NEW AREAS Mid-infrared Quantum Cascade Laser Integrated with Distributed Bragg Reflector Jun-ichi HASHIMOTO*, Hiroyuki YOSHINAGA, Yukihiro TSUJI, Hiroki MORI, Makoto MURATA, and Yasuhiro IGUCHI ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- To achieve a high facet reflectivity needed for the threshold current reduction (Ith) of a quantum cascade laser (QCL), we have developed an InP-based 7-µm Fabry-Perot (FP) QCL integrated with a distributed Bragg reflector (DBR). The DBR consists of semiconductor walls and air gaps which are alternately arranged by periodically etching the epitaxial layers of the air gap regions. The incorporation of a pair of 3λ/4 DBRs increased the facet reflectivity up to 66%, which was more than twice as high as that of a cleaved facet, and reduced Ith by 11%. This DBR-integrated FP-QCL succeeded in oscillation at up to 100°C in pulse operation and at up to 15°C in continuous wave operation, which is the first report on the operation of InP-based DBR-integrated QCLs. It also achieved sufficient output for sensing (up to dozens of mW). The DBR is expected to be used as a low-loss reflector suitable for front facet of QCLs. ---------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- Keywords: quantum cascade laser, QCL, mid-infrared, gas sensing, low power-consumption 1. Introduction tion(3)-(6) at room temperature (RT), single-mode opera- The mid-infrared (MIR) region (e.g., 3-20 μm) has tion(7)-(9) by introducing the distributed-feedback (DFB) many optical absorption lines attributed to fundamental structure that is required for gas detection, and so on, and vibrational resonances of the molecules of major industrial now QCLs have already been commercialized. and environmental gases (e.g., COx, NOx, SOx), and there- An outdoor measurement is one of the main potential fore it is called “the molecular fingerprint region.” applications of QCL gas sensors. These sensors must be The optical absorption attributed to fundamental battery-operated to reduce the size and weight for portability. vibrational resonance is several orders of magnitude higher Considering such battery operation, the overall power- than that attributed to higher-order harmonics in the near consumption of the sensor must be reduced to about 5 W, so infrared (NIR) region. This helps achieve a highly sensitive that the power-consumption of the QCL (light source) must (ppb – ppt) optical gas sensor.(1) be low, typically less than 1 W. However, the power- Recently, quantum cascade lasers (QCLs)(2) have been consumptions of currently available QCLs are still high at drawing attention as new light sources in the MIR region. several watts, and therefore most of these QCLs cannot be QCLs are semiconductor lasers developed in 1994 that can readily used for this application. To reduce the power- oscillate in the MIR region. There has been a growing consumption of QCLs, the threshold current must be reduced, effort to develop QCLs as compact, high-speed, and narrow for which it is effective to increase the reflectivity of the facet. linewidth MIR light sources. Table 1 shows the results of comparison of three Gas sensors using QCLs as the light source offer methods to increase the facet reflectivity. A typical method various advantages such as compactness, high-speed, and is to coat the facet with a high-reflective film, such as a high-sensitivity due to the characteristics of QCLs noted multilayer dielectric film or a metallic film. However a above. Such sensors are expected to play a key role as very thick film of around several μm is needed for the measuring instruments in various fields such as measuring multilayer dielectric film in the MIR region, resulting in a of process and exhaust gases at plants, monitoring of envi- difficulty in its formation. For this reason, a metallic film, ronmental gases, medical diagnosis (e.g., breath analysis), Au film in particular, is usually used as the high-reflective and detection of dangerous materials, and the market is film for the MIR region.(4),(8) As the calculation results in expected to expand rapidly. Fig. 1 show, the facet reflectivity increases rapidly with the The core region (light emitting region) of QCLs increase in Au film thickness; a high reflectivity close to consists of superlattice structure.*1 In the core region, a 100% can be easily attained. Thus, Au film is suitable as function peculiar to the superlattice is utilized for oscilla- tion in MIR-region: in the core region, radiative transition of carriers (electrons) between subbands of the conduction Table 1. Comparison of methods to increase the facet reflectivity band in the active region, and subsequent carrier transport Multilayer Method Au Film DBR to the next active region through the mini-bands of the Dielectric Film injector regions by tunneling effect makes the MIR oscilla- Suitable > 90% (Especially for Unsuitable tion possible, which was difficult to achieve in the conven- Facet Rear Facet) Unsuitable tional semiconductor laser. Reflectivity Suitable The technology has been improved since the first 50-80% Unsuitable (Especially for successful QCL oscillation in 1994,(2) leading to CW opera- Front Facet) SEI TECHNICAL REVIEW · NUMBER 85 · OCTOBER 2017 · 59 100 15 ] fabricating QCL chips, and therefore offers a significant ] % [ % 80 Reflectivity advantage in simplifying the manufacturing process [ e Transmittance 10 c compared to the Au coating in which a wafer must be n ity 60 a cleaved to form a chip and an Au film must be formed on v ti itt c 40 its facet. 5 m le f With laser diodes for optical communication, there ns e 20 λ: 7.54 μm a have been many reports of increasing the facet reflectivity R (10)-(12) 0 0 Tr by introducing a DBR, but there have been only a few 0 20 40 60 80 100 reports with QCLs.(13) As far as we know, there have been Au film thickness [nm] no such reports with InP-based QCLs. In this study, we tried to integrate a DBR with a 7 μm-band InP-based FP Fig. 1. Au film thickness vs. facet reflectivity and transmittance (calculation) (Fabry Perot) type QCL to verify its effectiveness. We succeeded in achieving oscillation of an InP-based DBR-QCL for the first time in the world.(14),(15) the total reflection film. It is also easy to form an Au film because a thin Au film of about 100 nm can increase the 2. Device Structure facet reflectivity sufficiently. Thus, Au film is suited to increasing the reflectivity of the rear facet that requires 2-1 Structure of the main body area high reflectivity close to the total reflection (e.g., 90% or Figure 2 shows the cross-sectional structure of the higher). mesa waveguide in the main body area of the On the other hand, to attain the output power required DBR-integrated FP-QCL in the direction in which the for a light source of sensing (typically several mW for gas waveguide extends.(14),(15) On the InP semiconductor sensing), a moderately high facet reflectivity (50%-80%) is substrate, an n-InP buffer layer, a core region (including 33 desirable for the front facet so as not to prevent the output laminations of a unit structure consisting of an active light from being reflected excessively by the front facet. region and injection region that were both made by AlInAs/ However, in the case of Au coating, an ultrathin Au film GaInAs superlattices), an n-InP cladding layer, and an (10 nm or less) must be used to attain the facet reflectivity n-GaInAs contact layer were successively grown in that in this range (see Fig. 1). This makes it difficult to control order. Then, the semiconductor laminate consisting of the the film thickness and quality of the Au film. above semiconductor layers was processed into a mesa- In addition, the transmittance decreases rapidly in the stripe, and it was buried by current-blocking layers in the MIR region when the film thickness is increased to about second growth to form a buried-heterostructure (BH) 10 nm, as shown in Fig. 1, because the optical absorption configuration. The current-blocking layer used InP, which of Au is high, so that the output power required for prac- is characterized by high thermal conductivity and low tical use cannot be attained. Thus, it is difficult to apply an optical absorption, to improve the heat dissipation of the Au film as the high-reflective film to the front facet. device and reduce the internal loss compared to other To address this issue, we decided to introduce a current confinement structures. The structure is considered distributed Bragg reflector (DBR) in place of Au coating, to be optimal for reducing the power-consumption of the as a means of increasing the facet reflectivity applicable to QCL. Finally, upper and lower electrodes were formed, and the front facet. A DBR consists of a high refractive index thus a device structure was completed. area and a low refractive index area, which are alternately 2-2 DBR structure arranged in a constant period in the direction in which a The DBR structure(14),(15) introduced in this time is also QCL waveguide extends. The facet reflectivity can be shown in Fig. 2. A DBR structure consisting of pairs of a increased in the oscillation wavelength region by inte- semiconductor wall (high refractive index area) and a gap grating the DBR on the facet of a QCL and optimally (low refractive index area) was integrated on the facet of setting the period of DBR so that Bragg reflection occurs there effectively at the QCL’s oscillation wavelength. The results of comparison between an Au film and Main Body Region DBR are summarized in Table 1.