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)