Evidence of Deep Ultraviolet Amplified Spontaneous Emission in Electron Beam Pumped AlGaN Multiple-Quantum-Well-based Structures A. Yu. Nikiforov, W. Zhang, J. Woodward, J. Yin, R. Paiella, K. F. Ludwig, Jr., and T. D. Moustakas Dept. of Electrical and Computer Engineering, Division of Materials Science and Engineering, Photonics Center Boston University Boston, MA, U. S. A. alnik@bu.edu L. Zhou and D. J. Smith Physics Department Arizona State University Tempe, AZ, U. S. A. Abstract—In this paper we report detailed cathodoluminescence (CL) studies of deep UV emitting AlGaN multiple quantum wells (MQWs) embedded in AlN cladding layers to simulate a deep UV laser structure. These structures were produced by plasma-assisted molecular beam epitaxy (MBE) on 6H-SiC substrates. The AlGaN QWs were grown under excess gallium conditions, a growth mode consistent with liquid phase epitaxy rather than physical vapor phase epitaxy. This growth mode leads to AlGaN films with compositional inhomogeneities and thus band structure potential fluctuations. The degree of such compositional inhomogeneities depends on the amount of excess gallium and employment of indium as a surfactant during growth. The structure and microstructure of these MQWs were investigated by x-ray diffraction and TEM/STEM. Their optical properties were investigated with spatially-resolved CL spectroscopy and mapping. E-beam pumping experiments were performed by irradiating the top surface and collecting the luminescence from the cleaved edge. The observed superlinear dependence of the QW CL intensity on beam current together with linewidth narrowing strongly suggests light amplification by stimulated emission. Index Terms—Deep UV emitting AlGaN quantum wells, molecular beam epitaxy, cathodoluminescence, electron beam pumping, stimulated emission, deep UV lasers I. INTRODUCTION Deep ultraviolet (UV) emitters are likely to find a large number of applications in air, water, and surface sterilization, identification of biological and chemical agents, curing, bioanalytical instrumentation, and free-space non-line-of-sight communications. Tunability of the energy gap of AlGaN alloys in the spectral region between 360 nm and 200 nm makes them ideal candidates for such devices. UV light- emitting diodes (LEDs) emitting below 300 nm have been developed on sapphire substrates by a number of groups [1-4]. Performance of LEDs grown on sapphire generally suffers from high density of threading defects due to the 14% lattice mismatch between sapphire and AlN. Recently, UV LEDs with significantly lower concentration of threading defects have also been demonstrated on AlN substrates [5] which are available, however, only in small sizes and are prohibitively expensive. There are a few reports on lasing in AlGaN multiple quantum well (MQW)-based structures by optical pumping [6, 7]. There exists also a report on electron beam (e- beam) pumping of AlGaN MQWs [8]. SiC is a good alternative substrate for the growth of AlGaN-based structures due to a small (1%) lattice mismatch to AlN, high thermal conductivity, and ability to form laser facets by cleaving. In this paper we report the growth of AlGaN MQWs by molecular beam epitaxy (MBE) on 6H-SiC substrates with various degrees of compositional inhomogeneities and provide evidence of stimulated emission by e-beam pumping. II. EXPERIMENTAL METHODS AlGaN samples were grown on the Si-face of (0001) 6H- SiC substrates by RF plasma-assisted MBE using the technique described in [9]. Growth of AlGaN alloys on SiC substrates provides a number of challenges, in particular the accidental nitridation of the SiC substrate prior to epitaxial growth and formation of stacking mismatch boundaries at the step edges due to the polytype difference between 2H-AlGaN and 6H- or 4H-SiC. The SiC substrates were degreased in organic solvents followed by dipping them into a heated H 2 SO 4 :H 2 O 2 (3:1) mix and then into a buffered HF to remove surface contaminants and oxides. Prior to growth, the substrates were exposed at high temperature to a gallium flux followed by desorption to remove residual oxygen through formation of volatile gallium oxides. Following this step, the SiC surfaces exhibit a clear √3×√3 R30° reconstruction in the reflection high-energy electron diffraction (RHEED) pattern [9]. First, a 500 nm thick AlN film was grown at substrate temperature of 800 °C, measured with a thermocouple positioned behind the substrate. Then the substrate temperature was lowered to 790 °C and 10 AlGaN MQWs consisting of 1.5 nm wells and 40 nm barriers were deposited. Two different structures were grown and investigated. 978-1-4673-2301-7/12/$31.00 ©2012 IEEE