PREPARATION AND CHARACTERIZATION OF TERNARY ALLOY CHALCOGENIDE SEMICONDUCTOR THIN FILMS PREPARED BY FLASH EVAPORATION TECHNIQUE By PRADYUMNA KUMAR SWAIN SUBMITTED IN FULFILLMENT OF THE REQUIREMENTS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY Department of Physics INDIAN INSTITUTE OF TECHNOLOGY, DELHI FEBRUARY 1995 csi,2sy I. 1. T. ClIE.0-41. Cir.-4 ,Z1... — ....2 oe CERTIFICATE This is to certify that the thesis entitled "Preparation and Characterization of Ternary Alloy Chalcogenide Semiconductor Thin Films prepared by Flash Evaporation Technique" being submitted by Mr. Pradyumna Kumar Swain to the Indian Institute of Technology, Delhi for the award of the degree of Doctor of Philosophy in Physics is a record of bonafide research work carried out by him. Mr. P.K. Swain has worked under my guidance and supervision for the submission of this thesis. The thesis or any part thereof has not been submitted to any other university or institute for the award of any degree or diploma. Prof. H.K. Sehgal Materials and Systems Laboratory Department of Physics Indian Institute of Technology New Delhi-110 016 DEDICATED TO MY PARENTS ACKNOWLEDGEMENTS I wish to express my deep sense of gratitude to Prof. H.K. Sehgal for his continuous and constructive guidance during the course of work reported in this thesis. His inspiration and kind encouragement were of great help to complete the present investigation. I am sincerely grateful to Prof. S.C. Mathur and Prof. D.C. Dube for providing facilities in their laboratory for electrical measurements. I am grateful to Dr. A.K. Sharma for his invaluable help and moral support throughout the research period. I would like to thank Dr. Preeti Kashyap for her constructive suggestions and helpful advice during the research period. I am grateful to Mr. V.D. Arora for his help with TEM analyses and Mr. H.S. Sharma for his help in absorption spectroscopic measurements. Sincere thanks are due to Mr. G.S. Pati for his valuable help and suggestions. I am thankful to my friends Ramkumar, Rabi, Pant, Ahuja, Jarwal, Kamran, Lalit, Pravin, Adda, Mishraji, Saytavir, Anil, Sudershan, Rajanesh and Pradhan for their help and moral support at the hours of need. I do not have words to express my gratitude to my parents and family members for their moral encouragement through out the period of research. P.K. Swain ABSTRACT Structural, optical and electrical properties of thin films of mercury manganese sulphide, mercury zinc sulphide, lead manganese sulphide and mercury manganese selenide systems, flash evaporated on optically polished quartz, glass micro slides and freshly cleaved single crystal KCl substrates maintained at different temperatures (Ts) are reported in this thesis. Results of investigations show that it is possible to grow single phase HgsMni _sS alloy films over the composition range 0.16x0.84. Structural investigations indicate the presence of f.c.c. lattices in these films. Variation of lattice parameter with composition can be expressed as per the relation: a(x)A = 5.63 + 0.53 x; for the Ts of 175°C films. Presence of a single well defined absorption edge which shifts its position with composition confirms the presence of a single phase alloy of the type HgxMn,_„S in the films. The bandgap is observed to increase with increase in mercury concentration 'x'. Variation of bandgap with 'x' is not linear, but shows a bowing typical of the pseudo-binary solid solutions. The observed structural and optical characteristics of the films can be explained on the basis of atom-by-atom condensation process. Increase in d.c. resistivity with increase in mercury concentration and substrate temperature is correlated to the increase in bandgap with increase in mercury concentration and increase in graillsize with substrate temperature (Ts). Investigations on the mercury zinc sulphide ternary systems suggest the possibility of obtaining uniphase Hg„Zn i _„S alloy films over a composition range of 0.16 5_ x < 0.84. Transmission electron microscopic investigations indicates the films to have a ii polycrystalline b.c.c. structure. The lattice parameter is observed to follow a linear relation, a(x) A = 5.37 - x; for the Ts of 175°C films. Presence of a single absorption edge in all the films confirms the existence of single phase ternary materials of the type HgsZn i _sS in each of the films. Optical bandgap (Eg) is observed to vary between 3.0 eV and 0.45 eV as the mercury concentration `x' is increased from 0.16 to 0.84. The observed structural and optical characteristics of the films can be explained if we consider the films to grow by the atom-by-atom condensation process. D.C. resistivity of these films decreases with increase in 'x' but increases with Ts. Resistive behavior could be understood by considering the grain-boundary conduction mechanism. PbsMni _sS films with lead concentration (x) varied in the range 0.16x 0.84, are analyzed for their structural, optical and electrical characteristics. Electron microscopic investigations indicated all films to have a single phase f.c.c. lattice and grow as epitaxial with <100> zone axes orientations of the grains suggesting the films to contain ternary materials of the type PbxMni_xS. Variation of lattice parameter with 'x' obeys Vegard's law as per the relation: a(x) A = 5.60 + 0.33 x; which gives the best fit for Ts of 165°C. Presence of a single absorption edge which shifts towards longer wavelengths confirms the existence of single phase PbsMni _sS materials in each of the film. The direct optical bandgap is observed to decrease from 2.26 eV and 0.52 eV as the lead concentration is changed from 0.16 to 0.84. Variation of Eg vs 'x' follows a parabolic relation given by: Eg(x) = 2.41 - 1.32 x - 1.1 x2 eV. Structural and optical properties can be explained on the basis of nucleation and growth of iii the films taking place by atom-by-atom condensation process. Room temperature d.c. resistivity of the films increases with T, and decreases with increase in lead concentration. Variation of resistivity is explained on the basis of 'Kane Model' and grain boundary trapping mechanism. Structural investigations carried out on the mercury manganese selenide system with mercury concentration (x) varied in the range 0.16x 5_ 0.84 indicate presence of single phase Hg„Mn i_sSe polycrystalline materials with f.c.c. structure in the films. Lattice parameter varies linearly with 'x' and can be expressed as per the relation; a(x)A = 5.82 + 0.27 x; for the Ts of 130°C films. Presence of a single absorption edge in the optical absorption investigations confirms the existence of a single phase ternary material of the type HgsMni_sSe in the films. Direct optical bandgap is observed to decrease linearly from Eg = 3.08 eV for x = 0.16 to Eg = 0.47 eV for x = 0.84; for T. of 225°C films. Structural and optical characteristics of the films can be explained on the basis of atom-by-atom condensation process. Room temperature d.c. resistivity decreases with increase in mercury concentration but increases with increase in T. Resistive behavior could be explained on the basis of grain boundary conduction mechanism. iv CONTENTS Page No. Acknowledgements Abstract fi Chapter-IIntroduction 1.1Historical Background 1.2Thin Film Deposition Techniques 1.2.1 Spray Pyrolysis 1.2.2 Solution Growth 1.2.3 Sputtering 1.2.4 Thermal Evaporation 1.2.5 Molecular Beam Epitaxy 1.2.6 Flash Evaporation 1.3The Binary Materials 1.4The Ternary Systems 1 2 2 3 3 3 4 4 5 6 Chapter-IIExperimental Technique 2.1 Introduction 2.2Deposition of Thin film samples by flash evaporation technique 2.2.1 Experimental set-up 2.2.1 (a) The vacuum system 2.2.1 (b) The flash evaporation assembly 2.2.1 (c) Evaporation Boat 2.2.1 (d) Substrate Heater 2.2.2 Preparation of the films 2.3Transmission Electron Microscopic Studies 2.3.1 Determination of average grainsize 2.4 Optical Bandgap Determination of the film 2.5Electrical Characterization Chapter-IIIStructural, Optical and Electncat Properties of Flash Evaporated HgxMni _xS films 3.1Introduction 3.2Sample Preparation 3.3Experimental Results 3.3.1 Transmission Electron Microscopic studies of the as grown films 3.3.1 (a) Ts of 65°C films 10 10 10 11 11 11 12 13 13 15 16 17 19 20 •21 21 21 3.3.1 (b) T, of 130°C films 3.3.1 (c) T, of 175°C and 225°C films 3.3.2 Variation of lattice parameter of the films 3.3.3 Variation of Average grainsize in the films 3.3.4 Effect of Electron beam heating on the as grown films 3.3.5 Optical Transmission and Reflection Measurements : Determination of Optical Energy Bandgap in the as grown films 3.3.6 Variation of Optical Bandgap with Composition 3.3.7 D.C. Electrical Resistivity Measurements Discussion of the results 3.4 3.4.1 The Atom-by-atom condensation process 3.4.2 Structural Properties 3.4.3 Optical Properties 3.4.4 Electrical Properties 21 22 23 23 23 24 24 25 25 25 28 28 29 Chapter-IV Structural, Optical and Electrical Properties of Mercury Zinc Sulphide Thin Films 4.1Introduction 4.2Preparation of Sample 4.3Experimental Results 4.3.1 Transmission Electron Microscopic studies of the as grown films 4.3.1 (a) Ts of 65°C films 4.3.1 (b) T, of 130°C films 4.3.1 (c) T, of 175°C films 4.3.1 (d) Ts of 225°C films 4.3.2 Variation of lattice parameter of the films 4.3.3 Variation of Average grainsize in the films 4.3.4 Effect of Electron beam heating on the as grown films 4.3.5 Optical Transmission and Reflection Measurements : Determination of Optical Energy Bandgap in the as grown films 4.3.6 Variation of Optical Bandgap with Composition 4. 3. 7 D.C. Electrical Resistivity Measurements 31 32 33 33 33 33 34 34 35 36 36 36 37 37 4.4Discussion of the results 4.4.1 Structural Properties 4.4.2 Optical Properties 4.4.3 Electrical Properties 38 38 39 40 Structural, Optical and Electrical Chapter-V Properties of Lead Manganese Sulphide Thin Films 5.1Introduction 5.2Preparation of Samples 5.3Experimental Results 5.3.1 Transmission Electron Microscopic studies of the as grown films 5.3.1 (a) Ts of 65°C films 5.3.1 (b) T, of 130°C films 5.3.1 (c) T, of 165°C films 5.3.1 (d) T, of 225°C films 5.3.2 Variation of lattice parameter of the films 5.3.3 Variation of Average grainsize in the films 5.3.4 Effect of Electron beam heating on the as grown films 5.3.5 Optical Transmission and Reflection Measurements : Determination of Optical Energy Bandgap in the as grown films 5.3.6 Variation of Optical Bandgap with Composition 5.3.7 D.C. Electrical Resistivity Measurements 5.4 Discussion 5.4.1 Structural Properties 5.4.2 Optical Properties 5.4.3 Electrical Properties 41 42 43 43 43 43 44 44 45 45 45 46 46 47 47 47 48 49 Chapter-VI Structural, Optical and Electrical Properties of Mercury Manganese Selenide Thin Films 6.1Introduction 6.2Preparation of Sample 6.3Experimental Results 6.3.1 Transmission Electron Microscopic studies of the as grown films 6.3.1 (a) Ts of 65°C films 6.3.1 (b) Ts of 130°C films 6.3.1 (c) T, of 175°C films 51 52 53 53 53 53 54. 6.3.1 (d) 'I', of 225°C films 6.3.2 Variation of lattice parameter of the films 6.3.3 Variation of Average grainsize in the films 6.3.4 Effect of Electron beam heating on the as grown films 6.3.5 Optical Transmission and Reflection Measurements : Determination of Optical Energy Bandgap in the as grown films 6.3.6 Variation of Optical Bandgap with Composition 6.3.7 D.C. Electrical Resistivity Measurements 6.4 Discussion 6.4.1 Structural Properties 6.4.2 Optical Properties 6.4.3 Electrical Properties Chapter-VII Conclusion and future scope of work 54 55 55 56 56 56 57 57 57 58 58 60 References 67 List of Publications 76
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