Hydrogen-Nitrogen Tailors Semiconductor Optoelectronics: The Case of Dilute Nitride III-V Alloys
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Hydrogen-Nitrogen Tailors Semiconductor Optoelectronics: The Case of Dilute Nitride III-V Alloys A. Janotti Metals and Ceramics Division, Center for Computational Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, USA ABSTRACT Hydrogen is an omnipresent impurity in semiconductors, often associated with other impurities and native defects, strongly affecting their electronic properties by passivating deep and shallow levels, or activating isoelectronic centers, and can be intentionally or unintentionally incorporated. On the other hand, nitrogen has profound effects on the electronic structure of conventional III-V compounds: just a few percent of N can drastically lower the band gap of GaAs making it suitable for long-wavelength optical devices; isovalent doping of GaP by N leads to a quasidirect band gap with enhanced optical functionality. The large difference in electronegativity between N and other group V elements is expected to couple with the high chemical activity of H, raising crucial questions about the behavior of H in dilute nitride alloys that theories of hydrogen in conventional semiconductors or in commom-anion nitrides are unable to answer. Here we show that N can qualitatively alter the electronic behavior of hydrogen: In GaAsN, an H atom bonds to N and can act as a donor in its own right, whereas in GaAs and GaN, H is amphoteric; Nitrogen also stabilizes the H∗2 complex, that is otherwise unstable against the formation of interstitial H2 molecules, reversing the effect of N on the band gap of GaAs, allowing us to interpret several recent experiments. INTRODUCTION Adding just a few atomic percent of N in GaAs can simultaneously reduces the lattice constant and causes a drastic reduction of the band-gap, offering unique opportunities in band-gap engineering for laser and photovoltaic applications[1–6]. In the case of GaP, the addition of N can lead to a quasi-direct band gap with enhanced optical functionality.[7] The growth processes of these dilute nitride alloys usually involve hydrogen[8], such as in metal organic chemical vapor deposition (MOCVD) or gas-source molecular beam epitaxy (MBE)[9–11]. The large electronegativity and size mismatch between the N and other groupV anions are expected to strongly couple with the high chemical activity of H, leading to new physical and chemical effects that cannot be understood on the basis of the current theory of hydrogen in either conventional semiconductors or in common-anion nitrides[12, 13]. Infra red spectroscopy measurements in N doped GaP revealed three distinct peaks at 2885.5, 2054.1, and 1049.8 cm−1 associated with the local vibrational modes of H-related complexes. Similar results were reported for N-doped GaAs and, to explain the data, a
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H-N-H dihydride model of trigonal symmetry was proposed where two H atoms are bonded directly to a N atom[14, 15]. There are, however, several difficulties with this model: first, the N is fivefold coordinated which has never been observed in any nitride; Second, the two H s
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