the etching of GaInAs and AlGaInAs layers for the first time. Using a three-step material-selective etching, a 3-μm-high mesa with about 1.5-μm-wide active layer and 3-μm-wide contact layer can be achieved with good reproducibility. Nearly flat surfaces were obtained after a two-step organometallic chemical vapor deposition growth. A low threshold current of 11 mA was obtained for a 570-μm-long cavity device.

Low-temperature operating life of continuous 300-K AlxGa1−xAs-GaAs quantum-well heterostructure lasers grown on Si

Data are presented on the continuous (cw) 77–200 K operational characteristics of cw 300-K AlxGa1−xAs-GaAs quantum-well heterostructure diode lasers grown on Si substrates. Operation is demonstrated for over 500 h with a junction temperature as high as ∼200 K for a diode previously operated cw 300 K for over 10 h with its junction side mounted away from the heat sink. The data indicate that longer cw 300-K lifetimes than previously demonstrated (17 h) may be possible. The effects of the optical power level on the degradation rate are examined, and it is shown that the maximum cw 300-K power output for these devices (∼30 mW/facet) is limited by catastrophic facet degradation. The effects of naturally occurring microcracks on device stability are also considered, and the effect of stress on the output polarization is measured and discussed.

Low-threshold disorder-defined native-oxide delineated buried-heterostructure AlxGa1−xAs-GaAs quantum well lasers

Impurity-induced layer disordering (IILD) along with oxidation (native oxide) of high-gap AlxGa1−xAs confining layers is employed to fabricate low-threshold stripe-geometry buried-heterostructure AlxGa1−xAs-GaAs quantum well heterostructure (QWH) lasers. Silicon IILD is used to intermix the quantum well and waveguide regions with the surrounding confining layers (beyond the laser stripe) to provide optical and current confinement in the QW region of the stripe. The high-gap AlxGa1−xAs upper confining layer is oxidized in a self-aligned configuration defined by the contact stripe and reduces IILD leakage currents at the crystal surface and diffused shunt junctions. AlxGa1−xAs-GaAs QWH lasers fabricated by this method have continuous 300 K threshold currents as low as 5 mA and powers ≳ 3l mW/facet for ∼ 3-μm-wide active regions.

Native-oxide stripe-geometry AlxGa1−xAs-GaAs quantum well heterostructure lasers

Data are presented on the room-temperature continuous (cw) operation of native-oxide single-stripe AlxGa1−xAs-GaAs quantum well heterostructure (QWH) lasers. The device quality native oxide is produced by the conversion of high Al composition AlxGa1−xAs (x∼0.8) confining layers via H2O vapor oxidation (400 °C) in a N2 carrier gas. The 10-μm-wide cw 300 K QWH lasers, which are fabricated by simplified processing, have excellent spectral quality and have been operated to powers in excess of 100 mW per facet.

Laser Quality AlGaAs-GaAs Quantum Wells Grown on Low Temperature GaAs

ABSTRACTIn this work we demonstrate that photopumped quantum wellheterostructure lasers with excellent optical quality can be grown ontop of a LT GaAs buffer layer by molecular beam epitaxy. Hightemperature thermal annealing of these lasers blue-shifts the laseremission wavelengths but the presence/absence of a LT GaAs layerhad little effect on the overall laser thresholds. Also, to first order itwas not necessary to include an AlAs barrier layer to preventadverse effects (as has been necessary in the gate stack of MESFETs to prevent carrier compensation).

Characterization of GaAs/AlGaAs graded index-separate confinement heterostructure lasers by Raman scattering

We report the first use of Raman scattering to study GaAs/AlGaAs graded index-separate confinement heterostructure quantum-well lasers. We have used a forward scattering geometry in which the waveguide is endfired, and the light emerging from the opposite end facet is collected and spectrally analyzed. The probe is confined by the waveguide and thus interacts with the entire laser cavity. Because Raman scattering occurs in all regions of the heterostructure to which the optical mode is confined, this technique is a useful indication of the mode profile in waveguide heterostructures. We observe inhomogeneously broadened longitudinal and transverse optical phonons in the graded region.

High-power disorder-defined coupled stripe AlyGa1−yAs-GaAs-InxGa1−xAs quantum well heterostructure lasers

Data are presented describing continuous (cw) room-temperature laser operation of AlyGa1−yAs-GaAs-InxGa1−xAs quantum well heterostructure (QWH) phase-locked arrays. The ten-stripe arrays have 3 μm emitters, with emitter to emitter spacing of 4 μm, and are patterned onto the QWH crystal using a self-aligned Si-O impurity-induced layer disordering (IILD) procedure. The IILD process is devised to provide limited layer intermixing to ensure optical coupling (across ∼1 μm). The coupled stripe QWH lasers exhibit narrow twin-lobed far-field patterns that show unambiguously phase locking in the highest order supermode. The cw output power of the lasers (differential quantum efficiency 52%) is shown from threshold (∼75 mA) to over 280 mW (both facets, no optical coatings).

Thermal behavior and stability of room-temperature continuous AlxGa1−xAs-GaAs quantum well heterostructure lasers grown on Si

Data are presented on the thermal characteristics of p-n AlxGa1−xAs-GaAs quantum well heterostructure (QWH) diode lasers grown on Si substrates. Continuous 300-K operation for over 10 h is demonstrated for lasers mounted with the junction side away from the heat sink (‘‘junction-up’’) and the heat dissipated through the Si substrate. ‘‘Junction-up’’ diodes that are grown on Si substrates have measured thermal impedances that are 38% lower than those grown on GaAs substrates, with further reductions possible. Thermal impedance data on ‘‘junction-down’’ diodes are presented for comparison. Measured values are consistent with calculated values for these structures. Low sensitivity of the lasing threshold current to temperature is also observed, as is typical for QWH lasers, with T0 values as high as 338 °C.

Anomalous threshold current and time delays in index-guided AlxGa1−xAs-GaAs quantum-well lasers

Anomalous threshold current (Ith) variation with temperature and with pulse length, and large delays (up to 6 μs) between excitation and the turn-on of stimulated emission are observed in index-guided AlxGa1−xAs-GaAs quantum-well heterostructure (QWH) lasers. These effects are found in laser diodes incorporating ‘‘spike’’ doping layers (δ-Mg and δ-Se) within the QWH active region and that are fabricated via laser-assisted Si impurity-induced layer disordering. The introduction of contaminants during the localized melting and regrowth of the laser-induced layer disordering, and the effect of these impurities (or defects) with the active region ‘‘spike’’ doping create traps. The traps cause Ith to increase (not decrease) at lower temperature and at shorter current pulses, and cause time delay in turn-on of the operation.

Coupled-stripe AlxGa1−xAs-GaAs quantum well lasers defined by vacancy-enhanced impurity-induced layer disordering from (Si2)y(GaAs)1−y barriers

Data are presented showing that high-performance (>100 mW) AlxGa1−xAs-GaAs quantum well heterostructure (QWH) coupled-stripe laser arrays can be realized by vacancy-enhanced impurity-induced layer disordering (IILD) employing an internal (Si2)y(GaAs)1−y barrier. This form of Si IILD (i.e., layer disordering from a ‘‘finite’’ internal Si source) results in coupled-emitter laser operation, even for relatively large stripe (7–10 μm) and spacing dimensions (2 μm), at currents just above threshold and higher. As a separate issue, the Si IILD QWH laser cross sections show that Si diffusion from a surface source is ‘‘slower’’ than the simultaneous vacancy diffusion from an SiO2 surface encapsulant (‘‘cap’’).

Supermodes of multiple-stripe quantum-well heterostructure laser diodes operated (cw, 300 K) in an external-grating cavity

The far-field supermode patterns of a phase-locked multiple-stripe quantum-well heterostructure (QWH) laser diode are described as a function of injection current and emission wavelength, the latter controlled by an external grating. The external-grating cavity is used to isolate single or multiple supermodes of the multiple-stripe QWH laser (Pout>170 mW cw, λ∼7400 Å). The progression of supermode patterns consists of a discrete set of mode configurations for each longitudinal mode of the spectrum. The progression is cyclic with a ∼2.8-Å period which corresponds to the longitudinal mode spacing of the diode. Under high gain conditions, i.e., near the center of the recombination-radiation spectrum or at higher current levels, continuous tunability is observed with gradual transitions between supermode eigenstates. As the gain is reduced (low current), the number of supermodes observed decreases until only the in-phase pattern, i.e., each emitter at the same phase, remains above threshold. The far-field patterns range from a double-lobe pattern with a 10° peak separation (5 μm between emitter phase reversals) to a narrow (<2° full angle half power) single-lobe in-phase pattern. The experimental data are compared to the results of coupled-mode analysis.

Hydrostatic pressure measurements (≲12 kbar) on single- and multiple-stripe quantum-well heterostructure laser diodes

Short wavelength Alx′Ga1−x′As-AlxGa1−xAs (x′∼0.85, x∼0.22) quantum-well heterostructure (QWH) laser diodes (well size Lz ≊400 Å) that operate continuously (cw) at 300 K are subjected to hydrostatic pressure (≲12 kbar). The emission spectrum and the light intensity versus current (L-I) curves are monitored to determine the pressure dependence of the direct (Γ) band gap and the threshold current. The band gap exhibits a linear pressure dependence with a noticeable change in slope at ∼4.5 kbar, similar to previously reported results for AlxGa1−xAs-GaAs QWH diodes. The threshold current increases monotonically with pressure, reflecting the increasing loss of carriers to the X and L bands. The short-wavelength cw limit of the system, i.e., a gain-guided laser with a 400-Å AlxGa1−xAs (x∼0.22) quantum well and no separate waveguide region, is determined to be ∼6980 Å.

IVA-6 high-energy (λ ≤ 7300 Å) 300-K operations of single- and multiple-stripe quantum-well heterostructure laser diodes in an external grating cavity

IVA-6 high-energy (λ ≤ 7300 Å) 300-K operations of single- and multiple-stripe quantum-well heterostructure laser diodes in an external grating cavity

Growth and characterization of AlGaAs/GaAs quantum well lasers

Quantum well heterostructure (QWH) lasers have unique and desirable characteristics. A review of a variety of QWH lasers, including infrared and visible, single and multiple stripe devices, will be discussed. Emphasis will be on advances made in the last three years, including broad band tuning with an external grating, wavelength modification by annealing, and index-guiding by impurity-induced disordering.

High-energy (λ≲7300 Å) 300 K operation of single- and multiple-stripe quantum-well heterostructure laser diodes in an external grating cavity

A set of high performance single- and multiple-stripe Alx′Ga1−x′As -AlxGa1−xAs quantum-well heterostructure (QWH) laser diodes coupled to an external grating cavity is used to demonstrate the tuning properties of a semiconductor laser at short wavelength (λ≲7300 Å). A single-stripe laser diode (6-μm stripe width) with a single AlxGa1−xAs (x∼0.22) quantum well of size Lz≊400 Å is broadly tunable (7080≤λ≤7370 Å, Δℏω∼70 meV) and delivers a single dominant longitudinal mode of moderate output power (Pout∼50 mW at 200 mA, pulsed). In continuous (cw) operation (I=135 mA) a single-stripe laser has a 36-meV tuning range, 7168≤λ≤7322 Å. Phase-locked twenty- and forty-stripe diodes (3.5-μm stripe width) from the same QWH wafer are capable of single-longitudinal-mode output at higher power (peak Pout∼1.6 W at a 8.0-A, 200-ns pulse) although at slightly longer wavelength and reduced tuning range (7225≤λ≤7425 Å). Data are presented illustrating the wavelength dependence of the gain and power output as well as the partial homogeneous broadening and phase-locked nature of the QWH laser arrays. The difference in performance of the multiple-stripe diodes compared with the single-stripe structure can be attributed to the internal coupling of the optical field in neighboring stripes and the reduced threshold current density.

Stripe-geometry AlGaAs-GaAs quantum-well heterostructure lasers defined by impurity-induced layer disordering

Stripe-geometry AlGaAs-GaAs single quantum-well heterostructure lasers are demonstrated in which the region complementary to the stripe (outside of and defining the stripe) is shifted to higher band gap, and lower refractive index, by low-temperature (600 °C) Zn diffusion. Impurity-induced Al-Ga interdiffusion causes the single GaAs quantum well (x=0, Lz≊80 Å) outside of the stripe region to be mixed (‘‘absorbed,’’ x→x′) into the Alx′Ga1−x′As (x′∼0.3, Lz′≊0.18 μm) bulk-layer waveguide of the crystal.

Wavelength modification (Δℏω=10–40 meV) of room temperature continuous quantum-well heterostructure laser diodes by thermal annealing

Data are presented showing that wavelength modification, of at least 210 Å (from 8180 to 7970 Å), of broad area room temperature pulsed quantum well heterostructure (QWH) laser diodes is possible by thermal annealing. Thermal annealing at 900 °C for 8 h results in only a minor change in the threshold current density, 385–425 A/cm2, thus making possible similar wavelength modification (8180–8080 Å) of continuous (cw) 300 K stripe-geometry QWH laser diodes.

Low threshold photopumped AlxGa1−xAs quantum-well heterostructure lasers

Data are presented on very low threshold photopumped separate-confinement quantum-well heterostructure (SC QWH) lasers grown by metalorganic chemical vapor deposition (MOCVD). An unusually thin single quantum well (size Lz≲ 60 Å) is employed in the QWH with the carriers confined (‘‘trapped’’) in the interior ‘‘cladding’’ region, which serves also as the optical waveguide. Excess carriers, which are photogenerated (or injected), are confined in the thin interior cladding region (size Lz′ ∼1000 Å) and, in this charge reservoir and waveguide region, are thermionically ‘‘emitted’’ back and forth across the well until scattered to lower energy in the well (ΔE∼ℏωLO) and collected. Continuous (cw) 300 K photopumed laser operation of these QWH’s is demonstrated for very short cavities. For one QWH wafer laser operation occurs at λ∼7730 Å with a photopumping threshold of 380 W/cm2 (Jeq ∼160 A/cm2) and for another wafer at λ∼7000 Å with threshold 103 W/cm2(Jeq∼ 410 A/cm2). The photopumped samples are as small as 20×40 μm, thus making these laser thresholds (for such short cavity lengths) a factor of 3–10 better than the lowest previously reported.

High pressure experiments on Al xGa 1−xAs-GaAs quantum-well heterostructure lasers

Hydrostatic pressure experiments on Al xGa 1−xAsGaAs quantum-well heterostructure (QWH) laser diodes are described. Data are presented giving ∼11.5meV/kbar for the bandgap vs. pressure coefficient at lower pressures, with a change to 8.5–9meV/kbar at higher pressures. We suggest that this behavior is caused by biaxial and shear stresses in the active region induced by doping or composition mismatch relative to the confining layers, or between the n and p confining layers themselves. A model consistent with the experimental data is presented.