Amorphous Semiconductors: Synthesis, Characterization and Applications

1. Amorphous Semiconductors:Synthesis, Characterizationand Applications Fei Wang May 19 2007

2. Background • Education --Ph.D. in Electrical Engineering (Electronic Materials and Devices)-- University of Cincinnati, College of Engineering (2005) • Academic Experience --Assistant Professor – Department of Electrical Engineering, California Polytechnic State Univ. (current) * Teaching -- Semiconductor Devices, Analog/Digital Electronic Design and Electromagnetic theory courses. * Research interest – Electronic material synthesis and characterization, Non-volatile memory device. * Publications – 15 journal/conference publications and 1 book chapter

3. Impact of material science on modern life • Ancient Age: -- Stone Age -- Bronze Age -- Iron Age • Modern life: -- Organic: Invention of plastic and synthetic fibers. -- Inorganic: Alloyed metal and Semiconductor

4. What does amorphous mean? • Amorphous materials are solids obtained by super-cooling liquid. It is also called glass. Glass-forming liquid is water quenched from a temperature above its liquidus. • Supercooling process prevents crystallization from happening. • Amorphous materials still have same short-range structure as its crystalline counterpart, but have distributed bond-length and bond-angle. (c-Si bond length: 2.33Å; bond-angle: 109.4o)

5. Structure of amorphous and crystalline silicon

6. Glass Forming Regions for selected system Ge-Se-I Ge-Se-Ag

7. Chalcogenide Glasses • Glasses containing Chalcogens (S, Se and Te) form a class of materials denoted asChalcogenide Glasses.

8. Applications of Chalcogenide Glasses- High Infra-red Transparency Infra-red Optical fibers/waveguide (2-12mm) – Ideal for remote chemical sensing.

9. Applications of Chalcogenide Glasses- High Infra-red Transparency • Sulfide glasses and telluride chalcogenide glasses are used as infra-red waveguide or fiber. • Halogen doped glasses, such as Ge-S-I and Ge-Se-I are also possible materials in infra-red fiber applications. • Slected halogen doped chalcogenide glasses display high optical nonlinearity. – all optical switching devices

10. Applications – High Photosensitivity • Mass information storage - Digital Video Disks (DVD) Active element is a GeSbTe film that can be photo-amorphourize into sub-micron sized amorphous grains.

11. Crucial Temperatures • Glass transition temperature: ---The temperature at which amorphous solids starts softening (Tg). • Crystallization temperature: ---The temperature at which amorphous material starts to crystallize. (Tc) • Melting temperature: ---The temperature at which the material melts. (Tm) Tg<Tc<Tm

12. Applications – Photosensor in imaging technologies Active layer -- photoconductivity Transparent Conductive Coating

13. Applications – Photosensor in imaging technologies

14. Applications –Switching Property • Switching property of selected glasses has been utilized in memory devices (Ovonic threshold switch-OTS). • The active elements consist largely of Telluride based glassy thin-films that have an on and off stage. • A filament like current saturation region starts to form when voltage applied hit a threshold. • Switch comes back to high resistance state when current drops below holding current.

15. Issues • Material selection: - good glass former (fiber, DVD) - minimal aging effect (life time of device). minimal internal network stress • Concept of intermediate phase (IP).1,2 1. P. Boolchand 2. J.C. Philips, M. Thorpe

16. Concept of three Elastic Phases • Our controlled experiments on glasses performed as a function of their connectivity ( or chemical composition) show, in general, three distinct elastic phases to occur. The opening of intermediate phases between floppy and stressed-rigid phases in glasses suggests that these glasses, strictly speaking, are not random. Intermediate phases may represent self-organized of disordered networks in which global connections between atoms are rigid but stress-free.

17. Experimental Methods--MDSC • MDSC-Temperature Modulated Differential Scanning Calorimetry. • Measure the heat flow response to the modulated heating rate. The total heat flow response can be separated into two useful parts: • Total = Reversing Heat Flow + Non-reversing Heat Flow • [glass transition temp] [Stress-releasing if any]

18. T-Modulated DSC Ge25Se75

19. 15 20 25 30 Ge Content x (%) Intermediate Phase in GexSe1-x -- T-modulated Differential Scanning Calorimetry (MDSC) Fei Wang et al. PRB71 , 17, 174201 (2005 )

20. Intermediate Phase in Ge25Se75-yIy -- T-modulated Differential Scanning Calorimetry (MDSC)

21. Raman Scattering • When light encounters molecules, the predominant mode of scattering is elastic scattering, called Rayleigh Scattering. • It is also possible for the incident photons to interact with the molecules in such a way that energy is either gained or lost so that the scattered photons are shifted in frequency. Such inelastic scattering is called Raman scattering.

22. Raman Scattering • Raman spectroscopy measures Raman scattering. Raman scattering modes are signatures of different molecular structures. • Powerful tool to study molecular structures.

23. Pressure Dependent Raman Measurements using Diamond Anvil Cell -- A direct method to probe stress

24. Diamond Anvil Cell Laser beam in Metal Gasket Diamond Anvils Side Top • Opening on the gasket is 200um. • Use Alcohol + Methanol mix (1:4) as pressure transfer media. • Ruby crystal is used to calibrate the pressure applied.

25. Threshold Pressure • Raman line-shapes of GexSe1-x glasses reveal that the frequency of the Corner-Sharing (CS) mode ( ~200 cm-1) usually blue shifts upon applying hydrostatic pressure (P), but only once P exceeds a threshold value( Pc).

26. Pc tracks the non-reversing enthalpy near Tg. * Fei Wang et al. Physical Review B, 2005 15 20 25 30 Ge Content (%)

27. Current Research • Resistance switching memory device-Programmable metallization cell (PMC) fabricated based on metal doped chalcogenides. • Thin films of metal doped Chalcogenides – Photodiffusion, photo-condensation and thermal annealing effects.

28. Current Research • Programmable Metallization Cell Devices—(New memory devices) Amorphous Ag-Ge-Se(S) thin film forms active layer. AgGeSe(S) * M.N. Kozicki and W.C. West, Programmable Metallization Cell, U.S. Patent 5,896,312(1999).

31. Issues about device based on Ag-Ge-S -Pros: --Sulfide glasses display better thermal stability comparing to corresponding selenides. (Ag-Ge-Se can not tolerate temperature beyond 200oC) --Ge-S network has less stress than Ge-Se network, so that more Ag can be doped into Ge-S glasses. -Cons: Since sulfur vaporize at extremely low temperature (57oC). Thin film fabrication of Ag-Ge-S is challenging.

32. Ag-Ge-S thin film fabrication • In order to avoid non-uniform film, Ag-Ge-S bulk material are placed in multiple evaporation boats to assure efficient heating. • We increase the temperature extremely fast to achieve flash deposition. • In order to prevent bulk material from spitting, we used tungsten mesh to cover evaporation boats.

33. Comparison of Raman line-shapes for thin film and bulk material Thin film sample Bulk sample

34. Device structure Cross-stripe structure

35. Future Plan • Using photo-diffusion method to assure the amount of Ag in the film. • Study light induced effect of Ag-chalcogenide thin film (i.e. Photo-condensation, aging etc). • Use other solid state electrolytes as active material for PMC memory cell (i.e. Cu-Ge-Se, Ag-As-Se etc)