XX--ray Probes of the Layer and Interface Structure ofray Probes of the Layer and Interface Structure of
NanoscaleNanoscale Films for Electronics and SpintronicsFilms for Electronics and Spintronics
Brian K Tanner (and others)
Bede plc, Belmont Business Park, Durham
and
Department of Physics, Durham University
2
Opto-electronic Materials Driver for High Resolution X-ray Diffraction
Development in 1980s
ØInxGa1-xAsyP1-y for PIN diodes for long-haul telecoms
ØAlxGa1-xAs for laser diodes
Measurement of composition
3
High Resolution X-ray Diffraction
X-ray tube
X-ray mirrors
Monochromator and conditioner crystals
Monochromatic, parallel beam
Composition of AlxGa1-xAs on GaAs
Measure splitting between substrate and layerAssume Vegard’s Law (linear interpolation) to get x
1000
10000
-100 0
Simulation with Lorentzian diffuse scatterSimulationExperiment
Specimen angle (arc sec)
Inte
nsity
(cps
)
Composition of AlxGa1-xAs on GaAs
Quadratic dependence 4 arc seconds ≅ 1% Al
B K Tanner et al Appl Phys Lett 59 (1981) 2272
S Gehrsitz et al Phys Rev B 60 (1999) 11601
0
100
200
300
400
0 0.2 0.4 0.6 0.8 1.0
r2 = 0.99933 linearr2 = 0.99984 quadratic
x
Bra
gg p
eak
split
ting
(arc
sec
onds
)
Takagi-Taupin formulationof X-ray Dynamical Diffraction Theory
hh DCDD
−+= χχ∂∂
πλ
000
0
i s
00 )(i
DCDD
hhhh
h χαχ∂∂
πλ
+−=s
αh is the deviation of the incident wave from the exact Bragg conditionso and sh are unit vectors in the directions ofKo and Kh
C is the polarisation factorχ0 and χh are the electric susceptibilities(these describe the crystal) χ
λπhe
h
rV
F= −2
Recursion relation format
A C h= −χ Bb h=
−+
( )12 2
0χ α π D = π
λγ 0
E Cb h= − γ F BB EA= −
where b= γo/γh.
Amplitude ratio X = Dh/Do,
X X' F BX'+E DF(z - w)
F AX'+B) DF(z - w)=
+ i( ) tan( ) - i( tan( )
z is depth above the depth w at which the amplitude ratio is the known value X'
M A G Halliwell, J Juler and A G Norman, Inst Phys Conf Ser 67 (1983) 365M G A Halliwell, M A G Lyons and M J Hill, J Crystal Growth 68 (1984) 523
Recursion relation format
Fit to a graded 2.3µm GaInAs layer, nominally lattice matched to InPNote mismatch of only several hundred ppm
M J Hill, B K Tanner, M A G Halliwell and M H Lyons J Appl Cryst 18 (1985) 446
9
Graded SiGe structure (blanket)
• QC200 diffractometer + bede RADS Mercury auto-fitting software
sample courtesy Hitachi-Kokusai Electric
010
110
210
310
410
510
-4000 -3000 -2000 -1000 0 1000 2000
Standardized Chi Squared: 0.013 R Factor: 8.82
Inte
nsity
Seconds
slot-23_0aa1.x01 temp.sim
Inte
nsity
(log
sca
le)
Ge profile for Slot 23
0
0.05
0.1
0.15
0.2
0.25
0.3
0 20 40 60 80 100 120
Depth / nm
Ge
frac
tion
Slot 23
10
Asymmetric relaxation: Reciprocal space maps of test structures: 10 µm × 0.5 µm
• (004) (224)GE (004)
(224)GESee Frontiers of Characterization and Metrology for Nanoelectronics Conference 2007
11
MgO Tunnel Junction for Spintronics Applications
SiSiO2
Co
NiFeMo
MgO
Ta
Co
Is there a change in the MgO interface structure when the bottom Co layer has a monolayer of CoO deposited?
12
First User Data from Diamond Synchrotron Light
Source
First User Data from Diamond Synchrotron
Light Source
13
X-ray Optics for Reflectivity
X-ray tube
X-ray mirrors
Monochromator and conditioner crystals
Monochromatic, parallel beam
14
X-ray Optics for Reflectivity
X-ray tube
X-ray mirrors
Slit monochromator
Monochromatic, parallel beam
ØRemove crystal monochromatorØUse slit to define wavelength
dispersion
Conditioner crystal
15
Ultra-thin transition metal films grown by Molecular Beam Epitaxy
Si (001)sample a sample b sample c
C u ( 9 0 0 Å )
N i ( 6 0 Å )Co (10Å)
C u ( 8 0 Å )
C o ( 1 0 Å )Cu (80Å)
C u ( 8 0 Å )
ØShadow masks used to create three samples with identical Cu and (where appropriate, Ni and Co) thicknessØPolarized neutron reflectivity analysis of magnetic momentsØNeed precise layer thickness to reduce number of free fitting parameters in PNR analysis
16
Ultra-thin Films - Specular Reflectivity
Si (001)sample a sample b sample c
C u ( 9 0 0 Å )
N i ( 6 0 Å )Co (10Å)
C u ( 8 0 Å )
C o ( 1 0 Å )Cu (80Å)
C u ( 8 0 Å )
100
102
104
106
108
0 0.05 0.10 0.15 0.20 0.25
C
B
A
qz (Å-1)
Cou
nt ra
te (c
.p.s
.)
840823830Cu thickness (Å)
-6164Ni thickness (Å)
1011-Co thickness (Å)
848275Cu cap thickness (Å)
Sample C
Sample BSample A
17
Scientific Conclusions
C.A.Vaz, G.Lauhoff, J.A.C. Bland, S.Langridge, D Bucknall, J Penfold,
J.Clarke, S.K.Halder and B.K.Tanner, J. Magn. Mag. Mater. 313 (2007) 89-97
ØPrecise measurement of magnetic moment possible due to restricting parameter spaceØPer Ni atom, 0.58±0.03µB and 0.59 ±0.03µB for Ni/Cu and Co/Ni/Cu samplesØNo effect of interface environment detected
Measurement of hot electron momentum relaxation times in metals by femtosecond ellipsometryV.V. Kruglyak, R.J. Hicken, M. Ali, B.J. Hickey, A.T.G.Pym and B.K.Tanner, Phys Rev B 71 (2005) 233104
Another example of need to determine layer thickness
Grazing incidence X-ray scattering• Fluorescence
Incident beam
Asymmetricdiffraction
Diffuse scatter
Diffuse scatter
Specular reflection
Evanescentwave
19
Principle of Grazing IncidenceIn-plane X-ray Diffraction
•
Beam measured in grazing incidence reflectivity
Diffracting planes
Beam measured in GIIXD
X-ray beam incident at grazing angle
Bragg angle, θ
2θ
•
Beam measured in grazing incidence reflectivity
Diffracting planes
Beam measured in GIIXD
X-ray beam incident at grazing angle
Bragg angle, θ
2θ
Beam measured in grazing incidence reflectivity
Diffracting planes
Beam measured in GIIXD
X-ray beam incident at grazing angle
Bragg angle, θ
2θ
Ø Bragg planes normal to the sample surfaceØ Probes in-plane lattice parameter (in-plane strain)Ø Probes in-plane mosaicØ Probes in-plane length scale
20
High Resolution GIIXD
0
0.35
0.70
-0.1 0.1
Optic + Ge crystalPolycapillary optic
Specimen rotation angle (°)
Nor
mal
ised
inte
nsity
(Rocking curve) scans of the Si specimen about its surface normal, with the detector set for the 220 reflection, with and without the Gemonochromator crystal. Solid lines are fits of Voigt functions to the data.
T. A. Lafford, P. A. Ryan, D. E. Joyce, M. S. Goorsky and B. K. Tanner, Phys. Stat. Sol (a) 195 (2003) 265
21
Tilt and Twist Mosaic
Tilt is misorientation out ofout of the wafer plane
Twist is the misorientation inin the wafer plane
22
Tilt and Twist Mosaic
Tilt is misorientation out ofout of the wafer plane
Twist is the misorientation inin the wafer plane
23
Twist and Tilt Mosaic as a Function of GaN Layer Thickness
0
0.2
0.4
0.6
0.8
1.0
0 5000 10000 15000 20000
Fit to Ayers' model and DE/D
S constant
Fit to Ayers' modelTwist mosaicTilt mosaic
Total thickness (Å)
Ro
ckin
g cu
rve
FWH
M (
°)
Ratio of number of threading screw to threading edge dislocations constant
T A Lafford, P J Parbrook and B K Tanner, Phys Stat Sol (c) 0 (2002) 542
24
Twist and Tilt Mosaic inSapphire/GaN/AlN/AlxGa1-xN
0.1
0.2
0.3
0.4
80 120 160 200
00.2 AlGaN (tilt)00.2 GaN (tilt)11.0 AlGaN (twist)
AlN interlayer thickness (Å)
FWH
M (
°)
0
0.1
0.2
0.3
0.4
0.5
0.1 0.2 0.3 0.4 0.5
00.2 AlGaN (tilt)00.2 GaN (tilt)11.0 AlGaN (twist)
Al fraction xFW
HM
(°)
T. A. Lafford, P. J. Parbrook and B. K. Tanner, Appl Phys Lett 83 (2003) 5434
AlN interlayer between micron thick GaN and AlxGa1-xN layers. MOVPE. No change in the tilt mosaic (threading screw dislocation density) with thickness or Al fraction x of the upper layer. A linear increase in the twist mosaic (threading edge dislocation density) was observed as a function of interlayer thickness and x. For all samples the twist mosaic of the AlGaN was significantly greater, by at least a factor of two, than that of the GaN layer.
25
Durham reflectometer with fluorescence attachment
26
Spin Valve Structure for Spintronics Applications
SiBuffer
Ni80Fe20
Cu
Ta
Ni80Fe20
MnFe
1nm
27
Grazing Incidence Fluorescencefrom Spin Valve Structures
• Deconvolution from Gaussian peaks• Measure area under peaks
0
0.2
0.4
5 6 7 8 9 10
IncidentBeam
Fe Kβ Cu K
β
Ni Kβ
Cu Kα
Ni Kα
Mn Kβ
Fe Kα
Mn Kα
Energy (keV)
Nor
mal
ised
Int
ensi
ty
28
Grazing Incidence FluorescenceAnalysis of Bi as a Surfactant
• Evanescent wave below critical angle• Used MoK radiation to excite Bi
0.00 0.25 0.50 0.75 1.00101
102
103
104
105
106
Cu Kα1 Mo Kα1
Pene
tratio
n D
epth
(Å)
α (deg)
Penetration depth L is
L = λ/{π(αc2 - α2)]½}
where αc is the critical angle for total external reflection
As the incidence angle rises, penetration of wave increases.
Around critical angle, very high sensitivity to depth
29
Normalized Grazing Incidence Fluorescence Data
Ø Ni fluorescence almost independent of Bi thicknessØ Cu fluorescence shows step at critical angle from
surface layer
30
Normalized Grazing Incidence Bi Fluorescence Data
Ø Low Bi thickness – fluorescence rises at lower angle compared to thick layer
Ø Peak shifted to higher angle for thick layer
50Å
Bi moves to the surface for monolayer coverageBi stays low for thick layer
31
Grazing Incidence FluorescenceBi above and below FeMn
• 25Å Bi below FeMn, 25Å low density Bi above
• Detailed fitting difficult
0
0.2
0.4
0.6
0.8
1.0
1.2
0 500 1000 1500
Top Bi layer: 100% density, 15Å thickTop Bi layer: 80% density, 18.75Å thickTop Bi layer: 60% density, 25Å thickTop Bi layer: 40% density, 37.5Å thickExperimental Data
Sample Angle (arc-seconds)
Inte
nsity
Sample A13 - Bi Fluorescence
0
0.2
0.4
0.6
0.8
1.0
1.2
0 500 1000 1500Sample Angle (arc-seconds)
Inte
nsity
Sample A13 - Mn Fluorescence
0
0.2
0.4
0.6
0.8
1.0
1.2
0 500 1000 1500Sample Angle (arc-seconds)
Inte
nsity
Sample A13 - Fe Fluorescence
32
Fits to simulation of fluorescence yield as function
of angle
0
1.5
3.0
4.5
6.0
7.5
0 1000 2000 3000 4000
Ni Kα (x 32)Cr Kα
Sample Angle (arc seconds)
Pea
k A
rea
(abr
itrar
y un
its)
Peak Areas from 200Å Cr Single Film
0
0.2
0.4
0.6
0 1000 2000 3000 4000
198.2Å Cr(1-x) Ni(x) (x=1.3%)198.2Å Cr / 3Á NiExperimental Data
Sample angle (arc seconds)
Inte
nsi
ty (
arb
itra
ry u
nit
s)
Simulations of Ni Fluorescence for Different Ni Contamination
33
Collaborators and Contributors
Bede plc
Paul RyanTamzin LaffordMatthew WormingtonStéphane GodnyKeith Bowen
Durham University
Tom HaseAlex PymDavid EastwoodStuart Wilkins (now at BNL)
Peter Parbrook (Univ of Sheffield)Mai Zhenhong (CAS, Beijing)Mark Goorsky (UCLA)Bill Egelhoff (NIST)Carlos Vaz (Univ of Cambridge)Tony Bland (Univ of Cambridge)
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