Kozlowska, Anna

Malag, Andrzej

Wesolowski, Marek

English

Università del Salento: ESE - Salento University Publishing

Effect of Al−content reduction in (AlGa)As cladding layers of MOVPE grownhigh−power laser diodesAndrzej Malag 1) ,  Anna Kozlowska  1) ,  Marek Wesolowski  1)1) Institute of Electronic Materials Technology, 133 Wolczynska Str., 01−919 Warsaw, Poland     It has been generally accepted to consider AlGaAs/GaAs heterostructures as lattice−matched. Due to the smalllattice constant difference between AlAs and GaAs however, the lattice−mismatch in e.g. Al0.7Ga 0.3As/GaAsheterojunction is a/a = 1E−3 [1]. This magnitude is, for instance, only one−fourth of that for doubtlesslattice−mismatched GaAs0.88P0.12/GaAs junction ( a/a = 4.3E−3, tensile−strained GaAsP layer with P−contentdesigned for  = 808 nm emission). This means that in heterostructures containing thick AlxGa1−xAs claddinglayers with high x−values one can expect some small perturbations during the epitaxial growth, leading to the QWactive region performance deterioration. This is in significant degree dependent on the optimisation of the MOVPEgrowth conditions, but in general, it would be a good idea to decrease the Al−content in claddings if possible. Thiswould be advantageous not only because of the lattice mismatch decrease, but also because of oxygen and carbonincorporation reduction, dopant activation energy decrease and so on.     This idea can be realised using the double−barrier SCH (DBSCH − MQW or SQW) heterostructure in the laserdiode (LD) design. The main feature of DBSCH is the presence of two thin, wide−gap (high x) AlxGa1−xAs barrierlayers inserted between waveguide and cladding layers which allows to set the x−values in the claddings relativelylow. The role of the barrier layers is twofold, to control the waveguiding properties and to maintain the carrierconfinement [2]. QW−DBSCH can be also considered as a conventional QW−SCH with x−values decreased inalmost whole cladding layers except some thin, high−x sublayers remaining at both cladding−waveguideinterfaces. This is shown in Fig.1.      Two design versions of SQWDBSCH are shown, differing toeach  o ther  in  the  bar r ie r  andwaveguide layers parameters: inthe version A (Fig.1) the barriert h i c k n e s s  t b =  1 5 0  n m ,  t h ew a v e g u i d e  t h i c k n e s s  a n dcomposition tw= 140 nm and xw=0.3, respectively, and, analogously,tb= 30 nm, tw= 130 nm and xw=0 .35  in  t he  ve r s ion  B .  O the rparameters ,  common for  bothversions are inserted in Fig.1. Thethird structure (S) shown in Fig.1 isconvent ional  SQW SCH withsimilar waveguide and active layer(tensile−strained GaAsP QW). Themain difference is in the parametersof the cladding layers: tc= 1.5 µm, xc= 0.6 for SCH and tc= 3 µm, xc= 0.45 for DBSCH. Calculated optical fieldintensity distributions of the fundamental (only maintained) transverse TM mode in the waveguide and at the LDfacets are also shown for three structures under investigation. TM polarised emission is the only one taken intoaccount as dominant in tensile−strained QW LDs. The difference between A and B profiles indicates the possibilityof the optical field distribution tailoring by DBSCH design.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.V.08Andrzej Malag et al      Electro−optical characteristics of the double−barrier heterostructure high power LDs (including a low beamdivergence perpendicular to the junction plane) have been described earlier [3−5]. Here we would like to presentthe influence of Al−contents in claddings on the quality of the laser heterostructure. One of the ways for that is bycomparison of spectral characteristics of LDs manufactured from the heterostructures grown according to the threedesign versions described above.     The GaAsP(QW)/AlGaAs SQW−SCH (ver. S) and two SQW−DBSCH (vers. A and B) heterostructures havebeen grown under the same conditions. The MOVPE growth was pursued in the horizontal flow AIXTRON R&D200 halogen heated reactor with TMGa, TMAl as group III sources, AsH3 and PH3 as group V source, DMZn asacceptor doping and SiH4 as donor doping sources. TMGa source was kept at 0°C. Applied growth temperaturewas 710°C, pressure 20 mbars and V/III ratios in the range 350−500. The highly p−doped GaAs cap layer wasgrown in lower temperature of 670°C to improve Zn incorporation. Growth interruptions for the gas flowstabilization at the GaAsP quantum well interfaces were minimized to 4s to avoid the interfacial defect generation.     Wide−stripe LDs of various cavity lengths have been fabricated using H+ implantation for the stripe definitionand CrPt and AuGeNi metallizations as p− and n−contacts, respectively. The pulsed (400 ns, 5 kHz) spectralcharacteristics were measured for sets of LDs made from each of the grown heterostructures and representativeresults are seen in Fig.2 and Fig.3. Exemplary spectrum of SCH LD shown in Fig.2 is rather wide and irregular.Sometimes LD spectra from this set consist of few separate groups. This indicates some nonuniformities in theactive region (spectra of CW operation are usually more regular due to stable excitation conditions). On thecontrary, spectra of DBSCH LDs are regular (single enveloped) for wide range of cavity lengths (L), as seen inFig.3. Comparison of spectral characteristics of DBSCH devices of the A and B versions (Fig.3a and Fig.3b,respectively) shows the best quality of the heterostructure with thinner barrier layers (B), that means, with lowestcumulative Al content in claddings (including barriers).     There is an interesting difference in the lasing spectrum shift as a function of cavity length (L) for both DBSCHLD versions. For the thin barrier (B) version the spectra of short cavity LDs are strongly short−wavelength shiftedindicating high quasi−Fermi levels separation and therefore high electronic threshold gain. The short−wavelengthshift for the A version DBSCH LDs of similar L is much weaker. This can be explained by the confinement factor( ) difference in the heterostructures design, as given in Fig.1. For the TM−polarised fundamental mode thecalculated = 0.037 for the A−version and = 0.022 for the B−version LDs. For a similar threshold modal gain(for similar L values) the electronic gain should be therefore distinctly higher in ver. B LDs, leading to a fastershort−wavelength shift. On the other hand, for the same QW thickness (dQW =15 nm for both versions), lowerd/  value for the version A should cause lower threshold current density (Jth). The measured differences inJth values are rather small however: approx. 800 Acm−2 (ver. A) and 900 Acm−2 (ver. B) for L= 0.25 mm LDs,while for L  J0 110 Acm−2 for both versions. This result, together with measured lower internal quantumefficiency of ver. A LDs ( i = 0.82) compared to those of ver. B ( i = 0.94) confirms the earlier conclusion aboutsuperior quality of the thinner barriers (lower cumulative Al−content) DBSCH.     Different d/  values should also result in different emitted beam divergences perpendicular to the junctionplane  (TM). In spite, the modelling results given in Fig.1 (the commercial "Photon Design" software [6] hasbeen used) predict similar beam divergences TM = 12° and 14° for the A and B versions, respectively. HereTM is FWHM of the far−field (FF) beam intensity distribution, which is proportional to the Fourier transform ofthe A and B optical field intensity profiles. Low TM despite low d/  value (0.4 µm) for the A−version is due tothe wide 'tails' at a low intensity level of the A field profile. This profile is 'irregular' in the sense that it is far fromGaussian−like profile of conventional SCH design (e.g. the profile S). The generally wider B profile of higher d/= 0.68 µm is more regular in this sense. The DBSCH design leading to similar TM with different d/  values10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.V.08Andrzej Malag et al 10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.V.08Andrzej Malag et al should result in different catastrophic optical damage (COD) levels.      Relatively good agreement between theoretical model and experimental results has been obtained for the Bversion, where = 17° (TM, CW) has been recorded (Fig.4a), close to best results for the double−barrier LDdesign [4,5]. The difference can be attributed to some small deviations of the experimental structure with respect tothe designed one (the transverse field distribution is quite sensitive to the barrier layers parameters) and to thethermal index guiding due to the temperature distribution perpendicular to the junction plane [7]. The later effect,not included in the modelling can be important in weakly guiding DBSCH. Concerning the A version, the recordedbeam divergence = 19° (TM; Fig.4b) is distinctly wider than predicted by modelling. This suggests that thelow d/  parameter is better indication in this case (the optical field profile far from the Gaussian), and especiallyfor TM modes, where modelling is generally less precise than for TE polarisation [6].      In conclusion, the DBSCH with possibly thin barrier layers of high Al−content (version B) seems to beappropriate solution for high power LDs not only because of potentially better growth quality (with respect toconventional SCH), but also because of high d/  value, resulting in lower beam divergence and higher COD level,still with acceptably low threshold current density.     References[1]    E.g. the EMF Band Gap Calculator, CD−ROM, EMF, April 2002.[2]    A. Malag, B. Mroziewicz, J. Lightwave Technology, 1996, 14, 6, pp.1514 − 1518.[3]    A. Malag, W. Strupinski, J. Crystal Growth, 1997, 170, pp.408 − 412. [4]    G. Lin et al., IEEE Photon. Technol. Lett.,1996, 8, pp.1588 − 1590.[5]    A. Malag, W. Strupinski , E. Jezierska , Proc. of  EW MOVPE VIII, Prague, 1999, p.253.[6]    Photon Design, http://www. photond.com[7]    E. Duda et al., IEEE J. Quant. Electron., 1979, QE−15, 8, pp.812−817.10th European Workshop on MOVPE, Lecce (Italy) 8−11 June 2003 PS.V.08Andrzej Malag et al 

Effect of Al-content reduction in (AlGa)As cladding layers of MOVPE grown high-power laser diodes

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Effect of Al-content reduction in (AlGa)As cladding layers of MOVPE grown high-power laser diodes

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