Sn-rich Conditions Research Articles

Point defects, doping and the path to n-type SnTe

Point defects in SnTe were studied using first-principles modeling. Sn vacancies were found to be spontaneously formed in SnTe and make it intrinsically a p-type semiconductor, in excellent accord with experiments. The calculated electronic structure for Sn vacancy reveals that the Fermi level is shifted down into the valence band and presents p-type behavior. In contrast, the Fermi level shifts near the conduction band minimum (CBM) for Te vacancy and presents an n-type behavior. All the interstitial sites were considered for a comprehensive intrinsic point defect study. The body-centered (tetrahedral) site was found to be a stable interstitial configuration for Sn, whereas the split<111> interstitial was preferred for Te. We found that the electronic structures due to the interstitial configurations were distinct. Sn interstitials defects, Fermi level was shifted inside the conduction band for body interstitial, near to CBM for split 111 interstitial, whereas inside the bandgap for the base and split 110 interstitial defects. Contrary results were obtained for Te interstitial defects; the Fermi level was found inside the valence band for body interstitial, close to VBM for split 111, while the Fermi level was obtained inside the bandgap for base and split 110 interstitial defects. In addition, extrinsic oxygen impurities and other possible dopants that can transform SnTe to n-type were investigated. Interstitial oxygen was found to be stable at the body-interstitial site and is incorporated easily into the material in both Sn- and Te-rich environments, leading the material to p-type. The Fermi level was obtained inside the bandgap for oxygen substituted in Te vacancy and showed no preferred conductivity behavior. Finally, we find that Sb, Bi, and halogens (in Sn-rich conditions) can significantly improve electron concentrations and be promising dopants to transform the material's intrinsic conductivity from p-type to n-type. Notably, the sample should be prevented from incorporating oxygen impurities while doping sample n-type.

Routes to High-Carrier-Density Doping in Thermoelectric SnSe

Ultralow thermal conductivity and distinct transportation properties of SnSe have attracted lots of interest in optimizing its thermoelectric properties. In this work, an extensive defects study of layered SnSe semiconductors was carried out using density functional theory (DFT) calculations. Results show that intrinsic SnSe is a p-type semiconductor under Sn-poor conditions because of the Sn vacancy and is slightly n-type under Sn-rich conditions. For extrinsic defects, doping effects of numerous elements, such as alkali metals, transition metals (Cu, Ag, and Au), boron and nitrogen group elements, and halogen elements on carrier densities are studied. In our calculations, ternary phases are included in the phase diagram analysis to improve the accuracy of SnSe dopability prediction. We found that Na exhibits extraordinary p-type doping efficiency with the hole concentration improved by around two orders of magnitude. Cl and Br enhance the electron concentration by four orders of magnitude, indicating they are promising n-type doping candidates.

Common Defects Accelerate Charge Carrier Recombination in CsSnI3 without Creating Mid-Gap States.

Lead-free metal halide perovskites are environmentally friendly and have favorable electro-optical properties; however, their efficiencies are significantly below the theoretical limit. Using ab initio nonadiabatic molecular dynamics, we show that common intrinsic defects accelerate nonradiative charge recombination in CsSnI3 without creating midgap traps. This is in contrast to Pb-based perovskites, in which many defects have little influence on and even prolong carrier lifetimes. Sn-related defects, such as Sn vacancies and replacement of Sn with Cs are most detrimental, since Sn removal breaks the largest number of bonds and strongly perturbs the Sn-I lattice that supports the carriers. The defects increase the nonadiabatic electron-vibrational coupling and interact strongly with free carrier states. Point defects associated with I atoms are less detrimental, and therefore, CsSnI3 synthesis should be performed in Sn rich conditions. The study provides an atomistic rationalization of why lead-free CsSnI3 exhibits lower photovoltaic efficiency compared to some lead-based perovskites.

Vacancies and dopants in two-dimensional tin monoxide: An ab initio study

Layered tin monoxide (SnO) offers an exciting two-dimensional (2D) semiconducting system with great technological potential for next-generation electronics and photocatalytic applications. Using a combination of first-principles simulations and strain field analysis, this study investigates the structural dynamics of oxygen (O) vacancies in monolayer SnO and their functionalization by complementary lightweight dopants, namely C, Si, N, P, S, F, Cl, H and H2. Our results show that O vacancies are the dominant native defect under Sn-rich growth conditions with active diffusion characteristics that are comparable to that of graphene vacancies. Depending on the choice of substitutional species and its concentration within the material, significant opportunities are revealed in the doped-SnO system for facilitating n/p-type tendencies, work function reduction, and metallization of the monolayer. N and F dopants are found to demonstrate superior mechanical compatibility with the host lattice, with F being especially likely to take part in substitution and lead to degenerately doped phases with high open-air stability. The findings reported here suggest that post-growth filling of O vacancies in Sn-rich conditions presents a viable channel for doping 2D tin monoxide, opening up new avenues in harnessing defect-engineered SnO nanostructures for emergent technologies.

Stoichiometry control in molecular beam epitaxy of BaSnO3

La-doped $\mathrm{BaSn}{\mathrm{O}}_{3}$ films were grown on $\mathrm{DySc}{\mathrm{O}}_{3}$ substrates by molecular beam epitaxy using La, Ba, and $\mathrm{Sn}{\mathrm{O}}_{2}$ sources with and without additional oxidant, respectively. Lattice parameter measurements as a function of growth conditions show a reduced lattice parameter that is likely due to substitution of $\mathrm{S}{\mathrm{n}}^{2+}$ on the Ba site. The propensity for the antisite defect is discussed as being due to the combination of oxygen-poor, Sn-rich conditions and the dual valence of Sn. Although electron mobilities are highest for films with reduced lattice parameters, antisite defects will pose an upper limit to thin film mobility. Less Sn-rich conditions lead to the formation of another defect that causes a lattice expansion. The combined effects of these defects on the lattice parameter can compensate each other and cause the appearance of a stoichiometric lattice parameter for nonstoichiometric films with poor electrical behavior.

Understanding the Effects of Cd and Ag Doping in Cu2ZnSnS4 Solar Cells

Cu2ZnSnS4-based solar cells, which constitute an inexpensive, beyond-Si photovoltaic technology, often suffer from low open-circuit voltage and efficiency. This drawback is often attributed to disorder in the Cu–Zn sublattice of the kesterite structure. While previous experiments have reported improved performances with isovalent substitution of Cd and Ag for Cu and Zn, respectively, the fundamental driving force for such improvements remains unclear. Here, we use density functional theory to study bulk stability, defect, and surface energetics, as well as the electronic structure of these dopants in Cu2ZnSnS4. We find that Cd and Ag can increase efficiencies, depending on the dopant concentration and Cu content used during synthesis. Most importantly, we find that a low level of Cd doping can suppress disorder in the kesterite phase across all Cu concentrations, while a low level of Ag doping can do so only when Zn- and Sn-rich conditions are employed. A higher Ag content is beneficial as it stabilizes t...

Strongly Quantum Confined Colloidal Cesium Tin Iodide Perovskite Nanoplates: Lessons for Reducing Defect Density and Improving Stability.

Within the last several years, metal halide perovskites such as methylammonium lead iodide, CH3NH3PbI3, have come to the forefront of scientific investigation as defect-tolerant, solution-processable semiconductors that exhibit excellent optoelectronic properties. The vast majority of study has focused on Pb-based perovskites, which have limited applications because of their inherent toxicity. To enable the broad application of these materials, the properties of lead-free halide perovskites must be explored. Here, two-dimensional, lead-free cesium tin iodide, (CsSnI3), perovskite nanoplates have been synthesized and characterized for the first time. These CsSnI3 nanoplates exhibit thicknesses of less than 4 nm and exhibit significant quantum confinement with photoluminescence at 1.59 eV compared to 1.3 eV in the bulk. Ab initio calculations employing the generalized gradient approximation of Perdew-Burke-Ernzerhof elucidate that although the dominant intrinsic defects in CsSnI3 do not introduce deep levels inside the band gap, their concentration can be quite high. These simulations also highlight that synthesizing and processing CsSnI3 in Sn-rich conditions can reduce defect density and increase stability, which matches insights gained experimentally. This improvement in the understanding of CsSnI3 represents a step toward the broader challenge of building a deeper understanding of Sn-based halide perovskites and developing design principles that will lead to their successful application in optoelectronic devices.

A comparative investigation of secondary phases and MoSe2 in Cu2ZnSnSe4 solar cells: Effect of Zn/Sn ratio

Our objective in this study was to elucidate the influence of the Zn/Sn ratio (ZTR) in the absorber layer on the formation of secondary phases, leakage current pathways, and the thickness of the MoSe2 layer in Cu2ZnSnSe4 (CZTSe) solar cells. Experiment results revealed that under Zn-rich conditions (ZTR ≥1.11), the bottom of the absorber layer contains small quantities of Cu2-XSe and SnSe2 secondary phases. Under Sn-rich conditions (ZTR ≤0.75), large quantities of SnSe, SnSe2, and Cu2SnSe3 secondary phases appeared on the surface of the absorber layer. Conductive atomic force microscopy (C-AFM) revealed that the presence of secondary phases caused leakage current over large areas at the surface of the absorber layer. We determined that Cu2-XSe is involved in leakage current at CZTSe grain boundaries, whereas SnSe2 or Cu2SnSe3 tend to promote leakage current in the grains (or near grain boundaries). A higher ZTR resulted in a thinner MoSe2 layer due to large quantities of Cu and Na at the bottom of Zn-rich ZTR specimens, which promoted the formation of Cu2-xSe and Na2Se phases. These phases competed with the Mo to react with the Se, which decreased the amount of the MoSe2 formed from Se and Mo, resulting in a thinner MoSe2 layer. The highest conversion efficiency (5.2%) was obtained from a Zn-rich specimen (ZTR1.43). In contrast, the Sn-rich specimens (ZTR0.75 and ZTR0.63) presented conversion efficiency of 0%. The influence of ZTR on device performance can be attributed to widespread leakage current due to excessive quantities of SnSe2 and Cu2SnSe3 secondary phases on the surface of the absorber layer. An overly thick MoSe2 layer was also shown to result in excessive series resistance.

Experimental and computational phase boundary mapping of Co4Sn6Te6.

Binary Co4Sb12 skutterudite (also known as CoSb3) has been extensively studied; however, its mixed-anion counterparts remain largely unexplored in terms of their phase stability and thermoelectric properties. In the search for complex anionic analogs of the binary skutterudite, we begin by investigating the Co4Sb12-Co4Sn6Te6 pseudo-binary phase diagram. We observe no quaternary skutterudite phases and as such, focus our investigations on the ternary Co4Sn6Te6 via experimental phase boundary mapping, transport measurements, and first-principles calculations. Phase boundary mapping using traditional bulk syntheses reveals that the Co4Sn6Te6 exhibits electronic properties ranging from a degenerate p-type behavior to an intrinsic behavior. Under Sn-rich conditions, Hall measurements indicate degenerate p-type carrier concentrations and high hole mobility. The acceptor defect SnTe, and donor defects TeSn and Coi are the predominant defects and rationally correspond to regions of high Sn, Te, and Co, respectively. Consideration of the defect energetics indicates that p-type extrinsic doping is plausible; however, SnTe is likely a killer defect that limits n-type dopability. We find that the hole carrier concentration in Co4Sn6Te6 can be further optimized by extrinsic p-type doping under Sn-rich growth conditions.

Thermodynamic, electronic, and magnetic properties of intrinsic vacancy defects in antiperovskite Ca3SnO

The density functional theory based total energy calculations are performed to examine the effect of charge neutral and fully charged intrinsic vacancy defects on the thermodynamic, electronic, and magnetic properties of Ca3SnO antiperovskite. The chemical stability of Ca3SnO is evaluated with respect to binary compounds CaO, CaSn, and Ca2Sn, and the limits of atomic chemical potentials of Ca, Sn, and O atoms for stable synthesis of Ca3SnO are determined within the generalized gradient approximation parametrization scheme. The electronic properties of the pristine and the non-stoichiometric forms of this compound have been explored and the influence of isolated intrinsic vacancy defects (Ca, Sn, and O) on the structural, bonding, and electronic properties of non-stoichiometric Ca3SnO are analyzed. We also predict the possibility of achieving stable ferromagnetism in non-stoichiometric Ca3SnO by means of charge neutral tin vacancies. From the calculated total energies and the valid ranges of atomic chemical potentials, the formation energetics of intrinsic vacancy defects in Ca3SnO are evaluated for various growth conditions. Our results indicate that the fully charged calcium vacancies are thermodynamically stable under the permissible Sn-rich condition of stable synthesis of Ca3SnO, while tin and oxygen vacancies are found to be stable under the extreme Ca-rich condition.

A first-principles study on the origin of magnetism induced by intrinsic defects in monolayer SnS2

The effect of intrinsic defects on the structural, electronic and magnetic properties of monolayer SnS2 has been studied using density functional theory. Among the possible intrinsic defects, S vacancy is the most energetically favorable intrinsic defect under the Sn-rich condition, while S-on-Sn anti-site defect is most likely to form under the S-rich condition. Besides, S-on-Sn anti-site defect can realize p-type semiconductor behavior. Additionally, Sn vacancy, Sn-on-S anti-site and S adsorption on the top of S atom from the upper triple layer defects can induce magnetism, which originates mainly from the nearest-neighbor S atoms close to the Sn vacancy, Sn substitutional atom and S adatom, respectively. Moreover, the room temperature ferromagnetism is possible to be realized in monolayer SnS2 with S adsorption on the top of S atom from the upper triple layer defects. These predictions give a new path to explore monolayer SnS2-based magnetic nanomaterials.

Theoretical perspective on the electronic, magnetic and optical properties of Zn-doped monolayer SnS2

The electronic, magnetic and optical properties of Zn-doped monolayer SnS2 have been theoretically investigated with the density functional theory. Numerical results reveal that monolayer SnS2 can be easily synthesized by cleaving its bulk crystal. Besides, the Zn doping in monolayer SnS2 is energetically favored under the S-rich with respect to the Sn-rich condition. The doped system exhibits the magnetic ground states due to the formation of defect states above the Fermi level, which are introduced by the hybridization between S-3p states and a small amount of Sn-4d states. The room temperature ferromagnetism can also be realized in Zn-doped monolayer SnS2. The injection of Zn can enhance the absorption efficiency of solar spectrum, especially in the near-infrared light region. Moreover, the Zn doping can enhance the photocatalytic activity for both the oxygen and hydrogen evolution reactions in the monolayer SnS2.

First-Principles Study on Doping of SnSe2 Monolayers.

Doping is a vitally important technique that can be used to modulate the properties of two-dimensional materials. In this work, by using first-principles density functional calculations, we investigated the electrical properties of SnSe2 monolayers by p-type/n-type and isoelectronic doping. Substitution at Sn/Se sites was found to be easy if the monolayer was grown under Sn-/Se-poor conditions. Substitutions at Sn sites with metallic atoms (e.g. Ga, Ge, In, Bi, Sb, Pb) resulted in positive substitution energies, which indicated that they were not effective doping candidates. For substitutions at Se sites with nonmetallic atoms, no promising candidates were found for p-type doping (e.g., N, P, As). Among these, N and As showed positive substitution energies. Although P had a negative substitution energy under Sn-rich conditions, it introduced trap states within the band gap. For n-type doping (e.g., F, Cl, Br), all the calculated substitution energies were negative under both Sn- and Se-rich conditions. Br was proven to be a promising candidate, because the impurity introduced a shallow donor level. Finally, for isoelectronic doping (e.g., O, S, Te), the intrinsic semiconducting features of the SnSe2 monolayer did not change, and the contribution from the impurity to the states near the band edge increased with the atomic number.

Influence of Defects and Synthesis Conditions on the Photovoltaic Performance of Perovskite Semiconductor CsSnI3

CsSnI3 is a prototype inorganic halide perovskite that has recently been proposed as a strong candidate for photovoltaic applications because of its unique semiconductor properties. Through first-principle calculations, we show that the concentration control of intrinsic defects is critical for optimizing the photovoltaic properties of CsSnI3. Under a Sn-poor condition, a high concentration of acceptor defects, such as Sn or Cs vacancies, can form easily and produce a high p-type conductivity and deep-level defects that can become electron–hole recombination centers, all with high energy. This condition is optimal for growing CsSnI3 as hole-transport material in solar cells. In contrast, when Sn becomes richer, the concentration of acceptor defects decreases; therefore, the p-type conductivity may drop to a moderate level, which can increase the shunt resistance and, thus, the efficiency of the solar cells with CsSnI3 as the light absorber material (LAM). However, under the Sn-rich condition, the concentr...

Structural stability and topological surface states of the SnTe (111) surface

We perform first-principles calculations to study the stability and electronic structure of the (111) surface of SnTe, a representative topological crystalline insulator (TCI). We find three stable surface phases, which support two qualitatively different types of topological surface states: type I with four Dirac points at $\overline{\ensuremath{\Gamma}}$ and three $\overline{\mathrm{M}}$ points and type II with two Dirac points nearby but not at $\overline{\ensuremath{\Gamma}}$. Their appearance can be controlled by varying growth conditions. Under an Sn-poor condition, the Te-terminated surface without reconstruction is stable, resulting in the type-I surface states. While under an Sn-rich condition, the (2$\ifmmode\times\else\texttimes\fi{}$1)-reconstructed Sn-terminated surface becomes more stable. The reconstruction folds the surface Brillouin zone and effectively induces interactions between the Dirac points at the $\overline{\ensuremath{\Gamma}}$ and $\overline{\mathrm{M}}$ points. Surface states thus change from type I to type II accompanied by a Lifshitz transition. Under intermediate growth conditions, the ($\sqrt{3}\ifmmode\times\else\texttimes\fi{}\sqrt{3}$)-reconstructed Sn-terminated surface gets stabilized, which recovers the type-I surface states. Our work suggests a promising alternative way to control the topological surface states of TCIs besides selecting different surface orientations.

Cathodoluminescence Studies of the Inhomogeneities in Sn-doped Ga2O3 Nanowires

Cathodoluminescence real-color imaging and spectroscopy were employed to study the properties of Ga(2)O(3) nanowires grown with different Sn/Ga ratios. The structures grown under Sn-rich conditions show large spectral emission variation, ranging from blue to red, with a green transition zone. Spectral emission changes correlate with changes in the chemical composition and structure found by energy dispersive spectroscopy and electron diffraction. A sharp transition from green to red emission correlates with a phase transition of beta-Ga(2)O(3) to polycrystalline SnO(2). The origin of the green emission band is discussed based on ab initio calculation results.