Long-wavelength lasers emitting in the mid-infrared are attractive light sources for gas-sensing applications using tunable diode-laser absorption spectroscopy (TDLAS), due to strong absorption bands in the mid-infrared [1]. For such spectroscopy systems, GaSb-based laser diodes are ideal candidates, as they can cover the wavelength range from 2 to 4 μm and have demonstrated the highest performance in the 2 to 3 μm wavelength range, most notably in terms of high output power and low threshold current densities [2]. Particularly, the wavelength region around 3 μm is of high interest because many technologically important gases (e.g., HCN, ${\mathrm{PH}}_3$, ${\mathrm{NH}}_3$, ${\mathrm{N}}_2\mathrm{O}$, ${\mathrm{C}}_2{\mathrm{H}}_2$) have strong absorption lines in this range, as can be extracted from HITRAN database [3]. Hitherto, research carried out on GaSb-based materials is concentrated on edge-emitting lasers [4–6]. For TDLAS applications, however, vertical-cavity surface-emitting lasers (VCSELs) are of particular interest because they are inherently longitudinally single-mode and have a more convenient circular beam shape as compared with edge-emitting lasers and lower power consumption, allowing them to be adopted in compact and portable systems [7]. Furthermore, VCSELs have wider tuning range and higher modulation frequency (FM tuning) compared with edge-emitting distributed feedback lasers, resulting in superior sensing performance when used in TDLAS systems [8]. On the other hand, the VCSEL performance strongly depends on the material parameters such as the refractive index and the absorption, which are less known for the mid-infrared semiconductor material systems, making design and fabrication of mid-infrared VCSELs challenging. Prior to this publication, electrically pumped GaSb-based VCSELs were reported for wavelengths up to 2.6 μm [9,10]. At longer wavelengths, only optically pumped devices were achieved, e.g., optically pumped semiconductor disk lasers at 2.8 μm [11] and optically pumped VCSELs at 2.9 μm [12]. In this Letter, we report the first electrically pumped VCSELs with an emission wavelength of 3 μm.

Figure 1 illustrates the optical intensity distribution in the device. The VCSEL consists of a 24-pair epitaxial ${\mathrm{AlAs}}_{0.08}{\mathrm{Sb}}_{0.92}/\mathrm{GaSb}$ bottom distributed Bragg reflector (DBR) and an amorphous Ge/ZnS top (outcoupling) DBR, surrounding the cavity. The cavity is dominated by $n\text{-}\mathrm{GaSb}$, modulation doped in the range from $1\times {10}^{17}{\mathrm{cm}}^{-3}$ to $2\times {10}^{18}{\mathrm{cm}}^{-3}$ with doping maxima in the nodes of the optical E-field. Due to high refractive index contrast of Ge and ZnS ($\mathrm{\Delta }n=2.02$), only five pairs are necessary to reach the reflectivity of 99.8% for the top DBR.

 

Fig. 1. Optical field intensity distribution overlaid with the refractive index profile inside the VCSEL (CL: contact layer; AR: active region; BTJ: buried tunnel junction).

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In previous GaSb-based VCSELs with a doped bottom DBR and a metal contact at the bottom of the substrate [9,10,13,14], the maximum achievable reflectivity of the epitaxial DBR is limited by the comparatively strong free-carrier absorption at such long wavelengths [13]. Therefore, even a very high number of mirror pairs do not increase the reflectivity of the doped DBR; for example, for homogeneous doping of $n=1\times {10}^{18}{\mathrm{cm}}^{-3}$, the reflectivity cannot be increased beyond 99.0%. To avoid this, the VCSEL concept with the undoped AlAsSb/GaSb DBR was developed. In this, only 24 pairs are needed to reach 99.94% reflectivity. The measured reflectivity spectra of the undoped DBR are in good agreement with the simulation; therefore, it can be concluded that the high reflectivity is also achieved experimentally. Table 1 shows a comparison of epitaxial DBRs from different laser designs. The calculated reflectivity shows that, for designs with the doped epitaxial DBR, the bottom mirror has three to five times higher optical losses than the outcoupling mirror. This causes a significant contribution to the total optical loss and leads to small external efficiency.

Table 1. Different AlAsSb/GaSb DBR Designs at 3 μm^a

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Due to the undoped DBR, current cannot be extracted through the bottom DBR and the substrate; therefore, an $n\text{-}{\mathrm{InAs}}_{0.91}{\mathrm{Sb}}_{0.09}$ ($n=1\times {10}^{19}{\mathrm{cm}}^{-3}$) current extraction layer is placed between the bottom DBR and the cavity. The $n\text{-}\mathrm{InAsSb}$ at the same time is a good contact layer (CL) for low resistive contacts [19]. It is placed in the first node of the field after the bottom DBR, to keep optical losses low. The resulting modal loss in the bottom contact layer is $0.15{\mathrm{cm}}^{-1}$, and the mirror loss of the undoped epitaxial DBR is $0.75{\mathrm{cm}}^{-1}$. The total loss in the device is $11.22{\mathrm{cm}}^{-1}$. On the other hand, for the doped DBR with 99.4% reflectivity, the mirror loss would be $7.61{\mathrm{cm}}^{-1}$.

The active region, consisting of six 10 nm thick GaInAsSb quantum wells separated by 8 nm thick AlGaInAsSb barriers, is placed in the antinode in the center of the cavity. The active region composition and strain are similar to the ones described in [20]. Current confinement is achieved by a buried tunnel junction (BTJ), which consists of a highly doped $p\text{-}\mathrm{GaSb}/n\text{-}{\mathrm{InAs}}_{0.91}{\mathrm{Sb}}_{0.09}$ ($n$, $p\sim 2\times {10}^{19}{\mathrm{cm}}^{-3}$) tunnel junction in the center of the laser, surrounded by a blocking $p\text{-}\mathrm{GaSb}/n\text{-}\mathrm{GaSb}$ diode. This not only effectively confines the current but also reduces the thickness of the $p$-doped material. This is beneficial for the electrical as well as optical losses because $p$-doped material has a higher optical absorption and lower conductivity, compared with $n$-doped material with the same doping. The BTJ is placed in the first node next to the active region, to not cause excess optical losses. More detailed information on this type of BTJ can be found in [13].

The device structure was grown in two steps on a Varian modular GEN II molecular beam epitaxy (MBE) system equipped with solid source cells. In the first step, the structure was grown from the epitaxial DBR to the BTJ. After this, the InAsSb was selectively etched, leaving only a small disk for current confinement and subsequently overgrown by the MBE, completing the cavity. The structure was completed with an $n\text{-}{\mathrm{InAs}}_{0.91}{\mathrm{Sb}}_{0.09}$ ($n=1\times {10}^{19}{\mathrm{cm}}^{-3}$) top CL. The thickness of the second epitaxial run was also adjusted to correct for minor deviations of the first run and to achieve the desired detuning of the active region and the cavity of approximately 6 meV.

After the second epitaxy, the circular mesas were defined by dry chemical etching using ${\mathrm{SiCl}}_4$ plasma, stopping in the $p$-doped layer between the BTJ and the active region. This step ensures that BTJs of different devices are not interconnected by conducting layers above and, therefore, prevents leakage currents. Next, a U-shaped structure, for back contacting, was etched around the mesa down to the bottom CL. This was done in combination of dry chemical etching and selective wet chemical etching. The devices were passivated using ${\mathrm{Si}}_3{\mathrm{N}}_4$. After uncovering both top and bottom contact layers, Ti/Pt/Au metal contacts were evaporated. Finally, the top Ge/ZnS DBR was evaporated. The final structure of the device is schematically shown in Fig. 2.

 

Fig. 2. Schematic structure of the GaSb-based VCSEL with undoped bottom DBR and intracavity contact.

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After fabrication, the devices were characterized using a thermoelectrically controlled stage with two needle contacts. The light output was focused on a thermoelectrically cooled InAs detector using off-axis parabolic mirrors. The power-current-voltage characteristics in continuous wave (CW) were measured with a Keithley digital multimeters. For pulsed measurements, a digital oscilloscope with multiple channels was used.

Figure 3 shows the optical output power as a function of current for different heatsink temperatures in CW (the BTJ diameter $D_{\mathrm{BTJ}}$ is 8 μm). The device is operating up to 5°C and exhibits lowest threshold currents at around $-10°\mathrm{C}$ in CW (as can be seen in Fig. 4). At this temperature, the cavity resonance overlaps optimally with the gain peak. The threshold current at this point is only 2.05 mA. This corresponds to the current density of $4.1\mathrm{kA}/{\mathrm{cm}}^2$. At 5°C, the threshold current density increases to $4.6\mathrm{kA}/{\mathrm{cm}}^2$. Both values are considerably lower than the ones so far reported for GaSb-based VCSELs with the emission wavelength of 2.6 μm ($9.8\mathrm{kA}/{\mathrm{cm}}^2$ at $-20°\mathrm{C}$ for 6 μm device in CW [9], $6.5\mathrm{kA}/{\mathrm{cm}}^2$ to $8.8\mathrm{kA}/{\mathrm{cm}}^2$ at room temperature for 25 μm device in quasi-CW [10]), which is primarily due to high reflectivities of both DBRs. The peak quantum efficiency of the device is ranging from $0.037\mathrm{W}/\mathrm{A}$ at $-40°\mathrm{C}$ down to $0.016\mathrm{W}/\mathrm{A}$ at 5°C. The maximum output power is 70 μW at $-40°\mathrm{C}$, this low value can be attributed to the high reflectivity of the outcoupling mirror and to the high voltage drop (around 3.8 V at threshold), which causes strong self-heating in the device. Therefore, the VCSELs are operating up to 50°C in pulsed mode with lowest threshold currents around 0°C. This confirms that the VCSEL performance is strongly deteriorated by self-heating.

 

Fig. 3. Power-current-voltage characteristic of a VCSEL at different temperatures in CW ($D_{\mathrm{BTJ}}=8\mathrm{\mu m}$, 5° temperature steps).

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Fig. 4. Pulsed and CW threshold current ($I_{\mathrm{th}}$) as a function of heat-sink temperature for a VCSEL with $D_{\mathrm{BTJ}}=8\mathrm{\mu m}$. The minimum threshold current is found around $-10°\mathrm{C}$ in CW (2.05 mA) and around 0°C in pulsed mode, respectively.

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Spectral characteristics of the VCSELs were measured using a Bruker Vertex 70 FTIR spectrometer. Figure 5 shows a typical emission spectrum of a device at thermal rollover in CW for $-15°\mathrm{C}$ heat-sink temperature. It is lasing single mode at 2.97 μm wavelength, with the side-mode suppression ratio to the next transverse mode of 30 dB. The devices show polarization along $〈110〉$ crystal direction. It should be noted, however, that no particular techniques, such as gratings or ellipticity, were applied to achieve a stable polarization.

 

Fig. 5. Spectrum of a VCSEL in CW at $-15°\mathrm{C}$ and 4.2 mA. BTJ diameter is 8 μm.

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One of the important laser parameters for TDLAS is the tuning range. The device offers 19.7 nm of continuous mode-hop-free current tuning at $-40°\mathrm{C}$ heat-sink temperature, as evident from Fig. 6. These spectra were obtained by increasing the current from 2.5 to 7 mA. This is, to our knowledge, the largest reported electro-thermal tuning range for monolithic VCSELs reported so far. Furthermore, tuning coefficients of $0.285\mathrm{nm}/\mathrm{K}$ and $0.639\mathrm{nm}/\mathrm{mW}$ were extracted for temperature and power tuning, respectively. From these values thermal resistance $R_{\mathrm{th}}=2.211\mathrm{K}/\mathrm{mW}$ can be extracted. This value is similar to the ones reported previously for GaSb VCSELs [9,14].

 

Fig. 6. Current-dependent single-mode laser spectrum of a VCSEL with $D_{\mathrm{BTJ}}=8\mathrm{\mu m}$ at $-40°\mathrm{C}$ heat-sink temperature. The current is varied from 2.5 to 7 mA.

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Although the output power might still be rather low, the high wavelength tuning range makes this type of laser a promising device for simultaneous sensing of multiple gases. The wavelength fits perfectly to the optical absorption spectra of hydrogen cyanide (HCN) and ammonia (${\mathrm{NH}}_3$), which are both toxic with permissible exposure limits of 10 and 50 ppm, respectively [21].

In summary, the first GaSb-based electrically pumped VCSEL emitting in the mid-infrared range around 3 μm was presented. For this VCSEL, a concept with the undoped epitaxial AlAsSb/GaSb bottom DBR and the intracavity back contact was shown. This device concept is promising for long wavelength VCSELs because it considerably reduces optical losses otherwise produced by the doped epitaxial mirror. The presented VCSELs exhibit the lowest CW threshold current of 2.05 mA, corresponding to $4.1\mathrm{kA}/{\mathrm{cm}}^2$, at $-10°\mathrm{C}$. They operate up to 5°C in continuous wave and up to 50°C in pulsed mode. The devices also have a continuous mode-hop-free electro-thermal tuning range of 19.7 nm at $-40°\mathrm{C}$. Because they are suffering from strong self-heating, it is expected that adjustments of the VCSEL design, especially reducing series resistance by an improved doping profile, will significantly improve the device performance leading to higher output powers. This will make the VCSEL highly useful for the sensitive detection of hazardous gasses like HCN and ${\mathrm{NH}}_3$.