Regrowth of quantum cascade laser active regions on metamorphic buffer layers

Abstract

Communicated by T. PaskovaKeywords: A3. Superlattice B3. Quantum cascade lasers A3. Metalorganic vapor phase epitaxy B2. Semiconducting III-V materials a b s t r a c t Metamorphic buffer layers (MBLs) were used as substrates with lattice constants selected for designing and fabricating intersubband transition sources involving strained superlattices (SLs) such as Quantum Cascade Lasers (QCLs). Chemical mechanical planarization (CMP) was used to prepare the InGaAs-based MBLs for epitaxial growth. Indium enrichment of the InGaAs layer on the MBL surfaces was observed when annealed at the regrowth temperatures. This post-anneal enhancement was eliminated by including a wet-etch treatment after CMP, which results in an epi-ready surface for regrowth. Ten stages of a QCL core region structure, designed for emission at a 3.4 μm wavelength are regrown on a surfaceoptimized MBL. Such structures exhibit well defined X-ray diffraction pendellösung fringes, and transmission electron microscopy confirms planar superlattice interfaces with layer thicknesses that are in good agreement with the design target.

Design considerations for λ ∼ 3.0- to 3.5-μm-emitting quantum cascade lasers on metamorphic buffer layers

“…Such a design, based on an MBL cap composition of In 0.22 Ga 0.78 As, with a 10-stage core region consisting of AlAs barriers and In 0.45 Ga 0.55 As wells, and designed to emit at 3.39 μm has been grown by using metalorganic chemical vapor deposition (MOCVD) of high crystalline quality. 20 The layers thicknesses (expressed in Å) within one stage of the QCL core region are as follows: 25/16/24/17/21/18/20/21/19/21/18/18/17/ 28/12/13/39/10/33/16. The bold-faced layers are the wells and the other layers are barriers, with the doped layers (n-type, 2 × 10 17 cm −3 ) indicated by italics.…”

“…The final layer was a constant-composition cap layer which is typically ∼15-μm thick to allow for surface preparation in order to perform the regrowth of strained layers atop with high fidelity. 20 The MBL cap is found to be nearly fully relaxed (>95%), owing to its thickness, and exhibits tilt with respect to the substrate, which is a function of composition and thickness. 13,21 The MBL enables trapping of misfit dislocations at the composition steps while forcing the threading dislocations to glide to the edges of the sample, thus giving us a virtual substrate with a threading-dislocation density of the order of ∼10 5 cm −2 .…”

“…An additional wet-chemical etching step introduced after the CMP resolved this issue and provided an epi-appropriate surface for regrowth of strained SL layers. 20 Ten stages of the QCL structure of the AR design for 3.4-μm emission target wavelength were grown with high fidelity on an MBL using the aforementioned optimized surface treatment, as confirmed by x-ray diffraction and transmission electron microscopy. 20 To enable growth of the complete laser design, several challenges remain such as the choice of cladding material and optical-waveguide design, which are being addressed in this study.…”

“…20 Ten stages of the QCL structure of the AR design for 3.4-μm emission target wavelength were grown with high fidelity on an MBL using the aforementioned optimized surface treatment, as confirmed by x-ray diffraction and transmission electron microscopy. 20 To enable growth of the complete laser design, several challenges remain such as the choice of cladding material and optical-waveguide design, which are being addressed in this study. Moreover, the structural, thermal, and optical characteristics of the ternary cladding material, with specific compositions that are lattice-matched to the MBL, have not been previously reported.…”

“…18 We have previously proposed the use of metamorphic buffer layers (MBLs) as the means to achieve highperformance low-strain QCLs at 3.0-to 3.5-μm emission wavelengths. 19,20 These "virtual substrates" under consideration were grown on (001) GaAs substrates by hydride vapor phase epitaxy. 21 They consisted of nine In x Ga 1−x As layers with linear grading of the In content in each of the nine ∼1.0-μm-thick steps.…”