bluuebunny/crossref_metadata_2025 huggingface dataset:
Deconvolution of topographic and ferroelectric contrast by noncontact and friction force microscopy
Authors: Eng L. M. , Friedrich M. , Fousek J. , Günter P.
Published: July 2002

We report on the recent investigations of the surface of ferroelectric crystals with scanning force microscopy. In situ investigations are demonstrated for temperatures T even above the transition temperature Tc for triglycine-sulphate (TGS) and barium-titanate (BaTiO3) (Tc,TGS=49 °C, Tc,BaTiO3=126 °C. We directly observed transition from the ferroelectric into the paraelectric phase. Our inspection of the ferroelectric crystals TGS and BaTiO3 allowed the simultaneous investigation of topographic surface features and ferroelectric domain structure in these crystals. Ferroelectric domains are clearly discernible in the noncontact mode as well as the friction force mode of imaging. The domain wall width in TGS was measured to be smaller than 8 nm. Furthermore, contact force microscopy indicated the domain wall to be atomically flat, exhibiting no surface step. While image contrast in noncontact scanning force microscopy pictures is found to be a convolution of both topographic and ferroelectric forces on TGS samples, the tip/sample interaction in BaTiO3 is dominated by van der Waals forces.
Electron scattering by electron-beam mask with tapered aperture in cell projection lithography
Authors: Yamashita Hiroshi , Ema Takahiro , Itoh Katsuyuki , Nozue Hiroshi , Nomura Eiichi
Published: July 2002

Electron scattering by stencil-type masks (electron-beam mask) of various thicknesses in the cell projection lithography has been studied by Monte Carlo simulation. We also prepared 20-μm-thick masks with various taper angles so that we could compare the exposed resist patterns. The simulation study revealed that 30% of the irradiated electrons pass through a 20-μm-thick mask with a 5 μm lines-and-spaces (L/S) aperture pattern, which corresponds to a 0.2 μm L/S pattern on the wafer. This result has shown that a stencil-mask forms an image on the wafer by scattering contrast even when its thickness is greater than the electron penetration depth. The simulation study also showed that 70% of incident electrons pass through a 10-μm-thick mask. The results have also indicated that more attention must be paid to mask profile, and that a forward tapered profile (in which the electron transmittance has a peak of 85%–90% at the mask taper edge) should be avoided because it may affect a beam profile. When the taper angle is 90° or more, on the other hand, the electron transmittance shows no peak at the mask taper edge and the energy distribution become wider. These transmitted electrons therefore cannot fully contribute to imaging. These scattered electrons, however, can be completely intercepted by an appropriate limiting aperture in order to maintain high contrast. A thinner mask is thus preferable for practical applications.
Low-energy ion damage in semiconductors: A progress report
Authors: Hu Evelyn L. , Chen Ching-Hui , Green Debora L.
Published: July 2002

There has been steady progress in understanding the propagation of low-energy, ion-induced damage into semiconductor substrates. The availability of specifically designed heterostructure substrates allows us to trace the profile of damage into the material. A number of experiments, together with theoretical simulations, have confirmed the important role played by fortuitous channeling of ions, deep into the material (e.g., >1000 Å deep for incident ions energies ∼300 eV). Recent experiments have also shown the importance of rapid diffusion of ion-created defects. Using a model that includes the effects of both channeling and defect diffusion, channeling and diffusion in ion damage (CHANDID), we have deduced a room-temperature diffusion constant of D∼1×10−15 cm2/s. This is an extremely high value for diffusion at room temperature, and is more characteristic of diffusion taking place at temperatures of a few hundred to a few thousand degrees centigrade. One cause of this high value of D may be attributed to radiation enhanced diffusion: the creation of excess electrons and holes during the etch process whose subsequent nonradiative recombination transfers momentum to the defects. Preliminary experiments, which monitor the effects of above band-gap illumination during ion bombardment, validate this picture. Such understanding, of intrinsic importance, can be used to design material and device structures in which the effects of ion damage may be mitigated.
Comparison of advanced plasma sources for etching applications. V. Polysilicon etching rate, uniformity, profile control, and bulk plasma properties in a helical resonator plasma source
Authors: Lee J. T. C. , Layadi N. , Guinn K. V. , Maynard H. L. , Klemens F. P. , Ibbotson D. E. , Tepermeister I. , Egan P. O. , Richardson R. A.
Published: July 2002

Etching of polysilicon features using a helical resonator plasma source is evaluated. Performance metrics consist of etching rate, etching rate uniformity, and profile control using HBr/O2–He gas-phase chemistry. The effect of source power, rf-bias power, and reactor pressure on etching rate and uniformity is examined using a response surface experiment. Feature profile control is determined by examining nested and isolated lines and trenches using oxide mask/polysilicon/oxide structures. Good uniformity and vertical profiles are obtained at low reactor pressures, high source power, and rf-bias between 50 and 60 W. The operating point for best uniformity is at 3.5 mTorr, 3000 W source power, and 53 W rf-bias power. At this point, the etching rate is 3700 Å/min and the nonuniformity is less than 1.0%, over 125-mm-diam wafers. Radial profiles of electron temperature and ion density near the wafer surface are presented as a function of source power, rf-bias power, and reactor pressure. The ion density was found to be in the mid-1011 cm−3 range and electron temperatures were 5–7 eV. An increase in source power and reactor pressure results in an increase in ion density; however, the electron temperature shows a weaker dependence. Finally, these results are compared to those using helicon and multipole electron cyclotron resonance plasma sources evaluated in previous studies. We found that all three plasma sources provide high ion density at low pressures to meet performance demands for polysilicon etching; however, the helical resonator source offers somewhat higher etching rate and better bulk plasma uniformity.
Correlation between dynamic magnetic hysteresis loops and nanoscale roughness of ultrathin Co films
Authors: Jiang Q. , Yang H.-N. , Wang G.-C.
Published: July 2002

Ultrathin Co films were epitaxially grown on a Cu(001) surface with different initial roughness created by Ar ion sputtering. The roughness of the Cu substrate and the Co film were characterized by high resolution low energy electron diffraction. The measured angular profiles were compared with a diffraction theory for rough surfaces and the roughness parameters were extracted quantitatively. Magnetic hysteresis loops of these characterized films were measured by surface magneto-optic Kerr effect. The hysteresis loop shape and loop area can be related to the nanoscale roughness in the Co films. For the roughest film with interface width ≊1.2t, where t is the single atomic step height, the magnetization is reduced several fold compared with that of smooth films with interface width ≊0.5t. Also, the coercivity in the roughest film is the highest and there exists a wide range of nucleation centers and coercive fields for magnetic domain reversals. These are related to the high step density in the rough substrate as the pinning centers. The hysteresis loop changes its shape and area under a sinusoidal external magnetic field as functions of frequency and field amplitude for all films. For the smooth films in the low frequency and low field regimes the loop area shows a 2/3 power law scaling behavior. The 2/3 value of scaling exponents are consistent with the prediction of a dynamic mean field theory with a double-well energy barrier. For a film with the same interface width but different step density and lateral correlation length the scaling exponents deviate from 2/3 value drastically.
A, B, and C characterization of gated field emission arrays for radio frequency device performance
Authors: Zaidman E. G. , Jensen K. L. , Kodis M. A.
Published: July 2002

A method for predicting the Fowler–Nordheim parameters A and B, and the capacitance for a unit cell of a gated field emission array is developed, applied, and integrated into end-to-end rf amplifier design. Density modulated rf amplifiers require a high frequency, well characterized electron emission source for efficient operation; a cathode constructed of field emission arrays is a potential candidate for insertion into the next generation of twystode and klystrode amplifiers. The A, B, and C parameterization of the emission from model unit cell geometries is done by a boundary element technique. The behavior of A, B, and C with variation in device material and geometry is illustrated with two sample case sets. Finally, the utility of A, B, and C in predicting rf device performance requirements is illustrated.
Preparation of sharp polycrystalline tungsten tips for scanning tunneling microscopy imaging
Authors: Zhang R. , Ivey D. G.
Published: July 2002

The fabrication of scanning tunneling microscopy (STM) tips by dc downward electrochemical polishing and ion milling has been investigated. The influence of parameters, such as voltage, immersion depth, cutoff time, and solution concentration, on the shape and sharpness of electropolished W tips are presented. Both electropolished and ion milled tips, which were characterized by transmission electron microscopy (TEM), were tested on Au films deposited on 〈100〉 oriented Si. The effects of tip radius on STM images are discussed thoroughly, and the results are also compared to atomic force microscopy (AFM) and TEM images.
Nanometer-scale lithography on Si(001) using adsorbed H as an atomic layer resist
Authors: Adams D. P. , Mayer T. M. , Swartzentruber B. S.
Published: July 2002

We describe nanometer-scale feature definition in adsorbed hydrogen layers on Si(001) surfaces by exposure to low energy electrons from a scanning tunneling microscope tip. Feature sizes range from <5 to ≳40 nm as a function of bias voltage (5–30 V) and exposure dose (1–104 μC/cm). We show that the cross section for electron stimulated desorption of hydrogen has a threshold at 6–8 eV and is nearly constant from 10 to 30 eV, so that above threshold the feature profiles are a direct reflection of the electron flux profile at the surface. Radial flux distributions are best fit by a simple exponential function, where the decay length is dependent primarily on the tip–sample separation. Low intensity tails at large radius are also observed for high bias emission. Comparison to field emission simulations shows that our tip has an ‘‘effective radius’’ of approximately 30 nm. Simulations demonstrate that tip geometry and tip–sample separation play the dominant role in defining the electron flux distribution, and that optimum beam diameter at the sample is obtained at small tip–sample separation (low bias) with sharp tips. We show that adsorbed hydrogen is a robust resist that can be used as a mask for selective area deposition of metals by chemical vapor deposition. Fe lines 10 nm wide are deposited by pyrolysis of Fe(CO)5 in areas where H has been desorbed, with minimal nucleation in the H-passivated areas.
Characterization of Si1−xGex epilayers grown using a commercially available ultrahigh vacuum chemical vapor deposition reactor
Authors: Lafontaine H. , Houghton D. C. , Elliot D. , Rowell N. L. , Baribeau J.-M. , Laframboise S. , Sproule G. I. , Rolfe S. J.
Published: July 2002

Si1−xGex epitaxial layers were grown at T=525 °C using a commercially available, ultrahigh vacuum chemical vapor deposition reactor. Various techniques, including cross-sectional transmission electron microscopy, Auger electron spectroscopy, secondary ion mass spectroscopy, double crystal x-ray diffraction, and photoluminescence (PL) are used to characterize this material. For the first time, phonon resolved PL is used to map out the composition uniformity obtained with this high throughput, production-ready technology. The composition variations along most of the wafer surface (except the flats) do not exceed ±0.15%. A discussion follows on the limitations of this technology, including the critical thickness for misfit strain relaxation, compared to other growth techniques such as molecular beam epitaxy and rapid thermal chemical vapor deposition. The material grown here exhibits characteristics that are very encouraging for the prospect of manufacturing high frequency devices and circuits.
Structural and electrical characterization of TiO2 grown from titanium tetrakis-isopropoxide (TTIP) and TTIP/H2O ambients
Authors: Yan J. , Gilmer D. C. , Campbell S. A. , Gladfelter W. L. , Schmid P. G.
Published: July 2002

Thin films of TiO2 have been deposited on silicon wafers from titanium tetrakis-isopropoxide (TTIP) and TTIP/H2O using low pressure metalorganic chemical vapor deposition. The films have been characterized by Rutherford backscattering spectrometry, forward recoil spectrometry, x-ray diffraction, and electrical techniques. Dielectric constants of approximately 30 were measured by high frequency C–V. Metal–oxide–semiconductor capacitors were fabricated using sputtered platinum as the upper electrode material. Films deposited in the presence of water vapor had a much lower quality interface with the silicon substrate. Normal thermionic emission was observed when the gate electrode was positively biased and barrier heights were extracted. Dramatically reduced leakage currents and increased barrier heights were seen after 750 °C anneals in dry oxygen. Using a ramped voltage test, capacitors were found to break down at applied fields of 3.0 MV/cm.
Novel conductive transparent tip for low-temperature tunneling-electron luminescence microscopy using tip collection
Authors: Murashita T.
Published: July 2002

Tip collection of luminescence is advantageous in low-temperature tunneling-electron luminescence microscopy because it can improve spatial resolution, luminescence collection yield, and thermal isolation. We have developed a novel conductive transparent (CT) tip that injects tunneling electrons into a sample and simultaneously collects tunneling-electron luminescence (TL). Essential features of the CT tip are a multimode fiber with a flame-shape tapered to a point, a gold-on-In2O3 double layer that serves as an optically transparent electrode to the apex, and a thick coaxial metal plating on the optical-fiber shaft that reinforces the CT tip allowing it to be firmly screwed onto a holder. The CT tip can easily be changed in a vacuum chamber like an all-metal tip. Using a TL microscope with the CT tip, TL spectra and TL images were successfully obtained on the cross section of GaAs(50 nm)/AlAs(50 nm) multiple quantum wells at 10 K.
Fabrication of one-dimensional nanowire structures utilizing crystallographic orientation in silicon and their conductance characteristics
Authors: Namatsu Hideo , Horiguchi Seiji , Nagase Masao , Kurihara Kenji
Published: July 2002

We propose a process for fabricating one-dimensional Si nanowires with a point contact. The nanowire structure can be easily obtained through two steps: KOH etching of a {110} Si layer of a silicon on insulator substrate and sufficient oxidation of the Si patterns formed by etching. In the etching process, vertical sidewalls comprised of {111} planes are formed into a wire. In addition, other {111} planes, projecting obliquely along the vertical sidewalls, spontaneously appear in the etched substrate. This is due to the fact that the etching proceeds as {111} planes appear because the etch rate of the {111} plane is the lowest of all planes. The bottom-corner region of two inclined {111} planes becomes a point-contact structure by making the distance between two inclined planes appropriate. The oxidation process converts the two-dimensional wire into one-dimensional nanowire by the stress-dependent oxidation phenomena of the Si wire. Consequently, a Si nanowire with a point contact can be formed in the bottom region. The Si nanowires fabricated through this process show clear conductance steps with little fluctuation on plateaus at 45 K. In addition, we discuss the conductance step characteristics in connection with a calculated energy level for a nanowire in (110) Si.
Development of ultrahigh vacuum-atomic force microscopy with frequency modulation detection and its application to electrostatic force measurement
Authors: Uchihashi Takayuki , Ohta Masahiro , Sugawara Yasuhiro , Yanase Yoshio , Sigematsu Tatsuhiko , Suzuki Mineharu , Morita Seizo
Published: July 2002

We succeeded in high resolution force measurements by using a noncontact ultrahigh vacuum-atomic force microscope (UHV-AFM) with frequency modulation (FM) detection. We clearly observed adatoms and corner holes on the Si(111)7×7 reconstructed surface. Then we applied the noncontact UHV-AFM with FM detection to the high resolution measurement of the electrostatic force. We prevented deterioration of the spatial resolution of the topography by isolating the electrostatic interaction from van der Waals interaction. By simultaneous measurements of the topography and electrostatic force on a silicon oxide, a spatial resolution ∼15 Å of the electrostatic force was achieved.
Integration issues for 850 nm optical modulators on Si electronics by direct epitaxy
Authors: Cunningham J. E. , Jan W. Y.
Published: July 2002

We report on the various strategies for integrating III–V semiconductors to very large scale integrated Si logic along with their strengths and limitations. We also detail the monolithic integration method involving direct epitaxial growth where several material science issues have arisen. They require fundamental investigation to further advance implementation. Those issues are (i) the need for a highly ordered array of bilayer steps as an initial Si surface condition for heteroepitaxy, (ii) lower Si oxide desorption temperature, e.g., via electron cyclotron resonance plasma treatments to preserve the bilayer step ordering, and (iii) thinner strain relief layers to reduce the III–V on Si topology mismatch.
Methods for fabricating arrays of holes using interference lithography
Authors: Fernandez A. , Decker J. Y. , Herman S. M. , Phillion D. W. , Sweeney D. W. , Perry M. D.
Published: July 2002

Optical interference lithography provides a robust patterning technology capable of achieving deep submicron resolution over extremely large field sizes (∼1 m2). Here, we compare two approaches for fabricating arrays of holes using interferometric techniques. We first show that, by applying an image reversal process to standard two-beam interference lithography, arrays of high aspect ratio holes (2:1) can be generated. This process scales well to submicron periods and allows holes as small as 0.1 μm to be patterned. Next, we present an analysis of the multiple-beam approach for patterning holes. This technique offers a potentially higher throughput process compared to other techniques. We demonstrate that, while the formation of higher contrast intensity profiles is possible by interfering four or more beams, the shape and modulation depth of such profiles are sensitive to relative phase variations. This dependence complicates the application of multiple-beam techniques for patterning large uniform arrays of resist structures.
Mask blanks for extreme ultraviolet lithography: Ion beam sputter deposition of low defect density Mo/Si multilayers
Authors: Kearney P. A. , Moore C. E. , Tan S. I. , Vernon S. P. , Levesque R. A.
Published: July 2002

We report on the growth of low defect density Mo/Si multilayer (ML) coatings. The coatings were grown in a deposition system specifically designed for extreme ultraviolet lithography mask blank fabrication. Complete, 81 layer, high reflectance Mo/Si ML coatings were deposited on 150 mm diam (100) oriented Si wafer substrates using ion beam sputter deposition. Process added defect densities correspond to 2×10−2/cm−2 larger than 0.13 μm as measured by optical scattering. This represents a reduction in defect density of Mo/Si ML coatings by a factor of 105.
Shallow junctions diffused from single- and coimplanted WSi2 films and integrated 0.2 μm complementary metal–oxide–semiconductor transistors
Authors: Liu C. T. , Luftman H. , Chang C. P. , Liu R. , Hillenius S. J.
Published: July 2002

Out-diffusion shallow junctions have several advantages for the deep-submicron very large scale integrated technologies. In this work, secondary ion mass spectrometry profiles of B, P, As, and BF2 are analyzed for 55 different junctions out-diffused from a WSi2 layer at 800, 850, or 900 °C. The results show important diffusion characteristics depending on various process parameters including implant dose, implant energy, and diffusion temperature. The junction depth varies from 50 to 100 nm for all species. In particular, in B and P coimplanted samples, a significant retardation of B diffusion in the presence of P is observed. Based on the experimental results and with the retardation phenomenon, 0.2 μm complementary metal–oxide–semiconductor transistors are fabricated with only one lithography step forming the lightly doped drain and source/drain junctions.
Electrical characterization of shallow pn junctions
Authors: Gaworzewski P. , Tittelbach-Helmrich K. , Jacob K. , Müller B.
Published: July 2002

In this article we present results of calculations and experiments concerning the characterization of pn junctions with depths xJ<100 nm. Simulations of differential Hall effect profiling with model dopant profiles as input demonstrate that this method can be used to analyze dopant profiles in pn structures, if it is known how the p and n regions of the pn junction are contacted. Results of differential Hall effect simulations obtained with the same model dopant profile as input strongly depend on the kind of the contact to the p and n regions of the junction. The results reveal that differential Hall effect analysis can be accurate with respect to the dopant profile and to the junction depth if Ohmic contacts are formed to both regions of the junction. The shift in the position of the apparent pn junction relative to the metallurgical pn junction is estimated for the case that contact is made to one of the regions. Spreading resistance, differential Hall effect measurements, and chemical staining of beveled samples are used to characterize pn junctions experimentally. Determining pn junction depths we find an error of about 5 nm using spreading resistance measurements and differences of more than 10 nm using conventional chemical staining.
Antenna sputtering in an internal inductively coupled plasma for ionized physical vapor deposition
Authors: Foster J. E. , Wang W. , Wendt A. E. , Booske J.
Published: July 2002

Ionized physical vapor deposition (IPVD) is an emerging technology for coating high aspect ratio vias and trenches for the microelectronics industry. Ionized physical vapor deposition systems typically utilize an inductive discharge generated by an internal antenna. Because the antenna is immersed in the plasma, the possibility of antenna material sputtering into the discharge is a contamination issue. In this investigation, optical emission spectroscopy is used to acquire spectra from an IPVD system to monitor the presence of antenna metal in the discharge. The observed presence of antenna material in the spectra confirms that antenna sputtering is occurring. Experimental sputter rates as determined from witness plate observations are in reasonable agreement with predictions of a simplified model of antenna sputtering, indicating that the sputtering results from large self-bias voltages on the rf antenna.
Vacuum analysis inside a field emission display panel: Experimental and Monte Carlo simulation results
Authors: Zoulkarneev A. , Park N. S. , Jung J. E. , Kim J. W. , Hong J. P. , Kim J. M.
Published: July 2002

Qualitative analysis of residual gases in fully sealed 4 in. color field emission display (FED) devices has been performed utilizing a home-made mass spectrometer. Relative molecule mass for each gas was measured as a function of time during an exhausting process. A real 4 in. panel experimentally exhibited several gases, such as H2O, H2, OH, CO, N2, and CO2, etc. Partial pressure of each residual gas was calculated as a function of exhausting time, resulting in an estimation of total pressure in a real panel. In addition, for simple comparison, vacuum properties inside the panel were analyzed using a Monte Carlo method, based on molecular flow conductance inside it with an exhausting pipe. Transmission probability of molecules at various geometrical and temporal parameters was simulated for pressure optimization inside the panel during exhausting. The results were used to calculate the pressure of the FED panel, resulting in comparison with experimental values. It would provide the basis in future design and fabrication of real FED panel.
High field breakdown characteristics of micrometric gaps in vacuum
Authors: Ma Xianyun , Sudarshan T. S.
Published: July 2002

In order to obtain the prebreakdown and breakdown data of plain vacuum gaps in the micrometric regime relevant to field emission displays, an automatic experimental system was developed. The dc breakdown characteristics of the gap in the micrometric regime, varying from 25 to 1000 μm, were extensively studied. The experimental results show that the highly finished broad-area metal electrodes withstand fairly high fields in relatively poor vacuum. The dc breakdown strength of a 50 μm gap was more than 220 V/μm at a pressure of 10−5 Torr, while for a 1000 μm gap, the breakdown strength was about 44.5 V/μm for highly polished chrome-steel electrodes, 2 cm in diameter. The breakdown of a narrow (<200 μm) vacuum gap resulted in rapid degradation, with the gap holding-off lower voltages after breakdown. However, a wider gap (500–1000 μm) was found to exhibit conditioning, with the dc breakdown voltage increasing after each successive breakdown. The breakdown characteristics of ideal gaps with very smooth solid electrodes, presented in this work, will define the theoretical limits to which actual gaps can be stressed in a field emission display.
Characterization of two by two electron-beam microcolumn array aligned with field emission array
Authors: Park Jeong-Young , Lera Jared D. , Choi H. J. , Buh G. H. , Kang C. J. , Jung J. H. , Choi S. S. , Jeon D. , Kuk Young
Published: July 2002

A two by two electron microcolumn array aligned with field emission array (FEA) was fabricated based on our electron-beam simulation. The spherical and chromatic aberrations, that affect the spot size of the e-beam, are highly dependent on the alignment of the electrostatic microlenses. A laser micromachining technique was used for making a self-aligned microcolumn. A FEA with a designed size and spacing was aligned and bonded to the microcolumn. The I–V and current stabilities of the microcolumn were measured and the field emission pattern of highly focused e-beam was obtained. The application of focused electron beam or ion beam for lithography and miniaturized scanning electron microscopy are suggested.
Reduction of work function on a W(100) field emitter due to co-adsorption of Si and Ti
Authors: Adachi Hiroshi , Ashihara Kazuto , Saito Yasushi , Nakane Hideaki
Published: July 2002

Selective reduction of work function on a W(100) field emitter was observed due to co-adsorption of Si and Ti, followed by angular confinement of field emission on the top (100) surface. Si was vacuum deposited on the side surface near the apex of the field emitter by an amount, which is enough for spreading by 1 ML. Then it was flash heated to spread the deposited Si over the apex surface. Ti was vacuum deposited over it from the opposite side and heat treated to spread over. The work function, which is determined from the Fowler–Nordheim plot, reduced to 3.3 eV. It is assumed that the work function of the clean tungsten is 4.5 eV. The experimental evidences suggest that there is a certain Ti/Si ratio for giving the minimum value of the work function.
Electron-beam lithography using a scanning transmission electron microscope CM12 (Philips)
Authors: Lohau J. , Friedrichowski S. , Dumpich G.
Published: July 2002

We report on the modification of a conventional scanning transmission electron microscope (CM12, Philips) for electron-beam lithography. The hardware as well as the entirely home designed lithography software are described in detail. Special attention is paid to the reduction of the proximity effect by means of our lithography software using a fairly simple exposure model. Various lithography results are shown including nonmagnetic as well as ferromagnetic nanostructures with linewidths as small as 20 nm. Problems occurring when fabricating magnetic nanostructures and their solutions are presented and discussed.
High mobility AlGaN/GaN heterostructures grown by gas-source molecular beam epitaxy
Authors: Li L. K. , Alperin J. , Wang W. I. , Look D. C. , Reynolds D. C.
Published: July 2002

We report on the growth of high electron mobility AlGaN/GaN heterostructures on sapphire substrates by gas-source molecular beam epitaxy (GSMBE) using ammonia as the nitrogen source. Improvements in structural, electrical, and optical properties of GaN and AlGaN layers have been made to achieve this goal. For the growth of AlGaN layers, the reflection high-energy electron diffraction revealed a twofold surface reconstruction, indicative of atomic smoothness of the film surface. High mobility two-dimensional electron gas has been achieved in both unintentionally doped (by piezoelectric effect induced by lattice mismatch strain) and modulation doped AlGaN/GaN heterostructures. The modulation-doped n+-Al0.2Ga0.8N/i-GaN heterojunction exhibited electron mobilities as high as 750 and 4070 cm2/V s at 300 and 77 K, respectively. Both values are the highest ever reported for the AlGaN/GaN heterostructures grown by MBE techniques.
Probing conducting particles buried in a Nix(SiO2)1−x composite by conducting atomic force microscopy
Authors: Luo E. Z. , Wilson I. H. , Xu J. B. , Ma J. X. , Yan X.
Published: July 2002

In this article, we present an experimental study on probing conducting particles buried in a Nix(SiO2)1−x composite with x around the percolation threshold xc by conducting atomic force microscopy (C-AFM). The buried conducting particles were “observed” via the electric current image of C-AFM at constant bias. The current from buried conducting particles originates from field assisted tunneling through the insulating layer. Examples of measuring the thickness of the insulating layer will be given. The analysis shows that it is possible to probe the buried metal particles as deep as several nanometers underneath the surface. By correlating the surface topographic and the current image, the profile of the metal-insulator interface can be measured. General issues on spatial resolution will also be discussed in this article.
Structural and electrical properties of chemical vapor deposition tungsten overgrowth on physical vapor deposited and metalorganic chemical vapor deposited TiN adhesion layers
Authors: Peng Y. C. , Chen L. J. , Hsieh W. Y. , Yang Y. R. , Hsieh Y. F.
Published: July 2002

The structure and electrical properties of chemical vapor deposited W (CVD-W) films on various physical vapor deposited or metalorganic chemical vapor deposited TiN films have been investigated. The growth orientations of the TiN adhesion layers were controlled by deposition method and film thickness. The growth orientations of CVD-W films were found to depend strongly on the microstructures of TiN. The grain sizes and electrical resistivity of CVD-W were found to increase and decrease, respectively, with the grain sizes of underlying TiN layers.
Advanced emitters for next generation rf amplifiers
Authors: Jensen K. L. , Yater J. E. , Zaidman E. G. , Kodis M. A. , Shih A.
Published: July 2002

Next generation rf amplifiers, in particular the inductive output amplifiers (IOAs), will require electron sources characterized by high current density, high brightness, low emittance, and the ability to be emission gated. The strong interaction between the beam and the resonant or synchronous electromagnetic field may enable compact, highly efficient, and moderate gain X-band power booster amplifiers. An analysis of amplifier demands on generic emitter candidates is provided. Of the emitter candidates available, two (namely, field emitter arrays and wide-band-gap semiconductors) are amenable to an analysis predicated on a simple model of field emission from hyperbolas and ellipsoids. The simple model is used to investigate two problems of critical concern: for field emitter arrays (FEAs), we will investigate the conditions under which important space charge effects exist, and from the model predict optimum FEA performance characteristics for rf IOAs; for wide-band-gap materials, the simple model identifies parameters which limit the maximum electron transport across a metal-semiconductor interface, thereby limiting current flow.
Ferromagnetic resonance imaging of Co films using magnetic resonance force microscopy
Authors: Suh B. J. , Hammel P. C. , Zhang Z. , Midzor M. M. , Roukes M. L. , Childress J. R.
Published: July 2002

Lateral one-dimensional imaging of cobalt (Co) films by means of microscopic ferromagnetic resonance (FMR) detected using the magnetic resonance force microscope (MRFM) is demonstrated. A novel approach involving scanning a localized magnetic probe is shown to enable FMR imaging in spite of the broad resonance linewidth. We introduce a spatially selective local field by means of a small, magnetically polarized spherical crystallite of yttrium iron garnet (YIG). Using MRFM-detected FMR signals from a sample consisting of two Co films, we can resolve the ∼20 μm lateral separation between the films. The results can be qualitatively understood by consideration of the calculated spatial profiles of the magnetic field generated by the YIG sphere.
Surface roughness, strain, and alloy segregation in lattice-matched heteroepitaxy
Authors: Priester Catherine , Grenet Genevieve
Published: July 2002

Spinodal decomposition of ternary semiconductor alloys during lattice-matched heteroepitaxy is considered here. It has been previously demonstrated that a perfectly flat surface (with no step) would forbid alloy demixing. The case of a rough surface is the purpose of this article. How the possibility of a better strain relaxation introduced by the surface roughness can favor alloy demixing is analyzed first. The present results are exemplified by the AlInAs lattice matched to the InP case. Second, a step-by-step model is proposed to simulate the growth process on a rough surface. This model leads to a description of the strain and alloy demixing during this growth. This study clearly shows how and why the atoms corresponding to binary materials with lower surface tension naturally tend to segregate towards bumped areas.
Emission observation of a microtip cathode array with an electrostatic-lens projector: Statistical approach
Authors: Constancias C. , Baptist R.
Published: July 2002

We report results on the field emitters’ electrical properties uniformity, directly measured by an electrostatic-lens projector apparatus. First, we describe the experimental setup and its resolution capability. Then, we present the emission characteristics of two 1 mm2 microtip arrays measured in ultrahigh vacuum and we compare these characteristics to the results of a numerical approach by taking into account a Gaussian distribution of β (the tip sharpness). From these experimental data, fitted by a numerical description of the probably emitting tips, it is possible to deduce the mean and the standard deviation of tip sharpness in our samples. A comparison between these experiments and microtip atomic force microscopy characterizations coupled to field-emission simulations, shows a good agreement. Finally, we show the effect of hydrogen on the tips’ emissive properties by using the standard Fowler–Nordheim analysis and we explore the induced changes on their statistical emission characteristics. All these results show that the probability density function of the emitting tips with respect to the applied voltage is uniform and very close to a top-hat function.
Localized interface states and the optical spectra of AlSb/InAs heterostructures
Authors: Shaw M. J. , Gopir G. , Briddon P. R. , Jaros M.
Published: July 2002

The existence of localized states at InSb-like interfaces of AlSb/InAs superlattices is predicted using empirical pseudopotential calculations. These predictions are shown to be in agreement with those of ab initio pseudopotential calculations performed using the local density approximation of density functional theory, demonstrating the ability of the empirical approach to describe the microscopic features of the interface. The frequency dependence of the absorption coefficient is calculated for a series of AlSb/InAs superlattices with differing interface configurations, and the role of the interface localization in determining the optical response is identified.
Visible light from Si/SiO2 superlattices in planar microcavities
Authors: Lockwood D. J. , Baribeau J.-M. , Sullivan B. T.
Published: July 2002

Bright quantum confined luminescence due to band-to-band recombination can be obtained from Si/SiO2 superlattices. To further enhance their light emitting properties, we have studied the effect of placing them in a one-dimensional optical microcavity. The Si/SiO2 superlattices were grown on various substrates in a magnetron sputtering system. The effect of the additional optical confinement on the photoluminescence (PL) results in a pronounced modulation of the PL intensity with emission wavelength, as a consequence of the standing wave set up between the substrate and air interfaces. The modulation in the case of a quartz substrate is much weaker, because of the low reflectance (∼1%) between the superlattice and the quartz. For a Si substrate, absorption of light reduces the PL efficiency, but for an Al-coated glass substrate the PL intensity is twice that of the quartz substrate case. These results show that a suitably designed planar microcavity cannot only considerably increase the efficiency of luminescence in Si/SiO2 superlattices but can also be used to decrease the bandwidth and selectively tune the peak wavelength.
Approaches to nanofabrication on Si(100) surfaces: Selective area chemical vapor deposition of metals and selective chemisorption of organic molecules
Authors: Abeln G. C. , Hersam M. C. , Thompson D. S. , Hwang S.-T. , Choi H. , Moore J. S. , Lyding J. W.
Published: July 2002

The selective removal of hydrogen from a passivated Si(100) surface with an ultrahigh vacuum scanning tunneling microscope (STM) allows nanometer-sized “templates” of clean Si(100) to be defined on an otherwise unreactive surface. Such depassivated areas have already been shown to react selectively with O2 and NH3 in preference to the surrounding H-terminated surface. This selectivity suggests two more sophisticated approaches to fabricating nanostructures with this technique: (1) selective metallization by thermal chemical vapor deposition, and (2) formation of ordered organic monolayers by reaction with specific organic molecules. In the first case, an intrinsic difference in the reaction rate of a metal precursor with the clean and H-terminated Si(100) surface results in selective deposition of a metal on the STM-patterned area. In order to prevent hydrogen desorption and loss of selectivity, the metal precursor must dissociate its ligands at relatively low temperatures. In the second case, the patterned surface is exposed to an organic molecule expected to react in a site specific manner with unsaturated Si dimer sites. An example of this site selective reaction is the [2+2] cycloaddition reaction between carbon–carbon double bonds and the Si dimer bond. Such reactions can result in the formation of spatially resolved nanometer-sized regions containing organic monolayers. In this article we describe progress toward the fabrication of nanostructures utilizing these two techniques. First, we discuss the use of a new amidoalane precursor for the selective chemical vapor deposition of aluminum on STM-patterned Si(100) surfaces, as well as the selective patterning of a nucleation promoter, TiCl4, commonly used to initiate aluminum film growth. We also discuss the selective chemisorption of norbornadiene (bicyclo[2.1.1] hepta-2,5-diene) on STM-patterned areas. STM images reveal the formation of a norbornadiene adlayer with indications of local ordering. Both of these methods show promise as techniques for the fabrication of nanostructures on Si(100) surfaces.
High-resolution resist etching for quartermicron lithography using O2/N2 supermagnetron plasma
Authors: Kinoshita Haruhisa , Yamauchi Atsushi , Sawai Mikio
Published: July 2002

For quartermicron lithography, fine pattern etching of bilayer resist was investigated using O2/N2 supermagnetron plasma. The maximum etch rate was obtained at a radio frequency (rf) phase difference of 150°–180° which was observed between two rf powers of the same frequency supplied to two parallel electrodes. The side etchings of resist walls were reduced by lowering the electrode (wafer) temperature to −30 °C. By the increment of N2 concentration from 0% to 60%, the etch rate decreased little, and the side etchings of resist walls became only ±0.015 μm at a self-bias voltage Vdc of −140 to −180 V. Microloading effect was greatly improved by lowering a wafer temperature to −30 °C and by increasing Vdc to −180 V.
Formation and shape of InAs nanoparticles on GaAs surfaces
Authors: Bottomley D. J.
Published: July 2002

The conclusions of published experimental work that InAs deposited under As-rich conditions on GaAs(001) at 770 K forms nanoparticles whose flat surfaces are the {136} family of planes, whereas that InAs remains flat on GaAs(110), on GaAs(111)A but not on GaAs(111)B, are considered. It is shown that these results are consistent with the behavior of the crystallographically anisotropic surface tension of the strain-free solid on a molten monolayer, the liquid phase being induced by solid phase heteroepitaxial stress. For the diamond structure, the surface tension is a minimum for (111) and a maximum for (001). Particle formation and shape depend on surface free energy minimization of the particle and substrate surfaces in conjunction. The molar entropy of liquid InAs at 770 K is calculated to be intermediate to that of the liquid and solid phases at the zero pressure melting point of 1215 K.
Defect reduction of ZnxCdyMg1−x−ySe based structures grown on InP by using Zn irradiation of the III–V surface
Authors: Zeng L. , Guo S. P. , Luo Y. Y. , Lin W. , Tamargo M. C. , Xing H. , Cargill G. S.
Published: July 2002

The quality of lattice-matched ZnxCdyMg1−x−ySe epitaxial layers and related quantum well laser structures grown on (001) InP substrates has been improved by using Zn irradiation of the III–V surface before the growth of II–VI materials. The full widths at half maximum of the double crystal x-ray rocking curves for ZnxCdyMg1−x−ySe epilayers with band gaps as high as 3.0 eV were reduced to about 50 arcsec. The defect density evaluated from the etch pit density and cathodoluminescence measurements was reduced from 106 to mid-104 cm−2. This result suggests that an initial reaction between Se and group-III (In and Ga) atoms is a main limit to the quality of ZnxCdyMg1−x−ySe grown on InP and the problem can be suppressed by using initial Zn irradiation of the III–V surface.
Cross-sectional scanning tunneling microscopy of InAsSb/InAsP superlattices
Authors: Zuo S. L. , Yu E. T. , Allerman A. A. , Biefeld R. M.
Published: July 2002

Cross-sectional scanning tunneling microscopy (STM) has been used to characterize compositional structures in InAs0.87Sb0.13/InAs0.73P0.27 and InAs0.83Sb0.17/InAs0.60P0.40 strained-layer superlattice structures grown by metal-organic chemical vapor deposition. High-resolution STM images of the (110) cross section reveal compositional features within both the InAsxSb1−x and InAsyP1−y alloy layers oriented along the [1̄12] and [11̄2] directions—the same as those in which features would be observed for CuPt–B type ordered alloys. Typically one variant dominates in a given area, although occasionally the coexistence of both variants is observed. Furthermore, such features in the alloy layers appear to be correlated across heterojunction interfaces in a manner that provides support for III–V alloy ordering models which suggest that compositional order can arise from strain-induced order near the surface of an epitaxially growing crystal. Finally, atomically resolved (11̄0) images obtained from the InAs0.87Sb0.13/InAs0.73P0.27 sample reveal compositional features in the [112] and [1̄1̄2] directions, i.e., those in which features would be observed for CuPt–A type ordering.
Mechanistic studies of silicon oxidation
Authors: Weldon M. K. , Queeney K. T. , Chabal Y. J. , Stefanov B. B. , Raghavachari K.
Published: July 2002

The microscopic mechanism of the formation of ultrathin oxides on Si(100) has been investigated using a combination of infrared spectroscopy and ab initio quantum chemical cluster calculations. The 0→2 monolayer oxide films are grown sequentially from the “bottom-up” using repeated water exposures and annealing cycles, with the partial pressure of water ranging from 10−10 to 10 Torr. The resultant films were then compared to the equivalent thicknesses of thermal and native oxide films. In this way, we obtain unprecedented insight into the essential chemical structures formed during the initial oxidation and subsequent layer growth of these technologically relevant films.
Plasma nitridation of thin Si layers for GaAs dielectrics
Authors: Wang Z. , Diatezua D. M. , Park D-G. , Chen Z. , Morkoç H. , Rockett A.
Published: July 2002

10–30 Å thick Si interface control layers (ICL) were deposited on GaAs epitaxial layers on GaAs substrates. These were then nitrided by exposure to an electron cyclotron resonance nitrogen plasma for varying times. The nitride thickness is shown to increase logarithmically with time. A model based on electron tunneling is proposed to explain the results. Capacitance–voltage measurements on resulting dielectric layers showed that, under optimal conditions, results adequate to fabrication of high-performance field-effect transistors are possible. The ICL thickness cannot be reduced to zero by nitridation because of damage to the underlying GaAs.
Model etch profiles for ion energy distribution functions in an inductively coupled plasma reactor
Authors: Chen Wenjing , Abraham-Shrauner Barbara , Woodworth Joseph R.
Published: July 2002

Rectangular trench profiles are modeled with analytic etch rates determined from measured ion distribution functions. The pattern transfer step for this plasma etch is for trilayer lithography. Argon and chlorine angular ion energy distribution functions measured by a spherical collector ring analyzer are fit to a sum of drifting Maxwellian velocity distribution functions with anisotropic temperatures. The fit of the model ion distribution functions by a simulated annealing optimization procedure converges adequately for only two drifting Maxwellians. The etch rates are proportional to analytic expressions for the ion energy flux. Numerical computation of the etch profiles by integration of the characteristic equations for profile points and connection of the profiles points is efficient.