A Quick and a Flexible Hydride Vapor Phase Epitaxy Process to Achieve Buried Heterostructure Quantum Cascade Lasers

Abstract

Quantum cascade lasers (QCL) first demonstrated by Faist and co-workers1 are attractive sources for spectroscopy, medical and biosensing, remote gas sensing, free-space communication and applications in defense security countermeasures as these cover a wide range of spectrum from near-infrared to mid THz2-3. Several of these application areas necessitate high power, good beam quality, efficient thermal dissipation and high wall plug efficiency (WPE). Together with the optimal basic design, buried heterostructure QCL (BH-QCL) with semi-insulating InP:Fe (SI-InP) regrowth offers unique possibility to achieve an effective thermal dissipation and lateral single mode with the above desired performance. So far only MOVPE (metal organic vapour phase epitaxy)4 and molecular beam epitaxy (MBE)5 have been used to realize BH-QCLs and not hydride vapour phase epitaxy (HVPE), an equilibrium process. The growth rate in HVPE is one order of magnitude higher than that in MOVPE and MBE. We demonstrate here buried heterostructure QCLs emitting at 4.7 µm with a single step regrowth of highly resistive (>1x108 ohm·cm) semi-insulating InP:Fe in less than 45 minutes in a HVPE reactor for burying 6-15 µm deep etched ridges. This has to be compared with the normally used growth time of several hours, e.g., in an MOVPE reactor. Maintenance of the QCL ridges at the regrowth temperatures for longer time to realize BH-QCL can cause severe inter well-barrier mixing6 leading to the loss of the original laser design characteristics and hence HVPE regrowth is advantageous. In addition, its planarizing capacity, total selectivity during the growth on patterned (planar or non-planar) surfaces, non-appearance of “rabbit ears” near the ridge (or mesa) edge, flexibility in growing around [110] and [-110] oriented ridges, both with or without mask overhang, and insensitivity to the profile of the etched ridge are its unique flexible features7. The HR-coated 5 mm long BH-QCLs of ridge widths 6-15 µm were characterized. Besides being spatially monomode, TM00, all the characterized lasers exhibited WPE of ~8-9% with an output power of 1.5 – 2.5 W at room temperature and under CW operation. The performance can be further improved by proper mounting and facet coating. As-cleaved BH-QCLs were also characterized and compared with the HR-coated ridge-alone QCLs; the former outperform the latter in their output power more than twice even though the latter were HR-coated. This indicates the good thermal dissipation in the BH-QCL with HVPE regrowth. Thus, we demonstrate a simple, flexible, quick, stable and single-step regrowth process with extremely good planarization for realizing buried QCLs leading to monomode, high power and high WPE. Acknowledgements This work was supported by EU FP7 MIRIFISENS Integrated Project 317884. Linné Center of Excellence, ADOPT is also acknowledged.