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Amorphous semiconductors KUGLER Sándor

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

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S. Kugler: Lectures on Amorphous Semiconductors 3 Historical Notes

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S. Kugler: Lectures on Amorphous Semiconductors 4 Scientific investigations started about 70 years earlier. Zachariasen (1932) proposed that SiO 2 structure can be described by a Continuous Random Network (CRN). Scientific investigations started about 70 years earlier. Zachariasen (1932) proposed that SiO 2 structure can be described by a Continuous Random Network (CRN).

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S. Kugler: Lectures on Amorphous Semiconductors 5

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7 (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.) 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) Z = N, if N<4. (additional rule) The consequence: glasses can NOT be doped! glasses can NOT be doped!

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

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

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

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S. Kugler: Lectures on Amorphous Semiconductors 11 Solid phase? -Liquid phase? How to distinguish between condensed phase and 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. 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. Glasses – usually said – are liquid having the atoms frozen the spatial positions.

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S. Kugler: Lectures on Amorphous Semiconductors 12 Solid to liquid “phase transition” A solid is a phase whose shear viscosity exceeds Ns/m 2. A solid is a phase whose shear viscosity exceeds Ns/m 2. Example: during a day a force of 100 N applied to 1 cm 3 of material having such shear viscosity yields a deformation of 0.02 mm. Example: during a day a force of 100 N applied to 1 cm 3 of material having such shear viscosity yields a deformation of 0.02 mm. Common liquids at room temperature are of the order of Ns/m 2. Common liquids at room temperature are of the order of Ns/m 2.

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S. Kugler: Lectures on Amorphous Semiconductors 13 What is amorphous? What is glassy? 1. Definition: Amorphous materials are in condensed phase and do not possess the long range translational order (periodicity) of atomic sites. 2. A glass is an amorphous solid which exhibits a glass transition (see later).

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S. Kugler: Lectures on Amorphous Semiconductors 14 Atomic Scale Ordering Usually we are speaking about three different orders (simplest definition): 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). 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. Medium range order is the order within the range of Å. Long range order means order over 100 Å. Long range order means order over 100 Å.

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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-S x Se 1-x (pure chalcogenide) b. a-As 2 Se 3, a-As 2 S 3, a-P 2 Se 3, etc. (pnictogen-chalcogen (V-VI)) b. a-As 2 Se 3, a-As 2 S 3, a-P 2 Se 3, etc. (pnictogen-chalcogen (V-VI)) c. a-GeSe 2, a-SiS 2, a-SiSe 2, etc. (tetragen- chalcogen (IV-VI)) c. a-GeSe 2, a-SiS 2, a-SiSe 2, etc. (tetragen- chalcogen (IV-VI))

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S. Kugler: Lectures on Amorphous Semiconductors 16 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 N c =2, Ar has N c =3, Si has N c =4, etc. 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 N c =2, Ar has N c =3, Si has N c =4, etc.

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

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

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S. Kugler: Lectures on Amorphous Semiconductors 19 If m>2.4, network is overconstrained (rigid) materials (a-Si,…) opposite cases 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-Ge x S (1-x) 4x+2(1-x)=2.4 => x=0.2 4x+2(1-x)=2.4 => x=0.2

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S. Kugler: Lectures on Amorphous Semiconductors 20 g-GeS 4, g-GeSe 4, g-SiS 4, g-SiSe 4, g-SiTe 4 g-GeS 4, g-GeSe 4, g-SiS 4, g-SiSe 4, g-SiTe 4 are the optimum composition, mechanically most stable. Do not forget that GeS 2 is the chemically stable composition. are the optimum composition, mechanically most stable. Do not forget that GeS 2 is the chemically stable composition. 2. V-VI systems such as g-AsS, g-AsTe, etc. V elements have 3 neighbours. a-As x S (1-x) V elements have 3 neighbours. a-As x S (1-x) 3x + 2(1-x) = 2.4 => x=0.4 g-As 2 S 3, g-As 2 Se 3, etc. are the optimum composition. g-As 2 S 3, g-As 2 Se 3, etc. are the optimum composition.

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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. 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.

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S. Kugler: Lectures on Amorphous Semiconductors 22

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

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

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

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

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S. Kugler: Lectures on Amorphous Semiconductors 27 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. 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. Microcrystalline silicon is similar containing µm size grains.

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S. Kugler: Lectures on Amorphous Semiconductors 28 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. 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.

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