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, Nonvolatile memory device.
* Publications – 15 journal/conference publications and 1 book chapter
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
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 shortrange 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
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 (212mm) – 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<Tc<Tm
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 stressedrigid 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
Ge25Se75
Intermediate Phase in GexSe1-x
-- T-modulated Differential Scanning Calorimetry (MDSC)
15
20
25
Ge Content x (%)
Fei Wang et al. PRB 71 , 17, 174201 (2005 )
30
Intermediate Phase in Ge25Se75-yIy
-- 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
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.
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).
Pc tracks the non-reversing enthalpy near Tg.
* Fei Wang et al. Physical
Review B, 2005
15
20
25
Ge Content (%)
30
Current Research
• Resistance switching memory deviceProgrammable 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)
* M.N. Kozicki and W.C. West, Programmable Metallization Cell, U.S. Patent
5,896,312(1999).
Amorphous AgGe-Se(S) thin
film forms active
layer.
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-GeS 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
Agx(Ge25S75)1-x
(x=10%)
Ge-S
CS (Q4)
30
Counts
25
20
Q1
Q2 Q3
S8
S8
Ge-S
ES
15
10
5
200
250
300
350
400
Frequency (cm-1)
Bulk sample
Thin film sample
450
500
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)