267 research outputs found
Anion vacancies as a source of persistent photoconductivity in II-VI and chalcopyrite semiconductors
Using first-principles electronic structure calculations we identify the
anion vacancies in II-VI and chalcopyrite Cu-III-VI2 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 VO in n-ZnO, and persistent hole photoconductivity (p-type PPC) caused
by the Se vacancy VSe in p-CuInSe2 and p-CuGaSe2. We find that VSe in the
chalcopyrite materials is amphoteric having two "negative-U" like transitions,
i.e. a double-donor transition e(2+/0) close to the valence band and a
double-acceptor transition e(0/2-) closer to the conduction band. We introduce
a classification scheme that distinguishes two types of defects (e.g., donors):
type-alpha, which have a defect-localized-state (DLS) in the gap, and
type-beta, 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 perturbed-host-state (PHS). Type alpha is
non-conducting, whereas type beta is conducting. We identify the neutral anion
vacancy as type-alpha and the doubly positively charged vacancy as type-beta.
We suggest that illumination changes the charge state of the anion vacancy and
leads to a crossover between alpha- and beta-type behavior, resulting in
metastability and PPC. In CuInSe2, the metastable behavior of VSe is carried
over to the (VSe-VCu) complex, which we identify as the physical origin of PPC
observed experimentally. We explain previous puzzling experimental results in
ZnO and CuInSe2 in the light of this model.Comment: submitted to Phys. Rev.
Polaronic hole localization and multiple hole binding of acceptors in oxide wide-gap semiconductors
Acceptor-bound holes in oxides often localize asymmetrically at one out of
several equivalent oxygen ligands. Whereas Hartree-Fock (HF) theory overly
favors such symmetry-broken polaronic hole-localization in oxides, standard
local density (LD) calculations suffer from spurious delocalization among
several oxygen sites. These opposite biases originate from the opposite
curvatures of the energy as a function of the fractional occupation number n,
i.e., d2E/dn2 0 in LD. We recover the correct linear
behavior, d2E/dn2 = 0, that removes the (de)localization bias by formulating a
generalized Koopmans condition. The correct description of oxygen
hole-localization reveals that the cation-site nominal single-acceptors in ZnO,
In2O3, and SnO2 can bind multiple holes
Defect complexes formed with Ag atoms in CDTE, ZnTe, and ZnSe
Using the radioactive acceptor Ag for perturbed --angular correlation (PAC) spectroscopy for the first time, defect complexes formed with Ag are investigated in the II-VI semiconductors CdTe, ZnTe and ZnSe. The donors In, Br and the Te-vacancy were found to passivate Ag acceptors in CdTe via pair formation, which was also observed in In-doped ZnTe. In undoped or Sb-doped CdTe and in undoped ZnSe, the PAC experiments indicate the compensation of Ag acceptors by the formation of double broken bond centres, which are characterised by an electric field gradient with an asymmetry parameter close to h = 1. Additionally, a very large electric field gradient was observed in CdTe, which is possibly connected with residual impurities
Identification of Ag-acceptors in Ag Cd doped ZnTe and CdTe
Nominally undoped ZnTe and CdTe crystals were implanted with radioactive Ag, which decays to Cd, and investigated by photoluminescence spectroscopy (PL). In ZnTe, the PL lines caused by an acceptor level at 121 meV are observed: the principal bound exciton (PBE) line, the donor-acceptor pair (DAP) band, and the two-hole transition lines. In CdTe, the PBE line and the DAP band that correspond to an acceptor level at 108 meV appear. Since the intensities of all these PL lines decrease in good agreement with the half-life of Ag of 178.8 h, both acceptor levels are concluded to be associated with defects containing a single Ag atom. Therefore, the earlier assignments to substitutional Ag on Zn- and Cd-lattice sites in the respective II-VI semiconductors are confirmed. The assignments in the literature of the S, S, and S lines in ZnTe and the X X/ C and C lines in CdTe to Ag-related defect complexes are not confirmed
The problem of a metal impurity in an oxide: ab-initio study of electronic and structural properties of Cd in Rutile TiO2
In this work we undertake the problem of a transition metal impurity in an
oxide. We present an ab-initio study of the relaxations introduced in TiO2 when
a Cd impurity replaces substitutionally a Ti atom. Using the Full-Potential
Linearized-Augmented-Plane-Wave method we obtain relaxed structures for
different charge states of the impurity and computed the electric-field
gradients (EFGs) at the Cd site. We find that EFGs, and also relaxations, are
dependent on the charge state of the impurity. This dependence is very
remarkable in the case of the EFG and is explained analyzing the electronic
structure of the studied system. We predict fairly anisotropic relaxations for
the nearest oxygen neighbors of the Cd impurity. The experimental confirmation
of this prediction and a brief report of these calculations have recently been
presented [P.R.L. 89, 55503 (2002)]. Our results for relaxations and EFGs are
in clear contradiction with previous studies of this system that assumed
isotropic relaxations and point out that no simple model is viable to describe
relaxations and the EFG at Cd in TiO2 even approximately.Comment: 11 pages, 8 figures, Revtex 4, published in Physical Review
Valency of rare earths in RIn3 and RSn3: Ab initio analysis of electric-field gradients
In RIn3 and RSn3 the rare earth (R) is trivalent, except for Eu and Yb, which
are divalent. This was experimentally determined in 1977 by perturbed angular
correlation measurements of the electric-field gradient on a 111Cd impurity. At
that time, the data were interpreted using a point charge model, which is now
known to be unphysical and unreliable. This makes the valency determination
potentially questionable. We revisit these data, and analyze them using ab
initio calculations of the electric-field gradient. From these calculations,
the physical mechanism that is responsible for the influence of the valency on
the electric-field gradient is derived. A generally applicable scheme to
interpret electric-field gradients is used, which in a transparent way
correlates the size of the field gradient with chemical properties of the
system.Comment: 10 page
- …