High continuous wave output power InGaAs/InGaAsP/InGaP diode lasers: Effect of substrate misorientation

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166

Al-free 980 nm InGaAs/InGaAsP/InGaP laser structures grown by low-pressure metalorganic chemical vapor deposition ͑LP-MOCVD͒ have been optimized for high cw output power by incorporating a broad waveguide design. Increasing the optical-confinement layer total thickness from 0.2 to 1.0 m decreases the internal loss fivefold to 1.0-1.5 cm Ϫ1 , and doubles the transverse spot size to 0.6 m ͑full width half-maximum͒. Consequently, 4-mm long, 100-m-aperture devices emit up to 8.1 W front-facet cw power. cw power conversion efficiencies as high as 59% are obtained from 0.5-mm long devices. Catastrophic-optical-mirror-damage ͑COMD͒ power-density levels reach 15.0-15.5 MW/cm 2 , and are found similar to those for InGaAs/AlGaAs facet-coated diode lasers. © 1996 American Institute of Physics. ͓S0003-6951͑96͒01237-5͔Diode lasers with reliable operation in the 980 nm wavelength range are needed for applications such as pump sources for solid-state lasers or rare-earth-doped fiber amplifiers, and medical therapy. The growth of InGaAsP alloys lattice-matched to a GaAs substrate is very attractive as an aluminum-free alternative to the conventional AlGaAs-based materials. The aluminum-free InGaAs͑P͒/InGaP/GaAs material system has several advantages over the GaAs/AlGaAs material system for the realization of reliable, high-power diode laser sources: ͑1͒ the low reactivity of InGaP to oxygen facilitates regrowth for the fabrication of single-mode index-guided structures, 1,2 ͑2͒ higher electrical 3,4 and thermal conductivity 5 compared with AlGaAs, ͑3͒ potential for improved reliability, 6 and ͑4͒ potential for growth of reliable diode lasers on Si substrates. 7 Here, we report on the optimization of InGaAs/InGaAsP/InGaP strained-layer quantum well laser structures by using the broad-waveguide concept, 8,9 for maximizing the cw output power. As a result, record cw performances ͑8.1 W front-facet power, cavity length Lϭ4 mm; and 59% wallplug efficiency, Lϭ0.5 mm͒ are obtained from broad-area ͑100-m wide stripe͒ devices. Catastrophic optical mirror damage ͑COMD͒ values from LR/HR facet-coated devices under cw operation ͑i.e., ϳ15 MW/cm 2 ) are found to be similar to those for InGaAs/ AlGaAs facet-coated lasers, indicating that the quantum-well material ͑i.e., strained-layer InGaAs͒, and not the cladding/ confinement layers material, primarily determines the COMD value.The cw output power of a diode laser is generally limited by either thermal rollover or COMD. Thermally limited power saturation can be eliminated by designing laser structures to have high total power conversion efficiencies, low threshold-current density, and weak temperature sensitivity for both the threshold current and the external differential quantum efficiency ͑i.e., high T 0 and T 1 values͒. 4 As previously reported, 4 the use of a double-quantum-well ͑DQW͒ InGaAs active region together with high-band-gap InGaAsP (E g ϭ1.62 eV͒ optical-confinement layers, leads to 0.98 m diode lasers with relatively temperature insensitive characteristics. Given a certain CO...

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8 W continuous wave front-facet power from broad-waveguide Al-free 980 nm diode lasers

Mawst

Bhattacharya

Lopez

et al. 1996

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166

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confidence: 99%

“…13 For example, for 500-mm-long devices, the relative decrease in d over the 20-55°C temperature range for lasers grown on 2°-off substrates is almost twice that of those grown on exact substrates. 13 The characteristic temperature coefficients for both threshold (T 0 ) and differential quantum efficiency (T 1 ) are higher for devices grown on exact ͑100͒ substrates, which can be related to the interface morphology of the quantum-well active region. While the exact mechanism is not fully understood, the roughness of the QW interface on the misoriented substrate leads to an increased carrier leakage from the quantum well into the confinement layers, resulting in a lowering of the efficiency and increased threshold current with increasing temperature.…”

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confidence: 99%

“…Cross-sectional TEM measurements are used to determine QW thickness. We have earlier reported 13 on the substrate dependence of the 12 K PL spectra from a single InGaAs QW. Sharp multiplet emission, with a FWHM of 6-10 meV, is observed from exactly ͑100͒ substrates whereas signals from misoriented substrates ͑2°, 6°, 10°off towards ͓110͔͒ are weaker, much broader, and shifted to lower energies.…”

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confidence: 99%

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Interface structures of InGaAs/InGaAsP/InGaP quantum well laser diodes grown by metalorganic chemical vapor deposition on GaAs substrates

Bhattacharya

Mawst

Nayak

et al. 1996

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We have studied the effects of substrate misorientation on the growth of strained-layer In 0.18 Ga 0.82 As quantum well laser structures with InGaAsP confinement layers and In 0.5 Ga 0.5 P cladding layers lattice matched to a GaAs substrate. Low-temperature photoluminescence ͑PL͒ and atomic force microscopy ͑AFM͒ provide evidence of a strong substrate-orientation dependence of the interface structure. The surface morphology of the InGaAs quantum well is found to be determined primarily by the underlying InGaAsP confinement layer. Structures grown on exact-͑100͒ oriented substrates exhibit three-dimensional island surface morphology, whereas growths on ͑100͒ substrates oriented 2°towards ͓110͔ exhibit high surface roughness, possibly due to step bunching. These observations correlate well with previously reported device performance from strained quantum well laser diodes in the InGaAs/InGaAsP/InGaP material system, and can serve as a tool to optimize device performance. © 1996 American Institute of Physics. ͓S0003-6951͑96͒03716-0͔The aluminum-free InGaAs/InGaAsP/InGaP material system is increasingly important for the fabrication of semiconductor lasers. The advantages of using the Al-free system for laser diodes over the conventional AlGaAs-based material system are: ͑1͒ low surface recombination 1 in this material system results in lower facet temperatures in laser diodes, permitting reliable operation at high output powers, ͑2͒ higher electrical, 2 and thermal 3 conductivity of InGaP cladding layers results in better total power-conversion efficiencies at high powers, and ͑3͒ novel index-guided structures can be easily fabricated due to the absence of aluminum containing compounds at the regrowth interface.4 Optimization of such devices however requires an understanding of the nature of quantum well ͑QW͒ growth in this material system. Recently, studies relating interface structure to photoluminescence ͑PL͒ spectra have been carried out for quantum well AlGaAs/GaAs, 5 strained InGaAs/GaAs ͑Ref. 6͒ and InGaAs/InP ͑Ref. 7͒ structures. Interface studies by atomic force microscopy ͑AFM͒ on MOVPE-grown ͑In,Ga͒͑As,P͒ materials, have mainly focused on growth mechanisms; few studies have correlated interface structure with device performance. Furthermore, the interfacial structure of strainedlayer InGaAs/InGaAsP QW structures has not been reported.Al-free lasers with GaAs or low-band-gap InGaAsP confinement layers exhibit strong carrier leakage from the InGaAs QW to the optical confinement layers, resulting in a strong temperature dependence of both threshold current and differential quantum efficiency. [8][9][10] To minimize this problem, higher band-gap InGaAsP confinement layers and multiple quantum wells are used to reduce carrier leakage, and thus reduce temperature sensitivity.9-12 The interface structure of InGaAs quantum wells on InGaAsP, integral to these improved structures, has however not been studied. In this letter, we report on the effects of substrate misorientation on the growth of strained-layer In 0....

“…P opt is a function of temperature since both the threshold current I th , as well as the external differential quantum efficiency D , are temperature sensitive, characterized 4,22 by the coefficients T 0 and T 1 , respectively,…”

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Design considerations and analytical approximations for high continuous-wave power, broad-waveguide diode lasers

Botez

1999

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118

Accurate analytical approximations are derived for the equivalent transverse spot size, d/⌫ ͑Ͻ5% error͒, and the transverse beamwidth 1/2 ͑Ͻ2% error͒, of broad-waveguide-type diode lasers, over a wide range in waveguide width: from the first-order-mode cutoff to the third-order-mode cutoff. The analytical formulas are found to be in good agreement with experimental values. For low-series-resistance and thermal-resistance devices, it is found that the junction-temperature rise ⌬T j in continuous wave ͑CW͒ operation is a strong function of both the characteristic temperature T 1 for the external differential quantum efficiency D as well as of the heatsink thermal resistance. If the device has relatively temperature-insensitive D ͑i.e., T 1 տ1000 K) the maximum CW power as well as the power density at catastrophic optical mirror damage, P COMD , are limited, for a given active-region material, only by the heatsink heat-removal ability. For large d/⌫, 0.97 m emitting, 100 m stripe InGaAs/InGaAs͑P͒/GaAs devices with T 1 ϭ1800 K, record-high CW and quasi-CW ͑100 s wide pulses͒ output powers are obtained. The ratio of quasi-CW to CW P COMD values is only 1.3, in contrast to devices of poor carrier confinement and subsequent low-T 1 values ͑ϳ140 K͒, for which the ratio is 1.9, and whose maximum CW powers are ϳ40% less than those obtainable from high-T 1 devices. © 1999 American Institute of Physics. ͓S0003-6951͑99͒03421-X͔In the quest for high continuous-wave ͑CW͒ powers from diode lasers, one key concept has been that of the broad-waveguide ͑BW͒ separate-confinement heterostructure ͑SCH͒;1,2 that is, a structure that concomitantly provides both a large equivalent ͑transverse͒ spot size 3 as well as low internal cavity loss, 1-3 ␣ i (р1 cm Ϫ1) with no sacrifice in wallplug efficiency at high drive levels. 4 As a result, record-high CW powers have been achieved from BW-type devices at wavelengths from the visible to the midinfrared. 4-9Here, we derive for BW-type devices accurate analytical expressions for the equivalent spot size, d/⌫ ͓d is quantumwell͑s͒ thickness and ⌫ the ͑transverse͒ optical-confinement factor͔ and the transverse beamwidth. In addition, we compare record-high CW and quasi-CW ͑QCW͒ data at ϭ0.97 m and determine which parameters are critical for achieving high CW power at catastrophic optical mirror damage ͑COMD͒.From the definition 7 of the internal optical-power density at COMD, P COMD , one can express the maximum CW power aswhere W is the stripe width and R the front-facet reflectivity. It has been established that for conventionally facetpassivated diodes P COMD is a function of the active-region material, 10 being in effect inversely proportional 11 to the surface-recombination velocity s as long as 12 sу10 5 cm/s. That is, for a given active-region material with sу10 5 cm/s and given stripe width, P max,cw directly scales with d/⌫. One main way to increase d/⌫ is to use BW-SCH structures. 3A schematic representation of the BW-SCH laser structure and its optical-mode profile is shown in Fig. 1. ...