Amorphous Semiconductors: Synthesis,

Characterization 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 science modern life on

•

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?

• Amorphous materials are

solids

obtained by super-cooling liquid. It is also called temperature above its liquidus.

glass . Glass-forming liquid is water quenched from a • 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: 2.33

Å; bond-angle: 109.4

o )

Structure of amorphous and crystalline silicon

Glass Forming Regions for selected system

Ge-Se-I Ge-Se-Ag

Chalcogenide Glasses

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

Applications of Chalcogenide Glasses-

High Infra-red Transparency

Infra-red Optical fibers/waveguide (2 12

m

m) – Ideal for remote chemical sensing.

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

•

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 .

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

Applications

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

Applications

– Photosensor in imaging technologies

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.

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

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.

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]

T-Modulated DSC

Ge 25 Se 75

Intermediate Phase in Ge x Se 1-x - T-modulated Differential Scanning Calorimetry (MDSC)

15 20 25 30 Ge Content x (%) Fei Wang et al. PRB 71 , 17, 174201 (2005 )

Intermediate Phase in Ge 25 Se 75-y I y - T-modulated Differential Scanning Calorimetry (MDSC)

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 .

Raman 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 Cell

Laser beam in Diamond Anvils Metal Gasket Side Top • • Opening on the gasket is

200um

.

• Use

Alcohol + Methanol mix (1:4)

transfer media.

Ruby crystal

applied.

as pressure is used to calibrate the pressure

Threshold Pressure

• Raman line-shapes of Ge x Se 1-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( P c ).

P c tracks the non-reversing enthalpy near T g

.

*

Fei Wang et al. Physical Review B, 2005 15 20 25 30 Ge Content (%)

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 Research

• Programmable Metallization Cell Devices —(New memory devices)

AgGeSe(S)

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).

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 200 o C) --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 (57 o C). 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

35 30 25 20 15 10 5 200 S8 Ge-S CS (Q4) Agx(Ge25S75)1-x (x=10%) Q1 Q2 Q3 S8 Ge-S ES 450 500 250 300 350 Frequency (cm -1 ) 400

Thin film sample Bulk sample

Device structure

Cross-stripe structure

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)