Xuxu Bai

Electronic structure of black phosphorus studied by angle-resolved photoemission spectroscopy

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.

Growth interface of In-doped CdMnTe from Te solution with vertical Bridgman method under ACRT technique

Abstract 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.

The study on the work function of CdZnTe with different surface states by synchrotron radiation photoemission spectroscopy

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.

Determination of trap levels in CZT:In by thermally stimulated current spectroscopy

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.

Electronic properties of aluminum/CdZnTe interfaces

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.

Electrical properties and electrical field in depletion layer for CZT crystals

Abstract Current—voltage ( I—V ) and capacitance—voltage ( C—V ) characteristics of Au/p-CZT contacts with different surface treatments on cadmium zinc telluride (CZT) wafers 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.

THE ADSORPTION AND DESORPTION OF OXYGEN ON CdZnTe (111)B-(2 × 2) SURFACE

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.

Non-destructive characterization and selection of X/γ-ray detector-grade CdZnTe crystals

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 cm2·V-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.