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**Amorphous semiconductors**

KUGLER Sándor

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**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

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**S. Kugler: Lectures on Amorphous Semiconductors**

Historical Notes S. Kugler: Lectures on Amorphous Semiconductors

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**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

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**S. Kugler: Lectures on Amorphous Semiconductors**

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**S. Kugler: Lectures on Amorphous Semiconductors**

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**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

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**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

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**S. Kugler: Lectures on Amorphous Semiconductors**

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

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**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

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**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

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**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

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**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

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**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

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**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

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**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

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**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

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**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

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**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

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**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

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**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

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**S. Kugler: Lectures on Amorphous Semiconductors**

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**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

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**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

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**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

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**Nanocrystalline? Microcrystalline? Polycrystalline?**

S. Kugler: Lectures on Amorphous Semiconductors

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**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

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**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

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