Papers

AVS1996 Session TF-ThA: Semiconductor Thin Films

Thursday, October 17, 1996 1:30 PM in Room 107B

Thursday Afternoon

Start	Invited?	Item
1:30 PM		TF-ThA-1 The Influence of Strain on Semiconductor Thin Film Epitaxy E. Fitzgerald, S. Samavedam, L. Giovane, M. Bulsara (Massachusetts Institute of Technology) Under typical semiconductor thin film growth conditions, strains at levels \>=\10\super -4\ are shown to influence thin film surface morphology, strain relaxation pathways, and dislocation morphologies in relaxed films. The sign of the strain in the film (i.e. compressive or tensile) as well as the magnitude have a tremendous influence on the film characteristics. We elucidate the myriad of microstructures that can form using the lattice-mismatched GeSi/Si and InGaAs/GaAs semiconductor systems. In relaxed GeSi/Si structures, we show that threading dislocation densities greater than 10\super 5\ cm\super -2\ are created from dislocation blocking (i.e. work hardening) due to a combination of surface morphology and dislocation morphology. In addition, three surface length scales can be identified which are related to different sources of strain: long wavelengths or undulations that are related to deeply-buried misfit dislocations; a more modest length scale related to either misfit dislocations lying close to the surface or threading dislocations intercepting the surface; and short ripples which are related to the sign and magnitude of unrelieved lattice-mismatch strain. Relaxed heterostructures in the InGaAs/GaAs system show similar features, with the additional complication that the strain-induced surface morphologies will lead to microscopic phase separation at higher growth temperature. InGaAs strained quantum wells grown on relaxed graded InGaAs/GaAs show a strong spatial-dependent luminescence correlated to the strain fields from the dislocations in the buried relaxed graded layer. Such quantum wells exhibit extremely luminescent wire-like features which are easily viewed with cathodoluminescence imaging. In both the GeSi/Si and InGaAs/GaAs systems, we emphasize the importance of controlling dislocation and surface morphology in optoelectronic devices which are fabricated from these materials.
2:10 PM		TF-ThA-3 Evolution of the Surface Morphology of MBE Grown Tensile Strain Si\sub 0\\sub .\\sub 5\Ge\sub 0\\sub .\\sub 5\/Ge(001) J. Van Nostrand, S. Chey, D. Cahill, I. Petrof, J. Greene (University of Illinois, Urbana) Significant surface roughening is commonly observed in compressively strained Si\sub x\Ge\sub 1-x\/Si(001) and In\sub x\Ga\sub 1-x\As/GaAs(001) thin films grown by MBE. Heteroepitaxy under tensile strain has been observed to be more stable (Xie et al., 1994). To better understand the interplay between surface morphology, strain, and the formation of extended defects, we use in-situ STM and ex-situ AFM to study the surface morphology of Si\sub 0.5\Ge\sub 0.5\/Ge(001) films (2% tensile strain) grown by MBE as a function of growth temperature (325\super o\C-500\super o\C), film thickness (5nm-10nm), and annealing temperature. At a deposition temperature of 325\super o\C, 5nm thick Si\sub 0.5\Ge\sub 0.5\/Ge(001) films exhibit a stacking fault density 10\mu\m\super -1\ determined by STM; a stacking fault intersecting the film surface produces a 90 degree rotation of the dimer rows on adjacent sides of the stacking fault. The Burgers vector are confirmed to be of 1/6<211> type by TEM. Tensile strain films show an increase in the density of antiphase boundaries, and therefore surface roughness, in comparison to homoepitaxial deposition. Appearance of significant roughening is observed at a deposition temperature of 400\super o\C for t\>=\ 10nm in the form of multilayer mounds. Annealing of 5nm Si\sub 0.5\Ge\sub 0.5\/Ge(001) films deposited at 400\super o\C at 625\super o\C enhances multilayer mound formation. The surface morphology is coupled to the locations of stacking faults. 10nm thick films annealed at 625\super o\C for 2 minutes produce 6\mu\m\super -2\ multilayer mounds that rise 5-15nm. Increasing the annealing temperature to 700\super o\C produces an ordered arrangement of rectangular pits, 185nm across.
2:30 PM		TF-ThA-4 Boron Segregation with and without Atomic Hydrogen during the Growth of Boron Doped Layers on Si(110), Si(100), and Si(111) C. Silvestre, G. Jernigan, P. Thompson (Naval Research Laboratory); D. Simons (National Institute of Standards & Technology) A series of 3 nm silicon layers, boron doped (2 x 10\super 19\ B/cm\super 3\ ), separated by 200 nm of undoped silicon were grown by solid source MBE on Si(110), Si(100), and Si(111) substrates at temperatures of 500 and 600C. These growths were done with and without an atomic hydrogen overpressure of 1.3 x 10\super -4\ Pa. Further, at 600C, without atomic hydrogen, the Si(110) showed an exponential boron tail toward the surface with a 22 nm decay length. Exposure to atomic hydrogen reduced this decay length to 13 nm, a decrease of 40%. Contrary to other reports in the literature,[1] no exponential tail in boron concentration was seen on Si(100) at either 500 or 600C. No boron segregation was seen on Si(111) at either temperature. The measurements of SIMS boron layer FWHM showed no significant difference between the Si(100) and Si(111) surfaces. For both surfaces at 500=B0C atomic hydrogen produced a FWHM of 7 nm. However, the Si(110) surface boron layer FWHM was found to be 14 nm when grown under the same conditions. [1]. Jorke, H. & Kibbel, H., APL 57, 1763(1990).
2:50 PM		TF-ThA-5 Low Temperature Polycrystalline Silicon Film Formation Technology by a Harmonization with Sputtering and Plasma CVD using a Low Kinetic Energy Particle Process G. Jong, S. Woo, W. Park, M. Takeya, Y. Jang, T. Ohmi (Tohoku University, Japan) Low kinetic energy particle process based on ultraclean technology has been successfully applied to low temperature polycrystalline silicon (poly-Si) formation for the fabrication of TFT-LCDs. By the precise control of plasma parameter and the catalytic contributions of hydrogen radicals, we have performed the growth of poly-Si films at 300$B!n(J using both sputtering and plasma CVD. The process conditions are as follows; input rf power and excitation frequency is 150W and 182.5MHz, working pressure is 10mTorr. From the Langmuir probe measurement, the mean ion energy of Ar and flux density impinging to the growing film surface is $B!A(J20eV and $B!A(J6$B!_(J10\super 16\/cm\super 2\, respectively. The crystallinity of the films has been characterized by RED and XRD. Good crystallinity films are obtained when hydrogen/Ar gas mixture ratio is 20-30%. The grain size were estimated to be $B!A(J200\Ao\ at the film thickness of 1000\Ao\. The resistivity of doped films decreases as hydrogen gas mixture ratio increase up to 20%. Hall mobility has been measured by Van der Pauw method and is up to about 10cm\super 2\/Vs. The hydrogen content in the films estimated by ir absorption spectra has been found to decrease abruptly as crystallization of the films is promoted when hydrogen gas mixture ratio is over 10%. It could be considered that the reaction precursors on the growing film surface can easily migrate due to the hydrogen passivation and the low kinetic energy ion bombardment, which results in the promotion of the film crystallization at low temperature.
3:10 PM		TF-ThA-6 The Effects of Ion Beam Pretreatment of SiO\sub 2\ Surfaces on the Nucleation and Surface Roughness of Polycrystalline Films Produced by Rapid Thermal Chemical Vapor Deposition C. Basa, Y. Hu, E. Irene, C. Zhao, W. Gao (University of North Carolina, Chapel Hill) The condition of a surface is crucial in low temperature silicon (Si) epitaxy. There have been many studies on surface pretreatments of Si surfaces on Si epitaxy, however, the effects of surface pretreatments on SiO\sub 2\ used in polycrystalline silicon (poly-si) growth are not well known. This research addresses the effects of SiO\sub 2\ surface pretreatments on poly-si nucleation and film surface roughness using low energy ion beams (200eV) of various ions (hydrogen, nitrogen, and helium) and gas (H\sub 2\, N\sub 2\, and He) annealing (850\super o\C) in a rapid thermal chemical vapor deposition sytem. In-situ real-time ellipsometry was used to monitor the surfaces during pretreatment and observe the nucleation. The microstructure and surface roughness of the deposited poly-si films are determined by analysis of in-situ spectro- scopic ellipsometry (SE) and atomic force microscopy (AFM) measurements. Ion beam pretreatments were found to produce higher nuclei density and smoother poly-si surface than non-pretreated substrates, and the oppposite was found for gas annealing giving lower nuclei density and rougher poly-si.
3:30 PM		TF-ThA-7 Microstructural Properties of Polycrystalline Silicon Solid-Phase-Crystallized from a-Si:H on Glass: The Effects of a Silicon Nitride Layer between a-Si and Glass Y. Wang, O. Awadelkarim, S. Fonash (Pennsylvania State University) Polycrystalline silicon (poly-Si) thin films with large grains and smooth surfaces are looked upon as promising materials for thin film transistor fabrication for flat panel display applications. In this study, 100nm-thick a-Si:H deposited from hydr ogen diluted silane below 320 degree C were used as precursor materials to produce poly-Si by the solid phase crystallization (SPC) technique of furnace annealing at 600 degree C. The precursor a-Si was PECVD deposited on either bare or 100-nm-silicon nit ride coated Corning 7059 glass substrates. The SPC poly-Si films were characterized using ultraviolet reflectance spectrometry to determine the annealing time for crystallization, transmission electron microscopy for grain size measurement, and atomic for ce microscopy (AFM) for surface roughness analysis. For a-Si deposition temperatures between 100 and 320 degree C, it was observed that the lower the deposition temperature the larger the average grain size in the SPC poly-Si film. This was the case irres pective of whether the a-Si was deposited on bare glass substrates or substrates coated with nitride. However the latter films needed longer annealing times, when compared with the former films, to full crystallize. Also, upon full crystallization, a-Si d eposited on bare glass was observed to yield grains that are larger than those in a-Si deposited in nitride coated glass. In addition, AFM analysis of the surface roughness in the poly-Si films have revealed that the surface of the poly-Si-on-nitride film was significantly smoother than the surface of the corresponding poly-Si film on bare glass. These marked differences in poly-Si film qualities are explained in terms of dissimilar Si/glass interface and Si/nitride interface properties.
3:50 PM		TF-ThA-8 Effect of Substrate Structure on CdTe OMVPE K. Yong, A. Gellman, P. Sides (Carnegie Mellon University) The effect of substrate misorientation on surface morphology and growth rate of CdTe epitaxial films deposited by organometallic vapor phase epitaxy (OMVPE) has been investigated by deposition onto CdTe(100) surfaces misoriented by 0\super o\, 2\super o\, 4\super o\, 6\super o\, and 8\super o\ toward the (111)\sub Te\ plane. Growth temperatures of 337, 370, 400, 425 \super o\C were used. Surfaces of CdTe epilayers grown on CdTe(100) had a high density of pyramidal hillocks. A remarkable improvement of surface morphology was observed when the epilayers were deposited on misoriented substrates. Hillock formation was suppressed and a mirror-like surface was observed for misorientation of 4\super o\. At higher misorientation angles the morphology was degraded and a different type of hillock was observed. When the growth temperature was increased, the sizes of hillocks decreased and the apparent number density of hillocks increased. A mirror-like finish was also observed for misorientation of 6\super o\ when the growth temperature was above 400 \super o\C. The film growth rate increased with increasing misorientation angles. The apparent activation energy was 15.2 kcal/mol for the growth on CdTe(100) and 13.6 ~ 14.6 kcal/mol on misoriented CdTe(100). The structure of the epilayers with a mirror-like surface has been studied using a cross-sectional transmission electron microscopy (TEM) and low energy electron diffraction (LEED).
4:10 PM		TF-ThA-9 The Effect of Temperature Profiling Techniques on the Properties of CdTe Thin Films P. Sheldon, X. Li, D. Rose, R. Dhere, H. Moutinho, R. Matson (National Renewable Energy Laboratory) We describe a substrate temperature profiling technique applied to CdTe thin films deposited by close-spaced sublimation. Previous work has shown that increased substrate temperatures (T\sub sub\) yield improved material properties, especially for temperatures \>=\600#161#C. However, soda-lime substrates, typically used for photovoltaic applications, can not tolerate these temperatures for extended periods of time. In order to maintain the advantages associated with higher T\sub sub\ and remain below the softening point of soda-lime glass, we have developed modified substrate temperature profiles with a reduced thermal budget. These profiles incorporate a brief, high temperature nucleation segment (t\<\1 min., T\sub n\ \>\ 600#161#C) after which the bulk of the film is deposited at lower temperatures. When T\sub n\ is increased from 620 to 645#161#C, films show a significant increase in grain faceting. For films that are deposited with a high T\sub n\ layer and the bulk of the film deposited at 600#161#C, we find that we can retain many of the benefits associated with the sustained high-temperature depositions. For example, AFM results show that, for films deposited at 600#161#C, increasing T\sub n\ from 625 to 645#161#C yielded a 100% increase in grain size. We have demonstrated that by optimizing the nucleation temperature profile, we can achieve CdTe films with increased grain sizes, reduced defect densities and improved electro-optical properties.
4:30 PM		TF-ThA-10 Highly Oriented Crystalline 2H-WS\sub 2\ Thin Films by Metal Organic Chemical Vapor Deposition J. Chung, K. Adib, F. Ohuchi (University of Washington) WS\sub 2\ is a two dimensional layered material with its layers parallel to the basal plane of its hexagonal structure. It has a lattice parameter of a=3.15 \Ao\ which has a mismatch of only 1.2% with AlN and GaN (a=3.11 \Ao\ , a=3.19 \Ao\, respectively). Suitably oriented WS\sub 2\ thin films are ideal candidates for use as substrates for the growth of III-nitride semiconductor thin films. We report depositing thin films of WS\sub 2\ using metal organic chemical vapor deposition (MOCVD). The precursors used were W(CO)\sub 6\ and H\sub 2\S. The thin films were grown on Si substrates using a variety of deposition pressures, precursor ratios and substrate temperatures. X-ray diffraction of the deposited films shows that the films are oriented with their (0002) crystal planes parallel to the substrate surface. The highly oriented crystals were obtained at substrate temperatures of 583K-723K at low reactor pressures. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) of the grown film show that the layered crystalline films are highly oriented and uniform. X-ray photoelectron spectroscopy (XPS) of the films reveals that the S/W ratio in the films is less than 2.0 indicating a need for subsequent compositional adjustment. The effects of reactor pressure, substrate temperature, precursor ratio and subsequent annealing on the growth rate, crystallinity and the crystal orientations of the films were investigated. It is observed that carbon contamination of the films is minimal compared to thermodynamic predictions. -------- * Research supported by AFOSR under grant F49620-95-1-0142