Publications

The quality of epitaxial layers in quantum cascade lasers (QCLs) has a primary impact on QCL performance, and establishing correlations between epitaxial growth and materials properties is of critical importance for continuing improvements. We present an overview of the growth challenges of these complex QCL structures; describe the metalorganic vapor phase epitaxy growth of AlInAs/GaInAs/InP QCL materials; discuss materials properties that impact QCL performance; and investigate various QCL structure modifications and their effects on QCL performance. We demonstrate uncoated buried-heterostructure 9.3-um QCLs with 1.32-W continuous-wave output power and maximum wall plug efficiency (WPE) of 6.8%. This WPE is more than 50% greater than previously reported WPEs for unstrained QCLs emitting at 8.9 um and only 30% below strained QCLs emitting around 9.2 um.

AlInAS/GaInAs superlattices (SLs) with barrier and well layers of various thicknesses were grown by organometallic vapor phase epitaxy to optimize growth of quantum cascade lasers (QCLs). High-resolution x-ray diffraction data of nominally lattice-matched SLs show a systematic shift toward more compressively strained SLs as the barrier/well layer thicknesses are decreased below about 10 nm. This shift is attributed to In surface segregation in both AlInAs and GaInAs. This shift is compensated for in the growth of ultra-thin layers in QCL structures. QCLs with tapered gain regions and emitting at 9.6 um are demonstrated with peak power as high as 5.3 W from one facet at 20 degrees C.

InP1-yAsy epitaxial layers grown lattice-mismatched (LMM) on InP substrates were investigated as a new materials system for multiplication layers in Geiger-mode avalanche photodiodes (GM APDs) for detection of photons in the range 1.6-2.5 mm. LMM InP1-yAsy epilayers were grown on semi-insulating (1 0 0) InP substrates misoriented 0.2 and 2 [1 1 0] by organometallic vapor phase epitaxy at a growth temperature of 580 1C. The growth scheme used for the InP1-yAsy buffer layer was optimized based on surface step structure and X-ray diffraction. It was found that step-flow growth is a minimum criterion for obtaining good material quality. A narrower XRD full-width at half-maximum values were measured for 21-miscut substrates compared to 0.21-miscut substrates. A highquality buffer was obtained by step-grading the InP1-yAsy composition in increments of y = 0.05 over a layer thickness of 0.5 mm to a final y = 0.25. The device performance of LMM GM APDs was compared to that of measured more traditional lattice-matched GaSbbased devices. At 77 K, dark count rates of LMM devices are ~50 kHz at 5V overbias, and are comparable to GaSb-based p-i-n diodes operated in Geiger mode, while reset times of 0.02 ms are approximately 3 orders of magnitude lower than GaSb-based GM APDs.