1. Amorphous semiconductors
KUGLER Sándor

2. S. Kugler: Lectures on Amorphous Semiconductors
Introduction
Amorphous materials: NOT NEW!
Iron reach siliceous glassy materials recovered from the Moon! (Apollo mission) Billion years old!
People has been preparing glassy materials (i.e. SiO2) for thousand of years.
S. Kugler: Lectures on Amorphous Semiconductors

3. S. Kugler: Lectures on Amorphous Semiconductors
Historical Notes
S. Kugler: Lectures on Amorphous Semiconductors

4. S. Kugler: Lectures on Amorphous Semiconductors
Scientific investigations started about 70 years earlier. Zachariasen (1932) proposed that SiO2 structure can be described by a Continuous Random Network (CRN).
S. Kugler: Lectures on Amorphous Semiconductors

5. S. Kugler: Lectures on Amorphous Semiconductors

6. S. Kugler: Lectures on Amorphous Semiconductors

7. S. Kugler: Lectures on Amorphous Semiconductors
(8 – N) rule N.F. Mott 1969
In a glass any atom is built in such a way that it retains its natural coordination (no dangling bonds). Z, the number of covalent bonds Z = 8 – N, where N is the number of valence electrons. (Original version, where we consider elements only in IV-VI. columns at the periodic table.)
Z = N, if N<4. (additional rule)
The consequence:
glasses can NOT be doped!
S. Kugler: Lectures on Amorphous Semiconductors

8. S. Kugler: Lectures on Amorphous Semiconductors
Chittick and coworkers at the Telecommunications Lab. in Harlow, England ( ) proposed first doping effect in glow discharge prepared amorphous silicon.
Mott’s (8-N) rule was strong enough to ignore this effect.
Six years later W.E. Spear and P.G. LeComber (Dundee group) could easily dope their film and it was thermally stable.
S. Kugler: Lectures on Amorphous Semiconductors

9. S. Kugler: Lectures on Amorphous Semiconductors
Definitions
Non-crystalline?
Amorphous?
Glassy?
Randomness?
Disorder?
Liquid?
Crystalline?
S. Kugler: Lectures on Amorphous Semiconductors

10. S. Kugler: Lectures on Amorphous Semiconductors
A perfect crystal is that in which the atoms are arranged in a pattern that repeats periodically in three dimensions to an infinite extent.
An imperfect crystal is that in which the atoms are arranged in a pattern that repeats periodically in three dimensions to a finite extent.
Real crystal: imperfect crystal having defects like vacancy, interstitial (foreign) atoms, dislocations, impurities, etc.
S. Kugler: Lectures on Amorphous Semiconductors

11. Solid phase? -Liquid phase?
How to distinguish between condensed phase and liquid phase?
How to distinguish between amorphous materials and liquids? They have very similar diffraction pattern. No long range order.
Glasses – usually said – are liquid having the atoms frozen the spatial positions.
S. Kugler: Lectures on Amorphous Semiconductors

12. Solid to liquid “phase transition”
A solid is a phase whose shear viscosity exceeds Ns/m2.
Example: during a day a force of 100 N applied to 1 cm3 of material having such shear viscosity yields a deformation of 0.02 mm.
Common liquids at room temperature are of the order of 10-3 Ns/m2.
S. Kugler: Lectures on Amorphous Semiconductors

13. What is amorphous? What is glassy?
Definition: Amorphous materials are in condensed phase and do not possess the long range translational order (periodicity) of atomic sites.
A glass is an amorphous solid which exhibits a glass transition (see later).
S. Kugler: Lectures on Amorphous Semiconductors

14. S. Kugler: Lectures on Amorphous Semiconductors
Atomic Scale Ordering
Usually we are speaking about three different orders (simplest definition):
Short range order means the order within the range of 0-10 Å (local order).
Medium range order is the order within the range of Å.
Long range order means order over Å.
S. Kugler: Lectures on Amorphous Semiconductors

15. Classification of amorphous semiconductors.
1. Tetrahedrally bonded amorphous semiconductors: a-Si, a-Ge, a-C(?) and their alloys like a-SiC, etc. (tathogen)
2. Chalcogenide glasses: a. a-S, a-Se, a-Te, a-SxSe1-x (pure chalcogenide)
b. a-As2Se3, a-As2S3, a-P2Se3 , etc. (pnictogen-chalcogen (V-VI))
c. a-GeSe2, a-SiS2, a-SiSe2, etc. (tetragen-chalcogen (IV-VI))
S. Kugler: Lectures on Amorphous Semiconductors

16. S. Kugler: Lectures on Amorphous Semiconductors
Glass formation
Glass forming ability has been discussed by Phillips (1979) in term of a constraint model. Most inorganic covalently bonded glasses have low values of atomic coordination number. An atom which has all covalent bonds satisfied, obeys the (8-N) rule i.e. Se has Nc=2, Ar has Nc=3, Si has Nc=4, etc.
S. Kugler: Lectures on Amorphous Semiconductors

17. S. Kugler: Lectures on Amorphous Semiconductors
For a binary alloy AxB1-x, the average coordination (m):
m = x Nc(A) + (1-x) Nc(B)
Phillips theory: the glass-forming tendency is maximized when the number of constraints is equal to the number of degrees of freedom, Nd. (usually Nd =3, 3D)
S. Kugler: Lectures on Amorphous Semiconductors

18. S. Kugler: Lectures on Amorphous Semiconductors
Constraints:
Bond stretching: m/2
Bond bending: m(m-1)/2, but only (2m–3) are linearly-independent bond angles Nc=m/2 + (2m – 3)
Nd = Nc
Solution: m = 2.4
(m is the average coordination number per atom)
S. Kugler: Lectures on Amorphous Semiconductors

19. S. Kugler: Lectures on Amorphous Semiconductors
If m>2.4, network is overconstrained (rigid) materials (a-Si,…) opposite cases m<2.4 underconstrained (floppy) materials.
Examples:
1. IV-VI systems such as g-GeS, g-GeSe, g-SiS, g-SiTe, etc. IV elements have 4 neighbours, VI elements have 2 neighbours. g-GexS(1-x)
4x+2(1-x)=2.4 => x=0.2
S. Kugler: Lectures on Amorphous Semiconductors

20. S. Kugler: Lectures on Amorphous Semiconductors
g-GeS4, g-GeSe4, g-SiS4, g-SiSe4, g-SiTe4
are the optimum composition, mechanically most stable. Do not forget that GeS2 is the chemically stable composition.
2. V-VI systems such as g-AsS, g-AsTe, etc.
V elements have 3 neighbours. a-AsxS(1-x)
3x + 2(1-x) = 2.4 => x=0.4
g-As2S3, g-As2Se3, etc. are the optimum composition.
S. Kugler: Lectures on Amorphous Semiconductors

21. Exception: SiO system Thorpe (1983)
Si-O-Si bond angle distribution is rather wide! The constraint associated with oxygen bond angles should be regarded as rather weak and should be neglected from consideration.
S. Kugler: Lectures on Amorphous Semiconductors

22. S. Kugler: Lectures on Amorphous Semiconductors

23. Exception: SiO system Thorpe (1983)
Let’s consider SixO(1-x). In 3=m/2+(2m–3)
equation the (2m–3) term associated with bond angles must be modified.
No bond angle constraint for in oxigen case:
x(2mSi-3) + (1-x)0 = x(2*4-3) = 5x ;
S. Kugler: Lectures on Amorphous Semiconductors

24. is the good glass-forming composition.
We must solve the following equations:
3 = m/2 + 5x, where
m = 4x + 2(1 –x). => x=1/3.
SiO2
is the good glass-forming composition.
S. Kugler: Lectures on Amorphous Semiconductors

25. S. Kugler: Lectures on Amorphous Semiconductors
Other exceptions
Some a-Ch materials show other property; m = The reason is the following: the constraint for an atom is 2D plane is define as
Nc=m/2 + (m – 1),
planar structure.
Nd = Nc = 3
see: Keiji Tanaka’s (Sapporo, Japan) works
S. Kugler: Lectures on Amorphous Semiconductors

26. Nanocrystalline? Microcrystalline? Polycrystalline?
S. Kugler: Lectures on Amorphous Semiconductors

27. S. Kugler: Lectures on Amorphous Semiconductors
Nanocrystalline silicon (nc-Si) - an allotropic form of silicon - is similar to amorphous silicon (a-Si), in that it has an amorphous phase. Where they differ, however, is that nc-Si has nm size grains of crystalline silicon within the amorphous phase.
Microcrystalline silicon is similar containing µm size grains.
S. Kugler: Lectures on Amorphous Semiconductors

28. S. Kugler: Lectures on Amorphous Semiconductors
Nanocrystalline silicon is in contrast to polycrystalline silicon (or polysilicon, poly-Si; Greek words: polys meaning many) which consists solely of crystalline silicon grains, separated by grain boundaries.
S. Kugler: Lectures on Amorphous Semiconductors