Cathodoluminescence from Polytype Layers in Silicon Carbide
http://img.cryst.bbk.ac.uk/www/kelly/homepage.htm
J.F. Kelly, P. Barnes Industrial Materials Group, School of Crystallography, Birkbeck College, University of London, Malet Street, London WC1E 7HX, U.K.
e-mail: kelly@img.cryst.bbk.ac.uk
Historical Background
Unique behaviour – Polytypism, semiconductor and gemstone
The word polytypie entered the literature in 1912 when Baumhauer attempted to describe materials that differ structurally only along one crystallographic direction. However polytypism did not attract widespread attention until studies in the 1940/50’s revealed new longer period polytypes (LPP’s). An improvement in the sublimation growth of SiC by Lely (1955) enabled its wide bandgap (Eg~3eV) semiconductor operation to be further explored. Manufacturing advances by GEC Hirst Research Centre U.K. in the 1960’s led directly to the availability of the crystals used in the present survey. Despite the potential of SiC as a high temperature/frequency power semiconductor (Janzén et al. 1994), the poor material quality has handicapped device development (Glass et al. 1997). One stumbling block appears to be a complete theoretical description of the prolific tendency for SiC to form so many polytypic modifications, including LPP’s (Trigunayat 1991). Recently the production of Moissanite gemstones has revived interest in this burgeoning material (Muller et al. 2000).
Barringer Meteorite Crater Diablo Canyon Arizona
On the origins of silicon carbide…….. Silicon carbide may be older than our solar system, having wandered through the Milky Way for billions of years as stardust generated in the atmospheres of carbon rich red giant stars. The gravitational coalescence of our solar system trapped these micron-sized SiC grains in meteorites. Recent analysis of carbonaceous chondrites has revealed that these starry messengers contain anomalous isotopic ratios of carbon and silicon, indicating an origin from outside the solar system (Clayton 1997). Originally an iron meteorite crater discovered at Diablo Canyon in Arizona provided material that was chemically analysed by Henri Moissan in 1905 and found to contain hexagonal platelets of silicon carbide. The idea that a silicon-carbon bond might exist in nature had been proposed as early as 1824, while carborundum was synthetically manufactured by Eugene Acheson in 1892.
Extract by the Authors from Ferroelectrics Journal 2000 (in press)
*Wide Indirect Bandgap SiC radiative recombination mechanisms are complex & phonon participation required
- Chronology of a semiconductor
1907 H.J. Round1 observed yellow luminescence in SiC
1955 Sublimation growth of SiC crystals improved by J. Lely2
1994 High power/frequency superiority cf. Si discussed by Janzen et al.3
1998 Cree Inc. manufacture SiC epitaxial blue LED’s
- Cathodoluminescence - light emission as a result of cathode ray bombardment
SiC represents a set of semiconductor light emitting diodes (LED) because Lely grown material contains a number of different polytypes with various bandgaps. The electrical & optical properties of materials are affected by crystal defects present within the lattice including dislocations & stacking faults which are common in SiC. Excitation caused by electron bombardment in a SEM provides the mechanism for the generation of electron-hole pairs with subsequent carrier recombination resulting in luminescence in the visible spectrum.
No general rule analogous to Moseley’s Law for X-rays applies to CL which would identify lines or bands in the CL spectrum. The absence of any general method of identification of an observed emission band limits CL as an analytical tool. Broad extrinsic CL bands are also temperature dependent, with phonon coupling responsible for broadening of the CL bands.
Edge Topograph Polytype Model J59 central 5270 reflection 15R+6H+LPP(146H/438R)+1DD(300?m) SEM micrograph Cathodoluminescence crystal morphology polytype/disorder interface
Experimental Techniques
- Synchrotron White Radiation X-ray Diffraction Topography (SRS-XRDT)
Daresbury SRS Station 7.6 Edge Geometry:
source-sample distance D ~ 80m sample-plate distance d ~ 50mm Nominally centred on 213l row ? = 1.5Å 2? = 1000
Dislocation mapping, polytype identification
- Scanning Electron Microscopy Cathodoluminescence (SEM-CL)
Jeol JSM-35CF electron microscope
35-CLD secondary electron detector photomultiplier tube (S-11).
wavelength range ? = 3000-6500 Å maximum sensitivity ? = 4400 Å.
Extrinsic recombination, Eg emission
Project aim - A comparative study of XRDT and SEM-CL from SiC edges
White beam synchrotron topography of multi-polytype SiC epitaxial device configurations has been undertaken by Dudley et al. (1995)4 while originally the technique was used for Lely SiC platelets by Fisher & Barnes (1990)5. Recently Kelly et al. (2000)6 have studied polytype coalescence neighbours, in particular long period polytypes (LPP’s) & one-dimensional disorder (1DD), using this technique.
Cathodoluminescence scanning electron microscopy of semiconductors (Yacobi & Holt 1986)7 is a well established technique and has been used by Saparin et al. (1996)8 to effectively study SiC polytype transformations in epitaxial layers and the spatial distribution of luminescence centres. The interface between polytypes in syntactic coalescence meanwhile has been studied by Din & Suleman (1993)9 using Auger Electron Spectroscopy (AES).
XRDT & CL
J52 model (American club sandwich)
6H+4H+6H+1DD+6H+1DD+6H+1DD+15R
Enhanced Cathodo-Luminescence observed at higher electron energies from region of 4H polytype.
Polytype Layers & SEM-CL
- Defect band observed at 6H/4H boundary in edge topograph.
Cathodoluminescence occurs in 6H polytype (lower region) and 40?m from the boundary in the 4H region.
J20 model (Asymmetric sandwich) 6H+4H+1DD(20?m)+4H+6H
Cathodoluminescence observed from regions in multipolytypic SiC crystals is correlated to the position of the various polytypes determined from XRDT edge topographs. Polytype sandwich models (accuracy ~ 5?m) have been obtained from 135 crystals.
Polytype model J77 6H+4H+LPP/1DD+6H
Synchrotron Edge Topograph
Defect layers in topographs appear as darker contrast in central 5270 reflections, these contain diffraction contrast from all polytypes.
Defect layer is coincident with one-dimensionally (1DD) disordered layer (both arrowed to left) and region of CL (shown on right).
J57 model 6H+15R+1DD(60?m)+4H
Topograph measured polytypes:
1DD layer & 4H polytype exhibit pronounced
cathodoluminescence (lower micrograph)
identified by bands (upper micrograph)
Edge calibration rotated 900
Disorder (1DD) & Luminescence
Defect bands at All boundaries 6H/15R, 15R/4H Asymmetric sandwich
Locating CL
- J40 shows Luminescence at the 6H / 15R boundary which contains a defect band present in the edge topograph.
- J47 shows Luminescence at the site of the 180 ?m disordered layer.
Disordered layer in 6H polytype
J40 model (Doubly filled sandwich)
6H+4H+LPP(106H/318R)+1DD(145?m)+15R+6H
Long period polytypes (LPP’s)6
High resolution SEM images of ZnS polytype layers have been obtained recently by Kozielski & Tomaszewicz (1999)10 in a statistical analysis of long period polytypes and disorder in the layer stacking sequences.
23/135 crystals have yielded LPP’s
78% contain disorder, 19 with a 1DD layer < 10 ?m
Summary SEM-CL, observed along silicon carbide crystal edges, is reported from regions containing defects & one-dimensionally (1DD) disordered layers. A qualitative description of the location of these features, the common 6H, 15R & 4H and long period polytypes (LPP’s) is possible using SRS XRDT as a comparative technique. Ordered 6H, 4H polytypes display CL. CL data is non quantitative.
SRS Daresbury Laboratory Cheshire, EPSRC for funding, painting by Rob Lewis
1. Round H.J., Electrical World (New York), 49, 309 (1907).
2. Lely J.A., Ber. Deutsch. Keram. Ges., 32, 229-231 (1955).
3.Janzén E., Kordina O., Henry A., Chen W.M., Son N.T., Monemar B., Sörman E., Bergman P., Harris C.I., Yakimova R., Tuominen M., Konstantinov A.O., Hallin C., Hemmingsson C., Physica Scripta, 54, 283-290 (1994).
4. Dudley M., Huang W., Wang S., Powell J.A., Neudeck P., Fazi C., J. Phys. D: Appl. Phys. 28, A56-A62 (1995).
5. Fisher G.R., Barnes P., Phil. Mag., B61, 217-236 (1990).
6. Kelly J.F., Barnes P., Fisher G.R., Ferroelectrics (2000) in press
7. Yacobi B.G., Holt D.B., J. Appl. Phys. 59, R1-R24 (1986).
8. Saparin G.V., Mokhov E.N., Obyden S.K., Roenkov A.D., Scanning, 18, 25-34 (1996).
9. Din S., Suleman M., Mat. Lett., 17, 241-245 (1993).
10. Kozielski M.J., Tomaszewicz A., Cryst. Res. Technol. 34, 1279-1285 (1999).
Acknowledgments: The Authors are indebted to Juliet Munn & Andy Beard for use of the SEM facilities Birkbeck College, London University