Nanometer-scale Semiconductor Research Articles

Imaging Defects in Nanometer-scale Semiconductor Crystals: Statistical Nucleation Events are Few in Small Crystals, but can Control Growth

Imaging Defects in Nanometer-scale Semiconductor Crystals: Statistical Nucleation Events are Few in Small Crystals, but can Control Growth

Nanometer-scale two-terminal semiconductor memory operating at room temperature

Based on a nanometer-scale semiconductor channel with an intentionally broken geometric symmetry, we have realized a type of memory device that consists of only two terminals, rather than the minimum of three terminals in conventional semiconductor memories. The charge retention time is at least 10 h at cryogenic temperatures and a few minutes at room temperature. Furthermore, the simplicity of the design allows the active part of the devices to be made in a single nanolithography step which, along with the planar structure of the device, provides promising possibilities for a high integration density. (C) 2005 American Institute of Physics.

Efficient light absorption in metal–semiconductor–metal nanostructures

A nanoscale metal–semiconductor grating is proposed for efficient and ultrafast photodetection. Theoretical and experimental results of efficient absorption in nanoscopic semiconductor wires are presented. The strong confinement of light in subwavelength metal–semiconductor gratings is achieved by Fabry–Pérot resonances involving vertical transverse magnetic surface-plasmon waves and transverse electric guided waves. Photodetectors have been fabricated with 40×100nm cross sections of Ag and GaAs wires. The reflectivity and photocurrent mesurements are in good agreement with theoretical estimates.

Diode-like characteristics of nanometer-scale semiconductor channels with a broken symmetry

We present a new type of nanometer-scale semiconductor nonlinear device, called self-switching device (SSD). The device was realized by simply etching insulating grooves into a semiconductor, between which a narrow channel with a broken symmetry was formed. Because of the asymmetry in the channel boundary, an applied voltage V not only changes the potential profile along the channel direction, but also either widens or narrows the effective channel width depending on the sign of V. This results in a diode-like current–voltage characteristic but without the use of any doping junction or barrier structure. The turn-on voltage can also be widely tuned from virtually 0 to more than 10 V by simply adjusting the channel width. Furthermore, only one lithography step was needed to fabricate SSDs. We used two different material systems, InGaAs–InP and InGaAs–InAlAs, to realize SSDs and the results at room temperature were compared. We also show that by adding a third terminal to an SSD as a gate, the turn-on voltage of the device could be tuned by the gate bias and the device functions either as a tunable diode or as a transistor.

Nanolasers: lasing from nanoscale quantum wires

Semiconductor lasers are in many ways second only to transistors as to their impact on today's high-tech industries. The unique characteristics, such as narrow emission wavelength, high frequency modulation, and device integratibility, make semiconductor lasers ideal photon sources for applications as diverse as telecommunication, signal processing, material characterisation, and medical diagnostics. Advances in material growth technologies, particularly molecular beam epitaxy, metal-organic chemical vapor deposition, and a suite of chemical synthesis techniques, make the fabrication of high quality nanometer scale semiconductor structures possible. Thanks to the quantum size effects that drastically modify the energy spectra of confined electrons in reduced dimensions, the population inversion necessary for lasing action occurs more efficiently as the active semiconductor gain medium is scaled down from the bulk to the nanometer scale. Consequently, semiconductor lasers built with nanoscale active media are expected to exhibit extraordinary features such as great color range, high optical gain, and low lasing threshold. Indeed, miniaturised lasers using nanoscale semiconductor gain media - two-dimensional quantum wells, one-dimensional quantum wires, and zero-dimensional quantum dots - have shown significant improvements in device performance. This article provides an overview of the physics and technologies behind the rapid progress of miniaturisation of semiconductor lasers, in particular the quantum wire lasers based on one-dimensional nanoscale optical gain media. Since the first report of lasing in quantum wires by Kapon and his co-workers, quantum wire lasers have evolved from "microlasers" in which the one-dimensional nanostructure is embedded in a micron size optical cavity, to "nanolasers" in which, as we recently demonstrated, the material gain and optical feedback are simultaneously achieved by individual nanoscale quantum wires. One-dimensional semiconductor fabrication technologies based on nanoscale lithography, self-organisation, selective growth, and chemical synthesis will be reviewed along with recent advances of quantum wire lasers built upon each of these fabrication technologies.

Simulation of nanometer-scale semiconductor devices considering quantum effects

It is necessary to take quantum effects into account in the design of nanometer-scale high electron mobility transistors (HEMTs) and heterojunction bipolar transistors (HBTs). To achieve this, an effective potential method is applied to drift-diffusion (DD)-based simulations of the threshold-voltage characteristics of nanometer-scale InP-based HEMTs and to ensemble Monte Carlo (EMC) simulations of transit times in InP-based HBTs. The simulated results are compared with the experimental results in order to validate the method. We conclude that device simulations incorporating the effective potential method are very useful for designing millimeter-wave HEMTs and HBTs. © 2004 Wiley Periodicals, Inc. Int J RF and Microwave CAE 14: 283–289, 2004.

Unidirectional electron flow in a nanometer-scale semiconductor channel: A self-switching device

By tailoring the boundary of a narrow semiconductor channel to break its symmetry, we have realized a type of nanometer-scale nonlinear device, which we refer to as self-switching device (SSD). An applied voltage V not only changes the potential profile along the channel direction, but also either widens or narrows the effective channel depending on the sign of V. This results in a diode-like characteristic but without the use of any doping junction or barrier structure. The turn-on voltage can also be widely tuned from virtually zero to more than 10 V, by simply adjusting the channel width. The planar and two-terminal structure of the SSD also allows SSD-based circuits to be realized by only one step of lithography.

Single electron tunneling of nanoscale TiSi2 islands on Si

Nanoscale TiSi2 islands are formed by electron beam deposition of a few monolayers of titanium on an atomically clean silicon surface followed by in situ annealing at high temperatures (800–1000 °C). The lateral diameter of typical islands are ∼5 nm, and they form a nanoscale metal–semiconductor interface. Direct probing of the electrical characteristics of these islands on both p- and n-type Si substrates was performed using ultrahigh vacuum scanning tunneling microscopy and scanning tunneling spectroscopy. With the vacuum between the tip and the island as a second tunnel junction, we thus form a double-junction system for observation of single electron tunneling (SET) effects. Moreover, the small dimensions of the system allow room temperature observation. The results showed features in the I–V spectra attributed to single electron tunneling. Features were more evident when the island–Si junction was in reverse bias. For substrates with a thin epitaxial layer of intrinsic Si, the tunneling related features were enhanced for both doping types. The experimental results are compared with the standard theory and numerical values from the fitting are in agreement with the experimental structures. The results indicate that the nanoscale Schottky barrier of the island–substrate interface can be employed as a tunnel barrier in SET structures.

Nano-island fabrication by electron beam lithography and selective oxidation of Al-rich AlGaAs layers for single electron device application

Abstract A nanometer-scale island embedded between two tunnel junctions constitutes the elementary cell of single electron devices (SED), such as single electron transistors (SET) and memories. In this work we report on a new method for the fabrication of a nanometer-scale semiconductor island through AlGaAs/GaAs epitaxial growth, electron beam lithography and selective oxidation of Al-rich AlGaAs layers. We have used a combination of high-resolution electron beam lithography and selective oxidation of Al-rich AlGaAs layers in order to fabricate a non-oxidized semiconductor island smaller than the electron beam lithography (EBL) defined size. The pattern was transferred down to the bottom AlAs layer and a selective oxidation of the Al-rich layers was performed in water vapor at a temperature of 300 °C, which led to the formation of the aluminum oxide. The higher oxidation rate of the AlAs compared to the Al 0.8 Ga 0.2 As layer, together with the strain accumulation in the Al 0.8 Ga 0.2 As, caused the formation of nanoscale semiconductor islands embedded in an aluminum oxide shell. After the oxidation and cleavage of the sample, selective wet etching has been performed in order to evidence the unoxidized region through an SEM inspection.

Size- and shape-controls and electronic functions of nanometer-scale semiconductors and oxides

Abstract Nanometer-scale semiconductor and oxide particles are synthesized by solution methods, i.e. emulsion and reverse micelle techniques. Particle sizes are not only controlled by synthesis conditions, but also selected by flotation and filtration. The physical and chemical properties of these particles are analyzed in finding materials functions appeared in nanometer sizes. In particular, II–VI semiconductor particles show high photo-chemical activities, such as photo-catalytic processes. They also show several size and shape dependent stabilities in diameters

Holography Measurement of Mean Inner Potential of Germanium

Abstract There is increasing interest in the quantitative characterization of nanometer-scale semiconductor alloy layers, such as those of SiGe or AlGaAs. Electron holography, which has the ability to detect small changes in mean inner potential Vo, can in principle provide information about local changes in composition. However, the measurement of absolute values of Vo is not always straightforward and values for only a few materials are known with confidence. In this study, we measure Vo for Ge using electron holography, and we compare our experimental data with calculations. The holographic approach that we use to measure Vo is described in the paper of Gajdardziska-Josifovska et al. The Ge samples were nominally 90° cleaved wedges from a <100> wafer. Holograms were recorded on a Philips CM200 FEG equipped with a Gatan 794 CCD camera, and data were obtained for several crystal tilts (determined from diffraction patterns) to assess dynamical contributions to the phases.

Cross-Sectional Scanning Tunneling Microscopy.

Since its invention in the early 1980, scanning tunneling microscopy1-4 (STM) and techniques such as atomic force microscopy5 (AFM) that have evolved from it have become tools of paramount importance in fundamental studies of surfaces. In addition, increasing effort has been directed towards the use of STM and related techniques to address problems of technological interest, particularly in the area of semiconductor materials and devices. The extremely high spatial resolution afforded by these techniques has led naturally to their application to a wide range of scientific and technological problems in semiconductor device physics. Indeed, their growing importance in semiconductor science and technology is a natural consequence of the desire to design and fabricate ever smaller structures to improve device performance or achieve new device functionality, and of the limitations of more traditional experimental techniques in performing detailed and comprehensive characterization at the nanometer to atomic scale. The unique and powerful advantage offered by STM and other scanning probe techniques is the ability to perform direct studies of structural, electronic, and other properties of materials with extremely high spatial resolution. Many other techniques, such as X-ray diffraction, electron diffraction, and transmission electron microscopy, typically provide only indirect information about sample structure and, while offering the ability to probe certain structural or compositional features at the atomic scale, inevitably average these properties over substantially larger areas or volumes. In scanning tunneling microscopy, however, direct imaging of features corresponding to individual atoms on a surface has been demonstrated for a wide range of materials. Other scanning probe techniques, while usually providing somewhat lower spatial resolution than STM, allow greater flexibility in samples that can be studied, imaging conditions that can be tolerated, and properties that can be characterized. Rapid progress in the development of new scanning probe techniques and in the commercial availability Edward Yu is Associate Professor in the Department of Electrical and Computer Engineering at the University of California, San Diego. He received his A.B. (summa cum laude) and A.M. degrees in Physics from Harvard University in 1986, and his Ph.D. degree in Applied Physics from the California Institute of Technology in 1991. While at Caltech he was the recipient of a National Science Foundation Graduate Fellowship and an ATT fundamental properties and applications of novel heterojunction material systems; and techniques for the fabrication and characterization of nanometer-scale semiconductor device structures. Recent activities in his research group at UCSD have included extensive studies using cross-sectional scanning tunneling microscopy of atomic-scale properties, and their relation to epitaxial growth conditions and device performance, of a wide range of III−V compound semiconductor heterostructures. Professor Yu has been the recipient of an Office of Naval Research Young Investigator Award, and Alfred P. Sloan Research Fellowship, and a National Science Foundation CAREER Award. 1017 Chem. Rev. 1997, 97, 1017−1044

Electronic structure of dual gate quantum wire

The electronic structure of a novel nanometer scale semiconductor quantum wire structure [S. Y. Chou and Y. Wang, Appl. Phys. Lett. 63, 788 (1993)] has been calculated self-consistently. The structure has two control parameters, the voltage applied to a split gate and the voltage applied to a wire gate. The influences of both the split gate and the narrow wire gate which is placed inside the gap of the split gate, on the electronic structure of the system are examined. We show that varying the voltage on either the split gate or the wire gate changes the induced quantum wire confinement potential profile, the energy level spacing, the channel electron density, and the effective channel width. Results for the ballistic conductance of the device as a function of the two control voltages are extracted from the electronic structure calculations and are found to be in satisfactory agreement with experimental data.

Nanoscale metal–semiconductor–metal photodetectors with subpicosecond response time fabricated using electron beam lithography

Metal–semiconductor–metal photodetectors (MSM PDs) with finger spacing and width as small as 25 nm have been fabricated using high-resolution electron beam lithography. Measurements using an electro-optic sampling system show that the fastest detector has a full width at half-maximum response time of 0.87 ps and a 3 dB bandwidth of 510 GHz. Monte Carlo simulation of detector response time is studied and compared with experimental data. Finally, scaling rules for high-speed MSM PDs are proposed.

A new sol-gel route for the preparation of nanometer-scale semiconductor particles that exhibit quantum optical behavior

A new sol-gel route for the preparation of nanometer-scale ZnS semiconductor particles that display quantum optical behavior is reported. Exploiting the scaling relationship between size and density of fractal polymeric precursors, films are prepared with narrow pore size distributions that serve as templates to constrain the growth of ZnS semiconductor particles. Blue shifts of the ZnS absorption threshold indicate that the particle size is reduced from 4.6 to 2.3 nm as the pore size is reduced from 3.5 to 12.0 nm. Results obtained using transmission electron microscopy are in general agreement with the spectroscopic results. This is a general approach for preparing thin-film nanocomposite materials with tailored optical and/or electronic properties.