Supplementary Material of “Ultrathin IBAD MgO films for epitaxial growth on
amorphous substrates and sub-50 nm membranes”
Siming Wang1,2 , C. Antonakos3, C. Bordel1,2,4, D. S. Bouma1,2, P. Fischer1,5, F. Hellman1,2
1
Materials Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California,
94720, USA
2
3
Department of Physics, University of California Berkeley, Berkeley, California, 94720, USA
Department of Chemistry, University of California Berkeley, Berkeley, California, 94720, USA
4
5
GPM, UMR 6634 CNRS-Université de Rouen, 76801 St. Etienne du Rouvray, France
Department of Physics, University of California Santa Cruz, Santa Cruz, California, 94056, USA
Experimental: RHEED, Stress measurement, AFM, and XRD
In-situ reflection high energy electron diffraction (RHEED) was used to monitor the
IBAD MgO texturing as shown in FIG. 1s. FIG. 1s(a) shows the RHEED image of the
amorphous substrate (a-SiNx in this case, similarly for a-SiOx) before the IBAD MgO deposition.
As expected for an amorphous substrate, no electron diffraction pattern was observed. After 45–
60s (~1 nm) IBAD MgO deposition, spots were observed as shown in FIG. 1s(b), showing
biaxial texturing of the MgO film.
The stress of the various MgO films grown on a-SiOx-covered [001] Si 2” wafers was
measured with a KLA-Tencor FLX-2320 Film Stress Measurement System. Wafers were
purchased from WRS Materials with 100 nm thermal oxide coating on both sides. The system
measures the radius of curvature of the wafer and the film stress is calculated with the following
equation:
𝐸
ℎ
2
1
1
𝜎 = 1−𝜈 ∗ 6𝑡 ∗ 𝑅
−𝑅
𝑓𝑖𝑙𝑚
𝑛𝑜 𝑓𝑖𝑙𝑚
(1s)
1
where 𝜎 is film stress in Pa,
!
!!!
is the biaxial elastic modulus of the substrate, equal to
1.805×1011 Pa for the [001] silicon wafer used, h is the substrate thickness, approximately 300
µm, t is the film thickness of the MgO, ranging from 1 to 16 nm, Rfilm is the radius of curvature
of the wafer after the film growth, and Rno film is the radius of curvature of the wafer before the
film growth. The radius of curvature and hence the stress was measured both parallel and
perpendicular to the direction of the Ar+ ion beam during growth.
The surface morphology of the various MgO films was characterized by atomic force
microscopy (AFM) with a vertical resolution better than 1 nm; the root mean square (rms)
roughness was determined across a linear profile of the image (shown in FIG. 1(c)). The full
images of IBAD-only and IBAD+15 nm homoepitaxial MgO before and after annealing are
shown in FIG. 2s, displaying clearly the reduced roughness after annealing. Optical microscope
images (shown in FIG. 2) were taken with a Nemarski contrast microscope of a-SiNx membranes
of nanocalorimetry devices with various MgO deposited on them to investigate the previous
“wrinkled membrane” problem.
X-ray diffraction (XRD) of the Fe films grown on the various MgO was carried out with
a PANalytical X’Pert Pro Diffractometer (λCu,Kα ≈ 1.54 Å). We performed 2θ/ω scans (FIG. 3(a))
and ω scans of the out-of-plane (002) peak and ϕ scans of the (101) peak of Fe (FIG. 3(b)) to
examine the epitaxial growth. The angular resolution of the XRD is 0.01°. All diffraction peaks
were fitted with a Gaussian function to extract the peak positions, the full width half maximum
(FWHM) of ω, 2θ, and ϕ (FIG. 3s(a,b,d)), and integrated intensity of 2θ (FIG. 3s(c)). The out-ofplane and in-plane coherence length of the Fe was estimated using the Scherrer equation:
𝐾∗𝜆
𝐺 = ∆ 2𝜃 𝐶𝑢,𝐾𝛼
∗cos 𝜃
(2s)
2
where G is the coherence length, K (≈ 0.94) is a dimensionless factor shape factor close to unity,
Δ(2θ) is the FWHM of the out-of-plane and in-plane scans.
Ion-to-atom ratio (IAR)
We define IAR as the ratio of the Ar+ ion flux and MgO flux. The ion flux FAr = 1.1875 ×
1015 ions/(cm2·s) is measured with a spherical Faraday cup (1 cm2 cross-sectional area). The
MgO flux FMgO = r · ρ · NA / M = 0.59915 × 1015 molecules/(cm2·s), where r = 1.12 Å/s is the
measured growth rate of MgO with the Ar gas turned on but the ion beam turned off, ρ = 3.58
g/cm3 is the bulk density of MgO, NA = 6.022 × 1023 molecules/mole is Avogadro constant, and
M = 40.3 g/mole is the molar mass of MgO. The ion-to-atom ratio IAR = FAr/FMgO = 1.98. Note
that Ref. [4] uses a different definition for “ion-to-atom ratio” (IAR*) by defining the MgO rate
as the deposition rate of the IBAD MgO. In our experiments, the IBAD MgO deposition rate is
0.15 Å/s, giving an IAR* = 14.8, nearly twenty times of that in Ref. [4] (IAR* = 0.8).
3
FIG. 1s. RHEED images (a) before and (b) after IBAD MgO deposition.
4
FIG. 2s. AFM images of IBAD-only MgO (a) as-deposited and (b) 500 °C annealed, and IBAD
+ 15 nm homoepitaxial MgO (c) as-deposited and (d) 500 °C annealed.
5
FIG. 3s. (a) FWHM of the ω scan, (b) FWHM and (c) integrated intensity of the 2θ peaks of the
Fe (002) peak, and (d) FWHM of the ϕ scan for the off-axis Fe (101) peak on various IBAD
MgO as in FIG. 3, and three single crystal MgO (001) substrates (blue crosses connected by
dashed dotted lines): pristine substrate, substrate annealed at 350 °C, substrate annealed at
500 °C.
6