IEEE JOURNAL OF QUANTUM ELECTRONICS, VOL. 46, NO. 12, DECEMBER 2010 1827 Patterned Quantum Dot Molecule Laser Fabricated by Electron Beam Lithography and Wet Chemical Etching V. B. Verma, Member, IEEE, U. Reddy, Student Member, IEEE, N. L. Dias, Student Member, IEEE, K. P. Bassett, X. Li, Senior Member, IEEE, and J. J. Coleman, Fellow, IEEE Abstract —We report on the fabrication and characterization of an edge-emitting semiconductor laser with a gain medium con- sisting of two layers of patterned, self-aligned, vertically coupled quantum dots (QDs) using a wet-etching and regrowth technique. A threshold current density of 300 A/cm 2 is demonstrated at 77 K. The presence of emission from QD excited states in both the spontaneous emission and laser spectra indicates 3-D quantum confinement in QDs fabricated using this technique. Index Terms—Coupled quantum dots, quantum dot laser, quantum dot molecule, quantum dots. I. Introduction Q UANTUM DOT (QD) semiconductor lasers have demonstrated desirable properties when compared with traditional quantum well (QW) based lasers such as ultralow threshold current density and reduced temperature sensitivity [1]–[6]. Typically multiple layers of QDs must be stacked in order to provide enough gain to overcome cavity losses and achieve laser threshold. Due to strain fields created by the first QD layer, the QDs in subsequently grown layers tend to self- align with QDs in the first layer, resulting in vertical electronic coupling of the QD layers for small spacer layer thicknesses [7]–[9]. The result is effectively an array of QD “molecules,” which have also been proposed as potential building blocks for solid state quantum information processing applications [10]–[12]. The most common technique for QD growth is the self- assembly technique, in which QDs form as a result of the large strain which exists between the QD material and substrate material [13]. This technique has the advantage of produc- ing QDs of high optical quality. However, the self-assembly technique results in QDs with random positions and a broad Manuscript received December 5, 2009; revised February 24, 2010; accepted March 7, 2010. Date of current version November 24, 2010. This work was supported in part by the Defense Advanced Research Projects Agency, under Grant No. 433 143-874a and in part by the National Science Foundation, under Grant No. 08-21 979. This paper was recommended by Associate Editor A. C. Bryce. V. B. Verma is with the National Institute of Standards and Technology, Boulder, CO 80305-3337 USA (e-mail: verma@illinois.edu). U. Reddy, N. L. Dias, K. P. Bassett, X. Li, and J. J. Coleman are with the Department of Electrical and Computer Engineering, Univer- sity of Illinois at Urbana-Champaign, Champaign, IL 61801 USA (e- mail: cvreddy@illinois.edu; ndiasan2@illinois.edu; kpbasset@illinois.edu; xi- uling@illinois.edu; jcoleman@illinois.edu). Digital Object Identifier 10.1109/JQE.2010.2047246 size distribution which increases inhomogeneous broadening, decreases peak gain, and increases threshold current density when incorporated into a laser structure. In addition, in QD molecules fabricated using the self-assembly technique, the height and diameter of QDs have been observed to vary between layers, with the general trend being a progressive increase in diameter and thickness for QDs in subsequently grown layers [14]–[16]. Many alternative techniques have been explored to achieve better control over QD properties, such as metalorganic chem- ical vapor deposition (MOCVD) or molecular beam epitaxy regrowth using a patterned SiO 2 growth inhibition mask [17], [18], prepatterning of InGaAs thin films [19], [20], growth in inverted pyramid shaped recesses [21], the preparation of preferential growth facets via selective area epitaxy [22], [23], manipulation of the local crystal plane step edge density [24], and etching of an existing QW or multiple QW structure [25]– [30]. To date there has only been one report of the use of patterned QDs in a semiconductor laser using the selective area epitaxy technique [31]. The primary challenges have been the development low-damage processing techniques which yield high QD densities as well as the development of barrier layer regrowth techniques. Recently, we presented photoluminescence characteristics of patterned QDs fabricated using electron beam lithography and wet chemical etching which allows for the precise positioning, both spatially and spectrally, of individual QDs [32]. Here we report on the incorporation of two vertically-coupled, self- aligned layers of wet-etched patterned QDs into the active layer of an edge-emitting laser. The proposed wet etching and regrowth technique is unique in that it allows for precise engineering of the discrete energy states of each component QD of the coupled QD molecule. This can be accomplished by adjusting the indium composition and thickness of each QW before wet etching is performed using the well-established accuracy and repeatability of 2-D planar growth methods. II. Experiment The epitaxial base structure was grown by MOCVD in a Thomas Swan reactor at atmospheric pressure. The structure was grown on an n+ (100) GaAs substrate, and consists of a 200nm n+ GaAs buffer layer, a 2 µm n-type Al 0.75 Ga 0.25 As 0018-9197/$26.00 c  2010 IEEE