bluuebunny/crossref_metadata_2025 huggingface dataset:
Reactive ion etching damage to GaAs layers with etch stops
Authors: Knoedler C. M. , Osterling L. , Shtrikman H.
Published: July 2002

The reactive ion etching damage occurring in GaAs/AlGaAs heterostructure devices etched in CCl2F2 and He at low-self-bias voltages (−85 V) was characterized by Schottky diodes. The effect of overetching was examined by etching down to a thin AlAs etch stop layer and electrically characterizing the underlying GaAs layers. Simultaneously etched bulk samples displayed less damage, indicating that bulk samples may not give a true indication of the damage in heterojunction samples. It was deduced that helium ions are most probably responsible for the damage observed.
Secondary ion mass spectroscopic studies of the atomic geometry of GaAs(110)
Authors: Blumenthal Rik , Donner S. K. , Herman J. L. , Trehan Rajender , Caffey K. P. , Furman Ehud , Winograd Nicholas , Weaver B. D.
Published: July 2002

The atomic geometry of the GaAs(110) surface has been determined by shadow-cone enhanced desorption. Ga+ secondary ion yields were analyzed without extensive calculations or assumptions of structure except that the second and all lower layer atoms are at bulk lattice sites. The structure determined from this simple analysis shows excellent qualitative agreement with most preceding models. The surface As atomic position is determined to be displaced laterally 1.28 Å toward the second layer Ga atom and is relaxed away from the surface by 0.69 Å, while the surface Ga atom is displaced 0.37 Å laterally in the same direction as the As atom and compressed 0.45 Å toward the surface. The resulting chain rotation is 29.3° and both surface As bond lengths are determined to be lengthened. A detailed discussion of the shadow-cone enhanced desorption method of structure analysis is presented along with areas of possible refinement. Also presented is a description of our experimental apparatus which was developed for the in situ characterization of molecular-beam epitaxially grown surfaces.
Observation of photoelectron oscillations during growth of GaAs
Authors: Eckstein J. N. , Webb C. , Weng S.-L. , Bertness K. A.
Published: July 2002

The use of reflection high-energy electron diffraction (RHEED) intensity oscillations to calibrate molecular-beam epitaxy (MBE) growth rates is an accurate and widely used technique. In order to actually monitor the accumulation of layers during growth, however, the orientation of the substrate must remain fixed. This typically results in much less uniform growth than is obtained with a rotating substrate. As an alternative we have studied near-threshold photoemission of electrons from a growing surface. Light from an ultraviolet lamp was focused through a sapphire window onto a wafer in a Varian Gen-II MBE system. A simple biased collector plate monitored the resulting photoemission current. Small oscillations were observed in the collected current after growth of GaAs was initiated. The photoemission oscillations occurred at the same frequency at which RHEED oscillations were subsequently observed. Since no azimuthal orientation is in principle required between the wafer, light source, and collector, with further optimization, this technique may allow for layer accumulation to be monitored during growth on a rotating substrate.
Investigations of the altered surface formed during the ion-assisted etching of titanium
Authors: O’Brien W. L. , Rhodin T. N.
Published: July 2002

The surface sensitive techniques of x-ray photoelectron spectroscopy and Auger electron spectroscopy have been used in combination with thermal desorption spectroscopy to investigate the altered surface formed on titanium during ion-assisted etching. The altered surface is formed primarily by the ion mixing processes of ion implantation and ion-induced diffusion. Ion-induced damage assists the formation of the altered surface to a lesser extent. Thermal desorption and surface analysis suggests that the altered surface is highly chlorinated and more readily removed than the unaltered surface. Two TiCl4 desorption peaks are found for the altered surface. The low-temperature peak is due to a disproportionation reaction in the altered surface. The high-temperature peak exhibits zero-order kinetics and is a result of the surface reaction TiCl+Cl→TiCl2.
Ag and Co cluster deposition on GaAs(110): Fermi level pinning in the absence of metal-induced gap states and defects
Authors: Waddill G. D. , Aldao C. M. , Vitomirov I. M. , Anderson Steven G. , Capasso C. , Weaver J. H.
Published: July 2002

A novel method of forming interfaces makes it possible to produce abrupt, defect-free metal/semiconductor boundaries. The procedure involves the growth of metal clusters on Xe buffer layers condensed on clean surfaces, and Xe sublimation brings the clusters into contact with the pristine surface. Clusters of Co and Ag produce unique, nearly coverage-independent Fermi level positions ∼0.32 and 1.0 eV below the conduction-band minimum for n- and p-GaAs (110), respectively. Detailed line shape analysis of the photoemission spectra shows no evidence for cluster-induced disruption or conventional defect formation. Instead, the EF position appears to be related to intrinsic surface states swept into the gap as a result of surface unrelaxation around the metal clusters. The results are contrasted to those for metal atom deposition at 60 and 300 K. For Co atom-by-atom deposition, substantial temperature dependences are observed in the Schottky barrier evolution, despite nearly equivalent adatom-induced surface disruption at both temperatures. Ag atom deposition at 300 K and Ag cluster deposition produce similar morphologies, but quite different EF positions. From the Ag results, it appears that cluster deposition produces strain at the interface, strain which is much less for atom deposition where the higher mobility of Ag allows better accommodation to the surface structure.
Abrupt interfaces on InP(110): Cases of Sb and Sn
Authors: Kendelewicz T. , Miyano K. , Cao R. , Lindau I. , Spicer W. E.
Published: July 2002

The formation of the Sb and Sn/InP(110) interfaces has been studied in situ by soft x-ray core level photoemission spectroscopy. It is found that in both cases the growth of the first uniform monolayer is followed by the formation of large overlayer islands (Stranski–Krastanov mode). The dramatic sharpening of the substrate core levels at one monolayer of Sb indicates the formation of an ordered epitaxial overlayer. No such effect is seen for Sn for which the In 4d core level shows an indication of surface disruption. Despite the lack of order, this interface also remains mostly nonreactive. A straightforward analysis of the data for these particularly simple interfaces indicates nonequivalent shifts in the P 2p and In 4d core levels which are taken as a measure of the band bending. The origin of this discrepancy is shown to be related to an additional interface component in the In 4d core level data. The results of this study indicate that even for nonreactive InP interfaces one must take this into account in using core level photoemission data for quantitative band bending determination. Interestingly, both Sb and Sn, although nonreactive, appear to form Ohmic contacts to InP(110).
The effects of subcutaneous oxidation at the interfaces between elemental and compound semiconductors and SiO2 thin films deposited by remote plasma enhanced chemical vapor deposition
Authors: Lucovsky G. , Kim S. S. , Tsu D. V. , Fountain G. G. , Markunas R. J.
Published: July 2002

We have observed that in the low-temperature deposition of thin films of SiO2 onto semiconductor surfaces by remote plasma enhanced chemical vapor deposition, oxidation occurs at the semiconductor–SiO2 interface. This subcutaneous process generates a thin film of native oxide, which can then play a significant role in determining the electrical properties of the semiconductor/oxide interface. The native oxide formed on Si is SiO2, and we have determined that the growth of this oxide does not interfere with the formation of device quality interfaces provided that the thickness of the deposited oxide layer is less than ∼250 Å. For an oxide thickness greater than ∼250–300 Å, trapping states associated with defects produced along with the subcutaneous oxidation degrade the properties of the Si/SiO2 interface. The native oxide that forms on Ge surfaces during SiO2 depositions is the water-soluble, rutile-structured form of GeO2, and the presence of this oxide significantly degrades the electrical quality of the Ge/dielectric interface. In the case of the compound semiconductor interfaces with deposited SiO2, native oxide formation also generally degrades the electrical characteristics. For example, the oxide that forms on GaAs is predominantly Ga2O3, whereas the oxide on CdTe displays both Cd–O and Te–O bonding. Interposing a thin layer of epitaxial, and pseudomorphic Si between a Ge, GaAs, or CdTe semiconductor surface and the deposited SiO2 film, can provide a viable approach to the formation of good metal–oxide-semiconductor (MOS) devices, provided that the subcutaneous oxidation of the Si layer that occurs during oxide deposition does not ‘‘punch through’’ that layer and thereby allow for the oxidation of the underlying Ge, GaAs, or CdTe substrate as well.
Interface dislocation structures in InxGa1−xAs/GaAs mismatched epitaxy
Authors: Breen K. R. , Uppal P. N. , Ahearn J. S.
Published: July 2002

The morphology of interfacial dislocations in molecular-beam epitaxy (MBE) grown GaAs/InxGa1−xAs/GaAs thin films (60–300 nm, x=0.15–0.40) on GaAs(100) undergoes a transition with increasing x from rectangular arrays, with dislocations lying in the 〈011〉 directions, to random tangled arrays, with a reduced preference for crystallographic orientation. Films with intermediate x values exhibit both types of morphology in separate areas. For a given x value the interfacial dislocation density increases with increasing thickness. A marked asymmetry in the dislocation density in the [01̄1] and [011] directions is observed in some of the films exhibiting rectangular arrays for specific values of InxGa1−xAs film thickness above the critical value. The asymmetry is not observed in thicker InxGa1−xAs films. This asymmetry is attributed to different mobility of α and β dislocations. The rectangular array morphology is found in a number of lattice-mismatched systems, including early observations in GaAsxP1−x/GaAs, while the random arrays are representative of recent observations in GaAs/Si. The measured dislocation densities in the InxGa1−xAs /GaAs system are generally inadequate to relax the interfacial strain, implying a significant partitioning of strain between elastic and plastic components. In 300 nm thick films, a dislocation pinning reaction was directly observed. This reaction is most probably responsible for (1) creating dislocation sources that facilitate strain relaxation in the early stages of strain relaxation, and (2) inhibiting dislocation motion in the latter stages of strain relaxation by pinning dislocations, thus producing a high density of threading dislocations. These observations were enabled by a new technique for preparing large-area electron transparent films, which required no thinning by ion milling or any other method.
Reflection high-energy electron diffraction oscillations during molecular-beam epitaxy growth of gallium antimonide, aluminum antimonide, and indium arsenide
Authors: Subbanna S. , Gaines J. , Tuttle G. , Kroemer H. , Chalmers S. , English J. H.
Published: July 2002

We report the observation of reflection high-energy electron diffraction (RHEED) intensity oscillations during the molecular-beam epitaxy growth of gallium antimonide (GaSb), aluminum antimonide (AlSb), and indium arsenide (InAs). For GaSb, these oscillations were not observed at the normal substrate temperature used for growth of these compounds (500–550 °C), but only at lower temperatures. For continuous growth of AlSb at lower temperatures, the RHEED oscillations persist for ∼40 monolayers. For AlSb and GaSb, the recovery of RHEED oscillation amplitude after growth interruption is quite different from the recovery, if any, of specular spot intensity. The RHEED oscillations for AlSb were observed at 570 °C after growth of as little as 350 Å of AlSb, which was nucleated directly on a GaAs substrate. Oscillations were seen in spite of the large lattice mismatch (7% with GaAs) and the large number of dislocations present (>109 cm−2). We have studied the effects of varying growth rates and flux ratios. The quality of the layers, as evidenced by the occurrence of RHEED oscillations and recovery of specular beam intensity, appears to be good provided that a thin (500 Å) buffer layer of AlSb is grown first.
Film redeposition on vertical surfaces during reactive ion etching
Authors: Allred D. , Jäckel S. , Mazuré C. , Barth H. J. , Cerva H. , Hösler W.
Published: July 2002

During reactive ion etching of tantalum silicide and polysilicon layers in a chlorine based chemistry it is observed that a film (15–20 nm) is deposited on vertical surfaces of etched structures. This film is even present after common resist strip processes. This film is analyzed by Auger electron spectroscopy and transmission electron spectroscopy, and the formation mechanism is investigated. It is shown it is composed of silicon dioxide sputtered from dielectric layers lying under the etched silicide and poly Si layers. Variables influencing the formation of the residual sidewall film are analyzed. It is demonstrated that the observed residual films are not involved in the anisotropic etch characteristics before the silicide or poly Si layer are etched through. It is further demonstrated that a carbon based polymer film or inhibition layer must be responsible for the anisotropic etch profile of the tantalum silicide layer. This second carbon based film is formed only with a sufficient amount of photoresist in the etching chamber.
Free standing silicon microstructures for soft x-ray masks and cold atom focusing
Authors: Tennant D. M. , Bjorkholm J. E. , O’Malley M. L. , Becker M. M. , Gregus J. A. , Epworth R. W.
Published: July 2002

Free standing microstructures become necessary when transparent materials are unavailable as supporting layers in transmission components. In this work we use electron beam lithography and reactive ion etching in crystalline silicon membranes to fabricate open stencil patterns for two novel applications: transmission reticles containing resolution test patterns for projection soft x-ray lithography at λ=36 nm; and a Fresnel zone plate for focusing beams of cold neutral atoms. Various schemes are described for making components with minimum zone plate feature sizes down to 0.23 μm and grating features below 0.1 μm. We also report on several materials and processing issues which limit feature sizes and distortions in these microstructures.
GaAs/Ge/GaAs heterostructures by molecular beam epitaxy
Authors: Strite S. , Ünlü M. S. , Adomi K. , Gao G.-B. , Agarwal A. , Rockett A. , Morkoç H. , Li D. , Nakamura Y. , Otsuka N.
Published: July 2002

We report high quality GaAs/Ge/GaAs heterojunctions grown by molecular-beam epitaxy. Antiphase domain (APD) free GaAs on Ge growth, as verified by high-energy electron diffraction and transmission electron microscopy, has been achieved for epitaxial (100) Ge surfaces misoriented towards [011] while APDs are observed to annihilate after roughly 500 Å of GaAs growth on nominally (100) epitaxial Ge. The outdiffusion of Ga and As into epitaxial Ge and outdiffusion of Ge into epitaxial GaAs was investigated by in situ x-ray photoemission spectroscopy and secondary ion mass spectroscopy. The electrical properties of the heterojunctions are studied and the transistor characteristics of a prototype N–Al0.22Ga0.78As/p+-Ge/n-GaAs heterojunction bipolar transistor is presented.
Inequivalence of normal and inverted interfaces of molecular-beam epitaxy grown AlGaAs/GaAs quantum wells
Authors: Köhrbrück R. , Munnix S. , Bimberg D. , Mars D. E. , Miller J. N.
Published: July 2002

Normal and inverted interfaces of GaAs/AlGaAs heterostructures are studied by comparing the luminescence of quantum wells (QWs) grown by molecular-beam epitaxy with a 10–300 s growth interruption either at one or both of the interfaces. The influence of growth interruption time on the incorporation of shallow impurities and traps and on the variation of the interface structure is investigated. GaAs surfaces become appreciably smoother with increasing interruption time, and increasing incorporation of shallow impurities, but no incorporation of traps at the growth surface is observed. At AlGaAs surfaces, the concentration of traps strongly increases with growth interruption time, but the surface does not become smoother. Additionally strong accumulation of shallow impurities at the inverted GaAs-on-AlGaAs interface is directly visualized.
Thermal and chemical stability of Se-passivated GaAs surfaces
Authors: Turco F. S. , Sandroff C. J. , Hedge M. S. , Tamargo M. C.
Published: July 2002

We report on the thermal and chemical stability of (100) GaAs interfaces passivated with aqueous, selenium-based reagents. Photoluminescence from the Se treated surfaces shows that they remain passivated after extended water rinses. The surfaces were studied under ultrahigh vacuum by reflection high-energy electron diffraction (RHEED) from room temperature to 620 °C. A streaky (1×1) RHEED pattern at low temperature gives way at 580 °C to a stable (4×1) reconstruction, suggesting strong bonding between Se and the GaAs surface. X-ray photoelectron spectroscopy confirms the presence of Se at the surface at high temperature, possibly in the form of gallium selenate. The stability of these Se-treated interfaces suggests that chemical treatments could be used in conjunction with molecular beam epitaxy to obtain high-quality regrown interfaces.
Effect of GaAs surface reconstruction on interface state density of epitaxial ZnSe/epitaxial GaAs heterostructures
Authors: Qiu J. , Qian Q.-D. , Kobayashi M. , Gunshor R. L. , Menke D. R. , Li D. , Otsuka N.
Published: July 2002

Epitaxial ZnSe/epitaxial GaAs interfaces have been grown using molecular beam epitaxy and evaluated by capacitance–voltage measurements and transmission electron microscopy. The GaAs surface stoichiometry was systematically varied prior to the nucleation of ZnSe. A significant reduction of interface state density occurred when the GaAs epilayer surface was As deficient. The resulting interface state densities of as-grown structures are comparable to values obtained with (Al,Ga)As/GaAs interfaces.
Tribological characteristics of amorphous carbon films investigated by point contact microscopy
Authors: Miyamoto T. , Kaneko R. , Miyake S.
Published: July 2002

Wear marks and frictional force distribution on amorphous carbon films, fluorinated amorphous carbon films, silicon-containing amorphous carbon films, and fluorinated silicon-containing amorphous carbon films were investigated by point contact microscopy. Film surfaces were also analyzed by x-ray photoelectron spectroscopy. The fluorinated silicon-containing (40%) amorphous carbon film has the highest strength of the tested films. CF4 plasma treatment fluorinated the silicon-containing (40%) amorphous carbon film, and significantly reduced its frictional coefficient to less than 0.3. The surface of fluorinated silicon-containing (40%) amorphous carbon film is composed of carbon, silicon, fluorine, and oxygen. Carbon, silicon, and fluorine show the plural chemical states of SiC and CFx. This increases film strength and enhances the lubricating effect of the fluorinated silicon-containing (40%) amorphous carbon film.
Micromachined silicon sensors for scanning force microscopy
Authors: Wolter O. , Bayer Th. , Greschner J.
Published: July 2002

We have developed a batch process for the microfabrication of silicon force sensors. A force sensor typically consists of a tip mounted onto a cantilever which is connected to a handling piece. The sensors are being used as microprobes and force transducers in scanning force microscopes. Our sensors are etched from single crystal silicon. The process, which mainly involves a combination of wet and dry etching techniques, results in cantilevers and tips suitable as general purpose force sensors. The newly developed batch microfabrication process is superior to the process which uses wet etching of individual metal wires and it differs substantially from the known process to produce thin film cantilevers with and without integrated tips. The sensors have been applied in various microscopes, and with different types of operation including the repulsive, attractive, and magnetic force mode.
Determination of acceptor densities in p-type Hg1−xCdxTe by thermoelectric measurements
Authors: Baars J. , Brink D. , Ziegler J.
Published: July 2002

The differential thermoelectric voltage of p-type bulk samples and epitaxial layers of Hg1−xCdxTe (MCT) (0.2<x<0.25) in the temperature range from 20 to 300 K is measured using two different experimental techniques, the hot point method, and the lateral gradient method. The samples were also examined by Hall-effect and conductivity measurements. In addition the Seebeck coefficient of p-MCT for acceptor densities 1014<NA<1017 cm−3 is calculated employing empirical relations for the energy gap, the intrinsic carrier density, the carrier mobilities, and the LO phonon frequencies. By fitting the calculated temperature dependence of the thermoelectric voltage to the experimental one, the lateral gradient method proved to be an adequate tool for determining the effective acceptor density in p-type MCT including surface inversion. The hot point method is found to be insensitive to surface inversion. It may be used for determining the temperature of zero thermoelectric power which directly yields a good estimate of the acceptor density.
Review of scanning force microscopy
Authors: Sarid Dror , Elings Virgil
Published: July 2002

We present an overview of scanning force microscopy with applications to electrostatic, magnetostatic, and atomic forces operating in the contact and noncontact mode, and highlight the main achievements in this field.
Molecular beam epitaxy growth and physical characterization of precise, narrow, triangular heterostructures using an analog grading algorithm
Authors: Giugni Stephen , Tansley T. L.
Published: July 2002

Precise control of the compositional profile in ternary semiconductors, with the resulting spatial variation of band structure, allows electronic and/or optical properties to be tailored for specific applications. An algorithm is presented, and its implementation described, for the growth of very narrow graded compositional GaAs/AlGaAs heterostructure wells and barriers by conventional solid-source molecular beam epitaxy. The Al source furnace temperature is controlled continuously to obtain linear analog compositional grading. Physical confirmation of the precision of the profiles has been obtained by a range of techniques including secondary ion mass spectrometry, secondary neutral mass spectroscopy, and compositional analysis from thickness fringes-transmission electron microscopy. The results show excellent linearity and symmetry in structures with compositional changes between 0% and 30% Al over 10-nm graded regions.
Split-gate electron waveguide fabrication using multilayer poly(methylmethacrylate)
Authors: Rooks M. J. , Eugster C. C. , del Alamo J. A. , Snider G. L. , Hu E. L.
Published: July 2002

We report on techniques for fabricating 20-nm scale ballistic electron devices and on techniques for imaging and characterizing these patterns in thin layers of poly(methylmethacrylate) (PMMA). A split-gate fabrication approach with nanometer-scale Schottky gates is used. Using a multiple layer PMMA resist technique, we have fabricated Au/Pd gates as narrow as 20 nm. In order to enhance the undercut profile a lower molecular weight PMMA is used as the bottom layer. We have also developed a resist stabilization technique which allows the viewing of 20 nm scale features in 0.1-μm thick PMMA resist under high magnification in a scanning electron microscope. These techniques have been used to fabricate ballistic electron devices which demonstrate quantum interference effects.
Intrinsic recombination and interface characterization in ‘‘surface-free’’ GaAs structures
Authors: Wolford D. J. , Gilliland G. D. , Kuech T. F. , Smith L. M. , Martinsen J. , Bradley J. A. , Tsang C. F. , Venkatasubramanian R. , Ghandi S. K. , Hjalmarson H. P.
Published: July 2002

We have conducted a thorough photoluminescence (PL) and PL time-decay study of the interfacial passivating effects of metalorganic chemical vapor deposition prepared Al0.3Ga0.7As, n+ -doped GaAs, and Na2S surface barriers on epitaxial, high-purity (n−) GaAs structures. We observe 300-K radiative lifetimes, in such 10-μm structures, of 2.5 μs, 800 ns, and 150 ns, respectively. These are to be compared with lifetimes of ∼2 ns for a single, ‘‘bare’’ GaAs surface with an Al0.3Ga0.7As rear surface barrier, and ∼0.5 ns for unpassivated epitaxial GaAs. Accompanying radiative efficiencies are 103–104 higher in all of these epibased structures, and 102 higher for Na2S, than for corresponding bare GaAs surfaces. Further, from detailed PL lifetime studies versus GaAs thickness, we find the lowest interfacial recombination velocities reported for any GaAs/AlxGa1−xAs structure, to date, of ≲40 cm/s, and, correspondingly, 0–1800 cm/s for n+/n−/n+ all-GaAs homostructures. Thus, virtually ‘‘surface-free’’ structures are now achievable. In comparison, we find, at best, ∼5500 cm/s for Na2S, and typically 34 000 cm/s for bare GaAs surfaces. We conclude, on the basis of our detailed experimental study of a wide variety of samples, that these values provide truly reliable measures of surface recombination velocities for both surface types. We find an unambiguous determination of surface recombination velocities requires detailed examination of the minority-carrier recombination kinetics versus temperature, together with minority-carrier spatial transport properties. After demonstrating that minority-carrier recombination kinetics in our ideal structures are truly ‘‘intrinsic,’’ and thus wholly unaffected by extrinsic processes, we examine the temperature dependence of band-to-band and free-exciton recombination. We fully explain all intrinsic, free-carrier recombination found in each structure, for temperatures of 40–300 K through rate equations appropriate to each structure. Similar low-temperature (1.8–40 K) studies confirm dominant decay proceeds, here, by intrinsic free excitons.
A new probe of metallization microstructure on semiconductor surfaces
Authors: Haugstad G. , Raisanen A. , Sorba L. , Vanzetti L. , Yu X. , Franciosi A.
Published: July 2002

Synchrotron radiation photoemission spectroscopy studies of the Xe 4d core level emission from Xe atoms physisorbed on unreactive Yb–GaAs(110) and reactive Yb–Hg1−xCdxTe(110) interfaces allowed us to detect the presence of metallic islands and follow the coverage-dependent evolution of island composition and morphology. Measurements of local island work function, and local work function of the semiconductor surface between the islands can be performed if the ionization energy of the adsorbed Xe atoms is known a priori. We conducted systematic studies of Xe physisorption on a variety of elemental metallic films and cleaved semiconductor substrates, and found that the 4d ionization energy of the first layer Xe atoms physisorbed on metals is relatively constant (65.7±0.1 eV). On cleaved semiconductor surfaces the apparent 4d ionization energy for Xe atoms adsorbed in the first of two physisorption sites is also relatively constant, but 0.6 eV higher than that observed on metals.
Scanning tunneling microscopy of molecular-beam epitaxially grown GaAs (001) surfaces
Authors: Tanaka Ichiro , Ohkouchi Shunsuke , Kato Takashi , Osaka Fukunobu
Published: July 2002

A multichamber ultrahigh vacuum (UHV) scanning tunneling microscope (STM) system which includes a molecular-beam epitaxy (MBE) growth chamber, as well as a STM chamber, has been constructed for the investigation of processed GaAs surfaces. We observed MBE-grown GaAs surfaces using this system. Samples were grown on GaAs (001) surfaces and cooled to certain temperatures in an As4 flux before being transferred into an UHV. STM images of c(4×4), 2×4, and mixed c(4×2) and 2×2 structures were obtained for the samples cooled to 330, 570, and 470 °C in an As4 flux, respectively. The mixed c(4×2) and 2×2 structure seems to be formed by desorption of As atoms from the c(4×4) surface in an UHV. Also, a new reconstructed structure which has a 2.8-nm periodicity along both the [110] and [11̄0] directions was observed. It seems to be a local structure which has a larger arsenic dimer density than that of the c(4×4) structure.
Study of gate oxide damage in an electron cyclotron resonance argon plasma
Authors: Felch S. B. , Salimian S. , Hodul D. T.
Published: July 2002

Charging damage of thin gate oxides is often a concern in plasma processing. This study examines the breakdown voltage characteristics of thin gate oxides in low-energy argon plasmas. An electron cyclotron resonance (ECR) system with a rf-biased electrode was used. The effects of rf bias, plasma exposure time, and gate oxide thickness on the gate oxide breakdown voltage were measured. The results show that the ECR plasma process with low, self-induced dc bias does not induce any charging damage for gate oxides as thin as 120 Å. More device failures are produced with higher bias and longer plasma exposure, while thicker gate oxides show less damage.
Dry etching bilayer and trilevel resist systems for submicron gate length GaAs based high electron mobility transistors for power and digital applications
Authors: Ren F. , Pearton S. J. , Tennant D. M. , Resnick D. J. , Abernathy C. R. , Kopf R. F. , Wu C. S. , Hu M. , Pao C. K. , Paine B. M. , Wang D. C. , Wen C. P.
Published: July 2002

Dry-etching technologies for submicron gate recess and resist pattern transfer are demonstrated with bilayer and trilevel resist systems for power and digital applications, respectively. For power applications, damage-free, dry-etched 0.25 μm T-shaped gate pseudomorphic InGaAs channel high electron mobility transistors (HEMTs) were fabricated. A Freon-12 based discharge was used in either electron cyclotron resonance or reactive ion etching systems to perform the gate recess process. Etching selectivity of more than 200 was obtained between the GaAs cap layer and the underlying AlGaAs donor layer. Self-bias voltages of −30 to −50 V were used in the etching process to minimize the damage. Pre- and postetch clean steps were utilized to achieve uniform etch and removal of any dry etch related residues. By using the dry etch for gate recess, very tight threshold voltage uniformity across a quarter of a three inch wafer of ±85 mV was obtained in comparison to ±500 mV with our conventional wet recess technology. The extrinsic transconductance was 437 mS/mm with output conductance of 10 mS/mm. With a trilevel resist system, AlGaAs/GaAs single and double heterostructure [high field effect transistors (HFETs) and single quantum wells (SQWs)] were also fabricated as well as pseudomorphic AlGaAs/InGaAs/GaAs [pseudomorphic HEMTs (PHEMTs)] heterojunction FETs to investigate the scaling of the direct-current performance with gate length for high speed digital applications. Electron-beam lithography was used to define the gate length from 1 to 0.1 μm. The PHEMT had a consistently higher voltage gain at all gate lengths, with a voltage gain of greater than 30 for a gate length of 0.1 μm.
Flux noise in effusion cells: A key to understanding oval defects?
Authors: Miller J. N.
Published: July 2002

The flux of Ga and Al from molecular-beam epitaxy effusion cells has a significant noise component. The magnitude of this noise has been measured to be as large as 8% of the average flux and the frequency distribution of this noise was centered near 0.1 Hz. The magnitude of the flux noise was found to depend on the amount of material in the cell, the element in the cell, and the thermal gradient in the cell. A correlation between the flux noise and the oval defect densities in GaAs grown with these effusion cells is observed. The defect densities range from 1000 cm−2 for a Varian 16 cc Ga cell to a low of less than 50 cm−2 for an EPI 60 cc cell. While no complete theory exists, there are two potential sources of flux noise: hydrodynamic instabilities of the melt and source ‘‘spitting.’’
Effect of surface reconstruction on Fermi-level pinning in the Sn on Si(111) system
Authors: Griffiths C. L. , Anyele H. T. , Matthai C. C. , Cafolla A. A. , Williams R. H.
Published: July 2002

We have performed detailed photoemission studies on the Si(111)(√3×√3)R30°-Sn and Si(111)(2√3×2√3)R30°-Sn reconstructed surfaces. Band bending is observed in both cases resulting in n-type Schottky barrier heights of (0.66±0.09) and (0.99±0.10) eV, respectively. In parallel, theoretical calculations using the self-consistent tight binding method in the extended Huckel approximation have been performed to determine the electronic structure and barrier heights at these reconstructed surfaces. The difference in barrier heights is found to be in excellent agreement with experiment.
Visualization and identification of intracellular structures by force modulation microscopy and drug induced degradation
Authors: Fritz M. , Radmacher M. , Petersen N. , Gaub H. E.
Published: July 2002

Magnetotactic bacteria and live human platelets were investigated by atomic force microscopy (AFM). Contributions to the measured height profiles due to different local mechanical properties were determined by force modulation microscopy. We found that in the case of the thick-walled bacteria the intracellular structures did not significantly influence the overall topology measured by AFM. The magnetosomes were only visible in the force modulation mode, in the case of the platelets the images were dominated by the cytoskeleton and the organelles. The local composition of the cytoskeleton was analyzed by subsequent steps of degradation of certain fibers with specific drugs during AFM imaging.
Energy distribution of tunneling emission from Si-gate metal–oxide–semiconductor cathode
Authors: Yokoo Kuniyoshi , Sato Shinji , Koshita Gen , Amano Isato , Murota Junichi , Ono Shoich
Published: July 2002

An Si-gate metal–oxide–semiconductor (MOS) electron tunneling cathode was fabricated and its emission characteristics were examined. The emission occurred at a gate voltage higher than the work function of the Si gate by electron tunneling through the potential barrier in the MOS cathode and was stable in the Si-gate cathode. The energy distribution of emitted electrons was measured and was confirmed to be mainly determined by the scattering process of hot electrons in the oxide. The emission current from the Si-gate MOS cathode was nearly independent of pressure.
Metal plasma immersion ion implantation and deposition using vacuum arc plasma sources
Authors: Anders André , Anders Simone , Brown Ian G. , Dickinson Michael R. , MacGill Robert A.
Published: July 2002

Plasma source ion implantation (PSII) with metal plasma results in a qualitatively different kind of surface modification than with gaseous plasma due to the condensable nature of the metal plasma, and a new, PSII-related technique can be defined: metal plasma immersion ion implantation and deposition (MPI). Tailored, high-quality films of any solid metal, metal alloy, or carbon (amorphous diamond) can be formed by MPI using filtered vacuum arc plasma sources, and compounds such as oxides or nitrides can be formed by adding a gas flow to the deposition. Here we describe the plasma formation at cathode spots, macroparticle filtering of the vacuum arc plasma by magnetic ducts, the underlying physics of MPI, and present some examples of MPI applications.
Investigation of the operating modes of gated vacuum field emitter arrays to reduce failure rates
Authors: Meassick S. , Xia Z. , Chan C. , Browning J.
Published: July 2002

The influence of the operating mode of arrays of gated vacuum field emitters on the intrinsic failure rate of emitter tips has been investigated. In particular, the failure rates of arrays of gated silicon vacuum field emitters operating in a dc and pulsed bias mode as a function of pulse duty cycle, pulse frequency, emitted current, and neutral gas pressure have been measured. There is a large reduction of the failure rate, per unit time that the array is emitting current, when emitters are operated in pulsed mode as compared to a dc mode of operation. This reduction in the failure rate is dependent on the duty cycle of the pulses that the array is operated at but is independent of the repetition frequency of the pulses. The failure rate is an exponential function of the emitted current, both for the dc bias and for the pulse bias case. The increase in failures with increasing current is more rapid for a dc bias than for a pulse bias. The failure rate of gated vacuum field emitters increases rapidly with the neutral gas pressure, with the rate of increase in failures identical for a dc and a pulsed bias.
Study of interface asymmetry in InAs–GaSb heterojunctions
Authors: Wang M. W. , Collins D. A. , McGill T. C. , Grant R. W. , Feenstra R. M.
Published: July 2002

We present reflection high energy electron diffraction, secondary ion mass spectroscopy, scanning tunneling microscopy and x-ray photoelectron spectroscopy studies of the abruptness of InAs–GaSb interfaces. We find that the interface abruptness depends on growth order: InAs grown on GaSb is extended, while GaSb grown on InAs is more abrupt. We first present observations of the interfacial asymmetry, including measurements of band alignments as a function of growth order. We then examine more detailed studies of the InAs–GaSb interface to determine the mechanisms causing the extended interface. Our results show that Sb incorporation into the InAs overlayer and As exchange for Sb in the GaSb underlayer are the most likely causes of the interfacial asymmetry.
100 years of x rays: Impact on micro- and nanofabrication
Authors: Smith Henry I.
Published: July 2002

In November of 1895 Wilhelm C. Röntgen observed fluorescence caused by invisible rays emanating from a discharge tube. This discovery of x rays was not a mere happenstance; Röntgen was a highly skilled experimentalist, well prepared to launch a series of clever experiments and startle the world with the announcement of ‘‘a new kind of ray.’’ The practice of medicine was transformed, and the era of modern physics inaugurated. X rays have proven essential to the probing of atomic, molecular, and solid-state structures. X-ray astronomy has revealed some of the most exotic and violent phenomena in the universe, including the black holes. X-ray microscopy provides information complementary to what is obtainable from optical and electron microscopies. In micro- and nanolithography x rays provide high quality aerial images and simple processing, in part because of an absence of spurious scattering, something that Röntgen himself observed. However, application in ultralarge scale integrated (ULSI) manufacturing is still uncertain for a variety of reasons, primarily because ultraviolet optical-projection lithography continues to satisfy commercial needs. X-ray lithography may find application in areas outside the ULSI industry, for example in optoelectronics, flat-panel displays, and magnetic data storage. It has the intrinsic resolution to reach the practical limits of the lithographic process, around 20 nm. However, to approach this limit diffraction blurring must be avoided by reducing the mask–sample gap below 5 μm, which may not be acceptable in manufacturing. In this event, and in order to retain the lithographic qualities of 1 nm wavelength radiation, projection with Fresnel zone plate arrays could be employed.
Fabrication of an ultrasharp and high-aspect-ratio microprobe with a silicon-on-insulator wafer for scanning force microscopy
Authors: Itoh Junji , Tohma Yasushi , Kanemaru Seigo , Shimizu Keizo
Published: July 2002

A microprobe having an ultrasharp and high-aspect-ratio stylus was made using a directly bonded silicon-on-insulator (SOI) wafer. The stylus and cantilever were made of 8 μm-thick (100)-Si film and 2 μm-thick dioxide film of the SOI wafer with sufficient reproducibility. The cantilever is 100 μm in length, 20 μm in width, and 1.6 μm in thickness; the stylus is 7 μm in height with an aspect ratio exceeding 2. The stylus was formed first by reactive ion etching and sharpened by orientation dependent etching with KOH solution. The apex of the stylus is 10 nm in radius of curvature. The present microprobe was found adequate for atomic force microscopy (AFM) measurement with spatial resolution better than 10 nm in noncontact-mode topographic imaging. The mode of fabrication, mechanical properties of the microprobe, and results of AFM measurement are discussed.
Scanning capacitance microscopy measurements and modeling: Progress towards dopant profiling of silicon
Authors: Kopanski J. J. , Marchiando J. F. , Lowney J. R.
Published: July 2002

A scanning capacitance microscope (SCM) has been implemented by interfacing a commercial contact-mode atomic force microscope with a high-sensitivity capacitance sensor. The SCM has promise as a next-generation dopant-profiling technique because the measurement is inherently two dimensional, has a potential spatial resolution limited by tip diameter of at least 20 nm, and requires no current carrying metal–semiconductor contact. Differential capacitance images have been made with the SCM of a variety of bulk-doped samples and in the vicinity of pn junctions and homojunctions. Also, a computer code has been written that can numerically solve Poisson’s equation for a model SCM geometry by using the method of collocation of Gaussian points. Measured data and model output for similar structures are presented. How data and model output can be combined to achieve an experimental determination of dopant profile is discussed.
Cold emission from the single-crystalline microparticle of diamond on a Si tip
Authors: Givargizov E. I. , Zhirnov V. V. , Kuznetsov A. V. , Plekhanov P. S.
Published: July 2002

Single crystalline diamond particles were grown on the ends of sharp silicon tips. The typical form of the particles was cubooctahedron with a (111) plane on the top. Field emission experiments of Si emitters with single crystalline diamond coating demonstrated hysteresis and normal emission modes. In the hysteresis mode, emission saturated at about 2 μA. Fowler–Nordheim plots for data points below the saturation limit were nonlinear. Transformation from hysteresis to normal behavior occurred with increasing voltage. A modified model of emission through dielectric layers was proposed for explanation of emission properties of diamond.
Characterization by x-ray photoelectron spectroscopy of the chemical structure of semi-insulating polycrystalline silicon thin films
Authors: Iacona Fabio , Lombardo Salvatore , Campisano Salvatore U.
Published: July 2002

The x-ray photoelectron spectroscopy (XPS) technique has been used to investigate the composition of semi-insulating polycrystalline silicon (SIPOS) films having oxygen contents of 10 and 35 at. % prepared by low pressure chemical vapor deposition. XPS analysis has demonstrated that the film compositions can be qualitatively described by means of the five Si-SixO4−x tetrahedra (with 0≤x≤4 and integer) predicted by the statistical random bonding model (RBM). However, the quantitative analysis of the XPS spectra has demonstrated that the concentrations of the various tetrahedra found in the SIPOS films are remarkably different from those predicted by a statistical approach, i.e., by assuming that each Si atom forms with equal probability bonds with either Si or O. We have also found that the composition of high temperature (up to 1000 °C) annealed films further departs from that predicted by the RBM model; indeed, the anneal promotes the decomposition of partially oxidized Si-SixO4−x tetrahedra in Si-Si4 tetrahedra (that form Si nanocrystals) and Si-O4 tetrahedra (that enrich the oxygen content of the amorphous phase).
High microwave power electron cyclotron resonance etching of III–V semiconductors in CH4/H2/Ar
Authors: Pearton S. J. , Lee J. W. , Lambers E. S. , Mileham J. R. , Abernathy C. R. , Hobson W. S. , Ren F. , Shul R. J.
Published: July 2002

Etch rates up to 7000 Å/min for InP and 3500 Å/min for GaAs are obtained for high microwave power (1000 W) CH4/H2/Ar electron cyclotron resonance plasma etching. Preferential loss of the group V element leads to nonstoichiometric, unacceptably rough surfaces on In-based binary semiconductors at microwave powers ≥400 W, regardless of plasma composition. Both Ga- and Al-based materials retain smooth, stoichiometric surfaces even at 1000 W, but the rates are still much slower than for Cl2 plasma chemistries. The results suggest that CH4/H2 plasmas are not well suited to electron cyclotron resonance systems operating at high powers.
Materials issues for optical components and photomasks in 157 nm lithography
Authors: Liberman V. , Bloomstein T. M. , Rothschild M. , Sedlacek J. H. C. , Uttaro R. S. , Bates A. K. , Van Peski C. , Orvek K.
Published: July 2002

Photolithography using 157 nm pulsed fluorine lasers has emerged as the leading candidate technology for the post-193-nm generation. Preliminary data have indicated that at 157 nm there are optical materials transparent enough to enable the fabrication of refractive elements, both in the projection and illumination part of the optical train. However, a number of critical issues still remain. Optical materials must show no appreciable degradation with laser irradiation. The availability of transparent photomask substrates must be ascertained. Optical coatings must be developed and qualified. At this short wavelength, interface effects, subsurface damage, and adsorbate effects become increasingly prominent. We present recent experimental results on the durability tests of calcium fluoride, modified fused silica, and optical coatings for 157 nm applications. Our initial assessment of several grades of modified fused silica demonstrates that at least one grade already meets transparency and durability requirements for reticle substrates for 157 nm applications. For both bulk calcium fluoride and antireflectance coatings our tests to date show no degradation for 300 million pulses at fluences up to 3 mJ/cm2/pulse. We do observe degradation of beam steering mirrors in our experimental setup. Detailed damage analysis of these coatings is presented.
In situ normal incidence reflectance study on the effect of growth rate of nucleation layer on GaN by metalorganic chemical vapor deposition
Authors: Kim Dong-Joon , Moon Yong-Tae , Ahn Kwang-Soon , Park Seong-Ju
Published: July 2002

The effect of the growth rate of the nucleation layer on the growth of a high temperature GaN layer has been studied by observing the in situ normal incidence reflectance during the growth of GaN by metalorganic chemical vapor deposition. This study revealed that the lateral growth and coalescence of a high temperature GaN layer was enhanced as the growth rate of the nucleation layer was increased. However, the measurement of (102) hk-circle scan using x-ray diffraction showed that the in-plane structural qualities of GaN were found to be deteriorated by increasing the growth rate of the nucleation layer. The electron mobility was also decreased by increasing the growth rate of the nucleation layer. These results suggest that the nucleation sites are well oriented on the sapphire substrate at a low growth rate of the nucleation layer but that the lateral growth and coalescence of the GaN layer is hindered due to the limited surface diffusion of adatoms at a fast growth rate of the nucleation layer.