Contains report on one research project.Joint Services Electronics Program (Contract DAAG29-78-C-0020

Kastner, Marc A.

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XVII. ELECTRONIC PROPERTIES OF CHARGED CENTERS IN SiO 2-LIKE GLASSESAcademic Research StaffProf. M.A. KastnerGraduate StudentsC.M. GeeJoint Services Electronics Program (Contract DAAG29-78-C-0020)Marc A. KastnerDefects in amorphous (a-) SiO 2 are thought to result in deleterious optical andelectronic effects. They may be responsible for limiting the transmission of opticalfibers; they may be the origin surface states at Si-Si0 2 interfaces which trap chargecarriers; they probably cause potential fluctuations which scatter mobile carriersmoving near the Si-SiO 2 interface. Both of the latter effects seriously limit theperformance of MOS devices.A wide variety of techniques has been employed to study the defects in a-SiO 2,and a great deal is known about them. However, the electronic structure and proper-ties of the intrinsic defects are not yet well established. On the other hand, muchprogress has been made in developing detailed models for intrinsic defects in nar-rower band gap chalcogenide glasses (e.g., Se, As2S3, AsxTey , etc.). The modelshave been successful in explaining Fermi-level pinning in the diamagnetic groundstate and a wide variety of excited-state properties. The model of Kastner, Adler,and Fritzsche strongly emphasizes that the nature of the defects is a result of thespecial chemical bonding in materials containing two-fold-coordinated group VI ele-ments. Therefore, it was proposed that defects similar to those in sulfides, sele-nides, and tellurides would be present in oxide glasses as well.The most valuable probe of defects in semiconducting glasses has been photolu-minescence (PL). The large PL Stokes shift and linewidth provided evidence for thestrong electron-phonon coupling necessary to explain the effective attraction betweenelectrons (Anderson's effective negative correlation energy). We therefore under-took a program solely under JSEP funding to study PL in a-Si0 2.PR No. 122(XVII. ELECTRONIC PROPERTIES OF CHARGED CENTERS IN SiO 2-LIKE GLASSES)This program has recently had its first success. We were the first to observeintrinsic photoluminescence in a-Si0 2. The results have just been published [Phys.Rev. Letters 42, 1765 (1979)]. Although some features of the PL are different fromthose in sulfide and selenide glasses, these differences are quantitative ratherthan qualitative. We found that the PL spectra of SiO 2 and other glasses scale withband gap. Neutron irradiation increases the PL intensity and increases the opticalabsorption in the excited band to levels higher than could arise from impuritiesshowing that the luminescence centers are intrinsic defects. The scaling of the PLand PLE spectra with band gap is strong evidence that the intrinsic defects in Si02have structure similar to those in other chalcogenide glasses.A second remarkable scaling was also discovered. The exponential decrease ofPL quantum efficiency with temperature for all chalcogenide glasses indicates thatthe activation energies A for nonradiative processes must have a distribution of theform-A/kTg(A) = e °0/kT .The spread of activation energies kT is found to be proportional to the glass tran-sition temperature T . Thus when temperature is normalized to Tg, the temperaturedependence of the quantum efficiencies of different glasses superpose. Experimentsare now under way to measure time-resolved PL spectra using a pulsed F2 laser(10-nsec pulse, 1 MW, 1575 A) as the excitation source.PR No. 122

Electronic Properties of Charged Centers in Si0₂-like Glasses

XVI. ELECTRONIC PROPERTIES OF CHARGED CENTERSIN SiO 2 -LIKE GLASSESAcademic Research StaffProf. Marc A. KastnerGraduate StudentsCaroline M. GeeJoseph W. OrensteinJoint Services Electronics Program (Contract DAAG29-78-C-0020)Marc A. KastnerWe have demonstrated that vacuum-ultraviolet excitation of pure silicon dioxidecauses luminescence. We believe that the luminescence centers are intrinsic defects.Excitation and luminescence spectra have been measured for suprasil (containing about103 ppm OH) and suprasil W (much lower OH concentration). Since the spectra andquantum efficiency are the same, OH is not important in the luminescence process. Theluminescence has a large Stokes shift: the excitation band is approximately 0.8 eV wide,centered at 7. 6 eV; the luminescence is approximately 1. 5 eV wide, centered at about4 eV. A 7. 6-eV optical absorption band is known to be enhanced by neutron damage,strongly suggesting that it arises from an intrinsic defect. Neutron irradiation of sam-ples to enhance the luminescence is now under way.PR No. 121

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Electronic Properties of Charged Centers in Si0₂-like Glasses

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