Computer simulation study on the atomistic and electronic structures of semiconductor heterostructures

The atomic and electronic structures of semiconductor heterostructures are investigated by using the tight-binding (TB) electronic theory, taking into account the realistic atomic irregularities, such as steps and misfit dislocations at the interface. Particular attention has been paid to the 1/2〈110〉(001) misfit dislocation of the wide gap II-VI semiconductor heterostructures, like ZnSe/GaAs(001) systems. It is shown that steps at the interface do not produce deep gap states, but they influence significantly the deep states associated with the misfit dislocations. The stability and degradation of II-VI semiconductor heterostructures are discussed in terms of the introduction of the misfit dislocations.     

Local density of states and interface effects in semimetallic ErAs nanoparticles embedded in GaAs

The atomic and electronic structures of ErAs nanoparticles embedded within a GaAs matrix are examined via cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/XSTS). The local density of states (LDOS) exhibits a finite minimum at the Fermi level demonstrating that the nanoparticles remain semimetallic despite the predictions of previous models of quantum confinement in ErAs. We also use XSTS to measure changes in the LDOS across the ErAs/GaAs interface and propose that the interface atomic structure results in electronic states that prevent the opening of a band gap.     

Overlayer systems: Suitable samples for probing heterojunction interface properties

In this paper we report electronic structure calculations for Ge-(110) GaAs heterostructures. Our results demonstrate that Ge-(110) GaAs overlayer s ystems should be suitable samples for experimental investigations of heterojunction interface properties. We find that a 3 Ge-(110) GaAs overlayer system shows all bound interface states characteristic for the (110) Ge-GaAs heterojunction. Interface-induced resonances or semiresonances occur strongly enhanced in the overlayer system as compared to the true heterojunction. The wavevector-resolved weighted layer density of states at the Brillouin zone center, for example, shows a salient structure of interface-induced density peaks which should readily be accessible to normal emission ARUPS experiments.     

Spin polarization of quantum-well and interface states of ultrathin films of Bi on w(110) with Ag interlayers

The electronic and spin structure of quantum-well and interface states, formed in the system, consisting of bilayer of Bi on 1 ML Ag/W(110) was investigated by angle- and spin- resolved photoelectron spectroscopy. It has been shown that interface states are formed in local surface-projected gap of W(110) and are characterized by spin polarization and spin-orbit splitting, corresponding to surface resonances with high density spin-polarized states near Fermi edge.     

GaAs(110)面的电子结构

本文研究GaAs(110)面旋转弛豫的电子结构,采用一个原子集团来模拟GaAs(110)面,在其内边界上用一些“类Ga”和“类As”原子来钝化伸向体内的悬挂键,以消除由于有限模型而引起的多余边界效应。用EHT方法计算集团的总能量,由能量极小定出GaAs(110)面最稳定的弛豫位置为表面旋转角ω=18°,表面Ga原子向体内下降0.33?,As原子上升0.13?,这与Pandey等人从光电子部分产额谱所得的结果基本一致。本文还计算了理想和弛豫的GaAs(110)面的态密度,发现对于理想的(110)面禁带中确实存在一个空的表面峰。弛豫后,该峰向上移动进入导带,禁带中不再出现表面峰,与实验结果相符。 关键词：     

Origin of HfO2/GaAs interface states and interface passivation: A first principles study

First principles calculations of HfO₂/GaAs interfaces indicate that the interface states originate from the charge mismatch between HfO₂ and GaAs surfaces. We find that a model neutral interface (HfO₂ and GaAs surfaces terminated with two O and one Ga atoms per surface unit cell) removes gap states due to the balance of the interface charge. F and H can neutralize the HfO₂/GaAs interface resulting in useful band offsets, thus becoming possible candidates to passivate the interface states.     

The electronic structure of Si/GaP(110) interface and superlattice

It is argued that the (110) interface between group IV and III–V semiconductors are more likely to lend themselves to fabrication via MBE techniques. Results for the electronic structure of Si-GaP(110) superlattice are reported for the first time. Both, interface states and two dimensionally confined states are found. Sensitivity of the energy and charge distribution of these states to the interfacial geometry and the band edge discontinuity is investigated. The results are contrasted with the situation found for lattice matched III–V compound semiconductor systems such as GaAs/AlAs and GaSb/InAs.     

A trend in the energy distributions of Ga dangling-orbital surface states on GaP,GaAs and GaSb (110)

Relaxed (110) surfaces of GaP, GaAs, and GaSb are studied by cluster calculations. Strongly localized surface states near the band gap are presented. It is shown that Ga dangling-orbital surface states can occur within the band gap as a result of a strong decrease in the anti-bonding character of surface Ga orbitals. A trend in the bond length on going from GaSb to GaP explains experimental data. Surface states associated with the group-V atoms are also described.     

THE ADSORPTION OF Ge ON GaAS(ll0)

In the present work, the chemisorption of Ge on GaAs(1lO) was studied by a cluster model using the charge self-consistent extended H ückel method (EHT) and the results thus obtained were compared with those derived from the experiments. It was shown from the calculation that after chemisorption, the substrate tends to relax back to the ideal unrelaxed state, after cleavage. The tilted angle of the As-Ga bond was reduced from the well khown 27° to 10°.The gap states which were driven into both the valence band and the conduction band due to relaxation will-be introduced into the gap after chemisorption. From the calculation of the total energy of the cluster, it was found that the Ge atom could bind to both Ga and As atoms although the binding of the latter was somewhat stronger. The gap states were derived from the p states of Ge and the s and p states of Ga and As atoms. The states will serve as donors and acceptors for p and n type materials respectively . From a comparison with the previous results concerning studies of group Ⅲ and Ⅴ metals on GaAs(110) surface, it seems that the change of surface relaxation after chemisorption might be the origin of gap states and the cause for Fermi level pinningr. The explanation was first put forward by Chen et al . from their experimental studies . The present calculation also gives the core shifts of Ga(3d) and As(3d) of the clean relaxation surfaces with respect to that of the bulk as well as the core shifts of Ga(3d) and As(3d) after the chemisorption of Ge atom. A chemisorption induced state which is attributed to a precursor state for a heterojunction interface state is also obtained at a binding energy near 7eV which is in fairly good agreement with the experimental result.     

Low-energy electron energy-loss spectroscopy with Ge:GaAs (110) heterostructures

Electronic properties and chemical composition of Ge films grown by molecular beam epitaxy on GaAs (110) surfaces were studied in situ by electron energy-loss spectroscopy. The loss peaks involving core-level excitations proved that As atoms segregate at the surface of the growing film. The well known 20 eV loss peak of the clean GaAs (110) surface, being attributed to transitions from Ga(3d) to Frenkel-type excitons of dangling-bond surface states, was found to persist, slightly shifted to 19.7 eV, with the growing Ge (110) film. Since the Ga coverage amounts to below approximately 0.05 of a monolayer this transition seems to contain a strong intraatom contribution.     

Electronic structure at the PTCDA/GaAs and NTCDA/GaAs interfaces

The formation of the interface between the GaAs(100) single-crystal surface and PTCDA and NTCDA organic semiconductors is investigated. The method of total current spectroscopy makes it possible to trace the formation of the interfacial electronic structure. The two organic materials and the GaAs substrate are bonded together when the π electron cloud of an aromatic ring spreads toward the substrate. This modifies the electronic states of interfacial organic molecules and generates a dipole at the interface.     

Electronic structure of the GeGaAs (111) and (111) heterojunctions

The electronic density of states for GeGaAs (111) and ($\text{1}$$\text{1}$$\text{1}$) heterojunctions has been calculated. No interface states in the fundamental gap are found. A sizeable density of interface states below the top of the valence band is found for GeGa bonds-(111) junctions-interface states in the ionic gap are reported. The effect of varying the amount of the valence band discontinuity across the interface is discussed.     

Epitaxy and magnetic properties of surfactant-mediated growth of bcc cobalt

High resolution core level photoemission spectroscopy, photoelectron diffraction, and x-ray magnetic circular dicroism (XMCD) have been used to characterize the structural and magnetic properties of bcc-cobalt films grown on GaAs(110) substrates by using Sb as a surfactant. We have unambiguously disentangled the surfactant role played by the Sb which improves the crystallinity and reduces the lattice distortion of the metallic films as well as changes the interdiffusion process at the interface compared to the Co/GaAs(110) system. As a consequence of these combined effects, an improvement on the magnetic response of the grown Co thin films has been observed by XMCD measurements.     

甲醇/TiO2(110)界面激发态的表征

用光电子能谱的方法研究了甲醇/TiO₂(110)界面的电子结构．在激发波长为400 nm的双光子光电子能谱(2PPE)中,探测到了一个末态能量在费米能级以上5.5 eV的共振信号．之前的研究[Chem. Sci. 1, 575 (2010)]表明,这个共振信号与甲醇在5配位的钛离子(Ti_5c)上的光催化解离相关．双光子光电子能谱同时携带初态和中间态的信息．为此设计了一个调谐激发光波长的2PPE实验以及一个单光子光电子能谱(1PPE)和2PPE对比的实验,结果一致表明这个共振信号来自于未占据的中间态,也就是激发态．能带色散关系测量表明这个激发态是局域的．时间分辨2PPE测得这个激发态的寿命是24 fs．     

Optical detection of surface states in GaAs(110) and GaP(110)

We present results on the optical detection of surface states in GaAs(110)and GaP(110)by the method of the fractional change of external reflectivity. Optical transitions are observed at ～3.1 eV m GaAs(110) and ～3.4 eV in GaP. A comparison with existing theories suggests a rotational relaxation model for the surface, with partial relaxation for GaAs(110) and full relaxation for GaP(110).     

ZnSe/GaAs(100)界面电子结构的计算

利用形式散射理论的格林函数方法，采用紧束缚最近邻近似下的sp³s模型，计算了ZnSe/GaAs(100)两类界面(Se/Ga和As/Zn界面)的电子结构.分别给出了两类界面的界面带结构和波矢分辨的层态密度及其分波态密度.计算结构表明，在ZnSe/GaAs(100)两类界面的禁带隙中均无界面态，而在其价带区均存在三条束缚的界面带和四条半共振带；通过比较，分析了两类界面的界面态特征及其来源. 关键词：     

用HREELS,XPS,LEED研究Al-GaAs(100)(4×1)界面反应

高分辨率电子能量损失谱(HRFELS)能直接判定Al-GaAS(100)界面反应的生成物。本文结合X射线光电子能谱(XPS)的测量,采用HREELS来测量界面的禁带宽度,研究了Al-GaAs(100)(4×1)界面的形成过程。结果判定了室温下Al-GaAs(100)界面生成的为AlAs。而退火后,界面上生成AlCaAs合金。实验中还用低能电子衍射(LEED)观察了室温下Al在GaAs(100)(4×1)面上的淀积过程,发现随着Al淀积量的增加,表面是从无序到有序转化的。 关键词：     

Optical characterization of InGaAsN/GaAsN/GaAs quantum wells with InGaP cladding layers

Optical properties of InGaAsN/GaAs and InGaAsN/GaAsN/GaAs quantum well structures with InGaP cladding layers were studied by photoreflectance at various temperatures. The excitonic interband transitions of the InGaAsN/GaAsN/GaAs QW systems were observed in the spectral range above hν=E_g(InGaAsN). The confinement potential of the system with strain compensating GaAsN barriers became one step broader, thus more quantum states and larger optical transition rate were observed. A matrix transfer algorithm was used to calculate the subband energies numerically. Band gap energies, effective masses were adopted from the band anti-crossing model with band-offset values adjusted to obtain the subband energies to best fit the observed optical transition features. A spectral feature below and near the GaAs band gap energy from GaAs barriers is enhanced by the GaAs/InGaP interface space charge accumulation induced internal field.     

Band Structures of Ultrathin Bi(110) Films on Black Phosphorus Substrates Using Angle-Resolved Photoemission Spectroscopy

The band structures of two-monolayer Bi(110) films on black phosphorus substrates are studied using angleresolved photoemission spectroscopy. Within the band gap of bulk black phosphorus, the electronic states near the Fermi level are dominated by the Bi(110) film. The band dispersions revealed by our data suggest that the orientation of the Bi(110) film is aligned with the black phosphorus substrate. The electronic structures of the Bi(110) film strongly deviate from the band calculations of the free-standing Bi(110) film, suggesting that the substrate can significantly affect the electronic states in the Bi(110) film. Our data show that there are no non-trivial electronic states in Bi(110) films grown on black phosphorus substrates.