Temperature-dependent spectral properties of (GaIn)As/Ga(AsSb)/(GaIn)As W-quantum well heterostructure lasers
Christian Fuchs, Ada Baeumner, Anja Brueggemann, Christian Berger, Christoph Moeller, Stefan Reinhard, Joerg Hader, Jerome V. Moloney, Stephan W. Koch, Wolfgang Stolz
IIEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. XX, NO. YY, DECEMBER 2019 1
Temperature-dependent spectral properties of(GaIn)As/Ga(AsSb)/(GaIn)As “W”-quantum well heterostructurelasers
Christian Fuchs , Ada B¨aumner , Anja Br¨uggemann , Christian Berger , Christoph M¨oller , Stefan Reinhard ,J¨org Hader , Jerome V. Moloney , Stephan W. Koch , Wolfgang Stolz Materials Sciences Center and Department of Physics, Philipps-Universit¨at Marburg, Renthof 5, 35032 Marburg, Germany. NAsP
III/V
GmbH, Hans-Meerwein-Strae, 35032 Marburg, Germany. College of Optical Sciences, University of Arizona, 1630 E. University Blvd., Tucson, AZ, 85721, USA. Nonlinear Control Strategies Inc., 7562 N. Palm Circle, Tucson, AZ, 85704, USA.
This paper discusses the temperature-dependent properties of (GaIn)As/Ga(AsSb)/(GaIn)As “W”-quantum well heterostructuresfor laser applications based on theoretical modeling as well as experimental findings. A microscopic theory is applied to discuss bandbending effects giving rise to the characteristic blue shift with increasing charge carrier density observed in type-II heterostructures.Furthermore, gain spectra for a “W”-quantum well heterostructure are calculated up to high charge carrier densities. At these highcharge carrier densities, the interplay between multiple type-II transitions results in broad and flat gain spectra with a spectralwidth of approximately
160 nm . Furthermore, the temperature-dependent properties of broad-area edge-emitting lasers are analyzedusing electroluminescence as well as laser characteristic measurements. A first indication for the theoretically predicted broad gainspectra is presented and the interplay between the temperature-dependent red shift and the charge carrier density-dependent blueshift is discussed. A combination of these effects results in a significant reduction of the temperature-induced red shift of the emissionwavelengths and even negative shift rates of (-0.10 ± nm / K are achieved. Index Terms —Semiconductor diode lasers, semiconductor optical amplifiers, telecommunications, novel materials, GaAs-substrate,fully-microscopic theory.
I. I
NTRODUCTION N EAR-infrared (NIR) semiconductor lasers are the dri-ving force behind the rapid progress in fiber-optic te-lecommunication [8]. InP-based materials systems such as(GaIn)(AsP)/InP [9] and (AlGaIn)As/InP [10], [11], [12], [13]have proven to be spectrally suitable choices as active media inlasers emitting in the O- ( . µ m to . µ m ) and C-band( . µ m to . µ m ). However, the performance of thesedevices is affected by Auger losses [14] and small band offsetscompared to materials systems based on GaAs substrate. Forexample, in case of the (GaIn)(AsP)/InP materials system,this results in poor device performances at high temperatures.Another advantage of GaAs-based technology is the availa-bility of (AlGa)As/GaAs alloys, which for example enablethe fabrication of high-quality distributed Bragg reflectors.While these advantages led to highly efficient lasers basedon (AlGaIn)As/GaAs and (GaIn)As/GaAs at wavelengths of . µ m and . µ m , respectively, the design of lasers forthe above-mentioned telecommunication bands beyond . µ m has proven to be challenging. Quantum well (QW) lasersbased on (GaIn)(NAs)/GaAs [15] and Ga(AsSb)/GaAs [16]for applications in the O-band were demonstrated but theircharacteristic properties are challenging in terms of fabrica-tion. Furthermore, Auger losses cannot be suppressed usingthese structures. Manuscript received November XX, 2019; revised December YY,2019. Corresponding author: C. Fuchs (email: [email protected]).
A potential active medium for semiconductor lasers ba-sed on GaAs substrate emitting in the O-band is the(GaIn)As/Ga(AsSb)/GaAs materials system which exhibits atype-II band alignment [17]. On the one hand, the type-IIband alignment enables a flexible band structure enginee-ring. On the other hand, type-II heterostructures offer thepossibility to suppress Auger losses [18], [14]. Thus, type-IIsemiconductor lasers are a promising candidate for moreefficient telecommunication lasers. Their fabrication, however,has proven to be challenging. Devices based on double QWheterostructures (QWH) exhibited low pulsed optical outputpowers of P opt, max =
140 mW per facet [19] and may easilyswitch from a type-II to a type-I transition under operatingconditions [20].A promising approach to optimize optoelectronic devi-ces based on type-II heterostructures is the application of“W”-QWHs as these heterostructures involve a more fa-vorable optical dipole matrix element [21]. In order toincrease the optical dipole matrix element in these sy-stems, one hole quantum well consisting of Ga(AsSb) isembedded between two electron quantum wells consistingof (GaIn)As in order to increase the confinement functionoverlap. Electrical injection lasing with differential efficien-cies of η d = .
12 W A − to .
22 W A − , internal losses of α i = − and an internal quantum efficiency of η i =
32 % at a wavelength of λ = . µ m was achieved using MBE-grown“W”-QWHs [22]. A characteristic temperature of T =
68 K was deduced from temperature dependent measurements inthe range of T = 10 to ° C . Furthermore, molecular beamepitaxy grown electrical injection lasers emitting at λ = . µ m a r X i v : . [ c ond - m a t . m e s - h a ll ] D ec EEE JOURNAL OF QUANTUM ELECTRONICS, VOL. XX, NO. YY, DECEMBER 2019 2 were reported [23].In recent investigations, we demonstrated that the photo-luminescence spectra of metal organic vapor phase epitaxy-grown “W”-QWHs can be reproduced by applying a fully mi-croscopic theory. Furthermore, significant material gain valueswere predicted based on this experiment-theory comparison[24]. Photomodulated reflectance (PR) spectroscopy was usedto confirm the fully microscopic model and to characterizethe transitions that contribute to these spectra [25]. Thesetransitions include the e1h1 transition between the electronand the hole ground states, the e2h2 transition between thefirst excited electron and hole states and the e1h3 transitionbetween the electron ground state and the second excited holestate.Additionally, we have demonstrated a vertical-external-cavity surface-emitting laser with Watt level output powers intransverse multimode operation at a wavelength of λ = . µ m [26] as well as fundamental transverse mode operation [27].The characterization of electrical injection lasers at . µ m yielded low threshold current densities of j th = . − and . − in case of µ m and µ m long laserbars, respectively. Furthermore, high differential efficienciesof η d =
66 % , low internal losses of α i = . − and opticaloutput powers of P opt, max = . per facet were demonstra-ted [28].As thermal stability is a key feature of telecommunica-tion lasers, the temperature dependent properties of electri-cal injection (GaIn)As/Ga(AsSb)/(GaIn)As “W”-QWH lasersare analyzed in the present publication. For this purpose,theoretical investigations of the gain and band structure pro-perties based on a microscopic approach are carried out inorder to provide an in-depth understanding of their chargecarrier density-dependent behavior. These results are usedto interpret experimental results obtained from temperaturedependent electroluminescence (EL) and laser characteristicmeasurements. These are carried out for heat sink temperaturesup to ° C resulting in a comprehensive understanding of thetemperature-dependent properties of this “W”-QWH system.II. M ETHODS
A. Theoretical modeling
In order to model the optical response of the W-type QWregion and thus simulate the material gain of the given de-vice, we use the well-known Semiconductor Bloch Equations(SBE) that has proven successful in many experiment- theorycomparisons [3], [4], [5] and design studies [1], [2].In this theoretical framework, the laser field is treatedclassically, while the field-induced optical excitations of thegain material are dealt with on a microscopic level to allowfor a highly accurate, quantitatively predictive analysis of theoptical properties. As typical, being interested in the laserbehavior around threshold, to keep the numerics feasible, weassume that Coulomb- and Phonon-Scattering events lead to athermalization of the excited semiconductor material. In thisquasi-equilibrium situation, the charge carriers follow Fermi-Dirac statistics and their detailed dynamics can be neglected.Higher order scattering terms relevant for the equation of motion for the microscopic polarization that characterizes thesystem enter on the level of the second Born approximation.The band structure and wavefunctions relevant for such asemiclassical theory are obtained from an 8x8 Luttinger kpmodel [7]. Additionally, to account for local charge inhomo-geneities in the complex heterostructure under consideration,a Schrdinger-Poisson Equation is solved [7], [6]. Intrinsicstatistical alloy fluctuations, layer variations and interfaceroughness get involved by convolving the optical gain spectracomputed on basis of all these model assumptions with aGaussian.
B. Experimental methods
All devices investigated in the present work are fabricatedusing metal organic vapor phase epitaxy [29] and processed asgain-guided broad-area edge-emitting lasers with stripe widthsof µ m [28].Device characterization is carried out in a p-side up geome-try on a copper heat sink allowing for a temperature variationbetween ° C and ° C using a Peltier device. All mea-surements are carried out under pulsed operating conditionsusing a pulse length of
400 ns and a repetition rate of
10 kHz resulting in a duty cycle of . . Spectral measurements areperformed by recording electroluminescence spectra using agrating monochromator in combination with a lock-in ampli-fier in case of measurements below laser threshold and usinga optical spectrum analyzer in case of measurements abovelaser threshold. Furthermore, laser characteristics are recordedusing a large-area germanium photodetector.III. T HEORETICAL INVESTIGATION
The following theoretical investigation outlines the con-finement potentials as well as material gain properties of(GaIn)As/Ga(AsSb)/(GaIn)As “W”-QWHs emitting at appro-ximately . µ m . This emission wavelength is obtained assu-ming (GaIn)As and Ga(AsSb) layer thicknesses of incombination with In- and Sb-concentrations of
28 % . This“W”-QWHs design results in a low-density luminescenceemission wavelength of approximately . A. Carrier density-dependence of the confinement potentials
While material gain calculations for(GaIn)As/Ga(AsSb)/(GaIn)As “W”-QWHs are availablein the literature [21], [23], [24], the confinement potentialsof these heterostructures are rarely discussed in moredetail. Thus, the starting point of the present work is aninvestigation of the charge-carrier density-dependence ofthe confinement potentials as well as the correspondingconfinement functions. The confinement potentials as wellas the confinement functions of the two energetically lowestelectron and hole states for the above-mentioned structure isshown in Fig. 1 a) assuming a low charge carrier densities of . × cm − . At this carrier density, the band edgesare box-like and both electron states (i.e. e1 & e2) are almostdegenerate with an energetic separation of only
21 meV .Furthermore, an energetic separation of
948 meV between theelectron and hole ground states (i.e. e1 & h1) is observed.
EEE JOURNAL OF QUANTUM ELECTRONICS, VOL. XX, NO. YY, DECEMBER 2019 3
These properties change when the charge carrier densityin the “W”-QWH is increased to . × cm − , whichwe consider a typical charge carrier density at which laseroperation is achieved in semiconductor diode lasers. Theconfinement potentials are strongly distorted due to the spatialseparation of electrons and holes. The spatial separation ofpositive and negative charge carriers results in an internalelectrical field caused by the Coulomb interaction betweenboth carrier species. Consequently, the overlap between theground state confinement functions as well as the energeticseparation between the electron states is increased to
32 meV as shown in Fig. 1 b). The energetic separation between bothground state levels is also is blue shifted by
37 meV up to atotal of
985 meV . Heavy-Hole BandLight-Hole Band Conduction Band
Thickness (nm) -5 0 5 10 15 20 25 30 35 40 B and E dge ( e V ) -0.50.00.51.01.5-1.0 n (10 cm -2 )0.002e1e2h1h2 m e V Thickness (nm) -5 0 5 10 15 20 25 30 35 40 B and E dge ( e V ) -0.50.00.51.01.5-1.0 n (10 cm -2 )5.000e1e2h1h2 m e V ( + m e V ) a)b) Abbildung 1. Theoretically calculated confinement potentials and functionsfor the “W”-QWH at charge carrier densities of a) . × cm − andb) . × cm − . B. Carrier density dependence of the material gain
In order to determine the implications of the above-mentioned behavior for semiconductor laser applications, gainspectra are calculated for charge carrier densities between . × cm − and . × cm − assuming an in-homogeneous broadening of
20 meV as shown in Fig. 2.While these gain spectra show similar characteristics com-pared to type-I heterostructures reaching transparency andbuilding up significant material gain values as the chargecarrier density is increased, two important differences areobserved. Firstly, the material gain of type-I heterostructuresis expected to be red shifted due to band gap renormalizationas the charge carrier density is increased. However, type-IIheterostructures exhibit a characteristic blue shift primarilycaused by the above-mentioned band bending, which wasalso outlined in previous studies [21], [23], [24]. This blueshift amounts to approximately
52 nm in case the presentlyinvestigated “W”-QWH corresponding to a shift of the gainpeak from at . × cm − to at . × cm − . Secondly, the material gain spectra areintitially dominated by the ground state transition between thee1 and h1 states. However, as the charge carrier density isincreased above . × cm − , the transition between thee2 and h2 states slowly starts to dominate the gain spectra.Due to the small energetic separation between the electronstates, the energetic separation between the hole states of
113 meV at . × cm − largely defines the separationbetween both transitions. Thus, an overlap between the e1h1and e2h2 contributions is observed resulting in an almostflat gain spectrum over a range of approximately
160 nm at . × cm − . This result highlights the importance ofa thorough theoretical design of these type-II heterostructu-res for actual device applications, because such a behaviormight result in semiconductor lasers switching to higher ordertransitions under certain operating conditions. However, theseproperties must also be considered as an unique opportunityfor the design of novel semiconductor devices such as semi-conductor optical amplifiers, where broad and flat gain spectraare highly beneficial. M a t e r i a l G a i n ( c m - ) -2000-1000010002000 Energy (eV)
Wavelength (nm) cm -2 )0.002 - 10.000 Abbildung 2. Theoretically calculated material gain spectra for the“W”-QWH at charge carrier densities of between . × cm − and . × cm − in steps of . × cm − . EEE JOURNAL OF QUANTUM ELECTRONICS, VOL. XX, NO. YY, DECEMBER 2019 4
C. Investigation of design-parameters for tailoring the ma-terial gain
While these results seem promising, it is also importantto show that the charge carrier density-dependence can betailored in order to be able to adapt the active regions forspecific device applications. Therefore, the gain calculationsare repeated for different “W”-QWH designs, where the(GaIn)As and Ga(AsSb) thicknesses are systematically variedbetween and while keeping either the (GaIn)As orGa(AsSb) thickness constant and maintaining constant In andSb concentrations. Furthermore, the “W”-QWH is assumed tobe symmetric in all calculations, i.e. both (GaIn)As quantumwells are assumed to exhibit the same thickness. Three im-portant properties of the flat gain spectra are monitored in thisinvestigation:1) The charge carrier density required to reach flat gainspectra n flat
2) The energetic witdh Δλ w in nm (or Δ E w in meV) ofthe flat gain region3) The plateau gain value g flat The investigation shows that increasing layer thicknessesof either (GaIn)As or Ga(AsSb) results in flat gain spectrawith smaller spectral widths λ w and a lower gain plateau g flat being obtained at lower charge carrier densities n flat . Despitethese comparably small changes, it is possible to modify allabove-mentioned properties by a factor of 2-3 compared tothe initial structure with thick (GaIn)As and Ga(AsSb)QWs. These observations are explained by multiple underlyingeffects including: • Reduced energetic separation between confined sub-bandstates as layer thicknesses are increased resulting inhigher occupation probabilities of excited states • Lower confinement function overlaps between electronand hole sub-band confinement functions • Smaller impact of of band bending effects due to largeractive regions • The changing density of states resulting from the largerlayer thicknessesIV. E
XPERIMENTAL FINDINGS
In addition to these theoretical investigations, a series oflaser samples emitting at approximately . µ m is investigatedexperimentally. The sample series was chosen in such a waythat the influence of the charge carrier density per active“W”-QWH can be investigated. In order to do so, sampleA and B were fabricated using the same growth conditions.However, the active region of sample A only includes a single“W”-QWH, while sample B includes two active “W”-QWHs.Additionally, the influence of interface modifications in theform of thin GaP layers surrounding the outer interfacesof the “W”-QWH are investigated. This modification aimsat improving electron-hole confinement function overlap byintroducing step-like barriers at the edge of the “W”-QWH.Relevant material properties of these samples are summarizedin Tab. I. A. Temperature-dependent spectral properties of(GaIn)As/Ga(AsSb)/(GaIn)As “W”-quantum well lasers
The present work aims at an in-depth inve-stigation of temperature-dependent properties of(GaIn)As/Ga(AsSb)/(GaIn)As “W”-QWH laserscomplementing previously published room temperature data[28]. In order to do so, the respective electroluminescencespectra below laser threshold are compared at differenttemperatures of ° C and ° C as shown in Fig. 3. Themeasurements verify that sample exhibits a significant blueshift as the current density is increased from .
11 kA / cm to .
29 kA / cm at a temperature of ° C [21], [23], [24].However, as the temperature is increased to ° C resultingin an increased threshold current density, the sample exhibitsa significant side peak which eventually starts to exceedthe fundamental type-II transition. As a consequence, laseremission is no longer based on the fundamental type-IItransition, but based on this excited type-II transition, whichis identified as e2h2 transition using the theoretical modeloutlined above. This finding indicates that the modal gaincontributed by the fundamental type-II transition eventuallysaturates prior to exceeding the total loss at this elevatedtemperature. Excess charge carriers start to populate thehigher order states until the modal gain contributed by thistransition eventually exceeds the losses and laser operation isobserved.These observations highlight the importance of a carefulmicroscopic design of these “W”-QWH active regions in orderto achieve stable operation across a desired temperature range.However, as laser operation based on both of these type-IItransitions is observed in case of this sample, these findingsare a first indication for the presence of the broad materialgain spectra described above. B. Temperature-dependent laser properties of(GaIn)As/Ga(AsSb)/(GaIn)As “W”-quantum well lasers
In order to investigate the feasibility of these “W”-QWHsfor laser applications, laser characteristics are recorded fortemperatures between ° C and ° C . The temperature-dependent behavior of the threshold current density as wellas differential efficiency are summarized in Fig. 4, wherefilled symbols indicate operation based on the fundamentaland open symbols indicate operation based on the excitedtype-II transition. Since operation based on the fundamentaltransition is desired for most laser applications, the followinganalysis excludes data points above the switching temperature.Samples A and B exhibit similar properties across the entiretemperature range for the threshold current density as well asthe differential efficiency. Exponential fits yield characteristictemperatures of T = (56 ± K and T = (105 ± K forsample A and T = (60 ± K and T = (107 ± K forsample B. Thus, these characteristic temperatures appear tobe independent of the charge carrier density per active “W”-QWH. However, the switching to a higher order transition isfully suppressed in this temperature range by the introductionof a second “W”-QWH as previously shown in the literature[30]. EEE JOURNAL OF QUANTUM ELECTRONICS, VOL. XX, NO. YY, DECEMBER 2019 5
Tabelle IS
UMMARY OF MATERIAL PROPERTIES OF SAMPLES
A, B
AND C INCLUDING THE CAVITY LENGTH (L),
CONTACT WIDTH (W),
AND EMISSIONWAVELENGTH ( λ ) AT A HEATSINK TEMPERATURE OF ° C .Sample Design d (GaIn)As (nm) c In d Ga(AsSb) (nm) c Sb L (µ m ) W (µ m ) λ ( nm )A Single “W”-QWH 6 0.2 4 0.2 930 100 1155B Double “W”-QWH 6 0.2 4 0.2 990 100 1186C Single “W”-QWH with GaP interlayers 6 0.2 4 0.2 970 100 1176 Lu m i ne sc en c e I n t en s i t y ( a r b . un i t s ) -1 -2 -3 -4 j (kA/cm )1.921.060.11 Sample AT = 97 °C
Energy (eV)
Wavelength (nm) Lu m i ne sc en c e I n t en s i t y ( a r b . un i t s ) -1 -2 -3 -4 j (kA/cm )0.290.11 Sample AT = 20 °C
Energy (eV)
Wavelength (nm) a)b)
Abbildung 3. Electroluminescence spectra below laser threshold of sampleA at a) ° C and b) ° C for different current densities. In case of sample C, the modification of the active regi-on using GaP interlayers resulted in a deterioration of thedevice properties. A significant increase in threshold cur-rent density and a decrease of its temperature-dependence(T = (50 ± K ) is observed. Furthermore, differential ef-ficiencies are decreased and a two slope exponential modelis required to characterize the temperature-dependence of thedifferential efficiency. This analysis yields T = (101 ± K and T = (24 ± K for temperatures below and above ° C , respectively. Further investigations are required in order toexplain these findings due to the complex nature of themicroscopic changes introduced in this structure. Possible rootcauses include among others the introduction of loss or leakagechannels through interface states, dissimilar injection efficien-cies for both charge carrier species as well as phosphorussegregation resulting in an asymmetry of the “W”-QWHs. Temperature (K)
280 300 320 340 360 380 T h r e s ho l d C u rr en t D en s i t y ( k A / c m ) -1 Temperature (°C) D i ff e r en t i a l E ff i c i en cy ( % ) η d j th Sample ASample BSample C T
56 K60 K50 K T
105 K107 K101 K/24 K
Abbildung 4. Temperature-dependence of the threshold current density andthe differential efficiency of sample A, B, and C. Open symbols indicate opera-tion based on an excited type-II transition. Exponential fits yield characteristictemperatures of T = (56 ± K and T = (105 ± K for sample A andT = (60 ± K and T = (107 ± K for sample B. While sample C alsoshows a single slope behavior in case of T = (50 ± K , different charac-teristic temperatures T = (101 ± K and T = (24 ± K are observedbelow and above ° C , respectively. In case of regular type-I QWH lasers, these findings couldbe interpreted as poor temperature stability. In type-II “W”-QWH lasers, however, low T values are particularly intere-sting due to the interplay between the charge carrier density-induced blue shift below threshold and the temperature-induced red shift of the emission wavelength. In order to inve-stigate this effect, laser spectra above threshold are recordedtogether with each laser characteristic used to determine thecharacteristic temperatures. Thermal shift rates of the emissionwavelength are determined for samples A, B and C up totheir respective temperature, where laser emission switchesfrom the fundamental to the excited transition. The presentdevices exhibit thermal shift rates of (0.04 ± nm / K (sample A), (0.17 ± nm / K (sample B), and (-0.10 ± nm / K (sample C) as shown in Fig. 5. The differencein shift rates between these samples can be explained by EEE JOURNAL OF QUANTUM ELECTRONICS, VOL. XX, NO. YY, DECEMBER 2019 6 the difference in microscopic as well as device design sincegrowth conditions were kept constant across devices. Theaddition of a second “W”-QWH to the active region in sampleB results in a distribution of carriers into both “W”-QWHs andas a result, the resulting blue shift is decreased and outweighedby the thermal red shift. In case of sample C, the additionof GaP interlayers appears to deteriorate the overall deviceperformance and significantly lowers T resulting in a strongercharge carrier density-induced blue shift as the temperature isincreased.These findings can be considered as unique property oftype-II heterostructure lasers and strongly differentiate themfrom regular type-I heterostructure lasers, where a thermalshift rate of approximately . / K is expected in thistemperature range. These findings open up novel device appli-cations, where type-II “W”-QWHs are tailored in such a waythat their thermal shift rate of the emission wavelength matchesthe respective device needs. Examples include distributedfeedback lasers, distributed Bragg reflector lasers or vertical-cavity surface-emitting lasers, where the thermal shift rate ofthe active region is matched to the grating or reflector inorder to improve the temperature stability of these devices.Furthermore, the interplay of resistive heating and the chargecarrier density-induced blue shift in devices operated undercontinuous wave operating conditions is expected to also resultin smaller thermal shift of the emission wavelength. Classicaldesign rules, which demand high characteristic temperaturesto lower the power dissipation of devices, can be circumventedsince the demand for external cooling can be significantlydecreased due to the more temperature stable emission wa-velength. Temperature (°C)
Temperature (K)
280 300 320 340 360 380 E m i ss i on W a v e l eng t h ( n m ) Sample A d λ /dT = 0.04 nm/KSample Bd λ /dT = 0.18 nm/K Sample C d λ /dT = -0.10 nm/K Abbildung 5. Temperature-dependence of the laser emission wavelength ofsample A, B, and C. Linear fits yield shift rates of (0.04 ± nm / K ,(0.17 ± nm / K , and (-0.10 ± nm / K , respectively. V. C
ONCLUSION
In conclusion, the theoretical and experimental investigationof type-II (GaIn)As/Ga(AsSb)/(GaIn)As “W”-quantum well heterostructures indicate a significant application potentialof these heterostructures. On the one hand, the microscopictheory predicts the existence of flat and broad gain spectrawith spectral widths exceeding
100 nm . First indications forthe existence of these broad gain spectra are being observedin temperature-dependent electroluminescence measurements.On the other hand, experimental findings show that it ispossible to use the well-understood charge carrier density-induced blue shift in these type-II “W”-quantum well hete-rostructures to compensate the temperature-induced red shiftof the emission wavelength in semiconductor lasers. Thus,it is possible to realize devices with thermal shift rates ofthe emission wavelength ranging from typical type-I valuesof . / K down to negative values of − . / K andpotentially even further. These findings could result in signi-ficant improvements of devices such as distributed feedback,distributed Bragg reflector and vertical-external-cavity surface-emitting lasers as well as semiconductor optical amplifiers andfrequency combs. Future research should focus on an in-depthunderstanding of loss and leakage mechanisms in these devicesin order to further tailor the device properties for specificapplications. A CKNOWLEDGMENT
The authors gratefully acknowledge the funding providedby Deutsche Forschungsgemeinschaft (DFG) in the frameworkof Sonderforschungsbereich 1083 - Structure and Dynamicof Internal Interfaces - and the framework of the ResearchTraining Group 1782 - Functionalization of Semiconductors.The Tucson work was supported by the Air Force Office ofScientific Research under award number FA9550-17-1-0246.L
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