Semicond. Sci. Technol. 15 (2000) L44–L46. Printed in the UK PII: S0268-1242(00)17233-5 LETTER TO THE EDITOR High-performance strain-compensated InGaAs/InAlAs quantum cascade lasers Feng-Qi Liu†§, Yong-Zhao Zhang†, Quan-Sheng Zhang†, Ding Ding†, Bo Xu†, Zhan-Guo Wang†, De-Sheng Jiang‡ and Bao-Quan Sun‡ † Laboratory of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, PO Box 912, Beijing 100083, People’s Republic of China ‡ National Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, PO Box 912, Beijing 100083, People’s Republic of China E-mail: fqliu@red.semi.ac.cn Received 19 September 2000, accepted for publication 12 October 2000 Abstract. We report on the realization of quantum cascade (QC) lasers based on strain-compensated In x Ga (1−x) As/In y Al (1−y) As grown on InP substrates using molecular beam epitaxy. X-ray diffraction and cross section transmission electron microscopy have been used to ascertain the quality of the QC laser materials. Quasi-continuous wave lasing at λ ≈ 3.54–3.7 µm at room temperature was achieved. For a laser with 1.6 mm cavity length and 20 µm ridge-waveguide width, quasi-continuous wave lasing at 34 ◦ C persists for more than 30 min, with a maximum power of 11.4 mW and threshold current density of 1.2 kA cm −2 , both record values for QC lasers of comparable wavelength. Quantum cascade (QC) lasers are a fundamentally new semiconductor laser source. They are not only renewing the field of mid-infrared injector lasers, but also represent a source of novel unconventional ideas for semiconductor lasers in general. QC lasers are based on electronic transitions between quantized conduction band states of a multiple quantum well structure and are grown by molecular beam epitaxy [1–3]. The wavelength of QC laser is essentially determined by the layer thickness of the active region rather than by the bandgap of the material. As such, it can be tailored over a wide range using the same heterostructure material [4–6]. QC lasers for the first atmospheric window (3–5 µm) are important for a variety of commercial and military applications. However, in intersubband QC lasers, the short-wavelength operation is limited by the size of the conduction band discontinuity E c , which exists between the two semiconductor materials. To achieve a QC laser with wavelength shorter than 4 µm, it is required to use strain-compensated In x Ga (1−x) As/In y Al (1−y) As (x> 53%, y < 52%) materials, which operate in the active region of the QC laser, because this strain-compensated material system gives enlarged conduction band discontinuity [6]. This approach adds flexibility in the QC laser design by allowing a selection of the desired discontinuity but also adds the constraint that the tensile strain balances the compressive § To whom correspondence should be addressed. strain in the structure. A strain-compensated InGaAs/InAlAs QC laser grown on InP was first demonstrated in 1998 to achieve a laser operating at a wavelength of 3.4 µm. However, the maximum operating temperature and output power of these strain-compensated InGaAs/InAlAs devices is limited relative to the corresponding lattice-matched devices. The most arduous problem is the difficulty of fabricating high-quality laser material. Reducing the threshold current density and enhancing the output power of a QC laser and its operating temperature are important for device applications. Here we demonstrate our results obtained on strain-compensated In x Ga (1−x) As/In y Al (1−y) As QC lasers, operating at a wavelength which is as short as 3.54–3.7 µm. Very low threshold current density of 1.2 kA cm −2 at 34 ◦ C is realized. The breakthrough is that quasi-continuous wave operation at 34 ◦ C with output power 11.4 mW persists for more than 30 min without obvious degradation. The strain-compensated InGaAs/InAlAs laser structures are grown by molecular beam epitaxy (MBE) on n-doped InP (Si, 1 × 10 18 cm −3 ) substrates in a Riber 32p MBE system. The active region of each laser structure consists of 25 superlattice periods, which are alternating n-doped injector regions and undoped triple-quantum-well (wafer B1143) or double-quantum-well (wafer B1146) active re- gions. The laser structures are similar to that described in [4, 7, 8]. The complete laser structures are schematized in figure 1. The active regions of QC lasers are designed 0268-1242/00/120044+03$30.00 © 2000 IOP Publishing Ltd