This dissertation has been microfilmed exactly as received 6 8-8827 G R E E N E , Lawrence Conde, 1913- SERIES ASSIGNMENTS IN THE FLUORESCENCE LINE SPECTRA OF HIGH PURITY CADMIUM SULFIDE. The Ohio State University, Ph.D„ 1967 Physics, solid state University Microfilms, Inc., Ann Arbor, Michigan SERIES ASSIGNMENTS IN THE FLUORESCENCE LINE SPECTRA OF HIGH PURITY CADMIUM SULFIDE DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosphy in the Graduate School of The Ohio State University by Lawrence Conde,Greene, B.S., M.A. * * * * * * The Ohio State University 1967 Approved by Department of Physics ACKNOWLEDGMENTS We wish to thank D, E, Johnson for his work in drawing preparation, H, A, Wilson for the crystal preparation, L, W. Croft for the spectro- graphy, as well as Professor Richard C, Nelson and the late Professor Charles H, Shaw for careful criticism of the original manuscript. VITA October 23. 1913 B o m - Milton, Wisconsin 1948 B.S., Western Michigan University, Kalamazoo, Michigan 1950 M,A,, Kalamazoo College, Kalamazoo, Michigan 1951-1967 .. Research Physicist, Aerospace Research Laboratories, Wright-Patterson AFB, Ohio PUBLICATIONS '’Emulsion Studies of Cosmic Ray Stars in Metal Foils", co-author I. Barbour, Phys, Rev. 79, 406 (15 Jul 1950). "Properties of a CdS Photorectifier", co-authors D. C. Reynolds and L. L. Antes, J. Chem, Phys. 25, 1177 (Dec 1956), "Crystal Growth Mechanism in Cadmium Sulfide Crystals" co-author D, C, Reynolds, J. Appl, Phys, 219, 559 (Mar 1958), "Growth and Properties of CdS Crystals", co-author D, C. Reynolds, pres. International Conf, on Solid State Physics, Brussels, Belgium (June 1958), "Method for Growing Large CdS and ZnS Single Crystals", co-authors D. C, Reynolds, S, Czyzak and W, M, Baker, J, Chem, Phys, 29, 1375 (Dec 1958), “ "An Improved Furnace for the Growth and Treatment of II-VI Compound Single Crystal Platelets", co-author Charles R. Geesner, J. Appl, Phys, 38, 3662 (Aug 1967). iii TABLE OF CONTENTS Acknowledgments.......................................... ii V i t a ............................. iii Tables ........................................ vi List of Figures . .vii SECTION I. INTRODUCTION....................................1 II. BACKGROUND..................................... 4 Excitons Phonon Assisted Edge Emission Crystal Growing III. CRYSTAL GROWTH AND CONTROL OF CRYSTAL DEFECT STRUCTURE..................................... 23 Apparatus The Edge Emission Bands Pure Argon as the Carrier Gas Argon and Hydrogen Sulfide as the Carrier Argon and Cadmium Vapor as the Carrier Argon and Oxygen as the Carrier IV. DISCUSSION OF DEFECT STRUCTURE AS RELATED TO CRYSTAL GROWTH........................................ 37 V. THE LOW TEMPERATURE EMISSION LINE SPECTRA OF CADMIUM SULFIDE .............................. 42 Experimental Procedure Results The I2JJ Line The S Series The T Series The V and W Series The U and U' Series iv Crystal Stoichiometry The Relation of the Emission Lines to the Defect Structure Concluding Remarks v TABLES Page TABLE I The S Series in the Fluorescence of Cadmium Sulfide at 4 . 2 ° K ....................... 52 TABLE II The T Series in the Fluorescence of Cadmium Sulfide at 4 . 2 ° K ...................... 54 TABLE III The V and W Series in the Fluorescence of Cadmium Sulfide at 4.2°K .............. 57 TABLE IV The U and U 1 Series in the Fluorescence of Cadmium Sulfide at 4 . 2 ° K .............. 58 vi LIST OF FIGURES Figure Page 1. Crystal Lattice for the Wurtzite Structure ......... 6 2. First Brillouin Zone for Cadmium Sulfide............. 8 3. Band Extrema in Cadmium Sulfide .......... 9 4. Cross Section Diagram of Platelet Furnace ...... .25 5. Furnace Temperature Profile Suitable for Growing Undoped CdS Crystals............................. 26 6. CdS Platelets ............ .... 28 7. Fluorescence Spectra of CdS Grown with Pure Argon as the Carrier, Quenched...........................30' 8. Fluorescence Spectra of CdS'.E^S, Quenched........... 32 9. Fluorescence Spectra of CdSjE^S, Slowly Cooled.. 33 10. Fluorescence Spectra of CdS:Cd, Quenched .......... 35 11. Fluorescence Spectra of CdS:Cd, Not Quenched ....... 36 12. Energy Level Diagram for CdS Showing Proposed Defect Level Assignments.......................... 38 13. Emission Spectrum of Cadmium Sulfide (4.2°K) Showing Dominant L i n e s ........................... 46 14. Emission Spectrum of Cadmium Sulfide (4.2°K) Showing I £g Line.................................. 49 15. Emission Spectrum of Cadmium Sulfide (4.2°K) Showing First Three Lines of S Series ........ 50 16 Emission Spectrum of Cadmium Sulfide (4.2°K) Showing First Three Lines of T Series ............ 53 17 Emission Spectrum of Cadmium Sulfide (4.2°K) Showing V and W Series......................... 56 vii Figures Page 18. Emission Spectrum of Cadmium Sulfide (4.2°K) Showing U and U1 Series............................. 59 19. Emission Spectra of Cadmium Sulfide (4.2°) Doped with Sulfur............................... 61 20. Emission Spectra of Cadmium Sulfide (4.2°K) Doped with Cadmium .................. 62 viii I. INTRODUCTION The subject of this dissertation is the experimental investigation of the line structure seen in the fluorescence spectra of single cry­ stals of cadmium sulfide. The origin of these lines has been explained in terms of the widely accepted exciton picture, which has been quite extensively investigated by optical reflection, absorption and Zeeman splitting measurements. In this paper the primary interest will be in the separation of these spectra into groups or series of lines of common origin and the study of the nature of the center associated with each series. The fluorescence spectra of many compounds having the wurtzite structure have been shown to lie in a region just to the long wavelength side of the absorption edge which contains a very large number of lines, many of them quite narrow. This structure has been explained largely on the basis of exciton decay, with phonon assistance. An exciton is the bound electron-hole system formed by a Coloumb attraction between an excited electron and a hole remaining in the valence band. Such a charge system, formed when an electron is excited from the valence band to the conduction band, may move away from the region of formation through normal diffusion processes, giving up its energy of formation on recombination, but cannot transport charge since it is electrically neutral. It can however transport energy and mass, Exciton lines are best seen at very low temperatures since they broaden rapidly with increasing temperature, and for most materials the excitons are thermal­ ly dissociated at roan temperature. 1 The exciton spectra in semiconductors and insulators can provide a great deal of information concerning the details of the electronic structure of the material. In the past a number of optical methods have been used in such investigations, Among these methods are trans­ mission, reflection, photoconductivity and fluorescence. Fluorescence at low temperatures has probably been the most widely used of these methods. It will be the purpose of this paper,' (1) to report on the detail­ ed structure of the fluorescence spectra near the absorption edge of cad­ mium sulfide in pure crystals, that is crystals containing no added foreign impurity; (2) to study the relationship between known defects and inpurities and the spectral line structure and from these relationships to make source assignments for certain line series whose origin has been in question. The line structure near the absorption edge in cadmium sulfide has been studied sporadically for a number of years. Some of the more prominant lines have been given assignments as due to intrinsic excitons, some as due to excitons bound to certain types of charged or neutral centers, and others as being phonon-assisted satellites. However, very little has been done toward making definite defect assignments for the centers associated with the bound excitons. The excitons responsible for the fluorescence lines studied in this work are considered to be those bound to lattice defects or for­ eign impurities. It is the purpose of this paper to relate these lines to specific defects by attempting to introduce or remove them 3 preferentially. Although these bound exciton lines have been under study for some time, it has been only recently that crystals of suffic­ iently high quality have become available to make possible a really definitive investigation. In these studies all the measurements were made on single crystal platelets of CdS grown from the vapor phase. The crystals were un­ doped, that is they contained no deliberately added impurity, but were grown under conditions such that the presence of native or stoich­ iometric defects was to be expected. II. BACKGROUND Cadmium sulfide in its crystalline form has received much atten­ tion because of its position midway between the ionic insulating cry­ stals such as the alkali halides and the covalent semiconductors such as silicon or germanium. Cadmium sulfide crystals of high purity and of a high degree of structural perfection may have a dark resistivity of as much as 10^ ohm-cm. On the other hand very small additions of suitable inpurities of structural defects may reduce the resistivity to less than an ohm-cm, thus giving it many of the properties of norm­ al semiconductors. It is as a photoconductor, however, that cadmium sulfide first attracted widespread attention. The presence of certain defects or inpurities