AVS2001 Session SC+SS-FrM: Growth and Epitaxy of Semiconductors

Friday, November 2, 2001 8:40 AM in Room 111

Friday Morning

Start	Invited?	Item
8:40 AM		SC+SS-FrM-2 STM Observations of the Initial Growth Process of Ge on Si(105) Surfaces: the Stability of Ge/Si(105) Surfaces Y. Fujikawa (University of Wisconsin-Madison); T. Sakurai (Tohoku University, Japan); M.G. Lagally (University of Wisconsin-Madison) The mechanism of the formation of self-organized SiGe quantum dots on the Si(100) surface has been intensively investigated because of the technological importance of achieving uniform size and high order in the dots. Although it has been demonstrated that the strain created by the lattice mismatch between Ge and Si plays an important role in the transition to 3D island growth and in 3D island ordering, there still is no satisfactory explanation of SiGe(105) facet formation, which creates the regular shape of the SiGe islands. We have used STM to observe the surface structures formed when Ge is deposited on Si(105) surfaces, in order to clarify the coverage dependence of the stability of the (105) surface. When the Ge coverage is greater than 0.5 ML, a p(2x1) reconstructed stable surface is observed, as previously reported.¹ At a coverage of ~ 0.5 ML or less, we observe formation of a p(4x1) structure over parts of the surface. The transformation to a p(4x1) structure is complete at 0.04 ML coverage. Decreasing the Ge coverage to 0.02 ML results in roughening of the Si(105) surface. During the transition from the p(2x1) to the p(4x1) structure, a significant change in surface morphology is observed. At coverages of 0.2 ML or less, the formation of the topmost layer tends to start from upper-step edges, while it starts from lower-step edges for coverages above 0.2 ML. As a result, defect arrays are frequently observed at the lower-step edges when the coverage is 0.2 ML or less, due to the incomplete formation of the topmost layer at the lower-step edges. This result clearly indicates the presence of increased strain on the Ge/Si(105) surface as the coverage of Ge is reduced. Ge thus stabilizes the surface. This work is partially supported by NSF and by the Ministry of Education, Culture, Sports, Science and Technology, Japan. ¹ Tomitori et al., Surf. Sci. 301 (1994) 214.
9:00 AM		SC+SS-FrM-3 Modulated Semiconductor Structures Studied by In Situ Scanning Tunneling Microscopy C.A. Pearson, C. Dorin, Y. Chen, J. Mirecki Millunchick, B.G. Orr (University of Michigan-Flint) Semiconductor structures known to exhibit compositional modulation are compared to similar structures that do not exhibit compositional modulation using in situ scanning tunneling microscopy. The structures are grown by molecular beam epitaxy on InP(001) following the growth of a lattice matched InGaAs buffer layer. InAs/GaAs and InAs/AlAs short period superlattices (SPSs) can be classified as having either rough or smooth interfaces. Rough interface SPSs exhibit compositional modulation. The buffer layer and smooth interface SPSs are structures that do not exhibit compositional modulation. Images of these structures are characterized by long wavelength surface undulations. This contrasts significantly from images of modulated structures, which show both long and short wavelength surface undulations. Another distinction is that the lattice planes on the modulated structures appear much more dendritic due to the short wavelength undulations. The relationship between the morphology of the buffer layer and the resulting SPS structure is discerned through these comparisons. Segregation of species is identified as well using scanning tunneling spectroscopy. Results from proximal probe experiments will increase the atomistic understanding of surface roughness and its role in compositional modulation.
9:20 AM		SC+SS-FrM-4 Scandium Gallium Nitride Alloys and ScN/GaN Heterostructures Grown by Molecular Beam Epitaxy A.R. Smith, H.A.H. Al-Brithen, D.C. Ingram (Ohio University) Scandium nitride is an interesting transitional metal nitride semiconductor with a simple crystal structure (rocksalt). Part of the interest in ScN is as a lower bandgap material (Eg = 2.1 eV) which can be combined with GaN to form novel heterostructures or alloys. In particular, GaN has a lattice constant close to ScN (both about 4.5Å ); it may therefore be possible to form an alloy with band gap in the range 2.1-3.4 eV. However, since ScN is rocksalt while GaN is zincblende or wurtzite, it is not clear that a simple alloy or interface can be formed. We have first investigated the growth of pure ScN on magnesium oxide (001) substrates by molecular beam epitaxy (MBE) using a RF-plasma source and a scandium effusion cell. The Sc/N flux ratio was found to be critical in determining the structural, optical, and electronic properties of the grown epitaxial layers. Under N-rich conditions, the growth is epitaxial, and we find atomically smooth terraces separated by steps having height a/2. Films grown N-rich are stoichiometric and transparent with a direct optical transition at 2.15 eV; further efforts are underway to experimentally verify the theoretically predicted indirect transition near 1 eV. We have begun to investigate the growth of GaN/ScN(001). First, we grew ScN(001) at growth temperature 850°C; next, we initiated GaN growth at reduced substrate temperature 550°C under Ga-rich conditions. After several minutes, the growth temperature was increased to 750°C, and the Ga flux was reduced but still maintaining Ga rich conditions. Reflection High Energy Electron Diffraction (RHEED) showed a pattern consistent with cubic GaN(001). X-ray diffraction showed only one peak beside the MgO substrate peak revealing that the GaN film is cubic having the same (002) orientation as the ScN(001) layer. A Sc-Ga-N layer was also grown on MgO(001) with growth temperature at 850°C. Structural and optical properties will be presented.
9:40 AM	Invited	SC+SS-FrM-5 Formation and Decay Processes of Silicon Mounds on Si(111) Surfaces A. Ichimiya, K. Hayashi (Nagoya University, Japan); E.D. Williams, T.L. Einstein (University of Maryland); M. Uwaha (Nagoya University, Japan) Two types of isolated single silicon pyramids on the Si(111) surfaces between 700K and 800K have been produced using an STM tip. Pyramids of 75% are normal stacking at the interface between the pyramid and the substrate (called type U). For pyramids of 25% which are in the twin relation of the type U pyramids, there is a stacking fault at the interface (called type F). The pyramids have certain facets just after the creation. Indices of main facets of the mounds on the Si(111) surface are {311}, and small facets are {221} for the type U and {331} for the type F. Just after the production, the pyramid begins to decompose. During the decay of the type U pyramid, the facets of the pyramid transform into multi-bilayer steps. Finally the mound becomes a bilayer mound (2D) with a truncated triangle shape. The longer edges of the 2D mounds are along the unfaulted halves of the DAS structure of the substrate, while these edges are the shorter one of the pyramid. The decay process of the type F pyramid is layer-by-layer without step bunching. Decay rates of the type F pyramids are about 3 times larger than those of the type U due to the stacking. The energy state of the type F pyramids is about 0.06eV higher than that of the type U. We have measured height evolution of the pyramids during decay, and found that the height decay is due to the power law , t^1/4, where t is the decay time. Such the simple law is predicted by Israeli and Kandel¹ for infinite cone decay. It should be noted that such the simple law is in very good agreement with the experimental results of silicon pyramid decay. The results are explained by attachment-detachment limited processes at step edges. The measured decay rates are in very good agreement with decay rates which are evaluated from those of 2D mounds.² ¹ N. Israeli and D. Kandel, Phys. Rev. Lett. 80, 556 (1998. ² A. Ichimiya, Y. Tanaka and K. Ishiyama, Phys. Rev. Lett. 76, 4721 (1996).
10:20 AM		SC+SS-FrM-7 Role of Surface Steps in the Arrangement of Silicon Nano-dots on Vicinal Si(111) Surfaces: STM Investigation J.S. Ha, K.-H. Park, Y.-J. Ko, K. Park (ETRI, Republic of Korea) Regular array of nano-structures such as dots and wires can be obtained by elaborate control of chemical reactions on stepped surfaces. In this work, we have investigated the role of surface steps in the arrangement of silicon nano-dots on vicinal Si(111) surfaces by scanning tunneling microscopy(STM). Nanometer sized silicon nitride islands were formed on a vicinal Si(111) surface, which was 1° off toward [112] direction, via thermal nitridation using N₂ gas. On the nitrided surface, oxygen gas was dosed at 700 °C to induce a local selective etching of silicon using silicon nitride islands as masks. The resultant surface showed one-dimensional arrangement of silicon nano-dots along the step edges of the silicon surface. The lateral size of the dot in the direction perpendicular to the step edges was restricted to the terrace width of the stepped Si(111) surface. Preferential growth of silicon nitride islands on the edges of single height steps is considered to be responsible for the arrangement of silicon nano-dots along the step edges of the 1 ° off vicinal Si(111) surface. Furthermore, comparative STM studies to investigate the role of surface steps, which were done on Si(111) surfaces with miscut angles of 0.1° and 4° off toward [110] direction, will be also discussed.
10:40 AM		SC+SS-FrM-8 Thermal Growth of High-density Silicon Quantum Dots on SiO₂/Si(001) with Submonolayer Silicon Adatom Predeposition J.G. Ekerdt, J.H. Zhu, W.T. Leach (University of Texas at Austin) A nonthermal method to facilitate nucleation and subsequent thermal chemical vapor deposition of Si quantum dots on SiO₂/Si(100) with high density and uniform size is demonstrated. Submonolayers (0.12 to 0.02 ML) of Si adatoms are predeposited on a room temperature to 825 K SiO₂/Si(001) substrate by cracking disilane on a high-temperature filament in a UHV chamber at pressures on the order of 10^-7 Torr. The quantum dots are grown at 825 K with a disilane pressure of 2x10^-6super Torr. The Si quantum dot density is increased and size distribution is narrowed by predeposition of Si adatoms when compared to thermal growth on bare SiO₂/Si(100). The dot density and size is controlled by the amount of Si adatom predeposition; 9.5x10¹¹ cm^-2 density and 5.5 nm size are demonstrated on SiO₂/Si(001). A surface kinetic model is also presented that accounts for dot nucleation, adatom formation during growth, diffusion of adatoms to growing dots and epitaxial growth of dots. The method provides an effective way to control the nucleation, and consequently the growth, of quantum dots on dielectric surfaces.
11:00 AM		SC+SS-FrM-9 Nucleationless Island Formation in SiGe/Si(100) Heteroepitaxy P. Zahl, P.W. Sutter (Colorado School of Mines) The self-assembly of quantum dot (QD) islands in lattice-mismatched heteroepitaxy has been studied extensively. One of the objectives of recent research is the creation of long-range ordered arrays of QDs of uniform size, a major technological milestone that would pave the way for application of these nanostructures in electronic and optoelectronic devices. The formation of epitaxial QD islands is generally assumed to involve nucleation, a statistical process that would severely impede QD organization. Our recent observations by low-energy electron microscopy, however, have demonstrated that QD islands in heteroepitaxial systems such as SiGe/Si(100) can self-assemble in a coninuous process that avoids nucleation.¹ With the observed nucleationless islanding, perfect periodic arrays of uniform QD islands may form spontaneously under suitable growth conditions or may be induced by weakly invasive growth modifiers that enhance existing ordering forces. We present a systematic study of the evolution of SiGe QD islands on Si(100) by combining growth with in-situ scanning tunneling microscopy. Our microscopic observations are analyzed to detect island interaction mechanisms that may act as driving forces for spontaneous spatial ordering in the nucleationless islanding process. First results on the use of growth modifiers to enhance or induce long-range order will be presented. ¹ P. Sutter and M. G. Lagally; Phys. Rev. Lett., 84(20):4637-4640, 2000.
11:20 AM		SC+SS-FrM-10 Electronic Structure of a Heteroepitaxially-Passivated Si(111) Surface: GaSe-terminated Si(111) M.A. Olmstead (University of Washington); R. Rudolf, C. Pettenkofer (Hahn-Meitner Inst., Germany); A.A. Bostwick, J.A. Adams (University of Washington); E. Rotenberg (Advanced Light Source, Berkeley); F.S. Ohuchi (University of Washington); R. Fritsche, A. Klein, W. Jaegermann (Tech. Univ. Darmstadt, Germany) Dissimilar materials heteroepitaxy often results in an interface reaction that passivates substrate dangling bonds. This forms a low energy surface that promotes islanded, rather than laminar, growth of the subsequent overlayer. GaSe-terminated Si(111) is an example of such a stable, low-diffusion barrier surface with no states in the Si band gap; it may serve as an ideal substrate for quantum dot growth on Si(111). We have performed detailed studies of the electronic and atomic structure of GaSe-terminated S i(111) using energy-dependent and angle-resolved valence band and core-level photoemission spectroscopy. The surface termination consists of a Ga layer bonded to the top-most Si atoms and a Se layer on top with each Se atom bonded to three Ga. Our photoelectron diffraction results show the Ga and Se atomic arrangement is identical to that of a half-sheet of the layered semiconductor GaSe. The surface thus contains no dangling bonds, but has fully-occupied lone-pair states similar to As-terminated Si(111). The observed dispersion of the energy bands is very close to those of a GaSe single crystal and is interpreted in terms of tight binding energy states. The dominance of the GaSe-derived states over those of Si-derived states is obvious and is different from As-terminated Si.
11:40 AM		SC+SS-FrM-11 Surface Reaction Study of TaN Growth from MOCVD Precursors on Cu(111) J.-B. Wu (National Chiao-Tung University, Taiwan); Y.W. Yang (Synchrotron Radiation Research Center, Taiwan); Y.-F. Lin, H.-T. Chiu (National Chiao-Tung University, Taiwan) Understanding the reaction pathway followed by MOCVD precursors during thin film growth is a scientifically challenging problem. Transition metal nitride films, due to their wide applications in semiconductor processing, e.g., in areas like diffusion barrier materials, have been extensively studied. However, the knowledge about detailed surface nitride chemistry has been relatively scarce. Here, we report on surface reaction studies of TaN CVD precursors carried out by using high-resolution XPS, TDS, and NEXAFS techniques. The precursors studied include some custom-synthesized compounds (^tBuN)Ta(NEt₂)₃ and (^tBuN)Ta(NEtMe)₃ that are characterized by the presence of absence of β-methyl groups and by whether singly or doubly-bonded nitrogen being attached to a Ta atom. The β-methyl group is known to influence the decomposition pathway of amido group, which in turn can determine the amount of carbon being incorporated into the films. XPS data show that the formation of nitride species starts at surface temperatures higher than 500 K and, as the reaction proceeds, graphic carbons are incorporated into TaN species and significant Ta oxides are formed due to favorable exothermicity. During the course of surface decomposition, multimass TDS data point to the evolution of hydrogen, hydrocarbon species, and possibly acetonitrile. Oxidation resistance and degree of carbon incorporation are found to vary with the precursors. Based on these results, possible reaction mechanisms and the effect of precursor are to be discussed.