Arthur H. Edwards

Semiempirical molecular orbital techniques applied to silicon dioxide: MINDO/3

Abstract We present a discussion of MINDO/3, a semiempirical molecular orbital program, which we have applied to clusters of atoms used to represent silicon dioxide. We have obtained MINDO/3 parameters for the Si-O interaction and have calculated a variety of observables for both molecules and solid-state clusters, for both open- and closed-shell systems. With some exceptions, we find very good agreement with other calculations and with experiment. In particular, our results for both E ′ 1 and E ′ 4 defects support existing models which feature asymmetric atomic relaxations.

Si/Pt Ohmic contacts to p-type 4H–SiC

In this letter, we report on the investigation of Si/Pt Ohmic contacts to p-type 4H–SiC. The contacts were formed by a vacuum annealing method at 1100 °C for 3 min, which resulted in specific contact resistivities in the low 10−4 Ω cm2 range. Auger analysis has shown that, at this anneal temperature, there was a uniform intermixing of the Si and Pt, migration of Pt into the SiC, and out-diffusion of C into the metallization layers. Overlayers of Au or Ni/Au on Si/Pt had the effect of decreasing the specific contact resistance and improving the surface morphology of the annealed contacts.

A high temperature vacuum annealing method for forming ohmic contacts to GaN and SiC

In this work we report on the high temperature annealing of Ti/Al and Cr/Al contacts on n-GaN by a vacuum annealing technique. This method consists of annealing the samples on a resistively heated tungsten strip, in vacuum, with an Ar background ambient of 5/spl times/10/sup -5/ Torr. Contact resistivities as low as 2.2/spl times/10/sup -5/ /spl Omega/-cm/sup 2/ and 4.1/spl times/10/sup -5/ /spl Omega/-cm/sup 2/ were obtained for Ti/Al and Cr/Al ohmic contact metallizations respectively. Auger profile analysis showed minimal oxidation of the Al surface in the vacuum annealed samples whereas high levels of O were observed in the furnace annealed case. A contact resistivity of 4.3/spl times/10/sup -4/ /spl Omega/-cm/sup 2/ was achieved for Si/Pt metallization on p-SiC.

Semiempirical Molecular Orbital Studies of Intrinsic Defects in a-SiO2

The intrinsic point defects in a-SiO2 have recently received considerable attention. Careful use of paramagnetic resonance, coupled with annealing and optical studies, has led to unambiguous identification of three fundamental defects. These are the E′ center, the superoxide radical and the nonbridging oxygen hole center (NBOHC). Theoretical studies of the first two defects have led to greater understanding of experiment through inclusion of atomic relaxation. Two models exist for the NBOHC. One, by Skuja and Silin, invokes a Jahn-Teller splitting to explain 2 eV optical transitions. The other, by Griscom, is an extension of a model devised for alkali silicate glasses and involves pairs of oxygens, one of which is adjacent to a proton. Using MOPN, a semiempirical spin-unrestricted molecular orbital program, we have done molecular orbital studies of both NBOHC models. Our results support the Griscom model and not the Skuja-Silin model. These results, coupled with our earlier calculations on the E′ and superoxide defects, allow us to address defect formation and transformation processes in a logical way. In particular, these results are consistent with our speculations on the sequential creation of NBOHC, superoxide precursor, and superoxide radical by hole trapping.

Theory of Oxygen-Vacancy Defects in Silicon Dioxide

Large, asymmetric atomic relaxations are important features of oxygen-vacancy-related defects in silicon dioxide. To investigate these defects we have adapted the MIND0/3 and MOPN semiempirical molecular structure methods. In several cases (the E 1 ′ , E 2 ′ , and E 4 ′ centers) the defect is paramagnetic and its primary characteristic is a single sp3 electron localized on one Si and oriented towards or away from the O vacancy. Different atomic relaxations and charge states, along with the presence or absence of atomic H, distinguish these defects from one another. One important type of relaxation appears to be the displacement of a Si into a “back-bonds” interstitial position.