bluuebunny/crossref_metadata_2025 huggingface dataset:
Evaluation of ohmic contacts to GaAs(100) and (111)B using InAs/GaAs short period superlattice and InGaAs intermediate layers
Authors: Hooper S. E. , Wilks S. P. , Elliot M. , Westwood D. I. , Woolf D. A. , Williams R. H.
Published: July 2002

The nature of InAs/GaAs short period superlattice (SPS) and InxGa1−xAs based ohmic contacts, grown by molecular-beam epitaxy, on GaAs(100) and (111)B surfaces have been investigated. The resistive nature of the contacts formed on GaAs(100), using an intermediate InAs/GaAs SPS between the metal and the substrate were found to be comparable with those obtained with an equivalent In0.33Ga0.67As interlayer. A model is described that accounts for the ohmic contact to the InxGa1−xAs structure and may apply to the SPS. Unlike the GaAs(100) configurations, similar contacts to GaAs(111)B were found to differ greatly in their resistive nature; this is a likely consequence of the laminar growth mode and poor crystalline quality of InAs epilayers grown on this substrate orientation, coupled to a higher barrier at the InAs–GaAs(111)B interface.
Experimental study of aerial images in x-ray lithography
Authors: Guo Jerry Z. Y. , Leonard Q. , Cerrina F. , Di Fabrizio E. , Luciani L. , Gentili M. , Frank J.
Published: July 2002

Depth-of-focus (gap latitude in proximity lithography) and exposure latitude are the two most important figures-of-merit for linewidth control in lithography. In a recent article, the preliminary result of the experimental verification of this theoretical study was presented—the linewidth variation as a function of gap. In this article, the experimental results of the exposure latitude are presented and discussed. Excellent agreement between experimental and simulation results are found. Optimized parameters based on verified model are presented here.
Patterning of x-ray masks using the negative-acting resist P(SI-CMS)
Authors: Mixon D. A. , Novembre A. E. , Tai W. W. , Jurgensen C. W. , Frackoviak J. , Trimble L. E. , Kola R. R. , Celler G. K.
Published: July 2002

The copolymer of trimethylsilylmethyl methacrylate with chloromethylstyrene [P(SI-CMS)] is a negative electron-beam and deep-UV resist which can withstand erosion in O2-containing plasma environments [J. R. Maldonado, J. Electron. Mater. 19, 6699 (1990)]. Manipulation of its composition and molecular weight allows control of the etch resistance and radiation sensitivity properties. Methods have been developed to provide reproducible synthesis of P(SI-CMS) with molecular weight and composition tailored to specific lithographic demands. For x-ray mask patterning, the copolymer having a 90:10 mole ratio of SI:CMS [P(SI90-CMS10)] and M̄w between 30 000 and 41 000 g/mol has been found to provide an optimal combination of resist sensitivity, dry-etching resistance, and pattern resolution. P(SI90-CMS10) is used to image the Cr–W–Cr metallization layer of a monolithic x-ray mask structure, by subtractive etching techniques [G. K. Celler et al., Appl. Phys. Lett. 59, 3105 (1991); G. K. Cellar et al., J. Vac. Sci. Technol. B 10, 3186 (1992); C. W. Jurgensen et al., J. Vac. Sci. Technol. B 9, 3280 (1991)]. The resist pattern is transferred into chromium with a O2–Cl2 plasma, and then into tungsten with a fluorine-based plasma. Resolution of 0.15 μm (line–space) in the resist has been achieved with 50 keV electron-beam exposure, and 0.20 μm patterns (line–space) have been successfully transferred into the tungsten of the x-ray mask.
Fabrication of 100 nm T-gates for monolithic microwave integrated circuits using x-ray lithography
Authors: Gupta Nitin , Hector Scott D. , Rhee Kee W. , Smith Henry I.
Published: July 2002

A process for fabricating T-gate structures using x-ray lithography is described along with experimental results comparable to the best reported for alternative processes. A trilayer of poly(methylmethacrylate)-based resists is used. Only one alignment, one exposure, and one development are required. Aerial image and development simulations confirm experimental results and enable the process to be optimized for any spectrum.
Electron cyclotron resonance ion stream etching of tantalum for x-ray mask absorber
Authors: Oda Masatoshi , Ozawa Akira , Yoshihara Hideo
Published: July 2002

Electron cyclotron resonance ion stream etching of Ta film was investigated for preparing x-ray mask absorber patterns. Ta is etched by the system at a high rate and with high selectivity. Using Cl2 as etching gas, the etch rate decreases rapidly with decreasing pattern width below 0.5 μm and large undercutting is observed. The problems are reduced by adding Ar or O2 gas to the Cl2. Etching with a mixture of Cl2 and O2 produces highly accurate Ta absorber patterns for x-ray masks. The pattern width dependence of the etch rate and the undercutting were simulated with a model that takes account of the angular distribution of active species incident on the sample. The experimental results agree well with those calculated assuming that the incidence angles are distributed between −36° and 36°. The addition of O2 or Ar enhances ion assisted etching.
Interface structure of the 1 monolayer (2×1)-Si/GaAs(001) system by x-ray photoelectron diffraction
Authors: Tran T. T. , Chambers S. A.
Published: July 2002

X-ray photoelectron diffraction was used to investigate the interface structure created by deposition of 1 monolayer-equivalent (ML-eq) of Si onto As-stabilized GaAs(001)-c(2×8)/(2×4). The system was grown via molecular-beam epitaxy at a substrate temperature of 450 °C. AlKα-excited azimuthal angle-resolved Si 2p photoelectron intensities were measured and compared with model angular distributions generated by means of spherical-wave, single-scattering cluster theory. Both the distribution of 1 ML-eq of Si atoms and the coordinates of atoms in the near-surface region were varied to optimize agreement with experiment. Substantial evidence was found for substitutional Si in the first three atomic layers. The extensive amount of diffraction modulation observed at polar angles up to 90° indicates that Si does not form a segregated ML at the surface, but rather, diffuses into the subsurface region. The best agreement between theory and experiment was obtained by modeling the ratio of Si fractional occupancies in the first, second, and third layers as 1.0:0.5:0.25. In addition, the (2×1) reconstruction geometry was modeled with a topmost Si atom displacement of 0.5 Å from bulk-truncated coordinates along [110] (the dimer bond direction), and a second-layer Si atom displacement of 0.4 Å along [1̄10]. Thus, the near-surface region is under considerable strain as Si atoms form surface dimers while being ‘‘anchored’’ to a lattice with dimensions that are 4% larger than those of bulk Si. These results provide an explanation for the unusual behavior of the AlAs/Si/GaAs heterojunction band offset.
Electronic properties of Sb monolayers on III–V(110) surfaces determined by resonance Raman scattering
Authors: Esser N. , Köpp M. , Haier P. , Kelnberger A. , Richter W.
Published: July 2002

Resonance Raman scattering (RRS) experiments have been performed to explore the electronic states via the vibrational modes of highly ordered (1×1) monolayers of Sb on InP(110) and GaAs(110) prepared by thermal annealing. In accordance to the CS symmetry of the two-atom unit cell of the monolayers three modes of A′ and one of A″ symmetry are observed by Raman scattering. The scattering intensities show pronounced resonances which are related to electronic transitions between the two-dimensional electronic surface bands. The resonances are found to be characteristic for each vibrational mode. This can be understood in terms of different electronic bands being involved in the scattering process. For Sb on GaAs(110), the resonance of the A″ mode yields a maximum between 2.2 and 2.3 eV. On InP(110), resonances at 2.55 eV for the 1A′, 2.3–2.6 eV for the 2A′, 1.9 eV for the 1A″ and ≥3.0 eV for the 3A′ modes occur. For both Sb on GaAs and InP the transition energies observed by RRS coincide with features reported by several other techniques. The resonance behavior of the ideal monolayers can be understood as a fingerprint of the joint density of states at the interface. In contrast, resonance Raman spectra taken after deposition of 1.5 ML Sb without annealing reveal a different resonance behavior. In this case, where the monolayer is not ideally ordered, the resonances are smeared out over an energy range between 2.3 and 2.7 eV revealing the influence of imperfections on the electronic band structure.
Scanning tunneling microscope study of GaAs(001) surfaces grown by migration enhanced epitaxy
Authors: Kim J. , Gallagher M. C. , Willis R. F. , Fu Jianming , Miller D. L.
Published: July 2002

We report an investigation of the morphology of p-type GaAs(001) surfaces using scanning tunneling microscopy (STM). The substrates were prepared using two methods: migration enhanced epitaxy (MEE) and standard molecular-beam epitaxy (MBE). The STM measurements were performed ex situ using As decapping. Analysis indicates that the overall step density of the MEE samples decreases as the growth temperature is increased. Nominally flat samples grown at 300 °C exhibited step densities of 10.5 steps/1000 Å along [110] dropping to 2.5 steps at 580 °C. MEE samples exhibited a lower step density than MBE samples. However as-grown surfaces exhibited a larger distribution of step heights. Annealing the samples reduced the step height distribution exposing fewer atomic layers. Samples grown by MEE at 580 °C and annealed for 2 min displayed the lowest step density and the narrowest step height distribution. All samples displayed an anisotropic step density. We found a ratio of A-type to B-type steps of between 2 and 3 which directly reflects the difference in the incorporation energy at steps. The aspect ratio increased slightly with growth temperature. We found a similar aspect ratio on samples grown by MBE. This indicates that anisotropic growth during MEE, like MBE, is dominated by incorporation kinetics. MEE samples grown at 580 °C and capped immediately following growth exhibited a number of ‘‘holes’’ in the surface. The holes could be eliminated by annealing the surface prior to quenching.
Novel Si-based superlattices consisting of alternating layers of crystalline Si and porous amorphous Si1−xGex alloys
Authors: Fathauer R. W. , George T. , Jones E. W. , Pike W. T. , Ksendzov A. , Vasquez R. P.
Published: July 2002

Superlattices consisting of alternating layers of crystalline Si and porous amorphous Si1−xGex have been studied. These are fabricated by immersing mesa structures of molecular-beam-epitaxy grown Si/Si0.7Ge0.3 superlattices in an HF:HNO3:H2O solution. A high selectivity in the pore formation leads to lateral penetration of pores ≊0.7 μm into 5-nm-thick Si0.7Ge0.3 layers. The effect of the etch on layers with differing alloy composition, thickness, and strain has been examined. Homogeneous strain has been identified as an important factor in establishing the selectivity of the etch, but other factors clearly play important roles as well.
Atomic force microscopy study of surface morphology of GaAs grown by atomic layer epitaxy using trimethylgallium and arsine
Authors: Park Seong-Ju , Ha Jeong Sook , Ro Jeong-Rae , Sim Jae-Ki , Lee El-Hang , Jeon Il Cheol
Published: July 2002

We have investigated the surface morphologies of GaAs epilayers grown by atomic layer epitaxy (ALE) with atomic force microscopy (AFM) in air. GaAs epitaxial layers were prepared by the ALE technique using trimethylgallium and arsine in an ultrahigh vacuum chemical vapor deposition system. AFM images of ALE epilayers grown at temperatures of 430 and 470 °C showed smooth surfaces with two-dimensional islands. Furthermore, surface morphology of ALE samples grown at 470 °C was rougher than that at 430 °C presumably due to the desorption of arsenics at higher temperature. These results suggest that the films have been grown in migration-enhanced epitaxylike mode rather than in an ideal self-limiting ALE mode.
Conformational differences in two mutant hinge IgG4 antibodies observed by scanning tunneling microscopy
Authors: Kreusel K. M. , Adair J. R. , Beeley N. R. A. , Davies M. C. , Jackson D. E. , Roberts C. J. , Tendler S. J. B. , Williams P. M.
Published: July 2002

Images have been obtained of two types of chimeric B72.3 anti-TAG-72 mutant hinge IgG4 antibodies using scanning tunneling microscopy. The antibodies were specifically engineered to show differences in their conformation due to site specific modifications in their hinge regions. To overcome initial difficulties due to tip sweeping effects, the method of immobilization employed was spray deposition on mica followed by rotary shadowing with platinum/carbon (Pt/C). Topographic data were obtained, and differences between the two mutant forms of the antibody were clearly visible.
Atomic force and scanning electron microscopy of Xenopus laevis oocytes
Authors: Solletti J.-M. , Kasas S. , Bertrand D. , Weisenhorn A. L.
Published: July 2002

The atomic force microscope (AFM) is increasingly used to image biological objects such as DNA, actin, membranes, and cells, in air or in liquid. It is very important that certain biological objects are observed in buffer, their ‘‘natural’’ environment. Xenopus oocytes form a good model system for expressing foreign genes, since their size easily allows the introduction of cDNA, coding for membrane proteins. Since oocytes are large objects, they have to be mounted so as to be immobilized during scanning by the AFM tip. We have used different supports for mounting Xenopus oocytes: pipettes, embedding in plexiglas, and gelatine. We have attempted to image the cell membrane of the oocyte with the AFM, but could only visualize the vitelline envelope and the yolk platelets. In the scanning electron microscope, we found that the cell membrane of the oocyte is rich in microvilli. The difficulty in imaging oocyte membrane with an AFM comes from the fact that the microvilli make the cell membrane of the oocyte very rough and deformable, thus creating instabilities for the tip during the imaging process. It thus seems impossible to image proteins that are inserted in the cell membrane of oocytes bearing microvilli. The use of oocytes with practically no microvilli may perhaps solve the problem of instability.
In situ characterization of the liquid–solid interface by scanning tunneling microscopy
Authors: Kim J. C. , Khang Y. H. , Shim T. E. , Lee D. H. , Jeon D. , Kuk Y.
Published: July 2002

A chemical cell with sorption and turbomolecular pumps was attached to an ultrahigh vacuum (UHV) scanning tunneling microscopy (STM) chamber to study the structure of surfaces etched or passivated by various chemicals. This system allows us to study various liquid–solid interfaces with little or no contamination that usually occurs in ambient pressure STM or during transportation of a sample in the air. Silicon surfaces treated by dilute HF, mixed solution of HF with NH4F and KOH were imaged by a UHV STM, revealing different surface structures. This system can be used as a new device for characterization of liquid–solid interfaces.
Thin-film induced stress in GaAs ridge-waveguide structures integrated with sputter-deposited ZnO films
Authors: Kim Hong Koo , Kleemeier Walter , Li Yabo , Langer Dietrich W. , Cassidy Daniel T. , Bruce Douglas M.
Published: July 2002

ZnO films were deposited on a GaAs ridge structure using radio-frequency (rf)-magnetron sputtering. A SiO2 thin buffer layer was introduced to alleviate a thermal mismatching problem between the ZnO film and the GaAs substrate. Deposition parameters such as rf power, distance, and gas composition/pressure were optimized to obtain highly c-axis oriented and highly resistive ZnO films. Postdeposition anneal treatment at 430 °C for 5–10 min was found to enhance c-axis orientation of the ZnO films dramatically and to reduce intrinsic stress significantly. Stress on the cleaved facet of the waveguide was imaged with a spatially resolved and polarization-resolved photoluminescence technique. The results showed that the GaAs mesa is stressed up to 1×109 dyn/cm2 (10−3 strain) due to residual stress from the ZnO/SiO2/GaAs structure.
Reactive ion etching of InP via holes
Authors: Hur Katerina Y. , Guerin Brian J. , Kazior Thomas E.
Published: July 2002

A high resolution dry etch process for InP via holes is presented. Using Cl2:HBr:BCl3:Ar reactive ion etching, etch rates in excess of 1 μm/min have been obtained on InP substrates with feature sizes as small as 15×15 μm2. The etch rate selectivity between InP and the photoresist was found to improve with the addition of HBr. The tapered sidewall profiles, suitable for subsequent metallization steps, can easily be achieved with this process.
Real space imaging of electron scattering phenomena at metal surfaces
Authors: Avouris Ph. , Lyo I.-W. , Walkup R. E. , Hasegawa Y.
Published: July 2002

Real space studies of the interaction of the two-dimensional electron gas provided by metal surface states with localized scatterers are presented. The results involve electron scattering by steps and point defects (adsorbates) at Au(111) and Ag(111) surfaces. These scattering events lead, through interference, to an oscillatory local density of states (LDOS), which is imaged in maps of (dI/dV)/(I/V). Analysis of the LDOS oscillations provides insights into the scattering phenomena involved. We show that the decay of the amplitude of the oscillations as a function of distance from the scatterer can be accounted for by a model that describes the loss of coherence as a result of the wave number (k∥) spread of the states probed by the STM. This model also explains the energy dependence of the amplitude of the oscillations and provides a basis for comparing results from different metal surfaces. Analysis of the properties of the oscillations shows that at low k∥, steps act very much like hard walls isolating the surface states of different terraces. At high k∥, however, the barriers become permeable. The circular LDOS waves that surround individual adsorbates on the terraces can be used to obtain information on their nature and charge state. Finally, we show that when the size of a terrace is decreased to a few coherence lengths, as in the case of small metal islands, quantum size effects appear. State densities of confined (‘‘particle in a box’’) states are revealed in the spectroscopic images.
Accelerated radiation damage studies of antireflection materials on SiC x-ray mask membrane
Authors: Shoki T. , Ohkubo R. , Kosuga H. , Yamaguchi Y. , Annaka N. , Wells G. M. , Yamazaki K. , Cerrina F.
Published: July 2002

The effect of antireflection (AR) coating on optical transparency for SiC membrane and the accelerated radiation damage of thicker AR film of 0.4 μm in thickness on a 2-μm-thick SiC membrane have been investigated in detail using SiO2, Al2O3, and ITO (indium tin oxide) films. SiO2, Al2O3, and ITO films were suitable for obtaining higher optical transparency and reducing the amplitude of interference fringes. The transmittance above 80% at 633 nm was achieved for 1-μm-thick as-deposited SiC membrane with these AR films on both sides. AR films showed film stress change toward the compressive direction by synchrotron radiation (SR) irradiation. ITO film was found to have the strongest durability against SR irradiation among the AR films studied. The measured maximum and 3σ values for X and Y coordinates of the SR-induced displacement of the SiC membrane with 0.4-μm-thick ITO film on one side were 30 nm and X=11 and Y=39 nm, respectively, after irradiation of 191 kJ/cm2. Optical transparency of the mask membrane with ITO film was not changed after SR irradiation.
Edge position measurement with a scanning probe microscope
Authors: Griffith J. E. , Marchman H. M. , Hopkins L. C.
Published: July 2002

A fundamental requirement in critical dimension measurement is determination of the edge positions. Edge position measurement can be degraded by many sources of error, the probe-sample interaction usually being the most important of them. In most cases a sharp conical probe is adequate to locate the top of an edge, and it can scan the entire edge if the walls are not too steep. We discuss aspects of the measurement that affect the uncertainty, such as tip radius, instrument drift, and surface roughness. Measurements with a conical probe of two samples are included. The first is very thin chrome on a quartz substrate, and the second sample is photoresist on a Si wafer.
Basis of macroscopic and microscopic surface shaping and smoothing by plasma assisted chemical etching
Authors: Zarowin C. B.
Published: July 2002

This is a summary of the basis of macroscopic shaping (or figuring) and microscopic pattern transfer and microsmoothing (or polishing) by plasma assisted chemical etching (PACE). The ability of PACE to precisely shape surfaces without mechanical contact stems from the reliable etch rate map of a plasma assisted chemical etch tool by programmed scanning of the etch tool’s profile over the surface to be shaped. For a given shape modification and the plasma tool removal rate map this programmed tool dwell time is obtained by deconvolution using Fourier transforms. The transforms of these quantities also yield their spatial frequency characteristics and clarify PACE’s figuring behavior, but do not explain the observed polishing. Both polishing and micropattern transfer etch profiles are governed by a differential surface evolution equation, whose behavior is reviewed here. We also show why the PACE process is able to etch directionally, produces negligible subsurface damage and vertical etch rates to 100 μm/min or volume removal rates of ∼10 mm3/min for a 1 cm diam plasma due to the atypical plasma regime employed.
Radical beam ion-beam etching of InAlAs/InP using Cl2
Authors: Yu D. G. , Hu E. L. , Hasnain G.
Published: July 2002

A Cl2 radical beam ion-beam etching (RBIBE) system was used to etch InP-based materials. In InAlAs/InAlGaAs heterostructures, vertical sidewalls, smooth surfaces, and no delineation of the epilayers resulted from etching at elevated temperatures (≳150 °C) and low ion-beam energies (≤300 eV). Rapid etch rates (≳1 μm/min) were also achieved under these conditions. This work demonstrates that reliable anisotropic and angled etching of InP-based III–V compound semiconductors is possible with the RBIBE system.
Vibrational and structural studies of oxygen adsorption on the Ag(110) surface
Authors: Hashizume T. , Rowe J. E. , Malic R. A. , Motai K. , Cho K. , Kishimoto J. , Sakurai T.
Published: July 2002

Adsorption of oxygen on the Ag(110)1×1 surface is studied by field ion-scanning tunneling microscopy (FI-STM) and high resolution electron energy loss spectroscopy (HREELS). Linear chain (added-row) formation in the [001] direction upon oxygen adsorption is observed. The step movement and step bunching are observed at the exposure lower than 60 L. Another type of chain formation are observed between the added rows with the local 3×1 configuration. The coverage and time-dependent HREELS data shows rather complicated dynamics in the case of room temperature adsorption. The HREELS data with the resolution of 4 meV (FWHM) shows an extra broad, weak peak at 15 meV, which has a maximum peak value at the exposure near 1800 L corresponding to the completion of the 3×1 phase.
Two-dimensional ordered Si3N4 nanoparticles observed by a scanning tunneling microscope
Authors: Zheng Z. J. , Zuo D. L. , Li D. H.
Published: July 2002

In this paper nanometer-sized particles of Si3N4 deposited on the surface of a highly oriented pyrolytic graphite (HOPG) under ultrahigh vacuum conditions have been observed. The reproducible scanning tunneling microscope (STM) topographic images show a two-dimensional ordered structure with a hexagonal close-pack lattice that covered a much smaller area of the HOPG surface. The lattice constant is about 24 nm, which is comparable to the sizes of the nanoparticles observed by transmission electron microscopy and is about 100 times that of the HOPG’s constant. A physical interaction is suggested to exist between the nanoparticles and substrate. The STM results demonstrate the anomalous electrical conductive mechanism during the experiment of the ceramic nanoparticles; however, Si3N4 in bulk form is an insulating material. The origin for the distortion of the images is discussed as well.
Tunneling-electron-induced adsorption of pyridine on a hydrogen-terminated silicon surface
Authors: Komiyama Masaharu , Kirino Manabu
Published: July 2002

Pyridine molecules were found to adsorb on a hydrogen-terminated silicon surface when electrons were tunneled through pyridine vapor to the silicon surface. The adsorption was apparently induced by the tunneling electrons, and its desorption also appeared to be induced by tunneling electrons. These findings consist of the first observation of tunneling-electron-induced adsorption–desorption on a solid surface.
Time-resolved atomic-scale modification of silicon with a scanning tunneling microscope
Authors: Grey F. , Huang D. H. , Kobayashi A. , Snyder E. J. , Uchida H. , Aono M.
Published: July 2002

Voltage pulses applied between the tip of a scanning tunneling microscope (STM) and a crystal surface often result in extraction or deposition of single atoms or clusters of atoms on the surface, which are detected by imaging the affected area with the STM after the pulse. Here, atomic-scale displacements of the STM tip during such surface modifications, due to structural changes occurring at the tunnel junction are studied. Using this technique, it is found that for the Si(111)7×7 surface and a W tip, modification during the pulse is field induced and not current induced, but that a considerable fraction of modification events occur shortly after a pulse. The ability to predict the type of modification from the direction of the tip displacement is investigated, and the applicability of this approach to high-speed nanofabrication is discussed.
Photon spectroscopy, mapping, and topography of 85% porous silicon
Authors: Dumas Ph. , Gu M. , Syrykh C. , Hallimaoui A. , Salvan F. , Gimzewski J. K. , Schlittler R. R.
Published: July 2002

We have used scanning tunneling microscopy simultaneously as a minority charge injection source for local cathodoluminescence measurements and as a topographic characterization technique to investigate porous silicon films on the nanometer scale. Photon spectra stimulated by field emission electrons show close similarities to photoluminescence spectra recorded on similarly prepared samples. Photon maps of the integral photon intensity show sharp contrast on the nanometer scale, and the surface topography reveals structural features also on that scale. We discuss our observations in terms of the proposed mechanism for the optical properties of this modification of Si. In particular, our results are found to be consistent with models involving quantum-sized effects which increase the band gap and shift it to a direct band gap at k=0.
Direct sublattice imaging of semiconductor materials
Authors: McGibbon A. J. , Chisholm M. F. , Pennycook S. J.
Published: July 2002

By employing the technique of Z-contrast imaging in a 300 kV scanning transmission electron microscope, we show that it is possible to obtain directly interpretable, atomic resolution images of interface structures in semiconductor materials. With particular emphasis on the study of the CdTe/GaAs(001) system, we demonstrate that such an approach enables the direct observation of the sublattice in compound semiconductors and, as a direct consequence of this, facilitates the observation of atomic arrangements (in some cases unexpected) at dislocation cores on the column-by-column level. In addition, we demonstrate that the technique can also be applied to the study of grain boundary structures, by showing examples taken from boundaries in Si and SrTiO3.
Optical studies of heterointerfacial growth interrupts in type-II GaAs/AlAs superlattices by time resolved photoluminescence imaging
Authors: Chang T. , Fu L. P. , Bacalzo F. T. , Gilliland G. D. , Wolford D. J. , Bajaj K. K. , Antonelli A. , Chen R. , Klem J. , Hafich M.
Published: July 2002

We have quantitatively characterized the heterointerfaces in type-II GaAs/AlAs short-period structures with various types of growth interrupts. The quality of the heterointerfaces is governed by two factors: the magnitude of the potential fluctuations or interface roughness and the density of nonradiative defect centers incorporated at the heterointerfaces during growth. We have quantified these aspects of the heterointerfaces through photoluminescence (PL), PL time-decay, and time-resolved PL-imaging measurements. We find that growth interrupts at the normal heterointerfaces significantly improves the quality of the interfaces over growth with no interrupts in regard to the interface roughness. Further, the transport of the excitons along the heterointerfaces may be explained by interface roughness induced scattering. We also find that growth interrupts at only the normal interfaces substantially reduces the nonradiative trap densities at the heterointerfaces.
Measurements of local strain variation in Si1−xGex/Si heterostructures
Authors: Bell L. D. , Kaiser W. J. , Manion S. J. , Milliken A. M. , Pike W. T. , Fathauer R. W.
Published: July 2002

The energy splitting of the conduction-band minimum of Si1−xGex due to strain has been directly measured by the application of ballistic-electron-emission microscope (BEEM) spectroscopy to Ag/Si1−xGex structures. Experimental values for this conduction-band splitting agree well with calculations. For Au/Si1−xGex, however, heterogeneity in the strain of the Si1−xGex layer is introduced by deposition of the Au. This variation is attributed to species interdiffusion, which produces a rough Si1−xGex surface. Preliminary modeling indicates that the observed roughness is consistent with the strain variation measured by BEEM.
Distributed microwave amplifier on field emitter arrays with a nonhomogeneous energy collector
Authors: Gulyaev Yu. V. , Nefedov I. S. , Sinitsyn N. I. , Torgashov G. V. , Zakharchenko Yu. F. , Zhbanov A. I.
Published: July 2002

The nonlinear characteristics of a new type of electrically tuned distributed amplifier on field emitter arrays are considered that involve an energy collector in the form of a periodically nonhomogeneous strip line. In contrast to standard amplifiers, this collector exhibits a higher gain and operates at lower currents.
Consequences of DX centers on the charge distribution of double quantum well, δ -modulation-doped heterostructures
Authors: Young A. P. , Chen Jianhui , Wieder H. H.
Published: July 2002

We have investigated the properties of modulation-doped, pseudomorphic, AlxGa1−xAs/ InyGa1−yAs heterostructures containing embedded, Si δ-doped GaAs wells. From galvanomagnetic and Shubnikov–de Haas measurements, we have determined the charge transport parameters before and after optically induced ionization of the DX centers in the structures. We find a DX center diffusion tail associated with the δ-doped donors extending into the undoped AlxGa1−xAs layers and conclude that for efficient modulation doping of a quantum well, δ doping and embedment of the donors within a GaAs well can reduce, but not entirely eliminate the effects of DX centers on the charge transport parameters.
Lithography by accelerated nanoparticle ions
Authors: Gspann Jürgen
Published: July 2002

A nanoparticle lithography process for microstructuring surfaces is realized by eroding an area selectively through impacts of accelerated nanoparticle ions. With singly charged nanoparticles of 1000 carbon dioxide molecules, acceleration by 100 kV leads to highly supersonic impacts which give rise to shock wave-induced craters in the target. First atomic force microscopy measurements after short-time bombardment are found to be in accordance with predicted crater radii of the order of several 10 nm. Prolonged erosion leads to pronounced smoothing of the eroded surfaces.
Effect of electron-beam parameters on critical-dimension measurements
Authors: Mizuno Fumio , Yamada Satoru
Published: July 2002

In critical-dimension (CD) measurements of resist patterns, CD deviation dependent on measurement time and adjacent pattern effect are major problems created by using an electron beam. We studied the mechanism of these effects. The dependence of CD deviation on measurement time may be caused as a result of compound effects of resist thickening due to surface contamination and resist thinning due to electron-beam induced resist contraction. The adjacent pattern effect is mainly produced by surface charging. The surface charge may distort the electron-beam trajectory.
Etching temperature dependence of the surface composition and reconstruction for Cl2-etched GaAs layers
Authors: Tanaka N. , López M. , Matsuyama I. , Ishikawa T.
Published: July 2002

In order to understand the mechanism of the Cl2-etching reaction with GaAs, the composition and reconstruction of in situ Cl2-etched GaAs surfaces were studied as functions of the etching temperature. From an Auger electron spectroscopy analysis and reflection high-energy electron-diffraction observations, it was shown that the GaAs surface changed from As stabilized to Ga stabilized during low-temperature (∼50 °C) etching, while it remained As stabilized during high-temperature (150–250 °C) etching. This result can be understood by considering the temperature dependence of the desorption rate of chloride compounds. At low temperature, the desorption of Ga chlorides is more suppressed than that of As chlorides, resulting in rough Ga-stabilized surfaces. At high temperature, the desorption of any chlorides is not suppressed. Thus, stoichiometric etching is realized, resulting in smooth As-stabilized surfaces, which are advantageous for high-performance microdevice fabrication.
Growth of vertical cavity on buried-oxide substrate by gas-source Si molecular-beam epitaxy and coupled mode luminescence of strained Si1−xGex/Si quantum wells
Authors: Fukatsu S. , Nayak D. K. , Shiraki Y.
Published: July 2002

Successful growth of a new class of Si-based vertical cavity structure is demonstrated using a buried-oxide substrate [separation-by-implantation-oxygen (SIMOX)] by gas source Si molecular beam epitaxy. The cavity is terminated with the Si/buried-oxide and top Si/air interface mirrors. Integration of strained Si1−xGex/Si quantum wells in the vertical cavity lead to the observation of spectral coupling of luminescence to the cavity longitudinal modes. Clear oscillations were observed in reflectance and photoluminescence excitation spectra.
Effect of Si+ and Ge+ ions on the growth of Si1−xGex films on Si(001) using potential-enhanced molecular-beam epitaxy
Authors: Yun Sun Jin , Lee Seung-Chang , Lee Jae-Jin , Park Sin-Chong
Published: July 2002

The effect of Si+ and Ge+ source ions on surface morphology and strain relaxation of Si1−xGex (0.25≤x≤1.0) film on Si (001) substrate was investigated using potential-enhanced molecular beam epitaxy. The growth temperature ranged from 450 to 710 °C. The applied potential to the substrate was varied between −2.0 and 1.5 kV. The acceleration of a small fraction of source ions toward the substrate suppressed three-dimensional nucleation mode, and dramatically improved the surface morphology. Contrary to the negative potential, the application of a positive potential did not contribute to improving the surface smoothness of Ge on Si. For the growth of Si0.5Ge0.5 film, the surface morphology was degraded further by applying a positive potential of 1.5 kV. Si0.75Ge0.25 films grown with a negative potential of −1.6 kV relieved the strain at the much earlier stage of heteroepitaxial growth than that of conventional molecular-beam epitaxy.
Molecular-beam epitaxy growth of II–VI light-emitting devices on GaAs substrates using valved sources for S and Se
Authors: Johnson M. A. L. , Yu Z. , Cook J. W. , Schetzina J. F.
Published: July 2002

II–VI light-emitting devices containing quaternary (ZnMgSSe), ternary (ZnSSe, ZnCdSe, ZnTeSe), and binary (ZnTe, ZnSe) epilayers have been successfully grown on (100) GaAs substrates by molecular-beam epitaxy (MBE) using valved sources specially designed for elemental S and Se, respectively. Use of S and Se valved sources permits the rapid adjustment of alloy compositions for lattice-matching of Zn1−xMgxSySe1−y and ZnSzSe1−z epilayers to GaAs as is needed for growth of light-emitting diode and laser diode structures. Flux measurements have shown that these valved sources are stable for time periods necessary for MBE growth of these II–VI device structures. Secondary ion mass spectroscopy analysis has shown that B contamination level, a major problem in earlier valved-source designs, is below the detection limit of the instrument in the II–VI epilayers grown by MBE using the present S and Se valved sources. Blue and green light-emitting diodes and room-temperature pulsed laser diodes have been successfully synthesized using the valved sources.
Graded-channel InGaAs–InAlAs–InP high electron mobility transistors
Authors: Streit Dwight C. , Block Thomas R. , Wojtowicz Michael , Pascua Dimas , Lai Richard , Ng Geok I. , Liu Po-Hsin , Tan Kin L.
Published: July 2002

We have fabricated pseudomorphic InGaAs–InAlAs–InP high electron mobility transistors by molecular beam epitaxy with InxGa1−xAs channels graded from x=0.60 to x=0.80. Compared with device profiles using flat x=0.80 channels the channel conductivity is improved by 24%. Hall mobility is improved from 10 750 to 12 200 cm2/V s at 295 K with sheet charge Ns of 3.6×1012 and 3.9×1012 cm−2 for the flat x=0.80 and graded-channel profiles, respectively. Graded-channel devices with 0.1 μm T gates obtained cutoff frequency fT=305 GHz and maximum frequency of oscillation fmax=340 GHz.
A novel technique for high aspect ratio high resolution patterning of membranes
Authors: Krasnoperova Azalia A. , Chen Zheng , DiFabrizio Enzo , Gentili Massimo , Cerrina Franco
Published: July 2002

This article presents a new technique for fabrication of high aspect ratio metal patterns on membranes. Two modifications of x-ray lithographic techniques have been employed: simultaneous exposure of a photoresist coated on both sides of the membrane, and membrane exposure from back side using a negative tone photoresist. Both methods provide self-aligned masks for pattern transfer into multiple metal layers. Those techniques have been used for fabrication of high aspect ratio Fresnel zone plates for hard x-ray microscopy. 0.15 μm design rule Fresnel zone plates, with gold structure of 1.6 μm thickness have been fabricated.
Comparison of different analytical techniques in measuring the surface region of ultrashallow doping profiles
Authors: Felch S. B. , Chapek D. L. , Malik S. M. , Maillot P. , Ishida E. , Magee C. W.
Published: July 2002

Accurate knowledge of the dopant surface concentration of ultrashallow junctions is very important, especially since high surface concentrations are often encountered and these could have serious consequences on device performance. Our group has studied several different analytical techniques to understand the boron surface concentration of plasma-doped p+–n junctions that are about 60 nm deep. Secondary ion mass spectrometry has been performed with both oxygen and cesium primary ions. The O+2 secondary ion mass spectrometry was carried out both in vacuo and with an oxygen bleed to minimize surface ion yield transients. In addition, the surface boron concentration has been measured by resonance ionization mass spectrometry and Auger electron spectroscopy. Finally, the surface electrical carrier concentration of these p+–n junctions has been determined by spreading resistance profiling, differential Hall effect profiling, and microwave surface impedance profiling. A comparison of the surface results of these various measurement techniques will be presented and discussed in this article.
Application of neural networks to plasma etch end point detection
Authors: Allen Ronald L. , Moore Randy , Whelan Mike
Published: July 2002

A popular technique for ending a plasma etch process is to monitor the optical emissions from reaction species for gross changes. This at first appears to be a simple real-time edge detection task. However, differences in plasma chemistry across runs, reactor chambers, and wafer patterns combine to make this control strategy quite problematic. Moreover, as exposed wafer areas continue to shrink, distinct end points become increasingly harder to identify. This new approach is simple, robust, and flexible. The application user trains elementary pattern detectors, or neurons, on a few representative process end point shapes. Software algorithms process the training end point data to find a range of resolutions for pattern recognition. The application then automatically builds a pattern detection network that tolerates signal noise, adapts to changing end point shape, and precisely registers process end points. The neural network end point detector has been successfully tested on a wide variety of data gathered over a period of many months. The application is currently calling end point in real time on several oxide etch processes at a major wafer fabrication facility.
Plasma-deposited silylation resist for 193 nm lithography
Authors: Horn Mark W. , Maxwell Brian E. , Goodman Russell B. , Kunz Roderick R. , Eriksen Lynn M.
Published: July 2002

Plasma-enhanced chemical-vapor-deposited carbon-based polymer films are examined for use as an all-dry positive-tone photoresist for 193 nm lithography. These films are designed to crosslink upon exposure to 193 nm radiation, enabling selective silicon uptake via reaction with hydroxyl groups. After oxygen plasma pattern transfer, features with resolution below 0.25 μm have been obtained.
Diagnostics of patterning mechanisms in chemically amplified resists from bake dependencies of images
Authors: Zuniga Marco A. , Neureuther Andrew R.
Published: July 2002

Measurement and analysis of bake dependencies of resist profiles are used to establish the critical role of diffusion and acid loss in lithographic performance. Imaging experiments carried out with APEX-E with postexposure bake temperatures above and below its glass transition temperature evidence widely dissimilar feature size dependencies attributable to different acid diffusion mechanisms in the glassy and the rubbery polymer, respectively. Examination of the mechanisms determining e-beam exposed resist image stability show that, when an acid loss mechanism is present, the initial amount of acid generated the reaction rate, and the diffusion mechanisms are essential in predicting feature size dependence on postexposure bake conditions.
Extendibility of x-ray lithography to ⩽130 nm ground rules in complex integrated circuit patterns
Authors: Hector Scott , Chu William , Thompson Matthew , Pol Victor , Dauksher Bill , Cummings Kevin , Resnick Doug , Pendharkar Sandeep , Maldonado Juan , McCord Mark , Krasnoperova Azalia , Liebmann Lars , Silverman Jerry , Guo Jerry , Khan Mumit , Bollepalli Srinivas , Capodieci Luigi , Cerrina Franco
Published: July 2002

Previous experimental and theoretical evidence indicates that x-ray lithography can be used to pattern ≤180 nm features. In order to be used in manufacturing, x-ray lithography of complex integrated circuit patterns (i.e., dense two-dimensional patterns) needs to be demonstrated with a practical proximity gap. However, no large body of experimental evidence exists for the extendibility of x-ray lithography for complex patterns with ground rules of ≤130 nm at gaps of 10–20 μm. Simulations of image formation and resist dissolution are shown to have good agreement with experimental results. These simulations are then used to predict exposure latitude and gap latitude for printing one-dimensional 75–125 nm patterns at 10–15 μm gaps. Simulations indicate that at least ±10% exposure dose latitude will exist for simple patterns at these gaps, but significant nested-to-isolated linewidth bias will exist. Gaps must be controlled to ±1 μm for ±10% dose latitude. More complex two-dimensional patterns have been shown to exhibit line end shortening [J. Maldonado, R. Dellaguardia, S. Hector, M. McCord, and L. Liebmann, J. Vac. Sci. Technol. B 13, 3094 (1995)] that simulations qualitatively indicate a rise in part due to image formation. Simple serifs on line ends may be needed to reduce line end shortening.
Initial images with a partially micromachined scanning electron microscope
Authors: Crewe D. A. , Ruffin M. M. , Feinerman A. D.
Published: July 2002

The focusing properties of a microfabricated silicon electrostatic electron lens have been tested in a machine tool fabricated assembly. Images of a 200 mesh gold transmission electron microscopy wire grid at a working distance of 4 mm are being obtained in transmission. The electron source is a zirconiated tungsten thermally assisted Schottky field emitter operating at 1800 K. The electron detector is a Faraday cup. The beam is scanned over the sample using parallel plate deflectors. The silicon lens is 1.64 mm long and consists of three silicon die separated by Pyrex optical fibers. Images of the grid at magnifications >10 000 × are being obtained.
Molecular beam epitaxial growth and properties of short-wave infrared Hg0.3Cd0.7Te films
Authors: Rajavel R. D. , Wu O. K. , Jamba D. M. , de Lyon T. J.
Published: July 2002

Hg1−xCdxTe films with cut-off wavelengths ranging from 1.25 to 1.65 μm (at 300 K) in the short wave infrared band were prepared by molecular beam epitaxy. The structural perfection of the Hg1−xCdxTe films was strongly dependent on the degree of lattice matching provided by the substrate. First reports of in situ p-type doping of Hg1−xCdxTe (x≊0.7) alloys by molecular beam epitaxy is presented here. Electron and hole concentration in excess of 1×1017 cm−3 were measured in In and As doped films, respectively. Analysis of variable temperature Hall effect data assuming singly ionized impurities provided activation energies of 28.5 meV for As acceptors, and 2.95 meV for In donors.
High quality CdTe/Cd1−xMgxTe quantum wells grown on GaAs (100) and (111) substrates by molecular-beam epitaxy
Authors: Xin S. H. , Hu B. H. , Short S. W. , Bindley U. , Yin A. , Dobrowolska M. , Furdyna J. K.
Published: July 2002

We report the successful molecular-beam epitaxy growth of high quality CdTe/Cd1−xMgxTe quantum wells (x=0.27–0.52) on GaAs (100) and (111)B substrates. The effectiveness of the two different buffer layers for providing nearly lattice-matched templates were verified by x-ray diffraction and photoluminescence. Optical properties of excitons in the above quantum wells were investigated. It is observed that, for the (100) sample, the exciton displays double peaks, the peak splitting originating from exciton localization on the monolayer fluctuation of the quantum well width. Our experiments indicate that the influence of such fluctuations can be reduced, and interfaces of exceptional perfection can be obtained by growth of the CdTe/CdMgTe quantum well structures on GaAs (111)B-oriented substrates or by a combined growth of conventional molecular-beam epitaxy and migration-enhanced epitaxy on GaAs (100)-oriented substrate. The photoluminescence spectra indicate that significant reduction of the influence of the monolayer fluctuation was indeed accomplished by these procedures.
Autocorrelation function of 1/f current fluctuations in vacuum microelectronics devices
Authors: Amirkhanov R. N. , Ghots S. S. , Bakhtizin R. Z.
Published: July 2002

An analysis has been done of the autocorrelation function properties for noise processes with power spectral density function S(f)∼1/fγ in a limited frequency range. A calculation technique is proposed to determine the spectral density index γ using the autocorrelation function. It has been found that the experimentally obtained autocorrelation function of low-frequency current fluctuations for p-type Si field emitters is approximated well by a logarithmic function in the range of small argument and the results of γ calculations is in good agreement with the experimental data.