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GaInAsSb hole-well laser diodes emitting near 2.93 μm
ARTICLE IN PRESS
Journal of Crystal Growth 301–302 (2007) 967–970 www.elsevier.com/locate/jcrysgro
Growth and characterization of GaInSb/GaInAsSb hole-well laser diodes emitting near 2.93 mm L. Ceruttia, G. Boissiera, P. Grecha, A. Peronaa, J. Angelliera, Y. Rouillarda, E. Tournie´a, F. Gentya,, G.C. Denteb, R. Kaspib a
Centre d’Electronique et de Microoptoe´lectronique de Montpellier, UMR CNRS 5507, CC67, Universite´ Montpellier 2, 34095 Montpellier, France b Air Force Research Laboratory, AFRL/DELS, Albuquerque, NM, 87111 USA Available online 9 January 2007
Abstract The growth by molecular-beam epitaxy of novel electrically pumped type-II multi-quantum well (MQW) Sb-based laser diodes in which only the holes are quantum confined was studied. These laser structures were fabricated on (0 0 1) GaSb substrates. In the MQW region, radiative recombinations originate from InGaSb hole wells embedded in InGaAsSb barriers lattice matched to GaSb. Two different laser structures were developed. The first one exhibited a well/barrier periodicity that was too short, which led to a laser emission near 2.65 mm originating from the waveguide rather than from the wells. With an improved well/barrier periodicity, the second structure exhibited laser emission up to 243 K at 2.93 mm in the pulsed regime (200 ns, 5 kHz). In this case, the laser photons were effectively produced by the hole-well active region. A minimum threshold of about 12.8 kA/cm2 at 80 K combined with a T0 around 70 K have been measured from this second structure. r 2006 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.07.St; 81.15.Hi; 42.55.Px Keywords: A3. Molecular beam epitaxy; B1. Antimonides; B2. Semiconducting III–V materials; B3. Mid-infrared devices; B3. Quantum well laser diodes
1. Introduction Semiconductor laser diodes emitting in the mid-infrared (MIR) wavelength range are being extensively developed because of the wide range of applications they allow. A number of applications, such as high-resolution trace gas detection by laser spectroscopy, free-space optical communications, military counter-measure systems or highly precise surgery, require compact and reliable MIR laser sources with low-power consumption and high output emission spectral quality. In particular, the MIR wavelength range contains strong absorption lines of almost all gas species of interest for atmospheric measurements. Laser diodes are particularly well adapted for molecular spectroscopy because their emission bands are narrower than the Doppler widths of absorption lines. For these applications Corresponding author. Tel.: +33 4 67 14 32 81; fax: +33 4 67 54 48 42.
E-mail address: [email protected] (F. Genty). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.057
and this wavelength range, the InAs–GaSb–AlSb system appears to be ideal in designing efficient devices. Indeed, a combination of (In, Ga, Al) and (As, Sb) species allows the fabrication of a number of alloys lattice matched to GaSb or InAs substrates that exhibit band alignments advantageous to emission in the mid-infrared (MIR), and the development of type-I, type-II, type-II broken gap multiquantum well (MQW) and quantum cascade laser (QCL) devices. The well-known AlGaAsSb/InGaAsSb type-I MQW system permits the fabrication of high-performance laser devices operating in continuous wave (CW) above room temperature (RT); however, the emission wavelength is limited to below 3.1 mm [1] due to the decrease in hole confinement and the increase of Auger recombination at longer wavelengths. When considering semiconductor lasers with emission wavelength above 3 mm, it appears that only Sb-based devices have shown promising results. For example, efficient devices were obtained with quantum cascade (InAs/AlSb) structures [2] and MQW structures
ARTICLE IN PRESS L. Cerutti et al. / Journal of Crystal Growth 301–302 (2007) 967–970
based on InAsSb/InAs [3], InAsSb/InAsP [4] or InAsSb/ InAlAsSb [5] systems. However, the leading Sb-based technology is currently the type-II ‘‘W’’ design with InGaSb hole QW surrounded by two InAs electron QWs embedded in AlAsSb barriers. Such a device demonstrated a laser operation up to 317 K in the pulsed regime with a T0 of about 40 K [6]. In this paper, growth and characterization of semiconductor laser sources operating under electrical injection and based on novel type-II InGaSb/InGaAsSb MQW active zones are described. The laser operation of devices including such an optically pumped MQW structure was recently demonstrated by researchers at AFRL, Albuquerque [7]. The novelty of such a configuration is that laser photons originate from the radiative recombinations between electrons from the InGaAsSb barriers and holes from the InGaSb wells. Moreover, the band alignment in such a hole-well laser lends itself well to improve the hole transport through the active layer leading to a better carrier injection. So, this type of active zone appears to provide another elegant solution for laser emission between 2 and 4 mm. But, as to date, only optically pumped devices were reported. 2. Laser heterostructures fabrication The typical structure was grown by molecular-beam epitaxy on (0 0 1) GaSb:Te substrates with a GEN II Varian system. Classical effusion cells were used for gallium, aluminium and indium, whereas valved cracking sources were used for producing As2 and Sb2 dimer fluxes. The heterostructure consists of a 100 nm thick Te-doped layer lattice matched on GaSb with a composition gradually varying from Al0.1Ga0.9As0.01Sb0.99 to Al0.9Ga0.1As0.08Sb0.92, a Te-doped 1.5 mm-thick cladding Al0.9Ga0.1As0.08Sb0.92, a first 420 nm thick waveguide of In0.3Ga0.7As0.26Sb0.74 followed by a Ga0.7In0.3Sb/In0.3Ga0.7As0.26Sb0.74 MQW zone with five InGaSb hole wells that are nominally 2.4 nm thick. A schematic band alignment structure of one hole well is represented by Fig. 1. Modelling using the superlattice pseudo-empirical method (SEPM) has shown that 2.4 nm-thick wells embedded in 50 nm-thick barriers should result in laser emission near 2.82 mm at 80 K. The SEPM model, which takes into account interfaces problems, is particularly well adapted for predictions of type-II lasers operation with thin wells. The upper part of the structure is similar to the lower one but with p-type doping (Be). A final 100 nmthick Be-doped GaSb is epitaxied as the contact layer. The first cladding was grown at 510 1C. The active region (waveguide+MQW) was grown at 420 1C and the second cladding and the top contact layer were grown at 440 1C to prevent intermixing in the active region. Particular attention was paid to the growth of the InGaAsSb quaternary alloy waveguide and barrier layers that are lattice matched to GaSb. According to published calculations [8], the intended alloy composition of In0.3Ga0.7As0.26Sb0.74 is
0.6 0.4 0.2 Energy (eV)
968
0.0 -0.2 -0.4 -0.6 -0.8 0
10
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40
Fig. 1. Schematic band diagram of type-II hole In0.3Ga0.7Sb wells embedded in In0.3Ga0.7AsSb barriers. The energy band gap of In0.3Ga0.7AsSb lattice matched on GaSb is estimated to be 0.47 eV. Modeling by SEPM method envisages an emission near 2.82 mm at 80 K.
indeed expected to be within the large miscibility gap of this alloy system. Therefore, a relatively low temperature of growth and the digital alloy growth technique [9] were employed to help improve the optical quality of these layers. 3. Hole-well laser diodes characterization Two different laser structures with a typical design described above were successively fabricated. In each structure, the In0.3Ga0.7As0.26Sb0.74 alloy was fabricated with the digital alloy technique. The energy band gap of this alloy is estimated to be 0.47 eV. The double-crystal X-ray diffraction analysis obtained from the first grown structure is reported in (Fig. 2(a)). This figure indicates clearly that the well/barrier periodicity was 42 nm, considerably thinner than the intended 52 nm. In this case, laser emission was observed from 15-mm-wide mesa stripes devices with 700 mm cavity lengths (Fig. 3(a)) near 2.65 mm up to 283 K in the pulsed regime (200 ns, 20 kHz) with a threshold current density varying between 11.2 kA/cm2 (at 237 K) and 38.1 kA/cm2 (at 283 K), which correspond to a characteristic temperature T0 of 32 K (inset in the Fig. 3(a)). These spectra were obtained on a Nicolet FTIR system with a cooled InSb detector at liquid nitrogen temperature. Considering the estimated value of the gap energy of the In0.3Ga0.7As0.26Sb0.74 alloy, 0.47 eV, it can be thought that, for this first structure, the laser emission was originating from electron–hole recombinations directly in the waveguide and not from quantum-confined holes in the InGaSb wells. Indeed, in this particular case, electrons that have a high mobility sweep across the cavity without being stopped. They recombine radiatively with holes (which have a low mobility) near the p-type cladding leading to an
ARTICLE IN PRESS L. Cerutti et al. / Journal of Crystal Growth 301–302 (2007) 967–970
GaSb substrate
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Fig. 2. DCXRD analysis of the type-II hole-well In0.3Ga0.7Sb/In0.3Ga0.7AsSb laser heterostructures. In each case, satellite peaks corresponding to the well/barrier periodicity are clearly visible: (a) a periodicity of 42 nm can be deduced for the first laser heterostructure grown, (b) a periodicity of 49 nm can be deduced for the second laser heterostructure grown.
emission at a wavelength corresponding to the gap energy of the waveguide, i.e. 2.65 mm. According to this first result, it appears that the quantum confinement of holes in InGaSb wells was not efficient enough to obtain type-II radiative recombinations. Therefore, a second laser structure, with the same design described previously, was then fabricated. As in the first structure, the quaternary InGaAsSb alloy was realized using the digital alloy technique. But for this new structure, a particular effort was made in order to improve the well/ barrier periodicity control. DCXRD analysis of this second structure is represented in Fig. 2(b). In this case, the pattern showed a well/barrier periodicity of 49 nm, closer to the intended value. This difference indicates that the InGaSb hole wells are also approximately 6% thinner than intended, which will unintentionally modify the hole energy levels in the well such that it will result in a shorter emission wavelength. Devices with similar characteristics than first ones (15-mm-wide mesa stripes devices with 700 mm cavity
2.6
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Fig. 3. Laser spectra of In0.3Ga0.7Sb/In0.3Ga0.7AsSb hole wells diodes: (a) an output peak emission around 2.65 mm is obtained for the first laser heterostructure. Such a wavelength corresponds to radiative recombinations in the quaternary waveguide rather than in hole well. In inset, a characteristic T0 temperature of 32 K was measured between 237 and 283 K, (b) For the second structure, the wavelengths of output peak emission vary from 2.88 mm up to 2.94 mm between 80 and 243 K. In this case, radiative recombinations originate from the type-II hole-well active region. In inset, a characteristic T0 temperature of 70 K was measured between 80 and 243 K.
lengths) were then characterized. As shown in Fig. 3(b), laser emission was observed in the pulsed regime (200 ns, 5 kHz) with peak emission wavelength varying from 2.88 up to 2.94 mm when the temperature is increased from 80 to 243 K. This corresponds to a wavelength chirp of 0.37 nm/K. For this new structure, the wavelength of emission is significantly longer than the wavelength of the first devices, suggesting that, in this case, the recombination well occurs between electrons in the barrier layers and holes that are confined in the wells. Moreover, with a similar technological process, the threshold current densities of laser diodes emitting near 2.93 mm are an order of magnitude higher than those of laser diodes emitting near 2.65 mm (13.2 kA/cm2 vs. 138.4 kA/cm2 at about 243 K). Such a result, if considering the weak overlap of electron
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L. Cerutti et al. / Journal of Crystal Growth 301–302 (2007) 967–970
and hole wavefunctions in the case of a type-II recombination between electrons in the barriers and holes in the wells, can confirm that lasing effect near 2.93 mm was obtained from radiative recombinations in the hole-well zone. A characteristic T0 temperature of 70 K, represented in the inset of Fig. 2, was demonstrated for this second diodes series. Such a characteristic temperature T0 is significantly higher than the T0 of ‘‘W’’ lasers (50 K) emitting at the same wavelength range and operating in the pulsed regime [10]. This high T0 temperature is thought to be related to a very low variation of the energy gap (i.e. the energy levels in the wells) with the temperature of InGaSb/InGaAsSb hole wells. This low variation is supported by the observed low wavelength chirp (0.37 nm/K) described in Fig. 2 (0.37 nm/K). For comparison, a wavelength chirp of 2 nm/K is typically reported for ‘‘W’’ structures for the same range of temperature in the pulsed operation mode [10]. Moreover, when comparing the characteristic temperatures of the two laser structures described in this paper, the T0 measured from the second one is much larger than the one measured from the first one (70 K vs. 32 K). Such a result is consistent with the traditional difference observed between T0 of MQW and double heterostructure laser diodes. So, the improved T0 observed from the second laser structure can confirm again that lasing effect in the second laser structure (emitting near 2.93 mm at 243 K) was really obtained from radiative recombinations in the hole-well active zone. 4. Conclusion In summary, we have developed a novel type-II MQW active zone that can provide another elegant solution for laser emission between 2 and 4 mm. Two different laser structures were MBE-grown and characterized. The first one showed a too short well/barrier periodicity (42 nm) to obtain an efficient hole confinement in the InGaSb wells, which lead to a laser emission at 2.65 mm up to 283 K originating from recombinations in the In0.3Ga0.7As0.26Sb0.74 waveguide. The second structure showed an improved well/barrier periodicity (49 nm). Pulsed laser emission (200 ns, 5 kHz) near 2.94 mm up to 243 K
originating from radiative transitions between holes in the wells and electrons in the barriers was demonstrated with this second structure. If the laser thresholds remain high (12.85 kA/cm2 at 80 K), a T0 characteristic temperature of 70 K and a wavelength chirp of 0.37 nm/K were measured. We have thus demonstrated that InGaSb/ InGaAsSb multi-hole quantum well active layer is a viable solution to fabricating electrically injected MIR laser diodes. A larger wavelength of laser emission is thought to be obtained with thicker wells while keeping the same well/barrier periodicity. Acknowledgements This work was partly supported by the EOARD grant no FA8655-05-1-3019. The authors want to thank the Languedoc Roussillon region. The authors also gratefully acknowledge M. L. Tilton from AFRL/DELS for SPEM calculations. References [1] C. Lin, M. Grau, O. Dier and M-C. Amann, in: Sixth International Conference on Mid-Infrared Optoelectronics Materials and Devices, St. Petersburg, 2004, p. 44. [2] R. Teissier, D. Barate, A. Vicet, C. Alibert, A.N. Baranov, X. Marcadet, C. Renard, M. Garcia, C. Sirtori, D. Revin, J. Cockburn, Appl. Phys. Lett. 85 (2004) 167. [3] A. Wilk, F. Genty, B. Fraisse, G. Boissier, P. Grech, M. El Gazouli, P. Christol, J. Oswald, T. Simecek, E. Hulicius and, A. Joullie´, J. Crystal Growth 223 (2001) 341. [4] B. Lane, Z. Wu, A. Stein, J. Diaz, M. Razeghi, Appl. Phys. Lett. 74 (1999) 3438. [5] H.K. Choi, G.W. Turner, M.J. Manfra, Electron. Lett. 32 (1996) 1296. [6] C.L. Canedy, W.W. Bewley, J.R. Lindle, I. Vurgaftman, C.S. Kim, M. Kim, J.R. Meyer, Appl. Phys. Lett. 86 (2005) 211105. [7] A.P. Ongstad, R. Kaspi, M.L. Tilton, J.R. Chavez and, G.C. Dente, J. Appl. Phys. 98 (2005) 043108. [8] V.S. Sorokin, S.V. Sorokin, A.N. Semenov, B.Ya. Meltser, S.V. Ivanov, J. Crystal Growth 216 (2000) 97. [9] R. Kaspi, G.P. Donati, J. Crystal Growth 251 (2003) 515. [10] W.W. Bewley, I. Vurgaftman, C.S. Kim, M. Kim, C.L. Canedy, J.R. Meyer, J.D. Bruno, F.J. Towner, Appl. Phys. Lett. 85 (23) (2004) 5544.
Journal of Crystal Growth 301–302 (2007) 967–970 www.elsevier.com/locate/jcrysgro
Growth and characterization of GaInSb/GaInAsSb hole-well laser diodes emitting near 2.93 mm L. Ceruttia, G. Boissiera, P. Grecha, A. Peronaa, J. Angelliera, Y. Rouillarda, E. Tournie´a, F. Gentya,, G.C. Denteb, R. Kaspib a
Centre d’Electronique et de Microoptoe´lectronique de Montpellier, UMR CNRS 5507, CC67, Universite´ Montpellier 2, 34095 Montpellier, France b Air Force Research Laboratory, AFRL/DELS, Albuquerque, NM, 87111 USA Available online 9 January 2007
Abstract The growth by molecular-beam epitaxy of novel electrically pumped type-II multi-quantum well (MQW) Sb-based laser diodes in which only the holes are quantum confined was studied. These laser structures were fabricated on (0 0 1) GaSb substrates. In the MQW region, radiative recombinations originate from InGaSb hole wells embedded in InGaAsSb barriers lattice matched to GaSb. Two different laser structures were developed. The first one exhibited a well/barrier periodicity that was too short, which led to a laser emission near 2.65 mm originating from the waveguide rather than from the wells. With an improved well/barrier periodicity, the second structure exhibited laser emission up to 243 K at 2.93 mm in the pulsed regime (200 ns, 5 kHz). In this case, the laser photons were effectively produced by the hole-well active region. A minimum threshold of about 12.8 kA/cm2 at 80 K combined with a T0 around 70 K have been measured from this second structure. r 2006 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.07.St; 81.15.Hi; 42.55.Px Keywords: A3. Molecular beam epitaxy; B1. Antimonides; B2. Semiconducting III–V materials; B3. Mid-infrared devices; B3. Quantum well laser diodes
1. Introduction Semiconductor laser diodes emitting in the mid-infrared (MIR) wavelength range are being extensively developed because of the wide range of applications they allow. A number of applications, such as high-resolution trace gas detection by laser spectroscopy, free-space optical communications, military counter-measure systems or highly precise surgery, require compact and reliable MIR laser sources with low-power consumption and high output emission spectral quality. In particular, the MIR wavelength range contains strong absorption lines of almost all gas species of interest for atmospheric measurements. Laser diodes are particularly well adapted for molecular spectroscopy because their emission bands are narrower than the Doppler widths of absorption lines. For these applications Corresponding author. Tel.: +33 4 67 14 32 81; fax: +33 4 67 54 48 42.
E-mail address: [email protected] (F. Genty). 0022-0248/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2006.11.057
and this wavelength range, the InAs–GaSb–AlSb system appears to be ideal in designing efficient devices. Indeed, a combination of (In, Ga, Al) and (As, Sb) species allows the fabrication of a number of alloys lattice matched to GaSb or InAs substrates that exhibit band alignments advantageous to emission in the mid-infrared (MIR), and the development of type-I, type-II, type-II broken gap multiquantum well (MQW) and quantum cascade laser (QCL) devices. The well-known AlGaAsSb/InGaAsSb type-I MQW system permits the fabrication of high-performance laser devices operating in continuous wave (CW) above room temperature (RT); however, the emission wavelength is limited to below 3.1 mm [1] due to the decrease in hole confinement and the increase of Auger recombination at longer wavelengths. When considering semiconductor lasers with emission wavelength above 3 mm, it appears that only Sb-based devices have shown promising results. For example, efficient devices were obtained with quantum cascade (InAs/AlSb) structures [2] and MQW structures
ARTICLE IN PRESS L. Cerutti et al. / Journal of Crystal Growth 301–302 (2007) 967–970
based on InAsSb/InAs [3], InAsSb/InAsP [4] or InAsSb/ InAlAsSb [5] systems. However, the leading Sb-based technology is currently the type-II ‘‘W’’ design with InGaSb hole QW surrounded by two InAs electron QWs embedded in AlAsSb barriers. Such a device demonstrated a laser operation up to 317 K in the pulsed regime with a T0 of about 40 K [6]. In this paper, growth and characterization of semiconductor laser sources operating under electrical injection and based on novel type-II InGaSb/InGaAsSb MQW active zones are described. The laser operation of devices including such an optically pumped MQW structure was recently demonstrated by researchers at AFRL, Albuquerque [7]. The novelty of such a configuration is that laser photons originate from the radiative recombinations between electrons from the InGaAsSb barriers and holes from the InGaSb wells. Moreover, the band alignment in such a hole-well laser lends itself well to improve the hole transport through the active layer leading to a better carrier injection. So, this type of active zone appears to provide another elegant solution for laser emission between 2 and 4 mm. But, as to date, only optically pumped devices were reported. 2. Laser heterostructures fabrication The typical structure was grown by molecular-beam epitaxy on (0 0 1) GaSb:Te substrates with a GEN II Varian system. Classical effusion cells were used for gallium, aluminium and indium, whereas valved cracking sources were used for producing As2 and Sb2 dimer fluxes. The heterostructure consists of a 100 nm thick Te-doped layer lattice matched on GaSb with a composition gradually varying from Al0.1Ga0.9As0.01Sb0.99 to Al0.9Ga0.1As0.08Sb0.92, a Te-doped 1.5 mm-thick cladding Al0.9Ga0.1As0.08Sb0.92, a first 420 nm thick waveguide of In0.3Ga0.7As0.26Sb0.74 followed by a Ga0.7In0.3Sb/In0.3Ga0.7As0.26Sb0.74 MQW zone with five InGaSb hole wells that are nominally 2.4 nm thick. A schematic band alignment structure of one hole well is represented by Fig. 1. Modelling using the superlattice pseudo-empirical method (SEPM) has shown that 2.4 nm-thick wells embedded in 50 nm-thick barriers should result in laser emission near 2.82 mm at 80 K. The SEPM model, which takes into account interfaces problems, is particularly well adapted for predictions of type-II lasers operation with thin wells. The upper part of the structure is similar to the lower one but with p-type doping (Be). A final 100 nmthick Be-doped GaSb is epitaxied as the contact layer. The first cladding was grown at 510 1C. The active region (waveguide+MQW) was grown at 420 1C and the second cladding and the top contact layer were grown at 440 1C to prevent intermixing in the active region. Particular attention was paid to the growth of the InGaAsSb quaternary alloy waveguide and barrier layers that are lattice matched to GaSb. According to published calculations [8], the intended alloy composition of In0.3Ga0.7As0.26Sb0.74 is
0.6 0.4 0.2 Energy (eV)
968
0.0 -0.2 -0.4 -0.6 -0.8 0
10
20 30 Position (nm)
40
Fig. 1. Schematic band diagram of type-II hole In0.3Ga0.7Sb wells embedded in In0.3Ga0.7AsSb barriers. The energy band gap of In0.3Ga0.7AsSb lattice matched on GaSb is estimated to be 0.47 eV. Modeling by SEPM method envisages an emission near 2.82 mm at 80 K.
indeed expected to be within the large miscibility gap of this alloy system. Therefore, a relatively low temperature of growth and the digital alloy growth technique [9] were employed to help improve the optical quality of these layers. 3. Hole-well laser diodes characterization Two different laser structures with a typical design described above were successively fabricated. In each structure, the In0.3Ga0.7As0.26Sb0.74 alloy was fabricated with the digital alloy technique. The energy band gap of this alloy is estimated to be 0.47 eV. The double-crystal X-ray diffraction analysis obtained from the first grown structure is reported in (Fig. 2(a)). This figure indicates clearly that the well/barrier periodicity was 42 nm, considerably thinner than the intended 52 nm. In this case, laser emission was observed from 15-mm-wide mesa stripes devices with 700 mm cavity lengths (Fig. 3(a)) near 2.65 mm up to 283 K in the pulsed regime (200 ns, 20 kHz) with a threshold current density varying between 11.2 kA/cm2 (at 237 K) and 38.1 kA/cm2 (at 283 K), which correspond to a characteristic temperature T0 of 32 K (inset in the Fig. 3(a)). These spectra were obtained on a Nicolet FTIR system with a cooled InSb detector at liquid nitrogen temperature. Considering the estimated value of the gap energy of the In0.3Ga0.7As0.26Sb0.74 alloy, 0.47 eV, it can be thought that, for this first structure, the laser emission was originating from electron–hole recombinations directly in the waveguide and not from quantum-confined holes in the InGaSb wells. Indeed, in this particular case, electrons that have a high mobility sweep across the cavity without being stopped. They recombine radiatively with holes (which have a low mobility) near the p-type cladding leading to an
ARTICLE IN PRESS L. Cerutti et al. / Journal of Crystal Growth 301–302 (2007) 967–970
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Fig. 2. DCXRD analysis of the type-II hole-well In0.3Ga0.7Sb/In0.3Ga0.7AsSb laser heterostructures. In each case, satellite peaks corresponding to the well/barrier periodicity are clearly visible: (a) a periodicity of 42 nm can be deduced for the first laser heterostructure grown, (b) a periodicity of 49 nm can be deduced for the second laser heterostructure grown.
emission at a wavelength corresponding to the gap energy of the waveguide, i.e. 2.65 mm. According to this first result, it appears that the quantum confinement of holes in InGaSb wells was not efficient enough to obtain type-II radiative recombinations. Therefore, a second laser structure, with the same design described previously, was then fabricated. As in the first structure, the quaternary InGaAsSb alloy was realized using the digital alloy technique. But for this new structure, a particular effort was made in order to improve the well/ barrier periodicity control. DCXRD analysis of this second structure is represented in Fig. 2(b). In this case, the pattern showed a well/barrier periodicity of 49 nm, closer to the intended value. This difference indicates that the InGaSb hole wells are also approximately 6% thinner than intended, which will unintentionally modify the hole energy levels in the well such that it will result in a shorter emission wavelength. Devices with similar characteristics than first ones (15-mm-wide mesa stripes devices with 700 mm cavity
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Fig. 3. Laser spectra of In0.3Ga0.7Sb/In0.3Ga0.7AsSb hole wells diodes: (a) an output peak emission around 2.65 mm is obtained for the first laser heterostructure. Such a wavelength corresponds to radiative recombinations in the quaternary waveguide rather than in hole well. In inset, a characteristic T0 temperature of 32 K was measured between 237 and 283 K, (b) For the second structure, the wavelengths of output peak emission vary from 2.88 mm up to 2.94 mm between 80 and 243 K. In this case, radiative recombinations originate from the type-II hole-well active region. In inset, a characteristic T0 temperature of 70 K was measured between 80 and 243 K.
lengths) were then characterized. As shown in Fig. 3(b), laser emission was observed in the pulsed regime (200 ns, 5 kHz) with peak emission wavelength varying from 2.88 up to 2.94 mm when the temperature is increased from 80 to 243 K. This corresponds to a wavelength chirp of 0.37 nm/K. For this new structure, the wavelength of emission is significantly longer than the wavelength of the first devices, suggesting that, in this case, the recombination well occurs between electrons in the barrier layers and holes that are confined in the wells. Moreover, with a similar technological process, the threshold current densities of laser diodes emitting near 2.93 mm are an order of magnitude higher than those of laser diodes emitting near 2.65 mm (13.2 kA/cm2 vs. 138.4 kA/cm2 at about 243 K). Such a result, if considering the weak overlap of electron
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L. Cerutti et al. / Journal of Crystal Growth 301–302 (2007) 967–970
and hole wavefunctions in the case of a type-II recombination between electrons in the barriers and holes in the wells, can confirm that lasing effect near 2.93 mm was obtained from radiative recombinations in the hole-well zone. A characteristic T0 temperature of 70 K, represented in the inset of Fig. 2, was demonstrated for this second diodes series. Such a characteristic temperature T0 is significantly higher than the T0 of ‘‘W’’ lasers (50 K) emitting at the same wavelength range and operating in the pulsed regime [10]. This high T0 temperature is thought to be related to a very low variation of the energy gap (i.e. the energy levels in the wells) with the temperature of InGaSb/InGaAsSb hole wells. This low variation is supported by the observed low wavelength chirp (0.37 nm/K) described in Fig. 2 (0.37 nm/K). For comparison, a wavelength chirp of 2 nm/K is typically reported for ‘‘W’’ structures for the same range of temperature in the pulsed operation mode [10]. Moreover, when comparing the characteristic temperatures of the two laser structures described in this paper, the T0 measured from the second one is much larger than the one measured from the first one (70 K vs. 32 K). Such a result is consistent with the traditional difference observed between T0 of MQW and double heterostructure laser diodes. So, the improved T0 observed from the second laser structure can confirm again that lasing effect in the second laser structure (emitting near 2.93 mm at 243 K) was really obtained from radiative recombinations in the hole-well active zone. 4. Conclusion In summary, we have developed a novel type-II MQW active zone that can provide another elegant solution for laser emission between 2 and 4 mm. Two different laser structures were MBE-grown and characterized. The first one showed a too short well/barrier periodicity (42 nm) to obtain an efficient hole confinement in the InGaSb wells, which lead to a laser emission at 2.65 mm up to 283 K originating from recombinations in the In0.3Ga0.7As0.26Sb0.74 waveguide. The second structure showed an improved well/barrier periodicity (49 nm). Pulsed laser emission (200 ns, 5 kHz) near 2.94 mm up to 243 K
originating from radiative transitions between holes in the wells and electrons in the barriers was demonstrated with this second structure. If the laser thresholds remain high (12.85 kA/cm2 at 80 K), a T0 characteristic temperature of 70 K and a wavelength chirp of 0.37 nm/K were measured. We have thus demonstrated that InGaSb/ InGaAsSb multi-hole quantum well active layer is a viable solution to fabricating electrically injected MIR laser diodes. A larger wavelength of laser emission is thought to be obtained with thicker wells while keeping the same well/barrier periodicity. Acknowledgements This work was partly supported by the EOARD grant no FA8655-05-1-3019. The authors want to thank the Languedoc Roussillon region. The authors also gratefully acknowledge M. L. Tilton from AFRL/DELS for SPEM calculations. References [1] C. Lin, M. Grau, O. Dier and M-C. Amann, in: Sixth International Conference on Mid-Infrared Optoelectronics Materials and Devices, St. Petersburg, 2004, p. 44. [2] R. Teissier, D. Barate, A. Vicet, C. Alibert, A.N. Baranov, X. Marcadet, C. Renard, M. Garcia, C. Sirtori, D. Revin, J. Cockburn, Appl. Phys. Lett. 85 (2004) 167. [3] A. Wilk, F. Genty, B. Fraisse, G. Boissier, P. Grech, M. El Gazouli, P. Christol, J. Oswald, T. Simecek, E. Hulicius and, A. Joullie´, J. Crystal Growth 223 (2001) 341. [4] B. Lane, Z. Wu, A. Stein, J. Diaz, M. Razeghi, Appl. Phys. Lett. 74 (1999) 3438. [5] H.K. Choi, G.W. Turner, M.J. Manfra, Electron. Lett. 32 (1996) 1296. [6] C.L. Canedy, W.W. Bewley, J.R. Lindle, I. Vurgaftman, C.S. Kim, M. Kim, J.R. Meyer, Appl. Phys. Lett. 86 (2005) 211105. [7] A.P. Ongstad, R. Kaspi, M.L. Tilton, J.R. Chavez and, G.C. Dente, J. Appl. Phys. 98 (2005) 043108. [8] V.S. Sorokin, S.V. Sorokin, A.N. Semenov, B.Ya. Meltser, S.V. Ivanov, J. Crystal Growth 216 (2000) 97. [9] R. Kaspi, G.P. Donati, J. Crystal Growth 251 (2003) 515. [10] W.W. Bewley, I. Vurgaftman, C.S. Kim, M. Kim, C.L. Canedy, J.R. Meyer, J.D. Bruno, F.J. Towner, Appl. Phys. Lett. 85 (23) (2004) 5544.