Realization of silicon quantum wires based on Si/SiGe/Si heterostructure

We report on the successful fabrication of silicon quantum wires with SiO2 boundaries on SiGe/Si heterostructures by combining Si/SiGe/Si heteroepitaxy, selective chemical etching, and subsequent thermal oxidation. The observational result of scanning electron microscope is demonstrated. The present method provides a well-controllable way to fabricate silicon quantum wires.     

Probing the lateral composition profile of self-assembled islands

We apply a selective etching procedure to probe the lateral composition profile of self-assembled SiGe pyramids on a Si(001) substrate surface. We find that the pyramids consist of highly Si intermixed corners, whereas the edges, the apex, and the center of the pyramids remain Ge rich. Our results cannot be explained by existing growth models that minimize strain energy. We use a model that includes surface interdiffusion during island growth, underlining the paramount importance of surface processes during the formation of self-assembled quantum dot heterostructures in many different material systems.     

Sharp n-type doping profiles in Si/SiGe heterostructures produced by atomic hydrogen etching

Surface segregation of group V dopant during thin film epitaxy of Si/SiGe heterostructures causes severe limitation on the sharpness of n-type doping profiles in pn junctions. Existing techniques for removal of surface segregated arsenic suffer from either high thermal budget or aggressive (ex situ) wet chemical etching. An in situ low temperature method is clearly desirable, particularly for device structures with high Ge content such as resonant tunnelling diodes, in order to minimize diffusion of the matrix elements as well as maintain structural integrity. In situ etching by atomic hydrogen is shown to be ideal for this purpose. The reaction mechanism ensures that this can only be a low temperature process and the method is shown to be highly effective and selective in the removal of surface segregated As. In comparison with other techniques, atomic hydrogen etching is also shown to be less aggressive and has a smaller impact on the surface/interface quality.     

Water-solid interfaces probed by high-resolution atomic force microscopy

Water-solid interfaces play important roles across a broad range of scientific and application fields. In the past decades, atomic force microscopy (AFM) has significantly deepened our understanding of water-solid interfaces at molecular scale. In this review, we describe the recent progresses on probing water-solid interfaces by noncontact AFM, highlighting the imaging of interfacial water with ultrahigh spatial resolution. In particular, the recent development of qPlus-based AFM with functionalized tips has made it possible to directly image the H-bonding skeleton of interfacial water under UHV environment. Based on high-order electrostatic forces, such a technique even enables submolecular-level imaging of weakly bonded water structures with negligible disturbance. In addition, the three-dimensional (3D) AFM using low-noise cantilever deflection sensors can achieve atomic resolution imaging at liquid/solid interfaces, which opens up the possibility of probing the hydration layer structures under realistic conditions. We then discuss the application of those AFM techniques to various interfacial water systems, including water clusters, ion hydrates, water chains, water monolayers/multilayers and bulk water/ice on different surfaces under UHV or ambient environments. Some important issues will be addressed, including the H-bonding topology, ice nucleation and growth, ion hydration and transport, dielectric properties of water, etc. In the end, we present an outlook on the directions of future AFM studies of water at interfaces and the challenges faced by this field, as well as the development of new AFM techniques.     

Free-standing Si/SiGe micro- and nano-objects

Free-standing SiGe/Si microtubes, microneedles, helical coils, bridges and submicron vertical rings have been fabricated from elastically strained SiGe/Si heterostructures grown by ultra-high vacuum chemical vapor deposition. Three-dimensional micro- and nano-objects have been formed by self-scrolling after electron beam lithography, reactive ion etching and wet selective etching. Vertical rings with very smooth sidewalls may have applications in hot embossing lithography as well as in microelectronics and micromechanics. By adjusting the SiGe/Si bilayer thickness or Ge concentration, the diameter of tube or ring could be decreased into the nanometer scale.     

AFM nanooxidation process - Technology perspective for mesoscopic structures

We study the technology of local anodic oxidation (LAO) by the AFM tip applied to semiconductor heterostructures with two-dimensional electron gas. The aim is to design mesoscopic rings with persistent current and one subband occupied. For this purpose the need is to oxidize narrow lines that represent energy barriers high enough. Using the electrostatic model, we explain the electric field distribution in the system tip-sample just before LAO starts. We study the influence of the conductivity of the cap layer on LAO and explain the origin of the saddle-like profile lines, observed in the experiment. Using Monte Carlo simulation we show that the carrier redistribution in the system with LAO energy barriers effectively lowers the barrier height. In the experimental part we have grown InGaP/AlGaAs/GaAs heterostructures by organometalic vapor phase epitaxy with an active layer only 31 nm below the surface. We have prepared oxide lines on the heterostructures by LAO and characterized them by the temperature-dependent transport measurement.     

Observations of fission-tracks in zircons by atomic force microscope

The fission-track (FT) method is a dating technique based on the observation of damage (tracks) by spontaneous fission of ²³⁸U left in a mineral. The date is calculated from the track density and the uranium concentration in the mineral. This is possible because the number of tracks is a function of uranium concentration and time since the start of track accumulation. Usually, the number of tracks is counted under an optical microscope after etching (chemical expansion of a track). However, as FT density per unit area rises, it becomes difficult to count the number of tracks. This is due to the fact that FTs overlap one another and are unable to be readily distinguished. This research examines the potential of atomic force microscope (AFM) for FT dating using zircons, which are likely to show higher FT density than other minerals due to their high U concentrations.To obtain an AFM image for a sample prepared for FT dating, removing the static electricity of the sample is essential to avoid an unexpected movement of the cantilever. A grain should be wider than about 30 μm to bring the cantilever on the mineral surface. Polishing with a fine grained compound is very important. There is not much difference in sharpness between images by AC mode (scanning with vibrating cantilever at a constant cycle) and Contact mode (scanning with the cantilever always in close contact with the surface). To confirm how tracks can be identified with the AFM, an AFM image was compared with an image obtained with the optical microscope. When change in the number of tracks and their shapes were observed through stepwise etching, the track expanded as the etching time increased. In addition, the etching rate was slower for large tracks than those for small tracks. This implied that the AFM can be used to observe etching of zircons with different degrees of nuclear fission damage. A track that could not be seen with the optical microscope due to insufficient etching could be observed by AFM methods, indicating the possibility of FT dating with high track densities using AFM after relatively short etching periods.     

An AFM study of the processing of hydrogen passivated silicon(1 1 1) of a low miscut angle

We present atomic force microscopy (AFM) measurements from a passivated silicon crystal miscut by 0.1° and show the etching regime to be significantly different from surfaces with a larger miscut angle. A simple kinetic model is developed to explain the results and is used to derive the optimal etching conditions for nominally flat Si(1 1 1)–(1×1)H. We show that small changes in miscut angle can alter the kinetic steady state and promote the formation of deep etch pits, even on the least stable, miscut surface. Collisions of steps with these pits result in arrays of stable, self-aligned ‘etch hillocks' over micron dimensions. Following preparation, we use AFM to observe the initial growth of native oxide on the Si(1 1 1)–(1×1)H surface, and demonstrate that AFM is a sensitive probe to surface oxidation in the sub-monolayer regime.     

A facile and efficient dry transfer technique for two-dimensional Van der Waals heterostructure

Two-dimensional(2D) Van der Waals heterostructures have aroused extensive concerns in recent years. Their fabrication calls for facile and efficient transfer techniques for achieving well-defined structures. In this work, we report a simple and effective dry transfer method to fabricate 2D heterostructures with a clean interface. Using Propylene Carbonate(PC)films as stamps, we are able to pick up various 2D materials flakes from the substrates and unload them to the receiving substrates at an elevated temperature. Various multilayer heterostructures with ultra-clean interfaces were fabricated by this technique. Furthermore, the 2D materials can be pre-patterned before transfer so as to fabricate desired device structures,demonstrating a facile way to promote the development of 2D heterostructures.     

Relative-thickness dependence of exchange bias in bilayers and trilayers

We consider the models of ferromagnetic (FM)/antiferromagnetic (AFM) bilayers and trilayers and perform a modified Monte Carlo method to study their exchange bias (EB) properties at low temperature after field cooling on increasing one component thickness at the expense of the other one. The results indicate that EB is insensitive to the thickness variations as the FM layer is thicker than the AFM one. Otherwise, it has a steep increase with the decrease of FM thickness, but the purely inverse proportion is no longer valid due to the dual influences of FM and AFM thicknesses. EB in trilayers should be approximately twice larger than that in bilayers because there is a double interfacial area in the trilayers compared with the bilayers, but the dispersed FM/AFM distributions may break this relation as a result of thermal destabilization. Moreover, EB is independent of FM/AFM stacking sequences probably because of the ideal interface between them. It has been clarified unambiguously that such control of EB through varying the FM/AFM dimensions in heterostructures is attractive for spintronics applications.     

Study into the shape of oxide lines formed by LAO - Influence of an oxidized material

Linear oxide patterns were formed by local anodic oxidation (LAO) using an atomic force microscope (AFM) on an n-doped GaAs substrate, a 10-nm-thick titanium layer, and on shallow GaAs/AlGaAs-based heterostructures capped either with a 5-nm-thick undoped GaAs layer or a 2-nm-thick undoped InGaP layer. Each heterostructures had a 2DEG buried at a specific depth between 22 and 45 nm. LAO was performed in contact and non-contact AFM modes with the aim to explain the phenomenon of single and double line formation depending on material oxidized. The occurrence of the phenomenon was also simulated. The results showed that the occurrence of the double lines is linked with the thickness of native oxides.     

Interface effects in spin-polarized metal/insulator layered structures

Recent advances in thin-film deposition techniques, such as molecular beam epitaxy and pulsed laser deposition, have allowed for the manufacture of heterostructures with nearly atomically abrupt interfaces. Although the bulk properties of the individual heterostructure components may be well-known, often the heterostructures exhibit novel and sometimes unexpected properties due to interface effects. At heterostructure interfaces, lattice structure, stoichiometry, interface electronic structure (bonding, interface states, etc.), and symmetry all conspire to produce behavior different from the bulk constituents. This review discusses why knowledge of the electronic structure and composition at the interfaces is pivotal to the understanding of the properties of heterostructures, particularly the (spin polarized) electronic transport in (magnetic) tunnel junctions.     

Combined cross-sectional STM/AFM in liquid: Study of InGaAs/GaAs heterostructures with quantum wells and dots

Combined scanning tunneling and atomic force microscopy (STM/AFM) of cross-sectional cleavages in a protective liquid medium (oil) is applied to study InGaAs/GaAs heterostructures with quantum wells and dots. It is shown that the quantum wells and dots can be visualized on cleavages in both AFM and STM modes and to measure the current-voltage characteristics of the contact between an AFM probe and the cleavage surface.     

Thermally induced quantum well shape modification in strained heterostructures

We have demonstrated that shallow ion (⁷⁵As⁺) implantation and rapid thermal annealing (RTA) of strained InGaAs/GaAs quantum well (QW) structures can modify the optical properties of these epitaxial semiconductor heterostructures in a spatially selective manner. After RTA, QW exciton energies, determined from peaks in the photoluminescence spectra, shifted significantly to higher values only in the implanted regions. The magnitudes of the shifts were dependent on QW widths, RTA temperatures, and ion implantation fluences. The shifts were interpreted as arising from the modification of the shapes of the as-grown QWs from square (abrupt interfaces) to rounded (gradual interfaces) due to enhanced indium diffusion out of the well layers in irradiated areas as a consequence of the in-diffusion of vacancies generated near the surface by the implantation. Except for QWs near the critical thickness boundary, the presence of strain in the quantum well layers due to the difference in the lattice constant of the well and barrier layers had negligible effect on the QW shape modification due to thermal processing.     

Response of the Dielectric Parameters of (110)SrTiO3 Films to the Formation of Ferroelectric Domains in Their Volume

Physics of the Solid State - Three-layer SrRuO3/SrTiO3/SrRuO3 heterostructures were grown by laser evaporation on (110)LaAlO3 substrates. Photolithography and ion etching are used to form...     

Ultrafast strain engineering in complex oxide heterostructures

We report on ultrafast optical experiments in which femtosecond midinfrared radiation is used to excite the lattice of complex oxide heterostructures. By tuning the excitation energy to a vibrational mode of the substrate, a long-lived five-order-of-magnitude increase of the electrical conductivity of NdNiO(3) epitaxial thin films is observed as a structural distortion propagates across the interface. Vibrational excitation, extended here to a wide class of heterostructures and interfaces, may be conducive to new strategies for electronic phase control at THz repetition rates.     

Track sensitivity and the surface roughness measurements of CR-39 with atomic force microscope

Atomic Force Microscope (AFM) has been applied to evaluate the surface roughness and the track sensitivity of CR-39 track detector. We experimentally confirmed the inverse correlation between the track sensitivity and the roughness of the detector surface after etching. The surface of CR-39 (CR-39 doped with antioxidant (HARZLAS (TD-1)) and copolymer of CR-39/NIPAAm (TNF-1)) with high sensitivity becomes rough by the etching, while the pure CR-39 (BARYOTRAK) with low sensitivity keeps its original surface clarity even for the long etching.     

Structure and morphology of Cu/Ni film grown by electrodeposition method: A study of neutron reflectivity and AFM

We present detailed study of structure and interface morphology of an electrodeposited Cu/Ni film using X-ray diffraction, X-ray reflectivity, neutron reflectivity and atomic force microscopy (AFM) techniques. The crystalline structure of the film has been determined by X-ray diffraction, which suggest polycrystalline growth of the film. The depth profile of density in the sample has been obtained from specular X-ray and neutron reflectivity measurements. AFM image of the air-film interface shows that the surface is covered by globular islands of different sizes. The AFM height distribution of the surface clearly shows two peaks and the relief structure (islands) on the surface in the film, which can be treated as a quasi-two-level random rough surface structure. We have demonstrated that the detailed morphology of air-film interfaces, the quasi-two-level surface structure as well as morphology of the buried interfaces can be obtained from off-specular neutron reflectivity data. AFM and off-specular neutron reflectivity measurements also show that the morphologies of electrodeposited surface is distinctively different as compared to that of sputter-deposited surfaces in the sample.     

Perspectives of cross-sectional scanning tunneling microscopy and spectroscopy for complex oxide physics

Complex oxide heterostructure interfaces have shown novel physical phenomena which do not exist in bulk materials. These heterostructures can be used in the potential applications in the next generation devices and served as the playgrounds for the fundamental physics research. The direct measurements of the interfaces with excellent spatial resolution and physical property information is rather difficult to achieve with the existing tools. Recently developed cross-sectional scanning tunneling microscopy and spectroscopy (XSTM/S) for complex oxide interfaces have proven to be capable of providing local electronic density of states (LDOS) information at the interface with spatial resolution down to nanometer scale. In this perspective, we will briefly introduce the basic idea and some recent achievements in using XSTM/S to study complex oxide interfaces. We will also discuss the future of this technique and the field of the interfacial physics.     

Ohmic contact formation to bulk and heterostructure gallium nitride family semiconductors

We describe experiments investigating the quality of ohmic contacts to both bulk GaN and to III-nitride heterostructures. Titanium-based contacts were investigated to assess the role of intermixing and surface impurities for contact formation to n-type GaN. Direct contact to the two-dimensional electron gas in GaN/AlGaN heterostructures was also studied. These contacts were made by photochemical etching of the samples to expose the heterointerface. It was observed that even in the latter case contact annealing leads to a lower contact resistance by consuming surface contaminants and promoting beneficial interfacial reactions. Various passivation techniques were tried to reduce surface leakage current between contact pads and PECVD-deposited silicon nitride was found to be the best material for this application.