Influence of holmium impurities on photoelectric properties of As2Se3 and (As2S3)0.3(As2Se3)0.7

The influence of a holmium impurity on the photoelectric properties of bulk and film As₂Se₃ and (As₂S₃)_0.3(As₂Se₃)_0.7 samples is studied. Measurements of the relative photoconductivity of bulk samples and the spectral distribution of the persistent photoconductivity in film samples showed an increase in the photoconductivity of materials doped with holmium to concentrations equivalent to 0.010–0.015 at %. The spectral distribution of the persistent photoconductivity and optical absorption showed that the band gap monotonically decreases from 1.88 to 1.85 eV for As₂Se₃ and from 2.05 to 2.00 eV for (As₂S₃)_0.3(As₂Se₃)_0.7 as Ho concentration increases to 0.015 at %, and then weakly increases to the values in initial pure materials.     

The effect of amorphization on the local structure of arsenic chalcogenides

The effect of amorphization on the symmetry of the local environment of chalcogen atoms in As₂S₃, As₂Se₃, and As₂Te₃ compounds has been investigated by ¹²⁹Te(¹²⁹I) Mössbauer spectroscopy. Three states of triply coordinated tellurium atoms are indistinguishable in the Mössbauer spectra of crystalline As₂Te₃. Amorphization of As₂Te₃ decreases the local symmetry of triply coordinated states of Te atoms and leads to the formation of doubly coordinated states in -As-Te-Te-As- chains. In the structure of crystalline As₂S₃ and As₂Se₃, two states of doubly coordinated chalcogen atoms X in -As-X-As- chains manifest themselves in the broadening of the Mössbauer spectra. Amorphization of As₂S₃ and As₂Se₃ is not accompanied by a change in the local symmetry of doubly coordinated chalcogen atoms in -As-X-As- chains; however, doubly coordinated states of S and Se atoms in -As-X-X-As chains are formed in the amorphous material.     

Doping in metal chalcogenide glasses

Chalcogenide glasses that contain sufficient concentrations of metal atoms can be p-type doped. This doping results in part because the local structural order is tetrahedral at the metal and the chalcogen sites in these glasses. At concentrations exceeding 10¹⁹ cm⁻³, oxygen promotes doping by increasing the densities of dangling-bond defects at threefold-coordinated chalcogens, which are the doping sites in these glasses. For oxygen concentrations below 10¹⁹ cm⁻³, the conductivity is independent of oxygen, and therefore is controlled by other mechanisms. The compositions Cu₆As₄S₉ and Cu₆As₄Se₉ are studied, because at these compositions the S (or Se) and Cu atoms are all tetrahedrally coordinated and there exist only Cu-S and As-S (or Cu-Se and As-Se) bonds. Fiz. Tekh. Poluprovodn. 32, 982–986 (August 1998) Published in English in the original Russian journal. Reproduced here with stylistic changes by the Translation Editor.     

Effect of tin impurity on the photoconductivity kinetics of thin amorphous layers of arsenic selenide

In thermally sputtered As₂Se₃ and As₂Se₃ + 0.1 at. % Sn films the tin impurity strongly influences the photoconductivity kinetics under stepped optical excitation. The tin quenches the “spike” on the section of increasing photocurrent, eliminates the dependence of the form of the decrease on the excitation intensity, and leads to a temperature-dependent delay in recombination onset. The effect of the impurity is attributed to an increase in trapping in deep localized states produced by the introduction of tin. Fiz. Tekh. Poluprovodn. 31, 836–840 (July 1997)     

Controlling the U −-center density in Se-As chalcogenide-glass semiconductors by doping with metals and halogens

Temperature dependences of the drift mobilities of electrons and holes are investigated in chalcogenide-glass semiconductors with composition Se₉₅As₅, both without impurities and with the impurities Ag and Br. The data obtained indicate that the localized states that control the transport of charge carriers are U ⁻-centers, and that the change in the magnitude of the drift mobility after doping is caused by a change in the concentration of these centers. Estimates of the concentrations of positive and negatively charged intrinsic defects show that their values are similar, equalling ∼10¹⁶ cm⁻³ in impurity-free glasses with the composition Se₉₅As₅ and lying in the range 10¹³–10¹⁷ cm⁻³ when these glasses are doped with Ag, Br, and Cl. It is established that halogen impurities change the concentration of U ⁻-centers most strongly (by two to three orders of magnitude). Analysis of the data obtained shows that the percentage of electrically active Br and Cl impurity atoms is 1%, while for Ag atoms it is 10⁻²%. Fiz. Tekh. Poluprovodn. 33, 866–869 (July 1999)     

Properties and structure of (As2Se3)1 ? z (SnSe2) z ? x (Tl2Se) x and (As2Se3)1 ? z (SnSe) z ? x (Tl2Se) x glasses

Tin is stabilized in the bivalent and tetravalent states in the structure of (As₂Se₃)_{1 − z} (SnSe₂) _{z − x} (Tl₂Se) _x and (As₂Se₃)_{1 − z} (SnSe) _{z − x} (Tl₂Se) _x glasses. The presence of bivalent tin in the structural network of a glass does not give rise to extrinsic conductivity. Dependences of density, microhardness, and the glass-transition temperature on the composition of the glasses are interpreted using a model according to which the structure of the glasses is composed of structural units that correspond to As₂Se₃, AsSe, TlAsSe₂, Tl₂Se, SnSe, and SnSe₂ compounds. Original Russian Text ? G.A. Bordovsky, A.V. Marchenko, E I. Terukov, P.P. Seregin, T.V. Likhodeeva, 2008, published in Fizika i Tekhnika Poluprovodnikov, 2008, Vol. 42, No. 11, pp. 1353–1356.     

Two-electron tin centers arising in glassy chalcogenides of arsenic due to nuclear reactions

Impurity ^119m Sn atoms arising as a result of radioactive decay of parent ^119mm Sn atoms in the structure of the glasses As₂S₃, As₂Se₃, and As₂Te₃ are part of the glass composition in the form of structural units corresponding to tetravalent tin. The impurity ^119m Sn atoms formed as a result of radioactive decay of ¹¹⁹Sb atoms in the structure of the As₂S₃ and As₂Se₃ glasses are localized at the arsenic sites and play the role of two-electron centers with a negative correlation energy. For the As₂Te₃ glass, similarly formed ^119m Sn atoms are electrically inactive. The greatest part of the daughter ^119m Sn atoms arising after radioactive decay of parent ^119m Te atoms are located at the chalcogen sites and are electrically inactive in the As₂S₃, As₂Se₃, and As₂Te₃ glasses. A significant recoil energy of daughter atoms in the case of the ^119m Te radioactive decay brings about the appearance of the ^119m Sn displaced atoms.     

The phase relations in the system Tl2Se-As2Se3 and the crystal growth of Tl3AsSe3

The phase diagram for the system T12Se-As2Se3 was studied by quenching and thermal analysis experiments, with emphasis on the composition range 15 to 35 mol % As₂Se₃, so as to determine the melting relations of Tl₃AsSe₃, a useful compound for optical and acousto-optical devices. Tl₃AsSe₃ melts congruently at 311 ± 2‡C, and the only other pseudobinary compound, TlAsSe₂, melts congruently at 272 ± 2‡C. Eutectics lie between Tl₂Se and Tl₃AsSe₃ (∿ 21 mol % As₂Se₃, 302 ± 2‡), between Tl₃AsSe₃ and TlAsSe₂ (∿ 32 mol % As₂Se₃, 238 ± 3‡), and between TlAsSe₂ and As₂Se₃ (∿ 72 mol % As₂Se₃, 249 ± 3‡). The maximum-melting Tl₃AsSe₃ composition lies at 24.62 + 0.13 mol % As₂Se₃. Crystals of Tl₃AsSe₃ were grown from three pseudobinary compositions with the best crystals obtained from melts containing 24.62 mol % As₂Se₃. This research was sponsored by the Advanced Research Projects Agency of the Department of Defense and was monitored by the Air Force Materials Laboratory, LP, under Contract No. F33615-72-C-1976     

Neutron transmutation doping of high purity GaAs

High purity n-type GaAs samples grown by a variety of epitaxial techniques were transmutation doped with a low fluence of thermal neutrons to produce Ge and Se impurities from the host Ga and As atoms, respectively. Samples were chosen having low concentrations of Se and Ge donors and Ge acceptors prior to doping. Photothermal ionization and photoluminescence spectra as well as Hall effect data were recorded before and after doping and subsequent low temperature anneals. The high purity of these samples and low neutron dose, together with the low noise and high resolu-tion of the photothermal ionization measurements, has allowed the precise identification of the Se and Ge donor peaks through an accurate determination of their ls-2p(m=−l) transition energies. Comparison of the relative concentra-tions of shallow donors and acceptors, obtained from the photothermal ionization and photoluminescence spectra, with N_D and N_A determined from the Hall effect data, allowed the activation of Se and Ge to be measured. The observation of Ge_Ass acceptors after doping and of the incomplete activation of Se as donors are interpreted in terms of transmutation induced recoil of the Se and Ge due to γ-pray and e -v_e pair emission.     

Raman scattering in As-Se-S and As-Se-Te Chalcogenide vitreous semiconductors

Chalcogenide vitreous semiconductors of As-Se-S and As-Se-Te compositions are synthesized, with thin films produced by thermal evaporation in vacuum. X-ray phase analysis and Raman spectroscopy are used to confirm their amorphous state and examine their structural specific features. It is shown that the matrices of both materials have a network-chain structure with covalent bonds between As, Se, S, and Te atoms. Structural units of the type of AsSe₃ and AsS₃ pyramids, AsSe_3/2 bipyramids, and α(β)-As₄S₄ molecules exist in the As-Se-S matrix, and AsSe₃, AsSe_3/2, As₂Se₃, and As₂Te₃ units, in As-Se-Te. The vibrational modes of Se-Se, Te-Te, As-As, As-S, Se-As-Se, As-Se-As, Se-As-S, Se-As-Te, and Se-Te bonds constituting both separate molecules and amorphous matrices are determined.     

Double Charge Polarity Switching in Sb-Doped SnSe with Switchable Substitution Sites

Tin mono-selenide (SnSe) is one of the most promising thermoelectric materials; however, it experiences difficulty in controlling the carrier polarity, which is inevitable for realizing p-n homojunction devices. Herein, double switching of charge polarity in (Sn_1−xSb_x)Se by varying x is reported; pure SnSe shows p-type conduction, whereas the polarity of (Sn_1−xSb_x)Se switches to n-type conduction for 0.005 < x < 0.05, and then re-switches to p-type conduction for x > 0.05. The major Sb substitution site switches from the Se (Sb_Se) to Sn site (Sb_Sn) with increasing x. Sb_Sn (Sb³⁺ at Sn²⁺) works as a donor, but Sb_Se (Sb³⁻ at Se²⁻) does not produce a hole because of the Sb–Sb dimer formation. The mechanism of double polarity switching is explained by native p-type conduction in pure SnSe due to Sn-vacancy formation, whereas (Sn_1−xSb_x)Se exhibits n-type behavior due to conduction through the Sb_Se impurity band formed above the valence band maximum, and finally re-switches to weak p-type, where the Fermi level approaches the midgap level between the Sb_Se band and conduction band minimum. Clarification of the Sb doping mechanism will provide a crucial guide for developing more sophisticated doping routes for SnSe and high-performance energy-related devices.     

Electrical properties and structure of chalcogenide glasses containing bivalent tin

Two valence states of tin atoms are identified by Mössbauer and X-ray photoelectron spectroscopy in (As₂Se₃)_1?z (GeSe) _z?x (SnSe) _x glasses; it is shown that the presence of bivalent tin in the structural network of a glass does not give rise to impurity conductivity and impurity optical absorption. It is suggested to regard (As₂Se₃)_1?z (GeSe) _z?x (SnSe) _x and (As₂Se₃)_1?z (GeSe₂) _z?x (SnSe₂) _x glasses as semiconductor solid solutions whose electrical properties depend both on the electrical properties of the starting components and on the composition of the solid solutions.     

The role of trapping levels of nonequilibrium electrons during the formation of pinning centers for domain walls in the magnetic semiconductor CdCr<Subscript>2</Subscript>Se<Subscript>4</Subscript>

A complex study of electrical conductivity and photoconductivity in constant and variable electric fields, thermally stimulated conductivity, and the photoferromagnetic effect are carried out in the temperature range 10–300 K for a CdCr₂Se₄ magnetic semiconductor with various concentrations of Ga impurity and Se vacancies (V_Se). The phenomenon of hopping photoinduced conduction is observed for the first time. It is shown that shallow donor levels that exchange photoelectrons trapped by them with Cr³⁺ magnetic ions may be responsible for the photoferromagnetic effect. As a result of this exchange, the Cr³⁺ ions acquire valence and spin instabilities, which lead to nonequilibrium pinning of the domain wall by these ions.     

Effect of Se content on the structural,morphological and optical properties of Bi2Te3−ySey thin films electrodeposited by under potential deposition technique

Bi₂Te_3−ySe_y thin films with different Se contents ranging from 0.3 to 2.5 were successfully electrodeposited by under potential deposition (UPD) technique onto gold foil substrates from an electrolyte consist of Bi(NO₃)₃, TeO₂, and SeO₂ at ambient conditions. The effects of Se content on structural, morphological and optical properties of the products were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and FT-IR spectroscopy, respectively. The XRD analysis revealed that the diffraction peaks positions of Bi₂Te_3−ySe_y thin films shifts gradually towards the higher angle side due to replacement of Te by Se atoms in the crystal structure with increasing Se content. The SEM results showed that the particle size of Bi₂Te_3−ySe_y thin films decreased as the Se content increased. The optical constants of ternary Bi₂Te_3−ySe_y thin films such as refractive index, extinction coefficient, and dielectric constant were obtained from the transmission spectra in the range of 2500–10.000 nm. The direct allowed band gap energies were estimated using Tauc equation and found to increase from 0.210 to 0.282 eV with increasing Se content from 0.3 to 2.5. The dispersion behavior of refractive index was studied by the single oscillator Wemple-DiDomenico model.     

Photoluminescence and composition of amorphous As<Subscript>2</Subscript>Se<Subscript>3</Subscript> films modified with Er(<Emphasis Type="Italic">thd</Emphasis>)<Subscript>3</Subscript> complex compound

The photoluminescence and composition of amorphous As₂Se₃ films modified with an Er(thd)₃ complex compound have been studied. A band centered at 1.54 μm, characteristic of photoluminescence from Er embedded in amorphous matrices, has been revealed at room temperature. The composition of thin amorphous As₂Se₃ films modified with an Er(thd)₃ complex compound has been examined by methods of nuclear microanalysis: Rutherford backscattering and nuclear resonant reactions. Dependences of the concentrations of Er ions, oxygen, and carbon on the growth conditions of the films are obtained. It is shown that the Er concentration in a thin film varies nonlinearly as the relative concentration of the starting complex compound increases. In addition, the increase in the Er content of a film is accompanied by a simultaneous rise in the content of such light elements as oxygen and carbon. Comparative analysis of the nuclear microanalysis data and IR spectra demonstrates that, in modification of As₂Se₃ with the Er(thd)₃ complex compound by the given method, the nearest environment of Er in the complex compound is partly preserved.     

Impurity centers of tin in glassy arsenic chalcogenides

¹¹⁹Sn atoms produced by radioactive decay of ¹¹⁹Sb impurity atoms in the structure of As _x S_{1 − x} and As _x Se_{1 − x} glasses are stabilized in the form of Sn²⁺ and Sn⁴⁺ ions at arsenic sites and correspond to ionized states of the amphoteric two-electron center with negative correlation energy (Sn²⁺ is an ionized acceptor, and Sn⁴⁺ is an ionized donor), whereas the neutral state of the Sn³⁺ center is unstable. The fraction of Sn⁴⁺ states increases with chalcogen content in glass. ¹¹⁹Sn atoms produced by radioactive decay of ^119m Te impurity atoms in the structure of As _x S_{1 − x} and As _x Se_{1 − x} glasses are stabilized at chalcogen sites (they are electrically inactive) and arsenic sites, and the fraction of arsenic atoms decreases with the chalcogen content in glass.     

Texture and morphology variations in (In,Ga)2Se3 and Cu(In,Ga)Se2 thin films grown with various Se source conditions

Texture and morphology variations in co‐evaporated (In,Ga)₂Se₃ and Cu(In,Ga)Se₂ (CIGS) films grown with various Se source conditions during growth were studied. The Se species of simply evaporated, large molecular Se (E‐Se, low‐sticking coefficient), and RF‐plasma cracked atomic Se (R‐Se, high sticking coefficient) were used in the present work. (In,Ga)₂Se₃ precursor films, which were prepared during the first stage of CIGS film growth by the three‐stage process, showed systematic variations in texture and Na distribution profile with varying evaporative Se (E‐Se) flux. The properties of CIGS films and solar cells also showed systematic variations, and the open‐circuit voltage (V_oc) and fill factor were found to be especially sensitive to the E‐Se flux. R‐Se grown (In,Ga)₂Se₃ precursor films featured granular morphology with strong (105) and (301) peaks in the diffraction pattern, and the texture was very similar to an E‐Se grown film fabricated with a Se to group III metal (In + Ga) flux ratio (P_{[Se]/[In + Ga]}) of about 6, although the nominal P_{[Se]/[In + Ga]} used for an R‐Se source was very small and less than 0.5. The R‐Se grown CIGS films displayed, however, highly dense surfaces and larger grain sizes than E‐Se grown CIGS films. The controllability of film morphology and the Na diffusion profile in (In,Ga)₂Se₃ and CIGS films with various Se source conditions are discussed. Copyright © 2011 John Wiley & Sons, Ltd.     

Spectra of optical parameters in bulk and film amorphous alloys of the Se<Subscript>95</Subscript>As<Subscript>5</Subscript> system containing samarium (Sm) impurities

Reflectance spectra of bulk and film amorphous alloys of the Se₉₅As₅ system containing samarium (Sm) impurities are studied in the energy range of 1–6 eV. Spectral dependences of optical constants and derivatives of optical dielectric functions are calculated by the Kramers-Kronig method. Changes in spectra of optical parameters depending on the content of impurities introduced into Se₉₅As₅ and conditions of their preparation are explained based on the cluster model. According to the latter, changes in the electron density of states depends on changes in atomic configurations in clusters, i.e., short-range order changes.     

Self‐Established Rapid Magnesiation/De‐Magnesiation Pathways in Binary Selenium–Copper Mixtures with Significantly Enhanced Mg‐Ion Storage Reversibility

Rechargeable magnesium/sulfur (Mg/S) and magnesium/selenium (Mg/Se) batteries are characterized by high energy density, inherent safety, and economical effectiveness, and therefore, are of great scientific and technological interest. However, elusive challenges, including the limited charge storage capacity, low Coulombic efficiency, and short cycle life, have been encountered due to the sluggish electrochemical kinetics and severe shuttles of ploysulfides (polyselenide). Taking selenium as model paradigm, a new and reliable Mg‐Se chemistry is proposed through designing binary selenium‐copper (Se‐Cu) cathodes. An intriguing effect of Cu powders on the electrochemical reaction pathways of the active Se microparticles is revealed in a way of forming Cu₃Se₂ intermediates, which induces an unconventional yet reversible two‐stage magnesiation mechanism: Mg‐ions first insert into Cu₃Se₂ phases; in a second step Cu‐ions in the Mg_2xCu₃Se₂ lattice exchange with Mg‐ions. As expected, binary Se‐Cu electrodes show significantly improved reversibility and elongated cycle life. More bracingly, Se/C nanostructures fabricated by facile blade coating Se nanorodes onto copper foils exhibit high output power and capacity (696.0 mAh g^?1 at 67.9 mA g^?1), which outperforms all previously reported Mg/Se batteries. This work envisions a facile and reliable strategy to achieve better reversibility and long‐term durability of selenium (sulfur) electrodes.     

Determination of the composition of multicomponent chalcogenide semiconductors by X-ray fluorescence analysis

The composition of As _x (Ge _y Se_{1 ? y} )_{1 ? x} glassy alloys is quantitatively determined by measuring the X-ray fluorescence spectrum of a Ge_0.2As_0.4Se_0.4 reference alloy. The atomic fractions of arsenic, germanium, and selenium are calculated from the X-ray fluorescence spectra and the x _RFA = f(x) and y _RFA = f(y) dependences are plotted. These dependences make it possible to determine the composition of the glasses with an accuracy of ±0.0005 for x and y. This procedure is effective for finding the concentration of the tin impurity in Pb_{1 ? x} Sn _x Se crystalline solid solutions. However, it is impossible to determine the content of tellurium in Te _x (As _y Se_{1 ? y} )_{1 ? x} glassy alloys because the alloy components have significantly different X-ray fluorescence characteristics.