Localized vibrational and electronic excitations of impurities in compound semiconductors

Gang Chen, Purdue University

Abstract

Two types of localized vibrational modes of oxygen substituting for Te in CdTe, i.e., OTe, are reported. In one, OTe is associated with a nearest neighbor (NN) vacancy as a (OTe - VCd) center and hence with C3v symmetry, with its uniaxial axis along ⟨111⟩, whereas in the other O Te is surrounded by all the four NN Cd's and thus possesses Td site symmetry. By an appropriate control of stoichiometry it is possible to reproducibly generate the formation of either (OTe - VCd) or OTe centers. These configurations are deduced from their ultrahigh resolution infrared signatures. For the (O Te - VCd) centers, consistent with their uniaxial symmetry, a pair of sharp local vibrational modes (LVM) are observed at [special characters omitted] = 1096.78 cm-1 and [special characters omitted] = 1108.35 cm-1, the latter nearly twice as intense as the former. In the LVM spectrum of OTe centers with the full complement of NN Cd's, consistent with its Td symmetry, only one LVM signature appears at [special characters omitted] = 349.79 cm-1. With the increasing temperature, [special characters omitted] and [special characters omitted] approach each other and coalesce into a single triply degenerate line at [special characters omitted] for temperature T ≥ T* ∼ 300 K; the uniaxial (C3v) symmetry of (OTe - VCd) transforms to T d symmetry at T* and above, acquired by the (OTe - VCd) centers due to the increasing rate of bond switching among the four possible OTe - VCd ⟨111⟩ directions as T approaches T*. The (OTe - VCd) centers also display a fascinating pair of second harmonics including a coalescence at T* and beyond. We have discovered two types of localized vibrational modes (LVMs) of oxygen related defect centers in stoichiometrically controlled CdSe, a wurtzite crystal. In one, oxygen substitutionally replaces Cd (OCd) as an anti-site impurity with C3v site symmetry. Consistent with its uniaxial configuration, under relatively low resolution a pair of sharp LVMs is observed at μ1 = 1991.77 cm-1 and μ2 = 2001.3 cm-1. Under high resolution, both μ1 and μ2 display a remarkable fine structure which can be traced to the motion of the nearest neighbor Se atoms surrounding the OCd in which oxygen replacing Cd is an anti-site impurity. The host isotopic fine structure is strongly reminiscent of the LVMs of CdSe observed with MgCd impurities. In both cases, the fine structure is associated with nearest neighbor Se atoms set in vibratory motion by the LVMs of OCd or MgCd. In the other, oxygen substitutionally replaces Se (OSe) in the vicinity of a Cd vacancy, labeled as (OSe - VCd) centers, in which the nearest neighbor Cd vacancy occurs on one of three vertical planes of reflection on which OSe also lies. The center displays a local C s symmetry resulting in three infrared absorption peaks at γ 1 = 1094.11 cm-1, γ2 = 1107.45 cm -1, and γ3 = 1126.33 cm-1. With increasing temperature, γ1 and γ2 approach each other and coalesce into a single doubly degenerate mode at T 1 ∼ 480 K; the Cs site symmetry of (OSe - VCd) thus transforms to C3v symmetry at higher temperatures, acquired by the increasing rate of bond switching among the three equivalent positions for VCd. At even higher temperatures, γ3 merges with γ1 and γ2 into a single mode at T2 ∼ 560 K; the acquired C 3v site symmetry of (OSe - VCd) further transforms to a quasi-Td symmetry due to the same mechanism. The group IB impurities (Cu, Ag, and Au) incorporated into II-VI zinc blende hosts of ZnTe and CdTe exhibit well resolved excitation lines followed by a photoionization continuum in their infrared absorption spectra. They are associated with transitions from a "1s-like" ground state to various "p-like" excited state characteristic of a hole bound to a Coulomb center. Their spacing agree well with those predicted in the effective mass theory for single acceptors as expected for group IB elements substitutionally replacing the group IIB cations of the host. The occurrence of the simultaneous excitation of the Lyman transitions in combination with the zone center longitudinal optical phonon and hence lying in the photoionization continuum and displaying Fano-like asymmetries are features described and interpreted.

Degree

Ph.D.

Advisors

Ramdas, Purdue University.

Subject Area

Condensed matter physics

COinS