Charged Defect States Research Articles

A STM study of the InP (110) surface

The energy of the surface states of the InP (110) surface and their positions in the surface Brillouin zone were determined by scanning tunneling microscopy (STM). There surface point defects influencing the electronic structure of the surface were studied. A positively charged P vacancy and its diffusion on the surface was examined and its diffusion constant calculated. A charge transition from a positive to a neutral charge state of another point defect was found to be tip-induced and accompanied by a symmetry-lowering of the defect. An explanation in terms of the Jahn-Teller effect is proposed. Finally, a positively charged point defect caused band bending which had been measured in I–V characteristics. It is shown that this point defect is probably an antisite defect P In.

The influence of antimony impurity on optical and electrical properties of amorphous selenium

Amorphous Se1-xSbx alloys were prepared using a conventional rapid quenching technique. It is found that antimony can only be substituted up to approximately 10 at.% in selenium to produce Se-Sb glassy alloys. The photoacoustic technique is employed for the first time to determine the optical energy gaps of Se1-xSbx glasses. The optical energy gap of amorphous selenium is found to be 1.99+or-0.02 eV. It reduces drastically on substitution of 2 at.% Sb after which its variation with x is small. The activation energies of Se1-xSbx glasses have been determined from the temperature dependence of their electrical resistivities. The activation energy as a function of x also shows a sudden decrease in the range 0.01<or=x<or=0.02. Plausible explanations have been suggested based on the Street-Mott model for charged defect states in amorphous chalcogenides.

Deep-level defects in high-resistivity GaAs grown by the horizontal Bridgman technique

In a horizontal Bridgman (HB) apparatus the authors have grown high-resistivity GaAs. With optical detection of electron spin resonance and optical measurements they detected both charge states of EL2, a Ga antisite-related defect, and the paramagnetic charge state of an oxygen-related defect as the dominant deep-level defects in this material. The latter is probably the off-centre substitutional oxygen defect with negative-U properties recently identified by local vibrational mode measurements. For the first time they observed a new paramagnetic As antisite-related defect besides EL2 in low-resistivity as-grown HB GaAs.

Positron Annihilation in Electron Irradiated GaAs: Atomic Structure and Charge State of the Defects

Positron Annihilation in Electron Irradiated GaAs: Atomic Structure and Charge State of the Defects

EBIC INVESTIGATIONS OF EXTENDED DEFECTS IN CdTe

The EBIC and remote contact EBIC (REBIC) techniques have been used to reveal grain boundaries and precipitates in CdTe crystals and to study their recombination contrast as a function of the electron beam parameters and temperature. The results obtained are discussed taking into account the defect charge state and the recombination properties of their environment.

Extrinsic transient diffusion in silicon

The effect of extrinsic background doping on the transient enhancement of dopant diffusion for an ion-implanted dopant is investigated to gain insight into the role of point defect charge states. The transient effect is found to be greatly increased for extrinsic background doping of the same type as the implanted ion, and reduced for background doping of the opposite type. Analysis of the relative enhancements for boron and phosphorous allows the position of the donor and acceptor levels for the silicon self-interstitial to be extracted. The results are in good agreement with earlier work based on extrinsic oxidation-enhanced diffusion.

Charge state of intrinsic interstitial defects and their substitution of boron atoms in the silicon lattice

The displacement of boron atoms from silicon lattice sites by silicon interstitial atoms was studied, by galvanomagnetic and x-ray methods. The substitution is controlled by the charge state of the interstitial defects. The charge transfer was effected by a change in the level of ionization during implantation by varying the ion-beam density and during annealing by additional electron bombardment.

Transient Phosphorus Diffusion Below the Amorphization Threshold

The transient‐enhanced diffusion of phosphorus in silicon has been investigated for doses and energies below the threshold for amorphization of the substrate. Transient‐enhanced diffusion occurs both for furnace and RTA annealing, and can increase the junction depth by 0.1–0.2 μm over that predicted by standard diffusion models. The magnitude of the transient is shown to depend on the implantation dose and energy, the anneal time and temperature, and the background doping level. A simple model of damage annealing is proposed which quantitatively describes the point defect enhancement of dopant diffusion and so allows each of these dependencies to be understood. In addition, these experiments provide a new means to characterize the diffusion and charge states of the point defects themselves.

Optical and magneto-optical determination of the EL2 concentrations in semi-insulating GaAs

A comparison is presented of two different optical methods to determine the concentrations of both charge states of EL2 defects. It is shown that a discrepancy exists for published optical cross sections and that optical absorption measurements are not suitable to determine the concentrations of the ionized EL2 defects with sufficient precision in semi-insulating GaAs wafers.

Electronic structure of the isolated vacancies and divacancy in InP.

Within the self-consistent semiempirical tight-binding theory, a study of the electronic structure of the undistorted vacancies and divacancy in InP has been performed using the Lanczos-Haydock recursion method. We predict the energy positions and the localizations of the deep levels for all possible charge states of these vacancy defects. In the cases of the isolated In and P vacancies, we find that with the Fermi level at energies above the midpoint of the fundamental band gap, the two isolated vacancies stay at opposite charge states and thus attract each other. In the case of the divacancy, we demonstrate that the defect can have many different charge states, and introduces many deep levels at energies around the middle of the fundamental band gap, revealing the complex nature of the defect. A model for the description of the electronic structure of the divacancy is presented. We discuss the complicated structure of the deep levels of the divacancy in all its charge states in terms of the calculated localizations of these levels. We compare our calculated results for the isolated In and P vacancies with local-density calculations. We show also that in the calculation for the divacancy, it is important for the self-interaction to be well accounted for.

Phenyl ring rotations, structural order and electronic states in polyaniline

Much progress has recently been made in the physics and chemistry of the polyanilines with respect to several central issues, such as the nature of the electron-phonon coupling in these materials, the stability of polarons versus bipolarons, and the role of disorder. The three primary oxidation states of polyaniline (leucoemeraldine, emeraldine and pernigraniline) have been synthesized. Photoinduced absorption studies on these materials reveal long-lived weak induced infrared activity and strong induced electronic transitions, demonstrating the importance of massive polarons (and other related defects) involving distortions of the ground state ring angles in these ring-containing polymers. The properties of recently synthesized ring derivatives of polyaniline, such as poly( o-toluidine), can also be understood by changes in ring-rotational properties. Localization effects are enhanced through chain derivatization, which increases the chain separation and thus reduces interchain interactions. The isolation of partially crystalline forms, fibers and oriented films of the emeraldine oxidation state of polyaniline has allowed not only the determination of structural information via X-ray diffraction, but also a correlation between the microscopic structure and the magnetic properties of the charged defect states of these materials. While a metallic polaron lattice is formed in protonated crystalline regions of emeraldine base, spinless defects form upon protonation of the amorphous regions.

Spectroscopy and defect states in polyaniline.

We report the far-infrared through visible photoinduced absorption (PA) spectra of polyaniline in the emeraldine-base and leucoemeraldine-base forms. The direct-absorption spectrum of emeraldine base (EB) has a broad absorption centered at 2 eV (the ``exciton'' band) and an absorption band at \ensuremath{\sim}3.6 eV (the \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ band gap). The PA spectra of EB for pumping into the ``exciton'' band and across the band gap are nearly identical, indicating the same types of charged defect states are created upon photoexcitation. The direct absorption spectrum of leucoemeraldine base (LB) shows only the \ensuremath{\pi}-${\mathrm{\ensuremath{\pi}}}^{\mathrm{*}}$ band-gap absorption at \ensuremath{\sim}3.6 eV. For pumping into this absorption band, the PA spectrum of LB is very similar to that of EB, although important differences result from the lack of quinoid structures in this material. Based on our results, we propose a model for the photocreation of defect states in leucoemeraldine base and emeraldine base. The central roles of phenyl-ring rotations and of massive polarons are discussed.

Ionization entropy and charge-state-controlled metastable defects in semiconductors

We propose a new theory of entropy-driven, charge-state-controlled metastability in semiconductors. The entropy change due to metastable defect ionization near 300 K reduces the Gibbs free energy by up to 0.1 eV. This affects equilibrium populations of the various defect charge states and configurations. An example is found in the experiment of Hamilton, Peaker, and Pantelides [Phys. Rev. Lett. 61, 1679 (1988)] in which a deep level transient spectroscopy signal suddenly disappears during cooling when the ionization entropy term causes a defect’s stable local energy minimum to become metastable.

Capacitance studies of deep defects in hydrogenated amorphous silicon

A number of different studies using junction capacitance and related techniques to deduce gap state properties in a-Si:H are reviewed. Included is a discussion of the DLTS and junction photocapacitance methods to deduce the density of states, lattice relaxation effects in transitions from deep states, ion-implantation induced impurity related defects, and the study of the c-Si/a-Si:H interface in heterojunction samples. Results using depletion width modulated ESR to deduce the charge state of deep defects in n-type doped and intrinsc samples are also discussed.

Inhomogeneous broadening and the breakdown of the quantum Hall effect in GaAs/AlGaAs

The critical current for the breakdown of the quantum Hall effect at fixed filling factors has been observed to drop by more than a factor of ten when a GaAs/AlGaAs heterostructure is subjected to thermal cycling after exposure to a light pulse. Concurrent changes in the transport parameters suggest that the metastable, charged deep defect states in the doped AlGaAs layer, which are also responsible for the persistent photoconductivity effect, tend to increase inhomogeneities of the carrier density with thermal cycling and reduce the effective width for the current path in the quantum Hall regime.

Ring-rotational defects in polyaniline

The effects of electron-lattice coupling via ring rotation on the electronic structure of leucoemeraldine base (LB) and its charged defects are investigated. We suggest that in LB and other phenyl-ring-containing polymers the charged defect states, particularly hole polarons, are associated with localized distortions of the ground state ring torsion angles toward a planar conformation, in contrast with the bond-alternation defects present in conducting polymers. The relevance of these defects to recent photoinduced optical experiments is discussed.

The Symmetry of the EL2 Defect in GaAs

AbstractLinear dichroism has been measured in the broad absorption band of the EL2 defect in GaAs under uniaxial stress. In addition, the splittings of the EL2 zero phonon line (ZPL) at 8378 cm-1 under uniaxial stress applied along [100], [111], and [110] directions have been measured. Splitting of the ZPL under [100] stress is over one order of magnitude smaller than under [111] stress, on the other hand, the linear dichroism in the broad absorption band is roughly equal for these two directions of stress. This is an evidence for the quenching of the coupling to tetragonal strains due to interaction with trigonal modes of the lattice (the Ham effect). Therefore, it is established, that the excited T2 state of EL2 is a localized state subject to dynamical Jahn-Teller coupling to trigonal modes of the lattice. The possibility that the excited T2 state has hydrogenic nature associated with the L minima is ruled out by the present results. The observed splittings of the ZPL together with polarization selection rules clearly indicate the tetrahedral Td symmetry of the EL2 defect ruling out any other point group in particular trigonal C3v In view of the presented experimental results, their interpretation, and recent theoretical investigations, the isolated arsenic antisite AsGa most successfully accounts for the properties of the neutral charge state of the EL2 defect.

Photoinduced Effects on Gap State Profiles and Recombination Processes in Amorphous Selenium Studied by Photoacoustic Spectroscopy and Xerographic Discharge Measurements

The effects of prolonged illumination on gap state profiles and recombination processes in amorphous selenium were studied by photoacoustic spectroscopy and xerographic discharge measurements. The photoacoustic signal decreases and the density of charged defect states increases with increasing illumination time. The experimental results show that after prolonged illumination the trapping at charged defects increases and recombination of holes with electrons at neutral defects decreases. We propose a model of photostructural change and recombination processes in amorphous selenium.

Study of the Density of Bulk Gap States in Undoped a-Si:H by Depletion Width Modulation Electron Spin Resonance

ABSTRACTThe density of gap states in the bulk of intrinsic a-SiH has been probed using depletion width modulation electron spin resonance spectroscopy (DWM ESR) as well as complementary capacitance techniques. Our sample consists of a 2.85 micron thick layer of undoped a-Si:H sandwiched between a large area Pd Schottky barrier contact and a p+ c-Si substrate, allowing charge-counting capacitance and spin-counting ESR measurements to be performed on the same sample. DWM ESR measures the spin change that occurs when electrons are removed from gap states by increasing reverse bias on a Schottky barrier. The phase of the signal indicates whether unpaired spins are created or destroyed with increasing bias, thus revealing the defect charge state at the Fermi level The DWM ESR signal from our sample indicates that removing electrons from gap states leads to a reduction of the spin signal. This is qualitatively consistent with a band of Do states near EFo. However, for all metastable states and at all temperatures studied, the magnitude of the DWM ESR signal account for only about 14% of the responding charge as determined by junction capacitance. This would occur only if the majority of the filled bulk gap states were doubly occupied with a small correlation energy Ueff. In that case the majority of transitions caused by depletion width modulation would be D−/D+ which would produce no spin change. Model calculations show that the small observed Do/D+ DWM ESR signal would indeed result from the occupation statistics for a near zero value of Ueff at the moderate temperatures of the experiment. Comparison with our data sets the overall limits If −20meV &lt; Ueff &lt; 50 meV. Implication for recent models of the density of states in intrinsic a-Si:H are discussed.

Alternating donorlike-acceptorlike configurationally bistable defect in irradiated phosphorus-doped silicon.

Using deep-level transient spectroscopy we have detected a new configurationally bistable defect in heavily phosphorus-doped silicon, electron irradiated at room temperature. The center has the unique property of being electron attractive in one of its configurations and hole attractive in the other. The activation energies for electron and hole emission for the corresponding traps are determined as 0.42 and 0.15 eV, respectively. The configurational transformation of this center can be activated by the cool-down of the sample under applied zero, forward, or reverse bias, but also by certain electrical pulsing schemes. A general model for the bistable defect as a close deep donor-acceptor pair is suggested, where the configurational transformation is accompanied by a partial internal charge transfer in the neutral charge state of the complex defect.