Polymer films studied by scanning tunneling microscopy

Polymer films with thicknesses up to 300 nm were investigated by scanning tunneling microscopy. It was demonstrated that the films contain areas whose images change depending on the scan parameters, which can be explained by the emission processes.     

GaAs/AlGaAs heterostructure nanowires studied by cathodoluminescence

In this report we explore the structural and optical properties of GaAs/A1GaAs heterostructure nanowires grown by metalorganic vapour phase epitaxy using gold seed-particles. The optical studies were done by low-temperature cathodo- luminescence （CL） in a scanning electron microscope （SEM）. We perform a systematic investigation of how the nanowire growth-temperature affects the total photon emission, and variations in the emission energy and intensity along the length of the nanowires. The morphology and crystal structures of the nanowires were investigated using SEM and transmission electron microscopy （TEM）. In order to correlate specific photon emission characteristics with variations in the nanowire crystal structure directly, TEM and spatially resolved CL measurements were performed on the same individual nanowires. We found that the main emission energy was located at around 1.48 eV, and that the emission intensity was greatly enhanced when increasing the GaAs nanowire core growth temperature. The data strongly suggests that this emission energy is related to rotational twins in the GaAs nanowire core. Our measurements also show that radial overgrowth by GaAs on the GaAs nanowire core can have a deteriorating effect on the optical quality of the nanowires. Finally, we conclude that an in situ pre-growth annealing step at a sufficiently high temperature significantly improves the optical quality of the nanowires.     

Multilayer contacts to AlGaAs/GaAs photodetector heterostructures

The processes occurring in a multilayer metallization during the fabrication of contacts to AlGaAs/GaAs quantum well heterostructures are analyzed using contact resistance measurements, microstructural analysis of the constituent metallic layers, and calculations of diffusion profiles. The influence of the diffusion processes and the structure of the layers on the contact resistance and reproducibility of contact characteristics is interpreted in terms of structural defects and chemical inhomogeneity of the layers.     

Investigation of MBE grown GaAs/AlGaAs/InGaAs heterostructures

This paper reports on the influence of the In mole fraction variation (0.1≤x≤0.25) of MBE grown pseudomorphic GaAs/Al_yGa_1−yAs/In_xGa_1−xAs heterostructures on the material quality and the performance of the fabricated devices. For x=0.1–0.15, the carrier mobility in the samples was 4500 cm² V⁻¹ s (at 300 K) and 37 000 cm²V⁻¹ s (at 77 K) and decreased significantly at low temperatures as x was increasing up to 0.25. Transistors with gate length of 0.8 m and In_0.1Ga_0.9As channels exhibited transconductances of 200–220 mSm mm⁻¹ and output conductances of 0.15–0.20 mSm mm⁻¹, while gate-source breakdown voltages were 27–28 V. Delay times of the designed and fabricated ICs frequency dividers by 2 were 130–140 ps.     

Investigation of the photoluminescence and modification of InGaP/GaAs/InGaAs heterostructures by near-field scanning microscopy

This study deals with the local spectroscopy and modification of semiconducting InGaP/GaAs/InGaAs quantum-well heterostructures by near-field scanning optical microscopy. The spatial distribution of the photoluminescence intensity in these structures is investigated and spatial nonuniformity of the photoluminescence is observed as a result of the nonuniform properties of the InGaP layers. It is shown for the first time that local quenching of the photoluminescence may be achieved by optically induced impurity diffusion near the quantum well, and this may be utilized to develop low-dimension semiconducting devices. Pis’ma Zh. Tekh. Fiz. 23, 20–25 (August 26, 1997)     

Laterally defined freely suspended quantum dots in GaAs/AlGaAs heterostructures

Free-standing beams containing a two-dimensional electron system are shaped from a GaAs/AlGaAs heterostructure. Quantum point contacts and (double) quantum dots are laterally defined using metal top gates. We investigate the electronic properties of these nanostructures by transport spectroscopy. Tunable localized electron states in freely suspended nanostructures are a promising tool to investigate the electron-phonon interaction.     

Assemblies of fluorine containing bent-shaped liquid crystal molecules studied by using scanning tunneling microscopy

In this work we prepared a fluorine containing bent-shaped liquid crystal from biphenyl as the central core and rod-like azobenzene mesogens as the side arms, namely 4',3-biphenyl bis[4-(4'-hexadecanloxy-3-fluorophenylazo)benzoate] (L104). The self-assembly behaviors of L104 molecules on graphite surface were investigated by using scanning tunneling microscopy (STM) under ambient conditions. The high-resolution STM images of L104 assemblies revealed three kinds of structures showing the joint effects of dipole-dipole interactions originated from the fluorine and the bent-core alignments for maximizing pi-pi interactions. These observations may be beneficial for understanding the assembly mechanism and designs for novel banana-shaped liquid crystal molecules.     

Epitaxial growth of p-sexiphenyl film on highly oriented pyrolytic graphite surface studied by scanning tunneling microscopy

The epitaxial growth of p-sexiphenyl (C₃₆H₂₆, 6P) on highly oriented pyrolytic graphite (HOPG) surface has been investigated by scanning tunneling microscopy (STM). 6P molecules prefer epitaxial growth with the long axis along the [110] direction (armchair direction) of the HOPG substrate, with the unit cell parameters b₁ = 0.67 ± 0.06 nm, b₂ = 5.97 ± 0.06 nm and angle of 88 ± 3° between them. The relation of the 6P overlayer lattice vectors with the HOPG substrate has also been deduced, i.e. the 5 × 1 supercell is in a point-on-point commensurate relation with respect to the HOPG substrate surface.     

Field emission property of hydrogenated chemical vapor deposited diamond films studied by scanning tunneling microscopy

By using scanning tunneling microscopy, the plots of tunneling current versus applied voltage, at the local points for hydrogenated and oxygenated chemical vapor deposited diamond films, were investigated. For comparison, the measurement points were adopted on the centers of the crystalline grains and at the grain boundaries, respectively. The results indicated that, for the hydrogenated chemical vapor deposited diamond, the field emission character is much better on the center of the crystalline grains than at the grain boundary. In contrast, for the oxygenated samples, the crystalline grains show a poor field emission character. The two diamond surfaces exhibit similar field emission characters at the grain boundaries. The surface emission mechanisms of the hydrogenated chemical vapor deposited diamond films were also discussed.     

Controlled positive and negative surface charge injection and erasure in a GaAs/AlGaAs based microdevice by scanning probe microscopy

In this paper, we show that positive and negative charges can be injected into the surface of SiO(2)/Si(3)N(4)/SiO(2)/GaAs/AlGaAs heterostructure material by using a biased tip of a scanning probe microscope. Furthermore, the injected charges can be erased with the same tip once grounded, working in slow scan and contact mode. Surface potential measurements by quantitative analysis of Kelvin probe force microscopy after drawing and erasing charges at room temperature are presented and discussed.     

All-optical modulator cells based on AlGaAs/GaAs/InGaAs 905-nm laser heterostructures

All-optical cells based on AlGaAs/GaAs/InGaAs laser heterostructures for a 905-nm wavelength have been developed, which operate in the regime of optical-power modulation by means of controlled generation switching between the Fabry–Perot cavity modes and high-Q closed mode. At a modulated power of 1.6 W, a mode-switching time of 1.2 ns and smaller is achieved.

     

Nanoscale dynamics probed by laser-combined scanning tunneling microscopy

With the miniaturization of functional devices, the difference in the electronic properties, for example, due to the structural nonuniformity in each element of nanoscale blocks has an ever more crucial influence on macroscopic functions. Laser-combined scanning tunneling microscopy, a potential method that enables us to probe the photoinduced carrier dynamics on the nanoscale, is reviewed with the latest results.     

Topography and transport properties of oligo(phenylene ethynylene) molecular wires studied by scanning tunneling microscopy

Conjugated phenylene(ethynylene) molecular wires are of interest as potential candidates for molecular electronic devices. Scanning tunneling microscopic study of the topography and current-voltage (I-V) characteristics of self-assembled monolayers of two types of molecular wires are presented here. The study shows that the topography and I-Vs, for small scan voltages, of the two wires are quite similar and that the electronic and structural changes introduced by the substitution of an electronegative N atom in the central phenyl ring of these wires does not significantly alter the self-assembly or the transport properties.     

Active nanocharacterization of nanofunctional materials by scanning tunneling microscopy

Recent developments in the application of scanning tunneling microscopy (STM) to nanofabrication and nanocharacterization are reviewed. The main focus of this paper is to outline techniques for depositing and manipulating nanometer-scale structures using STM tips. Firstly, the transfer of STM tip material through the application of voltage pulses is introduced. The highly reproducible fabrication of metallic silver nanodots and nanowires is discussed. The mechanism is thought to be spontaneous point-contact formation caused by field-enhanced diffusion to the apex of the tip. Transfer through the application of z-direction pulses is also introduced. Sub-nanometer displacement pulses along the z-direction form point contacts that can be used for reproducible nanodot deposition. Next, the discovery of the STM structural manipulation of surface phases is discussed. It has been demonstrated that superstructures on Si(001) surfaces can be reverse-manipulated by controlling the injected carriers. Finally, the fabrication of an atomic-scale one-dimensional quantum confinement system by single-atom deposition using a controlled point contact is presented. Because of its combined nanofabrication and nanocharacterization capabilities, STM is a powerful tool for exploring the nanotechnology and nanoscience fields.     

Active nanocharacterization of nanofunctional materials by scanning tunneling microscopy

Abstract

Recent developments in the application of scanning tunneling microscopy (STM) to nanofabrication and nanocharacterization are reviewed. The main focus of this paper is to outline techniques for depositing and manipulating nanometer-scale structures using STM tips. Firstly, the transfer of STM tip material through the application of voltage pulses is introduced. The highly reproducible fabrication of metallic silver nanodots and nanowires is discussed. The mechanism is thought to be spontaneous point-contact formation caused by field-enhanced diffusion to the apex of the tip. Transfer through the application of z-direction pulses is also introduced. Sub-nanometer displacement pulses along the z-direction form point contacts that can be used for reproducible nanodot deposition. Next, the discovery of the STM structural manipulation of surface phases is discussed. It has been demonstrated that superstructures on Si(001) surfaces can be reverse-manipulated by controlling the injected carriers. Finally, the fabrication of an atomic-scale one-dimensional quantum confinement system by single-atom deposition using a controlled point contact is presented. Because of its combined nanofabrication and nanocharacterization capabilities, STM is a powerful tool for exploring the nanotechnology and nanoscience fields.     

Study of overgrowth heterostructure InSb/GaAs by scanning electron acoustic microscopy

An overgrowth InSb epilayer on GaAs substrate with large lattice-mismatch was grown by metalorganic chemical vapor deposition (MOCVD), and the heterogeneous crystalline state was observed by scanning electron acoustic microscopy (SEAM). The middle stage of relaxation of the large mismatch InSb/GaAs epilayer is observed by SEAM images of crystalline state of the buried subsurfaces. A macroscopical heterogeneous distribution is formed by large compression stress fields. It was a very important result to observe and study semiconductor epitaxial heterostructures by SEAM uniquely imaging mechanism.     

Resonant electron tunneling through azurin in air and liquid by scanning tunneling microscopy

Electron transfer through the redox metalloprotein azurin immobilized on Au (111) by its disulphide bridge is studied by scanning tunneling microscopy (STM) in buffer solution and (for the first time) in air. STM analysis gives evidences of a stable and robust binding of the molecules in both cases. Bright spots, associated with azurin molecules, are clearly visible in STM images. The image contrast of adsorbed azurin is highly affected by the bias potential with proteins visible only for some well-defined voltage values. This experimental demonstration of the possibility to induce tunneling through azurin in air could disclose very interesting perspectives for the development of protein-based hybrid nanodevices operating in nonliquid environments.     

High-power single-mode 1.3-μm lasers based on InAs/AlGaAs/GaAs quantum dot heterostructures

Single-mode lasers operating in the 1.3 μ m wavelength range have been obtained with the active region based on InAs/AlGaAs/GaAs quantum dot heterostructures. A minimum threshold current of about 1.4 mA is reached, which is a record value for ridge waveguide lasers. The maximum efficiency and maximum output power in the cw lasing mode are 0.73 W/A and 120 mW, respectively.     

Atomic force microscopy and scanning tunneling microscopy studies of large-scale unstable growth formed during GaAs(001) homoepitaxy

Atomic force and scanning tunneling microscopy studies have been performed on GaAs(001) films grown by molecular beam epitaxy. Multilayered mounds are seen to evolve when the growth conditions favor island nucleation. As the epilayer thickness is increased, these features grow in all dimensions but the angle of inclination remains approximately constant at 1°. The mounding does not occur on surfaces grown in step flow. We propose that the multilayered features are due to an unstable growth mode which relies on island nucleation and the presence of a step edge barrier.