Near-field cathodoluminescence of nanoscopic diamond properties

Diamond and Related Materials 10 Ž2001. 1647᎐1651

Near-field cathodoluminescence of nanoscopic diamond properties R. Heiderhoff a,U , R.M. Cramer b, O.V. Sergeev c , L.J. Balk a a

Lehrstuhl fuer Elektronik (LFE), Fachbereich Elektrotechnik u. Informationstechnik, Bergische Uni¨ ersitaet Wuppertal, Fuhlrottstr. 10, 42097 Wuppertal, Germany b ALTIS Semiconductor, Corbeil-Essonnes Cedex, France c Belarussian State Uni¨ ersity of Informatics and Radioelectronics, Minsk, Belarus

Abstract Near-field detection cathodoluminescence ŽNF-CL. analyses on MWCVD diamond has been carried out in a scanning near-field optical microscope ŽSNOM.rscanning electron microscope hybrid system to ensure that even the smallest structures with modified material properties can be evaluated. The CL signal is detected directly with the SNOM tip above the recombination center while the sample is irradiated simultaneously as well as homogeneously with an electron beam. The achievable resolution of this NF-CL analyses has been improved to approximately 50 nm. In addition, it was possible to determine electronic properties such as the local diffusion lengths by keeping the position of the SNOM probe constant and scanning the electron beam. Using this technique, distributions of defects within small diamond grains became visible without any sample preparation. Dislocations, hydrogen plasma induced defects, and nitrogen impurities were separated on a H post-treated MW-CVD diamond single crystal. Clusters of the 575-nm luminescence system were detected. A shift from blue to green of the Band-A emission depicted a dislocation. 䊚 2001 Elsevier Science B.V. All rights reserved. Keywords: Cathodoluminescence; Diamond defects; Scanning electron microscopy; Spectroscopy

1. Introduction Cathodoluminescence from diamond has been studied for more than 50 years and is established in the characterization of natural, high-pressure synthesized, and CVD diamond. The spectral range covered by the luminescence band extends from 5.3 eV in the ultraviolet Žassociated with the ‘edge emission’ w1x. corresponding to the indirect energy gap to approximately 1.2 eV in the near-infrared w2x. More than 100 of the optical centers that have been documented for diamond give rise to luminescence. Most of them are listed by Clark

U

Corresponding author. Tel.: q49-202-439-2972; fax: q49-202439-3804. E-mail address: [email protected] ŽR. Heiderhoff..

et al. w3x. An overview on diamond luminescence can be found in the review articles by Heiderhoff and Balk w4x and Collins w5x. For the recognition of even the smallest defects in diamond and their subsequent characterization, analytical tools are needed that combine both requirements: nanometer resolution of material relevant parameters and a fast routine response without an essential need of specimen preparation. Scanning near-field microscope ŽSNOM. techniques can solve this problem as is obvious from the large number of publications covering fluorescence investigations on single molecules Žsee for instance w6x. or high-resolution photoluminescence w7x investigations of semiconductors, while simultaneously being able to analyze the sample topography with subnanometer resolution. Near-field cathodoluminescence investigations can be carried out by the implementation

0925-9635r01r$ - see front matter 䊚 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 5 - 9 6 3 5 Ž 0 1 . 0 0 4 4 8 - 4

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use of band pass filters. Due to the small aperture size the average number of photons we detected on the diamond was less than 1000 per point calculated from the PMT current, quantum efficiency, and magnification. The signal-to-noise ratio has been improved significantly by chopping the electron beam and using lock-in techniques. High resolution, up to 50 nm, has been achieved and, in addition, the diffusion length of minority carriers can be measured by keeping the probe at a fixed position while scanning the primary electron beam. Fig. 1. Scanning electron microscoperscanning near-field optical microscope hybrid system to perform high resolution cathodoluminescence.

of a scanning near-field microscope ŽSNOM. setup into the analysis chamber of a scanning electron microscope ŽSEM. using tapered, coated optical glass-fiber probes w8x. In a similar approach published by Troyon et al., Si 3 N4 cantilevers have been utilized as near-field probes w9x.

2. Experimental

To ensure that even the smallest defects in diamond can be evaluated by high-resolution near-field CL ŽNFCL. investigations, a compact scanning near-field optical microscope ŽSNOM. was developed and mounted into the specimen chamber of a scanning electron microscope Žsee Fig. 1. w8x. The vicinity of the tip-sample ‘contact’ area is irradiated homogeneously with primary electrons and the resulting luminescence is picked up directly above the recombination centers by the SNOM probe which is glued onto one leg of a tuning fork for distance regulation Ždistance probe-sample is a few nanometers. w10x. Coated fibers were used for the CL detection in order to reduce the scintillation of the fiber material generated by secondary or backscattered electrons. The incident angle of the primary electron beam is 60⬚ to guarantee that the generation volume is centered under the aperture of our probe for diamond at an acceleration voltage of 15 kV w11x. The optical fiber is fed out of the chamber and coupled to a photo-multiplier tube ŽPMT.. Spectrally resolved measurements are performed by introducing calibrated optical band pass filters Žfor instance RGB. into the optical path. However, because many of the emission bands are very broad it is difficult to isolate the emission originating from one type of defect by the

3. Results

In this study, we characterized by conventional and NF-CL analyses two MW-CVD diamond films deposited under the same conditions. One of these films has been post-treated in a hydrogen plasma after the deposition process. In Fig. 2 the results of these CVD diamond films obtained with conventional cathodoluminescence are illustrated. The broad Band-A and 575-nm system emissions are detectable on the untreated film. Small peaks appeared at 630 nm and 700 nm. The H plasma post-treated film has in contrast to the untreated diamond film, an additional broad luminescence centered at 550 nm. The origins of the 550-nm system and the Band-A luminescence which is located at dislocations are still not clear, because there is no theory which describes all observed phenomena w12x. The 575-nm Ž2.156 eV. system is most intense in the cathodoluminescence spectra of nitrogen-doped CVD diamonds following radiation damage and annealing. Collins and Lawson concluded that this system is due to a single nitrogen atom and a vacancy w13x. The panchromatic cathodoluminescence image of the untreated CVD film gained with the scanning electron microscope is illustrated in Fig. 2b. The luminescence is most intense on the facets of the diamond grains Žexplanation: see for example w14,15x. and twins are detectable. Distributions of defects within the diamond grains are not visible due to the small size of the particles and the large diffusion length of the minority carriers. The emission of each facet seems to be homogeneous also for the H plasma treated film. A difference within the micrographs of these untreated and post-treated MW-CVD films was not found. A lot of efforts were made to recognize and characterize even the smallest defects in diamond. TEM analyses superimposed with CL analyses needs an intensive destructive sample preparation to achieve a

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Fig. 2. Ža. Spectral behavior of an untreated and a H plasma posttreated CVD diamond film. Žb. Conventional panchromatic cathodoluminescence analyses of the untreated diamond film.

high spatial resolution w16,17x. A drawback of scanning probe microscopes is the restricted area of view w11x. The piezoelectric scanner does not allow a simple switching from a mesoscopic view to a nanoscopic view, the first necessary for orientation, the latter necessary for high spatially resolved measurements. To overcome these problems and to allow a comparison with conventional SEM measurements NF-CL investigations with the above mentioned system were carried out.

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The NF-CL results obtained at the H plasma posttreated film are illustrated in Fig. 3. We investigated a grain with a cross-section of approximately 3.5 ␮m. From the topography image Žsee Fig. 3a. a ‘bubble’ like structure with a height of approximately 100 nm is detectable. A similar morphology was found also for the untreated film, weakly detectable in the secondary electron image of Fig. 2. The luminescence of the so called Band-A emission, the broad H plasma induced peak at 550 nm, and the 575-nm multi-phonon system were separated spectrally using calibrated RGB filters. The related RGB luminescence regions, picked up directly above the recombination center and therefore independent of diffusion processes in contrast to conventional and TEM cathodoluminescence, are illustrated in Fig. 3b᎐d. Fig. 3e shows the integral NF-CL image Žsuperimposed RGB results .. The spectral dominating emission regions are depicted in Fig. 3f. White areas represent the Band-A emission regions. H plasma induced defects and the 575-nm multi-phonon system are illustrated by gray and dark-gray areas, respectively. As can be seen from these micro-graphs a lateral resolution of less than 100 nm is achievable also for these spectrally resolved measurements. A correlation of the topography and recombination centers was not found in these experiments. Dislocations are visible clearly in the integral NF-CL image and detectable weakly only from Fig. 3b,c. This decrease in the intensity effects therefore only the Band-A and hydrogen plasma induced luminescence but has no significant influence on nitrogen related defects. Furthermore, the Band-A and H plasma induced emission centers dominate in the vicinity of these dislocations and lead to a shift in the wavelength Žblue and green. obtainable from the marked areas in Fig. 3f. A similar behavior also appears on dislocations from natural diamond which is illustrated in Fig. 4. This alteration can be seen from green to blue as well as from blue to green. This spectral displacement was observed by timeresolved analyses for conventional CL analyses and has been shown for example in Heiderhoff and Balk w4x. But these conventional analyses do not allow a local separation of defects with a high resolution due to the large diffusion length of the minorities in diamond and the fact that the generation of electron᎐hole pairs and their recombination can be at different positions. From this NF-CL investigations two separated recombination centers are visible clearly now. Without taking into account the decrease in intensity at dislocations, the Band-A and the H plasma induced luminescence regions are spread randomly over the sample surface. A cluster structure can be detected in the emission image of the 575-nm multi-phonon system Žsee marked areas in Fig. 3d.. The luminescence of the

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Fig. 3. Ža. Topography of a H plasma treated diamond grain Ždifference in height is 600 nm.. Žb. Near-field cathodoluminescence of the so-called Band-A emission using a calibrated blue filter. Žc. Near-field cathodoluminescence of hydrogen plasma induced defects using a calibrated green filter. Žd. Near-field cathodoluminescence of nitrogen related defects using a calibrated red filter. Že. Integral near-field cathodoluminescence image of the hydrogen treated diamond film. Žf. Marked areas of Že. where the Band-A, H plasma induced, and N related emission dominated.

630- and 700-nm peaks are negligible in this micrograph due to the low intensity and small FWHM Žnumber of detected photons. of these centers. These luminescence islands have a diameter of approximately 200 nm and correlate in size and distribution with the observed ‘n-regions’ investigated by nano electron beam induced current Žnano-EBIC. characterizations w18x. Of course, cathodoluminescence investigations cannot distinguish directly between p- and n-behavior of a semiconductor, but allow analysis of local densities of defects and their binding energies. Therefore, the electronic structure detected by nano-EBIC can be explained by the coexistence of nitrogen and vacancies as presumed from panchromatic NF-CL investigations in Cramer et al. w11x. On the other hand, the electronic behavior can be analyzed indirectly by keeping the SNOM probe at a fixed position while scanning the primary electron beam if the diffusion lengths are known. An example of the contactless determination of the local minority diffusion length by the exponential decay of the CL signal extracted from a line scan is illustrated in Fig. 5. A micro-graph of a used glass-fiber tip is superimposed schematically on the luminescence image. Diffusion lengths of 1.8 and 1.6 ␮m were determined at the arbitrary positions. These values correspond with the results obtained by nano-EBIC investigations for

electrons and holes, respectively, and are therefore an additional indicator for the presumed electric model.

4. Summary Spectral near-field detection cathodoluminescence analyses on diamond showed an interesting perspective for the recognition of even the smallest defects and their subsequent characterization. No sample preparation is necessary so that the risk of changing relevant specimen properties during preparation is greatly reduced. The Band-A luminescence, hydrogen plasma induced defects, and nitrogen impurities were separated locally on a hydrogen plasma post-treated MW-CVD diamond single crystal by the use of calibrated RGB filters. It has been demonstrated that the Band-A and the 550-nm system depend on dislocations while no significant influence was found on the 575-nm system. A local shift from blue to green as well as from green to blue depicted these dislocations. Similar results were obtained for the Band-A emission of a natural diamond. Clusters of the 575-nm multi-phonon luminescence system with diameters of ; 200 nm were detected. The local separation of dislocations and nitrogen related centers as well as the resulting electronic model pre-

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Fig. 5. Contactless determination of local minority diffusion length by electron beam scanning Žoverlaid is schematically the position of the glass-fiber tip..

References

Fig. 4. Ža. Topography obtained on a natural diamond Ždifference in height is 150 nm.. Žb. Green-blue shift obtained with NF-CL analysis of dislocations on this natural diamond grain.

sumed by nano-EBIC analyses were verified by the determination of the local minority diffusion lengths.

Acknowledgements The authors gratefully acknowledge financial support of the present work by the Deutsche Forschungsgemeinschaft ŽDFG. ŽProject No. Ba805r4-3. carried out under the auspices of the trinational ‘D᎐A᎐CH’ German, Austrian, and Swiss cooperation on the ‘Synthesis of Superhard Materials’ and Dr M. Schreck, University of Augsburg, for the samples.

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