Currently I am working on iron (Fe) / silicon (Si) multilayers grown on SiO2 (glass) and on MgO (substrates). Specifically, I am using molecular beam epitaxy (MBE) in a reflection high energy diffraction (RHEED) chamber operating in the 10-10 Torr regime (ultra high vacuum). Dr. John Carlisle and I are investigating the properties of these multilayers as a function of Si layer thickness, substrate, and temperature. We will spend the first full week of August 1998 at the Advanced Light Source, beamline 8.0, performing fluorescent spectroscopy on these multilayers. Specifically of interest to us is the composition of the Si spacer layers. We would like to be able to control the diffusion of the Si into the Fe layers by variation of the parameters stated above. Previous experiments* by Dr. Carlisle and others have been performed on samples that were created by ion beam sputtering techniques, instead of MBE, which promises to allow much better control of the Si/Fe diffusion.
During the spring semester I worked breifly on Fe thin films on Si (111) 7x7 reconstruction. A paper on that subject follows. It is out intention to study Fe thin films on the Si (100) surface as well, as time allows.
*Carlisle, J. A., Chaiken, A., Michel, R. P., and Terminello,
L. J., Phys. Rev. B, Vol. 53, No. 14 (R8824), 1996.
and Chaiken, A., Michel, R.P., and Wall, M. A.,
Phys. Rev. B, Vol. 53, No. 9 (5518), 1996.
RHEED Studies of the Si(111) 7x7 Surface
Raina N. SmithPHY 450, Senior Physics Lab
Virginia Commonwealth University
May 5, 1998
Abstract: The silicon (111) 7x7 reconstructed surface has been
extensively studied, and it’s characteristics allow for calibration and
verification of data. Reflection High Energy Electron Diffraction
(RHEED) is a useful tool for examining the surface of smooth samples.
Introduction
The Silicon (111) 7x7 Surface
One of the Si(111) reconstructions is the 7x7, as shown in Figure 1. This is the surface which forms when a commercial Si wafer is ‘flashed’ briefly up to ~1200°C. I have added to this image an outline of the unit cell, a diamond shape with seven atoms along each edge, for two different orientations. The unit cell is repeated throughout the surface to form the 2-D lattice. Note that there is both a long axis and a short axis in the unit cell. The length of the unit cell edge is known as the lattice parameter a, and is a defining property of the surface.
Reflection High Energy Electron Diffraction (RHEED) Imaging
RHEED imaging utilizes a monoenergetic electron beam with primary energies between 10 and 100keV in an ultra high vacuum (UHV) environment (~ 10-10 Torr). The beam is directed toward the sample surface at an angle of less than 5°, and the diffracted beam is observed on a fluorescent screen (see Figures 2 and 3). The image is then captured using a video camera and transferred to a computer for analysis. Since the beam is incident at such a low angle RHEED allows for constant imaging during film growth on a substrate, whereas most imaging methods require that a film be laid down in one chamber, and imaged in another. It is easy to see the benefits that RHEED provides by allowing samples to remain in UHV without requiring multiple chambers for deposition and analysis.
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| Figure 2: Phosphor Screen on Chamber | Figure 3: Diffraction Pattern on Screen |
There are two types of lattices associated with every crystal: a crystal lattice and a reciprocal lattice2. A crystal lattice is a ‘real space’ mapping of the atoms in the lattice, whereas a reciprocal lattice maps the atoms in ‘reciprocal space.’ Scanning Tunneling Microscopy (STM) images as in Figure 1 are of the crystal lattice, and diffraction patterns such as RHEED (Figures 4 and 5) are images of the reciprocal lattice derived from the 2-D net of surface atoms3. A Fourier transform is used to go between points in real space and points in reciprocal space. Length in the reciprocal lattice has the dimensions of L-1, where L is the length in the crystal lattice, hence the name reciprocal lattice.
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| Figure 4: RHEED Image of Si(111) 7x7 along the long axis | Figure 5: RHEED Image of Si(111) 7x7 along the short axis |
Figures 4 and 5 show RHEED images of the Si(111) 7x7 surface along the two possible axes of the unit cell. In Figure 4, the electron beam is traveling parallel to the long axis, and perpendicular to the short axis. In reciprocal space the length of the short axis becomes larger, and the spots are further apart. In Figure 5, the beam is traveling perpendicular to the long axis, and the spots in reciprocal space are closer together.
The Ewald construction is a helpful method of determining where diffraction spots will occur in the reciprocal lattice (see Figure 7). A vector k is drawn along the direction of the electron beam a length of 1/(lambda) where (lambda) is the wavelength of the beam. Next a sphere (called the Ewald sphere) is drawn about k, and wherever the Ewald sphere intersects the reciprocal lattice, there will be a diffraction spot. The 3-D Fourier transform of a 2-D system of periodic points a distance a apart results in a system of periodic infinite rods normal to the original plane of points and located a distance 1/a apart. When the periodicity of the crystal lattice is not perfect, there are maxima and minima which occur on the rods (the maxima corresponding to the diffraction spots in Figure 7).
An interesting situation arises when you ‘rock’ the crystal sample in
a direction parallel to the beam. This shifts the Ewald, sphere down,
correspondingly shifting the intersections of the sphere with the reciprocal
lattice down. If you examine the RHEED pattern during the rotation,
the dots will move steadily downward, winking in and out of existence as
the Ewald sphere approaches and passes the diffraction spots.
Deposition of Thin Films of Iron on Si(111) 7x7 Substrate
Several variables affect the result of deposition of thin films on a substrate of Si(111). The temperature of the substrate, the thickness and composition of the film, and other factors determine the resulting structure. Figure 6 illustrates the findings of Le Thanh Vinh, J.Chevrier, and J. Derrien4. At substrate temperatures between 300 and 350°C approximately 3.75 monolayers (ML) of Fe produced FeSi (a 2x2 reconstructed surface). But only 50° warmer at 400°C, FeSi2 was formed which is a sqrt(3) x sqrt(3) reconstructed surface.
Experimental Method
RHEED was used to determine experimentally the lattice parameter of Si(111), to study the effects of ‘rocking’ the sample on the diffraction pattern of Si(111), and to survey the effect of molecular beam epitaxy (MBE) of up to 0.5 monolayers (ML) of Fe with the substrate at room temperature (RT) and up to 5 ML with the substrate at ~650°C. A schematic of the setup is shown in Figure 8.
For all experiments, a 10 keV beam of electrons was used, which has
a wavelength lambda of 1.228x10-11 m.
The distance L of the diffracted beam is 191 ± 2 mm.
Lattice Parameter of Si(111)
The lattice parameter a of a crystal can be calculated based on the distance xactual (in millimeters) between bulk spots in the RHEED pattern on the phosphor screen
where (lambda) is the wavelength in meters of the electron beam, and L is the distance the diffracted beam travels from the sample to the screen (in millimeters). A ruler is attached to the screen as a scale of distance, and the distance x in the RHEED image is adjusted to scale (xactual) before the lattice parameter is calculated.
Rocking RHEED Analysis
For ‘rocking’ RHEED, the sample manipulator is placed on low speed setting, and the sample is moved in one continuous direction parallel to the electron beam (see Figure 6) until the diffraction pattern fades away. A series of images were taken every 1/3 second during rocking of the sample for a duration of ~50 seconds (a total of 153 images). The images were then put into a video format and examined for characteristics of the reciprocal lattice rods.Molecular Beam Epitaxy of Fe on Si(111) Substrate
A thickness monitor is used to calibrate the Fe evaporator. It is moved in front of the sample, blocking it, and the evaporator is operated until it reaches a steady rate. That rate is then recorded, and ideally checked again after data is taken to assure that it remained constant during analysis. In our experiment, the rate of deposition was 1 angstrom / 64 seconds. Since the thickness of 1 ML of Fe is 1.34 angstroms, 0.1 ML was deposited every 8.6 seconds of continuous operation. 0.2 ML of Fe was first put down on the Si substrate at RT, and then it was annealed at ~500°C for one minute. Two subsequent annealings occured at ~600°C for one minute each. 0.3 ML of Fe was added to the sample with the substrate at RT, and four subsequent annealings were performed at ~650°C for one minute each. The sample was then annealed at ~900°C for 15 seconds. Next, approximately 4.5 ML of Fe was deposited onto the sample with the substrate kept at ~650°C.Images were taken of the RHEED patterns between all annealings and diffractions
compared.
Results and Analysis
Lattice Parameter of Si(111)The measured value for the distance x was 6.4 ± 0.2 mm, and the lattice parameter a was calculated to be 3.6 ± 0.4 angstroms, compared to the accepted value of 3.8 angstroms, which is a 5.6% difference and well within the expected value.
Rocking RHEED Analysis
Although the Si surface was not very smooth and the resulting diffraction pattern somewhat diffuse, the rocking pattern was exceptionally distinct, and clearly showed the expected winking in and out of the diffraction spots as the sample was rocked.Molecular Beam Epitaxy of Fe on Si(111) Substrate
Images of the RHEED pattern between annealings are shown below. The negative of the original images has been used to emphasize the pattern.
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| [1] Clean Si(111) sample
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[2] 0.5 ML of Fe after ~5 minutes of annealing
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| [3] 0.5 ML of Fe after ~7 minutes of annealing
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In Image [1], the large bulk spots are separated by six smaller spots evenly spaced (1/7th order) , characteristic of the Si(111) 7x7 surface. In Images [2] and [3], the 1/7th order spots are fading as Fe is deposited. In Image [4], weak 1/2 order spots are beginning to appear, and in Image [5], the central 1/7th order spots are reappearing. This could be due to prolonged conditions at 650°C. Silver on Si(111) forms a 7x7 surface between 600-800°C, so it is reasonable that Fe on Si(111) would exhibit that reconstruction.
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| [4]~3ML of Fe (substrate at 650°C)—weak 2x2
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| [5]~5ML of Fe (substrate at 650°C)—weak 7x7 |
Conclusion
The lattice parameter for the Si(111) 7x7 surface was experimentally determined to be 3.6 ± 4 angstroms, which is 5.6% difference from the accepted value of 3.8 angstroms. The rocking RHEED method produced dots which slowly winked in and out while shifting downward, representing the irregular sized reciprocal lattice rods of an imperfectly ordered crystal.Epitaxial layering of Fe on Si(111) 7x7 surface exhibited a weak 1/2 order spots when ~3ML of Fe had been deposited and the substrate was at ~650°C, and a return to a weak 1/7th order reconstruction when ~5ML of Fe had been applied and the substrate was at ~650°C.
More work is required to develop a complete phase diagram for Fe on Si(111). We will also be examining Fe-Si multilayers, as well as Fe on the Si(100) surface. Similar experiments will also be taken to the Advanced Light Source in California this August, and results should clarify our knowledge considerably.
1 Lüth, Hans,
Surfaces and Interfaces of Solid Materials, (Springer-Verlag, Berlin, 1998),
3rd corrected printing, p.88.
2Kittel,
Charles, Introduction to Solid State Physics, (John Wiley & Sons, NY,
1976), p.47.
3Carlisle,
John Arthur, Geometric and Electronic Structure of Reconstructed Semiconductor
Surfaces: Thesis, Univ. of Illinois at
Urbana-Champaign, (1993), p.11.
4Vinh,
Le Thanh, Chevrier, J. , and Derrien, J., Phy. Rev. B Vol. 46, No. 24,
15 946 (1992).
5Carlisle,
p.12.
Any comments would be appreciated. I can be reached via Email at raina@bitsmart.com.
This page last edited on July 9, 1997.