bluuebunny/crossref_metadata_2025 huggingface dataset:
Determination of the valence band offset at the asymmetric GaAs/In0.53Ga0.47As interface by ultraviolet photoemission spectroscopy
Authors: Williams M. D. , Chang T. Y.
Published: July 2002

Photoemission spectroscopy provides a method for determining the valence band offset of highly strained heterostructures. In situ ultraviolet photoemission spectroscopy shows that the A/B and B/A interfaces of the GaAs/In0.53Ga0.47As strained layer heterojunction are not abrupt and consist of a chemically reacted intermediate layer. The thickness of this layer depends strongly on the growth sequence and results in a strong asymmetry in the In profile between the two interfaces. Special considerations for determining the valence band offsets our outlined.
Lower-dimensional quantum structures by selective growth and gas-phase nucleation
Authors: Vahala Kerry J. , Saunders Winston A. , Tsai Charles S. , Sercel Peter C. , Kuech Tom , Atwater Harry A. , Flagan Richard C.
Published: July 2002

There is increasing interest in the potential application of quantum dots and quantum wires to various solid state devices. In this article, the physics of lower-dimensional quantum structures will be reviewed with emphasis on applications. In addition, two fabrication approaches under investigation at Caltech will be discussed. The first is based on selective nucleation of III–V semiconductors on a patterned host substrate and the second is gas-phase nucleation of III–V clusters. Recent results on formation of nanocrystal GaAs and silicon clusters in the gas phase are presented.
Effect of ordering, interface imperfections and clusters, and external electric fields on optical spectra of Si–SiGe heterostructures
Authors: Jaros M. , Beavis A. W. , Corbin E. , Hagon J. P. , Turton R. J. , Wong K. B.
Published: July 2002

A microscopic theory of optical transitions in Si–SiGe heterostructures which offers a direct comparison of the strength of the optical transitions in ordered and disordered structures is presented herein, and suggests a new enhancement mechanism via localization at interface imperfections and Ge islands, and shows that even ultrashort period Si–Ge superlattices exhibit a strong red Stark shift in place of the expected blueshift familiar from analogous GaAs–AlAs systems.
Surface-layer modulation of electron concentrations in InAs–AlSb quantum wells
Authors: Nguyen Chanh , Brar Berinder , Kroemer Herbert
Published: July 2002

A strong dependence of the electron concentration in not-intentionally doped InAs–AlSb quantum wells on the nature of the surface layer is demonstrated. We find that quantum wells capped with a GaSb layer result in much higher electron concentrations than wells capped with an InAs layer. The difference in concentration increases as the top barrier thickness of the well decreases. These changes are due to a shift in the pinning position of the Fermi level at the surface. For InAs surface layers, the Fermi-level pinning position also depends sensitively on the presence of residual Sb, which is found to move the pinning position further into the conduction band of InAs. The Fermi level is found to be pinned above the conduction band edge of bulk InAs by 80 meV for as-grown InAs surface layers, and 300 meV for etch-exposed InAs surface layers.
Optical properties of hydrogenated amorphous-carbon film for attenuated phase-shift mask applications
Authors: Callegari Alessandro , Pomerene Andrew T. , Hovel Harold J. , Babich Edward D. , Purushothaman Sampath , Shaw Jane M.
Published: July 2002

The optical properties of a plasma-deposited amorphous-carbon film have been investigated in the ultraviolet (365 nm) and deep ultraviolet range (248 nm). By varying process conditions, the optical transmission through the films was tuned from 4% to 20% at 365 nm and from 3% to 9% at 248 nm. This tuneability was related to the hydrogen content of the film as affected by the process parameters. The index of refraction n measured by spectroscopic ellipsometry is ∼2 at the wavelengths used. These optical properties make this film attractive for use in single layer attenuated phase-shift masks for potential application in 0.25 μm lithography at 365 and 248 nm.
Photoreflectance characterization of GaAs as a function of temperature, carrier concentration, and near-surface electric field
Authors: Badakhshan Ali , Durbin C. , Glosser R. , Alavi Kambiz , Pathak R.
Published: July 2002

Previous measurements of the photoreflectance line shape of GaAs at the E1 transition (2.9 eV) were extended. This study covers the combined effect of temperature and carrier concentration along with a discussion of the effect of the electric field intensity and the field inhomogeneity within a depth of 20 nm from the surface. A systematic study of changes in the line shape of the above band gap transition, E1 as a function of temperature (80–400 K) and carrier concentration (CC) (2–200×1016 cm−3) is presented and a model of the effect is discussed. It was found that as the carrier concentration increases beyond 1×1017 cm−3, the line shape changes in phase and in broadening in a characteristic way which depends on CC and temperature. A simple correlation is established between both the broadening and the line shape rotation as a function of temperature and CC. Using the Schottky relation and Fermi-level pinning, the observed effect qualitatively was explained. Also the effect of near-surface electric field on the line shape is discussed. The observed effect may be applied as an optical measurement of the carrier concentration or possibly the electric field within a depth of ≂20 nm from the surface/interface.
Selective and uniform high rate etching of polysilicon in a magnetically confined microwave discharge
Authors: Gadgil P. , Dane D. , Mantei T. D.
Published: July 2002

An electron cyclotron resonance plasma reactor with multipolar magnetic confinement has been characterized for potential applications in polysilicon gate patterning. A two-step, low pressure, 100% Cl2 etch process is used, in which a small substrate bias is applied only during the polysilicon etch step. This system etches anisotropic profiles into undoped polysilicon with an etch rate of 4000–4500 Å/min and polysilicon–oxide etch selectivities of 150–300. The downstream ion current density and plasma potential are radially uniform to within 1% (1σ) over a 200 mm diam. The polysilicon etch rate is radially uniform to within ±2% of the mean etch rate across a 150 mm wafer.
Ultrahigh vacuum scanning-tunneling microscope for in situ studies of annealing and electromigration behavior of thin films
Authors: Reiss G. , Levine L. E. , Smith D. A.
Published: July 2002

High resolution in situ imaging of metal surfaces during annealing and/or current transport is of great interest both for the application of metal lines in electronic devices and for extending our basic knowledge of the related complex physical phenomena. Due to its high resolution, the scanning tunneling microscope (STM) is a promising tool for in situ investigations of the structural changes of metals with respect to temperature (annealing) and current transport (electromigration). In this article, the instrumental modifications necessary for in situ scanning tunneling microscopy of heated and/or current carrying thin films under ultrahigh vacuum conditions are discussed. Using results obtained on Ag films, the ability of our STM to image the surface dynamically is demonstrated, while structural changes induced by the current transport are taking place.
Plasma etching of silylated photoresist: A study of mechanisms
Authors: Joubert O. , Pons M.
Published: July 2002

The parameters relevant to dry development of silylated polymers have been studied, in particular ion energy and atomic oxygen concentration. Critical dimension loss is demonstrated to depend upon silylation angle, sputtering rate of the silylated polymer, and development time. Anisotropic etch profiles are argued to be due to a compromise between critical dimension loss and isotropic etching of the unsilylated polymer.
High-speed λ=1.3 μm metal–semiconductor–metal photodetectors on GaAs
Authors: Sacks R. N. , Ade R. W. , Bossi D. E. , Basilica R. P. , Eichler D. W.
Published: July 2002

Metal–semiconductor–metal (MSM) photodetectors operating at λ=1.3 μm have been fabricated on (In,Ga)As grown on GaAs. The structure used was In0.4Ga0.6As/In0.2Ga0.8As/GaAs. The lattice-mismatched layers were designed to minimize dislocations in the optically active layer and near the carrier collection electrodes. To provide high enough Schottky barriers for fabricating MSMs without degrading carrier collection, a lattice-matched, graded In0.4(Ga,Al)0.6As cap was used. For devices fabricated on a wafer with a 0.5-μm-thick In0.4Ga0.6As layer, typical dark currents were <400 nA at 20 V. Carrier collection efficiency was 90%. Devices with 2 μm finger spacing exhibit a 3 dB bandwidth of 4.5 GHz.
Nature and origin of residual impurities in high-purity GaAs and InP grown by chemical beam epitaxy
Authors: Roth A. P. , Sudersena Rao T. , Benzaquen R. , Lacelle C. , Rolfe S.
Published: July 2002

In this article, we present the results of a systematic study of residual impurities in unintentionally doped GaAs and InP grown by chemical beam epitaxy. They are characterized using optical measurements and secondary-ion mass spectrometry which provide both the concentration and the chemical nature of the impurities. We have found that the residual carrier concentration in GaAs depends strongly on the arsine cracker cell temperature and that dominant n-type impurities such as sulfur and selenium originate from arsine. In contrast, InP is much less sensitive to the phosphine cracker cell temperature and the concentration of carbon is greatly reduced compared to GaAs.
Cleaning of silicon surfaces by argon microwave multipolar plasmas excited by distributed electron cyclotron resonance
Authors: Raynaud P. , Pomot C.
Published: July 2002

The cleaning of single-crystal silicon with an argon plasma produced by a microwave multipolar reactor has been studied as a function of the ion energy striking the surface (in the range 25–135 eV), the substrate temperature, and the ion dose. Two treatments have been applied. In the first one, the substrate is kept at room temperature, and the effect of the ion energy is studied before and after an anneal at 700 °C under high vacuum conditions. In the second case, the substrate is heated at 700 °C during the plasma treatment. In situ analysis (spectroscopic ellipsometry over the wavelength range 0.2–0.9 μm, low-energy electron diffraction, and Auger electron spectroscopy) and ex situ analysis (atomic force microscopy) allow us to define the surface state of the substrate and to follow its evolution as a function of the different parameters. We have shown that it is feasible to obtain a silicon surface exempt of contaminants (carbon and oxygen) by plasma treatment, even at low ion energy (<100 eV). However, the treated surface always shows a residual roughness due to the ion bombardment. The surface roughness increases with ion energy and treatment temperature. In the best case, the perturbed layer is still of the order of 30 Å. The temperature plays an important role as it can either increase or reduce the plasma-induced roughness according to the operating mode.
Dry surface cleaning of plasma-etched high electron mobility transistors
Authors: Pearton S. J. , Ren F. , Katz A. , Chakrabarti U. K. , Lane E. , Hobson W. S. , Kopf R. F. , Abernathy C. R. , Wu C. S. , Bohling D. A. , Ivankovits J. C.
Published: July 2002

Fabrication of submicron high electron mobility transistors (HEMTs) involves dry etch removal of GaAs from an underlying AlGaAs or InGaAs stop layer. The etch selectivity is achieved by formation of AlF3 on AlGaAs, or InCl3 and InF3 on InGaAs, which must be removed before processing can proceed. Wet chemical cleaning has difficulty in such a situation because of surface tension effects. We have investigated use of electron cyclotron resonance (ECR) H2 or Ar discharges, or hexafluoroacetylacetone (HFAC) vapor, for in situ dry etch cleaning of HEMTs exposed to low-bias BCl3/SF6 discharges. The HFAC vapor can remove most of the remnant fluorine, but is effective only when the sample is heated above ∼250 °C. This relatively high temperature is not compatible with in situ cleaning of the etched device. Low-bias (−75 V) sputter cleaning with an Ar discharge removes all remnant Cl and ∼40% of the F, but direct-current biases above −125 V are required for complete cleaning, and this ion bombardment can lead to damage in the HEMT. ECR H2 discharge exposure is effective in removing all Cl- and F-related residues in a short period (∼5 min) with low direct-current biases (−25 V) on the sample.
Modeling of parasitic barrier effects in silicide/Si1−xGex Schottky-barrier infrared detectors fabricated with a silicon sacrificial layer
Authors: Xiao X. , Sturm J. C. , Parihar S. R. , Lyon S. A. , Meyerhofer D. , Palfrey S. , Shallcross F. V.
Published: July 2002

By employing a thin silicon sacrificial cap layer for silicide formation, we have fabricated both Pd2Si/Si1−xGex and PtSi/Si1−xGex Schottky-barrier infrared detectors with extended cutoff wavelengths and near-ideal leakage characteristics. Substantial deviation in the spectral response from Fowler’s theory was observed in some silicide/Si1−xGex detectors fabricated using this technique. This deviation was attributed to a parasitic barrier at the interface due to the excess silicon sacrificial layer left unconsumed by the silicide formation process. A simple model which modifies Fowler’s theory to take into account the reduced injection efficiencies of the photoexcited holes through the Si barrier was proposed. Good agreement between this model and the experimental data has been achieved.
High-performance Si/SiGe heterojunction bipolar transistors grown by molecular-beam epitaxy
Authors: Gruhle A.
Published: July 2002

Si/SiGe heterojunction bipolar transistors have been fabricated by growing the complete layer structure with molecular-beam epitaxy. The precise control of composition and thickness of the different layers have yielded excellent device performance with high Early voltage and current gains. The best measured transit frequency was 73 GHz. The low base sheet resistance of about 1 kΩ per square led to an fmax of 55 GHz. Noise figures were 1.2 dB at 2 GHz and 2 dB at 10 GHz. In a first application a hybrid mm wave 24-GHz oscillator was built with a phase noise of −92 dB at 100 kHz off carrier.
Scaling behavior of delta-doped AlGaAs/InGaAs high electron mobility transistors with gatelengths down to 60 nm and source-drain gaps down to 230 nm
Authors: Van Hove M. , Zou G. , De Raedt W. , Jansen Ph. , Jonckheere R. , Van Rossum M. , Hoole A. C. F. , Allee D. R. , Broers A. N. , Crozat P. , Jin Yan , Aniel F. , Adde R.
Published: July 2002

Pseudomorphic delta-doped AlGaAs/InGaAs/GaAs high electron mobility transistors with gatelengths ranging from 60 to 250 nm have been fabricated in submicron source-drain gaps using high-resolution electron beam lithography. As a consequence of short channel effects, the maximum transconductance gm is not improved by decreasing the gatelength from 250 nm (950 mS/mm) to 60 nm (800 mS/mm). When comparing high-frequency (HF) and direct-current values of the peak transconductances for devices with gatelengths shorter than 100 nm, the HF performance is remarkably higher. The difference can be explained by the fact that short channel effects are less important at high frequencies. Due to the decrease of the gate capacitance, the transition frequency fT increases as the gatelength is reduced.
Spatial and angular nonuniformities from collimated sputtering
Authors: Dew S. K. , Liu D. , Brett M. J. , Smy T.
Published: July 2002

A Monte Carlo collimated sputtering model, SIMSPUD, has been extended to examine the effects of collimation on the uniformity of spatial and angular distributions of flux incident to the substrate. The model predicts that substantial periodic variations can occur in the film thickness due to the proximity of the collimator. Experimental measurements closely confirm these predictions. In addition, the model predicts considerable periodic nonuniformity in the angular distribution over the same lengthscale. This will cause substantial variations in sidewall and bottom coverage inside topography as demonstrated using the film growth simulation, SIMBAD. Finally, collimation is seen to effectively transfer nonuniform sputter target erosion profiles to film thickness profiles, especially at higher aspect ratios. All three of these nonuniform effects have the potential to adversely affect the yield, performance, and reliability of films deposited using collimated sputtering.
Stability of strained Si1−yCy random alloy layers
Authors: Powell A. R. , Eberl K. , LeGoues F. E. , Ek B. A. , Iyer S. S.
Published: July 2002

Si1−yCy random alloy material opens up new opportunities for heterostructures based on Si. It offers the possibility of tensile strain within the Si material system without the requirement for relaxed Si1−xGex buffer layers, and when combined with the Si1−xGex system allows some degree of independence between strain and band gap in Si-based semiconductors. Unlike the Si1−xGex system, the Si1−yCy system has a high misfit (52%) and low solubility (<10−6), with a propensity for compound formation. In order to prevent carbide formation, the structures are kinetically stabilized by low-temperature growth. In this work, we consider the metastable critical thickness that can be reached by the Si1−yCy alloys and find it to be considerably less than the comparable thicknesses obtained from Si1−xGex alloys on Si. In the Si1−yCy system, we have found two distinct relaxation regimes for the strained layers. These being low and high mismatch regions where relaxation occurs by the creation of 60° dislocations and microtwins, respectively.
Electron cyclotron resonance plasma oxidation studies of InP
Authors: Hu Y. Z. , Joseph J. , Irene E. A.
Published: July 2002

Electron cyclotron resonance plasma oxidation of InP was studied using both spectroscopic and single wavelength ellipsometry employed during the oxidation process. A two layer oxide was observed with the outer layer being In rich and the inner layer P rich as confirmed from x-ray photoelectron spectroscopy and etch rate studies. Optical models and oxidation kinetics are analyzed. From positive substrate bias effects on oxidation rates, negative ion oxidant species were identified as dominant and oxide etching was observed at negative bias. Plasma oxidation, like thermal oxidation, yielded excess P near the semiconductor surface which would degrade electronic properties.
Boron delta doping in Si and SiGe and its application toward field-effect transistor devices
Authors: Carns T. K. , Zheng X. , Wang K. L. , Wu S. L. , Wang S. J.
Published: July 2002

The mobility behavior of boron delta (δ)-doped Si, Si1−xGex(0≤x≤1) is investigated, which includes the first mobility measurements reported for B δ-doped Si1−xGex and Ge. The expected mobility enhancement from δ doping is not realized in Si:B due to the heavy effective mass of holes. However, some enhancement may be possible at lower doping levels for narrower wells. Indications of mobility enhancement have been realized for δ-doped Ge:B because of the relatively light effective mass. We also present the implementation of boron δ-doped layers in the fabrication of (i) the first SiGe:B δ-FET and (ii) the first coupled δ-layer Si:B δ-FET.
Optimization of an electron cyclotron resonance plasma etch process for n+ polysilicon: HBr process chemistry
Authors: Tipton G. D. , Blain M. G. , Westerfield P. L. , Trutna L. S. , Maxwell K. L.
Published: July 2002

Designed experiments were employed to characterize a process for etching phosphorus doped polycrystalline silicon with HBr in a close-coupled electron cyclotron resonance plasma reactor configured for 200 mm wafers. A fractional factorial screening experiment was employed to determine the principal input factors and the main etch effects. Linear models of the process responses indicate rf power, O2 flow rate, and the position of the resonance zone (with respect to the wafer) as the three strongest factors influencing process performance. Response surfaces generated using data from a follow-on response surface methodology experiment predicted an optimum operating region characterized by relatively low rf power, a small O2 flow, and a resonance zone position close to the wafer. One operating point in this region demonstrated a polysilicon etch rate of 270 nm/min, an etch rate nonuniformity of 2.2% (1 std. dev.), an etch selectivity to oxide greater than 100:1, and anisotropic profiles. Particle test results for the optimized process indicated that careful selection of the O2 fraction is required to avoid residue deposition and particle formation.
Evaluation and control of device damage in high density plasma etching
Authors: Gadgil P. K. , Mantei T. D. , Mu X. C.
Published: July 2002

The effects of polysilicon etch plasma conditions on metal–oxide–semiconductor (MOS) capacitor breakdown and n-channel MOS transistor (NMOS) performance have been investigated. A high density electron cyclotron resonance (ECR) plasma source with multipolar magnetic confinement was integrated into a full NMOS process flow. The polysilicon etch plasma process parameters for designed experiments were microwave power, overetch time, rf bias, and plasma radial uniformity. MOS capacitor leakage currents increase with longer polysilicon edge lengths on gate oxide, higher ion density, and higher ion energy during the polysilicon overetch step. Lower NMOS transistor transconductance and higher threshold voltages correlate with longer overetch times and high microwave power and rf bias during the overetch step. Device performance degradation increases with decreasing channel length, and the exposure of the source and drain oxide edges to a high density flux is thought to be the main cause for observed degradation. A high rate, selective, and low damage polysilicon etch process can be obtained using a high density (≳1011 cm−3) and moderate ion energy (<30 eV) ECR discharge, with moderate power and zero applied rf bias during the overetch step.
Detecting and controlling forces in atomic force microscopy with multidegrees of freedom
Authors: Kawakatsu Hideki , Saito Takashi , Kougami Hiroshi , Blanalt Pierre , Kawai Minoru , Watanabe Michihito , Nishioki Nobuhisa
Published: July 2002

An atomic force microscope (AFM) with the function of measuring both bend θx and twist θy of a rectangular-shaped cantilever, with its longitudinal axis pointing in the y direction, was used with a controller, where √a×θ2x+b×θ2y roughly corresponding to the vectorial magnitude of the force acting on the tip in the x-z plane was kept constant by integral control of the z scanner. The control was effective during scanning in the repulsive mode, and by changing the parameters a and b, fluctuation of θy corresponding to the lateral force could be maintained below a set threshold. Since the force acting on the end point of the tip changes its magnitude and orientation during scanning, the cantilever is expected to undergo various modes of deformation, resulting in relative displacement of the base of the cantilever and the end point of the tip. For better understanding of the imaging process of AFM, the trajectory of the end point of the tip was plotted during image acquisition. When scanning mica, it was found that the end point meanders by up to 0.3 nm from the axis of the scan. This implies that the conventional imaging technique of plotting acquired data along the straight movement of the xyz scanners may result in artifacts when the relative movement of the end point of the tip and the base of the cantilever is not negligible to the size of the features observed.
Plasma ion implantation technology at Hughes Research Laboratories
Authors: Matossian Jesse N.
Published: July 2002

The plasma ion implantation (PII) project at Hughes Research Laboratories (HRL) has as its main objective the evaluation and application of PII technology to improve the tribological properties of metal and nonmetal materials used in aerospace, defense, and commercial applications. The HRL PII facility consists of a 4-ft-diam×8-ft-long vacuum chamber capable of implanting objects weighing up to 7000 lbs, and a high-power (100-kW), high-voltage (100-kV) pulse modulator to provide voltage pulses for implantation. Advanced plasma sources have been developed to produce atomic, as well as molecular, nitrogen and oxygen ions, and PII processes have been developed to treat metal and nonmetal materials. The HRL PII facility has been operational since 1989 and has been used for prototype demonstrations of PII technology to achieve (1) a 2–3× improved wear life of Co/WC drill bits used for printed-wiring-board fabrication, (2) an 8× reduced wear rate for TiN-coated cutting tools, and (3) a 2× increased surface hardness for a 7000-lb polymer object, 3 ft by 5 ft by 1 ft.
Electroreflectance of Ag/GaAs
Authors: Wang Dong-Po
Published: July 2002

The electroreflectance (ER) spectra of Schottky-barrier Ag/GaAs have been measured at various dc bias voltages (Vbias). It is known that the surface electric field can be deduced from the period of the Franz–Keldysh oscillations (FKO) of ER. Therefore, the built-in voltage (Vbi) and the carrier concentration in the depletion region can be calculated by the assumption of a constant charge distribution in the depletion region. In this article, we show that Vbi can also be determined from the amplitude of the FKO as a function of Vbias in the forward biased region. The Vbi determined by the above two different methods have been verified to be in good agreement.
Structure of Fe/δ-Mn superlattices grown by molecular beam epitaxy
Authors: Pohl J. , Malang E. U. , Scheele B. , Köhler J. , Lux-Steiner M. Ch. , Bucher E.
Published: July 2002

We report on the successful epitaxial growth of the bcc high temperature modification δ-manganese in Fe/Mn multilayers on Ge(001) and GaAs(001) substrates at room temperature using molecular beam epitaxy. Both structure and growth mode were observed in situ by reflection high energy electron diffraction (RHEED) and Auger electron spectroscopy, respectively. RHEED and x-ray measurements confirmed the superlattices to grow coherently in the [001] orientation, given by the orientation of the substrates. The obtained relationship in orientation is (001)Fe/δ-Mn∥(001)Ge, [100]Fe/δ-Mn∥[100]Ge. RHEED revealed a slight tetragonal distortion of the cubic lattice parameters of Mn (a=2.86 Å, c=2.79 Å). The Fe/Mn multilayers form abrupt layers; the growth mode of the individual layers could be approached by the layer-by-layer model. No island formation could be detected.
Electromigration in AlSiCu/TiN/Ti interconnects with Ti and TiN additional layers
Authors: Sekiguchi Mitsuru , Sawada Kazuyuki , Fukumoto Masanori , Kouzaki Takashi
Published: July 2002

AlSiCu/Ti(TiN)//TiN/Ti interconnect structures, which have been fabricated by depositing a Ti or TiN additional layer on the TiN/Ti barrier exposed in atmospheric ambient and then by depositing AlSiCu on the additional layer without breaking vacuum, have shown improved electromigration lifetimes. These were 1.9 and 6.5 times longer than that of the conventional AlSiCu//TiN/Ti structure, for the TiN and Ti additional layer, respectively. From an analysis of these results, it has been assumed that elongated lifetimes were caused by an enhancement of the Al(111) crystallographic orientation in the AlSiCu/TiN//TiN/Ti structure, and caused by both a reduction of the Si nodule concentration in AlSiCu due to Al–Si–Ti ternary alloy formation and Al(111) texture further enhanced by the additional Ti underlayer in the AlSiCu/Ti//TiN/Ti structure.
Progress in mask technology for ion implantation based nanofabrication
Authors: Burkard M. , Griesinger U. A. , Menschig A. , Schweizer H. , Klein H. , Böhm G. , Tränkle G. , Weimann G.
Published: July 2002

Gold dot implantation masks with diameters down to 45 nm have been defined using a liftoff process and have been used to fabricate GaAs/AlGaAs quantum dots by the technique of masked-implantation-enhanced intermixing (MIEI). The photoluminescence of these quantum dots shows a high intensity down to the smallest diameters and a systematic blueshift due to the quantum size effect. Furthermore, a novel nanometer electroplating (NEP) technique for the fabrication of gold dot implantation masks down to 30 nm with high aspect ratios (≳1:3) is presented. It is expected that the NEP masks are superior to the liftoff masks for future application of the MIEI technique.
Interaction of C60 with the Au(111) 23×√3 reconstruction
Authors: Altman Eric I. , Colton Richard J.
Published: July 2002

The interaction of C60 with the 23×√3 Au(111) reconstruction was studied using UHV scanning tunneling microscopy (STM). Preferred adsorption at fcc or bulk terminated sites was observed on deposition and after annealing to partially desorb the C60 layer. Disruptions in the reconstruction were observed on the bare Au substrate neighboring the C60 layer with reconstruction ridges terminating at the layer edges rather than continuing through the adsorbed layer. When low C60 deposition rates were used, STM measurements indicated that virtually all Au atoms at the C60 interface occupy fcc bulk terminated sites indicating that C60, unlike ordinary hydrocarbons, interacts strongly enough with Au to lift the reconstruction. These results are contrasted with results obtained at much higher deposition rates that showed an altered reconstruction at the C60–Au(111) interface. This variation with deposition rate is attributed to the concerted motion of Au atoms required to lift the reconstruction creating a kinetic constraint.
Evolution of visible photoluminescence and surface morphology of ultrathin porous Si films imaged by scanning tunneling microscopy*
Authors: Enachescu M. , Hartmann E. , Koch F.
Published: July 2002

The surface morphology of various ultrathin porous Si (PS) films (∼20 nm in thickness) is systematically investigated by means of scanning tunneling microscopy (STM) and correlated with the respective photoluminescent properties. Using moderately doped Si(100) wafers, three classes of PS samples are fabricated by electrochemical etching at low current densities under different preparation conditions: in the dark (class A), under illumination with ultraviolet (UV) light (class B), and in the dark followed by a postphotochemical treatment (class C). Upon UV-light excitation, layers of class A do not emit light in the visible range, while weak and efficient photoluminescence signals are obtained from samples belonging to classes B and C, respectively. STM imaging reveals a remarkable decrease in the lateral size of characteristic surface features from approximately 10 nm for class A to roughly 2 nm for class C films, fitting excellently to the quantum confinement approach in describing the origin of luminescence. For comparison, mesoporous Si films are also employed, fabricated by electrochemical etching of highly doped Si wafers. Preparation in the dark leads to nonvisible light-emitting material and the surface morphology is comparable to that of class A films.
Spatial variation of 1/f current noise in scanning tunneling microscopes
Authors: Maeda K. , Sugita S. , Kurita H. , Uota M. , Uchida S. , Hinomaru M. , Mera Y.
Published: July 2002

To elucidate the origin of the 1/f fluctuations in tunneling current in scanning tunneling microscopes (STM), a systematic study was performed on combinations of Pt–Ir alloy tips and Au and graphite samples in air. The noise power increases quadratically with the tunneling current when the tunnel resistance is fixed constant, which indicates that the current fluctuations arise from the fluctuations of the tunnel resistance. The noise intensity increasing also with the tip-sample distance z rules out fluctuations of z from possible causes of the current fluctuations. Measurements of work function φ revealed a low-frequency fluctuation in φ as well. The noise power in a fixed bandwidth exhibited spatial variations on the Au surfaces, which have some correlation with the STM topography and more direct correlation with the spatial variation of φ. The experimental results are discussed in terms of desorption/adsorption of ambient gas molecules on the sample/tip surfaces.
Study of reconstruction at interfaces of CdSe/ZnTe superlattices by total energy calculations
Authors: Ren Shang-Fen , Gu Zong-Quan , Chang Yia-Chung
Published: July 2002

ZnTe/CdSe superlattices are II–VI heterostructures with a lattice mismatch less than 0.3%, so strain effects are expected to be negligible for ideal structures except for the two interface layers that consist of ZnSe and CdTe. Recent experiments indicated the exchange of whole atomic layers across the interfaces in ZnTe/CdSe superlattices despite the fact that more atomic layers near the interface would become strained. To understand such exchanges, first-principles pseudopotential studies of several different interface structures were performed. Total energies, band offsets, and band structures of superlattices with different atomic rearrangements at interfaces are studied and compared. Our results indicate that the exchange of cation atomic layers across an anion atomic layer in these superlattices is energetically favorable.
Strain dependence of the valence-band offset in arsenide compound heterojunctions determined by photoelectron spectroscopy
Authors: Ohler C. , Moers J. , Förster A. , Lüth H.
Published: July 2002

We have used ultraviolet and x-ray photoelectron spectroscopy to study the strain dependence of the valence-band offset (VBO) in situ for binary arsenide compound semiconductor heterojunctions grown by molecular beam epitaxy; i.e., we have measured the VBO of InAs/GaAs, InAs/AlAs, and AlAs/GaAs(100) heterojunctions as it changes when these heterojunctions are pseudomorphically strained to lattice constants ranging from 5.653 Å (GaAs) to 6.058 Å (InAs). Intermediate lattice constants have been realized by using fully relaxed InxGa1−xAs buffer layers as virtual substrates. The spectrum of two distinct core-levels, one from either side of the coherently strained junction, monitors the energetic alignment across the junction. To establish the band offset, one has to know in addition the binding-energy separation of the core-level to the respective valence-band maximum (VBM) for each of the junction’s two components and corresponding to their correct bulk strain state. Here, we have recorded valence-band and core-level spectra of strained ‘‘bulk’’ samples to determine this energy difference experimentally. We have accounted for the true shape of the density of states near the VBM by using results of k⋅p theory, thus avoiding a full pseudopotential calculation. GaAs/InAs and AlAs/InAs heterojunction VBOs are heavily strain dependent.

For example, one can alter the GaAs/InAs VBO by 480 meV on going from the GaAs to the InAs lattice constant, the InAs VBM being 0.04±0.10 eV lower than the GaAs VBM when InAs is strained to the GaAs lattice constant and 0.52±0.10 eV lower when the strain is vice versa. The AlAs/InAs heterojunction VBO changes from +0.29±0.11 eV (type-I), when InAs is strained to AlAs, to −0.10±0.17 eV (type-II) for the opposite case. The AlAs/GaAs heterojunction VBO is independent of strain within our limits of accuracy. The results are compared with recent theoretical data, both ‘‘model’’ theories and self-consistent calculations, shedding some light on the features that are important for the band alignment at polar heterojunctions. The results can be interpolated in a straightforward manner to estimate the VBO for the more realistic case of heterojunctions built up of any two ternary arsenide compounds strained to any reasonable lattice constant.
Thermal contraction of ultrahigh vacuum materials for scanning probe microscopy from 300 to 4 K
Authors: Nunes G. , Williams Dinsie
Published: July 2002

The total thermal contraction of UHV materials commonly used in the construction of scanning tunneling and scanning force microscopes has been measured at 195, 77, and 4.2 K. The measurements were made with a simple push-rod dilatometer whose accuracy was checked against the known thermal contraction of 304 stainless steel. The results of the measurements indicate that the total thermal contractions of Macor machinable ceramic and the lead–zirconate–titanate piezoceramic PZT-5A are closely matched and similar to the contractions of titanium and tantalum. The total thermal contraction of Torr Seal UHV compatible epoxy is substantially larger and is similar to that of other filled epoxies.
In-line holography using low-energy electrons and photons: Applications for manipulation on a nanometer scale
Authors: Schmid Heinz , Fink Hans-Werner , Kreuzer Jürgen
Published: July 2002

An in-line holography setup is used to obtain electron and photon holograms of high magnification from various samples. Electron holograms are obtained using the coherent beam of low-energy electrons that originates from the electron point-source tip. Light optical holograms are generated for comparison using a divergent laser beam. In both cases the numerical reconstruction of the holograms yields the wave front at the sample. The low-energy electron point source microscope has been modified by incorporating an additional tip for manipulating the sample under observation. With this manipulating tip, individual nanometer-sized wires have been contacted, and an electrical current has been passed through carbon fibers as well as through carbon nanotubes while the holographic electron pattern has been observed in situ.
Focused ion beam metrology
Authors: Wagner A. , Longo P. , Cohen S. , Blauner P.
Published: July 2002

The measurement of resist image size using a finely focused ion beam (FIB) is described. Comparisons of FIB and scanning electron microscope (SEM) measurements of resist features are presented. The short penetration range of an ion relative to an electron is shown to offer fundamental advantages for critical dimension metrology. Ion beams can induce less dimensional change (damage) than SEMs during the measurement process, potentially yielding improved measurement precision. Resist cross sectioning using FIB is demonstrated, and the factors contributing to quality of the cross section are presented. An H2O gas assisted etching process for polymer resists has been developed and is shown to significantly improve the quality of resist cross sections.
Feasibility study of photocathode electron projection lithography
Authors: Saville Gordon F. , Platzman P. M. , Brandes George , Ruel Rene , Willett Robert L.
Published: July 2002

Photocathode electron projection is an electron lithography technique that may be used to pattern semiconductors at the deep submicron level. Using a robust gold cathode, mask features in the range of 0.11–0.54 μm have been transferred to electron resist coated wafers with adequate depth of focus (≂5 μm) and large field of view (≂2 cm2). Low accelerating voltages ∼3 keV minimize proximity effects, and with a mask to wafer spacing of a few millimeters, the necessary magnetic field is ≂0.46 T.
Solid source molecular-beam epitaxial growth of Ga0.5In0.5P using a valved, three-zone phosphorus source
Authors: Hoke W. E. , Weir D. G. , Lemonias P. J. , Hendriks H. T. , Jackson G. S. , Colombo P.
Published: July 2002

Ga0.5In0.5P/GaAs heterojunction films were grown using a valved, three-zone phosphorus source and valved arsenic source. The design of the phosphorus source eliminates the flux bursts experienced upon valve opening with two-zone furnaces. Narrow x-ray linewidths with Pendellosung interference oscillations were observed in double crystal measurements. Chemically abrupt As/P and P/As interfaces were obtained using both valved sources. The optical band gap determined from photoluminescence was consistent with minimal ordering in the films. GaInP films were doped with silicon in the 1017 cm−3 range with good mobilities and negligible carrier freeze-out upon cooling. A GaInP(emitter)/GaAs(5×1019 cm−3 carbon doped base) heterojunction bipolar transistor exhibited a current gain of 30.
Low-temperature in situ cleaning of silicon (100) surface by electron cyclotron resonance hydrogen plasma
Authors: Tae Heung-Sik , Park Sang-June , Hwang Seok-Hee , Hwang Ki-Hyun , Yoon Euijoon , Whang Ki-Woong , Song Se Ahn
Published: July 2002

Low-temperature, defect-free, in situ cleaning of silicon prior to homoepitaxy is successfully developed by an electron cyclotron resonance hydrogen plasma treatment in an ultrahigh vacuum chamber. The plasma potential distribution was measured by a Langmuir probe method to understand the effect of the substrate dc bias during hydrogen plasma cleaning. It changes from downhill to uphill distribution as the dc bias changes from a negative to a positive value, which leads to a decrease in the ion number density arriving at the substrate and results in the complete suppression of the defect formation in the Si substrate. In situ hydrogen plasma cleaned Si wafer always resulted in higher quality epilayers than ones cleaned only by so-called hydrogen passivation after the HF dip. We found that there is a critical dose of the hydrogen ions during in situ plasma cleaning beyond which crystalline defects are observed in the Si substrate, subsequently leading to the poor crystallinity of the epilayers. The dose of the hydrogen ions during plasma cleaning can be effectively controlled by the substrate dc bias, the microwave power, the magnet current, and the cleaning time.
Properties of ultrathin films of porous silicon
Authors: von Behren J. , Üçer K. B. , Tsybeskov L. , Vandyshev Ju. V. , Fauchet P. M.
Published: July 2002

We have produced films of light emitting porous silicon (LEPSi) thinner than 1 μm, lifted them off the silicon wafer by an electropolishing step, and deposited them onto sapphire windows where they remain attached by van der Waals or electrostatic forces. Although free-standing LEPSi films had been obtained before, our films are one order of magnitude thinner, luminesce strongly, and have excellent mechanical properties because of the sapphire substrate. The important steps in this procedure are discussed, and the structural, chemical, and optical properties of these films as measured using a variety of probes are reported. These films are semitransparent in the visible and thus make several new optical measurements possible. In particular, the results of photoinduced absorption measurements performed with 100 fs time resolution are presented.
Atomic manipulation for patterning ultrathin films
Authors: Salling C. T. , Kravchenko I. I. , Lagally M. G.
Published: July 2002

Atomic manipulation with the scanning tunneling microscope offers a method for patterning with nanometer-scale resolution in the ultraclean environment available in ultrahigh vacuum. We have used atomic manipulation to write nanoscale trenches in Si(001) surfaces and thus define structures that have atomically straight edges. These experiments suggest that silicon films can be directly patterned with atomic resolution for films that are one atomic layer thick and with 2.4 nm resolution for films that are three atomic layers thick. We report on the growth and patterning of single atomic layers of electronic materials on Si(001).
Nanobeam process system: An ultrahigh vacuum electron beam lithography system with 3 nm probe size
Authors: Hiroshima H. , Okayama S. , Ogura M. , Komuro M. , Nakazawa H. , Nakagawa Y. , Ohi K. , Tanaka K.
Published: July 2002

We have constructed a ‘‘nanobeam process system’’ which is applicable to high resolution electron beam lithography using inorganic resists and is also compatible with electron beam induced surface reaction. It is a 50 kV electron beam lithography system with a gas introducible ultrahigh vacuum sample chamber using a double chamber stage system which isolates stage mechanisms from the sample chamber. The probe size measured with a knife edge method was 2.8 nm, where the probe current was 127 pA. The base pressure of the sample chamber was 3.5×10−7 Pa after baking. The pressure of the gun chamber did not vary at all and the pressure rise of the mechanism chamber was 3×10−6 Pa when the pressure of the sample chamber increased to 1×10−3 Pa during N2 gas introduction. Standard deviations of stitching and overlay accuracy were 14 and 18 nm, respectively. Line patterns with a width of about 5 nm and a pitch of 15 nm were delineated in SiO2 when used as a high resolution resist.
Low-temperature magnetic resonance force detection
Authors: Wago K. , Züger O. , Kendrick R. , Yannoni C. S. , Rugar D.
Published: July 2002

A low-temperature magnetic resonance force microscope (MRFM) has been built and a force sensitivity of 8×10−17 N/√Hz has been achieved using a single-crystal silicon cantilever with a Q of 200 000 at 6 K. Nuclear magnetic resonance (NMR) of 19F was measured via force detection in a 1% Nd-doped CaF2 sample mounted on the cantilever. Cyclic adiabatic inversion of the 19F nuclear spins was performed using a frequency modulated radio frequency field at 100 MHz, resulting in an oscillatory magnetic force that excited the cantilever vibration. NMR force signals were measured in the temperature range between 12 and 40 K. The MRFM technique was also used to measure some basic NMR properties such as spin-lattice relaxation time.
Effects of surface cleaning on electrical properties for Ni contacts to p-type ZnSe
Authors: Ishikawa H. , Tsukui K. , Koide Y. , Teraguchi N. , Tomomura Y. , Suzuki A. , Murakami Masanori
Published: July 2002

As a first step to achieve our goal to prepare Ohmic contacts to p-type ZnSe using a conventional deposition and annealing method, the microstructure and chemical composition of the ZnSe surface were analyzed by x-ray photoelectron spectroscopy, reflection high-energy electron diffraction, and reflection electron microscopy. A thin native oxide layer consisting of ZnO and SeOx and adsorbed carbons were observed on the ZnSe surface which was grown by the molecular beam epitaxy technique. Most of the oxide layer was removed by saturated bromine water solution. A strong correlation was observed between the surface composition and the electrical properties measured by a current-voltage method for Ni contacting to the N-doped p-type ZnSe substrates. The turn-on voltage (which corresponds to a breakdown voltage for back-to-back Schottky contacts) was significantly reduced by removing the oxide layer using saturated bromine water solution. The present experiment suggested that the interfacial oxide layer is an effective barrier for the carrier transport through the ZnSe/metal interface.
A triangle-shaped nanoscale metal–oxide–semiconductor device
Authors: Gondermann J. , Röwer Th. , Hadam B. , Köster Th. , Stein J. , Spangenberg B. , Roskos H. , Kurz H.
Published: July 2002

A new fabrication process for ultrasmall high-current-density metal–oxide–semiconductor transistors is proposed. The device fabrication is based on anisotropic wet etching of silicon using bonded and etched back silicon-on-insulator (BESOI) material and a self-adjusting polysilicon gate technology. The special feature in the design of the transistor is a small silicon wire with a triangular instead of a planar cross section connecting the source and drain areas. Applying a moderate voltage to the poly-gate, a high electric field is built up in the top of the triangle which increases the volume of the conducting channel below the gate. For normal operating conditions, the three-dimensional current flow allows smaller lateral dimensions of the device that offers higher integration. Because of the BESOI material, faster device performance is expected. The physical principle is discussed and the basic fabrication techniques are described. First electrical investigations of the transistors operating at room temperature are presented providing evidence that the current density is considerably higher than in planar transistors.