1:30PM, FF1+
"UHV-CVD of Doped and Undoped Epitaxial SiGeC:" S. JOHN, B. Ferguson, E. Quinones, C.B. Mullins, S.K. Banerjee, Microelectronics Research Center, Department of Electrical Engineering, University of Texas, Austin, TX 78712; Department of Chemical Engineering, University of Texas, Austin, TX 78712
Studies of column IV heterostructures in order to provide device properties that are enhanced above that of pure silicon have until recently concentrated only upon SiGe where germanium (typically less than 30%) is added to silicon in order to produce a reduced bandgap alloy. When grown upon silicon substrates, these thin films are free of misfit dislocations until a critical thickness (dependent upon the germanium mole fraction and growth conditions) is reached, at which point the film relaxes through defect formation. Adding carbon to the SiGe system allows the formation of the ternary alloy SiGeC where one C atom compensates the strain of about ten Ge atoms. This allows the growth of layers with increased thickness and Ge concentration while reducing the number of defects. Furthermore, substitutional carbon incorporation into the SiGe lattice allows the independent control of strain and Ge composition giving larger choices for the bandgaps and bandoffsets than the SiGe system.
Using a cold-walled, ultra-high-vacuum, stainless-steel chamber with single-wafer-processing capability, we have grown epitaxial SiGeC films. We have used a deposition temperature of 550deg.C and flow rates of 1-20 SCCM of Si2H6, 0.1-2 SCCM of GeH4, and 0.8-1.6 SCCM of CH3SiH3. We have studied the germanium and carbon incorporation as a function of growth conditions using EDS, FTIR, and SIMS. Furthermore, we have analyzed the crystallinity and surface morphology of these films using RHEED and AFM. EDS and SIMS indicate a germanium concentration of 4 to 40 at-% in these films. We have accomplished carbon incorporation of 2.6 at-% in Si and 1.4 at-% in SiGe, being limited by our gas concentrations. We have found that high germane flow rates decrease the incorporation of carbon by up to 50%. Using FTIR to analyze the Si-C vibrational bond at 607 cm-1 by subtracting out the Si two-phonon peak, we have found that the substitutional carbon incorporation in SiGe is a linear function of CH3SiH3 flow rate. RHEED studies indicate that all films analyzed are crystalline but three-dimensional islanding is observed in SiGeC films as the carbon concentration is increased above a C to Ge ratio of 1:10. AFM studies indicate a corresponding increase in the surface roughness with increasing the carbon concentration above 1x1020 atoms/cm3 at a Ge mole fraction of 0.2. An increase of the C concentration from 6.5x1019 to 2x1020 atoms/cm3 for a Ge mole fraction of 0.04 is accompanied by a decrease in growth rate of almost 50% presumably due to less hydrogen desorption from C sites compared to Ge sites. The in-situ doping of SiGeC has also been studied using B2H6 and PH3. As the phosphorous concentration is increased from 5x1017 to 5x1018 atoms/cm3, it is found that the carbon incorporation decreases by about 33% while also decreasing the growth rate. Boron does not have any noticeable effect upon the carbon incorporation even at a concentration of 3x1019 atoms/cm3 though the growth rate is increased.
1:50PM, FF2
"Growth and Properties of Si1-yCy Alloy Layer Pseudomorphically Strained on Si(001):" H.J. OSTEN, Myeongcheol Kim, G. Lippert, H. Rücker, P. Zaumseil, Institute of Semiconductor Physics, PO Box 409, D-15204 Frankfurt (0) Germany
The growth and properties of Si1-yCy alloys pseudomorphically strained on Si(001) will be critically reviewed. Although the bulk solubility of carbon in silicon is small, epitaxial layers which more than 1 at.% C can be fabricated by MBE and different CVD techniques. Local ordering effects of the incorporated carbon atoms due to atomic size differences and the growth on reconstructed surfaces will be presented.
One of the most crucial questions is the relation between substitutional and interstitial carbon incorporation, which has a large impact on all electrical and optical properties of these layers. We will show that the interstitial to substitutional carbon ratio is strongly influenced by the chosen growth conditions, like growth temperature and Si growth rate. There is no general road map for an enhancement in substitutional carbon incorporation. Both the reduction in growth temperature and the increase of the overall growth rate lead to an increase of the substitutional interstitial carbon ratio but might also cause some deterioration in crystal quality. Other strategies will be discussed that include a change in the surface kinetics by saturating free bonds like in CVD processes or by using surfactants in MBE growth of Si1-yCy layers.
2:10PM, FF3
"SiGeC Alloy Layer Formation by High-Dose C+ Implantations into Pseudomorphic Mestastable Ge0.08Si0.92on Si(100):" SEONGIL IM, J.H. Song, F. Eisen, H. Atwater, M.-A. Nicolet, M/S 116-81, California Institute of Technology, Pasadena, CA 91125
High-dose C+ implantation was performed into 260 nm thick undoped metastable pseudomorphic Si(100)/Ge0.08Si0.92 with 450 nm thick SiO2 capping layer to make SiGeC alloy layers by using double and triple implantations after TRIM simulation for uniform depth profile of carbon. For the double energy implantations, the energies of 150 keV and 220 keV were sequentially used with a same dose of 1x1016 cm-2 at room temperature and 100deg.C. Triple energy implantation sequentially using 150 keV, 220 keV, and 280 keV was also done at room temperature. Along with the energies, the chosen doses to obtain quite uniform C distribution were 6x1015, 7x1015, and 7x1015 cm-2 respectively. The SiO2 capping layer was then removed by chemical etching. All of the implanted samples were subjected to rapid thermal or steady state annealing at various temperatures over 550deg.C. Ge concentration, crystallinity, and strain have been characterized by Rutherford backscattering/channeling spectrometry (RBS) and double crystal x-ray diffractometry (DCD). C concentration was characterized by secondary ion mass spectroscopy (SIMS). Double energy implantation resulted in partial amorphization of the GeSi layer by a layer thickness of 150 nm measured by RBS. At elevated temperatures, the partially amorphized layer was shown to epitaxially grow with tensile strain due to extra carbon content overcompensating the pre-existing compressive strain characterized by RBS and DCD. However, the non-amorphized part of the film preserved additional compressive perpendicular strain by C+ irradiation damage, which was never recovered by annealing. Similar results were found from the samples implanted at 100deg.C. Observed from both as-implanted and annealed samples, it was clear that the additional strain by damage was never removed by annealing. These films showed no amorphization and either no tensile state of strain even after annealing at 700deg.C for 30 min.
The samples obtained by triple energy implantation showed amorphization through the whole thickness of the layer and epitaxial growth after annealing at 700deg.C for 30 min. Even though a nonuniform tensile state was shown in DCD, channeling yields measured from the samples were very low signifying good crystallinity of epitaxial thin films.
2:30PM, FF4
"Structural and Electronic Properties of Si1-x-yGexCy/Si Systems:" WOLFGANG WINDL, Center for Solid State Science, Arizona State University, Tempe, AZ 85287-1704; Otto F. Sankey, Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504
In recent years, a considerable amount of work has been spent on semiconductor alloys with variable bandgap depending on the composition. However, the quite attractive system Si1-xGex/Si turned out to be difficult to handle for higher germanium concentrations because of increasing strain. The alternative ternary alloy consisting of Si1-x-yGexCy/Si seemed to offer the possibility of an unstrained system with variable bandgap.
In order to understand the properties of this system, we have performed intensive theoretical studies on binary and ternary alloys of different composition within a self-consistent LDA electronic-structure approach using a local-orbital basis. We examined the compositional dependence of strain and electronic properties of the system and find, contrary to Si1-xGex systems, strong deviations from Vegard's law: Our results show a strong bowing of the strain-carbon content relation, which predicts systems containing substitutional carbon to be much more strained than expected by Vegard's law. In the case of the bandgap, our calculations show the gap to close with increasing amount of substitutional carbon in the region of small carbon concentrations. This conflicts with mean field models which assume the bandgap to become wider since the carbon bandgap is much larger than that in silicon or germanium. The physical origins of these fundamental differences between systems containing carbon and pure silicon-germanium alloys are discussed.
2:50PM, FF5+
"Synthesis and Optical Characterization of Epitaxial SnxGe1-x Alloy Thin Films:" GANG HE, Harry A. Atwater, Gerald E. Jellison, Jr., Thomas J. Watson Laboratory of Applied Physics, California Institute of Technology, Pasadena, CA 91125, Oak Ridge National Laboratory, Solid State Division, Oak Ridge, TN 37831-6056
The group IV metastable SnxGe1-x alloy system is an interesting semiconductor material with potential applications in the fabrication of Si-based infrared optoelectronic devices. The relatively low growth temperature of SnxGe1-x (approximately 200deg.C) opens possibility of direct monolithic integration of detector arrays on nearly fully-processed Si integrated circuits. Band structure calculations have suggested that the SnxGe1-x alloys have direct energy gaps continuously tunable from 0.55 eV to 0 eV for compositions x from 0.2 to 0.6 with very small electron effective masses. However, synthesis of SnxGe1-x alloy films in the direct gap composition range by conventional thin film growth techniques have not been successful due to the severe surface segregation of Sn during the film growth. We report the synthesis of epitaxial SnxGe1-x/Si(001) with compositions up to x=0.34 by electron cyclotron resonance (ECR) ion-assisted molecular beam epitaxy in the substrate temperature range of 120deg.C to 200deg.C. The ECR ion source produces 30-50 eV Ar+ ions with ion to atom flux ratios of the order of unity. The high flux low energy ion beam irradiation greatly inhibits Sn segregation without interrupting epitaxy. In situ reflection high energy electron diffraction as well as x-ray rocking curve indicated epitaxial SnxGe1-x alloy films, and Rutherford backscattering spectra confirmed the SnxGe1-x alloy compositions and indicated an absence of Sn segregation. Fourier transform infrared transmission spectroscopy in the spectral range of 0.2 eV to 0.9 eV showed decreased transmittance with increasing Sn concentrations from x=0 to 0.30 relative to Ge layers of the same thickness, which is consistent with decreased band gap. Transmission measurements were also performed on SnxGe1-x samples of the same composition but different thickness ranging from 50 nm to 300 nm to study the optical cavity effects in the SnxGe1-x layers. Spectroscopic ellipsometry measurements in the 1.5 eV to 5 eV range indicated a red shift of the fundamental gap with increasing Sn composition. Results of infrared photoconductivity measurements at 77 K will also be discussed.
3:30PM, FF6+
"Measurement of Band Offsets for Si/Si1-xGex and Si/Si1-x-yGexCy Heterojunctions:" B.L. STEIN, E.T. Yu, Department of Electrical and Computer Engineering, University of California at San Diego, 9500 Gilman Drive, Mail Code 0407, La Jolla, CA 92093-0407; E.T. Croke, A.T. Hunter, Hughes Research Laboratories, 3011 Malibu Canyon Road, Mail Stop Al64, Malibu, CA 90265; T. Laursen, J.W. Mayer, Center for Solid State Science, Arizona State University, Tempe, AZ 85287
Si1-yCy and Si1-x-yGexCy alloys in which C is incorporated substitutionally offer the possibility of considerably greater flexibility, compared to that available in Si/Si1-xGex heterostructures, to control strain and electronic properties in Group IV heterostructures. In particular, growth of Si1-x-yGexCy alloys with a Ge:C ratio of 8.21:1 offers the possibility of fabricating Group IV heterostructure devices lattice matched to Si. Realization of such devices will require accurate measurements of band offsets for Si/Si1-x-yGexCy heterojunctions.
We have used admittance spectroscopy to measure valence band offsets in Si/Si1-xGex and Si/Si1-x-yGexCy heterostructures grown by molecular beam epitaxy. The Si/Si1-xGex and Si/Si1-x-yGexCy samples consisted of 250 Å Si1-xGex or Si0.796Ge0.20C0.004 alternating with 350 Å Si for 10 periods, and both layers were doped p-type with dopant concentrations of 7.4x1016 cm-3 and 1x1017 cm-3, respectively. These heterostructures were grown on a 2000 Å Si buffer on Si substrates and capped with 2000 Å Si. X-ray diffraction, Rutherford backscattering (RBS), and secondary ion spectroscopy (SIMS) were used to determine the composition and confirm the high structural quality of this material. Measurements of conductance and capacitance as functions of temperature at various frequencies were used to determine the activation energy for thermal excitation over the Si barriers in the p-type MQW structures; band offsets were then obtained from the measured activation energies. For Si/Si0.75Ge0.25 and Si/Si0.80Ge0.20 heterostructures coherently strained to Si, we obtained valence band offsets of 198+/-12 and 160+/-20 meV, respectively, in good agreement with accepted values. For the Si0.796Ge0.20C0.004 heterostructure, we obtained a valence band offset of 118+/-10 meV. This value is slightly lower than the valence band offset of approximately 135 meV expected in a Si/Si0.833Ge0.167 heterojunction, for which the lattice mismatch is the same as in the Si/Si0.796Ge0.20C0.004 heterojunction. We have also performed preliminary measurements of the conduction and valence band offsets for Si/Si1-yCy heterostructures, obtaining values in the range of 30-40 meV for approximately 1.3-1.9% C.
3:50PM, FF7+
"Optical Properties of Si1-x-yGexCy Alloys:" J.D. LORENTZEN, M.A. Meléndez-Lira, J. Menéndez, Department of Physics and Astronomy, Arizona State University, Tempe, AZ 85287-1504
Alloys of Si, Ge and C have received considerable attention over the past few years. From the technological point of view, the incorporation of carbon into the Si-Ge system provides an additional degree of freedom which potentially allows for even more versatile band gap engineering. From a basic science perspective, the large size difference between C and the host atoms in Si-Ge alloys is expected to produce unusual physical properties. This has been confirmed by recent theoretical work, which predicts a reduction of the band gap as a function of the carbon concentration and significant deviations from Vegard's law.
Here we present extensive optical characterization results on Si1-x-yGexCy and Si1-yCy epitaxial films grown by atmospheric pressure CVD and direct carbon implantation followed by solid-phase epitaxial regrowth. By combining photoluminescence, Raman scattering and infrared absorption experiments, we address some of the key issues concerning Si1-x-yGexCy alloys, such as the degree of substitutionality of the carbon atoms, strain compensation, and the dependence of the band gap on composition. Our photoluminescence spectra show band-gap-like features corresponding to no-phonon and phonon-assisted transitions. The energy of these transitions is lower than the corresponding energy in SiGe alloys with the same Ge/Si ratio. After correcting for strain effects, the lowering of the band gap becomes even more significant. For samples containing a substitutional carbon fraction of 0.5%, the lowering of the band-gap-like emission energy is as large as 60 meV. These results are compared with recent theoretical predictions.
From our Raman data, we obtain the dependence of the phonon frequencies on the carbon concentration and compare our results with theoretical estimates. Recent theoretical calculations suggest a predominance of Si-C bonds over Ge-C bonds in samples with a Si/Ge ratio of unity. This is due to the higher energy of formation of the Ge-C bonds. Our Raman results, however, show evidence for Si-C as well as for Ge-C bonds, in apparent contradiction with theory.
4:10PM, FF8+
Electrical Properties of Si1-x-yGexCy Alloys: F. CHEN, B. Orner, D. Hits, K. Roe, M. Ahmed, J. Kolodzey, Electrical Engineering Department, 140 Evans Hall, University of Delaware, Newark, DE 19716
We have fabricated alloys of the Group IV elements on (100) Si substrates by molecular beam epitaxy (MBE). The non-equilibrium growth conditions promote the incorporation of C which has low equilibrium solubilities in Si and Ge. These alloys have been investigated for heterostructure devices compatible with Si circuit technology. From optical absorption measurements, we found that the bandgaps of Si1-x-yGexCy alloys varied with the composition by more than 300 meV. For device applications, it is necessary to control the alloy conductivity type by doping. We have fabricated p-type layers of Ge1-yCy using boron acceptors evaporated from a high temperature solid source. We report on comprehensive electrical characterizations of the doped alloys including Hall effect measurements and current-voltage characteristics.
The Si was evaporated from a solid thermal source using a pyrolytic graphite crucible. The Ge was evaporated from a solid thermal source using a pyrolytic boron nitride crucible. The C beam was produced by sublimation from a graphite filament. Elemental B was evaporated from a graphite crucible inserted into a tungsten jacket. Substrates were prepared by degreasing, etching and an HF dip. Growth occurred at substrate temperatures between 500 and 600deg.C. Electron diffraction indicated the two-dimensional growth of crystals oriented with the substrate. Layer thicknesses ranged from 0.01 to 0.5 um. Compositions were measured by Auger electron spectroscopy and secondary ion mass spectrometry (SIMS).
We obtained C fractions up to 3 at. %, and hole concentrations up to 3x1019 cm-3. Hall effect measurements indicated that the hole mobilities were a factor of 2 higher than in pure Si for the same doping level, and values up to 1000 cm2V-1s-1 were obtained. Junctions formed between p-type GeC alloys and n-type Si substrates showed rectifying current-voltage characteristics. Both Al and Ti were used to form ohmic contacts. We will report on junction ideality factors, current densities and breakdown voltages.
4:10PM, FF8+
"Electrical Properties of Si1-x-yGexCy Alloys:" F. CHEN, B. Orner, D. Hits, K. Roe, M. Ahmed, J. Kolodzey, Electrical Engineering Department, 140 Evans Hall, University of Delaware, Newark, DE 19716
We have fabricated alloys of the Group IV elements on (100) Si substrates by molecular beam epitaxy (MBE). The non-equilibrium growth conditions promote the incorporation of C which has low equilibrium solubilities in Si and Ge. These alloys have been investigated for heterostructure devices compatible with Si circuit technology. From optical absorption measurements, we found that the bandgaps of Si1-x-yGexCy alloys varied with the composition by more than 300 meV. For device applications, it is necessary to control the alloy conductivity type by doping. We have fabricated p-type layers of Ge1-yCy using boron acceptors evaporated from a high temperature solid source. We report on comprehensive electrical characterizations of the doped alloys including Hall effect measurements and current-voltage characteristics.
The Si was evaporated from a solid thermal source using a pyrolytic graphite crucible. The Ge was evaporated from a solid thermal source using a pyrolytic boron nitride crucible. The C beam was produced by sublimation from a graphite filament. Elemental B was evaporated from a graphite crucible inserted into a tungsten jacket. Substrates were prepared by degreasing, etching and an HF dip. Growth occurred at substrate temperatures between 500 and 600deg.C. Electron diffraction indicated the two-dimensional growth of crystals oriented with the substrate. Layer thicknesses ranged from 0.01 to 0.5 um. Compositions were measured by Auger electron spectroscopy and secondary ion mass spectrometry (SIMS).
We obtained C fractions up to 3 at. %, and hole concentrations up to 3x1019 cm-3. Hall effect measurements indicated that the hole mobilities were a factor of 2 higher than in pure Si for the same doping level, and values up to 1000 cm2V-1s-1 were obtained. Junctions formed between p-type GeC alloys and n-type Si substrates showed rectifying current-voltage characteristics. Both Al and Ti were used to form ohmic contacts. We will report on junction ideality factors, current densities and breakdown voltages.
4:30PM, FF9
Amorphous SiGe:H Black Matrix Material for Active-Matrix Liquid-Crystal Displays: STEVEN L. WRIGHT, Lauren F. Palmateer*, IBM Watson Research Center, PO Box 218, MS 10-212, Yorktown Heights, NY 10598; Hagen Klauk, Thomas N. Jackson, Electrical Engineering Department, The Pennsylvania State University, 228 Electrical Engineering West, University Park, PA 16803
Active matrix liquid crystal displays (AMLCDs) require an opaque "black matrix" (BM) material to block light leakage between pixels. Current practice employs a metal layer, such as Cr on the top color filter plate. Improved displays can be made by moving the BM material to the bottom active array glass plate. The reduced tolerance needed to align the front and back plates can then be used to increase the clear aperture for the display. Also, an integrated BM layer can be used to effectively shield the array thin film transistors (TFTs) from backscattered and ambient light, and to protect the TFTs from damage during processing. Attempts have been made to integrate conductive opaque materials into the active array plate, but capacitive coupling inherent in these configurations causes unacceptable crosstalk, particularly in high-resolution arrays. An optimal integrated BM material must be both an optical absorber and an electrical insulator that is compatible with display processing materials.
We have identified hydrogenated amorphous germanium silicon alloy films as a possible integrated black matrix material. The films were produced by sputter deposition using hydrogen and argon. Films with low Si content have low resistivity and films with a high Si content have large resistivity, with an abrupt, percolation-like transition between the two regions. The optical density gradually decreases as the silicon content increases. In the composition range between 40 and 60 percent silicon, the resistivity is greater than 108 W-cm, and for 500 nm thick films in this composition range, the single-number optical density is greater than 2. The optical and electrical characteristics are similar for films deposited by co-sputtering or sputtering from a composite mosaic target.
The characteristics of this sputtered material are markedly different than that for material produced by plasma-enhanced chemical vapor deposition. We believe the sputtered material is inhomogeneous, with small clusters of Ge-rich material. Transmission electron micrographs taken in a Z-contrast mode suggest the presence of clusters up to about 10 nm in size. The clusters create localized electronic states which allow for efficient light absorption, but are spaced far enough apart to inhibit electronic conduction. If this hypothesis is correct, other cluster materials should also make good opaque insulators. Possibilities include clusters of Ge or metals such as Sn or In in an insulated matrix such as SiO2, SiNx, or SiC :H.
Test TFT structures and prototype display arrays have been fabricated with an integrated SiGe black matrix. For the highest resistivity films, little or no TFT back-gating is observed, an arrays with high contrast and small crosstalk are obtained.
4:50PM, FF10
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