HIGH RESOLUTION STUDIES OF THE EXCITATION SPECTRA OF DONORS IN SILICON WITH AND WITHOUT UNIAXIAL STRESS

CHIRRAVURI JAGANNATH, Purdue University

Abstract

The excitation spectra of phosphorus, arsenic, isolated interstitial lithium, and lithium-oxygen donors in Si have been investigated with and without uniaxial stress under the high resolution (0.06 cm('-1)) of a Fourier transform spectrometer. Using a "strain free" mounting technique, the true line widths were observed to be much narrower than those reported earlier. An ingenious technique of introducing phosphorus in silicon is based on the nuclear transmutation of ('30)Si into ('31)P by the capture of a slow neutron followed by a (beta)('-) decay. The line widths and shapes as affected by the presence of charged defects produced by neutron irradiation were studied. An explanation for the observed line width and shape is proposed in terms of the electric fields due to charged impurities and defects. The effect of uniaxial stress on the excitation spectra of isolated, interstitial lithium donors (Li), and of lithium-oxygen donor complexes (Li-O) in silicon is studied using a quantitative stress cryostat. The ground state of interstitial lithium is anomalous in that it is a five-fold degenerate, ls(E+T(,2)), state close to the effective mass position, its site symmetry being T(,d); the ls(A(,1)) state lies above it. For compressive force, F, along {001} the site symmetry reduces to D(,2d) and ls(E+T(,2)) splits into the sub-levels ls(E+B(,1)), ls(A(,1)) and ls(B(,2)) in the order of decreasing energy. Following the <100>-conduction band valleys characteristic of Si, the p-states split into p(+) and p(-) levels, the + and - signs referring to the valleys having energies above and below their center of gravity, respectively. For F (VBAR)(VBAR) {001} the ls (--->) np transitions split into three components. The central component arises from the ls(B(,2)) (--->) p(-) and the ls(E+B(,1)) (--->) p(+) transitions. The central component occurs at the zero stress position even at the highest stress used--2 kbar. We therefore deduce that the shear deformation potential constant, (,u), characterizing ls(B(,2)) and the ls(E+B(,1)) is identical to that of the p-states and hence of the conduction band viz. (8.77 (+OR-) 0.07) eV. The value of (,u) deduced from the spacing of the low and high energy components, ls(A(,1)) (--->) p(+) and ls(A(,1)) (--->) p(-), also agrees with that of the conduction band. With increasing stress the intensity of ls(A(,1)) (--->) p(+) decreases dramatically while that of ls(A(,1)) (--->) p(-) increases. At the highest stress only the central and the low energy components survive. All the above striking behavior follows from the "inverted" ground state and the stress dependence of the ls(A(,1)) wavefunction for F (VBAR)(VBAR) {001}. Lithium introduced into crucible grown Si complexes with the dispersed oxygen; such centers are donors and with the "conventional" group V like ground state multiplet, with ls(A(,1)) below ls(T(,2)) and ls(E) sub-levels and exhibit the characteristic excitation spectra. More than one such species are formed depending upon the annealing history. Of the various donor species formed, the most prominent one, with ionization energy = 39.66 meV (series A), yields a value (,u) = (8.58 (+OR-) 0.12) eV. For compressive force, F, along {100} and {110}, the ls(A(,1)) (--->) 2p(,0), 2p(,(+OR-)) ep(,(+OR-)),... lines exhibit a splitting in addition to that originating from the regrouping of the conduction band minima along <100>. Such an effect is not found for F (VBAR)(VBAR) {111}. These results suggest that the donor species has a symmetry lower than T(,d) with a symmetry axis along <100>. The additional splitting observed under uniaxial stress arises from the lifting of the orientational degeneracy.

Degree

Ph.D.

Subject Area

Condensation

COinS