Defect-driven switchable polarization in SrTiO 3

Electron time-of-flight transient photocurrents have been investigated in stabilized a-Se as a function of electric field, annealing, aging (relaxation), and alloying with As and doping with Cl. The distribution of localized states (DOS) in stabilized a-Se has been investigated by comparing the measured and calculated transient photocurrents. The samples were prepared by conventional vacuum deposition techniques. The theoretical analysis of multiple-trapping transport has been done by the discretization of a continuous DOS and the use of Laplace transform formalism. The resulting DOS has distinct features: A first peak at ∼0.30eV below Ec with an amplitude ∼1017eV−1cm−3, a second small peak (or shoulder) at 0.45–0.50 eV below Ec with an amplitude 1014–1015eV−1cm−3, and deep states with an integral concentration 1011–1014cm−3 lying below 0.65 eV, whose exact distribution could not be resolved over the time scale of present experiments. The influence of doping, aging, annealing, and substrate temperature on the DOS distribution has been investigated. The doping with Cl does not affect the amplitudes of the first and second peaks while the concentration of deep states increases dramatically. The alloying with As reduces the density of deep states and seems to increase the amplitude of first and second peaks. The aging substantially reduces the deep states density and the amplitude of the second peak while the amplitude of the first one remains practically unchanged. The results have been interpreted primarily in terms of thermodynamic and intrinsic structural defects in the chalcogenide glass structure.

We investigate the distribution of occupied band-gap states in undoped, B-doped, and P-doped a-Si:H within the first \ensuremath{\sim}100 A\r{} of the surface using total-yield photoelectron spectroscopy in combination with the Kelvin probe. In clean, undoped a-Si:H the occupied density of states extracted from the measured yield spectrum consists of a linear valence-band edge, an exponential valence-band tail decreasing into the gap, and a broad band of deep defect states superposed on this tail. The deep-defect density of undoped a-Si:H measured by total yield is 2 orders of magnitude larger than that measured by photothermal deflection spectroscopy (PDS) on simultaneously prepared 1-\ensuremath{\mu}m-thick samples, from which we infer the existence of an excess density of deep defect states near the surface corresponding to an equivalent surface-state density of 3\ifmmode\times\else\texttimes\fi{}${10}^{11}$ ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}2}$.The addition of diborane (phosphine) to the glow-discharge plasma decreases (increases) the density of excess occupied near-surface defect states in the gap and shifts the Fermi energy towards the valence- (conduction-) band edge. A 0.5-eV increase in the work function over that in undoped a-Si:H accompanies the \ensuremath{\sim}98% reduction of these excess occupied near-surface deep defect states by ${10}^{\mathrm{\ensuremath{-}}5}$ B doping, from which we infer an (0.4--0.5)-eV downward band bending at the clean, undoped a-Si:H surface. The inverse logarithmic slope of the exponential intrinsic valence-band tail observed in B-doped a-Si:H is 45 meV, in excellent agreement with that inferred from dispersive-transport, electron-spin-resonance (ESR), and traveling-wave data. Dividing the occupied density of states of ${10}^{\mathrm{\ensuremath{-}}3}$ P-doped a-Si:H by the Fermi-Dirac distribution function, we observe an exponential conduction-band tail extending over more than 3 orders of magnitude in the density of states. Its inverse logarithmic slope of \ensuremath{\sim}35 meV agrees with that inferred from ESR and traveling-wave data on P-doped a-Si:H, but is slightly larger than the slope inferred from dispersive-transport measurements on undoped a-Si:H. This discrepancy arises from the presence of a large concentration of P donors which affects the deeper tail-state distribution observed in heavily-P-doped a-Si:H. The average density of occupied states at the Fermi energy ${E}_{F}$ for the 30 doped samples we have studied is 8(\ifmmode\pm\else\textpm\fi{}2)\ifmmode\times\else\texttimes\fi{}${10}^{15}$ states/eV ${\mathrm{cm}}^{3}$, in good agreement with the thermodynamic minimum density of defect states permitted in undoped a-Si:H. This implies that the Fermi level lies at a minimum in the a-Si:H density of states for all doping levels.From this result we infer that the position of the ${D}^{+}$ defect level in B-doped a-Si:H lies more than 0.5 eV above the ${D}^{\mathrm{\ensuremath{-}}}$ level in P-doped a-Si:H; an arrangement in conflict with a fixed energy distribution of deep defect states and with the generally accepted positive value of the defect correlation energy in a-Si:H. We resolve this seeming discrepancy by postulating that the amorphous network responds to changes in ${E}_{F}$ by changing its defect structure so as to minimize the total energy of the system. This postulate leads to a variable energy distribution of deep defect states that depends only on the position of ${E}_{F}$ and the defect-formation energies; intimate pairing of dopants and defects, which has been suggested to account for this discrepancy, is not required. The relation between the surface and bulk distributions of localized gap states in a-Si:H is discussed.

Impact of intrinsic point defect concentration on thermal transport in titanium dioxide

As a van der Waals magnetic semiconductor, chromium triiodide (CrI3) is widely considered for its high research value and potential applications. Defects in CrI3 are inevitably present and significantly alter the material properties. However, experimental identification of defects of CrI3 at the atomic level is still lacking. Here for the first time, we carried out a scanning tunneling microscopy (STM) study and density functional theory calculations to explore the intrinsic defects in monolayer CrI3 grown by molecular beam epitaxy. The three most common types of intrinsic point defects, i.e., I vacancy (VI), Cr vacancy (VCr), and multiatom CrI3 vacancy (VCrI3) with distinct spatial distributions of the localized defect states, are identified and characterized by high-resolution STM. Moreover, defect concentrations are estimated based on our experiments, which agree with the calculated formation energies. Our findings provide vital knowledge on the types, concentrations, electronic structures, and migration mechanism of the intrinsic point defects in monolayer CrI3 for future defect engineering of this novel 2D magnet.

Thin-film polycrystalline semiconductors are currently at the forefront of inexpensive large-area solar cell and integrated circuit technologies because of their reduced processing and substrate selection constraints. Understanding the extent to which structural and electronic defects influence carrier transport in these materials is critical to controlling the optoelectronic properties, yet many measurement techniques are only capable of indirectly probing their effects. Here we apply a novel photoluminescence imaging technique to directly observe the low temperature diffusion of photocarriers through and across defect states in polycrystalline CdTe thin films. Our measurements show that an inhomogeneous distribution of localized defect states mediates long-range hole transport across multiple grain boundaries to locations exceeding 10 μm from the point of photogeneration. These results provide new insight into the key role deep trap states have in low temperature carrier transport in polycrystalline CdTe by revealing their propensity to act as networks for hopping conduction.

The dark discharge of surface potential on corona or capacitively charged amorphous semiconductors via charge carrier emission from an energy distribution of deep bulk or surface localised states in the mobility gap has been recently studied by a number of authors. The demarcation energy concept has been widely used to extract the necessary theoretical expressions to analyse the surface potential decay experiments. The demarcation energy concept is analytically examined by the author to derive the conditions under which it remains valid. It is shown that for an arbitrary distribution of localised states, the fractional rate of change of the density of states function N(E) with respect to energy at the demarcation energy E'=kTIn vt, over approximately 1/2kT, must be less than unity. In the case of an exponentially decaying distribution of mid-gap states the demarcation energy approach predicts the correct functional time dependence of the rate of charge carrier emission. As an example, the decay of surface potential on an amorphous semiconductor due to field-enhanced surface generation from an exponential distribution of surface mid-gap states is considered and a theoretical expression is derived for the time evolution of the surface potential.

A theoretical description for recombination kinetics of charge carriers in a disordered system with a broad energy distribution of localized states (DOS) is suggested. This kinetics is governed by the exchange of carriers between transport states and traps. Concentration transients in systems with Gaussian DOS, typical for organic semiconductors, appear much steeper than those obtained for systems with exponential DOS. This difference in recombination kinetics is caused by the difference in thermalization kinetics for these two types of the DOS functions. The comparison of the recombination transients for mobile and trapped carriers in exponential and Gaussian DOS might help to distinguish between these two possible shapes of the DOS using experimental data for transient photoconductivity and photoabsorption.

The steady-state charge level reached after repeated contact of a single metal with a film of atactic polystyrene has been interpreted previously in terms of electron injection from the metal into the polymer. The charge level is a function of film thickness, the particular contacting metal, and the particular metallic film substrate. In this study different metals are contacted to the same film until a steady-state condition is reached for each metal. The net charge level at the end of the contact sequence is found to be approximately equal to the sum of the previously measured single-metal charge levels for an initially charge-neutral film and is independent of the order of the metals in the sequence. Therefore, charge injection does not follow a simple energetic relationship between the Fermi level of the metal and the energy of a surface or bulk band or localized level of the polymer. Rather, each metal communicates with a discreet portion of the distribution of localized polymeric electronic states. The energy selective feature of the charge-injection process converts the contact charge-exchange experiments into a spectroscopic tool for the study of the localized polymeric states. As a result the localized electronic-state distribution of the polymer can be derived for a range of electron energies related to that spanned by the Fermi levels of the contacting metals.

Analysis of the electronic structure of β-SiO2 intrinsic defects based on Density Functional Theory

On tunneling through inhomogeneous potential barriers showing resonances exemplified for Nb 2O 5 barriers

Non-stoichiometry and properties of ternary semiconductor phases of variable composition based on IV–VI compounds

Dislocations and the residual strain they produce are instrumental for the high thermoelectric figure of merit, zT ≈ 2, in lead chalcogenides. However, these materials tend to be brittle, barring them from practical green energy and deep space applications. Nonetheless, the bulk of thermoelectrics research focuses on increasing zT without considering mechanical performance. Optimized thermoelectric materials always involve high point defect concentrations for doping and solid solution alloying. Brittle materials show limited plasticity (dislocation motion), yet clear links between crystallographic defects and embrittlement are hitherto unestablished in PbTe. This study identifies connections between dislocations, point defects, and the brittleness (correlated with Vickers hardness) in single crystal and polycrystalline PbTe with various n‐ and p‐type dopants. Speed of sound measurements show a lack of electronic bond stiffening in p‐type PbTe, contrary to the previous speculation. Instead, varied routes of point defect–dislocation interaction restrict dislocation motion and drive embrittlement: dopants with low doping efficiency cause high defect concentrations, interstitial n‐type dopants (Ag and Cu) create highly strained obstacles to dislocation motion, and highly mobile dopants can distribute inhomogeneously or segregate to dislocations. These results illustrate the consequences of excessive defect engineering and the necessity to consider both mechanical and thermoelectric performance when researching thermoelectric materials for practical applications.

In this paper we present the results of research into a relation(s) between the bias voltage of an oxide/a-Si:H/c-Si sample during formation of very-thin and thin oxides and the resulting distribution of oxide/semiconductor interface states in the a-Si:H band gap. Two oxygen plasma sources were used for the first time in our laboratories for formation of oxide layers on a-Si:H: i) inductively coupled plasma in connection with its application at plasma anodic oxidation; ii) rf plasma as the source of positive oxygen ions for the plasma immersion ion implantation process. The oxide growth on a-Si:H during plasma anodization is also simply described theoretically. Properties of plasmatic structures are compared to ones treated by chemical oxidation that uses 68 wt% nitric acid aqueous solutions. We have confirmed that three parameters of the oxide growth process — kinetic energy of interacting particles, UV-VIS-NIR light emitted by plasma sources, and bias of the samples — determine the distribution of defect states at both the oxide/a-Si:H interface and the volume of the a-Si:H layer, respectively. Additionally, a bias of the sample applied during the oxide growth process has a similar impact on the distribution of defect states as it can be observed during the bias-annealing of similar MOS structure outside of the plasma reactor.

Intrinsic electronic defect states of anatase using density functional theory

The formation mechanism of the complex defect, consisting of a carbon (C) impurity and an atomic vacancy, and the corresponding effects on structure and properties of boron nitride nanotubes (BNNTs) are studied using density functional theory. It is found that the C impurity and the vacancy tend to bind together to form a pentagon-nonagon pair more stable than the isolated vacancy, because of geometrical and electronic effects. The combination of the cation vacancy and CB (C substitution for B) is preferable in N-rich conditions, while the combination of the anion vacancy and CN (C substitution for N) is favorable in B-rich conditions. The C-doping facilitates the formation of the vacancy in the growth of the BNNT. The interaction between the C impurity and the vacancy on the one hand quenches their respective local moments, making the system nonmagnetic; on the other hand substantially changes the density and distribution of localized gap states, reducing the activity. We suggest that the complex defect system is more appropriate for chemical applications, such as ion sieve and filter, than for physical applications.