- Email: [email protected]
AlAsSb structures for mid-infrared lasers
Journal of Crystal Growth 223 (2001) 341–348
MBE growth of InAs/InAsSb/AlAsSb structures for mid-infrared lasers A. Wilka, F. Gentya, B. Fraisseb, G. Boissiera, P. Grecha, M. El Gazoulia, P. Christolc, J. Oswaldd, T. Simecekd, E. Huliciusd, A. Joullie´a,* Centre d ’E´lectronique et de Micro-optoe´lectronique de Montpellier (CEM2), Unite´ Mixte de Recherche CNRS n85507, Universite´ de Montpellier II, Sciences et Techniques du Languedoc, case 067, 34095 Montpellier Cedex 05, France b Laboratoire des Agre´gats Mole´culaires et Mate´riaux Inorganiques (LAMMI), Universite´ de Montpellier II, Sciences et Techniques du Languedoc, case 412, 34095 Montpellier Cedex 05, France c Laboratoire de Physique des Mate´riaux (LPM), faculte´ des Sciences d ’Avignon, 33 rue Pasteur, F. 84000 Avignon, France d Institute of Physics, Academy of Sciences, Cukrovarnicka 10, 162 53 Prague, Czech Republic a
Received 7 July 2000; accepted 20 December 2000 Communicated by K.H. Ploog
Abstract The growth by solid source molecular beam epitaxy (MBE) of type-II InAsSb/InAs multi-quantum well laser diodes on InAs has been studied. Strained InAsSb/InAs quantum wells were sandwiched between two AlAs0.16Sb0.84 2 mmthick cladding layers, lattice-matched to InAs. The precise control of the composition of the thick AlAsSb ternary alloy was obtained using a quasi-stoichiometric growth (QSG) method, which requires a determination of the incorporation rate of each element. This rate was obtained from reflection high-energy electron diffraction (RHEED) intensity oscillations. Alloys composition was entirely controlled by Sb2 flux, suggesting a sticking coefficient close to unity. Mesa-stripe laser diodes processed from the epitaxied structures operated at 3.5 mm in pulsed regime up to 220 K, with a threshold current density of 130 A/cm2 at 90 K and a peak optical power efficiency of 50 mW/A/facet. # 2001 Elsevier Science B.V. All rights reserved. PACS: 42.55.Px; 68.55.Bd; 78.55.Cr; 85.30.z Keywords: A3. Molecular beam epitaxy; B1. Antimonides; B2. Semiconducting III–V materials; B3. Infrared devices; B3. Laser diodes
1. Introduction Semiconductor laser diodes emitting in the 2– 5 mm mid-infrared (MIR) wavelength range are *Corresponding author. Tel.: +33-46752-4368; fax: +3346754-4842. E-mail address: [email protected] (A. Joullie´).
being extensively developed because of the wide range of applications they allow. Different domains such as high-resolution gas spectroscopy, free-space optical communications, military counter-measure systems or highly precise surgery require compact and reliable infrared laser sources with low power consumption. In particular, the MIR wavelength range contains strong absorption
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 6 0 0 - 5
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A. Wilk et al. / Journal of Crystal Growth 223 (2001) 341–348
lines of almost all gas species of interest for atmospheric measurements. Laser diodes are particularly suitable for molecular spectroscopy because their emission bands are narrower than the Doppler widths of absorption lines. Nevertheless, high level of detectivity and selectively cannot be obtained without continuous-wave (cw) single-mode laser operation above 200 K, a performance that has not been reported yet above 3 mm whatever the system used. Although the first mid-infrared laser operation was demonstrated with the lead salt system [1], the realization of reliable MIR lasers is mainly concentrated on III–V semiconductors. Significant progress has recently been made in semiconductor lasers with emission wavelength above 3 mm using Sb-based compounds [2]. Efficient devices were obtained with quantum well (QW) lasers grown on GaSb and InAs substrates using InAsSb/InAs [3–5], InAsSb/InAsP [6] and InAsSb/InAlAsSb [7] systems. One of the major problems is to fabricate high quality cladding layers lattice-matched to InAs. These cladding layers must provide both high electrical and optical confinements in the laser structure in order to ensure low threshold operation [8]. The absorption of the emitted photon must be weak, both through the inter-valence band (aivb ) and the free carrier (afc ) absorption. An additional difficulty is to retain the favorable typeI band alignment between the cladding and active layers allowing an efficient carrier confinement. InP0.69Sb0.31 ternary alloy satisfies these requirements for confinement, but this material is grown inside a wide solid phase miscibility gap that makes difficult the growth of uniform layers [9]. Large elliptic defects due to solid phase separation in the InPSb cladding layers have been observed in MIR laser diodes grown by metal organic vapor phase deposition (MOCVD), limiting their performance [10]. Despite the fact that the control of two group-V elements is not trivial, higher band-gap lower refractive index AlAsSb alloys are preferable [11]. The growth of high quality AlAsSb has been reported in several papers [3,4,7,12,13]. This material was introduced in Bragg mirrors [14] or Al-containing active layers [15]. Moreover, n- and p-type doping of such an alloy can easily be
obtained by MBE for electrically pumped structures [16]. In this paper we report the growth on InAs substrate of thick high quality lattice-matched AlAsSb using a quasi stoichiometric growth (QSG) procedure which employs arsenic and antimony induced oscillations for the determination of group-V deposition rates [17,18]. The QSG procedure is then used to grow AlAs0.16Sb0.84 cladding layers in MIR laser structures with InAsSb/InAs quantum wells in the active region. Efficient lasers diodes have been obtained, which could operate pulsed up to 220 K.
2. Experimental procedure The layers and structures have been grown by MBE in a modified Varian Gen II 200 solid source machine. A valved cracker source was used for arsenic (As2) and an unvalved cracker for antimony (Sb2). The growth temperatures were measured with an optical pyrometer, calibrated on InSb melting point. MBE growths have been performed on n-type (1 0 0) InAs substrates prepared by solvent cleaning (CH3CHOHCH3) and chemical etching (40% HF), followed by Insoldering on molybdenum substrate holder. After outgassing in vacuum for several hours (about 12 h), the surface oxide on the substrate was desorbed at high temperature (5258C) under an As2 flux. In situ reflection high energy electron diffraction (RHEED) was employed to monitor the surface reconstruction. As the oxide was desorbed, the surface showed bulk streaked (1 3) patterns along the [1–10] azimuth instead of the classical (2 4) because of the presence of residual antimony in the growth chamber. Arsenic and antimony fluxes were calibrated using induced RHEED oscillations, a very precise method recently employed for the realization of GaAlAsSb/AlAsSb Bragg mirrors [14,19]. These induced oscillations were obtained in the following way: starting with an overpressure of the group V element, the effusion cell temperature was progressively decreased. When the V/III flux ratio became less than one, the induced RHEED oscillations related to V element were observable
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(inset of Fig. 1), allowing the precise determination of the Sb or As incorporation rates. Calibrations were performed at 4508C on GaSb or GaAs substrates. This low growth temperature led to low deposition rates and allowed the observation of RHEED oscillations over long enough time. The incorporation rates of arsenic and antimony were determined by measuring the growth rates of GaAs and GaSb binary compounds. During the measurements the grown surface deteriorated, so between each run it was necessary to grow a thick layer of good quality material to get good surface quality again. Fig. 1 shows arsenic and antimony incorporation rates as a function of the measured Sb2 and As2 pressures. A straight line is obtained, indicating that the growth is well controlled by the V element. The extrapolated lines intersect the X-axis at a low-pressure value (108 Torr) corresponding to the residual antimony (or arsenic) pressure in the growth chamber. It is very important to measure the As2 and Sb2 pressures for the AlAsSb growth precisely because a slight variation of the pressure can cause an important lattice mismatch.
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3. Growth of AlAs0.16Sb0.84 cladding layers One of the difficulties with the growth of AlAsSb is the large miscibility gap at the optimal growth temperature. This mainly explains why it is so difficult to obtain reproducible lattice matched layers. To find the lattice matching conditions with only one MBE run, three layers of AlAsSb on InAs were grown at a temperature of 5258C. This relatively high substrate temperature is recommended to achieve high crystalline quality while avoiding thermal degradation of the InAs substrate. First, the aperture of the arsenic valve (i.e. the arsenic partial pressure) was modified and the antimony pressure was kept constant. Only one X-ray diffraction peak appears on the highresolution X-ray diffraction (HRXRD) Y22Y scan (Fig. 2a). In contrast, when three other AlAsSb layers on InAs were grown keeping constant the arsenic pressure and varying the antimony one, three peaks corresponding to three different compositions appear on the HRXRD curve (Fig. 2b). This clearly demonstrates that antimony controls the layer composition.
Fig. 1. GaSb (and InAs) growth rates determining the antimony (and arsenic) incorporation rates, versus antimony (arsenic) beam equivalent pressure. In the inset antimony induced RHEED oscillations at the beginning of GaSb growth on GaSb substrate are shown.
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A series of AlAsSb layers was grown on InAs fixing the V/III flux ratio at a value slightly higher than 1 (typically 1.1) in order to approach the stoichiometry of the solid phase. Thick (1–2 mm) AlAs0.16Sb0.84 lattice matched layers were obtained by using deposition rates typically of 1 ML/s for aluminum, 0.84 ML/s for antimony, and slightly higher than necessary (e.g. 0.20 ML/s) for arsenic. Narrow diffraction peaks were obtained from HRXRD rocking curves with a full width at half maximum (FWHM) of 35 arcs for a 1 mm-thick layer, comparable to the best values published [13], and Pendello¨sung interference fringes were sometimes observed. Different AlAsSb layers were also grown using the classical high V/III flux ratio (typically 5–10), but lattice-matching was very difficult to find, and the reproducibility of latticematching conditions was not fulfilled. These results show that the quasi-stoichiometric growth procedure ensures a good control of the AlAsSb composition in an easier manner than for the conventional off-stoichiometry method.
4. InAsSb/InAs MQW Fig. 2. HRXRD scan of a stack of three 1 mm-thick AlAsSb layers, grown at 5258C: (a) with three different arsenic pressures [1 107, 3 107, 5 107 Torr]: only one large diffraction peak is obtained and (b) with three different antimony pressures [8.15 107, 8.4 107, 1.06 106 Torr]: three well resolved peaks are shown. The shoulder in the InAs diffraction peak is characteristic of the InAs buffer contaminated by residual antimony pressure.
This can be explained considering that the sticking coefficient of antimony is nearly 1. So the arsenic atoms fill up the lack of antimony atoms on the surface, which means that the ‘‘induced oscillations’’ method is applicable for antimony-rich layers because of the high amount of adsorbed antimony. It is different for arsenicrich layers because this element is very volatile. Contrary to Sb2 incorporation, the As2 incorporation strongly depends on the substrate temperature. That implies that growth rate calibrations, made at low temperature, can be used for antimony in a wide range of growth temperatures, which is not the case with arsenic.
InAsSb/InAs quantum wells are attractive for use in the active region of MIR lasers because InAsSb possesses the narrowest band gap of all III–V solid solutions and also because the active region of the laser structure can be aluminum free. The MBE growth of InAsSb on InAs substrates has been reported [13,20,21]. Optimal growth conditions were found to occur under group-V stabilized conditions at a growth rate of 0.4–0.6 mm/h with the substrate temperature 4808C [20] or 4308C [21]. We adopted this procedure for the growth of InAsSb/InAs quantum wells, using a substrate temperature of 4208C and a growth rate of 0.5 ML/s. Growth interruptions of 20 s under As2 and (As2+Sb2) fluxes were included to improve the interface quality by smoothing the surface. Fig. 3 shows the HRXRD scan for 15 periods of InAs0.94Sb0.06/InAs MQWs surrounded by 100 nm thick InAs with the corresponding simulated curve. There is a good agreement between the experimental and simulated curves. The presence of numerous thin
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Fig. 3. Experimental (Y22Y scan) and simulated XRD curves for 15 periods of (5 nm) InAs0.94Sb0.06/(20 nm) InAs quantum wells, surrounded by 100 nm-thick InAs.
Fig. 4. Low temperature photoluminescence spectrum of the same MQW structure as in Fig. 3.
satellites peaks demonstrates high crystalline quality and sharp interfaces. As another indication of the good quality of the MQW, strong photoluminescence (PL) intensity was obtained on this structure (Fig. 4) with an emission wavelength of around 3.25 mm at 7 K and an FWHM of only 6 meV.
5. Laser structure Both InAsSb/InAs MQW structure as active layer, and AlAsSb grown with the QSG method as
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cladding layers, were incorporated into a complete laser structure. Two 0.5 mm thick InAs spacer layers were also introduced on both sides of the MQWs in order to increase the optical confinement. Fig. 5 gives a schematic view of the grown structure with the corresponding calculated valence and conduction band edges. As the InAsSb/ InAs system exhibits type-II band alignment [22] the optical transitions occur from the fundamental electron level e1 in the InAsSb well to the fundamental hole level hh1 in the InAs barrier. In order to obtain a sufficient overlap of the electron and hole wave-functions, necessary for lasing operation [23], the width of the InAs barrier was reduced to 20 nm so that InAsSb wells are slightly coupled. Nevertheless, the 20 nm-thick InAs barriers maintain a two dimensional behavior of the carriers, which is essential for high differential gain. An InAs (Te-doped) buffer layer was first grown at 5008C. Then, the lower 2 mm-thick AlAs0.16Sb0.84 (Te) cladding layer was grown at 5258C. The active region (InAs spacers and MQWs) was grown at 4208C. In order to avoid any degradation of the InAsSb/InAs QW interfaces by thermal diffusion the upper 2 mm-thick AlAs0.16Sb0.84 (Be) cladding layer was grown at lower temperature than the first one, 4808C instead of 5258C. As the As2 incorporation rate is less important at lower temperatures the arsenic pressure was slightly raised (compared to the first cladding layer) during the growth of this layer. The contact layer is a 0.5 mm thick GaSb(As) Te-doped layer. The growth rates are 0.9 mm/h for both the InAs buffer layer and the AlAsSb cladding layers, and 0.5 mm/h for the active region. The grown structure was characterized by HRXRD Y22Y scanning (Fig. 6). The presence of several orders of satellite peaks shows that abruptness of QW interfaces was preserved during the growth of the upper layers. Fabry-Pe´rot laser diodes with uncoated facets and a mesa-stripe geometry were fabricated from these QW structures. Special attention had to be paid for handling the samples during processing to avoid oxidation of the AlAsSb cladding layers. The devices exhibited at low temperature (90 K) laser emission near 3.5 mm. This wavelength
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Fig. 5. MBE grown laser structure with the conduction and valence band profiles calculated using the method reported in Ref. [22].
corresponds to the calculated value from the fundamental ‘‘indirect’’ transition e1–hh1. Laser diodes could operate in pulsed regime up to 220 K, which is one of the highest maximum operating temperatures reported for antimony based lasers emitting above 3 mm [2]. Fig. 7 shows the peak optical power versus current for a 1 mm long device measured in pulsed regime at 90 K. The initial output power efficiency is 50 mW/A/facet corresponding to a differential quantum efficiency of 27.5%. The emission spectrum recorded at 150 mA is shown in the inset. The temperature dependence of the threshold current density Jth is shown in Fig. 8. The threshold current density increases from 130 A/cm at 80 K to 15 kA/cm at 220 K. At low temperatures, up to 120 K, it slowly increases, with a characteristic temperature T0 ¼ 110 K. Above 150 K, it rapidly increases, with T0 ¼ 20 K. The high characteristic tempera-
Fig. 6. Experimental (Y22Y scan) and simulated XRD profiles of the InAsSb/InAs MQW laser structure with AlAsSb claddings.
A. Wilk et al. / Journal of Crystal Growth 223 (2001) 341–348
Fig. 7. Light output power versus current for an InAsSb/InAs MQW device having a cavity length of 1 mm, measured at 90 K in pulsed regime. The inset shows the emission spectrum at 150 mA.
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preparation of long wavelength lasers with high optical and electrical confinement, was successfully demonstrated. This was made possible on InAs substrates using a quasi-stoichiometric growth (QSG) method, based on the use of V/III flux ratios close to 1 and on the knowledge of incorporation rates for group V elements. In the (425–5258C) growth temperature range the alloy composition was entirely controlled by the antimony flux, which suggests the sticking coefficient for antimony to be close to 1. AlAs0.16Sb0.84 layers were then incorporated as cladding layers in a diode laser structure containing ten (5 nm) InAs0.94Sb0.06/(20 nm) InAs MQWs and two InAs 0.5 mm thick spacers in the active region. Mesa stripe laser devices fabricated from the MBE wafers showed excellent performance, emitting around 3.5 mm up to 220 K.
Acknowledgements This work was supported by European Commission, BRITE-EURAM III, BRPR CT 97 0466 ADMIRAL.
References
Fig. 8. Temperature dependence of the threshold current density for a 1 mm long InAsSb/InAs MQW laser diode, measured in pulsed regime.
ture at 90 K is actually unusual for a MIR laser, which typically exhibits T0 values equal to 20–40 K [2]. The existence of a non radiative current, arising from deep centers located at the interfaces, which becomes dominant at low temperatures can explain the high T0 value [10].
6. Conclusion The reproducible MBE growth of high quality AlAsSb cladding layers, as a key issue of the
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[19] G. Almuneau, E. Hall, S. Mathis, L.A. Coldren, J. Crystal Growth 208 (2000) 113. [20] M.Y. Yen, B.F. Levine, C.G. Bethea, K.K. Choi, A.Y. Cho, Appl. Phys. Lett. 50 (14) (1987) 927. [21] G.W. Turner, H.K. Choi, H.Q. Le, J. Vac. Sci. Technol. B 13 (2) (1995) 699. [22] P. Christol, P. Bigenwald, A. Wilk, A. Joullie´, O. Gilard, H. Carrcˇre, F. Lozes-Dupuy, A. Behres, A. Stein, J. Kluth, K. Heime, E.M. Skouri, IEE Proc. Optoelectron. 147 (3) (2000) 181. [23] P. Christol, P. Bigenwald, A. Joullie´, Y. Cuminal, A.N. Baranov, N. Bertru, Y. Rouillard, IEE Proc. Optoelectron. 146 (1) (1999) 3.
MBE growth of InAs/InAsSb/AlAsSb structures for mid-infrared lasers A. Wilka, F. Gentya, B. Fraisseb, G. Boissiera, P. Grecha, M. El Gazoulia, P. Christolc, J. Oswaldd, T. Simecekd, E. Huliciusd, A. Joullie´a,* Centre d ’E´lectronique et de Micro-optoe´lectronique de Montpellier (CEM2), Unite´ Mixte de Recherche CNRS n85507, Universite´ de Montpellier II, Sciences et Techniques du Languedoc, case 067, 34095 Montpellier Cedex 05, France b Laboratoire des Agre´gats Mole´culaires et Mate´riaux Inorganiques (LAMMI), Universite´ de Montpellier II, Sciences et Techniques du Languedoc, case 412, 34095 Montpellier Cedex 05, France c Laboratoire de Physique des Mate´riaux (LPM), faculte´ des Sciences d ’Avignon, 33 rue Pasteur, F. 84000 Avignon, France d Institute of Physics, Academy of Sciences, Cukrovarnicka 10, 162 53 Prague, Czech Republic a
Received 7 July 2000; accepted 20 December 2000 Communicated by K.H. Ploog
Abstract The growth by solid source molecular beam epitaxy (MBE) of type-II InAsSb/InAs multi-quantum well laser diodes on InAs has been studied. Strained InAsSb/InAs quantum wells were sandwiched between two AlAs0.16Sb0.84 2 mmthick cladding layers, lattice-matched to InAs. The precise control of the composition of the thick AlAsSb ternary alloy was obtained using a quasi-stoichiometric growth (QSG) method, which requires a determination of the incorporation rate of each element. This rate was obtained from reflection high-energy electron diffraction (RHEED) intensity oscillations. Alloys composition was entirely controlled by Sb2 flux, suggesting a sticking coefficient close to unity. Mesa-stripe laser diodes processed from the epitaxied structures operated at 3.5 mm in pulsed regime up to 220 K, with a threshold current density of 130 A/cm2 at 90 K and a peak optical power efficiency of 50 mW/A/facet. # 2001 Elsevier Science B.V. All rights reserved. PACS: 42.55.Px; 68.55.Bd; 78.55.Cr; 85.30.z Keywords: A3. Molecular beam epitaxy; B1. Antimonides; B2. Semiconducting III–V materials; B3. Infrared devices; B3. Laser diodes
1. Introduction Semiconductor laser diodes emitting in the 2– 5 mm mid-infrared (MIR) wavelength range are *Corresponding author. Tel.: +33-46752-4368; fax: +3346754-4842. E-mail address: [email protected] (A. Joullie´).
being extensively developed because of the wide range of applications they allow. Different domains such as high-resolution gas spectroscopy, free-space optical communications, military counter-measure systems or highly precise surgery require compact and reliable infrared laser sources with low power consumption. In particular, the MIR wavelength range contains strong absorption
0022-0248/01/$ - see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 1 ) 0 0 6 0 0 - 5
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lines of almost all gas species of interest for atmospheric measurements. Laser diodes are particularly suitable for molecular spectroscopy because their emission bands are narrower than the Doppler widths of absorption lines. Nevertheless, high level of detectivity and selectively cannot be obtained without continuous-wave (cw) single-mode laser operation above 200 K, a performance that has not been reported yet above 3 mm whatever the system used. Although the first mid-infrared laser operation was demonstrated with the lead salt system [1], the realization of reliable MIR lasers is mainly concentrated on III–V semiconductors. Significant progress has recently been made in semiconductor lasers with emission wavelength above 3 mm using Sb-based compounds [2]. Efficient devices were obtained with quantum well (QW) lasers grown on GaSb and InAs substrates using InAsSb/InAs [3–5], InAsSb/InAsP [6] and InAsSb/InAlAsSb [7] systems. One of the major problems is to fabricate high quality cladding layers lattice-matched to InAs. These cladding layers must provide both high electrical and optical confinements in the laser structure in order to ensure low threshold operation [8]. The absorption of the emitted photon must be weak, both through the inter-valence band (aivb ) and the free carrier (afc ) absorption. An additional difficulty is to retain the favorable typeI band alignment between the cladding and active layers allowing an efficient carrier confinement. InP0.69Sb0.31 ternary alloy satisfies these requirements for confinement, but this material is grown inside a wide solid phase miscibility gap that makes difficult the growth of uniform layers [9]. Large elliptic defects due to solid phase separation in the InPSb cladding layers have been observed in MIR laser diodes grown by metal organic vapor phase deposition (MOCVD), limiting their performance [10]. Despite the fact that the control of two group-V elements is not trivial, higher band-gap lower refractive index AlAsSb alloys are preferable [11]. The growth of high quality AlAsSb has been reported in several papers [3,4,7,12,13]. This material was introduced in Bragg mirrors [14] or Al-containing active layers [15]. Moreover, n- and p-type doping of such an alloy can easily be
obtained by MBE for electrically pumped structures [16]. In this paper we report the growth on InAs substrate of thick high quality lattice-matched AlAsSb using a quasi stoichiometric growth (QSG) procedure which employs arsenic and antimony induced oscillations for the determination of group-V deposition rates [17,18]. The QSG procedure is then used to grow AlAs0.16Sb0.84 cladding layers in MIR laser structures with InAsSb/InAs quantum wells in the active region. Efficient lasers diodes have been obtained, which could operate pulsed up to 220 K.
2. Experimental procedure The layers and structures have been grown by MBE in a modified Varian Gen II 200 solid source machine. A valved cracker source was used for arsenic (As2) and an unvalved cracker for antimony (Sb2). The growth temperatures were measured with an optical pyrometer, calibrated on InSb melting point. MBE growths have been performed on n-type (1 0 0) InAs substrates prepared by solvent cleaning (CH3CHOHCH3) and chemical etching (40% HF), followed by Insoldering on molybdenum substrate holder. After outgassing in vacuum for several hours (about 12 h), the surface oxide on the substrate was desorbed at high temperature (5258C) under an As2 flux. In situ reflection high energy electron diffraction (RHEED) was employed to monitor the surface reconstruction. As the oxide was desorbed, the surface showed bulk streaked (1 3) patterns along the [1–10] azimuth instead of the classical (2 4) because of the presence of residual antimony in the growth chamber. Arsenic and antimony fluxes were calibrated using induced RHEED oscillations, a very precise method recently employed for the realization of GaAlAsSb/AlAsSb Bragg mirrors [14,19]. These induced oscillations were obtained in the following way: starting with an overpressure of the group V element, the effusion cell temperature was progressively decreased. When the V/III flux ratio became less than one, the induced RHEED oscillations related to V element were observable
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(inset of Fig. 1), allowing the precise determination of the Sb or As incorporation rates. Calibrations were performed at 4508C on GaSb or GaAs substrates. This low growth temperature led to low deposition rates and allowed the observation of RHEED oscillations over long enough time. The incorporation rates of arsenic and antimony were determined by measuring the growth rates of GaAs and GaSb binary compounds. During the measurements the grown surface deteriorated, so between each run it was necessary to grow a thick layer of good quality material to get good surface quality again. Fig. 1 shows arsenic and antimony incorporation rates as a function of the measured Sb2 and As2 pressures. A straight line is obtained, indicating that the growth is well controlled by the V element. The extrapolated lines intersect the X-axis at a low-pressure value (108 Torr) corresponding to the residual antimony (or arsenic) pressure in the growth chamber. It is very important to measure the As2 and Sb2 pressures for the AlAsSb growth precisely because a slight variation of the pressure can cause an important lattice mismatch.
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3. Growth of AlAs0.16Sb0.84 cladding layers One of the difficulties with the growth of AlAsSb is the large miscibility gap at the optimal growth temperature. This mainly explains why it is so difficult to obtain reproducible lattice matched layers. To find the lattice matching conditions with only one MBE run, three layers of AlAsSb on InAs were grown at a temperature of 5258C. This relatively high substrate temperature is recommended to achieve high crystalline quality while avoiding thermal degradation of the InAs substrate. First, the aperture of the arsenic valve (i.e. the arsenic partial pressure) was modified and the antimony pressure was kept constant. Only one X-ray diffraction peak appears on the highresolution X-ray diffraction (HRXRD) Y22Y scan (Fig. 2a). In contrast, when three other AlAsSb layers on InAs were grown keeping constant the arsenic pressure and varying the antimony one, three peaks corresponding to three different compositions appear on the HRXRD curve (Fig. 2b). This clearly demonstrates that antimony controls the layer composition.
Fig. 1. GaSb (and InAs) growth rates determining the antimony (and arsenic) incorporation rates, versus antimony (arsenic) beam equivalent pressure. In the inset antimony induced RHEED oscillations at the beginning of GaSb growth on GaSb substrate are shown.
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A series of AlAsSb layers was grown on InAs fixing the V/III flux ratio at a value slightly higher than 1 (typically 1.1) in order to approach the stoichiometry of the solid phase. Thick (1–2 mm) AlAs0.16Sb0.84 lattice matched layers were obtained by using deposition rates typically of 1 ML/s for aluminum, 0.84 ML/s for antimony, and slightly higher than necessary (e.g. 0.20 ML/s) for arsenic. Narrow diffraction peaks were obtained from HRXRD rocking curves with a full width at half maximum (FWHM) of 35 arcs for a 1 mm-thick layer, comparable to the best values published [13], and Pendello¨sung interference fringes were sometimes observed. Different AlAsSb layers were also grown using the classical high V/III flux ratio (typically 5–10), but lattice-matching was very difficult to find, and the reproducibility of latticematching conditions was not fulfilled. These results show that the quasi-stoichiometric growth procedure ensures a good control of the AlAsSb composition in an easier manner than for the conventional off-stoichiometry method.
4. InAsSb/InAs MQW Fig. 2. HRXRD scan of a stack of three 1 mm-thick AlAsSb layers, grown at 5258C: (a) with three different arsenic pressures [1 107, 3 107, 5 107 Torr]: only one large diffraction peak is obtained and (b) with three different antimony pressures [8.15 107, 8.4 107, 1.06 106 Torr]: three well resolved peaks are shown. The shoulder in the InAs diffraction peak is characteristic of the InAs buffer contaminated by residual antimony pressure.
This can be explained considering that the sticking coefficient of antimony is nearly 1. So the arsenic atoms fill up the lack of antimony atoms on the surface, which means that the ‘‘induced oscillations’’ method is applicable for antimony-rich layers because of the high amount of adsorbed antimony. It is different for arsenicrich layers because this element is very volatile. Contrary to Sb2 incorporation, the As2 incorporation strongly depends on the substrate temperature. That implies that growth rate calibrations, made at low temperature, can be used for antimony in a wide range of growth temperatures, which is not the case with arsenic.
InAsSb/InAs quantum wells are attractive for use in the active region of MIR lasers because InAsSb possesses the narrowest band gap of all III–V solid solutions and also because the active region of the laser structure can be aluminum free. The MBE growth of InAsSb on InAs substrates has been reported [13,20,21]. Optimal growth conditions were found to occur under group-V stabilized conditions at a growth rate of 0.4–0.6 mm/h with the substrate temperature 4808C [20] or 4308C [21]. We adopted this procedure for the growth of InAsSb/InAs quantum wells, using a substrate temperature of 4208C and a growth rate of 0.5 ML/s. Growth interruptions of 20 s under As2 and (As2+Sb2) fluxes were included to improve the interface quality by smoothing the surface. Fig. 3 shows the HRXRD scan for 15 periods of InAs0.94Sb0.06/InAs MQWs surrounded by 100 nm thick InAs with the corresponding simulated curve. There is a good agreement between the experimental and simulated curves. The presence of numerous thin
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Fig. 3. Experimental (Y22Y scan) and simulated XRD curves for 15 periods of (5 nm) InAs0.94Sb0.06/(20 nm) InAs quantum wells, surrounded by 100 nm-thick InAs.
Fig. 4. Low temperature photoluminescence spectrum of the same MQW structure as in Fig. 3.
satellites peaks demonstrates high crystalline quality and sharp interfaces. As another indication of the good quality of the MQW, strong photoluminescence (PL) intensity was obtained on this structure (Fig. 4) with an emission wavelength of around 3.25 mm at 7 K and an FWHM of only 6 meV.
5. Laser structure Both InAsSb/InAs MQW structure as active layer, and AlAsSb grown with the QSG method as
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cladding layers, were incorporated into a complete laser structure. Two 0.5 mm thick InAs spacer layers were also introduced on both sides of the MQWs in order to increase the optical confinement. Fig. 5 gives a schematic view of the grown structure with the corresponding calculated valence and conduction band edges. As the InAsSb/ InAs system exhibits type-II band alignment [22] the optical transitions occur from the fundamental electron level e1 in the InAsSb well to the fundamental hole level hh1 in the InAs barrier. In order to obtain a sufficient overlap of the electron and hole wave-functions, necessary for lasing operation [23], the width of the InAs barrier was reduced to 20 nm so that InAsSb wells are slightly coupled. Nevertheless, the 20 nm-thick InAs barriers maintain a two dimensional behavior of the carriers, which is essential for high differential gain. An InAs (Te-doped) buffer layer was first grown at 5008C. Then, the lower 2 mm-thick AlAs0.16Sb0.84 (Te) cladding layer was grown at 5258C. The active region (InAs spacers and MQWs) was grown at 4208C. In order to avoid any degradation of the InAsSb/InAs QW interfaces by thermal diffusion the upper 2 mm-thick AlAs0.16Sb0.84 (Be) cladding layer was grown at lower temperature than the first one, 4808C instead of 5258C. As the As2 incorporation rate is less important at lower temperatures the arsenic pressure was slightly raised (compared to the first cladding layer) during the growth of this layer. The contact layer is a 0.5 mm thick GaSb(As) Te-doped layer. The growth rates are 0.9 mm/h for both the InAs buffer layer and the AlAsSb cladding layers, and 0.5 mm/h for the active region. The grown structure was characterized by HRXRD Y22Y scanning (Fig. 6). The presence of several orders of satellite peaks shows that abruptness of QW interfaces was preserved during the growth of the upper layers. Fabry-Pe´rot laser diodes with uncoated facets and a mesa-stripe geometry were fabricated from these QW structures. Special attention had to be paid for handling the samples during processing to avoid oxidation of the AlAsSb cladding layers. The devices exhibited at low temperature (90 K) laser emission near 3.5 mm. This wavelength
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Fig. 5. MBE grown laser structure with the conduction and valence band profiles calculated using the method reported in Ref. [22].
corresponds to the calculated value from the fundamental ‘‘indirect’’ transition e1–hh1. Laser diodes could operate in pulsed regime up to 220 K, which is one of the highest maximum operating temperatures reported for antimony based lasers emitting above 3 mm [2]. Fig. 7 shows the peak optical power versus current for a 1 mm long device measured in pulsed regime at 90 K. The initial output power efficiency is 50 mW/A/facet corresponding to a differential quantum efficiency of 27.5%. The emission spectrum recorded at 150 mA is shown in the inset. The temperature dependence of the threshold current density Jth is shown in Fig. 8. The threshold current density increases from 130 A/cm at 80 K to 15 kA/cm at 220 K. At low temperatures, up to 120 K, it slowly increases, with a characteristic temperature T0 ¼ 110 K. Above 150 K, it rapidly increases, with T0 ¼ 20 K. The high characteristic tempera-
Fig. 6. Experimental (Y22Y scan) and simulated XRD profiles of the InAsSb/InAs MQW laser structure with AlAsSb claddings.
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Fig. 7. Light output power versus current for an InAsSb/InAs MQW device having a cavity length of 1 mm, measured at 90 K in pulsed regime. The inset shows the emission spectrum at 150 mA.
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preparation of long wavelength lasers with high optical and electrical confinement, was successfully demonstrated. This was made possible on InAs substrates using a quasi-stoichiometric growth (QSG) method, based on the use of V/III flux ratios close to 1 and on the knowledge of incorporation rates for group V elements. In the (425–5258C) growth temperature range the alloy composition was entirely controlled by the antimony flux, which suggests the sticking coefficient for antimony to be close to 1. AlAs0.16Sb0.84 layers were then incorporated as cladding layers in a diode laser structure containing ten (5 nm) InAs0.94Sb0.06/(20 nm) InAs MQWs and two InAs 0.5 mm thick spacers in the active region. Mesa stripe laser devices fabricated from the MBE wafers showed excellent performance, emitting around 3.5 mm up to 220 K.
Acknowledgements This work was supported by European Commission, BRITE-EURAM III, BRPR CT 97 0466 ADMIRAL.
References
Fig. 8. Temperature dependence of the threshold current density for a 1 mm long InAsSb/InAs MQW laser diode, measured in pulsed regime.
ture at 90 K is actually unusual for a MIR laser, which typically exhibits T0 values equal to 20–40 K [2]. The existence of a non radiative current, arising from deep centers located at the interfaces, which becomes dominant at low temperatures can explain the high T0 value [10].
6. Conclusion The reproducible MBE growth of high quality AlAsSb cladding layers, as a key issue of the
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