bluuebunny/crossref_metadata_2025 huggingface dataset:
Silicon molecular beam epitaxy: Use of a microion source for doping by low-energy implantation
Authors: de Carvalho R. P. , Djebbar N. , Gutierrez J. , Vapaille A.
Published: July 2002

N-type doped silicon films were grown by molecular beam epitaxy with low-energy (1000 eV) As ion implantation. The ion source used was of the ‘‘hot cathode electrostatic reflex’’ type. This source does not request carrier gas introduction, and consequently, no mass filtering and no differential pumping. The doping levels obtained range from 9×1016 to 2×1019 cm−3. The doping gradients have been evaluated at ∼100 Å/decade. The As concentration found by secondary ion mass spectroscopy and Rutherford backscattering spectroscopy analysis is in good agreement with values calculated from incident ion current and growth rate (all the implanted As is incorporated) and with electrically measured free-carrier concentration (all the incorporated As is electrically active).
Frequency effects and properties of plasma deposited fluorinated silicon nitride
Authors: Chang Chorng-Ping , Flamm Daniel L. , Ibbotson Dale E. , Mucha John A.
Published: July 2002

The properties of low-hydrogen, fluorinated plasma-enhanced chemical vapor deposition (PECVD) silicon nitride films grown using NF3/SiH4/N2 feed mixtures in 200 kHz and 14 MHz discharges were compared. High-energy ion bombardment at 200 kHz is expected to enhance surface diffusion and chemical reconstruction. Compared to fluorinated silicon nitride deposited at 14 MHz under otherwise comparable conditions, the 200 kHz films had a lower Si–H bond concentration (≲1×1021 cm−3), lower total hydrogen content (5–8×1021 cm−3), better resistance to oxidation, lower compressive stress (−0.7 to −1.5 Gdyne/cm), and higher density (3.1 g/cm3). The dielectric constant of better low-frequency Class I films was constant to 500 MHz, while that of high-frequency films fell up to 15% between 100 Hz and 10 MHz. The absorption edges of low-frequency PECVD fluorinated silicon nitride films were between 5.0 and 6.1 eV, which compare with 4.4 to 5.6 eV for the high-excitation frequency fluorinated material and 3 to 4 eV for conventional PECVD nitride. However high-frequency films may have fewer trap centers and a lower dielectric constant. 14 MHz p-SiN:F films grown with NH3 as an auxiliary nitrogen source showed absorption edges similar to low-frequency material grown from NF3/SiH4/N2, but they have substantially more N–H bonding. The dielectric constant and absorption edge of these films were comparable to those of low-frequency p-SiN:F from NF3/SiH4/N2.
Extraction performance of dual-grid ion optics: Simulation and experiment
Authors: Korzec D. , Schmitz K. , Engemann J.
Published: July 2002

A comprehensive comparison between experimental and simulated data for the ion beam characteristics of a two-grid extraction system is presented. An existing description for dc ion sources has been generalized and extended to capacitively coupled rf ion sources. The essential parameter in dc technology—the discharge voltage—was replaced by the plasma edge potential used also by simulation, allowing direct correlation between experimental and simulated data. Experimental results show 50% higher divergence angles if a part of the ion trajectories impinges on the accelerator grid. The reasons for this discrepancy are analyzed. Simple empirical formulas are proposed which describe the performance of the beamlet formed by one hole of a multiaperture extraction optic for a wide range of working conditions. These formulas permit better modeling of the broad beam and optimization of ion optics.
Investigations of double barrier resonant tunneling devices based on (AlGa)As/GaAs
Authors: Hughes O. H. , Henini M. , Alves E. S. , Eaves L. , Leadbeater M. L. , Foster T. J. , Sheard F. W. , Toombs G. A. , Celeste A. , Portal J. C.
Published: July 2002

A series of electrical measurements on double barrier resonant tunneling structures is described. It is shown that the bistability effect in the current–voltage characteristics of a double barrier structure can be removed by connecting a suitable capacitor to the external circuit. These measurements cast serious doubt on the recent interpretation of the bistability as an intrinsic space-charge effect. The temperature dependence of the tunneling current at voltages below threshold is also investigated. An expression for the temperature and voltage dependence of the thermally activated resonant tunneling process is compared with experiment. Finally, the effect of hydrostatic pressure (up to 11 kbar) on I(V) characteristics is studied. It is found that, whereas the pressure has little effect on the low-temperature I(V) characteristics, the peak/valley ratio at room temperature is significantly reduced. This behavior is attributed to the effect of the higher conduction band minima.
Boron doping of Si molecular-beam epitaxy layers: A new high-temperature effusion cell
Authors: Andrieu Stephane , Chroboczek J. A. , Campidelli Yves , André Elie , d’Avitaya François Arnaud
Published: July 2002

The high-temperature effusion cell (HTEC) presented in this paper is designed for materials requiring evaporation temperatures above 1000 °C. It employs a novel concept of charge heating, indirectly, through a graphite crucible, by electron bombardment combined with thermal radiation. The cell is particularly well suited to molecular-beam epitaxy (MBE) systems and we have used it as a source of B in the MBE growth and doping of Si, obtaining the resulting B concentration NB between 1015/cm3 and 5×1019/cm3. Some characteristics of Si:B layers thus fabricated are presented and it is shown that NB is: (i) an exponential function of Tcell ; (ii) inversely proportional to the growth rate; and (iii) does not depend on the substrate temperature. We conclude from (ii) and (iii) that B is totally incorporated in the crystal when coevaporated with Si. Because of the total incorporation of B and the absence of its surface segregation very abrupt doping profiles can be obtained. NB gradients as steep as one decade per 100 Å have been obtained. A comparison of the secondary ion mass spectroscopy (SIMS) data and electrical measurements results indicates that B impurities are all electrically active. The values of resistivity ρ, and the Hall mobility μ, at room temperature are both bulklike and both ρ(NB) and μ(NB) follow the standard bulk Si dependences. The MBE layers grown without intentional doping are n-type, and have an electron concentration of 5×1013/cm3 and μ=1450 cm2/V s at room temperature, like pure n-type Si.
The role of aluminum in selective reactive ion etching of GaAs on AlGaAs
Authors: Seaward K. L. , Moll N. J. , Stickle W. F.
Published: July 2002

We have studied the role of aluminum in the formation of an etch barrier at the GaAs/ Alx Ga1−x As interface during reactive ion etching in CCl2 F2 plasma. The minimum Alx Ga1−x As thickness needed to form the barrier is Al mole fraction dependent and was determined with etching experiments monitored by optical emission spectroscopy. Effective Alx Ga1−x As layers for forming an etch barrier are 275 Å for x=0.02, 22 Å for x=0.10, 15 Å for x=0.15, 12 Å for x=0.20, and 9 Å for x=0.30. For all Al mole fractions except x=0.02, these thicknesses correspond to a sheet dose equivalent to 3/4 of a monolayer of Al in the original Alx Ga1−x As layer. Barrier layers for x=0.02, 0.10, 0.25, and 0.30 were examined without air exposure by angle-dependent x-ray photoelectron spectroscopy. For samples that are not overetched, the surface is covered with ∼20 Å of AlF3 intermixed with a gallium halide containing chlorine and fluorine and is depleted of arsenic. For substantially overetched barriers, a 30 Å layer is formed with gallium halide present at the surface, AlF3 found farther in, and arsenic depletion throughout the barrier. During extreme overetch, barrier layers on the order of tens of Å in thickness were not etched away and yet did not completely prevent very slow etching of underlying GaAs. Barrier layers on the order of 60 Å in thickness did prevent etching of underlying GaAs. Collectively the data suggest that the role of Al is formation of AlF3 exclusively and that only this compound is responsible for stopping the GaAs etch.
Band bending and oxygen-induced defects in a-Si:H
Authors: Winer K.
Published: July 2002

Total yield photoelectron spectroscopy and Kelvin probe measurements are used to monitor the adsorption of activated oxygen onto clean hydrogenated amorphous silicon (a-Si:H) surfaces at submonolayer coverages. Oxygen adsorption induces up to 3×1012 cm−2 occupied near-surface gap states in a band centered 5.15 eV below the vacuum level and results in an 0.25-eV downward band bending at both slightly n-type and intrinsic a-Si:H surfaces. It is proposed that the downward band bending is due to the transfer of electrons from overcoordinated surface oxygen atoms to neutral defects native to a-Si:H, which form negatively charged deep states in a near-surface accumulation layer. The surface donor character of adsorbed oxygen is contrary to its usual strong electronegativity but is analogous to the donor character of overcoordinated phosphorus atoms in the a-Si:H bulk, which also lead to the creation of negatively charged deep states. Doping and deep defect formation in a-Si:H are discussed.
Decomposition of NH3 on Si(111) 7×7 studied using laser-induced thermal desorption
Authors: Koehler B. G. , Coon P. A. , George S. M.
Published: July 2002

The decomposition of ammonia on Si(111) 7×7 was studied using laser-induced thermal desorption (LITD), temperature-programmed desorption (TPD), and Auger electron spectroscopy (AES). Product yields versus NH3 exposure were determined using TPD and LITD measurements. Besides the NH3 multilayer desorption, the only TPD products were H2 and Si2 N, which desorbed at ∼800 and 1350 K, respectively. H2 was also observed in the LITD yield from NH3 on Si(111) 7×7. In addition, NH3 , SiNH2, and SiNH were detected in the LITD yield at temperatures above the NH3 multilayer desorption. After saturation NH3 exposures, the LITD desorption yield for NH3 and SiNH2 persisted until 700 K, and the SiNH LITD signals were observed up to 800 K. These SiNH2 and SiNH LITD signals were assigned to reaction intermediates on the Si(111) 7×7 surface because neither SiNH2 nor SiNH was observed in the TPD yield. The effect of preadsorbed deuterium on the reactive adsorption of NH3 was also investigated. Deuterium was observed to decrease the product yield from NH3 decomposition on Si(111) 7×7.
Scanning tunneling microscopy and spectroscopy of gold on the GaAs(110) surface
Authors: Feenstra R. M.
Published: July 2002

The geometric and electronic structure of Au on the GaAs(110) surface is studied with the scanning tunneling microscope. At low metal coverage, the binding site for the Au adsorbates is found to be beside a surface Ga atom. The adsorbates usually occupy every second unit cell on the surface. In tunneling spectroscopy, a characteristic band gap state is observed for the adsorbed Au atoms, located ∼1.0 eV above the valence-band maximum. A resonant state in the valence band is also observed. The valence-band resonance and band gap state are interpreted as the first and second electron states of the Au–Ga bond, respectively. Both states form bands, and the surface Fermi level is pinned between these bands. At higher metal coverage, the Au atoms form clusters, which have a distinct epitaxial relationship with the substrate. Spectroscopy at high metal coverage reveals a continuum of states throughout the band gap, associated with the metallic nature of the clusters.
An x-ray photoelectron spectroscopy study on ion beam induced deposition of tungsten using WF6
Authors: Xu Z. , Kosugi T. , Gamo K. , Namba S.
Published: July 2002

The deposition process of tungsten from WF6 during ion beam bombardment has been studied using in situ XPS measurements. The deposition was done using low energy beam of Ar+ and H+2, which is important to reduce damage in the substrate. The adsorption of WF6 on the Si3N4 surface shows a tendency to saturate at high exposure to WF6 gas without the ion bombardment, while metallic tungsten is basically formed on Si surfaces, indicating that WF6 is dissociatively chemisorbed on the Si surface. No fluorine desorbed spontaneously from the Si3 N4 surface. After irradiation with 500 eV Ar+ (0.1 uA/cm2) on the adlayer for 700 s, W(4f) peaks show a chemical shift by about 6 eV toward low binding energy and there is a decrease of the F(1s) peak intensity, indicating that metallic tungsten is formed on the surface by ion beam induced dissociation of WF6. The composition of the deposited layer is W: 93%, F: 5%, and O: 2%. The resistivity of a 50 nm thick deposited layer is about 1.5×10−5 Ω cm.
High accuracy electron trajectory plotting through finite-element fields
Authors: Khursheed A. , Dinnis A. R.
Published: July 2002

Computer programs have been written which plot high accuracy trajectory paths for electrons through irregular finite-element meshes. The technique used to achieve this revolves around mapping irregular regions onto a normalized plane, where the usual operations of locating the electron’s position and interpolating for field information is greatly simplified. Results show that if a method of bi-cubic Hermite interpolation is used to derive field information in the normalized plane, then the resulting error in an electron’s energy can typically be kept to below 20 mV, per 1 kV change in the electron’s kinetic energy, for electrostatic fields created on numerical meshes containing as few as 20 by 20 mesh lines.
Atomic geometry at the CoSi2/Si (111) interface
Authors: Santaniello A. , DePadova P. , Jin X. , Chandesris D. , Rossi G.
Published: July 2002

We present a polarization-dependent Co K-edge SEXAFS investigation on the local atomic geometry around Co atoms at the interface between epitaxial CoSi2 and Si(111). The Co interface atoms are found to be coordinated with eight Si atoms at CoSi2-like bond lengths.
A compact, low-cost system for sub-100 nm x-ray lithography
Authors: Moel A. , Schattenburg M. L. , Carter J. M. , Smith Henry I.
Published: July 2002

We have developed a compact, low-cost system for sub-100-nm x-ray lithography. This system, which provides mask alignment and exposure at atmospheric pressure, was designed for sub-100-nm electronics research where the process latitude and high throughput of x-ray nanolithography have proven to be especially valuable. The system includes a CuL electron bombardment x-ray source (λ=1.34 nm) and an exposure chamber. The exposure chamber is filled with 1 atm of helium and is separated from the vacuum chamber housing the electron bombardment source by a 2-cm-diam window of 1–2-μm-thick SiNx. The exposure chamber contains a pin chuck to keep the wafer flat, a mask holder, and piezoelectrically driven X, Y, and θ stages. In this initial design, mask-to-wafer alignment is performed using a high NA (0.95), 1000× optical microscope. Future designs will incorporate computer-controlled interferometric alignment, which is compatible with sub-10-nm precision. For sub-100-nm lithography, the mask and substrate are separated by less than ∼5 μm to avoid the deleterious consequences of diffraction at the CuL wavelength. The mask-to-wafer gap is adjusted using a piezoelectrically controlled mask holder. The system is modular, easily reproduced, and suitable for low-budget nanolithography.
Focused ion beam lithography
Authors: Huh J. S. , Shepard M. I. , Melngailis J.
Published: July 2002

Focused ion beams of Be and Si were used to expose the following resists: PMMA, HEBR-214, KTI 820, and Microposit 2400. The resist thickness remaining for various development times was measured. For example, the minimum dose of 200 keV Be ions needed to expose 6800 Å thick PMMA varied from 7×1012 to 2×1013 ions/cm2 depending on development time. HEBR was found to act as either a positive or negative resist depending on dose. Exposures were also carried out in resist whose thickness exceeded the ion range. The effective PMMA exposure ranges of 200 keV Be ions and 200 keV Si ions were found to be 1.2 and 0.45 μm, respectively.
Growth of Si1−xGex by rapid thermal chemical vapor deposition and application to heterojunction bipolar transistors
Authors: Sturm J. C. , Schwartz P. V. , Prinz E. J. , Manoharan H.
Published: July 2002

Rapid thermal chemical vapor deposition has been applied towards the growth of Si and Si1−xGex structures on a 100 Å scale. In this paper the relative merits of gas switching versus temperature switching for the growth of such structures are discussed. Active temperature control in the 600–700 °C range using infrared transmission for temperature measurement is demonstrated. The growth technique is applied to 45 Å period superlattices with individual layer temperature control, and to heterojunction bipolar transistors with near-ideal electrical characteristics.
Surface topography and nucleation of chemical vapor deposition diamond films on silicon by scanning tunneling microscopy
Authors: Turner K. F. , LeGrice Y. M. , Stoner B. R. , Glass J. T. , Nemanich R. J.
Published: July 2002

The surface topography of diamond thin films deposited on silicon have been charted by scanning tunneling microscopy (STM). This study addresses the initial nucleation of diamond growth on Si, and the faceted structure of the diamond films. The results were obtained from an in-air STM system with tunneling currents between 0.2 and 3 nA. For studies of the diamond nucleation, samples were prepared by timed growth up to 60 min in a microwave plasma chemical vapor deposition (CVD) system. Diamond films with thicknesses in the 1–2 μm range were prepared by hot-filament CVD and examined by STM to determine the morphology of the diamond growth surface. The presence of diamond was verified by Raman spectroscopy. The STM images show that the surface is uniformly affected in the first 30 min of growth and diamond nuclei are identified after 60 min of growth. The thick films showed topography with facets on the surface similar to those seen from scanning electron microscopy results. The surface of these facets have been examined as well as the area between the facets. Elongated ridge structures are observed between different facets on the surface.
Relaxed diffusion limited aggregation of Ag on Au(111) observed by scanning tunneling microscopy
Authors: Chambliss D. D. , Wilson R. J.
Published: July 2002

Novel properties of metal monolayers and superlattices are likely to depend on the detailed morphology of monolayer growth. We find, using scanning tunneling microscopy, that the ‘‘simple’’ system of Ag on room-temperature Au(111) exhibits a morphological instability in the formation of the first monolayer. Monolayer-high Ag protrusions grow outward from atomic steps onto atomic terraces. The structure of the deposits on a nanometer scale shows that Ag atoms diffuse freely on Au terraces but are not mobile along or across step edges that separate the terraces. The result is a complex growth front characteristic of diffusion-limited aggregation with local relaxation. This causes strong inhomogeneity in local coverage, so that even at 1.1 ml average coverage there are large areas [(1000 Å)2] of exposed Au. Experiments with step structures intentionally created by tip impact suggest that atoms on step edges are not highly mobile at room temperature, so that the dominant mechanism for local smoothing of these deposits is relaxation intrinsic to the aggregation process.
Growth on (001) and vicinal (001) GaAs surfaces in a combined scanning tunneling microscope/molecular-beam epitaxy system
Authors: Pashley M. D. , Haberern K. W. , Gaines J. M.
Published: July 2002

To study molecular-beam epitaxy of III–V surfaces, we have added a small growth chamber to our existing ultrahigh vacuum (UHV) scanning tunneling microscope (STM) system. The growth chamber accommodates four effusion cells with pneumatic shutters, and is equipped with a reflection high-energy electron diffraction (RHEED) system with the ability to measure RHEED oscillations. On completion of growth, the specimen is removed from the growth chamber and transferred under UHV into the analytical chamber for study by STM. We have compared a flat GaAs(001) surface grown under arsenic rich conditions with previously studied GaAs(001) surfaces that were grown in a separate system and arsenic capped. The (2×4) unit cell structure is the same, however, there are some interesting differences. First, the surface is generally much smoother with flat planes several hundred angstroms across. Second, the surface reconstruction is predominantly c(2×8) and not (2×4). Third, there are a large number of kinks in the missing dimer rows which are regularly spaced along the 2x direction, forming a domain like structure on the surface. We have obtained the first images from a vicinal GaAs(001) surface. GaAs was grown under step edge growth conditions on a substrate misoriented from the (001) plane by 2° towards the (111)A plane. The STM images show the steps to be relatively straight with some variation in the terrace width. The step structure is made up from complete (2×4) unit cells.
Fabrication of sub-50 nm finger spacing and width high-speed metal–semiconductor–metal photodetectors using high-resolution electron beam lithography and molecular beam epitaxy
Authors: Chou Stephen Y. , Liu Yue , Fischer Paul B.
Published: July 2002

Using high-resolution electron beam lithography, we have fabricated metal–semiconductor–metal photodetectors with sub-50 nm finger spacing and finger width on GaAs grown by molecular beam epitaxy, which are, to our knowledge, the smallest ever reported. dc measurements showed that they have low dark current and high sensitivity. Proper scaling of the detectors to reduce the finger resistance and detector capacitance and to increase detector speed was studied. The resistances of thin metal lines with various widths were measured and compared with the value calculated from resistivity for bulk metal. Monte Carlo simulation demonstrates that for the photodetectors with 30 nm finger spacing and width, the response time is below picosecond and the cut-off frequency is over 1 THz.
30 nm resolution zero proximity lithography on high-Z substrates
Authors: Atkinson G. M. , Stratton F. P. , Kubena R. L. , Wolfe J. C.
Published: July 2002

In order to fabricate masks for x-ray lithography, there is growing interest in subtractive patterning of high atomic mass (high-Z) materials such as tungsten or gold. Favorable writing speeds and sub-50 nm resolution without proximity effects combine to make heavy ion focused ion beam lithography an ideal candidate for this area of nanofabrication. Using a 50 keV Ga+ beam with an 8 nm spot diameter, we have exposed a variety of proximity effect test patterns in 60 nm thick PMMA on 0.5 μm thick tungsten films. The results indicate that 30-nm resolution or better is possible at line/space pitches as small as 80 nm. The test patterns show no apparent proximity effects at these dimensions. An anomalous ‘‘inverse proximity effect’’ was observed, and was determined to be an artifact of the scanning electron microscope technique used to observe the PMMA resist.
Growth of high quality strained AlxGa1−xAs/In0.26Ga0.74As/AlzGa1−zAs quantum wells and the effect of silicon nitride encapsulation and rapid thermal annealing
Authors: Kaviani Kian , Chen Jun , Hu Kezhong , Chen Li , Madhukar Anupam
Published: July 2002

We report on the realization of high quality single quantum wells (SQWs) in the highly strained AlxGa1−xAs/In0.26Ga0.74As/AlzGa1−zAs system with 0≤(x, z)≤1.0. Photoluminescence (PL) linewidths of 5.5±0.4 meV for 0.30≤(x and z)≤0.70 have been achieved. Use of both alloys and short period multiple layer structures as the well and/or barrier layers has been examined. The silicon nitride encapsulation as well as rapid thermal annealing (RTA) induced changes in the PL properties of the as-grown SQW structures have been examined. Deposition of the nitride is found to induce a blue shift in the exciton peak. RTA induces a further blue shift, though not as large as that induced by RTA of unencapsulated (i.e., as-grown) structures. The RTA induced changes indicate interdiffusion of the group III atoms at the GaAs/InGaAs and InGaAs/AlGaAs interfaces.
Migration and gettering of Si, Be, and ambient-related O in AlGaAs/GaAs laser structures
Authors: Chand Naresh , Chu S. N. G. , Jordan A. S. , Geva M. , Swaminathan V.
Published: July 2002

We have observed Be diffusion in an AlGaAs/GaAs quantum-well (QW) laser structure grown at ∼700 °C from the p-AlGaAs cladding layer into the adjacent graded index (GRIN) AlGaAs and GaAs regions and beyond. This displaces the p/n junction from the AlGaAs/GaAs heterojunction. The Si dopant in n-AlGaAs GRIN and cladding layers influences the diffusion of Be. When the GRIN regions on p and n sides are doped with Be and Si, respectively, the displacement of the p/n junction is significantly reduced. Oxygen, a common contaminant in AlGaAs irrespective of the growth technique, tends to accumulate at the inverted GaAs/AlGaAs heterointerface in the GRIN region. Furthermore, the Be-doped Al0.6Ga0.4As cladding layer has an increased amount of O compared to its Si-doped counterpart. In addition, cathodoluminescence (CL) intensity from the GaAs QW was more than an order of magnitude higher in structures with undoped GRIN regions compared to doped ones. The decreased CL and the increased amount of O in Be-doped layers suggest that oxygen related nonradiative centers are incorporated in the QW active region of lasers with an intentionally Be-doped p-side GRIN region. Note that Be coincorporates O from the ambient, but diffused Be does not carry O along. Therefore, we recommend that the Be doping in p-side GRIN region should be avoided and the n-side GRIN region should be doped with Si to minimize O contamination, Be diffusion, and p/n junction displacement. Although the results are presented for AlGaAs/GaAs QW lasers, similar arguments can be applied for the growth of strained InGaAs/AlGaAs QW lasers.
Growth and luminescence properties of GaP:N and G
a
P
1−
xNx
Authors: Baillargeon J. N. , Pearah P. J. , Cheng K. Y. , Hofler G. E. , Hsieh K. C.
Published: July 2002

Nitrogen-doped GaP and P-rich GaP1−xNx alloys (x<0.03) exhibiting 77 K photoluminescence (PL) in the yellow–green to red portion of the visible spectrum have been grown on GaP substrates by gas source molecular beam epitaxy. The growth of these compounds was accomplished using gaseous NH3 and PH3 and solid Ga as the source materials. In samples with nitrogen concentration [N] < 1020 cm−3, the dominant emission occurred at 569 nm, corresponding to the isoelectronic trap NN1. The PL intensity was greatest for [N]≂ 1020 cm−3. As the nitrogen concentration was increased beyond this value, a monotonic red shift was observed in the emission wavelength, while the intensity generally decreased with increasing [N]. The red shift in the band edge is explained in terms of severe bowing in the compositional dependence of the indirect band gap for the GaP1−xNx system, as predicted by the dielectric theory of electronegativity.
Molecular-beam epitaxial growth of heavily acceptor doped GaAs layers for GaAlAs/GaAs heterojunction bipolar transistors
Authors: Hoke W. E. , Lemonias P. J. , Weir D. G. , Brierley S. K. , Hendriks H. T. , Adlerstein M. G. , Zaitlin M. P.
Published: July 2002

Heterojunction bipolar transistor (HBT) structures with heavily carbon and beryllium doped base layers were grown in a conventional molecular-beam epitaxy (MBE) environment. The current gain in the carbon doped structures was reduced by enhanced bulk recombination. For a hole concentration of 5 × 1019 cm−3, the carbon concentration exceeded the hole concentration and is a likely cause of the enhanced recombination. Growth conditions were determined which minimized beryllium diffusion for HBT structures with base layers doped to 1.2 × 1020 cm−3. Lattice contractions were observed at high beryllium doping concentrations. The expected reduction in resistivity with increased beryllium base doping was obtained in the HBT structures. Using HBT structures doped with beryllium at 5 × 1019 cm−3, high-power performance was obtained at 10 GHz.
Interface states and the transport of two-dimensional interface excitons in AlGaAs/GaAs structures
Authors: Gilliland G. D. , Wolford D. J. , Northrop G. A. , Petrovic M. S. , Kuech T. F. , Bradley J. A.
Published: July 2002

Using an unique combination of time and spatially resolved all optical photoluminescence measurement techniques, we have studied the properties of intrinsic excitons localized in quantum interface potentials in symmetric GaAs/Al0.3Ga0.7As double heterostructures. We find the transport of these interface excitons to be extremely sensitive to the time-dependent, spatially varying conduction, and valence band potentials near the heterointerface. The observed dynamics have been modeled, relying heavily on our previous kinetic results, which proved conclusively that H-band emission in our samples arises from the radiative recombination of quasi-two-dimensional excitons localized at the heterointerfaces, and find a simple electrostatic model to be in excellent quantitative agreement with experiment.
Molecular-beam epitaxy and migration-enhanced epitaxy growth modes of GaAs on pseudomorphic Si films grown on GaAs(100) substrates
Authors: López M. , Yamauchi Y. , Kawai T. , Takano Y. , Pak K. , Yonezu H.
Published: July 2002

GaAs layers were grown by molecular-beam epitaxy and by migration-enhanced epitaxy (MEE) on pseudomorphic Si films grown on GaAs(100) substrates. The Si interlayer thickness (tSi) was varied from 0 to 3 monolayers (ML), and the effect on the GaAs growth mode was investigated by observing the behavior of the intensity of the specular spot of reflection high-energy electron diffraction patterns. From these measurements it was concluded that the surface migration of Ga atoms is disturbed by the Si atoms on the growing surface. The disturbance increased, at the growth temperature of 520 °C, with increasing the Si interlayer thickness to the point that for tSi≥0.4 ML, the two-dimensional (2D) growth changed to a three-dimensional one. By increasing the growth temperature, the growth mode improved but the Si surface segregation increased, as detected by secondary ion mass spectrometry. The effect of the thermally activated Si segregation process on the GaAs growth mode is discussed. Using MEE at a growth temperature of 480 °C, the Si segregation was eliminated and a 2D growth mode was obtained.
Molecular-beam epitaxial growth as a self-organized critical phenomenon
Authors: Das Sarma S.
Published: July 2002

Molecular-beam epitaxial (MBE) growth kinetics will be discussed from the viewpoint of nonequilibrium statistical mechanics by treating it as a self-organized critical phenomenon associated with the development of temporal and spatial scale invariance in the system. This scale invariance is characterized by dynamical critical exponents which determine the large scale and long time asymptotic properties of the growing self-affine fractal interface. The various atomistic growth rules associated with the universality classes determining these exponents, and their possible roles in MBE growth will be examined. The view is that MBE growth is dominated by crossover effects associated with the various dynamical growth universality classes due to the competition between the deposition rates and kinetic hopping rates. Also discussed is the Langevin type continuum stochastic differential equations which may provide a coarse-grained theoretical description of MBE growth. Experimental implications of the fractal MBE growth dynamics are pointed out.
Ordered phase in (Hg,Cd)Te grown by liquid-phase epitaxy on CdTe (111)B substrate
Authors: Chang K. T. , Goo E.
Published: July 2002

(Hg,Cd)Te films grown by liquid-phase epitaxy on CdTe (111)B substrates are observed by transmission electron microscopy. An ordered phase is observed giving rise to 1/2(111) reflections. Diffraction patterns are consistent with the CrCuS2 structure or L11 ordering. All four orientation variants of ordered phase coexist in the specimen. The selected-area diffraction pattern indicates the bond distances in basal plane decrease upon ordering.
Dopant diffusion in HgCdTe grown by photon assisted molecular-beam epitaxy
Authors: Myers T. H. , Harris K. A. , Yanka R. W. , Mohnkern L. M. , Williams R. J. , Dudoff G. K.
Published: July 2002

The results of a systematic study of low temperature diffusion of indium and arsenic under Hg-saturated conditions in HgCdTe epilayers grown by molecular-beam epitaxy (MBE) are reported. Anneal temperatures ranging from 250 to 400 °C were investigated. To our knowledge, this is the first report of indium and arsenic diffusion coefficients at temperatures less than 300 °C. Indium was determined to be more stable to redistribution than arsenic in the MBE layers at the temperatures investigated. During this study, evidence of p-type impurity outdiffusion from the substrate during anneal was discovered. Both copper and silver were observed to segregate into the HgCdTe epilayer during anneal. This latter effect represents a critical problem in the use of CdTe and CdZnTe substrates for material growth, and may be a major factor limiting yield of material and devices.
In situ Raman studies during the epitaxial growth of ZnSe layers on GaAs(110)
Authors: Nowak C. , Zahn D. R. T. , Rossow U. , Richter W.
Published: July 2002

ZnSe was deposited onto ultrahigh vacuum cleaved GaAs(110) using a single molecular beam source. The substrate temperature during growth was varied between room temperature (RT=300 K) and 820 K. In addition, some samples were annealed at various temperatures up to 870 K after the ZnSe deposition. Raman spectra were taken in situ after each deposition and after the annealing steps. Under resonant excitation strong scattering signals of the deposited ZnSe are observable for coverages as low as approximately 20 Å. Fröhlich induced scattering of the ZnSe longitudinal optical phonon dominates the spectra, while the deformation potential scattering of the ZnSe transverse optical phonon is very weak. For nominal equal exposure of approximately 1600 Å ZnSe, the layer thickness decreases dramatically with increasing substrate temperature Ts. For Ts ≳ 670 K no ZnSe is detectable, instead Raman features occur which are most probably due to the formation of Ga2Se3. Annealing of a deposited ZnSe layer leads to changes explainable by the presence of Ga2Se3 at the interface. From complementary ex situ spectroscopic ellipsometry measurements a dielectric function and thicknesses of the ZnSe films are derived.
Ballistic electron emission microscopy study of PtSi–n-Si(100) Schottky diodes
Authors: Niedermann Philipp , Quattropani Lidia , Solt Katalin , Kent Andrew D. , Fischer O/ystein
Published: July 2002

PtSi–n-Si(100) Schottky contacts have been studied by ballistic electron emission microscopy (BEEM) for PtSi layers from 30 to 195 Å thickness. Locally measured barrier heights were typically close to or above those determined from the diode I–V characteristics, with a tendency to higher barriers for the thinner silicide films. The ballistic transmission probability also showed local variations with a clear tendency to decrease for increasing PtSi layer thicknesses, which can be understood in terms of the mean free path of hot electrons in PtSi. From simultaneous measurements at several voltages over the same area, maps of the apparent barrier height and transmission rate have been obtained showing sometimes strong local variations of these quantities. Very low and very high apparent barriers that were occasionally observed seemed to show that the simple BEEM picture is not always applicable, possibly due to interface roughness.
Ellipsometric determination of the thickness and refractive index of silicon films
Authors: Li M. , Yakovlev V. A. , Wall J. , Irene E. A.
Published: July 2002

An ellipsometric method is demonstrated to measure film thickness and the complex index of refraction for Si films using infrared and visible light single wavelength ellipsometry. Errors are considered and optical models are recommended for different wavelength ranges using results from spectroscopic ellipsometry and atomic force microscopy.
Selective and blanket copper chemical vapor deposition for ultra-large-scale integration
Authors: Jain A. , Kodas T. T. , Jairath R. , Hampden-Smith M. J.
Published: July 2002

Selective and conformal chemical vapor deposition (CVD) of copper from (hfac)Cu(VTMS), where hfac=1,1,1,5,5,5-hexafluoroacetylacetonate, VTMS=vinyltrimethylsilane, has been studied. The compounds (hfac)CuL, L=VTMS, 1,5-COD, and 2-butyne, deposited copper on both SiO2 and W, nonselectively, under the conditions employed. Selective CVD onto W in the presence of SiO2 was obtained by passivating SiO2 surface hydroxyl groups via reaction with Me3SiCl in the liquid phase. However, selective deposition was maintained only for short periods (1 min), and loss of selectivity was attributed to the desorption of hydrogen-bonded Me3SiCl groups which exposed the reactive SiO2 surface sites (hydroxyl groups). To avoid this problem, gaseous (CH3)2SiCl2 was introduced during Cu CVD and resulted in selective deposition for longer periods at respectable rates (≳2500 Å/min at 170 °C). Highly conformal deposition was demonstrated on test structures with keyhole geometries and trenches with widths of 2.8–0.45 μm and aspect ratios of 0.35–1.40. Deposition rates were 1000–2500 Å/min at temperatures of 160–170 °C with (hfac)Cu(VTMS) partial pressures of 10–17 mTorr.
Defect coverage profile and propagation of roughness of sputter-deposited Mo/Si multilayer coating for extreme ultraviolet projection lithography
Authors: Nguyen K. B. , Nguyen T. D.
Published: July 2002

The presence of defects in multilayer reflective coatings to be used as mask blanks for extreme ultraviolet projection lithography can cause significant reduction in image intensity. The extent of the reduction depends on the size and position of the defects and also on the manner in which the deposition process covers the underlying defect. If the deposited film causes lateral propagation of defect geometry and does not reduce the step height, even a hundred angstrom-sized defect might cause an observable effect. However, if the deposition process can reduce the defects’ height and does not increase their effective size, the effect of defects on image would be less severe. Atomic force microscopy and transmission electron microscopy was used to study multilayer coating deposition profiles over programmed defects. It was found that the step height and edge slope is significantly reduced after film deposition with small lateral propagation of defect geometry. Roughness of the evaporated programmed defects was significantly reduced by the multilayer deposition process. A 200 nm Si buffer layer deposited before multilayer mirror deposition further smooth out the small features. However it also increases the roughness over the initially smooth wafer, resulting in lower achievable multilayer coating reflectivity.
Soft x-ray production from laser produced plasmas for lithography applications
Authors: Spitzer R. C. , Kauffman R. L. , Orzechowski T. , Phillion D. W. , Cerjan C.
Published: July 2002

Laser-produced plasmas are investigated as a source for soft x-ray projection lithography. The dependence of conversion efficiency on target material, intensity, wavelength, and pulse width is determined using absolutely calibrated detectors. Conversion efficiency greater than 1% into a 2.2 eV bandwidth is demonstrated for Sn targets, fulfilling the system source requirements.
Optical analysis of quantum confined Stark effect in overgrown InGaAs/InP quantum wires
Authors: Schilling O. , Forchel A. , Kohl A. , Brittner S.
Published: July 2002

The quantum confined Stark effect has been investigated in InGaAs/InP quantum wires with widths down to 20 nm. The wires were fabricated by electron beam lithography and wet chemical etching using titanium as mask material. By metalorganic vapor phase epitaxy overgrowth high quantum efficiencies are achieved even for very narrow wire structures. We observe blue shifts in the luminescence peak energy up to 20 meV due to lateral quantization. After evaporating a semitransparent 50 Å thick gold Schottky contact we observe red shifts of the luminescence energy up to 25 meV due to the quantum confined Stark effect by applying a reverse bias voltage of typically 1 V.
On the breakdown of layer-by-layer growth and the spontaneous nucleation of misfit dislocations in molecular-beam epitaxially grown GeSi/Si
Authors: Perovic D. D. , Houghton D. C. , Noël J.-P. , Rowell N. L.
Published: July 2002

In this study, it is considered that the transition from ideal two-dimensional layer-by-layer growth to the initial stages of a three-dimensional growth morphology is a precursor to misfit dislocation injection in GexSi1−x/Si heterostructures with x as low as 0.15. Transmission electron microscopy has been used to identify a new misfit dislocation nucleation mechanism, the ‘‘double half-loop’’ source. Furthermore, photoluminescence (PL) spectroscopy has also been used to study the effects of elastic strain-induced surface roughening at low misfits upon analyzing the change in PL behavior with increasing strained layer thickness.
Epitaxial surface of NiSi2 (001) studied with low-energy electron diffraction and scanning tunneling microscopy
Authors: Becker R. S. , Becker A. J. , Sullivan J. , Tung R. T.
Published: July 2002

The epitaxial surface of NiSi2 (100) has been examined using both low-energy electron diffraction and scanning tunneling microscopy. Under proper conditions, a surface structure with √34 ×√34 symmetry is found, consisting of silicon adatoms occupying nominal 1×1 sites. The superstructure comprises an ordered array of adatom vacancies. A disordered surface phase is observed under suboptimal sample preparation conditions which consists of the elements of the ordered phase arranged in a nonperiodic manner.
Effects of doping on growth in the dichlorosilane/germane system
Authors: Campbell S. A. , Liu W. H. , Leighton J. D.
Published: July 2002

The effects of diborane and arsine on GexSi1−x growth rate and doping were studied in this paper. Dilute flows of diborane increased the growth rate, but had little effect on the activation energy. The growth rate of undoped films was increased linearly with germane flow then saturated, in good agreement with the literature. Under a dilute arsine flow we have observed for the first time that this description is no longer valid. The growth rate for As-doped films increases sublinearly with the germane flow, and saturates in a manner consistent with a reaction that is second order in GeH4. The activation energy also changes from 22 (undoped) to 42 kcal/mol (doped). Possible mechanisms are discussed. The active carrier concentrations have also been measured for these doped films. Dopant incorporation in the B2H6/GeH4/SiH2Cl2 is well described by a simple kinetic model, as in silicon epitaxy. For arsenic doping with arsine, a simple kinetic model cannot be used to describe the incorporation. We believe instead that a site competition mechanism is operative.
Interfacial defects and morphology of InGaAs epitaxial layers grown on tilted GaAs substrates
Authors: Liliental-Weber Zuzanna , Chen Y. , Werner P. , Zakharov N. , Swider W. , Washburn J. , Klem J. F. , Tsao J. Y.
Published: July 2002

Systematic transmission electron microscopy studies of In0.2Ga0.8As layers grown by molecular-beam epitaxy on [001] GaAs substrates and on substrates tilted up to 10° toward [110], [120], [100], [1̄10], and [1̄20] are described. Three different layer thicknesses were investigated: 6, 20, and 40 nm. In the 40 nm layers misfit dislocations were formed which partially relaxed the elastic strain. Three-dimensional growth with an indication of microscale In segregation (dendritic growth) occurred for substrates tilted toward [1̄10] and [1̄20] directions. Smooth growth surfaces were observed when the substrates were tilted toward [110] and [120] showing that the chemistry of the predominant surface growth steps can play an important role in the growth mode and quality of the epitaxial layer. Indium segregation to misfit dislocation cores, the presence of In-rich platelets near the interface, and diffuseness of the interface are considered as evidence for short range migration of In during and after growth.
Cross-sectional scanning tunneling microscopy of epitaxial GaAs structures
Authors: Vaterlaus A. , Feenstra R. M. , Kirchner P. D. , Woodall J. M. , Pettit G. D.
Published: July 2002

The scanning tunneling microscope is used to study GaAs epitaxial structures, cleaved in ultrahigh vacuum, and viewed in cross section. Two applications are described: in the first, the net donor concentration in Si-doped GaAs is deduced by direct measurement of the depletion width at pn junctions. In the vicinity of the pn junctions, net donor concentrations of greater than 2×1019 cm−3 are observed. This interfacial donor activity is an order-of-magnitude higher than that found in the bulk. In the second application, low-temperature-grown and annealed GaAs is studied. Arsenic precipitates are observed in the material. The precipitates are found to produce electronic states within the GaAs band gap, and these states cause the Fermi level to be pinned near midgap.
Modulation-doped InAlAs/InGaAs quantum well structures for high electron mobility transistors
Authors: Klein W. , Böhm G. , Sexl M. , Grigull S. , Heiss H. , Tränkle G. , Weimann G.
Published: July 2002

Modulation-doped InAlAs/InGaAs heterostructures were grown on InP substrates by solid source molecular beam epitaxy. Lattice matching within ±10−3 was reproducibly obtained. The high quality of the ternary compounds is demonstrated by narrow photoluminescence linewidths of full width at half-maximum=2 and 13 meV for InGaAs and InAlAs, respectively. The transport properties are excellent, too, with a 4.2 K mobility of 84 000 cm2/Vs for two-dimensional electron gas concentrations of 1.3×1012 cm−2 in conventional single heterostructures. Optimized single quantum-well structures with high current carrying capacity due to modulation doping in both barriers have sheet carrier concentrations of 4.0×1012 cm−2 and 4.2 K mobilities of 28 000 cm2/Vs. Sub-μm-high electron mobility transistors processed from these structures show a maximum transconductance of 500 mS/mm at a drain current of 445 mA/mm. A very high maximum drain current of 1120 mA/mm is obtained, while maintaining cutoff frequencies of fT=118 GHz and fmax=191 GHz.
Molecular beam epitaxial growth of GaAs on CaF2/Si(111) substrate
Authors: Li Weidan , Anan Takayoshi , Schowalter Leo J.
Published: July 2002

In this work, molecular beam epitaxial (MBE) growth of GaAs on CaF2/Si(111) substrates has been studied with both Rutherford backscattering spectroscopy and transmission electron microscopy. It has been observed that, under certain conditions, a chemical reaction between As adatoms and CaF2 layers can be induced, by which a more stable As layer on the CaF2 surface is formed. The existence of the As layer improves the Ga wettability on the CaF2 surface and, if properly controlled, leads to two-dimensional (2D) nucleation of GaAs on the CaF2/Si(111) surface as opposed to the more commonly observed three-dimensional growth. In subsequent growth, two kinds of twins have been observed. All samples were observed to have microtwins near the GaAs/CaF2 interface. These twins can be suppressed during the first 1000 Å, if the layer is grown in a narrow optimal growth window. Otherwise, the growth will be in a 3D mode at lower temperature, or, it will suffer from the formation of big rotational twins at higher temperature. It has been observed that growth on the vicinal substrates tilted to [1̄1̄2] azimuth is helpful in suppressing the development of rotational twins so that growth on these substrates have a wider optimal growth window.