Anion vacancies as a source of persistent photoconductivity in II-VI and chalcopyrite semiconductors

Abstract

Using first-principles electronic structure calculations we identify the anion vacancies in II-VI and chalcopyrite Cu-III-VI 2 semiconductors as a class of intrinsic defects that can exhibit metastable behavior. Specifically, we predict persistent electron photoconductivity (n-type PPC) caused by the oxygen vacancy V O in n-ZnO, and persistent hole photoconductivity (p-type PPC) caused by the Se vacancy V Se in p-CuInSe 2 and p-CuGaSe 2 . We find that V Se in the chalcopyrite materials is amphoteric having two "negative-U" like transitions, i.e. a double-donor transition ε(2+/0) close to the valence band and a double-acceptor transition ε(0/2−) closer to the conduction band. We introduce a classification scheme that distinguishes two types of defects (e.g., donors): type-α, which have a defect-localized-state (DLS) in the gap, and type-β, which have a resonant DLS within the host bands (e.g., conduction band). In the latter case, the introduced carriers (e.g., electrons) relax to the band edge where they can occupy a perturbedhost-state (PHS). Type α is non-conducting, whereas type β is conducting. We identify the neutral anion vacancy as type-α and the doubly positively charged vacancy as type-β. We suggest that illumination changes the charge state of the anion vacancy and leads to a crossover between α-and β-type behavior, resulting in metastability and PPC. In CuInSe 2 , the metastable behavior of V Se is carried over to the (V Se -V Cu ) complex, which we identify as the physical origin of PPC observed experimentally. We explain previous puzzling experimental results in ZnO and CuInSe 2 in the light of this model.

Deep vs shallow nature of oxygen vacancies and consequentn-type carrier concentrations in transparent conducting oxides

“…If the ionization energies of these defects are low, the system will tend to be an intrinsic n-type conductor. Some early studies have indicated that vacancies are shallow donors in these oxides [29][30][31][32][33][34][35][36][37][38], but more recent analysis based on plane-wave density functional theory (DFT), first using a Hubbard U parameter then, in later studies, a hybrid functional, tends to place them as deep centers in many TCOs [20,[39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55]. Of those that have been shown to form shallow centers [41,[56][57][58][59], it has been argued that their compact wave functions preclude the possibility of them contributing to n-type conductivity [59][60][61].…”

Transition levels of defects in ZnO: Total energy and Janak's theorem methods

“…Earlier work mainly used local density approximation (LDA) or LDA+U (Ref. 2) Hamiltonians. These methods underestimate the band gaps of wide gap oxides 2 and so corrections to the band gap are necessary when calculating TL.…”

“…2) Hamiltonians. These methods underestimate the band gaps of wide gap oxides 2 and so corrections to the band gap are necessary when calculating TL. Here, we use the B3LYP hybrid density functional 9, 10 which predicts a single particle band gap for ZnO (3.17 eV, this work) in good agreement with experiment (3.47 eV (Ref.…”

Effects of annealing in N2 ambient on traps and persistent conduction in hydrothermally grown ZnO

“…2 A second mechanism involves photoexcitation of an electron ͑or multiple electrons͒ from a deep donor into the conduction band, followed by a reconfiguration of the lattice that moves the ionized state of the deep donor well into the conduction band, say, to an energy E C + ⌬E. Lany and Zunger 18 have proposed such a mechanism for the oxygen vacancy V O , which in thermal equilibrium is deep ͑roughly E C −2 eV͒ and neutral. Their theoretical calculations find that if the two valence electrons of V O are excited ͑in separate steps͒ to the conduction band ͑while the trap-filling light is on͒ the resultant state V O ++ +2e − is pushed by the lattice reconfiguration into the conduction band to about E C + 0.2 eV.…”