Second-harmonic spectroscopy of Ge/Si(001) and Si1-xGex(001)/Si(001)

0.9 Ge_0.1(001)/Si(001) films with SH photon energies 3.1<2hν<3.5 eV near the bulk E₁ critical point of Si(001) or Si_0.9Ge_0.1(001). Ge was deposited on Si(001) by using atomic layer epitaxy cycles with GeH₄ or Ge₂H₆ deposition at 410 K followed by hydrogen desorption. As Ge coverage increased from 0 to 2 monolayers the SH signal increased uniformly by a factor of seven with no detectable shift in the silicon E₁ resonant peak position. SH signals from Si_0.9Ge_0.1(001)/Si(001) were also stronger than those from intrinsic Si(001). Hydrogen termination of the Si_0.9Ge_0.1(001) and Ge/Si(001) surfaces strongly quenched the SH signals, which is similar to the reported trend on H/Si(001). We attribute the stronger signals from Ge-containingsurfaces to the stronger SH polarizability of asymmetric Ge-Si and Ge-Ge dimers compared to Si-Si dimers. Hydrogen termination symmetrizes all dimers, thus quenching the SH polarizability of all of the surfaces investigated. Received: 13 October 1998 / Revised version: 18 January 1999     

Highly polarized emission in spin resolved photoelectron spectroscopy of α-Fe(001)/GaAs(001)

Highly spin-polarized sources of electrons, integrated into device design, remain of great interest to the spintronic and magneto-electronic device community. Here, the growth of Fe upon GaAs(001) has been studied with photoelectron spectroscopy (PES), including Spin Resolved PES. Despite evidence of atomic level disorder such as intermixing, an over-layer with the spectroscopic signature of α-Fe(001), with a bcc real space ordering, is obtained. The results will be discussed in light of the possibility of using such films as a spin-polarized source in device applications.     

Adhesion at a heterophase interface: First-principles study of Mo(001)/MoSi2(001)

The bonding characteristics, interfacial energetics, and electronic structure associated with adhesion at the Mo-MoSi₂(001) heterophase interface are investigated using the first-principles, self-consistent local orbital method. We found both the adhesive energy and peak interfacial strength for the interface to be 10%-15% smaller than the respective values for cleavage along the (001) planes in crystalline Mo and MoSi₂. The equilibrium interlayer separation between Mo and MoSi₂ is found to lie between the interplanar spacings of crystalline Mo and MoSi₂. The interfacial adhesive bonding is attributable to the combination of a nearly uniform band of charge accumulation at the interface and directional charge accumulation between atoms across the interface. These first-principles calculations demonstrate that the universal-binding-energy relation can be extended to describe adhesion between dissimilar materials.     

Molecular orbital cluster model study of Cu(001)/Cl

The Cl-Cu metal chemisorption is studied using molecular orbital theory. The surface is represented by a Cu cluster. The Cu atoms are described by a relativistic effective core potential having only one (4s¹) valence electron. In particular, the Cl-Cu interlayer distance is calculated in order to complement the information obtained from recent LEED and SEXAFS studies and to calibrate the theoretical approach used.     

Energy band alignment of MgO (111)/ZnO (0002) heterojunction determined by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy (XPS) was used to measure the energy discontinuity in the MgO (111)/ZnO (0002) heterostructure. The valence band offset (VBO) was determined to be 1.22±0.23 eV and a type-I heterojunction with a conduction band offset (CBO) of 3.24±0.23 eV was obtained. The discrepancy of VBO values between MgO/ZnO and ZnO/MgO heterojunctions was mainly attributed to the internal electric field induced by spontaneous polarization effect in ZnO layer.     

Surface vibrations of Na(001) and K(001) surfaces

We present the results of a theoretical study of the dynamics of the atom motion of Na(001) and K(001) surfaces. The total electronic energy is calculated using a pseudopotential approach with a confined electron gas as unperturbed system. With this theory the dynamical matrix can he derived without resorting to empirical parametrizations. Surface phonon dispersion curves are reported for the high symmetry directions of the two-dimensional Brillouin zone for ideal and relaxed configurations. The calculated spectra are compared with the results of semi-empirical force constant calculations. The effects of single and multilayer relaxations on the location and the nature of the main surface bands are examined.     

Instability,intermixing and electronic structure at the epitaxial LaAlO3/SrTiO3(001) heterojunction

The question of stability against diffusional mixing at the prototypical LaAlO₃/SrTiO₃(001) interface is explored using a multi-faceted experimental and theoretical approach. We combine analytical methods with a range of sensitivities to elemental concentrations and spatial separations to investigate interfaces grown using on-axis pulsed laser deposition. We also employ computational modeling based on the density function theory as well as classical force fields to explore the energetic stability of a wide variety of intermixed atomic configurations relative to the idealized, atomically abrupt model. Statistical analysis of the calculated energies for the various configurations is used to elucidate the relative thermodynamic stability of intermixed and abrupt configurations. We find that on both experimental and theoretical fronts, the tendency toward intermixing is very strong. We have also measured and calculated key electronic properties such as potential energy gradients and valence band discontinuity at the interface. We find no measurable electric field in either the LaAlO₃ or SrTiO₃, and that the valence band offset is near zero, partitioning the band discontinuity almost entirely to the conduction band edge. Significantly, we find it is not possible to account for these electronic properties theoretically without including extensive intermixing in our physical model of the interface. The atomic configurations which give the greatest electrostatic stability are those that eliminate the interface dipole by intermixing, calling into question the conventional explanation for conductivity at this interface—electronic reconstruction. Rather, evidence is presented for La indiffusion and doping of the SrTiO₃ below the interface as being the cause of the observed conductivity.     

Characteristics of cylindrical surrounding-gate GaAs_xSb_(1-x)/In_yGa_(1-y)As heterojunction tunneling field-effect transistors

A Ⅲ-Ⅴ heterojunction tunneling field-effect transistor(TFET) can enhance the on-state current effectively,and GaAs_xSb_1_x/In_yGa_1_yAs heterojunction exhibits better performance with the adjustable band alignment by modulating the alloy composition.In this paper,the performance of the cylindrical surrounding-gate GaAs_xSb_1_x/In_yGa_1_yAs heterojunction TFET with gate-drain underlap is investigated by numerical simulation.We validate that reducing drain doping concentration and increasing gate-drain underlap could be effective ways to reduce the off-state current and subthreshold swing(SS),while increasing source doping concentration and adjusting the composition of GaAs_xSb_1_xIn_yGa_1_yAs can improve the on-state current.In addition,the resonant TFET based on GaAs_xSb_1_x/In_yGa_1_yAs is also studied,and the result shows that the minimum and average of SS reach 11 mV/decade and 20 mV/decade for five decades of drain current,respectively,and is much superior to the conventional TFET.     

a-Si(n)/c-Si(p)异质结太阳电池薄膜硅背场的模拟优化

采用AFORS-HET数值模拟软件,对不同带隙的薄膜硅材料在a-Si(n)/c-Si(p)异质结太阳电池上的背场效果进行了模拟,分析了影响背场效果的原因,得到了薄膜硅背场在a-Si(n)/c-Si(p)异质结太阳电池上的适用条件为薄膜硅材料是带隙16 eV,硼掺杂浓度在10¹⁸cm^-3以上的微晶硅材料,其最佳厚度在5nm左右. 这种背场从工艺上易于实现,并且,与常用的Al扩散背场相比,在相同的掺杂浓度下,电池效率可以大大提高. 关键词： 薄膜硅 背场 硅异质结太阳电池     

Structural phase transition on Si(001) and Ge(001) surfaces

Among a variety of solid surfaces, Si(001) and Ge(001) have been most extensively studied. Although they seem to be rather simple systems, there have been many conflicting arguments about the atomic structure on these surfaces. We first present experimental evidence indicating that the buckled dimer is the basic building block and that the structural phase transition between the low-temperature c(4x2) structure and the high-temperature (2x1) structure is of the order-disorder type. We then review recent theoretical work on this phase transition. The real system is mapped onto a model Ising-spin system and the interaction parameters are derived from total-energy calculations for different arrangements of buckled dimers. The calculated critical temperature agrees reasonably well with the experimental one. It is pointed out that the nature of the phase transition is crucially affected by a small amount of defects on the real surfaces.     

Ab-Initio Investigation of Metal-Ceramic Bonding: M(001)/MgAl2O4(001), M = Al, Ag

The optimised adhesion geometry and the work of adhesion were determined for the interface systems Al(001)/MgAl₂O₄(001) and Ag(001)/MgAl₂O₄(001) by ab-initio density-functional calculations. Al is bound strongly on-top of oxygen at reduced lattice spacing, whereas weak adhesion and no unique optimum structure are obtained for Ag. An analysis of the electronic structure shows that in both systems the metal-ceramic interaction is spatially confined to a narrow range along the interface layers. The weak adhesion of Ag on spinel is mediated by a polarisation of the metal film, which may be classified as an image-charge interaction. In the case of Al a weak image-charge contribution is dominated by a strong electron-density redistribution perpendicular to the interface, which leads to the formation of directional Al—O bonds.