BULLETIN OF THE POLISH ACADEMY OF SCIENCESTECHNICAL SCIENCESVol. 53, No. 2, 2005

High power QW SCH InGaAs/GaAs lasers for 980-nm band

M. BUGAJSKI∗, B. MROZIEWICZ, K. REGIŃSKI, J. MUSZALSKI, K. KOSIEL,M. ZBROSZCZYK, T. OCHALSKI, T. PIWOŃSKI, D. WAWER, A. SZERLING,

E. KOWALCZYK, H. WRZESIŃSKA, and M. GÓRSKAInstitute of Electron Technology 32/46 Lotników Ave., 02 668 Warszawa, Poland

Abstract. Strained layer InGaAs/GaAs SCH SQW (Separate Confinement Heterostructure Single Quantum Well) lasers weregrown by Molecular Beam Epitaxy (MBE). Highly reliable CW (continuous wave) 980-nm, broad contact, pump lasers werefabricated in stripe geometry using Schottky isolation and ridge waveguide construction. Threshold current densities of theorder of Jth ≈ 280 A/cm2 (for the resonator length L = 700 µm) and differential efficiency η = 0.40 W/A (41%) from onemirror were obtained. The record wall-plug efficiency for AR/HR coated devices was equal to 54%. Theoretical estimationsof above parameters, obtained by numerical modelling of devices were Jth = 210 A/cm and η = 0.47 W/A from one mirror,respectively. Degradation studies revealed that uncoated and AR/HR coated devices did not show any appreciable degradationafter 1500 hrs of CW operation at 35◦C heat sink temperature at the constant optical power (50 mW) conditions.

Key words: laser diodes, strained-layer semiconductor lasers.

1. IntroductionQuantum well separate confinement heterostructure lasers(QW SCH) are usually designed with single quantum well(SQW) or multiple quantum well (MQW) active region.Multiple quantum well active region leads to effective in-crease of the electrically pumped volume and thus al-lows for obtaining high powers which are attractive forsuch applications as diode pumped Nd:YAG lasers. Theprice one pays for higher power, however is an increasedthreshold current density, since for reaching a necessaryinversion level one has to fill with carriers a few quan-tum wells [1,2]. Available experimental data show thatalso the other parameters of the laser depend on whetherthe active region is single or multiple quantum well [3,4].Theoretical works published so far reflected in general ob-served experimental trends [5–9]. Nonetheless, since theyhave used threshold analysis of the laser, based on thebalance of gain and losses and employed a number of phe-nomenological parameters, in particular semi-empiricalgain model and simplified band structure, they were notbeen able to predict more sophisticated effects. It has tobe also stressed that models in question were originallydeveloped for double heterostructure (DH) lasers [10] andwere subsequently used, with only minor modifications,for quantum well lasers where they do not fully reflectphysical complexity of involved phenomena. The reasonfor that was their simplicity and low computational re-quirements. Assuming the technological importance ofsemiconductor lasers, it is crucial to be able to performpredictive modelling of new device designs before actualmanufacturing devices. In this work an attempt has beenmade to use more rigorous approach to laser modelling,

based on self-consistent solution of transport equationsand Schroedinger equation. The calculations have beenapplied to strained layer 980-nm InGaAs/GaAs SCH QWlasers.

The development of erbium-doped fiber amplifiers(EDFA) has enabled the proliferation of high-bandwidthdata networks. Er3+-doped fiber amplifiers coherentlyamplifying 1550-nm signals through the conversion of 980-nm pump laser light [11]. Because the process is all op-tical, many signals can be amplified simultaneously withno delay and minimal electronics is required. The useof 980-nm pump wavelength has also another advantageas no excited state absorption exists for this wavelength.The pump lasers powering EDFAs must be highly reli-able and at the same time have to provide maximum am-plification power. All practical 980-nm lasers are basedon the ternary AlGaAs and InGaAs alloys. The excel-lent lattice match, refractive index contrast, and thermalconductivity of AlGaAs give a freedom to optimize thevertical laser structure, while a single pseudomorphic In-GaAs quantum well active region produces enough gainand good electrical confinement leading to low-thresholdcurrent and high quantum efficiency [12,13]. Technologyof growth of SCH SQW structures has been developed topractical measures due to ours previous experiences withAlGaAs/GaAs semiconductor lasers fabricated at the In-stitute of Electron Technology [14,15].

2. Laser design and threshold analysisThe laser structures were designed for 980-nm operationat room temperature (T = 300 K). The required wave-length can be obtained by adjusting In content in the

∗e-mail: bugajski@ite.waw.pl

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active layer and the thickness of the quantum well. As-suming ∼ 20% In content in InGaAs QW, the thicknessof the quantum well has been estimated roughly to be inthe range between 60 Å and 100 Å to give the emissionin the 980-nm range. This has been generally verified byPL measurements on the grown test structures. The lasersimulation has been performed using commercial PICS 3Dsoftware package [16]. Typically, besides emission spectra,P-I (optical power-current) characteristics for lasers withstripe contact and different resonator lengths are calcu-lated. The calculated threshold current densities have tobe understood as a bottom limit; in actual devices oneshould expect the higher values, due to unavoidable tech-nological and processing faults and inaccuracies. Numeri-cal simulation gives us guidelines for designing lasers andoptimizing their performance and speeds up developmentof practical devices. Since laser performance optimizationhas to be subjected to numerous restrictions (material andtechnological), the numerical modelling is an indispens-able tool saving many efforts which would be otherwisespent on technological experiments.

2.1. Physical basis of laser symulator. Basic equa-tions describing operation of semiconductor laser are Pois-son’s equations:

−∇(εε0

e∇V

)=

− n + p + ND (1− fD)−NAfA +∑

j

Ntj (δj − ftj)(1)

and continuity equations for electrons and holes

∇Jn −∑

j

Rtjn −Rsp −Rst −Rau =

∂n

∂t+ ND

∂fD

∂t(2)

∇Jp +∑

j

Rtjp + Rsp + Rst + Rau = −∂n

∂t+ NA

∂fA

∂t(3)

The above equations govern electrical characteristics ofthe device (e.g., I–V characteristics). Analysis of the opti-cal characteristics requires the solution of Helmholtz equa-tion, describing optical field distribution in the resonator.

∇2W + k20

(ε− β2

)W = 0 (4)

The densities of electron and hole currents Jn and Jp canbe expressed as a function of free carrier concentrationsand variations of respective quasi-Fermi levels.

Jn = nµn∇Efn (5)

Jp = pµp∇Efp (6)

Laser simulator solves self-consistently the above set ofpartial differential equations for electrostatic potential V ,electron and hole concentration n and p, optical field dis-tribution W and photons number in the resonator S. Forthe analysis of semiconductor laser it is important to eval-uate the carrier density and the optical gain of a quantumwell. The standard approach to the modelling is based onthe parabolic band model. It is the most efficient model

and usually it reproduces the general trends with satis-factory accuracy. For more accurate calculations, espe-cially in the case of strained layer InGaAs/GaAs lasersone has to relay on more elaborate models. Inclusion ofthe biaxial strain in the design of quantum well semi-conductor lasers provides an additional degree of free-dom and produces some desirable effects, such as lowerthreshold current. The effects of strain are described the-oretically using k p description of the band structure inthe quantum well. This type of calculation up to nowhas limited capability in analyzing practical design issuesand optimization of laser geometry. The approximatetreatment of the strain is based on the approximation ofnon-parabolic band structure by an anisotropic parabolicone (with proper inclusion of strain-induced shifts andsplitting of light hole (LH) and heavy hole (HH) bands).The calculations are based on analytical approximation tothe band structure of strained quantum well, which hasbeen developed recently [17] using an efficient decouplingmethod to transfer the 4×4 valence band Hamiltonianinto two blocks of 2×2 upper and lower Hamiltonians. Asa result of the decoupling, analytical expressions for inplane valence sub-bands dispersion relations can be de-rived. Once the parabolic subbands are found, one canapply conventional approaches to treat carrier concentra-tion and the optical transition probabilities. The effectivemass perpendicular to the quantum well plane determinesthe quantum sub-band energies at k = 0, while the densi-ties of states for each sub-band are determined using thein plane effective masses. In the framework of the modeldeveloped in [17] it is also possible to account for anti-crossing behaviour of the valence band sub-bands (i.e.,valence band mixing).

With no carrier injection the active layer material isstrongly absorbing. With carrier injection we can invertthe carrier population near the band edge and convert ab-sorption into gain. The region of positive gain exists inlimited energy range above the bandgap of the material.It extends between the bandgap and the quasi-Fermi levelseparation: Eg < hν < 4Eg The spectral shape of theQW gain and peak gain on injected carrier density expres-sions differ considerably from that of bulk material. Thesedifferences are the consequences of the step-like density ofstates in QW material. Apart from this, the derivationof appropriate expression for gain follows usual treatment.The optical gain is calculated using standard perturbationtheory (Fermi’s Golden Rule). The spectrally dependentgain coefficient can be written in the form,

g(E) =q2|M |2

Eε0m2c}NLz×

∑

i,j

mr,ijCijAij [fc − (1− fv)] H (E − Eij)(7)

where: |M |2 – bulk momentum transition matrix element;Cij – spatial overlap factor between states i and j; Aij –anisotropy (polarization) factor for transition i, j; mr,ij

– spatially weighted reduced mass for transition i, j; Eij

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High power QW SCH InGaAs/GaAs lasers for 980-nm band

– transition energy between states i and j; N – effectiverefractive index; H – Heaviside step function; i, j – con-duction, valence (lh, hh) quantum numbers at Γ point.For the perfectly confined QW states ∆n = 0 selectionrule applies. The reduced mass parameter is given bym−1

r,ij = m−1i + m−1

j , where mi and mj are weighted (bywave function confinement factor in QW) averages of QWand cladding masses.

Momentum conservation restricts the energies of theinitial and final states. The bulk averaged momentum ma-trix element between the conduction and valence states is:

|M |2 =m2Eg (Eg +4)

6mc (Eg + 24/3)(8)

The angular anisotropy factor is normalized so that itsangular average (bulk limit) is unity. For the TE tran-sitions, with the electric field vector in the plane of theQW, its values are: Aij = 3/4

(1 + cos2θij

)for e-hh tran-

sitions and Aij = 1/4(5− 3cos2θij

)for e-lh transitions.

The bands are assumed to be parabolic in first approxima-tion, thus the occupation density of the i-th (conductionor valence) band is:

ni, pi =kTmc,ν

i

π}2Lzln

[1 + exp

(Ec,ν

f − Ec,νi

kT

)](9)

where the quasi-Fermi energies Ef and the quantum lev-els Ei are measured positive into respective band fromthe k = 0 band edge. We assume undoped QW withhigh injection, so the charge neutrality gives the conditionn = plh + phh. The carrier scattering processes are ac-counted for by introducing appropriate broadening of thequantum levels. The net effect of this broadening can befound by convoluting the Lorentzian shape function withgain distribution (g′(E) = g(E)×L(E)). The broadeningsignificantly reduces the local gain. The discussed rela-tions allow for calculating gain vs. injected carrier density.

The above equations give the material gain of quan-tum well in terms of carrier density, which is not di-rectly measurable. From the point of view of calculatingproperties of semiconductor injection laser the relationbetween the current and carrier density must be estab-lished by balancing current with total carrier recombi-nation rate which consists of radiative and non-radiativecomponents. The radiative component of carrier recom-bination is found from spectrally dependent spontaneousemission rate. The non-radiative contribution to the cur-rent comes mainly from thermal leakage current and fromAuger recombination. The most common method of esti-mating Auger recombination is to use experimentally ob-tained Auger coefficients in combination with calculatedcarrier density (RA = CN3). Theories can predict Augerrate to within an order of magnitude. In quantum-welllasers carriers can leak into separate confinement waveg-uiding layers as well as leaking out of the entire SCHwaveguide region into the doped cladding layers has to beconsidered. Carrier population in the SCH region leads torecombination giving leakage current density of the order

of 50 A/cm2 per 1017 cm−3 of carrier density. This showsthe importance of maintaining low carrier density in thewaveguide regions of the laser.

The theory of gain based on Fermi’s Golden Rule con-siders each electron in isolation as it interacts with elec-tromagnetic field, i.e., it is a single-particle theory andas such it neglects mutual interactions between electrons.The physical consequences of many-body effects in denseelectron plasma in QWs are basically of two types: ex-citonic effects and bandgap renormalization effects. Thefirst one will result essentially in the changes of the spec-tral shape of material gain curves and will be enhancedin quantum wells comparing to the bulk material. Thesecond will produce the bandgap shrinkage due to thecombined exchange and correlation effects. The net ef-fect of the bangap shrinkage is the noticeable red-shift ofgain spectrum accompanied by its reshaping and enhance-ment. This phenomenon is clearly observable in quantumwell lasers where the high threshold carrier density shiftsthe lasing wavelength beyond the known band edge wave-length of the quantum well. Nevertheless, all practicallaser simulators available now are based on free-carriergain model, which is easier to implement numerically. Thefull account of many-body laser theory can be found inthe monograph [18].

2.2. Numerical simulation of SCH SQW InGaAs/GaAs laser. The sketch of the typical device simulatedis shown in Fig.1. The sequence of layers consists of n-type GaAs buffer, the AlxGa1−xAs n-type barrier layer,undoped active layer and waveguide, the AlxGa1−xAs p-type barrier layer and p+-type GaAs sub-contact layer.Active layer is composed of InyGa1−yAs quantum wellenclosed by GaAs waveguide. The model was tested fordifferent values of structure parameters, i.e., thickness ofindividual layers, composition and doping. The indiumcontent in InGaAs QW was varied from y = 0.20 toy = 0.22 and the well thickness from 60 Å to 100 Å.The AlGaAs compositions x = 0.30 and x = 0.70 weretested. Finally we have studied the influence of the thick-ness of GaAs waveguide, which had been changed from0.1 µm on each side of QW to 0.3 µm, on the lasercharacteristics. The doping of both emitters was kepton 5×1017 cm−3 level for all simulations. Such complex

Fig. 1. SCH SQW InGaAs/GaAs strained layer laser structure

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M. Bugajski et al.

program of investigations would be difficult to real-ize by relaying exclusively on technological experiments.Nevertheless, the key numerical results were confrontedwith real experiments to verify calculations and pro-vide solid foundations for hypothesis derived from nu-merical experiments. Such combined approach provedto be very successful in developing 980-nm lasers.

Typical, calculated P-I (optical power-current) char-acteristics for lasers with stripe width W = 100 µmand resonator length L = 700 µm are shown in Fig. 2.Threshold current densities for modelled lasers are equalto 197 A/cm2 and 208 A/cm2, depending on construc-tion details, which are in agreement with values ob-tained experimentally by Coleman [12] for broad areastrained-layer InGaAs/GaAs lasers of similar geometrybut with higher Al content in the emitters. The ma-jority of early lasers were GRIN SQW type, while nowsimple SCH SQW or SCH MQW lasers, as studied inthis work, dominate, which in most cases makes compari-son of calculated results with available experimental dataonly approximate – but general trends are reproducedproperly. The calculated threshold current densities haveto be understood as a bottom limit; in actual devicesone should expect the higher values, due to unavoidabletechnological and processing faults and inaccuracies.

Fig. 2. Calculated P-I characteristics of SCH SQW In-GaAs/GaAs lasers

Application of narrower waveguiding layer (∼ 0.1 µmon each side of the quantum well) decreases threshold cur-rent by about 25% (cf. Fig. 3) but on the other handoptical power density in the resonator increases roughly3 times. This may lead to a faster degradation of lasers,

in particular to the lowering of COD (Catastrophic Opti-cal Damage) threshold level. The heat generated by theabsorption of laser radiation at the mirrors can result in ir-reversible damage of the laser. Since our primary concernwas durability of lasers the former design, with broaderwaveguide, was chosen although the penalty of slightlyhigher threshold had to be paid. Extremely low thresholdcurrent densities, of the order of 120 A/cm2, can be ob-tained using both narrow waveguide (∼ 0.1 µm) and highAl content (x = 0.70) in the emitters. Such constructioncan be useful for low power, high-speed lasers with nar-row stripes (5 µm–10 µm) operated at low drive currents.

Fig. 3. Calculated P-I characteristics of SCH SQW In-GaAs/GaAs lasers with different waveguide thickness

The influence of QW thickness, with the other con-struction details unchanged, on laser parameters has beenalso studied. The results of calculation show that differ-ential quantum efficiency of the laser (η) grows with de-creasing QW thickness, reaching 0.54 W/A (42.7%) forL = 60 Å. On the other hand, the threshold currentdensity J th reaches minimum 121 A/cm2 for L = 80 Å.The results suggest that a good optimization procedurewould be to choose QW thickness L = 80 Å and vary Incontent in the active region to get required wavelengthof laser emission, i.e., 980 nm. The thickness of bothemitters should be at least 1.0 µm each, preferably 1.5µm, to assure that optical field of the fundamental modedoes not penetrates the highly absorbing GaAs regions.

The final laser structure has been decided accord-ing to the simulation results. The thickness, com-position and doping of individual layers constitutingthe structure were chosen as listed in the Table 1.

Tabele 1Typical parameters of SCH SQW InGaAs/GaAs/AlGaAs laser structures

Buffor n-emiter Waveguide QW Waveguide p-emiter Cap1 µm GaAs:Si 1 µm – 1.5 µm 0.1–0.3 µm 80 Å 0.1–0.3 µm 1 µm – 1.5 µm 0.25 µmn = 2×1018cm−3 AlxGa1−xAs:Si GaAs InxGa1−xAs GaAs AlxGa1−xAs:Be GaAs:Be0.2 µm x = 0.3 x = 0.20–0.22 x = 0.3AlxGa1−xAs:Si n = 5×1017cm−3 undoped undoped undoped p = 5×1017cm−3 p = 3×1019cm−3

x = 0.0-0.3n = 5×1017cm−3

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High power QW SCH InGaAs/GaAs lasers for 980-nm band

SCH SQW laser structures were grown by molecular beamepitaxy (MBE) in Riber 32P reactor in a manner similarto previously described [19].

The structures were grown on (100) GaAs conduc-tive substrates. The sequence of layers for typical, op-timized for reliable high power CW operation, structureconsisted of n-type GaAs buffer, the Al0.3Ga0.7As n-typebarrier layer, undoped active layer and waveguide, theAl0.3Ga0.7As p-type barrier layer and p+-type GaAs sub-contact layer. Active layer and waveguide comprised ofIn0.21Ga0.79As 80 Å quantum well enclosed by 0.3 µmGaAs layers.

3. Strained-layer laser structures – MBEgrowth related issues

The growth of InGaAs/GaAs heterostructures is muchmore difficult than the growth of AlGaAs/GaAs ones.The reason for that is large lattice mismatch between sub-strate and the growing layer. The lattice constant of GaAsis equal to 5.6533 Å, whereas that of InAs equals 6.0584 Å,which results in 7% lattice mismatch between those twosemiconductors and precludes the growth of high indiumcontaining InGaAs on GaAs substrate. The room temper-ature band gap of InxGa1−xAs ternary alloy varies from1.424 eV (GaAs) to 0.36 eV (InAs). The energy rangeclose to 1.424 eV is attainable by using InxGa1−xAs layersgrown on GaAs substrates. This way the layers with in-dium content up to 0.2 and reasonable thickness up to 100Å can be grown [20]. The layer with different lattice con-stant than that of the substrate undergoes a tetragonaldeformation during the growth. Depending on whetherthe lattice constant of the layer is greater or smaller thanthe lattice constant of the substrate, we have biaxial com-pressive or biaxial tensile strain in the plane of the layerand appropriate deformation of the elementary lattice cellin perpendicular direction for which the strain is relaxed.With increase of the layer thickness, the elastic deforma-tion energy stored in the crystal grows and when its valueexceeds certain threshold value determined by the Hook’slaw the stress is released and the misfit dislocations areformed [21]. The thickness of the layer for which stressrelaxation occurs is called a critical thickness. It dependsmainly on the lattice mismatch between the layer and thesubstrate. For the materials with large lattice mismatch,such as InGaAs on GaAs, the critical thickness values arefew orders of magnitude smaller than that for AlGaAs onGaAs with a similar composition. The lattice mismatchis the main factor responsible for difficulties encounteredin the growth of InGaAs on GaAs. The growth of lat-tice mismatched layers can be realized only in the limitedrange of thickness and compositions and even then is adifficult task, requiring a precise knowledge of the phe-nomena occurring in strained materials.

Band structure of III–V semiconductor compoundschanges appreciably under biaxial strain originating inthin layers of these materials grown on lattice mismatched

substrates. The presence of strain removes degeneracy ofvalence band k = 0, changes band gap as well as dis-persion relation in the valence band. In quantum wellsthe influence of strain is even more complicated. Allthese changes can be positively exploited in designingquantum well lasers, resulting in improved device char-acteristics and flexibility in fabricated lasers parameters[22]. Penalty paid for this is difficult growth technology.Quality of interfaces and defects in strained layer semi-conductor structures greatly affect parameters of lasers.Roughness of surfaces is cause of dissipative loss of emis-sion and probable non-radiative recombination on defectsinvolved. Because of that total internal losses increasewhich leads to higher threshold current and decrease ofquantum efficiency of the lasers. This finally causes de-crease of external differential efficiency (slope of opticalpower vs. current characteristics is smaller). To achieveas smooth as possible and defect free interfaces (in partic-ular the most important are interfaces between quantumwell and waveguide) we have applied photoluminescencemeasurements performed on as grown structures [23].

The performance and reliability of semiconductorlasers depends critically on the crystal growth technique.In this respect, lasers are probably the most demandingIII–V minority carrier devices. Fabrication of high qual-ity laser structures by MBE needs a careful optimizationof the growth conditions. From the point of view of MBEtechnology several factors are of great importance: highpurity and structure perfection of undoped layers, the rel-evant profiles of dopant concentration, good quality of in-terfaces, high dopant concentration in contact layers, etc.Therefore, the optimization of the MBE process comprisesthe determination of the growth conditions for each layerof the laser structure. The growth must proceed withright combination of temperatures of substrate and ratiosof group V/III atomic beams, which guarantee appropri-ate reconstruction of surface and proper growth condi-tions for each layer, which indeed vary appreciably. Thisallows us to achieve layers of the best optical quality.

In general AlGaAs should be grown at as high tem-peratures as possible and at low V/III ratio (to minimizeoxygen content in the layers), whereas InGaAs preferssubstantially lower temperatures and higher V/III beamratios. Fulfilling these conditions requires abrupt changesof beam fluxes which is difficult to realize due to thermalinertia of effusion cells. To avoid process interruptions atthe surfaces we have used two arsenic cells preheated atdifferent temperatures. To monitor the state of the crys-tal surface at any stage of the growth process the RHEED(Reflection High Energy Electron Diffraction) system wasused. The RHEED patterns were registered by CCD cam-era and then processed in real time and recorded by acomputer acquisition system. The system enabled us toregister RHEED intensity oscillations and, as a result, todetermine the growth rate. As a result, we had at our dis-posal two independent methods of measuring the growthrate. The first one based on the measurement of atomic

Bull. Pol. Ac.: Tech. 53(2) 2005 117

M. Bugajski et al.

Table 2Optimized MBE growth conditions for InGaAs/GaAs laser structures

Material Layer T (◦C) Growth rate (µm/hr) V/III flux ratio ReconstructionGaAs buffer 580 0.8 4–5 (2×4)

waveguide 550–580subcontact 540 (3×1)

Al0.30Ga0.70As emitters 690 1.15 2.1 (3×1)In0.20Ga0.80As QW 550 1.0 4 (2×4)

fluxes and the second one based on registering theRHEED intensity oscillations. That additional possi-bility strengthened our control over growth process andturned out to be crucial in developing the technology oflaser structures. The analysis of RHEED diffraction pat-terns allowed us to determine substrate reconstruction forthe case of growth of different materials composing laserstructure, depending on the temperature and respectivebeam fluxes. It is well known that the quality of layersand their usefulness for certain applications can be linkedto the type of surface reconstruction during the growth.For GaAs there are known types of reconstruction whichproduce layers especially suitable for optical applications.In the case of InGaAs the subject is less studied but thegeneral trends are similar. Based on our previous expe-riences with GaAs/AlGaAs system and research done forthis work we have found the set of optimum growth condi-tions (in terms of beam fluxes, surface reconstruction andtemperatures) for the growth of InGaAs/GaAs/AlGaAsstrained layer structures. They are listed in Table 2.

Optimized MBE growth parameters allowed for thegrowth of defect free laser structures in the whole rangeof indium content in the active region studied (0.10–0.25mole fraction of In). Quality of the structures was rou-tinely studied by PL (photoluminescence), PR (photore-flectance) and occasionally by TEM.

4. Device fabrication and lasercharacteristics

The broad contact (100 µm stripe width) ridge-waveguidelasers were fabricated from SCH SQW structures follow-ing a standard processing procedures used previously forDH AlGaAs/GaAs lasers. The AuGeNi/Au contact withadditional thick Au layer was used for n-side of the de-vice. The p-contact comprised of the following sequenceof layers: Cr (50 nm), Pt (200 nm), Cr (50 nm) and Pt(150 nm). The individual lasers were In-soldered, p-sidedown, on copper blocks and contacted by a gold wire.The p-side down soldering of lasers has an advantage overn-side down mounting manifesting in better heat sinking,although it is much more difficult technologically and gen-erally results in lower yield. The laser chip and the struc-ture soldered to copper heat sink are shown in Fig. 4aand 4b, respectively. All laser chips were tested beforesoldering, using needle probe and micromanipulator. TheP-I characteristics and spectral characteristics were mea-sured. The CCD camera was used to observe near-field

picture of laser emission. Light from the laser was deliv-ered to the spectrometer using optical fiber. The measure-ments of laser characteristics, data acquisition and dataprocessing were controlled by PC computer. For selecteddevices angular distribution of laser emission in two per-pendicular directions (far-field pattern) was also recorded.Finally, devices were encapsulated in metal cases withwindow. Some of the lasers had antireflection (AR) andhigh-reflectivity (HR) coatings deposited on the front andrear facets respectively. One layer of SiO2 was used for ARcoating and Si/SiO2 multilayer was used for HR-coating.The AR, HR coatings were deposited on lines of lasers,before dividing them into individual chips.

Fig. 4. The laser chip (a) and the structure soldered to copperheat sink (b)

The preliminary results of works on 980-nm lasers havebeen published in series of recent papers of the authors[24–26]. Here we report updated results on degradationstudies and selected best characteristics of the lasers. Fab-ricated lasers exhibited similar characteristics to the otherstructures of this type published in the literature [27–31]. Threshold current densities of the order of Jth ≈ 280A/cm2 (for the resonator length L = 700 µm) and dif-ferential quantum efficiency η = 0.40 W/A (41%) wereobtained. The wall-plug efficiency of the lasers with-out AR coating reached 38%. The optical power-currentcharacteristics (P-I) for lasers fabricated from the samewafer showed almost equal thresholds and slightly differ-ent differential efficiencies. The linearity of the charac-teristics was good; there was no kinks and thermal roll-over for highest powers observed. The characteristics weretypically recorded for pulse operation with filling factorff = 0.1% (pulse length 200 ns, repetition 5 kHz).

Theoretical estimation of threshold current densityand differential efficiency obtained by numerical mod-elling of the devices were Jth ≈ 210 A/cm2 and η = 0.47

118 Bull. Pol. Ac.: Tech. 53(2) 2005

High power QW SCH InGaAs/GaAs lasers for 980-nm band

W/A, respectively. The obtained experimentally thresh-old currents are fully acceptable, whereas differential ef-ficiency could have been still improved. They lower thantheoretically predicted value is most probably caused ab-sorption losses in the cavity. Further work aimed on low-ering absorption losses is required. The lasers generatedat 980 nm – 990 nm wavelength range, depending on thepart of the wafer from which they have been made, withthe half-width of the emission band of the order of 1nm, just above the threshold and up to 3 nm at highcurrents (high optical power). Emission spectrum con-tained many well-distinguished longitudinal modes, be-longing to the fundamental transverse and lateral modes.As it has been mentioned earlier the requirement of pre-cise wavelength control demanded by application of thedevices as a pump source for Er3+-doped amplifiers is dif-ficult to fulfil but manageable. Besides the variation ofthe wavelength over the wafer, there are global variationsbetween different MBE runs. Usually, the best repro-ducibility is achieved when one grows a series of 2” waferswith laser structures, without interruption for other pro-cesses. That assures a maximum stability of growth pa-rameters.

Fig. 5. Comparison of P-I characteristics of lasers with orwithout AR/HR coatings

To force laser emission through one, selected mir-ror one has to apply AR/HR coating. The other ben-efits of dielectric passivation of the mirrors is theoreti-cally doubled differential efficiency of the laser and higherresistivity to degradation. The last particularly refersto increased COD level and greatly enhanced durabil-ity. According to recent reports [32], strained-layer In-GaAs/GaAs lasers without mirror coating lived in CWregime on average about 250 hrs, whereas the lifetimeof those with AR/HR coatings reached 5000–10000 hrs.Typical light-current characteristics of the lasers withoutAR/HR coatings and with AR/HR coatings, from one lot,are compared in Fig. 5. The threshold current of laserswith AR/HR coatings was unchanged comparing to un-coated ones but we have observed roughly twice increasein differential quantum efficiency. The record wall-plug

efficiency for AR/HR coated devices was equal to 54%.Which is among the best values obtained for that type oflasers.

For some lasers we have performed aging tests. Thelasers for the tests were selected on the basis of initialscreening based on threshold current determination. Onlythe lasers with threshold within the limit of up to twicethe average threshold have been subjected to lifetime test-ing. No special effort was made to select particularly gooddevices, rather we tried to chose a spectrum of differentinitial quality devices. Fabricated lasers showed in generalgood reliability. The uncoated devices, did not show anappreciable degradation after over 1000 hrs of CW opera-tion at 35◦C heat sink temperature, with 50 mW emittedpower (in a constant power mode). This result can be ex-trapolated to 106 hrs of pulse operation with ff = 0.1%.In the light of the results published in literature [32] thisis extremely good result, even having in mind that thelasers with whom we compare our results were operatedat 100 mW optical power. A total of 10 devices wereplaced on life test. The lasers with coated mirrors areat the moment of writing this paper still operated at theaging frame and their CW working time reaches 1500 hrs.Only one of them failed during the test and the rest main-tan basicaly unchanged current. This is to be comparedwith typical CW operating times of a few khrs, publishedin the literature [33–35]. The apparent immunity of in-vestigated lasers to sudden failure is striking and despitethe small statistical base it is undoubtedly an inherentproperty of strained-layer InGaAs/GaAs lasers as com-pared to conventional AlGaAs/GaAs lasers. Summariz-ing the results of aging test performed so far and those,which are in progress, we may conclude that they are inagreement with the similar studies for the state-of-the-art,InGaAs/GaAs lasers.

5. Thermal properties of lasers

The temperature at the facet has a critical role in devicereliability and performance. Catastrophic optical damage(COD) failure of a laser device occurs at the facet and iscaused by absorption of light at the facet which leads toa local band-gap reduction with consequent increased ab-sorption and temperature rise. The runaway effect leadsto device failure. Thus the local facet temperature is in-dicative of these processes. In addition the lateral tem-perature profile creates a refractive index profile whichhas a strong effect on device performance. The lateralrefractive index profile induced by a non-uniform junc-tion heating plays a dominant role in determining lateralmodes and emission characteristics of broad-area lasersduring continuous (CW) and long-pulse operation. Thisis due to thermal focusing caused by temperature inducedlateral index profile. Understanding and characterizingthese thermal effects is important to development of highpower CW lasers. We have developed an in-situ measure-ment technique for spatially resolved facet temperature

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measurements. The method allows for facet temperaturemapping under differing operating conditions and assess-ment of degradation of facets caused by high densitiesof optical field in the resonator. The measurements alsoqualify the bonding quality and optimum choice of sub-mount material for heat removal.

5.1. Temperature mapping system. Figure 6 showsthe experimental set-up developed for the thermore-flectance measurements of facet heating in semiconduc-tor lasers. The temperature induced changes of the probebeam reflectivity are generally small (∼ 10−5K−1) anddependent on the probe beam wavelength. We limit our-selves to single wavelength measurements and performmapping of the temperature distribution, using calibra-tion described in the subsequent section.

Fig. 6. The experimental set-up for thermoreflectance mea-surements of facet heating in semiconductor lasers

The dimensions that are of interest are the active re-gion (typically 1 µm high by 50–200 µm wide) and thesurrounding regions through which the heat is dissipated.This will amount to several 10 s of microns in the heightdirection. The spatial resolution of the system is deter-mined by: (1) probe beam focusing and (2) positioningaccuracy of translation stages. The probe beam can be fo-cused, using special techniques, down to single micron di-ameter. The piezoelectric transducers allow 300×300 µm2

scanning range with 0.2 µm positioning accuracy. Thevertical positioning is also done by piezoelectric trans-ducer. The positioning of the laser facet in the focal planeof the optical system is crucial for focusing. The probingbeam focusing on the sample is done with reflecting micro-scope objective. Because of its all reflecting constructionit is free from chromatic aberration. The objective con-sists of a small convex primary mirror and a larger concavesecondary mirror. The experiments showed that reflectingobjective has clear advantages over refracting objectivesof equivalent aperture and focal length. The software forthe controlling x-y-z microstages movement and data ac-quisition uses Lab View platform. The movement of thepiezoelectric stages is controlled by IEEE interface.

The thermoreflectance mapping system has beentested for various modes of operation. It can be usedfor simple reflectance (R) measurements, relative (differ-ential) reflectance measurements (∆R/R for given wave-length λ and spectral thermoreflectance measurements.The relative variation of reflectance (∆R/R is linear ver-sus the temperature variation ∆T :

∆R/R = k∆T, (1/k = CTR) (10)

with CTR being thermoreflectance coefficient. An accu-rate calibration method is an essential element of anyquantitative thermometry techniques. This is of particu-lar importance for the thermoreflectance studies of semi-conductor lasers, whose constituent materials have opticalproperties that are not well-characterized or can vary de-pending on the processing details. Very few data existfor the absolute values of CTR (thermoreflectance coeffi-cient) in the literature. Reported values on facet temper-ature scatter in the wide range under high power oper-ation, although each measurement technique is based onthe temperature dependence of inherent material param-eters, such as refractive index and energy gap. There-fore, the absolute facet temperatures reported are diffi-cult to compare. Due to the fact that the coefficient CTR

depends both on probed material, and the experimentalconditions, it should not be taken from the literature, butrather determined in-situ, on the probed sample itself.The value of the thermoreflectance constant for the probebeam wavelength (λ = 632.8 nm, used in this work equals8×103 K.

The example of temperature distribution maps for dif-ferent supply currents, for the laser mounted p-side downon SiC submount and subsequently on the copper heatsink is shown in Fig. 7. The temperature increase isnegligible, even at very high emitted powers, under CWoperation. This is because of the quality of the struc-ture and very efficient heat removal. However, for notoptimized devices the temperature increase on the mirrorcan easily exceed 100 K, which may eventually result inserious damage of the mirror and the whole laser.

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High power QW SCH InGaAs/GaAs lasers for 980-nm band

Fig. 7. The temperature distribution maps for different supply currents, for the laser mounted p-side down

6. Conclusions

We have developed MBE technology of strained layerCW InGaAs/GaAs SCH SQW lasers operating at 980-nm band. Broad contact, pump lasers were fabricated instripe geometry using Schottky isolation and ridge waveg-uide construction. Threshold current densities of the or-der of Jth ≈ 280 A/cm2 and differential quantum effi-ciency η = 0.40 W/A (41%) from one mirror were ob-tained. The wall-plug efficiency of the uncoated lasers was38%. The record wall-plug efficiency for AR/HR coateddevices was equal to 54%. Theoretical estimation of abovementioned quantities were Jth = 280 A/cm2 and η = 0.47W/A respectively. Degradation studies revealed that un-coated devices did not show any appreciable degradationafter 1000 hrs of CW operation with 50 mW of emittedpower. Similar life tests, with positive results, are ongo-ing for AR/HR coated devices already for 1500 hrs at thetime of writing this paper. These results are acceptableeven considering such demanding applications as pumpsource for EDFAs. There is however a need for more sys-tematic measurements in CW condition for both, struc-tures with and without coatings, as it is necessary to havebetter statistics of fabricated lasers reliability. We havealso developed new technique to monitor the laser facetsheating in real time and to correlate these measurementswith device performance and reliability. The method isbased on thermoreflectance, which is a modulation tech-nique relaying on periodic facet temperature modulationinduced by pulsed current supply of the laser. The pe-riodic temperature change of the laser induces variation

of the refractive index and consequently modulates probebeam reflectivity. The technique has a spatial resolutionof about ∼ 1 µ and temperature differences of a degreecan be measured. It can be applied to any kind of edgeemitting lasers or laser bars.

Acknowledgements. This work has been supported byKBN grant PBZ -MIN-009/T11/2003.

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