The Importance of In Situ Monitors in the Preparation of Layered Oxide Heterostructures by Reactive MBE
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105 Mat. Res. Soc. Symp. Proc. Vol. 619 © 2000 Materials Research Society
containing films, adsorption-controlled growth conditions were used to chemically limit the incorporation of these volatile elements [8-101. For all growths molecular beams of strontium, barium, and titanium were shuttered in a sequential manner to deliver monolayer-by-monolayer doses of atoms onto (100) SrTiO 3 [11], (100) LaAIO 3 [12], (110) NdGaO 3, or (100) LaAlO3 -Sr 2AITaO 6 (LSAT) substrates. Providing the growing film surface with the correct absolute dose of species to deposit a full stoichiometric monolayer is key to the successful synthesis of many of the layered structures described. For example, achieving the phase-pure synthesis of Sr, ,Ti,03,,. Ruddlesden-Popper compounds with n = 1 to 5 required that the doses supply both the correct ratio of cations as well as the correct absolute number of atoms to complete each monolayer. If either the cation ratio or absolute dose of cations were off by more than about 1%, phase-pure Sr,+iO,, phases were not attained [131. Achieving such composition control has been a major problem in prior oxide MBE work [14]. We have met this challenge using a combination of AA (calibrated in situ using a QCM) and by monitoring the RHEED intensity oscillations that occur during the monolayer-bymonolayer deposition of SrTiO 3 and BaTiO 3 calibration layers. These shuttered RHEED oscillations (which are different than the conventional RHEED oscillations that occur when species are codeposited) are used to fine-tune the relative and absolute doses supplied to the substrate surface in each monolayer with an accuracy of about 1% 1151. The films were characterized in situ by RHEED and ex situ by a Picker four-circle x-ray diffractometer, a Digital Instruments Multi-Mode atomic force microscope (AFM), and a JEOL 4000EX high-resolution transmission electron microscope (HRTEM) operated at 400 kV. RESULTS AND DISCUSSION
Metastable Superlattices and Digitally-GradedOxide Heterostructures To explore the ability of MBE to integrate known oxides on a nanometer scale, we have grown epitaxial superlattices and digitally-graded structures combining PbTiO, with SrTiO, and BaTiO 3 with SrTiO 3. Both of these systems form a solid solution over their entire composition range [16,171. Thus, PbTiO3 /SrTiO 3 as well as BaTiO3 / SrTiO 3 layered heterostructures are
Z
PbTiO•3 001 ,, SrTiO 001 3
105
o
0o
_
04
•
103 1O
o 10
433
ei
FWHM PbTiO 3 002 20 =0.440
0
1
101 0
20
10
30
20 (degrees) )1 011 superlattice. The 00W FIG. 1. 0-20 x-ray diffraction scan of a [(PbTiO 3) 10 / (SrTiO3A superlattice reflections, the 001 reflection of the thick PbTiO 3 buffer layer and overlayer, and 001 reflection of the SrTiO 3 substrate are labeled.
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metastable; it is energetically favorable for these oxides to dissolve into each other forming (Pb,Sr)TiO 3 and (Ba,Sr)TiO 3 solid solutions. The metastability of PbTiO 3 / Sri3 and BaTiO 3 / SrTiO 3 heterostructures is analogous to the situation for AlAs / GaAs heterostructure
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