Classification of Solid Structures

Transcription

1 Clssifiction of Solid Structures Represents n tom or molecule Amorphous: Atoms (molecules) bond to form very short-rnge (few toms) periodic structure. Crystls: Atoms (molecules) bond to form long-rnge periodic structure. The constnt bonds (coordintion), bond distnce nd ngles between bonds re the chrcteristics of crystl structure Polycrystlline: mde of pieces of crystlline structures (clled grin) ech oriented t different direction (intermedite-rnge-ordered)

2 Crystls An IDEAL CRYSTAL is constructed by the infinite repetition of identicl structurl units in spce.

3 (b) Crystlline Structures

4 Lttice Structures A LATTICE represents set of points in spce tht form periodic structure. Ech point sees exctly the sme environment. The lttice is by itself mthemticl bstrction. A building block of toms clled the BASIS is then ttched to ech lttice point yielding the crystl structure. LATTICE + BASIS = CRYSTAL STRUCTURE The identicl structure units tht hve smll volume re clled UNIT CELL.

5 Crystl lttices O' O A 2-D lttice showing trnsltion of unit cell by R = 4 + 2b

6 Lttice Structure ODEF is primitive unit cell nd PQRS is the conventionl unit cell of the bove lttice

7 Crystls The smllest unit cell is clled PRIMITIVE CELL nd the cell tht is most convenient to study crystl structure is clled CONVENTIONAL UNIT CELL. A unit cell contins ll the structurl nd physicl properties of the crystl. A unit cell IS NOT unique entity. The reltionship between unit cell nd lttice is chrcterized by three vectors,. Every equivlent lttice point in the 3-D crystl cn be found using the vector: R = p +qb + sc, p, q, s re integers.

8 Cubic P Cubic I Cubic F Tetrgonl P Tetrgonl I Orthorhombi c P Orthorhombi c C Orthorhombi c I Orthorhombi c F α b γ Monoclinic P Monoclinic C Triclinic c β Trigonl R Trigonl nd Hexgonl R

9 Lttice Structures Simple Cube SC Body Centred Cube BCC Fce Centred Cube FCC Lttice Constnt Side digonl 2 Body digonl 3

10 Exmples of SC & BCC Simple cubic (SC): Polonium (Po) Body centered cubic (BCC) unit cell: Alkli metls (Li, N, K), Cr, Mo, Mn Ref: S.O. Ksp, Principles of Electronic mterils nd devices

11 Exmple of FCC Structure () The crystl structure of copper is fce centered cubic (FCC). The toms re positioned t well defined sites rrnged periodiclly nd there is long rnge order in the crystl. (b) An FCC unit cell with closed pcked spheres. (c) Reduced sphere representtion of the unit cell. Exmples: Ag, Al, Au, C, Cu, Ni. Number of toms per unit cell 8 corners = 8 (1/8)=1 6 fces = 6 (1/2) = 3

12 Lttice Structure Miller Indices For ny cubic unit cell system: - build xes long the edges of unit cell - normlize intercepts: 3, 2, 1 - invert intercepts: 1/3, 1/2, 1 - djust: 2/6, 3/6, 6/6 - Miller Indices: (236) b z y x

13 Z Z y y X (100) X (110) Z Z y y X X (111) (200) Some populr lttice plnes

14 Crystls Prmeters tht chrcterize crystl structure: Lttice structure (e.g. cubic, tetrgonl, etc.). Bsis. Number of toms in unit cell. Crystl plnes e.g. {100}, {110}, {111}, Miller Indices. Number of toms in ech plne. Chemicl binding (e.g. metllic, covlent, ). Number of nerest (nn) nd next nerest (nnn) toms to ech tom.

15 Exmple of (100) (100) z c x y b

16 Exmple of (110) c z x y b

17 Exmple of (111) z c x y b

18 Dimond Structure Top view of n extended (100) plne of the dimond lttice structure.

19 b c x y z z y x The unit cell of dimond lttice structure. The position of ech lttice point is shown with respect to the 000 lttice point

20 Si (100) Crystl c Plne intercepts t x=1 z x y b Plne contins 2 unit cells (1+¼+¼+¼+¼)

21 Si (110) Crystl c z Plne intercepts t x=1, y=1 x y b 2 Plne contins 4 unit cells (2 + 2*½ + 4*¼)

22 Si (111) Crystl c z Plne intercepts t x=1, y=1, z=1 x y b Plne contins 2 unit cells (3*½ + 3*1/6) 2

23 Two Interpenetrting Fce-Centered Cubic Lttices Silicon Dimond Structure Common Plnes {100} Plne {110} Plne {111} Plne Lttice Constnt For Silicon = 5.34 A o

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25 Terms used often to chrcterize crystlline structure: 1) Reltionship between lttice constnt nd rdius of the toms 2) Atomic pcking fctor (APF) defined s: APF = Volume of toms in unit cell Volume of unit cell 3) Atomic surfce density (# of toms/unit of re) nd volume density (# of toms /unit of volume) 4) Distnce between nerest neighboring toms

26 Exmple 1.3. Consider BCC structure. Wht re the surfce density of toms for (100), (110) nd (111) plnes? (100) (110) (111) 2 A = 2 Answers: (100) (110) (111) A = Prctice: You do this for n FCC structure.

27 Silicon Production C SiC( solid) + SiO2 ( solid) Si( liquid) + SiO( gs) + CO( gs) Produced silicon t this stge is 98% pure nd is clled Metllurgicl Grde Silicon (MGS). For electronic purposes very high pure silicon - Electronic Grde Silicon (EGS) is needed. To obtin EGS from MGS the following rections re used: 300 C Si( solid ) + 3HCl( gs) SiHCl 3 ( gs) + H 2 + het 2 2 SiHCl 3( gs) + 2H ( gs) 2Si( solid) + 6HCl( gs) The bove process is clled chemicl vpour deposition (CVD) nd the produced Si is polycrystlline nd % pure.

28 Silicon crystl preprtion Single crystl Ingots or boules (bulk): A lrge single crystl Ingot grows from melt polycrystlline Si, using solidifiction on seed crystl. The seed is slowly pulled from the melt. As the seed is slowly pulled, solidifiction occurs long the plne between the solid-liquid interfce. Usully the crystl is lso rotted slowly to provide slight stirring ction to the melt, resulting more uniform temperture. Rottion RF heting Single crystl: cm in dimeter nd 1-2 m in length.

29 Silicon wfers production Single crystl Si ingot (bout 2 m) Ground edge or flt Flt (100) Plne Cut wfer [100] Direction

30 Silicon Wfer Type n-type (e.g.,as,p, Bi doping) P-type (B, G, In doping) Orienttion {100}, {111},