Measurements of beam stability for mid-infrared (IR)-emitting quantum cascade lasers (QCLs) are important for applications that require the beam to travel through air to remote targets, such as free-space communication links. We report beam-quality measurement results of narrow-ridge, 4.6 mu m-emitting buried-heterostructure (BH) QCLs fabricated using ICP etching and HVPE regrowth. Beam-quality measurements under QCW operation exhibit M-2 < 1.2 up to 1 W for similar to 5 mu m-wide ridges. 5 mu m-wide devices display some small degree of centroid motion with increasing output power (< 0.125 mrad), which corresponds to a targeting error of similar to 1.25 cm over a distance of 100 m.

Scaling the coherent power of mid-infrared (IR)-emitting quantum cascade lasers (QCLs) to the multi-watt range remains an important objective for applications where the laser beam needs to travel through air to remote targets, such as freespace communication links. For such applications requiring long-range pointing accuracy, measurements of beam stability are also important. We present beam-quality measurement results of narrow-ridge (4-5 μm), 4.6 μm-emitting buriedheterostructure (BH) QCLs. A 40-stage, step-tapered active-region (STA) structure was grown by MOCVD, and ICP etching was used to make deep ridges. InP:Fe was preferentially regrown in the field regions by using an SiO2 mask for ridge etching and Hydride Vapor Phase Epitaxy (HVPE). The HVPE process is attractive for selective regrowth, since high growth rates (0.2-0.3 μm/min) can be utilized, and highly planar top surfaces can readily be obtained. HVPE regrowth has been previously employed for BH devices of MBE-grown QCL ridges, but beam-stability measurements were not reported. HR-coated, 7.5 mm-long devices were measured under QCW operation (100 μsec pulse width, 0.5%-10% duty cycle) - very good beam quality factors, M2 < 1.2, were observed for both 4 μm and 5 μm ridge widths, but the narrower ridge exhibited better pointing stability. Collimated 5 μm-wide BH devices displayed some small degree of centroid motion with increasing power (< 0.125 mrad). This corresponds to a targeting error of ∼1.25 cm over a distance of 100 m. Significantly improved lateral-beam stability was observed for narrower ridge width, although at the expense of reduced output power. 

Heteroepitaxial growth of orientation‐patterned (OP) GaP (OP‐GaP) on wafer‐bonded OP‐GaAs templates is investigated by low‐pressure hydride vapor phase epitaxy for exploiting the beneficial low two‐photon absorption properties of GaP with the matured processing technologies and higher‐quality substrates afforded by GaAs. First, GaP homoepitaxial selective area growth (SAG) is conducted to investigate the dependence of GaP SAG on precursor flows and temperatures toward achieving a high vertical growth rate and equal lateral growth rate in the [110] and [-110]‐oriented openings. Deteriorated domain fidelity is observed in the heteroepitaxial growth of OP‐GaP on OP‐GaAs due to the enhanced growth rate on domain boundaries by threading dislocations generated by 3.6% lattice matching in GaP/GaAs. The dependence of dislocation dynamics on heteroepitaxial growth conditions of OP‐GaP on OP‐GaAs is studied. High OP‐GaP domain fidelity associated with low threading dislocation density and a growth rate of 57 μm h⁻¹ are obtained by increasing GaCl flow. The properties of heteroepitaxial GaP on semi‐insulating GaAs is studied by terahertz time‐domain spectroscopy in the terahertz range. The outcomes of this work will pave the way to exploit heteroepitaxial OP‐GaP growth on OP‐GaAs for frequency conversion by quasi‐phase‐matching in the mid‐infrared and terahertz regions.

Direct heteroepitaxy of GaAsP and GaP on GaAs and on Si by low-pressure hydride vapor phase epitaxy (HVPE) is investigated as prior studies for photovoltaics and non-linear optics applications. When growing GaAsP on GaAs, it is found that the ambient gas during substrate pre-heating influences the ternary composition as well as the crystalline quality of the subsequent growth. GaAs0.72P0.28 with bandgap energy of 1.76 eV has been achieved, which would be suitable for a top cell in Si tandem solar cell structures. Growth of GaP was investigated on planar GaAs as a prior study for realizing orientation patterned (OP) GaP on OP-GaAs. Threading dislocations caused by the 3.6% lattice mismatch between GaP and GaAs are suppressed by adjusting the GaCl flow, achieving a low full width at half maximum of 146 arcsec for the X-ray diffraction omega scan. Direct heteroepitaxy of GaAsP on Si aiming for achieving a GaAsP/Si dual junction solar cell is demonstrated. The inherent problem of initiating nucleation during the direct heteroepitaxy of III-V on Si by HVPE is overcome by utilizing the vapor mixing approach to grow a low-temperature GaP buffer layer on Si, followed by a GaAsP layer grown by conventional HVPE.

This work investigates the optical and interfacial properties of epitaxially fused direct GaInP/Si heterojunctions realized by the corrugated epitaxial lateral overgrowth (CELOG) approach. To provide a broad analysis of the above heterojunction, photoluminescence (PL), cathodoluminescence (CL), Raman, and high-resolution transmission electron microscopy (TEM) were employed in this study. The enhanced luminescence intensity was observed in the direct GaInP/Si heterojunction in the cross-sectional CL because of the reduced defect density in the CELOG GaInP. The spatial resolution dependent PL and CL spectra of GaInP on Si yielded the composition variation of GaInP arising from the anisotropic growth behavior of CELOG. The Ga composition, x, in GaxIn1-xP/Si at the interface deduced from the lattice constant measured by TEM has a good agreement with the results of PL and CL. Low thermal and lattice mismatch strain in CELOG GaInP on Si were revealed by the Raman spectra. TEM investigation further revealed the atomic structure of some planar defects in CELOG GaInP over Si. It is confirmed that although a thin atomic disorder was observed on the surface of Si substrate, an epitaxially fused GaInP/Si heterojunction with a reduced threading dislocation density of similar to 6.4x10(7) cm(-2) in comparison to similar to 4.8x10(8) cm(-2) in the InP seed on Si has been successfully fabricated by the CELOG technique despite about 4% lattice mismatch between GaInP and Si. The findings of this study demonstrate the great potential of the CELOG technique for promoting monolithic integration of III-V/Si-based optoelectronics.

We demonstrate a surface emitting 1.5 mu m multi-quantum well (MQW) light-emitting diode (LED) on a 3-inch epitaxial lateral overgrowth (ELOG) InP/Si wafer. The enhanced crystalline quality of ELOG InP/Si is revealed by various characterization techniques, which gives rise to a MQW with high photoluminescence intensity at 1.5 mu m and interference fringes arising from the vertical Fabry-Perot cavity. The LED devices exhibited strong electroluminescence intensity that increased with pump current. Moreover, transparency current measurements indicate optical gain in the 1.5 mu m MQW on InP/Si. The results are encouraging for obtaining wafer scale 1.5 mu m surface emitting laser structures on silicon with further optimization.

Quasi-phase-matched orientation-patterned GaP (OP-GaP) is an attractive technology for achieving mid-IR and terahertz (THz) radiation enabled by nonlinear frequency conversion, because of its low two-photon absorption (2PA) down to 1 pm. High crystalline quality of OP-GaP template and subsequent thick layer growth is required to exploit the benefit of low 2PA of GaP pumped with short wavelength lasers [1]. In this work, we present the development of heteroepitaxial growth of OP-GaP on wafer fused OP-GaAs template with high growth rate and good domain fidelity. The conductivity of the GaP grown on planar GaAs was investigated in the THz region.

Heteroepitaxial growth of orientation-patterned GaP on patterned GaAs template was developed by using hydride vapor phase epitaxy for quasi-phase-matching applications. We present the growth with well-defined periodic boundaries between (001) and (00 1) GaP domains. The GaP layer on planar GaAs was characterized by terahertz time-domain spectroscopy and the conductivity of GaP (0.16 S cm-1) was obtained in terahertz range. 

Room temperature surface emission is realized on a large area (1.5 mm x 1.5 mm) photonic crystal quantum cascade laser (PhC-QCL) driven under pulsed mode, at the wavelength around 8.75 mu m. By introducing in-plane asymmetry to the pillar shape and optimizing the current injection with a grid-like window contact, the maximum peak power of the PhC-QCL is up to 5 W. The surface emitting beam has a crossing shape with 10 degrees divergence.