1999 MRS Spring Meeting & Exhibit

Albert-Laszlo Barabasi 
Dept of Physics 
Univ of Notre Dame 
Notre Dame, IN 46556 
219-631-5767

 

Tom Pearsall
Dept Electrical Engineering
Univ of Washington
FB-10 Roberts Hall
Seattle, WA 98195
206-543-3063

 

Feng Liu 
Materials Research Group 
Univ of Wisconsin-Madison 
ERB-729 
Madison, WI 53706 
608-265-4024

 

George Maracas
Motorola Phoenix Corp
Research Labs 
MD EL508
Tempe, AZ 85284
602-413-6384

8:30 AM *V1.1 
FLUCTUATION MECHANISMS DURING SUBMONOLAYER EPITAXY. Mark F. Gyure and Christian Ratsch, HRL Laboratories, Malibu, CA; Russel E. Caflisch, Barry Merriman, Susan Chen, Myungjoo Kang and Stanley J. Osher, Department of Mathematics, UCLA, CA; Dimitri D. Vvedensky, The Blackett Laboratory, Imperial College, London, UNITED KINGDOM.

 

We describe a basic island dynamics model for irreversible epitaxial growth that is implemented using level set techniques [1]. We show how the effects of spatial and temporal fluctuations in deposition and nucleation can be isolated and studied systematically within this framework. We find that including spatial fluctuations in the nucleation of new islands is essential in order to reproduce the distribution of island sizes obtained from kinetic Monte Carlo simulations and experiments. We also discuss how different types of nucleation mechanisms affect the approach to the scaling limit of this distribution. We then outline how this model can be extended to include the effects of reversibility and discuss how this approach can be used to treat the effects of strain in a way that goes beyond what can be achieved with either Monte Carlo simulations, rate equations, or continuum equations of motion. 

9:00 AM V1.2 
SELF-CONSISTENT RATE-EQUATION APPROACH TO NUCLEATION AND GROWTH IN POINT/EXTENDED ISLAND MODELS OF 1-D HOMOEPITAXY. Mihail N. Popescu , Emory Univ., Dept. of Physics, Atlanta, GA; Jacques G. Amar, Univ. of Toledo, Dept. of Physics and Astronomy, Toledo, OH; Fereydoon Family, Emory Univ., Dept. of Physics, Atlanta, GA.

 

Although homoepitaxial submonolayer growth is usually two-dimensional, it is known that during heterogeneous deposition on a cleavage surface, nucleation occurs preferentially on steps leading to one-dimensional growth. The decoration of steps by clusters has often been used to study the behavior of surface steps, and recently it has been suggested that it offers the possibility of controlled growth of nanoclusters on cleavage steps. In the present work we consider both point and extended-island models of 1-d irreversible submonolayer growth using Kinetic Monte Carlo (KMC) simulations and a rate equation (RE) analysis. Our KMC results confirm the previously reported scaling behavior for the island-size distributions, average density of clusters N, and average monomer density N₁. We have also measured the average capture zone of clusters of various sizes and found that in both cases the average capture zones are strongly dependent on the cluster size, as in the case of two-dimensional growth. Using a self-consistent rate equation approach that explicitly takes into account the existence of gaps between clusters we can successfully predict the coverage dependence of the average densities of monomers and clusters. In particular, our RE analysis implies for the monomer capture number  an unusual dependence on the monomer density: , showing that at low coverages  the average density of clusters scales as , and leads to perfect agreement with KMC results for N and N₁ over the whole range of coverages. To obtain the island size-distribution, a second set of mean-field equations is used describing the evolution of the size-dependent capture zones and leading to explicit size- and coverage-dependent capture numbers. The solution of this fully self-consistent RE approach is then compared with KMC results.

 

9:15 AM V1.3 
MOLECULAR DYNAMICS STUDIES OF INTERLAYER MASS TRANSPORT AND DENDRITIC-TO-COMPACT MORPHOLOGICAL TRANSITIONS DURING SUBMONOLAYER GROWTH ON PT(111) SURFACES. Valeriu Chirita , Peter Munger, Lars Hultman, Thin Films Physics Division, Linkoping University, Linkoping, SWEDEN; Joseph E. Greene, Materials Science Department, University of Illinois, Urbana, IL.

 

We use embedded-atom method molecular dynamics simulations to investigate the kinetics of two processes which are critical in achieving the layer-by-layer growth mode: interlayer mass transport and dendritic-to-compact morphological transitions. The former investigation is carried out by following the dynamics of adatoms, vacancies and adatom-vacancy pairs within hexagonal Pt₁₉ clusters on Pt(111) at 1000 K, for simulation times totalling  135 ns. The latter study concentrates on the dynamics of Pt dendrites containing up to 25 atoms. The mapping of adatoms motion on clusters surfaces shows that prior to incorporation, adatoms are trapped near clusters edge for  80 of total simulation time. Cluster configurations with central vacancies are found to be quite stable. Adatom incorporation is observed to occur mainly via the two well-known mechanisms of hopping and push-outwith edge atoms. Our simulations for adatom-vacancy pairs within clusters, however, bring the first evidence that both mechanisms can also be active in the central region of the cluster, i.e., monovacancies are filled by adatom hopping or via exchanges with one of the atoms adjacent to the vacancy. The calculated activation barriers are comparable to the corresponding interlayer mass transport mechanisms observed at the cluster periphery. We have also followed the dynamics of Y-shaped Pt dendrites (for  150 ns), and found that, in agreement with previously proposed models, the dendritic-to-compact morphological transition proceeds via diffusion around branch corners, as well as edge diffusion. However, our simulations reveal two novel atomic processes playing a significant role in the morphology of dendrites; 1) the diffusion of the entire corner of a dendritic branch to adjacent terrace sites, via a concerted motion of the atoms forming the corner. The process has an activation energy similar to that of two-fold coordinated atoms diffusing around corners. 2) dendrites exhibit 2D3D2D transitions, involving the upward, respectively downward, diffusion of single atoms within their branches. Both mechanisms accelerate the transition towards compactness.

 

9:30 AM V1.4 
INTERMEDIATE SCALING REGIME FOR MULTILAYER EPITAXIAL GROWTH. Richard S. Ross and Mark F. Gyure, HRL Laboratories, Malibu, CA.

 

We have investigated a solid on solid kinetic Monte Carlo simulation of epitaxial growth in the multilayer regime, which we define as coverages between one monolayer and the crossover to the classical roughening regime. This regime, typically a few tens of monolayers, is of considerable practical importance for device applications, yet is not nearly as well-characterized as the submonolayer and kinetic roughening regimes where robust scaling theories have been developed. We compute surface roughness, height-height correlation functions and step-edge density for values of D/F in the range 10³ through 10⁸ where D is the diffusion constant of adatoms and F is the flux. In contrast to the surface roughness, simulated step edge densities achieve nearly constant asymptotic values and have a qualitatively different scaling behavior (as a function of D/F) than any other quantity directly related to height correlation functions. This scaling is explained by careful consideration of submonolayer nucleation and the evolution of surface roughness.

 

9:45 AM V1.5 
DIMER SHEARING AND DIFFUSION IN METAL(100) EPITAXIAL GROWTH. K. Wang , F. Family, Department of Physics, Emory University, Atlanta, GA; J. Amar, Department of Physics & Astronomy, University of Toledo, Toledo, OH.

 

Dimer-shearing was first proposed by Shi et al as a microscopic mechanism to explain an abrupt transition in the dependence of the island-density on deposition flux in experiments on Cu(100) submonolayer growth at 213 K. Using kinetic Monte Carlo (KMC) simulations of a model of metal (100) growth which includes dimer diffusion and shearing as well as monomer diffusion, we find that the scaling of the island density as well as the the shape of the island-size distribution and island morphology are strongly affected by the presence of dimer shearing. In particular, we find that the exponent  which describes the scaling of the island density with the ratio D/F of the diffusion coefficient D to the deposition flux Fhas a much larger value of , as compared to the value of 1/2 expected in the absence of dimer shearing. This implies that in agreement with recent experiments, the critical island size does indeed undergo a change from 1 to 8 as the temperature is increased. 

10:30 AM *V1.6 
MECHANISMS OF MOUND COARSENING IN UNSTABLE EPITAXIAL GROWTH. Jacques G. Amar , Department of Physics and Astronomy, University of Toledo, Toledo, OH.

 

During homoepitaxial growth on high-symmetry surfaces, the presence of a step-barrier to interlayer diffusion (Ehrlich-Schwoebel barrier) may lead to the formation of mounds which grow and coarsen with increasing film thickness h. In this work the dependence of the asymptotic mound-coarsening exponent n (where the typical mound size grows as hⁿ) on various island relaxation mechanisms is investigated using kinetic Monte Carlo simulations of growth on fcc(100) surfaces. The presence or absence of corner diffusion is found to play a crucial role in determining the asymptotic value of the coarsening exponent. In particular, for the case of no island-relaxation or island-relaxation via edge-diffusion but without corner diffusion, the asymptotic exponent is found to satisfy  in agreement with recent experiments on metal (100) surfaces. However, when rapid corner-diffusion is allowed, the asymptotic value of n is found to approach 1/3 even for the case of a finite step-barrier. A similarly large value is found for the case of a large step-barrier and island relaxation via adatom-island detachment. These results appear to account for the recent experimental observation of rapid angle-selection along with a larger-than-expected coarsening exponent () in epitaxial growth of Rh/Rh(111) at high temperature as well as the observation of  for growth on metal (100) surfaces.

 

11:00 AM *V1.7 
SELF-ORGANIZATION OF SURFACE PATTERNS DURING CLUSTERING. Martin Zinke-Allmang , Univ of Western Ontario, Dept of Physics and Astronomy, London, Ontario, CANADA; Sywert Brongersma, IMEC, Leuven, BELGIUM; Graham Carlow, JDS Fitel, Nepean, Ontario, CANADA.

 

Ostwald ripening and coalescence growth are the two well-characterized fundamental late-stage processes in the phase separation of thin film deposits on surfaces. Both processes have recently been studied in respect to ordering of the clustered phase. While coalescence growth is associated with random positions of the growing clusters, partial ordering is observed for mass conserved ripening systems. This partial order remains present in a self-similar fashion throughout the evolution of the pattern. The basic thermodynamic concepts of the self-organization of the clustered phase will be discussed. To obtain higher order of clustering, additional physical driving forces must be included in the process, such as strain effects in the now well-established coherent Stranski-Krastanov structures. Standard growth procedures for coherent SK morphologies will be reviewed. The I will discuss a novel approach where low density coherent clusters can be obtained in the silicide/silicon system through cluster growth from buried cobalt sources. These sources are obtained through cobalt ion beam implantation and connect to the surface through thermal etching pits. Due to strain effects during the coherent clustering, the resulting clusters are long, needle like structures, which are potentially useful for growth of quantum wires.

 

11:30 AM V1.8 
THE ROLE OF THE LATTICE STEP IN EPITAXIAL GROWTH. Tsu-Yi Fu , Tien T. Tsong, Institute of Physics, Academia Sinica, Taipei, TAIWAN, ROC.

 

Solid surfaces have many lattice steps. In epitaxy, aggregation of deposited atoms into islands or clusters during their diffusing can create many additional atomic steps. We study the effects of lattice steps on epitaxial growth in two aspects: 1. Movement across the step edge (including step up and down movements): a series of field ion microscope experiments reveal the importance of reflective and trapping properties of steps, and provide quantitative information that helps explain various growth modes observed in homoepitaxial growth. 2. Diffusion along the step edge (including along the straight edge and around the corner): a number of field ion microscope experiments are done to determine diffusion parameters of a ledge atom along the step edge, and to derive the potential-energy diagram along different diffusion paths that helps explain the growth morphology. 
During growth, an atom undergoes a number of elementary atomic processes. Each process is characterized by a few energy parameters for bonding and diffusion. The integrated effect of all of these processes determines the growth process. We provide reliable experimental data and find the temperature ranges where various atomic processes are important. Our result will be very valuable for developing kinetic simulations for growth phenomena.

 

11:45 AM V1.9 
RATCHET EFFECT IN SURFACE ELECTROMIGRATION: SMOOTHING SURFACES BY AN AC FIELD. Choongseop Lee , Albert-László Barabási, University of Notre Dame, Department of Physics, Notre Dame, IN; Imre Derényi, Department of Atomic Physics, Eötvös University, Budapest, Puskin, HUNGARY.

 

We demonstrate that for surfaces that have a nonzero Schwoebel barrier, the application of an AC field parallel to the surface will induce a net electromigration current. Most important, the direction of the net current will be always downhill; i.e. it will point in the step-down direction. The magnitude of this equilibrium current is calculated analytically, and compared with Monte Carlo simulations. A downhill current is known to smooth the surface, thus we suggest that the application of AC fields during annealing might aid the smoothing process and during growth it has to potential to slow or eliminate the Schwoebel barrier induced mound formation. 

1:30 PM *V2.1 
IN-SITU STUDIES OF THE FORMATION OF Ga AND Al WIRES ON Si(112) FACET SURFACES. S.M. Prokes and O.J. Glembocki, Naval Research Laboratory, Washington, DC.

 

We have studied the formation and growth of Ga and Al wires on facet Si(112) surfaces (consisting of (111) terraces and (001) steps), which were investigated using LEED, Auger spectroscopy and Reflectance Difference Anisotropy (RDA). Ga or Al chains form on this surface by a self-limiting process, which we can track from the rapid change from the (2x1) Si(112) reconstruction under sub-critical coverage, to chain formation leading to a 5x1 reconstruction followed by a 6x1 reconstruction, using RDA. Furthermore, AES and RDA results show the replacement of Ga atoms by Al atoms at the step edges during sequential deposition of Ga and Al, indicating a stronger Al-Si bond. Using RDA, we have also observed that depositions in the 300C range can lead to the formation of Ga metallic wires on the Si(111) terraces, while at higher temperatures, the chains form only along the step edges. Continued deposition to 9 monolayers at T below 250C leads to a large shift of the preferred polarization of the structure from along [112] to [1-10], while continued deposition in excess of 30 mn reverts back to a polarization along [112]. This result indicates that a very anisotropic but patterned Al or Ga structure in registry with the substrate forms at these low temperatures, retaining an unexpected large polarizability for coverages as large as 40mn. This does not occur for depositions near room temperature or above 350C. Using Monte Carlo techniques to model the time evolution of the deposition at various temperatures, we are also able to extract highly accurate values for the surface kinetic parameters involved in the formation of these nanostructures.

 

2:00 PM V2.2 
ELECTRICAL CHARACTERIZATION OF LOW TEMPERATURE MBE GROWN GaAs. Michael J. Cich , Robert C. Lutz, Rian Zhao, Petra Specht, Eicke R. Weber, University of California-Berkeley, Department of Materials Science and Mineral Engineering, Berkeley, CA.

 

The combined properties of high resistivity (>10⁴cm) and short carrier lifetime (< 1 ps) make low temperature MBE grown GaAs an attractive material in many device applications. Recently, greatly improved thermal stability by beryllium doping was demonstrated. The beryllium acceptor acts to electrically compensate arsenic antisites. This yields a higher charged antisite fraction, producing the short carrier lifetimes. In turn, the total arsenic antisite concentration may be reduced by using higher growth temperatures, thus reducing hopping conduction. However, reduced trap concentrations may result in lower voltages for abrupt current rise according to the basic trap-filled limit model. In this study, the current-voltage characteristics of n-i-n structures with the intrinsic layer consisting of low-temperature grown GaAs were evaluated. The films were grown between 200C and 300C with and without beryllium doping. The voltage of steep current rise showed a decrease on increasing the film growth temperature, as expected from the decreased trap concentrations. However, beryllium doping the LT-GaAs layer improved the breakdown characteristics at lower growth temperatures. A quantitative model of the dependence of the breakdown field on trap concentration and filling will be presented.

 

2:15 PM V2.3 
IN-SITU DOPING IN CVD EPITAXIAL Si_1-xGe_x: HEAVY-DOPING AND ELECTRICAL CHARACTERISTICS. Junichi Murota , Atsushi Moriya, Toshifumi Kikuchi, Takaaki Noda, Chunyang Deng, Masao Sakuraba, Takashi Matsuura, Tohoku Univ, RIEC, Sendai, JAPAN.

 

Exact control of the high concentration impurities and their electrical activation is important for realizing high performance Si_1-xGe_x/Si heterodevices. We have investigated doping and electrical characteristics of in-situ heavily P- and B-doped Si_1-xGe_x(0.15<x<0.85) films epitaxially grown at 550C on the Si(100) with a SiH₄-GeH₄-(PH₃ or B₂H₆)-H₂ gas mixture by using an ultraclean hot-wall LPCVD system. In the case of P doping, the P concentration increased up to a maximum value (around 210²⁰cm^-3) with increasing PH₃ partial pressure. In Si_1-xGe_x with the higher Ge fraction than 0.5, the electrically inactive P atoms were observed independently of the P concentration and the carrier concentration tended to saturate to about 10¹⁹cm^-3 at a higher P concentration up to 210²⁰cm^-3. This means that the P-doped Si_1-xGe_x film with the higher Ge fraction has the lower solid solubility of P. Thus, a lower Ge fraction x than 0.5 should be used for the lower electrical contact resistance between Si_1-xGe_x and metal. In the case of B doping, the B concentration increased nearly proportionally up to 10²²cm^-3 with increasing B₂H₆ partial pressure, nevertheless these films were single crystals. The carrier concentration was nearly equal to the B concentration up to about 210²⁰cm^-3, and it tended to saturate to about 510²⁰cm^-3 at a B concentration below 10²²cm^-3. Discrepancy of the lattice constants from Vegards law was observed at a higher B concentration in the order of 10²⁰cm^-3 and above, which corresponded with the saturation of the carrier concentration.

 

2:30 PM V2.4 
THE USE OF ELECTRON CHANNELING PATTERNS FOR PROCESS OPTIMIZATION OF LOW TEMPERATURE EPITAXIAL SILICON USING HOT WIRE CHEMICAL VAPOR DEPOSITION. R. Matson , R. Crandall, H. Mahan, E. Iwanikczo, National Renewable Energy Labaoratory, Golden, CO; J. Thiesen, U. of Colorado, Dept of E.E., Boulder, CO.

 

We demonstrate the first reported use of Electron Channeling Patterns - ECPs, for process optimization of epitaxial silicon. In an effort to fully characterize the new hot wire chemical vapor deposition -HWCVD, method of epitaxial growth recently discovered at the National Renewable Energy Laboratory - NREL, a large number of parameters with widely varying values needed to be addressed. To accomplish this we used the statistical design of experiments method. This allows one to limit the number of samples needed in parameter space reduction. The major problem with this technique in conjunction with epitaxial growth is, finding an effective quantitative analytical tool that allows one to perform the requisite statistical analysis. Transmission electron microscopy - TEM and secondary ion spectrometry - SIMS, could both be used for such work, but they are far too resource intensive to allow one to process the large number of samples produced in even in the moderated sample space. We will show how ECPs , can effectively be used to narrow the process space and to quickly and economically provide the process engineer with large amounts of information. When used in conjunction with other characterization techniques, the ECP technique then allows the epitaxial process space to be refined and the process optimized with the minimum waste of time and resources.

 

2:45 PM V2.5 
STABILIZATION OF ROCKSALT AlN IN EPITAXIAL AlN/TiN SUPERLATTICES AND ITS MECHANICAL PROPERTIES. Ilwon Kim , Murat U. Guruz, Vinayak Dravid, Anita Madan and Scott A. Barnett, Northwestern University, Department of Materials Science and Engineering, Evanston, IL; Harriet Kung and Michael A. Nastasi Materials Science and Technology Division, Los Alamos National Laboratory, Los Alamos, NM.

 

The epitaxial stabilization of high-pressure rocksalt phase of AlN in AlN/TiN superlattices was studied by varying the superlattice period (), AlN layer thickness (l_AlN), AlN layer thickness fraction (l_AlN/), and overall film thickness. Superlattices were grown by D.C. reactive magnetron sputtering on MgO (001) with , ranging from 1.8 nm to 9 nm and l_AlN/ varying from 0.1 to 0.8. For films with total thickness 100 nm, l_AlN/ 0.8, and l_AlN < 2 nm, strong satellite were observed in high- and low-angle x-ray diffraction (XRD) patterns indicating good planar layer structures. Dynamical and kinematical fits to the XRD patterns of these superlattices showed an AlN lattice constant close to that expected for the rocksalt structure. As l_AlN increased, superlattice reflections rapidly decreased in intensity, and an additional peak near wurtzite AlN (0002) appeared, suggesting that the AlN had undergone a transformation to the stable wurtzite structure. When the total thickness of the films was  1 m and l_AlN < 2 nm, good superlattice reflections were present only for l_AlN/ 0.5. This is probably because the coherency force by the MgO substrate diminishes as the total thickness of the film increases. Nanoindentation hardness and elastic modulus of the superlattices were measured. The superlattices with  2.5 nm showed a maximum hardness of  30 GPa. The mechanism of AlN phase transformation will be further analyzed using high resolution cross sectional transmission electron microscopy.

 

3:15 PM *V2.6 
CONNECTION BETWEEN STRUCTURE AND ELECTRONIC PROPERTIES IN EPITAXIAL MAGNETIC LAYERS. F.J. Himpsel , K.N. Altmann, J.A. Con Foo, J.F. Kelly, M. G. Lagally, J. McKay, W.L. O'Brien, D.Y. Petrovykh, University of Wisconsin, Madison, WI; J.E. Ortega, Univ. del Pais Vasco, San Sebastian, SPAIN.

 

This work explores the consequences of structure on the electronic properties of magnetic layer structures. The systems being studied are prototype structures for understanding oscillatory magnetic coupling, giant magnetoresistance (GMR), and magnetic tunneling. Such structures have found applications in sensors for reading heads of hard disks and in magnetic random access memory (MRAM). Epitaxial magnetic multilayers are grown in a new deposition system that makes it possible to deposit materials by sputter-deposition and by evaporation in the same system. This allows us to compare industrial fabrication with laboratory methods.