Band-gap engineering in amorphous/microcrystalline Sc x Ga 1-x N.
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Band-gap engineering in amorphous/microcrystalline ScxGa1-xN. M. E. Little, ICASE, NASA Langley Research Center Hampton, VA 23666 [email protected] M. E. Kordesch Condensed Matter and Surface Science Program Department of Physics and Astronomy Ohio University, Athens, OH 45701 Abstract Reactive sputtering was used to grow thin films of ScxGa1-xN with scandium concentrations of 20%-70% on quartz substrates at temperatures of 300-675 K. X-ray diffraction (XRD) of the films showed either weak or no structure, suggesting the films are amorphous or microcrystalline. Optical absorption spectra were taken of each sample and the optical band gap was determined. The band gap varied linearly with increasing Ga concentration between 2.0 and 3.5 eV. Ellipsometry was used to confirm the band gap measurements and provide optical constants in the range 250-1200 nm. ScN and GaN have different crystal structures (rocksalt and wurzite, respectively), and thus may form a heterogeneous mixture as opposed to an alloy. Since the XRD data were inconclusive, bilayers of ScN/GaN were grown and optical absorption spectra taken. A fundamental difference in the spectra between the bilayer films and alloy films was seen, suggesting the films are alloys, not herterogeneous mixtures. Introduction Nitride semiconductors have generated much research due to their optoelectronic applications. The ability to form alloys of GaN, InN, and AlN with continuously varying band gaps is one reason for the interest in these materials. By band gap engineering and subsequent creation of heterostructures, devices such as light-emitting diodes, laser diodes, and detectors with widely varying properties can be fabricated. High temperature applications for these devices have lead researchers to look for a replacement for InN (due to its relatively low melting temperature) in these alloys. Calculations by Drabold and Stumm suggest that amorphous GaN may serve as a useful electronic material due to its lack of midgap states [1]. More recently, amorphous AlxGa1-xN has been grown and shown to have a band gap linearly dependent on the alloy composition [2]. It has also been suggested [3] that ScN, because of its higher melting temperature and lattice spacing match (even though ScN is a rocksalt structure material, not wurtzite) with GaN, may serve as a useful replacement for InN. Controvery still exists over the direct or indirect nature of the optical gap in ScN. Experimental evidence and theoretical calculations suggest that crystalline ScN has a direct band gap between 2.0-2.4 eV, with 2.1 eV being the most accepted [4-7]. Moustakas, Molnar, and Dismukes [8] report a fundamental absorption edge at 2.1 eV for polycrystalline ScN but were unable to determine whether the gap was direct or indirect. A much weaker indirect transition between 0.9 and 1.3 eV is predicted [6,7] by band structure calculations. (Our measurements do I3.29.1
not extend to low enough energy to investigate this transition). In this paper the onset of strong absorption to be the optical band gap.
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