Semi-insulating InP:Fe for buried-heterostructure strain-compensated quantum-cascade lasers grown by gas-source molecular-beam epitaxy M.P. Semtsiv n , A. Aleksandrova, M. Elagin, G. Monastyrskyi, J.-F. Kischkat, Y.V. Flores, W.T. Masselink Department of Physics, Humboldt University Berlin, Newtonstrasse 15, D-12489 Berlin, Germany article info Available online 5 January 2013 Keywords: A3. Molecular-beam epitaxy B2. Semiconducting III–V materials B3. Heterojunction semiconductor devices B3. Infrared devices abstract We describe the realization of buried-heterostructure strain-compensated quantum-cascade lasers that incorporate a very high degree of internal strain and are grown on InP substrates using gas-source molecular- beam epitaxy (GSMBE). The active region of the lasers contains AlAs layers up to 1.6 nm thick with 3.7% tensile strain; restricting any post-growth processing to temperatures below 600 1C to avoid relaxation. We demonstrate that buried-heterostructure devices can be realized by using GSMBE to over-grow the etched laser ridge with insulating InP:Fe at temperatures low enough to preserve the crystal quality of the strain- compensated active region. Two distinct growth techniques are described, both leading to successful device realization: selective regrowth at 550 1C and non-selective regrowth at 470 1C. The resulting buried- heterostructure lasers are compared to a reference laser from the same wafer, but with SiO 2 insulation; all three have very similar threshold current densities, operational thermal stability, and waveguide losses. & 2013 Elsevier B.V. All rights reserved. 1. Introduction Since their invention in 1994, quantum-cascade lasers (QCLs), [1] have become the ultimate semiconductor laser sources from near-infrared [2] to middle THz [3] spectral range. QCLs are widely used now as sources of infrared laser emission for spectroscopy and chemical sensing. They also have the potential for application in military countermeasures and free-space communication. Several applications, for example photo-acoustic spectroscopy, require high average output power, which in turn requires efficient heat sinking out of the laser ridge. There has been enormous progress in QCL technology towards the optimization of the heat extraction and the best results are obtained by means of buried- heterostructure (BH) laser fabrication using semi-insulating InP:Fe overgrown by metal-organic vapor phase epitaxy (MOVPE) [4–8]. A successful QCL design for wavelengths between 3 and 5 mm is based on strain compensation with very high degrees of internal strain, including pure 1–2 nm AlAs barriers with 3.7% tensile strain to the InP substrate [9–12]; these structures, however do not withstand the typical MOVPE regrowth temperature of about 650 1C [13]. Thus, the growth of high-resistivity InP at low growth temperatures is required for these strain-compensated QCLs. Gas-source molecular-beam epitaxy (GSMBE) has already been used before to fabricate BH telecom lasers [14] and also BH QCLs with a lattice-matched active zone [13]. However, extending the GSMBE overgrowth technique to strain-compensated QCLs with highly strained alternating In 0.73 Ga 0.27 As and AlAs layers is not trivial and has not been demonstrated yet. Contrary to the MOVPE technique, GSMBE leaves the etched side walls of the laser ridge half a time without the group-V flux at elevated temperatures of 500–550 1C before the regrowth process is initiated. The situation arises because with a single gas injector and conventional sample rotation, the ridge shades each etched sidewall from the side- coming surface-stabilizing group-V flux during the half of the sample rotation. This is problematic because it can lead to material intermixing on the sidewalls due to strain-driven surface migra- tion, which can cause electrical shorts. In this paper we describe the successful InP:Fe regrowth of strain-compensated QCLs that include highly strained and relatively thick (up to 1.6 nm) AlAs barriers in the active zone. The regrowth can be done selectively around a dielectric mask, similar to the technique typically used with MOVPE, but at a significantly lower growth temperature of 550 1C. These BH QCLs, when compared to the reference QCLs with SiO 2 insulation of the ridges, show comparable threshold current densities, J th , and T 0 ; the confinement factor, G, is somewhat lower due to the smaller index discontinuity, and the waveguide losses, a w , are somewhat lower due to a superior interface. 2. QCL active region design The conduction band edge profile of the single active region cascade and the moduli square of the confined states, wavefunctions are shown in Fig. 1. The active region is designed to emit at the wavelength close to 4 mm at an operation voltage of approximately 13 V for 30 cascades. High values of T 0 and T 1 require the use of high Contents lists available at SciVerse ScienceDirect journal homepage: www.elsevier.com/locate/jcrysgro Journal of Crystal Growth 0022-0248/$ - see front matter & 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jcrysgro.2012.12.079 n Corresponding author. Tel.: þ49 30 20937919. E-mail address: semtsiv@physik.hu-berlin.de (M.P. Semtsiv). Journal of Crystal Growth 378 (2013) 125–128