Showing papers by "IBM published in 1998"

Quantum computation with quantum dots

Abstract: We propose an implementation of a universal set of one- and two-quantum-bit gates for quantum computation using the spin states of coupled single-electron quantum dots. Desired operations are effected by the gating of the tunneling barrier between neighboring dots. Several measures of the gate quality are computed within a recently derived spin master equation incorporating decoherence caused by a prototypical magnetic environment. Dot-array experiments that would provide an initial demonstration of the desired nonequilibrium spin dynamics are proposed.

Automatic subspace clustering of high dimensional data for data mining applications

Abstract: Data mining applications place special requirements on clustering algorithms including: the ability to find clusters embedded in subspaces of high dimensional data, scalability, end-user comprehensibility of the results, non-presumption of any canonical data distribution, and insensitivity to the order of input records. We present CLIQUE, a clustering algorithm that satisfies each of these requirements. CLIQUE identifies dense clusters in subspaces of maximum dimensionality. It generates cluster descriptions in the form of DNF expressions that are minimized for ease of comprehension. It produces identical results irrespective of the order in which input records are presented and does not presume any specific mathematical form for data distribution. Through experiments, we show that CLIQUE efficiently finds accurate cluster in large high dimensional datasets.

Electronic content delivery system

Abstract: Disclosed is a method and apparatus of securely providing data to a user's system. The data is encrypted so as to only be decryptable by a data decrypting key, the data decrypting key being encrypted using a first public key, and the encrypted data being accessible to the user's system, the method comprising the steps of: transferring the encrypted data decrypting key to a clearing house that possesses a first private key, which corresponds to the first public key; decrypting the data decrypting key using the first private key; re-encrypting the data decrypting key using a second public key; transferring the re-encrypted data decrypting key to the user's system, the user's system possessing a second private key, which corresponds to the second public key; and decrypting the re-encrypted data decrypting key using the second private key.

Decoherence-Free Subspaces for Quantum Computation

Abstract: Decoherence in quantum computers is formulated within the semigroup approach The error generators are identified with the generators of a Lie algebra This allows for a comprehensive description which includes as a special case the frequently assumed spin-boson model A generic condition is presented for errorless quantum computation: decoherence-free subspaces are spanned by those states which are annihilated by all the generators It is shown that these subspaces are stable to perturbations and, moreover, that universal quantum computation is possible within them

A security architecture for the Internet protocol

Abstract: In this paper we present the design, rationale, and implementation of a security architecture for protecting the secrecy and integrity of Internet traffic at the Internet Protocol (IP) layer. The design includes three components: (1) a security policy for determining when, where, and how security measures are to be applied; (2) a modular key management protocol, called MKMP, for establishing shared secrets between communicating parties and meta-information prescribed by the security policy; and (3) the IP Security Protocol, as it is being standardized by the Internet Engineering Task Force, for applying security measures using information provided through the key management protocol. Effectively, these three components together allow for the establishment of a secure channel between any two communicating systems over the Internet. This technology is a component of IBM's firewall product and is now being ported to other IBM computer platforms.

Efficiently mining long patterns from databases

Abstract: We present a pattern-mining algorithm that scales roughly linearly in the number of maximal patterns embedded in a database irrespective of the length of the longest pattern. In comparison, previous algorithms based on Apriori scale exponentially with longest pattern length. Experiments on real data show that when the patterns are long, our algorithm is more efficient by an order of magnitude or more.

A Practical Public Key Cryptosystem Provably Secure Against Adaptive Chosen Ciphertext Attack

Abstract: A new public key cryptosystem is proposed and analyzed. The scheme is quite practical, and is provably secure against adaptive chosen ciphertext attack under standard intractability assumptions. There appears to be no previous cryptosystem in the literature that enjoys both of these properties simultaneously.

Flocks, herds, and schools: A quantitative theory of flocking

Abstract: We present a quantitative continuum theory of ``flocking'': the collective coherent motion of large numbers of self-propelled organisms. In agreement with everyday experience, our model predicts the existence of an ``ordered phase'' of flocks, in which all members of even an arbitrarily large flock move together with the same mean velocity $〈\stackrel{\ensuremath{\rightarrow}}{v}〉\ensuremath{ e}0.$ This coherent motion of the flock is an example of spontaneously broken symmetry: no preferred direction for the motion is picked out a priori in the model; rather, each flock is allowed to, and does, spontaneously pick out some completely arbitrary direction to move in. By analyzing our model we can make detailed, quantitative predictions for the long-distance, long-time behavior of this ``broken symmetry state.'' The ``Goldstone modes'' associated with this ``spontaneously broken rotational symmetry'' are fluctuations in the direction of motion of a large part of the flock away from the mean direction of motion of the flock as a whole. These ``Goldstone modes'' mix with modes associated with conservation of bird number to produce propagating sound modes. These sound modes lead to enormous fluctuations of the density of the flock, far larger, at long wavelengths, than those in, e.g., an equilibrium gas. Our model is similar in many ways to the Navier-Stokes equations for a simple compressible fluid; in other ways, it resembles a relaxational time-dependent Ginsburg-Landau theory for an $n=d$ component isotropic ferromagnet. In spatial dimensions $dg4,$ the long-distance behavior is correctly described by a linearized theory, and is equivalent to that of an unusual but nonetheless equilibrium model for spin systems. For $dl4,$ nonlinear fluctuation effects radically alter the long distance behavior, making it different from that of any known equilibrium model. In particular, we find that in $d=2,$ where we can calculate the scaling exponents exactly, flocks exhibit a true, long-range ordered, spontaneously broken symmetry state, in contrast to equilibrium systems, which cannot spontaneously break a continuous symmetry in $d=2$ (the ``Mermin-Wagner'' theorem). We make detailed predictions for various correlation functions that could be measured either in simulations, or by quantitative imaging of real flocks. We also consider an anisotropic model, in which the birds move preferentially in an ``easy'' (e.g., horizontal) plane, and make analogous, but quantitatively different, predictions for that model as well. For this anisotropic model, we obtain exact scaling exponents for all spatial dimensions, including the physically relevant case $d=3.$

The object constraint language : precise modeling with UML

Abstract: (All chapters conclude with "Summary".) Foreword. Preface. Acknowledgments. Introduction. Who Should Read This Book. How This Book Should Be Used. Typeface Conventions. Information on Related Subjects. 1. Why Write Constraints? Definition of Constraint. Use of Constraints in Other Techniques. Design by Contract. Definition of Contract. Contents of a Contract. Advantages of Contracts. Preconditions and Postconditions. Invariants. Advantages of Constraints. Better Documentation. Improved Precision. Communication without Misunderstanding. Declarative or Operational Constraints. Advantages of a Declarative Language. Notation: Natural Language or Mathematical Expressions. Summary: Requirements for OCL. 2. OCL Basics. The "Royal and Loyal" System Example. Putting Invariants on Attributes. Putting Invariants on Associated Classes. Dealing with Collections of Objects. Sets, Bags, and Sequences. Inheritance. Working with Enumerations. Writing Preconditions and Postconditions. Where to Start Writing Invariants. Broken Constraints. Summary. 3. The Complete Overview of OCL Constructs. Types and Instances. Value Types and Object Types. OCL Expressions and OCL Constraints. The Context of an OCL Expression. The Context of an Invariant. The Context of a Pre- or Postcondition. The self Keyword. Basic Types and Operators. The Boolean Type. The Integer and Real Types. The String Type. Model Types. Attributes from the UML Model. Operations from the UML Model. Class Operations and Attributes from the UML Model. Associations and Aggregations from the UML Model. Association Classes from the UML Model. Qualified Associations from the UML Model. Using Package Names in Navigations. Using Pathnames in Inheritance Relations. Enumeration Types. The Set, Bag, and Sequence Types. Treating Instances as Collections. Flattening Collections. Operations on All Collection Types. Operations with Variant Meaning. Operations for the Set Type. Operations for the Sequence Type. Operations That Iterate over Collection Elements. The select Operation. The reject Operation. The collect Operation. Shorthand Notation for collect. The forAll Operation. The exists Operation. The iterate Operation. Constructs for Postconditions. Operations Defined on Every OCL Type. Types as Objects. Type Conformance Rules. Precedence Rules. Comments. Undefined. Summary. 4. Modeling with Constraints. Constraints in a UML Model. Invariants. Invariants for Derived Attributes or Associations. Preconditions and Postconditions. Guards in State Transition Diagrams. Using Guards and Events in Pre- and Postconditions. Change Events in State Transition Diagrams. Type Invariants for Stereotypes. Where OCL Expressions Can Be Used. Constraints and Inheritance. Styles for Specifying Constraints. Avoiding Complex Navigation Expressions. Choice of Context Object. Use of allInstances. Splitting and Constraints. Adding Extra Operations or Attributes. Using the collect Shorthand. Solving Modeling Issues with Constraints. Abstract Classes. Specifying Uniqueness Constraints. Adding Details to the Model versus Adding Constraints. Cycles in Class Models. Constraints on Associations. Multiplicity Constraints. The Subset Constraint. The Or Constraint. Optional Multiplicity in Associations. Summary. 5. Extending OCL. A Word of Caution. Extending the Standard OCL Types. Adding New OCL Types. Operational Use of Constraints. Generating Code for Constraints. When to Check Constraints. What to Do When the Constraint Fails. Summary. Appendix A. OCL Basic Types and Collection Types. Basic Types. OclType. OclAny. OclExpression. Real. Integer. String. Boolean. Enumeration. Collection-Related Types. Collection. Set. Bag. Sequence. Appendix B. Formal Grammar. Bibliography. Index. 0201379406T04062001

Latent semantic indexing: a probabilistic analysis

Abstract: Latent semantic indexing LSI is an information retrieval technique based on the spectral analysis of the term document matrix whose empirical success had heretofore been without rigorous prediction and explanation We prove that under certain conditions LSI does succeed in capturing the underlying semantics of the corpus and achieves improved retrieval performance We also propose the technique of random projection as a way of speeding up LSI We complement our theorems with encouraging experimental results We also argue that our results may be viewed in a more general framework as a theoretical basis for the use of spectral methods in a wider class of applications such as collaborative ltering

A practical public key cryptosystem provably secure against adaptive chosen ciphertext attack

Abstract: A new public key cryptosystem is proposed and analyzed. The scheme is quite practical, and is provably secure against adaptive chosen ciphertext attack under standard intractability assumptions. There appears to be no previous cryptosystem in the literature that enjoys both of these properties simultaneously.

The minimum description length principle in coding and modeling

Abstract: We review the principles of minimum description length and stochastic complexity as used in data compression and statistical modeling. Stochastic complexity is formulated as the solution to optimum universal coding problems extending Shannon's basic source coding theorem. The normalized maximized likelihood, mixture, and predictive codings are each shown to achieve the stochastic complexity to within asymptotically vanishing terms. We assess the performance of the minimum description length criterion both from the vantage point of quality of data compression and accuracy of statistical inference. Context tree modeling, density estimation, and model selection in Gaussian linear regression serve as examples.

Enhanced hypertext categorization using hyperlinks

Abstract: A major challenge in indexing unstructured hypertext databases is to automatically extract meta-data that enables structured search using topic taxonomies, circumvents keyword ambiguity, and improves the quality of search and profile-based routing and filtering. Therefore, an accurate classifier is an essential component of a hypertext database. Hyperlinks pose new problems not addressed in the extensive text classification literature. Links clearly contain high-quality semantic clues that are lost upon a purely term-based classifier, but exploiting link information is non-trivial because it is noisy. Naive use of terms in the link neighborhood of a document can even degrade accuracy. Our contribution is to propose robust statistical models and a relaxation labeling technique for better classification by exploiting link information in a small neighborhood around documents. Our technique also adapts gracefully to the fraction of neighboring documents having known topics. We experimented with pre-classified samples from Yahoo!1 and the US Patent Database2. In previous work, we developed a text classifier that misclassified only 13% of the documents in the well-known Reuters benchmark; this was comparable to the best results ever obtained. This classifier misclassified 36% of the patents, indicating that classifying hypertext can be more difficult than classifying text. Naively using terms in neighboring documents increased error to 38%; our hypertext classifier reduced it to 21%. Results with the Yahoo! sample were more dramatic: the text classifier showed 68% error, whereas our hypertext classifier reduced this to only 21%.

Damascene Copper electroplating for chip interconnections

Abstract: Damascene Cu electroplating for on-chip metallization, which we conceived and developed in the early 1990s, has been central to IBM's Cu chip interconnection technology. We review here the challenges of filling trenches and vias with Cu without creating a void or seam, and the discovery that electrodeposition can be engineered to give filling performance significantly better than that achievable with conformal step coverage. This attribute of superconformal deposition, which we call superfilling, and its relation to plating additives are discussed, and we present a numerical model that represents the shape-change behavior of this system.

The Enterprise Ontology

Abstract: This is a comprehensive description of the Enterprise Ontology, a collection of terms and definitions relevant to business enterprises. We state its intended purposes, describe how we went about building it, define all the terms and describe our experiences in converting these into formal definitions. We then describe how we used the Enterprise Ontology and give an evaluation which compares the actual uses with original purposes. We conclude by summarising what we have learned. The Enterprise Ontology was developed within the Enterprise Project, a collaborative effort to provide a framework for enterprise modelling. The ontology was built to serve as a basis for this framework which includes methods and a computer tool set for enterprise modelling. We give an overview of the Enterprise Project, elaborate on the intended use of the ontology, and give a brief overview of the process we went through to build it. The scope of the Enterprise Ontology covers those core concepts required for the project, which will appeal to a wider audience. We present natural language definitions for all the terms, starting with the foundational concepts (e.g. entity, relationship, actor). These are used to define the main body of terms, which are divided into the following subject areas: activities, organisation, strategy and marketing. We review some of the things learned during the formalisation process of converting the natural language definitions into Ontolingua. We identify and propose solutions for what may be general problems occurring in the development of a wide range of ontologies in other domains. We then characterise in general terms the sorts of issues that will be faced when converting an informal ontology into a formal one. Finally, we describe our experiences in using the Enterprise Ontology. We compare these with the intended uses, noting our successes and failures. We conclude with an overall evaluation and summary of what we have learned.

A portable information and transaction processing system and method utilizing biometric authorization and digital certificate security

Abstract: The present invention is a portable client PDA with a touch screen or other equivalent user interface and having a microphone and local central processing unit (CPU) for processing voice commands and for processing biometric data to provide user verification. The PDA also includes a memory for storing financial and personal information of the user and I/O capability for reading and writing information to various cards such as smartcards, magnetic cards, optical cards or EAROM cards. The PDA includes a Universal Card, which is common generic smartcard with a unique imprint provided by a service provider, on which selected financial or personal information stored in the PDA can be downloaded to perform certain consumer transactions. The PDA includes a modem, a serial port and/or a parallel port so as to provide direct communication capability with peripheral devices (such as POS and ATM terminals) and is capable of transmitting or receiving information through wireless communications such as radio frequency (RF) and infrared (IR) communication. The present invention is preferably operated in two modes, i.e., a client/server mode and a local mode.

Inferring Web communities from link topology

Abstract: The World Wide Web grows through a decentralized, almost anarchic process, and this has resulted in a large hyperlinked corpus without the kind of logical organization that can be built into more tradit,ionally-created hypermedia. To extract, meaningful structure under such circumstances, we develop a notion of hyperlinked communities on the www t,hrough an analysis of the link topology. By invoking a simple, mathematically clean method for defining and exposing the structure of these communities, we are able to derive a number of themes: The communities can be viewed as containing a core of central, “authoritative” pages linked togh and they exhibit a natural type of hierarchical topic generalization that can be inferred directly from the pat,t,ern of linkage. Our investigation shows that although the process by which users of the Web create pages and links is very difficult to understand at a “local” level, it results in a much greater degree of orderly high-level structure than has typically been assumed.

Automatic resource compilation by analyzing hyperlink structure and associated text

Abstract: We describe the design, prototyping and evaluation of ARC, a system for automatically compiling a list of authoritative Web resources on any (sufficiently broad) topic. The goal of ARC is to compile resource lists similar to those provided by Yahoo! or Infoseek. The fundamental difference is that these services construct lists either manually or through a combination of human and automated effort, while ARC operates fully automatically. We describe the evaluation of ARC, Yahoo!, and Infoseek resource lists by a panel of human users. This evaluation suggests that the resources found by ARC frequently fare almost as well as, and sometimes better than, lists of resources that are manually compiled or classified into a topic. We also provide examples of ARC resource lists for the reader to examine.

Mining process models from workflow logs

Abstract: Modern enterprises increasingly use the workflow paradigm to prescribe how business processes should be performed. Processes are typically modeled as annotated activity graphs. We present an approach for a system that constructs process models from logs of past, unstructured executions of the given process. The graph so produced conforms to the dependencies and put executions present in the log. By providing models that capture the previous executions of the process, this technique allows easier introduction of a workflow system and evaluation and evolution of existing process models. We also present results from applying the algorithm to synthetic data sets as well as process logs obtained from an IBM Flowmark installation.

The random oracle methodology, revisited (preliminary version)

Abstract: WC take a formal look at the relationship between the security of cryptographic schemes in the Random Oracle Model, and the security of the schemes which result from implementing the random oracle by so called “cryptographic hash functionon, The main result of this paper is a negative one: There exist signature and encryption schemes which are secure in the Random Oracle Model, but for which any implementation of the random oracle results in insecure schemes. In the process of devising the above schemes, me consider possible definitions for the notion of a “good implementatlon” of a random oracle, pointing out limitations and challenges,

Display screen and window size related web page adaptation system

Abstract: A web page adaptation system and method provides organization of viewing material associated with web sites for visual displays and windows on which these home pages are being viewed. A different viewing-access strategy is provided for such visual devices varying, for example, from standard PC monitors, laptop screens and palmtops to web phone and digital camera displays and from large windows to small windows. A new web site design incorporates features that permit automatic display of the content of home pages in the most friendly manner for a user viewing this content from a screen or window of a certain size. For example, if a size of a display screen or window allows, links are displayed with some text or pictures to which they are linked. Conversely, if a size of a screen or window does not allow display of all textual and icon information on a whole screen or window, the home page is mapped into hierarchically linked new smaller pages that fully fit the current display or window. The unique display strategy of the invention is provided by a web page adaptation scheme that is implemented on a web site server or is incorporated in a web browser (e.g., as a java appelet) or both. This adaptation strategy employs variables that provide size of screen and/or window information from which a call to a web site was initiated.

Mining Process Models from Workflow Logs

Abstract: Modern enterprises increasingly use the workflow paradigm to prescribe how business processes should be performed. Processes are typically modeled as annotated activity graphs. We present an approach for a system that constructs process models from logs of past, unstructured executions of the given process. The graph so produced conforms to the dependencies and past executions present in the log. By providing models that capture the previous executions of the process, this technique allows easier introduction of a workflow system and evaluation and evolution of existing process models. We also present results from applying the algorithm to synthetic data sets as well as process logs obtained from an IBM Flowmark installation.

Geometric compression through topological surgery

Abstract: The abundance and importance of complex 3-D data bases in major industry segments, the affordability of interactive 3-D rendering for office and consumer use, and the exploitation of the Internet to distribute and share 3-D data have intensified the need for an effective 3-D geometric compression technique that would significantly reduce the time required to transmit 3-D models over digital communication channels, and the amount of memory or disk space required to store the models. Because the prevalent representation of 3-D models for graphics purposes is polyhedral and because polyhedral models are in general triangulated for rendering, this article introduces a new compressed representation for complex triangulated models and simple, yet efficient, compression and decompression algorithms. In this scheme, vertex positions are quantized within the desired accuracy, a vertex spanning tree is used to predict the position of each vertex from 2,3, or 4 of its ancestors in the tree, and the correction vectors are entropy encoded. Properties, such as normals, colors, and texture coordinates, are compressed in a similar manner. The connectivity is encoded with no loss of information to an average of less than two bits per triangle. The vertex spanning tree and a small set of jump edges are used to split the model into a simple polygon. A triangle spanning tree and a sequence of marching bits are used to encode the triangulation of the polygon. Our approach improves on Michael Deering's pioneering results by exploiting the geometric coherence of several ancestors in the vertex spanning tree, preserving the connectivity with no loss of information, avoiding vertex repetitions, and using about three fewer bits for the connectivity. However, since decompression requires random access to all vertices, this method must be modified for hardware rendering with limited onboard memory. Finally, we demonstrate implementation results for a variety of VRML models with up to two orders of magnitude compression.

Deformation of carbon nanotubes by surface van der Waals forces

Abstract: The strength and effect of surface van der Waals forces on the shape of multiwalled and single-walled carbon nanotubes is investigated using atomic-force microscopy, continuum mechanics, and molecular-mechanics simulations. Our calculations show that depending on the tube diameter and number of shells, the van der Waals interaction between nanotubes and a substrate results in high binding energies, which has also been determined experimentally. Nanotubes on a substrate may consequently experience radial and axial deformations, which significantly modify the idealized geometry of free nanotubes. These findings have implications for electronic transport and the tribological properties of adsorbed nanotubes.

Locality-aware request distribution in cluster-based network servers

Abstract: We consider cluster-based network servers in which a front-end directs incoming requests to one of a number of back-ends. Specifically, we consider content-based request distribution: the front-end uses the content requested, in addition to information about the load on the back-end nodes, to choose which back-end will handle this request. Content-based request distribution can improve locality in the back-ends' main memory caches, increase secondary storage scalability by partitioning the server's database, and provide the ability to employ back-end nodes that are specialized for certain types of requests.As a specific policy for content-based request distribution, we introduce a simple, practical strategy for locality-aware request distribution (LARD). With LARD, the front-end distributes incoming requests in a manner that achieves high locality in the back-ends' main memory caches as well as load balancing. Locality is increased by dynamically subdividing the server's working set over the back-ends. Trace-based simulation results and measurements on a prototype implementation demonstrate substantial performance improvements over state-of-the-art approaches that use only load information to distribute requests. On workloads with working sets that do not fit in a single server node's main memory cache, the achieved throughput exceeds that of the state-of-the-art approach by a factor of two to four.With content-based distribution, incoming requests must be handed off to a back-end in a manner transparent to the client, after the front-end has inspected the content of the request. To this end, we introduce an efficient TCP handoflprotocol that can hand off an established TCP connection in a client-transparent manner.

High-aspect-ratio, ultrathick, negative-tone near-UV photoresist and its applications for MEMS

Abstract: Detailed investigations of the limits of a new negative-tone near-UV resist (IBM SU-8) have been performed. SU-8 is an epoxy-based resist designed specifically for ultrathick, high-aspect-ratio MEMS-type applications. We have demonstrated that with single-layer coatings, thicknesses of more than 500 μm can be achieved reproducibly. Thicker resist layers can be made by applying multiple coatings, and we have achieved exposures in 1200 μm thick, double-coated SU-8 resist layers. We have found that the aspect ratio for near-UV (400 nm) exposed and developed structures can be greater than 18 and remains constant in the thickness range between 80 and 1200 μm. Vertical sidewall profiles result in good dimensional control over the entire resist thickness. To our knowledge, this is the highest aspect ratio reported for near-UV exposures and the given range of resist thicknesses. These results will open up new possibilities for low-cost LIGA-type processes for MEMS applications. The application potential of SU-8 is demonstrated by several examples of devices and structures fabricated by electroplating and photoplastic techniques. The latter is especially interesting as SU-8 has attractive mechanical properties.

Combinatorial pattern discovery in biological sequences: The TEIRESIAS algorithm.

Abstract: Motivation The discovery of motifs in biological sequences is an important problem. Results This paper presents a new algorithm for the discovery of rigid patterns (motifs) in biological sequences. Our method is combinatorial in nature and able to produce all patterns that appear in at least a (user-defined) minimum number of sequences, yet it manages to be very efficient by avoiding the enumeration of the entire pattern space. Furthermore, the reported patterns are maximal: any reported pattern cannot be made more specific and still keep on appearing at the exact same positions within the input sequences. The effectiveness of the proposed approach is showcased on a number of test cases which aim to: (i) validate the approach through the discovery of previously reported patterns; (ii) demonstrate the capability to identify automatically highly selective patterns particular to the sequences under consideration. Finally, experimental analysis indicates that the algorithm is output sensitive, i.e. its running time is quasi-linear to the size of the generated output.

Experimental Implementation of Fast Quantum Searching

Abstract: Using nuclear magnetic resonance techniques with a solution of chloroform molecules we implement Grover's search algorithm for a system with four states. By performing a tomographic reconstruction of the density matrix during the computation good agreement is seen between theory and experiment. This provides the first complete experimental demonstration of loading an initial state into a quantum computer, performing a computation requiring fewer steps than on a classical computer, and then reading out the final state.

Electrical characteristics and efficiency of single-layer organic light-emitting diodes

Abstract: We have measured the electrical characteristics and the efficiencies of single-layer organic light-emitting diodes based on poly[2-methoxy-5-(2-ethylhexoxy)-1,4-phenylene vinylene] (MEH-PPV), with Au anodes and Ca, Al, and Au cathodes. We show that proper accounting of the built-in potential leads to a consistent description of the current-voltage data. For the case of Au and Al cathodes, the current under forward bias is dominated by holes injected from the anode and is space-charge limited with a field-dependent hole mobility. The Ca cathode is capable of injecting a space-charge-limited electron current.

Coarsening of Self-Assembled Ge Quantum Dots on Si(001)

Abstract: The size distribution of self-assembled heteroepitaxial islands is critical to their application as quantum dots in novel devices. In situ, real time UHV transmission electron microscopy studies of Ge island growth on Si(001) show that island coarsening occurs even during growth. With increasing volume, a shape transition from pyramids to domes gives rise to an abrupt change in chemical potential. This leads to a bifurcation in the size distribution and ultimately to a narrow size range. Simulations of coarsening in the presence of a shape transition are in good agreement with experimental results.