Table 6.1 Effects of deposition parameters on the pulsed laser ablation
process and the structure of thin films.
Parameter
Effect on Ablation Process and Structure of Thin Films
1. Thermal evaporation versus photo-dissociation
Laser Wavelength
2. Density and size of particulates on the film
3. The energy carrier by a photon transferred to the target
1. Kinetic energy of the ablated species
Energy Density
2. Ratio of neutral to ionic ablated species
3. Density and size of particulates on the film
Repetition Rate
Distance Between
Target and Substrate
1. Growth rate
2. Migration of adjacent clusters on the substrate
1. Density and size of particulates on the thin film
2. Ratio and number of ablated atomic species that reach the
substrate
1. Surface mobility of the ablated species
Substrate Temperature 2. Stoichiometric formation of the ablated species
3. Crystal orientation of the thin film
Background Gas
1. Oxidation of the ablated species
2. Contamination
1. Density and size of particulates on the thin film
Deposition Pressure
2. Oxygen content of the film
3. Spatial distribution of the plasma
4. Reduction of ionic species in the plasma after scattering
Ramp Down Pressure
Target Density
Laser Beam Angle on
Target
Oxygen content of the grown film
Density and size of particulates on the thin film
1. Tilting of the generated plume away from the normal of the
target
2. Angular distribution of thin film thickness
X-ray Diffraction
X-ray diffraction depicts the
Y
X
relation between the wave nature
of x-rays and the periodicity of the
X’
D
θ
arrangement of atoms within a
θ’
Y’
θ θ
crystal. In fact, diffraction is due
A
d
C
B
to the existence of certain phase
relations between two or more
Diffraction of x-rays by a crystal
waves that are scattered from a
crystal. The x-rays diffracted from a crystal will be detectable at certain diffraction angels
when the X-rays constructively interfere. This detectability condition is described in the
Figure, which leads to the derivation of Bragg Equation governing x-ray diffraction. A
section of a crystal is showed, its atoms arranged on a set of parallel planes, and every two
planes are separated by the lattice constant d, act as scattering centers for x-rays. The line
XX’ represents the x-ray source while the detector is marked by YY’. The source x-rays
with a wavelength λ are incident on two adjacent planes of the crystal, and then the
constructive interference occurs. It requires that the angle of incidence should be the same
with that of reflection, and the path difference of the two x-rays must be equal to an integer
of the x-ray wavelength. The path difference in Figure can be expressed as:
n  AB  BC
(6.1)
AB and BC can be expressed in terms of the incident angle θ as:
AB  d  sin  and BC  d  sin 
(6.2)
so the Eq.(6.1) becomes:
n  2d  sin 
(6.3)
which is known as Bragg’s law. From this equation, it can deduce that reflections only occur
when   2d . It can be utilized that to apply x-rays with a known wavelength then to
determine the lattice spacing of various planes in the crystal. This is known as structural
analysis.
X-ray structural analysis can be used to determine the epitaxial nature of various thin
films. Epitaxy general is a phenomenon where a relation between the structure of the film
and the substrate exists. In the common case, it refers to one set of crystalline planes in the
thin film whose normal is perpendicular to the plane of the film(c-axis) and there is locking
of the a- and b-axis, in the plane of the film, between grains and substrate. The epitaxial
conditions can be checked with several types of structural x-ray scans included θ-2θ scans,
ф-scans, and rocking curves, which are measured with an x-rays diffractometer.
θ-2θ scans are used to determine which crystalline axis is oriented perpendicular to the
surface of the thin films. The source and detector angles are locked. As the angle of the
incident x-ray beam is varied, the detector will pick up the constructive interference of the
reflected x-rays, when an angle corresponding to the crystalline lattice spacing of any
family of planes in the thin film or substrate is reached. Polycrystalline films have grains
with random orientation of the thin film unit cell so that all families of planes will be picked
up in the θ-2θ scans. However, c-axis orientation of a thin film refers to the fact that only
one family of planes, such as (00l), satisfy the Bragg condition during the θ-2θ scan. From
the θ-2θ scans, the effect of lattice mismatch strain can be traced in c-axis oriented thin
films as a function of the thickness of the film or as a function of the substrate. Actually, the
lattice constant of the thin film rarely matches that of substrate. Only when the lattice
mismatch between the substrate and the thin film is less than 10%, the growth of elastically
strained thin films is possible. There is typically stress in the plane of the thin film that
results in either the shortening or lengthening of the c-axis lattice constant, corresponding
to whether the stress is tensile or compressive.
Rocking curve can be used to determine the degree of c-axis orientation. The detector
angle is fixed to the value for a certain (00l) crystalline plane, such as (001), 2θ001, and the
source angle can be varied by as much as two degrees around θ001. The full width at half
maximum (FWHM) of the resulting peak can tell if each unit cell is completely aligned or if
there is some slight tilting of any of the unit cell. A FWHM of less than 0.5º indicates
superior alignment of the unit cells in the thin film.
Ф-scan is used to determine the locking of the a- and b-axis within the film and with the
substrate and then to examine if a thin film is epitaxial. By titling the crystal to a set of
planes away from those that have their normal perpendicular to the plane of the film, and
rotating the crystal in the plane of the film, the symmetry of the crystal and the orientation
of the a- and b-axis can be revealed. The thin films studied here can be approximated as
have a cubic unit cell, so how to detect a cubic unit cell will be described. For a cubic crystal
that is c-axis oriented, such as (00l) oriented, the crystal could be tilted for example to the
(112) family of planes in order to reveal the in-plane character of the film. The tilt angle is
determined by:
cos  
h1 h2  k1 k 2  l1l 2
(h12  k12  l12 )( h22  k 22  l 22 )
(2.4) [3]
The ф-scan should display four peaks, which are 90º apart, if the a- and b-axis are locked
within the film. These four peaks show the four-fold symmetry of the cubic crystal. Any
extra peaks in the ф-scan would indicate that the in-plane axes are not completely locked,
and the film would be oriented but not epitaxial.
Other varieties of X-ray diffraction include small incident angle reflection, two-dimensional
mapping, precise x-ray diffraction, etc.