2006. Fall Semester: Introduction to Electronic Materials & Devices (Prof. Sin-Doo Lee, Rm ,

Transcription

1 2006. Fll Semester: Introduction to Electronic Mterils & Devices (Prof. Sin-Doo Lee, Rm , Principles of Electronic Mterils nd Devices S. O. Ksp (McGrw ill, New York, 2000) Week Chpter Week Chpter

2 Chp. 1. Elementry Mterils : Science Concepts 1.1 Atomic Structure - Shell model bsed on "the Bohr model": L shell with two subshells Nucleus L K 1s 2s 2p 1s 2 2s 2 2p 2 or [e]2s 2 2p 2 Fig. 1.1: The shell model of the tom in which the electrons re confined to live within certin shells nd in subshells within shells.

3 1.2 Bonding nd Types of Solids Molecules nd Generl Bonding Principles - Net force = ttrctive nd repulsive Molecule r o r = + Seprted toms + Force Attrction 0 Repulsion r o F A = Attrctive force F N = Net force Intertomic seprtion, r F R = Repulsive force Potentil Energy, E(r) Repulsion 0 Attrction E o E R = Repulsive PE E = Net PE r o E A = Attrctive PE r () Force vs r (b) Potentil energy vs r Fig. 1.3: () Force vs intertomic seprtion nd (b) Potentil energy vs intertomic seprtion.

4 1.2.2 Covlently Bonded Solids: 2, C 4, dimond Covlent bond C L shell K shell C covlent bonds () (b) C (c) Fig. 1.5: () Covlent bonding in methne, C4, involves four hydrogen toms shring electrons with one crbon tom. Ech covlent bond hs two shred electrons. The four bonds re identicl nd repel ech other. (b) Schemtic sketch of C4 on pper. (c) In three dimensions, due to symmetry, the bonds re directed towrds the corners of tetrhedron.

5 1.2.3 Metllic Bonding: electron gs or cloud (collective shring of electrons) Positive metl ion cores Free vlence electrons forming n electron gs Fig. 1.7: In metllic bonding the vlence electrons from the metl toms form "cloud of electrons" which fills the spce between the metl ions nd "glues" the ions together through the coulombic ttrction between the electron gs nd positive metl ions.

6 1.2.4 Ioniclly Bonded Solids: slt (ction-nion) N Cl 3s 3s 3p Closed K nd L shells () Closed K nd L shells Cl- Cl- N+ F A F A 3s 3p N+ r (b) r o (c) Fig. 1.8: The formtion of n ionic bond between N nd Cl toms in NCl. The ttrction is due to coulombic forces.

7 - Potentil energy per ion pir in solid NCl Potentil energy E(r), ev/(ion-pir) nm Cohesive energy Cl - Cl r = 1.5 ev r = N + N Seprtion, r Cl - N + r o = 0.28 nm Fig. 1.10: Sketch of the potentil energy per ion-pir in solid NCl. Zero energy corresponds to neutrl N nd Cl toms infinitely seprted.

8 1.2.5 Secondry Bonding : hydrogen bonds (polr), vn der Wls bonds (induced dipolr) O () (b) Fig. 1.12: The origin of vn der Wls bonding between wter molecules. () The 2O molecule is polr nd hs net permnent dipole moment. (b) Attrctions between the vrious dipole moments in wter gives rise to vn der Wls bonding.

9 Time verged electron (negtive chrge) distribution Closed L Shell Ne Ionic core (Nucleus + K-shell) Instntneous electron (negtive chrge) distribution fluctutes bout the nucleus. A vn der Wls force B Synchronized fluctutions of the electrons Fig. 1.13: Induced dipole-induced dipole interction nd the resulting vn der Wls force.

10 1.3 Kinetic Moleculr Theory Men Kinetic Energy nd Temperture - Gs pressure: the collisions between the gs molecules nd the wlls of the continer Squre Continer Fce B Are A v y Fce A Gs toms v x Fig.1.15: The gs molecules in the continer re in rndom motion.

11 - The chnge in the momentum: nd Force = the chnge of momentum: Pressure - For ny molecule,, kinetic energy then - et cpcity of one mole of monotomic "gs" (not solid) t constnt volume: * Mxwell's principle of equiprtition of energy - n verge of to ech degree of freedom (ech degree of freedom hs n verge energy of nd thus the everge kinetic energy of monotomic molecule is ) 1.4, 1.5, 1.6 : Reding Assignment

12 1.7 The Crystlline Stte Type of Crystls : periodic rry of points in spce - lttice Lttice Crystl Bsis 90 Unit cell () (b) (c) Unit cell (d) Bsis plcement in unit cell (0,0) y (1/2,1/2) x Fig. 1.70: () A simple squre lttice. The unit cell is squre with side. (b) Bsis hs two toms. (c) Crystl = Lttice + Bsis. The unit cell is simple squre with two toms. (d) Plcement of bsis toms in the crystl unit cell.

13 UNIT CELL GEOMETRY CUBIC SYSTEM = b = c α = β = γ = 90 Mny metls, Al, Cu, Fe, Pb. Mny cermics nd semiconductors, NCl, CsCl, LiF, Si, GAs TETRAGONAL SYSTEM = b - c α = β = γ = 90 In, Sn, Brium Titnte, TiO 2 Simple cubic Body centered cubic Simple tetrgonl Fce centered cubic Body centered tetrgonl ORTOROMBIC SYSTEM - b - c α = β = γ = 90 S, U, Pl, G (<30 C), Iodine, Cementite (Fe 3 C), Sodium Sulfte Simple orthorhombic Body centered orthorhombic Bse centered orthorhombic Fce centered orthorhombic EXAGONAL SYSTEM = b - c α = β = 90 ; γ = 120 Cdmium, Mgnesium, Zinc, Grphite exgonl ROMBOEDRAL SYSTEM = b = c α = β = γ - 90 Arsenic, Boron, Bismuth, Antimony, Mercury (<-39 C) Rhombohedrl MONOCLINIC SYSTEM - b - c α = β = 90 ; γ - 90 TRICLINIC SYSTEM - b - c α - β - γ - 90 α Selenium, Phosphorus Lithium Sulfte Tin Fluoride Simple monoclinic Bse centered monoclinic Potssium dicromte Triclinic Fig. 1.71: The seven crystl systems (unit cell geometries) nd fourteen Brvis lttices.

14 - Fce-Centered Cubic (FCC) Structure: () FCC Unit Cell 2R (b) (c) Fig. 1.30: () 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 FCC unit cell. Exmples: Ag, Al, Au, C, Cu, γ-fe (>912 C), Ni, Pd, Pt, Rh

15 - Body-Centered Cubic (BCC) Structure: b Exmples: Alkli metls (Li, N, K, Rb), Cr, Mo, W, Mn, α-fe (< 912 C), β-ti (> 882 C). Fig. 1.31: Body centered cubic (BCC) crystl structure. () A BCC unit cell with closely pcked hrd spheres representing the Fe toms. (b) A reduced-sphere unit cell.

16 - exgonl Closed Pcked (CP) Structure: Lyer B () Lyer A Lyer B Lyer A Lyer A (b) Lyer A c (c) (d) Exmples: Be, Mg, α-ti ( < 882 C ), Cr, Co, Zn, Zr, Cd Fig. 1.32: The exgonl Close Pcked (CP) Crystl Structure. () The exgonl Close Pcked (CP) Structure. A collection of mny Zn toms. Color difference distinguishes lyers (stcks). (b) The stcking sequence of closely pcked lyers is ABAB (c) A unit cell with reduced spheres (d) The smllest unit cell with reduced spheres.

17 - Dimond & Zinc Blende Cubic Structure: C S Zn Fig. 1.33: The dimond unit cell is cubic. The cell hs eight toms. Grey Sn (α-sn) nd the elementl semiconductors Ge nd Si hve this crystl structure. Fig. 1.34: The Zinc blende (ZnS) cubic crystl structure. Mny importnt compound crystls hve the zinc blende structure. Exmples: AlAs, GAs, GP, GSb, InAs, InP, InSb, ZnS, ZnTe.

18 Tble 1.3 Properties of some importnt crystl structures Crystl nd R Structure (R is the rdius of the tom). Coordintion Number (CN) Number of toms per unit cell Atomic Pcking Fctor Exmples Simple = 2R None cubic BCC = 4R/ Mny metls: α-fe, Cr, Mo, W FCC = 4R/ Mny metls Ag, Au, Cu, Pt CP = 2R c = Mny metls: Co, Mg, Ti, Zn Dimond = 8R/ Covlent solids: Dimond, Ge, Si, α-sn. Zinc blende Mny covlent nd ionic solids. Mny compund semiconductors. ZnS, GAs, GSb, InAs, InSb NCl 6 4 ctions 4 nions CsCl 8 1 ction 1 nion 0.67 (NCl) Ionic solids such s NCl, AgCl, LiF MgO, CO Ionic pcking fctor depends on reltive sizes of ions. Ionic solids such s CsCl, CsBr, CsI

19 1.7.2 Crystl Directions nd Plnes c z Unit Cell Geometry z Unit cell x c β [001] O α γ b () A prllelepiped is chosen to describe geometry of unit cell. W e line the x, y nd z xes with the edges of the prllelepiped tking lower-left rer corner s the origin b y x c [111] x o z o P [121 b (b) Identifiction of direction in crystl y o y [010] [100] [111] [111] [010] [111] [111] [110] -y [110] - x y (c) Directions in cubic crystl system [111] [111] [111] [111] Fmily of <111> directions Fig. 1.39

20 - Directions of [uvw] Fmily of directions <100>; [100], [010], [001], [ 00], [0 0], [00 ] - Plnes of (hkl): Miller index, 1/intercept for ech xis Fmily of plnes {100}; (100), (010), (001), ( 00), (0 0), (00 ) x intercept t / 2 x z z intercept t z x Unit cell b c y y intercept t b () Identifiction of plne in crystl (010) y (010) (010) x z (010) Miller Indices (hk ) : 1 1 / (210) y (010) (001) (110) (111) (100) x (111) -z (b) Vrious plnes in the cubic lttice z y -y x z (110) Fig. 1.40: Lbelling of crystl plnes nd typicl exmples in the cubic lttice y

21 1.7.3 Three Phses of Crbon Covlently bonded lyer Cubic crystl Covlently bonded network of toms Lyers bonded by vn der Wl bonding Covlently bonded lyer exgonl unit cell () Dimond unit cell (b) Grphite The FCC unit cell of the Buckminsterfullerene crystl. Ech lttice point hs C 60 molecule (c) Buckminsterfullerene Fig. 1.42: The three llotropes of crbon. Buckminsterfullerene (C 60 ) molecule (the "buckybll" molecule)

22 1.8 Crystlline Defects Point Defects: vcncy concentrtion () Perfect crystl without vcncies (b) An energetic tom t the surfce breks bonds nd jumps on to new djoining position on the surfce. This leves behind vcncy. (c) An tom in the bulk diffuses to fill the vcncy thereby displcing the vcncy towrds the bulk. (d) Atomic diffusions cuse the vcncy to diffuse into the bulk. Fig. 1.43: Genertion of vcncy by the diffusion of n tom to the surfce nd the subsequent diffusion of the vcncy into the bulk.

23 () A vcncy in the crystl. (b) A substitutionl impurity in the crystl. The impurity tom is lrger thn the host tom. (c) A substitutionl impurity in the crystl. The impurity tom is smller thn the host tom. (d) An interstitil impurity in the crystl. It occupies n empty spce between host toms. Fig. 1.44: Point defects in the crystl structure. The regions round the point defect become distorted; the lttice becomes strined.

24 - Schottky defect (ction-nion pir missing) & Frenkel defect (host into interstitil position) Schottky defect Frenkel defect () Schottky nd Frenkel defects in n ionic crystl. Substitutionl impurity. Doubly chrged (b) Two possible imperfections cused by ionized substitutionl impurity toms in n ionic crystl. Fig. 1.45: Point defects in ionic crystls

25 1.8.2 Line Defects: Edge nd screw disloctions Edge disloction line () Disloction is line defect. The disloction shown runs into the pper. Compression Tension (b) Around the disloction there is strin field s the tomic bonds hve been compressed bove nd stretched below the isloction line Fig. 1.46: Disloction in crystl is line defect which is ccompnied by lttice distortion nd hence lttice strin round it.

26 A D C Disloction line () A screw disloction in crystl. Disloction line A B Atoms in the lower portion. Atoms in the upper portion. D (b) The screw disloction in () s viewed from bove. Fig. 1.47: A screw disloction involves shering one portion of perfect crystl with respect to nother portion on one side of line (AB). C

27 New molecule Fig. 1.49: Screw disloction ids crystl growth becuse the newly rriving tom cn ttch to two or three toms insted of one tom nd thereby form more bonds. Growth spirl on the surfce of polypropylene crystl due to screw disloction ided crystl growth. (SOURCE: Photo by Phillip Geil, Courtesy of Cse Western Reserve University.)

28 1.8.3 Plnr Defects: Grin boundries Nuclei Crystllite Liquid () (b) Grin Grin boundry (c) Fig. 1.50: Solidifiction of polycrystlline solid from the melt. () Nucletion. (b) Growth. (c) The solidified polycrystlline solid. For simplicity, cubes represent toms.

29 Foreign impurity Self-interstitil type tom Void, vcncy Strined bond Grin boundry Broken bond (dngling bond) Fig. 1.51: The grin boundries hve broken bonds, voids, vcncies, strined bonds nd "interstitil" type toms. The structure of the grin boundry is disordered nd the toms in the grin boundries hve higher energies thn those within the grins.

30 1.8.4 Crystl Surfces nd Surfce Properties 2 O Surfce Dngling bond Reconstructed surfce Absorbed Oxygen O 2 Surfce toms Bulk crystl Fig. 1.52: At the surfce of hypotheticl two dimensionl crystl, the toms cnnot fulfill their bonding requirements nd therefore hve broken, or dngling, bonds. Some of the surfce toms bond with ech other; the surfce becomes reconstructed. The surfce cn hve physisorbed nd chemisorbed toms.

31 1.10 Glsses nd Amorphous Solids Silicon (or Arsenic) tom Oxygen (or Selenium) tom () A crystlline solid reminiscent to crystlline SiO 2.(Density = 2.6 g cm -3 ) (b) An morphous solid reminiscent to vitreous silic (SiO 2 ) cooled from the melt (Density = 2.2 g cm -3 ) Fig. 1.56: Crystlline nd morphous structures illustrted schemticlly in two dimensions.

32 Dngling bond () Two dimensionl schemtic representtion of silicon crystl (b) Two dimensionl schemtic representtion of the structure of morphous silicon. The structure hs voids nd dngling bonds nd there is no long rnge order. (c) Two dimensionl schemtic representtion of the structure of hydrogented morphous silicon. The number of hydrogen toms shown is exggerted. Fig. 1.58: Silicon cn be grown s semiconductor crystl or s n morphous semiconductor film. Ech line represents n electron in bond. A full covlent bond hs two lines nd broken bond hs one line.

33 [omework] 1. Prob. # Prob. #1.18