1:30PM, V1
"BEEM and STM Studies of Self Assembled InAs Quantum Dots:" M.E. RUBIN, Center for Quantized Electronic Structures (QUEST) and Physics Department, University of California, UCSB, Santa Barbara, CA 93106; G. Medeiros-Ribeiro, J.J. O'Shea, P.M. Petroff, Materials Department, UCSB, Santa Barbara, CA 93106; M.A. Chin, V. Narayanamurti, ECE Department, UCSB, Santa Barbara, CA 93106
Ballistic Electron Emission Microscopy (BEEM) and STM have been used to study individual InAs self assembled quantum dots grown on a GaAs substrate capped with a 50Å or 75Å GaAs layer and 85Å Au layer. STM images show uniform surface features 1000Å in diameter. Corresponding BEEM images clearly show enhanced BEEM current near the centers of the features (where InAs islands are embedded below the surface). Local BEEM spectra show different current thresholds on and off the islands. The thresholds on the islands are below the known value for a Au/GaAs interface and are consistent with resonant tunneling through a 0D confined state in the dot. This is the first evidence of local electrical transport through a single InAs quantum dot.
1:50PM, V2+
"Imaging Dislocations In GaAs/InxGa1-xAs by Ballistic-Electron-Emission Microscopy (BEEM) and Scanning Tunneling Microscopy (STM):" S. BHARGAVA, E.Y. Lee, K. Pond, K. Luo, M.A. Chin, V. Narayanamurti, ECE Department, Materials Department, Mechanical Engineering Department, University of California, Santa Barbara, CA 93106
Partially relaxed GaAs/InxGa1-xAs/GaAs heterostructures were MBE-grown and their subsurface interfaces were analyzed using STM and BEEM, after deposition of Au overlayers. Simultaneous STM and BEEM imaging showed spatial variations that correlate with a single cross-hatch misfit dislocation core buried beneath the surface. The heterostructures were further analyzed using AFM, and TEM. Spatially resolved spectroscopy allowing for the study of local transport properties around the core was also performed. This work represents the first time that a defect in a semiconductor heterostructure has been identified by BEEM. This research was funded by AFOSR grant #442530-22502 and NSF #DMR91-20007 through NSF Science and Technology Center for Quantized Electronic Structures.
2:10PM, V3+
"Correlation of Atomic-Scale Interface Structure with Mobility Anisotropy in InAs/Ga1-xInxSb Superlattices:" A.Y. LEW, S.L. Zuo, E.T. Yu, Department of Electrical And Computer Engineering, University of California at San Diego, 9500 Gilman Dr., Mail Code 0407, La Jolla, CA 92093-0407; R.H. Miles, *Present address: SDL, Inc., San Jose, CA 95134, Hughes Research Laboratories, 3011 Malibu Canyon Rd., Malibu, CA 90265
We have used cross-sectional scanning tunneling microscopy (STM) to study
interface structure in InAs/Ga1-xInxSb superlattices grown by molecular-beam
epitaxy (MBE), and have correlated our STM results with Hall mobility studies.
The superlattice sample consisted of 50Å Ga0.75In0.25Sb alternating with
17Å InAs for 150 periods, and was grown at 410deg.C on a p-type GaSb
substrate. Samples for STM studies were cleaved under ultrahigh vacuum to
expose either the (110) or
cross-sectional face, on which STM measurements were performed. Atomically
resolved constant-current images of the samples exhibit electronically induced
topographic contrast between the InAs layers and the Ga1-xInxSb layers. The STM
data were used to obtain interface profiles along both the [110] and
directions. Fourier analysis was used to obtain roughness spectra of the
interfaces, and the spectra were fitted to a Lorentzian form to obtain
estimates of the roughness amplitude and correlation length. The data suggest
that interfaces parallel to the [110] direction, taken from the
cross-sectional images, have higher average roughness amplitude than those
parallel to the
direction (taken from the (110) cross-sectional images). The STM data correlate
well with data obtained from Hall electron mobility measurements along both the
[110] and
directions. The mobility data show that the mobility in the
direction is higher than that in the [110] direction. The mobility ratio
increases as the temperature is decreased, reaching as high as 55 at 15K. These
data suggest that the anisotropic interface roughness observed in the STM data
constributes to the anisotropic Hall mobilities observed.
2:30PM, V4
"Atomic-Scale Structure and Electronic Properties of GaAsN/GaAs Superlattices:" R.S. GOLDMAN, B.G. Briner*, R.M. Feenstra, Department of Physics, Carnegie Mellon University, Pittsburgh, PA 15213; M.L. O'Steen, R.J. Hauenstein, Department of Physics, Oklahoma State University, Stillwater, OK 74078; *Present address: Fritz-Haber-Inst. der Max-Planck-Gelleschaft, Faradayweg 4-6, D-14195, Berlin, Germany
We have investigated the atomic-scale structure and electronic properties of GaAsN/GaAs superlattices using cross-sectional scanning tunnelling microscopy and spectroscopy. The structures were produced by exposure of the molecular beam epitaxially grown GaAs surface to a nitrogen plasma from an electron cyclotron-resonance-source, without the co-deposition of Ga.[1] In-situ reflection high energy electron diffraction (RHEED) studies suggested the formation of coherently strained GaAsN films, approximately one monolayer thick.[1] Our cross-sectional studies indicate that the nitrided material is not a continuous film, but rather consists of groups of N atoms and larger clusters of GaAsN. In both cases, the extent of N incorporation in the growth direction was >20Å, considerably greater than the monolayer thickness suggested by RHEED. Spectroscopic studies on the N atoms reveal a state in the conduction band associated with an acceptor level of NAs in GaAs. In addition, spectroscopy on the GaAsN clusters reveals a significant upward shift of the valence band, possibily due to the high strain in the clusters. The effects of growth conditions including substrate temperature and GaAs doping concentration on the structure and electronic properties will also be discussed.
_______________________________________
[1] R. J. Hauenstein, D. A. Collins, X. P. Cai, M. L. O'Steen, and T. C.
McGill, Appl. Phys. Lett. 66, 2861 (1995).
* Present address: Fritz-Haber-Inst. der Max-Planck-Gelleschaft, Faradayweg
4-6, D-14195 Berlin, Germany
2:50PM, V5
"Atomic Force Microscopy of Organic Semiconductor Films:" S.F. NELSON, D.R. Foley, Y.-Y. Lin, D.J. Gundlach, T.N. Jackson, Department of Physics, Colby College, Waterville, ME 04937; Electronic Materials Processing Laboratory, Department of Electrical Engineering, The Pennsylvania State University, University Park, PA 16802
We have used atomic force microscopy (AFM) to explore the structural characteristics of evaporated films of [[alpha]]-sexithienyl, a conjugated polymer of interest for use as thin film transistors for display technology. The topology of the surface was found to be sensitive to solvent-related processing steps, in a way that correlates well with changes in the electrical properties of the films. Despite the fact that the solvents did not dissolve the polymer films, exposure to most solvents significantly degraded transistor characteristics. This study observed concurrent small-scale structural changes in the films.
While well layered structural organization has been demonstrated by x-ray diffraction in [[alpha]]-sexithienyl,1 evaporated films may be expected to be "polycrystalline" at best. AFM images of a layer deposited at room temperature, for instance, show closely packed rounded bumps, whose diameters vary from 50nm to 150nm, with the maximum height averaging about 20nm. From occasional flat, regular, terraced regions on the sides of the bumps we surmised that the bumps are regions of good molecular order, while the dips between bumps are essentially grain boundaries. Images of the same layer after soaking in toluene showed a significant difference; while the bumps seemed unchanged in shape and diameter distribution, there were now pits and crevices between the bumps. Our surmise that these in-between areas were grain boundaries was strengthened considerably by the fact that many of the newly etched crevices showed straight-line faceting between the rounded bumps. The maximum height of the films was also measured to be about 35nm, which is thicker than the nominal film thickness, suggesting that the etched boundaries might extend down to the silicon dioxide substrate.
These observations suggest that the degradation we have observed of [[alpha]]-sexithienyl transistor characteristics after solvent rinses may simply be related to decreased current transport through the boundary regions. This may also suggest that boundaries are limiting the transport in as-deposited films, thus limiting the measured mobility of the film.
_____________________________________
1 F. Garnier, A. Yassar, R. Hajlaoui, G. Horowitz, F. Deloffre, B. Servet, S. Ries, P. Alnot, J. Am. Chem. Soc. 115, 8716 (1993).
3:30PM, V6+
"Influence of Ga vs. As Prelayers on GaAs/Ge Growth Morphology:" QIN XU, Julia W.P. Hsu, Department of Physics, University of Virginia, Charlottesville, VA 22901; E.A. Fitzgerald, Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Ave., Cambridge, MA 02139; J.M. Kuo, Y.H. Xie, P.J. Silverman, AT&T Bell Labs, Murray Hill, NJ 07974
The surface morphology of GaAs films grown on Ge substrates (Ge deposited directly on Si) is studied by scanning force microscopy. Different prelayers (Ga as opposed to As) of GaAs films have dramatic effects on the surface morphology, including the formation of anti-phase boundaries (APBs), surface features near threading dislocations, and surface roughness. Our study includes films with different GaAs thicknesses: 100Å, 1000Å, and 1um.
Ga prelayer samples are generally smooth; the 1000Å film sample displays some APBs with predominantly one growth domain while the 1umm thick film displays the morphology of a homoepitaxial GaAs film. In contrast, the 1000Å As prelayer sample is rough with prevalent complicated APB structures, which can be attributed to the increase in single steps during the initial As2 deposition. We find that these APBs are depressed regions on the sample surface, and measure their depth and width quantitatively. The 1um thick As prelayer film, although without apparent APBs, shows characteristic deep wedge-shaped depression oriented in one direction. We find that As prelayer samples are rougher than those with Ga prelayers for the same thickness, most likely due to the formation of APBs during the initial growth. In the 100Å thick film study, we find that Ga and As prelayers also cause different GaAs growth structure around dislocations; Ga reduces the GaAs growth rate while As enhances it.
3:50PM, V7
"Surface Chemistry of GaS Overlayers on GaAs(001):" KYLE W. SELF, Sang I. Yi, Chan-Hwa Chung, K. Pond, W.H. Weinberg, Department of Chemical Engineering, University of California, Santa Barbara, CA 93106-5080
We discuss the results of surface passivation studies of GaAs (001) using a novel precursor, [GaSC(CH3)3]4. Using this precursor, thick films of GaS have been grown and tested as the gate passivant in GaAs-based field-effect transistors. The resulting transistors show excellent passivation properties. In this study we focus on the initial stages of formation of these films on GaAs (001) surfaces using scanning tunneling microscopy (STM), Auger electron spectroscopy (AES), and temperature programmed desorption (TPD). Surface coverage was calibrated using AES, and was analyzed as a function of temperature and flux. The results of investigations at low coverages on both vicinal and nominally flat surfaces have enabled us to identify the bonding sites and the reactivity at step edges and on terraces. Using TPD we have monitored the decomposition of the precursor molecule and subsequent desorption of decomposition products as a function of decomposition temperature. At higher coverages we have monitored the film morphology as a function of flux, temperature, and initial surface roughness. For the thickest films we have used atomic force microscopy to determine the surface roughness for layers too thick to be imaged using STM.
* Supported by NSF (grant DMR-9504400).
4:10PM, V8
"Resolution Limits of Localized Photoluminescence Measurements Using Near-Field Scanning Optical Microscopy:" JUTONG LIU, Syed A. Safvi, T. F. Kuech, Department of Chemical Engineering, University of Wisconsin, Madision, WI 53706
The high spatial resolution photoluminescence (PL) measurement using Near-Field Scanning Optical Microscopy (NSOM) is a powerful technique for the study of defect structures and the uniformity of semiconductor materials. PL from most semiconductors result from carriers which are mobile within the semiconductor, such as in band-to-band transitions, and defect related transitions. Due to the complicated photo-generated carrier transport and recombination in the three-dimensional media, the lateral spatial resolution limit of NSOM and the integrated PL intensity, however, are not easily determined. A three dimensional model of the carrier transport and recombination is needed to understand and interpret the NSOM measurements in a quantitative fashion.
We have developed a numerical model of the localized PL measurements by NSOM focusing on GaAs materials. Within the NSOM measurement, the NSOM tip is used as a localized sub-micron coherent light source. Photogenerated minority carrier transport in the lateral direction and the direction perpendicular to the surface. This transport was calculated using a quasi-three dimensional finite element model. The photo-generated carrier transport and the electrostatic band bending of the semiconductor materials are solved simultaneously. From the model, the lateral transport of the photogenerated carriers, their radial recombination, and the spatial resolution limit of the NSOM measurements were determined for variety of experimental conditions. The integrated PL intensity was also obtained and compared with a simple one-dimensional analytic solution. The results showed the carrier diffusion in the lateral direction can be shortened to sub-micron scale when a high surface recombination velocity is present, even though the intrinsic minority carrier diffusion length of GaAs is on the micron scale. This is consistent with our observations from NSOM experiments on the GaAs surfaces. The model predicts changes in the resolution limit and PL intensity for samples with different intrinsic minority carrier diffusion lengths and surface recombination velocities. We have performed NSOM measurements on GaAs and GaN samples with different dopant concentrations and surface treatments and compared the results with the data obtained from this modeling.
4:30PM, V9
"Characterization of Strain in Microelectronic Structures by Near Field Scanning Optical Microscopy:" WALTER M. DUNCAN, Corporate Research and Development, Texas Instruments, Inc., P.O. Box 655936, MS 147, Dallas, TX 75265
Near field scanning optical microscopy (NSOM) is an emerging method for characterization of advanced microelectronic materials and devices. Interest in NSOM is driven by the prospect of 20 to 50nm imaging capabilities while also providing optical, physical and chemical information. The present work concentrates on NSOM imaging modes with sensitivity to internal strain in SiO2/Si structures. All of our studies utilize a near field source and far field collection geometry. Approaches are sought that will allow measurements on structures containing surface topological features such as photolithographically defined mesas, via holes and trenches. Our scanned probe system allows simultaneous measurements of topological and NSOM images.
The first approach investigated in this work for imaging strain was strain induced birefringence. Whereas Si itself has no center of symmetry, hence does not exhibit birefringence, SiO2 possesses a center of symmetry, hence is birefringent when strained. Polarized intensity contrast was measured by placing an analyzer (a polarizer) before the collection detector. The near field source polarization was controlled with a wave plate before the NSOM source fiber. Whereas apparent strain fields are observed in flat regions of the structures studied, edge effects resulting from topology clearly dominate the polarized NSOM image. Hence, a second similar approach was investigated using polarization modulation instead of static polarization to yield a ratio of the polarization states. Images were obtained by recording the ratio of the time variant polarization signal to average intensity.
A second general approach studied for measuring strain was to measure the Raman spectra of Si in a NSOM sampling mode. Methods were developed for abating optical interferences due to fluorescence and Raman scattering in the NSOM tip. However, unacceptably long integration intervals make this approach impractical under most conditions for imaging applications.
4:50PM, V10
"Comparison of Two Dimensional Dopant Profiling Techniques: Differential Etching, Supreme Modeling, and Scanning Capacitance Microscopy:" ANDREW N. ERICKSON, Digital Instruments, 520 E. Montecito St., Santa Barbara, CA 93103; Roger Alvis, Scott Luning, Advanced Micro Electronics, One AMD Place, MS 32, Sunnyvale, CA 94088
Two-dimensional dopant metrology is a high priority on the national technology road map for semiconductors and is expected to become an enabling technology for next-generation integrated circuit manufacturing. Over the last few years, an increasing need has been identified for direct two-dimensional measurement of carrier densities in semiconductor devices to provide a means for the calibration of TCAD process modeling.
Until recently, quantitative 2D information could only be inferred from 1D dopant/carrier profiles provided by Secondary Ion Mass Spectroscopy (SIMS), Spreading Resistance Profilometry (SRP), and Capacitance-Voltage (CV) measurements. A number of different approaches have been taken to generate 2D information.[1] These include developments of different etching procedures which are sensitive to concentration and type of electrically active dopants.[1-3] Although at times providing high-quality data, these techniques are challenged by providing only indirect measurement of carrier densities and extensive and variable sample preparation.
Recently, self-aligned gate arsenic implant samples were generated with varying angles of incidence and dose. These samples were measured using differential etching techniques of TEM and AFM topography. We have used an SPM in contact mode to simultaneously measure topography (and therefore physical structure) and capacitance variations (due to an applied bias) of submicron MOSFETs on a nanometer scale. This technique is known as Scanning Capacitance Microscopy. [4,5] Comparison of the most popular 2D dopant profiling techniques sheds light in contrast of the various complementing strengths and weaknesses of the measurements.
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[1] R. Subrahmanyan, J. Vac. Sci. Technol. B 10(1), 358 (1992).
[2] H. Cerva, Proc. 1st Intern. Workshop on the Meas. and Char. of
Ultra-Shallow Doping Profiles, vol. 2, 286 (1991).
[3] R. Alvis, S. Luning, L. Thompson, R. Sinclair, and P. Griffin, Proc. 3rd
Intern. Workshop on the Meas. and Char. of Ultra-Shallow Doping Profiles, vol.
3 (1995).
[4] A. Erickson, L. Sadwick, G. Neubauer, J. Kopanski, D. Adderton, and M.
Rodgers, J. Elec. Mater. (Feb. 1996).
[5] Y. Huang, C. Williams, H. Smith, Proc. 3rd Intern. Workshop on the Meas.
and Char. of Ultra-Shallow Doping Profiles, vol. 3 (1995).
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