1998 MRS Spring Meeting & Exhibit

Chairs 

Julia Hsu 
Dept of Physics 
Univ of Virgina 
Charlottesville, VA 22901 
804-924-7956

Huub Salemink
IBM Zurich
Rueschlikon, SWITZERLAND
41-1-7248416

Clayton Williams 
Dept of Physics 
Univ of Utah 
302 James Fletcher Bldg 
Salt Lake City, UT 84112 
801-585-3226

Edward Yu
Dept of E&EC
Univ of California-San Diego
MC 0407
La Jolla, CA 92093
619-534-6619

Symposium Support 
*Academic Sinica 
*Digital Instruments, Inc. 
*Molecular Imaging Corporation 
*Omicron Associates 
*Park Scientific Instruments 
*TopoMetrix Corporation 

1998 Spring Exhibitor 

* Invited paper

TUTORIAL 

STS: ³BASIC PRINCIPLES AND APPLICATIONS OF SCANNING PROBE MICROSCOPY 
Monday, April 13, 8:00 a.m. - 4:00 p.m 
Salon 5/6 

scale surface characterization by Scanning Probe Microscopy (SPM) is providing new understanding to researchers in diverse areas of materials science, semiconductor technology, biophysics and mesoscopic systems. SPM techniques have been developed to measure almost every physical surface property on a 10 nanometer scale. This tutorial will provide a brief overview of the general principles of Scanning Probe Microscopy, followed by a detailed description of specific SPM techniques for measuring topographic, optical, thermal and electric properties on a nanometer scale. The basic tip/surface interaction for each technique will be discussed. Spatial resolution, topographic coupling, sensitivity and SPM artifacts will be examined. Local spectroscopies associated with the techniques will also be described. The applications of these probes to materials and devices will be detailed and illustrated with examples. 

The tutorial will include the following topics: 
Morning Session 8:00 - 12:00 Noon

• General SPM Principles
• Atomic Force Microscopy
• Near-field Scanning Optical Microscopy

Afternoon Session 1:30 - 4:00 p.m.

• General SPM Principles (repeat)
• Scanning Thermal Microscopy
• Scanning Capacitance/Electrostatic/Kelvin Probe Microscopy

Instructors: 
Joseph E. Griffith, Bell Laboratories, Lucent Technologies 
Hans Hallen, North Carolina State University 
Arun Majumdar, University of California 
Clayton C. Williams, University of Utah 

SESSION S1: SCANNING TUNNELING MICROSCOPY, BALLISTIC ELECTRON EMISSION MICROSCOPY, & CROSS-SECTIONAL SCANNING TUNNELING MICROSCOPY 
Chair: Huub W.M. Salemink 
Tuesday Morning, April 14, 1998 
Salon 5/6

8:30 AM *S1.1 
MBE UNDER THE MICROSCOPE: AN ATOMIC-SCALE VIEW OF  III-V SEMICONDUCTOR SURFACES AND INTERFACES. L.J. Whitman, Naval Research Laboratory, Washington, DC. 

The  III-V semiconductors, InAs, GaSb, and AlSb, can be combined via molecular beam epitaxy (MBE) into quantum well and superlattice structures that function as novel electro-optical and high speed devices, including infrared detectors, infrared lasers, high-frequency FET's, and resonant tunneling diodes. The performance of these devices is very sensitive to the morphology of the interfaces; characterizing, understanding, and ultimately controlling the atomic-scale structure of the interfaces are essential for the production of devices with optimal characteristics. I will review our recent studies of the (001) surfaces and interfaces of these structures using in-situ scanning tunneling microscopy/spectroscopy (STM/S) combined with a number of other surface characterization techniques and first-principles calculations. Topics to be discussed include: the unique reconstructions observed on the III-Sb surfaces (some of which are metallic); the role of the reconstructions in producing interface roughness; and the effects of different growth procedures on the surface morphology. I will describe how knowledge of the atomic-scale surface structures combined with an understanding of the competing kinetic and thermodynamic factors during growth can be used to tailor the morphology of the heteroepitaxial interfaces. 

9:00 AM *S1.2 
STABILITY AND AGING OF NANOSTRUCTURES ON Si. Ellen D. Williams, E.S. Fu, K.C. Lin, D. Liu, J.D. Weeks, University of Maryland, Materials Research Science and Engineering Center, College Park, MD. 

As the size-scale of practical devices shrinks into the sub-micron regime, the atomic characteristics of the component materials are increasingly likely to manifest themselves in the fabrication, properties and stability of the devices. Predicting and controlling this behavior is a challenging problem in statistical mechanics, which can be approached via a length-scaling approach called the continuum step model. The successful application of this model to developing and analyzing STM experiments on surface mass transport on Si will be presented. The relationship between equilibrium step fluctuations and large scale mass transport on Si surfaces will be demonstrated in quantitative measurements of the formation and decay of metastable structures. The use of this approach to determine the physical mechanisms underlying the spontaneous formation of metastable structures under a driving force induced by bulk electrical current, e.g. a surface electromigration effect, will be described. 

9:30 AM S1.3 
THE ATOMIC STRUCTURE OF STRAINED IN_xGA_1-xAS/GAAS(001) SURFACES. L. Li, B. -K. Han, Q. Fu, C. Li, R.F. Hicks, Chemical Engineering Department, University of California, Los Angeles, CA. 

Strained films of In-xGa-1-xAs (0.01<x<0.10) were grown on GaAs (001) substrates by metalorganic vapor-phase epitaxy (MOVPE). The film thickness and composition, and the degree of strain were determined by analysis of high-resolution x-ray diffraction spectra. In addition, the large-scale morphology and atomic structure of the film surfaces were characterized in situ after growth by low-energy electron diffraction, x-ray photoelectron spectroscopy, vibrational spectroscopy, and scanning tunneling microscopy. All the surfaces immediately following MOVPE are composed of atomically flat terraces that are separated by double height steps. The steps exhibit smooth, gradual undulations along the terrace edges. Close-up images of the terraces reveal a disordered c(4x4) pattern, which arises from a random mixture of arsenic dimers and alkyl groups adsorbed onto the top layer of arsenic atoms. Annealing the surfaces in vacuum desorbs the alkyl groups and then the arsenic, generating a variety of reconstructions at progressively higher gallium coverages. Many new reconstructions are observed on the surfaces of the strained InGaAs films, including the (2x3), (3x2) and other (nx2) unit cells, where n is an odd integer. Detailed characterization of these surfaces indicates that they do not obey the electron counting scheme that holds for unstrained GaAs(001) reconstructions, and may even be positively or negatively charged. These surfaces exhibit interesting chemical and physical properties that can be used to advantage in the MOVPE processing of semiconductor devices. The structures of the new surfaces and their properties will be described at the meeting. 

10:15 AM S1.4 
IMAGING DISLOCATIONS BY BALLISTIC-ELECTRON-EMISSION MICROSCOPY. Hans von Kanel, Thomas Meyer, Michaela Klemenc, Laboratorium fur Festkorperphysik, ETH Zurich, SWITZERLAND. 

Ballistic-electron-emission microscopy (BEEM) is one of few scanning probe techniques capable of providing detailed information on the electronic structure of buried interfaces [1]. We discuss the application of BEEM to epitaxial metal/semiconductor interfaces. Epitaxial films of CoSi₂ have been grown on Si(100) and Si(111) of both doping types by molecular beam epitaxy (MBE). For both substrate orientations, the defect structure has been studied before in detail by transmission electron microscopy [2]. BEEM images, obtained at 77 K in a UHV-STM chamber attached to the MBE system, exhibit clear contrast due to interfacial dislocations for both substrate orientations. The origin of the contrast is, however, different for Si(100) and Si(111). In the former case, the strain field around dislocations, associated with interfacial steps, affects the Schottky barrier height, leading to a pronounced shift of the BEEM threshold. At CoSi₂/Si(111) interfaces, however, the barrier height is not measurably altered at the dislocations. Here, we observe instead a huge scattering contrast, caused by point defects which have diffused into the dislocation strain field during layer growth [3]. 

10:30 AM S1.5 
A BALLISTIC ELECTRON-EMISSION MICROSCOPY (BEEM)-INVESTIGATION OF THE EFFECTS OF CHEMICAL PRETREATMENTS ON III-V SEMICONDUCTR SCHOTTKY BARRIERS. Roland Van Meirhaeghe, Greet Vanalame, Lieve Goubert, Felix Cardon, University of Gent, Dept of Solid State Science, Gent, BELGIUM; Peter Van Daele, Univ of Gent, INTEC, Gent, BELGIUM. 

Ballistic electron-emission microscopy (BEEM) has been applied to determine the Schottky barrier height change of Au-contacts on III-V semiconductors due to some classical chemical processing treatments. In contrast with the classical techniques (I-V, C-V, Fowler), BEEM allowed us to determine the distribution of Schottky barrier heights over the contact area on a nanometerscale. In the case of n-GaAs the influence of a HF-treatment was investigated. It was found that this process caused a Gaussian distribution with higher mean value than that of the reference samples. For p-InP, the same treatment was found to produce an opposite effect namely a Gaussian distribution with lower barrier height. A model is proposed based upon the presence of negative charges (F^-) at the metal-semiconductor interface introduced by the pretreatment. This model was further checked by measurements on other semiconductors : Si and Al_xGa_1-xAs and by using other processes (HCI treatment) that introduce negative interfacial charges (Cl^-). 

10:45 AM S1.6 
SCANNING TUNNELING MICROSCOPY AND BALLISTIC ELECTRON EMISSION MICROSCOPY STUDIES OF EPITAXIAL Pt/CaF₂/Si(111). Vincent LaBella, Y.S. Shusterman, L. Bennet, L.J. Schowalter, Rensselaer Polytechnic Institute, Dept. of Physics Applied Physics and Astronomy, Troy, NY; C.A. Ventrice, Jr., University of New Orleans, Dept. of Physics, New Orleans LA. 

The interest in understanding electron transport properties of ultra-thin insulating layers has prompted the study of Pt/CaF₂/Si(111) metal-insulator-semiconductor structures with scanning tunneling microscopy (STM) and ballistic electron emission microscopy (BEEM). BEEM is a scanning probe technique which is ideal for measuring local electron transport properties. The growth of these structures was performed via molecular beam epitaxy (MBE) and characterized in-situ with our STM which is capable of performing BEEM measurements. Thin 2-10 ML, relatively defect free CaF₂ layers were grown on both well-oriented and off-axis Si(111) wafers. Consistent with previous studies, the STM images of these structures show flat terraces and steps of CaF₂ as well as island formation and step bunching (for vicinal substrates). Platinum was deposited in-situ on top of the CaF₂/Si(111) structures with thicknesses ranging from sub-monolayer to 100 Å. The STM images of the Pt/CaF₂/Si(111) structures show the atomic steps of the underlying CaF₂ morphology, as well as the formation of Pt nodules. For the well oriented substrates, STM images for sub monolayer Pt coverages show the nodules nucleating on defect sites and at the step edges of the CaF₂ layer. The BEEM spectra show an onset near the conduction band minimum of the CaF₂, which is consistent with other BEEM studies of MIS structures. At certain tip locations an additional BEEM peak appears near 1.7 eV, which has not been previously observed in other metal/CaF₂/Si(111) studies. This low energy peak appears at an energy consistent with states in the band structure of CaF₂ with the first layer of fluorine desorbed, and has been attributed to an interaction of the ballistic electrons with localized F desorbed regions. In addition, the BEEM spectra are being used to measure the ballistic scattering lengths of the Pt and CaF₂layers. 

11:00 AM *S1.7 
SCANNNING TUNNELING MICROSCOPY OF III-NITRIDE MATERIALS GROWN BY MOLECULAR BEAM EPITAXY. R.M. Feenstra^a*, A.R. Smith^a, D.W. Greve^b, M.-S. Shin^c, M. Skowronski^c, J. Neugebauer^d, and J. Northrup^e, ^aDepartment of Physics, ^bDepartment of Electrical and Comp. Eng., ^cDepartment of Materials Science and Eng., Carnegie Mellon University, Pittsburgh, PA; ^dFritz-Haber Institut der MPG, Berlin, GERMANY; ^eXerox PARC, Palo Alto, CA. 

Results are presented from scanning tunneling microscopy (STM) studies of wurtzite GaN films grown on sapphire by molecular beam epitaxy. It is shown that the orientiation of the film - (0001) vs. (000) - can be determined from the STM results since completely different families of surface reconstructions are observed in each case. Complementary information from electron diffraction (LEED and RHEED) and Auger spectroscopy, together with first-principles total energy computations, are used to deduce structural models for the observed surface reconstructions. Novel results are found for these structures: for the (000) surface, the N-terminated GaN bilayers are covered with a monolayer of Ga, under very high stress. Additional Ga adatoms can be added to this 11 structure to form higher order reconstructions. Remarkably, it is also found that the (0001) Ga-terminated surface is, in the Ga-rich limit, covered with 2-3 monolayers of Ga. These results suggest that growth of the GaN in the usual Ga-rich conditions may be limited by the ability of impinging N atoms to penetrate the capping Ga layers[1]. Further conclusions regarding the growth of either (0001) or (000) surfaces, depending on the nucleation conditions, will also be discussed. 

11:30 AM S1.8 
SCANNING TUNNELING MICROSCOPY OF InAs_xP_1-x/InP and InN_yAs_xP_1-x-y/InP HETEROSTRUCTURES. S.L. Zuo, W.G. Bi, C. W. Tu, and E.T. Yu, Univ of California, San Diego, Dept of Electrical and Computer Engineering, La Jolla, CA. 

InAs_xP_1-x/InP and InN_yAs_xP_1-x-y/InP heterostructures have shown considerable promise for application in a variety of lasers, e.g., lasers operating at 1.3m and 1.55m. In these materials, phenomena such as ordering, clustering, and compositional grading may occur and will have considerable impact on the electronic and optical properties of heterostructure devices. Characterization of these phenomena at the atomic scale is therefore essential for understanding and optimization of device performance. We have performed cross-sectional scanning tunneling microscopy (STM) studies of InAs_0.35P_0.65/InP and InN_yAs_0.35P_0.65-y/InP (y0.01) multiple quantum wells grown by molecular beam epitaxy. The samples used for these studies consisted of 50  InAs_0.35P_0.65 alternating with 100  InP for five periods followed by 50  InN_yAs_0.35P_0.65-y alternating with 200  InP for five periods, grown on top of a 2500  InP buffer layer on n⁺ (001) InP substrates. The epilayers were doped n-type (n10¹⁶-10¹⁷) and grown at a substrate temperature of 460ºC. High resolusion  cross-sectional STM images show clear evidence of clustering of As and P in the InAs_xP_1-x alloy layers, and of compositional grading of As in both InAs_xP_1-x and InN_yAs_xP_1-x-y alloy layers. In addition, the boundaries between As-rich and P-rich regions in the (110) STM images of the InAs_xP_1-x layers appear to be preferentially oriented in <112> directions. Finally, a comparison of topographic contrast observed at InAs_xP_1-x/InP and InN_yAs_xP_1-x-y/InP heterojunction interfaces indicates that the valence-band offset is greater for the InN_yAs_xP_1-x-y/InP heterojunction than for InAs_xP_1-x/InP. 

11:45 AM S1.9 
III-V RESONANT TUNNELING DIODES OBSERVED BY CROSS-SECTIONAL STM. Jixin Yu, Meng Tao, Joseph W. Lyding, Beckman Institute, University of Illinois, Urbana, IL; Roger Lake, Texas Instruments, Dallas, TX. 

Resonant tunneling diodes are studied for their simpler IC architecture. III-V RTDs, such as GaAs/AlAs and InGaAs/InAlAs, are relatively easy to realize because lattice-matched systems can be found. Here we report our cross-sectional STM studies on InGaAs/InAlAs RTDs. The samples were grown by MBE on seminsulating InP substrates. Four RTDs were stacked on top of one another, separated by heavily-doped regions. The samples were then cleaved in UHV. The InAlAs barriers were observed. The uniformity of Al distribution in the barriers and the contamination of Al-rich regions by residue oxygen and water in the UHV system could be estimated from our STM images. Contrast in STM images caused by different doping levels was also observed in the regions between two RTDs, demonstrating the capability of cross-sectional STM in 
large-area dopant profiling. 

SESSION S2: NOVEL SCANNING PROBE MICROSCOPIES AND MAGNETIC FORCE MICROSCOPY 
Chair: Hans D. Hallen 
Tuesday Afternoon, April 14, 1998 
Salon 5/6

1:30 PM *S2.1 
SCANNING THERMAL MICROSCOPY AT NANOMETER SCALES. A. Majumdar, Dept. of Mechanical Engineering, University of California, Berkeley, CA. 

Measurement of temperature and thermophysical properties at nanometer scales can enable characterization of nanostructured materials and probing of microelectronic devices and structures. In addition, it allows one to perform photothermal microscopy and spectroscopy to measure optical properties of nanostructures. Traditional far-field optical techniques for temperature measurement are diffraction limited to a spatial resolution of about 1 micron and thereby incompatible for thermal probing of nanostructures. In the last few years, significant progress has been made in scanning thermal microscopy (SThM) where a sharp temperature-sensing tip is brought in close proximity to the sample surface and scanned laterally. This talk will focus on the latest developments of sensor fabrication, analysis of microscopic heat transfer mechanisms responsible for the nanometer-scale spatial resolution, as well as some results on probing electronic and optoelectronic devices. A new technique called scaninng Joule expansion microscopy (SJEM) has recently been developed to circumvent some of the problems encountered in SThM. The talk will discuss the basic principles of SJEM and present some recent results indicating its spatial resolution and potential applications. 

2:00 PM *S2.2 
SCANNING SINGLE-ELECTRON TRANSISTOR MICROSCOPY: FROM SINGLE ELECTRONS TO HALL POTENTIALS. H. F. Hess, PhaseMetrics, San Diego, CA; M.J.Yoo, Philips Research Labs, Briarcliff, NY, T.A. Fulton, A. Yacoby, R.L.Willett, L.N. Dunkelberger, R.J. Chichester, L.N. Pfeiffer, K.W.West, Lucent Technologies Bell Laboratories, Murray Hill, NJ. 

A single-electron transistor scanning electrometer (SETSE), a scanned probe microscope capable of mapping static electric fields and charges with 100 nanometer (nm) spatial resolution and charge sensitivity of a small fraction of an electron (0.01e) has been developed. The active sensing element of the SETSE, a single electron transistor fabricated at the end of a sharp glass tip, is scanned in close proximity across the sample surface. Images of the surface electric fields of a GaAs/Alx Ga1-xAs heterostructure sample show individual photoionized charge sites as well as 100-nm length scale fluctuations in the dopant and surface charge distribution. The SETSE has been used to image and measurement depleted regions, local capacitance, band bending, contact and hall potentials at submicrometer length scales on the surface of this semiconductor sample. 

2:30 PM S2.3 
APPLICATIONS OF NEAR-FIELD MICROWAVE IMPEDANCE MICROSCOPY. X.-D. Xiang, Chen Gao, I. Takeuchi, Yalin Lu, Fred Duewer, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA. 

Quantitative microscopy of complex electrical impedance is of great interest of both academic and industrial communities. We have recently developed a scanning tip near-field microwave microscope (STMNM) with resolution better than 100nm(/10⁶) and theories for quantitative imaging of microwave impedance of various materials[2]. The low temperature version of the STMNM has been used to study the microwave properties and uniformity of YBa₂Cu₃O_x patterned thin films[2] and other superconducting materials. A increase in surface conductivity was observed when the material undergoes a superconducting phase transition. It has also been used to characterize the dielectric/ferroelectric properties of doped (Ba_xSr_1-x)TiO₃ thin films and LiNbO₃ single crystals with periodic dielectric constant modulations and ferroelectric domain structures[3]. Both linear and nonlinear dielectric constant and loss tangent images with sub-micron spatial resolution have been obtained. ìEdge dislocationî defect included ì butterflyî strain field distribution has also been observed in LiNbO₃ single crystals, which is not observable by optical microscopy due to crystalís large birefringence. Potential applications in other area will be discussed. 

2:45 PM S2.4 
QUANTITATIVE NEAR-FIELD MICROWAVE MICROSCOPY. Chen Gao, and X.-D. Xiang, Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA. 

Quantitative characterization of microwave properties of materials with nanometer resolution is crucial for studying a wide range of materials. We have performed a theoretical model analysis for our recently developed scanning tip microwave near-field microscope (STMNM)[1, 2] and the results have enabled quantitative studies on dielectric, ferroelectric, electro-optical materials and superconductors. The field distribution around the tip and in the samples was obtained, and the tip-sample interaction was discussed in the near-field range using a quasi-static approximation and image charge approach[2]. Furthermore, evanescent waves refraction on the surface and decay/dissipation inside the conducting materials were examined and discussed. Analytic relationships between dielectric properties (i.e., dielectric constants and loss tangent) of insulating materials and the responses of the STMNM (i.e., frequency and quality factor shifts) were obtained for thick films or bulk samples, which allow the quantitative measurement and imaging of dielectric or ferroelectric properties of materials through proper calibration. Quantitative results can also been obtained for thin film dielectric samples and conducting materials. 50nm (/1X10^-6) resolution and 2X10^-3 sensitivity for dielectric constant can be expected in our current STMNM. Various experiments have been performed and found in good agreement with the theoretical results. 

3:30 PM S2.5 
Abstract Withdrawn. 

3:45 PM S2.6 
SCANNING HALL PROBE MICROSCOPY(SHPM) WITH 0.25m SPATIAL RESOLUTION. A. Oral, G.D. Howells, S.J. Bending, School of Physics, University of Bath, Claverton Down, Bath, UNITED KINGDOM; I.I. Kaya, Max-Plank-Institute fur Festkorperforschung, Stuttgart, GERMANY; M. Henini, Department of Physics, University of Nottingham, Nottingham, UNITED KINGDOM. 

We describe a low-noise Scanning Hall Probe Microscope having unprecedented magnetic field sensitivity (3x10^-8T/Hz^1/2 at 77K) and high spatial resolution, (0.25m) which can be operated in real-time (1frame/s) for studying magnetic flux profiles at surfaces. A submicron Hall probe manufactured in a GaAs/AlGaAs two dimensional electron gas (2DEG) is scanned over the sample to measure the surface magnetic fields using conventional scanning tunneling microscopy positioning techniques. The technique is quantitative and non-invasive compared to Magnetic Force Microscopy. We demonstrate high resolution SHPM images of 0.25m long bits in an ultra-high density hard disk sample. We have observed a tilt along the width of the bits. Those figure of merit have allowed us to image the speculated vortex ''melting'' transition in BSCCO single crystal directly with single vortex resolution. The implications of our results for the nature of this transition will also be discussed. 

4:00 PM *S2.7 
A MICROSCOPIC VIEW OF THE MAGNETIZATION PROCESS IN EPITAXIAL IRON FILMS. Dan Dahlberg, Sheryl Foss, Roger Proksch, George Skikmore, Chris Merton, and Jake Schmidt, Magnetic Microscopy Center, School of Physics and Astronomy, University of Minnesota, Minneapolis, MN.