bluuebunny/crossref_metadata_2025 huggingface dataset:
Ge growth on Si(001) studied by x-ray photoelectron spectroscopy peak shape analysis and atomic force microscopy
Authors: Schleberger M. , Cohen Simonsen A. , Tougaard S. , Hansen J. L. , Larsen A. Nylandsted
Published: July 2002

We investigated the nanostructure of Ge deposited on Si(001) at T=560 °C with x-ray photoelectron spectroscopy using peak shape analysis and with atomic force microscopy. Analyzing the Ge 2p as well as the Si KLL spectra, we find strong island growth. The Ge deposited is equal to a 14-monolayer-thick film. In agreement with the x-ray photoelectron spectroscopy results, the atomic force microscope pictures show strong island formation. About 50% of the Si surface is covered by Ge islands. The islands are distributed regularly and have an average height of 50 Å. We find some variation in height ranging from ≃20 to ≃80 Å.
Monte Carlo analysis of a hyperthermal silicon deposition process
Authors: Chen G. , Boyd I. D. , Roadman S. E. , Engstrom J. R.
Published: July 2002

Deposition of epitaxial silicon thin films through the seeding of silicon hydride molecules in a supersonic beam of light carrier gas is modeled using the direct simulation Monte Carlo technique. In this process, a hyperthermal collimated beam is formed by rapid expansion through a nozzle orifice and then refined through a skimmer. The fundamental characteristics of the process are evaluated quantitatively through a gas dynamics approach. General features of the internal supersonic flows are described. Detailed information is provided by the simulations on the beam properties, such as beam intensity, incident kinetic energy and angle as the precursor molecules impact the substrate surface. The thin film growth rates are quantified and film uniformity is discussed. Good agreement is achieved on the comparisons with measurements of the film growth rate and the quadrupole mass spectrometric measurements of the beam intensity. A wide range of geometric and physical parameter space is examined numerically. The effects of the skimmer interference, supersonic source temperature and size, and pumping conditions are also addressed with emphasis on the film deposition rate and uniformity.
Photoreflectance spectroscopy investigation of two-dimensional cesium metallic clusters on GaAs(100)
Authors: Paget D. , Kierren B. , Houdré R.
Published: July 2002

We have monitored, using photoreflectance spectroscopy, the change of photovoltage and of surface barrier induced by cesium adsorption at 90 K on GaAs(100). Over a total coverage range of only 0.1 monolayer (ML), near 0.6 ML, the photovoltage first increases abruptly by a factor of 5, and subsequently decreases by an even larger factor. The increase is due to the sudden change of surface density of states caused by the formation of cesium bidimensional clusters of size larger than the limit for metallicity. For further cesium exposure, the photovoltage decreases because, due to the increased cluster size, a significant fraction of these clusters overlap randomly-situated recombination centers, which efficiently discharge them through metallic transport in the cluster. Since at this temperature cesium does not diffuse on the surface, these clusters are disordered. Further analysis shows that the cesium metallic clusters are metastable and are irreversibly destroyed by annealing at room temperature. The disruption temperature is found to depend on cluster size, larger clusters being disrupted at a lower temperature. Based on simulations, we propose that the abrupt change of cesium sticking coefficient, which also occurs at a similar cesium coverage as the one of the photovoltage increase, reflects the change of the average coordinence of surface cesium.
Chemical alteration of the native oxide layer on InP(111) by exposure to hyperthermal atomic hydrogen
Authors: Wolan John T. , Hoflund Gar B.
Published: July 2002

A surface characterization study using x-ray photoelectron spectroscopy (XPS) and ion scattering spectroscopy (ISS) has been performed on polished, solvent-cleaned, undoped InP(111) substrates before and after room-temperature exposure to the flux produced by a novel atomic hydrogen source based on electron-stimulated desorption of hyperthermal (1–3 eV) hydrogen atoms from a Cu-alloy membrane. The native oxide layer on the solvent-cleaned InP(111) substrate is nonhomogeneous and contains primarily C, O, and In, and very little P. Indium is present in the near-surface region as InPO4, In(PO3)3, InPO3, InP, and a relatively small amount of In2O3 in the subsurface region. Phosphorus is present as InPO4, In(PO3)3, InPO3, InP, P2O5, and elemental P. For In and P, InP is the predominant form. Before H-atom exposure the C is present as hydrocarbons, alcohols, and carbide with hydrocarbons as the predominant chemical state. During room-temperature exposure to the hyperthermal H-atom flux for 90 min, removal of oxygen and carbon contamination occurs with the O content decreasing by about 28% and the C content decreasing by 93% according to the XPS data. The In:P ratio is initially 2.28 indicative of an In-rich near-surface region. After a 15 min and then 90 min H-atom exposure, the In:P ratio decreases to 1.30 and 1.11, respectively. Complex chemical changes occur during the H-atom exposures. The P2O5 is eliminated, the In phosphates are converted to biphosphates and hydroxide and all forms of C are removed. Some of the phosphates and In2O3 are decomposed by the H-atom flux, but these chemical reactions occur slowly at room temperature. According to ISS data, the H-atom flux is very effective in removing contamination at the outermost atomic layer which is crucial for epitaxial growth of device-quality thin films.
Evaluation of the ion bombardment energy for growing diamondlike carbon films in an electron cyclotron resonance plasma enhanced chemical vapor deposition
Authors: Kang Dae-Hwan , Ha Seung-Chul , Kim Ki-Bum , Min Seok-Hong
Published: July 2002

The amorphous hydrogenated carbon (a-C:H) films were deposited on (100) silicon substrates using a divergent Ar+CH4 electron cyclotron resonance plasma. During the deposition sequence, the rf bias was applied to the substrate to increase the ion bombardment energy. The results of the microhardness test and the Raman spectroscopy revealed that these films showed abrupt changes in the mechanical properties and in the bonding characteristics from polymerlike to diamondlike when the rf substrate bias (Vsub) was increased to −40 V. Such changes were attributed to the structural and compositional modifications of the growing film by the bombarding ions. In addition, the results of the plasma diagnostics and calculation showed that the required ion energy for growing a hard diamondlike carbon film was about 65 eV or higher. The bombardment by high energetic ions caused the a-C:H film to have diamondlike properties with a strong three-dimensional carbon network by dislodging hydrogen atoms from CH3 radicals on the growing surface, as was identified by Fourier transform infrared spectroscopy.
Etching properties of Pt thin films by inductively coupled plasma
Authors: Kwon Kwang-Ho , Kim Chang-Il , Yun Sun Jin , Yeom Geun-Young
Published: July 2002

The inductively coupled plasma etching of platinum with Ar/Cl2 gas chemistries is described. X-ray photoelectron spectroscopy (XPS) is used to investigate the chemical binding states of the etched surface with various Ar/(Ar+Cl2) mixing ratios. Atomic percentage of Cl element increases with increasing Ar/(Ar+Cl2) mixing ratio with the exception of Ar/(Ar+Cl2) mixing ratio of 1. At the same time, the peaks that seem to be subchlorinated Pt at XPS narrow scan spectra are found and Cl–Pt bonds rapidly increase at Ar/(Ar+Cl2) mixing ratio of 0.62. Quadrupole mass spectrometry (QMS) is used to examine the variations of plasma characteristics with various Ar/Cl2 gas chemistries. QMS results show that Cl2 molecules are converted to Cl radicals with adding Ar gas to Cl2 plasma. QMS results support the increased atomic percentages of Cl elements on the etched Pt surface. Single Langmuir probe measures ion current density with various Ar/Cl2 gas plasma. Ion current densities are used to investigate the ion bombardment effects on the etched surface. Thin film thickness measuring system, scanning electron microscope and a four-point probe are used to extract the Pt etching characteristics. The maximum etch rate of Pt is approximately 140 nm/min at the Ar/(Ar+Cl2) mixing ratio of 0.9. These results are consistent with XPS, QMS, and Langmuir probe data.
Positive ion species in high-density discharges containing chlorine and boron–trichloride
Authors: Woodworth J. R. , Nichols C. A. , Hamilton T. W.
Published: July 2002

We have used a quadrupole mass spectrometer to measure the positive ion species striking the wafer in a high-density plasma processing discharge containing chlorine and boron–trichloride. Since the relative transmission of our mass spectrometer versus ion mass is known, we are able to report the relative concentrations of the ion species striking the wafer. Our measurements were made in an inductively coupled Gaseous Electronics Conference Reference Cell operating at pressures between 15 and 50 mTorr and at electron densities up to 1011/cm3. The ion spectrum in these discharges is normally dominated by Cl+, Cl2+, BCl2+, and etch products. However, reactor wall or wafer surface conditions can strongly affect the ion species in the discharge. When a stainless steel “wafer” was replaced with a bare Si wafer, the dominant chlorine ion changed from Cl+ to Cl2+. A bare aluminum wafer strongly quenched both Cl+ and Cl2+ signals.
Continuity and topography of ultrathin diamond-like carbon films characterized by scanning electron microscopy/energy dispersive x-ray analysis and atomic force microscopy
Authors: Lemoine P. , Mc Laughlin J. M.
Published: July 2002

Diamond-like-carbon coatings used as wear and corrosion barriers for hard disk drives are very thin and extremely smooth layers. Good morphological characterization of these ultrathin films is crucial to the good performance of the drive and has been attempted by scanning electron microscopy, energy dispersive x-ray (EDX) analysis and contact mode atomic force microscopy (AFM). Pure secondary electron images were obtained by subtracting out the backscattered component of the detected signal. EDX analysis was used for estimating the electron stopping power and the thickness of the coatings. Run-on spot analysis mode, EDX also permitted to check the continuity of these thin films. Contact mode AFM microscopy was carried out for a range of force set points. The contrast was found independent of force set point. Continuum mechanics calculations and preliminary adhesion measurements indicate that the contrast observed is purely topographic (Ra=1 nm). Finally, comparing the results, we found that the three different techniques complement each other to yield a more complete morphological characterization of these ultrathin films.
Fundamental issues in wafer bonding
Authors: Gösele U. , Bluhm Y. , Kästner G. , Kopperschmidt P. , Kräuter G. , Scholz R. , Schumacher A. , Senz St. , Tong Q.-Y. , Huang L.-J. , Chao Y.-L. , Lee T. H.
Published: July 2002

Semiconductor wafer bonding has increasingly become a technology of choice for materials integration in microelectronics, optoelectronics, and microelectromechanical systems. The present overview concentrates on some basic issues associated with wafer bonding such as the reactions at the bonding interface during hydrophobic and hydrophilic wafer bonding, as well as during ultrahigh vacuum bonding. Mechanisms of hydrogen-implantation induced layer splitting (“smart-cut” and “smarter-cut” approaches) are also considered. Finally, recent developments in the area of so-called “compliant universal substrates” based on twist wafer bonding are discussed.
Ultrathin silicon oxide film on Si(100) fabricated by highly concentrated ozone at atmospheric pressure
Authors: Nakamura K. , Ichimura S. , Kurokawa A. , Koike K. , Inoue G. , Fukuda T.
Published: July 2002

We have investigated ultrathin silicon oxide film growth by highly concentrated ozone at atmospheric pressure. Oxide film >2 nm was grown on as-received Si(100) even at room temperature. The etching rate by dilute hydrofluoric acid solution of oxide fabricated on Si(100) at 350 °C by this method was almost the same as that of thermally grown oxide so that film density is equivalent to that of thermally grown oxide. The etching rate of this film also shows no dependence on the film depth. This is indicating that the transition layer due to the lattice mismatch of substrate and oxide is limited within a thinner region than that of thermally grown oxide. It also indicates that an oxide film with higher film density can be synthesized on the surface with preoxide film already formed to protect bare substrate surfaces.
Studies of ion bombardment in high density plasmas containing CF4
Authors: Olthoff J. K. , Wang Yicheng
Published: July 2002

We report ion energy distributions, relative ion intensities, and absolute total ion current densities at the grounded electrode of an inductively coupled Gaseous Electronics Conference radio-frequency reference cell for discharges generated in pure CF4, and in CF4:Ar and CF4:O2:Ar mixtures. Abundant ionic species, including secondary ions such as CO+ and COF+, were observed and their implications are discussed.
Conversion efficiency of graphite atomic-scale defects to etched pits in thermal oxidation reaction
Authors: Hahn J. R. , Kang H.
Published: July 2002

Thermal oxidation of atomic defects on a graphite surface, produced by low energy (50–200 eV) impact of Ar+ and Kr+ ions, leads to the formation of pits with a diameter of several nm. These pits are generated from both carbon vacancy defects and interstitial defects, the latter formed by trapping of incident ions in-between graphite layers. The probability of the pit formation is measured for each type of defect. Nearly all vacancy defects (>90%) are developed into pits by thermal oxidation in air at 560 °C, while only a small portion (<15%) of the interstitial defects are converted into pits.
Novel method for absolute quantification of the flux and angular distribution of a radical source for atomic hydrogen
Authors: Schwarz-Selinger T. , von Keudell A. , Jacob W.
Published: July 2002

In this article we present a novel method for absolute quantification of the flux and angular distribution of a beam source for atomic hydrogen or atomic deuterium. It is based on quantitative determination of the erosion of an amorphous, hydrogenated carbon (a-C:H) film. A heated tungsten capillary serves as the radical source. Atomic hydrogen is produced in this capillary by thermal dissociation of hydrogen molecules. A large-area a-C:H film is exposed to the source at a substrate temperature of 650 K. Interaction of atomic hydrogen with the a-C:H film causes erosion. From the spatial variation of the erosion rate one can deduce the angular distribution of the impinging hydrogen atoms. This angular distribution was also measured by mass spectrometry and showed excellent agreement with the erosion profile. The absolute flux of atomic hydrogen was also determined by mass spectrometry. With the absolute flux of atomic hydrogen known from mass spectrometry, measurement of the lateral variation of the erosion rate can be directly used as a probe for absolute quantification of the angular distribution of the impinging H (D) flux. The erosion yield is (2±0.7)×10−2, which is consistent with the microscopic erosion mechanisms of a-C:H by atomic hydrogen known from the literature.
High breakdown voltage Au/Pt/GaN Schottky diodes
Authors: Dang G. T. , Zhang A. P. , Mshewa M. M. , Ren F. , Chyi J.-I. , Lee C.-M. , Chuo C. C. , Chi G. C. , Han J. , Chu S. N. G. , Wilson R. G. , Cao X. A. , Pearton S. J.
Published: July 2002

Au/Pt/GaN Schottky diode rectifiers were fabricated with reverse breakdown voltage (VRB) up to 550 V on vertically depleting structures and >2000 V on lateral devices. The figure-of-merit (VRB)2/RON, where RON is the on-state resistance, had values between 4.2 and 4.8 MW cm−2. The reverse leakage currents and forward on-voltages were still somewhat higher than the theoretical minimum values, but were comparable to SiC Schottky rectifiers reported in the literature. These devices show promise for use in ultrahigh-power switches.
Thin film fabrication for the Josephson technology cross-sectional model
Authors: Bright A. A. , Greiner J. H. , Klepner S. P. , Wang R. H. , Warnecke A. J.
Published: July 2002

The cross-sectional model (CSM) is an experiment designed to test the electrical and technological issues that are representative of a proposed Josephson Signal Processor. This paper describes the thin films aspects of the CSM. The existing Pb-alloy technology for logic and memory circuits was developed further (for both chips and package parts) to meet the additional demands for the CSM. This included more extensive use of regulator junctions for on-chip power distribution, more precise target value objectives for current levels on chip, and extension of the thin film technology to new package part structures, including additional thermal and mechanical processing. The vacuum depositions and materials, photolithography, package parts diagnostics, and chip diagnostic aspects of the CSM are described.
Use of molecular beam epitaxy in research and development of selected high speed compound semiconductor devices
Authors: Eastman Lester F.
Published: July 2002

Molecular beam epitaxy can fill most of the requirements for high speed III–V compound semiconductor devices. Precise compositon and doping profiles, such as those in selectivity doped heterojunction transistors (SDHT) are examples. The inverted SDHT with the Al,GaAs under the GaAs is a future requirement. High quality, Al,GaAs buffer layers are a general requirement. Conduction band engineering, away from the active layer needs inventive research. The role of the superlattice or other means of improving the buffer needs effort. InP or Al,InAs buffer or confinement layers also need effort, along with improved GaInAs. The requirements of short, ballistic electron FETs, ballistic heterojunction GaAs bipolar transistors, and ballistic injecting and drift transistors will also be reviewed. High current density, exceeding 105 A/cm2, for very high frequency and speed, requires new levels of MBE performance for Ohmic contacts also. The relative merits of GaInAs and GaAs as active materials for high speed devices will also be covered, including applications to high speed photodetectors.
Enhanced optical nonlinearities in superlattices
Authors: Bloss W. L. , Friedman L.
Published: July 2002

The third order nonlinear susceptibility χ(3) due to band nonparabolicity is investigated for gallium arsenide–aluminum arsenide superlattices. Optimization of well width, superlattice period, and barrier height give rise to large nonparabolicities which yield nonlinear susceptibilities comparable to those of bulk indium antimonide and two orders of magnitude larger than those of bulk gallium arsenide.
Electronic and atomic structure of Ag–Si(111) and Ag–Ge(111)
Authors: Le Lay G.
Published: July 2002

The experimental and theoretical studies on the formation of the Ag/Si(111) and Ag/Ge(111) interfaces are reviewed in detail from the point of view of structural aspects and electronic properties. The discussion is focused on the local atomic arrangement and the abruptness of the junction.
Fermi-level pinning at heterojunctions
Authors: Allen Roland E. , Beres Richard P. , Dow John D.
Published: July 2002

We have extended our calculations for defect levels at semiconductor free surfaces to ideal semiconductor heterojunctions. We find that the Fermi-energy pinning observed for deposition of Ge and Si (as well as metals and oxygen) on GaAs(110) surfaces is explained very satisfactorily by free-surface antisite defect levels, but cannot be explained, even qualitatively, by antisite defect levels for bulk GaAs or for ideal GaAs/Ge or GaAs/Si interfaces. We conclude that when pinning occurs at heterojunctions and at other semiconductor/overlayer interfaces, the pinning defects are somehow ‘‘sheltered’’ by irregularities in the atomic configurations at the interface.
Electrical measurements of the conduction band discontinuity of the abrupt Ge–GaAs 〈100〉 heterojunction
Authors: Ballingall J. M. , Wood C. E. C. , Eastman L. F.
Published: July 2002

Current–voltage (I–V) measurements are used to measure the conduction band discontinuity of the n–n〈100〉 Ge–GaAs heterojunction grown by molecular beam epitaxy. All of the heterojunctions were grown at 300 °C to ensure abrupt interfaces. The free electron concentrations were varied on both sides of the heterojunctions by controlled doping of the Ge and GaAs epitaxial films. The current–voltage characteristics vary systematically with doping, temperature, and postgrowth heat treatments. The electrical barrier at the heterojunction is measured to be 80±10 meV. This value is considerably smaller than values obtained with vapor-phase-grown 〈100〉 n–n heterojunctions. The 80 meV value is in excellent agreement with theoretical predictions for the conduction band discontinuity at the abrupt, unreconstructed 〈100〉 interface. Postgrowth heat treatments show a decrease in the electrical barrier which is consistent with an interdiffusion model for the atomic species at the interface and contrary to previous anomalous results. The differences between the electrical barriers obtained with molecular beam and vapor phase epitaxial heterojunctions are apparently due to differences in substrate preparation procedures and results as well as the inherent differences in the two growth processes.
Study of the stepwise oxidation and nitridation of Si(111): Electron stimulated desorption, Auger spectroscopy, and electron loss spectroscopy
Authors: Knotek M. L. , Houston J. E.
Published: July 2002

Electron stimulated desorption, Auger line shape analysis, and electron loss spectroscopy measurements are reported for the electron activated stepwise oxidation and nitridation of the Si(111) surface. In ESD it is found that appreciable levels of surface hydrogen can be present which can lead to hydroxyl formation upon oxidation. The hydroxyl rich films are unstable in an electron beam, while surfaces oxidized with activated oxygen, where no OH is formed, are much more stable. The nitrided films are always stable in the electron beam even though there too hydrogen is always found. On the OH-free oxide, ESD shows two chemically distinct O species, one thought to be SiO2 and the other adsorbed O2 or a chemical intermediate. The Si(L23VV) Auger spectra for both the oxide and nitride are treated by background subtraction, integration, deconvolution, and subtraction of the elemental part of the spectrum, as a function of reaction time over a well controlled series of reaction steps. The Auger spectra for both oxide and nitride films suggest that in the earliest stages of reaction, the reacted film is made up of low coordination intermediates which gradually evolve to the stoichiometric compound as the coordination increases. In loss spectroscopy, both the Si(L23) core loss and the near elastic loss were measured. The L23 core loss shows the same gradual evolution to the oxide seen in the Auger results, with an intermediate oxidation state dominating in the early stages of reaction. The near elastic loss spectra, by contrast, quickly saturate in the early stages of reaction to the final oxide spectrum which is characterized by features both of the full oxide and a suboxide. Similar results are found for the nitride.
Structural and optical properties of GaAs–AlxGa1−xAs quantum wells
Authors: Woodbridge K. , Dawson P. , Gowers J. P. , Foxon C. T.
Published: July 2002

We report structural and optical data on single and multiple GaAs–AlxGa1−x As quantum well structures. The layers were grown in a fully automated system incorporating a high speed rotation stage of our own design. All layers were grown in the substrate temperature range 680–700 °C using an As2 source. Growth rates were normally about 1 μm/h for GaAs. The substrate rotation stage is capable of operation at up to 200 rpm compared to commercial equipment operating limits of about 10 rpm. This enables rotation of the substrate once during the deposition of each monolayer, thus avoiding the modulation of alloy composition found at lower rotation speeds. We present data showing the excellent layer uniformity obtained with this device. We have carried out TEM and photoluminescence studies on single quantum well structures grown with and without a prelayer of AlAs. The well with the prelayer shows a peak assigned to the n=1(e−hh) free exciton and a considerable decrease in impurity related luminescence at 6 K. We relate these results to improvements in structural properties seen in TEM. Multiple quantum well structures have been grown showing strong photoluminescence up to 300 K. Peaks assigned to the n=1(e−hh) and n=1(e−lh) transitions have been observed at room temperature and the possible excitonic nature of these peaks is discussed.
High resolution electron energy loss studies of Fermi level states of clean and metallized Si(111) surfaces
Authors: Demuth J. E. , Persson B. N. J.
Published: July 2002

High resolution electron energy loss spectroscopy has been performed as a function of temperature (15–300 K) to determine the electronic transitions of clean and metallized Si(111) surfaces. A quantitative analysis of these EELS results is used to delineate and identify the localized and delocalized states at the Fermi level. Si(111)-7×7 is found to have a 100 meV surface state band gap in which lies a narrow, half occupied state that determines the Fermi level. Hydrogen titration studies suggest densities of these states at ∼1.6×1013/cm2. In contrast, metal impurity stabilized Si(111)-1×1 surfaces are not found to have these narrow states at EF. Au overlayers on Si(111)-1×1 produce a metallic overlayer for coverages above ∼1.6×1015 Au/cm2. Pd reaction with Si(111)-1×1 forms a semimetal or semiconducting compound at coverages ≲1×1015 atoms/cm2 and appears to generate narrow partially occupied Si states at the interface. At higher Pd coverages a metallic silicide forms which uniformly covers the surface.
Theory of surface-defect states and Schottky barrier heights: Application to InAs
Authors: Allen Roland E. , Humphreys Terry J. , Dow John D. , Sankey Otto F.
Published: July 2002

Theoretical predictions of electronic energy levels associated with s- and p-bonded substitutional point defects at (110) surfaces of InAs and other III–V semiconductors are presented and discussed. The specific defects considered for InAs are: anion and cation vacancies, the (native) antisite defects InAs and AsIn, and 26 impurities. The predicted surface-defect deep levels are used to interpret Schottky barrier height data for (a) n- and p-(InAs) and (b) the alloys AlxGa1−xAs, GaAs1−xPx, In1−xGaxP, and In1−xGaxAs. The rather complicated dependence of the Schottky barrier height φB on alloy composition x provides a nontrivial test of the theory (and competing theories). The following unified microscopic picture emerges from these and previous calculations: (1) For most III–V and group IV semiconductors, Fermi-level pinning by native defects can explain the observed Schottky barrier heights. (2) For GaAs, InP, and other III–V semiconductors interfaced with nonreactive metals, the Fermi-level pinning is normally due to antisite defects. (3) When InP is interfaced with a reactive metal, surface P vacancies are created which pin the Fermi level. (4) Impurities and defect complexes are sometimes implicated. (5) At Si/transition-metal-silicide interfaces, Si dangling bonds pin the Fermi level. (6) These defects at the semiconductor/metal interfaces are often ‘‘sheltered’’ or ‘‘encapsulated.’’ That is, the states responsible for Fermi-level pinning are frequently ‘‘dangling-bond’’ states that dangle into a neighboring vacancy, void, or disordered region. The defects are partially surrounded by atoms that are out of resonance with the semiconductor host, causing the defect levels to be deep-level pinned and to have energies that are almost independent of the metal.
Study of symmetry and disordering of Si(111)-7×7 surfaces by optical second harmonic generation
Authors: Heinz T. F. , Loy M. M. T. , Thompson W. A.
Published: July 2002

A detailed study of the polarization dependences of the second-harmonic generation process from clean Si(111)-7×7 surfaces is presented and analyzed in terms of the symmetry properties of the surface. Results from real-time, in situ measurements on disordering of the Si(111)-7×7 surface by oxidation and Ar+ ion bombardment are also reported.
A microscopic model of metal–semiconductor contacts
Authors: Duke C. B. , Mailhiot C.
Published: July 2002

A one-dimensional local-density model of the charge density, electrostatic potential, and energetics of metal–semiconductor contacts is constructed. This model is an extension of analogous models of bimetallic junctions to include the gap in the semiconductor excitation spectrum, space charge effects in the semiconductor, and fabrication-induced charges near the interface. Self-consistent analysis of the valence-electron charge redistribution at the model metal–semiconductor interface relative to the corresponding vacuum surfaces reveals a cancellation which makes the height of the rectifying Schottky barrier directly proportional to the observed vacuum work function of the metal in the absence of additional charges induced by atomic relaxations or chemical reactions which occur during the fabrication of the interface. The experimentally observed stability of the Schottky barrier heights against changes is applied bias and semiconductor doping are predicted correctly. The occurrence of interfacial atomic rearrangements and/or chemical reactions is incorporated into the model via the inclusion of charge centers near the interface. Four types of centers are considered: donors, acceptors, and three-state centers with both positive (U>0) and negative (U<0) effective electron–electron interactions. Applications of the model to describe measured barrier heights on GaSe(0001) and GaAs(110) require approximately 1014 negative-U centers per cm2 except in the case of noble metals and Sn on GaSe(0001).
Enhanced sticking coefficients and improved profile control using boron and antimony as coevaporated dopants in Si–MBE
Authors: Kubiak R. A. A. , Leong W. Y. , Parker E. H. C.
Published: July 2002

Inefficient doping and poor profile control are generally associated with the use of coevaporated dopants in Si–MBE. However, two new techniques have been developed, one applicable to p-type and the other to n-type doping, which alleviate these problems. Coevaporated boron ( p-type) is shown to have a simple incorporation mechanism by which good doping and profile control can be achieved using the conventional MBE technique of changing the source cell temperature. For n-type doping, a method of enhancing the low incorporation efficiency of Sb has been found (potential enhanced doping) which is also ideally suited to profile control. Enhancements by a factor of up to 1000 times have been achieved. Material with bulklike mobilities has been obtained over the range of 1×1015 to 8×1019 cm−3 for p-type and up to 3×1019 cm−3 for n-type doping. Extremely sharp doping transitions down to <100 Å/decade have been obtained.
Effect of As/Ga flux ratio on the photoluminescence spectra of low donor concentration MBE GaAs
Authors: Salerno Jack P. , Koteles E. S. , Gormley J. V. , Sowell B. J. , Brody E. M. , Chi J. Y. , Holmstrom R. P.
Published: July 2002

Nominally undoped p-type GaAs with an unusually low shallow donor concentration has been grown by molecular beam epitaxy. The effect of the As/Ga flux ratio on the residual shallow donor and acceptor concentrations and defect density in this material was investigated by analyzing low temperature photoluminescence (PL) spectra. The PL spectra are indicative of low shallow donor concentrations, since they exhibit a single, narrow (FWHM=0.4 meV) free exciton peak and essentially no donor-related peaks. For increasing As/Ga flux ratio the donor concentration decreases, the defect concentration increases, and the acceptor concentration goes through a minimum. These results can be explained by a model based on the incorporation of Si and C into As and Ga sites and the formation of a carbon–Ga vacancy defect complex.
Photoluminescence studies of silicon molecular beam epitaxy layers
Authors: Robbins D. J. , Kubiak R. A. A. , Parker E. H. C.
Published: July 2002

Photoluminescence spectra are used to investigate inadvertent impurities in Si molecular beam epitaxy (MBE) layers. The spectra show free exciton and condensed exciton (electron-hole drop) features which are indicative of good crystalline quality. Electrically active shallow impurities are identified from near-gap bound exciton features. In one layer radiative recombination at dislocations is dominant. For the first time using an optical technique carbon is shown to be a persistent impurity in Si MBE material. This technique provides a test for inadvertent carbon incorporation at levels which may otherwise be difficult to detect in thin epitaxial layers.
An organosilicon novalac resin for multilevel resist applications
Authors: Wilkins C. W. , Reichmanis E. , Wolf T. M. , Smith B. C.
Published: July 2002

We have studied a low etch rate organosilicon novalac resin which can be used as the base resin component of thin-imaging resists in multilevel lithography schemes. The resin, a terpolymer of phenol, trimethylsilylphenol, and formaldehyde resists oxygen RIE and can be used in conventional resist formulations which employ o-quinonediazide solubility inhibitors. We report here the synthesis and RIE properties of resists based upon these resins. Multilevel techniques use a thick organic polymer film to form a relatively flat surface. A thin imaging resist to this film can be exposed and developed with minimal surface reflectivity and topography effects. The resin we have developed allows oxygen RIE to be used as the imaging transferring step.
Radiolysis and resolution limits of inorganic halide resists
Authors: Muray A. , Scheinfein M. , Isaacson M. , Adesida I.
Published: July 2002

Patterning at the 1–2 nm size scale has been demonstrated with self-developing metal halide resists using a subnanometer diameter 100 keV electron beam. Electron energy loss spectroscopy during lithographic exposure indicates removal of the halide ion first, followed by displacement of the metal ions. Under appropriate exposure of AlF3, we have demonstrated that aluminum metal structures can be fabricated in situ. Nanometer scale patterns have been replicated into Si3N4 via reactive ion etching using AlF3 as the resist mask.
Growth of semiconductor/insulator structures: GaAs/fluoride/GaAs (001)
Authors: Sullivan P. W. , Bower J. E. , Metze G. M.
Published: July 2002

Thin single-crystal layers (∼100–2000 Å) of (001) oriented SrF2 and CaF2 have been grown onto clean MBE-grown GaAs (001) surfaces. RED patterns suggest a significant difference in the interaction of CaF2 and SrF2 with clean reconstructed GaAs (001). Three-dimensional behavior is suggested in the initial nucleation stages of CaF2 growth, although significant ordering occurs for film thickness >100 Å. Nucleation of SrF2 on c(2×8) reconstructed GaAs surfaces results in a well-ordered reconstructed (4×2) surface up to film thicknesses of ∼100 Å, after which a disordered (1×1) unreconstructed surface is apparent. GaAs has been grown onto MBE prepared fluoride surfaces. At sufficiently low growth rates on CaF2, RED patterns indicate well ordered surfaces with (4×4) reconstruction. Increased deposition rates lead to twinned GaAs growth. Photoluminescence studies of undoped GaAs layers on SrF2 reveal a relatively sharp exciton structure with a full width at half-maximum (FWHM) of 4 meV. Auger depth profile experiments suggest little or no interdiffusion between the GaAs and SrF2 layers while the presence of both Ga and As is detected in the CaF2.
Observation of strong localization effects in (AlGa)As–GaAs two-dimensional electron gas structures at low magnetic fields
Authors: Foxon C. T. , Harris J. J. , Wheeler R. G. , Lacklison D. E.
Published: July 2002

Electrical measurements have been made on films of lightly Si-doped GaAs and on (AlGa)As/GaAs two-dimensional electron gas (2DEG) structures grown by MBE. These have shown residual donor and acceptor concentrations of ∼2×1014 cm−3 in the GaAs, and mobilities for 2DEG layers which varied systematically with carrier density ns and undoped spacer layer thickness d; the highest mobility obtained was 2.2×106 cm2 V−1 s−1 for a sample with ns=2×1011 cm−2 and d=800 Å. Analysis of the data shows that scattering by remote ionized impurities is the dominant mechanism in most of our 2DEG samples. In such layers, the mobility μ at 4 K varied with photoexcited carrier density ns approximately as ns1.5. This behavior is expected for photoexcitation of carriers from DX centers in the doped (AlGa)As, and results from increased screening in the 2DEG and increased scattering due to the greater number of charged impurities in the (AlGa)As. However, some samples showed a much steeper variation of μ with ns, and also different temperature dependences of μ for high and low ns values. Comparison with previously reported data on Si–SiO2 2DEG systems suggests that this effect is due to strong localization effects occurring at the (AlGa)As–GaAs interface.
Optical properties of GaAs/AlGaAs quantum wells grown by molecular beam epitaxy
Authors: Pearah P. J. , Masselink W. T. , Henderson T. , Peng C. K. , Morkoç H. , Sanders G. D. , Chang Yia-Chung
Published: July 2002

A detailed characterization of the optical properties of GaAs/AlGaAs multiple quantum well heterostructures has been performed at superfluid helium temperatures using photoluminescence, optical transmission, and reflectance techniques yielding complementary information. We present the first report of experimentally determined absolute absorption spectra obtained from optical transmission spectra. Exciton absorption features become strongly enhanced as the wells get narrower. Theoretical calculations of the quantum energy levels correlate well with experimental observations. Reflectance spectra revealing structure of greater detail than that observed in luminescence have permitted the observation of free exciton splitting attributed to submonolayer well width fluctuations.
Trapping mechanisms in device-quality molecular beam epitaxial AlxGa1−xAs and GaAs–AlxGa1−xAs modulation doped heterostructures
Authors: Dhar S. , Hong W. P. , Bhattacharya P. K.
Published: July 2002

The dominant electron trap D-X center in AlxGa1−xAs has been clearly identified by us to consist of two closely related levels with thermal activation energies of 0.48 and 0.52 eV. Photoionization properties of the two levels have been explicitly measured by us for the first time. A new model for capacitance transients from modulation doped (MD) structures, which takes into account the heterointerface capacitance, is presented to explain the observed anomalous behavior of trap emission in MD structures. Both the thermal and the optical ionization properties of the two D-X centers in MD structures are measured and discussed.
Molecular-beam epitaxial growth of AlGaAs/(In,Ga) As resonant tunneling structures
Authors: Lee J. W. , Reed M. A.
Published: July 2002

We have developed a routine molecular-beam epitaxial (MBE) growth technique to prepare room temperature operating AlGaAs/(In,Ga)As resonant tunneling diode (RTD) structures. To grow these structures the substrate temperature, growth rate, and Si-dopant profile were optimized in terms of the temperature dependent I–V characteristics of the RTDs. When a slow growth rate, two substrate temperatures, graded Si doping, and undoped spacer layers were combined, a 1.75:1 peak-to-valley ratio was achieved at room temperature in the AlGaAs/GaAs RTD structures. As a different approach, the AlGaAs barriers were replaced by all-binary AlAs/GaAs superlattices layers in order to reduce alloy scattering and a 1.8:1 peak-to-valley ratio was obtained. Also, for the first time, resonant tunneling through an AlGaAs/InGaAs strained layer structure was achieved up to room temperature.
Auger and Rutherford backscattering compositional analysis of GaInAs anodic oxide
Authors: Shanker Kartik , Fischer C. W.
Published: July 2002

Anodic oxidation of III–V semiconductors provides excellent control of oxide thickness and high chemical etch selectivity. Rutherford backscattering spectrometry (RBS) and Auger electron spectroscopy (AES) were used to characterize the oxides grown on GaInAs. Epilayer compositions with the Ga mole fraction ranging from 0.43 to 0.56 were used. The oxides were found to have uniform composition throughout their thickness, except when anodic growth was done using an electrolyte with tartaric acid buffered by NH4OH to a pH of 7.0. Under high pH (∼7) conditions, anodization was associated with nonuniform In or O concentrations. Compositional analysis of the AES sputter depth data was accomplished by using appropriate sensitivity factors, obtained from standards and by RBS measurement of the In/(Ga+As) ratios. RBS ratios were invaluable since there was strong correlation between the In concentration and its sensitivity factor, ranging (exponentially) from ∼0.75 at 6% In to ∼0.6 at 8% In. All the oxides had near stoichiometric O content but were In deficient, irrespective of the epilayer composition or the anodization conditions.
Valence-band discontinuity at the Ge/InP(110) interface
Authors: Mahowald P. H. , Kendelewicz Tom , Bertness K. A. , McCants C. E. , Williams M. D. , Spicer William E.
Published: July 2002

The valence band of the Ge overlayer is 0.65±0.1 eV higher than that of the InP(110) substrate when the interface is grown at 280 °C. The In 4d core level was curve fit to obtain a shift of −0.3 eV for surface atoms compared to the bulk at the lowest coverages and to separate the chemically reacted component at higher coverages to obtain an accurate band-bending determination. The interface is slightly intermixed and after the first two laminar layers, islands of Ge may be forming. Also, islands of In are formed at the surface of the overlayer. The interface was grown at 20 °C also, where less indium is intermixed in the overlayer, and more is surface segregated. The bulk component can still be obtained for the core levels, but the metallic indium emission obscures the valence-band maximum and a valence-band discontinuity value is not available for 20 °C.
Chemiluminescence from F and XeF2 etching reactions with silicon
Authors: Mitchell M. J. , Suto M. , Lee L. C. , Chuang T. J.
Published: July 2002

The ultraviolet-visible chemiluminescence spectra from the etching reactions of silicon with atomic fluorine and xenon difluoride were observed. In both reactions, the visible chemiluminescence arises mainly from excited SiF*3, which is produced from the reaction of SiF2+F in the gas phase. The XeF2 etching reaction yields additional bands from excited XeF and possibly from a second excited state of SiF3 which may be produced from the reaction of SiF2 with a hot F atom. When an inert gas is added to the XeF2 etching system, the SiF*3 emission is quenched and new emission bands appear. The mechanism for the production of chemiluminescence is discussed in accord with the chemiluminescence spectra and polarization observed.
Comparison of optical design approaches for use with liquid metal ion sources
Authors: Orloff J.
Published: July 2002

A comparison was made of the calculated performance of two optical systems for use with liquid metal ion sources and which use different approaches for varying the focused beam current. The purpose was to determine the difference in the performance at a given working distance between a system with a fixed aperture which changes the strength of a condenser lens to vary the beam current and a system which maintains a collimated beam and changes the physical aperture in order to vary the beam current. The former design generally has worse properties, but with proper design this limitation may not be significant.
A review of excimer laser projection lithography
Authors: Rothschild M. , Ehrlich D. J.
Published: July 2002

Excimer-laser projection lithography now appears to be in a position to extend production optical techniques to dimensions approaching 0.25 μm. Such methods could well be the basis for the bulk of the advanced manufacturing capability in microelectronics over the next decade. This technology is reviewed with an eye to the state of the art and to the optical-, resist-, and materials-engineering issues that it presents.
Pattern transfer by dry etching through stencil masks
Authors: Pang S. W. , Geis M. W. , Goodhue W. D. , Efremow N. N. , Ehrlich D. J. , Goodman R. B. , Randall J. N.
Published: July 2002

Anisotropic profiles and linewidths as small as 60 nm have been controllably achieved using pattern transfer by dry etching through stencil masks. This technique eliminates the conventional polymer resist and lithographic steps and could be useful in obtaining precisely reproducible submicrometer linewidths without variation due to run-to-run variability in resist processing. The stencil masks used in this study were 1-μm-thick SiNx membranes with transmission openings and are similar to those used for masked ion beam lithography. The dry etching techniques consisted of reactive ion etching, ion beam assisted etching, and hot jet etching. The profile and linewidth control depend on the divergence of the ion or reactive flux and the gap between the stencil mask and the sample.
Use of ion implantation to eliminate stress-induced distortion in x-ray masks
Authors: Ku Yao C. , Smith Henry I. , Plotnik Irving
Published: July 2002

One of the most important questions with regard to using x-ray lithography for integrated circuit manufacturing is whether distortion in the x-ray mask, caused by intrinsic stress in the absorber material, can be made sufficiently small for all types of patterns. Pattern features should deviate from their assigned in-plane positions by no more than a fraction of the minimum linewidth. Tungsten (W) is a more desirable absorber than gold because of its thermal stability and its closer match to the thermal expansion coefficient of Si. However, it is difficult to reproducibly deposit W films with zero stress. We describe a compensation technique for eliminating distortion, regardless of the initial absorber stress. Experiments were done on 0.3–0.4-μm-thick W patterned on 1-μm-thick Si and Si3N4 membranes, suitable for x-ray masks. Distortions caused by tensile stresses ∼109 dyns/cm2 can be eliminated by Si implantation at doses ∼1016 cm−2. Measurements of deflections at an edge were made using a Linnik interferometer, and from this the stress was calculated. Si ions were implanted to a projected range of 10 nm in the W at doses in the range 1015–1016 cm−2. Distortion correction takes place because the implantation produces compressive stress near the upper surface, resulting in a torque that balances a torque of opposite sign at the absorber–membrane interface due to tensile stress in the W. We present a theory for stress compensation together with an empirical prescription for eliminating stress-induced distortion. We also report crystallographic analysis of the W and describe a method for getting α-phase W, in either tensile or compressive stress, which appears to be highly stable.
Repair of electroplated gold masks for x-ray lithography
Authors: Weigmann U. , Petzold H.-C. , Burghause H. , Putzar R. , Schaffer H.
Published: July 2002

X-ray lithography now enters a stage where its potential is tested by making real circuits. Since defects on x-ray masks strongly affect the performance of x-ray lithography, the repair of mask defects becomes very important. This paper discusses results on the repair of opaque and clear defects on electroplated gold-on-silicon x-ray masks using a 100-kV focused ion beam system from Vacuum Generators and laser-induced photolytic deposition of tin from a Sn(CH3)4 atmosphere, respectively.