8. Epitaxy. - Extended single-crystal film formation on top of a crystalline substrate

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1 8. Epitaxy 1. Introduction επι(epi placed or resting upon) ταξιζ(taxis arrangement) - Extended single-crystal film formation on top of a crystalline substrate - Homoepitaxy : Film and substrate are the same material. > free of defects > purer than the substrate > doping layers independent of the wafer 1

2 - Heteroepitaxy : Film and substrate are the different material. GaAs/AlGaAs InGaAs/GaAs dislocation Si on Insulation (SOI) : 3-D IC Selective epitaxy & lateral growth Multilayer heterojunction composites : superlattices, quantum wells 2

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4 2. Structural Aspects of Epitaxial Films 2.1 Single crystal surfaces The loss of periodicity in one direction will tend to alter surface electronic properties and leave dangling bond to promote chemical reactivity relaxation reconstruction 4

5 Reconstruction of Si(III) Si(III)-7 7 (STM image of Si(111)-7 7, Courtesy of J.Ha, ETRI) STM LEED, RHEED 5

6 2-2. Surface crystallography 3D : 14 Bravais lattices 2D : 5 Bravais lattices 6

7 a b i δ δ i ij ij * a = 2π δ j * b = = 1 j 0 if if i i = ij = 2π δ j ij j P : unit cell is primitive C : Centered lattice R: overlayer is rotated w.r.t substrate coordinate 7

8 2.3 Lattice Misfit & Imperfections in Epitaxial Films An important quantity that characterizes epitaxy is the lattice misfit f. f a0( s) a0( f ) = : a a ( f ) 0 : unstrained lattice parameter 0 positive f epitaxial film in tension (tensile strain) negative f epitaxial film in compression (compressive strain) 8

9 In Fe-GaAs system, the misfit in the [001] direction is f = a 0 ( GaAs) 2a 2a 0 ( Fe) 0 ( Fe) = = % compressive strain Pseudomorphic Growth : < 9 % : very thin films are elastically strained to have the same interatomic spacing as the substrate Coherent 9

10 Critical layer thickness : d c With increasing film thickness, the total elastic strain energy increases exceeding the energy associated with misfit dislocation Beyond d c, misfit dislocations are introduced Misfit dislocations lie in planes parallel to the interface The edge dislocation results from wedging in an extra row of atoms : the screw dislocations require cutting followed by shearing of the perfect crystal lattice 10

11 The critical film thickness prior to misfit dislocation formation is minimization of elastic strain energy E c and dislocation energy E d Matthews (1975) d c = b 8π (1 + ν ) f d ln( b c + 1) In practice, d c is thicker than predicted Bean (1988) d c 2 ( 1 ν ) b (1 + ν ) 8πwf 2 d ln( b c ) b:burgers vector, misfit f ν : Poisson s ratio 11

12 2. 4 Sources of defects in epitaxial Films 12

13 3. laser diode 13

14 * VCSEL(Vertical-Cavity Surface Emitting Lasers) 14

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17 Liquid Phase Epitaxy(LPE) The supersaturated melts are in contact with the growing films. LPE is a low-cost method compared to CVD and MBE. Poor thickness uniformity and rough surface morphology, - CVD and MBE are superior to LPE 4. Epitaxial growth methods 17

18 Seeded Lateral Epitaxial Film Growth over Insulators 3-d VLSI, isolation of high-voltage devices, radiation-hardened devices LEGO(lateral epitaxial growth over oxide) 18

19 VPE(Vapor Phase Epitaxy)<chapter 6> Chloride, Hydride, MOCVD VLE(vapor levitation epitaxy) RTCVD(rapid thermal CVD) 19

20 MOCVD(MOVPE) For details, read MOCVD in Chapter 6 (CVD) -High quality epilayers -High throughput -Highly selective 20

21 MBE(molecular beam epitaxy) 21

22 MBE MOMBE GSMBE 22

23 MBE (Molecular Beam Epitaxy) 23

24 Near Surface Transition Layer Cation surface Kinetics is critical hops/sec for monolayer growth 24

25 Ga + As 2 Ga + As 4 25

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27 In-situ film characterization : surface structures Low energy electron diffraction ev Reflection high energy electron diffraction KeV 27

28 RHEED intensity oscillation (specular beam) 28

29 Transition from island growth to step edge growth mode For GaAs(100) 2x4 - Initial increase of intensity [110], but no effect along [-110] 29

30 PLE of a trilayer (GaAs) 3 (AlAs) 3 superlattice structure 30

31 Importance of abrupt heterointerface 31

32 Migration Enhanced Epitaxy (MEE) Fig.6.22(a): Surface flatness on the atomic scale deteriorates (layer growth up to ~ 20 layers) Fig.6.22(b): Rapid migration of Ga in a very low arsenic pressure (~ thousands of layers) 32

33 Atomic Layer Epitaxy (ALE) Experiment 33

34 Principle of ALE 34

35 GaAs ALE vs. Pseudo ALE 35

36 GaAs ALE 36

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38 Advantages of ALE from self-limiting epitaxy - digital epitaxy (one atomic layer unit) - perfect uniformity - independent of substrate shape - perfectly flat interface Disadvantages of ALE - slow growth rate - self limiting process is not easy to find - residual impurities 38

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40 Spotty diffraction pattern (bulk, 3-D island) Streaky diffraction pattern (2-D island) Finite widths of reciprocal lattice rods and Ewald sphere due to thermal vibration and electron energy distribution 40

41 Modifications of MBE Source Ⅴ (solid) Ⅲ (solid) MBE Ⅲ (MO gas) MOMBE (Metalorganic MBE) Ⅴ (hydride gas) GSMBE (gas source MBE) CBE (Chemical Beam Epitaxy) Research trends in epitaxy - Selective area epitaxy (SAE) - low temperature growth - atomic layer control (ALE) 41