MOVPE-Grown Quantum Cascade Laser Structures Studied by Kelvin Probe Force Microscopy

Home , Heterojunction, Quantum cascade laser

crystals

Article MOVPE-Grown Quantum Cascade Laser Structures Studied by Kelvin Probe Force Microscopy

Konstantin Ladutenko 1,2,*, Vadim Evtikhiev 1, Dmitry Revin 3 and Andrey Krysa 4,*

1 Semiconductor Luminescence and Injection Emitters Laboratory, Ioffe Institute, 26 Polytekhnicheskaya Str., 194021 St. Petersburg, Russia; [email protected]ffe.ru 2 Department of Physics and Engineering, ITMO University, 49 Kronverskii Ave., 197101 St. Petersburg, Russia 3 Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK; d.revin@sheffield.ac.uk 4 EPSRC National Epitaxy Facility, Department of Electronic and Electrical Engineering, University of Sheffield, Mappin Street, Sheffield S1 3JD, UK * Correspondence: [email protected] (K.L.); a.krysa@sheffield.ac.uk (A.K.)

Received: 31 January 2020; Accepted: 17 February 2020; Published: 20 February 2020

Abstract: A technique for direct study of the distribution of the applied voltage within a quantum cascade laser (QCL) has been developed. The detailed proﬁle of the potential in the laser claddings and laser core region has been obtained by gradient scanning Kelvin probe force microscopy (KPFM) across the cleaved facets for two mid-infrared quantum cascade laser structures. An InGaAs/InAlAs quantum cascade device with InP claddings demonstrates a linear potential distribution across the laser core region with constant voltage drop across the doped claddings. By contrast, a GaAs/AlGaAs device with AlInP claddings has very uneven potential distribution with more than half of the voltage falling across the claddings and interfaces around the laser core, greatly increasing the overall voltage value necessary to achieve the lasing threshold. Thus, KPFM can be used to highlight design and fabrication ﬂaws of QCLs.

Keywords: Kelvin probe force microscopy; quantum cascade laser; MOVPE

1. Introduction Metalorganic vapour phase epitaxy (MOVPE) of quantum cascade lasers (QCLs) [1,2] is a very demanding process. The interface abruptness, intentional and unintentional doping, layer thicknesses, and material compositions may critically affect the performance of any conventional laser diodes. Taking into account that thicknesses of individual layers in a QCL active region can be just a few atomic monolayers and the total number of individual layers is typically many hundreds, the control of the above parameters in QCLs is even more difficult. Various experimental techniques are used to assess the structural, electrical, optical and thermal properties of the as-grown material and fabricated devices, e.g., [3–19] in order to help to optimize the QCL design and related epitaxial growth technologies. In this paper, we present a technique for direct, high-spatial-resolution measurements of the voltage distribution across all the parts of QCL structures. The technique is based on Kelvin probe force microscopy (KPFM), which is widely used to study other electrically driven semiconductor devices [20–23]. It is usually assumed that the whole voltage that is applied to a QCL device is dropped only across the laser core, with negligible contribution from other parts such as top and bottom claddings or contact and spacer layers. Despite some agreement between the operating and predicted voltage values for working QCLs, there is high degree of uncertainty on how exactly the electric field is distributed across different layers of QCLs. In this study, QCL devices based on InP and GaAs substrates have

Crystals 2020, 10, 129; doi:10.3390/cryst10020129 www.mdpi.com/journal/crystals Crystals 2020, 10, 129 2 of 8 been tested, revealing examples of both almost ideal voltage distribution and the existence of the areas of excessive parasitic voltage drop.

2. Materials and Methods GaAs/AlGaAs on GaAs and InGaAs/InAlAs on InP QCL structures were grown by MOVPE in a horizontal flow reactor. Trimethylaluminium, trimethylgallium, trimethylindium, arsine and phosphine were used as the precursors of the group III and V atoms. Disilane was used for n-type doping. Hydrogen was used as a carrier gas. The total reactor pressure was 150 Torr. The orientation of the epitaxial surface of the InP substrates was (100) on-axis. The orientation of the GaAs substrates was (100), with a miscut angle of 10o towards <111> A in order to suppress the CuPt-type ordering in 18 3 Al(Ga)InP [24,25]. All substrates were n-type doped with n > 10 cm− . λ 9.7 µm GaAs/AlGaAs QCL wafer (sample MR2790) was grown with a 1 µm thick GaAs ≈ (n = 6 1018 cm 3) buffer layer. A 3.04 µm thick GaAs/Al Ga As laser core with 55 cascade × − 0.45 0.55 periods and average doping of n = 7 1016 cm 3 is surrounded by a 0.2 µm thick top and bottom × − GaAs (n = 5 1016 cm 3) spacer layers and 3 µm thick top and bottom In Al P (n = 5 1016 cm 3) × − 0.47 0.53 × − claddings. Finally, the structure was capped with a 1 µm thick GaAs (n = 6 1018 cm 3) top contact × − layer. The details of the laser core of this structure can be found in [26]. The design of the active region of the λ 5.7 µm InGaAs/InAlAs strain-compensated QCL ≈ wafer (sample MR2449) is very similar to that presented in [27]. The structure has a 2.125 µm thick In Ga As/In Al As laser core with 35 cascade periods and average doping of n = 2 1016 cm 3 0.6 0.4 0.42 0.58 × − surrounded by 0.3 µm thick In Ga As (n = 6 1016 cm 3) top and bottom spacer layers and 2.3 µm 0.53 0.47 × − thick InP (n = 1 1017 cm 3) top and bottom claddings. This structure has a 0.8 µm thick highly doped × − (n = 7 1018 cm 3) InP top contact layer. × − Fully processed 3 mm long lasers from both wafers operate above room temperature with a pulsed (50 ns pulse width at 5 kHz repetition rate) threshold current density of 4.4 kA/cm2 and 2.5 kA/cm2 at 300 K for the GaAs- and InP-based QCLs, correspondingly. The current–voltage (IV) characteristics for both QCL devices are presented in Figure1. The theoretical operating voltage (in Volts) at the threshold, assuming that the total voltage drops only across the laser core, can be estimated as Uth = N(Eem + Einj)/e, where N is the number of cascade periods, Eem is the laser emission energy in eV, e is the electron charge and Einj is the energy “width” of the miniband in the injector part of the cascade period at the alignment—often called the voltage defect. The theoretical values for the voltage at the laser threshold are 14.8 V and 10.6 V for MR2790 and MR2449, correspondingly. For the MR2449 device, the observed voltage is very similar to the predicted one, while the MR2790 laser requires a much higher voltage (27 V) to achieve lasing. It was suggested [26] that the high potential barrier in the conduction band offset (up to 0.3 eV) on the GaAs/InAlP interfaces can lead to an increased resistance and operating voltage. In an attempt to minimise this effect, a transitional layer between GaAs and InAlP materials is incorporated into the MR2790 QCL structure. The layer provides a step-wise, less abrupt change in the conduction band profile and consists of 25 nm of In0.49Ga0.51P followed by 15 nm of In0.49(Ga0.5Al0.5)0.51P in the direction from GaAs to InAlP, while the layer sequence is reversed for the direction from InAlP to GaAs. However, despite the introduction of this transitional layer, the operating voltage appears to still be higher than the predicted one. The measurements of voltage distribution across the cleaved QCL facets were carried out using an NT-MDT NTegra Aura atomic force microscopy (AFM) setup in the regime of amplitude modulated gradient scanning Kelvin probe force microscopy (AM-GKPFM) [28–30]. The microscopy head has a track system based on a λ = 1.3 µm infrared laser. The typical spatial resolution for the measured potential distributions and topography maps is better than 100 nm. Measurements were carried out using uncoated highly n-doped silicon NSG11 cantilevers, AM-GKPFM operated in two-pass lift mode; the amplitude of cantilever mechanical oscillations was 40 nm; lift 30 nm; cantilever bias voltage amplitude was 0.5 V, see [29] for detailed description of measurement method. The QCL chips were cleaved at atmospheric pressure outside the vacuum chamber, soldered epi-side up on copper Crystals 2020, 10, x; doi: FOR PEER REVIEW 3 of 8 Crystals 2020, 10, 129 3 of 8 relativeCrystals 2020 to , the10, x;earthed doi: FOR submount, PEER REVIEW was applied to the top of the laser ridge. The measurements were3 of 8 performed at room temperature and at reduced pressure (~0.1 mBar). The size of the cleaved ridge relativesubmounts to the and earthed then putsubmount, under vacuum. was applied The to negative the top cwof the bias, laser relative ridge. to The the measurements earthed submount, were devices used in the KPFM experiments is 0.27 mm long and 20 µm wide for wafer MR2790, and 2 performedwas applied at to room the top temperature of the laser and ridge. at reduced The measurements pressure (~0.1 were mBar). performed The size at room of the temperature cleaved ridge and mm long and 20 µm wide for wafer MR2449. devicesat reduced used pressure in the (~0.1KPFM mBar). experiments The size is of 0.27 the mm cleaved long ridge and devices20 µm wide used for in thewafer KPFM MR2790, experiments and 2 mmis 0.27 long mm and long 20 andµm 20wideµm for wide wafer for waferMR2449. MR2790, and 2 mm long and 20 µm wide for wafer MR2449. 40

35 T=300K MR2790 40 30 35 T=300K MR2790 25 30 20 25 laser thresholds 15

Voltage (V) 20 laser thresholds 10 15 MR2449 Voltage (V) 5 10 MR2449 0 5012345678 2 0 Current density (kA/cm ) 012345678 2 Current density (kA/cm ) Figure 1. Current–voltage characteristics for 3 mm long λ ≈ 9.7 µm GaAs/AlGaAs QCL (MR2790) and λFigure ≈ 5.7 µm 1. Current–voltage InGaAs/InAlAs characteristicsQCL (MR2449) for with 3 mm a pu longlsedλ threshold9.7 µm current GaAs/AlGaAs density QCL of 4.4 (MR2790) kA/cm2 and and Figure 1. Current–voltage characteristics for 3 mm long λ ≈ 9.7 µm GaAs/AlGaAs QCL (MR2790)2 and 2.5λ kA/cm5.7 µm2 at InGaAs 300 K,/InAlAs respectively. QCL (MR2449) with a pulsed threshold current density of 4.4 kA/cm and ≈ 2 λ2.5 ≈ 5.7 kA /µmcm2 InGaAs/InAlAsat 300 K, respectively. QCL (MR2449) with a pulsed threshold current density of 4.4 kA/cm and 2 3. Results2.5 kA/cm and Discussionat 300 K, respectively. 3. Results and Discussion 3. ResultsThe topography and Discussion map of the cleaved facet measured in the tapping mode of AFM [31] is The topography map of the cleaved facet measured in the tapping mode of AFM [31] is obtained obtained along with the voltage dependence. This helps to eliminate the possible features on voltage alongThe with topography the voltage dependence.map of the Thiscleaved helps facet to eliminate measured the in possible the tapping features mode on voltage of AFM distribution [31] is distribution graphs associated with unevenness or foreign particles that have appeared on the obtainedgraphs associated along with with the voltage unevenness dependence. or foreign This particles helps to that eliminate have appeared the possible on thefeatures cleaved on voltage surface. cleaved surface. The typical topography map of the part of the cleaved facet for the device from distributionThe typical topographygraphs associated map of thewith part unevenness of the cleaved or foreign facet for particles the device that from have MR2790 appeared is presented on the MR2790 is presented in Figure 2. The highest roughness in the cleaved surface flatness does not cleavedin Figure surface.2. The The highest typical roughness topography in themap cleaved of the surfacepart of the ﬂatness cleaved does facet not for exceed the device a few from nm. exceed a few nm. The GaAs/AlGaAs laser core is clearly visible—positioned approximately between MR2790The GaAs is/ AlGaAspresented laser in coreFigure is clearly2. The visible—positionedhighest roughness in approximately the cleaved surface between flatness the 3 anddoes 6 notµm the 3 and 6 µm marks on the horizontal scale. On both sides around the laser core there are darker exceedmarks ona few the nm. horizontal The GaAs/AlGaAs scale. On both laser sides core around is clearly the visible—positioned laser core there are darkerapproximately layers, which between are layers, which are about 200 to 250 nm wide, corresponding to the GaAs spacer layers. The white theabout 3 and 200 6 to µm 250 marks nm wide, on the corresponding horizontal scale. to the On GaAs both spacersides around layers. the The laser white core spots there are are probably darker spots are probably related to contaminating foreign particles on the facet surface. The larger spots layers,related which to contaminating are about 200 foreign to 250 particles nm wide, on corre the facetsponding surface. to the The GaAs larger spacer spots layers. appear The along white the appear along the interface between the core region and the spacers, while the smaller ones are spotsinterface are betweenprobably therelated core to region contaminating and the spacers, foreign while particles the smaller on the onesfacet aresurface. distributed The larger across spots the distributed across the bottom and top AlInP claddings. The surface of the core region part is clear appearbottom andalong top the AlInP interface claddings. between The surfacethe core of region the core and region the part spacers, is clear while from the such smaller particles. ones Such are a from such particles. Such a distribution is not yet fully understood; however, this can also be of a distributeddistribution across is not the yet fullybottom understood; and top AlInP however, claddings. this can The also surface be of aof chemical the core nature,region forpart example, is clear chemical nature, for example, selective oxidation of clusters formed in AllnP material. fromselective such oxidation particles. of Such clusters a distribution formed in AllnPis not material.yet fully understood; however, this can also be of a chemical nature, for example, selective oxidation of clusters formed in AllnP material.

FigureFigure 2.2. TopographicTopographic map map for for the the cleaved cleaved facet facet of the of GaAs the /AlGaAsGaAs/AlGaAs quantum quantum cascade lasercascade obtained laser obtainedin the tapping in the regimetapping of regime AFM. Theof AFM. laser The core lase is betweenr core is ~3 between and ~6 ~3µm and on ~6 the µm horizontal on the horizontal scale with Figure 2. Topographic map for the cleaved facet of the GaAs/AlGaAs quantum cascade laser scale0.2 µ mwith thick 0.2 GaAsµm thick spacers GaAs and spacers 3 µm wideand 3 AlInP µm wide claddings AlInP surroundingcladdings surrounding it on both sides.it on both The sides. GaAs obtained in the tapping regime of AFM. The laser core is between ~3 and ~6 µm on the horizontal Thesubstrate GaAs issubstrate on the right. is on the right. scale with 0.2 µm thick GaAs spacers and 3 µm wide AlInP claddings surrounding it on both sides. The GaAs substrate is on the right.

Crystals 2020, 10, 129 4 of 8

The scanning time for one line in the KPFM regime is just above 3 minutes, where half of this is spent for the actual measurement and another half for the back sweep. The contact potential difference profile under zero bias (CPD(0)) is recorded first. This CPD(0) value is then subtracted from the CPD(V) values measured at non-zero applied bias. The subtraction of CPD(0) values recorded at zero bias from all CPD(V) curves measured subsequently is used to partly eliminate the contribution from the static surface effects. However, if the charging/discharging of surface states have a relatively long-term dependence, then these effects should be studied separately and in much more detail. For each given applied voltage, just one CPD(V) scan was used. The voltage and the current values in these experiments are controlled with precision greater than 1 mV and 0.1 mA, correspondingly. The ∆CPD = CPD(V) CPD(0) profiles for the device from wafer MR2790 are presented in Figure3 − for the bias values up to 7 V (compare with IV curve for this laser in Figure1). These ∆CPD scans have been taken at the position on the facet close to the top of the topographic image in Figure2. The relative layout of the AlInP claddings, GaAs layers and the laser core in this figure is given as a reference. It should be noted that for the maximum applied bias used in these experiments, the detected shift of the sampleCrystals mounted 2020, 10 on, x; doi: the FOR copper PEER REVIEW holder due to thermal expansion was around 6 µm (the5 temperatureof 9 increased up to 30 K). Because of this temperature shift, all ∆CPD scans have been adjusted (with accuracy better than 0.1 µm) to be at the same position on the sample facet.

1 top laser core bottom buffer & cladding cladding substrate 0 0.2V -1 1V

-2 2V

3V -3 4V CPD (V)

Δ -4 5V -5 6V MR2790 T=300K -6 7V

-7 01234567891011121314 Distance (µm)

Figure 3. Contact potential diﬀerence proﬁles for the GaAs-based QCL with AlInP claddings measured

on the cleavedFigure laser 3. Contact facet by potential KPFM difference technique profiles for various for the appliedGaAs-based bias. QCL with AlInP claddings measured on the cleaved laser facet by KPFM technique for various applied bias. The applied voltage for the MR2790 device does not drop only across epitaxial layers. Up to 5% of the total bias hasThe been ΔCPD found = CPD(V) to fall− CPD(0) outside profiles of them, for the probably device from on wafer the electricalMR2790 are contacts presented or in the GaAs substrate. AlmostFigure 3 for half the of bias the values total up voltage to 7 V (compare drops on with the IV bottomcurve for AlInPthis laser cladding, in Figure 1). while These the ΔCPD drop on the scans have been taken at the position on the facet close to the top of the topographic image in Figure top AlInP cladding2. The relative is less;layout however, of the AlInP it claddings, is still excessive. GaAs layers Note,and the thatlaser topcore in and this bottom figure is given claddings as were designed anda reference. specified It should during be noted the growth that for the process maximum to have applied the bias same used composition in these experiments, and doping the level; however, KPFMdetected measurements shift of the sample show mounted that on thethe coppe voltager holder drop due across to thermal the expansion top and bottomwas around claddings 6 is different. Thisµm (the can temperature be caused increased by the up diff toerent 30 K). concentration Because of this temperature of localized shift, defects all ΔCPD and scans provides have a new been adjusted (with accuracy better than 0.1 µm) to be at the same position on the sample facet. topic for futureThe investigation. applied voltage for The the voltage MR2790 device distribution does not drop across only the across laser epitaxial core layers. is not Up precisely to 5% linear, resulting inof the the cascadetotal bias periodshas been found having to fall diff outsideerent levelsof them, of probably alignment on the at electrical any given contacts bias. or We the note that some unevennessGaAs substrate. in the surfaceAlmost half topography of the total voltage might drops contribute on the bottom to the AlInP appearance cladding, of while a slightly the drop non-linear voltage distributionon the top AlInP in the cladding laser core.is less; However,however, it is it still is unlikely excessive. asNote, the that position top and ofbottom the “inflection”claddings point were designed and specified during the growth process to have the same composition and doping on this dependencelevel; however, is shifting KPFM measurements with bias increase show that from the beingvoltage closer drop across to the the top top cladding and bottom towards the bottom cladding.claddings is different. This can be caused by the different concentration of localized defects and provides a new topic for future investigation. The voltage distribution across the laser core is not precisely linear, resulting in the cascade periods having different levels of alignment at any given bias. We note that some unevenness in the surface topography might contribute to the appearance of a slightly non-linear voltage distribution in the laser core. However, it is unlikely as the position of the “inflection” point on this dependence is shifting with bias increase from being closer to the top cladding towards the bottom cladding. Crystals 2020, 10, 129 5 of 8

After the ∆CPD scan was completed under the applied bias of 7 V, the voltage was decreased back to 4 V. Surprisingly, the re-measured potential profile does not exactly follow the one obtained for the same 4 V bias but from when the bias value had increased from zero volts (see Figure4). Such a hysteresis might indicate that the charge accumulated near the GaAs/AlInP interface at higher voltage and was still not dissipated when the voltage had dropped. Another indication of a slow discharge process in the area of the bottom cladding is the ∆CPD dependences (see Figure4) taken at zero bias straightaway after the main series of ∆CPD measurements had been finished. The ∆CPD graphs have been obtained in 3, 6 and 9 minutes after the bias was switched-off. Such extremely long relaxation time is very unlikely to be defined only by the processes happening in the volume of the semiconductor structure but also by kinetics of the surface states. On the other hand, deep traps for the electrons which might be formed in the energy gap of the Al-rich AlInP claddings and high conduction band discontinuity on GaAs/AlInP interfaces might contribute to such slow-discharging surface states. The charge accumulation is observed mainly in the bottom cladding and closer to the interface with the core. This behaviour can be explained partly by the different conditions on the interfaces between GaAs and AlInP in different parts of the epitaxial structure. It should not be an obstacle for the electrons to fall from the higher conduction band of AlInP when they move through AlInP/GaAs interfaces. To move through GaAs/AlInP interfaces is much harder even with the incorporated transitional layer. The higher doping level on the interface between the top contact GaAs layer and the top AlInP cladding should help to reduce the band offset by “levelling” the bottoms of their conduction bands. However, the conduction band profile on the GaAs/AlInP interface between the laser core and the bottom cladding may not be smooth enough because the doping level in this area is intentionally kept low to reduce possible free-carrier absorption, which can result in higher waveguide losses. The actual doping level in the AlInP claddings, which might end up being lower than intended due to possible formation of the deep traps, may also be the reason for an excessive voltage dropCrystals across 2020, 10 the, x; doi: claddings. FOR PEER REVIEW We believe that more growth runs and further optimization6 of 9 of the structural design are needed to reduce the voltage drop in the GaAs-based QCLs with AlInP claddings.

1 top laser core bottom buffer & cladding cladding substrate

0 9

3 6 -1 4V, (voltage on decreasing)

-2 CPD (V)

Δ 4V, (voltage on rising) -3 MR2790 T=300K

-4 01234567891011121314 Distance (µm)

Figure 4. Contact potential difference profiles for the GaAs-based QCL with AlInP claddings measured on the cleavedFigurelaser 4. Contact facet bypotential the KPFM difference technique. profiles for The th observede GaAs-based hysteresis QCL with (the AlInP voltage claddings was increased measured on the cleaved laser facet by the KPFM technique. The observed hysteresis (the voltage from 0 to 4 V for the first measurement and the voltage was decreased from 7 to 4 V for the second one) was increased from 0 to 4 V for the first measurement and the voltage was decreased from 7 to 4 V for and a verythe slow second time one) dependence and a very slow (measured time dependence in 3, 6 (measured and 9 minutes) in 3, 6 and after 9 mi switch-onutes) afterff switch-offmight indicate the existence ofmight charge indicate that the has existence accumulated of charge that around has accumulated the bottom around AlInP the cladding.bottom AlInP cladding.

After the ΔCPD scan was completed under the applied bias of 7 V, the voltage was decreased back to 4 V. Surprisingly, the re-measured potential profile does not exactly follow the one obtained for the same 4 V bias but from when the bias value had increased from zero volts (see Figure 4). Such a hysteresis might indicate that the charge accumulated near the GaAs/AlInP interface at higher voltage and was still not dissipated when the voltage had dropped. Another indication of a slow discharge process in the area of the bottom cladding is the ΔCPD dependences (see Figure 4) taken at zero bias straightaway after the main series of ΔCPD measurements had been finished. The ΔCPD graphs have been obtained in 3, 6 and 9 minutes after the bias was switched-off. Such extremely long relaxation time is very unlikely to be defined only by the processes happening in the volume of the semiconductor structure but also by kinetics of the surface states. On the other hand, deep traps for the electrons which might be formed in the energy gap of the Al-rich AlInP claddings and high conduction band discontinuity on GaAs/AlInP interfaces might contribute to such slow-discharging surface states. The charge accumulation is observed mainly in the bottom cladding and closer to the interface with the core. This behaviour can be explained partly by the different conditions on the interfaces between GaAs and AlInP in different parts of the epitaxial structure. It should not be an obstacle for the electrons to fall from the higher conduction band of AlInP when they move through AlInP/GaAs interfaces. To move through GaAs/AlInP interfaces is much harder even with the incorporated transitional layer. The higher doping level on the interface between the top contact GaAs layer and the top AlInP cladding should help to reduce the band offset by “levelling” the bottoms of their conduction bands. However, the conduction band profile on the GaAs/AlInP interface between the laser core and the bottom cladding may not be smooth enough because the doping level in this area is intentionally kept low to reduce possible free-carrier absorption, which Crystals 2020, 10, 129 6 of 8

The observed voltage distribution across the MR2449 device is much more predictable (see FigureCrystals5 )2020 compared, 10, x; doi: with FOR thePEER MR2790 REVIEW device. Almost all the applied voltage is dropped linearly across7 of 9 the laser core, demonstrating equal level alignment for all cascade periods. There is negligible voltage dropcan result across in the higher cladding waveguide and spacer losses. layers. The actual The di dopingﬀerence level between in the the AlInP predicted claddings, (10.6 which V) and might the observedend up being (10.8 V)lower voltage than at intended the threshold due isto minimal possible and formation depends of on the the deep slight traps, variation may inalso the be level the reason for an excessive voltage drop across the claddings. We believe that more growth runs and alignment and band bending due to charge redistribution in the active and injector regions, which further optimization of the structural design are needed to reduce the voltage drop in the was not included in the voltage calculations. Neither hysteresis nor slow time evolution of ∆CPD GaAs-based QCLs with AlInP claddings. dependences have been observed for the MR2449 device.

2 bottom sub- laser core 1 top cladding cladding strate spacer spacer

0 0.31V -1 0.99V -2 2.01V -3 3 V CPD, V Δ 3.51V MR2449 -4 T=300K 4.34V

-5 5.02V -6 6.68V -7 0123456789 Distance, µm

Figure 5. Contact potential difference profiles for the InP-based QCL measured on the cleaved laser facetFigure by KPFM5. Contact technique potential for difference various applied profiles bias. for the InP-based QCL measured on the cleaved laser facet by KPFM technique for various applied bias. 4. Conclusions The observed voltage distribution across the MR2449 device is much more predictable (see We have presented a technique for the detailed study of the spatial distribution of the applied Figure 5) compared with the MR2790 device. Almost all the applied voltage is dropped linearly voltage across the epitaxial structure of mid-infrared QCLs. The GaAs-based QCL with AlInP across the laser core, demonstrating equal level alignment for all cascade periods. There is negligible waveguides demonstrates an uneven potential drop across its laser core and very high parasitic voltage voltage drop across the cladding and spacer layers. The difference between the predicted (10.6 V) drops across the cladding layers. The developed method can be very valuable for the optimization of and the observed (10.8 V) voltage at the threshold is minimal and depends on the slight variation in the QCL design as well as the epitaxial growth technology of QCLs. The detailed knowledge of the the level alignment and band bending due to charge redistribution in the active and injector regions, voltage distribution might be particularly important for the broadband QCLs, where all sections in the which was not included in the voltage calculations. Neither hysteresis nor slow time evolution of laser core designed for different wavelengths should achieve optimum alignment at the same applied ΔCPD dependences have been observed for the MR2449 device. bias values.

Author4. Con Contributions:clusions All authors contributed to conceiving the experiment. K.L. and V.E. performed the Kelvin probe measurements. A.K. contributed to the design of QCLs, engineered the epitaxial process and grew the QCL wafers.We D.R. have designed presented the QCL a technique structures for and the measured detailed the st laserudy performance.of the spatial All distribution authors contributed of the applied to the datavoltage analysis across and discussion.the epitaxial K.L. structure and D.R. draftedof mid- theinfrared manuscript. QCLs. All The authors GaAs-based have read QCL and agreed with toAlInP the publishedwaveguides version demonstrates of the manuscript. an uneven potential drop across its laser core and very high parasitic Funding:voltage dropsThis work across was the supported cladding by layers. the EPSRC The UK, developed by the European method Unioncan be Marie very Curie valuable IAPP projectfor the “QUANTATEC”,optimization of K.L. the was QCL supported design by as Russian well as Science the epitaxial Foundation growth (Project technology 19-19-00683, of development QCLs. The ofdetailed KPFM measurement methodology). A considerable amount of the research presented in this paper was carried out outsideknowledge of the normalof the workingvoltage hours.distribution might be particularly important for the broadband QCLs, where all sections in the laser core designed for different wavelengths should achieve optimum Acknowledgments: A.K. would like to thank David Morris, Chief Research Technician, for his assistance in the MOVPEalignment lab. at the same applied bias values. Author Contributions: All authors contributed to conceiving the experiment. K.S.L. and V.P.E. performed the Kelvin probe measurements. A.B.K. contributed to the design of QCLs, engineered the epitaxial process and grew the QCL wafers. D.G.R. designed the QCL structures and measured the laser performance. All authors contributed to the data analysis and discussion. K.S.L. and D.G.R. drafted the manuscript. All authors reviewed and contributed to the final version of the manuscript. Crystals 2020, 10, 129 7 of 8

Conﬂicts of Interest: The authors declare no conﬂict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

References

1. Green, R.P.; Krysa, A.B.; Roberts, J.S.; Wilson, L.R.; Cockburn, J.W.; Revin, D.G.; Zibik, E.; Ng, W.H. Room-temperature operation of InGaAs/AlInAs quantum cascade lasers grown by metalorganic vapor phase epitaxy. Appl. Phys. Lett. 2003, 83, 1921. [CrossRef] 2. Krysa, A.B.; Roberts, J.; Green, R.; Wilson, L.; Page, H.; García, M.; Cockburn, J. MOVPE-grown quantum cascade lasers operating at 9 µm wavelength. J. Cryst. Growth 2004, 272, 682–685. [CrossRef] ∼ 3. Spagnolo, V.; Troccoli, M.; Scamarcio, G.; Gmachl, C.; Capasso, F.; Tredicucci, A.; Sergent, A.M.; Hutchinson, A.L.; Sivco, D.L.; Cho, A.Y. Temperature profile of GaInAs/AlInAs/InP quantum cascade-laser facets measured by microprobe photoluminescence. Appl. Phys. Lett. 2001, 78, 2095–2097. [CrossRef] 4. Offermans, P.; Koenraad, P.M.; Wolter, J.H.; Beck, M.; Aellen, T.; Faist, J. Digital alloy interface grading of an InAlAs/InGaAs quantum cascade laser structure studied by cross-sectional scanning tunneling microscopy. Appl. Phys. Lett. 2003, 83, 4131–4133. [CrossRef] 5. Revin, D.G.; Wilson, L.R.; Cockburn, J.W.; Krysa, A.B.; Roberts, J.S.; Airey, R.J. Intersubband spectroscopy of quantum cascade lasers under operating conditions. Appl. Phys. Lett. 2006, 88, 131105. [CrossRef] 6. Yu, N.; Diehl, L.; Cubukcu, E.; Pflügl, C.; Bour, D.; Corzine, S.; Zhu, J.; Höfler, G.; Crozier, K.; Capasso, F. Near-field imaging of quantum cascade laser transverse modes. Opt. Express 2007, 15, 13227–13235. [CrossRef] 7. Kuehn, W.; Parz, W.; Gaal, P.; Reimann, K.; Woerner, M.; Elsaesser, T.; Muüller, T.; Darmo, J.; Unterrainer, K.; Austerer, M.; et al. Ultrafast phase-resolved pump-probe measurements on a quantum cascade laser. Appl. Phys. Lett. 2008, 93, 151106. [CrossRef] 8. Wang, C.; Goyal, A.; Huang, R.; Donnelly, J.; Calawa, D.; Turner, G.; Sanchez-Rubio, A.; Hsu, A.; Hu, Q.; Williams, B. Strain-compensated GaInAs/AlInAs/InP quantum cascade laser materials. J. Cryst. Growth 2010, 312, 1157–1164. [CrossRef] 9. Grasse, C.; Katz, S.; Böhm, G.; Vizbaras, A.; Meyer, R.; Amann, M.-C. Evaluation of injectorless quantum cascade lasers by combining XRD- and laser-characterisation. J. Cryst. Growth 2011, 323, 480–483. [CrossRef] 10. Pier´sciński,K.; Pier´scińska,D.; Iwinska, M.; Kosiel, K.; Szerling, A.; Karbownik, P.; Bugajski, M. Investigation of thermal properties of mid-infrared AlGaAs/GaAs quantum cascade lasers. J. Appl. Phys. 2012, 112, 043112. [CrossRef] 11. Friedli, P.; Sigg, H.; Wittmann, A.; Terazzi, R.; Beck, M.; Kolek, A.; Faist, J. Synchrotron infrared transmission spectroscopy of a quantum cascade laser correlated to gain models. Appl. Phys. Lett. 2013, 102, 12112. [CrossRef] 12. Enobio, E.C.I.; Ohtani, K.; Ohno, Y.; Ohno, H. Detection and measurement of electroreflectance on quantum cascade laser device using Fourier transform infrared microscope. Appl. Phys. Lett. 2013, 103, 231106. [CrossRef] 13. Kubacka-Traczyk, J.; Sankowska, I.; Seeck, O.; Kosiel, K.; Bugajski, M. High-resolution X-ray characterization

of mid-IR Al0.45Ga0.55As/GaAs Quantum Cascade Laser structures. Thin Solid Films 2014, 564, 339–344. [CrossRef] 14. Michalowski, P.; Gutowski, P.; Pier´scińska,D.; Pier´sciński,K.; Bugajski, M.; Strupiński,W. Characterization of the superlattice region of a quantum cascade laser by secondary ion mass spectrometry. Nanoscale 2017, 9, 17571–17575. [CrossRef][PubMed] 15. Walther, T.; Krysa, A.B. Transmission electron microscopy of AlGaAs/GaAs quantum cascade laser structures. J. Microsc. 2017, 268, 298–304. [CrossRef] 16. Rajeev, A.; Chen, W.; Kirch, J.; Babcock, S.E.; Kuech, T.F.; Earles, T.; Mawst, L. Interfacial Mixing Analysis for Strained Layer Superlattices by Atom Probe Tomography. Crystals 2018, 8, 437. [CrossRef] 17. Lü, X.; Luna, E.; Schrottke, L.; Biermann, K.; Grahn, H. Determination of the interface parameter in terahertz quantum-cascade laser structures based on transmission electron microscopy. Appl. Phys. Lett. 2018, 113, 172101. [CrossRef] Crystals 2020, 10, 129 8 of 8

18. Becher, N.; Farzaneh, M.; Knipfer, B.; Sigler, C.A.; Kirch, J.; Boyle, C.; Botez, D.; Mawst, L.; Lindberg, D.F.; Earles, T. Thermal imaging of buried heterostructure quantum cascade lasers (QCLs) and QCL arrays using CCD-based thermoreflectance microscopy. J. Appl. Phys. 2019, 125, 033102. [CrossRef] 19. Khabibullin, R.; Shchavruk, N.; Ponomarev, D.; Ushakov, D.; Afonenko, A.; Maremyanin, K.; Volkov, O.; Pavlovskiy, V.; Dubinov, A. The operation of THz quantum cascade laser in the region of negative differential resistance. Opto-Electron. Rev. 2019, 27, 329–333. [CrossRef] 20. Weber, S.; Hermes, I.; Turren-Cruz, S.-H.; Gort, C.; Bergmann, V.W.; Gilson, L.; Hagfeldt, A.; Graetzel, M.; Tress, W.; Berger, R.; et al. How the formation of interfacial charge causes hysteresis in perovskite solar cells. Energy Environ. Sci. 2018, 11, 2404–2413. [CrossRef] 21. Chen, C.; Song, Z.; Xiao, C.; Zhao, D.; Shrestha, N.; Li, C.; Yang, G.; Yao, F.; Zheng, X.; Ellingson, R.J.; et al. Achieving a high open-circuit voltage in inverted wide-bandgap perovskite solar cells with a graded perovskite homojunction. Nano Energy 2019, 61, 141–147. [CrossRef] 22. Utama, M.I.B.; Kleemann, H.; Zhao, W.; Ong, C.S.; Da Jornada, F.H.; Qiu, D.Y.; Cai, H.; Li, H.; Kou, R.; Zhao, S.; et al. A dielectric-defined lateral heterojunction in a monolayer semiconductor. Nat. Electron. 2019, 2, 60–65. [CrossRef] 23. Ranjan, R.; Prakash, A.; Singh, A.; Singh, A.; Garg, A.; Gupta, R. Effect of tantalum doping in a TiO2 compact layer on the performance of planar spiro-OMeTAD free perovskite solar cells. J. Mater. Chem. A 2018, 6, 1037–1047. [CrossRef] 24. Suzuki, T.; Gomyo, A.; Iijima, S. Strong ordering in GaInP alloy semiconductors; Formation mechanism for the ordered phase. J. Cryst. Growth 1988, 93, 396–405. [CrossRef] 25. Minagawa, S.; Kondow, M. Dependence of photoluminescence peak energy of MOVPE-grown AlGaInP on substrate orientation. Electron. Lett. 1989, 25, 758. [CrossRef] 26. Atkins, C.; Krysa, A.B.; Revin, D.; Kennedy, K.; Commin, J.; Cockburn, J. Low threshold room temperature GaAs/AlGaAs quantum cascade laser with InAlP waveguide. Electron. Lett. 2011, 47, 1193. [CrossRef] 27. Zasavitskii, I.I.; Kovbasa, N.Y.; Raspopov, N.A.; Lobintsov, A.V.; Kurnyavko, Y.V.; Gorlachuk, P.V.; Krysa, A.B.; Revin, D.G. A GaInAs/AlInAs quantum cascade laser with an emission wavelength of 5.6 µm. Quantum Electron. 2018, 48, 472–475. [CrossRef] 28. Melitz, W.; Shen, J.; Kummel, A.C.; Lee, S. Kelvin probe force microscopy and its application. Surf. Sci. Rep. 2011, 66, 1–27. [CrossRef] 29. Ladutenko, K.S.; Ankudinov, A.V.; Evtikhiev, V.P. On the accuracy of quantitative measurements of the local surface potential. Tech. Phys. Lett. 2010, 36, 228–231. [CrossRef] 30. Girard, P.; Ramonda, M.; Saluel, D. Electrical contrast observations and voltage measurements by Kelvin probe force gradient microscopy. J. Vac. Sci. Technol. B Microelectron. Nanometer Struct. 2002, 20, 1348. [CrossRef] 31. Zhong, Q.; Inniss, D.; Kjoller, K.; Elings, V. Fractured polymer/silica fiber surface studied by tapping mode atomic force microscopy. Surf. Sci. Lett. 1993, 290, L688–L692.

© 2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).