Nanoscale characterisation of nitride semiconductor thin films using EBSD, ECCI, CL and EBIC

Lead Research Organisation: University of Oxford
Department Name: Materials

Abstract

The aim of this project is to produce a step change in the performance of the scanning electron microscopy techniques of electron backscatter diffraction (EBSD), and electron channelling contrast imaging (ECCI), and exploit these relatively new techniques together with cathodoluminescence imaging (CL) and electron beam induced current (EBIC) for the characterisation and hence the improvement of nitride semiconductor thin films. Such materials are used in the manufacture of UV/blue laser diodes, UV/visible LEDs and white LEDs. Nitride laser diodes are presently dictating the development of next generation DVDs and developments in printing and colour copying. Present applications of nitride LEDs extend from street lighting, to back lighting in mobile phones, to traffic lights. Future use of nitride-based LEDs promises to revolutionise lighting in the home and office. Nitrides are also being developed for the production of high frequency, high power electronic devices.

EBSD is an attractive technique with which to interrogate the crystallographic properties of materials because it can provide information on crystal orientation, polytype and strain with a resolution of tens of nanometres. In EBSD an electron beam is incident on a sample which is tilted at an angle of typically 70 degs. The impinging electrons are scattered inelastically through high angles forming a diverging source of electrons that can be diffracted. A simple description for the formation of an electron backscatter diffraction pattern (EBSP) presumes that electrons that satisfy the Bragg condition for a given plane emanate in diffraction cones from both the upper and lower surfaces of that plane. When these cones intersect a phosphor screen Kikuchi lines are observed. An EBSP consists of a large number of overlapping Kikuchi bands and is a 2-D projection of the crystal structure. Rotation of a crystal will produce a rotation of the EBSP; a tilt of a crystal will produce a shift in the EBSP, strain will produce distortion of the EBSP. EBSPs acquired from a mesh of points on a sample will produce a map of tilt, rotations or strain in that sample. In the course of our research we aim to attain much improved levels of sensitivity (less than 1 part in 10,000) to changes in tilt, rotation and strain to obtain detailed high resolution information on the crystallographic texture and strain distribution in nitride semiconductors.

ECCI can be used to reveal single crystallographic defects such as dislocations in semiconductor thin films. In ECCI the intensity of electrons backscattered from a suitably oriented sample depends on the relative orientation of planes in a crystal. Changes in crystallographic orientation or changes in lattice constant due to strain are revealed by changes in grey scale of an image constructed by monitoring the intensity of backscattered electrons as an electron beam is scanned over the sample. Defects are imaged due to lattice plane tilting and strain they produce. Our aim is develop the ECCI technique so that it may be used to fully characterise all defects present in our films.

We plan to apply these powerful techniques to optimise the quality of nitride semiconductor thin films, including: (i) non-polar and semi-polar nitride thin films, (ii) epitaxially overgrown nitride thin films and nitride thin films grown on native substrates, (iii) high aluminium content nitride thin films (including films grown on silicon), (iv) high indium content nitride thin films, and (v) zinc blende nitride thin films. Such novel nitride semiconductor structures will allow the production of higher power LEDs and extend their wavelength range to the red and infrared. They will also open up new applications for nitride-based devices - solar cells for example.

Throughout the project we will also establish with industrial partners the best routes for development and commercialisation of new instrumentation, and nitride thin film growth processes.

Publications

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Britton T (2016) Tutorial: Crystal orientations and EBSD - Or which way is up? in Materials Characterization

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Trager-Cowan C (2014) Electron Channeling Contrast Imaging of Defects in III-Nitride Semiconductors in Microscopy and Microanalysis

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Vilalta-Clemente A (2015) Analysis of Dislocation Densities using High Resolution Electron Backscatter Diffraction in Microscopy and Microanalysis

 
Description We have used electron backscatter diffraction (EBSD) to measure the variation in lattice rotations and strains in II-V nitride thin films grown in various collaborators labs.
In combination with electron channelling contrast imaging a methodology has been developed to allow quantification and separation of threading dislocation densities for screw edge and mixed defect types. This can be performed on the top surface of as grown structures removing the need for thin foil production and TEM observation and so allowing information to be provided to growers in a more quickly.

Advances have also be made toward use of simulated EBSD patterns as reference patterns. Methods for interpolating intensity distributions calculated in a full dynamical diffraction simulations to allow matching to experimental patterns at lower computational cost have been explored. As have new routes for calibrating the pattern centre and camera length.
Exploitation Route EBSD method is providing useful materials characterisation information that is being feedback to materials growers.
There is clearly scope for the method to be taken up for characterisation of other systems. SiC for power electronics being perhaps the most obvious target.

The advances made in understanding and improving the sensitivity of the method has impact on wider application of the method for example we have been able to take on new applications where the dislocation density is relatively low including Si for solar photovoltaics, and deformation of upper mantle rocks (NERC funding)
Sectors Aerospace, Defence and Marine,Electronics,Energy,Environment,Manufacturing, including Industrial Biotechology

URL https://omg.web.ox.ac.uk/
 
Description Better understanding of EBSD strain mapping method and visibility of the technique has improved both the quality and up-take of commercial products (hardware and software). Feedback of materials characterisation results has allowed growers to better understand materials being produced.
First Year Of Impact 2014
Sector Aerospace, Defence and Marine,Electronics,Energy,Environment,Other
Impact Types Economic

 
Description NERC Award
Amount £483,386 (GBP)
Funding ID NE/M000966/1 
Organisation Natural Environment Research Council 
Sector Public
Country United Kingdom
Start 01/2015 
End 01/2018
 
Description Platform Grant - Characterisation
Amount £1,094,904 (GBP)
Funding ID EP/K032518/1 
Organisation Engineering and Physical Sciences Research Council (EPSRC) 
Sector Public
Country United Kingdom
Start 06/2013 
End 05/2018
 
Description Earth Science 
Organisation University of Oxford
Department Department of Earth Sciences
Country United Kingdom 
Sector Academic/University 
PI Contribution We are transferring methodologies developed in Materials Science for EBSD mapping of lattice strain and dislocation densities to natural and synthetic minerals of interest to Earth Sciences.
Collaborator Contribution Identification of key Earth Science problems to which the methodology could be applied. In particular upper mantle rocks have been targeted as an initial application and a series of olivine samples have been worked on. Initial results were very promising and lead to a successful NERC funding application. The project is now fully active and funded and is likely to grow over the next few years.
Impact Multi-disciplinary collaboration involving transfer of methodologies from Materials Science to Earth Science Outputs to date have been conference presentations (no proceedings with ISSN) though two papers are currently under review
Start Year 2014