Xian Liang Wu

Negative capacitance at metal-semiconductor interfaces

A negative capacitance effect has been observed in metal‐semiconductor contacts. This phenomenon is explained by considering the loss of interface charge at occupied states below Fermi level due to impact ionization. A modified Shockley–Read treatment is proposed to interpret the experimental observations. In particular, a two‐energy‐level simplified model is presented to simulate the capacitance spectrum. The results are in good agreement with the experimental data.

Interfacial reactions and Schottky barriers of Pt and Pd on epitaxial Si1−xGex alloys

The evolution of interfacial reactions during the deposition of Pt and Pd on epitaxial Si1−xGex alloys was studied using x‐ray photoelectron spectroscopy (XPS) for metal coverage up to 10 A. Auger electron depth profiling was performed on a thicker metal overlayer before and after in vacuo annealing to study the redistribution of composition in the reactions. We have found that Pt and Pd react mainly with Si to form silicides at 350 °C, leaving some Ge to segregate at the surface. These results were correlated with Schottky barrier height measurements. We found that the Schottky barrier heights of Pt/n‐Si0.8Ge0.2 and Pd/n‐Si0.8Ge0.2 are about the same, pinned at 0.68 eV, which is much smaller than those of n‐Si. These barrier heights are quite stable up to 550 °C.

Structural and Electronic Role of Lead in (PbBi)2Sr2CaCu2O8 Superconductors by STM.

The structural and electronic effects of lead substitution in the high-temperature superconducting materials PbxBi2-xSr2CaCu2O8 have been characterized by scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). Large-area STM images of the Bi(Pb)—O layers show that lead substitution distorts and disorders the one-dimensional superlattice found in these materials. Atomic-resolution images indicate that extra oxygen atoms are present in the Bi(Pb)—O layers. STS data show that the electronic structure of the Bi(Pb)—O layers is insensitive to lead substitution within �0.5 electron volt of the Fermi level; however, a systematic decrease in the density of states is observed at ≈1 electron volt above the Fermi level. Because the superconducting transition temperatures are independent of x(Pb) (x ≤ 0.7), these microscopic STM and STS data suggest that the lead-induced electronic and structural changes in the Bi(Pb)—O layer do not perturb the electronic states critical to forming the superconducting state in this system.

Effect of Schottky barrier height on EL2 measurement by deep-level transient spectroscopy

A systematic study of EL2 midgap trap in GaAs using deep‐level transient spectroscopy (DLTS) is reported for contacts having a large range of Schottky barrier height. The results show that the DLTS signal of EL2 increases as the barrier height rises from 0.62 eV and saturates for barrier height above 0.83 eV. It is found, for the first time, that for Schottky barrier height lower than 0.62 eV the EL2 signal disappears. A model for calculation of the quasi‐Fermi level in the depletion region is used to explain the variation and disappearance of the EL2 signal. This model may also apply to other electron traps near midgap.

Scanning tunneling microscopy investigations of the local structure of Tl2Ba2CaCu2O8 single crystals

The in‐plane structure of single‐crystal samples of Tl2Ba2CaCu2O8 has been imaged at room temperature using a scanning tunneling microscope. Atomic‐resolution surface images exhibit areas in which the structure has tetragonal symmetry (peak spacing 2.5±0.2 A) and regions in which the structure is distorted from tetragonal symmetry. The lattice spacing indicates that the observed structure corresponds to the in‐plane thallium and oxygen positions. The observation of both sites also suggests that the Tl‐O band makes a significant contribution to the density of states near the Fermi level. In addition, a weak one‐dimensional superlattice (period 10±0.5 A) which shows short‐range order has been observed.

High Tc superconducting thin films by rapid thermal annealing of Cu/BaO/Y2O3 layered structures

Superconducting thin films of YBaCuO have been formed using rapid thermal annealing of Cu/BaO/Y2 O3 layered structures, which were deposited on MgO substrates by electron beam evaporation. The best film has an onset temperature of 94 K and zero resistance at 84 K. The dependence of the film characteristics and superconducting transition temperature on the annealing conditions has been studied. Auger depth profiling was used to examine the interdiffusion between the film and the MgO substrate.

Control of the Schottky barrier using an ultrathin interface metal layer

Fermi level movements at Pt/GaAs and Ti/GaAs interfaces have been investigated using a direct measurement of Schottky barrier heights in a bimetal Schottky structure. Using thin interfacial layers, the Schottky barrier was smoothly varied from the characteristic value of the thick metal to that of the interfacial metal. The variation of barrier height versus the inner metal thickness was found to exhibit an exponential behavior extending over a few monolayers coverage. This experiment indicates a new approach to the fundamental study of metal‐semiconductor interfaces and could be useful in device applications.

Variable temperature scanning tunneling microscopy studies of the charge density wave phases in tantalum disulfide

Variable temperature scanning tunneling microscopy has been used to elucidate details of the nearly commensurate charge density wave (CDW) phase in 1T‐TaS2. Large‐area images show that the nearly commensurate phase has a hexagonal domain structure, and that the domain period exhibits a strong temperature dependence. Real‐space and two‐dimensional Fourier transform analyses of atomic resolution images further demonstrate that within the domains the CDW is approximately commensurate with the atomic lattice, and that between domains the CDW amplitude decreases and the CDW phase changes. In addition, disorder observed in the hexagonal domain structure at low temperatures is suggested to be due to domain wall pinning.

Reply to ‘‘Comment on ‘Negative capacitance at metal‐semiconductor interfaces’ ’’ [J. Appl. Phys. 70, 1090 (1991)]

The origin of the excess admittance at a forward‐biased Schottky diode invokes a controversy among research workers. Werner commented on our papers [J. Appl. Phys. 70, 1090 (1991)], in which he believes that the excess admittance is caused by minority‐carrier extraction at defective back contacts rather than charge capture and emission at interface states. This reply answers the questions raised by Werner et al. [Phys. Rev. Lett. 60, 53 (1988)] and points out that the minority‐carrier effect cannot account for the experimental observations.