Vertical Bridgeman Method Research Articles

Investigation of group 13 elements as potential candidates for p-type dopants in the narrow-gap thermoelectric semiconductor α-SrSi2

AbstractTo investigate the possibility of p-type doping of α-SrSi2, a promising as an eco-friendly thermoelectric material, the energy changes of substitutions of the Si site of α-SrSi2 by group 13 elements were evaluated using first-principles calculations. It is found that Ga doping was the most energetically favorable dopant while In is the most unfavorable. We examined the synthesis of Ga- and In-doped α-SrSi2 using the vertical Bridgeman method and investigated their thermoelectric properties. The Ga atoms were doped to α-SrSi2 successfully up to 1.0 at. %, while In atoms could not be doped as suggested by calculations. For experimental prepared Ga-doped samples, the carrier density was observed to increase with Ga doping, from 3.58 × 1019 cm−3 for undoped α-SrSi2 to 4.49 × 1020 cm−3 for a 1.0 at. % Ga-doped sample at 300 K. The temperature dependence of carrier concentrations was observed to change from negative to positive with increasing Ga content. In addition, the temperature dependence of the Seebeck coefficient was also observed to change from negative to positive with increasing Ga content. The results indicate that α-SrSi2 undergoes a semiconductor–metal transition with Ga doping. The power factor for the undoped sample was quite high, at 2.5 mW/mK2, while the sample with 0.3 at. % Ga had a value of 1.1 mW/mK2 at room temperature.

Revealing the promising near-room-temperature thermoelectric performance in Ag2Se single crystals

With high carrier mobility and intrinsic low lattice thermal conductivity, Ag2Se compounds have attracted increasing attention for thermoelectric application near room temperature. Due to its phase transition at ∼406 K and resulting thermal volume expansion, the growth and thermoelectric properties of large-sized Ag2Se single crystals have seldom been reported so far. In this work, the vertical Bridgeman method was used for growing bulk Ag2Se single crystal, with an orientation preference along low-symmetric (201) plane. The Hall mobility as high as 2 000 cm2/(V·s) and weak electron-phonon coupling contributes to a high electronic quality BE of ∼7.0 in near-room-temperature β-Ag2Se single crystals, which is superior to the high-temperature phase α-Ag2Se. The observed low lattice thermal conductivity of 0.8 W/(m·K) at 300 K is due to the low group speeds and strong anharmonicity. A promising peak zT of 0.66 at 375 K and an average zT of 0.65 at 300–375 K were realized in β-Ag2Se crystals. The low Vickers hardness and good ductile properties were confirmed by experiment and theoretical analysis. This work not only synthesized large-sized and highly-orientated Ag2Se crystals, but also revealed its great potential of thermoelectric performance and mechanical properties for various applications near room temperature.

Study of deep levels in the Mg2Si grown by vertical Bridgeman method

The electrical characteristics of a Mg2Si p–n junction diode was investigated. The n-Mg2Si substrate was grown by using the vertical-Bridgeman method. A p–n junction was fabricated by the thermal diffusion of Ag as an acceptor. The reverse current of the Au/Ag electrode diode was larger than that of the Ag electrode diode. The trap levels in the n-Mg2Si bulk were investigated using deep-level transient spectroscopy. The magnitude of DLTS signal of E1 with the Au/Ag electrode diode was larger than that for the signals at the other trap levels. The E1 level corresponds to an Au-related trap that diffuses via thermal annealing during the alloying process. The open-circuit-voltage-decay study suggests the existence of a minority-carrier trap in n-Mg2Si. The minority-carrier lifetime was shorter for Au/Ag electrode diodes. Therefore, Au may be involved in the formation of minority-carrier traps, as well as in the formation of majority-carrier traps.

Effect of dimensional expansion on carrier transport behaviors of the hexagonal Bi-based perovskite crystals

All-inorganic Cs3Bi2I9 (CBI) halide perovskites are sought to be candidate for photoelectrical materials because of their low toxicity and satisfactory stability. Unfortunately, the discrete molecular [Bi2I9]3− clusters limit the charge-transport behaviors. Herein, the defect halide perovskite based on trivalent Bi3+ is expanded to Cs3Bi2I6Br3 (CBIB). Centimeter-size CBIB single crystal (Φ 15 × 70 mm3) was grown by the vertical Bridgeman method. The powder X-ray diffraction analysis shows that CBIB has P3¯m1 structure with lattice parameters of a = b = 8.223 Å, c = 10.024 Å, α = β = 90° and γ = 120°. The density functional theory (DFT) calculations demonstrate that the charge density distribution was enhanced after the dimensional expansion. The enhancement of carrier transport ability of (00l) in-plane is characterized before and after dimensional improvement. The obtained CBIB(001) exhibited an electron mobility up to 40.03 cm2 V−1 s−1 by time-of-flight (TOF) technique, higher than 26.46 cm2 V−1 s−1 of CBI(001). Furthermore, the X-ray sensitivity increases from 707.81 μC Gy−1 cm−2 for CBI(001) to 3194.59 μC Gy−1 cm−2 for CBIB(001). This research will deepen our understanding of Bi-based perovskite materials and afford more promising strategies for lead-free perovskite optoelectronic devices modification.

Tm3+: Bi4Si3O12crystal as a promising laser material near 2 μm: growth, spectroscopic properties and laser performance

We report on the crystal growth, spectroscopic properties and laser performance of Tm3+-doped Bi4Si3O12 (BSO) crystal. The crystal was grown by the vertical Bridgeman method. The spectroscopic properties are investigated based on absorption and luminescence spectroscopy. Judd - Ofelt (JO) analysis is performed to calculate the spontaneous emission probabilities, branching ratio and the radiative lifetimes. The absorption spectrum, emission spectrum and gain cross-section spectra of Tm3+: BSO crystal are determined for the 2 μm transition. Luminescence decay kinetic of 3F4 upper level was analysed in detail. The continuous-wave 2 μm laser with a maximum output power of 650 mW and a slope efficiency of 29.7% is demonstrated for the first time. The beam quality factor (M2) of Tm3+: BSO laser was about 1.03 at the maximum output level.

Synthesis, growth and mechanical properties of mid-infrared nonlinear optical crystal CdGeAs2

CdGeAs2 crystal is a promising nonlinear optical material for second harmonic generation (SHG) and optical parameter oscillation (OPO) applications in mid-infrared range. In this work, CdGeAs2 polycrystalline was synthesized using temperature oscillation method and CdGeAs2 single crystals with diameters of 30 mm were grown by modified vertical Bridgeman method. The synthesized polycrystalline was characterized by X-ray diffraction (XRD). The composition and transmission of wafers cut from a single crystal were characterized by energy dispersive spectrometer (EDS) and IR Prestige-21 spectrophotometer. The mechanical properties such as Young's modulus and hardness of (001), (100), (102) and (112) faces of CdGeAs2 were measured by nanoindentation technique. The measured Young's moduli established a relationship with the bonding direction, i.e., the closer the indentation direction was to the bonding direction, the stronger the stiffness will be.

Effects of growth technique on the microstructure of CuInSe2 ternary semiconductor compound

X-ray diffraction (XRD) and Energy-dispersive X-ray fluorescence spectrometer (EDXRF) are employed to investigate the microstructure of bulk CuInSe2 specimens grown through the Bridgman technique and traveling heater process, respectively. We investigate the lattice parameters, grain sizes, and microstrains of the two grown samples. For a crystal grown by the vertical Bridgeman method, the vacancy VCu serves as an acceptor, resulting in p-type conduction, whereas the vacancy VSe expected to serve as a donor, occurring in n-type conduction for the crystal grown via the traveling heater technique. The concentration of crystal grown via the VBM is determined to be p-type, whereas the concentration of that grown via the THM is n-type. Concerning CuInSe2 crystal, the determined crystallite sizes obtained to be 165 and 182 nm for the VBM and THM, respectively.

Trapping and Recombination Centers in Cesium Hafnium Chloride Single Crystals: EPR and TSL Study

Charge trapping phenomena and recombination centers were studied in three Cs2HfCl6 single crystals of slightly different stoichiometry grown by the vertical Bridgeman method. Electron paramagnetic ...

Optical and scintillation properties of Bi4Si3O12:RE (RE=Eu3+, Sm3+, Ho3+, Tb3+) single crystals

Bi4Si3O12:RE (BSO:RE, RE = Eu3+, Sm3+, Ho3+, Tb3+) crystals were grown by the modified vertical Bridgeman method, and doping effects on scintillation properties were investigated. Under γ-ray irradiation, the light yield of BSO doped with small doses of Eu3+ increases slightly, and the energy resolution improves significantly compared to pure BSO, therefore the ability of distinguishing between particles will be improved for BSO crystals with a small amount of Eu3+ dopant. The results show that a small amount of Eu3+ doping can sensitize the Bi3+ ions. The sensitization effect enables the reduction of intrinsic defects, and thus improves the scintillation properties. However, the relative light yield of BSO:Tb (1.0 mol%) crystal is 4.3%, which is smaller than 5.0% of pure BSO. The improved light yield and energy resolution in the BSO:Eu and BSO:Sm crystals are considered an impressive achievement in the optimization of this scintillator which is already suitable for applications such as dual-readout calorimeters and homogeneous hadron calorimeters.

Growth of lead iodide single crystals used for nuclear radiation detection of Gamma‐rays

PbI2 single crystals have been fabricated by vertical Bridgeman (VB) method and vertical gradient freezing (VGF) method. The crystal grown by the VGF method exhibited high quality with no stress cracks and high infrared transmission rate compared with the one grown by VB method. A transparent orange PbI2 single crystal with size of Φ20 mm×80 mm was obtained by the improved parameters. Electrical resistivity of the crystal was as large as 1.4×1011Ωcm, and the infrared transmission rate was as high as 53% in the wave‐numbers from 4000 to 400 cm−1. The wafer was deposited with graphite explode, and the photopeak was very sharp with the shaping time of 2.7 μs to detect 241Am@59.5 keV γ‐rays.

Cesium hafnium chloride scintillator coupled with an avalanche photodiode photodetector

Optical and scintillation properties of pure Cs2HfCl6 (CHC) single crystals were investigated. In particular, light output and energy resolution were measured using a Si avalanche photodiode (Si-APD), since the Si-APD has sufficient quantum efficiency of around 70 % at emission wavelength region of CHC around 420 nm. This CHC single crystal grown using the vertical Bridgeman method showed light output of 37,000± 2,000 photons/MeV . The FWHM energy resolution was determined to be 3.7± 0.5× (E/662 keV)−0.85± 0.03[%], where E [keV] is the gamma-ray energy. Moreover, the temperature dependence of the light output was stable from −5 to 30 ̂C, while the light output increased below −10 ̂C.

Improvement and luminescent mechanism of Bi4Si3O12 scintillation crystals by Dy3+ doping

Bi4Si3O12:Dy (BSO:Dy) crystals have been grown by the modified vertical Bridgeman method and doping effects on light yield have been investigated. Doped with small amount of Dy2O3 (0.05–0.3mol%), the light yield and energy resolution of BSO crystals were improved significantly. However, high concentrations of Dy2O3 doping resulted in the decrease of light yield. Pulse height measurement under γ-ray irradiation shows that 0.1mol% Dy2O3 doping can make the relative light yield of BSO from 24.6% to 35.8% of BGO crystal, with fast decay time of ~90ns. X-ray excited radioluminescence spectra showed Dy doping has an extra emission in the host emission band (Bi3+ emission) and acts as a sensitizer to the Bi luminescent center. These results indicate that BSO:Dy crystal could be one of promising candidates for replacing BGO in some application such as electromagnetic calorimeter and dual readout in nuclear or high energy physics.

Investigation of structural defects in In-doped CdZnTe under different in-situ annealing cooling rates

The In-doped Cd0.9Zn0.1Te (CZT) crystals were grown by the modified Vertical Bridgeman method and treated by in-situ annealing with six different cooling rates. Photo-Induced Transient Spectroscopy (PICTS) and IR microscopy were employed to investigate the evolution mechanism of point defects and bulk defects (mainly Te inclusions) in the CZT crystals with different cooling rates. Related optical and electrical properties were investigated by Fourier Transform Infrared Spectrometer (FTIR) and I–V measurements. The results indicated that cooling at slow rate (10–20K/h) could decrease the concentration of point defects, such as A center, Cd vacancy (VCd2−), Te antisite (TeCd2+) and so on, while the Te inclusions are of larger dimension and lower concentration. Otherwise, the faster cooling rate (50–60K/h), the higher concentration of these point defects, and Te inclusion present small size but much larger concentration. Moreover, cooling too fast or too slow significantly degrades the optical and electrical properties. When cooled at 30–40K/h, the concentration of internal point defects is the lowest, suggesting that VCd2− compensated with TeCd2+ to reach a new equilibrium corresponding to the theory of quasichemical defect reactions (QCDR). In addition, a certain concentration of TeCd2+ was beneficial to pin the Fermi-level, and the Te inclusions presented lowest total volume fraction, which made the crystals perform great with higher resistivity and infrared transmittance.

Device design of GaSb/CdS thin film thermal photovoltaic solar cells

Enthusiasm in the reflearch of thermo-photovoltaic (TPV) cells has been aroused because the low bandwidth semiconductors of III-V family are coming into use. GaSb, as a member of III-V family, has many merits such as high absorption coefficient, and low band gap of 0.725 eV at 300 K etc.. At preflent thermo-photovoltaic cells are usually based on GaSb wafer, and it can be manufactured by the vertical Bridgeman method. Thermo-photovoltaic cell based on GaSb films is one of the effective ways to reduce the cost of the thermo-photovoltaic system. GaSb polycrystalline films can be grown by physical vapor deposition (PVD) which has advantages in using fewer materials and energy, and also in doing little harm to the environment. Because of residual acceptor defects VGaGaSb, GaSb thin film is usually of p-type semiconductor. So we should find n-type semiconductor material to form pn junction. We choose CdS as the emission layer of a cell structure. CdS belongs to n-type semiconductor with a narrow band gap of 2.4 eV and high light transmissivity. CdS thin film grown by chemical bath deposition (CBD) has passivation properties for GaSb. CdS layers can remove native oxides from GaSb surface and reduce the surface recombination velocity of GaSb. This paper focuses on theoretical analysis of GaSb/CdS thin film photovoltaic structure. By way of AFORS-HET simulation, we analyze the defect state density and interface density in GaSb and CdS, and their effects on cell performance. According to the simulation, the defect density in GaSb absorption layer is the very important factors that affect cell performance. When GaSb defect increases, the major factor to affect the cell is the fill factor that leads to low efficiency. On condition that thereflexists high GaSb defect density, the thickness of GaSb should be kept at 1000 nm. GaSb with a thickness above 1000 nm can bring about a high recombination rate, which reduces the efficiency of the cell. As an emission layer, the defect density in CdS should not affect the cell performance obviously. When the increase of CdS defect density is of four orders of magnitude, the cell efficiency is only decreased by 0.11%. In order to demonstrate the interface between GaSb and CdS, we use an inversion layer n-GaSb according to the passivation properties of CdS thin film grown on GaSb. When the defect density of inversion layer increases, the efficiency of the cell will decrease rapidly. And the GaSb/CdS structure will act as a resistance when the defect density in the inversion layer reaches 1020 cm-3. So the defect density in GaSb layer and the interface is the very factor to affect thermo-photovoltaic performance.

Optical and scintillation properties of SrI2:Yb2+

The luminescence and scintillation properties of SrI2:0.5%Yb2+ have been investigated. SrI2:Yb single crystals were grown by the vertical Bridgeman method from the melt. They showed a light yield of 38,400ph/MeV and energy resolution of 12.5% for the 662keV full absorption peak. Yb2+ photoluminescence intensity and decay time were studied between 78 and 600K. Two emission bands centered at 418 and 446nm were observed and ascribed to spin-allowed and spin-forbidden Yb2+ 5d-4f transitions, respectively. Their corresponding room-temperature decay time constants are 710ns and 77μs. Both, the emission intensities and the decay time constants vary with temperature. The obtained results were interpreted using a model of self-absorption of Yb2+ emission and a model of non-radiative relaxation of the electron from the low spin to the high spin 4f135d Yb2+ excited states. The radiative lifetime of the low spin Yb2+ excited state was determined as 400ns.

Comparison of Sn-doped and nonstoichiometric vertical-Bridgman-grown crystals of the topological insulator Bi2Te2Se

A comparative study of the properties of topological insulator Bi2Te2Se (BTS) crystals grown by the vertical Bridgeman method is described. Two defect mechanisms that create acceptor impurities to compensate for the native n-type carriers are compared: Bi excess, and light Sn doping. Both methods yield low carrier concentrations and an n-p crossover over the length of the grown crystal boules, but lower carrier concentrations and higher resistivities are obtained for the Sn-doped crystals, which reach carrier concentrations as low as 8 × 1014 cm−3. Further, the temperature dependent resistivities for the Sn-doped crystals display strongly activated behavior at high temperatures, with a characteristic energy of half the bulk band gap. The (001) cleaved Sn-doped BTS crystals display high quality Shubnikov de Haas (SdH) quantum oscillations due to the topological surface state electrons. Angle resolved photoelectron spectroscopy (ARPES) characterization shows that the Fermi energy (EF) for the Sn-doped crystals falls cleanly in the surface states with no interference from the bulk bands, which the Dirac point for the surface states lies approximately 60 meV below the top of the bulk valence band maximum, and allows for a determination of the bulk and surface state carrier concentrations as a function of Energy near EF. Electronic structure calculations that compare Bi excess and Sn dopants in BTS demonstrate that Sn acts as a special impurity, with a localized impurity band that acts as a charge buffer occurring inside the bulk band gap. We propose that the special resonant level character of Sn in BTS gives rise to the exceptionally low carrier concentrations and activated resistivities observed.

The Optical Properties and the Structure of CuInS<sub>2</sub> Thin Films Deposited by Oblique Angle Deposition Technique

The oblique angle deposition (OAD) technique was used to deposit copper indium disulfide (CuInS2) films onto glass substrates by a vacuum thermal evaporation method. The powder source used in this work was obtained from the crushed CuInS2 polycrystalline ingot grown by Vertical Bridgeman method. In order to study the influences of depositing angle on the optical property and the structure of the CuInS2 films, the depositing angle θ was varied from 10° to 70° with a 30° gradient. These films were characterized by SEM, XRD and UV-Vis spectrophotometer. The results showed that the optical property and the structure of the films were closely related to the depositing angle θ. When the depositing angle θ increased, the band gap increased and the intensity of (112) characteristic peak of the CuInS2 films decreased.

Scintillation properties of LiF–SrF2 and LiF–CaF2 eutectic

Dopant free eutectic scintillators 6LiF–SrF2 and 6LiF–CaF2 were developed by the vertical Bridgeman method for the purpose of thermal neutron detection. The molar ratio of LiF and Ca/SrF2 was 4:1 on its eutectic composition. The α-ray induced radioluminescence spectra of the scintillators showed intense emission peak at 300nm due to the emission from the self-trapped exciton in Ca/SrF2 layers. When the samples were irradiated with 252Cf neutrons, 6LiF–SrF2 and 6LiF–CaF2 exhibited the light yields of 4700 and 9400ph/n, respectively. Scintillation decay times of 6LiF–SrF2 and 6LiF–CaF2 were accepted for scintillation detectors, 90 and 250ns, respectively.

Structural and magnetic properties of ZnMnTe semiconductor

The local structural properties of Zn 0.8 Mn 0.2 Te (ZMT) semiconductors were investigated by using extended X-ray absorption fine structure (EXAFS) measurements at Te, Zn, and Mn K-edges. The ZMT crystals were fabricated by an advanced vertical Bridgeman method. The EXAFS measurements revealed that the ZMT crystals have a well-ordered zinc-blende structure, and that manganese atoms were exactly substituted for the zinc atoms. The DC magnetization measurements of the Zn 0.8 Mn 0.2 Te crystals in the temperature range of 10–300 K showed that the crystals were paramagnetic. This implied that there was no natural charge mediating any ferromagnetism in the crystals.

Sensitivity of Zn<sub>1-x</sub>Cd<sub>x</sub> Te to Tera-Hertz Signals Enhanced by Structural Orderings

A better efficiency of sensing tera-hertz signals (T-rays) depends on a better crystalline structure which is obtained by replacing a small amount of Cd on Zn sites of ZnTe crystals. Zn1-xCdxTe (ZCT) crystals were fabricated with an advanced vertical Bridgeman method and the structural properties of the ZCT crystals were studied with high resolution x-ray diffraction (HRXRD) and x-ray absorption fine structure (XAFS) measurements. The Rietveld refinement to the HRXRD data showed that the ZCT crystals at x ~ 0.05 were slightly less disordered than the ZnTe crystals. These results were confirmed by the XAFS data analysis. The better structural orderings can be directly related to the sensing and emitting efficiencies of T-ray by the ZCT crystals.