Nanometer-scale Semiconductor Research Articles

Non-equipotential effect observed on the surface of Ti/Si under uniform illumination

In this letter, a non-equipotential surface photovoltaic effect is reported in nano metal–semiconductor structures. When the surface of the Ti/Si is uniformly illuminated by a beam of light, a controllable surface photovoltaic effect is observed on the metal side. The center of the surface presents a remarkably higher metallic potential than the surrounding region. The surface photovoltage is detected to be as high as 53 mV. Besides, it depends sensitively on the thickness and size of the metal films, demonstrating it is a unique feature of nano metal films. We ascribe this phenomenon to the boundary effect of photon-generated carriers in the ultrathin metal thickness. The theoretical calculations based on equivalent electron diffusion model are in great agreement with the experimental results. The results may promise some novel applications based on the nanoscale metal–semiconductor systems.

DNA origami mediated electrically connected metal—semiconductor junctions

DNA-based nanofabrication of inorganic nanostructures has potential application in electronics, catalysis, and plasmonics. Previous DNA metallization has generated conductive DNA-assembled nanostructures; however, the use of semiconductors and the development of well-connected nanoscale metal—semiconductor junctions on DNA nanostructures are still at an early stage. Herein, we report the first fabrication of multiple electrically connected metal—semiconductor junctions on individual DNA origami by location-specific binding of gold and tellurium nanorods. Nanorod attachment to DNA origami was via DNA hybridization for Au and by electrostatic interaction for Te. Electroless gold plating was used to create nanoscale metal—semiconductor interfaces by filling the gaps between Au and Te nanorods. Two-point electrical characterization indicated that the Au—Te—Au junctions were electrically connected, with current—voltage properties consistent with a Schottky junction. DNA-based nanofabrication of metal—semiconductor junctions opens up potential opportunities in nanoelectronics, demonstrating the power of this bottom-up approach.

Process Variation-Induced Contact Resistivity Variability in Nanoscale MS and MIS Contacts

This article presents contact resistivity variability (CRV) in metal-semiconductor (MS) and metal-interfacial layer (IL)-semiconductor (MIS) contacts on Ge and Si source/drain regions resulting from process-induced random dopant fluctuation (RDF) and IL thickness variation (ILTV). CRV is modeled by combining the effect of RDF and ILTV in statistical simulations incorporating Fermi-level pinning physics. CRV is also shown to have a direct and significant impact on 7-nm FinFET On-current variability. The choice of contact architecture depends strongly on the source/drain doping. RDF dominates at low doping and MIS contacts provide lower CRV as well as lower resistivity ( $\rho _{c}$ ) due to a lower contact barrier height. Impact of RDF decreases and that of ILTV increases with increasing doping resulting in lower CRV with comparable $\rho _{c}$ for MS contacts.

Evaluating nanoscale ultra-thin metal films by means of lateral photovoltaic effect in metal–semiconductor structure

Nanoscale metal–semiconductor (MS) structure materials occupy an important position in semiconductor and microelectronic field due to their abundant physical phenomena and effects. The thickness of metal films is a critical factor in determining characteristics of MS devices. How to detect or evaluate the metal thickness is always a key issue for realizing high performance MS devices. In this work, we propose a direct surface detection by use of the lateral photovoltaic effect (LPE) in MS structure, which can not only measure nanoscale thickness, but also detect the fluctuation of metal films. This method is based on the fact that the output of lateral photovoltaic voltage (LPV) is closely linked with the metal thickness at the laser spot. We believe this laser-based contact-free detection is a useful supplement to the traditional methods, such as AFM, SEM, TEM or step profiler. This is because these traditional methods are always incapable of directly detecting ultra-thin metal films in MS structure materials.

Electrostatic Doping in Semiconductor Devices

To overcome the limitations of chemical doping in nanometer-scale semiconductor devices, electrostatic doping (ED) is emerging as a broadly investigated alternative to provide regions with a high electron or hole density in a semiconductor device. In this paper, we review various reported ED approaches and related device architectures in different material systems. We highlight the role of metal and semiconductor workfunctions, energy bandgap, and applied electric field and the interplay between them for the induced ED. The effect of interface traps on the induced charge is also addressed. In addition, we discuss the performance benefits of ED devices and the major roadblocks of these approaches for potential future CMOS technology.

Aspect ratio tuned red-shift of photoluminescence emission of PbSe nanorods investigated by electron holography

The physical properties of nanometer scale semiconductors are known to be sensitively influenced by their aspect ratios, but the intrinsic mechanisms still remain unclear. Shape-controlled anisotropic PbSe nanorods were obtained by means of the addition of MnCl2, and the aspect ratio of the nanorods can be continuously tuned from 1 to 10 by simply modulating the amount of chloride ions. It was demonstrated that an optimized concentration of Cl− anions is about 0.04mmol, which controls the competition between thermodynamics and kinetics mechanisms. The emission peaks of the infrared absorbance and photoluminescence spectra were significantly tuned from 1664nm to 1840nm and from 1459nm to 1938nm only by the aspect ratios, respectively. A strong electric dipole phenomenon localized onside the surface of PbSe nanorods terminated by Pb2+ charge was found by using high-spatial-resolution off-axis electron holography, which was furthermore evidenced by the quantitative analysis of the mean inner potential and the surfaces charge. The charge intensity depended on the aspect ratio of PbSe nanorods. The results provide clear evidence that the energy gap interval reduces as a result of the increasing of conduction charge amounts. A novel strategy to facilely shift the peak position of absorbance and photoluminescence emission was therefore proposed.

나노 반도체 소자를 위한 펄스 플라즈마 식각 기술

As the size of the semiconductor devices shrinks to nanometer scale, the importance of plasma etching process to the fabrication of nanometer scale semiconductor devices is increasing further and further. But for the nanoscale devices, conventional plasma etching technique is extremely difficult to meet the requirement of the device fabrication, therefore, other etching techniques such as use of multi frequency plasma, source/bias/gas pulsing, etc. are investigated to meet the etching target. Until today, various pulsing techniques including pulsed plasma source and/or pulse-biased plasma etching have been tested on various materials. In this review, the experimental/theoretical studies of pulsed plasmas during the nanoscale plasma etching on etch profile, etch selectivity, uniformity, etc. have been summarized. Especially, the researches of pulsed plasma on the etching of silicon, <TEX>$SiO_2$</TEX>, and magnetic materials in the semiconductor industry for further device scaling have been discussed. Those results demonstrated the importance of pulse plasma on the pattern control for achieving the best performance. Although some of the pulsing mechanism is not well established, it is believed that this review will give a certain understanding on the pulsed plasma techniques.

Pinpointing the Cause of Platinum Tipping on CdS Nanorods

We computationally identify the precise mechanism by which metallic platinum aggregates at the tips of cadmium sulfide (CdS) nanostructures. Large-scale atomistic simulations of physically realistic nanorods are used to quantify the chemical, dispersive, and electrostatic contributions to platinum interaction with CdS. Crystallographic anisotropy as well as facet, edge, and tip effects are accounted for to show that Pt aggregation, known as “tipping”, is not due to the dynamics of adhesion and diffusion. Instead, efficient tipping is found to be due to long-range electrostatic interactions of metallic ions with polar tips set up by CdS surface stoichiometry. The results are used to stipulate the physical conditions by which metallic decoration of ionic nanostructures can be optimized. This is expected to be useful in the realization of nanoscale metal–semiconductor devices.

Quantum state tomography of large nuclear spins in a semiconductor quantum well: Optimal robustness against errors as quantified by condition numbers

We discuss methods of quantum state tomography for solid-state systems with a large nuclear spin $I=3/2$ in nanometer-scale semiconductors devices based on a quantum well. Due to quadrupolar interactions, the Zeeman levels of these nuclear-spin devices become nonequidistant, forming a controllable four-level quantum system (known as quartit or ququart). The occupation of these levels can be selectively and coherently manipulated by multiphoton transitions using the techniques of nuclear magnetic resonance (NMR) [Yusa et al., Nature (London) 434, 101 (2005)]. These methods are based on an unconventional approach to NMR, where the longitudinal magnetization $M_z$ is directly measured. This is in contrast to the standard NMR experiments and tomographic methods, where the transverse magnetization $M_{xy}$ is detected. The robustness against errors in the measured data is analyzed by using the condition number based on the spectral norm. We propose several methods with optimized sets of rotations yielding the highest robustness against errors, as described by the condition number equal to 1, assuming an ideal experimental detection. This robustness is only slightly deteriorated, as given by the condition number equal to 1.05, for a more realistic "noisy" $M_z$ detection based on the standard cyclically-ordered phase sequence (CYCLOPS) method.

Charge and Heat Transfer Mechanism in Directly Coupled CdSe–Metal Nanohybrids

The charge and heat energy transfer dynamics of directly coupled mixtures of CdSe quantum dots with metal nanoparticles have been studied using thermal lens and photoluminescence (PL) techniques, respectively. The PL of such nanohybrids is found to be quenched dramatically at a particular threshold. Fluorescence decay curves of the Au–CdSe nanohybrids and Ag–CdSe nanohybrids show distinct decay channels with the fastest one associated with transfer of electrons from the CdSe portion to the metal portion. A study on the influence of this charge transfer on the thermal diffusivity with respect to the emission wavelength of quantum dots has been carried out, which could lead to the design of modern photocatalysts and solar cells constructed from nanoscale metal–semiconductor hybrids.

Overcoated diamond tips for nanometer-scale semiconductor device characterization

Micromachined diamond tips have become the ultimate choice for the electrical probing of semiconductor devices at the nanometer scale because of the required high pressures in the GPa range. Although state-of-the-art full diamond tips (FDTs) show an ultra-high spatial resolution of 1nm, they are suffering from the limited electrical conductivity of the interfacial diamond layer resulting in an overall lower dynamic range. Conventional coated diamond tips (CDTs) on the other hand show a higher electrical conductivity but their core Si tips are prone to breaking off close to the apex due to the high lateral forces during scanning. Therefore, we developed in this work so-called overcoated diamond tips (ODTs) which are combining the advantages of the high mechanical stability of molded FDTs with the higher conductivity of CDTs. The key is the local underetching of a first molded diamond tip and the subsequent growth of a 50–150nm thin boron-doped diamond layer onto the interfacial diamond layer. The resulting ODT structure is attached to a metal cantilever and is used for electrical atomic force microscopy (AFM) measurements. The fabricated ODTs are mechanically stable and show a higher conductivity than FDTs. This work presents the probe and fabrication scheme, shows manufactured probe devices and demonstrates their performance in electrical AFM measurements.

Design of a Practical Nanometer-Scale Redundant Via-Aware Standard Cell Library for Improved Redundant Via1 Insertion Rate

Despite the rapid advances in process technology, via failure is still problematic in nanometer-scale semiconductor manufacturing. Adding redundant vias is a typical approach for improving yield and reliability. Cell-based design methodologies are widely adopted in the industry for application-specific integrated circuits. Standard cells are effective for increasing the insertion rate of redundant via1s in cell-based designs. This study proposes an efficient library check and staggered pin arrangement approach that compares redundant via1 insertion rate in different configurations such as double-via and rectangle-via. To compare the variability in standard cell (SC) libraries, accurate characterization results are provided. Moreover, the proposed SC library is easily implemented in all currently available routers. The experimental results reveal that the proposed library improves total inserted redundant vias, total inserted redundant via1s, and total run time by 20.2%, 51.9%, and 42.3%, respectively. In double-via pattern, the proposed approach improves average via1 insertion rate by 14.6%. In rectangle-via pattern, the proposed approach achieves a 100% via1 insertion rate.

High-Resolution Whole-MountIn SituHybridization Using Quantum Dot Nanocrystals

The photostability and narrow emission spectra of nanometer-scale semiconductor crystallites (QDs) make them desirable candidates for whole-mount fluorescent in situ hybridization to detect mRNA transcripts in morphologically preserved intact embryos. We describe a method for direct QD labeling of modified oligonucleotide probes through streptavidin-biotin and antibody-mediated interactions (anti-FITC and anti-digoxigenin). To overcome permeability issues and allow QD conjugate penetration, embryos were treated with proteinase K. The use of QDs dramatically increased sensitivity of whole-mount in situ hybridization (WISH) in comparison with organic fluorophores and enabled fluorescent detection of specific transcripts within cells without the use of enzymatic amplification. Therefore, this method offers significant advantages both in terms of sensitivity, as well as resolution. Specifically, the use of QDs alleviates issues of photostability and limited brightness plaguing organic fluorophores and allows fluorescent imaging of cleared embryos. It also offers new imaging possibilities, including intracellular localization of mRNAs, simultaneous multiple-transcript detection, and visualization of mRNA expression patterns in 3D.

Charge transfer and retention in directly coupled Au-CdSe nanohybrids

The energy and charge transfer dynamics of directly coupled Au-CdSe hybrid nanocrystals have been studied using time-resolved photoluminescence (PL) techniques. The PL of such nanohybrids was found to be quenched dramatically compared to that of both CdSe quantum dots and mixtures of CdSe quantum dots with Au nanoparticles. Fluorescence decay curves of the Au-CdSe nanohybrids show three distinct decay channels with the fastest one associated with the transfer of electrons from the CdSe portion to the Au portion. The holes on the CdSe portion created by such charge transfer were then quickly taken away by the solution, while the electrons on the Au portion slowly leaked into the solution as well, thus serving as a reductant for redox reactions. Using a model reaction based on the reduction of methylene blue by the leaking electrons, our photocatalytic experiments indicate that the electrons can be temporarily retained in the Au portion (most likely at the Au-capping agent interface) for a dramatically long timescale, up to 100 min. Finally, by merging all of the observations in the time-resolved PL measurements, we were able to figure out a relatively complete picture of charge transfer and retention in the Au-CdSe nanohybrids. This picture is expected to guide researchers in designing modern photocatalysts and solar cells constructed from nanoscale metal-semiconductor hybrids.

Imaging “Invisible” Dopant Atoms in Semiconductor Nanocrystals

Nanometer-scale semiconductors that contain a few intentionally added impurity atoms can provide new opportunities for controlling electronic properties. However, since the physics of these materials depends strongly on the exact arrangement of the impurities, or dopants, inside the structure, and many impurities of interest cannot be observed with currently available imaging techniques, new methods are needed to determine their location. We combine electron energy loss spectroscopy with annular dark-field scanning transmission electron microscopy (ADF-STEM) to image individual Mn impurities inside ZnSe nanocrystals. While Mn is invisible to conventional ADF-STEM in this host, our experiments and detailed simulations show consistent detection of Mn. Thus, a general path is demonstrated for atomic-scale imaging and identification of individual dopants in a variety of semiconductor nanostructures.

The Role of Particle Morphology in Interfacial Energy Transfer in CdSe/CdS Heterostructure Nanocrystals

Nanoscale semiconductor heterostructures such as tetrapods can be used to mimic light-harvesting processes. We used single-particle light-harvesting action spectroscopy to probe the impact of particle morphology on energy transfer and carrier relaxation across a heterojunction. The generic form of an action spectrum [in our experiments, photoluminescence excitation (PLE) under absorption in CdS and emission from CdSe in nanocrystal tetrapods, rods, and spheres] was controlled by the physical shape and resulting morphological variation in the quantum confinement parameters of the nanoparticle. A correlation between single-particle PLE and physical shape as determined by scanning electron microscopy was demonstrated. Such an analysis links local structural non-uniformities such as CdS bulbs forming around the CdSe core in CdSe/CdS nanorods to a lower probability of manifesting excitation energy-dependent emission spectra, which in turn is probably related to band alignment and electron delocalization at the heterojunction interface.

Photodeposition of Pt on Colloidal CdS and CdSe/CdS Semiconductor Nanostructures

Photodeposition of Pt on Colloidal CdS and CdSe/CdS Semiconductor Nanostructures Gordana Dukovic, Maxwell G. Merkle, James H. Nelson, Steven M. Hughes, and A. Paul Alivisatos* Department of Chemistry, University of California, Berkeley and Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA Semiconductor photocatalysis has been identified as a promising avenue for the conversion of solar energy into environmentally friendly fuels, most notably by the production of hydrogen from water.[1–5] Nanometer-scale materials in particular have attracted considerable scientific attention as the building blocks for light-harvesting applications.[6,7] Their desirable attributes include tunability of the optical properties with size, amenability to relatively inexpensive low-temperature processing, and a high degree of synthetic sophistication leading to increasingly complex and multi-functional architectures. For photocatalysis in particular, the high surface- to- volume ratios in nanoscale materials should lead to an increased availability of carriers for redox reactions on the nanoparticle surface. Recombination of photoexcited carriers directly competes with photocatalytic activity.[3] Charge separation is often achieved with multi-component heterostructures. An early example is the case of TiO2 powders functionalized with Pt and RuO2 particles, where photoexcited electrons are transferred to Pt (the reduction site) and holes to RuO2 (the oxidation site).[8] More recently, many colloidally synthesized nanometer-scale metal–semiconductor heterostructures have been reported.[7,9,10] A majority of these structures are made by thermal methods.[7,10] We have chosen to study photochemical formation of metal–semiconductor heterostructures. The detailed understanding of the mechanisms involved in photodeposition of metals on nanometer-scale semiconductors is necessary to enable a high degree of synthetic control. At the same time, because the results of metal deposition can be directly observed by electron microscopy, it can be used to understand how factors such as nanocrystal composition, shape, carrier dynamics, and surface chemistry influence the photochemical properties of semiconductor nanocrystals. In this communication, we report on the photodeposition of Pt on colloidal CdS and CdSe/CdS core/shell nanocrystals. Among the II–VI semiconductors, CdS is of particular interest because it has the correct band alignment for water photolysis[2] and has been demonstrated to be photocatalytically active.[11–16] We have found that the photoexcitation of CdS and CdSe/CdS in the presence of an organometallic Pt precursor leads to deposition of Pt nanoparticles on the semiconductor surface. Stark differences are observed in the Pt nanoparticle location on the two substrates, and the photodeposition can be completely inhibited by the modification of the semiconductor surface. Our results suggest that tuning of the semiconductor band structure, spatial organization and surface chemistry should be crucial in the design of photocatalytic nanostructures.

Numerical Study of Near-Infrared Photodetectors with Surface-Plasmon Antenna for Optical Communication

We propose an InP-based nanoscale metal–semiconductor–metal photodiode with a surface plasmon antenna composed of gold slit arrays for optical communication systems. Resonant modes of the surface plasmon antenna are numerically simulated by the finite-difference time-domain method. The calculation results suggest the advantage of the hybrid mode, which is produced by a Wood–Rayleigh anomaly mode and a vertical cavity mode. The hybrid mode enables a quantum efficiency of more than 50% when using a 250-nm-thick InGaAs absorption layer. A higher efficiency of about 95% can be achieved by combining a surface plasmon antenna with a distributed Bragg reflector consisting of 20 periods of quarter-wave InP/InGaAsP layers.

Ultrafast Electron Crystallography. 1. Nonequilibrium Dynamics of Nanometer-Scale Structures

In this paper, the first in a series, we give a full account of the development of ultrafast electron crystallography (UEC) and its applications in the study of nanometer-scale semiconductors and t...

Robust Read Channel System Directly Processing Asynchronous Sampling Data

In this study, we describe a robust read channel employing a novel timing recovery system and a unique Viterbi detector which extracts channel timing and channel data directly from asynchronous sampling data. The timing recovery system in the proposed read channel has feed-forward architecture and consists entirely of digital circuits. Thus, it enables robust timing recovery at high-speed and has no performance deterioration caused by variations in analog circuits. The Viterbi detector not only detects maximum-likelihood data using a reference level generator, but also transforms asynchronous data into pseudosynchronous data using two clocks, such as an asynchronous clock generated by a frequency synthesizer and a pseudosynchronous clock generated by a timing detector. The proposed read channel has achieved a constant and fast frequency acquisition time against initial frequency error and has improved its bit error rate performance. This robust read channel system can be used for high-speed signal processing and LSIs using nanometer-scale semiconductor processes.