1.
Introduction

Intersubband quantum cascade lasers (QCLs) based on InGaAs/InAlAs heterostructures have achieved remarkable device performances such as high output power, high efficiency, and continuous wave (cw) operation above room temperature at wavelengths longer than 4 μm^[1–3]. It is known that the shortwave mid-infrared spectral region in 3–4 μm is of special significance since the fundamental C–H, N–H, and O–H stretching modes have resonances in this region^{[4, 5]}. In order to lase in this region, the band offset of the InGaAs/InAlAs heterojunction in a QCL has to be enlarged, for example, by growing highly strained InGaAs/InAlAs superlattices (SLs)^{[6, 7]}, which is a challenge in material growth by molecular beam epitaxy (MBE). In comparison, the emission wavelength of an interband cascade laser (ICL) based on the type-II “W” shape InAs/GaInSb/InAs quantum well active region can be easily tailored to the range of 3–4 μm. Since being proposed in 1994 by Yang^[8], GaSb-based ICLs using InAs/AlSb SLs lattice-matched to the GaSb substrate as the optical cladding layers have been demonstrated in 3–4 μm for continuous wave operation above room temperature with low threshold current density by several groups^[9–12]. Meanwhile, InAs-based ICLs employing heavily doped InAs layers as the plasmon waveguides have also been demonstrated. The operation wavelengths have covered a wide spectral range from 3.3 to 11 μm^[13–18]. It is obvious that the growth of the plasmon bulk InAs waveguides is easier than the growth of the SL waveguides used in GaSb-based ICLs, since it is difficult to form atomically abrupt interfaces when the two constituent materials such as InAs and AlSb share neither common cations nor anions. Moreover, frequent shutter movement and possible strain accumulation during the growth of thick SL layers are difficult to control. The doping concentration of the plasmon InAs waveguides should be higher than 1 × 10¹⁹ cm^?3 to confine the shortwave mid-infrared optical modes at the center active core. Such highly doped InAs layers have a high absorption loss, which is detrimental to the performance of InAs-based ICLs. Recently, Li et al.^[14] achieved cw operation above room temperature near 4.8 μm in InAs-based ICLs by introducing an InAs/AlSb SL intermediate cladding layer. In this letter, we report InAs-based ICLs emitting in the 3–4 μm region using the InAs plasmon or the InAs/AlSb SL waveguides. The devices with SL waveguides have achieved room temperature pulsed operation near 4 μm.

2.
Experimental procedure

Two ICL structures with different optical cladding layers were grown using a Riber compact 21T MBE system on n-type InAs (001) substrates. Fig. 1 shows the two ICL structures. For growth of wafer C400, a 1.6-μm-thick heavily doped InAs (1 × 10¹⁹ cm^?3) was deposited as the bottom optical cladding layer. Then a 1.2-μm-thick nominally undoped InAs spacer layer was grown to prevent the impurities in the heavily doped bottom cladding layer from diffusing into the cascade region. The cascade region has 12 stages. Each stage consists of a type-II “W” shape InAs/Ga_0.7In_0.3Sb/InAs quantum well active region, a GaSb/AlSb hole injection region and a chirped InAs/AlSb SL electron injection region, as shown in Fig. 2. Then the top InAs spacer of 1.2 μm and the top InAs n⁺⁺ cladding of 1.1 μm thick were deposited subsequently.






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Figure2.
(Color online) Calculated band diagram for one cascade stage. The layer structure starting with the barrier separating the active region and the electron injector is as follows: 25 ? AlSb/21.5 ? InAs/29 ? Ga_0.7In_0.3Sb/19.5 ? InAs/12 ? AlSb/32 ? GaSb/12 ? AlSb/48 ? GaSb/21 ? AlSb/43 ? InAs/12 ? AlSb/33 ? InAs/12 ? AlSb/27 ?.






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Figure1.
Schematic diagram of two ICL structures with different waveguides.




Wafer C582 has 12 similar cascade stages with small adjustments in layer thicknesses. For changes, the thicknesses of both InAs spacer layers were reduced to 0.5 μm and the bottom (top) InAs n⁺⁺ layer was reduced to 0.6 μm (50 nm). Meanwhile, two 150-period 25 ? InAs/23 ? AlSb SL layers were inserted between the spacer and the InAs n⁺⁺ layers as optical cladding layers. The total epilayer thickness of wafer C400 is ~ 5.1 μm, which is reduced to ~ 3.1 μm for wafer C582. The epitaxial wafers were characterized by double crystal X-ray diffraction (DCXRD) measured with a Bede D1 high resolution X-ray diffractometer.



In a typical device process, a wafer was processed into ridge-waveguide lasers by standard photolithography and wet chemical etching. A 200-nm-thick SiO₂ layer was formed by plasma-enhanced chemical vapor deposition for electrical isolation. An electrical injection window of 3-μm-wide was opened on top of the ~ 14 μm wide ridge. For a 70-μm-wide ridge, the electrical injection window is 35 μm wide. The top contact of 40 nm/250 nm Ti/Au was deposited by electron-beam evaporation. An additional 5-μm-thick Au layer was electroplated for heat dissipation. Then the substrate was mechanically thinned to 120 μm and a bottom AuGeNi/Au contact was deposited by thermal evaporation. The processed wafers were typically cleaved into lasers with lengths from 1.5 to 5.0 mm, mounted epilayer-side down on copper heat sinks with indium solder and placed on the cold finger of a liquid nitrogen cooled cryostat with CaF₂ windows for electrical and spectral characterizations. For the pulsed measurements, the applied pulse width was 1 μs at a repetition rate of 5 kHz.

3.
Experimental results and discussion

The DCXRD pattern of wafer C400 with thicker InAs spacer and InAs cladding layers is shown in Fig. 3. The zeroth order satellite peak of the epilayer is almost overlapped with the InAs substrate, indicating negligible lattice mismatch between the epilayer and the substrate. As the total thickness of thecascade core is only ~ 0.5 μm, about one tenth of the entire laser epilayer structure, the strain accumulated in the SLs has a relatively small effect on the crystal quality of the entire epitaxial layer, which is an advantage of InAs-based ICL in growth. The typical full width at half maximum (FWHM) of the satellite peaks is 40 arcsec, which is close to our InGaAs/InAlAs superlattices matched to InP substrates. For comparison, Fig. 4 shows the DCXRD pattern of wafer C582 where two InAs/AlSb SL layers with total thickness of 1.5 μm were inserted as the optical cladding layers. As the thickness of InAs/AlSb SLs is about half of the entire laser structure, the interfaces of the SL structures were designed carefully to achieve strain balance. It is noted that even with a thick SL cladding layer the zeroth order peak still coincides with that of the substrate and the FWHM of the satellite peaks of C582 is the same as that of C400, implying good periodic arrangement of the superlattice.






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Figure4.
DCXRD patterns of wafer C582 with thinner InAs spacer layers and InAs/AlSb SL cladding layers.






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Figure3.
DCXRD patterns of wafer C400 with thicker InAs spacer and cladding layers.




Lasers made from wafer C400 with thick InAs n⁺⁺ cladding layer can operate in cw mode up to 210 K. Fig. 5 shows the cw lasing spectra from a 14.5-μm-wide and 2-mm-long device. The lasing wavelength changed from ~ 3.3 to ~ 3.6 μm when the temperature was varied from 85 to 200 K, similar to the sample R080 from Ref. [13]. The current–voltage–power (I–V–P) characteristics are shown in Fig. 6. The threshold current density at 85 K is 124 A/cm², which is higher than the 5.3 μm InAs-based plasmon waveguide ICLs^[15]. The cw output power of this 14.5-μm-wide device is 60 mW at 85 K with an injection current of 200 mA. The highest operation temperature in pulsed mode is about 255 K.






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Figure6.
(Color online) Measured I–V–P characteristics of a narrow ridge laser (14.5 μm × 2 mm) from wafer C400 in cw mode.






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Figure5.
(Color online) Temperature dependent emission spectra of a narrow ridge laser (14.5 μm × 2 mm) from wafer C400 in cw mode.




It is known that the heavily doped InAs plasmon waveguide can confine the optical wave due to its smaller refractive index than the active cascade core, which is more advantageous in long wavelength infrared lasers. While for wafer C400 emitting at around 3.6 μm, the refractive index contrast between the active core and the InAs plasmon cladding is rather small, thus the confinement factor is small. On the other hand, the waveguide loss is large due to heavily doping, which results in low operation temperatures of the lasers from wafer C400. In order to increase the operation temperature, we inserted 150-period InAs/AlSb SLs as optical cladding layers in wafer C582 and reduced the thickness of undoped InAs spacer and InAs n⁺⁺ layers. The refractive index and the calculated optical mode profiles along the growth direction for the two wafers are shown in Fig. 7. The confinement factors (Γ) of the active region for wafers C400 and C582 are about 27% and 37%, respectively.






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Figure7.
(Color online) Optical mode and refractive index profiles of wafers C400 and C582.




Fig. 8 shows the lasing spectra from a 70-μm-wide and 3-mm-long laser made from wafer C582 with an InAs/AlSb SL cladding layer, which can operate up to room temperature in pulsed mode. The emission peak red shifted by 1.8 nm/K from ~ 3.95 μm at 170 K to ~ 4.04 μm at 220 K at first, then blue shifted to ~ 3.99 μm at 240 K, which red shifted further by 1.2 nm/K to ~ 4.06 μm at room temperature. Such a blue shift has also been observed in the 5.3 μm InAs-based plasmon waveguide ICL^[15] at 280 K and was ascribed to band filling effects at high temperature. Room temperature pulsed operation of this device indicates that the InAs/AlSb SL cladding incorporated into wafer C582 enhanced the optical confinement significantly, consistent with the observations of Li et al.^[14].






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Figure8.
(Color online) Temperature dependent emission spectra of a wide ridge laser (70 μm × 3 mm) from wafer C582 in pulsed mode.




Fig. 9 shows the I–V–P curves of a narrow ridge device (18 μm × 3 mm) from wafer C582. The threshold current density at 77 K is 37 A/cm², which is greatly reduced as compared to wafer C400 even though the thick SL cladding in wafer C582 has small thermal conductivity. The cw output power is 230 mW at 77 K with an injection current of 400 mA, and cw operation can be observed up to 230 K.






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Figure9.
(Color online) Measured I–V–P characteristics of a narrow ridge laser (18 μm × 3 mm) from wafer C582 in cw mode.

4.
Summary

InAs-based ICLs emitting at wavelengths below 4.1 μm at room temperature have been demonstrated by using an InAs/AlSb SL cladding layer. The threshold current densities of these lasers were 37 A/cm² at 77 K in cw mode. The operation temperature reached room temperature in pulsed mode. Compared with the thick InAs n⁺⁺ plasmon cladding layer commonly used in InAs-based ICLs, the InAs/AlSb superlattice cladding layers have greater advantages for wavelengths less than 4 μm because in the short-wavelength region they have higher confinement factor of the active regions than InAs plasmon waveguides.

Acknowledgement

The reported laser structure of wafer C400 was designed by Prof. R. Q. Yang at the University of Oklahoma. The authors are grateful to Dr. Yang and Dr. Lu Li for their fruitful discussions. They would also like to thank P. Liang and Y. Hu for their technical support in the device processing.