bluuebunny/crossref_metadata_2025 huggingface dataset:
Directly identifying the order of layer switching in magnetic multilayers
Authors: Freeland J. W. , Chakarian V. , Idzerda Y. U. , Doherty S. , Zhu J. G. , Wende H. , Kao C.-C.
Published: July 2002

The determination of the order of layer switching for two different technologically interesting multilayer systems, NiFe/Cu/Co and Co/Cr/Co, is accomplished by monitoring changes in the angular dependence of the magnetic contributions to the soft x-ray resonant magnetic scattering as a function of applied field.
Optical properties of titania/silica multilayer filters prepared by helicon plasma sputtering
Authors: Wang X. , Masumoto H. , Someno Y. , Hirai T.
Published: July 2002

Seven-layer optical multilayer filters with alternating layers of titania and silica were fabricated by helicon plasma sputtering at room temperature. The reflectance of the filters around the design wavelengths of 800 and 1300 nm are up to 95.80% and 95.32%, respectively. Their reflected bands have the full widths at half maximum of about 360 and 600 nm, respectively. The measured transmittance spectra correspond well with the calculated results based on the optical multilayer film theory with measured refractive indices of 2.44 and 1.48 for TiO2 and SiO2, respectively. Microstructures of the TiO2 and SiO2 films and TiO2/SiO2 multilayer films were investigated.
Molecular beam epitaxial growth and structural properties of Bi1−xSbx alloy thin films on CdTe(111) substrates
Authors: Cho Sunglae , DiVenere Antonio , Wong George K. , Ketterson John B. , Meyer Jerry R.
Published: July 2002

We have successfully grown BiSb alloy thin films on CdTe(111)B over a wide range of Sb compositions using molecular beam epitaxy. Structural properties have been investigated using in situ reflection high-energy electron diffraction, x-ray diffraction, and atomic force microscopy. Adding Sb to Bi leads to a reduction of the lattice constant, poorer crystallinity, and a rougher surface morphology.
Electron spectroscopic study of C–N bond formation by low-energy nitrogen ion implantation of graphite and diamond surfaces
Authors: Gouzman I. , Brener R. , Hoffman A.
Published: July 2002

The effect of 500 eV N2+ irradiation of graphite and diamond surfaces has been investigated by in situ electron spectroscopies (Auger electron spectroscopy and x-ray photoelectron spectroscopy). The chemical state of the implanted nitrogen and carbon have been studied as a function of: (i) implantation temperature in the room temperature (RT) to 800 K range, (ii) annealing of the RT implanted layer up to 800 K, (iii) and ion dose. It is concluded that the implanted nitrogen is present in three different bonding states, denoted as α, β, and γ, for all implantation conditions. The distribution of these states was found to be affected by the substrate nature as well as by the temperature of implantation and annealing process. A chemical interconvertion model is proposed to explain the changes in population of the carbon–nitrogen bonding states as a function of annealing and implantation temperature. It is suggested that the β state includes nitrogen atoms in threefold configurations and may be related to an almost unpolarized carbon–nitrogen chemical bond, which is expected to be present in β-C3N4 phase. A predominant population of this state has been achieved in the case of nitrogen ion implantation into diamond. It has been demonstrated that hot nitrogen implantation results in the formation of the least polarized carbon–nitrogen bonding state [the β state which possess higher N(1s) binding energy] in all studied systems. The structure of the nitrogen implanted layers has been assessed by the analysis of the C(KVV) Auger line shape. Partial conservation of the initial substrate structure has been observed after hot nitrogen implantation of the diamond and graphite surfaces. Our model investigation of carbon nitride formation by low energy ion implantation strongly suggests that it is impossible to populate only one particular carbon–nitrogen bonding state in which carbon is in sp3 and nitrogen in sp2 hybridization state in the frame of the studied experimental conditions. However, this state was found to be formed among a variety of possible other carbon–nitrogen bonding states. The results presented in this work are of importance for understanding the fundamental processes involved in the formation of carbon nitride thin films by ion beam deposition methods.
Thermal Soret diffusion in the liquid phase epitaxial growth of binary III–V compounds
Authors: Chien Chung-Ping , Mattes Brenton L.
Published: July 2002

Steady-state liquid phase epitaxial nucleation and growth conditions have been achieved in the growth of III–V compounds that yield terrace and miniscus line-free layers and uniform submicron thickness control. A theoretical analysis of transient thermodynamic transport through super- and undersaturated melts appears to show that the thermal Soret effect is a very strong coupling force between solute and heat transport. When nucleation and growth are initiated at near steady-state equilibrium liquidus conditions, at the substrate interface with a temperature gradient, melt-back and spontaneous nucleation are minimized. This appears to indicate that near the interface localized nonuniformities exist in supersaturated melts that yield unstable conditions for nucleation and growth. The Soret thermal diffusion coefficient has been evaluated in terms of the solute and solvent masses and the temperature dependence of the diffusion coefficient. At a noninteracting interface the steady-state solute concentration is always greater than the equilibrium liquidus concentration with a positive temperature gradient to the III–V source. The increased concentration is a result due to zero net solute flux at this interface where equal but opposite driving forces are developed as steady-state conditions are approached. At a very critical time during melt saturation the equilibrium liquidus concentration is reached when the substrate should be brought into contact with the melt to avoid melt-back and spontaneous nucleation. The critical time determined in this analysis appears to be in agreement with the experimental results.
Preparation of low-reflectivity Al–Si film using dc magnetron sputtering and its application to multilevel metallization
Authors: Kamoshida K. , Makino T. , Nakamura H.
Published: July 2002

A low-reflectivity aluminum–silicon (LR Al–Si) film is prepared using a dc magnetron sputtering system under high Ar pressures. At a wavelength 4360 Å, the Al–2% Si film deposited at 3.99 Pa shows a specular reflectivity of 0.15 and a diffuse reflectivity of 0.10. The low-reflectivity is ascribable to the formation of columnar structures of several hundred angstroms with open boundaries. The formation is caused by thermalization of sputtered Al(Si) atoms and the suppression of surface migration by Si atoms. Microvoids formed between columnar crystallites are thought to absorb incident light. This film is applied to multilevel metallization, and the results obtained include improvement of patternability in photolithography, reduction of hillock formation, and thermal stability of contact characteristics. This film is thus attractive because it is simply prepared and processing procedures for multilevel metallization do not require any major modification of conventional processes.
Valence band offset in AlAs/GaAs heterojunctions and the empirical relation for band alignment
Authors: Wang W. I. , Stern Frank
Published: July 2002

The valence band offset in AlAs/GaAs heterojunctions is found to be 0.45±0.05 eV from analysis of charge transfer versus undoped spacer layer thickness. This result and several recent experiments on AlxGa1−xAs/GaAs heterojunctions indicate that the valence band offset is approximately linear in AlAs fraction x over the entire alloy composition range and more than twice the previously accepted value. The relation between heterojunction band offsets and Fermi level pinning for metals on semiconductors is discussed.
0.1 μ scale lithography using a conventional electron beam system
Authors: Dix C. , Flavin P. G. , Hendy P. , Jones M. E.
Published: July 2002

A commercial electron beam machine has been used in conjunction with high contrast resists to define patterns with dimensions close to the minimum beam diameter of 0.1 μ on a range of solid substrates. This has required a detailed knowledge of the beam-substrate interactions in order to control linewidth. The electron scattering profiles in thin resist layers have been experimentally determined for substrates of Si, GaAs, InP, and Au on Si, and for beam voltages between 10 and 50 kV. These experimental results have been correlated with empirical models of electron scattering. It has been found that the simple double Gaussian model is inadequate at these dimensions, and the addition of an exponential term is necessary to fit data adjacent to the primary beam. Exposures have been modeled using this three term approximation, and the resultant lithography predicted by taking account of resist contrast. Routine fabrication of asymmetric 0.46 μm period gratings on InGaAsP substrates, used in the manufacture of distributed feedback lasers, has been possible, while experimental nonperiodic structures of 0.1–0.2 μ dimensions have been made on Si and InP. The reduction of backscattering by a multilevel resist system on Si has been studied with planarizing layer thicknesses up to 2 μm, and beam voltages of 20 and 30 kV.
Characterization of thermal oxides grown on TaSi2/polysilicon films
Authors: Pawlik D. , Oppolzer H. , Hillmer T.
Published: July 2002

Oxidation of tantalum disilicide films deposited by cosputtering on highly phosphorus doped polycrystalline silicon was investigated as a function of various film preparation parameters. The thermal oxides, with a thickness of about 100 nm, were characterized by cross-sectional TEM, AES, and SIMS depth-profiling as well as breakdown strength measurements on small-area and large-area capacitors. No limitation on oxide growth for dry and steam oxidation was found in the investigated temperature range of 800 to 1000 °C. The removal of the native oxide on the poly-Si prior to TaSi2 deposition by an HF dip or by backsputtering is crucial for reproducible oxide growth. The nature of the oxide is similar to thermal SiO2 on (100) Si. The TaSi2 film integrity is maintained during oxidation. The high surface and interface roughnesses cause oxide thickness variations of about 50%. A high mass fraction of the order of 0.5 wt.% phosphorus was found in the oxides. The breakdown strength of both steam and dry oxides is about one order of magnitude lower than the values for thermal oxides on n+-poly-Si. These low breakdown strength values are attributed mainly to large pores in the oxides observed in the TEM. No restrictions on process compatibility were found.
Electron window cathode ray tube applications
Authors: Hanlon L. , Greenstein M. , Grossman W. , Neukermans A.
Published: July 2002

The refractory thin films of BN or SiC typically used in the fabrication of x-ray mask substrates are characterized by a very high yield strength, low mass density, and low atomic number. The deposition of these films by standard semiconductor techniques, low pressure chemical vapor deposition (LPCVD), coupled with orientation dependent etching of the substrate, allows the straightforward fabrication of defect-free, vacuum-tight membranes capable of supporting atmospheric pressure. Used as electron windows, these films are also distinguished by their high transmission of moderate energy electrons (typically 87% at 25 keV). The ease of deflection and modulation of electrons at these energies, and their availability in air or other media via one of these windows mounted on a cathode ray tube (CRT), allow a number of possible applications. Two such applications are reported here.
Lithographic fabrication of transmission electron microscopy cross sections in III–V materials
Authors: Dobisz E. A. , Craighead H. G. , Beebe E. D. , Levkoff J.
Published: July 2002

A procedure for forming specifically located transmission electron microscopy (TEM) cross section specimens of III–V semiconductor materials is described. By electron-beam lithography in a TEM/STEM followed by a lift-off technique, 40–100 nm wide metal lines were formed on the sample surface. Anisotropic reactive ion etching in a BCl3 and Ar plasma removed the material unprotected by metal lines to leave uniform thickness vertical sections of material. Details of the procedure and the TEM analysis of a sectioned GaAs/AlAs superlattice structure will be described. The ability to form a cross section from a specifically desired region of the sample and the ability to form long uniform sections makes this technique particularly applicable to analysis of devices in brittle semiconductor materials.
Azide–poly(methylmethacrylate) photoresist for ultraviolet lithography
Authors: Han C. C. , Corelli J. C.
Published: July 2002

Poly(methylmethacrylate) (PMMA) sensitized with an aromatic azide compound (4,4′-diazidodiphenyl sulfone), was prepared and evaluated as a negative UV resist. The resist was successfully applied to contact printings followed by wet development processing. In this study, both Xe–Hg and Hg lamps were used to evaluate the sensitivity of this new two-component resist. The sensitivity of this azide–PMMA resist depends on the azide concentration in PMMA solution, and the optimum azide concentration was found to range from 20 to 25 wt. % (based on PMMA weight). It was also found that postannealing of this resist after irradiation at 100 °C for 1/2 h can increase the sensitivity compared to no postannealing treatment. The addition of azide compound was observed to reverse the PMMA resist to negative tone for radiation doses 30 times lower than normally used in positive-tone PMMA. The range of deep-UV dose was 20 to 30 mJ/cm2 which gave sufficiently good resolution to clearly observe mask features of 1-μm lines and spaces.
Surface micromachining for microsensors and microactuators
Authors: Howe Roger T.
Published: July 2002

Micromechanical structures can be made by selectively etching sacrificial layers from a multilayer sandwich of patterned thin films. This paper reviews this technology, termed surface micromachining, with an emphasis on polysilicon microstructures. Micromechanical characteristics of thin-film microstructures critically depend on the average residual stress in the film, as well as on the stress variation in the direction of deposition. The stress in low-pressure chemical vapor deposition polysilicon varies with deposition temperature, doping, and annealing cycles. Applications of surface micromachining to fabricate beams, plates, sealed cavities, and linear and rotary bearings are discussed.
Projection patterned Si doping of GaAs in ambient SiH4 gas by a KrF excimer laser
Authors: Sugioka Koji , Toyoda Koichi
Published: July 2002

Potential application of a projection system with reduction optics to the patterned doping process has been demonstrated. A KrF excimer laser beam is projected on GaAs substrates enclosed in a SiH4 gas ambient cell to achieve the Si doping with a field size of 5×5 mm2. At a laser fluence of 380 mJ/cm2, the n-type conduction layer with a surface carrier density of 2.27×1014 cm−2 in the semi-insulating GaAs substrate and with an activation efficiency of ∼81% can be obtained. The minimum linewidth of 2.5 μm is discussed together with temperature profiles calculated by transient heat conduction in the substrate.
Proposal of a new visible light emitting structure: n-AlSb/p-ZnTe heterojunctions
Authors: McCaldin J. O. , McGill T. C.
Published: July 2002

From considerations of dopability, band offset, and lattice match, we find that of the various heterojunctions containing II–VI compounds, the n-AlSb/p-ZnTe heterojunction has the most promising properties for fabricating visible light emitters. The materials lattice match to 0.5%. Experimental evidence and theoretical predictions for the band offset blocking minority carrier injection indicate a range from zero to 0.3 eV maximum. This range of values is the lowest for heterojunctions involving wide-gap II–VI’s. Substantial electron injection into the p-ZnTe should be possible. Control of doping may suffice to suppress undesired hole current originating in the ZnTe.
Arsenic- and metal-induced GaAs interface states by low-energy cathodoluminescence spectroscopy
Authors: Viturro R. E. , Shaw J. L. , Brillson L. J. , Woodall J. M. , Kirchner P. D. , Pettit G. D. , Wright S. L.
Published: July 2002

We used optical emission techniques to obtain a direct measure of discrete electronic states at GaAs interfaces. These states depend on surface preparation, metallization, and semiconductor material. The energy of the discrete states found at metal/molecular-beam epitaxially grown GaAs(100) interfaces integrated within a Schottky formalism provides self-consistent results for the interface electrostatics. The presence of a large concentration of extrinsic defects causes the pinning behavior observed at metal/melt-grown GaAs(110) interfaces. These results provide the first direct experimental evidence for the origin of Fermi level ‘‘pinning’’ at metal/melt-grown GaAs junctions.
Crystal quality of solid phase epitaxially grown silicon
Authors: Ahlers E. D. , Ostrom R. M. , Datta P. , Allen F. G.
Published: July 2002

Solid phase epitaxial (SPE) films are formed in our molecular-beam epitaxy (MBE) system by depositing silicon on room temperature substrates and subsequently heating the substrates in vacuum in order to crystallize them. Using SPE growth, we have achieved doping levels >1020 cm−3 with both Ga and Sb, and also a minimum of dopant diffusion due to the low temperature of SPE growth. The chief disadvantage of SPE is a larger density of defects than is found in MBE silicon. We have conducted a transmission electron microscopy (TEM) study of SPE films grown at temperatures from 550 to 800 °C to determine the nature and approximate density of the dislocations and defects found in the films. For the optimal temperature of ≊650 °C, the defect density is quite low. Hall measurements show that the mobility of the SPE samples is comparable to, although somewhat lower than, that of bulk silicon.
Reflection high-energy electron diffraction studies of principal streak intensity profiles and adatom coverage of (100) GaAs grown by molecular-beam epitaxy
Authors: Farley C. W. , Streetman B. G.
Published: July 2002

The principal streak of reflection high-energy electron diffraction (RHEED) patterns of (100) GaAs grown by molecular-beam epitaxy (MBE) has been studied with an advanced imaging system which uses a charge-coupled device (CCD) camera and an array processor. Calculations by Fernandez suggest that the profile of the principal streak from MBE GaAs should be very sensitive to the relative coverage of Ga and As, as well as the degree of roughness of the GaAs surface. Observations of the principal streak profile under static and dynamic conditions confirm qualitatively the theoretical calculations of Fernandez. The principal streak profile changes dramatically as the relative coverage of Ga and As is changed. At approximately equal Ga and As coverage, the intensity at the specular position is relatively low, and increases as the degree of Ga or As coverage increases. Significant changes in principal streak intensity and shape are produced by changes in adatom coverage as small as 0.1 monolayer (ML), and these changes saturate as the surface reaches full Ga or As coverage. The streak profile is noticeably different for Ga- and As-stabilized surfaces, indicating that the (2×4) and (4×2) reconstructions are tetragonally distorted. Furthermore, the relative coverage of Ga and As adatoms during growth is strongly dependent on the ratio of the Ga and As fluxes. The lack of strong damping of oscillations in the specular spot intensity under optimized growth conditions is probably the result of relatively high Ga coverage and therefore higher Ga mobility on partially completed monolayers.
Exposure of ultrathin polymer resists with the scanning tunneling microscope
Authors: Zhang H. , Hordon L. S. , Kuan S. W. J. , Maccagno P. , Pease R. F. W.
Published: July 2002

The scanning tunneling microscope (STM) can be used to generate very narrow beams of low-energy electrons with extremely high current density. Because of this, it shows promise as a tool for lithography. We have previously demonstrated the capability of the STM in nanometer-scale lithography. Here we present new results using resist films prepared by the Langmuir–Blodgett (LB) technique, in which the polymer monolayer is spread on the water surface and is then transferred layer by layer onto the substrate. We have exposed both poly(methyl methacrylate) (PMMA) and poly(vinyl cinnamate). Patterns have been obtained in 5-monolayer (4.5 nm) and 17-monolayer (14 nm) LB PMMA films with linewidths down to 30 nm. For 17-monolayer PMMA, we have observed successful pattern transfer into the Cr layer for both polarities of tip-to-target voltage; smaller linewidths are observed for negative bias (target negative with respect to tip) than for positive bias (target positive with respect to tip). This is in agreement with our computer model, which predicts a smaller beam diameter for negative bias than for positive bias. For poly(vinyl cinnamate), patterns have been obtained in 11-monolayer (10 nm) films with linewidths down to 70 nm in the resist; the patterns transferred successfully into the Cr layer. Occasionally, with both materials, we have observed a ‘‘ghost image’’ of the pattern displaced from the primary image by 0.2–1 μm, with a reduced linewidth. We believe this is caused by two sharp protrusions on the tip, which may be separated by a comparable distance, acting as electron emission or collection sites.
Selective reactive ion etching for short-gate-length GaAs/AlGaAs/InGaAs pseudomorphic modulation-doped field-effect transistors
Authors: Ketterson A. A. , Andideh E. , Adesida I. , Brock T. L. , Baillargeon J. , Laskar J. , Cheng K. Y. , Kolodzey J.
Published: July 2002

Selective reactive ion etching of GaAs on AlGaAs in SiCl4/SiF4 plasma is reported. A selectivity ratio of 350:1 has been obtained at low power. A small decrease in the saturation current of gateless MODFET structures has been observed after etching the GaAs cap layer and has been ascribed to be due to low-power ion damage of the AlGaAs layer. This process was applied to the fabrication of 0.2 μm T-gate pseudomorphic MODFET’s. The dc and microwave performance of reactive-ion-etched devices and wet-etched devices were identical. However, for these short-gate-length devices a threshold voltage standard deviation of 30 mV was obtained for the reactive-ion-etched devices as compared to 230 mV for the wet-etched devices.
Effects of chromium on the reactive ion etching of steep-walled trenches in silicon
Authors: Maluf N. I. , Chou S. Y. , McVittie J. P. , Kuan S. W. J. , Allee D. R. , Pease R. F. W.
Published: July 2002

Formation of vertical-walled, high-aspect-ratio, narrow trenches in silicon is often required in submicron VLSI semiconductor processing. Present trench etching processes suffer from undercutting of the mask and sloped or bulged sidewalls; and when using chlorine-based reactive ion etching (RIE), there is frequently enhanced etching at the bottom corners of the trench. We report here the effects of chromium on the RIE of silicon. We found that, compared to using a SiO2 mask only, using a mask consisting of a chromium layer on top of SiO2, in a Cl2:SiCl4:N2 etching gas mixture, improves the trench sidewall angle relative to the vertical from 8° to 2°, and almost eliminates the enhanced etching at the bottom corners. Furthermore, we found that, by etching a sample partially covered with chromium in a Cl2:SiCl4:N2 gas mixture, the trench shape in the areas not covered with chromium depends on how far the trench is from the edge of the chromium layer. Those areas, within 1 cm of the edge of the chromium layer, showed quite steep sidewall angles that degraded (to 8°) at greater distances (few cm). Similarly, the size of the grooves due to the enhanced etching at the bottom corners is reduced as the trench gets closer to the edge of the chromium layer. It appears that the beneficial effect of the chromium is an indirect chemical action, based on the suppression of the concentration of neutrals or the formation of an inhibitor layer, rather than direct physical masking.
Strain relaxation kinetics in Si1−xGex/Si heterostructures
Authors: Hauenstein R. J. , Clemens B. M. , Miles R. H. , Marsh O. J. , Croke E. T. , McGill T. C.
Published: July 2002

Strain relaxation in Si1−xGex/Si superlattices and alloy films is studied as a function of ex situ anneal treatment with the use of x-ray diffraction and Raman spectroscopy. Samples are grown by molecular-beam epitaxy at an unusually low temperature (≊365 °C). This results in metastably strained alloy and superlattice films significantly in excess of critical thicknesses previously reported for such structures. Significant strain relaxation is observed upon anneal at temperatures as low as 390 °C. After a 700 °C, 2 h anneal, superlattices are observed to relax less fully (∼43% of coherent strain) than corresponding alloys (∼84% of coherent strain). Also, the strain relaxation kinetics of a Si1−xGex alloy layer is studied quantitatively. Alloy strain relaxation is approximately described by a single, thermally activated, first order kinetic process having activation energy Ea=2.0 eV. The relevance of our results to the microscopic mechanisms responsible for strain relaxation in lattice-mismatched semiconductor heterostructures is discussed.
The decomposition of triethylgallium on Si(100)
Authors: Lin R. , Gow T. R. , Backman A. L. , Cadwell L. A. , Lee F. , Masel R. I.
Published: July 2002

The decomposition of triethylgallium (TEG) on Si(100) is studied using temperature programmed desorption, x-ray photoelectron spectroscopy, and electron energy-loss spectroscopy. It is found that TEG adsorbs molecularly. It then decomposes mainly by a simple β-hydrogen elimination mechanism yielding ethylene and adsorbed hydrogen. The ethylene desorbs in a peak centered at 600 K while the hydrogen desorbs in a sharper peak at 820 K. There is some carbon incorporation which is attributed to adsorption of an ethylene impurity. However, there is no evidence for carbon deposition during the TEG decomposition process itself.
Fabrication of electron beam defined ultrasmall Ohmic contacts for III–V semiconductors
Authors: Randall J. N. , Yang C. H. , Kao Y. C. , Moore T. M.
Published: July 2002

The development of nanoelectronic devices calls for Ohmic contacts of very small dimensions. The size dependence of both alloyed and nonalloyed contacts to GaAs and InGaAs is investigated. It is found that alloyed contacts to GaAs fail to achieve Ohmic contact for contacts below 3000 Å in diameter, while molecular-beam epitaxy grown heavily doped InGaAs on GaAs nonalloyed Ohmic contacts can be formed with size as small as 1500 Å.
Silicon epitaxial growth on germanium using an Si2H6 low-pressure chemical vapor deposition technique
Authors: Fujinaga Kiyohisa , Takahashi Yasuo , Ishii Hiromu , Hirota Shoichi , Kawashima Izumi
Published: July 2002

Heteroepitaxial growth of silicon on a germanium surface at 580–800 °C is investigated using Si2H6/H2 gas low-pressure chemical vapor deposition processing. Crystalline Si grows on the Ge film surfaces at temperatures of 650–730 °C and, by contrast, polycrystalline Si is deposited at low temperatures of 580–630 °C. It is clarified that whether crystalline Si grows on the Ge surface depends only on substrate temperature, not on the Si2 H6 partial pressure (4×10−3–1×10−1 Torr) nor the Si growth rate (3–800 nm/min). The growth rate using Si2 H6 is more than one order higher than the SiH4 case. Its activation energy is 1.4 eV.
Application of reflection mass spectrometry to molecular-beam epitaxial growth of InAlAs and InGaAs
Authors: Brennan T. M. , Tsao J. Y. , Hammons B. E. , Klem J. F. , Jones E. D.
Published: July 2002

Reflection mass spectrometry (REMS) has been used to monitor In fluxes leaving the surface, and to optimize growth conditions during molecular-beam epitaxial (MBE) growth of InAlAs and InGaAs on InP. The optical, electrical, and structural properties of the films are shown to be a function of the measured indium signal. This optimization technique is independent of thermocouple or pyrometer readings to set the growth temperature, and has led to significant improvements in the optical and electrical properties of lattice matched InAlAs on InP. REMS-optimized unintentionally doped InAlAs has been grown with electron concentrations as low as 3×1015/cm3, electron mobilities as high as 4 000 cm2/V s at 77 K, and photoluminescence linewidths as narrow as 10 meV. The high quality of these layers is attributed to reduced alloy clustering and reduced ionized impurity scattering.
Technique for monolithically integrating GaAs/AlGaAs lasers of different wavelengths
Authors: Goodhue W. D. , Donnelly J. P. , Zayhowski J. J.
Published: July 2002

Lasers that operate at wavelengths differing by 40 nm have been fabricated from two different regions of a single wafer by a process using molecular-beam epitaxy. The process consists of controlling the thicknesses of the quantum wells in the regions which form the active areas of the lasers. The control is accomplished by manipulating the Ga sticking coefficient through local variations of the substrate temperature during the growth process. This process could lead to a variety of important developments in GaAs-based technology such as multiwavelength optical interconnects.
The addition of Ni in AuZn gate Ohmic contacts for InP junction field effect transistors
Authors: Boos J. B. , Kruppa W.
Published: July 2002

The fabrication and characteristics of AuZn/Ni/Cr/Au and AuZn/Ni/TiW/Au gate Ohmic contacts for InP junction field effect transistors (JFET’s) are reported. Auger electron spectroscopy sputter-depth profiles reveal substantial Ni interdiffusion at the metal–InP interface after heat treatment at 410 °C. The addition of a Ni layer significantly improves the adhesion of the contacts and a TiW barrier layer effectively maintains the conductivity of the gate metallizations after heat treatment. The specific contact resistance of the JFET gate structures is measured to be as low as 2×10−5 Ω cm2, which is the lowest value reported to date for contacts to p-type InP. InP JFET gate-drain diodes fabricated with the AuZn/Ni/TiW/Au gate contact exhibit a leakage current of ∼40 nA at −8 V for a 1μm×150 μm gate and a reverse breakdown voltage of 17.5 V at 25 μA. The contacts are shallow, as evidenced by the quality of the p+/n junctions fabricated, and thus are suitable for use in InP JFET structures where thin p+ regions are utilized.
Dry etch induced damage in GaAs investigated using Raman scattering spectroscopy
Authors: Lishan D. G. , Wong H. F. , Green D. L. , Hu E. L. , Merz J. L. , Kirillov D.
Published: July 2002

Raman scattering spectroscopy is used to probe the surface electrical properties (surface depletion width) of GaAs after dry etching in either a reactive ion etching (RIE) system or a remote plasma system. A variety of etch gases (argon, helium, Freon 12, HCl) and biases (−80 to −350 V) are used to examine physically and chemically induced damage. Through the use of wet etches, the dry etch induced damage was observed to be within ∼300 Å of the surface. In the RIE system, etching with argon and helium causes the depletion width to widen, with helium inducing a greater effect. A remote plasma does not cause a change in the depletion width, indicating an absence of energetically damaging species. In contrast to inert gases, etching with reactive gases in both systems reduces the effective surface charge.
Hot-electron magnetospectroscopy in resonant tunneling devices: A probe of conduction-band anisotropy
Authors: Hughes O. H. , Henini M. , Alves E. S. , Leadbeater M. L. , Eaves L. , Davies M. , Heath M.
Published: July 2002

We report a series of magnetotransport measurements on two double barrier resonant tunneling devices based on n-GaAs/(AlGa)As. The central GaAs wells have widths of 60 and 120 nm. Many well-defined quantum resonances are observed in the conductance even at room temperature and up to 70 resonances are observed at liquid helium temperatures. The existence of these resonances indicates that a significant fraction of the conduction electrons travel ballistically over distances >0.3 μm. When a magnetic field is applied in the plane of the well (B⊥J) tunneling takes places into two distinct types of quantum states: traversing states in which the electrons interact with both barriers and skipping states in which the electrons only interact with the emitter barrier. Rotation of the magnetic field in the (100) plane parallel to the barriers reveals the anisotropy in the Γ conduction band at high electron energies.
Temperature dependence of surface morphology of chemical vapor deposition grown Ge on Ge substrates
Authors: Aharoni Herzl
Published: July 2002

A step by step development of surface morphology as a function of deposition temperature (617–897 °C, by 40 °C steps) of Ge crystals grown by chemical vapor deposition using GeH4 in an He atmosphere is experimentally demonstrated. Three temperature regions are observed. A net-like rough surface morphology composed of many interconnected elongated crystals is observed in the low temperature range (617–697 °C). In the high temperature range (817–857 °C) isolated islands of three-dimensional structures having a pyramid-like geometry and their interactions are observed. The medium range (737–777 °C) is found to be a transition between the above ranges. As a whole, the degree of surface order obtained increases with the deposition temperature. The mechanisms influencing the morphology are explained by the temperature dependence of the GeH4 decomposition rate, Ge atom, surface mobility and migration length, nucleation and re-evaporation rates, activity of surface defects and nucleation sites, and finally, parasitic oxide formation.
Oxidation kinetics of plasma-enhanced chemical vapor deposition silicon nitride films deposited from SiH4/NH3/NF3/N2 mixtures
Authors: Gómez-Aleixandre C. , Sanchez Garrido O. , Albella J. M.
Published: July 2002

Fluorinated silicon nitride films have been deposited by plasma-enhanced chemical vapor deposition (PECVD) varying the relative [NF3]/([NH3]+[NF3]) flow ratio in the range 0–1. The as-deposited films show an increase in the –NH radicals and a decrease in the –SiH radicals when the flow ratio increases above 0.5. In addition, these films are very unstable in hot water as well as in normal air atmospheres. The oxidation kinetics has been explained in terms of a direct oxidation reaction of the silicon nitride lattice accompanied by the formation of fluorinated complexes which also react with moisture to give finally SiO2.
Molecular-beam epitaxial growth of pulse-doped pseudomorphic GaAlAs/GaInAs transistors with high gain and low noise properties
Authors: Hoke W. E. , Lyman P. S. , Labossier W. H. , Huang J. C. , Zaitlin M. , Hendriks H. , Flynn G.
Published: July 2002

Pulse-doped pseudomorphic Ga0.77Al0.23As/Ga0.85In0.15As/GaAs high electron mobility transistor structures have been grown by molecular-beam epitaxy. During film deposition, the substrate temperature is changed to improve the quality of the individual material layers in the structure. Although elevated growth temperatures are used following growth of the GaInAs channel layer, Auger electron spectroscopy indicates negligible indium migration. With pulse doping, sheet densities reaching 2.1×1012 cm−2 are obtained with mobilities of 7260 cm2 /V s at 300 K, 30 900 cm2 /V s at 77 K, and 38 300 at 12 K. Transistors with 0.5 μm gate lengths have been fabricated with excellent microwave performance. For this gate length on GaAs, a state-of-the-art current gain cutoff frequency of 62 GHz was obtained as well as a maximum available gain cutoff frequency in excess of 150 GHz. At 10 GHz, the minimum noise figure was 0.85 dB with an associated gain of 14 dB.
Synchrotron radiation x-ray lithography fabrication of 0.35 μm gate-length n-type metal–oxide–semiconductor transistors
Authors: Blackwell R. J. , Baker J. W. , Wells G. M. , Hansen M. , Wallace J. , Cerrina F.
Published: July 2002

Synchrotron radiation x-ray lithography, using commercial x-ray masks, has been used in a mix-and-match scheme with an optical stepper to fabricate functional 0.3 μm gate-length n-type metal–oxide–semiconductor devices. The x-ray exposures were done at the Center for X-ray Lithography (CXrL) at the University of Wisconsin-Madison. The gate-level mask was fabricated to our specifications by Perkin-Elmer Advanced Lithography Operations, Danbury, CT. All other processing and the optical stepper exposures were carried out at the McDonnell Douglas Electronics Systems Company—Microelectronics Center, St. Louis, MO. Processing results and device characteristics are discussed.
Metamorphic InyGa1−yAs/InzAl1−zAs heterostructure field effect transistors grown on GaAs(001) substrates using molecular-beam epitaxy
Authors: Grider D. E. , Swirhun S. E. , Narum D. H. , Akinwande A. I. , Nohava T. E. , Stuart W. R. , Joslyn P. , Hsieh K. C.
Published: July 2002

We present a novel technique for molecular-beam epitaxy (MBE) growth of InyGa1−yAs/InzAl1−zAs heterostructure field effect transistor (FET) material on GaAs (100) substrates, referred to as metamorphic InyGa1−yAs/InzAl1−zAs HFETs. This new approach uses an InyGa1−yAs step-graded buffer layer grown at low substrate temperature to accommodate lattice mismatch and to localize misfit dislocations below the active HFET device region grown at standard temperatures. Metamorphic InyGa1−yAs/InzAl1−zAs HFETs grown with a range of compositions (0.25<y,z<0.5) were found to exhibit 77 K Hall mobilities ranging from 22 000 to 27 450 cm2/V s and 77 K sheet carrier concentrations between 1.8 and 3.3×1012 cm−2. Transmission electron microscopy results from these HFETs show that dislocations generated by the lattice mismatch are essentially localized in the low-temperature InyGa1−yAs buffer. These results indicate that metamorphic InyGa1−yAs/InzAl1−zAs HFETs will exhibit excellent device performance.
Electron-beam lithography and chemically assisted ion beam etching for the fabrication of grating surface-emitting broad-area AlGaAs lasers
Authors: Tiberio R. C. , Porkolab G. A. , Johnson J. E. , Grande W. J. , Rathbun L. C. , Wolf E. D. , Craighead H. G. , Lang R. J. , Larsson A. , Forouhar S. , Cody J.
Published: July 2002

We report on the fabrication and characterization of broad-area, grating-coupled, distributed Bragg reflector, surface-emitting, AlGaAs/GaAs laser diodes. Electron-beam lithography and chemically assisted ion-beam etching (CAIBE) were used to fabricate both first-order (120-nm period) and second-order (240-nm period) gratings. Gratings were patterned by exposing PMMA resist using 50-keV electrons in a JEOL 5DIIU direct write electron-beam lithography system. The resist image was transferred into the AlGaAs layer by CAIBE. CAIBE was performed using chlorine gas in conjunction with a 500-eV argon-ion beam in a modified Technics Plasma GmbH RIB 160 etcher. Surface-emitting lasers using second-order gratings were fabricated on AlGaAs/GaAs asymmetric separate confinement double heterostructure layers grown by liquid-phase epitaxy. Wet chemical etching was used to remove the upper cladding layer, exposing the waveguide in selected areas. Gratings were then etched into the waveguide to produce surface emitting window regions. The far-field light intensity pattern shows a lateral angular spread of 2.4° for a 50-μm-wide stripe. This lateral angular distribution is the lowest reported for a broad-area (not array) grating surface-emitter.
Plasma-deposited organosilicon thin films as dry resists for deep ultraviolet lithography
Authors: Horn M. W. , Pang S. W. , Rothschild M.
Published: July 2002

Organosilicon thin films have been formed by plasma enhanced chemical vapor deposition and employed as deep-UV photoresists. Films 20–200 nm thick were deposited from liquid organosilicon sources onto carbon-based planarizing layers and patterned in projection with a 193 nm excimer laser. At fluences below ∼5 mJ/cm2/pulse, exposure to 193 nm radiation induced oxygen incorporation into the film. Following either a wet or dry development step, negative-tone imaging was achieved, with the remaining photooxidized film being highly resistant to the O2 reactive ion etching employed in the subsequent pattern transfer step. At higher fluences, ∼15 mJ/cm2, positive-tone imaging by self-development was obtained for single-pulse exposures.
The local piezoelectric activity of thin polymer films observed by scanning tunneling microscopy
Authors: Birk H. , Glatz-Reichenbach J. , Jie Li , Schreck E. , Dransfeld K.
Published: July 2002

Here we present first measurements of the local piezoelectric activity of thin vinylidene fluoride-trifluoroethylene (VDF-TrFE) copolymer films with a spatial resolution of about 10 nm. The average value of the piezoelectric constant d33 is −0.03 nm/V varying typically by about 20% across a distance of 10 nm depending on the poling state of the film.
Large-scale periodic features associated with surface boundaries in scanning tunneling microscope images of graphite
Authors: Buckley J. E. , Wragg J. L. , White H. W. , Bruckdorfer A. , Worcester D. L.
Published: July 2002

Large-scale periodic features (6–12 nm), which appear as two-dimensional waves, have been observed in four separate examples with scanning tunneling microscope imaging of pyrolytic graphite. The features have approximate hexagonal arrangement and cover regions of up to μm dimensions. At least one edge boundary was observed in all regions showing the large features. Atomic-scale imaging on the waves gave the usual atomic resolution image of graphite on some parts of the waves (near maxima), but substantially different images on other parts. The orientation of the large lattice with respect to the atomic lattice was found to be 29±1.5° and corresponds closely to the covalent bond directions. Damaging a 20×60 nm area by increasing the bias to 3 V removed the waves from that area, but not the surroundings. The nature of these periodic features is not yet established. IBM researchers have proposed that such large-scale waves are Moiré patterns due to rotational misorientation of the top two graphite atomic layers [APS Bulletin 35, 485 (1990)]. However, some features of the wave images presented here are not readily accounted for by rotational Moiré effects and thus indicate that additional phenomena may be present. Another possibility is a constrictional Moiré pattern caused by increased double bond character in the top layer, which has additional valence electrons from the broken bonds at the edges.
Impurities and metalorganic chemical-vapor deposition growth of mercury cadmium telluride
Authors: Easton B. C. , Maxey C. D. , Whiffin P. A. C. , Roberts J. A. , Gale I. G. , Grainger F. , Capper P.
Published: July 2002

The doping behavior of indium and iodine have been investigated for mercury cadmium telluride (MCT, Hg1−xCdxTe ) layers deposited by the interdiffused multilayer process procedure at 400 °C using diethyl tellurium and dimethyl cadmium. Trimethyl indium and solid iodine were used as dopant sources. Both elements exhibited donor behavior under the conditions employed. Secondary ion mass spectrometry profile analysis was used to demonstrate that indium required a relatively long period during growth to attain an equilibrium concentration in the layer; in addition a significant reactor system memory was observed allied to a relatively fast diffusion rate. Iodine showed encouraging dopant properties at low concentration levels, the chemical concentration of iodine was in good agreement with the free donor level from Hall measurements. The advantage and applicability of each of the three chemical analysis procedures used in this work are discussed together with comments on the residual impurity content and electrical properties of undoped layers.
A mechanical probe for accurate substrate temperature measurements in molecular beam epitaxy
Authors: Ekenstedt M. J. , Andersson T. G.
Published: July 2002

A mechanical probe for temperature measurements in the range 300–530 °C has been developed in order to control the substrate temperature in a molecular-beam epitaxy system. In this temperature range conventional measuring devices like standard thermocouples and pyrometers do not provide the absolute temperature of the substrate. The probe makes contact with the substrate in a nongrowth position. To balance the heat flow from the sample holder to the probe, two thermocouples and a compensating oven are integrated with the probe. A reproducibility of ±2 °C can be achieved when measuring the absolute temperature.
Ballistic electron emission microscopy studies of Au–CdTe and Au–GaAs interfaces and band structure
Authors: Fowell A. E. , Williams R. H. , Richardson B. E. , Cafolla A. A. , Westwood D. I. , Woolf D. A.
Published: July 2002

Ballistic electron emission microscopy (BEEM) is an exciting new technique for probing electrical barriers at semiconductor interfaces. It offers huge potential because of its high lateral resolution, which is of the order of 1 nm. We have successfully applied the method to Schottky barriers at Au/n-CdTe and Au/n-GaAs interfaces. Simultaneous imaging of the surfaces of the thin gold overlayers and the metal-semiconductor interfaces shows no correlation between the surface topography and the interface barrier heights. Measured variations in barrier heights are associated with local defects or contamination at the interfaces. On a nanometer scale, the interface barrier heights for the Au–CdTe show large variations (0.7–1.1 eV). We have observed small-scale regions where barrier heights are low. These are probably associated with tellurium precipitates at the interface. By contrast, barrier heights of the Au–GaAs system were exceptionally uniform. These samples were prepared by depositing gold onto atomically clean epitaxially grown GaAs(100) surfaces. Detailed investigations of the voltage dependence of the BEEM current allows the determination of minima in the conduction band of GaAs at the Γ, L, and X points.
High resolution patterning of high T
c superconductors
Authors: Kern D. P. , Lee K. Y. , Laibowitz R. B. , Gupta A.
Published: July 2002

A novel method for submicron patterning of high Tc superconductor thin films is presented. Specifically, we describe the patterning of the superconductor YBa2Cu3O7−x (YBCO) on single crystal SrTiO3 wafers by a selective epitaxy approach. A silicon nitride template is formed on the SrTiO3 using electron beam lithography and reactive ion etching. A thin film of YBCO is then deposited on the wafer, e.g., by laser ablation. Selective epitaxy occurs during the deposition process, the YBCO film grows epitaxially on SrTiO3, while the film depositing on the nitride forms insulating clusters. Good epitaxial films and patterns have been obtained as-deposited without the need for further annealing or other process steps. Lines as narrow as 130 nm have been fabricated which show no significant decrease in Tc as compared with the original blanket film.