Characterization of Thin Films by Low Incidence X-Ray Diffraction

doi:10.4236/csta.2012.13007

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Crystal Structure Theory and Applications, 2012, 1, 35-39

http://dx.doi.org/10.4236/csta.2012.13007 Published Online December 2012 (http://www.SciRP.org/journal/csta)

Characterization of Thin Films by Low Incidence X-Ray

Diffraction

Mirtat Bouroushian*, Tatjana Kosanovic

General Chemistry Laboratory, School of Chemical Engineering, National Technical University of Athens, Athens, Greece

Email: *[email protected]

Received September 20, 2012; revised October 22, 2012; accepted November 3, 2012

ABSTRACT

Glancing Angle X-ray Diffraction (GAXRD) is introduced as a direct, non-destructive, surface-sensitive technique for

analysis of thin films. The method was applied to polycrystalline thin films (namely, titanium oxide, zinc selenide,

cadmium selenide and combinations thereof) obtained by electrochemical growth, in order to determine the composition

of ultra-thin surface layers, to estimate film thickness, and perform depth profiling of multilayered heterostructures. The

experimental data are treated on the basis of a simple absorption-diffraction model involving the glancing angle of

X-ray incidence.

Keywords: Glancing Angle X-Ray Diffraction; Thin Films; Titanium Oxides; Metal Chalcogenides; Electrodeposition

1. Introduction

X-ray techniques hold a leading role as a tool for mate-

rial characterization. In this connection, X-ray powder

diffraction (XRD) generally employs the conventional θ -

2θ (or Bragg-Brentano) “reflection” geometry, in which

the incidence angle equals the angle of the diffracted

beam with respect to the inspected sample surface (Fig-

ure 1(a)). The device configuration ensures that a high

intensity beam diffracted from any particular set of crys-

talline planes of the sample will be focused onto a slit in

front of the rotating detector. However, X-rays with large

glancing angles of incidence will go through a few to

several hundred micrometers inside the material under

investigation, depending on its “radiation” density, so

that when it comes to thin film analysis the beam pene-

tration depth may be much greater than the sample thick-

ness. Hence, conventional XRD is rather not suitable for

detailed study of sub-micrometric layers in thin film

specimens. For instance, in the case of a multilayer elec-

trodeposited film, conventional XRD would only allow

for tracing the complete structure after deposition, while

the large number of overlapping peaks indicating the dif-

ferent crystallographic phases would make the spectrum

interpretation difficult.

Grazing incidence configurations have been developed

to overcome such limitations, e.g., to render the XRD

measurement more sensitive to the near surface region of

the sample and minimize the substrate contribution on

the diffraction response. In the glancing angle X-ray dif-

(a)

(b)

Figure 1. Schematic diagrams of (a) symmetric θ - 2θ, and

(b) asymmetric glancing angle XRD geometry.

fraction (GAXRD) technique, the Bragg-Brentano ge-

ometry is modified to provide an “asymmetric” diffrac-

tion result, which allows access to small depths in the

sample by varying the incidence angle. A parallel mono-

chromatic X-ray beam falls on the sample surface at a

fixed, low glancing angle, α (larger than the critical angle

for total reflection, αc, but usually smaller than 10˚) and

the diffraction profile is recorded by detector-scan only

(Figure 1(b); non-rotating sample). GAXRD makes pos-

*Corresponding author.

M. BOUROUSHIAN, T. KOSANOVIC

sible the use of X-rays for the characterization of sur-

faces, buried interfaces, and ultra thin films. By means of

this technique, valuable information can be obtained re-

garding the thickness and phase crystallography of the

sample [1-3], the change of its composition with depth

[4], and its microstructure (stress, preferred orientation,

etc.) [5].

In the present work, the GAXRD method is applied as

a depth-profiling tool for the identification of surface

oxide layers, the estimation of film thickness, and phase

characterization in multilayer deposits. Practice and theory

behind the evaluation is briefly discussed.

2. Experimental

An X-ray Siemens D5000 powder diffractometer with a

Soller collimator, a flat monochromator, and a fixed

CuΚα radiation (λ = 154.06 pm) was employed for film

characterization. The primary beam divergence was suf-

ficiently small to allow for the required resolution of low

glancing angles of incidence. The instrument was ope-

rated in a step-scan mode in increments of 0.02˚ 2θ, and

counts were accumulated for 1 s at each step. The angle

of incidence α was set to different values (0.2˚ - 10˚) in

order to vary the interaction length in each depth region

with scanning of the exit angle.

Titanium oxide films produced by anodization of me-

tallic titanium electrodes at various voltages in 1 M sul-

furic acid bath, as well as single and multi-layered thin

films of zinc and cadmium selenides, electrodeposited on

titanium and nickel electrodes from acidic aqueous solu-

tions [6,7], were used for the glancing angle XRD char-

acterization.

3. Results and Discussion

3.1. Identification of Thin Surface Layers

Thin titanium oxide films were formed by soft anodiza-

tion of titanium metal for few minutes at low oxidation

potential, and the electrode specimens were characterized

by conventional and grazing angle XRD. No oxide could

be detected by conventional XRD, since the strong con-

tribution from the Ti metal substrate overshadowed the

response from the upper oxide layer (Figure 2). By con-

trast, identification of the formed oxides down to very

thin surface layers could be achieved by reducing the

angle of incidence according to the GAXRD protocol, so

that titanium sub-oxides such as Ti2O, Ti2O3 were readily

detected (Figure 2).

3.2. Depth Profiling of Surface Oxide Films

Monitoring the distribution of the oxide phases along the

perpendicular, to the substrate, direction would be indis-

pensable in following the evolution of the anodic film

through the successive stages of electrolytic growth. In

this regard, results from grazing incidence experiments

as shown in Figure 3, for a sample obtained by anodiza-

tion of titanium metal at a voltage of 100 V for 2 h, per-

mit a semi-quantitative analysis of the composition pro-

file of the oxide layer. The depicted GAXRD sequence

yields information about the proportions of the oxide

phases at various depths in the sample, on the basis of the

recorded intensities of the main diffraction peaks. By

contrast, conventional XRD (not shown in figure, but

similar to the spectrum at α = 10˚) only indicates the

formation of both the common oxide (TiO2) phases,

namely rutile and anatase [6].

3.3. Estimation of Thin Film Thickness

The method we use, according to Nauer et al. [1], in order

Figure 2. Conventional and glancing angle XRD patterns of

Ti anodized at 5 V for 15 min. JCPDS #10 - 0063 and #11 -

0218 were used for Ti2O and Ti2O3 identification.

Figure 3. GAXRD diagrams of an oxide layer obtained by

anodization of Ti at 100 V for 2 h. Anatase, rutile, and Ti

substrate reflections are indicated. Note that the diffraction

signals from the substrate get weaker and disappear by

lowering α. Shallow angles give information about layers

closer to the sample surface.

M. BOUROUSHIAN, T. KOSANOVIC 37

to estimate the thin film thickness is based on the fol-

lowing assumptions: 1) at any low angle of incidence a

the attenuation of X-rays in the material (due to absorp-

tion, multiple scattering, etc.) follows the exponential

variation predicted by the Beer-Lambert law; and 2) the

whole of non-interacted radiation will be recorded at the

detector as a diffraction signal at the exit angle 2θ. In

Equation (1) (cf. [1]), f designates the intensity of inter-

acted radiation (lost for the detector at 2θ), normalized to

unity for complete attenuation when the transversed

depth x goes to infinity; d is the pursued thickness of the

film (= xsinα), and μ the linear absorption coefficient of

the target material.

1exp

sin

f



 











(1)

This expression is used to provide simulation curves

for variable α angle, having the thickness d as the unde-

termined parameter (μ-values are needed here for CuKα

radiation). Concurrently, diffraction peaks are recorded

at certain glancing angles of X-ray incidence for a spe-

cific crystallographic hkl plane of the investigated mate-

rial corresponding to the selected 2θ Bragg reflection,

e.g., anatase (101). The observed peak intensity values

(Iexp) are corrected for decreasing incident radiation due

to the lowering of angle a, and normalized to the maxi-

mum corrected intensity, which corresponds to near total

external reflection conditions (i.e., for α close to the

critical angle αc). Finally, a working experimental value

is obtained, as given by Equation (2).

exp

max sin



 (2)

The above procedure applies preferably for a charac-

teristic intense diffraction peak and can be repeated for

more than one such peaks of the spectrum, if present. It is

expected that fitting of the If values to the simulation

curves drawn by Equation (1) will give a measure of the

thickness of the examined film. In Figure 4, the results

of the above procedure are depicted for an anodic oxide

film on titanium, which is assumed to consist entirely

either of anatase or rutile phase. In fact, the oxide film

comprises a mixture of varying stoichiometry; however,

the absorption coefficients of the constituent phases do

not differ appreciably, thereby this approximation is not

critical for the result.

Statistical fitting of the experimental points to the

simulation curves indicates that according to this model

the thickness of the film lies in the range 300 ± 50 nm.

The accuracy of the estimation, although not verified

here by an independent measurement, corresponds to an

anodic growth rate of 3 ± 0.5 nm·V–1, which is fairly

consistent to the rate of 2.5 nm·V–1 reported for anodic

titanium oxide films grown potentiostatically at similar

conditions [8].

One can substantially simplify the above procedure by

assuming that the observed vanishing of the diffraction

response of the substrate (Ti metal) for a certain X-ray

grazing incidence practically occurs when the radiation

intensity penetrating the material becomes 1/e of its ini-

tial value. Then, according to the absorption law (Equa-

tion (1)) for the corresponding shallow incidence α, gi-

ven that the incoming radiation has interacted only with

the oxide upper layer, the interface between the latter and

the substrate can be considered as lying at a depth:

sin





 (3)

Assuming that the entire oxide layer consists of the

anatase phase, one has μ = 488 cm–1 (CuKα), and from

detailed spectra similar to Figure 3 the as-obtained layer

Figure 4. Solid lines: relative intensity (f) of radiation vs.

angle of incidence (α) as obtained by Equation (1), for dif-

ferent layer thicknesses of anatase and rutile. Black squares:

normalized experimental intensities of GAXRD signals for

anatase (101) at 2θ = 25.3˚, and rutile (110) at 2θ = 27.5˚, for

varying α.

M. BOUROUSHIAN, T. KOSANOVIC

thickness is estimated as 180 nm. This value deviates

from the above calculated, certainly as expected from the

simplistic argument we used, and can be trusted only as

an order-of-magnitude calculation. In any case, accurate

assignment of the α-angle marking the disappearance of

the substrate is essential for the calculation. Note also

that factors related to material density and surface rough-

ness account for limited accuracy as to the thickness

evaluation of the present anodic films. Actually, the up-

per values of the above estimation should be preferred

since the effective density of the anodic oxide layer is

less than the bulk value, due to a porous structure [6].

Clearly, independent measurements should be used in

order to verify the reliability of the presented procedure,

and possibly standardize a correction or scaling factor for

a range of thickness values. Along this line, our GAXRD

results for cadmium selenide (CdSe) films of sub micro-

metric thickness (electrodeposited on Ti or Ni electrodes

by various electrolysis charges) were found to be in fair

agreement with stylus profilometry measurements, namely,

within an accuracy of 50 nm (unpublished results). Else-

where also [1], application of the above model has been

shown to give results in consistence with other mea-

suring techniques.

3.4. Analysis of Multilayer Heterostructures

The present GAXRD protocol was employed for per-

forming analysis of a multilayer heterostructure, namely

the two-layer ZnSe/CdSe/Ni system, prepared by se-

quential cathodic electrodeposition of polycrystalline ZnSe

and CdSe on Ni substrate. The (111) reflection of the

upper ZnSe layer (2θ = 27.3˚) along with the CdSe(111)

(2θ = 25.3˚) were used for the estimation (Figure 5). The

GAXRD patterns of the system as given in Figure 5 re-

veal the layer sequence and the depth profile of the

stratified film. Note that upon decreasing the angle of

incidence the signals from the lowest lying phase (Ni

substrate; not shown in the figure) disappear first, follow-

ed by the complete vanishing of signals from the interme-

diate CdSe layer. Eventually, only the response from the

ZnSe layer remains, and no other phases are detected. It

can be concluded thus that appreciable intermixing of the

various phases can be excluded, that is, the electroche-

mical deposition sequence yields the intended stratified

structure with abrupt transition at the interfaces. In other

words, the very existence of a barrier-type upper layer

(ZnSe) is observed by the disappearance of the CdSe(111)

reflection at a certain low incidence angle (Figure 5: line

at 0.6˚).

The estimation of the thickness of the ZnSe layer is

demonstrated in Figure 6, where corrected and norma-

lized ZnSe(111) diffraction intensities obtained for X-ray

incidence angles of 0.2˚ - 4˚ are shown fitted to the rele-

vant simulation curves. By this graph, a thickness of

Figure 5. GAXRD diagrams of a ZnSe/CdSe bilayer elec-

trodeposited on Ni from acidic electrolyte [7]. ZnSe(111)

and CdSe(111) reflections are indicated.

Figure 6. Solid lines: relative intensity (f) of radiation vs.

angle of incidence (α) as obtained by Equation (1), for dif-

ferent layer thicknesses of ZnSe. Black squares: norma-

lized experimental intensities of GAXRD signals for

ZnSe(111) at 2θ = 27.3˚.

250 ± 50 nm could be ascribed to the ZnSe layer under

examination. At the same time, the application of the

simple formula of Equation (3) led to the quite consistent

result of 290 nm. It is believed that the better agreement

between these two values compared to the case of anodic

oxide layers of the preceding paragraph is due to the ex-

istence of a better defined interface in the present he-

terostructure between the epilayer and the substrate.

4. Conclusion

On the basis of the presented results, the low-glancing

angle incidence X-ray technique (GAXRD) has been

demonstrated to provide a simple and innovative tool for

studying the microstructure of polycrystalline films; in

particular for: 1) identification of crystalline surface

phases; 2) estimation of the thickness of crystalline thin

layers/films; and 3) depth profiling of multilayer struc-

tures.

M. BOUROUSHIAN, T. KOSANOVIC

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