Electron channelling contrast imaging (ECCI)

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Electron channelling contrast Imaging (ECCI) provides rapid and non-destructive characterisation of the structural properties of materials in a scanning electron microscope (SEM). Going beyond simple topographic microscopy, ECCI is used extensively for detailed characterisation of crystalline materials ranging from metals to ceramics and semiconductors [bibcite key=Wilkinson1997Micron28].

Electron channelling contrast images are produced when a sample is positioned so that a plane or planes may diffract the electrons incident on the sample. Changes in crystallographic orientation or changes in lattice constant due to local strain are revealed by changes in contrast in the channelling image constructed by monitoring the intensity of backscattered electrons as the electron beam is scanned over the sample. Extremely small changes in orientation and strain are detectable, revealing, for example, low angle tilt and rotation boundaries and atomic steps and enabling extended defects such as dislocations and stacking faults to be imaged [bibcite key=Naresh-Kumar2012PRL108,Naresh-Kumar2013APL102].

Our main application of ECCI is for the characterisation of electronic and optoelectronic devices as the reduction of defects such as threading dislocations in materials such as GaN, ZnO, SiC, and related materials is key to the optimisation of new devices. Threading dislocations act as scattering centres for light and charge carriers and give rise to nonradiative recombination and to leakage currents, severely limiting device performance [bibcite key=Look1999PRL82,Wu1996JAP80,Liu2007JAC40].

Vertical threading dislocations appear as spots with black-white (B-W) contrast; this is seen in Fig. 1, an electron channelling contrast image acquired from a nitride semiconductor thin film structure.

ECCI_fig1

Figure 1: Electron channelling contrast image from an AlGaN/GaN high electron mobility transistor structure. The image shows dislocations (spots with black-white contrast) and atomic steps (lines). Variations in grey scale indicate variations in tilt or twist.

Fig. 2 shows an electron channelling contrast image from m-plane GaN. Stacking faults are visible as lines in the image terminated by partial dislocations.

ECCI_fig2

Figure 2: Electron channelling contrast image from m-plane GaN on LiAlO2 (with a substrate offcut of 2.5° towards [010]). Some stacking faults are outlined in red.

Figure 3: ECCI geometry

Figure 3: ECCI geometry

The conditions required to resolve individual dislocations or stacking faults in an electron channelling contrast image are quite stringent: a small (nanometres), high brightness (nanoamps or higher), low divergence (a few mrad) electron beam is required [bibcite key=Trager-Cowan2007PRB75]. Such conditions are met in a field emission gun scanning electron microscope – we use the FEI Sirion. We use a forescatter geometry where the sample is tilted between 30° and 70° to the impinging electron beam and the backscattered electrons are detected by an electron-sensitive diode placed in front of the sample [bibcite key=Trager-Cowan2007PRB75,Simkin1999UM77]; see Fig. 3. It is also necessary to use a detection system that allows discrimination between electrons leaving the sample which carry channelling information and those which have been diffusely scattered by the sample. An amplifier which can offset the diffuse background signal and amplify the channelling signal is required. We a signal amplifier provided by K.E. Developments Ltd.

The acquisition of an electron channelling pattern (ECP) allows the set of planes from which the electrons are diffracted to be selected. This procedure is referred to as selecting g. An ECP is obtained when changes in the backscattered electron intensity are recorded as the angle of the incident electron beam is changed relative to the surface of a single crystal area of the sample—this is referred to as rocking the beam. When an image is acquired at low magnification, as the beam is scanned over the sample, it changes its angle with respect to the surface of the sample (in our case by around 2.5°) allowing an ECP to be obtained. When the beam is rocked over the sample, different planes of the crystal satisfy the Bragg condition, giving rise to the appearance of overlapping bands of bright and dark lines (Kikuchi lines [bibcite key=Williams2009]) superimposed on the image of the sample; this is the ECP, as seen in Fig. 4. The ECP is a 2-D projection of the crystal lattice. Tilting the sample moves the ECP up and down on the viewing screen and rotating the sample moves the ECP from left to right. The pattern centre (PC) of the ECP corresponds to the plane(s) from which the incident electrons are diffracted.

Figure 4: (a) Illustrating beam rocking (b) ECP from a GaN thin film

Figure 4: (a) Illustrating beam rocking (b) ECP from a GaN thin film

Combining ECCI with other techniques in the SEM such as energy dispersive X-ray mapping or cathodoluminescence mapping allows the correlation of structural and compositional properties, or structural and luminescence properties of materials. For example Fig. 5 below shows an electron channelling contrast image and a hyperspectral cathodoluminescence intensity map of the near band edge emission (NBE) from approximately the same region of a Al0.8Ga0.2N:Si thin film (~ 1.5 µm) grown on epitaxially laterally overgrown AlN on a patterned AlN/sapphire template. The NBE has a peak at around 5.3 eV for this sample.

ECCI_fig5

Figure 5: (a) Electron channelling contrast image (b) Cathodoluminescence NBE intensity image. Arrows indicate threading dislocations with a screw component which appear as dark spots in the cathodoluminescence image.

From the images presented in Fig. 5 above we can conclude that different crystalline domains are formed due to spiral growth around dislocations with a screw component which act as nonradiative recombination centres for NBE emission [bibcite key=Kusch2015APL107].

References

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