Se Single Crystals Research Articles

Crystal Growth, Thermal Treatment, and Characterization of Nonlinear Optical Crystal LiGa0.5In0.5Se2 for Mid-infrared Applications.

LiGa0.5In0.5Se2 is a new quaternary nonlinear optical crystal for the mid-IR application grown as a mixed crystal of the LiGaSe2-LiInSe2 solid-solution system. It is transparent in the 0.47-14 μm range and has an appropriate bandgap and a lower melting point than LiGaSe2 and LiInSe2. It is more technological about the growth process since its homogeneity range is broader in the phase diagram. In this work, we have synthesized the LiGa0.5In0.5Se2 polycrystal by the two-zone temperature method. LiGa0.5In0.5Se2 single crystals (Φ26 mm × 50 mm) were grown through the modified Bridgman method with the c-axis seed crystal which has the smallest thermal expansion coefficient of the three main axes in 293-773 K. The crystal structure was studied by X-ray diffraction and the Rietveld refinement method. Due to the low transmittance of the as-grown crystals, a systematic thermal treatment experiment was carried out. In the annealing experiment, the crystal surface is seriously enriched with selenium due to the thermal diffusion of selenium, resulting in the crystal opacity and cracking, while after vacuum quenching at 873 K, the transmittance of the LiGa0.5In0.5Se2 crystal wafer was greatly improved, the bandgap shows a large increase from 2.13 to 2.51 eV, and the quenched crystal shows strong SHG response (×1.91 LiGaSe2). The chemical states and vibration modes of surface elements for both conditions were characterized by X-ray photoelectron and Raman spectra. Density functional theory calculations were carried out to simulate the phonon spectrum and phonon density of states, which can help to study the phonon vibration modes in the lattice.

Salt-Assisted Low-Temperature Growth of 2D Bi2 O2 Se with Controlled Thickness for Electronics.

Bi2 O2 Se is the most promising 2D material due to its semiconducting feature and high mobility, making it propitious channel material for high-performance electronics that demands highly crystalline Bi2 O2 Se at low-growth temperature. Here, a low-temperature salt-assisted chemical vapor deposition approach for growing single-domain Bi2 O2 Se on a millimeter scale with thicknesses of multilayer to monolayer is presented. Because of the advantage of thickness-dependent growth, systematical scrutiny of layer-dependent Raman spectroscopy of Bi2 O2 Se from monolayer to bulk is investigated, revealing a redshift of the A1g mode at 162.4cm-1 . Moreover, the long-term environmental stability of ≈2.4nm thick Bi2 O2 Se is confirmed after exposing the sample for 1.5 years to air. The backgated field effect transistor (FET) based on a few-layered Bi2 O2 Se flake represents decent carrier mobility (≈287cm2 V-1 s-1 ) and an ON/OFF ratio of up to 107 . This report indicates a technique to grow large-domain thickness controlled Bi2 O2 Se single crystals for electronics.

High-temperature solution growth of large size chalcogenide FeTxSe (T: Fe, Co) superconducting single crystals

We report the growth of high quality Fe excess and Co doped, Fe T x Se single crystals in nominal ratio via slow cooling method of high temperature solution growth technique and, their structural and physical properties through the X-ray diffraction, Raman spectroscopy, magnetic and transports measurements. Selective area electron diffraction (SAED) patterns and X-ray diffraction of cleavage piece confirm the growth of single crystals in ( h0l ) orientations. Observations of phonon vibration modes (A 1g and B 1g ) in Raman spectroscopy measurements mark the qualitative analysis of these single crystals. Low temperature magnetic and electrical transport studies manifest the superconducting nature of both single crystals.

Tailoring of Bandgap and Spin-Orbit Splitting in ZrSe2 with Low Substitution of Ti for Zr

Tuning the bandgap in layered transition metal dichalcogenides (TMDCs) is crucial for their versatile applications in many fields. The ternary formation is a viable method to tune the bandgap as well as other intrinsic properties of TMDCs, because the multi-elemental characteristics provide additional tunability at the atomic level and advantageously alter the physical properties of TMDCs. Herein, ternary Ti x Zr1 – x Se2 single crystals were synthesized using the chemical-vapor-transport method. The changes in electronic structures of ZrSe2 induced by Ti substitution were revealed using angle-resolved photoemission spectroscopy. Our data show that at a low level of Ti substitution, the bandgap of Ti x Zr1 – x Se2 decreases monotonically, and the electronic system undergoes a transition from a semiconducting to a metallic state without a significant variation of dispersions of valence bands. Meanwhile, the size of spin-orbit splitting dominated by Se 4p orbitals decreases with the increase of Ti doping. Our work shows a convenient way to alter the bandgap and spin-orbit coupling in TMDCs at the low level of substitution of transition metals.

How much gallium do we need for a p-type Cu(In,Ga)Se2?

Doping in the chalcopyrite Cu(In,Ga)Se2 is determined by intrinsic point defects. In the ternary CuInSe2, both N-type conductivity and P-type conductivity can be obtained depending on the growth conditions and stoichiometry: N-type is obtained when grown Cu-poor, Se-poor, and alkali-free. CuGaSe2, on the other hand, is found to be always a P-type semiconductor that seems to resist all kinds of N-type doping, no matter whether it comes from native defects or extrinsic impurities. In this work, we study the N-to-P transition in Cu-poor Cu(In,Ga)Se2 single crystals in dependence of the gallium content. Our results show that Cu(In,Ga)Se2 can still be grown as an N-type semiconductor until the gallium content reaches the critical concentration of 15%–19%, where the N-to-P transition occurs. Furthermore, trends in the Seebeck coefficient and activation energies extracted from temperature-dependent conductivity measurements demonstrate that the carrier concentration drops by around two orders of magnitude near the transition concentration. Our proposed model explains the N-to-P transition based on the differences in formation energies of donor and acceptor defects caused by the addition of gallium.

Piezomodulation of electrical conductivity in TlIn0.9Ga0.1Se2 single crystals

In the present work, the piezoelectric properties of TlIn[Formula: see text]Ga[Formula: see text]Se2 single crystals are investigated. Single crystals were grown by the Bridgman–Stockbarger method. The results show that when periodic mechanical vibrations are excited in these crystals, the electrical conductivity is modulated at the same frequency. Measurements of the amplitude values of the piezoconductivity at the same degree of deformation in the dark and under different illumination revealed the presence of a linear relationship between them. The level of the piezosignal also changes linearly depending on the strength of the magnetic field. Thus, TlIn[Formula: see text]Ga[Formula: see text]Se2 crystals have a potential prospect of application in the field of highly sensitive magnetic and light sensors.

Bulk superconductivity in one-step grown Fe(Te,Se) crystals free of interstitial iron by minor Mn doping

The iron-based nontoxic chalcogenide superconductor Fe(Te,Se) has great potential for high magnetic field applications while it lacks a reliable method to produce bulk superconductor so far. Here we report a one-step synthesis method to grow high-quality Fe(Te,Se) single crystals free of interstitial iron atoms through minor Mn doping. Bulk superconductivity is revealed in the as-grown centimeter-sized crystals with the optimal doping level of 1% Fe atoms substituted by Mn, which is systematically demonstrated by sharp electrical resistivity and magnetic susceptibility transitions, and large specific heat jumps. Compared with the un-doped sample, the optimally doped one shows a significantly enhanced upper critical field, and a large self-field critical current density Jc of 4.5 × 105 A cm−2 at 2 K (calculated by the Bean model), which maintains large values under high fields. The absence of interstitial iron atoms is testified by the scanning tunneling microscopy, and the effect of Mn doping is discussed. Our results provide a practical method by minor Mn doping to directly synthesize high-performance Fe(Te,Se) bulks that allow for future high-field superconducting applications.

Charge-density-wave melted superconductivity in 1T-TiSe2

Understanding the origin of superconductivity and the interplay between superconductivity and charge-density wave (CDW) in transition-metal dichalcogenides (TMDs) is one of fundamental interests in scientific research as well as in applications. In this work, we have explored the inherent physical mechanism for the superconducting transition in Ta-doped 1T-TiTa x Se2 single crystals by combining density functional theory (DFT) calculations and experiment approaches. The theoretical calculation results reveal that with Ta doping, the CDW is suppressed by the raising of chemical potential, which is further confirmed by x-ray absorption near-edge structure (XANES) and low-temperature Raman spectra. And then, the superconductivity is motivated by the enhancement of density of states near the Fermi level. Our present results shed new light on understanding the emergence of superconductivity within the CDW phase, as well as the potential applications in electronic devices.

Studying the heterogeneity of the CrxTi1-xCh2 (Ch = S, Se) single crystals using X-ray scanning photoemission microscopy

The morphology of the heterogeneous CrxTi1-xSe2 and CrxTi1-xS2 single crystals has been studied using X-ray scanning photoemission microscopy (SPEM) and angular resolved photoemission spectroscopy (ARPES). A direct method of SPEM provided us the insight into the origin of the blurred ARPES images for Cr0.78Ti0.36Se2 single crystal. Using SPEM, we confirmed the formation of the CrSe2-based structural fragments inside the CrxTi1-xSe2 single crystals with x ≥ 0.75. The chemical composition of the forming structural fragments depends on the chalcogen (S, Se) forming the crystal lattice.

Effect of thallium (Tl) substitution for indium (In) on ellipsometric characteristics of TlInSe2 single crystals

TlMeSe2 (Me: Tl, In) semiconducting compounds exhibiting chain structure have been attractive ternary materials in various technological devices. In the TlMeSe2 structure, Tl1+ is monovalent while Me3+ is trivalent ions. The present paper reports the results of spectroscopic ellipsometry measurements performed on Tl1+(Tl0.2In0.8)3+Se2 (abbreviated as Tl1.2In0.8Se2) single crystals which were grown by substituting thallium for indium. The measurements were performed for orientations of E//c and E丄c (E: electric field and c: optical axis). The analyses of ellipsometry data considering air-sample optical model presented the spectral dependencies of dielectric function, refractive index and extinction coefficient in the 1.2–5.0 eV range. Critical point energies of studied single crystal were obtained by fitting second-energy derivative spectra of dielectric function. The determined energies were compared with those of TlInSe2 to understand the effect of thallium-indium substitution in the compound. The crystal structure and atomic compositions of the constituent elements were also reported throughout the paper.

Huge permittivity and premature metallicity in Bi2O2Se single crystals

Bi$_2$O$_2$Se is a promising material for next-generation semiconducting electronics. It exhibits premature metallicity on the introduction of a tiny amount of electrons, the physics behind which remains elusive. Here we report on transport and dielectric measurements in Bi$_2$O$_2$Se single crystals at various carrier densities. The temperature-dependent resistivity ($\rho$) indicates a smooth evolution from the semiconducting to the metallic state. The critical concentration for the metal-insulator transition (MIT) to occur is extraordinarily low ($n_\textrm{c}\sim10^{16}$ cm$^{-3}$). The relative permittivity of the insulating sample is huge ($\epsilon_\textrm{r}\approx155(10)$) and varies slowly with temperature. Combined with the light effective mass, a long effective Bohr radius ($a_\textrm{B}^*\approx36(2)$ $\textrm{nm}$) is derived, which provides a reasonable interpretation of the metallic prematurity according to Mott's criterion for MITs. The high electron mobility ($\mu$) at low temperatures may result from the screening of ionized scattering centers due to the huge $\epsilon_\textrm{r}$. Our findings shed light on the electron dynamics in two dimensional (2D) Bi$_2$O$_2$Se devices.

Resistance fluctuations in superconducting K x Fe2 – y Se2 single crystals studied by low-frequency noise spectroscopy* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11774303 and 11574373) and Joint Fund of Yunnan Provincial Science and Technology Department, China (Grant No. 2019FY003008).

Low-frequency resistance noise spectroscopy is applied to investigate bulk single crystals of the intercalated iron-selenide K x Fe2 – y Se2 superconductors with different iron vacancy orders. Based on a generalized fluctuation model, the well-observed resistance hump above 100 K is interpreted as an insulator–metal phase transition with a characteristic transition energy of 0.1–0.6 eV, indicating a highly inhomogeneous energy distribution configuration. In the superconducting transition regime, we find that the normalized resistance noise scales with resistance R excellently as SR /R 2 ∝ R l rs with the noise exponent l rs ≈ 1.4. With reduced iron vacancy disordering in enhanced superconductivity K x Fe2 – y Se2 crystals, the level of resistance fluctuations is greatly suppressed, suggesting a geometrical phase transition for conduction channel, which is directly related to the microstructure of the crystals.

The Effect of Potassium Fluoride Postdeposition Treatments on the Optoelectronic Properties of Cu(In,Ga)Se2 Single Crystals

The power conversion efficiency boost of Cu(In,Ga)Se2 in the past years has been possible due to the incorporation of heavy alkali atoms. Their addition through postdeposition treatments results in an improvement of the open‐circuit voltage, the origin of which has been associated with grain boundaries. Herein, the effect of potassium fluoride postdeposition treatments on the optoelectronic properties of a series of sodium‐free Cu(In,Ga)Se2 single crystals with varying Cu and Ga content is discussed. Results suggest that improvement of the quasi‐Fermi level splitting can be achieved in the absence of grain boundaries, being greater in low‐gallium Cu‐poor absorbers. Secondary ion mass spectrometry reveals the presence of potassium inside the bulk of the films, suggesting that transport of potassium can occur through grain interiors. In addition, a type inversion from n to p in potassium fluoride‐treated low‐gallium Cu(In,Ga)Se2 is observed, which along with study of the carrier lifetime demonstrates that potassium can act as a dopant. The fact that potassium on its own can alter the optoelectronic properties of Cu(In,Ga)Se2 single crystals demonstrates that the effect of postdeposition treatments goes beyond grain boundary passivation.

Non-hydrostatic pressure-dependent structural and transport properties of BiCuSeO and BiCuSO single crystals

High-pressure experiments usually expect a hydrostatic condition, in which the physical properties of materials can be easily understood by theoretical simulations. Unfortunately, non-hydrostatic effect is inevitable in experiments due to the solidification of the pressure transmitting media under high pressure. Resultantly, non-hydrostaticity affects the accuracy of the experimental data and sometimes even leads to false phenomena. Since the non-hydrostatic effect is extrinsic, it is quite hard to analyze quantitatively. Here, we have conducted high pressure experiments on the layered BiCuXO (X = S and Se) single crystals and quantitatively analyzed their pronounced non-hydrostatic effect by high throughput first-principles calculations and experimental Raman spectra. Our experiments find that the BiCuXO single crystals sustain the tetragonal structure up to 55 GPa (maximum pressure in our experiment). However, their pressure-dependent Raman shift and electric resistance show anomalous behaviors. Through optimization of thousands of crystal structures in the high throughput first-principles calculations, we have obtained the evolution of the lattice constants under external pressures, which clearly substantiates the non-hydrostatical pressure exerted in BiCuXO crystals. Our work indicates that the high throughput first-principles calculations could be a handy method to investigate the non-hydrostatic effect on the structural and electronic properties of materials in high pressure experiments.

The investigation of the acousto-photovoltaic effect in single crystals TlIn1−xGaxSe2

In this work, TlIn[Formula: see text]Ga[Formula: see text]Se2 single crystals were grown by zone recrystallization methods and their X-ray phase analysis was performed. The lattice parameters were determined and it was found that crystals based on the original TlIn[Formula: see text]Ga[Formula: see text]Se2 compound crystallized in the same tetragonal symmetry, the space group [Formula: see text]. It was revealed that with partial substitution of indium atoms by gallium atoms in the TlIn[Formula: see text]Ga[Formula: see text]Se2 lattice, the parameters of unit cells change according to the additivity law. First time it was found that with the simultaneous influence of electromagnetic and sound waves, an electromotive force (e.m.f.) occurs or short circuit current. The studies were carried out in the frequency range of electromagnetic radiation of 50–143 kHz. It was revealed that when the investigated crystals were irradiated with magnetic radiation in the absence of sound waves, there was no e.m.f. on the electrodes as well as memory when exposed only to sound. Based on the existing theoretical information, the nature of the revealed single crystal of the acousto-photovoltaic effect was explained.

Growth and Characterization of CuInGaSe2 Single Crystal

The chalcopyrite Cu(In,Ga)Se2 (CIGS) alloy system has received strong interest in recent years because of its potential for the development of high efficiency and low cost solar cells. The bandgap energy can be varied between 0.9 and 1.68 eV by composition ratio of In and Ga and the absorption coefficient is very high of abobe 105 cm-1. In our previous work1, I-III-VI2 chalcopyrite compound Cu(In0.2, Ga0.8)Se2 (CIGS) single crystal was successfully grown by traveling heater method (THM). The powder X-ray diffraction (XRD) pattern showed preferred orientations of (112), (220) and (312) planes, confirming the chalcopyrite structure. In Raman spectra, the A1 mode peaks expected for CIGS were observed, and no secondary phases were observed. The full-width at half-maximum (FWHM) of the X-ray rocking curve (XRC) for the (112) oriented CIGS single crystal is 103 arcsec. The good-quality single-phase CIGS single crystal can be obtained1). In this work, Ga-rich composition ratio CIGS single crystals could be also successfully grown by THM. At the ratio of Ga / (Ga + In) > 0.3, the CIGS changed from n-type conduction to p-type conduction. It was assumed that Se vacancies are reduced with increasing Ga ratio from the results of Energy dispersive X-ray spectroscopy. 1)Nagaoka, et al., Renewable Energy 2015, 79, 127-130.

Structural and electrical transport properties of Cu-doped Fe1–x Cu x Se single crystals* *Project supported by the National Key Research and Development of China (Grant No. 2018YFA0704200), the National Natural Science Foundation of China (Grant No. 11834016), and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB25000000).

We report the structural and electrical transport properties of Fe1–x Cu x Se (x = 0, 0.02, 0.05, 0.10) single crystals grown by a chemical vapor transport method. Substituting Cu for Fe suppresses both the nematicity and superconductivity of FeSe single crystal, and provokes a metal–insulator transition. Our Hall measurements show that the Cu substitution also changes an electron dominance at low temperature of un-doped FeSe to a hole dominance of Cu-doped Fe1–x Cu x Se at x = 0.02 and 0.1, and reduces the sign-change temperature (TR ) of the Hall coefficient (R H).

Microwave impedance study of superconducting (Li1-xFex)OHFeSe single crystal

Microwave (MW) surface impedance Zs =Rs +iXs of the recently discovered high-Tc iron-based superconductor (Li0.84Fe0.16)OHFe0.98Se single crystal was measured for the first time. The special X-band “hot finger" resonator allowed carrying out measurements when temperature of the sample under test can be varied from 2K to 90K and in the same time temperature of the resonator remained constant (4.2K). The unusual and important inequality Rs<Xs at T≥Tc =42K was found. The possible reason of the observed feature is considered. The temperature dependence of the quasiparticle σ1 and superfluid σ2 conductivities and structure of energy gap are analyzed. Experiments show the temperature dependence of the field penetration depth as Tn with n=2.8 at T<Tc /3. The temperature dependence of the superfluid density confirms fully gapped behavior of a superconductor with a complex gap.

Emergent vortex Majorana zero mode in iron-based superconductors

During the recent years, the iron-based superconductors with a topological band structure have attracted intensive attention from the science community as a new and promising platform for emerging Majorana zero modes in their vortex core. These topological iron-based superconductors possess all of the desirable properties, &lt;i&gt;i.e.&lt;/i&gt; single material, high-&lt;i&gt;T&lt;/i&gt;&lt;sub&gt;c&lt;/sub&gt; superconductivity, strong electron-electron correlation and topological band structure, thus successfully avoiding the difficulties suffered by previous Majorana platforms, such as intrinsic topological superconductors and multiple types of proximitized heterostructures. So far, one has observed pristine vortex Majorana zero modes in several different compounds of iron-based superconductors. The systematic studies performed on those systems show that the vortex Majorana zero modes are quite evident experimentally and very clear theoretically, leading to a bright future in applications. The vortex cores of iron-based superconductors can become one of the major candidates for exploring topological quantum computing in the future. In this review article, we will focus on Fe(Te, Se) single crystal, to introduce the original ideas and research progress of the new emerging “iron home” for Majorana zero modes. Having elabrated the basic band structures and the experimental facts of the observed vortex zero modes in Fe(Te, Se), we will systematically summarize the main observations and fundamental physics of vortex Majorana zero modes in Fe(Te, Se). First of all, with the help of the observed behavior of Majorana wavefunction and quasiparticle poisioning, we will analyze the emerging mechanism of vortex Majorana zero modes in Fe(Te, Se). Then we will elaborate the measurements on Majorana symmetry and topological nature of vortex Majorana zero modes, assisted by several existing Majorana theories. After that, we will switch our view angle from quantum physics to quantum engineering, and comprehensively analyze the fate of vortex Majorana zero modes in a real material under a real environment, which may benefit the potential engineering applications in the future. This review article follows the physical properties of vortex Majorana zero modes, and emphasizes the link between theories and experiments. Our goal is to bridge the gap between the classical Majorana theories and the new emerging Majorana platform in iron-based superconductors, and help the readers to understand the experimental observations of the newly discovered “iron home” for Majoranas.

Concentration dependences of electrical conductivity and the Hall effect of the CexSn1–xSe single crystals

The concentration dependences of the Hall coefficient, electrical conductivity, and charge carrier mobility in CexSn1–xSe (х ≤ 0.02) are studied over a temperature range of 77–400 K. In the considered samples, impurities are compensated with intrinsic defects. As the cerium concentration increases (x = 0.01; 0.015), the concentration of donor impurities grows, and a change in the type of conductivity is observed.