2-µm WAVELENGTH RANGE InGa(Al)As/InP-AlGaAs/GaAs WAFER FUSED VCSELs for SPECTROSCOPIC APPLICATIONS

Alexandru Mereuta, Vladimir Iakovlev, Andrei Caliman, Alexei Syrbu, Alok Rudra and Eli Kapon

Ecole Polytechnique Federale de Lausanne (EPFL), Laboratory of Physics of Nanostructures,

CH-1015 Lausanne, Switzerland e-mail: alexandru.mereuta@epfl.ch

Abstract—We demonstrate 2-µm wavelength, wafer-fused InGa(Al)As/InP-AlGaAs/GaAs vertical-cavity surface-emitting lasers (VCSELs) emitting single-mode power of 0.5 mW at room-temperature with a threshold current of 4mA and side-mode suppression ratio of over 30 dB. Emission wavelength can be continuously tuned with current by ~5 nm without mode hopping with a tuning rate of 0.31 nm/mA. These features demonstrate the long wavelength VCSELs potential for gas sensing and other optical spectroscopy applications.

Keywords- Vertical surface emitting lasers, tunable lasers, wafer bonding, gas sensing

I. INTRODUCTION Single mode laser diodes emitting around 2-µm wavelength are useful in optical spectroscopy, bio-medical and gas sensor applications [1]. So far, most of the work on such devices employed distributed feedback structures based on the InGaAsP/InP [2,3] or the GaInSb/AlGaAsSb [4,5] material systems with InGaAs or GaInSb strained quantum wells (QWs), respectively. Although capable of producing relatively high output power, of the order of 10 mW, these edge-emitting laser diodes do not exhibit mode-hop-free continuous wavelength tuning with current and/or temperature, are designed for narrow spectral intervals and suffer from high electrical power consumption, which prevents their efficient use in a number of spectroscopy applications. By contrast, VCSELs emit in a single transverse and longitudinal mode (i. e. single-frequency lasing), have a circular output beam, a lower sensitivity to optical feedback compared to DFB, both continuous and mode-hop-free wavelength tuning, as well as very low electrical power consumption [6-8]. They are therefore ideal and unique sources for spectroscopic gas analysis and the detection of many environmentally important and toxic gases. VCSELs emitting at ~2 µm with ~0.4 mW single-mode output power at room temperature have been previously realized in the InAlGaAs/InP material system using molecular beam epitaxy (MBE) [6]. These devices comprise an as-grown InAlAs/InAlGaAs distributed Bragg reflector (DBR) plus a dielectric DBR.

In this work we report on the fabrication and the performance of 2-µm wavelength range VCSELs,

incorporating an In(Al)GaAs/InP active cavity and fused top and bottom AlGaAs/GaAs DBRs, similar to wafer-bonded VCSELs emitting in the 1310- and the 1550-nm wavebands [8-10]. Here, we aimed in particular the specific 2003/2004-nm emission wavelengths for optical detection of water and carbon dioxide molecules by using one single VCSEL device.

II. DEVICE STRUCTURE AND FABRICATION The VCSEL structure is illustrated schematically in Fig. 1

and was fabricated by wafer fusion of InP active cavity and GaAs/AlGaAs-based DBRs [8]. The InP-based active cavity incorporates InGaAs/InAlGaAs QWs for optical gain, a p-n junction and a tunnel junction for carrier injection, a reverse biased junction for lateral current confinement, top and bottom intra-cavity contacting layers, and top and bottom structuring/cavity-adjustment layers. For spectroscopic applications laser fabrication with high precision of emission wavelength (± 2nm) and an accurate wavelength control is essential. The optical cavity structuring made possible by the wafer bonding process allows accurate setting of the emission wavelength, which increases the yield of devices applicable in spectroscopy at particular absorption lines. The AlGaAs/GaAs DBRs are undoped to achieve low absorption losses. All layers were grown by low pressure MOVPE in nitrogen using standard sources [9].

To obtain the photoluminescence (PL) with a peak in the 1.95-2.0-µm range at room temperature, the indium composition in the InGaAs QWs was increased up to 75-77%. At this composition the compressive strain of the InGaAs layers is ~1.6-1.7%, and was partially compensated using tensile strained InGaAlAs barriers. The growth conditions have been optimized in order to obtain smooth surfaces. Very

Fig. 1. Schematic cross section of the wafer fused VCSEL.

sharp and intense satellites peaks in the measured high-resolution X-ray diffraction curve (Fig.2) evidence a high InGaAs/InGaAlAs QWs/barriers structural quality. The zeroth order peak is shifted by 0.2% from the substrate peak indicating partial strain compensation. Test growths of the structures with higher indium content in the QWs (up to 82%) with the same barriers showed also perfect optical and structural properties with a PL peak up to ~2050 nm, demonstrating the possibility to achieve longer emission wavelengths.

Fig. 3. PL spectra of the InGaAs/InAlGaAs QWs in the 8-85°C temperature range.

The offset between the room temperature PL peak

wavelength and the 2-µm VCSEL cavity was set to be ~50-nm. Figure 3 depicts the PL spectra of the active cavity acquired at different temperatures. At room temperature, the PL peak is at 1950 nm, shifting at a rate of ~ 1nm/°C and reaching ~2000nm wavelength at 70°C. This offset of ~50nm between the room temperature PL peak wavelength and the target VCSEL wavelength was found suitable for adequate device performance at room temperature.

After the growth of an InP spacer, InGaAs/InAlGaAs QWs and tunnel junction layers, part of the tunnel junction was chemically etched to form a mesa, which defines the electrical

and optical aperture of the device. After etching, the tunnel junction mesa was re-grown with an n-InP spacer, which constitutes the top intra-cavity contacting layer (see Fig.1). This layer contacts the tunnel junction mesa and forms a blocking p-n junction around the mesa. Besides localized hole injection, the tunnel junction mesa introduces lateral refractive index variation for optical mode confinement. Additional index variation can be introduced also by patterning the structuring/cavity adjustment layers located on both sides of the cavity.

On the 2-inch InAlGaAs/InP active cavity wafer, 4 quarters with different cavity lengths were formed, by selectively etching some of the cavity adjustment layers. After wafer fusion, this procedure yielded 4 quarter wafer regions with different optical cavity lengths. The cavity mode wavelength was measured by optical pumping on the two-inch wafer, yielding the emission wavelengths listed in the Table 1. Quarters #2 and #4 of the fused wafer, which showed cavity resonance at ~1985 and ~2004nm were used for further VCSEL processing using standard processing methods including reactive ion etching of the top DBR, selective chemical etching to reach the bottom intra-cavity contact layer, passivation with silicon nitride and metal-contact deposition.

TABLE 1

EMISSION WAVELENGTH ON 2” VCSEL WAFER Quarter # 1 2 3 4 λ, nm 2011 2004 1996 1985

III. VCSEL CHARACTERISTICS Typical Light-Current-Voltage (LIV) characteristics of the

fabricated VCSELs emitting at ~1985 and 2004 nm are shown in the Fig. 4 (a and b). Devices have 9-µm aperture defined by the tunnel junction diameter. At room temperature both VCSELs have maximum emitting power of ~ 0.5 mW at 17-18 mA injection currents. The 1985-nm emitting device with lower offset between RT PL peak (gain) and optical cavity operates up to 30°C, while the 2004-nm VCSEL with higher (~ 50-nm) detuning is lasing up to 46°C. The threshold current of the 1984-nm device increases with temperature from 3.4 mA at 0°C to 11.5 mA at 30°C, whereas for 2004-nm laser the threshold current attains a minimum of 3.6 mA at 20°C and increases up to 8.7 mA at 46°C (Fig.5). Room-temperature emission spectra in the range of 4-18 mA are depicted on Fig.6a. Single mode operation with side-mode suppression ratio in excess of 30 dB is obtained at all currents in this range.

Emission wavelength of 2004-nm is obtained at 10 mA, and can be tuned within about 5-nm continuously (without mode hopping) by varying the diode current (Fig.6b). Wavelength tuning rates of 0.31 nm/mA with drive current and 0.14 nm/°C with temperature have been extracted from the measurements of the VCSEL emission wavelength at different current and

Fig. 2. Measured and simulated X-ray-diffraction spectra of a 7- InGaAs/InGaAlAs QW structure.

Fig. 5.Temperature variation of the threshold current of the 1985- and 2004 nm wavelength VCSELs.

temperature values. The results reported here show that wafer-fused VCSELs emitting at 2-µm have considerably lower emission power and differential efficiency than their 1310- and 1550-nm counterparts. In particular, the threshold current of the 2-µm devices with ~50nm (~16meV) offset between the emission wavelength and the PL peak at room temperature increases monotonically with increasing temperature. On the other hand, 1310- and 1550nm devices with similar wavelength offset (~38 and 28meV, respectively) show a minimum in threshold current at about 50°C [5-6]. This indicates that this offset is not optimal for high temperature operation at 2-µm. The relatively low emission power even at room temperature of the 2-µm devices could be explained by non optimized design of the VCSEL structure, higher optical losses and excessive Auger recombination as compared with 1.3- and 1.5µm wavelengths [11]. Thus, 2-µm wavelength VCSELs reaching higher values of power could be further optimized taking into account these considerations.

IV. CONCLUSION A first application of the wafer fusion approach to the

fabrication of VCSELs emitting at 2-µm wavelength has produced single mode devices with emission power of 0.5mW at room temperature and side mode-suppression ratio of more than 30 dB in the diode current range of 4-18 mA. Wavelength tuning with current and temperature at rates of 0.31 nm/mA

and 0.14nm/°C were measured, respectively. These devices are useful for optical spectroscopy applications, for example, for detection of water and carbon dioxide molecules in air.

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a) b)

Fig. 4. Light-current-voltage characteristics of the 1985- (a) and 2004-nm (b) wavelength VCSELs.

Fig. 6. Emission spectra (a), power and wavelength tuning (b) with current variation at 17°C.