Quantum cascade lasers (QCLs) are a type of unipolar light source based on electronic transitions between sub-bands in semiconductor-coupled quantum wells and phonon-resonant tunneling. Their small size, high power, and wide tunability render them highly versatile, with broad applications in biosensing, infrared spectroscopy, gas detection, and free-space communication. Currently, owing to its precise control over the material composition as well as its epitaxial layer thickness and sharp heterointerfaces, molecular beam epitaxy (MBE) serves as a critical technique for fabricating high-quality QCLs. However, limitations in production capacity and extended maintenance periods render MBE inadequate for meeting industrial demands. In this study, we report the successful growth of lattice-matched QCL material utilizing metal-organic chemical vapor deposition (MOCVD) on highly doped InP substrate, targeting an emission wavelength of 10.13 μm. The favorable performance of the device validates the potential of MOCVD for the epitaxial growth of long-wavelength infrared QCLs.

Mechods As the excitation wavelength and doping concentration both increase, the waveguide loss also increases, resulting in a higher threshold current density and deteriorated device performance. When the doping concentration in the active region is rationally adjusted and the doping of the waveguide layer is optimized, the waveguide loss can be reduced and materials with superior heterointerface quality can be grown. In this study, the optimized structure of a QCL was grown on a 2-inch InP substrate (Si doping concentration of 2×10¹⁸ cm^-3) using MOCVD at 100 mbar. The group III precursors were trimethyl indium (TMIn), trimethyl gallium (TMGa), and trimethyl aluminum (TMAl), and the group V precursors were arsine (AsH₃) and phosphine (PH₃). Silane (SiH₄, volume fraction of 0.02%) was employed as the N-type dopant, with the growth temperature maintained between 630 and 660 ℃. The growth structure consisted of the following sequential layers: a 500-nm InP buffer layer (Si doping concentration of 1×10¹⁷ cm^-3), 3-μm InP lower waveguide layer (Si doping concentration of 2×10¹⁶ cm^-3), 50-period lattice-matched active region, 3-μm InP upper waveguide layer (Si doping concentration of 2×10¹⁶ cm^-3), 0.5-μm grade-doped InP layer (Si doping concentration from 2×10¹⁶ to 5×10¹⁷ cm^-3), and 0.5-μm high-doped InP cap layer (Si doping concentration of 2×10¹⁸ cm^-3). The QCL core was fabricated into a ridge waveguide structure with an average ridge width of 15.2 μm using a semi-insulating InP buried heterostructure process (Fig. 2). This structure was then partitioned into devices with a cavity length of 4 mm, with a high-reflectivity (HR) coating applied to the rear facet and the front cavity facet left uncoated. The devices were epi-down on diamond submounts and subsequently mounted on copper heat sinks with indium-coated surfaces to enhance thermal dissipation.

High-reflection X-ray diffraction (HRXRD) measurements reveal a strong correspondence between the experimental data and theoretical simulations of the satellite peaks. The presence of sharp, well-defined, high-order satellite peaks with a narrow full width at half maximum (FWHM, 12 arcsec) underscores the uniformity of the chemical composition in the cascade structure and the quality of the heterointerfaces (Fig.1). The maximum peak output power achieves 0.96 W at 293 K, with a slope efficiency of 1.14 W/A and threshold current density of 0.68 kA/cm² in pulsed mode. For continuous wave operation, the highest output power is 0.52 W, with a peak current of 1.15 A, slope efficiency of 1.0 W/A, and threshold voltage of 9.5 V at 293 K, whereas the maximum wall plug efficiency (WPE) reaches 4% (Fig.3). A longer cavity length reduces the mirror loss, and lower doping concentrations in the waveguide layer and active region mitigate the free carrier absorption loss, thereby decreasing the overall absorption loss. In addition, the device grown by MOCVD exhibits a narrow FWHM in the HRXRD measurements, indicating superior quality at the heterointerfaces. This enhanced quality contributes to reducing interface roughness scattering, improving the excited-state electron lifetimes, and effectively suppressing nonradiative loss. Collectively, these factors result in a decrease in the threshold current density and an increase in the output power and WPE. The emitted laser spectrum has a center wavelength at 986.9 cm^-1 (10.13 μm) at an input current of 0.6 A in continuous wave mode. The beam image confirms that the device operates in transverse mode (TM₀₀), see Fig.4. Fitting the temperature dependence of the threshold current density and slope efficiency in the pulsed mode enables the characteristic temperatures of the threshold current density (T₀) and slope efficiency (T₁) to be determined as 156 K and 301 K, respectively (Fig. 5). Due to non-harmonic oscillators, a wider FWHM of the electroluminescence (EL) spectrum typically contributes to increasing interband loss, which in turn decreases the peak gain and increases the laser threshold current density. In our experiments, the narrow EL FWHM of 114.7 cm^-1 (17.2 meV) aligns with the previously recorded low threshold current density (Fig. 6).

This study reports a long-wavelength infrared QCL with a full structure grown using MOCVD. By optimizing the doping concentrations in the waveguide layers, we effectively reduce the waveguide loss and develop an active-region structure with high-quality heterointerfaces. The laser with a cavity length and ridge width of 4 mm and 15.2 μm, respectively, achieves a continuous wave output power of 0.52 W at room temperature. This is accompanied by a remarkably low threshold current density of 0.76 kA/cm² and maximum WPE of 4%. In the pulsed mode, a peak output power of 0.96 W is obtained, and the characteristic temperatures for the threshold current density and slope efficiency are 156 K and 301 K, respectively. The lasing wavelength is measured at 10.13 μm, with a clear TM₀₀ mode observed. The excellent performance of this device demonstrates the potential of MOCVD for epitaxial growth of long-wavelength infrared QCLs.