Lecture contents. Heteroepitaxy Growth technologies Strain Misfit dislocations. NNSE 618 Lecture #24

Transcription

1 1 Lecture contents Heteroepitaxy Growth technologies Strain Misfit dislocations

2 Epitaxy Heteroepitaxy 2 Single crystalline layer on Single crystalline substrate Strong layer-substrate interaction orientation Surface diffusion of adatoms Clean process semiconductors Heterostructures Major challenge: Tailor electronic and/or optical properties Bandgap engineering Carrier confinement (DHS, 2DEG...) Wavefunction engineering Creating wavefunction logic Optical elements engineering Integrated waveguides

3 Bulk semiconductor crystal growth methods 3 From khup.com

4 Chemical vapor epitaxy 4 Si epitaxy: Chlorine process Chemical reaction on a heated substrate - SiCl 4 process - Metal-Organic Chemical Vapor Deposition (MOCVD) - Low vapor pressure - Weak bond with metal (organic byproducts are pumped away) - Carbon and Hydrogen incorporation are sometimes a problem III-As MOCVD Trimethylaluminum (actually it exists as a dimer Al 2 (CH 3 ) 6

5 Commercial Thomas Swan MOCVD 5

6 Physical vapor epitaxy: Molecular Beam Epitaxy (MBE) Atomic structure of a 90 0 dialocation - UHV evaporator (~10-10 Torr) - Ultra-high purity materials (6N s) - Molecular beams due to UHV mean free path ~hundreds of meters) - Adatom diffusion on the substrate (hot substrate) for epitaxy and low defect density - Flux is controlled by effusion cell temperature and shutters - Elaborated cells: valved cracker fro As, Sb; e-beam evaporation cells, chemical cells. Cryo-shrouds Effusion Cells Loadlock Outgasing stage Sample holder 6

7 Commercial Veeco GEN2000 MBE system 7

8 Strain in heterostructures 8 Lattice mismatch: Strain relaxed: Strain: NNSE 618 Ref: Singh Lecture #24

9 Strain in Heterostructures 9 Misfit strain (0-10%) due to lattice mismatch accommodates at high (growth) temperatures a a s a s f Thermal strain (~ 10-3 ) due to difference in thermal expansion coefficients accommodates at lower temperatures s f T

10 How to Deal with Misfit Strain? Strain coherent energy : Prevent plastic deformation Allow plastic deformation Energy 2 t 2 cm 1 Decrease misfit and thermal strain Limit the thickness Strained layers/superlattices Limit lateral dimensions Quantum wires and quantum dots Increase the fraction of 90 0 dislocations Suppress threading dislocations Dislocation filters (MQW) Thick buffer layers

11 11 Heterostructures with Low Defect Density Low mismatch (<0.5-1%) High mismatch (>2%) Search for latticematched materials (for Si): old - GaP new - SiGeC Prevention from dislocation nucleation Reduction of dislocation density in the active region Reduction of thickness: strained layers Reduction of lateral size: 3D islands Thick buffer layers Graded buffer layers Usage of dislocation filters: SLS

12 Biaxial strain accommodation 12 Elastic: Roughness, Islanding Morphological instability (ratio of bulk and surface energy): 2 t 4 2 E 2 t Plastic: Introducing defects (Misfit Dislocations) Equilibrium separation: d b int b int a a f a s

13 Growth morphology 13 2D (layer-by-layer) growth => smooth surface morphology s 2 12 s 2 < s 1 3D (Volmer-Weber or Stranski-Krastanov) growth mode s 2 12 s 2 > s 1 is thermodynamically equilibrium for most heterostructures, in particular with high mismathch Ge on Si (4% mismatch) kinetics thermodynamics a poly 2D s 2 3D S 2 s 1 12 o 170 C o 350 C Islanding can be suppressed by kinetics => higher supersaturation (low temperature and high growth rate)

14 Dislocations: Equilibrium misfit dislocation structure 14 (001) - diamond and/or zinc-blende heterostructures (Si, Ge, III-V, II-VI semiconductors) Rectangular array of 90 o (Lomer) dislocations Low-energy dislocations in diamond and zinc-blende (001) structures [110] b=1/2[110] Also the most desirable defect structure: - most effective for MS relief - glide plane (001) => sessile dislocation - no dangling bonds => electrically inactive!?

15 15 Atomic structure of a 90 0 dislocation

16 Nucleation of misfit dislocations 16 - critical thickness half-loops 60 o misfit segments glide of threading segments [110] - Grown-in threading dislocations critical thickness Elongating 60 o misfit segment glide of threading segments b=1/2[011] - 3D growth critical thickness 90 o misfit segment at the island edge overgrowth and glide of threading segments

17 154 nm In 0.07 Ga 0.93 As on GaAs Critical thickness From force balance for dislocation bending 17 From Maree, 1987 From energy balance for dislocation array From energy balance From People and Bean, 1985

18 NNSE 618 Recap 18 Optics and recombination: (E), I(E) Defects: E D, E A Bandstructure: E(k) Statistics: f(e) Scattering: t(e) Bandengineering:, x, f Junctions: f, J