Electronic and vibrational spectroscopy of impurities in elemental and compound semiconductors

Abhilash J Mayur, Purdue University

Abstract

Fourier transform infrared spectroscopy, employed to investigate the electronic and vibrational excitations of impurities present in dilute concentrations in II-VI and in both natural and isotopically engineered group IV semiconductors, reveals many novel features accessible with the ultra-high resolution. The infrared active, high frequency localized vibrational modes (LVMs) of the substitutional 3d-transition metal ions (TMIs) in CdTe, ZnTe and CdSe appear with as many lines as there are isotopes of the impurity and with intensities proportional to their natural abundances. In CdSe, gap modes lying in the gap between the acoustic and optic phonon branches are also observed. Under high resolution, the sharp LVMs exhibit a rich fine structure originating in the nearest neighbor Te or Se isotopic disorder. Both the local and the gap modes exhibit a unique non-monotonic trend across the 3d-transition metal series, with a minimum at Mn; sp-d hybridization appears to be the underlying cause. Dispersed oxygen in Si and Ge leads to the formation of Si$\sb2$O and Ge$\sb2$O "quasi-molecules" which possess three infrared active normal modes of vibration. The asymmetric stretching mode $(\nu\sb3),$ exhibits a remarkable, temperature dependent fine structure. Coupling of $\nu\sb3$ with the low energy symmetric bending $\nu\sb2$ mode and isotope shifts associated with Si and Ge isotopes underlies the fine structure. By incorporating oxygen in monoisotopic $\sp{70}$Ge, $\sp{73}$Ge, $\sp{74}$Ge or $\sp{76}$Ge, the excitation spectrum of the $\nu\sb2+\nu\sb3$ coupling is selectively simplified. The $\sp{18}$O counterpart of the $\nu\sb2+\nu\sb3$ coupling excitation has also been observed. The binding energies of the effective mass like 1s(E) and 1s$(T\sb2)$ ground states of group V donors (P, As, Sb and Bi) in Si are deduced from the energies of 1s(E), 1s$(T\sb2)\to nP\sb0,$ $np\sb{\pm}$ transitions in the Lyman spectrum of the neutral donors. The 1s(E), $1s(T\sb2)\to np\sb0,\ np\sb{\pm}$ transitions appear on thermally populating 1s(E) and $1s(T\sb2).$ In Si(Sb), the transitions which originate from $1s(T\sb2:\Gamma\sb5)$ are doublets; this feature arises from the inclusion of the spin-orbit interaction. The corresponding observations on Si(Bi) show the spin-orbit splitting of $1s(T\sb2:\Gamma\sb5)$ in Bi and yield a binding energy of 30.1 meV for $1s(E:\Gamma\sb8).$

Degree

Ph.D.

Advisors

Ramdas, Purdue University.

Subject Area

Condensation

COinS