Strained Quantum Well Lasers Research Articles

Theoretical analysis of high-temperature characteristics of 1.3-μm InP-based quantum-well lasers

By taking into account the electrostatic deformation in the band profiles and the temperature dependence of the optical dephasing time, we study the temperature sensitivity of the differential gain, threshold carrier density, and radiative current density in 1.3-/spl mu/m InP-based strained-layer quantum-well (QW) lasers. Electrostatic deformation is analyzed by the self-consistent numerical calculation of Poisson's equation, the scalar effective-mass equation for the conduction band, and the multiband effective-mass equation for the valence band. The optical dephasing time is then obtained from the intrasubband scattering rates for electrons and holes within the fully dynamic random phase approximation including carrier-carrier and carrier-phonon interactions on an equal basis. It is clarified that the electrostatic band-profile deformation is one of the dominant mechanisms For determining the temperature sensitivity Of the differential gain, while the optical dephasing time has a pronounced influence on the transparent condition at elevated temperatures. We demonstrate that the electrostatic band-profile deformation and the temperature-dependent optical dephasing play essential roles in determining the high-temperature characteristics of InP-based QW lasers. >

Influence of strain on lasing performances of Al-free strained-layer Ga(In)As(P)-GaInAsP-GaInP quantum-well lasers emitting at 0.78>λ>1.1 μm

The influence of strain on lasing performances of Al-free strained-layer Ga(In)As(P)-GaInAsP-GaInP quantum-well lasers is investigated for the first time over a large emission range of 0.78</spl lambda/<1.1 /spl mu/m. GaAsP and InGaAs are used for tensile and compressive-strained quantum-well layers, respectively, while GaAs and GaInAsP lattice-matched to GaAs are applied for unstrained quantum wells. The laser structures were prepared by using gas-source molecular beam epitaxy, and broad-area and ridge waveguide Fabry-Perot laser diodes were fabricated. This study shows that applying both tensile and compressive strains in the quantum well reduces threshold current density for the Al-free strained-layer quantum-well lasers. However, it was found that the lattice relaxation set a limitation of maximum compressive strain (i.e., maximum lasing wavelength) for the compressive strained InGaAs lasers while the carrier confinement determined the acceptable maximum tensile strain (i.e., minimum lasing wavelength) and lasing performances for the tensile strained GaAsP lasers. Threshold current density as low as 164 A/cm/sup 2/ has been obtained for 1.4% compressive-strained InGaAs-GaInAsP-GaInP lasers having a 12-nm thick quantum well. However, excellent characteristics, such as low threshold current, high efficiency low internal loss, and high output power, have been achieved for the Al-free strained-layer quantum-well lasers.<<ETX>>

Orientation dependence of optical properties in long wavelength strained quantum-well lasers

The dependence of optical properties on crystal orientation is analyzed for long wavelength strained quantum-well (QW) GaAsP-InGaAsP lasers. The calculation is based on the multiband effective mass theory which enables us to consider the anisotropy and the nonparabolicity of the valence-band dispersions. It is found that the optical gain increases as the crystal orientation is inclined from [001] toward [110]. This is due to the reduced valence-band density of states. The differential gain is about 1.6 times larger for the [110]-oriented 1.55-/spl mu/m strained QW's than for equivalent [001]-oriented QW's. It is also shown that the threshold current density in 1.3-/spl mu/m strained QW lasers decreases to two-thirds of that in the [001]-oriented laser as the orientation is inclined away from [001] by 40/spl deg/-90.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Theoretical analysis of temperature sensitivity of differential gain in 1.55-μm InGaAsP-InP quantum-well lasers

We study the temperature sensitivity of the differential gain in InGaAsP-InP strained-layer (SL) quantum-well (QW) lasers operating at a wavelength of 1.55 μm. Electrostatic deformation in conduction-band and valence-band profiles is taken into account by solving Poisson's equation and the effective-mass equations for conduction and valence bands in a self-consistent manner. We demonstrate that electrostatic deformation in both band profiles plays a significant role in determining the temperature sensitivity of the differential gain in 1.55-μm InGaAsP-InP SL QW lasers. The physical mechanism for limiting the differential gain at elevated temperatures is also discussed.

InGaAs-AlGaAs-GaAs strained-layer quantum-well heterostructure square ring lasers

The processing and optical characteristics of an InGaAs-AlGaAs-GaAs strained-layer quantum-well heterostructure square ring laser (4 /spl mu/m width and 50-/spl mu/m/side length) with two output waveguides and two narrow physical gap couplers are described. The ridges and the total internal reflection (TIR) mirrors of the square ring lasers are fabricated by two photolithographic steps with a single SiO/sub 2/ mask and planarized with polyimide such that the TIR mirrors are etched through the active region while the ridge waveguides are not. Single longitudinal mode operation is observed with a side mode suppression ratio of 20 dB. Asymmetric characteristics of emission spectra from the two output waveguides demonstrates that the square ring lasers operate in traveling wave modes.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The effects of strain on intrasubband scattering rates in InP-based strained-layer quantum-well lasers

We analyze the strain effects (both compressive and tensile) on the intrasubband scattering rates for electrons and holes in InP-based strained-layer quantum-well (SL-QW) lasers. Carrier-carrier and carrier-LO-phonon interactions are taken into account on an equal basis within the fully dynamic random phase approximation for multi-subband structures at finite temperatures. The principal influences of the strain-induced changes in the valence band structures on the scattering rates are discussed in terms of the changes in the valence-band density of states and the phonon coupling with holes. We show that the hole-hole interaction plays a dominant role in determining the hole scattering rates regardless of the carrier energy. On the other hand, for electron scattering rates, the dominant scattering mechanism switches from electron-hole interaction to electron-electron interaction as the initial energy of electron increases. We also demonstrate that it is essential to take into account the mutual interactions among the individual scattering processes, such as carrier-carrier scattering and carrier-LO-phonon scattering, when evaluating the intrasubband scattering rates in SL-QW lasers.

Dependence of linewidth enhancement factor on crystal orientation in strained quantum well lasers

The linewidth enhancement factor a in strained quantum well (QW) lasers is estimated theoretically for various crystallographic directions. It is found that the a factor in a strained In/sub 0.7/Ga/sub 0.3/As-InP QW laser on a [111] substrate is less than 1.4, much lower than for conventional strained QW lasers on [001] substrates.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Characterization of effective masses in strained quantum-well laser structures.

Valence-band structures and hole effective masses for strained-layer quantum-well laser structures were calculated, using ${\mathrm{In}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$${\mathrm{Ga}}_{\mathit{x}}$As/InGaAsP material system as an example. A 4\ifmmode\times\else\texttimes\fi{}4 strain Hamiltonian with heavy-hole and light-hole band mixing was used in the calculation. Systematic numerical results have been presented for a large range of material parameters and quantum-well widths.

Low threshold current density 1.3-/spl mu/m strained-layer quantum-well lasers using n-type modulation doping

We have developed 1.3 /spl mu/m n-type modulation-doped strained-layer quantum-well lasers. Modulation-doped lasers with long cavities (low threshold gain) exhibit much lower threshold current densities than conventional lasers with undoped barrier layers. The lowest threshold current density we obtained was 250 A/cm/sup 2/ for 1500 /spl mu/m long lasers with five quantum wells. The estimated threshold current density for an infinite cavity length was 38 A/m/sup 2//well. This is the lowest value for InGaAsP-InGaAsP and InGaAs-InGaAsP quantum well lasers to our knowledge. >

High-power, high-temperature InGaAs-AlGaAs strained-layer quantum-well diode lasers

Output power of 2.26 W and maximum power conversion efficiency of 57% were obtained for InGaAs-AlGaAs strained-layer quantum-well lasers (25 °C, λ ≃ 1 µm). Output powers were 2.35 W (–55 °C) and 1.79 W (75 °C) with maximum power efficiencies of 51% (–55 °C) and 46% (75 °C).

Theoretical analysis of pure effects of strain and quantum confinement on differential gain in InGaAsP/lnP strained-layer quantum-well lasers

The pure effects of both strain and quantum confinement on differential gain of InGaAsP/InP strained-layer quantum-well lasers (SL-QWL's) are studied on the basis of valence band structures calculated by k/spl middot/p theory. Using an InGaAsP quaternary compound as an active layer makes it possible to distinguish the effect of strain (both tensile and compressive) from the quantum-confinement effect when keeping the emission wavelength constant. The essential features of strain-induced changes in the valence band structures are extracted from the k/spl middot/p results by four characterization parameters: the averaged density of states (DOS), the subband energy spacings, the joint density of electron and hole states, and the squared optical matrix elements. Each of them is then directly correlated to differential gain in SL-QWL's. In tensile-strained quantum wells, all of these factors are significantly improved compared with unstrained wells, while only the averaged DOS is improved in compressive-strained wells. Due to these characteristic features, it is concluded that the intrinsic potential of tensile-strained QWL's for improving differential gain is twice as high as that of compressive-strained ones. On the basis of the essential features of the strain-induced changes in valence band structures, we also discuss basic design principles for SL QWL's with larger differential gain.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Degradation behavior of 0.98-μm strained quantum well InGaAs/AlGaAs lasers under high-power operation

The degradation behavior of 0.98-/spl mu/m strained quantum well (QW) InGaAs/AlGaAs lasers with facet coating films is systematically clarified at high-power operations of more than 100 mW/facet. The degradation is mainly caused by the instability of the interface between the laser material (facet) and the antireflecting (AR) coating film. This phenomenon is associated with a high rate of facet oxidation and generation of instantaneous catastrophic optical damage (COD) at a relatively low optical output power in lasers without facet coating films. Throughout the clarification of those phenomena, the main reliability problem in 0.98-/spl mu/m strained QW lasers under high output power operation is clarified.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

The theory of strained-layer quantum-well lasers with bandgap renormalization

A theoretical model for the strained-layer quantum-well laser is presented taking into account the valence-band mixing and the bandgap renormalization. Our theoretical approach for the electronic properties is based on the Luttinger-Kohn Hamiltonian, including the strains and the carrier-induced bandgap shifts using the Hartree-Fock approximation. The effects of the biaxial compressive and tensile strains on the gain, the output characteristics, the bandgap renormalization, and the modulation response of strained-layer quantum-well lasers are studied. We present new results incorporating the many-body effects in the form of the bandgap renormalization with the valence-band mixing and the multisubband effects. It is found that the bandgap renormalization depends strongly on the nature of strain applied to the quantum well. The differential gain that determines the upper frequency limit of the direct current modulation is calculated from the total derivative of the equigain surface with respect to the carrier- and the photon-densities near the threshold condition. Our approach to the differential gain yields reasonable agreement between theory and experiment for the 3-dB modulation bandwidth. Both InGaAs-AlGaAs and InGaAs-InP strained quantum-well systems are considered.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Effects of well number in 1.3-μm GaInAsP/InP GRIN-SCH strained-layer quantum-well lasers

Effects of well number in 1.3-/spl mu/m GaInAsP/InP GRIN-SCH compressively strained-layer quantum-well lasers were investigated from the viewpoint of the crystal quality of the strained-layer quantum wells and low threshold current operation at high temperature. As a result, we find that the optimum well number that gives the minimum threshold current increases with operating temperature and it was limited to the degradation of crystal quality of the quantum wells due to the critical thickness. A very high CW operating temperature of 170/spl deg/C was obtained. A very high output power of over 300 mW was also achieved. >

5.5-MeV proton irradiation of a strained quantum-well laser diode and a multiple quantum-well broadband LED

The degradation under 5.5-MeV proton irradiation of two classes of quantum-well-based fiber-optic light sources was evaluated for satellite applications. The first was an InGaAs/GaAs strained-layer quantum-well (QW) laser, and the second was a broadband light-emitting diode (LED) based on dual asymmetric quantum wells in the InGaAs/GaAs/AlGaAs system. The QW LEDs were more tolerant of proton irradiation (-3 dB power at approximately 3*10/sup 13/ protons/cm/sup 2/) than the QW lasers (-3 dB power at approximately 3*10/sup 12/ protons/cm/sup 2/). The LEDs were operated far into gain saturation with a high-loss cavity structure, while the lasers were operated in a region where gain was more sensitive to current density. Therefore, atomic displacement-related recombination sites had a greater detrimental effect upon the lasers than the LEDs. The lasers held constant slope efficiency, and current thresholds increased linearly with proton fluence, while both LED power and slope efficiency decreased with proton fluence. The determined damage factors for both devices were found to fall within values predicted from a universal damage relation previously reported by G.P. Summers et al. (1988).< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Dependence of differential quantum efficiency on the confinement structure in InGaAs/InGaAsP strained-layer multiple quantum-well lasers

An internal efficiency of 91% was obtained with In/sub 0.7/Ga/sub 0.3/As/InGaAsP strained-layer multiple quantum well (MQW) lasers emitting at a wavelength of 1.5 mu m. The dependence of the reciprocal differential quantum efficiency on the length of the laser cavity shows that the absorption loss in the InGaAsP ( lambda =1.3 mu m) confinement layer caused by carrier overflowing into the confinement layer reduces the internal efficiency. >

Spectral dependence of differential gain, mode shift, and linewidth enhancement factor in a InGaAs-GaAs strained-layer single-quantum-well laser operated under high-injection conditions

Strained-layer quantum-well lasers, when operated at moderate injection levels, are known to exhibit a reduced linewidth enhancement factor, alpha , an important parameter in the design of semiconductor lasers. Under increased loss conditions, when spill-over of carriers into the barrier becomes significant, they can also be operated in a quasi-double-heterostructure mode at the barrier wavelength. Thus, by proper control of loss, lasing at different wavelengths is possible, at least in principle. An investigation has been carried out to determine explicitly the spectral dependence of the differential gain, spectral shift, and alpha of a narrow-stripe InGaAs-GaAs separate-confinement strained-layer single-quantum-well laser operated near the wavelength of the GaAs barrier under extremely high injection. Despite the carrier spill-over, alpha was found to be remarkably low, about 1.7 at the spectral gain peak (877 nm), which was also the lasing wavelength.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Envelope function calculations of linear and nonlinear optical gains in a strained-layer quantum-well laser

The linear and nonlinear optical gain of strained-layer InGaAs-AlGaAs quantum well (QW) lasers are studied theoretically, with band mixing effects taken into account. Effects of the biaxial compressive strain of the InGaAs-AlGaAs QW on the band structure are investigated by solving for the Pikus-Bir Hamiltonian. The biaxial compressive strain separates the HH and the LH subbands by pulling down the HH subbands and pushing the LH subbands away from the valence band edge. Since the C-HH transition is dominated by the TE polarization, it is expected that the TE mode gain would be substantially larger than the TM mode gain. The gain and the gain-suppression coefficient are calculated from the complex optical susceptibility obtained by the density matrix formalism. Optical output power is calculated by solving the rate equations for the stationary states with nonlinear gain suppression. The calculated L-I characteristics shows reasonable agreement with the experimental data.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

Wavelength control and residual oxygen in AlGaAs/InGaAs strained quantum-well heterostructures grown by molecular beam epitaxy

We have studied the residual oxygen in AlGaAs/GaAs/InGaAs heterostructures, and the effects of As2 and As4 species and growth temperature (TS) on optical properties of InGaAs quantum wells (QW) for AlGaAs/InGaAs strained QW lasers. In clean growth conditions, the optical properties of InGaAs QWs are insensitive to the As beam used, and the luminescence intensity does not increase with increasing TS from 570 to 630 °C or by postgrowth thermal annealing. Because of the very strong dependence of In sticking coefficient on TS, the TS needs to be precisely controlled to obtain the desired emission wavelength. A solubility limit of ∼8×1017 cm−3 for Be is measured in AlAS at 700 °C growth temperature. The residual oxygen level in undoped and Be-doped AlxGa1−xAs with x=0.35 was found to be higher by a factor 2 than in x=0.5, 0.6, and 1.0 compositions. In Si-doped AlxGa1−xAs, the O level is the lowest with no variation with x.

Carrier heating and spectral hole burning in strained-layer quantum-well laser amplifiers at 1.5 μm

We present the first polarization studies of femtosecond gain dynamics in strained-layer multiple-quantum-well laser amplifiers. We observe a response consistent with spectral hole burning when the diode is biased in the absorbing regime. In the gain regime, we show that the carriers are heated by free-carrier absorption and that there is a measurable delay (∼200 fs) in the thermalization of the hot-carrier distribution. Subsequent cooling to the lattice temperature follows with a time constant of ∼1 ps.