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Optical Materials Express

Optical Materials Express

  • Editor: David J. Hagan
  • Vol. 3, Iss. 10 — Oct. 1, 2013
  • pp: 1624–1631
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Interband cascade lasers with AlGaAsSb bulk cladding layers

Robert Weih, Adam Bauer, Martin Kamp, and Sven Höfling  »View Author Affiliations


Optical Materials Express, Vol. 3, Issue 10, pp. 1624-1631 (2013)
http://dx.doi.org/10.1364/OME.3.001624


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Abstract

Interband cascade lasers are promising candidates to cover a wide spectral range in the mid infrared spectral region with high performance devices. In this paper, we report on lasers where the cladding layers consist of quaternary bulk material (AlGaAsSb) instead of InAs/AlSb superlattices. The bulk claddings provide efficient mode confinement due to their low refractive index, comparable heat conductivity and a reduced current spreading. Broad area devices fabricated from laser layers with 5 cascades showed threshold current densities of 220 A/cm2 and narrow ridges operated up to 45 °C in continuous wave mode.

© 2013 OSA

1. Introduction

Since the beginning of research on interband cascade lasers (ICL) in the 90s, InAs/AlSb superlattices (SL) were proposed and implemented [1

1. J. R. Meyer, I. Vurgaftman, R. Q. Yang, and L. R. Ram-Mohan, “Type-II and type-I interband cascade lasers,” Electron. Lett. 32(1), 45–46 (1996). [CrossRef]

] as cladding material and are still present in current state of the art ICLs. Although they are not lattice matched to GaSb substrates, AlSb cladding layers have been utilized for type-II quantum well lasers [2

2. C.-H. Lin, S. J. Murry, D. Zhang, P. C. Chang, Y. Zhou, S. S. Pei, J. I. Malin, C. L. Felix, J. R. Meyer, C. A. Hoffman, and J. F. Pinto, “MBE grown mid-infrared type-II quantum-well lasers,” J. Cryst. Growth 175-176, 955–959 (1997). [CrossRef]

]. As an alternative route plasmon enhanced waveguides, which make use of the influence of free carriers from dopants on the refractive index, have been realized in long wavelength quantum cascade lasers (QCL) [3

3. C. Sirtori, P. Kruck, S. Barbieri, H. Page, J. Nagle, M. Beck, J. Faist, and U. Oesterle, “Low-loss Al-free waveguides for unipolar semiconductor lasers,” Appl. Phys. Lett. 75(25), 3911 (1999). [CrossRef]

,4

4. K. Ohtani and H. Ohno, “An InAs-based intersubband quantum cascade laser,” Jpn. J. Appl. Phys. 41(Part 2, No. 11B), L1279–L1280 (2002). [CrossRef]

] and ICLs [5

5. Z. Tian, R. Q. Yang, T. D. Mishima, M. B. Santos, R. T. Hinkey, M. E. Curtis, and M. B. Johnson, “InAs-based interband cascade lasers near 6 µm,” Electron. Lett. 45(1), 48–49 (2009). [CrossRef]

]. Quaternary AlGaAsSb cladding layers with high aluminum content are commonly used in GaSb based mid-infrared diode lasers [6

6. H. K. Choi and S. J. Eglash, “Room-temperature cw operation at 2.2 µm of GalnAsSb/AIGaAsSb diode lasers grown by molecular beam epitaxy,” Appl. Phys. Lett. 59(10), 1165 (1991). [CrossRef]

]. The material provides a low refractive index and can easily be lattice matched to GaSb by adjusting the group-V concentrations. In most cases, a Ga-content between 10 and 20% is used to prevent excessive oxidation that typically occurs for very high Al contents. In this work, we investigate ICLs with quaternary AlGaAsSb cladding layers. For completeness some alternative concepts within the field of mid infrared semiconductor lasers with recently published results are discussed in the following. QCLs grown on InP have already shown impressive results in the 3.4 µm region. Narrow ridge devices emit up to 400 mW at 3.39 µm in cw-mode at 25 °C and show very high characteristic temperatures of 166 K [7

7. N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi, “High power, continuous wave, room temperature operation of λ3.4μm and λ3.55μm InP-based quantum cascade lasers,” Appl. Phys. Lett. 100(21), 212104 (2012). [CrossRef]

]. Nevertheless, the threshold current and power density is comparatively high with room temperature values of 1100 A/cm2 and 15.4 kW/cm2 respectively which is disadvantageous regarding portable applications. GaSb based diode lasers with emission at 3.4 µm situate between the two cascaded concepts with Jth = 600 A/cm2 for a 2 mm long broad area device operated at 20 °C [8

8. K. Vizbaras, A. Vizbaras, A. Andrejew, C. Grasse, S. Sprengel, and M.-C. Amann, “Room-temperature type-I GaSb-based lasers in the 3.0 - 3.7 μm wavelength range,” Proc. SPIE 8277, 82771B, 82771B-7 (2012). [CrossRef]

]. Other concepts incorporate InAsN/GaAsSb [9

9. I. Vurgaftman, J. R. Meyer, N. Tansu, and L. J. Mawst, “InP-based dilute-nitride mid-infrared type-II “W” quantum-well lasers,” J. Appl. Phys. 96(8), 4653 (2004). [CrossRef]

] or InGaAs/GaAsSb [10

10. C. H. Pan and C. P. Lee, “Design and modeling of InP-based InGaAs/GaAsSb type-II “W” type quantum wells for mid-Infrared laser applications,” J. Appl. Phys. 113(4), 043112 (2013). [CrossRef]

] “W”-quantum wells whereas to our knowledge no room temperature lasing beyond 3 µm has been reported yet.

2. Waveguide design and MBE growth

The cladding material composition was chosen to Al0.85Ga0.15As0.07Sb0.93 to be lattice matched to the GaSb substrate and prevent excessive oxidation that typically occurs for very high Al contents. Furthermore, the quaternary material provides a low refractive index of approximately n = 3.30 [11

11. S. Adachi, “Band gaps and refractive indices of AlGaAsSb, GaInAsSb, and InPAsSb: Key properties for a variety of the 2–4μm optoelectronic device applications,” J. Appl. Phys. 61(10), 4869 (1987). [CrossRef]

], which is lower than the mean refractive index of the commonly used InAs/AlSb superlattice (n = 3.39). As a result, the confinement of the optical mode in the core of the waveguide is improved, which potentially allows a reduction of the lower cladding layer thickness that is required to prevent mode leakage into the substrate. A comparison of the optical mode profiles in ICLs with quaternary and superlattice claddings is shown in Fig. 1
Fig. 1 Refractive index and optical mode profile for an ICL with InAs/AlSb-Cladding (black line) and quaternary Al0.85Ga0.15As0.07Sb0.93 Cladding (red line). The structure incorporates 5 active cascades and two 220 nm thick GaSb separate confinement layers.
. The active region consists of 5 stages (see Fig. 2
Fig. 2 Band structure of one cascade in the active region. The structure was optimized for an electrical field of 90 kV/cm and five InAs injector quantum wells.
for details), sandwiched between two 220 nm thick separate confinement layers (SCL) made out of GaSb.

The mode intensity for the SL-cladding is normalized and the one for the quaternary cladding is scaled to yield the same area under the curve. As indicated, the peak mode intensity within the active region is increased by 14% in case of the quaternary cladding. This increases the confinement factor from 25.2% to 28.6% and reduces the overlap of the optical mode with the cladding. Furthermore, the mode decays faster, thus thinner claddings with better thermal conductance can be used.

The laser structure was grown in an EIKO molecular beam epitaxy reactor equipped with solid source effusion cells for Ga, Al and In and valved crackers for arsenic and antimony. After thermal oxide desorption, the growth was initiated with a 200 nm thick GaSb buffer layer in order to provide a smooth surface for the following layers. In order to flatten the conduction band offset between the GaSb buffer and the Al0.85Ga0.15As0.07Sb0.93 cladding, a 50 nm wide linear digital grading (LDG) was grown on top of the buffer layer. A 3 µm thick lower cladding was grown at a substrate temperature of 500 °C which was controlled with a pyrometer. Another LDG was inserted as a transition to the following 220 nm thick GaSb separate confinement layer (SCL) which was grown at 485 °C. The substrate temperature was ramped down to 450 °C within the last 15 nm of the SCL for the growth of the active region that consists of five cascades. Figure 2 shows the band structure of one and a half active stages.

3. Laser processing and device performance

In order to determine the laser characteristics, 150 µm wide broad area lasers were fabricated. An inductively coupled plasma reactive ion etch step was used to etch the devices through the active region and into the lower cladding layer. Afterwards, a wet chemical cleaning step in phosphoric acid was performed. The ridges were then coated with a passivation layer of 200 nm SiN and 200 nm SiO2. This insulating layer was then opened by optical lithography and a Ti/Pt/Au top contact was evaporated. After mechanically thinning of the substrate, an AuGe/Ni/Au contact was evaporated on the backside of the sample.

To avoid average heating of the sample, the basic characteristics were measured in pulsed mode with a pulse width of 300 ns and a repetition rate of 1 kHz. Figure 4
Fig. 4 I-V-P characteristics of a 5 stage broad area ICL with a threshold current density of 220 A/cm2. The emission wavelength centers around 3.40 µm as illustrated in the inset of the figure.
shows the I-V-P characteristics of the device. The threshold current density (Jth) of the 2 mm long and 150 µm wide broad-area device with uncoated facets was 220 A/cm2 at room temperature. This compares fairly well with values of state of the art ICLs with superlattice claddings where record values of 134 A/cm2 have been reached for a similar device emitting at 3.6 µm [13

13. W. W. Bewley, C. D. Merritt, C. S. Kim, M. Kim, C. L. Canedy, I. Vurgaftman, J. Abell, and J. R. Meyer, “Mid-IR Interband Cascade Lasers Operating with <30 mW of Input Power,” Proc. of SPIEVol. 8374, 83740H. [CrossRef]

]. Since the active region design is similar to previously published designs, the higher threshold current density might be due to optical losses that originate in the quaternary claddings. ICLs reported in [14

14. I. Vurgaftman, W. W. Bewley, C. L. Canedy, J. R. Lindle, C. S. Kim, M. Kim, and J. R. Meyer, “High-temperature interband cascade lasers,” U.S. Patent Application 12/402, 627, filed Mar. 12, 2009.

] grown at NRL generally incorporate thicker SCL layers (typically 500 nm instead of 220 nm for the given wavelength region) resulting in less optical mode penetration into the claddings. This option will be considered in future growth runs. Another possible reason for the higher threshold current density might be degradation of the active region due to the high substrate temperature (500 °C) in the upper cladding. The laser temperature stability represented by the characteristic temperature (T0) was determined to 47 K in a temperature range between 20 °C and 80 °C. Furthermore, it has been found that the current spreading in the cladding region is not as pronounced as in ICLs with SL-claddings. Shallowly etched devices (1.0 µm into the upper cladding) with the same dimensions had Jth of 290 A/cm2, whereas in ICLs with SL-claddings it is known that Jth is much higher due to excessive current spreading that results in larger pumped effective areas. From the slope efficiency of 274 mW/A the external quantum efficiency per stage can be extracted to 30.0%. The threshold voltage was 4.97 V which yields a threshold power density of 1.09 kW/cm2. The set in voltage determined from a fit to the linear part of the I-V-characteristic was 3.70 V, which corresponds to a voltage efficiency of 49.3%. This value is small compared to typical values of SL-cladding based ICLs and most probably caused by the additional voltage drops at the conduction band discontinuities between the quaternary cladding and the GaSb buffer, SCL and contact layer. However, the series resistance is in the range of 1-2 Ω similar to the values that were observed for ICL broad area lasers with superlattice claddings indicating a comparable conductivity.

For operation in continuous wave mode, narrow ridges were processed in a similar fashion as described above. An additional 5 µm thick layer of gold was electroplated on top of the ridge for improved heat dissipation. The gold layer is interrupted with regularly spaced 75 µm wide cleaving stripes. One facet of the cleaved ridges was coated with Al2O3 and Au for high reflectivity. Figure 5
Fig. 5 I-V-P characteristics of a narrow ridge device operated in cw-mode. The maximum cw-operation temperature was 45 °C. At room temperature the device emitted up to 18 mW of optical power from one facet. AC: as cleaved. HR: high reflection coated.
shows the I-V-P characteristics of a 3 mm long and 10.8 µm wide ridge at several temperatures. At room temperature 18 mW of optical power were emitted from the front facet and Jth was 282 A/cm2.

The maximum operation temperature in cw mode was 45 °C. This limitation is due to the high voltage that is necessary to operate the device. The I-V characteristic shown in Fig. 5 was measured at 20 °C. In Fig. 6
Fig. 6 Spectra of a narrow ridge laser operated at different temperatures with a driving current of 300 mA. The temperature induced shift of the wavelength is approximately 1.5 nm/K.
spectra of the narrow ridge driven with 300 mA at different temperatures are shown. The temperature induced red shift is estimated to be 1.5 nm/K. Regarding long term reliability no measurements were performed yet, but the measured devices showed no degradation while temperature dependent measurements were performed.

5. Conclusion

An alternative cladding material was successfully implemented into the waveguide structure of interband cascade lasers. Due to its low refractive index Al0.85Ga0.15As0.07Sb0.93 is suitable to confine the mode even better than the commonly used InAs/AlSb superlattices. High resolution X-ray diffraction indicates a good crystal quality of the layers. Broad area devices showed threshold current densities of 220 A/cm2 at room temperature and an emission wavelength of 3.40 µm. A 10.8 µm wide and 3 mm long narrow ridge device with HR coated facet operated up to 45 °C in cw-mode. This is limited by the power dissipation which in turn is affected by the additional voltage drops that occur in the device. With those being reduced, higher operation temperatures will be possible.

Acknowledgment

We thank the European Commission for financial support within the FP7 project ‘WideLase’ (no. 318798) and the Federal Ministry of Education and Research (BMBF) within the project ‘INBAKAS’ (FKZ: 13N12440).

References and links

1.

J. R. Meyer, I. Vurgaftman, R. Q. Yang, and L. R. Ram-Mohan, “Type-II and type-I interband cascade lasers,” Electron. Lett. 32(1), 45–46 (1996). [CrossRef]

2.

C.-H. Lin, S. J. Murry, D. Zhang, P. C. Chang, Y. Zhou, S. S. Pei, J. I. Malin, C. L. Felix, J. R. Meyer, C. A. Hoffman, and J. F. Pinto, “MBE grown mid-infrared type-II quantum-well lasers,” J. Cryst. Growth 175-176, 955–959 (1997). [CrossRef]

3.

C. Sirtori, P. Kruck, S. Barbieri, H. Page, J. Nagle, M. Beck, J. Faist, and U. Oesterle, “Low-loss Al-free waveguides for unipolar semiconductor lasers,” Appl. Phys. Lett. 75(25), 3911 (1999). [CrossRef]

4.

K. Ohtani and H. Ohno, “An InAs-based intersubband quantum cascade laser,” Jpn. J. Appl. Phys. 41(Part 2, No. 11B), L1279–L1280 (2002). [CrossRef]

5.

Z. Tian, R. Q. Yang, T. D. Mishima, M. B. Santos, R. T. Hinkey, M. E. Curtis, and M. B. Johnson, “InAs-based interband cascade lasers near 6 µm,” Electron. Lett. 45(1), 48–49 (2009). [CrossRef]

6.

H. K. Choi and S. J. Eglash, “Room-temperature cw operation at 2.2 µm of GalnAsSb/AIGaAsSb diode lasers grown by molecular beam epitaxy,” Appl. Phys. Lett. 59(10), 1165 (1991). [CrossRef]

7.

N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi, “High power, continuous wave, room temperature operation of λ3.4μm and λ3.55μm InP-based quantum cascade lasers,” Appl. Phys. Lett. 100(21), 212104 (2012). [CrossRef]

8.

K. Vizbaras, A. Vizbaras, A. Andrejew, C. Grasse, S. Sprengel, and M.-C. Amann, “Room-temperature type-I GaSb-based lasers in the 3.0 - 3.7 μm wavelength range,” Proc. SPIE 8277, 82771B, 82771B-7 (2012). [CrossRef]

9.

I. Vurgaftman, J. R. Meyer, N. Tansu, and L. J. Mawst, “InP-based dilute-nitride mid-infrared type-II “W” quantum-well lasers,” J. Appl. Phys. 96(8), 4653 (2004). [CrossRef]

10.

C. H. Pan and C. P. Lee, “Design and modeling of InP-based InGaAs/GaAsSb type-II “W” type quantum wells for mid-Infrared laser applications,” J. Appl. Phys. 113(4), 043112 (2013). [CrossRef]

11.

S. Adachi, “Band gaps and refractive indices of AlGaAsSb, GaInAsSb, and InPAsSb: Key properties for a variety of the 2–4μm optoelectronic device applications,” J. Appl. Phys. 61(10), 4869 (1987). [CrossRef]

12.

I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, J. R. Lindle, and J. R. Meyer, “Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption,” Nature Communications2, 585 (2011).

13.

W. W. Bewley, C. D. Merritt, C. S. Kim, M. Kim, C. L. Canedy, I. Vurgaftman, J. Abell, and J. R. Meyer, “Mid-IR Interband Cascade Lasers Operating with <30 mW of Input Power,” Proc. of SPIEVol. 8374, 83740H. [CrossRef]

14.

I. Vurgaftman, W. W. Bewley, C. L. Canedy, J. R. Lindle, C. S. Kim, M. Kim, and J. R. Meyer, “High-temperature interband cascade lasers,” U.S. Patent Application 12/402, 627, filed Mar. 12, 2009.

OCIS Codes
(140.3070) Lasers and laser optics : Infrared and far-infrared lasers
(140.5960) Lasers and laser optics : Semiconductor lasers
(160.6000) Materials : Semiconductor materials
(250.0250) Optoelectronics : Optoelectronics
(140.5965) Lasers and laser optics : Semiconductor lasers, quantum cascade
(250.5960) Optoelectronics : Semiconductor lasers

ToC Category:
Semiconductors

History
Original Manuscript: July 9, 2013
Revised Manuscript: August 25, 2013
Manuscript Accepted: August 26, 2013
Published: September 6, 2013

Virtual Issues
Mid-IR Photonic Materials (2013) Optical Materials Express

Citation
Robert Weih, Adam Bauer, Martin Kamp, and Sven Höfling, "Interband cascade lasers with AlGaAsSb bulk cladding layers," Opt. Mater. Express 3, 1624-1631 (2013)
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-3-10-1624


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References

  1. J. R. Meyer, I. Vurgaftman, R. Q. Yang, and L. R. Ram-Mohan, “Type-II and type-I interband cascade lasers,” Electron. Lett.32(1), 45–46 (1996). [CrossRef]
  2. C.-H. Lin, S. J. Murry, D. Zhang, P. C. Chang, Y. Zhou, S. S. Pei, J. I. Malin, C. L. Felix, J. R. Meyer, C. A. Hoffman, and J. F. Pinto, “MBE grown mid-infrared type-II quantum-well lasers,” J. Cryst. Growth175-176, 955–959 (1997). [CrossRef]
  3. C. Sirtori, P. Kruck, S. Barbieri, H. Page, J. Nagle, M. Beck, J. Faist, and U. Oesterle, “Low-loss Al-free waveguides for unipolar semiconductor lasers,” Appl. Phys. Lett.75(25), 3911 (1999). [CrossRef]
  4. K. Ohtani and H. Ohno, “An InAs-based intersubband quantum cascade laser,” Jpn. J. Appl. Phys.41(Part 2, No. 11B), L1279–L1280 (2002). [CrossRef]
  5. Z. Tian, R. Q. Yang, T. D. Mishima, M. B. Santos, R. T. Hinkey, M. E. Curtis, and M. B. Johnson, “InAs-based interband cascade lasers near 6 µm,” Electron. Lett.45(1), 48–49 (2009). [CrossRef]
  6. H. K. Choi and S. J. Eglash, “Room-temperature cw operation at 2.2 µm of GalnAsSb/AIGaAsSb diode lasers grown by molecular beam epitaxy,” Appl. Phys. Lett.59(10), 1165 (1991). [CrossRef]
  7. N. Bandyopadhyay, S. Slivken, Y. Bai, and M. Razeghi, “High power, continuous wave, room temperature operation of λ3.4μm and λ3.55μm InP-based quantum cascade lasers,” Appl. Phys. Lett.100(21), 212104 (2012). [CrossRef]
  8. K. Vizbaras, A. Vizbaras, A. Andrejew, C. Grasse, S. Sprengel, and M.-C. Amann, “Room-temperature type-I GaSb-based lasers in the 3.0 - 3.7 μm wavelength range,” Proc. SPIE8277, 82771B, 82771B-7 (2012). [CrossRef]
  9. I. Vurgaftman, J. R. Meyer, N. Tansu, and L. J. Mawst, “InP-based dilute-nitride mid-infrared type-II “W” quantum-well lasers,” J. Appl. Phys.96(8), 4653 (2004). [CrossRef]
  10. C. H. Pan and C. P. Lee, “Design and modeling of InP-based InGaAs/GaAsSb type-II “W” type quantum wells for mid-Infrared laser applications,” J. Appl. Phys.113(4), 043112 (2013). [CrossRef]
  11. S. Adachi, “Band gaps and refractive indices of AlGaAsSb, GaInAsSb, and InPAsSb: Key properties for a variety of the 2–4μm optoelectronic device applications,” J. Appl. Phys.61(10), 4869 (1987). [CrossRef]
  12. I. Vurgaftman, W. W. Bewley, C. L. Canedy, C. S. Kim, M. Kim, C. D. Merritt, J. Abell, J. R. Lindle, and J. R. Meyer, “Rebalancing of internally generated carriers for mid-infrared interband cascade lasers with very low power consumption,” Nature Communications2, 585 (2011).
  13. W. W. Bewley, C. D. Merritt, C. S. Kim, M. Kim, C. L. Canedy, I. Vurgaftman, J. Abell, and J. R. Meyer, “Mid-IR Interband Cascade Lasers Operating with <30 mW of Input Power,” Proc. of SPIEVol. 8374, 83740H. [CrossRef]
  14. I. Vurgaftman, W. W. Bewley, C. L. Canedy, J. R. Lindle, C. S. Kim, M. Kim, and J. R. Meyer, “High-temperature interband cascade lasers,” U.S. Patent Application 12/402, 627, filed Mar. 12, 2009.

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