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Amorphous Semiconductors:Amorphous Semiconductors:Synthesis, Synthesis, CharacterizationCharacterization
and Applications and Applications
Fei Wang
May 19 2007
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
Impact of material Impact of material sciencescience on on modern lifemodern life
• Ancient Age: -- Stone Age
-- Bronze Age
-- Iron Age
• Modern life: -- Organic: Invention of plastic and synthetic
fibers.
-- Inorganic: Alloyed metal and Semiconductor
What does amorphous mean?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)
Structure of amorphous and Structure of amorphous and crystalline siliconcrystalline silicon
Glass Forming Regions for Glass Forming Regions for selected systemselected system
Ge-Se-I Ge-Se-Ag
Chalcogenide GlassesChalcogenide Glasses• Glasses containing Chalcogens (S, Se and Te) form
a class of materials denoted as Chalcogenide Glasses.
Infra-red Optical fibers/waveguide (2-12m) – Ideal for remote chemical sensing.
Applications of Chalcogenide Applications of Chalcogenide Glasses- Glasses- High Infra-red TransparencyHigh Infra-red Transparency
Applications of Chalcogenide Applications of Chalcogenide Glasses- Glasses- High Infra-red TransparencyHigh 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
Applications – Applications – High PhotosensitivityHigh 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.
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
ApplicationsApplications – – Photosensor in imaging technologiesPhotosensor in imaging technologies
Active layer -- photoconductivityTransparent Conductive Coating
ApplicationsApplications – – Photosensor in imaging technologiesPhotosensor in imaging technologies
• 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.
Applications –Applications – Switching PropertySwitching Property
IssuesIssues• 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. Boolchand2. J.C. Philips, M. Thorpe
• Our controlled experiments on glasses performed as a function of their connectivity ( or chemical composition) show, in general, three distinct elastic phases to occur.
Concept of three Elastic PhasesConcept of three Elastic Phases
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.
Experimental Methods--MDSCExperimental 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]
T-Modulated DSCT-Modulated DSC
Ge25Se75
Intermediate Phase in GeIntermediate Phase in GexxSeSe1-x 1-x
-- -- T-modulated Differential Scanning Calorimetry (MDSC)T-modulated Differential Scanning Calorimetry (MDSC)
Ge Content x (%)
15 20 25 30
Fei Wang et al. PRB 71 , 17, 174201 (2005 )
Intermediate Phase in GeIntermediate Phase in Ge2525SeSe75-y75-yIIy y
-- -- T-modulated Differential Scanning Calorimetry (MDSC)T-modulated Differential Scanning Calorimetry (MDSC)
Raman ScatteringRaman 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.
Raman ScatteringRaman Scattering
• Raman spectroscopy measures Raman scattering. Raman scattering modes are signatures of different molecular structures.
• Powerful tool to study
molecular structures.
Pressure Dependent Raman Measurements using Diamond Anvil Cell
-- A direct method to probe stress
Diamond Anvil CellDiamond Anvil Cell
• 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.
TopSide
Laser beam in
DiamondAnvils
Metal Gasket
Threshold PressureThreshold 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).
PPcc tracks the non-reversing enthalpy near tracks the non-reversing enthalpy near
TTgg..
Ge Content (%) 15 20 25 30* Fei Wang et al. Physical
Review B, 2005
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.
Current ResearchCurrent Research
• Programmable Metallization Cell Devices—(New memory devices)
Amorphous Ag-Ge-Se(S) thin film forms active layer.
* M.N. Kozicki and W.C. West, Programmable Metallization Cell, U.S. Patent 5,896,312(1999).
AgGeSe(S)
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.
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.
Comparison of Raman line-shapes for thin film and bulk material
Frequency (cm-1)
200 250 300 350 400 450 500
Cou
nts
5
10
15
20
25
30
35
Agx(Ge25S75)1-x (x=10%)
S8
Q3Q2Q1
Ge-SES
S8
Ge-S CS (Q4)
Bulk sample Thin film sample
Device structure
Cross-stripe structure
Future PlanFuture 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)