GaAs Double-heterostructure Lasers Research Articles

ＭＯＣＶＤ法によるＳｉ基板上へのＧａＡｓ結晶成長とその評価

GaAs crystals were grown on Si substrates [(100) 2° off] by metalorganic chemical vapor deposition (MOCVD). The intermediate layers of GaP, (GaP/GaAsP) strained layer superlattice (SLS) and (GaAsP/GaAs) SLS were inserted to relax the lattice mismatch between GaAs and Si. They were characterized for different thickness of GaAs. The curvature radius of the wafer and the lattice constant perpendicular to the surface decreased with the increasing thickness. The etch pit density also decreased with the increasing thickness and was about 4000cm-2 for 8μm-thick GaAs. In this case no crack was observed on the surface.AlGaAs/GaAs double heterostructure laser diodes have been fabricated on GaAs/Si substrates by MOCVD. A threshold current density Jth at 16.5°C and a characteristic temperature To were 4.9kA/cm2 and 179K respectively.

High-speed modulation and switching with gain in a GaAlAs traveling-wave optical amplifier

We demonstrate optical amplification of 10-ps-long pulses in a traveling-wave type GaAs double heterostructure laser diode. A continuous train of mode-locked pulses from a dye laser focused end on into the GaAs active region is amplified by a factor of 10 under injection by a synchronous train of current pulses and attenuated by a factor of 10 in the absence of current injection. This gives a switch/modulator with modulation depths of 100, overall gain, and measured switching speed of many GHz. A cw beam can be modulated at similar gains and speeds and since the device is capable of integration the performance of many optical functions without loss is possible.

Carrier-Induced Energy-Gap Shrinkage in Current-Injection GaAs/AlGaAs MQW Heterostructures

Recombination radiation below the lowest confined particle transition energy observed in GaAs/AlGaAs multi-quantum-well heterostructure lasers was studied in comparison with that in conventional GaAs double-heterostructure lasers. The spontaneous emission shows lower-energy-side broadening in proportion to the 1/2.6 power of the injected carrier density for both the MQW structures and the conventional DHs. This carrier density dependence suggests that the observed low-energy-side broadening of the spontaneous emission is due to carrier-induced energy-gap shrinkage. The energy gap shrinkage in the MQW structures is considered theoretically by modifying the well-known bulk theory for gap shrinkage and taking the effective mass anisotropy into account. This theory is found to account satisfactorily for the observed lower-energy-side broadening. The results suggests that lasing below the lowest confined particle transition energy in MQW laser diodes is due to gap shrinkage rather than to LO-phonon participation as reported by other workers.

Temperature dependence of the lasing characteristics of the 1.3 µm InGaAsP-InP and GaAs-Al<inf>0.36</inf>Ga<inf>0.64</inf>As DH lasers

We report here our experimental observations on the temperature dependence of threshold current, carrier lifetime at threshold, external differential quantum efficiency, and gain of both the 1.3 μm InGaAsP-InP and GaAs-AlGaAs double heterostructure (DH) lasers. We find that the gain decreases much faster with increasing temperature for a 1.3 μm InGaAsP DH laser than for a GaAs DH laser. Measurements of the spontaneous emission observed through the substrate shows that the emission is sublinear with injection current at high temperatures for the 1.3 μm InGaAsP DH laser. Such sublinearity is not observed for GaAs DH lasers in the entire temperature range 115-350 K. The experimental results are discussed with reference to the various mechanisms that have been proposed to explain the observed temperature dependence of threshold of InGaAsP DH lasers. We find that inclusion of a calculated nonradiative Auger recombination rate can explain the observed temperature dependence of threshold current, carder lifetime at threshold, gain, and also the sublinearity of the spontaneous emission with injection current of the 1.3 μm InGaAsP-InP DH laser. Measurement of the nonradiative component of the carrier lifetime (τ <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">A</inf> ) as a function of injected carrier density ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">n</tex> ) shows that <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\tau_{A}^{-1} \sim n^{2.1}</tex> which is characteristic of an Auger process.

Calculation of the threshold current of stripe-geometry double-heterostructure GaAs-Ga1−xAlxAs lasers, including a self-consistent treatment of the current-temperature dependence

The threshold current density of a stripe-geometry GaAs double-heterostructure laser has been calculated taking into account the influence of the dynamical processes occurring along the junction plane. The calculation includes the effects of a temperature profile, current spreading, carrier diffusion and optical mode losses. The junction current density J, which causes heat generation, is assumed to be temperature dependent. The interdependence between them is taken into account in a self-consistent way. The temperature effect is shown to be particularly important for lasers with narrow stripe widths (&amp;lt;20 μm).

Abrupt degradation of three types of semiconductor light emitting diodes at high temperature

Abrupt degradation of three types of semiconductor light emitting diodes (LED’s), Ga1−xAlxAs double-heterostructure (DH) LED’s, Ga1−xAlxAs/ GaAs DH LED’s, and LED-operated Ga1−xAlxAs/ GaAs DH lasers operated at high temperature, was studied by electroluminescence topography, photoluminescence topography, and transmission electron microscopy. The following results were obtained from all types of the diodes. Dark regions which included 〈100〉 dark-line defects (DLD’s) and/or 〈110〉 DLD’s were often observed in the electronluminescence or photoluminescence patterns of the active regions. Two phenomena were associated with this type of degradation as follows: (i) generation of high density of dislocations and dislocation loops (in some cases, stacking faults) by dislocation glide motion due to the relaxation of the stress concentrated in the active region. (ii) subsequent development of dislocation dipoles from the glided dislocations (often accompanied with many small dislocation loops). In some of the degraded diodes, the phenomenon (ii) did not occur. There might be a certain threshold for the phenomenon (ii).

GaAs double heterostructure lasers fabricated by wet chemical etching

GaAs double heterostructure lasers compatible with monolithic intergration of optical devices have been fabricated by wet chemical etching. Results for a variety of laser orientations are reported. External differential quantum efficiencies as high as 18% have been achieved, as well as threshold current densities as low as 4.2 kA/cm2. An analysis for the evaluation of etched-mirror properties is presented. Using this analysis, it was found that etched mirrors had reflectivity R?0.22±0.05 and scattering losses S=0.3±0.1.

Light coupling problems for GaAs laser-multimode fibre coupling

Two methods of coupling a multimode fibre to a GaAs double-heterostructure laser are investigated: firstly direct coupling into a plane fibre end face, and secondly coupling via a cylindrical glass fibre lens. The power coupled into the fibre was measured for three different lasers (and with one laser as a function of its bias current). The measured coupling efficiency was compared with theoretical considerations. Optical feedback effects due to reflection of the fibre end face have been observed. These cause an oscillation of the coupled power, when the separation between the laser and the fibre end face is varied by only a few tenths of a micron. The alignment tolerances using a fibre lens were, as expected, more critical than for direct coupling.

Low-loss AlxGa1−xAs waveguides grown by molecular beam epitaxy

Low-loss AlxGa1−xAs waveguides with x=0.1 and x=0.25 have been fabricated by molecular beam epitaxy. Losses are determined for both n- and p-type waveguides using a combination of Raman scattering and transmission measurements. These waveguides have optical losses ⩽1.5 cm−1 between 1.1 and 1.4 eV, the energy of the GaAs double heterostructure laser, and are comparable to similar waveguide structures grown by liquid phase epitaxy.

I-3 laser-excited photoluminescence of three-layer GaAs double-heterostructure laser material

Summary form only. An abstract of the above-titled article taken from the Device Research Conference (24-26 June 1975, Carleton University, Ottawa, Ont., Canada) is presented.

Near-field emission from GaAs double-heterostructure laser mirrors

The near fields of GaAs double-heterostructure (DH) laser mirrors are studied both in photoluminescent (PL) excitation and in lasing emission. The various liquid-phase-epitaxial (LPE) layers of the laser diode are optically delineated, with \mp0.1-\mu m resolution, in a wavelength-selective PL detection system. The near fields of the transverse lasing modes are correlated with the LPE layers that constitute the optical waveguide. With special emphasis on the large-cavity fundamental-mode Ppn'N laser, it is found that small changes in refractive index within the waveguide have pronounced effects on the distribution of stimulated power within the LPE layers. The fundamental mode is found to be contained within the slightly higher refractive-index gain region. This explains the previously observed localization of catastrophic mirror damage and the anomalously large angles of beam divergence. High-order modes are also excited in the gain region when its thickness and refractive-index step within the waveguide exceed some prescribed limits.

Photon recycling in semiconductor lasers

In a suitably designed semiconductor laser, spontaneously emitted photons with energies above the absorption edge can be reabsorbed in the active layer and can decrease the current density that must be supplied to reach the lasing threshold. The magnitude of the threshold reduction is estimated to be about 20% in a GaAs double-heterostructure laser at room temperature.

Nature of optically induced defects in Ga1−xAlxAs–GaAs double-heterojunction laser structures

It is shown by transmission electron microscopy (TEM) investigation that the dark line defect (DLD) structure induced by optical pumping of undoped Ga1−xAlxAs–GaAs double-heterostructure (DH) laser material is identical to the DLD's which develop during laser diode operation of devices fabricated from similar p-n structure material. This confirms the DLD generation, and subsequent failure of cw laser diodes can be a growth-related property of Ga1−xAlxAs–GaAs DH laser material and is not necessarily associated with p-n junction dopants or contact metallization technology.

Mode control in GaAs large-cavity double-heterostructure lasers

Factors that are important in maintaining zero-order-mode operation at high power in large-cavity GaAs double-heterostructure lasers are described. In the first place, it had previously been shown that the electronic gain of the zero-order mode is larger than that of high-order modes when gain is restricted to two-thirds of the waveguide thickness, and the remaining one-third is kept lossy. In practice, this is realized by the LPE growth of a heteroboundary within the cavity such that (a) by the addition of a small amount of aluminum the band gap of the umpumped region is made larger (≈40 meV) than that of the gain region and (b) the umpumped region is doped with a donor concentration larger than the electron concentration in the gain region at lasing threshold. The second factor that is useful in maintaining zero-order-mode operation at high power levels is the use of a zero-order-mode antireflective (AR) coating that increases the mirror transmission loss for the undesired high-order modes. This zero-order-mode selective AR coating consists of layers of ZnS and Al2O3 whose thicknesses are accurately controlled to provide maximum transmission loss at some prescribed angle of emergence relative to the normal axis. Experimental results obtained on zero-order-mode large-cavity PpnN lasers substantiate these design considerations.

Zero-order transverse mode operation of GaAs double-heterostructure lasers with thick waveguides

Fundamental (TE00 and TM00) transverse mode operation has been obtained from stripe-geometry GaAs double-heterostructure injection lasers with optical waveguides which are 2 μm thick. Selection of the zero-order mode perpendicular to the junction plane is achieved by providing a spatially nonuniform gain distribution within the waveguide. The nonuniformity is realized by locating the p-n junction within the waveguide such that the ratio of the thicknesses of the p and n regions is 2/1. For this choice of gain profile, the electronic gain of the zero-order mode is greater than that for higher-order modes by an amount sufficient to overcome the higher mirror loss of the zero-order mode. Along the junction plane, the stripe-geometry configuration provides zero-order mode operation. As a result, pure fundamental mode operation has been obtained at current levels up to 2.2 times threshold. Measurements are also reported for the internal stimulated efficiency, the bulk loss, and the electronic gain at threshold of the zero-order mode relative to the higher-order modes. Single-ended pulse output powers as high as 0.7 W (4×106 W/cm2 at the mirror) have been obtained from AR-coated devices without observable mirror damage.

Behavior of threshold current and polarization of stimulated emission of GaAs injection lasers under uniaxial stress

The effect of uniaxial pressure perpendicular to the junction on the threshold current of GaAs double-heterostructure lasers and homostructure lasers operated at room temperature was studied. The threshold either first increases with pressure up to a certain critical pressure P o and then decreases, or decreases with pressure from the beginning, depending on whether the laser is operating in a TE or a TM mode with zero pressure. In the first case, the change in the threshold current behavior at P o is accompanied by a change of modes from TE to TM. This behavior is explained by a model, taking into account the splitting of the valence bands of GaAs on application of uniaxial pressure.

Gradual degradation of GaAs double-heterostructure lasers

Measurements have been made in the manner in which device parameters vary during forward-biased operation of GaAs double-heterostructure lasers. The threshold current increases, but the slope external efficiency remains relatively constant. A conclusion is that optically absorbing or scattering centers are not the cause of degradation. The increase of threshold is attributed to an increasing nonradiative recombination component, the defect responsible for which is not yet determined. The important question regarding the role of optical flux is examined and it is shown that this is not the cause of the observed degradation in device characteristics.