Hydrogenated and non-hydrogenated diamondlike films: Relationship to crystalline diamond

Hydrogenated and non-hydrogenated diamondlike films: Relationship to crystalline diamond

Workshop on diamond thin films SEMICONDUCTORS 765 HAVING WIDE BAND CAP The great interest in CVZ) diamond film aad also cBN is focused on uti~~ng ...

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Workshop on diamond thin films

SEMICONDUCTORS

765

HAVING WIDE BAND CAP

The great interest in CVZ) diamond film aad also cBN is focused on uti~~ng them as semiconducting materials, because of their wide band gap as compared with other materials. But these materials have some disadvantages. A p-type semiconductor of diamond is fabricated easil It is not easy to produce a n-type semiconductor having low reaistivitp. On the other hand, the d* vantage of cBN as compared with diamond is the ability to prepare p-type and n-type conductor of cBN and also a p-n junction diode. The disadvantage of cBN is that it is difficult to produce cBN films by CVD techniques. We are required to solve these problems in order to develop these materials as advanced devices. Especially, it is important to develop a technique for preparing cBN films at low pressure. I suppose that the technique for preparin cBN films depends on how to synthesize suitable volatile compounds, such as borazole or & tnc4l orborazole. The CVD technique has notwithstanding a versatile ability, but the technique is not fully developed except in the semiconductor field. That may be reason for the delay of development of suitable volatile compounds for CVD. References 1.

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Vol. 27, Part 1, p 982; Part 2, p 1096; 1973.

ed Diamondlike l?&nsz Hydrogenated aud Non-by ~atio~p to Crystalline Diamond Ytin

Waugt, John C. Angus*, Richard W. Hoffmant, Hsiung Chent and Susan Heidgert

The structure and properties of diamondlike hydrocarbon films (a-C:H) were studied using electron energy loss spectroscopy, chemical analysis and measurements of mass density. The a-C:H films were produced by RF self-bias plasma a&&d de~sition from pure methane. The density of the films inferred from the energy of the (x + u) lasmon is in agreement with independent sink-float determinations of the density. The energy oP the z plasmon decreases with increasing hydrogen content, a direct confirmation that hydra en reduces the extent of z bonding in the films. The relative numbers of r and (r electrons can be 1*bierred from the euergies of the (R + g) and R plasmons if assumptions are made about the effective number of electrons, neff, participating m the two plasmons. For the (x + g) plasmon the free electron value, neff = 1, is assumed; for the f plasmon the value for graphite, n,fr = 0.23, is assumed. Measurements of K-edge absorption of the diamondlike hydrocarbons were made as a function of hydrogen concentration in the films. The 1s - fl transition increased in energy from appro~mately 282 eV to 284 eV as the hydrogen content increased from 0.28 to 0.44 atom fraction. The values of average coordination number and sp3/sps ratio inferred from the EELS data are in general agreement with the predictions of the fully constrained non-crystalline network (FCN) model of a-C:H proposed by Angus and Jansen[l]. Ball-and-stick molecular models of fully constrained hydrocarbon networks have number and mass densities about 10 to 20% above measured values. The hardness of the a-C:B appears to arise from the full -constrained structure, which qualitatively may be viewed as a disordered thr~mension~ array of 7used 5, 6 and 7 member rings. The relationship of this structure to the ordered three-dimensionat array of six-membered rings in crystalline diamond is striking. The high state of internal compressive stress of the a-C:H may arise from steric interference involving the bonded hydrogen atoms. This is consistent with the observation that the compressive stress increases with increasing hydrogen content. A plot of fractional amount of sps carbon sites versus atom fraction hydrogen permits one to display the solid carbon and hydrocarbon phases of interest on a single field. See Figure 1. The FCN model divides this field into two sections, one in which the system is underconstr~ned and one in which the system is overconstrained[2 . Conventional h drocarbon pol mers, such as polyethylene, are underconstrained. The diamon dlike hydrocarbons Tindicated by t Ee data points) fall near the line predicted by the FCN model.

Workshopon diamondthin films

766

The non-hydrogenated diamondlike carbone a-C) are material8 in which approximately 49% of the carbon atoms are s 9 tetrahedral sites and w6,‘ch have mass densities about half way between graphite and ~~ond~3 P. The FCN model predicts that non~ydrog~ated nou~~st~ne networks with this amount of tetrahedral coordination are highly overconstrained and consequeutly would have unrealistically high values of strain energy. The exce68bonding constraint8 can be released by forming medium or long range order, for example, micro-crystallinity or micro-void& The structure resulting from such a relaxation would be conai8t~t with the propertiee reported by Cuomo[3]. References :: 3.

J-C. Angus and F. Jansen, J. Vat. Sci. Tech. A 6), 1778(Ma /June 1988). J.C. Angus, Proceedings NATO Advanced Stu dy Institute, E astelvecchio Pascoli, Italy, 1990. J.J. Cuomo, J.P. Doyle, J. Bruley, J.C. Liu, “Ion Beam Sputtered Diamondake Carbon with Densities of 2.9 g/cc,” to be published, 1990. (See also these abstracts.)

Acknowledgements: This research was supported by a National Science Foundation Materials Research Group grant, DMR-89-93527_

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ATOMIC FRACTlON HYDROC%EN Figure 1. Fraction of spr carbon versus atom fraction hydrogen. Data points are for dlaslOndhke hydrocarbons (a-C:H): ‘a-6 refers to the non-hydrogenated diamondlike films; PAH to polyaromatic hydrocarbon!?.