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Notes: The energetics of self-compensation through atomic relaxation around acceptor impurities in ZnSe were examined via first principles total energy calculations. We find large charge state and impurity-dependent lattice relaxations for As and P acceptors which can account for the experimentally observed difficulties in obtaining low-resistivity p-type ZnSe from these dopants. A much smaller relaxation is found for Li.

Notes: Transport measurements on large single crystals of Cd0.8Zn0.2Te:Cl indicate that Cl donors form DX centers in CdZnTe. We have observed persistent photoconductivity (PPC) with an annealing temperature Ta≈130 K. Hall-effect experiments indicate that the PPC arises from a persistent increase in the density of charge carriers; the saturation density is Nsat=6×1016 cm−3. The deep binding energy of the DX center is Ed=0.22 eV. © 1995 American Institute of Physics.

Notes: Electronic and ionic structures of lead vacancies in PbTiO3 were studied using an ab initio approach. Even though the lead vacancy is found to be an acceptor with stable charge states ranging from 2- to 4- it does not form a tightly bound defect pair with a double donor oxygen vacancy. The formation of distant and nearby lead-vacancy–oxygen-vacancy pairs is shown to be exothermic under certain growth conditions. © 2000 American Institute of Physics.

Notes: We have calculated the electronic and atomic structures for 180° domain walls in PbTiO3 using a first-principles total energy method. Domain walls are found to be Pb centered and extremely narrow with a width of only about two lattice constants. The energy density of a domain wall is calculated to be 0.1–0.2 J/m2. © 1999 American Institute of Physics.

Notes: We describe a family of reversible holographic storage materials which exploit the bistability of the crystal defect known as the "DX" center. Crystals containing these defects have the characteristics of local photorefractive materials in that their refractive index is modified in proportion to the local optical energy absorbed. This refractive index change, which results from the release of electrons from the DX deep trap states into the conduction band, is persistent at low temperatures due to a capture barrier, Ecap, which limits reformation of the DX centers. The effect is reversed by heating above an annealing temperature, which scales with Ecap and varies with the crystal host and active dopant. A number of DX materials have now been identified with long-term persistence temperatures ranging from 50 to 180 K. In this paper, we briefly review the physics of the DX center and present theoretical estimates of several important optical properties of these materials based on a simple model. We calculate spatial resolution, maximum refractive index shift, and sensitivity, and compare our predictions with measurements on one member of the DX family, AlGaAs:Te. In a 345 μm thick sample of this material doped at 9×1017 cm−3, we find a refractive index shift, Δn, of 2×10−3 and an exposure sensitivity, S, of 0.012 cm3/J. Our expectation that the maximum refractive index change scales linearly with the doping density is consistent with our previous measurement of Δn=1.1×10−2 obtained for a sample of AlGaAs:Si doped at 4×1018 cm−3. The measured values of Δn and S, are, respectively, two and three orders of magnitude larger than corresponding values for the photorefractive material LiNbO3, and are shown here to be independent of exposing irradiance from 10−3 to 108 W/cm2. At the latter irradiance, the refractive index shift is shown to occur with a material response time shorter than our measurement limit of several picoseconds. Thus, this material exhibits high sensitivity, large refractive index change, and fast write time, all desirable properties of an optical holographic storage medium. Phase gratings written in AlGaAs:Te using low-power (mW) beams from infrared diode lasers give diffraction efficiencies from 30% to 55% for grating periods from 0.13 to 15 μm. No degradation of sensitivity is observed after large numbers of exposure–erasure cycles. Experiments with multiple-hologram exposures show that the DX materials require no exposure schedule: equal strength holograms are obtained using equal exposures. Binary data have been stored in the form of multiplexed two-dimensional arrays of pixel bits. Required material and system parameters are estimated for a 1 Tbyte holographic storage device based on angle multiplexing in a DX material. © 1998 American Institute of Physics.

Notes: First-principles pseudopotential calculations are used in conjunction with extensive experimental data on P and As-derived acceptor states in ZnSe to develop a microscopic theory of their atomic and electronic properties. A structural model that explains the presence of both shallow and deep acceptor states, the thermal and optical quenching of photoluminescence lines, and the strong C3v symmetry of the electron-spin-resonance (ESR) active state is derived. The primary result of the calculations is that a neutral acceptor possesses two atomic configurations: a metastable effective-mass state with a small lattice relaxation labeled a0, and a deep A0 state with a large lattice distortion which is responsible for most of the observed properties of acceptors in ZnSe. Nitrogen impurities are proposed to give rise to a shallow acceptor state in either the small or large-lattice-relaxed limits. Extension of the results to ZnTe is discussed.

Notes: Experimentally identified deep levels in p-type GaN at approximately 0.9–1, 1.4, and 1.8–2 eV above the valence-band maximum have been attributed to Ga vacancies. From the results of first-principles calculations, we find that from both energetic and electronic level structure standpoints it is necessary to consider the structural modification VGa→Nanti+VN, resulting from the transfer of a nearest-neighbor N atom to a Ga-vacancy site (VGa) to explain the levels at 1 and 2 eV. Isolated N-antisite (Nanti) and nitrogen-vacancy (VN) defects are found to give rise to additional deep levels at 1.4 and 0.8 eV, respectively. © 1997 American Institute of Physics.

Notes: From the results of first-principles calculations, we find that substitutional Cl, and Br and I impurities in Si are stable shallow single donor dopants that are mostly immune to deactivation processes preventing superhigh n doping of Si with column V impurities. In the absence of acceptor impurities, such as H and F, room-temperature carrier densities above 1020/cm3 are proposed to be possible, particularly with Cl, which causes the least lattice strain. © 1997 American Institute of Physics.

Notes: The microscopic structures and binding energies of DX centers for column III and VII impurities in CdTe are determined through first-principles total energy calculations. The ionic displacements leading to DX formation for column VII impurities in II-VI semiconductors are found to be different than those for corresponding column VI impurities in III-V semiconductors. Three distinct types of structures with DX-like properties are found for column VII donors. The relative stability of these structures is impurity and pressure dependent. © 1995 American Institute of Physics.