Delayed crystallization of ultrathin Gd2O3 layers on Si(111) observed by in situ X-ray diffraction
© Hanke et al; licensee Springer. 2012
Received: 22 December 2011
Accepted: 29 March 2012
Published: 29 March 2012
We studied the early stages of Gd2O3 epitaxy on Si(111) in real time by synchrotron-based, high-resolution X-ray diffraction and by reflection high-energy electron diffraction. A comparison between model calculations and the measured X-ray scattering, and the change of reflection high-energy electron diffraction patterns both indicate that the growth begins without forming a three-dimensional crystalline film. The cubic bixbyite structure of Gd2O3 appears only after a few monolayers of deposition.
Binary rare-earth metal oxides became increasingly important as gate dielectrics in metal oxide semiconductor field-effect transistor technology [1, 2] and as catalysts. The application of these oxides as gate dielectrics is based on their high dielectric constants , large bandgaps , high thermal stability of their interfaces to silicon , and large band offsets with silicon. Gd2O3 is a promising candidate among the lanthanide oxides due to its close lattice match to silicon ( 2asi = 10.862 Å). Epitaxial growth of Gd2O3 has been demonstrated for a variety of substrates, including GaN , Si , and SiC  by atomic layer deposition  and molecular beam epitaxy (MBE) .
Here, we present an MBE growth study of Gd2O3 on Si(111), applying synchrotron-based X-ray diffraction with particular focus on the formation of the very first crystalline layers. These measurements were done during sample growth in situ, in a continuous way without any growth interruption.
Molecular beam epitaxy
All samples were grown in a dedicated MBE chamber under ultrahigh vacuum (UHV) conditions. The 2 × 2-cm2 Si(111) substrates were prepared by a 10-min dilute (5%) HF treatment, to remove the thermal oxide, followed by a 10-min H2O rinse. Next, they were loaded into the MBE system, degased at 300°C in the load lock for 20 min, and annealed in the growth chamber under UHV at 720°C (measured using a thermocouple between heater and substrate) for 60 min to prepare a (7 × 7) reconstructed Si(111) surface. The sample temperature was subsequently ramped to the growth temperature of 700 C. During the growth, Gd2O3 powder (99.999%) was evaporated from a special effusion cell (TUBO), which allows for cell temperatures in excess of 2,000°C . Temperature and flux from this cell can be adjusted more accurately than with conventional e-beam evaporators. Throughout the experiment, the cell temperature was kept constant at 1,650°C (thermocouple temperature; the actual temperature of the source material is higher), resulting in a growth rate of 0.13 Å/min. Molecular oxygen at 10-7 mbar was added 30 s prior to the growth sequence and throughout the entire layer deposition (OB, unpublished work).
The X-ray scattering experiments were performed at the dedicated beamline U125/2-KMC at the synchrotron BESSY II (Helmholtz-Zentrum Berlin HZB, Berlin, Germany). This experimental setup  combines the MBE system described above with the ability to perform high-resolution X-ray diffraction, which enables unique in situ growth studies. The primary X-ray beam with a size of 500 × 500 μm2 enters and exits the growth chamber via X-ray transparent Be windows, whereas the scattered intensity can be probed in an angular range of 0° to 120° in-plane and 0° to 50° out-of-plane. Both the sample and the detector can be precisely moved during the growth to probe scattered intensity in a wide range of reciprocal space. An X-ray energy E of 12 keV, selected by a Si(111) double crystal monochromator (ΔE /E = 10-4), is chosen as a compromise between the accessible area in reciprocal space and primary intensity. Throughout the experiment, the incidence angle was kept constant at α i = 0.2°.
Results and discussion
We index the reflections in surface (hexagonal) coordinates, marked with (HKL)hex, instead of using bulk indices, denoted by (HKL)cub . Referring to the symmetry of the Si(111)cub substrates, we define the surface unit cell by vectors a, b, and c with lengths a = b = 3.84 Å, c = 9.405 Å and α = β = 90°, γ = 120°. For example, the Si(022)cub bulk reflection refers to (104)hex in surface coordinates.
Post-growth X-ray diffraction
In-situ X-ray studies during growth
Layer formation at the interface
Atomic force microscopy of the pure Si(111) substrate (not shown) yields a mean terrace size of 600 nm, which does not change after the deposition of 1 and 2 ML of Gd2O3. The roughness of 0.46 nm is preserved during growth. Post-growth X-ray reflectivity measurements prove the nominal layer thickness with an RMS roughness of less than 1 ML.
The evolution of the reflection high-energy electron diffraction (RHEED) pattern (not shown) during the first MLs is consistent with the 'delayed crystallization' scenario: During deposition of the first ML, the 2D streaky RHEED pattern from the Si(111) surface disappeared completely and transformed into a diffuse RHEED pattern with almost vanishing specular spot intensity - indicative of the lack of crystal order . The specular spot intensity recovered after the deposition of (3 to 5 ML) together with the onset of pronounced RHEED oscillations.
For the final film, the fringes of CTRs (sensitive to crystalline film thickness only) and the X-ray reflectivity fringes (sensitive to total film thickness irrespective of crystalline order) correspond to the same thickness. Therefore, the crystalline order developed over the whole film thickness, without leaving a noncrystalline initial layer at the interface. This conclusion is in agreement with cross-sectional transmission electron micrographs in Figure 3b which demonstrates a pseudomorphic film and an atomically flat layer-substrate interface.
We have performed a synchrotron-based in situ X-ray diffraction study during the epitaxial growth of Gd2O3 on Si(111). By comparing measured crystal truncation rods to X-ray scattering calculations, we found that the crystallization into three-dimensional cubic bixbyite structure was delayed: it started at a layer thickness of about 2 ML and proceeded up to 4 ML.
crystal truncation rod
molecular beam epitaxy
reflection high-energy electron diffraction
transmission electron microscopy
We would like to thank C. Stemmler and S. Behnke (Paul-Drude-Institute, Berlin) for the technical support during the experiment and BESSY II (Helmholtz-Zentrum, Berlin) for granting the beamtime under project 100876. This work was supported by the Leibniz-Gemeinschaft under the project no. SAW-2011-PDI-230.
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