2. Semiconductor Crystals

Transcription

1 v.2018.aug 2. Semiconductor Crystals Contents 2.1 Semiconductors Elemental vs compound 2.2 Crystals lattice types directions & planes basic properties 2.3 Crystal Growth pulling & doping 2.4 Epitaxial Growth homo-, hetero-epitaxy lattice-match, -mismatch techniques: VPE, MBE 1

2 2.1 Semiconductors Conductivity s (-cm) Resistivity (r) and conductivity (s) of Solids Ag Cu 10 2 Au 20C Ge Si GaAs 10 5 r (-cm) s (-cm) -1 Electrical properties of semiconductors can be varied by orders of magnitude Resistivity r (-cm) diamond quartz W s, r are functions of - Impurity - Temperature - Electric Field - Optical Excitation Source: Science 281 (1998) 945 2

3 C o m p o u n d Semiconductors II(A) II(B) III(A) IV(A) V(A) VI(A) B C N O Mg Al Si P S Zn Ga Ge As Se Cd In Sn Sb Te Hg Pb Optoelectronics Elemental binary ternary IV-IV III-V II-VI IV-VI Si, Ge SiC, SiGe AlP, GaP, InP AlAs, GaAs, InAs AlSb, GaSb, InSb AlN, GaN, InN ZnO ZnS, CdS, HgS ZnSe, CdSe ZnTe, CdTe AlGaAs, AlInAs, GaAsP, GaInAs, GaInP, GaInN HgCdTe - SiGeC quarternary quinternary - - AlGaAsSb, GaInAsP AlInGaAsSb PbS PbSe PbTe Electronics/Power 5. Solar cells Classification: Elemental (ธาต ก งต วนา), Compound (สารประกอบก งต วนา) Applications/Industries: Electronics, Optoelectronics, New semiconductors New properties, applications, industries, lifestyles. 3

4 Energy Gap Electrons in semiconductors cannot occupy a certain energy range called energy gap or forbidden gap. E G T Semiconductors Insulators Ge Si GaAs Si 3 N 4 SiO ~ 5 ~ 9 Energy Gap (ev) (at 300 K) Electron energy at room temperature = k B T 26 mev Electron Volt (ev) Boltzmann s constant = 1.38x10-23 J/K 300 K k B T e J/C Energy obtained by an electron in a 1-V potential = 1 ev = 1.6x10-19 C V = 1.6x10-19 J. 26 mev note : unit check 2 1 Energy CV 2 Farad Volt electronic charge = 1.6x10-19 C Joule J F V J V C V 2 4

5 Energy Gap vs Electrical Properties Example: diode s built-in voltage (V 0 ) is approximately E G /e Energy Gap vs Optical Properties Example: LED color () is approximately 1.24/E G Photons in Energy = hf Absorption E C E V E G E C E V Emission In In Joules ev Photons out Energy = hf Planck: hc EG( J ) hf EG( ev ) ( m ) Direct GaAs (1.43 ev) 0.87 m III-V > Optoelectronics GaP (2.3 ev) 0.54 m LEDs Lasers Indirect (Electron) energy gap is related to photon energy through Planck relationship. Planck constant: h = J-s 5

6 Optical properties determined mainly by energy (E G ) and momentum (bandstructure, E-k) Electrical properties determined mainly by energy (E G ) and doping (controlled addition of impurities) The importance of doping: 1 ppm of impurity changes Si from bad to good conductor. (slight change in impurity --> large change in electrical properties) What happens at the atomic level? requires understanding of semiconductor crystals 2.2 Crystals most semiconductors most insulators most metals & alloys (a) (b) (c) SOLIDS: (Single-) Crystalline Amorphous Poly-Crystalline most stable (lowest energy) Atomic Arrangements: Periodic Random Grain Boundaries microscopic macroscopic periodic random 6

7 Examples 1. Solar Cells 2. MOSFET: TEM Transmission Electron Micrograph Metal-Oxide-Semiconductor Field-Effect Transistor (c) (b) (a) single crystal poly-crystal (c) (b) (a) single crystal, (b) amorphous, (c) polycrystal - all appear (needed) in semiconductor devices, but.. - device properties dictated by single crystal regions (a) 7

8 Definitions I (ก) lattice (ข) basis = (ค) Crystal structure (ก) (ข) (ค) (ก) แลตท ซ (ข) อะตอมม ลฐาน และ (ค) โครงสร างผล ก 8

9 Definitions II Periodic arrangment of atoms in a crystal > LATTICE A volume representative of entire lattice > UNIT CELL The smallest unit cell that can be repeated to form the lattice > PRIMITIVE CELL From the point of view of device applications, the unit cell is the easiest to work with. 9

10 Cubic Lattices simplest 3D lattice: cubic unit cell (ก) (ข) (ค) หน า ด าน a ม ม (1 atm, 20 degc)* Simple cubic (SC) Po Body-centered cubic (BCC) Cr, Fe, Na, K, W, V, Mo a = lattice constant, lattice parameter the spacing between atoms at one side of a cubic unit cell (~5-6 Å for typical semiconductors) Most metals (90%) crystallize into BCC, FCC, HCP Most semiconductors crystallize into diamond, zincblende (ZB), wurtzite (WZ) * allotropic transformation at non-standard conditions (elevated temperature, ) Face-centered cubic (FCC) Al, Ag, Au, Cu, Pt, Ni, Pb 10

11 Unit cell nearest atoms next nearest atoms Fraction of unit cell volume filled by atoms Periodicity attractive/repulsive FORCES DENSITY of solids allowed electron ENERGIES Lattice ---> MECHANICAL + ELECTRICAL properties We can analyze the crystal as a whole by investigating a representative volume (unit cell). 11

12 ตย. 2.1 : packing fraction / atomic packing factor (APF) - hard sphere atomic model - definition: volume of atoms in unit cell / volume of unit cell FCC 1 /8 th of an atom R Half of an atom a 2R R a a [fcc = 74%] [bcc = 68%] APF/Stability: SC (low), BCC (medium), FCC/HCP (high) The FCC unit cell. The atomic radius is R and the lattice parameter is a 12

13 Properties of crystals (metals, semiconductors) often depend on crystallographic orientation. Hence, need to specify planes / directions of crystals CRYSTALS : Plane ( ) ระนาบ find the intercepts 2, 4, 1 take reciprocal 1/2, 1/4, 1 reduce to smallest integers h, k, l 2, 1, 4 plane = (hkl) (214) h, k, l are called Miller indices Take reciprocal in order to avoid infinity in the notation; 0 thus means the plane is parallel to an axis. different planes different electrical / chemical properties: Chemical: etch rates Electrical: E br Si(100) for small-signal Si(111) for power click on (GaAs,Si).html 13

14 If a plane passes through the origin, translate it to a parallel position. Intercept at negative branch minus sign ( h -k l ) (110) (011) z x y ดกา น ด (111) (101) (002) (-100) Family of planes: { } วงศ ระนาบ { } = equivalent planes; e.g. {100} = (100), (-100), (010), (0-10), (001), (00-1) Interplanar spacing d: XRD - indirect determination of a STM - direct determination of a x z y [001] (001) d hkl [010] (010) (010) (100) ดกา น ด [100] h 2 d hkl a k 2 l 2 14

15 CRYSTALS : Direction [ ], family of directions < > ท ศ Direction is the same as vectors in that direction--reduced to smallest values and denoted in [ ]. วงศ ท ศ In cubic lattices, the direction [hkl] is perpendicular to the plane (hkl) < > = equivalent directions; e.g. <100> = [100], [-100], [010], [0-10], [001], [00-1] 15

16 LATTICES : Diamond & Zincblende most important lattice types: technologically, economically Diamond lattice can be thought of as an fcc structure with an extra atom placed at (a, b, c)/4 from each of the fcc atom = interpenetration fcc1 fcc2 2 sub-lattices : = Diamond (Si, Ge, C) Zincblende (GaAs) 16

17 electronics optoelectronics LATTICES : Si (Diamond) & GaAs (Zincblende) S a Zn a a The Zinc blende (ZnS) cubic crystal structure. Many important compound crystals have the zinc blende structure. Examples: AlAs, GaAs, GaP, GaSb, InAs, InP, InSb, ZnS, ZnTe. 17

18 Difficulty in growth/synthesis Degree of freedom Zincblende Compounds III-V Binary compounds: GaAs E g Ternary: Al x Ga 1-x As e.g. Al 0.3 Ga 0.7 As E g (x) GaAs Al x Ga 1-x As 0 x 1 AlAs Quarternary: In x Ga 1-x As y P 1-y E g (x,y) broad ranges of electronic and optical properties Compounds are widely employed in hetero-structures (C5, C8) 18

19 Most important graph for III-V optoelectronic material/device designs 19

20 ตย. 2.2 : Concentration / Density Silicon Smelter Given Si: a = cm how many Si atoms are there in 1 cm 2 on (100) plane? [areal concentration] how many Si atoms are there in 1 cm 3? [volume concentration] 1ppm corresponds to atoms/cm 3. Volume concentration (/cm 3 ) density (g/cm 3 ) Areal concentration (/cm 2 ) # surface state density, chemical reactivity Linear (/cm) [ atoms/cm 2 ] [ atoms/cm 3 ] [ atoms/cm 3 ] 20

21 ตย. 2.3 : Densities volume concentration Density = atoms / mole weight / mole (number of atoms / unit volume) (atomic weight) Avogadro s constant a S Zn = weight / volume The Zinc blende (ZnS) cubic crystal structure. Many i compound crystals have the zinc blende structure. Ex GaAs, GaP, GaSb, InAs, InP, InSb, ZnS, ZnTe. a a Vol. concentration (atoms/cm 3 ) Material Crystal structure Lattice constant (Å) Atomic weight (g/mole) Density (g/cm 3 ) Si diamond 5.43 Si (28.1) 2.33 GaAs zincblende 5.65 Ga (69.7) As (74.9) InP zincblende 5.87 In (114.8) P (31)

22 ELECTRONICS MATERIALS single-crystal Si - large single crystals device grade - high purity (0.1ppb) Electronic performance Economy of scale DEVICES junctions (p-n, metal-semiconductor) BJTs, FETs optoelectronics 22

23 2.3 Crystal Growth 1. 1,800 C SiO 2 + 2C > 2Si + 2CO quartz Raw Silicon (Si) 100s s ppm Metallurgical Grade Silicon impurity level (Fe, Al, etc.) MGS 5x10 16 cm Si + 3HCl > SiHCl 3 +H 2 + [FeCl ] MGS coke (from coal, wood chips) trichlorosilane liquid with BP = 32 C 3. SiHCl 3 + 2H > 2Si + 6HCl 5x10 ppb 13 cm impurity -3 level MGS main applications: Aluminum casting industry (Al-Si alloy), chemical industry (fumed silica) Arc Discharge chlorides of impurities with differrent BP Fractional Distillation Electronic Grade Silicon EGS poly-si 23

24 Single Crystal Ingots poly-si Polycrystalline EGS is slowly lowered into crucible: poly- upon pulling single-crystal automatic lowest energy configuration Note: Si 1,412C Slowly/rotating ingots > Czochrarlski (Si, Ge ) Slowly/rotating ingots + B 2 O > (e.g., GaAs) Liquid Encapsulated Czochrarlski (LEC) single-crystal Si 24

25 Single Crystal Ingots (contd.) wire saw machine Wafers becoming larger and moving towards 450 mm diameter now f= 300 mm (12 ) 25

26 Economy of Scale quartz (cents) Si Wafer ($ 100s) ~ 500 m thick Sawn into 100s of wafers consumers: (100)-Si industrial: (111)-Si Ingots / boules Scribed into 100s of chips CPU, DRAM, ASIC Each chip ($ 100s) 26

27 Doping : Intentional addition of impurity during Cz crystal growth Impurities redistribute themselves at solidifying interface สปส. การแยกต วสมด ล equilibrium segregation coefficient function of material, impurity, temperature at solid-liquid interface, growth rate k o C C ตย. 2.4 : Wanted: Si doped with phosphorus atoms / cm 3. Given: For P in Si, k o = 0.35 How much P (in atoms / cm 3 ) do we need in liquid? Have 5 kg of Si melt, how much P (in grams) should be added? Atomic weigth of P = 31 (g/mol) S L concentration of impurity in Solid (ingot) concentration of impurity in Liquid k d, k o : distribution coefficient [Ans: 3.2 mg] 27

28 2.4 Epitaxial Growth (on substrate) a means to change doping / material(s) after bulk crystal growth epi- on taxis- arrangement substrate film a 1 a 1 = a 2 a 2 substrate same or similar lattice structure to grown layer film Technique: Single-crystal layer growth on single-crystal substrate (same structure/orientation) The technique is called Epitaxial growth or Epitaxy The thin film layer is called Epilayer Growth can be done at low substrate temperatures Methods include growth from : vapour Chemical Vapour Deposition (CVD), Vapor Phase Epitaxy (VPE) melt Liquid Phase Epitaxy (LPE) evaporation Molecular Beam Epitaxy (MBE) homo-epitaxy: film = substrate; e.g. Si on Si (or GaAs on GaAs) hetero-epitaxy: film substrate; e.g. SiGe on Si (or AlGaAs on GaAs) 28

29 Hetero-epitaxy Epi-layer is different from substrate easy if a 1 a 2 a(gaas) ~ a(alas) ~ a(ge) ~ 5.65 Å a(gaas) ~ a(alas) Al x Ga 1-x As on GaAs Lattice match difficult if a 1 a 2 thin: pseudomorphic thick (> critical thickness) dislocations Lattice mismatch Heteroepitaxy is widely used (necessary) to grow optoelectronic materials Though lattice matching is preferred, lattice mismatch often cannot be avoided 29

30 Lattice Match GaP (I) (I) Commercial substrates are limited (by economy of scale) to: (I) 1-element: Si, Ge 2-element: GaAs, InP, GaP (D) (D) on GaAs: AlGaAs, In 0.5 Ga 0.5 P on InP: In 0.53 Ga 0.47 As, GaAsSb (D), AlAsSb (I) (D) (D) Direct (I) Indirect (D) Lattice-matched growth: limited number of materials Lattice-mismatched growth: broader range of materials, hence more useful, but 30

31 Critical thickness (nm) Lattice Mismatch ,000 Critical thickness (Å) Ge x Si 1-x /Si 100 Edge dislocation line (a) Dislocation is a line defect. The dislocation shown runs into 10 Ga 1-x In x As/GaAs Molar fraction (x) Compressi Tension - lattice mismatch may result in dislocations (b) Around the dislocation there is a strain field as the atomic bo been compressed above and stretched below the islocation line - dislocations trap carriers (non-radiative recombination centres), Dislocation in a crystal is a line defect which is accomp degrading electrical/optical properties distortion and hence a lattice strain around it. 31

32 Ex: growth of GaAs 0.6 P 0.4 on GaAs (for Red LED) m active / light-emitting layer buffer / grading layer GaAs substrate % P mechanical support / seeding layer 0 Cannot be grown directly on GaAs or GaP substrates Requires a grading layer: P gradually introduced till achieve desired P/As ratio (40/60) Desired epi-layer is then grown on the graded layer For the grading layer lower part matches substrate upper part matches the active layer (GaAs 0.6 P 0.4 ) Dislocations in, not active area is high quality Brief History of Light from solids: * incandescent (metal filaments) - Pt (1802), C (1880), W (1904), Ta (1908) * first electroluminescence in - IV-IV: carborundum (SiC) III-V: GaAsP

33 2.4.3 Vapour Phase Epitaxy (VPE)* Low temperatures and high purity epitaxial growth Best GaAs layers are grown by this method Si IC devices built in layers grown by VPE on Si wafers Sharp junctions possible with this method Reaction chamber / Reactor * General term is CVD but when growth results in single crystal, it is called VPE H 2 + SiCl 4 + other gases containing desired impurities rf coils furnace gas sources gas out wafers wafers holder valve UHV load chamber pump rf coils 33

34 VPE of Si SiCl 4 + 2H ๐ C deposit etch Process is reversible. Etching prior to deposition is possible ---> atomically clean surface. Stick to the substrate Si + 4HCl Low temperature process desirable (reduce dopant diffusion / cost): - SiH 2 Cl 2 ( C) - SiH 4 Si + 2H 2 ( C) Pyrolysis Remain gaseous at reaction temperature VPE of GaAs metal-organic (MOVPE) organometallic (OMVPE) or MOCVD (CH 3 ) 3 Ga + AsH 3 GaAs + 3CH 4 organic metal 34

35 2.4.4 Molecular Beam Epitaxy (MBE) gate valve high-energy electron gun ionization gauge UHV manipulator substrate main shutter fluorescent screen Substrate (GaAs) held at low temperatures (600 ๐ C). Ultra-High Vacuum (UHV) Melts in cells controlled by shutters. Slow growth rate ---> precise layer control (order of lattice constant) cryoshroud molecular beams cell shutters (ก) AlAs (ข) GaAs viewport Ga In (6N*) (6N) *6 nines ( %) (eye/pyrometer) Si (dopant) effusion cell As (7N) AlAs GaAs quantum wells InAs/GaAs quantum dots 35

36 2.5 Conclusions r, s of semiconductors vs metals, insulators semiconductors: elementals and compounds (binary, ternary, ) semiconductor properties: lattice: cubic, diamond, zincblende crystallography: planes ( ) { }, directions [ ] < >, concentration, density electrical/optical: energy gap, Planck semiconductor synthesis: growth: Czochralski, doping epitaxy: homo- vs hetero-, VPE (CVD), MBE 36