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Applied Physics Reviews — 2003

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Wide band gap ferromagnetic semiconductors and oxides

S. J. Pearton, C. R. Abernathy, M. E. Overberg, G. T. Thaler, D. P. Norton, N. Theodoropoulou, A. F. Hebard, Y. D. Park, F. Ren, J. Kim, and L. A. Boatner

J. Appl. Phys. 93, 1 (2003); http://dx.doi.org/10.1063/1.1517164 (13 pages)

Online Publication Date: 23 December 2002

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Recent advances in the theory and experimental realization of ferromagnetic semiconductors give hope that a new generation of microelectronic devices based on the spin degree of freedom of the electron can be developed. This review focuses primarily on promising candidate materials (such as GaN, GaP and ZnO) in which there is already a technology base and a fairly good understanding of the basic electrical and optical properties. The introduction of Mn into these and other materials under the right conditions is found to produce ferromagnetism near or above room temperature. There are a number of other potential dopant ions that could be employed (such as Fe, Ni, Co, Cr) as suggested by theory [see, for example, Sato and Katayama-Yoshida, Jpn. J. Appl. Phys., Part 2 39, L555 (2000)]. Growth of these ferromagnetic materials by thin film techniques, such as molecular beam epitaxy or pulsed laser deposition, provides excellent control of the dopant concentration and the ability to grow single-phase layers. The mechanism for the observed magnetic behavior is complex and appears to depend on a number of factors, including Mn–Mn spacing, and carrier density and type. For example, in a simple Ruderman–Kittel–Kasuya–Yosida carrier-mediated exchange mechanism, the free-carrier/Mn ion interaction can be either ferromagnetic or antiferromagnetic depending on the separation of the Mn ions. Potential applications for ferromagnetic semiconductors and oxides include electrically controlled magnetic sensors and actuators, high-density ultralow-power memory and logic, spin-polarized light emitters for optical encoding, advanced optical switches and modulators and devices with integrated magnetic, electronic and optical functionality. © 2003 American Institute of Physics.
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75.70.Ak Magnetic properties of monolayers and thin films
75.50.Pp Magnetic semiconductors
75.50.Dd Nonmetallic ferromagnetic materials
01.30.Rr Surveys and tutorial papers; resource letters
73.61.Ey III-V semiconductors
81.05.Ea III-V semiconductors
71.20.Nr Semiconductor compounds
68.55.-a Thin film structure and morphology
81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.15.Fg Pulsed laser ablation deposition
75.30.Et Exchange and superexchange interactions
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Nanoscale thermal transport

David G. Cahill, Wayne K. Ford, Kenneth E. Goodson, Gerald D. Mahan, Arun Majumdar, Humphrey J. Maris, Roberto Merlin, and Simon R. Phillpot

J. Appl. Phys. 93, 793 (2003); http://dx.doi.org/10.1063/1.1524305 (26 pages)

Online Publication Date: 27 December 2002

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Rapid progress in the synthesis and processing of materials with structure on nanometer length scales has created a demand for greater scientific understanding of thermal transport in nanoscale devices, individual nanostructures, and nanostructured materials. This review emphasizes developments in experiment, theory, and computation that have occurred in the past ten years and summarizes the present status of the field. Interfaces between materials become increasingly important on small length scales. The thermal conductance of many solid–solid interfaces have been studied experimentally but the range of observed interface properties is much smaller than predicted by simple theory. Classical molecular dynamics simulations are emerging as a powerful tool for calculations of thermal conductance and phonon scattering, and may provide for a lively interplay of experiment and theory in the near term. Fundamental issues remain concerning the correct definitions of temperature in nonequilibrium nanoscale systems. Modern Si microelectronics are now firmly in the nanoscale regime—experiments have demonstrated that the close proximity of interfaces and the extremely small volume of heat dissipation strongly modifies thermal transport, thereby aggravating problems of thermal management. Microelectronic devices are too large to yield to atomic-level simulation in the foreseeable future and, therefore, calculations of thermal transport must rely on solutions of the Boltzmann transport equation; microscopic phonon scattering rates needed for predictive models are, even for Si, poorly known. Low-dimensional nanostructures, such as carbon nanotubes, are predicted to have novel transport properties; the first quantitative experiments of the thermal conductivity of nanotubes have recently been achieved using microfabricated measurement systems. Nanoscale porosity decreases the permittivity of amorphous dielectrics but porosity also strongly decreases the thermal conductivity. The promise of improved thermoelectric materials and problems of thermal management of optoelectronic devices have stimulated extensive studies of semiconductor superlattices; agreement between experiment and theory is generally poor. Advances in measurement methods, e.g., the 3ω method, time-domain thermoreflectance, sources of coherent phonons, microfabricated test structures, and the scanning thermal microscope, are enabling new capabilities for nanoscale thermal metrology. © 2003 American Institute of Physics.
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65.80.-g Thermal properties of small particles, nanocrystals, nanotubes, and other related systems
63.22.-m Phonons or vibrational states in low-dimensional structures and nanoscale materials
01.30.Rr Surveys and tutorial papers; resource letters
68.65.Cd Superlattices
68.35.Ja Surface and interface dynamics and vibrations
72.20.Pa Thermoelectric and thermomagnetic effects
63.20.K- Phonon interactions
78.20.N- Thermo-optic effects
78.20.nb Photothermal effects
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Physics and materials challenges for lead-free solders

K. N. Tu, A. M. Gusak, and M. Li

J. Appl. Phys. 93, 1335 (2003); http://dx.doi.org/10.1063/1.1517165 (19 pages)

Online Publication Date: 6 February 2003

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At present, the electronic industry is actively searching for Pb-free solders due to environmental concerns over Pb-containing solders. Solder joints are widely used to bond chips to their substrates for electrical connection and packaging. Lacking reliability data, many electronic companies will be reluctant to adopt Pb-free solders in the advanced products. Hence, it is timely to review our understanding of structure-property relationship and potential reliability issues of Pb-free solders. A brief history of solder joint processes in electronic manufacturing is presented to serve as a background for the review. It emphasizes the unique phenomenon of spalling of interfacial intermetallic compound in solder reactions. Challenges for Pb-free solders from the point of view of physics and materials are given since the reliability issues of solder joints will remain with us when advanced Cu/low k dielectric interconnect technology is introduced into microelectronic devices. © 2003 American Institute of Physics.
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85.40.Qx Microcircuit quality, noise, performance, and failure analysis
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Vibrational lifetimes of hydrogen in silicon

G. Lüpke, N. H. Tolk, and L. C. Feldman

J. Appl. Phys. 93, 2317 (2003); http://dx.doi.org/10.1063/1.1517166 (20 pages)

Online Publication Date: 4 March 2003

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Characterization of defect and impurity reactions, dissociation, and migration in semiconductors requires a detailed understanding of the rates and pathways of vibrational energy flow, of the energy transfer channels, and of the coupling mechanisms between local modes and the phonon bath of the host material. Significant progress in reaching this goal has been accomplished in recent landmark studies exploring the excitation and dynamics of vibrational states associated with hydrogen in silicon. The lifetime of the Si–H stretch mode is found to be extremely dependent on the local solid-state structure, ranging from picoseconds for interstitial-like hydrogen, hundreds of picoseconds for hydrogen–vacancy complexes, to several nanoseconds for hydrogen bonded to Si surfaces—over three orders of magnitude variation. Such large variations in lifetime (transition probability) are extraordinarily rare in solid-state science. The level of theoretical investigation into the vibrational lifetime of the Si–H oscillator is less advanced. This state of affairs is partly because of the difficulties in explicitly treating slow relaxation processes in complex systems, and partly because, as suggested by experiment, a highly anharmonic coupling mechanism is apparently responsible for the (multiphonon) relaxation process. Even more importantly, because of the high frequency of the Si–H stretching motion, a quantum mechanical treatment of the Si–H oscillator is required. A combination of Bloch–Redfield theory and molecular dynamics simulation seems promising in describing the relaxation process of the Si–H vibrational modes. It is the aim of this review article to present a comprehensive overview of the recent accomplishments, current understandings, and future directions in this emerging field of time-resolved vibrational spectroscopy of point defects in solids. © 2003 American Institute of Physics.
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63.20.Pw Localized modes
61.72.J- Point defects and defect clusters
63.20.Ry Anharmonic lattice modes
01.30.Rr Surveys and tutorial papers; resource letters
61.72.Yx Interaction between different crystal defects; gettering effect
63.20.K- Phonon interactions
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Small molecular weight organic thin-film photodetectors and solar cells

Peter Peumans, Aharon Yakimov, and Stephen R. Forrest

J. Appl. Phys. 93, 3693 (2003); http://dx.doi.org/10.1063/1.1534621 (31 pages)

Online Publication Date: 21 March 2003

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In this review, we discuss the physics underlying the operation of single and multiple heterojunction, vacuum-deposited organic solar cells based on small molecular weight thin films. For single heterojunction cells, we find that the need for direct contact between the deposited electrode and the active organics leads to quenching of excitons. An improved device architecture, the double heterojunction, is shown to confine excitons within the active layers, allowing substantially higher internal efficiencies to be achieved. A full optical and electrical analysis of the double heterostructure architecture leads to optimal cell design as a function of the optical properties and exciton diffusion lengths of the photoactive materials. Combining the double heterostructure with novel light trapping schemes, devices with external efficiencies approaching their internal efficiency are obtained. When applied to an organic photovoltaic cell with a power conversion efficiency of 1.0%±0.1% under 1 sun AM1.5 illumination, devices with external power conversion efficiencies of 2.4%±0.3% are reported. In addition, we show that by using materials with extended exciton diffusion lengths LD, highly efficient double heterojunction photovoltaic cells are obtained, even in the absence of a light trapping geometry. Using C60 as an acceptor material, double heterostructure external power conversion efficiencies of 3.6%±0.4% under 1 sun AM1.5 illumination are obtained. Stacking of single heterojunction devices leads to thin film multiple heterojunction photovoltaic and photodetector structures. Thin bilayer photovoltaic cells can be stacked with ultrathin ( ∼ 5 Å), discontinuous Ag layers between adjacent cells serving as efficient recombination sites for electrons and holes generated in the neighboring cells. Such stacked cells have open circuit voltages that are n times the open circuit voltage of a single cell, where n is the number of cells in the stack. In optimized structures, the short circuit photocurrent remains approximately constant upon stacking thin cells, leading to higher achievable power conversion efficiencies, as confirmed by modelling optical interference effects and exciton migration. A 2.5%±0.3% power efficiency under 100 mW/cm2 AM1.5 illumination conditions is obtained by stacking two ∼ 1% efficient devices. Alternatively, when the contact layers between the stacked cells are eliminated, a multilayer structure consisting of alternating films of donor and acceptor-type materials is obtained. Since the thicknesses of the individual layers ( ∼ 5 Å) can be substantially smaller than the exciton diffusion length, nearly 100% of the photogenerated excitons are dissociated, and the resulting free charges are detected. In addition, the ultrathin organic layers facilitate electron and hole transport through the multilayer stack by tunneling. When these devices are operated as photodetectors under applied fields >106 V/cm, the carrier collection efficiency reaches 80%, leading to external quantum efficiencies of 75%±1% across the visible spectrum in cells containing the thinnest layers. We find that due to the fast carrier tunneling process, the temporal response of these multilayer detectors is a direct measure of exciton dynamics. Response times of 720±50 ps are achieved, leading to a 3 dB bandwidth of 430±30 MHz. A summary of representative results obtained for both polymer and small molecule photovoltaic cells and photodetectors is included in this review. Prospects for further improvements in organic solar cells and photodetectors are considered. © 2003 American Institute of Physics.
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84.60.Jt Photoelectric conversion
85.60.Gz Photodetectors (including infrared and CCD detectors)
73.61.Ph Polymers; organic compounds
71.35.-y Excitons and related phenomena
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Quantum well photoconductors in infrared detector technology

A. Rogalski

J. Appl. Phys. 93, 4355 (2003); http://dx.doi.org/10.1063/1.1558224 (37 pages)

Online Publication Date: 28 March 2003

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The paper compares the achievements of quantum well infrared photodetector (QWIP) technology with those of competitive technologies, with the emphasis on the material properties, device structure, and their impact on focal plane array (FPA) performance. Special attention is paid to two competitive technologies, QWIP and HgCdTe, in the long-wavelength IR (LWIR) and very-long-wavelength IR (VLWIR) spectral ranges. Because so far, the dialogue between the QWIP and HgCdTe communities is limited, the paper attempts to settle the main issues of both technologies. Such an approach, however, requires the presentation of fundamental limits to the different types of detectors, which is made at the beginning. To write the paper more clearly for readers, many details are included in the Appendix. In comparative studies both photon and thermal detectors are considered. Emphasis is placed on photon detectors. In this group one may distinguish HgCdTe photodiodes, InSb photodiodes, and doped silicon detectors. The potential performance of different materials as infrared detectors is examined utilizing the α/G ratio, where α is the absorption coefficient and G is the thermal generation rate. It is demonstrated that LWIR QWIP’s cannot compete with HgCdTe photodiodes as single devices, especially at higher operating temperatures (>70 K). This is due to the fundamental limitations associated with intersubband transitions. The advantage of HgCdTe is, however, less distinct at temperatures lower than 50 K due to problems inherent in the HgCdTe material (p-type doping, Shockley–Read recombination, trap-assisted tunneling, surface and interface instabilities). Even though QWIP is a photoconductor, several of its properties, such as high impedance, fast response time, long integration time, and low power consumption, comply well with the requirements imposed on the fabrication of large FPA’s. Due to a high material quality at low temperatures, QWIP has potential advantages over HgCdTe in the area of VLWIR FPA applications in terms of array size, uniformity, yield, and cost of the systems. The performance figures of merit of state-of-the-art QWIP and HgCdTe FPA’s are similar because the main limitations come from the readout circuits. Performance is, however, achieved with very different integration times. The choice of the best technology is therefore driven by the specific needs of a system. In the case of readout-limited detectors a low photoconductive gain increases the signal-to-noise ratio and a QWIP FPA can have a better noise equivalent difference temperature than an HgCdTe FPA with a charge well of similar size. Both HgCdTe photodiodes and QWIP’s offer multicolor capability in the MWIR and LWIR range. Powerful possibilities offered by QWIP technology are associated with VLWIR FPA applications and with multicolor detection. The intrinsic advantage of QWIP’s in this niche is due to the relative ease of growing multicolor structures with a very low defect density. © 2003 American Institute of Physics.
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85.60.Gz Photodetectors (including infrared and CCD detectors)
85.35.Be Quantum well devices (quantum dots, quantum wires, etc.)
85.60.Dw Photodiodes; phototransistors; photoresistors
01.30.Rr Surveys and tutorial papers; resource letters
07.57.Kp Bolometers; infrared, submillimeter wave, microwave, and radiowave receivers and detectors
73.63.Hs Quantum wells
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Frontiers of silicon-on-insulator

G. K. Celler and Sorin Cristoloveanu

J. Appl. Phys. 93, 4955 (2003); http://dx.doi.org/10.1063/1.1558223 (24 pages)

Online Publication Date: 16 April 2003

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Silicon-on-insulator (SOI) wafers are precisely engineered multilayer semiconductor/dielectric structures that provide new functionality for advanced Si devices. After more than three decades of materials research and device studies, SOI wafers have entered into the mainstream of semiconductor electronics. SOI technology offers significant advantages in design, fabrication, and performance of many semiconductor circuits. It also improves prospects for extending Si devices into the nanometer region (<10 nm channel length). In this article, we discuss methods of forming SOI wafers, their physical properties, and the latest improvements in controlling the structure parameters. We also describe devices that take advantage of SOI, and consider their electrical characteristics. © 2003 American Institute of Physics.
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85.40.-e Microelectronics: LSI, VLSI, ULSI; integrated circuit fabrication technology
85.30.Tv Field effect devices
73.40.Qv Metal-insulator-semiconductor structures (including semiconductor-to-insulator)
01.30.Rr Surveys and tutorial papers; resource letters
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Low dielectric constant materials for microelectronics

K. Maex, M. R. Baklanov, D. Shamiryan, F. lacopi, S. H. Brongersma, and Z. S. Yanovitskaya

J. Appl. Phys. 93, 8793 (2003); http://dx.doi.org/10.1063/1.1567460 (49 pages)

Online Publication Date: 19 May 2003

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The ever increasing requirements for electrical performance of on-chip wiring has driven three major technological advances in recent years. First, copper has replaced Aluminum as the new interconnect metal of choice, forcing also the introduction of damascene processing. Second, alternatives for SiO2 with a lower dielectric constant are being developed and introduced in main stream processing. The many new resulting materials needs to be classified in terms of their materials characteristics, evaluated in terms of their properties, and tested for process compatibility. Third, in an attempt to lower the dielectric constant even more, porosity is being introduced into these new materials. The study of processes such as plasma interactions and swelling in liquid media now becomes critical. Furthermore, pore sealing and the deposition of a thin continuous copper diffusion barrier on a porous dielectric are of prime importance. This review is an attempt to give an overview of the classification, the characteristics and properties of low-k dielectrics. In addition it addresses some of the needs for improved metrology for determining pore sizes, size distributions, structure, and mechanical properties. © 2003 American Institute of Physics.
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85.40.Ls Metallization, contacts, interconnects; device isolation
77.55.-g Dielectric thin films
77.22.Ch Permittivity (dielectric function)
85.40.Sz Deposition technology
61.43.Gt Powders, porous materials
81.05.Rm Porous materials; granular materials
68.35.Fx Diffusion; interface formation
01.30.Rr Surveys and tutorial papers; resource letters
68.60.Bs Mechanical and acoustical properties

Negative bias temperature instability: Road to cross in deep submicron silicon semiconductor manufacturing

Dieter K. Schroder and Jeff A. Babcock

J. Appl. Phys. 94, 1 (2003); http://dx.doi.org/10.1063/1.1567461 (18 pages)

Online Publication Date: 20 June 2003

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We present an overview of negative bias temperature instability (NBTI) commonly observed in p-channel metal–oxide–semiconductor field-effect transistors when stressed with negative gate voltages at elevated temperatures. We discuss the results of such stress on device and circuit performance and review interface traps and oxide charges, their origin, present understanding, and changes due to NBTI. Next we discuss the effects of varying parameters (hydrogen, deuterium, nitrogen, nitride, water, fluorine, boron, gate material, holes, temperature, electric field, and gate length) on NBTI. We conclude with the present understanding of NBTI and its minimization. © 2003 American Institute of Physics.
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85.30.Tv Field effect devices
85.40.-e Microelectronics: LSI, VLSI, ULSI; integrated circuit fabrication technology
01.30.Rr Surveys and tutorial papers; resource letters
81.05.Cy Elemental semiconductors
85.30.De Semiconductor-device characterization, design, and modeling

Indium nitride (InN): A review on growth, characterization, and properties

Ashraful Ghani Bhuiyan, Akihiro Hashimoto, and Akio Yamamoto

J. Appl. Phys. 94, 2779 (2003); http://dx.doi.org/10.1063/1.1595135 (30 pages)

Online Publication Date: 19 August 2003

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During the last few years the interest in the indium nitride (InN) semiconductor has been remarkable. There have been significant improvements in the growth of InN films. High quality single crystalline InN film with two-dimensional growth and high growth rate are now routinely obtained. The background carrier concentration and Hall mobility have also improved. Observation of strong photoluminescence near the band edge is reported very recently, leading to conflicts concerning the exact band gap of InN. Attempts have also been made on the deposition of InN based heterostructures for the fabrication of InN based electronic devices. Preliminary evidence of two-dimensional electron gas accumulation in the InN and studies on InN-based field-effect transistor structure are reported. In this article, the work accomplished in the InN research, from its evolution to till now, is reviewed. The In containing alloys or other nitrides (AlGaInN, GaN, AlN) are not discussed here. We mainly concentrate on the growth, characterization, and recent developments in InN research. The most popular growth techniques, metalorganic vapor phase epitaxy and molecular beam epitaxy, are discussed in detail with their recent progress. Important phenomena in the epitaxial growth of InN as well as the problems remaining for future study are also discussed. © 2003 American Institute of Physics.  
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81.15.Hi Molecular, atomic, ion, and chemical beam epitaxy
81.15.Gh Chemical vapor deposition (including plasma-enhanced CVD, MOCVD, ALD, etc.)
81.15.Kk Vapor phase epitaxy; growth from vapor phase
68.55.A- Nucleation and growth
73.61.Ey III-V semiconductors
78.55.Cr III-V semiconductors
78.66.Fd III-V semiconductors
81.05.Ea III-V semiconductors
01.30.Rr Surveys and tutorial papers; resource letters
72.20.Ee Mobility edges; hopping transport
72.20.My Galvanomagnetic and other magnetotransport effects
78.20.Ci Optical constants (including refractive index, complex dielectric constant, absorption, reflection and transmission coefficients, emissivity)
85.30.Tv Field effect devices
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Band parameters for nitrogen-containing semiconductors

I. Vurgaftman and J. R. Meyer

J. Appl. Phys. 94, 3675 (2003); http://dx.doi.org/10.1063/1.1600519 (22 pages)

Online Publication Date: 29 August 2003

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We present a comprehensive and up-to-date compilation of band parameters for all of the nitrogen-containing III–V semiconductors that have been investigated to date. The two main classes are: (1) “conventional” nitrides (wurtzite and zinc-blende GaN, InN, and AlN, along with their alloys) and (2) “dilute” nitrides (zinc-blende ternaries and quaternaries in which a relatively small fraction of N is added to a host III–V material, e.g., GaAsN and GaInAsN). As in our more general review of III–V semiconductor band parameters [I. Vurgaftman et al., J. Appl. Phys. 89, 5815 (2001)], complete and consistent parameter sets are recommended on the basis of a thorough and critical review of the existing literature. We tabulate the direct and indirect energy gaps, spin-orbit and crystal-field splittings, alloy bowing parameters, electron and hole effective masses, deformation potentials, elastic constants, piezoelectric and spontaneous polarization coefficients, as well as heterostructure band offsets. Temperature and alloy-composition dependences are also recommended wherever they are available. The “band anticrossing” model is employed to parameterize the fundamental band gap and conduction band properties of the dilute nitride materials.
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71.20.Nr Semiconductor compounds
01.30.Rr Surveys and tutorial papers; resource letters
71.18.+y Fermi surface: calculations and measurements; effective mass, g factor
77.84.Bw Elements, oxides, nitrides, borides, carbides, chalcogenides, etc.
71.70.Ej Spin-orbit coupling, Zeeman and Stark splitting, Jahn-Teller effect
71.70.Ch Crystal and ligand fields
62.20.D- Elasticity
77.65.Bn Piezoelectric and electrostrictive constants
77.22.Ej Polarization and depolarization

Fluorescence intensity ratio technique for optical fiber point temperature sensing

S. A. Wade, S. F. Collins, and G. W. Baxter

J. Appl. Phys. 94, 4743 (2003); http://dx.doi.org/10.1063/1.1606526 (14 pages)

Online Publication Date: 30 September 2003

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The fluorescence intensity ratio technique for optical fiber-based point temperature sensing is reviewed, including the materials suitable for this technique. The temperature dependence of the fluorescence intensity ratio has been studied using thermally coupled energy levels in seven different rare earth ions doped into a variety of glasses and crystals. Sensor prototypes developed using Pr3+:ZBLANP, Nd3+-doped silica fiber and Yb3+-doped silica fiber as the sensing material have been used to measure temperatures covering the range of approximately −50 to 600 °C with a resolution of the order of 1 °C. © 2003 American Institute of Physics.
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42.81.Pa Sensors, gyros
07.60.Vg Fiber-optic instruments
07.20.Dt Thermometers
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Recent advances on electromigration in very-large-scale-integration of interconnects

K. N. Tu

J. Appl. Phys. 94, 5451 (2003); http://dx.doi.org/10.1063/1.1611263 (23 pages)

Online Publication Date: 23 October 2003

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Today, the price of building a factory to produce submicron size electronic devices on 300 mm Si wafers is over billions of dollars. In processing a 300 mm Si wafer, over half of the production cost comes from fabricating the very-large-scale-integration of the interconnect metallization. The most serious and persistent reliability problem in interconnect metallization is electromigration. In the past 40 years, the microelectronic industry has used Al as the on-chip conductor. Due to miniaturization, however, a better conductor is needed in terms of resistance–capacitance delay, electromigration resistance, and cost of production. The industry has turned to Cu as the on-chip conductor, so the question of electromigration in Cu metallization must be examined. On the basis of what we have learned from the use of Al in devices, we review here what is current with respect to electromigration in Cu. In addition, the system of interconnects on an advanced device includes flip chip solder joints, which now tend to become weak links in the system due to, surprisingly, electromigration. In this review, we compare the electromigration in Al, Cu, and solder on the basis of the ratio of their melting point to the device operating temperature of 100 °C. Accordingly, grain boundary diffusion, surface diffusion, and lattice diffusion dominate, respectively, the electromigration in Al, Cu, and solder. In turn, the effects of microstructure, solute, and stress on electromigration in Al, Cu, and solder are different. The stress induced by electromigration in Cu/low-k interconnects will be a very serious issue since the low-k dielectric (with a value of k around 2) tends to be weak mechanically. In a multilevel interconnect, a electromigration force due to current crowding, acting normal to current flow, has been proposed to explain why many electromigration induced damages occur away from the high current density region. In mean-time-to-failure analysis, the time taken to nucleate a void is found to be much longer than the growth of the void in Al and solder interconnects. This is not the case for Cu interconnects for the nucleation of a void on a surface. On accelerated tests of electromigration in Cu interconnects, the results gathered above 300 °C will be misleading since the mass transport will have a large contribution of grain boundary diffusion, which is irrelevant to electromigration failure in real devices induced by surface diffusion. © 2003 American Institute of Physics.
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85.40.Ls Metallization, contacts, interconnects; device isolation
68.35.Fx Diffusion; interface formation
61.72.Mm Grain and twin boundaries
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Strained Si–O–Si bonds in amorphous SiO2 materials: A family member of active centers in radio, photo, and chemical responses

Koichi Awazu and Hiroshi Kawazoe

J. Appl. Phys. 94, 6243 (2003); http://dx.doi.org/10.1063/1.1618351 (20 pages)

Online Publication Date: 31 October 2003

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Amorphous SiO2 (a-SiO2), such as bulk silica glasses and thin films has been one of the key materials in modern optoelectronic industries. These materials are currently used in communication technologies as optical fibers, thin films for electrical insulation in dynamic random access memories (DRAM), and optical lenses for excimer laser lithography, for example. The property essential for these applications is the wide band gap amounting to ∼9 eV. However, bulk silica glasses commercially available and silica thin films show photoresponses to subband gap lights in the vicinity of 5 eV and unexpected trapping of charges, and the behavior has a strong dependency on the preparation history. A number of studies were carried out to clarify the relationship between the properties and structural imperfections in the materials and the formation mechanisms of the defects. There are two categories of the imperfections: one is dopant- or impurity-related imperfections and the other is nonstoichiometry related defects. These defects constitute gap states in a-SiO2. The structural identification was usually performed by absorption and emission spectroscopy in the visible–ultraviolet (UV) region and electron spin resonance (ESR). The experimentally proposed models were compared with the predictions by theoretical calculations of energy levels. Recent development of the excimer laser lithography technique led us to recognize that a latent member, which has been unnoticed because of no response to the optical absorption or emission in the visible-UV range and ESR absorption, exists in the family of active centers in a-SiO2, that is a strained Si–O–Si bond originating from the planar three membered ring. In contrast, the puckered four membered ring is unstrained. Although it has been pointed out that there was a wide distribution in Si–O–Si bond angle from 90° to 180° by x-ray analysis or 29Si solid state nuclear magnetic resonance, the physical, and chemical responses of the Si–O–Si bonds with a particular bond angle could not be differentiated. Very recently it was clarified that a strained Si–O–Si bond, in other words chemically excited bonds, has an optical absorption locating on the band edge. The chemically excited bond can be scavenged by fluorine doping, because it is chemically reactive. In the present review we show that the unresolved optical and electric responses of silica glasses can be comprehensively understood by taking the presence of the strained bonds into consideration. © 2003 American Institute of Physics.
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61.43.Er Other amorphous solids
78.40.Pg Disordered solids
76.30.Mi Color centers and other defects
71.55.Jv Disordered structures; amorphous and glassy solids
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Solid state quantum computer development in silicon with single ion implantation

T. Schenkel, A. Persaud, S. J. Park, J. Nilsson, J. Bokor, J. A. Liddle, R. Keller, D. H. Schneider, D. W. Cheng, and D. E. Humphries

J. Appl. Phys. 94, 7017 (2003); http://dx.doi.org/10.1063/1.1622109 (8 pages)

Online Publication Date: 10 November 2003

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Spawned by the finding of efficient quantum algorithms, the development of a scalable quantum computer has emerged as a premiere challenge for nanoscience and nanotechnology in the last years. Spins of electrons and nuclei in 31P atoms embedded in silicon are promising quantum bit (qubit) candidates. In this article we describe single atom doping strategies and the status of our development of single atom qubit arrays integrated with control gates and readout structures in a “top down” approach. We discuss requirements for 31P qubit array formation by single ion implantation, and integration with semiconductor processing.
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61.72.uf Ge and Si
03.67.Lx Quantum computation architectures and implementations
85.40.Ry Impurity doping, diffusion and ion implantation technology
84.30.Sk Pulse and digital circuits
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