Te-rich Conditions Research Articles

Vapor–liquid–solid synthesis of Ag2Te using chemical vapor deposition method

Silver telluride, Ag2Te, has been selectively synthesized by conventional chemical vapor deposition (CVD) via the vapor–liquid–solid growth mechanism. The pre-deposited Ag film on the SiO2/Si substrate was chemically reacted with vaporized Te atoms and transformed into liquid-phase Ag–Te during the CVD process. The appropriate supply of Te vapor to the Ag film influenced the stoichiometry of Ag–Te compounds, and the formation of stoichiometric Ag–Te compounds was well-explained by the phase diagram of the Ag–Te system. We found that Ag2Te was grown in the liquid of Ag–Te under the Te-poor condition, while Ag5Te3 and Te were simultaneously solidified under the Te-rich condition. The high-temperature synthesis of Ag2Te showed higher crystallinity and better stoichiometry than the low-temperature synthesis. This study demonstrates that Ag2Te can be selectively synthesized by conventional CVD via delicate control over the phases of the complex Ag–Te system.

Effects of electrode fabrication on electrical properties of CdMgTe room temperature radiation detectors

Cd1-xMgxTe (CdMgTe) is an ideal alternative material for room temperature radiation detectors. In this paper, the effects of different electrode structures on the performance of CdMgTe detectors were studied. The electrodes were prepared by vacuum evaporation and chemical precipitation method using Au and Al materials. The results indicated that CdMgTe detectors with Au/CdMgTe/Al electrode structure prepared by vacuum evaporation method had the best electrical properties. For the crystals grown by Cd-excess and Te-rich conditions, the resistivity were 2.89 × 1011 Ω cm and 3.8 × 1011 Ω cm, the leakage current (under 100 V bias voltage) were 4 nA and 3 nA, and the Hall mobility were 2.73 × 103 cm2 V−1 s−1 and 3.69 × 102 cm2 V−1 s−1, respectively. The conductive type of the crystals were weak n-type. 241Am @ 5.48 MeV α particle spectra of the detectors showed that the optimal energy resolution were 8.66% and 8.62%, and the carrier mobility-lifetime products (μτ)e were 1.47 × 10−3 cm2/V and 1.03 × 10−3 cm2/V, respectively. Moreover, the carrier mobility obtained by time-of-flight (TOF) test were 15.01 cm2/(V·s) and 77.79 cm2/(V·s), respectively.

An analysis of point defects in ZnTe using density functional theory calculations

Zinc Telluride, a wide bandgap semiconductor, is a promising candidate material for a number of optoelectronic applications. It is important to accurately characterize the effects of point defects on this material and how they may impact its performance. In this work, GGA + U density functional theory calculations were employed to study point defects in ZnTe. Vacancies, interstitials, and anti-site defects were all considered and Jahn-Teller distortions were reported. For each point defect five charge states were considered ranging from + 2 to − 2. It was found that VZn−2, Zni2+ and TeZn2+ were the most stable defects. The zinc vacancy acted as a double acceptor introducing shallow acceptor levels 0.05 eV away from the valence band maximum mediating p-type conduction. The equilibrium concentration of holes was found to be about four orders of magnitude bigger than the free electron concentration under Te-rich conditions. An analysis of the kinetics of the stable defects showed that VZn0 was the fastest diffusing defect with a migration energy of 0.97 eV. This study suggests that zinc vacancies and interstitials are the most relevant defects to account for in radiation damage studies on ZnTe and that devising strategies to limit the formation of such defects may be useful for extending the operational lifetime of the semiconductor device. This work also provides useful input parameters for higher scale defect evolution models to enable improved prediction of radiation damage which may ultimately inform the design for appropriate extenuation measures.

Structure and morphology of 2H-MoTe2 monolayer on GaAs(111)B grown by molecular-beam epitaxy

The structure and morphology of monolayer 2H-MoTe2 on GaAs(111)B grown by molecular-beam epitaxy have been studied using scanning tunneling microscopy, electron diffraction, and X-ray photoelectron spectroscopy. The MoTe2 film grown and annealed under the Te-rich condition is mainly composed of a pure 2H phase, while, under the Te-deficient conditions, the 2H-MoTe2 phase begins to evolve into nanowire-like structures owing to the desorption of Te. The 2H-MoTe2(0001) film on GaAs(111)B exhibits two types of energetically favorable epitaxial orientations; one is a perfect alignment of [11overline{2}0]MoTe2 // [1overline{1}0]GaAs, and the other shows a slight in-plane rotation of ± 0.77∘, which reduces the effective lattice mismatch between MoTe2 and GaAs.

Molecular beam epitaxy and polarized excitonic emission of layered GaTe/GaAs thin films

This paper reports on molecular beam epitaxy of GaTe thin films grown on GaAs(001) and GaAs(111)B substrates at temperatures of TS = 450–550 °C under weak Te-rich conditions (Te/Ga≈10–18) as well as studies of their structural and optical properties. The results obtained by transmission electron microscopy and X-ray diffraction techniques have established a correlation between the growth conditions and the GaTe polymorphic transition from hexagonal to monoclinic phase. The critical temperature of the polymorphic transition was found to be ∼540–550 °C. The monoclinic GaTe layers demonstrate an excellent excitonic emission that was confirmed by both micro-photoluminescence and time-resolved photoluminescence techniques. The strong anisotropy of the GaTe excitonic emission has been demonstrated.

Improvement of thermoelectric performance in Sb2Te3/Te composites

Te-impurity-incorporated ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$, i.e., ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}+x\phantom{\rule{0.16em}{0ex}}\mathrm{mol}\phantom{\rule{0.16em}{0ex}}%$ Te ($x=0$, 4, 6, and 9) composites were synthesized by solid-state reaction technique. Analysis of x-ray diffraction indicates not only Te impurity as a second phase but also doping of Te via suppression of inherent Te vacancies in the ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ matrix. As a result of this doping and of the change in formation energy of different types of native defects in ${\mathrm{Sb}}_{2}{\mathrm{Te}}_{3}$ due to synthesis in a Te-rich condition, carrier concentration $({n}_{\mathrm{H}})$ lower than the pristine sample was observed. Low ${n}_{\mathrm{H}}$ along with gradual convergence of valence bands due to progressive suppression of Te vacancies increases the Seebeck coefficient $(S)$ in Te-incorporated samples. Even though Te impurities increase electrical resistivity (\ensuremath{\rho}), enhanced texturing of lattice planes ensures that charge carrier mobility does not degrade due to Te addition. As a result, a maximum power factor $=17\phantom{\rule{0.16em}{0ex}}\ensuremath{\mu}\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$ at $T=480$ K for $x=6$ has been achieved. In addition, Te addition strengthens phonon scattering via an increase of phonon-phonon Umklapp scattering and point-defect-induced scattering of phonons. Due to such a strong phonon scattering, thermal conductivity $(\ensuremath{\kappa})$ decreases, and a reduced lattice thermal conductivity $({\ensuremath{\kappa}}_{\mathrm{L}})$, as low as $0.28\phantom{\rule{0.16em}{0ex}}\mathrm{W}\phantom{\rule{0.16em}{0ex}}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}1}$ at 500 K for $x=6$, has been achieved. As a result of simultaneous increase of $S$ and decrease of \ensuremath{\kappa}, a high $ZT\ensuremath{\sim}0.87$ at 480 K, almost 33% higher than that of the host material, has been achieved.

Tuning the magnetic properties of FeTe2 monolayer doped by (TM: V, Mn, and Co)

Based on density-functional calculations with Hubbard U correction for onsite Coulomb interactions, the structural, electronic, and magnetic properties of V, Mn, and Co-doped FeTe2 monolayers were investigated. Doping is more preferred in Fe-rich conditions than in Te-rich conditions, while Mn inclusion is the most thermodynamically stable in any environment, according to the formation energy. In all doped systems, the energy bandgap was widened, and the electron transport properties were improved. According to our predictions, the V, Mn, Co-doped FeTe2 monolayers are half metal in their ground states with enhanced magnetic moments. These V, Mn, and Co-doped FeTe2 monolayers with interesting electronic and magnetic properties can achieve novel spintronic functionalities.

Impact of growth conditions on the structural and magnetic properties of (Zn,Fe)Te thin films grown by molecular beam epitaxy (MBE)

We have investigated the effect of growth conditions on the structural and magnetic properties of (Zn,Fe)Te. Thin films of Zn1-xFexTe are grown by molecular beam epitaxy (MBE) with an almost same Fe composition, x = 1.4% but under two different growth conditions: (a) Te-rich (Te/Zn flux ratio of 2.3) and (b) Zn-rich (Te/Zn flux ratio of 0.7). Structural analysis by using x-ray absorption fine structure (XAFS) reveals that both films are mainly composed of pure diluted phase with substitutional Fe atoms on the Zn-site, but the valence state of Fe atoms in the film grown under Zn-rich condition has deviated more from isoelectronic Fe2+ as compared to the one grown in Te-rich condition. Accordingly, the magnetization measurement using SQUID confirms a significant difference in magnetic properties among these films; a linear dependence of magnetization on magnetic field (M−H), typical of van-Vleck paramagnetism under Te-rich growth condition turns into ferromagnetic behaviors with hysteretic M−H curve in the Zn-rich condition. The ferromagnetism exhibited by the Zn-rich growth film may reflect the deviation of substitutional Fe valence state from Fe2+ to Fe2+/3+ mixed states.

Molecular beam epitaxy growth of monolayer hexagonal MnTe2 on Si(111) substrate* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11604366, 11634007, 21872099, and 22072102) and the National Natural Science Foundation of Jiangsu Province, China (Grant No. BK 20160397). F. S. L. acknowledges support from the Youth Innovation Promotion Association of Chinese Academy of Sciences (Grant No.

Monolayer MnTe2 stabilized as 1T structure has been theoretically predicted to be a two-dimensional (2D) ferromagnetic metal and can be tuned via strain engineering. There is no naturally van der Waals (vdW) layered MnTe2 bulk, leaving mechanical exfoliation impossible to prepare monolayer MnTe2. Herein, by means of molecular beam epitaxy (MBE), we successfully prepared monolayer hexagonal MnTe2 on Si(111) under Te rich condition. Sharp reflection high-energy electron diffraction (RHEED) and low-energy electron diffraction (LEED) patterns suggest the monolayer is atomically flat without surface reconstruction. The valence state of Mn4+ and the atom ratio of ([Te]:[Mn]) further confirm the MnTe2 compound. Scanning tunneling spectroscopy (STS) shows the hexagonal MnTe2 monolayer is a semiconductor with a large bandgap of ∼ 2.78 eV. The valence-band maximum (VBM) locates at the Γ point, as illustrated by angle-resolved photoemission spectroscopy (ARPES), below which three hole-type bands with parabolic dispersion can be identified. The successful synthesis of monolayer MnTe2 film provides a new platform to investigate the 2D magnetism.

Stresses caused by Cd and Te inclusions in CdMnTe crystals and their impact on charge carrier transport

We present a defect analysis in Cd and Te inclusion regions within Bridgman-grown Cd0.95Mn0.05Te crystals for nuclear detectors. Crystal growth was performed under Cd- or Te-rich conditions so that Cd and Te inclusions could be investigated, respectively. Cathodoluminescence (CL) maps confirm that a Te inclusion does not generate defects in its vicinity, in contrary to a Cd inclusion, which induces microcracks in its neighborhood in the CdMnTe matrix. This originates from the higher thermal expansion coefficient ratio of Cd to the CdTe than Te to CdTe. Next, the relative stress analysis was carried out using the electron backscatter diffraction (EBSD) technique, which shows that a Te inclusion causes higher stresses in its environment than a Cd inclusion does. The reason for this is that the Cd inclusion releases the stresses due to a creation of the tensile cracks. Despite the fact that a Te inclusion stresses the crystal lattice more, it can have a smaller impact on charge carrier transport in nuclear detectors than a Cd inclusion. This effect we confirm by μτ product measurements, which determine charge carrier transport properties.

Growth and characterization of room temperature radiation detection material Cd0.95Mg0.05Te

Recently, researchers were interesting in exploring alternative materials for well-studied CdZnTe which was the key material of radiation detection. Cadmium Magnesium Telluride (Cd1-xMgxTe) crystal was regarded as promising one for room temperature X-ray and gamma-ray detectors due to some potential advantages. In this paper, two large-size Cd0.95Mg0.05Te ingots with 30 mm diameter and over 100 mm length were successfully grown by a modified vertical Bridgman method. Two modifications, including Cd excess or Te-rich condition and the accelerated crucible rotation technique (ACRT), were applied in growing process to decrease defects, enhance resistivity and homogenize component distribution. The results indicated that both ingots had a cubic zinc-blende structure. The distribution of Mg element in the ingots along axial direction was very homogeneous. The energy band gap of Cd0.95Mg0.05Te was close to Cd0.9Zn0.1Te. Low density of Te inclusions (103 cm−3 order of magnitudes) and high IR transmittance (over 60%) showed the good crystal quality. Raman scattering spectroscopy also showed a good crystal quality for Cd0.95Mg0.05Te crystals. Thus, Cd0.95Mg0.05Te crystals could be a promising radiation detector material.

Enhanced thermoelectric properties of n-type Bi2Te2.7Se0.3 for power generation

The abundant waste heat (below 500 K) from industry and automobile exhaust stimulate the development of thermoelectric materials for power generation. The commercial Bi2Te3-based alloys are generally used near room temperature. Herein, n-type Bi2Te2.7+xSe0.3 was prepared by a method of mechanical alloying combined with hot pressing, which is more efficient, time-saving, and energy-saving than the method of melting and spark plasma sintering. The n-type Bi2Te2.7Se0.3 being suitable for power generation (below 500 K) is obtained by manipulating the intrinsic point defects to enhance carrier concentration and suppress the intrinsic excitation at elevated temperature using a tiny amount of excess Te. Enhanced carrier concentration improves electrical conductivity, while enhanced phonon scattering reduces lattice thermal conductivity, originating from increased anti-site defects Te.Bi in Te-rich condition. Consequently, the enhanced ZT peak gradually shifts to higher temperature while increasing the excess Te content. The maximum ZT peak of 0.9 at 373 K and an average ZT of 0.82 between 300 and 498 K are obtained in Bi2Te2.71Se0.3, indicating potential for thermoelectric power generation (below 500 K).

Effects of vacancy defects on electronic properties of 2D group-IV Tellurides (XTe, X = Si, Ge, Sn and Pb)

Abstract Vacancy defects are almost inevitable in the synthesis process and have important effects on the physical properties of two-dimension (2D) materials. Using first-principles calculations based on the density functional theory (DFT), we investigated the effects of vacancy defects (X-vacancy and Te-vacancy) on the geometry structures, mechanical and electronic properties of 2D group-IV Tellurides (XTe, X = Si, Ge, Sn and Pb). Results demonstrate that the X-vacancy is prone to form than Te-vacancy under Te-rich growth condition. The X-vacancy and Te-vacancy defects can be avoided under X-rich and Te-rich growth conditions, respectively. Moreover, vacancy defects have weak effects on the crystal structures and mechanical properties of XTe monolayers. It should also be noticed that the XTe monolayers with X-vacancy show p-type doping characteristics due to extra holes. However, the Te-vacancy XTe monolayers maintain indirect bandgap semiconductors except for SiTe monolayer. Particularly, the SiTe monolayer can be converted from semiconductor to semimetal by Te-vacancy. Furthermore, the uniaxial strain can also effectively regulate the electronic properties of perfect and defective XTe monolayers. These theoretical results provide valuable insights into identification of vacancy defects in the fabrication process and applications of 2D Tellurides.

Effect of Heating Rate on Bulk Density and Microstructure in Bi2Te2.7Se0.3 Sintering

Bi-Te is a thermoelectric material used at room temperature. It is typically fabricated as a single-crystal ingot, making processing difficult and material loss very high. Pressureless sintering processes have been studied to address these issues; however, they typically result in lower-density material. Methods such as hot isostatic pressing and spark plasma sintering have mainly been studied. However, there has been limited research on pressureless sintering without additional steps such as pressurization or charging. The aim of this study is to obtain a dense Bi2Te2.7Se0.3 sintered body by combining rapid sintering with air sintering by modifying a typical sintering furnace. The effect of heating rate on densification and phase change was also investigated. Bi2Te2.7Se0.3 powder was first synthesized by the mechanochemical method. The synthesized powder was then heated at a rate of 5°C/s to 20°C/s and finally sintered at a temperature of 570°C for 10 min in H2/N2 mixture under Te-rich conditions. These conditions produced a sintered body with relative density of at least 90% of the theoretical density, and a bulk density of 6.98 g/cm3 was obtained when temperature was increased at a rate of 20°C/s. Observation of the microstructure showed that porosity decreased as the heating rate was increased. The results of this study confirm that the heating rate during rapid sintering can affect the density of the sintered body.

Self-compensation in chlorine-doped CdTe

Defect energetics, charge transition levels, and electronic band structures of several Cl-related complexes in CdTe are studied using density-functional theory calculations. We investigate substitutional chlorine (ClTe and ClCd) and complexes formed by ClTe with the cadmium vacancy (ClTe-VCd and 2ClTe-VCd) and the TeCd antisite (ClTe-TeCd). Our calculations show that none of the complexes studied induce deep levels in the CdTe band gap. Moreover, we find that ClTe-VCd and ClTe are the most stable Cl-related centers in n-type and p-type CdTe, under Te-rich growth conditions, showing shallow donor and acceptor properties, respectively. This result suggests that the experimentally-observed Fermi level pinning near midgap would be originated in self-compensation. We also find that the formation of the ClTe-TeCd complex passivates the deep level associated to the Te antisite in neutral charge state.

Molecular beam epitaxy of superconducting PdTe2 films on topological insulator Bi2Te3

Majorana fermions have been observed in topological insulator/s-wave superconductor heterostructures. To manipulate Majorana fermions, superconducting materials should be deposited on the surfaces of topological insulators. In this study, high-quality superconducting PdTe2 films are deposited on the topological insulator Bi2Te3 surface using molecular beam epitaxy. The surface topography and electronic properties of PdTe2/Bi2Te3 heterostructures are investigated via in situ scanning tunneling microscopy/spectroscopy. Under Te-rich conditions, the Pd atoms presumably form PdTe2 film on Bi2Te3 surface rather than diffuse into Bi2Te3. The superconductivity of the PdTe2/Bi2Te3 heterostructure is detected at a transition temperature of ~1.4 K using the two-coil mutual inductance technique. This study proposes a method for fabricating superconducting materials on topological insulator surfaces at low doping levels, paving ways for designing nanodevices that can manipulate Majorana fermions.

Gd doping effect in p-type Bi2Te3 single crystals

It is generally accepted that Bi2Te3 prepared from stoichiometric melts has p-type charge carriers generated from BiTe-type antisite defects, while Bi2Te3 grown under Te-rich condition becomes n-type due to another type of TeBi antisite defects. We report the magnetic and transport properties of GdxBi2-xTe3 prepared from stoichiometric melts, where p-type charge carriers are dominant. The physical properties of all the samples have no significant changes with varying the nominal Gd composition up to x = 0.2. Compared with n-type GdxBi2-xTe3 samples grown under Te-rich condition, we find low solubility for all the samples and no clear signature of antiferromagnetic order. These results suggest that the Gd doping rate in GdxBi2-xTe3 is governed by the type of antisite defects and charge carriers, so that the antiferromagnetic ordering is not eventually introduced.

Enhanced electrical transport properties via Pb vacancies in single crystalline PbTe prepared by Te-flux method

In PbTe system, Pb vacancies (VPb) are preferentially formed compared with other intrinsic point defects (anti-sites TePb and interstitials Tei) under the Te-rich condition. In order to explore the effect of VPb on electrical transport properties of PbTe thermoelectric material, the electronic structures of PbTe with VPb are determined by the first principle calculations. Theoretical calculations reveal that the VPb can modify the band structure near the Fermi level, which can increase the number of carrier pockets thus increase the carrier concentration. Based on the calculated results, single crystalline PbTe samples with VPb have been prepared by Te-flux method according to the stoichiometric ratio of PbTe1+x (x = 2.0, 2.5, 3.0 and 3.5). The experimental results show that carrier concentration is optimized significantly by tuning Te-flux content, and it varies from 2.94 × 1018 to 8.01 × 1018 cm−3. The highest electrical conductivity reaches 3.1 × 104 S m−1 at 300 K for x = 3.5. A large Seebeck coefficient of 562 μV/K is obtained at 480 K for x = 2.0. Due to the synergistic effect of the Seebeck coefficient and the electrical conductivity, the highest power factor of 3.17 × 10−3 W m−1 K−2 is achieved at 450 K for x = 3.0.

Bonding mechanisms and electronic properties of HgIn2Te4 with Au doping: First-principles study

HgIn2Te4 (Mercury indium telluride, MIT) is a promising mid-infrared CO2 laser candidate material. Through combining the calculation results of structural relaxation, Bader charge, electronic localization function, formation energy, and density of states, we systematically explored the stability and doping efficiency of gold (Au) in MIT. The results show that the Au-Te bond has a similar polar covalent characteristic as Hg-Te bond, which indicates the relative stability of Au dopant in MIT. In addition, two defect states were formed through the hybridization between the Au impurity and its nearest neighboring Te atoms in the substitutional doping systems of AuHg and AuIn. We found that the acceptor transition levels in AuHg and AuIn are 0.095 eV and 0.265 eV above the valance band maximum, respectively, whereas the donor transition level in the substitutional doping system of AuTe and interstitial system of AuI are 0.894 eV and 0.322 eV below the conduction band minimum, respectively. Meanwhile, in the Hg-rich condition, the Fermi level would be pinned about 0.511 eV above the valence band maximum due to the self-compensation effect; however, this effect will be efficiently reduced in the Te-rich condition.

Enhancement of Thermoelectric Properties in n-Type Cu0.01Bi2Te2.3+xSe0.7 (0 ≤ x ≤ 0.7) Compounds with Te-Excess

The fine control of antisite defects for Bi2Te3-based materials is necessary to improve their thermoelectric performance using the optimization of a carrier concentration. In this work, we attempted to tune the n-type carrier concentration by forming antisite TeBi defects under a Te-rich condition for Cu0.01Bi2Te2.3+xSe0.7 samples (0 ≤ x ≤ 0.7). The electrical resistivity decreases with increasing the amount of excess Te in the sample of Cu0.01Bi2Te2.3+xSe0.7, which is originated from the increase in the electron carrier concentration for the Te-excess samples. The highest power factor of 2.72 mW/m K2 is obtained at 323 K for Cu0.01Bi2Te2.4Se0.7, which is enhanced by ~ 20% compared to the x = 0 sample. The highest ZT of 0.92 is achieved at 473 K for Cu0.01Bi2Te2.4Se0.7, which is 11% higher than that of x = 0 sample (ZT = 0.83). We demonstrate that the optimization of n-type carrier concentration by forming antisite TeBi defects in n-type Bi2Te3-based materials should be effective for enhancing their thermoelectric performance.