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MOVPE-grown quantum cascade lasers operating at ∼9 μm wavelength
ARTICLE IN PRESS
Journal of Crystal Growth 272 (2004) 682–685 www.elsevier.com/locate/jcrysgro
MOVPE-grown quantum cascade lasers operating at 9 mm wavelength A.B. Krysaa,, J.S. Robertsa, R.P. Greenb, L.R. Wilsonb, H. Pagec, M. Garciac, J.W. Cockburnb a
EPSRC National Centre for III–V Technologies, University of Sheffield, Sheffield S1 3JD, UK b Department of Physics and Astronomy, University of Sheffield, Sheffield S1 3JD, UK c Thales Research and Technology, Domaine de Corbeville, 91404 Orsay Cedex, France
Abstract We report entirely MOVPE-grown QCL lasers operating around 9 mm. High-resolution X-ray diffraction analysis and TEM data confirm precise thickness and composition control for the individual layers. Consequently, these MOVPE-grown QCLs demonstrate a pulsed mode performance comparable to similarly designed MBE-grown devices. QCLs having a conventional three-well active region design exhibit a threshold current density of 1.5 kA cm 2 at 12 K and continue to lase up to room temperature. Furthermore, QCLs designed with a four-QW double-phonon active region show a threshold current density as low as 880 A cm 2 at 12 K and 3.75 kA cm 2 at 300 K with peak powers in excess of 1 W. r 2004 Elsevier B.V. All rights reserved. PACS: 42.55Px; 42.60By; 73.21Ac; 81.15Gh Keywords: A3. Metalorganic vapor phase epitaxy; B3. Quantum cascade laser
1. Introduction Quantum cascade lasers are very demanding structures in terms of their precise thickness requirements, interface abruptness and low oxygen contamination, when Al-containing layers are involved. The availability of low oxygen-containCorresponding author. Fax: +44-114-272-6391.
E-mail address: a.krysa@sheffield.ac.uk (A.B. Krysa).
ing TMA and TMI precursors, (e.g. Epichem) have particularly been important for the development of all MOVPE-grown QCLs. Since their first experimental demonstration in 1994 [1], MBE, including gas source MBE [2], has been considered to be the only viable growth technique for this class of device. The role of MOVPE was essentially limited to the re-growth of high thermal conductivity InP upper cladding layers for some highperformance InP-based QCLs [3]. Consequently,
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.066
ARTICLE IN PRESS A.B. Krysa et al. / Journal of Crystal Growth 272 (2004) 682–685
applying MOVPE growth to the whole structures essentially simplifies the manufacture of InP clad QCLs. In addition, there are other potential advantages of MOVPE over MBE, such as the higher available growth rates and scaling the reactors from single to multi-wafer deposition; essential for the mass production of QCL devices. Recently, the first all-MOVPE QCL based on the GaAs/AlGaAs material system has been reported [4]. In this paper, we present results on InGaAs/ AlInAs/InP QCLs grown by low-pressure MOVPE.
2. Growth details The QCL structures were grown on (1 0 0), misoriented by 0.251 to [1 1 0], InP:S substrates in a horizontal reactor at a pressure of 150 Torr and temperature of 690 1C. TMAl, TMGa and TMIn were used as precursors of the group III elements, and arsine and phosphine as precursors of the group V elements. The system pipe-work, transporting the alkyls to the reactor, as well as switching of the reagents between run/vent lines, were considered to be a critical features of the growth system. Our MOVPE system featured heated stainless pipes to reduce the adsorption of the alkyls. Furthermore, the zero dead-space and cross-purged design of the individual run/vent valves is presently the subject of a patent application.
683
predicted laser emission wavelength was 8.8 mm at room temperature. TEM characterisation reveals a good resolution of the layers and a superlattice (SL) period of 485 A˚, which compares well with the intended period. X-ray diffraction measurements using the (0 0 4) reflection showed a low lattice mismatch for the AlInAs/InGaAs SL of Da/a10 4. The SL period obtained was also in agreement with that derived from a bright-field TEM image of the QCL gain region. This structural characterisation indicates a high level of accuracy, both for the layer thicknesses and the compositions within the active region. A portion of the TEM image is shown in Fig. 1(a), demonstrating the high degree of contrast available for AlInAs/GaInAs material system. Following growth, the sample was processed into ridge wave-guide lasers using conventional techniques. These were then cleaved into 1.3 mm long Fabry–Perot cavities and mounted epilayer side up onto copper submounts for characterisation. Laser emission was observed up to room temperature, with threshold current densities of Jth (12 K)1.5 kA cm 2 and Jth (290 K)9 kA/cm 2, which compares well with the MBE-grown QCL reported in Ref. [6]. Emission spectra measured at both low and room temperature show a good agreement between the predicted and measured laser wavelengths. Fig. 1(b) shows the emission
Energy (meV) 141
140
139
138
Intensity (a.u.)
3. Experimental results and discussion 3.1. Three-well active region design Samples with two different designs have been grown and characterised. The first design is known as the three-well active region QCL [5]. The exact layer structure has been published in reference [5] and consisted of 25 repeats of AlInAs/InGaAs layers with the following nominal thickness: 40/25/ 15/74/11/60/24/39/11/34/11/34/12/37/17/41 A˚, where AlInAs is in bold and underlined layers were Si-doped to a level of 2.5 1017 cm 3. The
8.8
(a)
(b)
8.85
8.9
8.95
9
Wavelength (µm)
Fig. 1. (a) Part of TEM image of the three-well active region sample. (b) Room temperature emission spectrum.
ARTICLE IN PRESS A.B. Krysa et al. / Journal of Crystal Growth 272 (2004) 682–685
684
spectrum measured at room temperature for this sample. The light/current (L/I) data for this QCL is shown in Fig. 2 for various temperatures up to 280 K, including 77 K. 3.2. Double-phonon active region design The previous QCL sample was constructed with a three-well active region; however, improved performance can be obtained using a four-well, or ‘double phonon resonance’ design [7] to optimise the depopulation of the lower laser level. The use of such an active region places even more stringent requirements on the crystal growth, by reducing the thickness of the barriers within the active region to 7–9 A˚, making the device performance even less tolerant to small errors in the thickness of the barriers. Such errors can arise from either the timing uncertainties during growth or interfacial grading effects [4]. A structure designed for 9.1 mm emission (RT) was grown with a design similar to that reported in reference [7] and consisted of 35 repeats of the following sequence of AlInAs/InGaAs layers: 40/ 19/7/58/9/57/9/50/22/32/15/31/19/30/23/29/25/ 29 A˚, where AlInAs is in bold and underlined layers were Si-doped to a level of 2 1017 cm 3. High resolution X-ray diffraction rocking curves were collected about the (0 0 4) reflection using a Bede D1 diffractometer. In order to eliminate the
influence of the intentional wafer misorientation (0.251 towards [1 1 0]) the wafer was rotated by 1801 between the measurements, with values of superlattice period and mismatch averaged between these two measurements. One of these rocking curves is shown in Fig. 3, together with a simulated curve generated using dynamical diffraction theory, using commercially available software. A superlattice period of 625 A˚ was obtained, slightly longer than the intended value of 598 A˚, with a mismatch Da/a less than 6 10 4 between the InGaAs/AlInAs SL and InP substrate. We expect a thickness variation for the SL of 4% across the wafer. This value was based on X-ray diffraction in combination with a wafermapping technique for a similar QCL sample, designed for a slightly longer wavelength of 10.3 mm (to be published elsewhere). Strong laser emission up to and above room temperature was observed, after processing of the wafer into the Fabry–Perot laser design described above. The low temperature threshold current density was reduced to Jth880 A cm 2, increasing to 3.75 kA cm 2 for room temperature operation. This value compares well with that reported
400
16
70K Voltage (V)
10 100K 200
8 6 4
160K 190K 220K
2
Power (mW)
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12
100 280K(x5)
0 0
2
4
6
Intensity
14
8
-4000
-2000 0 2000 angle (arc seconds)
4000
10
Current density (kA cm-2)
Fig. 2. Light-intensity versus current characteristics for the three-well active region sample. The dotted line represents the I–V relation measured for the device at 77 K.
Fig. 3. (upper) High-resolution X-ray diffraction rocking curve of the double-phonon active region sample measured about the (0 0 4) reflection for the double-phonon active region sample. (lower) Simulation of the XRD rocking curve using dynamical diffraction theory.
ARTICLE IN PRESS A.B. Krysa et al. / Journal of Crystal Growth 272 (2004) 682–685
Power (mW)
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50 190K 220K 320K
0 0
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of thin layers with abrupt interfaces and good thickness and composition control. Consequently, there is close correlation between the emission wavelengths recorded in this study and the predicted value calculated for different QCL designs. The threshold and power output of the MOVPE-grown QCLs compare well with the previously published ‘‘state-of-the-art’’ MBEgrown QCLs having a similar design. Our work also suggests that limited availability of QCL material could be overcome by adopting MOVPE growth and multi-wafer production style reactors.
3
Current (A)
Fig. 4. LI curves measured for the double-phonon active region sample. (inset) Emission spectra measured at 1.1 Jth and temperatures of 12 and 320 K.
for an MBE grown sample of a similar design [7]. L/I curves measured at various temperatures up to 320 K (the maximum temperature of the cryostat) can be seen in Fig. 4. Emission spectra measured at 12 K and 320 K are also presented in the inset to Fig. 4. The close correspondence between predicted and measured emission wavelengths (8.8 mm at 12 K, 9.3 mm at 320 K) confirms the quality of crystal growth, implying that the thin active region barriers are well resolved.
4. Conclusions We have developed an MOVPE growth technology for AlInAs/InGaAs QCLs. X-ray diffraction measurements and TEM confirm the growth
Acknowledgements This work was supported by EPSRC, Grant no. GR/R25576. References [1] J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L. Hutchinson, A.Y. Cho, Science 264 (1994) 553. [2] S. Slivken, C. Jelen, A. Rybaltiwski, J. Diaz, M. Razeghi, Appl. Phys. Lett. 71 (1997) 2593. [3] D. Hofstetter, M. Beck, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, H. Melchio, Appl. Phys. Lett. 78 (2001) 1964. [4] J.S. Roberts, R.P. Green, L.R. Wilson, E.A. Zibik, D.G. Revin, J.W. Cockburn, R.J. Airey, Appl. Phys. Lett. 82 (2003) 4221. [5] C. Gmachl, F. Capasso, J. Faist, A.L. Hutchinson, A. Tredicucci, D.L. Sivco, J.N. Baillargeon, S.N.G. Chu, A.Y. Cho, Appl. Phys. Lett. 72 (1998) 1430. [6] C. Gmachl, F. Capasso, D.L. Sivco, A.Y. Cho, Rep. Prog. Phys. 64 (2001) 1533. [7] M. Beck, D. Hofstetter, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, H. Melchior, Science 295 (2002) 301.
Journal of Crystal Growth 272 (2004) 682–685 www.elsevier.com/locate/jcrysgro
MOVPE-grown quantum cascade lasers operating at 9 mm wavelength A.B. Krysaa,, J.S. Robertsa, R.P. Greenb, L.R. Wilsonb, H. Pagec, M. Garciac, J.W. Cockburnb a
EPSRC National Centre for III–V Technologies, University of Sheffield, Sheffield S1 3JD, UK b Department of Physics and Astronomy, University of Sheffield, Sheffield S1 3JD, UK c Thales Research and Technology, Domaine de Corbeville, 91404 Orsay Cedex, France
Abstract We report entirely MOVPE-grown QCL lasers operating around 9 mm. High-resolution X-ray diffraction analysis and TEM data confirm precise thickness and composition control for the individual layers. Consequently, these MOVPE-grown QCLs demonstrate a pulsed mode performance comparable to similarly designed MBE-grown devices. QCLs having a conventional three-well active region design exhibit a threshold current density of 1.5 kA cm 2 at 12 K and continue to lase up to room temperature. Furthermore, QCLs designed with a four-QW double-phonon active region show a threshold current density as low as 880 A cm 2 at 12 K and 3.75 kA cm 2 at 300 K with peak powers in excess of 1 W. r 2004 Elsevier B.V. All rights reserved. PACS: 42.55Px; 42.60By; 73.21Ac; 81.15Gh Keywords: A3. Metalorganic vapor phase epitaxy; B3. Quantum cascade laser
1. Introduction Quantum cascade lasers are very demanding structures in terms of their precise thickness requirements, interface abruptness and low oxygen contamination, when Al-containing layers are involved. The availability of low oxygen-containCorresponding author. Fax: +44-114-272-6391.
E-mail address: a.krysa@sheffield.ac.uk (A.B. Krysa).
ing TMA and TMI precursors, (e.g. Epichem) have particularly been important for the development of all MOVPE-grown QCLs. Since their first experimental demonstration in 1994 [1], MBE, including gas source MBE [2], has been considered to be the only viable growth technique for this class of device. The role of MOVPE was essentially limited to the re-growth of high thermal conductivity InP upper cladding layers for some highperformance InP-based QCLs [3]. Consequently,
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.066
ARTICLE IN PRESS A.B. Krysa et al. / Journal of Crystal Growth 272 (2004) 682–685
applying MOVPE growth to the whole structures essentially simplifies the manufacture of InP clad QCLs. In addition, there are other potential advantages of MOVPE over MBE, such as the higher available growth rates and scaling the reactors from single to multi-wafer deposition; essential for the mass production of QCL devices. Recently, the first all-MOVPE QCL based on the GaAs/AlGaAs material system has been reported [4]. In this paper, we present results on InGaAs/ AlInAs/InP QCLs grown by low-pressure MOVPE.
2. Growth details The QCL structures were grown on (1 0 0), misoriented by 0.251 to [1 1 0], InP:S substrates in a horizontal reactor at a pressure of 150 Torr and temperature of 690 1C. TMAl, TMGa and TMIn were used as precursors of the group III elements, and arsine and phosphine as precursors of the group V elements. The system pipe-work, transporting the alkyls to the reactor, as well as switching of the reagents between run/vent lines, were considered to be a critical features of the growth system. Our MOVPE system featured heated stainless pipes to reduce the adsorption of the alkyls. Furthermore, the zero dead-space and cross-purged design of the individual run/vent valves is presently the subject of a patent application.
683
predicted laser emission wavelength was 8.8 mm at room temperature. TEM characterisation reveals a good resolution of the layers and a superlattice (SL) period of 485 A˚, which compares well with the intended period. X-ray diffraction measurements using the (0 0 4) reflection showed a low lattice mismatch for the AlInAs/InGaAs SL of Da/a10 4. The SL period obtained was also in agreement with that derived from a bright-field TEM image of the QCL gain region. This structural characterisation indicates a high level of accuracy, both for the layer thicknesses and the compositions within the active region. A portion of the TEM image is shown in Fig. 1(a), demonstrating the high degree of contrast available for AlInAs/GaInAs material system. Following growth, the sample was processed into ridge wave-guide lasers using conventional techniques. These were then cleaved into 1.3 mm long Fabry–Perot cavities and mounted epilayer side up onto copper submounts for characterisation. Laser emission was observed up to room temperature, with threshold current densities of Jth (12 K)1.5 kA cm 2 and Jth (290 K)9 kA/cm 2, which compares well with the MBE-grown QCL reported in Ref. [6]. Emission spectra measured at both low and room temperature show a good agreement between the predicted and measured laser wavelengths. Fig. 1(b) shows the emission
Energy (meV) 141
140
139
138
Intensity (a.u.)
3. Experimental results and discussion 3.1. Three-well active region design Samples with two different designs have been grown and characterised. The first design is known as the three-well active region QCL [5]. The exact layer structure has been published in reference [5] and consisted of 25 repeats of AlInAs/InGaAs layers with the following nominal thickness: 40/25/ 15/74/11/60/24/39/11/34/11/34/12/37/17/41 A˚, where AlInAs is in bold and underlined layers were Si-doped to a level of 2.5 1017 cm 3. The
8.8
(a)
(b)
8.85
8.9
8.95
9
Wavelength (µm)
Fig. 1. (a) Part of TEM image of the three-well active region sample. (b) Room temperature emission spectrum.
ARTICLE IN PRESS A.B. Krysa et al. / Journal of Crystal Growth 272 (2004) 682–685
684
spectrum measured at room temperature for this sample. The light/current (L/I) data for this QCL is shown in Fig. 2 for various temperatures up to 280 K, including 77 K. 3.2. Double-phonon active region design The previous QCL sample was constructed with a three-well active region; however, improved performance can be obtained using a four-well, or ‘double phonon resonance’ design [7] to optimise the depopulation of the lower laser level. The use of such an active region places even more stringent requirements on the crystal growth, by reducing the thickness of the barriers within the active region to 7–9 A˚, making the device performance even less tolerant to small errors in the thickness of the barriers. Such errors can arise from either the timing uncertainties during growth or interfacial grading effects [4]. A structure designed for 9.1 mm emission (RT) was grown with a design similar to that reported in reference [7] and consisted of 35 repeats of the following sequence of AlInAs/InGaAs layers: 40/ 19/7/58/9/57/9/50/22/32/15/31/19/30/23/29/25/ 29 A˚, where AlInAs is in bold and underlined layers were Si-doped to a level of 2 1017 cm 3. High resolution X-ray diffraction rocking curves were collected about the (0 0 4) reflection using a Bede D1 diffractometer. In order to eliminate the
influence of the intentional wafer misorientation (0.251 towards [1 1 0]) the wafer was rotated by 1801 between the measurements, with values of superlattice period and mismatch averaged between these two measurements. One of these rocking curves is shown in Fig. 3, together with a simulated curve generated using dynamical diffraction theory, using commercially available software. A superlattice period of 625 A˚ was obtained, slightly longer than the intended value of 598 A˚, with a mismatch Da/a less than 6 10 4 between the InGaAs/AlInAs SL and InP substrate. We expect a thickness variation for the SL of 4% across the wafer. This value was based on X-ray diffraction in combination with a wafermapping technique for a similar QCL sample, designed for a slightly longer wavelength of 10.3 mm (to be published elsewhere). Strong laser emission up to and above room temperature was observed, after processing of the wafer into the Fabry–Perot laser design described above. The low temperature threshold current density was reduced to Jth880 A cm 2, increasing to 3.75 kA cm 2 for room temperature operation. This value compares well with that reported
400
16
70K Voltage (V)
10 100K 200
8 6 4
160K 190K 220K
2
Power (mW)
300
12
100 280K(x5)
0 0
2
4
6
Intensity
14
8
-4000
-2000 0 2000 angle (arc seconds)
4000
10
Current density (kA cm-2)
Fig. 2. Light-intensity versus current characteristics for the three-well active region sample. The dotted line represents the I–V relation measured for the device at 77 K.
Fig. 3. (upper) High-resolution X-ray diffraction rocking curve of the double-phonon active region sample measured about the (0 0 4) reflection for the double-phonon active region sample. (lower) Simulation of the XRD rocking curve using dynamical diffraction theory.
ARTICLE IN PRESS A.B. Krysa et al. / Journal of Crystal Growth 272 (2004) 682–685
Power (mW)
320K
100K
9.0 9.5 Wavelength (µm)
160K
12K
150
100 8.5
50 190K 220K 320K
0 0
1
2
685
of thin layers with abrupt interfaces and good thickness and composition control. Consequently, there is close correlation between the emission wavelengths recorded in this study and the predicted value calculated for different QCL designs. The threshold and power output of the MOVPE-grown QCLs compare well with the previously published ‘‘state-of-the-art’’ MBEgrown QCLs having a similar design. Our work also suggests that limited availability of QCL material could be overcome by adopting MOVPE growth and multi-wafer production style reactors.
3
Current (A)
Fig. 4. LI curves measured for the double-phonon active region sample. (inset) Emission spectra measured at 1.1 Jth and temperatures of 12 and 320 K.
for an MBE grown sample of a similar design [7]. L/I curves measured at various temperatures up to 320 K (the maximum temperature of the cryostat) can be seen in Fig. 4. Emission spectra measured at 12 K and 320 K are also presented in the inset to Fig. 4. The close correspondence between predicted and measured emission wavelengths (8.8 mm at 12 K, 9.3 mm at 320 K) confirms the quality of crystal growth, implying that the thin active region barriers are well resolved.
4. Conclusions We have developed an MOVPE growth technology for AlInAs/InGaAs QCLs. X-ray diffraction measurements and TEM confirm the growth
Acknowledgements This work was supported by EPSRC, Grant no. GR/R25576. References [1] J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L. Hutchinson, A.Y. Cho, Science 264 (1994) 553. [2] S. Slivken, C. Jelen, A. Rybaltiwski, J. Diaz, M. Razeghi, Appl. Phys. Lett. 71 (1997) 2593. [3] D. Hofstetter, M. Beck, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, H. Melchio, Appl. Phys. Lett. 78 (2001) 1964. [4] J.S. Roberts, R.P. Green, L.R. Wilson, E.A. Zibik, D.G. Revin, J.W. Cockburn, R.J. Airey, Appl. Phys. Lett. 82 (2003) 4221. [5] C. Gmachl, F. Capasso, J. Faist, A.L. Hutchinson, A. Tredicucci, D.L. Sivco, J.N. Baillargeon, S.N.G. Chu, A.Y. Cho, Appl. Phys. Lett. 72 (1998) 1430. [6] C. Gmachl, F. Capasso, D.L. Sivco, A.Y. Cho, Rep. Prog. Phys. 64 (2001) 1533. [7] M. Beck, D. Hofstetter, T. Aellen, J. Faist, U. Oesterle, M. Ilegems, E. Gini, H. Melchior, Science 295 (2002) 301.