1. Introduction

Since the beginning of research on interband cascade lasers (ICL) in the 90s, InAs/AlSb superlattices (SL) were proposed and implemented [1] 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]. 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) [3,4] and ICLs [5]. Quaternary AlGaAsSb cladding layers with high aluminum content are commonly used in GaSb based mid-infrared diode lasers [6]. 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]. 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 J_th = 600 A/cm² for a 2 mm long broad area device operated at 20 °C [8]. Other concepts incorporate InAsN/GaAsSb [9] or InGaAs/GaAsSb [10] “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 Al_0.85Ga_0.15As_0.07Sb_0.93 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], 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. The active region consists of 5 stages (see Fig. 2 for details), sandwiched between two 220 nm thick separate confinement layers (SCL) made out of GaSb.

 

Fig. 1 Refractive index and optical mode profile for an ICL with InAs/AlSb-Cladding (black line) and quaternary Al_0.85Ga_0.15As_0.07Sb_0.93 Cladding (red line). The structure incorporates 5 active cascades and two 220 nm thick GaSb separate confinement layers.

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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.

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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 Al_0.85Ga_0.15As_0.07Sb_0.93 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.

It is built around a W-quantum well (W-QW) and contains 5 electron injector quantum wells. The four wells next to the W-QW were heavily n-doped to 5x10¹⁸ cm⁻³ in order to rebalance the electron and hole density, which has proven to strongly improve the laser performance [12]. One cascade comprises the following layer sequence: 2.5 nm AlSb/1.73 nm InAs/3.0 nm Ga_0.65In_0.35Sb/1.37 nm InAs/1.0 nm AlSb/2.7 nm GaSb/1.0 nm AlSb/4.6 nm GaSb/2.5 nm AlSb/4.2 nm InAs/1.2 nm AlSb/3.2 nm InAs/1.2 nm AlSb/2.4 nm InAs/1.2 nm AlSb/1.7 nm InAs/1.2 nm AlSb/1.7 nm InAs. The laser transition energy is 350 meV, which corresponds to a wavelength of 3.5 µm. Immediately after the growth of the active region, the substrate temperature was ramped up to 485°C for the growth of the adjacent GaSb SCL. Another LDG served as a connection to the quaternary upper cladding which was grown at a substrate temperature of 500 °C. After another LDG the growth was finalized with a 50 nm thick heavily doped (6x10¹⁸ cm⁻³) GaSb layer. Figure 3 shows a high resolution X-ray diffraction (HR-XRD) spectrum ([004] direction) of the grown structure. The GaSb substrate peak and the peak arising from the quaternary cladding are indicated with arrows. The close spacing (0.0371°) between those peaks indicates almost perfect lattice matching of the cladding layers to the substrate. This is also confirmed by the mirror like surface of the sample, without dislocation stripes that typically occur if the cladding layers are too strained. The satellite peaks are associated with the periodicity of the cascaded active region. They can be observed up to the 15th order which indicates a high crystal quality and well defined interfaces. From the spacing of those satellite peaks the thickness of one cascade can be determined to 40 nm which is almost identical with the design value.

 

Fig. 3 HR-XRD spectrum ([004] direction) of the grown structure. The peak corresponding to the quaternary cladding layer is located on the left of the GaSb substrate peak. The close spacing indicates almost perfect lattice matching. Satellite peaks on both sides of the substrate peak are associated with the periodicity of the cascaded active region and correspond to a cascade length of 40 nm.

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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 shows the I-V-P characteristics of the device. The threshold current density (J_th) 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]. 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] 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 J_th of 290 A/cm², whereas in ICLs with SL-claddings it is known that J_th 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.

 

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.

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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 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 J_th was 282 A/cm².

 

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.

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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 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.

 

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.

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5. Conclusion

An alternative cladding material was successfully implemented into the waveguide structure of interband cascade lasers. Due to its low refractive index Al_0.85Ga_0.15As_0.07Sb_0.93 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.