THE IONIZATION ENERGY OF NITROGEN DONORS IN 6H AND 15R SiC

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1 VOL. 25 No. 1 FEBRUARY 1970 Philips Research Reports EDITED BY THE RESEARCH LABORATORY OF N.V. PHILIPS' GLOEILAMPENFABRIEKEN. EINDHOVEN. NETHERLANDS R711 Philips Res. Repts 25, 1-7, 1970 THE IONIZATION ENERGY OF NITROGEN DONORS IN 6H AND 15R SiC by S. H. HAGEN and C. J. KAPTEYNS Abstract The results of Hall-effect and conductivity measurements performed on single crystals of n-type 6H and 15R SiC doped with nitrogen show that there is a considerable difference in the ionization energy of nitrogen centres between the two polytypes. Measured values ofmobilities which agree with already published results are also presented. 1. Introduction Patriek et al. 1.2) concluded from an analysis of luminescence spectra that the ionization energy of nitrogen donors in SiC is somewhat different for donors occupying inequivalent lattice sites. They gave values ofo'17, 0 20 and 0 23 ev for 6H SiC and 0'14, 0'16, 0 16 and 0 20 ev for 15R SiC. Up to now no work has been published concerning a comparison between the ionization energies resulting from Hall-effect measurements on single crystals of these two polytypes 3.4). In this paper we report on measurements of the conductivity and the Hall effect of very pure n-type 6H and 15R single crystals which clearly show a difference in ionization energy between the polytypes mentioned. In view of the evidence for a difference in ionization energy of inequivalent nitrogen donors it was considered necessary to compare crystals with equal impurity concentrations. As the preparation of the crystals always resulted in batches containing 6H and 15R crystals suitable samples could be obtained by preparing homogeneously doped batches and taking 6H and 15R crystals from the same batch. We also present values of the mobilities of conduction-band electrons which confirm earlier results obtained by Van Daal 5) and by Barrett and Campbell 6). '. 2. Preparation of the crystals The crystals were prepared by an improved modified Lely process 7). A smaller furnace allowing a higher purity than previously attainable was used

2 J 2 S. H. HAGEN nnd C. J. KAPTEYNS for the crystal growth, The. crystals were grown on a wall of pyrolytic SiC in a graphite crucible. This crucible was' sur;ounded by a thick~walled cnicible of pyrolytic graphite. The crucible system was placed in a water-cooled silica vessel and was heated by induction. Both crucibles had been previously purified by baking at 2300 C in vacuum (10-6 Torr). The polycrystalline SiC used as a starting material was prepared by pyrolysing methyltrichlorosilane in hydrogen. The growth of the crystals took place in an ambient of pure helium. The helium gas was obtained by evaporation of liquid helium. The impurity content of this gas. was checked with a gas chromatograph and proved to be below the detection limit, i.e. ~. 5 ppb O 2 and ~ 10 ppb N 2 Very pure crystals were prepared by this procedure. The impurity content of the crystals was below the detection limit of spectrochemical analysis, using dissolution and preconcentration techniques. From results of activation analysis and mass speetrometry we estimate that the impurity content is less than 0 5 ppm. A detailed description of the preparation procedure will be given in a future publication 8). The crystals were doped with nitrogen by adding a controlled amount of nitrogen to the helium. 3. Selection of the samples The crystals are obtained in the form of hexagon-shaped plane-parallel platelets. It is a well-known fact 9) that in single crystals of SiC barriers are often present parallel to the {OOI}basal faces of the crystal. These barriers are sometimes connected with a syntactic growth of lamellae of different polytype. When four small gold-tantalum ohmic contacts are alloyed on one basal face at the edge, conductivity and Hall-effect measurements can be performed using Van der" Pauw's method 10). The presence of the barriers mentioned then becomes immediately evident from the fact that the Hall voltage does not change when the thickness of the crystal is diminished by grinding away part of the crystal starting from the other basal face. The measurements reported here refer only to samples from which such barriers, if present, had been removed by grinding. The crystal structure was determined from X-ray rotation diagrams. Samples were selected that were strictly of one polytype. 4. Experimental results As it is not readily possible to correct for the influence of scattering mechanisms on the Hall factor 5) the carrier concentration was calculated by means of the relation n = I/RH q, where RH is the Hall constant and q is the unit charge. Figure 1 shows the temperature dependence of the carrier concentration n for two 6H and two 15R crystals from batch A which was intentionally doped

3 IONIZATION ENERGY OF NITROGEN DONORS'IN 6H AND 15R SiC mwr ~ ~--~--~~~~---, I.. Fig. 1. Carrier concentration in samples of batch A as a function of temperature. PH (cm 2 /Vs) T(oK) Fig. 2. Hall mobility in samples of batch A as a function of'temperature,

4 4 S. H. HAGEN and C. J. KAPTEYNS n (cm-3) 1 10 '7 r , Fig. 3. Carrier concentration in samples of batch B as a function of temperature. PH (cm 2 /Vs) 4000', , 1 '000 15R 100 1~ T (ok) Fig. 4. Hall mobility in samples of batch B as a function of temperature.

5 IONIZATION ENERGY OF NITROGEN DONORS IN 6H AND lsr SiC 5 with nitrogen. The measurements clearly show a difference in the ionization energy of the donor centres between the two polytypes. Figure 2 shows the corresponding Hall mobility zq, of two samples of different polytype. In figs 3 and 4 results are shown for two 6H and two 15R samples from batch B, which was not intentionally doped. These crystals were also n-type. Electron-spin-resonance measurements revealed that these crystals also contain nitrogen as the dominant impurity. The difference in ionization energy between the two polytypes found for the crystals from batch A is also present in this case. One 6H crystal (sample 3) shows a temperature-independent carrier concentration in the low-temperature region. The fact that it is accompanied by an anomalously large change in the Hall mobility indicates that it is not caused by the presence of a shallow donor impurity 11). At present we have no explanation for the deviation. All measurements except those on samples I and 2 at the highest temperatures could be fitted very well to the theoretical net) relation for one kind of partly compensated donors 12). The curves of the not intentionally doped samples I and 2 (fig. 3) show at high temperatures some structure which indicates the presence of a comparatively deep centre in a concentration comparable to that of the nitrogen donors. The concentrations of the nitrogen donors (N d ) and compensating acceptors (Na) and the ionization energies (LIE) resulting from an analysis of the results are presented in table I. We also determined the concentration of the uncompensated donor centres in single crystals of batch B from the slope of the I/C2 vs V plot of a surface barrier (SiC-Au) 13); C is the capacitance of the metal-semiconductor junction and V is the applied voltage. We found values for (Nd - Na) of cm=", i.e. concentrations in the same range as found TABLE I Donor concentration, compensating acceptor concentration and ionization energy for the samples of batches A and B batch sample polytype Ndx cm- 3 NaX cm- 3 LIE (ev) A 8 6H A 7 6H B 4 6H A 5 15R A 6 15R B 2 15R B I 15R

6 6 S. H. HAGEN and C. J. KAPTEYNS from the Hall measurements. As deep donor centres in the depletion layer are lifted above the Fermi level and therefore contribute to the capacitance of the juriction, we may conclude that deep centres 'are not present in concentrations largely exceeding that of the uncompensated nitrogen donors. 5. Discussion From the measurements the following conclusions can be drawn. (1) There is a considerable difference between the ionization energy of nitrogen donor centres in 6H and 15R SiC. Using a hydrogenic model for the centres, which is certainly not quite correct, this difference could be understood from the fact that the effective mass of conduction-band electrons is smaller for 15R than for 6H. 4 ). (2) As a small difference in ionization energy between inequivalent nitrogen donors cannot readily be determined from Hall-effectmeasurements, ourresults are on this point not inconsistent with those obtained by Patriek et al. 1.2). There is, however, a large discrepancy between the magnitude of their ionization energies and our LlE values. As the amount of compensating impurities in our crystals is rather small, this discrepancy cannot in our opinion be explained by the interaction between ionized donor centres and free carriers 14). The results of Patriek et al. seem to be supported by thermoluminescence experiments performed by Gorban et al. 15) who found for nitrogen donors in 6H SiC ionization energies of 0,18, 0 21 and 0 24 ev. According to the interpretation ofthe latter authors the thermoluminescence peaks are caused by thermal depopulation of the nitrogen donor levels. The ionization energies determined in this way should therefore be directly comparable with results obtained by Hall-effect measurements. Gorban et al. did not present results of such measurements on their samples. As far as we know from published Hall-effect measurements, ionization energies as large as the aforementioned values have never been found. As, however, they seem to emerge directly from luminescence measurements on samples of different sources, the question must be raised whether the derivation of these quantities from luminescence spectra should be reconsidered. Anyhow a discrepancy seems to exist which in our opinion cannot be explained in a simple way. (3) The measured Hall mobilities are in good agreement with results published by Van Daal 5) and by Barrett and Campbell 6). This agreement suggests-that, at least for 15R and 6H SiC, a considerable improvement of the mobility is not to be expected from a further improvement in the usual technology of crystal growing. Acknowledgement The authors wish to thank Ir J. A. W. van der Does de Bye for the computer program used for the analysis of the Hall-effect measurements, Ir. P. P. _J. van,

7 IONIZATION ;ENERGY OF NITROGEN DONORS IN 6H AND ISR S,. _c 7 Engelen for performing the paramagnetic-resonance measurements and J. van Werkhoven and A. H. F. Toonders for their assistance in the experimental ~k. Eindhoven, August 1969 REFERENCES 1) D. R. Hamilton, W. J. Choyke and L. Patrick, Phys, Rev. 131, 127, ) L. Patrick, D. R. Hamilton and W. J. Choyke, Phys. Rev. 132, 2023, ) D. L. Barrett, J. electrochem. Soc. 113, 1215, ) B. Ellis and T. S. Moss, Proc. roy. Soc. (London) 299, 393, ) H. J. van Daal, Philips Res. Repts Suppl. 1965, No. 3. 6) D. L. Barrett, R. B. Campbell, J. appl. Phys. 38,53, ) W. F. Knippenberg, Philips Res. Repts 18, 161, ) C. J. Kapteyns and W. F. Knippenberg, to be published. 9) G. Bosch, J. Phys. Chem. Solids 27, 795, ) L. J. van der Pauw, Philips Res. Repts 13, 1, ) G. A. Lomakina, Sov. Phys, solid State 8, 1038, ) R. A. Smith, Semiconductors, Cambridge University Press, 1959, p ) S. H. Hagen, J. appl. Phys. 39, 1458, ) P. P. Debye and E. M. ConweIl, Phys, Rev. 93, 693, ) J. S. Gorban, A. F. Gumenyuk and Vu. M. Suleimanov, Sov. Phys, solid State 8, 2746, 1967.