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Type II quantum wells on GaSb substrate designed for laser-based gas sensingapplications in a broad range of mid infrared

M. Motyka a,⇑, K. Ryczko a, G. Sek a, F. Janiak a, J. Misiewicz a, A. Bauer b, S. Höfling b, A. Forchel b

a Institute of Physics, Wrocław University of Technology, Wybrze _ze Wyspianskiego 27, 50-370 Wrocław, Polandb University of Würzburg, Technische Physik and Wilhelm-Conrad-Röntgen-Research Center for Complex Material Systems, Am Hubland, D-97074 Würzburg, Germany

a r t i c l e i n f o

Article history:Received 31 July 2011Received in revised form 5 January 2012Accepted 6 January 2012Available online 9 February 2012

Keywords:Type II quantum wellsInterband cascade laserMid-infraredGas sensing

a b s t r a c t

Optical properties of InAs/GaInSb/InAs type II quantum wells grown on GaSb substrate have been studiedby Fourier transformed photoreflectance and photoluminescence supported by electronic structure cal-culations. Such a broken gap material system is utilized for the active region of interband cascade lasersand further for laser-based gas sensors operating at room temperature. Based on the measured absorp-tion-like and emission-like spectra in the range from about 2 to above 5 lm, we indicate the potential ofsuch type II structures for detecting such environmentally relevant gasses as HCl, CO2, N2O, and NH3

which have their absorption lines at wavelengths longer than about 3.5 lm, i.e. beyond the alreadyexplored range characteristic for hydrocarbons. We investigate the issue of the type II transition oscillatorstrength versus the InAs well width and temperature for two different quantum well layer structures. Sig-nificant enhancement of the type II transition intensities could be predicted for W-like design of the welland increasing with temperature, as a consequence of various thermal energy gap coefficients of theinvolved materials and weakening of the confinement for electrons. The concept of compensating theelectric field effect in the real operational device, affecting the transition probability, by intentionallyintroducing an asymmetry of the double quantum well structure has been shown to be functional for var-ious emission wavelengths. Reasonable values of the transition oscillator strengths could still be demon-strated at about 5 lm.

� 2012 Elsevier B.V. All rights reserved.

1. Introduction

The market of gas sensors and the related disciplines and indus-tries as for instance medical diagnosis in e.g. cancer tissue discov-ery by breath composition analysis [1] or environment protectionin tracing the air pollution [2], etc., demand more and more sensi-tive detector systems with many other functionalities as tunability,portability or real time response. All these can be realized in opticalsensors which are based on semiconductor laser sources withwavelengths typical for vibrational–rotational absorption bandsof the relevant gases. One possible solution is the use of so calledinterband cascade lasers (ICLs) [3] utilizing usually the GaSb-basedtype II InAs/GaInSb quantum wells in the active region [3–6]and up to now have been demonstrated as very competitive espe-cially in the range of 3–4 lm with a focus on detection of hydrocar-bons. All concepts developed so far for semiconductor diodes andlasers for wavelengths in the mid infrared and beyond suffer fromsevere limitations. Devices based on interband transitions arehampered by high internal losses. Auger recombination losses areparticularly strong in materials with small bandgaps, usually

preventing room-temperature operation. On the other hand, indevices based on intraband transitions (quantum cascade lasers –QCLs) Auger processes tend to be unimportant because no holesare present and the electron densities are low. However, QCLs havedifficulties performing well at wavelengths shorter than about5 lm since this requires extremely deep quantum wells and veryoften complex multinary compound semiconductor materialscombinations. The type-II QW design (or its double electron wellversion of W-like shape) intends to overcome these limitations.Since it is based on interband transitions, it does not have thewavelength limitations of QCLs. At the same time, the type-IIW-design leads to a partial local separation of electrons and holes.The reduced electron–hole wave function overlap decreases theAuger coupling strength. This reduces the nonradiative carrierlosses in ICLs. It has indeed been shown that, in comparison to typeI-QW-based devices, type II structures exhibit much lower Augercoefficients [7–9]. Hence, due to almost independent Augercoefficients of the emission wavelength this kind of structuremight be a promising candidate for laser-based gas sensing appli-cations in the range beyond 4 lm, where there also exist strongabsorption bands of many industrially and environmentallyrelevant gasses as HCl, CO2, CO, N2O, OCS, NH3, NO and CH2O [9].And indeed, room temperature continuous wave operation has

0925-3467/$ - see front matter � 2012 Elsevier B.V. All rights reserved.doi:10.1016/j.optmat.2012.01.012

⇑ Corresponding author. Tel.: +48 71 320 29 86; fax: +48 71 328 36 96.E-mail address: marcin.motyka@pwr.wroc.pl (M. Motyka).

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already been demonstrated by interband cascade lasers with ‘‘W’’active regions [10]. There are, however, several additional factors,which have to be taken into account considering further develop-ment of mid infrared emission sources using low dimensionalstructures, especially in case of the type II wells. In this work, weelaborate on the optical properties and electronic structure of thatkind of broken gap system designed as the so called W-shapedquantum wells. In order to predict on their potential for the appli-cations in gas sensing and possible practical implementations inthe view of wavelength range broader than previously, especiallyto further infrared, there have been investigated such crucial prop-erties as freedom in the wavelength tunability and its conse-quences to the optical transitions oscillator strength and therelated changes of the electronic structure. Also the effect of theexternal electric field and compensation of the introduced systemasymmetry induced by an external bias and its influence on the en-ergy and intensity of the type II transitions have been studied.

2. Experimental details

The investigated structures were grown on (100) oriented GaSbsubstrates by a solid source molecular beam epitaxy systemequipped with valved cracking cells for both antimony and arsenic.The ‘‘W’’-shaped active part of the structure consists of two InAs lay-ers confining the electrons and a 3 nm wide Ga0.7In0.3Sb layer inbetween for the hole confinement. These layers are surrounded by2 nm thick AlSb barriers, preceded and followed by a 300 nm GaSbbuffer and a 50 nm GaSb cap, respectively. In this paper, we haveinvestigated a set of samples with different InAs widths (labeledd1) residing in the range of �(1–3) nm. In some cases, we also referto common type II structures with a single InAs QW layer [5].

In order to measure the Fourier transformed photoreflectance(FTPR) [11–14] and photoluminescence (FTPL) spectra we used aBruker spectrometer Vertex 80v equipped with both rapid-scanand step-scan modes, and an external evacuated chamber forexperiments with an additional modulated beam. The 660 nmsemiconductor laser diode pump beam affecting/modulating thereflectivity coefficient and liquid nitrogen cooled MCT and InSbphotodiodes as detectors were used (see Refs. [13,14] for moredetails).

3. Results and discussion

Fig. 1 shows room temperature Fourier transformed photore-flectance (blue line) and photoluminescence (red line) spectra of atypical W-shaped sample consisting of GaSb/AlSb(2.0 nm)/InAs(2.3 nm)/GaInSb(3.0 nm)/InAs(2.3 nm)/AlSb(2.0 nm)/GaSb (25.0 nm)five times repeated for an enhancement of the optical response.The PL peak is associated with the fundamental transition in theinvestigated type II quantum wells (i.e. the transition between H1and E1 levels of heavy hole and spatially separated electron states,respectively). The obtained features in the PR spectra can be dividedinto three parts. The first one is connected with two features asso-ciated with the optical transition in the active region between theconfined states in the well (called ‘‘QW region’’ in Fig. 1). The secondpart is a feature connected with the energy gap of the GaSb sub-strate (at 0.72 eV). And the last part (called above barrier region),which features are connected with transitions involving the reso-nant states above the band edge of GaSb (mostly in the conductionband). The latter, has been the subject of a previous publication[15], and therefore is not a scope of the current paper. Instead, inthis work we have focused on the confined QW states region onlyas the one directly related to device performances.

Fig. 2 shows PL and PR spectra for ‘‘W’’-shaped GaSb/AlSb/InAs/GaInSb/InAs/AlSb/GaSb quantum wells with different width of the

InAs layer: 1.5 nm (panel a), 1.7 nm (panel b), 2.3 nm (panel c),2.5 nm (panel d), 2.9 nm (panel e). As it can be seen by changingthe InAs layer width we can tune the fundamental type II transitionthrough a broad spectral range of interest from the point of view ofdetecting various gases, i.e. from about 2.5 to 5.5 lm. Examples of

Fig. 1. Fourier transformed PL (red line) and PR (blue line) room temperature spectraof GaSb/AlSb(2.0 nm)/InAs(2.3 nm)/GaInSb(3.0 nm)/InAs(2.3 nm)/AlSb(2.0 nm)/GaSbW-shaped quantum well. (For interpretation of the references to color in this figurelegend, the reader is referred to the web version of this article.)

Fig. 2. Room temperature FTPL (red line) and FTPR (blue lines) spectra for the W-shaped GaSb/AlSb/InAs/GaInSb/InAs/AlSb/GaSb quantum wells with various thick-nesses of the InAs layers changed from d1 = 1.5 nm (panel a) to d1 = 2.9 nm (panele). (For interpretation of the references to color in this figure legend, the reader isreferred to the web version of this article.)

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gases having their absorption lines in the respective spectral win-dows are indicated by gray areas (e.g. CO2, CH4, NH3).

The origin of the fundamental type II transitions’ spectral shiftin such QWs has also been analyzed theoretically by calculatingthe respective energy levels within the eight-band k � p formalism[16] usefulness of which has already been demonstrated previ-ously for the wavelength range below 4 lm [5,6]. The energiesand wave functions of holes and electrons were calculated by solv-ing the Schrödinger equation within the envelope function approx-imation. The used eight-band k � p Hamiltonian includes the straineffects [17], actually important mostly for the GaInSb layer, be-cause the InAs layer is almost lattice matched to GaSb. The result-ing Hamiltonian matrix for envelope wave functions is then solvedby using the finite difference method [18]. All the material param-eters have been taken from Ref. [19] and interpolated linearly forthe ternary compound, except its band gap for which the bowingparameter of 0.415 eV has been used. Fig. 3 shows a comparisonof the experimental data (energies of optical transitions from roomtemperature PL and PR spectra) and the calculated dependence ofthe type II transition energy vs the thickness of the InAs layer forthe W-shaped structures as in Fig. 2. Our calculations have con-firmed that indeed the shift of the spectral features related to thefundamental type II transition is driven by the change of the InAswell width, which allows the wavelength tuning in a broad rangeof 2 to above 5 lm, and further beyond that range even. If for in-stance, the InAs layer thickness is increased above 3 nm thenpotentially wavelengths longer than 6 lm become reachable. Gen-erally, one could consider also other ways of tuning the emissionwavelength by changing, for instance, the thickness of the GaInSblayer (the width of the well in the valence band). But, based on theresults of our calculations (not shown here), the shift is muchsmaller, which actually should be expected, mainly due to muchshallower potential well for holes and their larger effective masses,which makes the valence levels much less sensitive to QW modifi-cations. The obtained energy shift of the fundamental transition isthen about 50 meV only, when the GaInSb layer thickness is chan-ged from 2.5 to 4 nm. This corresponds to about 0.5 lm total wave-length shift in that spectral range, i.e. almost an order of magnitudeless while tailoring the InAs well widths. Thus, this kind of shift, asless efficient, could rather be used for a fine tuning of the targetedemission wavelength.

Tuning the type II QW emission to long wavelengths is a neces-sary but not the sufficient condition to obtain operational inter-band cascade lasers above 5 lm. The ICL concept bases in part ona large enough overlap of the electron and hole wave functions.Therefore, we have calculated the overlap integrals, squared valueof which reflects the transition oscillator strength (transitionintensity), as a function of the InAs thickness for two types of thewells, a common asymmetric (single InAs layer) type II wells,and the W-shaped ones, both at low and room temperature,respectively. In order to simulate properly the effect of tempera-ture on the band structure of the investigated type II system wehave assumed that the band gap changes induced by temperaturecause the shift of the conduction band edges, whereas the energyposition of the valence band edges have been kept constant [20].In Fig. 4 we show the band gap lineup (and the confined stateenergy levels) for two temperatures of 10 K and 300 K, to showmainly that the confinement energy of electrons is significantlydecreased at higher temperature (as the effect of various bandgap shifts of the materials). The latter causes important conse-quences to the probability of the electron wave function to pene-trate the GaInSb barrier (and enhancing the overlap with thehole wave function), which will be discussed below.

As expected and shown in Fig. 5, the oscillator strengths are lar-ger for the W-like shaped wells due to just doubled InAs layer, i.e.approximately doubled the contribution of the electron state to the

overlap with the hole state. A comparison of the respective wavefunctions (their squared values actually, i.e. the probability densi-ties) is shown in Fig. 6 for two type II QW layouts, i.e. with a single,and a double InAs layer.

The transition intensity decreases significantly when the widthof the InAs well is increased (Fig. 5). This is consequence of stron-ger confinement of the electrons in the well and hence a smallerpart of the wave function leaking into the GaInSb layer, which de-cides on the value of the overlap with the hole wave function. InFig. 7, we show a comparison of the calculated probability densi-ties for two cases of a thin (1 nm) and thick (3 nm) InAs layers ina common type II QW design. A change in the wave function leak-age into the GaInSb layer is clearly visible, which explains thedependences in Fig. 5.

Further, which is much less intuitive, there has been observedthat the overlap integrals and hence oscillator strengths increaseat room temperature, when compared to low temperature data,which is accidentally beneficial from the point of view of the laseroperation. This is a consequence of various thermal coefficients ofthe energy gaps for the InAs and GaInSb layers mainly (smaller forInAs), which causes that at increased temperatures the electron

Fig. 3. Comparison of the room temperature experimental and calculated transitionenergies as a function of the InAs layer thickness in the investigated W-shapedQWs.

Fig. 4. Band structure lineup and the confined state energy levels for twotemperatures of 10 and 300 K. The gray arrows indicate that in spite of a lightdecrease of the electron level energy its confinement energy (energy separation tothe edge of the GaInSb barrier) decreases significantly. (For interpretation of thereferences to color in this figure legend, the reader is referred to the web version ofthis article.)

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wave function leaks more into the GaInSb layer enhancing theoverlap with the hole wave function (confined within the GaInSblayer) – see Fig. 4 with the band lineup changes as a function oftemperature. The latter effect is obviously sensitive to the thick-ness of the InAs layer – the electron wave function leaks out stron-ger for thinner InAs when the confined level is lifted up within thepotential well.

It is also worth noting that in spite all the calculations havebeen performed for the interfaces between the layers assumed tobe perfectly sharp and for rectangular shape of the QWs, it is ex-pected that including into consideration effects as intermixingshould not cause qualitative changes to the obtained results. Gen-erally, if even some interdiffusion of the materials occurs it willcause a smoothing of the potential well shapes, which, as one of

the consequences, means an effective weakening of the confine-ment and hence a stronger penetration of the carrier wave functioninto the subsequent layers. This could even increase slightly theoscillator strength values in the real structures when comparedto the numbers obtained by us in the modeling used here. Also,the spatial inhomogeneity of the layers, which is mainly reflectedin the spectral linewidths of the PL and PR features seen in Fig. 2,does not disqualify any of the conclusions. The full widths at halfmaximum are in the range of 30–50 meV, but after considering avery strong change of the energy with the thickness of the InAslayer (see Fig. 3) it corresponds to the well width fluctuations ofabout 1–2 monolayers. The latter suggests a rather good qualityof the interfaces which is in agreement with previous studies[21,22] for full ICL structures or other structures containing thesame material combinations and grown under very similar condi-tions to the ones used here.

These results in Fig. 5 demonstrate that there is no severe det-rimental decrease of the type II transition intensity when going tostructures emitting above 5 lm, compared to these at shorterwavelengths (at approx. 3.5 lm) for which room temperature laseremission has been reported [9,10,22], even operating continueswave in single mode [23], and for which, an InAs layer thicknessof about 2 nm is typically used. The transition oscillator strengthdecreases by only about 20% for the wells with 2 nm and 3 nmthick InAs in the W-like structure at room temperature. Therefore,it should not be the factor limiting the possible ICL operation atlong wavelengths. The latter supports the idea of using the ICL de-vices for applications in mid infrared of the range much broaderthan till now, i.e. further beyond the 3–4 lm, where the main chal-lenge seems to be expanding the wavelengths farther into theinfrared, because below the 3 lm emission wavelength the wavefunctions overlap naturally increases significantly. Of course, forreal devices other limitations need to be considered and structuresoptimized with respect, for instance, to the carrier losses which canbecome important when changing the confinement (e.g. for thin-ner InAs layers the electrons become less confined).

Another issue, which cannot be neglected, is the fact of biasingthe laser with an external voltage and hence the existence of an

Fig. 5. Calculated transition intensity (oscillator strength) in sense of squared wavefunction overlap integrals of the fundamental transition vs the InAs layer thicknessfor two QW designs (common type II with a single InAs layer, and the W-like onewith two InAs layers) and two temperatures – room temperature (RT) and 5 K (LT).

Fig. 6. Calculated electron and hole wave functions for the ground states in therespective wells for two designs: common type II with a single InAs layer, and W-shaped with two InAs layers shown on the potential profile. The dotted-dashedlines represent the energy levels.

Fig. 7. Calculated electron and hole wave functions for the ground states in therespective wells for two thicknesses of the InAs layer in a common type II QW: (a)d1 = 1 nm; (b) d1 = 3 nm.

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electric field applied in the active region. It will of course affectboth the electronic levels and the wave functions (and their over-laps). Therefore, the electric field effect needs to be compensatedaccordingly. In case of W-like designed structures this is accom-plished by using InAs layers of two slightly differing thicknesses– see Fig. 8a showing the scheme of such asymmetric confining po-tential profile (without the electric field). Typically, the electricfield in this kind of devices is of about 75 kV/cm [21]. How it doesaffect the electronic structure is shown in Fig. 8b, where the calcu-lated transition energy and the overlap integral are plotted againstthe thickness of one of the two InAs wells, whereas the other one iskept constant with a width of d1 = 2 nm. One can see, that themaximal transition intensity (overlap integral and oscillatorstrength) is obtained for a value of about 1.7 nm for the secondwell thickness, which corresponds to an expected emission wave-length of 3.4 lm. Therefore, this particular design would be wellsuited for applications related to the sensing of hydrocarbons,and actually based on such room temperature emitting ICLs havebeen demonstrated [9,21–23]. Similar calculations have been per-formed for a larger value of the first QW width d1 = 2.9 nm (in or-der to get electric field compensation and the optimal conditions atlonger wavelengths). The maximum in the overlap integral thenoccurs for a second well width of 2.4 nm, and the correspondingwavelength is approx. 4.75 lm. Such an active region band struc-ture optimization can be performed for the entire range to be pos-sibly covered, i.e. 2–6 lm, or even beyond.

4. Conclusions

The optical properties of type II InAs/GaInSb quantum wellswith respect to their potential for mid-infrared laser emitters ina broad range of wavelengths exceeding significantly 3–4 lm andutilizing the concept of interband cascade laser have been investi-gated. Characteristic features measured by photoreflectance andphotoluminescence have been recognized and analyzed as associ-ated with fundamental type II transitions in investigated GaSb/AlSb/InAs/GaInSb/InAs/AlSb/GaSb W-shaped quantum wells prov-ing the emission wavelength tuning by changing the thickness of

InAs layer. We have for the first time investigated systematicallythe issue of the type II transition oscillator strength, especially withrespect to longer wavelength applications. On one hand, it hasbeen shown that the a change in the electron and hole wave func-tions overlap integral can be tailored by the structure details, e.g.replacing the common type II well with the W-shaped double welldesign can increase the transition intensity by even a factor of 2.Additional increase can be obtained by elevating the structuretemperature due to the weakening of the electron confinement dri-ven by various thermal coefficients of the respective energy gaps.All these appear to compensate for the natural decrease of the tran-sition oscillator strength with increasing emission wavelengthmaking the investigated system usable in further infrared. Eventu-ally, we have shown that negative effect of the electric field can beefficiently reduced by an application of an asymmetric QW andthat this concept can be exploited in any range of the emissionaccessible by this type II system (2–6 lm, and beyond). We haveshown the prospects and evidenced the potential of the brokengap InAs/GaInSb material combination as the active region forthe laser-based sensing of such environmentally important gasesas HCl, CO2, N2O, NH3, and many others.

Acknowledgements

This work has been supported by the European Commissionwithin the FP7 Project ‘‘SensHy’’ No. 223998, by the PBZ-MNiSW-02/I/2007 Project of Polish Ministry of Science and Higher Educa-tion, and by Foundation for Polish Science (FNP) and DeutscheForschungsgemeinschaft (DFG) – COPERNICUS Award. M.M. wouldlike to also acknowledge the Ministry of Science and Higher Educa-tion for financial support from the Iuventus Plus program.

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Fig. 8. Schematic diagram of the confined profile for electrons in asymmetric W-shaped type II QW (panel a). Overlap integral as a function of the second QW widthin case of the first QW width d1 = 2 nm (panel b) and d1 = 2.9 nm (panel c).

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