Recombination and trapping centers in pure and doped TlBr crystals

TlBr is a promising wide-gap semiconductor for developing γ-radiation detectors. One of the limiting factors in developing the technology of detectors is the lack of experimentally determined trapping and recombination centers. In this paper, a generalized model of the formation and behavior of intrinsic defects in pure and doped TlBr single crystals is presented. The relation of intrinsic defects to growth conditions and electrical properties is determined. The previously obtained temperature dependences of the photoconductivity, the data of current deep level transient spectroscopy and microcathodoluminescence, and the kinetic characteristics of the photoconductivity are used as objects of analysis. It is shown that the compensation of charged centers control the transport properties of charge carriers. In compensated doped TlBr crystals, the product of the mobility and lifetime can reach μτ = 5 × 10^?4 cm² V^?1. The energy-level diagram of local levels in pure and doped TlBr crystals is proposed. The ionization energies of major structural and impurity defects in TlBr, i.e., the anion vacancy V _a ⁺ , cation vacancy V _c ^? , and Pb²⁺, O^2?, S^2? ions, are determined. The energy position of a single anion vacancy V _a ⁺ is E _c ? 0.22 eV. The energy level of the cation vacancy is E _v + 0.85 eV for a single cation vacancy and E _v + 0.58 eV for a vacancy incorporated into the {Pb²⁺ V _c ^? }⁰ complex. The ionization energy of the Pb²⁺ Coulomb trap is E _c ? 0.08 eV in doped TlBr crystals.     

The nature of point defects in CdTe

High temperature in-situ Hall effect measurements in CdTe single crystals grown by different techniques at 500–1,200 K under well-defined Cd and Te vapor pressure were made. It was established that native point defects (PD) Cd _i ²⁺ and V _Te ²⁺ are dominant at T>770 K in Cd-rich CdTe. Their formation enthalpies were determined (ΔH _VTe ²⁺ =1.3 eV; ΔH _Cdi ²⁺ =2.5 eV). It was shown that V _Te ²⁺ dominate at low temperatures and Cd _i ²⁺ begin to prevail at T>930 K. In Te-saturated CdTe at heating up to ～800 K, the hole density was Te vapor pressure independent. At higher temperatures (HT), the conductivity changed to intrinsic type, turning then into n-type. The results were explained in the framework of Kröger’s quasichemical formalism assuming the presence of an electrically active foreign point defect, the oxygen acceptor. The PD structure modeling demonstrated satisfactory agreement with experimental results both for temperature and component vapor pressure dependencies.     

Reflection spectra of two polymorphic modifications of cadmium arsenide

The optical properties of cadmium arsenide are studied. Reliable new data on the reflection spectra are obtained owing to the use of perfect crystals and high-precision spectral equipment. The reflection spectra of the polymorphic modifications α″-Cd₃As₂ (space group D _4h ¹⁵ ) and α′-Cd₃As₂ (space group ) are D _4h ¹¹ are recorded at room and liquid-nitrogen temperatures in polarized light (E ⊥ c, E ‖ c) at incident-light photon energies of 1–5 eV. For the α′ modification, anisotropy is observed in the reflection spectra for the first time. The obtained results are compared with the known experimental and theoretical data.     

I–V characteristics and the spectrum width during electron tunneling through nanosandwiches W-WO2-(Au 147 ? )-Al2O3-Al and Nd-Nd2O3-(Au 55 ? )-Nd2O3-Nd. Part I: Quantum-chemical calculation of energies of orbitals for anions of nanoclusters Au55 and Au147

This work is devoted to the investigation of electron tunneling through ??magical?? nanoclusters Au₅₅ and Au₁₄₇. Using the quantum-chemical calculation, it is shown that the energy of the highest occupied molecular orbital (HOMO) level is ${E_{HOMOAu5{5^ - }}} = - 3.2eV$ and E _HOMOAu147 = ?4.4 eV, respectively. It is established that the difference between energy E _LUMO ^? of the lowest unoccupied level LUMO and energy E _LUMO ^? for the anions of clusters (Au ₅₅ ^? and Au ₁₄₇ ^? ) is 0.86 and 0.24 eV, respectively. Based on the results of the calculations, it is assumed that nanosandwiches W-WO₂-(Au ₁₄₇ ^? )-Al₂O₃-Al can be promising structures for implementation of metal nanodiodes based on them, while nanosandwiches Nd-Nd₂O₃-(Au ₅₅ ^? )-Nd₂O₃-Nd can be promising for implementation of energy filters not producing hot electrons and allowing the fabrication of the ring Aharonov-Bohm interferometers based on normal metals at liquid-helium temperatures.     

Electrical characteristics of CdTe:Pb single crystals at high temperatures

The electrical properties of CdTe:Pb single crystals at high temperatures (400–900°C) and under controlled Cd vapor pressure (0.001–3 atm) were investigated for the first time. The temperature and baric dependences of the conductivity and Hall coefficient were measured. Low (in comparison with undoped CdTe) electron concentration indicates an increase in the number of impurity point defects related to the Pb impurity. The results obtained are explained within the Kröger theory of quasi-chemical reactions of defect formation on the assumption that lead may exist in the isolated state (Pb _Cd ⁺ ) and as a component of (Pb _Cd ⁺ V _Cd ^2? )^? associates.     

Emission associated with extended defects in epitaxial ZnTe/GaAs layers and multilayer structures

It was shown by the methods of X-ray diffraction and photoluminescence that the use of a thin intermediate recrystallized ZnTe layer between the ZnTe buffer layer obtained by molecular-beam epitaxy and GaAs substrate, as well as an increase in the thickness of the epilayer result in the improvement of the structure (enhancement of the mosaic size) and an increase in the intensity of exitonic bands. It is established that a number of characteristics of the I ₁ ^C bands with hν≈2.361 eV, which are observed in the samples with quantum wells and superlattices, differ from the corresponding features of the emission lines of free and bound excitons and those lines typical of dislocation-related radiation in II-VI single crystals. It is assumed that the I ₁ ^C band is associated with the subblocks boundaries, which comprise the mosaic structure of epitaxial layers.     

Origin of an absorption band peaked at 5560 cm−1 and related to divacancies in Si1−x Gex

It is found that two types of centers are formed in Si_1?x Ge_x single crystals as a result of irradiation with fast electrons: divacancies (V ₂) characteristic of silicon and the V ₂ ^* centers; the latter are complexes of divacancies V ₂ with germanium atoms (V ₂Ge). It is shown that an absorption band peaked at about 5560 cm^?1 is a superposition of two absorption bands that correspond to the above centers. The V ₂ divacancies diffuse during isochronous heat treatment and interact with germanium atoms, thus giving rise to additional V ₂ ^* centers. The latter have a higher thermal stability than the V ₂ centers do, and their annealing temperature increases with increasing content of germanium.     

Study of the spectrum of CH3 35C1 between 4250 and 4600 CM−1: thev 2+v 4 andv 4+v 5 rovibrational bands

About 2500 lines of CH₃ ³⁵Cl have been assigned. The strong xy Coriolis resonance between thev ₂ andv ₅ modes is quite visible between thev ₄+v ₄ ^±1 perpendicular band, centered around 4383 cm^?1, and thev ₄ ^±1 +v ₅ ^±1 perpendicular component, centered around 4475 cm^?1, with a crossing of upper energy levels allowing the observation of lines which are normally forbidden. Although not yet observed with certainty, because of the great density of lines of the spectrum, thev ₄ ^?1 +v ₅ ^±1 parallel component is nevertheless detectable by its effects onv ₂+v ₄ ^±1 which is linked by Coriolis resonance to both components ofv ₄+v ₅. Moreover the spectrum is much complicated by many other resonances with weak bands which occur at level crossings: it is the case ofv ₂+3v ₆ ^±1 , connected tov ₂+v ₄ ^±1 by the well known Darling Dennison resonance which couplesv ₄ ^±1 and 3v ₆ ^±1 , and also ofv ₅ ^±1 +3v ₆ ^±1 connected tov ₄ ^±1 +v ₅ ^±1 by the same resonance; but this last case is complicated by an anharmonic resonance betweenv ₅ ^±1 +3v ₆ ^±1 and 2v ₃+3v ₆ ^?1 . Two more perturbations occur on the ΔK=?1 side ofv ₂+v ₄: a weak Coriolis resonance gives rise to one subband ofv ₁+v ₂ at a level crossing withv ₂+v ₄, and thev ₁+v ₅ band (linked of course tov ₁+v ₂ by the Coriolis resonance between thev ₂ andv ₅ modes) is quite visible and perturbs several subbands ofv ₂+v ₄ of high values of K through an anharmonic resonance. Moreover, the complex (3v ₅ ^±1 ,v+2v ₅ ⁰ , 2v ₂+v ₅ ^±1 , 3v ₂,v ₂+2v ₅ ^±2 , 3v ₅ ^±3 ) system of Coriolis-connected bands is linked to the bands studied in the present work by two Fermi resonances: one betweenv ₂+2v ₅ ⁰ andv ₁+v ₂, and the other one betweenv ₁+v ₅ and 3v ₅ ^±1 , whose several subbands have been observed on the low part of the spectrum. The values of all the band centres and of the different coupling constants have been estimated, but all these interactions make the line assignments and the interpretation of the spectrum very difficult.     

Investigation of vacancy-type complexes in GaN and AlN using positron annihilation

The momentum distribution of electron-positron pairs in GaN and AlN has been investigated for the first time by measuring the one-dimensional angular correlation of the annihilation radiation (1D-ACAR). The characteristic parameter of the electron density r _s ^′ (the radius of the sphere occupied by an electron) estimated from the experimental data differs noticeably from r _s calculated in terms of a standard model of noninteracting particles: r _s ^′ (AlN)≈1.28r _s, and r _s ^′ (GaN)≈1.66r _s, where r _s(AlN)≈1.6091 au, and r _s(GaN)≈1.6568 au. The chemical nature of atoms in the environment of the annihilating positron in AlN and GaN was identified from the electron-positron ionic radius of Al³⁺, Ga³⁺, and N⁵⁺ cores, which were determined from the characteristics of the high-momentum component of 1D-ACAR curves. The analysis was based on a comparison of the electron-positron ionic radii with those considered standard for Al and Ga metals and the related compounds GaP, GaAs, and GaSb. A conclusion is made that positron annihilation dominates in the regions of vacancy-type “nitrogen antisite”-“vacancy” complexes, [N _Ga ⁺ V _Ga] and [N _Al ⁺ V _Al], in GaN and AlN, respectively.     

Molecular — impurity absorption in KC1 for infrared laser windows

Residual impurity absorption in the 10μm region severely limits the usefulness of KCl as a high-power CO₂ laser window. In order to investigate possible origins for this absorption, KCl was doped with carbonate and hydroxyl impurities, and grown in various atmospheres. Characteristic absorption bands in the infrared and ultraviolet were used to determine relative dopant concentrations. Growth conditions introducing CO/ ₃ ⁼ exclusively, led to relatively low and featureless absorption spectra in the 10.6μm region, while conditions introducing both CO/ ₃ ⁼ and OH⁻ led to enhanced absorption. (Additionally, a correlation is indicated between the strength of absorption at 10.6μm and that of the oxygen band at 0.26μm in samples grown in atmospheres in which oxygen was present). Our observations suggest the possibility of absorption due to CO/ ₃ ^≽ complexes, NO ₃ ⁻ , HCO ₃ ⁻ and/or ClO ₃ ⁻ in the 10μm region. While the effects of ultralow concentration of these impurities remain uncertain, the present results suggest that their presence may well contribute to extrinsic absorption in the 10μm region. Rsch supported by AFCRL(AFSC) - Contr F19628-72-C-0286     

On the photoconductivity of TlInSe<Subscript>2</Subscript>

The current–voltage (I–V) and photocurrent–light intensity (I _pc –Φ) characteristics and the photoconductivity relaxation kinetics of TlInSe₂ single crystals are investigated. Anomalously long relaxation times (τ ≈ 10³ s) and some other specific features of the photoconductivity are observed, which are explained within the barrier theory of inhomogeneous semiconductors. The heights of the drift and recombination barriers are found to be, respectively, E _dr ≈ 0.1 eV and E _r ≈ 0.45 eV.     

Electronic Processes in CdIn<Subscript>2</Subscript>Te<Subscript>4</Subscript> Crystals

The results of investigations of electrical, optical, and photoelectric properties of CdIn₂Te⁴ crystals, which were grown by the Bridgman method are presented. It is shown that electrical conductivity is determined mainly by electrons with the effective mass m_n = 0.44m₀ and the mobility 120–140 cm²/(V s), which weakly depends on temperature. CdIn₂Te₄ behaves as a partially compensated semiconductor with the donor-center ionization energy E_d = 0.38 eV and the compensation level K = N_a/N_d = 0.36. The absorption-coefficient spectra at the energy hν < E_g = 1.27 eV are subject to the Urbach rule with a typical energy of 18–25 meV. The photoconductivity depends on the sample thickness. The diffusion length, the charge-carrier lifetime, and the surface-recombination rate are determined from the photoconductivity spectra.     

TE On o -to- mode transducer designmode transducer design

This paper gives a structure on the mode converter from TE _0n ^O mode of gyrotron output to the main mode of the circular groove guide. The design consists of two parts: the first part is to convert TE _0n ^O to TE ₀₁ ^O using an advanced periodic wall perturbation technology, the second part is to convert TE ₀₁ ^O to mode basing on the method of gradually changing the wave guide cross section. An example at 37.5Ghz is given. For the first part, the efficiency is 94.71%, the length is 1.868m. For the second part, the theoretic figure of reflectance coefficient vs. frequency is given.     

Native and impurity defects in ZnSe:In single crystals prepared by free growth

The optical absorption and photoluminescence spectra and the Hall effect were studied in ZnSe:In single crystals. The presence of electrically active In _Zn ⁺ donor centers responsible for the impurity absorption and electrical conduction of crystals is established. It is shown that the conduction compensation in the ZnSe:In crystals is effected by cationic vacancies. The In _Zn ⁺ donors and cationic vacancies form associative defects responsible for long-wavelength ZnSe:In luminescence. A high crystal conductivity (～5 Ω^?1 cm^?1) is achieved as a result of ZnSe:In annealing in the zinc melt, which results in the extraction of cationic vacancies. The electron mobility in high-conductivity crystals is limited by scattering at the LO phonons and macrodefects formed due to the reduction of In solubility in crystals by their annealing in zinc.     

The contact of metal with silicon carbide: Schottky barrier height in relation to SiC polytype

The results obtained in measuring the Schottky barrier height Φ _b ⁿ for a chromium contact with 8H-, 6H-, 15R-, 27R-and 4H-SiC silicon carbide polytypes with n-type conduction are analyzed in terms of a simple model. It is shown that the Φ _b ⁿ value is proportional to the concentration of silicon vacancies in the polytypes. The results of Φ _b ⁿ measurements for palladium and platinum contacts to silicon carbide polytypes are discussed.     

On measuring the permittivity of “living” wood for microwave remote sensing

A procedure for measuring the permittivity of “living” wood is proposed. The permittivity of the trunk of a growing tree ε ₁ ^* has been measured by this procedure. Simultaneously, samples of wood of the same tree have been taken and their permittivities ε ₀ ^* determined under laboratory conditions. The obtained values has been compared, and considerable disagreement between measured ε ₁ ^* and ε ₀ ^* has been revealed. The study has been performed to model the reflection of microwaves from the forest.     

Optical properties and electronic structure of mercury iodide

The spectra of sets of optical functions of a HgI₂ crystal for the polarization E ⊥ c are determined in the energy range of 0–20 eV. The ?₂ and ?Im?^?1 spectra are decomposed into elementary components with establishment of their principal parameters. The calculations are carried out based on the experimental reflection spectrum at 100 K for E ⊥ c and computer programs composed with the help of integral Kramers-Kronig relations, analytical formulas, and the method of combined Argand diagrams. The principal features of the ?₂ and ?Im?^?1 spectra of optical functions and parameters of components of the decomposition bands are established.     

Analysis and application of NRD-guide excitors

In this paper, the NRD-guide excitors using a length of coaxial line which consists of a rectangle external conductor and a thin round inner conductor are presented, and the characteristics of the input impedances of LSE ₁₁ ^o mode and LSM ₁₁ ^o mode at the insertion points are analysed and calculated by using the method of the rector Green's function respectively. The computation results show that the mode excitors are able to improve the operating modes matching between NRD-guide and the quasi-TEM transmission line. The structures are suitable for fabrication LSE ₁₁ ^o mode oscillator and LSM ₁₁ ^o mode oscillator to compose millimeter wave NRD-guide integrated front ends. An example of an LSE ₁₁ ^o mode oscillator in Ka-band is given. Its oscillation frequency and output power could be easily adjusted by changing the parameters of the excitor.     

Long-range effects of ion irradiation, chemical etching, and mechanical grinding on relaxation of a solid solution of iron in gallium phosphide

Electron spin resonance was used to study directly the influence of external factors on a system of defects in gallium phosphide crystals doped nonuniformly with iron. The Fe _s ³⁺ (Ga) ions replacing Ga atoms (the A centers) and Fe _i ⁰ interstitial atoms (the B centers) are considered as indicators and participants in rearrangement of the crystal’s defect system. Long-range effects of ion irradiation with argon, chemical etching, or mechanical grinding on the A and B centers are observed. The long-range effect is explained by plastic deformation of the GaP:Fe crystal and by interaction of dislocations with A and B centers when the crystal is modified by ion irradiation or by removal of a layer that is saturated with Fe _s ³⁺ (Ga) centers and gives rise to stresses from one of the sample surfaces. In the case of ion irradiation, the role of elastic waves that are generated in the stopping zone of argon ions and that interact with B centers and dislocations is apparently important.     

Donor–Acceptor–Donor Modular Small Organic Molecules Based on the Naphthalene Diimide Acceptor Unit for Solution-Processable Photovoltaic Devices

Two novel solution-processable small organic molecules, 4,9-bis(4-(diphenylamino)phenyl)-2,7-dioctylbenzo[3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone (S6) and 4,9-bis(benzo[b]thiophen-2-yl)-2,7-dioctylbenzo[3,8]phenanthroline-1,3,6,8 (2H,7H)-tetraone (S7), have been successfully designed, synthesized, characterized, and applied in solution-processable photovoltaic devices. S6 and S7 contain a common electron-accepting moiety, naphthalene diimide (NDI), with different electron-donating moieties, triphenylamine (S6) and benzothiophene (S7), and are based on a donor–acceptor–donor structure. S7 was isolated as black, rod-shaped crystals. Its triclinic structure was determined by single crystal x-ray diffraction (XRD): space group \(P\bar{1}\) , Z = 2, a = 9.434(5) Å, b = 14.460(7) Å, c = 15.359(8) Å, α = 67.256(9) degrees, β = 80.356(11) degrees, γ = 76.618(10) degrees, at 150 Kelvin (K), R = 0.073. Ultraviolet–visible absorption spectra revealed that use of triphenylamine donor functionality with the NDI acceptor unit resulted in an enhanced intramolecular charge transfer (ICT) transition and reduction of the optical band gap compared with the benzothiophene analogue. Solution-processable inverted bulk heterojunction devices with the structure indium tin oxide/zinc oxide (30 nm)/active layer/molybdenum trioxide (10 nm)/silver (100 nm) were fabricated with S6 and S7 as donors and (6,6)-phenyl C₇₀-butyric acid methyl ester (PC₇₀BM) as acceptor. Power conversion efficiencies of 0.22% for S6/PC₇₀BM and 0.10% for S7/PC₇₀BM were achieved for the preliminary photovoltaic devices under simulated AM 1.5 illumination (100 mW cm^?2). This paper reports donor–acceptor–donor modular small organic molecules, with NDI as central accepting unit, that have been screened for use in solution-processable inverted photovoltaic devices.