Sn Vacancies Research Articles

High Thermoelectric Performance of p-Type SnTe via a Synergistic Band Engineering and Nanostructuring Approach

SnTe is a potentially attractive thermoelectric because it is the lead-free rock-salt analogue of PbTe. However, SnTe is a poor thermoelectric material because of its high hole concentration arising from inherent Sn vacancies in the lattice and its very high electrical and thermal conductivity. In this study, we demonstrate that SnTe-based materials can be controlled to become excellent thermoelectrics for power generation via the successful application of several key concepts that obviate the well-known disadvantages of SnTe. First, we show that Sn self-compensation can effectively reduce the Sn vacancies and decrease the hole carrier density. For example, a 3 mol % self-compensation of Sn results in a 50% improvement in the figure of merit ZT. In addition, we reveal that Cd, nominally isoelectronic with Sn, favorably impacts the electronic band structure by (a) diminishing the energy separation between the light-hole and heavy-hole valence bands in the material, leading to an enhanced Seebeck coefficient, and (b) enlarging the energy band gap. Thus, alloying with Cd atoms enables a form of valence band engineering that improves the high-temperature thermoelectric performance, where p-type samples of SnCd(0.03)Te exhibit ZT values of ~0.96 at 823 K, a 60% improvement over the Cd-free sample. Finally, we introduce endotaxial CdS or ZnS nanoscale precipitates that reduce the lattice thermal conductivity of SnCd(0.03)Te with no effect on the power factor. We report that SnCd(0.03)Te that are endotaxially nanostructured with CdS and ZnS have a maximum ZTs of ~1.3 and ~1.1 at 873 K, respectively. Therefore, SnTe-based materials could be ideal alternatives for p-type lead chalcogenides for high temperature thermoelectric power generation.

Ab initio study of native defects in SnO under strain

Tin monoxide (SnO) has promising properties to be applied as a p-type semiconductor in transparent electronics. To this end, it is necessary to understand the behaviour of defects in order to control them. We use density functional theory to study native defects of SnO under tensile and compressive strain. We show that Sn vacancies are less stable under tension and more stable under compression, irrespectively of the charge state. In contrast, O vacancies behave differently for different charge states. It turns out that the most stable defect under compression is the charged O vacancy in an Sn-rich environment and the charge neutral O interstitial in an O-rich environment. Therefore, compression can be used to transform SnO from a p-type into either an n-type or an undoped semiconductor.

Density-functional study on the robust ferromagnetism in rare-earth element Yb-doped SnO2

So far, little has been known about the ferromagnetism induced by p–f hybridization. We investigate the magnetic properties of Yb-doped SnO2 by first-principles calculations. We find that the doped system favors the ferromagnetic state and a room-temperature ferromagnetism can be expected in it. The origin of ferromagnetism can be attributed to the p–f hybridization between Yb impurity and its surrounding oxygen atoms. The formation energy of defect complex is calculated and the magnetic mediation of intrinsic vacancies is studied. Our results reveal that the formation energy of the defect complex with Sn vacancy is about 7.3eV lower in energy than that with oxygen vacancy. This means Sn vacancy is much easier to form than oxygen vacancy in the presence of Yb substitution. The ferromagnetism of the doped system is greatly enhanced in the presence of Sn vacancies.

Microscopic origin of thep-type conductivity of the topological crystalline insulator SnTe and the effect of Pb alloying

The interest in SnTe has recently increased due to its topological crystalline insulator nature, despite the fact that SnTe is always heavily $p$ type. Here, using first-principles calculations, we identify the microscopic origin of the $p$-type conductivity of SnTe. It is found that the negatively charged Sn vacancy (${V}_{\mathrm{Sn}}^{2\ensuremath{-}}$) dominates the electronic properties of SnTe: regardless of the growth conditions, ${V}_{\mathrm{Sn}}^{2\ensuremath{-}}$ always has a negative formation energy within the band gap, which forces the Fermi level below the valence band maximum (VBM), leading to degenerate $p$-type doping. In contrast, the deeper VBM of PbTe increases the formation energy of the Pb vacancy (${V}_{\mathrm{Pb}}^{2\ensuremath{-}}$), allowing for either $n$-type or $p$-type PbTe. So even though standard n doping of SnTe is very difficult, alloying with Pb can achieve $n$ doping by lowering the VBM, thereby inhibiting the formation of cation vacancy and, thus, probably producing a topological crystalline insulator with the Fermi level located near the Dirac point.

Magnetron sputtered Cu doped SnS thin films for improved photoelectrochemical and heterojunction solar cells

This work describesex situCu-doped SnS demonstrating a largeJph= 3.2 mA cm−2as a photocathode and enhancedVOC= 465 mV in a hetero-junction solar cell.

First principles study of point defects in SnS.

Photovoltaic cells based on SnS as the absorber layer show promise for efficient solar devices containing non-toxic materials that are abundant enough for large scale production. The efficiency of SnS cells has been increasing steadily, but various loss mechanisms in the device, related to the presence of defects in the material, have so far limited it far below its maximal theoretical value. In this work we perform first principles, density-functional-theory calculations to examine the behavior and nature of both intrinsic and extrinsic defects in the SnS absorber layer. We focus on the elements known to exist in the environment of SnS-based photovoltaic devices during growth. In what concerns intrinsic defects, our calculations support the current understanding of the role of the Sn vacancy (VSn) acceptor defect, namely that it is responsible for the p-type conductivity in SnS. We also present calculations for extrinsic defects and make extensive comparison to experimental expectations. Our detailed treatment of electrostatic correction terms for charged defects provides theoretical predictions on both the high-frequency and low-frequency dielectric tensors of SnS.

Electron spin resonance parameters of cation vacancies in tin dioxide doped with fluorine and hydrogen

The spin Hamiltonian parameters of intrinsic defects in tin dioxide (SnO2) doped with fluorine or hydrogen are examined through the first-principles electronic structure calculations based on density functional theory (DFT). The electron paramagnetic resonance signals with g-tensor value in the range of 1.89–1.94 were found for tin vacancy (VSn) and its complex with oxygen vacancy (VSn-VO) associated with a donor like interstitial hydrogen or fluorine, substituting oxygen. The calculated parameters are consistent with experimental observations, which indicate that Sn vacancies may be present in SnO2 at essentially higher concentration than it is predicted by DFT based on the formation energies calculations. Within the second coordination shell of the tin vacancy, the VO may stabilize in the singly ionized charge state, which is otherwise considered to be unstable for isolated oxygen vacancy in the bulk of SnO2.

Density-functional study on the ferromagnetism of Mn-doped SnO2

So far, both room-temperature ferromagnetism and paramagnetism have been reported in Mn-doped SnO2 films and their magnetic properties remain still puzzling. We have systematically investigated the magnetic properties of Mn-doped SnO2 by first-principles calculations. Our results reveal that the magnetic coupling is too weak and consequently paramagnetism is only found in the doped system when Mn substitutes Sn atoms. The formation energy of Mn impurity is much higher without the presence of vacancy, which prevents their substitutions significantly. In the presence of vacancies, the formation energy of Mn is greatly reduced and they can substitute Sn heavily. Compared to Sn vacancies, oxygen vacancies mediate the ferromagnetism much more effectively. The origin of ferromagnetism in Mn-doped SnO2 can be attributed to the oxygen-vacancy mediated RKKY interaction.

Ambipolar doping in SnO

SnO is a promising oxide semiconductor that can be doped both p- and n-type, but the doping mechanisms remain poorly understood. Using hybrid functionals, we find that native defects cannot account for the unintentional p-type conductivity. Sn vacancies are shallow acceptors, but they have high formation energies and are unlikely to form. Unintentional impurities offer a more likely explanation for p-type doping; hydrogen is a likely candidate, and we find that it forms shallow-acceptor complexes with Sn vacancies. We also demonstrate that the ambipolar behavior of SnO can be attributed to the high position of the valence-band on an absolute energy scale.

Electronic Structure of Defects in SnTe

Myab initioelectronic structure calculations inRSn2n-1Te2n, n=16, R = a vacancy, Cd, and In show that when Sn atom is substituted by R, the Density of State (DOS) of the valence and conduction bands get strongly perturbed. There are significant changes near the band gap region. Sn vacancy causes very little change near the bottom of the conduction band DOS whereas there is an increase in the DOS near the top of the valence band. Results for In impurity shows that, unlike PbTe, the deep defect states in SnTe are resonant states near the top of the valence band. In PbTe these deep defect states lie in the band-gap region (act asn-type). This fundamental difference in the position of the deep defect states in SnTe and PbTe explains the experimental anomalies seen in the case of In impurities (act asn-type in PbTe andp-type in SnTe).

Deep electron traps and origin ofp-type conductivity in the earth-abundant solar-cell material Cu2ZnSnS4

Using hybrid functional calculation, we identify the key intrinsic defects in Cu${}_{2}$ZnSnS${}_{4}$ (CZTS), an important earth-abundant solar-cell material. The Sn-on-Zn antisite and the defect complex having three Cu atoms occupying a Sn vacancy are found to be the main deep electron traps. This result explains the optimal growth condition for CZTS, which is Cu poor and Zn rich, as found in several recent experiments. We show that, under the growth condition that minimizes the deep traps, Cu vacancy could contribute the majority of hole carriers, while the Cu-on-Zn antisite will become the dominant acceptor if the growth condition favors its formation.

Atomic insight into Ge1−xSnx using atom probe tomography

Ge(1-x)Sn(x) is receiving a growing interest in the scientific community, as it has important applications in opto-electronic devices, ( as stressor) Source/Drain materials for Ge and SiGe MOSFETS. It is predicted that at 10% Sn concentration or even lower, unstrained Ge(1-x)Sn(x) will exhibit a direct band gap. Moreover, in strained Ge(1-x)Sn(x) the expected concentration of Sn for this cross-over is even lower. As the theoretical Sn incorporation in Ge(1-x)Sn(x) is less than 1%, and Ge(1-x)Sn(x) is prone to relaxation, routes towards the growth of metastable strained films has been extensively explored. Although Ge(1-x)Sn(x) films (with x up to 10%) have been grown using various methods like molecular beam epitaxy, CVD growth etc. there remain issues with tendency of these layers to relax. Detailed studies on the relaxation mechanisms and effects on the Sn-atoms require suitable characterization techniques. Various techniques have been used to study the surface of the film, crystallography or concentration of Sn in the film but none of them provides information at the atomic scale as they average over many layers and atoms. Atom probe tomography (APT) analysis, on the other hand, is one such method that can provide atomic scale resolutions (∼0.3 nm) due to its ability to perform atom by atom analysis. In this paper we explore the use of APT for characterizing Ge(1-x)Sn(x) layers. We comment on the difference of field evaporation values of Ge and Sn in Ge(1-x)Sn(x) layer by taking a closer look at the co-evaporation of the two elements and comment on the accuracy of depth reconstruction of APT for Ge(1-x)Sn(x) layer. Comparing the Sn-distributions and their local surroundings we saw a tendency for the Sn to locally enrich forming Sn clusters. Higher order clusters were observed for the relaxed sample.

Origin of ferromagnetism in Cu-doped SnO2: A first-principles study

We investigate the magnetic properties of Cu-doped SnO2, which was reported recently in experiments to possess ferromagnetism at room temperature, by first-principles calculations. Our results show that the doped system prefers the antiferromagnetic state for only Cu substitution. The oxygen vacancies are more inclined to form on the sites nearest to Cu defects. However, only those oxygen vacancies away from Cu defects contribute to the ferromagnetism significantly. Our results reveal that Sn vacancies are much easier to form than oxygen vacancies for their much lower formation energy. Moreover, the Cu-doped system always prefers the ferromagnetic state in the presence of Sn vacancies. Therefore, the origin of ferromagnetism in Cu-doped SnO2 can be attributed to the magnetic mediation of Sn vacancies.

Annealing Effects on Electrical Properties of Pure and Tin-Doped Indium Oxide Thin Films

The annealing effects on the properties of ITO and pure In2O3 thin films have been investigated. The thin films were deposited with various O2 flow ratios to total gas flow by pulsed dc magnetron sputtering. The post-deposition annealing of the thin films was carried out for 30 minutes at various temperatures ranging up to 500 degrees C in air. It was found through the comparison of the carrier density of ITO and In2O3 thin films that the carrier electrons of the ITO thin films came from both of the dopant Sn and oxygen vacancies under the annealing less than 400 degrees C. Therefore, the ITO thin films deposited with lower O2 flow ratio exhibited higher carrier density due to many oxygen vacancies; in consequence, they exhibited lower resistivity at the annealing up to 400 degrees C. On the other hand, the carrier density of ITO thin films was almost identical regardless of O2 flow ratio when they were annealed at 500 degrees C. This fact indicates that most carrier electrons of the ITO thin films were brought by the dopant Sn at the annealing temperature of 500 degrees C. However, the ITO thin films deposited with lower O2 flow ratio exhibited higher Hall mobility; as a result, they showed lower resistivity at the annealing of 500 degrees C. Atomic force microscope, X-ray diffraction and X-ray reflectivity measurements revealed that the ITO thin films deposited with lowe O2 flow ratio exhibited dense structure even after they were annealed at 500 degrees C. Hence, the carrier electrons of the dense ITO thin films deposited with low O2 flow ratio can conduct better, as a result, the ITO thin films exhibited high Hall mobility and low resistivity.

Conduction-type control of SnSx films prepared by the sol–gel method for different sulfur contents

The effect of S content on the electrical property of the sol–gel SnSx films was examined. The observed conduction-type changes are related to the different ratios between the concentrations of Sn4+ and Sn2+. The experimental identification confirms that n-type conversion is due to an increase in the atomic concentration ratio of Sn4+/(Sn4++Sn2+) in the S-rich film. The probability of having formed Sn vacancies (VSn) should be high under S-rich growth conditions. Transformation from VSn to VSn2- is accompanied by lattice relaxation. Therefore, transformation from Sn2+ to Sn4+ is an offset to lattice relaxation under S-rich growth conditions, increasing the electron density and producing n-type conversion. A suitable sulfur concentration is an important issue for tuning conduction type of SnSx.

The effect of defects on the electronic and magnetic properties of the Co2VSn full Heusler alloy: Ab-initio calculations

Density Functional Theory (DFT) calculations are performed using full potential linearized augmented plane wave (FP-LAPW) method to investigate the effect of defects on the electronic and magnetic properties of Co2VSn full Heusler alloy. The formation energies are calculated for antisite, swap and vacancy defects. The Vsn antisite, V, Co and Sn vacancies have relatively low formation energies with high probability to occur. The half metallicity is maintained in all structures with band gaps smaller than that of the perfect alloy except for CoSn, SnCo antisite and Co–Sn swap, which exhibit a metallic behavior.

Intrinsic magnetism in nanosheets of SnO2: A first-principles study

We propose intrinsic magnetism in nanosheets of SnO2, based on first-principles calculations. The electronic structure and spin density reveal that p orbitals of the oxygen atoms, surrounding Sn vacancies, have a non-itinerant nature which gives birth to localized magnetism. A giant decrease in defect formation energies of Sn vacancies in nanosheets is observed. We, therefore, believe that native defects can be stabilized without any chemical doping. Nanosheets of different thicknesses are also studied, and it is found that it is easier to create vacancies, which are magnetic, at the surface of the sheets. SnO2 nanosheets can, therefore, open new opportunities in the field of spintronics.

CsSnI3: Semiconductor or Metal? High Electrical Conductivity and Strong Near-Infrared Photoluminescence from a Single Material. High Hole Mobility and Phase-Transitions

CsSnI(3) is an unusual perovskite that undergoes complex displacive and reconstructive phase transitions and exhibits near-infrared emission at room temperature. Experimental and theoretical studies of CsSnI(3) have been limited by the lack of detailed crystal structure characterization and chemical instability. Here we describe the synthesis of pure polymorphic crystals, the preparation of large crack-/bubble-free ingots, the refined single-crystal structures, and temperature-dependent charge transport and optical properties of CsSnI(3), coupled with ab initio first-principles density functional theory (DFT) calculations. In situ temperature-dependent single-crystal and synchrotron powder X-ray diffraction studies reveal the origin of polymorphous phase transitions of CsSnI(3). The black orthorhombic form of CsSnI(3) demonstrates one of the largest volumetric thermal expansion coefficients for inorganic solids. Electrical conductivity, Hall effect, and thermopower measurements on it show p-type metallic behavior with low carrier density, despite the optical band gap of 1.3 eV. Hall effect measurements of the black orthorhombic perovskite phase of CsSnI(3) indicate that it is a p-type direct band gap semiconductor with carrier concentration at room temperature of ∼ 10(17) cm(-3) and a hole mobility of ∼585 cm(2) V(-1) s(-1). The hole mobility is one of the highest observed among p-type semiconductors with comparable band gaps. Its powders exhibit a strong room-temperature near-IR emission spectrum at 950 nm. Remarkably, the values of the electrical conductivity and photoluminescence intensity increase with heat treatment. The DFT calculations show that the screened-exchange local density approximation-derived band gap agrees well with the experimentally measured band gap. Calculations of the formation energy of defects strongly suggest that the electrical and light emission properties possibly result from Sn defects in the crystal structure, which arise intrinsically. Thus, although stoichiometric CsSnI(3) is a semiconductor, the material is prone to intrinsic defects associated with Sn vacancies. This creates highly mobile holes which cause the materials to appear metallic.

Defect levels in SnS thin films prepared using chemical spray pyrolysis

AbstractThe origin of various defect levels in the SnS thin films deposited using chemical spray pyrolysis (CSP) technique has been explored in this manuscript, by employing low‐temperature photoluminescence (PL) technique. Concentration of Sn in the samples was varied purposefully by ex situ diffusion in order to alter the defect levels. The acceptor level obtained at 0.22 eV from the Arrhenius plot, has been assigned as the defect level caused by the Sn vacancies present in the lattice. Two shallow donor levels are conclusively identified and their activation energies have been estimated. The present study could also unearth a trap level in the forbidden energy gap which was due to the oxygen contaminant occupied by the vacancy of Sn. This trap level could be removed by annealing the sample in vacuum or through the ex situ diffusion of Sn. Employing Kelvin probe force microscopy (KPFM), the work‐function of SnS was obtained as 4.925 eV, from which the position of the Fermi level could be assigned. Based on the present work, an energy level scheme for SnS thin films is proposed outlying origin of various defect levels.

Positron Annihilation Lifetime Studies of Nb‐Doped TiO2, SnO2, and ZrO2

Positron annihilation lifetime spectroscopy (PALS) has indicated that Nb2O5‐doped TiO2 samples treated in either Ar or air at 1400°C for 48 h are both charge‐compensated by Ti vacancies, but with statistically significant differences between the results. A Ta2O5‐doped air‐sintered TiO2 sample also showed behavior similar to the Nb2O5‐doped air‐sintered TiO2. In addition, Nb2O5‐doped SnO2 samples sintered in air at 1400°C for 1 h showed evidence of Sn vacancy cluster formation, with no indications of Sn2+ compensation from ultraviolet irradiation studies. PALS studies of air‐sintered ZrO2 did not show any evidence of Zr vacancies after doping with Nb2O5.