Direct gap Group IV semiconductors for next generation Si-based IR photonics

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Direct gap Group IV semiconductors for next generation Si-based IR photonics John Kouvetakis1, James Gallagher2 and José Menéndez 2 1 Department of Chemistry and Biochemistry, Arizona State University, Tempe, AZ 85207, U.S.A. 2 Department of Physics, Arizona State University, Tempe, AZ 85207, U.S.A. ABSTRACT This paper presents synthesis and optical properties of mono-crystalline Ge1-ySny and Ge1-xySixSny semiconductor alloys grown on Si/Ge platforms via purposely designed CVD routes using highly reactive Si/Ge/Sn hydrides including Ge3H8, Ge4H10, Si4H10 and SnD4. The Ge1-ySny materials are shown to exhibit strong and tunable photoluminescence induced by the substitution of sizable Sn concentrations in the Ge diamond lattice ultimately leading to an indirect-to-direct band gap crossover at y= 0.08-0.09. The optical data indicate that the IR coverage of the alloy extends well beyond that of elemental Ge into the broader long wavelength range suggesting a variety of applications in Si-based photonics. Ge1-x-ySixSny alloys represent the first viable ternary semiconductor among group IV elements with independently tunable lattice parameter and electronic structure. Studies of the compositional dependence of direct and indirect edges in these alloys using photoluminescence and photocurrent measurements are reviewed. The optical results show band gap variation over a wide range above and below that of Ge from 1.1 to 0.5 eV and provide the first demonstration of direct gap behavior in this semiconductor system. INTRODUCTION The successful epitaxial growth and stabilization of diamond-cubic α-Sn in the early 1980’s1 raised expectations that crystalline Ge1-ySny alloys might also achievable by epitaxial stabilization on suitable substrates.2,3 This system had attracted considerable attention since the unraveling of the semimetallic band structure of α-Sn,4 because a simple interpolation between α-Sn and Ge suggested that the alloy, unlike the parent compounds, should be a direct gap semiconductor over a broad compositional range.5 Subsequent theoretical calculations within the virtual crystal approximation gave results similar to the linear interpolation, predicting a direct band gap for 0.2 < y < 0.6.6,7 Epitaxial stabilization and non-equilibrium growth are critical for Ge1-ySny alloys because the phase diagram of this system reveals a vanishing solid solubility of the two elements.8 This is in sharp contrast with its fully miscible Ge-Si counterpart, a prototypical alloy system with widespread technological applications.9 After several attempts with mixed success in the 19871997 decade, mostly using molecular beam epitaxy (MBE) and related techniquies,10-13 our group at ASU introduced a CVD approach to Ge1-ySny alloys based on reactions of deuterated stannane (SnD4), and digermane (Ge2H6) at low temperatures between 250°C and 350°C. This method led to films with atomically flat surfaces and high structural quality, as evidenced by atomic force microscopy (AFM), electron microscopy, x-ray, and Rutherford Backscattering (RB