Morphology of twinned diamond particles
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John C. Angus Depurtment of Chemical Engineering, Case Western Reserve University, Cleveland, Ohio 44106-7217
David Aue Digital Instruments, Inc., 520 E. Montecito Street, Santa Barbara, California 93103 (Received 14 April 1995; accepted 22 August 1995)
Morphology of twinned diamond particles grown by chemical vapor deposition was characterized by atomic force microscopy in both contact and tapping modes. Quantitative angle measurements using a surface normal algorithm were performed on untwinned crystals, penetration twins, re-entrant corners, and fivefold dimples. Tip-sample interaction is discussed. The morphology of the penetration twins and some of the re-entrant corners can be explained by low order X3 twins and flat crystallographic surfaces. Abnormally shallow re-entrants with large vicinal faces are attributed to rapid nucleation of new layers at a point along the re-entrant intersection.
1. INTRODUCTION
Twinning is one of the most important processes during diamond growth by chemical vapor deposition (CVD). It gives rise to re-entrant corners, penetration twins, and fivefold dimples. Re-entrant corners provide preferred sites for the nucleation of new layers,' thus changing the diamond growth rate and film morphology. The shapes of penetration twins have been related to the growth parameter,2 a , and it has been suggested that penetration twins are a dominant factor in determining the morphology of diamond films.3 These twin-related features can be observed with scanning electron microscopy: but quantitative measurement of angles between different crystal surfaces is difficult and time consuming. In this paper, we present quantitative angle measurements of twin-related features on CVD diamond films using a simple algorithm for measuring surface normals with an atomic force microscope. II. EXPERIMENTAL The diamond films were grown on Si substrates by biased microwave assisted CVD. The process began with negative biasing of the substrate for 25 min at 205 V with 2% CH4 in H2 at 25 Torr and a substrate temperature of 900 "C, followed by 9 h deposition with 5% CH4 and 5 % CO in H2 at a substrate temperature of 810 "C and a pressure of 35 Torr. The AFM images were taken using a multimode Nanoscope 111 (Digital Instruments, Santa Barbara, CA) in both contact and tapping modes. In contact mode, commercial Si3N4tips were used with minimized contact force to reduce tip wear. For tapping mode, commercial etched Si tips were used. To allow direct angle measurements between surface planes, a surface normal function was incorporated into the AFM J. Mater. Rss., Vol. 10, No. 12, Dec 1995 http://journals.cambridge.org
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software by Digital Instruments at the suggestion of one of the authors (JCA). The function makes planar leastsquare fits of two user-defined surface elements in the image and then calculates the angle, 8, between the two surface normals. Surface normal measurements can also be obtained by exporting AFM images to a commercial image processing program.5 Also, angle measurements
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