- Email: [email protected]
MOVPE growth of AlGaInAs–InP highly tensile-strained MQWs for 1.3 μm low-threshold lasers
ARTICLE IN PRESS
Journal of Crystal Growth 272 (2004) 543–548 www.elsevier.com/locate/jcrysgro
MOVPE growth of AlGaInAs–InP highly tensile-strained MQWs for 1.3 mm low-threshold lasers J. Decobert, N. Lagay, C. Cuisin, B. Dagens, B. Thedrez, F. Laruelle Alcatel R&I, OPTO+, Route de Nozay, F91460 Marcoussis, France Available online 12 October 2004
Abstract The low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) of tensile AlGaInAs multi-quantum wells (MQWs) for transverse magnetic (TM) 1.3 mm emitting lasers is presented. Al-containing wells have been mostly studied with compressive strain for transverse electric (TE) lasers. In this study, we report on highly tensile-strained AlGaInAs well layers ( 0.72 to 1.65%) grown with compressive-strained AlGaInAs barrier layers (0.64%). The good agreement of high-resolution X-ray curves and simulated curves indicates that good crystalline quality and abrupt heterointerfaces are obtained. An enhanced separation between light hole and heavy hole transitions is clearly observed by room-temperature photoluminescence as the strain increases. From broad-area laser results, it was observed that the strain had a low impact on the laser internal loss, the quantum efficiency and the transparency current density, which was as low as 0.32 A/cm2 for a 6 QW structure. On the opposite, a doubling of the gain parameter g0 when the strain increases from 0.72 to 1.65% was clearly observed. This result is associated with a 40% threshold density reduction on 300 mm long lasers. These investigations show that highly tensile-strained layers are very promising for the realisation of high-speed lasers. r 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.07.St; 81.15.Kk; 85.60. q Keywords: A1. X-ray diffraction; A2. Tensile strain; A3. Metalorganic vapor phase epitaxy; B1. AlGaInAs/InP; B3. Quantum-well lasers
1. Introduction Quantum-well (QW)-based optoelectronic devices are currently of considerable interest for Corresponding
author. Tel.: +33 1 6963 1143; fax +33 1 6963 1785. E-mail address: [email protected] (J. Decobert).
:
application in optical communication systems. Bulk or lattice-matched QW laser diodes are known to suffer from large nonradiative recombination that degrades device performance, both at 1.3 and 1.55 mm wavelength. By straining the QW, enhancement of laser performances such as reduced threshold current, higher modulation speed, smaller linewidth enhancement factor and
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.032
ARTICLE IN PRESS 544
J. Decobert et al. / Journal of Crystal Growth 272 (2004) 543–548
reduced temperature sensitivity was expected theoretically and demonstrated experimentally [1–3]. Up to now, either unstrained or compressively strained QW lasers have been mainly studied. Nevertheless, theoretical studies predict superior characteristics of tensile-strained QW lasers, such as larger radiative recombination coefficient and higher differential gain as compared to compressive-strained QW lasers [4–7]. In tensile-strained QW lasers, transverse magnetic field (TM) mode gain is significantly enhanced and the peak gain is larger than that of the transverse electric field (TE) mode of compressively strained material. Furthermore, a novel concept has been described in the recent literature, which allows for monolithic integration of an optical isolator with a distributed feedback (DFB) InPbased laser [8]. The optical isolator requests a TM polarisation, which implies the use of tensile strained QWs. The AlGaInAs MQW active material system leads to superior laser properties as compared to the more conventional GaInAsP MQW system [9–11]. This is due to the large conduction-band offset and to the small valence-band offset, which improve the electron confinement and the hole density uniformity in the QWs, respectively. Furthermore, the AlGaInAs quaternary alloy system presents some advantages in terms of flexibility to control the heterostructure. Indeed, if we assume the (AlzGa1 z)uIn1 uAs notation, the layer strain is mostly controlled by 1 u (the indium mole fraction) while the bandgap wavelength can be adjusted easily by controlling z (the Al/Ga ratio). Furthermore, the presence of only one V element, here the arsenic, is a clear advantage to obtain abrupt interfaces, which is not so easy in the GaInAsP system where some As–P exchanges are often observed. In this work, AlGaInAs/AlGaInAs tensilestrained QW laser structures emitting at the wavelength of 1.3 mm were grown by MOVPE and fabricated into a set of broad area lasers. The QW tensile strain was varied from 0.72% to 1.65% partially compensated by the fixed +0.64% barrier compressive strain. The performances of the differently strained QW lasers were compared as a function of strain.
2. Experimental procedure and material results All layers were grown at 650 1C in an AIX200/4 LP-MOVPE reactor designed for three 2-inch wafers [12,13]. Trimethylaluminum (TMA), trimethylgallium (TMG), trimethylindium (TMI), arsine (AsH3), phosphine (PH3), diethylzinc (DEZn) and silane (SiH4) are the source materials. Purified hydrogen was employed as a carrier gas. Five MQW structures (here designed by samples A, B, C, D and E) were grown with increasing well strain, keeping constant the well and barrier thicknesses and the barrier composition. The structures of the proposed devices are shown in Fig. 1. The active region consists of 6 periods of 10-nm-thick, undoped AlxGayIn1 x yAs tensilestrained QWs partially compensated by 20-nmthick Al0.30Ga0.08In0.62As barriers (+0.64% compressively strained). The well indium mole fraction (1 x y) was varied from 42.7% (sample A) to 29.5% (sample E), while the well aluminum content was adjusted from 8.0% to 6.4%, to keep a constant 1.3 mm wavelength bandgap. Therefore, from samples A to E, the QW tensile strain was 0.72, 0.94, 1.16, 1.40 and 1.65%, respectively. Lattice-matched, undoped 40 nm thick GaInAsP and AlGaInAs quaternary layers (1.06 eV) were grown as separate confinement heterostructures (SCH) below and above the active region, respectively. The active layers were sandwiched between p-type and n-type doped InP cladding layers. In addition, the structure was
Fig. 1. Schematic structure of the AlGaInAs MQW tensilestrained lasers. The five samples (named A,B,C,D and E) have different AlxGayIn1 x yAs well composition: the (x, y) values are chosen to vary the tensile strain from 0.72% to 1.65%, keeping the wavelength bandgap at 1.3 mm constant.
ARTICLE IN PRESS J. Decobert et al. / Journal of Crystal Growth 272 (2004) 543–548
topped by a p+ GaInAs contact layer. The choice of SCH layers has been optimised for better carrier injection in the active region from the SCH and the doped InP cladding layers (Fig. 1). The conduction band discontinuity DE c between lattice-matched GaInAsP/InP on the one hand, and AlGaInAs/ InP on the other hand, is assumed to be 0.4 and 0.75 of the bandgap difference DE g ; respectively. Therefore, AlGaInAs and GaInAsP materials allow better barrier height for electrons and holes, respectively. The surface morphology of the samples was mirrorlike, even at the highest well strain. The high resolution X-ray rocking curves around (4 0 0) diffraction of the five MQW structures are shown in Fig. 2. The period of the MQW structure, i.e. the sum of the barrier and well thicknesses, can be deduced from the spacing of satellite peaks. Thicknesses together with strain values of the well and barrier layers can be estimated from the intensity and the diffraction angle of the envelop shape of the satellite peaks. One can see in Fig. 2, the envelope shape shift of the tensile-strained well, from samples A to E, while decreasing the indium content. It should be noted that no degradation of the X-ray profile is observed even at the highly tensile strain of 1.65%, which claims for high crystalline quality of the heterostructure. Fig. 3 shows the high-resolution X-ray rocking curve together with the best-fitting simu-
545
Fig. 3. Experimental (upper trace) and simulated (lower trace) high-resolution X-ray rocking curves around (4 0 0) diffraction of sample C.
Fig. 4. Room-temperature PL spectra of the AlGaInAs/ AlGaInAs tensile-strained MQWs (samples A,B,C,D and E) with strain varying from 0.72 to 1.65%. The energy splitting between light-hole (LH) and heavy-hole (HH) is clearly enhanced by strain. Measured splitting values are reported.
Fig. 2. High-resolution X-ray rocking curve of the MQW heterostructures (samples A,B,C,D and E). The envelope shape of the tensile-strained QWs is shown (dotted line) as a visual guide.
lated curve of sample C. The good agreement of the simulated curve with the measured one indicates high heterostructure quality with abrupt interfaces and perfect periodicity. Fig. 4 shows the results of photoluminescence (PL) measurements on the tensile-strained QWs at room temperature. Two peaks are clearly observed corresponding to n=1 electron-light hole (E1–LH1) and n=1 electron-heavy hole
ARTICLE IN PRESS 546
J. Decobert et al. / Journal of Crystal Growth 272 (2004) 543–548
Fig. 5. The E1–LH1 and E1–HH1 calculated transitions are plotted against tensile strain for different well thicknesses (8–12 nm). Enhancing the well tensile strain, the composition is adjusted to keep constant the E1–LH1 transitions. The change from E1–HH1 to E1–LH1 transition appears around a tensile strain of 0.3%.
(E1–HH1) transitions, respectively. The separation between E1–LH1 and E1–HH1 transitions as determined from the figure varies from 28.5 (sample A) to 73.2 meV (sample E). All the peak wavelength values are extracted from the deconvolution of the PL spectra. The E1–LH1 transition is dominant in AlGaInAs tensile-strained material. The Al content in the QW material has been slightly adjusted to keep the E1–LH1 transition constant at 1.3 mm. Fig. 5 gives the E1–LH1 and E1–HH1 transitions calculated against tensile strain for the well thickness (10 nm) of the samples and for other thicknesses (8,9,11 and 12 nm) for comparison, keeping almost constant the E1–LH1 transition values. The change from HH to LH transition appears around a tensile strain of 0.3%. It should be noted that a tensile strain above 0.77% results in a type II band line-up for heavy holes. This means that heavy holes are located in the barrier. In Fig. 6, the energy splitting is plotted against tensile strain together with a calculated estimation for each structure. Experimental results are well supported by the theoretical estimation. The slight differences between measured and calculated values can be attributed to a nonnegligible deviation of the conduction band offset DE c =DE g
Fig. 6. The energy separation between LH and HH emissions is plotted against tensile strain (dark squares). Theoretical separation is calculated for the different sample compositions extracted from the X-ray diffraction measurements (solid line).
from 0.75 (the only value used here for calculation) for highly strained heterostructures. Their composition is far from those of the lattice-matched ones and the ‘‘true’’ band offset should be estimated for better precision [14].
3. Laser results and discussion Broad-area lasers have been processed from each of the five samples, in order to evaluate and compare the properties of tensile-strained InGaAlAs/InP structures. The broad-area laser stripes are 100 mm wide, and they were cleaved at different lengths. Both the threshold and the efficiency were recorded for each length at different controlled temperatures. The characteristic temperature T0 was highest for the larger tensile-strained lasers, with a value of 80 K for 600 mm long lasers. The T1 value characterizing the efficiency drop was over 200 K. The high T0 characteristic is a specificity of Al-based structure: although phosphorus-based tensile-strained laser emitting at 1.3 mm present a low transparency current density per well of 100 A/ cm2 at room temperature, limitations occur when operation temperature increases with a characteristic temperature lower than 67 K [6]. In Fig. 7, the inverse of the total laser efficiency is given as a function of the device length for
ARTICLE IN PRESS J. Decobert et al. / Journal of Crystal Growth 272 (2004) 543–548
Fig. 7. Inverse broad-area laser efficiency versus device length for tensile strain between 0.72% and 1.65%. The slope indicates an internal efficiency of 0.7 W/A and internal losses of 16 cm 1.
measurements performed at 25 1C. The slope of the curve indicates internal losses of 16 cm 1, while the extrapolation at L=0 cm gives an internal efficiency around 0.7 W/A. The fit is the same for the different wafers, showing the independence of the efficiency with respect to the amount of strain. From additional experiments, a large part of the loss is actually attributed to free carrier absorption in the cladding. A logarithmic gain model was used to fit the threshold dependence with respect to the inverse length. The extrapolation for infinite length provides a room temperature transparency current density which is weakly dependent on strain, and a value below 60 A/cm2 per well for the 1.65% strain. Fig. 8 shows the extracted gain parameter g0 which is strongly enhanced by the tensile strain amount and reaches values near 90 cm 1. The threshold current density for a 300 mm long laser is also plotted, showing a 40% decrease when the strain is increased from 0.72% to 1.65%. The high g0 value range, associated with low threshold current densities around 0.6 kA/cm2, is very promising for the realisation of uncooled highspeed lasers. In particular, directly modulated 10 Gbit/s InGaAlAs uncooled devices with tensile strain should show very competitive performances with respect to their compressive strain counterparts. Our study also indicates that further improvements should be achieved by increasing the strain beyond 1.65%.
547
Fig. 8. Evaluation of broad-area laser versus active layer tensile strain. Doubling of the gain parameter g0 altogether with a 40% threshold density reduction is achieved by increasing the strain from 0.72% to 1.65%.
4. Summary and conclusions Tensile-strained AlGaInAs/AlGaInAs MQWs, grown by LP-MOVPE for TM 1.3 mm emitting lasers, were investigated. The tensile-strained AlGaInAs QWs were designed with strains from 0.72% to 1.65%, keeping constant the electron–light hole transition around 1.3 mm. All samples contain 6 wells. X-ray diffraction and room-temperature PL measurements indicate a high crystalline quality of the heterostructures. The energy splitting enhancement against strain between the LH and HH transitions is clearly observed and are well fitted by simulation. The laser characteristics show that the internal efficiency around 0.7 W/A is weakly dependent on strain. Conversely, the gain parameter g0 is strongly enhanced with strain and reaches near 90 cm 1. These high g0 values lead to a low threshold current density of 100 A/cm2 per well on 300 mm long lasers. All those characteristics are promising for the realisation of highspeed lasers.
References [1] P.J.A. Thijs, L.F. Tiemeijer, J.J. Binsma, T. van Dongen, IEEE J. Quantum Electron. 30 (1994) 477–499. [2] A. Mathur, J.S. Osinski, P. Grodzinski, P. Daniel Dapkus, IEEE Phot. Tech Lett. 5 (7) (1993) 753.
ARTICLE IN PRESS 548
J. Decobert et al. / Journal of Crystal Growth 272 (2004) 543–548
[3] C.E. Zah, R. Bhat, B. Pathak, C. Caneau, F.J. Favire, N.C. Andreadakis, D.M. Hwang, M.A. Koza, C.Y. Chen, T.P. Lee, Electron. Lett. 27 (1991) 1414–1416. [4] E.P. O’Reilly, G. Jones, A. Ghiti, A.R. Adams, Electron. Lett. 27 (1991) 1417. [5] S. Seki, T. Yamanaka, W. Lui, Y. Yoshikuni, K. Yokoyama, IEEE J. Quantum Electron. 30 (1994) 500. [6] N. Yokouchi, N. Yamanaka, N. Iwai, Y. Nakahira, A. Kasukawa, IEEE J. Quantum Electron. 32 (12) (1996) 2148. [7] S.W. Ryu, W.G. Jeong, B.D. Choe, IEEE J. Quantum Electron. 35 (1999) 1207–1212. [8] M. Vanwolleghem, W. Van Parys, D. Van Thourhout, R. Baets, F. Lelarge, O. Gauthier-Lafaye, B. Thedrez, R. Wirix-Speetjens, J. de Boeck, Post-deadline, Th4.4.2, ECOC-IOOC 2003, Rimini, Italy, September 21–25, 2003.
[9] Z. Wang, D.B. Darby, P. Whitney, R. Panock, D. Flanders, Electron. Lett. 13 (17) (1994) 1413. [10] C.E. Zah, R. Bhat, F.J. Favire, M. Koza, T.P. Lee, D. Darby, D.C. Flanders, J.J. Hsieh, Electron. Lett. 28 (25) (1992) 2323. [11] R. Paiella, P.A. Kiely, A. Ougazzaden, J.W. Stayt, A. Sirenko, G. Derkits, K. Glogovsky, C.C. Wamsley, C.W. Lentz, L.E. Eng, L.J.P. Ketelsen, Electron. Lett. 39 (23) (2003). [12] J. Decobert, G. Patriarche, J. Appl. Phys. 92 (10) (2002) 5749–5755. [13] J. Decobert, D. Herrati, V. Colson, D. Leclerc, L. Goldstein, J. Crystal Growth 248 (2003) 390–394. [14] S.-H. Wei, A. Zunger, Appl. Phys. Lett. 72 (16) (1998) 2011–2013.
Journal of Crystal Growth 272 (2004) 543–548 www.elsevier.com/locate/jcrysgro
MOVPE growth of AlGaInAs–InP highly tensile-strained MQWs for 1.3 mm low-threshold lasers J. Decobert, N. Lagay, C. Cuisin, B. Dagens, B. Thedrez, F. Laruelle Alcatel R&I, OPTO+, Route de Nozay, F91460 Marcoussis, France Available online 12 October 2004
Abstract The low-pressure metalorganic vapor-phase epitaxy (LP-MOVPE) of tensile AlGaInAs multi-quantum wells (MQWs) for transverse magnetic (TM) 1.3 mm emitting lasers is presented. Al-containing wells have been mostly studied with compressive strain for transverse electric (TE) lasers. In this study, we report on highly tensile-strained AlGaInAs well layers ( 0.72 to 1.65%) grown with compressive-strained AlGaInAs barrier layers (0.64%). The good agreement of high-resolution X-ray curves and simulated curves indicates that good crystalline quality and abrupt heterointerfaces are obtained. An enhanced separation between light hole and heavy hole transitions is clearly observed by room-temperature photoluminescence as the strain increases. From broad-area laser results, it was observed that the strain had a low impact on the laser internal loss, the quantum efficiency and the transparency current density, which was as low as 0.32 A/cm2 for a 6 QW structure. On the opposite, a doubling of the gain parameter g0 when the strain increases from 0.72 to 1.65% was clearly observed. This result is associated with a 40% threshold density reduction on 300 mm long lasers. These investigations show that highly tensile-strained layers are very promising for the realisation of high-speed lasers. r 2004 Elsevier B.V. All rights reserved. PACS: 81.05.Ea; 81.07.St; 81.15.Kk; 85.60. q Keywords: A1. X-ray diffraction; A2. Tensile strain; A3. Metalorganic vapor phase epitaxy; B1. AlGaInAs/InP; B3. Quantum-well lasers
1. Introduction Quantum-well (QW)-based optoelectronic devices are currently of considerable interest for Corresponding
author. Tel.: +33 1 6963 1143; fax +33 1 6963 1785. E-mail address: [email protected] (J. Decobert).
:
application in optical communication systems. Bulk or lattice-matched QW laser diodes are known to suffer from large nonradiative recombination that degrades device performance, both at 1.3 and 1.55 mm wavelength. By straining the QW, enhancement of laser performances such as reduced threshold current, higher modulation speed, smaller linewidth enhancement factor and
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.032
ARTICLE IN PRESS 544
J. Decobert et al. / Journal of Crystal Growth 272 (2004) 543–548
reduced temperature sensitivity was expected theoretically and demonstrated experimentally [1–3]. Up to now, either unstrained or compressively strained QW lasers have been mainly studied. Nevertheless, theoretical studies predict superior characteristics of tensile-strained QW lasers, such as larger radiative recombination coefficient and higher differential gain as compared to compressive-strained QW lasers [4–7]. In tensile-strained QW lasers, transverse magnetic field (TM) mode gain is significantly enhanced and the peak gain is larger than that of the transverse electric field (TE) mode of compressively strained material. Furthermore, a novel concept has been described in the recent literature, which allows for monolithic integration of an optical isolator with a distributed feedback (DFB) InPbased laser [8]. The optical isolator requests a TM polarisation, which implies the use of tensile strained QWs. The AlGaInAs MQW active material system leads to superior laser properties as compared to the more conventional GaInAsP MQW system [9–11]. This is due to the large conduction-band offset and to the small valence-band offset, which improve the electron confinement and the hole density uniformity in the QWs, respectively. Furthermore, the AlGaInAs quaternary alloy system presents some advantages in terms of flexibility to control the heterostructure. Indeed, if we assume the (AlzGa1 z)uIn1 uAs notation, the layer strain is mostly controlled by 1 u (the indium mole fraction) while the bandgap wavelength can be adjusted easily by controlling z (the Al/Ga ratio). Furthermore, the presence of only one V element, here the arsenic, is a clear advantage to obtain abrupt interfaces, which is not so easy in the GaInAsP system where some As–P exchanges are often observed. In this work, AlGaInAs/AlGaInAs tensilestrained QW laser structures emitting at the wavelength of 1.3 mm were grown by MOVPE and fabricated into a set of broad area lasers. The QW tensile strain was varied from 0.72% to 1.65% partially compensated by the fixed +0.64% barrier compressive strain. The performances of the differently strained QW lasers were compared as a function of strain.
2. Experimental procedure and material results All layers were grown at 650 1C in an AIX200/4 LP-MOVPE reactor designed for three 2-inch wafers [12,13]. Trimethylaluminum (TMA), trimethylgallium (TMG), trimethylindium (TMI), arsine (AsH3), phosphine (PH3), diethylzinc (DEZn) and silane (SiH4) are the source materials. Purified hydrogen was employed as a carrier gas. Five MQW structures (here designed by samples A, B, C, D and E) were grown with increasing well strain, keeping constant the well and barrier thicknesses and the barrier composition. The structures of the proposed devices are shown in Fig. 1. The active region consists of 6 periods of 10-nm-thick, undoped AlxGayIn1 x yAs tensilestrained QWs partially compensated by 20-nmthick Al0.30Ga0.08In0.62As barriers (+0.64% compressively strained). The well indium mole fraction (1 x y) was varied from 42.7% (sample A) to 29.5% (sample E), while the well aluminum content was adjusted from 8.0% to 6.4%, to keep a constant 1.3 mm wavelength bandgap. Therefore, from samples A to E, the QW tensile strain was 0.72, 0.94, 1.16, 1.40 and 1.65%, respectively. Lattice-matched, undoped 40 nm thick GaInAsP and AlGaInAs quaternary layers (1.06 eV) were grown as separate confinement heterostructures (SCH) below and above the active region, respectively. The active layers were sandwiched between p-type and n-type doped InP cladding layers. In addition, the structure was
Fig. 1. Schematic structure of the AlGaInAs MQW tensilestrained lasers. The five samples (named A,B,C,D and E) have different AlxGayIn1 x yAs well composition: the (x, y) values are chosen to vary the tensile strain from 0.72% to 1.65%, keeping the wavelength bandgap at 1.3 mm constant.
ARTICLE IN PRESS J. Decobert et al. / Journal of Crystal Growth 272 (2004) 543–548
topped by a p+ GaInAs contact layer. The choice of SCH layers has been optimised for better carrier injection in the active region from the SCH and the doped InP cladding layers (Fig. 1). The conduction band discontinuity DE c between lattice-matched GaInAsP/InP on the one hand, and AlGaInAs/ InP on the other hand, is assumed to be 0.4 and 0.75 of the bandgap difference DE g ; respectively. Therefore, AlGaInAs and GaInAsP materials allow better barrier height for electrons and holes, respectively. The surface morphology of the samples was mirrorlike, even at the highest well strain. The high resolution X-ray rocking curves around (4 0 0) diffraction of the five MQW structures are shown in Fig. 2. The period of the MQW structure, i.e. the sum of the barrier and well thicknesses, can be deduced from the spacing of satellite peaks. Thicknesses together with strain values of the well and barrier layers can be estimated from the intensity and the diffraction angle of the envelop shape of the satellite peaks. One can see in Fig. 2, the envelope shape shift of the tensile-strained well, from samples A to E, while decreasing the indium content. It should be noted that no degradation of the X-ray profile is observed even at the highly tensile strain of 1.65%, which claims for high crystalline quality of the heterostructure. Fig. 3 shows the high-resolution X-ray rocking curve together with the best-fitting simu-
545
Fig. 3. Experimental (upper trace) and simulated (lower trace) high-resolution X-ray rocking curves around (4 0 0) diffraction of sample C.
Fig. 4. Room-temperature PL spectra of the AlGaInAs/ AlGaInAs tensile-strained MQWs (samples A,B,C,D and E) with strain varying from 0.72 to 1.65%. The energy splitting between light-hole (LH) and heavy-hole (HH) is clearly enhanced by strain. Measured splitting values are reported.
Fig. 2. High-resolution X-ray rocking curve of the MQW heterostructures (samples A,B,C,D and E). The envelope shape of the tensile-strained QWs is shown (dotted line) as a visual guide.
lated curve of sample C. The good agreement of the simulated curve with the measured one indicates high heterostructure quality with abrupt interfaces and perfect periodicity. Fig. 4 shows the results of photoluminescence (PL) measurements on the tensile-strained QWs at room temperature. Two peaks are clearly observed corresponding to n=1 electron-light hole (E1–LH1) and n=1 electron-heavy hole
ARTICLE IN PRESS 546
J. Decobert et al. / Journal of Crystal Growth 272 (2004) 543–548
Fig. 5. The E1–LH1 and E1–HH1 calculated transitions are plotted against tensile strain for different well thicknesses (8–12 nm). Enhancing the well tensile strain, the composition is adjusted to keep constant the E1–LH1 transitions. The change from E1–HH1 to E1–LH1 transition appears around a tensile strain of 0.3%.
(E1–HH1) transitions, respectively. The separation between E1–LH1 and E1–HH1 transitions as determined from the figure varies from 28.5 (sample A) to 73.2 meV (sample E). All the peak wavelength values are extracted from the deconvolution of the PL spectra. The E1–LH1 transition is dominant in AlGaInAs tensile-strained material. The Al content in the QW material has been slightly adjusted to keep the E1–LH1 transition constant at 1.3 mm. Fig. 5 gives the E1–LH1 and E1–HH1 transitions calculated against tensile strain for the well thickness (10 nm) of the samples and for other thicknesses (8,9,11 and 12 nm) for comparison, keeping almost constant the E1–LH1 transition values. The change from HH to LH transition appears around a tensile strain of 0.3%. It should be noted that a tensile strain above 0.77% results in a type II band line-up for heavy holes. This means that heavy holes are located in the barrier. In Fig. 6, the energy splitting is plotted against tensile strain together with a calculated estimation for each structure. Experimental results are well supported by the theoretical estimation. The slight differences between measured and calculated values can be attributed to a nonnegligible deviation of the conduction band offset DE c =DE g
Fig. 6. The energy separation between LH and HH emissions is plotted against tensile strain (dark squares). Theoretical separation is calculated for the different sample compositions extracted from the X-ray diffraction measurements (solid line).
from 0.75 (the only value used here for calculation) for highly strained heterostructures. Their composition is far from those of the lattice-matched ones and the ‘‘true’’ band offset should be estimated for better precision [14].
3. Laser results and discussion Broad-area lasers have been processed from each of the five samples, in order to evaluate and compare the properties of tensile-strained InGaAlAs/InP structures. The broad-area laser stripes are 100 mm wide, and they were cleaved at different lengths. Both the threshold and the efficiency were recorded for each length at different controlled temperatures. The characteristic temperature T0 was highest for the larger tensile-strained lasers, with a value of 80 K for 600 mm long lasers. The T1 value characterizing the efficiency drop was over 200 K. The high T0 characteristic is a specificity of Al-based structure: although phosphorus-based tensile-strained laser emitting at 1.3 mm present a low transparency current density per well of 100 A/ cm2 at room temperature, limitations occur when operation temperature increases with a characteristic temperature lower than 67 K [6]. In Fig. 7, the inverse of the total laser efficiency is given as a function of the device length for
ARTICLE IN PRESS J. Decobert et al. / Journal of Crystal Growth 272 (2004) 543–548
Fig. 7. Inverse broad-area laser efficiency versus device length for tensile strain between 0.72% and 1.65%. The slope indicates an internal efficiency of 0.7 W/A and internal losses of 16 cm 1.
measurements performed at 25 1C. The slope of the curve indicates internal losses of 16 cm 1, while the extrapolation at L=0 cm gives an internal efficiency around 0.7 W/A. The fit is the same for the different wafers, showing the independence of the efficiency with respect to the amount of strain. From additional experiments, a large part of the loss is actually attributed to free carrier absorption in the cladding. A logarithmic gain model was used to fit the threshold dependence with respect to the inverse length. The extrapolation for infinite length provides a room temperature transparency current density which is weakly dependent on strain, and a value below 60 A/cm2 per well for the 1.65% strain. Fig. 8 shows the extracted gain parameter g0 which is strongly enhanced by the tensile strain amount and reaches values near 90 cm 1. The threshold current density for a 300 mm long laser is also plotted, showing a 40% decrease when the strain is increased from 0.72% to 1.65%. The high g0 value range, associated with low threshold current densities around 0.6 kA/cm2, is very promising for the realisation of uncooled highspeed lasers. In particular, directly modulated 10 Gbit/s InGaAlAs uncooled devices with tensile strain should show very competitive performances with respect to their compressive strain counterparts. Our study also indicates that further improvements should be achieved by increasing the strain beyond 1.65%.
547
Fig. 8. Evaluation of broad-area laser versus active layer tensile strain. Doubling of the gain parameter g0 altogether with a 40% threshold density reduction is achieved by increasing the strain from 0.72% to 1.65%.
4. Summary and conclusions Tensile-strained AlGaInAs/AlGaInAs MQWs, grown by LP-MOVPE for TM 1.3 mm emitting lasers, were investigated. The tensile-strained AlGaInAs QWs were designed with strains from 0.72% to 1.65%, keeping constant the electron–light hole transition around 1.3 mm. All samples contain 6 wells. X-ray diffraction and room-temperature PL measurements indicate a high crystalline quality of the heterostructures. The energy splitting enhancement against strain between the LH and HH transitions is clearly observed and are well fitted by simulation. The laser characteristics show that the internal efficiency around 0.7 W/A is weakly dependent on strain. Conversely, the gain parameter g0 is strongly enhanced with strain and reaches near 90 cm 1. These high g0 values lead to a low threshold current density of 100 A/cm2 per well on 300 mm long lasers. All those characteristics are promising for the realisation of highspeed lasers.
References [1] P.J.A. Thijs, L.F. Tiemeijer, J.J. Binsma, T. van Dongen, IEEE J. Quantum Electron. 30 (1994) 477–499. [2] A. Mathur, J.S. Osinski, P. Grodzinski, P. Daniel Dapkus, IEEE Phot. Tech Lett. 5 (7) (1993) 753.
ARTICLE IN PRESS 548
J. Decobert et al. / Journal of Crystal Growth 272 (2004) 543–548
[3] C.E. Zah, R. Bhat, B. Pathak, C. Caneau, F.J. Favire, N.C. Andreadakis, D.M. Hwang, M.A. Koza, C.Y. Chen, T.P. Lee, Electron. Lett. 27 (1991) 1414–1416. [4] E.P. O’Reilly, G. Jones, A. Ghiti, A.R. Adams, Electron. Lett. 27 (1991) 1417. [5] S. Seki, T. Yamanaka, W. Lui, Y. Yoshikuni, K. Yokoyama, IEEE J. Quantum Electron. 30 (1994) 500. [6] N. Yokouchi, N. Yamanaka, N. Iwai, Y. Nakahira, A. Kasukawa, IEEE J. Quantum Electron. 32 (12) (1996) 2148. [7] S.W. Ryu, W.G. Jeong, B.D. Choe, IEEE J. Quantum Electron. 35 (1999) 1207–1212. [8] M. Vanwolleghem, W. Van Parys, D. Van Thourhout, R. Baets, F. Lelarge, O. Gauthier-Lafaye, B. Thedrez, R. Wirix-Speetjens, J. de Boeck, Post-deadline, Th4.4.2, ECOC-IOOC 2003, Rimini, Italy, September 21–25, 2003.
[9] Z. Wang, D.B. Darby, P. Whitney, R. Panock, D. Flanders, Electron. Lett. 13 (17) (1994) 1413. [10] C.E. Zah, R. Bhat, F.J. Favire, M. Koza, T.P. Lee, D. Darby, D.C. Flanders, J.J. Hsieh, Electron. Lett. 28 (25) (1992) 2323. [11] R. Paiella, P.A. Kiely, A. Ougazzaden, J.W. Stayt, A. Sirenko, G. Derkits, K. Glogovsky, C.C. Wamsley, C.W. Lentz, L.E. Eng, L.J.P. Ketelsen, Electron. Lett. 39 (23) (2003). [12] J. Decobert, G. Patriarche, J. Appl. Phys. 92 (10) (2002) 5749–5755. [13] J. Decobert, D. Herrati, V. Colson, D. Leclerc, L. Goldstein, J. Crystal Growth 248 (2003) 390–394. [14] S.-H. Wei, A. Zunger, Appl. Phys. Lett. 72 (16) (1998) 2011–2013.