AVS2001 Session SC-TuA: Semiconductor Heterojunctions

Tuesday, October 30, 2001 2:00 PM in Room 124

Tuesday Afternoon

Start	Invited?	Item
2:00 PM	Invited	SC-TuA-1 Characterization of SiGe/Si Heterostructures with Abrupt Interfaces Y. Shiraki (The University of Tokyo, Japan)
2:40 PM		SC-TuA-3 Indium Segregation and its Effect on Interfacial Bonding at the GaSb-on-InAs Heterojunction: A Cross-Sectional Scanning Tunneling Microscopy Study¹ J. Steinshnider, M. Weimer (Texas A&M University); E.M. Shaw, Z. Mi, T.C. Hasenberg (University of Iowa); R. Kaspi (Air Force Research Laboratory) We describe how scanning tunneling microscopy (STM) may be used to determine the chemical composition across the nearly-lattice-matched, non-common-atom GaSb-on-InAs heterojunction with atomic-scale precision. An ideal, compositionally-abrupt GaSb-on-InAs interface is composed of either InSb-like or GaAs-like bonds whose character is easily distinguished with STM.² Indium segregation, on the other hand, leads to compositional grading within the subsequent GaSb layers that compromises interfacial abruptness. We have quantified the indium fraction in successive gallium planes and find this compositional grading is described by the same microscopic model previously applied to antimony segregation at the InAs-on-GaSb interface.³ We discuss how indium segregation at the GaSb-on-InAs heterojunction is linked with the surface reconstruction of the underlying InAs template and consider the effect of this segregation on the interfacial bonding. ¹Work supported by the National Science Foundation (Division of Materials Research) and Air Force Research Laboratory ²J. Steinshnider, M. Weimer, R. Kaspi, and G.W. Turner, Phys. Rev. Lett., 85, 2953 (2000). ³J. Steinshnider, J. Harper, M. Weimer, C.-H. Lin, S.S. Pei, and D.H. Chow, Phys. Rev. Lett., 85, 4562 (2000).
3:00 PM	Invited	SC-TuA-4 Dislocations and Microstructure Evolution in Semiconductor Thin Films A. Sakai (Nagoya University, Japan) The utilization of high-quality heteroepitaxial films is the key to realizing high performance optoelectronic and electronic semiconductor devices. In general, a lattice mismatch between a heteroepitaxial film and a substrate induces strain into the film and the strain relaxation is achieved by the introduction of misfit dislocations. This, in most cases, results in threading dislocations in the film, which severely degrade the properties required for the device operation. In order to reduce the threading dislocation density, we have performed novel heteroepitaxy which is based on the idea that misfit dislocations are confined at the hetero interface regions without leaving their threading arms in the film. Two successful demonstrations for GaN and SiGe thin films are presented. 1) Facet-initiated epitaxial lateral overgrowth (FIELO) allows us to obtain GaN films on sapphire substrates with threading dislocation densities on the order of 10⁷ cm^-2 which is two orders of magnitude smaller than that of the conventional epitaxy. Transmission electron microscopy analyses revealed that the reduction of the threading dislocation density was mainly due to dislocation bending in the FIELO GaN layer. Mechanisms of dislocation propagation which is closely related to the appearance of the facets early in ELO are discussed. 2) Strain-relaxed SiGe buffer layers on Si(001) substrates with low threading dislocation densities have been grown. The process consists of annealing of the first low-temperature-grown SiGe layer and growth of the second SiGe layer on the first layer. A thin capping Si layer formed before the annealing effectively suppressed surface roughening during the annealing. Periodic undulation was formed on the second layer surface, conformably to the alignment of interface misfit dislocations. This undulation plays an important role in introducing the dislocations uniformly and in suppressing the entanglement of threading arms of the dislocations.
3:40 PM		SC-TuA-6 The Strain Relaxation Mechanism of SiGe Growth with a Low Temperature Si Buffer Layer by Molecular-beam Epitaxy S.W. Lee (National Tsing Hua University, Taiwan, R.O.C.); Y.H. Peng (National Taiwan University, Taiwan, R.O.C.); H.C. Chen (National Tsing Hua University, Taiwan, R.O.C.); H.H. Cheng, C.H. Kuan (National Taiwan University, Taiwan, R.O.C.); L.J. Chen (National Tsing Hua University, Taiwan, R.O.C.) Recently, the use of the low temperature Si (LT-Si) buffer layer to achieve dislocation-free SiGe films was found to be effective to share the mismatch strain in epilayers. However, the mechanism of strain relaxation in a LT-Si buffer layer has not been well understood. In the present work, the growth of 300-nm-thick Si_0.7Ge_0.3 films with a LT-Si buffer layer grown at 550°C~350°C and with thickness of 50nm~250nm have been carried out by molecular-beam epitaxy. The SiGe films were characterized by transmission electron microscopy (TEM), double-axis x-ray diffraction (DAXRD), atomic force microscopy (AFM) and photoluminescence (PL). From DAXRD measurement, Si_0.7Ge_0.3 films with a 100-nm-thick LT-Si buffer layer grown at different temperatures were found to be fully relaxed (100%). However, Si_0.7Ge_0.3 films became partially relaxed with increased thickness of LT-Si buffer layers. From cross-section TEM (XTEM) observation, the microstructures of LT-Si buffer layers change with deposition temperature and thickness of LT-Si layers. XTEM images showed that the distribution of dislocations formed in the LT-Si buffer layer is correlated with the degree of relaxation. The strain relaxation mechanism is explained in terms of the compliant effect of LT-Si buffer. A novel method based on this mechanism using a thin Ge layer interposed below the LT-Si buffer layer for Si_0.7Ge_0.3 growth is demonstrated. The interposed Ge layer plays a critical role in leading the misfit dislocations to transverse along the LT-Si/Si interface. Controlling misfit dislocations in LT-Si buffers was achieved. The interposed Ge layer was expected to promote the relaxation of the top SiGe films.
4:00 PM	Invited	SC-TuA-7 Heteroepitaxy of Highly Mismatched Systems and the Role of Coincidence Lattice K.H. Ploog (Paul Drude Institute for Solid State Electronics, Germany) While till mid 1980 a good lattice match of substrate and constituent layers in most semiconductor heterostructures was considered to be mandatory for successful device operation, this constraint has since become more relaxed. Today not only semiconductor materials with considerable lattice mismatch are explored in devices, but also heterostructures combining materials very dissimilar in structure, bonding, and chemical properties play an ever increasing role in the development of novel device concepts. In the heteroepitaxy of such highly mismatched systems, the existence of a "coincidence" lattice at the interface often leads to a unique epitaxial alignment and misfit accommodation in the early stages of epitaxy. This structural coincidence between the adjacent lattices helps to generate a low-energy interface. Using functional selforganized molecular beam epitaxy (MBE), even the epitaxy of metastable phases (like cubic GaN-on-GaAs), of M-plane oriented GaN[GaN(1-100) on g-LiAlO2(100)], and of layers with a symmetry different from the substrate (like hexagonal MnAs on cubic GaAs) can be obtained and the resulting nanostructures at the respective interface can be controlled in a reproducible manner. The M-plane group-III nitride heterostructures are of great importance for highly efficient blue/UV light emitters, and ferromagnetic MnAs on GaAs heterostructures are paving the way to spin-electronics operating at room temperature.
4:40 PM		SC-TuA-9 Nanoscale Dislocation Patterning in PbTe/PbSe (001) Lattice-mismatched Heteroepitaxy G. Springholz, K. Wiesauer (University of Linz, Austria) Molecular beam epitaxy of PbTe on 5.2% lattice-mismatched PbSe (100) is studied using scanning tunneling microscopy. It is found that at a critical thickness of 0.8 monolayers, pure edge type misfit dislocations are formed at the layer/substrate interface. In the STM images these misfit dislocations appear as dark lines that run over the epitaxial surface along the four-fold <011> directions. From atomically resolved lattice images, the dislocation Burgers vector is found to be ½<001> parallel to the interface. This unusual misfit dislocation configuration is explained by the fact that the dislocation are formed by climb rather than glide processes. From detailed investigations of the early relaxation stages, we find that all misfit dislocations are all injected from monolayer step edges on the surface, which greatly reduces the nucleation barrier of dislocation half loops. As the layer thickness increases, the dislocation density drastically increases and a nearly perfect quadratic grid of dislocations with an average spacing of 10 nm is formed, indicating that at 10 ML more than 90% of the misfit strain is relaxed. In addition, the course of strain relaxation is found to be in well agreement with the Frank-van-der-Merwe model. This surprising result is explained by the reduction of the dislocation nucleation barrier by the edge injection mechanism. The remarkable uniformity of the dislocation array is evidenced by the appearance of satellite peaks in the FFT power spectra of the STM images due to the dislocation superstructure. From a statistical analysis we find a variation of the lateral dislocation spacing of only 12%, which is better than the typical size uniformity of self-assembled quantum dots. Thus, these structures could serve as templates for the deposition of self-organized ordered nanostructures.
5:00 PM		SC-TuA-10 STM-Controlled Epitaxy of Cobalt-Semiconductor Compounds I. Goldfarb (Tel Aviv University, Israel); G.A.D. Briggs (Oxford University, UK) Metal-semiconductor compounds play a key role in micro- and optoelectronic devices, mostly as contacts and interconnects. At present, the most popular are the silicides of Ti, and even more so of Co (due to its suitability for self aligned process). However both CoSi₂ and TiSi₂ are usually used in a form of polycrystalline thin films. While monocrystalline epitaxial growth of TiSi₂ on silicon is impeded by its large lattice mismatch between them, it could have been expected for the CoSi₂ in view of the latter's low mismatch with silicon. Obviously, monocrystalline contacts with improved electrical characteristics are highly desirable. Yet, CoSi₂ does not grow on silicon as a moncrystalline two-dimensional layer, at least not on the Si(001) surface, where CoSi₂ forms misoriented three-dimensional islands. CoGe₂ is another interesting metal-semiconductor compound that can be used, for example, as contacts to SiGe alloys and GaAs. CoGe₂ forms three-dimensional islands on Ge/Si(001), which are very similar to the CoSi₂ ones on Si(001). In this work we investigate the mechanisms of CoSi₂ and CoGe₂ growth by carefully-controlled e-beam evaporation of Co onto Si(001) substrates, as monitored in situ, from the very initial submonolayer stages, by scanning tunneling microscopy (STM) and reflection high-energy electron diffraction (RHEED). In order to affect the resultant epilayer morphologies, we use flat and vicinal surfaces, and two different ways of synthesis: reactive deposition (where Co is deposited onto hot substrate), and solid-phase reaction (where Co is deposited at lower, or room temperature). We attempt to account for the observed morphological differences of the epilayers by correlating them with the above-mentioned parametric differences.
5:20 PM		SC-TuA-11 Electron-beam Patterning of Cobalt Fluoride on 10-nm Length Scale M. Malac, Y. Zhu, M. Schofield (Brookhaven National Laboratory) Electron-beam modification of a precursor material can be utilized to fabricate metallic structures on single-digit nanometer dimensions. Whereas, reliable fabrication of magnetic nanostructures is essential for study of fundamental processes in magnetism, cobalt fluoride (CoF2) precursor is a candidate for such fabrication of magnetic (cobalt) nanostructures. We have studied in situ electron beam patterning of CoF2 using a JEOL 3000F transmission electron microscope. The microscope is equipped with a Gatan Image Filter allowing for electron-energy-loss chemical analysis, and equipped for electron holography for mapping of magnetic fields. Our results on electron beam (<10 nm probe) modification of cobalt fluoride show that electron dose typically of 900 C/cm2 is needed for complete removal of fluorine at temperature 570 K. However, cobalt particles about 3 nm in diameter start to form at electron doses on the order of 200 C/cm2. Nucleation of cobalt particles initiates at the grain boundaries of the CoF2 precursor. The Co particles grow during exposure resulting, after complete exposure, in structures composed of separated, faceted cobalt particles typically 5 - 15 nm in size. The cobalt particles are either c-axis parallel or perpendicular to the substrate plane. Elevated sample temperature during exposure was necessary to eliminate buildup of microscope-related contamination. Cooling the sample to liquid nitrogen temperature during exposure also resulted in elimination of microscope contamination, but the resulting cobalt structures were composed of individual, separated particles. We believe that the non-continuous nature of the final cobalt structures stems from the surface energetics of high surface energy metal (cobalt) on low surface energy substrate (amorphous carbon). A real-time high-resolution TEM movie of the exposure process will be presented to provide insight to the exposure process.