Contains report on one research project.Joint Services Electronics Program (Contract DAAG29-80-C-0104

Kastner, Marc A.

Contains report on one research project.Joint Services Electronics Program (Contract DAAG29-83-K-0003

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Electronic Properties of AmorphousSilicon Dioxide10. Electronic Properties of Amorphous SiliconDioxideAcademic and Research StaffProf. M.A. KastnerGraduate StudentsJ.H. StathisJoint Services Electronics Program (Contract DAAG29-83-K-0003)Marc A. KastnerWe have discovered that exposure to below-bandgap (i.e., weakly absorbed) photons changesthe charge state of some or all of the intrinsic defects in amorphous SiO 2. This change ismanifested in several ways: (a) a mid-gap absorption band (4.8 eV); (b) a large enhancement ofthe intensity of the 1.9 eV photoluminescence band; and (c) the appearance of anelectron-spin-resonance (ESR) signal. The discovery of photoinduced ESR in a-SiO2 is the mostsignificant development. ESR provides a local probe of the structure of the defects which iscomplementary to the optical probes (absorption, luminescence). Experiments now in progresswill enable us to correlate the photoinduced ESR and photoinduced optical effects, giving usspecific information about the structure and properties of the defects intrinsic to SiO 2 . Althoughsimilar studies have been performed in the past to correlate the ESR and optical measurements,with some success, these studies have relied on ionizing radiation (gamma or x-ray) to generatesignificant numbers of paramagnetic centers from the normally diagmagnetic ground state of theintrinsic defects. Photoinduced ESR has the advantage of providing another parameter, namelythe photon energy. This gives us a new spectroscopic tool which we are using to measure theenergy needed to ionize the intrinsic defects in SiO 2.By changing the photon energy and annealing temperature we have been able to resolve thephotoinduced ESR into several components. We have found that at the highest photon energycurrently available to us, 7.9 eV, the ESR spectrum is very similar, although not identical, to thatobsrvcd after gamma irradiation, nd 10-10 paramagnetic centers per cubic centimetercan be photogenerated, the same number as produced by gamma rays. When lower photonenergies are used, the relative intensities of the components are changed. Paramaqnetic centerscan be generated by photon energies at least as low as 5 eV, corresponding to roughly half theband-gap. The centers responsible for the 1.9 eV luminescence are also created by these lowerphoton energies, but the centers giving the mid-gap absorption are not. We also found that theRLE P.R. No. 127Electronic Properties of AmorphousSilicon Dioxidephotoresponse of a-SiO2 depends markedly on the amount of OH present in the material. SiO2which is specially prepared in a way which prevents the incorporation of OH is much moresensitive to ultraviolet light than material containing -1200 ppm OH impurity.RLE P.R. No. 127

Electronic Properties of Amorphous Silicon Dioxide

Contains a report on one research project.Joint Services Electronics Program (Contract DAAG29-80-C-0104

87 Electronic Properties of Amorphous SiliconDioxide15. Electronic Properties of Amorphous SiliconDioxideAcademic and Research StaffProf. M.A. Kastner, K. MurayamaGraduate StudentsR. Johnson, D. Monroe, J.H. StathisJoint Services Electronics Program (Contract DAAG29-80-C-0104)Marc A. KastnerAmorphous SiO 2 (a-SiO 2) is one of the most important materials for electronic and opticalapplications. The thermal oxide which provides high electrical insulation and dielectric strength forintegrated circuits and most other Si devices is a-Si0 2. One might say, therefore, that the entireelectronic revolution rests on a thin layer of a-SiO 2 . The newest revolution in the technology ofcommunication, optical fibers, also depends on a-SiO, for this material is the low-loss medium ofwhich the fibers are made.It is surprising, therefore, how little is really understood about the optical and electronic propertiesof this very important material. The paucity of understanding does not reflect a lack of effort. On thecontrary, there is a long history of experiments, dating back at least to the mid 1950's, and a gooddeal of factual information is available. However, very few of the simplest fundamental issues havebeen resolved. For example, the magnitude of the energy gap between valence and conductionbands is not completely agreed upon.The difficulty in deciding such fundamental points in the case of a-SiO 2 arises from two of itsproperties. First, the band gap of a-Si0 2 is very large which means that vacuum ultraviolet excitationis needed for optical experiments, making some of them quite difficult. Second, the fact that thematerial is amorphous allows for a wide variety, and greater number, of atomic-scale defects thanwould be accommodated by the ordered structure of a crystal. As a result, many of the optical andelectronic properties of a-SiO 2 are dominated by these defects which have, therefore, been the focusof much of the research on this material. The defects may be extremely important, by the way, in bothelectronic and optical applicadtions of a-SiO2 . Defects in thermal oxides on Si devices, especially ifcharged, can influence carrier mobilities near the surface as well as the threshold of field-effecttransistors. Defects in optical fibers may limit the ultimate optical attenuation either by absorption orscattering.It is because we have had some success in understanding the electronic and optical properties ofRLE P.R. No. 12488 Electronic Properties of Amorphous SiliconDioxidedefecls in other glasses. that we have been emboldened to attack the problem of defects in a-SiO . Inthe chalcogenide glasses, sulfur, selenium or tellurium takes the place of oxygen as the two-foldcoordinated element which forms the backbone of the glass network structure. These glasses havemuch narrower band gaps (1-2 eV) than SiO 2 so that they can be photoexcited with visible lasers anda wide variety of experiments can be carried out. We have found1 that the response of thechalcogenide glasses to pulsed laser excitation provides a wealth of new information about theirelectronic structure. The experiments that have provided great insight are time-resolvedphotoluminescence, photoconductivity, and photoinduced optical absorption.Our study of photoluminescence in a-SiO 2 began with experiments using a continuous and weakexcitation source, a H2 discharge lamp. These early experiments proved very informative. We found 1that a-SiO 2 , like the narrower-gap glasses, has a photoluminescence (PL) band peaked near mid-gapwhen excited in the region of the optical absorption edge. By showing that the PL band increases inintensity after neutron-irradiated, we demonstrated that the PL band increases in intensity afterneutron irradiation and that crystalline SiO 2  shows 2  no PL until it is neutron-irradiated, wedemonstrated that the PL centers are defects, as is thought to be the case in the other glasses.Furthermore, the temperature dependence of all the glasses could be explained in a simple way,3which indicates that the nonradiative mechanism in all the glasses, including a-SiO 2, have a commonorigin.In the past two years we have begun time-resolved experiments on the PL of amorphous andneutron-irradiated crystalline SiO 2. The high power of our pulsed F2 laser allowed us to see 4 PLbands which were buried in noise when using the H2 lamp.The time evolution of the PL spectrum is not yet understood. Part of the problem is that the variousbands we observe may arise from electronic transitions at several different types of defect. Thequestion of which PL bands come from the same defect will be partly resolved by comparing glassesprepared in different ways. However, a discovery we have made in the past few months may provide abreakthrough in our ability to study the PL processes.The results of our measurements with the continuous excitation source led us to believe that onlyvacuum-ultraviolet excitation had adequate photon energy to excite the photoluminescence. Wehave found, however, that near-ultraviolet excitation wi5th high-power excimer laser lines can alsoexcite the PL. We are now trying to determine whether this is a one-or two-photon process. In anyevent, excitation in the near-ultraviolet can bee made tunable, and we have found tunable excitationindispensable in our studies of narrower-gap materials. Furthermore, near-ultraviolet excitationwould allow us to dispense with the high-vacuum requirements which have made our measurementsarduous.Our long-term effort will take two directions. First, we hope by improving our experiment to be ableRLE P.R. No. 12489 Electronic Properties of Amorphous SiliconDioxideto decide which PL bands arise at particular defects. In addition. we will begin to study the PL bandsin a-SiO2 thermally grown on Si surfaces and in optical fibers. Second, we will explore the possibilityof measuring transient photocurrent and photoinduced optical absorption, two experiments whichhave provided great insight into the nature of defect states in the narrower-gap glasses.References1. C.M. Gee and M. Kastner. Phys Rev. Letter. 42, 1765 (1979).2. C.M. Gee and M. Kastner, J. Non-Crystalline Solids 35-36, 927 (1980); J. Non-Crystalline Solids 40,577 (1980).3. M. Kastner, J. Phys. C13, 3319 (1980).4. C.M. Gee and M. Kastner, in G. Lucovsky, S.T. Pantelides, and F.L. Galeener (Eds.), The Physics ofMOS Insulators(Pergamon, 1980), p. 132.RLE P.R. No. 124

Contains summary of research on one research project.Joint Services Electronics Program (Contract DAAG29-83-K-0003

Electronic Properties of AmorphousSilicon Dioxide15. Electronic Properties of Amorphous SiliconDioxideAcademic and Research StaffProf. M.A. Kastner, S. OdaGraduate StudentsJ.H. StathisJoint Services Electronics Program (Contract DAAG29-83-K-0003)Marc A. KastnerSince last January a number of significant changes and advancements have taken place in ourinvestigation of defects in amorphous SiO 2 (a-SiO 2). These advancements have been facilitated by amove to a larger lab and the acquisition of several new pieces of equipment.Although the initial plan was to probe the intrinsic defects using photoluminescence, this has beensuperceded, for the time being, as a result of a remarkable discovery. We have found that the chargestate of some of or all of the intrinsic defects in a-SiO2 is changed, in a metastable way, by exposureto sub-bandgap ultraviolet light. This change is manifested in several ways: (a) a mid-gap absorptionband (4.8 eV); (b) a large enhancement of the intensity of the 1.9 eV photoluminescence band; and (c)the appearance of an electron-spin-resonance (ESR) signal. The discovery of photoinduced ESR ina-SiO2 is perhaps the most significant development. ESR provides a local probe of the structure ofthe defects which is complementary to the optical probes (absorption, luminescence). Experimentsnow in progress will enable us to correlate the photoinduced ESR and photoinduced optical effects,giving us specific information about the structure and properties of the defects instrinsic to SiO 2 .Although similar studies have been performed in the past to correlate ESR and optical measurements,with some success, these studies have relied on ionizing radiation (gamma or x-ray) to generatesignificant numbers of paramagnetic centers from the normally diagmagnetic ground state of theintrinsic defects. Photoinduced ESR has the advantage of providing another parameter, namely thephoton energy. This gives us a spectroscopic tool which we are using to measure the energy neededto change the charge state of the defect.By changing the photon energy we have been able to resolve the ESR into several components. Wehave found that at the highest photon energy currently available to us, 7.9 eV, the ESR spectrum isvery similar, although not identical, to that observed after gamma irradiation, and that 1016-1017paramagnetic centers per cubic centimeter can be photogenerated, the same number as produced bygamma rays. When lower photon energies are used part of the spectrum is absent, and the relativeintensities of other components are changed. Paramagnetic centers can be generated by photonRLE P.R. No. 126Electronic Properties of AmorphousSilicon Dioxideenergies at least as low as 5 eV, corresponding to roughly half the band-gap. The centersresponsible for the 1.9 eV luminescence are also created by these lower photon energies, but thecenters giving the mid-gap absorption are not.Our investigation of the photo-generation of the 4.8 eV absorption and the 1.9 eV luminescencecenters has been accepted for publication. We demonstrated that these two optical transitions ariseat different types of defects, belying previous speculations that a single defect was responsible. Wealso found that the photoresponse of a-SiO 2 depends markedly on the amount of OH present in thematerial. SiO 2 which is specially prepared in a way which prevents the incorporation of OH is muchmore sensitive to ultraviolet light than material containing -1200 ppm OH impurity.During this past year we also initiated experiments to study electrical transport in SiO 2 . Theseexperiments use the phenomenon of internal photoemission to inject electrons or holes into theconduction or valence band of thin SiO 2 samples, where their subsequent transport and interactionwith traps can be studied using the time-of-flight technique.RLE P.R. No. 126

Electronic Properties of Amorphous Silicon Dioxide16. Electronic Properties of Amorphous SiliconDioxideAcademic and Research StaffProf. M.A. Kastner, S. OdaGraduate StudentsJ.H. StathisJoint Services Electronics Program (Contract DAAG29-80-C-0104)Marc A. KastnerThe objective of this program is to study the electronic properties of amorphous SiO 2 (a-Si0 2)using pulsed laser excitation. There is a great deal of evidence that many of the optical and transportproperties of this technologically important material are dominated by defects. Electron spinresonance experiments have identified a number of radiation-induced defects and certain types ofdefects present in unirradiated bulk a-SiO 2 and films of a-SiO2 thermally grown on Si. Our work overthe past several years has identified a number of photoluminescence (PL) bands which probablyoriginate at intrinsic defects. The PL may be a more sensitive probe of the defects than othertechniques.One major goal of our current research is to determine which of the several PL bands we observearises at the same, and which at different, PL centers. To do this we are carrying out time-resolvedPL measurements at a variety of temperatures for a-SiO2 prepared in a number of different ways. Thetime-resolved spectroscopy allows us to unambiguously identify PL bands by their different decayrates. The temperature dependence of the PL decay is important because it can provide informationabout whether two different PL bands can originate at the same defect. The excitation-energydependence of the intensity of the various PL bands will also help to differentiate among PL centers.The preparation of the glass, however, is the most direct probe of which bands arise at differentcenters. Neutron irradiation, for example, increases the intensity of some PL bands but not others.During the past year we discovered that our molecular flourine laser causes photo-induced opticalabsorption (PA) in a-SiO2 and simultaneously enhances some of the PL bands. This may provideanother means of correlating the various bands. We are also preparing to measure the timedependence of PA. Such measurements have proved very important in the study of the narrowerband-gap chalcogenide glasses.We have also begun studies on thermally grown oxide films. We have made a preliminaryobservation of PL from a 1 j.m thick film of a-SiO2 grown on single crystal Si at Lincoln LaboratoryRLE P.R. No. 125Electronic Properties of Amorphous Silicon DioxideTransient photoconductivity experiments on a-SiO 2 using the same films as well as bulk samples arebeginning. Theoretical work on the interpretation of such photoconductivity experiments has led tointeresting predictions for the experiments.PublicationsKastner, M.A., "Dielectric Relaxation and Delayed-Collection Field Experiments in AmorphousSemiconductors," Solid State Comm. 45, 191 (1983).RLE P.R. No. 125

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Electronic Properties of Amorphous Silicon Dioxide

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