Grazing incidence x-ray diffraction

What is grazing incidence X-ray diffraction?

Grazing incidence XRD (GIXRD) is a modification of the X-ray diffraction (XRD) analytical method. With XRD, the crystalline phases present in a sample can be identified and their crystallinity, crystallite size, lattice paraments and strain of the phases can be determined.

When X-ray diffraction (XRD) analysis is performed on thin films or coatings (1–1,000 nanometers thick) using conventional scanning parameters, the surface layer typically produces a weak signal, while the underlying layers generate a much stronger response (Scenario a in the below figure). This imbalance makes it challenging to accurately examine the top layers with conventional XRD setup. To overcome this, measurements can be conducted with the X-ray source positioned at a fixed, small grazing incidence angle relative to the sample (Scenario b in the below figure). This technique, known as Grazing Incidence X-ray Diffraction (GIXRD), enhances surface sensitivity and allows for more precise characterization of thin films.

You can find more information about the working principles of XRD on our XRD method page.

What is GIXRD commonly used for?

GIXRD analysis reveals the phase(s) present in the sample and reveals its crystallinity, crystallite size, lattice paraments and strain of the phase. GIXRD is used to characterize crystalline thin films and coatings on solid substrates.

GIXRD is widely used by thin film and coatings manufacturers when they are developing new manufacturing techniques and need information on the crystalline properties of the materials produced. The most common use case for GIXRD is the characterization of thin films (both inorganic and organic-hybrid) deposited on typical substrates, such as silicon (Si), gallium nitride (GaN), silicon carbide (SiC), gallium arsenide (GaAs) or indium phosphide (InP). The method can also be used to study the effect of different surface modifications on the above-mentioned bare wafers. Other common use cases include the identification of unknown coatings and thin films as well as investigating the effect that certain processing conditions have on coatings.

Suitable samples and limitations of GIXRD?

GIXRD is best suited for the characterization of thin and smooth films and coatings. As a rule of thumb, samples with thicknesses ranging from just a few nanometers up to a micrometer with less than 10 nm RMS surface roughnesses are best suited for GIXRD analysis. Thicker and rougher samples can also be analyzed, but the quality of the data tends to weaken when the thickness and roughness increase.

Only samples consisting of crystalline matter are suitable for detailed GIXRD analysis, as amorphous matter yields practically no signals in GIXRD. However, GIXRD is sometimes intentionally used to examine amorphous thin films or coatings, as it provides a clear demonstration of their lack of crystallinity.

Samples containing many different crystalline components are difficult to analyze with GIXRD as the diffractograms tend to be very complex and it might be impossible to determine the crystalline parameters. When a layer of unknown material is to be identified, the layer should be homogenous, or its composition should be consistent. A successful identification of an unknown material also requires it to exist in a reference data library.

How to properly adjust the incident angle of the X-rays?

In GIXRD, the incident angle of the X-ray beam hitting the sample is adjusted relative to the critical angle of the reflected X-rays in the same way as in X-ray reflectivity (XRR), with which the structural properties of thin films can be determined. The critical angle of the reflected X-ray beam is unique for every material and is generally very small.

The higher the incident angle relative to the critical angle of the material, the deeper the X-rays go down into the material. Therefore, if the incident angle of the X-rays rises above the critical angle, the depth of the penetrating X-rays increases rapidly. When the incident angle is smaller than the critical angle, the X-rays penetrate the sample only to a depth of a few nanometers.

The surface sensitivity of GIXRD

The critical angle phenomenon enables grazing incidence XRD to measure the crystal structure of thin films and coatings by using small incident angles of the X-ray beam. Below the critical angle of the surface material, only an evanescent wave of the reflected X-rays is established for a short distance, and the wave is exponentially damped. Therefore, the reflections in the diffraction pattern are only coming from the surface structure in GIXRD.

The incident angle is usually chosen to be slightly above the critical angle of the material to obtain a total reflection of the whole surface layer. Still, penetration of the X-ray beam into the bulk material is prevented and limited only to the surface layer, making the diffraction phenomenon surface sensitive. Because the overlapping peaks in the diffractogram coming from different depths of the sample are avoided with GIXRD, the examination of thin surface films in film stacks is much easier.

GIXRD also amplifies the weak diffraction signal coming from ultra-thin films and thus optimizes the intensity of the reflected X-rays. In conclusion, GIXRD is a method that combines the best of two techniques: the analysis of crystal structure (XRD) and the examination of thin films (XRR).

GIXRD under non-ambient conditions

Grazing incidence XRD (GIXRD) can be performed under normal (ambient) or controlled (non-ambient) conditions. In non-ambient GIXRD (NA-GIXRD), the sample is analyzed while being subjected to specific environmental changes, such as variations in temperature, pressure, humidity, gas composition, mechanical stress, or electromagnetic fields.

Modifying these parameters in NA-GIXRD leads to structural changes in the material, which can be observed in real time. This technique is useful for studying transformations that occur during processes like operation, heat treatment, calcination, sintering, hydration, and dehydration.

One widely used NA-GIXRD technique is high-temperature XRD (HT-GIXRD), which helps investigate phase transitions, decomposition reactions, and the material's behavior at elevated temperatures. Certain instruments enable temperature-programmed experiments, where the temperature is gradually increased while continuously recording XRD data, allowing researchers to monitor structural evolution under heat exposure.

Measurlabs offers NA-GIXRD under a wide variety of different conditions.