1. Introduction

The mid-infrared (MIR) wavelength (2 – 5 µm) range is rich of interest due to numerous absorption lines of pollutants such as, e.g.,CO₂, CH₄, NH₃,which is very useful for trace gas sensing with Tunable Diode Laser Absorption Spectroscopy (TDLAS) [1]. Wavelengths around 2.3 µm are particularly attractive thanks to strong absorption of alkanes, including methane, and weak absorption of water vapor in this spectral range. GaSb-based materials allow covering this wavelength range by exploiting the efficient GaInAsSb/AlGaAsSb type-I quantum well (QW) system.

The TDLAS technique requires single-mode laser emission and large electro-thermal tunability without mode hops. Distributed feed-back (DFB) diode lasers are the most widespread technology to get such performances but they rely on complex technology [2–4]. Vertical-Cavity Surface Emitting Lasers (VCSELs) fulfill all the requirements and, moreover, exhibit low laser thresholds. MIR GaSb-VCSELs operating in continuous wave (CW) at room temperature (RT) were first optically pumped devices [5–7]. Indeed, unlike GaAs or InP based materials, the realization of electro-optical confinement with oxidation of an Al-rich layer or ion implantation [8] is not possible with GaSb-based materials [9].

Moreover, p-type AlAsSb/GaSb Distributed Bragg Reflectors (DBRs) exhibit poor conductivity and high free-carrier absorption losses [10] which limits electrically-pumped p-n junction VCSELs to pulsed operation at RT [11]. Insertion of a low-resistivity type-III InAs/GaSb tunnel junction (TJ) has been proposed to overcome this issue [12–15]. In the last few years, hybrid semiconductor/dielectric structures using buried tunnel junction (BTJ) processing allowed demonstrating single mode operation in CW up to 75°C around 2.3 µm [16, 17] and up to 50°C around 2.6 µm [18]. Output powers around 300 µW and wavelength tunability around 10 nm have been achieved at 2.3 µm at RT [16–18]. However, processing of these devices is complex and relies on epitaxial re-growth where high temperature is required for oxide desorption. This raises the TJ resistivity [12] and blue-shifts the active-zone emission [19].

A monolithic approach has also been developed using two n-type DBRs and a TJ which allowed fabricating VCSELs operating at RT up to 2.3 µmin CW mode [20] and up to 2.63 µm in pulsed mode [21]. However, this technology was based on large-area etched mesas and lacked efficient electro-optical confinement, leading to multimode emission and high threshold currents. Recently, we have shown the possibility of selective lateral etching of the InAs/GaSb TJ while keeping a low TJ resistivity around 2 x10⁻⁵ Ω.cm⁻² [22]. Such a technique has previously been proved efficient to realize single-mode InP VCSELs [23, 24] operating in CW up to 70°C in the 1.3-1.6 µm wavelength range.

In this letter, we report the technology and characterizations of a single-mode monolithic GaSb-VCSEL emitting CW at 2.3 µm with a current aperture formed by selective lateral wet-etching of the TJ.

2. VCSEL structure and fabrication

The device structure is illustrated in Fig. 1 . The epitaxial stack is grown in a single run by solid source molecular beam epitaxy on an (001) n-doped GaSb substrate. The 3λ/4 cavity contains five 10-nm wide Ga_0.68In_0.32As_0.08Sb_0.92QWs embedded in 15-nm-thick Al_0.35Ga_0.65As_0.03Sb_0.97 barrier layers. The QWs are 1.5% compressively strained and have been designed for an emission wavelength of 2.3 µm. The TJ is positioned at the second standing wave null-position above the QWs in order to reduce free-carrier absorption losses. The TJ is realized with a 10¹⁹cm⁻³ n⁺⁺-InAs/10¹⁹ cm⁻³ p⁺⁺-GaSb heterostructure. Both InAs and GaSb layers are 20nm thick. Amphoteric Si is used as dopant in the TJ for both p-type GaSb and n-type InAs layers.

 

Fig. 1 Schematic diagram of the processed structure, with Φ₁ the external diameter of the etched mesa,Φ₂ the internal diameter of the output VCSEL and Φ₃the effective diameter of the TJ.

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This ensures high doping levels in both layers and prevents dopant interdiffusion at the interface [13, 15]. The active region is embedded between two Te-doped lattice-matched AlAsSb/GaSb DBRs made of 23 and 21 quarter-wavelength pairs for the bottom and the top mirror, respectively. Both AlAsSb and GaSb layer in the DBRs are Te-doped with a concentration of 10¹⁸ cm⁻³ in order to improve their electrical conduction [10].

Device fabrication involved wet etching of the top DBR with aCrO₃:HF:H₂O solution. The InAs layer of the TJ plays the role of an etch-stop layer. Then, InAs is selectively etched with a solution of citric acid and hydrogen peroxide to form the thin air-gap aperture. The process set-up for the lateral etching of the TJ is described in ref [22]. Figure 2 presents a cross-section scanning electron microscope (SEM) picture of a selectively etched TJ in a monolithic VCSEL structure. Deep lateral etching can be achieved.

 

Fig. 2 Cross-section SEM picture of selectively etched InAs/GaSb tunnel-junction in a monolithic GaSb-VCSEL.

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Devices with 35 µm pillar diameter (Φ₁) and a top aperture of 25 µm (Φ₂) have been fabricated. Using the method described above to form the aperture, we realized VCSELs with a 6µm TJ effective diameter (Φ₃). After the lateral etching step of the TJ the devices were passivated with the AZ4533photoresistand annealed in an oven to form a solid passivation layer. Before metallization, the samples were etched with HCl (1:2) in order to remove native oxide. The top ring contact is made with sputtered Pd/Au/Ge/Ni. The substrate was thinned down to 300 µm and Au/Ge/Ni back contact was deposited on the GaSb substrate. The whole sample was then annealed around 200°C to form top and bottom ohmic contacts [25].

3. Results and discussions

Typical light-current (L-I) and voltage-current (V-I) characteristics obtained in CW at various temperatures are reported in Fig. 3 . CW operation is obtained up to a heat-sink temperature as high as 70°C.

 

Fig. 3 L-I and V-I characteristics (same color code) taken at various temperatures in CW for a monolithic GaSb-VCSEL with a 6 µm TJ effective diameter.

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The maximum output power is around 100 µW. This rather low value can partly be explained by the very high reflectivity of the top DBR. Indeed, the calculated reflectivity is around 99.8%, taking into account 7 cm⁻¹ optical absorption losses in n-type DBR [11].The I-V characteristic at 20°C exhibits a turn-on voltage of 3V. This value which is twice the value previously obtained with large-area monolithic VCSELs can be attributed to the narrow aperture of the TJ combined with the total thickness of the structure (~16 µm). Such an increase of the turn-on voltage has already been observed with InP-based apertured TJ VCSEL [27]. It also limits the output power as we discuss later in the paper.

Figure 4 presents the variation of the threshold current with the temperature. The gain shifts faster with the temperature than the microcavity resonance which results in this parabolic variation of the threshold current with the temperature. Notice that the threshold current is almost constant in the 10 – 40°C temperature range. Still, a minimum value of 1.9 mA is observed between 30°Cand 40°C which is thus the range of temperature at which the modal gain is maximum [27, 28].

 

Fig. 4 Threshold current versus temperature of a monolithic GaSb-VCSEL with a 6 µm TJ effective diameter.

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Figure 5 presents CW laser emission spectra (measured with a FTIR) taken at 20°C under various drive currents. The laser exhibits single-mode emission with a Side Mode Suppression Ratio (SMSR) around 25 dB in the whole range of drive current. Single-mode emission is also achieved in the whole temperature range up to 70 °C (Fig. 5). Single-mode emission arises from narrow TJ effective diameters [29]. VCSELs with TJ effective-diameter larger than 8 µm exhibit multimode emission, as also reported in Ref [29].

 

Fig. 5 Laser emission spectra taken at 20 °C under different CW drive currents for a monolithic GaSb-VCSEL with a 6 µm TJ effective diameter.

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Figure 6 reports the wavelength tunability of the VCSEL as a function of the drive current and heat-sink temperature. The wavelength shifts at a rate of 2.7 nm/mA at constant heat-sink temperature (Fig. 6(a)) and at a rate of 0.21 nm/K at constant drive current (Fig. 6(b)).These electro-thermal effects allow shifting the laser emission continuously in a wavelength range as large as 14 nm without mode hop. This wavelength agility demonstrates that such devices are well suited to scan several gas absorption lines as required for TDLAS applications.

 

Fig. 6 Wavelength tunability of the VCSEL with 6 µm TJ effective diameter: (a) evolution of the emitted wavelength with drive current at different temperatures (20, 30, 40, 50 and 60 °C); (b) evolution of the emission wavelength with heat-sink temperature at different drive currents (3, 4 and 5 mA).

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To estimate the thermal resistance of the device, we calculated a power tenability of 0.47 nm/mW, taking into account the applied voltage. The thermal resistance for a constant temperature and an electrical power is given by the following relation [8]: $R_{th}=\frac{\Delta \lambda /\Delta {\rm P}}{\Delta \lambda /\Delta {\rm T}}$

We deduced a thermal resistance of 2240 K/W, which is comparable to the value obtained for the same diameter with buried-TJ VCSELs emitting in the same wavelength range [17]. This explains also that we observe similar wavelength tunability.

Based on this calculated value, we estimate the temperature rise in the active region of the device to be around 20 K at threshold and 65 K at the thermal rollover which appears only 4 mA above threshold (Fig. 3). The rapid occurrence of the thermal rollover arises from the high voltage (~4 V) at laser threshold and high series resistance of DBRs. This limits the maximum operating temperature and, together with the high top-DBR reflectivity, the output power which is a factor of ~4 lower than that of BTJ VCSELs at 2.3 µm and 20 °C [16–18]. To increase the output power it will be necessary to reduce both the turn-on voltage and serial resistance of the devices. Several ways can be explored, such as improving the InAs/GaSb TJ resistivity [15] or developing intra-cavity contacts in order to avoid driving the current through the DBRs.

3. Conclusion

In this paper, we have reported the fabrication and the characterization of a monolithic GaSb VCSEL emitting at 2.3 µm with selective lateral etching of the TJ. This process allows simultaneous electrical and optical confinement. A small TJ effective diameter of 6 µm allowing low current threshold has led to CW operation up to 70°C. Identified limitations of this device are high turn-on voltage and serial resistance which reduce the operating current range and the output optical power. Improvement of the TJ and development of intracavity contact should allow reaching higher output powers. Single mode operation in the whole range of current and temperature is demonstrated for the first time with electrically-pumped monolithic GaSb VCSELs. This work shows that this technology is viable for developing mid-IR photonic devices and systems.