Room-temperature Threshold Current Research Articles

High-power low-threshold graded-index separate confinement heterostructure AlGaAs single quantum well lasers on Si substrates

A high-power low-threshold graded-index separate confinement heterostructure AlGaAs single quantum well laser on Si substrates has been demonstrated for the first time by a hybrid growth of migration-enhanced molecular beam epitaxy followed by metalorganic vapor phase epitaxy. The quantum well laser showed an output power of more than 400 mW per facet under pulsed conditions. A room-temperature threshold current of 300 mA was obtained with a differential quantum efficiency of 40% without facet coating. The threshold current density was 550 A/cm2 for a cavity length of 500 μm. These results show the highest peak power reported to date for low-threshold lasers on Si substrates. The full width at half maximum of the far-field pattern parallel to the junction was 6°. Threshold current densities as low as 250 A/cm2 were obtained for lasers on GaAs substrates.

High-Power AlGaAs/GaAs DH Stripe Laser Diodes on GaAs-on-Si Prepared by Migration-Enhanced Molecular Beam Epitaxy

We report on high-power low-threshold AlGaAs/GaAs double heterostructure (DH) stripe laser diodes on Si substrates by a hybrid growth of migration-enhanced molecular beam epitaxy (MEMBE) and metalorganic chemical vapor deposition (MOCVD). MEMBE-grown GaAs-on-Si had excellent surface morphology and showed much lower dislocation density than that of normal two-step MBE-grown or in situ annealed layers. The MEMBE growth and material characterization are described in detail. The 6-µm-wide oxide stripe lasers showed the highest peak power of up to 184 mW per facet reported to date, room-temperature pulsed threshold currents as low as 150 mA, and differential quantum efficiencies as high as 30% without mirror facet coating. The intrinsic threshold current density has been estimated to be about 2 kA/cm2 when taking current spreading and lateral diffusion effects into account. For comparison, the room-temperature pulsed threshold current density of 1.1 kA/cm2 was obtained for the broad-area DH laser diode fabricated on GaAs substrates in the same epitaxial deposition.

GaAs single-quantum well GRINSCH ridge lasers grown on InP by MOVPE

In this letter, lasing action of GaAs graded index separate confinement heterostructure single quantum well lasers grown by MOVPE on InP substrates is reported. Stripe lasers without facet coating show a room temperature thresholdcurrent down to 54 mA under pulsed condition. The differential quantum efficiency attains 20% per facet but preliminary aging measurements indicate a short lifetime.

High-peak-power low-threshold AlGaAs/GaAs stripe laser diodes on Si substrates grown by migration-enhanced molecular beam epitaxy

A high-peak-power low-threshold AlGaAs/GaAs double-heterostructure stripe laser diode on Si substrates grown by hybrid migration-enhanced molecular beam epitaxy (MEMBE) and metalorganic chemical vapor deposition (MOCVD) has been demonstrated for the first time. These devices showed the highest peak powers of up to 184 mW per facet reported so far for double-heterostructure stripe laser diodes on Si substrates, room-temperature pulsed threshold currents as low as 150 mA, and differential quantum efficiencies as high as 30% without mirror facet coating. An intrinsic threshold current density has been estimated to be about 2 kA/cm2 when taking current spreading and lateral diffusion effects into account. Low dislocation density shows that MEMBE can be a useful method to grow high quality GaAs and AlGaAs/GaAs layers on Si substrates by combining with MOCVD.

1.5-μm GaInAsP planar buried heterostructure lasers grown using chemical-beam-epitaxial base structures

GaInAsP/InP double heterostructures grown by chemical-beam epitaxy have been used in conjunction with liquid-phase-epitaxial regrowth to fabricate high-performance buried heterostructure lasers operating at a wavelength of 1.5 μm. These lasers show room-temperature threshold currents as low as 12 mA, external quantum efficiencies as high as 0.2 mW/mA per facet, and, in general, linear output power up to ∼10 mW/facet. The 3-dB bandwidth at optimal biasing is about 8 GHz and is believed to be limited by the heatsink stud. The relative intensity noise is low, <−150 dB/Hz at 1 GHz for bias currents from 50 mA to above 150 mA.

Defect mechanisms in degradation of 1.3-μm wavelength channeled-substrate buried heterostructure lasers

Channeled-substrate buried heterostructure (CSBH) lasers which were purged from populations undergoing high reliability qualification have been studied in detail. Gradual and rapid degradation mechanisms leading to accelerated aging failure modes have been analyzed by transmission electron microscopy, convergent beam electron diffraction, electroluminescence, energy dispersive x-ray analysis, and chemical etching. The gradual degradation mode of CSBH lasers is characterized by (1) a gradual increase in room-temperature threshold current; (2) a decrease in external quantum efficiency, typically a drop in peak value of dL/dI greater than 25%; (3) a drop in forward voltage at low current, indicating a change in junction characteristics; (4) a large peak in I(dV/dI) below threshold (at around 3 mA); and (5) an enhancement in the peak in I2(d2V/dI2) at laser threshold. A defect mechanism associated with the gradual degradation begins with a nucleation of extrinsic dislocation loops along the V-groove {111} p-n–type sidewall interfaces between the Cd-diffused p-InP and liquid-phase-epitaxial-grown n-InP buffer inside the groove. These dislocation loops subsequently grow out of the interfaces into the n-InP buffer region in the direction of minority-carrier injection, indicating a nonradiative recombination-assisted defect growth process. For those loops which enter the quaternary active region near the tip of the active crescent, the growth rate along the (001) and (010) planes is greatly enhanced and the loops eventually cut across the active stripe and become dark-line defects, as confirmed by electroluminescence. Nucleation of dislocation loops is not observed along the {111} p-p–type sidewall interfaces above the active stripe. The fact that the dislocation loops are all extrinsic in nature implies that the {111} sidewall interfaces as well as the quaternary active region contain a high density of interstitials. The possible causes for the generation and growth of the dislocation loops and the high density of point defects are discussed. The rapid degradation mode of the CSBH laser is characterized by a sudden drop in light intensity during the aging process. The associated defect mechanism starts with localized melting at the mirror facet or inside the lasing cavity. A metal-rich droplet subsequently forms which propagates along the center of the active stripe in the direction towards the cavity center via a meltback-regrowth process; i.e., material melts in front of the droplet and regrows after it propagates by. The nonideal condition of regrowth results in the formation of a wormlike defect composed of a cylinder of defective materials bounded by an off-stoichiometric interface. The wormlike defect is dark under electroluminescence. Complicated dislocation structures can also be grown from the wormlike defect under a nonradiative recombination-assisted defect growth process. These phenomena are presented and discussed.

Material properties of high-quality GaAs epitaxial layers grown on Si substrates

We report an investigation of the materials properties of GaAs on Si epitaxial layers. By using properly oriented substrates, we have found that a substantial reduction in the density of threading dislocations can be achieved. In the presence of steps, dislocations with their Burgers vectors in the (100) substrate plane are preferentially generated, which are more effective in accommodating lattice mismatch and do not thread into the epitaxial layer. We have also found that the density of threading dislocations can be reduced significantly by the use of GaAs/InGaAs pseudomorphic superlattices. Using these techniques, dislocation densities of as low as 103–104 cm−2 have been achieved in 2-μm-thick GaAs on Si epitaxial layers. In growth on nominal (100) orientations, where it is known that single atomic steps dominate, we have found no evidence of antiphase domains by transmission electron microscopy or chemical etching. This result suggests that it may not be energetically favorable for antiphase domains to form in these samples. Alternatively antiphase domains may propagate along the {111} directions and annihilate one another. For GaAs/(Al,Ga)As double heterojunction (DH) laser structures on Si substrates, the dislocation control techniques have made possible electroluminescence intensity (spontaneous emission) within a factor of 2 of state-of-the-art DH lasers on GaAs (which for reference lased at a current threshold density of 600 A/cm2). These results compare to within experimental determination. Electroluminescence intensities were also found to increase with increasing initial growth temperature. Stripe geometry lasers with room-temperature pulsed threshold currents as low as 170 mA for 10×240 μm2 stripe have been obtained using these techniques.

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

Two different quantum well heterostructure wafers are used to fabricate buried-heterostructure AlxGa1−xAs-GaAs quantum well lasers using Si-induced layer disordering (via Si diffusion). In contrast to the first wafer (QWH1), the second quantum well wafer (QWH2) utilizes Zn instead of Mg as the p-type dopant in the top AlxGa1−xAs confining layer and yields, because of concentration mismatch in acceptor and donor doping in the confining layers (nZn>nSe), inferior laser diodes owing to Zn diffusion from the p-type to the n-type confining layer during high temperature processing (850 °C Si diffusion). The first quantum well heterostructure, however, employs a lower concentration Mg doping for its p-type confining layer (nMg<nSe) and yields high performance devices when used with the Si-induced layer-disordering process. For QWH1 the p-n junction and injection is not displaced (as for QWH2) from the QW active region during Si-induced layer disordering (850 °C annealing). A fabrication process is presented in which quantum well laser diodes are built with active regions as narrow in width as 0.6 μm, cw room-temperature laser threshold currents as low as 3 mA, and pulsed current thresholds as low as 1.5 mA.

Hybrid growth of InGaAsP double-channel planar buried heterostructure lasers

Separate confinement InGaAsP structures grown by gas-source molecular beam epitaxy have been used in conjunction with liquid phase epitaxial regrowth to fabricate high-performance buried heterostructure lasers operating at a wavelength of 1.52–1.55 μm. These lasers show room-temperature threshold currents as low as 17 mA, external quantum efficiency as high as 27% per facet, and modulation bandwidth of 1.7–2.0 GHz. Single longitudinal mode operation under cw injection can be maintained up to 8 mW.

Cw operation of 1.57-μm GaxIn1−xAsyP1−yInP distributed feedback lasers grown by low-pressure metalorganic chemical vapor deposition

Continuous wave operation of 1.57-μm distributed feedback lasers fabricated on material grown by two-step low-pressure metalorganic chemical vapor deposition growth process is reported for the first time. Room-temperature continuous wave threshold currents as low as 60 mA have been measured for devices with cavity length of 300 μm and stripe width of 5 μm. Single longitudinal mode operation at fixed mode was obtained under the continuous wave condition, in the temperature range 9–90 °C, with the wavelength shift of 0.9 Å/°C. A stop band of 25 Å in which no resonance mode emission existed, was observed in the output spectrum of the distributed feedback laser.

Ridged substrate internally diffused stripe AlGaAs laser emitting in the visible wavelength region

The ridged substrate internally diffused stripe (RIDS) laser, a new gain-guided AlGaAs diode laser structure emitting light in the short wavelength region, is described. The five-layer structure, including an intermediate n-type Ga1−xAlxAs current blocking layer, is grown by one-step liquid phase epitaxy on a p-type substrate with a mesa etched in the stripe direction. Above the mesa a narrow current injection path is created by local conversion of the blocking layer due to outdiffusion of p-type dopants from the neighboring p-type cladding layer and substrate. Lasers emitting at 770 nm have thus been obtained with cw room-temperature threshold currents of 70 mA and operating stably in the fundamental transverse mode up to pulsed output powers of 100 mW.

Reduced threshold current temperature dependence in double heterostructure lasers due to separate p-n and heterojunctions

The reduced and even negative temperature dependence of the lasing threshold current that has been observed around room temperature for some (Al, Ga)As double heterostructure lasers grown by molecular beam epitaxy (MBE) can be explained by a separation of the p-n junction from the active layer heterojunctions. This separation requires minority carrier diffusion into the active layer for lasing and also allows majority carrier current flow out of the active layer. The latter current does not contribute to lasing and adds to the current necessary to achieve lasing. First-order calculations of the temperature dependences of these effects are presented, showing good agreement with measured threshold current and junction voltage dependences. The separation of the p-n and heterojunctions in the lasers with a reduced temperature dependence is supported by secondary-ion mass spectroscopy, electron beam induced current, and photoluminescence results. Besides the negative temperature dependence of the threshold current for the anomalous devices, the same remote junction effect can explain variability in the threshold current temperature dependence of seemingly normal devices and wafer-to-wafer variability in room temperature threshold currents. An improved understanding of these effects can be important in the design of lasers and optoelectronic systems.

Monolithic integration of InGaAsP heterostructure lasers and electrooptical devices

Monolithically integrated InGaAsP heterostructure lasers ( <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">\lambda = 1.3 \mu</tex> m) coupled to low-loss InGaAsP waveguides are described. The integrated InGaAsP etched-mirror lasers are fabricated using liquid-phase epitaxy and a new material-selective chemical etching technique. The integrated devices have been operated as optically coupled lasers, photodiodes, and resonant optical amplifiers. For lasers with two etched mirrors, differential transfer efficiencies as high as 8 percent have been measured for waveguide-coupled laser-photodiode pairs. The waveguide-coupled InGaAsP lasers with <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12.5 \times 300 \mu</tex> m contact stripes and two etched mirrors have room-temperature threshold currents as low as 580 mA pulsed. Similarly fabricated etched-mirror lasers without the waveguide layer have threshold currents as low as 280 mA pulsed (25 μm stripewidth). In order to obtain lower threshold currents and stable transverse-mode characteristics, InGaAsP buried-heterostructure (BH) lasers with chemically etched mirrors have been fabricated. The etched-mirror BH lasers have threshold currents as low as 160 mA pulsed. Evaluation of the etched-mirror reflectivity for the material-selective-etched lasers yielded an estimated reflectivity <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R = 0.01-0.03</tex> and scattering loss <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S = 0.6-0.8</tex> . Similar evaluation of a BH laser with non-material-selective-etched mirrors gave an estimated reflectivity <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">R = 0.11</tex> and scattering loss <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S = 0.4</tex> . Analysis of the mode reflectivity of nonideal double-heterostructure laser mirrors shows that the reflectivity is limited by misalignment (tilt) of the etched mirrors.

The electro-optic applications of InP

The electro-optic applications of InP and GaInAsP are reviewed. For fiber optics applications, diode lasers have been developed with low cw room-temperature threshold currents (25 mA), high modulation frequencies (>2 GHz) and long cw room-temperature lifetimes (>10 4 h). Avalanche photodiodes which have the p-n junction avalanche region in InP and the photosensitive region in GaInAsP have been made with peak avalanche gains of several hundred and with very low leakage currents (0.2 nA) at a gain of ten. For signal-processing applications and possibly for communications applications, an external-cavity mode-locked laser has been operated with pulse widths of ∼18 ps, an integrated Q-switched laser modulator has been operated at 2.2 GHz, and InP optoelectronic switches with very fast (<50 ps) response times have been demonstrated.

Channelled-substrate narrow-stripe GaAs/GaAlAs injection lasers with extremely low threshold currents

Channelled-substrate GaAs/GaAlAs injection lasers with very narrow current-confining stripe contacts are described. They have threshold currents less than half those previously reported for this type of device. The best devices had a room-temperature threshold current of only 12 mA pulsed and 14 mA c.w. The lasers also have very good high-temperature performance and c.w. operation has been obtained up to 160°C.

Low-threshold 1.25-μm vapor-grown InGaAsP cw lasers

Vapor-grown double-heterojunction lasers of InGaAsP/InP have been prepared with cw room-temperature threshold currents of 85 mA and differential quantum efficiencies exceeding 50% at 1.25 μm. From several lasers, fundamentaal-lateral- and fundamental-longitudinal-mode operation have been observed over moderate current ranges. Over 1000 h of room-temperature cw operation has been observed to date without significant degradation.

Barrier-controlled low-threshold p n p n GaAs heterostructure laser

Incorporation of GaAlAs potential barrier layers into the active regions of a heterostructure pnpn injection laser makes it possible to design Shockley diode lasers with low (3 kA/cm2) room-temperature threshold currents.

Etched buried heterostructure GaAs/GaAlAs injection lasers

An injection laser is described in which a semicircular GaAs active region is completely surrounded by Ga1−xAlxAs by using selective etching and a single liquid-phase epitaxial growth.The active region dimensions can be made much more symmetric than those obtained with conventional stripe geometry lasers resulting in improved optical characteristics and low room-temperature threshold currents.

Striped-substrate double-heterostructure lasers

A different type of stripe-geometry laser is demonstrated in which current confinement is achieved by selective diffusion of the n-type substrate prior to growth of the GaAs/GaAlAs heterostructure layers. This structure exhibits low room-temperature threshold currents resulting from the small lateral-current spreading from the edges of the stripe. Lowest order transverse-mode operation is achieved for stripe widths of 10μm.