∼8.5 μm-emitting InP-based quantum cascade lasers grown on GaAs by metal-organic chemical vapor deposition

Abstract

Room-temperature, pulsed-operation lasing of 8.5 μm-emitting InP-based quantum cascade lasers (QCLs), with low threshold-current density and watt-level output power, is demonstrated from structures grown on (001) GaAs substrates by metal-organic chemical vapor deposition. Prior to growing the laser structure, which contains a 35-stage In0.53Ga0.47As/In0.52Al0.48As lattice-matched active-core region, a ∼2 μm-thick nearly fully relaxed InP buffer with strained 1.6 nm-thick InAs quantum-dot-like dislocation-filter layers was grown. A smooth terminal buffer-layer surface, with roughness as low as 0.4 nm on a 10 × 10 μm2 scale, was obtained, while the estimated threading-dislocation density was in the mid-range × 108 cm−2. A series of measurements, on lasers grown on InP metamorphic buffer layers (MBLs) and on native InP substrates, were performed for understanding the impact of the buffer-layer's surface roughness, residual strain, and threading-dislocation density on unipolar devices such as QCLs. As-cleaved devices, grown on InP MBLs, were fabricated as 25 μm × 3 mm deep-etched ridge guides with lateral current injection. The results are pulsed maximum output power of 1.95 W/facet and a low threshold-current density of 1.86 kA/cm2 at 293 K. These values are comparable to those obtained from devices grown on InP: 2.09 W/facet and 2.42 kA/cm2. This demonstrates the relative insensitivity of the device-performance metrics on high residual threading-dislocation density, and high-performance InP-based QCLs emitting near 8 μm can be achieved on lattice-mismatched substrates.

Quantum Cascade Lasers Grown by Metalorganic Chemical Vapor Deposition on Foreign Substrates with Large Surface Roughness

“…In addition, similar broad satellite superlattice (SL) peaks were also observed, circled in Figure 4, which have also been reported previously for MBE-grown QCLs on foreign substrates with miscut angles [3,8]. In contrast, well-defined and distinguishable active region SL peaks were reported for MOCVD-grown QCLs on foreign substrates without miscut [4][5][6]. Thus, the use of miscut substrates and a non-optimized core region growth condition are believed to play a role in the large fringe broadening in Figure 4.…”

“…Furthermore, we observed a higher maximum peak height of 103.1 nm for the buffer on Si, and this value was only 49.4 nm for the buffer on GaAs. These 3D roughness values indicate that the quality of the buffer on either GaAs or Si was quite poor, and as such, these RMS roughness values are generally thought to be not suitable for any laser growth, as they were significantly larger compared with prior reports of graded InAlAs buffer on Si with 3.3 nm over 400 µm 2 area [10] and the InP buffer on GaAs with an RMS roughness of 0.4 nm over 100 µm 2 area [4].…”

“…Surprisingly, the QCL devices grown on those unoptimized templates still show lasing characteristics with reasonably low threshold current densities and pulsed power output. To further investigate the impact of the buffer's surface roughness on QCL's device performance, the above device results were compared with that from our prior research on the work of QCLs on on-axis GaAs [4] and Si [5] with the same active region design, as shown in Table 1. By contrast with the devices on the miscut Si and GaAs, the RMS surface roughness for the devices on on-axis Si and GaAs of the same active region design were measured and found to be as low as 1.53 nm and 0.4 nm, respectively.…”

“…The band structure for the lattice-matched QCL active region utilized in this study is based on a two-phonon resonance design [8]. This is also the same QCL structure previously adopted for realizing QCLs on on-axis GaAs and Si by MOCVD, and the growth details and full laser structure can be found here [4,5]. As shown in Figure 2, the QCL on the Si wafer was fabricated into a deep-etched ridge waveguide with an average core region width of ~25 µm.…”

“…Recently, a room-temperature continuous wave operation for MBE-grown QCLs on miscut Si substrates with an output power greater than 0.7 W and a low threshold current (1.3 kA/cm 2 ) has been achieved [3]. Meanwhile, our recent work shows that the InP-based QCLs on foreign substrates grown by metalorganic chemical vapor deposition (MOCVD) can also have a comparable device performance to its counterpart on native substrates, even in the presence of a high dislocation density [4][5][6]. However, these prior studies have not elucidated the role of the root-mean-square (RMS) surface roughness of buffer layers on subsequent device performances.…”