Band alignment and optical absorption in Ga(Sb)N alloys

Abstract

We extend the theory of band alignment proposed by Harrison to ternary and quaternary heteropolar semiconductors. Combining this with first-principles density functional theory incorporating the LDA/GGA+U formalism (LDA: local density approximation; GGA: generalized gradient approximation) can result in useful electronic structure predictions for new alloys. The practicality of this is demonstrated by application to the Ga(Sbx)N1-x alloys, where the feasibility of water splitting reaction under visible light irradiation is discussed.

Band gap engineering via doping: A predictive approach

“…For example, if we assume that d GaÀSb ¼ 0:5ðd GaN þ d GaSb Þ, then the Sb-impurity level can be located at approximately 1.0-1.5 eV above the E GaN VBM , and therefore, can reduce the energy gap of GaN by an appreciable amount. 13,14 Our ab initio results do confirm the above predictions. In particular, we find that even at a very small Sb-concentrations (x Sb 1%) impurity bands are formed within the energy gap of GaN, "pinning" the Fermi energy leading not only to a significant reduction of the band gap value but also turning the direct nature of the GaN gap into an indirect one.…”

“…13 GaN is a wide band gap material with a fundamental band gap of %3.4 eV. 12,24 The experimental band gap of bulk GaSb is 0.73 eV.…”

“…This means that the impurity levels which are introduced by the Sb-impurities can indeed shift E F above the E GaN VBM and lead, thus, to an appreciable reduction in the energy gap of GaN. 13,14 The exact location of the Sb-impurity level depends on the Ga-Sb bond-length, d GaÀSb , that will be developed. For example, if we assume that d GaÀSb ¼ 0:5ðd GaN þ d GaSb Þ, then the Sb-impurity level can be located at approximately 1.0-1.5 eV above the E GaN VBM , and therefore, can reduce the energy gap of GaN by an appreciable amount.…”

“…If the impurity states are formed within the energy range of the energy gap, this will cause its reduction and most probably will shift the Fermi energy, E F , 13 into the impurity bands. On the other hand, if the impurity states are created outside the gap, it is expected that they will not affect the gap value in an appreciable way.…”

“…In particular, we presented an extension of Harrison's theory applicable to ternaries and quaternaries within a virtual crystal type of approximation (VCA). 13 This is a fundamental step in verifying whether the gap of a doped semiconductor straddles the redox potentials of water (i.e., it satisfies the (ii) of the prerequisites for PEC applications mentioned above). Furthermore, we used our proposed theory to justify that the doping of GaN with Sb can indeed lead to band gap narrowing in agreement with ab initio and experimental results.…”