bluuebunny/crossref_metadata_2025 huggingface dataset:
Ultraviolet quantum well structures based on ZnS/ZnS1−xSex and ZnS/Zn1−xCdxS multilayers grown by molecular-beam epitaxy
Authors: Vaudo R. P. , Eason D. B. , Bowers K. A. , Gossett K. J. , Cook J. W. , Schetzina J. F.
Published: July 2002

Multilayers of ZnS/ZnS1−xSex and ZnS/Zn1−xCdxS were successfully grown by molecular-beam epitaxy using elemental sources. Photoluminescence (PL) experiments were performed at 4.2 K to evaluate the optical quality and carrier-confinement properties of these multiple-quantum-well structures. In general, the emission peaks from the ZnS/Zn1−xCdxS samples were much sharper and more intense than those from ZnS films or ZnS/ZnS1−xSex multilayers. These PL characteristics indicate improved electron confinement in the Cd-based wells, implying that an appreciable conduction band offset exists between ZnS and Zn1−xCdxS. This work suggests that ZnS/Zn1−xCdxS quantum-well structures may play an important role in the development of ultraviolet (UV) and near-UV light emitting structures.
Sb surface segregation during heavy doping of Si(100) grown at low temperature by molecular beam epitaxy
Authors: Hobart K. D. , Godbey D. J. , Thompson P. E. , Simons D. S.
Published: July 2002

Sb surface segregation and doping during Si(100) molecular beam epitaxy at low temperature have been studied by depth profiling with secondary ion mass spectrometry, x-ray photoelectron spectroscopy, and conductivity measurements. We find that at a growth temperature near 320 °C complete donor activation and doping densities near 5×1020 cm−3 are obtainable. For temperatures between 320–500 °C, measurements point to the existence of distinct surface segregation regimes. For dilute surface and bulk Sb concentrations the measurements reveal a region where the surface segregation is constant and bulk and surface concentrations are linearly related. For bulk concentrations near the solid solubility limit, the surface segregation increases rapidly with increasing bulk concentration for temperatures between 350–450 °C. Finally, for Sb-saturated surfaces the surface segregation actually decreases, and doping levels exceeding the solid solubility limit are possible.
Effect of heavy doping on band gap in SiGe base regions
Authors: Manning B. M. , Peters C. J. , Tarr N. G. , Noel J.-P. , Houghton D. C.
Published: July 2002

The band gap in heavily boron-doped base regions of SiGe heterojunction bipolar transistors is determined from the temperature dependence of the collector saturation current JC0 in experimental devices. It is found that heavy boron doping leads to a significant further shrinkage of band gap relative to that expected from the incorporation of Ge alone. Experimental results are in good agreement with the recent theory of Jain and Roulston.
Detection and reduction of indium segregation during molecular-beam epitaxial growth of InGaAs/GaAs using in situ reflection mass spectrometry
Authors: Kao Y. C. , Celii F. G. , Liu H. Y.
Published: July 2002

The use of reflection mass spectrometry (REMS) as an in situ diagnostic for surface segregation of indium during InGaAs layer growth is reported. InGaAs growth at substrate temperatures above 500 °C yields a REMS signature which indicates a thickness-dependent surface In content. Adapting a simple segregation model [K. Muraki, S. Fukatsu, Y. Shiraki, and R. Ito, Appl. Phys. Lett. 61, 557 (1992)], the segregation ratio R was extracted under various deposition conditions in real time. The segregation ratio obtained during InGaAs/GaAs multiple quantum well growth at 500–530 °C suggests the presence of InGaAs composition grading near interfaces, and agrees qualitatively with ex situ characterization by x-ray diffraction. The high values of R (0.7–0.8) observed under normal device-layer growth produces a surface layer with high In content (i.e., InAs). The segregation ratio was not sensitive to unrelaxed layer strain, but showed a large increase under conditions which probably produce island (three-dimensional) growth. Methods for preparation of high-quality InGaAs layers with more abrupt AlGaAs/InGaAs/GaAs heterointerfaces are discussed, such as use of a two-step growth procedure or the incorporation of thin ‘‘cap layers’’ prior to high temperature AlGaAs growth.
Triode characteristics and vacuum considerations of evaporated silicon microdevices
Authors: Aslam M. , Klimecky P. , Myers G. P. , Busta H. H. , Zimmerman B. J. , Artz B. E. , Cathey L. W. , Elder R. E.
Published: July 2002

Fowler–Nordheim plots of evaporated Si cold emitter arrays, tested in triode configuration at 42 and 300 K, are found to be temperature independent. Fluctuations in emission current decrease with decreasing temperature. Pressure dependence of collector current, measured in a diode configuration for collector voltage range of 90–130 V, shows a sharp decrease if the pressure is raised above 2×10−4 Torr.
Self-aligned silicon field emission cathode arrays formed by selective, lateral thermal oxidation of silicon
Authors: Spallas J. P. , Das J. H. , MacDonald N. C.
Published: July 2002

This article reports on the fabrication of single crystal silicon, field emission tip arrays. The tips are formed by exposing a ‘‘capped’’ silicon pedestal to a lateral high temperature thermal oxidation. Tips formed using this process have uniform height and profile, and have small radii of curvature—typically less than 20 nm. We have fabricated tip arrays with up to 2500 elements and with the tip-to-tip spacing ranging from 1.0 to 5.0 μm. The thermal SiO2 has a nominal thickness of 600 nm and serves as insulation between the extraction grid and the substrate. The extraction grid is a self-aligned, sputter deposited Ti0.1W0.9 film. Extraction grid apertures range from 900 nm to 1.7 μm. The process sequence is flexible allowing the fabrication of tips with different heights. The vertical position of the tip apex with respect to the extraction grid aperture, therefore, is a controllable parameter. The preliminary emission measurements are presented here for arrays containing 400 elements. All testing is done in ultrahigh vacuum (typically less than 5.0×10−10 Torr). The areas of the arrays fabricated range from 40 μm×40 μm to 100 μm×100 μm. The tip heights range from 1.0 to 1.4 μm above the substrate silicon. The maximum current produced from a 400 element array is only 0.45 μA at 140 Vdc. The low array current indicates that only a small number of tips are active at a given voltage.
Variations in substrate temperature induced by molecular-beam epitaxial growth on radiatively heated substrates
Authors: Shanabrook B. V. , Waterman J. R. , Davis J. L. , Wagner R. J. , Katzer D. S.
Published: July 2002

Large changes in substrate temperature that occur during molecular-beam epitaxial growth of materials with energy gaps smaller than that of the substrate have been observed. These changes in temperature, which are not detected by the conventional thermocouple, have been measured by observing the changes in the infrared transmission spectrum of the radiatively heated substrate. In addition to describing the experimental arrangement used for these measurements, the advantages and disadvantages of using this technique for substrate temperature determination are discussed.
Etching of InP(100) 4×2 and molecular-beam epitaxially grown GaAs(100)-c(4×4) with atomic hydrogen
Authors: Schaefer J. A. , Stietz F. , Woll J. , Wu H. S. , Yu H. , Lapeyre G. J.
Published: July 2002

Phonon, plasmon, and vibrational modes of clean and hydrogenated molecular-beam epitaxially grown As-rich GaAs(100)-c(4×4) and ion-bombarded and annealed InP(100) 4×2 have been studied with high-resolution electron energy-loss spectroscopy. The plasmon energy at the interface of depletion layer and bulk enabled us to determine the carrier concentration for Si-doped GaAs samples. With increasing hydrogen exposure characteristic changes in plasmon energy, its intensity and the intensity of the surface optical phonons have been observed. This is explained by changes in the space charge regime. Using hydrogen as a local probe provides sensitivity to the stoichiometry and to the chemical bonding situation at the uppermost surface layer. It is shown that the As-terminated GaAs(100) surface can be converted finally into a Ga-terminated surface by etching with atomic hydrogen. For InP, for the very first time an interface plasmon was observed at 90 meV, due to high S doping. In contrast to GaAs, sequential hydrogenation results in the formation of metallic droplets (In), which increase in size and concentration with increasing hydrogen exposure. Consequently, the phonon and plasmon intensities are reduced drastically. Models of the atomic surface structures for different stages of hydrogenation are discussed.
Electron energy loss spectroscopy on FeSi2/Si(111) heterostructures grown by gas source molecular-beam epitaxy
Authors: Rösen B. , Schäfer H. Ch. , Dieker Ch. , Lüth H. , Rizzi A. , Gerthsen D.
Published: July 2002

The growth of FeSi2 on Si(111) by gas source molecular-beam epitaxy is reported. Fe(CO)5 and SiH4 are used as source gases for the silicide growth on the heated substrate. A wide range of growth temperatures, 400–800 °C, has been examined and the thickness of the grown layers ranges between 50 and 200 nm. High resolution electron energy loss spectroscopy, with its sensitivity to the electronic properties of heterostructures in a range of several hundreds angstroms below the surface, turns out to be very effective for the in situ characterization of the growth. Photoelectron spectroscopy and ex situ high-resolution transmission electron microscopy complete this study. Three main growth modes are identified. For Tgrowth≤500 °C, different phases are growing, with an early stage of β-FeSi2 nucleation. Above 550 °C only the semiconducting β phase is formed with increasing tendency towards three dimensional growth as the temperature increases. In the range 500–550 °C the β-FeSi2 layer grows uniformly and the presence of a p++ accumulation layer, ∼100 Å thick, at the surface is inferred. This accumulation layer is removed by room-temperature adsorption of dissociated SiH4.
Surface studies during growth of Si1−xGex/Si from gaseous Si and Ge hydrides
Authors: Mokler S. M. , Ohtani N. , Xie M. H. , Zhang J. , Joyce B. A.
Published: July 2002

The epitaxial growth of Si and Si1−xGex alloys from molecular beams of gaseous Si (Si2H6) and Ge (GeH4) hydrides on Si(001) substrates (Si gas-source molecular-beam epitaxy) has been studied using reflection high-energy electron diffraction (RHEED). Diffraction patterns reveal that Ge deposited on Si results in a Stranski–Krastanow growth mode with a change in surface reconstruction prior to the onset of three-dimensional growth. RHEED intensity oscillations measured during Si1−xGex alloy growth indicate that below 600 °C, the addition of GeH4 flux to the substrate results in an enhanced growth rate (GR), but above 600 °C this phenomenon is not observed and the GR remains proportional to the Si2H6 flux. Further, a temperature and flux dependent change in the growth rate, ΔGR, is observed at the Si/Si1−xGex interface. At low temperatures (<580 °C), the growth of several monolayers precedes the establishment of a constant alloy GR, indicative of a graded Ge composition in the interface region. This phenomenon is discussed in terms of the temperature dependent reaction kinetics in this system during growth between 520–640 °C.
Effect of the proximity of an ex situ patterned interface on the quality of two-dimensional electron gases at GaAs/AlGaAs heterojunctions
Authors: Grimshaw M. P. , Ritchie D. A. , Burroughs J. H. , Jones G. A. C.
Published: July 2002

High-quality two-dimensional electron gases (2DEGs) were regrown onto ex situ patterned substrates and the effect of the distance between the 2DEG and the contaminated regrowth interface was investigated. At a separation of 220 nm mobilities of 7.1×105 cm2 V−1 s−1 in the dark and 1.3×106 cm2 V−1 s−1 after illumination with a red light-emitting diode (LED) were measured with corresponding carrier concentrations of 2.4×1011 and 4.3×1011 cm−2. The quality of this 2DEG was equivalent to that attained in conventionally grown 2DEGs at the time, indicating that the 2DEG was not affected by presence of the regrowth interface in this case. For smaller separations a degradation in the 2DEG quality was observed. Thus, at a separation of only 60 nm, a mobility of 4.1×105 cm2 V−1 s−1 at a carrier concentration of 4.2×1011 cm−2 was measured after illumination. The use of an independently contacted n+ layer in the patterned substrate as a backgate was also demonstrated.
Factors affecting the temperature uniformity of semiconductor substrates in molecular-beam epitaxy
Authors: Johnson S. R. , Lavoie C. , Nodwell E. , Nissen M. K. , Tiedje T. , Mackenzie J. A.
Published: July 2002

The temperature of GaAs substrates is profiled in a molecular-beam epitaxy system with a spatial resolution of 3 mm and a thermal resolution of 0.4 °C, respectively. The effects of substrate doping, back surface textures, thermal contact to the holder, and a pyrolytic boron nitride diffuser plate, on the temperature uniformity, are explored for indium-free mounted substrates. Both positive and negative curvature temperature profiles are observed.
Atomic nitrogen production in a molecular-beam epitaxy compatible electron cyclotron resonance plasma source
Authors: Vaudo R. P. , Cook J. W. , Schetzina J. F.
Published: July 2002

The first high-resolution study of the optical emission from nitrogen plasmas produced by an ASTeX compact electron cyclotron resonance (ECR) microwave plasma source is reported. The spectroscopic results clearly show that the ECR plasma source generates an appreciable flux of nitrogen atoms, as indicated by strong atomic emission lines in the near-infrared spectral region, in addition to various species of molecular nitrogen.
Reflection high-energy electron diffraction intensity oscillations during molecular-beam epitaxy on rotating substrates
Authors: van der Wagt Jan P. A. , Harris James S.
Published: July 2002

We report the gated detection of reflection high-energy electron diffraction (RHEED) signal intensity oscillations during molecular-beam epitaxy (MBE) while the wafer is being rotated at high speed (≳100 rpm). In general, a larger number of oscillation periods are observed with this technique than during stationary measurement because of the very high growth rate uniformity across the sample. We found that the average over all azimuths of the RHEED specular spot intensity shows the same growth induced oscillations. This allowed us to replace the gated detection by an averaging detection method: a low-pass filter suppresses fast variations due to rapid changes in azimuth, while passing low-frequency (<1 Hz) growth related oscillations. This simplifies the detection system and at the same time reduces noise related to wafer motion, 60 Hz, etc. These techniques have potential use for in situ monitoring of thickness and composition of device wafers.
Exciton dynamics in multiquantum well CdTe–Cd1−xMnxTe systems
Authors: Stirner T. , Hagston W. E. , O’Neill M. , Harrison P.
Published: July 2002

The exciton dynamics associated with the transfer of energy from the light-hole state to both the heavy-hole state and to a donor bound exciton (D0X) are investigated utilizing time-resolved spectroscopy, in a multiquantum well sample of CdTe–Cd1−xMnxTe containing 75 Å wells and 150 Å barriers. The results show that the relaxation time of the light-hole to the heavy-hole state occurs, at low temperature, in a time ≤40 ps whereas the combined drift/diffusion of the excitons in the plane of the quantum well and their subsequent trapping at donor states occurs in a time ∼80 ps. Theoretical calculations show that the light-hole–heavy-hole carrier relaxation time is consistent with confined (acoustic) phonon relaxation and that the equivalent in-plane mobility is consistent with interface roughness scattering.
Gate oxide damage in a high density inductively coupled plasma
Authors: Gabriel Calvin T. , Melaku Yosias
Published: July 2002

A high density inductively coupled plasma (ICP) polycide etcher was studied by incorporating it into a 0.5 μm complementary metal–oxide–semiconductor process flow having a nominal 10 nm gate oxide. Polysilicon: oxide etch selectivity, sidewall profile, and critical dimension control were found to be good. Gate oxide damage was studied in detail using both antenna and large capacitor test structures in area- and edge-intensive configurations. No charge buildup was detected using the antenna structures, and damage measured using large capacitors was also very low. Damage was found to be insensitive to bias power, while the sequence of extinguishing the plasma had a small effect. An experimental hardware modification significantly reduced the level of subintrinsic failures of gate edge-intensive capacitors. Split-lot comparisons were made of several etchers using standard hardware and processes, and the rf ICP etcher was found to result in significantly lower oxide damage than the split-power rf triode, plasma-mode rf diode, or microwave electron cyclotron resonance etchers studied. The lowest damage occurred in the ICP, followed by the diode. Langmuir probe measurements were made of plasma potential and ion current for the bulk etch and overetch processes used in the ICP etcher; the results are compared to wafer maps of oxide damage. Plasma potential and ion current are very uniform in this plasma, and no pattern is evident from the wafer maps of breakdown failures. The low oxide damage measured using the ICP etcher probably results from the combination of hardware and process design that ensures a uniform plasma.
Performance of the plasma deposited tungsten nitride barrier to prevent the interdiffusion of Al and Si
Authors: Lee Chang Woo , Kim Yong Tae , Lee Choochon , Lee Jeong Yong , Min Suk-Ki , Park Young Wook
Published: July 2002

It is proposed that the plasma enhanced chemical vapor deposited tungsten nitride (PECVD-W–N) thin film is used as a diffusion barrier to prevent the interdiffusion between Al and Si during postannealing process. The atomic concentration of N in W–N film deposited with NH3/WF6 partial pressure ratio of 0.5 is about 33 at. % and its resistivity is 90–110 μΩ cm. The Rutherford backscattering spectrometry, Auger electron depth profiles, x-ray diffraction, and scanning electron micrographs show that 900 Å PECVD-W67N33 film interposed between Al and Si is more impermeable than PECVD-W film due to N atoms and it also keeps its chemical integrity during the postfurnace annealing at 600 °C for 30 min in Ar ambient.
Growth studies on In0.5Ga0.5As/AlGaAs quantum wells grown on GaAs with a linearly graded InGaAs buffer
Authors: Chui H. C. , Harris J. S.
Published: July 2002

High indium content InGaAs/AlGaAs quantum wells (QWs) are useful for modulator and intersubband applications. Growth of these highly strained QWs on GaAs has been facilitated by the use of a linearly graded InGaAs buffer. We present here growth studies performed on these QW structures. Buffer parameters including buffer substrate temperature, buffer grading rate, and final buffer indium composition are studied. QW parameters including QW substrate temperature, the use of GaAs interface smoothing layers, and barrier substrate temperature are also investigated. Using near optimized growth conditions, narrow linewidth intersubband transitions are demonstrated.
Resist heating effect in electron beam lithography
Authors: Yasuda M. , Kawata H. , Murata K. , Hashimoto K. , Hirai Y. , Nomura N.
Published: July 2002

Resist heating effects on pattern shapes in electron beam lithography have been studied. The beam current density and dose partition dependences of the pattern shape are explained by comparison between the calculated spatial distribution of resist temperature and experimental resist pattern profiles. From direct heating of the resist using a tungsten heater during electron beam irradiation, the temperature effect on the resist sensitivity and pattern shape has been investigated. The pattern shape distortion is evaluated by a simulation considering the temperature effect.
High performance self-aligned sub-100 nm metal–oxide-semiconductor field-effect transistors using x-ray lithography
Authors: Yang Isabel Y. , Hu Hang , Su Lisa T. , Wong Vincent V. , Burkhardt M. , Moon Euclid E. , Carter J. M. , Antoniadis D. A. , Smith Henry I. , Rhee Kee W. , Chu W.
Published: July 2002

Recent studies have shown that high performance 0.1 μm complementary metal–oxide semiconductor (CMOS) can be achieved with proper channel and source/drain engineering. Specifically, retrograde channel doping and shallow source/drain junctions with counterdoping implant (halo) allow the threshold voltage to be kept low while maintaining acceptable short-channel behavior. These studies have certainly demonstrated the feasibility of CMOS technology scaled down to 0.1 μm from a device design point-of-view. However, the main challenge to the lithography technology is to fabricate 0.1 μm metal-oxide-semiconductor field-effect transistor (MOSFET) devices with high yield and high throughput, as required in manufacturing. X-ray lithography is a technology that can potentially meet this challenge. This work shows the results of integrating a low cost x-ray technology with standard IC processing to fabricate high performance 100 nm and even sub-100 nm MOSFETs.
Ion projection lithography over wafer topography
Authors: Brünger W. H. , Buchmann L.-M. , Torkler M. A. , Finkelstein W.
Published: July 2002

Ion projection lithography has been used to structure resist lines running over 0.5-μm-high surface steps on a wafer. Line profiles were straight and did not change at the surface step consisting of SiO2 on Si. 0.2-μm resolution was found in 0.7-μm-thick positive and negative tone resist. In the list of tested resists [poly(methylmethacrylate) (PMMA), Ray PN, and AZ 5206], the novolack resist AZ 5206 showed better edge roughness compared to Ray PN due to the lower sensitivity of AZ. Exposure times range from 300 ms to 3 s. The recorded penetration depth in PMMA was 0.8 μm using H+ ions at 70 keV. A variation of ion energy gives the possibility to adjust the penetration depth to the required resist thickness and minimize radiation damage in the substrate.
Advanced fabrication techniques of three-dimensional microstructures for future electronic devices
Authors: Ugajin Ryuichi , Ishibashi Akira , Mori Yoshifumi
Published: July 2002

Methods to fabricate three-dimensional microstructures for mesoscopic devices are discussed. Reactive ion etching and metalorganic chemical vapor deposition experiments were performed to obtain structures with various potential applications. For example, a structure with a pyramid shape is suitable for a field emitter, and a structure with an X-shaped oblique groove is suitable for a vacuum microtransistor. The results are also applicable to fabrication of mesoscopic three-dimensional structures based on GaAs/AlGaAs heterostructures.
Effect of acid diffusion on performance in positive deep ultraviolet resists
Authors: Fedynyshyn T. H. , Thackeray J. W. , Georger J. H. , Denison M. D.
Published: July 2002

Two methods to measure acid diffusion in positive acid catalyzed resists are described. The first method employs a spectrophotometric titration to determine the acid concentration ([H+]) followed by measuring the ion conductivity (σ) of the resist film to determine the diffusion coefficient (D). This method allows the diffusion coefficient of acid in the resist to be determined at different temperatures ranging from room temperature to different post-exposure bake (PEB) temperatures. The second method is based on the threshold acid density theory of image formation, which assumes that when a critical concentration of acid is reached, the developer solubility of the resist is changed. With this method, a constant level of acid can be followed at different PEB times and the diffusion coefficient determined. A comparison of the two methods to measure the acid diffusion coefficient will be made and the temperature dependence of diffusion for different types of organic acids will be presented. Based on a previously described reaction–diffusion model that predicts relative deblocking levels, evidence is presented for room temperature diffusion of acid to be a possible explanation for delay instability in positive deep ultraviolet resists.
Modeling and simulations of a positive chemically amplified photoresist for x-ray lithography
Authors: Krasnoperova A. A. , Khan M. , Rhyner S. , Taylor J. W. , Zhu Y. , Cerrina F.
Published: July 2002

This article presents the results of the experimental and modeling study of a positive tone, chemically amplified photoresist, in application to x-ray lithography. Spectrophotometric titration, Fourier transform infrared spectroscopy (FTIR), and development rate monitor data were acquired and used as inputs for the modeling of the processes and pattern simulations. The exposure model assumes monomolecular decomposition upon radiation and corresponds to Dill’s model for a nonbleaching photoactive compound. The post-exposure bake (PEB) model is based on formal kinetic equations which include a term for photoacid loss (or side reactions) during the post-exposure bake process in a generalized way. The effective kinetic order of the photoacid loss reaction is derived from the FTIR absorbance data obtained for different PEB times and exposure doses. For patterned exposures, a diffusion term for the local photoacid concentration is included. The photoacid and tert-butoxycarbonyloxystyrene concentration profile changes with PEB time have been simulated for a 0.25 μm line and spaces pattern. It is shown that a nonlinear PEB photoacid reaction kinetics, rather than the photoacid diffusion, dominates the linewidth decrease during PEB for the photoresist studied.
Fabrication of diamond tips by the microwave plasma chemical vapor deposition technique
Authors: Liu N. , Ma Z. , Chu X. , Hu T. , Xue Z. , Jiang X. , Pang S.
Published: July 2002

Scanning tunneling electron microscopy (STM) has recently shown great promise for surface modification in the nanometer regime. As is well known, the normal STM tips are easily damaged by mechanical contact with the sample surface in most cases. In this paper we describe a new type of diamond STM tip prepared by the microwave plasma chemical vapor deposition technique. The reactant is a gas mixture of methane and hydrogen. Diamond crystals are grown on a tungsten tip. The diamond tips are very hard and always have enough electrical conductivity for STM experiments. They can be used as a machining tool to scratch the metal surfaces.
Scanning tunneling microscopy study of the growth of Cr/Fe(001): Correlation with exchange coupling of magnetic layers
Authors: Stroscio Joseph A. , Pierce D. T. , Unguris J. , Celotta R. J.
Published: July 2002

Scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED) were used to study the epitaxial growth of Cr on Fe(001) whiskers as a function of the Fe whisker temperature during growth. The STM images give real space views of the morphology of Cr growth, which can be correlated with the nature of the RHEED intensity oscillations. Layer by layer growth is found for Cr deposition on an Fe(001) surface at 300 °C, and very rough growth, limited by diffusion kinetics, is observed at lower temperatures. The variation in the interlayer exchange coupling in Fe/Cr/Fe sandwiches as a function of the thickness of the Cr interlayer, which has been found to depend strongly on the growth temperature of the Cr interlayer, can be explained by the thickness fluctuations determined from the STM measurements of Cr films grown at different temperatures.
Scanning tunneling microscopy investigation of monolayer of metal complexes adsorbed on highly oriented pyrolytic graphite
Authors: Miyamura Kazuo , Kimura Masahiro , Okumura Akihiko , Gohshi Yohichi
Published: July 2002

Monolayer of nickel(II) complexes coordinated with N,N′-disalicylideneethylenediamine (salen) substituted by dodecyl side chains have been prepared at a water–air interface using the Langmuir–Blodgett method. The monolayer of the complex thus formed is scooped onto the cleaved surface of highly oriented pyrolitic graphite, and imaged by scanning tunneling microscopy.
Scanning tunneling microscopy study of solid-phase epitaxy processes of argon ion bombarded silicon surface and recovery of crystallinity by annealing
Authors: Uesugi K. , Yoshimura M. , Yao T. , Sato T. , Sueyoshi T. , Iwatsuki M.
Published: July 2002

The solid-phase epitaxy (SPE) processes of amorphized Si surfaces by Ar+-ion bombardment are investigated in situ using a scanning tunneling microscope. As-bombarded Si(001) surfaces consist of grains 0.63–1.6 nm in diameter. The grains gradually coalesce and forms clusters 2–3.6 nm in diameter at an annealing temperature of 245 °C. Prolonged annealing at this temperature promotes crystallization of the amorphous layers; (2×1) and (1×2) reconstructed regions surrounded by amorphous regions are partially observed on the surface, which suggests the onset of SPE. Successive observation reveals that the smoothing of the surface occurs layer by layer. As the annealing temperature is raised to 445 °C, the amorphous layer epitaxially crystallizes up to the topmost surface, and a (2×1) reconstructed surface with monatomic height steps is observed. The smoothing of the step edges is observed during annealing at 500 °C. The behavior of the defects during the recovery of crystallinity is also described.
Tunneling through an epitaxial oxide film: Al2O3 on NiAl(110)
Authors: Bertrams Th. , Brodde A. , Neddermeyer H.
Published: July 2002

By controlled oxidation of a clean NiAl(110) surface an epitaxial thin layer of Al2O3 may be grown. The oxide layer shows a sharp low-energy electron diffraction pattern of two domains of nearly rectangular unit cells. A stable tunneling current was obtained for a wide range of sample bias voltages U (from a few mV up to nearly 10 V). For U in the mV range the images essentially contain contributions of the interface layer, which can be measured with atomic resolution. For large U (e.g., 4 V) the electronic states of the oxide film contributed additionally to the substrate electrons, which allows an accurate analysis of the growth mode of the oxide film.
Characterization of metal–oxide–semiconductor capacitors with improved gate oxides prepared by repeated rapid thermal annealings in N2O
Authors: Wu You-Lin , Hwu Jenn-Gwo
Published: July 2002

Gate oxides prepared by repeated rapid thermal annealings of the thermal oxide in N2O ambient is proposed and metal–oxide–semiconductor (MOS) capacitors with these newly proposed gate dielectrics are studied. It is found that both the charge-to-breakdown and the oxide breakdown field shift to higher values as the number of repeated annealings and the annealing time increase. All the samples that have been subjected to repeated N2O rapid thermal annealings exhibit improved Fowler–Nordheim constant current stress resistance and radiation hardness when compared to the fresh sample and the conventional one-time N2O-annealed sample with a comparable annealing time. It is believed that this multiple N2O annealing method can be another promising candidate for fabricating hot-carrier resistant and radiation-hard MOS devices.
Scanning tunneling microscopy observation of Al-induced reconstructions of the Si(111) surface: Growth dynamics
Authors: Yoshimura M. , Takaoka K. , Yao T. , Sueyoshi T. , Sato T. , Iwatsuki M.
Published: July 2002

The atomic structure of α-7×7 and γ-phase structure has been investigated with use of scanning tunneling microscopy. Structural models for both phases are presented where both structures are based on the dimer adatom stacking-fault structure. Continuously linked boundaries are observed between these phases, in contrast to those between α-7×7 and √7×√7 phase. It is found that the properties of the boundaries are closely related to the basement structure. Phase transitions appearing at higher or lower temperature are discussed in terms of growth dynamics.
Scaling of Si/SiO2 interface roughness
Authors: Yoshinobu T. , Iwamoto A. , Sudoh K. , Iwasaki H.
Published: July 2002

The spatial scaling law of the Si/SiO2 interface roughness was investigated with atomic force microscopy. Scaling behavior was observed on smaller scales, where the root-mean-square (RMS) roughness increased as a power of the scale of observation. When viewed as a fractal geometry, such a structure is characterized as a self-affine fractal. On larger scales, the roughness was no more dependent on the scale, showing the (macroscopic) RMS roughness in the conventional sense. The observed structure (self-affine fractal with a finite-length cut-off) is consistent with the prediction of the theory of kinetic roughening in a far-from-equilibrium growth, where the fluctuation on smaller scales evolves into roughness on larger scales. Statistical description of the Si/SiO2 interface roughness was also given in terms of autocorrelation function and power spectral density. It was found that the autocorrelation function of the Si/SiO2 interface roughness is well approximated by an exponential form rather than a Gaussian form. Numerical simulation was carried out to explain the kinetic roughening in the oxidation process with relation to the dependence on the oxide thickness.
Hydrogen-induced modification of the optical properties of the GaAs(100) surface
Authors: Esser N. , Santos P. V. , Kuball M. , Cardona M. , Arens M. , Pahlke D. , Richter W. , Stietz F. , Schaefer J. A. , Fimland B. O.
Published: July 2002

The influence of hydrogen adsorption on the surface order, dielectric function, and dielectric anisotropy was investigated for the three main reconstructions of the GaAs(100) surface [c(4×4), (2×4), (4×2)] by low-energy electron diffraction, spectroscopic ellipsometry, and reflectance difference spectroscopy. For all reconstructions, low hydrogen exposure removes the surface dimers, whereas surface roughening is observed at high hydrogen exposures. However, detailed differences can be resolved depending on the surface reconstruction. On the As-rich c(4×4) surface, the outermost As-dimers are removed at low H-exposures leaving behind a still As-rich surface of (1×1) symmetry. Thereafter, more As is removed and a new surface anisotropy develops due to the exposed Ga-dimers arranged in a mixture of (1×2) and (√2×√2)-reconstructions. For H-adsorption on the (2×4)- and (4×2)-surfaces, in contrast, only the subsequent reduction of the surface reconstruction is observed with increasing exposure, but no structural rearrangement. The removal of the As- [Ga-] dimers modifies the surface symmetry from (2×4) [(4×2)] through (1×4) [(4×1)] to (1×1). Finally, for large exposures, inhomogeneous surface etching by hydrogenation leads to rough, disordered surfaces for all three reconstructions.
Fabrication of sub-50-nm gate length n-metal–oxide–semiconductor field effect transistors and their electrical characteristics
Authors: Ono Mizuki , Saito Masanobu , Yoshitomi Takashi , Fiegna Claudio , Ohguro Tatsuya , Momose Hisayo Sasaki , Iwai Hiroshi
Published: July 2002

A method of fabricating 40-nm gate-length n-metal–oxide–semiconductor field effect transistors (MOSFETs) is described in detail. The fabrication of MOSFETs with gate lengths of this order poses two major problems: how to fabricate such small-geometry gate electrodes, and how to fabricate the ultrashallow source and drain junctions required. Two special techniques are used to overcome these problems: a resist-thinning process using isotropic oxygen plasma ashing for the fabrication of the gate electrodes, and a process of solid-phase diffusion from phosphorus-doped silicated-glass gate sidewalls for fabrication of the source and drain junctions. The resulting 40-nm gate electrodes and ultrashallow 10-nm junctions have an adequate impurity concentration and have been shown to function successfully. It has been confirmed that these 40-nm gate-length n-MOSFETs have good electrical characteristics at room temperature. Certain details of small-geometry MOSFET electrical characteristics are studied using these devices.
Measurement of AlxGa1−xAs temperature dependent optical constants by spectroscopic ellipsometry
Authors: Kuo C. H. , Anand S. , Fathollahnejad H. , Ramamurti R. , Droopad R. , Maracas G. N.
Published: July 2002

The optical constants of AlxGa1−xAs as a function of temperature in the range 1.24–5.00 eV have been measured using spectroscopic ellipsometry (SE) in situ immediately after growth in a specially designed molecular-beam epitaxy chamber. Because of a lack of native oxides, capping layer, and other surface adsorbates a simple two-phase (thin film/vacuum) model was used to calculate the optical constants directly from the SE data. In the photon energy region where the Alx Ga1−xAs layer became transparent, the dielectric function oscillates due to interference effects between the substrate and the thin film. The ε2 value in this oscillating region was assigned a value of zero and the Kramers–Kronig dispersion relation was then used to calculate ε1 for ε2. The dielectric functions, ε1 and ε2, of AlxGa1−xAs around the band gap region was determined by the three phase model (vacuum/thin film/substrate) with the knowledge of the layer thickness from transmission electron microscopy.
Comparison of the electrical characteristics of Si metal–insulator–semiconductor tunnel diodes with interfacial layer grown by rapid thermal oxidation of Si in O2 and in N2O
Authors: Eftekhari G.
Published: July 2002

The electrical properties of Si metal–insulator–semiconductor tunnel diodes with the interfacial layer grown by rapid thermal oxidation in oxygen [case (a)] and in N2O [case (b)] were analyzed. The interface state density in case (b) was found lower than that in case (a) by a factor of 4. The reverse current in case (b) was decreased by more than an order of magnitude. The interface state density is further reduced after high-temperature annealing. Diodes in case (b) showed better thermal stability and reproducibility. The formation of Si–N bonds and variation in trap density and fixed charges were used to explain the observations.
Field emission from ion-milled diamond films on Si
Authors: Asano Tanemasa , Oobuchi Yoshimichi , Katsumata Satoshi
Published: July 2002

Diamond grains were grown on Si substrates by plasma-enhanced chemical vapor deposition. Ar ion milling was applied to the diamond/Si structures. It has been found that sharp diamond cones can be formed by ion milling if diamonds are in the form of isolated grains. It has also been found that the field emission current from diamond/Si samples is drastically increased by Ar ion milling and subsequent heat treatment in vacuum.
Sequential tunneling through n-type GaAs/AlGaAs multi-quantum-well structures with Schottky and ohmic contacts
Authors: Han Z. Y. , Yoon S. F. , Radhakrishnan K. , Zhang D. H.
Published: July 2002

The sequential tunneling of electrons through n-type GaAs/AlGaAs multiple-quantum-well (MQW) structures with Schottky and ohmic contacts is reported. Two series of tunnelings corresponding to those between the ground state and the first and second excited states were observed in low-temperature current–voltage (I–V) characteristics in 200-Å-period and 240-Å-period MQWs. The oscillations in the conductance (G) and steplike features in the I–V characteristics were found to be persistent up to 200 K. Unlike previous investigations using structures with nominally undoped MQW regions, S-shaped steplike features in the I–V characteristics corresponding to the negative differential resistance (NDR) effect, were observed at low temperature in our MQW structures with Schottky contacts and uniformly doped n-type MQW regions. The electron tunneling process through the MQW structures is discussed in terms of the creation and propagation of high electric field domains as a function of the applied bias.
Characterization of the Si/GaAs(110) interface by soft x-ray surface x-ray absorption fine structure
Authors: Hasnaoui M. L. , Flank A. M. , Delaunay R. , Lagarde P.
Published: July 2002

Surface x-ray absorption fine structure experiments have been carried out on the system Si/GaAs(110) at coverages ranging from 0.8 to around 2 monolayers (ML) and at room temperature. Polarized experiments have allowed us to propose a model for the silicon adsorption site that has the initial silicon atoms sitting just above the middle of the As–As and Ga–Ga bond along the [001] direction. Above one monolayer, silicon atoms begin to be ordered with a siliconlike structure. At thicker coverages (above 3 ML) the silicon layer is amorphous, which permits a determination of the thickness probed by x-ray absorption using the total electron yield technique.
Micromachined field emission cathode with an integrated heater
Authors: Das John H. , MacDonald Noel C.
Published: July 2002

We report on the fabrication and operation of integrated micromachined and suspended field emission cathode tips. The emitter tips are suspended at the center of a silicon beam using multiple patterning methods of high aspect ratio silicon structures using a single layer silicon dioxide etch mask. The integrated, suspended silicide microheater is used to clean and to activate the cathodes. The silicon tips are subsequently modified to improve their emission properties. The modified cathodes exhibit a significantly lower turn on voltage of about 50 V (∼100 V for silicon cathodes), and need no chemical tip cleaning prior to emission. A much larger emission current with reduced emission noise has been observed when a single tip is cycled through the tip forming process.
Miniature Schottky electron source
Authors: Kim H. S. , Yu M. L. , Kratschmer E. , Hussey B. W. , Thomson M. G. R. , Chang T. H. P.
Published: July 2002

A miniature Schottky electron source has been developed and evaluated for applications in a new generation of scanning tunneling microscope aligned field emission microcolumns. Both the physical dimensions and the heating power of this source have been significantly reduced from a conventional source of the same kind. Operating parameters for such a source in a microcolumn environment in terms of emission characteristics, suppressor operating range, etc., have been evaluated. Test results show that very good emission stability at ≥100 μA emission current over several hours, and axial angular current densities in excess of 100 μA/sr can be obtained. Energy distributions have been measured using a carefully calibrated analyzer, and the results show a full width at half-maximum of 0.4 to 0.76 eV for a 0.3 μm radius Schottky source operating over an angular current density range of 1 to over 100 μA/sr. A significant change in the shape of the energy distribution was observed over this range of operation, indicating evidence of tunneling currents at the high angular current density regime.
Reactive ion etching of silicon stencil masks in the presence of an axial magnetic field
Authors: Pendharkar S. V. , Wolfe J. C. , Rampersad H. R. , Chau Y.-L. , Licon D. L. , Morgan Mark D. , Horne William E. , Tiberio R. C. , Randall J. N.
Published: July 2002

A reactive ion etching (RIE) system with a magnetic field, mainly parallel to the dark space electric field on the etch electrode, is described. This axial-field RIE system has been used with a molecular bromine (Br2) plasma to fabricate Si stencil masks for ion beam lithography. Silicon dioxide (SiO2) films were used as masking layers, and the etching process was optimized for smallest silicon wall angle at a constant Si/SiO2 selectivity of 50. The optimized process was used to fabricate 0.75–1.0-μm-thick Si masks with sub-50 nm resolution. Over a thickness of 0.75 μm, change in linewidth was found to be less than 10 nm, corresponding to a wall angle of less than 0.2° or 3 mrad.
Toward accurate metrology with scanning force microscopes
Authors: Martin Yves , Wickramasinghe H. Kumar
Published: July 2002

Scanning force microscopes (SFMs) have become valuable instruments for inspection of surfaces on a submicron scale in the semiconductor industry. Their ability to track the position of a surface with an accuracy of the order of 1 nm opens the door to applications in micro- or nanometrology. Standard SFMs, using a conical shaped tip, are being applied to measurements of the height or depth of shallow features, such as via holes or small lines and trenches, that cannot readily be characterized by scanning electron microscopy or optical techniques. Recent technical developments of the SFM have provided new capabilities for accurate measurement of the line and trench width using a flared tip and an improved scanning and tracking method for the tip. Laboratory tests as well as industrial tests have revealed that repeatability in measuring critical dimensions approaches 1 nm. If careful attention is given to issues of calibration, particularly for the motion of the tip and for dimensioning of the tip itself, SFM techniques may attain accuracies that approach their measurement precision.
Lithography using electron beam induced etching of a carbon film
Authors: Wang D. , Hoyle P. C. , Cleaver J. R. A. , Porkolab G. A. , MacDonald N. C.
Published: July 2002

A focused electron beam was found, in the presence of oxygen gas, to induce the etching of a plasma enhanced chemical vapor deposited amorphous hydrogenated carbon (PECVD a-C:H) film. This reaction was used to pattern the film directly, eliminating the need for subsequent development of the exposed areas (as required for conventional resists). Pattern transfer from the film into gold and into silicon was investigated. Submicrometer patterns have been transferred from a 50-nm-thick PECVD a-C:H film into a gold film by wet chemical etching. With a silicon substrate, the exposure process produced an etch-resistant layer on the surface.
Surface topography of phosphorus doped polysilicon
Authors: Hegde Rama I. , Paulson Wayne M. , Tobin Philip J.
Published: July 2002

The surface topography of doped polysilicon films was investigated by atomic force microscopy for a wide range of doping and process conditions. These low-pressure chemical vapor deposition silicon films were approximately 350 nm thick. The amorphous films were in situ phosphorus doped during deposition at 550 °C, while the crystalline films were deposited at 625 °C and subsequently diffusion doped using either PH3 or POCl3 gases. Measured resistivities ranged from 700 to 10 000 μΩ cm corresponding to secondary ion mass spectrometry phosphorus concentrations that ranged from 8.45 to 0.95×1020 cm−3. In situ doped films exhibited the smoothest surface topography with a peak-to-valley surface roughness of 11 nm. The surface roughness values were 50 nm for PH3 doped poly films, and as high as 135 nm for the POCl3 doped films. Atomic force microscopy grain size analysis showed uniform distributions for the in situ and PH3 doped films with grain sizes of 130 and 200 nm, respectively. POCl3 doped poly-Si showed bimodal grain size distributions, with the secondary grains measuring 500 nm in size and the normal grains averaging 225 nm. These secondary grains increased the surface roughness and their occurrence correlates with chlorine concentration. The number of secondary grains and their size increases with higher phosphorus content. Following the polyoxide growth, the surface roughness increased 3× to 5× with POCl3 doping, but the surface topography increased only slightly for PH3 and in situ doped poly-Si. After removing the polyoxide, the surface roughness decreased for the diffusion doped films. In situ doped films retained their smooth surface following the oxidation and removal of the oxide.