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Structural and photoemission investigations of a new pseudo binary semimagnetic semiconductor: Sn1−xMnxSe2
Abstract
Sn1−xMnxSe2 crystals have been grown by a modified Bridgman method aiming to form a new semimagnetic semiconductor whose particularity will be to have a layered CdI2-type crystal structure. This paper reports the first detailed structural and photoemission study of this new material. X-ray diffraction analysis performed on two different samples showed that only one had the expected lattice symmetry, the other one being a mixture of SnSe and α-MnSe phases. Although the preparation method of these specimen was the same they differed by the manganese quantity inserted to substitute tin in the SnSe2 lattice. This study also reveals that Mn-solubility into this lattice is not high. Photoelectron spectroscopy using X-ray and synchrotron radiation were also carried on. As it was expected, chemical states of tin and selenium in layered Sn1−xMnxSe2 are Sn4+ and Se2−, respectively. Manganese is present as Mn2+ ion, like it is the case in II1−xMnxVI materials. Investigations of layered Sn1−xMnxSe2 valence band revealed that the Mn 3d density of states is constituted of localized states at 3.5 eV of binding energy with delocalized states at lower and higher energy. Differences in Mn 3d density of states have been observed with respect to a biphased Sn1−xMnxSe2 compound.
... The 2p 3/2 peak position at 641.2 eV may correspond to the Mn-Se bonding as expected (640.5-641.0 eV in [57] and 640.95 eV in [56]), but it could also be a Mn-O signature which is very close in terms of binding energy [41]. From a quantitative analysis of XPS spectra taking into account all the Mo components (including the ones from the undoped MoSe 2 template layer), we find the following atomic composition: Mo 0.89±0.03 ...
Large-area growth of continuous transition metal dichalcogenides (TMDCs) layers is a prerequisite to transfer their exceptional electronic and optical properties into practical devices. It still represents an open issue nowadays. Electric and magnetic doping of TMDC layers to develop basic devices such as p-n junctions or diluted magnetic semiconductors for spintronic applications are also an important field of investigation. Here, we have developed two different techniques to grow MoSe 2 mono- and multi-layers on SiO 2 /Si substrates over large areas. First, we co-deposited Mo and Se atoms on SiO 2 /Si by molecular beam epitaxy in the van der Waals regime to obtain continuous MoSe 2 monolayers over 1 cm ² . To grow MoSe 2 multilayers, we then used the van der Waals solid phase epitaxy which consists in depositing an amorphous Se/Mo bilayer on top of a co-deposited MoSe 2 monolayer which serves as a van der Waals growth template. By annealing, we obtained continuous MoSe 2 multilayers over 1 cm ² . Moreover, by inserting a thin layer of Mn in the stack, we could demonstrate the incorporation of up to 10% of Mn in MoSe 2 bilayers.
X-ray photoelectron (XPS) studies of core-levels in Sn1−xMnxTe (x < 0.1) semimagnetic semiconductors have been performed. The spectra were acquired under UHV conditions from the clean (as-cleaved or in-situ scraped) crystal surface. The single-phase NaCl structure of the alloys studied was verified by X-ray diffraction (XRD). The structure of Sn 3d and Te 3d core-levels in SnMnTe was found fully consistent with that of SnTe. Remarkable qualitative similarity of the Mn 2p spectrum of Sn1−xMnxTe (x = 0.09) with the case of zinc-blende MnTe [R.J. Iwanowski, M.H. Heinonen, E. Janik, Chem. Phys. Lett. 387 (2004) 110] has been shown: (1) the same binding energies (BEs) of the main contributions to the Mn 2p3/2 line, related to Mn2+ state of the bulk MnTe bond; (2) occurrence of low BE component in the Mn 2p spectrum, indicative of clean-surface species containing reduced-valence Mn ions (i.e. Mnq+, where 0 < q < 2); (3) strong satellites of the 2p3/2 (Mn2+ related) parent lines. In SnMnTe, the highest intensity ratio of the satellite to main peak (ever reported for Mn 2p photoelectron spectrum) was revealed; this was interpreted in terms of the so-called charge-transfer model.
Electronic structure of NiAs-type MnTe (H-MnTe) has been studied by X-ray photoelectron spectroscopy (XPS). Among the core-level spectra, analysis of the Mn 2p XPS spectrum revealed a prevailing contribution of the Mn2+ cationic state, accompanied by minor spectral component of low binding energy (BE) indicative of reduced-valence Mnq+ ions (0 < q < 1). The latter one was attributed to the Mn–Te bonds that were broken during mechanical cleaning (scraping) of crystal surface. Occurrence of the high-intensity satellite of Mn 2p3/2 (Mn2+-related) parent line confirmed that H-MnTe belongs to the class of charge-transfer compounds. Evidence of plasmon loss satellite in Mn 2p spectrum led to an estimate of the bulk plasmon energy in H-MnTe. The valence-band (VB) XPS spectrum of H-MnTe exhibited a well-pronounced structure of two peaks (at about 3.8 eV and 2.6 eV below VB maximum). Compared with the band theory [S.-H. Wei, A. Zunger, Phys. Rev. B 35 (1987) 2340] they were assigned to the predominant Mn 3d majority-spin contribution to the total VB density of states.
We analyze theoretically X-ray photoemission spectra (XPS) from Mn 2p
and 3p core-levels in Mn compounds and Mn Kβ X-ray emission spectra
(XES) by means of an MX6 cluster model.In the calculation of
3p-XPS and Kβ-XES, we take into account the effect of the
term-dependent lifetime of the Mn 3p core-hole due to the 3p-3d3d
super-Coster-Kroning decay. The Kβ-XES is treated as a coherent
second-order optical process, and the excitation spectra of Kβ-XES,
which reflect the spin-dependent Mn 1s absorption, are also calculated.
It is shown that the effect of the core-hole potential plays an
essential role in the excitation spectra of MnF2 and MnO. The
importance of the term-dependent lifetime of the final states is also
pointed out.
Highly oriented pyrolytic graphite was fluorinated at room temperature with molecular fluorine at 2.5 atm for 8 days to 2 months without addition of any catalyst. Under these mild conditions we obtained graphite-fluorine intercalation compounds in a wide range of stoichiometry 64 ⩾ C/F ⩾ 4. Except for the highest fluorine content (C/F ⩽ 4) all our samples are better conductors than graphite. A thorough XPS study of clean cleaved internal surfaces of the oriented samples shows a new kind of carbon-fluorine bonding. The binding energies of the C1s and F1s photoemission peaks are 287.3 ± 0.2 and 685.7 ± 0.2 eV. These values differ from those of typical covalent bonds (290.4 and 689.6 eV) and the nature of the bond is probably different from that of covalent CF.
Passive film formation as a function of applied potential on tin in citrate buffer solution was studied by X-ray photoelectron spectroscopy (XPS) and cyclic voltammetry. In order to avoid air oxidation and to minimise contamination of the samples, an electrochemical preparation and transfer system attached to the ultrahigh vacuum system was used. Quantitative evaluation of the electrochemical and XPS data showed the characteristic changes of the spectra in the prepassive and in the passive potential range. The prepassivation correlated with the clear presence of Sn0, Sn2+ and Sn4+ species, while true passivation correlated with the presence of Sn4+ species only, as expected on thermodynamic grounds. Three different oxygen-containing species were found in both potential ranges: oxide, hydroxide and water. The hydroxyl oxygen peaks (oxygen atoms from tin-hydroxide species) were dominant in the O ls spectra due to electrochemical conditions used (first scan) to produce samples for XPS examination.
Magnetoabsorption experiments have been performed at 4. 2 and 2 K in the spectral region 220-370 MeV, on zero gap Hg//1// minus //kMn//kTe alloys. The temperature and the composition dependence of the GAMMA //6 yields GAMMA //8 spectrum provides evidence of striking modifications in the GAMMA //8 and GAMMA //6 spin sublevels due to exchange interactions between localized d electrons and mobile carriers. The theoretical interpretation of the magnetooptical spectra of alloys of low Mn composition is carried out, using the virtual crystal and the molecular field approximations, by including the s-d and p-d exchange interactions within the framework of the Pidgeon and Brown model. By a fitting procedure of the GAMMA //6 yields GAMMA //8 transition energies, the magnetization is determined at 4. 2 and 2 K as a function of the magnetic field and the exchange parameters for the GAMMA //6 and GAMMA //8 levels are evaluated. The Landau level energy spectrum established, at k//H equals 0, for alloys of composition up to 1. 5% exhibits a SM yields SC transition induced by the magnetic field.
Interband magnetooptical studies are performed on Hg//1// minus //xMn//xSe alloys at temperatures from 10 to 40 K and compositions x up to 11. 5%. The obtained spectra are analyzed using the Pidgeon-Brown model modified by exchange interaction. Band parameters, including exchange integrals, are determined by a fitting procedure of the interband transition energies.
The chemical shifts of the Ge 3d, 3p3/2,1/2 and Se 3d,
3p3/2,1/2 photoelectron lines were measured for the amorphous
Ge-Se system and those of the Ge photoelectron lines are corrected for
Auger parameter shifts. According to the valence shell potential model,
the ratio of the chemical shift in the amorphous Ge-Se system to that in
stoichiometric GeSe2 can be approximated by the ratio of the
Ge-Se bond number in the Ge-Se system to that in GeSe2. The
chemical shift ratios evaluated from the experimental results reveal
bond structures at non-stoichiometric compositions. In the excess-Ge
range, GeSe is composed of atomic clusters of three-fold co-ordinated Ge
and Se atoms, and Ge2Se3 contains atomic clusters
of Se3Ge-GeSe3 units. In the excess-Se range,
GeSe3 includes GeSe4 tetrahedral units, and Se-Se
chains and/or Se8 rings.
Ultraviolet and X-ray photoemission spectroscopy was carried out to study a semimagnetic semiconducting tin compound. The lamellar compound was obtained by random partial substitution of tin by manganese ions. The spectroscopic analysis was carried out to identify the different electronic states within the valence band. Two contributions of manganese 3d electrons to the valence band were present which include a contribution of unhybridized states located at 3.5 eV below the valence band and the contribution that ranges from 7 to 11 eV binding energy due to hybridization between manganese 3d orbitals and selenium 4p orbitals. The binding energy was determined and the result indicated some signs of oxidation.
The recent development of so-called van der Waals epitaxy offers a whole new variety of material combinations for semiconductor heterojunctions. In this paper we investigate the morphology and the growth modes of the layered semiconductors SnSe2 (Eg=1.03 eV) and SnS2 (Eg=2.18 eV) on a variety of layered single crystalline substrate materials (highly oriented pyrolytic graphite, MoTe2, MoS2, WSe2, GaSe, SnS2 and SnSe2). The growth modes were investigated by low energy electron diffraction, photoemission spectroscopy and scanning tunnelling microscopy. We found that both of the materials grow in a two-dimensional layer-by-layer fashion with three-dimensional nucleation. However, despite the strong similarities of both of the materials the results indicate that SnS2 grows in a more ideal layer-by-layer mode than SnSe2. This is attributed to different surface diffusion rates due to different growth temperatures. The difference in empirically determined optimized growth temperatures are explained by the different thermal stabilities of the materials.
Electronic structures of Zn1−xMnxTe films (0 ≤ x ≤ 0.7) have been investigated by means of in situ measurements of inverse-photoemission and photoemission spectra. The Mn 3d spin-exchange splitting energy of 7.2 ± 0.2 eV is evaluated from emission peaks due to the Mn 3d↓ and 3d↑ states with eg symmetry. On the other hand, the whole spectrum including multielectron satellite compares well with the Mn 3d inverse-photoemission and photoemission spectra calculated using a configuration-interaction theory.
Valence-band and conduction-band structures of NiAs-type MnTe have been investigated by means of ultraviolet photoemission and inverse-photoemmion epectroscopies. Based on the comparison with the results of band-theory, features observed at −3.7 and 2.9 eV relative to the valence-band maximum (VBM) are assigned to emission from the Mn 3d↑ and 3d↓ states with fairly localized character, providing a spin-exchange splitting energy of 6.6 ± 0.2 eV. On the other hand, the Mn 3d photoemission and inverse-photoemission spectra have been calculated in terms of a configuration interaction theory using a MnTe6 model cluster, to interpret the whole features of the experimental spectra including multielectron satellites. The Mn 3d spectral features at −12 to −6, −6 ∼ 0 and 2.9eV relative to the VBM are attributed predominantly to transitions into the d4, d5L and d6 final state configurations, respectively, where L represents a ligand hole.
Mn 3d states in Cd1-xMnxY (Y = S, Se and Te have been investigated by using synchrotron radiation photoemission. The Mn 3d partial density-of-states spreads over the whole valence bands as a result of strong p-d hybridization with a prominent peak at 3.3–3.4 eV below the valence band maximum. Degree of hybridization between the majority-spin Mn 3d and anion p states increases on going from Te to S.
A detailed account of the electrochemistry involved in the deposition of Sn, Se and SnSe films is presented. The redox reactions and the polarization curves of Sn and Se were studied to fix the pH and potential values V (NHE) to get uniform deposition. Films were cathodically deposited at 55°C. XRD studies show an orthorhombic structure. Films showed an indirect band-gap of 1.05 eV. Surface morphological studies were carried out using SEM, and the stoichiometry was estimated from XPS analysis. Effect of annealing in air at 200°C has been reported.
The s,p–d exchange interaction in GaMnAs is discussed in view of different charge states of Mn impurities present in this material. The ferromagnetic to antiferromagnetic crossover of p–d exchange is discussed on the grounds of varying concentration of neutral and ionized Mn acceptor centers in bulk, Mn-doped GaAs and GaMnAs epilayers.
We report on the growth of Ga1-xFexSb, a new diluted magnetic semiconductor by the Bridgman method. XRD studies and the observed change of the lattice parameter with x (0.0001<x<0.0075) confirmed the formation of the material. A scanning electron microscopy study was done to understand the growth profile of this material. The change in morphology of the wafers from different parts of the ingot is shown. The type of defects and defect density indicates growth process deficiencies. On the basis of these results the desirable parameters for better crystal growth are presented. SEM results also showed the upper limit of iron substitution in GaSb using equilibrium growth methods. The cause of low iron incorporation in GaSb has been interpreted.
Synchrotron-radiation photoemission and inverse photoemission spectroscopies are powerful techniques to investigate occupied and unoccupied electronic states of solids. In this paper, we demonstrate that in situ measurements of ultraviolet photoemission and inverse photoemission spectra are successfully applied to studies of semiconductors. Our experimental results for Cd1–xMnxTe, zincblende (ZB–) and hexagonal NiAs-type (H–) MnTe, trigonal (t-) and glassy (g-) Se, and g-GexSe1−x and g-AsxSe1−x are presented to discuss the Mn 3d spin-exchange splitting energy and degree of hybridization between Mn 3d and sp-band states in Cd1−xMnxTe, ZB– and H–MnTe, dominant source of the distinct difference of electronic structures between t- and g-Se, and percolation phenomenon in non-crystalline covalent networks in g-GexSe1−x and g-AsxSe1−x, respectively.
The high resolution Mn and FeKα X-ray emission spectra (XES), and Mn and Fe2p X-ray photoelectron spectra (XPS) for manganese and iron oxides were measured. The spectra were compared with those of [MnO4]−, [Fe(CN)6]4− and [Fe(CN)6]3− ions. As the electronic structure of the latter compounds do not change with electron hole creation in the core levels, satellite peaks due to charge transfer are not observed in the 2p XPS spectra, and the peak profiles of metal 2p XPS and Kα XES are governed by the exchange splitting between 2p and valence electrons. The metal 2p XPS spectra of the oxides had satellite peaks, but the XES spectra had no satellites. FWHMs of the metal 2p3/2 main peaks of the compounds being low spin states are smaller than those of metal Kα1 XES spectra. However, FWHMs of Mn2p3/2 of the manganese oxide were nearly equal to those of Mn Kα1 XES spectra, and those of Fe2p3/2 XPS spectra of the iron oxides are greater than those of Fe Kα1 XES spectra.
Photoelectron-spectroscopic studies (XPS and UPS) have been carried out to investigate electronic structure and chemical bonding in the monoselenides of nickel, manganese and cobalt. Binding-energy values and chemical shifts are reported from the XPS measurements for Ni 2p and 3s, Mn 2p and 3s, Co 2p and 3s and Se 3p and 3d. The electronic structure and bonding in the monoselenides are interpreted from an analysis of the valence-band spectra. The combined XPS and UPS data suggest transfer of electrons from the metal orbitals (Ni, Mn and Co) to selenium, contrary to bonding schemes proposed previously in the literature. Magnetic-susceptibility measurements performed by us indicate NiSe to be diamagnetic and MnSe and CoSe to be highly paramagnetic at room temperature. The presence of shake-up satellites in the XPS spectra for MnSe and CoSe and their absence for NiSe are discussed in relation to these magnetic properties.
The electronic structure of pyrite-type MnTe2 has been investigated by photoemission spectroscopy. The Mn 3d partial density of states was obtained from resonance photoemission experiments in the Mn 3p-->3d core excitation region. The experimental results were compared with a linear-muffin-tin-orbital atomic-sphere-approximation ab initiocalculation, and some discrepancy was found. To reveal a role of electron correlation, the results were analyzed in terms of the configuration-interaction model by considering a (MnTe6)10- cluster, and parameter values for a model Hamiltonian were estimated (U=5.5 eV, Delta=1.5 eV and T=1.4 eV). X-ray photoemission spectra of the Mn 2p core level were also measured to confirm this assignment.
Von polykristallinen Proben der Legierungssysteme Cd1-xMnxTe 0 < x ⩽ 0.1 und Sn1-xMnxTe 0 < x ⩽ 0.4 werden die magnetische Suszeptibilität und Magnetisierung angegeben. Die magnetischen Messungen wurden im Temperaturbereich 2.3 K ⩽ T ⩽ 300 K bei einem äusseren Magnetfeld 0 ⩽ H ⩽ 11 kOe durchgeführt. Für hinreichend hohe Temperaturen kann die Suszeptibilität mit einem Curie-Weiss-Gesetz beschrieben werden. Im System Sn1-xMnxTe gilt θp #62; 0 mit einem gradlinigen Zusammenhang θp ∞ x und θp(0.4) = 49 K. In der Reihe Cd1-xMnxTe wechselt θp in Abhängigkeit von x sein Vorzeichen. Für 0 < x ⩽ 0.04 ist θp #62; 0 mit einem Maximum von θp ≈ 10 K bei x = 0.02. Im Bereich x #62; 0.04 ist θp < 0 und erreicht bei x = 0.1 den Wert -35 K. In allen Fällen bestimmen sich die effektiven Spins für Mangan zu Seff #62; 5/2. Die Experimente wurden zur Überprüfung der Annahme durchgeführt, dass in den betrachteten Telluriden des Mangans bei einem kürzesten Abstand dMnMn #62; 3.4 Å ferromagnetische Kopplung auftreten kann. Die Arbeitshypothese wurde bestätigt. Als Austauschmechanismus im Falle θp #62; 0 wird in Teilen eine RKKY-Kopplung angenommen.
The optical properties of the semiconducting compounds 0953-8984/9/41/020/img12 (Me = Ti, V, Cr, Mn, Fe, Co, and Ni) crystallizing in the zinc-blende structure have been investigated. Reflectivity spectra of these materials have been taken at room and liquid nitrogen temperatures. The measurements have been performed in the 4 - 25 eV energy range using synchrotron radiation from the ADONE storage ring in Frascati. Comparisons between the reflectivity spectra of the ternary systems and those of the host crystal ZnSe have been made, and models explaining the influence of the transition metal ions on the electronic structure of the host crystal ZnSe have been discussed.
The paramagnetic resonance has been measured on Mn ions (0.02-2.2 at%) in p-type SnTe crystals with carrier density 1.2-2× 1020 cm-3, and partly in n-type PbTe crystals, over the temperature range 120-480 K. For low Mn contents the hyperfine interaction constants A were found to be 56.0± 0.2× 10-4 cm-1 for SnTe and 61.0± 0.2× 10-4 cm-1 for PbTe, and the Debye temperatures theta were estimated from their temperature dependence; theta{=}200 K for SnTe and 110 K for PbTe. It was found that the g-values of SnTe (approximately g{=}2) were almost independent of temperature and Mn content. Finally, the susceptibility data of SnTe with 0.88 and 2.2 at% Mn are presented to show some possibility of magnetic inclusions in the grown crystal.
Electronic structures of Cd1-xMnxTe films (0x0.7) grown epitaxially on GaAs(100) substrates have been investigated by means of in situ measurements of conduction-band inverse-photoemission and valence-band photoemission spectra. With increasing Mn concentration, the energy position of the conduction-band minimum shifts almost linearly toward higher energy relative to the valence-band maximum (VBM) as a result of an increasing contribution of the higher-lying Mn 4s level relative to the Cd 5s level. Features observed at 3.6 and -3.4 eV relative to the VBM are ascribed to emission from the Mn 3d and 3d states with eg symmetry, respectively, providing a spin-exchange splitting energy of 7.00.2 eV. This value compares well with the predicted value in the theoretical investigation of electronic structures and magnetic properties of Cd1-xMnxTe. Apart from one-electron band theory, the whole spectrum including multielectron satellites in the energy region between -5 and -9 eV is found to be in good agreement with that calculated in terms of the configuration-interaction theory using a Mn2+(Te2-)4 model cluster.
K, Rb and Cs graphite intercalation compounds have been studied by photoelectron spectroscopy both in U.P.S. and in X.P.S. The study in high vacuo purity conditions and on well characterized samples ensures the experimental results to be relative to clean and not deteriorated surfaces. This certainty has been verified by comparison with spectra obtained on the same surfaces after a well-controlled oxidization. Core and valence spectra of carbon and alkaline species obtained at three different energies (1253.6, 21.2 and 40.8 eV) at room and at low temperature are analyzed in terms of stoichiometry and surface cleanness.Observing the surface of the first stage Cs-, Rb- and K- graphite intercalation compounds, the XC8 composition has been obtained provided the bulk is stoichiometric and the ultra-high vacuo of good quality. Criteria for surface cleanness are described and the work functions given.
The local electronic structure of 3d transition metal compounds is characterized by a few parameters, namely the ligand p to cation d charge transfer energy Δ, the dd Coulomb repulsion energy U, and the pd transfer integrals T. Values for these parameters deduced from the cluster model analysis of cation core level photoemission spectra are shown to exhibit systematic chemical trends as functions of cation atomic number, ligand, and cation valence. Physical properties of these compounds such as the magnitudes of the band gaps, pd covalency and the character of doped carriers, however, are not necessarily smooth functions of those variables but depend also on the nominal d electron number n due to the multiplet effects leading to the stabilization of the Hund's rule ground state. As an illustrative example, the electronic structure of valence-control Mn and Fe oxides is discussed.
Photoemission, optical-absorption, and isochromat spectra of NiO and NiCl2 are studied theoretically by the consideration of configuration interactions within the metal-ligand cluster. It is shown that the satellites in the valence-band photoemission spectra contain significant d7 final-state components produced by photoemission of a d electron from the largely d8-like ground state and that final states giving the main lines are predominantly d8-like resulting from ligand →3d charge-transfer transitions following the d-electron emission. This identification differs markedly from the traditional one, according to which the main lines are due to d7 final states and the satellites are produced by ligand →3d shakeup transitions. The crystal-field splitting and the apparent reduction of Racah parameters are shown to be due to hybridization between different configurations. The resonance enhancement of the satellites rather than the main lines at the 3p→3d photoabsorption threshold is attributed partly to covalency and partly to the small number of 3d holes in the nickel compounds as compared to other 3d transition-metal compounds. Excitation energies for ligand p→Ni 3d charge-transfer optical absorption are calculated and it is shown that the fundamental absorption edge of NiO at ~4 eV is not due to the p→d charge-transfer transitions. Instead, d→d charge-transfer transitions are proposed as the origin of the NiO fundamental edge. Energy levels involved in the intra-atomic d→d optical absorption are also calculated by the configurationinteraction approach and good agreement with experiment and energy levels calculated by the ligand-field theory is obtained. Finally the isochromat spectrum of NiO is discussed, based on the same approach.
Core-level and valence-band spectra have been obtained by means of x-ray photoemission spectroscopy for the group IV, V, and VI elements Ge, Sn, Pb, As, Sb, Bi, S, Se, and Te and the group IV-VI compounds GeS, GeSe, GeTe, SnS, SnSe, SnTe, PbS, PbSe, and PbTe. These results, taken under ultrahigh-vacuum conditions with unmonochromatized x rays, are presented and discussed in terms of the bonding in materials with an average valence of 5. The effects of relaxation on the chemical shifts are found to be relatively small. The chemical shifts are found to vary in a manner similar to that expected from the magnitude of the elemental electronegativities except for the ordering of the shifts of GeS and GeSe. Relative charge transfers are calculated from the chemical shifts and are found to be in general agreement with ionicities calculated using the Phillips-Van Vechten theory although there is some disagreement as to their magnitudes. A consideration of the structures of the compounds relative to the charge transfers demonstrates the importance of metallic as well as covalent and ionic bonding in determining the most stable structure. The value of critical ionicity carried over from the average-valence-4 materials does not apply to the average-valence-5 materials and this concept does not appear useful in understanding their bonding because of the increased importance of nondirectional metallic bonding.
A systematic study of the low-dc-field magnetic susceptibility and the specific heat has been carried out on mixed Hg1-xMnxTe crystals, in the composition range 0<~x<~0.35. The alloy with x=0.35 showed spin-glass behavior below T=10.9 K. At this Mn concentration the sample is a very poor conductor at low temperatures, so that the Ruderman-Kittel-Kasuya-Yosida (RKKY) mechanism cannot be responsible for the spin-glass transition. Also, since the Mn ions interact only antiferromagnetically, the observed spin-glass phase does not result from competition between ferro- and antiferromagnetic interaction. It must therefore be ascribed to the frustration of the antiferromagnetic interactions inherent in an fcc sublattice over which the Mn ions are distributed. For x<~0.25, the Hg1-xMnxTe samples remain paramagnetic down to 1 K. Experimental results for the specific heat and the susceptibility for x<0.1 are discussed in terms of a cluster model which leads to an estimated value of the antiferromagnetic exchange constant J/k≈ -0.7±0.3 K. When a random distribution of Mn ions over the fcc sublattice is assumed, calculated values for the specific heat and the susceptibility differ substantially from the experimental results for the low Mn concentration. This therefore indicates that in real crystals the distribution of Mn ions is very different from random. To obtain agreement between calculated and experimental results, the number of single ions has to be reduced to less than 30% of the number corresponding to a purely random distribution, leading to the conclusion that the magnetic ions prefer to cluster rather than to remain isolated in Hg1-xMnxTe. For HgTe, the Debye temperature ΘD is 141 K at T=0 K, and goes through an anomalously low minimum value of ΘD=77 K at 7 K. A very small linear term in the low-temperature specific heat of HgTe gives an estimate of the electric charge carrier density in the 1018 cm-3 range.
A measure of the Mn 3d partial density of states (DOS) of NiAs-type MnTe has been obtained from resonant photoemission experiments in the Mn 3p-3d core excitation region. On the basis of an analysis of the Mn 3d partial DOS spectrum in terms of a configuration interaction on a Mn2+ (Te2-)6 model cluster, we find that the spectral intensities in the top 6 eV are predominantly due to transitions to the d5L, final-state configuration, where L represents a ligand hole. On the other hand, those at 6-14 eV originate mainly from transitions to the d4 configuration.
Angle-resolved photoelectron spectroscopy has been applied to study atomically clean surfaces of SnSe2 prepared by cleavage in vacuum. The polar variation of the emission, as well as its dependence on the azimuthal angle, has been measured at room temperature. The results may be easily understood in terms of the band structure of the material and are compared with theoretical calculations.
The authors have conducted reflectivity measurements of the layer materials SnS2, SnSe2, CdI2, PbI2, BiI3 and BiOI between 8 and 40 eV. Band structures have been calculated for SnS2, CdI2, PbI2 on the basis of an LCAO approach and these are compared with the experimental data. Transitions from the metal d core states are discussed in terms of an atomic-like picture.
Interband magnetooptical studies are performed on Hg1−xMnxSe alloys at temperatures from 10 to 40 K and compositions x up to 11.5%. The obtained spectra are analysed using the Pidgeon-Brown model modified by exchange interaction. Band parameters, including exchange integrals, are determined by a fitting procedure of the interband transition energies.
Magnetooptische Interbanduntersuchungen werden an Hg1−xMnxSe-Legierungen bei Temperaturen von 10 bis 40 K und Konzentrationen x bis zu 11,5% durchgeführt. Die erhaltenen Spektren werden mit dem durch Austauschwechselwirkung modifizierten Pidgeon-Brown-Modell analysiert. Die Bandparameter, einschließlich der Austauschintegrale, werden durch ein Anpaßverfahren der Interbandübergangsenergien bestimmt.
The ablation technique was applied to thin film deposition from SnSe powders. A mechanism for epitaxial growth as characterized by X-rays and scanning electron microscope analysis is suggested. Plume analysis of the ablated material is also reported.
Pseudoternary alloys {(GeTe)1−y(PbTe)y}1−x(MnTe)x have been prepared and the magnetic and electrical properties of these alloys have been studied. The magnetic interaction in (PbTe)1−x(MnTe)x is very weak, but it has been found that the {(GeTe)1−y(PbTe)y}1−x (MnTe)x alloys (y ≦ 0.1) are ferromagnetic. In the latter, the increase of the amount of Pb substitution makes both the paramagnetic Curie temperature and the carrier density lower. These experimental results are discussed by using the RKKY interaction.
The treated surfaces of artificial heart valves made of silicon alloyed pyrolytic carbon were compared to similarly treated SiC using x‐ray photoelectron spectroscopy (XPS) and secondary ion mass spectrometry (SIMS). The materials were exposed to water, isopropanol, and acetone cleaning, diamond and alumina polishing, and sterilization. Three chemical states of carbon, three states of silicon, and low concentrations of polishing contaminants were identified and related to surface treatment. Characterization of the heart valve surfaces will assist in determining appropriate manufacturing processes required for assuring blood compatibility of the devices.
SnS 2 and SnSe 2 thin films were deposited by molecular beam epitaxy (MBE) methods on a variety of layered semiconductor substrates (freshly cleaved SnS 2 , SnSe 2 , WSe 2 , MoS 2 , MoTe 2 , GaSe, InSe) and cleaved mica, for investigation of the interfaces formed as a result of MBE growth. These ultrathin films were characterized in situ by x‐ray photoelectron spectroscopy and surface reflection high energy and low energy electron diffraction techniques. The growth modes were verified ex situ by scanning tunneling microscopy and/or atomic force microscopy [scanning probe microscopies (SPM)]. Despite the chemical and structural similarities between SnS 2 and SnSe 2 , SPM measurements show that the two materials as ultrathin films have different growth morphologies in the first few monolayers. Photoelectron spectroscopy (PES) measurements lead to the conclusion that both materials grow on the basal planes of most layered substrates in an epitaxial, layer‐by‐layer mode. Small deviations from ideal Frank–van der Merwe growth for these thin films could be determined from PES intensity ratios versus coverage. Deposition of both materials on freshly cleaved mica, where the lattice mismatch between the substrate and the overlayer surface unit cell dimension exceeds 40%, yields exclusively 500–1000 Å diameter epitaxial islands with threefold symmetry. © 1995 American Vacuum Society
The detailed design of a beamline, monochromator and photoelectron spectrometer for surface science studies in the photon energy range 15–200 eV is described. The photon flux from the monochromator has been measured as a function of energy. Adequate intensity and resolution are available for a wide range of photoemission studies.
Photoelectron spectra of the synthetic compounds FeS2, CoS2, NiS2, MnSe2, CoSe2, and NiSe2 and of a natural crystal of MnS2, all with the pyrite structure, are reported. The sulfur 3s and selenium 4s contributions are split into peaks for bonding and antibonding orbitals due to the covalent bonding in the molecular anion pairs. The difference in lineshape of the peaks for the bonding and antibonding orbitals is attributed to vibronic effects. The metal spectra show the effects of multiplet splitting and satellites due to shake-up or shake-off processes. The valence band spectra consist of slightly overlapping contributions of anion p and metal 3d electrons. The metal 3d spectrum of FeS2 has a single strong peak of width 0.9 eV. The 3d spectra of the other compounds show structure due to several final state configurations.
A direct comparison of the splitting of free exciton states with the magnetic moment of the Mn2+ ion system shows excellent agreement with simple exchange interaction model throughout the composition range studied (0.005 < x < 0.3) at liquid helium temperature in magnetic field up to 5.6 T. Measurements of magnetic moments up to 15.5 T at 1.5 K show paramagnetic behaviour of samples with low manganese contents. With increasing amount of manganese mile fraction, strong influence of interaction of antiferromagnetic type between Mn2+ ions is observed. Comparison of optical and magnetic data yields refined values of exchange integrals of Mn2+ ions with conduction and valence electrons: 0.22 and −0.88 eV, respectively.
The main achievements in the study of X-ray photoemission of MnO, FeO, CoO, and NiO, single crystals are discussed. For these compounds the oxygen 1s, the cation 2s, 2p, and 3s core line spectra and the one-electron removal valence band spectra are reported. The unresolved problems in the understanding of the fine structure present in the X-ray photoemission spectra are evidenced. © 1999 Elsevier Science B.V. All rights reserved. Keywords: X-ray photoemission spectra; Fine structure; Core line spectra; One-electron removal valence band spectra.
The polycrystalline semimagnetic semiconductor Pb1−xMnxTe (x = 0.02–0.08 mol%) was prepared and characterized by powder X-ray diffraction, magnetic susceptibility and X-ray photoelectron spectroscopy. The compound was single phase homogeneous and has cubic structure. The magnetic susceptibility increases with increasing concentration of Mn. The multiplet splitting, ΔE3s, also increases with increasing Mn concentration. The observed asymmetry in the Mn3s satellite structure is attributed to configuration interaction or interatomic relaxation.
We have studied the contribution of Mn 3d states to the valence bands of Cd0.35Mn0.65Te by means of the resonant enhancement of the Mn 3d photoionization cross section near the Mn 3p53d6( 6P) core excitation (homega=50 eV). The cross section of selected valence-band areas has a characteristic resonance shape as a function of photon energy that is well described by a Fano-type profile. From the strength of the resonance we conclude that there is appreciable hybridization with Mn 3d states throughout the valence bands. In addition, a new interpretation of the photoemission intensity with strong Mn 3d character that lies in the ionic gap of the CdTe valence bands is given. Based on an analysis of the final-state structure in terms of a configuration-interaction calculation performed on a MnTe4 6- cluster, we ascribe these features between 5 and 9 eV binding energy to d4 final states (satellites), whereas the photoemission between 0 and 5 eV is due to Mn 3d multiplets where the d4 final state is screened by charge transfer from the Te-derived valence-band states.
The satellite structure in both core-level and valence-band photoemission spectra from single-crystal MnO(100) has been studied by using both x-ray photoemission spectra and resonant photoemission. Only very weak satellites are present in the Mn 2p core-level spectra, while no satellite structure is evident in the Mn 3p and 3s core-level spectra. The small intensity of the satellite peaks in MnO is consistent with the trend predicted by a recent ligand charge-transfer screening model; such screening is quite weak in MnO in comparison to the heavier transition-metal oxides. The valence-band spectrum, however, exhibits a stronger satellite peak than predicted by the model, and its origin has been uncertain. We have found that, while the satellite in the valence-band spectrum can be resonantly enhanced across the Mn 3p-->3d optical excitation threshold, its intensity depends strongly upon surface treatment. The satellite disappears with only 0.2-1-langmuir exposure of oxygen at 340 °C, confirming that it is essentially a surface-related feature. We suggest that it is associated with point defects on the surface, presumably surface Mn vacancies.
The electronic structure of MnO has been investigated using high-energy (x-ray photoelectron and bremsstrahlung-isochromat) spectroscopies. An experimental gap of 3.9 eV is found. By comparing the experimental results to a configuration-interaction cluster model, values for the different parameters in a model Hamiltonian are found [U=8.5 eV, Δ=8.8 eV, and (pdσ)=1.3 eV]. These parameter values place MnO in the intermediate region of the Zaanen-Sawatzky-Allen phase diagram. By using the same parameters, the d-d forbidden optical-absorption energies can be calculated, and good agreement with experiment is found.
The electronic properties of substitutional 3d transition-metal impurities in II-VI semiconductors have been studied using the cluster and Anderson impurity models with configuration interaction. It is shown that the photoemission and inverse-photoemission spectra, d-d optical-absorption spectra, exchange interaction between the 3d magnetic moment and the host band states, and donor and acceptor ionization energies can be reproduced with the same set of parameters, which show systematic variation with expected chemical trends. The importance of multiplet effects in the formation of donor and acceptor levels within the band gap is demonstrated.