Dynamic probe of ZnTe(110) surface by scanning tunneling microscopy

Abstract

The reconstructed surface structure of the II–VI semiconductor ZnTe (110), which is a promising material in the research field of semiconductor spintronics, was studied by scanning tunneling microscopy/spectroscopy (STM/STS). First, the surface states formed by reconstruction by the charge transfer of dangling bond electrons from cationic Zn to anionic Te atoms, which are similar to those of IV and III–V semiconductors, were confirmed in real space. Secondly, oscillation in tunneling current between binary states, which is considered to reflect a conformational change in the topmost Zn–Te structure between the reconstructed and bulk-like ideal structures, was directly observed by STM. Third, using the technique of charge injection, a surface atomic structure was successfully fabricated, suggesting the possibility of atomic-scale manipulation of this widely applicable surface of ZnTe.     

Contrast Reversal in Scanning Tunneling Microscopy and Its Implications for the Topological Classification of SmB6

SmB₆ has recently attracted considerable interest as a candidate for the first strongly correlated topological insulator. Such materials promise entirely new properties such as correlation-enhanced bulk bandgaps or a Fermi surface from spin excitations. Whether SmB₆ and its surface states are topological or trivial is still heavily disputed however, and a solution is hindered by major disagreement between angle-resolved photoemission (ARPES) and scanning tunneling microscopy (STM) results. Here, a combined ARPES and STM experiment is conducted. It is discovered that the STM contrast strongly depends on the bias voltage and reverses its sign beyond 1 V. It is shown that the understanding of this contrast reversal is the clue to resolving the discrepancy between ARPES and STM results. In particular, the scanning tunneling spectra reflect a low-energy electronic structure at the surface, which supports a trivial origin of the surface states and the surface metallicity of SmB₆.     

Study of polycrystalline ZnTe (ZnTe:Cu) thin films for photovoltaic cells

In our work, polycrystalline ZnTe and ZnTe:Cu thin films were deposited by vacuum co-evaporation technology. The conductivity–temperature relationship was measured. And the properties of films were studied by X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and differential scanning calorimety (DSC). The results show that the as-deposited films are cubic and that the films annealed at 185°C are cubic and hexagonal. Cu _x Te are observed by XRD and XPS. DSC shows ZnTe:Cu film has a peak of decalescence at 170°C, which means that there can be a change. Therefore we assume ZnTe:Cu thin films have structure changes at 185°C and the existence of Cu _x Te leads to the abnormal conductivity–temperature relationship. During annealing, copper diffuse from grain boundary to lattices. 1/C ²-V curves show that Cu _x Te can form tunneling junction with CdTe, which can improve the back contact.     

Structure and optical properties of polycrystalline Cd1−xZnxTe thin films prepared by simultaneous evaporation

Cd_1–xZn_xTe films were deposited by simultaneous evaporation of CdTe and ZnTe. These Cd_1–xZn_xTe films were of cubic phase, and strongly (1 1 1) oriented as deposited. Predominant direct optical transitions were observed and the band gap varied with zinc content in a non-linear way. A structure development of CdS/CdTe/ZnTe : Cu solar cells with a Cd_1–xZn_xTe buffer layer was proposed.     

Emergent multistability in assembled nanostructures

Scanning tunneling microscopy (STM) at 5 K reveals that native atoms in the surface layer of a semiconductor crystal become bistable in vertical height when a nanostructure is assembled nearby. The binary switching of surface atoms, driven by the STM tip, changes their charge state. Coupling is facilitated by assembling adatom chains, allowing us to explore the emergence of complex multiple switching. Density-functional theory calculations rationalize the observations and a lattice-gas model predicts the cooperative behavior from first principles.     

STM Observations of 2D Kr and Xe Adsorbed on Graphite

Crystalline structures of two dimensional rare-gas solids physisorbed on a graphite surface are studied with a low-temperature scanning tunneling microscope (STM) at T=4 K. We have obtained atomically resolved STM images of monolayer krypton (Kr) for the first time as well as those of xenon (Xe). It was observed that the 2D structure of Xe is destroyed with small tip-sample separation. Distinct changes in the local density of states were observed in tunneling spectra after the adsorption. For a multi-layer Xe film, a characteristic telegraph pattern of the tunneling current was also observed, which probably indicates single atom dynamics.     

Microscale study of poly(vinyl alcohol) as an effective additive for inhibiting recrystallization in ice slurries

Scanning tunneling microscopy (STM) was used to investigate the surface morphology of ice crystals containing adsorbed poly(vinyl alcohol) (PVOH) molecules inside a cold room at −7.0°C. PVOH was used as a substitute for antifreeze protein (AFP) type I, which is an effective additive for making ice slurries resistant to recrystallization. The STM images revealed microscale grooves on ice crystals made from PVOH solutions, indicating that PVOH molecules significantly influence the surface structure of the ice crystal. The length of each groove was similar to that of a PVOH molecule, indicating that PVOH molecules were adsorbed on the ice crystal surfaces. The interaction force between PVOH molecules and the ice surface was discussed by assuming a molecular structure of PVOH on the ice crystal surface, and the depression of the local freezing point was analyzed based on the surface curvature of ice revealed by STM.     

温度梯度溶液法生长的Cr掺杂的ZnTe晶体的表征

以CrTe作为掺杂源、以Te作为溶剂, 用温度梯度溶液法生长了Cr掺杂的ZnTe晶锭。晶锭头部的晶粒尺寸较大(>10 mm×10 mm), 且Te夹杂相较少。Te夹杂相的大小、形状和分布可以反映晶锭中的温场分布。晶锭的径向非对称温场导致富Te相沿径向非对称分布。Te夹杂相在温度梯度作用下的热迁移会导致其相互融合长大、变长。Te夹杂相也会在晶体中引入裂纹和空洞等缺陷。部分未被掺入ZnTe中的CrTe富集于固液界面处, 表明温度梯度溶液法生长晶体时具有一定的排杂作用。Cr掺杂的ZnTe晶体的电阻率(约1000 Ω·cm)高于未掺杂的ZnTe(约300 Ω·cm)。Cr掺杂晶体在约1750 nm处的吸收峰表明Cr²⁺离子被成功地掺入了ZnTe中。但是Cr掺杂后晶体的红外透过率降低, 表明Cr掺杂引入了较多的缺陷。     

Charge sensing of precisely positioned p donors in Si

Real-time sensing of (spin-dependent) single-electron tunneling is fundamental to electrical readout of qubit states in spin quantum computing. Here, we demonstrate the feasibility of detecting such single-electron tunneling events using an atomically planar charge sensing layout, which can be readily integrated in scalable quantum computing architectures with phosphorus-donor-based spin qubits in silicon (Si:P). Using scanning tunneling microscopy (STM) lithography on a Si(001) surface, we patterned a single-electron transistor (SET), both tunnel and electrostatically coupled to a coplanar ultrasmall quantum dot, the latter consisting of approximately four P donors. Charge transitions of the quantum dot could be detected both in time-averaged and single-shot current response of the SET. Single electron tunneling between the quantum dot and the SET island on a time-scale (τ ～ ms) two-orders-of-magnitude faster than the spin-lattice relaxation time of a P donor in Si makes this device geometry suitable for projective readout of Si:P spin qubits. Crucial to scalability is the ability to reproducibly achieve sufficient electron tunnel rates and charge sensitivity of the SET. The inherent atomic-scale control of STM lithography bodes extremely well to precisely optimize both of these parameters.     

How important is the interfacial chemical bond for electron transport through alkyl chain monolayers?

We show that the electrode/molecule chemical bond does not change the tunneling barrier for charge transport through alkyl chain monolayers sandwiched between Si and Hg electrodes. This observation can be understood if the interfacial bond mainly governs the monolayer's structure, while the electronic states due to molecule-electrode bonding do not contribute significantly to tunneling. Yet, the nature of the bond affects the Schottky barrier inside the semiconductor due to changes in the interface dipole.     

Magnetic-Field-Induced Level Crossing and the Spin Dynamics of Excitons in Zn1?xMnxTe/ZnTe Quantum Wells

Magnetic-field-induced level crossing and the spin dynamics of excitons in a Zn_1–x Mn _x Te/ZnTe single quantum well are studied. The circularly-polarized photoluminescence (PL) shows that the down spin branch of the Zn_1–x Mn _x Te exciton overlaps with both the up and down spin branches of the ZnTe exciton at a crossing field (H _c) of 4 T, due to the giant Zeeman shift of Zn_1–x Mn _x Te. The PL intensities and lifetimes in each layer become gradually equal toward H _c, which shows the mixing of wavefunctions of the excitons generated in each layer. Above H _c, each branch of the spin-polarized exciton separates again. The lifetimes of the spin-polarized exciton PL reflect the spin-flip relaxation in ZnTe and the spin mixing between Zn_1–x Mn _x Te and ZnTe layers.     

Atomic imaging of macroscopic surface conductivity

Scanning tunneling microscopy (STM) enables direct imaging of surface-state bands, through which electrical conduction occurs, confirmed by direct measurements with the four-point probe method. STM images also exhibit voltage drops along a surface due to electrical resistance of the surface states (scanning tunneling potentiometry). Scanning micro-four-point probes and multi-tip STM are newborn techniques for much more direct mapping of the conductivity. Such capability of imaging provides direct insights on carrier scattering at atomic scales.     

Heterostructures with CdTe/ZnTe quantum dots for single photon emitters grown by molecular beam epitaxy

We report on the molecular beam epitaxy (MBE) of heterostructures with CdTe/ZnTe quantum dots (QDs) with relatively low surface density, which could be used as single-photon emitters. The QDs were formed on the surface of a 3.1- to 4.5-monolayer-thick two-dimensional strained CdTe layer by depositing amorphous Te layer and its fast thermal desorption. Subsequent thermal annealing of the surface with QDs in the absence of external Te flux led to strong broadening and short-wavelength shift of the QD photoluminescence (PL) peak. Measurement of the micro-PL spectra of individual CdTe/ZnTe quantum dots in fabricated mesastructures with a diameter of 200—1000 nm allowed estimation of the QD surface density as ~10¹⁰ cm^–2.     

Metal-free phthalocyanine (H2Pc) molecule adsorbed on the Au(111) surface: formation of a wide domain along a single lattice direction

Using low-temperature scanning tunneling microscopy (STM), we observed the bonding configuration of the metal-free phthalocyanine (H₂Pc) molecule adsorbed on the Au(111) surface. A local lattice formation started from a quasi-square lattice aligned to the close-packed directions of the Au(111) surface. Although we expected the lattice alignment to be equally distributed along the three crystallographically equivalent directions, the domain aligned normal to the ridge of the herringbone structure was missing in the STM images. We attribute this effect to the uniaxial contraction of the reconstructed Au(111) surface that can account for the formation of a large lattice domain along a single crystallographical direction.     

Mechanistic consideration of ZnTe microspheres formation in a PVP-contained polyol system via hot injection method

Zinc telluride (ZnTe) microspheres have been synthesized in a polyol system with adding various amount of poly(N-vinyl-2-pyrrolidone) (PVP, K-90) at 200 °C via a hot injection method. The smallest sized ZnTe microspheres obtained in this study had an average size of 634 nm through reaction for 48 h using 518 mg of PVP. A retrospective study was conducted to evaluate the formation process of ZnTe microspheres by conducting experiments from 0.2 h to 48 h. Nucleation and growth of ZnTe at the tips of Te rods in a PVP-contained polyol system were observed in sample reacted less than 1 h. More ZnTe microspheres and less Te rods were observed when the reaction was proceeded for longer time, suggesting that Te rods were the sacrificial template for the growth of ZnTe. Amount of PVP is the key factor of controlling dimensions of Te rods formed at the initial stage as well as the sizes of ZnTe microspheres by influencing the nucleation and growth rate of ZnTe. The obtained ZnTe microspheres at 48 h consisted of voids, which were originated by the detachment of ZnTe microspheres from Te rods after their growth.     

Bistable charge configuration of donor systems near the GaAs(110) surfaces

In gated semiconductor devices, the space charge layer that is located under the gate electrode acts as the functional element. With increasing gate voltage, the microscopic process forming this space charge layer involves the subsequent ionization or electron capture of individual dopants within the semiconductor. In this Letter, a scanning tunneling microscope tip is used as a movable gate above the (110) surface of n-doped GaAs. We study the build-up process of the space charge region considering donors and visualize the charge states of individual and multi donor systems. The charge configuration of single donors is determined by the position of the tip and the applied gate voltage. In contrast, a two donor system with interdonor distances smaller than 10 nm shows a more complex behavior. The electrostatic interaction between the donors in combination with the modification of their electronic properties close to the surface results in ionization gaps and bistable charge switching behavior.     

Metal-free phthalocyanine (H2Pc) molecule adsorbed on the Au(111) surface: formation of a wide domain along a single lattice direction

Abstract

Using low-temperature scanning tunneling microscopy (STM), we observed the bonding configuration of the metal-free phthalocyanine (H₂Pc) molecule adsorbed on the Au(111) surface. A local lattice formation started from a quasi-square lattice aligned to the close-packed directions of the Au(111) surface. Although we expected the lattice alignment to be equally distributed along the three crystallographically equivalent directions, the domain aligned normal to the ridge of the herringbone structure was missing in the STM images. We attribute this effect to the uniaxial contraction of the reconstructed Au(111) surface that can account for the formation of a large lattice domain along a single crystallographical direction.     

Studying local surface electric conductivity of a ZnSe/CdSe/ZnSe heterostructure by scanning tunneling microscopy in the field electron emission regime

The surface morphology and local electric conductivity of a ZnSe/CdSe/ZnSe nanoheterostructure have been studied by scanning tunneling microscopy (STM) in the field electron emission regime. The homogeneity of the local conductivity distribution in a near-surface layer has been evaluated. The main parameters of a potential barrier in the local field contact are determined. It is shown that the STM probe can be used for creating local regions with nonequilibrium carrier concentration in a semiconductor.     

The use of a scanning tunneling microscope (STM) for investigation of local photoconductivity of quantum-dimensional semiconductor structures

The possibility is demonstrated of using the STM for recording spectra of photoconductivity of quantum-dimensional semiconductor structures with a high spatial resolution. Studies are made into the local photoconductivity of GaAs/In_xGa_1−x As based quantum wells and quantum points as a function of the depth of location of the quantum-dimensional structure relative to the surface space charge region. For quantum points in the vicinity of the sample surface, spectra are obtained which are characterized by features associated with the individual energy spectrum of those points.     

Scanning tunnelling microscopy and tunnelling spectroscopy of titanium before and afterin vitro immersion

In an effort to understand thein vivo interactions of titanium and its alloys with a biological environment, surface science methods have been used on specimens retrieved fromin vitro andin vivo experiments. A relatively new technique that has the potential to further our knowledge of the oxide-solution interface is scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (TS). This work documents the use of STM/TS in the study of titanium thin films before and after immersion in anin vitro solution. Titanium thin films were fabricated using a procedure which produced an oxide that had minimal contaminants. Half of the thin films were immersed in an electrolyte. STM/TS was performed immediately after the immersion period. Constant current images were obtained. Current-voltage characteristics were recorded at regions of interest. The topography of the nonimmersed films revealed that the surface was qualitatively the same as other sputter deposited metal films. I–V curves showed little spatial variation. The topography of the immersed film showed little change from the nonimmersed ones. However, significant spatial variation of the local electronic structure was noted. This indicates that titanium surface-fluid interactions do not occur uniformly on the film.