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Low Schottky barrier electrode has great significance on CdZnTe semiconductor nuclear radiation detectors. In this work, synchrotron radiation photoemission spectroscopy (SRPES) has been used to st...

CdMnTe (CMT) crystals were grown from Te solution with vertical Bridgman method under accelerated crucible rotation (ACRT) technique. Ingot in diameter of 30 mm and length of 60 mm was obtained. The result shows that as-grown CMT has fewer twins compared with the one grown by conventional vertical Bridgman method. However, IR microscopy shows that the microscopic growth interface morphology is non-uniform and irregular, which is attributed to higher Te inclusions density. Meanwhile, the laser confocal microscope images reveal that the Te phases are deposited randomly in the Te-rich CMT region with irregular shapes and voids. By optimizing the growth parameters to obtain a smooth interface, the Te solution vertical Bridgman technique can effectively reduce the twins in CMT crystal.

Many defects in semi-insulating (SI) cadmium zinc telluride (Cd1-xZnxTe or CZT) ingots grown by the melt methods act as trapping centers to introduce deep levels in the band gap, which has strong effects on CZT detection properties. The thermally stimulated current (TSC) spectroscopy was used to measure these traps, and the initial rise method and the simultaneous multiple peaks analysis (SIMPA) method were introduced to characterize trap levels in SI-CZT:In. The results show that there is a larger error in the determination for the trap peaks with the initial rise method due to the interference of overlapping peaks, while the SIMPA method demonstrates a better performance in resolving these overlapping peaks simultaneously for a full characterization of trap levels. On this basis, a theoretical SIMPA fitting, which is composed of ten trap levels and a deep donor level EDD dominating the dark current in SI-CZT:In, is achieved. Furthermore, the reason of high resistivity in CZT:In was explained by the relationship between EDD level and Fermi level.

Synchrotron radiation photoemission spectroscopy (SRPES) has been used to study the electronic structure of the Au/CdZnTe(111)A for Au coverage ranging from about 0.3 up to 20 monolayers (ML). It is found tha a Schottky barrier with a height of 0.82 eV is formed at the initial deposition of Au. This barrier decrease gradually with increasing Au coverage, which can be ascribed to band bending caused by charge redistribution at the interface and the formation of a positive interface dipole introduced by Cd diffusion. After an annealing process, a signal due to the formation of Au-Cd alloy caused by exquisite Cd diffusion into Au overlayer i observed, and simultaneously the Schottky barrier height (SBH) reduces to 0.32 eV. The present work indicate that cation diffusion into metal overlayer plays a critical role in controlling the SBH.

Adsorption of water molecules on the CdZnTe (1 1 1) B surface has been studied at room temperature using synchrotron radiation ultraviolet photoemission spectroscopy (SRUPS), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). After water exposure, the Cd 3d core level is chemically shifted by −0.5 eV, and in the valence band, three water-induced orbital levels are found at −3.6, −4.8 and around −8.5 eV below the Fermi level. They can be assigned to water molecules adsorbed on Cd atoms. Together with the observed work function decrease, we suggested that water is adsorbed oxygen-end-down.

Synchrotron radiation ultraviolet photoemission spectroscopy (SRUPS) and X-ray photoelectron spectroscopy (XPS) have been applied to investigate oxygen adsorption on a cadmium zinc telluride (CZT) (1 1 1)A surface. The surface chemical composition and the surface oxidation process were monitored by recording the Te 3d, O 1s, Zn 2p, Cd 4d core level peaks, and the Cd MNN Auger peak. The CZT (1 1 1)A surface was effectively oxidized by dosing oxygen directly. The typical surface state of the clean CZT (1 1 1)A surface was identified. After oxygen exposure, this surface state disappeared and a signal due to the formation of O–CZT appeared. In addition, the work function of CZT decreased with the increasing oxygen exposure.

The electrical properties of different metal–cadmium–zinc–telluride (CdZnTe) contacts produced by sputtering deposition method are investigated. The results of current–voltage and SEM analyses show that Au is the most suitable electrical contact material, which forms nearly ideal Ohmicity contact with high resistivity p-CdZnTe:In crystals and a 0.95 ± 0.02 eV barrier height with low resistivity ones. Passivation treatment can decrease the leakage current. XPS analyses show that Au atoms diffused into CdZnTe:In during annealing, meanwhile Cd and Te atoms diffuse into the Au contact. Diffused Au atoms do not form any compound with any element in CdZnTe:In crystal, but subsitute Cd sites or occupy Cd vacancies as acceptors. Thus, a heavy p-type doping layer is formed, and M–p + –p Ohmic contact is obtained.

The indium-doped CdZnTe crystal was characterized by PL spectra at 10 K and IR transmission spectra, as well as its current–voltage behavior at room temperature. The results showed that indium atoms substituted for the Cd vacancy and produced ionized donors [In Cd ] + , i.e. indium doping element recombined with [V cd ] 2− and formed the singly negative defect complex A-center [ In Cd + V Cd 2 - ] - and the neutral ones [ 2 In Cd + V Cd 2 - ] 0 and [ In cd + ( In Cd + V Cd 2 - ) - ] 0 . Indium doping also increased the IR absorption of the CdZnTe crystal due to the lattice absorption and free-carrier absorption. Meanwhile, indium doping increased the electrical resistivity and decreased the leakage current of CdZnTe crystals remarkably.

Current—voltage ( I—V ) and capacitance—voltage ( C—V ) characteristics of Au/p-CZT contacts with different surface treatments on cadmium zinc telluride (CZT) wafer's surface were measured with Agilent 4339B high resistance meter and Agilent 4294A precision impedance analyzer, respectively. The Schottky barrier height was 0.85±0.05, 0.96±0.05 eV for non-passivated and passivated CZT crystals by I—V measurement. By C—V measurement, the Schottky barrier height was 1.39±0.05, 1.51±0.05 eV for non-passivated and passivated CZT crystals. The results show that the passivation treatment can increase the barrier height of the Au/p-CZT contact and decrease the leakage current. The main reason is that the higher barrier height of Au/p-CZT contacts can decrease the possibility for electrons to pass through the native T e O 2 film. Most of the applied voltage appears on the depleted layer and there is only a negligible voltage drops across the nearly undepleted region. Furthermore, the electric field in the depleted layer is not uniform and can be calculated by the depletion approximation. The maximum electric field of CZT crystals is E ml =133 V/cm at x =0 for non-passivated CZT crystal and E m2 =55 V/cm for passivated CZT crystal, respectively.

Layered materials, such as graphite/graphene, boron nitride, transition metal dichalcogenides, represent materials in which reduced size, dimensionality, and symmetry play critical roles in their physical properties. Here, we report on a comprehensive investigation of the phonon properties in the topological insulator ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ and ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ two-dimensional (2D) crystals, with the combination of Raman spectroscopy, first-principles calculations, and group theory analysis. Low frequency $(l30\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1})$ interlayer vibrational modes are revealed in few-quintuple-layer (QL) $\mathrm{B}{\mathrm{i}}_{2}\mathrm{T}{\mathrm{e}}_{3}\text{/}\mathrm{B}{\mathrm{i}}_{2}\mathrm{S}{\mathrm{e}}_{3}$ 2D crystals, which are absent in the bulk crystal as a result of different symmetries. The experimentally observed interlayer shear and breathing mode frequencies both show blueshifts, with decreasing thickness in few-QL ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$ (down to 2QL) and ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ (down to 1QL), in agreement with first-principles calculations and a linear chain model, from which the interlayer coupling force constants can be estimated. Besides, an intense ultralow $(l12\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1})$ frequency peak is observed in 2--4QL ${\mathrm{Bi}}_{2}{\mathrm{Te}}_{3}$, which is tentatively attributed to a substrate-induced interface mode supported by a linear chain model analysis. The high frequency Raman peaks exhibit frequency shifts and broadening from 3D to 2D as a result of the phonon confinement effect. Our studies shed light on a general understanding of the influence of dimensionality and crystal symmetry on the phonon properties in layered materials.

Electronic structures of single crystalline black phosphorus were studied by state-of-art angleresolved photoemission spectroscopy. Through high resolution photon energy dependence measurements, the band dispersions along out-of-plane and in-plane directions are experimentally determined. The electrons were found to be more localized in the ab-plane than that is predicted in calculations. Beside the kz-dispersive bulk bands, resonant surface state is also observed in the momentum space. Our finds strongly suggest that more details need to be considered to fully understand the electronic properties of black phosphorus theoretically.

The clean and ordered surfaces of CdZnTe (111)B grown by the Bridgman method were obtained by Ar ion bombardment and thermal annealing in situ in an ultrahigh vacuum. The surface atomic structures of CdZnTe (111)B after annealing at different temperature were observed by low-energy electron diffraction (LEED). The valence band and work function of CdZnTe (111)B surfaces were determined by synchrotron radiation photoemission spectroscopy. The order of CdZnTe (111)B after annealing at 350 °C will worsen, and the (111)B-(2 × 2) local reconstruction will be formed. The work function of CdZnTe (111)B after annealing at 350 °C is 0.8 eV higher than that of CdZnTe (111)B-(1 × 1), and the local reconstruction may be induced by Te adatoms on top of the ideal truncation.

Angle-resolved photoemission spectroscopy (ARPES) was used to characterize the surface state of clean CdZnTe (1 1 0) surface. The surface state of CdZnTe with the peak at 0.9 eV below the Fermi level is identified. Meanwhile, Photoluminescence (PL) spectrum confirmed that there existed a surface trap state which introduced a deep-level peak at 1.510 eV. The surface trap states can be decreased by aging in dry-air. The surface leakage current was measured also by I–V measurements. After aging, the leakage current was decreased remarkably, which suggested that the aging treatment is an effective method to decrease the surface trap state.

Infrared (IR) transmission spectra and IR microscopy images were measured to evaluate the sliced CdZnTe crystals grown under different starting charges using modified vertical Bridgman method. Upon comparing the corresponding electric properties and charge transport performance, IR absorption within the wave-number range from 500 to 2500 cm-1 was potentially attributed to the free carrier absorption caused by the ionized impurities. The size and density of Te particles were not sensitive to IR transmission spectra over the same wave-number range. However, the electric field was modified around isolated Te particles, in such a way that the impurities gettering in the Te inclusions. With respect to the high resistive CdZnTe crystals, IR transmission measurements demonstrated that the mean transmittance is higher than 60% in the wave-number region from 500 to 4000 cm-1. IR microscopy shown the typical diameters of Te particles present in the material were in the range of 6-9 μm, and the density of the particles was 1-4×105 cm-3. The obtained electron mobility lifetime product (μτ)e value was in the range of 1-3×10-3 cm2V-1 by using well-known alpha particle spectra at room temperature. The fabricated CdZnTe thin planar detector showed the typical energy resolution was approximately 5.7% for the 59.5 keV peak at room temperature, without any additional signal processing.

In the light of hybrid functional density-functional theory calculations of the core-excited and core-ionized states, this paper discusses original N 1s x-ray photoemission spectroscopy (XPS) and near-edge x-ray absorption fine-structure (NEXAFS) experimental data on the single-domain (vicinal) Si(001)-2x1 surface saturated by NH3 at 300 K. The theoretical approach enables to discuss the vibrational shape of the N 1s XPS spectrum and quantifies the binding-energy splitting due to intrarow and inter-row hydrogen bondings between amine pairs. The observed N 1s NEXAFS peaks are interpreted through the analysis of the contour maps of the antibonding Kohn-Sham orbitals of the NH2 adsorbate and through the Delta Kohn-Sham calculation of the transition energies, laying a theoretical basis for a discussion of the adsorbate spatial orientation. Finally, suggestions are given for future directions of research to get a further knowledge of a system that has potential applications as a template for molecular layer deposition and supramolecular assembly.

The oxygen adsorption and desorption on the CdZnTe (111) B -(2×2) surface were studied with synchrotron radiation ultraviolet photoemission spectroscopy (SRUPS) and X-ray photoelectron spectroscopy (XPS). The results show that a surface state of clean CdZnTe (111) B -(2×2) surface appears at 0.5 eV below Fermi level ( E F), which disappears after the oxygen exposure, and shows again after annealing in UHV. The surface work function of CdZnTe (111) B -(2×2) decreases after the oxygen exposure, and futher reduces after annealing.

Understanding complex correlations between the macroscopic device performance and the contact formation on the atomic level in CdZnTe radiation detectors remains an enormous challenge. In this work, an effort towards bridging that macro-nano knowledge gap is made by systematic study of the electronic structures in the interface of Al/CdZnTe(111)A and Al/CdZnTe(111)B with Al coverage from sub-monolayer to multilayers using photoemission spectroscopy. Remarkable difference of the electronic states was found in these two interfaces. A strong interaction between Al and CdZnTe(111)A was observed at room temperature and thick interface layers (>12 nm) formed. In contrast, an intermix layer with a thickness of about one atomic layer (∼0.3 nm) was formed at Al/CdZnTe(111)B interface.

The oxygen adsorption and desorption on the CdZnTe(111)B-(2×2) surface were studied with synchrotron radiation ultraviolet photoemission spectroscopy (SRUPS) and X-ray photoelectron spectroscopy (XPS). The results show that a surface state of clean CdZnTe(111)B-(2×2) surface appears at 0.5 eV below Fermi level (EF), which disappears after the oxygen exposure, and shows again after annealing in UHV. The surface work function of CdZnTe(111)B-(2×2) decreases after the oxygen exposure, and futher reduces after annealing.

The work functions of CdZnTe single crystal with different surface states were measured using synchrotron radiation photoemission spectroscopy. The work function of CdZnTe after mechanical and chemical polishing is higher than that of clean and ordered surface. The damage layer deduced by Ar ion sputtering increases the work function. The Te adatoms of the CdZnTe (111)B (2×2) reconstruction form the interface dipole and increase the work function. The effects of H2O adsorption and Au evaporation on the work function of CdZnTe were investigated. The relationships among work function, surface dipole, valence band bending, ionization energy, and electron affinity of CdZnTe with different surface states were discussed.