Development of CMR Manganite Thin Films for Bolometeric Detector Application in the Linear Coherent Light Source ( LCLS) Free Electron Laser (FEL)

Project Summary

Phase –1 Tasks

(i) Development of Epitaxial thin films on Si

We will grow epitaxial thin films of Nd1-xSrx MnO3 (NSMO) on SrTiO3 buffered Si substrates using Pulsed Laser Deposition (PLD). Crystalline quality of the thin films will be analyzed using a 4-circle x-ray diffractometer, employing ?-2 ? scans to determine phase purity, rocking angle scans to determine the quality of out-of-plane alignment and ?-scans to determine in-plane epitaxy. Electrical resistivity will be measured to determine the temperature coefficient of resistance (TCR). These characterization results will be used as a feed back to optimize the growth parameters such as laser energy density, deposition temperature, oxygen pressure during film growth and thermal kinetics of the growth process. Post-annealing processes will be optimized as needed to obtain the best detector performance. Based on the results of this effort, schemes to introduce additional template layers on SrTiO3 buffer or schemes to employ alternate buffer layers will be developed as needed. Initial effort will be geared towards demonstrating desired detector performance and characteristics and uniformity of film properties in small samples (0.25” x 0.25 “ or smaller). The task of scaling up the process to larger area samples will be undertaken in phase-2. However a few large area (1 sq. inch) samples ( not guaranteed to be uniform) will be grown at the end of phase-1 to begin development of the detector fabrication process at LLNL.
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(ii) Exploration of other ‘low Z’ substrates:

Since recent simulations at LLNL have indicated some potential problems with Si due to the possibility of local melting in 1900 eV energy range, there is concern that Si may not be able to withstand the LCLS X-ray laser pulse for an extended period of time without some undesirable attenuation. This scenario calls for the development of other low Z substrates which can withstand the x-ray pulse at these energies. Based on preliminary discussions with LLNL researchers, we have identified some possible choices which include BN, SiC and BeO. However, there has been no previous efforts at growing CMR thin films on any of these substrates. As a first step in this direction we will investigate this list of substrates (with possible additions in consultation with LLNL) in terms of investigating the chemical compatibility, lattice match and thermal expansion match with CMR manganites, high temperature stability of the substrates, thermal transport characteristics, and commercial availability. After having identified suitable low Z substrate candidates, we will explore suitable chemical barrier layers and/or buffer and template layers as needed for CMR film growth.

Phase –2 Tasks

(i) Scale up of the film growth on Si for large area uniformity.

Scaling up the growth process towards larger ( 1 sq. inch and larger) to areas ensure good material properties and uniformity of the material requires two modifications of PLD porocess. These are: (i) Use of laser beam scanning optics which enables rastering of the plasma plume over the desired area for coating and (ii) Use of a radiative substrate heater in place of the conductive one that is currently used for smaller area film growth.
Uniformity over 1 sq. inch can be obtained using beam scanning alone
while uniformity over larger areas (up to 2 inch dia wafers) requires the
use of a radiative heater in addition to beam scanning. We will use these
methods to scale up our film growth on Si, initially over 1 sq. inch area
and subsequently on 2 ‘ dia wafers. The 1 sq. inch samples will be grown
on the existing SrTiO3 buffered Si substrates. The 2 “ dia wafers will be
grown on our in-house developed buffer layers due to the limitations in the
availability of STO buffered large area substrates.

(ii) Development of High Quality CMR films on the new low Z substrate(s):

Following on the phase-1 results, optimization efforts will be undertaken on one or two of the most promising low Z substrates. This task will involve surface preparation of the substrate, optimization of the growth parameters of the buffer and template layers as needed and optimization of the CMR film to obtain the best detector performance characteristics. Detailed structural analysis of the multi layers (including buffer, template and the CMR film) will be undertaken and the results will be used to optimize the film growth process to obtain good crystalline quality. Electrical resistivity and TCR of the CMR layer will be measured for each sample and the growth process will be further tuned to obtain the desired device performance characteristics.

Subsequent to optimizing the properties on small area samples, we will begin scale up of the process to large areas based on substrate availability.