- Email: [email protected]
High power 3–12μm infrared lasers: recent improvements and future trends
Physica E 11 (2001) 233–239
www.elsevier.com/locate/physe
High power 3–12 m infrared lasers: recent improvements and future trends M. Razeghia; ∗ , S. Slivkena , A. Tahraouia , A. Matlisa , Y.S. Parkb a Department
of Electrical Engineering, Center for Quantum Devices, Northwestern University, 2145 N. Sheridan Road, Evanston, IL 60208, USA b O*ce of Naval research, 800 North Quincy Street, Arlington, VA 22217, USA
Abstract In this paper, we discuss the progress of quantum cascade lasers (QCLs) grown by gas-source molecular beam epitaxy. Room temperature QCL operation has been reported for lasers emitting between 5 –11 m, with 9 –11 m lasers operating up to 425 K. Laser technology for the 3–5 m range takes advantage of a strain-balanced active layer design. We also demonstrate record room temperature peak output powers at 9 and 11 m (2:5 and 1 W, respectively) as well as record low 80 K threshold current densities (250 A=cm2 ) for some laser designs. Preliminary distributed feedback (DFB) results are c 2001 Elsevier Science B.V. All also presented and exhibit single mode operation for 9 m lasers at room temperature. rights reserved. PACS: 73.21.Cd Keywords: Quantum cascade laser; Wavefunction engineering; InP-based material; Gas-source molecular beam epitaxy
1. Introduction Compact, high-power lasers in the mid-to farinfrared wavelength range (3–20 m) are in great demand for many laser-based applications areas such as free-space communication, chemical sensing, and medicine. Gas lasers and parametric oscillators are bulky and require precise alignment of specialized optics. The semiconductor laser, on the other hand, ∗
Corresponding author. Tel.: +1-847-491-7251; fax: +1-847467-1817. E-mail address: [email protected] (M. Razeghi).
is inherently compact and durable, with an internal optical cavity and low voltage=current operation. Up until recently, the only commercially available semiconductor laser in the mid- and far-infrared wavelength range has been based on lead-salt technology, which has both low power output and high sensitivity to temperature. The quantum cascade laser (QCL) is a unipolar semiconductor laser [1]. Rather than relying on intrinsic band gaps, quantum eFects are exploited in order to produce designer energy levels and transitions within the conduction band of a complex heterostructure. The beneGt of this approach is a widely
c 2001 Elsevier Science B.V. All rights reserved. 1386-9477/01/$ - see front matter PII: S 1 3 8 6 - 9 4 7 7 ( 0 1 ) 0 0 2 1 0 - 7
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M. Razeghi et al. / Physica E 11 (2001) 233–239
Fig. 1. (a) Schematic of SLQCL active region and (b) miniband dispersion.
variable transition energy dictated primarily by layer thicknesses. Typical emission wavelengths vary from 3.4 –17 m. Further, because the primary limitation to radiative eIciency is based on phonons rather than Auger processes, the device performance is much less sensitive to temperature change than competing technology. The primary requisite is a heterostructure with a large conduction band oFset and well-controlled interfaces. In other words, it now becomes practical to use mature GaAs- and InP-based technology to produce long wavelength emitters. Driven by a number of emerging applications, QCL technology is improving at a phenomenal rate. In this paper, recent, state-of-the-art advances are presented which help establish the QCL as a compact, durable source of 3–12 m laser emission. SpeciGcally, the discussion centers on lasers produced at Northwestern University’s Center for Quantum Devices (CQD) based on InGaAs=AlInAs=InP heterostructures on InP substrates. All laser structures are grown in a single step by gas-source molecular beam epitaxy (GSMBE). 2. Laser design 2.1. Cascade active layers The Grst step in producing a QCL is the proper design of the active layers. Layer thicknesses are carefully chosen to produce the proper transition energies, transition probabilities, and transport
characteristics. The active region is designed with either a multi-quantum well (MQW) or superlattice (SLQCL) basis. Electron relaxation is dominated by polar optical phonon emission. The principle of operation for a MQW-QCL is described in detail in Ref. [1]. A SLQCL [2] uses minibands to enhance transport and provide an eIcient relaxation path. A typical superlattice active region is shown in Fig. 1a. DeGning z as the growth direction, an approximate dispersion for this type of active region is given in Fig. 1b. The electron relaxation is still dominated by optical phonon processes, but the intraminiband relaxation is intrinsically faster than the interminband transition. This causes a buildup of carriers at the superlattice Brillouin zone edge (kz = =d) of the upper subband. The enhanced transport and deep electron reservoir should theoretically give the SLQCL an advantage over the MQW design in terms of maximum current density and operating temperature. The only disadvantage is the maximum emission energy is reduced due to the wide minibands, limiting emission wavelengths in lattice-matched GaInAs=AlInAs structures to ¿ 7 m. Another way to reach shorter wavelength emission is by incorporating strained heterostructures, which intrinsically have higher conduction band oFsets. However, in order to avoid unwanted misGt dislocations, the net strain should be zero. This is accomplished by proper selection of materials and accurate control over layer composition. For the Gax In1−x As=Aly In1−y As QCL, x and y are chosen such that compressive
M. Razeghi et al. / Physica E 11 (2001) 233–239
strain of the Gax In1−x As quantum wells are balanced throughout the entire device with an equal and opposite tensile strain due to Aly In1−y As quantum barriers. The conduction band oFset can be calculated using a method such as model solid theory [3]. Using this technique, the conduction band oFset can be increased at least 50% with near zero net strain, allowing laser emission as low as 3:4 m [4]. Regardless of the speciGc emitting region design, the QCL is designed in such a way as to allow an electron to emit multiple photons while traversing the waveguide core. This is accomplished through use of a carefully designed injector which connects adjacent emitting regions in series. The use of cascaded active regions allows higher power output and a higher optical conGnement factor. The downside is an operating voltage and thermal resistance that increase with the number of stages (active=injector pairs). 2.2. Waveguide design As the quantum cascade laser often emits at longer wavelengths (3–12 m) than traditional semiconductor lasers, special considerations must be made during laser waveguide design. One such consideration is free carrier absorption. Emitted light, instead of being collected, is absorbed by plasma-like oscillations of free carriers in the semiconductor. The absorption coeIcient is proportional to the number of free-carriers and is strongly dependent on the frequency of light. In the mid- to far-infrared, the absorption coeIcient can be experimentally Gtted to give a 2−3 dependence. In order to reduce the loss, the doping should be kept to a minimum wherever the photon density is high. Another consideration that becomes particularly important as the wavelength increases is coupling of the laser TM modes (primary emission modes for an intersubband laser) to the surface plasmon mode [5]. The surface plasmon mode is very lossy, and exists at the interface between the contact metal and the cap layer of the waveguide. In order to reduce the coupling to the surface plasmon mode, we are using a thick cladding and a highly doped cap layer. The cap layer has an extremely low index of refraction. It serves as a plasma mirror, which separates the optical mode from the lossy surface plasmon mode and reduces the overall waveguide loss.
235
2.3. Design summary The above design principles were used to produce a variety of QCLs. Our preliminary eForts were devoted towards long wavelength ( ¿ 7 m) lattice-matched lasers. Both multi-quantum well and superlattice lasers were designed in order to compare performance in actual devices. The goal was to realize room temperature operation from 7– 11 m. In parallel, the doping of the active layers and waveguide was optimized for individual structures. Using this doping structure, we then investigated SLQCL performance in terms of the number of cascade stages. More recently, strain-balanced laser structures have also been utilized. As mentioned, this gives access to shorter emission wavelengths. 3. Experiment All material growth is done using a EPI Mod Gen II GSMBE reactor. This type of growth technique allows a wide variety of heterostructures to be grown, with very accurate control over layer thickness and interfaces. All of the work described here is based on GaInAs=AlInAs=InP heterostructures. Typically, 25 – 60 Gax In1−x As=Aly In1−y As emitting regions are connected in series and surrounded by 0:3 m of low n-doped (8 × 1016 cm−3 ) Ga0:47 In0:53 As to make up the laser waveguide core. The n-InP substrate (1 × 1017 cm−3 ) and InP buFer layer (7×1016 cm−3 ) serve as the lower waveguide cladding. The top cladding is also InP, with n-type doping graded from 1–3 × 1017 cm−3 over 2–3 m. The cap layer is typically a 1 m thick layer of highly n-doped (2 × 1019 cm−3 ) Ga0:47 In0:53 As. Structural quality, layer thickness, and interface smoothness are very important to QCL device performance. Typical X-ray diFraction for lattice-matched and strain-compensated laser structures (Fig. 2) exhibit clear satellite peaks over 4 degrees. X-ray simulation shows excellent agreement between the ideal and as-grown diFraction spectra. Surface quality is conGrmed with atomic force microscopy (AFM). Typical strain-compensated laser structures exhibit an rms roughness of less than 0:2 nm over a 50 m square area.
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Fig. 2. Typical omega=2 theta X-ray diFraction for (a) lattice-matched and (b) strain-balanced quantum cascade lasers. Simulated diFraction plotted along with experimental data shows the excellent epitaxial control and material quality.
The wafers are processed into a ridge waveguide geometry, which is used to conGne the injected current and optical mode along the lateral direction. A 20-m wide ridge is deGned by optical contact lithography and electron cyclotron resonance reactive ion etching (ECR-RIE) for precise control of the ridge proGle. The center-to-center distance between adjacent stripes is 300 m and a 6 –8 m deep groove is etched between the stripes for optical isolation. Our results show that lasers produced with dry etching show extremely uniform performance over large areas. Plasma-enhanced chemical vapor deposition (PECVD) SiO2 is used as insulation between stripes, and Ti=Au is used as the contact metal for both the top surface and substrate. For pulsed operation, the lasers are typically driven with 5-s current pulses with a repetition rate of 200 Hz. Powers are reported per 2 facets for uncoated lasers. 4. Results Much of our initial work, as contracted, was focused on the 8–12 m wavelength region. This led to the Grst demonstration of QCLs grown by GSMBE [6]. During this work we also demonstrated several records for low threshold current density at room temperature [2,7]. The laser structures were grown lattice-matched to InP substrate. By optimizing the doping level it is
possible to come up with a laser that exhibits both a low threshold current density (1:95 kA=cm2 ) and high peak power (700 mW) at room temperature. These results have been published elsewhere for a 9 m laser structure [8]. Impressive room temperature performance has been observed out to 11 m, with over 1 W peak power observed for a 20 m by 3 mm cavity at room temperature [9]. As part of the work that led to Ref. [8], it was found that by decreasing the doping level in the injector from 2:5 × 1017 cm−3 to 1 × 1017 cm−3 , the threshold current density could be further reduced. In fact, at room temperature alone, the threshold current density decreased to 1:8 kA=cm2 and the laser exhibited a T0 of 220 K. The same laser operated in continuous wave up to 160 K without the beneGt of facet coating or epilayer side down heat sinking. In order to test the scalability of the cascade design and epitaxial process, we also varied the number of emitting stages for a series of otherwise identical lasers. As expected, and as shown in Fig. 3a, the peak output power did increase with the number of stages. No saturation was observed up to 60 periods. The output characteristics for a typical = 9:4 m 60 period laser at room temperature are shown in Fig. 3b. 2:5 W of peak power was observed with a threshold current density of 2:2 kA=cm2 and a threshold voltage of 12:4 V. This is the highest reported QCL power at room temperature in this wavelength range [10]. Lasing action persists in pulsed mode up to 425 K
M. Razeghi et al. / Physica E 11 (2001) 233–239
237
Fig. 3. (a) Peak power output at 150 and 300 K as a function of the number of emitting stages for ∼ 9 m SLQCLs. (b) Room temperature emission characteristics (peak power vs. current, inset spectrum) of a typical 60-period 9:4 m SLQCL. Peak power up to 2.5 W is observed, with a slope eIciency of 1:17 W=A.
Fig. 4. (a) Output power vs. current characteristic of = 9 m Fabry–Perot QCL at 300 K. Inset shows lasing spectra at 3.0 A. (b) Output power vs. current characteristic of = 9 m DFB-QCL at 300 K. Inset shows single mode lasing spectra at 3:0 A.
(limited by package design), with 720 mW peak power observed at 400 K. In order to obtain single-wavelength emission, distributed feedback (DFB) lasers have also been fabricated. Using e-beam lithography, gratings were exposed on top of a = 9 m laser structure with a period of approximately 1:36 m. The pattern was etched into the cap layer just before normal ridge waveguide laser processing began. A comparison of ridges from the same wafer is shown in Fig. 4. The Fabry– Perot laser on the left shows a strong multimode output at high currents. The DFB laser on the right
exhibits a clear single mode emission at the same current. In order to realize 5 m lasers, a strain-balanced MQW structure was employed. Results are very promising, with a record low threshold current density of only 292 A=cm2 at 80 K and a peak power of 2 W for a 20 m by 3 mm cavity. In continuous wave at 80 K, power output reached 400 mW at 0:8 A. The low temperature laser characteristics are highlighted in Fig. 5. The same laser operates up to room temperature with 280 mW of peak power and a threshold current density of only 2:35 kA=cm2 .
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Fig. 5. (a) 80 K pulsed operation of a 5:2 m strain-balanced MQW-QCL. Threshold current density is only 292 A=cm2 with a maximum power of 2 W. (b) 80 K continuous wave operation of the same laser. Power output increases up to 400 mW at 0:8 A.
Highly reQective (HR) mirror facet coating has also been investigated to help increase the amount of usable output from our devices. Using 2 pairs of SiO2 =Y2 O3 , we can achieve reQectivity up to 95%. The quality of the layers are demonstrated by a concomitant reduction in the laser threshold current density. For 9 m lasers at room temperature, the coatings have successfully reduced the threshold current density from 1.95 to 1:8 kA=cm2 . More recently, a = 5 m strain-balanced structure with a HR coating exhibited an 80 K threshold current density of only 250 A=cm2 .
lem, especially at higher duty cycle operation. Heat dissipation is a serious issue, and can be partially alleviated by using advanced processing techniques such as a buried ridge heterostructure and=or epilayer down bonding to a highly thermally conducting heat sink. Another approach that is physically obvious, but technologically challenging, is the use of a lower-dimensional active layer design. The primary source of nonradiative relaxation, as well as heat generation, is polar optical phonon scattering. The use of quantum wire and quantum dot intersubband emitters should theoretically increase the device eIciency by an incredible amount, as the optical phonon relaxation becomes dramatically less probable.
5. Discussion After almost 7 years of development, the QCL has now become a viable source of mid- and far-infrared radiation. Since the start of this research, the threshold current density has come down at least an order of magnitude, while peak power output has increased by an order of magnitude. Unfortunately, the overall goal of a zero threshold laser in this system is not achievable with current technology, even at low temperature. However, our demonstrated threshold current density of only 250 A=cm2 is certainly impressive considering the excited electron lifetime is only a few picoseconds. While a lot of work has been covered, a lot more effort is required to see the full capabilities of the quantum cascade design. Due to the limited eIciency and thick waveguide core, heat buildup is a deGnite prob-
6. Conclusion This paper has been a summary of the technological development and state-of-the-art performance of quantum cascade lasers produced by gas-source molecular beam epitaxy. Room temperature QCL operation has been reported for lasers emitting between 5 –11 m, with 9 –11 m lasers operating up to 425 K. Laser technology for the 3–5 m range takes advantage of a strain-balanced active layer design. We also demonstrate record room temperature peak output powers at 9 and 11 m (2:5 and 1 W, respectively) as well as record low 80 K threshold current densities (250 A=cm2 ) for some laser designs. Preliminary distributed feedback (DFB) results are
M. Razeghi et al. / Physica E 11 (2001) 233–239
also presented and exhibit single mode operation for 9 m lasers at room temperature. Finally, some of the current limitations to laser eIciency have been explored, as well as a means to combat them. Acknowledgements This work is supported by DARPA=US Army contract # DAAD19-99-1-0217. We would like to acknowledge the continued support of Dr. L.N. Durvasula, Dr. H.O. Everitt, Dr. A. Morrish, and Dr. S. Meth. References [1] J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L. Hutchinson, A.Y. Cho, Science 264 (1994) 553.
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[2] S. Slivken, A. Matlis, A. Rybaltowski, Z. Wu, M. Razeghi, Appl. Phys. Lett. 74 (19) (1999) 2758. [3] C.G. Van de Walle, Phys. Rev. B 39 (3) (1989) 1871. [4] J. Faist, F. Capasso, D. Sivco, A. Hutchinson, S. Chu, A. Cho, Appl. Phys. Lett. 72 (6) (1998) 680. [5] C. Sirtori, J. Faist, F. Capasso, D. Sivco, A. Hutchinson, A. Cho, Appl. Phys. Lett. 66 (1996) 3242. [6] S. Slivken, C. Jelen, A. Rybaltowski, J. Diaz, M. Razeghi, Appl. Phys. Lett. 71 (1997) 2593. [7] M. Razeghi, S. Slivken, A. Matlis, A. Rybaltowski, C. Jelen, J. Diaz, LEOS newsletter 12 (6) (1998) 5. [8] A. Matlis, S. Slivken, A. Tahraoui, K.J. Luo, J. Diaz, Z. Wu, A. Rybaltowski, C. Jelen, M. Razeghi, Appl. Phys. Lett. 77 (12) (2000) 1741. [9] A. Tahraoui, A. Matlis, S. Slivken, J. Diaz, M. Razeghi, High performance quantum cascade lasers ( ∼ 11 m) operating at high temperature (T ¿ 425 K), Appl. Phys. Lett. 78(4) (2001) 416. [10] M. Razeghi, presented at the Solid State and Diode Laser Technology Review, Albuquerque, NM, June 5 –8, 2000.
www.elsevier.com/locate/physe
High power 3–12 m infrared lasers: recent improvements and future trends M. Razeghia; ∗ , S. Slivkena , A. Tahraouia , A. Matlisa , Y.S. Parkb a Department
of Electrical Engineering, Center for Quantum Devices, Northwestern University, 2145 N. Sheridan Road, Evanston, IL 60208, USA b O*ce of Naval research, 800 North Quincy Street, Arlington, VA 22217, USA
Abstract In this paper, we discuss the progress of quantum cascade lasers (QCLs) grown by gas-source molecular beam epitaxy. Room temperature QCL operation has been reported for lasers emitting between 5 –11 m, with 9 –11 m lasers operating up to 425 K. Laser technology for the 3–5 m range takes advantage of a strain-balanced active layer design. We also demonstrate record room temperature peak output powers at 9 and 11 m (2:5 and 1 W, respectively) as well as record low 80 K threshold current densities (250 A=cm2 ) for some laser designs. Preliminary distributed feedback (DFB) results are c 2001 Elsevier Science B.V. All also presented and exhibit single mode operation for 9 m lasers at room temperature. rights reserved. PACS: 73.21.Cd Keywords: Quantum cascade laser; Wavefunction engineering; InP-based material; Gas-source molecular beam epitaxy
1. Introduction Compact, high-power lasers in the mid-to farinfrared wavelength range (3–20 m) are in great demand for many laser-based applications areas such as free-space communication, chemical sensing, and medicine. Gas lasers and parametric oscillators are bulky and require precise alignment of specialized optics. The semiconductor laser, on the other hand, ∗
Corresponding author. Tel.: +1-847-491-7251; fax: +1-847467-1817. E-mail address: [email protected] (M. Razeghi).
is inherently compact and durable, with an internal optical cavity and low voltage=current operation. Up until recently, the only commercially available semiconductor laser in the mid- and far-infrared wavelength range has been based on lead-salt technology, which has both low power output and high sensitivity to temperature. The quantum cascade laser (QCL) is a unipolar semiconductor laser [1]. Rather than relying on intrinsic band gaps, quantum eFects are exploited in order to produce designer energy levels and transitions within the conduction band of a complex heterostructure. The beneGt of this approach is a widely
c 2001 Elsevier Science B.V. All rights reserved. 1386-9477/01/$ - see front matter PII: S 1 3 8 6 - 9 4 7 7 ( 0 1 ) 0 0 2 1 0 - 7
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M. Razeghi et al. / Physica E 11 (2001) 233–239
Fig. 1. (a) Schematic of SLQCL active region and (b) miniband dispersion.
variable transition energy dictated primarily by layer thicknesses. Typical emission wavelengths vary from 3.4 –17 m. Further, because the primary limitation to radiative eIciency is based on phonons rather than Auger processes, the device performance is much less sensitive to temperature change than competing technology. The primary requisite is a heterostructure with a large conduction band oFset and well-controlled interfaces. In other words, it now becomes practical to use mature GaAs- and InP-based technology to produce long wavelength emitters. Driven by a number of emerging applications, QCL technology is improving at a phenomenal rate. In this paper, recent, state-of-the-art advances are presented which help establish the QCL as a compact, durable source of 3–12 m laser emission. SpeciGcally, the discussion centers on lasers produced at Northwestern University’s Center for Quantum Devices (CQD) based on InGaAs=AlInAs=InP heterostructures on InP substrates. All laser structures are grown in a single step by gas-source molecular beam epitaxy (GSMBE). 2. Laser design 2.1. Cascade active layers The Grst step in producing a QCL is the proper design of the active layers. Layer thicknesses are carefully chosen to produce the proper transition energies, transition probabilities, and transport
characteristics. The active region is designed with either a multi-quantum well (MQW) or superlattice (SLQCL) basis. Electron relaxation is dominated by polar optical phonon emission. The principle of operation for a MQW-QCL is described in detail in Ref. [1]. A SLQCL [2] uses minibands to enhance transport and provide an eIcient relaxation path. A typical superlattice active region is shown in Fig. 1a. DeGning z as the growth direction, an approximate dispersion for this type of active region is given in Fig. 1b. The electron relaxation is still dominated by optical phonon processes, but the intraminiband relaxation is intrinsically faster than the interminband transition. This causes a buildup of carriers at the superlattice Brillouin zone edge (kz = =d) of the upper subband. The enhanced transport and deep electron reservoir should theoretically give the SLQCL an advantage over the MQW design in terms of maximum current density and operating temperature. The only disadvantage is the maximum emission energy is reduced due to the wide minibands, limiting emission wavelengths in lattice-matched GaInAs=AlInAs structures to ¿ 7 m. Another way to reach shorter wavelength emission is by incorporating strained heterostructures, which intrinsically have higher conduction band oFsets. However, in order to avoid unwanted misGt dislocations, the net strain should be zero. This is accomplished by proper selection of materials and accurate control over layer composition. For the Gax In1−x As=Aly In1−y As QCL, x and y are chosen such that compressive
M. Razeghi et al. / Physica E 11 (2001) 233–239
strain of the Gax In1−x As quantum wells are balanced throughout the entire device with an equal and opposite tensile strain due to Aly In1−y As quantum barriers. The conduction band oFset can be calculated using a method such as model solid theory [3]. Using this technique, the conduction band oFset can be increased at least 50% with near zero net strain, allowing laser emission as low as 3:4 m [4]. Regardless of the speciGc emitting region design, the QCL is designed in such a way as to allow an electron to emit multiple photons while traversing the waveguide core. This is accomplished through use of a carefully designed injector which connects adjacent emitting regions in series. The use of cascaded active regions allows higher power output and a higher optical conGnement factor. The downside is an operating voltage and thermal resistance that increase with the number of stages (active=injector pairs). 2.2. Waveguide design As the quantum cascade laser often emits at longer wavelengths (3–12 m) than traditional semiconductor lasers, special considerations must be made during laser waveguide design. One such consideration is free carrier absorption. Emitted light, instead of being collected, is absorbed by plasma-like oscillations of free carriers in the semiconductor. The absorption coeIcient is proportional to the number of free-carriers and is strongly dependent on the frequency of light. In the mid- to far-infrared, the absorption coeIcient can be experimentally Gtted to give a 2−3 dependence. In order to reduce the loss, the doping should be kept to a minimum wherever the photon density is high. Another consideration that becomes particularly important as the wavelength increases is coupling of the laser TM modes (primary emission modes for an intersubband laser) to the surface plasmon mode [5]. The surface plasmon mode is very lossy, and exists at the interface between the contact metal and the cap layer of the waveguide. In order to reduce the coupling to the surface plasmon mode, we are using a thick cladding and a highly doped cap layer. The cap layer has an extremely low index of refraction. It serves as a plasma mirror, which separates the optical mode from the lossy surface plasmon mode and reduces the overall waveguide loss.
235
2.3. Design summary The above design principles were used to produce a variety of QCLs. Our preliminary eForts were devoted towards long wavelength ( ¿ 7 m) lattice-matched lasers. Both multi-quantum well and superlattice lasers were designed in order to compare performance in actual devices. The goal was to realize room temperature operation from 7– 11 m. In parallel, the doping of the active layers and waveguide was optimized for individual structures. Using this doping structure, we then investigated SLQCL performance in terms of the number of cascade stages. More recently, strain-balanced laser structures have also been utilized. As mentioned, this gives access to shorter emission wavelengths. 3. Experiment All material growth is done using a EPI Mod Gen II GSMBE reactor. This type of growth technique allows a wide variety of heterostructures to be grown, with very accurate control over layer thickness and interfaces. All of the work described here is based on GaInAs=AlInAs=InP heterostructures. Typically, 25 – 60 Gax In1−x As=Aly In1−y As emitting regions are connected in series and surrounded by 0:3 m of low n-doped (8 × 1016 cm−3 ) Ga0:47 In0:53 As to make up the laser waveguide core. The n-InP substrate (1 × 1017 cm−3 ) and InP buFer layer (7×1016 cm−3 ) serve as the lower waveguide cladding. The top cladding is also InP, with n-type doping graded from 1–3 × 1017 cm−3 over 2–3 m. The cap layer is typically a 1 m thick layer of highly n-doped (2 × 1019 cm−3 ) Ga0:47 In0:53 As. Structural quality, layer thickness, and interface smoothness are very important to QCL device performance. Typical X-ray diFraction for lattice-matched and strain-compensated laser structures (Fig. 2) exhibit clear satellite peaks over 4 degrees. X-ray simulation shows excellent agreement between the ideal and as-grown diFraction spectra. Surface quality is conGrmed with atomic force microscopy (AFM). Typical strain-compensated laser structures exhibit an rms roughness of less than 0:2 nm over a 50 m square area.
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Fig. 2. Typical omega=2 theta X-ray diFraction for (a) lattice-matched and (b) strain-balanced quantum cascade lasers. Simulated diFraction plotted along with experimental data shows the excellent epitaxial control and material quality.
The wafers are processed into a ridge waveguide geometry, which is used to conGne the injected current and optical mode along the lateral direction. A 20-m wide ridge is deGned by optical contact lithography and electron cyclotron resonance reactive ion etching (ECR-RIE) for precise control of the ridge proGle. The center-to-center distance between adjacent stripes is 300 m and a 6 –8 m deep groove is etched between the stripes for optical isolation. Our results show that lasers produced with dry etching show extremely uniform performance over large areas. Plasma-enhanced chemical vapor deposition (PECVD) SiO2 is used as insulation between stripes, and Ti=Au is used as the contact metal for both the top surface and substrate. For pulsed operation, the lasers are typically driven with 5-s current pulses with a repetition rate of 200 Hz. Powers are reported per 2 facets for uncoated lasers. 4. Results Much of our initial work, as contracted, was focused on the 8–12 m wavelength region. This led to the Grst demonstration of QCLs grown by GSMBE [6]. During this work we also demonstrated several records for low threshold current density at room temperature [2,7]. The laser structures were grown lattice-matched to InP substrate. By optimizing the doping level it is
possible to come up with a laser that exhibits both a low threshold current density (1:95 kA=cm2 ) and high peak power (700 mW) at room temperature. These results have been published elsewhere for a 9 m laser structure [8]. Impressive room temperature performance has been observed out to 11 m, with over 1 W peak power observed for a 20 m by 3 mm cavity at room temperature [9]. As part of the work that led to Ref. [8], it was found that by decreasing the doping level in the injector from 2:5 × 1017 cm−3 to 1 × 1017 cm−3 , the threshold current density could be further reduced. In fact, at room temperature alone, the threshold current density decreased to 1:8 kA=cm2 and the laser exhibited a T0 of 220 K. The same laser operated in continuous wave up to 160 K without the beneGt of facet coating or epilayer side down heat sinking. In order to test the scalability of the cascade design and epitaxial process, we also varied the number of emitting stages for a series of otherwise identical lasers. As expected, and as shown in Fig. 3a, the peak output power did increase with the number of stages. No saturation was observed up to 60 periods. The output characteristics for a typical = 9:4 m 60 period laser at room temperature are shown in Fig. 3b. 2:5 W of peak power was observed with a threshold current density of 2:2 kA=cm2 and a threshold voltage of 12:4 V. This is the highest reported QCL power at room temperature in this wavelength range [10]. Lasing action persists in pulsed mode up to 425 K
M. Razeghi et al. / Physica E 11 (2001) 233–239
237
Fig. 3. (a) Peak power output at 150 and 300 K as a function of the number of emitting stages for ∼ 9 m SLQCLs. (b) Room temperature emission characteristics (peak power vs. current, inset spectrum) of a typical 60-period 9:4 m SLQCL. Peak power up to 2.5 W is observed, with a slope eIciency of 1:17 W=A.
Fig. 4. (a) Output power vs. current characteristic of = 9 m Fabry–Perot QCL at 300 K. Inset shows lasing spectra at 3.0 A. (b) Output power vs. current characteristic of = 9 m DFB-QCL at 300 K. Inset shows single mode lasing spectra at 3:0 A.
(limited by package design), with 720 mW peak power observed at 400 K. In order to obtain single-wavelength emission, distributed feedback (DFB) lasers have also been fabricated. Using e-beam lithography, gratings were exposed on top of a = 9 m laser structure with a period of approximately 1:36 m. The pattern was etched into the cap layer just before normal ridge waveguide laser processing began. A comparison of ridges from the same wafer is shown in Fig. 4. The Fabry– Perot laser on the left shows a strong multimode output at high currents. The DFB laser on the right
exhibits a clear single mode emission at the same current. In order to realize 5 m lasers, a strain-balanced MQW structure was employed. Results are very promising, with a record low threshold current density of only 292 A=cm2 at 80 K and a peak power of 2 W for a 20 m by 3 mm cavity. In continuous wave at 80 K, power output reached 400 mW at 0:8 A. The low temperature laser characteristics are highlighted in Fig. 5. The same laser operates up to room temperature with 280 mW of peak power and a threshold current density of only 2:35 kA=cm2 .
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Fig. 5. (a) 80 K pulsed operation of a 5:2 m strain-balanced MQW-QCL. Threshold current density is only 292 A=cm2 with a maximum power of 2 W. (b) 80 K continuous wave operation of the same laser. Power output increases up to 400 mW at 0:8 A.
Highly reQective (HR) mirror facet coating has also been investigated to help increase the amount of usable output from our devices. Using 2 pairs of SiO2 =Y2 O3 , we can achieve reQectivity up to 95%. The quality of the layers are demonstrated by a concomitant reduction in the laser threshold current density. For 9 m lasers at room temperature, the coatings have successfully reduced the threshold current density from 1.95 to 1:8 kA=cm2 . More recently, a = 5 m strain-balanced structure with a HR coating exhibited an 80 K threshold current density of only 250 A=cm2 .
lem, especially at higher duty cycle operation. Heat dissipation is a serious issue, and can be partially alleviated by using advanced processing techniques such as a buried ridge heterostructure and=or epilayer down bonding to a highly thermally conducting heat sink. Another approach that is physically obvious, but technologically challenging, is the use of a lower-dimensional active layer design. The primary source of nonradiative relaxation, as well as heat generation, is polar optical phonon scattering. The use of quantum wire and quantum dot intersubband emitters should theoretically increase the device eIciency by an incredible amount, as the optical phonon relaxation becomes dramatically less probable.
5. Discussion After almost 7 years of development, the QCL has now become a viable source of mid- and far-infrared radiation. Since the start of this research, the threshold current density has come down at least an order of magnitude, while peak power output has increased by an order of magnitude. Unfortunately, the overall goal of a zero threshold laser in this system is not achievable with current technology, even at low temperature. However, our demonstrated threshold current density of only 250 A=cm2 is certainly impressive considering the excited electron lifetime is only a few picoseconds. While a lot of work has been covered, a lot more effort is required to see the full capabilities of the quantum cascade design. Due to the limited eIciency and thick waveguide core, heat buildup is a deGnite prob-
6. Conclusion This paper has been a summary of the technological development and state-of-the-art performance of quantum cascade lasers produced by gas-source molecular beam epitaxy. Room temperature QCL operation has been reported for lasers emitting between 5 –11 m, with 9 –11 m lasers operating up to 425 K. Laser technology for the 3–5 m range takes advantage of a strain-balanced active layer design. We also demonstrate record room temperature peak output powers at 9 and 11 m (2:5 and 1 W, respectively) as well as record low 80 K threshold current densities (250 A=cm2 ) for some laser designs. Preliminary distributed feedback (DFB) results are
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also presented and exhibit single mode operation for 9 m lasers at room temperature. Finally, some of the current limitations to laser eIciency have been explored, as well as a means to combat them. Acknowledgements This work is supported by DARPA=US Army contract # DAAD19-99-1-0217. We would like to acknowledge the continued support of Dr. L.N. Durvasula, Dr. H.O. Everitt, Dr. A. Morrish, and Dr. S. Meth. References [1] J. Faist, F. Capasso, D.L. Sivco, C. Sirtori, A.L. Hutchinson, A.Y. Cho, Science 264 (1994) 553.
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[2] S. Slivken, A. Matlis, A. Rybaltowski, Z. Wu, M. Razeghi, Appl. Phys. Lett. 74 (19) (1999) 2758. [3] C.G. Van de Walle, Phys. Rev. B 39 (3) (1989) 1871. [4] J. Faist, F. Capasso, D. Sivco, A. Hutchinson, S. Chu, A. Cho, Appl. Phys. Lett. 72 (6) (1998) 680. [5] C. Sirtori, J. Faist, F. Capasso, D. Sivco, A. Hutchinson, A. Cho, Appl. Phys. Lett. 66 (1996) 3242. [6] S. Slivken, C. Jelen, A. Rybaltowski, J. Diaz, M. Razeghi, Appl. Phys. Lett. 71 (1997) 2593. [7] M. Razeghi, S. Slivken, A. Matlis, A. Rybaltowski, C. Jelen, J. Diaz, LEOS newsletter 12 (6) (1998) 5. [8] A. Matlis, S. Slivken, A. Tahraoui, K.J. Luo, J. Diaz, Z. Wu, A. Rybaltowski, C. Jelen, M. Razeghi, Appl. Phys. Lett. 77 (12) (2000) 1741. [9] A. Tahraoui, A. Matlis, S. Slivken, J. Diaz, M. Razeghi, High performance quantum cascade lasers ( ∼ 11 m) operating at high temperature (T ¿ 425 K), Appl. Phys. Lett. 78(4) (2001) 416. [10] M. Razeghi, presented at the Solid State and Diode Laser Technology Review, Albuquerque, NM, June 5 –8, 2000.